Wei-Xin Ni

Enhanced photoresponse of a metal-oxide-semiconductor photodetector with silicon nanocrystals embedded in the oxide layer

The authors report a two-terminal metal-oxide-semiconductor photodetector for which light is absorbed in a capping layer of silicon nanocrystals embedded in a mesoporous silica matrix on p-type silicon substrates. Operated at reverse bias, enhanced photoresponse from 300to700nm was observed. The highest optoelectronic conversion efficiency is as high as 200%. The enhancements were explained by a transistorlike mechanism, in which the inversion layer acts as the emitter and trapped positive charges in the mesoporous dielectric layer assist carrier injection from the inversion layer to the contact, such that the primary photocurrent could be amplified.

Synthesis design of artificial magnetic metamaterials using a genetic algorithm

In this article, we present a genetic algorithm (GA) as one branch of artificial intelligence (AI) for the optimization-design of the artificial magnetic metamaterial whose structure is automatically generated by computer through the filling element methodology. A representative design example, metamaterials with permeability of negative unity, is investigated and the optimized structures found by the GA are presented. It is also demonstrated that our approach is effective for the synthesis of functional magnetic and electric metamaterials with optimal structures. This GA-based optimization-design technique shows great versatility and applicability in the design of functional metamaterials.

Electrical properties of Si films doped with 200‐eV In+ ions during growth by molecular‐beam epitaxy

A single‐grid ultra‐high‐vacuum‐compatible ion source was used to provide accelerated In+‐dopant beams during Si(100) growth by molecular‐beam epitaxy. Indium incorporation probabilities σ, determined by secondary ion mass spectrometry, in films grown at Ts=800 °C were too low to be measured for thermal In (σIn was 550 °C) . However, for accelerated In+ doping, σIn+ at 800 °C ranged from 0.03 to ∼1 for In+ acceleration energies EIn+ between 50 and 400 eV. Temperature‐dependent Hall‐effect and resistivity measurements were carried out on In+‐doped Si films grown at Ts =800 °C with EIn+=200 eV . Indium was incorporated substitutionally into electrically active sites over a concentration ranging from 2×1015−2×1018 cm−3, which extends well above reported equilibrium solid‐solubility limits. The acceptor‐level ionization energy was 156 meV, consistent with previously published results for In‐doped bulk Si. Room‐temperature hole mobilities μ were in good agreement with the best reported data for B...

X-ray reciprocal space mapping studies of strain relaxation in thin SiGe layers (≤ 100 nm) using a low temperature growth step

Relaxation of thin SiGe layers (~90 nm) grown by molecular beam epitaxy using a low temperature growth step (120-200°C) has been investigated using two-dimensional reciprocal space mapping of X-ray diffraction. The samples studied have been divided in two groups, depending on the substrate cooling process during the growth of the low temperature layer. It has been found that a higher degree of relaxation was easily achieved for the sample group without growth interruption. A process window for full relaxation of the Si0.74Ge0.26 layer has been observed in the range of 140-150°C.

Spatially direct and indirect transitions observed for Si/Ge quantum dots

The optical properties of Ge quantum dots embedded in Si were investigated by means of photoluminescence, with temperature and excitation power density as variable parameters. Two different types of recombination processes related to the Ge quantum dots were observed. A transfer from the spatially indirect to the spatially direct recombination in the type-II band lineup was observed with increasing temperature. A blueshift of the spatially indirect Ge quantum-dot-emission energy with increasing excitation power is ascribed to band bending at the type-II Si/Ge interface for high carrier densities. Comparative studies were performed on uncapped Ge dot structures.

Toward Silicon-Based Lasers for Terahertz Sources

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser

Room temperature 1.54 μm light emission of erbium doped Si Schottky diodes prepared by molecular beam epitaxy

Schottky-type light emitting devices have been fabricated on Er-oxide doped Si layers grown by molecular beam epitaxy, in order to study the light emission process of Er-doped Si structures. By applying a reverse bias on the Schottky junction, Er ions incorporated within the depletion layer can be electrically excited via a hot electron impact process. Rather intense electroluminescence (EL) at a wavelength of 1.54 μm has been observed at room temperature. The optoelectronic properties of the devices have been characterized by both input-power dependent and temperature dependent EL measurements. An activation energy value of ∼160 meV responsible for luminescence thermal quenching has been obtained.

Hall factor and drift mobility for hole transport in strained Si1−xGex alloys

Hole transport in B-doped strained Si1−xGex has been studied using Hall measurements for boron concentrations from 2×1018 to 7.5×1018 cm−3 with Ge content 0⩽x⩽0.36. By keeping the B flux constant during the molecular beam epitaxy growth of sets of samples and only varying the Si and Ge fluxes, we were able to prepare samples for an accurate determination of the Hall factor based on using the established relationship between B-doping concentration and resistivity for pure Si. It was found that the Hall factor drops considerably when the Ge content is increased. Determined Hall factor values are compared with calculated values taking into account the full valence band structure and various scattering mechanisms. The hole drift mobility has been derived from our measured Hall mobility using the determined Hall factor for the corresponding Ge content. We find that, depending on the doping concentration, the drift mobility can be higher for strained layers containing Ge. The grown layers were also characterize...

Growth of high quality Ge films on Si(111) using Sb as surfactant

High quality, completely relaxed Ge films have been grown on Si(111) using Sb as surfactant at the initial stage of growth. After desorbing the surface Sb layer, cusplike reflection high‐energy electron diffraction intensity oscillations indicated excellent Ge layer‐by‐layer epitaxy. High resolution x‐ray diffraction analysis showed a very high crystalline quality and well resolved thickness fringes consistent with a flat relaxed Ge layer. Chemical preferential etching experiments revealed a defect density of down to ∼3×104 cm−2.

SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3-1.55 µm

The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–1.55 μm. The MBE ...