Thermal stability of a partly Fe-intercalated GaSe film

Applied Surface Science 181 (2001) 160±165

Thermal stability of a partly Fe-intercalated GaSe ®lm M. Zerroukia,b, J.P. Lacharmea,*, M. Ghamniaa,b, C.A. SeÂbennea, B. Abidria,b a

Lab. MineÂralogie±Cristallographie, UMR-CNRS 7590, Univ. P. et M. Curie, 4 pl. Jussieu, 75252 Paris Cedex 05, France b Lab. des Surfaces des MateÂriaux, Institut de Physique, Univ. Oran es-seÂnia, 31000 Oran, Algeria Received 28 February 2001; accepted 18 June 2001

Abstract A single crystal ®lm of layered GaSe epitaxially grown onto a Si(1 1 1) substrate has been partly intercalated at room temperature under ultra-high vacuum. Then it was vacuum annealed sequentially at increasing temperatures up to 8508C and studied at each step by low energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy. It is shown that the Fe-intercalated GaSe is perfectly stable until above 4008C. At higher temperatures, the intercalated Fe is destabilised and is fully trapped into the Si substrate, restoring an Fe free GaSe crystalline ®lm which decomposes above 6008C. # 2001 Elsevier Science B.V. All rights reserved. PACS: 68.35Fx; 73.30.‡y Keywords: Intercalation; Iron on gallium selenide; Metal±semiconductor interface

1. Introduction During the last decade several studies have been devoted to the interaction of layered materials such as GaSe or InSe with alkali metal compounds (NaNO2, KNO2) in order to investigate new photoelectric processes, polarisability properties and more generally transport phenomena [1,2]. Lithium-intercalated InSe and GaSe electrical properties were also examined to explore new applications for performant solid electrolytes [3]. During these interactions which occur at temperatures much lower than the dissociation temperature of the layered material (which is about 5008C for InSe and 6008C for GaSe), the phenomenon is mainly governed by the layered character of the III±VI compound. It is easy to propose that the interlayer *

Corresponding author. Tel.: ‡33-144275226; fax: ‡33-144273785. E-mail address: [email protected] (J.P. Lacharme).

gaps, where only weak, van der Waals-like, bonds are present, can easily accept foreign atoms such as alkali metals. The ®eld of application of the layered character of GaSe and InSe for intercalation purposes is not limited to alkali metals. It has been shown that Cu atoms can easily enter the interlayer space, offering interesting possibilities in the case of InSe for photovoltaic conversion of solar energy [4]. In order to get a better understanding of the intercalation processes into these materials, we had deposited a small amount of Cu atoms, from an effusion cell in ultra-high vacuum, onto the clean and passive face of an InSe ®lm, and we had shown that the intercalation process took place even at room temperature (RT) without perturbing the overall crystalline structure within each layer of the lamellar material [5]. A similar experiment was then performed at RT on the system Fe/GaSe, using the same procedure, leading us to propose a model in which Fe atoms deposited onto the layered material surface ®ll, in the early stage of

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 7 6 - 2

M. Zerrouki et al. / Applied Surface Science 181 (2001) 160±165

the deposition, the interlayer planes where van der Waals (vdW) bonding forces are present [6]. According to the model, it was proposed that the ®rst Fe atoms were intercalated between the ®rst and second Ga2Se2 elementary layers up to a saturation evaluated at 2 ML, where 1 ML ˆ 8  1014 Fe atoms/cm2, the atomic density of the perfect GaSe(0 0 1) surface plane. Then, the intercalation went on in a similar way into successively the second, the third and so on until all the vdW interlayers of the GaSe ®lm have been occupied, each by 2 ML of Fe, depending of course on the amount of deposited Fe. In the present experiments, the total amount of RT deposited Fe was limited to 8 ML in order to ®ll only the ®rst four vdW interlayers of a GaSe ®lm made of 11 or 12 Ga2Se2 elementary layers. The Fe±GaSe sample was then sequentially annealed at increasing and well controlled temperatures in order to check the thermal stability of the system. The experiments consisted in the study of the Fe-interacted GaSe surface after each annealing, using the following surface analysis techniques: Auger electron spectroscopy (AES), to control the Fe amount compared to Ga and Se in the sample near the surface, low energy electron diffraction (LEED), to know the changes in the surface structure, and photoemission yield spectroscopy (PYS) which gives some convenient electronic properties of the surface region. 2. Experimental procedure Measurements were performed onto 10 nm thick GaSe ®lms grown by MBE on the hydrogenated 1  1-reconstructed Si(1 1 1) surface. The latter was obtained from an electronic grade wafer which had been chemically treated by HF and NH4F according to a now well-established procedure [7]. The layered GaSe ®lm was made of 11 or 12 Ga2Se2 elementary Ê thick, as determined previously [8], layers, each 8 A which con®rmed that the layered ®lm had grown epitaxially in large single domains with its c-axis perpendicular to the (1 1 1) plane of the Si substrate surface. The samples were cut from the 0.5 mm thick GaSecovered wafer into 20  5 mm2 parallelepipeds. One was then mounted into a previously described ultrahigh vacuum (UHV) chamber [9] in which, once

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evacuated and baked, a stable pressure lower than 5  10 10 Torr was established. The sample holder was ®xed in such a way as to permit a controlled electrical current to run through the silicon sample. Annealing treatments could then be carefully performed at temperatures ranging from 200 to 8008C, as measured through an infrared pyrometer. It could ®rst be used for sample degassing below 3008C before Fe deposition. The latter was made in the UHV chamber from a source made of a high purity Fe wire which was Joule-heated at a temperature near 10508C. The Fe ¯ux reaching the sample (equal to 1:6  1013 Fe atoms/(cm2 s)) was known from measurements using a quartz balance. Taking into account the atomic density of the 1  1-reconstructed GaSe(1 1 1) surface equal to 8  1014 atom=cm2 , the Fe ¯ux corresponds then to 2  10 2 ML=s. The results reported here were obtained in a twostep experiment performed on the same GaSe sample. In the ®rst step, isochronous annealing were applied to the ®lm sequentially exposed to small doses of Fe up to a total dose of 4 ML, at which then, according to the intercalation model, the two vdW gaps closest to the surface were initially ®lled, each with 2 ML of Fe; the annealing temperatures ranged from RT to 4008C, at most. During this ®rst set of measurements the stability of the Fe/GaSe system could therefore be tested at low temperatures. In a second step, the sample was again exposed at RT to an additional Fe dose of 4 ML, ®lling then two more vdW gaps closest to the surface, before being submitted to a new set of annealing ranging this time from RT to 9008C. Each annealing cycle kept the sample at a carefully controlled and stable temperature for 4 min. Back to RT after each annealing, the GaSe surface was studied by AES, LEED and PYS. AES concerned the Ga, Se and Fe Auger main lines, respectively, at 1064, 1308 and 698 eV, as obtained in the derivative mode with a cylindrical mirror analyser and a 2 keV normal incidence electron gun; the SiLVV line at 91 eV of the substrate was also each time looked for LEED patterns were observed at an electron beam energy of 65 eV. The PYS gave spectra of the form Y…hn† ˆ Ne =Nph , where Ne is the number of electrons photoemitted by the sample and Nph the number of incident photons at energy hn. The PYS spectral range covered 3:8 eV < hn < 6:7 eV by steps of 0.025 eV. The Y(hn) curve gives some electronic features of the

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surface such as the work function f and the ionisation energy Ei and indications on gap state density, as explained in a previous paper [9]. 3. Results and discussion 3.1. Annealing at a low temperature (4008C) During the ®rst set of annealing, the GaSe ®lm which had been previously exposed to an Fe dose of 4 ML was heated, respectively, at 150, 220, 280, 330, 360, and 4008C. After each new heating treatment, AES showed that the peak to peak height of the Ga, Se and Fe Auger lines stayed at a constant value equal to the ones measured just after the Fe deposition at RT. The initial LEED diagram did not suffer at all from the successive heatings; the pattern remained clearly observed as sharp spots corresponding to the typical hexagonal lattice of the clean GaSe surface. The PYS curve, as for AES and LEED results, remained almost

unaffected by these successive annealing performed at T  400 C. Only a slight effect could be noticed during the ®rst annealing treatment performed at 1508C, which consisted in a decrease of the photoyield by about 10% of its initial value without any change of the electronic properties of the surface. The photoyield spectrum was then very similar to the one presented in Fig. 2, which corresponds to a 8 ML dose of Fe deposited under similar conditions. These results show that the GaSe ®lm with two Fe-intercalated vdW gaps along its surface remains perfectly stable upon vacuum annealing up to 4008C. 3.2. Annealing at a temperature higher than 4008C As seen in Section 2, the 4 ML Fe-covered GaSe ®lm once annealed at 4008C received at RT a second 4 ML Fe dose. The effect of annealing was then studied varying the temperature from RT to 8508C, showing that in the higher temperature range, above 4008C, considerable changes had happened to the

Fig. 1. Ga, Se, Fe and Si Auger peak-to-peak intensities, respectively, at 1064, 1308, 698 and 91 eV for the Fe±GaSe/Si(1 1 1) system as a function of increasing temperatures of vacuum annealing. The changes in the LEED observations are also reported in the upper part of the ®gure.

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Fe/GaSe/Si system, as revealed by the different surface analysis and detailed below. AES results are summarised in Fig. 1. The peak-topeak Auger lines for Fe, Ga, Se and Si, when observable, for the Si substrate, are reported against the temperature of annealing. It should be noticed that the ®ne structure and the energy position of the different Auger peaks did not change all over the treatments. LEED observations are summarised in the upper part of Fig. 1. The evolutions of the PYS spectra are shown in Fig. 2. No change occurred in the PYS spectrum 2 upon annealing below 5208C. For annealing ranging from 520 to 5908C, the changes in the PYS curve were noticeable as compared to the unannealed sample

Fig. 2. Semilogarithmic plot of a set of photoemission yield spectra: the clean GaSe/Si(1 1 1) surface (1), the same exposed to 8 ML Fe at 258C (2), then UHV annealed at successively 520 (3), 550 (4), 570 (5), and 5908C (6). The valence band contribution of the clean GaSe surface starting at an ionisation energy of 5.85 eV (see text) is also reported (curve BV).

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(curve 2). The spectrum of the clean GaSe ®lm prior Fe deposition is shown in curve 1. The spectra obtained after annealing beyond 6008C are not displayed: they wereprecognised  p as the well-known ones of ®rst Si…1 1 1† 3  3-Ga, then, after annealing above 650±7008C, somewhat perturbed Si…1 1 1†7  7. Looking also at Fig. 1, no change is observed in LEED as well as in the different AES peak intensities upon annealing at temperatures lower than about 400± 4508C. It shows that increasing the Fe amount deposited onto the GaSe ®lm at RT has no in¯uence on the thermal stability of the Fe/GaSe system up to at least 4008C since a similar behaviour was observed for the GaSe ®lm exposed to a 4 ML Fe dose. It shows also that the system remains stable in spite of an Fe dose of 8 ML which has been kept lower than the 22±24 ML needed to Fe-saturate all the 11 or 12 vdW gaps of the GaSe ®lm. From about 400 to 5708C, the iron AES signal decreases continuously and totally disappears at 5708C while Ga and Se Auger intensities may show a slight increase as Fe decreases. It should be noticed that the corresponding LEED observations do not show any change. From 570 to 6208C, the AES spectra are characterised by a Se signal keeping the same intensity, while the Ga one starts decreasing to reach an intensity about one-third lower than the initial one without any signal of Fe or Si. Similar results were observed previously upon vacuum annealing performed on a clean GaSe single crystal ®lm free of any metal deposited onto its surface [10]. This corresponds in fact to the temperature range where the GaSe ®lm is no longer chemically stable as already explained in [11]. It is of course further con®rmed by the LEED observations which show a vanishing GaSe 1  1 pattern into an increasing background. The next annealing, performed at 6508C, provokes a total change of the Auger intensities: Se is gone, some Ga is left, an Fe signal is restored and a large signal coming from the silicon substrate is observed. The Fe signal has a lower intensity than prior annealing above 4008C. The remaining Ga Auger intensity, compared to the pSi  one, pis compatible with the LEED observation of a 3  3-R30 , as referenced to the ideal Si(1 1 1) surface, which characterises the one-third of an ML Ga-covered Si(1 1 1) ordered surface [12]. Annealing at 8508C removes the remaining gallium and forms

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the well-known 7  7 reconstructed Si(1 1 1) surface in spite of the persisting presence of some Fe. The PYS spectra reported in Fig. 2 are characterised by the nearly constant value of the photoemission threshold throughout the experiment. Then, the work function f deduced from a ®t between the Y(hn) curve in the threshold region and the shape of the integrated Fermi distribution is found equal to 5:00  0:05 eV. It means that the GaSe surface work function f keeps the same value when the Fe/GaSe system is annealed up to 5908C. f keeps its value until annealing higher than 5908C, when the GaSe ®lm reaches its dissociation threshold. Besides the threshold analysis, the photoemission yield spectra show very instructive features. In order to discuss the behaviour of these spectra upon annealing in the 500±6008C temperature range where Fe leaves the uppermost vdW gaps and restores a ``pure'' GaSe top layer, curve 1 (pure GaSe before Fe deposition) and curve 2 (unannealed GaSe after Fe deposition) must be understood. Considering ®rst the pure GaSe, spectrum 1 in Fig. 2 has two parts, above and below about 5.9 eV. The high energy part corresponds to the valence states contribution which starts when the photon energy passes the ionisation energy (valence band edge to vacuum level) of the semiconductor. It is well ®tted by a power law shown as curve BV in Fig. 2 which places the ionisation energy at 5:7  0:1 eV. The low energy part of curve 1 corresponds to occupied states in the 2.0 eV band gap of the semiconductor coming from bulk and surface defects. This tail of states goes up to Fermi level, at 5.0 eV, and covers therefore 0.7 eV, about 13 of the total band gap. Looking now at curve 2, the effect of Fe deposition is to induce a large density of states starting steeply at Fermi level and masking the GaSe contribution. These new states are located near the top of the GaSe valence band and they originate from the electron transfer between Fe atoms and the host (GaSe) at the vdW gap. It is now easy to analyse the effects of annealing in the 500± 6008C temperature range. Upon 520 and 5508C annealing, the respective spectra 3 and 4 remain dominated by the density of states induced by the intercalated Fe atoms still present in the vdW gap near the surface. Then, upon 570 and 5908C annealing, as seen in curves 5 and 6, the Fe contribution, respectively, decreases signi®cantly then vanishes. It leaves in curve 6 a spectrum in which the high energy part ®ts correc-tly

the GaSe valence state contribution, however it shows a low energy tail of occupied states about 10 times larger than in curve 1: the density of defects in the Ga Se ®lm has been strongly increased by the Fe cycle. Moreover, spectrum 6 shows that all the intercallated Fe atoms have left the three or four vdW gaps closest to the surface because (i) it is very sensitive to Fe around 5.8±5.9 eV photon energy, (ii) the electron escape depth in PYS is about 1.5 nm, that is twice larger than in AES. Then, after 5908C annealing, the GaSe ®lm is essentially restored and the formerly intercalated Fe atoms have diffused into the Si substrate. 4. Conclusions The main conclusions which can be drawn from the above experimental results and analysis can be summarised as follows: 1. An Fe-intercalated single crystal ®lm of layered GaSe is thermally perfectly stable until 4008C. This ®rst point may give some interest to the Fe± GaSe system which appears as a kind of superlattice made of alternate GaSe layers and Fe double atomic layers, the magnetic properties of which should be studied. 2. After annealing above 4008C, the study of a partly intercalated ®lm shows that the Fe interlayer starts to destabilise and Fe atoms move towards less occupied vdW gaps, deeper in the GaSe ®lm. In the present case where the GaSe ®lm was epitaxially grown onto an Si(1 1 1) substrate, the latter plays the role of an Fe sink, as expected knowing the strong reactivity between Fe and Si in that temperature range. 3. The removal of intercalated Fe restore a crystallised GaSe ®lm with a higher density of defects than before intercalation. These new defects very likely come from the RT Fe intercalation process itself. The restored GaSe ®lm behaves exactly as a fresh one under annealing at dissociative and destructive temperatures. References [1] V.V. Netyaga, Grigorchak II, Z.D. Kovalyuk, Semiconductors 27 (1993) 673.

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