Daniel A. Haeger

Continuous-wave Operation of AlGaN-cladding-free Nonpolar m-Plane InGaN/GaN Laser Diodes

We demonstrate continuous-wave (CW) operation of nonpolar m-plane InGaN/GaN laser diodes without Al-containing waveguide cladding layers. Thick InGaN quantum wells (QWs) are used to generate effective transverse optical mode confinement, eliminating the need for Al-containing waveguide cladding layers. Peak output powers of more than 25 mW are demonstrated with threshold current densities and voltages of 6.8 kA/cm2 and 5.6 V, respectively. The unpackaged and uncoated laser diodes operated under CW conditions for more than 15 h.

AlGaN-Cladding Free Green Semipolar GaN Based Laser Diode with a Lasing Wavelength of 506.4 nm

We demonstrate electrically driven InGaN based laser diodes (LDs), with a simple AlGaN-cladding-free epitaxial structure, grown on semipolar (2021) GaN substrates. The devices employed In0.06Ga0.94N waveguiding layers to provide transverse optical mode confinement. A maximum lasing wavelength of 506.4 nm was observed under pulsed operation, which is the longest reported for AlGaN-cladding-free III-nitride LDs. The threshold current density (Jth) for index-guided LDs with uncoated etched facets was 23 kA/cm2, and 19 kA/cm2 after application of high-reflectivity (HR) coatings. A characteristic temperature (T0) value of ~130 K and wavelength red-shift of ~0.05 nm/K were confirmed.

Low-threshold-current-density AlGaN-cladding-free m-plane InGaN/GaN laser diodes

We demonstrate AlGaN-cladding-free m-plane InGaN/GaN laser diodes with threshold current densities that are comparable to state-of-the-art c-plane InGaN/GaN laser diodes. Thick InGaN waveguiding layers and a relatively wide active region with three 8 nm quantum wells were used to provide adequate refractive index contrast with the GaN cladding layers, thus eliminating the need for AlGaN cladding layers. Despite the large active region volume, lasing was achieved at a threshold current density of 1.54 kA/cm2, suggesting that the realization of even lower threshold current densities should be possible by reducing the number of quantum wells in the active region.

Blue-Green InGaN/GaN Laser Diodes on Miscut m-Plane GaN Substrate

Nonpolar blue-green (481 nm) InGaN/GaN laser diodes (LDs) were realized by using m-plane GaN substrates with a misorientation angle of approximately 1° toward [0001] direction. The laser diode structures grown on miscut m-plane GaN substrates showed significantly smoother surface morphology compared to structures grown on nominally on-axis m-plane GaN substrates. It is contributed to lower threshold current densities, longer lasing wavelength, and higher lasing yield than those on on-axis substrates grown by the same growth condition. Material improvement based on the control of substrate misorientation is essential to achieve high performance LDs beyond the blue spectral range.

Origin of pyramidal hillocks on GaN thin films grown on free-standing m-plane GaN substrates

The relationship between pyramidal hillocks and dislocations was studied for GaN thin films grown by metalorganic chemical vapor deposition on low-dislocation-density free-standing m-plane GaN substrates. Four-sided pyramidal hillocks were observed on GaN thin films grown on nominally on-axis m-plane GaN substrates. Cathodoluminescence measurements revealed the presence of a dislocation at the apex of each pyramidal hillock. High-resolution atomic force microscopy images showed a pinned step at the apex of each pyramidal hillock and a spiral ramp around the termination of the step, indicating that the pyramidal hillocks arise from spiral growth around screw-component dislocations intersecting the surface of the crystal.

Nonpolar AlGaN-Cladding-Free Blue Laser Diodes with InGaN Waveguiding

We have demonstrated AlGaN-cladding-free m-plane InGaN-based blue laser diodes (LDs) using a novel structure that employs 50-nm-thick n- and p-type InxGa1-xN (x = 5–10%) as waveguiding layers. The thick, high In content InGaN waveguiding layers provided significant refractive index contrast to the GaN cladding layers, thereby eliminating the need for AlGaN cladding. Under pulsed operation, lasing was achieved at 442 nm with a threshold current density of 10 kA/cm2.

Determination of internal parameters for AlGaN-cladding-free m-plane InGaN/GaN laser diodes

The dependence of device characteristics on cavity length is used to determine the injection efficiency, internal loss, and material gain of electrically injected AlGaN-cladding-free m-plane InGaN/GaN laser diodes. Estimates for the transparency carrier density are discussed in the context of recombination coefficients that have been reported for c-plane InGaN-based light-emitting devices.

High-power blue-violet AlGaN-cladding-free m-plane InGaN/GaN laser diodes

We demonstrate AlGaN-cladding-free (ACF) m-plane InGaN/GaN laser diodes (LDs) with peak output powers and estimated front facet optical power densities that are comparable to the state-of-the-art single-stripe emitter c-plane LDs. The threshold current density, slope efficiency, and peak output power were 4.66 kA/cm2, 1.29 W/A, and 1.6 W, respectively, after facet coating and under pulsed conditions. Catastrophic optical mirror damage was not observed up to an estimated optical power density of 51.2 MW/cm2 at the front mirror facet, indicating the potential for using ACF m-plane InGaN/GaN LDs in high-power LD applications.

InGaN/GaN Blue Laser Diode Grown on Semipolar (3031) Free-Standing GaN Substrates

We demonstrate the first electrically-injected InGaN/GaN laser diodes (LDs) grown on semipolar (3031) free-standing GaN substrates. The lowest threshold current density (Jth) was 5.6 kA/cm2 with a clear lasing peak at 444.7 nm. The peak electroluminescence (EL) wavelength blue-shifted 4 nm below threshold and the characteristic temperature was ~135 K. These results suggest that the semipolar (3031) plane may be a potential candidate for growing high performance nitride-based LDs.

384 nm laser diode grown on a (202¯1) semipolar relaxed AlGaN buffer layer

We demonstrate an electrically injected semipolar (202¯1) laser diode grown on a partially relaxed AlGaN buffer layer. The coherency stresses are relaxed by misfit dislocations at the GaN/AlGaN heterointerface which form by glide of preexisting threading dislocations along the (0001) basal plane. The defects are confined to the heterointerface which allows the growth of high aluminum composition films with threading dislocation densities of less than 108 cm−2. The lasing wavelength was 384 nm with a threshold current density of 15.7 kA/cm−2. UV lasers grown on semipolar relaxed AlGaN buffers provide an alternative to devices grown on AlN or sapphire.