GaAs(110) interface formation

509

Surface Science 126 (1983) 509-517 North-Holland Publishing Company

INVESTIGATION

OF THE Cu/GaAs(llO)

D. BOLMONT b, V. MERCIER, C.A. SEBENNE

P. CHEN

Laboratoire de Physique des Solides, associP au CNRS, Jussieu, F-75230 Paris Cedex 05, France Received

24 August

1982; accepted

for publication

INTERFACE ‘, H. LOTH

FORMATION

a

d and

UniversitP Pierre et Marie Curie, 4 Place

8 October

1982

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss spectroscopy (ELS) and photoemission yield spectroscopy (PYS) measurements are performed on a set of UHV cleaved n- and p-GaAs( 110) crystal surfaces as a function of Cu coverage f? obtained by in-situ evaporation in the range 10m3 5 0 5 200 monolayers (ML). A model of interface formation is developed where, first, a uniform, non-metallic double layer, including a two-dimensional compound associating Cu. As and Ga atoms, forms to be completed below 2 ML; then, growth of metallic Cu islands occurs. It explains the full set of experimental results, including the observed electron state density changes at the interface and the characters of the Schottky barrier in formation.

1. Introduction Several models are used to explain the Schottky barrier properties of GaAs( 110): three main ideas are presently considered to justify the presence of electron states in the gap of the semiconductor which are needed to induce the observed Fermi level pinning at the interface with any metal. The first involves the formation of new bonds at the interface [1,2], the second involves the formation of surface defects such as vacancies or adatoms [3-51, and the third involves the metal-induced deformation of the GaAs surface structure [6,7]. The three effects are not mutually exclusive, and any one of them is insufficient to fully explain the electronic properties of any given system. Moreover the local atomic arrangement at the interface may differ widely depending on the nature of the metal and the thermal conditions of preparation, and metals ’ Work supported in part by the Direction Gtnerale des Telecommunications, 81-35-053-00-790-92-45-BCZ. b On leave from Conservatoire National des Arts et Metiers, Paris, France. ’ ‘On leave from Fudan University, Shanghai, People’s Republic of China. d Permanent address: 2. Physikalische Institut der Rheinisch-Westfalischen schule, D-5100 Aachen, Fed. Rep. of Germany.

0039-6028/83/0000-0000/$03.00

0 1983 North-Holland

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D. Boimonr et al. / Cu/GaAs(llO)

interface formation

such as Au [2] and Ag [7] have quite different behaviour when deposited, even at room temperature on the clean cleaved GaAs( 110) surface. In the present work, Cu, the lightest of the three transition metals, with Ag and Au of the first column in the periodic table, is deposited on GaAs( 110) and its effect studied in the same way as the effect of Ag in an earlier work [7]. The Cu/GaAs( 110) interface formation has not been studied up to now and a set of different techniques in surface physics has been used here to get as detailed an information as possible on both structural and electronic properties, limiting the investigation to the case where the substrate is kept at room temperature during the full cycle of preparation, metallization and measurements. In the present paper, after briefly indicating some specific details of the experimental procedure, the results obtained from low energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron loss spectroscopy (ELS) and photoemission yield spectroscopy (PYS) are successively presented before discussing a comprehensive model of the Cu/GaAs( 110) interface.

2. Experimental procedure The multiple purpose ultra-high vacuum (UHV) apparatus used for the experiments has been described elsewhere [8]. The cylindrical mirror analyser with coaxial electron gun used for AES was also used here for ELS measurements. A set of monocrystalline GaAs bars was studied, the free carrier densities of which were respectively n = 2 x 1016 cme3, nsi = 1.5 x 10” cmp3 and pZn = 1.6 X lOI cmW3. The sample, 6 X 4 mm* in cross section, was cleaved under UHV. It gave a sharp 1 x 1 LEED pattern and no trace of impurity at the sensitivity of AES. Copper was evaporated from a Knudsen-type cell with a graphite crucible. The deposition rate was carefully measured in absolute value, using a quartz balance calibrated by interferometric measurements. The stability and reproducibility of the Cu flux were regularly controlled with the quartz balance which could replace the sample at its very position. Depending on the coverage range under study, from less than low3 to more than lo2 ML, four deposition rates were used, roughly 10p4, 10v3, lo-* and 10-l ML s- ‘, where 1 ML (monolayer) corresponds to 8.8 X lOI atoms cm-*, that is, the number of Ga and As atoms in the (110) atomic plane. The sample remained close to room temperature during Cu deposition. The photoemission yield versus photon energy between 4 and 6.5 eV was recorded after cleavage and after each Cu deposition. LEED, AES and ELS measurements were performed at some intermediate coverages but usually neither after cleavage nor after coverages below 10-l ML. During evaporation as well as during measurements the total pressure in the vessel remained below 4 X 1O- lo Torr. The contamination by residual gases was carefully checked and the time schedule of a whole experiment was chosen to make the effect of contamination negligible.

D. Bofmontet al. / Cu/GoAs(l10) 3.

interfaceformation

511

Results

3.1. LEED As Cu is deposited, the sharp 1 X 1 diagram of the clean GaAs( 110) surface fades away slowly. It is still visible at 20 ML coverage but with a very low intensity and high background. It is not observable anymore beyond 30 ML. Contrary to the case of Ag [7], no evidence of an epitaxial growth of CU metal is found from LEED measurements and the Cu film is certainly highly polycrystalline.

3.2. AES The typical ways of behaviour of the low energy Auger peak relative intensities of Ga, As and Cu are shown in fig. 1 where somewhat arbitrary curves have been drawn simply to underline the different variations which are observed and can be summarized as follows. Comparison with an exponential law shows that the As signal decreases first up to one to two ML of Cu as if the system were progressively covered with a uniform layer, and then decreases more slowly as in the case of an inhomogeneous film formation. The Ga signal decreases slower than As from 0 to 0.5 ML, and then faster from roughly 0.5 to 1.5 ML of Cu where it starts to follow the same curve as As. The Cu signal behaviour is complementary to that of Ga: it shows up very slowly in the spectra, up to 0.5 ML, and then up to almost 2 ML it increases linearly before tending progressively towards saturation. If compared to the behaviour of Ag and Ga signals in the case of Ag deposition [‘7], the Cu and Ga evolutions differ markedly in the first stage, up to 0.5 ML, and then become qualitatively similar. -1 3

4

1 .a

ab, !i Y

Cu _GaAs (1101

e

% 80

1

2

3

4

5

6

@(ML) Fig. 1. Relative peak to peak Auger signals of Cu (62 eV), Ga (48 eV) and As (31 eV) under primary

beam energy of 2 keV as a function

of Cu coverage

on a clean cleaved GaAs( 1 10) surface.

512

D. Belmont et al. /

Cu /GaAs(I

10) interface formation

3.3. ELS Fig. 2 shows the doubly differentiated electron energy cleaved GaAs(l10) surface covered with Cu layers of from 0 to 200 ML. The spectrum at zero coverage is well understood [lo-121: losses occur at 3.6, 5.9, 10.4 (with 12.2 eV), 16.3, 19.9, 21.3 and 23.4 eV. For Cu coverages

Fig. 2. Doubly differentiated different thicknesses.

loss spectra of a clean cleaved GaAs(ll0)

loss spectra of a clean different thicknesses, known and fairly well shoulders at 8.4 and up to about 6 ML, the

covered with Cu layers of

Il. Bolnmr et al. / Cu/GaAs(l

IO) interface formation

513

Fig. 3. First derivative of the photoemission yield spectra with respect to photon energy, called effective density of filled states N*(E), versus photon energy for an n-type Gags sample at different Cu coverages. Both semilogarithmic and linear plots of the same curves are presented.

main loss peaks of GaAs remain well-pronounced; only the shoulders at 8.4 and 12.2 eV have disappeared and at 6 ML coverage, an additional loss at 4 eV, due to Cu, has grown on top of the 3.6 eV loss of the clean surface. This Cu loss becomes the most prominent feature for coverages from 10 ML. An additional Cu characteristic loss at 7 eV becomes observable in the 10 ML spectrum while, beyond 20 ML, the GaAs loss peaks vanish. At 200 ML of Cu, only features associated with the metal are observed [13]. As in Auger spectra, the changes in electron loss spectra upon Cu deposition differ from the case of Ag where significant variations are already observed in the 2 ML coverage range [ 111. 3.4. PY?s As in our previous studies of similar systems [6-Q the results of PYS measurements are presented in terms of effective densities of filled states

514

D. Belmont et al. / Cu/GaAs(llO)

interfaceformation

Cu-GaAs (110)

Fig. 4. Variations of the work function +,, (n-type substrate), +,, (p-type substrate) and ionization energy @ of GaAs(l IO) versus Cu coverage as deduced from N,*(E) curves such as in fig. 3.

N,*(E) which are deduced from the yield spectra by taking the first derivative with respect to photon energy. In fig. 3, a selected set of typical state densities obtained after increasing Cu coverages are shown in the case of an n-type GaAs substrate. Similar data are also obtained for p-type substrates. Both the semilogarithmic and linear plots are presented to show more clearly the behaviour of the curves respectively at low and high photon energies in the studied range. The work function $J governs the low energy threshold of the yield curves, from which it can be deduced. Comparison of the measured densities of filled states on both n- and p-type substrates leads to a good estimate of the ionization energy @ of the semiconductor at least in the low coverage range. These determinations have been discussed in earlier publications [6-81 and the results in the case of the present system are summarized in fig. 4. On n-type GaAs substrates, the work function $” switches at coverages in the lop3 ML range from its value for the clean surface to a value of about 4.9 f 0.1 eV. Since @ is found to be constant up to a coverage slightly higher than 1 ML, where it ceases to be safely determined, it means that the pinning effect which is usually observed on GaAs upon contamination is also effective in the case of Cu and the barrier height establishes at 0.9 k 0.1 eV. On p-type substrates, the work function $r does not exhibit significant changes below 0.5 ML and then it decreases at higher coverages. At high coverages, the work function of polycrystalline Cu is found to be: +cU = 4.95 f 0.05 eV.

D. Bolntonf et al. / Cu/GaAsjiiO)

interface formation

515

If the changes of the density of filled states are now considered, besides the obvious increase observed in the low energy region (labeled a in fig. 3) corresponding to the gap of bulk GaAs, the decrease on the high energy region (labeled b in fig. 3), corresponding to bulk valence states of GaAs, should be noted: such a decrease at low metal coverage has been observed neither in the case of Ag [7] nor with other metals such as Ga [8]. A more detailed analysis of the state density changes will be proposed in the next section.

4. Discussion In order to discuss the set of experimental results which have been presented, the approach will be the following: considering the AES results, a picture of the Cu/GaAs( 110) interface formation will be established and then it will be confronted to the ELS and PYS observations. The behaviour of the Auger signals shows that three successive steps can be ~stinguished in the Cu deposition process. First, from 0 to roughly 0.5 ML, the delayed decrease of Ga associated with the delayed increase of Cu and a linear decrease of As can be explained by the formation of a two-dimensional compound with supposedly one Cu atom per GaAs surface unit cell. The structure of the new Cu/GaAs surface unit cell should be such that the Ga atom remains on the outside of the system while the Cu atom goes inside, the As atom remaining in between. Then, roughly from 0.5 to about 1.5 ML or more, Cu progressively forms a uniform monolayer bonded to the already formed two-dimensional compound. The association of these two parts forms the double layer which constitutes the final interface. This corresponds to the coverage range where the three Auger signals exhibit essentially a linear variation. Then the Auger signals all go slowly towards either zero for Ga and As or saturation for Cu at a coverage of a few lo2 ML, which is typical of the growth of Cu islands progressively covering up the whole surface when their thickness reaches the 100 A range. The discussion of the loss spectra will be limited to their variations at intermediate Cu coverages since the interpretation of the spectra of both the clean GaAs surfaces [lo- 121 and polycrystalline Cu [ 131 is known. Up to 2 ML of Cu, the loss spectrum remains practically the same as for the clean substrate: it is simply attenuated and the features are slightly widened. In particular, the structure at 10.4 eV (surface plasmon) and it shoulders, associated with surface states, are the most affected and no effect associated with metallic copper is observed: this behaviour is compatible with the picture given above where Cu, below 2 ML, forms a double layer with the first atomic plane of GaAs. Then, at higher Cu coverage, losses associated with metallic copper progressively appear and dominate the spectra while the main GaAs features

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D. Belmont et al. / Cu/Ga~s(lI0)

interface formation

remain observable up to about 10 ML: this is in agreement with the growth of Cu islands leaving shrinking areas of the substrate with a low Cu coverage. Therefore the changes of the electron loss spectra upon Cu deposition agree with the picture of the interface formation deduced from Auger measurements. Looking now at the photoemission yield spectra, the main point which makes the case of Cu different from Ag [7], Ga [8] or In [9] is the important decrease of the density of filled states at “high” photon energy: for example at the point labeled b in fig. 3 (hv - 6.2 eV, about 0.8 eV below the valence band edge of GaAs), the decrease is about 40% at 1 ML and almost 60% at 2 ML. Moreover the decrease rate below 1 ML is about three times higher than between 1 and 2 ML. In order to explain such a strong decrease, it is necessary to admit that a number of valence states have vanished from the energy range under study, which means that Ga-As bonds have been replaced by other bonds, where Cu atoms must be associated, which generate new electron states located deeper into the valence band of GaAs beyond the investigated energy range. The model given to explain the AES results appears appropriate to explain at least qualitatively the preceding photoemission result. If the Cu atoms form a two-dimensional compound with the first layer of GaAs it means that new and likely stronger bonds are formed. So not only GaAs valence states are suppressed, but also the layer formed attenuates the contribution of the GaAs valence states which remain beneath. Consequently, if the composition of the two-dimensional compound associates one Cu atom per Ga-As couple of atoms (it may be a rough approximation), the attenuation factor is three times larger than with a simple Cu overlayer. This is no longer true for the next Cu layer which attenuates the bulk signal as a uniformly growing layer, and justifies the smaller decrease rate of the photoemission signal between 1 and 2 ML. The other features deduced from the PYS measurements, namely the growing density of filled states in the lower part of the gap of GaAs (see the point labeled a in fig. 3), and the band bending behaviour shown in fig. 4, resemble qualitatively those observed in our previous studies with other metals [7-91. Thus the model which has been discussed then [7-91 should remain qualitatively valid. However, some quantitative differences show up in the case of Cu which may bring out some interesting remarks. In our model, the early pinning of the Fermi level on n-type substrates is explained by the penetration in the gap of the empty surface state band associated with Ga atoms upon removal of the clean surface reconstruction. In the case of Cu, the Fermi level pinning position is lower in the gap (EF is about 0.9 eV below the conduction band edge, compared to 0.7 eV in most other cases). This can be understood if the effect of Cu is to “force out” a little the Ga atoms off the surface plane, and then the corresponding surface state band penetrates deeper into the gap and places its acceptor levels closer to the valence band edge, the Fermi level being therefore lowered in the gap.

D. Belmont et al. / Cu/GaAs(IlO)

interface formation

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The same effect explains why the band bending appears only in the 1 ML range for Cu on p-type substrates. The small tail of interface states associated with the bonds between metal atoms and substrate (a in fig. 3) acts as a growing density of deep donors with Cu coverage: it has to be large enough to compensate the tail of acceptor states and start an overall positive surface charge leading to a significant band bending in p-type GaAs. In summary, the deposition in ultra high vacuum of Cu atoms on the clean cleaved (110) face of GaAs at room temperature leads to the formation of a uniform, non-metallic double layer made of a two-dimensional compound, the composition of which being roughly estimated to be one Cu atom per GaAs surface unit cell plus at least one monolayer of Cu atoms. Then, once this double layer has been completed, metallic Cu islands start to form and progressively cover the substrate. The double layer governs the electronic properties of the interface, which are settled when it is completed. The origin of the electron states which give the donor and acceptor levels commanding the Fermi level position in the gap at the interface appears to be the same as in less structurally perturbed systems.

Acknowledgments Many fruitful discussions with Dr. F. Proix and the able technical of Mr. B. Htlie are gratefully acknowledged.

assistance

References [I] [2] [3] [4] [S] [6] [7] [8] [9] [IO] [l l] [ 121 [13]

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