Institute of High Pressure Physics

This Review provides a thorough description of the experimental progress on the InN family and other relevant compounds. Although InN is of great interest, many of its properties are not well understood and are still puzzling researchers with a number of unexpected effects. These include a surprisingly small energy gap, sensitivity to applied pressure in terms of lattice stability, and poor miscibility with compounds with smaller lattice parameters, such as GaN and AlN. Special features of InN under pressure are highlighted, such as the effect of conduction band filling and the strong pressure dependence of the effective mass. Several negative and positive effects due to the presence of In have been observed. We highlight their implications for InN-based alloys and quantum structures, which are crucial materials in modern optoelectronics (light emitting diodes and laser diodes). These effects include In clustering, large piezoelectricity resulting in strong internal electric fields that reduce the optical gain in nitride heterostructures, and difficulties in growing high-In superlattices and other quantum structures. All of these effects pose challenges that need to be addressed. We show that theoretical explanations allow for the clarification of puzzling experimental observations. Discussed are (i) a reformulation of the rule describing the bandgap dependence on pressure in all III–V semiconductors; (ii) the very large bandgap curvatures in nitride alloys; and (iii) the discrepancies between theory and experiment in the optical emission from InN/GaN superlattices, leading to the conclusion that epitaxial growth of high In content In_xGa_1−xN (x > 0.3) quantum wells on GaN is not possible.

The names of the individual files correspond to the numbering of the figures in the paper I. Gorczyca, T. Suski, P. Perlin, I. Grzegory, G. Staszczak, M. Aktas; Special role of indium nitride in the properties of related compounds and quantum structures. AIP Advances 1 April 2024; 14 (4): 040704. https://doi.org/10.1063/5.0198117.

Files included in this collection:

FIG. 1. Equilibrium N2 pressure over III–V nitrides.

FIG. 2. Pressure coefficients of the direct gap of various III–V compounds as a function of the ratio between their lattice constant and ionicity f, as calculated in Ref. 49. Based on Fig. 6 of Ref. 49.

FIG. 3. The CB energy vs free carrier concentration (solid line) in comparison with a parabolic model (dotted line). The experimental points (dots) represent the values of the absorption edges obtained in Ref. 14. Based on Fig. 5 of Ref. 56.

FIG. 4. The calculated effective mass of InN as a function of the electron concentration for two pressure values: 0 and 10 GPa. Based on Fig. 6 and 7(a) of Ref. 56.

FIG. 5. Schematic representation of the influence of hydrostatic pressure on the optical bandgap E_opt in the case of a high-electron concentration. Gray areas have identical surfaces, expressing the fact that the amount of electrons does not vary with pressure. The application of pressure leads to a larger bandgap, and the reduced interaction between CB and VB results in a flatter CB. The increase of E_opt and also E_PL is then adjusted with respect to the bandgap increase by the amount E_g − E_F. This is stated explicitly by Eq. (3). Based on Fig. 10 of Ref. 56.

FIG. 6. Bowings of the (a) InGaN bandgap [experimental points are from (a) Ref. 19 and (b) Ref. 45], (b) InAlN bandgap [experimental points are from (a) Ref. 72 and (b) Ref. 73], (c) dE_g/dp of InGaN [experimental points are from (a) Ref. 45 and (b) Ref. 67], and (d) dE_g/dp of InAlN [experimental points are from Ref. 67]. Two arrangements of In atoms: uniform and clustered are shown. (a) and (b) Based on Figs. 3 and 2 of Ref. 70. (c) and (d) Based on Figs. 8 and 9 of Ref. 67.

FIG. 7. Schematic arrangement of atoms for In_0.25Al₀.₇₅ N and In_0.25Ga₀.₇₅ N in the uniform and clustered cases; see the text for discussion. Nitrogen is in the center, and its four nearest neighbor cations are shown. The calculated equilibrium bond lengths in Å are given, and only the configuration around the nitrogen atoms with the largest number of In atoms as nearest neighbors is shown. Based on Figs. 10 and 11 of Ref. 70.

FIG. 8. Valence band DOS of In_0.25Al₀.₇₅N for two cases: uniform: black solid line and clustered In distribution: red dashed line. Based on Fig. 4 of Ref. 70.

FIG. 9. Valence band DOS for In_0.25Al₀.₇₅ N (red solid line) for the case of a clustered In distribution case. The decomposition into Al, green dashed line; In, blue short dashed line; and N, black solid line; partial DOS is shown. The inset shows a further decomposition of the nitrogen partial DOS into the contributions from N2 (indium nearest neighbors) and N1 (others). Based on Fig. 5 and 6 of Ref. 70.

FIG. 10. Calculated bandgaps, Eg, for mInN/nGaN SLs (denoted briefly as m/n) vs In cation fraction, x = m/(m + n), compared with calculations performed for In_xGa_1−xN alloys with uniform (solid curve) and clustered (dashed curve) In distribution. Lines are spline fits to the calculated values. Based on Fig. 1 of Ref. 78.

FIG.11. The valence- and conduction-band edge profiles in the 5InN/5GaN SL. Based on Fig. 3 of Ref. 78.

FIG. 12. Calculated bandgaps (dots) of 1In0.33Ga0.67N/nGaN SLs and InN/nGaN SLs compared with experimental data (open circles). Values of n are given. The lines are spline fits to guide the eye. Based on Fig. 4 of Ref. 84.

FIG. 13. Comparison of the calculated SL bandgaps E_g (solid lines) and PL emission energies (symbols). For SLs, we use the short notation: x/y instead of mIn_xGa_1−xN/nIn_yGa_1−yN. SLs 33/16.5 and 33/25 on buffers: GaN, In_0.165Ga_0.835N, and In_0.33Ga_0.67N. Dots, stars, and triangles represent experimental results for the sets of samples investigated. Based on Fig. 9 of Ref. 89.