Richard Haight

Band alignment at the Cu2ZnSn(SxSe1−x)4/CdS interface

Energy band alignments between CdS and Cu2ZnSn(SxSe1−x)4 (CZTSSe) grown via solution-based and vacuum-based deposition routes were studied as a function of the [S]/[S+Se] ratio with femtosecond laser ultraviolet photoelectron spectroscopy, photoluminescence, medium energy ion scattering, and secondary ion mass spectrometry. Band bending in the underlying CZTSSe layer was measured via pump/probe photovoltage shifts of the photoelectron spectra and offsets were determined with photoemission under flat band conditions. Increasing the S content of the CZTSSe films produces a valence edge shift to higher binding energy and increases the CZTSSe band gap. In all cases, the CdS conduction band offsets were spikes.

Electron dynamics at surfaces

Abstract The physics of electron dynamics at surfaces and interfaces encompasses a rich variety of processes ranging from electron scattering and diffusion to electron-hole recombination and surface state and defect trapping. This review includes an overview of many of these important interactions and a detailed discussion of the experimental approaches used to study them. Particular attention is given to the surface-sensitive technique of angle-resolved photoemission carried out with nanosecond, picosecond and femtosecond laser sources which have revealed new insights into the modes of dynamical electronic properties at surfaces. A range of experiments carried out on semiconductor and metal surfaces will be described.

Cd-free buffer layer materials on Cu2ZnSn(SxSe1−x)4: Band alignments with ZnO, ZnS, and In2S3

The heterojunctions formed between Cu2ZnSn(SxSe1−x)4 (CZTSSe) and three Cd-free n-type buffers, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission and photovoltage spectroscopy. The electronic properties including the Fermi level location at the interface, band bending in the CZTSSe substrate, and valence and conduction band offsets were determined and correlated with device properties. We also describe a method for determining the band bending in the buffer layer and demonstrate this for the In2S3/CZTSSe system. The chemical bath deposited In2S3 buffer is found to have near optimal conduction band offset (0.15 eV), enabling the demonstration of Cd-free In2S3/CZTSSe solar cells with 7.6% power conversion efficiency.

Electronic and elemental properties of the Cu2ZnSn(S,Se)4 surface and grain boundaries

X-ray and femtosecond UV photoelectron spectroscopy, secondary ion mass spectrometry and photoluminescence imaging were used to investigate the electronic and elemental properties of the CZTS,Se surface and its oxides. Oxide removal reveals a very Cu poor and Zn rich surface relative to bulk composition. O and Na are observed at the surface and throughout the bulk. Upward bending of the valence bands indicates the presence of negative charge in the surface region and the Fermi level is found near the band gap center. The presence of point defects and the impact of these findings on grain boundary properties will be described.

Growth system, structure, and doping of aluminum-seeded epitaxial silicon nanowires.

We have examined the formation of silicon nanowires grown by self-assembly from Si substrates with thin aluminum films. Postgrowth and in situ investigations using various Al deposition and annealing conditions suggest that nanowire growth takes place with a supercooled liquid droplet (i.e., the vapor-liquid-solid system), even though the growth temperatures are below the bulk Al/Si eutectic temperature. Wire morphology as a function of processing conditions is also described. It is shown that when Al environmental exposure is prevented before wire growth a wide process window for wire formation can be achieved. Under optimum growth conditions, it is possible to produce excellent crystal quality nanowires with rapid growth rates, high surface densities, low diameter dispersion, and controlled tapering. Photoelectron spectroscopy measurements indicate that the use of Al leads to active doping levels that depend on the growth temperature in as-grown nanowires and increase when annealed. We suggest that these structural and electronic properties will be relevant to photovoltaic and other applications, where the more common use of Au is believed to be detrimental to performance.

Band alignment of SnS/Zn(O,S) heterojunctions in SnS thin film solar cells

Band alignment is critical to the performance of heterojunction thin film solar cells. In this letter, we report band alignment studies of SnS/Zn(O,S) heterojunctions with various compositions of Zn(O,S). Valence band offsets (VBOs) are measured by femtosecond laser pump/probe ultraviolet photoelectron spectroscopy (fs-UPS) from which conduction band offsets (CBOs) are calculated by combining with band gaps obtained by optical transmission/reflection measurements. The SnS/Zn(O,S) heterojunctions with S/Zn ratios of 0.37 and 0.50 have desirable small positive CBOs, while a ratio of 0.64 produces an undesirable large positive CBO. The results are consistent with the device performance of SnS/Zn(O,S) solar cells.

Role of oxygen vacancies in V FB /V t stability of pFET metals on HfO 2

We demonstrate experimentally that the flatband/threshold voltages (V/sub FB//V/sub t/) of pFET metal gates are strongly dependent on the post-deposition annealing conditions of the gate stacks. By varying the temperature and the O/sub 2/ partial pressure during post-deposition N/sub 2//O/sub 2/ and/or forming gas annealing (FGA) with Re, Ru and Pt, the gate stack V/sub FB/ can change by as much as 750 mV. However, using Re as an example, it is shown that conditions can be optimized and V/sub FB//V/sub t/-tuning for pFETs can be achieved for aggressively scaled stacks. It is proposed that charge transfer from oxygen vacancies to the gate electrode, possible only for high workfunction metal gates, leads to the formation of a dipole layer near the gate which can shift V/sub FB//V/sub t/. The results indicate that V/sub FB//V/sub t/ control remains a formidable processing challenge with high workfunction metals on HfO/sub 2/.

Unoccupied molecular orbital states of tris (8-hydroxy quinoline) aluminum: Observation and dynamics

Electrons photoexcited into the normally unoccupied states of vacuum deposited thin films of tris (8-hydroxy quinoline) aluminum (Alq) have been studied with excite-probe harmonic laser photoemission. Both valence and transiently excited empty states are observed. A surface recombination velocity of 75±30 cm/s has been determined.

Cr‐ and Cu‐polyimide interface: Chemistry and structure

High‐resolution x‐ray and ultraviolet photoemission spectroscopies have been applied to study the initial formation of the Cr‐ and Cu‐polyimide interfaces. A model for the growth modes of these two interfaces is proposed based on comparison of experimental results with trends derived from quantum‐chemical calculations.

Diffusion of metals into organic films

The temporal evolution of Ga and Ca vacuum deposited onto thin films of tris (8-hydroxy quinoline) aluminum (Alq) was studied with valence and atomic core level photoelectron spectroscopy. It was found that Ga, which exhibits weak reactivity with the Alq diffused into the organic at room temperature. Conversely Ca, a significantly more reactive metal, shows strong interaction with the organic surface. Effects of the initial morphology of the deposited materials on diffusivity will be described.