p-GaSb(110) interface formation and band bending at low temperature

Applied Surface Science 68 (1993) 427-432 North-Holland

applied surface science

Synchrotron radiation study of Rb/p-GaSb(110) interface formation and band bending at low temperature J.J. B o n n e t a L. S o o n c k i n d t and P. Soukiassian b

a,

K.M. S c h i r m

b

S. Nishigaki

b,l, K.

Hricovini c, J.E. B o n n e t c

a Laboratoire d'Etudes des Surfaces, Interfaces et Composants, Unir,ersitd de MontpeUier II, 34095 Montpellier Cedex 5, France b Commissariat h l'Energie Atomique, Service de Recherche sur les Surfaces et l'Irradiation de la Mati~re, Bdtiment 462, Centre d'Etudes de Saclay, 91191 Gif sur YL'ette Cedex, France and D~partement de Physique, Unieersitd de Paris-Sud, 91405 Orsay Cedex, France c Laboratoire pour l'Utilisation du Rayonnement Electromagndtique, Universitd de Paris-Sud, 91405 Orsay Cedex, France Received 18 December 1992; accepted for publication 11 March 1993

The formation of the R b / p - G a S b ( l l 0 ) interface at liquid-nitrogen temperature is investigated by soft X-ray photoemission spectroscopy using synchrotron radiation. We use the Ga 3d and Sb 4d core levels to monitor the band bending during the interface formation. The deposition of Rb induces an overshoot of 0.3 eV with a final pinning position near the valence-band maximum. The band bending could be explained by the existence of donor levels located at about 0.3 eV above the valence-band maximum. The interface formed at low temperature was found to be much less reactive than observed previously in room-temperature studies.

1. Introduction

M e t a l / I I I - V semiconductor interfaces are subject of great activity in fundamental research since several decades [1,2]. A growing amount of applications in device technology and some specific properties of the clean cleaved I I I - V semiconductors like the lack of surface states in the fundamental band gap [3,4] motivated systematic studies of the initial adsorption, growth and electronic properties of those systems [5]. Alkali metals are known as low electronegativity and very low work function metals with the simplest electron configuration. They are often used as model systems in comparison with theoretical studies [6]. Additionally, they are prominent adsorbates due to their unique properties in forming negativeelectron-affinity electrodes [7] and their role as promoters of passivating reactions [6,8]. The ad-

1 Present address: D e p a r t m e n t of Electronics, Kyushu Institute of Technology, Kitakyushu 804, Japan.

sorption of alkali metals on I I I - V semiconductors has been studied with different surface-sensitive techniques like core-level and valence-band photoemission spectroscopy [9-11], scanning tunneling microscopy [12], electron energy-loss spectroscopy [13,14], and photoemission extended Xray absorption fine structure (PEXAFS) [15]. There are also some theoretical investigations using tight-binding or pseudopotentiai approaches [6,16,17]. The formed interfaces were mainly found to be non-reactive at room temperature [6,9-14] with the growth of two-dimensional overlayers [6,12]. Interestingly, it was also found that, at very low coverages, the alkali-metal atoms have two adsorption sites for G a A s ( l l 0 ) [11] and I n P ( l l 0 ) surfaces [18]. So far however, these studies have been generally focused on large-band-gap I I I - V semiconductors such as GaAs and InP. GaSb is a narrow-band-gap semiconductor with a small heat of formation (10 kcal/mol). Its 0.72 eV gap at room temperature (0.81 eV at 80 K) makes it technologically promising for opto-electronical

0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

J.J, Bonnet et al. / Rb /p-GaSb(110) interlace formation and band bending

428

applications. The room (RT) and low temperature (LT) electronic and chemical properties of GaSb interfaces were investigated for the Agand In-covered surfaces [19-22], allowing a general understanding of GaSb interface formation and making possible correlations between the Fermi-level pinning and overlayer metallization. In contrast to G a A s ( l l 0 ) and InP(ll0), there are only very few investigations on alkali-metal/GaSb interfaces including the pioneer total yield photoemission work of Viljoen, Jazzar and Fischer [19]. We have also recently investigated the R b / G a S b ( l l 0 ) interface formation and alkalim e t a l - G a S b ( l l 0 ) - p r o m o t e d oxidation at room temperature [24]. We found the adsorption of an alkali metal such as Rb on G a S b ( l l 0 ) surfaces [24] to be highly disruptive. In contrast to the general non-reactive behavior of alkali metals on other I I I - V compound semiconductor substrates [6,9-11], we found the R b / G a S b ( l l 0 ) interface to be strongly reactive at room temperature, depending on defects and dopant nature [24]. In fact, the R b / p - t y p e G a S b ( l l 0 ) interfaces were shown to be significantly more reactive than the corresponding R b / n - t y p e G a S b ( l l 0 ) [24]. This adsorbate-substrate reaction appeared to be esliq. N 2 temperature ~L

sential for the alkali-metal-promoted oxidation of GaSb(110), in contrast to the behavior of elemental semiconductors where the corresponding promoted oxidation was based on non-reactive interfaces [25]. In these views, it is interesting to investigate the formation of the R b / G a S b ( l l 0 ) interface at low temperatures (liquid N 2) since the similar alkali-metal/GaAs(110) interfaces are known to be non-reactive at this temperature [101. In this paper, we investigate the formation of the Rb/p-GaSb(llO) interface at liquid N 2 temperature by core-level soft X-ray photoemission spectroscopy using synchrotron radiation and compare our previous results obtained on the same interface at room temperature [24]. We show that, in contrast to the case of GaAs(ll0), the R b / p - G a S b ( 1 1 0 ) interface is still weakly reactive at low temperature.

2. Experiment

The photoemission measurements were performed at Laboratoire d'Utilisation du Rayonnement Electromagnfitique (LURE), Universitd liq. N 2 temperature

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Fig. 1. (a) Rb/p-GaSb(110): Ga 3d core-level spectra for various Rb exposures at low temperature. The photon energy was 82 eV. (b) Rb/p-GaSh(110): Sb 4d core-level spectra for various Rh exposures at low temperature. The photon energy was 82 cV.

J.J. Bonnet et al. / Rb / p-GaSb( l l O) interface formation and band bending

oration conditions (meaning same geometry and same current controlled by a digital a m p e r e m e ter) as for our room-temperature studies where the saturation coverage was reached at one monolayer (1 ML). One Rb monolayer corresponds to the completion of a full "physical" Rb layer corresponding approximately to a coverage of 0 = 0.5. Different Rb coverages between 0 and 1 ML were obtained by varying the time of evaporation. All the other experimental and data analysis details can be found elsewhere [24].

de P a r i s - S u d / O r s a y . The radiation emitted by the 800 MeV SuperAco storage ring was dispersed by a 3 m toroidal grating monochromator (3m TGM). The photoelectrons were collected at normal emission and their energy analyzed by an angle-resolved hemispherical electrostatic analyzer. The overall resolution (monochromator and analyzer) was better than 0.25 eV. In order to minimize surface photovoltage effects, we have used only highly doped degenerate GaSb samples (Zn doped, p = 6.7 x 10 is cm -3 at 300 K) which were cleaved in-situ at low temperature (T = 80 K) in an ultra-high vacuum chamber at pressures better than 2 x 10 - l ° Torr. To avoid charging effects, contacts were made along the side of the whole bars by evaporating, under vacuum conditions, Au on p-type crystals. The Au deposition was followed by a thermal annealing at 260°C in a mixed hydrogen-nitrogen ambient atmosphere. The deposition of pure Rb was performed by using a SAES Getters (Italy) chromate source which was very carefully outgassed. No pressure rise was observed during Rb deposition. The Rb coverage was determined by using the same evapliq. N 2 t e m p e r a t u r e

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3. Results and discussion We first examine the substrate core-level changes upon increasing Rb deposition at low temperature (80 K) on the p - G a S b ( l l 0 ) surface. Figs. la and lb display the G a 3 d and Sb4d core levels for Rb coverages between 0 and 1 Rb ML. For a coverage of 0.25 ML Rb, the Ga 3d peak is shifted by 0.3 eV to lower kinetic energy. With additional Rb deposition up to a Rb coverage of 1 ML, the Ga 3d is shifted back to higher kinetic liq. N 2 t e m p e r a t u r e

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Fig. 2. (a) G a 3d core-level d e c o m p o s i t i o n at various R b c o v e r a g e s for the R b / p - t y p e GaSb(110) l o w - t e m p e r a t u r e interface: b o t t o m p a n e l - clean; m i d d l e - 0.25 M L Rb; top - 1 M L Rb. The p h o t o n e n e r g y was 82 eV. (b) S b 4 d core-level d e c o m p o s i t i o n at various R b c o v e r a g e s for the R b / p - t y p e G a S b ( l l 0 ) l o w - t e m p e r a t u r e interface: b o t t o m p a n e l - clean; m i d d l e - 0.25 M L Rb; top - 1 M L Rb. T h e p h o t o n e n e r g y was 82 eV.

430

J.J. Bonnet et al. / Rb /p-GaSb(l lO) interface formation and band bending

energy and is stabilized at the initial kinetic energy position corresponding to the clean surface. These changes result only from band bending during R b / G a S b ( l l 0 ) interface formation since the influence of surface photovoltage is likely to be negligible because, as mentioned above, the GaSb sample is highly doped. They are further confirmed by the observation of the same rigid shifts at the Sb4d core level for the same Rb deposition sequence (fig. lb). Figs. 2a and 2b show respectively the decomposition of the Ga 3d and Sb 4d core-level spectra for the clean and Rb-covered (at coverages of 0.25 and 1 ML) G a S b ( l l 0 ) low-temperature surface into bulk (B), surface (S) and reacted (R) components. A least-squares deviation between spectra and fit is reached by a variation of position and intensities of the components. The fitting parameters like theoretical and Gaussian linewidth, spin-orbit splitting and branching ratio are kept fixed and are in very good agreement with previous work [26] for the clean surface spectra. For the G a 3 d spectrum at 0.25 ML Rb

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coverage, a third component (labeled R) has to be introduced to obtain fitting of the data. It is likely to result from a reaction between the Rb adsorbate and the Ga. Peak R is shifted by 0.3 eV to higher kinetic energy and is the first sign of a reaction between Rb and the G a S b ( l l 0 ) surface. This reacted component (peak R) is growing further in intensity at a Rb coverage of 1 ML (fig. 2a). We now study the same sequence on the corresponding Sb4d presented in fig. 2b. The Sb4d doublet also exhibits a reacted component (R) but only at the monolayer coverage with no reacted component at a Rb coverage of 0.25 ML like on G a 3 d (fig. 2a). However, at the monolayer coverage, this Sb4d reacted component is shifted by 0.9 eV to higher kinetic energy, which indicates a reaction between the Rb adsorbate and Sb. During A g / G a S b ( l l 0 ) interface formation, a similar " r e a c t e d " component shifted by less than 0.5 eV was interpreted in terms of migrating metallic Sb [20]. In order to compare this reactional behavior at low temperature with the results obtained in the room-temperature in-

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Fig. 3. (a) G a 3 d core-level decomposition at various Rb coverages for the R b / p - t y p e G a S b ( l l 0 ) room-temperature interface: bottom panel - clean; middle 0.1 ML Rb; top - 1 ML Rb. The photon energy was 100 eV. After ref. [24]. (b) Sb4d core-level decomposition at various Rb coverages for the R b / p - t y p e G a S b ( l l 0 ) room-temperature interface: bottom panel - clean; middle 0.1 ML Rb; top - 1 ML Rb. The photon energy was 100 eV. After ref. [24].

J.J. Bonnet et al. / Rb /p-GaSb(110) interface formation and band bending

terface formation, we have shown in figs. 3a and 3b the decomposition of G a 3d and Sb 4d spectra for the clean and Rb-covered surfaces at coverages ranging from 0.1 to 1 monolayer after ref. [24]. As can be seen by following the development of the reacted component R in fig. 3a for Ga 3d and fig. 3b for Sb 4d with increasing Rb deposition, the reaction has already started at 0.1 ML Rb coverage in both G a 3d and Sb 4d spectra for the r o o m - t e m p e r a t u r e deposition, indicating a reactivity significantly increased at room temperature when compared to the low-temperature results presented here. For the monolayer coverage with Rb, the reacted peaks are the dominating component features in the spectra of both Ga 3d and Sb4d core levels, which indicates that the reaction is significantly activated at room temperature for comparable Rb coverages. This behavior of the low-temperature G a S b ( l l 0 ) surface upon Rb deposition significantly differs from G a A s ( l l 0 ) where no reaction at all was reported with Cs or Rb adsorbate in the same temperature conditions [10]. Concerning the energetic positions of the reacted components (R), the G a 3 d reacted part grows with the same energy shift at room temperature and low temperature (0.3 eV), while the shift of the Sb 4d component appears to be larger than 1 eV at room temperature instead of 0.9 eV at low temperature. The energy position of the bulk component (B) in the decomposed spectra for the low-temperature R b / G a S b ( l l 0 ) interface has been used to evaluate the band bending due to Rb deposition at low temperature. The surface Fermi level position EFsp as a function of the Rb coverage is shown in fig. 4. There is first an initial increase in the energy position of the surface Fermi level EFsp as compared to the clean surface position under deposition of Rb. EFsp moves up to 0.25 eV above the valence-band maximum (VBM) at a Rb coverage of 0.25 ML and drops back toward the VBM for higher coverage with the existence of a clear overshoot. It reaches a pinning position near the VBM, the final EFsp pinning position occurring at about 0.05 eV below the VBM, although the uncertainty in the Ga peak position makes an accurate determination of this energy position rather difficult. The upward EFsp varia-

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tion is likely to be due to donor states located about 0.3 eV above the VBM. In the case of A g / G a S b ( l l 0 ) , it has been shown that several electronic surface states were involved in the process of formation of the interface at low temperature [22]. For a small metal coverage, Klepeis and Harrison [16] predict adsorption on the anion with transfer of an electron to the substrate and subsequent formation of a positively charged a d a t o m - s u r f a c e complex. The donor states pull EFsp above the VBM. For higher coverages, the effect of adatom proximity becomes important. Due to the very low electronegativity of alkali metals such as Rb (as compared to the Ga and Sb one), the formation of a dipole layer can be expected which will shift electrostatically donor levels [16,27] downward, resulting in the motion of the EFsp downward.

4. Conclusions

In conclusion, our results indicate that the reactivity of Rb on GaSb is strongly reduced at low temperature, which is in qualitative agreement with the behavior of other alkali-metal/ G a A s ( l l 0 ) interfaces. However, while there is no reaction at all at low temperature in the case of alkali metals on a GaAs substrate, a small reac-

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J.J. Bonnet et al. / Rb /p-GaSb(110) interface Jbrmation and band bending

tion is still taking place in the case of the G a S b ( l l 0 ) surface. The growth of the reacted G a 3 d and Sb4d components at lower binding energy always indicates the presence of surface disruption with formation of a R b - s u b s t r a t e complex. The extent of the reactions is shown to be significantly affected by the temperature. For the Rb monolayer deposition, which corresponds to the pinning position, Ga 3d and Sb 4d core-level spectra are not dominated by the reacted components, as previously reported at room temperature [24]. This feature clearly suggests that the breaking of the G a - S b bond by the Rb adsorbate is significantly influenced by the temperature. Finally, there also is no significant difference concerning the band bending when comparing our results to previous low-temperature experiments for other metals such Ag [20] or In [21]. The final Fermi-level pinning position remains practically the same for these metals while no overshoot is observed on p-type at room temperature, in significant contrast to the case of Rb [24].

Acknowledgments We are grateful to Drs. A. Gouskov and L. Gouskov for providing GaSb ingots and for Halleffect measurements, and to Ph. Brun, V. Coronato and the staff of Laboratoire d'Utilisation du Rayonnement Electromagndtique, Universit6 de Paris-Sud/Orsay for technical assistance.

References [1] L.W. Wilmsen, J. Vac. Sci. Technol. 19 (1981) 279, and references therein. [2] E.H. Rhoderick and R.H. Williams, Metal-Semiconductor Contacts (Oxford Science, Oxford, 1988), and references therein. [3] X. Zhu, S.B. Zhang, S.G. Louie and M.L. Cohen, Phys. Rev. Lett. 63 (1989) 2112, and references therein. [4] H. Carstensen, R. Claessen, R. Manzke and M. Skibowski, Phys. Rev. B 41 (1991) 880.

[5] I.P. Batra, Ed., Metallization and Metal-Semiconductor Interfaces, N A T O Advanced Studies Institute Ser. B 195 (Plenum, New York, 1989), and references therein. [6] H.P. Bonzel, A.M. Bradshaw and G. Ertl, Eds., Physics and Chemistry of Alkali Metal Adsorption, Vol. 57 of Materials Science Monographs (Elsevier, Amsterdam, 1989), and references therein. [7] R.L. Bell, Negative Electron Affinity Devices (Clarendon, Oxford, 1973). [8] P. Soukiassian, M.H. Bakshi and Z. Hurych, J. Appl. Phys. 61 (1987) 2679: P. Soukiassian, H.I. Starnberg, T. Kendelewicz and Z. Hurych, Phys. Rev. B 42 (1990) 3769. [9] M. Prietsch, C. Laubschat, M. Domke and G. Kaindl, Europhys. Lett. 6 (1988) 451. [10] M. Prietsch, M. Domke, C. Laubschat, T. Mandel, C. Xue and G. Kaindl, Z. Phys. B 74 (19891 21. [11] T. Kendelewicz, P. Soukiassian, M.H. Bakshi, Z. Hurych, I. Lindau and W.E. Spicer, Phys. Rev. B 38 (19881 7568. [12] L.J. Whitman, J.A. Stroscio, R.A. Dragoset and R.J. Celotta, Phys. Rev. B 44 (1991) 5951: Phys. Rev. Lett. 66 (1991) 1338. [13] S. Valeri, M. Lolli and P. Sberveglieri, Surf. Sci. 238 (19901 63. [14] N.J. DiNardo, T. Maeda Wong and E.W. Plummer, Phys. Rev. Lett. 65 (1990) 2177. [15] K.M. Choudhary, P.S. Mangat, H.I. Starnberg, Z. Hurych, D. Kilday and P. Soukiassian, Phys. Rev. B 39 (1989) 759. [16] J.E. Klepeis and W.A. Harrison, J. Vac. Sci. Technol. B 7 (1989) 964. [17] G. Allan, M. Lannoo and C. Priester, J. g a c . Sci. Technol. B 8 (19901 980. [18] T. Kendelewicz, P. Soukiassian, M.H. Bakshi, Z. Hurych, 1. Lindau and W.E. Spicer, J. Vac. Sci. Technol. B 6 (19881 133l. [19] P.E. Viljoen, M.S. Jazzar and T.E. Fischer, Surf. Sci. 32 (1972) 506. [20] D. Mao, A. Kahn and L. Soonckindt. Phys. Rev. B 40 (1989) 5579. [21] Z.M. Lu, D. Mao, L. Soonckindt and A. Kahn, J. Vac. Sci. Technol. A 8 (1990) 1988. [22] J.J. Bonnet and A. Doukkali, J. Vac. Sci. Technol. A 9 (19911 2239. [23] Y. Chang, D. Mao, A. Kahn, J.J. Bonnet, L. Soonckindt and G. Lelay, J. Vac. Sci. Technol. B 9 (1991) 2349. [24] K.M. Schirm, P. Soukiassian, P.S. Mangat, Z. Hurych, L. Soonckindt and J.J. Bonnet, J. Vac. Sci. Technol. B 10 (1992) 1867. [25] P.S. Soukiassian, T.M. Gentle, M.H. Bakshi and Z. Hurych, J. Appl. Phys. 60 (1986) 4339. [26] D.E. Eastman, T.C. Chiang, P. Heimann and F.J. Himpsel, Phys. Rev. Lett. 45 (19801 656. [27] J.J. Bonnet, L. Soonckindt, A. Ismail and L. Lassabat~re, Thin Solid Films 151 (1987) 1(13.