Hartmut Höchst

Electronic decoupling of an epitaxial graphene monolayer by gold intercalation

The application of graphene in electronic devices requires large-scale epitaxial growth. The presence of the substrate, however, usually reduces the charge-carrier mobility considerably. We show that it is possible to decouple the partially sp(3)-hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp(2)-hybridized graphene layer with improved electronic properties.

ZnTe: A potential interlayer to form low resistance back contacts in CdS/CdTe solar cells

We studied the structural and electronic properties of the ZnTe/CdTe(100) interface with reflection high‐energy electron diffraction and angle‐resolved synchrotron radiation photoemission spectroscopy (ARPES). ZnTe overlayers grown at 300 °C on CdTe(100) were fully strained and pseudomorphic up to ≊16 A. Beyond this coverage the ZnTe film starts to gradually relax the 6.6% in‐plane lattice strain. Complete relaxation is reached at a ZnTe coverage of ∼300 A. A valence‐band offset of ΔEv=0.00±0.05 eV was measured with ARPES at the Γ point. This propitious band lineup may allow for the use of a ZnTe intermediate layer at metal/CdTe structures to induce ohmic back contacts in CdS/CdTe heterojunction solar cells.

A photoemission determination of the band diagram of the Te/CdTe interface

Two experiments designed to assist in understanding the physics of certain back contacts on p‐type CdTe solar‐cell devices are described. In the first experiment, x‐ray photoelectron and Auger electron spectroscopies are used to show that etching CdTe in HNO3:H3PO4 results in a Te layer on the CdTe surface. In the second experiment, photoemission spectroscopy is used to explore the electronic properties of evaporated Te deposited on thin‐film, polycrystalline p‐CdTe in an effort to develop a band diagram for the Te/p‐CdTe interface. The motivation for developing the band diagram derives from previous observations that chemically etching polycrystalline p‐CdTe solar‐cell device material before application of the back contact reduces the series resistance of the device. The key results are that the evaporated Te overlayer is p type and that the valence‐band offset between Te and p‐CdTe is favorable for low‐series‐resistance contact, ΔEv=0.26±0.1 eV.

Angular resolved photoemission of InSb(001) and heteroepitaxial films of α-Sn(001)

Abstract Angular resolved ultraviolet photoemission spectra (ARUPS) of InSb(001) and of heteroepitaxial grown α-Sn(001) films are reported for the first time. Results from normal emission spectra are explained by direct transitions into a parabolic free-electron-like final states band. The experimentally obtained dispersion E ( k ) along ΓX agrees well with the non-local Pseudopotential band structure calculation by Chelikowsky and Cohen. Other band structure calculations of α-Sn exhibiting smaller dispersion of the first valence band towards the X 5 point fail to explain the photoemission data.

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...

Spin-dependent band structure, Fermi surface, and carrier lifetime of permalloy

Angle-resolved photoemission is used to determine the energy bands of permalloy (Ni0.8Fe0.2) and compare them to Ni, Co, and Cu. The energy and momentum resolution (≈0.01 eV and ≈0.01 A−1) is high enough to resolve the magnetically split bands at the Fermi level that are responsible for spin-dependent conductivity and tunneling. For the Σ1 band we find the magnetic exchange splittings δEex=0.27 eV (0.23 eV for Ni), δkex=0.16±0.02 A−1 (0.12±0.01 A−1 for Ni), the Fermi velocity vF↑=(0.22±0.02)106 m/s (0.28×106 m/s for Ni, 0.33×106 m/s for fcc Co), and the widths δk↑⩽0.11 A−1 and δk↓=0.24 A−1. Compared to Ni, permalloy features a 27% larger magnetic splitting of the Fermi surface and an extremely short mean free path of 4–8 A for minority spins.

Theoretical and experimental studies of the ZnSe/CuInSe2 heterojunction band offset

We report first‐principles band structure calculations that show that ZnSe/CuInSe2 has a significant valence band offset (VBO, ΔEv): 0.70±0.05 eV for the relaxed interface and 0.60±0.05 eV for the coherent interface. These large values demonstrate the failure of the common anion rule. This is traced to a stronger Cu,d‐Se,p level repulsion in CuInSe2 than the Zn,d‐Se,p repulsion in ZnSe. The VBO was then studied by synchrotron radiation soft x‐ray photoemission spectroscopy. ZnSe overlayers were sequentially grown in steps on n‐type CuInSe2(112) single crystals at 200 °C. In situ 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 Zn 3d core lines. Results of these measurements reveal that the VBO is ΔEv=0.70±0.15 eV, in good agreement with the first‐principles prediction.

Study of the phase transition in heteroepitaxially grown films of αSn by Raman spectroscopy

Abstract The transition in heteroepitaxially grown films of tin from the semiconducting (α) phase to the metallic (β) phase depends on the film thickness and the surface orientation. Investigations of the optical Raman phonon of α-Sn demonstrate that αSn films grown on InSb(001) surfaces show a sharp α ↔ β transition at a temperature T∗ which depends on the film thickness. For a film thickness of less than 60 nm the transformation occurs at T ∗ = 115°C , which is much higher than the bulk T∗ value of 13.2°C. Evidence for the formation of β-Sn on (110) and (111) surfaces is given from Raman and reflection high energy electron diffraction investigations.

A photoemission investigation of the SnO2/CdS interface: A front contact interface study of CdS/CdTe solar cells

A frequently used front contact in CdS/CdTe heterojunction solar cells is SnO2. We have performed a soft x‐ray synchrotron radiation photoemission investigation of the formation and thermal stability of the SnO2/CdS interface in an attempt to understand how device processing influences this interface. The most important results are that (1) the CdS and SnO2 do not interact chemically, even after annealing to 400 °C, (3) the first ∼16 A CdS deposited on polycrystalline SnO2 grows in a layer‐by‐layer mode, (3) subsequent CdS layers agglomerate, forming a topologically rough surface, (4) diffusion of Sn, Cd, and S across the interface does not occur, (5) annealing a CdS thin film grown at room temperature on SnO2 to 400 °C enhances the agglomeration, and (6) the near coincidence of the conduction‐band minimum across the interface facilitates ohmic contact between n‐type CdS and n‐type SnO2.

Electron-Phonon Coupling in Crystalline Pentacene Films

The electron-phonon (e-p) interaction in pentacene (Pn) films grown on Bi(001) was investigated using photoemission spectroscopy. The spectra reveal thermal broadening from which we determine an e-p mass enhancement factor of lambda=0.36+/-0.05 and an effective Einstein energy of omega{E}=11+/-4 meV. From omega{E} it is inferred that dominant contributions to the e-p effects observed in angle-resolved photoemission spectroscopy come from intermolecular vibrations. Based on the experimental data for lambda we extract an effective Peierls coupling value of g{eff}=0.55. The e-p coupling narrows the highest occupied molecular orbital bandwidth by 15+/-8% between 75 and 300 K.