John L. Stickney

Electrochemical atomic layer epitaxy (ECALE)

Abstract Electrodeposition holds promise as a low cost, flexible room temperature technique for the production of II-VI compound semiconductors. Previous studies, however, have resulted in the production of polycrystalline deposits in every case. This paper describes a new method, developed in this laboratory, for depositing these materials epitaxially. The method involves the alternate deposition of the component elements a monolayer at a time. To limit deposition to a monolayer, underpotential deposition (UPD) is employed. UPD occurs because of the enhanced stability provided by bond formation between the II and VI elements, relative to formation of bulk elemental deposits. This method is the electrochemical equivalent of atomic layer epitaxy (ALE), and is thus referred to as “electrochemical atomic layer epitaxy” (ECALE). This paper describes the first example of the ECALE method, involving the thin-layer electrodeposition of CdTe on a Au polycrystalline electrode.

Conditions for the Deposition of CdTe by Electrochemical Atomic Layer Epitaxy

In this paper the method of electrochemical atomic layer epitaxy (ECALE) is described. It involves the alternated electrochemical deposition of atomic layers of elements to form compound semiconductors. It is being investigated as a method for forming epitaxial thin films. Presently, it appears that the method is applicable to a wide range of compound semiconductors composed of a metal and one of the following main group elements: S, Se, Te, As, Sb, or Br. Initial studies have involved CdTe deposition. Factors controlling deposit structure and composition are discussed here. Preliminary results which show that ordered electrodeposits of CdTe can be formed by the ECALE method are also presented. Results reported here were obtained with both a polycrystalline Au thin-layer electrochemical cell and a single-crystal Au electrode with faces oriented to the (111), (110), and (100) planes. The single-crystal electrode was contained in a UHV surface analysis instrument with an integral electrochemical cell. Deposits were examined without their exposure to air using LEED and Auger electron spectroscopy. Coverages were determined using coulometry in the thin-layer electrochemical cell.

Formation of Thin Films of CdTe, CdSe, and CdS by Electrochemical Atomic Layer Epitaxy

Thin films of CdTe, CdSe, and CdS have been electrodeposited by electrochemical atomic layer epitaxy (ECALE), using an automated electrochemical deposition system. Previous reports of an automated system for forming ECALE deposits involved use of a small thin layer flow cell, which revealed several drawbacks. Conversion of the thin layer cell to a thick layer design resulted in greatly improved deposit quality and reproducibility. Deposits were analyzed using electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and grazing incident X-ray diffraction (XRD). The results were consistent with a layer by layer growth mode and the principles of atomic layer epitaxy. CdTe films were grown using up to 1000 ECALE cycles, and were stoichiometric through 500. The 1000 cycle films were a few percent rich in Te, under the conditions used. CdSe and CdS films formed also contained some excess chalcogenide, probably the result of less then ideal deposition parameters. Increasing amounts of particulates and surface roughening were observed for the 500 and 1000 cycle CdTe and CdSe films, relative to the 200 cycle deposits normally formed. This roughening may result from the excess chalcogenide. XRD of the films indicated cubic crystal structures with preferred (111) orientations for all three compounds

Superlattices formed by electrodeposition of silver on iodine-pretreated Pt(111); Studies by leed, auger spectroscopy and electrochemistry

Reported are studies by LEED and Auger spectroscopy of silver layers electrodeposited on well-characterized Pt(111) surfaces from aqueous solution. Prior to electrodeposition. the Pt(111) surface was treated with I2 vapor to form the Pt(111) (7 × 7)R19.1°-I superlattice which protected the Pt and Ag surfaces from attack by the electrolyte and residual gases. Electrodeposition of silver occurred in four distinct ranges of electrode potential. Ordered layers having (3 × 3) and (18 × 18) (coincidence lattice) LEED patterns were formed at all coverages from the onset of deposition to the highest coverages studied, about twenty equivalent atomic layers. Formation of ordered Ag layers has therefore been demonstrated, at least for deposits of limited thickness. Auger spectra revealed that for deposits of a few atomic layers. The iodine layer remained attached to the surface during multiple cycles of electrodeposition and dissolution of silver from iodine-free solution. Each peak of the voltammetric current-potential scan produced a change in the LEED pattern.

Electrodeposition on a well-defined surface: Silver on Pt(111)(√7×√7)R19.1°−I

Abstract An exploration is reported of the structures formed by electrodeposition of silver on well-defined Pt(111) surfaces in various amounts up to a few monolayers. Prior to electrodeposition, the Pt(111) surface was treated with I2 vapor to form a Pt(111)(√7×√7)R19.1°−I superlattice which effectively protected the Pt and Ag surfaces from attack by the aqueous HClO4 electrolyte and residual gases. Silver electrodeposited in three widely separated underpotential deposition stages, forming distinct lattice structures having (3×3) or (√3×√3)R30° LEED patterns at all coverages studied. Formation of ordered Ag layers has therefore been demonstrated. Measurements of Auger electron spectroscopic current for Pt, Ag and I revealed that the silver was located underneath the iodine atomic layer, which remained attached during multiple cycles of electrodeposition and dissolution of silver from iodine-free solutions.

Preliminary Studies of GaAs Deposition on Au(100), (110), and (111) Surfaces by Electrochemical Atomic Layer Epitaxy

The development of a new method for epitaxial growth of compound semiconductors is briefly described: electrochemical atomic layer epitaxy (ECALE). ECALE is based on the successive underpotential deposition (UPD) of atomic layers of different elements to form a compound. Preliminary studies of the ECALE deposition of GaAs were performed in a thin-layer electrochemical cell with a polycrystalline Au electrode. Potentials required for oxidative UPD of As and reductive UPD of Ga were evaluated with different solution concentrations and pHs

Preparation of well-defined surfaces at atmospheric pressure: Studies of structural transformations of I, Ag-adlattices on Pt(111) by LEED and electrochemistry

Pt(111) surfaces disordered by ion-bombardment or electrochemical oxidation were converted to well-defined, ordered states by annealing in iodine vapor at atmospheric pressure. A structure not obtainable in vacuum was formed, Pt(111)(33 × 93)R30°-I, containing 0.62 I atoms per surfa ce Pt atom in a slightly distorted hexagonal array. The I-I interatomic distances in this structure, 0.33 and 0.36 nm, were less than the Van der Waals distance, 0.43 nm. Gentle heating of this structure under pure Ar yielded I2 molecules, I atoms and a series of structures: Pt(111)(33 × 9 3)R30°-I(3 × 3)R30°-IPt(111) (clean surface). The Pt(111)(7 × 7 )R19.1°-I adlattice proved to be identifiable from its distinctive electrochemical behavior in electrodeposition of Ag from aqueous solutions of AgClO4, which consists of three prominent structural transitions. Kinematic calculations of the directions and qualitative intensities of the LEED beams at selected kinetic energies has led to proposed structures consisting of Ag atoms close-packed in registry with the three-fold sites of Pt but with I atoms substituted for Ag atoms at the (3 × 3)R30° positions. Phase boundaries caused by reversals of the two packing sites of the 3 unit mesh at intervals 17 Pt unit vectors divide the surface into hexagonal antiphase domains.

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.

Se adlattices formed on Au(100), studies by LEED, AES, STM and electrochemistry

Abstract Ordered selenium atomic layers have been formed electrochemically on Au(100) at a series of coverages. Cyclic voltammetry and coulometry were used to study the deposition process, and to determine the corresponding coverages of a number of Se structures. Structures, with Se coverages of 0.25, 0.33, 0.5, and 0.89 monolayers, were identified using ultra high vacuum — electrochemical techniques as well as scanning tunneling microscopy. The corresponding unit cells of those structures were: p(2 × 2), (2 × √10), c(2 × 2), and a mostly (3 × √10), composed of close-packed Se 8 rings. Pit formation, associated with the formation of the densely packed Se 8 ring structure, was observed. They are reminiscent of pits observed in self-assembled monolayers of alkane thiols on Au surfaces. The pits disappeared as the structure, composed of Se rings, was converted to lower coverage structures, such as the 0.25 monolayer p(2 × 2), via anodic stripping. Se atomic layers were formed electrochemically in three ways: direct reduction from a HSeO − 3 solution; anodic stripping of previously formed bulk Se; or cathodic stripping of previously formed bulk Se. All three methods resulted in equivalent atomic layer structures on the Au(100) surface, but with some variation in the homogeneity and distribution of particular structures.

Thin-layer electrochemical studies of the underpotential deposition of cadmium and tellurium on polycrystalline Au, Pt and Cu electrodes

Abstract Thin-layer electrochemical studies of the underpotential deposition (UPD) of Cd and Te on polycrystalline Au, Pt, and Cu substrates have been performed. These studies were done in order to investigate the initial stages of the electrodeposition of CdTe. Tellurium deposition on Cu electrodes is very rapid at potentials between hydrogen evolution and Cu dissolution; as a result, the amount of electrodeposited Te cannot be suitably controlled with potential. Subsequent removal of Te on Cu also proved difficult by standard electrochemical cleaning procedures. Tellurium UPD and stripping on Pt occurred simultaneously with Pt oxide reduction and Pt oxidation, respectively. In addition, cadmium UPD on Pt is ill-defined, resulting in three peaks overlapping with the hydrogen wave voltammetry. It is presumed that the hydrogen waves are suppressed by the deposited Cd, although this is not definite. The most informative results were obtained with Au because of its broad double-layer window, which is free of complications from surface-specific faradaic reactions. Deposition of Te from TeO 2 solutions on Au was shown to require 3.9 ± 0.1 electrons per deposited Te. A substantial loss of electrodeposited Te was observed when the potential was cycled into the oxidation region on both the Au and Pt electrodes. Quantitation of the charge on Au indicated this loss was the result of oxidation of the Te(IV) to a Te(VI) compound which is difficult to reduce back to Te(IV) prior to hydrogen evolution on Au or Pt. At potentials between Au oxidation and Te UPD, the soluble Te species, HTeO + 2 , was shown to adsorb reversibly on the Au surface. In studies of the alternated deposition of Cd and Te, initially deposited Cd is displaced by Te at potentials positive of −0.35 V. Subsequent UPD of Cd on the Te-covered Au resulted in one Cd atom for each deposited Te.