Lewis M. Fraas

GaSb booster cells for over 30% efficient solar‐cell stacks

The fabrication of GaSb infrared‐sensitive photovoltaic cells designed to boost the energy‐conversion efficiency in tandem solar cell stacks is reported. Located behind GaAs solar‐cells in 50× concentrated light configurations, these GaSb cells will boost the stack efficiency by 6.5 percentage points for space (AM0) and 7.0 percentage points for terrestrial (AM1.5D) applications. Assuming a GaAs cell efficiency of 26.7% (AM1.5D, 50×) as recently reported, the GaAs on GaSb stack efficiency will be 33.7%. Reduced series resistance in future GaSb cells will allow tandem‐stack energy‐conversion efficiencies over 35%.

A new low temperature III–V multilayer growth technique: Vacuum metalorganic chemical vapor deposition

A new technique for multilayer growth by metalorganic chemical vapor deposition is described. The vacuum metalorganic chemical vapor deposition technique combines the low‐temperature growth capability of molecular beam epitaxy with the source handling system of chemical vapor deposition. The viability of the new technique is demonstrated by the growth of high‐mobility layers of GaAs, GaAs(1−x)P(x), and Ga(1−x)In(x)As at 570 °C. Room‐temperature mobilities of GaAs films as high as 4990 cm2/V s are obtained. Doping of both p‐type and n‐type films is demonstrated. GaAs shallow homojunction solar cells fabricated with this technique are described. Active‐area solar cell efficiencies as high as 19.6% are obtained with 6 ’’suns’’ AM2 concentrated light. This multilayer growth technique is particularly suited to the fabrication of multicolor solar cells.

26.1% solar cell efficiency for Ge mechanically stacked under GaAs

We have processed a diffused Ge wafer into a Ge concentrator solar cell and mechanically stacked it under a GaAs cell fabricated by Varian. We measured this stack’s efficiency to be 26.1% for terrestrial air mass 1.5 direct (AM1.5D) conditions at a 285× concentration ratio. We showed that this efficiency is limited by optical absorption in the Varian GaAs cell caused by high 2–4 (1018) cm−3 substrate doping. We used a 2×1017 cm−3 doped GaAs filter to estimate the stack efficiency as 27.4%, which would be achieved with the same Varian GaAs cell formed on a lower doped substrate. We project efficiencies assuming the best properties reported for a GaAs device. This gives a 29.6% efficiency for an improved, planar Ge cell and 31.6% efficiency for a proposed point contact geometry for the Ge cell. The corresponding space (AM0) efficiencies at a 159× concentration ratio range from the 23.4% value we measured on the stack up to 28.4% projected for the point contact Ge place under the best GaAs cell. We showed th...

Epitaxial films grown by vacuum MOCVD

Abstract We are developing high efficiency multicolor solar cells for terrestrial applications using a novel MOCVD growth technique, vacuum MOCVD, for growing the sequential epitaxial layers. We believe the vacuum MOCVD growth technology offers several advantages for production scale-up including the more efficient use of the metal alkyl source materials. In addition, the use of stainless steel rather than glass offers important safety advantages. For two color cell applications, we are currently developing the GaAs1−xPx and GaAs1−ySby ternary alloys. In this paper, we first describe our vacuum MOCVD system and then the current status of our vacuum MOCVD grown GaAs1−ySby materials. Triethyl-Sb and trimethyl-Sb are compared as sources of Sb, and dicyclopentadienyl-Mg and diethyl-Zn are compared as p-type dopant sources

GaAs films grown by vacuum chemical epitaxy using thermally precracked trimethyl‐arsenic

Trimethyl‐arsenic (TMAs) is used as a source of arsenic for GaAs film growth. In the process used, vacuum chemical epitaxy, TMAs is thermally decomposed into arsenic upstream in a hot cracker furnace. The arsenic and stable hydrocarbons are then transported in vacuum without condensation to the epitaxial growth zone. The hole carrier concentration and carbon content in grown films are studied via Hall, electrochemical profile, and secondary ion mass spectroscopy as a function of cracker furnace design. It is shown that when the TMAs decomposition efficiency is poor, the carbon content can be as high as 1019/cm3 but for a more efficient cracker, the carbon content can be reduced into the 1016/cm3 range. Toxic injury hazards can be reduced substantially by substituting TMAs for the more widely used arsine in GaAs growth systems.

A new sequentially etched quantum‐yield technique for measuring surface recombination velocity and diffusion lengths of solar cells

We have developed a new technique to characterize the individual layers of high‐efficiency solar cells. In general, the technique allows one to set lower bounds for diffusion lengths and upper and lower bounds for interface recombination velocity. This is sufficient to determine which parameter limits performance, and often the actual parameter values are also determined accurately. We obtain this information by fitting a theoretical model to quantum‐yield spectra measured on a sample in its initial state, and after its window passivation and top active layers are sequentially etched away. With such data on two p on n GaAs solar cells with AlxGa1−xAs passivation, we determined minority‐carrier hole diffusion lengths of 1.0±0.2 and 0.2±0.05 μ in the Te‐doped n layers for first and second samples, respectively. We found lower limits for the minority‐carrier electron diffusion lengths in the top p layers of 2.0 μ in the carbon‐doped first sample and 4.0 μ in the Mg‐doped second sample. We determined interfac...

Vacuum chemical epitaxy utilizing organometallic sources

Herein, we describe a process, Vacuum Chemical Epitaxy (VCE), which incorporates some of the principle advantages of molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) systems while rejecting their disadvantages and limitations. In this process, multiple group(III)-alkyl molecular beams are directed through a water cooled gas distribution block onto wafers providing for the growth of uniform films over large areas with high group(III)-alkyl utilization efficiency. The group(V) source, on the other hand, is injected at a single point on one side of the deposition zone. The group(V) molecules are confined and undergo molecular flow across the deposition zone. A variety of group(V) source molecules are used including the group(V) hydrides (AsH3 and PH3) and elementary group(V) molecules (As2 and P2). In the work presented here, the elemental group(V) molecules are generated by thermally cracking the hydrides. However, the use of conventional MBE elemental group(V) evaporative sources is also possible thereby eliminating the safety issues associated with the hydride source gases. In this paper, our VCE reactor is described in some detail along with the properties of III-V films grown with this equipment. The fabrication of a GaAsSb solar cell with an active area energy conversion efficiency of 26.7% demonstrates that Vacuum Chemical Epitaxy has the capability of producing high performance devices.

Growth of GaAs in a rotating disk MOCVD reactor

We report the growth of GaAs homoepitaxial films from trimethylgallium and arsine in a multi-wafer rotating disk reactor. In this configuration the substrates are mounted on a disk that is spun at high speed (> 1000 rpm) in a sub-atmospheric (<100 Torr) environment. The spinning disk pumps the reactant and carrier gases radially outwards; under optimum conditions, convective recirculating cells are avoided, thus facilitating rapid transitions in doping and composition in the grown layers. In this paper we look at the morphology, growth rate and electrical properties of the GaAs epitaxial layers as a function of substrate temperature, V/III ratio, dopant type, spin speed and hydrogen carrier flow conditions. These results are compared with those obtained in conventional MOCVD reactors. Silicon and tellurium doping over a wide range of carrier concentrations has been achieved with excellent mobilities and uniformity across the wafers. Preliminary results on MESFETs fabricated from n+/n/buffer structures show good device characteristics.

GaSb films grown by vacuum chemical epitaxy using triethyl antimony and triethyl gallium sources

GaSb films have been grown using triethyl‐Ga and triethyl‐Sb sources. In a hot‐wall reaction chamber located within a high‐vacuum chamber, multiple group‐III alkyl molecular beams are directed into the reaction chamber onto wafers. The group‐V molecules are injected from the perimeter of the reaction chamber and undergo molecular flow across the deposition zone. The utilization efficiency of the group‐V source material is enchanced by the use of a thermal cracker located at the point of group‐V gas injection and by the use of the hot‐wall chamber. Both unintentionally doped p‐type and Te doped n‐type GaSb films are grown and characterized. GaSb p‐n junction photodiodes are also reported with internal quantum yields as high as 85%. Unintentionally doped films were shown to have background carrier concentrations of 4×1016 cm3 by capacitance versus voltage measurement.

Measurement of a long diffusion length in a GaAs film improved by metalorganic-chemical-vapor-deposition source purifications

The vacuum metalorganic‐chemical‐vapor‐deposition (Vacuum MOCVD) process was combined with two source purifications to grow p‐GaAs epitaxial films of high quality. Theoretical modeling of quantum yield spectra measured on a specially configured n+‐p sample determined the minority‐carrier electron diffusion length to be 10 μm to within a factor of 2 in the p layer. The p doping was reduced to the 5×1017 cm−3 level to avoid suppression of the diffusion length by Auger recombination. Multiple vacuum sublimations of dicyclopentadienyl magnesium (CP2Mg), the source of Mg for p doping, reduced the contamination by air and by cyclopentadiene (CP) by an order of magnitude. A dry ice/acetone cold trap was operated at slightly below 180‐Torr pressure to reduce the water vapor content of arsine, used as the As source, from the hundreds of ppm down level down to the 2 ppm range. The vacuum growth process reduced residual gas contamination. These techniques were combined to grow a p on n GaAs solar cell with an effici...