Effect of substrate temperature on the growth and photoluminescence properties of vertically aligned ZnO nanostructures

Vertically well-aligned ZnO nanoridge, nanorod, nanorod-nanowall junction, and nanotip arrays have been successfully synthesized on Si (100) substrates using a pulsed laser deposition prepared ZnO film as seed layer by thermal evaporation method. Experimental results illustrated that the growth of different morphologies of ZnO nanostructures was strongly dependent upon substrate temperature. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies showed that the ZnO nanostructures were single crystals with a wurtzite structure. Compared with those of the other nanostructures, the photoluminescence (PL) spectrum of nanorod-nanowall junctions showed the largest intensity ratio of ultraviolet (UV) to yellow-green emission and the smallest full-width at half-maximum (FWHM) of the UV peak, reflecting the high optical quality and nearly defect free of crystal structure. The vertical alignment of the nanowire array on the substrate is attributed to the epitaxial growth of the nanostructures from the ZnO buffer layer. The growth mechanism was also discussed in detail. (c) 2006 Elsevier B.V. All rights reserved.

Preparation and AFM characterization of self-ordered porous alumina films on semi-insulated gaas substrate

Self-ordered porous alumina films on a semi-insulated GaAs substrate were prepared in oxalic acid aqueous solutions by three-step anodization. The I-t curve of anodization process was recorded to observe time effects of anodization. Atomic force microscopy was used to investigate structure and morphology of alumina films. It was revealed that the case of oxalic acid resulted in a self-ordered porous structure, with the pore diameters of 60-70 nm, the pore density of the order of about 10(10) pore cm(-2), and interpore distances of 95-100nm. At the same time the pore size and shape change with the pore widening time. Field-enhanced dissolution model and theory of deformation relaxation combined were brought forward to be the cause of self-ordered pore structure according to I-t curve of anodization and structure characteristics of porous alumina films. (c) 2006 Elsevier Ltd. All rights reserved.

Biaxial stress dependence of the electrostimulated near-band-gap spectrum of GaN epitaxial film grown on (0001) sapphire substrate

The biaxial piezospectroscopic coefficient (i.e., the rate of spectral shift with stress) of the electrostimulated near-band-gap luminescence of gallium nitride (GaN) was determined as Pi=-25.8 +/- 0.2 meV/GPa. A controlled biaxial stress field was applied on a hexagonal GaN film, epitaxially grown on (0001) sapphire using a ball-on-ring biaxial bending jig, and the spectral shift of the electrostimulated near-band-gap was measured in situ in the scanning electron microscope. This calibration method can be useful to overcome the lack of a bulk crystal of relatively large size for more conventional uniaxial bending calibrations, which has so far hampered the precise determination of the piezospectroscopic coefficient of GaN. The main source of error involved with the present calibration method is represented by the selection of appropriate values for the elastic stiffness constants of both film and substrate. The ball-on-ring calibration method can be generally applied to directly determine the biaxial-stress dependence of selected cathodoluminescence bands of epilayer/substrate materials without requiring separation of the film from the substrate. (c) 2006 American Institute of Physics.

Selective growth of InAs islands on patterned GaAs (100) substrate

By a combination of prepatterned substrate and self-organized growth, InAs islands are grown on the stripe-patterned GaAs (100) substrate by solid-source molecular beam epitaxy. It is found that the InAs quantum dots can be formed either on the ridge or on the sidewall of the stripes near the bottom, depending on the structure of the stripes on the patterned substrate or molecular beam epitaxy growth conditions. When a InxGa(1-x)As strained layer is grown first before InAs deposition, almost all the InAs quantum dots are deposited at the edges of the top ridge. And when the InAs deposition amount is larger, a quasi-quantum wire structure is found. The optical properties of the InAs dots on the patterned substrate are also investigated by photoluminescence. (c) 2005 Elsevier Ltd. All rights reserved.

P-doped p-type ZnO films deposited on Si substrate by radio-frequency magnetron sputtering

P-doped ZnO films were deposited on n-Si substrate by radio-frequency magnetron sputtering. Hall measurements revealed that the films annealed in situ at 750 degrees C in an oxygen ambient at a pressure of 1.3x10(-3)-3.9x10(-3) Pa showed p-type behavior with a hole concentration of 2.7x10(16)-2.2x10(17) cm(-3), a mobility of 4-13 cm(2)/V s, and a resistivity of 10.4-19.3 Omega cm. Films annealed at 750 degrees C in a vacuum or in oxygen ambient at higher pressures (5.2x10(-3) and 6.5x10(-3) Pa) showed n-type behavior. Additionally, the p-ZnO/n-Si heterojunction showed a diodelike I-V characteristic. Our results indicate that P-doped p-type ZnO films can be obtained by annealing in oxygen ambient at very low pressures. (c) 2006 American Institute of Physics.

Kinetic Monte Carlo simulation of spatially ordered growth of quantum dots on patterned substrate

We report the growth of well-ordered InAs QD chains by molecular beam epitaxy system. In order to analyze and extend the results of our experiment, a detailed kinetic Monte Carlo simulation is developed to investigate the effects of different growth conditions to the selective growth of InAs quantum dots (QDs). We find that growth temperature plays a more important role than growth rate in the spatial ordering of the QDs. We also investigate the effect of periodic stress on the shape of QDs in simulation. The simulation results are in good qualitative agreement with our experiment. (c) 2006 Elsevier Ltd. All rights reserved.

Influence of the cavity on the low-temperature photoluminescence of SiGe/Si multiquantum wells grown on a silicon-on-insulator substrate

The influences of the cavity on the low-temperature photoluminescence of Si0.59Ge0.41/Si multiquantum wells grown on silicon-on-insulator substrates are discussed. The positions of the modulated photoluminescence (PL) peaks not only relate to the nature of SiGe/Si multiquantum wells, but also relate to the characteristic of the cavity. With increasing temperature, a redshift of the modulated PL peak originating from the thermo-optical effect of the cavity is observed.

Study of nucleation positions of InAs islands on stripe-patterned GaAs(100) substrate

By combination of prepatterned substrate and self-organized growth, InAs islands are grown on the stripe-patterned GaAs (100) substrate by solid soul-cc molecular beam epitaxy. Four [011] stripe-patterned substrates different in pitch, depth, and sidewall angle, respectively, are used in this work. The surface morphology obtained by atomic force microscopy shows that the InAs quantum dots can be formed either on the ridge or on the sidewall of the stripes near the bottom, depending on the structure of the stripes on the patterned substrate. The mechanism determining the nucleation position of the InAs dots is discussed. The optical properties of the InAs dots on the patterned substrates are also investigated by photo luminescence. (c) 2005 Elsevier B.V. All rights reserved.

Growth of low-density InAs/GaAs quantum dots on a substrate with an intentional temperature gradient by molecular beam epitaxy

InAs was deposited by molecular beam epitaxy (MBE) on a GaAs substrate with an intentional temperature gradient from centre to edge. Two-dimensional (2D) to three-dimensional (3D) morphology evolution was found along the direction in which the substrate temperature was decreasing. Quantum dots (QDs) with density as low as similar to 8 x 10(6) cm(-2) were formed in some regions. We attribute the morphological evolution to the temperature-dependent desorption of deposited indium and the intermixing between deposited indium and gallium from the buffer.

Effect of a low-temperature thin buffer layer on the strain accommodation of In0.25Ga0.75As grown on a GaAs(001) substrate

Various low-temperature (LT) ultra-thin buffer layers have been fabricated on the GaAs (001) substrate. The buffer layer is decoupled from the host substrate by introducing low-temperature defects. The 400 nm In0.25Ga0.75As films were grown on these substrates to test the 'compliant' effects of the buffer layers. Atomic force microscopy, photoluminescence, double crystal x-ray diffraction and transmission electron microscopy were used to estimate the quality of the ln(0.25)Ga(0.75)As layer. The measurements indicated that the misfit strains in the epilayer can be accommodated by the LT ultra-thin buffer layer. The strain accommodation effects of the LT defects have been discussed in detail.

Strain accommodation of 3C-SiC grown on hydrogen-implanted Si (001) substrate

The hydrogen-implanted Si substrate has been used for the fabrication of the "compliant substrate", which can accommodate the mismatch strain during the heteroepitaxy. The compliance of the substrate can be modulated by the energy and dose of implanted hydrogen. In addition, the defects caused by implantation act as the gettering center for the internal gettering of the harmful metallic impurities. Compared with SiC films growth on substrate without implantation. all the measurements indicated that the mismatch strains in the SiC films grown on this substrate have been released and the crystalline qualities have been improved. It is a practical technique used for the compliant substrate fabrication and compatible with the semiconductor industry. (C) 2003 Elsevier B.V. All rights reserved.

In0.25Ga0.75As films growth on the thin GaAs/AlAs buffer layer on the GaAs(001) substrate

Thin GaAs/AlAs and GaAs/GaAs buffer layer structure have been fabricated on the GaAs(001) substrate. The top GaAs buffer layer is decoupled from the host substrate by introduction of a low temperature thin interlayer (AlAs or GaAs), which was mechanically behaved like the compliant substrate. Four hundred nanometer In0.25Ga0.75As films were grown on these substrates and the traditional substrate directly. Photoluminescence (PL), double-crystal X-ray diffraction (DCXRD) and atomic force microscopy (AFM) measurements were used to estimate the quality of the In0.25Ga0.75As layer and the compliant effects of the low temperature buffer layer. All the measurements shown that the qualities of epilayer have been improved and the substrate have been deteriorated severely. The growth technique of the thin GaAs/AlAs structure was found to be simple but very powerful for heteroepitaxy. (C) 2003 Elsevier Science B.V All rights reserved.

Effects of reactor pressure on GaN nucleation layers and subsequent GaN epilayers grown on sapphire substrate

The influence of reactor pressure on GaN nucleation layer (NL) and the quality of subsequent GaN on sapphire is studied. The layers were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrates and investigated by in situ laser reflectometry, atomic force microscope, scanning electron microscope, X-ray diffraction and photoluminescence. With the increase of reactor pressure prior to high-temperature GaN growth, the size of GaN nuclei formed after annealing decreases, the spacing between nucleation sites increases and the coalescence of GaN nuclei is deferred. The optical and crystalline qualities of GaN epilayer were improved when NLs were deposited at high pressure. The elongated lateral overgrowth of GaN islands is responsible for the quality improvement. (C) 2003 Elsevier Science B.V. All rights reserved.

Influence of heated catalyzer on thermal distribution of substrate in HWCVD system

Based on Stefan-Boltzman and Lambert theorems, the radiation energy distribution on substrate (REDS) from catalyzer with parallel filament geometry has been simulated by variation of filament and system layout in hot-wire chemical vapor deposition. The REDS uniformity is sensitive to the distance between filament and substrate d(f-s) when d(f-s) less than or equal to 4 cm. As d(f-s) > 4 cm, the REDS uniformity is independent of d(f-s) and is mainly determined by filament number and filament separation. Two-dimensional calculation shows that the REDS uniformity is limited by temperature decay at filament edges. The simulation data are in good agreement with experiments. (C) 2003 Elsevier Science B.V. All rights reserved.