Anisotropy and temperature dependence of small angle neutron scattering from silicon-oxide precipitates in silicon

Czochralski grown silicon crystals contain interstitially dissolved oxygen which diffuses on heating to form precipitates of silica. We have examined these precipitates by small angle neutron scattering (SANS) in the Q-range 0.05 Å^–1<Q<0.4 Å^–1. The obtained SANS patterns reveal pronounced anisotropic intensity distributions which resemble the symmetry of the host crystal. The SANS spectra show an anisotropic central peak at Q<0.1 Å^–1 due to the single particle shape and a number of weak intensities for larger Q-values. These weak side maxima are considered correlation peaks or quasi-elastic interference peaks. They show, however, an unexpected and distinct temperature dependence: with decreasing temperature below values of 220 K their intensity is lost slowly, but reversibly. At T = 50 K only the central peak from the single-particle scattering remains unchanged. Upon heating, the correlation peaks regain their former value of intensity and Q-position without any evidence of thermal hysteresis.     

Oxygen in Ge crystals grown by the B2O3 encapsulated Czochralski method

Local vibrations of oxygen in Ge crystals grown by the Czochralski method adopting liquid-B₂O₃ encapsulation and GeO₂ powder doping were investigated by Fourier-transform infrared spectroscopy. Strong absorption peaks at 855 cm⁻¹, originating in local vibration of interstitially dissolved oxygen O_i as Ge–O_i–Ge quasi-molecules, developed depending on the doped amount of GeO₂. Similarly, an absorption peak related to the combined vibration of Ge–O_i–Ge was found at 1264 cm⁻¹ and the conversion factor from the peak intensity to the oxygen concentration was evaluated to be 1.15×10¹⁹ cm⁻². By prolonged annealing at 350 °C an absorption peak developed at 780 cm⁻¹, indicating formation of oxygen-related thermal donors. From the variations of carrier density and oxygen concentration, one donor was found to possess about 15 O_i atoms.     

Electrical properties and defect model of tin-doped indium oxide layers

Tin-doped In₂O₃ layers were prepared by the spray technique with doping concentrationsc _Sn between 1 and 20 at. % and annealed at 500 °C in gas atmospheres of varying oxygen partial pressures. The room-temperature electrical properties were measured. Maximum carrier concentrationsN=1.5×10²¹cm^–3 and minimum resistivities =1.3×10^–4 cm are obtained if the layers are doped withc _Sn9 at. % and annealed in an atmosphere of oxygen partial pressurep _O2 10^–20 bar. At fixed doping concentration, the carrier mobility increases with decreasing oxygen pressure. The maximum obtainable mobility can be described in terms of electron scattering by ionized impurities. From an analysis of the carrier concentration and additional precision measurements of the lattice constants and film thicknesses, a defect model for In₂O₃:Sn is developed. This comprises two kinds of interstitial oxygen, one of which is loosely bound to tin, the other forming a strongly bound Sn₂O₄ complex. At low doping concentrationc _Sn4 at. % the carrier concentration is governed by the loosely bound tin-oxygen defects which decompose if the oxygen partial pressure is low. The carrier concentration follows from a relationN=K ₁ ·p _O2 ^–1/8 ·(3 ×10¹⁰ × c_Sn –N)^1/4 with an equilibrium constantK ₁=1.4×10¹⁵ cm^–9/4bar^1/8, determined from our measurements.     

Deep bandtail states picosecond intensity-dependent carrier dynamics of GaN epilayer under high excitation

Picosecond carrier dynamics of deep bandtail states (3.1 eV) in an unintentionally n-doped GaN epilayer at room temperature under high excitation densities (i.e., N ₀ = 1.0× 10¹⁹– 1.1× 10²⁰ cm^–3) have been investigated with nondegenerate femtosecond pump–probe (267/400 nm) reflectance ( R/R ₀). All R/R ₀ traces possess a 2 ps buildup time that represents an overall time for the initial non-thermal carrier population to relax towards the continuum extremes and then into the probed tail states. We observe a saturation of R/R ₀ initial (first 10 ps) recovery rate _i at a density of 5– 6×10¹⁹ cm^–3 close to the Mott transition threshold obtained from time-integrated PL measurements. Such a saturation phenomenon has been identified as the trap-bottleneck due to the bandtail states and deep traps. As N ₀ is further increased, _i accelerates due to the onset of Auger recombination as the trap-bottleneck becomes effective. The best fit by the Auger model for N ₀ in the range of the mid-10¹⁹–10²⁰ cm^–3 yields an Auger coefficient of C _a 5.0× 10^–30 cm⁶ s^–1.     

Correlations between frequency of infra-red active vibrational modes and copper-oxygen distance in copper oxides. Application to superconductors

The infra-red spectra of a large number of ternary Cu(II) oxides with at least a quasi square-planar coordination of oxygen around the copper ions have been studied. The frequency of the bands with the highest frequency,v _max, is found to correlate extremely well with the shortest Cu–O distance.v _max increases at an impressive rate of 20 cm^–1 per 0.01 Å when the Cu–O distance becomes less than 1.97 Å, which is the Cu²⁺–O^2– distance in square-planar CuO₄ complexes as obtained from empirical ionic radii considerations. The marked sensitivity may be used as a titration procedure not only to assign bands but also to obtain diagnostic information about local coordination in compounds derived, for example, from the YBa₂Cu₃O_7–d structure such as LaCaBaCu₃O_7–d . The only example where this correlation fails is in the two-layer non-superconducting oxides derived from La₂(Ca, Sr)Cu₂O₆. The significance of this result is discussed. The marked dependence of frequency on the bond-distance is qualitatively examined in terms of an increased electron-phonon coupling to account for the observed tendency of the superconducting transition temperature to go through a maximum as the average basal plane Cu–O distance is decreased.     

Supersaturated Si-As alloy formation by ion implantation and pulsed electron beam annealing

Supersaturated surface alloys produced by very high dose (0.8–2.6×10¹⁷cm^–2) implantation of As-ions into silicon and subsequent pulsed electron-beam annealing have been investigated by means of the channeling technique. The maximum solubility limit of 7×10²¹ As/cm³ has been determined. It exceeds the equilibrium solubility limit by more than a factor of 4. Angular scan measurements indicated that for doses above 1×10¹⁷cm^–2 As atoms are displaced by about 0.12 Å from the regular lattice sites.     

Shallow acceptor population and free hole concentration in Si: In and Si:Ga with IR-photoexcitation

Hall measurements at low temperaturesT<50 K have been performed on Si:In (N _In10¹⁷ cm^–3) and Si:Ga (N _Ga10¹⁸ cm^–3) with infrared photoexcitation of holes into the valence band. It is shown in quantitative agreement with a theoretical model that the population of shallow acceptors, e.g. B and Al, which are present as impurities in concentrations ofN _B,Al10¹²-10¹⁴ cm^–3 strongly affects the photoexcited hole concentration. Photo-Hall measurements can, therefore, serve as a tool for the determination of low impurity acceptor concentrations in the case of high In- or Ga-doping. Hole capture coefficientsB _In=6×10^–4 (T/K)^–1,8 cm³ s^–1 andB _Ga=2×10^–4 (T/K)^–1 cm³ s^–1 have been determined.     

Collisional excitation X-ray laser experiments in plasmas produced by 0.53-μm and 0.35-μm laser light

Recent Ne- and Ni-like X-ray laser experiments carried out at the Centre d'Etudes de Limeil-Valenton (CEL-V) are reviewed. A variety of experiments in Ne-like X-ray lasers were performed; here we discuss measurements of soft X-ray amplification in Ge (Z=32) and Sr (Z=38) plasmas. In Ge plasmas produced by 0.53-m laser light at an irradiance of 6.0×10¹³ W/cm², gains between 2.2–2.5 cm^–1 on the 232.2 and 236.2 Å J=2–1 lines and a gain of 1.0 cm^–1 on the 196.1 Å J=0–1 line were measured. In addition, gains of 4.4 cm^–1 and 4.0 cm^–1 have been demonstrated on the J=2–1 transitions at 164.1 and 166.5 Å in Nelike Sr at laser intensities of 1.3×10¹⁴ W/cm². The effects of pumping the Ne-like Se X-ray laser with 0.35-m laser light have also been investigated; the Se lasing spectra is similar to that obtained with 0.53-m light. Experiments have also been carried out to optimize the gain of the 50.3 Å Ni-like Yb (Z=70) J=0–1 line. For Yb, no significant increase in gain over that previously reported was seen, but the time history of the Ni-like Yb X-ray laser was measured for the first time. Finally, attempts to extrapolate the Ni-like results to shorter wavelength were made using Ta (Z=73), W (Z=74), and Re (Z=75). No definitive observation of the Ni-like J=0–1 lasing lines was made in these experiments.     

Trap spectrum of the “new oxygen donor” in silicon

Electronic properties of thenew oxygen donor generated in phosphorus-doped Czochralski-silicon at 650C are investigated by deep level transient spectroscopy. A continuous distribution of trap states (10¹⁴–10¹⁶ cm^–3 eV^–1) is detected in the upper half of the band gap with increasing values towards the conduction band. The magnitude of the state density observed increases with the oxygen content, the heat duration, and a preanneal at temperatures lower than 650C. The continuous trap spectrum of thenew donor is explained by interface states occuring at the surface of SiO _x precipitates.     

A superheated Na cell for X-ray photoionization experiments

The performance of a superheated alkali vapor cell, with a geometry suitable for x-ray photoionization experiments, is described. An internal heater was used to superheat sodium vapor in a split-wick heat pipe. At a Na pressure of 20 Torr, the transmission through the cell in the region of theC–X molecular band of Na₂ increased to 50% from the 5% observed when the cell was operated in a conventional heat-pipe mode. In the presence of a Na atom density of 2×10¹⁷ cm^–3, the average molecule density over the length of the cell was measured to be 2.5×10¹⁵ cm^–3. In the hot central region of the superheated cell, the molecule density was predicted to have been reduced by a factor of 50 to 3×10¹⁴ cm^–3.     

Amorphous to crystalline transition in Dy-Fe thin films

The crystallization of vacuum-deposited amorphous Dy-Fe thin films was studied by transmission electron microscopy and electron diffraction. The effect of thickness, deposition rate and substrate temperature on the crystallization process have been investigated. The results show that the crystallization thicknessd _c decreases with increasing deposition rate and substrate temperature. The number density of Dy-Fe islands were found to be almost constant at (4–5)×10¹¹ cm^–2 in the thickness range 20 Å<d <50 Å. The number density decreases with increase ind _c .     

Fourier Transform Spectroscopy of theYΣ–XΠiTransition of CuO

TheY²Σ⁺–X²Π_inear-infrared electronic transition of CuO was observed at high resolution for the first time. The spectrum was recorded with the Fourier transform spectrometer associated with the McMath–Pierce Solar Telescope at Kitt Peak. The excited CuO molecules were produced in a low pressure copper hollow cathode sputter with a slow flow of oxygen. Constants for theY²Σ⁺states of CuO are:T₀= 7715.47765(54) cm⁻¹,B= 0.4735780(28) cm⁻¹,D= 0.822(12) × 10⁻⁶cm⁻¹,H= 0.46(10) × 10⁻¹⁰cm⁻¹, γ = −0.089587(42) cm⁻¹, γ_D= 0.1272(79) × 10⁻⁶cm⁻¹,b_F= 0.12347(22) cm⁻¹, andc= 0.0550(74) cm⁻¹. ImprovedX²Π_iconstants are also presented.     

TEM characterization of yttrium silicide layers synthesized by ion implantation

The structure of Si implanted with high doses of yttrium has been investigated by varying implantation doses and energies. As implantation doses increase into the low 10¹⁷ cm^–2 range, silicide precipitates form. The precipitates are thin and long and lie parallel to {111} planes in the Si matrix. As dopant concentrations increase, the precipitates themselves become more equiaxed, aspect ratios decrease, and precipitates densities increase until the precipitates coalesce to form a continuous buried layer of yttrium silicide within the Si matrix. The layer thickness is relatively uneven. As implant doses increase to 4×10¹⁷ cm^–2, the layer thicknesses become more uniform although there are still defects present. As the implant doses increase further, the precipitate bands on either side of the continuous layer decrease due to gettering of yttrium to the layer. As the energy of the implant is increased, the appearance of the sample is similar to that of the lower energy implants except that the layer is buried deeper in the Si matrix. Observations of the silicide are consistent with its having the AlB₂ structure with ordered vacancies on the Si sublattice.Address from July 1, 1992: Arizona State University, Tempe, AZ     

A polarized CARS investigation of the fine structure population inversion of Cl atoms from laser photolysis of ICl and the quenching of Cl(2 P 1/2) by O2

The time resolved polarized CARS technique has been used to detect Cl atoms produced by photolysis of ICl in the presence and absence of O₂. A population inversion was observed between the ground state electronic levels Cl(² P _1/2) and Cl(² P _3/2). The rate constant for Cl(² P _1/2) decay (quenching + reaction) in ICl was determined to be (3.2±0.2)×10^–13 cm³/molecule×s; the rate constant for Cl(² P _3/2) reaction with ICl was determined to be (7.8±0.5)×10^–12 cm³/molecule×s; and the rate constant for Cl(² P _1/2) quenching by O₂ was determined to be (1.9±0.2)×10^–13 cm³/molecule×s.     

Multilevel systems and generalizations of the superalgebra

A new approach to constructing the various generalizations of the one-dimensional supersymmetric quantum mechanics is proposed, including the parasupersymmetric quantum mechanics constructed by Rubakov and Spiridonov as the special case. In particular, we derive the generalized superalgebra, which possesses the features both of the familiar superalgebra and of the parasuperalgebra. Namely, the generalized supercharges Q_i ^± and the Hamiltonian H forms the generalized superalgebra, where Q_i ^±2=0 (as for ordinary superalgebra), but the triple products of generalized supercharges obey the relations Q₁ ⁺Q_j ^–Q_j ⁺=Q_i ⁺H (i, j=1, 2) and Q_i ⁺Q_i ^–Q_j ⁺=(1/4)kQ_i ⁺, Q_i ⁺Q_i ^–Q_j ⁺=(1/4)kQ_i ⁺(i, j=1, 2; ij) (analogous to the parasuperalgebra). Furthermore, the generalized supercharges are conserved, i.e. [H, Q_i ^±]=0.Presented at the International Workshop on Squeezed and Correlated States, Moscow, December 3–7, 1990.     

Surface resistance and far infrared absorptivity at T=7 K of highTc YBaCuO superconductors

A surface resistance as small as R_S=3×10^–2 Ohms has been computed at T=8 K, for v=0.6 Thz (v=20 cm^–1), for an YBaCuO film deposited on an MgO substrate. The calculations are made with the refractive index computed from the Far IR transmission spectra of a 400 Å thick film.Still lower values are obtained with another sample, 300 Å thick of exceptional quality (R_S=1.2×10^–2 Ohms).     

Excitation of the 7s2P1/2, 3/2 levels of Xe+ due to charge exchange in the negative glow of a glow discharge in a neon-xenon mixture

It is shown that in the negative glow of a glow discharge in mixtures of neon with xenon (xenon contents of 5.5·10^–3 to 2% of the total pressure P of the mixture) with P=15 torr and discharge currents i=40 mA (cathode diameter 30 mm), the 7s²P_{1/2, 3/2} levels of the Xe⁺ ion are excited by the charge exchange reaction Xe ^m + Ne⁺ Xe^+* + Ne.The effective cross section Q_3/2 for excitation of the 7s²P_3/2 level is estimated to lie between 10^–14 and 10^–15 cm² and Q_3/26.5 × Q_1/2, where Q_1/2 is the effective cross section for excitation of the 7s²P_1/2 level.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 3, pp. 96–99, March, 1979.     

Electrical properties of p +/n + In0.53Ga0.47As tunnel diodes grown by liquid phase epitaxy

p ⁺/n ⁺ In_0.53Ga_0.47As tunnel diodes were prepared by liquid phase epitaxy and their electrical properties were characterized. These devices exhibit large forward conductances (2.59×10^{3 –1} cm^–2), high peak current densities (793 A/cm²) and large peak to valley current ratios (16.2). These devices offer great promise as intercell ohmic contacts (IOCs) for InP-based, onolithic multijunction solar cells.     

The magnetic susceptibility of p-type CdSb

The paper gives the measurements of the magnetic susceptibility of p-type CdSb at 77°K on samples crystallographically oriented and cut from single crystals having an acceptor concentration of 2.3×10¹⁵cm^–3, 2.4×10¹⁶ cm^–3 and 1.5×10¹⁷ cm^–3. The anisotropy of the lattice and hole gas contribution was found and the ratio of the hole effective mass obtained from measurement of the transversal magnetoresistivity in p-type CdSb at 77°K [3] was used to determine their absolute values:m _a=0.48m ₀=m _b=0.44m ₀,m _c=0.17m ₀.     

TDPAC studies of internal AgIn alloy oxidation

The internal oxidation of dilute AgIn alloys in open air versus In concentration and oxidation temperature was investigated by the TDPAC technique. The¹¹¹In interaction described by the broad quadrupole frequencies distribution aroundv _Q = 115 MHz and =0.5–0.7 was observed and interpreted as due to nonstoichiometric In–O_X cluster formation. For higher temperatures and more concentrated alloys additionally quasiunique frequencyv _Q 157 MHz with 0.28 was evidenced and ascribed to¹¹¹In interaction inside In₂O₃ precipitates. The internal oxidation under reduced oxygen pressures was studied and the thermal stability of the complex formed was tested. The effect of surface preparation on internal oxidation was observed.