Indium Doping Of Cd_1-xZn_xTe Crystals And Au-Cd_1-xZn_xTe:In Contact Properties

The compound semiconductor Cd_1-xZn_xTe possesses excellent opto-electronic properties and is, therefore, thought as the most promising material for room temperature nuclear radiation detector. Although the study on Cd_1-xZn_xTe has lasted for a long period, there still exist some problems in the preparation of Cd_1-xZn_xTe crystal with high resistivity, the growth of large diameter Cd_1-xZn_xTe ingots and metal-Cd_1-xZn_xTe contact properties. The three problems will be studied in this work.Due to the high Cd partial pressure, Cd evaporates seriously from the raw materials during the modified Bridgman growth process, which in turn results in high Cd vacancy density in the as-grown Cd_1-xZn_xTe crystals and severely degrades the opto-electronic properties of Cd_1-xZn_xTe. In this dissertation, two approaches, i. e. the doping growth and the doping annealing, are adopted to improve the crystal properties. The investigation indicates that both the doping growth and the doping annealing can improve the resistivity of Cd_1-xZn_xTe effectively. By the former method, the resistivity of Cd_1-xZn_xTe is increased three orders, from 3×10⁷Ωcm to 5×10¹⁰Ωcm. By the latter method, the resistivity of Cd_1-xZn_xTe is increased two orders, from 5×10⁷Ωcm to 3×10⁹Ωcm. In addition, both methods also result in the reduction of IR transmission of Cd_1-xZn_xTe crystal, ascribed to the strengthening of free carrier absorption and crystal lattice absorption. After the doping growth, the dislocation density and the full with at half maximum (FWHM) of X ray rocking curve of Cdl_xZnxTe change slightly, which implies that the doping growth do not affect the crystalline quality of Cd_1-xZn_xTe. While for the doping annealing, the dislocation density of Cd_1-xZn_xTe is reduced from 6.2×10⁴cm^-1 to 1.5×10⁴ cm^-1 after annealing and the FWHM of X ray rocking curve is also lowered from 125"to78", implying the improvement of the crystalline quality. Photoluminescence (PL) spectra measurement shows that the defect-related region is extended from a weak hump into an obvious broad peak after the growth doping. It indicates that the defect-related center (2Incd⁺-Vcd^<sub>2-)⁰ is formed in the doped crystal. At the same time, the donor-accepter pair recombination DAP peak is strengthened and the corresponding phonon replica DAP-LO appears. The disappearance of the neutral accepter-bound exciton (A⁰, X) peak means that Cd vacancies are compensated effectively. After the doping annealing, the FWHM of neutral donor-bound exciton (D⁰, X) peak is minished and the free excitonic FE peak appears on the high energy shoulder of (D⁰, X) peak. Meanwhile, the intensity of DAP peak is improved distinctly and the intensity of D_complex peak is also increased to some extent.To simplify the traditional characterization methods of Cd_1-xZn_xTe: In crystal, a non-destructive but effective way based on PL measurement is established for the crystal evaluation of Cd_1-xZn_xTe: In. For high quality Cd_1-xZn_xTe: In, the neutral donor-bound exciton (D⁰, X) peak dominates the near bandgap region and the free exciton FE peak can be distinguished easily. While for low quality Cd_1-xZn_xTe: In, the neutral accepter-bound exciton (A⁰, X) peak becomes the main peak of near bandgap region and the FE peak disappears. In addition, the increase of the FWHM of (D⁰, X) peak corresponds to higher Te inclusion/precipitates density. The relative intensity of D_complex peak can be also used as an indicator of dislocation density. The higher I_Dcomplex/I_{D⁰, x} is, the higher the dislocation density will be. Moreover, when the relative intensity of DAP peak is augmented and the relative intensity of D_complcx peak is reduced, the resistivity of Cd_1-xZn_xTe: In falls.In this work, we successfully grow Cd_1-xZn_xTe: In crystals with the diameter up to 60 mm by applying the modified Bridgman method with an optimized temperature field. The variation of the Cd_1-xZn_xTe: in ingot diameter changed the crystal quality and thus influenced the optical and electronic properties of Cd_1-xZn_xTe: In. When the diameter of Cd_1-xZn_xTe: In ingot is enlarged from 30ram to 60mm, the dislocation density is increased an order and the FWHM of X ray diffraction peak is also increased obviously, which means the degrading of crystalline quality. However, the enlarging of ingot diameter also increased the IR transmission of Cd_1-xZn_xTe: In to some extent. In addition, the D_complex peak was intensified clearly and the DAP peak was weakened.To ameliorate the contact properties, a vacuum annealing is applied for Au-Cd_1-xZn_xTe: In contact. The investigation Suggests that the annealing can reduce the leakage current effectively and the reduction extent enlarges with the increase of the annealing temperature. An annealing below 200℃can upgrade the ohmic properties of Au-Cd_1-xZn_xTe: In contact, which is, however, degraded following the annealing at above 300℃. The variation of cohesive strength of Au film on Cd_1-xZn_xTe: In are responsible for the change. Additionally, the annealing can also improve the stability of leakage current. PL measurement reveals that the D_complex peak is intensified after the annealing, indicating that the complex 2[Au]³⁺-3[V_cd]^2- is formed due to the diffusion of Au into Cd_1-xZn_xTe: In.