Vertical Gradient Freeze Technique Research Articles

Electrical field enhanced thermal quenching of a prominent thermally stimulated current peak in semi-insulating GaAs

Detailed experimental results are presented for a ‘‘thermal quenching’’ of thermal stimulated current signals in the most prominent trap in undoped semi-insulating (SI) GaAs, T5 with an activation energy of 0.27–0.31 eV. A possible model for the thermal quenching of T5 is discussed, emphasizing the thermally stimulated nature of the quenching process, the effect of electric field and the formation of high-field domains. The thermal quenching of T5 can frequency be observed in SI GaAs grown by the vertical gradient freeze (VGF) technique, or by the liquid encapsulated Czochralski (LEC) technique under certain conditions.

Twins in GaAs Crystals Grown by the Vertical Gradient Freeze Technique

AbstractThe growth of facets and the generation of twins on <100> VGF (vertical gradient freeze technique) grown GaAs were investigated using DSL (diluted Sirtl‐like etchant with light) photoetching and transmission X‐ray topography. Due to the polarity of the (111) plane in GaAs, As facets are larger and more irregular than Ga facets and twins always occur on As facets. Twins are initiated at the change of boundary condition which is affected by temperature gradient and crucible shape. The mechanism of twin generation is explained by considering the edge concavity at the solid‐liquid interface and the supercooling required for initial nucleation of a facet. Twins are more often produced in Si‐doped crystals than undoped ones due to the constitutional supercooling.

Growth of Semiinsulating GaAs Crystals by Vertical Gradient Freeze Technique

AbstractGaAs crystals having dislocation densities of 1–2 103 cm−2 were grown using VGF technique. In the grown crystals SiGa is the dominant donor and CAs the dominant acceptor. Theoretical and experimental investigations have shown the possibilities to influence on the silicon and carbon content in GaAs. Based on these results, semiinsulating properties in the crystals could be achieved reproducibly.

Growth and characterization of 2 in InP crystals by the vertical gradient freeze technique

Using the example of a {100};, 2 in InP crystal grown with the vertical gradient freeze (VGF) technique, the advantages of this method are demonstrated. It shows the possibility of growing more uniform {;100}; InP, with lower etch pit density (EPD) compared with material which is grown using the liquid encapsulated Czochralski (LEC) technique. The VGF growth was performed in a cylindrical pyrolytic boron nitride (pBN) crucible, without a cone region. A 2 in diameter, LEC-grown crystal was prepared as a seed. The interface between the seed and the grown VGF crystal exhibited the main property changes between LEC- and VGF-grown material. At the seeding interphase, a reduction of half an order of magnitude in EPD was seen, along with the disappearance of the striations which are characteristic of LEC-grown material. Photoluminescence intensity mapping over a {;100}; VGF wafer showed a variation of about 1.8% with no detectable striations.

Undoped GaAs grown by the vertical gradient freeze method: growth and properties

Undoped GaAs crystals were grown by the vertical gradient freeze technique in a multizone furnace. By optimization of the growth equipment and the thermodynamic conditions, crystals were obtained with a low density of microdefects and dislocations (etch pits density about 10 3 cm −2) as well as semi-insulating behaviour. Residual impurities and defect levels were investigated by optical (local vibrational-mode and IR absorption) and Hall effect measurements.

Characterization of VGF melt-grown YBCO

In melt-textured YBaCuO (YBCO), prepared by the vertical gradient freeze (VGF) technique, the influence of a significantly larger cooling rate through the peritectic reaction was investigated. A three times faster cooling rate results mainly in a larger amount of cracks and in a size reduction of the textured domains. As in the case of smaller cooling rates, however, the c-axis is found to be almost parallel to the external temperature gradient.

Vertical gradient-freeze growth of aluminate crystals

Single crystals of LaAlO3 and DyAIO3 up to 3 cm in diameter and crystals of NdAIO3 up to 5 cm in diameter have been grown by the vertical gradient-freeze (VGF) technique in sealed tungsten crucibles. The LaAlO3 crystals are water white, while Czochralski-grown LaAIO3 crystals are generally light yellow or tan. The LaAIO3 and NdAIO3 crystals are highly twinned, but the DyAlO3 crystals are twin-free. An effort to grow single crystals of Sr2AlTaO6 and Sr2AlNbO6 by the VGF technique has not been successful, because spontaneously nucleated crystallites tend to propagate upward along the length of the boule without much change in their lateral dimensions, forming a very irregular assembly of rod-like grains.

The growth of a cubic, single phase, Cd0.6Mn0.4Te single crystal by the vertical gradient freeze method

The growth of CdMnTe single crystals faces some difficulties due to the phase transition in the solid state at high temperatures, just after solidification. To overcome this problem, it has been suggested to grow the crystals from a Te-rich solution by the traveling heater method at a lower temperature than that of growth from the melt, or alternatively to grow the crystal from the melt at high axial thermal gradient near the interface. In this paper, we present for the first time the growth of cubic, single phase, Cd x Mn 1− x Te single crystals by the vertical gradient freeze technique under a very low axial thermal gradient in the melt (3°C/cm) and high manganese content ( x ≈ 40%). The solidification stage is characterized by a rapid cooling of the crystal through the phase transition temperature region, followed by annealing at about 800°C. The crystal exhibits a very high crystalline perfection, as measured by the double-crystal X-ray rocking curves with a full width at half maximum of 30 arc sec and low optical losses of about 1 dB/mm between 650 and 820 nm with λ-cutoff at 630 nm. Further magneto-optical properties are currently examined for a possible application in devices.

Thermal stress theory of dislocation reduction in the vertical gradient freeze (VGF) growth of GaAs and InP

We have developed the quasi-steady state heat transfer/thermal stress model for dislocation generation in the vertical gradient freeze growth (VGF) of GaAs and InP. Dislocation density contour maps for 2 to 4 inch diameter boules reveal a dramatic reduction in dislocation density for both compounds grown by VGF compared to the standard LEC process. For example, we have determined that for 3 inch diameter undoped GaAs grown by VGF the defect density at the edge of the wafer is reduced by at least a factor of six compared to undoped or even In-alloyed LEC material. The theoretical calculations are in satisfactory agreement with etch-pit density (EPD) data for both compunds. In particular, at a temperature gradient of −5 K/cm the calculated results track the EPD counts in both the 〈100〉 and 〈110〉 directions for 3 inch diameter undoped GaAs grown by the VGF technique.

Growth of 2 inch Ge: Ga crystals by the dynamical vertical gradient freeze process and its numerical modelling including transient segregation

A process model of the dynamical vertical gradient freeze technique for heat transfer and segregation is introduced. Model predictions for favorable growth conditions are verified in a new specially designed multizone furnace by 2 inch Ge: Ga crystal growth experiments. The controlled establishment of a convex solid-liquid interface shape is demonstrated. Measured dopant distribution shows a good correlation with the results of the segregation model.

A comparison of reducing the dislocation densities in GaAs grown by the vertical gradient freeze and liquid-encapsulated Czochralski techniques

We have recently applied the quasi-steady state heat transfer/thermal stress model for dislocation generation to the vertical gradient freeze (VGF) process for GaAs, permitting a direct comparison with the original treatment of liquid-encapsulated Czochralski (LEC) growth. Very recent high temperature critical resolved shear stress (CRSS) data on undoped VGF and In-doped LEC specimens were used. We show that the ∼threefold increase in CRSS with In is sufficient to inhibit defect formation in the central ∼75% of 3 in. diameter LEC wafers grown in a high ambient temperature gradient, duplicating the etch-pit density (EPD) data. Undoped VGF wafers are predicted to be nearly dislocation-free. The theoretical results on 3 in. material track the low EPD counts in both the 〈100〉 and 〈110〉 directions in a 5 K/cm gradient imposed on the crystals’ surface. We also discuss the origin of dislocations in regions free of thermal stresses and propose their suppression by the addition of a small amount of In in VGF experiments.

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.

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.

X-ray diffraction with a four-reflection monochromator

A formula is derived for simulating rocking-curve measurements made with an X-ray diffractometer fitted with a four-reflection monochromator. The derivation is carried out both graphically and algebraically. Results of a simulation using this formula are then compared with experimentally obtained rocking curves. The rocking curves were obtained using a diffractometer with a four-reflection monochromator that uses 440 reflections from two channel-cut germanium crystals. The experimental data comprise 200, 400, 600, 511, 711, 622, 422 and 444 reflections from thick single-crystal indium phosphide grown by the vertical-gradient freeze technique. The simulated data correlate well with the experimental data, although the simulations consistently show somewhat higher reflectivities and narrower linewidths than the experiment, indicating the existence of broadening mechanisms not included in the simulation that are affecting the experiment.