Bruce M. Clemens

Engineering light absorption in semiconductor nanowire devices

The use of quantum and photon confinement has enabled a true revolution in the development of high-performance semiconductor materials and devices. Harnessing these powerful physical effects relies on an ability to design and fashion structures at length scales comparable to the wavelength of electrons (approximately 1 nm) or photons (approximately 1 microm). Unfortunately, many practical optoelectronic devices exhibit intermediate sizes where resonant enhancement effects seem to be insignificant. Here, we show that leaky-mode resonances, which can gently confine light within subwavelength, high-refractive-index semiconductor nanostructures, are ideally suited to enhance and spectrally engineer light absorption in this important size regime. This is illustrated with a series of individual germanium nanowire photodetectors. This notion, together with the ever-increasing control over nanostructure synthesis opens up tremendous opportunities for the realization of a wide range of high-performance, nanowire-based optoelectronic devices, including solar cells, photodetectors, optical modulators and light sources.

CRYSTALLITE COALESCENCE : A MECHANISM FOR INTRINSIC TENSILE STRESSES IN THIN FILMS

We examined the stress associated with crystallite coalescence during the initial stages of growth in thin polycrystalline films with island growth morphology. As growing crystallites contacted each other at their bases, the side-walls zipped together until a balance was reached between the energy associated with eliminating surface area, creating a grain boundary and straining the film. Our estimate for the resulting strain depends only on interfacial free energies, elastic properties, and grain size and predicts large tensile stresses in agreement with experimental results. We also discuss possible stress relaxation mechanisms that can occur during film growth subsequent to the coalescence event.

Plasmon Enhanced Solar-to-Fuel Energy Conversion

Future generations of photoelectrodes for solar fuel generation must employ inexpensive, earth-abundant absorber materials in order to provide a large-scale source of clean energy. These materials tend to have poor electrical transport properties and exhibit carrier diffusion lengths which are significantly shorter than the absorption depth of light. As a result, many photoexcited carriers are generated too far from a reactive surface and recombine instead of participating in solar-to-fuel conversion. We demonstrate that plasmonic resonances in metallic nanostructures and multilayer interference effects can be engineered to strongly concentrate sunlight close to the electrode/liquid interface, precisely where the relevant reactions take place. On comparison of spectral features in the enhanced photocurrent spectra to full-field electromagnetic simulations, the contribution of surface plasmon excitations is verified. These results open the door to the optimization of a wide variety of photochemical processes by leveraging the rapid advances in the field of plasmonics.

Structure and Strength of Multilayers

Nanometer-scale multilayer materials exhibit a wealth of interesting structural and mechanical property behaviors. Physical-vapor-deposition technology allows almost unlimited freedom to choose among elements, alloys, and Compounds as layering constituents and to design and produce materials with compositional and structural periodicities approaching the atomic Scale. These materials have tremendous interface area density, approaching 10 6 mm/mm 3 , so that a Square centimeter area of a one-micron-thick multilayer film with a bilayer period of 2 nm has an interface area of roughly 1,000 cm 2 . Hence interfacial effects can dominate multilayer structure and properties leading to unusually large strains and frequently stabilization of metastable structures. The atomic-scale layering of different materials also leads to very high hardnesses and good wear resistance. These materials are a test-bed for examination of the fundamental aspects of phase stability and for exploring mechanical strengthening mechanisms. They are also becoming increasingly interesting for applications such as hard coatings, x-ray optical elements, in microelectromechanical Systems (MEMS), and in magnetic recording media and heads. In this article, we review some of the interesting structures and mechanical properties that have been observed in nanometer-scale artificial multilayer structures. Superlattice thin films are readily deposited by vapor-phase techniques such as sputter deposition, evaporation, and chemical vapor deposition, as well as by electrochemical deposition. Superlattice deposition Systems are similar to conventional film deposition Systems, except for the provision to modulate the fluxes and thereby produce alternating super-lattice layers.

Resonant Germanium Nanoantenna Photodetectors

On-chip optical interconnection is considered as a substitute for conventional electrical interconnects as microelectronic circuitry continues to shrink in size. Central to this effort is the development of ultracompact, silicon-compatible, and functional optoelectronic devices. Photodetectors play a key role as interfaces between photonics and electronics but are plagued by a fundamental efficiency-speed trade-off. Moreover, engineering of desired wavelength and polarization sensitivities typically requires construction of space-consuming components. Here, we demonstrate how to overcome these limitations in a nanoscale metal-semiconductor-metal germanium photodetector for the optical communications band. The detector capitalizes on antenna effects to dramatically enhance the photoresponse (>25-fold) and to enable wavelength and polarization selectivity. The electrical design featuring asymmetric metallic contacts also enables ultralow dark currents (approximately 20 pA), low power consumption, and high-speed operation (>100 GHz). The presented high-performance photodetection scheme represents a significant step toward realizing integrated on-chip communication and manifests a new paradigm for developing miniaturized optoelectronics components.

Investigating the Role of Grain Boundaries in CZTS and CZTSSe Thin Film Solar Cells with Scanning Probe Microscopy

The holy grail of solar cell technology would be a low-cost, high efficiency solar cell composed of earth-abundant, non-toxic elements. A thin film Cu2ZnSnS4 (CZTS) solar cell is proposed as the technology to achieve this holy grail and thus it has received much attention in recent years. CZTS’s earth-abundant and non-toxic elemental composition makes it an ideal candidate to replace Cu(In,Ga)Se2 (CIGS) and CdTe solar cells which face material scarcity and toxicity issues. CZTS is reported to have a bandgap of 1.32–1.85 eV and a band edge absorption coefficient of above 104 cm−1 [1–3] which makes it highly attractive as a single junction solar cell material. Efficiencies of up to 8.4% and 10.1% have been achieved for CZTS[4] and Cu2ZnSn(S,Se)4 (CZTSSe)[5] solar cells, respectively. The question that remains is whether these solar cells are able to reach higher efficiencies that are comparable to or higher than CIGS and CdTe solar cells. To answer this question, we need to understand what enables CIGS and CdTe solar cells to achieve their high efficiencies and whether CZTS and CZTSSe have the same beneficial property. This is the aim of this paper. Polycrystalline CIGS and CdTe solar cells have achieved 20.3% and 16.7% efficiencies, respectively,[6] while their singlecrystal counterparts have surprisingly only achieved 12% and 13.5%, respectively.[7,8] Typically, semiconductor materials for optoelectronic or transistor applications perform worse in their polycrystalline form because of carrier recombination at the grain boundaries (GBs). However, for polycrystalline CIGS and CdTe solar cells, this is not the case. This begs the question of why these materials perform better in the polycrystalline form rather than their single-crystal form. Studies have identified GBs as the source of high efficiency for polycrystalline CIGS and CdTe solar cells.[9–13] These GBs enhance minority carrier collection and provide a current pathway for minority carriers to reach the n-type CdS and ZnO layers and be collected.[10] Minority carrier collection is enhanced by the presence of an electric field in the vicinity of the GBs, which increases charge separation. This electric field

Epitaxial PtFe(001) thin films on MgO(001) with perpendicular magnetic anisotropy

We report a technique for producing the ordered PtFe intermetallic compound with perpendicular magnetic anisotropy and a preferred c‐axis orientation perpendicular to the film plane. PtFe alloys possess high magneto‐optic Kerr rotations and magnetizations, suggesting them as likely candidates for magneto‐optic and perpendicular magnetic recording.

Stress Determination in Textured Thin Films Using X-Ray Diffraction

Thin film stresses are important in many areas of technology. In the semiconductor industry, metal interconnects are prone to stress voiding and hillock formation. Stresses in passivation layers can lead to excessive substrate curvature which can cause alignment difficulty in subsequent lithographic processing. In other thin film applications, stresses can cause peeling from mechanical failure at the film-substrate interface. Beyond these issues of reliability, stress and the resulting strain can be used to tune the properties of thin film materials. For instance, strain, coupled with the magnetostrictive effect, can be utilized to induce the preferred magnetization direction. Also, epitaxial strains can be used to adjust the bandgap of semiconductors. Finally, the anomalous mechanical properties of multilayered materials are thought to be partially due to the extreme strain states in the constituents of these materials. To fully optimize thin film performance, a fundamental understanding of the causes and effects of thin film stress is needed. These studies in turn rely on detailed characterization of the stress and strain state of thin films. X-ray diffraction and the elastic response of materials provide a powerful method for determining stresses. Stresses alter the spacing of crystallographic planes in crystals by amounts easily measured by x-ray diffraction. Each set of crystal planes can act as an in-situ strain gauge, which can be probed by x-ray diffraction in the appropriate geometry. Hence it is not surprising that x-ray diffraction is one of the most widely used techniques for determining stress and strain in materials. (For reviews of this topic, see References 5–7.) This article is a tutorial on the use of x-ray diffraction to extract the stress state and the unstrained lattice parameter from thin films. We present a handbook of useful results that can be widely applied and should be mastered by anyone seriously interested in stresses in crystalline thin films with a crystallographic growth texture.

Understanding Interactions between Manganese Oxide and Gold That Lead to Enhanced Activity for Electrocatalytic Water Oxidation

To develop active nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER), a limiting reaction in several emerging renewable energy technologies, a deeper understanding of the activity of the first row transition metal oxides is needed. Previous studies of these catalysts have reported conflicting results on the influence of noble metal supports on the OER activity of the transition metal oxides. Our study aims to clarify the interactions between a transition metal oxide catalyst and its metal support in turning over this reaction. To achieve this goal, we examine a catalytic system comprising nanoparticulate Au, a common electrocatalytic support, and nanoparticulate MnOx, a promising OER catalyst. We conclusively demonstrate that adding Au to MnOx significantly enhances OER activity relative to MnOx in the absence of Au, producing an order of magnitude higher turnover frequency (TOF) than the TOF of the best pure MnOx catalysts reported to date. We also provide evidence that it is a local rather than bulk interaction between Au and MnOx that leads to the observed enhancement in the OER activity. Engineering improvements in nonprecious metal-based catalysts by the addition of Au or other noble metals could still represent a scalable catalyst as even trace amounts of Au are shown to lead a significant enhancement in the OER activity of MnOx.

Epitaxial Pt(001), Pt(110), and Pt(111) films on MgO(001), MgO(110), MgO(111), and Al2O3(0001)

We have grown epitaxial Pt films, both in oxidizing and nonoxidizing environments, using planar magnetron sputtering onto heated substrates. The out‐of‐plane orientation relationships we report are Pt(001)∥MgO(001), Pt(110)∥MgO(110), Pt(111)∥MgO(111), and Pt(111)∥Al2O3(0001). We also report a seeded epitaxy technique using Fe for lower temperature epitaxial growth of Pt(001)∥MgO(001).