Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-Si wafers

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/1T0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 mu m)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm(2). (c) 2006 Elsevier B.V. All rights reserved.

Valence band offset of MgO/TiO2 (rutile) heterojunction measured by X-ray photoelectron spectroscopy

The valence band offset (VBO) of MgO/TiO2 (rutile) heterojunction has been directly measured by Xray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 +/- 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 +/- 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO2 based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO2 based field-effect transistors and dye-sensitized solar cells.

Growth and characterization of InAsSb alloys by metalorganic chemical vapour deposition

Low pressure metalorganic chemical vapour deposition (LP-MOCVD) growth and characteristics of InAssb on (100) Gasb substrates are investigated. Mirror-like surfaces with a minimum lattice mismatch are obtained. The samples are studied by photoluminescence spectra, and the output is 3.17 mu m in wavelength. The surface of InAssb epilayer shows that its morphological feature is dependent on buffer layer. With an InAs buffer layer used, the best surface is obtained. The InAssb film shows to be of n-type conduction with an electron concentration of 8.52 x 10(16) cm(-3).

INVESTIGATION OF GAAS/SI MATERIAL BY X-RAY DOUBLE-CRYSTAL DIFFRACTION

GaAs epilayer films on Si substrates grown by molecular-beam epitaxy were investigated by the x-ray double-crystal diffraction method. The rocking curves were recorded for different diffraction vectors of samples. The results show that the unit-cell volumes of GaAs epilayers are smaller than that of the GaAs bulk material. The strained-layer superlattice buffer layer can improve the quality of the film, especially in the surface lamella. The parameter W' = W(expt)/(square-root \gamma-h\/gamma-0/sin 2-theta-B) is introduced to describe the quality of different depths of epilayers. As the x-ray incident angle is increased, W' also increases, that is, the quality of the film deteriorates with increasing penetration distance of the x-ray beam. Therefore, W' can be considered as a parameter that describes the degree of perfection of the epilayer along the depth below the surface. The cross-section transmission electron microscopy observations agree with the results of x-ray double-crystal diffraction.

Growth of high-quality relaxed In0.3Ga0.7As/GaAs superlattices

With a low strained InxGa1-xAs/GaAs(x similar to 0.01) superlattice (SL) buffer layer, the crystal quality of 50 period relaxed In0.3Ga0.7As/GaAs strained SLs has been greatly improved and over 13 satellite peaks are observed from X-ray double-crystal diffraction, compared with three peaks in the sample without the buffer layer. Cross-section transmission electron microscopy reveals that the dislocations due to superlattice strain relaxation are blocked by the SLs itself and are buried into the buffer layer. The role of the SL buffer layer lies in that the number of the dislocations is reduced in two ways: (1) the island formation is avoided and (2) the initial nucleation of the threading dislocations is retarded by the high-quality growth of the SL buffer layer. When the dislocation pinning becomes weak as a result of the reduced dislocation density, the SLs can effectively move the threading dislocations to the edge of the wafer.

Some properties of gallium nitride films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy

The Raman and photoreflectivity spectra of gallium nitride (GaN) films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy (GSMBE) have been investigated. The Raman spectra showed the presence of the E-2(high) mode and a shift in the wavenumber of this mode with respect to the GaN epilayer thickness. The Raman scattering results suggest the presence of stress due to lattice and thermal expansion misfit in the films, and also indicate that the buffer layer play an important role in the deposition of high quality GaN layers. The residual stress changes from tensile to compressive as the epilayer thickness increases. Samples subjected to anneal cycles showed an increase in the mobility due probably to stress relaxation as suggested by an observed shift in the E-2(high) mode in the Raman spectra after annealing.

GaN m-i-n LED grown by MOVPE

Undoped and Zinc-doped GaN films have been grown using TMGa, DEZn and Ammonia by MOVPE. The GaN blue-green LEDs of m-i-n structure have been fabricated. They can be operated at forward bias less than 5 volts. The EL peak wavelengths was from 455 nm to 504 nm.

Assessment of the structural properties of GaAs/Si epilayers using X-ray (004) and (220) reflections

We improved the method previously used to determine the lattice constants and misorientation of GaAs/Si by recording the patterns of X-ray (004) and (220) reflections. The (220) reflection was measured from the (110) cross section of a GaAs/Si epilayer. The structural properties of the GaAs/Si epilayers grown by metal-organic chemical-vapor deposition (MOCVD) using an ultrathin a-Si buffer layer were investigated. The rotation angle of GaAs/Si epilayers grown by MOCVD using an a-Si buffer layer is very small and the lattice constants of these GaAs/Si epilayers agree quite well with elastic theory.

METAL-ORGANIC CHEMICAL-VAPOR-DEPOSITION GROWTH OF GAN

Single-crystal GaN films have been deposited on (01 (1) over bar 2) sapphire substrates using trimethylgallium (TMGa) and NH3 as sources. The morphological, crystalline, electrical and optical characterizations of GaN film are investigated. The carrier concentration ofundoped GaN increases with decreasing input NH3-to-TMGa molar flow ratio.

Compressively Strained InGaAs/InGaAsP Quantum Well Distributed Feedback Laser at 1.74μm

The compressively strained InGaAs/InGaAsP quantum well distributed feedback laser with ridge-wave- guide is fabricated at 1.74μm. It is grown by low-pressure metal organic chemical vapor deposition（MOCVD）. A strain buffer layer is used to avoid indium segregation. The threshold current of the device uncoated with length of 300μm is 11.5mA. The maximum output power is 14mW at 100mA. A side mode suppression ratio of 35.5dB is obtained.

X-Band GaN Power HEMTs with Power Density of 2.23W/mm Grown on Sapphire by MOCVD

The growth,fabrication,and characterization of 0.2μm gate-length AlGaN/GaN HEMTs,with a high mobility GaN thin layer as a channel,grown on (0001) sapphire substrates by MOCVD,are described.The unintentionally doped 2.5μm thick GaN epilayers grown with the same conditions as the GaN channel have a room temperature electron mobility of 741cm2/(V·s) at an electron concentration of 1.52×1016 cm-3.The resistivity of the thick GaN buffer layer is greater than 108Ω·cm at room temperature.The 50mm HEMT wafers grown on sapphire substrates show an average sheet resistance of 440.9Ω/□ with uniformity better than 96%.Devices of 0.2μm×40μm gate periphery exhibit a maximum extrinsic transconductance of 250mS/mm and a current gain cutoff frequency of77GHz.The AlGaN/GaN HEMTs with 0.8mm gate width display a total output power of 1.78W (2.23W/mm) and a linear gain of 13.3dB at 8GHz.The power devices also show a saturated current density as high as 1.07A/mm at a gate bias of 0.5V.

Growth of Space Ordered on GaAs（100） Vicinal 1.3μm InAs Quantum Dots Substrates by MOCVD

Space ordered 1.3μm self-assembled InAs QDs are grown on GaAs（100） vicinal substrates by MOCVD. Photoluminescence measurements show that the dots on vicinal substrates have a much higher PL intensity and a narrower FWHM than those of dots on exact substrates, which indicates better material quality. To obtain 1.3μm emissions of InAs QDs, the role of the so called InGaAs strain cap layer （SCL） and the strain buffer layer （SBL） in the strain relaxation process in quantum dots is studied. While the use of SBL results only in a small change of emission wavelength,SCL can extend the QD＇s emission over 1.3μm due to the effective strain reducing effect of SCL.

Preparation of 50mm 3C-SiC/Si(111) as Substrates Suited for Ⅲ-Nitrides

50mm SiC films with high electrical uniformity are grown on Si(111) by a newly developed vertical low-pressure chemical vapor deposition (LPCVD) reactor.Both in-situ n- and p-type doping of 3C-SiC are achieved by intentional introduction of ammonia and boron into the precursor gases.The dependence of growth rate and surface morphology on the C/Si ratio and optimized growth conditions is obtained.The best electrical uniformity of 50mm 3C-SiC films obtained by non-contact sheet resistance measurement is ±2.58%.GaN films are grown atop the as-grown 3C-SiC/Si(111) layers using molecular beam epitaxy (MBE).The data of both X-ray diffraction and low temperature photoluminescence of GaN/3C-SiC/Si(111) show that 3C-SiC is an appropriate substrate or buffer layer for the growth of Ⅲ-nitrides on Si substrates with no cracks.

Metal-organic Vapor Phase Epitaxy Growth and Characterization of InAlGaN Epilayers

In order to improve crystal quality for growth of quaternary InAlGaN, a series of InAlGaN films were grown on GaN buffer layer under different growth temperatures and carrier gases by low-pressure metal-organic vapor phase epitaxy. Energy dispersive spectroscopy (EDS) was employed to measure the chemical composition of the quaternary, high resolution X-ray diffraction (HRXRD) and photoluminescence (PL) technique were used to characterize structural and optical properties of the epilayers, respectively. The PL spectra of InAlGaN show with and without the broad-deep level emission when only N2 and a N2+H2 mixture were used as carrier gas, respectively. At pressure of 1.01×104 Pa and with mixed gases of nitrogen and hydrogen as carrier gas, different alloy compositions of the films were obtained by changing the growth temperature while keeping the fluxes of precursors of indium (In), aluminum (Al), gallium (Ga) and nitrogen (N2) constant. A combination of HRXRD and PL measurements enable us to explore the relative optimum growth parameters-growth temperature between 850℃ and 870℃,using mixed gas of N2+H2 as carrier gas.

Reduction in crystallographic tilt of lateral epitaxial overgrown GaN by using new patterned shape mask

We proposed a new method to suppress the crystallographic tilt in the lateral epitaxial overgrowth of GaN by using an oxide mask with a newly designed pattern. A rhombus mask with edges oriented in the direction of <10 - 10>(GaN) was used instead of the traditional stripe mask. The morphology evolution during the LEO GaN with the rhombus mask was investigated by SEM, and the crystallographic tilt in the LEO GaN was measured by DC-XRD. It is found that using the new rhombus mask can decrease the crystallographic tilt in the LEO GaN. In addition, this method makes the ELO GaN stripes easy to coalesce. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.