Structural, electrical, and luminescence properties of (0001) ZnO epitaxial layers grown on c-GaN/sapphire templates by pulsed laser deposition technique

Abstract

A systematic study of growth, structural, electrical, and luminescence properties of zinc oxide (ZnO) layers grown on c-oriented GaN/sapphire templates by the pulsed laser deposition technique is carried out. A thorough high-resolution x-ray diffraction study reveals that c-ZnO films with high crystalline quality can be grown under certain growth conditions. Screw and edge dislocation densities in these films are found to be as low as 7×108 and 3×1010cm−2, respectively. All layers are found to be unintentionally n-type with ∼1019cm−3 electron concentration and mobility as high as ∼50 cm2 V−1 s−1. Temperature and excitation intensity dependent photoluminescence (PL) studies as functions of the growth conditions are carried out to identify the transition processes behind various luminescence features found in these samples. At low temperatures, PL spectra are marked by sharp neutral donor bound excitonic transitions, their phonon replicas, and two broad luminescence bands at 2.2 and 2.9 eV. These broad bands are attributed to transitions from the (2+/0) oxygen vacancy (VO) and (2+/+/0) zinc-interstitial (Zni) levels, respectively, to the valence band. Thermal energy needed to depopulate these defects is found to be 11 and 385 meV, respectively, for the (2+/0) VO and (2+/+/0) Zni levels. Low temperature PL spectra for the samples grown with relatively high oxygen pressures are featured by the Zn-vacancy (VZn) related neutral acceptor bound excitonic transition, its phonon replicas, and a broad band at 2.75 eV. This band diminishes with increasing temperature and, instead, another broad feature appears at ∼2.1 eV. Our study attributes the 2.75 eV band to transition from the conduction band to (0/−) VZn levels and the 2.1 eV feature to the transition between (−/2−) VZn levels and the valence band. It has been found that all the defect related features can be minimized by adjusting the growth conditions.

Study of lithium incorporation in (111) NiO epitaxial layers grown on c-sapphire substrates using the pulsed laser deposition technique

“…While the base broadening is associated with a much broader tilt distribution of the overgrown crystallites. The dislocation density (ρ) is calculated using equation, ρ = (FWHM) 2 4.35×b 2 , where the burger vector ⃗ b for screw and edge dislocations are 1 2 a ⟨111⟩ and 1 2 a ⟨ 1 10 ⟩ , respectively [30,31]. The density of screw dislocations is found to range between 5 and 7 × 10 7 cm −2 for samples grown at T G > 100 • C. However, this value increases Figure 3(a) shows the in-plane 2θ χ − ϕ scans for the samples grown at different temperatures.…”

“…Wide bandgap oxide semiconductors such as ZnO, Ga 2 O 3 and In 2 O 3 have tremendous potential for high temperature, high power, high frequency electronics as well as for ultra-violet (UV) emitters and detectors. However, unintentional generation of certain donor type of defects in the film during growth, makes it challenging to achieve stable p-type doping in these materials [1][2][3]. Nickel oxide (NiO) with a direct bandgap of 3.6-4 eV [4] is one of the few wide bandgap semiconductors, where stable p-type conductivity has often been reported.…”