Columnar structures and stress relaxation in thick GaN films grown on sapphire by HVPE

Thick GaN films with high quality are directly grown on sapphire in a home-built vertical hydride vapour phase epitaxy (HVPE) reactor. The optical and structural properties of large scale columnar do-mains near the interface are studied using cathodoluminescence and micro-Raman scattering. These columnar do-mains Show a strong emission intensity due to extremely high free carrier concentration up to 2 x 10(19) cm(-3), which are related with impurities trapped in structural defects. The compressive stress in GaN Elm clearly decreases with increasing distance from interface. The quasi-continuous columnar domains play an important role in the stress relaxation for the upper high quality layer.

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.

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.

Liquid-phase-epitaxy-grown InAsxSb1-x/GaAs for room-temperature 8-12 mu m infrared detectors

High-quality InAsxSb1-x (0 < x <= 0.3) films are grown on GaAs substrates by liquid phase epitaxy and electrical and optical properties of the films are investigated, revealing that the films exhibit Hall mobilities higher than 2x10(4) cm(2) V-1 s(-1) and cutoff wavelengths longer than 10 mu m at room temperature (RT). Photoconductors are fabricated from the films, and notable photoresponses beyond 8 mu m are observed at RT. In particular, for an InAs0.3Sb0.7 film, a photoresponse of up to 13 mu m with a maximum responsivity of 0.26 V/W is obtained at RT. Hence, the InAsxSb1-x films demonstrate attractive properties suitable for room-temperature, long-wavelength infrared detectors. (c) 2006 American Institute of Physics.

Investigation of Mn-doped Si films prepared by magnetron cosputtering

Mn-doped Si films were prepared on Si(001) substrates by magnetron cosputtering and post-annealing process. The structural, morphological and magnetic properties of the films have been investigated. X-ray diffraction results show that the as-prepared film is amorphous. By annealing at 800 degrees C, however, the film is crystallized. There is no secondary phase found except Si in the two films. Chemical mapping shows that no segregation of the Mn atoms appears in the annealed film. Atomic force microscopy images of the films indicate that the annealed film has a granular feature that covers uniformly the film surface while there is no such kind of characteristic in the as-prepared film. The field dependence of magnetization was measured using an alternating gradient magnetometer, and it has been indicated that the annealed film shows room-temperature ferromagnetism. (c) 2006 Elsevier B.V. All rights reserved.

Room-temperature ferromagnetism in zinc-blende and deformed CrAs thin films

We try to clarify the controversy about the origin of room-temperature ferromagnetism in a CrAs compound. Two kinds of CrAs thin films were grown on GaAs by molecular-beam epitaxy. Structural analyses confirm that the as-grown CrAs film is a pure zinc-blende phase. Magnetic measurements suggest that room-temperature ferromagnetism exists in zinc-blende CrAs. In contrast, the CrAs film turns into a mixture of zinc-blende and deformed CrAs after annealing. A ferromagnetic signal measured at room temperature demonstrates that zinc-blende CrAs remains room-temperature ferromagnetism even when it is partly deformed into a non-zinc-blende phase. (c) 2006 American Institute of Physics.

High-precision determination of lattice constants and structural characterization of InN thin films

X-ray diffraction and Rutherford backscattering/channeling were used to characterize the crystalline quality of an InN layer grown on Al2O3(0001) Using metal-organic chemical-vapor deposition. A full width at half maximum of 0.27 degrees from an InN(0002) omega scan and a minimum yield of 23% from channeling measurements show that this 480-nm-thick InN layer grown at low temperature (450 degrees C) has a relatively good crystalline quality. High-resolution x-ray diffraction indicates that the InN layer contains a small fraction of cubic InN, besides the predominant hexagonal phase. From this InN sample, the lattice constants a=0.353 76 nm and c=0.570 64 nm for the hexagonal InN and a=0.4986 nm for the cubic InN were determined independently. 2 theta/omega-chi mapping and a pole figure measurement revealed that the crystallographic relationship among the cubic InN, the hexagonal InN, and the substrate is: InN[111]parallel to InN[0001]parallel to Al2O3[0001] and InN{110}parallel to InN{1120}parallel to Al2O3{1010}, and that the cubic InN is twinned. Photoluminescence measurements indicate that the band-gap energy of this sample is approximately 0.82 eV. (c) 2006 American Vacuum Society.

Structural and photoluminescent properties of ternary Zn1-xCdxO crystal films grown on Si(111) substrates

The ternary Zn1-xCdxO (0less than or equal toxless than or equal to0.6) alloying films with highly c-axis orientation have been deposited on Si(111) substrates by direct current reactive magnetron sputtering method. X-ray diffraction measurement indicates that the wurtzite-type structure of ZnO can be stabilized up to nominal Cd content x similar to 0.6 without cubic CdO phase separation. The lattice parameter c of Zn1-xCdxO increases almost linearly from 5.229 Angstrom (x = 0) to 5.247 Angstrom (x = 0.6), indicating that Cd substitution takes place on the Zn lattice sites. The photoluminescence spectra of the Zn1-xCdxO thin films measured at 12 K display a substantial red shift (similar to0.3 eV) in the near-band-edges (NBEs) emission of ZnO: from 3.39 eV of ZnO to 3.00 eV of Zn0.4Cd0.6O. The direct modulation of band gap caused by Zn/Cd substitution is responsible for the red shift effect in NBE emission of ZnO. (C) 2003 Elsevier Science B.V. All rights reserved.

Structural and optical characterization of Zn1-xCdxO thin films deposited by dc reactive magnetron sputtering

Zn1-xCdxO crystal thin films with different compositions were prepared on silicon and sapphire substrates by the dc reactive magnetron sputtering technique. X-ray diffraction measurements show that the Zn1-xCdxO films are of completely (002)-preferred orientation for x less than or equal to 0.6. For x = 0.8, the Elm is a mixture of ZnO hexagonal wurtzite crystals and CdO cubic crystals. For pure CdO, it is highly (200) preferential-oriented. Photoluminescence spectrum measurement shows that the Zn1-xCdxO (x = 0.2) thin film has a redshift of 0.14 eV from that of ZnO reported previously.

Preparation and characterization of the stable nc-Si/a-Si : H films

High-quality nc-Si/a-Si:H diphasic films with improved stability were prepared by using the plasma-enhanced chemical vapor deposition technology. In comparison with typical amorphous silicon, the diphasic silicon films possess higher photoconductivity (two orders larger than that of the amorphous silicon film) and fairly good photosensitivity(the ratio of the photo-to dark-conductivity is about 10) and higher stability (the degradation of the photoconductivity is less than 10% after 24h long light soaking with 50 mW/cm(2) intensity at room temperature). In addition, the diphasic silicon film has a better light spectra response in the longer wavelength range. The improvement in photoelectronic properties may be attributed to: the existence of the disorder within the amorphous matrix, which breaks the momentum selection rule in the optical transition and, consequently, results in the large light absorption coefficient and high photosensitivity; the improved medium range order and low gap states density. Excess carriers generated in the amorphous matrix tend to recombine in the embedded crystallites, which suppresses nonradiative recombination within the amorphous matrix and reduces the subsequent defect creation.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Nitrogen vacancy scattering in GaN grown by metal-organic vapor phase epitaxy

Electron mobility limited by nitrogen vacancy scattering was taken into account to evaluate the quality of n-type GaN grown by metal-organic vapor phase epitaxy. Two assumptions were made for this potential for the nitrogen vacancy (1) it acts in a short range, and (2) does not diverge at the vacancy core. According to the above assumptions, a general expression to describe the scattering potential U(r) = - U-0 exp[- (r/beta)(n)], (n = 1, 2,...,infinity) was constructed, where beta is the potential well width. The mobilities for n = 1, 2, and infinity were calculated based on this equation, corresponding to the simple exponential, Gaussian and square well scattering potentials, respectively. In the limiting case of kbeta << 1 (where k is the wave vector), all of the mobilities calculated for n = 1, 2, and infinity showed a same result but different prefactor. Such difference was discussed in terms of the potential tail and was found that all of the calculated mobilities have T-1/2 temperature and beta(-6) well width dependences. A mobility taking account of a spatially complicate scattering potential was studied and the same temperature dependence was also found. A best fit between the calculated results and experimental data was obtained by taking account of the nitrogen vacancy scattering. (C) 2002 Elsevier Science Ltd. All rights reserved.

Excitonic photoluminescence properties of nanocrystalline GaSb and Ga0.62In0.38Sb embedded in silica films

The GaSb and Ga0.62In0.38Sb nanocrystals were embedded in the SiO2 films by radio-frequency magnetron co-sputtering and were grown on GaSb and Si substrates at different temperatures. We present results on the 10K excitonic photoluminescence (PL) properties of nanocrystalline GaSb and Ga0.62In0.38Sb as a function of their size. The measurements show that the PL of the GaSb and Ga0.62In0.38Sb nanocrystallites follows the quantum confinement model very closely. By using deconvolution of PL spectra, origins of structures in PL were identified. (C) 2002 Elsevier Science B.V. All rights reserved.

Structural characterization of stable amorphous silicon films

A kind of hydrogenated diphasic, silicon films has been prepared by a new regime of plasma enhanced chemical vapor deposition (PECVD) in the region adjacent to the phase transition from amorphous to crystalline state. The photoelectronic and microstructural properties of the films have been investigated by the constant photocurrent method (CPM), Raman scattering and nuclear magnetic resonance (NMR). Our experimental results and corresponding analyses showed that the diphasic films, incorporated with a subtle boron compensation, could gain both the fine photosensitivity and high stability, provided the crystalline fraction (f) was controlled in the range of 0 < f < 0.3. When compared with the conventional hydrogenated amorphous silicon (a-Si:H), the diphasic films are more ordered and robust in the microstructure, and have a less clustered phase in the Si-H bond configurations. (C) 2002 Elsevier Science Ltd. All rights reserved.

Structural evaluation of polycrystalline silicon thin films by hot-wire-assisted PECVD

Tungsten wires were introduced into a plasma-enhanced chemical vapor deposition (PECVD) system as a catalyzer: we name this technique 'hot-wire-assisted PECVD' (HW-PECVD). Under constant deposition pressure (p(g)), gas flow ratio and catalyzer position, the effects of the hot wire temperature (T-f) on the structural properties of the poly-Si films have been characterized by X-ray diffraction (XRD), Raman scattering and Fourier-transform infrared (FTIR) spectroscopy. Compared with conventional PECVD, the grain size, crystalline volume fraction (X-e) and deposition rate were all enhanced when a high T-f was used. The best poly-Si film exhibits a preferential (220) orientation, with a full width at half-maximum (FWHM) of 0.2 degrees. The Si-Si TO peak of the Raman scattering spectrum is located at 519.8 cm(-1) with a FWHM of 7.1 cm(-1). The X-c is 0.93. These improvements are mainly the result of promotion of the dissociation of SiH4 and an increase in the atomic H concentration in the gas phase. (C) 2001 Elsevier Science B.V. All rights reserved.