University of Groningen
Growth and nanostructure of tellurides for optoelectronic, thermoelectric and phase-change
applications
Vermeulen, Paul Alexander
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Publication date: 2019
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Vermeulen, P. A. (2019). Growth and nanostructure of tellurides for optoelectronic, thermoelectric and phase-change applications.
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Chapter 3. Pulsed laser deposition of
tellurides.
Basic principles and results for the growth optimization of telluride thin films are presented.
3.1
Introduction
While extensive research has been performed on oxide growth using PLD (where it is in fact the preferred tool), the growth of (telluride) chalcogenides has received far less attention. Consequently, the literature is relatively scarce, and reports on process parameters and/or achieved layer quality were incomplete or conflicting. The aim of the present thesis has been to grow textured (at least out-of plane), flat, stoichiometric, and thickness controlled films of single-and multi-component films. To this end, a large number of basic experiments was carried out to get a basic understanding of the PLD process and the growth of these materials. Three key deposition stages can be identified:
1. Ablation
2. Transport (to the substrate) 3. Growth
All steps allow interdependent tuning of parameters, and therefore some rudimentary understanding (if not full characterization) of all three steps is needed to attain control of layer growth.
3.2
Ablation
Target surface analysis
The source material for PLD comes from targets which were obtained commercially (Ktech). The targets are obtained using a powder sintering process providing a highly dense target which is able to withstand repeated laser pulses without cracking. A target is generally chosen to have the stoichiometry which is desired in the film, but sometimes one might deviate from this, when the film turns out to be off-stoichiometric. Since a target is hit repeatedly (thousands of pulses per deposition), it is moved to prevent excessive erosion of one particular spot. This erosion usually alters the local composition, and may also result in an increase of particulates on the film (that is the result of a process called ‘splashing’). To study this erosion phenomenon, as well as determine the spot size on the target, several lens positions were used to make spots of various sizes. Similarly, different numbers of pulses were used to obtain a qualitative understanding of the speed of
degradation. Figure 1 shows the Bi2Te3 target with many different spot marks. Note
that in this case target scanning was intentionally not used.
Figure 1. Optical image of the Bi2Te3 target (diameter 2.54 cm). Different spot
sizes can be observed, as well as progressively eroded spots due to increasing pulse number (bottom row).
The spots were studied using SEM and are shown in figure 2. Although the number of pulses might seem low compared to an actual deposition, one has to keep in mind the target is generally scanned, so we expect a single area to be hit less than 20 times during a deposition of 4000 pulses. As the same area is hit consecutively, the area is roughened considerably, and edge features start to develop. While edges are not a problem by itself, this is indicative of partial melting and/or element specific evaporation. When these effects can accumulate over many depositions, film stoichiometry will degrade.
3.2 Ablation
Figure 2. Spots made with the same spotsize and laser energy ( 1 Jcm-2) on a
Bi2Te3 target. The number gives the amount of pulses. The spot morphology
clearly changes, large-scale structures seem to emerge, and the edges of the spot become slanted, which is related to the 45° incidence angle of the laser beam. Stoichiometric ablation
In a regular deposition, the spot is continuously scanned, which yields an ablation track as shown in figure 3. The surface looks smooth (except for some small edge effects), indicative of proper ablation and local melting, but no element specific evaporation or pillar formation. Using EDS, the ablation track is confirmed to still possess the Bi2Te3 stoichiometry (Bi 41.3 at.%, Te 58.7 at.%) within the
measurement accuracy, which means the initial ablation plume contains the correct stoichiometry as well. Similar results were obtained for Sb2Te3.
Figure 3. a) Secondary-Electron image of the ablation track on Bi2Te3. The
ablation track is smooth and faint lines 100 μm apart indicate successive ablation spots. The edge is slightly rougher. b) An EDS spectrum taken of the ablation area confirms Bi and Te are still present with 40 at.% and 60 at%. respectively.
Fluence
The fluence is defined as the energy per unit of area per pulse deposited by the laser on the substrate. For well-known materials, the optimal fluence for deposition is well defined, but for the tellurides this was not the case. The fluence should be above the ablation threshold: the energy required to generate a plasma plume. Generally, an increase of fluence increases the amount of material ablated, but can also cause the splashing effect: small particulates are expelled from the surface, and arrive at the substrate, roughening the film.
The fluence can be tuned in two ways: either by changing the mask and lens positions or by changing the discharge energy of the laser. Generally changing the laser discharge energy is preferable, since it does not change the size of the ablation spot on the target. To probe more than one order of magnitude, it was necessary to change the spot size, however. Figure 4 shows ablation on a single spot (without scanning), using different laser energy/spotsize pairs, changing the fluence
between 0.2 – 3.7 Jcm-2. The ablation effect is most pronounced for higher fluences,
and the feature size increases for higher fluence. Figure 2 shows that more pulses produce a similar effect. Therefore we speculate that the relevant parameter is the total amount of energy that hits the target, or the amount of material removed over time. The corners and edges are rounded for the smallest spot size which introduces an error in fluence calculation. Since one may assume that the spotsize does not change in-between depositions, this should not be an issue.
3.2 Ablation
Figure 4. SEM images of the Bi2Te3 target for increasing Fluence (Jcm-2) using the
same number of pulses. For the top- and bottom row a respectively higher and lower spotsize than for the two middle rows was used to reach the desired fluence. The middle area of the spot shows that higher fluence shows large-scale features.
3.3
Transport
While the PLD technique is often seen as a reliable way to convey the target stoichiometry due to the physical processes involved. This premise was tested for
Bi2Te3 and GST targets to verify the maximum area in which homogeneous
deposition could be expected. Recall that we showed stoichiometric ablation was achieved from the target, loss of stoichiometry therefore should be attributed to re-evaporation or a different plume propagation. Large wafers (3 inch diameter) were inserted in the PLD system and a deposition was performed. Using SEM-EDS, the stoichiometry and layer thickness (Te spectral peak height) was determined on a large number of locations. Since thin-film characterization is not very accurate using SEM (although it is precise), we will not comment on absolute values. It seems both films show homogeneous coverage within the 1x1 cm substrate area, and composition seems relatively constant as well. Outside the 1x1 cm area, layer thickness rapidly diminishes, while composition seems constant. In the outer regions, the BiTe film seems less Te deficient, which may be consistent with a lower wafer temperature on the outer regions.
3.3 Transport
Figure 6. The film thickness (parametrized by the Te signal intensity) is shown as a contour plot (top row), and versus the radial distance from the plume center (middle row). The red line designates the outer corner of a 1x1 cm substrate. Layer thickness is homogeneous within this substrate area for both GST and BiTe. The stoichiometry as radial distance is determined (bottom row). Both films seem to be slightly Te-deficient within the plume center. GST composition seems stable, while for BiTe composition shows a slight gradient.
3.4
Growth
Arguably the most direct way to modify the deposition, assuming a stoichiometric plume of ablated material was created, is to modify the temperature and substrate to force the desired crystal structure and texture.
Seed Layers
Although vdWaals materials such as Sb2Te3 prefer to crystallize with an
out-of-plane texture, their deposition on amorphous substrates still yields a poorly textured film. Several authors1,2 have described the best method to obtain a
textured layers is through a so-called seed layer. This seed layer can in fact be a thin film of the material itself, prepared using a specific thermal treatment. Furthermore, other hexagonal materials such as GeTe (with <111> axis out of plane) can be crystallized or grown on such a seed, retaining the original texture. The process is shown in figure 4 and comprises the following steps:
Figure 4. The growth scheme to obtain out-of-plane textured films grown on amorphous substrates.
1. An amorphous film of the material is deposited at room temperature. 2. The sample is annealed to the deposition temperature. Depending on the
thickness more time is needed to obtain a flat, textured film. According to Saito et al., an optimum layer thickness of 3 nm exists, which has lowest roughness for Sb2Te3.
3. When growth is continued at the high temperature, subsequent material layers grow homoepitaxially.
While films grown using this technique exhibit excellent out-of-plane texture, they do not possess in-plane texture as is shown in figure 5. When instead a single-crystal mica substrate is used, the films possess excellent texture both in-plane and
out-of-plane. The same effect can be obtained by growth on passivated Si, 3
although the formed films are not actually single crystals as is shown in figure 5, because a relatively high density of small angle tilt boundaries are present.
3.4 Growth
Figure 5. Several thermal treatments and substrates yield different layer
morphologies. Row 1: Room temperature deposition yields an
amorphous/nanocrystalline layer: Distinct (but somewhat blurred) rings are visible in diffraction. Row 2: Increasing substrate temperature yields clearly facetted crystals and an extremely rough surface. The SAED shows many different planes, indicating poor texture. Row 3: by employing the seed-layer method, the a smoother surface is grown, as evidenced by AFM and RHEED. The SAED pattern only shows rings corresponding to c-axis texture. Row 4: Growth on mica without seed layer still yields smooth and textured films. SAED and RHEED reveal a single-crystal like diffraction pattern.
Growth Temperature and Crystal Size
After establishing a procedure to grow seed layers, one still may change the growth temperature to tune the features of the layer. Several layers were grown on
SiO2 and investigated using AFM. The initial seed layer crystallized at 150 °C, and
the RHEED pattern was still comparatively weak. Above 300 °C evaporation was expected to be too high to deposit a layer. The results are shown in figure 6.
For Si-SiO2 substrates,at the lowest temperature (150 °C), surface mobility is
extremely low. The film is quite smooth, except for some spikes where material has accumulated faster. Increasing the temperature leaves the smooth film, and the peak features disappear. Increasing the temperature to 250 °C shows the grains become larger, clearly facetted and terraced, which increases roughness. Finally at 300 °C, the film is no longer closed, but grows as large, clearly facetted islands. For mica substrates, the same temperature dependent increase in feature size and roughness can be observed.
Depending on the needs, temperature is an effective parameter to tune the film morphology. In this thesis work, the standard temperature chosen is 210 °C, since for the growth of multilayers smoothness of interfaces is of highest importance. Furthermore, as will be shown in the next sections, using a higher growth temperature reduces growth rates due to re-evaporation, and can also influence the stoichiometry of the films. Finally, when growing multilayers intermixing of the layers has to be prevented. As was shown by several authors, these processes can occur for temperatures as low as 250 °C.4–6
Figure 6. AFM scans (1x1 μm) of films grown at different temperatures on two different substrates. Feature size and roughness increase with temperature.
3.4 Growth Growth Rates
Sb2Te3 and GeTe evaporation becomes significant at 250-300 °C as has been
shown by Perumal et al. and Thelander et al..7,8 Since these temperatures are close
to the ones of the growth regime, thickness control has to be implemented. In chapter 2, a method for monitoring layer thickness during growth using RHEED intensity oscillations was described. However, these oscillations are only present for a certain growth regime, and when the diffracted signal has a high signal-to-noise ratio. Since a majority of films was grown on SiO2, there was no in-plane
texture, and the RHEED signal was relatively weak since only a limited number of grains was oriented in the diffraction condition. Therefore, post-deposition analysis was required to determine the growth rate per pulse. A relatively quick and reliable way was to make a cut through the film, and measure the thickness using AFM. In our experience the thickness obtained agreed quite well with XRR and cross-sectional TEM analysis (figure 7). The results are shown in Table 1. The growth rate of all materials slightly reduces when deposition is performed at 210 °C instead of room temperature, which is expected due to lowered sticking coefficient. The deposition rate for Bi2Te3, Sb2Te3 and Ge2Sb2Te5 remains fairly constant when
deposited homo- or hetero-epitaxially, but the GeTe system shows a sharp reduction in growth rate. We speculate this is due to the chemically unreactive surface of Bi2Te3. Growth Rate nm / 100 pulses Substrate or sublayer SiO 25 °C 210 °C SiO Bi210 °C 2Te3 210 °C GeTe Sb210 °C 2Te3 Ma te ri a l to de p os it Bi2Te3 2.2 2.0 x 2.1 2.0 GeTe 2.1 1.8 1.5 x Ge2Sb2Te5 2.3 1.8 1.9 Sb2Te3 2.1 1.9 1.9 x
Table 1. Growth Rate of several materials under standard conditions (10.4 mJ/pulse, spotsize 3 (1.3 mm2), 0.12 mBar Argon, 1 Hz repetition rate) obtained
from AFM analysis. Error from AFM thickness measurement is estimated at ±0.1 nm/100 pulses. Increasing the temperature reduces growth rate slightly.
Figure 7. Thickness of the PLD-grown films was determined using several different methods. (a) shows a TEM cross-section of the film, where even the quintuple layers are visible. The thickness can be obtained by taking an intensity profile across the film, shown in (c). (b) shows an AFM scan with the edge of the scratch clearly visible. The flat area on the left shows bare substrate. by measuring the step height (d) we obtain film thickness. e.) shows an XRR scan with characteristic thickness fringes. The thickness can be directly obtained by plugging the oscillation frequency into Bragg’s law.
3.5 Literature
3.5
Literature
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3 R.WANG,J.E.BOSCHKER,E.BRUYER,D.DI SANTE,S.PICOZZI,K.PERUMAL,A.GIUSSANI,H. RIECHERT AND R.CALARCO,J.PHYS.CHEM.C,2014,118,29724–29730.
4 A.-L.HANSEN,T.DANKWORT,M.WINKLER,J.DITTO,D.C.JOHNSON,J.D.KOENIG,K.
BARTHOLOMÉ,L.KIENLE AND W.BENSCH,CHEM.MATER.,2014,26,6518–6522.
5 A.KUMAR,P.A.VERMEULEN,B.J.KOOI,J.RAO,L.VAN EIJCK,S.SCHWARZMÜ,O.OECKLER AND
G.R.BLAKE,INORG.CHEM.,,DOI:10.1021/ACS.INORGCHEM.7B02433.
6 R.WANG,V.BRAGAGLIA,J.E.BOSCHKER AND R.CALARCO,CRYST.GROWTH DES.,2016,16,3596– 3601.
7 M.T.K.PERUMAL,THESIS WORK. 8 E.THELANDER,THESIS WORK.
