The study of HgCdTe MBE-grown structure with ion milling
|Malgorzata M. Pociask|
University of Rzeszow, Institute of Physics, Rejtana 16, Rzeszów 35-310, Poland
Ion milling (IM) is emerging as a unique tool to reveal electrically active defects and complexes at MCT. It is known that IM is capable of strongly affecting electrical properties of MCT, up to conductivity type conversion in p–type material. It appears, that strongly non-equilibrium processes which take place under IM, when material is oversaturated with interstitial mercury atoms HgI generated near the surface, lead to the appearance of specific defect complexes, which may not form under other conditions. These complexes may also comprise intrinsic point and extended defects that normally do not show their presence due to their electrical neutrality or compensation. By measuring the parameters of the crystal before and after the milling, and following the disintegration of defects with time after IM (‘relaxation’), one can detect and identify these defects.
The applying IM for the study of defects in LWIR and MWIR MCT films grown by MBE on GaAs substrates with ZnTe/CdTe buffer layers is reported. The results obtained on the MBE films grown on GaAs are compared to those acquired on the MBE films grown on CdZnTe substrates, and on MCT bulk crystals and films grown by liquid (LPE) and isothermal vapor (ISOVPE) phase epitaxy.
Nominally un-doped MBE films with composition x=0.22 (LWIR) and x=0.30 (MWIR) have been studied, and those in situ doped with an acceptor (As with concentration 1015–1016 cm–3) or a donor (In with concentration 5∙1014–1017 cm‑3), both as-grown and after various kinds of post-growth annealing. The thickness of the MBE films was 7 to 11 μm, and they were protected by ~ 0.3 mm–thick (top) and ~ 1.0 mm–thick (bottom) graded–gap films with x increasing up to ~ 0.5 near the both interfaces. Straight after IM the electron concentration at T=77 K n77(i) in all the MBE films (~1017 cm-3 for LWIR and ~5∙1016 cm-3 for MWIR films) greatly exceeded that in the bulk, LPE and ISOVPE samples.
The n77(i) in the MBE films depended only on x (i.e., growth conditions), and neither on substrate type, nor on the type and concentration of dopants, nor on the terms of post-growth annealing. Since the n77(i) in the MBE films was 1 to 2 orders higher than the concentration of residual/intentionally introduced dopants in the samples. It’s high value could not be explained solely by the interaction of HgI with electrically active species, which is typical for IM. It was concluded that in our MBE films IM led to the formation of a donor center, which resulted from the interaction of HgI with a certain defect that had been electrically neutral before the milling. The analysis showed that the most probable candidate was an initially neutral impurity (e.g., oxygen), or a defect complex involving excessive tellurium atoms. The exact nature of the defect and its relation to the MBE growth conditions will be discussed in details.
Using IM, was established donor and acceptor background doping level in the MBE films and made estimates of their electrical compensation. The electron concentration as measured after the ‘relaxation’ of defects formed under the milling, n77(f), appears to be similar in all the HgCdTe samples, except for those strongly doped with In, ~(1‑4)×1015 cm-3. The other exception was MWIR MBE films, where the n77(f) comprised (4‑7)×1015 cm-3. This feature is related to the MBE growth conditions that allow for a variety of Te-related defects, which will be discussed in the paper. Important is that n77(f) in un-doped samples was much higher than the n77 in the as-grown samples (the latter comprised ~1013 cm-3 in the MWIR films and ~1014 cm-3 in the LWIR films). This testifies to the fact that some of the as-grown samples were heavily compensated for, and the reason of such a high compensation degree will be also discuss.
Presentation: Oral at E-MRS Fall Meeting 2009, Symposium D, by Malgorzata M. Pociask
See On-line Journal of E-MRS Fall Meeting 2009
Submitted: 2009-06-14 09:11 Revised: 2009-08-13 17:30