Kechang Song

Quantum well intermixing

Embodiments of a method of quantum well intermixing (QWI) comprise providing a wafer (1) comprising upper and lower epitaxial layers (10, 13), which each include barrier layers, and a quantum well layer (11) disposed between the upper and lower epitaxial layers (10, 13), applying at least one sacrificial layer (21) over the upper epitaxial layer, and forming a QWI enhanced region and a QWI suppressed region by applying a QWI enhancing layer (31) over a portion of the sacrificial layer, wherein the portion under the QWI enhancing layer (31) is the QWI enhanced region, and the other portion is the QWI suppressed region. The method further comprises the steps of applying a QWI suppressing layer (41) over the QWI enhanced region and the QWI suppressed region, and annealing at a temperature sufficient to cause interdif fusion of atoms between the quantum well layer (11) and the barrier layers of the upper epitaxial layer and the lower epitaxial layer (10, 13).

500-nm Optical Gain Anisotropy of Semipolar (1122) InGaN Quantum Wells

We studied the effect of carrier population on light emission polarization of green InGaN quantum wells (QWs) on the semipolar (1122) plane. The 3 nm thick QWs emitting light at about 540 nm at low pumping power have electrical field (E) component E∥[1123] stronger than that E∥[1100]. However, we found that increasing the pumping power changed the sign of the polarization ratio. Using the varied stripe length (VSL) method, we measured the optical gain for light propagating ∥[1123] direction to be ~2 times that of light propagating ∥[1100] direction. We explain this behavior by inhomogeneous QW state filling.

Carrier Transport in InGaN MQWs of Aquamarine- and Green-Laser Diodes

We studied experimentally and theoretically the substrate-orientation impact on carrier transport and capture in InGaN multiple quantum well (MQW) laser diodes (LDs) with emission in the aquamarine-green spectral range. A new simulation approach was developed to analyze this behavior of LEDs and LDs emitting at these wavelengths. We show that due to deep carrier confinement, the thermal escape from a QW in such devices is negligible. The carrier distribution among QWs is therefore determined by the carrier transport and capture rates. We also show that the ballistic transport mechanism is dominant in this type of MQW active region. In c-plane structures, this mechanism is tunneling-assisted, and therefore, the transport is much slower than in nonpolar and semipolar structures. Because of this, a strong carrier injection nonuniformity observed in c-plane LDs, causes the threshold current increase when number of QWs is >;2. This effect is not observed in semipolar LDs because the carrier transport rate is faster than the capture rate.

107-mW low-noise green-light emission by frequency doubling of a reliable 1060-nm DFB semiconductor laser diode

We have generated 107-mW green-light emission by frequency doubling of a reliable 1060-nm distributed feedback (DFB) laser diode using a periodically poled MgO-doped lithium niobate waveguide in the most compact single-pass configuration. The green power variation is lower than 1% at frequencies below 82 kHz. The relative intensity noise of -150 dB/Hz has been measured at 100 MHz. We also report 5000-h life-test results of 1060-nm DFB lasers at 80/spl deg/C.

High-power high-Modulation-speed 1060-nm DBR lasers for Green-light emission

We report on the static and dynamic performance of high-power and high-modulation-speed 1060-nm distributed Bragg reflector (DBR) lasers for green-light emission by second-harmonic generation. Single-wavelength power of 387 mW at 1060-nm wavelength and green power as high as 99.5 mW were achieved. A thermally induced wavelength tuning of 2.4 nm and a carrier-induced wavelength tuning of -0.85 nm were obtained by injecting current into the DBR section. Measured rise-fall times of 0.2 ns for direct intensity modulation and 0.6 ns for wavelength modulation make the lasers suitable for >50-MHz green-light modulation applications

304 mW green light emission by frequency doubling of a high-power 1060-nm DBR semiconductor laser diode

We report for the first time, to the best of our knowledge, 304 mW green light emission generated by frequency doubling of the output from a 1060-nm DBR semiconductor laser using a periodically poled MgO-doped lithium niobate waveguide in a compact single-pass configuration. The excellent performance of these DBR lasers, including a kink-free power greater than 750 mW, single-spatial-mode output beam, single-wavelength emission spectra, and high wavelength-tuning efficiency, plays an important role in the generation of high-power green light.

Impact of Carrier Transport on Aquamarine-Green Laser Performance

We studied the carrier transport phenomena of the multiple-quantum-well (MQW) active region and their impact on the performance of aquamarine and green laser diodes (LDs) grown on polar and semipolar planes. The ballistic carrier transport mechanism was found to be dominant in the MQW region. For the c-plane, because of the high hole capture probability and slow escape rate, mainly the quantum wells (QWs) positioned close to the p-side are electrically pumped. The optical loss induced by the underpumped QWs further away from the p-side leads to significantly higher laser threshold current density and a longer lasing wavelength with increased number of QWs. These effects are not significant for semipolar LD structures.

60 mW Pulsed and Continuous Wave Operation of GaN-Based Semipolar Green Laser with Characteristic Temperature of 190 K

We studied characteristic temperatures (T0) of laser diodes (LDs) grown on semipolar GaN substrates and emitting in the green spectral range. For several semipolar laser designs with and without an electron blocking layer (EBL), T0 remains higher (161–246 K) than that typically reported for c-plane green LDs. The slope efficiency measured in the pulsed regime is nearly temperature independent. These observations indicate that T0 is mainly determined by intrinsic quantum well (QW) properties, such as higher differential gain. A high T0 and a sufficient injection efficiency allow the achievement of a continuous wave output power of 60 mW for an LD without an EBL.

Design and implementation of metallization structures for epi-down bonded high power semiconductor lasers

High power semiconductor lasers have found increasing applications in industrial, military, commercial and consumer products. The thermal management of high power lasers is critical since the junction temperature rise resulting from large heat fluxes strongly affects the device characteristics, such as wavelength, kink power, threshold current and efficiency, and reliability. The epitaxial-side metallization structure of epi-down bonded lasers has significant impact on the thermal performance and reliability of the high power semiconductor lasers. In this paper, the influence of the epitaxial-side metal (p-metal) on the thermal behavior of a GaAs-based high power single-mode laser, mounted epi-side down, is studied using finite element analysis. Metallization structures having different diffusion barriers for eutectic AuSn solder are designed and implemented, and the metallurgical stability of the four metal systems, Ti/Pt/thick Au (2-3 /spl mu/m thick), Ti/Pt/thick Au/Ti/Pt/Au, Ti/Pt/thick Au/Ti/Ni/Au, and Ti/Pt/thick Au/Ti/Cr/Au, are reported.

Internal Optical Waveguide Loss and p-Type Absorption in Blue and Green InGaN Quantum Well Laser Diodes

We present a new characterization method for internal optical waveguide loss of blue, aquamarine, and green group-III–nitride laser diodes from as-grown wafers without need for further fabrication. This approach relies on excitation-position dependent polarization-resolved photoluminescence spectra collected from the edge of the planar waveguide. The high measurement accuracy of ±1 cm-1 enables for the first time determination of the mechanisms for p-layer optical loss from the waveguide loss difference before and after Mg dopant activation. Temperature-dependent measurements show that the dominant optical loss mechanism is absorption by acceptor-bound holes. This absorption mechanism does not depend significantly on light polarization.