S. J. Chua

Effects of substrate on the structure and orientation of ZnO thin film grown by rf-magnetron sputtering

X-ray diffractions, Nomarski microscopy, scanning electron microscopy, and photoluminescence have been used to study the effects of substrate on the structure and orientation of ZnO thin films grown by rf-magnetron sputtering. GaAs(001), GaAs(111), Al2O3(0002) (c-plane), and Al2O3(11¯02) (r-plane) wafers have been selected as substrates in this study. X-ray diffractions reveal that the ZnO film grown on GaAs(001) substrate is purely textured with a high c-axis orientation while that grown on GaAs(111) substrate is a single ZnO(0002) crystal; a polycrystalline structure with a large-single-crystal area of ZnO(0002) is obtained on a c-plane Al2O3 substrate while a ZnO(112¯0) single crystal is formed on an r-plane Al2O3 substrate. There is absence of significant difference between the photoluminescence spectra collected from ZnO∕GaAs(001), ZnO∕GaAs(111), and ZnO∕Al2O3(0002), while the photoluminescence from ZnO∕Al2O3(11¯02) shows a reduced intensity together with an increased linewidth, which is, likely, due...

Characterization of inductively coupled plasma etched surface of GaN using Cl2/BCl3 chemistry

We have investigated plasma-induced damage of the inductively coupled plasma (ICP) etched surface of n-type GaN using Cl2/BCl3 chemistry. The surface morphology of the etched GaN under different plasma conditions is analyzed by atomic force microscopy. X-ray photoelectron spectroscopy is used to correlate the chemical changes induced by plasma etching of the GaN surface. We have carried out photoluminescence measurements of etched GaN surfaces subjected to varying ICP conditions. The intensity of the band-edge and yellow luminescence transitions was used to evaluate the damage introduced into the semiconductor during dry etching.

Annealing effects on electrical and optical properties of ZnO thin-film samples deposited by radio frequency-magnetron sputtering on GaAs (001) substrates

The effects of thermal annealing on Hall-effect measurement and photoluminescence (PL) from undoped n-type ZnO/GaAs thin-film samples have been studied. The evolutions of carrier concentration, electrical resistivity, and PL spectrum at various annealing conditions reveal that the dominant mechanism that affects the electrical and PL properties is dependent on the amount of thermal energy and the ambient pressure applied during the annealing process. At low annealing temperatures, annihilation of native defects is dominant in reducing the carrier concentration and weakening the low-energy tail of the main PL peak, while the GaAs substrate plays only a minor role in carrier compensations. For the higher temperatures, diffusion of Ga atoms from the GaAs substrate into ZnO film leads to a more n-type conduction of the sample. As a result, the PL exhibits a high-energy tail due to the high-level doping.

Effects of oxygen on low-temperature growth and band alignment of ZnO∕GaN heterostructures

Continuous ZnO thin films have been grown at low temperature (400°C) on GaN∕c-sapphire substrates in a radio-frequency magnetron-sputtering chamber employing a substoichiometric ZnO target with and without extra oxygen feeding. The effects of oxygen on the growth and band alignment of the ZnO∕GaN heterostructures were investigated by using scanning-electron microscopy, x-ray diffraction, photoluminescence and transmittance/absorbance, ultraviolet-resonant Raman scattering, and x-ray photoelectron spectroscopy. Very remarkable changes of the structural and optical properties resulted from the introduction of oxygen: the surface hexahedral facets were diminished; the size of the surface islands and, hence, the compressive strains were reduced; ultraviolet transparency of the ZnO film was enhanced, together with an increased band gap due to the reduced intrinsic shallow-donor defects; and hence, the free-electron concentration. The offset in valence bands of the ZnO∕GaN heterostructure was increased by ∼90me...

Effect of high temperature and interface treatments on photoluminescence from InGaN/GaN multiple quantum wells with green light emissions

InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.

Generation of amber III-nitride based light emitting diodes by indium rich InGaN quantum dots with InGaN wetting layer and AlN encapsulation layer

Indium rich InGaN nanostructures grown by metalorganic chemical vapor deposition were incorporated in InGaN/GaN quantum wells for long wavelength generation. These results were achieved by optimizing the growth temperature of the nanostructures, InGaN quantum well, the AlN capping layer and the GaN barrier layers. Before the growth of nanostructures, a thin InGaN wetting layer was included to reduce the lattice mismatch as well as to enhance the deposition of indium-rich InGaN nanostructures These individual quantum wells were each subsequently capped with an AlN layer which better preserved the In-rich phase in the nanostructures and prevented the indium interdiffusion between the InGaN/GaN heterojunctions. The AlN capping layer also reduces the effect of piezeoelectric field in the active layers of the light emitting diodes as seen from the reduction in the blueshift in the electroluminescence peaks with higher injection currents. The energy band profile of such a structure is discussed.

Strong screening effect of photo-generated carriers on piezoelectric field in In0.13Ga0.87N/In0.03Ga0.97N quantum wells

We report the strong screening effect of photo-generated carriers on piezoelectric field induced by lattice mismatch strain in In0.13Ga0.87N/In0.03Ga0.97N multiple quantum wells. Blue shifts as large as 83 meV and 120 meV of intrinsic transitions at 4 K and 80 K, respectively, are observed when excitation power is increased by two orders of magnitude. Self-consistent numerical calculations were carried out that involved simultaneously solving Schrodingers and Poissons equations in order to interpret the experimental data. Efficient screening of the huge piezoelectric field by photo-generated carriers in quantum well regions is demonstrated both experimentally and theoretically.

Crystallographically tilted and partially strain relaxed GaN grown on inclined {111} facets etched on Si(100) substrate

High resolution X-ray diffractometry (HR-XRD), Photoluminescence, Raman spectroscopy, and Transmission electron microscope measurements are reported for GaN deposited on a conventional Si(111) substrate and on the {111} facets etched on a Si(100) substrate. HR-XRD reciprocal space mappings showed that the GaN(0002) plane is tilted by about 0.63° ± 0.02° away from the exposed Si{111} growth surface for GaN deposited on the patterned Si(100) substrate, while no observable tilt existed between the GaN(0002) and Si(111) planes for GaN deposited on the conventional Si(111) substrate. The ratio of integrated intensities of the yellow to near band edge (NBE) luminescence (IYL/INBE) was determined to be about one order of magnitude lower in the case of GaN deposited on the patterned Si(100) substrate compared with GaN deposited on the conventional Si(111) substrate. The Raman E2(high) optical phonon mode at 565.224 ± 0.001 cm−1 with a narrow full width at half maximum of 1.526 ± 0.002 cm−1 was measured, for GaN deposited on the patterned Si(100) indicating high material quality. GaN deposition within the trench etched on the Si(100) substrate occurred via diffusion and mass-transport limited mechanism. This resulted in a differential GaN layer thickness from the top (i.e., 1.8 μm) of the trench to the bottom (i.e., 0.3 μm) of the trench. Mixed-type dislocation constituted about 80% of the total dislocations in the GaN grown on the inclined Si{111} surface etched on Si(100).

Channeling contrast microscopy of GaN and InGaN thin films

Abstract The recent development of blue and green light emitting diodes (LED) based on single quantum well structures made from GaN and related materials (AlGaN, InGaN) has created many efforts to achieve a complete characterisation of devices grown under various conditions. Here we report on CCM measurements on GaN thin films (d=0.7–3.0 μm) grown by metal organic vapour phase epitaxy (MOVPE) and on 500 A InGaN films grown epitaxially on top of the GaN thin films. The samples were analysed by broad beam channeling and channeling contrast microscopy (CCM), using 1–2 MeV H+ and He+ ions. Generally, very low minimum yields were found (χmin=2–4%), indicating nearly perfect crystal structures. The susceptibility to ion–beam induced damage was assessed by random and channeled 1 MeV He+ irradiation and subsequent CCM analysis. CCM also revealed the presence μm-sized regions in the InGaN films with increased In signal strength. The channeling PIXE data for 500 A thin films are found to be in excellent agreement with the corresponding RBS results, allowing the determination of channeling yields of elements for which RBS data is difficult to obtain.

Growth and Characterization of High Quality Continuous GaN Films on Si-Doped Cracked GaN Templates

It was found that continuous GaN films could be grown on cracked Si-doped GaN templates. Plan-view and cross section micrographs showed that no cracks occurred in the top GaN layer although it was grown on a cracked Si-doped GaN layer. The continuous GaN films on the cracked Si-doped GaN have been characterized by X-ray diffraction and Raman spectroscopy. It was shown that the top GaN film is of high crystal quality. A good surface morphology has been proven by scanning electron microscopy observations. Transmission electron microscopy analysis showed that the screw and/or mixed dislocation density in the top GaN film has been greatly reduced. However, pure edge dislocation density in the film remains unchanged.