Electronic properties of cleaved (110) and MBE-grown (100) InAs surfaces, clean and covered with an ultra-thin Ag adlayer

Applied Surface Science 70/71 (1993) 502-506 North-Holland

applied surface science

Electronic properties of cleaved (110) and MBE-grown (100) InAs surfaces, clean and covered with an ultra-thin Ag adlayer G. L e Lay, V . Y u . A r i s t o v 1 Centre de Recherche sur les Mecanismes de la Croissance, Cristalline-CNRS, Campus de Luminy, Case 913, F-13288 MarseiUe Cedex 9, France

J. K a n s k i , P . O . N i l s s o n Department of Physics, Chalmers University of Technology, S-41296 Goteborg, Sweden

U.O. Karlsson Materials Science, Royal Institute of Technology, S-10044 Stockholm, Sweden

K. H r i c o v i n i a n d J.E. B o n n e t LURE Bat 209 D, Centre Universitaire Paris-Sud, 91405 Orsay, France Received 31 August 1992; accepted for publication 22 November 1992

The initial electronic structure of the pseudomorphic InAs/GaAs(100) heterostructure as well as that of the A g / I n A s ( l l 0 ) interface at 20 K have been studied by synchrotron radiation photoelectron spectroscopy. In the first case we find that the valence band spectra show no evidence for the formation of bulk-like energy bands. In the second case we prove for the first time that upon deposition of minute amounts of Ag at low temperature onto cleaved InAs(ll0) substrates one induces a giant movement of the Fermi level well into the conduction band thus creating a strong two-dimensional electron channel at the surface.

1. Introduction One knows that an electron accumulation layer is easily formed on InAs surfaces [1]. This property makes the study of the initial formation of InAs heterojunctions [2] and m e t a l / I n A s interfaces very attractive [3]. In view of recent results [4] a thin InAs film on GaAs(100) a p p e a r e d to us as a most suitable system to test for the first time for this couple how strain affects the electronic states in pseudomorphic heterostructures, while the unreactive A g / I n A s ( 1 1 0 ) interface [5] was an

1 Permanent address: Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 14232, Russia.

ideal candidate to study at very low t e m p e r a t u r e the movement of the Fermi level E v at the early stages of the formation of a Schottky barrier. Synchrotron radiation (SR) valence band (VB) and core-level (CL) spectroscopy - with photoelectron mean free paths typically in the range 5 - 1 0 A - is a most valuable tool to analyse these problems.

2. Experiment InAs films were grown on a GaAs(100) substrate at the Swedish National SR facility MAX, where a dedicated M B E system has been attached to an analysis chamber housing an angle-

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

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resolving spectrometer at the T G M beam line. The Ag/InAs(110) system was studied at the SU6 beam line of the SuperACO storage ring at the L U R E , Orsay. InAs bars were cleaved in situ, they could be cooled to 20 K. The Ag deposition rate was calibrated with a quartz thickness monitor. In both places the total instrumental energy resolution was kept in the range 0.25-0.35 eV. _=

3. Coherently strained heterostructures

3.1. Pseudomorphic growth It has been proved recently that atomically smooth InAs films of well-defined thickness in the ML regime, buried in GaAs, can be synthesized by MBE at 420°C [4] by modulating the growth temperature during the interface formation. This study shows that the tetragonal distortion of the InAs unit cell quickly agrees with the continuum elastic theory but fails for 1 M L InAs where the lattice distortion is much larger than expected. As a matter of fact, in other works, only about 2 ML could be grown monolayer-bymonolayer before 3D island formation [6-9]. However, the use of Te as a surfactant permits to sustain the 2D coherent growth up to 6 ML [10]. We have prepared InAs films, 1 and 2 monolayers thick, on top of GaAs(100)-c(2 × 8) substrates in unusual conditions at high temperature ~ 550°C. These films were of high quality since we observed, respectively, 3 × 1 and 4 × 1 superstructures, both by R H E E D during growth and L E E D after transfer to the photoemission chamber, instead of the 2 × 3 [6] or 2 × 4 [8] usual reconstructions. These superstructures insure pseudomorphic growth and coherently strained InAs films. Ga3d, As3d and In4d CL spectra recorded at varying polar angles of emission O confirm this view. They were synthesized with Voigt functions using least-squares curve fitting with the same spin-orbit splittings, Lorenztian widths used for the cleaved (110) surfaces of GaAs [11] and InAs [12]. As an illustration we show in fig. 1 the deconvolution of a Ga + In spectrum for the 1 M L film. We note that the Ga

14

15

16 17 18 19 Kinetic Energy (eV) Fig. 1. Synthesisof a CL spectrum for 1 ML InAs/GaAs(100).

line is perfectly fitted with a single component with a Gaussian width just slightly broader (by 0.05 eV) than our experimental resolution. This proves that the GaAs topmost layer has recovered a bulk environment and that the interface is very sharp. Two components must be consistently introduced to fit the In 4d CL: Inl situated at the lowest binding energy (BE) is at 1.52 eV from the Ga line, which coincides with the position of one M L of In absorbed on GaAs(100) [11]. We thus assign it to the first atomic In plane at the abrupt interface. In2, situated at higher BE, shows that our nominal 1 M L film in fact is thicker than 1 ML, meaning that a second M L has started to form. This point agrees with the fact that for spectra taken at increasing polar angles the relative weight I n 2 / I n l increases markedly, while correlatively the relative weight G a / ( I n l + In2) strongly decreases, as expected for a sharp interface. Apart from a higher weight (Inl + I n 2 ) / G a and I n 2 / I n l the decomposition of the Ga + In CL spectra from our 2 M L film is identical and follows the same trend with varying 0. This coverage is thus correct: if a third layer had already started to grow we would have resolved a third component or noticed some broadening; on the

G. Le Lay et al. / Cleaved (110) and MBE-grown (100) InAs

504

contrary, the same Gaussian width G ( I n ) = 0.42 eV results from the fit. 3.2. Electronic states

We have studied for the first time the electronic structures of these highly strained (7.2% misfit) layers by AR-VB photoemission. In spite of the different thickness and surface structure of the 1 and 2 M L films which can be associated with quite different distorsions of the InAs unit cell, we note no pronounced difference in their normal emission VB spectra. We display in fig. 2a a set of such spectra for a series of photon energies, for the 2 M L film. Among the genuine VB features, two structures, B1 and B2, show clear dispersion with photon energy while two others S1 and $2 appear stationary. The two dispersing peaks can be immediately interpreted as due to interband transitions from valence bands along the F X direction in the Brillouin zone. The question we address now is to what extend these bands can be associated with the substrate and overlayers materials. In a previous work concerning a thin InAs film on InP(100) [2] it was found

that the dispersive states were those of InP unaffected by the InAs overlayer but the strain was much smaller than in the present case, 3.2% instead of 7.2%. Despite the much larger misfit we find here an identical situation: the energy separation between the X 3 and X 5 critical points, which is a direct quantitative characteristics of the band structure, can be determined, within 0.1 eV, from the data presented in fig. 2a since it is defined by the turning points of the dispersing bands. We find AE(X 3 - X 5) = 4.0 eV which is just the value determined experimentally for GaAs, while for InAs it is 3.3 eV [13]. Besides, the positions of these points, and that of ~nin seen as a small peak at - 4 eV, in the hu = 20 eV spectrum, are fully consistent with those measured [13] and calculated [14] for GaAs, but not InAs. Furthermore the dispersion of B2 nicely follows the experimental band structure of GaAs along the direction F X [15]. Non-dispersing structures can be either due to surface states or regions of high density of bulk states. The shoulder-like structure S1 near the VBM is very likely to be surface related, since the bulk density of states in this energy region is

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G. Le Lay et aL / Cleaved (110) and MBE-grown (100) InAs

quite low; this is also the case for the broad p e a k $2. Some further insight into the nature of the surface states can be gained by studying the angular dependence of the VB emission. In fig. 2b we show a set of spectra for the 1 M L film measured along the direction of the threefold reconstruction. We see that S1 disperses downwards along the FJ direction of the 1 X 1 unreconstructed SBZ and,becomes a sharp p e a k around the J1xl point. $2 can be followed up to about 0 = 18 ° and shows a symmetric dispersion around the J3×1 point of the 3 × 1 SBZ and is thus related to this reconstruction. For spectra recorded in the onefold direction of the surface unit cell ([110] azimuth), we only note the dispersion of S1 in the 1 x 1 BZ. The same situation occurs for the 2 M L film in both orientations: probably due to the smaller reciprocal unit cell and the weakness of $2 no dispersion within the 4 x 1 SBZ can be traced. To summarize the above discussion we have shown that the strained overlayer is not thick enough to develop bulk-like properties in the direction perpendicular to the interface. It ap-

0.8

pears thus not very meaningful to try to extract from the data quantities like band offsets for such thin layers. However, genuine surface states have been identified for the first time for each 3 x 1 and 4 x 1 reconstruction and their dispersion has been studied in the SBZ.

4. Ag Schottky barrier formation on InAs(llO) We just display in fig. 3 the most striking new results which were obtained upon Ag condensation at 20 K [12]: the EF position at the surface of n- and p-type I n A s ( l l 0 ) depends on the coverage in a dramatic fashion; it passes through a sharp, extremely high maximum at 0.01-0.1 ML. This giant (0.8 eV on p-type samples) movement of E F which goes well into the conduction band (0.4 eV above the CBM) thus creates a strong two-dimensional electron channel. We assign this spectacular overshoot at very low coverages to donor-like atom-induced states (AIS), as suggested by M6nch [16], that must be present at 0.4 eV above the CBM to pin E F at this position. At higher coverages one observes the onset of met-

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2 3 4 5 Ag coverage (ML) Fig. 3. E F positions on cleaved InAs(ll0) as a function of Ag deposition at LT.

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G. Le Lay et al. / Cleaved (110) and MBE-grown (100) ln,4s

allisation a n d A g clusters p r o d u c e m e t a l - i n d u c e d gap states, M I G S [17], which p i n E F a r o u n d the charge n e u t r a l i t y level of I n A s n e a r the C B M [18]. W e n o t e that previous studies of clean a n d a d s o r b a t e covered I n A s surfaces [3,19-21] have given i n d i c a t i o n s of the m o v e m e n t of the F e r m i level into the c o n d u c t i o n b a n d , b u t the giant initial overshoot m e a s u r e d h e r e c a n only b e comp a r e d with the similar j u m p f o u n d also with A g a d s o r b e d at 10 K o n cleaved I n S b ( l l 0 ) surfaces [221.

5. Summary U p o n studying the e l e c t r o n i c p r o p e r t i e s at the early stages of the f o r m a t i o n of two sharp interfaces involving I n A s as o n e of the m a t e r i a l s we have f o u n d striking results: (i) T h e electronic states at the highly s t r a i n e d I n A s / G a A s ( 1 0 0 ) h e t e r o j u n c t i o n s are d o m i n a t e d by the G a A s substrate b u l k states, b u t g e n u i n e surface states associated with different reconstructions are identified. (ii) A tiny a m o u n t of A g o n well cleaved I n A s ( l l 0 ) surfaces i n d u c e s at L T a giant downwards b a n d b e n d i n g ; this creates a strong 2D electron c h a n n e l at these surfaces.

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