Isoris S. Gergis

Electron mobility enhancement in epitaxial multilayer Si‐Si1−xGex alloy films on (100) Si

Enhanced Hall‐effect mobilities have been measured in epitaxial (100)‐oriented multilayer n‐type Si/Si1−xGex films grown on single‐crystal Si substrates by chemical vapor deposition. Mobilities from ∼20 to 40% higher than that of epitaxial Si layers and ∼100% higher than that of epitaxial SiGe layers on Si were measured for the doping range ∼8×1015–∼1017 cm−3. The mobilities of multilayer Si/SiGe films approach that of single‐crystal Si films at ∼2×1017 cm−3. No mobility enhancement was observed in multilayer p‐type (100) films and n‐type (111)‐oriented films. Experimental studies included the effects upon film properties of layer composition, total film thickness, doping concentrations, layer thicknesses, and growth temperature.

Reproducible growth of high quality YBa2Cu3O7−x film on (100) MgO with a SrTiO3 buffer layer by pulsed laser deposition

Epitaxial YBa2Cu3O7−x(YBCO) films were grown on SrTiO3‐buffered (100) MgO substrates by pulsed laser deposition. The SrTiO3 layer of about 500 A thick serves a dual purpose to provide a better lattice match to the YBCO films as well as a diffusion barrier to prevent interaction between YBCO and MgO. These YBCO films exhibit excellent structural, electrical, and microwave properties with Tc≳89 K and Tc=2.9×106 A/cm2 at 77 K (1×106A/cm2 at 84 K). X‐ray pole figure measurements showed that over 99.92% of the volume fraction of the c‐axis oriented film has the a‐b axis aligned parallel to the [010] and [001] direction of the substrate.

Reactive ion beam deposition of thin films in the bismuth‐calcium‐strontium‐copper oxide ceramic superconductor system

Reactive ion beam deposition has been used to grow c‐axis‐oriented superconducting thin films in the Bi‐Ca‐Sr‐Cu‐O (BCSCO) system around the cation ratio 1:1:1:1 on single‐crystal (001) MgO. The films show a single superconducting transition with an initial onset near 85 K and a critical current of 5×104 A/cm2 at 10 K. Two different BCSCO‐containing phases have been identified in the thin films: one with a tetragonal pseudo‐body‐centered subcell, c=24.4 A, which is not superconducting above 28 K, and a second with c=30.6 A, which is responsible for the superconductivity. Electron diffraction measurements on the 30.6 A phase are consistent with those previously reported for the bulk ceramic.

55 GHz microstrip bandpass filter made from YBa2Cu3O7−x superconductor thin film

We report the first mm wave passive component made with high temperature superconducting thin films. Microstrip bandpass filters, with bandwidth of 2.2 GHz and a center frequency of 55 GHz, were made using YBa2Cu3O7−x filter patterns on MgO substrates with Au ground planes. The best results were obtained for a film grown with a thin SrTiO3 buffer layer and had Tc=89 K and Jc=106 A/cm2 at 84 K. The filter showed insertion loss of 1.7 db at 77 K, and 1.5 db at 70 K, compared to 7.3 db for Au filters at 77 K. Subtracting the losses in the Au ground plane, we estimated that the superconductor surface resistance is lower than one fourth that of Au at 77 K and 55 GHz.

Planar weak-link devices from YBaCuO and BiSrCaCuO films

Weak‐link devices were fabricated from polycrystalline thin films of YBaCuO and BiSrCaCuO. Two distinct types of weak links have been found in these devices. The first type, found in all of the YBaCuO and most of the BiSrCaCuO devices, has relatively low dynamic resistances and critical currents with nearly linear temperature dependences. The second type of weak links, found in some BiSrCaCuO films, has much larger dynamic resistances and critical current temperature dependences similar to that of a tunnel junction. Superconductivity quantum interference devices (SQUID’s) fabricated from BiSrCaCuO showed a periodic response to applied fields that is consistent with the magnitude of the flux enclosed by the SQUID loop. Significant hysteresis was found in the SQUID response, apparently indicating flux penetration into the films at relatively low fields.

Gap tolerant bubble propagation circuit

A new bubble propagation pattern for field access devices has been developed which has a period to gap ratio of 8:1. In this pattern of semicircular elements (half-disk) the gaps are situated between essentially parallel poles in contrast to the TI pattern where gaps are located between orthogonal poles. The bubble, therefore, comes under the influence of two strong parallel poles causing it to stretch across the gap. The energy barrier that would normally be encountered thus virtually disppears. Devices of 32, 18, and 10 μm periods have been designed and fabricated. The results show a typical margin of at least 20% of the bias field under normal operating conditions. The operating drive field is relatively low for small bubbles. The minimum drive field for a 10 μm period pattern is only 13 Oe.

The properties of Si/Si1-xGex films grown on Si substrates by chemical vapor deposition

A growth parameter study was made to determine the proper of a SiGe superlattice-type configuration grown on Si substrates by chemical vapor deposition (CVD). The study included such variables as growth temperature, layer composition, layer thickness, total film thickness, doping concentrations, and film orientation. Si and SiGe layers were grown using SiH4 as the Si source and GeH4 as the Ge source. When intentional doping was desired, diluted diborane for p-type films and phosphine for n-type films were used. The study led to films grown at ∼1000°C with mobilities from ∼20 to 40 percent higher than that of epitaxial Si layers and ∼100 percent higher than that of epitaxial SiGe layers grown on (100) Si in the same deposition system for net carrier concentrations of ∼8x1015 cm-3 to ∼2x1017 cm-3. Enhanced mobilities were found in multilayer (100)-oriented Si/Si1-xGex films for layer thicknesses ≥400A, for film thicknesses >2μm, and for layers with x = 0.15. No enhanced mobility was found for (111)-oriented films and for B-doped multilayered (100)-orlented films.

High-performance 5-um 640 x 480 HgCdTe-on-sapphire focal plane arrays

A high-performance 5-μm 640 X 480 HgCdTe/CdTe/Al2O3 infrared focal plane array (FPA) that offers full TV-compatible resolution with excellent sensitivity at temperatures below 120 K has been developed. Mean FPA D* at 95 K and background of 1014 photons/cm2 s is background-limited at ~1 x 1012 cm Hz1/2/W for the typical mean quantum efficiency of 60 to 70%. The key technology making this large, high-sensitivity device producible is the epitaxial growth of HgCdTe on a rugged CdTe-buffered sapphire substrate. Mean camera noise-equivalent temperature difference NEΔT of 13 mK has been achieved at ≤ 120 K operating temperature and 3.4- to 4.2-μm passband; this is about an order of magnitude better than similar currently available cameras, which use PtSi FPAs and require cooling to ≤ 77 K to maintain performance at low scene temperatures.

64K fast access chip design

A 65,664 bit, 4 μm, minor loop chip of 128 loops of 513 bits is described. The goals of fast read and write access, and high average data rates are shown to lead to a decision to use a replicate switch of compact design to minimize loop length and data housekeeping. Operation of the device is described, followed by an example of a read and write of a single block of data and examples of read/write of multiple blocks. Use of modulo 128 and modulo 151 block address assignments illustrate means of obtaining either gapless and gapped record delivery.

The effect of DC in plane field on the operation of field access bubble memory devices

The effect of a small dc in-plane field on the start-stop operation of field access bubble devices has been studied. Experimental results show that the bias margin in this mode is very sensitive to the magnitude of the field and its orientation relative to the start-stop direction of the drive field. In a T-I circuit a complete margin loss was observed for an in-plane field of 3 Oe oriented antiparallel to the start-stop direction. For parallel orientations of the in-plane field the start-stop margin improved and approached that of the continuous propagation margin at an in-plane field of approximately 6 Oe. Dependence of the start-stop margin on the orientation of the start-stop direction relative to the pattern was also observed. Measurements of the bubble collapse field at various points in the pattern show a very strong dependence on the in-plane field and the permalloy geometry. The collapse-field results and magnetostatic energy considerations which take into account local field variations and bubble-bubble interactions provide a basis for understanding the experimentally observed start-stop margins. These results show that a small tilt (2 to 3°) should be introduced in the bias field to overcome normal alignment tolerances and ensure that a favorable in-plane field is always present. This assures reliable start-stop operation.