Schottky Contact and Ohmic Contact

Questions:

1. Scaling of nano-Schottky-diodes
2. Enhanced tunneling across nanometer -scale metal- semiconductor interfaces
3. Electrical contacts to one- and two- dimensional nanomaterials

Answers:

1: Scaling of nano-Schottky-diodes

Metal-semiconductor contact is an important component for any nano material (sized between 1-100 nm with unique optic-electronic properties).  To create a metal-semiconductor junction one important problem is the Fermi energy (The energy difference between the lowest and highest particle state at absolute zero) mismatch of these two types of materials. The large mismatch in Fermi energy level in metal (the difference between the highest single state at T=0) and semiconductor (known as fermions and at T=0 no electron can achieve enough energy to reach above the surface) creates a high energy rectifying contact. To create an Ohomic contact the choice of metal for semiconductor is important. The right choice of materials can result in a low resistance Ohomic contact.

2: Enhanced tunneling across nanometer -scale metal- semiconductor interfaces

According to Sasaki (2013), high doping (semiconductor) and increase in the temperature can improve the condition. Doping in semiconductors means adding impurities to increase the conductance in semiconductors. Light and moderate doping makes the intrinsic semiconductor extrinsic as well as high doping can change the semiconductor properties and make it act like a conductor. Highly doped semiconductors are known as degenerate semiconductors.  In n type of semiconductors, the semiconductors are doped with donor atoms (column V impurities). Column III impurities are acceptor impurities and used to create p type of semiconductors.  N type of impurities creates additional band near conduction band (lowest range of vacant electron) whereas acceptor impurities creates additional band near the valence band (highest range of electrons at T=0). The carrier concentration excluding Pauli’s exclusion principal can be given as follows:

n_e= N_c (T) exp ((E_F-E_C)/kT)

n_h=N_v (T)exp ((E_v-E_f)/kT)

where E_F, E_C and E_V are Fermi level, minimum energy in conduction band and maximum energy in valence band respectively (Scotognella et al.,2013).  The Ohomic contact depends upon the Schottky barrier height  Φ_B  (characteristic for potential energy barrier of rectifying diode). If  Φ_B  is zero or negative the electrons can move freely in or out the semiconductor thus creating a successful Ohomic contact. Tunnel contact is a practical approach in metal- semiconductor contacts (Zhong & Zheng ,2012).  The Schottky barrier height is positive in this case however this contact needs high doping in the semiconductor to create a thin barrier (>=3nm) in metal semiconductor junction.

3: Electrical contacts to one- and two- dimensional nanomaterials

In the case of nano Schottky diode the thin barrier, increase the tunneling without the reduction of Schottky barrier height (Yang et al.,2013).  If the SBH (Schottky barrier height) is enough larger than the thermal energy kT , the semiconductor depletes (insulating region of a doped semiconductor where the charge carriers diffused away by some external  factor; electric field ) near the metal to create the Schottky barrier. The lower value of SBH does not allow depletion near the metal thus creating the Ohomic contact. Thus if the Fermi level is between valence and conduction band, the condition forms the Schottky barrier. The ohomic contact shows the Fermi level just below valence or conduction band. In the case of metallic nanostructure, the tunneling effect is another issue. The charge neutrality level in the semiconductor metal junction creates a local charge in semiconductor and image charge ( replacement of certain element by imaginary charges; electrostatics) in metal.

References:

Sasaki, K., Higashiwaki, M., Kuramata, A., Masui, T., & Yamakoshi, S. (2013). Si-ion implantation doping in β-Ga2O3 and its application to fabrication of low-resistance ohmic contacts. Applied Physics Express, 6(8), 086502.

Scotognella, F., Della Valle, G., Kandada, A. R. S., Zavelani-Rossi, M., Longhi, S., Lanzani, G., & Tassone, F. (2013). Plasmonics in heavily-doped semiconductor nanocrystals. The European Physical Journal B, 86(4), 1-13.

Yang, H., Heo, J., Park, S., Song, H. J., Seo, D. H., Byun, K. E., … & Kim, K. (2012). Graphene barristor, a triode device with a gate-controlled Schottky barrier. Science, 336(6085), 1140-1143.

Zhong, H., Fu, J., Wang, X., & Zheng, S. (2012). Measurement of laser activated electron tunneling from semiconductor zinc oxide to adsorbed organic molecules by a matrix assisted laser desorption ionization mass spectrometer. Analytica chimica acta, 729, 45-53.