Development of high-responsivity Ge:Ga photoconductors

Czochralski-grown Ga-doped Ge (Ge:Ga) single-crystal samples, with a compensation of 10⁻⁴ have been modified by the indiffusion of Cu to produce photoconductors which provide NEPs comparable to current optimum Ge:Ga detectors, but exhibit responsivities a factor of 5–6 times higher when tested at a background photon flux of 10⁸ photon/s at λ = 93 μm. The introduction of Cu, a triple acceptor in Ge which acts as a neutral scattering center, reduces carrier mobility and extends the breakdown field significantly in this ultra-low compensation material.     

Stressed Ge:Ga photoconductor with a compact and stable stressing assembly

Sensitive far infrared stressed Ge:Ga photoconductors which have response up to 200μm wavelength have been fabricated and studied to apply them to astronomical and atmospherical observations using balloons and satellites. The stressed Ge:Ga photoconductors have shown very high detectivity, i.e. responsivity of 15 A/W and NEP = 2 × 10⁻¹⁸ W/√Hz when operated at 2 K and at photon fluxes 1.1 × 10⁶ photons/s, and the stable stress operation has been achieved by using the compact stressing mechanism with cone disk springs. The good performance has been confirmed by balloon experiments to detect astronomical CII line at 157.7 μm.     

Calculations of the energy spectra of Zn,Ga, and Ge isotopes by the shell model

The effective Hamiltonian which was determined empirically by Koops and Glaudemans is tested in shell model calculations for the^65–68Zn,^67–69Ga, and^68–70Ge nuclei in the full (1p _3/2,0f _5/2,1p _1/2) ⁿ space. The resulting energy spectra are compared with the experimental spectra and results of previous calculations. The overall agreement with experiment is as satisfactory for these nuclei as for the Ni and Cu isotopes, by which the Hamiltonian was determined. It is noticed that the spectra of⁶⁷Zn and^67,69Ga calculated in this work are similar to those provided by the Alaga model.     

Ion-implanted Ge:B far-infrared blocked-impurity-band detectors

Ge blocked-impurity-band (BIB) photoconductors have the potential to replace stressed Ge:Ga photoconductors for far-infrared astronomical observations. A novel planar BIB device has been fabricated in which ion-implanted boron is used to form the blocking contact and absorbing layers of necessary purity and compensation. The effect of doping in the infrared active layer on the far-infrared photoconductive response has been studied, and the optimum doping concentration is found to be ∼4 × 10¹⁶ cm⁻³. Devices doped near this concentration show good blocking characteristics with low dark currents. The spectral response extends to ∼45 cm⁻¹, clearly showing the formation of an impurity band. Under low background testing conditions these devices attain a responsivity of 0.12 A/W and NEP of 5.23 × 10⁻¹⁵ W/Hz^1/2.     

Impact of O2 exposure on surface crystallinity of clean and Ba terminated Ge(1 0 0) surfaces

In analogy with the case of Sr on Si [Y. Liang, S. Gan, M. Engelhard, Appl. Phys. Lett. 79 (2001) 3591], we studied surface crystallinity and oxidation behaviour of clean and Ba terminated Ge(1 0 0) surfaces as a function of oxygen pressure and temperature. The structural and chemical changes in the Ge surface layer were monitored by LEED, XPS and real-time RHEED. In contrast to the oxidation retarding effect, observed for 1/2 monolayer of Sr on Si, the presence of a Ba termination layer leads to a pronounced increase in Ge oxidation rate with respect to clean Ge. In fact, while the Ge(1 0 0) surface terminated with 1/2 ML Ba amorphizes for a pO₂ of 10⁻² Torr, LEED indicates that clean Ge forms a thin (4.5 Å), 1 × 1 ordered oxide upon aggressive O₂ exposure (150 Torr, 200 °C, 30 min). We briefly discuss the origins for the difference in behaviour between Ba on Ge and Sr on Si.     

PAL spectroscopy of rare-earth doped Ga–Ge–Te/Se glasses

Positron annihilation lifetime (PAL) spectroscopy was applied for the first time to study free-volume void evolution in chalcogenide glasses of Ga–Ge–Te/Se cut-section exemplified by glassy Ga₁₀Ge₁₅Te₇₅ and Ga₁₀Ge₁₅Te₇₂Se₃ doped with 500 ppm of Tb³⁺ or Pr³⁺. The collected PAL spectra reconstructed within two-state trapping model reveal decaying tendency in positron trapping efficiency in these glasses under rare-earth doping. This effect results in unchanged or slightly increased defect-related lifetimes τ₂ at the cost of more strong decrease in I₂ intensities, as well as reduced positron trapping rate in defects and fraction of trapped positrons. Observed changes are ascribed to rare-earth activated elimination of intrinsic free volumes associated mainly with negatively-charged states of chalcogen atoms especially those neighboring with Ga-based polyhedrons.     

Excited states of100Tc from100Mo(p,nγ)100Tc reaction and the parabolic rule

γ-spectra and excitation functions of the¹⁰⁰Mo(p, nγ)¹⁰⁰Tc reaction were measured in the 1.2–3.6 MeV proton energy range by using thick, enriched targets, Ge(Li) and low energy photon (hyperpure Ge) spectrometers. These detectors were used inγγ-coincidence experiments, too. Conversion electron spectrum measurements were performed by means of a superconducting magnet transporter Si(Li) spectrometer (SMS) atE _p =4 MeV and multipolarities of some transitions have been determined. Based on the experimental results a level scheme of¹⁰⁰Tc has been constructed. Level energies of¹⁰⁰Tc were calculated on the basis of the parabolic rule, derived from the cluster-vibration model.     

Electron density of states and superconducting Tc in A15-compounds

We compare the superconductivity parameters of V₃Ga and V₃Ge by the method of McMillan with the help of new neutron diffraction data obtained by P. Schweiss. We consider the strongly varying density of states in V₃Ga at the Fermi energy, a complication suggested by Labbé, Barisic, and Friedel. We find that increased electronic density of states contributes about equally with lattice softening to the enhancement of T_c in V₃Ga as compared to V₃Ge, although the values of N(O)J² = Mλ〈ω²〉 are roughly consistent with those obtained for some other metals by McMillan. The reason N(O) is not more effective in raising T_c is the partial compensation obtained from averaging over the narrow density-of-states peak which gives N ? 0.6N(O).     

Structural investigation of Te-based chalcogenide glasses using Raman spectroscopy

Structural changes of metals (Zn, Sb, In, Ga) and metal halides (AgI, ZnI₂, CdI₂, PbI₂, BiI₃) modified GeTe₄ glasses were investigated with the aid of Raman spectroscopy. The Raman spectra of these glasses in the frequency region between 100 cm^?1 and 300 cm^?1 display four main bands at about 124, 140, 159 and 275 cm^?1 which are contributed by Ge–Te, Te–Te, Te–Te and Ge–Ge vibration modes. The intensity of 159 cm^?1 and 275 cm^?1 bands vary with the addition of different glass modifiers. While the relative intensity of the 124 cm^?1 and 140 cm^?1 bands are insensitive to composition changes. Glass modifiers like Zn, In and Sb act as glass network unstabilizer which will disorganize the glass network by opening up the chain structures of Ge–Te and Te–Te. In the case of Ga and metal halides, Ga can open up Ge–(Te–Te)_4/2 tetrahedra and form Ga–(Te–Te)_3/2 triangle. Iodine can form covalent bonds with tellurium and decrease the tendency of microcrystal formation. Thus both Ga and iodine ultimately act as glass network stabilizer.     

Analysis of temperature and concentration dependence of flow stress in KCl–KBr single crystals with special reference to solute distribution

Available data on the temperature and concentration dependence of critical resolved shear stress (CRSS) of KCl–KBr solid-solution crystals containing 9, 17, 27 and 45?mol% KBr in the temperature range 77–230?K have been analyzed within the framework of the kink-pair nucleation model of plastic flow in solid- solution crystals. It is found that CRSS τ decreases with increasing temperature T in accordance with the model relation lnτ?=?A???BT, where A and B are positive constants. The CRSS τ at a given temperature depends on solute concentration c as τ?∝?c^p , where exponent p has a value between 0.33 and 0.57 as temperature T rises from 0 to 230?K. The model parameter W _o, i.e. binding energy between the edge-dislocation segment L _o involved in the unit activation process and the solute atoms close to it (T?→?0?K), which is inversely proportional to B, increases with solute concentration c monotonically as W _o?∝?c ^0.33 up to a critical value c _m?=?35?mol% KBr, which is in reasonable agreement with the model prediction W _o?∝?c ^0.25. However, W _o decreases with an increase in c beyond c _m, which indicates somewhat ordered distribution of solute in the host lattice of concentrated KCl–KBr solid solutions with c?>?c _m.     

New type of photo-induced photoluminescence fatigue in glassy GeSe2

A new type of photo-induced photoluminescence (PL) fatigue in glassy semiconductors of the system GeSe in the temperature range 80–300 K has been observed. The duration of this effect t_f is less than 30 s and it varies with temperature and the energy of exciting light quanta E_ex. After fatigue the intensity of PL returns to its original value if the sample is kept in the dark. The time of complete restoration is τ_o ≥ 20 s and depends on temperature. The results are interpreted by comparing the probabilities of radiative and non-radiative transitions of quasi-excitons formed during irradiation.     

Superconducting fluctuations,weak anti-localization and interaction effects in Nb0.53Ti0.47-Ge multilayers

The effect of weak anti-localization, electron interactions and superconducting fluctuations on the transport properties of disorder Nb_0.53Ti_0.47-Ge multilayers were studied. The temperature dependence of the inelastic scattering time was found to be τ_in ∼ T^−1±0.25 and τ_in ∼ T^{−1.75±0.25} for temperatures lower and higher than ∼ 6K, repectively for a sample with thick Ge layers. The effect of the Ge thickness on the prefacter A in the expression R□ ∼ ln T may arise from an interlayer electron-phonon process.     

Investigation of22Na excited states

Fourteen²¹Ne(p, γ)²²Na resonances have been observed in the rangeE _p =300–1,300 keV. Theγ-decay of all these resonances has been investigated by means of a 38 cm³ Ge(Li) detector. Energies and branching ratios of several bound states have been determined. TheQ-value was determined as 6,738.5±1.7 keV. Lifetimes of seven states were determined with the Doppler-shift attenuation method. The observed upper limit (τ _m ≦4 fs) of the lifetime for the 4,071 keV state, regarded as the analogue of the third excited state in²²Ne, and the transition observed from this state to the 1,528 keV state do not support the proposed rotational band structure of the²²Na low-lying states.     

TSC study of electric dipole relaxations in chlorapatite

Electric dipole relaxations in chlorapatite, Ca₅(PO₄)₃Cl, have been studied with the fractional polarization mode of the thermally stimulated currents (TSC) method. Fifty-one of the fifty-seven sets of data obtained in the range 10–443°K fell naturally into four groups yielding compensation temperatures T_C of T_C1, = 202°C, T_C2: = 202°C, T_C3 = 420°C and T_C4= 644°C, with estimated error < 10°C, and characteristic relaxation times τ_C of τ_C1 = 1.3 × 10^?7s, τ_C2 = 3.2 × 10^?6s, τ_C3 = 8.8 × 10^?5s and τ_C4 = 2.3 × 10^?4s. Atomic-scale physical models involving Cl^? ion motion are offered for the 202°C compensation at the temperature of the reported monoclinic-to-hexagonal phase transition and for the 420°C compensation, at which temperature the Cl^? ions individually are thought to have enough thermal energy to maintain the hexagonal form dynamically.     

Invasion percolation between two sites in two, three, and four dimensions

The mass distribution of invaded clusters in non-trapping invasion percolation between an injection site and an extraction site has been studied, in two, three, and four dimensions. This study is an extension of the recent study focused on two dimensions by Araújo et al. [A.D. Araújo, T.F. Vasconcelos, A.A. Moreira, L.S. Lucena, J.S. Andrade Jr., Phys. Rev. E 72 (2005) 041404] with respect to higher dimensions. The mass distribution exhibits a power-law behavior, P(m)∝m^−α. It has been found that the index α for p_e<p_c, p_c being the percolation threshold of a regular percolation, appears to be independent of the value of p_e and is also independent of the lattice dimensionality. When p_e=p_c, α appears to depend marginally on the lattice dimensionality, and the relation α=τ−1, τ being the exponent associated with cluster size distribution of a regular percolation via n_s∝s^−τ, appears to be valid.     

The 16O(γ, po) reaction at intermediate photon energies

The ¹⁶O(γ, p_o) reaction has been investigated between photon energies E_γ = 40 and 105 MeV using a bremsstrahlung beam, full angular distributions being obtained at E_γ = 60, 80 and 100 MeV. While cross-section calculations which introduce a residual interaction of Yukawa form agree well with the data, the need for further work is indicated.     

High-velocity DSA measurements in the 1–20 ps lifetime region: Comparison between DSA and RD

Mean lives in the range 1–20 ps of low-lying states of ¹³C, ¹⁶N, ²⁰O and ³⁶Cl have been measured with the Doppler-shift attenuation method by heavy-ion bombardment of ²H and ³H targets. The recoils are slowed down in Mg, Al, Cu, Ag and Au. The γ-ray patterns are observed with a large Ge(Li) detector at 0° in coincidence with protons; for ¹³C the patterns are measured in singles with a Compton-suppression spectrometer. Analysis of the γ-ray patterns with ⁴He-scaled stopping power data of Northcliffe and Schilling yields the following results: ¹³C, τ_m(3.85 MeV) = 12.6 ± 0.3ps; ¹⁶N, τ_m(0.40 MeV) = 5.1 ± 0.3 ps; ²⁰O, τ_m(1.67 MeV) = 9.8 ± 0.7 ps; ³⁶Cl, τ_m(0.79 MeV) = 19.9 ± 1.7 ps, τ_m(1.16 MeV) = 9.2 ± 0.6 ps and τ_m(1.60 MeV) = 0.94 ± 0.06 ps. A comparison with results obtained with the recoil-distance method shows agreement to about 10 %, with a slight tendency to somewhat longer lifetimes for the recoil-distance technique. The above stopping power is also used to reanalyze our previously published measurements. The new mean lives differ less than 4.5 % from the previous results.     

Collisional-radiative model for non-Maxwellian inductively coupled argon plasmas using detailed fine-structure relativistic distorted-wave cross sections

Our recently developed collisional-radiative model which included fine-structure cross sections calculated with a fully relativistic distorted-wave method [R.K. Gangwar, L. Sharma, R. Srivastava, A.D. Stauffer, J. Appl. Phys. 111, 053307 (2012)] has been extended to study non-Maxwellian inductively coupled argon plasmas. We have added more processes to our earlier collisional-radiative model by further incorporating relativistic distorted-wave electron impact cross sections from the 3p ⁵4sJ = 0, 2 metastable states, (1s ₃, 1s ₅ in Paschen’s notation) to the 3p ⁵5p (3p _i ) excited states. The population of various excited levels at different pressures in the range of 1–25 mTorr for an inductively coupled argon plasma have been calculated and compared with the recent optical absorption spectroscopy measurements as well as emission model results of Boffard et al. [Plasma Sources Sci. Technol. 19, 065001 (2010)]. We have also calculated the intensities of two emission lines, 420.1 nm (3p ₉ → 1s ₅) and 419.8 nm (3p ₅ → 1s ₄) and compared with measured intensities reported by Boffard et al. [J. Phys. D 45, 045201 (2012)]. Our results are in good agreement with the measurements.