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NbN ultra-thin films for HEB-based THz systems

Sascha Krause

Published Jan 1, 2013• 0 citations
OpenAlex

Abstract

The key technology for low-noise THz receivers operating above 1 THz, lies inevitably in the employment niobium nitride superconductors with critical bulk temperature of 16-17K. The enhancement of HEB’s limited IF bandwidth has been a relevant topic for two decades and is associated with the quality, hence critical temperature, Tc, of the 3.5-6nm thin NbN films. This thesis revisits the concept of employing buffer-layers which have not been utilized yet to deposit NbN onto. As a result, state-of-the-art 5.5nm thin NbN films have been successfully deposited exhibiting Tc as high as 13.2K and RRR-figure-of-merit close to unity. These films show considerable potential for highly-integrated THz electronics.

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•Jan 1, 2013•0 citations

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New developments in textured and epitaxial NbN superconducting layers for ultimate sensors and RSFQ digital circuits

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Nitride superconductors and specially niobium nitride are key materials for developing high performance optoelectronic and digital circuits. We are presenting a way to achieve such high frequency devices on R-plane sapphire or MgO substrates. Deposition of thin and flat NbN films with Te above 10 K, low and reproducible penetration depth ( L 250 nm) and surface resistance (Rs) values up to I THz, is required and obtained by sputtering on a substrate heated in the 300-600 °C range. Simple sub-micrometer size HEB bridge structures where patterned even in a very thin (2-5 nm thick) NbN layers offering relaxation times below 30 ps. It is then possible to achieve fast optoelectronic data links and sensors on-chip with high clock frequency NbN RSFQ digital circuits.

Journal de Physique IV (Proceedings)•May 1, 2002•4 citations
The key technology for low-noise THz receivers operating above 1 THz, lies inevitably in the employment niobium nitride superconductors with critical bulk temperature of 16-17K. The enhancement of HEB’s limited IF bandwidth has been a relevant topic for two decades and is associated with the quality, hence critical temperature, Tc, of the 3.5-6nm thin NbN films.
This thesis revisits the concept of employing buffer-layers which have not been utilized yet to deposit NbN onto. As a result, state-of-the-art 5.5nm thin NbN films have been successfully deposited exhibiting Tc as high as 13.2K and RRR-figure-of-merit close to unity. These films show considerable potential for highly-integrated THz electronics.
The key of improving hot-electron bolometer (HEB) mixer performance lies inevitably in the quality of ultra-thin NbN films itself. This work presents a thorough investigation of crucial process parameters of NbN films deposited by means of reactive DC-sputtering on Si-substrates at elevated temperatures up to 750°C.The polycrystalline NbN films with thickness of 4 to 10nm were characterized by DC resistivity measurements, ellipsometry and high resolution transmission electron microscopy (HRTEM) in order to confirm thickness and film structure. Since the macroscopic properties such as critical temperature, thickness as well as the transition width to the superconducting state are directly linked to HEB mixer noise temperature and IF bandwidth, a set of experiments were conducted to enhance aforementioned properties. We considered deposition temperature, RF biasing, nitrogen and argon partial and total pressure during deposition as major process variable parameters. Careful optimization of the deposition conditions allowed setting up a process resulting in high-quality NbN ultra-thin films with thickness of 5.5nm exhibiting Tc of 10.5K. Moreover, the transition width could be kept as low as 1.4K. The produced films were stored at ambient conditions and re-characterized over a period of 4 month without measurable degradation.
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 2æm 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 600ʿC 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 æm x 4 æm. 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 ...
Experimental work on the development of a quasi-optical heterodyne receiver for the THz frequency range is reported. The mixing device is a superconducting hot electron bolometer (HEB) made from ultrathin, d = 3 - 5 nm, niobium nitride (NbN) films. Work described in this thesis comprises: The design of a prototype receiver using quasi-optical coupling. Device fabrication involving reactive magnetron sputtering of NbN films and lithographic definition of the antenna integrated mixer. An investigation of the receiver performance at frequencies between 0.6 THz and 2.5 THz. The best performance is obtained with HEB mixers made from NbN films with a thickness d = 3-3.5 nm and a critical temperature T c = 9 - 9.5 K. Double-side band receiver noise temperatures are 530 K at 0.6 THz, 650 K at 1.6 THz and 1100 K at 2.5 THz. Corrected for losses in the receiver input optics, these values amount to only 10 times the theoretical quantum limit. MgO as the substrate material has been shown to provide improved acoustic coupling between NbN film and the substrate compared to other materials such as Si or sapphire. A maximum IF gain bandwidth of 4.5 GHz has been measured under the same operating conditions which yield lowest noise temperature. The local oscilltor power requirement is ~250 nW absorbed in the bolometer. Quasi-optically coupled HEB mixers have become the device-of-choice for radio astronomical applications in the frequency regime at and above 1 THz.
We present recently obtained values for gain bandwidth of NbN HEB mixers for different substrates and film thicknesses and for MgO buffer layer on Si at LO frequency of 0.85-1 THz. The maximal bandwidth, 5.2 GHz, was achieved for the device on MgO buffer layer on Si with a 2 nm thick NbN film. Functional devices based on NbN films of such thickness were fabricated for the first time due to an improvement of superconducting properties of NbN film deposited on MgO buffer layer on Si substrate. Introduction. In the last years, the development of HEB mixers has reached such a high level that these devices can be successfully utilized in heterodyne receivers of THz frequency range. Their high sensitivity and low LO power make them suitable for radioastronomic observations on space- and airbom platforms. An NbN HEB mixer seems to be a
Hot electron bolometer mixers (HEBM) are expected to be used as low-noise heterodyne mixers in the terahertz frequency range. We are developing quasi-optical HEBMs for applications such as atmospheric remote sensing and radio astronomy in the terahertz frequency range. Therefore, we developed a fabrication method using niobium nitride (NbN) ultrathin films deposited on a silicone (Si) substrate for terahertz applications. First, a magnesium oxide buffer layer 50 nm thick was deposited on a Si substrate at approximately 525 °C. After deposition of this layer, NbN ultrathin films were deposited at ambient temperature. Using the buffer layers, we found the transition temperature of the 3.5-nm-thick NbN thin films to be approximately 10 K. The same method was applied to fabricate NbN-HEBMs on a Si substrate. The fabricated quasi-optical mixers were designed to operate at 3.1 THz. The receiver noise temperature was found to be approximately 1930 K (DSB) at 3.1 THz.
Nitride superconductors and specially niobium nitride are key materials for developing high performance optoelectronic and digital circuits. We are presenting a way to achieve such high frequency devices on R-plane sapphire or MgO substrates. Deposition of thin and flat NbN films with Te above 10 K, low and reproducible penetration depth ( L 250 nm) and surface resistance (Rs) values up to I THz, is required and obtained by sputtering on a substrate heated in the 300-600 °C range. Simple sub-micrometer size HEB bridge structures where patterned even in a very thin (2-5 nm thick) NbN layers offering relaxation times below 30 ps. It is then possible to achieve fast optoelectronic data links and sensors on-chip with high clock frequency NbN RSFQ digital circuits.