In this study, the fabrication and characterization of AlGaInP-based light-emitting diodes (LEDs) with further improvement by the design of a SiO2 current blocking layer were described. It was found that with the SiO2 CBL, the injected current can be forced to spread outside instead of flowing directly downward under the p-pad electrode. At 20mA, as compared to traditional LEDs, the optical output power for novel LEDs is increased by about 30%. We found that the novel LEDs have better saturation characteristics, this improvement is contributed to more uniform of injection current and less heat generation in the novel LED chips.
For developing the tunable performance and stability, we present a widely tunable 850nm-range VCSEL structure based on the voltage-dependent birefringence of liquid crystal. An intracavity liquid crystal layer is imbedded between the top DBR (Distributed Bragg Reflector) and the Half VCSEL as an electro-optic index modulator. An Al0.98Ga0.02As oxidization layer was grown above the active region for current and optical confinement. By the calculation, we found tuning efficiency increased after thickening the liquid crystal layer. However, the optical loss in resonance cavity also increased simultaneously. For compromise, we got that 1837nm is the most suitable thickness. And the tuning efficiency is obviously larger than the electrostatic method. Then, we calculated the electric field intensity distribution, the gain characteristics of GaAs/Al0.3Ga0.7 As quantum wells and the threshold features when thickness of liquid crystal layer is 1837nm. By analyzing these results, tuning efficiency of 5.4nm/V and 15nm tuning range are obtained at last. Our study could provide insight into tunable VCSELs design and optimization.
Low threshold, continuous wavelength tuning micro-electro-mechanically system (MEMS) tunable
vertical-cavity surface-emitting lasers (VCSELs) operating at 980 nm are demonstrated. The device utilizes the
two-chip concept with a MEMS membrane mirror suspended by an air gap above a VCSEL amplifier. Output
power of 4.56 mW with 14μm diameter oxide aperture in continuous operation and tuning range of 6.8nm are
obtained. Due to the low optical loss in resonance cavity results in threshold current as low as 0.6 mA at room
temperature. Theoretical calculation for the threshold gain as a function of the wavelength and air gap is obtained
respectively, which provides the design strategies to improve device performance.
Based on the band gap theory, a dual-wavelength VCSELs with same direction, equal-intensity,
high-Q program is presented. The wavelengths of the VCSEL can be located with the aid of the
Al0.8Ga0.2As defect layer in 1D photonic crystal structure. The results indicated that
one-dimensional PC with a sheet of defect layer provides a parent structure on which laser beam
can be well engineered without the expense of the macroscopic structural integrity.
In this study, the indium tin oxide (ITO) was etched by an inductively coupled plasma (ICP) etcher using
Cl2/C2H4/Ar as the etching gases. The etch rates were studied as a function of RF power, inductively coupled plasma
(ICP) source power, working pressure and gas mixing ratio. We found that the RF Power plays a dominant role in the
elevation of the etch rates, which indicates that the ITO etching in this study is a physical mechanism dominated etching.
The state of the photoresist (PR) was observed after each experiment. As expected, the high power, which generally
leads to a high etch rate, would distort the PR mask, resulting in a bad etched profile of the sample. Through research, we
got a relatively high etch rate (~ 200nm/min) with the shape of PR mask remain undistorted. The profile, which includes
the morphology of the film sidewall and surface after the etching process, was sensitive to the gas mixing ratio. We
adjusted the gas mixing ratio, and observed the profile using a scanning electron microscope (SEM), then optimal
morphology was get as the Cl2/C2H4/Ar at the value of 30sccm/7sccm/50sccm.
As one of the most rapidly growing areas of the tunable semiconductor diodes, tunable vertical
cavity surface-emitting lasers (VCSELs) have been focused on and the wavelength tuning
characteristics have been investigated in detail. By applying an electrostatic bias between the
semiconductor and the cantilever which is an external reflector apart from the device, the stimulated
wavelength can be continuously modulated with a continuous tuning over a 10nm spectral range. It is
found that, as the tunable VCSEL operate in its stable regime, the displacement of the cantilever will
result in a periodic variation in the intensity and wavelength with a period of half the resonant
wavelength. We also found the same phenomenon in the tunable RCLED. In addition, based on our
observations, precise analysis is presented from the Lang and Kobayashi model. Our results show an
adequate match between theory and experiments for the detailed tunable spectra. These results can be
used to greatly enhance the performance of tunable VCSEL and RCLED.
The relationship between the wavelength shift and the thickness of the air gap was investigated by the Optical Standing
Wave Method; the modal characteristics of the MEMS tunable Vertical Cavity Surface Emitting Lasers were also
analysed in the Dielectric Cylindrical Waveguide by the Improved Effective Index Model.
The problems and fabrication difficulties for the conventional semiconductor LDs (laser diodes), VCSELs
(vertical cavity surface emitting lasers) and LEDs (light emitting diodes) were analyzed. The high quantum
efficiency transverse optical coupled LDs, longitudinal optical coupled VCSELs with multi-active region structure
and high internal and external quantum efficiency high brightness LEDs with small size were proposed and
fabricated; they have showed the excellence performance. The external and differential quantum efficiency are 3.3
and 3.8 W/A, and the output light power is as high as ~ 6.6W when the injecting current equals 2A for the four
active regions 980nm strained InGaAs/GaAs QW lasers; the highest pulse and CW light power output are 13.1mW
and 9mW of the 980nm longitudinal optical coupled VCSELs; the on-axis luminous intensity of the tunneling
regenerated multi-active region LEDs will increase linearly with the number of active regions approximately. The
on-axis luminous intensity for the high external quantum efficiency 622nm LEDs with small size 8milx×8mil is as
high as 150-200mcd at 20mA injecting current.
The theory of Light Emitting Diodes(LEDs) life tests and mathematic model of life tests were introduced. The performance of LEDs was affected by the drive current and by the ambient temperature. Life tests of tunnel junction regenerated AlGaInP LEDs were performed at different currents and ambient temperatures. On axis output intensity of tunnel junction regenerated LED had decreased 35.53% after 5203 hours at 30mA and 25°C. At the ambient temperature of 80°C, on axis output intensity of tunnel junction regenerated LED had degraded 19.26% after 3888 hours at 20mA. According to the results mentioned above, the normal working lifetime of tunnel junction regenerated LEDs were concluded. Moreover, the main Failure Mechanisms of it were described. Our work reviews the failure analysis that was performed on the degraded LEDs and the degradation mechanisms that were identified. The results show a thermal degradation mechanism that dominates degradation at high ambient temperatures.
A Novel structure of dual-wavelength semiconductor laser diode is proposed and fabricated. Two laser structures based on two kinds of materials AlGaAs and AlGaInP active layer, which are cascaded by a high doping tunnel junction during the epitaxial growth. The lasers can emit at wavelength of 699nm and 794nm at the same time. Without face coating, the output power of the dual-wavelength laser is high as 50mW at 220mA. And the slope efficiency of these devices is about 0.42A/W.
A Novel structure of high power dual-wavelength semiconductor laser diode is proposed and fabricated. Two laser structures are cascaded by a high doping tunnel junction during the epitaxial growth. The lasers can emit at wavelength of 951nm and 987nm at the same time. Without facet coating, the output power of the dual-wavelength laser is as high as 3.1W at 3A. And the slope efficiency of these devices is about 1.21A/W. Much higher output power can be reached for those dual-wavelength lasers when modifying the structure. The external differential quantum efficiency of different cavity length devices is analyzed.
Tunnel cascaded and coupled multi-active regions laser diodes are novel high power laser diodes. This kind of laser diodes can achieve high output power at relatively low current density and overcome the main hindrance of the normal high power semiconductor lasers: catastrophic optical damage (COD) by increase the size of facula. Transient thermal property of these laser diodes has been calculated by using finite element method (FEM). Three kinds of laser diode structures, one active region, two active regions with one tunnel junction and three active regions with two tunnel junctions, are simulated. The calculated results are in agreement with the measured data. The result indicates that for tunnel cascaded and coupled multi-active regins laser diodes, temperature rising of the active region near the substrate is a little higher than that near heat sink. With active region number increasing, the temerature of the laser diodes rises but multi-active regions were fabricated on the uniform substrate, its thermal resistance is still smaller than that of series with the same number normal laser diodes.
For increasing laser diode (LD) output power, improving laser beam quality and enhancing the light emitting diode (LED) brightness, the coupled large optical cavity semiconductor lasers and multi-active LEDs with tunneling- regenerated current transport have been presented and experimented. Both the external and differential quantum efficiency and both the LD's output power and LED's brightness are together increasing approximately with the number of the active regions. The very high power Lds and and the very high brightness LEDs working the low injecting current and also the fundamental mode stimulated light with good beam quality have been achieved in our laboratory.
Novel multi-active region semiconductor lasers with large coupled optical cavity and high quantum efficiency, and new mechanism tunneling-regenerated multi-active region light emitting diodes with high quantum efficiency and high brightness have been proposed and fabricated. The external and differential quantum efficiency are 2.9 and 3.0 W/A, and the output light power as high as approximately 5 W when the injecting current equals 2A for the four active region 980 nm strained InGaAs/GaAs QW lasers. The fundamental mode light output with perpendicular angle <EQ 17 degrees for this type of large coupled optical cavity laser has been achieved. The on-axis luminous intensity of the new mechanism 620 nm AlGaInP/AlInP LEDs with two active regions is more than 5 cd. It was theoretically and experimentally resulted in that the electro-luminescence efficiency and the on-axis luminous intensity are linearly increasing approximately with the number of the active regions.
A new mechanism of tunneling-regenerated multi-active region LEDs with high quantum efficiency and high brightness has been presented. The layer structure, MOCVD growth, device technology, a several of measured curves and their analysis of these new mechanism LEDs were shown in the paper. It was theoretically and experimentally resulted in that efficiency of the electro-luminescence and the on-axis luminous intensity can linearly increase approximately with the number of active regions.
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