Lei- Ying

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.

Threshold Current Density Reduction by Utilizing High-Al-Composition Barriers in 3.7 THz GaAs/AlxGa1-xAs Quantum Cascade Lasers

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.

Quantum dot vertical-cavity surface-emitting lasers covering the ‘green gap’

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.

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.

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.

Coplanar metal–semiconductor–metal light-emitting devices with an n++ InGaN layer and their application to display

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.

Well-width dependence of the emission linewidth in ZnO/MgZnO quantum wells

Photoluminescence (PL) spectra were measured as a function of well width (LW) 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 LW was studied. The inhomogeneous linewidth obtained at 5 K was found to decrease with increasing LW from 1.8 to 3.3 nm due to the reduced potential variation caused by the LW fluctuation. Above 3.3 nm, however, the linewidth became larger with increasing LW, 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 ΓLO and impurity scattering coefficient Γimp were deduced from the temperature dependence of the linewidth of the PL spectra. Evident reduction of ΓLO with decreasing LW was observed, which was ascribed to the confinement-induced enhancement of the exciton binding energy. Different from ΓLO, a monotonic increase in Γimp was observed with decreasing LW, which was attributed to the enhanced penetration of the exciton wave function into the barrier layers.

Ag–Metal Bonding Conditions for Low-Loss Double-Metal Waveguide for Terahertz Quantum Cascade Laser

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.

Auto-Split Laser Lift-Off Technique for Vertical-Injection GaN-Based Green Light-Emitting Diodes

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.