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.     

Undoped semi-insulating GaAs crystals grown by a modified low pressure LEC method

A modified low pressure in-situ synthesis LEC method of growing undoped SI (semi-insulating) GaAs crystals has been established. The key points for controlling melt composition and As evaporation during synthesis and growth have been described. Using this novel approach, crystals are able to be grown from the nominal melt composition in the range of 0.491–0.499 As fraction with high reproducibility. Some characteristics of the undoped SI crystals grown by the present work including electrical properties, dislocation density, carbon and EL2 concentrations and thermal annealing effects have been studied.     

Effect of the vacancy composition of GaAs single crystals on optical quenching of luminescence through <Emphasis Type="Italic">EL</Emphasis>2 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.     

On the Reasons Leading to an Appearance of Correlated and Anticorrelated Dependences of the Minority Carrier Lifetime on the Dislocation Density in Undoped Semi-Insulating “Stoichiometric” GaAs Crystals

It is shown that in undoped semi-insulating GaAs crystals grown under the stoichiometric conditions both correlated and anticorrelated dependences of the minority carrier lifetime τ on the dislocation density N_d could be observed. The above-pointed effect is connected with a slight excess of Ga atoms (then the correlated dependence τ vs Nd appears) or of As atoms (then the anticorrelated dependence τ vs N_d appears) which inevitably exists even in “stoichiometric” GaAs crystals (i.e. in GaAs crystals of “stoichiometric” composition).     

Investigation of semi-insulating oxygen-doped GaAs

We found Oxygen-doped GaAs crystals to be suitable materials for CO₂ laser optical component preparation, with application at 10.6 μm. An optical transmission of 55% in the IR spectrum range, between 2 and 15 μm has been reached for such a GaAs type material. The GaAs crystals that we have analysed were grown by two procedures: Horizontal Bridgman (HB) and Liquid Encapsulated Czochralski (LEC). The HB method has been used for obtaining pure (undoped) crystals, while the oxygen-doped GaAs ingots were grown by LEC technique. The two types of samples processed in the same manner as regards mechanical polishing and chemical etching, which were investigated by Hall measurements, optical transmission spectrometry and elastic recoil detection analysis (ERDA) technique. The GaAs:O (LEC) has near semi-insulating properties as can be observed from the results of the electrical resistivity and Hall effect measurements. The ERDA spectrum shows an intense signal of oxygen in the bulk of GaAs:O (LEC) crystals, while the oxygen signal is not present in the ERDA spectrum of the undoped GaAs (HB). We consider that these results could recommend the ERDA technique as a possible qualitative and quantitative analysis in an ion-beam accelerator for oxygen content in oxygen-doped GaAs crystals. The analysis is not sensitive to the native oxide, as could be seen by measuring GaAs (HB) undoped crystals.     

The role of dislocations in the formation of mechanical stresses during annealing of gallium arsenide single crystals

The effect of dislocations on the change of mechanical stresses in undoped semi-insulating gallium arsenide single crystals has been studied during their annealing in vacuum and in the arsenic atmosphere. The phenomena observed are explained by the effect of dislocations playing the role of channels for arsenic diffusion on the concentration of intrinsic point defects in the crystal regions surrounding dislocations. The mechanism of arsenic diffusion over dislocations allows one to consider dislocations as sources and sinks of arsenic without the translational and twinning processes and, thus, makes the well-known data on the reactions of dislocation interaction with point defects and the experimental structural data for single crystals more consistent.     

The role of structural defects in diffusion of elements of group VI in semi-insulating gallium arsenide crystals

The combined effect of the changes in the number and type of vacancies and dislocation density on selenium and sulfur diffusion in single crystals of undoped semi-insulating gallium arsenide has been studied. The differences in the diffusion mechanisms in the subsurface region of samples with an initial deficiency in gallium or arsenic are established as well as the dependence of the effective radius of arsenic trapping by dislocations on the ratio of the concentrations of gallium and arsenic vacancies.     

Thermochemical analysis of native point defects and substitutional carbon and boron in GaAs crystal growth

The phase extent of GaAs has been analyzed and compared with published phase diagrams as related to total and partial point defect equilibria including charged and uncharged Frenkel, Schottky, antisite defects and substitutional carbon and boron on both sublattices. The well-known transition between semiconducting and semi-insulating behaviour at 300 K as a function of melt stoichiometry in LEC crystals can be reproduced in our model in which complete equilibrium exists above, only electronic equilibrium below a freeze-in temperature of 1100 K. The corresponding model standard enthalpy of formation of neutral Schottky defects is 4.0 eV, of a pair of neutral uncorrelated antisite defects 3.8 eV, of neutral Ga Frenkel defects 4.1 eV and of neutral As Frenkel defects 3.6 eV. Defect reactions in cooling processes after crystal growth are discussed and shown to be quite different for crystals with high or low dislocation density. Semi-insulating behaviour requires the existence of carbon acceptors if dislocations provide internal sources and sinks for point defects. For ideal crystals carbon would not be necessary. The possible site distribution of C and B is analyzed in its dependence on temperature and chemical potential of As. Constitutional supercooling is negligible in LEC growth. Macrosegregation is severe if the As fraction in the melt deviates more than ±0.02 from the stoichiometric value 0.5.     

A Study of Convection,Striations and Interface Shape in InP Crystals Grown by the Double-Crucible LEC Technique

A doublecrucible (DC) Czochralski setup has been used for the pulling of semi-insulating Fe-doped InP. The level of the Fe-doped melt in the growth crucible is replanished with the undoped InP from the reservoir crucible. In this way, since the distribution coefficient of iron is very small (about 0.001), the Fe concentration in the growth crucible is virtually unchanged during the pulling so that the axial Fe concentration is much more uniform than in standard LEC crystals. The use of two concentric crucibles has great implications in terms of convective flows, stability of the melt temperature and interface shape. In this paper we report the results of a study on striations and structural defects in InP grown from a double-crucible LEC arrangement. A dimensionless relationship for correlating striation features with melt motions is also proposed.     

Radiation hardening studies in KBr-KI mixed crystals

Radiation hardening studies have been made in KBr, KI and in different compositions of KBr-KI mixed crystals, grown from melt by Kyropoulos technique. The irradiation hardness in mixed crystals is found to vary non-linearly with composition, attaining a minimum value at intermediate compositions. Dislocation density measurements have shown a high concentration of dislocations and grain boundaries in mixed crystals as compared to end crystals. The results obtained on radiation hardening of mixed crystals were explained in terms of dislocations present in them.     

Tin segregation and donor compensation in melt-grown gallium arsenide

Radio-tin-doped single crystals of GaAs have been grown by the LEC technique from melts of varying composition. Carrier concentration and Sn distribution determined by radio-counting and autoradiography are reported and analysed to show that Sn-related acceptors are incorporated to give a compensation ratio of N_A/N_D = 0.24+-0.03 independent of doping level and of melt composition. These concentrations are significantly in excess of a non-Sn-related residual acceptor - believed to be C_As - which is shown to be present in the crystals at a level of 1.6x10¹⁶ cm^-3. Modified Sheil plots are used to show that the melt composition appears to move progressively toward As-richness as growth proceeds. The distribution coefficient for tin in crystals growing from a stoichiometric melt is determined to be 4.0x10^-3.     

Reduction of grown-in dislocation density in Ge Czochralski-grown from the B2O3-partially-covered melt

Germanium (Ge) single crystals with an extremely low density of grown-in dislocations were grown by the Czochralski (CZ) technique with boron oxide (B₂O₃) liquid. Because attachment of particles floating on the melt surface to a growing Ge crystal leads to generation of dislocations during the growth, partial covering of the Ge melt surface with B₂O₃ liquid was attempted. Such attachment of particles was drastically suppressed or the particles were caught by the introduction of B₂O₃ liquid, and a particle-free Ge melt was realized in the central region of the melt surface. Ge single crystals were successfully grown from such melt, the grown-in dislocation density being 0–1×10³ cm⁻², which was remarkably lower than that in Ge crystals grown by a conventional CZ technique. The contaminations by B and O atoms of the grown crystal detected by SIMS analysis were very low. These Ge crystals have the potential for application to be applied as high-quality, dislocation-free substrates of GaAs solar cells for various usages including in space.     

The incorporation of Cr into gallium arsenide grown by the LEC method

To investigate the incorporation of Cr into LEC GaAs, we have grown nine Cr-doped GaAs crystals where the dopant is labelled with radio-tracer ⁵¹Cr. A thermodynamic model is developed to describe the segregation behaviour in which the distribution coefficient of the Cr is a function of the arsenic activity in the melt. The analysis shows that either the congruent melt is ˜ 2% richer in Ga than the stoichiometric composition or, much more probably, that a significant amount of Ga is lost into the B₂O₃ encapsulant. The latter postulate is shown to be consistent with recent studies by Emori et al. [Japan. J. Appl. Phys. 24 (1985) L291].     

Photorefraction Dependent on Composition in LiNbO3 Crystals with and without MgO Doping

The photorefraction in LiNbO₃ single crystals dependent on the melt composition with and without MgO doping was investigated. It was found that 1 mol% MgO-doped crystal with nearly stoichiometric composition has a strong photorefraction resistance. compared with a congruent composition. In an undoped crystal, the photorefraction was shown to be pronounced as incrcasing Li content of a melt from 48.6 to 58 mol% Li₂O. These results were discussed from a viewpoint of the relationship between the photoconductivity and the concentration of cation-site vacancies or Nb on Li-site.     

Effect of thermal annealing conditions on the microdefect formation in undoped GaAs single crystals grown by Czochralski method

The effect of thermal annealing conditions on the microdefect formation in undoped GaAs single crystals grown by the Czochralski method has been studied by X-ray diffraction and metallographic analyses. It is found that the standard heat treatments performed with the aim to relieve elastic stresses and to increase the homogeneity of wafers substantially affect microdefects formed in the crystal. Upon annealing, the micro-defects in ingots and wafers exhibit different behavior. Prolonged annealing leads to an increase in the sizes of large microdefects but does not suppress the formation of small-sized microdefects. The latter defects are formed at T< 950°C upon cooling from the annealing temperature, and their number strongly depends on the density of dislocations, which serve as sinks for intrinsic point defects.     

Effects of doping of Ca and Ba on the dislocation distribution in melt grown KCl crystals

The effects of doping of Ca and Ba (∼ 0.02 at %) upon the dislocation distribution in melt grown KCl crystals without cellular segregation were studied by an etch pit method in connection with the dispersion state of the dopants, which was examined by the dark field illumination. – (1) In the Ca-doped crystal, in which the dopants were distributed uniformly, the formation of subboundaries was completely suppressed and dislocations were distributed at random. (2) In the Ba-doped crystal, however, the dopants were non-uniformly distributed and the dislocations tended to form subboundaries. The dislocations within the subgrains were distributed in a non-uniform manner. (3) The dislocation density in the Ba doped crystal was higher than that of undoped or Ca-doped crystal. This was explained by the mechanism similar to the Tiller's prediction. – Finally, the stress distribution due to the concentration fluctuation was examined by the birefringence patterns.     

Etch Pit Density Distributions in Undoped and Doped GaAs Single Crystals

The undoped and doped (Si, N, In) GaAs single crystals are grown by horizontal Bridgman technique. The etch pit density (EPD) profiles were measured in transverse direction for each crystal. It was found that the shape of EPD profiles differs in dependence on the used dopant.     

Melt nonstoichiometry and defect structure of ZnGeP<Subscript>2</Subscript> crystals

The defect structure of ZnGeP₂ crystals grown from a melt by the vertical Bridgman method has been investigated. A deviation of the melt composition from stoichiometric leads to the formation of striations and the inclusions of other phases which are observed as structures (chains) oriented parallel to the growth axis. According to the microanalysis data, the inclusion composition corresponds to a mixture of ZnGeP₂, Zn₃P₂, and Ge. Nanoinclusions of germanium phosphide are detected by transmission electron microscopy. X-ray topography reveals defects of four types. The main defects in the central part of an ingot are related to the composition fluctuations, and the newly formed dislocations are basically single ones. Most dislocations are formed at the crystal periphery.     

Extended defects in bulk GaN and III-nitrides grown on this substrate

A short review of the structural perfection of high-pressure grown bulk crystals is given. As-grown undoped and Mg-doped crystals are described. The dependence of defect arrangement and quality of the surface on growth polarity is described. A high perfection of homoepitaxial layers grown on these substrates is shown. However, growth of thick layers by HVPE may lead to the formation of differently arranged dislocations and the formation of low angle grain boundaries associated with cracks. It is shown that the introduction of dopant or growth of mismatched layers on undoped high-pressure substrates may lead to the formation of additional defects.     

Röntgenographische Untersuchungen zur Kristallperfektion von GaAs-Einkristallen und -Epitaxieschichten. Will Kleber zum Gedenken

The defect structure of bulk GaAs and homoepitaxial layers has been characterized by means of reflection methods of x-ray diffraction topography (doublecrystal topography, Berg-Barrett method) and in addition by rocking-curve measurements. Besides of dislocations, long-range distortions of the single crystals of GaAs are investigated and determined quantitatively. The measured widths of the rocking curves approach those expected for perfect crystals. Single and double twinning in growth hillocks on epitaxial GaAs is found in connection with a higher degree of imperfection. The perfection of single-crystal layers is similar to that of the bulk crystals. Based on contrasts observed in double crystal topographs and energetic discussions, statements are made one defect geometry of stacking-fault tetrahedra in epitaxial layers.