Effect of Fast Electron Irradiation and Annealing on the Luminescence in GaAs(Zn) crystals. Irradiation-induced 1.26 and 1.39 eV Emission Bands

The effect of 2.2 MeV electron irradiation and subsequent annealings on the photoluminescence in zinc-doped p-type GaAs crystals is studied and analyzed. Rather strong emission bands peaked at hv_m (77 K) near 1.26 eV (induced by electron irradiation) and 1.39 eV (induced by annealing of irradiated crystals) are observed. Evidence is presented that the 1.26 and 1.39 eV emission bands occur due to radiative electronic transitions in As_iZn_Ga and V_AsZn_Ga pairs induced by irradiation and annealing of irradiated crystals, accordingly. The observed variations in the intensities of the 1.26 and 1.39 eV emission bands upon irradiation and subsequent annealings of GaAs(Zn) crystals are explained in terms of irradiation and annealing-induced variations in the amount of 1.26 and 1.39 eV radiative centres resulting from: a) the effective interaction of mobile radiation-induced defects in the arsenic sublattice with zinc atoms leading to the formation of As_iZn_Ga and V_AsZn_Ga pairs; b) the thermal dissociation of As_iZn_Ga and V_AsZn_Ga pairs on individual components.     

Electrical Characterization of Irradiation plus Annealing – induced VAsZnGa Pairs in p-type GaAs (Zn) Crystals

Effect of fast electron irradiation (E =2.2 Mev, ϕ_c = 1 × 10¹⁶ el/cm²) and subsequent annealings (T = 150 to 350 °C, t = 10 to 600 min) of zinc-doped p-type GaAs crystals on the formation and dissociation of V_AsZn_Ga, pairs is studied. An analysis of the formation and dissociation kinetics of V_AsZn_Ga pairs permitted to find the diffusion coefficient of radiation-induced arsenic vacancies D(D = 1.5 × 10⁻¹⁸, 1 × 10⁻¹⁷ and 5 × 10⁻¹⁷ cm²/s at 150, 175 and 200 °C accordingly), their migration energy ϵ_m(ϵ_m = 1.1 eV), the binding energy of V_AsZn_Ga, pairs ϵ_b(ϵ_b = 0.5 eV), and also their dissociation energy ϵ_d(ϵ_d = 1.6 eV).     

Generation of VGaTeAsVAs Complexes in Tellurium-doped n-type GaAs by Low-temperature Annealing

It is shown that low-temperature annealing (T= 425 to 625 °C, t ⩾ 0.5 h) of tellurium-doped n-type GaAs crystals (n₀ = 2 × 10¹⁸ cm⁻³) leads to a generation of V_GaTe_AsV_As complexes as a result of a diffusion of arsenic vacancies to V_GaTe_As complexes or arsenic and gallium vacancies to isolated tellurium atoms. The observed regularities of generation of V_GaTe_AsV_As complexes as the annealing temperature and the annealing time are varied are well explained by the proposed model of diffusion-limited formation of V_GaTe_AsV_As complexes.     

Effect of Neutron Irradiation and Annealing on the Intensity of the Copper Related 1.01 eV Emission Band in n-type GaAs

Effect of neutron irradiation (E = 2 MeV, ϕ ≤ 10¹⁵ n/cm²) and subsequent annealing (T ≤ 700 °C, t = 30 min) on the intensity of the copper-related peaked at hvm =1.01 eV emission band in n-type GaAs (n₀ = 2 × 10¹⁸ cm⁻³) is studied. A strong irradiation-induced increase of the above emission intensity was observed testifying about the irradiation-stimulated growth in the concentration of copper-related 1.01 eV radiative centres (Cu_GaV_As pairs). A model is presented to explain this effect.     

Native Point Defects and Nonstoichiometry in GaAs (II) Mechanism of Formation and Degradation of Semiinsulating Properties of Undoped Gallium Arsenide Crystals

The nature of the main electron trap, EL2, in undoped semiinsulating GaAs crystals have been studied both qualitatively and quantitatively. It has been shown that point defects which, most probably, are responsible for the EL2 level are antisite As_Ga defects or (As_i V_Gs) complexes which are indistinguishable from the thermodynamic standpoint. The enthalpy and entropy of As_Ga formation according to the reaction As_As + V_Ga ⇄ As_Ga + V_As are equal to 0.5 eV and −7k, respectively.     

The Nature of As-related Defects in Semi-Insulating GaAs Bulk Crystals

The behaviour of As-related defects in SI (semi-insulating) GaAs are studied from the viewpoint of the origination of As-related precipitates, generation and evolution of EL2 during postgrowth and the possible defect interactions involved EL2 variation under As overpressure annealing. The precipitates were identified as elemental As and As-rich GaAs polycrystalline grains and their features depend on the growth process. A defect interaction model has been proposed on the generation/annihilation of EL2 assigned as As_Ga V_As V_Ga complex respect to non-stoichiometry such as As_i V_Ga and V_As.     

The study of the influence of deep level defects on GaAs epilayer electrophysical parameters with electron beam and photoelectron paramagnetic resonance methods

Deep level point defects in the GaAs p⁺-p^ˆ−n^ˆ structures, grown by liquid-phase epitaxy, were investigated with electron-beam methods (electron-beam induced current and modulation method) and photo-EPR. It was shown that during the growing process the donor As_Ga antisite defects with levels ev + 0.52 eV and ec — 0,75 eV and acceptor defect with the level ev + 0.44 eV have appeared. The interconnection between the thickness of p^ˆ-layer, the concentration of deep levels defects, diffusion length of the minority carriers in the p° and n°-layers was found.     

Quenching by Copper Atoms of the EL2-induced Luminescence in GaAs

It is shown that the introduction of copper atoms into GaAs crystals containing antistructure defects EL2 (isolated arsenic atoms on gallium sites As_Ga) leads to a practically complete disappearance of the EL2-induced luminescence bands peaked at 0.63 and 0.68eV. This effect is connected with the passivation of the EL2 defects (i.e. with the substantial decrease in their concentration) because of their interaction with copper atoms (they become bound by copper atoms) resulting in an appearance of electrically inactive AS_GaCU_Ga complexes.     

Network topological thresholds in gallium doped As–Te glasses – Electrical and thermal investigations

Electrical switching and differential scanning calorimetric studies are undertaken on bulk As₂₀Te_80−xGa_x glasses, to elucidate the network topological thresholds. It is found that these glasses exhibit a single glass transition (T_g) and two crystallization reactions (T_c1 & T_c2) upon heating. It is also found that there is only a marginal change in T_g with the addition of up to about 10% of Ga; around this composition an increase is seen in T_g which culminates in a local maximum around x = 15. The decrease exhibited in T_g beyond this composition, leads to a local minimum at x = 17.5. Further, the As₂₀Te_80−xGa_x glasses are found to exhibit memory type electrical switching. The switching voltages (V_T) increase with the increase in gallium content and a local maximum is seen in V_T around x = 15. V_T is found to decrease with x thereafter, exhibiting a local minimum around x = 17.5. The composition dependence of T_c1 is found to be very similar to that of V_T of As₂₀Te_80−xGa_x glasses. Based on the present results, it is proposed that the composition x = 15 and x = 17.5 correspond to the rigidity percolation and chemical thresholds, respectively, of As₂₀Te_80−xGa_x glasses.     

Structural and electrical properties of CuIn0.7Ga0.3Se2 epitaxial layers on GaAs substrates

Epitaxial layers of CuIn_0.7Ga_0.3Se₂ could be prepared by flash evaporation onto (111)A-oriented GaAs substrates in the substrate temperature range T_sub = 745 … 870 K. At T_sub = 745 … 820 K the films had the chalcopyrite structure, at T_sub = 820 … 845 K an additional pseudohexagonal phase was found. Indications to the presence of a sphalerite phase were found at high substrate temperatures. Films grown at T_sub ≦ 750 K were always n-type conducting and showed a largely pronounced impurity band conduction effect. At T_sub ≧ 860 K the films were always p-type conducting and two acceptor states with ionization energies of some 10⁻³ eV and of 125 … 140 meV were found.     

Growth and electrical properties of epitaxial CuInS2 thin films on GaAs substrates

CuInS₂ thin films with thicknesses in the range of 500 Å were deposited onto semi-insulating (111) A-oriented GaAs substrates by flash evaporation in the substrate temperature range T_sub = 570 … 870 K. Epitaxial growth begins at T_sub = 645 K. The films had always the chalcopyrite structure. Indications to a transition from the chalcopyrite phase to the sphalerite phase were observed at T_sub = 870 K. Films grown at T_sub ≦ 800 K showed n-type conductivity whereas at growth temperatures T_sub ≧ 850 K the films were always p-type conducting. A donor level and an acceptor level with ionization energies of 0.24 eV and 0.22 eV, respectively, were found from an analysis of the electrical measurements.     

The field effect in amorphous chalcogenides: An investigation of localized states and electronic transport

The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As₂Te₃, field effect screening by localized states at the Fermi level at low temperatures (～ 10¹⁹ cm^?3 eV^?1) and by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ～320 K; (2) in As₂SeTe₂, screening by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) with strongly type conductivity; (3) in As₂Se₂Te, screening by localized states at the Fermi level (～ 10¹⁹ cm^?3 eV^?1) with strongly p-type conductivity; and (4) in Sb₂Te₃, a very high density of localized states at the Fermi level (～ 2 × 10²⁰ cm^?3 eV^?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As₂Te₃ is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.     

Space-Charge-Limited Current Effects in p-Type CuIn0.8Ga0.2Se2/In Schottky Diodes

Schottky diodes, fabricated by local in diffusion of indium onto p-type single crystals of CuIn_0.8Ga_0.2Se₂ have been shown to exhibit space-charge-limited current effects under forward bias. Two regions are clearly observed in the current-voltage characteristics of the device; the first one associated with the saturation-velocity regime and the second region that corresponds to the ballistic regime with a V^3/2 dependence (Child-Langmuir law). The observed characteristic was used to calculate the semiconductor potential (E_i ≈︂ 0.15 eV). By means of photovoltaic effects the barrier height (φ_B ≈︂ 0.74 eV) and the ideality factor (n = 1.7) of the diode were also obtained.     

Concentration dependence of the electron diffusion length in p-type GaAs

The concentration dependence of the electron diffusion length in p-type GaAs is analyzed based on experimental values reported in the literature and recent studies of minority carriers mobility in GaAs. The dependence of L_n with doping impurity concentration is determined for τ_nr ≫ τ_r and good agreement is found with highest reported experimental values of L_n; considerations are made about τ_nr. The dependence of electron mobility, diffusion constant, and radiative recombination lifetime on concentration are also given.     

Epitaxial layers of CuInTe2 on GaAs

CuInTe₂ thin films were deposited onto semi-insulating (111)A-oriented GaAs substrates by flash evaporation. Epitaxial growth was found in the substrate temperature range T_sub = 620 … 790 K. Above 750 K a second phase was found besides the chalcopyrite phase. All epitaxial films were p-type conducting and showed an increase of the hole concentration with rising substrate temperature above T_sub = 720 K. Three acceptor states with ionization energies of 350 … 500 meV, 135 … 150 meV, and of some 10⁻³ eV were found.     

Thermal diffusivity measurements on As-Te-Ga glasses by photo-thermal deflection technique: Composition dependence and topological thresholds

The thermal diffusivity (α) of As₂₀Te_80−xGa_x glasses (7.5 ? x ? 18.5) has been measured using photo-thermal deflection (PTD) technique. It is found that the thermal diffusivity is comparatively lower for As₂₀Te_80−xGa_x glasses, which is consistent with the memory type of electrical switching exhibited by these samples. Further, the thermal diffusivity of As₂₀Te_80−xGa_x glasses is found to increase with the incorporation of gallium initially (for x ? 9), which is consistent with the metallicity of the additive. This increase in α results in a maximum at the composition x = 9; beyond x = 9, a decrease is seen in α leading to a minimum at the composition x = 15. The observed composition dependence of thermal diffusivity of As₂₀Te_80−xGa_x glasses has been found to be similar to that of Al₂₀As_xTe_100−x glasses, based on which it is proposed that As₂₀Te_80−xGa_x glasses exhibit an extended stiffness transition with compositions x = 9 and x = 15 being its onset and completion, respectively. Also, the composition x = 17.5 at which a second maximum is seen in the thermal diffusivity has been identified to be the chemical threshold (CT) of the As₂₀Te_80−xGa_x glassy system, as at CT, the glass is configurationally closest to the crystalline state and the scattering of the diffusing thermal waves is minimal for the chemically ordered phase.     

MOVPE growth of spontaneously ordered (GaIn) and (AlIn)P layers lattice matched to GaAs substrates

A systematic study of the metal-organic vapour-phase epitaxial growth of (GaIn)P and (AlIn)P layers deposited on GaAs substrates with (001) and (110) orientation is presented. Special attention has been paid to the growth on (001)-oriented wafers with different misorientations to the growth direction. The influence of the growth conditions on the properties of the epitaxial layers such as lattice mismatch, alloy composition, photoluminescence (PL) wavelength, FWHMs of PL peaks and atomic ordering is discussed. Layers with mirrorlike surfaces and various degrees of order could be deposited at growth temperatures T_g ranging from 595 °C to 750 °C for (GaIn)P and 720 °C to 800 °C for (AlIn)P. In addition to the influence of T_g on the Ga incorporation during the (GaIn)P growth we found the Ga distribution coefficient k_Ga to be affected by the misorientation of the substrates. k_Ga correlates presumably with the number of kinks and steps on the substrate surface. Transmission electron diffraction (TED) and PL investigations show that the degree of order — often described by the ordering paramter η — depends strongly on T_g the ordering is more pronounced when the layers are deposited on substrates misoriented towards the (1 11) lattice plane. Strong ordering has been observed for (GaIn)P samples grown at 680 °C on substrates 2° misoriented towards the [1 10] direction and at 650 °C on substrates 6° misoriented towards the same direction. For the (AlIn)P samples striking ordering has been found when they were grown at 720 °C.     

Optical properties of thin films of system (As2Se3)80−x(As2Te3)x(SnTe)20 prepared by PLD

Thin amorphous films from system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ were prepared by pulsed laser deposition (PLD) from their bulk glasses and their optical properties were studied by spectral ellipsometry. Spectral dependencies of refractive index, absorption and extinction coefficient and optical gap (1.41–1.66 eV for (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 20 resp. x = 0) were calculated from optical tansmittance, from ellipsometric data by Tauc method. High values of refractive index n₀ (2.49–2.60) and of non-linear χ⁽³⁾ coefficient of index of refraction (4.9–7.5 × 10⁻¹² esu for the glass (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 0 resp. x = 20) made studied thin films of system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ promising candidates for application in optics and optoelectronics.     

Low temperature investigations of e−-irradiated GaAs

Positron lifetime measurements have been performed on electron irradiated chromium doped semi-insulating GaAs as a function of annealing between 139 and 908 K. Trapping at negatively charged Ga_As defects and by a vacancy mixture dominated by trivacancies took place in the as-irradiated state. Upon annealing around 210 K divacancies were formed from the trivacancies as caused by migrating arsenic interstitials. Complete trapping prevailed up to 530 K but decreased then rapidly at higher temperatures due to the migration of divacancies which also remove the Ga_As defects.     

Electrical properties of AgGaSe2 epitaxial layers

Epitaxial layers of AgGaSe₂ with thicknesses in the range from 50 to 70 nm were deposited onto (111)A-oriented semi-insulating GaAs substrates by flash-evaporation in the substrate temperature range T_S = 825 … 900 K. Films grown at T_S ≦ 850 K are n-type conducting whereas p-type conductivity was observed at T_S ≧ 875 K. In the p-type samples two acceptor states with ionization energies of 60 and 410 meV were found from an analysis of the electrical measurements.