Juergen Schubert

Fabrication and Characterization of Enhancement-Mode High-

In this paper, we investigate the enhancement-mode (E-mode) LaLuO3 (LLO)-AlGaN/GaN metal-insulator-semiconductor high-electron mobility transistors (MIS-HEMTs) fabricated using fluorine (F) plasma ion implantation with a gate-dielectric-first planar process. The E-mode MIS-HEMTs exhibit a threshold voltage (VTH) of 0.6 V, a peak transconductance of ~ 193 mS/mm, a small hysteresis of 0.04 V in linear region characterized by a pulse-mode current-voltage measurement, and significantly suppressed current collapse under high-drain-bias switching conditions. X-ray photoelectron spectroscopy and secondary ion mass spectrometry analyses manifest that the negatively charged F ions penetrating into the (Al)GaN barrier layer serve as the primary VTH modulation mechanism, whereas the F ions in the fluorinated LLO film form chemical bonds with La/Lu atoms and become charge-neutral. The suppressed current collapse is verified as an advantageous byproduct of the F plasma ion implantation that also fluorinated the SiNx sidewalls in the vicinity of the gate electrode, and therefore, suppress electron injection to the gate-drain access region.

\kappa~{\rm LaLuO}_{3}

The effect of the superconductivity transition on the thermal crosstalk in YBa2Cu3O7-x edge-transition bolometer arrays is investigated for DC to midrange modulation frequency infrared radiation. The bolometers in the arrays were designed with various distances on SrTiO3 (100) substrates. We have observed a change in the thermal crosstalk between neighbor devices through the superconductivity transition temperature range. Superconductivity transition dependence of the thermal coupling between the devices was measured by utilizing the thermal conductance measurement methods developed for the bulk material. The knee points in the magnitude of the response versus modulation frequency curves of the devices were also determined by illuminating one of the bolometers in the arrays and measuring the response of the neighbor devices. By using the knee frequency and the distance between the bolometers, the modulation frequency criterion for crosstalk-free response at the transition region in various array structures is found and the results of the thermal coupling measurements are further studied and presented here

-AlGaN/GaN MIS-HEMTs

We have investigated the dependence of the optimal non destructive evaluation (NDE) characterization on different excitation parameters in an eddy current SQUID NDE system for samples with known flaw depths. The considered parameters in our study include the configuration of the excitation-coil and its current frequency. The system is based on a high-TC YBCO gradiometer RF-SQUID sensor with a flux noise level below 100 muPhi0/radicHz at 100 Hz in an unshielded environment, while being shielded against external RF EMI (electromagnetic interference). According to experimental results and the associated numerical analysis using finite element (FEM) simulations, we have derived the optimized parameters for the maximum sensitivity of the system. The optimized parameters are found using a new model for excitation coil and eddy current anomalies caused by the flaws. Very good agreement between experimental and numerical approaches confirmed our model and the resultant implemented optimization method.

Superconductivity transition dependence of the thermal crosstalk in YBa/sub 2/Cu/sub 3/O/sub 7-x/ edge-transition bolometer arrays

Step edge grain boundary (GB) junctions and rf-SQUIDs have been made using pulsed laser deposited Y-Ba-Cu-O films on crystalline LaAlO/sub 3/ substrates. The steps were developed using various ion-beam etching processes resulting in sharp and ramp type step structures. Sharp step based GB junctions showed behavior of serial junctions with resistively shunted junction (RSJ)-like I-V characteristics. The ramped type step structures resulted in relatively high critical current, I/sub c/, junctions and noisy SQUIDs. The sharp steps resulted in low noise rf-SQUIDs with a noise level below 140 fT/Hz/sup 1/2/ down to few Hz at 77 K while measured with conventional tank circuits. The I/sub c/ of the junctions and hence the operating temperature range of the SQUIDs made using sharp steps was controlled by both the step height and the junction widths. The junction properties of the SQUIDs were also characterized showing RSJ-like characteristics and magnetic field sensitivities correlated to that of the SQUIDs. Two major low and high background magnetic field sensitivities have been observed for our step edge junctions and the SQUIDs made on sharp steps. High quality step edge junctions with low magnetic field sensitivities made on clean sharp steps resulted in low 1/f noise rf-SQUIDs proper for applications in unshielded environment.

Optimization of NDE Characterization Parameters for a RF-SQUID Based System Using FEM Analysis

The application of sensitive layers for chemical microsensors consisting of multicomponent compositions and dielectric materials requires specific deposition techniques, since the different chemical and physical properties of the respective components can be significantly disturbed during the deposition process. To avoid this drawback, the pulsed laser deposition technique is suggested as a novel thin film preparation method for such sensor devices.

Junction characteristics and magnetic field dependencies of low noise step edge junction rf-SQUIDs for unshielded applications

Incorporating an analytical approach to simulate the interaction of a series of long cracks and the induced current of a double-D excitation coil, we have developed a model-based method to do precise detection of the positions of the cracks in a metallic structure by using eddy-current superconducting quantum interference device (SQUID) nondestructive evaluation (NDE) measurements. Conventionally, the structure of the defects is found by iteratively solving a numerical forward problem, which is usually based on finite-element, boundary-element, or volume-integral method. This, however, incurs a heavy numerical burden, as every time the forward problem is to be solved, a rigorous numerical model should be inevitably employed to extract the complex distribution pattern of the induced current encountering defects of the structure. In this paper, an analytical approach is used for the modeling of the interaction of the induced current and a series of cracks in the sample. It duly considers the distribution of the induced current in the flawed samples, does not call for extremely high computational resource, and thus permits efficient NDE as the forward problem can be solved within a reasonable time. Here, a high-Tc first-order radio-frequency SQUID gradiometer is employed as the magnetic sensor of the NDE system to scan the samples with different cracks. The accuracy of the proposed algorithm is verified by having the extracted shape of the defects obtained by applying the proposed algorithm on the SQUID NDE measurements against the actual cracks.

Pulsed-laser deposition as a novel preparation technique for chemical microsensors

Detectivity optimization of large-area freestanding-type YBCO superconducting bolometric detectors is theoretically and experimentally investigated. The effect of the device thermal parameters on the theoretical total noise-equivalent power based on background radiation, thermal fluctuation, and Johnson noise is considered. By analytical optimization of the total noise-equivalent power (NEP) of the device with respect to the thermal parameters and radiation modulation frequency, the maximum sensitivity and optimal thermal parameters of the bolometric detector are obtained. Relating device thermal parameters to the special case of the large substrate dimensions, the optimum device geometry is calculated. In addition, several devices with different geometries are fabricated to experimentally test the analysis. By performing experimental optical responsivity and detectivity measurements for these devices, the effect of the geometrical parameters on the maximum sensitivity of bolometric detectors is also empirically obtained. The analytical and experimental results are compared and presented in this paper.

Analytical Model for the Extraction of Flaw-Induced Current Interactions for SQUID NDE

Incorporating an efficient approach for the finite-element simulation of eddy current superconductive quantum interface device (SQUID) nondestructive evaluation (NDE) systems, an appropriate finite-element method (FEM) has been presented for simulating and analyzing such systems. We have introduced a new model for the planar double-D coils, which are used as the excitation source in eddy current SQUID NDE systems, and also another model for the description of the flaw effect on the induced current. We have also examined our simulation results with their associated measurements. Our system is based on a high-TC YBCO gradiometer RF-SQUID sensor with a flux noise level below 100 ¿¿0/ ¿Hz at 100 Hz in an unshielded environment while being shielded against external RF electromagnetic interference. The very good agreement between experimental and numerical approaches confirmed our model for the 3-D FEM simulation of the system, which is being done in reasonable time and using reasonable computer resources.

Detectivity Analysis and Optimization of Large-Area Freestanding-Type HTS Bolometers

Bolometric response and noise characteristics of YBCO superconductor transition edge IR detectors with relatively sharp transition and its resulting detectivity are investigated both theoretically and experimentally. The magnitude of response of a fabricated device was obtained for different bias currents and modulation frequencies. Using the measured and calculated bolometric response and noise characteristics, we found and analyzed the device detectivity versus frequency for different bias currents. The detectivity versus chopping frequency of the device did not decrease following the response strongly, due to the decrease of the noise at higher frequencies up to 1 kHz, resulting in maximum detectivity around the modulation frequency of 100 Hz. We also improved the responsivity of the device through the increase of the surface absorption by using a novel infrared absorber, which is made of a copper-carbon composite, coated in a low-temperature process. Within the modulation frequency range studied in this paper, comparison of device detectivity before and after coating is also presented.

An Efficient Finite-Element Approach for the Modeling of Planar Double-D Excitation Coils and Flaws in SQUID NDE Systems

We have investigated the rf SQUID (radio-frequency superconducting quantum interference device) and its coupling to tank circuit configurations to achieve an optimal front-end assembly for sensitive and high spatial resolution magnetic imaging systems. The investigation of the YBCO rf SQUID coupling to the conventional LC tank circuits revealed that coupling from the back of the SQUID substrate enhances the SQUID signal while facilitating the front-end assembly configuration. The optimal thickness of the substrate material between the SQUID and the tank circuit is 0.4 mm for LaAlO3 resulting in an increase of the SQUID flux–voltage transfer function signal, Vspp ,o f 1.5times, and 0.5 mm for SrTiO3 with an increase of Vspp of 1.62 times compared to that for direct face to face couplings. For rf coupling with a coplanar resonator, it has been found that the best configuration, in which a resonator is sandwiched between the SQUID substrate and the resonator substrate, provides a Vspp about 3.4 times higher than that for the worse case where the resonator and the SQUID are coupled back to back. The use of a resonator leads to a limitation of the achievable spatial resolution due to its flux focusing characteristics. This resulted in a favouring of the use of the conventional tank circuits when considering the desired high spatial resolution. The effect of the YBCO flip chip magnetic shielding of the SQUIDs in the back-coupling with the LC tank circuit configuration has also been investigated, with a view to reducing the SQUID effective area to increase the spatial resolution and also for studying the effect of the coupling of various kinds of transformers to the SQUIDs. It is revealed that there is no very considerable change in the flux–voltage transfer function signal level with respect to the effective shield area, while the lowest working temperature of the SQUIDs was slightly shifted higher by a couple of degrees, depending on the shield area. (Some figures in this article are in colour only in the electronic version)