Integrating efficient W-superlattices and cavities into mid-wave LEDs and VCSELs
Creators: name: Montealegre, David Alexander
Publisher: name: University of Iowa
Year: 2022
Subjects: GaSb
LED
mid-Infrared
Semiconductor
Superlattice
VCSEL

description: There are many degrees of freedom in the design and incorporation of superlattices into mid-infrared light emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSELs). The combination of different semiconductor layers can yield different bandgaps and change the location of higher energy states by changing layer thicknesses, strain, and carrier confinement. These changes are important for increasing the internal quantum efficiency of the LEDs and the gain in lasers. The very high index of materials used to make mid-infrared (midIR) LEDs makes extraction of internally produced light challenging due to total internal reflection. For array style mid-infrared VCSELs, challenges include achieving a high quality factor due to low-index contrast of Bragg layers, high reflection distributed Bragg reflectors (DBRs) due to their thickness, a single-infrared-mode cavity tuned to the correct resonance wavelength, and low contact resistance while simultaneously keeping free carrier absorption low. These challenges will be addressed in this thesis.
description: The first project was to increase the radiance or output power of mid-IR LEDs. Mid-IR LEDs can be used for thermal scene projection and sources for detectors in gas sensing. Increased radiance was accomplished by making use of an efficient W-superlattice that had 77% internal quantum efficiency, which was over a 2x enhancement to the 29% internal quantum efficiency seen in the previous best performing InAs/GaSb mid-IR LED. To improve the fraction of light that can escape, the LEDs were thinned and roughened to increase the light that can escape by a factor of 2-3x. In total, the radiance reached a total of ~4.5x improvement using the W-superlattice and thinning and roughening, while wallplug efficiency improved by over a factor of 10x.
description: These improved mid-infrared LEDs had high surface defect densities impacting performance initially. To resolve the issue, Al-treated gallium crucibles were introduced, which suppress Ga-related oval defect formation. After the Al-treatment, the surfaces improved and improved to the light output further by a factor of &gt;2x through, surprisingly, suppression of Auger scattering.
description: Building on advancements in the single-color high-power LEDs, we revisited midinfrared two-color LEDs, which are important in advanced thermal scene projectors and nondispersive, infrared gas (NDIR) sensors. Two challenges in operating two color LEDs are: making sure the two colors do not overlap to ensure truly independent two-color operation while simultaneously realizing high in-band light output. This second project mapped out the behaviors of the LEDs in single and two color operation to show under which conditions the two-color LED met application requirements. For single-color operation, both LEDs were able to achieve temperature goals set in three color bands within the mid-infrared. A desired improvement of our initial, non-cavity two-color LEDs was to further increase the light output of the longer wavelength LED.
description: To make the longer wavelength LED brighter, we sought to narrow and re-center the emission. The longer wavelength, broader LED was hampered by color overlap, requiring it to be shifted out in wavelength, limiting the max light output in the waveband of interest. The method chosen to narrow the emission was to create a multimode cavity. With this multimode cavity, the longer wavelength LED achieved similar temperatures as without the cavity, while having low color overlap throughout most of the operating conditions. Another success was that the shorter wavelength LED’s spectrum was not affected much by the cavity. This means that with proper band structure engineering, a multimode cavity can be used to enhance emission within narrow bands while not affecting other bands.
description: The last project was to find a path to the creation of a flip-chipped VCSEL using Wsuperlattices, which have potentially high gain-thickness product. A high reflection semiconductor distributed Bragg reflector and lattice matched cavity were successfully grown and fabricated into a functioning device. The cavity was made up of a 4-stage gain region, and utilized a top dielectric DBR in combination with a ring contact. The device exhibited cavityenhanced spontaneous emission when operated below threshold, achieved relatively low RsA values while keeping free carrier absorption low by avoiding p-doped regions by using a novel approach, and diode-like performance as evidenced in the IV characteristics. Lasing was likely not achieved in this pathfinder, however, and investigations were carried out to correct the issues found (insufficient gain).