Universität zu Köln

Development of Ultrathin Niobium Nitride and Niobium Titanium Nitride Films for THz Hot-Electron Bolometers

Bedorf, Sven Holger (2005) Development of Ultrathin Niobium Nitride and Niobium Titanium Nitride Films for THz Hot-Electron Bolometers. PhD thesis, Universität zu Köln.

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    Abstract

    The main focus of this work is the development of ultrathin NbN and NbTiN films. A reproducible and reliable deposition process for ultrathin NbN and NbTiN films for the use in phonon-cooled HEB devices was established. The ultrathin films were deposited on silicon (Si) substrates and on 2µm Si3N4 membranes by DC reactive magnetron sputtering. A method for the precise control of the nitrogen partial pressure by monitoring the target voltage has been introduced to deposit high quality, ultrathin NbN (3-4nm, Tc=8.5 K) and NbTiN (4-5nm, Tc=8K) films. Substrate heating of at least 600°C during the deposition is essential for the fabrication of ultrathin NbN and NbTiN films on Si substrates and Si3N4 membranes. The fabrication process required for HEB devices to be used in a quasi-optical mixer was developed. The ultrathin film was patterned by electron beam lithography (EBL), resulting in bolometer devices that measure areas of about 0.4 µm x 4 µm. The nature of the contact determines the interface transparency between the bolometer and the contact structure. Different cleaning processes have been performed and the influence on the contact resistance has been instigated. A better interface transparency gives less RF losses and could improve the HEB sensitivity and local oscillator (LO) requirement. A better control of the interface transparency also leads to a better reproducibility in values of the normal state resistance of the HEB devices. Heterodyne measurements were performed at 0.8 THz and 1.6 THz. For the NbTiN HEB devices, the double sideband (DSB) receiver noise temperature at 0.8 THz was found to be 2500 K for at 1.2 GHz IF. The main problem with HEB mixers is the limitation in the IF bandwidth. The measured noise bandwidth was about 1.5 GHz. The DSB receiver noise temperature at 1.6 THz LO frequency measured at Chalmers University, Sweden, was found to be 1600 K at 1.5 GHz IF. This result shows that the noise of these NbTiN HEB devices is comparable with the NbN HEB mixers fabricated at the Chalmers University. For the NbN HEB devices the DSB receiver noise temperature Trec was 1344 K at 0.8 THz with an absorbed LO power of 55 nW, estimated using the isothermal method. This receiver noise temperature is higher than the state-of-the-art receiver noise temperature at this frequency. This is possibly due to the losses in the optics. It was not possible to determine the noise bandwidth of this device, because the noise temperature did not increase by the factor of two in the bandwidth of the isolator. But the noise bandwidth is estimated to be about than 1.8 GHz. Although the true bandwidth of the NbTiN and NbN HEB devices could only be measured with an appropriate IF system, the combination of these results show that NbTiN is possibly inferior in bandwidth to NbN used for the HEB fabrication.

    Item Type: Thesis (PhD thesis)
    Creators:
    CreatorsEmail
    Bedorf, Sven Holgerbedorf@ph1.uni-koeln.de
    URN: urn:nbn:de:hbz:38-16225
    Subjects: Physics
    Uncontrolled Keywords:
    KeywordsLanguage
    Dünnfilmentwicklung, NbN, NbTiN, PVD, HEB, THzGerman
    thin films, NbN, NbTiN, PVD, HEB, THzEnglish
    Faculty: Mathematisch-Naturwissenschaftliche Fakultät
    Divisions: Mathematisch-Naturwissenschaftliche Fakultät > I. Physikalisches Institut
    Language: English
    Date: 2005
    Date Type: Completion
    Date of oral exam: 07 December 2005
    Full Text Status: Public
    Date Deposited: 25 Jan 2006 16:00:09
    Referee
    NameAcademic Title
    Stutzki, JürgenProf. Dr.
    URI: http://kups.ub.uni-koeln.de/id/eprint/1622

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