Native deep level defects in ZnO single crystal grown by CVT method - art. no. 68410I

Hall effect, photoluminescence (PL), infrared absorption, deep level transient spectroscopy (DLTS), and Raman scattering have been used to study property and defects of ZnO single crystal grown by a chemical vapor transport method (CVT). As-grown ZnO is N type with free electron density Of 10(16)-10(17)cm(-3). It has a slight increase after 900 degrees C annealing in oxygen ambient. The DLTS measurement revealed four deep level defects with energy at 0.30eV, 0.50eV, 0.68eV and 0.90eV in the as-grown ZnO sample, respectively. After the high temperature annealing, only the 0.5eV defect survive and has a concentration increase. PL results of the as-grown and annealed ZnO indicate that the well-known green emission disappear after the annealing. The result suggests a correlation between the 0.68eV defect and the green PL peak. Results of P-doped ZnO were also compared with the undoped ZnO sample. The nature of the defects and their influence on the material property have been discussed.

Characterization of AlGaN on GaN template grown by MOCVD - art. no. 68410K

AlxGa1-xN layer was grown on sapphire substrate with GaN template by Metal Organic Chemical Vapor Deposition system (MOCVD). High temperature A1N (HT-A1N) interlayer was inserted between AlxGa1-xN layer and GaN template to solve the cracking problem that often appears on AlxGa1-xN surface when directly grown on high temperature GaN template. Optical microscope, scanning electron microscopy (SEM), atomic force microscope (AFM), high resolution x-ray diffraction (HRXRD) and cathodoluminescence (CL) were used for characterization. It was found that the cracking was successfully eliminated. Furthermore, the crystalline quality of AlxGa1-xN layer with HT-A1N interlayer was much improved. Interference fringes were found in the HRXRD images. CL test showed that yellow emission was much reduced for AlGaN layer with HT-A1N interlayer.

AlGaN layers grown on AlGaN buffer layer and GaN buffer layer using strain-relief interlayers - art. no. 68410S

We investigated AlGaN layers grown by metalorganic chemical vapor deposition (MOCVD) on high temperature (HT-)GaN and AlGaN buffer layers. On GaN buffer layer, there are a lot of surface cracking because of tensile strain in subsequent AlGaN epilayers. On HT-AlGaN buffer layer, not only cracks but also high densities rounded pits present, which is related to the high density of coalescence boundaries in HT-AlGaN growth process.The insertion of interlayer (IL) between AlGaN and the GaN pseudosubstrate can not only avoid cracking by modifying the strain status of the epilayer structure, but also improved Al incorporation efficiency and lead to phase-separation. And we also found the growth temperature of IL is a critical parameter for crystalline quality of subsequent AlGaN epilayer. Low temperature (LT-) A1N IL lead to a inferior quality in subsequent AlGaN epilayers.

High-brightness GaN-based blue LEDs grown on a wet-patterned sapphire substrate - art. no. 68410T

Patterning sapphire substrate can relax the stress in the nitride epilayer, reduce the threading dislocation density, and significantly improve device performance. In this article, a wet-etching method for sapphire substrate is developed. The effect of substrate surface topographies on the quality of the GaN epilayers and corresponding device performance are investigated. The GaN epilayers grown on the wet-patterned sapphire substrates by MOCVD are characterized by means of scanning electrical microscopy (SEM), atomic force microscopy (AFM), high-resolution x-ray diffraction (HRXRD), and photoluminescence (PL) techniques. In comparison with the planar sapphire substrate, about a 22% increase in device performance with light output power of 13.31 mW@20mA is measured for the InGaN/GaN blue LEDs grown on the wet-patterned sapphire substrate.

Zinc phthalocyanine (ZnPc) incorporated into silicon matrix grown by plasma enhanced chemical vapor deposition (PECVD)

Hybrid composites composed of zinc phthalocyanine embedded in silicon matrixes have attracted attention because of the potential for solar energy conversion. We produce hybrid composites by thermal evaporation for the plithalocyanine and PECVD (Plasma Enhanced Chemical Vapor Deposition) for the silicon matrix. Deposition of ZnPc/a-Si(amorphous silicon) composites was achieved in a sequential manner. The compound films were characterized by optical transmittance spectra and photoconductivity measurement. The optical transmittance measurements were carried out in the visible region (500 - 800 nm). Compared to pure silicon film, the photosensitivity of compound functional films was enhanced by one order of magnitude. This demonstrates the Si sensitized by adding ZnPc.

High Quality AlGaN Grown on GaN Template with HT-AlN Interlayer

When AlGaN is grown on GaN template, crack networks invariably generate when the thickness of the AlGaN layers over GaN exceeds the critical value. We used thin high temperature deposited AlN layer (HT-AlN) as the interlayer between GaN template and AlGaN epilayer which was very effective in eliminating the cracks in AlGaN epilayer. AlGaN layers with high Al mole fractions were also grown. Characterization showed that the crystalline quality of AlGaN epilayer was fairly good even when the At mole fraction was high.

AlGaN/AlN/GaN/InGaN/GaN DH-HEMTs with improved mobility grown by MOCVD

AlGaN/AlN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) structures with improved buffer isolation have been investigated. The structures were grown by MOCVD on sapphire substrate. AFM result of this structure shows a good surface morphology with the root-mean-square roughness (RMS) of 0.196 nm for a scan area of 5 mu mx5 mu m. A mobility as high as 1950 cm(2)/Vs with the sheet carrier density of 9.89x10(12) cm(-2) was obtained, which was about 50% higher than other results of similar structures which have been reported. Average sheet resistance of 327 Omega/sq was achieved. The HEMTs device using the materials was fabricated, and a maximum drain current density of 718.5 mA/mm, an extrinsic transconductance of 248 mS/mm, a current gain cutoff frequency of 16 GHz and a maximum frequency of oscillation 35 GHz were achieved.

Long-Wavelength Emission InAs Quantum Dots Grown on InGaAs Metamorphic Buffers

In this work, InAs quantum dots (QDs) grown on a linear graded InGaAs metamorphic buffer layer by molecular beam epitaxy have been investigated. The growth of the metamorphic buffer layers was carefully optimized, yielding a smooth surface with a minimum root mean square of roughness of less than 0.98 nm as measured by atomic force microscopy (AFM). InAs QDs were then grown on the buffer layers, and their emission wavelength at room-temperature is 1.49 mu m as measured by photoluminescence (PL). The effects of post-growth rapid thermal annealing (RTA) on the optical properties of the InAs QDs were investigated. After the RTA, the PL peak of the QDs was blue-shifted and the full width at half maximum decreased.

In-Situ Boron and Aluminum Doping and Their Memory Effects in 4H-SiC Homoepitaxial Layers Grown by Hot-Wall LPCVD

The in-situ p-type doping of 4H-SiC grown on off-oriented (0001) 4H-SiC substrates was performed with trimethylaluminum (TMA) and/or diborane (B2H6) as the dopants. The incorporations of Al and B atoms and their memory effects and the electrical properties of p-type 4H-SiC epilayers were characterized by secondary ion mass spectroscopy (SIMS) and Hall effect measurements, respectively. Both Al- and B-doped 4H-SiC epilayers were p-type conduction. It was shown that the profiles of the incorporated boron and aluminum concentration were in agreement with the designed TMA and B2H6 flow rate diagrams. The maximum hole concentration for the Al doped 4H-SiC was 3.52x10(20) cm(-3) with Hall mobility of about 1 cm(2)/Vs and resistivity of 1.6 similar to 2.2x10(-2) Omega cm. The heavily boron-doped 4H-SiC samples were also obtained with B2H6 gas flow rate of 5 sccm, yielding values of 0.328 Omega cm for resistivity, 5.3x10(18) cm(-3) for hole carrier concentration, and 7 cm(2)/VS for hole mobility. The doping efficiency of Al in SiC is larger than that of B. The memory effects of Al and B were investigated in undoped 4H-SiC by using SIMS measurement after a few run of doped 4H-SiC growth. It was clearly shown that the memory effect of Al is stronger than that of B. It is suggested that p-type 4H-SiC growth should be carried out in a separate reactor, especially for Al doping, in order to avoid the join contamination on the subsequent n-type growth. 4H-SiC PiN diodes were fabricated by using heavily B doped epilayers. Preliminary results of PiN diodes with blocking voltage of 300 V and forward voltage drop of 3.0 V were obtained.

Low threading-dislocation-density Ge film on Si grown on a pitting Ge buffer layer

A Ge layer with a pitting surface can be obtained when the growth temperature is lowered to 290 degrees C. On the low temperature Ge buffer layer with pits, high quality Ge layer was grown at 600 degrees C with a threading dislocation density of similar to 1x10(5)cm(-2). According to channeling and random Rutherford backscattering spectrometry spectra, a chi(min) value of 10% and 3.9% was found, respectively, at the Ge/Si interface and immediately under the surface peak. The root-mean-square surface roughness of Ge film was 0.33nm.

Investigations on oxide-free InGaAlAs waveguides grown by narrow stripe selective MOVPE

Oxide-free InGaAlAs waveguides have been grown on the InP substrates patterned with pairs of SiO2 mask stripes using narrow stripe selective MOVPE. The mask stripe width is varied from 0 to 40 pm, while the window region width between a pair of mask stripes is fixed at 1.5, 2.5 and 3.5 mu m, respectively. Smooth surface s and flat interfaces are obtained in the selectively grown InQaAlAs waveguides. There exhibit strong dependences of the thickness enhancement ratio and the photoluminescence (PL) spectrum on the mask stripe width and the window region width for the InGaAlAs wavegwdes. A large PL peak wavelength shift of 79 nm and a PL full width of at half maximum (FWHM) of less than 64 meV are obtained simultaneously. Some possible interpretations for our investigations are presented by considering both the migration effect from a masked region (MMR) and the lateral vapor diffusion effect (LVD).

Room temperature mobility above 2100 cm2/Vs in Al0.3Ga0.7N/AIN/GaN heterostructures grown on sapphire substrates by MOCVD

Al0.3Ga0.7N/AlN/GaN HEMT structures with significantly high mobility have been grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. At room temperature (RT) a Hall mobility of 2104 cm(2)/Vs and a two-dimensional electron gas (2DEG) density of 1.1x10(13) cm(-2) are achieved, corresponding to a sheet resistance of 277.8 Omega/sq. The elimination of V-shaped defects were observed on Al0.3Ga0.7N/AlN/GaN HEMT structures and correlated with the increase of 2DEG mobility. (c) 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim.

Optical and structural properties of ZnO films grown on Si(100) substrates by MOCVD - art. no. 60290G

High quality ZnO films have been successfully grown on Si(100) substrates by Metal-organic chemical vapor deposition (MOCVD) technique. The optimization of growth conditions (II-VI ratio, growth temperature, etc) and the effects of film thickness and thermal treatment on ZnO films' crystal quality, surface morphology and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectrum, respectively. The XRD patterns of the films grown at the optimized temperature (300 degrees C) show only a sharp peak at about 34.4 degrees corresponding to the (0002) peak of hexagonal ZnO, and the FWHM was lower than 0.4 degrees. We find that under the optimized growth conditions, the increase of the ZnO films' thickness cannot improve their structural and optical properties. We suggest that if the film's thickness exceeds an optimum value, the crystal quality will be degraded due to the large differences of lattice constant and thermal expansion coefficient between Si and ZnO. In PL analysis, samples all displayed only ultraviolet emission peaks and no observable deep-level emission, which indicated high-quality ZnO films obtained. Thermal treatments were performed in oxygen and nitrogen atmosphere, respectively. Through the analysis of PL spectra, we found that ZnO films annealing in oxygen have the strongest intensity and the low FWHM of 10.44 nm(106 meV) which is smaller than other reported values on ZnO films grown by MOCVD.

Deep levels in high resistivity GaN epilayers grown by MOCVD

Undoped high resistivity (HR) GaN epilayers were grown on (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Thermally stimulated current (TSC) and resistivity measurements have been carried out to investigate deep level traps. Deep levels with activation energies of 1.06eV and 0.85eV were measured in sample 1. Gaussian fitting of TSC spectra showed five deep levels in different samples. (c) 2006 WILEY VCH Vertag GmbH & Co. KGaA, Weinheim

Research on the band-gap of InN grown on siticon substrates

Photoluminescence (PL) and absorption experiments were carried out to examine the fundamental band-gap of InN films grown on silicon substrates. A strong PL peak at 0.78 eV was observed at room temperature, which is much lower than the commonly accepted value of 1.9 eV. The integrated PL intensity was found to depend linearly on the excitation laser intensity over a wide intensity range. These results strongly suggest that the observed PL is related to the emission of the fundamental inter-band transitions of InN rather than to deep defect or impurity levels. Due to the effect of band-filling with increasing free electron concentration, the absorption edge shifts to higher energy. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.