Development of ultraviolet electroabsorption modulators and light emitting diodes based on AlGaN alloys
Embargo Date
Indefinite
OA Version
Citation
Abstract
The research in this dissertation addressed the development of ultraviolet (UV) electroabsorption modulators and ultraviolet light emitting diodes (UV-LEDs), covering the spectral range from 360 to 265 nm. The materials system for both types of devices is the AlGaN alloys, either in bulk or quantum well (QW) form, grown by plasma-assisted molecular beam epitaxy (MBE). Potential applications of these devices either individually or in combination include UV non-line-of-sight free-space-optical communications, UV sensing and spectroscopic systems, Q-switched pulsed lasers, water/air purification and various medical applications.
Optical modulators based on cubic III-V semiconductors have been the subject of
extensive research over the past several years. Such devices are typically based on the
quantum-confined Stark effect to modify the absorption spectrum of multiple-quantumwell
active regions. On the other hand, in wurtzite III-Nitride semiconductors, strong
electric fields are already present in the quantum wells due to intrinsic and piezoelectric
polarizations; as a result, an even greater change in absorption is achievable, especially if the internal fields are compensated by the external bias so that the net field in the
quantum wells is reduced. A number of UV electroabsorption modulators based on
Schottky barriers on bulk GaN and GaN /AlGaN multiple quantum wells (MQWs) were
designed, fabricated and characterized. Record modulation ratio of 30 % was obtained
from bulk GaN Schottky barrier modulators at the excitonic resonant energy of 3.45 eV
(360 nm) upon the application of 12 V reverse bias. Similarly, record modulation ratio of
43% was obtained from GaN I AlGaN MQWs Schottky barrier modulators at the
excitonic resonant energy of 3.48 eV (356 nm) upon the application of 17 V reverse bias.
The external quantum efficiency (EQE) of AlGaN based deep UV LEDS is
relatively low ( ~ 1% at 270 nm). This is generally attributed to the poor internal quantum
efficiency (IQE) of this material system due to the high concentration of line and point
defects. In the current work the deep UV-LED structures were grown on inexpensive and
widely available sapphire substrates, which resulted in materials with dislocation density of 1010 cm-2. To prevent the non-radiative recombination of the injected electron-hole
pairs, the active region of the devices were grown under conditions which lead to band
structure potential fluctuations, which lead to exciton localization and thus efficient
radiative recombination. Using such a growth method AlGaN MQWs emitting at 265 nm
with an IQE as high as 58.8% were demonstrated. Using such QWs a number UV LEDs
emitting in the spectral region from 340 to 265 nm were fabricated and evaluated at the
die level. A number of milliwatt output power LEDs emitting at 280 nm were
demonstrated.
Description
Thesis (Ph.D.)--Boston University
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.