Art J. Nelson

Composition and structure of thermal oxides of indium phosphide

The thermal oxidation of InP has been studied for growth temperatures of 400 to 700 °C using x‐ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) profiles and x‐ray diffraction. For T 650 °C the oxide composition is primarily InPO4 throughout the layer although the roughness of the high temperature oxide may distort the XPS sputter profiles. From x‐ray diffraction data, the oxides grown at T<650 °C appear noncrystalline. At T=700 °C the oxide is crystalline InPO4.

Photoemission investigation of the electronic structure at polycrystalline CuInSe2 thin‐film interfaces

The surface versus bulk composition and electronic structure of polycrystalline CuInSe2 thin‐film interfaces were studied by synchrotron radiation soft‐x‐ray photoemission spectroscopy. An n‐type In2Se3/CuIn3Se5 surface layer forms on enhanced‐grain polycrystalline thin‐film p‐type CuInSe2 during fabrication. Enhanced‐grain CuInSe2 films were sputter etched (500 V Ar) and analyzed in situ to determine core‐level binding energies and Fermi‐level positions for the n‐type surface and the p‐type CuInSe2 bulk within ±0.1 eV. The transition between the n‐type surface and the p‐type bulk was experimentally observed by noting the change in the position of the valence‐band maximum relative to the Fermi level EF. From these measurements, the valence‐band offset ΔEv between the layers was determined to be 0.50 eV. Measurement of the work functions φ was also completed and reveals φ=4.75 eV for the In2Se3 (CuIn3Se5) surface layer and φ=4.04 eV for the bulk CuInSe2. Combining these results allows construction of a sur...

Valence‐band electronic structure of Zn3P2 as a function of annealing as studied by synchrotron radiation photoemission

Ultraviolet photoemission (UPS) utilizing synchrotron radiation has been used to characterize changes in the valence‐band electronic structure of crystalline Zn3P2 as a function of annealing temperature. The Zn3P2 crystal was etched in bromine‐methanol prior to analysis and annealing was performed in vacuum at 300 and 350 °C after sputter cleaning. The UPS spectra for the virgin material are qualitatively similar to the photoemission results for various II‐VI Zn compound semiconductors and a comparison of the Zn 3d binding energies with respect to the valence band maximum is presented. The results for the virgin material and the 300 °C anneal are further compared with the theoretically predicted band structure of Zn3P2 as determined by a pseudopotential energy band calculation. Loss of phosphorus from the surface and the presence of elemental zinc on the surface after the 350 °C anneal is evident.

Photoemission characterization of the H2 plasma etched surface of InP

Synchrotron radiation soft x‐ray photoemission spectroscopy was used to characterize the surface chemistry of InP before and after exposure to a H2 plasma. The low‐power H2 plasma was generated with a commercial electron cyclotron resonance plasma source using a mixture of H2 and Ar with the plasma exposure being performed at ambient temperature. Plasma species were identified with quadrupole based mass spectrometry and optical emission spectroscopy. Photoemission measurements were acquired after each plasma exposure in order to observe changes in the valence‐band electronic structure as well as changes in the In 4d and P 2p core lines. The results were correlated in order to relate the plasma species and characteristics to changes in surface chemistry.

Novel method for growing CdS on CdTe surfaces for passivation of surface states and heterojunction formation

Large-grain polycrystalline CdTe was subjected to in situ H2S plasma processing for <30 min at 100 and 200 °C. High-resolution x-ray photoemission measurements on the Cd 4d, Te 4d, and S 2p core levels, and the valence band were used to determine the resultant chemical environment of S and the electronic structure at the CdTe surface following plasma processing. Auger electron spectroscopy compositional depth profiling was also used to determine the distribution of S in the near surface region. Furthermore, time-resolved photoluminescence was used to measure carrier lifetimes and, thus, determine the degree of passivation of CdTe surface states. These results provide evidence for a CdS/CdS1−xTex/CdTe heterojunction device structure, as a result of the H2S plasma processing, and a reduction in the surface recombination velocity through passivation of surface states. This is a novel method of fabricating CdS/CdTe heterojunction devices.

Processing and characterization of large‐grain thin‐film CdTe

Basic material studies addressing the growth and processing of CdTe have resulted in dense, defect‐free as‐grown CdTe films on 7059 glass with initial grain sizes of ≊0.2 μm. Innovations in postdeposition processing (no CdCl2) have resulted in films with ≳50 μm grain sizes. Scanning electron microscopy analyses confirm film density while concurrent cathodluminescence reveals a change in the recombination efficiency. Transmission electron microscopy analyses reveal that films grown below 300 °C are defect‐free, while films grown above 300 °C contain defects. Photoluminescence lifetime measurements reveal a fivefold increase in lifetime following postdeposition processing of these films. These results were correlated with x‐ray photoemission measurements of the Te 4d, Cd 4d, and valence band. This indicates that grain boundaries are the main factor limiting lifetimes. Based on these results, we have developed an understanding of the effects of oxygen and grain boundary oxides on postdeposition processing an...

Formation and Schottky barrier height of Au contacts to CuInSe2

Synchrotron radiation soft x‐ray photoemission spectroscopy was used to investigate the development of the electronic structure at the Au/CuInSe2 interface. Au overlayers were deposited in steps on single‐crystal p and n‐type CuInSe2 at ambient temperature. Reflection high‐energy electron diffraction analysis before and during growth of the Au overlayers indicated that the Au overlayer was amorphous. Photoemission measurements were acquired after each growth in order to observe changes in the valence band electronic structure as well as changes in the In 4d and Se 3d core lines. The results were used to correlate the interface chemistry with the electronic structure at these interfaces and to directly determine the Au/CuInSe2 Schottky barrier height.

Low temperature oxidation of plutonium

The initial oxidation of gallium stabilized δ-plutonium metal at 193 K has been followed using x-ray photoelectron spectroscopy. On exposure to Langmuir quantities of oxygen, plutonium rapidly forms a trivalent oxide followed by a tetravalent plutonium oxide. The growth modes of both oxides have been determined. Warming the sample in vacuum, the tetravalent oxide reduces to the trivalent oxide. The kinetics of this reduction reaction have followed and the activation energy has been determined to be 38.8 kJ mol−1.

Surface topography of oxides on InP thermally grown at high temperatures

The surface topography of the thermal oxides on InP are shown to blister for growth temperatures somewhat above 600 °C. It appears that the oxide softens sufficiently to allow the trapped phosphorus at the interface to expand, thus causing the surface bubbles to form. The size of the bubbles varies considerably across a wafer and they tend to increase in size with temperature. At 780 °C one bubble had a radius of ≊100 μ. At T≲650 °C the elemental phosphorus at the interface is the source of the P and the underlying substrate remains smooth. Above this temperature the InP substrate provides the P as shown by the pits in the substrate. There appears to be a relationship between the surface roughening and a change in oxide layer composition.

Measurement of the Auger parameter and Wagner plot for uranium compounds

In this study, the photoemission from the U 4f7/2 and 4d5/2 states and the U N6O45O45 and N67O45V x-ray excited Auger transitions were measured for a range of uranium compounds. The data are presented in Wagner plots and the Auger parameter is calculated to determine the utility of this technique in the analysis of uranium materials. It was demonstrated that the equal core-level shift assumption holds for uranium. It was therefore possible to quantify the relative relaxation energies, and uranium was found to have localized core-hole shielding. The position of compounds within the Wagner plot made it possible to infer information on bonding character and local electron density. The relative ionicity of the uranium compounds studied follows the trend UF4 > UO3 > U3O8 > U4O9/U3O7 ≈ UO2 > URu2Si2.