S. P. Łepkowski

Piezoelectric field and its influence on the pressure behavior of the light emission from GaN/AlGaN strained quantum wells

We have studied the influence of hydrostatic pressure on the light emission from a strained GaN/AlGaN multiquantum well system. We have found that the pressure coefficients of the photoluminescence peak energies are dramatically reduced with respect to that of GaN energy gap and this reduction is a function of the quantum well thickness. The decrease of the light emission pressure coefficient may be as large as 30% for a 32 monolayer (8 nm) thick quantum well. We explain this effect by the hydrostatic-pressure-induced increase of the piezoelectric field in quantum structures. Model calculations based on the k×p method and linear elasticity theory reproduce the experimental results well, demonstrating that this increase may be explained by small anisotropy of the wurtzite lattice of GaN and a specific interplay of elastic constants and values of the piezoelectric tensor.

Universal behavior of photoluminescence in GaN-based quantum wells under hydrostatic pressure governed by built-in electric field

Correlation between the photoluminescence (PL) energy at ambient pressure and the pressure coefficient of photoluminescence is studied in quantum wells (QWs) based on nitride alloys, such as InGaN/GaN, GaN/AlGaN, and GaN/InAlN, grown along the polar direction [0001] of the wurtzite structure. Analyzing previously published and new experimental data, we have found that for InGaN/GaN QWs independent of In content (in the range between 6% and 25%) and also QW number and QW width, a linear relationship between these two parameters occurs. The presented experimental results are in agreement with numerical calculations carried out in the framework of the k→⋅p→ method with excitonic effects, provided that nonlinear piezoelectricity and nonlinear elasticity are taken into account. The performed analytical analysis indicates that the slope of the linear relationship between the pressure coefficient of photoluminescence and the photoluminescence energy at ambient pressure is determined by the logarithmic derivative...

Inapplicability of Martin transformation to elastic constants of zinc-blende and wurtzite group-III nitride alloys

The applicability of the Martin transformation [R. M. Martin, Phys. Rev. B 6, 4546 (1972)] to the elastic constants of wurtzite and zinc-blende group-III nitride alloys is examined using density functional theory calculations. The composition dependencies of the elastic constants in InGaN, AlGaN, and InAlN are determined by means of ab-initio calculations and compared with the results obtained from the Martins method. A detailed analysis reveals that the Martin transformation can approximate reasonably well the dependence of the elastic constants on composition in wurtzite InGaN alloys, except for the case of C33 where it predicts too small bowing. However, it fails to reproduce correctly the composition dependencies of C13 and C33 in wurtzite InAlN and C13, C33, and C44 in wurtzite AlGaN. In order to identify the origin of the failure of the Martin transformation, the effective elastic constants of strained wurtzite alloys with the ideal value of the lattice axial ratio c/a have been investigated. It is...

Strong electric field and nonuniformity effects in GaN∕AlN quantum dots revealed by high pressure studies

The photoluminescence (PL) from GaN quantum dots (QDs) embedded in AlN has been investigated under hydrostatic pressure. The measured pressure coefficient of emitted light energy [dEE∕dP] shows a negative value, in contrast with the positive pressure coefficient of the GaN band gap. We also observed that increasing pressure leads to a significant decrease of the light emission intensity and an asymmetric broadening of the PL band. All these effects are related to the pressure-induced increase of the built-in electric field. A comparison is made between experimental results and the proposed theoretical model which describes the pressure behavior of nitride QDs.

Anomalous Rashba spin–orbit interaction in electrically controlled topological insulator based on InN/GaN quantum wells

We study theoretically the topological phase transition and the Rashba spin-orbit interaction in electrically biased InN/GaN quantum wells. We show that that for properly chosen widths of quantum wells and barriers, one can effectively tune the system through the topological phase transition applying an external electric field perpendicular to the QW plane. We find that in InN/GaN quantum wells with the inverted band structure, when the conduction band s-type level is below the heavy hole and light hole p-type levels, the spin splitting of the subbands decreases with increasing the amplitude of the electric field in the quantum wells, which reveals the anomalous Rashba effect. Derived effective Rashba Hamiltonians can describe the subband spin splitting only for very small wave vectors due to strong coupling between the subbands. Furthermore, we demonstrate that for InN/GaN quantum wells in a Hall bar geometry, the critical voltage for the topological phase transition depends distinctly on the width of the structure and a significant spin splitting of the edge states lying in the 2D band gap can be almost switched off by increasing the electric field in quantum wells only by a few percent. We show that the dependence of the spin splitting of the upper branch of the edge state dispersion curve on the wave vector has a threshold-like behavior with the on/off spin splitting ratio reaching two orders of magnitude for narrow Hall bars. The threshold wave vector depends weakly on the Hall bar width, whereas it increases significantly with the bias voltage due to an increase of the energetic distance between the s-type and p-type quantum well energy levels and a reduction of the coupling between the subbands.

Topological phase transition and evolution of edge states in In-rich InGaN/GaN quantum wells under hydrostatic pressure

Combining the k · p method with the third-order elasticity theory, we perform a theoretical study of the pressure-induced topological phase transition and the pressure evolution of topologically protected edge states in InN/GaN and In-rich InGaN/GaN quantum wells. We show that for a certain range of the quantum well parameters, thanks to a negative band gap pressure coefficient, it is possible to continuously drive the system from the normal insulator state through the topological insulator into the semimetal phase. The critical pressure for the topological phase transition depends not only on the quantum well thickness but also on the width of the Hall bar, which determines the coupling between the edge states localized at the opposite edges. We also find that in narrow Hall bar structures, near the topological phase transition, a significant Rashba-type spin splitting of the lower and upper branches of the edge state dispersion curve appears. This effect originates from the lack of the mirror symmetry of the quantum well potential caused by the built-in electric field, and can be suppressed by increasing the Hall bar width. When the pressure increases, the energy dispersion of the edge states becomes more parabolic-like and the spin splitting decreases. A further increase of pressure leads to the transition to a semimetal phase, which occurs due to the closure of the indirect 2D bulk band gap. The difference between the critical pressure at which the system becomes semimetallic, and the pressure for the topological phase transition, correlates with the variation of the pressure coefficient of the band gap in the normal insulator state.

Magnetoconductance in InN/GaN quantum wells in topological insulator phase

We present a theoretical study of the magnetic-field effect on the electronic properties of the two-dimensional, hypothetical topological insulator based on the InN/GaN quantum well system. Using the effective two-dimensional Hamiltonian, we have modelled magneto-transport in mesoscopic, symmetric samples of such materials. It turns out that, as in the case of the other two-dimensional topological insulators, the magnetoconductance in such samples is quantized due to the presence of helical edge states for magnetic fields below a certain critical value and for fairly small disorder strength. However, in our case the helical edge transport is much more prone to the disorder than, for example, in the case of topological insulators based on the HgTe/CdTe quantum wells. At low enough level of disorder and for the Fermi energy located in the energy gap of an infinite planar quantum well, we may expect an interesting phenomenon of non-monotonic dependence of the conductance on the magnetic field caused by the complicated interplay of couplings between the heavy hole, light hole and conduction subbands.

Optical anisotropy in [0001] oriented GaN/AlxGa1−xN quantum wells under pressure

We investigate the influence of external pressure on optical anisotropy of GaN/AlxGa1−xN quantum wells (QWs) grown along the c-crystallographic direction. Our theoretical study reveals that for sufficiently narrow GaN/AlxGa1−xN QWs, lattice matched to GaN substrate a pressure-dependent switching of polarization of emitted light occurs. This switching of polarization is manifested by the change of sign of the degree of polarization of photoluminescence spectra. We note that the results of our model critically depend on the deformation potential values and therefore can be used for verification of existing literature values of these parameters.We investigate the influence of external pressure on optical anisotropy of GaN/AlxGa1−xN quantum wells (QWs) grown along the c-crystallographic direction. Our theoretical study reveals that for sufficiently narrow GaN/AlxGa1−xN QWs, lattice matched to GaN substrate a pressure-dependent switching of polarization of emitted light occurs. This switching of polarization is manifested by the change of sign of the degree of polarization of photoluminescence spectra. We note that the results of our model critically depend on the deformation potential values and therefore can be used for verification of existing literature values of these parameters.

Modeling of elastic, piezoelectric and optical properties of vertically correlated GaN/AlN quantum dots

We theoretically investigate elastic, piezoelectric and optical properties of wurtzite GaN/AlN quantum dots, having hexagonal pyramid-shape, stacked in a multilayer. We show that the strain existing in quantum dots and barriers depends significantly on the distance between the dots i.e. on the width of AlN barriers. For typical QDs, having the base diameter of 19.5nm, the drop of the electrostatic potential in the quantum dot region slightly decreases with decreasing of the barrier width. This decrease is however much smaller for QDs than for superlattice of GaN/AlGaN quantum wells, with thickness similar to the height of QDs. Consequently, the band-to-band transition energies in the vertically correlated GaN/AlN QDs show unexpected, rather weak dependence on the width of AlN barriers. Increasing the QD base diameter leads to stronger decreasing dependence of the band-to-band transition energies vs. the width of AlN barriers, similar to that observed for superlattieces of QWs.