Electron Spin Resonance (ESR), temperature dependent Hall effect measurements and photoluminesence (PL) are used to examine the assumption that the residual donor in β-SiC films is nitrogen. At low temperatures the ESR has a three line isotropic spectrum which is characteristic of a central hyperfine interaction with nitrogen. The temperature dependence of the intensity of the nitrogen ESR signal correlates with the concentration of un-ionized donors measured by the Hall effect. Donor-Acceptor pair PL spectra are used to establish that the binding energy of the dominant donor in the films is the same as the nitrogen donor observed in Lely-grown samples. Neither PL nor ESR provide any evidence for the presence of a shallower donor.

The conversion of semi-insulating GaAs to p type as a result of heat treatment in H2 was studied by photoluminescence (PL), secondary-ion mass spectrometry (SIMS), and transport measurements. The SIMS measurements resulted in the direct chemical identification of Mn near the heated surface. The correlation of the SIMS profiles with the results of PL and transport measurements indicated that Mn acceptors are responsible for the type conversion, and that substantial concentrations of Mn(?1017/cm3) are found in thin (1–3 μm) layers near the surface. The results of studies of samples heated under several different conditions showed that the Mn layers were not introduced by contamination from external sources during heat treatment, but were probably due to the presence of a bulk Mn concentration (<3×1015/cm3): during heat treatment the Mn diffuses to the surface, probably assisted by the in diffusion of Ga vacancies, as suggested by Zucca. Photoluminescence profiling measurements and the correlation between the SIMS and PL results indicate that the 1.41-eV PL band that is associated with the type-converted surface is due to recombination at a Mn acceptor on a Ga site, and not at a next-nearest-neighbor arsenic vacancy-amphoteric acceptor complex.

The Hall coefficient ($R$) for homogeneous potassium single crystals was measured at 4.2\ifmmode^\circ\else\textdegree\fi{}K in magnetic fields ($H$) up to 85 kG. Above 15 kG, $R$ approached $H$-independent values in excellent agreement with free-electron calculations. Thus, the Fermi surface must be simply connected. Below 15 kG, $R$ exhibited weak $H$ dependence stemming almost unambiguously from scattering anisotropy (due predominantly to phonons). At low $H$, $R$ is compared with pseudopotential calculations. Correlations between $R$ and magnetoresistance are identified for either scattering or inhomogeneity phenomena.

A new model is proposed in which the observed magnetoresistance curves for potassium are qualitatively analyzed as a synthesis of four superposed behaviors: (1) a phonon saturating behavior exhibiting the predicted temperature dependence for umklapp scattering, (2) a new residual saturating behavior due to impurities and defects, (3) a negative saturating behavior agreeing quantitatively with size-effect theory, and (4) a newly identified low-field quadratic behavior which becomes asymptotically linear at high fields due to macroscopic inhomogeneities.

The theoretically predicted and long-sought intrinsic saturation of the transverse magnetoresistance of the alkali metals has finally been observed after sufficiently reducing the masking effect of the large superimposed linear magnetoresistance which is attributed to such classical considerations as geometry, Hall angle and non-uniform current distribution. The observed saturation values were about 100 times smaller than those observed previously in complicated metals but were still much too large to be explained only by Fermi surface anisotropy. The saturation phenomena contradict Kohler's rule but can be plausibly explained by mean free path anisotropy due predominantly to electron-phonon scattering mechanisms.

Received 25 September 1967DOI:https://doi.org/10.1103/PhysRevLett.19.1428©1967 American Physical Society

Conduction band structure in n-type strontium titanate investigated by measuring oscillatory magnetoresistance in high magnetic fields

The magnetoresistance of semiconducting SrTi${\mathrm{O}}_{3}$ has been investigated in high magnetic fields (up to 150 kOe). In the temperature range 1.4-2.1 \ifmmode^\circ\else\textdegree\fi{}K, for fields of more than 50 kOe, well-developed Shubnikov-de Haas-type oscillations have been observed. The data support a conduction band consisting of spheroids along the $〈100〉$ crystalline axes, having 3 minima at the points ${X}_{3}$. The periods of oscillation as well as the temperature dependence of the amplitude and the magnetic fields saturation lead to the following values for the transverse and longitudinal effective masses: ${m}_{t}=1.5{m}_{0}\ifmmode\pm\else\textpm\fi{}15%$; ${m}_{l}=6.0{m}_{0}\ifmmode\pm\else\textpm\fi{}30%$.

Received 2 February 1965DOI:https://doi.org/10.1103/PhysRevLett.14.360©1965 American Physical Society

The quenching curves of some high-field superconductors have been measured for the first time in steady transverse magnetic fields up to 150 kOe. The quenching curves of Nb3Sn diffusion-layers on single niobium wires show the same slope in the high-field region as they do in lower fields. On the other hand, the quenching curves of “Nb-Sn multiwires” with a synthetic filamentary structure of many interior Nb3Sn diffusion-layers in the core of this material are nearly horizontal in the high-field region with very high critical current values. The quenching curves of V3Ga diffusion-layers and of V3Ga core-wires show an unexpected inversion at about 90 and 100 kOe respectively with increasing critical currents in higher fields.

The magnetoresistive properties of a thin sodium wire have been studied at 1\ifmmode^\circ\else\textdegree\fi{}K in transverse magnetic fields (${H}_{T}$) up to 60 000 gauss. This study was undertaken in order to determine whether magnetoresistive oscillations of the de Haas-van Alphen type could be detected in the vicinity of 60 000 gauss. Although no such oscillations were found, the magnetoresistance for ${H}_{T}$ below 15 000 gauss exhibited a completely new type of oscillatory phenomena. These new oscillations are periodic in $H$ with a decreasing amplitude in increasing magnetic fields, whereas the de Hass-van Alphen oscillations would be periodic in ${H}^{\ensuremath{-}1}$ with an increasing amplitude in increasing magnetic fields. The period of these new oscillations is in excellent agreement with the period of the oscillatory behavior predicted theoretically by Sondheimer for the magnetoresistance due to surface scattering of thin metallic films in ${H}_{T}$. From this period, a value for the electronic momentum was obtained. The significance of these new oscillations is discussed.

S>The transverse magnetoresistance of Al was studied. The measurements were made on six samples of 99.999% pure Al at 4.2 deg K in magnetic fields H up to 120000 gauss, the samples having been annealed, and in two cases coldworked, so as to be in various states of strain. Valid data were obtained from four of the samples, for which the ratios (at H = 0) of room temperature resistivity R/ sub 300 deg K/ to the resistivity R at 4.2 deg K were 1530, 1l80, 1075, and 333. Since R/sub 300 deg K/ is essentially the same for all the samples, the ratios are proportional to the R values, which for two of the samples can be seen to be in the ratio 1530/333 or approximately 4.6. At H = 80,000 gauss, the ratio was still 4.4, showing that almost all of the resistivity decrease gained at H = 0 by using a less strained metal is preserved at high magnetic fields for a metal whose magnetoresistance exhibits a predominantiy saturating behavior. The data from the four samples were normalized fairly well by Kohler's rule, which is DELTA R/R = f(HR/sub 300 deg K//R), where DELTA R is themore » increment to R due to the magnetic field. This indicates that further reductions in the power necessary to operate cryogenic magnets can be profitably achieved by using purer and purer Al. (auth)« less