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Single photon avalanche diodes (SPADs) are key enabling technologies for a wide range of applications in the near-infrared wavelength range. Recently, AlAs0.56 Sb0.44 (hereafter AlAsSb) lattice-matched to InP has been demonstrated for extremely low excess noise avalanche photodiodes (APDs) due to its large disparity between electron and hole ionization coefficients ( $\alpha $ and $\beta $ respectively). The $\alpha /\beta $ ratio also plays a role in Geiger mode operation as it affects the avalanche breakdown probability and hence detection efficiency. In this work, we theoretically investigate the performance of AlAsSb based SPADs. The probability of breakdown for electron-initiated Geiger mode operation increases more sharply with multiplication region width due to progressively more dissimilar ionization coefficients. In comparison with other common avalanche materials, such as InAlAs, InP and Si, our result also suggests that SPADs based on AlAsSb have a sharper breakdown probability than the other three materials under similar low overbias ratio. The calculated breakdown probability of 0.81 in AlAsSb is 0.18 and 0.28 higher than that of InAlAs/Si and InP respectively at 5% overbias ratio and with avalanche region width of 1500 nm.

Sunlight driven formic acid decomposition has great potential to supply high-purity H2 without consuming fossil fuel-derived energy. However, a trace amount of CO invariably exists in the obtained H2 and the H2 production rate is always lower than 278 mmol g−1 h−1. Here, we found that high quality MoS2 grown on graphene decorated on Ni foam (Ni/G/MoS2) was active and stable for H2 production from thermocatalytic formic acid decomposition without CO presentation and first principles calculation confirmed that the perfect surface terminating sulfur of MoS2 changed the reaction path of intermediates, thus inhibiting the production of CO. Furthermore, a reaction device constructed with Cu2Se can heat catalysts to 120 and 260 °C under 0.25 kW m−2 and 1 kW m−2 (1 Sun) of irradiation, respectively. By using the system of the Cu2Se based reaction device and Ni/G/MoS2, a CO free H2 production rate of 982 mmol g−1 h−1 was achieved under 0.6 Sun of irradiation, 3.5 times higher than the previous record of photocatalytic formic acid decomposition. Therefore, this work provides a new viewpoint for large scale CO free H2 production in a sustainable and green way.

The defects into the hexagonal network of a sp2-hybridized carbon atom have been demonstrated to have a significant influence on intrinsic properties of graphene systems. In this paper, we presented a study of temperature-dependent Raman spectra of G peak and D’ band at low temperatures from 78 to 318 K in defective monolayer to few-layer graphene induced by ion C+ bombardment under the determination of vacancy uniformity. Defects lead to the increase of the negative temperature coefficient of G peak, with a value almost identical to that of D’ band. However, the variation of frequency and linewidth of G peak with layer number is contrary to D’ band. It derives from the related electron-phonon interaction in G and D’ phonon in the disorder-induced Raman scattering process. Our results are helpful to understand the mechanism of temperature-dependent phonons in graphene-based materials and provide valuable information on thermal properties of defects for the application of graphene-based devices.

Possing excellent catalytic performance about counter electrode (CE) is the prerequisite for obtaining high performance dye sensitized solar cells (DSSCs). Herein, The NiCo2S4/carbon nanofibers (NiCo2S4/CNFs) are successfully synthetised via hydrothermal and electrospinning method, and then directly used in DSSCs. Owing to the synergistic effect between the catalytic of NiCo2S4 materials and the electrical conductivity of CNFs, All chemical measurements reveal that NiCo2S4/CNFs applied in DSSCs exhibit good power conversion efficiency (9.0%) and stability, which surpass that of DSSCs with Pt CE (7.48%), indicating that NiCo2S4/CNFs composite would be helpful for the low-cost and efficient application of DSSCs.

Fast, sensitive avalanche photodiodes (APDs) are required for applications such as high-speed data communications and light detection and ranging (LIDAR) systems. Unfortunately, the InP and InAlAs used as the gain material in these APDs have similar electron and hole impact ionization coefficients (α and β, respectively) at high electric fields, giving rise to relatively high excess noise and limiting their sensitivity and gain bandwidth product1. Here, we report extremely low excess noise in an AlAs0.56Sb0.44 lattice matched to InP. A deduced β/α ratio as low as 0.005 with an avalanche region of 1,550 nm is close to the theoretical minimum and is significantly smaller than that of silicon, with modelling suggesting that vertically illuminated APDs with a sensitivity of −25.7 dBm at a bit error rate of 1 × 10−12 at 25 Gb s−1 and 1,550 nm can be realized. These findings could yield a new breed of high-performance receivers for applications in networking and sensing.

This paper presents the electron and hole avalanche multiplication and excess noise characteristics based on bulk AlAs0.56Sb0.44 p-i-n and n-i-p homojunction diodes lattice matched to InP, with nominal avalanche region thicknesses of 0.6 -1.5 μm. From these, the bulk electron and hole impact ionization coefficients (α and β respectively), have been determined over an electric field range of 220-1250 kV/cm for α and from 360-1250 kV/cm for β for the first time. Excess noise characteristics suggest an β/α ratio as low as 0.005 for an avalanche region of 1.5 μm in this material, close to the theoretical minimum and significantly lower than AlInAs, InP, or even silicon. This material can be easily integrated with InGaAs for networking and sensing applications, with modeling suggesting that a sensitivity of -32.1 dBm at a bit-error rate (BER) of 1×10-12 at 10 Gb/s at 1550 nm can be realized. This sensitivity can be improved even further by optimizing the dark currents and by using a lower noise transimpedance amplifier.

Topological insulators, such as Bi2Te3, have been confirmed to exhibit plasmon radiation over the entire visible spectral range. Herein, we fabricate bullseye nanoemitters, consisting of a central disk and concentric gratings, on the Bi2Te3 nanoflake. Due to the existence of edge plasmon modes, Bi2Te3 bullseye nanostructures are possible to converge light towards the central disk. Taking advantage of the excellent spatial resolution of cathodoluminescence (CL) characterization, it has been observed that plasmonic behaviors depend on the excitation location. A stronger plasmonic intensity and a wider CL spectral linewidth can be obtained at the edge of the central disk. In order to further improve the focusing ability, a cylindrical Pt nanostructure has been deposited on the central disk. Additionally, the finite element simulation indicates that the electric-field enhancement originates from the coupling process between the plasmonic emission from the Bi2Te3 bullseye and the Pt nanostructure. Finally, we find that enhancement efficiency depends on the thickness of the Pt nanostructure.

AlAs0.56Sb0.44 is a promising avalanche material which can be grown lattice-matched to InP and therefore use InGaAs as the absorption region in a Separate Absorption and Multiplication APD (SAM-APD). The electron and hole ionisation coefficients in this material are very dissimilar and our experiments show that this leads to AlAs0.56Sb0.44 having the lowest excess noise performance of any InP based material system (F = 2.2 at M = 40) reported to date. Simulations suggest that operation at 1550 nm and 25 GB s-1 with a sensitivity of -25.7 dBm is possible in a normal incidence SAM-APD.

The In(Ga)As/GaAs surface quantum dots (SQDs) have become appealing recently due to the special surface sensitivity property and consequently the advantage in developing sensors for humidity detection. This research reveals that the InGaAs/GaAs single-layer SQDs exhibit carrier separation due to Fermi level pinning, where electrons are pinned at surface but holes are confined inside the SQDs. Thus, photoluminescence (PL) features corresponding to carrier indirect transition are observed through a prominent blue-shift of ∼ 31 meV for the PL band that obeys a cube root power law over four orders of magnitude change in excitation power. The indirect transition of these SQDs is further characterized through time-resolved PL studies showing a biexponential decay with a relatively elongated carrier lifetime of ∼1.35 ns. This is the first time that a concept of type-II characteristic is proposed for the InGaAs SQDs, which will help to optimize the design and working principles for developing sensors with surface-sensitive InGaAs SQDs.

In this research Raman spectroscopy and photoluminescence (PL) spectroscopy measurements reveal different optical characteristics between monolayer (1 L) and bilayer (2 L) WS2 due to the interlayer interaction. First, ultralow frequency (ULF) Raman spectra indicate that 2 L WS2 shows additional shear mode (S) and layer-breathing (LB) mode in comparison with 1 L WS2. The 2 L WS2 has the in-plane vibration mode E 2 g 1 red-shifted while the out-plane vibration mode A 1 g blue-shifted. Then, both direct transition (DT) and indirect transition (IT) emission are observed in the PL spectra for 2 L WS2 while only DT for 1 L WS2. The PL signal corresponding to localized excitons, that is measured as a low energy side tail with the DT emission, is more prominent for 1 L WS2 at low temperature of 7 K. The DT emission and IT emission show opposite variation trend as the temperature increases from 7 K to 300 K, while the average phonon energy in 2 L WS2 is estimated to be almost twice of that for 1 L WS2. These observations illustrate that the interlayer interaction not only change the phonon characteristics for WS2, but also impact the energy band structure and the exciton dynamics, including exciton radiative recombination and exciton-phonon interaction.

Carrier dynamics including carrier relaxation and tunneling within a coupled InAs quantum dot (QD) – In0.15Ga0.85As quantum well (QW) hybrid nanostructure are investigated using photoluminescence (PL) spectroscopy. This coupled hybrid nanostructure shows a single PL peak from the QD emission at low excitation intensity and a band filling behavior is observed as the excitation intensity increases, suggesting that there exists a channel to capture carriers from the QW to the QDs. Time resolved PL (TRPL) measurements extract a carrier tunneling time of 103.7 ps, which is only one third of the theoretical prediction. A double-channel resonant carrier tunneling mechanism from the QW to the wetting layer and to the fifth excited state of the QDs and then carrier relaxation into lower discrete QD energy states is proposed to explain this fast carrier tunneling. The double-channel resonant carrier tunneling mechanism is qualitatively supported through the analysis of the excitation-dependent PL spectra as well as the PL excitation spectra. These results enrich our understanding of carrier dynamics in coupled QD and QW hybrid structures.

Solar cells based on Cu2ZnSnS4 (CZTS) thin-film light absorber are promising photovoltaic (PV) technologies to achieve cost-effective solar electricity. One of the challenges in CZTS PV technology is its low open-circuit voltage ( V oc), which is normally less than 600 mV. In this paper, we enhanced V oc of CZTS solar cells to above 700 mV by constructing a four-layer stacking precursor of Zn/Sn/Cu/Zn (ZTCZ). Compared with the ordinary three-layer Zn/Sn/Cu (ZTC) stacking precursor, the efficiency of CZTS solar cells prepared by the ZTCZ precursor of the same stoichiometry was significantly enhanced. Cross-sectional microscopic characterization revealed that a compact bilayer structured CZTS with Zn-rich surface was formed using the ZTCZ precursor while lots of voids were observed at the bottom of the CZTS film produced using the ZTC precursor. The mechanism is proposed for the high V oc achieved through a Zn-rich surface in the CZTS. Photoluminescence and time-resolved photoluminescence measurements confirmed that the device made from the four-layer ZTCZ stacking precursor possess higher charge separation efficiency and lower radiative recombination compared with the ZTC stacking precursor. These factors have greatly contributed for the high V oc and superior efficiency of the CZTS thin-film solar cells.

Recently, vertically oriented few-layer 2D materials prepared by chemical vapor deposition (CVD) have attracted much attention due to their attractive properties. However, the integration of these materials as heterojunctions in optoelectronic sensors has been rarely reported. In this paper, large-area, high crystalline quality, and vertically oriented few-layered MoS2 (V-MoS2) nanosheets were synthesized and transferred successfully onto a silicon substrate to form a V-MoS2/Si heterojunction photodetector. The photodetector exhibits high photoelectric performances in a wide broadband ranging from visible to near-infrared with a photoresponsivity of up to 908.2 mA W−1, and a detectivity of up to 1.889 × 1013 Jones. More importantly, an unprecedented response speed of (rise time ∼ 56 ns, fall time ∼ 825 ns) was found in the photodetector by time response measurements, which is the best result achieved so far in any other 2D-based photodetectors or phototransistors. These excellent performances can be ascribed to the strong light absorption and the quick longitudinal intralayer carrier transport speed of the V-MoS2 nanosheets as well as the good heterojunction formed between V-MoS2 and Si. This intriguing vertical oriented structure in combination with both ultrafast time response and ultrahigh detectivity in the V-MoS2/Si heterojunction provide great potential for application in optoelectronic devices.

Tin selenide (SnSe) has recently attracted great attention as a new promising thermoelectric material. In this paper, a-axis inclined oriented SnSe thin films have been fabricated by pulsed laser deposition for the development of the transverse thermoelectric effect. Large open-circuit voltage signals along the film plane are all clearly observed when the film surface is illuminated by different lasers with the wavelength ranging from 532 to 2200 nm with no external bias or a built-in electric field, and the amplitude of the output voltage can be enhanced by tuning the inclination angle of the films or the applied temperature difference across the film thickness. Moreover, transverse open-circuit voltage signals are also detected when the film surface is irradiated by a thermal heater. This work demonstrates the novel potential application of SnSe for self-powered broad-band light detectors or thermal sensors.

Synthesis of photothermal materials with high efficiency and anti-corrosion stability is key for photothermal corrosive-water evaporation, but remains a great challenge. Herein, for the first time, we developed mesoporous Fe3Si aerogel as a new photothermal material for remarkably efficient and stable corrosive-water evaporation. The as-prepared Fe3Si aerogel presented narrow mesopores with a size of 2–3 nm that could greatly reduce the thermal conductivity to 0.04 W m−1 K−1, which is the lowest reported value so far, and depressed the latent heat of water evaporation. The obtained Fe3Si aerogel supported on melamine foam demonstrated outstanding seawater evaporation rate of 2.08 kg m−2 h−1 and high solar-thermal utilization efficiency of 91.8% under one solar level irradiation, exceeding those of most of the photothermal materials. Notably, owing to the strong corrosion resistance, the support-free hydrophobic Fe3Si aerogel displayed utrastable solar-thermal water evaporation performance, with more than 1 kg m−2 h−1 of water evaporation and outstanding cycling stability in two highly corrosive solutions, namely, 0.5 M H2SO4 and 1 M NaOH. Therefore, we believe that the Fe3Si aerogels have great potential for wide industrial applications such as seawater desalination, salt production, brine management and water purification.

Time-resolved photoluminescence (TRPL) has been implemented experimentally to measure the carrier lifetime of semiconductors for decades. For the characterization of nanowires, the rich information embedded in TRPL curves has not been fully interpreted and meaningfully mapped to the respective material properties. This is because their three-dimensional (3-D) geometries result in more complicated mechanisms of carrier recombination than those in thin films and analytical solutions cannot be found for those nanostructures. In this work, we extend the intrinsic power of TRPL by developing a full 3-D transient model, which accounts for different material properties and drift-diffusion, to simulate TRPL curves for nanowires. To show the capability of the model, we perform TRPL measurements on a set of GaAs nanowire arrays grown on silicon substrates and then fit the measured data by tuning various material properties, including carrier mobility, Shockley-Read-Hall recombination lifetime, and surface recombination velocity at the GaAs-Si heterointerface. From the resultant TRPL simulations, we numerically identify the lifetime characteristics of those material properties. In addition, we computationally map the spatial and temporal electron distributions in nanowire segments and reveal the underlying carrier dynamics. We believe this study provides a theoretical foundation for interpretation of TRPL measurements to unveil the complex carrier recombination mechanisms in 3-D nanostructured materials.

Thermal properties of sp2 systems such as graphene and hexagonal boron nitride (h-BN) have attracted significant attention because of both systems being excellent thermal conductors. This research reports micro-Raman measurements on the in-plane E2g optical phonon peaks (~ 1580 cm−1 in graphene layers and ~ 1362 cm−1 in h-BN layers) as a function of temperature from − 194 to 200 °C. The h-BN flakes show higher sensitivity to temperature-dependent frequency shifts and broadenings than graphene flakes. Moreover, the thermal effect in the c direction on phonon frequency in h-BN layers is more sensitive than that in graphene layers but on phonon broadening in h-BN layers is similar as that in graphene layers. These results are very useful to understand the thermal properties and related physical mechanisms in h-BN and graphene flakes for applications of thermal devices.

We have determined a reproducible set of growth conditions for the self-assembly of tensile-strained In 1−xGa xAs quantum dot (QD) nanostructures on (111)A surfaces. During molecular beam epitaxy, In 1−xGa xAs islands form spontaneously on InAs(111)A when the Ga content x≥50%. We analyze the structure and composition of InGaAs/InAs(111) samples using atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy. We demonstrate control over the size and areal density of the islands as a function of In 1−xGa xAs coverage, In 1−xGa xAs composition, and substrate temperature. We calculated the conduction and valence band energy values for these QDs in an InAs matrix. This work supports the efforts to establish InAs(111)A as a platform for future incorporation with other (111)-oriented materials from the 6.1 A family of semiconductors.

Digital alloy Al0.85Ga0.15As0.56Sb0.44 and random alloy Al0.79In0.21As0.74Sb0.26 avalanche photodiodes with a 1-μm multiplication layer exhibit low excess noise under 543-nm laser illumination comparable to Si avalanche photodiodes. This has motivated a study of the optical characteristics of these materials. The absorption coefficients and complex refractive indices were extracted via variable-angle spectroscopic ellipsometry. Features of three semiconductor layer fitting approaches are compared, and a Kramers–Kronig-consistent basis spline function was chosen due to its flexibility and accuracy of approximating optical constants of quaternary materials. The external quantum efficiency has been calculated based on the extracted absorption coefficients and is shown to agree well with the measured external quantum efficiency.

Two samples with [001] orientated rhenium disulfide (ReS2) nanowalls (NWs) grown above and in front of precursor (NH4ReO4) by chemical vapor deposition were investigated. The temperature-dependent photoluminescence (PL) indicated that the PL peak exhibited linear blue-shift at a rate of ∼0.24 meV K-1with increasing the temperature from 10 to 300 K, while the linewidth monotonically increased due to the exciton-phonon interaction. This abnormal blue-shift of PL emission energy, which is explained by a competition between the band gap shrinkage and the energy level degeneracy with respect to the increase of temperature and lattice constant, enables ReS2NWs to possess great potential for development of thermal sensors. In addition, exciton localization effect in the ReS2NWs from abundant edges and weak interlayer interaction was also observed to be related to the height and density of ReS2NWs. These results not only enrich the understanding for exciton dynamics in ReS2NWs, but also help to exploit ReS2NWs for device applications.

Ternary I-III-VI (such as CuInSe2 and AgInS2 ) quantum dots (QDs) have been increasingly studied in the field of photoelectric conversion applications. Here, we prepare a relatively less toxic Cu–In–Sn–Se (CISSe) QD by an organic high-temperature hot injection method, and this material is subsequently applied as a functional sensitizer to produce QD-sensitized solar cells (QDSSCs). Because of the effect of Sn doping and ZnS passivation , the TiO 2/CISSe/ZnS photoanode obtains the greatest composite resistance (985.5 Ω·cm2) and highest electron lifetime (120.23 ms) among the experimentally compared materials, showing the further inhibition of charge recombination when compared with bare Cu–In–Se solar cells while effectively enhancing the collection efficiency of photogenerated electrons. Using CISSe/ZnS QDs as a sensitizer, the power conversion efficiency of the QDSSC reaches 6.7%, of which Voc, Jsc, and FF reach 0.559 V, 22.93 mA/cm2, and 0.52, respectively. It provides a new possibility for the development of QDSSCs.

Dedicated Raman spectroscopy investigations in the temperature range from 78 K to 298 K were performed for the graphene suspended above round holes on SiO2/Si substrate (the suspended graphene, SUG), in comparison with the surrounding graphene adhered to solid SiO2/Si substrate (the substrate supported graphene, SSG). The results confirmed that one-layer SUG (1L-SUG) was subjected to strains originated from the surrounding 1L-SSG due to the different area and shape around the hole along different directions. In further, the split-up of G band and 2D band were observed at low temperature because of the enhanced anisotropic strains in 1L-SUG due to the mismatch of thermal expansion coefficient (TEC) between the 1L-SSG and SiO2/Si substrates. The temperature-dependent (T-dependent) frequency shifts for the G band and 2D band in 1L-SUG and 1L-SSG were consistent with previous Raman investigations. Therefore, the micro-hole boundary effect in suspended graphene play a significant role in determination of the properties for graphene materials and graphene-based flexible devices.

Optical response of the indium arsenide (InAs)/gallium arsenide (GaAs) quantum dots (QDs) with a bimodal distribution is investigated through varying an initial GaAs capping layer between 35 and 18 monolayers. Photoluminescence (PL) measurements confirm that a thinner initial capping layer can reduce the QD dimensions and also modify the population ratio between the large QDs and the small QDs. Therefore, PL quenching related to QD dimension and carrier transfer between the bimodal QD populations has been affected. Manipulation of the initial GaAs capping layer provides a feasible approach to tailor the formation and optical performance of InAs QDs for optoelectronic device applications.

An antimony sulfide (Sb2S3) semiconductor is appealing as a promising light absorber due to its suitable bandgap (1.5–1.7 eV), ‘one dimensional’ crystal structure and non-toxic constituents. In this work, the orientation preferences of the Sb2S3 thin films grown on the CdS layer and bare glass substrates were compared by SEM and XRD measurements. The growth direction of (Sb4S6)n ribbons was found to be substrate-dependent, and the growth mechanism was discussed. Thin film solar cells in the configuration of glass/(SnO2:F) FTO/CdS/Sb2S3/Au were fabricated and the highest conversion efficiency reached 3.01%. The defects and recombination losses in the thin film solar cells were investigated by voltage-dependent quantum efficiency and time-resolved photoluminescence measurements. The CdS/Sb2S3 (n–p) heterojunction solar cells showed a little decrease by about 0.2% in conversion efficiency to 2.84% after three month storage in ambient air without encapsulation.

Driven by tensile strain, GaAs quantum dots (QDs) self-assemble on In0.52Al0.48As(111)A surfaces lattice-matched to InP substrates. In this study, we show that the tensile-strained self-assembly process for these GaAs(111)A QDs unexpectedly deviates from the well-known Stranski-Krastanov (SK) growth mode. Traditionally, QDs formed via the SK growth mode form on top of a flat wetting layer (WL) whose thickness is fixed. The inability to tune WL thickness has inhibited researchers’ attempts to fully control QD-WL interactions in these hybrid 0D-2D quantum systems. In contrast, using microscopy, spectroscopy, and computational modeling, we demonstrate that for GaAs(111)A QDs, we can continually increase WL thickness with increasing GaAs deposition, even after the tensile-strained QDs (TSQDs) have begun to form. This anomalous SK behavior enables simultaneous tuning of both TSQD size and WL thickness. No such departure from the canonical SK growth regime has been reported previously. As such, we can now modify QD-WL interactions, with future benefits that include more precise control of TSQD band structure for infrared optoelectronics and quantum optics applications.

In this paper, the flexibility of long-wavelength Type-II InAs/GaSb superlattice (Ga-containing SL) is explored and investigated from the growth to the device performance. First, several samples with different SL period composition and thickness are grown by molecular beam epitaxy. Nearly strain-compensated SLs on GaSb exhibiting an energy band gap between 105 to 169 meV at 77K are obtained. Second, from electronic band structure calculation, material parameters are extracted and compared for the different grown SLs. Finally, two p-i-n device structures with different SL periods are grown and their electrical performance compared. Our investigation shows that an alternative SL design could potentially be used to improve the device performance of diffusion-limited devices for long-wavelength infrared detection., Comment: 8 Figures

This work investigates midwave infrared Type-II InAs/GaSb superlattice (SL) grown by molecular beam epitaxy on GaSb substrate. In order to compensate the natural tensile strain of the InAs layers, two different shutter sequences have been explored during the growth. The first one consists of growing an intentional InSb layer at both interfaces (namely GaSb-on-InAs and InAs-on-GaSb interfaces) by migration enhanced epitaxy while the second uses the antimony-for-arsenic exchange to promote an InSb-like interface at the GaSb-on-InAs interface. SLs obtained via both methods are compared in terms of structural, morphological and optical properties by means of high-resolution x-ray diffraction, atomic force microscopy and photoluminescence spectroscopy. By using the second method, we obtained a nearly strain-compensated SL on GaSb with a full width at half maximum of 56 arcsec for the first-order SL satellite peak. Relatively smooth surface has been achieved with a root mean square value of about 0.19 nm on a 2 $\mu m$ x 2 $\mu m$ scan area. Finally, a p-i-n device structure having a cut-off wavelength of 5.15 $\mu m$ at 77K has been demonstrated with a dark-current level of $8.3 * 10^{-8} A/cm^2$ at -50 mV and a residual carrier concentration of $9.7 * 10^{14} cm^{-3}$, comparable to the state-of-the-art., Comment: 8 figures

The GaSb quantum dots (QDs) with type II band alignment have attracted great attention recently. They are predicted to be optimizing active region materials for achieving high efficient intermediate-band solar cells and for obtaining ultra-long storage time for memory cells. In this research, GaSb QDs sandwiched inside InAlAs matrix lattice-matched to InP (001) substrate have been obtained via droplet epitaxy. The droplet epitaxy enable us to achieve low density (~2.6 x 10^9/cm^2) and large size (average height ~6.5nm) for the QDs while the lattice mismatch between the GaSb and InAlAs matrix is only ~4%. PL measurements reveal a type-II band alignment for these GaSb/InAlAs/InP QDs. The PL peak energy of QDs shows a blue-shift of >100 meV when the laser intensity increases by six orders of magnitude. Time-resolved PL measurements further confirm the type-II band alignment for the QDs by showing a maximum carrier lifetime of ~4.5 ns. The abnormal dependence of peak energy of QD PL band on the temperature in together with the special PL decay curve indicate that these GaSb/InAlAs QDs likely have different physics mechanism from common GaSb/GaAs type-II QDs. This study provide useful information for understanding the band structure and carrier dynamics of the GaSb/InAlAs QDs grown on InP surface.

In this work, the electronic band structure of the InAs/GaSb superlattice (SL) is calculated using a commercial 8-band k⸳p solver and the electrical performance of longwave nBp device structure evaluated with Atlas from Silvaco software. By taking into account an InSb interface layer and the interface matrix (formulated by P.C. Klisptein), the model can predict the measured energy band gap of different InAs/GaSb SLs having different period composition and thickness (7/4, 10/4, 12/4, 14/4 and 14/7 SLs) within an error corresponding to the ±

The post deposition treatment of CsF (CsF-PDT) plays an important role in improving the performance of CIGS solar cells. However, the doping mechanism of Cs-PDT is controversial. To further clarify it, the material properties of the CIGS thin film and photovoltaic characteristics of the devices with different amount of CsF doping are investigated in this paper. It can be demonstrated that the surface morphology, chemical properties and electrical structure of the absorber after CsF-PDT are similar to those of samples after KF-PDT and RbF-PDT. Especially, analogous wide band gap compound CsInSe2 is formed at the surface of the absorber, which can significantly reduce the interfacial recombination between the buffer layer and absorber. However, the measurement results indicate that the distribution and role of Cs in the absorber are somewhat different from those previously reported. Firstly, the entry of Cs does not promote the reduction of Na, but makes more Na enter the absorber. Secondly, the Cs atoms entering the film not only locate at the grain boundary but also enter the grains. Therefore, they can passivate the defects both at the grain boundary and grain interior, improving the hole carrier concentration and minority carrier lifetime. Finally, when excess Cs atoms enter the grain, the lattice constants and texture of the film transform significantly. In this case, the new defects (40 meV and 420 meV) will be generated in the film, which deteriorates the device performance.

The two-dimensional transition metal dichalcogenides (TMDCs) have been considered as promising candidates for developing a new generation of optoelectronic devices. Accordingly, investigations of exciton dynamics are of great importance for understanding the physics and the performance of devices based on TMDCs. Herein, after exposure to ambient environment for six months, monolayer tungsten disulfide (WS2) shows formation of localized states. Photoluminescence (PL) and time-resolved PL (TRPL) spectra demonstrate that these localized states have significant impacts on the exciton dynamics, including energy states filling, thermal activation and redistribution, and the decay behavior of excitons. These observations not only enrich the understanding for localized states and correlated exciton dynamics of aged monolayer WS2, but also reveal a possible approach to modulate the optical properties of TMDCs via the aging process.

With the development of nanoengineering and nanotechnology, numerous emerging materials and constructions are being presented in optoelectronics to challenge traditional photoelectric effects and detecting techniques, and are inspiring innovation and growth in photoelectric research fields. Recently, due to its unique working mechanism, the lateral photovoltaic effect (LPE) has been identified as an indispensable and effective method of studying the properties of novel materials and also shows promising application in position sensors and photodetection. This article will present a comprehensive review of the recent progress on the LPE in novel materials, including metal nanomaterials, oxide semiconductors, organic semiconductors, two-dimensional layered materials, and perovskites. Moreover, it is demonstrated that the LPE is also very sensitive to different constructions or external modulations and can be substantially tuned by changing the structure or morphology of materials, adding external fields, and utilizing plasmon resonance. This summary may provide a comprehensive picture regarding the recent achievements of the LPE in novel materials and external modulations. Finally, we also present a discussion of the challenges and potential trends of the LPE in the near future.

The twisted bilayers of MoS2 with van der Waals coupling have attracted much attention of researchers for providing multiple degrees of freedom to engineer optical and electronic properties. In this work, partial monolayer MoS2 flakes are folded onto the monolayer MoS2 flake itself to form folded MoS2 bilayers during the chemical vapor deposition (CVD) growth process. Lattice vibrations and photon transitions of folded MoS2 bilayers with variable interfacial coupling are revealed by Raman and Photoluminescence (PL) spectroscopy. It is found that interlayer twist angle provides novel physical properties, e.g., the occurring of folded phonons and the evolution of exciton transitions, for promising application of optoelectronic devices.

The optical properties are investigated by spectroscopic characterizations for bilayer InGaAs/GaAs quantum dot (QD) structures consisting of a layer of surface quantum dots (SQDs) separated from a layer of buried quantum dots (BQDs) by different GaAs spacers with thicknesses of 7 nm, 10.5 nm and 70 nm. The coupling from the BQDs to SQDs leads to carrier transfer for the two samples with thin spacers, 7 nm and 10.5 nm, in which QD pairs are obtained while not for the 70 nm spacer sample. The carrier tunneling time is measured to be 0.145 ns and 0.275 ns from BQDs to SQD through the 7 nm and 10.5 nm spacers, respectively. A weak emission band can be observed at the wavelength of ∼ 960 nm, while the excitation intensity dependent PL and PLE spectra show that this is from the wetting layer (WL) of the SQDs. This WL is very important for carrier dynamics in bilayer structures of BQDs and SQDs, including for carrier generation, capture, relaxation, tunneling, and recombination. These results provide useful information for understanding the optical properties of InGaAs SQDs and for using such hybrid structures as building blocks for surface sensing devices.

GaSb quantum dots (QDs) have been grown by droplet epitaxy within InAlAs barrier layers on an InP (001) substrate. The droplet growth mode facilitates a larger size (average height ∼4.5 nm) and a lower density (∼6.3 × 109 cm-2) for the QDs than would be expected for the 4% lattice mismatch between GaSb and InAlAs. A type-II band alignment between the GaSb QDs and the InAlAs barriers is revealed by photoluminescence (PL) through a prominent blue-shift of ∼0.11 eV resulting from a six orders of magnitude increase in excitation power. Further confirmation of the type-II nature of these QDs is found through time-resolved PL studies showing a biexponential decay with a long carrier lifetime of ∼10.9 ns. These observations reveal new information for understanding the formation and properties of GaSb/InAlAs/InP QDs, which may be an optimum system for the development of both efficient memory cells and photovoltaic devices.

Energy transfer (ET) is an important strategy to lower threshold and to extend output spectrum range for organic semiconductor lasers. The key issue of ET assisted lasing is to balance ET rate kET and stimulated emission rate kSE of donor. In this research we studied ET process between Oligo(Phenylene Vinylene), 1,4-dimethoxy-2,5-di[4′-(methylthio)styryl]benzene (TDSB) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in the microcrystal. The stimulated emission rate kSE of TDSB is estimated to be 7.6 × 10 10 s∧{-1}, and the ET rate kET between TDSB and DCM is calculated on the order of ~ 109 s∧{-1}. In addition, we experimentally investigated the influences of DCM doping concentration and temperature on the ET process between TDSB and DCM, showing that the ET rate kET increases with high DCM doping concentration. But in microcrystal matrix DCM was prone to non-uniform distribution in high doping region to deteriorate the ET rate kET. We also compared the ET rate kET and ET efficiency at different temperatures, and found the ET efficiency at room temperature is nearly twice of that at 35 K. The dependence of ET process on temperature can be well explained with exciton diffusion assisted ET mode.

A recently developed growth technique enables the self-assembly of defect-free quantum dots on (111) surfaces under large tensile strains. We demonstrate the use of this approach to synthesize germanium (Ge) quantum dots on In0.52Al0.48As(111)A with >3% residual tensile strain. We show that the size and areal density of the tensile-strained Ge quantum dots are readily tunable with growth conditions. We also present evidence for an unusual transition in the quantum dot growth mode from Stranski-Krastanov to Volmer-Weber as we adjust the substrate temperature. This work positions Ge quantum dots as a promising starting point for exploring the effects of tensile strain on Ge’s band structure.

In(Ga)As surface quantum dots (SQDs) are promising candidates for surface-sensitive detection applications. As such, studies of the carrier dynamics are indispensable when attempting to understand both the physics and the device performances based on such SQD structures. Here, lateral carrier transfer among neighboring quantum dots (QDs) has been investigated through spectroscopic measurements for high density InGaAs/GaAs SQDs in a hybrid nanostructure with similar buried QDs (BQDs). Photoluminescence (PL) and temporal-resolved PL (TRPL) spectra demonstrate that the SQDs exhibit intra-layer exciton transfer and also strong carrier transfer to the surface states at 10 K. This imparts different optical characteristics to the SQDs with regards to the band filling and emission-energy-dependent carrier lifetime. In addition, PL excitation (PLE) spectra reveal a wetting layer absorption feature for the SQDs, which provides a channel for thermally activated carriers to transfer among the SQDs and to ultimately escape from the SQD layer. These investigations enrich our understanding of carrier dynamics for InGaAs SQDs, which may help to develop novel surface sensitive sensors.

We investigate the optical properties of InGaAs surface quantum dots (SQDs) in a composite nanostructure with a layer of similarly grown buried quantum dots (BQDs) separated by a thick GaAs spacer, but with varied areal densities of SQDs controlled by using different growth temperatures. Such SQDs behave differently from the BQDs, depending on the surface morphology. Dedicated photoluminescence (PL) measurements for the SQDs grown at 505 °C reveal that the SQD emission follows different relaxation channels while exhibiting abnormal thermal quenching. The PL intensity ratio between the SQDs and BQDs demonstrates interplay between excitation intensity and temperature. These observations suggest a strong dependence on the surface for carrier dynamics of the SQDs, depending on the temperature and excitation intensity.

The performance of nanowire-based devices is predominantly affected by nonradiative recombination on their surfaces, or sidewalls, due to large surface-to-volume ratios. A common approach to quantitatively characterize surface recombination is to implement time-resolved photoluminescence to correlate surface recombination velocity with measured minority carrier lifetime by a conventional analytical equation. However, after using numerical simulations based on a three-dimensional (3D) transient model, we assert that the correlation between minority carrier lifetime and surface recombination velocity is dependent on a more complex combination of factors, including nanowire geometry, energy-band alignment, and spatial carrier diffusion in 3D. To demonstrate this assertion, we use three cases—GaAs nanowires, InGaAs nanowires, and InGaAs inserts embedded in GaAs nanowires—and numerically calculate the carrier lifetimes by varying the surface recombination velocities. Using this information, we then investigate the intrinsic carrier dynamics within those 3D structures. We argue that the conventional analytical approach to determining surface recombination in nanowires is of limited applicability, and that a comprehensive computation in 3D can provide more accurate analysis. Our study provides a solid theoretical foundation to further understand surface characteristics and carrier dynamics for 3D nanostructured materials.

As part of a study on radiation effects in optoelectronic materials, we exposed a series of AIGaAs/GaAs double heterostructures grown by molecular beam epitaxy and metalorganic chemical vapor deposition to electron and proton radiation. The active regions of the test articles were either p-, n-, or unintentionally doped. Steady state and time resolved photoluminescence spectroscopy were used to characterize radiation induced changes to the band structure and carrier dynamics. The effects of electron radiation on low temperature photoluminescence spectra and room temperature carrier dynamics varied with dopant type and density. Steady-state photoluminescence reveals distinct proton exposure effects for p-type materials grown by molecular beam epitaxy compared to n-type structures. Both the steady state and time resolved results suggest that n-type materials are more radiation hard to the effects of 1 MeV electrons than p-type materials.

Recently, to sandwich InAs QDs with GaAs1-xSbx layers have attracted enormous attention. It is expected to achieve a transition from type-I to type-II band alignment at ~ x=12%. The type-II InAs/GaAs1-xSbx QDs are predicted to be one of the optimizing active region materials for achieving high efficient intermediate-band solar cells. We first investigate PL properties of InAs/GaAs1-xSbx QD structures of different Sb compositions (x=0, 0.15, and 0.25) in the GaAs1-xSbx capping layer. By capping the InAs QDs with GaAs0.85Sb0.15 and GaAs0.75Sb0.25 layer, we are able to gain type II QDs. These type II QDs exhibit a clear multi-peaks characteristic under increasing laser excitation intensity, which stems from different carriers recombination routes due to the combination of InAs QDs with GaAs1-xSbx capping layer. Time-resolved PL measurements further confirm our assignment for the mnulti-peaks in the PL spectra. We then study carrier coupling inside vertically aligned InAs/GaAs and InAs/GaAs1-xSbx QD pairs. The features of InAs/GaAs1-xSbx QD pairs are very different from the traditional InAs/GaAs QD-pairs. Again, clear multi-peaks characteristic are observed under stronng laser excitation intensity, which stems from different carriers recombination routes from the top InAs/GaAs1-xSbx QDs. Our investigations indicate that the optical behavior and carrier dynamics in type-II InAs/GaAs1-xSbx QDs and QD-pairs are much more complicated than the InAs/GaAs QDs counterparts. This study provide useful information for understanding the band structure and carrier dynamics of the InAs/GaAs1-xSbx QDs for high efficiency solar cell applications.

The optical properties have been investigated for the GaAs/Al0.2Ga0.8As quantum dot-ring hybrid nanostructures grown by droplet epitaxy, in which each nanostructure consists of four quantum dots (QDs) sitting on a distinct ring of GaAs. A blueshift and narrowing of the photoluminescence (PL) spectra along with the nonlinear decay of the time-resolved PL curves of the QDs have been observed. These abnormal PL behaviors are caused by the unique state filling effect correlated with the quantum dot-ring structure feature, which is strongly affected by carrier transfer from smaller dots to larger dots via the wetting ring in the GaAs/Al0.2Ga0.8As hybrid structure.

To characterize surface recombination of nanowires, time-resolved photoluminescence (TRPL) is commonly implemented to correlate measured lifetime with the nonradiative effect at surface. In this work, we develop a threedimensional transient model to perform a numerical analysis of surface recombination for InGaAs nanowires on GaAs substrates. By mimicking a complete TRPL measurement process, we computationally calculate optical generation and emission of InGaAs nanowires, and numerically probe the carrier dynamics inside nanowires. It is found that the TRPL spectra are determined by a complex convolution of surface recombination velocity and incident wavelengths. In addition, we show that due to the three-dimensional geometry of nanowire, using a typical analytical equation to extract surface recombination velocity might be no longer valid. We believe these results provide an alternative approach for the computational analysis of TRPL measurements and surface properties for three-dimensional nanostructured devices.

Single layer self-assembled InGaAs quantum dots (QDs) are manipulated by using different arsenic species on GaAs (100) surface. The As4 molecules are experimentally observed to be more promising than As2 to promote the formation of one-dimensionally-aligned QD-chain arrays. The lateral alignment of QDs and the corresponding formation of dot chains are explained by the anisotropic surface kinetics in combination with the different reactivities of the two molecules with bonding sites on the GaAs (100) surface. Photoluminescence (PL) measurements demonstrate that the spectra of the QD-chains broaden to higher energy and increases in intensity with increasing excitation laser power. The PL band of the QD-chains also exhibits a 9 meV reduction in linewidth as temperature increases starting from 8 K. These observations confirm an efficient lateral coupling between neighboring QDs and thereafter polarized QD emission, whereas the randomly distributed QDs grown with As2 show no preferential polarization. Such QD-chains exhibiting anisotropic properties have the potential for nanophotonics applications like electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation.

We investigate stacked structures of InAs/AlAsSb/InP quantum dots using temperature- and power-dependent photoluminescence. The band gap of InAs/AlAsSb QDs is 0.73 eV at room temperature, which is close to the ideal case for intermediate band solar cells. As the number of quantum dot layers is increased, the photoluminescence undergoes a blue-shift due to the effects of accumulated compressive strain. This PL red shift can be counteracted using thin layers of AlAs to compensate the strain. We also derive thermal activation energies for this exotic quantum dot system.

III-(As, Sb) alloys are building blocks for various advanced optoelectronic devices, but the growth of their ternary or quaternary materials are commonly limited by spontaneous formation of clusters and phase separations during alloying. Recently, digital alloy growth by molecular beam epitaxy has been widely adopted in preference to conventional random alloy growth because of the extra degree of control offered by the ordered alloying. In this article, we provide a comparative study of the optical characteristics of AlAsSb alloys grown lattice-matched to GaSb using both techniques. The sample grown by digital alloy technique showed stronger photoluminescence intensity, narrower peak linewidth, and larger carrier activation energy than the random alloy technique, indicating an improved optical quality with lower density of non-radiative recombination centers. In addition, a relatively long carrier lifetime was observed from the digital alloy sample, consistent with the results obtained from the photoluminescence study.