Effect of arsenic precipitates on Fermi level in GaAs grown by molecular-beam epitaxy at low temperature

A simple model is presented to discuss the effect of As precipitates on the Fermi level in GaAs grown by molecular-beam epitaxy at low temperature (LT-GaAs). This model implements the compensation between point defects and the depletion of arsenic precipitates. The condition that the Fermi level is pinned by As precipitates is attained. The shifts of the Fermi level in LT-GaAs with annealing temperature are explained by our model. Additionally, the role of As precipitates in conventional semi-insulating GaAs is discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)09905-9].

Doping during low-temperature growth of materials for n-p-n Si/SiGe/Si heterojuction bipolar transistor by gas source molecular beam epitaxy

In situ doping for growth of n-p-n Si/SiGe/Si heterojuction bipolar transistor (HBT) structural materials in Si gas source molecular beam epitaxy is investigated. We studied high n-type doping kinetics in Si growth using disilane and phosphine, and p-type doping in SiGe growth using disilane, soild-Ge, and diborane with an emphasis on the effect of Ge on B incorporation. Based on these results, in situ growth of n-p-n Si/SiGe/Si HBT device structure is demonstrated with designed structural and carrier profiles, as verified from characterizations by X-ray diffraction, and spreading resistance profiling analysis. (C) 2000 Elsevier Science B.V. All rights reserved.

Ultrafast low-temperature grown AlGaAs/GaAs photorefractive quantum wells using point defects as capture centers

At a medium substrate temperature of 400 degrees C and a lower As flux, we have grown an ultrafast AlGaAs/GaAs photorefractive multiple quantum well (MQW) structure by molecular beam epitaxy. The as-grown sample exhibits strong photorefractive effect under the transverse Frantz-Keldysh geometry. A peak electroabsorption of 2100 cm(-1) is measured in the as-grown sample in an 11 kV/cm dc electric field, and the peak photorefractive diffraction efficiency can be 1.2%. After postgrowth annealing, the photorefractive effect becomes weak and disappears in samples annealed above 700 degrees C. Using optical transient current spectroscopy, deep levels are measured in these samples. It is found that deep levels are stable against annealing until 700 degrees C. Using a pump-probe technique, carrier lifetimes are measured at room temperature. We find that the as-grown sample has a lifetime of 20 ps, while the 700 degrees C annealed sample has a lifetime of more than 200 ps. The ultrafast lifetime in the as-grown sample is caused by point defects, not by As clusters. Our result show that AlGaAs/GaAs MQW structure grown around 400 degrees C has better performance of the photorefractive effect. (C) 1999 American Institute of Physics. [S0003-6951(99)04036-X].

Influence of (001) vicinal GaAs substrates on the optical properties of defects in low-temperature grown GaAs/AlGaAs multiple quantum wells

Photoluminescence (PL) spectroscopy and carrier lifetime measurement has been used to characterize optical properties of defects in the low-temperature (LT) grown GaAs/AlGaAs multiple quantum well structures. Two sets of samples were grown at 400 degrees C by molecular beam epitaxy on nominal (001) and miscut [4 degrees off (001) towards (111) A] GaAs substrates, respectively. After growth, samples were subjected to 30 s rapid thermal annealing at 600-800 degrees C. It is found that after annealing, two defect-related PL features appear in the samples grown on nominal (001) GaAs substrates, but not in those grown on miscut (001) GaAs substrates. The carrier lifetimes are about 31 and 5 ps in as-grown samples grown on nominal and miscut (001) GaAs substrates, respectively. The different PL spectra and carrier lifetimes in two sets of samples are attributed to different structures of the As-Ga-like defects formed during LT growth. (C) 1999 American Institute of Physics. [S0003-6951(99)00230-2].

Strain relaxation of GeSi alloy with low dislocation density grown on low-temperature Si buffers

We have developed a low-temperature (LT) growth technique. Even with Ge fraction x upto 90%, the total thickness of fully relaxed GexSi1-x buffers can he reduced to 1.7 mu m with dislocation density lower than 5 x 10(6) cm(-2). The surface roughness is no more than 6 nm. The strain relaxation is quite inhomogeneous From the beginning. Stacking faults generate and form the mismatch dislocations in the interface of GeSi/LT-Si. (C) 1999 Elsevier Science B.V. All rights reserved.

Enhanced hydrogen desorption from Si sites during low-temperature Si1-xGex growth by disilane and solid-Ge molecular beam epitaxy

A phenomenological model is proposed to explain quantitatively the interesting compositional dependence on the Ge incorporation rate during low-temperature growth of Si1-xGex by disilane and solid-Ge molecular beam epitaxy, based on enhanced hydrogen desorption from Si sites due to the presence of Ge atoms. The hydrogen desorption rate constant for disilane on Si sites is fitted to an exponential function of Ge incorporation rate and a possible physical explanation is discussed. Simulated results are in excellent agreement with experimental data. (C) 1999 American Institute of Physics. [S0021-8979(99)02109-X].

Low-temperature growth properties of Si1-xGex by disilane and solid-Ge molecular beam epitaxy

Low temperature (similar to 500 degrees C) growth properties of Si1-xGex by disilane and solid-Ge molecular beam epitaxy have been studied with an emphasis on surface morphology and growth kinetics. It is found that low-temperature growth(<500 degrees C) is in layer-by-layer mode and atomically-smooth surfaces have been obtained in as-grown samples with large Ge composition (>0.5). Ge composition dependence on substrate temperature, Ge cell temperature and disilane flow rate have been investigated. It is found that in low-temperature growth (less than or equal to 500 degrees C) and under large disilane flux, Ge composition increases with the increase of Ge flux and further increase of Ge flux leads to the saturation of Ge composition. Similar compositional dependence has been found at different growth temperatures. The saturated composition increases with the decrease of substrate temperature. The results can be explained if H desorption is assumed to occur from both Si and Ge monohydrides without diffusional exchange and the presence of Ge enhances H desorption on a Si site. (C) 1998 Elsevier Science B.V. All rights reserved.

Low-temperature growth of cubic GaN by metalorganic chemical-vapor deposition

Low-temperature growth of cubic GaN at 520 degrees C was achieved using CCl4 as an additive by metalorganic chemical-vapor deposition (MOCVD) on GaAs substrate. X-Ray measurement confirmed that the films are single-phase cubic GaN. Scanning electron microscopy (SEM) and reflection high-energy electron diffraction (RHEED) were also used to analyze the surface morphology and the quality of films. The evolution of surface morphology suggests that CCl4 can reduce the hopping barrier and thus Ga adatoms are able to diffuse easily on the GaN surface. (C) 1998 Elsevier Science S.A. All rights reserved.

Temperature dependence of the Fermi level in low-temperature-grown GaAs

A variable-temperature reflectance difference spectroscopy study of GaAs grown by molecular beam epitaxy at low-temperature GaAs (LT-GaAs) shows that the Fermi level is mostly determined by the point defects in samples annealed at below 600 degrees C and can be shifted by photoquenching the defects. The Fermi level is otherwise almost temperature independent, leading to an estimated width of the defect band of 150 meV in the as-grown sample, For LT-GaAs annealed at 850 degrees C, the Fermi level is firmly pinned, most Likely by the As precipitates. (C) 1998 American Institute of Physics.

Fabrication of Nano-structured VOx Film by Low Temperature Ion Beam Sputtering and Reductive Annealing

VOx thin films have been fabricated by low temperature ion beam sputtering and post reductive annealing process. Semiconductor-metal phase transition is observed for the film annealed at 400 degrees C for 2 hours. The film also shows a polycrystal structure with grain size from 50nm to 150nm. The VOx thin films fabricated by this process have a TCR up to -2.7% at room temperature. Our results indicate a promising fabrication method of the nano-structured VOx film with relatively high TCR and semiconductor-metal phase transition.

Low Temperature Deposited Nano-structured Vanadium Oxide Thin Films for Uncooled Infrared Detectors

A novel process of room temperature ion beam sputtering deposition of vanadium oxide films and low temperature post annealing for uncooled infrared detectors was proposed in this work. VOx thin films with relatively low square resistance (70 K Omega / square) and large temperature coefficient of resistance (more than 3%/K) at room temperature were fabricated using this low temperature process which was very compatible with the process of uncooled infrared detectors based on micromachined technology. Furthermore, chemical composition and film surface have been characterized using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) respectively. The results showed that the main composition of the processed thin films was V2O5 and the thin films were in the process of crystallization.

Ge composition saturation behavior during low-temperature Si1-xGex growth by disilane and solid Ge molecular beam epitaxy

Ge composition dependence on the Ge cell temperature has been studied during the growth of Si1-xGex by disilane and solid Ge molecular beam epitaxy at a substrate temperature of 500 degrees C. It is found that the composition x increases and then saturates when the Ge cell temperature increases, which is different from the composition-dependent behavior in growth at high temperature as well as in growth by molecular beam epitaxy using disilane and germane. The enhanced hydrogen desorption from a Ge site alone cannot account for this abnormal composition-variation behavior. We attribute this behavior to the increase of rate constant of H desorption on a Si site when the Ge cell temperature increases.