Microstructures of microcrystalline silicon thin films prepared by hot wire chemical vapor deposition

Undoped hydrogenated microcrystalline silicon (mu c-Si:H) thin films were prepared at low temperature by hot wire chemical vapor deposition (HWCVD). Microstructures of the mu c-Si:H films with different H-2/SiH4 ratios and deposition pressures have been characterized by infrared spectroscopy X-ray diffraction (XRD), Raman scattering, Fourier transform (FTIR), cross-sectional transmission electron microscopy (TEM) and small angle X-ray scattering (SAX). The crystallization of silicon thin film was enhanced by hydrogen dilution and deposition pressure. The TEM result shows the columnar growth of mu c-Si:H thin films. An initial microcrystalline Si layer on the glass substrate, instead of the amorphous layer commonly observed in plasma enhanced chemical vapor deposition (PECVD), was observed from TEM and backside incident Raman spectra. The SAXS data indicate an enhancement of the mass density of mu c-Si:H films by hydrogen dilution. Finally, combining the FTIR data with the SAXS experiment suggests that the Si--H bonds in mu c-Si:H and in polycrystalline Si thin films are located at the grain boundaries. (C) 2000 Elsevier Science S.A. All rights reserved.

Annealing behavior of hexagonal phase content in cubic GaN thin films grown on GaAs (001) by MOCVD

The annealing behavior of the hexagonal phase content in cubic GaN (c-GaN) thin films grown on GaAs (001) by MOCVD is reported. C-GaN thin films are grown on GaAs (001) substrates by metalorganic chemical vapor deposition (MOCVD). High temperature annealing is employed to treat the as-grown c-GaN thin films. The characterization of the c-GaN films is investigated by photoluminescence (PL) and Raman scattering spectroscopy. The change conditions of the hexagonal phase content in the metastable c-GaN are reported. There is a boundary layer existing in the c-GaN/GaAs film. When being annealed at high temperature, the intensity of the TOB and LOB phonon modes from the boundary layer weakens while that of the E-2 phonon mode from the hexagonal phase increases. The content change of hexagonal phase has closer relationship with annealing temperature than with annealing time period.

Growth and transport properties of InAs thin films on GaAs

High-quality InAs epitaxial layers have been grown on (1 0 0) oriented semi-insulating GaAs substrates by MBE. The transport properties of largely lattice mismatched InAs/GaAs heterojunctions have been investigated by Hall effect measurements down to 10 K. In spite of a high dislocation density at the heterointerface, very high electron mobilities are obtained in the InAs thin films. By doping Si into the layer far from the InAs/GaAs interface, we found that the doped samples have higher electron mobility than that of the undoped samples with the same thickness. The mobility demonstrates a pronounced minimum around 300 K for the undoped sample. But for Si-doped samples, no pronounced minimum has been found. Such abnormal behaviours are explained by the parallel conduction from the quasi-bulk carriers and interface carriers. These high-mobility InAs thin films are found to be suitable materials for making Hall elements. (C) 1998 Elsevier Science B.V. All rights reserved.

Effect of reflectivity at the interface of oxide layer on transverse mode control in oxide confined vertical-cavity surface-emitting lasers

The transverse mode control in oxide confined vertical-cavity surface-emitting lasers is discussed by modeling the dielectric aperture as a uniform waveguide and an extra reflectivity at the oxide layer. The phase of the extra reflectivity and the refractive index step can be adjusted to change the mode threshold gain. We calculate the lateral refractive index step from the mode wavelength difference between aperture and perimeter modes, and compare it with that obtained from the weighted average index. The mode reflectivity in terms of the lateral optical confinement factor at the oxide layer is considered in calculating the threshold gain for transverse modes. The numerical results show that higher transverse modes can be suppressed by adjusting the position of a thin AlAs-oxide layer inside a three-quarter-wave layer in the distributed Bragg reflector. (C) 1998 American Institute of Physics. [S0021-8979(98)04007-9].

Wurtzite GaN epitaxial growth on a Si(001) substrate using gamma-Al2O3 as an intermediate layer

Wurtzite GaN films have been grown on (001) Si substrates using gamma-Al2O3 as an intermediate layer by low pressure (similar to 76 Torr) metalorganic chemical vapor deposition. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements revealed that the thin gamma-Al2O3 layer of "compliant" character was an effective intermediate layer for the GaN film grown epitaxially on Si. The narrowest linewidth of the x-ray rocking curve for (0002) diffraction of the 1.3 mu m GaN sample was 54 arcmin. The orientation relationship of GaN/gamma-Al2O3/Si was (0001) GaN parallel to(001) gamma-Al2O3 parallel to(001) Si, [11-20] GaN parallel to[110] gamma-Al2O3 parallel to[110] Si. The photoluminescence measurement for GaN at room temperature exhibited a near band-edge peak of 365 nm (3.4 eV). (C) 1998 American Institute of Physics.

Correlation Between Lattice Strain and Energy Gap Bowing of AlxGa1-xN Epitaxial Thin Films

The correlation between the energy band-gap of AlxGa1-xN epitaxial thin films and lattice strain was investigated using both High Resolution X-ray Diffraction (HRXRD) and Spectroscopic Ellipsometry (SE). The Al fraction, lattice relaxation, and elastic lattice strain were determined for all AlxGa1-xN epilayers, and the energy gap as well. Given the type of intermediate layer, a correlation trend was found between energy band-gap bowing parameter and lattice mismatch, the higher the lattice mismatch is, the smaller the bowing parameter (b) will be.

Heteroepitaxial Growth of 3C-SiC on Si (111) Substrate using AlN as a Buffer Layer

Using AlN as a buffer layer, 3C-SiC film has been grown on Si substrate by low pressure chemical vapor deposition (LPCVD). Firstly growth of AlN thin films on Si substrates under varied V/III ratios at 1100 degrees was investigated and the (002) preferred orientational growth with good crystallinity was obtained at the V/III ratio of 10000. Annealing at 1300 degrees C indicated the surface morphology and crystallinity stability of AlN film. Secondly the 3C-SiC film was grown on Si substrate with AlN buffer layer. Compared to that without AlN buffer layer, the crystal quality of the 3C-SiC film was improved on the AlN/Si substrate, characterized by X-ray diffraction (XRD) and Raman measurements.

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.

Quantifying the effectiveness of SiO2/Au light trapping nanoshells for thin film poly-Si solar cells

In order to enhance light absorption of thin film poly-crystalline silicon (TF poly-Si) solar cells over a broad spectral range, and quantify the effectiveness of nanoshell light trapping structure over the full solar spectrum in theory, the effective photon trapping flux (EPTF) and effective photon trapping efficiency (EPTE) were firstly proposed by considering both the external quantum efficiency of TF poly-Si solar cell and scattering properties of light trapping structures. The EPTF, EPTE and scattering spectrum exhibit different behaviors depending on the geometric size and density of nanoshells that form the light trapping layer. With an optimum size and density of SiO2/Au nanoshell light trapping layer, the EPTE could reach up to 40% due to the enhancement of light trapping over a broad spectral range, especially from 500 to 800 nm.

Spectroscopic ellipsometry study of In2O3 thin films

In2O3 films grown by helicon magnetron sputtering with different thicknesses were characterized by spectroscopic ellipsometry in the energy range from 1.5 to 5.0 eV. Aside from one amorphous sample prepared at room substrate temperature, polycrystalline In2O3 films with cubic crystal structure were confirmed for other four samples prepared at the substrate temperature of 450 A degrees C. Excellent SE fittings were realized by applying 1 and/or 2 terms F&B amorphous formulations, building double layered film configuration models, and further taking account of void into the surface layer based on Bruggeman effective medium approximation for thinner films. Spectral dependent refractive indices and extinction coefficients were obtained for five samples. The curve shapes were well interpreted according to the applied dispersion formulas. Almost similar optical band gap values from 3.76 to 3.84 eV were obtained for five samples by Tauc plot calculation using extinction coefficients under the assumption of direct allowed optical transition mode.

Enhanced photocatalytic activity of TiO2 nano-structured thin film with a silver hierarchical configuration

TiO2 sol-gels with various Ag/TiO2 molar ratios from 0 to 0.9% were used to fabricate silver-modified nano-structured TiO2 thin films using a layer-by-layer dip-coating (LLDC) technique. This technique allows obtaining TiO2 nano-structured thin films with a silver hierarchical configuration. The coating of pure TiO2 sol-gel and Ag-modified sol-gel was marked as T and A, respectively. According to the coating order and the nature of the TiO2 sol-gel, four types of the TiO2 thin films were constructed, and marked as AT (bottom layer was Ag modified, surface layer was pure TiO,), TA (bottom layer was pure TiO,, surface layer was Ag modified), TT (pure TiO, thin film) and AA (TiO, thin film was uniformly Ag modified). These thin films were characterized by means of linear sweep voltammetry (LSV), X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy and transient photocurrent (I-ph). LSV confirmed the existence of Ago state in the TiO, thin film. SEM and XRD experiments indicated that the sizes of the TiO,, nanoparticles of the resulting films were in the order of TT > AT > TA > AA, suggesting the gradient Ag distribution in the films. The SEM and XRD results also confirmed that Ag had an inhibition effect on the size growth of anatase nanoparticles. Photocatalytic activities of the resulting thin films were also evaluated in the photocatalytic degradation process of methyl orange. The preliminary results demonstrated the sequence of the photocatalytic activity of the resulting films was AT > TA > AA > TT. This suggested that the silver hierarchical configuration can be used to improve the photocatalytic activity of TiO2 thin film.

Study on pulsed laser ablation and deposition of ZnO thin films by L-MBE

ZnO, as a wide-band gap semiconductor, has recently become a new research focus in the field of ultraviolet optoelectronic semiconductors. Laser molecular beam epitaxy (L-MBE) is quite useful for the unit cell layer-by-layer epitaxial growth of zinc oxide thin films from the sintered ceramic target. The ZnO ceramic target with high purity was ablated by KrF laser pulses in an ultra high vacuum to deposit ZnO thin film during the process of L-MBE. It is found that the deposition rate of ZnO thin film by L-MBE is much lower than that by conventional pulsed laser deposition (PLD). Based on the experimental phenomena in the ZnO thin film growth process and the thermal-controlling mechanism of the nanosecond (ns) pulsed laser ablation of ZnO ceramic target, the suggested effective ablating time during the pulse duration can explain the very low deposition rate of the ZnO film by L-MBE. The unique dynamic mechanism for growing ZnO thin film is analyzed. Both the high energy of the deposition species and the low growth rate of the film are really beneficial for the L-MBE growth of the ZnO thin film with high crystallinity at low temperature.

Investigation on the strain relaxation of InGaN layer and its effects on the InGaN structural and optical properties

The evolution of strain and structural properties of thick epitaxial InGaN layers grown on GaN with different thicknesses are investigated. It is found that, with increase in InGaN thickness, plastic relaxation via misfit dislocation generation becomes a more important strain relaxation mechanism. Accompanied with the relaxation of compressive strain, the In composition of InGaN layer increases and induces an apparent red-shift of the cathodoluminescence peak of the InGaN layer. On the other hand, the plastic relaxation process results in a high defect density, which degrades the structural and optical properties of InGaN layers. A transition layer region with both strain and In composition gradients is found to exist in the 450-nm-thick InGaN layer.

A transition layer model and its application to resonant tunneling in heterostructures

A transition layer model is proposed and used to calculate resonant tunneling in a double-barrier quantum well system. Compared with the ideal step of the potential at the interface, the studied system has transition layers that are composed by many thin rectangular barriers with a random height. It is found that these transition layers can improve the peak-to-valley ratio of the tunneling current and change the negative differential conductance.

Boron-doped silicon film as a recombination layer in the tunnel junction of a tandem solar cell

Boron-doped hydrogenated silicon films with different gaseous doping ratios (B_2H_6/SiH_4) were deposited in a plasma-enhanced chemical vapor deposition (PECVD) system. The microstructure of the films was investigated by atomic force microscopy (AFM) and Raman scattering spectroscopy. The electrical properties of the films were characterized by their room temperature electrical conductivity (σ) and the activation energy (E_a). The results show that with an increasing gaseous doping ratio, the silicon films transfer from a microcrystalline to an amorphous phase, and corresponding changes in the electrical properties were observed. The thin boron-doped silicon layers were fabricated as recombination layers in tunnel junctions. The measurements of the Ⅰ-Ⅴ characteristics and the transparency spectra of the junctions indicate that the best gaseous doping ratio of the recombination layer is 0.04, and the film deposited under that condition is amorphous silicon with a small amount of crystallites embedded in it. The junction with such a recombination layer has a small resistance, a nearly ohmic contact, and a negligible optical absorption.