Pyrolytic Boron Nitride Crucibles Research Articles

Arsenic-rich melt effect on threshold voltage scattering for Si-implanted GaAs metal-semiconductor field-effect transistor

The microscopic electrical activation efficiency of implanted Si atoms in undoped, liquid-encapsulated Czochralski-grown GaAs substrates was investigated by means of the threshold voltage (Vth) of implanted metal-semiconductor field-effect transistors as a function of stoichiometry and impurities, especially of carbon as determined by Fourier transform infrared (FT-IR) spectroscopy (≤2.5×1016 cm−3). The Vth scattering, which is related to the electrical activation efficiency dispersion, was small for an As-rich substrate grown from an As-rich melt, whether or not there existed a horizontal 3400-Oe magnetic field. The smallest amplitude of the Vth scattering occurred for the As-rich melt and did not vary with magnetic field. The Vth scattering with magnetic field was independent of carbon concentration (≤2.5×1016 cm−3). However, without magnetic field, an effect clearly appeared at low carbon concentration (nearly nondetectable). It seems that the Vth scattering or the electrical activation efficiency dispersion is dominated by the dispersion of stoichiometry, that is, composition, of the GaAs substrate because the electrical activation efficiency depends on the ratio of the Ga vacancy concentration and As vacancy concentration and the magnitude of their concentrations at the annealing temperature for the Si-implanted GaAs substrate. Therefore small Vth scattering means that the dispersion of the composition (Ga and As) is small. The composition dispersion for the most As-rich melt is small because the solidification is controlled by the straight As-rich side solidus line of the Ga:As composition-temperature phase diagram, and thus temperature variations of the GaAs melt in the pyrolytic boron nitride crucible are not as important. Also, there was no correlation between Vth scattering and dislocation density.

Residual stress and strain in pyrolytic boron nitride resulting from thermal anisotropy

Residual stresses in chemically deposited pyrolytic boron nitride (PBN) crucibles caused by thermal expansion anisotropy during cooling immediately following deposition are analysed. The calculations reveal that radial tension and combined tangential tension and compression exist in the crucible. The maximum stresses increase with an increase in the thickness of the crucible. Furthermore, while the outer wall of the crucible always shrinks upon cooling, the inner wall may expand, due to the residual stress states, resulting in a negative effective thermal expansion coefficient in the tangential direction. The influence of the PBN attachment to the mandrel on which it is deposited is also considered. Specifically, the radial tensile stress in the crucible is shown to increase due to this attachment, which in turn, enhances the delamination of the crucible.

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.

Thermal detection of electron spin resonance in the persistent photoconductive state of semi-insulating GaAs crystals grown from pyrolytic BN crucibles

Thermally detected electron spin resonance (TDESR) has been applied to paramagnetic defects characteristic of semi-insulating (SI) GaAs crystals grown from pyrolytic boron nitride (pBN) crucibles. A strong isotropic TDESR signal near g=2.2 and two anisotropic lines at higher fields were observed at T=4.2 K and 13 GHz after in situ optical excitation. Spin-dependent scattering of electrons, in their persistent photoconducting state, by paramagnetic defects is discussed as a possible cause of these resonances. Unsuccessful searches for this TDESR signal in semi-insulating GaAs crystals grown under non-pBN ambient conditions point to paramagnetic defects owing to the inclusion of some crucible material, e.g., nitrogen, as the source of the TDESR spectrum in pBN-GaAs.

Identification of a vanadium-related level in liquid encapsulated Czochralski-grown GaAs

We present the first positive identification of a vanadium-related electron trap in V-doped GaAs crystals grown by the liquid encapsulated Czochralski technique in pyrolytic boron nitride crucibles. Detailed deep level transient spectroscopy and capacitance transient analysis yielded a trap energy of 0.15±0.01 eV below the conduction band and an electron capture cross section of about 2×10−14 cm2. Optical absorption and mobility data show that this level corresponds to the ionized acceptor state V2+(3d3) of substitutional vanadium. No midgap levels other than EL2 could be detected in these V-doped crystals showing that doping with vanadium plays no direct role in the compensation process in semi-insulating GaAs crystals.

Quartz versus PBN—The effect of crucible type on undoped LEC GaAS

Undoped semi-insulating GaAs crystals were grown in a low pressure LEC system using quartz and pyrolytic boron nitride (PBN) crucibles. Crystals grown in PBN crucibles are consistently semi-insulating from the seed end to the tail end. Crystals grown in quartz crucibles have lower Hall mobility. Other properties such as dislocation etch-pit of conductive material beginning from the seed end of the crystal, and have lower Hall mobility. Other properties such as dislocation etch-put density, implant profile, and thermal conversion characteristics of the two types of crystal are about the same.

Effects of stoichiometry on thermal stability of undoped, semi-insulating GaAs

We provide independent evidence of the effects of melt composition on the semi-insulating behavior and thermal stability of undoped GaAs crystals pulled from pyrolytic boron nitride crucibles by the liquid-encapsulated Czochralski technique. Semi-insulating crystals pulled from stoichiometric or As-rich melts show excellent stability to heat treatments, in contrast to semi- insulating Ga-rich material which exhibits p-type conversion after prolonged annealing at 860 °C. Deep level transient spectroscopy and Hall-effect measurements suggest that p-conversion is caused by outdiffusion of native As and/or Ga defects associated with the EL2 deep donors, resulting in uncompensated residual acceptors, predominantly carbon impurities at the surface. The measured mobility of undoped, semi-insulating GaAs is found empirically to be a sensitive parameter for qualifying substrates for direct ion implantation.

Boron contamination and precipitation during the growth of InP

We have examined polycrystalline and single crystal, liquid encapsulated Czochralski (LEC) grown InP for evidence of boron contamination using secondary-ion mass spectrometry (SIMS) and photoluminescence. Precipitates of boron or a boron compound have been found in InP grown by the LEC method with boric oxide (B 2O 3) encapsulation and pyrolytic boron nitride (pBN) crucibles. The density of precipatates appears to increase towards the last-to-freeze end of the boules. Neither B 2O 3 or pBN appears to be solely responsible for the observed phenomenon. At present, the source of boron contamination most consistent with our observations is the result of interaction between B 2O 3 and pBN. A diffusion coefficient of D(750°C) < × 10 -14 cm 2 s -1 for boron in InP has been determined in this work.

Growth and characterization of large diameter undoped semi-insulating GaAs for direct ion implanted FET technology

The growth of large diameter, semi-insulating GaAs crystals of improved purity by Liquid Encapsulated Czochralski (LEC) pulling from pyrolytic boron nitride (PBN) crucibles and characterization of this material for direct ion implantation technology, is described. Three-inch diameter, 〈100〉-oriented GaAs crystals have been grown in a high pressure Melbourn crystal puller using 3 kg starting charges synthesized in-situ from 6/9s purity elemental gallium and arsenic. Undoped and Cr-doped LEC GaAs crystals pulled from PBN crucibles exhibit bulk resistivities in the 10 7 and 10 8 Ω cm range, respectively. High sensitivity secondary ion mass spectrometry (SIMS) demonstrates that GaAs crystals grown from PBN crucibles contain residual silicon concentrations in the mid 10 14 cm −3 range, compared to concentrations up to the 10 16 cm −3 range for growths in fused silica containers. The residual chromium content in undoped LEC grown GaAs crystals is below the SIMS detection limit for Cr (4 × 10 14 cm −3). The achievement of direct ion implanted channel layers of near-theoretical mobilities is further evidence of the improved purity of undoped, semi-insulating GaAs prepared by LEC/PBN crucible techniques. Direct implant FET channels with (1–1.5) × 10 17 cm −3 peak donor concentrations exhibit channel mobilities of 4,800–5,000 cm 2/V sec in undoped, semi-insulating GaAs substrates, compared with mobilities ranging from 3,700 to 4,500 cm 2/V sec for various Cr-doped GaAs substrates. The concentration of compensating acceptor impurities in semi-insulating GaAs/PBN substrates is estimated to be 1 × 10 16 cm −3 or less, and permits the implantation of 2 × 10 16 cm −3 channels which exhibit mobilities of 5,700 and 12,000 cm 2/V sec at 298K and 77K, respectively. Discrete power FET's which exhibit 0.7 watts/mm output and 8 dB associated gain at 8 GHz have been fabricated using these directly implanted semi-insulating GaAs substrates.

High-purity semi-insulating GaAs material for monolithic microwave integrated circuits

Liquid-Encapsulated Czochralski (LEC) growth of large-diameter bulk GaAs crystals from pyrolytic boron nitride (PBN) crucibles has been shown to yield high crystal purity, stable high resistivities, and predictable direct ion-implantation characteristics. Undoped (≲low 1014cm-3chromium) and lightly Cr-doped (low 1015cm-3range) -GaAs crystals, synthesized and pulled from PBN crucibles contain residual shallow donor impurities typically in the mid 1014cm-3, exhibit bulk resistivities above 107Ω . cm, and maintain the high sheet resistances required for IC fabrication (>106Ω/□) after implantation anneal. Direct29Si channel implants exhibit uniform (± 5 percent) and predictable LSS profiles, high donor activation (75 percent), and 4800- to 5000-cm2/V . s mobility at the (1 to 1.5) × 1017cm-3peak doping utilized for power FET's. It has also been established that LEC crystals can provide the large-area, round

Monolithic microwave amplifiers formed by ion implantation into LEC gallium arsenide substrates

A fabrication procedure for broad-band monolithic power GaAs integrated circuits has been demonstrated which includes formation of via holes through the 100-µm-thick GaAs substrate. A selective implant of29Si ions into the GaAs substrate is used to dope the FET channel region to 1.2 × 1017cm-3. The ohmic contacts are AuGe/Ni/Pt and the gates are Ti/Pt/Au. A 1.5-µm-thick circuit pattern is achieved using metal rejection assited by chlorobenzene treatment of AZ1350J photoresist. Using undoped Czochralski wafers of GaAs pulled from a pyrolytic boron nitride crucible, integrated amplifiers have been produced which deliver 28 ± 0.7 dBm from 5.7 to 11 GHz. These chips are 2 mm × 4.75 mm × 0.1 mm thick.

Incorporation of boron during the growth of GaAs single crystals

A study of GaAs prepared by conventional Bridgman techniques and by liquid-encapsulated Czochralski methods reveals that only small amounts of boron are incorporated from the boric oxide encapsulant. When a pyrolytic boron nitride crucible is used, there is a 100-fold increase in the amount of incorporated boron, suggesting some decomposition of the boron nitride.

The growth of single crystal lithium-aluminum, 7LiAl

Single crystal lithium-aluminum has been prepared as chemically and isotopically pure 7LiAl. Growth has been by the Stockbarger technique in a low-pressure bomb. Pyrolytic boron nitride crucibles have proven to be nonreactive and noncontaminating containers for pyrometallurgial synthesis and for crystal growth from the melt. The chemical and physical quality of the monocrystalline boules have been documented. The lattice constant varies with composition across the β-phase and can be used as a probe to evaluate stoichiometry and phase homogeneity.

Permeability, magnetomechanical coupling and magnetostriction in grain-oriented rare earth–iron alloys

Grain-oriented samples of highly magnetostrictive rare earth–iron compounds have been prepared. These samples possess lower inhomogeneous strains than found in the random polycrystalline RFe2 compounds, resulting in much higher values of relative permeability (μr) and magnetomechanical coupling (k33). A partially oriented Th.20Dy.22Ho.58Fe1.95 sample was prepared using a pyrolytic Bridgman type boron nitride crucible. At a bias field of 100 Oe k33=.73, which is considerably larger than found in the random polycrystal of the same composition. A relative permeability of 36 occurs in this same sample when a low ac drive of 1.6 Oe rms is used. A second fabrication method using a horizontal zone technique with a supporting ’’cold finger’’ was employed to grow the ternary Tb.27 Dy.73Fe1.98. The low ac drive values of k33 and μr were .74 and 19 respectively. The permeability at low bias was found to possess a sharp ac drive dependence. Near zero bias, when the drive was changed from 1.6 Oe rms to 13 Oe rms, μr in the quaternary increased from 36 to 98. In the ternary the values of μr near zero bias increased from 19 to 61. Magnetostriction measurements on both samples show a significant increase in dλ/dH and λs over the random polycrystals.

Evaluation of Pyrolytic Boron Nitride Crucible Thimble in Carrier Gas Fusion Determinations of Gases in Metals.

Evaluation of Pyrolytic Boron Nitride Crucible Thimble in Carrier Gas Fusion Determinations of Gases in Metals.

New Pair Spectra in Gallium Phosphide

Crystals of GaP doped with Zn+Te, Zn+Se, Cd+Te, and Cd+Se were grown from gallium-rich solutions contained in pyrolytic boron nitride crucibles, using vapor-grown GaP as source material. The low-temperature photoluminescence of these crystals reveals the presence of new pair spectra involving zinc or cadmium on gallium sites and tellurium or selenium on phosphorus sites (type II spectra). The values of (${E}_{A}+{E}_{D}$), the sum of the acceptor and donor binding energies, derived from the spectra are in excellent agreement with the values predicted from previously observed pair spectra. The data on all the previously observed spectra have also been recalculated to yield more consistent values of (${E}_{A}+{E}_{D}$) and the Van der Waals parameter.