Sergiy N. Levytskyi

Laser-Induced Doping of CdTe Crystals in Different Environments

Different procedures of laser-induced doping of the surface region of semi-insulating CdTe semiconductor are discussed. CdTe crystals pre-coated with an In dopant film were subjected to irradiation with nanosecond laser pulses in different environments (vacuum, gas or water). The dopant self-compensation phenomenon was overcome under laser action and In impurity with high concentration was introduced in a thin surface layer of CdTe. In the case of a thick (300-400 nm) In dopant film, laser-induced shock wave action has been considered as the mechanism of solid-phase doping. Formed In/CdTe/Au diode structures showed high rectification depending on the fabrication procedure. Diodes with low leakage current were sensitive to high energy radiation.

Features of characteristics and stability of CdTe nuclear radiation detectors fabricated by laser doping technique

Native and impurity point defects, complexes and extended defects which are formed during CdTe crystal growth and fabrication of diode structure are crucial for CdTe-based X-ray and gamma-ray detectors, cause deterioration of parameters and limit widespread practical application. Therefore, control of defect formation in CdTe crystals and device structures is important to achieve excellent charge collection efficiency and high energy resolution. Photoelectric, electrical and spectral properties of M-p-n structured CdTe diodes fabricated by the optimized excimer laser doping technique have been studied at different conditions. To make the diodes, a relatively thick In film was deposited on the surface of CdTe crystal and then it was irradiated with a laser pulse. The film served as an n-type dopant source as well as an electrode after laser irradiation. A Schottky contact was deposited on the opposite side of CdTe crystals. The In/CdTe/Au detectors have showed promise for nuclear radiation devices. However, the variations of I-V and C-V characteristics, fluctuations of time dependences of leakage current and degradation of spectral characteristics were evidences that electrically active defects cause non-uniform carrier trapping and induce excessive noise, deteriorating the detector performance.

Surface processing of CdZnTe crystals

The Cd1-xZnxTe (x = 0.1) crystals from two different manufacturers were studied by photoconductivity (PC) measurements. The samples 1 and 2 were subjected to chemical etching and irradiation with nanosecond laser pulses. The IR transmittance spectra of the crystals before and after laser irradiation were also monitored. The PC spectrum of the sample 1 had a typical one-band shape while the spectrum of the sample 2 exhibited two bands roughly corresponding to the bandgaps of CdTe and Cd1-xZnxTe that could be attributed to inhomogeneities in the surface region of the crystal. The positions of the maximum and red edge of the PC spectra did not correspond to the component compositions x in the bulk of Cd0.9Zn0.1Te crystals, however chemical polishing etching of the samples in a brominemethanol solution or/and laser irradiation led to this correspondence. Moreover, depending of laser pulse energy density, irradiation of Cd1-xZnxTe crystals resulted in a short-wavelength shift of the PC spectra, transformation of two bands to one in the case of the sample 2, and an increase in the photosensitivity of the semiconductor. The laser processing provided equalization of parameters in the surface and bulk regions.

CdTe diode detectors with a p-n junction formed by laser-induced doping

The procedures of fabrication of X- and γ-ray CdTe-based diode detectors with a p-n junction formed by laser-induced doping of the surface region of high resistivity p-like crystals in different environments are considered. The use of a relatively thick In dopant film and laser irradiation of In/CdTe structure in liquid allowed to apply higher laser energy densities and multiple irradiation of an In film. Finally, the room temperature In/CdTe/Au diode detectors with extremely high energy resolution have been obtained.

The COCAE detector: An instrument for localization — Identification of radioactive sources

The COCAE project develops an instrument for localization and identification of radioactive sources. For the localization task it will exploit the Compton scattered photons within its detecting layers. It is based on pixilated Cd(Zn)Te matrices in a stacked configuration. Progress has been achieved in all the components necessary for this technology. CdZnTe crystals have been grown of up to 75mm in diameter. A wafer level pixelization process has been tested and its results will be presented. Very good energy resolution is necessary for the performance of the instrument. For this reason we have investigated planar M-p-n diodes fabricated by using laser-induced solid phase doping and Schottky diodes created by Arion etching of the Cd(Zn)Te surfaces before deposition of Ni electrodes. The energy resolution achieved is better than 1% FWHM @662KeV. The development of a pixel CMOS readout integrated circuit has been undertaken. It outputs in digital format address, energy and time information for the pixels which have collected charge above a given threshold. Simulation studies of the minimum detection limits, of the Compton sequencing algorithms, of the angular resolution and of the reconstruction of the radioactive source position have been performed. Experiments with a precursor setup using an existing hybrid detector are underway.

Simulation studies of CdTe pixel detectors

We have performed simulation studies of CdTe pixel detectors using the commercial SDEVICE simulator by SYNOPSYS. We have incorporated energy levels and concentrations of defects in the band gap using the information by published compensation models. We have performed I–V simulation experiments. We simulated the transient behavior of the detectors due to the bombardment with alpha particles and X-ray photons. The simulation of the interaction with alpha particles required the description of the dependency on energy of the Bragg peak in CdTe, which we included in the material database file of CdTe. We investigated the relation of the charge collected at neighbor pixels with the depth of interaction of X ray quanta. The study has the following purposes: (a) To provide realistic current waveforms as input to electronics simulations needed for the development of pixel readout electronics. (b) To evaluate methods for the extraction of the depth of interaction. (c) To incorporate to this device simulator as much detailed information as possible about the material, about the electrodes and about the defects that degrade its performance and explore through numerical simulation their effect on charge collection.

Modification of surface state of CdZnTe crystals and contact formation

Different surface treatments such as chemical etching in bromine-containing solutions and nanosecond laser annealing were applied to provide the appropriate surface states of the CdZnTe (CZT) crystals prior to formation of metal-semiconductor contacts and electrode deposition. The effects of chemical etching and laser irradiation with different energy densities on the characteristics of the surface region of CZT solid solutions were studied. Depending on number and energy density of laser pulses and irradiation ambient conditions, photoelectric, electrical and optical properties of CZT crystals are changed and can be modified. Laser processing of CZT samples with the certain energy densities resulted in an increase in photosensitivity and transformation of photoconductivity (PC) spectrum profile. This was attributed to an increase in homogeneity in the crystal surface area and equalization of structural characteristics in the subsurface region and bulk of the samples. Irradiation of CZT crystals pre-coated with an electrode metal film allowed us to modify the I-V characteristics that can be employed in the fabrication of CZT-based X-ray and γ-ray detectors.

Laser-Induced Modification of the Surface State and Optical Properties of CdTe Crystals

Optical properties and surface state of semiconductor CdTe crystals subjected to irradiation with nanosecond laser pulses were studied. Ellipsometric parameters Δ and Ψ were measured on two opposite surfaces of (111) oriented CdTe wafers before. The samples were subjected to polishing chemical etching and laser irradiation with energy densities lower and higher than the melting threshold of CdTe. The morphology and structure of CdTe crystals were monitored before and after treatments. Irradiation formed a thin modified surface layer on the both sides of CdTe crystals (Cd- and Te-terminated) with the similar optical constants n and k, respectively. However, the effective thicknesses of such modified layers differed and depended on the chemical and laser treatment conditions and also on the faces of CdTe(111) crystals.

Formation of CdTe diode detectors by laser irradiation in water

The method of laser-induced solid phase doping of a thin surface layer of high resistivity CdTe crystals has been developed and M-p-n structured In/CdTe/Au diodes have been fabricated. Semi-insulating p-like CdTe crystals pre-coated with an In dopant film were irradiated with nanosecond laser pulses in water environment that made it possible to introduce and activate In dopant atoms with high concentration and form a steep p-n junction in the surface region of the crystal. Multiple laser irradiation of the samples from the In-coated side increased forward current of the diodes and decreased reverse one. The fabricated diode detectors demonstrate high energy resolution.