G. Asa

Recent progress in n-type CdZnTe arrays for gamma-ray spectroscopy

Abstract This study focuses on the characterization of n-type CdZnTe gamma-ray spectrometers, which are designed for pixelated imaging array. The intentionally doped n-type CdZnTe crystals are grown by IMARAD Imaging Systems by a modified horizontal Bridgman technique. Imaging arrays, which are produced by IMARAD with ohmic contacts are compared with arrays with different types of contacts, fabricated at Technion-Israel Institute of Technology. We investigate the performance of n-type MSM spectrometers fabricated with several types of contacts: ohmic anodes and cathode, rectifying anodes and cathode as well as mixed (i.e., ohmic anodes and rectifying cathode). The DC characteristics are correlated with the detector dark noise and the spectral performance. The study presents the significant recent advance in the technology of room-temperature semiconductor gamma-ray spectrometers. It also gives physical insight to the parameters that determine the performance of n-type CdZnTe spectrometers. The comparison between spectrometers with different contacts may indicate whether the high performance of IMARAD arrays depends upon electron injection from the cathode, as previously suggested, or to adequate hole mobility-lifetime products in the n-type CdZnTe material, as suggested here.

The interface of metalorganic chemical vapor deposition-CdTe/HgCdTe

The metalorganic chemical vapor deposition (MOCVD) growth of CdTe on bulk n-type HgCdTe is reported and the resulting interfaces are investigated. Metalinsulator-semiconductor test structures are processed and their electrical properties are measured by capacitance-voltage and current-voltage characteristics. The MOCVD CdTe which was developed in this study, exhibits excellent dielectric, insulating, and mechano-chemical properties as well as interface properties, as exhibited by MIS devices where the MOCVD CdTe is the single insulator. Interfaces characterized by slight accumulation and a small or negligible hysteresis, are demonstrated. The passivation properties of CdTe/ HgCdTe heterostructures are predicted by modeling the band diagram of abrupt and graded P-CdTe/n-HgCdTe heterostructures. The analysis includes the effect of valence band offset and interface charges on the surface potentials at abrupt hetero-interface, for typical doping levels of the n-type layers and the MOCVD grown CdTe. In the case of graded heterojunctions, the effect of grading on the band diagram for various doping levels is studied, while taking into consideration a generally accepted valence band offset. The MOCVD CdTe with additional pre and post treatments and anneal form the basis of a photodiode with a new design. The new device architecture is based on a combination of a p-on-n homojunction in a single layer of n-type HgCdTe and the CdTe/HgCdTe heterostructure for passivation.

Single crystalline CdTe solar cells grown by MOCVD

Abstract CdTe has unique advantages for space applications solar cells. Previous structures were not able to yield the full potential of single crystalline solar cells. The limitations of these structures are discussed. We then report a new structure that is being studied with numerical simulations and preliminary experimental results. The actual fabrication and characterization of the cells under study are discussed in detail.

Barrier formation at graded HgTe/CdTe heterojunctions

Numerical calculations of graded HgTe/CdTe heterojunction (HJ) band diagrams at equilibrium are presented and discussed. The calculations are performed in the entire compositional range (0<x<1), using a nonparabolic conduction band and Fermi‐Dirac statistics. The dependence of barrier formation at graded HJs are examined as a function of the graded region width and the graded region doping profiles. The graded region width and doping profiles were found to be the two main factors that determine whether barriers are formed as well as their shape and magnitude. The calculated results indicate that epitaxial ohmic HgTe contacts to extrinsic CdTe are possible, provided that the graded region is wider than one micron, and that it has the same doping type as the doping of the substrate with equal or higher absolute value. Further numerical calculations take into consideration the possible existence of distributed interface charges in the graded region of the HJ. It is shown that by assuming a classical transpor...

Properties of metal‐organic chemical‐vapor‐deposition mercury telluride contacts on p‐type cadmium telluride

The semimetal‐semiconductor HgTe/CdTe (p‐type) graded heterostructures were formed by the epitaxial growth of HgTe on 〈111〉‐oriented CdTe substrates using metal‐organic chemical‐vapor‐deposition. These graded heterostructures are lattice matched, have a high degree of structural perfection, and the semimetallic HgTe provides both the high electron affinity and high carrier concentration required for ohmic contacts. Unlike the abrupt HgTe/CdTe heterostructure which is rectifying due to the valence‐band offset and interface charges that reside on the polar surfaces of 〈111〉‐oriented CdTe, the graded heterostructures form ohmic contacts. The contact specific resistance can be as low as 0.4 Ω cm2 and is determined by the transmission line method on HgTe/Ti/Au stripes processed by ion‐beam milling (on CdTe bulk with resistivity of 10 Ω cm). The contacts are characterized electrically by current‐voltage and dynamic resistance‐voltage characteristics. Auger depth profile and morphology of contacts grown on the A...

Photocurrent in CdTe NIP solar cells

Abstract A simple analytical model for the voltage dependence of the photocurrent in CdTe n–i–p solar cells is presented. The physical model is corroborated with a numerical solution of Poisson and the two continuity equations under illumination and shows excellent agreement with the numerical data. The new model is compared with previously reported models of Bube and Crandall. The new model illuminates the loss mechanism of carriers near the front interface of CdTe solar cells. It is shown that the photocurrent is high when the carrier velocity due to the electric field is higher than the product of the absorption coefficient and the diffusion coefficient, almost regardless of the interface recombination velocity value. At lower electric field values, the interface recombination velocity has a stronger effect on the photocurrent. Experimental conditions leading to a reduction of the electric field and a corresponding decrease in the collection efficiency in CdTe n–i–p solar cells are discussed.