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Abstract: The carrier recombination dynamics of InGaAsP material with a bandgap energy of 1eV for quadruple-junction solar cells grown by solid-source molecular beam epitaxy have been investigated by the employment of time-resolved photoluminescence (PL) measurement. For the nominally undoped material, the PL decay time increases with increasing temperature, which indicates that radiative recombination dominates the recombination process. The radiative and the nonradiative recombination time constants were calculated on the basis of the temperature-dependent PL decay time and the integrated PL intensity. With the incorporation of Be (as the p-type dopant) into the material, the PL decay time decreases with increasing temperature, and a double-exponential PL decay curve is observed in the case of the material with a higher doping density. An InGaAsP-based single-junction photovoltaic device with a bandgap of 1eV was fabricated, and an efficiency of 16.4% was obtained under the AM1.5G solar spectra. [Copyright &y& Elsevier]

Abstract: There are many difficulties limiting the further development of monolithic multi-junction solar cells, such as the growth of lattice-mismatched material and the current matching constraint. As an alternative approach, the light-splitting photovoltaic system is investigated intensively in different aspects, including the energy loss mechanism and the choice of energy bandgaps of solar cells. Based on the investigation, a two-dual junction system has been implemented employing lattice-matched GaInP/GaAs and InGaAsP/InGaAs cells grown epitaxially on GaAs and InP substrates, respectively. [ABSTRACT FROM AUTHOR]

Abstract: To investigate factors limiting the performance of a GaAs solar cell, genetic algorithm is employed to fit the experimentally measured internal quantum efficiency (IQE) in the full spectra range. The device parameters such as diffusion lengths and surface recombination velocities are extracted. Electron beam induced current (EBIC) is performed in the base region of the cell with obtained diffusion length agreeing with the fit result. The advantage of genetic algorithm is illustrated. [ABSTRACT FROM AUTHOR]

Glass spherical microcavities containing CdSSe semiconductor quantum dots (QDs) of a few microns in diameter are fabricated using a physical method. When a single glass microspherical cavity is excited by a laser beam at room temperature, very strong and sharp whispering gallery modes are shown on the background of PL spectra of CdSSe QDs, which confirms that coupling between the optical emission of embedded QDs and spherical cavity modes is realized. For a glass microsphere only 4.6 μm in diameter, it was found that the energy separation is nearly up to 26 nm both for TE and TM modes. With the increasing excitation intensity, the excitation intensity dependence of the emission intensity is not linear in the double-logarithmic scale. Above the threshold value, the linewidths of resonance modes become narrower. The lasing behavior is achieved at relatively low excitation intensity at room temperature. High optical stability and low threshold value make this optical system promising in visible microlaser applications. [Copyright &y& Elsevier]

Due to significant response contradictions, unidirectional carrier transmission of typical semiconductor p‐n junctions limits integrated sensors and artificial synapses in a monolithic device. In this work, a bipolar p‐n junction designed by employing the hydrogel/p‐GaN local contact interface with p‐GaN/(In,Ga)N heterojunction, demonstrating the bidirectional photocurrent and sensor/artificial synapse dual‐mode device successfully. After modifying Au nanoparticles, the negative (positive) photocurrent is increased by 900% (300%) under 365 nm (520 nm) light to achieve a more balanced bipolar carrier dynamic. Such a regulation of photocurrent is found to be attributed to the promotion of hydrogen evolution reaction (HER) at the hydrogel/Au/p‐GaN interface. Thus, the device achieves the ultrahigh paired‐pulse facilitation (PPF) index of 243% under self‐powered condition. Moreover, the implementation of plasticity from short‐term to long‐term demonstrates its adaptability in neural morphology visual recognition. Beyond independent working mode, the function of five classic logic gates is achieved under three‐terminal operation. Finally, an encrypted wireless communication system using dual‐channel light signals demonstrates the extensive application potential of the monolithic device. Therefore, this work presents a viable construction blueprint to meet the demands of next‐generation all‐in‐one optoelectronics for intricate application scenarios. [ABSTRACT FROM AUTHOR]

Highlights • Nanocolumn selective-area growth was achieved by nanosphere lithography and etching. • The shadow effect and Ti growth suppression contribute to selective-area growth. • InGaN sections selective-area grown could be beneficial to uniform In distribution. Abstract In this paper, the selective-area growth (SAG) of GaN-based nanocolumns (NCs) on silicon (Si) has been investigated experimentally. A low-cost and facile method has been proposed and implemented to fabricate the SAG NCs, which contains the processes of nanosphere lithography (NSL) and plasma-assisted molecular beam epitaxy (MBE). SAG NCs grown by this method indicates a higher ability to modulate the NC distributions than those grown by the conventional patterned method. According to photoluminescence results, InGaN/GaN NCs grown on Si nanopillars produced by NSL and etching show the more uniform indium (In) distribution than those grown on the openings of patterned Si substrate. Furthermore, according to the kinetic processes during MBE growth, it is proposed that the SAG of NCs results from the combination of the shadow effect of Si nano-pillars and the growth suppression of titanium (Ti) film. [ABSTRACT FROM AUTHOR]

InGaN quantum dots (QDs) have attracted significant attention as a promising material for high-efficiency micro-LEDs. In this study, plasma-assisted molecular beam epitaxy (PA-MBE) was used to grow self-assembled InGaN QDs for the fabrication of green micro-LEDs. The InGaN QDs exhibited a high density of over 3.0 × 1010 cm−2, along with good dispersion and uniform size distribution. Micro-LEDs based on QDs with side lengths of the square mesa of 4, 8, 10, and 20 μm were prepared. Attributed to the shielding effect of QDs on the polarized field, luminescence tests indicated that InGaN QDs micro-LEDs exhibited excellent wavelength stability with increasing injection current density. The micro-LEDs with a side length of 8 μm showed a shift of 16.9 nm in the peak of emission wavelength as the injection current increased from 1 A/cm2 to 1000 A/cm2. Furthermore, InGaN QDs micro-LEDs maintained good performance stability with decreasing platform size at low current density. The EQE peak of the 8 μm micro-LEDs is 0.42%, which is 91% of the EQE peak of the 20 µm devices. This phenomenon can be attributed to the confinement effect of QDs on carriers, which is significant for the development of full-color micro-LED displays. [ABSTRACT FROM AUTHOR]

The charge carrier concentration profile is a critical factor that determines semiconducting material properties and device performance. Dielectric force microscopy (DFM) has been previously developed to map charge carrier concentrations with nanometer-scale spatial resolution. However, it is challenging to quantitatively obtain the charge carrier concentration, since the dielectric force is also affected by the mobility. Here, we quantitative measured the charge carrier concentration at the saturation mobility regime via the rectification effect-dependent gating ratio of DFM. By measuring a series of n-type GaAs and GaN thin films with mobility in the saturation regime, we confirmed the decreased DFM-measured gating ratio with increasing electron concentration. Combined with numerical simulation to calibrate the tip–sample geometry-induced systematic error, the quantitative correlation between the DFM-measured gating ratio and the electron concentration has been established, where the extracted electron concentration presents high accuracy in the range of 4 × 1016 – 1 × 1018 cm−3. We expect the quantitative DFM to find broad applications in characterizing the charge carrier transport properties of various semiconducting materials and devices. [ABSTRACT FROM AUTHOR]

• For the first time, OISF-free Si polished wafer were employed as substrates for GaN-epitaxial wafers. • The surface cracks of 8-inch epitaxial wafers were removed fully and the bow value was decreased to 14.3 μm. • The yields of high electron mobility transistors grown on GaN wafers were improved up to 97%. The oxygen induced stacking fault (OISF) in heavy boron-doping Si substrates causes severe warpage and surface cracks of GaN epitaxial wafers, which will lower the production yield of GaN-based power devices. In this work, high-quality 8-inch Si substrates were fabricated by using rapid thermal annealing (RTA) to eliminate OISF. RTA at 1200 °C effectively eliminated OISF, then surface cracks of 8-inch epitaxial wafers were removed fully and its bow value was decreased markedly from 45.7 μm to 14.3 μm. Consequently, the yields of high electron mobility transistors based on GaN wafers were improved up to 97 %. [ABSTRACT FROM AUTHOR]

Metal modulation epitaxy (MME) is a technique in which metal beams (Al, Ga, In, and Mg) are switched on and off in short periods in an RF MBE system while a continuous nitrogen plasma beam is kept on. We systematically studied the effect of periodic duty cycling on the morphology, crystalline quality, Mg doping concentration, and electrical properties of GaN:Mg films grown by MME. When the metal shutter duty cycling is 20 s open/10 s close, the sample has smooth surface with clear steps even with Mg doping concentration higher than 1 × 1020 cm−3. The RMS roughness is about 0.5 nm. The FWHM of (002) XRD rocking curve is 230 arcsec and the FWHM of (102) XRD rocking curve is 260 arcsec. As result, a hole concentration of 5 × 1018 cm−3 and a resistivity of 1.5 Ω·cm have been obtained. The hole concentration increases due to the incorporation of surface accumulated Mg dopants into suitable Ga substitutional sites with minimal formation of compensatory defects. [ABSTRACT FROM AUTHOR]

Due to the emerging requirements of miniaturization and multifunctionality, monolithic devices with both functions of lighting and detection are essential for next-generation optoelectronic devices. In this work, based on freestanding (In,Ga)N films, we demonstrate a monolithic device with two functions of lighting and self-powered detection successfully. The freestanding (In,Ga)N film is detached from the epitaxial silicon (Si) substrate by a cost-effective and fast method of electrochemical etching. Due to the stress release and the lightening of the quantum-confined Stark effect (QCSE), the wavelength blueshift of electroluminescent (EL) peak is very small (<1 nm) when increasing the injection current, leading to quite stable EL spectra. On the other hand, the proposed monolithic bifunctional device can have a high ultraviolet/visible reject ratio (Q = 821) for self-powered detection, leading to the excellent detection selectivity. The main reason can be attributed to the removal of Si by the lift-off process, which can limit the response to visible light. This work paves an effective way to develop new monolithic multifunctional devices for both detection and display. [ABSTRACT FROM AUTHOR]

Owing to its broadband absorption, ultrafast recovery time, and excellent saturable absorption feature, graphene has been recognized as one of the best candidates as a high-performance saturable absorber (SA). However, the low absorption efficiency and reduced modulation depth severely limit the application of graphene-based SA in ultrafast fiber lasers. In this paper, a single-layer graphene saturable absorber mirror (SG-SAM) was coated by a quarter-wave SiO2 passivated layer, and a significantly enhanced modulation depth and reduced saturation intensity were obtained simultaneously compared to the SG-SAM without the SiO2 coating layer. In addition, long-term operational stability was found in the device due to the excellent isolation and protection of the graphene absorption layer from the external environment by the SiO2 layer. The high performance of the SAM was further confirmed by the construction of a ring-cavity EDF laser generating mode-locked pulses with a central wavelength of 1563.7 nm, a repetition rate of 34.17 MHz, and a pulse width of 830 fs. [ABSTRACT FROM AUTHOR]

Self-assembled growth of blue-green-yellow-red InGaN quantum dots (QDs) on GaN templates using plasma-assisted molecular beam epitaxy were investigated. We concluded that growth conditions, including small N2 flow and high growth temperature are beneficial to the formation of InGaN QDs and improve the crystal quality. The lower In/Ga flux ratio and lower growth temperature are favorable for the formation of QDs of long emission wavelength. Moreover, the nitrogen modulation epitaxy method can extend the wavelength of QDs from green to red. As a result, visible light emissions from 460 nm to 622 nm have been achieved. Furthermore, a 505 nm green light-emitting diode (LED) based on InGaN/GaN MQDs was prepared. The LED has a low external quantum efficiency of 0.14% and shows an efficiency droop with increasing injection current. However, electroluminescence spectra exhibited a strong wavelength stability, with a negligible shift of less than 1.0 nm as injection current density increased from 8 A/cm2 to 160 A/cm2, owing to the screening of polarization-related electric field in QDs. [ABSTRACT FROM AUTHOR]

In this paper, we report bidirectional negative differential resistance (NDR) in Al(Ga)N/GaN/AlN resonant tunneling diodes grown on free-standing GaN substrates by RF-plasma assisted molecular beam epitaxy. Bidirectional NDR has exhibited a current–voltage (I–V) characteristic in both forward and reverse biases at room temperature. The positive peak current density is 160 kA/cm2 with a peak to valley current ratio (PVCR) of 1.34, and the negative peak current density is 112 kA/cm2 with a high PVCR of 1.56. The bidirectional NDR is attributed to the change in the polarization field in the active region, which is caused by the asymmetric barrier component. [ABSTRACT FROM AUTHOR]

Due to the wide application of UV-A (320 nm–400 nm) and UV-C (200 nm–280 nm) photodetectors, dual-wavelength (UV-A/UV-C) photodetectors are promising for future markets. A dual-wavelength UV photodetector based on vertical (Al,Ga)N nanowires and graphene has been demonstrated successfully, in which graphene is used as a transparent electrode. Both UV-A and UV-C responses can be clearly detected by the device, and the rejection ratio (R254 nm/R450 nm) exceeds 35 times at an applied bias of –2 V. The short response time of the device is less than 20 ms. Furthermore, the underlying mechanism of double ultraviolet responses has also been analyzed systematically. The dual-wavelength detections could mainly result from the appropriate ratio of the thicknesses and the enough energy band difference of (Al,Ga)N and GaN sections. [ABSTRACT FROM AUTHOR]

The crystal structure, light transmittance, and contact resistivity of transparent conducting indium‐tin‐oxide (ITO) films for a III–V solar cell are studied. The optical transparency of ITO can reach 96% under appropriate annealing conditions. ITO and n+‐GaAs can form Ohmic contacts having a lower contact resistance than those formed with ITO and n+‐GaInP. A specially designed solar cell structure is fabricated to grow the ITO film as the front electrode and thus prevent oxidation of the AlInP window layer. Three different GaInP solar cells with pure ITO and ITO/metal grid electrode structures are designed and compared. The results indicate that the ITO electrode has good carrier‐collection ability. After irradiation with 1 MeV energy electrons with an electron fluence of 1 × 1015 e cm−2, the remaining factor of the GaInP solar cells with a pure ITO electrode is ≈98%. The results indicate that ITO films have significant application potential in space solar cell manufacturing. [ABSTRACT FROM AUTHOR]

As the key device in a UV warning system, a solid‐state UV photodetector has attracted great attention. Herein, a new UV photodetector structure based on vertical (Al, Ga)N nanowires with graphene electrode and Si substrate is designed and demonstrated. By graphene/vertical (Al, Ga)N nanowire array heterojunction, the rectifying characteristics of I–V curve are formed, and the dark current is 54 nA at −2 V bias. The fabricated device exhibits a responsivity of 0.176 mA W−1 at the bias of −2 V, as well as a stable switching characteristic. It is proposed that the photogenerated electrons and holes can reach positive and negative electrodes by diffusion or tunneling effect, even with thick AlN sections. This method can promote the implementation of low cost for UV photodetector with adjustable detection range nowadays. [ABSTRACT FROM AUTHOR]

• Growth of mismatched InGaAs on Si is an important method to improve integration. • Investigated the effect of the low temperature buffer layer on the quality of InGaAs grown on the GaP/Si template. • Better dislocation relaxation can be achieved by using the digital alloy InAs/InP as the buffer layer. In this paper, the effect of low-temperature buffer layer on the quality of InGaAs grown on GaP/Si templates is investigated. Three different materials, InP, GaAs/InAs and InAs/InP, were grown on GaP/Si templates by molecular beam epitaxy (MBE). The interfacial states between the above materials and the GaP layer were calculated using first principles methods. It was found that the interfacial structure formed with the InAs/InP material was the most robust due to the shorter bond lengths and stronger interaction forces between the interfaces. Theoretical calculations are in agreement with the experimental results that the InGaAs grown using the InAs/InP digital alloy structure as a low temperature buffer layer has the best material quality. [ABSTRACT FROM AUTHOR]

Highlights • InGaN film with high-In-content of 51.9% has been obtained by MBE MME-growth. • MME-growth technique can improve crystal quality of high-In content InGaN. • The effect of MME time ratio on the properties of InGaN material was investigated. • MME technique can effectively release the strain of In-rich InGaN materials. Abstract We investigated the growth and characterization of InGaN films with high In content up to 51.9% obtained by MBE MME-growth. Comparing the structural and optical properties of InGaN films grown using common growth method and MME technique, we could acquire the good quality of InGaN sample by MME-growth method with lower dislocation density, better composition uniformity and smoother surface. These experimental results suggest that MME has an enormous potential to promote the quality of high In-content InGaN materials and the development of InGaN-based devices. Besides, further study shows the process of MME growth, the parameters of material growth have opposite effects on the background carrier concentration and carrier mobility. [ABSTRACT FROM AUTHOR]

Highlights • The multi-junction solar cells were grown by MBE. • A higher growth temperature is good for GaInP and InGaAsP. • Multi-junction solar cells were fabricated by wafer bonding technique. • The optimiztion process of four-junction solar cell was discussed. Abstract We report on the room-temperature wafer-bonded InGaP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid-source molecular beam epitaxy (MBE). The material growth and structure optimization of multi-junction solar cell were investigated. The effect of growth temperature on phosphide-related material of the solar cell was discussed. The lattice-matched InGaP/GaAs tandem cell on GaAs substrate and InGaAsP- or InGaAsP/InGaAs-tandem cell on InP substrate were grown separately by MBE. Room-temperature wafer bonding technique was used to integrate the subcells into multi-junction solar cells. The current limiting of multi-junction cells was analysed and the device structure of four-junction solar cell was optimized. The best wafer-bonded four-junction solar cell reached a conversion efficiency of 42% under concentration. [ABSTRACT FROM AUTHOR]

Highlights • TOECs are obtained by using the strain-energy relation method on the GGA level. • Successfully analyzed the nonlinear response of unordered epitaxial layers. • The nonlinear correction rate of strain energy of epitaxial GaN on AlN is 18.5%. • The nonlinear correction rate of internal stress of epitaxial GaN on AlN is 29.4%. Abstract The mechanical properties of Al 1−x Ga x N epitaxial layers grown on GaN and AlN substrates were studied by using density functional theory and anharmonic elastic calculation method. Accurate theoretical calculation of nonlinear elastic responses is a basic and important scientific question, which is the key to understand and further explore the potential synthesis and design of new solids. In this paper, detailed numerical calculations were carried out for the nonlinear mechanical properties of würtzite GaN and AlN compounds. The variation of strain energy and stress of disordered Al 1−x Ga x N epitaxial layer with gallium composition was analyzed. We found that in the epitaxial structure of AlN substrate, with the increase of gallium content, the influence of nonlinear effect on internal stress reached 29.4%, which proved the great influence of nonlinear mechanical response on semiconductor alloy material and heterostructure. [ABSTRACT FROM AUTHOR]

Highlights • The AlGaInAs graded buffer layer is grown to relax the strain of InGaAs with GaAs. • The solar cell achieves rapid removal of the substrate by a GaInP etch stop layer. • The Cu thin film serve as not only the flexible support but also the electrode. • The photovoltaic conversion efficiency of the thin film solar cell is 33.13%. Abstract Highly efficient, lightweight, and flexible III-V compound multi-junction solar cells play an important role in the wearable electronic field. GaInP/GaAs/InGaAs inverted metamorphic (IMM) solar cells directly grown on an N-doped GaAs substrate by a GaInP etch stop layer were fabricated. The AlGaInAs graded buffer layer is grown to relax the strain of InGaAs with GaAs. Most misfit dislocations are localized at the internal interfaces of the graded buffer and do not appear to propagate. Then, the solar cells are fabricated by the copper-plated process, and Cu thin films serve as not only the flexible support but also the P-type electrode. The device had a total thickness of less than 35 μm, a mass density of less than 0.03 g/cm2, and can exhibit excellent bendability. Under the AM1.5G solar illumination, the thin film solar cell has the best photovoltaic conversion efficiency of 33.13% with the Voc of 2.86 V, a Jsc value of 13.55 mA/cm2, and a FF of 85.49%. This method can be used to prepare flexible thin film solar cells simply and efficiently. [ABSTRACT FROM AUTHOR]

Highlights • Deep-UV emission at 260 nm has been obtained from AlGaN-based MQW structures. • High AlGaN and AlGaN/AlN MQW structures has been grown by PA-MBE. • Larger Al flux ratio and higher growth temperature contribute to increasing Al content of AlGaN. • The emission wavelength was tuned by controlling the thickness of AlGaN quantum well. Abstract Deep ultraviolet (UV) emission at 260 nm has been obtained from high Al content AlGaN/AlN multi-quantum well (MQW) structures by plasma-assisted molecular beam epitaxy (PA-MBE) on AlN template on sapphire substrates. The effect of III/V ratio and growth temperature on structural and optical properties of AlGaN was studied. It was observed that Al content in AlGaN material increased with increasing Al flux ratio or growth temperature. By altering growth conditions, AlGaN with high Al content of 47–80% was obtained. Further studies indicate that AlGaN layers have excellent surface morphology and optical properties grown in slightly metal-rich condition. As for AlGaN/AlN MQW, the emission wavelength was tuned by controlling the thickness of AlGaN quantum well, and deep ultraviolet emission below up to 260 nm was obtained from AlGaN (2.4 nm)/AlN(6.9 nm) MQW at room temperature. [ABSTRACT FROM AUTHOR]

Multi-functional photodetector is very important in spectral information acquisition but is difficult to implement. In this work, a self-driven multifunctional ultraviolet photodetector based on (Al,Ga)N nanowire/hydrogel interface is proposed and demonstrated successfully. It is found that the conventional photodetector has two functional interfaces with the full contact between nanowires and liquid-electrolyte, which is very difficult to modulate the competition carrier transport process. Thanks to the design of local contact between (Al,Ga)N nanowires and quasi-solid hydrogel, only one functional interface operates in the photodetector, achieving the slow/fast response speed over 20 times difference at 0 V bias (1080/1217 ms under 310 nm illumination and 46/45 ms under 365 nm illumination). Such unique behavior of switchable response speed is mainly attributed to the balance of dynamic carrier transport across the (Al,Ga)N/GaN and nanowire/hydrogel local contact interfaces. Moreover, the local contact interface proves excellent reusability and the photocurrent density remains at 94.7% after preserving for 2 months. Particularly, taking advantage of the slow/fast response characteristic, the proposed self-driven ultraviolet photodetector is demonstrated to have the ability to distinguish light wavelengths and can be integrated with hydrogel into an array detection system for the image display. [Display omitted] • A novel photodetector with (Al,Ga)N nanowire/hydrogel interface is realized. • The self-driven photodetector shows a special switchable response speed. • The use of hydrogel breaks the limitation of traditional liquid-electrolyte and cell. • The response speed is fast for PEC-type devices with nanostructure. • Self-driven photodetector array can distinguish light wavelengths and display images. [ABSTRACT FROM AUTHOR]

We demonstrate both theoretically and experimentally that the power density of resonant tunneling diode (RTD) can be enhanced by optimizing emitter spacer layer thickness, in addition to reducing barrier thickness. Compared to the widely used epitaxial structure with ultrathin emitter spacer layer thickness, appropriate increasing the thickness will increase the voltage drop in accumulation region, leading to larger voltage widths of negative differential resistance region. By measuring J-V characteristics, the specific contact resistivity, and the self-capacitance, we theoretically analyze the maximum output power of the fabricated RTDs. It shows that the optimized In 0.8 Ga 0.2 As/AlAs RTD with 20 nm emitter spacer thickness and 5 μm 2 mesa area theoretically possesses the capability to reach 3.1 mW at 300 GHz and 1.8 mW at 600 GHz. [ABSTRACT FROM AUTHOR]

Comparative study of GaAs and CdTe solar cell under low-intensity light irradiance was carried out to study the cell device performance in response to the changed light irradiance intensity. For highly efficient GaAs solar cell, the series R s and the shunt R p resistance were found to be low/high enough to have almost undetectable negative effect on the cell device performance at low-intensity light irradiance. The robust diode parameters guaranteed good cell performance at low-intensity light irradiance. An ideal logarithmic function was established to describe the variation of cell efficiency with light irradiance intensity for GaAs solar cell. For CdTe polycrystalline solar cell, the relatively large series resistance R s and small shunt resistance R p , compared to that of the GaAs solar cell, significantly decreased the cell fill factor. With increased light illumination intensity, both the diode ideality factor A and the reverse saturation current density J 0 were increased due to the high density of interface states at the CdS/CdTe junction. The comparative study and conclusions drawn in this work provide device fabrication improvement direction for the CdTe solar cell. [ABSTRACT FROM AUTHOR]

For application in space environments, the effect of 1-MeV electron irradiation on wafer-bonded GaInP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid state molecular beam epitaxy was studied. After exposure to 1-MeV electron irradiation at 1×15 e/cm 2 , an end of life remaining factor of approximately 85% was obtained. The wafer bonding interface was studied by spectral response and transmission electron microscopy. 1-MeV electron irradiation was conducted on the individual InGaAsP and InGaAs single junction cell, respectively. The degradation of the four-junction cell was mainly due to damage on the InGaAsP and InGaAs subcells rather than the bonding interface. [ABSTRACT FROM AUTHOR]

Rapid thermal annealing (RTA) has been performed on InGaAsP solar cells with the bandgap energy of 1 eV grown by molecular beam epitaxy. With the employment of RTA under an optimized condition, the open voltage was increased from 0.45 to 0.5 V and the photoelectric conversion efficiency was increased from 11.87–13.2%, respectively, which was attributed to the crystal quality improvement of p-type InGaAsP and therefore a reduced recombination current inside depletion region. The integral photoluminescence (PL) intensity of p-type InGaAsP increased to 166 times after annealing at 800 °C and its PL decay time increased by one order of magnitude. While the changes of nominally undoped and n-doped InGaAsP were negligible. The different behaviors of the effect of RTA on InGaAsP of different doping types were attributed to the highly mobile “activator” – beryllium (Be) atom in p-type InGaAsP. [ABSTRACT FROM AUTHOR]

We have investigated the electron spin relaxation in undoped InGaAsP grown on an InP substrate. Time-resolved spin-dependent pump and probe reflection measurements revealed a spin relaxation behavior between 10 and 300 K. We have observed the electron spin relaxation times of 980 ps at 10 K and 95 ps at room temperature. The observed presence of the carrier density dependence and the weak temperature dependence of spin relaxation time imply that the Bir-Aronov-Pikus (BAP) process is effective at 10-30 K. At 100-300 K, the observed presence of the temperature dependence and the absence of the carrier density dependence of spin relaxation time indicate that the D'yakonov-Perel' (DP) and Elliott-Yafet (EY) processes are effective. [ABSTRACT FROM AUTHOR]

We report on the study of InGaAsP solar cells grown by solid-state molecular beam epitaxy (MBE) on InP. The effect of growth temperature on device performance is investigated. Under standard one-sun air-mass 1.5 global (AM1.5) illumination, an efficiency of 18.8% has been obtained for In 0.78 Ga 0.22 As 0.48 P 0.52 single-junction solar cells grown at high temperature. Time-resolved photoluminescence (PL) results demonstrate that the In 0.78 Ga 0.22 As 0.48 P 0.52 optical quality is greatly improved in the case of a high growth temperature. A longer PL decay time of In 0.78 Ga 0.22 As 0.48 P 0.52 /InP in contrast to In 0.78 Ga 0.22 As 0.48 P 0.52 /In 0.52 Al 0.48 As indicates that InP is more promising as the back surface field for future solar cell performance improvements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Self-assembled growth of blue-green-yellow-red InGaN quantum dots (QDs) on GaN templates using plasma-assisted molecular beam epitaxy were investigated. We concluded that growth conditions, including small N2 flow and high growth temperature are beneficial to the formation of InGaN QDs and improve the crystal quality. The lower In/Ga flux ratio and lower growth temperature are favorable for the formation of QDs of long emission wavelength. Moreover, the nitrogen modulation epitaxy method can extend the wavelength of QDs from green to red. As a result, visible light emissions from 460 nm to 622 nm have been achieved. Furthermore, a 505 nm green light-emitting diode (LED) based on InGaN/GaN MQDs was prepared. The LED has a low external quantum efficiency of 0.14% and shows an efficiency droop with increasing injection current. However, electroluminescence spectra exhibited a strong wavelength stability, with a negligible shift of less than 1.0 nm as injection current density increased from 8 A/cm2 to 160 A/cm2, owing to the screening of polarization-related electric field in QDs. [ABSTRACT FROM AUTHOR]

Underwater wireless optical communication (UWOC) is a wireless communication technology using visible light to transmit data in an underwater environment, which has wide applications. Based on lift-off (In,Ga)N nanowires, this work has proposed and successfully demonstrated a self-powered photoelectrochemical (PEC) photodetector (PD) with excellent transmissivity. The transparent functionality of the PD is critical for 360° omnidirectional underwater detection, which was realized by detaching the (In,Ga)N nanowires from the opaque epitaxial substrates to the indium tin oxide (ITO)/glass. It was also found that the insulating SiO2 layer can enhance the photocurrent by about 12 times. The core–shell structure of the nanowires is beneficial for generating carriers and contributing to the photocurrent. Furthermore, a communication system with ASCII code is set to demonstrate the PD detection in underwater communication. This work paves an effective way to develop 360° omnidirectional PDs for the wide applications in UWOC system and underwater photodetection. [ABSTRACT FROM AUTHOR]

• Reduction of cantilever crosstalk in amplitude-modulation scanning Kelvin probe microscopy. • Verification of accuracy of measurements using interdigitating electrodes calibration samples and numerical simulation. • Determination of built-in potential of wafer-bonded GaAs/GaN heterojunction using quantitative AM-SKPM. Amplitude modulation scanning Kelvin probe microscopy (AM-SKPM) is widely used to measure the contact potential difference (CPD) between probe and samples in ambient or dry inert atmosphere. However, AM-SKPM is generally considered quantitatively inaccurate due to crosstalk between the cantilever and the sample. Here we demonstrate that the accuracy of AM-SKPM-based CPD measurements is drastically improved by exciting the SKPM probe at its second eigenmode. In the second eigenmode of oscillation, there exists a stationary node at the cantilever towards its free end, across which the displacement bears opposite signs; therefore driving the SKPM probe at its second eigenmode helps to partially cancel the virtual work done by the cantilever and reduce the crosstalk effect. The improvement in accuracy is experimentally confirmed with interdigitating electrodes calibration samples as well as practical samples such as the cross-section of wafer-bonded GaAs/GaN heterojunction. [ABSTRACT FROM AUTHOR]

• Nanowire films have been detached successfully by the acid solution. • Nanowires remain stable after the electrochemical etching. • Etching and incident light angle are the key reasons for peak shifts. In this work, the acid solution has been demonstrated in detaching transparent (In,Ga)N nanowire films. The lift-off (In,Ga)N nanowire film remains stable after the electrochemical etching. Without obvious damages, the etching time of acid solution is possible to be longer than that of alkaline solution, which is beneficial to maintain the stability of etching. In addition, the photoluminescence (PL) peak shifts exist between the samples before and after etching. By the numerical simulations and optical experiments, it is proposed that both etching process and incident angle of the lighting source mainly contribute to the shift. When the nanowires are immersed in an acid solution, the top and edge of nanowires with low components of indium (In) are partially dissolved, leading to the increase of the overall In component. Meanwhile, different incident angles lead to the heterogeneous absorption of light by different In components within (In,Ga)N section, which should be another reason for the shift of peak position. This study is beneficial for promoting the lift-off method in the applications of transparent nanowire films. [ABSTRACT FROM AUTHOR]

We have investigated the exciton and carrier spin relaxations in InGaAs lattice-matched to Ge substrates. Time-resolved spin-dependent pump and probe reflectance measurements revealed a spin relaxation behavior between 10 and 300 K. The presence of the carrier density dependence of spin relaxation time at 10-200 K implies that the Bir-Aronov-Pikus process is effective. At 250-300 K, the strong temperature and weak carrier density dependences of spin relaxation time show that the D'yakonov-Perel' process is dominant. The longest observed spin relaxation time of 2.6 ns at 77 K is explained by the decrease in the spatial overlap of electrons and holes. [ABSTRACT FROM AUTHOR]

• Tune the luminescence spectra of InGaN nanocolumns by a low-cost method. • Uniform microhole arrays show the ability in tuning the luminescence spectra. • Microhole size and flat area are found to have obvious effects in spectra tuning. In this paper, the molecular beam epitaxy of InGaN nanocolumns on silicon-based microhole arrays has been investigated experimentally. A low-cost method has been designed, which contains the processes of photolithography and wet etching. Tuning uniform microhole arrays show the ability in modulating the luminescence spectra of InGaN nanocolumns, which could lead to the modulation of tristimulus coordinates and correlated color temperatures. Due to the shadow effect, it is found that the size of microholes and the area of flat Si surface have more obvious effects, while the configuration of microhole arrays has limited effect on the modulation of peak wavelength. [ABSTRACT FROM AUTHOR]

A flexible four-junction (4 J) AlGaInP/AlGaAs/In 0.17 Ga 0.83 As/In 0.47 Ga 0.53 As solar cell with the band gap energy of 1.92/1.53/1.18/0.82eV was fabricated. Taking advantages of aluminum (Al)-contained material as well as metamorphic growth, the design of multiple junction solar cells became much easier and more flexible. In order to accommodate lattice mismatch between two InGaAs sub cells, compositionally step-graded AlGaInAs buffer layers were applied to release the mismatch strain during the inverted metamorphic (IMM) 4 J solar cells growth. A flexible 4 J solar cell on a 50 μm thick polyimide (PI) film was successfully fabricated by using temporary bonding and epitaxial layer lift-off via selective wet chemical etching. A conversion efficiency of 25.76% (AM1.5G) with an open circuit voltage of 3.46V, a short-circuit current density of 9.07 mA/cm2 and a fill factor of 82.14% was obtained without anti-reflection coating (ARC), and it would be higher than 32% by ARC integration. The mass density of the 4 J flexible solar cell was only 467 g/m2, and the specific power was up to 550 W/kg. • A flexible four-junction AlGaInP/AlGaAs/In 0.17 Ga 0.83 As/In 0.47 Ga 0.53 As solar cell on polyimide film was fabricated. • Temporary bonding and epitaxial layer lift-off methods were applied to fabricate a flexible solar cell. • A conversion efficiency of 25.76% without anti-reflection coating was obtained. • Al-contained material and metamorphic growth make the solar junction solar cell easier and more flexible. [ABSTRACT FROM AUTHOR]

Many flexible electrode materials with good electrochemical properties show poor sustainability due to the complex stress field inside the electrodes. Here, an analytical model is established to describe the ratcheting behaviors of the flexible electrode during electrochemical cycle with the presence of bending deformation, and is verified by finite element simulation. Under the impact of bending stress, the evolution of plastic strain during cyclic deformation is discussed and the effects of material softening due to the diffusion of Li-ions are considered. It is found that the electrode material accumulates a certain amount of plastic strain during each electrochemical cycle even under small bending deformation. Therefore, the flexible electrode material continuously accumulates plastic strain during cycling and becomes ineffective after several cycle times. Further, a strain hardening model is developed to discuss the possibility of mitigating ratcheting behavior. The increased yield strength in the cycling is found to be effective in suppressing the accumulation of plastic strain. The present work gives the new explanation for the poor sustainability of flexible batteries and provides a theoretical basis for further improvement. • A ratcheting model of the flexible planar electrode is developed. • The evolution of plastic strain during cyclic deformation is discussed. • The effect of material softening due to the diffusion of Li-ions is considered. • A strain hardening model is applied to mitigate the strain accumulation. [ABSTRACT FROM AUTHOR]

Broadband ultraviolet (BUV) photodetectors responding to the multiband spectrum can effectively reduce false alarm rates and improve the accuracy and versatility of detection systems in various situations. A high‐responsivity BUV photodetector based on vertical Ga2O3/GaN nanowire array is proposed and demonstrated. Ga2O3/GaN nanowires are obtained by partially thermally oxidizing GaN nanowires grown by molecular beam epitaxy and used to combine with a monolayer graphene film to form graphene/Ga2O3/GaN heterojunction. Moreover, the oxidation mechanism of GaN nanowires is further investigated by the developed thermal oxidation model. The fabricated devices exhibit excellent performance with a broadband spectral response of exceeding 550 A W−1 at −5 V and a fast‐response speed in the millisecond range, which can be attributed to the optical properties of vertical nanowire array structure and the internal gain mechanism of graphene/Ga2O3/GaN heterojunction. A high‐responsivity broadband ultraviolet photodetector based on vertical Ga2O3/GaN nanowire array is proposed and demonstrated. The nanowires are obtained by partial thermal oxidation and form a graphene/Ga2O3/GaN heterojunction. The fabricated devices exhibit a great broadband spectral response with the high responsivity exceeding 550 A W−1. Meanwhile, an oxidation model is proposed to explain the crystalline transformation of GaN nanowires in thermal oxidation. [ABSTRACT FROM AUTHOR]