Lian Ji

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.

High-efficiency GaAs and GaInP solar cells grown by all solid-state molecular-beam-epitaxy.

We report the initial results of GaAs and GaInP solar cells grown by all solid-state molecular-beam-epitaxy (MBE) technique. For GaAs single-junction solar cell, with the application of AlInP as the window layer and GaInP as the back surface field layer, the photovoltaic conversion efficiency of 26% at one sun concentration and air mass 1.5 global (AM1.5G) is realized. The efficiency of 16.4% is also reached for GaInP solar cell. Our results demonstrate that the MBE-grown phosphide-contained III-V compound semiconductor solar cell can be quite comparable to the metal-organic-chemical-vapor-deposition-grown high-efficiency solar cell.

Room-temperature GaAs/InP wafer bonding with extremely low resistance

Low-temperature direct wafer bonding is a promising technique for fabricating multijunction solar cells with more than four junctions in order to obtain high conversion efficiencies. However, it has been difficult to reduce the bond interface resistance between a GaAs-based subcell wafer and an InP-based subcell wafer. We found that a novel bonding structure comprising heavily Zn-doped (1 x 10(19) cm(-3)) p(+)-GaAs and S-doped (3 x 10(18)cm(-3)) n-InP had an interface resistance of 2.5 x 10(-5) Omega.cm(2), which is the lowest value ever reported. This result suggests that the newly developed room-temperature wafer bonding technique has high potential to realize high-efficiency multijunction solar cells

Investigation of InGaAs thermophotovoltaic cells under blackbody radiation

The efficiency calibration of In GaAs thermophotovoltaic (TPV) cells with band gap energies of 0.6 and 0.74 eV under blackbody radiation is performed on the basis of the combination of measurement with theoretical calculation. Efficiencies of 19.1% for the 0.6 eV InGaAs cell and 16.4% for the 0.74 eV InGaAs cell are obtained at the radiation temperature of 1323 K. The notable differences in reverse saturation current density and ideality factors under the blackbody radiation and standard solar spectrum illumination indicate the significant effect of the radiation spectrum on the detailed understanding of devices

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.

III–V compound semiconductor multi-junction solar cells fabricated by room-temperature wafer-bonding technique

We have developed III-V compound semiconductor multi-junction solar cells by a room-temperature wafer-bonding technique to avoid the formation of dislocations and voids due to lattice mismatch and thermal damage during a conventional high-temperature wafer-bonding process. First, we separately grew an (Al) GaAs top cell on a GaAs substrate and an InGaAs bottom cell on an InP substrate by metal solid source molecular beam epitaxy. Thereafter, we successfully bonded these sub-cells by the room-temperature wafer-bonding technique and fabricated (Al) GaAs parallel to InGaAs wafer-bonded solar cells. To the best of our knowledge, the obtained GaAs parallel to InGaAs and AlGaAs parallel to InGaAs wafer-bonded solar cells exhibited the lowest electrical and optical losses ever reported. The AlGaAs parallel to InGaAs solar cells reached the maximum efficiency of 27.7% at 120 suns. These results suggest that the room-temperature wafer-bonding technique has high potential for achieving higher conversion efficiencies

Transparent conducting indium-tin-oxide (ITO) film as full front electrode in III–V compound solar cell*

The application of transparent conducting indium-tin-oxide (ITO) film as full front electrode replacing the conventional bus-bar metal electrode in III–V compound GaInP solar cell was proposed. A high-quality, non-rectifying contact between ITO and 10 nm -GaAs contact layer was formed, which is benefiting from a high carrier concentration of the terrilium-doped -GaAs layer, up to 2 . A good device performance of the GaInP solar cell with the ITO electrode was observed. This result indicates a great potential of transparent conducting films in the future fabrication of larger area flexible III–V solar cell.

Effect of high temperature rapid thermal annealing on optical properties of InGaAsP grown by molecular beam epitaxy

The effect of rapid thermal annealing (RTA) on the optical properties of InGaAsP with band-gap energy of around 1.05 eV for quadruple-junction solar cells grown by molecular beam epitaxy (MBE) has been investigated. The photoluminescence (PL) spectrum of InGaAsP film annealed at 800 °C has strong integrated intensity and low activation energy of band-tail states. The time-resolved PL measurement shows that the decay time of the InGaAsP annealed at 800 °C and as-grown one are 11.6 ns and 3.0 ns at 10 K, respectively. An S-shape PL decay time as a function of temperature for the InGaAsP annealed at 800 °C is observed and is explained by the carrier relaxation dynamics. The RTA process induces reorganization of In and Ga inside the alloy due to the existence of miscibility gap in InGaAsP grown by MBE owing to the Be diffusion at high temperature and results in an increased composition uniformity and an improved PL intensity.

The influence of Sb content and dots size of InAs/GaAs(1-x)Sbx quantum dot on type I-type II band alignment and carrier dynamics

The band structure and carrier dynamics of InAs/GaAs(1-x)Sbx/GaAs quantum dots (QDs) are investigated by the eight-band model and by looking at the optical matrix elements of the envelope functions. Simulation solutions were obtained for different Sb concentrations in the cover layer and different QD heights. The results indicated that the conversion process from type I to type II caused by the Sb content could be divided into three stages: type I (less than 8%), the turning process (8%–14%), and type II (more than 14%). The influence of QD height on the wave function of carriers can be seen only in the second stage. It reverses the effect of Sb content by influencing the quantum confinement of hole states. Additionally, the potential radiation recombination channels are changed by these two factors. Unlike the first stage (where the ground-state transition is mainly channel), the first two hole levels and the first two electron levels are involved in radiation recombination in the second stage. So two re...