Wen-Jie Liu

Room temperature continuous wave lasing of electrically injected GaN-based vertical cavity surface emitting lasers

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.

Investigation of InGaN p-i-n Homojunction and Heterojunction Solar Cells

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.

Fabrication and Characterization of High-Quality Factor GaN-Based Resonant-Cavity Blue Light-Emitting Diodes

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.

Spectral dynamics of picosecond gain-switched pulses from nitride-based vertical-cavity surface-emitting lasers

Short pulses generated from low-cost semiconductor lasers by a simple gain-switching technique have attracted enormous attention because of their potential usage in wide applications. Therein, reducing the durations of gain-switched pulses is a key technical point for promoting their applications. Therefore, understanding the dynamic characteristics of gain-switched pulses is highly desirable. Herein, we used streak camera to investigate the time- and spectral-resolved lasing characteristics of gain-switched pulses from optically pumped InGaN single-mode vertical-cavity surface-emitting lasers. We found that fast initial components with ultra-short durations far below our temporal resolution of 5.5 ps emerged on short-wavelength sides, while the entire pulses were down-chirped, resulting in the simultaneous broadening of the spectrum and pulse width. The measured chirp characteristics were quantitatively explained using a single-mode rate-equation model, combined with carrier-density-dependent gain and index models. The observed universal fast short-wavelength components can be useful in generating even shorter pulses from gain-switched semiconductor lasers.

On the importance of cavity-length and heat dissipation in GaN-based vertical-cavity surface-emitting lasers.

Cavity-length dependence of the property of optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) with two dielectric distributed Bragg reflectors was investigated. The cavity lengths were well controlled by employing etching with inductively coupled plasma and chemical mechanical polishing. It was found that the lasing characteristics including threshold, slope efficiency and spontaneous emission coupling factor were substantially improved with reducing the cavity length. In comparison with the device pumped by a 400 nm pulsed laser, the lasing spectrum was featured by a red shift and simultaneous broadening with increasing the pumping energy of a 355 nm pulsed laser. Moreover, the lasing threshold was much higher when pumped by a 355 nm pulsed laser. These were explained by taking into account of the significant heating effect under 355 nm pumping. Our results demonstrate that a short cavity length and good heat-dissipation are essential to GaN-based VCSELs.

Low Threshold Lasing of GaN-Based VCSELs With Sub-Nanometer Roughness Polishing

Low threshold lasing at room temperature was achieved in optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) with sub-nanometer roughness polishing. The cavity region sandwiched by two dielectric distributed Bragg reflectors, incorporating InGaN/GaN multiquantum wells, a p-type AlGaN layer, and n- and p-type GaN layers, is a typical structure for electrically driven VCSELs. We observed lasing at a wavelength of 431.0 nm with a low threshold pumping energy density of ~ 3.2 mJ/cm2 and a high spontaneous emission coupling factor of ~ 0.09. These results were attributed to the significant reduction of the internal cavity loss by the removal of the high-dislocation GaN region, the reduction of cavity length, and the achievement of sub-nanometer level surface roughness (root mean square roughness of 0.3 nm) via inductively coupled plasma etching and chemical mechanical polishing. The loss mechanism is discussed and loss is quantitatively calculated in this letter.

Dependence of the Property of InGaN p-i-n Solar Cells on the Light Concentration and Temperature

The effects of the light concentration and temperature on the performance of InGaN p-i-n homojunction solar cells were investigated experimentally. With the increase in the light concentration, the short-circuit current density Jsc increases linearly, and the open-circuit voltage Voc increases logarithmically. However, the fill factor FF and the relative efficiency η increase first and then decrease. On the other hand, the performance of the solar cell becomes worse with the increase in temperature. The temperature coefficients of important parameters related with the cell property were discussed. The red shift of the peak external quantum efficiencies was observed with increasing temperature due to the band-gap shrinkage. In addition, the ideality factor n and the reverse saturation current density Js were also investigated to reveal the intrinsic mechanism of temperature dependence and the effect of crystalline quality. This paper provides the variation trends of device characteristics, which are useful references concerning the reliability of GaN-based solar cells.

Performance enhancement of GaN-based light emitting diodes by transfer from sapphire to silicon substrate using double-transfer technique

GaN-based light emitting diodes (LEDs) fabricated on sapphire substrates were successfully transferred onto silicon substrates using a double-transfer technique. Compared with the conventional LEDs on sapphire, the transferred LEDs showed a significant improvement in the light extraction and thermal dissipation, which should be mainly attributed to the removal of sapphire and the good thermal conductivity of silicon substrate. Benefited from the optimized wafer bonding process, the transfer processes had a negligible influence on electrical characteristics of the transferred LEDs. Thus, the transferred LEDs showed a similar current–voltage characteristic with the conventional LEDs, which is of crucial importance for practical applications. It is believed that the double-transfer technique offers an alternative way to fabricate high performance GaN-based thin-film LEDs.

Optical properties of ZnO/MgZnO quantum wells with graded thickness

National Natural Science Foundation of China [60876007, 10974165]; State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University) [KF2010-ZD-08]

Direct generation of 2-ps blue pulses from gain-switched InGaN VCSEL assessed by up-conversion technique

Ultra-short pulses in blue region generated from compact and low-cost semiconductor lasers have attracted much attention for a wide variety of applications. Nitride-based vertical-cavity surface-emitting lasers (VCSELs), having intrinsic high material gain and short cavities, favor the generation of ultra-short blue pulses via a simple gain-switching technique. In this study, we fabricated a single-mode InGaN VCSEL consisting of 10-period InGaN/GaN quantum wells (QWs). The output pulses were evaluated accurately with an up-conversion measurement system having time resolution of 0.12 ps. We demonstrated that ultra-short blue pulses, as short as 2.2 ps at 3.4 K and 4.0 ps at room temperature, were generated from the gain-switched InGaN VCSEL via impulsive optical pumping, without any post-processing. The gain-switched pulses we obtained should greatly promote the development of ultra-short blue pulse generation. In addition, this successful assessment demonstrates the up-conversion techniques usefulness for characterizing ultra-short blue pulses from semiconductor lasers.