(Invited) Development of Ultraviolet Vertical-Cavity Surface-Emitting Light Sources

Abstract

Nitride-based vertical-cavity surface-emitting lasers (VCSELs) at wavelengths below 400 nm have drawn tremendous interest as they are potentially utilized in various applications as highly-efficient and temperature-insensitive optical sources. One example is for a compact, energy efficient, portable ytterbium-ion-based atomic clock that requires a high-stability CW optical pump at near ultraviolet (UV) wavelengths. In this work, we have used metalorganic chemical vapor deposition (MOCVD) to grow optically pumped UV VCSEL epitaxial structures and UV resonant-cavity light-emitting diodes (RCLEDs) to demonstrate several design concepts targeting the ~360-370nm UVA spectral range. The UV VCSELs are designed using a proprietary III-N device simulation tool developed at Georgia Institute of Technology. Previously, we have demonstrated a variety of optically pumped III-N VCSELs lasing at ~375 nm that showed the feasibility of building the near-UV coherent light source on GaN materials platform. The test structures consisted of a five-pair InGaN/Al0.15Ga0.85N multiple-quantum-well (MQW) active region embedded in Al0.1Ga0.9N spacers with the total optical cavity thickness of 3λ. The lowest threshold incident power density for optically pumped VCSELs was estimated to be ~270 kW/cm2 under pulsed conditions at the room temperature. In further work, we implemented a hybrid distributed Bragg reflectors (DBR) combination to demonstrate vertical resonant cavity light emitting diodes (RCLEDs) targeting at 370-nm peak emission wavelengths. The hybrid mirrors use a dielectric DBR with twelve pairs of ¼-wavelength HfO2/SiO2 layers as the topside mirror, and a semiconductor-based mirror composed of five periods of ¾λ air-gap/Al0.05Ga0.95N DBR at the bottom side of the vertical cavity. The RCLED structure comprises three distinct regions: (1) bottom DBR mirror of five-pair 3λ/4-air-gap/ Al0.045Ga0.95fN, (2) a 6λ-cavity LED active region of n-Al0.08Ga0.92N (n~8x1018cm-3) spacer/InGaN-AlGaN MQW/p-AlGaN electron blocking/p-Al0.08Ga0.92N (p~3x1017cm-3) spacer/graded p +-AlGaN, and (3) a 12-pair λ/4 HfO2/SiO2 dielectric DBR as the top-side mirror. The device fabrication employed a nitrogen ion-implantation isolation to form a current confinement aperture. The mesa-type devices have the anode and the cathode electrodes on the topside of the wafer. The air-gap DBRs were formed using optical lithography and subsequent inductively coupled plasma dry etching to create a deep trench, followed by laterally removing sacrificial layers using a conductivity-selective electrochemical etching process. Once formed, this airgap DBR provides a high reflectivity of ~99.9% covering the spectral range from 360 to 400 nm while the dielectric DBR has a reflectivity of >99% covering the range from 350 to 405 nm. The fabricated RCLEDs can be operated at a current density >50 kA/cm2 in the pulsed current mode with a peak emission wavelength of ~375 nm at the room temperature. Details of MOCVD growth, device fabrication, and characterization will be presented.