Detailed structural study of low temperature mixed-phase Si films by X-TEM and ambient conductive AFM

Transcription

1 Journal of Non-Crystalline Solids 352 (2006) Section 6. Order disorder, defects and electronic structure Detailed structural study of low temperature mixed-phase Si films by X-TEM and ambient conductive AFM T. Mates a, *, P.C.P. Bronsveld b, A. Fejfar a, B. Rezek a, J. Kočka a, J.K. Rath b, R.E.I. Schropp b a Institute of Physics, Academy of Sciences of the Czech Republic, Department of Thin Films, Cukrovarnická 10, Praha 6, Czech Republic b Utrecht University, Debye Institute, SID-Physics of Devices, P.O. Box , 3508 TA Utrecht, The Netherlands Available online 29 March Abstract Microcrystalline silicon (lc-si:h) thin films prepared by plasma enhanced chemical vapour deposition (PECVD) at 37 C has been studied by cross-sectional TEM and ambient conductive AFM. We have succeeded in the combined measurement of topography and local conductivity under standard ambient conditions, overcoming the surface native oxide by more sensitive (pa range) current detection. We observed the columnar structure of the amorphous phase in the TEM micrograph and related it to the surface corrugation of the same size detected by AFM. Ó 2006 Elsevier B.V. All rights reserved. PACS: Ps; Ac; Bd; Gh Keywords: Silicon; Solar cells; Plasma deposition; Atomic force and scanning tunneling microscopy; TEM/STEM 1. Introduction Microcrystalline silicon (lc-si:h) is the key material for thin film Si solar cells. Recent efforts to make this material even more affordable include high-rate [1] or low temperature [2] depositions. Besides this, there are still many open questions concerning the structure and electrical properties of lc-si:h, which is characterized by a wide variety of structures depending on the deposition conditions [3]. Previous studies indicate that samples prepared at the border between amorphous and microcrystalline growth (mixedphase) exhibit improved photovoltaic performance compared to fully amorphous or microcrystalline thin silicon films [2,4]. * Corresponding author. Tel.: address: mates@fzu.cz (T. Mates). 2. Experimental In this work we present the study of intrinsic mixedphase thin Si film prepared by Plasma Enhanced CVD (PECVD) at very low deposition temperature of T s =37 C, hydrogen dilution ratio r H =[H 2 ]/[SiH 4 ]=40 and f exc = 50 MHz. Thickness of the deposited Si film was around 900 nm. Sample A was deposited on Corning 1737F glass and the same deposition was repeated with Cr coated glass substrate (sample B) to enable the conductive AFM measurements. Two pieces of the sample A were cut and glued together with Si films facing each other. Then a thin slice was sawed-off, ground, polished and ion milled to reduce the thickness below 450 nm. Finally, the thin cross-sectional slice was glued on a copper grid to prevent charging during the cross-sectional (X-TEM) measurements, which were done in the bright field configuration with electrons of 100 kev. The remaining part of the sample A and sample B were measured by atomic force microscope (AFM) Veeco /$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.jnoncrysol

2 1012 T. Mates et al. / Journal of Non-Crystalline Solids 352 (2006) Dimension 3100 in tapping mode. The contact mode conductive AFM (C-AFM) measurement was performed in the same experimental configuration as in [5] to obtain topography image simultaneously with the local current map. The AFM was equipped with the extended TUNA (Tunneling AFM) optional module to provide very sensitive current detection in the pa range. For the measurement in contact mode we used soft, Si etched cantilevers with Pt/It metal coating, resonant frequency around 13 khz and applied normal force around 25 nn and scanning speed 500 nm/s. Power spectral density (PSD) function [6,7] was computed [8] from the AFM topography images in order to provide typical lateral dimension of the surface features. AFM data were further processed in WSxM software [9]. Evaluations of crystallinity (X C ) from Raman and hydrogen content from FTIR measurements is described elsewhere [10]. 3. Results The sample, we decided to study in detail, was carefully selected from the series of samples with variable dilution r H, where the structure of samples changed from amorphous to microcrystalline [10]. This particular sample with r H = 40 exhibits the best opto-electronic properties, i.e., the highest activation energy around 0.7 ev together with the highest photoresponse (ratio of dark and photo-conductivity r D /r ph 10 5 ). The crystallinity (X C ) of this sample measured by Raman spectroscopy is around X C =30%. This is the only sample of the whole series having the X C value between amorphous (X C = 0%) and fully microcrystalline values (X C 60%). The first topography measurement of both samples revealed isolated microcrystalline grains of similar diameter embedded in amorphous phase, see Fig. 1. The PSD analysis provides average diameter of the microcrystalline grain 570 ± 75 nm for sample A and 570 ± 45 nm for sample B. Both samples differ only in the density of these grains, influenced by the substrate material, on which the Si film was grown. Next, the surface topography of sample A is compared with the X-TEM image. This direct comparison of two different microscopic methods applied to one sample shows very good agreement of sizes of surface caps of microcrystalline grains as indicated by the dotted lines in Fig. 2. The other grains shown in the X-TEM micrograph in Fig. 2(b) appear a bit smaller in size since the sample was probably not cut through the centers of all the cones. However, a closer look at the X-TEM image in Fig. 2(b) brought our attention to another interesting feature of this sample indication of a columnar structure of the amorphous phase emphasized at the top, where the X-TEM sample is expected to be quite thin. PSD analysis of a selected part of the X-TEM image reveals typical column width of 54 ± 8 nm. Since the digital sampling limit of the AFM images in Fig. 1 makes the comparison of such small structures impossible, we performed the AFM measurements in a smaller scale, this time in contact mode. Topography images obtained for sample A [11] and sample B (Fig. 3(a)) are very similar. Both show corrugated amorphous surrounding, which seems to correspond to the heads of the columns observed in X-TEM (Fig. 2(b)). PSD numerical comparison of typical lateral sizes reveals 51 ± 3 nm for sample A and 52 ± 3 nm for sample B, which is in very good agreement with the value obtained from X-TEM image. Fig. 1. AFM surface topography of Si film deposited on (a) glass sample A and on (b) Cr coated glass sample B. The white rectangle in (a) indicates the area, which is plotted in 3D in Fig. 2(a). Fig. 2. (a) 3D plot of AFM surface topography of selected area of sample A with color scale representing the local height and (b) X-TEM image of film cross-section. Both images are in the same scale for comparison purposes.

3 T. Mates et al. / Journal of Non-Crystalline Solids 352 (2006) (c) (a) (b) and a broader one with the maximum at pa representing the amorphous and microcrystalline phases respectively. Another C-AFM measurement of even smaller area ( nm 2 ) is shown as a merged 3D image in Fig. 4. One interesting profile containing part of the microcrystalline grain and part of its amorphous surrounding is plotted in Fig. 4(b). In this profile, the microcrystalline grain is represented by bumps in topography and a higher signal of local current. The amorphous tissue is rather flat both in topography and local current. There is just one exception (indicated by the arrows), which is discussed in Section Discussion Fig. 3. Conductive AFM measurement of sample B showing (a) surface topography together with (b) local current map obtained at U bias =2V. Double peaked distribution of local current values (c) with a-si:h and lc- Si:H maximum at and pa, respectively. The black square in (a) and (b) indicates the area, which is plotted in 3D in Fig. 4(a). By applying a bias voltage (U bias ) between the bottom Cr electrode (sample B) and the conductive cantilever as in [5], we detected the local current shown in Fig. 3(b). The histogram of local current values (Fig. 3(c)) shows two peaks narrow one with the maximum at pa (a) (b) Fig. 4. Edge of the microcrystalline grain measured by conductive AFM at U bias = 4 V. The image (a) shows merged topography (3D effect) with local current values (color scale). The height and current profiles along the black line are shown in (b). Note the higher local current signal of an amorphous bump indicated by arrows in (b) and brighter spot in (a). The comparison of topography images of samples A and B in Fig. 1 as well as the typical grain diameters evaluated by PSD indicate that the repeated deposition was successful and that both samples could be considered structurally quite similar. The presence of Cr bottom electrode resulted in slightly different nucleation density, but did not change the growth regime of microcrystalline grains, resulting in similar surface grain diameters. X-TEM image provided an illustrative insight into the evolution of mixed-phase structure. Microcrystalline grains typically start the growth on the top of an incubation layer. Based on the deposition conditions very thin incubation layers of few nm [12] or thicker [13] up to several hundreds of nm were observed. The desired structure of mixed phase sample requires very fine tuning of deposition conditions, most often performed by the variation of r H [3]. Our sample has shown typical features of mixed phase films, i.e., incubation layer (100 nm) and conical shape of microcrystalline grains. The C-AFM measurement, which we demonstrate in Figs. 3 and 4, was considered not to be feasible for Si thin films in the ambient atmosphere. Since the first work [5] it was supposed, that UHV conditions ( in situ samples) are essential to prevent the surface from oxidation. However, here we firstly demonstrate that the microscope can be operated in ambient conditions (air) and similar results can be obtained. Generally, the values of local current are very small even in the case of measurement on naked Cr bottom contact with Pt it coated cantilever the local current fluctuates around 10 na at U bias = 0.5 V. For the measurement of ex situ Si films, the occurrence of native oxide on surface is expected, so the current detection must be extremely sensitive. We performed the C-AFM measurement of sample B with U bias =2V and typical current of 0.05 pa (Fig. 3) just 2 weeks after the deposition of the Si film. Another C-AFM measurement was repeated six months later and a bit higher U bias =5V had to be applied to obtain similar contrast in current image. For the testing purposes we also examined (U bias = 3 V) another mixed phase sample deposited five years ago with no major obstacles. In spite of the presence of SiO 2 on surface of measured films, the current flowing through a-si and lc-si:h remains

4 1014 T. Mates et al. / Journal of Non-Crystalline Solids 352 (2006) distinct enough to provide the same qualitative information as in the case of measurements performed in UHV. Quantitatively, the current signal drops down by about three orders of magnitude from na range at U bias =3 7 V in UHV [5] into the pa range at the same U bias voltage in ambient conditions. The pa current range and given values of U bias fully correspond to STM measurement conditions [14], i.e., the C-AFM is in this range measuring tunneling current through the thin surface oxide barrier. We also tested the influence of major scanning parameters on the absolute value and spatial distribution of the local current. The increase of scanning speed from 125 to 2000 nm/s in five steps brought only a slightly decreased homogeneity of current distribution for the speeds above 500 nm/s. The contact force value has much stronger impact. With a very soft contact force (10 nn), the current is detected only at grain boundaries, i.e., on sloped areas. This measurement artefact can be suppressed by the increase to 25 nn (standard measuring value). Contact forces above 50 nn give slightly better homogeneity of local current on rough surfaces, but may have a harmful effect on the life of cantilever metal coating [15], thus complicating the measurement reproducibility. Due to the agreement of surfaces of both samples (Fig. 1) we can expect also similar cross-sectional structure of sample B. This is important for the understanding of the histogram in Fig. 3(c). The typical local current of microcrystalline grain is just about 4 higher than for the amorphous surrounding. This is a small difference compared to five orders of magnitude difference of macroscopic conductivities of a-si:h and lc-si:h [16]. The small difference in the histogram values indicates that the current is limited by the a-si:h phase present as the incubation layer and preventing the lc-si:h grain from direct electrical contact to the bottom electrode. Our sample A indicates the presence of columnar structure of the amorphous phase [11]. A survey of various deposition conditions (dilution, power, bias voltage, temperature, thickness) [17 19], indicates that the columnar structure is typically formed just before the transition to microcrystalline growth occurs. There are just a few observations of this structure by TEM, e.g., in [17] the samples prepared from argon diluted silane were studied. This work is, to our knowledge, the first X-TEM observation of these columns for the sample prepared from silane and hydrogen only. The low deposition temperature, which is responsible for quite high hydrogen content (22.5%) in the film made the X-TEM observation a bit easier. The contrast between amorphous columns and their boundaries is probably enhanced by the concentration of hydrogen to the boundaries, thus making the difference in the cross-section for the electron beam. Finally, we focused on electrical properties in the high resolution C-AFM image (Fig. 4). Some of the a-si:h columns in the neighbourhood of the lc-si:h grain exhibit higher current clearly corresponding to the topography of the a-si:h column, see the arrows in Fig. 4. This finding indicates that the current probably passes through the column and enters the lc-si:h grain. The other columns are then isolated by the boundaries between the columns supporting the hypothesis of hydrogen rich column boundaries acting as potential barriers for the electronic transport. This behaviour is in a sense similar to the active role of grain boundaries in lc-si:h. 5. Conclusion In this paper we report on the columnar structure of a- Si:H for samples prepared from silane and hydrogen at low deposition temperatures. We have successfully measured AFM topography and local current under ambient conditions utilizing very sensitive current detection. The first direct observation of amorphous columns by X-TEM was correlated with C-AFM measurement of the same sample with very good agreement. The results of high resolution C-AFM indicate that some a-si:h films (close to a- Si:H/lc-Si:H boundary) may be considered structurally and electrically inhomogeneous and, surprisingly, serve as an optimal photovoltaic material [10]. Acknowledgements We thank Jeroen Francke for the deposition of the layers. This research has been supported by AV0Z , VaV SN/172/05, GAAV IAA , IAA and GA ER 202/05/H003. References [1] C. Niikura, M. Kondo, A. Matsuda, J. Non-Cryst. Solids (2004) 42. [2] M. Ito, C. Koch, V. Svrcek, M.B. Schubert, J.H. 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