Program Listings

R1D Growth of CdTe-based Materials

Monday, Nov. 2 16:30-17:50 California

Session Chair: Andrea Zappettini, IMEM-CNR, Italy

(16:30) R1D-1, invited, New Insights on Mechanisms That Limit Growth Rates During THM Growth of Cadmium Zinc Telluride

J. J. Derby, J. H. Peterson, Z. Li, A. Yeckel

Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, U.S.A.

The traveling heater method (THM) has enjoyed particular success for growth of large crystals of cadmium zinc telluride (CZT). In THM, crystalline CZT is grown from a liquid phase that contains excess tellurium. This solvent phase is produced in a liquid zone that is simultaneously dissolving a feed solid while growing single-crystal material as it is moved via a traveling heater. While THM has proven to grow better quality material than competing methods, growth rates in THM are orders of magnitude smaller. In this presentation, we present a comprehensive finite-element model for the THM process, employing a rigorous phase diagram within the framework of a classical crystal growth model that accounts for heat and species transfer, fluid mechanics, and moving, phase-change interfaces. We examine parametric process sensitivity to factors such as growth rate, heater profile, and size of the melt zone. In particular, we focus on how the interaction of species segregation and thermal gradients near the solidification interface leads to conditions of morphological instability that limit stable growth. Specifically, we find supercooled liquid regions that arise when mass transport is modified via convective flows driven by a flow structure triggered by the Brunt-Väisälä instability. This classical instability is associated with atmospheric flows that oscillate within a vertically stratified fluid, forming structures such as lee waves behind a mountain range. In the case of THM, there is a stabilizing vertical stratification in the lower portion of the liquid zone that is established during growth by the axial thermal profile and fluid density. The onset of constitutional supercooling is associated with a change in the structure of the fluid flow shown in the figure below, where computations are presented for increasing zone size under nearly constant thermal conditions. Approaches aimed at ameliorating this instability and achieving faster growth rates are discussed.

(16:50) R1D-2, Large Volume Uniform Single Crystal Growth of Detector Grade CdZnTe Implementing pBN Crucible in Vertical Bridgman Growth Setup

S. K. Swain, J. J. McCoy, R. Rao, K. G. Lynn

Center for Materials Research, Washington State University, Pullman, Washington, USA

CdZnTe (10% Zn) crystals have been grown by modified vertical Bridgman technique, implementing pBN crucible, in order to avoid deleterious effects of crucible melt interaction in CZT melt growth and hence enable faster production of large volume uniform detector grade crystals. Single crystal wafers of 50x50 mm^2 have been reproducibly obtained in pBN crucible grown ingots. The grown crystals have been evaluated with respect to Zn distribution, which indicates nearly uniform radial Zn profile, and axial Zn variation between 8% to 12% throughout the ingot volume. Additionally, planar detectors were fabricated to perform transport characteristics and spectrometric performance. The crystals were very well compensated with resistivity >10^10 ohm.cm and electron µt > 1x10^-3 cm2/v. Second phase defect distribution in the crystals indicates mean inclusion size < 3 micron. In addition, etch pit density studies have been performed on (111) oriented single crystals by Everson etching process, which yields EPD values in the range of (3-5) x10^4 cm-2, suggesting minimized dislocation in the crystals.

(17:05) R1D-3, Role of Selenium in CdTe-Based Host Materials for Radiation Detector Applications

U. N. Roy, A. E. Bolotnikov, G. S. Camarda, Y. Cui, R. Gul, A. Hossain, R. Tappero, G. Yang, R. B. James

NN, Brookhaven National Laboratory, Upton, USA

CdTe-based materials face long-standing material challenges due to a high concentration of crystallographic defects resulting from its poor thermo-physical properties. A high concentration of Te-rich secondary phases in the range of 2-8x105 cm-3, dislocation walls and cellular structures of sub-grain boundary networks, and non-uniformity of Zn concentration in CdZnTe caused by its high segregation coefficient (~1.35) are major problems in today’s CdTe-based materials. They severely hinder the yield of high-quality detector grade material. All these defects govern the charge characteristics of the material, viz. they severely degrade the µt value. Although the quality of CdTe-based ingots has improved substantially during the last few years, increasing the yield of large-volume detector grade material to lower cost is still a challenge. Since the poor thermo-physical properties of CdTe are intrinsic to the material, it is perhaps not possible to considerably improve the material quality by modifying the growth parameters alone. The CdTe-based materials thus demand an alternative path, such as the study of the effects of solid solution with an isoelectronic element in order to improve the crystalline quality of the grown ingots resulting in an increase in the yield of large-volume high quality detector grade material. In this present work, we have chosen selenium as the isoelectronic element to investigate the solid solution effects in CdTe and CdZnTe. The effects of selenium on the structural, concentration of secondary phases, electronic defects and charge transport characteristics on CdTe-based materials will be discussed in detail.

(17:20) R1D-4, A Simulation Study about the Use of Fixed and Mobile Heat Exchangers Systems for Modifying the Solid Liquid Interface in the Growth of CdZnTe Bulk Crystals in a Vertical Gradient Freeze Equipment.

E. Dieguez¹, E. Repiso¹, A. Corrochano¹, S. Rubio¹, J. Plaza¹, F. Sizov², A. Shevehik-Shekera²

¹Depart. Physics of Materials, Universidad Autónoma de Madrid, UAM, Madrid, SPAIN
²Physics, Institute of Semiconductor Physics, ISP, Kiev, Ukraine

CdTe and related alloys based materials are extensively used on security and medical applications as bulk material, being the counter partner CdZnTe (Zn= 0.1%) (CZT) deeply applied in the field of medical applications. An approach to improve the solid liquid interface (SLI), is the use of heat exchanger systems. In this work the effect of fixed or mobile heat exchangers systems on the shape of the SLI is studied, and applied on a VGF experimental furnace. The approach has been done using CrysMAS code simulation facilities, with the appropriate experimental conditions of the VGF furnace. The shape of the SLI has been studied as the critical parameter to decide the experimental heat exchanger system. For fixed heat exchanger system a hollow cylinder located on the bottom of the ampoule growth was used, and the simulation was done with PBN, SiC, Pt and W heat exchanger. While for mobile heat exchanger system, a hollow core simultaneously displaced with the SLI was used for the simulation. A thermal study was done with all the systems described above, and both the shape of the SLI and the growth velocity were obtained, considering constants all the boundary conditions and temperature gradients. Without cold finger, the interface is ever slightly concave. Nevertheless the use of a cold finger drastically influences the shape of the interface, and with higher thermal conductivity material promotes a more convex interface, although only during the half section of the whole ingot. For this reason a mobile heat exchanger is proposed as a mean for a better control of the solid liquid interface shape. In this case the shape of the solid-liquid interface can be more easier controlled, being the mobile heat conductive core displaced along the growth ampoule following the position of the solid liquid interface, a possibility experimentally possible in our VGF equipment. Results about the shape of the interface and the growth velocities will be presented and discussed.

(17:35) R1D-5, The Effects of Zn Concentration on Detector Performance of Modified Horizontal Bridgeman CdZnTe

A. Ofan, Y. Glazer, P. Rusian, A. Shahar

DCD, GE Healthcare, Rehovot, Israel

The high resistance close to intrinsic CdZnTe (CZT) semiconductor is an excellent material candidate for high efficiency, high-resolution room-temperature nuclear radiation detectors thanks to its physical properties. Indeed, CZT crystals are available commercially and CZT based room-temperature radiation detectors and detector arrays are steadily gaining acceptance in many medical, industrial, security, safeguards and scientific X-ray and ?-ray imaging and spectroscopic applications. Modified horizontal Bridgeman (MHB) crystal growth method, in which the CZT melt is solidified gradually along the CZT ingot, is the growth method used to grow CZT at GE Healthcare DCD Rehovot. The MHB method is fast compare to other growth methods and gives good detection yields; as a result it produces excellent CZT detectors for the best market price. MHB growth methods of directional solidification along with a segregation coefficient larger than unity dictate variation in the Zn concentration along the CZT ingot. In this study we examined the effect of the different Zn concentration in MHB grown CZT on its electrical and spectral parameters. MHB grown CZT were cut into segment and then sliced into wafers. Wafers from different segments were analyzed for their Zn concentration using mass spectrometry and IR absorption measurement. The wafers were processed to fabricate detectors and their electrical conductivity, electron mobility-lifetime (µt), energy resolution and detection efficiency (sensitivity) were measured. The results show decrease in the Zn concentration along the solidification trajectory, as expected by its Zn solubility. The decrease in Zn concentration was found to correlate with lower resistivity, lower electron mobility-lifetime, lower energy resolution but higher detection efficiency. The wide range of Zn concentration and the resulting performance enable to sort different regions in the CZT ingot for different purposes.