H.M. Hobgood

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

4H-SiC MESFET's with 42 GHz f/sub max/

We report for the first time the development of state-of-the-art SiC MESFETs on high-resistivity 4H-SiC substrates. 0.5 /spl mu/m gate MESFETs in this material show a new record high f/sub max/ of 42 GHz and RF gain of 5.1 dB at 20 GHz. These devices also show simultaneously high drain current, and gate-drain breakdown voltage of 500 mA/mm, and 100 V, respectively showing their potential for RF power applications.

DEEP LEVEL TRANSIENT SPECTROSCOPIC AND HALL EFFECT INVESTIGATION OF THE POSITION OF THE VANADIUM ACCEPTOR LEVEL IN 4H AND 6H SIC

Hall effect, deep level transient spectroscopy (DLTS) and optical absorption measurements were employed in concert to determine the position of the vanadium acceptor level in vanadium and nitrogen doped 6H and 4H SiC. Hall effect results indicate that the acceptor position in 4H SiC is at 0.80 eV beneath the conduction band edge, and 0.66 eV for the 6H polytype. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 0.80 eV and a capture cross section of 1.8×10−16 cm−2. The optical absorption measurements proved that the levels investigated are related to isolated vanadium, and therefore the vanadium acceptor level. Based on the DLTS measurements and secondary ion mass spectroscopy data, the maximum solubility of vanadium in SiC was determined to be 3.0×1017 cm−3. At these incorporation limits and with the depth of the level, the vanadium acceptor level could be used in the creation of semi‐insulating silicon carbide.

RF performance of SiC MESFET's on high resistivity substrates

State-of-the art SiC MESFETs showing a record high f/sub max/ of 26 GHz and RF gain of 8.5 dB at 10 GHz are described in this paper. These results were obtained by using high-resistivity SiC substrates for the first time to minimize substrate parasitics. The fabrication and characterization of these devices are discussed.<<ETX>>

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.

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.

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.