D.L. Barrett

Growth of large SiC single crystals

We have grown 6H-polytype SiC single crystal boules up to 60 mm in diameter by the physical vapor transport process at 2300 o C. [0001] oriented substrate wafers prepared from these undoped crystals exhibit resistivities of up to 10 5 Ω cm and etch pit defect densities of 10 4 -10 5 cm -2 . Epitaxially-grown microwave MISFIT structures exhibit 5 GHz cutoff frequency; the highest reported to date

Large diameter 6H-SiC for microwave device applications

6H-polytype SiC single crystals with diameters up to 50 mm and lengths up to 75 mm have been grown in the c-and a-axis directions by physical vapor transport (PVT) at growth rates of 0.25 to 1 mm h -1 . Undoped crystals grown from purified source material reveal residual impurity concentrations in the 10 16 cm -3 range and resistivities up to 1000 Ω-cm. N + crystals with resistivities < 0.02 Ω-cm have been produced by controlled nitrogen doping. PVT-grown SiC crystals are characterized by dislocation densities of 10 4 to 10 5 cm -2 and can also exhibit micropipe defects in the 10 2 to 10 3 cm -2 range

SiC boule growth by sublimation vapor transport

Abstract Silicon carbide is an attractive candidate for high power and high temperature electronics due to its inherent high thermal conductivity, large saturated drift velocity, high breakdown strength and large bandgap. A review of the material properties which influence semiconductor device characteristics is presented, and recent advances in crystal growth technology leading to the preparation of 25 mm and larger wafers for “silicon-like” device fabrication processes are reviewed. A sublimation vapor transport system is described and preliminary results on growth of 6H-SiC boules are presented.

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.

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.

Status of device-qualified GaAs substrate technology for GaAs integrated circuits

A review is presented of the current technical status of large-diameter GaAs crystal growth, the effects of residual impurities, stoichiometric defects and crystalline imperfections on the electrical properties of undoped semi-insulating GaAs, and the effectiveness of Group III and V isovalent, lattice-hardening dopants in yielding dislocation-free, semi-insulating GaAs crystals. Factors related to crystal growth, postgrowth annealing, and the preparation of ultraflat, damage-free GaAs wafers, which can significantly improve the performance and yields of directly implanted devices and monolithic circuits are discussed. >

Chapter 1 High-Purity LEC Growth and Direct Implantation of GaAs for Monolithic Microwave Circuits†

Publisher Summary This chapter discusses the establishment of a reproducible gallium arsenide (GaAs) materials base to realize the full potential of direct ion implantation as a reliable, cost-effective fabrication technology of high-performance GaAs metal–semiconductor field effect transistor (MESFET) devices and integrated circuits (IC). The considerable efforts directed at improving basic GaAs materials and processes result from the strong interdependence of high-frequency GaAs circuit performance upon substrate quality. Many of the conventional wafer preparation techniques used today in silicon have been applied on a laboratory scale to large liquid-encapsulated Czochralski (LEC)-grown GaAs crystals. The low-breakage processing of GaAs demands the development of special handling techniques based on the automated cassette and wafer transport methods that are being utilized in silicon IC manufacturing.

Silicon carbide devices for radiation hard applications

Silicon carbide has long been recognized as a favorable material for applications at high temperatures and in radiation environments, but device development has been hindered by lack of adequate substrates. This paper reviews the current Westinghouse material development effort aimed at the growth of high quality 6H boules and describes 6H SiC devices fabricated on Westinghouse substrates. MESFET and MOSFET transistors were made in a microwave power design layout. The MESFET and MOSFET transistors were subjected to a total gamma irradiation of 1 megaGray (100 megarad) and exhibited threshold voltage shifts of about 0.4 and 1.2 Volts respectively with little change in bulk material parameters.