Electrical properties and electrical field in depletion layer for CZT crystals

LI Qiang(李  强), JIE Wan-qi(介万奇), FU Li(傅  莉), YANG Ge(杨  戈),

ZHA Gang-qiang(查钢强), WANG Tao(王  涛), BAI Xu-xu(白旭旭)

College of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Current—voltage (I—V) and capacitance—voltage (C—V) characteristics of Au/p-CZT contacts with different surface treatments on cadmium zinc telluride (CZT) wafer’s surface were measured with Agilent 4339B high resistance meter and Agilent 4294A precision impedance analyzer, respectively. The Schottky barrier height was 0.85±0.05, 0.96±0.05 eV for non-passivated and passivated CZT crystals by I—V measurement. By C—V measurement, the Schottky barrier height was 1.39±0.05, 1.51±0.05 eV for non-passivated and passivated CZT crystals. The results show that the passivation treatment can increase the barrier height of the Au/p-CZT contact and decrease the leakage current. The main reason is that the higher barrier height of Au/p-CZT contacts can decrease the possibility for electrons to pass through the native TeO₂ film. Most of the applied voltage appears on the depleted layer and there is only a negligible voltage drops across the nearly undepleted region. Furthermore, the electric field in the depleted layer is not uniform and can be calculated by the depletion approximation. The maximum electric field of CZT crystals is E_m1=133 V/cm at x=0 for non-passivated CZT crystal and E_m2=55 V/cm for passivated CZT crystal, respectively.

Key words: current—voltage characteristic; capacitance—voltage characteristic; depletion layer; electrical field

1 Introduction

Cadmium zinc telluride (CZT) has shown great promise for the application in large volume room- temperature X-ray and gamma-ray spectrometers [1, 2]. However, the performance of X-ray and gamma-ray spectrometers fabricated from CZT detector crystals is often limited by surface leakage currents which act as an important source of noise. The surface leakage current is mainly due to the presence of the low-resistivity surface layer that has a different chemical composition and band structure in comparison with the bulk material. Surface damaged layers caused by mechanical polishing can be removed with 5% Br-methanol solution (Br-MeOH)[3]. Although the chemical etching can produce a smooth surface, the surface is non-stoichiometric and has been observed to be Te-enriched for CZT. MONTGOMERY [4] demonstrated that a thick layer of amorphous tellurium could be chemically produced on the surface of CdTe, and this layer had indeed a very low resistivity because of the narrow bandgap of Te, 0.33 eV. Therefore, the process of passivation has been typically required to reduce the conductivity of Te-riched surface layer and decrease the surface leakage current. Surface passivation technologies include the deposition of dielectric materials (ZnS, SiO₂), coating the surface with the native films (oxides, sulphides, fluorides), and in-situ growth of heterostuctures of wide band gap Ⅱ-Ⅵ compound [5]. WRIGHT et al[6] evaluated NH₄F/H₂O₂ effectiveness as a surface passivation agent for CZT crystals and found that NH₄F/H₂O₂ surface passivation significantly improved the sensitivity and energy resolution of CZT detectors.

In this work, the electrical properties of Au/p-CZT contact were analyzed by I—V and C—V measurements.

2 Experimental

2.1 Sample preparation

A boule of Cd_1-xZn_xTe (x=0.1) grown by Bridgman method was cut into the wafers of 10 mm×10 mm×2.5 mm. These wafers were mechanically polished using diamond paste and MgO powders of 0.5 μm in diameter. The wafers were chemically etched with 5% Br- methanol solution (Br-MeOH). Then these samples were rinsed with methanol and dried with N₂. For non- passivated Au/p-CZT contacts, a 100 nm-thick layer of gold with the size of 10 cm×10 cm was sputtered by KYKY-121 after etching.

For passivated samples, the prepared wafers were first passivated with 10%NH₄F-10%H₂O₂-H₂O solution for 15 min and rinsed with de-ionized water. And then Au contact was coated by the same way as non-passivated samples.

2.2 Measurements

The current-voltage measurements were performed at room temperature using Agilent 4339B. The Agilent 4339B high resistance meter was capable to detect the current down to picoamperes. The capacitance—voltage characteristic was measured by Agilent 4294A Precision impedance analyzer at room temperature with the frequency of 1 MHz.

3 Results and discussion

3.1    I—V characteristic

One of the most widely used techniques to measure the Schottky barrier height (SBH) is the current—voltage (I—V) technique[7]. In particular, the forward-bias portion of the I—V characteristic has often been used to deduce the magnitude of the SBH[1]. The transport of carriers across a MS interface is sensitively dependent on the magnitude of the barrier height. The current based on just the flow of electrons from the semiconductor into the metal can be written as I_s→m. The current for electrons flowing from the metal into the semiconductor is given by I_m→s. Assuming that thermionic emission is the only transport mechanism with a barrier height greater than kT/q and independent of the bias, and the series resistance and ideal factor n are negligible. The net current at a given bias V can then be expressed by Eqn.(3).

            (1)

                   (2)

                       (4)

where  q is the electron charge; is the Schottky barrier height; V is the applied voltage; S is the effective contact area; k is the Boltzmann’s constant; T is the temperature in Kelvin; A^* is the effective Richardson constant; I₀ is the saturation current and is derived from the straight line intercept of at V= 0.

At large forward bias, V>3kT/q, the normal 1 in the square bracket can be ignored, and the current should have an exponential dependence on the applied bias. The barrier height can be calculated by Eqn.(3) and the results listed in Table 1 are determined.

Fig.1 I—V character of passivated and non-passivated p-CZT wafer

3.2  C—V characteristic

Another often used and convenient technique to measure the SBH is the capacitance—voltage (C—V) technique. The depletion layer in MIS capaci­tance per unit area can be expressed as

                             (5)

                         (6)

where  V_bi is the barrier built-in potential; V is the applied voltage; ε₀ is the permittivity of vacuum; ε_S is the dielectric constant of CZT(about 11); N_A is the doping level of the substrate. Eqn.(6) shows that the relationship of C^-2—V is a convenient method to deduce the doping profile of the semiconductor. In the case that the doping profile is homogeneous throughout the space charge region, a C^-2—V plot should yield a straight line in the reverse bias region with a slope of –2/(qε₀ε_SN_A) and intercept with the bias voltage axis of V_int. The value of this intercept, known as the built-in potential, can be used to deduce the barrier height through the following relationship:

                     (7)

where

                         (8)

V_int is the intercept in the V-axle of C^-2—V curve, N_v is the state density in the valance band of CZT. Fig.2 shows the C^-2—V curve of CZT wafer before and after passivation. The calculation results are listed in Table 1.

Fig.2 C^-2—V character of p-CZT wafer: (a) Non-passivation; (b) Passivation

Table 1 Barrier height for p-CZT wafers with different methods

Compared to the I—V measurements, the C—V curves get a higher Schottky barrier height values. The discrepancy of barrier heights obtained from the two methods can be attributed to inhomogeneous distribution of barrier heights at metal/semiconductor interface, including a combination of the interfacial oxide layer composition, non-uniformity of the interfacial layer thickness and distribution of interfacial charges[8]. In C—V measurements, the capacitance is insensitive to potential fluctuations at a width less than the space charge region. To the I—V measurements, the current across the interface depends exponentially on and the barriers lowering effect of the interfacial oxide layer or interface states. Consequently, for an inhomogeneous interface, space changes of band bending V_bi and  result in different Schottky barrier height for current and capacitances [9, 10].

Whichever the measurement is chosen, the barrier height increases after passivation. The surface leakage current of p-CZT decreases as the oxide thickness increases, because the increasing barrier height reduces the possibility for electrons to pass through the native TeO₂ film [11].

   For non-passivated p-CZT crystal, the holes recombine with electrons at interface states or in the valance band depletion region, and therefore greater surface leakage current is caused. However, for passivated CZT crystal with thicker oxides film, the higher interface barrier caused by TeO₂ film restrains the holes from tunneling through the barrier.

3.3 Depletion layer width and applied electric field calculation

For the lower resistivity CZT crystals(ρ＜      10⁸ Ω?cm), assume the hole mobility μ=100   cm²/(V?s), the applied voltage is in the range of  0-100 V, the depletion layer width can be expressed as

                       (9)

For non-passivated CZT wafer, assuming the hole mobility μ=100 cm²/(V?s), N_A=5.35×10¹⁰ cm^-3 (corre- sponding to ρ=1.17×10⁶ Ω?cm), the depletion layer width extends between 0.15mm(at V=0, assuming V_bi=1) and 1.5 mm at V= 100 V. Therefore, it is not fully depleted to 2.5 mm-thick detector of such material at 100 V.

For passivated CZT wafer, assuming the hole mobility μ=100 cm²/(V?s), N_A=9.17×10⁹ cm^-3 (corre- sponding to ρ=6.82×10⁶ Ω?cm), the depletion layer width extends between 0.36mm(at V=0, assuming V_bi=1) and 2.5 mm at V= 47 V. Therefore, a 2.5 mm thick detector of such material exhibits both regions of partial depletion (below 47 V) and over depletion (47 V＜V＜100 V).

Most of the applied voltage appears on the depleted layer and there is only a negligible voltage drops across the nearly undepleted region. Furthermore, the electric field in the depleted layer is not uniform and can be calculated by the depletion approximation. Fig.3 shows the applied field diagram of nonpassivated and passivated CZT crystal, which E_m1 and E_m2 are maximum electric field of CZT crystals at x=0, respectively. The results of depth of the depletion layer and electric field calculation are shown in Table 2.

Fig.3 Diagram of applied electric field for non-passivated (E_m1) and passivated (E_m2) CZT crystals

Table 2 Results of depth of depletion layer and electric field calculation

                      (10)

The maximum electric field E_m at x=0 is

   (11)

The voltage of reach-through (V_RT) bulk material is

                              (12)

For V＞V_RT, the effect of the overvoltage V-V_RT is to increase the field by a constant amount (V-V_RT)/L everywhere within the detector.

                  (13)

4 Conclusions

1) The Schottky barrier height is 0.85±0.05, 0.96±0.05 eV for non-passivated and passivated CZT crystals by I—V measurement. By C—V measurement, the Schottky barrier height is 1.39±0.05, 1.51±0.05 eV for non-passivated and passivated CZT crystals. The C—V measurements obtain a higher Schottky barrier height value than those of the I—V measurements. The reason is related to the inhomogeneous distribution of barrier heights at metal/semiconductor interface.

2) The depletion layer width is 0.15 mm for non-passivated CZT crystal and 0.36mm for passivated CZT crystal, respectively. The maximum electric field of CZT crystals is E_m1=133 V/cm at x=0 for non-passivated CZT crystal and E_m2=55 V/cm for passivated CZT crystal, respectively.

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(Edited by LI Xiang-qun)

Foundation item: Project(50336040) supported by the National Natural Science Foundation of China

Corresponding author: LI Qiang; Tel: +86-29-88486065; E-mail: jerrylee57318@hotmail.com