S. X. Li

Structure and electronic properties of InN and In-rich group III-nitride alloys

The experimental study of InN and In-rich InGaN by a number of structural, optical and electrical methods is reviewed. Recent advances in thin film growth have produced single crystal epitaxial layers of InN which are similar in structural quality to GaN films made under similar conditions and which can have electron concentrations below 1 × 1018 cm−3 and mobilities exceeding 2000 cm2 (Vs)−1. Optical absorption, photoluminescence, photo-modulated reflectance and soft x-ray spectroscopy measurements were used to establish that the room temperature band gap of InN is 0.67 ± 0.05 eV. Experimental measurements of the electron effective mass in InN are presented and interpreted in terms of a non-parabolic conduction band caused by the k · p interaction across the narrow gap. Energetic particle irradiation is shown to be an effective method to control the electron concentration, n, in undoped InN. Optical studies of irradiated InN reveal a large Burstein–Moss shift of the absorption edge with increasing n. Fundamental studies of the energy levels of defects in InN and of electron transport are also reviewed. Finally, the current experimental evidence for p-type activity in Mg-doped InN is evaluated.

Universal bandgap bowing in group III nitride alloys

The energy gaps of MBE-grown wurtzite-structure In{sub 1-x}Al{sub x}N alloys with x {le} 0.25 have been measured by absorption and photoluminescence experiments. The results are consistent with the recent discovery of a narrow bandgap of {approx}0.8 eV for InN. A bowing parameter of 3 eV was determined from the composition dependence of these bandgaps. Combined with previously reported data of InGaN and AlGaN, these results show a universal relationship between the bandgap variations of group III nitride alloys and their compositions.

Group III-nitride alloys as photovoltaic materials

The direct gap of the In1-xGaxN alloy system extends continuously from InN (0.7 eV, in the near IR) to GaN (3.4 eV, in the mid-ultraviolet). This opens the intriguing possibility of using this single ternary alloy system in single or multi-junction (MJ) solar cells. A number of measurements of the intrinsic properties of InN and In-rich In1-xGaxN alloys (0 < x < 0.63) are presented and discussed here. To evaluate the suitability of In1-xGaxN as a material for space applications, extensive radiation damage testing with electron, proton, and alpha particle radiation has been performed. Using the room temperature photoluminescence intensity as a indirect measure of minority carrier lifetime, it is shown that In1-xGaxN retains its optoelectronic properties at radiation damage doses at least 2 orders of magnitude higher than the damage thresholds of the materials (GaAs and GaInP) currently used in high efficiency MJ cells. Results are evaluated in terms of the positions of the valence and conduction band edges with respect to the average energy level of broken-bond defects (Fermi level stabilization energy EFS). Measurements of the surface electron concentration as a function of x are also discussed in terms of the relative position of EFS. The main outstanding challenges in the photovoltaic applications of In1-xGaxN alloys, which include developing methods to achieve p-type doping and improving the structural quality of heteroepitaxial films, are also discussed.

Properties of native point defects in In1―xAlxN alloys

The electrical and optical properties of the In-rich InAlN alloys are strongly influenced by native point defects. Here the effects of the defects are studied using 2?MeV He+ irradiation to vary the defect concentration. Localized native defects in In1?xAlxN(x < 0.45) are predominantly donors, with energy levels located above the conduction band edge. Accordingly, the electron concentration increases and the optical absorption edge blue shifts with increasing irradiation fluence before saturating at high fluences. Saturation occurs when the Fermi level reaches the Fermi level stabilization energy, which is the average energy of localized native defects in semiconductors, at 4.9?eV below the vacuum level. The energy position of the native defects also explains the initial increase followed by the quenching of the photoluminescence (PL) intensity, as well as the blue shift in the PL peak, with increasing irradiation fluence.