Multiband Effective-mass Theory Research Articles

Tuning the Photoluminescence Anisotropy of Semiconductor Nanocrystals.

Semiconductor nanocrystals are promising optoelectronic materials. Understanding their anisotropic photoluminescence is fundamental for developing quantum-dot-based devices such as light-emitting diodes, solar cells, and polarized single-photon sources. In this study, we experimentally and theoretically investigate the photoluminescence anisotropy of CdSe semiconductor nanocrystals with various shapes, including plates, rods, and spheres, with either wurtzite or zincblende structures. We use defocused wide-field microscopy to visualize the emission dipole orientation and find that spheres, rods, and plates exhibit the optical properties of 2D, 1D, and 2D emission dipoles, respectively. We rationalize the seemingly counterintuitive observation that despite having similar aspect ratios (width/length), rods and long nanoplatelets exhibit different defocused emission patterns by considering valence band structures calculated using multiband effective mass theory and the dielectric effect. The principles are extended to provide general relationships that can be used to tune the emission dipole orientation for different materials, crystalline structures, and shapes.

Enhanced Light Emission from Type-II Red InGaN/GaNSb/GaN Quantum-Well Structures

Electronic and optical properties of type-II InGaN/GaNSb/GaN quantum-well (QW) structures are investigated by using the multiband effective mass theory for potential applications in red light-emitting diodes. The heavy-hole effective mass around the topmost valence band is not affected much by the insertion of the GaNSb layer, and the optical matrix elements are greatly increased by the inclusion of the GaNSb layer in the InGaN/GaN QW structure. As a result, the type-II InGaN/GaNSb/GaN QW structure shows a much larger emission peak than the conventional type-I QW structure owing to the decrease in spatial separation between electron and hole wavefunctions, in addition to the reduction of the effective well width. It is also observed that the In content in InGaN well can be significantly reduced for the type-II QW structure with a large Sb content, compared to that for the type-I QW structure.

Recombination Rate and Internal Field Efficiency of Staggered InGaN/GaN Light-Emitting Diodes with Left and Right Steps

In this study, the recombination rate and internal field efficiency of staggered InGaN/GaN quantum well lightemitting diodes with left and right steps were investigated using the multiband effective mass theory. The left step quantum well structure shows a larger optical matrix element than that of the right step quantum well structure. In addition, the heavy-hole effective mass around the uppermost valence band for the left step quantum well structure is slightly larger than that for the right step quantum well structure. Consequently, the staggered left step quantum well structure shows a much larger radiative recombination rate and internal quantum efficiency than that of the right step structure.

Optical anisotropy in type-II (110)-oriented GaAsSb/GaAs quantum wells

The optical anisotropy of zinc-blende type-II GaAsSb/GaAs quantum wells (QWs) with (110) crystal orientation was investigated by using the multiband effective-mass theory. The optical matrix element for the y′([11̄0])-polarization rapidly decreases with increasing in-plane wave vector. On the other hand, in the case of the x′([001])-polarization, the optical matrix element is nearly independent of the in-plane wave vector. The optical anisotropy gradually increases with increasing carrier density. This can be explained by the fact that, at a higher carrier density, carriers occupy higher states above k||=0 and the optical matrix element for the x′-polarization becomes larger than that for the y′-polarization at high k||.

Electronic and Optical Properties of Staggered ZnO/ZnO1−xSx-ZnO1−ySy/ZnO Quantum Wells for Bluish-Green Light-Emitting Diodes

The optical and electronic properties of staggered ZnO/ZnO1−xSx-ZnO1−ySy/ZnO (for x > y) bluish-green light-emitting diodes with quantum well (QW) structures are investigated using the multiband effective mass theory. The obtained results are compared with those of conventional ZnO1−xSx/ZnO QW structures, and the staggered QW structures are found to have a comparatively larger optical matrix, showing a substantial, yet gradual, reduction in the internal field effect. Thus, the staggered ZnO/ZnO1−xSx-ZnO1−ySy/ZnO QW structure design leads to a significantly higher spontaneous emission compared to that of conventional ZnO1−xSx/ZnO QW structures. This analysis indicates the potential of using a staggered QW structure for high-efficiency ZnO-based bluish-green LEDs.

Anisotropic strain effects on light emission characteristics of CdZnO/ZnO quantum well structures

Abstract Anisotropic strain effects by strain relaxation on TE-polarized light emission characteristics of c-plane CdZnO/ZnO quantum well (QW) structures were theoretically investigated by using the multiband effective-mass theory. The CdZnO/ZnO QW structure with anisotropic strain has much larger emission intensity than conventional CdZnO/ZnO QW structure without the strain relaxation. In the case of the strain relaxation along x(or y)-direction, the x(or y)-polarized light emission is observed to be larger than the x(or y)-polarized light emission. In particular, in the case of the strain relaxation along both x- and y-directions, the increase in the spontaneous emission peak is significant. This can be explained by the fact that the internal field is reduced owing to the decrease in the piezoelectric field by the strain relaxation.

Theoretical study of optical properties of non-polar BAlGaN/AlN quantum wells lattice-matched to AlN

Light emission characteristics of non-polar ultraviolet (UV) BxAlyGa1−x−yN/AlN quantum well (QW) structures lattice-matched to AlN substrate were theoretically investigated using the multiband effective-mass theory. The peak intensity of the non-polar BAlGaN/AlN QW structure lattice-matched to AlN is found to be much larger than that of the c-plane BAlGaN/BAlGaN QW structure lattice-matched to AlN. Also, the lattice-matched BAlGaN/AlN QW structure shows a large polarization ratio and its value changes from 0.7 to 0.9 in an investigated range of carrier density. We expect that non-polar BAlGaN/BAlGaN QW structures lattice-matched to AlN could be used as a UV light source with a higher crystal quality.

P-type wurtzite GaN/AlGaN quantum well structures for normal incidence inter-subband photodetectors at 1.55 µm

The inter-subband (ISB) absorption of wurtzite (WZ) p-type GaN/AlGaN quantum well (QW) structures was theoretically investigated as a function of Al content in the barrier by using the multiband effective mass theory. The GaN/AlGaN QW structure with x = 0.7 shows the peak wavelength of the TE-polarized absorption spectrum at 1.55 µm. The peak intensity of the TE-polarized absorption spectrum is much higher than that of the TM-polarized absorption spectrum. We expect that a p-type WZ GaN/AlGaN heterostructure with a relatively low Al content (x = 0.7) will be attractive for a normal incidence photodetector application for fiber-optic communications because the free hole concentration of about 1 × 1017 cm−3 is expected to be possible on the experimental side.

Strain and built-in fields in wurtzite GaN/AlxIn1−xN quantum wells and quantum dots

Strain and built-in fields in wurtzite (WZ) GaN/AlxIn1−xN quantum wells (QWs) and quantum dots (QDs) with piezoelectric (PZ) and spontaneous (SP) polarizations were investigated by using a multi-band effective mass theory. In the case of GaN/AlInN QW structures, the built-in field in the well nearly becomes zero for the QW structure with x = 0.7 while the potential well depth in the conduction band is very small. However, the GaN/AlInN QD structures show that the built-in field in the GaN dot does not become zero in a range of an investigated Al content and the carrier confinement is possible even for the QD structure with x = 0.7. The potential profiles of both QW and QD structures change from type-I to type-II at the Al content of x = 0.7. These results can be used as a design guide for fabrications of QD-based optoelectronic devices with a high emission intensity.

Intersubband absorption of p-type wurtzite GaN/AlN quantum well for fiber-optics telecommunication

The intersubband transition of wurtzite (WZ) p-type GaN/AlN quantum well (QW) structures grown on GaN substrate was investigated theoretically using the multiband effective-mass theory. The peak value of the TE-polarization absorption spectrum is found to be similar to that of the TM-polarization absorption spectrum. The absorption coefficients for TE- and TM-polarizations are mainly attributed to the absorption from the ground state (m1 = 1) because holes are mainly confined in ground states near the band-edge in an investigated range of the carrier density. We observe that a transition wavelength of 1.55 μm can be obtained for the QW structure with a relatively thin (∼16 Å) well width. Thus, we expect that a p-type WZ AlN/GaN heterostructure is applicable for a photodetector application for fiber-optic communications with normal incidence of wave.

Optical gain characteristics of a-plane GaN/AlGaN quantum well lasers grown on strain-engineered MgZnO layer

Optical gain characteristics of a-plane (11 2¯ 0) AlGaN/GaN quantum well (QW) lasers grown on a GaN buffer with strain anisotropy using a strain-engineered MgZnO layer were investigated using the multiband effective mass theory. The calculated transition energies for QW structures grown on MgZnO layer are in good agreement with experimental results. The optical gain of the QW structure grown on the MgZnO substrate is dominated by the z′-polarization because the dominant states constituting the topmost valence subband for the QW structure changes from |Y′>−to|Z′>-like and carriers occupy higher states above k∥=0 at a higher carrier density. On the other hand, the optical gain of the QW structure grown on conventional GaN buffer is dominated by both y′- and z′-polarizations. Thus, the optical polarization characteristics of a-plane AlGaN/GaN QW lasers can be engineered by using MgZnO substrate.

Dip-Shaped AlGaN/AlN Light-Emitting Diodes With Delta-Layer Containing Boron

Effects of a BAlGaN delta-layer on light emission characteristics of ultraviolet AlGaN/AlN quantum-well (QW) structures in light-emitting diodes were investigated using the multiband effective-mass theory. The reduction in the spatial separation between electron and hole wavefunctions is found to be significant with the inclusion of the BAlGaN delta-layer. The light emission intensity rapidly increases with increasing B content x <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> in the delta-layer. The peak intensity of the QW structure with the boron content of 0.06 appears to be two and half times larger than that of the QW structure with the conventional AlGaN delta-layer. On the other hand, the transition wavelength is nearly not affected by the inclusion of the BAlGaN delta layer.

Theoretical Studies on TM-Polarized Light Emission for Ultraviolet BAlGaN/AlN Optoelectronic Devices

The transverse-magnetic (TM) light emission characteristics of ultraviolet B x Al y Ga1– x – y N/AlN quantum well (QW) structures with several different Al contents ( $x = 0.7-0.9$ ) were theoretically investigated using the multiband effective-mass theory. The TM-polarized spontaneous emission peak of BAlGaN/AlN QW structures is found to be significantly enhanced with the inclusion of the boron. In particular, in the case of the QW structure with a relatively low Al content ( $y=0.7$ ), the light intensity is about three times larger than for the conventional QW structure. This can be explained by the decrease in the internal field, in addition to the change of the characteristics of the topmost valence subband.

Theoretical studies on ultraviolet nearly lattice-matched BAlGaN/BAlGaN quantum well structures with quaternary BAlGaN barriers

Light emission characteristics of BxAlyGa1−x−yN/BAlGaN quantum well (QW) structures with quaternary BAlGaN barrier were investigated using the multiband effective-mass theory. The internal field in the BAlGaN well of the BAlGaN/AlN QW structure is shown to be large (∼10 MV/cm) under the lattice-matched condition (%). On the other hand, the internal field is significantly reduced by using BAlGaN barrier and the peak intensity of the spontaneous emission coefficient of the lattice-matched BAlGaN/BAlGaN QW structure is comparable to that of the conventional BAlGaN/AlN QW structure. We observe that the light intensity of the lattice-matched BAlGaN/BAlGaN QW structure is comparable to that of the conventional BAlGaN/AlN QW structure. Also, we observe that the nearly lattice-matched BAlGaN/BAlGaN QW structure with small strain (0.5%) has about 3 times larger peak intensity than the conventional QW AlGaN/AlN structure. Hence, BAlGaN/BAlGaN QW system could be used as a UV light source with a higher light emission and a higher crystal quality, compared to conventional AlGaN/AlN QW structures with larger strain (1.78%).

Theoretical studies on light emission characteristics of high-efficiency BInGaN/GaN quantum well structures with blue spectral range

Light emission characteristics of blue BxInyGa1−x−y N/GaN quantum well (QW) structures were using the multiband effective-mass theory. The In composition (y) to give the transition wavelength of 440 nm is reduced with the inclusion of the B composition (x). We find that the spontaneous emission peak rapidly increases with increasing x. In particular, the peak intensity of the BInGaN/GaN QW structure with x = 0.18 and y = 0.008 is about twice larger than that of the conventional InGaN/GaN QW structure. This can be explained by the fact that the internal field (−0.40 MV/cm) in the BInGaN well is In greatly reduced, compared to that (−1.04 MV/cm) in the conventional InGaN well. Also, the strain (−1.7%) in the BInGaN well is shown to be reduced slightly, compare to that (−2.0%) in InGaN well. Hence, we expect that BInGaN/GaN QW structures with a reduced strain can be used as a light source with a high efficiency.

Optical polarization characteristics of m-plane GaN/AlGaN quantum well structures grown on m-plane SiC substrate

Abstract Optical polarization characteristics of m-plane GaN/AlGaN QW structures grown on m-plane SiC substrate were theoretically investigated using the multiband effective-mass theory. The QW structure grown on SiC substrate shows much larger in-plane optical polarization than that grown on GaN substrate. This is attributed to the fact that the QW structure grown on SiC substrate has larger y ′ -polarized optical emission and smaller x ′ -polarized optical emission than the QW structure grown on GaN substrate. Also, the magnitude of the optical polarization is found to depend on the carrier density and decrease gradually with increasing carrier density. This can be explained by the fact that, with increasing k ∥ , the x ′ -polarized matrix element increases while the y ′ -polarized matrix element rapidly decreases.

Polarization characteristics of 480–680 nm (1 1 [formula omitted] 2) InGaN/GaN quantum well structures with strain relaxation effects

The polarization characteristics as a function of the In composition for semipolar (112¯2) InGaN/GaN quantum well (QW) structures with strain relaxation effects are investigated by using the multiband effective-mass theory. We assumed that the strain relaxation ratio ∊y′y′0-∊y′y′/∊y′y′0 is linearly proportional to the difference in the lattice constants between the well and the substrate. The energy spacing between the first two subbands increases with increasing In composition, and the average hole effective mass of the first subband is reduced for a QW structure with a larger In composition. The [1¯1¯23] x′-polarized matrix elements are shown to be much larger than the [11¯00] y′-polarized matrix elements. As a result, the x′-polarized light emission is shown to be dominant and a weak function of the In composition. On the other hand, the light emission for y′-polarizations rapidly decreases with increasing In composition, mainly due to a decrease in the matrix elements. Thus, the absolute value of the polarization ratio rapidly increases with increasing transition wavelength.

Quaternary AlInGaN/InGaN quantum well on vicinal c-plane substrate for high emission intensity of green wavelengths

Electronic and optical properties of non-trivial semipolar AlInGaN/InGaN quantum well (QW) structures are investigated by using the multiband effective-mass theory and non-Markovian optical model. On vicinal c-plane GaN substrate miscut by a small angle (θ &amp;lt; 40°) from c-plane, the AlInGaN/InGaN system is shown to have ∼3 times larger spontaneous emission peak intensity than the conventional InGaN/GaN system at green wavelength. It is attributed to much larger optical matrix element of the quaternary AlInGaN/InGaN system, derived from the reduction of internal electric field induced by polarizations. This effect exceeds the performance-degrading factor of smaller quasi-Fermi-level separation for the quaternary AlInGaN/InGaN system than that for the conventional InGaN/GaN system. Results indicate that the use of quaternary III-nitride QWs on vicinal substrates may be beneficial in improving the performance of optical devices emitting green light.

Effects of AlGaN delta-layer insertion on light emission characteristics of ultraviolet AlGaN/AlN quantum well structures

The light emission characteristics of ultraviolet (UV) AlGaN/AlN quantum well (QW) structures with the AlGaN delta-layer in the active region were investigated using the multiband effective-mass theory and non-Markovian gain model. In a wavelength range of 220–280 nm, AlGaN/AlN QW structures with the ultrathin AlGaN layer are shown to have much larger light emission than conventional AlGaN/AlN QW structures. This can be explained by the fact that the internal field in the well is reduced and the optical matrix element is greatly enhanced with the inclusion of the low-bandgap AlGaN delta-layer. We expect that AlGaN/AlN QW structures with the ultrathin AlGaN layer can be used as a light source with a high efficiency in UV region.

High-efficiency BAlGaN/AlN quantum well structures for optoelectronic applications in ultraviolet spectral region.

Light emission characteristics of ultraviolet (UV) B(x)Al(y)Ga(1-x-y)N/AlN quantum well (QW) structures were using the multiband effective-mass theory. The TE-polarized spontaneous emission is found to be significantly improved owing to the decrease in the lattice-mismatch between the well and the substrate with the inclusion of boron. However, the spontaneous emission peak begins to decrease when the boron composition exceeds a critical value (x = 0.08 for y = 0.2), which is mainly due to an increase in the heavy-hole effective mass. In addition, in the case of QW structures with higher Al composition (y > 0.5), the light emission is shown to decrease with increasing the boron composition because the characteristic of the topmost valence subband is changed to the crystal-field splitoff hole band. Hence, we expect that B(x)Al(y)Ga(1-x-y)N/AlN QW structures with y < 0.5 can be used as a TE-polarized light source with a high efficiency.