Role of surface antisite defects in the formation of heterojunctions

~

Solid State Communications, Printed in Great Britain.

Vol.44,No.8,

pp.1231-1234,

1982.

0038-1098/82/44123-04503.OO/0 Pergamon Press Ltd.

ROLE OF SURFACE ANTISITE DEFECTS IN THE FORMATION OF HETEROJUNCTIONS A. D. Katnani and G. Margaritondo, Department

of Physics, University

of Wisconsin,

Madison, Wi 53706,

R. E. Allen, Department of Physics, Texas A&M University,

College Station, Texas 77843, and

J. D. Dow, Department

of Physics, University of Illinois at Urbana-Champaign, IIi0 W. Green St., Urbana, Ii 61801, (Received

August

25, 1982 by F. Bassani)

Fermi level pinning positions were measured for Si overlayers on cleaved, n-type GaAs and GaP substrates. The observed positions are 0.73 ± 0.i eV and 0.98 ± 0.i eV above the top of the substrate valence band. Theoretical calculations of the surface antisite acceptor levels predict the same chemical trend on going from GaAs to CaP. Thls agreement and other arguments suggest an important role for surface defects in the formation of semiconductor heteroJunctions.

measured for GaAs-Ge. 5 That pinning position is reasonably close to the theoretical antisite acceptor levels for the GaAs(llO) surface and the measured pinning position for SI on n-type GaP is close to the theoretical antisite acceptor levels for GaP(IIO). Much more important, theory and experiment give the same chemical trend of the pinning position on going from GaAs to GaP. The experiments were performed at the University of Wisconsin Synchrotron Radiation Center with a Brown-Lien-Pruett "Grasshopper" monochromator. The experimental chamber and the experimental procedure were described in detail elsewherell, 12 and we shall give here only a short outline. Photoelectron energy distribution curves (EDC's) were taken with a double-pass cylindrical mirror anal~zer on freshly cleaved, n-type (n = 1018 cm -~) GaAs and GaP, and then on the same surfaces covered with Si overlayers of increasing thickness. The Si overlayers were deposited on room-temperature substrates by means of a miniaturized electron-bombardment source and their thickness was measured by a quartz-crystal monitor. The experiments were entirely performed under ultrahigh vacuum conditions at pressures of 3.5-20 x i0 -II torr. For the clean surfaces the top of the valence band, Ev, was given by the linearly extrapolated hlgh-energy edge of the corresponding EDC's. The clean-surface position of E F in the gap was given by the distance in energy between E v and the hlgh-energy cutoff of a freshly deposited AI or Au film EDC. After Si deposition the position of E v changed due to changes in the band bending. These changes were estimated from the shift in energy of the Ga 3d core level EDC peak. For low Si coverages these shifts coincided with those of the valence-band spectral features (which became invisible at high coverages). In contrast, the shift of the anion core-level features did not coincide with

We present evidence that surface antisite defects play an important role in the formation of abrupt heteroJunctions involving cleaved GaAs and GaP substrates interfaced with Si. In 1978 Spicer and his collaborators I proposed that Fermi-level pinning 2 at semiconductorsolid interfaces can be interpreted in terms of defect levels of the clean semiconductor surface. This hypothesis is supported by the fact that the pinning position is often insensitive to the nature of the other solid and is often the same for coverages ranging from a fraction of a monolayer to many monolayers. The surface antisite defects are the most satisfactory candidates for a defect-induced Fermi-level pinning since tbey are the most thermodynamically probable3, 4 and the most plausible on other grounds as well. 4 The experimental tests of the defect model have primarily involved metal-semlconductor interfaces. I Recently, however, M6nch et al. 5 found evidence that the defect model could be extended to semiconductor heteroJunctlons -and more specifically to the Fermi-level pinning during the deposition of Ge on cleaved GaAs. We present here some crucial tests that support M~nch's conclusion. From these results it appears that surface defects play an important role in the establishment of the most fundamental interface parameters, the built-ln potential and the interface band discontinuities. 6-I0 Our study involves synchrotron-radlatlon photoemlssion measurements of the pinning position of the Fermi level E F at GaAs(llO)-Si and GaP(IIO)-Si interfaces together with theoretical calculations of the levels associated with antisite defects at GaAs(llO) and GaP(II0) surfaces. The most important results supporting a defect-level pinning of E F are the following. The measured pinning position for Si on n-type cleaved GaAs, 0.73 eV above the top of the valence band Ev, coincides within the experimental uncertainty with that 1231

1232

SURFACE ANTISITE DEFECTS

the common shift of the valence-band and cation core-level features. This shows that the anion core-level peaks are strongly perturbed by the changes in chemical shift due to the formation of chemisorption bonds -- similar to what happens during the deposition of Ge on III-V and II-VI substrates. 1 2 Figure 1 shows the evolution of the position of E F in the gap for a GaAs-Si interface and for a GaP-SI interface. Notice in particular that the clean-surface position of E F in the GaAs gap corresponds to little or no band bending while that for GaP corresponds to a band bending of a few tenths of an eV. This is consistent with the experimental evidence that most III-V cleavage surfaces do not have intrinsic surface states in the gap -while GaP is an exception to that rule.12,14 For comparison, Norman et al. 13 found E F to be pinned 1.5 eV above E v on cleaved, n-type GaP. Data like that of Fig. 1 taken on several interfaces gave the following average distances between E F and EV: 0.73 eV for GaAs-Si and 0.98 eV for GaP-SI. A conservative estimate gives an experimental uncertainty of 0.I eV. As suggested by M~nch et el. 5, a possible cause of systematic errors in E F - E v is the band bending occurring within the substrate region probed by photoemission. This effect was minimized in our case by the use of 90 eV photons to excite photoelectrons from the valence-band and Ga 3d states to kinetic energies corresponding to a very short electron escape depth of the order of 6 A. From the bulk doping level we estimate the corresponding systematic error in the E F - Ev energy difference not to exceed 0.015 eV, which is negligible with respect to the overall experimental uncertainty. We emphasize that the above value for E F - Ev at GaAs-Si interfaces coincides with that measured by M~nch et el. 5 for both amorphous and ordered Ge overlayers on cleaved GaAs wlth low density of cleavage steps, 0.75 eV. We also emphasize that the increase in E F - E v on going from GaAs-Si to G a P - S i 0.25 eV, is well beyond our experimental uncertainty. The theoretical calculations for GaP were performed in the same way as those for GaAs4: First the bulk Green's function G O is evaluated by means of the analytic representation. 15 Next, the surface Green's function G is obtained from the one-electron Dyson's equation, G = G O + G0VG , where V is the perturbation of the Hamiltonlan when the crystal is theoretically cleaved and the surface atoms are allowed to relax. Finally, the defect levels are obtained from the Koster-Slater-Lifshitz equation, det(l - GVd) = O, where V d is the perturbation due to the defect. The bulk electronic structure is treated with the sp3s * empirical tight-blnding model of Vogl et al. 16 This model incorporates chemical trends and, because of the s* orbital, provides a reasonable description of the lowest conduction bands as well as the relevant valence bands. Defect potentials are determined from the scaled-atomlc-energy model of HJalmarson et el. 17 The atomic relaxation at the surface is taken from experiment. 18 It was argued earlier 19 that there was no reason

Vol. 44, No. 8

Ec >

I

LLI

~ ' ~ _ ~

....

~_#_F_

O_

0.5 GaAsCIIO)-Si

O=E,

GaP(llO)-Si 1.5

EF

0.5

O= Ev |

0

I

20 4O Si OVERLAYER THICKNESS (~,)

Fig. 1 - Shift in energy of the Fermi level at a cleaved GaAs or GaP surface as a function of the nominal thickness of a Si overlayer deposited on it. E v and E c are the valence and conduction band edges and the band gaps are shown with their room-temperature width. The clean-surface position of E F was obtained from the hlgh-energy cutoff of the EDC of freshly evaporated Au or A1 film and from the linearly extrapolated edge of the clean-substrate valence band EDC's. The subsequent shift in energy was deduced from the spectral shift of the Ga 3d core level. This shift is consistent with that of the valence-band features observed at very low coverages (open circles). The average final values for E F - E v obtained from this data and from other sets of data are 0.73 ± 0.I eV for GaAs-Si and 0.98 ± 0.I eV for GaP-SI.

to conclude that GaP has a relaxation substantially different from that of GaAs. A subsequent careful analysis of the LEED data showed that the relaxations for these two materials are indeed nearly identical. 18 Except for the measured surface relaxation, 18 the present theory of surface defects contains no adjustable parameters. I.e., the only further inputs are the same bulk electronic structure and table of atomic energies that are used in the calculations of bulk defect levels. 17

Vol. 44, No. 8

1233

SURFACE ANTISITE DEFECTS

In Fig. 2, we show the deep levels predicted for antlslte defects at the (Ii0) surfaces of GaAs and GaP, together with an indication of their occupancy in the neutral defect. This occupancy is obtained from simple electron-counting (i.e., from the fact that As or P has 2 more valence electrons than Ga) and from our ability, with the present theoretical method, to follow a level as it is continuously pulled down from the conduction bands (as Ga develops into the more electronegatlve As or P) or pushed up from the valence bands (as As or P ÷ Ga). There are two important features in the results of Fig. 2: First, there is both an acceptor level and a donor level for As or P on the surface Ga site, but only an aceeptor level for Ga on the surface As or P site. (As emphasized previously, an antlslte level is either empty or fully occupied in the neutral defect. An "acceptor" level is one that is empty, and a "donor" level is one that is occupied by two electrons having opposite spins.) In a degenerately-doped n-type semiconductor, the Fermi energy at the surface can be "pinned" at the lowest acceptor level. In a degenerately-doped p-type semiconductor, on the other hand, pinning can occur at the highest donor level. The theoretical results consequently predict that the Ga-slte defect can produce pinning on both n- and p-type semiconductors, but that the anlon-site defect can pin the surface Fermi energy only on n-type GaAs and GaP. The second important feature is the chemical trend on going from GaAs to GaP: All three of the predicted defect levels are higher in GaP, relative to the valence band edge. In particular, the pinning energies for n-type are raised from I.I (As on Ga site) and 0.85 eV (Ga on As site) to 1.4 (P on Ga site) and 1.25 eV (Ga on P site). (The pinning energy for p-type is raised from 0.7 eV, for As on the Ga site, to 1.0 eV, for P on the Ga site.) i.e., the pinning energy is predicted to he 0.3 or 0.4 eV higher for n-type GaP than for n-type GaAs, relative to the valence band edge, regardless of which antlslte is responsible. It should he emphasized here, as it has been elsewhere,4,17 that the theory is more reliable for predictions of chemical trends than for predictions of absolute energies, since uncertainties of several tenths of an eV can result from the simplicity of the model. In particular, charge-state spllttlngs and lattice relaxation around the point defect are omitted (although the stronger surface relaxation is included). The theoretically predicted trend -- a 0.3 - 0.4 eV increase in the pinning position as one goes from n-GaAs to n-GaP -- is in rather satisfactory agreement with the trend in the measurements described above. We have also calculated the levels for antislte defects at GaAs - Si and GaP - Si (II0) Interfaces. 20 The predicted chemical trends are similar to those for GaAs and GaP free surfaces, but the energy levels themselves appear to be too high by ~ 0.8 eV to explain the measured Fermi level pinning positions. This leads us to hypothesize that the defects responsible for pinning are somehow "sheltered"

GaP E=

GaAs Ec

hi


0

---<)-----O----0---

AsGQ

GaAs

Ev

PG(,

~ Ga.p

Ev

F i g . 2 - Deep e l e c t r o n i c energy levels predicted for the two surface antlslte defects in GaAs and GaP -- ASGa or PGa (anion on the Ga site) and GaAs or Gap (Ga on the anion site). The levels marked with solid circles are completely filled in the neutral defect (by two electrons having opposite spins), so that these levels can produce Fermi-level pinning in degenerately doped p-type semiconductors. The levels marked with open circles are completely empty, and can produce pinning in degenerately-doped n-type semiconductors. The lowest level for GaP is marginal; i.e., it could easily lie within the valence band as a "deep resonance" because of uncertainty in the theory (In GaAs, this level is already predicted to be such a resonance). The theoretical band gaps have the low-temperature values of 1.55 eV (GaAs) and 2.35 eV (GAP).

from interaction with the overlayer; i.e., they appear to sense a local environment that resembles a free surface more closely than an intlmately-bonded interface. This notion of sheltering of pinning defects appears to be required in order to explain why the Fermi level pinning position is so insensitive to the nature of the overlayer. The comparison between theory and experiment does not enable us to make a definitive assignment as to which defect is responsible for Fermi-level pinning in these materials, since the theory predicts two antislte acceptor levels in the same range of energies, both displaying nearly the same trend as the anion is changed from As to P. One of these levels is produced by P on the Ga site, and the other by Ga on the P site. The measured pinning positions are reasonably close in energy to the theoretical levels of both kinds of defects if one considers the experimental and theoretical uncertainties. Therefore, the most significant result of our study is the similarity of the chemical trends exhibited by the theoretical defect levels and the measured interface Fermi energy. This similarity, and the coincidence between the GaAs-Si and GaAs-Ge interface E F pinning positions, suggest that surface antlslte defects play an important role in the Fermi level pinning at III-V/SI interfaces. Acknowledgments - This work was supported by the NSF, grant DMR 82-00518 and by the Office

1234

SURFACE ANTISITE DEFECTS

of Naval Research (N00014-77-C-0537 and NO0014-82-K-0447). We are grateful to W. M~nch, W. E. Splcer, H. Wieder, R. N. Willlams, L. J. Brillson, and R. S. Bauer for illuminating discussions. Our experimental

Vol. 44, No. 8

work was greatly helped by the expert assistance of the staff of the University of Wisconsin Synchrotron Radiation Center (supported by the NSF, grant DMR 76-15089) and of Te-Xiu Zhao.

References.

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(1980). 18. S. Y. Tong, A. R. Lublnsky, B . J . Mrstlk and M . A . van Hove, Phys. Rev. B 17, 3303 (1978); A. Kahn, E. So, P. Mark and C. B. Duke, J. Vac. Sei. Teehnol. 15, 580 (1978); C. B. Duke, A. Paton, W. K. Ford, A. Kahn and J. Carelll, Phys. Rev. B 24, 562 (1981). 19. R. E. Allen, H . P . Hjalmarson and J. D. Dow, Surf. Sci. ii0, L625 (1981). 20. R. P. Beres et al., to be published.