Kuniyoshi Okamoto

Continuous-Wave Operation of m-Plane InGaN Multiple Quantum Well Laser Diodes

Continuous-wave (CW) operation of nonpolar m-plane InGaN/GaN laser diodes (LDs) with the lasing wavelengths approximately 400 nm was demonstrated. The threshold current was 36 mA (4.0 kA/cm2) for the CW operation [28 mA (3.1 kA/cm2) for pulsed mode], being comparable to that of conventional c-plane violet LDs. Both the LDs with the stripes parallel to a- and c-axes showed TE mode operation, according to the polarization selection rules of the transitions in strained InGaN. The c-axis stripe LDs exhibited lower threshold current density, since the lowest energy transition is allowed. As is the case with the m-plane light emitting diodes fabricated on the free-standing m-plane GaN bulk crystals [Okamoto et al.: Jpn. J. Appl. Phys. 45 (2006) L1197], the LDs shown in this paper did not have distinct dislocations, stacking faults, or macroscopic cracks. Nonpolar m-plane GaN-based materials are coming into general use.

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.

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m-Plane (10-10) nonpolar InGaN-based light emitting diodes (LEDs) with no threading dislocations or stacking faults have been realized on m-plane GaN single crystals by conventional metal organic vapor phase epitaxy. The crystalline properties of the material, together with the structures of the LED devices, have been observed by scanning transmission electron microscopy. It is shown that dislocation-free nonpolar nitride layers with smooth surfaces can be obtained under growth conditions involving high V/III ratios, which are the optimized growth conditions for c-plane GaN. The peak wavelength of the electroluminescence emission obtained from the finished devices is 435 nm, which is in the blue region. The output power and the calculated external quantum efficiency are 1.79 mW and 3.1%, respectively, at a driving current of 20 mA.

-Plane Gallium Nitride with InGaN Waveguiding Layers

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.

Dislocation-Free m-Plane InGaN/GaN Light-Emitting Diodes on m-Plane GaN Single Crystals

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.

Temperature dependence of polarized photoluminescence from nonpolar m-plane InGaN multiple quantum wells for blue laser diodes

An accurate method of estimating polarized light emission was presented for nonpolar m-plane InGaN-based blue light emitting diodes, where the unpolarized component caused by unintentional light scattering was eliminated as noise. The polarization ratios of electroluminescence (EL) at 300 and 100K were 0.85 and 0.98, respectively. The energy difference between the highest and the second highest valence bands was estimated to be 129meV from the temperature dependence of the spectrally integrated EL intensities under the assumption of Fermi statistics. This value agreed with the one (=118meV) obtained directly from the difference of the EL peak energies between two polarized components, the electric fields perpendicular and parallel to the c axis.

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

Polarized electroluminescence (EL) from m-plane InGaN-based blue light emitting diodes (LEDs) has been investigated. These LEDs comprised a ZnO transparent electrode as an anode, polished mirror-like sidewalls, and a polished backside with the reflective mirror to remove the light scattering due to the surface roughness. The polarization ratio at room temperature was 0.80, which was relatively high compared to known values and was considered to be close to the true value. The reduction of the light scattering is essential for the accurate measurement of the polarized emission. The EL spectra from different three planes, i.e., m-, c-, and a-planes, have been measured. The peak energies of spectra polarized parallel to m-plane, emitted from a- and c-plane sidewalls, agree with the values of the two components emitted from top m-plane surface. The strongest component was E⊥c while the second strongest was E∥c. The E∥m component was extremely low compared with E∥c.

Temperature dependence of polarized electroluminescence from nonpolar m-plane InGaN-based light emitting diodes

Quantum-confined Stark effects (QCSEs) in a polarization-free m-plane In0.15Ga0.85N∕GaN multiple quantum well (MQW) blue light-emitting diode fabricated on the low defect density (DD) freestanding GaN substrate were investigated. The electroluminescence (EL) peak at 2.74eV little shifted to the higher energy with the increase in current because of the absence of the polarization fields. The effective radiative lifetime increased and the nonradiative lifetime decreased with the increase in the junction field, and the results were quantitatively explained in terms of field-induced QCSE including tunneling escape of holes from the MQW. As a result of the use of the low DD substrate, the equivalent internal quantum efficiency, which was approximated as the spectrally integrated EL intensity at 300K divided by that at 150K, of 43% was achieved.