Hall mobility doping effects

The Hall effect in the temperature range where conduction is by variable-range hopping is not well understood. Evidence from the early work by Fritzsche is discussed by Shklovksii and Efros (1984), who come to the conclusion that the Hall mobility must be small. Hopkins et al (1989) have investigated the behaviour of heavily doped Ge Sb, pushed into the non-metallic regime by magnetic fields up to 7 T, at temperatures down to 100 mK. Below 1 K the... 

Measurements of mobility in PS suffer from the fact that the number of free charge carriers is usually small and very sensitive to illumination, temperature and PS surface condition. Hall measurements of meso PS formed on a highly doped substrate (1018 cm3, bulk electron mobility 310 cm2 V-1 s-1) indicated an electron mobility of 30 cm2 V 1 s 1 and a free electron density of about 1013 cm-3 [Si2]. Values reported for effective mobility of electron and hole space charges in micro PS are about five orders of magnitude smaller (10-3 to 10 4 cm2 V 1 s ) [PelO]. The latter values are much smaller than expected from theoretical investigations of square silicon nanowires [Sa9]. For in-depth information about carrier mobility in PS see [Si6]. 

Detailed reports of n-type Si-doping of AlxGai.xN were also limited until recently. Details of the electrical and other properties of these doped films are usually limited to a few selected carrier concentrations and alloy compositions. Murakami et al [10] achieved Si-doping of Alo.1Gao.9N by MOVPE with carrier concentrations of 6 x 1017 and 2 x 101 cm 3, with mobilities (calculated from resistivity data) of 40 and 180 cm2/V s. Khan et al [11] also demonstrated MOVPE growth of Alo.09Gao.91N doped with silicon. Hall-effect measurements of these films revealed a mobility of 35 cm2/V s and a donor concentration of 5 x 101 cm 3. 

Hall effect measurements were used to investigate the electrical properties of the poly-Si films formed by the ALILE process. Due to the incorporated Al, the poly-Si films are always p-type. At room temperature, a hole concentration of 2.6 x 1018 run 3 and a hole mobility of 56.3 cm2 V 1 s 1 were determined [16]. Temperature dependent Hall measurements revealed both valence band conduction and defect band conduction (two-band conduction). For such highly doped material, the presence of a defect band conduction is expected. The Al concentration in the poly-Si films was measured by secondary ion mass spectroscopy (SIMS). An Al concentration of about 3 x 1019 cm 3 was found, which is about a factor of 10 larger than the... 

Generally, the aim of doping a semiconductor is to control the electronic properties exclusively by shifting the Fermi energy. In the study of a-Si H, the question arose early as to whether the incorporation of dopants causes side effects as well. The formation of a phosphorus impurity band 0.13 eV below the conduction-band mobility edge E. has been proposed by LeComber et al (1977) from their results of Hall effect experiments. An arsenic impurity level 0.35 eV below E,. and a boron impurity level 0.42 eV above the valence-band mobility edge Ey have been inferred by Jan et al (1979,... 

Further information on the transport processes in a-Si H and on the influence of doping can be obtained, e.g., from measurements of the drift mobility (Allan et al., 1977 Moore, 1977), of the photoconductivity (Rehm et al., 1977 Anderson and Spear, 1977), as well as of the magnetic field dependence of the photo- and dark conductivity (Weller et al., 1981). In this chapter, however, we shall confine ourselves mainly to results of conductivity and thermopower measurements. Some results from Hall effect and photoconductivity studies are also discussed. 

The electrical properties of the diamond films or free-standing discs are largely determined by the boron-doping level. Resistivities of useful diamond OTEs are in the range of 0.5-0.05 H-cm. Boron-doping levels associated with this resistivity are ca. 1-10 x 10 B/cm, as determined by boron nuclear reaction analysis measurements. Very preliminary Hall effect measurements for the diamond/quartz (Fig. 23A, 2) and diamond/ Si (Fig. 23B, 7) OTEs have revealed carrier concentrations between 10 and 10 cm and carrier mobilities (holes are the majority carrier in boron-doped films) of 1-100 cm /V-s. 

One can easily see that if n p, Eh reduces to -1/ne. Hall measurements are easiest to interpret in doped materials when either n 3> p orn < p. Otherwise one is faced with four unknowns, which require other measurements to resolve. For example, except for the difference between electron and hole mobilities, the Hall effect would be zero for intrinsic materials. One can also see that doing Hall measurements as a fimction of temperature offers a means of determining the occupancy number and energy levels of the various impurity states in the freeze-out region through Equation 20.22. 

Doping experiments, thermoelectric and Hall effect experiments provide powerful methods for separating the charge carrier concentration and mobility. 

---