Schottky barrier internal photoemission

Internal Photoemission Measurements of Schottky Energy Barriers... 

In this section the electronic structure of metal/polymcr/metal devices is considered. This is the essential starting point to describe the operating characteristics of LEDs. The first section describes internal photoemission measurements of metal/ polymer Schottky energy barriers in device structures. The second section presents measurements of built-in potentials which occur in device structures employing metals with different Schottky energy barriers. The Schottky energy barriers and the diode built-in potential largely determine the electrical characteristics of polymer LEDs. 

It is difficult to measure metal/polymer Schottky energy barriers smaller than about 0.5 eV using internal pholoemission. Small Schotiky energy barriers lead to thermal emission currents produced by the absorption of light in the metal which are difficult to separate from true photocurrents 134]. If the structure is cooled to try to improve this contrast, it is often found that the significant decrease in the electrical transport properties of the polymer [27 [ makes it difficult to measure the internal photoemission current. To overcome this limitation, internal photoemission and built-in potential measurements are combined to measure small Schottky energy barriers, as described below. 

Electrons are optically excited from the metal Fermi energy over the Schottky barrier by the internal photoemission mechanism. The carriers are collected at the opposite contact and detected as a photocurrent when the junction is held in reverse bias. The energy required to excite carriers is less than the band gap energy, so that the internal photoemission spectrum is distinguished from bulk band-to-band carrier generation. Models of the effect predict that the photoresponse spectrum is... 

Fig. 6. Schematic of the optical transitions that contribute to the photoresponse of a-Si H Schottky-barrier diodes. A, internal photoemission B, optical band-to-band absorption C, localized to extended state absorption D, absorption in the doped layer.

Here, A is a constant determined by the absorption of the metal and the probability of photoemission into the semiconductor. Thus by plotting versus photon energy, extrapolation to zero will display the barrier height. The first internal photoemission results for a-Si H Schottky barriers were reported by Wronski et al. in 1980, and their results are shown in Fig. 7. The spectrum has two components the strong absorption at >1.4 eV due to band-to-band transitions and the weaker feature at <1.4 eV due to the internal photoemission. By plotting the square root of the internal photoemission in Fig. 8, the barrier height can be obtained. As is evident from the... 

Thus, more measurements are necessary to sort out the actual mechanism responsible for the transition from Schottky barrier to ohmic contact. Key among these would be depletion-layer profiling and built-in potential correlated with barrier-height variations. Furthermore, detailed field-dependent internal photoemission measurements may also help in solving the problem. 

Photoemissive Schottky detector (metal-silicide detector) Utilizes internal photoexcitation at a Schottky barrier, e.g., at PtSi... 

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