William H. Hartwig

Use of Superconducting Cavities to Resolve Carrier Trapping Effects in CdS

The excellent frequency stability and cryogenic environment of a superconducting resonant cavity provides a sensitive method for observing trap‐filling in CdS and similar materials. When used with thermally stimulated conductivity and dc photoconductivity, it is possible to solve for trap energy, capture cross section, density of trap states, and free‐carrier lifetime. The technique is that used by Arndt, Hartwig, and Stone to observe optically induced changes in the complex dielectric constant by inertia forces on free carriers in Si and other indirect‐gap semiconductors. Using TSC, pure CdS crystals showed very weak trapping effects and CdS: Al displayed electron traps at 0.52, 0.35, and a group at 0.265, 0.20, and 0.15 eV. A quenching, or hole trapping, level was seen at 1.6 eV below the conduction band. Deep electron trap densities were about 1015 cm−3 and the shallow set was about 1017 cm−3 each. Hole trap density was slightly in excess of 1018 cm−3. In CdS: Al, the photodielectric frequency shift of...

Performance of Superconducting Oscillators and Filters

With improved understanding of loss mechanisms in superconducting resonant circuits (SRC), it is possible to design a variety of devices which take advantage of the high Q. Superconducting surface resistance, trapped‐flux loss, and residual losses which are annealable can all be related to the superconductor itself. Dielectric dissipation is well enough understood to permit this source of loss to be minimized. Coupling and radiation losses can be made negligible. SRCs are described with a variety of geometries to fill the range from 10 MHz to 10 GHz. Type‐I superconductors provide the highest Qs and are readily fabricated by several techniques. The superconducting resonator can be tuned accurately in frequency by temperature control of the surface reactance as well as the dielectric constant of liquid helium. An external magnetic field can be used to tune the bandwidth as well as the resonant frequency. A quarter‐wave reentrant cavity is ideally suited for use as an optically tuned frequency‐control ele...

PHOTODIELECTRIC DETECTOR USING A SUPERCONDUCTING CAVITY.

Photoinduced free‐carrier processes in Si and Ge have been studied in superconducting cavities to exploit their excellent frequency stability and negligible dissipation. Using the narrow bandwidth and very high unloaded Q, it is possible to detect changes in the complex dielectric constant by observing the shift in resonant frequency. Simultaneous measurement of relative power absorption gives additional and confirming data. An increase in frequency of several kHz/mW and a linear power absorption is observed for low values of light. At higher intensity both responses become nonlinear due to the growth of the plasma frequency and nonuniform carrier density distribution. For a 525 Ω·cm sample of n‐type Si in a cavity with resonances at 290 and 810 MHz, data analysis showed the thermal carrier density at 4.2°K was 109 cm−3, relaxation time was 1.44×10−10 sec, the Fermi level was 0.00305 eV above the donor level, and the unilluminated plasma frequency was 148 MHz. The product of conversion efficiency and free...

Radio-frequency losses in the superconducting penetration depth

Trapped flux has been shown to be responsible for a large part of the residual ac losses in both types I and II superconductors. The authors have made theoretical and experimental investigations of losses in the 40–400 MHz range to establish a more detailed low‐field model. The surface resistance rs, as given by Pippard, is exceedingly small for Sn, Pb, and other pure metals below about 0.95 Tc at these frequencies. As a consequence, trapped‐flux effects provide the dominant losses in rolled‐foil resonant circuits. The theoretical model is simply Ohmic losses in the normal regions of trapped fluxoids bounded by the penetration depth and the surface. This gives an added resistance rh=rh(0)V(t), where the temperature function is V(t) = (1−t4)−1/2(1−t2)−1. The magnitude of the loss is predicted to be proportional to the density of fluxoids trapped, which in turn is assumed proportional to the background magnetic field for low fields. The experimental technique consisted of pulse determinations of circuit Q i...

Material‐Properties Analyzers Using Superconducting Resonators

A low‐temperature material‐properties analyzer, using a superconducting microwave resonant cavity, is discussed. Placing semiconductor or dielectric material samples in the cavity perturbs the resonant frequency, absorbed microwave power, and cavity Q. Additional perturbations occur when the sample complex dielectric constant is altered by a thermal, nuclear radiation, or optical stimulus. In samples such as Si, GaAs, CdS, and CdTe, these perturbations have been used to determine such material properties as relaxation time, lifetime, Fermi level, trap ionization energy, trap density, capture cross section, free‐carrier density, and trap population. A contactless experimental technique similar to the thermally stimulated conductivity experiment is proposed. The contactless ac measurement system is shown to be sensitive, accurate, useful with randomly shaped or powdered samples, and applicable to many types of insulators and semiconductors.

Dielectric Dissipation in NaCl and KCl below 4.2°K

Dielectric loss tangents for NaCl and KCl single crystals are presented for the frequency range 40–1000 Mc/sec from 4.2° to 1.8°K. The tan δ‐frequency‐reciprocal temperature surface shows considerable structure with loss tangents in the range of 2–9×10−5. Debye relaxation associated with crystal defects appears to be the dominant mechanism, with individual peaks becoming resolved above the background caused by peaks at other frequencies.A NaCl sample shows a loss peak near 300 Mc/sec at 4.2°K, with its high‐frequency shoulder distorted by an adjacent peak. At lower temperatures the peaks move closer and to lower frequencies. The activation energy is estimated to be 0.0011 eV and occurs at approximately one lattice site in 5×1010. The loss mechanism suggested is electron relaxation between neighboring multivalent impurities. The loss would be indistinguishable at room temperature, but is calculated to occur at 4.6 Gc/sec with a peak loss tangent of 3.2×10−7.

Automatic tuning of a superconducting cavity using optical feedback.

The resonant frequency of a superconducting cavity has been photodielectrically controlled by a feedback loop which contains an optical path. The cavity is an 870‐MHz lead‐plated quarter‐wave reentrant structure. A 19 000 Ω·cm silicon wafer terminates the quarter‐wave stub. The loaded Q is approximately 105 indicating the losses in the semiconductor dominate the superconducting surface resistance. The high Q provides a large phase error between the input and output voltage when the driving oscillator drifts or is modulated away from the very stable resonant frequency of the cavity. The error is processed by a wide‐bandwidth phase detector that drives a gallium arsenide diode to follow the frequency excursion of the oscillator by photodielectric tuning. The 9000 A light is the optical feedback signal which corrects the phase error. The system response typically provides a frequency deviation of ±50 kHz at a 0.2 MHz rate. Higher cavity frequency and purer semiconductor samples extend the deviation limits an...

Characteristics of Photodielectric Optical Detectors Using Superconducting Cavities

Superconducting cavities loaded with various semiconductor crystals form a unique class of digital optical transducers. There are several low‐temperature photodielectric effects in semiconductors. They are used to tune superconducting cavity resonators and have been demonstrated as laser detectors and for optical feedback control of the resonant frequency. The free‐carrier PDE is due to inertia forces on mobile carriers and the trapped carrier PDE is due to polarizability changes on entering a trap. Recent work by Hartwig, Hinds, and Khambaty (unpublished) shows most semiconductors display mixtures of the two effects. The present state of knowledge supports a preliminary design rationale for radiation detectors. This paper establishes the link between the several photodielectric mechanisms, the properties of the superconducting cavity, and accepted standards of qualifying a radiation detector.