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| IFGARD QD. Comparison between a conventional quantum dot (QD) structure (1st row) and a QD comprising the Internal Field Guarded Active Region Design (IFGARD, 2nd row). From left to right: the particular layer sequence is exemplified for the GaN/AlN case as illustrated in the corresponding 2D-scans in (a) and (d). The contour-plots in (b) and (e) show the sum of the piezo-and pyroelectric potential for the conventional and the IFGARD QD structure. As a consequence of such particular potential distributions, different conduction and valence band edge profiles are obtained for a linear scan through the QD centre along the c-axis as depicted in (c) and (f). While the conventional QD structure exhibits a prominent potential gradient (yellow à black) inside of the QD (b), the IFGARD QD features a constant potential inside of the QD as evidenced by the purple colouring in (e). As a result, flat-band conditions are achieved inside of the IFGARD QD in contrast to a strong band-edge tilt for the conventional case (f, c). Consequently, the potential gradient inside of the conventional QD structure separates the charge carriers as shown by the electron (blue) and hole (red) density of states in (c). In contrast, a drastically increased electron-hole overlap is obtained for the IFGARD QD (f) causing a beneficial boost in electron-hole oscillator strength and recombination rate.
Source publication
Modern opto-electronic devices are based on semiconductor heterostructures employing the process of electron-hole pair annihilation. In particular polar materials enable a variety of classic and even quantum light sources, whose on-going optimisation endeavours challenge generations of researchers. However, the key challenge - the inherent electric...
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... IFGARD is based on a simple, but at the same time counterintuitive idea. Commonly, the emissive, active material region (e.g. QD, QW) of devices features a smaller band gap than the matrix material in its surrounding providing the beneficial carrier confinement effect. In this situation, as exemplified in Fig. 1 a, any conventional device design strives to avoid additional layers that solely comprise the material of the active region to avoid light reabsorption. Hence, it appears as a ludicrous design to encapsulate this sandwich by the active region material as shown in Fig. 1 d. In this counterintuitive design a significant fraction of the ...
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... beneficial carrier confinement effect. In this situation, as exemplified in Fig. 1 a, any conventional device design strives to avoid additional layers that solely comprise the material of the active region to avoid light reabsorption. Hence, it appears as a ludicrous design to encapsulate this sandwich by the active region material as shown in Fig. 1 d. In this counterintuitive design a significant fraction of the emitted light gets reabsorbed by the, so-called, guard layers, casting doubt on the usability of the device -at first sight. However, guard material thicknesses below the emitted wavelength only absorb a well tolerable amount of emitted photons as discussed in the ...
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... order to exemplify the IFGARD, we first choose a GaN QD embedded in AlN -a selection that does not restrict the general applicability of the entire concept to a specific material system and/or nanostructure. Figure 1 summarizes the major differences between a conventional GaN QD and its IFGARD counterpart in the first and second row, focussing from left to right on the composition, the polarisation fields, and the band structure. Here, the horizontal c-axis denotes the most favourable, natural [0001] growth direction of III-nitrides. ...
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... nanostructure. Figure 1 summarizes the major differences between a conventional GaN QD and its IFGARD counterpart in the first and second row, focussing from left to right on the composition, the polarisation fields, and the band structure. Here, the horizontal c-axis denotes the most favourable, natural [0001] growth direction of III-nitrides. Fig. 1 a shows a GaN QD with a height along this c-axis of 2 nm (dark grey) embedded in a matrix of AlN (light grey), while the IFGARD equivalent features thin AlN barriers and additional GaN guard layers as depicted in Fig. 1 d. A significant interface charge built-up occurs at the AlN/GaN/AlN interfaces, yielding a huge polarisation ...
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... the band structure. Here, the horizontal c-axis denotes the most favourable, natural [0001] growth direction of III-nitrides. Fig. 1 a shows a GaN QD with a height along this c-axis of 2 nm (dark grey) embedded in a matrix of AlN (light grey), while the IFGARD equivalent features thin AlN barriers and additional GaN guard layers as depicted in Fig. 1 d. A significant interface charge built-up occurs at the AlN/GaN/AlN interfaces, yielding a huge polarisation gradient with a potential drop of ≈ 1.7 V for the conventional case - right across the QD as shown in the colour-coded image of Fig. 1 b. Naturally, the associated polarisation potential overlays the band structure, provoking the ...
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... while the IFGARD equivalent features thin AlN barriers and additional GaN guard layers as depicted in Fig. 1 d. A significant interface charge built-up occurs at the AlN/GaN/AlN interfaces, yielding a huge polarisation gradient with a potential drop of ≈ 1.7 V for the conventional case - right across the QD as shown in the colour-coded image of Fig. 1 b. Naturally, the associated polarisation potential overlays the band structure, provoking the band edges to be tilted right along the horizontal c-axis as shown in Fig. 1 c. As a result, not only a red-shift of the emission wavelength occurs, but also the electron and hole are spatially separated along the c-axis, lowering their overlap ...
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... interfaces, yielding a huge polarisation gradient with a potential drop of ≈ 1.7 V for the conventional case - right across the QD as shown in the colour-coded image of Fig. 1 b. Naturally, the associated polarisation potential overlays the band structure, provoking the band edges to be tilted right along the horizontal c-axis as shown in Fig. 1 c. As a result, not only a red-shift of the emission wavelength occurs, but also the electron and hole are spatially separated along the c-axis, lowering their overlap and subsequently the electron- hole pair annihilation rate. Figure 1 c illustrates this matter based on the electron (blue) and hole (red) density of states (profiles along ...
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... a result, not only a red-shift of the emission wavelength occurs, but also the electron and hole are spatially separated along the c-axis, lowering their overlap and subsequently the electron- hole pair annihilation rate. Figure 1 c illustrates this matter based on the electron (blue) and hole (red) density of states (profiles along the c-axis through the QD centre) and the corresponding overlap (coloured in green). The entire phenomenon that counteracts the confinement-induced blue- shift of the QD emission is known as the Quantum-Confined Stark Effect (QCSE) and has been studied in great profusion in the last decades. ...
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... the same charge built-up occurs for the IFGARD case depicted in Fig. 1 d. However, due to the inclusion of the guard layers, the polarisation potential gradient is now suspended from the QD. By adding two additional GaN/AlN interfaces as described by the IFGARD, one can suppress the electric field inside of the QD as depicted in Fig. 1 e. Here, the constant purple colouring of a major fraction of the QD ...
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... the same charge built-up occurs for the IFGARD case depicted in Fig. 1 d. However, due to the inclusion of the guard layers, the polarisation potential gradient is now suspended from the QD. By adding two additional GaN/AlN interfaces as described by the IFGARD, one can suppress the electric field inside of the QD as depicted in Fig. 1 e. Here, the constant purple colouring of a major fraction of the QD approves constancy for the sum of the piezo-and pyroelectric polarisation potential -the main benefit of the IFGARD. As a result, one obtains flat conduction and valence band edges within the QD, a strongly reduced charge carrier separation along the c-axis, and, as a ...
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... of the QD approves constancy for the sum of the piezo-and pyroelectric polarisation potential -the main benefit of the IFGARD. As a result, one obtains flat conduction and valence band edges within the QD, a strongly reduced charge carrier separation along the c-axis, and, as a direct consequence, an enhanced electron-hole overlap as shown in Fig. 1 f. Therefore, in comparison to the conventional case, the IFGARD raises the directly related oscillator strength by a factor of 20 for the common QD dimensions assumed in Fig. 1. This improvement directly translates to a factor of 20 in the rate of emitted, single photons from such a GaN QD. Please note that all detailed information ...
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... band edges within the QD, a strongly reduced charge carrier separation along the c-axis, and, as a direct consequence, an enhanced electron-hole overlap as shown in Fig. 1 f. Therefore, in comparison to the conventional case, the IFGARD raises the directly related oscillator strength by a factor of 20 for the common QD dimensions assumed in Fig. 1. This improvement directly translates to a factor of 20 in the rate of emitted, single photons from such a GaN QD. Please note that all detailed information regarding the simulations (8-band-kp implementation for wurtzite materials like nitrides) can be found in the SI, cf. Fig. S1. Here, also the particle interactions are considered ...
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... by a factor of 20 for the common QD dimensions assumed in Fig. 1. This improvement directly translates to a factor of 20 in the rate of emitted, single photons from such a GaN QD. Please note that all detailed information regarding the simulations (8-band-kp implementation for wurtzite materials like nitrides) can be found in the SI, cf. Fig. S1. Here, also the particle interactions are considered for the aforementioned electron-hole pairs (e) show the sum of the piezo-and pyroelectric potential for the conventional and the IFGARD QD structure. As a consequence of such particular potential distributions, different conduction and valence band edge profiles are obtained for a ...
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... However, some fine tuning of the AlN barrier thicknesses is needed in order to reach a fully optimised field cancellation for QDs not only due to their top and bottom facets of different size but also due to their inclined side facets. Please note that these top and bottom facets correspond to the left and right GaN/AlN interface of the QD in Fig. 1 and 2 in order to allow a convenient comparison. Figure 2 focuses on the influence of structural IFGARD parameters on the polarisation potential within another, here, 3- nm-high (h) QD shown in Fig. 2a. By varying the top AlN barrier thickness (t, red) above and the bottom barrier thickness (b, blue) below the QD, the gradient of the ...
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... inversion of the IFGARD stack (t ↔ b) does not significantly alter the PD value as indicated by the double triangle in Fig. 2 c (green and black) and the potential scans in Fig. 2 b that are coloured accordingly. Figure 2 c proves the fact that both, negative and positive PD values are accessible by the presented concept allowing the IFGARD to reach the desirable flat-band condition (compare Fig. 1 f) under any reasonable operating voltage in case of electrically driven devices. ...
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... by a factor of 100 if compared to the conventional, 3-nm-high QD embedded in AlN. In other words, the photon rate provided by each of such GaN QDs is increased by two orders of magnitude. Nevertheless, the advantages of the IFGARD even go beyond such a tremendous increase in overall QD brilliance. The absence of the QCSE for the IFGARD case in Fig. 1 d leads to a QD emission energy of 4.2 eV, which is now exclusively governed by the confinement, whereas the conventional QD from Fig. 1 a emits at 3.5 eV due to the red-shift induced by the additional QCSE. In direct comparison to the 50%-higher QD from Fig. 2 with emission energies of 2.9 eV and 4.0 eV, for the respective conventional and ...
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... the advantages of the IFGARD even go beyond such a tremendous increase in overall QD brilliance. The absence of the QCSE for the IFGARD case in Fig. 1 d leads to a QD emission energy of 4.2 eV, which is now exclusively governed by the confinement, whereas the conventional QD from Fig. 1 a emits at 3.5 eV due to the red-shift induced by the additional QCSE. In direct comparison to the 50%-higher QD from Fig. 2 with emission energies of 2.9 eV and 4.0 eV, for the respective conventional and optimum IFGARD constellations, the QD size dependence of the emission energies is reduced by a factor of three from 3.5 eV - 2.9 eV = 0.6 eV to 4.2 eV -4.0 eV = 0.2 eV. ...
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... in this figure, the GaN IFGARD QD from Fig. 1 d got replaced by a GaN QW, again exhibiting a horizontal orientation of the polar c-axis. Similar to the QD case in Fig. 1 e, interface-charges build up at each of the GaN/AlN or AlN/GaN interfaces of the IFGARD QW structure as illustrated in Fig. 3 a by the + (red) or -(black) signs. Due to this particular, reverse interface sequence ...
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... in this figure, the GaN IFGARD QD from Fig. 1 d got replaced by a GaN QW, again exhibiting a horizontal orientation of the polar c-axis. Similar to the QD case in Fig. 1 e, interface-charges build up at each of the GaN/AlN or AlN/GaN interfaces of the IFGARD QW structure as illustrated in Fig. 3 a by the + (red) or -(black) signs. Due to this particular, reverse interface sequence of the IFGARD it is now feasible to achieve flat-band conditions inside of this single-QW as shown in Fig. 3 b -top (black ...
Citations
... Generally, approaches to avoid the preferential [0001] wurtzite crystal growth are challenging as they often produce a reduced crystal quality frequently combined with comparably lower deposition rates [30][31][32][33][34][35] . A more promising method to control the internal electric field is the Internal-Field-Guarded-Active-Region Design (IFGARD) 36 , theoretically developed by Hönig et al. 37 . As described in detail in ref. 37 , a conventional structure of a GaN quantum well (QW) embedded in AlN barriers is complemented in the IFGARD structure by additional GaN guard layers enclosing the AlN barriers (see Fig. 1a-c). ...
... A more promising method to control the internal electric field is the Internal-Field-Guarded-Active-Region Design (IFGARD) 36 , theoretically developed by Hönig et al. 37 . As described in detail in ref. 37 , a conventional structure of a GaN quantum well (QW) embedded in AlN barriers is complemented in the IFGARD structure by additional GaN guard layers enclosing the AlN barriers (see Fig. 1a-c). This is not intuitive as the additional GaN guard layers reabsorb a certain percentage of the photons generated in the QW. ...
... This is not intuitive as the additional GaN guard layers reabsorb a certain percentage of the photons generated in the QW. But as discussed by Hönig et al. 37 , the overall gain based on the elimination of the polarization field in the QW and the resulting boost in the exciton recombination probability can overcompensate the reabsorption losses by the guard layers, if the thickness of the guard layer in the emission direction is below the emitted wavelength [38][39][40] . It was numerically demonstrated that this concept leads to a constant piezo-and pyroelectric polarization potential inside of a QW, which results in flat conduction-and valence-band edges therein (Fig. 1c). ...
Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or the related growth procedures. In this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the quantum-confined Stark effect in strongly polar [000-1] wurtzite GaN/AlN nanodiscs as evidenced by a reduction of the exciton lifetimes by up to four orders of magnitude. Furthermore, the tapered geometry of the utilized nanowires (which embed the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement and Stark emission energy shifts. Due to the IFGARD, both effects become independently adaptable.
... 31 Generally, approaches to avoid the preferential [0001] wurtzite crystal growth are challenging, slow, and they often produce a reduced crystal quality. [32][33][34][35][36][37] A more promising method to control the internal electric field is the Internal-Field-Guarded-Active-Region Design (IFGARD), 38,39 theoretically developed by Hönig et al.. 40 As described in detail in Ref. 40, a conventional structure of a GaN quantum well (QW) embedded in AlN barriers is complemented in the IFGARD structure by additional GaN guard layers enclosing the AlN barriers [see Fig. 1(a) + (c)]. This is not intuitive as the additional GaN guard layers reabsorb a particular percentage of the photons generated in the QW. ...
... 31 Generally, approaches to avoid the preferential [0001] wurtzite crystal growth are challenging, slow, and they often produce a reduced crystal quality. [32][33][34][35][36][37] A more promising method to control the internal electric field is the Internal-Field-Guarded-Active-Region Design (IFGARD), 38,39 theoretically developed by Hönig et al.. 40 As described in detail in Ref. 40, a conventional structure of a GaN quantum well (QW) embedded in AlN barriers is complemented in the IFGARD structure by additional GaN guard layers enclosing the AlN barriers [see Fig. 1(a) + (c)]. This is not intuitive as the additional GaN guard layers reabsorb a particular percentage of the photons generated in the QW. ...
... This is not intuitive as the additional GaN guard layers reabsorb a particular percentage of the photons generated in the QW. But as discussed by Hönig et al., 40 the overall gain based on the elimination of the polarization field in the QW and the resulting boost in the exciton recombination probability, can overcompensate the reabsorption losses by the guard layers, if the thickness of the guard layer in the emission direction is below the emitted wavelength. [41][42][43][44] It was numerically demonstrated that this concept leads to a constant piezo-and pyroelectric polarization potential inside of a QW, which results in flat conduction-and valence-band edges therein [ Fig. 1 (c)]. ...
Recently, we suggested an unconventional approach [the so-called Internal-Field-Guarded-Active-Region Design (IFGARD)] for the elimination of the crystal polarization field induced quantum confined Stark effect (QCSE) in polar semiconductor heterostructures. And in this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the QCSE in strongly polar [000-1] wurtzite GaN/AlN nanodiscs while reducing the exciton life times by more than two orders of magnitude. The IFGARD based elimination of the QCSE is independent of any specific crystal growth procedures. Furthermore, the cone-shaped geometry of the utilized nanowires (which embeds the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement- and QCSE-induced emission energy shifts. Due to the IFGARD, both effects become independently adaptable.
Quantum-confined Stark effects (QCSEs), where external or built-in electric fields modify optical transition energies ¹⁻⁶ , have garnered significant interest due to their potential for tuning emission energies to couple with quantum dots, metasurfaces and cavities, etc 1-3,7,8 . However, only external electric-field-enabled QCSEs in 2D semiconductors have been reported so far ¹⁻³ , owing to the challenges posed by small and uncontrollable built-in electric fields 2,3 , as well as charge modulation effects ⁹⁻¹⁴ . Here we report the first observation of giant built-in electric field-enabled QCSEs in 1L WSe2/1L graphene heterostructure (HS), based on chemical potential calculations. Electrical control of QCSEs demonstrates a maximum Stark shift of ~56.97 meV. This significant shift is attributed to enhanced built-in electric fields, resulting from the increased chemical potential difference induced by electrostatic doping. While increasing optical doping or reducing the interlayer distance, QCSEs weaken due to the reduced built-in electric fields. By leveraging efficient exciton dissociations from built-in electric fields 15,16 , the responsivity ( R ) and response speed of HS photodetectors increase by 6 orders of magnitude and 3 folds, respectively, compared to 1L WSe 2 . Our results offer a new avenue for expanding the tunability for excitons and exploiting the application potentials for 2D material in photodetectors, polariton transistors and quantum light sources.
