CRYSTAL GROWTH & DESIGN
Correlation between Ingot Diameter and Crystal Properties of CdZnTe:In Grown by the Modified Bridgman Method
2007 VOL. 7, NO. 2 435-438
Ge Yang,* Wanqi Jie, Tao Wang, Guoqiang Li, Wenwei Li, and Hui Hua State Key Laboratory of Solidification Processing, Northwestern Polytechnical UniVersity, Xi’an 710072, People’s Republic of China ReceiVed July 18, 2006; ReVised Manuscript ReceiVed October 8, 2006
ABSTRACT: The compound semiconductor CdZnTe:In is the most promising material for room temperature nuclear radiation detectors. Enlarging the diameter of CdZnTe:In ingot can improve the yield and reduce the production cost, which, however, affects the properties of CdZnTe:In to some extent. Recently, CdZnTe:In ingots of 30 mm and 60 mm diameter were grown by the modified vertical Bridgman method in our laboratory. The crystal properties of both ingots were compared according to etch pit density (EPD) of dislocation, X-ray rocking curve, IR transmission spectra, and photoluminescence (PL) spectra. For the CdZnTe:In ingot of 30 mm diameter, the EPD of dislocation was 2.1 × 104 cm-2, and was up to 1.9 × 105 cm-2 when the CdZnTe:In ingot diameter was enlarged to 60 mm. The full width at half-maximum (FWHM) of X-ray rocking also increased from 0.02014° to 0.02864° at the same time. The two phenomena imply that the crystalline quality of CdZnTe:In deteriorated with the enlarging of the ingot diameter. In addition, when the CdZnTe:In ingot diameter was varied from 30 to 60 mm, the average IR transmittance increased from 27 to 33%. The augmentation of dislocation was responsible for the change, which was confirmed by the improvement of the intensity of the dislocation-related D peak in the PL spectra. 1. Introduction Recently, the compound semiconductor CdZnTe:In has attracted much attention due to its wide applications in the nuclear radiation detection.1-5 The Bridgman method is widely used to produce CdZnTe:In crystals. However, due to the bad yield of this method, it is still a challenge to achieve detector-grade CdZnTe:In single crystals with low cost.1,2 Researchers have kept looking for new ways to ameliorate the present situation, and enlarging the diameter of the CdZnTe:In ingot is thought to be a very promising approach. Nevertheless, considering the physical properties of CdZnTe:In, such as the low thermal conductivity, the low stacking fault energy, and the hardcontrolled liquid-solid interface, it is difficult to achieve CdZnTe:In single crystals of large diameter. Recently, using the modified Bridgman method, we successfully grew CdZnTe: In single crystals whose diameter is up to 60 mm. The variation of the CdZnTe:In ingot diameter can cause changes in the crystal quality and thus influence the optical and electrical properties of CdZnTe:In. However, there are few reports thus far regarding the effects of different ingot diameters on the properties of CdZnTe:In. In this work, CdZnTe:In ingots of 30 mm and 60 mm diameter were compared according to etch pit density (EPD) of dislocation, X-ray rocking curve, IR transmission spectra, and photoluminescence (PL) spectra. The effects of ingot diameter variation on the properties of CdZnTe:In were analyzed further. 2. Experimental Procedures During our experiments, high purity raw materials of Cd (7N), Zn (7N), Te (7N), and In (7N) were used to prevent unintended impurities. Two modifications were applied to the vertical Bridgman method for the growth of CdZnTe:In, which include the addition of excess Cd into the stoichiometric starting raw materials to compensate for Cd * To whom correspondence should be addressed. School of Materials Science and Engineering, Northwestern Polytechnical University (NWPU), No. 127, Youyixi Road, Xi’an 710072, Shaanxi Province, People’s Republic of China. Tel: 0086-29-88486065. Fax: 0086-29-88495414. E-mail:
[email protected].
evaporation and the introduction of the accelerated crucible rotation technique (ACRT). The mass of excess Cd depends on how much Cd is evaporated from raw materials, which can be estimated by the Clayeyron equation
PV ) nRT )
m′Cd RT MCd
(1)
or
m′Cd )
MCd PV ‚ R T
(2)
where P is the Cd equilibrium partial pressure at growth temperature, V is the spare volume above the CdZnTe:In melt in the sealed quartz crucible, R is the gas constant, T is the growth temperature of CdZnTe: In, n and m′Cd are the mole number and the mass of Cd evaporated from raw materials, and MCd is the mole mass of Cd. In addition, ACRT was also adopted to stabilize the growth velocity, homogenize the concentration distribution, control the shape of liquid-solid interface, and depress segregation. The control curve in a cycle for our ACRT is shown in Figure 1. It has two reverse directional rotation processes, each of which includes various periods of acceleration, stable rotation, and deceleration. The more specific growth procedures are described as follows. First, raw materials with an excess Cd of m′Cd were synthesized in carbon-coated quartz crucibles at 1400 K using a rocking furnace. Next, using these synthesized polycrystals, crystal growth was undertaken at the withdrawal rate of 0.8 mm h-1 and the temperature gradient of 13.0 K cm-1 in an ACRT furnace with the control curve mentioned in Figure 1. By this modified Bridgman method, CdZnTe: In ingots of 30 mm and 60 mm diameter were grown for comparison. The mole ratio of Cd, Zn, and Te in both ingots was 0.9:0.1:1.0. The concentration of In was 15 ppm. Both CdZnTe:In ingots produced in this way were cut along (111) faces with a dimension of 10 × 10 × 2 mm3. X-ray rocking curves of CdZnTe:In crystals were measured by a PANalytical X’Pert Pro diffractometer. IR transmission tests were carried out at room temperature by a Nicolet Nexus 670 spectrometer. In the PL measurements, the samples were attached on a cold copper finger in a closed-cycle cryostat with grease to keep the sample temperature at 10 K. An argon ion laser with the wavelength of 488 nm was used to excite the PL spectrum. A Triax 550 tri-grating monochromator with a photomultiplier tube (PMT), whose spectral
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436 Crystal Growth & Design, Vol. 7, No. 2, 2007
Figure 1. Control curve of the accelerated crucible rotation technique (ACRT) during the growth of CdZnTe:In.
Yang et al.
Figure 3. X-ray rocking curves of CdZnTe:In ingots of 30 mm and 60 mm diameter.
Figure 4. IR transmission spectra of CdZnTe:In ingots of different diameter. The solid line is from CdZnTe:In ingot of 30 mm diameter, and the dashed line is from CdZnTe:In ingot of 60 mm diameter.
Figure 2. The photographs of the two CdZnTe:In ingots with different diameters. (a) CdZnTe:In ingot with 30 mm diameter. (b) CdZnTe:In ingot with 60 mm diameter.
resolution was better than 0.3 nm, was employed to collect and analyze the signals emitted from the samples.
3. Results and Discussion Figure 2 shows the photographs of the CdZnTe:In ingots of 30 mm and 60 mm diameter. As it shows, the surfaces of both ingots are smooth and no obvious holes and gas bubbles exist. It is generally accepted that dislocation is one of the main defects in CdZnTe:In crystals.1 Here, the wafers from the two CdZnTe: In ingots were etched first by E (HNO3/deionized water/K2CrO7 ) 10 mL/20 mL/4 g) then Eg (E solution/AgNO3 ) 10 mL/0.5 mg) solution to cause the dislocation to emerge. The etch pit densities (EPD) of dislocation were counted in several regions, which were selected randomly on the wafers. Their mean values were adopted in our analysis. For the CdZnTe:In ingot of 30 mm diameter, the EPD of dislocation was 2.1 × 104 cm-2, and was up to 1.9 × 105 cm-2 for the CdZnTe:In ingot of 60 mm diameter. X-ray rocking curves of CdZnTe:In ingots are given in Figure 3. The FWHM of the X-ray rocking
curve of the CdZnTe:In ingot of 30 mm diameter was 0.02014°, whereas this value was increased to 0.02864° for the CdZnTe: In ingot of 60 mm diameter. In addition, a slight splitting in the X-ray rocking curve existed for the latter. The increasing of EPD of dislocation and the broadening of FWHM of X-ray rocking curve indicated that the crystalline quality of CdZnTe:In deteriorated when the ingot diameter was enlarged. It is mainly because the enlarging of the ingot diameter increased the curvature of the thermal field, which resulted in higher stress in the CdZnTe:In crystal.5,6 As we know, high stress causes the plastic deformation of the growing crystal, giving rise to the multiplication of dislocation and lowering the crystalline quality of CdZnTe:In. IR transmission spectroscopy is also a powerful tool to elevate the quality of CdZnTe:In.2,7 Figure 4 gives the typical IR transmission curves of CdZnTe:In ingots of 30 mm and 60 mm diameter. The average IR transmittance was increased from 27 to 33% when the ingot diameter was varied from 30 to 60 mm. The phenomenon can be interpreted according to absorption behaviors in the crystals. As we know, the transmission of CdZnTe:In can be expressed by the result of Sen et al.7
T)
(1 - R)2e-Rd 1 - R2e-2Rd
(3)
Ingot Diameter and Crystal Properties of CdZnTe:In
Crystal Growth & Design, Vol. 7, No. 2, 2007 437
Figure 5. Photoluminescence (PL) spectra of CdZnTe:In ingots of 30 mm diameter and 60 mm diameter. The measurement temperature is 10 K.
where R is the reflectivity, d is the sample thickness, and R is the absorption coefficient. For CdZnTe:In samples, the thickness d is 2 mm and the reflectivity R is equal to 0.21.8 The absorption coefficient, R, is mainly influenced by the following factors: (1) free carrier absorption; (2) precipitates absorption; (3) impurity absorption. In the wavenumber domain of 1000 to 4000 cm-1 investigated in this work, the free carrier absorption dominates. The scattering by precipitates occurs primarily in the longer range, and impurity absorption only takes effect under low temperature. Therefore, the absorption coefficient, R, can be expressed by the following equation.9
R)
N2λ2 m*8π2nc3τ
of the ingot diameter. Here, the intensity of the (D0, X) peak was used as a reference. ID/I(D0,X), the ratio of the intensity of the D peak to that of the (D0,X) peak, was approximately equal to 1.3018 in the case of the CdZnTe:In ingot of 30 mm diameter, while the value increased to 1.8080 for the CdZnTe:In ingot of 60 mm diameter. In our opinion, the D peak, centered at 1.5067 eV, is ascribed to extended dislocations in CdZnTe:In. The dislocations bound the free excitons and formed so-called “dislocation-bound” excitons. When these “dislocation-bound” excitons were released from the defect states, a radiative recombination process happened, which brought on the D luminescence. This assignment is strongly supported by the temperature-dependence luminescence experiment of Hildebrandt in CdTe.13 When CdZnTe:In ingot diameter varied from 30 to 60 mm, the augmentation of the dislocation resulted in more “dislocation-bound” excitons produced and, therefore, strengthened the intensity of the D peak. In addition, it was also found that IDAP/I(D0,X), the ratio of the intensity of DAP peak to that of the (D0,X) peak, was reduced from 0.5299 to 0.4837 with the enlarging of the ingot diameter. Furthermore, the phonon replica of DAP, DAP-LO, became undistinguishable. The DAP peak is closely related to the ionized impurities such as Al and Ag, which act as the donors in the DAP transition.14,15 As is well-known, the impurities tend to gather around the dislocations, which reduce the strain near the dislocation and lower the free energy of the system.16 Hence, the increase of dislocation density with the enlarging of ingot diameter meant that more impurities were bound near the dislocations. It reduced the amount of donors involved in the DAP transition and, thus, weakened the intensity of DAP peak. 4. Conclusion
(4)
where N is the concentration of free carrier, λ is the wavelength of IR emission, m* is the effective mass of a free carrier, n is the refractive index, and τ is the relaxation time. As analyzed above, after the CdZnTe:In ingot diameter was enlarged from 30 to 60 mm, the dislocations of the CdZnTe:In crystal were augmented to a great degree. In fact, dislocations can produce half-atom surfaces in crystals and, therefore, results in unsaturated dangling bonds, which trap free carriers to make themselves saturated.10-12 Hence, the increase of dislocation density meant that more free carriers were trapped. As a result, the concentration of free carrier was reduced with the enlarging of ingot diameter, which brought on the fall of absorption coefficient according to eq 4. Judging from eq 3, the declining absorption coefficient resulted in the increase of IR transmittance of CdZnTe:In. PL measurement is sensitive to perceive the energy levels of defects and impurities in materials and, therefore, is often used to resolve different energy states of semiconductors. Figure 5 shows the low-temperature PL spectra of the CdZnTe:In ingots of 30 mm and 60 mm diameter. Both spectra can be divided into three regions: region (I), the near band-edge region, which involves the neutral donor bound exciton (D0, X) peak and the neutral acceptor bound exciton (A0, X) peak; region (II), covering the donor-acceptor pair (DAP) recombination and its phonon replicas (DAP-LO); region (III), showing the region associated with some defect energy levels. Here, the band located in the defect-related region is named as the D peak. Comparing the curves in Figure 5, we noticed that the intensity of the D peak was strengthened remarkably with the enlarging
For the CdZnTe:In ingot of 30 mm diameter, the EPD of dislocation was 2.1 × 104 cm-2, while this value was up to 1.9 × 105 cm-2 when the CdZnTe:In ingot diameter was enlarged to 60 mm. The full width at half-maximum (FWHM) of X-ray rocking also increased from 0.02014 ° to 0.02864° at the same time. The two phenomena imply that the crystalline quality of CdZnTe:In deteriorated with the enlarging of the ingot diameter. In addition, when the CdZnTe:In ingot diameter was varied from 30 to 60 mm, the average IR transmittance value was increased from 27 to 33%. This change is ascribed to the augmentation of dislocation, which was confirmed by the improvement of the intensity of the dislocation-related D peak in the PL spectra. Acknowledgment. This work was supported by the National Natural Science Foundations of China under Grant No. 50336040. References (1) Schlesinger, T. E.; Toney, J. E.; Yoon, H.; Lee, E. Y.; Brunett, B. A.; Franks, L.; James, R. B. Mater. Sci. Eng. Rep. 2001, 32, 103189. (2) Yang, G.; Jie, W.; Li, Q.; Wang, T.; Li, G.; Hua, H. J. Cryst. Growth 2005, 283, 431-437. (3) Seto, S.; Suzuki, K.; Abastillas, V. N. Jr.; Inabe, K. J. Cryst. Growth 2000, 214/215, 974-978. (4) Suzuki, K.; Inagaki, K.; Seto, S.; Tsubono, I.; Kimura, N.; Sawada, T.; Imai, K. J. Cryst. Growth 1996,159, 388-391. (5) Garandet, J. P. J. Cryst. Growth 1989, 96, 680-684. (6) Boiton, P.; Giacometti, N.; Duffar, T.; Santailler, J. L.; Dusserre, P.; Nabot, J. P. J. Cryst. Growth 1999, 206, 159-165. (7) Sen, S.; Rhiger, D. R.; Curtis, C. R.; Kalisher, M. H.; Hettich, H. L.; and Currie, M. C. J. Electron Mater. 2001, 30, 611-618.
438 Crystal Growth & Design, Vol. 7, No. 2, 2007 (8) Zhu, J.; Chu, J.; Zhang, X.; Li, B.; Cheng, J. Chin. J. Semicond. 1997, 18, 782-786. (9) Zong, X.; Wong, Y. The Foundations of Material Physics; Fudan University Press: Shanghai, 2001, p 706. (10) Gurusinghe, M. N.; Andersson, T. G. Phys. ReV. B. 2003, 67, 235208-1-235208-7. (11) Li, G.; Zhang, X.; Jie, W. Semicond. Sci. Technol. 2005, 20, 8689. (12) Pearson, G. L.; Read, W. T., Jr.; Morin, F. J. Phys. ReV. 1954, 93, 666-668.
Yang et al. (13) Hildebrandt, S.; Uniewski, H.; Schreiber, J.; Leipner, H. S. J. Phys. III 1997, 7, 1505-1514. (14) Yang, G.; Jie, W. Q. Appl. Phys. A [Online early access]. DOI: 10.1007/s00339-006-3701-2. Published online: September 21, 2006. http://www.springerlink.com/content/341nnj1478881157/. (15) Zelaya-Angel, O.; Garcia-Rocha, M.; Mendoza-Alvarez, J. G. J. Appl. Phys. 2003, 94, 2284-2288. (16) Geipel, H. J.; Tice, W. K. Appl. Phys. Lett. 1977, 30, 325-327.
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