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We report on full wafer and small area photoluminescence topography investigations of VGF GaAs:Si wafers. The wavelength-specific images exhibit various correlations and anti-correlations. Intensity variations due to competitive radiative and non-radiative recombination processes are mainly due to stoichiometric fluctuations and can be distinguished from those generated by the variation of the silicon dopant concentration. X-ray transmission topograms allow to identify grown-in defects like precipitates and dislocations and to correlate these with the observed macro- and microscopic luminescence variation patterns.

Parylene C was investigated as anti-reflection coating for silicon at terahertz frequencies. Measurements with a Fourier-transform spectrometer show that the transmittance of pure silicon can be improved by about 30% when applying a layer of Parylene C with a quarter wavelength optical thickness. The 10% bandwidth of this coating extends from 1.5 to 3 THz for a center frequency of 2.3–2.5 THz, where the transmittance is constant. Heterodyne measurements demonstrate that the noise temperature of a hot-electron-bolometric mixer can be reduced significantly by coating the silicon lens of the hybrid antenna with a quarter wavelength Parylene C layer. Compared to the same mixer with an uncoated lens the improvement is about 30% at a frequency of 2.5 THz.

Diffusion of platinum at low temperatures is a convenient way to characterize vacancy profiles in silicon. This article summarizes the experiments performed to find a standard procedure, discusses the pitfalls and limitations, and shows the applicability of the method. The results of experiments with float-zone and Czochralski-grown samples in the temperature range from 680 to 842 °C were found to disagree with the predictions of models published in the literature. Therefore, parameters governing the diffusion of point defects and platinum in silicon were determined for this temperature range.

Stimulated emission has been obtained from intra-center donor transitions in silicon monocrystals doped by arsenic. The Si:As laser was optically excited by radiation from a CO2 laser. The emission spectrum consists of two lines corresponding to the 2p±→1s(E) and 2p±→1s(T2) intra-center arcenic transitions. The population inversion is formed due to fast 2s→1s(A1) electron relaxation assisted by intervalley longitudinal acoustic f-phonon emission. This keeps the excited donor states below the 2p± state unpopulated. Thus population inversion occurs between the 2p± state and the 1s(E), 1s(T2) states.

The effects of the terahertz-stimulated emission of Group-V donors (phosphorus, antimony, arsenic, bismuth) in uniaxially stressed silicon, excited by CO2 laser radiation are experimentally studied. It is shown that uniaxial compressive stress of the crystal along the [100] direction increases the gain and efficiency of stimulated radiation, significantly decreasing the threshold pump intensity. The donor frequencies are measured and active transitions are identified in stressed silicon. The dependence of the residual population of active donor states on the uniaxial compressive stress along the [100] direction is theoretically estimated.

Tomographic scanners are classified by various parameters, and two which are important are the geometric resolution and the contrast resolution. A numerical study has been reported previously on the application of certain estimates of theoretical error in the tomographic inversion formula. This study deals with the experimental aspects of those error estimates, and further consolidates the findings of the previous work in representing contrast by a mathematical quantity related to the object cross-section. Thin films of a diandicyanato bisphenol A (DCBA) prepolymer on silicon substrates have been investigated. Angle dependend X-ray photoelectron spectroscopy reveals some thickness-dependent features, which lead to an adsorption model for the DCBA prepolymer molecules. The adsorption of the first layer is governed by the interaction of the triazine rings with the substrate surface.

Silicon membranes are widely used for sensor applications because of their mechanical and electrical properties. Because of the high heat conductance of silicon, they are often required for thermal insulation of sensor devices like gas-sensing elements and electronic circuits. In the present paper a method to improve the thermal insulation of single-crystal silicon membranes is presented. SIMOX technology is employed for the fabrication of silicon membranes with a thickness of 5 μm, where the buried oxide layer is used as a stop for backside etching. This technique allows batch processing and is compatible to a standard CMOS process, so that electronic devices can be placed directly on the membrane. Improvement of the lateral thermal insulation is achieved by etching trenches into the outer region of the silicon membrane and filling them with a thermally grown oxide. For a typical 600 μm×600 μm silicon membrane, the measured thermal resistance amounts to about 200 K/W. By the use of 18 trenches the thermal resistance can be increased to 1700 K/W (960 nm oxide). For comparison, a pure oxide membrane shows a thermal resistance of about 3500 K/W. These experimental data are in good agreement with simulations done by the ANSYS finite-element program.

With the further shrink of IC dimensions, metallization becomes the most crucial layer because conductivity and contact resistivity determine the RC constants and thus the speed of the circuits. With our Q-switched Nd:YAG laser we have evaporated different materials (Al, Ti, W, Pt, Au), alloys (Ta-Si) and dielectrics (ZrO 2 , Al 2 O 3 ). We also produced sandwich layers (Al-Au, Ti-Al). The layers were investigated with regard to deposition rate, homogeneity, adhesion, step coverage and surface roughness. Deposition rates in the order of 60 nm/min were achieved. At a power of 10 W and a repetition rate of about 5 kHz we could form ohmic contacts to silicon with a good step coverage in the contact.

Recent experimental and theoretical results of impurity doped germanium and silicon terahertz lasers are reviewed. Three different laser mechanisms exist in p-type germanium. Depending on the operating conditions and the properties of the crystal, laser transitions can occur between light- and heavy-hole subbands, between particular light-hole Landau levels or between impurity states. Electric and magnetic fields are required for laser operation. In n-type silicon lasing originates solely from impurity transitions of group-V donors, which are optically excited. The properties of these lasers depend upon the chemical nature of the impurity centre and the properties of the host material. The principles of operation are discussed in terms of their basic physical concepts. The state-of-the-art performance of these lasers is summarized.

The results of gain measurements for THz silicon lasers based on shallow donor intracenter optical transitions are presented. The experiments were performed under optical excitation of neutral donor centers by free electron (FELIX) and TEA carbon dioxide laser radiation using oscillator-amplifier scheme. Experimental data are compared with the theoretical estimates.

Optical pumping of excited donor states in silicon crystals doped by antimony, phosphor, arsenic, and bismuth impurity centers by emission from a free electron laser yields stimulated terahertz emission (4-7 THz). The detailed study of silicon emission spectra provides information on intracenter phonon-assisted relaxation mechanisms in silicon at low lattice temperatures as well as on binding energies of excited states of donor centers.

The operating principles and recent results on THz lasers based on intracenter optical transitions of shallow donors in silicon under optical excitation are reviewed. The aspects of THz lasing from double donor and acceptor centers in bulk silicon as well as electrical current excitation by resonant tunneling in Si/SiGe heterostructures are discussed.

Photoexcitation of shallow impurity centers in silicon at low temperatures (

Terahertz stimulated emission of phosphorus donors in silicon optically excited by radiation from the free-electron laser FELIX has been studied. It is found that a spectral line of the Si:P laser emission depends on pump frequency. Stimulated emission arises on the 2p0→1s(E) intra-centre transition (21.2 meV) under resonant pumping of the 2p0 state and on the 2p0→1s(T2) transition (22.3 meV) under pumping of the 2p± or higher odd-parity donor states. The line shift is attributed to the Auger redistribution of the 1s(E)- and 1s(T2)-state populations.

The observation of population inversion and far-infrared emission from shallow donor transitions in silicon is reported. Silicon crystals doped with phosphor and bismuth have been optically excited by radiation from a CO2 laser. For Si : P, the population inversion is due to the suppression of acoustical phonon assisted intracenter relaxation between the 2p0 and 1s(E) states and stimulated emission from the transition between these states has been observed. For Si : Bi, the population inversion is due to a resonance interaction with optical phonons which deplete the 2p0 state. In this case only spontaneous emission was detected. The experimental results are compared with theoretical calculations and found to be in good agreement. # 2001 Elsevier Science B.V. All rights reserved.

Semiconductor sources of stimulated emission which are based on intraband optical transitions of highly nonequilibrium charge carriers are the object of an extensively developing field in physics. The new approaches in the development of unipolar active media are promising to bridge the wavelength range from l 1 m mu p to l 1000 mm of the electromagnetic spectrum. Examples of sources based on bulk material are germanium hot hole lasers [1] and Er-doped silicon lasers [2]. Emitters based on the properties of a two-dimensional electron gas in different kinds of heterostructures are quantum well “cascade” [3] and “fountain” lasers [4] as well as miniband Bloch oscillators in superlattices [5]. We propose a new far-infrared (FIR) (55 mm ,l

We report on the determination of valley-orbit splittings of nitrogen in different SiC polytypes by exploiting the temperature dependence of the ESR signal amplitudes. The effect is shown to be caused by an Orbach spin-lattice relaxation process and, therefore, enables us to measure relaxation times. The reliability of the method was tested by detection of the is ground state splitting of phosphorus and arsenic in silicon and it is applied to other defects in Si:P as -well as to the GaAs antisite related defect in GaAs. New results on hyperfine splittings are presented, too.

Spark source mass spectrometry (SSMS) is used to analyze metallurgical grade silicon powder which has been submitted to different purification steps. The principal advantage of this method in comparison to other analytical methods is the broad range of impurity elements detected simultaneously in each analysis. The silicon powder is mixed with gallium in a weight-ratio of 5∶1 and pressed in a moulding die to solid rods. Detection limits are in the range of 0.02–1 ppma with the exception of some few elements contained in the gallium or introduced during sample preparation. By using self-prepared standards relative sensitivity factors for 35 impurity elements have been determined to enhance the accuracy of the results. Concentration values for most elements are expected to be accurate within a factor of 2. As an example the results of SSMS-measurements referring to one special purification step (acid-treatment) are shown.

The ESR of a new Fe-S center in Si is reported. It is shown that the g-values of three of the known Fe-S pairs are determined by exchange interaction of the angular momentum of Feoi with that of nearby S=12 ion, which is likely to be S+ or (S-S)+. The analysis uses an analogy to the O2--centers in the alkali halides. Orbach relaxation of the ESR of the new Fe-S center shows that an excited state lies 8.4 meV above the groundstate.

The annealing behavior of arsenic‐implanted silicon using Nd:YAG laser irradiation was investigated. The annealing process depended upon the scanning speed and the power density. The carrier concentration at the surface was higher by factor of approx. 1.5 to 3 than in the bulk. Multiple irradiation resulted in a profile broadening and a reduced carrier concentration in the bulk. The dependence of the electron mobility upon the electron concentration up to 1.5×1021 cm−3 was measured. The experimental results show that the electron mobility depends only on the electron concentration, and is independent of the conditions of annealing.