Recent advances in the growth, doping and characterization of III–V nitride thin films

Abstract

Boron nitride thin films have been grown on the (100) surfaces of Si and diamond via ion beam assisted deposition (IBAD) using electron beam evaporation of B in tandem with N and Ar ion bombardment within the ranges of substrate temperature and ion flux of 200–700°C and 0.20–0.30 mA/cm², respectively. Fourier-transform infrared spectroscopy (FTIR) and high resolution transmission electron microscopy (HRTEM) revealed a growth sequence of amorphous (a-BN), hexagonal (h-BN) and cubic (c-BN) layers under most conditions. This sequence is attributed primarily to increasing biaxial compressive stress with film thickness due to ion bombardment and some interstitial Ar incorporation. A minimum substrate temperature of 200–300° C is required for nucleation and growth of single phase c-BN by this technique. The initial stage of AlN film growth on α(6H)-SiC(0001) substrates by plasma-assisted, gas source molecular beam epitaxy has been investigated in terms of growth mode and interface defects. Essentially atomically flat AlN surfaces, indicative of two-dimensional growth, were obtained using on-axis substrates. Island-like features were observed on the vicinal surfaces. The coalescence of latter features gave rise to double positioning boundaries as a result of the misalignment of the Si/C bilayer steps with the Al/N bilayers in the growing films. The quality of the thicker AlN films was strongly influenced by the concentration of these boundaries. Monocrystalline GaN and Al_xGa_1−x N(0001) (0≤x≤1) films, void of oriented domain structures and associated low-angle grain boundaries and with smooth surface morphologies, have been grown via OMVPE on high-temperature monocrystalline AlN(0001) buffer layers, previously deposited on vicinal α(6H)−SiC(0001) wafers, using TEG, TEA and ammonia in a cold-wall, vertical, pancake-style reactor. Abrupt heterojunctions were demonstrated. The PL spectrum of the pure GaN showed strong near band-edge emissions with a FWHM value of 4 meV. Cathodoluminescence spectra of Al_xGa_1−x N films for x<0.5 also showed intense near band-edge emission. The dislocation density within the first 0.5 μm was ≈1×10⁹ cm⁻²; it decreased substantially with increasing film thickness. Double-crystal XRC measurements indicated a FWHM value of 66 arc sec for the pure GaN(0004) reflection; the value of this parameter increased with increasing values of x. Controlled n-type Si-doping of pure GaN has been achieved for net carrier concentrations ranging from approximately 1×10¹⁷ cm⁻³ to 1×10²⁰ cm⁻³. As-deposited Si-doped Al_0.75Ga_0.25N exhibited negative electron affinity. Mg-doped, p-type GaN was achieved with n _A−n _D≈3×10¹⁷ cm⁻³, ρ≈7 Ω·cm and μ≈3 cm²/V·s.