Optimizing the GaAs capping layer growth of 1.3 mu m InAs/GaAs quantum dots by a combined two-temperature and annealing process at low temperatures

We report on optimizing the GaAs capping layer growth of 1.3 mu m InAs quantum dots (QDs) by a combined two-temperature and annealing process at low temperatures using metalorganic chemical vapor deposition. The initial part (tnm) of the capping layer is deposited at a low temperature of 500 degrees C, which is the same for the growth of both the QDs and a 5-nm-thick In0.15Ga0.85As strain-reducing capping layer on the QDs, while the remaining part is grown at a higher temperature of 560 degrees C after a rapid temperature rise and subsequent annealing period at this temperature. The capping layer is deposited at the low temperatures (<= 560 degrees C) to avoid postgrowth annealing effect that can blueshift the emission wavelength of the QDs. We demonstrate the existence of an optimum t (=5 nm) and a critical annealing time (>= 450s) during the capping, resulting in significantly enhanced photoluminescence from the QDs. This significant enhancement in photoluminescence is attributed to a dramatic reduction of defects due to the optimized capping growth. The technique reported here has important implications for realizing stacked 1.3 mu m InAs/GaAs QD lasers. (C) 2008 Elsevier B.V. All rights reserved.

Structual and optoelectronic properties of polycrystalline silicon thin films prepared by hot-wire chemical vapor deposition at low temperatures

Polycrystalline silicon thin films were prepared by hot-wire chemical vapor deposition ( HWCVD) on glass at 250 degreesC with W or Ta wire as the catalyzers. The structual and optoelectronic properties as functions of the filament temperature, deposition pressure and the filament-substrate distance were studied, and the optimized polycrystalline silicon thin films were obtained with X-c > 90 % ( X-c denotes the crystalline ratio of the film), crystal grain size about 30-40nm, R-d approximate to 0.8nm/s, sigma(d) about 10(-7) - 10(-6) Omega(-1) cm(-1), Ea(a) approximate to 0.5eV and E-opt less than or equal to 1.3eV.

Improved neutron radiation hardness for Si detectors: Application of low resistivity starting material and or manipulation of N-eff by selective filling of radiation-induced traps at low temperatures

Radiation-induced electrical changes in both space charge region (SCR) of Si detectors and bulk material (BM) have been studied for samples of diodes and resistors made on Si materials with different initial resistivities. The space charge sign inversion fluence (Phi(inv)) has been found to increase linearly with the initial doping concentration (the reciprocal of the resistivity), which gives improved radiation hardness to Si detectors fabricated from low resistivity material. The resistivity of the BM, on the other hand, has been observed to increase with the neutron fluence and approach a saturation value in the order of hundreds k Omega cm at high fluences, independent of the initial resistivity and material type. However, the fluence (Phi(s)), at which the resistivity saturation starts, increases with the initial doping concentrations and the value of Phi(s) is in the same order of that of Phi(inv) for all resistivity samples. Improved radiation hardness can also be achieved by the manipulation of the space charge concentration (N-eff) in SCR, by selective filling and/or freezing at cryogenic temperatures the charge state of radiation-induced traps, to values that will give a much smaller full depletion voltage. Models have been proposed to explain the experimental data.

Properties of GaAs single crystals grown by molecular beam epitaxy at low temperatures

Properties of GaAs single crystals grown at low temperatures by molecular beam epitaxy (LTMBE GaAs) have been studied. The results shaw that excessive arsenic atoms of about 10(20) cm(-3) exist in LTMBE GaAs in the form of arsenic interstitial couples, and cause the dilation in lattice parameter of LTMBE GaAs, The arsenic interstitial couples will be decomposed, and the excessive arsenic atoms will precipitate during the annealing above 300 degrees C. Arsenic precipitates accumulate in the junctions of epilayers with the increase in the temperature of annealing. The depletion regions caused by arsenic precipitates overlap each other in LTMBE GaAs, taking on the character of high resistivity, and the effects of backgating or sidegating are effectively restrained.

Excess arsenic in GaAs grown at low temperatures by molecular beam epitaxy

The structural properties of GaAs grown at low temperatures by molecular beam epitaxy (LTMBE GaAs) were studied. The excess arsenic atoms in LTMBE GaAs exist in the form of arsenic interstitial couples (i,e, two ns atoms share the one host site), and cause an increase in the lattice parameter of LTMBE GaAs. Annealing at above 300 degrees C, the arsenic interstitial couples decomposed, and As precipitates formed, resulting in a decrease in the lattice parameter.

Detection of Terahertz Photon by GaAs-Based Single Electron Transistor at Low Temperatures

A reproducible terahertz (THz) photocurrent was observed at low temperatures in a Schottky wrap gate single electron transistor with a normal-incident of a CH_3OH gas laser with the frequency 2. 54THz.The change of source-drain current induced by THz photons shows that a satellite peak is generated beside the resonance peak. THz photon energy can be characterized by the difference of gate voltage positions between the resonance peak and satellite peak. This indicates that the satellite peak exactly results from the THz photon-assisted tunneling. Both experimental results and theoretical analysis show that a narrow spacing of double barriers is more effective for the enhancement of THz response.

Improved neutron radiation hardness for Si detectors: Application of low resistivity starting material and or manipulation of N-eff by selective filling of radiation-induced traps at low temperatures

Radiation-induced electrical changes in both space charge region (SCR) of Si detectors and bulk material (BM) have been studied for samples of diodes and resistors made on Si materials with different initial resistivities. The space charge sign inversion fluence (Phi(inv)) has been found to increase linearly with the initial doping concentration (the reciprocal of the resistivity), which gives improved radiation hardness to Si detectors fabricated from low resistivity material. The resistivity of the BM, on the other hand, has been observed to increase with the neutron fluence and approach a saturation value in the order of hundreds k Omega cm at high fluences, independent of the initial resistivity and material type. However, the fluence (Phi(s)), at which the resistivity saturation starts, increases with the initial doping concentrations and the value of Phi(s) is in the same order of that of Phi(inv) for all resistivity samples. Improved radiation hardness can also be achieved by the manipulation of the space charge concentration (N-eff) in SCR, by selective filling and/or freezing at cryogenic temperatures the charge state of radiation-induced traps, to values that will give a much smaller full depletion voltage. Models have been proposed to explain the experimental data.