Distorted wurtzite unit cells: Determination of lattice parameters of non-polar a-plane AlGaN and estimation of solid phase Al content

Unlike c-plane nitrides, ``non-polar" nitrides grown in e.g. the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (x_solid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of non-polar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for x_solid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well

Anisotropic structural and optical properties of a-plane (11-20) AlInN nearly-lattice-matched to GaN

We report epitaxial growth of a-plane (11-20) AlInN layers nearly-lattice-matched to GaN. Unlike for c-plane oriented epilayers, a-plane Al_{1-x}In_{x}N cannot be simultaneously lattice-matched to GaN in both in-plane directions. We study the influence of temperature on indium incorporation and obtain nearly-lattice-matched Al_{0.81}In_{0.19}N at a growth temperature of 760^{o}C. We outline a procedure to check in-plane lattice mismatch using high resolution x-ray diffraction, and evaluate the strain and critical thickness. Polarization-resolved optical transmission measurements of the Al_{0.81}In_{0.19}N epilayer reveal a difference in bandgap of ~140 meV between (electric field) E_parallel_c [0001]-axis and E_perpendicular_c conditions with room-temperature photoluminescence peaked at 3.38 eV strongly polarized with E_parallel_c, in good agreement with strain-dependent band-structure calculations

Demonstration of Acceptor-Like Traps at Positive Polarization Interfaces in Ga-Polar P-type (AlGaN/AlN)/GaN Superlattices

The shortcomings with acceptors in p-type III-nitride semiconductors have resulted in not many efforts being presented on III-nitride based p-channel electronic devices (here, field effect transistors (FETs)). The polarization effects in III-nitride superlattices (SLs) lead to the periodic oscillation of the energy bands, exhibiting enhanced ionization of the deep acceptors (Mg in this study), and hence their use in III-nitride semiconductor-based light-emitting diodes (LEDs) and p-channel FETs is beneficial. This study experimentally demonstrates the presence of acceptor-like traps at the positive polarization interfaces acting as the primary source of holes in Ga-polar p-type uniformly doped (AlGaN/AlN)/GaN SLs with limited Mg doping. The observed concentration of holes exceeding that of the dopants incorporated into the samples during growth can be attributed to the ionization of acceptor-like traps, located at 0.8 eV above the valence band of GaN, at positive polarization interfaces. All samples were grown using the metal organic vapor phase epitaxy (MOVPE) technique, and the materials’ characterization was carried out using X-ray diffraction and Hall effect measurements. The hole concentrations experimentally measured are juxtaposed with the calculated value of hole concentrations from FETIS®, and the measured trends in mobility are explained using the amplitude of separation of the two-dimensional hole gas in the systems from the positive polarization interfaces

Metal Organic Vapor Phase Epitaxy of Thick N-Polar InGaN Films

Hillock-free thick InGaN layers were grown on N-polar GaN on sapphire by metal organic vapor phase epitaxy using a digital growth scheme and H2 as surfactant. Introducing Mg to act as an additional surfactant and optimizing the H2 pulse time, In compositions up to 17% were obtained in 100 nm thick epilayers. Although Mg adversely affected the In incorporation, it enabled maintenance of a good surface morphology while decreasing the InGaN growth temperature, resulting in a net increase in In composition. The parameter space of growth temperature and Mg precursor flow to obtain hillock-free epilayers was mapped out

Demonstration of HCl-Based Selective Wet Etching for N-Polar GaN with 42:1 Selectivity to Al_0.24Ga_0.76N

A wet-etching technique based on a mixture of hydrochloric (HCl) and nitric (HNO3) acids is introduced, demonstrating exceptional 42:1 selectivity for etching N-polar GaN over Al0.24Ga0.76N. In the absence of an AlGaN etch stop layer, the etchant primarily targets N-polar unintentionally doped (UID) GaN, indicating its potential as a suitable replacement for selective dry etches in the fabrication of GaN high-electron-mobility transistors (HEMTs). The efficacy and selectivity of this etchant were confirmed through its application to a gate recess module of a deep-recess HEMT, where, despite a 228% over-etch, the 2.6 nm AlGaN etch stop layer remained intact. We also evaluated the proposed method for the selective etching of the GaN cap in the n+ regrowth process, achieving a contact resistance matching that of a BCl3/SF6 ICP process. These findings underscore the applicability and versatility of the etchant in both the electronic and photonic domains and are particularly applicable to the development of N-polar deep-recess HEMTs