A. E. Kibbler

29.5%‐efficient GaInP/GaAs tandem solar cells

We report on multijunction GaInP/GaAs photovoltaic cells with efficiencies of 29.5% at 1‐sun concentration and air mass (AM) 1.5 global and 25.7% 1‐sun, AM0. These values represent the highest efficiencies achieved by any solar cell under these illumination conditions. Three key areas in this technology are identified and discussed; the grid design, front surface passivation of the top cell, and bottom surface passivation of both cells. Aspects of cell design related to its operation under concentration are also discussed.

Effect of growth rate on the band gap of Ga0.5In0.5P

The band gap of Ga0.5In0.5P is reported as a function of growth rate and growth temperature. The Ga0.5In0.5P is grown lattice matched to 2°‐off (100) GaAs substrates by atmospheric pressure organometallic chemical vapor deposition using an inlet group V/III ratio of 65. The variation of the band gap is surprisingly complex, taking five different functional forms within the two‐dimensional parameter space. These include regions in which the band gap (1) increases with growth rate, (2) decreases with growth rate, (3) is independent of both growth rate and temperature, (4) is independent of growth rate, but dependent on growth temperature, and (5) is not measurable since three‐dimensional, instead of two‐dimensional, growth is observed. The behavior can only be explained by a theory involving competing processes. One such theory is described.

Carbon doping and etching of MOCVD-grown GaAs, InP, and related ternaries using CC14

Abstract Pure carbon tetrachloride is shown to be an excellent carbon-doping source for MOCVD-grown GaAs, but an etchant, rather than a doping source, for InP. Similar to the equilibria used in chloride vapor phase epitaxy, the etching (or reduced growth rate) is believed to occur because of the formation of InC1. In the case of Ga 0.5 In 0.5 P, this etching is manifested by a change in the alloy composition, since the InP etches faster than the GaP. The composition of Ga 0.5 In 0.5 As is not changed significantly for comparable carbon tetrachloride flows. The doping efficiency of CC1 4 for GaAs is shown to decrease sharply with the growth temperature and to be inversely proportional to the V/III ratio. The hole mobilities for carbon-doped GaAs are shown to be as higher than for zinc-doped GaAs. Compensation is not a problem, even at hole concentrations exceeding 10 19 cm -3 . However, the quality of carbon-doped Ga 0.5 In 0.5 P films is significantly lower than that of zinc-doped Ga 0.5 In 0.5 P, and hole concentrations could not be increased above 10 17 cm -3 . The carrier concentration of InP films is unaffected by the flow of carbon tetrachloride.

MOCVD growth and characterization of GaP on Si

Abstract GaP and GaP/GaAsP epitaxial layers have been grown on Si substrates by metalorganic chemical vapor deposition (MOCVD), and characterized by SEM and TEM plan-view and cross-sectional examination. At growth temperatures ranging from 600 to 800°C, the initial stages of growth were dominated by three-dimensional nucleation. TEM studies showed that the nuclei were generally misoriented with respect to each other, yielding, upon coalescence, polycrystalline layers. The growth of single-crystal layers could be achieved only by nucleating a 30–50 nm layer of GaP at 500°C, followed by continued growth at 750°C. A high density of structural defects (∼10 11 cm −2 ) was observed at the Si/GaP interface, decreasing to ∼10 8 cm −2 , 2 μ away from the interface. The use of 2° off (100) Si substrates resulted in GaP layers free of antiphase domains. These results and their implications are discussed.

Monolithic, Ultra-Thin GaInP/GaAs/GaInAs Tandem Solar Cells

We present here a new approach to tandem cell design that offers near-optimum subcell bandgaps, as well as other special advantages related to cell fabrication, operation, and cost reduction. Monolithic, ultra-thin GaInP/GaAs/GaInAs triple-bandgap tandem solar cells use this new approach, which involves inverted epitaxial growth, handle mounting, and parent substrate removal. The optimal ~1-eV bottom subcell in the tandem affords an -300 mV increase in the tandem voltage output when compared to conventional Ge-based, triple-junction tandem cells, leading to a potential relative performance improvement of 10-12% over the current state of the art. Recent performance results and advanced design options are discussed

High efficiency GaAs solar cells using GaInP/sub 2/ window layers

GaAs single-junction solar cells using Ga/sub 0.5/In/sub 0.5/P and having 1 sun, an air mass (AM) of 1.5, and global efficiencies of 25.0-25.7% are reported. The open-circuit voltage (V/sub oc/), short-circuit current (J/sub sc/), and fill factor (ff) for the 25.7% efficient cell were 1.039 V, 28.5 mA/cm/sup 2/, and 86.8%, respectively. The devices were grown at 700 degrees C using conventional atmospheric pressure metalorganic chemical vapor deposition (MOCVD). The antireflection coating is a double layer of MgF/sub 2/ and ZnS. V/sub oc/s as high as 1.055 V were obtained for some of the devices. This high V/sub oc/ is attributed to the low interface recombination velocity of the Ga/sub 0.5/In/sub 0.5/P-GaAs heterointerface. Factors that affect the efficiency of this device, including the thickness and composition of the Ga/sub 0.5/In/sub 0.5/P window layer, are presented and discussed.<<ETX>>

Ordering and disordering of doped Ga 0.5 In 0.5 P

The band gap of Ga0.5In0.5P is reported as a function of doping level and growth rate. The lowest band gaps are obtained for hole concentrations of about 2 × 1017 cm−3. For samples doped p-type above 1 × 1018 cm−3, the band gap increases dramatically, regardless of growth rate. This effect is shown to be the result of disordering during growth rather than a change in the equilibrium surface structure with doping. The doping level dependence of the band gap of Ga0.5In0.5P samples grown at higher and lower growth rates differs for selenium and zinc doping even though the effects of high doping are the same for both dopants.

Surface topography and ordering‐variant segregation in GaInP2

Using transmission electron diffraction dark‐field imaging, atomic force microscopy (AFM), and Nomarski microscopy, we demonstrate a direct connection between surface topography and cation site ordering in GaInP2. We study epilayers grown by organometallic vapor‐phase epitaxy on GaAs substrates oriented 2° off (100) towards (110). Nomarski microscopy shows that, as growth proceeds, the surface of ordered material forms faceted structures aligned roughly along [011]. A comparison with the dark‐field demonstrates that the [111] and [111] ordering variants are segregated into complementary regions corresponding to opposite‐facing facets of the surface structures. This observation cannot be rationalized with the obvious but naive model of the surface topography as being due to faceting into low‐index planes. However, AFM reveals that the facets are in fact not low‐index planes, but rather are tilted 4° from (100) towards (111)B. This observation explains the segregation of the variants: the surface facets a...

Transmission electron microscopy study of the effect of selenium doping on the ordering of GaInP 2

Selenium doped Ga0.51In0.49P films have been grown by metalorganic chemical vapour deposition at 600, 670 and 740° C. The extent of ordering of the Group III sublattice has been monitored by transmission electron microscopy. Ordering disappears at carrier concentrations on the order of 1018 cm−3 for samples grown at 600 and 740° C although a small degree of ordering persists in the samples grown at 670° C up to a carrier concentration of 1019 cm−3. At each growth temperature, the ordering observed decreased and the bandgap measured increased with increasing Se doping.

Passivation of Interfaces in High-Efficiency Photovoltaic Devices

Solar cells made from III-V materials have achieved efficiencies greater than 30%. Effectively ideal passivation plays an important role in achieving these high efficiencies. Standard modeling techniques are applied to Ga0.5In0.5P solar cells to show the effects of passivation. Accurate knowledge of the absorption coefficient is essential (see appendix). Although ultralow (<2 cm/s) interface recombination velocities have been reported, in practice, it is difficult to achieve such low recombination velocities in solar cells because the doping levels are high and because of accidental incorporation of impurities and dopant diffusion. Examples are given of how dopant diffusion can both help and hinder interface passivation, and of how incorporation of oxygen or hydrogen can cause problems.