Optical properties and origin of infrared light scattering centers in undoped semi-insulating GaAs crystals

The wavelength dependence and polarization characteristics of the infrared light scattered from an undoped GaAs crystal were investigated in the 90° angle infrared light scattering configuration. The scattering is Rayleigh scattering from scatterers which are always associated with the dislocations, and they are classified into three types,S, L _A , andL _G scatterers, according to their polarization characteristics. TheS, L _A , andL _G -scatterers are thought to be small As clusters, large As precipitates and large Ga precipitates, respectively.     

Laser melting of GaAs covered with thin metal layers

GaAs melting simulations with a pulsed ruby laser are reported. The presence of a thin metal layer deposited on the GaAs surface gives rise to a reduction in the melting threshold and to an increase of melted depths when compared with nude GaAs surfaces. Melting thresholds around 0.3 J/cm² for nude GaAs surface and slightly below 0.25 J/cm² for GaAs covered with a 120 Å tin layer are predicted in reasonable agreement with experimental results.     

Ge diffusion into GaAs by pulsed laser irradiation

Ge diffusion into GaAs from thin evaporated layers as sources is reported. Irradiation with aQ-switched ruby laser gives rise ton-type diffused layers of a thickness from 240 to 710 Å. A strong compensation of the diffused layers, that cannot be removed by thermal annealing, was observed. From the present experimental results it can be inferred that the diffusion coefficient increases at the melting point by 5 to 6 orders of magnitude.     

Electrical activity and precipitation behavior of copper in gallium arsenide

The electrical properties and preferred lattice site of Cu in GaAs were investigated combining electrical and optical measurements with ion beam and structural analysis. From this comprehensive study it was determined that Cu introduces two levels in the band gap, that the concentration of electrically active centers introduced by Cu diffusion is considerably smaller than the total Cu concentration, that this ratio of electrically active to total Cu concentration depends strongly on the cooling speed after diffusion, and that the portion of Cu that remains electrically inactive forms Cu-Ga precipitates.     

Raman and point contact current-voltage characterization of laser-induced diffusion in GaAs

Raman scattering and point contact current-voltage (PCIV) measurements were used as characterization tools of tin-diffused GaAs layers. Diffusion was induced by irradiating GaAs substrates covered with thin tin layers single pulses of a ruby laser. Samples processed with the lowest energies show strong damage and incomplete electrical activation as deduced from Raman and PCIV measurements, respectively. Raman microprobe in depth analysis and PCIV profiles also suggest the presence of a damaged region with incomplete electrical activation at the boundary between the molten layer and the solid substrate.     

Enhancement of minority carrier lifetime in GaAs1? x P x ( x =0.4; 0.65) by nitrogen implantation

Minority carrier lifetimes in nitrogen implanted GaAs_1-x P _x (x=0.4; 0.65) were measured at 77K by an optical phase shift method as a function of nitrogen dose and annealing temperature in order to investigate the dependence of the lifetime on the concentration of nitrogen isoelectronic traps. A large increase in the lifetime was observed after nitrogen implantation followed by annealing at and above 800°C. The maximum lifetimes were 22ns for GaAs_0.35P_0.65 and 6.7 ns for GaAs_0.6P_0.4. They were obtained by implantation to a dose of 5×10¹³ cm⁻² in GaAs_0.35P_0.65 and 10¹³ cm⁻² in GaAs_0.6P_0.4. The lifetime after nitrogen implantation followed by annealing was longer by a factor of 6–7 than that of the unimplanted sample.     

Temperature dependence of persistent photo-conductivity due to DX centers in AlxGa1−xAs:Si

Conductivity and Hall effect measurements were performed on molecular beam epitaxy grown Al_xGa_1–xAs:Si samples, which show a large persistent photoconductivity effect. We observe one, two, and three minima in the temperature-dependent carrier concentration during the heating process after having first illuminated the samples (x=0.25, 0.30, and 0.37, respectively) at low temperature. We interpret this structure in terms of the existence of different types of large lattice relaxation DX centers.     

Mechanism for the creation of antisite defects,during combined climb-glide motion of dislocations in sphalerite-structure crystals

Processes are considered which lead to a formation of irregular jogs in a dislocation core, e.g., when in GaAs two As atoms are absorbed at an already existing jog. Then, after glide, the dislocation leaves an arsenic antisite defect behind. This mechanism can give an account of a correlation between the spatial distributions of EL2 donors (that are most likely related to the As_Ga defects) and dislocations in melt-grown GaAs crystals.     

Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was 0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×10¹⁸ cm^–3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.     

Investigation of transient transport and recombination phenomena in semiinsulating GaAs

Photoconductivity and Hall voltage kinetics were measured simultaneously in SI GaAs monocrystals, using the pulsed neodimium laser excitation. The scattering and recombination centres were found to have a different influence at different time intervals of the transients (from 10 ns to some seconds). It is shown that in GaAs the photoconductivity relaxation in some time intervals can be interpreted correctly only by taking into account the mobility changes. The obtained resuls are explained in terms of recharging of the scattering centres and variations of the capture cross-section of charge carries on the local centres.     

Defect study of proton-irradiated liquid-encapsulated Czochralski GaAs using the positron-annihilation technique

The positron lifetime of undoped Liquid-Encapsulated Czochralski (LEC)-GaAs and Si-doped (1.3×10¹⁸ cm^–3) LEC-GaAs was measured before and after irradiation with protons (dose 1×10¹⁵/cm², 15 MeV). In Si-doped GaAs, the decrease of positron lifetime at temperatures between 10 and 300 K are due to the decrease of the positron-diffusion length and the increase of the effective shallow traps such as antisite Ga_As. The annealing stage of the proton-irradiation-induced defects which show the different behavior from that of electron-irradiation-induced defects suggests that proton irradiation creates more complicated defect complexes, containing vacancies rather than isolated vacancy-type defects or simple complexes which have been observed during electron-irradiation processes. Above 700 K, proton-irradiation-induced defects such as vacancy-type defects and simple vacancy complexes are almost annealed out, while Si-induced defects such as Si_Ga-V_Ga complexes cannot be annealed out above 973 K.     

The relation between lattice-parameter and particle size in ZnS:Cu Phosphor

The effect of particle size on life time in electroluminescent phosphor was investigated using fractionated samples. The life time of large-particle size phosphors was longer than that of small-particle size phosphor. It was found that particle size has a close relation with the lattice-parameter magnitude; the lattice parameter was larger in a small-particle size phosphor when the activator and coactivator content were the same. It was concluded that the lattice parameter was an important brightness life-time factor.     

Photo-magneto-electric effect in semi-insulating GaAs: carrier lifetimes and influence of the defect structure

We report the investigation of the Photo-Magneto-Electric effect (PME) in semi-insulating Liquid-Encapsulated (LEC-) grown GaAs crystals, using both intrinsic and impurity excitation. The role of the majority and minority carriers on the conductivity phenomena was evaluated and the lifetimes of electrons and holes were determined depending on excitation conditions. Anomalously high PME voltages, reaching in some cases some volts, were measured, which demonstrate a sharp drop in the temperature region 320–360K. The observed changes are discussed supposing that the influence of the non-homogeneous defect structure of the samples is essential.     

Photoelectrochemical etching of compound semiconductors: Wavelength dependence

Argon-ion-laser photoetching was performed at various wavelengths, around the absorption edge of ZnSe and CdS. The surface etch pit density is observed to decrease with increasing penetration depth of the light. This observation is explained in terms of the recent theory of non-uniform charge flow within semiconductor junctions.Incumbent of Helen and Milton A. Kitmmelman Career Development Chair in perpetuity, established by Helen and Milton A. Kimmelman, N.Y.Formerly: Department of Plastics ResearchIncumbent of Jacob and Alphonse Laniado (Montreal, Canada) Career Development Chair     

Semi-insulating electrical properties of undoped inp after heat treatment in a phosphorus atmosphere

Nominally undoped InP wafers have been annealed in a phosphorus atmosphere under a pressure of about 5 bar at temperatures of 900 °C for about 80 h. It was found that the electrical properties of the samples changed considerably after this treatment. A room temperature resistivity of up to 2×10⁷cm (semi-insulating behaviour) was obtained in the bulk of the samples. The resistivity finally obtained depends on the starting carrier concentration of the untreated samples. The Hall coefficient and Hall mobility have been measured up to 600 °C. The results can be interpreted in terms of a deep electronic level (E _A=0.63 ... 0.65 eV below the conduction band). The Hall coefficient was always found to be negative resulting in a Hall mobility of 1.4 to 4.9×10³ cm²/Vs. The highest resistivity in nominally undoped bulk InP so far reported in the literature [1] was =3.6 × 10⁵cm. Therefore, this paper demonstrates for the first time that a really semi-insulating behaviour of >10⁷ cm can be achieved for bulk InP with the purity of nominally undoped material (10¹⁵ to 10¹⁶cm^–3).     

Microwave waveguide method for the measurement of electron mobility and conductivity in GaAs/AlGaAs heterostructures

The microwave waveguide method for contactless determination of the electron mobility and conductivity of thin active layers is reported. The method is based on relative measurements of the magnetic field dependences of the derivative of the reflection coefficient with respect to the magnetic field from a semiconductor wafer bridging the waveguide.Experiments are performed on GaAs/AlGaAs heterostructures at microwave frequency = 36.4 GHz and liquid nitrogen temperature. For the analysis of the experimental data the theoretical basis for arbitrary frequencies is developed. The main advantage of the proposed method is that this method enables one to determine material parameters - mobility and conductivity - without careful calibration of the microwave system and does not require the accurate measurements of the absolute values of the reflection coefficient and phase of the reflected wave.     

Incorporation of Sn into epitaxial GaAs grown from the liquid phase

The incorporation of Sn into LPE GaAs was studied as a function of the atomic fractionx _Sn ^l of Sn in the liquid (1.6×10⁻⁴≤x _Sn ^l ≤0.54), the growth temperatureT _K and the cooling rate α. The diffusion coefficient of As in Ga for moderate Sn-doping was deduced from the growth velocities to beD _As (760° C)=(3.3±1.0)×10⁻⁵ cm²/s. The epitaxial layers were analyzed after van der Pauw with special emphasis on the sources of experimental error. With the aid of current mobility theories the concentrations of the ionized donors and acceptors were derived. From their dependence onx _Sn ^l , on α and onT _K combined with the Schottky-barrier model of Sn incorporation it can be concluded that the melt and the growing crystal surface were in thermal equilibrium. The diffusion coefficient of Sn in GaAs is about 8×10⁻¹⁴ cm²/s at 760° C. The distribution coefficient for Sn increases from 4.4×10⁻⁵ to 12.3×10⁻⁵ in the temperature range from 690 to 800° C. The total Sn incorporationx _Sn ^s was measured using the atomic absorption spectroscopy for the first time down tox _Sn ^s =10¹⁷/cm³. From these data it can be concluded that up tox _Sn ^l =0.54 the dopant Sn is incorporated as donor and as acceptor only and that within the experimental scatter there is no indication of incorporation as a neutral species.     

Temperature-Dependent Electron Transport in In0.5Ga0.5 P/GaAs Grown by MOVPE

Hall effect measurements in undoped In0.5Ga0.5 P/GaAs allo grown by metal organic vapour-phase epitaxy （MOVPE） have been carried out in the temperat.ure range 15-350K. The experimenta.1 results are analysed using a two-band model including conduction band transport calculated using an iterative solution of the Boltz- mann equation. A good agreement was obtained between theory and experiment. The impurity contents of In0.5Ga0.5 P/GaAs alloy, such as donor density ND, acceptor density NA and donor activation energy εD, were also determined.     

Light-intensity dependence of slow-relaxation phenomena in semi-insulating GaAs

Quenching-only and quenching-enhancement phenomena in low-temperature photoconductivity (PC) of SI GaAs have been studied as a function of light intensity for photons in 1.0–1.2 eV energy range. Quenching-only of PC occurs only at high light intensities (above 10¹⁴ photons/cm² s) and reflects well-known bleaching of EL2 defects. On the contrary, the quenching-enhancement effect can be observed only for several orders of magnitude lower light intensities and neither the quenching nor the enhancement part of low-temperature evolution of PC is directly connected with EL2 defects, but reflects the time evolution of the occupancy of deep traps other than EL2. It was also found that bleaching of EL2 is quite an unefficient process.     

Carrier injection level dependence of post-breakdown metastability in semi-insulating GaAs

Recently, a room temperature electrically induced metastability in semi-insulating (SI)-GaAs has been reported in which the normally high resistance state of SI-GaAs converts into a low resistance state when breakdown electric fields are applied to the metal/SI-GaAs/metal system. The low resistance state continues to persist when the electric field is lowered below the breakdown bias and as such may be treated as metastable state of the material. This phenomenon is believed to be closely related to the ‘lock-on’ effect utilized in high power photoconductive semiconductor switches made from SI-GaAs. The present study seeks to understand the mechanism for this electrically induced metastability by studying the influence of the injection current on the metastable phase. The data favor an interpretation of the high current state of the SI-GaAs in terms of double carrier injection and the sustaining of an internal electron-hole plasma in the material.