David W. Niles

Na impurity chemistry in photovoltaic CIGS thin films: Investigation with x-ray photoelectron spectroscopy

Thermal processing of Cu(In1−xGax)Se2 thin-films grown as part of photovoltaic devices on soda-lime glass leads to the incorporation of Na impurity atoms in the Cu(In1−xGax)Se2. Na contamination increases the photovoltaic efficiency of Cu(In1−xGax)Se2-based devices. The purpose of this investigation is to develop a model for the chemistry of Na in Cu(In1−xGax)Se2 in an effort to understand how it improves performance. An analysis of x-ray photoelectron spectroscopy data shows that the Na concentration is ∼0.1 at. % in the bulk of Cu(In1−xGax)Se2 thin films and that the Na is bound to Se. The authors propose a model invoking the replacement of column III elements by Na during the growth of Cu(In1−xGax)Se2 thin films. Na on In and Ga sites would act as acceptor states to enhance photovoltaic device performance.

Direct observation of Na and O impurities at grain surfaces of CuInSe2 thin films

The authors use field-emission Auger electron spectroscopy to investigate the spatial nature of trace Na and O impurities in thin films of photovoltaic-grade CuInSe2 thin films. They give the first direct proof that Na and O reside at grain surfaces and not in the grain interiors of CuInSe2 (CIS) thin films, and discuss the improvement in photovoltaic conversion efficiency of CIS with Na.

EFFECT OF NITRIC-PHOSPHORIC ACID ETCHES ON MATERIAL PROPERTIES AND BACK-CONTACT FORMATION OF CDTE-BASED SOLAR CELLS

Forming a low-resistance contact to p-type CdTe is a critical issue for successful commercialization of CdTe-based photovoltaic devices. One solution to this problem has been to incorporate surface pretreatments to facilitate contact formation. In this article, the effects of a nitric–phosphoric (NP) acid pretreatment on material properties and device performance are investigated for polycrystalline CdTe-based devices. We demonstrate that the NP acid pretreatment, when applied to CdTe thin films, forms a thick, highly conductive Te layer on the back surface of the film and on exposed grain boundaries. When etched under optimal conditions, this results in CdS/CdTe devices with reduced series resistance and enhanced performance. On the other hand, we find that the NP etch preferentially etches grain boundaries. Overetching can result in complete device failure by forming shunt paths that extend to the heterointerface. Therefore, carefully controlling the etch concentration and duration is critical to optimi...

Heterojunction Band Discontinuity Control by Ultrathin Intralayers

We present evidence that the band lineup at a semiconductor‐semiconductor heterojunction interface can be changed and potentially controlled by an ultrathin metal intralayer. Synchrotron‐radiation photoemission experiments demonstrate that 0.5–2‐A‐thick Al intralayers increase the valence‐band discontinuity of CdS‐Ge and CdS‐Si heterojunctions by 0.15 eV on the average.

Ethylene and Acetylene Adsorption on Cleaved Si - a Photoemission-Study with Synchrotron Radiation

Abstract Our data reveal a close analogy between the adsorption states of ethylene and acetylene on cleaved silicon. The adsorption mechanisms confirm that the cleaved Si surface is a good π donor.

Understanding and controlling heterojunction band discontinuities

We discuss two recent results on the microscopic nature and control of the band lineup at semiconductor–semiconductor interfaces. First, we identified a correlation between measured heterojunction band discontinuities and Schottky barrier heights of the corresponding semiconductors, as predicted by several theoretical models. Second, we found that ultrathin metal intralayers modify the band lineup of polar interfaces by several tenths of an electron volt. At least in principle, this degree of freedom can be exploited to tailor heterojunction devices.

Thermal oxides of In0.5Ga0.5P and In0.5Al0.5P

An investigation of the chemical composition of wet thermal oxides grown on In0.5Ga0.5P and In0.5Al0.5P is reported. The oxides were grown in the temperature range 500–650 °C. An estimate of the expected oxide composition was obtained by the construction of three-dimensional phase diagrams of the In–Ga–P–O and In–Al–P–O systems. These diagrams indicate that under thermodynamic equilibrium, the oxide layers should be composed primarily of mixtures of InPO4 and GaPO4 on In0.5Ga0.5P and InPO4 and AlPO4 on In0.5Al0.5P. X-ray photoemission spectroscopy (XPS) sputter profiles were used to determine the distribution of elements in the oxide layers and to identify the chemical compounds. The binding energy shifts observed in the XPS data suggests that the oxides are composed primarily of metal phosphates with low concentrations of the metal trioxides. At lower growth temperatures, the oxides composition is uniform with depth, but there is an increasing depletion of In near the substrate interface as the growth te...

The interface formation and thermal stability of Ag overlayers grown on cubic SiC(100)

We have used photoemission spectroscopy with synchrotron radiation in the energy range hν=40–140 eV to study the interface formation of Ag overlayers on cubic SiC(100). Our data are consistent with a three‐dimensional island growth mode of Ag at room temperature. The substrate attenuation curve supports a model of reduced lateral growth rates for the Ag islands. Annealing of a 340‐A‐thick Ag overlayer in the temperature range 400–600 °C causes the film to coalesce, forming clusters which leave large parts of the SiC surface uncovered. Thermal treatment above 600 °C most likely leads to evaporation of Ag from the surface. At ∼1000 °C the surface is essentially free of Ag. The Ag/SiC(100) interface is nonreactive for Ag films deposited at 25 °C. It is equally stable at temperatures up to 1000 °C.

Interface and growth studies of α‐Sn/CdTe(110) superlattices

Angular‐resolved synchrotron radiation photoemission spectroscopy and reflection high‐energy electron diffraction (RHEED) are used to study the molecular‐beam epitaxy of CdTe/ α‐Sn(110) interfaces and superlattices. Core level photoemission spectra indicate that both sides of the interface are stable, nonreactive, and abrupt for growth temperatures up to 100 °C. At the α‐Sn/CdTe interface, the valence band maximum at Γ is at EV=1.1 eV below the Fermi level. This gives a valence band offset of ΔEV=1.1 eV, assuming zero band gap for the Sn. Stable superlattices of α‐Sn/CdTe(110) have been grown at 100 °C. The surface quality of the superstructure degrades after the growth of several α‐Sn/CdTe periods. After the growth of ten periods each 50‐A thick, the RHEED pattern shows mainly three‐dimensional bulk diffraction, indicating increased surface, and interface roughness.

Nonuniform strain profiles in cubic CdS/GaAs films measured by reflection high energy electron diffraction and Raman spectroscopy

We report a study of the strain relaxation in zinc‐blende CdS films grown on GaAs by molecular beam epitaxy. We use Raman spectroscopy to determine the depth dependence of the strain profile. This dependence is compared with the instantaneous surface strain relaxation measured by reflection high energy electron diffraction during growth. The Raman results show that the strain profile determined by RHEED remains largely ‘‘frozen’’ in the final sample. It is only near the heterostructure interface, where the strain is large, that evidence of additional relaxation is found in the Raman spectrum.