G.W. Eldridge

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.

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.

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.

Large Diameter, Undoped Semi-Insulating GaAs for High Mobility Direct Ion Implanted FET Technology

The growth of 2 and 3 inch diameter, oriented semi-insulating GaAs crystals of improved purity by liquid encapsulated Czochralski (LEC) growth from silicon-free, pyrolytic boron nitride (PBN) crucibles in a high pressure Melbourn crystal puller, is described. Undoped and Cr-doped LEC GaAs crystals pulled from PBN crucibles exhibit bulk resistivities in the 107–108 and 108–109 ohm-cm ranges, respectively. High sensitivity SIMS demonstrates that GaAs crystals grown from PBN crucibles contain residual silicon concentrations in the low 1015 cm-3 range, compared to concentrations up to the 1016 cm-3 range for growths in silica containers. The residual chromium content in LEC/PBN-grown crystals is below the l05 cm-3 range.