Structural Characterization of Amorphous Silicon Bianca Haberl A thesis submitted for the degree of Doctor of Philosophy of The Australian National University December 2010
Chapter 5 Mechanical Properties
Chapter 5: Mechanical Properties 5.1 Introduction Nanoindentation is a powerful tool when probing the characteristics of a-si as it is not only sensitive to the state of the amorphous network, but also to more subtle differences between different amorphous networks which may result in different mechanical properties. The sensitivity to the state of an amorphous network is clearly evident for example for ion-implanted a-si: whereas the unrelaxed (as-implanted) state deforms via plastic flow, the relaxed state will phase transform in the same manner as c-si [21, 22]. As detailed in Section 2.1.2 this distinct difference in mechanical response is in most cases already evident from the load-displacement curves due to the occurrence of pop-in and pop-out or elbow events, but even more so from subsequent Raman microspectroscopy and/or TEM analysis. Examples of cross-sectional TEM (XTEM) micrographs of indents made in as-implanted and relaxation annealed implanted a-si have been shown in Fig. 1.4 in Chapter 1. Most techniques traditionally used for the investigation of the state of a-si such as differential scanning calorimetry [13] or X-ray diffraction [14] require a sufficiently large volume of a-si. This however, is not easily accomplished in the case of the laser-quenched or PI a-si. Therefore, nanoindentation, especially in conjunction with the precise positioning capability of the Hysitron TriboIndenter, is a unique tool for the investigation of the state of these various forms of a-si. Not only can differences in the overall state of a-si be detected, but more subtle differences can be observed when probing for the indentation hardness of a film. As detailed in Section 2.1.1 indentation hardness is not a true materials property but can be defined as the the resistance of a material to deformation by another material [110]. Indentation in the plastic regime results in a residual impression after indentation unloading and the larger this residual impression, the smaller the indentation hardness. Since the nature of the plastic deformation (e.g. plastic flow or phase transformation) determines the size of the residual impression and thus the indentation hardness, further insight into the material can be gained from such hardness measurements. Moreover, the indentation hardness is not only sensitive to deformation via plastic flow vs. phase transformation, but also to more subtle differences which may impede an easier pathway for deformation such as voids, impurities, inhomogeneities or even an increased mass density. However, the very nature of the sensitivity of the indentation hardness to the mode of plastic deformation necessitates great care when interpreting results. This is especially evident when phase-transforming forms of a-si are considered as these forms effectively consist of a composite of materials of different hardness while pressurized, as the metallic Si-II phase is considerably softer than any form of silicon stable at ambient. Therefore when necessary, the indentation hardness was not only determined using the Oliver-Pharr method [113, 114] detailed in Section 2.1.1, but also by direct scanning of residual indent 93
Section 5.2: Deformation Behaviour impressions with a scanning electron microscope (SEM). In this chapter the mechanical properties of the various forms of a-si subjected to different thermal histories are described. All films, i.e. the PECVD grown, magnetronsputtered, laser-quenched and pressure-induced a-si, are investigated for the possible occurrence of phase transformation before and after thermal annealing. This deformation behaviour will be presented in the first half of the chapter. However, as the PECVD grown and laser-quenched a-si cannot be investigated for their structural characteristics or even indentation hardness, the results for these two forms of a-si will only be briefly summarized and the detailed results are presented in Appendices D and E, respectively. The indentation hardness of the more uniform films prepared by magnetron-sputtering and by pressure-induced phase transformation is also investigated. These results are then compared to ion-implanted a-si, which is well characterized by indentation [20 22, 213]. The results obtained from the hardness measurements will then be presented in the second half of the chapter. 5.2 Deformation Behaviour 5.2.1 Experimental Details The deformation behaviour of the various films was investigated using either the UMIS with a 5 µm spherical tip or the Hysitron TriboIndenter with a Berkovich tip. In all cases the unloading rate was kept at a ( slow ) rate that ensures a phase transformation to the crystalline high-pressure phases. In the case of the UMIS a maximum load of 80 mn with the continuous loading cycle detailed in Section 2.1.3 with loading and unloading rates of 0.6 mn/s was used. In order to achieve such a slow loading and unloading rate, 50 increments were used. These conditions ensure the formation of high-pressure phases in the case of c-si and avoids cracking. The loading time when using the Hysitron TriboIndenter was 30 s, whereas the unloading time was 60 s. This also results in phase transformation in c-si, although at low loads the formation of PI a-si rather than of the high-pressure phases is more likely. The deposited films, PECVD grown and magnetron-sputtered, as-prepared and thermally annealed, were investigated for their deformation behaviour using the UMIS. This was done as these indentation conditions result in larger residual indents thus simplifying the subsequent analysis by Raman microspectroscopy and XTEM. Under the loading conditions used the indentation tip often penetrated to 100% of the layer thickness at maximum load rather than just probing the top surface. These conditions were used to further the likelihood of phase transformation as even as-implanted a-si is known to phase transform under high-load conditions [22]. The Hysitron TriboIndenter was employed in the cases of the laser-quenched and the 94
Chapter 5: Mechanical Properties PI a-si as these forms of a-si require precise positioning of the indentation tip. Such positioning is achieved by imaging the surface with the indentation tip itself. The maximum load in the case of PI a-si was 5 mn which results in a maximum penetration depth of 30% of the film thickness. The maximum load in the case of the laser-quenched a-si was 7 mn resulting in both, low and high-load conditions due to the non-uniform film thickness. The occurrence of phase transformation was then studied using Raman microspectroscopy and further XTEM of residual indent impressions. The cross-sections of such residual indents were also prepared using the dualbeam focused-ion-beam (FIB) system. The use of the dualbeam FIB allowed precise positioning of the cross-section and hence enables the direct correlation between the load-displacement curve of an indent and the TEM analysis. 5.2.2 Results Non-Uniform Films As detailed in the previous chapter both, the PECVD and laser-quenched a-si contain large crystalline volume fractions. Thus the detailed results of the indentation testing are shown in Appendix D and E, respectively, since these films do not contribute to the main scope of this thesis. Nonetheless, these findings are summarized here. In the case of the PECVD a-si the film deposited for 6 minutes was studied in its as-prepared and relaxation annealed form. Analysis of the load-displacement curves as well as consecutive Raman microspectroscopy and XTEM revealed the presence of highpressure phases and also significant slip in the underlying c-si. The a-si layer however, did not display any evidence of phase transformation in both cases. Additionally, a residual indent in the as-prepared polycrystalline film deposited for 30 minutes at elevated temperatures was investigated. In this case no evidence of phase transformation was observed in the film or the substrate although slip was evident in the substrate. This clearly shows that PECVD a-si cannot phase transform under indentation testing suggesting that even after thermal annealing its state is still unrelaxed, a behaviour which may be attributed to the high impurity content. (For further details refer to Appendix D.) For the laser-quenched a-si two laser-modified spots without craters were investigated for their deformation behaviour. One of these spots was subjected to a relaxation anneal prior to annealing. Various indents at various distances from the edge of the laser-modified spot were investigated and a wide variation was observed in penetration depth. However, no evidence of phase transformation was observed in any of these cases before and after annealing. This lack of phase transformation even in the annealed case is most likely due to the low density and non-uniform appearance of the a-si layer and this material will deform plastically via flow or densification and not via phase transformation. This 95
Section 5.2: Deformation Behaviour suggests that even pure a-si cannot undergo structural relaxation if the film is not uniform. For further details on these indentation results refer to Appendix E, Section E.1. However, it might be possible to induce phase transformation in a laser-quenched zone by first compressing the laser-quenched material with large scale spherical indentation tip in order to densify the film and thereafter subject the film to thermal annealing before re-indenting it on a small scale. To date this was not done as the high-load nanoindentation system at the ANU does not allow precise enough positioning to compress a laser-quenched spot in such a way. Magnetron-Sputtered Amorphous Silicon Fig. 5.1(a) shows a typical load-displacement curve from an indentation performed on assputtered a-si. A smooth curve without any evidence of a pop-in or pop-out event is observed indicating deformation via plastic flow. The deformation behaviour was further investigated using XTEM. A bright-field image of the cross-section made from this indent Figure 5.1: (a) Load-displacement curve of an indentation performed in as-sputtered a- Si and (b) a bright-field micrograph of an TEM cross-section prepared from the same indent. The SADP taken directly from beneath the residual indent impression is shown as an inset. 96
Chapter 5: Mechanical Properties impression is shown in Fig. 5.1(b) with an SADP taken from beneath the impression as an inset. Again, no evidence of phase transition is observed, not even in the underlying c-si. Fig. 5.2(a) shows a load-displacement curve from an indentation performed in annealed sputtered a-si. Interestingly, as marked by an arrow a pop-in event is observed upon loading. However, no pop-out or elbow events are observed making it less likely that this event is correlated to phase transition behaviour. As above, the indent impression was further investigated by XTEM. A bright-field image of the cross-section is shown in Fig. 5.2(b) with an SADP taken from beneath the impression as an inset. Again, no evidence of phase transition is observed, not even in the underlying c-si. Contrary to the as-sputtered case not only are distinct slip planes visible in the underlying c-si, the entire c-si section beneath the indent impression appears to have moved downwards between the slip planes. This moved section is also marked by arrows in Fig. 5.1(b). No evidence of phase transition was found in the volume of c-si trapped between the slip planes. The probable explanation seems to be that this c-si is significantly placed under Figure 5.2: (a) Load-displacement curve of an indentation performed in annealed sputtered a-si and (b) a bright-field micrograph of an TEM cross-section prepared from the same indent. The SADP taken directly from beneath the residual indent impression is shown as an inset. 97
Section 5.2: Deformation Behaviour compression (giving rise to slip) although the pressure was not sufficiently high to induce phase transformation. No significant amount of pile-up is observed on either side of the residual impression and thus the question arises as to where the missing volume of material from beneath the residual indent impression has disappeared to. To answer this question, the pristine a-si layer as well as the layer the indent impression was investigated in more detail. A brightfield image of the pristine, unindented as-sputtered a-si beside the impression is shown in Fig. 5.3(a). Clearly nanopores running vertically through the entire film thickness are visible in the film extending up to 400 nm in length with a diameter of 1 nm. Interestingly, these nanopores seem to be absent in the a-si layer beneath the residual impression as shown in Fig. 5.3(b). Note that in both these bright-field images the brightness and contrast balance were intentionally over-saturated in order to make the nanopores visible in print. To investigate this further the normalized bright-field image intensity according to Equation 2.11 was measured using the program ImageJ [116] and is summarized in Table 5.1. The contrast of the a-si film on both sides of the impression is significantly higher than beneath the impression. Usually a higher bright-field contrast indicates a Figure 5.3: Bright-field images of the as-sputtered film taken (a) next to the indent and (b) beneath the residual indent with a CCD camera. Position Brightfield Contrast Beneath indent 0.530±0.007 Pristine film 0.590±0.003 Table 5.1: Comparison of normalized bright-field image contrast from beneath the residual indent impression and from the pristine film. 98
Chapter 5: Mechanical Properties thinner region of a-si. However, as this is a single cross-section of uniform thickness it can be assumed that the change in contrast is not due to thickness changes, but to density changes. Thus the a-si beneath the indent impression has a higher mass density than the original film, which is consistent with the disappearance of the nanopores. Moreover, from the change in bright-field contrast the volume of nanopores and thus also mass density of the deformed film may be estimated. Usually, the change in brightfield contrast is approximately inversely proportional to the change in film thickness, t [50, Chap. 3]. Therefore, the ratio of the normalized bright-field image contrast from beneath and beside the indent of 0.898 (taken from Table 5.1) can be used to estimate the change in mass density beneath and beside the indent. This is possible as both areas have the same actual film thickness and the same size area was measured in both cases. Thus the same volume of material was probed. Therefore, the mass density of the material beneath the impression, ρ indent, can be estimated from this ratio and the mass density of the material beside the impression, ρ pristine, to: ρ indent = ρ pristine 0.898 (5.1) Using the mass density gained from the RBS measurement summarized in Table 4.1, the mass density of the as-sputtered film beneath the residual indent impression may be estimated to be 5% higher than the mass density of c-si. This also explains why no pile-up was observed as the material is compressed during the indentation rather then deforming via plastic flow out from under the indentation tip. The same behaviour was observed for the annealed case with the pristine film revealing the presence of nanopores, which disappear upon indentation as shown in Fig. 5.4. Note that as in the as-sputtered case the bright-field images are intentionally over-saturated. Figure 5.4: Bright-field images of the annealed sputtered film taken (a) next to the indent and (b) beneath the residual indent with a CCD camera. 99
Section 5.2: Deformation Behaviour A similar densification as observed in the as-sputtered case is thus expected. However, this could not be confirmed as a result of non-uniform cross-sections. Therefore, clearly the as-sputtered as well as annealed sputtered films do not phase transform upon indentation testing. Intriguingly, rather then deforming via plastic flow, they appear to deform via a significant densification of the film beneath the residual indent impression. The nanopores present in the pristine films clearly disappear upon indentation and in the as-sputtered case the mass density of the deformed film may be estimated to be 5% higher than that of c-si. Pressure-Induced Amorphous Silicon The deformation behaviour of as-prepared and relaxation annealed PI a-si was investigated by small-scale Berkovich indentation. An example of a load-displacement curve performed in such an as-indented PI microindent is shown in Fig. 5.5(a). The unloading rate used was slow enough to form crystalline high-pressure end phases if Si-II was formed during loading. However, no events are observed upon loading or unloading, indicating deformation via plastic flow in this case. The deformation behaviour was further investigated using Raman microspectroscopy. In Fig. 5.5(b) spectra of (1) pristine c-si, of (2) an as-indented microindent and of (3) a nanoindent (the small-scale Berkovich indent) are shown. No evidence of high-pressure phases can be observed, the only difference being the larger prominence of the Si-I TO peak in the case of the nanoindent. This is due to the decreased layer thickness and thus greater penetration depth of the Raman laser into the crystalline substrate. The occurrence of phase transformations was further investigated by XTEM and an example of a whole as-indented microindent containing a nanoindent is shown in Fig. 5.5(c). A close-up of the nanoindent is shown in Fig. 5.5(d) with an SADP taken from beneath the residual nanoindent as an inset. No evidence of phase transformation is observed with no crystallinity present in either the bright-field image or the SADP. This situation is very different when performing mechanical testing on an annealed microindent. An example of a load-displacement curve is shown in Fig. 5.6(a). As marked with an arrow, in this case a pop-out event is clearly visible upon unloading. This indicates the formation of the high-pressure phases and therefore the prior formation of Si- II. This phase transformation behaviour was confirmed using Raman microspectroscopy and XTEM. Fig. 5.5(b) shows Raman spectra of (1) pristine c-si, of (2) annealed PI a- Si and of (3) a nanoindent made in annealed PI a-si. Additional bands at 350 cm 1 and 395 cm 1 appeared after re-indentation of the annealed PI a-si. Such bands can be attributed to the presence of Si-XII [97, 131]. An example of a cross-section of a whole microindent is shown in Fig. 5.5(c), whereas the nanoindent is shown in Fig. 5.5(d). The SADP taken from beneath the residual nanoindent, shown as an inset in Fig. 5.5(d), clearly reveals reflections within the first amorphous ring which cannot be labelled as Si-I. Both 100
Chapter 5: Mechanical Properties Figure 5.5: (a) Load-displacement curve of a nanoindent made in as-indented PI a-si. (b) Raman spectra of (1) c-si, (2) a microindent and (3) a nanoindent. (c) XTEM of the microindent containing the nanoindent and of (d) the respective nanoindent. The SADP taken from beneath the residual nanoindent is shown as an inset. 101
Section 5.2: Deformation Behaviour Figure 5.6: (a) Load-displacement curve of a nanoindent made in annealed PI a-si. (b) Raman spectra of (1) c-si, (2) a microindent and (3) a nanoindent. (c) XTEM of the microindent containing the nanoindent and of (d) the respective nanoindent. The SADP taken from beneath the residual nanoindent is shown as an inset. 102
Chapter 5: Mechanical Properties these reflections correlate to a d-spacing of 4.7 Å, which indicates the presence of either Si-III or Si-XII. In summary, indentation data as well as Raman microspectroscopy and XTEM reveal the deformation via plastic flow in the as-indented case and deformation via phase transformation in the annealed PI case. Therefore, indentation reveals that PI a-si undergoes structural relaxation upon annealing in a similar manner to ion-implanted a-si. Summary of Deformation Behaviour Clearly, no evidence of phase transformation was observed for the deposited films, PECVD grown as well as magnetron-sputtered, before and after annealing. This indicates that these films do not undergo structural relaxation in the same way as the pure ion-implanted a-si does. Furthermore, the laser-quenched a-si did not undergo phase transformation after annealing despite being pure, which is probably due to the very high density of largescale voids. Therefore, this low-density, pure, but porous film cannot undergo sufficient ordering during annealing towards a homogeneous CRN. The behaviour of the PI a-si is very different. While the as-indented PI a-si was also observed to deform via plastic flow, the annealed form of PI a-si undergoes a phase transformation in the same manner as c-si or relaxed ion-implanted a-si. This phase transformation behaviour of PI a-si suggests that it has undergone structural relaxation upon annealing. Therefore, for the remainder of this study, the annealed PI a-si will be referred to as relaxed PI a-si and annealed ion-implanted a-si as relaxed implanted a-si. Meanwhile, the films which underwent plastic deformation via plastic flow (or compression) after the relaxation anneal will be referred to as annealed sputtered/deposited a-si. 5.3 Indentation Hardness 5.3.1 Experimental Details The indentation hardness of the more uniform films, i.e. the magnetron-sputtered and PI a-si, was determined using the Hysitron TriboIndenter with a Berkovich tip and compared to ion-implanted a-si as well as c-si. To ensure the greatest possible accuracy, the frame compliance and tip area function were very carefully calibrated for each day a measurement took place. The loading time applied was 30 s, whereas the unloading time was 60 s. However, a variety of maximum loads was used depending on which condition proved most suitable for a particular film. In the case of the magnetron-sputtered film deposited at room temperature, the assputtered and relaxation annealed films, as well as pre-deformed films were studied. 103
Section 5.3: Indentation Hardness This latter process was done since, as shown in the previous section, indentation, or predeformation, of some of the films leads to a removal of the nanopores and hence densification of the film. The pre-deformation of the film was performed in the UMIS using the 18 µm spherical tip applying a maximum load of 700 mn and a loading and unloading rate of 4 mn/s, which results in a pre-deformed region of 8 µm diameter. This loading-/unloading-rate was achieved using 50 increments in a continuous loading cycle. Additional to the relaxation anneal a so-called argon anneal at 600 C was performed in order to drive out the argon prior to indentation a. A portion of these 600 C annealed samples were then, in turn, pre-deformed and/or relaxation annealed. For the testing of the indentation hardness of these magnetron-sputtered films (pristine, annealed and pre-deformed) the Hysitron TriboIndenter applying a maximum load of 9 mn was used. Although this yields a penetration depth of 40% of the film thickness, such a high load was chosen in order to minimize the effect of the surface roughness on the indentation data. The indentation hardness was then calculated using the software of the Hysitron TriboIndenter which employs the Oliver-Pharr method [113, 114] detailed in Section 2.1.1. For each measurement five indentations were performed and the indentation hardness was averaged for each material. The error denoted is the standard error from these five different indents. In the case of the magnetron-sputtered film deposited at 300 C, the as-sputtered and relaxation annealed film were studied. A maximum load of 2.5 mn was applied resulting in a maximum penetration depth of 100 nm or 20% percent of the film thickness. This was done in order to minimize the influence of the substrate, and, although it is still slightly more than the 10% recommended for very soft films on hard substrates [214], the films of a-si on c-si studied here are of only slightly lower hardness than the substrate. Ion-implanted a-si, as-prepared and relaxed, as well as c-si were also investigated as above for comparison. As in the case of the RT deposition, the indentation hardness was calculated using the software of the Hysitron TriboIndenter. For each case 60 indentations were performed and the indentation hardness quoted is the average of these. The error is the standard error of the 60 measurements. In the case of the PI a-si the same indentation conditions as when probing for the deformation behaviour were applied, i.e. a maximum load of 5 mn resulting in a maximum penetration depth of 30% of the layer thickness. This load was chosen as a compromise between avoiding the influence of the c-si matrix and the surface roughness. As in the sputtered cases, the indentation hardness was analysed using the Hysitron TriboIndenter software employing the Oliver-Pharr method. Additionally, as-implanted and relaxed implanted a-si as well as c-si were measured the same way for comparison. Again, for each material five indentations were performed and the indentation hardness quoted is the average with the error denoted being the standard error. a See Section 3.6 for details on the thermal annealing. 104
Chapter 5: Mechanical Properties In the case of PI and ion-implanted a-si, the indentation hardness of phase transforming systems (i.e. c-si, relaxed ion-implanted and PI a-si) is compared to that of non-phase transforming systems (i.e. as-prepared ion-implanted and PI a-si). This different mode of deformation behaviour could influence the analysis of the indentation hardness by the Oliver-Pharr method significantly. Therefore, the projected area was directly determined by scanning the residual indent impressions using an SEM. The SEM used was a Zeiss UltraPlus analytical FESEM operated at 3 kv with a 10 µm aperture using the secondary electron In-Lens detector. The image size of each scan was 2048 1536 pixels and the scan time was 4 mins. The projected area was defined as the area within the triangle formed by the apexes of the residual Berkovich impressions [215]. From this projected area and the applied maximum load, the indentation hardness was calculated for each respective nanoindent using Eq. 2.1. Again, the indentation hardness was averaged for each material and the error denoted is the standard error from these five different indents. In summary, the indentation hardness of the sputtered films in all their variations was determined using the Oliver-Pharr method, whereas for PI a-si the Oliver-Pharr method, as well as direct scanning of residual indent impressions was used. As the ion-implanted a-si as well as c-si were measured for comparison with the other forms of a-si, they were always measured in the same manner as the film they are compared to. 5.3.2 Results Magnetron-Sputtered Amorphous Silicon The indentation hardness of the as-sputtered and annealed films deposited at room temperature and 300 C was compared to ion-implanted a-si. Since the indentation conditions used to study the room temperature film differ from those used to study the 300 C film, the indentation hardness is quoted as a percentage of the c-si value. These results are shown in Fig. 5.7. Examples of the load-displacement curves used to determine the indentation hardness are shown in Appendix F. Interestingly, the indentation hardness of all sputtered films exceeds that of as-implanted a-si and is even higher or equal to the relaxed implanted film. However, note that the relaxed implanted a-si is a phase transforming form of a-si and hence the indentation hardness measured by the Oliver-Pharr method, as a consequence, may not be accurate. Nonetheless, the lower mass density of the sputtered films would have been expected to also yield a lower indentation hardness, and thus this is somewhat surprising. However, such behaviour might be due to the porous microstructure in the film providing an inhibition to plastic flow or alternatively to an impurity-mediated hardening. Interestingly, this behaviour indicates that the indentation hardness of a material and its mass density are not necessarily directly correlated: in fact, porosity and associated impurity content may lead to a higher indentation hardness. Furthermore, the film sputtered at 300 C displays 105
Section 5.3: Indentation Hardness Figure 5.7: Indentation hardness of as-prepared and annealed ion-implanted and magnetron-sputtered a-si deposited at room temperature and 300 C determined in all cases by the Oliver-Pharr method. a considerably higher indentation hardness than the film sputtered at room temperature. This behaviour might be due to dynamic annealing occurring during the deposition and a microstructural hardening. As detailed in Section 5.2.2 plastic deformation obviously changes the mass density of the film. In order to investigate how this densification influences the indentation hardness additional pre-deformed sputtered films with various thermal history (relaxation anneal and/or argon anneal) were investigated. This yielded an additional six samples, four predeformed films (as-sputtered, relaxation annealed, argon annealed and both, relaxation and argon annealed) and two more undeformed annealed films (argon annealed and both, relaxation and argon annealed). The indentation hardness of all these films was then determined using the Hysitron Triboindenter and is summarized together with the indentation hardness of the films deposited at 300 C in Table 5.2. Again, the indentation hardness is denoted as a percentage of that of c-si, to ensure direct comparability of the different measurements. The error denoted is, therefore, the propagating error from the standard error of the indentation measurement. More details such as the load-displacement curves of these pre-deformed and annealed films are given in Appendix F. Note that no evidence of phase transformation could be seen in any of the displacement-curves and even the pre-deformed films appear to deform via plastic flow. In the case of the room temperature deposition, annealing as well as pre-deformation increased the indentation hardness. Interestingly, the film displaying the highest indentation hardness is the pre-deformed as-sputtered film. Moreover, its indentation hardness 106
Chapter 5: Mechanical Properties Deposition Annealing Further Indentation Hardness Temp. [ C] Temp. [ C] Treatment [% of c-si] RT - - 86.1±0.6 RT - pre-deformed 102.6±1.0 RT 450-88.4±0.9 RT 450 pre-deformed 95.2±1.2 RT 600-89.6±0.7 RT 600 pre-deformed 97.3±0.7 RT 600 and 450-92.1±0.7 RT 600 and 450 pre-deformed 98.3±1.0 300 - - 95.2±1.2 300 450-95.0±0.5 Table 5.2: Comparison of the indentation hardness of the differently treated films of sputtered a-si. Note that the 600 C anneal is the argon anneal and the 450 C anneal is the relaxation anneal. decreases upon annealing which indicates that this highly densified form of a-si is not thermally stable. This is in contrast to the behaviour observed for the films that had undergone the argon anneal at 600 C, in which case the consecutive relaxation anneal (i.e. 450 C) after pre-deformation increased the indentation hardness of the pre-deformed region. Therefore, it is possible to conclude that considerable changes had occurred to the film when subjected to the argon anneal. Interestingly, the deposition at 300 C yields a film of higher indentation hardness than any of the non-deformed room temperature films. Once more, this might indicate that dynamic annealing occurring during the deposition is influencing the structural properties of the film significantly more than the thermal annealing performed after deposition. Pressure-Induced Amorphous Silicon The indentation hardness of PI a-si was compared to that of ion-implanted a-si. As detailed in the experimental section, the indentation hardness was determined by two different methods. Firstly, it was calculated using the Hysitron TriboIndenter employing the Oliver-Pharr method [113, 114] and secondly by direct determination of the projected area employing an SEM. These results are summarized as H OP M for the determination by the Oliver-Pharr method and as H SEM for direct scanning in Table 5.3. The Oliver-Pharr method gives for both ion-implanted cases, as-implanted and re- 107
Section 5.3: Indentation Hardness laxed, the same indentation hardness. However, from inspection of the load-displacement curves this seems unlikely as the maximum penetration depth of the as-implanted material is slightly increased over the relaxed material. One explanation for this discrepancy might relate to the complications occurring when comparing two different mechanical systems such as a phase transforming one and a non-phase transforming one. In the as-implanted case the plastic deformation and resulting indentation hardness is based on plastic flow out of the indent volume and possibly compression below the indenter, whereas in the relaxed case the indentation hardness is based on a phase transformation between the soft metallic Si-II phase and the crystalline high-pressure phases, which have been reported to be even harder than c-si [216]. These differences are especially evident during the unloading part of the indentation experiment and hence the stiffness used for the indentation hardness (that relies on the unloading slope) is especially susceptible to these differences. Therefore, it is questionable if the comparison of indentation hardness of as-implanted and relaxed a-si measured by the Oliver-Pharr method is accurate. A previous study observed the same hardness for relaxed a-si (11.4 GPa at a depth of 450 nm) and c-si (11.5 GPa at a depth of 400 nm) [20]. In this previous study however, the indentation hardness was measured employing the Field-Swain method [115], which is based on partial fast unloading and consecutive re-loading using a spherical tip. During this fast unloading, PI a-si forms under the indentation tip in the relaxed case thus increasing the indentation hardness upon re-loading. Since this phase transition process is the same as observed for the crystalline case, the indentation hardness is expected to be similar. In contrast, for the Oliver-Pharr method the indentation hardness is influenced significantly by the phase transformation to Si-II upon loading as the maximum penetration depth will be determined by the amount of Si-II formed under the tip. The transformation from this Si-II phase to further end phases will then determine the stiffness and thus the indentation hardness. If the phase transformation of c-si to Si-II takes place at the same hydrostatic stress under the tip as the transformation from relaxed a-si to Si-II, the same Material H OP M [GPa] H SEM [GPa] c-si 12.38 ± 0.09 11.26 ± 0.05 as-implanted 10.70 ± 0.04 9.55 ± 0.05 relaxed implanted 10.71 ± 0.04 10.42 ± 0.08 as-indented PI 11.01 ± 0.27 11.87 ± 0.38 relaxed PI 9.61 ± 0.19 10.98 ± 0.29 Table 5.3: Comparison of the indentation hardness of c-si, as-prepared and relaxed PI and ion-implanted a-si determined by the Oliver-Pharr method and by SEM scanning. 108
Chapter 5: Mechanical Properties volume of Si-II under the tip and hence the same indentation hardness would be expected. Therefore, the lower indentation hardness of relaxed ion-implanted a-si observed in the present study by the Oliver-Pharr method might be an indication of a different onset pressure for phase transformation as compared to the crystalline case. This observation might be consistent with the further observation that, under the same loading and unloading indentation conditions, the high-pressure phases form more readily in relaxed a-si than in c-si [217]. Interestingly, the indentation hardness of as-indented PI a-si is found to be increased over all other a-si cases, and opposite to the ion-implanted case, it is observed to decrease upon annealing. However, as in the ion-implanted case comparison is complicated by the fact that only one of these systems is phase transforming, whereas the other is deforming plastically via flow. Furthermore, in the PI case a rough rather than a smooth surface is studied. This latter effect might lead to an underestimation of the indentation hardness since an increased maximum penetration depth arising from this surface roughness will lead to an increase in stiffness by the Oliver-Pharr method and hence decrease in the indentation hardness. Therefore, not only the indentation hardness of the as-indented and relaxed PI cases are hard to compare to each other, also the indentation hardness of the PI cases is not directly comparable to the ion-implanted cases using the Oliver-Pharr method. In order to take account for all these factors the indentation hardness was directly measured by imaging the residual indent impression with an SEM. This direct measurement is expected to be significantly more accurate [110]. Examples of such residual indent impressions are shown for c-si, as-implanted a-si and relaxed a-si in Fig. 5.8(a), (b) and (c), respectively. The projected area defined by the apexes is marked with dotted lines in each case. Note that the micrograph size is the same in all these cases. The results for the indentation hardness obtained by SEM, H SEM, are summarized in the second column of Table 5.3. The indentation hardness of relaxed ion-implanted a-si is increased compared to as-implanted a-si by 10%, but is 8% lower than that observed for c-si. This result confirms that indentation hardness values of as-implanted and relaxed a-si are not directly comparable by the Oliver-Pharr method since the deformation processes are different. Additionally, these results (the H SEM values) indicate that the phase transformation of relaxed a-si to the softer Si-II takes place at a lower hydrostatic stress than the transition from c-si to Si-II. This lower transformation pressure is consistent with simulations predicting an onset of phase transformation for a-si to Si-II at vastly lower pressures [218, 219]. An example of a nanoindent made in a microindent of as-indented PI a-si is shown in Fig. 5.8(d). Clearly, the nanoindent is considerably smaller than the microindent suggesting that the surrounding matrix of c-si will not influence the measurement. Examples of nanoindents made in as-indented and relaxed PI a-si are shown in Fig. 5.8(e) and (f), 109
Section 5.3: Indentation Hardness Figure 5.8: SEM scans of (a) a nanoindent in c-si, (b) a nanoindent in as-implanted a-si, (c) a nanoindent in relaxed implanted a-si, (d) an as-indented microindent with a nanoindent, (e) a nanoindent in as-indented PI a-si and (f) a nanoindent in relaxed PI a-si. Note that the black spots in the case of relaxed PI a-si in (f) appeared during the SEM scanning and are most likely due to carbon contamination in the SEM. 110
Chapter 5: Mechanical Properties Figure 5.9: Indentation hardness of c-si, as-prepared and relaxed ion-implanted and PI a-si measured by the Oliver-Pharr method [113] and SEM scanning. The phase transforming materials are marked in the plot. respectively. Again the projected area defined by the apexes is marked by the dotted lines. As the size of the micrographs is the same as for the previous nanoindents in part (a) through to (c) of the figure, already visual comparison shows that the nanoindent in as-indented PI a-si is considerably smaller than the other nanoindents indicating a significantly higher indentation hardness. The results for the PI a-si cases are also given in Table 5.3. Similar trends as observed by the Oliver-Pharr method are observed, namely an increased indentation hardness of as-indented PI a-si over all other forms of a-si and a reduction in its indentation hardness upon annealing. Interestingly, as-indented PI a-si is not only significantly harder than all other forms of a-si, but also than c-si. This may indicate that as-indented PI a-si cannot compress any further and will solely deform via plastic flow out from under the indenter. Furthermore, the indentation hardness decreases upon annealing to a value closer to that of relaxed implanted a-si indicating the formation of a relaxed network rather than of a compressed, dense network. However, the indentation hardness of relaxed PI a-si appears slightly increased over relaxed ion-implanted a-si. This might be due to an underestimation of the projected area in the PI case due to the surface roughness. These differences in indentation hardness obtained by the Oliver- Pharr method and by direct determination of the projected area are shown in Fig. 5.9. Not withstanding this small difference in the relaxed cases, it is clear that, whereas the indentation hardness increases upon annealing in the ion-implanted case, it will decrease upon annealing in the PI case. Furthermore, the as-indented PI case displays a 111
Section 5.4: Summary very high indentation hardness which is probably due to the high mass density observed for this form of a-si. 5.4 Summary The deformation behaviour of all the various forms of a-si has been studied using nanoindentation. Furthermore, the indentation hardness of the more uniform films was determined and these results are summarized in Table 5.4. No evidence of phase transformation was observed for the deposited films, PECVD grown as well as magnetron-sputtered, and for the laser-quenched a-si (both, as-prepared or annealed). Thus, the preferred mode of deformation of all these films appears to be plastic flow or compression of the film under the indenter against the substrate. In the case Form of a-si Indentation Hardness Phase [% of c-si] Transformation as-prepared PECVD - no annealed PECVD - no as-sputtered RT 86.1±0.6 a no as-sputt. RT deformed 102.6±1.0 a no annealed sputtered RT 88.4±0.9 a no ann. sputt. RT deformed 95.2±1.2 a no as-sputtered 300 C 95.3±0.5 a no annealed sputtered 300 C 95.0±0.5 a no as-quenched - no annealed quenched - no as-implanted 84.8±0.8 b no relaxed implanted 92.6±1.1 b yes as-ind. PI 105.4±3.8 b no relaxed PI 97.5±3.0 b yes Table 5.4: Comparison of the indentation hardness and transformation behaviour of the different forms of a-si. a Determined by the Oliver-Pharr method. b Determined by SEM analysis. 112
Chapter 5: Mechanical Properties of magnetron-sputtered a-si, a significant densification of the film is observed, noticeable from the absence of the nanopores after deformation. The behaviour of PI a-si is very different. While the as-indented PI a-si was also observed to deform via plastic flow, the annealed form of PI a-si phase transforms in the same manner as c-si or relaxed ion-implanted a-si. Therefore, in order to enable this phase transformation, the PI a-si has most likely undergone structural relaxation upon annealing. The indentation hardness of some forms of magnetron-sputtered a-si were found to be increased over as-implanted a-si and this was attributed to structural differences in the sputtered films. However, all the densified pre-deformed films display a higher indentation hardness with the pre-deformed as-sputtered film even exceeding that of c-si. Whereas the indentation hardness of as-implanted films generally increases upon annealing, the opposite behaviour occurs for PI a-si. As-prepared PI a-si also exhibited a high indentation hardness exceeding that of c-si and this high indentation hardness decreased upon thermal annealing to a similar value as observed for relaxed implanted a-si. This strongly indicates that the dense, compressed forms of a-si, as-indented PI and also pre-deformed as-sputtered a-si, are not structurally stable upon annealing. Therefore, based on the above results there is clearly no direct correlation between indentation hardness and deformation behaviour in the case of the various forms of a-si. Moreover, this distinct lack of correlation indicates strongly that the ability of an a-si film to phase transform must be tied to other structural properties such as, for example, the structural order of the amorphous network. Therefore, this topic will be explored in depth in the next two chapters. 113
Chapter 6 Medium-Range Order
Chapter 6: Medium-Range Order 6.1 Introduction Fluctuation electron microscopy (FEM) is a powerful technique when probing an amorphous material for its order on the length-scale of 1-3 nm. While diffraction techniques, and also Raman microspectroscopy, give access to the nearest-neighbour characteristics, FEM is to date the only technique that gains access to the pair-pair atomic correlation functions and hence into the realm beyond these nearest-neighbour characteristics. How FEM gains this insight into the realm of medium-range order (MRO) is detailed in Section 2.3.3 in Chapter 2, whereas an example of previous data obtained from sputtered and ion-implanted a-si has been shown in Fig. 1.3 in Chapter 1. For the case of amorphous semiconductors the magnitude of the variance has been correlated with the degree of MRO by interpreting a higher variance as indicative of a higher degree of MRO [47, 48, 55, 149] and this interpretation will be adopted throughout this work. This degree of MRO depends on a number of interconnected factors such as the number of regions of correlated structure, the size of these regions and the relative order within such a region. Moreover, the positions of the peaks in the variance plot give some additional insight into the local bonding and the type of MRO within the regions of correlated structure [45, 150]. Additionally, the ratio of the magnitude of the second to the first peak can give insight into the nature of the order present when FEM is performed in a STEM [152]. In this chapter the results for magnetron-sputtered and PI a-si probed for their MRO characteristics will be presented. The MRO-scale characteristics probed for by FEM are then compared to ion-implanted a-si in both cases. The use of a standard was necessary as the FEM employed in this study measures relative changes in MRO rather than absolutes since FEM is dependent on the exact experimental parameters used. Ion-implanted a-si was chosen as the standard as it is currently the only form of a-si known to undergo structural relaxation to a CRN. This relaxation is also observable by FEM as the degree of the MRO of the ion-implanted network is known to decrease significantly upon annealing (as seen in Fig. 1.3 in Chapter 1). However, the current study has also revealed some exciting new insights into the nature of the order present in as-implanted a-si, as indicated in this chapter. The other two forms of a-si probed by FEM are sputtered deposited a-si, with low but not negligible impurity content and a somewhat lower density, and the PI a-si, another pure form of a-si with uniform film characteristics. No studies of the MRO characteristics of PI a-si have previously been reported and thus this study provides the opportunity to explore the difference in the structure of different forms of a-si in depth. In contrast, sputtered a-si is the most studied form of a-si by FEM (see for example Refs. [15, 50, 51, 148] and references therein) thus enabling some comparison with the literature. However, in all these previous studies of magnetron-sputtered a-si, the TEM foils had 117
Section 6.2: Experimental Details been prepared by deposition of 20 nm layers on rock salt which was then dissolved in de-ionized water or by direct deposition onto TEM grids. In the present study, sputtered a-si specimens were prepared for TEM by back-thinning of a thick a-si layer deposited onto c-si similar to the ion-implanted specimens. Additionally, these three forms of a-si, magnetron-sputtered, PI and ion-implanted, are investigated in both their as-prepared and their annealed forms. Although the influence of ex situ thermal annealing has previously been studied in the case of ion-implanted a-si [46, 55], it has not been studied for sputtered a-si. Two different techniques, the tilted dark-field (DF) method as well as the newer STEM-FEM are employed. The ion-implanted a-si standard was studied by both techniques in order to facilitate comparison. The sputtered a-si was studied by STEM-FEM and the PI a-si by the tilted DF method. These two different FEM methods were performed on different microscopes and with very different experimental conditions and hence the results are not directly comparable without the ion-implanted standard. Such comparison however, is desirable since FEM variations between different a-si samples illuminate details of the structure that are influenced by the formation method or the influence of a thermal anneal. 6.2 Experimental Details The sputtered a-si and the ion-implanted a-si were studied using the STEM-FEM technique as detailed in Section 2.3.3. Approximately five areas were measured for each film, whereby 100 nanodiffraction patterns make up the variance of one area. The size of the probe used for this study was 1.6 nm. The film used was deposited at 300 C as the higher uniformity (as indicated by the increased indentation hardness for example) of the film grown under these conditions resulted in better TEM foils. In contrast, the PI a-si (again as well as ion-implanted a-si) was studied using the tilted DF method, also detailed in Section 2.3.3. In this case 10 areas were measured for each form of a-si. The forms of a-si studied by these two different types of FEM are summarized in Table 6.1. Note that all forms had undergone the same relaxation anneal. Only the ionimplanted and PI a-si however, were found to phase transform upon annealing under indentation testing. Therefore, only these two will be referred to as relaxed, whereas the sputtered film will be referred to as annealed. All TEM foils were prepared in the plan-view geometry by back thinning as detailed in Section 2.3.1. It should be noted however, that the ratio of the etchent used varied for the two sets of experiments. For the sputtered and first set of ion-implanted a-si, a ratio of the HNO 3 :HF:CH 3 COOH mixture of 7:1:1 was used as this resulted in better TEM foils for the sputtered samples. In the case of the PI a-si and second set of ion-implanted samples a ratio for the HNO 3 :HF:CH 3 COOH mixture of 5:1:1 was used to avoid crystallization of 118