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High efficient luminescence in type-II GaAsSb-capped InAs quantum dots upon annealing
Abstract
The photoluminescence efficiency of GaAsSb-capped InAs/GaAs type II quantum dots (QDs) can be greatly enhanced by rapid thermal annealing while preserving long radiative lifetimes which are ∼20 times larger than in standard GaAs-capped InAs/GaAs QDs. Despite the reduced electron-hole wavefunction overlap, the type-II samples are more efficient than the type-I counterparts in terms of luminescence, showing a great potential for device applications. Strain-driven In-Ga intermixing during annealing is found to modify the QD shape and composition, while As-Sb exchange is inhibited, allowing to keep the type-II structure. Sb is only redistributed within the capping layer giving rise to a more homogeneous composition.
... To address this issue, tin (Sn 2+ ) has emerged as a promising alternative to lead (Pb 2+ ) due to its similar electronic properties and environmentally friendly nature [11][12][13]. Numerous studies have probed into the intriguing properties of 0D tin halide perovskites, including Cs 4 SnX 6 (where X = Br, Rapid thermal treatment (RTT) is a widely used technique to modify the micronanostructure and enhance the optoelectronic properties of materials [29]. RTT involves rapid heating and cooling, characterized by short heating times and accelerated cooling rates. ...
... The excitation power-dependent PL intensity is commonly employed to determine the underlying mechanism of light emission in semiconductors. As per the literature [29,30], the PL intensity (I) can be described by the equation I = ηI k 0 , where I 0 denotes the excitation power, η symbolizes the emission efficiency, and the exponent k is affiliated with the radiative recombination process. A linear fit of ln(I/η) in contrast to ln(I 0 ) allows the estimation of the k parameter values as 1.11 and 1.17 for the pristine sample and sample S-120 s, respectively. ...
... The excitation power-dependent PL intensity is commonly employed to determine the underlying mechanism of light emission in semiconductors. As per the literature [29,30], the PL intensity (I) can be described by the equation I ηI , where I0 denotes the excitation power, η symbolizes the emission efficiency, and the exponent k is affiliated with the radiative recombination process. A linear fit of ln(I/η) in contrast to ln(I0) allows the estimation of the k parameter values as 1.11 and 1.17 for the pristine sample and sample S-120 s, respectively. ...
Zero-dimensional (0D) tin halide perovskites feature extraordinary properties, such as broadband emission, high photoluminescence quantum yield, and self-absorption-free characteristics. The innovation of synthesis approaches for high-quality 0D tin halide perovskites has facilitated the flourishing development of perovskite-based optoelectronic devices in recent years. However, discovering an effective strategy to further enhance their emission efficiency remains a considerable challenge. Herein, we report a unique strategy employing rapid heat treatment to attain efficient self-trapped exciton (STE) emission in Cs4SnBr6 zero-dimensional perovskite. Compared to the pristine Cs4SnBr6, rapid thermal treatment (RTT) at 200 °C for a duration of 120 s results in an augmented STE emission with the photoluminescence (PL) quantum yield rising from an initial 50.1% to a substantial 64.7%. Temperature-dependent PL spectra analysis, Raman spectra, and PL decay traces reveal that the PL improvement is attributed to the appropriate electron–phonon coupling as well as the increased binding energies of STEs induced by the RTT. Our findings open up a new avenue for efficient luminescent 0D tin-halide perovskites toward the development of efficient optoelectronic devices based on 0D perovskites.
... We have proven in the past that a rapid thermal annealing treatment (RTA) is beneficial for these nanostructures, not only blueshifting their fundamental energy and narrowing and boosting their emission as shown in Fig. 1(b), but also maintaining the type-II band alignment. [23] The same RTA treatment was applied in this case. Regarding the QD structural properties, transmission electron microscopy reveals that the QDs become flatter (more quantum disk-like) after annealing. ...
... (b) Photoluminescence spectrum obtained at 15 K in a similar sample before and after applying a rapid thermal annealing treatment. [23] using a 9 T superconducting magnet. In the latter case, light polarization was also analyzed in the σ + and σ − basis. ...
... We obtain long decay times, in excess of 6 ns, confirming that carriers exhibit type-II confinement after the RTA. [23] The full evolution of the decay time with the external electric field is represented in Figure 3(b). For each point, the decay time and its statistical error were estimated at the maximum of the emission following the Stark shift of the ground state. ...
We present an experimental and theoretical study about the carrier confinement geometry and topology in InAs/GaAsSb quantum dots. The investigated sample consists of a field-effect device embedding a single layer of dot-in-a-well InAs/GaAsSb nanostructures. These nanostructures exhibit large electron-hole dipole moments and radiative lifetimes under externally applied electric fields. Both phenomena are related to the type-II band alignment existing between the two materials which, in principle, could also result in a change of the hole orbital confinement topology from simply to doubly connected. The latter aspect will be confirmed by ensemble magnetophotoluminescence experiments at 4.2 K. The oscillations observed in the photoluminescence intensity and degree of circular polarization will be described by an axially symmetric \(\mathbf {k}\cdot \mathbf {p}\) model combining vertical electric and magnetic fields. Due to the large spin-orbit coupling of III-Sb nanostructures, the modulation of the orbital confinement geometry and topology reported here shall open a venue to control the spin dynamics by external voltages. This exciting idea will be theoretically discussed through band-effective models including spin-orbit coupling and anisotropic confinement effects.
... The ground state wave function of holes in type-II InAs quantum dots (QDs) with a GaAs 1−y Sb y capping layer (CL) resides outside of the dot volume and in general has the form of two mutually perpendicular pairs of segments [1,2]. Remarkably, the pair oriented along the [110] crystallographic direction is positioned close to the QD base, while the other, oriented along [1][2][3][4][5][6][7][8][9][10], is located above the dot [1][2][3][4][5]. Both the vertical position and the orientation of the hole wave function have been recently discussed in the literature [3][4][5][6][7][8]. ...
... Remarkably, the pair oriented along the [110] crystallographic direction is positioned close to the QD base, while the other, oriented along [1][2][3][4][5][6][7][8][9][10], is located above the dot [1][2][3][4][5]. Both the vertical position and the orientation of the hole wave function have been recently discussed in the literature [3][4][5][6][7][8]. It has been shown elsewhere [1,5] that the potential minimum for holes in this system results from a delicate interplay between quantum size effect and the piezoelectric potential and is, thus, sensitive to the thickness d of CL [3,5]. ...
... However, further increase of d results in an increase ofz hole and C max . Also a flip of the orientation α max to [1][2][3][4][5][6][7][8][9][10] direction for d c is observed and this is attained for even larger values of d. While α max is identical with the orientation of the dominant pair of segments of the hole wave function, i.e., those with the largest overlap with the electrons, C max corresponds to the ratio of overlaps of both pairs with them, see also Fig. 1(a). ...
We study the polarization response of the emission from type-II GaAsSb capped InAs quantum dots. The theoretical prediction based on the calculations of the overlap integrals of the single-particle states obtained in the (k) over right arrow . (p) over right arrow framework is given. This is verified experimentally by polarization resolved photoluminescence measurements on samples with the type-II confinement. We show that the polarization anisotropy might be utilized to find the vertical position of the hole wave function and its orientation with respect to crystallographic axes of the sample. A proposition for usage in the information technology as a room temperature photonic gate operating at the communication wavelengths as well as a simple model to estimate the energy of fine-structure splitting for type-II GaAsSb capped InAs QDs are given.
... An alternative approach to control the Sb distribution is the use of postgrowth thermal annealing that can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAsSb type II QDs [18]. Recently, we have reported about the effect of rapid thermal annealing (RTA) on this kind of sample where large blueshifts and a strong enhancement of emission efficiency were observed while preserving the long radiative lifetimes [19]. However, there was a lack of knowledge about the processes involved during annealing. ...
... Previous results had shown that the incorporation of a high Sb content in the SRL extended the emission wavelength of InAs QDs to 1345 nm (at 15 K) and induced a type II band alignment, as revealed by the large radiative lifetime of 14.2 ns (further details of PL and time-resolved PL of these samples can be found in [19]). More important, the RTA retained the type II emission (radiative lifetime of 11.8 ns) while producing a significant enhancement of the PL, increasing both the integrated intensity (by a factor of 3.3) and monochromaticity (FWHM reduced from 97 meV to 32 meV, i.e., by 67%). ...
... In these images, the wetting layer (WL) and QDs can be clearly distinguished with a darker contrast than GaAs, while the capping layer exhibits a brighter contrast. Our measurement on more than 60 QDs [19] showed that the RTA gave way to an important reduction of the average height (from 3.3 ± 0.6 to 2.3 ± 0.4 nm) together to an increase of the base diameter (from 16 ± 4 nm to 22 ± 7 nm). In addition, the thickness of the SRL was reduced by approximately 1 nm, i.e., from 6.5 to 5.5 nm in areas far from the QDs. ...
Type II emission optoelectronic devices using GaAsSb strain reduction layers (SRL) over InAs quantum dots (QDs) have aroused great interest. Recent studies have demonstrated an extraordinary increase in photoluminescence (PL) intensity maintaining type II emission after a rapid thermal anneal (RTA), but with an undesirable blueshift. In this work, we have characterized the effect of RTA on InAs/GaAs QDs embedded in a SRL of GaAsSb by transmission electron microscopy (TEM) and finite element simulations. We find that annealing alters both the distribution of Sb in the SRL as well as the exchange of cations (In and Ga) between the QDs and the SRL. First, annealing causes modifications in the capping layer, reducing its thickness but maintaining the maximum Sb content and improving its homogeneity. In addition, the formation of Sb-rich clusters with loop dislocations is noticed, which seems not to be an impediment for an increased PL intensity. Second, RTA produces flatter QDs with larger base diameter and induces a more homogeneous QD height distribution. The Sb is accumulated over the QDs and the RTA enlarges the Sb-rich region, but the Sb contents are very similar. This fact leaves the type II alignment without major changes. Atomic-scale strain analysis of the nanostructures reveal a strong intermixing of In/Ga between the QDs and the capping layer, which is the main responsible mechanism of the PL blueshift. The improvement of the crystalline quality of the capping layer together with higher homogeneity QD sizes could be the origin of the enhancement of the PL emission.
... For example, the introduction of varying metal ions into 0D metal halide perovskites can amplify the distortion of [BX6] 4− octahedra and bolster electron-phonon coupling, consequently enhancing the density of STE states and boosting luminescence efficiency [26]. In previous work, we successfully expanded the emission spectra and amplified the emission efficiency of STEs in Cs4SnBr6 through an innovative Mn 2+ doping strategy [28]. This approach imbued the Mn 2+ -doped Cs4SnBr6 with remarkably enhanced PL QY of up to ~75%, a broader emission spectrum, and increased thermal stability. ...
... The Raman spectrum shown in the inset of Figure 7(c) reveals two dominant phonon modes, corresponding to the Sn-Br stretching vibrational modes at approximately 130 cm -1 and 220 cm -1 [37,38], respectively, which may be involved in the electron-phonon coupling and thus result in the S value as large as 63.7. After annealing the sample at a RTT temperature of 200°C for 120 s, the value of S is significantly reduced to 46.1, which closely resembles that observed in Cs4SnBr6 [28]. This reduction can be attributed to the disappearance of the Sn-Br stretching vibrational mode near 240 cm -1 (see the inset of Figure 7(d)) resulting from the enhanced crystallinity of the Cs4SnBr6 powders after the RTT process, as evidenced by the X-ray diffraction (XRD) patterns (refer to Figure 5). ...
Zero-dimensional (0D) tin halide perovskites feature extraordinary properties, such as broadband emission, high photoluminescence quantum yield, and self-absorption-free characteristics. The innovation of synthesis approaches for high-quality 0D tin halide perovskites has facilitated the flourishing development of perovskite-based optoelectronic devices in recent years. However, discovering an effective strategy to further enhance their emission efficiency remains a considerable challenge. Herein, we report a unique strategy employing rapid heat treatment to attain efficient self-trapped exciton (STE) emission in Cs4SnBr6 zero-dimensional perovskite. Compared to the pristine Cs4SnBr6, rapid thermal treatment (RTT) at 200°C for a duration of 120 seconds results in an augmented STE emission with photoluminescence (PL) quantum yield rising from an initial 50.1% to a substantial 64.7%. Temperature-dependent PL spectra analysis, Raman spectra, and PL decay traces reveal that the PL improvement is attributed to the appropriate electron-phonon coupling as well as the increased binding energies of STEs induced by the RTT. Our findings open up a new avenue for efficient luminescent 0D tin-halide perovskites toward the development of efficient optoelectronic devices based on 0D perovskites.
... Consequently, due to the rotational symmetry along the direction of growth, a necklace richer in Sb is formed around the vertex. This behaviour has recently been foreseen for similar structures, especially for the case of annealed samples [11,12]. ...
... Thus, it is said that Sb limits the In mobility, having a shield effect on the QD erosion and avoiding the QD coalescence, which are supposed to be the origins of In-rich agglomerations [21,24]. However, though many initial studies reported of a QD protection effect of Sb by limiting the In outdiffusion [11,12,26], there are other recent outcomes pointing to the opposite [42,43]. The reason comes from the fact that there are at least two atomic-scale mechanisms that could operate under the term "surfactant" [44e46], having opposite effects regarding the In mobility. ...
The implementation of GaAs0.8Sb0.2 as CL to obtain type-II strain-coupled InAs MQD structures has been examined and compared to similar structures without Sb or without strain coupling. First, it has been demonstrated that capping with GaAsSb prevents the formation of In-rich agglomerations that hampered the QD formation as it has been observed in the sample without Sb. Instead, it promotes the vertical alignment (VA) of almost all QDs with a high density of QD columns. Second, there is a preferential Sb accumulation over the dots together with an undulation of the growth front, contrary to the observed in the uncoupled structure. In case of a deficient covering of GaAsSb, as occurs for giant QDs, In-rich agglomerations may develop. Each VAQD column consists of a sequence of alternating quantum blocks of pyramid-shaped In(Ga)As separated by GaAsSb blocks that rest over them. These Sb-rich blocks are not homogeneous accumulating around the pyramidal apex like a collar. Between the columns, there is an impoverishment of In and Sb compared to the uncoupled sample. These columns can behave as self-aligned nanowires with type II band alignment between self-assembled InAs and GaAsSb quantum blocks that opens new opportunities for novel devices.
... After the formation of the QDs, they are covered by a 6-nm-thick GaAs x Sb 1−x layer with 28% nominal content of Sb. More details about the growth recipe and the morphological changes induced by rapid-thermalannealing (RTA) treatment of these QDs can be found elsewhere [40]. After the RTA treatment, mesas of different sizes and ohmic contacts are defined by conventional optical-lithography techniques. ...
... The numerical results are obtained for a quantum dot of the same size and composition as those analyzed in our previous studies [34,40]. The geometry of the In x Ga 1−x As QD is a lens shape of radius R QD = 11 nm and height H QD = 3 nm. ...
We investigate how the voltage control of the exciton lateral dipole moment induces a transition from singly to doubly connected topology in type-II InAs/GaAsxSb1−x quantum dots. The latter causes visible Aharonov-Bohm oscillations and a change of the exciton g factor, which are modulated by the applied bias. The results are explained in the frame of realistic k→⋅p→ and effective Hamiltonian models and could open a venue for new spin quantum memories beyond the InAs/GaAs realm.
... We obtain long decay times, in excess of 6 ns, confirming that carriers exhibit type II confinement after the rapid thermal annealing treatment. 29,30 The full evolution of the decay time with the external electric field is represented in Figure 3(b). For each point, the decay time and its statistical error were estimated at the maximum of the emission, i.e. following the Stark shift of the ground state. ...
... where we introduce the electron effective mass of compressed InAs, m * e = 0.1, the GaAs band edge at the QD base, E c = 0.765 eV, H = 2.3 nm, for the QD height determined independently, 29 and E e (F ) for the electron ground state energy given by the model. The resulting τ tun is depicted by the dotted line in Figure 3(b). ...
External control over the electron and hole wavefunctions geometry and
topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb
quantum structure with type II band alignment. We find highly tunable exciton
dipole moments and largely decoupled exciton recombination and ionization
dynamics. We also predicted a bias regime where the hole wavefunction topology
changes continuously from quantum dot-like to quantum ring-like as a function
of the external bias. All these properties have great potential in advanced
electro-optical applications and in the investigation of fundamental spin-orbit
phenomena.
... Further improvements in the material quality involve using rapid thermal annealing (RTA) for enhancing the optical properties of the InAs/GaAs(Sb) QD structures through the reduction of the crystal defect density. Although examinations of the effects of the RTA process on a single-layer InAs/GaAs(Sb) QD structure have been completely investigated [27,28], studies regarding the effects of thermal annealing on the vertically-aligned InAs/GaAsSb QD structure are scarce. In this study, the columnar dot structure was combined with GaAsSb strain-reducing layers (SRLs) as a vertically aligned InAs/GaAsSb columnar QD structure with ten dot layers. ...
... Typically, the energy band alignment of single InAs/GaAsSb QD layer is tailored from staggered (type-II) to straddling (type-I) gap by In-Ga intermixing after high-temperature annealing [27,28]. However, the time-resolved photoluminescence (TRPL) results in this study showed a noticeable extension of the carrier lifetime from 4.7 to 9.4 ns, and the power-dependent PL (PDPL) results reveal a band bending behavior in the annealed columnar InAs/GaAsSb QD structure with an Sb content of 10%, indicating the tailoring of the energy band alignment from type-I to type-II by RTA, which was less discussed previously. ...
This study presents an band-alignment tailoring of a vertically aligned InAs/GaAs(Sb) quantum dot (QD) structure and the extension of the carrier lifetime therein by rapid thermal annealing (RTA). Arrhenius analysis indicates a larger activation energy and thermal stability that results from the suppression of In-Ga intermixing and preservation of the QD heterostructure in an annealed vertically aligned InAs/GaAsSb QD structure. Power-dependent and time-resolved photoluminescence were utilized to demonstrate the extended carrier lifetime from 4.7 to 9.4 ns and elucidate the mechanisms of the antimony aggregation resulting in a band-alignment tailoring from straddling to staggered gap after the RTA process. The significant extension in the carrier lifetime of the columnar InAs/GaAsSb dot structure make the great potential in improving QD intermediate-band solar cell application.
... This is due to the absence of In-related defects and lower N content to achieve the same bandgap. Furthermore, the conduction and valence band energies can be adjusted independently while it is lattice-matched to GaAs [5,7]. Another favourable aspect concerns the fabrication. ...
Dilute nitrides are semiconductor alloys, obtained from the conventional III-V compounds by incorporating a small amount of nitrogen. In this work, we focus on GaAsSbN considered as a perspective material for incorporation in multijunction solar cells. Nitrogen creates a localized level inside the conduction band continuum. The interaction of this level with the conduction band is usually described by the single band anti-crossing (BAC) model. The double BAC model of GaAsSbN considers both the N and the Sb localized levels in the conduction and the valence band, respectively. We calculate the bandgap energy of GaAsSbN employing the double BAC model for different concentrations of Sb and N. Parameters of the BAC model taken from different literature sources are used in the calculations and their influence on the final result is explored. Finally, the calculated bandgap energies are compared to experimental data of GaAsSbN layers grown on n-GaAs substrates by low-temperature liquid phase epitaxy. These data include the optical absorption edge of the material determined by surface photovoltage spectroscopy and the energy position of the photoluminescence peak at room temperature.
... Most of the present applications in optics are based on so-called type-I QDs, which show direct electron-hole recombination in both real and k space, as for In(Ga)As QDs embedded in a GaAs matrix. Much less attention has been given to type-I indirect and/or type-II QDs, particularly antimony-based ones, like In(Ga)As QDs overgrown by a thin Ga(AsSb) layer [26][27][28][29][30][31], or In(Ga)Sb QDs in a GaAs matrix [32][33][34][35][36], which show spatially indirect optical transitions. Such structures generally require more challenging growth * steindl@physics.leidenuniv.nl ...
The optical response of (InGa)(AsSb)/GaAs quantum dots (QDs) grown on GaP (001) substrates is studied by means of excitation and temperature-dependent photoluminescence (PL), and it is related to their complex electronic structure. Such QDs exhibit concurrently direct and indirect transitions, which allows the swapping of Γ and L quantum confined states in energy, depending on details of their stoichiometry. Based on realistic data on QD structure and composition, derived from high-resolution transmission electron microscopy (HRTEM) measurements, simulations by means of k·p theory are performed. The theoretical prediction of both momentum direct and indirect type-I optical transitions are confirmed by the experiments presented here. Additional investigations by a combination of Raman and photoreflectance spectroscopy show modifications of the hydrostatic strain in the QD layer, depending on the sequential addition of QDs and capping layer. A variation of the excitation density across four orders of magnitude reveals a 50-meV energy blueshift of the QD emission. Our findings suggest that the assignment of the type of transition, based solely by the observation of a blueshift with increased pumping, is insufficient. We propose therefore a more consistent approach based on the analysis of the character of the blueshift evolution with optical pumping, which employs a numerical model based on a semi-self-consistent configuration interaction method.
... Most of the present applications in optics are based on socalled type-I QDs, which show direct electron-hole recombination in both real and k-space, as for In(Ga)As QDs embedded in a GaAs matrix. Much less attention has been given to type-I indirect and/or type-II QDs, particularly antimony-based ones, like In(Ga)As QDs overgrown by a thin Ga(AsSb) layer [26][27][28][29][30][31], or In(Ga)Sb QDs in a GaAs matrix [32][33][34][35][36], which show spatially indirect optical transitions. Such structures generally require more challenging growth processes, but bring new and improved characteristics, for example intense room temperature emission [37], naturally low fine-structure splitting (FSS) [38], increased tuneability of the exciton confinement geometry and topology [24,[39][40][41], radiative lifetime [42,43] and magnetic properties [44][45][46]. ...
The optical response of (InGa)(AsSb)/GaAs quantum dots (QDs) grown on GaP (001) substrates is studied by means of excitation and temperature-dependent photoluminescence (PL), and it is related to their complex electronic structure. Such QDs exhibit concurrently direct and indirect transitions, which allows the swapping of $\Gamma$ and $L$ quantum confined states in energy, depending on details of their stoichiometry. Based on realistic data on QD structure and composition, derived from high-resolution transmission electron microscopy (HRTEM) measurements, simulations by means of $\mathbf{k\cdot p}$ theory are performed. The theoretical prediction of both momentum direct and indirect type-I optical transitions are confirmed by the experiments presented here. Additional investigations by a combination of Raman and photoreflectance spectroscopy show modifications of the hydrostatic strain in the QD layer, depending on the sequential addition of QDs and capping layer. A variation of the excitation density across four orders of magnitude reveals a 50 meV energy blueshift of the QD emission. Our findings suggest that the assignment of the type of transition, based solely by the observation of a blueshift with increased pumping, is insufficient. We propose therefore a more consistent approach based on the analysis of the character of the blueshift evolution with optical pumping, which employs a numerical model based on a semi-self-consistent configuration interaction method.
... These values of composition and size parameters are found in this type of QDs after a rapid thermal annealing treatment as explained elsewhere. [19] The QDs are covered with a conformal GaAs 0.8 Sb 0.2 layer of varying thickness, d. Finally, both QDs and overlayer are embedded on GaAs. ...
InAs self‐assembled quantum dots capped with GaAsSb exhibit type‐II band alignment and singly or doubly connected hole wave‐function topology depending on the thickness of the capping layer or the Sb concentration. A configuration interaction analysis of the excitons complexes X⁰, X⁻¹, and X⁺¹ for different overlayer thicknesses is presented. A characteristic double line structure of the X⁺¹ recombination is predicted for the doubly connected topology. A hallmark is established to identify the optical Aharonov–Bohm transition in addition to the well‐known intensity fade‐out of the X⁰.
... Amongst the possible III-V-based dilute nitride materials, GaAsSbN system is a promising one for such technologies [11,12]. This system can be grown with narrow band gap staying lattice matched to GaAs [13] while it also adjusts independently both conduction and valence-band energies by controlling the N and Sb content, respectively [14,15]. Besides, it presents better thermal stability by having only one element of group III, and it needs a lower N composition to reach for the same band gap (and therefore it shows fewer N-related defects) than the most intensely studied GaInAsN material [16]. ...
As promising candidates for solar cell and photodetection applications in the range 1.0–1.16 eV, the growth of dilute nitride GaAsSbN alloys lattice matched to GaAs is studied. With this aim, we have taken advantage of the temperature gradient in the molecular beam epitaxy reactor to analyse the impact of temperature on the incorporation of Sb and N species according to the wafer radial composition gradients. The results from the combination of X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopies (EDS) show an opposite rate of incorporation between N and Sb as we move away from the centre of the wafer. A competitive behaviour between Sb and N in order to occupy the group-V position is observed that depends on the growth rate and the substrate temperature. The optical properties obtained by photoluminescence are discussed in the frame of the double-band anticrossing model. The growth conditions define two sets of different parameters for the energy level and the coupling interaction potential of N, which must be taken into account in the search for the optimum compositions 1–1.15-eV photonic applications.
... Remarkably, in this sample, not only the PL band is narrower but the integrated PL emission is the most intense despite the longer carrier lifetime (see Figure 3a). [26] This is a clear indication of the improved crystal and interface quality produced by our method, also underlined by the fact that the PL of bulk GaAsSbN layer, despite having twice as much active material, is much weaker. Figure 4a. ...
We propose type-II GaAsSb/GaAsN superlattices (SLs) lattice-matched to GaAs as a novel material for the 1 eV sub-cells present in highly efficient GaAs/Ge-based multi-junction solar cells. We demonstrate that, among other benefits, the spatial separation of Sb and N allows a better control over composition and lattice matching, avoiding the growth problems related to the concomitant presence of both elements in GaAsSbN layers. This approach not only reduces clustering and improves crystal quality and interface abruptness, but also allows for additional control of the effective bandgap in the 1.0-1.15 eV spectral region through the SL period thickness. The optimized SL structure exhibits a type-II band alignment and strong electronic coupling at 0 V. Both effects cooperate to increase the minority carrier collection and leads to a net strong enhancement of the external quantum efficiency (EQE) under photovoltaic conditions with respect to bulk layers of equivalent thickness.
... This is particularly due to the smaller overlap of the quasiparticle wavefunctions leading to considerably reduced emission intensity and rate [12][13][14][15] . Moreover, the emission from type-II QDs is considerably and inhomogeneously broadened 16,17 compared to type I, and a large blue-shift of the emission energy with increasing laser pumping has been observed 18,19 . Apart from a rich physics, the spatial separation of the carriers in systems with type-II confinement provides several advantages compared to type I. ...
In this work we study theoretically and experimentally the multi-particle structure of the so-called type-II quantum dots with spatially separated electrons and holes. Our calculations based on customarily developed full configuration interaction approach reveal that exciton complexes containing holes interacting with two or more electrons exhibit fairly large antibinding energies. This effect is found to be the hallmark of the type-II confinement. In addition, an approximate self-consistent solution of the multi-exciton problem allows us to explain two pronounced phenomena: the blue-shift of the emission with pumping and the large inhomogenous spectral broadening, both of those eluding explanation so far. The results are confirmed by detailed intensity and polarization resolved photoluminescence measurements on a number of type-II samples.
... This reduces the oscillator strength and, therefore, increases the radiative carrier lifetime [6][7][8]. Nevertheless, these type-II structures can show improved luminescence and a highly tunable excitonic geometry and topology [9,10]. ...
This work reports on the benefits from using thin GaAsSb capping layers (CLs) on InAs/GaAs quantum dot (QD) solar cells. The application of such CLs allows the tunability of the QD ground state, switching the QD-CL band alignment from type I to type II for high Sb contents and extending the photoresponse beyond 1.5 µm. Two different structures with ~10% and ~20% Sb contents in the CL (type-I and type-II band alignments, respectively) are explored, leading to efficiency improvements over a reference InAs/GaAs QD solar cell of 20% and 10%, respectively. In general, a significant increase in short-circuit current density (Jsc) is observed, partially due to the extended photocurrent spectrum and the additional contribution of the CL itself. Particularly, for a moderate Sb content, an improved carrier collection efficiency is also found to be a main reason for the Jsc increase. Calculations from an 8×8 k·p method suggest the attribution of such an improvement to longer carrier lifetimes in the wetting layer-CL structure due to the transition to a type-II band alignment. Open-circuit voltages (Voc) exceeding that of a reference QD solar cell are demonstrated under light concentration using GaAsSb CLs, which proves that the Voc is not limited by the low bandgap CLs. Moreover, the highest value is obtained for the high Sb content type-II structure, despite the higher accumulation of strain and the lower effective bandgap. Indeed, the faster Voc increase with light power found in the latter case leads to an Voc even larger than the effective bandgap.
... Its thickness was 14 nm in the areas where the dot was not present and 6 nm above the QD apex. The particular choice of both structures is based on experimental studies [17][18][19][20]. ...
We have studied theoretically the type-II GaAsSb capped InAs quantum dots for two structures differing in the composition of the capping layer, being either (i) constant or (ii) with Sb accumulation above the apex of the dot. We have found that the hole states are segmented and resemble the states in the quantum dot molecules. The two-hole states form singlet and triplet with the splitting energy of 4 mu eV/325 mu eV for the case (i)/(ii). We have also tested the possibility to tune the splitting by vertically applied magnetic field. Because the predicted tunability range was limited, we propose an approach for its enhancement.
... The PL energy shift for this sample (163 meV) is approximately the same as the one resulting from the addition of those in samples GaAsSb10 (93 meV) and GaAsN (64 meV). This would confirm that the effect of Sb and N on the band structure are cumulative and, therefore, the electron and hole confinements can be independently tuned in stacked QD layers using GaAsSbN CLs [31]. The PL emission of samples capped with the quaternary alloy can be modified by using alternative approaches, as shown in samples GaAsSbN-SL and GaAsSbN-thin. ...
performances. Inordertocontrolthebandoffsetsandtheaccumulatedstrain,theapplicationofcapping
layers(CLs)madeofGaAs-basedalloysisbecomingacommonapproachthatshowspromisingadvan-
tages forSCapplications.However,uptonowthereareveryfewstatisticallyreliablestudiesoftheir
impact ontheQDmorphologies.OurworkbyTEMtechniquesovermorethanahundredofInAsQDshas
evaluatedabroadrangeofCLmaterialsandstructureswithSband/orNwithasingularaccuracy.Our
outcomes showedthattheSbcouldplayantagonisticrolesregardingtheQDdecompositiondepending
on thegrowthconditions.Unlikeitoccursatcommongrowthrates(1ML/s),athighgrowthratesof
2 ML/stheSbdoesnotprotecttheQDs,butevenintensifies theirerosion.Ontheotherside,Nprovides
the bestresultsintermsofmaintainingtheoriginalshapeoftheQDs.Inthecaseofasimultaneous
presence ofSbandN,asynergeticeffectintheerosionstrengthisobserved.Nevertheless,avoidingthe
concomitance presenceofNandSbonthegrowthfrontbygrowingashort-periodGaAsSb/GaAsN
superlattice CLsmoderatesthisbehavior.Theresultsarediscussedintermsofprocessessuchassur-
factant, intermixing,segregation,bondstrengthorstrain-enhanceddiffusionthatcouldoperateduring
the cappingprocess.Thestrongmodifications ontheQD–CL morphology,thatweshowareexpectedto
haveasignificant impactonQDSCperformance,cannotbedisregarded.
& 2015PublishedbyElsevierB
... The PL energy shift for this sample (163 meV) is approximately the same as the one resulting from the addition of those in samples GaAsSb10 (93 meV) and GaAsN (64 meV). This would confirm that the effect of Sb and N on the band structure are cumulative and, therefore, the electron and hole confinements can be independently tuned in stacked QD layers using GaAsSbN CLs [31]. The PL emission of samples capped with the quaternary alloy can be modified by using alternative approaches, as shown in samples GaAsSbN-SL and GaAsSbN-thin. ...
... The photoluminescence of GaAs 1−y Sb y capped QDs is rather intense despite the type-II confinement with the radiative lifetimes as low as 10 ns. 26 The strain-reducing effect of GaAs 1−y Sb y layer together with the surfacting effect of antimony allow to increase the emission wavelengths of standard InAs QDs and reach the telecommunication wavelength of 1.3 and 1.55 µm. 27,28 Various shapes of GaAs 1−y Sb y QDs have been reported, including a lens 29 or a pyramid with a graded In concentration. ...
Excitonic fine structure splitting in quantum dots is closely related to the
lateral shape of the wave functions. We have studied theoretically the fine
structure splitting in InAs quantum dots with a type-II confinement imposed by
a GaAsSb capping layer. We show that very small values of the fine structure
splitting comparable with the natural linewidth of the excitonic transitions
are achievable for realistic quantum dot morphologies despite the structural
elongation and the piezoelectric field. For example, varying the capping layer
thickness allows for a fine tuning of the splitting energy. The effect is
explained by a strong sensitivity of the hole wave function to the morphology
of the structure and a mutual compensation of the electron and hole
anisotropies. The oscillator strength of the excitonic transitions in the
studied quantum dots is comparable to those with a type-I confinement which
makes the dots attractive for quantum communication technology as emitters of
polarization-entangled photon pairs.
... 9,10 Although the type-II InAs/GaAsSb QDs are promising for memory 11 and photovoltaic devices, 12,13 the degraded recombination efficiency is however detrimental for light emitting devices. Several works have been devoted to the tailoring of the optical properties of GaAsSb-capped InAs QDs, such as varying the Sb composition in the GaAsSb CL, 6,7 post-growth thermal treatments, 14,15 varying the GaAsSb CL thickness, 16 graded Sb content in CL, 17 or using quaternary GaAsNSb. 18 However, since the effects of strain reduction and decomposition suppression are proportional to the Sb content in the CL, 6 it seems unlikely to take the advantages of GaAsSb CL while retaining a type-I QD band alignment. ...
We investigate the optical properties of InAs quantum dots (QDs) capped with a thin AlxGa1−xAsSb layer. As evidenced from power-dependent and time-resolved photoluminescence (PL) measurements, the GaAsSb-capped QDs with type-II band alignment can be changed to type-I by adding Al into the GaAsSb capping layer. The evolution of band alignment with the Al content in the AlGaAsSb capping layer has also been confirmed by theoretical calculations based on 8-band k⋅p model. The PL thermal stability and the room temperature PL efficiency are also improved by AlGaAsSb capping. We demonstrate that using the quaternary AlGaAsSb can take the advantages of GaAsSb capping layer on the InAs QDs while retaining a type-I band alignment for applications in long-wavelength light emitters.
Recently, very thin AlAs capping layers (CLs) have been proposed as a useful tool to increase the performance of InAs/GaAs quantum dot (QDs) devices. However, the structure of QDs after AlAs deposition remains poorly understood and the mechanisms to explain it are often contradictory. In this work, the structural and compositional changes of InAs QDs using different AlAs CL thicknesses have been studied by state-of-the-art STEM-related techniques. First, the heights and In contents of InAs QDs progressively increase with the CL thickness, demonstrating that the AlAs capping produces a strong shielding effect against the decomposition of QDs. However, QD populations for CL thicknesses above 5 ML split into a bimodal distribution in which smaller lenticular QDs cohabit with bigger truncated pyramids. Second, the actual Al contents around the QDs are well below the nominal design, but increasing for thicker CLs. Its distribution is initially non-uniform, tending to accumulate on the flanks of the QDs to the detriment of the apex. Only for thicknesses above 2 ML the Al contents around the QDs start to be similar to those in the regions between the QDs, behaving as a continuous film without irregularities from 5 ML onwards.
This chapter looks at the integration of telecoms wavelength emitting InAs/GaAs quantum dots, with silicon (Si). Photoluminescence (PL) measurements on two samples were performed, with one grown on silicon with defect filter layers (DFLs) grown underneath the active region, and the other grown on a Gallium Arnside (GaAs) substrate. Individual quantum dots on both samples were isolated using both micropillars and gold apertures.
The effects of thermal annealing treatment on a vertically aligned InGaAs/GaAs(Sb)/AlGaAsSb quantum dot (QD) structure with the purpose of tailoring energy band alignment are studied. In contrast to the typical blueshift in the emission upon annealing because of In-Ga intermixing in the typical InGaAs/GaAs QDs, thermally annealed InGaAs/GaAs(Sb)/AlGaAsSb QDs exhibit a redshift upon annealing at 700 oC, owing to Sb aggregation on top of the InGaAs QDs, resulting in tailoring of the band alignment and strain reduction for the reduced emission energy. Power-dependent and time-resolved photoluminescence were utilized herein to extend the carrier lifetime from 1.63 to 6.38 ns and to elucidate mechanisms of the aggregation of antimony that cause the energy band alignment modifying from type-I to type-II after rapid thermal annealing. In addition, the thermal stability of the columnar QDs was improved by capping QDs with a GaAsSb overgrown layer, because In-Ga intermixing was suppressed, helping to preserved QD heterostructures. Therefore, the flexible modulation of energy band alignment for columnar InGaAs/GaAs(Sb)/AlGaAsSb QD structures as type-I or type-II by thermal annealing has potential and flexible application to versatile QD-related optoelectronic devices.
This study demonstrated the feasibility of fabricating a highly stacked vertically aligned InGaAs/GaAs(Sb) quantum dot (QD) structure with an AlGaAsSb spacer layer for improving the device performance of QD intermediate-band solar cell (QD-IBSC) devices. The power-dependent photoluminescence measurements of the proposed structure revealed a blueshift in the QD ground-state emissions when the excitation power was increased, indicating the formation of an intermediate band inside the QD structure. Capping the InGaAs QDs with a GaAsSb layer prevented the QDs from collapsing because there was less In–Ga intermixing between the QDs and GaAsSb layer. In addition to maintaining the QD structure, the carrier lifetime was extended by tuning the energy band alignment of the InGaAs/GaAsSb QD structure. Inserting the AlGaAsSb layer into the spacer layer increased the band gap, which in turn increased the open-circuit voltage of the QD-IBSC. The QD-IBSC in this work shows an extension of external-quantum efficiency by up to 1200 nm (compared with a GaAs reference cell) through the absorption by QDs and increased the open-circuit voltage from 0.67 to 0.70 V by adopting the AlGaAsSb spacer layer. These results confirm that adopting a columnar InGaAs/GaAs(Sb) QD structure with a AlGaAsSb spacer layer can enhance the performance of QD-IBSC devices. Copyright
We have theoretically studied type-I and type-II confinement in InAs quantum dots with GaAs1-y Sb-y capping layer. The character of the confinement can be adjusted by the Sb content. We have found that upon the transition from type-I to type-II confinement the hole wave functions change the topology from a compact shape to a two-segment shape, resulting in the complex changes in the exciton fine structure splitting with zero values at particular compositions. Additionally, a high exciton radiative recombination probability is preserved even in type-II. This allows to design strongly luminescent quantum dots with naturally low fine structure splitting, which could serve as sources of entangled photon pairs for quantum communication.
On the basis of optical characterization experiments and an eight band
kp model, we have studied the effect of Sb incorporation on the
electronic structure of InAs quantum dots (QDs). We have found that Sb
incorporation in InAs QDs shifts the hole wave function to the center of
the QD from the edges of the QD where it is otherwise pinned down by the
effects of shear stress. The observed changes in the ground-state energy
cannot merely be explained by a composition change upon Sb exposure but
can be accounted for when the change in lateral size is taken into
consideration. The Sb distribution inside the QDs produces distinctive
changes in the density of states, particularly, in the separation
between excitation shells. We find a 50% increase in the thermal escape
activation energy compared with reference InAs quantum dots as well as
an increment of the fundamental transition decay time with Sb
incorporation. Furthermore, we find that Sb incorporation into quantum
dots is strongly nonlinear with coverage, saturating at low doses. This
suggests the existence of a solubility limit of the Sb incorporation
into the quantum dots during growth.
It is demonstrated that the emission of InAs quantum dots (QDs) capped with GaAsSb can be extended from 1.28 to 1.6 μm by increasing the Sb composition of the capping layer from 14% to 26%. Photoluminescence excitation spectroscopy is applied to investigate the nature of this large redshift. The dominant mechanism is shown to be the formation of a type-II transition between an electron state in the InAs QDs and a hole state in the GaAsSb capping layer. The prospects for using these structures to fabricate 1.55 μm injection lasers are discussed.
We report the effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs1−xSbx layer. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAsSb layer. Evidences of the evolution from type-II to type-I band alignments are obtained from time-resolved and power-dependent photoluminescence measurements. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAsSb type-II QDs.
We report on optically pumped semiconductor lasers emitting near 3.8 μm that exhibit high power and low output divergence. The lasers incorporate multiple InAs/InGaSb/InAs type-II wells imbedded in an InGaAsSb waveguide that is designed to absorb the pump emission. When operated at 85 K, 0.25 mm×2.5 mm broad area devices produce >5 W of peak power under long pulse conditions. Moreover, these extremely bright devices exhibit a fast axis divergence of only ∼ 15° full width at half maximum (FWHM), coupled with a slow axis divergence of ∼ 6° FWHM. The first is due to the reduced optical confinement in the transverse direction, while the latter is attributed to the suppression of filament formation, which is another beneficial consequence of the low optical confinement. © 2002 American Institute of Physics.
Microdisk lasers with active region made of type-II GaSb/GaAs quantum dots on the GaAs substrate have been demonstrated. A microdisk cavity with diameter of 3.9 μm was fabricated from a 225-nm-thick GaAs layer filled with GaSb quantum dots. Lasing at wavelengths near 1000 nm at 150 K was achieved for this microdisk. A high threshold characteristic temperature of 77 K was also observed. It is found that the lasing wavelength matches closely with the first-order whispering-gallery mode of the cavity as obtained from the finite-element method simulation.
The excitation power dependence of the ground and excited state transitions in type-II InAs-GaAs0.78Sb0.22 quantum dot structure has been studied. Both transitions exhibit a strong blueshift with increasing excitation power but their separation remains constant. This behavior indicates a carrier-induced electric field oriented predominantly along the growth axis, which requires the holes to be localized in the GaAsSb above quantum dots. An accelerated blueshift of the ground state emission is observed once the excited state in the dots starts to populate. This behavior can be explained by a smaller spontaneous recombination coefficient for the excited state transition.
The influence of a GaAsSb capping layer on the structural properties of self-assembled InAs/GaAs quantum dots (QDs) is studied on the atomic scale by cross-sectional scanning tunneling microscopy. QDs capped with GaAs0.75Sb0.25 exhibit a full pyramidal shape and a height more than twice that of the typical GaAs-capped QDs, indicating that capping with GaAsSb suppresses dot decomposition. This behavior is most likely related to the reduced lattice mismatch between the dot and the capping layer. (c) 2007 American Institute of Physics.
The authors report the optical characteristics of GaSb/GaAs self-assembled quantum dots (QDs) embedded in an InGaAs quantum well (QW). Variations in the In composition of the QW can significantly alter the emission wavelength up to 1.3 μm and emission efficiency. Lasing operation at room temperature is obtained from a 2-mm-long device containing five stacked GaSb QDs in In0.13Ga0.87As QWs at 1.026 μm with a threshold current density of 860 A/cm2. The probable lasing transition involves electrons and holes confined in the QW and QDs, respectively, resulting in a large peak modal gain of 45 cm−1. A significant blueshift of the electroluminescence peak is observed with increased injection current and suggests a type-II band structure.
We present a comprehensive, up-to-date compilation of band parameters for the technologically important III{endash}V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other. {copyright} 2001 American Institute of Physics.
The electronic structure of InAs quantum dots covered with the GaAs(1-y)Sb(y)
strain reducing layer has been studied using the k.p theory. We explain
previous experimental observations of the red shift of the photoluminescence
emission with increasing y and its blue shift with increasing excitation power.
For y>0.19 type-II dots are formed with holes localized in the GaAsSb close to
the dot base; two segments at opposite sides of the dot, forming molecular-like
states, result from the piezoelectric field. We also propose an experiment that
could be used to identify the hole localization using a vertical electric
field.
The effect of postgrowth rapid thermal annealing (RTA) on GaAs proximity-capped structures with self-assembled InAs/GaAs quantum dots (QDs) is investigated using transmission electron microscopy (TEM) and photoluminescence (PL). As can be seen from the TEM images, QDs increase their lateral sizes with increasing annealing temperature (up to 700°C). QDs cannot be distinguished after RTA at temperature 800°C or higher, and substantial thickening of the wetting layer can be seen instead. The main PL peak blueshifts as a result of RTA. We propose that in the as-grown sample as well, as in samples annealed at temperatures up to 700°C, the peak is due to the QDs. After RTA at 800°C and higher the PL peak is due to a modified wetting layer. Relatively fast dissolution of QDs is explained in terms of strain-induced lateral Ga/In interdiffusion. It is proposed that such a process may be of importance in proximity-capped RTA, when no group-III vacancy formation takes place at the sample/capping interface.
We discuss an improved mid‐wave infrared diode laser structure based on InAs‐Ga 1-x In x Sb‐ InAs‐Ga 1-x Al x Sb Type‐II multiple quantum wells. The proposed design combines strong optical coupling, 2D dispersion for both electrons and holes, suppression of the Auger recombination rate, and excellent electrical and optical confinement. © 1995 American Institute of Physics.
Laser emission at 4.2-4.5 /spl mu/m has been observed at temperatures up to 310 K in pulsed optical pumping experiments on type-II quantum-well (QW) lasers with four constituents in each period (InAs-Ga/sub 1-x/In/sub x/Sb-InAs-AlSb). The characteristic temperature, T/sub 0/, is 41 K, and a peak output power exceeding 2 W/facet is observed at 200 K. The power conversion efficiency per facet of /spl ap/0.2% up to 200 K is within a factor of 2 of the theoretical value. The 300 K Auger coefficient of 4/spl times/10/sup -27/ cm/sup 6//s extracted from the threshold pump intensity demonstrates that Auger losses have been suppressed by a factor of four.
nextnano is a semiconductor nanodevice simulation tool that has been developed for predicting and understanding a wide range of electronic and optical properties of semiconductor nanostructures. The underlying idea is to provide a robust and generic framework for modeling device applications in the field of nanosized semiconductor heterostructures. The simulator deals with realistic geometries and almost any relevant combination of materials in one, two, and three spatial dimensions. It focuses on an accurate and reliable treatment of quantum mechanical effects and provides a self-consistent solution of the Schrodinger, Poisson, and current equations. Exchange-correlation effects are taken into account in terms of the local density scheme. The electronic structure is represented within the single-band or multiband kldrp envelope function approximation, including strain. The code is not intended to be a ldquoblack boxrdquo tool. It requires a good understanding of quantum mechanics. The input language provides a number of tools that simplify setting up device geometry or running repetitive tasks. In this paper, we present a brief overview of nextnano and present four examples that demonstrate the wide range of possible applications for this software in the fields of solid-state quantum computation, nanoelectronics, and optoelectronics, namely, 1) a realization of a qubit based on coupled quantum wires in a magnetic field, 2) and 3) carrier transport in two different nano-MOSFET devices, and 4) a quantum cascade laser.
The origin of the modified optical properties of InAs/GaAs quantum dots (QD) capped with a thin GaAs1−xSbx layer is analyzed in terms of the band structure. To do so, the size, shape, and composition of the QDs and capping layer are determined through cross-sectional scanning tunnelling microscopy and used as input parameters in an 8 × 8 k·p model. As the Sb content is increased, there are two competing effects determining carrier confinement and the oscillator strength: the increased QD height and reduced strain on one side and the reduced QD-capping layer valence band offset on the other. Nevertheless, the observed evolution of the photoluminescence (PL) intensity with Sb cannot be explained in terms of the oscillator strength between ground states, which decreases dramatically for Sb > 16%, where the band alignment becomes type II with the hole wavefunction localized outside the QD in the capping layer. Contrary to this behaviour, the PL intensity in the type II QDs is similar (at 15 K) or even larger (at room temperature) than in the type I Sb-free reference QDs. This indicates that the PL efficiency is dominated by carrier dynamics, which is altered by the presence of the GaAsSb capping layer. In particular, the presence of Sb leads to an enhanced PL thermal stability. From the comparison between the activation energies for thermal quenching of the PL and the modelled band structure, the main carrier escape mechanisms are suggested. In standard GaAs-capped QDs, escape of both electrons and holes to the GaAs barrier is the main PL quenching mechanism. For small-moderate Sb (<16%) for which the type I band alignment is kept, electrons escape to the GaAs barrier and holes escape to the GaAsSb capping layer, where redistribution and retraping processes can take place. For Sb contents above 16% (type-II region), holes remain in the GaAsSb layer and the escape of electrons from the QD to the GaAs barrier is most likely the dominant PL quenching mechanism. This means that electrons and holes behave dynamically as uncorrelated pairs in both the type-I and type-II structures.
The hole confinement of self-organized GaSb/GaAs quantum dots embedded in n+p-diodes is investigated experimentally by admittance spectroscopy. The highest thermal activation energy obtained, 400 meV, refers to only weakly charged quantum dots. Detailed bias-dependent investigations allow to study state- filling and Coulomb charging effects. State filling lowers the activation energy down to 150 meV in quantum dots charged with the maximum number of about 15 holes. The observed thermal activation barrier for GaSb/GaAs quantum dots is about twice as high as for structurally comparable InAs/GaAs quantum dots.
The Sb-induced changes in the optical properties of GaAsSb-capped InAs/GaAs quantum dots (QDs) are shown to be strongly correlated with structural changes. The observed redshift of the photoluminescence emission is shown to follow two different regimes. In the first regime, with Sb concentrations up to approx12%, the emission wavelength shifts up to approx1280 nm with a large enhancement of the luminescence characteristics. A structural analysis at the atomic scale by cross-sectional scanning tunneling microscopy shows that this enhancement arises from a gradual increase in QD height, which improves carrier confinement and reduces the sensitivity of the excitonic band gap to QD size fluctuations within the ensemble. The increased QD height results from the progressive suppression of QD decomposition during the capping process due to the presence of Sb atoms on the growth surface. In the second regime, with Sb concentrations above approx12%, the emission wavelength shifts up to approx1500 nm, but the luminescence characteristics progressively degrade with the Sb content. This degradation at high Sb contents occurs as a result of composition modulation in the capping layer and strain-induced Sb migration to the top of the QDs, together with a transition to a type-II band alignment.
Carrier lifetimes have been measured for long-wavelength emitting InAs quantum dots (QDs) capped with a thin GaAsSb layer. Above a critical Sb composition, a type-II system is formed, resulting in an increase in the carrier lifetime. The carrier lifetime in a strongly type-II structure is increased by a factor ∼ 54 in comparison to the lifetime in a type-I structure. In addition, the type-II carrier lifetime varies across the inhomogeneously broadened ground-state emission, while the type-I QD lifetime is invariant.
We report structural and optical properties of GaSb/GaAs self-assembled quantum dots (QDs) grown by molecular beam epitaxy. The QDs, with nanometer-scale dimensions, were characterized by atomic force microscopy. Furthermore, in photoluminescence (PL) measurements the feature from the QDs was observed at ∼1.1 eV, clearly separated from that of the wetting layer at ∼1.3 eV. With increasing excitation power, the peak from the QDs displayed a large shift towards higher energy. In addition, the temperature dependence of PL yielded a large thermal activation energy, 130 meV, confirming the strong localization of excitons in the QDs. © 1999 American Institute of Physics.
A microscopic laser theory is used to investigate gain and threshold properties in a GaAsSb quantum-well laser. Depending on the geometry of the type-II quantum-well gain region, there may be appreciable band distortions due to electron–hole charge separation. The charge separation and accompanying band distortions lead to interesting optical behaviors, such as excitation-dependent oscillator strength and band edge energies. Implications to laser operation include significant blueshift of the gain peak with increasing injection current, and inhibition of spontaneous emission, which may result in threshold current reduction. © 2001 American Institute of Physics.
A series of light-emitting diodes (LEDs) with active layers based on InAs quantum dots (QDs) covered by GaAsSb capping layers is presented. Varying the Sb content in the capping layer from similar to 2 to similar to 28%, room temperature electroluminescence (EL) from 1.15 to 1.5 mu m is obtained. The external efficiency of the devices, eta(ext), increases as the Sb is increased up to similar to 15% and then decreases for higher Sb contents, consistently with the reported increase of QD height with the Sb content up to similar to 15% and the band alignment transition from type I to type II above similar to 15% Sb. An analysis of the EL and photocurrent spectra shows that the emission from type I LEDs originates from the recombination between electrons and holes confined in the QDs. On the other hand, the EL from the type II devices is the combination of two different processes. First, recombination between electrons confined in the QDs and holes at the capping layer. Second, a type I-like recombination of electrons from the QDs and holes residing in extended levels of the quantum well composed by the capping layer and the QDs. The mechanisms responsible for the thermal quenching of the EL are also studied. Escape of holes from the QD to the capping layer is identified as the dominant mechanism for the type I devices, whereas in type II structures it is the escape of electrons from QD excited levels to the barrier which dominates.
The possibility of an independent tuning of the electron and hole confinement in InAs/GaAs quantum dots (QDs) by using a thin GaAsSbN capping layer (CL) is studied. By controlling the Sb and N contents in the quaternary alloy, the band structure of the QDs can be broadly tuned and converted from type-II in the valence band (high Sb contents) to type-I and to type-II in the conduction band (high N contents). Nevertheless, the simultaneous presence of Sb and N is found to induce strain and composition inhomogeneities in the CL and to degrade the photoluminescence of the structure.
A thermal activation energy of 710 meV for hole emission from In As / Ga As quantum dots (QDs) across an Al <sub>0.9</sub> Ga <sub>0.1</sub> As barrier is determined by using time-resolved capacitance spectroscopy. A hole storage time of 1.6 s at room temperature is directly measured, being three orders of magnitude longer than a typical dynamic random access memory (DRAM) refresh time. The dependence of the hole storage time in different III–V QDs on their localization energy is determined and the localization energies in GaSb-based QDs are calculated using eight-band k ∙ p theory. A storage time of about 10<sup>6</sup> years in Ga Sb / Al As QDs is extrapolated, sufficient for a QD-based nonvolatile (flash) memory.
The authors report an enhanced infrared spectral response of GaAs-based solar cells that incorporate type II GaSb quantum dots (QDs) formed using interfacial misfit array growth mode. The material and devices, grown by molecular beam epitaxy, are characterized by current-voltage and spectral response characteristics. From 0.9 to 1.36 μ m , these solar cells show significantly more infrared response compared to reference GaAs cells and previously reported InAs QD solar cells. The short circuit current density and open circuit voltages of solar cells with and without dots measured under identical conditions are 1.29 mA / cm <sup>2</sup> , 0.37 V and 1.17 mA / cm <sup>2</sup> , 0.6 V , respectively.
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