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Surface, Interface and Dislocation Behaviors of CdZnTe Single Crystals

Author: ChaGangQiang
Tutor: JieWanQi
School: Northwestern Polytechnical University
Course: Materials Science
Keywords: CdZnTe surface leakage current IR transmittance surface relaxation surface reconstruction surface electronic structure work function interface chemical reaction Schottky barrier ohmic contact dislocations
CLC: TN304
Type: PhD thesis
Year: 2007
Downloads: 394
Quote: 3
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Abstract


CdZnTe is the most promising material for room temperature X-ray andgamma-ray detectors, as well as epitaxial substrate for HgCdTe and other infraredmaterials. Although the researches on CdZnTe lasted for over 20 years, there are stillmany unknows about the surface treatment, metal-CdZnTe contact properties anddislocations behaviors of CdZnTe crystal, which are investigated in this work.The surface treatments of CdZnTe crystals, such as mechanical polishing, chemicalpolishing and passivation, were first studied. The characters of CdZnTe surfaces withdifferent treatments, such as the surface atomic structure, electronic structure and workfunction were analyzed with variant advanced technologies. The interface behaviorbetween CdZnTe and metal contacts, including the interface chemical reaction, Schottkybarrier heights, and electroless ohmic contact were studied in detail. At last, thebehaviors and the effects of dislocations in CdZnTe single crystals were investigated.Mechanical polishing produces damaged layer on CdZnTe surface. By comparingthe weight lose and FWHM of X-ray rocking curves after different etching time, thethickness of the surface damaged layer is determined to be about 13μm. The damagedlayer results in high surface leakage current and IR absorption. Etching with Br-MeOHis an effective method to remove the surface damaged layer, and in return produces Teenriched surface. Te enriched layer results in high surface leakage current, but has littleeffect on IR transmittance. Te enriched layer could be deprived by chemo-mechanicalpolishing. Surface passivation can decrease the surface leakage current, but has greatnegative effect on IR transmittance.A new passivation solution, i.e. Br2 aqueous solution was adopted to replace theconventional NH4F+H2O2 solution, and produced the better passivation efficiency,because TeO2 passivation layer formed in’stead of TeCdO3.The atomic structure of the clean CdZnTe surface obtained by Ar+ etching andannealing in-situ in ultrahigh vacuum is observed by Low energy electron diffraction.The CdZnTe (110) surface was determined to be unreconstructed, and the surfacerelaxation was identified by X-ray photoemission spectroscopy. Both theunreconstructed (111)A-(1×1) surface and reconstructed (111)A-(31/2×31/2)R30°structure were found on the CdZnTe (111)A surface. On CdZnTe (111)B surface, theunreconstructed (111)B-(1×1) surface and (111)B-(2×2) reconstructed structure werefound. The reconstruction mechanisms are different for different surface.(111)A-(31/2×31/2)R30°reconstruction was induced by Cd vacancy, while (111)B-(2×2)was induced by Te adatoms on top of the ideal truncation. Angle resolved photoemission spectroscopy was used to characterize the surfacestate of the clean CdZnTe surface, by which the density of dangling-bonds on theCdZnTe (110) surface is determined to be 6.9×1014/cm2, approximately one electron persurface atom. The valance band of (111)A-(31/2×31/2)R30°bends down compared to(111)A unreconstructed surface due to the donor surface state caused by the higher Cdvacancy at reconstruction surface. The valance band of (111)B-(2×2) bends up, ascribedto the acceptor of Te adatoms.The work function of CdZnTe surfaces was determined by ultravioletphotoelectron spectroscopy. The results suggested that the work function of CdZnTe(111)A surface is higher than that of (111)B. Gas and gold adsorption increases the workfunction of CdZnTe, while clean and ordered surface presents lower work function. Thework function of (111)A-(31/2×31/2)R30°reconstructed surface is higher than that ofunreconstructed one due to Cd vacancy on the reconstructed surface. The work functionof (111)B-(2×2) reconstructed surface is higher than that of (111)B-(1×1), ascribed tothe surface dipole layer caused by Te adatoms.Schottky barrier height of Au/CdZnTe interface on CdZnTe (110) surface is thehighest, on (111)B surface is lower, and that on CdZnTe (111)A is the lowest. Theresults can be explained as follows. The (110) surface is a stabilized one with the lowestsurface energy, so it is not easy for chemical reaction between Au and CdZnTe to takeplace, and preserved the high Schottky barrier. (111)A surface is a polarized one, wherethe Te atoms are easy to react with gold to reduce the barrier. The reaction wasconfirmed by ultraviolet photoelectron spectroscopy. The interface reaction between Agand clean CdZnTe is more obvious, which results in a low Schottky barrier height ofabout 0.2 eV. While the Schottky barrier height between Ag and etched CdZnTe surfaceis as high as 0.67 eV, ascribed to the interception of the oxide and Te enriched layer,which stoped the chemical reaction between Ag and CdZnTe.On the reconstructed CdZnTe (111)A-(31/2×31/2)R30°surface, the Au atoms fill-upCd vacancies and bond with around Te atoms, which promotes the interface reactionand diffusion of the gold into the crystal. So the Schottky barrier height on Au/CdZnTe(111)A-(31/2×31/2)R30°surface is lower than that of unreconstructed one. The Schottkybarrier height on Au/CdZnTe (111) B-(2×2) surface is lower than that of Au/CdZnTe-(111)B-(1×1), which is determined by the top two layer Te atoms on (111)B-(2×2)surface, where Te is easy to react with gold to form the alloyed interface layer anddecreases Schottky barrier height. Annealing at 350℃for one hour in vacuum couldimprove the ordering of gold film and decrease the barrier height to about 0.15 eV.Au ohmic electrode was also prepared by the reaction of gold chloride solution onCdZnTe surface. The component of the interface was studied with X-ray photoelectronspectroscopy, and CdTeO3 interface layer was found. A new model was proposed to explain the ohmic contact mechanism, and explain experiment results of otherresearchers.Cd(g) and Te(g) dislocations were introduced into CdZnTe by the means ofbending deformation at an elevated temperature. The average IR transmittance ofCdZnTe was decreased apparently after deformation, ascribed to the polarizationabsorption of danling-bond electrons in the dislocations. The leakage current of CdZnTeafter deformation increased largely, and Poole-Frenkel effect should be responsible forthe increase. The effect of dislocations on the carriers density, was analyzed by I-Vcharacters, photoluminescence spectra and Hall effect. It was found that, Cd(g)dislocations act as acceptors, scatter hole and decrease the mobility of hole, while Te(g)dislocations act as donors, promote dispersion of hole and increase the mobility of hole.

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