J. W. Garland

Single-crystal II-VI on Si single-junction and tandem solar cells

CdTe is one of the leading materials used in solar photovoltaics. However, the maximum reported CdTe cell efficiencies are considerably lower than the theoretically expected efficiencies for the ∼1.48 eV CdTe band gap. We report a class of single crystal CdTe-based solar cells grown epitaxially on crystalline Si that show promise for enhancing the efficiency and greatly lowering the cost per watt of single-junction and multijunction solar cells. The current-voltage results for our CdZnTe on Si solar cells show open-circuit voltages significantly higher than previously reported for any II-VI cells and as close to the thermodynamic limit as the best III-V-based cells.CdTe is one of the leading materials used in solar photovoltaics. However, the maximum reported CdTe cell efficiencies are considerably lower than the theoretically expected efficiencies for the ∼1.48 eV CdTe band gap. We report a class of single crystal CdTe-based solar cells grown epitaxially on crystalline Si that show promise for enhancing the efficiency and greatly lowering the cost per watt of single-junction and multijunction solar cells. The current-voltage results for our CdZnTe on Si solar cells show open-circuit voltages significantly higher than previously reported for any II-VI cells and as close to the thermodynamic limit as the best III-V-based cells.

Study of the interface of undoped and p‐doped ZnSe with GaAs and AlAs

We have used electrolyte electroreflectance (EER) to characterize ZnSe/GaAs and ZnSe/AlAs interfaces. The great sensitivity of EER to interface space‐charge regions enabled us to detect both interface crossover transitions and transitions to triangular‐well interface states. The observation of these transitions provides the first unambiguous proof that the ZnSe/GaAs interface is type I and allowed us to determine the band offsets and band bendings, the diffusion lengths across each interface, and the amount of interdiffusion.

Study of mercury cadmium telluride epilayers grown by metalorganic vapor‐phase epitaxy

We report on the growth and characterization of HgCdTe epilayers grown by metalorganic vapor‐phase epitaxy. The transport properties indicate that it is possible to grow epilayers with mobilities comparable to those of the best bulk‐grown materials. Electrolyte electroreflectance studies have yielded depth profiles for the alloy composition, the strains, and the density of polarizable defects. These latter results indicate very clearly that the interface alloy/substrate is very sharp and does not extend over more than 2000 A. Moreover, in the bulk of the epilayers the strains are low and comparable to those found in high‐quality bulk single crystals.

Line shape of the optical dielectric function

A systematic study of the optical dielectric function by spectroscopic ellipsometry and electroreflectance has shown that the proper functional form for the Green’s function for an electron‐hole pair in GaAs or CdTe is primarily Gaussian, not Lorentzian as is commonly assumed, although it is primarily Lorentzian for Hg1−xCdxTe. The Lorentzian part of the broadening is shown to measure the alloy, impurity, and defect scattering.

Arsenic activation in molecular beam epitaxy grown, in situ doped HgCdTe(211)

Photovoltaic p-n junctions are the most significant active components of both current infrared photodetectors and advanced ones being developed. It is of the utmost importance to control both p- and n-type extrinsic doping. This letter addresses the issue of activating arsenic as a p-type dopant of Hg1−xCdxTe at temperatures sufficiently low that the integrity of p-n junctions and the intrinsic advantages of molecular beam epitaxy as a growth technique will not be compromised. The p-type activation of arsenic in (211)B Hg1−xCdxTe is reported after a two-stage anneal at temperatures below 300 °C for Cd compositions suitable for the sensing of long wavelength infrared radiation.

Electron beam electroreflectance studies of GaAs and CdTe surfaces

An electron beam electroreflectance (EBER) system has been built and used to investigate GaAs and CdTe sample surfaces. EBER permits one to go to low temperatures, to couple electroreflectance with high‐vacuum techniques and to study the effects of different surface treatments. Analysis shows that the EBER technique leads to only negligible thermal modulation of the sample for low‐energy electron beams such as that used here. The results of our measurements before and after sputtering with 2 keV O+2 ions clearly show that the measurements are very sensitive to the condition of sample surfaces. Analysis of the results shows the existence of distinct surface and bulk signals and gives detailed information about the surface before and after sputtering. Comparison of our EBER results with room‐temperature electrolyte‐electroreflectance results on both GaAs and CdTe confirms the analysis and establishes that electroreflectance is a surface‐sensitive technique.

Next-generation multijunction solar cells: The promise of II-VI materials

High concentration photovoltaic (HCPV) systems offer the highest photovoltaic (PV) conversion efficiencies. Also, as production is beginning to ramp up, HCPV is becoming cost competitive with thin-film poly-CdTe and crystalline Si systems in high solar insolation regions. High solar concentrations, X ∼ 500, are used to increase cell efficiencies and greatly reduce the cell area per unit of incident solar radiation, thereby greatly reducing the cell cost per watt. The monolithic three-junction (3J) solar cells presently used in HCPV systems typically consist of two epitaxial III-V homojunctions, such as GaInP and GaInAs, grown on an active Ge substrate by metal-organic chemical vapor deposition (MOCVD). The III-V bandgaps are chosen to match the currents generated in each junction and minimize the energy lost to thermalization of the electron-hole pairs generated, subject to the constraint of approximate lattice matching. We propose using cells consisting of one or more CdTe-based II-VI homojunctions grown...

Evidence that arsenic is incorporated as As4 molecules in the molecular beam epitaxial growth of Hg1−xCdxTe:As

Molecular arsenic, As4, is commonly used as the source for in situ p-type doping of Hg1−xCdxTe grown by molecular beam epitaxy. As incorporated, the arsenic is strongly self-compensated, requiring annealing for its p-type electrical activation. Here, a quasithermodynamic model is used to interpret the dependence of the arsenic concentration, cAs, as measured by secondary ion mass spectroscopy, on the incident As4 and Hg fluxes during growth. The results strongly suggest that the As4 is absorbed in its molecular form rather than being dissociated on the growth surface, as has previously been assumed. This clearly is relevant to the self-compensation of the arsenic in as-grown Hg1−xCdxTe.

Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrates

The efficiencies of monolithic single-crystal II–VI and III–V, two-junction and three-junction solar cells are calculated. The structures consist of II–VI or III–V homojunctions grown on an active-junction substrate (silicon for II–VI and germanium for III–V) and of inverted three-junction II–VI or III–V structures, with the band gaps chosen to maximize the efficiencies. Our calculations for the II–VI cells give theoretical efficiencies up to 44% under 1 sun and 50% under 500 suns, ∼3% absolute higher than for the III–V cells. Maximum obtainable laboratory and production-line efficiencies for multijunction II–VI cells are predicted. Preliminary laboratory results for II–VI two-junction cells are also presented.

The improvement of phase modulated spectroscopic ellipsometry

Spectroscopic ellipsometry using photoelastic modulator [phase modulated spectroscopic ellipsometry (PMSE)] has been improved in the spectral range. Spectroscopic ellipsometry using the rotating analyzer [rotating analyzer spectroscopic ellipsometry (RASE)] has demonstrated its capability of measuring the reflectivity ratio, ρ(ω), from 1.5 to 6 eV with a single scan, but PMSE has not been able to do so. We demonstrate that PMSE also can measure ρ(ω) from 1.5 to 6 eV with a single scan. We discuss the problems and show their solutions to achieve this goal. We also discuss the accuracy of our PMSE by comparing the spectral data by RASE with those by our PMSE. We find that the simplest possible procedure with our system provides reasonably accurate values, after including the zone average which is easy to perform with our system. The extension of the spectral range is a decisive advantage, especially in studying the E1 structure of the wide band gap materials such as ZnSe.