Zengcheng Li

Carriers capturing of V-defect and its effect on leakage current and electroluminescence in InGaN-based light-emitting diodes

The effect of quantum well (QW) number on performances of InGaN/GaN multiple-quantum-well light-emitting diodes has been investigated. It is observed that V-defects, originated from various InGaN well layers intercepted by threading dislocations (TDs), increase in density and averaged size with more periods of QWs, resulting in larger reverse-bias leakage current and lower emission efficiency of light-emitting diodes. Conductive atomic force microscopy measurements demonstrate that V-defects may preferentially capture carriers, subsequently enhance local current and nonradiative recombinations at associated TD lines, which suggest that TD lines with V-defects at vertex have larger influence on emission efficiency than those without V-defects.

Suppression of thermal degradation of InGaN/GaN quantum wells in green laser diode structures during the epitaxial growth

Local InGaN quantum well (QW) decomposition and resultant inhomogeneous luminescence in green laser diode (LD) epitaxial structures are investigated using micro-photoluminescence, Z-contrast scanning transmission electron microscopy, and high-resolution transmission electron microscopy. The local InGaN QW decomposition is found to happen during p-type layer growth due to too high thermal budget and may initiate at the InGaN/GaN QW upper interface probably due to the formation of In-rich InGaN clusters there. Reducing thermal budget and optimizing InGaN/GaN QW growth suppress the local InGaN QW decomposition, and green LD structures with homogeneous luminescence and bright electroluminescence (EL) intensity are obtained.

GaN-on-Si blue/white LEDs: epitaxy, chip, and package*

The dream of epitaxially integrating Ⅲ-nitride semiconductors on large diameter silicon is being fulfilled through the joint R&D efforts of academia and industry, which is driven by the great potential of GaN-on-silicon technology in improving the efficiency yet at a much reduced manufacturing cost for solid state lighting and power electronics. It is very challenging to grow high quality GaN on Si substrates because of the huge mismatch in the coefficient of thermal expansion (CTE) and the large mismatch in lattice constant between GaN and silicon, often causing a micro-crack network and a high density of threading dislocations (TDs) in the GaN film. Al-composition graded AlGaN/AlN buffer layers have been utilized to not only build up a compressive strain during the high temperature growth for compensating the tensile stress generated during the cool down, but also filter out the TDs to achieve crack-free high-quality n-GaN film on Si substrates, with an X-ray rocking curve linewidth below 300 arcsec for both (0002) and (1012) diffractions. Upon the GaN-on-Si templates, prior to the deposition of p-AlGaN and p-GaN layers, high quality InGaN/GaN multiple quantum wells (MQWs) are overgrown with well-engineered V-defects intentionally incorporated to shield the TDs as non-radiative recombination centers and to enhance the hole injection into the MQWs through the via-like structures. The as-grown GaN-on-Si LED wafers are processed into vertical structure thin film LED chips with a reflective p-electrode and the N-face surface roughened after the removal of the epitaxial Si(111) substrates, to enhance the light extraction efficiency. We have commercialized GaN-on-Si LEDs with an average efficacy of 150-160 lm/W for 1 mm 2 LED chips at an injection current of 350 mA, which have passed the 10000-h LM80 reliability test. The as-produced GaN-on-Si LEDs featured with a single-side uniform emission and a nearly Lambertian distribution can adopt the wafer-level phosphor coating procedure, and are suitable for directional lighting, camera flash, streetlighting, automotive headlamps, and otherlighting applications.

GaN-based green laser diodes

Recently, many groups have focused on the development of GaN-based green LDs to meet the demand for laser display. Great progresses have been achieved in the past few years even that many challenges exist. In this article, we analysis the challenges to develop GaN-based green LDs, and then the approaches to improve the green LD structure in the aspect of crystalline quality, electrical properties, and epitaxial layer structure are reviewed, especially the work we have done.

Strong localization effect and carrier relaxation dynamics in self-assembled InGaN quantum dots emitting in the green

Strong localization effect in self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition has been evidenced by temperature-dependent photoluminescence (PL) at different excitation power. The integrated emission intensity increases gradually in the range from 30 to 160 K and then decreases with a further increase in temperature at high excitation intensity, while this phenomenon disappeared at low excitation intensity. Under high excitation, about 40% emission enhancement at 160 K compared to that at low temperature, as well as a higher internal quantum efficiency (IQE) of 41.1%, was observed. A strong localization model is proposed to describe the possible processes of carrier transport, relaxation, and recombination. Using this model, the evolution of excitation-power-dependent emission intensity, shift of peak energy, and linewidth variation with elevating temperature is well explained. Finally, two-component decays of time-resolved PL (TRPL) with various excitation intensities are observed and analyzed with the biexponential model, which enables us to further understand the carrier relaxation dynamics in the InGaN QDs.

Realization of InGaN laser diodes above 500 nm by growth optimization of the InGaN/GaN active region

Two-step growth was employed to grow GaN quantum barriers (QBs) in InGaN green LD structures. A cap layer was grown at the same temperature as an InGaN quantum well (QW), and the temperature was then raised by around 130 °C to grow GaN QBs. The effects of low-temperature-grown cap (LT-cap) layers on the optical properties and microstructures of green LD structures were investigated. It was found that the LT-cap layer with an optimal thickness can improve the luminescence homogeneity and suppress the thermal decomposition of InGaN QWs. C-plane ridge waveguide laser diodes lasing above 500 nm were realized.

High efficient GaN-based laser diodes with tunnel junction

High-efficient GaN-based laser diodes (LDs) with tunnel junction are designed by replacing conventional p-type AlGaN cladding layers and p-type GaN contact with lower-resistant n-type AlGaN cladding layers and n-type GaN contact. In addition, the characteristics of the LDs with tunnel junction are numerically investigated by using the commercial software lastip. It is found that the performance of these LDs is greatly improved. As a comparison, the absorption loss and non-radiative recombination are greatly reduced. The threshold current and series resistance are decreased by 12% and 59%, respectively, and the slope efficiency is raised up by 22.3%. At an injection current of 120 mA, the output power and wall-plug-efficiency are increased by 34% and 79%, respectively.

Design Considerations for GaN-Based Blue Laser Diodes With InGaN Upper Waveguide Layer

Effects of an inserted InGaN interlayer between active region and p-AlGaN electron blocking layer on electrical and optical characteristics of GaN-based blue laser diodes are numerically investigated. It is found that the inserted InGaN interlayer reduces the barrier height for hole injection into multiple quantum wells. Moreover, it is found that the background electron concentration of the undoped InGaN plays a critical role in the LD performance. A background electron concentration higher than 1 × 1017 cm-3 may induce undesired electron-hole recombination in this layer. In addition, we have calculated the dependences of optical confinement factor and internal absorption loss (IAL) on location, In composition, and thickness of the InGaN layer. A significant increase in OCF and a decrease in IAL are obtained by inserting the InGaN layer.

Thermal characterization of GaN-based laser diodes by forward-voltage method

An expression of the relation between junction temperature and forward voltage common for both GaN-based laser diodes (LDs) and light emitting diodes is derived. By the expression, the junction temperature of GaN-based LDs emitting at 405 nm was measured at different injection current and compared with the result of micro-Raman spectroscopy, showing that the expression is reasonable. In addition, the activation energy of Mg in AlGaN/GaN superlattice layers is obtained based on the temperature dependence of forward voltage.

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.