Te-rich Surface Research Articles

Investigating the impact of growth temperature on the direct integration of pure phase 2H-MoTe2 with Si(1 1 1) using molecular beam epitaxy

Group-VI transition metal dichalcogenides (TMDCs) have garnered significant attention due to their novel properties in the two-dimensional realm. Among these TMDCs, MoTe2 is a particularly intriguing material with a small difference between Gibbs free energies of its semiconducting and semimetallic phases, rendering it suitable for a wide array of applications. In this study, we report the molecular beam epitaxy (MBE) growth of 2H-MoTe2 directly on a silicon (111) substrate, which holds great significance due to its compatibility with the existing integrated circuit industry. By employing a growth strategy involving a Te-rich silicon surface, we achieved the growth of high-quality, phase-pure 2H-MoTe2 films. The effect of growth temperature on the film quality was investigated, revealing that the growth at 450 °C led to highly crystalline and layered growth. The thickness, roughness and stoichiometry of the grown films were characterized using spectroscopic ellipsometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy confirmed the presence of the desired 2H-phase for the grown films. The direct integration of MoTe2 with silicon opens up exciting possibilities for its utilization in nanoelectronics and optoelectronics devices.

Passivation effect on Cd0.95Mn0.05Te0.98Se0.02 radiation detection performance

CdTe-based detectors have the problem of Te-rich surface layers caused by Br etching, which is one of fabrication steps. Te-rich layer acts as a trapping center and serves as an additional source of charge carriers, thereby degrading transport property of charge carriers and enriching leakage current on surface of detector. To solve this problem, we introduced sodium hypochlorite (NaOCl) as a passivant, and investigated its effect on Cd0.95Mn0.05Te0.98Se0.02 (CMTS), by analyzing chemical state of surface and its performance. After passivation with NaOCl, the results of X-ray photoelectron spectroscopy (XPS) shows the formation of tellurium oxide and elimination of water on CMTS surface, and CMTS presented enhanced performance with Am-241 radioisotope. Consequently, it is demonstrated that the passivation with NaOCl reduces leakage current, compensates defect, and elevates transport of charge carriers, thereby decreasing charge loss of carriers and improving performance of CMTS detector.

Wet chemical etching of cadmium telluride photovoltaics for enhanced open-circuit voltage, fill factor, and power conversion efficiency

Abstract

Ammonium Fluoride Passivation of CdZnTeSe Sensors for Applications in Nuclear Detection and Medical Imaging.

Cadmium zinc telluride selenide (Cd1−xZnxTe1−ySey or CZTS) is one of the emerging CdTe-based semiconductor materials for detecting X- and gamma-ray radiation at or near room temperature (i.e., without cryogenic cooling). Potential applications of CZTS sensors include medical imaging, X-ray detection, and gamma-ray spectroscopy. Chemical passivation of CZTS is needed to reduce the conductivity of Te-rich surfaces, which reduces the noise and improves the device performance. In this study, we focus on the effect of surface passivation of CZTS using a 10% aqueous solution of ammonium fluoride. The effects of the chemical treatment were studied on the leakage current, charge transport measured as the electron mobility-lifetime (µτ) product, and the spectral resolution measured as the full-width at half-maximum (FWHM) of specific peaks. After passivation, the leakage current increased and began to decrease towards pre-passivation levels. The energy resolutions were recorded for eight applied voltages between −35 V and −200 V. The results showed an average of 25% improvement in the detector’s energy resolution for the 59.6 keV gamma peak of Am-241. The electron µτ product was unchanged at 2 × 10−3 cm2/V. These results show that ammonium fluoride is effective for chemical passivation of CZTS detectors.

Surface analysis and electrical measurement of the ohmic contact on p-CdZnTe (111)B face with Au/Cd composite electrode

CdZnTe crystal has proved to be an excellent material to detect X-ray and gamma ray. Ohmic electrode on CdZnTe wafer is one of the key factors during the fabrication of CdZnTe detector. In this work, the effect of Au/Cd composite electrode on the (111)B (Te-rich surface) of CdZnTe wafers with p-type conductivity (p-CdZnTe) was investigated. The Au/Cd electrode was deposited on the (111)B of CdZnTe wafers by vacuum evaporation method. For comparison, the Au/Zn and Au electrodes were also deposited on CdZnTe (111)B surfaces. The surface and structural properties of the composite electrodes on the (111)B surface of CdZnTe were characterized by the AFM, XPS and SEM. The electrical properties of the electrodes were evaluated by Current-Voltage (I-V) test. The results show that the surfaces of the Au/Cd and Au/Zn electrodes on (111)B surface were more smooth with less foreign impurities among the interface of the electrode and CdZnTe, as compared to the Au electrode on (111)B surface. The Au/Cd composite electrode can obtain a more ideal ohmic contact than Au/Zn composite electrode on the (111)B surface of CdZnTe wafer. The Au/Cd composite electrode on (111)B surface has the lowest Schottky barrier height, which is attributed to the reduction of the influence of Te enriched surface on the metal-semiconductor contact.

Interface phenomena between CdTe and ZnTe:Cu back contact

Thin film technology has reached a maturity to achieve conversion efficiencies of the order of 22%. Among thin films, CdTe-based photovoltaic modules represent 80% of the total production. Nonetheless, some issues concerning back-contact are still open. In industrial process a chemical etching is required in order to make the CdTe film surface rich in Te. The Te-excess is fundamental in order to form a stable telluride compound with copper and to obtain an ohmic, low-resistance back-contact. Moreover, the Te-excess hinders the fast diffusion of copper in CdTe and its achievement of the junction region, preventing the destruction of the device. In this paper we study a ZnTe:Cu buffer layer deposited onto a CdTe film, characterized by a naturally Te-rich surface obtained with a particular chlorine heat treatment without any chemical etching. Copper diffusion and the CdTe/ZnTe:Cu interface were studied by x-ray photoemission spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF- SIMS) to deeply analyze the intermixing phenomena and copper behavior inside polycrystalline CdTe film.

Selective Cd Removal From CdTe for High-Efficiency Te Back-Contact Formation

The presence of a Te-rich surface or an elemental Te layer is beneficial for the formation of a low-barrier back contact for high-efficiency CdTe solar cells. Etching processes are widely used to form Te-rich CdTe surfaces, while deposition processes such as evaporation are used to form elemental Te layers. Here, we show that a reaction between methylammonium iodide (CH3NH3I, MAI) and CdTe can be used to simply and controllably produce elemental Te over a wide processing window. Both X-ray diffraction and Raman spectroscopy confirmed the formation of a Te layer. The MAI-produced Te layer reduces the Schottky barrier height, improves the open-circuit voltage ( V OC) and fill factor, and outperforms contacts formed with the evaporated Te. We examined the effect of MAI reaction temperature and the amount of Cu needed to optimize the device. CdS/CdTe stacks that were treated with a 125 mM MAI solution and heated to 125 °C for 10 min showed the best power conversion efficiency (PCE) of 14.1%, while the best efficiency of a standard device without treatment was 13.0%, and the best PCE of an evaporated Te layer was 13.8%. Notably, the improved efficiency for the MAI-treated devices was achieved with less Cu than was required for the standard device. With an indium tin oxide back electrode, the PCE was also improved from 11.0% to 12.2% with MAI treatment, providing a potential route for fabricating high-efficiency transparent or bifacial CdTe solar cells.

Low energy ion scattering as a depth profiling tool for thin layers - Case of bromine methanol etched CdTe

We have investigated the properties of the Te-rich surface layer formed after a bromine-methanol etch of CdTe single crystal by two methods: Angle Resolved X-Ray Photoelectron Spectroscopy (ARXPS) and Low Energy Ion Scattering (LEIS) in the Dynamic mode. We compare the acquisition time of each method. The results showed similar, exponential decay of the Te/Cd ratio to a depth of 6 nm. At the depths higher than 6 nm, the substrate becomes stoichiometric. Dynamic LEIS provided more detailed information about composition at depths lower than the probing depth of ARXPS. The Dynamic LEIS measurements suggest that the composition of the outermost layer of CdTe after bromine-methanol etching consists of CdTe4.

Band alignment of CdTe with MoOx oxide and fabrication of high efficiency CdTe solar cells

The band alignment of CdTe/MoOx hetero-interface contact at back electronic electrode in CdTe thin film solar cell was quantitatively characterized by photon electron emission spectroscopy. The experimental results showed that depending on the CdTe surface chemical state and the stoichiometric value of x in MoOx, energy barrier with a value as low as 0.23–0.39 eV was developed at the CdTe/MoOx contact interface for a MoOx buffer layer with an x value of 2.86–3. CdTe solar cell with relatively high efficiency can be fabricated by using a MoOx as a back contact buffer, which is allowed to have a relatively large stoichiometric x range of 2.86–3. The Te-rich CdTe surface obtained by chemical etching induced a reduction reaction upon the deposition of a MoOx thin layer, leading to the formation of reduced Mo states and other defect states below the Fermi level, which assisted hole carrier transport.

Strong Photothermoelectric Response and Contact Reactivity of the Dirac Semimetal ZrTe5

The family of three-dimensional topological insulators opens new avenues to discover novel photophysics and to develop novel types of photodetectors. ZrTe5 has been shown to be a Dirac semimetal possessing unique topological, electronic, and optical properties. Here, we present spatially resolved photocurrent measurements on devices made of nanoplatelets of ZrTe5, demonstrating the photothermoelectric origin of the photoresponse. Because of the high electrical conductivity and good Seebeck coefficient, we obtain noise-equivalent powers as low as 42 pW/Hz1/2, at room temperature for visible light illumination, at zero bias. We also show that these devices suffer from significant ambient reactivity, such as the formation of a Te-rich surface region driven by Zr oxidation as well as severe reactions with the metal contacts. This reactivity results in significant stresses in the devices, leading to unusual geometries that are useful for gaining insight into the photocurrent mechanisms. Our results indicate that both the large photothermoelectric response and reactivity must be considered when designing or interpreting photocurrent measurements in these systems.

Effect of vacuum annealing on structural, electrical and thermal properties of e-beam evaporated Bi2Te3 thin films

Nanocrystalline thin films of a V-VI compound Bi2Te3 are fabricated with uniform thickness by e-beam evaporation at room temperature. The as-deposited films are stoichiometric, monophasic, highly strained and polycrystalline. We studied the effect of vacuum annealing (at a pressure of ~3×10−6mbar) on composition, structure, optical and electrical properties of these films. It is observed that, as the annealing temperature increases (from 100°C to 300°C), the crystallites grow with a preferential orientation along (110) planes with slight increase in the crystallite size from ~14nm to 30nm. This is associated with the breaking of quintuple layers and rearrangement of crystallographic planes in the crystallites with Te rich surface emerging on vacuum annealing as evidenced from the XRD, Raman and high-resolution TEM studies. The direct bandgap (0.116eV) of as-deposited Bi2Te3 changes from 0.092eV to 0.113eV on annealing at 100°C to 300°C, respectively. Interestingly, we observe a gradual change from a semiconductor to metallic behavior on annealing the samples from 100°C to 300°C. Such a transition from negative temperature coefficient (NTC) to positive temperature coefficient (PTC) is seen mainly due to the percolation of Te - rich crystallite surfaces, which evolve as the annealing temperature increases. While the films annealed at 200°C and 250°C shows a broad semiconductor to metallic transition at ~150K and 200K respectively, the thin films annealed at 300°C are found to exhibit complete metallic behavior below room temperature. The electrical property and Seebeck coefficient studies with power factors in the range of ~4 to 12×10−4W/K2m for films annealed above 200°C suggest that the vacuum annealed Bi2Te3 thin films are favorable for thermoelectric applications.

Improving the Performances of CdTe Gamma Ray Detectors by H2/Ar ECR Plasma Processing

H2/Ar electron cyclotron resonance plasma processing of the high-resistivity (111) CdTe crystals was studied to find its effect on the leakage current and the gamma ray detection properties of the detectors. All the crystals were chemically etched in a bromine–methanol (BM) solution prior to the plasma processing to obtain a smooth surface. A remarkable reduction in both the surface and the bulk leakage current was observed in the plasma processed detectors when compared with the unprocessed one. The gamma detection property of the detectors, both the spectral resolution and the peak shape, was also significantly improved. X-ray photoelectron spectroscopy confirmed that an excess Te-rich surface layer was present on the BM etched crystal, however, such layer was removed, and the surface atomic ratio was modified after the plasma processing. The improvement in the detector performance could be related to the change in the surface chemical states of the crystals. Hence, this could be a promising process for improving the detectors performance and their stabilities.

Intrinsic surface passivation of CdTe

Recombination is critically limiting in CdTe devices such as solar cells and detectors, with much of it occurring at or near the surface. In this work, we explore different routes to passivate p-type CdTe surfaces without any intentional extrinsic passivation layers. To provide deeper insight into the passivation routes, we uniquely correlate a set of characterization methods: surface analysis and time-resolved spectroscopy. We study two model systems: nominally undoped single crystals and large-grain polycrystalline films. We examine several strategies to reduce surface recombination velocity. First, we study the effects of removing surface contaminants while maintaining a near-stoichiometric surface. Then we examine stoichiometric thermally reconstructed surfaces. We also investigate the effects of shifting the surface stoichiometry by both “subtractive” (wet chemical etches) and “additive” (ampoule anneals and epitaxial growth) means. We consistently find for a variety of methods that a highly ordered stoichiometric to Cd-rich surface shows a significant reduction in surface recombination, whereas a Te-rich surface has high recombination and propose a mechanism to explain this. While as-received single crystals and as-deposited polycrystalline films have surface recombination velocities in the range of 105–106 cm/s, we find that several routes can reduce surface recombination velocities to <2.5 × 104 cm/s.

A Simple Sb2Te3 Back-Contact Process for CdTe Solar Cells

CdTe solar technology has proved to be a cost-efficient solution for energy production. Formation of the back contact is an important and critical step in preparing high-efficiency, stable CdTe solar cells. In this paper we report a simple CdTe solar cell (Sb2Te3) back contact-formation process. The CdS and CdTe layers were deposited by close-space sublimation. After CdCl2 annealing treatment, the CdTe surface was etched by use of a mixture of nitric and phosphoric acids to obtain a Te-rich surface. Elemental Sb was sputtered on the etched surface and successive post-annealing treatment induced Sb2Te3 alloy formation. Structural characterization by x-ray diffraction analysis confirmed formation of the Sb2Te3 phase. The performance of solar cells with nanoalloyed Sb2Te3 back contacts was comparable with that of reference solar cells prepared with sputtered Sb2Te3 back contact from a compound sputter target.

Observation of Distinct Bulk and Surface Chemical Environments in a Topological Insulator under Magnetic Doping

The influence of magnetic dopants on the electronic and chemical environments in topological insulators (TIs) is a key factor when considering possible spintronic applications based on topological surface state properties. Here we provide spectroscopic evidence for the presence of distinct chemical and electronic behavior for surface and bulk magnetic doping of Bi2Te3. The inclusion of Mn in the bulk of Bi2Te3 induces a genuine dilute ferromagnetic state, with reduction of the bulk band gap as the Mn content is increased. Deposition of Fe on the Bi2Te3 surface, on the other hand, favors the formation of iron telluride already at coverages as low as 0.07 monolayer, as a consequence of the reactivity of the Te-rich surface. Our results identify the factors that need to be controlled in the realization of magnetic nanosystems and interfaces based on TIs.

An XPS study of bromine in methanol etching and hydrogen peroxide passivation treatments for cadmium zinc telluride radiation detectors

The performance of single crystal CdZnTe radiation detectors is dependent on both the bulk and the surface properties of the material. After single crystal fabrication and mechanical polishing, modification of the surface to remove damage and reduce the surface leakage current is generally achieved through chemical etching followed by a passivation treatment. In this work, CdZnTe single crystals have been chemically etched using a bromine in methanol (BM) treatment. The BM concentrations employed were 0.2 and 2.0 (v/v) % and exposure times varied between 5 and 120s. Angle resolved XPS and sputter depth profiling has been employed to characterize the surfaces for the different exposure conditions. A Te rich surface layer was formed for all exposures and the layer thickness was found to be independent of exposure time. The enriched Te layer thickness was accurately determined by calibrating the sputter rate against a CdTe layer of known thickness. For BM concentrations of 0.2 (v/v) % and 2 (v/v) %, the Te layer thickness was determined to be 1.3±0.2 and 1.8±0.2nm, respectively. The BM etched surfaces have subsequently been passivated in a 30wt.% H2O2 solution employing exposure time of 15s. The oxide layer thickness has been calculated using two standard XPS methodologies, based on the Beer–Lambert expression. The TeO2 thickness calculated from ARXPS data are slightly higher than the thickness obtained by the simplified Beer–Lambert expression. For BM exposures of 30–120s followed by a passivation treatment of 30wt. % H2O2 solution employing an exposure time 15s, the ARXPS method gave an average TeO2 thickness value of 1.20nm and the simplified Beer–Lambert expression gave an average thickness value of 0.99nm.

Reprint of: Segregation of Te at the back contact in electrochemically deposited CdTe thin film solar cells

One significant problem associated with thin film CdTe solar cells is the reverse biased Schottky barrier at the back contact (CdTe/metal interface), caused by the high electron affinity of p-CdTe, along with the non-availability of metals with adequate work functions. This Schottky barrier increases contact resistance, thereby reducing cell performance. In order to eliminate this barrier, attempts to create a tunnelling contact are being made by etching the CdTe surface, to form a Te rich degenerate semiconductor surface. The Te rich surface layer usually contains increased conductivity and is p+-type. Electrochemically deposited CdTe thin films are naturally Te-rich, but significantly, in this work, we report the segregated concentration of Te at the back contact resulting from the controlled addition of Te to the deposition electrolyte. This segregation of Te is notable as it minimises the use of acidic etchants to create a Te rich surface. Raman and XRD spectra confirm Te segregation at the surface of the CdTe, which one would expect to produce a good ohmic back contact. Effectively, high-quality layers and cells can be obtained from this process, which can be scaled up to an industrial level, inline and without the use of etching baths or external CdCl2 treatment.Photocurrent spectroscopy was undertaken on CdTe thin film solar cells to investigate their interfacial and structural properties, both of which play a crucial role in determining photovoltaic performance. High chopping frequencies were employed in order to obtain reliable incident photon to current conversion efficiency (IPCE) data, in the cases where surface recombination was rapid. The absorption edge at the low energy end of the IPCE spectrum corresponds to an interfacial bandgap of 1.45eV, which in turn corresponds to interdiffusion of about 10% of S into CdTe, according to bowing equations found in literature.

Segregation of Te at the back contact in electrochemically deposited CdTe thin film solar cells

One significant problem associated with thin film CdTe solar cells is the reverse biased Schottky barrier at the back contact (CdTe/metal interface), caused by the high electron affinity of p-CdTe, along with the non-availability of metals with adequate work functions. This Schottky barrier increases contact resistance, thereby reducing cell performance. In order to eliminate this barrier, attempts to create a tunnelling contact are being made by etching the CdTe surface, to form a Te rich degenerate semiconductor surface. The Te rich surface layer usually contains increased conductivity and is p +-type. Electrochemically deposited CdTe thin films are naturally Te-rich, but significantly, in this work, we report the segregated concentration of Te at the back contact resulting from the controlled addition of Te to the deposition electrolyte. This segregation of Te is notable as it minimises the use of acidic etchants to create a Te rich surface. Raman and XRD spectra confirm Te segregation at the surface of the CdTe, which one would expect to produce a good ohmic back contact. Effectively, high-quality layers and cells can be obtained from this process, which can be scaled up to an industrial level, inline and without the use of etching baths or external CdCl 2 treatment. Photocurrent spectroscopy was undertaken on CdTe thin film solar cells to investigate their interfacial and structural properties, both of which play a crucial role in determining photovoltaic performance. High chopping frequencies were employed in order to obtain reliable incident photon to current conversion efficiency (IPCE) data, in the cases where surface recombination was rapid. The absorption edge at the low energy end of the IPCE spectrum corresponds to an interfacial bandgap of 1.45 eV, which in turn corresponds to interdiffusion of about 10% of S into CdTe, according to bowing equations found in literature.

Study of effects of polishing and etching processes on Cd 1− xZn xTe surface quality

The effect of surface preparation on CdZnTe properties was investigated. Surface etching using bromine solutions enhances Te elemental composition, resulting in a Te rich surface layer that is prone to oxidize. This oxidation degrades the performance of the fabricated CZT gamma detector. Roughness results were identical for samples polished with 1 and 3 μm and subsequently etched in 2% Br-MeOH. The optimal concentration of etching was 2% Br-MeOH.

Surface leakage current control with heterojunction-type passivation in semi-insulating CdZnTe material

Inter-pixel resistance in the planar structure X-ray or gamma-ray detector has severe effects on signal cross-talk and the signal integration time. Generally, Br-MeOH etching in Cd-based compounds leaves a Te-rich or Cd-rich surface resulting from selective etching. The etching then converts the Te-rich or Cd-rich surface in air into TeO 2 or CdO oxides, which exhibit low resistance. To reduce the surface recombination, we adopted (NH 4) 2S for surface passivation in the polycrystalline CdZnTe:Cl grown by thermal evaporation method. The optimization of passivation was confirmed by I–V measurement and X-ray photoemission spectroscopy (XPS) analysis. From the I–V curve, we confirmed that the surface resistance was considerably increased after passivation with (NH 4) 2S. XPS data showed that (NH 4) 2S passivation removed conductive TeO 2 layers and induced formation of insulating CdTeO 3 and CdS layers. A heterojunction by the thin CdS layer and CdZnTe was formed.