Study On Interfaces Of Cd(Zn)Te With Metal And Semiconductor

Cd?Zn?Te is considered as the best room temperature radiation detection material.The detectors fabricated by CdZnTe crystal and film have excellent properties for the?-ray and X-ray detection,respectively.The CdZnTe and CdTe film also have advantage such as large area and low cost.However except for material itself,the interfaces of Cd?Zn?Te with metal and semiconductor substrate have significant influence on the performance of the detector.Although there are some researches about the two kinds of interfaces,the relation among the microscopic physical essence,interface potential barrier and photoelectric properties are still not clear.Firstly,four metal electrodes were deposited by magnetron sputtering on the surface of CdZnTe crystal,including high work function metals such as Au and Ni,and low work function metals such as Cr and Al.The effects of metal/CdZnTe interface on the electrical properties and detector performance were studied.Then,the interface properties between Au,Al and Cr metal with CdZnTe were investigated,using Synchrotron radiation photoelectron spectroscopy.In addition,the influence of interface between CdZnTe film and FTO substrate on the electrical properties of film detector has been studied.At last,the mechanism between CdTe/GaAs interface and potential barrier has been researched.The metal electrodes were prepared on the surface of p-type CdZnTe crystal using magnetron sputtering method.The powers of radio frequency,working pressure of Ar and substrate temperature were optimized.The leakage current is turned to increase at the power of 60 W and 80 W at the bias larger than 50 V.The current of detector increases sharply when the bias increasing up to 700 V at Ar pressure of 0.2 Pa and 0.3 Pa,which is attributed to the large energy of sputtering atom or ion.For the substrate temperature 723 K,the bulk resistivity of CdZnTe is lower,due to the stoichiometric ratio deviation.The electrical properties of CdZnTe detectors with Au,Ni,Cr and Al electrodes are compared at the voltage range from 0.01 to 1000 V.The I-V curve can be divided into four regions,i.e.,a first linear region,a sublinear region,a second linear region and a superlinear region.The first linear region means the Schottky barrier has little influence on the detector.The sublinear region means that the width of barrier region is smaller than the thickness of CdZnTe crystal,and the current transportation is mainly limited by the carrier diffusion at the barrier region and neutral?bulk?region.The effective barrier is reduced by the voltage drop on the interface layer.The second linear region reflects the fully depleted CdZnTe crystal.The current of detector is mainly limited by the carrier diffusion at the barrier region and thermal emission at the interface.The voltage drop on the interface layer and transmission prevention coefficient commonly affects the electrical properties of the detector.The superlinear region usually occurs at high bias,which means the voltage drop on the interface layer is the main factor of barrier decrease.The Schottky barrier height is calculated through fitting the I-V curve at sublinear region of Au/CdZnTe/Au and Ni/CdZnTe/Ni detectors.The Schottky barrier height of Ni/CdZnTe interface is lower than that of Au/CdZnTe.Due to the large carrier transmittance prevention effects of interface,Ni/CdZnTe/Ni detector shows lower current and better detector performance for ²⁴¹Am radioactive source.The Ni/CdZnTe/Ni detector shows larger current at superlinear region because of the barrier lower effects.The composition of Au/CdZnTe and Ni/CdZnTe interface layer is TeO₂ and NiTe/TeO₂,with a thickness of 8 nm and 29 nm,respectively.The order of electrodes according to the Schottky barrier height formed with CdZnTe is Al>Au>Ni>Cr.The order of electrodes according to the barrier lower effects is Ni>Cr>Au>Al.The CdZnTe detector with Al electrode has the smallest leakage current,the energy resolution of which for ¹³⁷Cs can reach a good value of 1.45%.The compositions of Cr/CdZnTe and Al/CdZnTe interface layer is CrO/Te and TeO₂,with a thickness of 16 nm and 9 nm,respectively.The Ni and Cr metal can be chosen as electrode for the CdZnTe crystal with large resistivity or low carrier mobility life product.Otherwise the Au and Al can be chosen.Au,Al and Cr layer were deposited by MBE on the atomically clean surface of p-type CdZnTe crystal.The synchrotron radiation photoelectron spectroscopy was used to investigate the interface properties.There is no chemical reaction occurs at Au/CdZnTe and Al/CdZnTe interface.The band bending happens at interface.At the initial stage of Cr deposition,Cr-Te reaction happens,then Cr-Te bond break,for which the elemental Cr and Te both exits at the interface.Te diffuses to the surface layer of CdZnTe crystal and forms the dipole layer pointing from Te??⁺?to CdZnTe Cr??^-?with a thickness about 23?.The Schottky barrier height hence reduces from 0.82 eV to 0.48 eV.Also due to the interface dipole layer,the surface work function reduces from 5.8 eV to 5.58 eV.The effects of CdZnTe/FTO interface on the CdZnTe film and detector properties were studied.The interface shows column morphology.The deposited CdZnTe films process stable cubic zinc-blende structure with?111?preferred orientation.Due to the large roughness of FTO,the texture coefficient of?111?surface of CdZnTe film has been weakened as the Ar pressure decrease.The Zn and Cd contents of CdZnTe film are larger than that of target.The carrier concentration and mobility of CdZnTe film detector is 6.25×10⁷/cm² and930 cm²/?V·s?.The detector shows uniform electrical properties which is ascribed to the good contact between CdZnTe and FTO.CdTe thin film was deposited by MBE on the atomically clean surface of GaAs single crystal.The synchrotron radiation photoelectron spectroscopy was used to investigate the chemical reaction,inter diffusion,interface layer and potential barrier height of CdTe/GaAs heterointerface.The results show that As-Te and Ga-Te chemical reaction happens successively during CdTe deposition.The dipole moment caused by As-Te reaction leads to the out diffusion of Ga.The Ga-Te interface layer finally exists with a thickness of 30?.Ga-Te interface layer forms the dipole moment pointing from Ga-Te??⁺?to GaAs??^-?.The potential barrier hence decreases from 0.46 eV to 0.08 eV.The potential barrier height is inversely proportional the density of Ga-Te bonds and thickness of Ga-Te interface layer.