Semiconductor dependence

Theory predicts that buckytubes can either be metals, semimetals, or semiconductors, depending on diameter and degree of helicacy. The purpose of our study is to give a preliminary answer to this question. A detailed analysis has been published elsewhere[39]. In this section, we present mainly experimental results. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

The position of the Fermi level in an extrinsic semiconductor depends upon the dopant concentrations and the temperature. As a rough guide the Fermi level can be taken as half way between the donor levels and the bottom of the valence band for n-type materials or half way between the top of the valence band and the acceptor levels for p-type semiconductor, both referred to 0 K. As the temperature rises the Fermi level in both cases moves toward the center of the band gap. 

The most probable donor level, ered, the most probable acceptor level, eox, and the standard Fermi level, e redox) of redox electrons are characteristic of individual redox particles but the Fermi level, e m dox), of redox electrons depends on the concentration ratio of the reductant to the oxidant, which fact is similar to the Fermi level of extrinsic semiconductors depending on the concentration ratio of the donor to the acceptor. 

The conductive properties of SWCNTs were predicted to depend on the helicity and the diameter of the nanotube [112, 145]. Nanotubes can behave either as metals or semiconductors depending upon how the tube is rolled up. The armchair nanotubes are metallic whereas the rest of them are semiconductive. The conductance through carbon nanotube junctions is highly dependent on the CNT/metal contact [146]. The first measurement of conductance on CNTs was made on a metallic nanotube connected between two Pt electrodes on top of a Si/Si02 substrate and it was observed that individual metallic SWCNTs behave as quantum wires [147]. A third electrode placed nearby was used as a gate electrode, but the conductance had a minor dependence on the gate voltage for metallic nanotubes at room temperature. The conductance of metallic nanotubes surpasses the best known metals because the... 

FIGURE 11.2 The band gap of a semiconductor depends on its size. See colour insert following page 356. 

It is appropriate to review the various major commercial applications of amorphous semiconductor devices and to indicate the extent to which the nsefnlness of amorphons semiconductors depends on an optimization of their electronic transport properties. 

Recently, the electron-transfer kinetics in the DSSC, shown as a schematic diagram in Fig. 10, have been under intensive investigation. Time-resolved laser spectroscopy measurements are used to study one of the most important primary processes—electron injection from dye photosensitizers into the conduction band of semiconductors [30-47]. The electron-transfer rate from the dye photosensitizer into the semiconductor depends on the configuration of the adsorbed dye photosensitizers on the semiconductor surface and the energy gap between the LUMO level of the dye photosensitizers and the conduction-band level of the semiconductor. For example, the rate constant for electron injection, kini, is given by Fermi s golden rule expression ... 

In Fermi-Dirac statistics, g is the Fermi energy Er, which is such that the probability, that a state of energy is occupied, is 1/2. States with energies higher than Et have a smaller probability of being occupied, those with lower energy, a higher probability. The position of the Fermi level in a semiconductor depends markedly on the temperature and on the concentration of impurities. The Fermi levels of two conductors in electrical contact and in thermal equilibrium are the same. 

It is clear from Equation 11.3 that resistivity should approach within 10% of the bulk value when the film thickness exceeds about four times the mean free path. The better the conductor, the smaller the mean free path. Thus, the resistivity approaches the bulk value as the film thickness reaches typical values of 100-200 nm for metallic conductors, or perhaps as much as several micrometers for semiconductors, depending on the intrinsic or doped carrier density. For sufficiently thick metallic films with K 1, the temperature coefficient of resistivity becomes positive, as bulk electron-phonon scattering becomes the primary contribution to resistivity [5]. Conduction in semiconductor films remains activation-limited, and retains a negative temperature coefficient. Figure 11.1 illustrates the dependence of resistivity on film thickness for sputtered... 

We note that the presence of a chemical species, if it affects the double layer, can cause an increase or decrease in the barrier height 4>b in the semiconductor depending on the interaction between the species and the C-S structure. Those species that cause the barrier to increase would enhance rectifying current-voltage behavior in the C-S structure those species that cause the barrier to decrease would enhance ohmic (lack of rectification) current-voltage behavior in the C-S structure. 

Hashimoto, K. Hiramoto, M. Sakata, T. Photo-induced electron transfer from adsorbed rhodamine B to oxide semiconductor substrates in vacuo Semiconductor dependence, Chem. Phys. Lett. 1988, 148, 215. 

Lattice parameters (LPs) of a semiconductor depend on the following factors [1] (i) chemical composition (including deviation from stoichiometry), (ii) presence of free-charge acting via the deformation potential of the energy-band extremum occupied by this charge, (iii) presence of foreign atoms and defects, (iv) external stresses (for example, exerted on a heteroepitaxial layer by its substrate), and (v) temperature. These factors are not independent [1], For nitrides, studies of such factors are in a state of infancy. 

It is worth noting that the lattice parameters of GaN (as for other direct-gap semiconductors) depend on the concentration of free electrons, which expand the lattice. According to our experience [7], this is a major factor in the lattice expansion of GaN (other nitrides have not yet been examined). 

Thermal expansion of a semiconductor depends on its microstructure, i.e. stoichiometry, presence of extended defects, ffee-carrier concentration. For GaAs [24] it was shown that for samples of free-electron concentrations of about 1019 cm"3, the thermal expansion coefficient (TEC) is bigger by about 10% with respect to the semi-insulating samples. Different microstructures of samples examined in various laboratories result in a large scatter of published data even for such well known semiconductors as GaP or GaAs. For group III nitrides, compounds which have been much less examined, the situation is most probably similar, and therefore the TECs shown below should not be treated as universal values for all kinds of nitride samples. It is especially important for interpretation of thermal strains (see Datareview A 1.2) for heteroepitaxial GaN layers on sapphire and SiC. 

In order to understand the performance of the tandem device, low temperature transport studies are a valuable tool. Diodes made from pristine MDMO-PPV and in composites with PTPTB are compared. ITO/PEDOT and Au electrodes are chosen to guarantee hole-only devices. This special choice of the electrodes is a successful technique for improving our understanding of transport failures. The proper choice of contacts allows us to produce p-type or n-type diodes from the same semiconductor, depending on the selectivity of the contact. For instance, Au is a hole-injection contact for most of the polymeric semiconductors, while Ca is an electron-injection... 

Fig. 3.18 Types of space-charge region in an n-type semiconductor, dependent on the potential applied relative to the flat band potential, Un,. U represents potential (V) and Ec sur the electronic energy corresponding to Ec close to the surface, (a) c,sur = E no space-charge region (b) c,sur> E (U < U ) formation of an accumulation layer (c) c,sur formation of a depletion layer (d) c,sur efb (U U ) formation of an inversion layer.

Single-wall nanotubes retain some of these features, however, in contrast to graphene, they are either metals or semiconductors, depending on the helicity [125,139,141-143]. This property is a consequence of the symmetry [124]. In particular, if (n-m) is a multiple of 3, then the tube is metallic (i.e. will have a non-zero density of states at the Fermi level), otherwise there is a gap between the highest occupied and lowest unoccupied levels. 

Insulators. In insulators the number of pre-existing carriers is always very small. As a consequence, considerable effects are to be expected. The irradiated sohd may behave transiently as an extrinsic or an intrinsic semiconductor, depending upon whether the trapping of one or the other type of carrier modifies or not, to any appreciable extent, the relative concentration of both types of carriers. Moreover, structural imperfections created by irradiation may play here an important role, either because of their own character or in consequence of their intervention in the phenomenon of trapping of the excess carriers. 

Three types of space charge layers, namely, depletion layer, accumulation layer, and inversion layer, may occur in a semiconductor depending on the bias and equilibrium conditions as shown in Fig. 1.7. 

The choice of impurity source and experimental setup for doping a semiconductor depends on the impurity and on factors such as vapor pressure and purity of the impurity source, solid solubility in the semiconductor, and alloying or compound formation on the semiconductor surface. 

---