Gradient Freeze Technique Research Articles

High Quality CdTe Growth by Gradient Freeze Method

ABSTRACTThe vertical Bridgman method, a gradient freeze technique, is feasible for the growth of high quality, large CdTe crystals. The equi-composition contour of Zn in doped crystals has been used to reveal the solid-liquid interface shape in the growth process. A slow cooling rate is necessary to obtain a convex interface shape. A low temperature gradient at the solid-liquid interface, down to 2 °C/cm, and Zn doping are found to be effective to reduce the etch pit density to 5×103/cm2 (minimum) to 104/cm2 (average). The crystallographic quality has been evaluated by means of X-ray diffraction, X-ray topography, etch pit delineation with the Nakagawa etchant, infrared measurements, photoluminescence and Hall effect measurements. CdTe crystals are found to be free from subgrain structure, Te precipitates and deep levels, and have high electron mobility.

Photoelectrochemistry of CuIn5 S 8 and HgIn2 S 4

was prepared by gradient freeze techniques. Doping was done, and annealing was carried out under various sulfur atmosphere vapor pressures and vacuum. was prepared by chemical vapor phase transport and was annealed under various sulfur vapor pressures. Both materials showed increased photocurrent density with photoetching at short circuit, which was particularly dramatic for . Purely n‐type response was seen for samples as well as for , except where phosphorus plus sulfur dopants were used (both n‐ and p‐type responses). Quantum efficiencies of carrier collection were 10–15% at short circuit for (maximum of 77%) and ∼22% for (maximum of 83%) in a polysulfide couple. Considerable increase in the short‐circuit current density and open‐circuit photovoltage could be obtained for in a couple, but this was offset by substantial photocorrosion. Power efficiencies up to ∼0.4% and ∼0.2% were obtained for and , respectively. Stability in a polysulfide couple was up to ∼97% for and >99% for .

Effects of thermal annealing on semi-insulating undoped GaAs grown by the liquid-encapsulated Czochralski technique

Recently, a number of studies have reported a correlation between variations in threshold voltage of field effect transistors and the nonuniformity of the luminescence efficiency of semi-insulating GaAs crystals grown by the liquid-encapsulated Czochralski technique. The changes in luminescence efficiency and subsequently the variations in threshold voltages were dramatically reduced by postgrowth annealing of the GaAs crystals under a variety of conditions. In this study, we employ the technique of spatially resolved cathodoluminescence (CL) to carefully examine the changes in luminescence efficiency due to postgrowth annealing. In agreement with previous work, we find that the CL variations are greatly reduced from a factor of ∼2 to ∼5% by thermal annealing at 800 °C for 30 h or at 1200 °C for 6 h followed by slow cooling. The latter thermal treatment is the same as that experienced by crystals during growth by the horizontal gradient freeze (HGF) technique. The extremely uniform luminescence efficiency of HGF crystals is thus believed to be a result of the thermal treatment during growth. Using evacuated sealed ampoule annealing in the temperature range of 550–650 °C, we show for the first time that improvements in CL uniformity are a result of a diffusion process which involves an As vacancy. Due to the dependence on As loss, values of the diffusivity (D) depend on the surface conditions. Values of D=9×106 exp(−2.6 eV/kT) cm2/s and D=1×107 exp(−2.5 eV/kT) cm2/s are obtained for polished and as-cut surfaces, respectively.

Some aspects of the recent work on InP crystal growth in China

InP synthesis and LEC growth of single crystals in China is reviewed. Three methods have been adopted in InP synthesis from the starting elements, namely direct synthesis inside the puller, vapor stream stirring method and conventional gradient freeze technique. InP twin-free crystals, 300–1000 g (30–60 mm diameter × 100–150 mm length) can be routinely grown using LEC technique. The quality of these crystals meet requirements for device fabrication.

Uniformity characterization of semi-insulating GaAs by cathodoluminescence imaging

Miyazawa et al. [Appl. Phys. Lett. 43, 853 (1983)] have recently established a spatial correlation between variations in field-effect transistor performance and nonuniformities in the cathodoluminescence (CL) efficiency of semi-insulating (SI) GaAs substrates. In this study, we compare the CL uniformity of both Cr-doped and undoped SI GaAs crystals grown by the liquid-encapsulated Czochralski (LEC) technique with undoped SI crystals grown by the horizontal gradient freeze (HGF) technique. In contrast to the LEC crystals, HGF GaAs has extremely uniform CL characteristics which should result in uniform device performance.

Numerical simulation of the horizontal Bridgman growth of a gallium arsenide crystal

Two-dimensional numerical simulations have been developed which represent the thermomechanical behavior of semiconductor melts in horizontal crucibles. These computer models are based on time-dependent finite-difference and finite-element codes, capable of simulating steady and transient melt convection. At moderate Rayleigh numbers, one finds time-invariant solutions which typically involve several primary and secondary vortices, depending on aspect ratio. Above a critical Rayleigh number, these steady solutions are replaced by oscillatory melt convection, as suggested by earlier experimental and analytic studies. The finite element code was extended to include a solid-liquid interface, which permits the study of actual crystal growth processes. In the presence of thermal oscillations, one finds periodic freezing and remelting at the interface, which may be related to the observed striations in crystals grown by the gradient freeze technique. The amplitude of these oscillations depends on the boundary conditions and the Stefan number. Three-dimensional extensions of the steady-state code are under way to determine the effects of finite crucible width, as encountered in experiments.

A modified horizontal gradient-freeze technique for growth of dislocation-free Si-doped GaAs single crystals

A modified horizontal gradient-freeze (GF) technique with quasi-double parabolic temperature profile has been developed for the growth of Si-doped GaAs crystals. Using this technique and taking measures to prevent wetting of the quartz boat by the GaAs melt, dislocation-free (DF) Si-doped GaAs single crystals with <111>B and <211>B oriented seeds covering the carrier concentration (n) range of 1.8 × 1017ȁ7.6 × 1018 cm-3 have been successfully grown. A preliminary study has been made of the homogeneities of the crystals. It is suggested that the SiGaVGa complex could play a role in influencing the homogeneity and dislocation density (EPD) of Si-doped crystals.

The effect of silicon doping on the lattice parameter of gallium arsenide grown by liquid-phase epitaxy, vapour-phase epitaxy and gradient-freeze techniques

An extensive study of lattice parameters of silicon doped gallium arsenide grown by liquid-phase epitaxy, vapour-phase epitaxy and gradient-freeze techniques has been undertaken. Lattice parameters were measured by the Bond techniques to a precision of 1 ppm, and the material studied was also characterised independently for carrier concentration, carrier mobility, and, in some cases, for silicon site distribution by infra-red LVM measurements. It is concluded that the p-type LPE layers show a very large lattice contraction compared with undoped material, while VPE and gradient-freeze material have parameters much closer to the undoped value. The lattice contraction in Si-doped LPE layers is about 6 × 10 -4 Å at hole concentration levels of 6 × 10 18 cm -3, and the contraction is roughly proportional to the free-hole concentration from 1 to 6 × 10 18 cm -3. It is concluded that the p-type Si-doped LPE layers studied contain a high concentration of a contracting defect associated with silicon whose concentration is related to the free-hole concentration. The most likely defect responsible is Si Ga, present in high concentrations as a compensating donor, although other defects cannot be ruled out.

Growth of GaAs ingots with high free electron concentrations

A horizontal gradient-freeze technique has been developed for growth of GaAs ingots with high free electron concentrations while minimising the crystal defects associated with large quantities of dopant. With Si as dopant satisfactory crystal quality was maintained at carrier concentrations up to 4 × 10 18 cm -3. Attempts to extend this range upwards by supplementing the Si doping with other elements showed that the incorporation of one dopant is influenced by the presence of others. Higher carrier concentrations were obtained with Si alone than with Si + Se together.

The growth of In-rich bulk single crystals of Ga xIn 1− xP yAs 1− y via the gradient freeze method

Bulk single crystals of Ga x In 1− x P y As 1− y were grown from the melt via the gradient freeze technique in the composition range 0 ⩽ x ⩽ 0.07, 0 < y ⩽ 0.80. The crystals have nominally undoped a net electron concentration of 8 × 10 15 ⩽ N D − N A ⩽ 4 × 10 16 cm -3 corresponding to electron mobilities 1600 ⩽ μ e ⩽ 2500 cm 2V -1s -1 at room temperature. P-type crystals with net acceptor concentrations 3 × 10 17 < N A − N D <3 × 10 18 cm -3 and 17 < μ h K 55 cm 2 V -1s -1 were grown from Zn doped melts. The dislocation density revealed by etch pits on the (111)-A face is typically ⪅10 6 cm 2. Photoluminescence peaks recorded at room temperature from different spots on a nominally undoped crystal were located at 1.24 ± 0.08 μm.

Growth of Ni-doped MgF 2 crystals in self-sealing graphite crucibles

Large Ni-doped MgF 2 single crystals of excellent optical quality have been grown in self-sealing graphite crucibles by a vertical gradient freeze technique. This relatively simple technique always yields single crystals having excellent optical quality and should be applicable to the melt growth of other crystals that are too volatile to permit the use of open systems.

The influence of gravity on crystal defect formation in InSb-GaSb alloys

InxGa1−xSb melts with x = 0.5, 0.3 and 0.1 were solidified by a gradient freeze technique on earth and in Skylab 3 and Skylab 4. The three ingots processed in Skylab 3 had diameters less than those of their carbon-coated silica ampoules, while those processed in Skylab 4 (with a higher initial heater temperature) completely filed their ampoules. All six of the earth-processed ingots were sufficiently bireflectant that grains and twins were easily observed with incident polarized light. This was true of only space-processed ingot, the others requiring sandblasted surfaces and oblique lighting to observe twins and grains. Twinning was 70% less in the six Skylab ingots than in the corresponding six earth-processed ingots. Microcracks were more numerous in inhomogeneous regions of the ingot. The number of grain boundaries (exclusive of twin boundaries) was about 18% less in the Skylab ingots. The distribution of voids was more uniform in the Skylab ingots.

Influence of gravity-free solidification on solute microsegregation

Single crystalline germanium has been grown for the first time in a zero gravity environment. The crystal growth experiment, designed to characterize the influence of gravity-free solidification on microsegregation of a semiconductor material, contained three crystals. One was doped with Ga with an initial doping concentration of 8 × 10 16 atoms/ cm 3, the second with Sb, with 4 × 10 14 atoms⧸ cm 3, and the third with B, with 2 × 10 15 atoms⧸ cm 3. Portion of each germanium crystal was remelted and subsequently resolidified at 5 μm per second under controlled conditions by the gradient freeze technique. The space grown crystals (grown aboard Skylab Mission No. 2) were compared with identical germanium crystals resolidified in a terrestial environment in horizontal and vertical positions. Detailed spreading resistance measurements made on the Ga doped Ge crystals indicate that microsegregation in space is reduced by four to five-fold in the bulk and by eight-fold near the surface, while macrosegregation in space is reduced by six-fold in the bulk. The effective segregation coefficient for Ga in Ge is analyzed extensively and is found to be larger for space solidification. This has the implication that the solute boundary layer at the growth interface in space is correspondingly thicker. The solidification interface is significantly smoother and is found to be initially concave toward the melt in space.

ChemInform Abstract: PHASE EQUILIBRIA AND VAPOR PRESSURES OF PURE PHOSPHORUS AND OF THE INDIUM/PHOSPHORUS SYSTEM AND THEIR IMPLICATIONS REGARDING CRYSTAL GROWTH OF INP

AbstractDer Dampfdruck als Funktion der Temperatur wird von kommerziellen roten P‐Produkten und einem speziell hergestellten monoklinen Material gemessen.

Phase relations, crystal growth and heteroepitaxy in the quaternary system Cd, Sn, In and P

New data on the liquidus of the pseudobinary system InP/Sn are reported and the growth of graded quaternary epilayers from solutions in the tin-rich corner of the quaternary system Cd, Sn, In and P on p-type InP substrates is described. Also, we present preliminary results on the preparation of InP substrates crystals via zone melting and LEC pulling techniques. The properties of such crystals are characterized and compared to previously reported results for InP crystals grown by the gradient freeze technique.

Phase Equilibria and Vapor Pressures of Pure Phosphorus and of the Indium/Phosphorus System and Their Implications Regarding Crystal Growth of InP

The vapor pressure as a function of temperature of several commercial grades of red phosphorus and a specially prepared crystalline sample has been measured. For crystalline red P we obtain a heat of sublimation, . The commercial grades yield vapor pressures, which above 450°C are described by the equation In . The sublimation equilibrium of red P can be utilized for controlling phosphorus pressures, , with a reproducibility of 10% if commercial P of at least 5N purity is used. Measurements of the pVT relation of phosphorus vapor in the regimes and reveal deviations up to 10% from ideal gas behavior. Excellent accuracy in describing the behavior of phosphorus vapor is obtained when van der Waals equation is used in conjunction with the critical data of Marckwald and Helmholz and a critical coefficient of 0.375 for calculation of both the co‐volume and the interaction parameter. Consideration of the pressure dependence of the equilibrium constant Kp for the reaction yielded approximate values for the fugacity constant at 1000° and 1100°C. The pTx relation at the liquidus in the vicinity of stoichiometric InP for indium‐rich and phosphorus‐rich compositions were established by simultaneous vapor pressure and DTA measurements. The melting point of stoichiometric InP is and the corresponding equilibrium phosphorus pressure is . T−x liquidus data calculated with the assumption of ideal behavior of the liquid are in good agreement with our experimental data. Interpretation of the liquidus pressure data yields values of the standard enthalpies and entropies at 298° K for the reaction and for the reaction These results are in excellent accord with the standard enthalpies reported by Panish and Arthur and agree within 3% with literature data on the standard entropies of the reactants. The growth of n‐ and p‐type InP crystals via the gradient freeze technique is discussed and the degree of control of the liquidus concentration is evaluated for various conditions of crystal growth.

The growth of InP crystals from the melt

The growth of InP crystals from the melt is discussed on the basis of new data on the pTx relations for the In/P system in the vicinity of stoichiometric InP. The preparation of nominally undoped n-type material by the gradient-freeze technique with visual control of the seeding process is described. Zn- and Cd-doped p-type crystals were prepared in a modified almost-isothermal apparatus in order to obtain uniform stoichiometry and regular dopant distribution. The crystals were characterized in terms of their chemical purity, dopant distribution, physical defects and electrical properties.

Some properties of Bi12TiO20 and the system Bi2O3-TiO2

Bi12TiO20 has been prepared by the fluxed horizontal gradient freeze technique. Some physical properties are given. That it is an incongruently melting compound is argued.

Synthesis and melt growth of doped ZnSe crystals

Single crystals of ZnSe have been grown by the gradient freeze technique from a stoichiometric melt in a sealed hot-walled system. This allows the incorporation of dopants of high vapor pressure and maintainance of stoichiometry. Samples are subsequently shown not to be entirely self-compensated.

Cristallisation du phosphure de gallium a partir d'une fonte stoechiometrique et en solution de gallium

Gallium phosphide crystals grown by a gradient freeze technique in a high temperature, high pressure apparatus are compared with material obtained by a thermal gradient technique in gallium rich solution. In view of the crystallinity and of the electrical properties of the crystals the interest of the solution growth is pointed out for obtaining pure, single gallium phosphide crystals.