Q. Huang

Low‐temperature buffer layer for growth of a low‐dislocation‐density SiGe layer on Si by molecular‐beam epitaxy

A method using a low‐temperature Si (LT‐Si) buffer layer is developed to grow a SiGe epilayer with low density of dislocations on a Si substrate by molecular‐beam epitaxy. In this method, a LT‐Si layer is used to release the stress of the SiGe layer. The samples have been investigated by x‐ray double‐crystal diffraction and transmission electron microscopy. The results indicate that the LT‐Si is effective to release the stress and suppress threading dislocations.

Substrate doping effects on Raman spectrum of epitaxial graphene on SiC

In this paper, we reported a Raman scattering study of epitaxial graphene on different doped 6H-SiC (0001) substrates and investigated the substrate induced charge-transfer doping to the epitaxial graphene. We found that the charge carrier type and concentration of epitaxial graphene can be altered by SiC substrates with different doping level and doping type. This effect is comparable to that obtained by electrochemical doping. As Raman scattering is very sensitive to the doping level, the charge carrier concentration of epitaxial graphene can be estimated by the Raman G-peak shift. Our results are fundamental and may have implications for future epitaxial-graphene-based micro/nanoelectronic devices.

Resonant photorefractive AlGaAs/GaAs multiple quantum wells grown by molecular beam epitaxy at low temperature

The resonant photorefractive devices using low temperature AlGaAs/GaAs multiple‐quantum‐well structures in a parallel field geometry are demonstrated. The samples are semi‐insulating as grown. The AsGa‐related defects incorporated into the samples during low temperature growth provide the required deep centers. No proton implantation, Cr doping, or annealing is needed for device fabrication. In the photorefractive wave mixing experiment, an output diffraction efficiency higher than 0.84% and a two‐wave‐mixing gain of more than 3000 cm−1 are obtained under a dc electric field of 15 kV/cm.

Optical transition in SiGe self-organized dots

It is found that the SiGe alloy self-organizers into uniform quantum dots embedded in the Si layer during the growth of a strained Si/Ge short period superlattice on a Si(001) substrate at a temperature of 800u2009°C by molecular beam epitaxy. The peak of photoluminescence from the quantum dots is at an energy higher than the band gap of Si. The intensity is two orders of magnitude higher than that of SiGe/Si quantum well.

Influence of growth conditions on Al-Ga interdiffusion in low-temperature grown AlGaAs/GaAs multiple quantum wells

Low-temperature growth and subsequent rapid thermal anneal were used to intermix Al and Ga atoms in AlGaAs/GaAs multiple quantum wells (QWs). The intermixed samples were characterized by photoluminescence (PL) spectroscopy, and the observed blue shifts in PL energies are interpreted as the result of modification of the QW shape due to the enhanced Al-Ga interdiffusion in the samples. The enhancement of interdiffusion was found to be strongly dependent on the growth and annealing conditions. In addition, the saturation behavior of Al-Ga interdiffusion was also observed.

Temperature induced blue shift and broadening of intersubband absorption and photocurrent spectra in GaAs/Al0.3Ga0.7As multiple quantum wells

The temperature induced blue shift of the intersubband absorption peak in GaAs/Al0.3Ga0.7As multiple quantum wells has been observed and calculated as a function of well thickness (Lw). As the temperature is lowered from 290 to 10 K, the absorption peak shifts to the higher energy by 7.1 meV. Calculation shows that the change in the barrier height dominates the blue shift at small Lw while exchange interaction is important for large Lw. At 80 K, the photocurrent spectrum (PCS) at 0 V bias (Vb) is gradually reduced and cut off by the barrier. The bias induced forbidden transition from the ground state, E1, to the second excited state, E3, is observed in PCS. The broadening of full width at half maximum of PCS is related to the bias induced forbidden transition at small bias (0<Vb<0.2 V) and the reduction of the barrier height and tunneling lifetime at large bias (0.2<Vb<2 V).

Low‐temperature AlGaAs/GaAs multiple quantum well structure and its application to photorefractive devices

We report the low‐temperature (LT) growth of a AlGaAs/GaAs multiple quantum well (MQW) structure by molecular beam epitaxy and demonstrate its application to photorefractive devices. The samples are semi‐insulating as grown, and show large electro‐optic effect. Resistivity up to 108 Ωu2009cm has been observed. The electroabsorption approaches 3000 cm−1 and the electrorefraction is higher than 1.5% for dc electric field of 15 kV/cm. The effect of annealing on electrical and optical properties of LT AlGaAs/GaAs MQW has also been investigated.

Influence of probe beam size on signal analysis of modulated free carrier absorption technique

A three-dimensional modulated free carrier absorption (MFCA) model taking into account the size of a Gaussian-profile probe beam, referred as the accurate model, is developed to describe the influence of the probe beam size on signal analysis of the MFCA technique. Numerical simulations are presented to investigate the influences of different probe beam radii on the amplitude and phase of the laterally resolved MFCA (LR-MFCA) signal at 2kHz and 200kHz, respectively. The MFCA signals obtained by this accurate model are compared to those obtained by a model assuming a point detection (referred as the point model). The conditions that the probe beam size can be neglected are given.

Effect of thermal annealing on optical emission properties of low‐temperature grown AlGaAs/GaAs multiple quantum wells

We have investigated the effect of thermal annealing on optical emission properties of low‐temperature (LT) grown AlGaAs/GaAs multiple quantum wells (MQWs) by using photoluminescence (PL) spectroscopy. For comparison, the results on normal‐temperature (NT) grown MQWs implanted with protons are also presented. The LT sample was grown by molecular beam epitaxy at 310u2009°C. The as‐grown LT‐MQWs show moderately strong PL. Upon annealing at 600u2009°C, the PL intensity of the LT‐MQWs is dramatically quenched, in sharp contrast to the large increase in PL intensity of the implanted NT‐MQWs. The quenching of PL intensity in the LT‐MQWs is attributed to the formation of arsenic clusters that fast trap photoexcited carriers. In addition, an enhancement in the interface intermixing and roughening induced by thermal annealing has also been observed in the LT‐MQWs.

Conversion of step configuration induced by strain in Si1-xGex layers deposited on vicinal Si(001) surface

Reflection high energy electron diffraction (RHEED) intensity measurements reveal that the strain in a Si1−xGex layer on a vicinal Si(001) surface converts a (1×2) domain dominated step configuration to a (2×1) domain dominated one. The dependence of the effect on the Ge content is similar to the dependence of the critical thickness of the pseudomorphic growth of Si1−xGex layers on Ge content. No conversion effect has been observed on exact Si(001) substrates.