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InGaN p-i-n homojunction (HOJ) and heterojunction (HEJ) solar cells (SCs) with similar width of depletion region are investigated. Through comparison of both the material property and device performance, it is demonstrated that HEJ exhibits much better results than HOJ, indicating that HEJ is preferred for fabrication of InGaN SCs. Some suggestions are proposed for the development of InGaN SCs in the future.

We demonstrated InGaN/GaN multiple-quantum-well solar cells with enhanced photovoltaic characteristics using vertical configuration. As compared to the conventional solar cell device, the vertical device with a bottom Al reflector and a top textured surface exhibits much improved performances, including a short-circuit current density of 1.2 mA/cm2, an open-circuit voltage of 1.9 V, a fill factor of 57.6 % and a conversion efficiency of 1.3 %. The bottom Al reflector could reflect a large amount of photons back into the absorption layer, which led to the improvement of light absorption. In addition, due to the top surface texturing, reflection is reduced that further improved the absorption. The combination of a bottom Al reflector and a top textured surface, which yields much higher conversion efficiency than a conventional device, shows great promise for future photovoltaic applications.

We demonstrated InGaN/GaN multiple quantum well (MQW) solar cells with vertical structure using aluminum (Al) reflectors in this study. The Al reflectors at the bottom of the solar cells can reflect large amount of photons back into the absorber layer, which greatly enhanced the photovoltaic properties of the MQW solar cells. These solar cell devices exhibit excellent performances, including an open-circuit voltage of 1.9 V, a short-circuit current density of 1.6 mA/cm2 with a fill factor of 74.6%. The conversion and external quantum efficiencies of the solar cell were 2.3% and 41.4%, respectively. The extremely high conversion efficiency of the vertical-type solar cell device shows high potential for future photovoltaic applications.

We reported a new method of enhancing the quantum efficiency (n) of InGaN/GaN multiple quantum well (MQW) solar cells using resonant cavity. The resonant cavity enhanced (RCE) structure is designed with a top mirror and a bottom distributed Bragg reflector (DBR). The origin of the enhancement in n is the resonance-induced increase of the optical field which causes more photons to be absorbed in the InGaN absorption layers. We simulated the photovoltaic performances of InGaN/GaN MQW solar cells by comparing RCE-type and conventional solar cells. The results show the RCE-type solar cells exhibit enhancements in short-circuit current density by 2.12 times and conversion efficiency by 2.13 times, as compared to those of conventional solar cell.

In recent decades, literatures about visible vertical cavity surface emitting lasers (VCSELs) have been reported. However, due to high optical loss in the cavity, lasing from deep ultraviolet (DUV) VCSEL was still rarely achieved. The optical loss in nitride DUV microcavity was analyzed in detail. DUV nitride vertical Fabry–Perot microcavity with active layer of AlGaN-based quantum dots and double-side HfO2 / SiO2 distributed bragger reflectors was fabricated. Optical losses with of the order of 103 cm − 1 were deduced from the Q value of the cavity modes. The main origination of optical loss in DUV cavity was calculated and ascribed to the interface scattering. The interface roughness appearing after laser lift-off process and overlap between rough interface and standing optical wave were two key parameters that contributed to interface scattering loss. We believe that our results will provide useful information for improving DUV VCSEL devices.

InGaN/GaN multiple quantum well-solar cells with different barrier thicknesses have been comparatively studied, and it is demonstrated that a thinner barrier facilitates the transport of photogenerated carriers. Meanwhile, an abnormal but regular staircase-like I-V curve is observed. The induction factors and microscopic physical process relevant to this abnormal feature are analyzed in detail, which is found closely related to the inadequate internal field and special epitaxial structure. Furthermore, the formation conditions of I-V curves with various types of inflections are also proposed. This work provides insight into the origin of device problems, which can be applicable to other types of solar cells.

Thermal characteristics of GaN-based vertical cavity surface emitting lasers (VCSELs) with three typical structures were investigated both theoretically and experimentally. The simulation results based on a steady state quasi three-dimensional cylindrical model show that the thermal resistance (R th) is affected by cavity length, mesa size, as well as the bottom distributed Bragg reflector (DBR) size, and the detail further depends on different structures. Among different devices, GaN VCSEL with a hybrid cavity formed by one nitride bottom DBR and another dielectric top DBR is featured with lower R th, which is meanwhile affected strongly by the materials of the epitaxial bottom DBR. The main issues affecting the thermal dissipation in VCSELs with double dielectric DBRs are the bottom dielectric DBR and the dielectric current-confinement layer. To validate the simulation results, GaN-based VCSEL bonded on a copper plate was fabricated. R th of this device was measured and the results agreed well with the simulation. This work provides a better understanding of the thermal characteristics of GaN-based VCSELs and is useful in optimizing the structure design and improving the device performance.

High-quality factor (Q >; 1700) GaN-based blue resonant-cavity light-emitting diodes (RCLEDs) incorporating an InGaN/GaN multiquantum well active region, two high-reflectivity dielectric-distributed Bragg reflectors, and a thin indium tin oxide (ITO) layer are fabricated by a two-step substrate transfer technique. Electroluminescence measurements showed a narrow linewidth of 0.26 nm at the wavelength of 450.6 nm by precisely placing the ITO layer at the node position of the electric field, corresponding to a high Q-value of 1720. Further, adopting a chemical-mechanical polishing (CMP) technique to polish the GaN surface after the removal of sapphire substrate, an even higher Q-value of 2170 was obtained. This improvement was attributed to the exclusion of the defect-rich buffer layer and the achievement of a smooth surface with a root mean square roughness below 1 nm. The integrated electroluminescence intensity was enhanced by 40% as compared with the RCLEDs without CMP at a current density of 8 kA/cm2.

National Natural Science Foundation of China [91023048, 61106044, 10974165]; Doctoral Program Foundation of Institutions of Higher Education of China [20110121110029]

An electrically pumped wavelength-tunable InGaN quantum dot (QD) based microcavity (MC) lighter emitter with a large tuning range of 129 nm was demonstrated. The multi-mode emission spectrum was tuned by injected current from 564 nm (yellow-green) to 435 nm (violet). The MC light emitter is featured with a double dielectric distributed Bragg reflector structure and a copper substrate fabricated using substrate transfer and laser lift off techniques. By utilizing an InGaN QD active layer with a tunable broad emission spectrum and a Fabry-Perot cavity which allows multi-longitudinal mode resonating, the emission spectrum could be tuned among several particular cavity modes, which are decided by the gain enhancement factor. In addition, both the enhancement and suppression of MC emission modes caused by the gain enhancement factor were observed in a single MC device. As the first electrically driven III-V nitride semiconductor based tunable MC light emitter with a tuning range of 129 nm, the device is promis...

Using a quantum dot in quantum well (QD-in-QW) active region, current injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) lasing simultaneously in blue and green were achieved at room temperature (RT). Lasing was first achieved at 545 nm with a threshold current of ~2 μA, and with a further increase of current, another lasing peak at 430 nm came out with a threshold current of ~5 mA. The energy difference between the two lasing peaks originating from QDs and QWs is 609 meV. According to the spontaneous emission spectra measured under different injected currents, a model of energy states in QD-in-QW is proposed to describe the distribution of carriers in this structure. Using this model, the evolution of emission intensity, shift of peak energy, variation of linewidth and the lasing characteristics can be well explained.

In the development of high-performance terahertz quantum cascade lasers (THz-QCLs), the use of a Ag double-metal waveguide (DMW) is attractive because it has low propagation-loss and high thermal conductivity. In this study, we investigated the Ag–metal bonding conditions for the DMW of THz-QCL. GaAs wafers were bonded together with Ag–metal layers by applying high-pressure and heat. We achieved successful Ag bonding at 400 °C with an applied pressure of 38.5 kg/cm2 for 30 min in N2 ambient. A thin Ti adhesion layer was inserted between Ag and GaAs. The Ti layer (thickness ≥ 10 nm) was, in addition, found to act as a barrier preventing Ag diffusion into GaAs at temperatures below 400 °C.

Semiconductor vertical-cavity surface-emitting lasers (VCSELs) with wavelengths from 491.8 to 565.7 nm, covering most of the ‘green gap’, are demonstrated. For these lasers, the same quantum dot (QD) active region was used, whereas the wavelength was controlled by adjusting the cavity length, which is difficult for edge-emitting lasers. Compared with reports in the literature for green VCSELs, our lasers have set a few world records for the lowest threshold, longest wavelength and continuous-wave (CW) lasing at room temperature. The nanoscale QDs contribute dominantly to the low threshold. The emitting wavelength depends on the electron–photon interaction or the coupling between the active layer and the optical field, which is modulated by the cavity length. The green VCSELs exhibit a low-thermal resistance of 915 kW−1, which benefits the CW lasing. Such VCSELs are important for small-size, low power consumption full-color displays and projectors. Researchers have used vertically oriented devices containing tiny quantum dots to improve the production of green laser light. While powerful red and blue lasers can be easily manufactured using multilayered semiconductors, finding material combinations that generate green wavelengths is trickier. Baoping Zhang from Xiamen University in China and co-workers cracked this problem by turning to vertical-cavity surface-emitting lasers (VCSELs) whose emission wavelength can be controlled by adjusting their cavity length. Fabricating a VCSEL with an active layer composed of indium–gallium–nitrogen quantum dots reduced the threshold currents needed to emit green light, thanks to the strong quantum confinement effects. The team demonstrated VCSELs with different wavelengths that covered the ‘green gap’ and used heat-dissipating materials to produce a continuous-wave device that operates at room temperature.

Low threshold continuous-wave (CW) lasing of current injected InGaN quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs) was achieved at room temperature. The VCSEL was fabricated by metal bonding technique on a copper substrate to improve the heat dissipation ability of the device. For the first time, lasing was obtained at yellow-green wavelength of 560.4 nm with a low threshold of 0.61 mA, corresponding to a current density of 0.78 kA/cm2. A high degree of polarization of 94% were measured. Despite the operation in the range of "green gap" of GaN-based devices, single longitudinal mode laser emission was clearly achieved due to the high quality of active region based on InGaN QDs and the excellent thermal design of the VCSELs.

We demonstrated a new kind of InGaN/GaN multi-quantum well (MQW) light-emitting diode (LED) with simplified fabrication processes, where only one round of photolithography and electrode deposition is necessary. The electrical and optical properties of this LED, which is defined as a coplanar metal–semiconductor–metal (CMSM) LED, were characterized. The electroluminescence spectrum only exhibits blue emission at 450 nm, meaning that the light comes exclusively from MQWs. The optical output at 20 mA was comparable to that of conventional LEDs. A shunt circuit model with a surface thin film resistance, an n++ InGaN/p-GaN/n-GaN structure and a p-GaN/n-GaN junction was proposed to explain the working mechanism of the CMSM LED. A proof-of–concept display was demonstrated, exploiting the promising application of CMSM LED to display.

Photoluminescence (PL) spectra were measured as a function of well width (L W) and temperature in ZnO/Mg0.1Zn0.9O single quantum wells (QWs) with graded thickness. The emission linewidth (full width at half maximum) was extracted from the emission spectra, and its variation as a function of L W was studied. The inhomogeneous linewidth obtained at 5 K was found to decrease with increasing L W from 1.8 to 3.3 nm due to the reduced potential variation caused by the L W fluctuation. Above 3.3 nm, however, the linewidth became larger with increasing L W, which was explained by the effect related with defect generation due to strain relaxation and exciton expansion in the QW. For the homogenous linewidth broadening, longitudinal optical (LO) phonon scattering and impurity scattering were taken into account. The LO phonon scattering coefficient Γ LO and impurity scattering coefficient Γ imp were deduced from the temperature dependence of the linewidth of the PL spectra. Evident reduction of Γ LO with decreasing L W was observed, which was ascribed to the confinement-induced enhancement of the exciton binding energy. Different from Γ LO, a monotonic increase in Γ imp was observed with decreasing L W, which was attributed to the enhanced penetration of the exciton wave function into the barrier layers.

The temperature dependent of threshold current density (J th ) of GaAs/Al x Ga 1−x As terahertz quantum cascade lasers (THz QCLs) with different Al barrier composition are studied and reported here. One of the most important issues of the recent THz QCLs is their limited operation temperature. Increasing the Al composition in longitudinal optical (LO) phonon depopulation design THz QCLs tend to reduce J th and improve current dynamic range of lasers, which is expected to increase the maximum operation temperature (T max ).

In diffusion to blue light-emitting diode (LED) wafers is performed by the inductive coupled plasma (ICP) treatment of a covering layer of indium tin oxide (ITO) on the wafer surface. The electrical property of the p-type contact is improved and the redshift of photoluminescence (PL) from the InGaN quantum well of the wafer is found. Measurements by x-ray photoelectron spectroscopy (XPS) demonstrate that In atoms have diffused into p-GaN. Reflectance spectra of the sample surface reveal the variation caused by the ICP treatment. A model of compensation of the in-plane strain of the InGaN layer is used to explain the redshift of the PL data. Finally, LEDs are fabricated by using as-grown and ICP-treated wafers and their properties are compared. Under an injection current of 20 mA, LEDs with ICP-induced In doping show a decrease of 0.3V in the forward voltage and an increase of 23% in the light output, respectively.

Continuous wave (CW) lasing of electrically injected GaN-based vertical cavity surface emitting lasers (VCSELs) was achieved at room temperature. First, a high quality factor (Q) VCSEL-structured device with very narrow linewidth of 0.12 nm, corresponding to a Q-value of 3570 was obtained through two-step substrate transfer technique. However, poor heat dissipation ability prevented the device from lasing. Based on the high-Q resonant cavity design, we further fabricated vertical-structured VCSELs through metal bonding technique on Si substrate. CW lasing from vertical-structured VCSELs was observed with threshold current of density of 1.2 kA/cm2 and lasing linewidth of about 0.20 nm.

National Natural Science Foundation of China [61106044, 61274052]; Specialized Research Fund for the Doctoral Program of Higher Education of China [20110121110029]; Fundamental Research Funds for the Central Universities of Ministry of Education of China [2013121024]; Natural Science Foundation of Fujian Province of China [2013J05096]

An auto-split laser lift-off (LLO) method for fabrication of vertical-injection GaN-based green light-emitting diodes (ASV-LEDs) is demonstrated. The ASV-LEDs exhibited a significant improvement in the light output and thermal dissipation, as compared with that of conventional LEDs on sapphire. The intrinsic physical mechanism of the auto-split LLO technique is studied by a Frank-Read dislocation clustering model. The laser energy density and mesa spacing are shown to be key factors in the auto-split LLO method. It is believed that this method offers an alternative way to fabricate high-performance GaN-based thin-film LEDs.

A tunable grating-coupled external cavity (EC) laser is realized by employing a GaN-based laser diode as the gain device. A tuning range of 4.47 nm from 403.82 to 408.29 nm is achieved. Detailed investigations reveal that the injection current strongly influences the performance of the EC laser. Below the free-running lasing threshold, EC laser works stably. While above the free-running lasing threshold, a Fabry—Perot (F-P) resonance peak in the emission spectrum and a smooth kink in the output power-injection current characteristic curve are observed, suggesting the competition between the inner F-P cavity resonance and EC resonance. Furthermore, the tuning range is found to be asymmetric and occurs predominantly on the longer wavelength side. This is interpreted in terms of the asymmetric gain distribution of GaN-based quantum well material.

Optical properties and carrier dynamics of InGaN/GaN asymmetric coupled quantum wells (ACQWs) are studied by excitation-power-dependent photoluminescence (PL), photoreflectance (PR) and time-resolved PL (TRPL) experiments. Under weak excitations, only the emission from the widest well is observed due to the tunneling from narrower to wider wells. Under strong excitations, the carrier distribution becomes more uniform and an enhanced emission from the mid well (2.5 nm well) is observed. Dependence of the PL intensity on excitation power is well explained by a rate equation model. The energy levels in the ACQW structure are clearly revealed by PR measurements and are in good agreement with calculations. Our results indicate that the enhanced emission from the mid well is ascribed to "reverse tunneling" from 3.0 to 2.5 nm well, which is confirmed by TRPL experiments.

Laser lift-off (LLO), by which GaN is separated from sapphire, is demonstrated to be a promising technique for advanced GaN-based optoelectronic devices. Its physical insight, however, is still not fully understood. We study systematically the effect of laser pulse width on the LLO process and the property of GaN. To estimate accurately the temperature distribution and the decomposed thickness of GaN, fluctuation in the pulse laser energy is taken into account. It is found that the temperature at the interface is increased in a higher speed for a narrower pulse width. In addition, less damage to the GaN film is expected for a narrower pulse width owing to the smaller heated area, lower transient temperature and lower N2 vapor pressure encountered during LLO. Some experimental results reported in literature are explained well. Our results are useful in understanding the effect of laser pulse width and can be taken as references in LLO of GaN/sapphire structures.

The temperature dependence of threshold current density (Jth) for GaAs/AlxGa1-xAs terahertz quantum cascade lasers (THz-QCLs) with different Al barrier compositions is studied. We achieved a maximum operation temperature (Tmax) of 143 K for a 3.7 THz QCL by employing a longitudinal-optical (LO) phonon depopulation scheme. High-Al-composition barriers are used for increasing averaged LO-phonon energy. A significant reduction in Jth of approximately 30% was obtained by increasing the Al composition from 15 to 35%, when we used the same energy separation of the depopulation stats (E21). Tmax could be increased by using high-Al-composition structures due to the reduction of thermally-activated LO-phonon scattering.