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