H. Tanoto

Growth of GaAs on vicinal Ge surface using low-temperature migration-enhanced epitaxy

In this article, we demonstrate the influence of substrate temperature during migration-enhanced epitaxy (MEE) process of GaAs epitaxy on a vicinal surface of Ge (100), 6° offcut towards the (111) plane. It was found that the offcut surface is not the sufficient condition for suppressing the formation of antiphase domains at the GaAs∕Ge interface. Rather, it has to be complemented by low substrate temperature during the MEE process. GaAs grown at 250°C, the lowest temperature among all the samples, exhibits the smoothest surface and best structural and optical qualities, as characterized by atomic force microscopy, cross-sectional transmission electron microscopy, and low-temperature photoluminescence, respectively. At this substrate temperature, As dimers are adsorbed onto the substrate surface more readily with negligible reevaporation, ensuring complete coverage on the Ge surface with double-atomic steps. Complete coverage by As proved to be crucial in preventing the occurrence of inversion boundaries,...

Micromachined switchable metamaterial with dual resonance

We experimentally demonstrate a micromachined switchable metamaterial with dual mode resonance which is induced at THz regime under oblique incidence. Here, we explore, both theoretically and experimentally, the dynamic dual mode switching by reshaping metamaterial elements using micromachined actuators. The mode switching allows robust control over the transmission and the reflection of the metamaterial at 0.76 THz and 1.16 THz. Such switchable dual mode metamaterial promises wide applications in optical switches, tunable filters, and THz detectors.

Polarization dependent state to polarization independent state change in THz metamaterials

We experimentally demonstrated a polarization dependent state to polarization independent state change in terahertz (THz) metamaterials. This is accomplished by reconfiguring the lattice structure of metamaterials from 2-fold to 4-fold rotational symmetry by using micromachined actuators. In experiment, it measures resonance frequency shift of 25.8% and 12.1% for TE and TM polarized incidence, respectively. Furthermore, single-band to dual-band switching is also demonstrated. Compared with the previous reported tunable metamaterials, lattice reconfiguration promises not only large tuning range but also changing of polarization dependent states, which can be used in photonic devices such as sensors, optical switches, and filters.

Nano-antenna in a photoconductive photomixer for highly efficient continuous wave terahertz emission

We report highly efficient continuous-wave terahertz (THz) photoconductive antenna based photomixer employing nano-gap electrodes in the active region. The tip-to-tip nano-gap electrode structure provides strong THz field enhancement and acts as a nano-antenna to radiate the THz wave generated in the active region of the photomixer. In addition, it provides good impedance matching to the THz planar antenna and exhibits a lower RC time constant, allowing more efficient radiation especially at the higher part of the THz spectrum. As a result, the output intensity of the photomixer with the new nano-gap electrode structure in the active region is two orders of magnitude higher than that of a photomixer with typical interdigitated electrodes. Significant improvement in the THz emission bandwidth was also observed. An efficient continuous wave THz source will greatly benefit compact THz system development for high resolution THz spectroscopy and imaging applications.

Selective electroless silver plating of three dimensional SU-8 microstructures on silicon for metamaterials applications

We report a method for selective silver coating of SU-8 structures on Si substrates by treating the sample with radio frequency plasma prior to electroless plating. Silver films with high conductivity of 9 × 10−8Ω.m and low surface roughness of 9 nm have been obtained. When combined with two-photon lithography, this process can be utilized for three-dimensional metamaterials applications. Unlike previous work on selective coating, our process can coat directly on SU-8 photoresist that is widely used for two-photon lithography and does not require any resin modification.

Resonance Switchable Metamaterials Using MEMS Fabrications

This paper reports a switchable metamaterial working at terahertz (THz) regime, which is realized by using MEMS fabrications. The resonance induced by 2.60-THz incidence is switched by reconfiguring the metamaterial element, which is analyzed using the coupled oscillators model. The resonance switching of the metamaterial has potential applications, such as optical switching, tunable filtering, THz sensing, etc.

Electroluminescence and structural characteristics of InAs/In0.1Ga0.9As quantum dots grown on graded Si1−xGex/Si substrate

We studied the electroluminescence and structural characteristics of five-layer stacked self-assembled InAs/In0.1Ga0.9As quantum dot (QD) structures grown on graded Si1−xGex/Si substrate. The QD was found to take on a lens shaped structure with aspect ratio of 0.23±0.05. Room-temperature electroluminescence at 1.29 μm was observed from the QD structures. The external quantum efficiency as function of injected current was investigated and the dominant carrier recombination processes were identified from analysis of the current-optical power relationship.

Structural and optical properties of stacked self-assembled InAs∕InGaAs quantum dots on graded Si1−xGex∕Si substrate

We have investigated the effects of InAs monolayer coverage on the structural and optical characteristics of stacked InAs quantum dot (QD) layers on graded Si1−xGex∕Si substrate. No preferential InAs QDs nucleation was observed along the cross-hatched lines on the graded Si1−xGex∕Si substrate. Employing alternate-beam molecular beam epitaxy, InAs QDs with areal density as high as 7×1010cm−2 was achieved. Temperature dependence of the InAs QDs optical properties is discussed. The InAs QDs show room-temperature photoluminescence at 1.3μm with full width at half-maximum of 65nm. The results are significant for potential realization of III-V QD devices on silicon-based platforms.

The role of ions and reaction sites for electrochemical reversible charge cycling in mesoporous nickel hydroxides

Layered nickel hydroxide thin films with mesoporous structure were prepared on ITO substrates by a facile chemical bath deposition method. The electrochemical properties of the nickel hydroxide/oxyhydroxide films were investigated in both potassium hydroxide and lithium perchlorate in propylene carbonate electrolytes. We show that the high reversible charge cycling capability of the material is enabled by the redox reaction involving hydroxyl ions, especially in the oxidative cycle. Li ion reversible charge cycling requires available reaction or adsorption sites that can be provided by sub-stoichiometry or defective films. Raising the defect concentration, the cyclic reversibility is shown to increase by ∼5 times. The effects of intercalated water in the interlayers are also discussed. We show that the presence of water in the interlayers can lead to a passivating reaction during the charge cycling and also cause optical efficiency losses through unwanted charge trapping.

High gain AlGaAs∕GaAs heterojunction bipolar transistor fabricated on SiGe∕Si substrate

High gain AlGaAs∕GaAs heterojunction bipolar transistors grown on SiGe∕Si substrate have been fabricated. Measured peak dc current gain of ∼100 is obtained for a device with emitter area of ∼1.6×103μm2, with base concentration of 1×1019cm−3. The dominant base current component is discussed and determined. The breakdown characteristic is studied and compared with that of the device grown on GaAs substrate. Our experimental results demonstrate that SiGe∕Si substrate could provide a robust method for monolithic integration of high speed GaAs-based electronic devices with silicon-based circuitry.