H.Q Zheng
Nanyang Technological University
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Featured researches published by H.Q Zheng.
Applied Physics Letters | 2000
H.Q Zheng; K. Radhakrishnan; H. Wang; K. Yuan; S. F. Yoon; G. I. Ng
InP/InGaAs double-heterojunction bipolar transistor (HBT) structures were grown metamorphically on GaAs substrates by solid-source molecular-beam epitaxy. A linearly graded InxGa1−xP (x varying from 0.48 to 1) buffer layer was used to accommodate the strain relaxation. The crystallinity of the buffer layer and the HBT structure was examined by x-ray diffractometry. Devices with 5×5 μm2 emitter area showed a typical peak current gain of 40, a common-emitter breakdown voltage (BVCEO) higher than 9 V, a current gain cut-off frequency (fT) of 46 GHz, and a maximum oscillation frequency (fmax) of 40 GHz.
Journal of Crystal Growth | 1999
W. Shi; D. H. Zhang; H.Q Zheng; S. F. Yoon; Chan Hin Kam; A Raman
Abstract We report the effects of arsenic beam equivalent pressure on lattice mismatch, electrical properties, surface roughness and morphology of InGaAsP grown by solid source molecular beam epitaxy using valve arsenic and phosphorous cracker cells with continuous white phosphorous production. Arsenic is found to have a higher sticking coefficient than phosphorous in almost all arsenic pressure employed in the growth. The incorporation of arsenic is found to fit a polynomial expression, Y =1.56 R −0.59 R 2 , with the beam equivalent pressure ratio R = f As /( f As + f P ). The incorporated arsenic elements significantly affect lattice mismatch and electrical properties. They also dominate surface construction of the quaternary material.
Journal of Crystal Growth | 2000
D. H. Zhang; W. Shi; H.Q Zheng; S. F. Yoon; Chan Hin Kam; Xiujuan Wang
InGaAsP films grown on InP substrate by solid source molecular beam epitaxy (SSMBE) using a valve phosphorous cracker cell are investigated. It is found that the films grown at flux ratios f As /(f As + f p ) from 0.45 to 0.50 show a superior quality. It is also found that As pressure plays a crucial role in the scattering process; for the films grown at higher arsenic beam pressure (BEP), the Hall mobility μ is dominated by impurity scattering, polar phonon scattering and alloy scattering. For the films with high quality, optical scattering and alloy scattering dominate the mobility. The exponent of T for the films grown at low BEP is found to be as high as 2.54, which cannot be explained by impurity scattering alone. It is believed that, in addition to the impurity-related scattering, some defects associated with As vacancies also significantly contribute to the scattering, especially at low temperatures.
Journal of Applied Physics | 2000
H.Q Zheng; K. Radahakrishnan; S. F. Yoon; G. I. Ng
We report on the electrical and optical properties of silicon (Si)-doped InP layers grown by solid-source molecular beam epitaxy using a valved phosphorus cracker cell. Within the range of Si effusion cell temperatures investigated (900–1200 °C), the highest electron concentration obtained was 1.1×1020 cm−3. A saturation phenomenon was observed for the electron concentration at higher Si cell temperatures. 300 and 77 K Hall mobility data were used to determine the compensation ratios by comparing them with the theoretical data. Although the Hall data show that the compensation ratio increases with the increase in carrier concentration, the exact values are not certain because the theoretical calculation overestimates the mobility values at higher carrier concentrations. The saturation phenomenon of electron concentration in InP may be considered due to the Si atoms occupying both the In and P lattice sites, or Si donors located at the interstitial sites. The 300 K Hall mobility and the concentration data ...
Journal of Applied Physics | 1999
S. F. Yoon; K.W Mah; H.Q Zheng
We report the transport and photoluminescence (PL) properties of silicon-doped GaInP layers grown on GaAs (100) substrate using a valved phosphorus cracker cell in solid source molecular beam epitaxy. Within the range of silicon (Si) effusion cell temperature investigated (900–1200 °C), the highest electron concentration obtained was 7.7×1018 and 3.2×1018 cm−3 at room temperature and 77 K, respectively. The concentration decreased with further increase in the silicon cell temperature. The Hall mobility at 300 K varied from 356 to 1720 cm2/V s within the range of electron concentration measured (4.5×1016–7.7×1018 cm−3). Except for the sample grown at the highest silicon cell temperature (1200 °C), the PL spectrum of other samples showed a dominant peak attributed to Si donor-to-band transition (D–B), which shifted to higher energy following an increase in the electron concentration. This phenomenon was attributed to the Burstein–Moss effect. The blueshift of the (D–B) transition peak at increasing temperat...
Materials Science and Engineering B-advanced Functional Solid-state Materials | 2000
Soon Fatt Yoon; H.Q Zheng; Adele H.T Kam
Abstract In 0.48 Ga 0.52 P/In 0.20 Ga 0.80 As/GaAs pseudomorphic high electron mobility transistor (p-HEMT) structures were grown by solid-source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell. The sheet carrier density at room temperature was 3.3×10 12 cm −2 . A peak transconductance ( G m ) of 267 mS mm −1 and peak drain current density ( I ds ) of 360 mA mm −1 were measured for a p-HEMT device with 1.25 μm gate length. A high gate-drain breakdown voltage (BV gd ) of 33 V was measured. This value is more than doubled compared to that of a conventional Al 0.30 Ga 0.70 As/In 0.20 Ga 0.80 As/GaAs device. The drain-source breakdown voltage (BV ds ) was 12.5 V. Devices with a mushroom gate of 0.25 μm gate length and 80 μm gate width achieved a peak transconductance ( G m ) of 420 mS mm −1 and drain current density of nearly 500 ma/mm. A high cut-off frequency ( f T ) of 58 GHz and maximum oscillation frequency ( f max ) of 120 GHz were obtained. The results showed that the In 0.48 Ga 0.52 P/In 0.20 Ga 0.80 As/GaAs material system grown by SSMBE using the valved phosphorus cracker cell for the In 0.48 Ga 0.52 P Schottky and spacer layers is a viable technology for high frequency p-HEMT device applications. The performance of the 0.25-μm gate length device was simulated using a two-dimensional device simulator, MEDICI®, which incorporates physical models such as Shockley–Read Hall recombination, Auger recombination, Fermi-Dirac statistics and field-dependent mobility.
Journal of Alloys and Compounds | 1998
S. F. Yoon; K.W Mah; H.Q Zheng
We report the effect of elastic strain on the optical properties of In1−xGaxP grown using a valved phosphorus cracker cell in solid source molecular beam epitaxy (SSMBE). Sample characterization was carried out using photoluminescence (PL) and double-axis X-ray diffraction (XRD). All the In1−xGaxP epilayers prepared in this study are gallium-rich (tensile strained) with composition x in the range of 0.516<x<0.559. The unstrained plot of the band-gap as a function of composition x was deduced from the tensile-strained data obtained experimentally by assuming that the valence band splitting is due only to biaxial elastic strain. The unstrained plot of band-gap vs. composition x for our MBE-grown samples was about 10 meV and 20 meV lower than those reported for samples grown by organometallic vapour phase epitaxy (OMVPE) and liquid phase epitaxy (LPE) techniques, respectively. The photoluminescence (PL) full width at half-maximum (FWHM) of the InGaP samples was higher as the composition x increases due to an increase in the lattice mismatch. Compared to other growth techniques involving the use of higher substrate temperature, InGaP of comparable optical quality can be grown using the valved phosphorus cracker cell SSMBE technique.
Materials Science in Semiconductor Processing | 2001
K. Yuan; K. Radhakrishnan; H.Q Zheng; S. F. Yoon
Abstract Compositionally graded InxGa1−xP (x=0.48→x=1) metamorphic layers have been grown on GaAs substrate by solid source molecular beam epitaxy using a valved phosphorus cracker cell. Three series of samples were grown to optimize the growth temperature, V/III ratio and grading rate of the buffer layer. X-ray diffraction (XRD) and photoluminescence (PL) were used to characterize the samples. The following results have been obtained: (1) XRD measurement shows that all the samples are nearly fully strain relaxed and the strain relaxation ratio is about 96%; (2) the full-width at half-maximum (FWHM) of the XRD peak shows that the sample grown at 480°C offers better material quality; (3) the grading rate does not influence the FWHM of XRD and PL results; (4) adjustment of the V/III ratio from 10 to 20 improves the FWHM of XRD peak, and the linewidth of PL peak is close to the data obtained for the lattice-matched sample on InP substrate. The optimization of growth conditions will benefit the metamorphic HEMTs grown on GaAs using graded InGaP as buffer layers.
Solid-state Electronics | 1999
S. F. Yoon; H.Q Zheng; Kian Siong Ang; Hao Wang; G. I. Ng
Abstract In0.48Ga0.52P/In0.20Ga0.80As/GaAs pseudomorphic high electron mobility transistor (p-HEMT) structures were grown by solid-source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell. Device with a mushroom gate of 0.25 μm gate length and 80 μm gate width achieved a peak transconductance (Gm) of 420 mS/mm and drain current density of nearly 500 mA/mm. A high cut-off frequency (fT) of 58 GHz and maximum oscillation frequency (fmax) of 120 GHz were obtained. The results showed that the In0.48Ga0.52P/In0.20Ga0.80As/GaAs material system grown by SSMBE using the valved phosphorus cracker cell for the In0.48Ga0.52P Schottky and spacer layers is clearly a viable technology for high frequency p-HEMT device applications.
Microelectronics Journal | 1999
S. F. Yoon; Adele H.T Kam; H.Q Zheng; G. I. Ng
Abstract The performance of In 0.48 Ga 0.52 P/In 0.2 Ga 0.8 As/GaAs pseudomorphic high electron mobility transistor (p-HEMT) was simulated using a two-dimensional device simulator, MEDICI [(Two-Dimensional device Simulation Program, Technology Modeling Associates Inc., Sunnyvale, CA, 1997)1] . Physical models used in the simulation include Shockley–Read Hall recombination, Auger recombination, Fermi–Dirac statistics and field-dependent mobility. Key results presented include the transconductance, current gain cut-off frequency and current–voltage ( I – V ) characteristics. We compared the simulated performance to two fabricated devices of different gate lengths and obtained a good match between our simulation results and measured data. These results show that the chosen physical models applied by the two-dimensional device simulation program is viable for a fast turn-around study and development of p-HEMT devices prior to fabrication.