STRUCTURE OF THE Si-SiO2 INTERFACE BY INTERNAL PHOTOEMISSION

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STRUCTURE OF THE S i - S i 0 2 INTERFACE BY INTERNAL PHOTOEMISSION

T. H. DiStefano IBM T. J. Watson Research Center, Yorktown Heights, NY 10598 ABSTRACT Details of the S i 0 2 conduction band near the Si-Si0 2 interface were examined by the technique of field-dependent internal photoemission. From a measurement of the threshold Φ(Ε) for internal photoemission for fields up to 8.17 x 10 6 V/cm, the conduction band bottom ψ(χ), as a function of position x in the S i 0 2 was determined to within approximately two lattice units of the interface. In calculating the effective potential, corrections were made for the effects of the photon induced tunneling. The results of the threshold measurements show only small deviations from the flat band o

model, about 0.06 eV ± 0.03 eV, up to a distance 4.5A from the silicon surface. There is no indication of a large ionic charge or of a greatly reduced S i 0 2 bandgap to within about two lattice units of the interface. INTRODUCTION The interface between a given crystallographic surface of Si and an amorphous S i 0 2 layer may not be precisely smooth on an atomic scale.[l] Evidence [2, 3] based on He backscattering data suggests that S i 0 2 which has been thermally grown on silicon contains excess silicon near the Si-Si0 2 interface. However, ion backscattering measurements are known to be somewhat insensitive and difficult to interpret because of the background from the substrate, even if channeling techniques are employed [4]. Several sputter-etch profiling techniques have been used to examine the Si-Si0 2 interface, including Auger [5] and ESCA [6] measurements. Data from the sputter-etch measurements show some irregularities near the Si-Si0 2 interface which have been variously interpreted as excess Si in the S i 0 2 , a graded bandgap interface Si-SiO x -Si0 2 , free silicon inclusions in the S i 0 2 , or an undulating interface. However, the data are difficult to interpret unequivocally due to nonuniformities in the microscopic sputter-etch rate in S i 0 2 , which cause the interface to appear distorted because some portions of the silicon surface are exposed by the sputter-etch process before others. Additionally, these profiling techniques are complicated by details of the electron escape mechanism and by the physical damage induced by the sputtering process itself. In this paper, we provide information on the local microscopic structure of the Si-Si0 2 interface obtained by the field-dependent internal photoemission technique [7]. The electric field dependence of the threshold for the internal photoemission is measured and used to determine the conduction band energy in the insulator as a function of distance away from the interface. In this way, the S i 0 2 conduction band was probed from 4.5A to 12A from the Si-Si0 2 interface, and the curvature of the conduction band was determined [8]. FIELD DEPENDENT INTERNAL PHOTOEMISSION The Si-Si0 2 interface has been extensively studied by internal photoemission measurments [9]. The interface barrier is determined by finding the threshold for photoemission over the barrier from spectral photoresponse measurements. The threshold is determined by extrapolating a power law fit to the photoresponse, where the best fit is obtained with power of 2 or 3 [10]. Typically, the

363 threshold Φ determined from a third power fit is 0.15-0.2 eV lower than that obtained from a second power fit. The field dependence of the internal photoemission from Si into S i 0 2 was used to determine the variation of the conduction band potential φ with distance from the interface of the S i 0 2 with the Si. Any curvature of the S i 0 2 conduction band near the interface would result in a deviation from simple Schottky barrier reduction. This deviation Δφ is used to determine the conduction band potential φ(χ) as a function of the distance x. The potential φ(χ), together with the image potential and the applied field, produce a total barrier which is approximately the photoemission threshold Φ(Ε). At one particular value of field, the position x of the maximum of the barrier and the potential φ(χ) can be obtained from the dependence of the photoemission threshold φ(Ε) upon electric field. An increase in the applied electric field moves the position of the barrier maximum closer to the interface and lowers the total barrier height. By using this field dependence of the photoemission barrier, the S i 0 2 band curvature was measured as a function of distance away from the silicon surface. MEASUREMENTS The measurements were performed on samples formed by the oxidation of 10 ohm-cm n-type Si (100) in dry 0 2 at 1150°C. The thickness of the SiO^, determined by the ellipsometric measurements, was 1090 A. A semitransparent electrode of 160A of aluminum, 0.3 cm in diamter, on top of the S i 0 2 was used to apply the electric field and to collect the photocurrent. Photoemission measurements were made up to the highest electric field at which the sample was stable. Three independently fabricated samples were measured, with similar results. The thresholds were determined by extrapolation from a plot of the half-power of the photoyield at a given applied field. At the higher electric fields, the photoyield near threshold was complicated by a component due to the photoemission of electrons from the conduction band of the inverted silicon surface. The second power, rather than the third power, was chosen because it facilitated a separation of the photoyield due to emission from the Si conduction band from that from the valence band. The photoelectric threshold Φ(Ε) for emission from Si into S i 0 2 field in Fig. 1.

is shown as a function of electric

Fig. 1. The threshold for internal photoemission from Si into S i 0 2 as a function of electric field.

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The experimental points were determined from a fit of the spectral photoemissive yield to a parabolic dependence on photon energy. When extrapolated to zero electric field, the threshold indicates an

364 interface barrier of about 4.47 eV ± 0.03 eV. This is somewhat higher than the 4.35 eV found from a third power fit, but this shift does not significantly influence the band bending determination. The barrier Φ is fit to a simple Schottky barrier reduction at the low field, using εχ = 2.15 for Si0 2 . A very small correction due to photon assisted tunneling is not included; this correction is -0.025 eV at the highest fields. Also, because of an uncertainty in the parameters, no corrections are included at this point for the effects of field penetration into silicon, imperfect screening at the silicon surface, and phonon scattering energy loss. The potential of the conduction band bottom in the S i 0 2 , determined from the electric field dependence of Φ(Ε), is shown in Fig. 2. The band curvature is small in comparison to the uncertainties in the measurement and to the corrections which have not been included. However, the band bending in the S i 0 2 near the silicon surface is small, only about 0.06 eV for distances up to 4.5Â from the silicon surface.

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Fig. 2. The potential at the bottom of the S i 0 2 conduction band as a function of distance from the interface.

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Fig. 3. Schematic represenation of the electronic bands near the Si-Si0 2 interface: (a) conic Si in Si0 2 ; and (b) graded Si-SiO x -Si0 2 bandgap.

DISCUSSION The lack of any significant bending of the conduction band of the S i 0 2 near a silicon surface indicates that any substantially graded bandgap does not extend beyond about 4À from the silicon surface. If there were a non-stoichiometric layer of S i 0 2 at the interface, the layer would influence the curvature of the conduction bond, as illustrated in Fig. 3. The dashed curve "a" shows the influence of excess ionic silicon, while curve "b" shows a graded bondgap due to a layer of covalently bonded SiO x . Since no significant band bending is seen, there is no direct evidence of a nonstoichiometric layer beyond 4A from the interface. l a l u Z T ,κ ? ! P e u n e t r a t l o n · s c r e e n i " g . Phonon scattering and photon assisted tunneling Γ"η ,κ ë , , 6 C a l c " l a t e d b a n d c u r v a t u ' e somewhat. Inclusion of the corrections would also change the calculated shape of the conduction band, but they would not introduce the sort of gross g curvature expected from a non-stoichiometric layer.

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REFERENCES 1

S. T. Pantelides and M. Long, Continuous-Random Network Models for the Si-Si0 2 Interface, Proceedings of the International Topical Conference on the Physics of Si02 and its Interfaces (1978).

2

(a) J. A. Davies, J. Denhartog, L. Eriksson, and J. W. Mayer Can. J. Phys. 45, 407 (1967) (b) T. M. Buck and G. H. Wheatley, Surface Science 33, 35 (1972) (c) W. F. van der Weg, W. H. Kool, H. E. Rosendaal, and F. W. Saris Radiât. Eff (1973).

17, 245

3

T. W. Sigmon, W. K. Chu, E. Lugujjo, and J. W. Mayer, Appl. Phys. Lett. 24, 105 (1974).

4

W. K. Chu, J. W. Mayer, M. A. Nicolet, T. M. Buck, G. Amsel, and F. Eisen, Thin Solid Films 17, 1 (1973)

5

J. S. Johannessen, W. E. Spicer, and Y. E. Strausser, J. Vac. Sei. and Tech. 13, 849 (1976); C. M. Garner, I. Lindau, J. N. Miller P. Pianetta, and W. E. Spicer, J. Vac. Sei. Tech. }A 372 (1977)

6

S. I. Raider, R. Flitch, and M. J. Palmer, J. Electrochem. Soc. 122, 413 (1975); S. I. Raider and R. Flitsch, / . Vac. Sei. Tech. B , 856 (1976).

7

C. G. Wang and T. H. DiStefano, Crit. Rev. Sol. State Sei. 5, 327 (1975).

8

T. H. DiStefano, J. Vac. Sei. Technol 11, 996 (1976)

9

R. Williams, "Injection by Internal Photoemission," Chapter 2 in Semiconductors Semimetals, Vol. 6, (Ed. R. K. Willi ardson, Academic Press, New York, 1970)

10

B. E. Deal, E. H. Snow, and C. A. Mead, J. Phys. Chem. Solids 27, 1873 (1966).

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