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NASA NTRS1985

Vacuum MOCVD fabrication of high efficience cells

Vacuum metal-organic-chemical-vapor-deposition (MOCVD) is a new fabrication process with improved safety and easier scalability due to its metal rather than glass construction and its uniform multiport gas injection system. It uses source materials more efficiently than other methods because the vacuum molecular flow conditions allow the high sticking coefficient reactants to reach the substrates as undeflected molecular beams and the hot chamber walls cause the low sticking coefficient reactants to bounce off the walls and interact with the substrates many times. This high source utilization reduces the materials costs power device and substantially decreases the amounts of toxic materials that must be handled as process effluents. The molecular beams allow precise growth control. With improved source purifications, vacuum MOCVD has provided p GaAs layers with 10-micron minority carrier diffusion lengths and GaAs and GaAsSb solar cells with 20% AMO efficiencies at 59X and 99X sunlight concentration ratios. Mechanical stacking has been identified as the quickest, most direct and logical path to stacked multiple-junction solar cells that perform better than the best single-junction devices. The mechanical stack is configured for immediate use in solar arrays and allows interconnections that improve the system end-of-life performance in space.

Partain, L. D.↗

NASA NTRS1987

P/N InP homojunction solar cells by LPE and MOCVD techniques

P/N InP homojunction solar cells have been prepared by using both liquid phase epitaxy (LPE) and metallorganic chemical vapor deposition (MOCVD) growth techniques. A heavily doped p-In sub 0.53Ga sub 0.47As contacting layer was incorporated into the cell structure to improve the fill factor and to eliminate surface spiking at the front surface. The best conversion efficiencies (total area) obtained under AM 1 illumination are 14.2 percent for a LPE cell and 15.4 percent for a MOCVD cell.

Choi, K. Y.↗

NASA NTRS1990

Photoreflectance for in-situ characterization of MOCVD growth of semiconductors under micro-gravity conditions

A contactless electromodulation technique of photoreflectance (PR) was developed for in-situ monitoring of metal-organic chemical vapor deposition (MOCVD) semiconductor growth for micro-gravity applications. PR can be employed in a real MOCVD reactor including rotating substrate (approximately 500 rev/min) in flowing gases and through a diffuser plate. Measurements on GaAs and Ga(0.82)Al(0.18)As were made up to 690 C. The direct band gaps of In(x)Ga(1-x)As (x = 0.07 and 0.16) were evaluated up to 600 C. In order to address the question of real time measurement, the spectra of the direct gap of GaAs at 650 C was obtained in 30 seconds and 15 seconds seems feasible.

Pollak, Fred H.↗

NASA NTRS1989

Effect of crystal orientation on anisotropic etching and MOCVD growth of grooves on GaAs

Grooves can be formed on GaAs by wet-chemical anisotropic etching of surfaces masked by photoresist stripes. The effect of crystal orientation on the shape of the grooves etched and on subsequent epitaxial growth by MOCVD is presented. The polar lattice increases the complexity of the etching and growth processes. The slow-etch planes defined by anisotropic etching are not always the same as the growth facets produced during MOCVD deposition, especially for deposition on higher order planes.

Bailey, Sheila G.↗

NASA NTRS1991

Temperature coefficients and radiation induced DLTS spectra of MOCVD grown n(+)p InP solar cells

The effects of temperature and radiation on n(+)p InP solar cells and mesa diodes grown by metallorganic chemical vapor deposition (MOCVD) were studied. It was shown that MOCVD is capable of consistently producing good quality InP solar cells with Eff greater than 19 percent which display excellent radiation resistance due to minority carrier injection and thermal annealing. It was also shown that universal predictions of InP device performance based on measurements of a small group of test samples can be expected to be quite accurate, and that the degradation of an InP device due to any incident particle spectrum should be predictable from a measurement following a single low energy proton irradiation.

Walters, Robert J.↗

NASA NTRS1999

Influence of Natural Convection and Thermal Radiation Multi-Component Transport in MOCVD Reactors

The influence of Grashof and Reynolds number in Metal Organic Chemical Vapor (MOCVD) reactors is being investigated under a combined empirical/numerical study. As part of that research, the deposition of Indium Phosphide in an MOCVD reactor is modeled using the computational code CFD-ACE. The model includes the effects of convection, conduction, and radiation as well as multi-component diffusion and multi-step surface/gas phase chemistry. The results of the prediction are compared with experimental data for a commercial reactor and analyzed with respect to the model accuracy.

Lowry, S.↗

DOE OSTI2021

Pinning Characteristics of Zr and Hf- Added REBCO Coated Conductors Made by Advanced MOCVD in Low-to-High Magnetic Fields

BaMO 3 (M=Zr, Hf) pinning centers introduced in REBa 2 Cu 3 O 7-x (REBCO and RE = rare earth) coated conductors yield superior performance in high magnetic fields. We present the critical current density J c over a temperature range of 4.2-77 K and magnetic fields of 0-14 T (B || c-axis), of 5-15 mol.% Zr- and Hf-added REBCO 4+ μm thick film tapes fabricated by advanced metal organic chemical vapor deposition (A-MOCVD). The morphology of self-assembled BMO nanorods aligned along c-axis is found to be dependent on the Zr/Hf content. We also observe a correlation between the density of RE 2 O 3 in-plane nano-precipitates and the continuity and concentration of BMO nanorods in films of different Ba content. It is found that the (Ba+M)/Cu (M=Zr, Hf) content in REBa 2 Cu 3 O 7-x affects the shape of pinning force density curves over a wide magnetic field range and temperatures below 20 K. Another remarkable observation is the similarity in the critical current properties of Hf and Zr -added REBCO films as function of (Ba+M)/Cu content at intermediate to high range of (Ba+M)/Cu content. As a result, a quantitative analysis of pinning efficiency and correlation with the microstructure of Zr and Hf- added REBCO coated conductors is discussed.

2G high-temperature superconductor (2G-HTS)↗

DOE OSTI2021

Correlation of Early-Stage Growth Process Conditions with Dislocation Evolution in MOCVD-Based GaP/Si Heteroepitaxy

To identify the complex relationships between early-stage growth processes and the resultant defect microstructure in GaP/Si heteroepitaxy, a holistic study of several key metal-organic chemical vapor deposition (MOCVD) parameters was conducted, focusing on Si surface preparation and GaP atomic layer epitaxy (ALE) based nucleation processes. Here, crystalline defects related to the lattice mismatch and/or interfacial heterovalency, namely misfit dislocations (MD), threading dislocations (TD), and stacking fault pyramids (SFP), were quantitatively characterized via electron channeling contrast imaging (ECCI) and correlated against the different process variations. Choice of Si surface preparation method between the two examined (dilute SiH4 annealing versus Si2H6 based homoepitaxy) had little impact on resultant GaP film morphology and defect content, whereas differing GaP ALE nucleation conditions produced much more substantial changes. In particular, the initial precursor species (tert-butylphosphine versus triethygallium) and ALE cycle purge times both yielded significant influence over threading dislocation densities (TDD) in thin (100 nm), post-critical thickness GaP/Si films, with TDD spanning two orders of magnitude, from 6.7×10 7 cm -2 to 7.1×10 5 cm -2 , depending on the specific process conditions employed. SFP densities were also found to follow a similar trend, ranging from 2.0×10 7 cm -2 to 1.8×10 5 cm -2 , but with no apparent causal relationship between SFP density and TDD. To help explain the dramatic differences observed, detailed, large-area MD network characterization was used to provide statistically-relevant quantitative analyses of the critical dislocation dynamics (introduction rates and glide velocities) associated with the different process variants. These extracted values are then correlated against the ALE process variants to provide insight into the potential mechanistic roles of the different growth processes.

36 MATERIALS SCIENCE↗

DOE OSTI2023

High-current, double-sided REBCO tapes by advanced MOCVD

Abstract An approach to fabricate double-sided, high-current RE-Ba 2 Cu 3 O 7− δ (REBCO) tapes for use in ultrahigh-field superconducting magnets operating at and above 20 T is demonstrated. In this work, we modified our advanced metal organic chemical vapor deposition (A-MOCVD) reactor to deposit REBCO films on each side of a double-sided buffer substrate in a single pass. The composition, thickness, and critical current ( I c ) of 1 µ m-thick REBCO films on each side of the double-sided tapes are compared. Double-sided REBCO tape with 2 µ m-thick films on each side showed self-field critical current density ( J c ) over 2.6 MA cm −2 at 77 K and 0 T. 15%-Zr-added double-sided REBCO tape showed I c over 530 A/4 mm at 20 K, 20 T.

Paidpilli, M.↗

DOE OSTI2020

Quantitative defect analysis in MOCVD GaN-on-GaN using cathodoluminescence

Cathodoluminescence (CL) is used as a quantitative characterization technique to probe impurities at the metal-organic chemical vapor deposition (MOCVD) grown GaN-on-GaN homoepitaxial interfaces. CL intensity contrast shows a strong correlation with the interfacial impurity concentrations. Based on the analysis of recombination mechanisms of electron beam induced non-equilibrium carriers, an analytical model is proposed to quantitatively determine the impurity concentrations from CL intensity. The extracted interfacial impurity concentrations from the analytical model show a good agreement with the compensation levels obtained from capacitance-voltage measurement, signifying the potential of CL for probing the quantitative impurity levels in GaN-on-GaN structures. This approach can also be extended to be applied in other material systems.

Chen, Zhaoying↗

DOE OSTI2021

Spatially-resolved measurements of spin valley polarization in MOCVD-grown monolayer WSe 2

Time-resolved Kerr rotation microscopy is used to generate and measure spin valley polarization in MOCVD-grown monolayer tungsten diselenide (WSe 2 ). The Kerr signal reveals bi-exponential decay with time constants of 100 ps and 3 ns. Measurements are performed on several triangular flakes from the same growth cycle and reveal larger spin valley polarization near the edges of the flakes. This spatial dependence is observed across multiple WSe 2 flakes in the Kerr rotation measurements but not in the spatially resolved reflectivity or microphotoluminescence data. Time-resolved pump-probe overlap measurements further reveal that the Kerr signal’s spatial dependence is not due to spin diffusion on the nanosecond timescale.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2023

GaAs-based superlattice photocathodes manufactured using MOCVD provide spin polarization > 90% and quantum efficiency > 1%

Spin-polarized electron beams are important for nuclear physics research performed at electron accelerators. Over many years, beam polarization has increased significantly, from 35% provided by unstrained bulk GaAs, to ~ 75% using single strained-layer GaAs, to ~ 90% using strained-superlattice GaAs photocathodes grown using molecular beam epitaxy (MBE). Commercial vendors once provided high-polarization photocathodes but today there is little interest in supporting this small market. Here, we report successful results fabricating high polarization GaAs/GaAsP superlattice photocathodes using metal organic chemical vapor deposition (MOCVD), a method dismissed by some as not providing sufficient control of layer thickness and uniformity. The best samples provide spin polarization greater than 90% and quantum efficiency (QE) exceeding 2%, with QE enhanced using a distributed Bragg reflector (DBR) incorporated into the structure. Together, these quantities represent some of the best results ever reported for high polarization photocathodes. Enhanced QE via DRB is very important for future work at CEBAF/Jefferson Lab because it will simplify the production of polarized electron beams at milliampere current needed for a polarized positron source.

Poelker, Matt↗

NASA NTRS1990

In-situ integrated processing and characterization of thin films of high temperature superconductors, dielectrics and semiconductors by MOCVD

In this strategy of depositing the basic building blocks of superconductors, semiconductors, and dielectric having common elements, researchers deposited superconducting films of Y-Ba-Cu-O, semiconductor films of Cu2O, and dielectric films of BaF2 and Y2O3 by metal oxide chemical vapor deposition (MOCVD). By switching source materials entering the chamber, and by using direct writting capability, complex device structures like three-terminal hybrid semiconductors/superconductors transistors can be fabricated. The Y-Ba-Cu-O superconducting thin films on BaF2/YSZ substrates show a T(sub c) of 80 K and are textured with most of the grains having their c-axis or a-axis perpendicular to the substrate. Electrical characteristics as well as structural characteristics of superconductors and related materials obtained by x-ray defraction, electron microscopy, and energy dispersive x-ray analysis are discussed.

Singh, R.↗

NASA NTRS1989

The use of metalorganics in the preparation of semiconductor materials. VIII - Feasibility studies of the growth of Group III-Group V compounds of boron by MOCVD

The MOCVD growth of B-As and B-P films on Si, sapphire, and Si-on-sapphire substrates is described; in this process, trimethylborane (TMB) or triethylborane (TEB) is pyrolyzed in the presence of AsH3 or PH3 in an H2 atmosphere. The procedures employed are outlined, and the results are presented in graphs, tables, and micrographs. It is found that the growth rate of the primarily amorphous films is dependent on the TMB or TEB concentration but approximately constant for TEB and AsH3 at 550-900 C. The nominal compositions of films grown using TMB are given as B(12-16)As2 and B(1-1.3)P. Carbon impurities and significant stress, bowing, and crazing are observed in the films grown on Si substrates, with the highest carbon content in the films grown from TMB and PH3.

Manasevit, H. M.↗

NASA NTRS1990

Three-dimensional numerical modelling of transport in horizontal MOCVD reactors

A systematic three-dimensional study for the MOCVD of gallium arsenide is presented. Numerical solutions, obtained for the system of equations governing the steady state heat, momentum, and mass transfer in horizontal reactors with large and small aspect ratio, are compared with experimental growth rate distributions. The effects on layer thickness uniformity of lateral thermal boundary conditions and susceptor tilt are explored. The validity of a parabolic flow approximation of the equations is also investigated by comparison with the complete three-dimensional treatment.

Ouazzani, Jalil↗

NASA NTRS1990

Three-dimensional modelling of horizontal chemical vapor deposition. I - MOCVD at atmospheric pressure

A systematic numerical study of the MOCVD of GaAs from trimethylgallium and arsine in hydrogen or nitrogen carrier gas at atmospheric pressure is reported. Three-dimensional effects are explored for CVD reactors with large and small cross-sectional aspect ratios, and the effects on growth rate uniformity of tilting the susceptor are investigated for various input flow rates. It is found that, for light carrier gases, thermal diffusion must be included in the model. Buoyancy-driven three-dimensional flow effects can greatly influence the growth rate distribution through the reactor. The importance of the proper design of the lateral thermal boundary conditions for obtaining layers of uniform thickness is emphasized.

Ouazzani, Jalil↗

NASA NTRS1991

Characterization of MOCVD fluid dynamics by laser velocimetry

Three-component laser velocimetry techniques are applied to characterize the fluid dynamics of an MOCVD reactor. These methods provide three-dimensional quantitative measurements of the gas velocities inside the reactor at typical growth temperatures. The effects of buoyancy-induced convection are examined by comparing data from the hot reactor with cold reactor results. It was found that thermal convection dominated the fluid dynamics of the reactor at growth temperatures. Cold reactor tests showed unstable flow patterns that were subject to fluidic switching effects.

Johnson, E. J.↗

NASA NTRS1991

MOCVD manifold switching effects on growth and characterization

A combined modeling and experimental approach is used to quantify the effects of various manifold components on the switching speed in metalorganic chemical vapor deposition (MOCVD). In particular, two alternative vent-run high-speed switching manifold designs suitable for either continuous or interrupted growth have been investigated. Both designs are incorporated in a common manifold, instrumented with a mass spectrometer. The experiments have been performed using nitrogen as the transport gas and argon as the simulated source gas. The advantages and limitations of two designs are discussed. It is found that while constant flow manifold switching systems may have fluid dynamic advantages, care must be taken to minimize sections of the supply manifold with low flow rates if rapid changes in alloy composition are required.

Clark, Ivan O.↗