CdZnTe(111)A

Sep 14, 2010 - Shouzhi Xi , Wanqi Jie , Gangqiang Zha , Wenhua Zhang , Junfa Zhu , Xuxu Bai , Tao Feng , Ning Wang , Fan Yang , and Rui Yang...
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J. Phys. Chem. C 2010, 114, 16426–16429

Interface Dipole and Schottky Barrier Formation at Au/CdZnTe(111)A Interfaces Xuxu Bai,† Wanqi Jie,*,† Gangqiang Zha,† Wenhua Zhang,‡ Junfa Zhu,‡ Tao Wang,† Yanyan Yuan,† Yuanyuan Du,† Yabin Wang,† and Li Fu† State Key Laboratory of Solidification Processing, Northwestern Polytechnical UniVersity, Xi’an 710072, P. R. China, and National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei 230029, P. R. China ReceiVed: April 12, 2010; ReVised Manuscript ReceiVed: August 19, 2010

Synchrotron radiation photoemission spectroscopy (SRPES) has been used to study the electronic structure of the Au/CdZnTe(111)A for Au coverage ranging from about 0.3 up to 20 monolayers (ML). It is found that a Schottky barrier with a height of 0.82 eV is formed at the initial deposition of Au. This barrier decreases gradually with increasing Au coverage, which can be ascribed to band bending caused by charge redistribution at the interface and the formation of a positive interface dipole introduced by Cd diffusion. After an annealing process, a signal due to the formation of Au-Cd alloy caused by exquisite Cd diffusion into Au overlayer is observed, and simultaneously the Schottky barrier height (SBH) reduces to 0.32 eV. The present work indicates that cation diffusion into metal overlayer plays a critical role in controlling the SBH. 1. Introduction CdZnTe (CZT) single crystals have important scientific and technological applications in many fields including medical imaging, homeland security inspection, and spaceborne X-ray and gamma-ray astronomy.1-5 These applications rely on several interesting properties of CZT crystals, i.e., large adsorption coefficient, high resistivity, and room temperature operation capability.6 The most common configuration of CdZnTe-based radiation detectors is a metal-semiconductor-metal (MSM) structure. Even if a CZT-based device is fabricated with high quality materials, the nonoptimized contacts will result in an inferior performance.7 Therefore, it is meaningful to investigate the optimum condition of metal/CZT contact, where the chemical composition, the interdiffusion between the metal and CZT crystal, the defect formation, and their interaction may significantly affect the practical performance of the devices.8-10 A survey of published work shows that the SBH at the AuCdTe contact depends on the device annealing process and the formation of an intermediate Au-Te compound.11,12 However, little attempt has been made to understand the effect of metallization, interface diffusion, and thermal annealing.13-15 Likewise, many theoretical models have been developed to explain the formation of Schottky barrier, such as the Schottky-Mott model,16 the unified defect model (UDM),17 and the metal-induced gap states model (MIGS).18-20 In addition, the Coulomb buffer/interface model,21 the surface dipole model,22 and bond-polarization theory23 were also used to explain the formation of Schottky barrier. These models all focus on a particular aspect of the interface density of states, providing a simple picture of the SBH formation and neglecting other features of the spectrum. In this paper, SRPES was used to investigate the interface electronic structure of Au/CdZnTe(111)A as Au was deposited on CdZnTe(111)A at room temperature from submonolayer to multilayer coverages, and the change of the interface after annealing. The core level spectra indicate the charge transfer * To whom correspondence should be addressed: [email protected]. † Northwestern Polytechnical University. ‡ University of Science and Technology of China.

from Au to CdZnTe(111)A upon the formation of interface. A cation-diffusion controlled Model was used to explain the formation of the interface. The goal of this paper is to establish a more fundamental understanding of the mechanism controlling the SBH at the Au/CZT interface. 2. Experimental Section The experiments of SRPES were performed at the Surface Physics Station of the National Synchrotron Radiation Laboratory (NSRL) of China. The endstation equipment has been described in detail previously.24 Briefly, the systems work under UHV conditions (better than 2 × 10-10 mbar). The beamline covers the energy range from 10 to 250 eV, and the energy resolution (E/∆E) is better than 1000. The analysis chamber is equipped with a VG ARUPS10 electron energy analyzer for SRPES and XPS, a twin-anode X-ray gun, a retractable fourgrid optics for low energy electron diffraction (LEED), and an Ar+ sputter gun. The MBE chamber comprises a quartz crystal microbalance (QCM) for monitoring the deposition rates. The undoped Cd0.9Zn0.1Te crystal was grown by the modified Bridgman method in our laboratory.25 CZT samples were cut into the wafers of 8 × 8 × 1.5 mm3 with the surface parallel to (111) planes. Then CdZnTe samples were first mechanically polished using 0.5 µm size magnesia suspension then chemically polished with a 2% Br-MeOH solution for 5 min. Afterward, there were rinsed in deionized water and finally dried with nitrogen.26 The standard procedure of surface cleaning and surface structure ordering were done by soft Ar+ sputtering (1.0 kV, 3 µA, 1 × 10-5 Torr of Ar), followed by sample annealing in UHV at 200 °C for more than 1 h. After this treatment, neither the C 1s nor the O 1s peaks were found by XPS measurement and sharp lattice spots were observed by LEED. The clean sample was kept in the ultrahigh vacuum system with the pressures in the low 10-10 Torr for subsequent Au deposition, which was performed in the MBE chamber. The deposition rate was monitored by QCM. Pressures during the deposition ranged from the mid 10-9 to low 10-8 Torr. No contamination was detected after Au deposition. Au coverage was determined from the amount of Au incident on the sample

10.1021/jp1032756  2010 American Chemical Society Published on Web 09/14/2010

Properties at Au/CdZnTe(111)A Interfaces

Figure 1. Te 4d core level and the valence band of the Au/ CdZnTe(111)A interfaces for various Au coverage, recorded at normal emission with photon energy of 150 and 28 eV, respectively.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16427

Figure 2. (a) Au 4f core levels with different Au coverage. (b) Difference valence band spectra obtained by subtracting from the spectra in Figure 1b properly attenuated spectra of the clean substrate.

surface assuming unity sticking coefficient with one monolayer (ML) defined to be one Au atom per surface Cd and Te atom, i.e., 1 ML equals 8.37 × 1014 atoms/cm2. Accordingly, monolayer thickness corresponds to 1.5 Å of metallic Au. The valence band spectra were taken with the photon energy of 28 eV. The Au 4f and Te 4d core levels were taken with the photon energy of 150 eV to get the minimum mean free path of the electrons.27 All of the binding energies reported in this work were referenced to the Au 4f7/2 (84.0 eV) which was acquired from an Au foil attached to the sample holder just below the CZT sample. 3. Results and Discussion Figure 1 shows Te 4d core level and valence band spectra for Au on CdZnTe(111)A with different Au coverages. As can be seen, Te 4d and Cd 4d core levels shift gradually toward lower binding energy with increasing Au coverage, and a value of 0.12 eV is reached at 3.4 ML, which suggests that a band bending occurs at the Au/CdZnTe(111)A interface. Meanwhile, a clear emission of metallic Fermi edge (EF) can be detected at Au coverage as low as 1.8 ML and confirms the formation of the Au cluster. Beyond 3.4 ML, no additional shift is observed in Te 4d and Cd 4d core levels, and the FHWM (full width at half-maximum) almost does not change. Whereas, plus changes in the Cd 4d peak shape, which may result from Cd diffuse into Au overlayer. At 20 ML Au coverage, the spin-orbit splitting of Cd 4d can be clearly resolved, indicates that the distribution of the environments in which Cd is found becomes more atom-like, which differs from that of the substrate where the k-space dispersion of the shallow Cd core levels obscured the spin-orbit splitting. The fwhm of Au 4f peaks decrease monotonically with increasing Au coverage, shown in Figure 2a, which may be ascribed to the distribution of the number of Te and Cd atoms seen by a given Au atom becomes more uniform. The similar result was found in Au/GaAs,28 Au/InSb29 and Au/InSe30 interface. During Au deposition, the Au 4f core level always shows a value close to that expected for metallic Au.31 Although the clusters would have a higher binding energy, the band bending covered this movement. The effects of Au coverage on the valence band of the CZT were depicted with the difference spectra, where the subtraction of the CZT substrate signal took into consideration the attenuation induced by the Au overlayer, as shown in Figure 2b. The intensity of the Au 5d peaks increase and the fwhm of them

Figure 3. Te/Cd and Au/Cd ratios for Au/CdZnTe (111)A interface, as a function of the thickness of Au layers.

decrease, when Au coverage increases from the submonolayer to the multilayer. At Au coverage as low as 0.3 ML, the separation between the two Au 5d peaks is around 1.85 eV, which changes to 2.4 eV with 20 ML Au coverage. At the initial deposition of Au, Au on the surface act atomic-like. With Au coverage increases, the Au 5d level changes from an atomiclike spin-orbit split doublet into bulk Au metal as the number of nearest neighbor Au atoms increases.32,33 The binding energy of the two Au 5d band components exhibits different behaviors. The Au 5d3/2 component is shifted up by about 0.25 eV, whereas the Au 5d5/2 component is shifted down by more than 0.3 eV. The shifts of the two peaks may be ascribed to differential relaxation, exchange and correlation effects.34 To reveal details about the formation of Au/CdZnTe(111)A interface, the Te/Cd and Au/Cd ratios with different Au coverage were obtained, as shown in Figure 3. At the initial deposition of Au, the Te/Cd and Au/Cd ratios increase near exponentially, which may be caused by the formation of Au cluster and the diffusion of Cd element into Au cluster.35 Between 1.8 and 6 ML, the Te/Cd ratio decreases, meanwhile Au/Cd ratio increases slowly, which may due to further deposition of Au incline to form a uniform overlayer and the Cd-out-diffusion into surface region of Au overlayer. Beyond 6 ML, the Te/Cd ratio decreases successively, whereas, Au/Cd ratio increases dramatically, which is in consistent with the report of Friedman et al.9 that Cd-Te bond stronger than the Cd-Au bond.

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Bai et al.

Figure 4. Schematic representation of diffusion processes near the interface: (a) clean and ordered CZT surface; (b) 0.3-3.4 ML Au; (c) 6-20 ML Au; and (d) Au/CZT after annealing process.

Figure 6. Valence band taken at two different photoemission angles.

Figure 5. Schottky barrier height decreases as a function of the Au coverage.

According to the former discussion, the scheme of the Au deposited on the CdZnTe(111)A surface process is shown in Figure 4. In Figure 4a, clean and ordered CZT surface was shown. Between 0.3 and 3.4 ML, Cd out diffusion into Au cluster is shown in Figure 4b. Beyond 6 ML, the density of dissolved Cd tends to constant, as shown in Figure 4c. Figure 4d shows the interface region after annealing. The SBH with different Au coverage was also measured by SRPES, and the details of the measurement were described in our previous paper.26,36 In the measurement, the Fermi level is considered to be fixed at Au Fermi edge. As shown in Figure 5, after 0.3 ML Au deposition, Au/CdZnTe(111)A interface forms a Schottky barrier with a height of 0.82 eV, then the SBH decreased gradually with increasing Au coverage. Beyond 6 ML, the SBH becomes stable. The deposition of Au on the CZT surface results in a charge transfer across the interface, which leads to interfacial electric dipole layers and a band bending in the semiconductor. According to small band bending (0.12 eV) and large SBH reduction (0.33 eV) during the Au deposition, indeed, the interface dipole caused a potential drop. Cd diffusion into Au overlayer leads to an electric field pointing from Au (δ+) toward the CZT substrate (δ-), resulting in a positive dipole on the interface. The SBH as a function of Cd diffuses into the Au region, a cation-controlled SBH model was built to explain the variation of SBH. The SBH to a p-type semiconductor,Φ0B,p, can be written as23 0 ΦB,p ) Eg + (χS - φM) + eVint

(1)

where φM is the work function of the metal, χS is the electron affinity of the semiconductor, and Vint is the voltage drop due

to an interface dipole. We assumed that the Cd diffusion leads to a homogeneous interface structure, so that the electronic properties are also homogeneous for the entire Au/CZT interface. And the interface assumed to be atomically abrupt. In order to simplify the model, the diffusion caused charge atoms only interacted with nearest neighbors. The chemical potential of the metal atom at the interface is equal to that of a semiconductor atom. According to Tung,37 one thus obtains 0 ΦB,p ) γB(χS - φM) + (1 + γB)Eg /2

(2)

where

γB ) 1 -

e2dMSNB εit(Eg + κ)

(3)

where κ is the sum of all of the hopping interactions and dMS is the distance between metal and semiconductor atoms at the interface. εit is dielectric constant of interface region. NB is the density of interface chemical bonds, which is related to the diffusion of Cd into Au overlayer. φM of Au is 5.1 eV, and Eg equals to 1.6 eV. χS of the CZT surface can be calculated by χS ) φS - Eg/2 is 4.9 eV. Due to screening, κ is small compared with typical band gaps. Based on above assumptions, εit and dMS are nearly constant during the deposition of Au. Bring all parameters in eqs 2, ΦB,p ) 1.4 - 3e2dMSNB/5εitEg, was obtained. Where, ΦB,p is a function of NB. Between 0.3 and 3.4 ML, the SBH decreases with NB increasing. The SBH tends to be a constant beyond 6 ML, as the Cd-Te bond is greater than Cd-Au, the density of NB becomes invariable. In order to find a more fundamental understanding of the role of Cd atoms in controlling the SBH, valence band spectra were recorded at two different emission angles (Au coverage 20 ML) for the surface of as deposited and annealed at 350 0 for 1 h, shown in Figure 6. After annealing process, for both emission angles, the relative intensity of low binding energy Cd 4d splitting part increases, which we assumed to be Cd dissolved into Au overlayer to form Au-Cd alloy. At the emission angle of 45°, which is supposed to be more surface sensitive, a signal

Properties at Au/CdZnTe(111)A Interfaces

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16429 50902114, and 50772091), the 111 Project (B08040), and the Foundation for Fundamental Research of (Northwestern Polytechnical University (JC200928). The authors are grateful to all of the members of NSRL for their help with the experiments. References and Notes

Figure 7. Cd 4d and Te 4d core levels taken from Au/CZT (Au, 20 ML) for as deposited and after annealing.

due to the formation of Cd-Au alloy appears at 10.5 eV below EF, in good agreement with literature.38,39 A clear signal from Cd alloyed with the overlayer has been seen for Pt and Cu overlayers on CdTe by Alnajjar et al.40 and Friedman et al.41 The Cd 4d and Te 4d spectra of as deposited (Au, 20 ML) and annealed interface taken with photon energy of 150 eV are shown in Figure 7. It shows that the intensity of Cd 4d increases after annealing, whereas the intensity of Te 4d decreases, which coincides with the appearance of signal due to the formation of Cd-Au alloy in the valence band. In addition, after annealing, the Te 4d and Cd 4d core levels shift slightly toward the low binding energy. The measured SBH decreased to 0.32 eV, which is consistent with the further band bending and increasing in density of interface dipole. These phenomena give a hint that the formation of Au-Cd alloy, which is related with Cd out-diffusion, plays an important role in controlling of the barrier height, when a heating process is involved. Further reduction in the SBH makes improvement of Au/CZT interface performances possible, and it is very meaningful in fabricating suitable electrode for CZT detector. 4. Conclusions The electronic properties of the Au/CdZnTe(111)A interface has been studied by synchrotron radiation photoemission measurements. During the initial deposition of Au, it was found that Au film grown on the CdZnTe(111)A in the form of cluster and Cd diffuses into Au cluster. A classical Schottky barrier was formed with a height of 0.82 eV at the initial deposition of Au. The SBH reduces as increasing Au coverage, that caused by band bending and the formation of a positive interface dipole related to Cd diffusion into Au overlayer. A cation-controlled SBH model was built to explain this decreasing. After annealing, the diffusion of Cd into Au overlayer becomes intense, a signal due to the formation of Cd-Au alloy appears in the valence band and the barrier height decreased to 0.32 eV, which confirms the cation-controlled SBH model. We conclude that the cation diffused into metal region plays an important role in controlling the barrier height. Further adjustment in the SBH makes improvement of metal/semiconductor interface performances possible, and it is very meaningful in fabricating suitable electrodes. Acknowledgment. This work has been supported by the Project of National Natural Science Foundation of China (50902113,

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