Tokuya Kozaki

InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate

InGaN multi-quantum-well-structure laser diodes with Al0.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN (ELOG) substrate was demonstrated to have a lifetime of more than 1150 h under room-temperature continuous-wave operation. After 4 μm etching of the ELOG substrate, the etch pit density was about 2×108 cm2 in the region of the 4-μm-wide stripe window, but almost zero in the region of the 7-μm-wide SiO2 stripe.

InGaN/GaN/AlGaN-Based Laser Diodes with Modulation-Doped Strained-Layer Superlattices

InGaN multi-quantum-well-structure laser diodes with Al0.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN substrate were demonstrated to have an estimated lifetime of more than 10000 h under continuous-wave operation at 20° C. Under operation at a high temperature of 50° C, the lifetime was longer than 1000 h. With the operating current increasing to above the threshold, a self-pulsation with a high frequency of 3.5 GHz was observed. The carrier lifetime was estimated to be 1.8 ns from the pulsed modulation of the LDs.

Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates

InGaN multi-quantum-well-structure laser diodes (LDs) grown on GaN substrates were demonstrated. The LDs showed a small thermal resistance of 30 °C/W and a lifetime longer than 780 h despite a large threshold current density of 7 kA/cm2. In contrast, the LDs grown on a sapphire substrate exhibited a high thermal resistance of 60 °C/W and a short lifetime of 200 h under room-temperature continuous-wave operation.

High-Power, Long-Lifetime InGaN/GaN/AlGaN-Based Laser Diodes Grown on Pure GaN Substrates

Epitaxially laterally overgrown GaN on sapphire was used to reduce the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. The GaN layer above the SiO2 mask area surrounding the window, corresponding to the lateral overgrowth, was nearly free of the threading dislocations. A high density of threading dislocations was observed in the vicinity of GaN grown in the window regions. InGaN multi-quantum-well-structure laser diodes (LDs) grown on pure GaN substrates, which were fabricated by removing the sapphire substrate, were demonstrated. The LDs with an output power of 5 mW exhibited a lifetime of more than 290 h and an estimated lifetime of 10,000 h despite a relatively large threshold current density. The far-field pattern of the LDs with a cleaved mirror facet revealed single-mode emission without any interference effects.

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.

High-Power and Long-Lifetime InGaN Multi-Quantum-Well Laser Diodes Grown on Low-Dislocation-Density GaN Substrates

Epitaxially laterally overgrown GaN on free-standing GaN was used to reduce the threading dislocations which were widely scattered over the entire surface. The threading dislocation density of the GaN layer above the window area surrounding the SiO2 mask was reduced to 5×107/cm2, and that of the GaN layer above the SiO2 mask area was reduced to 7×105/cm2. InGaN multi-quantum-well laser diodes (LDs) grown on low-dislocation-density GaN substrates were demonstrated. LDs with an output power of 30 mW exhibited an estimated lifetime of 15,000 h at a case temperature of 60°C. At a case temperature of 25°C, the current at the output power of 30 mW and at the lasing threshold current were 42 mA and 23 mA, respectively. For comparison, LDs were grown on different substrates, and the dependence of their characteristics on the substrates was examined.

Violet InGaN/GaN/AlGaN-Based Laser Diodes Operable at 50°C with a Fundamental Transverse Mode

A violet InGaN multi-quantum-well (MQW)/GaN/AlGaN separate-confinement-heterostructure laser diode (LD) was grown on epitaxially laterally overgrown GaN on sapphire. The threshold current density was 3.9 kA/cm2. The LDs with cleaved mirror facets showed an output power as high as 30 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode in the near-field patterns was observed at an output power up to 30 mW. The lifetime of the LDs at a constant output power of 5 mW was more than 1,000 h under CW operation at an ambient temperature of 50°C. The estimated lifetime was approximately 3,000 h under these high-power and high-temperature operating conditions.

INGAN/GAN/ALGAN-BASED LASER DIODES WITH CLEAVED FACETS GROWN ON GAN SUBSTRATES

After obtaining an epitaxially laterally overgrown GaN on sapphire by the metalorganic chemical vapor deposition method, GaN growth was continued up to a thickness of 200 μm by a hydride vapor phase epitaxy method. The InGaN multi-quantum-well-structure laser diode (LD) was grown on a free-standing GaN substrate, which was obtained by removing the sapphire substrate. The LDs with cleaved mirror facets showed an output power as high as 160 mW under room-temperature continuous-wave (CW) operation. The fundamental transverse mode was observed up to an output power of 80 mW. The lifetime of the LDs at a constant output power of 5 mW was about 180 h under CW operation at an ambient temperature of 50 °C, due to a high threshold current density of 14 kA/cm2.

InGaN/GaN/AlGaN-Based Laser Diodes Grown on GaN Substrates with a Fundamental Transverse Mode.

An InGaN multiquantum-well (MQW)-structure laser diode (LD) was grown on an epitaxially laterally overgrown GaN on sapphire. The lowest threshold current densities between 1.2 and 2.8 kA/cm2 were obtained when the number of InGaN well layers was two. The InGaN MQW LD was grown on a free-standing GaN substrate that was obtained by removing the sapphire substrate. The LDs with cleaved mirror facets showed an output power as high as 30 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed by reducing the ridge width to a value as small as 2 µm. The lifetime of the LDs at a constant output power of 5 mW was about 160 h under CW operation at an ambient temperature of 50°C, due to a high threshold current density of 6 kA/cm2.

Present status of InGaN/GaN/AlGaN-based laser diodes

InGaN multi-quantum-well-structure (MQW) laser diodes (LDs) with Al 0.14 Ga 0.86 N/GaN modulation-doped strained-layer superlattice (MD-SLS) cladding layers grown on an epitaxially laterally overgrown GaN (ELOG) substrate was demonstrated to have a lifetime of more than 1150 h under room-temperature continuous-wave operation. The lasing was a single mode emission at a wavelength of 403.7 nm. The use of the M D-SLS was effective in reducing the operating voltage of the LDs. The ELOG substrate was used to reduce the number of threading dislocations in the InGaN MQW structure. After μm etching of the ELOG substrate, the etch pit density was about 2x10 7 /cm 2 in the region of the 4 μm wide stripe window, but almost zero in the region of the 8 μm wide SiO 2 stripe.