José
Figueiredo
BSc in Physics
(Optics & Electronics)
MSc in Optoelectronics and Lasers
Ph.D. in Physics (Electronics and Optoelectronics)
PhD/Doutoramento
Universidade
do
Porto, em
co-tutela
com a Universidade de Glasgow
Thesis: Optoelectronic
Properties
of Resonant Tunneling Diodes
ABSTRACT
This
thesis reports an investigation of the optoelectronic properties of
unipolar
semiconductor optical waveguides incorporating a double barrier quantum
well
resonant tunnelling diode (DBQW-RTD), implemented successively in the AlGaAs/GaAs and the InGaAlAs/InP
material
systems, and showing typical DBQW-RTD behaviour (negative differential
conductance, NDC). The material systems employed allow operation at
wavelengths around
900 nm and 1300/1550 nm, respectively. This unipolar diode combines
waveguide
optical confinement with the electrical gain and potential wide
bandwidth properties of the DBQW-RTD. Research concentrated on the
demonstration of
its potential as an optical modulator based on the Franz-Keldysh effect: the resonant tunnelling diode
electro-absorption
modulator (RTD-EAM). A reliable RTD-EAM fabrication process was
established
for both material systems.
The
AlGaAs/GaAs
RTD-EAM
consists of an unipolar AlGaAs-GaAs-AlGaAs waveguide embedding a GaAs/AlAs DBQW-RTD.
This device presented NDC with a peak-to-valley
current ratio around 1.6, peak current densities up to 13.5 kAcm-2
and peak voltages in the range 1.5 V – 3.2 V. From the devices
current-voltage characteristic and spectral behaviour, an optical
modulation depth of around
14 dB was estimated. The electro-absorption modulation was
characterized
using a streak camera, and modulation depths up to 18 dB were measured
in
The
RTD-EAM implemented in the InGaAlAs
material
system, lattice matched to InP,
operated at around 1560 nm. This device configuration consists of an unipolar InAlAs-InGaAlAs-InP
waveguide incorporating, in the InGaAlAs
core
region, an In0.53Ga0.47As/AlAs
DBQW-RTD. The InGaAlAs RTD-EAM showed
larger
NDC (peak-to-valley current ratio up to 7 and peak current density as
high as 18 kAcm-2) than the GaAs
device.
A low frequency electrical signal with an amplitude
of 1 V induced an optical modulation depth as high as 28 dB at around
1565
nm. The electro-absorption response showed a change in absorption of 5
dB
for a 1 mV variation in the bias voltage within the NDC region. A total
change
in absorption of 13 dB was observed as the RTD-EAM bias voltage was
swept
through the NDC region. This device configuration proved its potential
for
applications in high frequency optoelectronic communication systems.
InGaAlAs-InP RTD optical waveguide Gama-band profile.
A tipical IV curve of a
InGaAs-AlAs double-barrier quantum well
RTD.
©
2002 José
Figueiredo