Doped semiconductor diamond

In this section, unlike the previous one, we deal with less heavily doped, semiconductor diamond. Quantitative studies of reaction kinetics have been performed in Fe(CN)63 -/4, quinone/hydroquinone (recall that this is an inner-sphere reaction), and Ce3+/4+ systems [94, 104, 110]. Potentiodynamic curves recorded in solutions containing only one (either reduced or oxidized) component of a redox system are shown on Figs. 22a and b the dependences of anodic and cathodic current peak po-... 

The plasma jet can be cooled rapidly prior to impact on the substrate surface by mixing with a cold inert gas fed into an annular fixture. Gaseous boron or phosphorous compounds can be introduced into the gas feed for the deposition of doped semiconductor diamond,... 

Owing to its extraordinary chemical stability, diamond is a prospective electrode material for use in theoretical and applied electrochemistry. In this work studies performed during the last decade on boron-doped diamond electrochemistry are reviewed. Depending on the doping level, diamond exhibits properties either of a superwide-gap semiconductor or a semimetal. In the first case, electrochemical, photoelectrochemical and impedance-spectroscopy studies make the determination of properties of the semiconductor diamond possible. Among them are the resistivity, the acceptor concentration, the minority carrier diffusion length, the flat-band potential, electron phototransition energies, etc. In the second case, the metal-like diamond appears to be a corrosion-stable electrode that is efficient in the electrosyntheses (e.g., in the electroreduction of hard to reduce compounds) and electroanalysis. Kinetic characteristics of many outer-sphere... 

It is crucial to dope the diamond with enough boron to impart sufficient electrical conductivity on the matrix that the material can be considered metal-like. It is thus essential to consider how the boron dopant density, denoted in B atoms per cubic centimeter, affects the electrical resistivity of the material [14], as shown in Figure 5.1a. Note that diamond contains 2 x 10 C atoms per cubic centimeter. Boron doping introduces a band acceptor level 0.37 eV above the valence band. In the boron dopant range 10 -10 B atoms per cubic centimeter (i.e., 1 in 2 X10 - 1 in 2 X 10 C atoms are replaced with B), the material shows resistivity changes in accordance with p-type semiconductor behavior. The implications... 

There are several challenges associated with the synthesis of BDD suitable for electrochemistry. Since diamond is a semiconductor with exceptional properties, precise control of dopant impurities and extended defects is required to dope the diamond lattice with sufficient boron to make the material conduct. However, as the boron levels increase, it can be harder to maintain crystallinity and control the amount of nondiamond carbon (NDC) both within crystal defects and at grain boundaries. While NDC can increase material conductivity, it is be detrimental to a diamond electrochemist, as the widely recognized electrochemical properties of BDD (wide solvent window, low background currents, reduced susceptibility to electrode fouling, corrosion resistance) are impaired and the electrochemical response becomes more akin to glassy carbon. If the presence of NDC is unaccounted for, electrical resistivity measurements will mislead the user into believing that there is more boron than actually present in the matrix. 

Diamond can be changed from an intrinsic to an extrinsic semiconductor at room temperature by doping with other elements such as boron and phosphorus.f lf l This doping can be accomplished during the synthesis of diamond either by high pressure or especially by CVD (see Ch. 13, Sec. 4.4). Doped natural diamond is also found (Type lib) but is rare. 

The second group of methods evolved from a solid source of hydrocarbon species formed during etching of graphite by hydrogen see Fig. 2. Using this method, homoepitaxial, doped films have been obtained to demonstrate that semiconductor diamond films can be grown from the... 

The development of low-pressure synthesis methods for diamond, such as the chemical vapor deposition (CVD) technique, has generated enormous and increasing interest and has extended the scope of diamond applications. Highly efficient methods have been developed for the economical growth of polycrystalline diamond films on non diamond substrates. Moreover, these methods allow the controlled incorporation of an impurity such as boron into diamond, which in this case forms a ptype semiconductor. By doping the diamond with a high concentration of boron (B/C = O.Ol), conductivity can be increased, and semi-metallic behavior can be obtained, resulting in a new type of electrode material with all of the unique properties of diamond, such as hardness, optical transparency, thermal conductivity and chemical inertness [1,2]. 

Depending on the doping level, diamond exhibits properties either of a semiconductor (e.g., at boron content from 10 to 1000 ppm) or a poor metal (with up to 10,000 ppm of B or even higher). It is the heavily doped diamond that is used as an electrode material in electrosynthesis, electroanalysis, etc. 

More precisely, the potential drop in the Helmholtz layer in a redox electrolyte increases with increasing doping. Therefore, the reaction rate reflects the Helmholtz potential drop, rather than the charge carrier concentration. This is the reason why the behavior of semiconductor diamond is far from ideal in particular, no current rectification is typically observed on (rather heavily doped) diamond electrodes, as mentioned above. 

On metal electrodes, the transfer coefficients typically approach 0.5. Generally, the transfer coefficients for redox reactions on moderately doped diamond electrodes are smaller than 0.5 their sum a +p, less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, on p-type semiconductors, reactions proceeding via the valence band have the transfer coefficients a = 0, P = 1, and thus, a +p = 1 [7]. Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials manufactured by use of advanced technologies ( like germanium, silicon, gallium arsenide, etc.). The departure from the ideal semiconductor behavior is likely to be caused by the fact that the interfacial potential drop appears essentially localized, even in part, in the Helmholtz layer, due, e.g., to a high density of surface states, or the surface states directly participate in the electrochemical reactions. As a result, the transfer coefficients a and p have intermediate values, between those characteristic of semiconductors (O or 1) and metals (-0.5). Semiconductor diamond falls in with this peculiarity. However, for heavily doped electrodes, the redox reactions often proceed as reversible, and the transfer coefficients approach 0.5 ( metaMike behavior). 

Ion implantation has been successfully used in doping semiconductors such as silicon and gallium arsenide. In particular, applications of ion implanted diamond have recently come to light. In these studies, the electrical conductivity and other physical properties could be controlled by ion-implanting diamond. However, only a few applications for electrochemical uses by preparing conductive electrodes have been reported [16,17]. 

Electrical and Electronic. Diamond is an electrical insulator (-- lO H/cm) unless doped with boron when it becomes ap-ty e semiconductor with a resistivity in the range of 10 to 100 Q/cm. n-Ty e doping has often been claimed but is less certainly estabUshed. The dielectric constant of diamond is 5.58. 

The compact structure of diamond accounts for its outstanding properties. It is the hardest of all materials with the highest thermal conductivity. It is the most perfectly transparent material and has one of the highest electrical resistivities and, when suitably doped, is an outstanding semiconductor material. The properties of CVD and single-crystal diamonds are summarized in Table 7 2.[1][18]-[20]... 

The semiconductors that have been the subject of numerous investigations in bulk, alloyed, or nanocrystalline form include Si, Ge, doped diamond, SiC, (B, Al, Ga, In)(N, P, As, Sb), and (Zn, Cd, Hg, Pb)(0, S, Se, Te). Nature has been exceptionally benign in providing NMR-active isotopes at natural abundances exceeding 4% for all of the preceding elements except in the cases of 13C, 33S, and 170, and enrichment with isotopic-labels has become more common. 

The size of the bandgap can vary from a fraction of an eV (in the IR region of the spectrum) to ca. 4 eV or more (wide-bandgap semiconductors). The upper limit is somewhat arbitrary a substance commonly thought of as an insulator such as diamond has a large bandgap of 5.5 eV, but it can nevertheless be doped with elements such as B, N, or P to become an electrically-conducting semiconductor. 

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