Uwe Happek

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.

Electrochemical formation of a III–V compound semiconductor superlattice: InAs/InSb

Abstract We report on the use of electrochemical atomic-layer epitaxy (EC-ALE) to grow thin-films of the III–V compounds InAs, InSb, and an InAs x Sb 1− x superlattice. EC-ALE is a method for forming compound semiconductors with improved control, compared to other electrodeposition methodologies. It involves the use of surface limited reactions to form deposits an atomic layer at a time, in a cycle. The EC-ALE cycle uses underpotential deposition (upd) to form atomic layers of each of the component elements. One cycle ideally produces one monolayer (ML) of the desired compound. Studies to optimize the InAs cycle are reported, specifically the dependence on the In and As deposition potentials. These studies show that the potentials must be adjusted for each of the first 25 or more cycles, as a contact potential between the Au substrate and the growing semiconductor develops. After deposition of this initial ‘buffer layer’, steady state conditions are reached, and the same potentials can be used without change, for the remaining cycles. The formation of InSb has also been investigated, and the EC-ALE growth of InSb deposits is reported for the first time. Due to a 6% lattice mismatch, and a less than fully optimized cycle, the InSb deposits on Au appear composed of 70 nm particles. By combining the InAs and InSb programs, a superlattice was formed with 41 periods, where each period involved ten cycles of InAs followed by ten cycles of InSb. X-ray diffraction (XRD) indicated a period of 5.5 nm, whereas a 7.4 nm period was expected, based on 1 ML/cycle and the (111) interplanar spacing, derived from the lattice constants for InAs and InSb. Given the stoichiometry of the resulting deposit, and the shorter periodicity observed, it appears that 1 ML/cycle of InAs was formed, while only a 1/2 ML/cycle of InSb was obtained. IR absorption measurements indicate that the deposit was red shifted relative to the lower bandgap compound, InSb (0.17 eV), which is consistent with a type II superlattice. If an alloy had been formed, the bandgap should have been a linear function of the bandgaps and relative mole fractions of InAs and InSb, or about 0.31 eV, twice the observed bandgap.

Formation of In2Se3 thin films and nanostructures using electrochemical atomic layer epitaxy

Abstract The formation of the III–VI compound In 2 Se 3 , at room temperature by electrochemical atomic layer epitaxy (EC-ALE) is reported here. EC-ALE involves the use of surface limited reactions to form atomic layers of the elements making up a compound (In and Se) in a cycle. In electrodeposition, surface limited reactions are referred to as under potential deposition, and generally result in the formation of an atomic layer of the depositing element. These layers are deposited alternately in a cycle, resulting in the formation of a one monolayer of the compound, In 2 Se 3 . Cyclic voltammograms were used to determine approximate deposition potentials for each element. An automated deposition program was used to form thin films of In 2 Se 3 , with from up to 350 cycles. Electron probe microanalysis was performed to determine the stoichiometry of the thin films. The atomic ratio of Se/In in the thin films was found to be 3/2. X-ray diffraction of 350 cycle films indicated the deposits contained beta phase In 2 Se 3 . Band gaps were determined by FT-IR reflection absorption measurements, and found to be 1.73 eV. The surface morphology was determined by atomic force microscopy (AFM), suggesting that the deposits consist of 100 nm crystallites. Deposits on rougher substrates resulted in still smaller crystallites, and a blue shift in the band gap, possibly due to quantum confinement. Photoelectrochemical measurements suggested a band gap of 1.82 eV. In 2 Se 3 nanostructures were electrodeposited inside the pores (200 nm) of commercial polycarbonate membrane using EC-ALE. AFM images indicated that nanostructures were higher then expected, for 200 cycles of deposition. Studies of the Au vapor-deposited on the membrane showed that it had ingressed into the holes, accounting for most of the extra height. Microprobe data suggested that the total coverage was 1/6th that observed for a thin film, consistent with the observed coverage of nanostructures.

Charge creation, trapping, and long phosphorescence in Sr2MgSi2O7:Eu2+, RE3+

This report describes experiments that elucidate the phosphorescence mechanism in Sr2MgSi2O7:Eu2+, RE3+ phosphors. The first step of phosphorescence, the storage of carriers in traps, is traced to thermally assisted photoionization of Eu2+ ions, supported by thermal quenching studies that give the thermal ionization threshold of Eu2+. The trapping of these delocalized electrons has been detected using the spectroscopic signature of Sm2+ ions created by electron capture by Sm3+ in samples doped with Eu2+ and Sm3+ ions. The final steps for phosphorescence, liberation of trapped carriers and recombination with the luminescence center, have also been studied by monitoring the Eu2+ luminescence induced by optical excitation into the Sm2+ 4f6→4f55d1 absorption band. These results show that the phosphorescence in this system starts with the promotion of electrons into the conduction band and subsequent reduction of RE3+ ions, not from hole formation and RE3+ oxidation, as has been reported for similar systems. F...

Nature of luminescent centers in cerium-activated materials with the CaFe2O4 structure

Abstract The cerium luminescence in SrY 2 O 4 shows two bands emitting in the blue and green regions of the optical spectrum. Based on standard emission and excitation spectroscopy combined with time-resolved emission studies, we can attribute the blue emission to a Ce 3+ 5d–4f transition and tentatively assign the green emission to a “Ce 4+ –O 2− ”charge transfer transition.

Formation of HgSe Thin Films Using Electrochemical Atomic Layer Epitaxy

The growth of HgSe using electrochemical atomic-layer epitaxy (EC-ALE) is reported. EC-ALE is the electrochemical analog of ALE, where electrochemical surface-limited reactions referred to as underpotential deposits, generally result in the formation of an atomic layer of an element, under controlled potential. HgSe thin films were formed on gold substrates using two reactant solutions: a solution of Hg 2 + complexed with ethylenediaminetetraacetic acid and a HSeO - 3 ion solution. X-ray diffraction analysis showed a zinc blende structure for the deposits, with a strong (111) preferred texture, and an average grain size of 425Δ. Electron probe microscope analysis showed near-stoichiometric deposits. Fourier transform infrared spectroscopy reflection absorption measurements suggest two bandgaps: 0.42 and 0.88 eV.

Optimized Phosphors for Warm White LED Light Engines

The objective of this program is to develop phosphor systems and LED light engines that have steady-state LED efficacies (using LEDs with a 60% wall-plug efficiency) of 105–120 lm/W with correlated color temperatures (CCT) ~3000 K, color rendering indices (CRI) >85, <0.003 distance from the blackbody curve (dbb), and <2% loss in phosphor efficiency under high temperature, high humidity conditions. In order to reach these goals, this involves the composition and processing optimization of phosphors previously developed by GE in combination with light engine package modification.

Phosphor Systems for Illumination Quality Solid State Lighting Products

The objective of this program is to develop phosphor systems that will enable LED lamps with 96 lm/W efficacy at correlated color temperatures, (CCTs) ~3000 K, and color rendering indices (CRIs) >80 and 71 lm/W efficacy at CCT<3100 K and CRI~95 using phosphor downconversion of LEDs. This primarily involves the invention and development of new phosphor materials that have improved efficiency and better match the eye sensitivity curves.

MORPHOLOGY IN ELECTROCHEMICAL ATOMIC LAYER EPITAXY

Compound semiconductors are an important group of materials, used in a wide variety of optoelectronic devices, including detectors, displays and photovoltaics. They are generally used in the form of thin films, deposited by molecular beam epitaxy (MBE), chemical vapor deposition (CVD), or one of a variety of low-tech methods, such as chemical bath deposition (CBD) or electrodeposition. Compound electrodeposition has been well reviewed by a number of workers1–5.