Promoting strain relaxation of Si0.72Ge0.28 film on Si (100) substrate by inserting a low-temperature Ge islands layer in UHVCVD

A flat, fully strain-relaxed Si0.72Ge0.28 thin film was grown on Si (1 0 0) substrate with a combination of thin low-temperature (LT) Ge and LT-Si0.72Ge0.28 buffer layers by ultrahigh vacuum chemical vapor deposition. The strain relaxation ratio in the Si0.72Ge0.28 film was enhanced up to 99% with the assistance of three-dimensional Ge islands and point defects introduced in the layers, which furthermore facilitated an ultra-low threading dislocation density of 5 x 10(4) cm (2) for the top SiGe film. More interestingly, no cross-hatch pattern was observed on the SiGe surface and the surface root-mean-square roughness was less than 2 nm. The temperature for the growth of LT-Ge layer was optimized to be 300 degrees C. (C) 2008 Elsevier B.V. All rights reserved.

Comment on

In a recent letter, Hsieh reported the growth of high-quality Ge epilayers with a SiGe buffer thickness of only 0.45 mu m, a surface root-mean-square roughness of less than 0.4 nm, and a threading dislocation of 7.6 x 10(6) cm(-2) on Si+ pre-ion-implantation Si substrate utilizing of strain relaxation enhancement by point defects and interface blocking of the dislocations. Our comment has focused on x-ray diffraction data shown in Fig. 3 of Ref. 1. We demonstrate that the strain in Ge epilayers is tensile, rather than compressive as misunderstood by the authors. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3003873]

The influence of low-temperature Ge seed layer on growth of high-quality Ge epilayer on Si(100) by ultrahigh vacuum chemical vapor deposition

High-quality Ge epilayer on Si(1 0 0) substrate with an inserted low-temperature Ge seed layer and a thin Si0.77Ge0.23 layer was grown by ultrahigh vacuum chemical vapor deposition. The epitaxial Ge layer with surface root-mean-square roughness of 0.7 nm and threading dislocation density of 5 x 10(5) cm(-2) was obtained. The influence of low temperature Ge seed layer on the quality of Ge epilayer was investigated. We demonstrated that the relatively higher temperature (350 degrees C) for the growth of Ge seed layer significantly improved the crystal quality and the Hall hole mobility of the Ge epilayer. (C) 2008 Elsevier B.V. All rights reserved.

Influence of V/III ratio on the structural and photoluminescence properties of In0.52AlAs/In0.53GaAs metamorphic high electron mobility transistor grown by molecular beam epitaxy

A series of metamorphic high electron mobility transistors (MMHEMTs) with different V/III flux ratios are grown on GaAs (001) substrates by molecular beam epitaxy (XIBE). The samples are analysed by using atomic force microscopy (AFM), Hall measurement, and low temperature photoluminescence (PL). The optimum V/III ratio in a range from 15 to 60 for the growth of MMHEMTs is found to be around 40. At this ratio, the root mean square (RMS) roughness of the material is only 2.02 nm; a room-temperature mobility and a sheet electron density are obtained to be 10610.0cm(2)/(V.s) and 3.26 x 10(12)cm(-2) respectively. These results are equivalent to those obtained for the same structure grown on InP substrate. There are two peaks in the PL spectrum of the structure, corresponding to two sub-energy levels of the In0.53Ga0.47 As quantum well. It is found that the photoluminescence intensities of the two peaks vary with the V/III ratio, for which the reasons are discussed.

An internally-matched GaN HEMTs device with 45.2 W at 8 GHz for X-band application

Optimized AlGaN/AlN/GaN high electron mobility transistor (HEMT) with high mobility GaN channel layer structures were grown on 2-in. diameter semi-insulating 6H-SiC substrates by MOCVD. The 2-in. diameter GaN HEMT wafer exhibited a low average sheet resistance of 261.9 Omega/square, with the resistance un-uniformity as low as 2.23%. Atomic force microscopy measurements revealed a smooth AlGaN surface whose root-mean-square roughness is 0.281 nm for a scan area of 5 x 5 mu m. For the single-cell HEMTs device of 2.5-mm gate width fabricated using the materials, a maximum drain current density of 1.31 A/mm, an extrinsic transconductance of 450 mS/mm, a current gain cutoff frequency of 24 GHz and a maximum frequency of oscillation 54 GHz were achieved. The four-cell internally-matched GaN HEMTs device with 10-mm total gate width demonstrated a very high output power of 45.2 W at 8 GHz under the condition of continuous-wave (CW), with a power added efficiency of 32.0% and power gain of 6.2 dB. To our best knowledge, the achieved output power of internally-matched devices are the state-of-the-art result ever reported for X-band GaN-based HEMTs. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

Growth and characterization of AlGaN/GaN heterostructure using unintentionally doped AlN/GaN superlattices as barrier layer

AlGaN/GaN heterostructure using unintentionally doped AlN/GaN superlattices (SLs) as barrier layer is grown on C-plane sapphire by metal organic vapor deposition (MOCVD). Compared with the conventional Si-doped structure, electrical property is improved. An average sheet resistance of 287.1 Omega/square and high resistance uniformity of 0.82% are obtained across the 2-inch epilayer wafer with an equivalent Al composition of 38%. Hall measurement shows that the mobility of two-dimensional electron gas (2DEG) is 1852 cm(2)/V s with a sheet carrier density of 1.2 x 10(13) cm(-2) at room temperature. The root mean square roughness (RMS) value is 0.159 nm with 5 x 5 mu m(2) scan area and the monolayer steps are clearly observed. The reason for the property improvement is discussed. (c) 2008 Elsevier Ltd. All rights reserved.

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.

1-mm gate periphery AlGaN/AIN/GaN HEMTs on SiC with output power of 9.39 W at 8 GHz

AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with high mobility GaN channel layer were grown on 50 min diameter semi-insulating (SI) 6H-SiC substrates by metalorganic chemical vapor deposition and large periphery HEMT devices were fabricated and characterized. High two-dimensional electron gas mobility of 2215 cm(2)/V s at room temperature with sheet electron concentration of 1.044 x 10(13)/cm(2) was achieved. The 50 mm diameter HEMT wafer exhibited a low average sheet resistance of 251.0 Omega/square, with the resistance uniformity of 2.02%. Atomic force microscopy measurements revealed a smooth AlGaN surface with a root-mean-square roughness of 0.27 nm for a scan area of 5 mu mi x 5 pm. The 1-mm gate width devices fabricated using the materials demonstrated a very high continuous wave output power of 9.39 W at 8 GHz, with a power added efficiency of 46.2% and power gain of 7.54 dB. A maximum drain current density of 1300 mA/mm, an extrinsic transconductance of 382 mS/mm, a current gain cutoff frequency of 31 GHz and a maximum frequency of oscillation 60 GHz were also achieved in the same devices. (C) 2007 Elsevier Ltd. All rights reserved.

Growth of GaSb layers on GaAs (001) substrate by molecular beam epitaxy

GaSb 1 mu m-thick layers were grown by molecular beam epitaxy on GaAs (001). The effects of the growth conditions on the crystalline quality, surface morphology, electrical properties and optical properties were studied by double crystalline x-ray diffraction, atomic force microscopy, Hall measurement and photoluminescence spectroscopy, respectively. It was found that the surface roughness and hole mobility are highly dependent on the antimony-to-gallium flux ratios and growth temperatures. The crystalline quality, electrical properties and optical properties of GaSb layers were also studied as functions of growth rate, and it was found that a suitably low growth rate is beneficial for the crystalline quality and electrical and optical properties. Better crystal quality GaSb layers with a minimum root mean square surface roughness of 0.1 nm and good optical properties were obtained at a growth rate of 0.25 mu m h(-1).

Growth and characterization of semi-insulating GaN films grown by MOCVD

High resistivity unintentionally doped GaN films were grown on (0001) sapphire substrates by metalorganic chemical vapor deposition. The surface morphology of the layer was measured by both atomic force microscopy and scanning electron microscopy. The results show that the films have mirror-like surface morphology with root mean square of 0.3 nm. The full width at half maximum of double crystal X-ray diffraction rocking curve for (0002) GaN is about 5.22 arc-min, indicative of high crystal quality. The resistivity of the GaN epilayers at room temperature and at 250 degrees C was measured to be approximate 10(9) and 10(6) Omega(.)cm respectively, by variable temperature Hall measurement. Deep level traps in the GaN epilayers were investigated by thermally stimulated current and resistivity measurements.

Investigation of a chemically treated InP(100) surface during hydrophilic wafer bonding process

Surface micro-roughness, surface chemical properties, and surface wettability are three important aspects of wafer surfaces during a wafer cleaning process, which determine the bonding quality of ordinary direct wafer bonding. In this study, InP wafers are divided into four groups and treated by different chemical processes. Subsequently, the characteristics of the treated InP surfaces are carefully studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements. The optimal wafer treatment method for wafer bonding is determined by comparing the results of the processes as a whole. This optimization is later evaluated by a scanning electronic microscope (SEM), and the ridge waveguide 1.55 mu m Si-based InP/InGaAsP multi-quantum-well laser chips are also fabricated. (c) 2005 Elsevier B.V. All rights reserved.

AlGaN/GaN/InGaN/GaN DH-HEMTs structure with an AlN interlayer grown by MOCVD

A novel AlGaN/GaN/GaN/GaN double heterojunction high electron mobility transistors (DH-HEMTS) structure with an AlN interlayer on sapphire substrate has been grown by MOCVD. The structure featured a 6-10 nm In0.1Ga0.9N layer inserted between the GaN channel and GaN buffer. And wer also inserted one ultrathin. AlN interlayer into the Al/GaN/GaN interface, which significantly enhanced the mobility of two-dimensional electron gas (2DEG) existed in the GaN channel. AFM result of this structure shows a good surface morphology and a low dislocation density, with the root-mean-square roughness (RMS) of 0.196 nm for a scan area of 5 mu m x 5 mu m. Temperature dependent Hall measurement was performed on this sample, and a mobility as high as 1950 cm(2)/Vs at room temperature (RT) was obtained. The sheet carrier density was 9.89 x10(12) cm(2), and average sheet resistance of 327 Omega/sq was achieved. The mobility obtained in this paper is about 50% higher than other results of similar structures which have been reported. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

AN EFFICIENT ITERATIVE DFT-BASED CHANNEL ESTIMATION FOR MIMO-OFDM SYSTEMS ON MULTIPATH CHANNELS

In this paper, an efficient iterative discrete Fourier transform (DFT) -based channel estimator with good performance for multiple-input and multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems such as IEEE 802.11n which retain some sub-carriers as null sub-carriers (or virtual carriers) is proposed. In order to eliminate the mean-square error (MSE) floor effect existed in conventional DFT-based channel estimators, we proposed a low-complexity method to detect the significant channel impulse response (CIR) taps, which neither need any statistical channel information nor a predetermined threshold value. Analysis and simulation results show that the proposed method has much better performance than conventional DFT-based channel estimators and without MSE floor effect.