Aspects of Schottky Barriers

The second part of this chapter will describe some of the general aspects of the Schottky barrier and properties specific to a-Si H. The third part is devoted to the transport mechanisms and measurements, and the fourth describes the effects due to atomic structural properties of the interface. Next, the effects due to doping are addressed, and following that the origin of the Schottky barrier is discussed. By presenting the question of the origin of the barrier near the end of this chapter, we imply that it is an unsolved problem. The chapter is concluded with comments about applications and important problems to be addressed for further understanding. 

Consider now the charge distribution of ionized centers in the depletion region. For comparison, two cases can illustrate the difference between crystalline and a-Si H. The two cases are for n-type semiconductors, one with a single band of donor levels and the other with a uniform density of donor levels throughout the band gap. These two cases are illustrated in Fig. 2. In these cases, Poisson s equation can be explicitly solved. The solutions yield parabolic bands for the case with a single donor band and an exponential behavior for the continuous uniform-state density. The field dependences of the two cases differ also. For the discrete level, a linear dependence results, whereas an exponential behavior is obtained in the uniform-state case. The most striking difference is in the density of ionized states, which is uniform 

It should be noted that a-Si H exhibits a density of states that is neither constant nor discrete. Deep-level transient spectroscopy (DLTS) (Lang et al, 1982), optical absorption (Jackson and Amer, 1982), and photoconductivity (Jackson et al, 1983b) measurements have indicated that a rather broad defect band lies in the gap and is centered below the Fermi energy. This means that for cases in which the built-in potential is less than the width of 

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The properties of metals on doped a-Si H are sure to be an important area of research. Conflicting models have already been di ussed. Sorting this problem out will be important for both research applications and technology. Lastly, the actual details of Schottky-barrier formation remain a mystery, The previous studies seem to indicate a strong similarity in many aspects of Schottky barriers on a-Si H and crystalline Si. 

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