P. Höschl

Type conversion of p-(HgCd)Te using and Ar reactive ion etching

Hydrogen/methane gas mixtures and pure argon were used for reactive ion etching (RIE) of ( and 0.28). The effect of the ratio on the depth of the etched surface and the depth of the p - n junction created under the etched surface were studied for the RIE process. It was found that the etch depth reaches a maximum at an ratio and the depth of the p - n junction decreases with increasing fraction in the mixture. The roughness of the etched surface is smallest using a gas mixture with a small amount of (20 - 30%). For the pure Ar RIE process the etch and p - n junction depths were studied as functions of etch time, Ar pressure and rf power. Clear evidence for the creation of p - n junctions using various kinds of Ar RIE processes is found.

High-temperature electron and hole mobility in CdTe

We have developed a novel technology for forming high-temperature stable ohmic contacts on CdTe, and we have measured the Hall mobilities of electrons at temperatures between 300–1300 K and holes between 225–600 K. The temperature interval also covers the region 300–600 K, where no data have yet been presented. The measured mobilities agree very well with theoretical calculations.

Determination of the migration energy of Hg interstitials in (HgCd)Te from ion milling experiments

Abstract The influence of temperature on processes occurring during and after ion beam milling with low energy Ar ions (500–1000 eV) on Hg 1− x Cd x Te samples ( x ≈ 0.2) was demonstrated. Nearly exponential dependence of the depth of the p-n junction created during ion milling on temperature in the range 77–300 K was found. Theoretical fits based on the presented model yield a migration energy of the mercury interstitials of E M = 120 ± 30 meV.

Deep level defects in CdTe materials studied by thermoelectric effect spectroscopy and photo-induced current transient spectroscopy

The deep levels in semi-insulating CdTe crystals grown by vertical gradient freezing technique were studied using thermoelectric effect spectroscopy (TEES) and photo-induced current transient spectroscopy (PICTS). The measurement of TEES spectra in the temperature range 90–400 K was performed at different heating rates. The positions of the levels and their capture cross-sections were obtained by using the heating rate method. It was found that near midgap levels in samples doped with shallow donors (Cl, In) have a low value of capture cross section (≈10−15 − 10−17 cm2) and are hole traps. Samples doped with a deep donor (Ge, Sn) have a much higher capture cross section of the midgap level (≈10−13 cm2), which acts as an electron trap. Comparison of the TEES results with the PICTS method has shown that while the evaluated values of ionization energies are comparable using both methods, the PICTS technique gives systematically approximately two orders of magnitude higher capture cross sections than TEES.

CdTe and CdZnTe crystals for room temperature gamma-ray detectors

Abstract CdTe(Cl) detectors from CdTe single crystals, grown by the Bridgman method from Te-rich melt, were fabricated. The quality of the detectors was tested with 57Co and 241Am sources. In the 57Co spectrum low noise is demonstrated by the presence of a 14 keV peak and good resolution ≈7 keV (FWHM) evident from the separation of 122 and 136 keV peaks. A review is given of the state-of-the-art properties of (CdZn)Te single crystals prepared for substrates in the Institute of Physics of Charles University. The quality of samples is tested by measurements of the diffusion length of minority carriers, from which the mobility–lifetime product is evaluated.

Deep p-n junction in Hg1-xCdxTe created by ion milling

Thick n-type layers near the surface of p-Hg1-xCdxTe produced as a result of low energy ion milling have been investigated. EBIC measurements of cleaved cross sections show the depth and shape or the p-n junction under the surface of (HgCd)Te. An analysis of the n-type layers by differential Hall effect measurements is reported. It was found that the electron concentration in the n-type layer is of the order of 1015 cm-3 and is nearly constant from the surface to the p-n junction. A diminishing density of etch pits was found on the surfaces influenced by ion milling. A model for the p-n conversion during ion milling is discussed.

Determination of energy gap in Cd1-xZnxTe (x = 0-0.06)

A dependence of energy gap on composition x for Cd1-xZnxTe (x = 0-0.06) was found combining x-ray diffraction and low temperature photoluminescence measurements (free exciton energy) on Cd1-xZnxTe single crystals. The use of this dependence for evaluation of Zn content (x) in the crystals shows very good agreement with the near infrared absorption method. Published dependences Eg(x) are reviewed and compared with the dependence Eg(x,T = 4 K) = 1.606 + 0.520x + 0.254x2 derived in this work.

Dynamics of native point defects in H2 and Ar plasma-etched narrow gap (HgCd)Te

We report on a long-run evolution of electrical conductivity s(77 K) of n-Hg1� xCdxTe (x=0.21) samples which were createdby reactive ion-etching process (RIE) in hyd rogen andargon plasma. We show that after storing at room temperature s(77 K) decreases to less than one half of its initial value in a time interval � 2 � 10 5 s. The time of the relaxation is about 1000 times longer than the etching time. The increase of the storing temperature to 323 K results to about 5 times faster rate. In the coincidence with our previous model we interpret the effects as a result of release and diffusion out of donor-like mercury interstitials (HgI) captured on defects inside the sample during RIE. The numerical solution of the diffusion equation allows to estimate the diffusion constant of HgI at room temperature > 10 � 8 cm 2 s � 1 , concentrations of two trap levels � 10 16 and � 10 15 cm � 3 andrespective formation free energies �� 500 and �� 600 meV. The traps are interpretedas complexes which are formedby extrinsic acceptors andbound Hg interstitials. The method is proposed to be a proper tool to study the defect structure also in other semiconductors. # 2001 Elsevier Science B.V. All rights reserved.

Preparation of cadmium telluride single crystals for nuclear detectors

Cadmium telluride single crystals, both undoped and doped with chlorine, bromine and iodine, were grown from tellurium melt. The electrical conductivity, Hall effect and photo-Hall effect were investigated. Acceptor levels located at 0·15 eV, 0·5 eV and 0·9 eV above the top of the valence band were found in samples of this semi-insulating material. A model is proposed which describes the compensation of cadmium vacancies and complexes by chlorine atoms. Chlorinedoped CdTe single crystals have proved suitable for preparation of the detectors of gammaradiation.

Defect structure of high resistive CdTe:In prepared by vertical gradient freeze method

High resistive and photosensitive CdTe doped with In aimed for fabrication of X- and gamma-ray detectors was produced by vertical gradient freeze method. A complex investigation of defects and compensation by a number of optical and photoelectrical mapping methods was performed. A model of energy levels dominating the recombination processes in the material was elaborated, where the role of In, and related complexes as well as native defects (Cd vacancy and its competes) is discussed.