Santosh K. Swain

Study of Different Cool Down Schemes During the Crystal Growth of Detector Grade CdZnTe

Cd0.9Zn0.1Te crystals were grown using a modified vertical Bridgman growth technique to produce radiation detector grade material. Motivated by the importance of the retrograde solubility problem in II-VI compound semiconductors, particularly CZT, different cool down techniques were used to observe the effects on the secondary phase (SP) size, distribution, density and resulting detector properties. Presented are four of the cool down schemes performed and the results in terms of the secondary phases and charge transport characteristics of the grown crystals. The cooling of the grown ingot to ambient temperatures was done over various lengths of time. The differences between the cool down methods are in the rates of cooling through the retrograde solubility phase. Apart from these cool downs, quenching studies were also performed on some crystal growths giving unique results in terms of the secondary phase distribution and characteristics as radiation detectors.

Relationship of Open-Circuit Voltage to CdTe Hole Concentration and Lifetime

We investigate the correlation of bulk CdTe and CdZnTe material properties with experimental open-circuit voltage (Voc) through fabrication and characterization of diverse single-crystal solar cells with different dopants. Several distinct crystal types reach Voc > 900 mV. Correlations are in general agreement with Voc limits modeled from bulk minority-carrier lifetime and hole concentration.

Fabrication of single-crystal solar cells from phosphorous-doped CdTe wafer

A phosphorous doped CdTe boule was grown from melt using the modified vertical Bridgman technique. Single crystals prepared from the boule showed hole density of (5-10) × 1015 cm-3, resistivity ~(10-50) Ω.cm and mobility of ~50 cm2/v.s determined by Hall measurement. The two-photon excitation time-resolved photoluminescence (2PE-TRPL) measurement revealed a bulk minority carrier lifetime of ~47 ns. A 500μm thick single-crystal wafer was prepared and CdS/CdTe heterojunction solar cells were fabricated with ITO/CdS/CdTe/Cu/Au structure where all relevant thin film layers were deposited by DC/RF magnetron sputtering. An efficiency of ~11% and short circuit current density of as high as ~25 mA/cm2 has been obtained on a cell upon annealing at 320°C. The high short circuit current density could be attributed to the superior quality of the CdTe material with a relatively longer minority carrier lifetime.

Interface Characterization of Single-Crystal CdTe Solar Cells With VOC > 950 mV

Advancing CdTe solar cell efficiency requires improving the open-circuit voltage (VOC) above 900 mV. This requires long carrier lifetime, high hole density, and high-quality interfaces, where the interface recombination velocity is less than about 104 cm/s. Using CdTe single crystals as a model system, we report on CdTe/CdS electrical and structural interface properties in devices that produce open-circuit voltage exceeding 950 mV.

Excitation-dependent carrier lifetime and diffusion length in bulk CdTe determined by time-resolved optical pump-probe techniques

We applied time-resolved pump-probe spectroscopy based on free carrier absorption and light diffraction on a transient grating for direct measurements of the carrier lifetime and diffusion coefficient D in high-resistivity single crystal CdTe (codoped with In and Er). The bulk carrier lifetime τ decreased from 670 ± 50 ns to 60 ± 10 ns with increase of excess carrier density N from 1016 to 5 × 1018 cm−3 due to the excitation-dependent radiative recombination rate. In this N range, the carrier diffusion length dropped from 14 μm to 6 μm due to lifetime decrease. Modeling of in-depth (axial) and in-plane (lateral) carrier diffusion provided the value of surface recombination velocity S = 6 × 105 cm/s for the untreated surface. At even higher excitations, in the 1019–3 × 1020 cm−3 density range, D increase from 5 to 20 cm2/s due to carrier degeneracy was observed.

Defects in Undoped p-type CdTe Single Crystals

Recently, the device performance of Cadmium Telluride (CdTe) solar cells have improved significantly. However, little progress has been made in understanding the defected structures responsible for carrier trapping, which have limited further cell efficiency improvements. In order to better understand the underlying electrical compensation mechanism, an undoped CdTe bulk crystal boule was grown from melt with Te-rich stoichiometry using the Vertical Bridgman technique, where the growth process was controlled to maximize net acceptor density. The samples prepared from the boule indicate low resistivity of 100-500 Ω·cm, and Hall measurements provided net acceptor density of (1-2) × 1015 cm-3, which are comparable to relevant values reported from the CdTe films after CdCl2 treatment. Infrared microscopy suggests there are significant amounts of secondary phases (SPs) present in the samples, where the SPs are believed to be Te inclusions and precipitates. The point defects are characterized by thermoelectric effect spectroscopy (TEES), where we observed six TEES current peaks associated with the defect levels ranging from 110 to 700 mV within the bandgap.

Modified vertical bridgman growth of Cd 1−x Zn x Te detector grade crystal in a 4 inch EDG furnace

Achieving a high yield of detector grade CdZnTe single crystals is one of the greatest challenges in CdZnTe crystal growth. Over 85, 3.5-4 inch long CdZnTe crystals were grown in a 43 zone modified low pressure Vertical Bridgman Electro-Dynamic Gradient (EDG) Freeze 3 in. Mellen Sunfire furnace. The diameter of the grown ingots has been increased using a new Electro-Dynamic Gradient (EDG) Freeze 4 in. Mellen Sunfire Platinum coil furnace to visualize the effects of increased ampoule diameter on the grain structure and spectroscopic properties of CdZnTe. The reproducibility of detector grade crystals using the 4 in. furnace is discussed and some properties of the fabricated detectors are presented.

Influence of accelerated crucible rotation on defect distribution and detector characteristics of melt grown CdZnTe(Conference Presentation)

Non-stoichiometry related extended defects in CdTe/CZT, such as tellurium inclusions and precipitates are known to be detrimental bulk defects in detector grade cadmium zinc telluride. In our attempt to minimize the size of tellurium inclusions we have employed accelerated crucible rotation technique in modified vertical Bridgman growth configuration. Acceleration and deceleration rate as high as 900 rpm2 was successfully applied during superheated melt mixing and growth. By comparing growths with and without ACRT under otherwise identical growth conditions, it was observed that the average inclusion size reduced by more than 50 percent due to ACRT. Additionally, we will discuss the effect of forced melt convection on the axial zinc and dopant segregation profile. Electrical characterization, spectrometric performance and purity analysis of the grown crystals will be presented.

Correlations of secondary phases (SPs) with mobility lifetime (μτe) of the electrons in CZT crystals using IR microscopy

Cadmium Zinc Telluride (CdZnTe/CZT) crystals were grown using a modified vertical Bridgman growth technique with 10 % Zn concentration at Washington State University (WSU). Analyses of the effects of volume (vol.) %, number density (cm-3), mean diameter (μm) of secondary phases (SPs) and thickness (mm) of the CZT crystals on single crystal properties such as carrier mobility lifetime (μτe) and resistivity (ρ) were performed. Some correlations were observed between μτe values of different CZT crystals and vol. %, number density, mean diameter of SPs and thickness of the crystals. High μτe and lower SP vol. % values were obtained for the ingots grown with rapid cool down times and with no intentional amounts of excess Te/Cd. For the selected samples, the effects of the SPs on the μτe values were established for the SPs whose mean diameters were ≤4 μm and >4 μm. These studies indicate vol. % and mean diameter of SPs are the important parameters for CZT crystal performance as a radiation detector.

Effects of impurities and secondary phases on the performance of CdZnTe radiation detectors

CdZnTe (10 % Zn) crystals for nuclear radiation detectors were grown using modified vertical Bridgman growth technique. Analyses of the effects of impurities and secondary phases on the detector properties were performed. We have attempted to establish a correlation between total impurities and μτe. Also presented are the effects of volume % of secondary phases on the CdZnTe detector performance. μτe values of 1E-2 cm2/V have been obtained from ingots with lower volume % of Te secondary phases. However, no correlation was observed between the volume % of secondary phases and μτe indicating that both impurities and secondary phases effect the μτe value. Results documenting the simultaneous avoidance of impurities and secondary phases are presented. We have shown that post processing CdZnTe is an effective means of reducing volume % of secondary phases.