Thomas Colpitts

MOCVD of Bi2Te3, Sb2Te3 and their superlattice structures for thin-film thermoelectric applications

The characteristics of metalorganic chemical vapor deposition (MOCVD) of Bi2Te3, Sb2Te3 and their superlattice structures are discussed in this paper. We have grown c-oriented films on both hexagonal sapphire and fcc GaAs substrates, with specular morphology and occasional stacking faults. Single crystallinity was confirmed by X-ray diffraction and low-energy electron diffraction (LEED). The stoichiometry (Bi:Te = 2:3, Sb:Te = 2:3) of the films were confirmed by X-ray photo-emission spectroscopy (XPS) and Rutherford back-scattering. We have also attempted to grow short-period (∼ 10 to 80 A) superlattice structures in the Bi2Te3Sb2Te3 materials system. X-ray diffraction data indicating the quality of these layered structures is presented. The advantages offered by the nature of chemical bonding in these materials, along the growth direction, for obtaining abrupt interfaces is discussed. The electrical transport properties of the MOCVD-grown p-type Bi2Te3Sb2Te3 structures and other thermoelectric properties including thermal conductivity and Seebeck coefficient are discussed. The initial results on the performance parameter known as figure-of-merit of the superlattice structures, measured parallel to the plane of the superlattice interfaces, are significantly higher than in conventional bulk materials. These initial results suggest a significant potential for MOCVD-based materials technology for high-performance, thin-film, thermoelectric refrigeration.

Enhanced thermoelectric performance in PbTe-based superlattice structures from reduction of lattice thermal conductivity

We have fabricated two-dimensional n-type PbTe∕PbTe0.75Se0.25 structures using an evaporation process. In optimized films exhibiting a high-quality superlattice structure, a significant reduction in lattice thermal conductivity has been experimentally measured. The reduction would indicate enhanced thermoelectric device performance compared to standard PbTeSe alloys given that the electrical components, specifically, the Seebeck coefficient and electrical resistivity, were not observed to deteriorate from bulk values. The analysis of these films shows continuous layers with a true two-dimensional superlattice structure, as opposed to the PbTe∕PbSe system that exhibits zero-dimensional structures from self-assembly. The room-temperature measurement of cross-plane figure-of-merit in a n-type PbTe∕PbTe0.75Se0.25 device structure by the transient method has been combined with temperature-dependent measurements of in-plane resistivity and Seebeck coefficient to yield evidence of enhanced thermoelectric perform...

Low-temperature organometallic epitaxy and its application to superlattice structures in thermoelectrics

We describe a simple, yet phenomenologically very different, low-temperature modification to the conventional metal–organic chemical vapor deposition. It has been applied to the epitaxy of hexagonal-phased Bi2Te3/Sb2Te3 superlattices on zinc-blende GaAs substrates. The modification enables a two-dimensional, layer-by-layer, epitaxy instead of a three-dimensional islanded growth. Therefore, this approach is of generic importance to the epitaxy of many electronic and magnetic materials and their superlattices. High-resolution transmission electron microscopy studies indicate that the interface between the GaAs substrate and Bi2Te3 film is qualitatively defect free and that periodic structures are formed in the Bi2Te3/Sb2Te3 superlattices, with one of the individual layers as small as 10 A. Such ultra-short-period superlattices offer significantly higher carrier mobilities than their respective solid-solution alloys, apparently due to the elimination of alloy scattering and the minimal effects of random inte...

An inverted-growth approach to development of an IR-transparent, high-efficiency AlGaAs/GaAs cascade solar cell

An approach for inverted-grown AlGaAs/GaAs cascade cells, where the AlGaAs top cell is grown first at high temperatures, placing the surface to be illuminated nearest to the substrate, is presented. Following the growth of the top cell, the GaAs tunnel interconnect and the bottom cell are grown at lower temperatures. After the inverted growth, the AlGaAs/GaAs cascade structure is selectively removed from the parent substrate, Ge in this case. Advantages of the inverted-growth approach are discussed.<<ETX>>

Measurement of AlGaAs/AlGaAs interface recombination velocities using time‐resolved photoluminescence

Time‐resolved photoluminescence has been used to examine AlxGa1−xAs/AlyGa1−yAs interfaces, focusing on the recombination velocity. For an Al0.08Ga0.92As/Al0.88Ga0.12As interface, important for solar cells, recombination velocities are about 104 cm/s with the growth conditions used in this study. Several types of interface passivation were attempted, but the most successful was the insertion of thin Al0.14Ga0.86As layers between the other two alloys. Using this technique, a 16‐fold increase (to ∼20 ns) of the minority‐carrier lifetime was measured in a 0.8‐μm‐thick Al0.08Ga0.92As layer in which interface recombination would normally have limited the lifetime to about 1–2 ns. Compositional grading was found to be ineffective at passivating the interfaces.

Selective plasma etching of Ge substrates for thin freestanding GaAs-AlGaAs heterostructures

Selective plasma etching of Ge with a CF4/O2 mixture is used to produce freestanding GaAs‐AlGaAs thin films. The etch rate of Ge substrates is as high as 150 μm/h at temperatures of 75 °C under optimized conditions and with a negligible etch rate for GaAs and AlGaAs materials. This plasma etching technique, combined with a lattice‐matched growth of GaAs‐AlGaAs structures on Ge substrates, has a variety of potential device applications. The characteristics of this etch process and the photoluminescence of freestanding AlGaAs‐GaAs structures are presented.

Preparation of clean Bi2Te3 and Sb2Te3 thin films to determine alignment at valence band maxima

The thermoelectric application of Bi2Te3 and Sb2Te3 thin film structures relies on the relative alignment of the valence band maxima for good electrical conduction. In order to determine the valence band maxima of the bulk films, the authors propose a simple repeatable treatment of a chemical etching in dilute hydrochloric acid solution and a subsequent annealing at ∼150 °C under ultrahigh vacuum to prepare clean surfaces of Bi2Te3 and Sb2Te3 thin films. High-resolution photoemission spectroscopy using synchrotron radiation is used to investigate the chemical states of epitaxial Bi2Te3 and Sb2Te3 thin films grown on GaAs by low-temperature metal-organic chemical vapor deposition. The valence band and core-level photoemission spectra indicate that the surface contaminations and oxides were removed. After chemical etching in acid solution, elemental Te was observed on the surface; a follow-up anneal in ultrahigh vacuum creates a stoichiometric oxide-free surface.

GROWTH OF GAINASP USING ETHYLDIMETHYLINDIUM AND TERTIARYBUTYLPHOSPHINE

The growth of GaInAsP lattice matched to GaAs by organometallic vapor phase epitaxy using ethyldimethylindium and tertiarybutylphosphine is reported for the first time. The composition of the films is approximately Ga0.87In0.13As0.75P0.25, giving a band gap of about 1.52 eV. Intrinsic films are n type. Both n‐ and p‐type doped layers have been prepared. Photoluminescence measurements on intrinsic films give spectral half widths of about 70 meV. Capping the films with AlGaAs window layers and using n+‐n, high‐low junctions at the GaInAsP/GaAs interface greatly improve the photoluminescent properties. A minority‐carrier lifetime of 35 ns in an intrinsic film has been measured and suggests that the quaternary is equivalent to AlGaAs with the same band gap.

Advances in the development of an AlGaAs/GaAs cascade solar cell using a patterned germanium tunnel interconnect

Abstract In this paper, we discuss various aspects of the development of an inverted-grown AlGaAs/GaAs cascade solar cell incorporating a patterned germanium tunnel junction. Topics include the development of the Al 0.37 Ga 0.63 As top cell, the growth of the GaAs bottom cell over the patterned germanium tunnel junction, and a technique for selective removal of thin AlGaAs/GaAs heterostructures after lattice-matched growth of germanium substrates. The problems to be overcome for the achievement of around 30% efficiencies in the AlGaAs/GaAs cascade cell under concentrator applications are also discussed.

GaInAsP lattice matched to GaAs for solar cell applications

Initial results in the development of GaInAsP (1.55 eV) solar cells lattice matched to GaAs are presented. The films are grown by organometallic vapor phase epitaxy (OMVPE) on GaAs and on Ge substrates. As-grown films are n-type, with minority carrier lifetimes of up to 35 ns. p-on-p solar cells have been prepared, with AlGaAs window layers. Under AM1.5 direct light (100 mW/cm/sup 2/), a cell has been measured with a V/sub oc/ of 1.08 V, a J/sub sc/ of 20.6 mA/cm/sup 2/, a fill factor of 81.7%, and an active area efficiency of 18.2%.<<ETX>>