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Understanding the Evolution of the Pop-out Effect in Si-based Structures for Photovoltaics
E. E. Hareaa,b, K. E. Aifantisb,c
aInstitute of Applied Physics, ASM, Chisinau, Republic of Moldova, b Lab of Mechanics and Materials, Aristotle University, Thessaloniki, Greece, cCivil Engineering-Engineering Mechanics, University of Arizona, Tucson, AZ, USA e-mail:aifantis@email.arizona.edu
The most interesting phenomenon observed during nanoindentation of Si is the strain burst which occurs during unloading. This feature is referred to as a “pop-out” effect, and it is linked to the phase transformation that occurs underneath the indenter at high stresses. One of the peculiarities of this effect is the observed linear dependence between the depth at which the “pop-out” effect occurs and the maximum penetration depth. By performing a systematic study on the doped Si as well as on photovoltaic structures such as ITO/Si and SnO2/Si, it is shown that this linearity holds for different unloading rates, which strongly affects the “pop-out” appearance and contact pressure. Furthermore, it is illustrated that that phase transformation in the doped Si is delayed when it is coated with a thin film due to the tension preservation in the imprint under the film coating. This observation suggests that the mechanical properties at the interface between thin films and Si substrates in coated systems (MEMs and photovoltaics) should be further investigated.
Keywords: pop-out effect, Si, nanoindentation, silicon structures, photovoltaics.
УДК 538.9:539.2/.6:548.1
INTRODUCTION
One of the most common features observed in load-displacement curves recorded during nanoindentation are strain bursts. Such strain bursts, referred to as “pop-in” effects, take place during loading and indicate dislocation motion [1, 2], or fracture of surface oxides [3, 4]. A peculiar exception to this rule is Si that exhibits a strain burst during unloading [5-9], which is termed a “pop-out”. This effect has been widely studied and there are several interpretations for its nature. Pharr [6] attributed the “pop-out” effect to the crystal lateral cracks that are formed under the imprint of the indenter during unloading and can lead to a sudden expulsion of the indenter from Si. However, if this were indeed the case, it can not explain why other materials do not exhibit such strain bursts during unloading.
Another theory, which is more widely accepted, is the emergence of the “pop-out” effect due to a phase transformation that occurs under the indenter [10, 11]. During loading, at pressures between 11.3-12.5 GPa, the density of Si-I increases by 22% and it transforms to Si-II ф-Sn type of structure) [11]. As Si-II is metallic, it has mechanical and electrical properties similar to those of metals. However, this phase is unstable and during unloading it transforms into Si-XII and Si-III, as well as amorphous a-Si. The imprint region, along with the above mentioned crystalline phases, contains some amorphous silicon (a-Si). The mixture of crystalline and amorphous phases in the deformed zone under the imprint depends on the magnitude of the maximum applied
load [7, 11 and 12]. At very low loads of ~ 20 mN the amorphous phase is formed predominantly under the indenter, which is shown on the load/displacement curve, P(h), as an “elbow” during unloading, rather than a distinct strain burst. At a maximum applied load of ~ 50 mN, the volume of the mixed amorphous-polycrystalline structures increases. With a further increase of the maximum applied load the quantity of crystalline phases Si-III and Si- XII also increases. In such cases the load/displacement curves P(h) exhibit a distinct “pop-out”.
Numerous studies suggest that the final mixture of Si phases during unloading depends not only on the magnitude of the applied load but also on the unloading rate [8, 11, 13-17]. Particularly, it was shown that a slow unloading rate (6-120 mN/min) stimulates the formation of Si-III and Si-XII, while a high unloading rate (> 100 mN/min) favored the formation of the amorphous silicon (a-Si), which strongly affects the occurrence of pop-outs. These works motivated the present study whose purpose is to understand the sensitivity of the indentation depth at which the pop-out occurs to the nanoindentation parameters, such as the maximum applied load, penetration depth, as well as unloading rates. A wide range of maximum loads were examined (20-500 mN), while the loading and unloading rates were either kept constant or varied, between 40-1000 mN/min in order to observe how the depth at which the pop-out occurred would be affected.
It is well known that the mechanical properties in thin film/substrate systems are different from those
© Harea E.E., Aifantis K.E., Электронная обработка материалов, 2014, 50(6), 44-50.
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of their bulk counterparts. Hence, the question arises regarding the evolution of pop-outs in the coated silicon systems. This is why in addition to testing «-Si, coated silicon systems such as ITO/Si and SnO2/Si used as photovoltaic cells were examined and «-Si was coated with a thin film (~ 350 nm) of ITO or SnO2. These films with significant differences in their mechanical properties (especially adhesion), belong to the class of ceramic materials. The ITO films were our choice because they are used in different devices since they have outstanding physical properties such as: thermal, electrical and chemical stability, adhesion, transparency, thermoelectricity, conductivity, and piezoresistivity. Furthermore, the ITO films are used as transparent contacts for solar cells [18], active elements for temperature measurements [19], pressure sensors [20] and are promising gauge sensors that can work in elevated temperature conditions (up to 1500°C) [21]. So we try to answer some fundamental questions about Si using coated silicon systems that are fabricated for practical applications as solar cells.
EXPERIMENTAL PROCEDURE
Indentation was performed, using the trapezoidal loading sequence on a CSM nanoindenter. The maximum applied load (Pmax) varied from 20 mN to 500 mN, as shown in Table 1, and a hold time of 20 seconds was used in all cases. Different conditions allowed for the loading/unloading rates were as follows: (i) the loading and unloading rates were taken equal to each other, and were twice the maximum load/minute for each maximum load, (ii) the loading rate varied as in (i) but the unloading rate was kept constant at 80 mN/min (slow rate) for all maximum loads, (iii) the loading rate was varied as in (i) and (ii), but the unloading rate was kept constant at 600 mN/min (high rate) for all maximum loads.
As mentioned in the Introduction, these experiments were repeated for three Si samples: «-type Si, ITO/Si, and SnO2/Si. The structures of ITO/Si and SnO2/Si are promising for converting solar energy into electricity. These structures were obtained by pyrolytic spraying of the alcoholic solution of indium chloride and tin chloride (InCl3:SnCl4) in the case of ITO films, and the alcoholic solution of tin chloride (SnCl4) in the case of SnO2 films, on a heated phosphorus doped-silicon substrate, with a (100) crystallographic orientation [22]. As a result, initially a thin layer of SiO2 of about 3-5 nm formed, followed by a polycrystalline film of In2O3:SnO2 (ITO) or SnO2, with a ~ 350 nm thickness. Since ITO and SnO2 have significantly different mechanical properties it is important to investigate how they affect the substrate mechanical behavior during indentation.
The surface roughness is an important parameter that can affect the determination of mechanical parameters during nanoindentation [23]. Based on the atomic force microscopy (AFM) measurements the average roughness for ITO was Ra = 6.4 ± 0.5 nm, and for the SnO2 film it was Ra = 10.5 ± 1.2 nm. These parameters were taken into consideration during the hardness and elastic modulus measurements (maximum penetration depth was chosen hc ~ 50 nm in accordance with ISO 14577-4, where the surface roughness vs. indentation depth should be Ra < 5% hc and hc< 10-15% of the coating thickness).
The hardness and elastic modulus of the samples were automatically calculated by the nanotester software using the Oliver-Pharr method [24].
The adhesive properties of coated systems were assessed using the method proposed by Rosenfeld et al. [25], which is based on the indentation data. The energy (G) necessary to delaminate the coating from the substrate can be determined from the following expression:
0.627H
G =--------------- 2
Ef D + V + 2 ()H,a,/ P_x ]
where Ef and Vf are the Young’s modulus and Poisson’s ratio of the coating, respectively, Hf is the hardness of the coating, a is the measured diameter of the delaminated area, t is the film thickness and Pmax is the value of the applied load.
RESULTS
Coatings strongly affect the mechanical properties of structures. Figure 1 shows that the maximum penetration depth in Si, ITO/Si and SnO2/Si structures depends on the maximum applied load.
For SnO2/Si it can be seen that after Pmax = 60 mN, its hm(Pmax) plot is nearly parallel with the hm(Pmax) curve for the doped Si. This suggests that the SnO2 volume that is compressed from the indenter deforms until delamination and the circular crack formation occurs at loads between 0 < Pmax < 60 mN. The moment at which film delamination takes place can be determined from the large “pop-in” that appears in the load-penetration curves (Fig. 2e and f) [26]. This “pop-in” occurs, since during delamination the bonding forces between the film and the substrate are suddenly broken. Particularly, the film compresses the substrate material around the indenter until delamination, but after delamination it can move easier and form for example pile-ups. Hence, fast lattice reorganization takes place and the indenter tip “falls” into the material at a constant load, resulting in a “pop-in”. As the hmax vs Pmax plots for «-Si and SnO2/Si are parallel after delamination in the latter, it can be assumed that for
Ч1 -v2)
(1)
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Table 1. Loading-unloading parameters
^max, mN Case (i) Case (ii) Case (iii)
Loading rate=unloading rate, mN/min Loading rate, mN/min Unloading rate, mN/min Loading rate, mN/min Unloading rate, mN/min
20 40 40 80 40 600
40 80 80 80
60 120 120 120
80 160 160 160
100 200 200 200
200 400 400 400
300 600 600 600
400 800 800 800
500 1000 1000 1000
Fig. 1. Maximum penetration depth in Si, ITO/Si and SnO2/Si structures depending of maximum applied load.
Fig. 2. Load-displacement curves for: (a) and (b) doped Si, (c) and (d) ITO/Si, (e) and (f) SnO2/Si.
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Table 2. hpop-out (hm) function constants and mechanical properties determined from nanoindentation and AFM
Material Constants Hardness H, GPa Young’s modulus E, GPa Roughness Ra, nm Adhesion G, J/m-2
A B
Si 0.88 -72 13.2 ± 0.2 150 ± 10 1.2 ± 0.5 -
ITO 0.88 -123 9.6 ± 0.4 126 ± 16 6.4 ± 0.5 9.5 ± 0.8
SnO2 0.81 -89.5 7.5 ± 0.3 146 ± 11 10.5 ± 1.2 0.11 ± 0.05
Pmax > 60 mN, the SnO2 film consistently affects the penetration of the indenter into the Si substrate regardless of the maximum applied load. It can be supposed that the silicon substrate is indented by a modified indenter system, which is the Berkovich diamond indenter covered by the stretched SnO2 film. On the other hand, for the ITO/Si structure starting from Pmax = 40 mN a monotonous increase is observed of the maximum penetration depth versus the maximum applied load in the coated samples compared with the uncoated Si. This suggests that the influence of the ITO film on the mechanical properties of the ITO/Si structure decreases continuously as the maximum applied load increases. This may be due to the fact that as the penetration increases the thickness of the stretched ITO film that covers the Berkovich diamond tip significantly decreases, making the existence of ITO negligible for applied high loads, whereas the thickness of the SnO2 most likely remained more uniform with penetration depth. In Table 2 it can be seen that the elastic modulus of ITO is lower than that of SnO2, which suggests that ITO bends more and therefore with increasing indentation depths (applied high loads) the thickness of ITO covering the tip would be less.
The nanoindentation experiments on all tested samples (Si wafers doped with phosphorus, the ITO/Si and SnO2/Si structures) showed the existence of the anticipated “pop-out” (Fig. 2). The phenomenon had a probabilistic nature and occurred in about 80% of the total number of experiments depending on the unloading rate and applied load. The ITO and SnO2 films had a significant influence on the emergence and development of the effect.
In the ITO/Si samples the “pop-out” began to appear when the maximum applied load was Pmax = 40 mN or higher, while in the SnO2/Si samples it began at loads of Pmax = 60 mN and higher (Fig. 2). This is directly related to the presence of the ITO or SnO2 thin film on the Si surface. In the uncoated samples the “pop-out” appeared starting with the load of 20 mN. When the load was not high enough to produce a distinct burst, an “elbow” feature was observed.
The depth at which the “pop-out” occurred (hpop-out) was defined as (hi - h2)/2+h2 (Fig. 3), where hi was the depth at which the burst began and h2 the
depth at which it ended. The values of hm and hr are the maximum and residual depth, respectively.
Fig. 3. Definition of indentation depths considered to characterize the occurrence of pop-out effect.
In Fig. 4 the maximum penetration depth is plotted as a function of the depth at which the “pop-out” occurred. It was very interesting to observe that the relationship between hm and hvov.out is quite linear not only for Si [12] but also for the coated Si systems (Fig. 4).
It is worth noting that in all experiments the relationship between hpop.out and hm is linear regardless of the conditions for the unloading rates.
DISCUSSION
The fact that the strain bursts (Fig. 2) occurred at higher loads for the coated samples was expected as the thin film coatings limit the deformation that the substrate can undergo, and in order for the “pop-out” to occur a certain amount of pressure had to be exerted on Si. Particularly, in order to deform the silicon substrate to a depth necessary for the “pop-out” effect to take place, the indenter had to overcome the mechanical resistance of the film
The “pop-out” is affected by the unloading rate, but the hpop-out vs hm relationship remains linear, and fitted lines tilt angle shows a very small variation depending on the unloading rate. This feature of the silicon “pop-out” effect can be used to study the thin films mechanical characteristics. If it is assumed that only Si can produce “pop-outs”, the hpop.out for equal depths of deformation of Si should be the same in both uncoated and coated samples. But further studies indicated that this is not the case. Linear curves of Fig. 4 are described by the following equation:
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doped with phosphorus (substrate), as well as of ITO/Si and SnO2/Si.
Fig. 5. hpop-out vs. hm for cases (i), (ii) and (iii) for doped Si.
200 400 600 800 1000 1200 1400 1600 hm, nm
(a)
(b)
Fig. 6. hpop-out vs. hm for cases (ii) and (iii) for (a) ITO/Si and (b) SnO2/Si structures.
(c)
1
- 3
t;
Fig. 7. (a) “Pop-out” effect when Pmax = 60 mN for (1) Si doped with phosphorus (substrate), (2) for ITO/Si and for (3) SnO2/Si; case (i) from Table 1; (b) indenter displacement in unloading stage for studied samples for equal time sequence; (c) hm - hpop-out distance for the same applied load.
Table 3. Examples of Eq. (3).
hm hpop-out, nm- Si ITO/Si SnO2/Si
hm, nm hpop-out nm- hm, nm hpop-out nm hm, nm hpop-out nm
270 1672 1402 1230 960 959 689
250 1450 1200 1073 823 830 580
hpop-out Ahm+B (2)
where A and B are constants which depend on Si and the coating materials properties (Table 2). The estimation of A and B is important as they allow predicting the depth at which the “pop-out” will occur once the maximum penetration depth is known. This implies that one can predict the indenter displacement depth when the phase transition (Si-II trans-
forms into Si-XII, Si-III and a-Si mixture [27]) takes place during unloading. The physical interpretation of these constants is not clearly understood and is still being studied.
To better understand the phenomena that occur under the indenter during the unloading stage, we will examine the indentation curves for Pmax = 60 mN (case (i) from Table 1) for all investi-
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gated samples (Fig. 7a). First of all, our attention is captured by the significant difference between the maximum penetration depth in each sample. This is due to the presence of a thin film (~ 350 nm thick), which has a different Young’s modulus and fracture toughness. The second difference can be observed when we compare the indenter motion speed during unloading. Fig. 7b illustrates the imprint recovery distance for the same period of time. It is interesting that in the SnO2/Si sample the elastic recovery is faster.
The recovery process in imprints for the uncoated Si wafer is different from that of the coated systems. Even regardless of the equal unloading rates for all samples, in the uncoated Si the imprint depth decreased slower during the unloading stage. Fig. 7b presents the indenter motion distance as the indenter tip withdrew from the samples during the load decrease from 60 to 30 mN, within 15 sec (the unloading ratio is 120 mN/min according to case (i) from Table 1). The recovery speed for this case is: 5.5 nm/s for Si, 5.7 nm/s for Si covered by SnO2 and 6.7 nm/s for Si coated by ITO. So it can be assumed that coatings (i) preserve tension in Si under the in-denter and favor elastic recovery of the indented volume, (ii) delay significantly the phase transformation evolution during the unloading stage (Fig. 7c) and change the phase mixture proportions responsible for the “pop-out” effect [27] (see pop-out shape in Fig. 7a).
There appears a question: when the elastic recovery distance until the “pop-out” effect is the same in coated and uncoated Si samples? The answer may be: this will occur when the distances illustrated in Fig. 7c are equal. It is very difficult to achieve this in a “real life” experiment, but we can easily determine it from the hm vs hp0p-out graph of Fig. 4. We can find the distance at which the indenter had to be “removed” from the sample upon reaching the maximum depth, before the “pop-out” occurs for all of the investigated samples. An interesting observation is when
hm(Si) - hpop-out(Si) = hm(ITO/Si) - hpop-out(ITO/Si) = = hm (SnO2/Si) - hpop-out(SnO2/Si). (3)
Some results of it are presented in Table 3.
It can be clearly seen that for the same recovery depth until the “pop-out” occurred (hm - hpop-out) the maximum penetration depth and therefore the maximum applied load are lower in the coated systems than in the uncoated ones. The connection between recovery distance until the “pop-out” occurrence and phase transformation in Si is not clearly understood. These results indicate the difference and complexity of mechanical processes that occur in coated substrates subjected to a concentrated load action.
CONCLUSIONS
In the present study it was shown that the depth at which the strain burst (“pop-out”) observed in «-Si, ITO/Si and SnO2/Si during unloading strongly depends on the maximum applied load and is linearly proportional to the maximum indentation depth. In particular, the relationship of hpop.out vs. hm remains linear, and the fits in Figs. 5 and 6 show a very small variation depending on the unloading rate.
Some new features of the “pop-out” effect for the coated «-Si structures used in photovoltaics were depicted and analyzed. It was established that the phase transformations in «-Si are delayed and that the phase mixture proportions responsible for the “pop-out” effect change due to the tension preservation in the imprint under the film coating. Furthermore, by comparing the indentation behavior between the ITO/Si and SnO2/Si samples, significant differences in the mechanical properties between the two coatings can be understood.
ACKNOWLEDGEMENTS
The present study was performed in the frame of the KEA's European Research Council Starting Grant 211166 MINATRAN.
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Received 05.03.14 Accepted 19.06.14
Реферат
Наиболее интересным явлением, замеченным в наноиндентировании кремния, считается разрыв деформационной кривой на стадии разгрузки. Данная особенность названа «pop-out» эффектом и относится к фазовым превращениям, имеющим место под ин-дентором при большом давлении. Одним из свойств данного эффекта является обнаруженная нами линейная зависимость между глубиной, на которой «pop-out» эффект, и максимальной глубиной проникновения индентора. Систематическое изучение легированного Si, а также фотопреобразователей типа ITO/Si и SnO2/Si показало, что данная линейность сохраняется при разных скоростях разгрузки индентора, которые сильно влияют как на появление «pop-out» эффекта, так и на контактное давление. Более того, показано, что фазовые превращения в легированном кремнии, покрытом тонкой плёнкой, заторможены из-за сохранения механического напряжения в отпечатке под пленкой. Эти наблюдения предполагают необходимость дальнейших исследований механических свойств на границе раздела между плёнкой и кремниевой подложкой в плёночных структурах (МЭМС и ФЭП).
Ключевые слова: «pop-out» эффект, кремний, наноиндентирование, кремниевая структура, фотопреобразователи.
