Chemical Treatment Of And Ohmic Contact On TlBr Wafers For Detectors

Among the wide band gap semiconductor, thallium bromide(TlBr) crystal is of great promise for room-temperature nuclear radiation detector. Its high atomic number(Tl:81 and Br:35) and large density(7.56 g/cm3) can produce a better stopping power(absorption depth is 0.32 mm at 100 keV) and detection efficiency for the X- and gama-ray. Meanwhile,the wide band gap(2.68 eV) of TlBr crystal enables the device to work at room temperature with low noise signal. Besides, since TlBr crystal has a simple cubic crystal structure(CsCl type) and a relatively low melting point(460 ?), it is convenient to grow TlBr single crystals by melt-based methods.The integrity of TlBr single crystal and surface quality of TlBr wafer are two key factors which will determine the TlBr detector performance. As the mechanical damages,which can seriously affect the electrode quality and detector performance, will be introduced on the surface of TlBr wafers during the mechanical procedure, this paper focused on a systematic study of the chemical polishing process on TlBr wafer. On this basis, we also researched the capability of ohmic contact with two different electrode materials(Au and Ti). The main contents are as follows:TlBr wafers were prepared by the same mechanical procedure in this paper. After that,different chemical treatments were introduced on these samples. Comparing the surface morphology, optical property, V-I characteristic and detection performance of these TlBr wafers, we found the optimal chemical process parameters as follows: concentration of Br2-CH3 OH solution is 5%, and the etching time should be 60 s. A typical sample treated by this chemical process has a roughness of 10.978 nm, an average infrared transmittance of64.1%,equivalent resistivity of 9.93×1010 ?·cm and energy resolution for 241 Am sources of29.93%. The mechanism of chemical polishing with Br2-CH3 OH solution on TlBr surface was explained by analyzing the X-ray photoelectron spectroscopy(XPS) results of etched and unetched samples.In addition, We applied the vacuum evaporation method to fabricate the Ti/TlBr/Ti and Au/TlBr/Au planar detectors, and studied their capability of ohmic contact. The I-V, I-t curve and energy response of detectors with different electrode structure have been analyzed. The results showed that the detector of Ti/TlBr/Ti exhibited a better energy resolution of 26.48% for 241 Am sources, while the detector of Au/TlBr/Au had better stability at room temperature. the differences between Ti/TlBr/Ti and Au/TlBr/Au detectors had been discussed by analyzing XPS results of the interface of metal-TlBr.