This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1−xN and AlyGa1−yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty et al., Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1−xN and AlyGa1−yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point ≈45° tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1−xN, but the exact value depends on the In content. For AlyGa1−yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to ∼70° from c-axis oriented films. These calculations were performed u...

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Strain-induced polarization in wurtzite III-nitride semipolar layers

Growth of InGaN/GaN light-emitting devices on nonpolar or semipolar planes offers a viable approach to reducing or eliminating the issues associated with polarization-related electric fields present in c-plane III-nitride heterostructures. Although progress in device performance has been rapid since the introduction of high-quality free-standing nonpolar and semipolar GaN substrates, a full appreciation of the materials challenges unique to nonpolar and semipolar III-nitride semiconductors has been slower to emerge. Only recently have researchers begun to understand issues such as the origins of the pyramidal hillocks typically observed on nominally on-axis m-plane GaN films, the effects of m-plane substrate misorientation on surface morphology and device performance, the mechanics of anisotropic cracking in tensile strained m-plane AlGaN films, the formation of basal-plane stacking faults in long-wavelength m-plane InGaN quantum wells, and the mechanisms for stress relaxation in semipolar AlGaN and InGaN films. In this paper, we review the materials and growth issues unique to high-performance nonpolar and semipolar light-emitting devices grown on high-quality free-standing GaN substrates and provide an outlook for the opportunities and challenges that lie ahead. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0268-1242/27/2/024001/pdf

Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices

We have made AlGaN∕GaN high electron mobility transistors with a Si3N4 passivation layer that was deposited in situ in our metal-organic chemical-vapor deposition reactor in the same growth sequence as the rest of the layer stack. The Si3N4 is shown to be of high quality and stoichiometric in composition. It reduces the relaxation, cracking, and surface roughness of the AlGaN layer. It also neutralizes the charges at the top AlGaN interface, which leads to a higher two-dimensional electron-gas density. Moreover, it protects the surface during processing and improves the Ohmic source and drain contacts. This leads to devices with greatly improved characteristics.

Improvement of AlGaN∕GaN high electron mobility transistor structures by in situ deposition of a Si3N4 surface layer

III-nitride nanostructures have recently emerged as promising materials for new intersubband (ISB) devices in a wide variety of applications. These ISB technologies rely on infrared optical transitions between quantum-confined electronic states in the conduction band of GaN/Al(Ga)N nanostructures, namely quantum wells or quantum dots. The large conduction band offset (about 1.8 eV for GaN/AlN) and sub-picosecond ISB relaxation of III-nitrides render them appealing materials for ultrafast photonic devices in near-infrared telecommunication networks. Furthermore, the large energy of GaN longitudinal-optical phonons (92 meV) opens prospects for high-temperature THz quantum cascade lasers and ISB devices covering the 5?10 THz band, inaccessible to As-based technologies due to phonon absorption. In this paper, we describe the basic features of ISB transitions in III-nitride quantum wells and quantum dots, in terms of theoretical calculations, material growth, spectroscopy, resonant transport phenomena, and device implementation. The latest results in the fabrication of control-by-design devices such as all-optical switches, electro-optical modulators, photodetectors, and lasers are also presented.

https://iopscience.iop.org/article/10.1088/0268-1242/28/7/074022/pdf

III-nitride semiconductors for intersubband optoelectronics: a review

We present a methodology to determine critical dimensions for coherently strained coaxial nanowire heterostructures based on a well-known formalism used to determine the critical thickness in planar epitaxial growth. The unique geometry of the nanowire structure along with the volumetric similarity of the core and shell regions give rise to a number of possible stable core-shell configurations for a given choice of materials. We show that a unique critical core radius and critical shell thickness, dependent on core radius, can quantify these configurations. Illustrative calculations are presented for various nitride semiconductor-based core-shell structures. It is anticipated that this model will serve as a guide to determine the feasibility of specific coherently strained nanowire heterostructure designs.

Critical dimensions in coherently strained coaxial nanowire heterostructures

Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3⟨1123⟩∕{1122} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mec...

P. Provencio

Papers

Misfit dislocation formation in the AlGaN∕GaN heterointerface