Shen-Jie Wang

Growth of GaN on ZnO for Solid State Lighting Applications

In this work, ZnO has been investigated as a substrate technology for GaN-based devices due to its close lattice match, stacking order match, and similar thermal expansion coefficient. Since MOCVD is the dominant growth technology for GaN-based materials and devices, there is a need to more fully explore this technique for ZnO substrates. Our aim is to grow low defect density GaN for efficient phosphor free white emitters. However, there are a number of issues that need to be addressed for the MOCVD growth of GaN on ZnO. The thermal stability of the ZnO substrate, out-diffusion of Zn from the ZnO into the GaN, and H2 back etching into the substrate can cause growth of poor quality GaN. Cracks and pinholes were seen in the epilayers, leading to the epi-layer peeling off in some instances. These issues were addressed by the use of H2 free growth and multiple buffer layers to remove the cracking and reduce the pinholes allowing for a high quality GaN growth on ZnO substrate.

Metalorganic chemical vapor deposition of InGaN layers on ZnO substrates

InGaN layers have been grown on (0001) ZnO substrates by metalorganic chemical vapor deposition utilizing a low temperature grown thin GaN buffer. Good quality InGaN films with a wide range of In composition were confirmed by high-resolution x-ray diffraction. Even at high indium concentrations no In droplets and phase separation appeared, possibly due to coherent growth of InGaN on ZnO. Photoluminescence showed broad InGaN-related emissions with peak energy lower than the calculated InGaN band gap, possibly due to Zn/O impurities diffused into InGaN from the ZnO substrate. An activation energy of 59 meV for the InGaN epilayer is determined.

Development of new substrate technologies for GaN LEDs: atomic layer deposition transition layers on silicon and ZnO

Al2O3 layers have been deposited by atomic layer deposition (ALD) on both silicon and zinc oxide (ZnO) substrates as a transition layer for MOCVD growth of GaN. These Al2O3 layers have been shown to reduce tensile strain and cracking in GaN thin films on Si, and they have also been shown to help suppress impurity diffusion from the ZnO substrate into the GaN layers. Surface morphology of the ALD-grown layers was investigated using scanning electron microscopy (SEM), and structural properties were studied using high resolution x-ray diffraction (HR-XRD). GaN thin films were then grown on these layers to determine the effects of the Al2O3 layer on subsequent GaN quality. The optical and structural properties of these films were studied, as well as surface morphology. GaN layers grown using the Al2O3 layers on Si in particular exhibit structural and optical properties approaching those of typical GaN thin films on sapphire, which shows significant promise for high performance GaN-based devices on Si substrates.

Metalorganic chemical vapour deposition of GaN layers on ZnO substrates using α-Al2O3 as a transition layer

This work addresses the instability of a ZnO substrate during metalorganic chemical vapour deposition (MOCVD) growth of GaN by using Al2O3 films deposited by atomic layer deposition (ALD) as a stabilizing transition layer on the Zn face of ZnO (0 0 0 1) substrates. A systematic study of Al2O3 films of different thicknesses (2–90 nm) under different ALDs and post-annealing conditions was carried out. However, this paper focuses on as-deposited 20 and 50 nm Al2O3 films that were transformed to polycrystalline α-Al2O3 phases after optimal annealing at 1100 °C for 10 min and 20 min, respectively. GaN layers were grown on ZnO substrates with these α-Al2O3 transition layers by MOCVD using NH3 as a nitrogen source. Wurtzite GaN was observed by high resolution x-ray diffraction only on 20 nm Al2O3/ZnO substrates. Field-emission scanning electron microscopy showed a mirror-like surface, no etch pits and no film peeling in these samples. Room temperature photoluminescence showed a red-shift in the near band-edge emission of GaN, which may be related to oxygen incorporation forming a shallow donor-related level in GaN. Raman scattering also indicated the presence of a well-crystallized GaN layer on the 20 nm Al2O3/ZnO substrate.

Metalorganic Chemical Vapor Deposition of GaN and InGaN on ZnO Substrate using Al 2 O 3 as a Transition Layer

Al2O3 films were deposited on the Zn face of ZnO (0001) substrates as a transition layer by atomic layer deposition (ALD). The as-deposited 20 and 50nm Al2O3 films were transformed to polycrystalline α-Al2O3 phase after optimal annealing at 1100°C after 10 and 20 minutes, respectively, as identified by high resolution x-ray diffraction (HRXRD). Furthermore, GaN and InGaN layers were grown on annealed 20 and 50nm Al2O3 deposited ZnO substrates by metalorganic chemical vapor deposition (MOCVD) using NH3 as a nitrogen source at high growth temperature. Wurtzite GaN was only seen on the 20nm Al2O3/ZnO substrates. Room temperature photoluminescence (RT-PL) shows the near band-edge emission of GaN red-shifted, which might be from oxygen incorporation forming a shallow donor-related level in GaN. Raman scattering also indicated the presence of a wellcrystallized GaN layer on the 20nm Al2O3/ZnO substrate. InGaN was grown on bare ZnO as well as Al2O3 deposited ZnO substrates. HRXRD measurements revealed that the thin Al2O3 layer after annealing was an effective transition layer for the InGaN films grown epitaxially on ZnO substrates. Auger Electron Spectroscopy (AES) atomic depth profile shows a decrease in Zn in the InGaN layer. Moreover, (0002) InGaN layers were successfully grown on 20nm Al2O3/ZnO substrates after 10min annealing in a high temperature furnace.

Growth of InGaN with high indium content on ZnO based sacrificial substrates

ZnO has been considered as a substrate for epitaxial growth of III-Nitrides due to its close lattice and stacking order match. This paper will cover growth of InxGa1-xN epitaxial layer on lattice-matched ZnO substrates by metal-organic chemical vapor deposition (MOCVD). InGaN of various indium compositions from different growth temperatures were well controlled in the InGaN films on ZnO substrates. High-resolution X-ray diffraction (HRXRD) confirmed the epitaxial growth of InGaN film on ZnO. The optical and structural characterization of InGaN epilayer on ZnO substrates was measured by room temperature photoluminescence, temperature-dependent photoluminescence, and field-emission secondary electron microscope. In addition, a transition layer of Al2O3 on ZnO substrates have been employed for InGaN growth to help prevent Zn and O diffusion into the epilayers as well as assist nitride epilayer growth. HRXRD results show a single crystal InGaN film has been successfully grown on annealed Al2O3 coated ZnO substrates.

MOCVD Growth of High-Hole Concentration (>2×10 19 cm −3 ) P-Type InGaN for Solar Cell Application

InGaN alloys are widely researched in diverse optoelectronic applications. This material has also been demonstrated as a photovoltaic material. This paper presents the study to achieve optimum electrically active p-type InGaN epi-layers. Mg doped InGaN films with 20% In composition are grown on GaN templates/sapphire substrates by MOCVD. It is found that the hole concentration of p-type InGaN depends strongly on the Mg flow rate and V/III molar ratio and hole concentration greater than 2×10 19 cm −3 has been achieved at room temperature. The optimum activation temperature of Mg-doped InGaN layer has been found to be 550-600°C, which is lower than that of Mg-doped GaN. A solar cell was realized successfully using the InGaN epi-layers presented here.