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Published in Zbigniew Galazka, Transparent Semiconducting Oxides, 2020
Etching (100)-oriented crystal samples (in 10 wt.% KOH solution for 90 min at 90°C) of the early crystals revealed that the etch pit density (EPD) in the crystal obtained with much thinner neck (0.16 mm2) was lower as compared to the crystal with a thicker neck (0.63 mm2), 5 × 104 versus 4 × 105 cm−2, respectively. The same relates to the FWHM of the rocking curve, which was 75 and 160 arcsec for the crystals with thinner and thicker neck, respectively. The crystals reported by Mu et al. [266] had the FWHM of the rocking curve of about 43–55 arcsec.
Crystal Structure
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
A line defect can be thought of as a one-dimensional array of point defects, which are out of position in the crystal structure and occur when the crystal is subjected to stresses beyond the elastic limit of the material. These stresses generally arise from thermal and mechanical processing during the growth and detector fabrication process. The energy to create a dislocation is of the order of ~100 eV per mm of dislocation line, whereas it takes only a few eV to form a point defect, which is a few nm in extent. Thus, forming a number of point defects is energetically more favourable than forming a dislocation. Dislocations interact with chemical and other point defects. The presence of a dislocation is usually associated with an enhanced rate of impurity diffusion leading to the formation of diffusion pipes. This effect gives rise to the introduction of trapping states in the bandgap, altering the electrical properties of the devices. In addition, dislocations move when a stress is applied, leading to slip and plastic deformation of the lattice. Note that a line of dislocation cannot end within a crystal unless it forms a complete loop. Generally, it ends at the sample surface, in which case it can be visualized by etching (acids preferentially etch the intersections of dislocations with the crystal surface), infrared, X-ray or electron transmission techniques (see Fig. 3.15). In fact, etching is routinely used to quickly assess material quality in terms of number of dislocations at a surface18 – the result is usually expressed in terms of the Etch Pit Density. Dislocations can also be visualized by decoration (metallic impurities will tend to precipitate on dislocations forming so-called decorated boundaries). Depending on the size of the dislocation, these can be easily observed using optical microscopy.
Semiconductor Detectors
Published in Douglas S. McGregor, J. Kenneth Shultis, Radiation Detection, 2020
Douglas S. McGregor, J. Kenneth Shultis
The introduction of Zn in the growth process of CdTe, nominally between 2% to 15%, has led to the production of CdZnTe detectors.32 Overall, CdZnTe detectors have the same advantages as CdTe detectors with several added benefits. By adding a small amount of ZnTe to the melt, many important semiconductor properties are drastically improved. The band gap of CdZnTe increases with Zn concentration, with band-gap energy of Cd1−xZnxTe at 300 K is well approximated by [Olego et al. 1985] Eg(x)eV=(1.51±0.005)+(0.606±0.01)x+(0.139±0.01)x2eV. The band gap ranges from 1.52 to 1.64 eV for Zn contents ranging from x = 0.02 to x = 0.2; hence the detectors can operate at room temperature without serious leakage current concerns. With an increased band-gap energy, the intrinsic free carrier concentration diminishes, thereby increasing the resistivity while reducing detector leakage current. Butler et al. [1992] report that adding x = 0.2 amount of Zn changes the resistivity of CdTe from 3 × 109 Ω cm to 2.5 × 1011 Ω cm for Cd0.8Zn0.2Te. The dielectric constant is approximately 10.6 for CdZnTe, although this property is also a function of the Zn concentration. The addition of Zn increases the hardness of CdZnTe over CdTe and decreases the dislocation density [Anand 2013]. The etch pit density (EPD) is a measure of the dislocation defect density. Butler et al. [1993] show a marked reduction in the EPD as the Zn content was increased, with the densities of 1.5 × 105 cm−2, 104 cm−2, and 5 × 103 cm−2 for Zn contents of 0, 0.04, and 0.2, respectively. Further, CdZnTe detectors do not exhibit the polarization phenomenon at low irradiation levels, which are often observed with CdTe detectors. However, Wang et al. [2013] report the observation of polarization from CdZnTe detectors under high irradiation conditions. An additional benefit of incorporating Zn is that CdZnTe devices, because of the increased band gap and resistivity, can be operated at higher temperatures than CdTe devices, and they can also resolve lower energy photon energies (x rays and gamma rays) traditionally difficult to observe with CdTe detectors. CdZnTe detectors are presently used in handheld gamma-ray spectrometers and for smaller medical imaging apparatuses.
Unidirectional seeded growth of l -Glutamic acid hydrobromide single crystal and its characterization
Published in Phase Transitions, 2020
M. Senthilkumar, Pramod K. Singh, Vijay Singh, R. Sathyalakshmi, K. Pandiyan, R. K. Karn
The growth analysis and defect pattern can be observed during the crystal growth stage and the observations are recorded in the chemical etching process under optical microscope [26]. The etching process is the removal of top layers from the as-grown samples for a given time period results in the fresh layers of the surface of the crystal available for the analysis. Also, etching analysis is one of the important aspects to analyze the defects like point or line defects and various dislocations in single crystals. Selection of etchants and optimizing the etching condition to identify the defects in the form of etch pits are some of the key factors in finding the growth pattern of the samples. Also, the defect distribution and defect densities can also be studied by calculating the etch pit density in a crystal which translates and reveals the growth pattern. In addition to that the dislocations observed in the crystals, using this analysis, one can able to identify and find the influence of the crystal hardness in the as-grown samples.