PhD Thesis Abstract

José Figueiredo

Faculdade de Ciências e Tecnologia
Universidade do Algarve
E-mail: jlongras@ualg.pt
URL: http://w3.ualg.pt/~jlongras/

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 4 um × 200 um active area AlGaAs/GaAs RTD-EAMs. Optical modulation associated with RTD-EAM self-oscillation was also demonstrated, showing modulation depth up to 11 dB. From analysis of the operation in the relaxation oscillation mode, a bandwidth higher than 30 GHz is to be expected. Knowledge of the electrical properties of the devices based on the AlGaAs/GaAs system allowed a simple operation model to be established, which was employed to help the design of InGaAlAs/InP wafers presenting significant NDC.

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 In_0.53Ga_0.47As/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.

http://w3.ualg.pt/~jlongras/JLFPhDThesis.pdf.

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