Specific heat measurements in small samples at low temperature

We describe a calorimeter which has been used to measure the heat capacity of small samples weighing 50 to 200 mg in the temperature range of 1.5 K to 20 K. We have used the thermal relaxation method as well the heat pulse method for our measurements with the same experimental set-up. An accuracy of better than 5% is obtained with this system.     

Uniaxial stress measurements on n-type GaAs

Measurement of the influence of 〈100〉 stress on the low-field resistivity of n-type GaAs shows a linear change to 10 k-bar and no influence of the X_1c minima. This removes the apparent discrepancy between high-stress and electro-reflectance data for the conduction band structure of GaAs.     

Optical absorption of silver colloids in KCl crystals at low temperature

The observed increase in the intensity of the optical absorption curve at low temperature, due to Ag colloidal particles in KCl crystals, is quantitively explained by the effect of temperature on the collision frequency of the free electrons with electrons and phonons in silver.     

Optical measurements of silicon wafer temperature

An optical technique for precise, non-contact, and real time measurement of silicon wafer temperature that uses the polarized reflectivity ratio R_p/R_s is described. The proposed method is based on temperature dependence of the optical functions of silicon. Expected strong temperature sensitivity is obtained near band gap. Simultaneous monitoring of temperature and oxide layer thickness is discussed using measurements at four wavelength 365 nm, 405 nm, 546 nm, and 820 nm.     

GaAs在光通讯波段的超快非线性光学研究

采用Z-扫描和泵浦-探测技术,在光通讯波段对砷化镓(GaAs)单晶进行了非线性动力学以及非线性光学的实验研究.飞秒泵浦-探测实验结果表明,三阶非线性光学效应源于砷化镓单晶对飞秒激光的瞬态双光子吸收,而五阶非线性光学效应源于砷化镓单晶双光子吸收诱导的自由载流子吸收效应.通过Z扫描实验,得到了关于GaAs单晶所有的非线性光学参数,包括双光子吸收系数、三阶非线性折射系数、双光子吸收诱导的自由载流子吸收截面以及双光子吸收诱导的自由载流子折射截面.结果表明,砷化镓单晶在制造光限幅器件和光电探测器方面具有良好的发展前景.     

超低温高电场下GaAs的电子太赫兹功耗谱的研究

利用自由空间太赫兹电光取样方法,测量了在高电场下,GaAs中受飞秒激光脉冲激发的电子所辐射出的太赫兹电磁波,发现从样品中辐射出的和电子加速度成正比的太赫兹电磁波电场强度E_THz（t）,表现出双极特性.通过分析GaAs中辐射出的太赫兹电磁波的傅里叶变换谱,首次实验上得到在阶跃电场下的GaAs的电子太赫兹功耗谱.研究发现,当电场小于50 kV/cm时,由电子谷间散射引起的负功耗（即增益）的截止频率ν_c,随着电场的增大而增大;当电场大于50 kV/cm时,负功耗的截止频率ν_c开始在750 GHz（10 K）附近饱和. 关键词： 太赫兹 非平衡载流子 功耗谱 谷间散射     

Micropulsation measurements at low latitudes

Summary Measurements of the geomagnetic field variations performed. at L'Aquila in the range of frequency of micropulsations suggest that the most important events mainly perturb, during daytime intervals, the H-component and show typical periods asT ₁=(25±5)s andT ₂=(13±2)s. These results can be interpreted in terms of a source spectrum peaked atT ₁ together with a resonant mechanism atT ₂ which might correspond to the fundamental eigenperiod of the local field line. Night-time events, with typical periods longer than 40s, basically consist of few cycles of irregular Pi2 pulsations. A comparison with measurements performed at different sites suggest a UT dependence at periods longer than 45s while, at higher frequencies, the power of pulsations might rather be driven by more local magnetosphere and ionosphere conditions. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Finite element analysis of stress and strain distributions in InAs/GaAs quantum dots

In this paper, we perform systematic calculations of the stress and strain distributions in InAs/GaAs truncated pyramidal quantum dots (QDs) with different wetting layer (WL) thickness, using the finite element method (FEM). The stresses and strains are concentrated at the boundaries of the WL and QDs, are reduced gradually from the boundaries to the interior, and tend to a uniform state for the positions away from the boundaries. The maximal strain energy density occurs at the vicinity of the interface between the WL and the substrate. The stresses, strains and released strain energy are reduced gradually with increasing WL thickness. The above results show that a critical WL thickness may exist, and the stress and strain distributions can make the growth of QDs a growth of strained three-dimensional island when the WL thickness is above the critical value, and FEM can be applied to investigate such nanosystems, QDs, and the relevant results are supported by the experiments.     

Non trivial overlap distributions at zero temperature

We explore the consequences of Replica Symmetry Breaking at zero temperature. We introduce a repulsive coupling between a system and its unperturbed ground state. In the Replica Symmetry Breaking scenario a finite coupling induces a non trivial overlap probability distribution among the unperturbed ground state and the one in presence of the coupling. We find a closed formula for this probability for arbitrary ultrametric trees, in terms of the parameters defining the tree. The same probability is computed in numerical simulations of a simple model with many ground states, but no ultrametricity: polymers in random media in 1+1 dimension. This gives us an idea of what violation of our formula can be expected in cases when ultrametricity does not hold. Received 16 June 2000     

Oxidation of Si and GaAs in air at room temperature

The oxidation of Si and GaAs single-crystals, both cleaved and etched, has been studied at room temperature in air. We found experimentally that the oxidation of GaAs follows the direct exponential law at least within a 2 years time interval. The same also holds for both cleaved and etched Si single-crystals until the oxide film reaches a thickness of about 35–40 Å: beyond this thickness the cubic law starts to dominate the oxidation rate. An interpretation of the observed experimental results is given.     

AES measurements of adsorption isobars for oxygen on tungsten at low pressure and high temperature

Auger electron spectroscopy (AES) has been employed to determine the relative coverage of oxygen on polycrystalline tungsten at high temperatures (1200 ?T ? 2500 K) and low O₂ pressures (5 × 10^?9 ?po₂ ?5 × 10^?6 Torr). We believe that this is the first demonstration that chemical analysis of solid surfaces by AES is possible even at temperatures as high as 2500 K. It is assumed that the relative oxygen coverage is directly proportional to the peak-to-peak amplitude of the first derivative of the 509 eV oxygen Auger peak. The experimental results illustrate the dependence of coverage on temperature and pressure, and it is shown that the results for low coverages may be described reasonably well by a simple first-order desorption model plus a semi-empirical expression for the equilibration probability (or sticking coefficient). On the basis of this approximate model, the binding energy of oxygen on tungsten is estimated as a function of coverage, giving a value of ～ 140 $\text{kcal}\text{mole}$ in the limit of zero coverage.     

Doppler-broadening measurements of microvoids at the Au/GaAs interface

Recent positron lifetime studies made on the Au/GaAs interface with an applied electric field returning a significant fraction of bulk implanted positrons to the interface have revealed the presence of microvoids( 1 nm diameter) at the interface. In this work an attempt has been made to study these microvoids by observing the Doppler broadening on the annihilation radiation coming from them. This is done both by observing theS-parameter as a function of applied bias and by applying the generalized least-squares method to the deconvolution of the annihilation radiation lineshape. The general conclusion is that the Doppler-broadened data are consistent with the majority of positrons trapping into microvoids, probably associated with grain boundaries. The data suggest that these open volume defects are more associated with the Au film rather than the Au-Ga alloyed interfacial region.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Optical characterization of doped and undoped GaAs at 300 K

Infrared reflectance spectroscopy was employed for the optical characterization of undoped GaAs, as well as for GaAs doped with Si, Zn and Cr. Dielectric constants, refractive indices and extinction coefficients were determined using the classical two oscillator model.     

Electrooptic sampling of low temperature GaAs pulse generators for oscilloscope calibration

Optoelectronic generation of well characterized ultrashort electrical pulses plays an important role in the calibration of fast-sampling oscilloscopes. In this paper the authors describe the National Physical Laboratory (NPL) pulse generator, comprising a femtosecond laser and an ultrafast photoconductive switch which can generate electrical pulses as short as 650 fs. The photoconductive switch consists of a GaAs substrate with a top GaAs layer grown under low temperature conditions to ensure a subpicosecond recombination rate. The technique of electrooptic sampling is used to measure pulses on planar transmission lines, such as coplanar waveguide and coplanar stripline. Good agreement is shown between electrooptic sampling measurement and the modelling of pulse propagation along a coplanar waveguide, enabling one to optimize a design of a calibration test source. The use of a pulse generator to calibrate a 50 GHz sampling oscilloscope is described.     

Johnson noise at low temperature

It has long been accepted that Johnson noise in metals is proportional to absolute temperature, and is materials-independent. This is a semi-classical result, based on the equipartition of energy. Johnson noise in copper below 10 mK suggests a departure from this relation. The data have been fitted empirically to T_J = T_o coth $\text{T}{}_{\mathrm{<mtext>o</mtext>}}\text{T}$, where T_J is the “Johnson noise” temperature, T is measured by CMN, and T_o is chosen for best fit and is about 2 mK. T_o is now identifiable with a zero-point frequency of electronic charge imbalance, which moves with average Fermi velocity ν from end to end of the resistor of length l. $\text{T}{}_{\mathrm{<mtext>o</mtext><mtext>\; =\; </mtext><mtext>h\nu </mtext><mtext>2kl\; \approx \; 2\; </mtext><mtext>mK</mtext>}}$, and so is materials- and geometry-dependent.     

干涉法测量气体低压下声速的实验研究

介绍了干涉法测量气体声速的试验原理和变程干涉声速测量的实验系统,使用本套测量系统测得的气体声速的精度可达5×10-4。根据热力学原理,分析了气体声速与理想气体比定压热容之间的物理关系。在本套实验系统上测得了一批高精度的新环保制冷剂的气相声速数据,并可进一步确定其理想气体比定压热容.     

Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

ZnS thin films grown by atomic layer deposition on GaAs and HgCdTe substrates at very low temperature

ZnS films grown on GaAs and HgCdTe substrates by atomic layer deposition (ALD) under very low temperature were investigated in this work. ZnS films were grown under several temperatures lower than 140 °C. The properties of the films were investigated with high-resolution X-ray diffraction (HRXRD), scanning electron microscope (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The results showed the ZnS films were polycrystalline. The growth rate monotonically decreased with temperature, as well as the root mean square (r.m.s) roughness measured by AFM. XPS measurement revealed the films were stoichiometric in Zn and S.