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Physica B 393 (2007) 285–291 www.elsevier.com/locate/physb
Transport properties of Cd0.8Zn0.2Te crystals G.A. Gamal, M. Abou Zied, A.A. Ebnalwaled Physics Department, Faculty of Science, South Valley University, Qena 83523, Egypt Received 1 July 2006; received in revised form 11 January 2007; accepted 23 January 2007
Abstract After Cd0.8Zn0.2Te (CZT) crystals were prepared by a special design based on Bridgman technique, the transport properties were investigated. The dependence of electrical conductivity, Hall effect, Hall mobility, charge carriers concentration and thermoelectric power on temperatures was carried out in the temperature range (228–500 K). The relaxation time for both majority and minority carriers was estimated. The scattering mechanism for carrier transport was discussed in the same temperature range. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 81.10.h; 72.80.Ey; 05.60.Cd Keywords: Crystal growth; Semiconductors; Transport properties; Scattering mechanism; CdZnTe
1. Introduction Although the potential of CdZnTe for producing relatively large-volume gamma-ray spectrometers has been well demonstrated [1], numerous problems remain associated with growth process [2–6]. Crystal growth of CdZnTe is performed mainly by high-pressure vertical Bridgman (HPVB) and low-pressure horizontal or vertical Bridgman (LPB) method [7–13]. Undoped CdZnTe crystal is regarded as one of the most promising materials for room temperature nuclear detector [14]. Several reports have been made [15,16] to elucidate the nature of the defects, which have decisive role for the carrier lifetime. However, the dominant scattering mechanism for carrier transport and/or the value of the mobility for CdZnTe system still have some open questions. Very high mobilities of 1350 cm2 V1 s1 (electron) and 120 cm2 V1 s1 (hole) for Cd0.8Zn0.2Te at room temperature were reported [17]. These values are higher than those for Cd0.9Zn0.1Te [14] (electron 1000–1100 cm2 V1 s1, hole 50 cm2 V1 s1). Further, the values are even much higher than those reported for chlorine-doped CdTe [18].
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[email protected] (A.A. Ebnalwaled). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.01.016
Since, up to our knowledge, many parameters are not known for CZT crystals, like transport parameters and their temperature’s dependence, we undertook such work to report the growth of Cd0.8Zn0.2Te crystals and the results of temperature dependence of the electrical conductivity, Hall mobility, thermoelectric power and the charge carrier’s density for Cd0.8Zn0.2Te crystals. 2. Experimental details The feed materials were synthesized with 6N-source material of Cd, Zn and Te. The chemicals were obtained from Aldrich. The percentage of the charge elements are 39% Cd, 5.7% Zn and 55.3% Te, the total mass of the components are (5.85 g Cd, 0.855 g Zn and 8.295 g Te). Small Cd excess was attached to this percentage, to grow crystals under Cd-annealing according to the previous recommendation [19]. The chemicals were introduced in the ampoule, which was then evacuated to 106 Torr and sealed under this vacuum. The growth experiment took place in a modified vertical Bridgman technique {traveling solvent method (TSM)}(see Fig. 1). The present technique introduces a suitable solution to the presence of the pulleys and hence their problems. In this apparatus the ampoule, with its charge, was supported in its holder where the holes were placed in a three-zone tube furnace. This technique
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intensity (arb. units)
(220)
(111)
(311) (331)
(422) (511)
20
30
40
50 2θ (deg.)
60
70
80
Fig. 2. XRD pattern for Cd0.8Zn0.2Te crystals.
Fig. 1. Schematic of a modified Bridgman growth system.
was employed for growing Cd0.8Zn0.2Te crystals. Before each experiment, the furnace was calibrated. From previous work [20,21] the temperature profile was adjusted to melt the feed materials. The temperature gradient was measured with accurate K-type thermocouples. The ampoule was heated up to 900 1C within 7 h and then kept for 24 h to guarantee a homogeneous distribution of the materials in the melt. The melting point according to Ref. [22] depends on the percentage of Zn in the CdZnTe crystals. Due to the small amount of Zn in our crystals the melting point according to phase diagram [22] is 819.4 1C. We have already published results for Cd0.78Zn0.22Te, which was grown at the same temperature [23]. After that period and for the growth process, the ampoule was pulled down with 3 mm h1, without ampoule rotation. X-ray diffraction (see Fig. 2) shows that our sample is single-phase Cd0.8Zn0.2Te crystals with cubic structure and has lattice parameters, a ¼ 6.456 A˚. For the electrical conductivity and Hall effect measurements, the sample dimensions were adjusted to be 6.5 mm 2 mm 2 mm with the aid of razor blade and fine polishing papers (0.5 Leco mark, USA). The electrical conductivity and Hall effect measurements were measured in the direction perpendicular to the crystallographic c-axis. For the purpose of thermoelectric measurements the length was adjusted to be 5 mm, while the crystal cross-section was 8 mm in diameter.
The silver conducting paste contacts were soldered on the Cd0.8Zn0.2Te to carry out the electrical conductivity, Hall effect and thermoelectric measurements. We put a small point of silver paste on each contact area of a polished Cd0.8Zn0.2Te. The contacts were let to dry off in air and after that the specimen was annealed in evacuated atmosphere at100 1C. From the current–voltage characteristics, the ohmicity coefficient has the nearest value of 1. In this experiment, three different cuts from the virgin ingot were prepared for both electrical and thermoelectric power measurements. The results and the general behavior were the same. However, some upward or downward shift was observed. 3. Results and discussions 3.1. Electrical properties for Cd0.8Zn0.2Te The present electrical conductivity work was carried out in a temperature range extending from 217 up to 500 K. Fig. 3 shows the electrical conductivity s vs. 103/T for Cd0.8Zn0.2Te crystals. As shown, in the investigated temperature range, the logarithm of the conductivity showed a linear dependence on the temperatures with two modes of conduction in addition to the transition region that appeared between them. From the relationships between the conductivity and the temperature, the energy gap is deduced to be 1.35 eV while the ionization energy is 0.25 eV. The increase of s in the intrinsic part (above 393 K) is regarded as a result of excitation of the carriers from the valence band to the conduction band. However, in the extrinsic part (below 272 K), s increment is regarded as a result of ionization of impurity atoms. The transition region (272–393 K) is characterized by a slight increase of s as the temperature increases. This is due to the dominant charge carrier concentration effect in this range, as we will see. At room temperature, s has the value 3.1 104 O1 cm1. The Hall effect measurements were performed in the same temperature range. The sign of RH indicates that
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0.0 -0.5 -1.0 Lg [σ(Ohm. cm)-1]
-1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
103 / T (k-1) Fig. 3. Temperature dependence of electrical conductivity for Cd0.8Zn0.2Te crystals.
10.35
Lg [ RH. T3/2 (K3/2.cm3/ C)]
9.90 9.45 9.00 8.55 8.10 7.65 7.20 6.75 6.30 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
103 / T (k-1) Fig. 4. Plot of log(RH T3/2) vs. 103/T for Cd0.8Zn0.2Te crystals.
Cd0.8Zn0.2Te behaves as a p-type semiconductor. The relationship between RHT3/2 and 103/T was plotted in Fig. 4. From the figure, the forbidden gap width is obtained to be 1.42 eV, whereas the ionization energy as calculated from the same curve (at low temperature) is 0.3 eV. Simultaneous measurements of the electrical conductivity and Hall effect permit us to investigate the influence of temperature on Hall mobility. This is typically presented in Fig. 5. At low temperatures, the mobility increases with temperature (up to T ¼ 338 K). Above this temperature, the Hall mobility decreases with temperature. The work of the Hall mobility enabled us to gain a good interpretation to the scattering mechanism of the charge carriers. From the results we noted that: 1. The relationship between m and T in the extrinsic region of temperature justifies the following relationship m p T3.8.
Such dependence leads to the assumption that the scattering mechanism is due to the impurities in this range. 2. In high temperature range (intrinsic), the mobility obeys the power law m p T3.4. Based on this, we consider that the effect of the short-range scattering optical phonon is the main reason for the scattering mechanism in this range. The m–T dependence observed in the Cd0.8Zn0.2Te crystals is similar to that reported for other semiconductor materials [23–25]. Fig. 6 depicts the results of charge carrier density vs. reciprocal temperature. There are three regions of this curve, namely the extrinsic (low temperature side 222–273 K), the transition (273–348 K) and the intrinsic region (above 348 K). We calculated the energy gap width from the slope of Fig. 6 in the intrinsic region. It is found to
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2.6 2.4
Lg [ μ(cm2/ V. s)]
2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 2.35
2.40
2.45
2.50
2.55 Lg [ T (K)]
2.60
2.65
2.70
2.75
Fig. 5. Hall mobility as a function of temperature for Cd0.8Zn0.2Te crystals.
17.0 16.5 16.0
Log P (cm-3)
15.5 15.0 14.5 14.0 13.5 13.0 12.5 12.0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
103 / T (k-1) Fig. 6. Variation of the carrier density vs. reciprocal of temperature.
0.07
α(mV / K.)
0.06
0.05
0.04
0.03 1.5
2.0
2.5
3.0
3.5
4.0
4.5
1000/T (K-1) Fig. 7. Thermoelectric power vs. 103/T for Cd0.8Zn0.2Te crystals.
5.0
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3.2. Thermoelectric properties for Cd0.8Zn0.2Te
be 1.42 eV. Also the calculation in the extrinsic region showed an acceptor level of impurities at 0.3 eV. The room temperature hole concentration is 3.3 1013 cm3.
The thermoelectric power measurements were carried out as a complementary part to the electrical conductivity and Hall effect work to: 1. Illustrate the importance of combination of the electrical and thermal measurements. 2. Gain a more definite idea about the semiconductivity and the behavior of Cd0.8Zn0.2Te.
0.07
0.06 α(m V / K)
289
We measured the thermoelectric power when the temperature gradient was produced in a direction perpendicular to the c-axis by the differential method in a wide temperature range from 250 up to 475 K. Fig. 7 shows the relationship between a and 103/T for Cd0.8Zn0.2Te. From the figure, we can find that the sign of a is positive indicating that Cd0.8Zn0.2Te is p-type semiconductor. The value of a is found to decrease continuously with increasing temperature in the range from 250 up to 338 K. This leads to the assumption of
0.05
0.04
0.03 -10
-8
-6
-4
-2
0
[Ln {σ(Ohm. cm)-1}] Fig. 8. Relation between thermoelectric power and electrical conductivity.
0.07
α(m V / K)
0.06
0.05
0.04
0.03 28
29
30
31
32
33
34
35
36
37
38
39
40
Ln {P(cm-3)} Fig. 9. Relation between thermoelectric power and charge carrier density.
0.07
α(m V / K)
0.06
0.05
0.04
0.03 5.4
5.5
5.6
5.7
5.8
5.9 6.0 Ln [T (K)]
6.1
Fig. 10. Plot of thermoelectric power and ln(T).
6.2
6.3
6.4
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presence of trapping centers or some crystal defects in the direction of the carrier flow. From 338 to 372 K a is increased with temperature. This is attributed to the thermal activation of the charge carriers in this range. Up to 372 K, the value of a decreases as the temperature increases. This is a result of the compensation process, which occurs in this temperature range. The value of a at room temperature is (47 mV K1). For more definite understanding of the behavior of TEP we used our electrical conductivity and Hall effect data to construct Figs. 8 and 9. From Figs. 8 and 9 and their comparison with Fig. 7 we can conclude that the concentration of the charge carriers is the dominant factor that governs a. 3.3. Transport parameters for Cd0.8Zn0.2Te Some major semiconductor parameters for Cd0.8Zn0.2Te crystals, such as the electron-to-hall mobility ratio, relaxation time of majority and minority carriers, diffusion coefficient and diffusion length can be estimated by using the formulas suggested in Refs. [26,27]. So, we plot Figs. 7 and 10 to show the relation between a and 103/T, a and ln(T), respectively. The values of the relaxation time were calculated to be tp ¼ 5.4 1011 s and tn ¼ 4.9 1011 s, for holes and electrons, respectively. The data for relaxation times was different compared to those published. The possible reasons for this discrepancy are detrapping of charge carriers and non-uniformity of an electric field [28,29] or Zn concentration [30,31]. The diffusion constants for electrons and holes were calculated to be Dp ¼ 1.5 cm2 s1 and Dn ¼ 1.7 cm2 s1. The diffusion length for holes is Lp ¼ 9 106 cm, while for electrons is Ln ¼ 9.2 106 cm. We computed the ratio mn =mp to be 1.05. Finally, the straight-line slope of a plot of a against ln T, for a given sample was found to be (129 mV K1). This value is typically the theoretical prediction (129 mV K1), which indicates that the effective masses of the charge carriers are temperature independent for Cd0.8Zn0.2Te crystals. 4. Conclusion In the present work, single-phase Cd0.8Zn0.2Te crystals were grown by using the modified vertical Bridgman technique. The present experimental results can be summarized as follows: 1. The electrical conductivity at room temperature has the value 3.1 104 O1 cm1. 2. The energy gap is deduced to be 1.39 eV while the ionization energy is 0.28 eV. 3. The sign of RH indicates that Cd0.8Zn0.2Te behaves as a p-type semiconductor. The room temperature hole concentration is 3.3 1013 cm3. 4. The scattering mechanism in low-temperature range is
due to the impurities. While in high-temperature range (intrinsic), the effect of the short-range scattering optical phonon is the main reason for the scattering mechanism. 5. The sign of a for Cd0.8Zn0.2Te is positive indicating that the conducting mechanism in this sample is of p-type. The value of a at room temperature is 47 mV K1. 6. We computed the ratio mn =mp to be 1.05. Also the ratio mn =mp was found to be 0.84 for Cd0.8Zn0.2Te crystals. 7. The effective masses of the charge carriers are temperature independent for Cd0.8Zn0.2Te crystals.
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