Near-surface defects in amorphous semiconductors related to hydrogen incorporation

]OURNAL OF

Journal of Non-Crystalline Solids 137&138 (199l) 327-330 North-Holland

NON-CITILLINESOLIDS

NEAR-SURFACE DEFECTS IN AMORPHOUS SEMICONDUCTORS RELATED TO HYDROGEN INCORPORATION Shu JlN7 Samer ALJlSHltand Lothar LEY Universit~t Erlangen, Institut {fir Technische Physik, Erwin-Rommel-Str.1, D-8520 Erlangen, Federal Republic of Germany Using total yield photoelectron spectroscopy we demonstrate that the room temperature (RT) hydrogenation of amorphous germanium (a-Ge) surfaces leads to the creation of a deep defect band 0.45 eV above £~ that anneals out at ~ 300°C. We argue that this defect is related to the high concentration of hydrogen at the growth surface and is responsible for the high surface defect density (~ 10is cm -3) in g.d. a-Ge:H and a-Si:H as derived from yield and PDS measurements. We show that the surface defect density in g.d. a-Si:H can be reduced through hydrogen dilution and through post-deposition anneals but not through post-hydrogenation. Drawing from the analogy with c-Si we propose that a small fraction (~ 10-4) of hydrogen in the bond center position is responsible for the observed defect which is different from the more common dangling bond defect.

I.

INTRODUCTION

A minimum concentration of 5 - 10 at. % hydrogen is needed to lower the density of deep defects in a-Si or a-Oe from 1019- 102o cm -a to an astounding 1015 - 1016 cm -3. Hydrogen is, however, only a necessary but not a sufficient condition for low defect densities as illustrated by the high defect density (~ I0 Is cm -3) in RT g.d. a-Si:H with a hydrogen content of about 30 at. % (Re{. i). More important, even a-Si:H prepared so as to have the lowest possible bulk defect density has a near-surface defect density of typically 1018 cm -a as derived by PDS and yield measurements 2'a'4. This high defect density also appears to be connected with a high concentration of hydrogen at the growth surface, bonded in the form of "polyhydricles" [both multiply bonded X H ~ ( X = Si or Ge and * = 2 and 3) and polyhydride (XH2)2] as revealed by photoemission and electron loss enegy spectroscopy s'6'r. We have investigated the connection between defect density and hydrogen content at the surface of tetrahedrally bonded amorphous semiconductors by measuring the nearsurface electronic structure of evaporated a-Ge as a function of hydrogen content using total yield photoelectron spectroscopy in combination with the Kelvin probe, a-Ge was chosen for experimental reasons (lower evaporation temperature and less reactive than a-Si) but the results pertain just as well to a-Si and we shall use the analogy between the two materials freely in what follows. A full account of this work

is given in Re{. 8. 2.

EXPERIMENT Evaporation, g.d. deposition, hydrogenation by a hydrogen plasma, the determination of the sample work function by the Kelvin method, and the yield measurements of a-Ge(:H) and a-Si:H were all done in a series of interconnected ultra high vacuum chambers. The evaporated a-Ge films were hydrogenated in a rf H2 plasma with hydrogen flow and pressure at 5 sccm and 0.6 mbar, respectively, aSi:H films were prepared by standard capacitive rf (13.56 M Hz) glow discharge decomposition of silane and hydrogen gas mixtures. The deposition parameters were: substrate temperature 250°C, gas flow rate 5 sccm, and rf power density 40 m W / c m -2. The photoelectric yield Y(h~), is defined as the number of electrons emitted into vacuum per incident photon of energy hw. In amorphous solids }'(h~') is simply the integral over the occupied density of states (DOS), 9~, from the vacuum level E~.~cto an energy Ev~c-hw and 9v (Ev~c-hu.,) is retrieved by differentiating Y(h~,) with respect to h~,,. The probe depth of the method is 5 0 - i 0 0 . 4 . More details may be found in Re{. 4. 3.

RESULTS AND DISCUSSION Figure 1 shows yield spectra of a-Ge and RT hydrogenated a-Ge:H before and after a series of 30 min

*Present address: Department of Physics, Boston University, Boston, MA 02215. tpresent address: Department of Physics, University of Bahrain, Bahrain. 0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

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Taw ( e V ) FIGURE i Yield spectra of evaporated a-Ge after different treatments. Curve 1: freshly evaporated and after exposure to molecular hydrogen; Curve 2: after exposure to atomic hydrogen at RT; Curves 3 to 5: after 30 rain annealing steps at the indicated temperatures; Curve 6: after rehydrogenation with atomic hydrogen at RT. The vertical marks in the low energy region indicate the position of E] as obtained by the Kelvin method.

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FIGURE 3 Occupied density of states of undoped a-gi:H prepared under various hydrogen dilution ratios.

Fermi level moves closer to the top of valence bands. Rehydrogenation at RT restores the defect band to its original strength (curve 6 is identical to curve 2) without affecting the region above 5.0 eV significantly. From these and further arguments detailed in Ref. 8 we conclude that there is a hydrogen-induced defect state in the gap of a-Ge:H and by analogy in a-Si:H as well. It is distinct from the usual dangling bond state on the account of its energy and annealing behavior. We further maintain that this defect contributes at least partially for the high defect density found in RT g.d. a-Si:H and at the surface of g.d. a-Si:H where yield measurements give an effective density of 3xi018 cm -3 over the sampling depth 4. In agreement with our present findings this high surface defect density can be reduced by post-deposition annealing (compare Fig. 2) or by hydrogen dilution of Sill4 during growth (see Fig. 3) albeit not through post-hydrogenation (refer to Fig. 4 in Ref. 4). Hydrogen dilution reduces the surface defect density from 3×I018cm -3 for tile undiluted a-Si:H to 3x10~'cm -3 for 1:20 diluted one with the amorphous structure confirmed

329

by both ellipsometric and Raman spectroscopy, and at the same time suppresses the near-surface hydrogen incorporation as witnessed by the shift of the valence band edge towards lower energy. The result points again towards the connection of defect density and hydrogen content alluded to in the introduction. Johnson et al (Ref. 9) have reported hydrogen-induced defects in c-Si that can be removed partially upon 350°C annealing. Electronic structure calculationsI° show that the bond center (BC) or bridging position is the most stable site for neutral hydrogen. In this three center bond configuration a single electron occupies a non-bonding orbital which falls into the gap of c-Si and thus gives rise to the observed paramagnetic deep defect level I]. If only about a fraction of 10-4 of the hydrogen atoms in the near-surface layer or in the RT g.d. a-Si:H would occupy such a bridging position it would account for the observed defect density as well as for its paramagnetic character. It is worth pointing out that neutral hydrogen in the BC position turns into H+ when the Fermi level drops below the defect level in p-type material. This would account for the conspicuous drop in near-surface defect density seen in the yield spectra of B-doped a-Si:H (refer to Ref. 4). ACKNOWLEDGMENTS The authors would like to thank Wolfgang Stiepany, H. B. Rose and A. Dittrich for their technical assistance. S. J. acknowledges support by the Max-Plank-Society. This work was supported in part by the Bundesminister fiir Forschung and Technologie under contract No. 0328962A. REFERENCES I. J. C. Knights, G. Lucovsky, and R. J. Nemanich, J. Non-Cryst. Solids 32 (1979) 393. 2. H. Curtains and M. Favre, in Advances in Amorphous Silicon and Related Materials, edited by H. Fritzsche (World Scientific, Singapore, 1988), p. 329. 3. A. Maruyama, J. Z. Liu, V. Chu, D. S. Shen, and S. Wagner, J. Appl. Phys. 69 (1991) 2346. 4. K. Winer and L. Ley, as in Ref.2, p. 365. 5. L. Ley, in Hydrogenated Amorphous Semiconductors, Semiconductors and Semimetals Vol. 21B, ed. J. I. Pankove (Academic Press, Orlando, 1984) p. 385.

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8. S. Jin and L. Ley, Phys. Rev. B 44 (1991) 1066.

10. C. G. Vad de Walle, in Hydrogen in Semiconductors, Semiconductors and Semimetals Vol. 34, eds. J. I. Pankove and N. M. Johnson (Academic Press, Boston 1991) ,. 585.

9. N. M. Johnson, F. A. Ponce, R. A. Street, and R. J. Nemanich., Phys. Rev. B 35 (1987) 4166.

11. Y. V. Gorelkinskii and N. N. Nevinnyi, Engl. Transl. Soy. Tech. Phys. Lett. 13 (1987)45.

7. H. Sasaki, M. Deguchi, K. Sate, and M.. Aiga, J. NonCryst. Solids 114 (1989) 672.