Brian W. Gregory

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.

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.

Formation of compound semiconductors by electrochemical atomic layer epitaxy

Abstract : A method for the electrochemical formation of epitaxial deposits of compound semiconductors is being developed. It is referred to as Electrochemical Atomic Layer Epitaxy (ECALE). The method is the electrochemical analog of Atomic Layer Epitaxy (ALE), where ALE is a method used to form compounds by alternately depositing atomic layers of the constituent elements. Atomic layers are formed in ECALE by using Underpotential Deposition (UPD). UPD is a phenomena where an atomic layer of an element deposits at a potential prior to that needed to deposit the bulk element, due to the increased stability afforded by reaction with a second element present at the substrate surface. This paper describes the structure of the first monolayer of Te formed on a Au(100) surface and the structure of a monolayer of CdTe, subsequently formed by deposition of an atomic layer of Cd. Deposits have been formed and analyzed in a UHV surface analysis instrument directly coupled to an electrochemical cell. LEED and Auger electron spectroscopy have been used to follow the structures and compositions of deposits after various steps in the ECALE cycle. As well, some initial studies of the atomic arrangements have been performed using scanning tunneling microscopy.

Metastability in Monolayer Films Transferred onto Solid Substrates by the Langmuir-Blodgett Method: IR Evidence for Transfer-Induced Phase Transitions

The rate at which a monomolecular film is deposited onto a solid substrate in the Langmuir-Blodgett process of preparing supported monolayer films influences the final structure of the transferred film. Attenuated total reflectance infrared spectroscopic studies of monolayers transferred to germanium substrates show that the speed at which the substrate is drawn through the air/water interface influences the final conformation in the hydrocarbon chains of amphiphilic film molecules. This transfer-induced effect is especially evident when the monolayer is transferred from the expanded region of surface-pressure-molecular-area isotherms at low surface pressures; the effect is minimized when the film molecules are transferred from condensed phases at high surface pressures. This phenomenon has been observed for both a fatty acid and a phospholipid, which suggests that these conformational changes may occur in a variety of hydrocarbon amphiphiles transferred from the air/water interface. This conformational ordering may be due to a kinetically limited phase transition taking place in the meniscus formed between the solid substrate and aqueous subphase. In addition, the results obtained for both the phospholipid and fatty acid suggest that the structure of the amphiphile may help determine the extent and nature of the transfer-speed-induced structural changes taking place in the monomolecular film.

Anodic passivation of tin by alkanethiol self-assembled monolayers examined by cyclic voltammetry and coulometry.

The self-assembly of medium chain length alkanethiol monolayers on polycrystalline Sn electrodes has been investigated by cyclic voltammetry and coulometry. These studies have been performed in order to ascertain the conditions under which their oxidative deposition can be achieved directly on the oxide-free Sn surface, and the extent to which these electrochemically prepared self-assembled monolayers (SAMs) act as barriers to surface oxide growth. This work has shown that the potentials for their oxidative deposition are more cathodic (by 100-200 mV) than those for Sn surface oxidation and that the passivating abilities of these SAMs improve with increasing film thickness (or chain length). Oxidative desorption potentials for these films have been observed to shift more positively, and in a highly linear fashion, with increasing film thickness (~75 mV/CH2). Although reductive desorption potentials for the SAMs are in close proximity to those for reduction of the surface oxide (SnOx), little or no SnOx formation occurs unless the potential is made sufficiently anodic that the monolayers start to be removed oxidatively. Our coulometric data indicate that the charge involved with alkanethiol reductive desorption or oxidative deposition is consistent with the formation of a close-packed monolayer, given uncertainties attributable to surface roughness and heterogeneity phenomena. These experiments also reveal that the quantity of charge passed during oxidative desorption is significantly larger than what would be predicted for simple alkylsulfinate or alkylsulfonate formation, suggesting that oxidative removal involves a more complex oxidation mechanism. Analogous chronocoulometric experiments for short-chain alkanethiols on polycrystalline Au electrodes have evidenced similar oxidative charge densities. This implies that the mechanism for oxidative desorption on both surfaces may be very similar, despite the significant differences in the inherent dissolution characteristics of the two materials at the anodic potentials employed.

Formation of CdTe and GaAs by Electrochemical Atomic Layer Epitaxy (ECALE)

The principles of Atomic Layer Epitaxy (ALE) have been applied to the formation of compound semiconductors by an electrochemical technique, referred to as Electrochemical Atomic Layer Epitaxy (ECALE). Atomic layers of the component elements are alternately electrodeposited at underpotential (UPD) from separate solutions and at separate potentials. Results are presented concerning the structures of both CdTe and GaAs deposits formed by ECALE. Studies were performed using singlecrystalline Au electrodes in a UHV surface analysis instrument coupled directly with an electrochemical cell. This instrument was used in order to prevent corruption by contact with air during transfer to the surface analysis environment.

Preparation of atomically smooth Ge substrates for combined IR spectroscopy and scanning probe microscopy of organic monolayers

We have investigated whether large ATR crystals of Ge can be made atomically flat and resistant to atmospheric oxidation, thus enabling us to use them simultaneously as an IR- transparent ATR waveguide and a scanning probe conducting substrate. A protocol has been developed for producing atomically flat Ge substrates resistant to oxidation; this method involves UHV annealing and the electrochemical deposition of a well-ordered, passivating atomic layer of tellurium (Te) onto the Ge surface in order to eliminate further reoxidation of the clean, ordered, Ge substrate. We have obtained STM images in air of the Ge:Te surface and found no sign of disordered oxide formation. IR spectroscopy of monolayer films transferred onto the Sb-doped Ge crystal show little difference with spectra obtained from normal, monocrystalline Ge ATR crystals.

Structural characterization and nanometer-scale domain formation in phospholipid model membranes by infrared spectroscopy and scanning tunneling microscopy

Research in this laboratory is presently being conducted towards the structural characterization of well-ordered molecular monolayers using an integrated spectroscopic/imaging approach. We are currently using infrared spectroscopy (IR) in combination with scanning tunneling microscopy (STM) on both Langmuir-Blodgett (L-B) and self-assembled (SA) monolayer films in order to obtain compositional, conformational, and topographic information about these films. We have constructed organized lipid bilayer assemblies using a combination of SA and L-B methods which were studied using this combined IR/STM approach. These studies showed acyl chain tilt angles of 5 - 10 degree(s) within the DPPG monolayer by IR and STM. In addition, nanometer-size DPPG with hexagonal-type grain boundaries, and molecular resolution of phosphoglycerol headgroup structure within these domains, have been observed by STM. Phospholipid headgroup areas are in excellent agreement with previous x-ray diffraction studies of this system.