Beryllium Doping in MBE-grown GaAs and AlGaAs
- PDF / 267,991 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 31 Downloads / 189 Views
BERYLLIUM DOPING IN MBE-GROWN GaAs AND AlGaAs Joseph Pellegrino, National Institute of Standards and Technology, Gaithersburg, Md.; James Griffin, Leary Myers, and Michael Spencer, Howard University, Materials Science Research Center of Excellence, Washington, D.C. ABSTRACT Beryllium is an effective p-dopant in GaAs and AlGaAs and plays an important role in device characterizations of hetero bipolar transistors. This work addresses the doping and mobility properties for two series of beryllium-doped samples: GaAs and AlGaAs. Within each series the doping ranged between 3X10 15 cm- 3 to levels of 5x101 9 cm- 3 . Mobility and carrier concentrations were obtained through Hall and Polaron measurements. The doping concentration results suggest the onset of carrier compensation at higher doping levels. One possible explanation is that for high doping levels, Be is incorporated as interstitial donors. A thermodynamic model is used to explain the observations. INTRODUCTION AND THEORY Heavily doped p-type GaAs and AlGaAs have important technological applications. These include heterojunction bipolar transistors and heterostructure lasers. In this paper we characterize the electrical behavior of heavily doped p-type GaAs and Al1 Gal- As and to understand how this behavior can be correlated with the various mechanisms of beryllium incorporation in the gallium arsenide and aluminum gallium arsenide systems. Previous studies of beryllium incorporation in GaAs and AlGaAs have shown the role of interstitial Be at high doping levels('-5).
Kirchner'
et al.
used a thermodynamic model for highly n-doped GaAs where the ratio of acceptor to donor concentrations was determined by the Fermi level during growth. We investigate the applicability of a similar model for the case of heavy beryllium-doped GaAs and Al.Gal1 -As. Beryllium can be an acceptor in GaAs if it occupies a gallium site. But beryllium can also behave as a donor if it is incorporated into an interstitial site. At equilibrium the ratio of ionized acceptors to donors is given by: NA/ND = K(T)[P(A.(,))]( 1 /2 )][Ni] 2 /[P]2
[1]
where K(T) is the temperature dependent rate constant. [Ni/P] is the carrier concentration ratio; Ni, the intrinsic carrier concentration which can be expressed as (NcN,,)("/ 2 )exp(-Eg/2KT); N,, effective density of states in the conduction band; and N5 , effective density of states in the valence band. P is the free hole concentration. Thus, P is proportional to a term which varies as exp(-E 9 /2KT) where E. = energy gap and T = temperature. EXPERIMENTAL Molecular-beam-epitaxial GaAs and Al.Gal-.As doped with beryllium is grown with elemental sources at a growth rate of lym/h using (100) GaAs substrates. The
Mat. Res. Soc. Symp. Proc. Vol. 163.
1990 Materials Research Society
882
MBE system is a Varian Gen 1.5.* During growth the pressure in the growth chamber is approximately 10-8 Torr. The GaAs is grown at a substrate temperature of 580 °C, and AlGaAs at 680 'C unless otherwise stated. The material is electrically characterized using Pol
Data Loading...
