Ji Zheng-Ming

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.

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+.

C-Axis Current-voltage Characteristics of Mesa Structures on Bi2Sr2CaCu2O8+δ Single Crystals Fabricated by a Simple Technique Without Photolithography

A simple technique is reported for fabricating the mesa structure on Bi2Sr2CaCu2O8+δ single crystal. In the patterning process, metal masks are used instead of photolithography and argon ion milling is applied to form the small mesa on the Bi2Sr2CaCu2O8+δ crystal surface. Real four-probe transport measurements are made on the mesa structure and typical c-axis current-voltage (I - V) characteristics of the intrinsic Josephson effect have been observed. The superconducting gap parameter can be extracted from the multi-branch structure in the I - V characteristics. Additionally, from the strong hysteresis in the I - V characteristics, the capacitance CJ of the unit intrinsic Josephson junction has been estimated to be 2.3 pF, which is in good agreement with that evaluated from the geometric parameters of the unit junction between the two copper oxide layers.

Injection Efficiency of Spin-Polarized Quasi-particles in Y-Ba-Cu-O Thin Film

The injection efficiency of spin-polarized quasi-particles in a superconducting Y-Ba-Cu-O (YBCO) film is investigated by means of current injection into the Nd0.7Sr0.3MnO3/SrTiO3/YBa2Cu3O7-δ heterostructures. We have successfully prepared six injection junction windows in the YBCO thin film strip, which are all 56 µm wide, but 80 µm, 40 µm, 20 µm, 10 µm, 5 µm, and 2 µm long, respectively. At 16 K, our YBCO thin film strip (80 nm thick) has Jc = 2×105A/cm2. Under the spin-polarized injection current of Iinj = 0.5 mA, with decreasing injection junction length L, injection efficiency η (η≡ΔJc/ΔJinj; ΔJc: the suppression of critical current density due to finite ΔJinj) increases gradually, until L is not more than 20 µm. In that case, η hardly varies with the change of L, reaching up to about 6. We think that these results are related to the spin diffusion length of spin-polarized quasi-particles in the superconducting thin film. In our heterostructures, the YBCO thin film keeps good superconductivity and the injection junction windows are of proper size, therefore, we have obtained relatively high injection efficiency of spin-polarized quasi-particles.

Phase-Locking of a Gunn Oscillator at 3 mm Waveband Using High Temperature Superconducting Josephson Junction Harmonic Mixer

A Gunn oscillator at 3 mm waveband has been phase-locked to a synthesizer local oscillator using a high Tc YBa2Cu5O7-δ Josephson junction operated at 77 K as the harmonic mixer, with harmonic numbers up to 52. The observed spectrum of the phase-locked output at the intermediate frequency (IF) shows clearly that the quality of the phase-locking is very good. To drive the analogue phase locking loop successfully, it is essential to have IF output and signal-to-noise ratio both as high as possible.

Experimental Study of the Plasma Fluorination of Y-Ba-Cu-O Thin Films

We have experimentally studied the surface modifications of Y-Ba-Cu-O (YBCO) thin films using CF4 plasma. The intensity of the plasma fluorination was controlled by changing the biasing voltage and the time of the plasma treatment. Microstructural analyses reveal that the oxygen content of the YBCO thin films was changed. Transport measurements of sufficient fluorinated YBCO films imply that the films changed totally into an oxygen-deficient semi-conducting state. From these experimental results, we believe that plasma fluorination is quite a useful method to form controllable a thin barrier layer in fabricating interface engineered junctions and to form a stable narrow weak-link region in fabricating planar superconductor-normal-superconductor junctions.

Highly Oriented HgBaCaCuO Superconducting Thin Films Prepared by Laser Ablation

Pb-doped HgBa2CaCu2Oy films were prepared on the (100) SrTiO3 substrate by using laser ablation followed by post-Hg-vapor annealing. These films are dominated by c-axis-oriented Hg-1212 phase as indicated by x-ray diffraction and electromagnetic measurements. The transition temperature Tc0 is 110 K. The critical current density Jc achieved 106 A/cm2 at 77 K and zero applied field.