Gradient Freeze Technique Research Articles

Modelling of Thermal Conditions in GaAs Crystal Growth by the Gradient Freezing Technique (I) Fundamentals

AbstractIn the gradient freezing process crystal growth is fully controlled by influencing the thermal conditions. The method described in this publication combines measurements with Si phantoms with their modelling by calculation and the extrapolation of the material properties to GaAs. Results show that the gradients in time and space and the crystallization rate are interrelated and influenced by technological and design factors. For process control in the gradient freezing process, accurate knowledge of the differences between the time‐temperature programs given by the automatic control system and the actual thermal regime in the crystal is required.

Modelling of Thermal Conditions in GaAs Single Crystal Growth by the Gradient Freezing Technique (II) Technology

AbstractThe fundamentals of the gradient freezing process were dealt with in the first part of this publication. The second part describes aspects of the thermal technology of the process. Temperature measurements in model crystals and temperature field calculations show how the thermal conditions are influenced by the quality of furnace design, the size and position of the charge, installations in the furnace, and the effect of the melting enthalpy. A characteristic example of the gradient freezing technique is derived from the results.

Compensation mechanisms in nominally undoped semi-insulating InP and comparison with undoped InP grown under stoichiometry control

Nominally undoped semi-insulating InP can be prepared reproducibly by annealing under controlled phosphorus pressure. We present the electrical properties of a large series of undoped InP samples before and after annealing. Spectroscopic investigations show that the specimens are contaminated by iron during annealing, but native defects have to be taken into consideration in order to explain the electrical data. Annealed InP specimens are characterized by electrical and optical measurements and the results are compared to those obtained from InP crystals grown under stoichiometry control by the horizontal gradient freeze technique.

Growth of InP crystals by the horizontal gradient freeze technique

Abstract Undoped InP single crystals were grown by the horizontal gradient freeze technique with 〈100〉 seed orientation. Growth was performed inside thick-walled silica ampoules positioned in a gradient furnace without a pressure vessel. Boats made of SiO 2 and pyrolytic BN were used. The stoichiometry of the melt was controlled by a separately heated source of red phosphorus. The properties of the crystals were analysed by Hall effect 3easurements and by chemical etching to reveal dislocation densities (EPD).

Comparison of deep centers in semi-insulating liquid-encapsulated Czochralski and vertical-gradient freeze GaAs

Three-inch, semi-insulating (SI) GaAs, grown by the vertical gradient freeze (VGF) technique, has been studied by IR absorption, temperature-dependent dark current and Hall-effect, thermally stimulated current (TSC), and photoinduced current transient spectroscopy and has been compared with undoped, SI GaAs, both As-rich and Ga-rich, grown by the high-pressure liquid-encapsulated Czochralski method. The results clearly indicate that (1) the VGF GaAs contains less EL2, which suggests a less As-rich crystal stoichiometry; (2) in some VGF samples activation energies of 0.43 or 0.46 eV are deduced from temperature-dependent carrier concentration or resistivity measurements, respectively, and (3) VGF samples often show a thermal quenching behavior in the TSC peak T5. There is evidence to suggest that the 0.43 eV center is related to VAs, and T5 to VGa.

Effects of total liquid encapsulation on the characteristics of GaAs single crystals grown by the vertical gradient freeze technique

Total liquid encapsulation with B 2O 3 has been used to grow 50 mm diameter GaAs single crystals in PBN crucibles in a vertical gradient freeze configuration. The B 2O 3 layer efficiently prevents direct contact between the crucible and the GaAs charge and reproducible growth of single crystals can be achieved. The effect of B 2O 3 water content on the structural and electrical characteristics of the crystals was investigated. Water vapor can be trapped betwen the crystal and the crucible affecting the surface morphology of the crystals. The water content of the B 2O 3 encapsulant was found to affect the electrical properties of the crystals in a manner similar to what is observed for growth of GaAs crystals by the liquid encapsulated czochralski technique. Crystals grown encapsulated with dry B 2O 3 have been ion-implanted with silicon. The implant activations are comparable to those obtained on LEC grown crystals. Total liquid encapsulation in vertical gradient freeze can be used to produce device quality substrates.

Vertical seeded melt growth of GaAs

The growth of GaAs single crystals, 37, 50 and 62.5 mm in diameter, by the dynamic gradient freeze technique is described. Both quartz and pyrolitic boron nitride crucibles have been used successfully. Characterization of the crystals, 37 mm in diameter, grown in quartz is presented. Using very shallow temperature gradients over the melt and slow growth rates, the crystal-melt interface can be maintained flat over about 75% of the diameter. Through control of stoichiometry and with a thermal environment designed to reduce thermal stresses, about 50% of the crystals have a dislocation density of less than 1000 cm -2. Finally, we present the results from post-growth annealing.

The dynamic gradient freeze growth of (Ti + Zn) doped semi-insulating InP

High quality single crystals of semi-insulating InP were grown by the electrodynamic gradient freeze technique. The material is co-doped with the shallow acceptor zinc and the deep donor titanium. Resistivities in the 10 5−10 6 Ω cm range were measured with corresponding mobilities of 2500 cm 2 V −1s −1. Variable temperature Hall measurements resulted in an activation energy 0.61 ± 0.03 eV below the conduction band. The dislocation density is less than 500 cm −2 and is the lowest ever reported for comparably sized semi-insulating InP bulk crystals. Precipitates containing titanium and phosphorus were found when the titatium concentration exceeded 3 × 10 16 cm −3. A distribution coefficient of 4 × 10 −4 was measured for titanium in InP. The precipitates have been identified for the first time as hexagonal TiP using X-ray powder diffraction.

Cdte and CdZnTe crystal growth by horizontal bridgman technique

By means of the horizontal modified Bridgman technique, CdTe and CdZnTe crystals have been grown. The conditions of synthesis have been described and a modified gradient freeze technique has been experimented and proposed. By modifying the transverse gradient, nature of the boat, gas overpressure and temperature cycles, different growths have been compared. A computerized power recording was found to be extremely sensitive in spotting any thermal accidents. With an optimization of these different factors, we have obtained large single crystals: they gave (111) substrates up to 28 cm 2. A new and fast optical technique with a He-Ne laser has been used for studying all orientation parameters. The substrates have been characterized by double X-ray diffraction, photoluminescence and electrical measurements.

Thermoelastic study of GaAs in vertical gradient freeze configuration: Limits to the optimum growth rate and approaches to its augmentation

The influence of growth rate, crucible thickness and thermal conductivity, and furnace axial thermal gradients on the morphology of the growth interface and the associated stresses in 5 and 7.6 cm diameter GaAs crystals grown by the vertical gradient freeze technique is numerically investigated. It is shown that release of heat of solidification and the presence of the crucible mandate axial temperature gradients of the order of 3–5°C/cm and growth rates close to 0.1 cm/h for inhibition of plastic flow in the crystal. Variations in the thickness and thermal conductivity of the crucible are found to have a minimal effect on increasing these low growth rates, indicating the over-riding influence of release of solidification energy on solid-liquid interface shape. Modulation of the furnace temperature profile, as a means for reducing the nonplanarity of the growth interface, is shown to be an effective approach for substantial augmentation of the admissible growth rates.

Synchrotron section topographic study of defects in InP substrates and quaternary laser structures

Indium phosphide single crystals grown by the liquid encapsulated Czochralski (LEC) and vertical gradient freeze (VFG) techniques have been studied with X-ray diffraction topography using synchrotron radiation. Section topographs show that while the VGF substrates are of a better quality than the LEC ones, the defect level in epitaxial layers is more strongly determined by the quaternary layer composition and lattice mismatch than by threading dislocations from the substrate.

The impact of various annealing methods on silicon implant activation properties for GaAs crystals

Wafers obtained from undoped, semi-insulating GaAs crystals grown by liquid-encapsulated Czochralski and vertical gradient freeze techniques were implanted with 29Si at a dose of 3×1012 ions/cm2 and an energy of 60 keV. Activation of the implanted region was carried out by furnace or rapid thermal annealing methods, with and without the use of an SiO2 encapsulant. Profiles of the electron concentration and distribution were obtained by the capacitance-voltage method. Significant variations in the donor distributions were found to be dependent upon the annealing technique employed. The dissimilarities in implant activation properties for GaAs wafers grown by the two techniques, and subjected to identical annealing cycles, have been attributed to variations in melt stoichiometry and native point defects.

The growth of low dislocation densityp-lnP single crystals

High qualityp-type InP is critical for devices ranging from high power injection lasers to space-based solar cells. The growth of 50 mm diameter, low defect density,p-type, Zn:InP substrates has been achieved for the first time at doping levels below 1018 cm−3. The 600 gram 〈111〉 B-seeded crystals were grown by the vertical dynamic gradient freeze technique. Dislocation densities are more than an order of magnitude below those achieved in comparable LEC-growth material. These range from 300 cm−2 at the seed end to 1200 cm−2 in the 50 mm diameter portion of the crystal. Single crystals were grown with carrier concentrations ranging from 1–5 × 1017 cm−3 as determined by Hall measurements. Hole mobilities as high as 100 cm2 volt−1 sec−1 were achieved. The in-corporation of the zinc dopant follows normal freezing and a distribution coefficient of 0.67 ± .09 was determined. Infrared transmission imaging shows a lower level of stria-tion contrast relative to that observed for sulfur doped InP.

ChemInform Abstract: Growth and Characterization of Large Size, High Quality, InP Single Crystals.

AbstractSingle‐crystal InP wafers grown by both the vertical gradient freeze (VGF) technique (described in detail) and the liquid encapsulated Czochralski (LEC) method are studied by transmission X‐ray topography, transmission cathodoluminescence, and defect revealing etching.

The Growth and Characterization of Large Size, High Quality, InP Single Crystals

Single‐crystal wafers grown by both the vertical gradient freeze (VGF) technique and the liquid encapsulated Czochralski (LEC) have been studied and compared in terms of overall quality. The techniques used to characterize the defect structure consist of (i) defect‐revealing etch, (ii) transmission x‐ray topography (TXRT), and (iii) transmission cathodoluminescence (TCL). Subsequent to etching, the VGF substrates were found to have a uniformly low (<100/cm2) EPD from the wafer edge to the center with no slip observed. The low defect density is independent of the sulfur doping level in the range and is attributed to the reduced axial and radial temperature gradients during growth. In contrast, the Sn‐ and Fe‐doped LEC material had overall EPD levels exceeding 104/cm2. For heavily sulfur‐doped LEC material considerable slip and dislocations remain at the periphery. The TXRT studies of the VGF material show large areas (>10 cm2) that are dislocation free, whereas the LEC material shows dislocation tangles even in the relatively low EPD region. TXRT and TCL reveal the growth striations due to nonhomogeneous dopant incorporation. In the case of the LEC material, they are strongly concave, indicating a nonplanar liquid/solid growth interface and there is sharp contrast in the striations due to large fluctuations in dopant incorporation. The VGF striations are found to be planar and show very weak contrast.

Uniformity of threshold voltage for MESFETs fabricated on VGF GaAs substrates

Improved uniformity of threshold voltage is shown for MESFETs fabricated on GaAs substrates grown by a novel vertical gradient freeze technique when compared to devices fabricated on LEC GaAs substrates. The improved uniformity is most likely related to the decreased dislocation density and reduced impurity clustering in the VGF material.

Vertical gradient freeze growth of large diameter, low defect density indium phosphide

The demand for larger diameter, low dislocation density InP substrates has traditionally been filled by highly doped LEC-grown material. We describe the use of the vertical gradient freeze (VGF) technique to produce uniformly high quality wafers from 〈111〉 seeded, 750 g, 50 mm diameter single crystals. Typical dislocation levels are in the low 10 2 cm -2 range, independent of doping level. The dislocations are shown to be uniformly distributed across a wafer and from seed to tail. This adaptation of the VGF technique features seeded growth in pyrolytic boron nitride crucibles under low axial and radial thermal gradients as well as independent stoichiometry control. The reduction of thermally generated strain eliminates the need for impurity hardening of InP to achieve low defect levels. Comparisons with typical LEC results will be made in terms of growth striae, defect distribution and incidence of slip, as well as stoichiometry control. The design of the crystal growth equipment, the growth sequence and aspects of the process control will be discussed. Impurity incorporation will be discussed in terms of spark source mass spectrometry and low temperature compensation ratio estimates. Comparisons of room temperature mobilities are made with other high purity InP material.

Different annealing behaviour of the metastable state in GaAs

We have studied the temperature dependence of the stable state (EL2) recovery after photoquenching in two different materials, grown by the LEC (material A) and the horizontal gradient freeze (B) techniques, by means of optical absorption measurements. The temperature of annealing of the metastable state is 110–120 K in material A and 40–50 K in material B, suggesting the existence of two different defects belonging to the EL2 family. Furthermore, one sample from the B ingot, which has been platically deformed, shows the two thresholds of temperature recovery (50 and 120 K) and also an increase of the EL2 concentration.

Composition gradients and segregation in Hg1−xMnxTe

We have studied the growth of Hg1−xMnxTe crystals, using both vertical Bridgman and vertical gradient freeze techniques. Due to the disparity between the size and weight of the mercury and manganese ions, the segregation of Mn in HgTe is quite large. This segregation can lead to large changes in the composition of the material along the length of the ingot. We have found that both the size of these composition gradients and the fraction of the ingot affected are closely related to the speed with which the melt is solidified, but only slightly related to the growth technique used. These results agree with earlier work on the Hg1−xCdxTe system, and tend to indicate that the crystallography of the two systems are quite similar.

Synthesis and photoelectrochemistry of In 2S 3

In 2S 3 was prepared by chemical vapor phase transport and gradient freeze techniques. A variety of differently doped samples were also synthesized. Both n, and n plus p photoresponses were observed depending on the dopant, quantity of excess sulfur, or sulfur atmosphere annealing. Photoetching significantly increased the photocurrent density at short circuit. In a polysulfide couple, the maximum power efficiency obtained with white light was 0.3% and the quantum efficiency of carrier collection at short circuit was 43%. Photocurrent onset occured near - 1.1 V which was shifted significantly positive in nonsulfide-sulfur containing couples. Band gap determinations were made. Stability studies as measured by In 3+ in solution indicate a high (to 100%) stabilization which appears to be in conflict with visual observations. The behavior of photocurrent with time varies depending upon whether a negative or positive value of applied potential is involved.