Deep defects in n-type high-purity germanium: quantification of optical variants of deep level transient spectroscopy

Abstract

The characterization of high-purity (HP) Ge for the fabrication of γ-ray detectors poses very specific demands due to the high degree of purity of the material (shallow concentration of the order 109–1010cm−3). Deep level transient spectroscopy (DLTS) may still be applied to this kind of material since the sensitivity is relative to the shallow doping concentration. In contrast with p-type HP Ge which was characterized extensively in the 1980s, very little is known about deep defects in n-type HP Ge. Two optical variants of DLTS have been applied to n-type HP Ge and quantified for the first time. Several deep minority carrier traps are detected and identified as mainly Cu-related traps with concentrations in the 106–108cm−3 range. These Cu-related traps, which are well known as the majority carrier traps appearing in typical p-type HP Ge, are thus present as minority carrier traps in typical n-type HP Ge. The conclusion that deep-level defects in n- and p-type HP Ge are very similar could be expected from the similarity in growing conditions for the two types of materials. In the first DLTS variant, known as optical DLTS or ODLTS the deep levels are filled by optical injection (with light of above bandgap energy) at the back ohmic contact of a reverse biased diode. The spectrum is generated by the capacitance transients following the optical excitation. In the second variant, known as photo induced (Current) transient spectroscopy or PI(C)TS the deep levels are also filled optically with intrinsic light, but here a neutral structure is used with two ohmic contacts in sandwich configuration. The spectrum is generated by current transients instead of capacitance transients. This method is especially suited for high-resistivity or semi-insulating materials which cannot be measured with capacitance-based DLTS. PICTS was applied to n-type Ge with a shallow concentration as low as 109cm−3.