Development of a vertical gradient freeze process for low EPD GaAs substrates

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Abstract

The main characteristics of low EPD 4 in semi-insulating and Si-doped conductive GaAs single crystals grown by a proprietary low thermal gradient vertical gradient freeze (VGF) technique on an industrial scale are presented.

Introduction

Heterojunction Bipolar Transistor (HBT) and related bi-polar devices are the latest products of the GaAs-technologies utilized by the mainstream industry [1]. The reliability of these minority-type device structures especially at higher temperatures is assumed to be determined by the same degradation mechanisms [2] as for laser and luminescence diodes: non-radiative recombination and defect generation at lattice defects like dislocations and vacancies (so-called dark point defects) accelerated by residual strain in the active layer, high dopant concentrations and precipitates.

Due to this an increasing demand is observed for semi-insulating (SI) and semi-conducting (SC) substrates with a low dislocation density and, correspondingly, a low EPD, which will continue according to recent forecasts in the next years [3]. For laser application low-EPD substrates are even an indisputable requirement.

The low thermal gradient VB/VGF (vertical Bridgman/vertical gradient freeze) growth method is an appropriate method for the commercial production of SI and SC GaAs single crystals as already demonstrated by others [4], [5].

This paper presents typical features of 4 in. (100 mm) low-EPD wafer mass production established by Freiberger during the last years.

Section snippets

Crystal growth and characterization

SI and SC GaAs-single crystals (<100>-seeded 4 in. (100 mm)) are grown by a proprietary VGF-process [6] using cone-shaped pBN crucibles and pre-synthesized polycrystalline charges. A B2O3 liquid encapsulation is applied to suppress a charge-crucible contact harmful for single crystal growth. Carbon is added for semi-insulating and Si for conductive crystals.

The furnace has been designed for crucibles from 3 in. to 150 mm in diameter and up to 360 mm in length. Its construction and the

Structural properties

A typical EPD-topogram of an undoped 4 in. SI GaAs-wafer with EPD=1326 cm−2 and EPD <5000 cm−2 on 95% of the wafer area is shown in Fig. 1. At this EPD-level a cellular distribution can be recognized in the central and peripheral part of the wafer similar to LEC-grown GaAs but with a much greater cell size (>2 mm), partly fragmented cell walls and a lower dislocation density in the boundaries. Between these regions fragments of cells are observed.

The average EPD of 4 in. Si-doped GaAs is

Summary

A VGF growth equipment, the corresponding growth technology and a boule annealing procedure (SI) have been proved to be capable for mass-production of 4 in. (100 mm) state-of-the-art SI and Si-doped SC GaAs crystals.

The production process for 150 mm SI VGF GaAs crystals is under development. Preliminary results regarding EPD and electrical properties are comparable to those of 100 mm VGF GaAs.

Acknowledgements

The authors wish to thank Dr U. Zeimer of the Ferdinand-Braun-Institut fuer Hoechstfrequenztechnik, Berlin, for the double-crystal rocking-curve measurements and Dr R. Bindemann for the critical reading of the manuscript.Financial support by the Bundesministerium fuer Verteidigung and the Bundesministerium fuer Bildung und Forschung (13N7225/2) are gratefully acknowledged.

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