Ryan Matthew France

National Renewable Energy Laboratory | NREL · National Center for Photovoltaics

We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcel...

Nonradiative recombination in inverted GaInP junctions is dramatically reduced using a rear-heterojunction design rather than the more traditional thin-emitter homojunction design. When this GaInP junction design is included in inverted multijunction solar cells, the high radiative efficiency translates into both higher subcell voltage and high lum...

It has been theoretically predicted that isolated Bi forms a resonant state in the valence band of the dilute bismide alloy, GaAs1-xBix. We present ultrafast pump-probe reflectivity measurements of this interesting alloy system, which provide experimental evidence for the resonant state. The reflectivity transients for pump/probe wavelengths lambda...

Lattice-matched and pseudomorphic tunnel junctions have been developed in the past for application in a variety of semiconductor devices, including heterojunction bipolar transistors, vertical cavity surface-emitting lasers, and multijunction solar cells. However, metamorphic tunnel junctions have received little attention. In 4-junction Ga0.51In0....

The metamorphic growth of lattice-mismatched materials has allowed optimizing the bandgap combination in multijunction solar cells for the solar spectrum under consideration. Buffer structures are used to accommodate the lattice-mismatch by introducing dislocations and relaxing the material in a controlled way. However, the metamorphic buffers typi...

We have used femtosecond laser pulses to generate coherent acoustic phonons in the dilute Bismide alloy, GaAs1−xBix. The observed oscillation periods match well with the oscillation periods calculated using the propagating strain pulse model. We attribute the generation process predominantly to electronic stress due to the absorption of the laser p...

The optical and electrical properties of many III-V alloys change with the degree of CuPt atomic ordering, which is very sensitive to growth conditions. The bulk ordered alloy is elongated along the normal to the ordered planes, and is asymmetrically strained when coherent to a cubic substrate. Here, we demonstrate in situ measurement of the anisot...

A novel bonding approach with an interface consisting of a metal and dielectric is developed, and a “pillar-array” metal topology is proposed for minimal optical and electrical loss at the interface. This enables a fully lattice-matched two-terminal, four-junction device that consists of an inverted top two-junction (2J) cell with 1.85 eV GaInP/1.4...

Ordering-induced effects on dislocations in metallic alloys have been extensively studied due to their importance in technology applications. We demonstrate that dislocation behavior in ordered III-V semiconductor alloys can be drastically different. This is because ordering in bulk metallic alloys is generally stable, whereas the surface-stabilize...

The three-junction 1.8/1.4/1.0-eV inverted metamorphic multijunction solar cell can be extended to four junctions by adding another lattice-mismatched GaInAs junction with a bandgap of 0.7 eV. However, this requires a significant amount of mismatch to GaAs substrates, i.e., 3.8%, which is difficult to obtain while maintaining high-quality material....

The occurrence of single variant CuPtB ordering during growth of InGaP graded buffer layer structures on offcut (001) GaAs substrates for inverted metamorphic solar cells is found to have a strong influence on strain relaxation mechanisms. Since the surface-induced CuPtB ordering is metastable in the bulk of the material, a strong preference is obs...

Metal-interconnected multijunction solar cells offer one pathway toward efficiencies in excess of 50%. However, if a three- or four-terminal configuration is used, optical losses from the interfacial grid can be considerable. Here, we examine an alternative that provides an optimal interconnection for two-terminal bonded devices. This “pillar-array...

A unique aspect of the inverted metamorphic multijunction (IMM) solar cell is the bandgap tunability of each junction, creating extremely flexible device designs. The optimal structure has subcell photocurrents that are matched for a given spectrum. However, the subcell photocurrents depend on the cell operating temperature, and therefore, the band...

High-quality, direct-bandgap solar cells emit significant luminescence at their band edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected I...

Four different band gap combinations of triple-junction inverted metamorphic solar cells are characterized as a function of temperature and concentration up to 120°C and ~1000 suns. We demonstrate that the standard 1.82/1.40/1.00 eV combination is an excellent choice for typical operating conditions of 1000 suns and 75°C. Improved metal grids and t...

We show a strong relationship between CuPt atomic ordering and misfit dislocation glide plane preference during strain relaxation. A miscut substrate creates an asymmetry in the resolved mismatch stress between {111} glide planes, causing a preference for one glide plane that results in a systematic tilt of the epilayer relative to the substrate. H...

Surface crosshatch roughness typically develops during the growth of lattice-mismatched compositionally graded buffers and can limit misfit dislocation glide. In this study, the crosshatch roughness during growth of a compressive GaInP/GaAs graded buffer is reduced by increasing the phosphine partial pressure throughout the metamorphic growth. Chan...

Atomic force microscopy, wavelength-dispersive x-ray spectroscopy and photoemission electron microscopy were used to study the contact formation of Au/V/Al/V-based contacts on n-type GaN. After a rapid thermal annealing contact formation step, we find that the surface is composed of dendritic structures. The dendrites are Au-rich, while the voids b...

Initial tests are performed regarding the degradation of lattice-mismatched GaInAs solar cells. 1eV metamorphic GaInAs solar cells with 1-2×106 cm-2 threading dislocation density in the active region are irradiated with an 808 nm laser for 2 weeks time under a variety of temperature and illumination conditions. All devices show a small degradation...

The energy and composition of the direct to indirect bandgap crossover in GaxIn1-xP significantly influences its potential for optoelectronic devices, such as solar cells and light emitting diodes, however considerable discrepancies still remain in the literature with regard to the precise value of the crossover composition xc. We revisit this issu...

Several benefits of in situ wafer curvature monitoring on simple structures with low misfit are discussed. The misfit of lattice-mismatched layers is measured during pseudomorphic growth, allowing for experiments that test relationships between misfit and growth conditions. As an example, Bi incorporation in GaAs is quantified by varying the substr...

Transmission electron microscopy studies of GaAs1-xBix layers grown at low temperature by molecular beam epitaxy have revealed evidence of both atomic ordering and phase separation. In layers containing up to ∼10% Bi, the two variants of CuPtB-type atomic ordering on {111}B planes were observed and this is believed to be associated with the surface...

The strain relaxation of GaAsBi is studied in order to determine both the maximum thickness before dislocations form for various misfits and the potential of GaAsBi for usage in the compositionally graded buffer of lattice-mismatched devices. Low-misfit GaAsBi epilayers are grown and compared with GaInAs, a well-studied material currently used in c...

We discuss uniaxial optical anisotropy in single-crystal BiFeO3 determined by spectroscopic ellipsometry from 1.0 to 5.5 eV. The dielectric function ε = ε1 + iε2 and refractive index N = n + ik spectra of BiFeO3 are extracted for the tensor components along its ordinary and extraordinary principal axes. Using the standard line-shape analysis, we al...

ABSTRACT: Transmission electron microscopy studies of GaAs1-xBix layers grown at low temperature by molecular beam epitaxy have revealed evidence of both atomic ordering and phase separation. In layers containing up to ∼10% Bi, the two variants of CuPtB-type atomic ordering on {111}B planes were observed and this is believed to be associated with t...

In this paper, we report the growth and fabrication of non-polar A-plane AlGaN multiple quantum well based ultraviolet light emitting diodes (UV-LEDs). The LEDs were grown on R-plane sapphire substrates using molecular beam epitaxy (MBE). The Current-voltage characteristics of the fabricated devices demonstrated rectifying behavior with a series re...

We have investigated the chemical interaction between a Au V Al V layer structure and n type AlN epilayers using soft x ray photoemission, x ray emission spectroscopy, and atomic force microscopy. To understand the complex processes involved in this multicomponent system, we have studied the interface before and after a rapid thermal annealing step...

AlInP commonly serves as the window layer in high bandgap III-V solar cells where it is responsible for reducing surface recombination by reflecting minority carriers. It must be optically transparent and conductive to majority carriers, and so is typically thin, 25 nm, and doped. It is the semiconductor layer most exposed to the environment during...

A high quality solar cell with a bandgap in the range of 2.0-2.1 eV may enable the development of four- and five-junction solar cells for terrestrial and space applications. In this paper we describe a set of 2.0-2.1 eV nVp solar cells fabricated from Gaxln1-xP and grown on compositional step-grades of GaAs1-yPy, on GaAs substrates. Cells were grow...

Results from our study of carrier mobilities in doped GaAs1-xBix epilayers will be presented and compared with the dilute nitride alloy, GaAsN. We observed no significant degradation in the electron mobility with Bi incorporation in GaAs, up to a concentration of 1.2%. At higher Bi concentration (>=1.6%) some degradation of the electron mobility wa...

A series of GaAs1-xBix (0.00 <= x <= 0.13) alloy thin films has been grown by molecular beam epitaxy on GaAs(001). Structural properties were characterized by high-resolution x-ray diffraction and cross-sectional transmission electron microscopy. Above-bandgap optical properties were determined by spectroscopic ellipsometry at room temperature. In...

A high quality solar cell with a bandgap in the range of 2.0-2.1 eV may enable the development of four- and five-junction solar cells for terrestrial and space applications. In this paper we describe a set of 2.0-2.1 eV nVp solar cells fabricated from Gaxln1-xP and grown on compositional step-grades of GaAs1-yPy, on GaAs substrates. Cells were grow...

We present measurements of the electron Hall mobility in n -type GaAs _1-x Bi _x epilayers. We observed no significant degradation in the electron mobility with Bi incorporation in GaAs, up to a concentration of 1.2%. At higher Bi concentration (≥1.6%) some degradation of the electron mobility was observed, although there is no...

Dilute III–V alloys containing N or Bi share many features that are common, but some that are distinct. In GaP and GaAs, both the substituent species N and Bi behave as isoelectronic impurity traps and both lead to a giant bandgap bowing phenomenon. The isolated N and Bi impurities generate bound states in GaP but resonant states in GaAs. N impurit...

We will present photoluminescence measurements of GaAs(1-x)Bix thin films containing dilute concentration (x

In this work we report a large effect due to relativistic corrections in the electronic structure of very dilute GaAs1−xBix (x < 0.0025) thick epitaxial layers. The variation of the spin–orbit split-off band for x as small as 0.0001 is reported. Very thick (2–3 µm) epilayers were grown by molecular-beam epitaxy to isolate the transitions between th...

We report on photoluminescence measurements of GaAs _(1-x) Bi _x thin films containing dilute concentration (x≤0.045%) of isoelectronic impurity Bi. At a temperature of 4 K, we observed a sharp emission line at ∼1.510 eV and a series of undulations in an energy range of ∼20 meV below it. We attribute the sharp line at ∼1.510 eV...

The interface between n-type GaN and V-based contacts was characterized by soft x-ray spectroscopy. We have investigated the chemical interface structure before and after a rapid thermal annealing (RTA) step, which is crucial for the formation of an Ohmic contact. X-ray photoelectron and x-ray excited Auger electron spectra suggest that RTA induces...

The alloying of GaAs with small amounts of N or Bi results in a large reduction of the fundamental band gap, leading to the so called ``giant band gap bowing''. GaAs1-xNx has been the subject of intense investigation in recent years; however the lower mobility of the dilute nitride alloys limits its use for device applications. Bi incorporation is...

In this work we will present the electronic and optical properties of dilute GaAs1-xBix epitaxial layers for the range of samples with concentration up to ˜3%. Variation of fundamental band gap (E0) and the transition from the spin-orbit split off valance band (E0+delta) using the contactless modulated electroreflectance will be presented as a func...

Self-assembled InGaN quantum dots were grown in the Stranski–Krastanov mode by plasma-assisted molecular beam epitaxy. The average dot height, diameter and density are 3 nm, 30 nm and 7 × 1010 cm–2, respectively. The dot density was found to decrease as the growth temperature increases. The cathodoluminescence emission peak of the InGaN/GaN multipl...

The authors report on the formation and evaluation of V-based Ohmic contacts to n- Al Ga N films in the entire alloy composition. The films were produced by plasma assisted molecular beam epitaxy and doped n -type with Si. The conductivity of the films was determined to vary from 10³ to 10^-2 (Ω cm )^-1 as the AlN mol...

In this paper, we report the development of blue-green-red LEDs based on InGaN quantum dots (QDs) and quantum wells in the active region, and GaN QDs in the nucleation layer for dislocation filtering, by plasma assisted molecular beam epitaxy. Self-assembled InGaN QDs and GaN QDs were grown in the Stranski-Krastanov mode. For the GaN QDs grown at 7...

Much of the work on III-Nitride-based LEDs that has been published and applied commercially has been done using metal-organic chemical vapor deposition (MOCVD) as a method of film growth. We report on the growth and fabrication of visible light emitting diodes, by combining hydride vapor-phase epitaxy (HVPE) and rf plasma-assisted MBE (PAMBE) metho...

Acoustic pressures may generally be measured with much greater sensitivity, dynamic range, and frequency response than acoustic currents. Consequently, most measurements of acoustic impedance consist of comparison with standard impedances. The method reported here uses a semi-infinite waveguide as the reference because its impedance is purely resis...