Search

N-polar GaN film was obtained by using a high-temperature AlN buffer layer. It was found that the polarity could be inverted by a thin low-temperature AlN interlayer with the same V/III ratio as that of the high-temperature AlN layer. Continuing to increase the V/III ratio of the low-temperature AlN interlayer, the Ga-polarity of GaN film was inverted to N-polarity again but the crystal quality and surface roughness of GaN film greatly deteriorated. Finally, we analyzed the chemical environment of the AlN layer by x-ray photoelectron spectroscopy (XPS), which provides a new direction for the control of GaN polarity. [ABSTRACT FROM AUTHOR]

Overlapping between the quantum-well structure and p–n junction plays an important role in the development of high-efficiency GaAs-based light-emitting diodes (LEDs). In this work, we apply p–n junction engineering to study electro-photoluminescence spectra under open-circuit, forward voltage, and reverse voltage conditions. Through modifying the doping of p-layer or n-layer, luminescent characteristics are apparently improved as a result of the more effective overlapping of the quantum-well structure and p–n junction caused by energy band modification, with the more balanced carrier and stronger confinement in quantum wells. Furthermore, the electroluminescence spectra of all LEDs studied herein are consistent with the photoluminescence results. This work provides an explicit way to tune p- and n-doping to increase emission efficiency, which is important for research and industrial applications. [ABSTRACT FROM AUTHOR]

To increase the efficiency of silicon heterojunction (SHJ) solar cells (SCs), it is paramount to enhance the utilization of sunlight by light management. In this study, the dependences of weighted reflectance and thus generation current (J G) for SHJ SCs on different anti-reflective structures are displayed by OPAL2 simulation tool. According to this, SiN x and SiO2 films are deposited on front of indium tin oxide (ITO) as multilayer anti-reflection coatings (ARCs). It is demonstrated experimentally that the photovoltaic performance of SHJ SCs can be significantly improved by multilayer ARCs. Especially, with 90/21/40 nm SiO2/SiN x /ITO anti-reflective layer structure, the reflectance of SHJ SC is reduced as low as 0.94%, and the current density is shown to be increased by 4.34% compared to the common SCs. This work shows a promising and cost-effective way to achieve higher light utilization and thus promotes photovoltaic characteristics for SHJ SCs. [ABSTRACT FROM AUTHOR]

Recent experiments have shown that more than 85% of photo-generated carriers can escape from multiple quantum wells (MQWs) sandwiched between p-type and n-type layers (PIN). In this work, we quantitatively analyze the relationship between the energy of carriers and the height of potential barriers to be crossed, based on the GaAs/InGaAs quantum well structure system, combined with the Heisenberg uncertainty principle. It was found that that the energy obtained by electrons from photons is just enough for them to escape, and it was found that the energy obtained by the hole is just enough for it to escape due to the extra energy calculated, based on the uncertainty principle. This extra energy is considered to come from photo-generated thermal energy. The differential reflection spectrum of the structure is then measured by pump–probe technology to verify the assumption. The experiment shows that the photo-generated carrier has a longer lifetime in its short circuit (SC) state, and thus it possesses a lower structure temperature than that in open circuit (OC). This can only explain a thermal energy reduction caused by the continuous carrier escape in SC state, indicating an extra thermal energy transferred to the escaping carriers. This study is of great significance to the design of new optoelectronic devices and can improve the theory of photo-generated carrier transports. [ABSTRACT FROM AUTHOR]

In this work, we used the Direct Laser Writing Grayscale Photolithography technology to fabricate a normally-off p-GaN gate high-electron-mobility transistor with the air-bridge source-connection. The air-bridge source-connection was formed using the Direct Laser Writing Grayscale Photolithography, and it directly connected the two adjacent sources and spanned the gate and drain of the multi-finger p-GaN gate device, which featured the advantages of stable self-support and large-span capabilities. Verified by the experiments, the fabricated air-bridge p-GaN gate devices utilizing the Direct Laser Writing Grayscale Photolithography presented an on-resistance of 36 Ω∙mm, a threshold voltage of 1.8 V, a maximum drain current of 240 mA/mm, and a breakdown voltage of 715 V. The results provide beneficial design guidance for realizing large gate-width p-GaN gate high-electron-mobility transistor devices. [ABSTRACT FROM AUTHOR]

InGaN-based light-emitting diodes (LEDs) have higher luminescence efficiency than other materials used for the blue and green LEDs in spite of their relatively high dislocation density. Localization theory has been used to explain this phenomenon, but the direct observation of localization states in the InGaN active region has been rarely reported. In this paper, we propose an LED structure to obtain higher luminescence efficiency in the yellow-green LEDs and directly observe the transition of carriers between different localization states. The localization states were investigated and confirmed by temperature-dependent photoluminescence and excitation power-dependent photoluminescence. The value of the external quantum efficiency also exhibited a higher radiative efficiency of the quantum well with a higher degree of localization states. These results offer a promising means of realizing high-luminescence LEDs. [ABSTRACT FROM AUTHOR]

Despite the large misfit dislocation densities, indium gallium nitride (InGaN) demonstrates high luminous efficiency both for electroluminescence and photoluminescence. The mechanism behind it has been interpreted as the existence of potential minima (i.e., localized states), which will screen the non-radiative recombination centers to avoid carriers being trapped by the defects. The existence of localized states has been testified by many experiments. However, almost all of the observations are indirect observations, and some experiments, such as those focused on whether the indium clusters observed by transmission electron microscopy are localized states, still remain controversial. Here, we report the direct observation of carrier transportation between localized states driven by temperature-dependent photoluminescence (TDPL) and excitation power-dependent PL in InGaN quantum wells. This enriches the experimental evidence on the existence of localized states. [ABSTRACT FROM AUTHOR]

Plasmonic photodetection based on the hot-electron generation in nanostructures is a promising strategy for sub-band detection due to the high conversion efficiencies; however, it is plagued with the high dark current. In this paper, we have demonstrated the plasmonic photodetection with dark current suppression to create a Si-based broadband photodetector with enhanced performance in the short-wavelength infrared (SWIR) region. By hybridizing a 3 nm Au layer with the spherical Au nanoparticles (NPs) formed by rapid thermal annealing (RTA) on Si substrate, a well-behaved ITO/Au/Au NPs/n-Si Schottky photodetector with suppressed dark current and enhanced absorption in the SWIR region is obtained. This optimized detector shows a broad detection beyond 1200 nm and a high responsivity of 22.82 mA/W at 1310 nm at −1 V, as well as a low dark current density on the order of 10−5 A/cm2. Such a Si-based plasmon-enhanced detector with desirable performance in dark current will be a promising strategy for realization of the high SNR detector while keeping fabrication costs low. [ABSTRACT FROM AUTHOR]

Dual-wavelength multiple quantum wells (MQWs) have great potential in realizing high quality illumination, monolithic micro light-emitting diode (LED) displays and other related fields. Here, we demonstrate a single chip white light indium gallium nitride (InGaN) LED via the manipulation of the dual-wavelength MQWs. The MQWs contain four pairs of blue light-emitting MQWs and one pair of green light-emitting QW. The fabricated LED chips with nickel/gold (Ni/Au) as the current spreading layer emit white light with the injection current changing from 0.5 mA to 80 mA. The chromaticity coordinates of (0.3152, 0.329) closing to the white light location in the Commission International de I'Eclairage (CIE) 1931 chromaticity diagram are obtained under a 1 mA current injection with a color rendering index (CRI) Ra of 60 and correlated color temperature (CCT) of 6246 K. This strategy provides a promising route to realize high quality white light in a single chip, which will significantly simplify the production process of incumbent white light LEDs and promote the progress of high-quality illumination. [ABSTRACT FROM AUTHOR]

High-temperature nitridation is commonly thought of as a necessary process to obtain N-polar GaN films on a sapphire substrate. In this work, high-quality N-polar GaN films were grown on a vicinal sapphire substrate with a 100 nm high-temperature (HT) AlN buffer layer (high V/III ratio) and without an intentional nitriding process. The smallest X-ray full width at half maximum (FWHM) values of the (002)/(102) plane were 237/337 arcsec. On the contrary, N-polar GaN film with an intentional nitriding process had a lower crystal quality. In addition, we investigated the effect of different substrate treatments 1 min before the high-temperature AlN layer's growth on the quality of the N-polar GaN films grown on different vicinal sapphire substrates. [ABSTRACT FROM AUTHOR]

The absorption coefficient is usually considered as a constant for certain materials at the given wavelength. However, recent experiments demonstrated that the absorption coefficient could be enhanced a lot by the PN junction. The absorption coefficient varies with the thickness of the intrinsic layer in a PIN structure. Here, we interpret the anomalous absorption coefficient from the competition between recombination and drift for non-equilibrium carriers. Based on the Fokker–Planck theory, a non-equilibrium statistical model that describes the relationship between absorption coefficient and material thickness has been proposed. It could predict the experimental data well. Our results can give new ideas to design photoelectric devices. [ABSTRACT FROM AUTHOR]

SiGe spheres with different diameters are successfully fabricated on a virtual SiGe template using a laser irradiation method. The results from scanning electron microscopy and micro-Raman spectroscopy reveal that the diameter and Ge composition of the SiGe spheres can be well controlled by adjusting the laser energy density. In addition, the transmission electron microscopy results show that Ge composition inside the SiGe spheres is almost uniform in a well-defined, nearly spherical outline. As a convenient method to prepare sphere-shaped SiGe micro/nanostructures with tunable Ge composition and size, this technique is expected to be useful for SiGe-based material growth and micro/optoelectronic device fabrication. [ABSTRACT FROM AUTHOR]

InAsSb/AlGaSb systems have potential in mid-wavelength infrared detection and laser fields. Thus, their heteroepitaxial crystal quality and heterointerface are of great importance. Herein, the quantum well structure based on InAs0.8Sb0.2/Al0.2Ga0.8Sb was grown by interruption with and without Sb2 soaking methods by molecular beam epitaxy to optimize the interface quality and adjust the heterostructure strain. Narrow and well-defined satellite peaks in high-resolution x-ray diffraction patterns manifest good crystal quality of the sample with 15 s Sb2 soaking interruption. The relaxation of sample A without Sb2 soaking interruption is ∼23% calculated by the reciprocal space mappings, while there is no relaxation found in sample B with 15 s Sb2 soaking. High-resolution transmission electron microscopy and energy-dispersive spectroscopy were carried out. They showed sharp and coherent heterointerfaces generated by adding interruptions with an Sb2 overpressure before and after InAsSb layer growth. Furthermore, the atomic force microscopy images of a 5 × 5 μm2 scan area show that the surface of sample B with Sb2 soaking contains atomic steps with a root-mean-square roughness of 1.44 Å. The photoluminescence peaks of the samples located in the range of 3–4 µm show that these InAsSb/AlGaSb material systems have potential applications in mid-wavelength optoelectrical devices. [ABSTRACT FROM AUTHOR]

The contact characteristic between p-InP and metal plays an important role in InP-related optoelectronic and microelectronic device applications. We investigate the low-resistance Au/Pt/Ni and Au/Ni ohmic contacts to p-InP based on the solid phase regrowth principle. The lowest specific contact resistivity of Au(100 nm)/Pt(115 nm)/Ni (50 nm) can reach 2.64 × 10−6 Ω ⋅ cm2 after annealing at 380 °C for 1 min, while the contact characteristics of Au/Ni deteriorated after annealing from 340 °C to 480 °C for 1 min. The results of scanning electron microscopy, atomic force microscopy and x-ray photoelectron spectroscopy show that the Pt layer is an important factor in improving the contact characteristics. The Pt layer prevents the diffusion of In and Au, inhibits the formation of Au3In metal compounds, and prevents the deterioration of the ohmic contact. The metal structures and optimized annealing process is expected to be helpful for obtaining high-performance InP-related devices. [ABSTRACT FROM AUTHOR]

Relevant experiments have shown that more than 90% of photo-generated carriers can escape from multiple quantum wells (MQWs) sandwiched between p-type and n-type layers (a PIN structure). The escape time of the photo-generated carriers is on the femtosecond scale, much less than their relaxation time. In contrast, photo-generated carriers cannot escape from MQWs sandwiched between two n-type layers (a NIN structure) with bias. We found that there is a high barrier for holes and that the electric field intensity in QWs is close to zero, resulting in holes accumulating and a low escape efficiency in a NIN structure with bias. [ABSTRACT FROM AUTHOR]

In this paper, a virtual GeSi template has been successfully fabricated by Ge filling the porous silicon (PSi) prepared by electrochemical etching (EC etching). The microstructure quality measured with cross-sectional scanning electron microscopy and photoluminescence spectra reveal that the PSi structure is fully filled with uniformly distributed Ge. Furthermore, Raman and X-ray diffraction results also show that the template has a typical GeSi feature, which originates from Ge interdiffusion with PSi during the Ge deposition. This design and structure can be expected to be useful as a template for Ge-related materials growth and device fabrication. [ABSTRACT FROM AUTHOR]

We have successfully prepared GaN based high electron mobility transistors (HEMTs) on metallic substrates transferred from silicon substrates by electroplating technique. GaN HEMTs on Cu substrates are demonstrated to basically have the same good electric characteristics as the chips on Si substrates. Furthermore, the better heat dissipation of HEMTs on Cu substrates compared to HEMTs on Si substrates is clearly observed by thermoreflectance imaging, showing the promising potential for very high-power and high-temperature operation. This work shows the outstanding ability of HEMT chips on Cu substrates for solving the self-heating effect with the advantages of process simplicity, high yield, and low production requirement. [ABSTRACT FROM AUTHOR]

We studied the non-polar a-plane GaN directly grown on an SiO2 stripe-patterned r-plane sapphire substrate along the [0001] crystallographic direction of GaN. The XRC-FWHM values of the optimized a-plane GaN were 386 arcsec along the c-axis and 289 arcsec along the m-axis. Detailed scanning electron microscopy (SEM), X-ray diffraction (XRD) reciprocal space maps (RSMs) and Raman scattering studies were performed to investigate the growth kinetics. It was found that the increase in mosaic block dimension could be the primary reason for the improved crystal quality. This work shows a very promising and simple method to achieve high-quality a-plane GaN films. [ABSTRACT FROM AUTHOR]

The characteristics of a-plane GaN films directly grown on silicon dioxide (SiO2) hole-array patterned r-sapphire substrates (HPSS) were investigated in this work. Various pattern sizes of SiO2 HPSS were prepared to obtain fully coalescent a-plane GaN films with high crystalline quality. The anisotropic behavior and crystalline quality characterized by X-ray diffraction (XRD) were significantly improved in a-plane GaN grown on optimized micro-HPSS (MHPSS) of 3 μm pattern with 1 μm diameter, in which the X-ray rocking curve-full width at half maximum (XRC-FWHM) values were 513 arcsec along the m-axis and 531 arcsec along the c axis. Scanning electron microscopy (SEM) results suggested that the presence of the SiO2 mask might influence the dominant fast-growing facets. The different sizes of patterns governed the lateral growth length of a-plane GaN islands, which increased the mosaic size along the m direction and eliminated the anisotropic behavior. In addition, the modified Williamson–Hall (W–H) analysis showed that the density of basal stacking faults (BSFs) was reduced by an order of magnitude in a-plane GaN using MHPSS, as compared with a-plane GaN grown on planar r-sapphire. This work provides a simple and effective method without interruption and regrowth to solve two important problems in a-plane GaN, in which not only was the anisotropic behavior almost eliminated, but also the BSF density was greatly reduced. [ABSTRACT FROM AUTHOR]

In this paper, we report the fabrication of flexible ZnO thin-film transistors (TFTs) on the 25- $\mu \text{m}$ -thick copper substrate. The Cu substrate compatible with high-temperature processes was deposited by electroplating, which led to a smooth surface. The ZnO layer was deposited at room temperature, and a thermal annealing process was conducted at 300 °C in the air after the fabrication of TFTs. The threshold voltage, hysteresis voltage, and contact resistance are improved after annealing, whereas the mobility, subthreshold swing, and ON/OFF ratio are barely changed. The stabilities of ZnO TFTs are also enhanced, evidenced by the smaller threshold voltage shifts induced by positive and negative gate bias stresses. The field-effect mobility, threshold voltage, and subthreshold swing are stable even when exposed to a mechanical tensile strain up to 1.25% and 10 000 bending cycles. [ABSTRACT FROM AUTHOR]

SU-8 was found to be a good candidate for passivation of GaSb diodes, because of its stable chemical properties and high resistivity. However, stresses might be induced during the SU-8 solidification process, which could affect the performance of GaSb diode. Here, we measured the dark currents of devices with different sizes of mesa area and thicknesses of passivation layer, found that the thickness of passivation layer is a key factor for the performance of GaSb based devices. The dark current density of the optimum passivated diode is $9.80\,\,\times \,\, 10^{-6}$ A/cm2 (at −0.3 V bias) and the calculated surface resistivity based on dynamic resistance-area product (R0A) of the passivated diode is $854~\Omega \cdot \text {cm}$ , which are about twentyfold decrease and sixfold increase than that for unpassivated diodes at 77 K, respectively. The calculated quantum efficiency is 36.3% for the passivated device, whereas the value for the unpassivated device is 19.2%. [ABSTRACT FROM AUTHOR]

A method is suggested to improve the characteristics of light emitting diodes (LEDs) by using p-GaN structure with heavily Mg-doping grown at low temperature in full N2 environment. The LED exhibits a 36.4% enhancement in light output power (LOP) at 222 A cm−2 due to the higher hole concentration as compared to the normal p-GaN layer. Meanwhile, the experimental results prove that the optimized LED structure shows reduced efficiency droop behavior, smaller peak wavelength blueshift and narrower electroluminescence (EL) spectrum broadening. [ABSTRACT FROM AUTHOR]

InGaN-based light-emitting diodes with graded indium composition p-type InGaN hole reservoir layer (HRL) are numerically investigated using the APSYS simulation software. It is found that by gradient increasing indium composition in growth direction of the p-InGaN HRL can improve light output power, lower current leakage and efficiency droop. Based on numerical simulation and analysis, these improvements on the electrical and optical characteristics are attributed mainly to tailoring energy band in p-n junction vicinal region, and finally enhanced the hole injection efficiency and electron blocking efficiency. [ABSTRACT FROM AUTHOR]

The impact of nanoporous SiN interlayer growth position on high-quality GaN epitaxial film was elucidated from the behavior of dislocations. The best quality GaN film was achieved when a nanoporous SiN interlayer was grown on a rough layer, with the high-resolution X-ray diffraction rocking curve full width at half maximum for ( $1\bar 102$) reflection decreasing to 223 arcs, and the total dislocation density reduced to less than 1.0×10 cm. GaN films were grown on sapphire substrates by metal organic chemical vapor deposition. The quality of these films was investigated with high-resolution X-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. A preference for the formation of half-loops to reduce threading dislocations was observed when an SiN interlayer was grown on a rough layer. A growth mechanism is proposed to explain this preference. [ABSTRACT FROM AUTHOR]