|
|
Universidade do Porto, em co-tutela com a Universidade de Glasgow
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.
|
|
© 2000 José Figueiredo