Yuh Shiuan Liu

Deep-ultraviolet lasing at 243 nm from photo-pumped AlGaN/AlN heterostructure on AlN substrate

Deep-ultraviolet lasing was achieved at 243.5 nm from an AlxGa1−xN-based multi-quantum-well structure using a pulsed excimer laser for optical pumping. The threshold pump power density at room-temperature was 427 kW/cm2 with transverse electric (TE)-polarization-dominant emission. The structure was epitaxially grown by metalorganic chemical vapor deposition on an Al-polar free-standing AlN (0001) substrate. Stimulated emission is achieved by design of the active region, optimizing the growth, and the reduction in defect density afforded by homoepitaxial growth of AlN buffer layers on AlN substrates, demonstrating the feasibility of deep-ultraviolet diode lasers on free-standing AlN.

Sub-250 nm low-threshold deep-ultraviolet AlGaN-based heterostructure laser employing HfO2/SiO2 dielectric mirrors

We report a sub-250-nm, optically pumped, deep-ultraviolet laser using an AlxGa1−xN-based multi-quantum-well structure grown on a bulk Al-polar c-plane AlN substrate. TE-polarization-dominant lasing action was observed at room temperature with a threshold pumping power density of 250 kW/cm2. After employing high-reflectivity SiO2/HfO2 dielectric mirrors on both facets, the threshold pumping power density was further reduced to 180 kW/cm2. The internal loss and threshold modal gain can be calculated as 2 cm−1 and 10.9 cm−1, respectively.

Strain management of AlGaN-based distributed Bragg reflectors with GaN interlayer grown by metalorganic chemical vapor deposition

We report the crack-free growth of a 45-pair Al0.30Ga0.70N/Al0.04Ga0.96N distributed Bragg reflector (DBR) on 2 in. diameter AlN/sapphire template by metalorganic chemical vapor deposition. To mitigate the cracking issue originating from the tensile strain of Al0.30Ga0.70N on GaN, an AlN template was employed in this work. On the other hand, strong compressive strain experienced by Al0.04Ga0.96N favors 3D island growth, which is undesired. We found that inserting an 11 nm thick GaN interlayer upon the completion of AlN template layer properly managed the strain such that the Al0.30Ga0.70N/Al0.04Ga0.96N DBR was able to be grown with an atomically smooth surface morphology. Smooth surfaces and sharp interfaces were observed throughout the structure using high-angle annular dark-field imaging in the STEM. The 45-pair AlGaN-based DBR provided a peak reflectivity of 95.4% at λ = 368 nm with a bandwidth of 15 nm.

Sub 250 nm deep-UV AlGaN/AlN distributed Bragg reflectors

Deep-UV distributed Bragg reflectors (DBRs) operating at λ = 220–250 nm with reflectivity close to unity were produced using epitaxial AlxGa1-xN/AlN superlattice structures grown on AlN/sapphire templates via metalorganic chemical vapor deposition. Owing to the near-bandedge excitonic resonance in the AlxGa1-xN layers, the AlN mole fractions, x, were regulated to keep the reflective plateau within the enhanced refractive index contrast region between AlGaN and AlN of approximately 7%–11%. For DBRs incorporating high-index layers of AlGaN grown via a flow-rate modulated epitaxy technique, a reflectivity of 97% was achieved with a total pair number of 30.5 which was much smaller than number of pairs needed for the DBRs with conventionally grown AlGaN layers. The stopbands of these DBRs were about 6–9 nm.

Inverse-Tapered p-Waveguide for Vertical Hole Transport in High-[Al] AlGaN Emitters

We report a high-aluminum-containing ([Al] ~ 0.6) AlGaN multiple-quantum well (MQW) double-heterojunction (DH) emitter employing an inverse-tapered-composition AlGaN:Mg p-type waveguide grown on a c plane Al-polar AlN bulk substrate. Using numerical simulations, we have determined that the inverse-tapered p-type waveguide design is necessary for high [Al] containing p-n junction devices as any valence band discontinuity at the junction will limit the vertical hole transport and induce a larger voltage-drop across the structure. The fabricated ultraviolet MQW DH emitter can sustain a DC current of at least 500 mA and a pulsed current of at least 1.07 A, which corresponds to a current density of 10 and 18 kA/cm2 at maximum measured voltage of 15 and 20 V with the measured series resistance of 15 and 11 Ω, respectively.

Optically pumped vertical-cavity surface-emitting laser at 374.9 nm with an electrically conducting n-type distributed Bragg reflector

An optically pumped vertical-cavity surface-emitting laser with an electrically conducting n-type distributed Bragg reflector was achieved at 374.9 nm. An epitaxially grown 40-pair n-type AlGaN/GaN distributed Bragg reflector was used as the bottom mirror, while the top mirror was formed by a dielectric distributed Bragg reflector composed of seven pairs of HfO2/SiO2. A numerical simulation for the optical mode clearly demonstrated that a high confinement factor was achieved and the threshold pumping power density at room temperature was measured as 1.64 MW/cm2. The achieved optically pumped laser demonstrates the potential of utilizing an n-type distributed Bragg reflector for surface-emitting optical devices.

Optically pumped deep-ultraviolet AlGaN multi-quantum-well lasers grown by metalorganic chemical vapor deposition

A 245.3 nm deep ultraviolet optically pumped AlGaN based multiple-quantum-well laser operating at room temperature is described. Epitaxial growth was performed by metalorganic chemical vapor deposition on a c-plane bulk AlN substrate at a growth temperature of ~ 1130 °C. The wafer was fabricated into cleaved bars with a cavity length of ~1.45 mm and the lasing threshold was determined to be 297 kW/cm2 under pulsed 193 nm ArF excimer laser excitation. A further ~20% reduction in threshold pumping power density was observed with six pairs of SiO2/HfO2 distributed Bragg reflector deposited at the rear side of facets.

High Reflectivity Hybrid AlGaN/Silver Distributed Bragg Reflectors for use in the UV-Visible Spectrum

Indium-free AlGaN-based distributed Bragg reflectors (DBRs) in the UV spectrum are known to have very low reflectivities due both to the low refractive index contrast as well as limitations imposed by the critical thickness of AlGaN alloys (tensile strain of ~2.41% for AlN on GaN). Near-bandedge excitonic resonances influence the real part of AlGaN’s dielectric function, which sharply increases its refractive index as the photon energy approaches the bandgap. Furthermore, heavy doping (Si:

Optically pumped low-threshold UV lasers

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Room-temperature optically pumped AlGaN-AlN multiple-quantum-well lasers operating at <260nm grown by metalorganic chemical vapor deposition

cm−3) can modify the plasma frequency of AlGaN, leading to a reduction in its refractive index. Hence, judiciously choosing the high index material to exploit excitonic resonances and using heavy doping to reduce the refractive index of the low index material can enhance the index contrast and enable growth of epitaxial DBRs with higher reflectivities. We have demonstrated this technique both experimentally and by simulations for wavelengths ranging from 240 to 370 nm. Typically, over 50 epitaxial pairs are needed to achieve a mirror whose reflectivity exceeds 99%, but this can be shrunk down to 20–30 epitaxial pairs by depositing silver/aluminum underneath the epitaxial DBR stack. Silver and aluminum exhibit >90% reflectivity at the AlGaN/metal interface between wavelengths ranging from >360 to 180–670 nm, respectively. A thinner DBR stack also reduces the thermal resistance, which would allow the VCSEL to achieve higher peak output powers, and simultaneously reduce overall tensile strain.