Volume 32, Issue 10, 01 October 1961
Index of content:
- 3–5 COMPOUNDS: BAND STRUCTURE AND TRANSPORT PROPERTIES
32(1961); http://dx.doi.org/10.1063/1.1777018View Description Hide Description
In discussing the galvanomagnetic effects of semiconductors with high electron mobility one has to distinguish four main groups of parameters which influence these effects: (1) For InSb with practically no magnetoresistive effect on a long rod, an increase in resistance by a factor 38 can be reached in 10 000 gauss with appropriate shape (field disk). (2) Layers periodically changing their electron concentration produce an anisotropy of magnetoresistance. For certain specimen orientations the Hall coefficient depends on the magnetic field and a planar Hall effect is observed. Near a step of concentration one measures apparent negative resistances which are caused not by a retrograde current, but merely by rotation of the current lines. (3) If there are more than one type of charge carriers, it is difficult to know the concentration and mobility characteristic of a special type. Because of the high mobility ratio in InSb it is possible to state a hole mobility of 620 cm2/v sec in 150 000 gauss for pure material at room temperature. (4) After elimination of the influences of the above‐mentioned points 1 to 3 one cannot find in InSb any magnetoresistive effect of electrons in the conduction band up to 150 000 gauss. The Hall coefficient is magnetic field independent up to this value of the magnetic induction for n‐type InSb.
32(1961); http://dx.doi.org/10.1063/1.1777019View Description Hide Description
Most samples of GaAs show properties similar to those of germanium and silicon, but it is possible to prepare GaAs with a resistivity at room temperature greater than 106 ohm‐cm, and the electrical properties are then more like those of the wide band gap II–VI compounds, such as CdS. This form of material, known as semi‐insulating GaAs, previously has not been studied thoroughly, partly because homogeneous samples were not available.
Measurements have now been made on semi‐insulating GaAs, and results are reported for carrier concentration and mobility as a function of temperature. The interpretation of the results is sometimes complicated because even at room temperature the activation energy is about half of the intrinsic activation energy, and the carrier concentration can be close to the intrinsic concentration.
The dominant lattice scattering mechanism in GaAs is believed to be polar scattering, but even in the purest samples of semiconducting GaAs made thus far, impurity scattering is observed at room temperature. In a highly compensated material like semi‐insulating GaAs, neither the Brooks‐Herring nor the Conwell‐Weisskopf theory of impurity scattering is likely to be valid. An initial study of carrier scattering has been made using measurements of transverse magnetoresistance and the field dependence of Hall coefficient. Some values for carrier lifetime are also reported.
32(1961); http://dx.doi.org/10.1063/1.1777020View Description Hide Description
The effect of hydrostaticpressure on the transport properties of n‐type GaSb,InP,GaP, and p‐type PbTe was measured to study their band structure. (1) The Seebeck coefficient, Hall coefficient, and resistance of three n‐GaSb samples were measured as a function of hydrostaticpressure up to 17 000 atm between 200° and 400°K. The Seebeck coefficient α increased with pressure and approached a constant value at about 10 000 atm. The saturation value of α does not follow the simple lnT relation for any of the samples; e.g., for a sample with RH (77°K) ≈95 coul−1 cm3, the saturation value of α decreases with temperature. The contribution due to the phonon‐drag effect has been considered as a possible explanation for this phenomenon. (2) The conductivity of p‐PbTe increased almost exponentially with pressure both at 300° and 194°K; the Hall coefficient at 300°K decreased by about 5% at 8000 atm, while the conductivity increased by 55% at this pressure. (3) The resistance of n‐InP samples increased with pressure; the pressure coefficient was found to be bigger for samples with higher impurity contents. (4) The resistance of an n‐GaP sample decreased by about 3% at 10 000 atm.
32(1961); http://dx.doi.org/10.1063/1.1777021View Description Hide Description
Measurements of the voltage and temperature dependence of tunneling in Ge and GaSb are presented which confirm the close proximity of the (000) and (111) conduction band edges in these materials. In the case of Ge, the energy separation of these edges is found to increase with increasing donor concentration.
Tunneling in the indirect semiconductorGaP shows no evidence for indirect (phonon‐assisted) tunneling transitions. It is believed that tunneling in the junctions which we studied proceeds via deep‐level impurities rather than between conduction and valence bands directly, thereby eliminating the requirement of wave number conservation.
Revised values for the zone‐center longitudinal optical phonon energies as deduced from tunneling data in 3–5 and lead salt semiconductors are presented.
32(1961); http://dx.doi.org/10.1063/1.1777022View Description Hide Description
Three types of conduction band extrema in the (000), (100), and (111) directions in k space seem to determine many of the properties of the group 4 and group 3–5 semiconductors. Early experimental work on the pressure coefficients of the energy separations of these extrema from the valence band maximum energy, carried out on Ge (111), (000), (100), Si (100), and InSb (000), suggested that the three pressure coefficients might be independent of the specific element or compound in the group 4 and group 3–5 series. This work is discussed in detail, and the theoretical basis is briefly considered. All of the completed pressure measurements on these compounds are critically reviewed, and the correlation of unique pressure coefficients with specific band edges examined. It is demonstrated that pressure experiments can be planned to show up details of the band structure unavailable for study at atmospheric pressure. Particular attention is paid to GaP, and a new model for excess absorption occurring in n‐type samples of this compound and in Si, GaAs, and AlSb is suggested. The application of similar techniques to PbS, PbSe, and PbTe is discussed, and results of electrical and optical measurements of energy gap and electron and hole mobilities presented.
32(1961); http://dx.doi.org/10.1063/1.1777023View Description Hide Description
Resistivity, Hall coefficient, and magnetoresistance were studied for n‐ and p‐type GaSb. The infrared absorption edge was investigated using relatively pure p‐type, degenerate n‐type, and compensated samples. Infrared absorption of carriers and the effect of carriers on the reflectivity were studied. The magnetoresistance as a function of Hall coefficient for n‐type samples at 4.2°K gave clear evidence for a second energy minimum lying above the edge of the conduction band; the energy separation is equal to the Fermi energy for a Hall coefficient of 5 cm3/coulomb. The shift of absorption edge in n‐type samples showed that the conduction band has a single valley at the edge, with a density‐of‐state mass m d1=0.052 m. By combining the results on the edge shift, magnetoresistance, and Hall coefficient, it was possible to deduce: the density‐of‐states mass ratio m d2/m d1=17.3, the mobility ratio μ2/μ1=0.06, and the energy separation Δ=0.08 ev between the two sets of valleys at 4.2°K. Anisotropy of magnetoresistance, observed at 300°K, showed that the higher valleys are situated along (111) directions. The infrared reflectivity of n‐type samples can be used to deduce the anisotropy of the higher valleys; tentative estimates were obtained. Infrared reflectivity gave an estimate of 0.23 m for the effective mass of holes. The variation of Hall coefficient and transverse magnetoresistance with magnetic field and the infrared absorptionspectrum of holes showed the presence of two types of holes. Appreciable anisotropy of magnetoresistance was observed in a p‐type sample, indicating that the heavy hole band is not isotropic; this was confirmed by the infrared absorptionspectrum of holes. The results on the absorption edge in various samples seemed to indicate that the maximum of the valence band is not at k=0. However, it appears likely that transitions from impurity states near the valence band produced absorption beyond the threshold of direct transitions.
32(1961); http://dx.doi.org/10.1063/1.1777024View Description Hide Description
A series of detailed measurements of the lattice absorption bands of gallium arsenide has been made over the wavelength range 10–40 μ and over the temperature range 20–292°K. These results can be interpreted in terms of multiple phonon interactions involving five characteristic phonon energies. These results, along with the known elastic constants, have enabled us to supply all the relevant data for a computation of the complete phonon spectrum using an extension of the shell model.
32(1961); http://dx.doi.org/10.1063/1.1777025View Description Hide Description
The influence of various structural characteristics in the III–V compounds on galvanomagnetic properties is discussed. Evidence for the scattering of charge carriers by polar optical modes is reviewed, and the behavior of Hall and magnetoresistance coefficients is examined in regard to the conduction band structure. Unique characteristics, imparted by light masses in certain bands, include high mobilities and large magnetoeffects associated either with transport in the band or with ionization energies of the impurity centers. The importance of avoiding inhomogeneities, either in specimen or in magnetic field, when measuring Hall coefficient or magnetoresistance in high‐mobility materials is emphasized. Illustrations are given of the effects of nonuniformities in carrier concentration or in applied magnetic field on various galvanomagnetic phenomena.