CdZnTe Surface Research Articles

Effectiveness of parylene coating on CdZnTe surface after optimal passivation

Parylene coating was adopted on CdZnTe (CZT) detector as a mechanical protection layer after wet passivation with hydrogen peroxide (H2O2) and ammonium fluoride (NH4F). Wet chemical passivant lose their effectiveness when exposed to the ambient conditions for a long time. Parylene coating could protect the effectiveness of passivation, by mechanically blocking the exposure to the ambient conditions. Stability of CZT detector was tested with the measurement of leakage current density and response to radio-isotopes. When the enough thickness of parylene (>100 μm) is adopted, parylene is a promising protection layer thereby ensuring the performance and long-term stability of CZT detectors.

Surface Planarization of CdZnTe Wafers: Effect of Slurry Formulation and CMP Processing Parameters on Surface Planarity

This study investigates the effect of chemical mechanical planarization (CMP) processing parameters such as platen velocity, the concentration of the oxidizer and abrasive nanoparticle, slurry pH and surfactant types on the surface roughness of cadmium zinc telluride (CdZnTe) substrate. It was found that these parameters have a significant effect on the quality of the polished surfaces. It was found that lower platen velocity, lesser concentration of abrasive particles, basic slurry pH, and addition of anionic surfactant (SDS) into the CMP slurry solution showed improved surface planarity. Optical Surface Profiler and atomic force microscopy (AFM) techniques were used to monitor the surface topography before and after polishing. A notable root-mean-square surface roughness, (Rq), ∼0.9 nm, has been obtained on the polished CdZnTe (CZT) surface over a scan area of 481 × 361 μm2 under the optimized conditions of 60 rpm relative velocity, slurry pH of 9, 3.75 vol% of oxidizer (H2O2) and 1.25 wt% of abrasive (SiO2 nanoparticle). A probable mechanism of the present CMP surface planarization of CZT substrate has been proposed. Unlike the conventional surface planarization processes, which involve two-step lapping followed by CMP for the CZT surfaces, we have developed a single step CMP process to obtain good surface planarity.

Design and development of double-layer anti-reflection coating for HgCdTe based mid-wave infrared detector

A novel design and development of double-layer antireflection coating (ARC) is presented here for back side illuminated HgCdTe based mid- wave infrared (MWIR) detector. Reflectance of infrared radiation in the CdZnTe/HgCdTe structure is simulated to achieve optimum material and thickness of antireflection coating layers for mid- wavelength range. A polishing process for the surface qualification of CdZnTe material prior to deposition of layers has been developed using different chemical treatments and ellipsometric characterisation techniques. The desired ellipsometry parameters (i.e. 4.0<Ψ < 4.6 and 145<Δ<150) of CdZnTe surface is achieved with the polishing of Br2-metanol solution. The optimized ARC layers of YF3 and ZnS are deposited on the back side of CdZnTe surface and measured the reflectance by Fourier-Transform Infrared (FTIR) technique. The reflectance of below 2% is observed in the backside illuminated YF3/ZnS/CdZnTe/HgCdTe structure for mid-wave infrared range, which is similar with the simulated results. Thus, an improvements as well as new functionalities are implemented in infrared sensors by depositing optimum stack of thin film layers.

Effects of the inductively coupled Ar plasma etching on the performance of (111) face CdZnTe detector

The effect of inductively coupled Ar plasma etching (ICP-Ar) instead of traditional Br–MeOH etching on the performance of CdZnTe detectors was studied. The optimal ICP etching parameters were determined by experiments. The XPS results indicated that the surface composition of CdZnTe etched by ICP-Ar had no obvious TeOx peak and was closer to stoichiometric ratio than that etched by Br–MeOH. The leakage current of ICP-Ar etched CdZnTe surface was significantly reduced, and the detection performance with the energy resolution was improved by 12%, as compared to Br–MeOH etched sample. Moreover, ICP-Ar etching abandoned the drawbacks of Br–MeOH by producing corrosive gases, making it safer and more environmentally friendly. Therefore, inductively coupled Ar plasma etching is expected to replace Br–MeOH etching and become an effective method for CdZnTe surface treatment.

Characterization of (111)B and (211)B CdZnTe Substrates for HgCdTe Growth

We have studied state-of-the-art CdZnTe (211)B and (111)B substrates and compared them to each other and to substrates from an alternative vendor. The CdZnTe surface has been characterized both as-received and after growth preparation procedures using dark field optical microscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, Fourier transform infrared transmission spectroscopy, and secondary ion mass spectroscopy. From these measurements we have obtained densities of particles and morphological defects with size > 0.5 μm, densities of small particles/features with size ≤ 100 nm, and substrate/film interface impurities. The state-of-the-art substrates all have remains of silica grit polishing particles around the edges. The as-received (111)B substrates had very few particles or morphological defects on the surface, but a Br:methanol surface preparation etch exposed a large number of features with sizes ≤ 100 nm. Some of these were silica grit particles, but most of them were Te precipitates. The (111)B substrate from an alternative vendor had a lot of different particles, stains, and voids. It improved significantly after a fine-polishing procedure, but still had the surface with most defects after surface preparation. The state-of-the-art (211)B had the largest as-received density of small features, but after Br:methanol etching this density (of Te precipitates and silica grit particles) was the same as for the etched (111)B substrate. SIMS measurements showed that some of the polishing grit particles ended up on the top surface after surface preparation. Hydrofluoric acid can be used to etch away the silica particles.

Studies on Cr electrode of CdZnTe detector for high energy radiation detection

Electrode with proper contact properties is of vital importance to CdZnTe detector for high energy radiation detection. In this work, Au and Cr electrodes are prepared on CdZnTe surface using magnetron sputtering method. The surface morphology and the interface structure are analyzed using atomic force microscopy and X-ray photoelectron spectroscopy (XPS). No reaction is found at Au/CZT interface. But for Cr/CdZnTe, interface layers of CrO and Te were found, which forms a structure of Cr/Cr–O/Te/CdZnTe with a metal-insulator-metal tunnel junction. Due to this interface structure, Schottky Barrier height was tuned, which enhanced the collection efficiency and energy resolution for high energy irradiation. The energy resolution of CdZnTe detector with Cr electrode for 137Cs isotope with the energy of 662 keV reaches about 1.3%. The adhesive force of Cr is also enhanced because of the interface reaction. The results prove Cr to be a proper electrode material for CdZnTe detectors.

Hypersound waves at the surface of CdZnTe crystals: The role of surface orientation and scattering at twin boundaries

The patterns of propagation of gigahertz surface acoustic waves (SAWs) are obtained for cuts of CdZnTe single crystals with different crystallographic orientations. By comparing the experimental patterns with the calculated ones, the existence of at least two SAW modes, one of which is a Rayleigh mode, is demonstrated. It is shown that the anisotropy of propagation of different SAW modes makes it possible to determine the local crystallographic orientation of the CdZnTe surface and detect local imperfections of the crystal structure. Strong anisotropic scattering of the Rayleigh wave by coherent twin boundaries is found.

Optical and electrical study of CdZnTe surfaces passivated by KOH and NH4F solutions

Performance of CdZnTe-based detectors is highly related to surface preparation. Mechanical polishing, chemical etching and passivation are routinely employed for this purpose. However, the relation between these processes and the detector performance in terms of underlying physical phenomena has not been fully explained. The dynamics and properties of CdZnTe surface oxide layers, created by passivation with KOH and NH4F/H2O2 solutions, were studied by optical ellipsometry and X-ray photoelectron spectroscopy (XPS). Thicknesses and growth rates of the surface oxide layers differed for each of the passivation methods. Leakage currents which influence the final spectral resolution of the detector were measured simultaneously with ellipsometry. Results of both optical and electrical investigation showed the same trends in the time evolution and correlated to each other. NH4F/H2O2 passivation showed to be a method which produces the most desirable properties of the surface oxide layer.

Characterization of the metal–semiconductor interface of gold contacts on CdZnTe formed by electroless deposition

Fully spectroscopic x/γ-ray imaging is now possible thanks to advances in the growth of wide-bandgap semiconductors. One of the most promising materials is cadmium zinc telluride (CdZnTe or CZT), which has been demonstrated in homeland security, medical imaging, astrophysics and industrial analysis applications. These applications have demanding energy and spatial resolution requirements that are not always met by the metal contacts deposited on the CdZnTe. To improve the contacts, the interface formed between metal and semiconductor during contact deposition must be better understood. Gold has a work function closely matching that of high resistivity CdZnTe and is a popular choice of contact metal. Gold contacts are often formed by electroless deposition however this forms a complex interface. The prior CdZnTe surface preparation, such as mechanical or chemo-mechanical polishing, and electroless deposition parameters, such as gold chloride solution temperature, play important roles in the formation of the interface and are the subject of the presented work. Techniques such as focused ion beam (FIB) cross section imaging, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS) and current − voltage (I–V) analysis have been used to characterize the interface. It has been found that the electroless reaction depends on the surface preparation and for chemo-mechanically polished (1 1 1) CdZnTe, it also depends on the A/B face identity. Where the deposition occurred at elevated temperature, the deposited contacts were found to produce a greater leakage current and suffered from increased subsurface voiding due to the formation of cadmium chloride.

Analysis of Defects on Chemically-Treated CdZnTe Surfaces

In this work, we focused on investigating the various defects that extend into the near-surface region of CdZnTe (CZT) crystals, and on exploring processing techniques for producing a smooth, non-conductive surface that is ideal for growing thin films and depositing contacts. We determined the surface’s features and the chemical species present using atomic-force microscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy (SEM), coupled with energy-dispersive spectroscopy. We revealed crystallographic defects, e.g., sub-grains and dislocations on the CZT crystals’ surfaces, after employing selected chemical etchants, and then characterized them using optical microscopy, SEM and optical profilometer. Our experimental data imply that the surface defects and chemical species induced by chemical processing may alter the material’s interfacial behavior, and ultimately significantly influence the performance of radiation detectors.

Impact of Tellurium Precipitates in CdZnTe Substrates on MBE HgCdTe Deposition

State-of-the-art (112)B CdZnTe substrates were examined for near-surface tellurium precipitate-related defects. The Te precipitate density was observed to be fairly uniform throughout the bulk of the wafer, including the near-surface region. After a molecular beam epitaxy (MBE) preparation etch, exposed Te precipitates, small pits, and bumps on the (112)B surface of the CdZnTe wafer were observed. From near-infrared and dark field microscopy, the bumps and small pits on the CdZnTe surface are associated with strings of Te precipitates. Raised bumps are Te precipitates near the surface of the (112)B CdZnTe where the MBE preparation etch has not yet exposed the Te precipitate(s). An exposed Te precipitate sticking above the etched CdZnTe surface plane occurs when the MBE preparation etch rapidly undercuts a Te precipitate. Shallow surface pits are formed when the Te precipitate is completely undercut from the surrounding (112)B surface plane. The Te precipitate that was previously located at the center of the pit is liberated by the MBE preparation etch process.

Structural and Electronic Properties of Gold Contacts on CdZnTe with Different Surface Finishes for Radiation Detector Applications

State-of-the-art room-temperature, high-resolution x-ray and gamma-ray semiconductor detectors can be fabricated from CdZnTe (CZT) crystals. The structural and electronic properties of the CZT surface, especially the contact interfaces, can have a substantial effect on radiation detector performance, for example leakage current, signal-to-noise ratio, and energy resolution, especially for soft x-rays and large pixilated arrays. Atomically smooth and defect-free surfaces are desirable for high-performance CZT-based detectors; chemo-mechanical polishing (CMP) is typically performed to produce such surfaces. The electrical behavior of the metal/CZT interface varies substantially with surface preparation before contact deposition, and with choice of metal and deposition technique. We report a systematic study of the structural and electronic properties of gold (Au) contacts on CZT prepared with different surface finishes. We observed subsurface damage under Au contacts on CMP-finished CZT and abrupt interfaces for Au on chemically-polished (CP) CZT. Schottky barrier formation was observed for Au contacts, irrespective of surface finish, and less charge trapping and low surface resistance were observed for CP-finished surfaces. Pre-deposition surface treatment produced interfaces free from oxide layers.

Impact of Surface Treatment on the Structural and Electronic Properties of Polished CdZnTe Surfaces for Radiation Detectors

We present the effects of surface treatments on the structural and electronic properties of chemomechanically polished Cd0.9Zn0.1Te before contact deposition. Specifically, polished CdZnTe (CZT) samples were treated with four distinct chemical etchants: (1) bromine methanol (BM), (2) bromine in lactic acid, (3) bromine in methanol followed by bromine–20% lactic acid in ethylene glycol, and (4) hydrochloric acid (HCl). The surface structure and surface electronic properties were studied with atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). AFM images showed that three of the four etchants significantly altered the surface morphology and structure of CZT. All etchants created smoother surfaces; however, all except HCl also introduced high densities of defects. HCl was found to not affect the surface structure. XPS measurements indicated that a thick, ∼3 nm to 4 nm, TeO2 layer formed about 1 h after etching; hence, it is very important to process devices immediately after etching to prevent oxide formation.

A Slightly Oxidizing Etchant for Polishing of CdTe and CdZnTe Surfaces

New iodine-evolving etching solutions were developed for surface treatment of CdTe and CdZnTe crystals. The nature of the chemical dissolution of CdTe samples with (100), (110), and (111) orientations, and also of (211) Cd0.96Zn0.04Te samples, in KIO3-KI-lactic acid solution was studied. It was established that this dissolution is diffusion controlled. Surface roughness measurements revealed highly polishing properties of the etchant. A study of the chemical composition and structure of the (211)B Cd0.96Zn0.04Te surfaces etched under different conditions was carried out. The thickness of the Te oxide layer was evaluated in the course of heating in vacuum. KIO3-KI-lactic acid treatment appears to be preferable in terms of elimination of Te oxides from the surface as compared with standard Br2-methanol solution. The etching compositions were shown to be useful for chemical polishing of CdTe-based surfaces.

Influence of Surface Polishing on the Structural and Electronic Properties of CdZnTe Surfaces

The structural and electronic properties of CdxZn1−xTe (CZT) surfaces have a significant impact on CZT-based radiation detector performance since they can significantly affect a detector’s signal-to-noise ratio. Atomically smooth and defect-free surfaces are desirable for high-performance CZT-based detectors. We studied in this work the effects of chemomechanical polishing on the structural and electronic properties of Cd0.9Zn0.1Te. CZT samples were polished using two different-sized abrasives and then purely chemically to create atomically smooth surfaces. The surface structure and surface electronic properties were characterized by atomic force microscopy, x-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). Surface roughness of about 1 nm was routinely achieved using 1.0-μm- and 0.05-μm-sized aluminum abrasives followed by chemical polishing. XPS indicates that the polished surfaces are not stoichiometric and have ∼1-nm-thick layers of TeO2 on them. PL indicates high-quality CZT at the surface region with full-width at half-maximum of 0.363 meV for the neutral donor exciton peak.

Slow-Polishing Iodine-Based Etchant for CdTe and CdZnTe Single Crystals

We report on new etching solutions for treatment of CdTe and CdZnTe surfaces based on the iodine-emerging etchant composition KIO3–KI–citric acid (C6H8O7). CdTe samples with (111), (110), and (100) orientations, and also Cd1−xZnxTe (x = 0.04, 0.1) samples with (111), (110), (100), and (211) orientations were investigated. The dissolution rate was determined as a function of solution composition, etchant storage time, disc rotation speed, and temperature. It was established that this chemical dissolution is diffusion controlled. Study of the chemical composition and structure of (211)B Cd1−xZnxTe surfaces etched under different conditions was carried out. x-Ray photoelectron spectroscopy measurements showed that a stoichiometric surface was achieved after briefly heating the etched surface in a vacuum. Reflection high-energy electron diffraction measurements revealed a high-quality single-crystalline surface layer in samples etched with KIO3–KI–citric acid solutions as compared with those etched with a bromine–methanol treatment. The etching compositions were shown to be useful for controlled removal of semiconductor material, and also for chemical polishing of CdTe-based surfaces.

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 75 mm in diameter. A wafer level pixelization process has been tested and its results will be presented. Angular resolution for directional identification of the order of 6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> requires energy resolution better than 5%. For this reason we have investigated planar metal-p-n diodes fabricated by using laser-induced solid phase doping and Schottky diodes created by Ar-ion etching of the Cd(Zn)Te surfaces before deposition of Ni electrodes. The energy resolution achieved is better than 1% FWHM @662 KeV. 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.

Synchrotron study of charge transport in a CZT ring-drift detector

We report on experimental and theoretical results obtained with a CdZnTe (CZT) detector with a versatile read-out scheme: the ring detector. This detector has been tested extensively both in our laboratories and at the HASYLAB synchrotron facility and has demonstrated excellent energy resolution of up to 0.73%, over a wide dynamic range covering 6−662keV. The key feature in this design is carefully managed charge transport, which eliminates the hole contribution, and optimizes the electron collection. We have developed a theoretical model of this detector, to enable the optimization of the read-out structure as a function of detector dimensions. A comparison between X-ray scans across the detector and model calculations shows that the model already captures all the major detector features, but still leaves a few challenging questions related to CZT surface conductivity. A defect in the detector, which breaks the expected cylindrical symmetry of the charge transport, is examined in detail by differential spectrometry, a powerful technique, which exploits differences in X-ray absorption between bulk and defect.

The effect of etching time on the CdZnTe surface

The surface quality of CdZnTe plays an important role in the performance of sensors based on this material. In this paper the effect of chemical etching on Cd0.9Zn0.1Te sensor performance was examined. Sample surfaces were treated with the same concentration 2% Br-MeOH for different etching times (30s, 2, 4, 6, 8min). The surfaces were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and I–V Measurement. These results demonstrate that the best surface quality can be obtained by chemical etching for 30s. Under these experimental conditions, the surface composition Te/Cd+Zn approaches 1, the roughness is lower than 3nm, and the leakage current shows a value lower than 10nA.

Morphology and electronic states of chemically treated CdZnTe surfaces

The morphology and electronic structure of chemically treated CdZnTe surfaces has been studied using atomic force microscopy and scanning tunneling microscopy/spectroscopy. Studies have been performed on wet chemically etched and passivated surfaces. Etching in bromine methanol solutions effectively removes surface damage after polishing and produces a hillock like structure. A number of surface electronic states are introduced, both donor-like and acceptor-like, and are attributed to various surface defects. Because of the amorphous nature of the surface, these states have tails that extend into the band gap and produces a very narrow surface band. Charge hopping across the narrow band gap is proposed to explain the high surface conductivity. Surfaces passivated in hydrogen peroxide show increased roughness and using atomic force microscopy, the evolution of the formation of an oxide layer has been observed. Additional acceptor-like surface states of higher energy are introduced and the surface band gap is found to increase, reducing the surface conductivity.