Te Atomic Layer Research Articles

ChemInform Abstract: Sc9Te2: A Two-Dimensional Distortion Wave in the Scandium-Richest Telluride.

Sc9Te2 was prepared by high-temperature solid-state techniques, and the structure was determined by single-crystal X-ray diffraction to be monoclinic, Cc (No. 9, Z = 8) with a = 7.7576(1) A, b = 15.654(3) A, c = 17.283(3) A, and β = 90.01(3)° at 23 °C. The structure of metallic Sc9Te2 can be viewed as columns built either from distorted 3 × 3 bcc metal atoms or from distorted edge-sharing octahedra along a that are joined via intercolumn bonds on opposite sides into two-dimensional layers along b. A corrugated layer of Te atoms separates the metal layers in the c direction. This structure derives from the Ti9Se2 structure, with doubled a and b dimensions and a lower symmetry. Magnetic susceptibility measurements on Sc9Te2 show a substantially temperature-independent paramagnetism that is much smaller per metal atom than those for other scandium-rich tellurides or scandium metal. Extended Huckel band calculations reveal a clear distinction between high overlap populations for Sc−Sc interactions in the in...

Synthesis and characterization of Sb2Te3 nanostructures

Single-crystalline Sb2Te3 nanostructures, of which the bulk is one of the best thermoelectric materials at room temperature, are synthesized by chemical vapor deposition. The composition, crystal structure, and growth mechanism of the sample are investigated. According to our experimental results, Sb2Te3 normally grows into hexagonal nanoplates without using catalyst, but single-crystalline nanowires can be fabricated with Au nanoparticles as the catalyst. The growth mechanism of Sb2Te3 nanostructures is closely related to its anisotropic crystal structure. Sb2Te3 has a rhombohedral structure, which exhibits a layered anisotropy with the Te and Sb atom layers arranged along the c-axis. Moreover, there are two adjacent Te layers connected by van der Waals bonds. Therefore, Sb2Te3 prefers to grow into hexagonal plates in the ab-plane. When Au nanoparticles are used as the catalyst, the growth direction of the precipitated Sb2Te3 is restricted, leading to the formation of Sb2Te3 nanowires.

Electrodeposition of Tellurium Atomic Layer on n-GaAs(100) Substrate

This paper describes the electrodeposition of Tellurium (Te) atomic layer on n-type GaAs(100) substrates. As-received n-GaAs(100) substrates were treated in 10 % HF and ultraviolet (UV) ozone cleaned. The substrate was then transferred to the ultrahigh vacuum (UHV) electrochemistry chamber and cleaned by Ar+ ion bombardment. The clean substrate was then transferred into the attached electrochemistry ante-chamber, and immersed in a tellurite solution. A number of deposition potentials were investigated. The resulting Auger peak height ratios, Te/Ga, were plotted versus the Te deposition potential. From the Auger ratio plot, below -0.8 V, a reduction feature was observed, at −0.9 V, due to the reduction of Te to Telluride ion (Te0 + 2e− → Te2−). At potentials of -0.9 and below, only a surface limited atomic layer of Te was left on the GaAs surface. An initial investigation of In deposition on this Te coated GaAs surface was also performed.

Deposition of antimony telluride thin film by ECALE

The process of Sb2Te3 thin film growth on the Pt substrate by electrochemical atomic layer epitaxy (ECALE) was studied. Cyclic voltammetric scanning was performed to analyze the electrochemical behavior of Te and Sb on the Pt substrate. Sb2Te3 film was formed using an automated flow deposition system by alternately depositing Te and Sb atomic layers for 400 circles. The deposited Sb2Te3 films were characterized by XRD, EDX, FTIR and FESEM observation. Sb2Te3 compound structure was confirmed by XRD pattern and agreed well with the results of EDX quantitative analysis and coulometric analysis. FESEM micrographs showed that the deposit was composed of fine nano particles with size of about 20 nm. FESEM image of the cross section showed that the deposited films were very smooth and dense with thickness of about 190 nm. The optical band gap of the deposited Sb2Te3 film was determined as 0.42 eV by FTIR spectroscopy, and it was blue shifted in comparison with that of the bulk Sb2Te3 single crystal due to its nanocrystalline microstructure.

CdTe Electrodeposition on InP(100) via Electrochemical Atomic Layer Epitaxy (EC-ALE): Studies Using UHV-EC

The II-VI compound semiconductor CdTe was electrodeposited on InP(100) surfaces using electrochemical atomic layer epitaxy (EC-ALE). CdTe was deposited on a Te-modified InP(100) surface using this atomic layer by atomic layer methodology. The deposit started with formation of an atomic layer of Te on the InP(100) surface, as Cd was observed not to form an underpotential deposition (UPD) layer on InP(100), although it was found to UPD on Te atomic layers. On the In-terminated 'clean' InP(100) surface, Te was deposited at -0.80 V from a 0.1 mM solution of TeO2, resulting in formation of a Te atomic layer and some small amount of bulk Te. The excess bulk Te was then removed by reduction in blank solution at -0.90 V, leaving a Te atomic layer. Given the presences of the Te atomic layer, it was then possible to form an atomic layer of Cd by UPD at -0.58 V to complete the formation of a CdTe monolayer by EC-ALE. That cycle was then repeated to demonstrate the applicability of the cycle to the formation of CdTe nanofilms. Auger spectra recorded after the first three cycles of CdTe deposition on InP(100) were consistent with the layer-by-layer CdTe growth. It is interesting to note that Cd did not form a UPD deposit on the In-terminated InP(100) surface and only formed Cd clusters at an overpotential. This issue is probably related to the inability of the Cd and In to form a stable surface compound.

Multiparametric electrochemical characterisation of Te–Cu–Pb atomic three-layer structure deposition on polycrystalline gold

Successive underpotential deposition of Te ad, Cu ad and Pb ad adlayers in the potentiodynamic mode was used for Te ad/Cu ad/Pb ad atomic three-layer structure assembling on polycrystalline Au support. Layer-by-layer deposition was characterised in each of the stages with potentiodynamic electrochemical impedance spectroscopy (PDEIS) by variations, with the electrode potential E, of double layer pseudocapacitance Q dl, charge transfer resistance R ct and Warburg coefficient A of diffusion impedance. Q dl( E) has shown a strong decrease in the cathodic scan during Te atomic layer deposition on Au, a sharp peak in the subsequent Cu ad deposition on Au/Te ad and an increase in Pb ad deposition on Au/Te ad/Cu ad. The inverses of charge transfer resistance and Warburg coefficient have shown characteristic maxima in R ct - 1 ( E ) and A −1( E) dependencies in all the three stages of Te ad/Cu ad/Pb ad three-layer assembling. The characteristic features in the dependences on the potential of the ac response parameters compensated for insufficient self-descriptiveness of the dc part of the nonstationary interface response in the layer-by-layer deposition. High information density of PDEIS spectra provides better control of electrochemical processes on nonstationary interfaces and this new opportunity can enable wider use of electrochemical approaches for thin-layer structures preparation.

EC-STM Studies of Te and CdTe Atomic Layer Formation from a Basic Te Solution

The cyclic voltammetry of Te on Au is markedly affected by pH. This fact can be used to advantage when designing an electrodeposition cycle for CdTe. For instance, if a pH 2 Te solution is used, the underpotential deposition (UPD) potential for Te is 0.8 V positive of that for Cd. However, if a pH 9.2 Te deposition solution is used, the potential for Te UPD coincides with that for Cd, greatly simplifying the development of an electrochemical atomic layer epitaxy (EC-ALE) cycle. This report describes electrochemical scanning tunneling microscopy (EC-STM) studies of Te deposition on Au(111) and Au(100) from basic media. Several structures were observed on Au(111): a tellurite adlayer which spontaneously adsorbed prior to Te formation, a 1/4 coverage and a 1/3 coverage While on Au(111), a 1/3 coverage with light domain walls, and two structures, with coverages of 4/9 and 5/9, were observed. Results of the formation of the first CdTe compound monolayer using an EC-ALE cycle which includes Te deposition from a basic solution are also included. © 2004 The Electrochemical Society. All rights reserved.

Atomic layer epitaxy of CdTe using an automated electrochemical thin-layer flow deposition reactor

A number of different cycle chemistries, along with an automated thin-layer flow cell electrodeposition system, are described for the formation of CdTe thin film deposits using electrochemical atomic layer epitaxy (EC-ALE). Atomic layer epitaxy (ALE) involves the deposition of a compound one atomic layer at a time, via surface limited reactions, in a repeating cycle. In EC-ALE, underpotential deposition (upd) is used to form the atomic layers. Previous studies of the EC-ALE growth of CdTe have involved a cycle where Cd was deposited by reductive upd, followed by oxidative upd of Te from an acidic (pH 2.0) solution. In the present study, basic (pH 10.2) tellurium solutions were investigated in an attempt to use direct reductive upd of Te, as well as reductive upd of Cd. The idea was to simplify the cycle. The deposition in the basic solution is shifted dramatically negative, such that surface limited reductive deposition of Te appears to coincide with potentials used for reductive Cd upd, thus allowing both elements to be reductively deposited in a cycle at similar potentials. Improvements have been made relative to previous deposits reported by this group, such as an increase in the amount deposited per cycle. The old cycle and the H-cell design produced only 0.4 ML per cycle, while our new cycle deposits the expected 1 ML per cycle. However, there were some drawbacks to the new cycle, which was based on the reductive upd formation for both Cd and Te. Even though voltammetry for Te deposition on Au suggests that Te deposits by a surface limited process, it in fact deposits at an overpotential. Therefore, some bulk Te is inevitably deposited along with each Te atomic layer. The amount of bulk deposited is a function of convection in the cell, and thus leads to inhomogeneity in the deposit, something not expected for a purely surface limited process. In order to avoid the traces of bulk Te, the best deposits were formed when the reductive deposition of Te was combined with a bulk Te stripping step to remove excess material. This process is referred to here as oxidative Te upd. The resulting deposits evidenced a predominant [111] orientation for zinc blende CdTe (from XRD), and a band gap of 1.55 eV (from reflection adsorption measurements), consistent with the literature bandgap for CdTe.

Formation of the first monolayers of CdTe on Au(111) by electrochemical atomic layer epitaxy (EC-ALE): studied by LEED, Auger, XPS, and in-situ STM

The two-step alternated electrodeposition of Cd and Te atomic layers to form CdTe monolayers, electrochemical atomic layer epitaxy (EC-ALE), was studied on Au(111) using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), in-situ scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS). Well ordered (√7×√7) R19.1°-CdTe and (3×3)-CdTe structures were formed using either Te or Cd as the first layer, and model structures are proposed for both. STM images suggest that previously proposed hexagonal structures based on a plane of zinc blende CdTe may be incorrect. A chain structure is suggested to account for the (√7×√7) R19.1°, based on 3/7 (0.43) ML each of Cd and Te. The (3×3)-CdTe structure results from the deposition of a CdTe sandwich: first an atomic layer of Cd on Au, followed by a layer of Te, and then a second Cd layer. Based on this three layer model for the (3×3), a three step deposit was formed, starting with Cd, then Te, and finally Cd. This resulted in an excellent quality LEED pattern, suggesting a well-ordered (3×3)-CdTe deposit and strongly supporting the proposed model. The importance of deposit stoichiometry was also investigated using STM, which indicated that too low a coverage in the first atomic layer resulted in cluster formation and the degradation of surface morphology.

Clustering of Zirconium Atoms in Zr5Te6: A Novel NiAs-Type-Related Telluride with Ordered Vacancies

Zr5Te6 has been synthesized and its structure determined by means of single crystal X-ray diffraction to be trigonal, P3m1, Z=3, Pearson symbol hP33, a = 1172.8(2) pm, c = 707.0(1) pm. Zr5Te6 adopts a metal-deficient, vacancy-ordered 3a x 3a x 1c superstructure of the NiAs type structure. In the Zr atom layers, alternately one and two out of nine Zr atoms are missing. The less densely populated layers (7/9) consist of star-shaped Zr7 clusters with intracluster contacts of 351.1 pm; the shortest Zr-Zr intercluster distance is 470.5 pm. In the more densely populated Zr atom layers (8/9), three quarters of the Zr atoms are arranged to pairs (326.4 pm). The distinctive distribution of the vacancies affords a topologically uniform fivefold Zr coordination (283.5 - 302.6 pm) for all three crystallographically inequivalent Te atoms. They are shifted towards the vacancies in the Zr atom layers. The associated corrugation of the Te atom layers is characterized by an amplitude of 28 pm. The Te-Te contacts are > or =368.1 pm. According to extended Hückel calculations, the defects in the Zr atom layers lead to a reduction in overall Zr-Te bonding interactions relative to ZrTe (NiAs). However, through the clustering the total attractive intralayer Zr-Zr interactions increase considerably, thus providing decisive stabilization of the structure. As revealed by thermal analyses, Zr5Te6 undergoes a reversible phase transition at 1,513 +/- 5 K. On the Zr-rich side, Zr5Te6 coexists with ZrTe (WC), and, above 1,438 +/- 5 K with the hitherto unknown ZrTe (MnP). Zr5Te6 exhibits temperature independent paramagnetic properties (chimol = 0.7 x 10(-3) cm3 mol(-1)) that are typical for a metallic conductor. An abrupt increase in the magnitude of the diamagnetic susceptibility below 2.2 K in a weak magnetic field indicates a superconducting transition.

Clustering of Zirconium Atoms in Zr5Te6: A Novel NiAs-Type-Related Telluride with Ordered Vacancies

Zr5Te6 has been synthesized and its structure determined by means of single crystal X-ray diffraction to be trigonal, , Z=3, Pearson symbol hP33, a=1172.8(2) pm, c=707.0(1) pm. Zr5Te6 adopts a metal-deficient, vacancy-ordered 3a×3a×1c superstructure of the NiAs type structure. In the Zr atom layers, alternately one and two out of nine Zr atoms are missing. The less densely populated layers (7/9) consist of star-shaped Zr7 clusters with intracluster contacts of 351.1 pm; the shortest Zr–Zr intercluster distance is 470.5 pm. In the more densely populated Zr atom layers (8/9), three quarters of the Zr atoms are arranged to pairs (326.4 pm). The distinctive distribution of the vacancies affords a topologically uniform fivefold Zr coordination (283.5–302.6 pm) for all three crystallographically inequivalent Te atoms. They are shifted towards the vacancies in the Zr atom layers. The associated corrugation of the Te atom layers is characterized by an amplitude of 28 pm. The Te–Te contacts are ≥368.1 pm. According to extended Huckel calculations, the defects in the Zr atom layers lead to a reduction in overall Zr–Te bonding interactions relative to ZrTe (NiAs). However, through the clustering the total attractive intralayer Zr–Zr interactions increase considerably, thus providing decisive stabilization of the structure. As revealed by thermal analyses, Zr5Te6 undergoes a reversible phase transition at 1513±5 K. On the Zr-rich side, Zr5Te6 coexists with ZrTe (WC), and, above 1438±5 K with the hitherto unknown ZrTe (MnP). Zr5Te6 exhibits temperature independent paramagnetic properties (χmol=0.7×10−3 cm3 mol−1) that are typical for a metallic conductor. An abrupt increase in the magnitude of the diamagnetic susceptibility below 2.2 K in a weak magnetic field indicates a superconducting transition.

Clustering of zirconium atoms in Zr5Te6: a novel NiAs-type-related telluride with ordered vacancies.

Zr5Te6 has been synthesized and its structure determined by means of single crystal X-ray diffraction to be trigonal, P3m1, Z=3, Pearson symbol hP33, a = 1172.8(2) pm, c = 707.0(1) pm. Zr5Te6 adopts a metal-deficient, vacancy-ordered 3a x 3a x 1c superstructure of the NiAs type structure. In the Zr atom layers, alternately one and two out of nine Zr atoms are missing. The less densely populated layers (7/9) consist of star-shaped Zr7 clusters with intracluster contacts of 351.1 pm; the shortest Zr-Zr intercluster distance is 470.5 pm. In the more densely populated Zr atom layers (8/9), three quarters of the Zr atoms are arranged to pairs (326.4 pm). The distinctive distribution of the vacancies affords a topologically uniform fivefold Zr coordination (283.5 - 302.6 pm) for all three crystallographically inequivalent Te atoms. They are shifted towards the vacancies in the Zr atom layers. The associated corrugation of the Te atom layers is characterized by an amplitude of 28 pm. The Te-Te contacts are > or =368.1 pm. According to extended Hückel calculations, the defects in the Zr atom layers lead to a reduction in overall Zr-Te bonding interactions relative to ZrTe (NiAs). However, through the clustering the total attractive intralayer Zr-Zr interactions increase considerably, thus providing decisive stabilization of the structure. As revealed by thermal analyses, Zr5Te6 undergoes a reversible phase transition at 1,513 +/- 5 K. On the Zr-rich side, Zr5Te6 coexists with ZrTe (WC), and, above 1,438 +/- 5 K with the hitherto unknown ZrTe (MnP). Zr5Te6 exhibits temperature independent paramagnetic properties (chimol = 0.7 x 10(-3) cm3 mol(-1)) that are typical for a metallic conductor. An abrupt increase in the magnitude of the diamagnetic susceptibility below 2.2 K in a weak magnetic field indicates a superconducting transition.

Sc9Te2: A Two-Dimensional Distortion Wave in the Scandium-Richest Telluride

Sc9Te2 was prepared by high-temperature solid-state techniques, and the structure was determined by single-crystal X-ray diffraction to be monoclinic, Cc (No. 9, Z = 8) with a = 7.7576(1) Å, b = 15.654(3) Å, c = 17.283(3) Å, and β = 90.01(3)° at 23 °C. The structure of metallic Sc9Te2 can be viewed as columns built either from distorted 3 × 3 bcc metal atoms or from distorted edge-sharing octahedra along a⃗ that are joined via intercolumn bonds on opposite sides into two-dimensional layers along b⃗. A corrugated layer of Te atoms separates the metal layers in the c direction. This structure derives from the Ti9Se2 structure, with doubled a and b dimensions and a lower symmetry. Magnetic susceptibility measurements on Sc9Te2 show a substantially temperature-independent paramagnetism that is much smaller per metal atom than those for other scandium-rich tellurides or scandium metal. Extended Hückel band calculations reveal a clear distinction between high overlap populations for Sc−Sc interactions in the interior of the condensed metal structure and lower values for those on the exterior that have Te neighbors. The distortion generally enhances the overlap populations of occupied states.

Phase transitions in the electrodeposition of tellurium atomic layers on Au(100)

Abstract This paper describes the electrochemical formation of tellurium atomic layers on Au(100) and the phase transitions associated with those layers. Voltammetry in HTeO2+ shows two clear sub-monolayer reduction features prior to the onset of bulk Te deposition. In situ electrochemical scanning tunneling microscopy (STM) has shown that a 0.25 coverage (2×2) structure is the first Te adlayer formed by scanning into the first reduction feature. The (2×2) unit cell was also observed using low energy electron diffraction (LEED). As the potential was shifted through the first deposition feature, the (2×2) converted to a 0.33 coverage (2× 10 ) structure, by the random introduction of Te atom chains in the initial (2×2) layer. Te–Te distances in the chains were consistent with Te atoms at their van der Waals diameter, 0.44 nm. The chains can be thought of as boundaries between domains of the (2×2), shifted by 2 from each other. The (2× 10 ) structure consists of a complete layer of these chains. Further deposition, past the first reduction feature, resulted in a 0.375 coverage (2×4) structure. The (2×4) also appears to consist of chains of Te atoms, but with every third atom set in a lower site where they are not easily imaged under some tunneling conditions. Scanning into the second reduction feature, a shoulder on bulk Te deposition, atoms in the (2×4) structure became compressed, converting to a structure with a coverage of 0.67 and a ( 2 × 5 ) unit cell. At this stage, the Te atoms were no longer separated by a distance consistent with their van der Waals diameter, but with one closer to that of the underlying Au atoms, 0.288 nm. The Te atoms appear to have covalently bonded to each other, simultaneously inducing a roughening transition in the surface. The transition may result from stress induced via bonding between the Au substrate and this covalently bound Te atomic layer. All the structures described above are single atomic layers. Three-dimensional nucleation progressed at potentials associated with the bulk Te deposition feature. Needles, or rectangular Te atom domains with high aspect ratios, propagating in the 〈100〉, were formed in a second layer, apparently on domains of the ( 2 × 5 ) structure.

Adsorption and dimer exchange processes in surfactant-mediated epitaxial growth of GaAs/InAs

We investigate, using first-principles total energy calculations, the initial steps of adsorption and dimer-exchange mechanisms in the surfactant-mediated epitaxial growth of InAs over GaAs(001) surface with Te as a surfactant. We find that on the Ga-terminated GaAs(001) surface, covered by Te atoms, the dimer exchange between the layer of Te atoms and an overlayer of As atoms, based on a first neighbour exchange process, is only partial, in agreement with recent experimental results. On the other hand, if the exchange process between As and Te atoms occurs after the deposition of In and As atoms, in a second neighbour exchange process, our results indicate a complete segregation of Te atoms, reducing the surface total energy. The Te atoms also prevent the adsorption of As 2 molecule on neighbour sites, avoiding the formation of As clusters.

A theoretical study of the stability and electronic properties of a GaAs/Te/InAs interface

We report first-principles studies of the stability and electronic properties of a layer of Te atoms at an InAs/GaAs(001) interface. The calculations are performed within the LDA approximation. Our results indicate that the formation of half a monolayer of Te atoms is energetically unstable against phase segregations, indicating that the Te atoms do not reduce the GaAs/InAs interface energy. However, a GaAs/Te(0.5)/InAs interface can be formed on the basis of a kinetic process. The Te atoms occupying As sites have strongly covalent character, and give rise to a half-filled band at 0.15 eV below the conduction band. The calculated valence-band offset, with the inclusion of half a monolayer of Te atoms, presents a good agreement with experimental results.

Thin Layer Electrochemical Studies of ZnS, ZnSe, and ZnTe Formation by Electrochemical Atomic Layer Epitaxy (ECALE)

ABSTRACTThin-layer electrochemical studies of the underpotential deposition (UPD) of Zn, Te, Se, and S on polycrystalline Au substrates have been performed. These studies were initiated to investigate the electrodeposition of ZnTe, ZnSe, and ZnS by electrochemical ALE (ECALE). Zn UPD on Au begins at -0.5 V and results in a coverage of 0.47 monolayer (ML). Te and Se atomic layers were formed using a two step process where bulk chalcogenide was removed by reduction, leaving the atomic layer. The reduction of the last atomic layer of Te or Se was not observed, regardless of how negative the potential was scanned. Sulfur atomic layers were spontaneously deposited below -0.6 V from a sulfide solution. Thermodynamic effects are clearly evident during the first monolayer of deposition. Zinc deposition onto Te, Se, and S coated electrodes occurs at progressively more positive potentials as the stability of the zinc compounds increase.This initial information was used to develop ECALE cycles for the compounds, and thin-films were formed by repeated application of the cycles. The dependence of the deposit coverage on the deposition potentials was examined and found to display the characteristic “S” curve of a surface limited process. In addition, the dependence of the coverage on the number of ECALE cycles performed was found to be near the ideal 0.5 ML per cycle for ZnSe and ZnS. The ZnTe coverage per cycle was less than expected indicating that further optimization of deposition conditions is needed.

Electrochemical Atomic Layer Processing

Abstract Atomic layer processing with electrochemical control is discussed. A method for the electrodeposition of compound semiconductors based on the principles of atomic layer epitaxy (ALE) is reported, with specific reference to the formation of ZnTe. This method is referred to as electrochemical atomic layer epitaxy (ECALE). A number of II-VI compounds have been formed using this method, including: CdTe, CdSe, CdS, ZnTe, ZnSe, ZnS and HgSe. Initial studies of GaAs and PbSe have also been pursued. A computer-controlled electrochemical flow deposition system is described. The system has been constructed to form thin-films of the compounds listed above using the ECALE methodology. In addition, an analogous digital electrochemical etching procedure has been developed, and used to etch CdTe substrates. The etching cycle consists of oxidizing off the top atomic layer of Cd atoms at a relatively positive potential, followed by reducing off the top layer of Te atoms at a relatively negative potential. Atomic Force Microscopy (AFM) has been used to image the resulting features. ECALE and the digital electrochemical etching process are both based on selecting potentials where an atomic layer of an element is deposited, or removed, in a surface limited reaction. The potentials used are referred to as underpotentials in the electrochemical literature. The atomic layer deposition process is referred to as underpotential deposition (UPD).

Studies of the surface structures formed by the alternated electrodeposition of Cd and Te on the low-index planes of Au: II. STM studies

Scanning tunneling microscopy (STM) studies of the alternated deposition of Te and Cd atomic layers on the low-index planes of Au are presented here. These investigations were designed to gain an understanding of the surface chemistry involved in the deposition of CdTe by electrochemical atomic layer epitaxy (ECALE). ECALE is the electrochemical analog of atomic layer epitaxy (ALE). In the present study, the structures formed by the alternated electrodeposition of Te and Cd atomic layers, at underpotential, were examined by STM on the low-index planes of Au. Deposition of atomic layers of each element were achieved by underpotential deposition (UPD). Studies of the UPD of Cd by STM were not performed due to Cd's instability in aqueous solutions after withdrawal from solution and loss of potential control. Studies of Te UPD were performed, however, since Te is stable in aqueous media without potential control. Ordered Te structures were observed using STM on all three Au low-index planes. Atomic resolution imaging of Cd UPD on Te atomic layers proved more difficult than imaging simple Te deposits, however, ordered CdTe structures were observed on each Au plane. Higher bias voltages were generally required to image the CdTe deposits. On Au(100) the CdTe deposit appears to adopt a structure equivalent to a (100) slice of bulk CdTe (zinc blende), but contracted 10% due to the lattice missmatch on the Au surfaces. Similar structures were observed on the other two low index planes, as well; although Cd-Cd distances were closer to those found in bulk CdTe.

Reconstructions and Dynamic Processes on CdTe Surfaces Studied by UHV High-Resolution Electron Microscopy

Surface profile imaging at resolutions of better than 2Å is highly suitable for studies of surface structures and reactions. In the case of semiconductor materials, the main challenge is to prepare surfaces free of any contamination. The technique has previously been used to study surface reconstructions of Si and CdTe. In our previous observations, clean surfaces of CdTe were obtained by careful control of the incident electron beam within a JEM-4000EX high resolution electron microscope with a pressure of 10-7 torr. In the present study, observations of reconstructions and dynamic phenomena on CdTe surfaces were carried out with a Phillips-430ST, modified for Ultra-High Vacuum in the vicinity of the specimen and equipped with an in situ heating facility. The base vacuum in the region of the sample could usually reach ∼3×l0-9 torr after baking the microscope column at ∼120°C for 36 hours. The CdTe specimen was prepared by cutting a large single crystal into 3mm discs in a [110] direction, then mechanically polishing to a thickness of ∼20 microns, and finally ion milling to perforation.When viewed along a [110] projection, the CdTe sample was found to be dominated by clean or nearly clean (111) and (110) surfaces(with amorphous materials less than 5Å) whilst the (001) surface was usually very short and rough. A completely clean surface was obtained by in situ annealing of the crystal to about 200°. The (110) surface was then found to be reconstructed with a very characteristic chevron appearance in the manner described previously. Long and flat CdTe(OOl) surfaces were obtained by insitu annealing of the crystal at ∼510°C at which temperature edges of the crystal started to gradually sublime. Characterization of the surface structure was then possible when the crystal was cooled back down to temperatures below about 300°C. It was found that the (001) surface had a (2×1) reconstruction at temperatures below about 200°C which transformed reversibly into a (3×1) reconstruction over the approximate temperature range of 200°C<T<300°C. Figures la and lb show the (2×1) and (3×1) reconstructed (001) surfaces, viewed along the [110] projection, which were recorded at temperatures of 140°C and 240°C respectively. Structural models for the (2×1) and (3×1) reconstructions, obtained directly on the basis of the experimental images, are shown in Figs.2a and 2b respectively. The (2×1) reconstruction involves a 1/2 monolayer of Cd vacancies and a very large inward contraction of the remaining Cd surface atoms, which then displace the second layer of Te atoms as indicated. This model is similar to that proposed by Chadi for the Ga-rich (2×1) reconstructed GaAs(100) surface. The (3×1) reconstruction involves both the formation of surface dimers and the presence of vacancies at the surface. Every third atomic-pair is missing along the [1,-1,0] direction, and the remaining two atom pairs at the surface form the surface dimer. Although the (3×1) reconstruction has a larger number of electrons in dangling bonds, a surface with vacancies can be relaxed to reduce the strain energy due to the surface dimers. The directions of the atomic displacements away from the ideal dimer positions are indicated in the figure. Relatively large atomic displacements for several layers into the bulk are clearly visible in experimental images, as seen in Fig.lb. Further details of the surface reconstructions can be found elsewhere.