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a) Temperature dependences of the PL intensities of the 1 ML GaN/AlN (0001) QW and 1.5 nm thick AlxGa1−xN/AlN (0001) QW emitting at a similar wavelength of 230 nm. Symbols and solid curves denote the experimental results and those of the fits using Equation (1), respectively. b) PL decay curves of these QWs acquired at RT.
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GaN/AlN ultrathin quantum wells (QWs) emitting in the deep UV spectral range are fabricated by metalorganic vapor phase epitaxy. The GaN thickness is automatically limited to the monolayer (ML) scale due to the balance between crystallization and evaporation of Ga adatoms. This growth characteristic facilitates the fabrication of highly reproducibl...
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Citations
... As a consequence of its continuous decrease, the valence band maximum at the Γ point may change, 4 leading to different light emission directions and polarization dependence on the electric field direction in the crystal, 5 as well as to low emission intensity. 6 Taking this specific property into account, it was shown that by performing adequate epitaxial growth engineering, e.g., by using short-period AlN/GaN superlattices, 7 compressive strain and/or narrow quantum wells, 8 and/or using high Miller index growth planes, 9 strong modifications of the light polarization can be obtained that leads to improved light emission efficiency. ...
The luminescence efficiency of AlxGa1−xN quantum dots (QDs) and quantum wells (QWs), buried in AlN cladding layers and emitting in the ultraviolet range between 234 and 310 nm, has been investigated. The growth and optical properties have been done using similar aluminum composition (varying from 0.4 to 0.75) for both QDs and QWs. In order to compare as much as possible the optical properties, the QWs were fabricated with a growth time tuned such that the QW width is similar to the average height of the QDs. The photoluminescence (PL) showed emission ranging from 4 to 5.4 eV, putting into evidence differences in terms of full width at half maximum, PL intensity, and asymmetry of the line shape between QDs and QWs. The results show shorter wavelengths and a slightly narrower PL linewidth for QWs. To determine the light emission dependence with the electric field direction in the crystal, the evolutions of the emission diagrams for all samples were recorded along two orthogonal directions, namely, the “in-plane” (growth) and the “on-side” directions, from which the light emission was collected. For the whole QDs and QWs samples' series, the shapes of the emission diagram indicate emission in both in-plane and on-side directions, as evidenced by intra-valence band mixings caused by strain effects combined with the anisotropic Coulomb interactions that are particularly contributing to the polarization at wavelengths below 260 nm. Furthermore, the degree of polarization, determined for each sample, showed good agreement with results from the literature.
... Semiconductor heterostructures, stacked by two or more dissimilar materials, have been extensively explored in controlling band structure (1,2), polarization field (3,4), strain distribution (5,6), charge carrier confinement and mobility (7,8), and excitonic oscillator strength (9,10), which can provide material properties that are superior or not possible otherwise (11)(12)(13). A significant challenge in unlocking the potential of atomic-scale heterostructures is the presence of atomic substitution processes among cations with different ionicity at the heterointerface (14), which leads to interfacial composition gradients or formation of nanoclusters, preventing the realization of atomically ordered quantum heterostructures (15)(16)(17). A diffusive interface causes a reduction in electron mobility due to alloy scattering in high electron mobility transistors (18,19) and significantly broadened emissions and slow radiative recombination in light-emitting diodes (17,20). ...
Interface engineering in heterostructures at the atomic scale has been a central research focus of nanoscale and quantum material science. Despite its paramount importance, the achievement of atomically ordered heterointerfaces has been severely limited by the strong diffusive feature of interfacial atoms in heterostructures. In this work, we first report a strong dependence of interfacial diffusion on the surface polarity. Near-perfect quantum interfaces can be readily synthesized on the semipolar plane instead of the conventional c -plane of GaN/AlN heterostructures. The chemical bonding configurations on the semipolar plane can significantly suppress the cation substitution process as evidenced by first-principles calculations, which leads to an atomically sharp interface. Moreover, the surface polarity of GaN/AlN can be readily controlled by varying the strain relaxation process in core–shell nanostructures. The obtained extremely confined, interdiffusion-free ultrathin GaN quantum wells exhibit a high internal quantum efficiency of ~75%. Deep ultraviolet light-emitting diodes are fabricated utilizing a scalable and robust method and the electroluminescence emission is nearly free of the quantum-confined Stark effect, which is significant for ultrastable device operation. The presented work shows a vital path for achieving atomically ordered quantum heterostructures for III-nitrides as well as other polar materials such as III-arsenides, perovskites, etc.
... The spectrum marked by «*» was measured for the structure grown at m = 1.5 ML, n = 22 ML and N = 120, as described in [110]. (b) Literature review of the dependence of the energy positions of CL and PL peaks on the nominal QW thickness in MQW structures [106,107,123,128,129]. (c) Dependence of the CL intensity on the QW thickness in the MQW structures whose spectra are shown in (a). ...
... The spectrum marked by «*» was measured for the structure grown at m = 1.5 ML, n = 22 ML and N = 120, as described in [110]. (b) Literature review of the dependence of the energy positions of CL and PL peaks on the nominal QW thickness in MQW structures [106,107,123,128,129]. (c) Dependence of the CL intensity on the QW thickness in the MQW structures whose spectra are shown in (a) [124]. ...
Powerful emitters of ultraviolet C (UVC) light in the wavelength range of 230-280 nm are necessary for the development of effective and safe optical disinfection technologies, highly sensitive optical spectroscopy and non-line-of-sight optical communication. This review considers UVC emitters with electron-beam pumping of heterostructures with quantum wells in an (Al,Ga)N material system. The important advantages of these emitters are the absence of the critical problem of p-type doping and the possibility of achieving record (up to several tens of watts for peak values) output optical power values in the UVC range. The review consistently considers about a decade of world experience in the implementation of various UV emitters with various types of thermionic, field-emission, and plasma-cathode electron guns (sources) used to excite various designs of active (light-emitting) regions in heterostructures with quantum wells of AlxGa1-xN/AlyGa1-yN (x = 0-0.5, y = 0.6-1), fabricated either by metal-organic chemical vapor deposition or by plasma-activated molecular beam epitaxy. Special attention is paid to the production of heterostructures with multiple quantum wells/two-dimensional (2D) quantum disks of GaN/AlN with a monolayer's (1 ML~0.25 nm) thickness, which ensures a high internal quantum efficiency of radiative recombination in the UVC range, low elastic stresses in heterostructures, and high-output UVC-optical powers.
... Ultrathin quantum wells (QWs) GaN/(Al,Ga)N with thicknesses of 1-2 monolayers (MLs) are the structures of choice for the development of ultraviolet (UV) light-emitting devices with operating wavelengths (λ) in the highly demanded UVC and UVB ranges [1][2][3][4][5][6][7][8][9][10][11][12][13]. In particular, when using GaN/AlN multiple QWs (up to 400 periods), electron-beampumped UVC emitters with maximum peak output optical powers of 50 W for λ = 265 nm and 10 W for λ = 238 nm were demonstrated [12]. ...
GaN/AlN heterostructures with thicknesses of one monolayer (ML) are currently considered to be the most promising material for creating UVC light-emitting devices. A unique functional property of these atomically thin quantum wells (QWs) is their ability to maintain stable excitons, resulting in a particularly high radiation yield at room temperature. However, the intrinsic properties of these excitons are substantially masked by the inhomogeneous broadening caused, in particular, by fluctuations in the QWs’ thicknesses. In this work, to reduce this effect, we fabricated cylindrical nanocolumns of 50 to 5000 nm in diameter using GaN/AlN single QW heterostructures grown via molecular beam epitaxy while using photolithography with a combination of wet and reactive ion etching. Photoluminescence measurements in an ultrasmall QW region enclosed in a nanocolumn revealed that narrow lines of individual excitons were localized on potential fluctuations attributed to 2-3-monolayer-high GaN clusters, which appear in QWs with an average thickness of 1 ML. The kinetics of luminescence with increasing temperature is determined via the change in the population of localized exciton states. At low temperatures, spin-forbidden dark excitons with lifetimes of ~40 ns predominate, while at temperatures elevated above 120 K, the overlying bright exciton states with much faster recombination dynamics determine the emission.
... The spectrum marked by «*» was measured for the structure grown at m = 1.5 ML, n = 22 ML, and N = 120 as described in [106]. (b) dependence of the CL peak energy positions on the nominal QW thickness in MQW structures in (a) and described in [103,104,106,120,124,125]. (c) Dependence of the CL intensity on the QW thickness in the MQW structures whose spectra are shown in (a) [120]. ...
Powerful emitters of the ultraviolet C (UVC) light in the wavelength range of 230-280nm are necessary for the development of effective and safe optical disinfection technologies, high-sensitive optical spectroscopy and non-line-of-sight optical communication. This review considers such UVC-emitters with electron-beam pumping of heterostructures with quantum wells in the (Al,Ga)N material system. The important advantages of these emitters include the absence of the critical problem of p-type doping and the possibility of achieving record (up to several tens of watts for peak values) output optical power values in the UVC range. The review consistently considers about a decade of world experience in the implementation of various UV emitters with various types of thermionic, field-emission, and plasma-cathode electron guns (sources) used to excite various designs of active (light-emitting) regions in heterostructures with quantum wells AlxGa1-xN/AlyGa1-yN (x = 0 − 0.5, y = 0.6 − 1) fabricated either by metal-organic chemical vapor deposition or by plasma-activated molecular beam epitaxy. Special attention is paid to the production of heterostructures with multiple quantum wells/two-dimensional(2D) quantum disks GaN/AlN with a monolayer (1ML ~ 0.25nm) thickness, which ensures a high internal quantum efficiency of radiative recombination in the UVC range, low elastic stresses in heterostructures, and a high output UVC-optical powers.
... Page 10 of 17 AUTHOR SUBMITTED MANUSCRIPT -NANO-134470 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 The results of the present study for the emission energies are plotted in Figure 7 along with a review of data reported in the literature, which refer to GaN/AlN superlattices in layers [11,12,[27][28][29][19][20][21][22][23][24][25][26] and NWs [30,31]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 13 (SPSLs) can be viewed either as digital alloys or as a collection of GaN wells into AlN barriers. As theoretically established by Sun et al in the case of layers, AlN barriers above a threshold thickness of 7-9 MLs behave similarly to infinite ones leading to a saturation of GaN QW emission energy. ...
Molecular beam epitaxy growth and optical properties of GaN quantum disks in AlN nanowires were investigated, with the purpose of controlling the emission wavelength of AlN nanowire-based light emitting diodes. Besides GaN quantum disks with a thickness ranging from 1 to 4 monolayers, a special attention was paid to incomplete GaN disks exhibiting lateral confinement. Their emission consists of sharp lines which extend down to 215 nm, in the vicinity of AlN band edge. The room temperature cathodoluminescence intensity of an ensemble of GaN quantum disks embedded in AlN nanowires is about 20 % of the low temperature value, emphasizing the potential of ultrathin/incomplete GaN quantum disks for deep UV emission.
... 7 The same growth control and reproducibility concerns have led to DA intentionally being grown while managing additional difficulties to achieve ML-scale growth using the MOVPE technique. 8 ML-scale growth can be achieved by the development of delayed sequential growth process to assure sharp interfaces. 9 This is the author's peer reviewed, accepted manuscript. ...
The growth of non-polar AlGaN digital alloy (DA) is achieved by metal-organic vapor phase epitaxy using GaN microwire m-facets as the template. This AlGaN DA consisting in 5 periods of 2 monolayers-thick layers of GaN and AlGaN (approximately 50% Al-content) is integrated into the middle of an n-p GaN/AlGaN junction to design core-shell wire-μLED. The optical emission of the active zone investigated by 5 K cathodoluminescence is consistent with the AlGaN bulk alloy behavior. Several contributions from 295 to 310 nm are attributed to the lesser thickness and/or composition fluctuations of AlGaN DA. Single-wire μLED is fabricated using a lithography process and I-V measurements confirm a diode rectifying behavior. Room
temperature UV electroluminescence originating from m-plane AlGaN DA is accomplished at 310 nm.
... [1][2][3] As a relatively new application, far UV-C light (200-240 nm) is focused on for sterilization without a significant damage on animal skins or eyes, [4][5][6][7] and AlGaNbased far UV-C LEDs have been widely studied. [8][9][10][11][12][13][14][15][16][17] However, far UV-C LEDs are still not practical because of their low output power, high operating voltage, and short lifetime compared to commonly used visible or UV LEDs (260$ nm). External quantum efficiency (EQE) of UV-C LEDs drastically drops at shorter emission wavelengths, especially those shorter than 230 nm. ...
... From X-ray reciprocal space mapping images acquired around the (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) plane of the 226-nm and 229-nm LEDs, all AlGaN layers were coherently grown on the AlN substrate. FWHMs at peaks of XRC spectra of n-AlGaN around (002) and (104) directions were 68 and 88 arcsec, respectively, indicating high crystal quality. ...
We fabricated sub-230-nm (far UV-C) light emitting diodes (LEDs) on a single-crystal AlN substrate. With 20 quantum well cycles implemented to enhance carrier injection into the active layers, over 1-mW output power (1.4 and 3.1 mW for 226- and 229-nm LEDs, respectively) was obtained under 100-mA operation. The maximum output power reached 21.1 mW for the single-chip 229-nm LED operating at 700 mA, without significant drooping. The forward voltage for both sub-230-nm LEDs operating at 100 mA was low (5.9 V) due to their low resistances and ideal Ohmic contacts between metal and semiconductor components. Additionally, wall plug efficiencies were 0.24% and 0.53% for the 226- and 229-nm LEDs, respectively. The lifetime of the 226-nm LED while operating at 25 °C reached over 1500 h and did not show current leakage, even after 1524 h. This long lifetime will be achieved by improving carrier injection due to many quantum wells, using a high-quality AlN substrate and achieving high wall plug efficiency.
... (¯) planes, respectively, owing to the balance between crystallization and the evaporation of Ga adatoms at the surface. 12) The formation of GaN/AlN QWs results in higher luminescence efficiencies than conventional Al x Ga 1−x N QWs. Although the balance is recognized as a consequence of self-limiting growth of GaN on a AlN surface, the atom-scale mechanism of the self-limiting growth of GaN on a AlN surface is still unknown. ...
... Therefore, the growth of GaN layers after one GaN bilayer on the AlN(0001) surface is suppressed by relatively large adsorption energies leading to the selflimiting growth in the MOVPE. 12) Although detailed growth kinetics such as surface lifetime and diffusion length of the Ga adatom should be verified, the difference in the behavior of the Ga adatom could affect the growth of GaN layers on the AlN(0001) surface. The mechanism proposed in this study will be supported by further investigations of the surface stability and adsorption behavior on 1102 (¯) planes, in which two-bilayer GaN are formed on AlN 1102 (¯) surface by the MOPVE. ...
... The mechanism proposed in this study will be supported by further investigations of the surface stability and adsorption behavior on 1102 (¯) planes, in which two-bilayer GaN are formed on AlN 1102 (¯) surface by the MOPVE. 12) ...
The GaN thickness dependence of surface structural stability and adsorption behavior of Ga adatom in GaN layers on AlN(0001) surface are investigated on the basis of first-principles calculations to clarify the self-limiting growth on AlN(0001) surface during metal organic vapor phase epitaxy. The calculations demonstrate that the stability of reconstructed GaN layers on AlN(0001) surface is similar to that of GaN(0001) surface irrespective of the GaN film thickness. Furthermore, we find that the adsorption of Ga adatom on AlN(0001) surface easily occurs compared with that of on GaN layers on AlN(0001) surfaces. The difference in the adsorption behavior implies that the growth of GaN layers on AlN(0001) surface is suppressed. The calculated results provide a theoretical guidance for understanding the self-limiting growth of GaN layers, resulting in to the formation mechanism of GaN quantum wells.
... 6 As an alternative, in the last few years, attention has been focused on gallium nitride layers with extremely reduced dimensionality, that is, ultrathin or quasi-2D GaN QWs, that can exhibit high quantum confinement, enhanced carrier localization, and a blue shift of their bulk optical emission when inserted in AlGaN matrix. These active regions have recently been demonstrated to be beneficial for efficient UV emission in LEDs 7,8 and laser devices. 9 Interesting features of excitonic properties have been theoretically anticipated and experimentally observed in these systems, such as high exciton binding energy, 10,11 high radiative recombination rate, 9,10 and large energy splitting between dark and bright excitons. ...
