Erin R. Cleveland

Atomic Layer Deposition of Al2O3 on GaSb Using In Situ Hydrogen Plasma Exposure

In this report, we study the effectiveness of hydrogen plasma surface treatments for improving the electrical properties of GaSb/Al2O3 interfaces. Prior to atomic layer deposition of an Al2O3 dielectric, p-GaSb surfaces were exposed to hydrogen plasmas in situ, with varying plasma powers, exposure times, and substrate temperatures. Good electrical interfaces, as indicated by capacitance-voltage measurements, were obtained using higher plasma powers, longer exposure times, and increasing substrate temperatures up to 250 °C. X-ray photoelectron spectroscopy reveals that the most effective treatments result in decreased SbOx, decreased Sb, and increased GaOx content at the interface. This in situ hydrogen plasma surface preparation improves the semiconductor/insulator electrical interface without the use of wet chemical pretreatments and is a promising approach for enhancing the performance of Sb-based devices.

Physical properties of nanometer graphene oxide films partially and fully reduced by annealing in ultra-high vacuum

The properties of reduced graphene oxide (GO) are reported from a non-chemical reduction method. Ultra-high vacuum annealing of GO films in the thickness of 1–80 nm was studied by XPS, AFM, UV-Vis-NIR, Raman, and TEM to observe the controlled removal of oxygen. We observed the loss of hydroxyl (C-OH) at low temperatures (<600 °C) followed by the complete loss of carbonyls (C = O) and epoxy (C-O-C) species by 1200 °C. As oxygen was removed, we observed a decrease in the layer spacing between the GO sheets and a concurrent decrease in the film resistance. While the Raman spectroscopy showed no change with reduction, indicating no change in the overall defect density or the general structure of the GO, the transmission spectra showed a shift in the transmission minimum from 245 nm to 260 nm, and a total decrease in transmission above 800 nm occurs as the films visibly darken. TEM indicated that there is turbostratic stacking of the graphene layers as the reduction occurs, leading us to conclude that at a cer...

Integration of atomic layer deposited high-k dielectrics on GaSb via hydrogen plasma exposure

In this letter we report the efficacy of a hydrogen plasma pretreatment for integrating atomic layer deposited (ALD) high-k dielectric stacks with device-quality p-type GaSb(001) epitaxial layers. Molecular beam eptiaxy-grown GaSb surfaces were subjected to a 30 minute H2/Ar plasma treatment and subsequently removed to air. High-k HfO2 and Al2O3/HfO2 bilayer insulating films were then deposited via ALD and samples were processed into standard metal-oxide-semiconductor (MOS) capacitors. The quality of the semiconductor/dielectric interface was probed by current-voltage and variable-frequency admittance measurements. Measurement results indicate that the H2-plamsa pretreatment leads to a low density of interface states nearly independent of the deposited dielectric material, suggesting that pre-deposition H2-plasma exposure, coupled with ALD of high-k dielectrics, may provide an effective means for achieving high-quality GaSb MOS structures for advanced Sb-based digital and analog electronics.

Hole-selective molybdenum oxide as a full-area rear contact to crystalline p-type Si solar cells

We examine thermally evaporated MoO x films as a full-area rear contact to crystalline p-type Si solar cells for efficient hole-selective contacts. Prior to front- and rear-metallization, the implied open-circuit voltage (iV oc) is evaluated to be 646 mV with implied fill factor (iFF) of 82.5% for the tunnel SiO x /MoO x rear contacted cell structure with the passivated emitter on the textured surface, showing it is possible to achieve an implied 1-sun efficiency of 20.8%. Numerical simulation reveals that the electron affinity (χ) of the MoO x material strongly influences the performance of the MoO x contacted p-Si cell. Simulated band diagrams show that the values in χ of the MoO x layer must be sufficiently high in order to lower junction recombination, indicating that the highest efficiency of 21.1% is achievable for a high χ of 5.6 eV of MoO x films and back surface recombination velocity of <100 cm/s at p-Si/MoO x .

Electronic properties of atomic-layer-deposited high-k dielectrics on GaSb(001) with hydrogen plasma pretreatment

In this report, the authors investigate the use of H2/Ar-plasma exposure as a means for achieving high-quality electrical interfaces between p-type GaSb and atomic-layer-deposited Al2O3 dielectric films. Dry in-situ plasma treatments are shown to reduce the estimated density of interface states by over two orders of magnitude compared to a standard wet HCl-treatment, without increasing gate leakage. The chemical compositions of the natively oxidized and treated GaSb surfaces are analyzed via x-ray photoemission spectroscopy (XPS). XPS spectra indicate that the native GaSb oxide is segregated, with Sb-oxide compounds localized at the air interface. Effective H2/Ar-plasma treatments act to remove the Sb-oxide, resulting in a surface Ga-oxide layer enriched in Ga2O3.

Plasma enhanced atomic layer deposition of silver thin films for applications in plasmonics and surface enhanced Raman scattering

We have employed plasma-enhanced atomic layer deposition (PEALD) as a means to create multi-layered nanocomposite structures in order to enhance the plasmonic behavior and SERS response in the detection of benzenethiol (BZT). Ag PEALD films were deposited within nanoporous anodic aluminum oxide (AAO) templates of various pore depths, using Ag(fod)(PEt3)(fod=2,2-dimethyl-6,6,7,7,8,8,8-heptafluorooctane-3,5-dionato) as the precursor. We have examined the polycrystalline microstructure and conformality of the Ag films across the surface of an AAO template as well as into the pores, which varies significantly as thicknesses decrease. Furthermore, we investigated the plasmonic behavior of these films by performing SERS as a function of the Ag microstructure and conformality within the nanopores, using a 785 nm laser excitation and BZT as a test molecule, which forms a self-assembled monolayer on the Ag surface.

Novel optical properties of Ag films deposited by plasma enhanced atomic layer deposition (PEALD)

We have deposited Ag metal via plasma enhanced atomic layer deposition (PEALD) and we investigated the novel optical behavior of this material. We have found that as-deposited flat PEALD Ag films exhibit unexpected plasmonic properties and the plasmonic enhancement can differ markedly, depending on the microstructure of the Ag film. Electromagnetic field simulations indicate that this plasmonic behavior is due to air gaps that are an inherent property of the mosaiclike microstructure of the PEALD-grown Ag film. We show that this material is plasmonic by itself, and when combined with previously developed dielectric core nanowires, it can produce enhancements which are two orders of magnitude greater than those reported using electroless Ag or Ag produced by e-beam deposition. We have also investigated the effect of substrate on the plasmonic enhancement, as well deposition on fabric, which results in a flexible plasmonic material.

Reticulated shallow etch mesa isolation for controlling surface leakage in GaSb-based infrared detectors

Longwave infrared detectors using p-type absorbers composed of InAs-rich type-II superlattices (T2SLs) nearly always suffer from high surface currents due to carrier inversion on the etched sidewalls. Here, we demonstrate reticulated shallow etch mesa isolation (RSEMI): a structural method of reducing surface currents in longwave single-band and midwave/longwave dual-band detectors with p-type T2SL absorbers. By introducing a lateral shoulder to increase the separation between the n+ cathode and the inverted absorber surface, a substantial barrier to surface electron flow is formed. We demonstrate experimentally that the RSEMI process results in lower surface current, lower net dark current, much weaker dependence of the current on bias, and higher uniformity compared to mesas processed with a single deep etch. For the structure used, a shoulder width of 2 μm is sufficient to block surface currents.

Growth of ZnO nanowires on retroreflector microspheres and the resulting light channeling and plasmonic properties.

We have investigated the growth of ZnO nanowires on curved BaTiO3 retroreflector beads, as well as growth of ZnO nanowires on flat substrates. Results indicate that the growth of ZnO aligned nanowire arrays occurs farther away from the Zn source in the retroreflectors, while the results are opposite for the flat Si substrates. In the case of the ZnO nanowires on flat Si, the nanowires formed in nearly aligned arrays are short and significantly thicker, suggesting that the growth occurs both longitudinally and laterally in this process, which is not the case for the growth on the retroreflector beads. The SERS response of the nanowire arrays on the retroreflectors has been compared to random nanowires on flat Si substrates, and results show that the signal strength is 29 times greater in the case of the wires grown on the retroreflectors. Since one would only expect a factor of 4 enhancement due to the light reflecting properties of the retroreflector, it is believed that the enhancement in the SERS signal is due to light channeling by the aligned nanowire arrays.