Low noise MgB2 terahertz hot-electron bolometer mixers

Abstract

We report on low noise terahertz bolometric mixers made of MgB 2 superconducting thin films. For a 10-nm-thick MgB 2 film, the lowest mixer noise temperature was 600 K at 600 GHz. For 30 to 10-nm-thick films, the mixer gain bandwidth is an inverse function of the film thickness, reaching 3.4 GHz for the 10-nm film. As the critical temperature of the film decreases, the gain bandwidth also decreases, indicating the importance of high quality thin films for large gain bandwidth mixers. The results indicate the prospect of achieving a mixer gain bandwidth as large as 10-8 GHz for 3 to 5-nm-thick MgB 2 films. Superconducting NbN hot-electron bolometer (HEB) 1 mixers are widely used for high resolution terahertz radio astronomy. 2 Such mixers have superior performance over other types of mixers (e.g., SIS, Schottky diodes) 2 at frequencies higher than 1.2 THz. 3-5 A large RF bandwidth, a low noise temperature, and low LO power requirements determined the choice of NbN HEB mixers for the Herschel space observatory. 6,7 In contrast to SIS mixers, the useful IF bandwidth of NbN HEB mixers is practically limited to 3-5 GHz, 8 as the noise temperature rises drastically at higher intermediate frequencies. The reason for this is that the HEB mixer gain rolls off as the IF exceeds the mixer's 3 dB gain bandwidth (GBW). The GBW is determined by two consequent processes in the electron energy relaxation: the electron-phonon interaction and the phonon energy relaxation. The second process mainly occurs via acoustic phonon escape into the substrate. The electron-phonon interaction time is usually a function of the temperature. For HEB mixers, the relevant electron temperature is the critical temperature of the superconducting film, T c , to which the electrons are heated by a combination of the LO power and the dc bias current. In NbN thin films, the electron-phonon interaction time is approximately s e-ph % 12 ps at 10 K. 9,10 The phonon escape time is on the order of s esc % 40 ps for NbN films as thin as 3-4 nm. 8

Fabrication of superconducting nanowires from ultrathin MgB2 films via focused ion beam milling

“…In particular, the T c of above 34 K of the nanowires, which surpassed the values of about 20 K for MgB 2 nanowires obtained by other groups, 11,13 opens the door for developing MgB 2 superconducting devices such as submicron-HEBs and SSPDs with higher operating temperatures at 15-20 K than that currently available. [18][19][20][21] In practically realizing such devices, we note that further refinement to the present FIB milling procedure may be needed. For instance, before the deposition of the conductive Ti/Au layers, one could deposit an insulating passivation layer first in the area to fabricate nanowires on the MgB 2 ultrathin film, 20 which would help isolate the obtained nanowires from directly contacting with the Ti/Au layers.…”

“…Despite the above significant advances, it has to be pointed out that so far the fabricated MgB 2 nanowires mostly show T c 's considerably lower than the value of 39 K in bulk MgB 2 samples, 11,13,[18][19][20][21] primarily owing to the fact that the T c 's of the precursor MgB 2 ultrathin films used to fabricate these nanowires are low, showing values only around or below 20 K in films of 10 nm thick or less. [10][11][12][13] It is obvious that with such markedly decreased T c of the nanowires, the initial expectation is difficult to be fulfilled to develop MgB 2 -based HEBs or SSPDs with operating temperatures apparently higher than that of NbN-based devices.…”

“…It is worth pointing out that the T c 's of the present nanowires are considerably higher than the values of about 20 K as reported by other groups 11,13 and indicate that it is indeed possible to make MgB 2 -based nano-devices such as HEBs and SSPDs with high operating temperatures at 15-20 K. Also, the small ∆T c 's of the nanowires would be beneficial to ensure the high performance of the devices, for instance, to lower the noise temperature of the HEB mixers. 18 To check whether the FIB milling process has influenced the superconductivity of the MgB 2 nanowires, we have compared the superconducting transition of the nanowire with that of the MgB 2 microstrip fabricated on the same film without the FIB process, as shown in Fig. 3(b).…”

“…Based on this, the fabrication of MgB 2 nanowires has been further explored by using, for instance, e-beam lithography (EBL) in tandem with Ar ion milling 11 or electron cyclotron resonance etching. 13 It is more encouraging to see that the HEB mixers and SSPDs made from MgB 2 microstrips or nanowires have appeared recently, [18][19][20][21] which demonstrate the prospects of MgB 2 in developing low noise and wide gain bandwidth mixers or high-performance single-photon detectors.…”

“…[10][11][12][13] It is obvious that with such markedly decreased T c of the nanowires, the initial expectation is difficult to be fulfilled to develop MgB 2 -based HEBs or SSPDs with operating temperatures apparently higher than that of NbN-based devices. 18,19 In order to fulfil the expectation and therefore to take full advantage of MgB 2 in making these devices, it is obligatory to obtain ultrathin MgB 2 films and nanowires with T c 's as high as possible, preferably close to its bulk value of 39 K. In this regard, we note that in recent experiments ultrathin MgB 2 films grown by the HPCVD method on various substrates have shown decently high T c 's, with values above 35 K for films in thicknesses around 10 nm. [14][15][16][17] It is therefore desired to know whether one could obtain MgB 2 nanowires from these ultrathin films while maintaining the high T c , which has not yet been explored experimentally.…”