Chemical Treatment And Ohmic Contact Property Of CdZnTe Wafer Surface

Chemical polishing and passivation of mechanically polished CdZnTe wafersurfaces were studied in this paper. Based on the results, the fabrication principles andmethods of ohmic electrodes on the pre-treated CdZnTe surfaces were investigated.By comparing with several other etchants, it was found that 2ï¼…Br-MeOH etchingproduced the brighter and smoother CdZnTe surfaces, meanwhile the erosion rate wasstable and easy to control. After Br-MeOH etching, the (111) Cd polar surface will becovered by the polycrystalline Te layer and become the Te-rich nonpolar face. PLspectra showed that after the etching the deep-level-energy defects were eliminated,surface impurities and defect state density were decreased, and surface lattice perfectionwas improved. Compared with the mechanically polished CdZnTe surface, Br-MeOHetched CdZnTe surface could form better ohmic contact with Au electrodes with theohmicity close to 0.991. The Te-rich surface after Br-MeOH etching increased theleakage current 3～4 orders. Prolonging of the holding time of etched CdZnTe surface inair, the leakage current is gradually decreased and tends to a stable value after 2 weeks,but it is still 2～3 orders higher than that of the mechanically polished sample withoutetching because the element Te on CdZnTe surface is not fully oxidized to TeO₂ in air.The viewpoint is also proved by the fact that the Te⁴⁺/Te⁰ ratio of CdZnTe surfaceplaced in air for 2 weeks after Br-MeOH etching is only 0.668. Another reason is thatthe surface with active Te after Br-MeOH etching easily adsorbs the impurities in air,which also increases the leakage current and deteriorates the detector performance.Therefore, the subsequent passivation is an important step after Br-MeOH etching. Inorder to obtain excellent ohmic contacts, metal electrodes should be immediatelydeposited on Br-MeOH etched surface and the un-deposited surface should besubsequently passivated during 0.5 h.In the research about CdZnTe passivation, it was found that all the five passivantsof 10wtï¼…NH₄F/10wtï¼…H₂O₂, 15wtï¼…KOH, 15wtï¼…H₂O₂, 15wtï¼…H₂O₂ after 15wtï¼…KOH,15wtï¼…KOH/15wtï¼…H₂O₂ aqueous solutions could reduce the leakage current by 1～2orders compared with the mechanically polished sample and by 4～6 orders comparedwith Br-MeOH etched sample. After the passivation treatments, the excellent ohmiccontact between electrodes and 2ï¼…Br-MeOH etched CdZnTe surfaces could be obtained. Through XPS analysis, it was found that the CdZnTe surface passivated inweak acid solutions, such as, NH₄F/H₂O₂ or H₂O₂ was Te-rich, and was Cd-rich in basicsolutions, such as, KOH or KOH/H₂O₂. Additionally, the most effective path is throughthe formation of TeO₃^2- in basic solutions to climate the enriched Te and the secondaryeffective way is through the formation of TeF₆ in weak acid solutions. Compared withother four passivants, NH₄F/H₂O₂ solution produced the best passivation efficiencybecause Te enriched on the passivated surface was fully oxidized to TeO₂, which resultsin the thickest oxide layer and the least leakage current among the above mentioned fivepassivants. Additionally, Cd/Zn/Te atomic ratio on the surface reaches 0.884/0.05/1,which is most close to the stoichiometric proportion (Cd/Te/Zn=0.9/1/0.1) ofCd_0.9Zn_0.1Te wafer. AFM measurements showed that the average roughness of CdZnTesurface passivated in NH₄F/H₂O₂ solution was lower than others. PL spectra showedthat NH₄F/H₂O₂ passivation could also reduce deep-level-energy defects and impurities.Additionally, there existed an acceptor energy level E_A of 0.113 eV above the valenceband for three CdZnTe surfaces with the mechanical polishing, Br-MeOH etching andsubsequent NH₄F/H₂O₂ passivation, which was caused by Cd vacancies (V_Cd) formedfrom the crystal growth.Through XPS analysis, it was also found that the (111) A face of mechanicallypolished CdZnTe wafer was Cd-rich and the (111) B face was Te-rich. But the two facesbecame Te-rich due to the deposition of Te on the surface after etched in Br-MeOHsolution and passivated in NH₄F/H₂O₂ solution.â… -â…¤measurements found that Br-MeOHetching was beneficial to increase the critical field intensity of undoped p-Cd_0.9Zn_0.1Tewafer to 333 V/cm from 66.7 V/cm.In the research of annealing, it was found that Au could effectively diffuse intoCdZnTe bulk crystal 200℃. Au/p-CdZnTe contacts with low barrier and excellentohmic-property were rapidly obtained after 20min annealing. After the low-temperatureannealing of Au/p-CdZnTe in air at 60℃for 2 hrs, the excellent ohmic contact was alsoobtained. In the annealing process, Au atoms diffused into the region near the CdZnTesurface, but did not form any compound with any element in CdZnTe crystal. Cd, Zn andTe atoms weakly disffused into Au layer, meanwhile Cd atoms were easier to diffuseinto Au layer than Te atoms.For Au/p-CdZnTe contacts, Au replaced Cd sites or occupied Cd vacancy asacceptors, which resulted in the heavy p-type doping on CdZnTe surface. Thus, M-p⁺-p ohmic contact was formed. For Au/n-CdZnTe contacts, n-CdZnTe near the surface wascompensated by the diffusion of Au as acceptors after the annealing. Diffused Au withhigh concentration became the complex centre and the excellent ohmic-contact was alsoobtained. Au was diffused into the undoped p-CdZnTe with the slower displacementdiffusion, most of which replaced Cd vacancies as acceptors to form [Au_Cd^-]. While Auwas diffused into the n-CdZnTe doped by In with the faster interstitial diffusion, most ofwhich was recombined to form [In_Cd⁺-Au^-] complex.For Au/TeO₂/p-CdZnTe(111)B structure, the contact barrier between p-CdZnTe(111)B and Au was degraded from 0.75eV to 0.68eV after 100℃×1h annealing. ForAu/TeO₂/n-CdZnTe(111)B structure, the contact barrier between n-CdZnTe (111)B andAu was 0.68eV.After 20min annealing at 200℃or 1h annealing at 100℃, Au did not form anycompound with any element in CdZnTe crystal. Additionally, the (531) plane is also acandidate surface for Au contact preparation.Through UV spectra analysis, it was found the optical band gap of Cd_0.9Zn_0.1Te at303K was 1.492eV, which was independent on the crystal orientation and surface state.TG-DTG analysis showed that Cd_0.9Zn_0.1Te began to lose the weight at 200℃and theweight loss amounted to the maximum value of 2.2% at 484.82℃. The oxidization rateof Cd_0.9Zn_0.1Te was increased quickly from 655℃and reached the maximum at698.82℃. The weight growth amounted to the maximum value, of 13.7% at 748.82℃.There was the obvious loss of weight due to the evaporation of TeO₂, CdO and CdTeover 748.82℃.