Xue-Qin Lv

Broadband external cavity tunable quantum dot lasers with low injection current density

Broadband grating-coupled external cavity laser, based on InAs/GaAs quantum dots, is achieved. The device has a wavelength tuning range from 1141.6 nm to 1251.7 nm under a low continuous-wave injection current density (458 A/cm(2)). The tunable bandwidth covers consecutively the light emissions from both the ground state and the 1st excited state of quantum dots. The effects of cavity length and antireflection facet coating on device performance are studied. It is shown that antireflection facet coating expands the tuning bandwidth up to ~150 nm, accompanied by an evident increase in threshold current density, which is attributed to the reduced interaction between the light field and the quantum dots in the active region of the device.

Broadband Emitting Superluminescent Diodes With InAs Quantum Dots in AlGaAs Matrix

Superluminescent diodes were fabricated by using InAs-AlGaAs self-assembled quantum dots (QDs) as the active region. The ultrawide emitting spectrum of 142 nm was achieved. The short migration length of indium adatoms on AlGaAs surface increases the size dispersion of InAs QDs, resulting in the broadening of optical gain spectrum.

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.

A high-performance quantum dot superluminescent diode with a two-section structure

Based on InAs/GaAs quantum dots [QDs], a high-power and broadband superluminescent diode [SLD] is achieved by monolithically integrating a conventional SLD with a semiconductor optical amplifier. The two-section QD-SLD device exhibits a high output power above 500 mW with a broad emission spectrum of 86 nm. By properly controlling the current injection in the two sections of the QD-SLD device, the output power of the SLD can be tuned over a wide range from 200 to 500 mW while preserving a broad emission spectrum based on the balance between the ground state emission and the first excited state emission of QDs. The gain process of the two-section QD-SLD with different pumping levels in the two sections is investigated.

III-Nitride-Based Quantum Dots and Their Optoelectronic Applications

During the last two decades, III-nitride-based quantum dots (QDs) have attracted great attentions for optoelectronic applications due to their unique electronic properties. In this paper, we first present an overview on the techniques of fabrication for III-nitride-based QDs. Then various optoelectronic devices such as QD lasers, QD light-emitting diodes (LEDs), QD infrared photodetectors (QDIPs) and QD intermediate band (QDIB) solar cells (SCs) are discussed. Finally, we focus on the future research directions and how the challenges can be overcome.

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.

Broadly Tunable Grating-Coupled External Cavity Laser With Quantum-Dot Active Region

A broadly tunable and high-power grating-coupled external cavity laser with a tuning range of more than 200 nm and a ~ 200-mW maximum output power was realized, by utilizing a gain device with the chirped multiple quantum-dot (QD) active layers and bent waveguide structure. The chirped QD active medium, which consists of QD layers with InGaAs strain-reducing layers different in thickness, is beneficial to the broadening of the material gain spectrum. The bent waveguide structure and facet antireflection coating are both effective for the suppression of inner-cavity lasing under large injection current.

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.

Improved Continuous-Wave Performance of Two-Section Quantum-Dot Superluminescent Diodes by Using Epi-Down Mounting Process

Using AlN submounts with metal patterns made by a photolithographic process, the two-section quantum-dot superluminescent diodes were mounted epi-down on the heatsinks. It is demonstrated that the epi-down mounting process can offer improved heat dissipation and optical performance in the two-section devices. Under continuous-wave operation without temperature control, by properly controlling the pump levels in the two sections, a broad flat-top-like emission spectrum of 112 nm with a high output power of 107 mW is obtained from a two-section quantum-dot superluminescent diode with an epi-down mounting structure.