Aiqin Tian

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.

Investigation of InGaN/GaN laser degradation based on luminescence properties

Degradation of InGaN/GaN laser diode (LD) is investigated based on the luminescence properties. Gradual degradation of the LD is presented with the threshold current increase and the slope efficiency decrease. The cathodoluminescence and photoluminescence characterizations of the LD show a dislocation independent degradation of the active region under the ridge. Detailed studies on the temperature-dependent micro-photoluminescence and the electroluminescence indicate that the degradation of the LD is attributed to the generation of non-radiative recombination centers in the local multiple quantum well regions with lower indium content. The activation energy of the non-radiative recombination centers is about 10.2 meV.

Identification of degradation mechanisms of blue InGaN/GaN laser diodes

A comprehensive analysis of the degradation mechanism of blue InGaN/GaN laser diodes (LDs) is carried out by investigating the electrical and optical characteristics. The increase in the leakage current as well as decrease in the slope efficiency is observed. The luminescence properties of the active region at different aging stages are studied by means of cathodoluminescence. Significant degradation of the active region is observed on the room temperature cathodoluminescence while the low temperature cathodoluminescence shows almost no degradation, indicating that the degradation of the LDs is due to generation of low temperature frozen point defects. Furthermore, the generation of the defects follows a kinetic mechanism enhanced by electron-hole non-radiative recombination which explains the acceleration of time degradation in our LDs.

Green laser diodes with low threshold current density via interface engineering of InGaN/GaN quantum well active region

By observing the morphology evolution of green InGaN/GaN quantum well (QW) and studying the catholuminescence (CL) property, we investigate indium-segregation-related defects that are formed at green InGaN/GaN QW interfaces. Meanwhile, we also propose the approach and suggest the mechanism to remove them for green InGaN/GaN QW grown on both GaN templates and free-standing GaN substrates. By engineering the interface of green InGaN/GaN QWs, we have achieved green laser diode (LD) structure with low threshold current density of 1.85 kA cm-2. The output power of the green LD is 58 mW at a current density of 6 kA cm-2 under continuous-wave operation at room temperature.

Hole transport in c-plane InGaN-based green laser diodes

Hole transport in c-plane InGaN-based green laser diodes (LDs) has been investigated by both simulations and experiments. It is found that holes can overflow from the green double quantum wells (DQWs) at high current density, which reduces carrier injection efficiency of c-plane InGaN-based green LDs. A heavily silicon-doped layer right below the green DQWs can effectively suppress hole overflow from the green DQWs.

Conductivity enhancement in AlGaN:Mg by suppressing the incorporation of carbon impurity

Growth conditions were explored to suppress the carbon impurity incorporation in AlGaN:Mg grown at low temperatures. Electrical properties of Al0.07Ga0.93N:Mg samples with various carbon concentrations were investigated by Hall measurements. A clear correlation between carbon concentration and electrical conductivity has been found. By reducing the carbon concentration from 2 x 10(18) to 5 x 10(16) cm(-3), the resistivity of p-Al0.07Ga0.93N decreases from 7.4 to 2.2 Omega.cm. From the results of the analysis of the charge neutrality equation, we found that the carbon concentration is close to the compensating donor concentration in the AlGaN:Mg samples, which suggests that carbon acts as the main compensating donor in AlGaN:Mg

Optical characterization of InGaN/GaN quantum well active region of green laser diodes

We performed the optical characterization of InGaN/GaN quantum well (QW) active regions of green laser diodes (LDs) with different threshold current densities by temperature-dependent photoluminescence (PL) analysis. The internal quantum efficiency (IQE) was evaluated to be 39 and 59% for green LDs with threshold current densities of 8.50 and 1.85 kA cm−2, respectively. Additional nonradiative recombination centers with an activation energy of 10 meV were found in the sample with the lower IQE, which is attributed to defects located at the interface of InGaN/GaN QWs.

Asymmetrical quantum well degradation of InGaN/GaN blue laser diodes characterized by photoluminescence

The temperature, power, and voltage dependent photoluminescence spectra are studied in InGaN/GaN double quantum well blue laser diodes. Emissions from the two quantum wells can be distinguished at low temperature at low excitation power density due to the different built-in electric field in the two quantum wells. This finding is utilized to study the degradation of InGaN/GaN blue laser diodes. Two peaks are observed for the non-aged laser diode (LD), while one peak for the aged LD which performed 3200 h until no laser output is detected. The disappearance of the high energy peak in the photoluminescence spectra indicates a heavier degradation of the quantum well on the p-side, which agrees with our previous observation that both the linewidth and the potential fluctuation of InGaN quantum wells (QWs) reduced for the aged LDs.

Enhanced temperature characteristic of InGaN/GaN laser diodes with uniform multiple quantum wells

Temperature-dependent electroluminescence (EL) of two high-power blue InGaN/GaN laser diodes (LDs) is studied. An enhanced temperature characteristic is observed in one LD, having a smaller Shockley–Read–Hall non-radiative recombination coefficient and a smaller full-width-at-half-maximum (FWHM) of the spontaneous EL spectra. The scanning transmission electron microscopy image of this LD shows a better uniformity of the multiple quantum wells (MQWs), indicating that the uniform MQWs contribute to the stable temperature characteristic of the LD.

Identification of Degradation Mechanisms Based on Thermal Characteristics of InGaN/GaN Laser Diodes

A method of identifying laser degradation mechanism is established based on thermal characteristic analysis of InGaN/GaN laser diodes (LDs). Both steady and transient thermal characteristics of LDs are calculated by the finite-element analysis method, the results show that thermal resistance and thermal time constant of each layer are determined by the corresponding structure and material. Experiments on thermal characteristics of LDs are carried out, degradation induced heat transport irregularities are localized by comparing the transient cooling curves of the virgin, and degraded LDs and further analyses give more information on degradation mechanisms of the LDs. The results confirm that this identification method is an effective method, which gives specific guidance for the analysis of the laser degradation mechanisms.