Space-charge-limited-trap-limited-currents in anthracene crystals

Space-charge-limited-trap-limited hole currents in anthracene crystals are interpreted in terms of a trap distribution which starts at a discrete trap level at 0·6–0·8 eV and in which the trap density falls off exponentially with energy relative to this level. It is suggested that exponential hole trap distributions are produced by physical perturbations of the crystal lattice introduced by the same impurities which give rise to the discrete hole trap level at 0·6–0·8 eV (revealed by thermally stimulated current studies). The results indicate that a perturbed molecule is introduced for every 10,000 impurity molecules and the total number of perturbed molecules is constant at 10¹⁷ m^?3 for melt-grown and vapor-grown crystals. The parameter kT_c varies from crystal to crystal, indicating that the relative contributions of highly perturbed and slightly perturbed molecules is influenced by crystal growth conditions.     

Optical absorption edge in SnS

Optical absorption in single crystals of tin sulfide has been studied at many temperatures between 100 and 300 °K, in the wavelength range 2·2–0·8 μ. From the interference fringe patterns the absorption coefficient, reflection coefficient and index of refraction as a function of wavelength were determined for two light polarizations (ε∥a and ε∥b). From an analysis of the data, indirect band gaps of 1·142 and 1·095 eV were found for the two directions of polarization. Also it was found that the phonon assisted transitions required the participation of two phonons at different energy thresholds with energies 0·033 or 0·038 eV and 0·082 or 0·113 eV, with reference to the two axis. The temperature dependence of the indirect band gap for each direction of light polarization is linear with a slope ?4·05 × 10^?3eV and ?4·37 × 10^?3 eV respectively.     

Relation entre proprietes electriques et defauts de reseau dans ZnSiAs2 Partie I—analyse de l'energie d'ionisation

The electrical properties of crystals of the ternary semiconducting compound ZnSiAs₂ grown either by horizontal vapor phase transport or by the Bridgman method are reported.The resistivity and the Hall coefficient in the temperature range 77–300 K or 4.2–300 K have been measured and explained taking into account the asymmetry of the effective mass. All crystals arep-type with only one acceptor energy level between 7 × 10⁻³ eV and 0.32 eV, and are generally highly compensated. Annealing in zinc or arsenic vapor at 873 K induces a shift of the energy level vs annealing time.The position of the energy level vs annealing time has been explained in terms of intrinsic defects such as Schottky defects (As or Zn vacancies). The latter are accompanied by anti-structural disorder on the zinc sites. The different values of the energy levels created by the Schottky defects are attributed to a concentration effect; this may lead to the formation of an impurity band in some samples.     

Indirect electronic transitions in alkali halate crystals-I (sodium and potassium chlorate crystals)

The long wavelength tail of the fundamental absorption in NaClO₃ and KClO₃ crystals has been analysed based on the theory of band to band transitions of Bardeen et al.[8] developed in the case of semi-conducting crystals. Evidence of phonon involvement in the transitions giving an indirect band gap is observed. The energies of the phonons involved in the process are the same for both the crystals, and agree well with combinations of prinicple frequencies of ClO₃^? ion, their overtones and also lattice phonons. The indirect band gap in these crystals varies with temperature more or less linearly and the rate of variation is ?3·8 × 10^?4 eV/K and ?5·0 × 10^?4 eV/K for sodium chlorate and potassium chlorate respectively.     

Electrical conductivity,absorption and luminescence measurements in cubic boron nitride

Electrical and optical measurements were carried out on tiny crystals of cubic boron nitride. The dark current i_D was found to change exponentially with T, with activation energies in the range 0·2–0·4 eV. A red electroluminescence, of intensity i_EL. proportional to i_D was observed. Upon illumination at low temperatures a photocurrent i_p proportional to the square root of the excitation intensity appeared. It varied exponentially with T, with an activation energy of 0.05 ± 0.01 eV. The crystals exhibited a red thermoluminescence with several unresolved peaks covering the temperature range 100–400K, and having activation energies in the range 0·15–0·40 eV.     

Diffusionskonstante und charakteristische Absorption vor der Bandkante von Mn in ZnO-Kristallen

ZnO single crystals were doped with Mn and Co by diffusion. In the temperature range from 1400–1600 K the Mn and Co-diffusion-constants were determined:D _Mn=3.2 · 10^?3 exp (?2.87 eV/kT) cm² sec^?1 andD _Co=1·10exp(?3.98 eV/kT) cm² sec^?1. The Mn doped ZnO crystals show a characteristic colour due to an absorption near the intrinsic absorption edge. The corresponding absorption spectra were measured forE⊥c andE∥c. A discussion of different absorption mechanism shows that a charge-transfer transition is responsible for this absorption.     

Ionic conductivity of pure and doped Li3N

The ionic conductivity of Li₃N crystals doped with various metal ions (magnesium, copper and aluminum) or hydrogen has been investigated. The metal ions have a negative effect on the conductivity whereas hydrogen increases it. The intrinsic Li⁺ ionic conductivity of pure Li₃N is (2·-4)×10^-4Ω^-1cm^-1 at room temperature with an activation energy of 0.26?0.27 eV. Doping with hydrogen to a maximum level of 0.5?1.0 atom% results in a conductivity of 6×10^-3Ω^-1cm^-1 and an activation energy which has been lowered to 0.20 eV. A model is proposed for the action of hydrogen whereby the Li-N bonds next to an NH^2- group are weakened thereby facilatating the creation of Li⁺ Frenkel defects and the vacancy migration. Hydrogen-doped Li₃N is termed an enhanced intrinsic conductor.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

Deep level structure and compensation mechanism in In-doped CdTe crystals

Deep Level Transient Spectroscopy (DLTS) and Optical Deep Level Transient Spectroscopy (ODLTS) experiments have been conducted on a series of In-doped CdTe crystals grown by the Bridgman or the travelling heater (THM) methods using Te as the solvent. The THM samples are n-type but strongly compensated. Annealing at 700°C under high Cd vapour pressure leads to a decompensation of the crystals. The electron concentration is then a measure of the donor (In) concentration, which was in the range 3 × 10¹⁶−1.5 × 10¹⁸cm⁻³. Six and eight electron traps are, respectively detected in the non-annealed and annealed samples at a concentration level 10²–10³ times below the net electron concentration. They cover the energy range 0.2–0.8 eV. Similar traps are found in both types of crystals, the concentration of which increases with In content and after annealing. The presence of In interstitial-type defects is suggested. A main hole trap at 0.12eV is detected by ODLTS in compensated samples with a concentration close to the donor concentration. Low temperature electrical measurements show that this trap is ionized under equilibrium conditions. It appears to be the main compensating acceptor centre. A plausible microscopic structure is In_cdV_cd. This study shows that In-doped CdTe grown by THM is electrically compensated and that In-containing neutral associates or precipitates play a minor role in the compensation mechanism.     

Amorphous germanium III. Optical properties

The optical properties of thick sputtered films (～30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T _sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T _s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (10²?α?10⁴ cm^?1) but shifts monotonically to higher energy and increases in slope with increasing T _s. Similarly, the peak in ε₂ grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω²ε₂ occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T _s. The sum rules for n _eff(ω) and ε_0,eff(ω) are evaluated for ▄ω?5 eV and vary monotonically with T _s. These trends are neither compatible with Galeener's void resonance theory nor with changes in the oxygen content of the films, determined by the examination of absorption peaks at 0·053 eV and 0·09 eV. An explanation, suggested here and expanded in I, is based on the observed changes in the structure of the network and voids.     

Impurity photoconductivity in epitaxial layers of gallium arsenide

Impurity photoconductivity spectra in the range 0.5–1.5 eV are studied in epitaxial layers of n-GaAs grown on substrates of semiconductive GaAs withρ > 10⁶ Ω·cm in the system Ga-AsCl₃ -H₂. The effect of uncontrolled acceptor impurities on the impurity photoconductivity spectrum is evaluated.     

Iodine as a donor in CdS

Iodine doped single crystals of CdS were grown from the vapor phase. High temperature Hall effect measurements for the crystals equilibrated with Cd and S₂ vapors at temperatures between 700 and 1000°C gave the free electron concentration as a function of p_Cd or p_S2 and temperature. The results can be explained on the basis of a model in which the CdS is saturated with iodine at low p_Cd (=high p_S2) but unsaturated at high p_Cd.The solubility of iodine in CdS is given by c_t=1·73×10²²p_S2^?1/8 exp (?1·045 eV/kT) cm^?3 atm^?1/8=4·62×10¹⁹p_Cd^1/4 exp (?0·195 eV/kT) cm^?3 atm^1/4The formation of pairs (I_SV_Cd)′ from I_S^· and V_Cd″ is governed by the equilibrium constant K_{P(I, V)}=4 exp (≤1·1 eV/kT)If Cd diffusion occurs primarily by free vacancies, the Cd^* tracer self diffusion leads to a vacancy mobility of (1·2±0·5)×10^?5 cm² sec^?1 at 900°C, in agreement with results reported by Woodbury [12], but (7±3) times larger than reported by Kumar and Kroger [10].     

Self-diffusion of potassium in potassium azide

The tracer sectioning technique was used to study the diffusion of potassium ions in melt grown single crystals of potassium azide. The temperature dependence of the diffusion coefficients in the range 85–254°C can be represented by D=(0·19±0·03) exp [(?0·80±0·06) eV/kT] cm²/sec. It appears that the cations are the predominant mobile species and that they diffuse by a vacancy mechanism with an enthalpy of migration of 0·80±0·06 eV.     

The behavior of copper impurity in CdSnAs2

Single crystals of n-type CdSnAs₂ with a carrier concentration of 2 ·10¹⁷–4 ·10¹⁸ cm^–3 and mobility (3 to 6) · 10³ cm²/V ·sec were copper doped by diffusion saturation at temperatures from 400 to 570C. As a result of the study of the electrical properties of the doped crystals it was established that the copper in CdSnAs₂ is a fast-diffusing acceptor impurity. The solubility of Cu depends primarily on the donor-center concentration and has clearly a retrograde character. Low-temperature heat treatment (over the 200–400C range) of the Cu-doped specimens results in an increase in the acceptor concentration. The form of the log R(10³/T) curve indicates the existence of acceptor centers with an ionization energy of 0.05 eV in the Cu-doped CdSnAs₂ specimens.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 7, pp. 39–44, July, 1973.     

Microwave electrophysical properties of superionic crystals Ag2HgI4

The paper concerns itself with the investigation results of temperature dependencies of electric conduction and dielectric properties of Ag₂Hgl₄ crystals in the frequency range of 10⁷–7,8·10¹⁰ Hz. The obtained data have shown that in α-phase at T=326 K, the electric conductivity σ is proportional v^0,28 in the frequency range of 10⁷–10⁹ Hz and σ is proporti onal v^0,5 in the range of (1,1–78)·10⁹ Hz. The dependence σ(v) in the range of (1,1–78)·10⁹ Hz may be conditioned by the jumping mechanism of the conductivity and low frequency oscillations of the crystal lattice. It is believed, tha in the σ-phase of Ag₂Hgl₄ a condition of the existence of the ionic polaron is satisfied. The activation energy of the polaron is ΔE_p=0,09 eV.     

Letters to the editor

Abstract

Optical absorption spectra of TeO₂ crystals, irradiated at room temperature by up to 2·10¹⁸ cm^?2 10-MeV electrons and subsequently annealed to 575K, are studied. The dependence of Urbach absorption edge parameters in TeO₂ on the electron beam fluence is discussed. The irradiation-induced near-edge broad (2.5–3.5eV) absorption band is shown to be related to oxygen vacancies, annealing at 475–525K.     

Zeeman spectroscopy of the 4 Eu → 2 B 1g phosphorescence of copper porphin in an n-alkane single crystal

The phosphorescence spectrum of the metastable ⁴ E_u state of copper porphin in single crystals of n-octane (C₈) and n-decane (C₁₀) has been studied between 2·3 and 35 K, with and without a magnetic field B. The crystal field splitting between the orbital components observed at 35 K is δ = 30·3 ± 0·3 (C₈), 13·8 ± 0·2 cm^-1 (C₁₀). From the Zeeman shifts we derive the effective orbital angular momentum Λ′ = 0·8 ± 0·2 (C₁₀), the spin-orbit coupling parameter |Z′| = 1·5 ± 1·0 cm^-1 (C₁₀), the spin-spin dipolar interaction parameters D = -0·1 ± 0·2 cm^-1 (C₈, C₁₀) and |E| = 0·31 ± 0·03 cm^-1 (C₈, C₁₀), and the g-factors g _∥ = 2·14 ± 0·04 (C₈, C₁₀) and g _⊥ = 2·00 ± 0·03 (C₈, C₁₀).     

Radiation defects in Si of high purity

Abstract

Radiation defects created by γ-irradiation of Co⁶⁰ and fast neutrons in high purity p-Si (p = 5×10³ to 4 × 10⁴ Ω.cm) and n-Si (p = 4 × 10² to 5 × 10³ Ω.cm) are investigated by measurements of Hall effect, resistivity and minority carrier lifetime. The oxygen concentration in the crystals is in the range of 5 × 10¹⁴ to 5 × 10¹⁵ cm^?3.

It is shown that stable γ-defects at 300 °K are divacancies and complexes of vacancies with donor or acceptor impurities. Divacancies introduced by γ-irradiation are the secondary defects. They become predominant after ‘exhaustion’ of the dopant. When divacancies become the predominant defects the Fermi level occupies its boundary position E_v +0.39 eV in the gap. At low doses (Φ<10¹⁶ photons/cm²) vacancy-impurity complexes and at heavy doses (Φ>10¹⁷ photons/cm²) divacancies play the main role in the recombination process.

In neutron irradiation disordered regions are introduced and the level at E_v +0.35 eV is observed. The Fermi level in both n- and p-Si shifts to the middle of the gap. At the annealing of disordered regions in the interval 200 to 250 °C the level at E_v +0.27 eV appears and Fermi level occupies its boundary position at E_v +0.39 eV. This indicates that divacancies become the predominant defects which can be formed as secondary defects at the destruction of the disordered regions.     

Association reactions between Ni2+ ions and vacancies in NaCl crystals

The association of Ni²⁺ cations with cation vacancies was followed by measuring the electric conductivity of NaCl: NiCl₂ crystals. It was found that this association reaction may be described over the whole temperature range of 20 to 620 ?C by a simple Stasiw-Teltow model with an association enthalpy of 0·62 eV and association entropy of 6·3 k. The precipitation effects in crystals were negligible above the room temperature up to the Ni²⁺ concentration of 100 ppm.