Room-temperature detection of mobile impurities in compound semiconductors by transient ion drift

Abstract

We show that the transient ion drift (TID) method, which is based on recording junction capacitance under constant reverse bias [A. Zamouche, T. Heiser, and A. Mesli, Appl. Phys. Lett. 66, 631 (1995)], can be used not only for measurements of the diffusion coefficient of mobile impurities, but also to estimate the concentration of mobile species as part of the total dopant density. This is illustrated for CdTe, contaminated by Cu, and intentionally doped by Li or Ag and for CuInSe2. We show also that, with some restrictions, the TID method can be used if the mobile ions are major dopants. This is demonstrated using Schottky barriers on CdTe, and p–n junction devices in (Hg,Cd)Te, and CuInSe2. The values that we obtain for the diffusion coefficients (for Li, Ag, and Cu in CdTe and for Cu in CuInSe2) agree well with measured or extrapolated values, obtained by other methods, as reported in the literature. Furthermore, we could distinguish between diffusion and chemical reactions of dopants, as demonstrated for the case of Cu in CdTe and Ag-doped (Hg,Cd)Te. In the former case this allows us to separate copper-free from contaminated CdTe samples.

Identification of Ag-acceptors in 111Ag/111Cd doped ZnTe and CdTe

“…Normalising the data to other lines in the PL spectrum yields similar results, but with a larger scatter of the extracted intensities. In accordance with the literature [3,14], the diffusion occurring during the annealing process resulted in a low concentration of 111 Ag, which is concluded from the low intensities of the Ag-related lines. They are not stronger than the lines related to the Li and Cu impurities, which are usually present in the order of 10 15 cm -3 .…”

“…In order to remove implantation-induced damage, the ZnTe crystals were annealed at 700 K for 30 min in a Te atmosphere and the CdTe crystals were annealed at 620 K in vacuum. Since Ag is a fast diffuser in both ZnTe and CdTe [3,14], the diffusion occurring during this annealing process resulted in a broad depth distribution and, as a consequence, in a low concentration of the 111 Ag atoms.…”

Anomalous Diffusion Profiles of Ag in CdTe due to Chemical Self-Diffusion

“…2(b) and 2(d) showed that a high V Cd concentration drastically reduces the diffusivity of Ag as is reflected by the large fraction of the Ag atoms remaining within a layer of about 20 m below the surface. Also the results for the undoped CdTe obtained by Lyubomirsky et al [15] and the p-type CdTe investigated by Kovalets et al [16] may be well understandable in view of the presence of a high and a low concentration of V Cd vacancies, respectively.…”

Polycrystalline Thin-Film Cadmium Telluride Solar Cells Fabricated by Electrodeposition; Final Technical Report, 20 March 1995-15 June 1998

“…There is evidence that Cu diffused from the ZnTe dopes the CdTe. It was shown that the Cu presence is essential not only for the small initial series resistance but also for the degradation (see sec.4.2) The Cu diffusion constant in single crystal CdTe is very high, about 3x10 -12 cm 2 /s [29] at room temperature. It is believed that the high diffusivity is due to the motion of interstitial Cu.…”

“…According to [29] the diffusion coefficient of Cu, D Cu , in single crystal CdTe at T=300 K is about 3x10 C should be significantly higher, suggesting that the Cu concentration gradient in the CdTe adjacent to the ZnTe becomes small in a short time and no considerable additional diffusion of Cu into CdTe can take place in the following tens and hundreds of hours. Thus it is hard to expect a change in Cu concentration in the vicinity of the CdTe/ZnTe interface which can influence considerably the Schottky barrier properties.…”