Deep Level Traps And Annealing Effects In Indium And Lead Co-doped Cd_0.9Zn_0.1Te Single Crystal

Cadmium Zinc Telluride(Cd_1-xZn_xTe or CZT)with several superiority electrical and optical properties have attracted more and more attention in recent years.Especially for x-ray and ?-ray spectroscopy detection,CZT possesses high charge collection efficiency and energy resolution at room temperature.However because of non-stoichiometric growth conditions,as grown CdZnTe single crystal contains variant defects,such as cadmium vacancies,Te inclusions and dislocations etc.,which applies negative effect on the properties of CZT crystal,such as high leakage current,poor detector performance and low energy resolution etc.Doping is an efficient way to improve the crystal quality.Therefore,many researchers have used different dopants for CZT crystal,like indium and aluminum of group ?,lead of group ? and chlorine of group ?,which have improved the resistivity,and also the detector performance of CZT crystal.However,dopants will create deep traps in the crystal band gap,which may result in low resistivity as well as low electron mobility life time product.Therefore,more detail work is necessary.This work mainly focuses on CZT co-doped with indium and lead and grown by modified vertical Bridgman?MVB?method.The purpose of co-doping is to improve crystal quality to minimize point defects,such as Cd-vacancies and other complexes,because indium is shallow level donor,while lead is deep level donor.It is expected to get some new results by co-doping with the two elements.This thesis focus mainly on electrical and optical properties of Cd_0.9Zn_0.1Te:?In,Pb?wafers obtained from the tip,middle and tail of the same ingot grown by modified vertical Bridgman method,using ?-? measurement,Hall measurement,IR Transmittance,IR Microscopy and Photoluminescence?PL?spectroscopy.?-?results show that the resistivity of tip,middle and tail wafers are 1.8×10¹⁰,1.21×10⁹ and 1.2×10¹⁰?.cm respectively,reflecting native deep level defects dominating in tip and tail wafers with high resistivity,comparing to the middle one.Hall measurement shows that the conductivity type changes from n at the tip to p at the tail in growth direction.IR transmittance of tail,middle and tip wafers is 58.3%,55.5% and 54.1%,respectively,whereas IR microscopy shows the density of Te inclusions at tip,middle and tail to be 1x10³,6x10² and 15x10³/cm² respectively.Photoluminescence?PL?reflects that neutral acceptor exciton?A⁰,X?and neutral donor exciton?D⁰,X?of tip and tail wafers have high intensity corresponding to their high resistivity,compared to the middle wafer,which have a little lower resistivity.Indium and Lead co-doped Cd_0.9Zn_0.1Te?CZT:?In,Pb??were characterized by using ?-?measurement,thermally stimulated current?TSC?spectroscopy and time-of-flight?TOF?.The concentration of doping level of In and Pb was 10 ppm and 2 ppm respectively.?-?curves showed that CZT:?In,Pb?possessed the resistivity as high as 1.8×10¹⁰ ?.cm,and the mobility???of about 868 cm²/Vs,which was considered acceptable for detector's fabrication.However,the carrier life time???was only 9.44 ×10^-7 s.Therefore,???mobility life time product?value was low.TSC results showed thirteen different trap levels,which were much more than that in Indium doped CZT crystal.Several special traps associated with Lead were found,which might be the reason for the low carrier life time.This thesis also focuses on comparative studies of In doped CdZnTe and In and Pb co-doped CdZnTe by using thermally stimulated current?TSC?and photoluminescence?PL?measurements.Although the resistivity for the two different wafers are in the region 10⁹¹0¹⁰ ?cm,In doped CdZnTe have seven different traps,but In and Pb co-doped CdZnTe have thirteen different traps,which are almost double of CdZnTe:In.PL spectra shows that neutral acceptor exciton?A⁰,X?and neutral donor exciton?D⁰,X?have higher intensity for co-doped CdZnTe:?In,Pb?,while the donor acceptor pair?DAP?have lower intensity for Cd ZnTe:In.Channel numbers of CdZnTe:In is higher compared to co-doped CdZnTe:?In,Pb?,reflecting better detector performance.This thesis also studies the annealing effects under Te-rich conditions on the properties of Indium and Indium and Lead co-doped Cd_0.9Zn_0.1Te.Cd_0.9Zn_0.1Te:In and co-doped Cd_0.9Zn_0.1Te:?In,Pb?were comparatively studied by Thermally stimulated current?TSC?,Photoluminescence and at last the performance of detector.For Cd_0.9Zn_0.1Te:In Interestingly,the number of traps levels was six before annealing and after annealing of about sixty hours,it increased to ten but for Cd_0.9Zn_0.1Te:?In,Pb?the traps levels were recorded as 13 but after annealing the leakage current become so high and were unable to do the TSC test for it.Photoluminescence study showed that?D⁰,X?peak,which was there before annealing,have vanished completely after annealing.At last detector performance showed that before annealing the full width at half maximum?FWHM?of co-doped Cd_0.9Zn_0.1Te was 8.37%,while after annealing it was improved to 4.35%,which is decreased for almost 50%.Interestingly after annealing of Cd_0.9Zn_0.1Te:?In,Pb?the intensity of DAP have increased and?A⁰,X?and?D⁰,X?which were visible before have merged together into one peak because of Te atmosphere.Shallow level donor increased because of it resistivity decreased of co-doped crystal after annealing.