Deep Level Defects in CdTe
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DEEP LEVEL DEFECTS IN CdTe
V. B. LEIGH AND R. E. KREMER Oregon Graduate Center,
19600 NWVon Neumann Dr., Beaverton, OR
97006
ABSTRACT We
levels
have
in
used
CdTe
two
single
complementary
crystals.
techniques
Samples
of
to study deep trapping
undoped, semi-insulating
material and p-type material doped with phosphorus or cesium have been Both capacitance techniques. examined using transient spectroscopic and photocurrent transients (PITS) have been measured. transients (DLTS) The DLTS measurements showed several trapping levels in all of the specimens, while the PITS data usually revealed only a single level. INTRODUCTION CdTe is a material that has shown promise for a wide variety of Among these are use in gamma ray detection, solar cells, applications. For and, more recently, as a substrate for the epitaxial growth of HgCdTe. any of these applications, material that is as defect-free as possible is the CdTe is to be used as a required. This is especially important if as defects in the substrate tend to substrate for epitaxial growth, propagate up into the epitaxial layer during the growth. Knowledge of what then, is defects are present in the material, and to what they are due, necessary before they can be controlled and eliminated. In CdTe, as in most wide gap II-VI compounds, the defect structure is such as Cd or Te strongly dependent on the chemistry of native defects, vacancies. The concentration of these native defects can be affected by the activity of Cd or Te during growth or some postgrowth anneal process 11,21. The position of the energy levels corresponding to these defects and their relative concentrations determine basic material properties such as the conductivity type, and properties such as absorption spectra, or the sample resistivity. For wide bandgap semiconductors such as CdTe, the concentration of shallow and deep levels are often found to be related. This complicates the identification of an electron or hole trap as being either impurity or native defect related, since either impurities or native defects may act as shallow levels in CdTe. For example, a change in the concentration of certain deep levels may be observed as CdTe is annealed in various Te or Cd the But this change can be accomplished in two ways: overpressures. anneal can change the native defect concentration in the material, or the shallow impurity can be changed due to dopant out-diffusion during the anneal. The actual case may even be a combination of the effects. We have used the complementary techniques of deep level transient spectroscopy (DLTS) and photo-induced current transient spectroscopy (PITS) to study trapping levels due to defects and impurities that lie deep in the The samples examined have come from undoped, band of the material. semi-insulating ingots, and from p-type ingots doped with phosphorus and cesium. In all cases, the ingots were grown in quartz ampoules, using the vertical Bridgman process. The samples were not annealed, to ensure that the same Cd and Te activity was present for all
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