János Volk

Band-Gap Deformation Potential and Elasticity Limit of Semiconductor Free-Standing Nanorods Characterized in Situ by Scanning Electron Microscope–Cathodoluminescence Nanospectroscopy

Modern field-effect transistors or laser diodes take advantages of band-edge structures engineered by large uniaxial strain εzz, available up to an elasticity limit at a rate of band-gap deformation potential azz (= dEg/dεzz). However, contrary to aP values under hydrostatic pressure, there is no quantitative consensus on azz values under uniaxial tensile, compressive, and bending stress. This makes band-edge engineering inefficient. Here we propose SEM-cathodoluminescence nanospectroscopy under in situ nanomanipulation (Nanoprobe-CL). An apex of a c-axis-oriented free-standing ZnO nanorod (NR) is deflected by point-loading of bending stress, where local uniaxial strain (εcc = r/R) and its gradient across a NR (dεcc/dr = R(-1)) are controlled by a NR local curvature (R(-1)). The NR elasticity limit is evaluated sequentially (εcc = 0.04) from SEM observation of a NR bending deformation cycle. An electron beam is focused on several spots crossing a bent NR, and at each spot the local Eg is evaluated from near-band-edge CL emission energy. Uniaxial acc (= dEg/dεcc) is evaluated at regulated surface depth, and the impact of R(-1) on observed acc is investigated. The acc converges with -1.7 eV to the R(-1) = 0 limit, whereas it quenches with increasing R(-1), which is attributed to free-exciton drift under transversal band-gap gradient. Surface-sensitive CL measurements suggest that a discrepancy from bulk acc = -4 eV may originate from strain relaxation at the side surface under uniaxial stress. The nanoprobe-CL technique reveals an Eg(εij) response to specific strain tensor εij (i, j = x, y, z) and strain-gradient effects on a minority carrier population, enabling simulations and strain-dependent measurements of nanodevices with various structures.

Nanoimprint for Fabrication of Highly Ordered Epitaxial ZnO Nanorods on Transparent Conductive Oxide Films

We established a process of growing large-scale highly ordered ZnO nanorod arrays on a transparent conductive oxide layer. Nanoimprint lithography was employed specifically to produce large-scale ordered ZnO nanorod arrays. The growth conditions were examined by comparing nanorod growth on different substrates, namely, aluminum-doped zinc oxide (AZO) and indium tin oxide (ITO). Structural characterization revealed that the crystallinity and flatness of the ZnO seed layer were key factors in improving uniformity as regards the diameter, height, and orientation of the ZnO nanorods.

Improving the performance of inorganic-organic hybrid photovoltaic devices by uniform ordering of ZnO nanorods and near-atmospheric pressure nitrogen plasma treatment

We investigated the performance of hybrid photovoltaic devices composed of ZnO and poly(3-hexylthiophene) (P3HT). The uniform ordering of ZnO nanorods (NRs) and nitrogen plasma treatment at near-atmospheric pressure offer advantages in modifying the ZnO NR surface. Uniform ordering of the ZnO NRs promoted the effective infiltration of P3HT, increasing the donor–acceptor interface area, which is directly related to short-circuit current density (JSC). Near-atmospheric pressure treatment compensated carriers to form a highly resistant interlayer at the ZnO surface, which reduced carrier recombination and, as a result, increased the open circuit voltage (VOC). Combining these two approaches achieved five-fold increase in JSC compared to that of the planar heterojunction, while the VOC was increased up to 0.71 V.

Arbitrary cross-section SEM-cathodoluminescence imaging of growth sectors and local carrier concentrations within micro-sampled semiconductor nanorods.

Future one-dimensional electronics require single-crystalline semiconductor free-standing nanorods grown with uniform electrical properties. However, this is currently unrealistic as each crystallographic plane of a nanorod grows at unique incorporation rates of environmental dopants, which forms axial and lateral growth sectors with different carrier concentrations. Here we propose a series of techniques that micro-sample a free-standing nanorod of interest, fabricate its arbitrary cross-sections by controlling focused ion beam incidence orientation, and visualize its internal carrier concentration map. ZnO nanorods are grown by selective area homoepitaxy in precursor aqueous solution, each of which has a (0001):+c top-plane and six {1–100}:m side-planes. Near-band-edge cathodoluminescence nanospectroscopy evaluates carrier concentration map within a nanorod at high spatial resolution (60 nm) and high sensitivity. It also visualizes +c and m growth sectors at arbitrary nanorod cross-section and history of local transient growth events within each growth sector. Our technique paves the way for well-defined bottom-up nanoelectronics.

Band gaps in photonic crystals with dispersion

Purpose – The paper presents the band gap computation in one‐ and two‐dimensional photonic crystals built up from porous silicon. The frequency dispersion of the dielectric materials is taken into account.Design/methodology/approach – The behavior of the light in a photonic crystal can be well described by the Maxwell equations. The finite difference time domain (FDTD) method is applied to determine the band structure. The frequency dependence of the dielectric constant is taken into account by a sum of second‐order Lorenz poles. The material parameters are determined applying a conjugate gradient‐based minimization procedure. Passing a light pulse of Gaussian distribution through the photonic crystal and analyzing the transmitted wave can explore the photonic bands.Findings – The realized simulations and visualizations can lead to a much better understanding of the behavior of electromagnetic waves in dispersive photonic crystals, and can make possible to set up experimental conditions properly. The obta...

Picosecond reflectance recovery dynamics of porous silicon multilayer

Light-induced reflectance dynamics of a porous silicon multilayered structure via the picosecond pump-probe technique is investigated. Fast recovery dynamics is inspected for a range of excitation energy densities (2.4-24 mJ/cm2), incident angles (0°-25°), and polarizations. Modeling performed using the transfer matrix method revealed the photonic origin of the phenomena observed. The spectral shift of the reflectance spectrum, i.e., photonic bandgap, responsible for the positive signal originates from the change of photoinduced carrier concentration with linear and nonlinear character for different porosity layers. The influence of temperature on the refractive index in this thermally nonconductive material is taken into account.

Spiral-Shaped Piezoelectric MEMS Cantilever Array for Fully Implantable Hearing Systems

Fully implantable, self-powered hearing aids with no external unit could significantly increase the life quality of patients suffering severe hearing loss. This highly demanding concept, however, requires a strongly miniaturized device which is fully implantable in the middle/inner ear and includes the following components: frequency selective microphone or accelerometer, energy harvesting device, speech processor, and cochlear multielectrode. Here we demonstrate a low volume, piezoelectric micro-electromechanical system (MEMS) cantilever array which is sensitive, even in the lower part of the voice frequency range (300–700 Hz). The test array consisting of 16 cantilevers has been fabricated by standard bulk micromachining using a Si-on-Insulator (SOI) wafer and aluminum nitride (AlN) as a complementary metal-oxide-semiconductor (CMOS) and biocompatible piezoelectric material. The low frequency and low device footprint are ensured by Archimedean spiral geometry and Si seismic mass. Experimentally detected resonance frequencies were validated by an analytical model. The generated open circuit voltage (3–10 mV) is sufficient for the direct analog conversion of the signals for cochlear multielectrode implants.

Enhanced pressure response in ZnO nanorods due to spontaneous polarization charge

We present measurements of the induced charge flow when a compressive force is applied by contacting to a ZnO nanowire (NW). The measured charge transfer from the NW is over 104 times larger than expected from the strain-induced piezoelectric response, and is comparable in magnitude to the spontaneous polarization charge, associated with an ideal ZnO NW. A model is presented that compares the total energy of an isolated NW and external capacitor with the total energy when the capacitor and NW form a closed circuit. The analysis shows that it is possible, for realistic values of surface defect creation energy, to have spontaneous polarization charge transferred from a NW to an external capacitor when a circuit is completed between them. We propose that it is possible to use spontaneous polarization charge to get a significantly enhanced response in ZnO-based NW pressure sensors.

Engineered ZnO nanowire arrays using different nanopatterning techniques

The impact of various masking patterns and template layers on the wet chemically grown vertical ZnO nanowire arrays was investigated. The nanowires/nanorods were seeded at nucleation windows which were patterned in a mask layer using various techniques such as electron beam lithography, nanosphere photolithography, and atomic force microscope type nanolithography. The compared ZnO templates included single crystals, epitaxial layer, and textured polycrystalline films. Scanning electron microscopy revealed that the alignment and crystal orientation of the nanowires were dictated by the underlying seed layer, while their geometry can be tuned by the parameters of the certain nanopatterning technique and of the wet chemical process. The comparison of the alternative nanolithography techniques showed that using direct writing methods the diameter of the ordered ZnO nanowires can be as low as 30-40 nm at a density of 100- 1000 NW/μm2 in a very limited area (10 μm2-1 mm2). Nanosphere photolithography assisted growth, on the other hand, favors thicker nanopillars (~400 nm) and enables large-area, low-cost patterning (1-100 cm2). These alternative lowtemperature fabrication routes can be used for different novel optoelectronic devices, such as nanorod based ultraviolet photodiode, light emitting device, and waveguide laser.

Design of UV-crosslinked polymeric thin layers for encapsulation of piezoelectric ZnO nanowires for pressure-based fingerprint sensors

The demand for biometric identification, and more particularly fingerprint recognition, has significantly increased in recent years in domains such as national security, controlled access to health care, banking and leisure. Typical resolution of current fingerprint sensors constitutes 500 dpi, which conforms to the FBI standard for the detection of level 1 (pattern) and level 2 (minutiae) features. However, at least 1000 dpi is required to extract the tiniest level 3 features such as shape of pores and ridge edges. To address an increasing need for the highest reliability in fingerprint identification, this work presents the elaboration of a specific encapsulation polymeric layer for a new design of a pressure sensor which is expected to provide higher resolution and minimal volume occupation compared to already existing setups. The sensor comprises a periodic array of individually contacted piezoelectric ZnO nanowires (NWs), which generate piezopotentials when deformed (each electrically addressable NW constitutes a pressure-sensitive pixel). Fragile NWs cannot undergo direct contact with the finger, therefore a polymeric encapsulation layer is required to simultaneously provide physical protection and to transfer the force from the finger to the NWs. To ensure sensor robustness, the polymeric layer must also exhibit appropriate chemical inertness, and water- and oil-repellency. To achieve this aim, novel formulations of UV-crosslinkable polymeric materials were developed, prepared and deposited as thin layers on the NWs using spin-coating by following the recommendations derived from numerical simulations. An ideal balance between polymer formulation processability and final thin layers characteristics (Youngs modulus, thickness) was identified. The resulting encapsulation layer was proved to properly protect the piezoelectric ZnO nanowires from breaking under pressure forces and from being contaminated with external impurities while enabling successful collection of electrical signals in a bottom–bottom contacted NW configuration.