Si-doped GaAs Crystals Research Articles

Crack Behavior of Si-Doped GaAs Crystals Grown by Pulling-Down Method

The crack behavior of Si-doped GaAs crystals produced by a novel pulling-down method was investigated. Some cracks were observed in the crystal tail part and no twins or polycrystals were observed. The cracking mechanism was discussed considering the growth parameters, such as the pulling-down rate, annealing time and cooling speed of the furnace. The crystal was easily broken if the cooling rate was too fast. To avoid cracking, the temperature profile and the growth parameters had been optimized. The cleavage property of the GaAs crystal was strongly related to its atomic arrangement and corresponding electron density map. Ultrasonic vibration or mechanical machine would make the crystal cleaved along (110) plane. GaAs crystal displayed a strong anisotropic crack property under the force of microindentation test.

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)

Investigation of residual dislocations in VGF-grown Si-doped GaAs

The VGF-method was applied for the growth of Si-doped GaAs crystals of 3 and 4 inch diameter ( n>10 18 cm −3). When the average dislocation density in these crystals falls below 200 cm −2 the stress-induced 60°-dislocations disappear and other dislocation types dominate the crystal. These remaining dislocations are classified and the conditions for their occurence are analyzed. Close to the crystal axis, dislocation walls are found which extend along [1 1 0]- and [ 1 1 ¯ 0 ] -directions and grow through the whole crystal. These walls are formed by edge dislocations belonging to the (1 1 0) 1 2 [ 1 ¯ 1 0 ] - or the ( 1 ¯ 1 0 ) 1 2 [1 1 0]-glide system. Apart from the dislocation walls, a dislocation arrangement in the form of a cross is observed in Si-doped GaAs. The orientation of the Burgers vector relative to the dislocation line of this dislocation type was determined. They include an angle close to 45°. Dislocations of this type belong to the (0 1 0) 1 2 [1 0 1] or the (0 1 0) 1 2 [ 1 0 1 ¯ ] -glide system. The examination of the growth in the seed well shows that the dislocation structures found in the crystal are partly already formed in the seed well. The growth in the seed well leads to a reduction of the EPD although the solid–liquid interface is concave. A reduction of the EPD in the seed well is accomplished by annihilation of dislocations.

Optimization of VGF-growth of GaAs crystals by the aid of numerical modelling

The VGF growth of Si-doped GaAs crystals is improved considerably by optimizing the design of the crucible support and the temperature profile during the growth run. Inverse simulation with the software program CrysVUN++ was used for this procedure. The criteria for the optimized conditions are flat phase boundaries and low thermal stress during the whole growth run. The crystals which were grown according to the simulated conditions indeed showed flat phase boundaries and a very low EPD (<100 cm −2) within the whole crystal. It is shown that the growth conditions in the seed well and conical part of the crystal have a major influence on the dislocation density in the whole crystal.

Anomalous interface shapes in the seed well during vertical gradient freeze growth of Si-doped GaAs

The growth mode in the seed well during vertical gradient freeze growth of Si-doped GaAs crystals has been studied via DSL etching and spatially resolved Fourier transform infrared spectroscopy. Pronounced faceted growth occurs and the shape of the phase boundary deviates remarkably from the isotherm. With ongoing solidification even melt inclusions are observed, which result in precipitates probably formed by Si. By taking into account segregation phenomena in the seed well, the observations are correlated to different enrichment of Si in front of the rough and the faceted parts of the phase boundary. The segregation is treated by a one-dimensional diffusion boundary layer model for both the rough and the atomically smooth interface. Interdiffusion between these two regions is treated as a perturbation by Fick's second law.

Growth of 3″ and 4″ gallium arsenide crystals by the vertical gradient freeze (VGF) method

Results of the vertical gradient freeze (VGF) growth of Si-doped (3″) and semi-insulating (4″) GaAs crystals are shown. The VGF process conditions were optimised with the aid of numerical simulations using the so-called inverse modelling. Experimental results and predictions from the computer simulation (software CrysVUN++) are quantitatively compared with respect to the power versus time profiles of the heaters, and qualitatively with respect to the shapes of the solid–liquid interface and the growth rate.

Donor Neutralization by Fluorine Containing Plasmas in Si-Doped n-Type GaAs Crystals

CF4, SF6 and CF4/O2 plasma-induced damage in silicon-doped n-type GaAs crystals have been investigated by using Hall measurements, X-ray Photoelectron Spectroscopy (XPS) analysis and Secondary Ion Mass Spectrometry (SIMS) analysis. We observed that the carrier density degraded after plasma exposure and post annealing. The degree of deactivation of carrier density with post annealing was strongly dependent on the plasma chemistries and the self-bias Vdc of the plasmas. From SIMS analysis, fluorine contamination was observed in the surface region of GaAs crystals after CF4 plasma exposure. The internal diffusion of fluorine atoms and the localization of fluorine atoms in n-type GaAs layers were observed after post annealing at 400°C. From these results, we proposed a mechanism of plasma induced damage in which internally diffused fluorine atoms neutralize the donor silicon by forming a Si–F bond. We also evaluated the effective diffusion coefficient of fluorine in GaAs as 1.5×10-11 cm2/s (at 400°C).

Optical and electrical non-uniformity around dislocations in silicon doped GaAs

Highly spatially resolved mappings of photoluminescence (PL) and capacitance-voltage (C-V) measurements have been used to characterise the optical and electrical non-uniformity around dislocations in Si doped GaAs crystals. A broad emission band with a peak at 0.95 eV commonly appears in Si doped GaAs wafers with carrier concentrations ranging from 3 × 1017 to 4 × 1018 cm-3. The average intensity of the 0.95 eV band is lower in the dislocated area than in the dislocation free area. The microscopic intensity pattern of the band agrees with the dislocation pattern. The intensity increases in the core region of dislocations and decreases in the surrounding area. The band edge emission shows a complementary intensity pattern. The C-V mapping of a wafer indicates that the carrier concentration is higher in the dislocated area, correlating with the PL intensity pattern. These results suggest that the microdefects responsible for the 0.95 eV band act as acceptors compensating Si donors and that they are gettered by dislocations.

Low dislocation density Si-doped GaAs single crystal grown by the vapor-pressure-controlled Czochralski method

Si-doped GaAs substrates have been widely used for optical devices. Recently, as the device fabrication process has changed, larger diameter substrates can be used. Almost dislocation-free Si-doped GaAs crystals of 75 mm diameter, which are suitable for the fabrication of optical devices, have been successfully grown using our vapor-pressure-controlled Czochralski (VCZ) method. The generation of slip dislocations could be suppressed and the growth of the dislocation-free crystal was achieved by decreasing the thickness of B2O3. The VCZ crystals contained a large amount of unintentionally doped boron (B) of about 1018 cm−3. By comparing the electrical data of the VCZ crystals with those of the gradient freeze (GF) crystal free from B, it is found that B acceptors decrease the intrinsic compensation ratio ([SiAs][SiGa]).

Structure of planar aggregates of si in heavily si-doped gaas

Abstract Planar defects on {111} planes in heavily Si-doped boat-grown GaAs crystals have been studied with a high-resolution technique in transmission electron microscopy and energy-dispersive X-ray analysis. There are two sizes of defect, both of which are considered to be Si aggregates, precipitated in a GaAs matrix and forming interstitial-type stacking faults. The Burgers vector of the defects are of (a/3) , the same as that of an extrinsic Frank loop. The larger loops form triangles with such orientations as to constitute the surfaces of a tetrahedron. This effect is discussed by considering the type of partial dislocations around the defect.

Transmission electron microscope investigation of dislocation loops in Si-doped GaAs crystals

Dislocation loops in Si-doped GaAs crystals have been studied by conventional transmission electron microscopy (CTEM) and high-resolution transmission electron microscopy (HRTEM). In addition to the prismatic dislocation loops lying on (110) planes and the faulted loops lying on (111) planes, the existence of prismatic dislocation loops lying on (111) planes with 1/2 (110) Burgers vectors and of interstitial character was identified for the first time. Some of the dislocation loops exhibited partly faulted and partly prismatic characteristics. This observation indicates that the prismatic dislocation loops lying on (111) planes may be converted from faulted loops by the generation and movement of two 1/6 (112) Shockley partial dislocations across the stacking fault. Because all the dislocation loops studied by HRTEM have an extra plane, it was concluded that interstitials are responsible for the formation of dislocation loops.

Raman-scattering tomography studies on semiconductors

Distributions of phonons in ZnSxSe1-x, ZnTexSe1-x mixed crystals were observed by Raman-scattering tomography as stripes which were caused by the lack of some elements of the Raman tensor due to rotational twins around the (111) axis. The technique was also applied to the analysis of phonons on the surfaces of Si-doped GaAs crystals, Si-implanted GaAs crystals and epitaxial layers of ZnSe on GaAs crystals.

Spatially resolved study of dislocations in Si-doped LEC GaAs by DSL, PL and EBIC

Three high-spatial-resolution methods, namely DSL (dilute Sirtl with light) photoetching, electron-beam-induced current (EBIC) and photoluminescence (PL), were used to study GS defects (grown-in dislocations which have moved from their original position leaving behind a Cottrell atmosphere and a trace) in Si-doped GaAs crystals grown from Ga-rich melts by the LEC method. A peculiar behaviour of these defects during DSL photoetching was revealed by interference contrast microscopy and surface profiling of the local etch rates: the start and end points of the GS defects were etched more quickly and the trace more slowly than the defect-free matrix. EBIC measurements showed a decrease in the net ionised impurity concentration at the Cottrell atmospheres but were ineffective in determining any compositional changes at the traces of the GS dislocations. PL mapping showed all components of the defects as well as remarkable spectral shifts at their different parts. On the basis of these results and previous data a model of the point defect equilibrium within the Cottrell atmospheres is proposed. The possible mechanisms of movement of the dislocations are considered, taking into account three different processes: climb, glide and climb/glide. It is argued that antisite AsGa defects (or related complexes) are formed during movement of the dislocations independently of the deviation from stoichiometry.

Silicon incorporation anomaly in LEC grown GaAs

The growth of Si doped LEC GaAs shows a significant deviation in the Si incorporation from the prediction of a classic segregation model. As a rule, it is expected that for a given impurity segregation coefficient, the dopant incorporation throughout the crystal may be calculated with fairly good accuracy. In fact, Te-doped GaAs LEC doping profiles show a close approximation to this classic model. However, in Si doped GaAs LEC growth, the doping distribution in the crystals deviates significantly from the segregation model. In some cases, this deviation is of several orders of magnitude. The degree of deviation was found to be dependent on the growth conditions. This work was performed to understand the source of the deviation from the model and to allow for an accurate account of the Si incorporation. In this work, Si doped GaAs crystals were grown by low pressure LEC. These crystals were targeted for doping in the 1016 to low 1018 cm-3 range. The actual dopant incorporation was significantly lower than the segregation model predicted, and the axial doping variation was much too flat. Moreover, in spite of an intentional Si doping, a significant number of crystals were semi-insulating. Analysis of the experimental data has shown that this doping behavior cannot be explained by the assumption of an unknown compensating impurity. A different model has been developed that is consistent with all of our observations and that provides an accurate account of the Si incorporation. Understanding the Si incorporating anomaly has allowed us to make successful process changes and improvements. This model explains the observed doping anomaly by taking account of the diffusion of Si between the GaAs melt and the B2O3 encapsulant, as well as some permanent Si trapping in the B2O3. When the interaction of the Si with B2O3 is taken into account, the Si incorporation follows the segregation model and the anomaly vanishes.

Influence of through-window radiation on the horizontal Bridgman process for rectangular shaped GaAs crystals

Si-doped GaAs crystals with a rectangular cross section have been grown using the modified two-temperature horizontal Bridgman (M2T-HB) technique [1]. A viewing window was opened on top of the furnace to monitor the growth process of rectangular GaAs crystals. By controlling the position and shape of the solid/liquid interface underneath the viewing window, lineage defects were eliminated and crystals of low dislocation density were successfully grown. Numerical analysis based on a semi-implicit finite difference scheme was performed with a radiative boundary condition imposed on the surface of solid crystal. The effect of radiative heat loss through the viewing window is discussed.

The growth and characterization of GaAs single crystal by a modified horizontal bridgman system

A modified two-temperature zone horizontal Bridgman (M2T-HB) system to grow high quality GaAs crystals is described in detail. GaAs crystals of 2 inch diameter and 30 cm length can be routinely grown by the M2T-HB technique. Keeping the solid/melt interface flat with the specially designed liner construction and viewing window, as well as eliminating the sticking between the quartz boat and GaAs crystals, lineage defects can be eliminated and the dislocation density can be greatly reduced. Moderately Si-doped GaAs crystals have etch pit densities (EPDs) lower than 2000 cm -2. For moderately Si-doped GaAs crystals, the best result shows that EPDs are lower than 700 cm -2 in the whole ingot. Nearly dislocation free Si-doped GaAs crystals with high carrier concentration of (1−4)×10 18 cm -3 can also be grown by this technique. Zn-doped GaAs crystals with carrier concentration in the range of (1−5)×10 19 cm -3 have EPD lower than 3000 cm -2. The carrier concentrations of undoped GaAs crystals are in the range of (5−10)×10 15 cm -3. The result indicated that high purity GaAs crystals with low Si contamination can be grown by M2T-HB technique.

Study of decoration microdefects in lec-grown Si-doped GaAs crystals by selective photoetching and laser scattering tomography

The status of decoration microdefects along the grown-in dislocations was studied in Si-doped LEC GaAs by means of selective DSL photoetching and laser scattering tomography (LST). It will be shown that the existence and type of microdefects in GaAs doped with silicon to give n = 10 18 cm -3 is strongly dependent on the melt stoichiometry. In As-rich material numerous arsenic decoration precipitates are present. In Ga-rich material complex decoration defects are formed while in stoichiometric samples no decoration effect could be recognized neither by DSL photoetching nor by LST. In addition in the latter material the presence of very fine matrix microdefects is evidenced by DSL and LST.

Characteristics of facets in Si-doped GaAs crystals grown by horizontal Bridgman technique

Dislocation free Si-doped GaAs crystals, 2 inches in diameter, 30 cm in length and carrier concentration of 10 18 cm -3, have been growm by horizontal Bridgman (HB) technique. Large facets were found to be more prone to form under a longitudinal temperature gradient less than 1°C/cm. Characteristics of facets in these HB grown GaAs crystals have been investigated. The occurence of large facets resulted in the lifting of the top surface of the crystal. Facets were found to be more prone to form by reducing the temperature gradient, increasing the Si-dopant concentration and the growth rate. Perpendicular twins, which were related to the (1̄, 1̄, 1̄) facet growth, were generated at the positions where the facet size varied drastically. These twins were identified to be 60° rotation twins by chemical etching method. The carrier concentration of the facet area, as measured by Van der Pauw and spreading resistance methods, was found to be 1.7–2.0 times higher than that of the non-facet area. The dislocations in the non-facet area were observed to terminate at the boundary of facet and non-facet areas.

Complementary DSL, EBIC and PL study of grown-in defects in Si-doped GaAs crystals grown under Ga- and As-rich conditions by LEC method

Local environments (Cottrell atmospheres) of grown-in dislocations in Si-doped LEC GaAs crystals were examined by three independent high spatial resolution techniques: DSL (diluted Sirtl-like etchant with light) photoetching combined with surface step profiling, photoluminescence (PL) and energy-dependent electron beam induced current (EBIC) mode of the scanning electron microscope (SEM). The GaAs samples were taken from two ingots grown under Ga- and As-rich conditions both with comparable Si doping level (4×10 17 cm -3). The chemical composition and consequently the point defect equilibrium within the Cottrell atmospheres was found to be essentially different in the two crystals. It has been clearly recognized by step profiling after DSL photoetching that in As-rich crystals elevated plateaus are formed whereas in Ga-rich GaAs complex depressions appear in the vicinity of dislocations. This corresponds to a decrease and increase of the etching rate, respectively, i.e., indicates essential difference in the number of holes available for surface reaction during DSL photoetching. On the basis of data obtained from EBIC measurements, PL mappings and spectral analyses, models are proposed which explain the observed morphological differences after photoetching.

The complexing of silicon impurities with point defects in plastically deformed and annealed GaAs

Si-doped GaAs crystals grown by the horizontal Bridgman method were plastically deformed at 450 °C and then annealed at various temperatures up to 800 °C. Localized vibrational mode (LVM) absorption due to Si-related defects was investigated using infrared Fourier-transform spectroscopy with the samples at liquid He temperature. Two LVM lines near 367 cm−1 due to a complex labeled Si-Y increased in strength in deformed samples which were subsequently annealed around 650–700 °C for 1 h. A new result was the observation of an LVM line from Si-Y superposed on the absorption from SiAs. Thus three separate lines occur at 366.8, 367.5, and 397.8 cm−1. It is proposed that the Si-Y center is a SiGa−VGa complex.