Man-Fang Huang

Tunable Cr/sup 4+/:YSO Q-switched Cr:LiCAF laser

Tunable passive Q-switching (781 nm to 806 nm at 300 K) of a flash-lamp pumped Cr/sup 3+/:LiCaAlF/sub 6/ (Cr:LiCAF) laser with a Cr/sup 4+/:Y/sub 2/SiO/sub 5/ (Cr/sup 4+/:YSO) broad-band solid-state saturable absorber has been realized. Typical pulse widths of the Q-switched laser output ranged from 40 ns to 80 ns, depending on the lasing wavelength. Spectral narrowing and reduced beam diameter with the use of the saturable absorber were observed. The ground state and the excited state absorption cross sections of the Cr/sup 4+/:YSO absorber were found by bleaching experiments to be (7.0/spl plusmn/1.4)/spl times/10/sup -19/ cm/sup 2/ and (2.3/spl plusmn/0.5)/spl times/10/sup -19/ cm/sup 2/ at 694 nm, respectively. Numerical simulation was utilized to simulate the Cr:LiCAF passive Q-switching with Cr/sup 4+/:YSO solid-state saturable absorber. >

AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding

An AlGaInP light-emitting diode (LED) with a Au/AuBe/SiO2/Si mirror substrate has been fabricated using wafer bonding. The bonded mirror-substrate LED is capable of emitting luminous intensity of 90 and 205 mcd under 20 and 50 mA injection, respectively. The emission wavelength was found to be independent of the injection current. This feature is attributed to the Si substrate providing a good heat sink.

Optimization of the active-Layer structure for the deep-UV AlGaN light-emitting diodes

The dependence of the active-layer structure on the performance of the deep-UV AlGaN light-emitting diodes (LEDs) was theoretically investigated with an APSYS simulation program. Several structure parameters such as well width, well number, barrier height, barrier width, and doping type were employed to study how these parameters change the band structures as well as the carrier distributions. The band offset and bowing parameter used in the theoretical analysis were extracted from the experimental results. Theoretical analysis shows that the nonuniform carrier distributions as well as the low hole concentrations, which caused by polarization-induced tilted band structures, play important roles in improving the performance of the AlGaN LEDs. Compensating this asymmetric band structure and increasing the hole density are the important keys to improve the AlGaN LED performance. Numerical simulation results suggest that the higher output power can be obtained when the active layer consists of only one quantum well with a width of 1-3 nm and two thicker n-doped barriers with a small Al composition

AlGaInP/AuBe/glass light-emitting diodes fabricated by wafer bonding technology

An AlGaInP/AuBe/glass light-emitting diode (LED) was fabricated by a wafer bonding technique. The AlGaInP LED was grown on a temporary GaAs substrate by metalorganic vapor phase epitaxy. By bonding the AuBe/glass substrate on top of epitaxial layers, the temporary GaAs substrate was removed. The luminance of this wafer-bonded device is about 3050 cd/m2 (600 nm wavelength) at an operating current of 20 mA. It is about three times brighter than a conventional device with an absorbing GaAs substrate. This could be due to the fact that the AuBe/glass substrate serves as a reflective mirror, improving the light extraction efficiency.

Ho:CaF 2 solid-state saturable-absorber Q switch for the 2-μm Tm,Cr:Y 3 Al 5 O 12 laser

The holmium-doped calcium fluoride (Ho:CaF(2)) crystal was shown to be an effective solid-state saturable-absorber Q-switch for a flash-lamp-pumped Tm,Cr:Y(3)Al(5)O(12) laser at 2.017 µm. With a 1-cm-thick Ho(0.5%),Er(5%):CaP(2) saturable absorber and a 6.3% output coupler, a single Q-switched laser pulse of 51 mJ in energy and 60 ns in duration was obtained at a flash-lamp input energy of 85 J. With a 14.6% output coupler, a typical Q-switched laser pulse of 84 mJ and 82 ns was observed.

Room temperature optical absorption characteristics of GaAs/AlGaAs multiple quantum well structures under external anisotropic strain

We report the room temperature anisotropic absorption characteristics of a GaAs/AlGaAs multiple quantum well structure under biaxial anisotropic strain that was achieved by bonding to an x‐cut LiTaO3 substrate at 150 °C, with the [110] and [1■10] directions of the multiple quantum well along the y and z axes of LiTaO3, respectively. A stronger heavy hole excitonic feature can be observed when the polarization of the incident light is in the compression direction, which thus results in larger quantum confined Stark effect. An absorption coefficient change of 0.6 μm−1 was observed for an applied field of 7.4 V/μm. This change in absorption coefficient is 1.5 times the value obtainable from a multiple quantum well without strain.

Wafer bonding of 50 mm diameter mirror substrate to AlGaInP light-emitting diode wafer

The feasibility of bonding 50-mm-diameter Si with a Au/AuBe mirror to AlGaInP light-emitting diode (LED) wafers is demonstrated. Wafer bonding over the entire wafer area is achieved while the metallic mirror still maintains high reflectivity. Using this technique, the mirror-substrate AlGaInP LEDs are fabricated across an entire 50-mm wafer. The test data show that 98% of the dice with operating voltages <2.2 V at 20 mA and 85% of the dice with luminous intensity in the 130∼140 mcd region. The wafer-bonded mirror-substrate LED lamps operating at 626 nm can emit 3 lm at 20 mA with a forward voltage of 2 V, corresponding to a luminous efficiency of 74 lm/W. Moreover, they present a peak power efficiency of 21% with 4 mW output at 10 mA (1.9 V). Essentially no degradation is observed for these LEDs after 2000 h stress at 80°C and 50 mA (55.6 A/cm2). The results indicate the mirror-substrate AlGaInP LEDs of highly reliable and efficient performance.

High-Brightness Wafer-Bonded Indium-Tin Oxide/Light-Emitting Diode/Mirror/Si

Indium-tin oxide (ITO) used as the window layer and current-spreading layer for wafer-bonded AlGaInP/mirror/Si light-emitting diodes (MS LEDs) has been reported. The ITO films prepared by sputtering have low resistivity (2.1×10-4 Ω-cm) and high transmittance (>90% in the visible region). The MS LEDs incorporating the ITO layer and In/ITO provide higher light output than ITO-free MS LEDs. They also exhibit a linear increase of a uniform distributed light output without radiation saturation as the injection current increases. Moreover, the MS LED, ITO/MS LED and ITO/In/MS LEDs provide 2.8, 3.0 and 3.4 times improvement in external power efficiency, respectively, as compared with the absorbing-substrate LED fabricated from the same AlGaInP LED wafer. Due to the inserted In layer that reduces the contact resistance between ITO and the GaAs contact layer without causing obvious absorption of the emitting light, the ITO/In/MS LEDs can achieve the highest power conversion efficiency among the LEDs studied.

Wafer-bonded AlGaInP/Au/AuBe/SiO2/Si light-emitting diodes

An AlGaInP light-emitting diode (LED) with a Au/AuBe/SiO2/Si mirror substrate has been successfully fabricated by the wafer bonding technique. The mirror-substrate LED is capable of emitting luminous intensity of 90 mcd under 20 mA injection. It is clearly far superior to the conventional absorbed-substrate LED with a distributed Bragg reflector structure (≤1 mcd). This device exhibits normal p-n diode behavior with a forward voltage of less than 2 V operating at 20 mA. The leakage current is about 3 nA under a 36 V reverse bias. This result indicates that the wafer bonding technique does not adversely affect the I–V characteristic of the LED device. Moreover, the emission wavelength of the bonded LED was independent of the injection current. The low forward series resistance and the Si substrate providing a good heat sink solve the joule heating problem inherent in conventional LEDs.

Absorption Anisotropy for Lattice Matched GaAs/AlGaAs Multiple Quantum Well Structures under External Anisotropic Biaxial Strain: Compression along [110] and Tension along [1̄10]

We report theoretical studies on the optical absorption anisotropy for excitonic transitions in lattice-matched GaAs/AlGaAs multiple quantum well (MQW) structures under simultaneous compression and tension applied along the [110] and [10] directions of the MQW, respectively. The analyses are based on a model that includes both the 4×4 kp Hamiltonian and the strain Hamiltonian. The wave functions, found by solving the eigenvalue equations, are used to calculate the dipole matrix elements for excitonic transitions and evaluate the anisotropic absorption properties. The effect of variation of parameters such as well width and barrier height on the performance of the strained GaAs/AlGaAs MQW electroabsorption modulators is discussed.