Current Issues of High-Pressure Bridgman Growth of Semi-Insulating CdZnTe
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applications. The applications range from nuclear diagnostics, digital radiography, highX-ray and y-ray imaging, industrial web gauging and nuclear resolution astrophysical 2 nonproliferation.'These crystals offer a remarkable combination of electrical properties, notably sufficiently high electrical resistivity and acceptable carrier transport properties. In contrast to SI CdTe crystals grown by conventional Bridgman technique3 '4 or the traveling heater method5 , the HPB CdZnTe crystals also show improved long term stability and much better polarization properties. Indeed, the introduction of Zn into the CdTe matrix strengthens the lattice due to the shorter bond length (0.2643 nm in ZnTe vs. 0.2794 nm in CdTe), less ionicity (0.49 for ZnTe vs. 0.56 for CdTe), and higher binding energy (-4.7 eV for ZnTe vs. -4.3 eV for CdTe). The stronger lattice and the randomness introduced by the Zn atoms slow down the long-range diffusion of native defects and impurities in the ternary compound, that is often associated with the slow deterioration of CdTe radiation detectors over long periods of time. The introduction of Zn also widens the band gap of the material increasing the intrinsic resistivity and shifts the characteristic temperature of polarization phenomena from around room temperature in CdTe to the -50°C to 70 *C temperature range for Cd0.gZno. 0 Te. The typical electrical properties of HPB SI CdZnTe crystals used in radiation detector applications are listed in Table I. These crystals are commonly grown with 10% Zn content. Since the segregation coefficient of Zn is greater than 1, the first to solidify section of the ingot 309
Mat. Res. Soc. Symp. Proc. Vol. 484 ©1998 Materials Research Society
(tip) contains Zn in excess of 10%. The Zn concentration gradually decreases along the growth axis of the ingot and drops well below 10% in the last to solidify section (heel). Because of this long-range Zn composition variation a range of characteristic physical properties exists within the same CdZnTe ingot as indicated in Table I. Table I. Typical properties of HPB-grown SI CdZnTe crystals. Property Nominal Zn composition (%) 10 Band gap at room temperature (eV) 1.56 Intrinsic resistivity (x 10'0 facm) 3.0 j.tor(x103 cm 2/V) 2.5 tt
5
2
I- h (x10- cm /V)
2.0
Actual 5 -13 1.53 - 1.58 1.7 -4.0 0.5 - 5.0 0.2 - 5.0
The remarkable electrical properties of HPB SI CdZnTe crystals, however, are at present accompanied with rather poor crystalline perfection and unfavorable mechanical properties. The ingots are prone to macroscopic cracking and are often fragile. The crystals contain significant concentrations of large defects such as hollow pipes, grain boundaries, twins and Te rich inclusions and Te precipitates. In addition to the long-range Zn distribution, short-range Zn concentration variations are also often observed in the material. Most of the observed macro defects have significant influence on the electrical properties of the material and limit the yield of homogeneous detector crystals from the ingots. In add
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