SiO2 interface

Solid State Communications, Vol. 57, No. 8, pp. 615-617, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .00 Pergamon Press Ltd.

THE OPTICALLY DETECTED MAGNETIC RESONANCE OF DANGLING BONDS AT THE Si/SiO2 INTERFACE K.M. Lee and L.C.K.imerling AT & T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey, 07974, USA and B.G. Bagley and W.E. Quinn Bell Communications Research, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA

(Received 23 October 1985 by J. Tauc) The optically detected magnetic resonance (ODMR) observation of dangting bonds at the Si/SiO2 interface (Pb centers) is reported in this Communication. A luminescence quenching signal is identified as arising from the Pb center through its axially symmetry g tensor along the ( 1 1 1 ) direction. The negative sign observed for both the Pb center and the neutral phosphorus donor resonances allows us to interpret the recombination mechanism as a spin dependent electron transfer from the phosphorus donor to the Pb center. THE DANGLING BOND is an important structural defect in silicon and it has been intensively studied for many years. The EPR spectrum of the Pb center was first observed by Nishi [1] and later identified by Poindexter et al. [2, 3] as arising from a dangling bond at the Si/SiO2 interface. A recent EPR study by Brower [4] reported observations of the 298i hyperffme structure in the Pb spectrum. His analysis showed that this center is 12% s-like, 88% p-like, and 80% localized on the Si atom and therefore clearly exhibits the nature of a dangling bond. Correlative spectral studies using optical spectroscopy [5] and spin-dependent measurements of DLTS [6] and photocurrent [7] have been applied to the Si/SiO2 interface. This prior work, however, has not established the specific role of the dangling bond in determining interfacial electrical properties. This communication reports on an optically detected magnetic resonance (ODMR) observation of dangling bonds at the Si/SiO2 interface. These measurements unambiguously establish a microscopic recombination mechanism involving this interfacial defect. The sample used in this study is a ~ 1000 ohm-cm P doped (1 1 1 ) silicon wafer. Initially both sides of the surface were covered with a native oxide typically 1.2 nm thick [8]. One side of the wafer, 0.35 mm thick, was irradiated with a 250V argon ion beam for 10min at a current density of 1.1 macm -2 (4 x 101BAr cm-2). The irradiation affected the wafer surface in two ways: it stripped the oxide and also damaged the underlying silicon. The other wafer surface was not irradiated with the argon beam and, therefore, maintained its oxide. The sample, mounted on a wall-less microwave cavity [9] of a 24GHz optically detected magnetic 615

resonance (ODMR) spectrometer, was excited through the oxide layer with 488nm Ar ion laser light. The sample was in contact with pumped liquid helium (~ 2 K). The luminescence from the sample was detected by a Ge detector (North Coast). Synchronous changes in the luminescence due to microwave chopping were recorded as a function of the magnetic field for the ODMR studies. The magnetic field was produced by a split-pair superconducting magnet in an optical cryostat (Oxford Instruments). The angular dependence of the ODMR line positions was obtained by rotating the sample about its (1 1 0) or (2 1 1) directions in the { 1 1 1 } Si wafer surface with the magnetic field always perpendicular to the rotation axis. This procedure permits a determination of the g tensor of the defect. The low temperature luminescence spectrum is shown in Fig. 1. The sharp features near the energy band gap are known to arise from the radiative recombination of free excitons (FE) [10] and bound excitons (BE) at the neutral P donors [11]. The observation of a strong FEro line as compared to the BE(P) lines indicates that the free exciton recombination is dominant, consistent with the high resistivity ("~ 1000ohm-cm) of the sample [12]. The broad band peaking at 1.45/~m is of unknown origin but is produced by the low energy ion damage in the opposite face of the wafer. Figure 2(a) shows the ODMR spectrum in the region near g of 2 as obtained with the static magnetic field along a (1 1 0) crystallographic direction. The spectrum consists of four distinct resonances: a broad triangularly shaped feature (A), a sharp positive signal (D), a broader negative resonance (Pb-center) and the two sharp negative lines (P donor). Figure 2(b) illustrates line positions as a

MAGNETIC RESONANCE OF DANGLING BONDS AT THE Si/SiO2 INTERFACE Vol. 57, No. 8

616

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Fig. 1. Photoluminescence spectrum at 2 K (uncorrected) obtained by exciting the argon bombarded sample through the oxide layer on the undamaged side. The intense peak at 1.13~m is the TO phonon replica of the FE emission. The BE(P)To line is buried in this peak. function of the crystallographic orientations of the sample with respect to the magnetic field. Differences were not observed when the magnetic field was rotated in either the{1 1 0} or{2 1 1} planes. The g-tensor of the resonance labeled Pb, revealed by the angular dependence studies, is axially symmetric about a (1 1 1) axis with g~ = 2 . 0 0 2 ( 2 ) ,

g± = 2.009(2). This g tensor is in excellent agreement with published results for the dangling bond at the Si/SiO2 interface [ 1 - 4 ] . The pair of sharp negative signals is isotropic within experimental error. The g value for the center of mass position [ g = 1.9985(1)] and the 4.2roT separation identify the equally intense two negative lines as the P donor resonances [13]. (The isotope alp is 100% naturally abundant and its nuclear spin I = 1/2 provides a doublet hyperfine spectrum. Our observed splitting of 4.2 mT is fully consistent with the previously reported hyperfine constant, A = 39.17 x 10 -4 cm -1.) The g values for the two resonances A and D are 2.005(5) and 1.9992(5), respectively. They arise from bulk defects of, at present, unknown origins produced by the low energy Ar irradiation. When the sample was excited on the damaged (oxide free) surface, the luminescence spectrum was the same as that obtained by exciting through the oxide layer. However, a different ODMR spectrum was observed. Only the A and D resonances were detected. This observation clearly indicates that the defects responsible for the A and D resonances are bulk defects and that the Pb centers exist only at the oxide interface. A simple

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Fig. 2. (a) A typical ODMR spectrum at B 11(1 1 0) or (2 1 1). (b) Angular dependent line positions. The magnetic field was rotated in the {1 1 0} and {2 1 1} planes. spectral dependence study, done by placing optical filters prior to the photodetector, showed that the A and D resonances are largely associated with the broad deep luminescence band while the Pb center and P donor resonances are dominated by the FE and phosphorus BE emission. The Pb centers, being on the undamaged side of the wafer, are not associated with the damage; but they are made more easily detected because, we believe, the effective carrier lifetime is shortened by the deep level recombination paths produced by the damage. The positive sign observed for the A and D resonances is consistent with an assumption that the damage produced defects are directly participating in the radiative recombination processes for the deep emission band. The negative sign observed for the Pb center and P donor resonances, on the other hand, suggests that the recombination process for these defects reduces the other luminescent processes, such as the phosphorus BE emission and the deep damage-produced band. Because the sample was excited through the oxide layer, the optical pumping will produce a high density of electron-hole pairs (X) near the interface. Possible recombination processes include the free exciton (FE) decay, the recombination of the bound exciton (BE) at the P donor, and processes involving the deep centers. The P donor BE emission can be described by X + eO

, pO(BE)~pO"

(1)

Vol. 57, No. 8 MAGNETIC RESONANCE OF DANGLING BONDS AT THE Si/SiO2 INTERFACE The neutral-charge-state P donor, pO, serves as a core for binding an exciton. This bound exciton decays radiatively by emitting the BE luminescence. The carriers not involved in either the process of equation (1) or the FE decay will diffuse deep into the bulk, and they will recombine at the radiation damage produced defects. The deep luminescent processes at the damage produced defects will compete with the BE emission. An additional recombination process occurs near the interface. If the photo-induced carriers recombine only at the Pb centers at the interface, the intensity of the FE, BE and deep emissions would be quenched. The presence of the P donor resonance provides evidence for a spin dependent recombination process involving both the Pb center and P donor. Among the three charge states for the dangling bond, only the neutral charge state (singly occupied state) is paramagnetic. Therefore the process becomes TO(S) + pO(,~) _+ T-(I"~) + P+.

reduce the FE and BE emissions as well as the deep luminescence. If the process in equation (2) is radiative, the photon energy of the resulting emission should be less than 0.7 eV, which is the detection limit of the Ge detector, and therefore would have gone undetected by us. The overall scheme is consistent both with our observations and with the energy level schemes as published [5, 6]. In conclusion, the Pb center was observed by ODMR as an Si/SiO2 interface recombination center. It was established that the recombination mechanism is a spin dependent electron transfer from the shallow P donor to the neutral charge state of the deep dangling bond. This process results in a decrease in the net photoinduced carrier flux to the radiative recombination channels, such as the exciton related emission and the damage-produced deep center luminescence.

(2)

Here T represents the dangling bond and the arrow(s) in the parenthesis indicate(s) the spin state(s) of the bound electron(s). Since both electrons in the left hand side of equation (2) have spin of 1/2, they will align along the static magnetic field; in other words, both spins are "down." By flipping either one of them by means of an electron spin resonance, the electron transfer in equation (2) will be enhanced. On the other hand, the BE recombination process in equation (1) will be quenched because of the reduced p0 concentration. This explains why both resonances (the Pb center and P donor) are observed as negative signals. One would predict chain processes such as P+ + X -+ pO + h,

(3)

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

and (4)

12.

will exist and thereby complete the optical pumping cycle. Both spin independent processes will further

13.

T- + h ~ T °,

617

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