Surface Treatment, Metal Electrode Contact And Indium Doping Behavior Of Cd_1-xZn_xTe Crystals

Cd_1-xZn_xTe(CZT) possesses excellent optoelectronic properties and is thereforethe most promising material for room temperature X-ray and gamma-ray detectors.Although the study on CZT has lasted for a long period, there still exist someproblems in the surface treatment of CdZnTe crystal, metal-CdZnTe contact propertiesand indium doping behaviors of CZT, which are investigated in this work.The process of passivation had been typically required to reduce the conductivityof Te-riched surface layer of CZT crystal and decrease the surface leakage current.Passivation is a chemical and /or physical process that renders the surface of amaterial chemically and/or electrically inert to its environment. Typical surfacepassivation technologies include the deposition of dielectric materials (ZnS, SiO₂),coating the surface with the native inert films, such as oxides, sulphides and fluorides,and in-situ growth of heterostuctures of wide band gapâ…¡-â…¥compound. The oxideformed on CZT surface through the passivation treatment with NH₄/H₂O₂ agent isTeO₂, which offers an inert surface. After passivation,([Cd]+[Zn])/[Te] ratioapproches to 1, which means the stoichiometry composition is restored and thecrystallinity, is improved near the surface. Photoluminescence(PL) spectra confirmedthat the passivation treatment minimized the surface trap state density and decreasedthe deep level defects related to recombination of Cd vacancies. I-V and C-Vcharacteristics of Au/CZT contacts with different surface treatments on CZT wafer'ssurface were measured with Agilent 4339B High Resistance Meter and Agilent 4294APrecision Impedance Analyzer, respectively. It was shown that the passivationtreatment increased the barrier height of the Au/CZT contact and decreased theleakage current. The main reason is that higher barrier of Au/CZT contacts decreasesthe possibility for electrons to pass through the sandwich TeO₂ layer between CZTand Au contact.The interface barrier between Au/CZT contact was studied by synchrotron-basedX-ray photoemission spectroscopy(SXPS). The interface barrier was determined to be 0.88±0.02eV for Au/CZT without passivation and 1.17±0.02eV for Au/CZT afterpassivation, respectively. Schottky barrier height was determined to be 0.85±0.02eVwithout passivation and 0.96±0.02eV after passivation by current-voltagemeasurement. However, 1.39±0.02eV without passivation and 1.51±0.02eV withpassivation were obtained according to the capacitance-voltage measurement.The electrical properties of different metal-CZT contacts produced by sputteringdeposition method are investigated. The results of current-voltage and SEM analysesshow that Au is the most suitable electrical contact materials, which forms a nearlyideal Ohmicity contact with high resistivity CZT crystals and 0.95±0.02 eV barrierwith low resistivity ones. XPS analyses show that Au atoms diffuse into highresistivity CZT crystal during annealing, meanwhile Cd and Te atoms diffuse into Aucontact. Diffused Au did not form any compound with any element in CZT crystal.Thus, the heavy doped layer is formed and Ohmic contact is obtained. PL spectraanalysis results of CZT crystals with deposited Au layer show that the inter-diffuseddonors[Au]³⁺ recombine with acceptors [V_Cd]^2-during sputtering process.Meanwhile, the intensity of (D_complex) peak of CZT surface with Au contact increasessharply in comparison with un-deposited CZT crystal. Meanwhile, donor [Au]³⁺ and[Au³⁺·V_Cd^2-]⁺ compensates Cd vacancy [V_Cd]^2- and produce different defectcomplexes.The indium doped CZT crystal was characterized by PL spectra at 10 K and IRtransmission spectra at room temperature. The results showed that indium atomssubstituted Cd vacancy and produced ionized donor [In_Cd]⁺, i. e. indium dopingelement recombined with [V_Cd]^2- and formed the single negative defect complexA-center[In_Cd⁺·V_Cd^2-]^- and the neutral ones [2In_Cd⁺·V_Cd^2-]⁰ and [In_Cd⁺·(In_Cd⁺·V_Cd^2-)^-]⁰.According to shallow level DAP peak at 1.6014 eV in high resistivity In-doped CZT,shallow donors level [In_Cd]⁺ and shallow acceptors level [V_Cd-In_Cd]^- are EDS=13.3meV, E_AS=29.5 meV, respectively. According to the measurements of temperaturedependence of CZT:In crystal from 10 K to 60 K, the ionization energies of deepdonors level [Te_Cd]⁴⁺ and deep acceptor level [V_Cd]^2- are determined to be 65.5 meV and 87.4 meV, respectively.IR transmission spectrum of the CZT crystal without indium dopant is about58ï¼…. The IR transmission spectra of CZT:In crystal with the concentrations of indiumdopant 10ppm, 15ppm and 30ppm are about 37ï¼…, 21ï¼…and 4ï¼…in the range from4000cm^-1 to 500cm^-1, respectively. The reason for IR absorption of CZT crystal maybe attributed to the lattice absorption and the free-carrier absorption.