E. J. Haugland

Critical currents in A-15 structure Nb3Al converted from cold-worked bcc structure

Nb3Al prepared in the ductile bcc phase by quenching, followed by mechanical working, followed by conversion to the A‐15 structure is found to carry currents above 109 A/m2 in fields near 20 T. These critical currents are comparable to those found in Nb3Ge and V3Ga, the closest competing materials for use in high fields. Further enhancement of Jc is possible if thermal treatments are optimized.

Transport properties of epitaxial lift-off films

Transport properties of epitaxially lifted‐off (ELO) films were characterized using conductivity, Hall, and Shubnikov–de Haas measurements. A 10%–15% increase in the two‐dimensional electron gas concentration was observed in these films as compared with adjacent conventional samples. We believe this result to be caused by a backgating effect produced by a charge buildup at the interface of the ELO film and the quartz substrate. This increase results in a significant decrease in the quantum lifetime in the ELO samples, by 17%–30%, but without a degradation in carrier mobility. Under persistent photoconductivity, only one subband was populated in the conventional structure, while in the ELO films the population of the second subband was clearly visible. However, the increase of the second subband concentration with increasing excitation is substantially smaller than anticipated due to screening of the backgating effect.

Room‐temperature determination of two‐dimensional electron gas concentration and mobility in heterostructures

A technique for determination of room‐temperature two‐dimensional electron gas (2DEG) concentration and mobility in heterostructures is presented. Using simultaneous fits of the longitudinal and transverse voltages as a function of applied magnetic field, we were able to separate the parameters associated with the 2DEG from those of the parallel layer. Comparison with the Shubnikov–de Haas data derived from measurements at liquid helium temperatures proves that the analysis of the room‐temperature data provides an excellent estimate of the 2DEG concentration. In addition we were able to obtain for the first time the room‐temperature mobility of the 2DEG, an important parameter to device application. Both results are significantly different from those derived from conventional Hall analysis.

Enhancement of Shubnikov-de Haas Oscillations by Carrier Modulation

A drastic enhancement of the Shubnikov–de Haas (SdH) pattern is obtained by recording the changes in the quantum oscillations of magnetoresistance due to modulation of the carrier concentration. The technique enables measurement of the SdH waveform at relatively high temperatures and in samples with moderate mobilities. The modulated waveform shows selective enhancement of the low‐frequency SdH oscillations associated with the upper subband. Thus, we were able to record very clear oscillations generated by a carrier concentration well below 5×1010 cm−2. The theory for this selective enhancement is provided.

Electronic properties of carbon fibers intercalated with copper chloride

Copper chloride intercalated pitch-based carbon fibers are found to have electrical resistivities as low as 12.9 micro-ohm-cm, and are air, and thermally stable at and above room temperature. This is therefore a good candidate system for conductor application. In addition, we find Shubnikov-deHaas quantum oscillatory effects, and electronic properties of the intercalated fiber are studied using magnetic fields to 20 tesla.

Characterization of the transport properties of channel delta‐doped structures by light‐modulated Shubnikov–de Haas measurements

The transport properties of channel delta‐doped quantum‐well structures were characterized by conventional Hall effect and light‐modulated Shubnikov–de Haas (SdH) effect measurements. The large number of carriers that become available due to the delta doping of the channel, leads to an apparent degeneracy in the well. As a result of this degeneracy, the carrier mobility remains constant as a function of temperature from 300 down to 1.4 K. The large amount of impurity scattering, associated with the overlap of the charge carriers and the dopants, resulted in low carrier mobilities and restricted the observation of the oscillatory magnetoresistance used to characterize the two‐dimensional electron gas by conventional SdH measurements. By light modulating the carriers, we were able to observe the SdH oscillation at low magnetic fields, below 1.4 T, and derive a value for the quantum scattering time. Our results for the ratio of the transport and quantum scattering times are lower than those previously measur...

Subband Quantum Scattering Times for Algaas/GaAs Obtained Using Digital Filtering

In this study we investigate both the transport and quantum scattering times as a function of the carrier concentration for a modulation doped Al0.3Ga0.7As/GaAs structure. Carriers in the well are generated as a result of the persistent photoconductivity effect. When more than one subband becomes populated, digital filtering is used to separate the components for each of the excited subbands. We find that the quantum scattering time for the ground subband increases initially as the carrier concentration is increased. However, once the second subband becomes populated, the ground subband scattering time begins to decrease. The quantum scattering time for the excited subband is also observed to decrease as the concentration is increased. Our results are consistent with the theoretical results by A. Isihara and L. Smrcka [J. Phys. C 19, 6777 (1986)]. Finally, from the ratio of the transport and quantum scattering times, it is seen that the transport in the well becomes more isotropic as the concentration is ...

Hall effect and magnetoresistivity in the ternary molybdenum sulfides

Abstract We have measured the Hall coefficient and magnetoresistance of sputtered films of CuXMo6S8 and PbMo6S8, as well as the magnetoresistance in sintered samples of the same materials. Assuming a single band model, net carrier densities and mean mobilities are determined.

Two carrier analysis of persistent photoconductivity in modulation‐doped structures

A simultaneous fit of Hall and conductivity data gives quantitative results on the carrier concentration and mobility in both the quantum well and the parallel conduction channel. In this study this method was applied to reveal several new findings on the effect of persistent photoconductivity (PPC) on free‐carrier concentrations and mobilities. The increase in the two‐dimensional electron‐gas (2DEG) concentration is significantly smaller than the apparent one derived from single carrier analysis of the Hall coefficient. In the two types of structures investigated, delta doped and continuously doped barrier, the apparent concentration almost doubles following illumination, while analysis reveals an increase of about 20% in the 2DEG. The effect of PPC on mobility depends on the structure. For the sample with a continuously doped barrier the mobility in the quantum well more than doubles. This increase is attributed to the effective screening of the ionized donors by the large electron concentration in the barrier. In the delta doped barrier sample the mobility is reduced by almost a factor of 2. This decrease is probably caused by strong coupling between the two wells, as is demonstrated by self‐consistent analysis.

Mixed Carrier Conduction in Modulation-doped Field Effect Transistors

The contribution of more than one carrier to the conductivity in modulation‐doped field effect transistors (MODFET) affects the resultant mobility and complicates the characterization of these devices. Mixed conduction arises from the population of several subbands in the two‐dimensional electron gas (2DEG), as well as the presence of a parallel path outside the 2DEG. We characterized GaAs/AlGaAs MODFET structures with both delta and continuous doping in the barrier. Based on simultaneous Hall and conductivity analysis we conclude that the parallel conduction is taking place in the AlGaAs barrier, as indicated by the carrier freezeout and activation energy. Thus, simple Hall analysis of these structures may lead to erroneous conclusions, particularly for real‐life device structures. The distribution of the 2D electrons between the various confined subbands depends on the doping profile. While for a continuously doped barrier the Shubnikov–de Haas analysis shows superposition of two frequencies for concent...