Microstructure and electronic properties of InGaN alloys

Abstract

The In x Ga 1-x N system has electronic band gaps extending from under 1eV to 3.4 eV, and as such they are used as the active layer in commercially available visible-light emitting devices. There are many interesting features that make these nitride semiconductor alloys especially useful for efficient light emitters. It has been conjectured that the combination of piezoelectric fields and local composition inhomogeneities may be responsible for the observed high emission efficiencies, in spite of their characteristic high dislocation densities. But it is very difficult to grow In x Ga 1-x N layers with high indium composition. This paper presents an overview of the properties of In x Ga 1-x N epilayers based on a systematic study of thick layers and of quantum well structures. We find that the microstructure of thick films varies significantly with indium composition. For x < 0.08, the composition is uniform and unperturbed by dislocations. For 0.10 < x < 0.20, secondary phases nucleate at threading dislocations. For x > 0.20, spontaneous phase separation occurs resulting in a polycrystalline, inhomogeneous layer. A correlation between optical properties and microstructure is presented. It is observed that the misfit strain is affected by threading dislocations. Mechanisms of misfit strain relaxation are presented for In x Ga 1-x N layers grown on standard GaN on sapphire and on epitaxial-lateral-overgrowth GaN layers. In addition, we have studied the properties of quantum well structures using several novel techniques. The electrostatic fields across the wells have been profiled using electron holography in the TEM. The effect of well thickness on the strength of the fields is reported. The effects of localization by compositional fluctuations and of internal field screening have been studied using time-resolved cathodoluminescence spectroscopy. In spite of significant progress that has been made in the last ten years, much work remains ahead in order to master the science and technology of these alloys.

Intrinsic degradation mechanism of nearly lattice-matched InAlN layers grown on GaN substrates

“…31,54 The degradationobservedwith increasing thickness is generally attributed to strain relaxation, 31,34 and/or to V-defects due to threading dislocations, 34 and/or to phase separation due to spinodal decomposition. 55 The presentwork points out that even in the absence of stress and threading dislocations, In-rich nitride alloys grown with the (0001) orientation have a strong tendency to form V-defects, which can eventually lead to phase separation.This phase separation is not due to spinodal decomposition (In 0.17 Al 0.83 N was shown to be stable up to 960°C) 56 but to a different incorporation of indium on concave or convex shape surfaces of the V-defects. …”

Does In form In-rich clusters in InGaN quantum wells?

“…Thermodynamic calculations then showed that there is a miscibility gap in the In x Ga 1-x N system and that for typical compositions (x~0.1 for blue emission, x~0.2 for green) and typical InGaN growth temperatures (600-800 deg C) the unstrained homogeneous alloy is unstable, leading to decomposition into In-rich and In-poor regions [5]. It is not clear how applicable these equilibrium thermodynamic calculations are to the InGaN layers grown by MOVPE or Molecular Beam Epitaxy (MBE)(see later), but Ponce et al [6] have reported phase separation in MOVPE grown thick (200 nm) epilayers of InGaN with In fractions of 0.1 and above.…”

Defects, strain relaxation, and compositional grading in high indium content InGaN epilayers grown by molecular beam epitaxy

“…1,2 However, due to the immiscibility of alloy constituents, chemically induced phenomena, such as phase separation and compositional pulling, may take place, resulting in complex defect microstructures that are sensitive to the growth conditions. [3][4][5][6] The fluctuations in the indium concentration can be either long-or short-range in character. Short-range ones cause the local depletion of indium around regions that exhibit higher InN molar fraction.…”