Lifeng Bian

Stable few-layer MoS2 rectifying diodes formed by plasma-assisted doping

We present a method for making stable MoS2 rectifying diodes using selected-area plasma treatment. The transport and X-ray photoelectron spectroscopic characterizations of MoS2 transistors treated with different plasmas confirm that the rectifying characteristics of MoS2 diodes are attributed to plasma-induced p-doping and p-n junctions in MoS2. Such plasma-doped diodes exhibit high forward/reverse current ratios (∼104 for SF6-treated diodes) and a superior long-term stability. They can play an important role in the development of nanoelectronic devices. In addition, the presented plasma-assisted doping process could be also used for making ambipolar MoS2 transistors and functionalizing other emerging two-dimensional materials.

The effect of inserting strain-compensated GaNAs layers on the luminescence properties of GaInNAs/GaAs quantum well

Photoluminescence (PL) properties of GaInNAs/GaAs quantum wells (QWs) with strain-compensated GaNAs layers grown by molecular beam epitaxy are investigated. The temperature-dependent PL spectra of GaInNAs/GaAs QW with and without GaNAs layers are compared and carefully studied. It is shown that the introduction of GaNAs layers between well and barrier can effectively extend the emission wavelength, mainly due to the reduction of the barrier potential. The PL peak position up to 1.41 mum is observed at the room temperature. After adding the GaNAs layers into QW structures, there is no essential deterioration of luminescence efficiency. N-induced localization states are also not remarkably influenced. It implies that with optimized growth condition, high-quality GaInNAs/GaAs QWs with strain-compensated GaNAs layers can be achieved

Selective growth and piezoelectric properties of highly ordered arrays of vertical ZnO nanowires on ultrathin alumina membranes

A well controlled and cost effective method of fabricating highly ordered arrays of vertical zinc oxide (ZnO) nanowires or nanopores is demonstrated where an ultrathin alumina membrane (UTAM) itself is utilized as a substrate for the selective growth of the ordered arrays. A thin film of gold was thermally evaporated on the UTAM followed by the growth of highly regular ZnO nanowires using chemical vapor deposition (CVD). Alternatively, highly ordered ZnO nanopores arrays were also grown by CVD on the bare UTAM. Additionally, piezoelectric currents were generated from the ZnO nanowires during the conductive atomic force microscopy probe tip scan across the array.

Optical investigation on the annealing effect and its mechanism of GaInAs/GaNAs quantum wells

Thermal annealing of GaInAs/GaNAs quantum wells (QWs) as well as other nitrogen- and indium-contained QW structures grown by molecular beam epitaxy and its effect on optical properties are investigated. The photoluminescence (PL) and photovoltaic (PV) spectra of annealed GaInAs/GaNAs QWs show that the luminescence properties become degraded due to the N diffusion from the GaNAs barrier layers to the GaInAs well layer. Meantime, the annealing-induced blueshift of the PL peak in this QW system is mainly induced by the change of In distribution, suggesting that the In reorganization is greatly assisted by the N-induced defects. The elucidation of annealing effect in GaInAs/GaNAs QW samples is helpful for a better understanding to the annealing effect in the GaInNAs/GaAs QWs

The effects of concomitant In and N incorporation on the photoluminescence of GaInNAs

We investigated the effects of concomitant In- and N-incorporation on the photoluminescence (PL) of GaInNAs grown by molecular beam epitaxy. In comparison with the N-free GaInAs epilayer, the PL spectra of the GaInNAs epilayer exhibit an anomalous S-shape temperature dependence of dominant luminescence peak. Through further careful inspection, two PL peaks are clearly discerned and are associated with the interband excitonic recombinations and excitons bound to N-induced isoelectronic impurity states, respectively. By comparing the PL spectra of GaInNAs/ GaAs quantum wells (QWs) with those of In-free GaNAs/GaAs QWs grown under similar conditions, it is found that the concomitant In- and N-incorporation reduces the density of impurities and has an effect to improve the intrinsic optical transition of GaInNAs, but also enhance the N-induced clustering effects. At last, we found that rapid thermal annealing can significantly reduce the density of N-induced impurities

The effects of rapid thermal annealing on the optical properties of Zn1−xMnxSe epilayer grown by MOCVD on GaAs substrate

Zn1-xMnxSe thin films with different Mn compositions are grown by metal-organic chemical vapor deposition on GaAs substrate. Good crystallinity of sample is evidenced by X-ray diffraction and rocking-curve measurements. Photoluminescence (PL) properties were carefully studied. A dominant PL peak close to the band edge is observed at low temperature for samples with higher Mn concentration. The temperature-dependent PL and time-resolved photoluminescence show that this emission peak is associated with the recombination of exciton bound to Mn-induced impurity bound states. It is found that rapid thermal annealing can induce reorganization of Mn composition in alloys and significantly reduce the density of impurity induced by Mn incorporation and improve the intrinsic interband transition

Room-temperature wafer bonded InGaP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all-solid state molecular beam epitaxy

An InGaP/GaAs tandem cell on a GaAs substrate and an InGaAsP/InGaAs tandem cell on an InP substrate were grown separately by all-solid-state molecular beam epitaxy. A room-temperature direct wafer-bonding technique was used to integrate these subcells into an InGaP/GaAs//InGaAsP/InGaAs wafer-bonded solar cell, which resulted in an abrupt interface with low resistance and high optical transmission. The current-matching design for the base layer thickness of each cell was investigated. The resulting efficiency of the four-junction solar cell was 42.0% at 230 suns, which demonstrates the great potential of the room-temperature wafer-bonding technique to achieve high conversion efficiency for cells with four or more junctions.

A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy

We report the recent result of GaAs/GaInP dual-junction solar cells grown by all solid-state molecular-beam-epitaxy (MBE). The device structure consists of a GaIn0.48P homojunction grown epitaxially upon a GaAs homojunction, with an interconnected GaAs tunnel junction. A photovoltaic conversion efficiency of 27% under the AM1.5 globe light intensity is realized for a GaAs/GaInP dual-junction solar cell, while the efficiencies of 26% and 16.6% are reached for a GaAs bottom cell and a GaInP top cell, respectively. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-containing III—V compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.

Forward bias voltage controlled infrared photodetection and electroluminescence from a p-i-n quantum dot structure

We report a specially designed p-i-n quantum dot (QD) device in which both the intersubband and the interband transitions of the QD are controlled by the applied forward bias voltage. The design is achieved by using asymmetric AlGaAs barriers on both sides of only one QD layer and by placing two-dimensional electron gas structure on one side of the QD layer. Experimentally, at 77 K, the device can detect normal incidence infrared light of about 5.6 µm when the forward bias voltage is in between about 3 and 7 V, while it emits electroluminescence of about 1 μm when the applied forward bias voltage is larger than 11 V.

Lasing of CdSexS1-x quantum dots in a glass spherical microcavity

A glass spherical microcavity only a few microns in diameter embedded with CdSexS1-x quantum dots (QDs) was fabricated using a physical method; it exhibited good optical stability under continuous-wave laser excitation with high power. We investigated the excitation power dependences of the emission intensity and the linewidth of both transverse electric and transverse magnetic resonance peaks of whispering gallery modes. Stimulated emission behaviour of multi-frequency modes is observed at room temperature. The low threshold value and large mode separation makes QD-containing microspheres promising for visible microlaser applications.