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Nitride Processing

Gallium nitride and its alloys are desirable materials to work with because of their wide direct bandgap, which spans across the visible range and into the UV. These materials are useful for a variety of optical devices, from short wavelength blue lasers for data storage to white LEDs for visible lighting. However, III-nitrides are difficult to work with for many reasons, including their chemical stability, which makes dry etching difficult and conventional wet etching impossible, and their relatively high defect density because of the lack of a lattice-matched substrate for material growth. We focus on nanofabrication techniques that can be used to realize a variety of nitride optical devices.

One aspect of our research involves photoelectrochemical (PEC) etching of GaN and InGaN, which allows us to wet etch nitride material with bandgap selectivity, dopant selectivity, and defect selectivity. PEC etching is photoassisted in nature, allowing GaN, a chemically inert material, to be wet etched under illuminated conditions. This technique allows us to form undercut structures such as microdisk lasers, air-gap distributed bragg reflectors, and photonic crystal membranes, among other applications.

Microdisk resonators consist of a disk-shaped semiconductor region surrounded by air. Light is confined to the structure by total internal reflection in whispering gallery modes, with light emission occurring in-plane, radially in all directions. GaN-based microdisk resonators, fabricated using PEC etching, have been used to realize room temperature, continuous-wave blue lasing with a record low threshold. Advances in III-nitride processing have also led to the air-gap distributed bragg (DBR) reflectors and freestanding photonic crystal (PC) membrane nanocavities. The selective removal of embedded materials is obtained by PEC etching, allowing larger index contrast in a DBR structure because the layers are comprised of air and remaining GaN material. This leads to stronger Bragg scattering and an electrical aperture for efficient current injection.

High-quality (Q) PC nanocavities can lead to low-threshold lasers with the possibility of high-density integration with filters and waveguides. The strong coupling regime can be achieved, considering the large oscillator strength of InGaN quantum well excitons and the high Q/V of PC nanocavities. Furthermore, GaN-based quantum dots embedded in high-Q PC nanocavities will facilitate the study of cavity quantum electrodynamics and single photon sources at room temperature.

GaN has also become the prominent material for blue-green light emitting diodes (LEDs). The development of nitride LEDs has had great impact on the lighting industry by enabling the production of full color displays and white light sources. With further improvements, nitride LEDs have the potential to replace traditional light sources due to their high efficiency and long life times. The limiting factor to achieving the high efficiencies needed for such applications is the external efficiency due to poor extraction of the emitted light from the device.

In GaN LEDs, photonic crystals can be used to increase light extraction and control the emission profile by coupling guided modes in the material into extracted modes in air. Electron beam lithography is used to pattern 2D PCs into the LED. New patterning techniques, such as imprint lithography, are also being developed so that PCs can be patterned over large areas more efficiently and inexpensively. Etching of the PCs into the LED is being developed to reliably control PC properties. Challenges presented by the integration of the PCs into LEDs are being addressed, such as etch damage to critical LED material and electrical contact incorporation. GaN/InGaN photonic crystal LEDs have been fabricated, and increases in light extraction in addition to controllable emission profiles have been demonstrated.

Sample Publications