Vertical Gradient Freeze Technique Research Articles

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.

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.

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.

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.

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.