P. S. McLeod

Surface chemistry of CuxS and CuxS/CdS determined from x‐ray photoelectron spectroscopy

X‐ray photoelectron spectroscopy spectra measured on copper sulfide (CuxS) films showed that a thin surface reaction product containing Cu in the +2 valence state was formed on CuxS films exposed to air for 46 h at 40 °C and 90% relative humidity. An entirely different CuxS surface reaction product layer was formed in dry air at 170 °C for 30 min and it contained sulfur in the +6 valence state. The copper (Cu) valence state in CuxS was not found to be +2 even when the x value was less than 1.9. When the argon sputter‐cleaned surface of CuxS or CuxS/CdS films was exposed to room‐temperature air for 10 min, cadmium (Cd) atoms appeared on the CuxS surface. X‐ray powder diffraction patterns showed that CuO and CdS reacted at 500 °C in flowing nitrogen to form Cu2S and CdO. This cation exchange between CdS and copper oxide may explain the surface Cd on the CuxS films. The standard free energy of reaction between CuO and CdS is positive while that between Cu2O and CdS is negative. These results indicate a metho...

Degradation of a CuxS/CdS solar cell in hot, moist air and recovery in hydrogen and air

A polycrystalline, thin‐film CuxS/CdS solar cell was exposed to air saturated with water vapor at temperatures between 27 and 67 °C for up to 6 1/2 h. The short‐circuit current decreased progressively from 11.7 to 1.02 mA/cm2. Subsequent heating in hydrogen at 150 °C for 680 h and in 170 °C air for 5 h restored the short‐circuit current to 14.4 mA/cm2. Modeling of the measured quantum yield indicates that the degradation could be quantitatively explained by two effects: (1) the CuxS minority carrier electron diffusion length decreasing from 0.23 to 0.02 μm (±20%) and (2) the CuxS optical band gap increasing from 1.16 to 1.46 eV (±3%). The recovery was quantitatively modeled by the CuxS diffusion length increasing back to 0.24 μm and the CuxS band gap returning to 1.16 eV. A Burstein–Moss analysis shows the band‐gap shift is due to the Fermi level penetration of the valence band as the measured hole concentration increased from 1.03(1020) cm−3 to 4.62(1021) cm−3 during degradation. A new band structure is ...

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.

Photoconductivity of sputtered CuxS films

The optical band edge of reactively sputtered CuxS films has been determined to be 1.18±0.03 eV using a technique in which the conductance of the films with respect to the wavelength of the incident light was measured. These results were found to confirm optical absorption data on CuxS films. Also, the efficiency of a 6.0% solar cell which was made using this sputtering technique is reported.

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.

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...