Shaun L. McCarthy

Enhancement of light emission from metal‐insulator‐metal tunnel junctions

Light emission from metal‐insulator‐metal tunnel junctions is enhanced using two schemes for improving photon‐surface plasmo coupling. Results reported here establish that the light emission arises from radiative decay of junction surface plasmon modes excited by inelastic electron tunneling. The enhanced light‐emitting tunnel junctions have an external quantum efficiency in the 10−5 range.

Intermittence detection in fretting corrosion studies of electrical contacts

The occurrence of intermittences in electrical circuits can result in serious reliability problems particularly where low signal voltages and currents are used. It is well known that differential thermal expansion or vibration in electrical connectors can result in micromotion resulting in fretting corrosion. This can lead to an increase in contact resistance and eventual loss of electrical contact. The occurrence of electrical intermittences has been considered a precursor to contact failure associated with fretting corrosion and can cause disruption in digital circuit signals. We report the development of instrumentation that simultaneously measures the occurrence of electrical intermittences along with contact resistance during the fretting of electrical contacts. In addition, the measurement system records contact friction and normal force dynamically. Intermittences can be counted and timed with durations from 20 ns to milliseconds as a function of fretting cycles and correlated with the increase in contact resistance. All systems are integrated under LABVIEW computer control software. Measurements were made on Cu-Cu and Sn-Sn rider/flat combinations. Results will be interpreted in terms of the influence of wear debris on the electrical properties of the contacts.

High cycle fretting corrosion studies on tin-coated contact materials

The high-cycle (above 20,000) fretting contact characteristics of matte tin-plated copper are reported in this paper. Current loads extended from 0.1 mA to 100 mA with an open circuit voltage of 12 V. As observed in the past, there is a plateau in the contact resistance versus fretting cycles above 20,000 cycles. Here, we report on changes in the plateau as a function of load current. Box plots of the median values of the distribution of contact resistances show a dramatic decrease with increasing current. Contact voltages are also seen to range from 1 to 12 V, depending on the current. These results are discussed in terms of changes in the granular interface between the contacts with increasing current.

LEIT effect in metal‐insulator‐semiconductor tunnel junctions

Light emission via inelastic tunneling (LEIT) has been observed from the metal‐insulator‐semiconductor tunnel‐junction Al‐Al2O3‐ Sn‐doped indium oxide (ITO). The spectra exhibit the bias‐voltage‐dependent upper‐frequency quantum cutoff relation. These devices were stable at room temperature for months at a time while biased to 3 V. The external quantum efficiency of this metal‐insulator‐semiconductor system is about 0.1 that of the metal‐insulator‐metal system Al‐Al2O3‐Ag. Enhancement of the light emission using silver particle resonators is also reported.

Thin‐film electroluminescence in impurity‐doped Al2O3

The discovery of a new thin‐film electroluminescence system based on impurity‐doped anodic aluminum oxide is reported. Average brightnesses of 30 f L have been observed for devices at room temperature operating at 1 kHz biased at 60 V rms, resulting in 7 mA electronic current and a 10% duty cycle. A new stabilizing layer of manganese oxide permits long‐term operation without catastrophic breakdown. While electrical stability is achieved, loss in electroluminescence efficiency is apparent, possibly due to electromigration of the impurity ions.

Lubricant-induced corrosion in copper electrical contacts

We report that a high performance switch grease tested by the standard ASTM method for corrosivity to copper produced little visual tarnish. However, when tested in thin layers, as would occur in the application, corrosion layers occur in excess of that seen using the ASTM method. Optical and electrical measurements were used to characterize the layer. Contact resistance measurements with and without wiping action are reported. Infrared absorption spectra of the thin layer of lubricant on copper show changes in the lubricant after thermal aging as compared with aged material not in close proximity with the metal. It will be discussed that for a thin layer of lubricant on copper experiences a more accelerated corrosion.

Contact resistance comparison of good and bad crimp joints with tinned wires under thermal shock

#16 wire gauge crimp joints of good quality were made tinned wires in the conventional way. Some of them then subjected to twisting in gap between the two wings. All of them were heat-soaked at 135/spl deg/C for 300 hours. They were then into a thermal-shock furnace for thermal cycling from -40/spl deg/C to 125/spl deg/C, each cycle lasting one hour for a total of 570hours. The four-leads method was used to measure the contact resistance for every cycle under the dry circuit condition. The contact resistance of good crimps generally increased at a slower pace over time than the bad ones. The resistance of some good and bad crimps would tend to more or less off after 100-200 cycles while others would continue to increase their resistance without leveling off and some of them might show a sudden jump. Nevertheless, the measured resistance history showed that a crimp with a tinned wire exhibited accepted performance whether it was good or not so good a crimp. The results also showed that the temperature history would effect the contact resistance measured at a later time. For example, the resistance of a crimp at 125/spl deg/C would be somehow different depending on weather the previous temperature started from -40/spl deg/C or from -17/spl deg/C. Data analysis showed that even with severe thermal shocks for 570 hours and possible consequence of repeated shearing, the contact in the crimp was still essentially a metallic one.

Thin‐film light emitting display devices

A review article is presented of the author’s work on light emission from electron tunneling junctions. The light emission arises from the radiative decay of junction surface plasmon modes excited by tunneling electrons. The coupling out of the light is assisted by roughness and resonant particle scattering schemes. As a consequence of the quantum nature of the light generation process, there is a fundamental relation between the applied voltage and the observed emission spectrum. Because of the limited tunneling electron‐surface plasmon coupling, these thin‐film light sources have photometric brightnesses of only a few foot‐Lamberts. Other avenues of research for brighter thin‐film light sources are: development of metal electrodes having a lower surface plasmon frequency excitation of electronic transitions.

Light Emission From Inelastic Tunneling — LEIT

It has been shown that inelastic tunneling excitation of surface plasmon modes can result in light emission when electrodes are properly prepared. The light emission establishes a fundamental quantum relationship between the maximum optic frequency and the applied voltage. Such a process can be used as a basis for spectroscopy when modulation-derivative techniques are used. The derivative method is very similar to that used in IETS except it is a second derivative of a photo detection current that is measured. We term this LEIT spectroscopy (light emission by inelastic tunneling). We will discuss the physical processes underlying the LEIT effect as well as possible applications. Some of the key factors of this light source are: 1) the precision of the relationship between voltage and maximum optical frequency; 2) mechanics for coupling out light for excited surface plasmon modes; 3) the optical range over which such sources can be operated.