U.N. Roy a, C.T. Trinh b, H. Htoon b, A.C. Jones b, R. Cortes Robles b f, A.A. Egarievwe c, G.S. Camarda d, S.U. Egarievwe e, M. Drabo c, R.B. James a
a Savannah River National Laboratory, Aiken, SC 29808, USA
b Center for Integrated Nanotechnologies, Materials, Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
c Alabama A&M University, Normal, AL 35762, USA
d Brookhaven National Laboratory, Upton, NY 11973, USA
e Morgan State University, Baltimore, MD 21252, USA
f Department of Physics, University of Puerto Rico, Rio Piedras, San Juan, PR 00931, USA
## Abstract
The quaternary compound CdZnTeSe (CZTS) has emerged as a next-generation detector material. Unlike CdZnTe (CZT), CZTS has the distinct advantages of little or no sub-grain boundary networks and a much lower concentration of Te inclusions, plus better compositional homogeneity. Thus, the material is expected to offer better spatial charge-transport homogeneity compared to CZT. However, an inhomogeneous distribution of point defects in both CZT and CZTS, such as Cd vacancies and residual impurities, can impose additional spatial inhomogeneity of the resistivity, carrier mobilities, and carrier lifetimes of the material. To investigate the spatial distribution of such defects in CZTS, low-temperature photoluminescence (PL) spectroscopic studies were performed at different positions along a single crystalline CZTS sample surface, which was grown by the traveling heater method (THM). The intensity variation of the PL emission of excitons bound to a neutral acceptor defect (A0, X), which is likely dominated by an acceptor like Cu-related level, with respect to the PL emission line from a neutral donor-bound exciton (D0, X) exhibited significant spatial variation, while the peak energy positions were approximately uniform due to the high compositional and bandgap homogeneity of the material.
Graphical Abstract
## Introduction
The development of Cd1-xZnxTe (CZT) as a room temperature X- and gamma-ray detector has proven to be pivotal for a wide range of applications ranging from homeland security, non-proliferation, high energy physics, medical imaging, gamma telescope, X-ray astronomy, and environmental remediation [1], [2], [3], [4], [5], [6], [7]. Due to the favorable opto-electronic properties and high atomic number of the constituent elements, CZT with the preferred composition of 10 at% Zn (Cd0.9Zn0.1Te) has become the leading ternary II-VI compound semiconductor, dominating the commercial supply of compound semiconductor radiation detectors for about three decades. Despite the continuous improvement of material quality, widespread deployment has been limited due to CZT’s high cost of production, owing to the low yield of high-quality detector grade material. The spatial nonuniformity of the charge transport properties has been the major cause for the sub-optimal detector performance and lower yield of high-quality material suitable for high-energy-resolution spectroscopic applications. These defects are typically generated during the growth and subsequent cooling process due to the inherent poor thermophysical properties of CdTe/CdZnTe [1], [8], [9]. Three groups of defects are known to limit the detector performance and yield of CZT. They are compositional inhomogeneity due to non-unity segregation of Zn in the CdTe matrix, high concentrations of Te inclusions, and the presence of sub-grain boundary networks [10], [11], [12]. Often, the sub-grain boundaries are decorated with Te inclusions12. The roles of Te inclusions and sub-grain boundaries on charge transport and detector performance have been measured and modeled [12], [13], [14]. The random distribution of these defects in the CZT matrix imposes spatial non-uniformity in the charge transport characteristics, and the performance degradation of the devices becomes even more pronounced with increased detector thickness. It is thus clear that eliminating or reducing the presence of these defects, especially the sub-grain boundary network and Te inclusions, can further improve the detector energy resolution and efficiency, plus provide for an enhanced yield of material suitable for device fabrication and wider use of the technology.
In the recent past, the new quaternary Cd1-xZnxTe1-ySey (CZTS) has been emerging as an improved next-generation detector material capable of room temperature operation [15], [16], [17], [18], [19], [20], [21], [22]. The new material has been found to mitigate most of the long-standing issues pertaining to extended defects in present day CZT. In contrast to CZT, the CZTS material is free from sub-grain boundary networks and possesses a reduced concentration of Te inclusions [23], [24], [25], [26]. Previously, the CZTS (Cd0.9Zn0.1Te1-ySey) quaternary containing 1.5 at% of Se has been found to be free from sub-grain boundary networks [16], [23]. Very recently, Naydenov and Pritula [24] predicted the optimum concentration of selenium to be ∼ 1.8 % for effective solid solution hardening via thermodynamic modeling, which validates our experimental findings. Additionally, the CZTS ingots offer better compositional homogeneity for growth by the Traveling Heater Method (THM) as well as melt growth techniques [15], [27], [28], [29]. The density of electrically active defects and related trap states responsible for degradation of detector performances has also been reported as less than CZT [30], [31], [32]. Higher hardness of CZTS [33] is also an added advantage for handling the material during the device fabrication processes and the robustness during use.
The quaternary material was also found to surpass commercial CZT material for stability during high flux X-ray applications, such as those involved in medical imaging [31], [34]. The observed lower concentrations of performance-limiting extended defects and higher uniformity of the charge transport properties for CZTS are expected to lead to a higher yield of high-quality detectors, especially for larger detectors with higher efficiency, and potentially a lower cost of production [35].
An inhomogeneous distribution of electrically active point defects/impurities can also impose severe spatial nonuniformity in the charge-transport properties. In this paper, we report an effort to understand the spatial distribution of electrically active defects originating from extrinsic impurities. Low temperature micro-photoluminescence (PL) was used to investigate the spatial variation of different radiative recombination centers for THM-grown Cd1-xZnxTe1-ySey with x = 0.1 and y = 0.015.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/spatial-variation-of-the-low-temperature-micro-photoluminescence-of-thm-grown-cdzntese.html