Electronic Transport in Bulk and Quantized Low Dimensional Semiconductor Systems

Abstract

It is very interesting and important to study electronic transport parameters of bulk and low dimensional systems. Bulk material, such as GaN, a III+V compound semiconductor is a very useful material for electronic device applications due to its direct and wide band gap and also due to the strong bond strength. However, a non availability of single crystalline form of GaN and perfectly matched substrates are always problems for GaN. Hence, GaN, grown by various chemical vapor deposition techniques on a substrate such as sapphire is having large dislocations at the interface, the layer between the bulk layer of GaN and the sapphire due to the mismatch in the lattice constants. Such interfacial layer significantly affects the transport parameters of the material, where the transport properties are high dominated by scattering due to dislocations.

The author calculated various transport parameters such as ac/dc mobilities, Seebeck coefficient, thermal conductivity and the thermoelectric figure of merit considering two layer model formulated by Look and the results are also found to be agreement with the experimental values.

In a junction between high and low band gap semiconductors, the carriers are transferred from higher band gap to the lower band gap due to which a space charge is generated in the higher band gap. At the same time, charge gets accumulated at the lower band gap forming an accumulation of charges. It causes the band bending and the formation of a triangular quantum well. The carriers accumulated at the interface of the junction get quantized into various energy levels. Such system behaves as quantum-two dimension (Q2D) because the carriers are free to move on a plane, perpendicular to the junction. The mobility of such a system is very high due to reduction of the scattering mechanisms as well as due to charge getting separated from the parent donors.

Further, by confining the one more dimension of the heterojunction by taking an infinite potential well causes the carriers to move only on one dimension and the system is called quasi-one dimensional (Q1D) system. Such Q1D system is also called quantum well wire (QWW) because; the carries are restricted to move on only one dimension instead of a plane, which was in the case of Q2D system. The author has carried out comparative studies on Q2D and Q1D ac/dc mobilities in the heterojunctions of AlGaAs/GaAs and GaInAs/InP. The effect of various low temperature phonon and nonphonon scattering mechanisms on the systems are also observed.

In addition, the Q1D system formed by magnetically confined system is also attracting attention of researchers in device application because it is capable of over looking the various techniques of fabrication difficulties and defects created by such fabrication techniques.

In the presence of a high magnetic field, the transverse component of the energy dispersion relation gets quantized into various equally spaced energy levels called Landau levels and the motion of the carriers is completely restricted. However, the longitudinal component along the field is still free to move. The mobility of such system is enhanced when a low effective mass semiconductors n-HgCdTe (Mercury Cadmium Telluride) is used. The band structure of n-HgCdTe is found to be nonparabolic due to its low band gap according to Kane [Phadke and Sharma, 1975]. Recent publications, based on experimental verifications of transport coefficient of n-HgCdTe of Chen and Sher [Chen and Sher, 1982] show that the band structure of MCT (Mercury Cadmium Telluride) is more hyperbolic in nature rather than nonparabolic. The author has compared the effect of band structures on the various transport properties of MCT such as mobility, Seebeck coefficient, thermal conductivity, figure of merit ( )Z etc. The figure of merit is a very important property of a material to be used in thermoelectric devices, such as cooler, refrigerator etc.

Emission of photoelectron takes place when a monochromatic photon of energy ωh is incident on a sample. These photoelectrons will have the information about the quantized states they left. Depending upon the ii strength of the application of the photon energy, the transverse components of the energy dispersion relation with various quantized energy levels in the case of magnetic confinement take part in the photoemission process. Hence, the photoemission causing photo electric current density is found to be step like nature due to the participation of various energy levels. Comparative study of the photo emission due to geometrically confined system of dimension exactly as that of the magnetically confined structure is another interesting study presented in the thesis.

When such a magnetically confined Q1D system be replaced by a thin film of thickness equal to that of the deBroglie wave length, the longitudinal component of the energy dispersion relation also gets quantized and the system behaves as quantum-zero dimensional (Q0D) system. The comparative studies of such magnetically and geometrically confined systems have also been formulated in the thesis.

Due to the presence of impurities in semiconductors quantized energy levels get broadened. Hence, the photoelectric current density gets modified due to broadening. The author has observed and compared the effect of broadening of the quantized energy levels on the photoelectric emission due to magnetically and geometrically confined systems. It is found that the characteristic nature of the photoelectric current density remains the same but the values of the current density decreases with the inclusion of the broadening parameter.

Phadke U. P. and Sharma S., J. Phys. Chem. Sol. 36, 1 (1975). Chen A.B. and Sher A ,J. Vac. Sci. Technol. 21, 138 (1982).