Takashi Miyoshi

510–515 nm InGaN-Based Green Laser Diodes on c-Plane GaN Substrate

We succeeded in developing InGaN-based green laser diodes (LDs) with a wavelength of 515 nm under continuous-wave (cw) operation by improving the growth condition of epitaxial layers and structures of LDs. The LD structures were grown on conventional c-plane free-standing GaN substrates by metal organic chemical vapor deposition (MOCVD). The threshold current and threshold voltage at 515 nm were 53 mA and 5.2 V, respectively. The lifetime of 510–513 nm LDs was estimated to be over 5000 h under cw operation with an optical output power of 5 mW at 25 °C.

Recent development of nitride LEDs and LDs

We fabricated the high efficiency white LEDs. The white LEDs, the yellow YAG-phosphors-coated small-size (290µm × 500µm) blue LED, designed for minimizing forward voltage. At a forward current of 20mA, the luminous flux, the forward voltage (Vf), the correlated color temperature (Tcp), the luminous efficiency, and the wall-plug efficiency (WPE) are 9.5lm, 2.8V, 5193K, 169Lm/W, and 50.8%, respectively. The high-power white LEDs were fabricated from the larger-size (1mm × 1mm) blue LED chips with the output power of 651mW at 350mA. Flux, Vf, Tcp, luminous efficiency, and WPE of the high-power white LED are 145Lm, 3.09V, 4988K, 134Lm/W, and 39.5%, respectively, at 350mA. This power white LEDs showed total flux of 361Lm at 1A. Moreover, we succeeded in developing high-power and high-efficiency blue laser diodes (LDs) with an emission wavelength at 445nm range by using GaN-based materials. This achievement leads to the full-color laser display applications. We fabricated multi-transverse mode LDs by a single emitter, and adopting φ9mm packages for the reduction of the thermal resistance. The typical optical-output power, voltage and wall-plug efficiency of the LDs at forward current of 1.0A at a temperature of 25ºC was 1.17W, 4.81V and 24.3%, respectively. The catastrophic optical damage at the facets of these LDs did not appear up to 3W in the optical output power. The estimated lifetime of the LDs at a temperature of 25ºC under continuous-wave operation 1.0A in automatic current control condition was over 30,000 hours.

Recent status of white LEDs and nitride LDs

We fabricated two types of high luminous efficiency white light emitting diodes (LEDs). The first one is the white LED, which had a high luminous efficiency (ηL) of 161 Lm/W with the high luminous flux (φv) of 9.89 Lm at a forward- current of 20 mA. Used blue LED had a high output power (φe) of 42.2 mW and high external quantum efficiency (ηex) of 75.5%. The second one is the luminous-efficiency-maximized white-LED with a low forward-bias voltage (Vf) of 2.80 V, which is almost equal to the theoretical limit. ηL and wall-plug efficiency (WPE) were 169 Lm/W and 50.8%, respectively, at 20 mA. They were approximately twice higher than those of a tri-phosphor fluorescent lamp (90 Lm/W and 25%). Moreover, we succeeded in fabricating longer wavelength laser diodes (LDs) with an emission wavelength of 488 nm under CW current condition by optimizing the growth conditions and structure of LDs. To our knowledge, this wavelength is the longest for under CW current condition in GaN-based LDs.

Remarkably Suppressed Luminescence Inhomogeneity in a (0001) InGaN Green Laser Structure

The optical properties of InGaN-based green laser structures fabricated on (0001) GaN substrates are investigated using photoluminescence (PL) spectroscopy. Both macroscopic and microscopic measurements demonstrate that the potential fluctuations are drastically suppressed in state-of-the-art (0001) green InGaN laser structures. Time-resolved PL suggests that InGaN quantum wells (QWs) become thinner than conventional QWs, which is compensated for by higher In compositions. Such QWs increase the radiative recombination probability due to a greater overlap between the electron and hole wavefunctions. The suppressed luminescence inhomogeneity and increased radiative recombination probability may be responsible for the recent remarkable reduction in the lasing threshold.

67.4: GaN-Based High-Power Blue Laser Diodes for Display Applications

We succeeded fabricating high power blue (445nm) laser diodes (LDs) with an output power of 500mW. The operating current, voltage and wall-plug efficiency of these LDs were 480mA, 4.8V, and 21.7%, respectively. Estimated Lifetime of these LDs was over 10,000h under continuous-wave operation at 25 °C.

GaN‐based high‐output‐power blue laser diodes for display applications

— We succeeded in fabricating high-output-power blue (445 nm) laser diodes (LDs) with an output power of 500 mW. The operating current, voltage, and wall-plug efficiency of these LDs were 480 mA, 4.8 V, and 21.7%, respectively. The lifetime of these LDs was estimated to be 10,000 hours under continuous-wave operation at 25°C. From examination of the degradation mode, we found that the operating current seriously affects the lifetime of LDs. In the next stage, we will focuse on the optimization and sophistication of the manufacturing processes to fabricate longer-lifetime (>30,000 hours) blue LDs.

Optical Gain Spectra of a (0001) InGaN Green Laser Diode

The optical gain properties of InGaN-based green (512 nm) laser diodes fabricated on (0001) GaN substrates are investigated. Fitting simulations to the experimental gain spectra provides a Gaussian inhomogeneous broadening of 95 meV, an optical confinement factor of 0.006, and an internal loss as low as ~10/cm. The remarkable suppression of inhomogeneous broadening and internal loss compensate for the low optical confinement, leading to a low threshold current density of 2.75 kA/cm2. The suppressed inhomogeneity contributes to the highly linear gain increase with the injection carrier, while the low optical confinement results in a relatively low differential mode gain.

1 W A1InGaN based green laser diodes

The AlInGaN based green laser diodes were grown on c face GaN substrates by a metal organic chemical vapor deposition method. As a result, we succeeded in demonstrating 1W 525 nm Green LDs. The optical output power, voltage and the wall-plug efficiency at the forward current of cw 1.5 A were 1.01 W, 4.76 V and 14.1% at 25°C, respectively. The lifetime was estimated to be over 15,000 h by the lifetime test which was carried out under the condition of a constant current of 1.5 A at 50°C for 1000 h.

52.3: High - Power InGaN Blue - Laser Diodes for Displays

We have developed high power blue laser diodes (LDs) with an emission wavelength of 445 nm by using GaN material. The operating current and voltage of these LDs at an output power of 200mW were 274 mA and 4.9 V, respectively. The estimated lifetime was over 10,000h under 25 °C continuous-wave operation at an output power of 200 mW.

Blue and green laser diodes for large laser display

Watt-class AlInGaN blue and green laser diodes (LDs) on c face GaN substrate are fabricated. The optical output powers of blue LDs and green LDs were 3.75 W and 1.01 W, and the wall plug efficiencies were 38.5% and 14.1%, respectively. The lifetimes were estimated to be over 30,000 and 15,000 hours at a case temperature of 50°C.