Temperature Dependent Current-Voltage Characteristics in Thin SiO 2 Films
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TEMPERATURE DEPENDENT CURRENT-VOLTAGE CHARACTERISTICS IN THIN SiO 2 FILMS JIN ZHAO AND N. M. RAVINDRA Microelectronics Research Center, New Jersey Institute of Technology, Newark, NJ 07102
ABSTRACT An analysis of the Fowler-Nordheim tunneling (FNT) theory and its application to temperature dependent current-voltage characteristics, of very thin films of Si0 2 on silicon, is presented. The final results are believed to provide the most complete examination of FN emission theory and predict the breakdown electric field in thin Si0 2 films. The role of
the roughness, at the Si-Si0 2 interface, in determining the FNT current in these structures is also discussed.
Oxidized silicon surfaces are used on virtually all of today's integrated circuits and silicon devices. It is very important to understand the electrical properties of SiO 2 for making high quality devices. From past studies, it is well known that the FNT contributes to current conduction in SiO 2 films. In the present work, an investigation of the currentvoltage characteristics, in the temperature range of 100 - 350'K, of thermally grown Si0 2 films on silicon, is presented. At room temperature, our experimental data is in accordance with the FNT equation
[1]:
(1)
J = CF 2e-01 F q~mo 2
C = 167r hmOb,
4m(2m.. __ 3'
/
(2))/2
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Where, F is the uniform electric field, q is the electronic charge, mi, m.., are the electron mass in free space and in the oxide respectively, 21rh is planck's constant, and Ob is the barrier height. The FN plot parameters based on static current-voltage characteristics in this study are comparable to the other independent experimental results available in the literature. This is presented in table I. TABLE I. Tunneling results from present and past work at 300 0K. Negative electrodes are n-type Si< 100 >. Parameters
Present Work
Ravindra[2]
Weinberg[3]
Krieger[4]
Osburn[5]
Slope(MV/cm)
270
260
238.5
237
246
O'b(eV)
3.05
3.05
2.9
2.89
2.96
mr,/mo
0.46
0.45
0.5
0.36
0.5
However, at temperatures other than 300°K, our data cannot fit the simplified FNT equation (1). This is because the barrier height and effective mass decrease with increasing temperature [5-7]. Such a variation leads to increase in the value of C given by equation (2). Using the earlier reported results of high resolution transmission electron microscopy (HRTEM) [8, 9], a discussion of breakdowns caused by interface protrusions is presented.
Mat. Res. Soc. Symp. Proc. Vol. 159. 01990 Materials Research Society
218
Theory The high field imposed across the oxide, which is necessary for tunneling (approxiniately in the range of 7.5 MV/cm-10 MV/cm), results in a large density of electrons which are confined to a narrow potential well at the interface. This leads to quantization of the energy to the interface [10]. See Fig. 1(a). E. remains discrete while the higher energy levels are merged into a continuum beginning with El. The continuum is assumed to obey bulk statistics and tunneling from it will be small compared to tunneling from E
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