Concept: Semiconductor materials
One important use of layered semiconductors such as molybdenum disulfide (MoS2) could be in making novel heterojunction devices leading to functionalities unachievable using conventional semiconductors. Here we demonstrate a metal-semiconductor-metal heterojunction photodetector, made of MoS2 and amorphous silicon (a-Si), with rise and fall times of about 0.3 ms. The transient response does not show persistent (residual) photoconductivity, unlike conventional a-Si devices where it may last 3-5 ms, thus making this heterojunction roughly 10X faster. A photoresponsivity of 210 mA/W is measured at green light, the wavelength used in commercial imaging systems, which is 2-4X larger than that of a-Si and best reported MoS2 devices. The device could find applications in large area electronics, such as biomedical imaging, where a fast response is critical.
Fast flexible electronics operating at radio frequencies (>1 GHz) are more attractive than traditional flexible electronics because of their versatile capabilities, dramatic power savings when operating at reduced speed and broader spectrum of applications. Transferrable single-crystalline Si nanomembranes (SiNMs) are preferred to other materials for flexible electronics owing to their unique advantages. Further improvement of Si-based device speed implies significant technical and economic advantages. While the mobility of bulk Si can be enhanced using strain techniques, implementing these techniques into transferrable single-crystalline SiNMs has been challenging and not demonstrated. The past approach presents severe challenges to achieve effective doping and desired material topology. Here we demonstrate the combination of strained- NM-compatible doping techniques with self-sustained-strain sharing by applying a strain-sharing scheme between Si and SiGe multiple epitaxial layers, to create strained print-transferrable SiNMs. We demonstrate a new speed record of Si-based flexible electronics without using aggressively scaled critical device dimensions.
An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state. We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness. Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier. They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively. Such devices have potential for high-frequency operation and large-scale integration.
Despite their technological importance, lead sulfide (PbS) nanocrystals have lagged behind nanocrystals of cadmium selenide (CdSe) and lead selenide (PbSe) in terms of size and energy homogeneity. Here we show that the ratio of lead to sulfur precursor available during nucleation is a critical parameter affecting subsequent growth and monodispersity of PbS nanocrystal ensembles. Applying this knowledge, we synthesize highly monodisperse (size dispersity < 5%) PbS nanocrystals over a wide range of sizes (exciton energies from 0.70-1.25 eV, or 1000-1800 nm) without the use of size-selective precipitations. This degree of monodispersity results in absorption peak half width at half max (HWHM) values as small as 20 meV, indicating an ensemble that is close to the homogeneous limit. Photoluminescence emission is correspondingly narrow and exhibits small Stokes shifts and quantum efficiencies of 30-60%. The nanocrystals readily self-assemble into ordered superlattices and exhibit exceptional air stability over several months.
Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.
A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga(0.97)Mn(0.03)As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga(0.97)Mn(0.03)As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga(1-x)Mn(x)As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material.
We report the use of black silicon (bSi) as a growth platform for III-V nanowires (NWs), which enables low reflectance over a broad wavelength range as well as fabrication of optoelectronic devices by metalorganic vapor phase epitaxy. In addition, a new isolated growth regime is reported for self-catalyzed InAs NWs at record-low temperatures of 280 °C-365 °C, where consistently rectangular [-211]-oriented NWs are obtained. The bSi substrate is shown to support the growth of additionally GaAs and InP NWs, as well as heterostructured NWs. As seed particles, both ex-situ deposited Au nanoparticles and in-situ deposited In droplets are shown feasible. Particularly the InAs NWs with low band gap energy are used to extend low-reflectivity wavelength region into infrared, where the bSi alone remains transparent. Finally, a fabricated prototype device confirms the potential of III-V NWs combined with bSi for optoelectronic devices. Our results highlight the promise of III-V NWs on bSi for enhancing optoelectronic device performance on the low-cost Si substrates, and we believe that the new low-temperature NW growth regime advances the understanding and capabilities of NW growth.
Strain engineering is seen as a cost-effective way to improve the properties of electronic devices. However, this technique is limited by the development of the Asarro Tiller Grinfeld growth instability and nucleation of dislocations. Two strain engineering processes have been developed, fabrication of stretchable nanomembranes by deposition of SiGe on a sacrificial compliant substrate and use of lateral stressors to strain SiGe on Silicon On Insulator. Here, we investigate the influence of substrate softness and pre-strain on growth instability and nucleation of dislocations. We show that while a soft pseudo-substrate could significantly enhance the growth rate of the instability in specific conditions, no effet is seen for SiGe heteroepitaxy, because of the normalized thickness of the layers. Such results were obtained for substrates up to 10 times softer than bulk silicon. The theoretical predictions are supported by experimental results obtained first on moderately soft Silicon On Insulator and second on highly soft porous silicon. On the contrary, the use of a tensily pre-strained substrate is far more efficient to inhibit both the development of the instability and the nucleation of misfit dislocations. Such inhibitions are nicely observed during the heteroepitaxy of SiGe on pre-strained porous silicon.
Compositional abruptness of the interfaces is one of the important factors to performance of Group IV semiconductor heterojunction (Si/Ge or Si/SiGe) nanowire devices. However, forming abrupt interfaces in the nanowires using the common Vapor-Liquid-Solid (VLS) method is restricted, because large solubility of Si and Ge in the Au eutectic liquid catalyst makes gradual composition change at the heterojunction after switching the gas phase components. According to the VLS growth mechanism, another possible approach to the formation of an abrupt interface is making a change of the semiconductor concentration in the eutectic liquid before precipitation of the second phase. Here we use low Ge concentration (≤6%) SiGe nanowires to show that the composition in the AuSiGe eutectic liquid on nanowire tips can be altered by thermal oxidation at 700°C - only Si is oxidized on the surface of the eutectic liquid and the Ge concentration ratio in the eutectic liquid is correspondingly increased. The subsequently precipitated SiGe alloy at the liquid/solid interface has a higher Ge concentration (~20%), and a compositionally abrupt interface is therefore produced in the nanowires. The composition change is in one atomic plane. The growth mechanism of the heterojunction includes diffusion of Si and Ge atoms on nanowire surface into the AuSiGe eutectic liquid and step nucleation at the liquid/nanowire interface.
The micro-luminescence spectra of the diluted magnetic semiconductor (DMS) can reflect the spin-exciton interaction and related relaxation process. Here the micro-photoluminescence (micro-PL) spectra and PL lifetime measurements have been done on an individual ferromagnetic-coupled cobalt (Co) doped zinc selenide (ZnSe) nanowire. There occurs a double-peak profile in its near bandedge emission spectrum: the first peak is from free exciton (FX) and the second comes from magnetic polaron (MP). In their temperature dependent PL spectra, the MP emission peak demonstrates obviously temperature-independent behavior, in contrast to the behaviors of free exciton and reported exciton magnetic polaron (EMP) in nanobelt. It is found that in this Co(II) doped ZnSe nanowires, this MP’s temperature-independent emission is related to the coupling between exciton and a ferromagnetic nanocluster ( ). The nanocluster is likely due to the interaction of Se vacancies of the wide bandgap semiconductors with the antiferromagnetic arrangement transition metal ions in these Se-deficient Co doped ZnSe nanowires. These results reflect that the antiferromagnetic coupling TM ions pair can give rise to ferromagnetic behavior with the involvement of positive charge defect, also indicating that the micro-luminescence detection can be used to study the magnetic coupling in DMS.