Tomoya Yanamoto

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.

Wavelength Dependence of InGaN Laser Diode Characteristics

InGaN multi-quantum-well-structure laser diodes (LDs) with an emission wavelength of longer than 420 nm were grown on both an epitaxially laterally overgrown GaN (ELOG) substrate and an ELOG on a free-standing GaN substrate by a metaorganic chemical vapor deposition method. The wavelength dependence of InGaN LD characteristics was investigated. It was found that there was a strong relationship between the threshold current density and the emission wavelength of LDs. The LDs with the emission wavelength of 450 nm grown on the ELOG on a free-standing GaN substrate were demonstrated. The threshold current density and voltage of these LDs were 2.8 kA/cm2 and 4.5 V, respectively. The estimated lifetime was approximately 5000 h under 50°C continuous-wave operation at an output power of 5 mW.

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.

365 nm Ultraviolet Laser Diodes Composed of Quaternary AlInGaN Alloy

Ultraviolet (UV) laser diodes (LDs), whose active layer were quaternary AlxInyGa1-x-yN single-quantum well structure, were grown on epitaxial lateral overgrown GaN substrates by a metalorganic chemical vapor deposition method. We investigated the lasing wavelength dependence of UV LDs on the threshold current density. As a result, we succeeded in fabricating 365 nm UV LDs which are useful for various industrial uses because their wavelength is similar to that of the i-line of high-pressure mercury vapor lamps. The threshold current and voltage of the 365 nm UV LD under continuous wave (cw) operation at 25°C were 50 mA and 4.8 V, respectively. The estimated lifetime of the 365 nm UV LDs was approximately 2000 h at an output power of 3 mW under cw operation at 30°C. The shortest lasing wavelength was achieved at 354.7 nm under pulse current injection at 25°C.

Characteristics of InGaN laser diodes in the pure blue region

InGaN multi-quantum-well-structure laser diodes (LDs), whose emission wavelengths are in the pure blue region, were grown on epitaxially laterally overgrown GaN on a free-standing GaN substrate by the metaorganic chemical-vapor deposition method. The wavelength dependence of the InGaN LD characteristics was investigated. These results indicated that there is a strong relationship between the threshold current density and the emission wavelength of LDs. LDs with an emission wavelength of 460 nm were demonstrated. The threshold current density and voltage of these LDs were 3.3 kA/cm2 and 4.6 V, respectively. The estimated lifetime was approximately 3000 h under 50 °C continuous-wave operation at an output power of 5 mW.

Ultraviolet GaN Single Quantum Well Laser Diodes

The ultraviolet laser diodes (LDs) whose active layers consisted of binary GaN were grown on epitaxially laterally overgrown GaN substrates by a metalorganic chemical vapor deposition method. For the first time, we observed the lasing emission from binary GaN active layer by current injection. The emission wavelength of GaN single quantum well LDs was 366.9 nm under pulsed current injection and 369.0 nm under continuos-wave (cw) operation at room temperature. The threshold current density and voltage of these LDs under the 25°C cw operation were 3.5 kA/cm2 and 4.6 V, respectively. The estimated lifetime was approximately 2000 h under 25°C cw operation at an output power of 2 mW.

Study of GaN-based laser diodes in near ultraviolet region

Ultraviolet (UV) laser diodes (LDs) were grown on epitaxially laterally overgrown GaN substrates by a metalorganic chemical vapor deposition method. We fabricated three types of UV LDs whose active layers were (I) ternary InGaN, (II) quaternary AlInGaN and (III) binary GaN single quantum well (SQW) structures. We investigated the LD characteristics in detail in the near UV region. LDs whose active layers were quaternary Al0.03In0.02Ga0.95N were demonstrated under the pulsed-current-biased conditions at room temperature. The emission wavelength of these LDs was the shortest wavelength (366.4 nm) in our experiments. We also demonstrated binary GaN SQW LDs for the first time. The threshold current density, voltage and emission wavelength of these LDs under 25°C continuous-wave (cw) operation were 3.5 kA/cm2, 4.6 V and 369.0 nm, respectively. The estimated lifetime was approximately 2000 h under 25°C cw operation at an output power of 2 mW.

Characteristics of Ultraviolet Laser Diodes Composed of Quaternary AlxInyGa(1-x-y)N

Ultraviolet (UV) laser diodes (LDs) whose active layers were composed of quaternary AlxInyGa(1-x-y)N were grown on epitaxially laterally overgrown GaN substrates by a metalorganic chemical vapor deposition method. We investigated the Al and In mole fractions dependence of LD characteristics in the UV region. The emission wavelength of the LDs whose active layers consisted of Al0.03In0.02Ga0.95N single quantum well (SQW) was 366.4 nm under pulsed current injection at room temperature. The lasing wavelength was the shortest ever reported for III-V nitride-based LDs. The Al0.03In0.03Ga0.94N SQW UV LDs were demonstrated under 25°C continuous-wave (cw) operation. The threshold current density and voltage of these LDs were 3.5 kA/cm2 and 4.8 V, respectively. The estimated lifetime was approximately 500 h under 25°C cw operation at an output power of 2 mW.

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.

GaN‐Based Light‐Emitting Diodes and Laser Diodes, and Their Recent Progress

It is recognized that GaN-based semiconductor is the most excellent material for short wavelength emitting devices. In this paper, we review the development of InGaN light emitting diodes (LEDs) and laser diodes (LDs). Additionally, purely-blue LDs are demonstrated, and the emission-wavelength dependence of the threshold current density is studied.