Merits and demerits of (Cd,Mn)Te crystals will be compared with those, well known, of Cd,Te and (Cd,Zn)Te, and the differences in the crystal growth technologies, suitable for each type of crystals, will be discussed.
It is known that the presence of grain boundaries and twinning planes plagues the CdTe and (Cd,Zn)Te crystals. For (Cd,Mn)Te the tendency to form those defects is significantly smaller and large (e.g. 40 × 40 × 3 mm3 ), monocrystalline plates can be obtained.
In the (Cd,Zn)Te crystals the segregation of Zn occures for all the growth methods. While the segregation of Mn does occur during the growth of (Cd,Mn)Te by the travelling heater method (THM), it does NOT occur for the Bridgman method.
All types of crystals - (Cd, Mn)Te, CdTe and (Cd,Zn)Te always contain tellurium precipitates (the size below 1 µm) and inclusions (the size between 1µm and 50µm). Analyses show that inside those objects the concentrations of intentional and unintentional impurities are larger than in the surrounding crystal.
Removal of a significant part of precipitates and inclusions is performed by various annealings, which are applied to the crystals, also in order to lower (annealing in the cadmium vapours) the concentration of the cadmium vacancies, which are present, especially in the crystals grown by low-pressure Bridgman method. The cadmium vacancies act as double acceptors with small and medium first and second ionization energy of holes, respectively. Thus, to obtain semiinsulating crystals for detector applications we have to eliminate cadmium vacancies, and, as this is not enough, to add compensating dopants. Usually chlorine, indium, gal, or the transition metal - vanadium are used.
To characterize the manufactured (Cd, Mn)Te crystals from the point of view of the detector applications, and to confront the results with those for CdTe and (Cd,Zn)Te, the following investigations were performed:
- contactless mapping of the resistivity and μτ product
- microscopic visual observation of the precipitates and inclusions with infra-red and visible light
- investigating the surface with scanning electron microscope (SEM)
- measurements of the concentrations of impurities (both intentional and unintentional) with secondary ions mass spectroscopy (SIMS)
- low-temperature photoluminescence (PL) measurements
- low-temperature paramagnetic resonans (EPR) measurements
- low-temperature time-resolved photoluminescence (TRPL) measurements
The results will be shown and discussed. |