Thermal stability of a partly Fe-intercalated GaSe film
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 500°C for InSe and 600°C for GaSe), the phenomenon is mainly governed by the layered character of the III–VI compound. It is easy to propose that the interlayer gaps, where only weak, van der Waals-like, bonds are present, can easily accept foreign atoms such as alkali metals. The field 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 film, 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 fill, in the early stage of the deposition, the interlayer planes where van der Waals (vdW) bonding forces are present [6]. According to the model, it was proposed that the first Fe atoms were intercalated between the first and second Ga2Se2 elementary layers up to a saturation evaluated at 2 ML, where 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 film 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 fill only the first four vdW interlayers of a GaSe film 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.
Section snippets
Experimental procedure
Measurements were performed onto 10 nm thick GaSe films 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 film was made of 11 or 12 Ga2Se2 elementary layers, each 8 Å thick, as determined previously [8], which confirmed that the layered film had grown epitaxially in large single domains with its c-axis
Annealing at a low temperature (≤400°C)
During the first set of annealing, the GaSe film which had been previously exposed to an Fe dose of 4 ML was heated, respectively, at 150, 220, 280, 330, 360, and 400°C. 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
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 film of layered GaSe is thermally perfectly stable until 400°C. This first 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 400°C, the study of a partly intercalated film
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