Taketoshi Tanaka

Nonpolar m-plane InGaN multiple quantum well laser diodes with a lasing wavelength of 499.8 nm

We demonstrated nonpolar m-plane InGaN multiple quantum well laser diodes (LDs) under continuous-wave (cw) operation with a lasing wavelength of 499.8 nm, which is the longest reported for GaN-based LDs. A maximum optical output power of 15 mW was achieved, with the threshold current and the corresponding threshold current density (Jth) of 46 mA and 3.1 kA/cm2, respectively. The correlation between lasing wavelength shift and electrical input power (Pin) under cw operation was investigated using LDs of which reflectivity of front facet were varied from 70% to 97%. The lasing wavelength increased with increasing Pin with a slope of 4.56 and 4.34 nm/W for 70% and 97% mirror, respectively. The result suggested that the redshift due to self-heating is more predominant than the blueshift due to band filling above Jth even at near green region for nonpolar GaN-based LDs and reduction in Pin is indispensable to improve wavelength stabilization.

Pure Blue Laser Diodes Based on Nonpolar

Blue laser diodes (LDs) based on m-plane gallium nitride were demonstrated by using m-plane GaN substrates. The lasing wavelength and the threshold current under pulsed operation were 451.8 nm and 134 mA (22.3 kA/cm2), respectively. The device structures consisted of InGaN-based multi-quantum wells, InGaN guiding layers, and Al-containing cladding layers. The InGaN guiding layers play two roles; as appropriate optical waveguides for longer lasing wavelengths and for the prevention of macroscopic cracks parallel to the c-plane. The latter is an indispensable technology in order to fabricate nonpolar LDs for longer wavelengths beyond the blue region.

m

We demonstrated nonpolar m-plane GaN-based blue-green laser diodes (LDs) under continuous-wave (cw) operation with a lasing wavelength of 481 nm. A maximum output power of more than 20 mW was achieved, for which the threshold current and the corresponding threshold current density (Jth) were 61 mA and 6.1 kA/cm2, respectively. The value of Jth and the electroluminescence peak wavelength shift until lasing did not change with lasing wavelength in the range from 459 to 481 nm, though the reflectivities of the cavity facets were fixed for each LD. In addition, the slope efficiency increased with increasing lasing wavelength, from 0.37 W/A at 459 nm to 0.49 W/A at 481 nm. This is the remarkable advantages of nonpolar GaN-based material compared to c-plane material for the realization of green LDs.

-Plane Gallium Nitride with InGaN Waveguiding Layers

Polarized photoluminescence (PL) spectra from nonpolar m-plane InGaN multiple quantum wells (MQWs) in blue laser diode wafers fabricated on m-plane GaN substrates were measured as a function of temperature. The polarization ratio (ρ) and the energy difference between the highest and the second highest valence bands estimated from the energy difference between PL peaks (ΔE) increased with increasing InN molar fraction x (or the estimated anisotropic compressive strain along the m-axis eyy) in the MQWs. The values of ρ at 300K and ΔE were 0.71 and 76meV for the case of 430nm PL peak (x=0.104, eyy=+0.75%) and 0.92 and 123meV for the case of 485nm PL peak (x=0.175, eyy=+1.26%). These results suggest that the preferred stripe direction is the c axis for nonpolar m-plane laser diodes in the region from violet to near green.

High-Efficiency Continuous-Wave Operation of Blue-Green Laser Diodes Based on Nonpolar m-Plane Gallium Nitride

The continuous-wave (cw) operation of m-plane InGaN-based blue (460 nm) laser diodes (LDs) has been achieved. The threshold current and the corresponding threshold current density were 40 mA and 5.0 kA/cm2, respectively, with a 459 nm lasing wavelength under cw operation. The electroluminescence peak wavelength shift in pulsed mode was only 10 nm (58 meV), from spontaneous emission (at 0.3 mA) to stimulated emission (at 32 mA), which is extremely small when compared with that of c-plane blue LDs. This is first clear experimental demonstration of the advantage in fabricating nonpolar InGaN-based LDs beyond the blue region.