Wu Peiheng

Nb5N6 microbolometer arrays for terahertz detection

A novel room-temperature microbolometer array chip consisting of an Nb5N6 thin film microbridge and a dipole planar antenna, which is used as a terahertz (THz) detector, is described in this paper. Due to the high-temperature coefficient of the resistance of the Nb5N6 thin film, which is as high as −0.7% K−1, such an antenna-coupled microbolometer is ideal for detecting signals in a frequency range from 0.22 THz to 0.33 THz. The dc responsivity, calculated from the measured I–V curve of the Nb5N6 microbolometer, is about −760 V/W at a bias current of 0.19 mA. A typical noise voltage as low as 10 nV/Hz1/2 yields a low noise equivalent power (NEP) of 1.3 × 10−11 W/Hz1/2 at a modulation frequency above 4 kHz, and the best RF responsivity, characterized using an infrared device measuring method, is about 580 V/W, with the corresponding NEP being 1.7 × 10−11 W/Hz1/2. In order to further test the performance of the Nb5N6 microbolometer, we construct a quasi-optical type receiver by attaching it to a hyperhemispherical silicon lens, and the result is that the best responsivity of the receiver is up to 320 V/W. This work could offer another way to develop a large scale focal-plane array in silicon using simple techniques and at low cost.

Noise Behaviour of a THz Superconducting Hot-Electron Bolometer Mixer

A quasi-optical superconducting NbN hot-electron bolometer (HEB) mixer is measured in the frequency range of 0.5-2.5 THz for understanding of the frequency dependence of noise temperature of THz coherent detectors. It has been found that noise temperature increasing with frequency is mainly due to the coupling loss between the quasi-optical planar antenna and the superconducting HEB bridge when taking account of non-uniform distribution of high-frequency current. With the coupling loss corrected, the superconducting HEB mixer demonstrates a noise temperature nearly independent of frequency.

Growth and Characterization of a Kind of Nitrogen-Rich Niobium Nitride for Bolometer Applications at Terahertz Frequencies

Niobium is sputtered onto a single crystalline silicon substrate in N2:Ar = 4:l gas mixture at the total pressure of 2Pa. The temperature coefficient of resistance of the sample is about 0.5% at 300K, and up to 7% at 77K, indicating the possibility of using it to make room-temperature bolometers with performances better than those based on Pt, Bi, or Nb. For a 60-nm-thick sample, the rms surface roughness is 0.45 nm over an area of 2μm × 2 μm. Analyses based on x-ray diffraction and x-ray photoelectronic spectroscopy indicate that the samples are Nb5N6 thin films in which there is a combination of Nb3+ and Nb5+, or Nb4+.

Acid etching process for fabrication of Bi2Sr2CaCu2O8+x stack

We adopted a new method, acid etching process, to fabricate the intrinsic Josephson junctions based on the Bi2Sr2CaCu2O8+x single crystals. By soaking the crystals into the dilute hydrochloric acid, we fabricated a junction stack successfully, and meantime made the surrounding area insulated. A certain concentration of hydrochloric acid was used to maintain the roughness of the modified layer. The current-voltage characteristic was achieved through the four terminal measurement. We could control the junctions’ number by changing the concentration and the soaking time. We also found that the thickness of the stack was equal to the average height of the insulation layer. Such a simple, convenient and controllable fabrication method with a high yield might widen the applications of the intrinsic Josephson junctions.

Extraction of material parameters of a bi-layer structure using Terahertz time-domain spectroscopy

For a bi-layer structure consisting of a film deposited on a substrate, a new extraction method is proposed using which we can extract both the material parameters of the film and the thickness of the substrate from the measured Terahertz transmission through it, so long as the complex refractive index of the substrate is known beforehand. This method is applicable to a range of refractive indices of the film less than the refractive index of the substrate and a very wide range of layer thicknesses from 20 μm to at least 200 μm. It is very useful in some cases where the thickness of the substrate cannot be determined using conventional methods such as a Vernier caliper or micrometer screw.

One-dimensional chain structure based on bifurcated hydrogen bonding

The structure of complex [PyH][Ni(mnt)2], as determined by X-ray single crystal analysis, consists of pyridinium cations and [Ni(mnt)2]− anions. Crystallographic data for the complex: monoclinic P21/c, a = 16.5430(18) Å, b = 6.3847(9) Å, c = 17.430(2) Å, β = 118.040(7)°, V = 1625.4(3) Å3, Z = 4. The [Ni(mnt)2]− anion is not a planar and the CN groups bend away from the S4 plane. The interactions between anions and cations via the formation of bifurcated N–H···N≡C hydrogen bonds give rise to an 1D zigzag chain of alternating cations and anions.

Tuning an rf-SQUID flux qubit system's potential with magnetic flux bias

At an extremely low temperature of 20 mK, we measured the loop current in a tunable rf superconducting quantum interference device (SQUID) with a dc-SQUID. By adjusting the magnetic flux applied to the rf-SQUID loop (Φf) and the small dc-SQUID (Φfcjj), respectively, the potential shape of the system can be fully controlled in situ. Variation in the transition step and overlap size in the switching current with a barrier flux bias are analyzed, from which we can obtain some relevant device parameters and build a model to explain the experimental phenomenon.

Dielectric Properties of GaN in THz Frequencies

The complex refractive indices and the dielectric function of GaN for frequencies ranging from 0.25 to 1.22 THz are obtained using THz time-domain spectroscopy. The real part of the dielectric function first decreases from 0.25 to 0.42 THz and then oscillates from 0.42 to 1.22 THz, whereas the imaginary part of the dielectric function is oscillating within the whole range of frequency. The simple Drude model is extended to take into account the effect of defects on the dielectric function. The extended model is in agreement with the experimental data.

Matching effect in superconducting NbN thin film with a square lattice of holes

A square hole array is fabricated over a micro-bridge of NbN thin film by electron beam lithography and reactive ion milling. Magneto-resistance is measured across the micro-bridge filled with a hole array near the superconducting transition temperature. It is found that magneto-resistance minima occur when the number of vortices is an integer multiple or a fractional multiple of the number of holes. The temperature and the current dependences of the matching effect are studied.

Barrier Formation on a YBa2Cu3Oy Thin Film Using CF4 Plasma Fluorination

We investigate the surface structure and composition of a YBa2Cu3Oy (YBCO) thin film modified by CF4 plasma fluorination. In addition to the absorption of hydrocarbons, chemical reactions of the YBCO surface take place during CF4 plasma treatment. Various x-ray photoelectron spectroscopic data are reported and discussed. The existence of a thin barrier is confirmed, which homogeneously covers the edge of the base YBCO film in our interface engineering Josephson junction. Measurements of Auger electron spectroscopic data and the resistance versus temperature indicate that the barrier is a controllable-insulating layer.