Doping for n-type Diamond


I learn something new today, a diamond chemistry.

Originally, diamond is an inert material, it has no any conductivity capabilities. A highly electronegative atom has usually been used to dope it, hence the doped diamond will being conductive enough to become able to be used as an electrode for the electrochemical measurement. This is highly beneficial for the electrochemical research. How can the diamond be doped with the electronegative atom? It’s explained as bellow.

Most semiconductor devices are manufactured from impure material, where the atoms in the host crystal are replaced by elements which have either more or fewer valence electrons. This is termed doping. At a simplistic level, one can view the impurity with one extra electron than the host (such as phosphorus in silicon) as being able to give up this electron to the conduction band (donate an electron) and then take no other role in the materials properties. Similarly, one fewer electron corresponds to a hole in the valence band, so the impurity accepts additional electrons. Donors give rise to so-called n-type material and acceptors p-type, with “n” and “p” standing for negative and positive, respectively.

Since the energy required to ionize a donor (E in Figure 2) may be very small, the number of conduction electrons can be made very large even at modest temperatures, and this process can turn an insulating material, such as diamond, into a conductive medium.

However, the presence of an impurity with an excess or deficit of electrons is by no means a guarantee of enhanced conductivity. For instance, nitrogen is one place to the right of carbon on the peroidic table, and hence has one more valence electron. One might then anticipate that substituting carbon with nitrogen would make diamond conductive, via the motion of free electrons in the conduction band. However, nitrogen substituting for carbon undergoes a distortion which generates a deep donor, i.e. the energy E in Figure 2 is large on a scale of kBT, a characteristic for available thermal energy.

donor

The origin of the distortion may be viewed as a the preferential formation of lone-pair on the nitrogen and a dangling bond on one of its four carbon neighbours.

One important fact is that even for species with a smaller covlant radius than carbon, the surrounding lattice is pushe outwards. The greater the deformation of the bond-lengths (and angles) in the vicinity of the impurity, the greater the energy required to form the defect. This leads to the idea that the equilibrium solubility of dopants in bulk diamond is often very low.

However, phosphorus has been used successfully to produce n-type diamond, in the sense that the donor level (E in Figure 2) is much shallower than any other donor which can be reproducibly included in diamond. This qualification of the term success is very important: the donor level of P is relatively deep, at 0.6 eV below the conduction band. Therefore at room temperature the number of conduction electrons will be small as the fraction of donors that will be ionized depends on the exponential of the ratio E/kBT.

Source: http://www.staff.ncl.ac.uk/j.p.goss/Research/NtypeDiamond/

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