DOI: 10.18150/vkiqyi
title: Hydrostatic Pressure as a Tool for the Study of Semiconductor Properties—An Example of III–V Nitrides

description: This review highlights the unique role of hydrostatic pressure in studying semiconductor properties, focusing on III–V nitrides (GaN, AlN, InN) crystallizing in a wurtzite structure. Starting with high-pressure techniques for bulk nitride crystal growth, it examines how hydrostatic pressure aids experimental and theoretical investigations of nitride compounds, alloys, and quantum structures. Special attention is given to hydrostatic pressure and strain effects in short-period nitride superlattices. Discrepancies between theory and experiment in optical emission and its pressure dependence in InN/GaN superlattices led to the conclusion that InN growth on GaN substrates is not feasible. The role of built-in electric fields in InGaN/GaN and AlGaN/GaN heterostructures is discussed, showing how hydrostatic pressure modifies these effects and helps clarify their origins.The names of the individual files correspond to the numbering of the figures in the paper Gorczyca, I.; Suski, T.; Perlin, P.; Grzegory, I.; Kaminska, A.; Staszczak, G. Hydrostatic Pressure as a Tool for the Study of Semiconductor Properties—An Example of III–V Nitrides. Materials 2024, 17, 4022. https://doi.org/10.3390/ma1716402.Files included in this collection:Figure 1. Free energy of GaN and the system of its constituents at 1 bar and 10 kbar N2 pressures. Adapted from Ref. [4].Figure 2. Equilibrium N2 pressure for the nitrides considered. The maximum pressure and temperature available in the system are indicated by dashed lines. Adapted from Ref. [4].Figure 3. Pressure coefficients of the direct gaps of various III–V compounds as a function of the ratio between their ionicity fi and lattice constant a. Ionicity values are taken from Ref. [34].Figure 4. Bulk modulus for typical III–V compounds and nitrides as (a) function of their lattice constant, and (b) their lattice constant to ionicity ratio.Figure 5. Calculated total energies for (a) GaN and (b) AlN in wurtzite (wz) and rock salt (rs) structures as functions of the relative volume V/V0.Figure 6. Calculated InN effective masses as a function of electron concentration for two pressures: 0 and 10 GPa. Red lines represent p = 0 GPa, dashed blue lines represent p = 10 GPa. Based on Figures 6 and 7a in Ref. [48].Figure 7. Effect of pressure on the optical band gap Eopt at high-electron concentration. Black areas cover identical areas, as the number of electrons does not vary with pressure. Based on Figure 4 in Ref. [42].Figure 8.dEg/dp as a function of x in InxAl1−xN and InxGa1−xN. Symbols represent the experimental results. Solid and dashed lines represent the uniform and clustered arrangements of indium atoms, respectively. The experimental results for InxGa1−xN (stars) are obtained from Ref. [5] and for InxAl1−xN (circles) from Refs. [6,56].Figure 9. Schematic illustration of the emergence of the resonant DX state of oxygen (high doping level) in the gap under pressure.Figure 10. Schematic configuration coordinate diagram for oxygen displacement in AlGaN. The diagram includes the optical ionization energy (Eopt) as well as the capture (Ec) and emission (Ee) barriers. Based on the calculations in Ref. [65].Figure 11. Comparison of the band structure profiles along the c-axis of the wurtzite structure for a single QW without (left) and with (right) an internal electric field. F denotes the internal electric field. EL denotes the luminescence energy.Figure 12. (Color online). Schematic dependence of the PL peak position on pressure in GaN/Al0.3Ga0.7N for different QW thicknesses. Two cases are illustrated: QWs grown along the nonpolar (dashed lines) and polar (solid lines) crystallographic directions. Based on Figure 6 in Ref. [70].Figure 13. Dependence of the dEPL/dp on the QW width for QWs in GaN/AlxGa1−xN with different Al concentrations, x (Ref. [76]).Figure 14. Pressure dependencies of the EPL for three samples (A–C) of GaN/Al0.88In0.12N QWs. The values of the QW widths and pressure coefficients are given in the plots. Based on Figure 8 in Ref. [70].Figure 15. PL pressure coefficients of wurtzite (Ref. [75]) and cubic (Ref. [107]) In0.20Ga0.80N/GaN QWs as a function of the QW width.Figure 16. Comparison of the dEPL/dp for InGaN alloy and for InGaN/GaN QWs for different QW widths (as described in the Figure). Experimental data are from Ref. [5] (for the InGaN alloy) and Ref. [75] (for InGaN/GaN QWs). The polynomial fit to the theoretical dEg/dp values for the InGaN alloy is shown by the solid line.Figure 17. Photoluminescence spectra of (a) green SQW LED. (b) blue SQW LED measured at different pressures. Spectra are normalized for clarity. (c) PL peak positions as a function of pressure for blue (squares) and green (triangles) SQW LEDs compared to the pressure dependence of the InN and GaN band gaps.Figure 18. Band gap pressure coefficients of 1InxGa1−xN/nGaN SLs versus In content, x. Calculated results are indicated by diamonds, and experimental data by circles. The values of n are indicated. Lines are spline fits for guidance. Reproduced with permission from Ref. [116] (Figure 27). Copyright 2018 IOP Publishing CC BY licence.Figure 19. Schematic structures of (a) LED with tunnel junction above the QW and (b) LD structure with AlGaN cladding to form the waveguide. In both structures, the active layer consists of In0.17Ga0.83N QW.Figure 20. (a) EL spectra for LED1, LED2, and LED3 measured at different driving currents, (b) EL energy vs. current density for the studied LEDs. Different colors correspond to transitions between different electronic states.Figure 21. (a) Normalized EL spectra measured in LED3 for three selected values of the applied hydrostatic pressure: 0.1 GPa, 0.5 GPa, and 1 GPa. (b) Pressure dependence of the maxima of the EL spectra for the pressure range of 0.1–1.0 GPa.Figure 22. Pressure coefficient of the EL emission energy vs. driving current density in the studied LEDs. The energetic states of electrons and holes involved in radiative recombination are shown.Figure 23. Current density dependence of pressure coefficients for two LDs with QWs of different widths: (a) 2.6 nm and (b) 10.4 nm. The light green horizontal bars represent the pressure coefficient values corresponding to a fully screened built-in field. The violet bars represent the driving current range above the laser threshold.