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Hofstetter, Daniel

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Hofstetter, Daniel
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https://libra.unine.ch/handle/123456789/472

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  • Publication
    Accès libre
    Design and characterisation of far- and mid-infrared quantum cascade detectors
    (2008)
    Graf, Marcel
    ;
    Hofstetter, Daniel 
    Cette thèse traite un type nouveau de photodétecteurs infrarouges, basés sur les transitions inter-sous-bande dans les matériaux semi-conducteurs III-V (AlAs/GaAs sur GaAs et InAlAs/InGaAs sur InP). S'adressant à une des sources bruyantes principales de photodétecteurs photoconducteurs, le bruit de courant d'obscurité, ce design comprend un gradient potentiel intérieur en forme de cascade quantique, donc le nom détecteur à cascade quantique (QCD). Cette approche permet l'opération sans mise sous tension et sans courant d'obscurité, en conséquence. L'extraction d'électrons excités est fournie par cette cascade. La fabrication et l'opération de tels détecteurs ont été accomplis avec succès, dans la gamme de l'infrarouge moyen (détection aux longueurs d'ondes de 5.3 µm et de 9 µm) et dans l'infrarouge lointain (la région THz) à 84 µm, au-dessous de l'énergie de phonons optiques longitudinales (LO). Additionellement, il est démontré que ces QCDs sont rapides, comparables avec des photodétecteurs infrarouges à puits quantiques (QWIP). Jusqu'à la limite de fréquence superieure de 23 GHz, imposée par l'électronique de haute fréquence disponible pendant ces mesures, la diminution en performance observée est entièrement expliquée par l'amortissement à cause du circuit RLC, créé par la technique d'assemblage simple. En plus du design, la fabrication et les mesures expérimentales, cette thèse essaie aussi d'expliquer deux propriétés importantes, variant en fonction de température de détecteur. Les changement fortes en résistance électrique et la diminution en réponse des détecteurs à cascade quantique en infrarouge moyen sont numériquement calculés, basés sur un modèle de transitions d'électrons entre les différents états de la cascade quantique, assistées par des phonons optiques longitudinales., This thesis covers a novel type of infrared photodetectors, based on intersubband transitions in III-V semiconductors (AlAs/GaAs on GaAs and InAlAs/InGaAs on InP). To address one of the main noise sources of photoconductive photodetectors, dark current noise, this design comprises an internal potential gradient in form of a quantum cascade, hence the name quantum cascade detector (QCD). This approach allows bias-less and therefore dark current-less operation, the extraction of excited electrons is provided by the cascade. Successful fabrication and operation of such detectors was achieved, both in the mid-infrared range (detectors at 5.3 µm and 9 µm wavelength) and in the far-infrared (THz region) at 84 µm, below the energy of LO-phonons. It is also shown that these QCDs are fast, comparable to quantum well infrared photodetectors (QWIP). Up to the limit of the high frequency electronics available for this measurement, 23 GHz, the observed decrease in performance is entirely due to the RLC damping, caused by the simple device mounting technique. In addition to the design, fabrication and the experimental measurements, this thesis also attempts to explain two important properties as function of detector temperature. The strongly varying device resistance and the drop in responsivity of the mid-infrared quantum cascade detectors are numerically calculated based on a modell describing LO-phonon assisted transitions between the different involved states of the quantum cascade.
  • Publication
    Accès libre
    Design, fabrication, and testing of intersubband infrared photodetectors operating at wavelengths between 2 ųm and 17 ųm
    (2007)
    Giorgetta, Fabrizio
    ;
    Hofstetter, Daniel 
    Intersubband (ISB) photon detectors based on photon – electron interactions between quantized electron subbands in the conduction band of semiconductor heterostructures are presented. As opposed to interband devices, the operating wavelength of ISB devices is set by choosing appropriate layer thicknesses of the heterostructure and is not fixed by the semiconductor material system. As only electrons are involved in ISB detection, ISB detectors are potentially faster compared to semiconductor interband detectors involving both conduction and valence band; the speed of the latter is limited by the slower holes in the valence band. This work focuses on novel quantum cascade detectors (QCDs). In contrast to common photoconductive ISB photodetectors, QCDs do not require an external bias voltage due to their asymmetric conduction band profile. This results in a favorable noise behavior, reduced thermal load, and simpler readout circuits. Several QCDs with detection wavelengths ranging from 2 µm to 17 µm are designed, grown, processed, characterized, and discussed. Using the In0.53Ga0.47As / In0.52Al0.48As semiconductor system, QCDs detecting at 17 µm, 10 µm, 7.5 µm, and 4.7 µm are presented. The specific detectivity D* of the 17 µm QCD is 11011Jones at an operating temperature of 5 K; this result is close to the background limited D*BLIP=1.41011 Jones at 17 µm. As the shortest ISB wavelength between bound states in a heterostructures is determined by the conduction band offset at the interface between two semiconductors, In0.53Ga0.47As / In0.52Al0.48As QCDs can only be realized down to about 4.5 µm. To obtain shorter wavelengths, QCDs based on two alternative semiconductor heterostructures are presented. An In0.61Ga0.39As / In0.45Al0.55As QCD operating at 4 µm is demonstrated. At 100 K, its specific detectivity is D*=1.91011 Jones. This value compares favorably to commercial semiconductor photodetectors in this wavelength range; at 4 µm, the common MCT detectors reach specific detectivities around 11011 Jones at operating temperatures of 77 K. To obtain even shorter wavelengths, InAlAs is replaced by AlAs0.56Sb0.44. Based on this system, a QCD detecting at 2 µm is presented.
  • Publication
    Accès libre
    Near infrared intersubband absorption and photovoltaic detection in GaN/AIN multi quantum well structures
    (2007)
    Baumann, Esther
    ;
    Hofstetter, Daniel 
    Due to the large conduction band offset of nearly 2 eV between GaN and AlN, group III nitride semiconductor heterostructures are of great interest for optoelectronics based on intersubband transitions (ISBTs). These properties allow the extension of the ISBT wavelength range into the near infrared. In this work near infrared ISBTs in GaN/AlN heterostructures were investigated for samples grown by both plasma assisted molecular beam epitaxy (PAMBE) and metal-organic vapor-phase epitaxy (MOVPE). It was shown in the experiment that a 15 Å thick quantum well (QW) absorbs light around 800 meV (1.5 μm). The highest ISB absorption energy for PAMBE samples in this work is 890 meV (1.39 μm) and 834meV (1.49 μm) for MOVPE. Those results show that both growth techniques are appropriate to achieve short ISBT wavelengths. As group III nitrides are non-centrosymmetric, strong piezo- and pyro-electric fields occur in GaN/AlN heterostructures; they lead to intrinsic asymmetries in the electronic potential of a multi quantum well (MQW) structure and give rise to nonlinear optical phenomena. Based on the photoinduced polarization detection mechanism, ISB detectors were designed and investigated. Photovoltaic responses, which originate both from linear absorption as well as from step two-photon absorption were measured. These prototype photoinduced polarization photodetectors work up to room temperature. The frequency response obtained with a modulated telecommunication laser diode showed a signal up to 3 GHz. A narrow-band electro-optic modulator prototype working around 1.5 μm based on the depletion of a GaN/AlN MQW region is also presented. Those results on modulation and fast photovoltaic detection at 1.55 μm illustrate the application potential of group III nitride ISB devices as electro-optic detectors and modulators for long-haul optical fiber telecommunication