ETD

Archivio digitale delle tesi discusse presso l'Università di Pisa

Tesi etd-06032019-121012


Tipo di tesi
Tesi di laurea magistrale
Autore
GERMANESE, GAIA
URN
etd-06032019-121012
Titolo
Mid-Infrared Quantum Cascade Detector with Graphene Field-Effect read-out
Dipartimento
FISICA
Corso di studi
FISICA
Relatori
relatore Prof. Tredicucci, Alessandro
Parole chiave
  • Quantum Cascade Detectors
  • Intersubband Photodetectors
  • Graphene-Field Effect Transistors
  • Graphene
Data inizio appello
24/06/2019
Consultabilità
Completa
Riassunto
Although Intersubband (ISB) photodetectors were demonstrated more than thirty years ago, they remain at the center of scientific and practical interest for their unique properties and applications in spectroscopy and pyrometry.
The Quantum Cascade Detector (QCD) has been introduced recently as an alternative photovoltaic version to the QuantumWell Infrared Photodetectors (QWIP), suppressing the dark current and avoiding the capacitance saturation in the readout circuit, thus, allowing longer integration time. The QCD is a unipolar device, which operates in the near-, mid-, and far- infrared ranges, by ensuring room
temperature operation, high detection speed and excellent Johnson noise limited detectivity.
By analogy with the Quantum Cascade Laser structure, from which it derives its name, the device is based on an asymmetric active region of semiconductor quantum wells (QWs) and barriers, where it is possible to identify a single unit, called period, repeated multiple times. The widths of the QWs are designed in
order to generate a ladder of energy levels: under illumination, electrons localized on the lowest energy level of the first active QW of each period are excited to a higher subband and extracted through the cascade, emitting Longitudinal Optical(LO)-phonons. The unidirectional carrier transport induces a displacement of electrons, generating a net photocurrent, which flows through each period. Consequently, a potential difference is produced at the opposite sides of the heterostructure in the open circuit configuration.
The idea of this thesis is to investigate and verify the behaviour of a Quantum Cascade photoDetector, exploiting the extraordinary properties of graphene, an innovative bidimentional material. Its discovery in 2004 by A. Geim and K. Novoselov has revolutionized the world of electronics, transforming Field-Effect Transistor devices. Its zero-bandgap structure leads to very high mobility and to easily
tunable Fermi energy and carrier density.
Graphene Field-Effect Transistors (G-FET) consist of three terminals: the Source, the Drain and the Gate. The current, flowing in the graphene channel applying a constant source-drain potential difference, is modulated by the electric field generated by the gate at a certain applied voltage.
The novelty of this work is to use the Quantum Cascade Detector as a back-gate for G-FET. The exfoliated graphene monolayer, transferred on top of the detector, acts as the readout of the potential difference, created across the heterostructure. A consequent charge density variation in the graphene channel should be appreciated.
Preliminary simulations of the light propagation along the growth direction of the heterostructure (z-axis) and its correlated Poynting vector have established that the radiation incident at 45° with respect to the z-axis on a lateral side of the detector does not interfere destructively with the reflected beam by the graphene sheet. Therefore, all periods of the QCD contribute to generate the photocurrent.
The realization of this particular device was divided into two major-steps: the preparation of the QCD substrate and the fabrication of the G-FET. The first one consisted of the wet chemical Etching [5] and the Hafnium oxide Atomic Layer Deposition (ALD) on top of N1021 InGaAs/InAlAs-QCD, previously grown and
characterized by the research group at ETH in Zurich (M.Beck). The second one consisted into mechanical exfoliation of Graphite on a stack of PolyMethyl MethAcrylate(PMMA AR-P 679.04)/PolyVinyl Alcohol(PVA)/Silica(SiO2)/Silicium(Si) to obtain vary large graphene flakes(~ 50-100 um), the transfer of selected graphene flakes onto the QCD substrate, the Electron Beam Lithography(EBL) of the contacts
and pads patterning and their metallization, the lapping and polishing of one lateral side of the sample at 45° with respect to the z-axis, the manual wire bonding.
The graphene flakes were optically characterized pre- and post-transfer by Raman Spectroscopy, estimating the strain and the doping, and electronically checked by micro-manipulator miBot.
Eight devices were fabricated following this procedure.
Opto-electric measurements were performed on three samples, exploiting the globar of a Fourier Transform Infrared Spectroscopy (FT-IR) as Source to illuminate the QCDs, which operate at lambda = 7.5 um, and Keithley Source-Meter for I-V characterization of the G-FET. All measurements were performed at cryogenic temperature (~ 87 K) under high vacuum (~ 10^-6 mbar).
Illuminating the QCD with p-polarized radiation, as required by the selection rules of ISB transitions, we expected to observe a variation of resistance or current in the graphene channel due to the gating-effect attributable to the voltage difference across the heterostructure. This effect was not clearly visible in the measured samples, probably due to the direction of the cascade, which transports the electrons away from the graphene. Despite the weakness of this effect, we found a very low photo-response in the leakage current of the QCD-FET, applying a bias voltage to the bottom of the heterostructure. Moreover, investigating directly the QCD behaviour, we verified the increasing of the resistance of the detector lowing the temperature.
On the contrary, an evident photo-conductive effect in the QCD around the energy gap of InGaAs (lambda = 1.5 um), due to the Interband transitions, occurred, leading to a significant photo-gating effect on the graphene channel.
In summary, this thesis explored an unprecedented way to operate ISB photodetectors, exploiting the properties of graphene for FETs, offering a starting point for further optimization of the device architecture.
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