M. K. Kelly

Photoemission studies of atomic redistribution at gold–silicon and aluminum–silicon interfaces

We have used soft x‐ray photoemission spectroscopy (SXPS) to monitor the rearrangement of Si and metal atoms during the initial stages of Au or Al interface formation with UHV‐cleaved (111) or (100) Si surfaces. From Si 2p core level spectra as a function of metal overlayer thickness and as a function of incident photon energy, we obtain evidence for strong Si bond changes at submonolayer Au coverages but only weak Al–Si interactions. Marker experiments reveal that Au diffuses into Si (Si diffuses into Al) with the first few deposited metal overlayers. We examine our new information on the interface evolution at room and elevated temperatures in relation to the corresponding bulk phase diagrams.

Absence of Fermi level pinning at metal‐InxGa1−xAs(100) interfaces

Soft x‐ray photoemission spectroscopy measurements of clean, ordered InxGa1−xAs (100) surfaces with Au, In, Ge, or Al overlayers reveal an unpinned Fermi level across the entire In alloy series. The Fermi level stabilization energies depend strongly on the particular metal and differ dramatically from those of air‐exposed interfaces. This wide range of Schottky barrier height for III‐V compounds is best accounted for by a chemically induced modification in metal‐alloy composition.

Fermi level pinning and chemical interactions at metal–InxGa1−xAs(100) interfaces

Soft x‐ray photoemission spectroscopy (SXPS) measurements of metals on clean, ordered InxGa1−xAs(100) surfaces reveal that Fermi level stabilization energies depend strongly on the particular metal, i.e., the Fermi level is not pinned. For InxGa1−xAs, x>0, the range of Fermi level movement is comparable to or greater than the semiconductor band gap. For the same metal on different alloys, we observe regular trends in stabilization energies. The trend for Au is strikingly different from previous, air‐exposed values. Our results challenge Schottky barrier models based on simple native defects, metal‐induced gap states, or the ‘‘common‐anion’’ rule. Observed variations in semiconductor outdiffusion provide a chemically‐modified interface work function model which accounts for the data across the alloy series.

Reduction of silicon‐aluminum interdiffusion by improved semiconductor surface ordering

Aluminum overlayers on highly ordered single‐crystal silicon (100) and (111) surfaces in ultrahigh vacuum exhibit characteristic interface widths less than tens of angstroms at room temperature and hundreds of angstroms at 400 °C—orders of magnitude more abrupt than conventionally prepared Al‐Si contacts. We demonstrate that surface disorder plays a critical role in promoting Si diffusion into the Al overlayer.

Reconstruction of Aluminum and Indium Overlayers on Si(111) - a Systematic Study with High-Resolution Electron-Energy Loss Spectroscopy and Low-Energy Electron-Diffraction

We describe a detailed study of the different reconstructions obtained by depositing Al or In overlayers on Si(111)2×1 and Si(111)7×7 substrates, and of the reconstructions obtained by thermal annealing of these overlayers. We correlate the information obtained by low‐energy electron diffraction (LEED) with high‐resolution electron energy loss spectra (HREELS).

Near‐ideal Schottky barrier formation at metal‐GaP interfaces

Soft x‐ray photoemission measurements of ultrahigh‐vacuum‐cleaved GaP (110) surfaces with In, Al, Ge, Cu, and Au overlayers reveal Fermi level stabilization over a wide energy range and a near‐ideal correlation between Schottky barrier height and metal work function. Coupled with recent findings for InAs (110) and InxGa1−xAs (100) (x>0) surfaces, these results demonstrate that Fermi level pinning in a narrow energy range is not representative of metal/III‐V compound semiconductor interfaces.

Unpinned Schottky-Barrier Formation at Metal-Gap Interfaces - a Representative Iii-V Compound Interface

Metal–semiconductor interfaces obtained by deposition of In, Al, Cu, Ge, and Au onto ultrahigh‐vacuum cleaved GaP surfaces have been studied by photoemission with synchrotron radiation. The results indicate a wide range of Schottky barrier height. A plot of the Schottky barrier values versus metal work function reveals an almost ideal Schottky‐like behavior. Among current alternative models of metal–semiconductor interfaces, only the effective‐work‐function model is compatible with the experimental data. The GaP data plus the absence of Fermi level pinning recently observed for In‐based semiconductors indicate that GaP may be more representative than GaAs of metal/III–V compound interfaces in general.

Direct confirmation of the conduction‐band lineup in the CuInSe2‐CdS heterojunction solar cell

We applied to the CuInSe2‐CdS system the Katnani–Margaritondo rule for estimating heterojunction‐band discontinuities from photoemission measurements. We found that the CuInSe2 conduction‐band edge is 0.2 eV above the CdS conduction‐band edge. This band‐edge lineup explains the efficiency of p‐CuInSe2/n‐CdS solar cells.

Thiophene on Si(111)2×1: Synchrotron radiation study of a desulfurization process

Abstract The adsorption and desulfurization of thiophene on cleaved silicon was studied at different temperatures. For substrate temperatures of 60–85 K, we found the co-existence of two different adsorption states at low exposures, which yield to a condensed thiophene multilayer at high exposures. For room-temperature substrates, we observed a desulfurization process. The process is probably followed by further fragmentation, and the fragmentation path depends on the substrate preparation process.

Interfacial Deep-Level Formation and Its Effect on Band Bending at Metal Cdte Interfaces

We present soft x‐ray photoemission spectra (SXPS) of cleaved (110) CdTe interfaces with Au and In measured as a function of coverage and pulsed laser annealing. These spectra show a wide range of Fermi‐level positions determined by the degree of interdiffusion or reaction at the interface. Photoluminescence (PL) spectra of the CdTe surface versus the processed Au/CdTe interface reveal that the metallization and processing produce large increases in the luminescence intensity from deep‐level defects. The energies of the intensified PL transitions correlate with the Fermi‐level positions determined from SXPS, suggesting that bulklike defects, enhanced in concentration at the interface by metallization and processing, determine the Fermi‐level position.