Shinichi Takigawa

High-Extraction-Efficiency Blue Light-Emitting Diode Using Extended-Pitch Photonic Crystal

We have integrated the surface photonic crystal (PhC) on GaN-based blue light-emitting diodes (LEDs) for the first time in order to enhance the extraction efficiency of the LEDs. With the finite-difference time-domain method, we have calculated 3.6-fold enhancement in light output. The theoretical calculations have revealed that the optimum pitch of the PhC is much longer than the emission wavelength when the distance between the PhC and the active layer of LEDs is short. This design enables PhC formation on chemically stable GaN surfaces. In addition, an indium tin oxide (ITO)-based transparent electrode is formed directly on the surface of PhC to realize light emission from the whole area of the LED. The fabricated PhCs have increased the light output of blue LEDs by 1.5 times compared with the LEDs without PhC. We have demonstrated that PhC will realize highly efficient solid-state lighting with GaN-based LEDs.

Investigation of the deep level involved in InGaN laser degradation by deep level transient spectroscopy

This paper reports an extensive analysis of the properties of the deep level responsible for the degradation of InGaN-based laser diodes. The analysis is based on combined optical measurements and Deep-Level Transient Spectroscopy (DLTS) investigation. Results indicate that stress induces a significant increase in threshold current of the devices, which is strongly correlated to the increase in the concentration of a deep level (DL) detected by DLTS. The DL involved in the degradation process is located 0.35–0.45 eV below the conduction band. 2D simulation indicates that degradation occurs within the quantum-well region.

Degradation of InGaN-based laser diodes analyzed by means of electrical and optical measurements

In this paper we present a detailed analysis of the degradation of InGaN-based laser diodes carried out by means of electrical and optical techniques. The study is based on the comparison between the degradation kinetics of laser diodes and light-emitting diode (LED)-like samples, i.e., devices with the same epitaxial structure as the lasers, but with no ridge and facets. Results described in the following indicate that degradation of lasers and LED-like samples is due to the same mechanism, possibly involving the generation of point defects within the active region of the devices. Furthermore, since degradation occurs both in lasers and in LED-like samples (i.e., structures with no current confinement), results suggest that degradation of lasers is not correlated with the geometry of the devices, nor to worsening of current confinement under the ridge.

Enhanced responsivity in a novel AlGaN / GaN plasmon-resonant terahertz detector using gate-dipole antenna with parasitic elements

Plasmon-resonant terahertz (THz) detection using heterojunction field effect transistors (HFETs) is a promising method to enable compact and efficient THz detectors which can be applied to real-time imaging systems or THz spectroscopic analysis [1–2]. So far, the plasmon-resonant detectors which receive sub-THz and THz radiation at a gate bonding-wire or an external antenna have been reported [3–5]. However, the signal transmission from the antenna to the FET causes large propagation loss which degrades the sensitivity. In this paper, we present a novel AlGaN / GaN heterojunction FET which can detect THz radiation directly at a gate electrode with high responsivity.

Analysis of Diffusion-Related Gradual Degradation of InGaN-Based Laser Diodes

This report reveals that diffusion of hydrogen induces gradual degradation in InGaN-based laser diodes (LDs). The increase in nonradiative recombination centers (NRCs) in the LDs has been attributed to diffusion-related phenomena. Factors other than NRCs, such as the threshold carrier density Nth, can increase threshold current Ith. Those factors, however, were not fully investigated. Moreover, the diffusant responsible for the degradation of the LDs has not been univocally identified yet. To separately evaluate the roles of NRCs and Nth in increasing Ith, this report analyzes the stress-induced variation of nonradiative recombination lifetime τnr and lasing wavelength λl. It is revealed that the density of NRCs increases at the first stage of gradual degradation, followed by a rise in Nth. In addition, this report proposes a novel model for the time-variation of 1/τnr to investigate the diffusion-related degradation. By using this model, we extrapolate the value of the diffusion coefficient of diffusants involved in the degradation in InGaN-based LDs. The proposed analysis methods and obtained results are useful for understanding the physics of LD degradation.

Wavelength stabilization of a distributed Bragg reflector laser diode by use of complementary current injection

We have demonstrated wavelength stabilization in an 821-nm AlGaAs three-section tunable distributed Bragg reflector (DBR) semiconductor laser diode (LD) that consists of active, phase-controlled, and DBR regions. We injected two separate, complementary currents into the active and the phase-controlled regions in the DBR-LD to suppress wavelength shift. This modulation method was applied to the LD fundamental wave in a second-harmonic-generation (SHG) laser, and the oscillating wavelength was maintained within the phase-matching acceptance range of the SHG device during modulation. A peak blue-violet light power of 62 mW was obtained for the ideal modulation waveform.

High power terahertz emission from a single gate AlGaN/GaN field effect transistor with periodic Ohmic contacts for plasmon coupling

We report on room temperature terahertz (THz) emission by a single, short gate AlGaN/GaN field effect transistor with grating Ohmic contacts. The fingers of metal contacts are fabricated at the nanoscale in length and spacing in order to work as a radiation coupler of electron plasmons in the THz range. Spectrum analysis revealed a broadband emission centered at 1.5 THz with a controlled polarization by the grating contacts. The measured output power is linearly increased with the drain input power and reached up to 1.8 μW.

Optical-loss suppressed InGaN laser diodes using undoped thick waveguide structure

We propose optical-loss suppressed thick-optical-waveguide (TOW) InGaN laser diodes (LDs) without operatingvoltage increase. A record high continuous-wave (CW) output of 7.2W for a single-emitting InGaN LD is achieved without thermal peak-out in the light-current curve. The TOW enables to confine major part of the propagating light into a transparent undoped region, and thus significantly reduces the optical-loss. An electron-overflow-suppression (EOS) layer placed between the waveguide layer and a p-cladding layer plays an important role to reduce the operating voltage after introduction of the undoped TOW layer. We executed a self-consisted calculation of voltage-current characteristics taking into account Schrödinger and Poisson equations in conjunction with a carrier continuity equation. The calculation result indicates possible presence of conductivity-modulation in the waveguide filled with electrons reflected backward by the EOS layer and holes injected from the p-type cladding layer. We successfully demonstrated the optical-loss suppressed operation resulting in the slope efficiency (SE) increase from 2.0W/A to 2.5W/A. It is noted that the operating voltage of the TOW LD is nearly identical to the conventional LD thanks to the above conductivitymodulation phenomenon. The presented result suggests that our TOW structure can overcome the optical-loss drawback of the InGaN LDs, and hence will lead them to the applications requiring high wattage light sources.

Degradation of InGaN-Based Laser Diodes Related to Nonradiative Recombination

We present a detailed study of the degradation of InGaN-based laser diodes submitted to electrical stress tests, which is aimed at understanding the role of nonradiative recombination in determining the worsening of the properties of the devices. The analysis, which is carried out by means of optical techniques, indicates that stress determines an increase in the threshold current of the devices without strong modifications in the slope efficiency. For the first time, we give an experimental demonstration of the fact that the threshold current increase is correlated to the increase in the nonradiative recombination rate of the carriers in the active layer. This result has been verified in a wide range of operating current levels; furthermore, the results of stress tests carried out at different current levels support the hypothesis that current is a significant driving force for the analyzed degradation process.

50 mW stable single longitudinal mode operation of 780 nm GaAlAs DFB laser

The achievement of stable single-longitudinal-mode (SLM) operation of a 780 nm GaAlAs distributed-feedback (DFB) laser with output power as high as 50 mW is discussed. The laser employs the buried twin-ridge substrate structure which allows stable fundamental spatial mode operation even at high power levels. The designed coupling strength is 0.5 from the viewpoint of obtaining a low operation current at 50 mW. SLM operation was maintained for powers up to 50 mW at room temperature and in the temperature range from -17 to 37 degrees C at 50 mW. The maximum power attained was 62 mW. >