Internal Photoemission Measurements for the Determination of Schottky Barrier Height on a-Si,Ge:H,F Alloys
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INTERNAL PHOTOEMISSION MEASUREMENTS FOR THE DETERMINATION OF SCHOTTKY BARRIER HEIGHT ON a-Si,Ge:H,F ALLOYS*) V. CHU, S. ALJISHI, D. SLOBODIN, AND S. WAGNER Department of Electrical Engineering Princeton University, Princeton, New Jersey 08544
ABSTRACT We report measurements of internal photoemission from Ni, Au, and Pd contacts into a-Si,Ge:H,F alloys. The alloys were prepared by d.c. glow discharge decomposition of either SiF 4 or SiH 4 and GeF 4, and H2. The sharp exponential drop in subgap absorption in these alloys, measured by the Constant Photocurrent Method (CPM), allows the determination of barrier heights using internal photoemission thresholds. The barrier heights of Ni, Au and Pd contacts are presented as a function of alloy composition. We find Ni has the lowest barrier heights while Au shows the highest barrier heights over the entire range of Eopt. We also 4 find that for the Ni and Au contacts, oB varies as 1/2 the optical gap. In the case of Pd, ( B shows a dependence of 1/3 the optical gap. We observed an increase in (PB for Pd contacts when etched with a diluted HF solution prior to metallization. A similar increase in 4lDwas not observed for the Au and Ni contacts.
INTRODUCTION The growing importance of a-(Si,Ge):H alloys for use in high-efficiency multilayer solar cells has made it necessary to address the question of what happens at the interface between metal contacts and these alloys. Do metal/a-Si,Ge:H interfaces form rectifying barriers similar to those in c-Si? If so, how does this barrier height vary with Ge content? Because electronic states of a-Si:H can be modeled by energy bands, the formation of a rectifying Schottky barrier can be treated in analogy to c-Si. However, because of the high density of states in the gap in a-Si:H and the use of intrinsic material (as opposed to the doped material used in c-Si Schottky diodes), the two cases are quite different in many respects [11. It has been shown that the optical gap, Ept, can be controlled by the alloying of a-Si:H with Ge to obtain values ranging from -1.0 eV for pure a-Ge:H to 1.7 eV for pure a-Si:H [2]. In this paper, the variation of of the Schottky barrier height with Eopt will be examined.
THEORY The most direct method of measuring the Schottky barrier height is the method of internal photoemission. The theory was formulated originally by Fowler in 1931 [3]. The theory states that the photoemission yield, Y, defined as the photocurrent per incident pho-
*) Funded by the Electric Power Research Institute
Mai. Res. Soc. Symp. Proc. Vo4.70. ' 1986 Materials Research Society
296
ton flux, exhibits a wavelength dependence described by: Y(hv) = A[h V--FB] 2,
= 0,
(hv> 4 B)
(1)
(hv< IDB)
where A is a constant determined by the absorption of the metal and the probability of photoemission into the semiconductor. The barrier height, %B can be obtained by plotting the square root of the yield versus energy for low photon energies (energies above the barrier height, but below the energy of strong absorption in the semiconducto
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