J. F. Butler, C. L. Lingren and F. P. Doty Aurora Technologies Corporation

7408 Trade Street, San Diego, Ca 92121

Absfract

Results of an effort to improve the performance at CdTe detectors by addressing starting element purity and crystal- linity are described. Structural perfection was improved by alloying with ZnTe to form crystals of Cd,,Z%Te. Crystals were grown by a high pressure Bridgman method. Evidence for significant enhancements of the p t products resulting from these efforts is presented. Features of C&,Zn,,,Te detectors include: energy resolutions at 122 KeV e 7%; resistivity approximately ohm-cm; no polarization effects; and temperature for useful operation up to 1OOC. The large sizes (e.g., 3 Kg, 7.5-cm diameter) and excellent homogeneity of the crystals make it possible to produce detectors and imaging arrays with areas of several square inches.

I. INTRODUCTION

The potential benefits of CdTe detectors, such as room temperature operation, high counting rates, small size, high stability and solid-state reliability and ruggedness make these devices attractive candidates for applications in areas as diverse as medical instrumentation, industrial gaging and hazardous waste management. However, CdTe detectors are actually in very limited use compared to other detector types, such as NaI(T1) scintillators and G-M tubes, even though they have been available for several decades. The lack of interest in CdTe detectors can probably be ascribed to their generally high prices and to various technical defects, such as polarization effects; such problems are believed to stem from factors related to production of the detector crystals. An earlier publication described this company’s use of a high pressure Bridgman (HPB) approach to crystal growth which has the potential for overcoming many of these problems 111. This paper discusses further developments in the application of the HPB method, including the growth of high quality Cd,.J%Te crystals for use as gamma ray detectors.

11. BACKGROUND

The free carrier mobility-lifetime (pe) products are critical performance limiting parameters of semiconductor detectors since they determine the fraction of the charge released by a gamma event that contributes to an output charge pulse. One important result of finite pt products is a dependence of the output pulse magnitude on the position of the gamma event within the detector, which may lead to a degradation of the energy resolution. In general, the bias voltage required to achieve any performance criterion is

inversely proportional to the p t products. There is thus considerable interest in pursuing approaches designed to increase the p t products of semiconductor detectors.

Free carrier lifetimes of high resistivity semiconductors are largely governed by recombination and trapping states within the forbidden gap associated with impurities and lattice defects whose concentrations may vary by large amounts; mobilities are also dependent on impurity concen- trations, although generally to a lesser degree [2]. A central approach to improving semiconductor detector performance is thus to attempt to enhance the purity and lattice perfec- tion of the detector crystal. This paper describes preliminary results of an experimental program designed to improve the performance of CdTe detectors by such an approach. In addition to addressing the purity of the starting elements employed in crystal growth, the program included an effort to improve lattice perfection by alloying ZnTe with CdTe to form the mixed crystal Cd,,ZqTe. It was anticipated that, in addition to improving the pt products, alloying ZnTe with CdTe would lead to other benefits such as lower leakage currents and higher operating temperatures.

111. MATERIALS PROCESSING

The CdTe and Cd,,Z%Te crystals for this program were grown by the HPB approach. In this method, crystals are grown near stoichiometry with respect to the metal-chalcoge- nide ratio, at temperatures near the maximum melting point. The method provides both a favorable growth temperature profile and an exceptionally clean, quartz-free growth environment. New furnaces employing this method allow us to grow crystals up to more than 10 cm in diameter and 5 Kg in mass. The growth process includes pre-reaction of the elements, and no dopants are added to the growth charge. The resistivities of crystals grown by this method are quite high (in the range of 5 X lo1’ to 5 X lo1, ohm-cm for C&.,Z%,,Te, for example) and extremely uniform throughout each boule. This high degree of uniformity is also exhibited by the pr products, as deduced from the relative perfor- mance of detectors fabricated from samples taken along the total length and diameter of a boule [3].

The relationships between impurities and free carrier lifetimes have been extensively studied for the 11-VI com- pounds [4] and other semiconductors since the early days of semiconductor device technology, and general principles are well established. It is known that elements with empty inner shell orbitals such as those of the Fe group and Au and Cu, as well as lattice vacancies, the ubiquitous 0 and other ele- ments form deep levels that may be highly efficient recombi-

0018-9499/92$03.00 0 1992 IEEE

nation or trapping sites in virtually all semiconductors. An examination of the supplier furnished analysis of the purest available Cd, Te and Zn shows that the as-purchased elements contain impurities strongly suspected of causing recombination or trapping sites at levels in the order of 10l6 Accordingly, the program reported here included steps to lower the overall background level of impurities in the as-purchased elements prior to their use in the growth process. The purification process was based on segregation at a solid-liquid interface, and one-to-three passes were made for each element.

The application of Cd,-jT%Te to replace CdTe as sub- strate material in the manufacture of infrared focal plane arrays has become common practice in recent years. A primary motivation for this approach is the superior structur- al properties found to result from the alloy effect. It is found, for example, that defect densities in Cd,-.J%Te, as determined by etch-pit density (EPD) measurements are typically an order of magnitude lower than that of CdTe [3]. This beneficial result of alloying has been explained on the basis of an enhancement of the covalent component of the interatomic bonding in the crystal [5]. We anticipated that the reduced lattice defect density would lead to an enhance- ment of pr products and hence to improved gamma ray detector performance.

The essential effects on detector performance of both purification and alloying are brought out in Fig. 1, which shows the dependence of counting rate on bias voltage for two purity levels of CdTe and for purified Cd&%Te. The

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bre 1. Dependence of counting rate on voltage for CdTe an C&Z%Te ofdiffering purities. Imp&ed performancedue to purification and alloying is evident [4].

detectors for this study had identical dimensions and were operated under essentially identical conditions. As an aid to interpreting Fig. 1, it is shown in the appendix that, under appropriate restrictive assumptions, the total counting rate can be expressed as a function of bias voltage V by the following equation:

where CO is the absorption rate of gamma photons in the detector, E is photon energy absorbed per event (assumed to be single-valued), L the detector thickness, pe the electron mobility, T is defined as ~~+r,,/b, where b is the ratio of electron to hole mobility, bE is the standard deviation of the electronic noise expressed in energy equivalent units and V,, is given by

in Eq. 2, W is the electron-hole impact ionization energy and ~m is the minimum countable charge magnitude as estab- lished by a discriminator.

From Eq. 1, it follows that Vo is the bias voltage at which the counting rate is one-half of its maximum, a parameter which can be estimated experimentally. With the other parameters known, Eq. 2 can then be applied to obtain a value for p e ~ . Equations 1 and 2 predict that the counthg rate curves shift to lower values of voltage with increasing pr products. Applying these considerations to Fig. 1, the shift of the curves with, first, increasing purity and, second, use of an alloy composition is consistent with a signifcant increase in free carrier lifetimes with purity and alloying.

Note that Eq. 1 was presented in a somewhat different form in Ref. 1. The earlier derivation was based on the assumption that the primary relevant broadening mechanism is a spatial variation in lifetime values. Subsequent measure- ments have indicated that this assumption is not valid and that the dominant broadening mechanism is electronic noise associated with the preamp input and leakage current.

Iv. Cd,+Z\Te DETECTOR CHARACTERIZATION

A. Energy Spectrum

Figure 2 shows an energy spectrum of nCo, measured with a typical C4,Zq,,Te detector. Its dimensions were 10 X 10 mm2 in area and 1.7 mm thickness. The bias voltage was 450 volts, The radiation source had an intensity of 1 pCi and was located 1 mm from the detector face. Under the conditions of this measurement, the energy resolution was governed to a great extent by the electronic noise of the preamplifer which was determined by injection of a test pulse to be about 6 KeV. The noise band'at the lower energy range has been discriminated out. Note that the "CO, x-ray line at 14 KeV is clearly resolved while the 7 KeV line is partially masked by preamplifier noise.

COUNTS/CHANNEL (Thousands) I 5 1

0 160 200 3M) 400 500 600 700 CHANNEL NO.

F@re 2. Energy spectrum of %on measured with a Cdo,8Zno,,Te detector at a bias of 450 volts. Detector dimensions are 10 X 10 X 1.7 "3. Energy equivalent noise bandwidth of approximately 6 KeV contributes to line- width.

B. Polarization

A considerable body of literature exists concerning the phenomenon of polarization in CdTe detectors [6]. In view of its importance to any application of these devices, a systematic study of its possible occurrence was included in the program. Counting rates of detectors were routinely monitored for extended periods of time after the application of bias voltage, and bias voltages up to 500 volts were applied. No decrease over time was observed in any instance; Fig. 3 shows counting rates for a detector biased at 50 volts and exposed to a constant radiation flux for a time period of one week. The scatter in counting rates apparent in Fig. 3 is completely within the range expected from counting statis- tics. We conclude that Cd,J%Te detectors fabricated from HPB grown crystals do not exhibit polarization effects.

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C. Temperature Dependence

The wider bandgap of Cd,J%Te was anticipated to result in detectors with better high temperature performance than CdTe devices. In order to evaluate the dependence of detector performance on temperature, a shielded detector enclosure separated from a preamplifier by a 30-cm length of shielded cable was devised. This arrangement allowed the detector to be installed in a small laboratory furnace and be heated independently of the electronic instrumentation. The preamplifier available for this preliminary study was not optimized for the large capacitance of the shielded cable, hence this system was quite noisy. System noise was estimat- ed to be equivalent to about 34 KeV.

In the first experiment, the leakage current at a bias of 50 volts was monitored with a picommeter as the tempera- ture was increased from room temperature to 100 C. Results are shown in Fig. 4, where ln[I(nA)] is plotted against 1000/T(K) to allow the activation energy of the charge generation process to be estimated from the slope of the curve. Actual current values ranged from 6 nA at 22 C to 710 nA at 100 C. Note that the value of 710 nA at 100 C is about an order-of-magnitude less than is typically projected for commercial CdTe detectors. The activation energy deduced from Fig. 4 is 0.5 eV. Since the bandgap of C&.&q,MTe is 1.4 eV, the activation energy for intrinsic resistivity would be 0.7 eV. Thus, the experimentally ob- served value indicates that the resistivity is governed by a donor or acceptor level 0.5 eV below a band edge.

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@re4. Dependence of leakage current on temperature for a CdossZno,,Te detector biased at 50 volts. The end points are 6 nA at 22.5 C and 710 nA at 1OOC.

Energy spectral characteristics were studied at elevated temperatures by observing the response of a detector to s7Co, emission. Spectra measured at detector temperatures of 100 C (solid curve) and 22.5 C (broken curve) are shown

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connecting the preamp to the detector installed within the furnace. Temperature driiing at the higher temperature made it necessary to limit the accumulation times to 5 minutes, which resulted in the obvious scatter in the curves. The spectral linewidths for both curves are dominated by the 34 KeV-wide electronic noise. The shift in the peak to lower energies in going from room temperature to 100 C indicates a lowering of the pr products. The increase in noise is probably due to the increased leakage current, as seen in Fig. 4. Even though the p~ products are reduced and the noise is higher, there is clearly useful spectral information at 1OOC. Operation at 100 C had no observable detrimental effect on the detector.

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Fire 5. Performance at elevated temperatures. Spectrum of using a Cd,,$n,,Te detector at 100 volts bias, with low energy noise discrimi- nated out. Resolution limiting noise includes a 34 KeV contribution from signal cable leading into furnace.

D. Large Area Structures

The high degree of uniformity in Cd,..JqTe crystals of more than 7.5 cm in diameter makes it feasible to fabricate detector structures of large area. Figure 6, for example, shows two types of stripe array configurations produced on Cd,,,,ZQ,Te wafers. A BNC coupling connector 3.1 cm in length is included in the figure for size calibration. The device on the right is a 32 X 32 element matrix array designed for two-dimensional gamma ray imaging. The smaller device is a simple linear array for one-dimensional imaging. Gamma ray imaging is an important technique in medical diagnostics, space sciences, various defense related activities and other areas. The use of Cd,-,J%Te detector arrays offers the prospect of sigaifcantly improved spatial resolution, excellent long term stability and other benefits.

Large area wafers can also be employed to fabricate single-element detectors with areas of several square inches to be used, for example, in low-background measurements. The high and uniform resistivity of Cd,,ZqTe is an impor- tant factor in making this application feasible. Another

sigdicant aspect of producing large area wafers with a lugh yield of detector quality material is the potential for using large-volume manufacturing processes that could eventually lead to lower unit prices.

Figure 6. Photograph of two gamma ray imaging arrays fabricated on Cdl-z15;re wafers. The square array is 3.7 cm on a side.

V. CONCLUSIONS

It has been demonstrated that operating characteristics of CdTe detectors can be improved by enhancing crystal quality through p&cation of the elements used in crystal growth and alloying with ZnTe. In addition to exhibiting improved performance, detectors fabricated from purified Cd&%Te do not exhibit polarization and can be operated at tempera- tures up to 100 C. Use of the HPB growth method yields large boules possessing excellent uniformity and a high yield of detector quality material, making it feasible to develop large area Cd,.&Te detectors and imaging arrays and to consider employing large volume detector manufacturing techniques.

ACKNOWLEDGMENT

We wish to acknowledge the valuable contributions of Mr. B. Apotovsky in crystal growth and detector fabrication.

APPENDIX: DERIVATION OF EQS. (1) AND (2)

The major assumptions used in deriving Eqs. (1) and (2) were that 1) the entire quantum energy is translated to electron-hole pair production with each absorption event in the detector, 2) the spatial extent of the electron and hole charge clouds remain much less than any relevant detector dimensions and 3) there are no coincident events. The first two assumptions are required for the validity of well-known

609

Hecht equation [7], which can be expressed for the present case as

Nuclear Radiation Detectos," 1.m Trans. Nucl. Science, Vol. NS-23, pp. 159-170, Feb. 1976.

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where q is the charge at the detector terminals resulting from the absorption of a gamma ray in the crystal a distance x from the negative electrode. Equation (Al) ignores any contribution from noise sources.

If it is assumed that a gaussian noise is superimposed on the gamma ray induced signal, we may write

where 8q is the standard deviation of the noise distribution, and AC(q') is the contribution to the counting rate due to charges in the range of q' to q'+ Aq. The total counting rate, then is obtained by integrating Eq. (A2) over all q' greater than %. Carrying out this integration results in

In many detector applications, including materials evaluation discussed in Section 11, qm is set to be much less than the maximum output charge, eE/W. Since 8q is generally also a very small quantity, the detector achieves nearly its full counting rate for very small values of q. In the limit of small q, Eq.(Al) reduces to

p e ~ eEV

wL2 ' 044) q=

Substituting Eq. (A4) into Eq. (A3) and setting 8E = W8q leads to Eqs. (1) and (2) in the text.

REFERENCES

E. Raiskin and J.F. Butler, "CdTe Low Level Gamma Detectors Based on a New Crystal Growth Method," IEEE Truns. on Nucl. Science, vol. NS-35, No. 1, pp. 82-84, February, 1988. for example, see C. Kittel, Innoduction to Solid State Physics, John Wiley and Sons, N.Y., 1953, pp. 361-371. F.P. Dofy, J.F. Butler, KA. Bowers and J.F. Schetzina, "Properties of CdZnTe Crystals Grown by a High Pressure Bridgman Method," I. Vac. Sci. Technol. vol. B10, Jul/Aug 1992 (in press). C. H.Bube, The Physics und Chemistty of II-M Compounds, M. Aven and J. S. Prener, eds. (John Wiley and Sons, N.Y., 1967) pp. 659-705. A. Sher, A. Chen, W.E. Spicer and C. Shih, "Effects Influencing the structural integrity of semiconducton and their alloys," I . Vac. Sci. Technol. vol. A3, pp. 105-111, Jan/Feb 1985. P. Siffert, J. Berger, C. Scharager, A. Comet, R Stuck, RO. Bell, H.B. Serreze and F.V. Wald, "Polarization in Cadmium Telluride