Structural Defects in Mg-Doped GaN: TEM Analysis

Abstract

Crystallographic defects can seriously affect the electronic properties of p-type GaN. This chapter describes the structural analysis of such defects using transmission electron microscopy (TEM). First, the structure of pyramidal inversion domains (PIDs) in heavily magnesium (Mg)-doped GaN epitaxial layers is described. Atomic resolution TEM shows that Mg atoms segregate to form an atomic layer at the (0001) boundary and substitute 1/4 of Ga atoms in the neighboring Ga layers. This structure agrees with that proposed from the first-principles calculations, and indicates that the segregated Mg atoms are electrically inactive. Moreover, the estimated concentration of segregated Mg indicates that the Mg segregation plays a key role in free-carrier reduction in heavily Mg-doped GaN. Second, we describe the defects in Mg ion-implanted and annealed GaN. The 1573-K annealing produces defects with Mg segregated boundary, whose structure is the same as that of PIDs. Moreover, the 1753-K annealing dissociates the Mg segregation and forms vacancy-type defects. Formation of the vacancy-type defects likely reduces the concentration of vacancies, which compensate Mg acceptors. This explains the significant increase in hole-concentrations in Mg ion-implanted GaN by annealing at elevated temperatures. Thus, atomic resolution TEM allows determining defect structures and locations of dopants in extended defects.