The temperature dependence f photoconductivity in a-Si

The photoconductivity and its dependence on light intensity have been investigated in a-Si as a function of temperature between 100 and 500 K. In most experiments a photon energy of 2 eV was used. Specimens were deposited on to a substrate held at a temperature between 300 and 600 K by the r.f. decomposition of silane. Graphs of the photocurrent versus 1/T show a photoconductive maximum and the general features of the curves are similar to those found for the chalcogenide glasses. The main emphasis of the paper lies in the interpretation of the results in the light of the information on transport properties and the density of state distribution obtained from drift mobility and field effect experiments. It is shown that recombination takes place predominantly between two groups of localized states, which have been identified in the previous work. The initial state at ?_A is situated about 0.18 eV below ge_C in the electron tail states, the final state lies in a density of state maximum, 0.4 eV above ?_V. Above about 250 K, the photocurrent is carried by electrons in extended states, but below this temperature transport is by phonon assisted hopping through states near ?_A. A recombination process involving two states of a structural defect centre is discussed on the basis of the results and appears to be a feasible interpretation.     

A measurement of B(E3;0+ → 31−) and some E2 transition probabilities in 132, 134, 136, 138Ba using Coulomb excitation

Reduced transition probabilities, B(E2) and B(E3), have been measured for low-lying 2⁺ and 3^? states in ^{132, 134, 136, 138}Ba using Coulomb excitation by 40 MeV ¹²C ions. The B(E2) values are in general consistent with previous measurements and the B(E3;0⁺ → 3₁^?) values are 0.176 ± 0.022, 0.148 ± 0.018, 0.155 ± 0.018 and 0.133 ± 0.013 e² · b³ for ^{132, 134, 136, 138}Ba respectively. These B(E3) values correspond to about 24 to 17 W.u. and such enhancements suggest that these 3^? states have an essentially collective character which may be attributed to octupole vibrations.     

The quadrupole moment of 26Mg as a test of shell-model interactions

The static quadrupole moment Q_21⁺, and the B(E2; 0₁⁺ → 2⁺₁) value of the first excited state of²⁶Mg have been measured using the reorientation effect in Coulomb excitation of ²⁶Mg projectiles. It is found that Q_21⁺ = ?13.6±3.0 (?9.5±3.0) e ·fm² and B(E2; 0⁺₁ → 2⁺₁) = 322±16 (328±16) e² ·fm⁴ for constructive (destructive) interference from higher states. The result for Q₂ clearly differentiates among several alternative effective interactions which have been used in shell-model calculations.     

Electric moments of the first excited state of 18O

The static quadrupole moment Q_2⁺ and the B(E2; 0⁺ → 2⁺) value of the first excited state of ¹⁸O at e_x = 1.982 MeV have been determined using the reorientation effect in Coulomb excitation. Surface-barrier detectors at laboratory angles of 90° and 174° were used to detect ¹⁸O ions elastically and inelastically scattered from ²⁰⁸Pb. At both angles, we determined experimentally the maximum bombarding energies at which nuclear interference effects were negligible. It is found that Q_⁺ = ?0.023 ± 0.021 e · b (?0.052 ± 0.021 e · b) for destructive (constructive) interference from higher states. This result is in good agreement with theoretical expectation. For the transition moment we find B(E2; 0⁺ → 2⁺) = 0.00390 ± 0.00018 e² · b² (0.00371 ± 0.00018 e² · b²) for destructive (constructive) interference.     

A cusp in the 54Cr(p, γ)55Mn reaction

Absolute cross sections for the reactions ${}^{54}\text{Cr}\text{(}\text{p}\text{, γ)}{}^{55}\text{Mn and}{}^{54}\text{Cr}\text{(}\text{p, n}\text{)}{}^{54}\text{Mn}$ are presented for effective bombarding energies E_p from 0.830 to 3.606 MeV. A substantial cusp is observed in the ${}^{54}\text{Cr}\text{(}\text{p}\text{, γ)}{}^{55}\text{Mn}$ excitation function. The data are compared with the predictions of global Hauser-Feshbach models in order to evaluate their applicability to nucleosynthesis calculations.     

Hole transport in solid xenon

The hole mobility μ_η in solid Xe has been investigated by drift mobility techniques in the temperature range from 110K to 160K. μ_η = 1.8 × 10⁻²cm²Sec⁻¹V⁻¹ at 160K and has a temperature dependence of the form μ_η ∞ T^−1.6. It is concluded that a non-adiabatic hopping transport with a small polaron binding energy and small intermolecular overlap is the most likely transport mechanism.     

Doped amorphous semiconductors

The article reviews the advances that have been made during the past two years in the new field of substitutionally doped a-semiconductors, particularly a-Si. After introducing some of the basic concepts used in the treatment of a-materials, the preparation and doping of a-semiconductors from the gas phase and the role of hydrogen are discussed in some detail. Recent results on doping by ion implantation have also been included. The following sections are concerned with the effect of n and p-type impurities on the electronic transport properties of a-Si and a-Ge. The discussion is based on results from conductivity, drift mobility, Hall effect and thermoelectric power measurements and leads to the main conclusion that the donors introduce a new hopping path through the system which begins to dominate over tail-state hopping when their density exceeds about 10¹⁸ cm^-3. Recent photo-conductivity and luminescence results on doped a-Si are then discussed; they give information on recombination and also demonstrate the pronounced effect of doping on these properties. In the final section amorphous p-n junctions are considered as well as their possible applications in photovoltaic solar energy conversion.     

Application of amorphous silicon field effect transistors in integrated circuits

The work described in this paper is concerned with the possible applications in integrated circuits of thin-film field effect transistors (FETs) made from glow discharge amorphous (a-) silicon and silicon nitride. The construction and performance of inverter circuits, employing integrated a-Si load resistors, are described in some detail. The extension of this basic circuit to NAND and NOR gates, to a bistable multivibrator and to a shift register is reported. Based on the excellent photoconductive properties of a-Si an integrated addressable photosensing element has been constructed, which could have potential applications in imaging arrays.     

Quadrupole moment of the first excited state of 32S

The static quadrupole moment and B(E2; 0⁺ → 2⁺) value for the first excited state of ³²S were measured using the reorientation effect in Coulomb excitation of ³²S projectiles. Silicon surface barrier detectors were used to detect ³²S ions scattered from ²⁰⁸Pb at mean laboratory angles of 90° and 174°, for bombarding energies in the range 122 to 160 MeV. The results for constructive (destructive) interference from the second 2⁺ state in ³²S are QP_2⁺ = ?16.0±2.2 (?13.3±2.2) e·fd² and B(E2; 0⁺ → 2⁺) = 292±17 (307±17) e²·fm⁴     

Transport properties of amorphous germanium prepared by the glow discharge technique

The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures T_d of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at T_d = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?_C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?_f lies near a density of state minimum. The density of states at ?_f is appreciably higher than that in similarly prepared Si specimens.