Undoped GaAs Crystals Research Articles

Absorption spectra of some radiation-doped A3B5 compounds

Temperature dependences of the absorption coefficient in A3B5 crystals before and after irradiation by electrons with an energy of 6 MeV and a dose of Ф = 2 × 1017 electron/cm2 are studied. A low-lying Ev + 0.4 eV center of a nonimpurity origin is found in both undoped GaAs crystals and those doped with various impurities (Te, Zn, Sn, Ga1–xInxAs, InP, and InP〈Fe〉).

Second-Harmonic Generation Induced by Electric Currents in GaAs

We demonstrate a new, nonlinear optical effect of electric currents. First, a steady current is generated by applying a voltage on a doped GaAs crystal. We demonstrate that this current induces second-harmonic generation of a probe laser pulse. Second, we optically inject a transient current in an undoped GaAs crystal by using a pair of ultrafast laser pulses and demonstrate that it induces the same second-harmonic generation. In both cases, the induced second-order nonlinear susceptibility is proportional to the current density. This effect can be used for nondestructive, noninvasive, and ultrafast imaging of currents. These advantages are illustrated by the real-time observations of a coherent plasma oscillation and spatial resolution of current distribution in a device. This new effect also provides a mechanism for electrical control of the optical response of materials.

Влияние размеров кристаллов на краевое излучение в нелегированном арсениде галлия

The sizes of surface of the samples, which were cut out from undoped GaAs crystals with surplus of gallium and with surplus of arsenic, when influence of mechanical stress on energy position of the maximum of the edge radiation band at 77 K can be neglected are determined

Impurity photovoltaic effect in p-i-n structures of undoped GaAs

Photo-emf in the range hv = 0.76 − 1.35 eV was found in p-i-n structures produced from undoped GaAs crystals with known parameters; the current sensitivities in the impurity and intrinsic (hv > 1.35 eV) regions were comparable. It was proven that the impurity photovoltaic effect results from EL2 and EL3 structural defects creating deep donor levels in the forbidden zone. Calculations were performed that justified the possibility of observing this effect on the investigated structures.

Homogeneity assessment of large‐diameter III‐V compound semiconductor substrates by near‐infrared transmittance measurement

The light absorption mechanisms below the fundamental energy gap in undoped and Si-doped GaAs crystals have been investigated to assess EL2 defects and the carrier concentration distribution in III-V compound semiconductor substrates. It is demonstrated, that the near-infrared transmittance mapping equipment consisting of a high-brigthness LED, a high-sensitivity CCD camera, and a digital image processing system is capable to reveal surface scratches and local crystal imperfections such as inclusions, voids, and twining inside the substrate, besides EL2 defects and the carrier concentration distribution. Hence, the homogeneity of III-V compound semiconductor substrates with diameter up to 6 inches can be assessed in less than several seconds with high spatial resolution of 640×480 pixels. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation patterning during crystal growth of semiconductor compounds (GaAs)

The phenomenon of dislocation patterning during melt growth of semiconductor compound crystals is discussed. The paper is focused on the formation of cellular structures in GaAs driven by thermomechanical stresses during the growth process. Undoped GaAs crystals show dislocation cell patterns with cell dimensions in the range 0·1–2 mm, orders of magnitude larger than in deformed metals. However, the correlation between cell diameter and dislocation density follows the same relation. Experimental results on the dislocation structure in GaAs were correlated with simulations of the resolved shear stress distribution based on a global temperature field simulation and scaled with the Taylor relation d=Kρ−½ and the similitude principle d=αKGbτ−1 (d is cell diameter, ρ is dislocation density, G is shear modulus, b is Burgers vector, τ is resolved shear stress, and α and K are proportionality factors). GaAs samples grown from a Ga rich melt with in situ control of stoichiometry show almost no dislocation patterning.

Effect of the vacancy composition of GaAs single crystals on optical quenching of luminescence through EL2 defects

Optical quenching of luminescence through EL2 defects in single crystals of undoped semi-insulating gallium arsenide is investigated. It is shown that the minimum energy of light photons providing the transition of such defects into the metastable state depends on the vacancy composition of the crystals. It is suggested that the nature of the effect revealed is related to the existence of a set of EL2 defects with different configurations. An optical method is proposed to determine the vacancy composition of undoped semi-insulating GaAs crystals.

Scaling of dislocation cells in GaAs crystals by global numeric simulation and their restraint by in situ control of stoichiometry

Undoped GaAs crystals show dislocation cell patterns with cell dimensions of some hundred micrometer orders of magnitude larger than in metals. Nevertheless, the correlation between cell diameter and dislocation density obeys the same Holt relation. Experimental results of dislocation analysis in GaAs were correlated with global computation of the resolved shear stress distribution at growth relevant temperatures and scaled with the universal function d = Kρ −1/2 = αKGbτ −1, where d is the cell size, ρ the dislocation density, K, α the constants, G the shear modulus, b the magnitude of Burgers vector and τ is the resolved shear stress. Samples grown with in situ control of stoichiometry show nearly no dislocation patterning due to the minimised native point defect content needed for dislocation climb.

Growth of GaAs crystals from Ga-rich melts by the VCz method without liquid encapsulation

The growth experiments and properties of undoped GaAs crystals grown by the vapour pressure controlled Czochralski (VCz) method without boric oxide encapsulant from Ga-rich melts are presented. Morphological and structural features were investigated by sensitive etching procedures. The precipitate content was analysed by laser scattering tomography. The mean scattering intensity of As precipitates decreases with increasing Ga excess in the melt. An enhanced probability of Ga-rich inclusion incorporation has been observed. In 3-in crystals, the mean etch pit density was decreased down to 5×10 3 cm −2 with improved radial homogeneity. The etch pits did not show cell patterning. Markedly reduced contents of boron and hydrogen have been detected. No oxygen defects and related complexes independently on the carbon concentration were ascertained by local vibrational mode. For the first time, the carbon concentration in the grown crystals was reduced by carefully controlling the CO content of the growth atmosphere in a VCz arrangement without boric oxide encapsulant. Such experiments without B 2O 3 encapsulant are of fundamental character and offer numerous information about the point defect situation in as-grown crystals.

Anti-Stokes Bands of Intrinsic Radiation in Undoped GaAs Crystals

It has been established that undoped gallium crystals exhibit anti-Stokes radiation with energy corresponding to the interband recombination of these crystals. Its appearance is most probably determined by EL2 defects, and its intensity depends on the product of the cross sections for photoionization of electrons and the holes from these defects.

Effect of Impurities and Structural Defects on the Transport Properties of n-Type GaAs Crystals

The carrier mobility and concentration in n-type GaAs single crystals, undoped, doped during growth (Te, Sn, and Ge), and neutron-transmutation doped (NTD) with Se and Ge, were measured as a function of temperature. Although the donor concentration was the same in all of the doped crystals, the carrier mobility and concentration in the GaAs crystals were found to be lower than those in the GaAs , GaA , and NTD GaAs crystals by about a factor of three and one order of magnitude, respectively. These results are interpreted in terms of the compensation ratio and predominant scattering processes in the crystals. The large compensation ratio in the GaAs crystals is due to the high acceptor concentration. The low carrier mobility in GaAs is associated with the strong scattering of charge carriers by ionized defects, because of their high density, and also, in contrast to the undoped, GaAs , and GaAs , and NTD GaAs crystals, with additional scattering by nonuniformly distributed structural defects. In calculations of the compensation ratio and analysis of the scattering mechanisms, the Brooks–Herring or Cornell–Weisskopf approximation was used, depending on the compensation ratio in the material.

Synchrotron X-ray topography of undoped VCz GaAs crystals

For the first time vapour pressure controlled Czochralski (VCz) monocrystals of semi-insulating (SI) GaAs, grown at IKZ Berlin, have been investigated by synchrotron X-ray topography. The X-ray topographs of a typical VCz sample, taken from the cylindrical part, show dislocation images resembling those of SI vertical gradient freeze-grown GaAs crystals. From the disappearance of the dislocation image in selected topographs it is concluded that the Burgers vector for most dislocations is parallel to 〈1 1 0〉. The main part proves to be of 60° type. The cellular structure, typical for liquid encapsulated Czochralski material, is not seen in the VCz samples. Large volumes up to 0.5×0.5×0.5 mm 3 are dislocation-free. The results are compared with etch pit density (EPD) measurements from the same crystals. The average EPD is (1–2)×10 4 cm −2. The minimum value along 〈1 1 0〉 is 2×10 3 cm −2.

An analysis of the shape of a luminescence band induced by transitions of free electrons to carbon atoms in semi-insulating undoped GaAs crystals

The shape of a photoluminescence band observed due to recombination of free electrons at shallow-level acceptors (carbon atoms) in semi-insulating undoped GaAs crystals was analyzed at various temperatures (T=4.8–77 K). It is shown that at low temperatures the shape observed essentially differs from the theoretical one, while at high temperatures theory and experiment agree closely for radiative transitions of free electrons to isolated shallow-level acceptors. The difference between the experimental and theoretical shapes of the photoluminescence band is associated with the broadening of carbon-induced acceptor levels (i.e., with the formation of the acceptor impurity band), resulting from the effect of electric fields of randomly distributed ionized acceptors and donors on “isolated” carbon atoms. Coincidence of the shapes is associated with a considerable increase in the energy of free carriers (to values up to and above the width of the acceptor impurity band).

Influence of heat treatment on luminescence of semi-insulating undoped GaAs crystals

Variations in the spectra of edge-emission photoluminescence of semi-insulating undoped GaAs crystals as a result of heat treatment for 20–90 min at 900°C were studied. It is shown that heat treatment substantially affects (transforms) the excitonic component of the spectrum. The observed changes in the spectra are related to variations in the impurity composition of the crystals studied. These variations are shown to be caused by heat treatment.

Carbon-stimulated increase in the concentration of gallium divacancies in semi-insulating undoped GaAs crystals

It is shown that an increase in carbon content in semi-insulating undoped GaAs crystals leads to a substantial rise of the concentration of gallium divacancies in these crystals. This effect seems to be related to the process of carbon atoms occupying the arsenic vacancies involved in the As-divacancy-Ga-divacancy complex.

Analysis of the Low-Temperature (77 K) Near-Band-Edge Luminescence in Undoped Semi-Insulating GaAs

The dependences of the peak position and of the half-width of the near-band-edge luminescence band in nominally undoped semi-insulating GaAs crystals on the content of residual C and Si shallow impurities are studied at 77 K. It is shown that a rise in the impurity concentration leads to a shift of the peak position to lower energies and to an increase of the half-width. The observed effect is mainly connected with a broadening of the excitonic component of the near-band-edge luminescence band resulting from an increase in the efficiency of radiative recombination of free excitons due to their interaction with C and Si shallow impurities.

Spatial variation of activation energy in undoped high-resistivity bulk GaAs

In undoped semi-insulating GaAs grown by the liquid encapsulated Czochralski or vertical gradient freezing technique mesoscopic electrical nonuniformities correlated to the cellular structure of dislocations exist which are mainly caused by the enrichment of the deep defect level EL2 in the dislocation-rich cell walls. In undoped GaAs crystals with a transition semi-insulating/medium resistivity in the transition region, the resistivity fluctuations between cell walls and cell interiors are much more pronounced (up to three orders of magnitude) and must be caused by different donors. A point contact technique developed for the detection of such nonuniformities was used to measure activation energies separately in cell walls and in cell interiors. In this way, it could be shown that also other defects or impurities than the EL2 are accumulated in the cell walls so that different donor species dominate the electrical properties of cell walls and of cell interiors.

Analysis of changes in the intensity of intrinsic luminescence after diffusion of copper into semi-insulating undoped gallium arsenide crystals

The effect of copper diffusion into semi-insulating undoped GaAs crystals on the intensity of intrinsic luminescence is analyzed. It is shown that diffusion of copper into semi-insulating undoped GaAs crystals can lead either to an increase or a decrease in the intensity of intrinsic luminescence. Analytic relations, which connect the magnitude and sign of the effect with recombination parameters in these crystals, and also with the intensity of the excitation luminescence, are obtained.

Analysis of Changes in the Intensity of the Intrinsic Luminescence after the Copper Diffusion into Semi-Insulating Undoped GaAs Crystals

An analysis is made of the effect of the copper diffusion into semi-insulating undoped GaAs crystals on the intensity of the intrinsic luminescence. It is shown that the copper diffusion into semi-insulating undoped GaAs crystals could lead both to an increase and to a decrease in the intrinsic luminescence intensity. Analytical expressions connecting the value and the sign of the effect observed with the recombination parameters of crystals pointed and also with the intensity of luminescence excitation are obtained.

Effect of crystal shape on etch-pit-density distribution in undoped low dislocation density GaAs crystals grown by the As-ambient liquid encapsulated Czochralski technique

The dislocation density for conventional liquid encapsulated Czochralski (LEC) GaAs crystals such as those of 3–4 inch in diameter is too high to clarify the dislocation generating conditions. Therefore, growing almost dislocation free crystals and clarifying the dislocation generating conditions individually is necessary before the crystal size is enlarged. To reduce the dislocation density, crystals were grown by the As-ambient LEC (As-LEC) technique under low temperature gradient along the pulling direction. An etch pit density (EPD) at the center of the 20 mm diameter crystal of the order of 10 3 cm −2 was obtained. The EPD distribution and crystal shape in the pulling direction correlate for low EPD crystals and show that crystal shape control is necessary for obtaining lower EPD crystals. The EPD distribution perpendicular to the pulling axis, however, exhibited the so-called “W”-shape. Even though the temperature gradient along the pulling axis was reduced, dislocations which were considered to be generated by excessive radial thermal gradient, remained in the crystals.