Elsevier

Physica B: Condensed Matter

Volumes 308–310, December 2001, Pages 808-811
Physica B: Condensed Matter

Incorporation and thermal stability of defects in highly p-conductive non-stoichiometric GaAs : Be

https://doi.org/10.1016/S0921-4526(01)00809-2Get rights and content

Abstract

Non-stoichiometric GaAs is known to contain a high concentration of native point defects. The dominant defect in the epilayers is the arsenic antisite (AsGa), a deep double donor, which is incorporated at low growth temperatures (commonly between 200°C and 300°C) in molecular beam epitaxy (MBE). Consequently, p-doping of non-stoichiometric GaAs is difficult because large concentrations of acceptors are compensated by the AsGa. Recently, we found that despite this compensation effect we can achieve p-conductive GaAs : Be with almost one order of magnitude higher Be-doping than previously obtained in MBE grown GaAs. The kinetics of dopant incorporation during MBE growth at these low growth temperatures seem to allow pushing the doping concentration further beyond thermal equilibrium. The epilayers, which are about 1 μm thick, are pseudomorphic with a lattice mismatch to the substrate of up to Δc/c=–0.4%. They remain free of structural defects such as dislocations and stacking faults. After annealing at 600oC only the highest doped epilayers show a reduction in the Be concentration although the layers remain ultrahigh p-conductive. The increased incorporation of Be as well as its unusual stability in non-stoichiometric GaAs is likely influenced by the native defects in these layers, double positively charged AsGa defects and probably neutral gallium vacancies (VGa). This novel material which is solely achievable through low-temperature growth may significantly enhance III–V semiconductor applicability due to ultrahigh doping capability with increased thermal stability.

Introduction

Low-temperature (LT) molecular beam epitaxy (MBE)-grown III–V semiconductors have found many applications as highly resistive buffer layers for FETs [1], as radiation hardened layer for satellite technology [2] and in ultrafast opto-electronics [3], [4], [5], [6]. Conductive LT-layers, however, never reached satisfying conduction because of the presence of electrically active, native defects, namely arsenic antisites (AsGa) and gallium vacancies (VGa). The AsGa defects, electrically active as deep double donors, which can be incorporated in concentrations as high as 1020/cm3 [7] compensate p-dopants (commonly Be or C) and also lead to a reduced mobility of the active carriers, usually electrons [8]. N-type conduction was identified to be dominated by nearest-neighbor hopping [9], [10]. P-conduction was observed in highly compensated epilayers with low dopant activation or in the so-called stoichiometric LT-GaAs with Be doping concentrations up to 1019/cm3 only [7], [11].

The addition of Beryllium to a GaAs epilayer leads to a smaller lattice constant for high doping concentrations because the Beryllium which is commonly incorporated on the Ga-sublattice is smaller than the host atom Ga. At low growth temperatures this effect is compensated by the introduction of larger native point defects, the AsGa antisite defects. These defects are also located in the Ga-sublattice and dilate the epilayer lattice if present in large concentrations [7]. The strain compensation between the small BeGa and the larger AsGa defects is expected to thermally stabilize the defects [12]. Even more important than this stabilization is the fact that the maximum concentration of Be which can be incorporated into LT-GaAs epilayers is largely enhanced. In this contribution, the electronic properties of the highly strained epilayers with several 1020/cm3 Be incorporation are investigated. It is studied how the maximum free hole concentration and their thermal stability as well as the stability of the Be dopant atoms depend on the growth conditions.

Section snippets

Experimental

Be-doped LT-GaAs layers were grown by MBE on (1 0 0) GaAs wafers grown by Vertical Gradient Freeze (VGF substrates, AXT, Fremont, CA) at As/Ga beam equivalent pressure ratios of 20 (As-rich conditions) and a growth rate of 1 μm/h. The growth temperature measured by diffuse reflectance spectroscopy (Thermionics NW, Hayward, CA) was varied from 210°C to 300°C. Nominal Be doping levels between 1×1019 and 2×1021/cm3 were attempted. Be concentrations were determined by SIMS analysis at Applied

Results and discussion

The lattice mismatch between the GaAs substrate and the epilayer gives a first indication for the large concentrations of incorporated Be acceptors in LT-GaAs : Be. With increasing Be concentration the epilayer lattice constant decreases resulting in lattice matching to the substrate [15]. For higher doping concentrations the lattice mismatch becomes negative and duplicates approximately the data from GaAs : Be grown at 580°C if extrapolated to high Be concentrations as can be seen in Fig. 1. It

Summary

Ultrahigh p-conductive LT-GaAs : Be epilayers were grown with free hole concentrations as high as 7×1020/cm3. After annealing at 600°C about 2×1020/cm3 holes remain in the epilayers which are highly strained. The effect of ultrahigh Be-doping is fairly growth temperature insensitive within at least a 40°C temperature range, although the maximum amount of free holes is dependent on the growth temperature. A suppressed Be diffusion is observed, which is likely to be caused by residual AsGa antisite

Acknowledgments

This work has been supported by the Air Force Office of Scientific Research under grant no. F49620-98-1-0135. We appreciate the use of the Integrated Materials Laboratories at UC Berkeley. Positron Annihilation Spectroscopy was performed at the Martin-Luther Universität in Halle, Germany. We thank Dr. R. Krause-Rehberg for his support. One of us, J.G., acknowledges a Feodor-Lynen Fellowship of the Alexander von Humboldt Foundation.

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