R.N. Thomas

Low dislocation, semi-insulating In-doped GaAs crystals

Abstract Elemental indium doping of GaAs melts to a concentration in the 10 20 cm -3 range was found to be highly effective in reducing dislocation densities in large diameter GaAs crystals grown by the high-pressure liquid encapsulated Czochralski technique. Nominally 50 mm diameter 〈 100 〉-grown crystals exhibit dislocation densities of less than 500 cm -2 over 80% of the central crystal diameter compared to densities greater than 10 4 cm -2 in undoped GaAs crystals. In other respects, In-doped GaAs grown from stoichiometric or slightly As-rich melts are indistinguishable from undoped GaAs, showing stable resistivities in the 10 7 to 10 8 ohm cm range, measured mobilities approaching 5000 cm 2 /V·s and only slightly modified 29 Si implantation characteristics.

Compensation of residual boron impurities in extrinsic indium-doped silicon by neutron transmutation of silicon

Infrared‐sensitive focal plane arrays based on extrinsic silicon, which integrate the detection and signal‐processing functions onto a single chip, are currently being developed at several laboratories. For imaging in the 3–5‐μm atmospheric window, highly doped Si : In is a leading candidate due to its spectral range, quantum efficiency, and moderate cooling requirements (50–60 K). The effects of residual boron impurities in the Si : In detector must, however, be compensated by donor concentrations to achieve these operational temperatures, so that precision compensation is a key factor for the production of uniform high‐responsivity detector material. We report here the successful use of thermal‐neutron irradiation for transmuting a small fraction of the silicon atoms into a known concentration of phosphorus donors in order to compensate Si : In detector material. Czochralski‐grown Si : In starting material of 〈100〉 orientation was evaluated by variable‐temperature Hall effect studies to contain NIn=2.5×...

ZnGeP2 grown by the liquid encapsulated Czochralski method

The growth of ZnGeP2 by the liquid encapsulated Czochralski method is reported for the first time herein. Large boules of ZnGeP2, with diameters up to 40 mm and weights up to 400 gm were grown by Czochralski pulling from B2O3 encapsulated melts under high pressure (20 atm Ar) using axial gradients ≤120 °C/cm. Boules pulled at ≤4 mm/h exhibited large (50×20×15 mm3) monocrystalline grains of α‐phase ZnGeP2 with room temperature electrical properties of p‐type conduction, carrier concentrations ranging from 1012 to 1016 cm−3, and mobilities of 20 cm2/V s or less. Optical samples exhibited broad IR transmission (0.7 to 12.5 μm), second harmonic generation at 4.7 μm with 7.2% conversion efficiency, a broad subband gap photoluminescence signature, and near band‐edge absorption similar to that observed in Bridgman‐grown ZnGeP2.

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 FETs. It has also been established that LEC crystals can provide the large-area, round

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

Abstract 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 FETs 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.

Growth and properties of large-diameter indium lattice-hardened GaAs crystals

The liquid-encapsulated Czochralski (LEC) growth of In-doped GaAs from 3 kg melts in a Melbourn high-pressure puller has resulted in low-dislocation, large-diameter crystals. Post-growth boule annealing at 950°C for 18 h is found to be an effective stress-relief treatment. This allows high wafer yields, comparable to those from undoped GaAs crystals to be obtained from In-doped boules, with the added advantage of greatly improved uniformity in electrical properties. These substrates are semi-insulating, thermally stable (ρ ≥ 107 ω cm and μH ⩾ 5000 cm2/V · s with a ± 10% or better radial uniformity), and contain low residual impurity concentrations ( ⩽ mid-1015 cm-3) as determined by secondary ion mass spectroscopic (SIMS) analysis. In this study the effectiveness of various concentrations of indium in reducing the dislocation density in GaAs have been explored. A comparison of the thoretically calculated thermal stress experienced by a crystal during LEC growth, with observed reductions in dislocation etch pitch density, indicate an apparent 28-fold increase in the critically resolved shear stress (CRSS) of In-doped over undoped GaAs for 8 x 1019 cm-3 In in the solid. Polished substrates obtained from these crystals show minimal subsurface damage, believed to be related to the increased hardness of this material, and are now approaching high-quality silicon wafers in this respect.

Effects of indium lattice hardening upon the growth and structural properties of large-diameter, semi-insulating GaAs crystals

The high‐pressure liquid encapsulated Czochralski growth of indium lattice‐hardened GaAs, from 3 kg melts, has resulted in low‐dislocation, large‐diameter crystals which exhibit thermally stable, semi‐insulating properties. Post‐growth boule annealing is found to be an effective stress‐relief treatment, which assures high wafer yields and extremely uniform electrical properties. Observed reductions in dislocation density for mid 1019 cm−3 In‐doped GaAs substrates indicate an apparent 28‐fold increase in the critically resolved shear stress of this material over undoped GaAs near the melting point. Polished substrates obtained from these crystals exhibit very little subsurface damage, approaching high‐quality silicon wafers in this respect.

Meeting device needs through melt growth of large-diameter elemental and compound semiconductors*

High-quality, large-diameter semiconductor wafers are required by the device engineer because of the well-known yield advantages of large-area wafer processing. Yet the growth of large semiconductor single crystals with high compositional purity, low concentrations of stoichiometric and point defects, and high crystalline perfection becomes increasingly difficult as one progresses from elemental Si and Ge, through the III-V compounds to the II-VI compounds. Limitations are imposed by the fundamental thermophysical constants of latent heat, thermal conductivity, and critical resolved shear stress and present significant challenges to the crystal grower. This presentation reviews progress made in the melt growth of these semiconductors as large-diameter crystals.

Impurities in commercial-scale magnetic czochralski silicon: Axial versus transverse magnetic fields

Abstract Magnetic Czochralski (MCZ) silicon crystals, 4 inch diameter, were grown in a HAMCO CG-2000 puller, modified to accommodate either a 5000 G superconducting axial field magnet or a 1500 G transverse field electromagnet. The effect of field strength, crystal and crucible rotation rates on the oxygen concentration and distribution are reported for both field configurations. Results for the axial case demonstrate that the oxygen concentration increases in field strength and crystal rotation rate, while crucible rotation rate has only a small effect. In the case of transverse field, it is shown that the oxygen concentration decreases with increase in field strength and decrease in crucible rotation rate. Crystal rotation rate has negligible effect. The macroscopic radial distribution of oxygen using the transverse field is very uniform under most conditions, compared to the axial field and is comparable to the zero field case. While the application of axial field causes a dramatic increase in the effective segregation coefficient of gallium, no such effects were observed (at least for the low fields used) in the segregation behavior of phosphorous in the transverse field. Probable causes for the various observed differences between the two field configurations are discussed.

Czochralski growth of CdTe and CdMnTe from liquid encapsulated melts

CdTe and CdMnTe boules with diameters up to 50 mm and weighing up to 1 kg have been grown by liquid-encapsulated Czochralski (LEC) pulling from B2O3-encapsulated melts. The severe heat-transfer problems associated with the low thermal conductivity of CdTe were circumvented by suitable modification of a high-pressure Melbourn puller to accomodate > 400°C/cm axial gradients, convex radial temperature distributions at the encapsulant/melt interface, and helium overpressures of 75 atm. LEC CdTe boules were always characterized by a high incidence of twinning and polycrystallinity. However, by alloying with manganese at compositions up to 0.20 mole fraction MnTe, growth proceeding by means of alternating bands or lamellas perpendicular to the 〈111〉 growth axis was achieved. The lamellae consisted of twinned {111} sections of crystal corresponding to twinning on the (111) plane perpendicular to the pulling direction. These crystals exhibited semiinsulator behavior, dislocation densities in the 105 to 106 cm-2 range, and subgrain misorientations within the twin lamellae of 300 arc sec to 1° from the 〈111〉 growth direction. The major chemical impurity incorporated in these growths was boron in the form of electrically inactive inclusions presumably associated with the encapsulant. Preliminary attempts to control melt convection by the application of magnetic fields to LEC CdTe and CdMnTe melts indicate that field intensities much greater than 2000 G will be required.