Electrical conduction mechanism and carrier trapping in β-metal free phthalocyanine single crystals

The dark electrical conductivity of β-metal free phthalocyanine single crystals has been investigated over the temperature range 273–600°K, at a reduced pressure of 10^?7 torr. The results obtained are in accordance with the model proposed by Barbe and Westgate[5] for this material, in which the energy gap between the top of the valence band and the bottom of the conduction band is determined to be 2·00 eV. At temperatures below about 410°K, the conduction process is consistent with the presence of an electron trapping level located 0·32 eV below the conduction band edge, with a density of 7×10¹⁶ cm^?3, and a donor level of density 2×10⁷ cm^?3 at the same energy. Above about 410°K, there is evidence to suggest that the conduction process is intrinsic.     

Shubnikov-de haas effect in n-CdSnAs2

Shubnikov-de Haas oscillations in the transverse magnetoresistance of single-crystalline n-type CdSnAs₂ have been recorded at temperatures between 2 and 25 K in magnetic fields up to 5T. The electron concentration of the samples ranged from 2 × 10¹⁷ to 2 × 10¹⁸ cm^?3. The angular dependences of the oscillation periods and cyclotron effective masses showed that the conduction band exhibits an energy dependent anisotropy, obeying the Kildal band structure model. For the low-temperature values of the band parameters we found: a band gap E_g = 0.30 eV, a spin-orbit splitting Δ = 0.50 eV, a crystal field splitting parameter δ = ?0.09 eV, and an interband matrix element P = 8.5 × 10^?8eV cm. This simple four-level model was found to be not adequate to describe quantitatively the observed electronic effective g-factor for a sample with low electron concentration.     

Electron traps in GaS

Measurements performed on n-GaS by means of the space-charge limited current method indicate the presence of a deep trap for electrons, at 0.57 eV from the conduction band and with a density of about 2.3 × 10¹³cm^?3. Another deeper trap at 0.63 eV and with a density of 6 × 10¹²cm^?3 is probably also present. The results seem to confirm the validity of a new direct method of analysis. Traps are tentatively attributed to compensated sulphur vacancies.     

EHD-photoluminescence kinetics in doped Ge

Measurements of the LA-phonon assisted line kinetics of the EHD photoluminescence in As- and Sb-doped germanium with impurity concentrations n_D = 10¹⁵ ? 10¹⁷ cm^?3 are presented. These kinetics are found to be strongly dependent on the excitation level at 4.2 K. From the experimental results and a simplified kinetic equation the EHD “diffusion length” in Ge:As sample with n_D = 2 × 10¹⁶ cm^?3 is estimated to be L_D ? 0.34 mm which is consistent with previous results.     

Electrical properties of n-type vapor-grown gallium nitride

Hall measurements are reported for undoped and Zn-doped vapor-grown single crystal GaN on (0001) Al₂O₃ layers with 298 K carrier concentrations (n-type) between 1·4×10¹⁷cm^?3 and 9×10¹⁹ cm^?3. Then n～10¹⁷ cm^?3 crystals (undoped) have mobilities up to μ～440 cm²/V sec at 298 K. Their conduction behavior can be described by a two-donor model between 150 and 1225 K and by impurity band transport below 150 K. Crystals with n≥8×10¹⁸ cm^?3 show metallic conduction with no appreciable variation in n or μ between 10 and 298 K.Results of mass spectrographic analyses indicate that the total level of impurities detected is too low to account for the observed electron concentration at the n～10¹⁹ cm^?3 level, and suggest the presence of a high concentration of native donors in these crystals. No significant reduction in carrier concentration was achieved with Zn doping up to concentrations of ～10²⁰ cm^?3 under the growth conditions of the present work, and no evidence was found to indicate that high conductivity p-type behavior may be achieved in GaN. The influence of factors such as growth rate, crystalline perfection and vapor phase composition during growth on the properties of the layers is described.     

Effect of uniaxial stress on InSbtunnel junctions

The deformation potential constants of heavily doped InSb have been determined by applying uniaxial stress to p-n tunnel junctions formed below the surface of rectangular InSb bars. Stress applied parallel to the major axes of these bars and parallel to the junction plane resulted in a linear decrease in tunnel current as a function of stress. The percentage decreases were 24·5 × 10^?3, 9·5 × 10^?3 and 4·5 × 10^?3cm²kg^?1 for stress applied parallel to [010], [11¯0], and [111] directions respectively. The observed changes in tunnel currents are attributed to a decrease in the tunnel probability produced by a shift in the conduction band edge relative to that of the valence band. Our data is consistent with a hydrostatic deformation potential of — 10 eV, and valence band deformation potentials b = ? 1·3 eV and d = ?7·4 eV.     

Exciton spectra and electrical conductivity of epitaxial silicon-doped GaN layers

Optical spectra and electrical conductivity of silicon-doped epitaxial gallium nitride layers with uncompensated donor concentrations N _D — N _A up to 4.8 × 10¹⁹ cm^?3 at T ≈ 5 K have been studied. As follows from the current-voltage characteristics, at a doping level of ～3 × 10¹⁸ cm^?3 an impurity band is formed and an increase of donor concentration by one more order of magnitude leads to the merging of the impurity band with the conduction band. The transformation of exciton reflection spectra suggests that the formation of the impurity band triggers effective exciton screening at low temperatures. In a sample with N _D — N _A = 3.4 × 10¹⁸ cm^?3, luminescence spectra are still produced by radiation of free and bound excitons. In a sample with N _D — N _A = 4.8 × 10¹⁹ cm^?3, Coulomb interaction is already completely suppressed, with the luminescence spectrum consisting of bands deriving from impurity-band-valence band and conduction-band-valence band radiative transitions.     

Photoelectronic investigations of semi-insulating p-type GaAs:Cr containing neutral chromium acceptors

A photoelectronic analysis of p-type GaAs:Cr, i.e. measurements of thermally stimulated currents and the dependences of photoconductivity and photo-Hall effect on photon energy, temperature and light intensity, have enabled trap locations and densities as well as properties of neutral chromium acceptors to be determined. Hole traps proved to be located at 0.15 and 0.23 eV above the valence band, and their densities have been estimated to be 10¹⁵ cm^?3 and 5 × 10¹⁶ cm^?3 respectively. Their occurrence is related to the presence of copper in the samples investigated. Neutral chromium acceptors are located at 0.77 eV above the valence band and are at a constant distance from the conduction band. Their photoionization cross-section is 3 × 10^?17 cm² while the photoexcited electron escape cross-section is about 10^?20 cm². The potential of a neutral Cr acceptor is of the delta function type with weak coulombic tails. The maximum radius of the Bohr orbit of an electron in the ground state is 4 atomic units.     

Analysis of the 2ν3 band of 12CD3F at 5.06 μm recorded under high resolution

The 2ν₃(A₁) band of ¹²CD₃F near 5.06 μm has been recorded with a resolution of 20–24 × 10^?3 cm^?1. The value of the parameter (α^B ? α^A) for this band was found to be very small and, therefore, the K structure of the R(J) and P(J) manifolds was unresolved for J < 15 and only partially resolved for larger J values. The band was analyzed using standard techniques and values for the following constants determined: ν₀ = 1977.178(3) cm^?1, B″ = 0.68216(9) cm^?1, D″_J = 1.10(30) × 10^?6 cm^?1, α^B = (B″ ? B′) = 3.086(7) × 10^?3 cm^?1, and β^J = (D″_J ? D′_J) = ?3.24(11) × 10^?7 cm^?1. A value of α^A = (A″ ? A′) = 2.90(5) × 10^?3 cm^?1 has been obtained through band contour simulations of the R(J) and P(J) multiplets.     

Magnetization,AC susceptibility and microwave absorption in Tb and Gd70Tb30

Measurements of magnetization, ac susceptibility and microwave absorption at 9.5 and 35 GHz, have been carried out with single crystals of Tb and Gd₇₀Tb₃₀. The critical temperatures for Tb were in agreement with previous published results. The Curie Temperature for Gd₇₀Tb₃₀ was measured to be (275 ± 2) K. Anisotropy constants have been derived from the microwave measurements. The value of K₂ at 0 K is estimated to 8.8 × 10⁸ erg cm^-3 (Tb) and 1.3 × 10⁸ erg cm^-3 (Gd₇₀Tb₃₀). At 77 K, the value of K₆ is estimated to be 6 × 10⁴ erg cm^-3 (Gd₇₀Tb₃₀.     

Study of hydrogen chemisorbed on Raney-nickel by neutron inelastic spectroscopy

The motion of chemisorbed hydrogen on the Raney-nickel surface was studied by neutron inelastic spectroscopy. The peaks found at low energy transfers (below 320 cm^?1) are nearly identical to the spectrum of lattice frequencies of pure nickel. This means that each hydrogen atom is bound to only one nickel atom. The mean square amplitude of the bound proton was found to exceed that of nickel by 0.04 ± 0.02 Ų. A broad band found at 1120 cm^?1 (800 cm^?1 in the case of deuterium) is attributed to motions of hydrogen atoms relative to the nickel surface. An interpretation of this band is given in terms of harmonic approximation. An analysis of the shape of the elastic line has shown that no broadening could be detected with our instrument. This leads to an upper limit for the diffusion constant of the protons, D<5×10^?7 cm²/s, at room temperature.     

Absorption spectra of C6H6 and C6D6 near 340 nm

The absorption spectra of C₆H₆ and C₆D₆ in the liquid phase have been studied near 340 nm. The absorption spectrophotometric mounting was a sequential double-beam attachment with linear response to energy on scanning of the spectrum before the exit slit and an electronic device which gives directly either the absorbance or the integrated absorbance of a transition and, consequently, its oscillator strength.The oscillator strength measured for the band of C₆H₆ is 8×10^?8, which corresponds to a dipole moment of 2.4×10^?3 Debye; this value is of the same order as a theoretical value calculated by Tsubomura and Mulliken (3.8×10^?3 Debye) for a transition between states ³F and ³A of an oxygen-benzene pair. This agreement corroborates the hypothetical existence of such a transition.The first vibrational band is at 28553 cm^?1 for C₆H₆; this band is not observed in the vapor or solid phase. It corresponds probably to the transition 0-0, which is considered in the literature to be near 29500 cm^?1. The isotopic shift measured for this first band is 164 cm^?1. The vibrational frequencies are, respectively, 910 cm^?1 for C₆H₆ and 889 cm^?1 for C₆D₆.     

Fundamental reflection spectrum and the electronic band structure of chlorine-doped CdTe

The effect of chlorine impurity on the fundamental reflection spectrum and the electronic band structure of cadmium telluride crystals has been studied. At the impurity concentration N _Cl>5.0×10¹⁹ cm^?3, a peak appears in the reflectance spectra. This peak is due to electron transitions at the X point of the Brillouin zone from the upper split valence band to Cl levels lying 0.05 eV above the Γ minimum of the conduction band. The other features in the reflectance spectra and band structure are explained as being due to the effect of spin-orbit splitting at the X point and to indirect electronic transitions from the Cl levels to the Γ minimum.     

Elastic scattering of the conduction electrons by adsorbed hydrogen

The cross section of adsorbed hydrogen for the conduction electrons is evaluated according to the Boltzmann-Fuchs equation, the Greene and Soffer theories for surface scattering of the conduction electrons and to the change of the electrical resistivity due to hydrogen adsorbed on evaporated nickel films obtained experimentally by Suhrmann et al. The calculated cross sections are 2.0 × 10^?15 cm² and 1.8 × 10^?15 cm² at 273°K and 90°K respectively at a low coverage of hydrogen, which are consistent with the theoretical value 3.0 × 10^?15 cm² by Toya and reasonable compared with 0.9 × 10^?15 cm², the cross section of a gaseous hydrogen atom. The cross section decreases with increase of the coverage. This change is considered to be closely related to that of the heat of adsorption.     

Wechsellichtmethode zur Messung von Haftstellen-Einfangquerschnitten beim Cadmiumsulfid

An extension of a method previously described is used to determine capture crosssections for shallow traps of CdS-crystals in the vicinity of the conduction band. The data are obtained by measurement of photoconductivity induced by sinusoidally modulated light ofλ ≈ 600 mμ. The results are as follows:σ ≈ 1,5 to 6 · 10^?15 cm² for traps at 0.12 ev (below the conduction band);σ ≈ 5 to 9 · 10^?15 cm² for traps between 0.2 and 0.6 ev;σ ≈ 10^?14 to 10^?13 cm² for traps at 0.7 to 0.8 ev.     

High resolution measurements of the line positions and strengths of the 2ν2 band of H2CO

Spectra of the 2ν₂ band of formaldehyde have been obtained with high resolution (0.035 cm^?1). Measurements were made with path lengths of 8, 16, and 24 m and at sample pressures from 0.1 to 0.3 mm Hg at room temperature (～296°K). From these data, the following constants were determined for the 2ν₂ band in wavenumber units: v₀=3471.718±0.004,A=9.3958±030013,B=1.28100±0.00024,C=1.11662±0.00024, Tbbb=-12.8±0.5×10^-6,Taabb=60±5×10^-6. The line strengths were also obtained from the data. The strengths were analyzed to determine the band strength and the rotational factors. At 296°K, the strength of the 2ν₂ band was found to be 15.5 ± 0.9 cm^?1/(cm·atm).     

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C-V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10^?3 eV wide with acceptor type states 0.18 eV below the intrinsic level E_i and a density of 10¹⁷ states/cm² eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E_i, and a density of 4 × 10¹⁴ states/cm²eV.     

Auger recombination of electron-hole drops

Phonon-assisted Auger recombination is calculated for indirect band gap semiconductors in the strongly degenerate case. It follows a reciprocal lifetime τ^?1=Cn² with C=7.19×10³¹ cm⁶ sec^?1 for Si and C=2.94× 10^?31 cm⁶ sec^?1 for Ge. These results are in good agreement with experimental values of the decay of electron-hole drops. Therefore one can conclude that phonon-assisted Auger recombination is the essential nonradiative recombination process in this case.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

The 3D electronic band calculation of nickel by the extended Hu¨ckel method

An expression for the calculation of 3d electronic band structures of metallic nickel is given in the tight-binding scheme using the extended Hu¨ckel method. An explicit formulation in the nearest neighbor approximation is given and the accuracy of the nearest neighbor approximation is discussed. It is found that when a combination of two Slater wave functions is used for each 3d orbital, and when overlap matrix elements between neighbors are included until convergence is achieved, the resulting 3d band structures and band width agree well with those obtained from ab initio tight-binding calculations. For nickel, this calculation gives a 3d band width of 2.48 eV, a degeneracy temperature of 2.8 × 10³ K and a linear, low temperature electronic heat coefficient of 1.9 × 10^?3 cal mol^?1 deg^?2.