ABSTRACT
CONTENTS 22.1 Introduction ...................................................................................................................... 671 22.2 Defects in Bulk Crystals.................................................................................................. 672 22.3 Defects in Standard Epitaxial Layers............................................................................ 674 22.4 Defects in Low-Temperature MBE Grown Layers ..................................................... 675 22.5 Defects in Ferromagnetic Mn-Doped Low-Temperature MBE Grown Layers ...... 677 22.6 Conclusions....................................................................................................................... 680 References.................................................................................................................................... 680
GaAs is an important electronic material for high-frequency and optoelectronic applications that are not achievable with Si-based microelectronics. The two main advantages of GaAs over Si are: (a) GaAs has a considerably higher electron mobility, and (b) since GaAs is a direct bandgap material, it allows efficient light emission, a prerequisite for applications in optoelectronics. Both semi-insulating (SI) bulk crystals and highly conductive n-type bulk crystals and n-and p-type epitaxial layers are needed for different applications and can be obtained with specific growth techniques. It is possible to obtain an intrinsic material of high resistivity (larger than 106 Vcm) at room temperature due to the presence of a dominating defect associated with a midgap level that compensates the residual donors and acceptors. In bulk GaAs this defect can be introduced through the control of the growth conditions; it is the so-called EL2 defect, a deep double donor with a first 0=þ ionization state at EC 0.86 eV. In contrast, high conductivities are required in other applications and can be obtained by doping with Si for n-type or Zn for p-type properties. More recently, it has been shown that Mn doping in the atomic% range adds an additional functionality to this material: it gives rise to the formation of ferromagnetic thin films with interesting fundamental properties. In this brief review, we describe the present state of knowledge of the electrically active
point defects in GaAs. Since their natures and concentrations vary with the growth process, we treat separately the following three categories of crystals: bulk crystals grown from the melt, high-temperature epitaxially grown layers, and low-temperature molecular beam epitaxial (MBE) grown thin films. An extensive list of references on defects in GaAs, GaN, and ZnO materials is provided in Appendix A, as well.
