D. Rioux

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.

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.

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.

Photoemission study on the formation of Mo contacts to CuInSe2

Synchrotron radiation soft‐x‐ray photoemission spectroscopy was used to investigate the development of the electronic structure at the Mo/CuInSe2 interface. Mo overlayers were e‐beam deposited in steps on single‐crystal n‐type CuInSe2 at ambient temperature. 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, Se 3d, and Mo 4d core lines. Photoemission measurements on the valence‐band and core lines were also obtained after annealing. The results were used to correlate the interface chemistry with the electronic structure at this interface and to directly determine the maximum possible Schottky barrier height φb to be ≤0.2 eV at the Mo/CuInSe2 junction before annealing, thus showing that this contact is essentially ohmic.

Sb as an indium-absorptive barrier in ZnSe/InP(100) heterostructures

Antimony layers, sandwiched between a ZnSe/InP(100) heterostructure interface, can be used as a barrier preventing outdiffusion of In into ZnSe epilayers. Through core level photoemission spectroscopy and reflection high‐energy electron diffraction investigations, we found that the Sb layer must be annealed in order to prevent In outdiffusion. Barrier action occurs by means of InSb formation. A single pseudomorphic monolayer was insufficient to prevent In from outdiffusing. A thicker Sb layer greatly reduced In outdiffusion but through misfit dislocation the initially strained InSb film relaxed to its bulk lattice constant and did not allow for subsequent pseudomorphic epitaxy of strained ZnSe overlayers.

Understanding and Controlling Semiconductor Interfaces - Spectroscopy and Spectromicroscopy

A series of events has revolutionized the research on semiconductor interfaces. One of the most interesting is the discovery that heterojunction band lineups can be modified by manipulating the interface on an atomic scale. Specifically, it has been possible to modify heterojunction valence band discontinuities by inserting ultrathin intralayers and to create homojunction band discontinuities by inserting double atomic layers. These successes also have an impact on the fundamental understanding of the interface parameters. In parallel, progress has been made in the understanding of Schottky barriers by studying silicon and germanium films on metal substrates; these experiments have clarified the relative role of the local chemistry and morphology in determining the barrier height. Additional, exciting possibilities are opened up by the advent of lateral resolution in some of the techniques, like photoemission, that are most extensively used in semiconductor interface research: this makes it possible to study the lateral dependence of the interface parameters, a problem that has been largely ignored in the past.

Soft x‐ray photoemission investigation on the effect of In doping in CdS/CuInSe2 heterojunction formation

Synchrotron radiation soft x‐ray photoemission spectroscopy was used to investigate the development of the electronic structure at the CdS(In)/CuInSe2 heterojunction interface. In‐doped CdS overlayers were deposited in steps on single‐crystal n‐type CuInSe2 at 250 °C. Results indicate that the CdS(In) grows in registry with the substrate, initially in a two‐dimensional growth mode followed by three‐dimensional island growth as is corroborated by reflection high‐energy electron diffraction analysis. Photoemission measurements were acquired after each growth in order to observe changes in the valence‐band electronic structure. The results were used to correlate the interface chemistry with the electronic structure at these interfaces and to directly determine the CdS(In)/CuInSe2 heterojunction valence‐band discontinuity and the consequent heterojunction band diagram as a function of In dopant concentration. We measured a valence‐band offset ΔEv=0.3 eV, independent of In doping.

Search for the 1+ State of Copper in the Electron Superconductor Nd2-Xcexcuo4

We used the resonant photoemission technique to search for Cu1+ resonances in the spectra of superconducting and non‐superconducting compounds in the Nd2−xCexCuO4 family. No evidence for such resonances was found. The possible implications of this somewhat suprising result are discussed.