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Defect elimination in solid-phase crystallised Si thin films by line-focus diode laser annealing
Thin Solid Films 576 (2015) 42–49
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Defect elimination in solid-phase crystallised Si thin films by line-focus diode laser annealing Wei Li ⁎, Sergey Varlamov, Jialiang Huang School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 1 May 2014 Received in revised form 16 December 2014 Accepted 16 December 2014 Available online 19 December 2014 Keywords: Polycrystalline silicon Defect annealing Laser annealing Thin film Stress
a b s t r a c t A high density of intragrain defects in solid-phase crystallised Si thin films results in poor electronic properties and impedes their use for thin-film solar cell or thin-film transistor applications. This paper demonstrates that a high-power line-focus diode laser can eliminate intragrain defects (microtwins and dislocations) in polycrystalline Si films while maintaining the smooth defect-free surface. Improved electronic properties of ultra-thin polycrystalline Si thin films are thus achieved. To alleviate crack formation during diode laser annealing, a rapid-thermal pre-treatment at 800 °C for 60 s is introduced since it effectively relieves the tensile stress in the films and thus generates a more stable precursor material for subsequent laser annealing. The film thickness plays an important role in diode laser annealing. The films thinner than 100 nm show relatively smaller improvement due to the limited absorption of 808 nm laser radiation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Polycrystalline silicon (poly-Si) thin films on glass have attracted considerable research interest in the fields of thin-film solar cells and liquid crystal displays (LCDs) [1,2] Solid phase crystallisation (SPC) of amorphous Si is an established method to obtain poly-Si thin films that comprise crystal grains of up to several microns in size [3]. However, the SPC process mediated by formation of micro-twins results in a large residual density of these defects [4]. Microtwins are a source for other intragrain defects (such as dislocations) which all lead to inferior electronic properties of SPC poly-Si [4]. Consequently, these defects limit applications of SPC poly-Si thin films as a material for thin film solar cells or as pixel switch and driver elements for high-speed, highdefinition, and low-power smart displays [5,6]. Microtwins in SPC poly-Si films are unstable and their density can be effectively reduced at a relatively high temperature (N 750 °C) [4]. Above 900 °C, a partial removal of the defects at grain boundaries occurs due to the crystallographic relaxation [7]. Even higher temperatures above 1170 °C allow dislocation movement unconstrained by crystallographic glide planes, leading to dislocation annihilation [8]. Thus, a higher annealing temperature should result in a lower defect density and hence a better structural and electronic quality of SPC poly-Si thin film. However, the limited thermal stability of glass limits the maximum possible temperature by conventional furnace or rapid thermal annealing [9].
⁎ Corresponding author. E-mail address: [email protected] (W. Li).
http://dx.doi.org/10.1016/j.tsf.2014.12.033 0040-6090/© 2014 Elsevier B.V. All rights reserved.
Laser annealing can quickly heat up a poly-Si film to the temperature close to the Si melting point (1414 °C) while a glass substrate remains at a much lower temperature of about 650 °C (i.e. the sample stage temperature). During this process, intragrain defects density is greatly reduced and a better electronic quality is obtained [10]. Excimer-laser annealing (ELA) has been used for defects removal in poly-Si films from 1990s [10,11]. However, very fast cooling (at a rate of ~1010 K/s) after a short pulse (in the range of several ns) of an excimer-laser results in a hillock formation at grain boundaries [12] and a large number of vacancies in grains [13] leading to a higher bulk deep trap density. Diode laser annealing is a much slower process, of an order of tens of microseconds, with an accordingly lower cooling rate which is expected to result in a lower intragrain defect density [13]. A line-focus beam is particularly useful for any practical application as it allows fast large area processing [14]. In this paper, a high-power continuous-wavelength diode laser with a line-focus beam (12 mm × 181 μm) is used to anneal intragrain defects in SPC poly-Si films. It can effectively eliminate intragrain defects while keeping the smooth planar film surface without any damage (in contrast to ELA) and greatly shortening the processing time as compared with a conventional continuous-wavelength pointfocus laser. 2. Experimental details The sample structure (80 nm SiNx / 80–160 nm a-Si:H with phosphorous concentration of 1.7 × 1020 cm−3 / 150 nm SiOx) was deposited on a planar glass substrate (Schott Borofloat33) by plasma-enhanced chemical vapour deposition. SiNx acts as a barrier and anti-reflection layer and SiOx is a capping layer protecting Si films from atmospheric
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contamination during thermal annealing. The samples were processed by the following three steps: 1. SPC: all samples were crystallised in a tube furnace purged with nitrogen at 600 °C for 24 h. 2. Rapid thermal annealing (RTA): some of the crystallised poly-Si films were additionally annealed in a belt furnace heated by halogen lamps and purged with nitrogen at 800 °C for 1 min for stress relief. 3. DLA: following the RTA treatment, a part of the samples was annealed by scanning with the line-focus diode laser beam (LIMO 450-L12 × 0.3-DL808-EX937) for an exposure time of 136 ms at a range of energy densities. The samples were preheated to 650 °C. In this paper, micro-Raman spectroscopy (Renishaw inVia Raman Microscope with 442 nm laser and a spot size of 1.5 μm) was used to non-destructively characterise the stress and structure quality in the poly-Si thin films [15]. The peak position and full-width at halfmaximum (FWHM) of the optical-phonon mode were estimated by fitting the mixed Gaussian–Lorentzian function to the spectra. The peak shift and the peak FWHM were averaged over nine different positions taken over the area of 25 × 25 μm2 for each sample. The film surface roughness was analysed by Atomic Force Microscope (AFM, Veeco Digital Instruments Multimode SPM) in a PeakForce Tapping mode with a ScanAsyst-air tip (tip radius: 2 nm). The surface morphology of the film was also observed by an optical microscope (Olympus, BH-2) in a transmission mode. A focused ion beam (FIB) system (the xT Nova NanoLab 200) operating at 30 keV is used to prepare cross-sectional transmission electron microscopy (XTEM) specimens from the samples studied in this work. In this method, a focused beam of Ga+ ions is used to mill away small portions of the sample. Ultimately, a 100–200 nm thick, 22 μm long and electron-transparent membrane specimen is produced which is then removed from the sample surface ex-situ lifted out and attached to a Cu grid. Prior to FIB milling, the sample pieces were placed on Al stubs with C adhesive and subsequently sputter coated with ~ 20 nm of Au. The thin Au film was used to make the surface conductive for imaging and for surface protection during the initial stages of preparation. Subsequently, a layer of ~1 μm thick Pt was FIB-deposited over the area of interest to protect the area from further beam damage during the main stages of sample preparation. Thus, negligible FIB-induced damage to the specimens was experienced [16]. A Philips CM200 TEM operated at 200 kV was used to analyse the intragrain defects in the Si films. The sheet resistance, carrier concentration and electron mobility of each sample are determined by Hall Effect measurements. The electrode for Hall measurements was a Van der Pauw configuration. The thin film sample was laser isolated into the cross pattern and the characterisation area sets to 400 μm × 400 μm. Electrical contacts were made by using a GaAl alloy. The intensity of the magnetic field was 1 T, and the temperature was 300 K. The results were averaged over three different positions and the error bars shown in the figures are one standard deviation of the mean.
Fig. 1. Typical Raman spectra of the optical phonon mode obtained for 120 nm SPC poly-Si films prepared under different conditions, and c-Si reference: (a) the poly-Si film after SPC only; (b) the poly-Si film after SPC and RTA; (c) the poly-Si film after SPC, RTA and DLA; and (d) the c-Si reference.
3. Results and discussions
result from the volumetric contraction of a Si film when it transforms from the amorphous into crystalline phase, which is denser. This contraction is retarded by the substrate, which imposes a tensile stress on the film [18]. It is reported that a tensile stress can lead to film cracking but an excessive compressive stress can result in film buckling [19]. In either case, the residual stress can cause poor performance of a poly-Si thin film device or impede further processing on this stressed layer [19]. Therefore, it is of importance to release the residual stress in the films before the DLA treatment. The stress can be relieved by loose adhesion to the substrate [18] which is due to the breaking of bonds at the interface between a polySi film and the underlayer [20]. The stress in the poly-Si films is also relieved once the glass substrate softens at higher temperatures. Conventional furnace annealing can partially relax a residual stress in poly-Si thin films but it takes quite a long time (~2 h) at a high temperature (N1000 °C) [21] which is unsuitable for the glass. It was reported that RTA can effectively reduce the residual stress in poly-Si thin films [18,22]. Therefore, RTA is a better process for stress relaxation but RTA plateau temperatures should be limited such that to maintain a smooth and even film surface. Afterwards, before DLA processing, all SPC poly-Si films are treated by RTA with a plateau temperature of 800 °C (close to 820 °C of the softening point of Schott Borofloat33 glass) for 60 s. After RTA the film surface remains smooth and suitable for following DLA processing. As shown in Fig. 2(a), after RTA processing the Raman peak shifts to a higher wavenumber indicating that the residual stress in the SPC polySi films is relieved. The magnitude of the residual stress can be estimated from the stress-induced wave-number shift of the TO–LO peak compared to the Raman line of the stress-free single crystal. By assuming biaxial isotropic stress, the stress (δ) can be expressed as follows [23],
3.1. Stress relief by RTA
δ ¼ 250 Δω MPa
Typical Raman spectra of the optical-phonon mode for poly-Si films and single crystalline Si (c-Si) used as the standard are shown in Fig. 1. The Raman peaks of the SPC poly-Si thin films appear at wavenumbers lower than the standard c-Si peak position. The strain in the crystalline lattice distorts the atomic distances and causes a shift in the energies of the vibrational modes. Hence, the first order Raman line, which is located at ~520 cm−1 at the neutral stress level, shifts to lower or higher wavenumbers with tensile or compressive stress, respectively [17]. Raman spectra in Fig. 1 indicate the presence of a tensile stress in the SPC poly-Si films. During the SPC process, a biaxial tensile stress usually develops. Tensile stress evolution is considered to
where Δω (cm−1) is the phonon shift calculated from (ωs − ω0), ω0 is the Raman shift of stress-free single crystalline Si and ωs is the Raman shift of the film under measurement. The estimated residual stress in the differently processed poly-Si films is shown in the Fig. 2(b). The RTA plateau temperature of 800 °C results in partial softening of the glass substrate. During this process, most of the residual stress in the SPC poly-Si thin films is relaxed without glass deformation. Fig. 2(b) also reveals that as the film becomes thinner, the residual stress can be relieved to a smaller value by RTA. All samples of different thicknesses show no change in the stress level before and after the DLA treatment. It indicates that DLA does not introduce extra stress to the poly-Si films.
ð1Þ
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3.2. Effects of laser dose density on annealing of the SPC poly-Si thin films
Fig. 2. Comparison of Raman shifts (a) and the residual stress (b) in poly-Si films of different thicknesses after processing by SPC, RTA, and DLA.
It has to be noted that a doping concentration can also influence the LO–TO phonon peak shift in poly-Si thin films. It was reported that in P-doped poly-Si films the wavenumber of the LO–TO phonon line occurs at 517 cm− 1 for P concentrations of up to 4×1019 cm− 3 [24] which is similar to our samples. Moreover, with an increasing P concentration, the LO–TO phonon line shifts to a lower wavenumber and a low-energy tail extending from 400 to 500 cm−1 develops [25]. A high concentration of the free electrons in the conduction band interacting with the phonon deformation-potential results in a small shift in the position of the phonon to lower frequencies due to the Fano effect [26,27]. This interaction takes place only above a certain free-carrier concentration for n-type Si [26] which is estimated to be about 4×1019 cm−3 [24]. In our case, the free carrier concentration in the poly-Si film is higher than this threshold value. After the RTA treatment, the free carrier concentration in the poly-Si thin film increases due to the dopant activation. Consequently, the LO–TO peak of a Pdoped Si film should move to a lower wavenumber due to the Fano effect. In contrast, after RTA the LO–TO peak shifts in the opposite direction, to a higher wavenumber, indicating that the stress relief due to the RTA treatment overweighs the doping effect. Because of the above, the residual stress in RTA or DLA treated poly-Si film is actually lower than the value shown in Fig. 2. Highly stressed SPC poly-Si films can develop cracks during DLA processing. However, after stress relief, the films are less likely to crack. Additionally, the stress can directly trigger formation and propagation of dislocations which is detrimental to the device performance [15]. RTA at 800 °C for 1 min effectively relieves the residual stress in SPC poly-Si thin films and generates a stress-relaxed precursor for DLA processing.
The 120 nm thick SPC poly-Si thin films are annealed by the linefocus diode laser at a range of energy doses. As the laser dose density is lower than 724.3 J/cm2, DLA does not generate any damage to polySi films as shown in the optical transmission microscope (OTM) image of Fig. 3(a). However, the laser dose in excess of 724 J/cm2 can cause damage to the films as shown in Fig. 3(b). The carrier concentrations and electron mobilities in the 120 nm thick SPC poly-Si films versus the laser dose density are plotted in Fig. 4. As the laser dose density increases from 0 to 696 J/cm2, carrier concentration continuously increases from 7.3 × 1019 to 8.2 × 1019 cm−3. Intragrain defects which are capable of trapping free carriers limit their active concentration. A higher free carrier concentration indicates a lower intragrain defect density at a higher laser dose. At the laser dose density above 696 J/cm2, the carrier concentration reaches the maximum value of 8.2 × 1019 cm−3. It means that most of the dopants have already been activated. As the laser dose density increases to 686 J/cm2, electron mobility declines slightly but then rises at the laser dose densities above 686 J/cm2. The mobility is influenced by two factors: a free carrier concentration and a density of defects acting as traps. Reduction of the defect density can improve the mobility and also increase the free carrier concentration as reported above. However, at the same time, at high free carrier concentrations the mobility can decrease due to the carrier scattering by ionised dopant atoms [28]. Therefore, the electron mobility as a function of the laser dose density combines the effects of the two above factors. The electron mobility firstly declines due to the free carrier scattering effect [28]. Afterwards, the electron mobility increases quickly due to the annealing of intragrain defects. The sheet resistance of the 120 nm thick SPC poly-Si film as a function of the laser dose density is shown in Fig. 5. The sheet resistance decreases from 86.7 Ω/□ to 72.5 Ω/□ as the laser dose density increases to 724 J/cm2. The sheet resistance is determined by both the active doping concentration and the mobility. Therefore, the sheet resistance versus the laser dose density can be interpreted in terms of electronic activation of P atoms and a mobility improvement. Fig. 6(a) reveals the surface root-mean-squared roughness (RMS) measured by AFM of the 120 nm thick SPC poly-Si film as a function of the laser dose density. RMS roughness (Rq) of the film surface, defined as the standard deviation of the elevation, Z values, within the scanned area, is calculated from the equation below, Rq ¼
qX ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ðZ i −Z ave Þ =N
ð1Þ
where Zave is the average of the Z values within the scanned area, Zi is the Z value for a given point, and N is the number of points within the scanned area. When the laser dose density is lower than 667.7 J/cm2, the film roughness does not change much and it remains as smooth as for the sample without DLA processing. At the laser dose density in the range from 696 J/cm2 to 707.3 J/cm2, the surface roughness increases from 1.6 nm to 2.62 nm. The film is not as smooth as it was before DLA processing. However, when the laser dose is about 724 J/cm2, it is quite noticeable that the surface roughness decreases abruptly back to 1.6 nm. The smooth planar surface after DLA processing at 724 J/cm2 is also observed in the cross-sectional focused ion beam image in Fig. 6(b). Due to the very short solidification time, the excimer laser annealing leads to a hillock formation at grain boundaries which is shown in the Fig. 2(c) of reference [12]. The roughness due to the hillocks increases the density of the bulk deep traps and causes scattering of carriers [12]. However, the silicon surface after the DLA treatment remains as smooth as it was before the treatment which is evident from a crosssectional focused ion beam image in Fig. 6(b) and the TEM images in Fig. 9. One possible reason is that the cooling rate during diode laser
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Fig. 3. OTM images of DLA treated 120 nm SPC poly-Si film: (a) at laser dose density of 724.3 J/cm2 and (b) at laser dose density of N724.3 J/cm2.
annealing is much lower than that during ELA, so hillocks do not form at grain boundaries as shown by Hara et al. [29]. Although the surface roughness increases when the film starts melting, the film turns smooth again when full, or mostly full melting is reached. Furthermore, the SiOx capping layer also helps to reduce the film roughness [30]. Because a SiOx film is physically hard and the melting temperature of SiOx is much higher than that of Si, the SiOx cap behaves as a mould for a liquefied Si thin film [30]. Finally, the average roughness can also be reduced at a lower scanning speed, due to longer solidification [30]. 3.3. Effects of the film thickness on properties of SPC, RTA and DLA processed poly-Si films The FWHM of the transverse optical (TO) peak in a Raman spectrum is directly related to the structural quality of poly-Si [31]. The better the structural quality, the smaller the FWHM. The FWHM of the Raman TO peak of the SPC, RTA and DLA treated poly-Si thin films are shown in Fig. 7. After the SPC poly-Si thin film is processed by RTA its FWHM becomes smaller than that of the poly-Si film after SPC only. FWHM decreases further after DLA processing. FWHM of SPC poly-Si films is the largest which is due to the high intragrain defect density which will be shown by the TEM images in the following sections. RTA at 800 °C for 1 min results in the improvement of the structural quality while also avoiding excessive heating of the glass substrate. This RTA temperature and duration are suitable for stress relief but not sufficient to remove all microtwins and other intragrain defects from the interior of the grains [4]. DLA processing can heat up poly-Si films to temperature close to the Si melting point leading to a further reduction of the intragrain defect density. On the one hand, FWHM decreases further after DLA processing due to the structural quality improvement. On the other hand, DLA processing can also increase the free carrier concentration leading to larger FWHM due to the Fano effect [25–27,32]. Although DLA-treated polySi films combine effects from both defect elimination and dopant activation, the improvement of structural quality overweighs the doping effect. Fig. 7 reveals that the film thickness has a positive effect on the structural quality of SPC poly-Si films. As the DLA treated poly-Si film thickness increases, Raman FWHM decreases, indicating the structural quality improvement. As the light absorption by the poly-Si film is proportional to its thickness, the annealing temperature also depends on the film thickness. Although higher laser dose densities are applied to the thinner films, due to the limited absorption of 808 nm laser radiation, the thinner SPC poly-Si films achieve lower maximum temperatures during DLA hence a relatively smaller improvement in Raman FWHM as compared to the thicker films. XTEM images are taken to investigate the crystal quality of the polySi films. Fig. 8(a) and (b) show a highly defective grain in the SPC poly-Si film and its diffraction pattern respectively. Microtwins are one of the main defects in the SPC poly-Si thin films and they always occur on
(111) planes. Microtwins can be clearly seen under [110] direction in TEM [33] so in this study the TEM specimen was tilted so that the [011] crystallographic direction of the grain was parallel to the electron beam. From the diffraction pattern in Fig. 8(b), the periodic extra spots in the reciprocal lattice are due to the twins on (111) plain so the parallel stripes in Fig. 8(a) are twins growing along [112] direction. Fig. 9 shows a XTEM image of a DLA-processed poly-Si film with the thickness from 80 nm to 160 nm, in which the 80 nm thick poly-Si film contains more intragrain defects than the 120 and 160 nm thick SPC poly-Si films. By comparison of Fig. 8(a) with Fig. 9(c), the poly-Si film after DLA contains much fewer defects than the poly-Si film after SPC only. During the DLA process, the poly-Si films were heated up to the temperature close to the Si melting point. Therefore, most of the microtwins were absorbed by larger twins [4] and dislocations are annihilated through movement unconstrained by crystallographic glide planes [8]. Moreover, the film is partially melted so the defects in the melted part are annihilated. Thus, the interior of the grains appears essentially free of defects after DLA processing. In Fig. 9, the clear interface between the glass and the SiNx buffer layer is visible indicating that DLA does not cause any damage. The poly-Si thin films are seen to have the smooth planar surface after the DLA treatment which is consistent with the AFM results. If the laser dose increases to the extent that the Si film is fully melted, the Si films thinner than about 400 nm tend to disintegrate because of dewetting from the substrate [34,35]. In order to prevent dewetting, the film should avoid complete melting but leave a thin unmelted solid part. If the temperature in the thin unmelted layer is sufficiently high, the defect will be mostly eliminated. This unmelted thin layer will then act as a defect-free seed for epitaxial recrystallisation of the melted part of the film. Grains free from defects are thus formed. A similar result was reported by using excimer laser annealing of SPC poly-Si thin films [10]. Additionally, the grain size after DLA is determined by the grain size of the unmelted part so the grain size is not appreciably affected from comparison between Fig. 8(a) and Fig. 9(c). It is possible that some of the twins are still left in the unmelted layer and they can propagate into crystallising Si during the solidification process. Nevertheless, as shown in Fig. 9, they completely penetrate the whole crystal without introducing other electric active defects so they should not be detrimental to the electronic quality of annealed poly-Si thin films [36]. However, the diode laser radiation at 808 nm wavelength is not well absorbed by the thinner poly-Si films leading to lower achieved maximum temperatures. In this case, some intragrain defects are not well removed as shown in Fig. 9(c). Nevertheless, the 80 nm DLA treated polySi film still contains much fewer intragrain defects than the SPC poly-Si without DLA processing. To summarise, due to the transient high temperature DLA treatment, most of the intragrain defects such as microtwins and dislocations are removed. Therefore, the structural quality of SPC poly-Si thin films is effectively improved and better electronic characteristics are obtained.
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Fig. 4. Carrier concentration and electron mobility in the 120 nm SPC poly-Si film as a function of the laser dose density. (“Zero” energy density means the film without DLA treatment).
The carrier concentration, electron mobility and resistivity of the Pdoped poly-Si films versus the different annealing processes are presented in Fig. 10. The poly-Si film after SPC has the lowest carrier concentration while the DLA treated poly-Si film has the highest carrier concentration. As discussed above, the higher free carrier concentration indicates fewer intragrain defects after thermal processing which is consistent with the findings from TEM analysis. The peak RTA temperature is limited by stability of the glass substrate which impedes more complete elimination of intragrain defects and obtaining a better film quality. DLA can increase the annealing temperature up to near the Si melting point without overheating the glass substrate. Therefore, due to more efficient defect elimination by DLA, a higher degree of dopant activation is achieved. In Fig. 10(b), most of the poly-Si films after SPC only have the lowest electron mobilities while the DLA treated poly-Si films have the highest electron mobility. According to Haji's work [4], the low mobility in SPC poly-Si films is due to their defective nature. However, the residual stress can also influence mobility. Thompson et al. reported that strain could enhance the electron and hole mobility by strain-induced energy-level splitting, inversion-layer quantum confinement energylevel shifts, average mass change due to repopulation and band warping, two-dimensional density of states, and interband scattering changes due to band splitting [37]. Relieving the stress by RTA can thus decrease the electron mobility. On the other hand, the intragrain defect elimination by RTA will improve the Hall mobility. As a result of two opposite effects, the RTA treated poly-Si films do not show a
Fig. 5. Measured sheet resistance of the 120 nm SPC Si thin film as a function of the laser dose density.
Fig. 6. (a) AFM surface roughness of 120 nm SPC Si thin films irradiated at different laser dose densities, (b) cross-sectional FIB image of a DLA treated 120 nm SPC Si thin film with laser dose of 724 J/cm2. (The Pt layer was coated for protection from ion beam damage).
significant improvement in the electron mobility compared with the poly-Si films after SPC only. The poly-Si films after DLA processing have higher Hall mobility than those after RTA. It can be explained by further intragrain defect elimination at a higher temperature provided by DLA processing. Although reducing intragrain defect density improves the electron mobility, the free carrier concentration caused by better dopant activation can also result in lowering the electron mobility due to carrier scattering by the ionised dopant atoms [28]. Consequently, the two opposing effects can balance each other leading to an insignificant electron mobility improvement after DLA processing.
Fig. 7. Effects of SPC, RTA, and DLA on the FWHM of Raman peak for 80 nm, 120 nm and 160 nm thick SPC poly-Si films.
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Fig. 8. (a) Dark field image and (b) diffraction pattern of a 80 nm poly-Si film after SPC only. (The Pt layer was coated against the ion beam damage during TEM sample preparation).
Fig. 10(c) shows a decrease in the resistivity of poly-Si films after RTA and particularly after DLA processing. Lower resistivity can be interpreted in terms of activation of P atoms and electron mobility improvement. Because the DLA treated poly-Si films have higher free carrier concentration and higher electron mobility, their resistivity is much lower than those after SPC or RTA. The film thickness effects on the carrier concentration, the electron mobility and the resistivity are also shown in Fig. 10. The thicker polySi films have higher carrier concentrations, higher Hall mobility and lower resistivity. It is possible that thinner films are more likely to be influenced by defects at the interface between SiNx and the poly-Si film. Moreover, due to the limited absorption of the 808 nm laser radiation, lower maximum temperatures are reached causing incomplete defect elimination in the thinner poly-Si films after DLA as shown in Fig. 9. Therefore, as the film thickness increases from 80 nm to 160 nm, better electronic properties are obtained for the DLA treated poly-Si films. Such low-defect laser annealed poly-Si films can be used as high quality seed layers for epitaxial thickening for solar cell and thin-film transistor applications.
Fig. 9. Dark-field cross-section TEM micrograph of the DLA SPC poly-Si films with different film thicknesses, (a) 160 nm, (b) 120 nm and (c) 80 nm. (The diffraction pattern of each grain is shown in the inset and SiOx capping layer was removed by HF dip so it is not seen in these figures).
4. Conclusion SPC poly-Si contains a high density of intragrain defects which limit its electronic properties. In this paper, effects of DLA processing on
electrical and structural properties of ultra-thin SPC films are presented. The films obtained by SPC are always under tensile stress which leads to cracking during DLA. By RTA at 800 °C for 60 s, the residual stress in SPC
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not accepted by the Australian Government. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Electron Microscope Unit, The University of New South Wales (UNSW). Wei Li would like to appreciate the Ph.D. scholarship from UNSW and China Scholarship Council (CSC). References
Fig. 10. Effects of SPC, RTA, and DLA on (a) the carrier concentration, (b) electron mobility and (c) resistivity of the 80 nm, 120 nm and 160 nm thick P-doped SPC poly-Si films.
poly-Si thin films can be effectively relieved thus providing a stable precursor material for laser annealing. TEM, Raman, and Hall measurements showed that DLA significantly improves the structural and electronic properties of poly-Si, by reducing the intragrain defect density. AFM proves that a DLA-treated SPC film retains a smooth planar surface as before DLA processing. Film thickness plays an important role in diode laser annealing. Due to the limited absorption of 808 nm laser radiation, the quality of thinner SPC poly-Si films improves less than that for the thicker films. Acknowledgement This programme has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) (LP0883548) and Australian Research Council (ARC) (2A014A). Responsibility for the views, information or advice expressed herein is
[1] S. Varlamov, J. Dore, R. Evans, D. Ong, B. Eggleston, O. Kunz, U. Schubert, T. Young, J. Huang, T. Soderstrom, K. Omaki, K. Kim, A. Teal, M. Jung, J. Yun, Z.M. Pakhuruddin, R. Egan, M.A. Green, Polycrystalline silicon on glass thin-film solar cells: a transition from solid-phase to liquid-phase crystallised silicon, Sol. Energy Mater. Sol. Cells 119 (2013) 246. [2] S. Inoue, S. Utsunomiya, T. Saeki, T. Shimoda, Surface-free technology by laser annealing (SUFTLA) and its application to poly-Si TFT-LCDs on plastic film with integrated drivers, IEEE Trans. Electron. Devices 49 (2002) 1353. [3] T.L.Y.M. Keevers, U. Schubert, R. Evans, R.J. Egan, M.A. Green, 10% efficient CSG minimodules, Proceedings of the 22nd EUPVSEC, Milan, Italy, 2007, p. 1783. [4] L. Haji, P. Joubert, J. Stoemenos, N.A. Economou, Mode of growth and microstructure of polycrystalline silicon obtained by solid-phase crystallization of an amorphous silicon film, J. Appl. Phys. 75 (1994) 3944. [5] Y. Masayuki, K. Shin-Ichiro, S. Tadashi, K. Koji, High-performance poly-Si thin film transistors with highly biaxially oriented poly-Si thin films using double line beam continuous-wave laser lateral crystallization, Jpn. J. Appl. Phys. 53 (2014) 03CC02. [6] W. Li, S. Varlamov, M. Jung, J. Huang, Vapour-phase and solid-phase epitaxy of silicon on solid-phase crystallised seed layers for solar cells application, Int. J. Photoenergy 2014 (2014) 234602. [7] J.P. Colinge, E. Demoulin, F. Delannay, M. Lobet, J.M. Temerson, Grain size and resistivity of LPCVD polycrystalline silicon films, J. Electrochem. Soc. 128 (1981) 2009. [8] K. Hartman, M. Bertoni, J. Serdy, T. Buonassisi, Dislocation density reduction in multicrystalline silicon solar cell material by high temperature annealing, Appl. Phys. Lett. 93 (2008) 122108. [9] W. Li, S. Varlamov, J. Dore, M. Green, Defect annealing in ultra-thin polycrystalline silicon films on glass: rapid thermal versus laser processing, Mater. Lett. 107 (2013) 1. [10] R. Carluccio, J. Stoemenos, G. Fortunato, D.B. Meakin, M. Bianconi, Microstructure of polycrystalline silicon films obtained by combined furnace and laser annealing, Appl. Phys. Lett. 66 (1995) 1394. [11] S. Friligkos, V. Papaioannou, J. Stoemenos, R. Carluccio, S. Cina, G. Fortunato, Structural characterization of α-Si films crystallized by combined furnace and laser annealing, J. Cryst. Growth 182 (1997) 341. [12] M. Miyasaka, J. Stoemenos, Excimer laser annealing of amorphous and solid-phasecrystallized silicon films, J. Appl. Phys. 86 (1999) 5556. [13] K. Kitahara, Y. Ohashi, Y. Katoh, A. Hara, N. Sasaki, Submicron-scale characterization of poly-Si thin films crystallized by excimer laser and continuous-wave laser, J. Appl. Phys. 95 (2004) 7850. [14] G. Andrä, J. Bergmann, F. Falk, Laser crystallized multicrystalline silicon thin films on glass, Thin Solid Films 487 (2005) 77. [15] I. DeWolf, Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits, Semicond. Sci. Technol. 11 (1996) 139. [16] W. Li, J. Chen, C. Yan, F. Liu, X. Hao, Transmission electron microscopy analysis for the process of crystallization of Cu2ZnSnS4 film from sputtered Zn/CuSn precursor, Nanotechnology 25 (2014) 195701. [17] F. Cerdeira, C.J. Buchenauer, F.H. Pollak, M. Cardona, Stress-induced shifts of first-order Raman frequencies of diamond- and zinc-blende-type semiconductors, Phys. Rev. B 5 (1972) 580. [18] A.K. Kalkan, S.J. Fonash, Stress and dopant activation in solid phase crystalized Si films, in: B.R. Chalamala, R.H. Friend, T.N. Jackson, F.R. Libsch (Eds.), Flat-Panel Displays and Sensors: Principles, Materials and Processes, Materials Research Society, Warrendale, 2000, p. 225. [19] X. Zhang, T.-Y. Zhang, M. Wong, Y. Zohar, Residual-stress relaxation in polysilicon thin films by high-temperature rapid thermal annealing, Sensors Actuators A Phys. 64 (1998) 109. [20] K. Kitahara, Y. Ohashi, K. Yamamoto, N. Sasaki, Electrochemical and Raman-scattering characterizations of defects in polycrystalline silicon thin films formed by excimerlaser annealing, solid-phase crystallization, and continuous-wave laser lateral crystallization, Jpn. J. Appl. Phys. 48 (2009) 021205. [21] D. Maierschneider, J. Maibach, E. Obermeier, D. Schneider, Variations in Young's modulus and intrinsic stress of LPCVD-polysilicon due to high-temperature annealing, J. Micromech. Microeng. 5 (1995) 121. [22] Z. Xin, Z. Tong-Yi, W. Man, Z. Yitshak, Rapid thermal annealing of polysilicon thin films, J. Microelectromech. Syst. 7 (1998) 356. [23] V. Paillard, P. Puech, M.A. Laguna, P. Temple-Boyer, B. Caussat, J.P. Couderc, B.D. Mauduit, Resonant Raman scattering in polycrystalline silicon thin films, Appl. Phys. Lett. 73 (1998) 1718. [24] N.H. Nickel, P. Lengsfeld, I. Sieber, Raman spectroscopy of heavily doped polycrystalline silicon thin films, Phys. Rev. B 61 (2000) 15558. [25] R. Saleh, N.H. Nickel, Raman spectroscopy of doped and compensated laser crystallized polycrystalline silicon thin films, Surf. Coat. Technol. 198 (2005) 143. [26] M. Chandrasekhar, J.B. Renucci, M. Cardona, Effects of interband excitations on Raman phonons in heavily doped n-Si, Phys. Rev. B 17 (1978) 1623. [27] U. Fano, Effects of configuration interaction on intensities and phase shifts, Phys. Rev. 124 (1961) 1866.
W. Li et al. / Thin Solid Films 576 (2015) 42–49 [28] S.M. Sze, M.-K. Lee, Semiconductor Devices: Physics and Technology, 3rd ed. John Wiley & Sons, Inc., 2012.. [29] A. Hara, M. Takei, F. Takeuchi, K. Suga, K. Yoshino, M. Chida, T. Kakehi, Y. Ebiko, Y. Sano, N. Sasaki, High performance low temperature polycrystalline silicon thin film transistors on non-alkaline glass produced using diode pumped solid state continuous wave laser lateral crystallization, Jpn. J. Appl. Phys. 1 Regul. Pap. Short Notes Rev. Pap. 43 (2004) 1269. [30] S. Fujii, S.I. Kuroki, M. Numata, K. Kotani, T. Ito, Roughness reduction in polycrystalline silicon thin films formed by continuous-wave laser lateral crystallization with cap SiO2 thin films, Jpn. J. Appl. Phys. 48 (2009) 04C129. [31] D. Song, D. Inns, A. Straub, M.L. Terry, P. Campbell, A.G. Aberle, Solid phase crystallized polycrystalline thin-films on glass from evaporated silicon for photovoltaic applications, Thin Solid Films 513 (2006) 356. [32] M. Chandrasekhar, M. Cardona, E.O. Kane, Intraband Raman scattering by free carriers in heavily doped n-Si, Phys. Rev. B 16 (1977) 3579.
49
[33] X.-Z. Bo, N. Yao, S.R. Shieh, T.S. Duffy, J.C. Sturm, Large-grain polycrystalline silicon films with low intragranular defect density by low-temperature solid-phase crystallization without underlying oxide, J. Appl. Phys. 91 (2002) 2910. [34] G. Andrä, C. Lehmann, J. Plentz, A. Gawlik, E. Ose, F. Falk, Varying the layer structure in multicrystalline LLC-silicon thin-film solar cells, 33rd IEEE Photovoltaic Specialists Conference, IEEE, San Diego, California, vol. 1–4, 2008, p. 1830. [35] J. Dore, D. Ong, S. Varlamov, R. Egan, M.A. Green, Progress in laser-crystallized thin-film polycrystalline silicon solar cells: intermediate layers, light trapping, and metallization, IEEE J. Photovolt. 4 (2014) 33. [36] J. Chen, D. Yang, Z. Xi, T. Sekiguchi, Recombination activity of Sigma 3 boundaries in boron-doped multicrystalline silicon: influence of iron contamination, J. Appl. Phys. 97 (2005) 033701. [37] S.E. Thompson, S. Guangyu, C. Youn Sung, T. Nishida, Uniaxial-process-induced strained-Si: extending the CMOS roadmap, IEEE Trans. Electron. Devices 53 (2006) 1010.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Defect elimination in solid-phase crystallised Si thin films by line-focus diode laser annealing Wei Li ⁎, Sergey Varlamov, Jialiang Huang School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 1 May 2014 Received in revised form 16 December 2014 Accepted 16 December 2014 Available online 19 December 2014 Keywords: Polycrystalline silicon Defect annealing Laser annealing Thin film Stress
a b s t r a c t A high density of intragrain defects in solid-phase crystallised Si thin films results in poor electronic properties and impedes their use for thin-film solar cell or thin-film transistor applications. This paper demonstrates that a high-power line-focus diode laser can eliminate intragrain defects (microtwins and dislocations) in polycrystalline Si films while maintaining the smooth defect-free surface. Improved electronic properties of ultra-thin polycrystalline Si thin films are thus achieved. To alleviate crack formation during diode laser annealing, a rapid-thermal pre-treatment at 800 °C for 60 s is introduced since it effectively relieves the tensile stress in the films and thus generates a more stable precursor material for subsequent laser annealing. The film thickness plays an important role in diode laser annealing. The films thinner than 100 nm show relatively smaller improvement due to the limited absorption of 808 nm laser radiation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Polycrystalline silicon (poly-Si) thin films on glass have attracted considerable research interest in the fields of thin-film solar cells and liquid crystal displays (LCDs) [1,2] Solid phase crystallisation (SPC) of amorphous Si is an established method to obtain poly-Si thin films that comprise crystal grains of up to several microns in size [3]. However, the SPC process mediated by formation of micro-twins results in a large residual density of these defects [4]. Microtwins are a source for other intragrain defects (such as dislocations) which all lead to inferior electronic properties of SPC poly-Si [4]. Consequently, these defects limit applications of SPC poly-Si thin films as a material for thin film solar cells or as pixel switch and driver elements for high-speed, highdefinition, and low-power smart displays [5,6]. Microtwins in SPC poly-Si films are unstable and their density can be effectively reduced at a relatively high temperature (N 750 °C) [4]. Above 900 °C, a partial removal of the defects at grain boundaries occurs due to the crystallographic relaxation [7]. Even higher temperatures above 1170 °C allow dislocation movement unconstrained by crystallographic glide planes, leading to dislocation annihilation [8]. Thus, a higher annealing temperature should result in a lower defect density and hence a better structural and electronic quality of SPC poly-Si thin film. However, the limited thermal stability of glass limits the maximum possible temperature by conventional furnace or rapid thermal annealing [9].
⁎ Corresponding author. E-mail address: [email protected] (W. Li).
http://dx.doi.org/10.1016/j.tsf.2014.12.033 0040-6090/© 2014 Elsevier B.V. All rights reserved.
Laser annealing can quickly heat up a poly-Si film to the temperature close to the Si melting point (1414 °C) while a glass substrate remains at a much lower temperature of about 650 °C (i.e. the sample stage temperature). During this process, intragrain defects density is greatly reduced and a better electronic quality is obtained [10]. Excimer-laser annealing (ELA) has been used for defects removal in poly-Si films from 1990s [10,11]. However, very fast cooling (at a rate of ~1010 K/s) after a short pulse (in the range of several ns) of an excimer-laser results in a hillock formation at grain boundaries [12] and a large number of vacancies in grains [13] leading to a higher bulk deep trap density. Diode laser annealing is a much slower process, of an order of tens of microseconds, with an accordingly lower cooling rate which is expected to result in a lower intragrain defect density [13]. A line-focus beam is particularly useful for any practical application as it allows fast large area processing [14]. In this paper, a high-power continuous-wavelength diode laser with a line-focus beam (12 mm × 181 μm) is used to anneal intragrain defects in SPC poly-Si films. It can effectively eliminate intragrain defects while keeping the smooth planar film surface without any damage (in contrast to ELA) and greatly shortening the processing time as compared with a conventional continuous-wavelength pointfocus laser. 2. Experimental details The sample structure (80 nm SiNx / 80–160 nm a-Si:H with phosphorous concentration of 1.7 × 1020 cm−3 / 150 nm SiOx) was deposited on a planar glass substrate (Schott Borofloat33) by plasma-enhanced chemical vapour deposition. SiNx acts as a barrier and anti-reflection layer and SiOx is a capping layer protecting Si films from atmospheric
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contamination during thermal annealing. The samples were processed by the following three steps: 1. SPC: all samples were crystallised in a tube furnace purged with nitrogen at 600 °C for 24 h. 2. Rapid thermal annealing (RTA): some of the crystallised poly-Si films were additionally annealed in a belt furnace heated by halogen lamps and purged with nitrogen at 800 °C for 1 min for stress relief. 3. DLA: following the RTA treatment, a part of the samples was annealed by scanning with the line-focus diode laser beam (LIMO 450-L12 × 0.3-DL808-EX937) for an exposure time of 136 ms at a range of energy densities. The samples were preheated to 650 °C. In this paper, micro-Raman spectroscopy (Renishaw inVia Raman Microscope with 442 nm laser and a spot size of 1.5 μm) was used to non-destructively characterise the stress and structure quality in the poly-Si thin films [15]. The peak position and full-width at halfmaximum (FWHM) of the optical-phonon mode were estimated by fitting the mixed Gaussian–Lorentzian function to the spectra. The peak shift and the peak FWHM were averaged over nine different positions taken over the area of 25 × 25 μm2 for each sample. The film surface roughness was analysed by Atomic Force Microscope (AFM, Veeco Digital Instruments Multimode SPM) in a PeakForce Tapping mode with a ScanAsyst-air tip (tip radius: 2 nm). The surface morphology of the film was also observed by an optical microscope (Olympus, BH-2) in a transmission mode. A focused ion beam (FIB) system (the xT Nova NanoLab 200) operating at 30 keV is used to prepare cross-sectional transmission electron microscopy (XTEM) specimens from the samples studied in this work. In this method, a focused beam of Ga+ ions is used to mill away small portions of the sample. Ultimately, a 100–200 nm thick, 22 μm long and electron-transparent membrane specimen is produced which is then removed from the sample surface ex-situ lifted out and attached to a Cu grid. Prior to FIB milling, the sample pieces were placed on Al stubs with C adhesive and subsequently sputter coated with ~ 20 nm of Au. The thin Au film was used to make the surface conductive for imaging and for surface protection during the initial stages of preparation. Subsequently, a layer of ~1 μm thick Pt was FIB-deposited over the area of interest to protect the area from further beam damage during the main stages of sample preparation. Thus, negligible FIB-induced damage to the specimens was experienced [16]. A Philips CM200 TEM operated at 200 kV was used to analyse the intragrain defects in the Si films. The sheet resistance, carrier concentration and electron mobility of each sample are determined by Hall Effect measurements. The electrode for Hall measurements was a Van der Pauw configuration. The thin film sample was laser isolated into the cross pattern and the characterisation area sets to 400 μm × 400 μm. Electrical contacts were made by using a GaAl alloy. The intensity of the magnetic field was 1 T, and the temperature was 300 K. The results were averaged over three different positions and the error bars shown in the figures are one standard deviation of the mean.
Fig. 1. Typical Raman spectra of the optical phonon mode obtained for 120 nm SPC poly-Si films prepared under different conditions, and c-Si reference: (a) the poly-Si film after SPC only; (b) the poly-Si film after SPC and RTA; (c) the poly-Si film after SPC, RTA and DLA; and (d) the c-Si reference.
3. Results and discussions
result from the volumetric contraction of a Si film when it transforms from the amorphous into crystalline phase, which is denser. This contraction is retarded by the substrate, which imposes a tensile stress on the film [18]. It is reported that a tensile stress can lead to film cracking but an excessive compressive stress can result in film buckling [19]. In either case, the residual stress can cause poor performance of a poly-Si thin film device or impede further processing on this stressed layer [19]. Therefore, it is of importance to release the residual stress in the films before the DLA treatment. The stress can be relieved by loose adhesion to the substrate [18] which is due to the breaking of bonds at the interface between a polySi film and the underlayer [20]. The stress in the poly-Si films is also relieved once the glass substrate softens at higher temperatures. Conventional furnace annealing can partially relax a residual stress in poly-Si thin films but it takes quite a long time (~2 h) at a high temperature (N1000 °C) [21] which is unsuitable for the glass. It was reported that RTA can effectively reduce the residual stress in poly-Si thin films [18,22]. Therefore, RTA is a better process for stress relaxation but RTA plateau temperatures should be limited such that to maintain a smooth and even film surface. Afterwards, before DLA processing, all SPC poly-Si films are treated by RTA with a plateau temperature of 800 °C (close to 820 °C of the softening point of Schott Borofloat33 glass) for 60 s. After RTA the film surface remains smooth and suitable for following DLA processing. As shown in Fig. 2(a), after RTA processing the Raman peak shifts to a higher wavenumber indicating that the residual stress in the SPC polySi films is relieved. The magnitude of the residual stress can be estimated from the stress-induced wave-number shift of the TO–LO peak compared to the Raman line of the stress-free single crystal. By assuming biaxial isotropic stress, the stress (δ) can be expressed as follows [23],
3.1. Stress relief by RTA
δ ¼ 250 Δω MPa
Typical Raman spectra of the optical-phonon mode for poly-Si films and single crystalline Si (c-Si) used as the standard are shown in Fig. 1. The Raman peaks of the SPC poly-Si thin films appear at wavenumbers lower than the standard c-Si peak position. The strain in the crystalline lattice distorts the atomic distances and causes a shift in the energies of the vibrational modes. Hence, the first order Raman line, which is located at ~520 cm−1 at the neutral stress level, shifts to lower or higher wavenumbers with tensile or compressive stress, respectively [17]. Raman spectra in Fig. 1 indicate the presence of a tensile stress in the SPC poly-Si films. During the SPC process, a biaxial tensile stress usually develops. Tensile stress evolution is considered to
where Δω (cm−1) is the phonon shift calculated from (ωs − ω0), ω0 is the Raman shift of stress-free single crystalline Si and ωs is the Raman shift of the film under measurement. The estimated residual stress in the differently processed poly-Si films is shown in the Fig. 2(b). The RTA plateau temperature of 800 °C results in partial softening of the glass substrate. During this process, most of the residual stress in the SPC poly-Si thin films is relaxed without glass deformation. Fig. 2(b) also reveals that as the film becomes thinner, the residual stress can be relieved to a smaller value by RTA. All samples of different thicknesses show no change in the stress level before and after the DLA treatment. It indicates that DLA does not introduce extra stress to the poly-Si films.
ð1Þ
44
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3.2. Effects of laser dose density on annealing of the SPC poly-Si thin films
Fig. 2. Comparison of Raman shifts (a) and the residual stress (b) in poly-Si films of different thicknesses after processing by SPC, RTA, and DLA.
It has to be noted that a doping concentration can also influence the LO–TO phonon peak shift in poly-Si thin films. It was reported that in P-doped poly-Si films the wavenumber of the LO–TO phonon line occurs at 517 cm− 1 for P concentrations of up to 4×1019 cm− 3 [24] which is similar to our samples. Moreover, with an increasing P concentration, the LO–TO phonon line shifts to a lower wavenumber and a low-energy tail extending from 400 to 500 cm−1 develops [25]. A high concentration of the free electrons in the conduction band interacting with the phonon deformation-potential results in a small shift in the position of the phonon to lower frequencies due to the Fano effect [26,27]. This interaction takes place only above a certain free-carrier concentration for n-type Si [26] which is estimated to be about 4×1019 cm−3 [24]. In our case, the free carrier concentration in the poly-Si film is higher than this threshold value. After the RTA treatment, the free carrier concentration in the poly-Si thin film increases due to the dopant activation. Consequently, the LO–TO peak of a Pdoped Si film should move to a lower wavenumber due to the Fano effect. In contrast, after RTA the LO–TO peak shifts in the opposite direction, to a higher wavenumber, indicating that the stress relief due to the RTA treatment overweighs the doping effect. Because of the above, the residual stress in RTA or DLA treated poly-Si film is actually lower than the value shown in Fig. 2. Highly stressed SPC poly-Si films can develop cracks during DLA processing. However, after stress relief, the films are less likely to crack. Additionally, the stress can directly trigger formation and propagation of dislocations which is detrimental to the device performance [15]. RTA at 800 °C for 1 min effectively relieves the residual stress in SPC poly-Si thin films and generates a stress-relaxed precursor for DLA processing.
The 120 nm thick SPC poly-Si thin films are annealed by the linefocus diode laser at a range of energy doses. As the laser dose density is lower than 724.3 J/cm2, DLA does not generate any damage to polySi films as shown in the optical transmission microscope (OTM) image of Fig. 3(a). However, the laser dose in excess of 724 J/cm2 can cause damage to the films as shown in Fig. 3(b). The carrier concentrations and electron mobilities in the 120 nm thick SPC poly-Si films versus the laser dose density are plotted in Fig. 4. As the laser dose density increases from 0 to 696 J/cm2, carrier concentration continuously increases from 7.3 × 1019 to 8.2 × 1019 cm−3. Intragrain defects which are capable of trapping free carriers limit their active concentration. A higher free carrier concentration indicates a lower intragrain defect density at a higher laser dose. At the laser dose density above 696 J/cm2, the carrier concentration reaches the maximum value of 8.2 × 1019 cm−3. It means that most of the dopants have already been activated. As the laser dose density increases to 686 J/cm2, electron mobility declines slightly but then rises at the laser dose densities above 686 J/cm2. The mobility is influenced by two factors: a free carrier concentration and a density of defects acting as traps. Reduction of the defect density can improve the mobility and also increase the free carrier concentration as reported above. However, at the same time, at high free carrier concentrations the mobility can decrease due to the carrier scattering by ionised dopant atoms [28]. Therefore, the electron mobility as a function of the laser dose density combines the effects of the two above factors. The electron mobility firstly declines due to the free carrier scattering effect [28]. Afterwards, the electron mobility increases quickly due to the annealing of intragrain defects. The sheet resistance of the 120 nm thick SPC poly-Si film as a function of the laser dose density is shown in Fig. 5. The sheet resistance decreases from 86.7 Ω/□ to 72.5 Ω/□ as the laser dose density increases to 724 J/cm2. The sheet resistance is determined by both the active doping concentration and the mobility. Therefore, the sheet resistance versus the laser dose density can be interpreted in terms of electronic activation of P atoms and a mobility improvement. Fig. 6(a) reveals the surface root-mean-squared roughness (RMS) measured by AFM of the 120 nm thick SPC poly-Si film as a function of the laser dose density. RMS roughness (Rq) of the film surface, defined as the standard deviation of the elevation, Z values, within the scanned area, is calculated from the equation below, Rq ¼
qX ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ðZ i −Z ave Þ =N
ð1Þ
where Zave is the average of the Z values within the scanned area, Zi is the Z value for a given point, and N is the number of points within the scanned area. When the laser dose density is lower than 667.7 J/cm2, the film roughness does not change much and it remains as smooth as for the sample without DLA processing. At the laser dose density in the range from 696 J/cm2 to 707.3 J/cm2, the surface roughness increases from 1.6 nm to 2.62 nm. The film is not as smooth as it was before DLA processing. However, when the laser dose is about 724 J/cm2, it is quite noticeable that the surface roughness decreases abruptly back to 1.6 nm. The smooth planar surface after DLA processing at 724 J/cm2 is also observed in the cross-sectional focused ion beam image in Fig. 6(b). Due to the very short solidification time, the excimer laser annealing leads to a hillock formation at grain boundaries which is shown in the Fig. 2(c) of reference [12]. The roughness due to the hillocks increases the density of the bulk deep traps and causes scattering of carriers [12]. However, the silicon surface after the DLA treatment remains as smooth as it was before the treatment which is evident from a crosssectional focused ion beam image in Fig. 6(b) and the TEM images in Fig. 9. One possible reason is that the cooling rate during diode laser
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Fig. 3. OTM images of DLA treated 120 nm SPC poly-Si film: (a) at laser dose density of 724.3 J/cm2 and (b) at laser dose density of N724.3 J/cm2.
annealing is much lower than that during ELA, so hillocks do not form at grain boundaries as shown by Hara et al. [29]. Although the surface roughness increases when the film starts melting, the film turns smooth again when full, or mostly full melting is reached. Furthermore, the SiOx capping layer also helps to reduce the film roughness [30]. Because a SiOx film is physically hard and the melting temperature of SiOx is much higher than that of Si, the SiOx cap behaves as a mould for a liquefied Si thin film [30]. Finally, the average roughness can also be reduced at a lower scanning speed, due to longer solidification [30]. 3.3. Effects of the film thickness on properties of SPC, RTA and DLA processed poly-Si films The FWHM of the transverse optical (TO) peak in a Raman spectrum is directly related to the structural quality of poly-Si [31]. The better the structural quality, the smaller the FWHM. The FWHM of the Raman TO peak of the SPC, RTA and DLA treated poly-Si thin films are shown in Fig. 7. After the SPC poly-Si thin film is processed by RTA its FWHM becomes smaller than that of the poly-Si film after SPC only. FWHM decreases further after DLA processing. FWHM of SPC poly-Si films is the largest which is due to the high intragrain defect density which will be shown by the TEM images in the following sections. RTA at 800 °C for 1 min results in the improvement of the structural quality while also avoiding excessive heating of the glass substrate. This RTA temperature and duration are suitable for stress relief but not sufficient to remove all microtwins and other intragrain defects from the interior of the grains [4]. DLA processing can heat up poly-Si films to temperature close to the Si melting point leading to a further reduction of the intragrain defect density. On the one hand, FWHM decreases further after DLA processing due to the structural quality improvement. On the other hand, DLA processing can also increase the free carrier concentration leading to larger FWHM due to the Fano effect [25–27,32]. Although DLA-treated polySi films combine effects from both defect elimination and dopant activation, the improvement of structural quality overweighs the doping effect. Fig. 7 reveals that the film thickness has a positive effect on the structural quality of SPC poly-Si films. As the DLA treated poly-Si film thickness increases, Raman FWHM decreases, indicating the structural quality improvement. As the light absorption by the poly-Si film is proportional to its thickness, the annealing temperature also depends on the film thickness. Although higher laser dose densities are applied to the thinner films, due to the limited absorption of 808 nm laser radiation, the thinner SPC poly-Si films achieve lower maximum temperatures during DLA hence a relatively smaller improvement in Raman FWHM as compared to the thicker films. XTEM images are taken to investigate the crystal quality of the polySi films. Fig. 8(a) and (b) show a highly defective grain in the SPC poly-Si film and its diffraction pattern respectively. Microtwins are one of the main defects in the SPC poly-Si thin films and they always occur on
(111) planes. Microtwins can be clearly seen under [110] direction in TEM [33] so in this study the TEM specimen was tilted so that the [011] crystallographic direction of the grain was parallel to the electron beam. From the diffraction pattern in Fig. 8(b), the periodic extra spots in the reciprocal lattice are due to the twins on (111) plain so the parallel stripes in Fig. 8(a) are twins growing along [112] direction. Fig. 9 shows a XTEM image of a DLA-processed poly-Si film with the thickness from 80 nm to 160 nm, in which the 80 nm thick poly-Si film contains more intragrain defects than the 120 and 160 nm thick SPC poly-Si films. By comparison of Fig. 8(a) with Fig. 9(c), the poly-Si film after DLA contains much fewer defects than the poly-Si film after SPC only. During the DLA process, the poly-Si films were heated up to the temperature close to the Si melting point. Therefore, most of the microtwins were absorbed by larger twins [4] and dislocations are annihilated through movement unconstrained by crystallographic glide planes [8]. Moreover, the film is partially melted so the defects in the melted part are annihilated. Thus, the interior of the grains appears essentially free of defects after DLA processing. In Fig. 9, the clear interface between the glass and the SiNx buffer layer is visible indicating that DLA does not cause any damage. The poly-Si thin films are seen to have the smooth planar surface after the DLA treatment which is consistent with the AFM results. If the laser dose increases to the extent that the Si film is fully melted, the Si films thinner than about 400 nm tend to disintegrate because of dewetting from the substrate [34,35]. In order to prevent dewetting, the film should avoid complete melting but leave a thin unmelted solid part. If the temperature in the thin unmelted layer is sufficiently high, the defect will be mostly eliminated. This unmelted thin layer will then act as a defect-free seed for epitaxial recrystallisation of the melted part of the film. Grains free from defects are thus formed. A similar result was reported by using excimer laser annealing of SPC poly-Si thin films [10]. Additionally, the grain size after DLA is determined by the grain size of the unmelted part so the grain size is not appreciably affected from comparison between Fig. 8(a) and Fig. 9(c). It is possible that some of the twins are still left in the unmelted layer and they can propagate into crystallising Si during the solidification process. Nevertheless, as shown in Fig. 9, they completely penetrate the whole crystal without introducing other electric active defects so they should not be detrimental to the electronic quality of annealed poly-Si thin films [36]. However, the diode laser radiation at 808 nm wavelength is not well absorbed by the thinner poly-Si films leading to lower achieved maximum temperatures. In this case, some intragrain defects are not well removed as shown in Fig. 9(c). Nevertheless, the 80 nm DLA treated polySi film still contains much fewer intragrain defects than the SPC poly-Si without DLA processing. To summarise, due to the transient high temperature DLA treatment, most of the intragrain defects such as microtwins and dislocations are removed. Therefore, the structural quality of SPC poly-Si thin films is effectively improved and better electronic characteristics are obtained.
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Fig. 4. Carrier concentration and electron mobility in the 120 nm SPC poly-Si film as a function of the laser dose density. (“Zero” energy density means the film without DLA treatment).
The carrier concentration, electron mobility and resistivity of the Pdoped poly-Si films versus the different annealing processes are presented in Fig. 10. The poly-Si film after SPC has the lowest carrier concentration while the DLA treated poly-Si film has the highest carrier concentration. As discussed above, the higher free carrier concentration indicates fewer intragrain defects after thermal processing which is consistent with the findings from TEM analysis. The peak RTA temperature is limited by stability of the glass substrate which impedes more complete elimination of intragrain defects and obtaining a better film quality. DLA can increase the annealing temperature up to near the Si melting point without overheating the glass substrate. Therefore, due to more efficient defect elimination by DLA, a higher degree of dopant activation is achieved. In Fig. 10(b), most of the poly-Si films after SPC only have the lowest electron mobilities while the DLA treated poly-Si films have the highest electron mobility. According to Haji's work [4], the low mobility in SPC poly-Si films is due to their defective nature. However, the residual stress can also influence mobility. Thompson et al. reported that strain could enhance the electron and hole mobility by strain-induced energy-level splitting, inversion-layer quantum confinement energylevel shifts, average mass change due to repopulation and band warping, two-dimensional density of states, and interband scattering changes due to band splitting [37]. Relieving the stress by RTA can thus decrease the electron mobility. On the other hand, the intragrain defect elimination by RTA will improve the Hall mobility. As a result of two opposite effects, the RTA treated poly-Si films do not show a
Fig. 5. Measured sheet resistance of the 120 nm SPC Si thin film as a function of the laser dose density.
Fig. 6. (a) AFM surface roughness of 120 nm SPC Si thin films irradiated at different laser dose densities, (b) cross-sectional FIB image of a DLA treated 120 nm SPC Si thin film with laser dose of 724 J/cm2. (The Pt layer was coated for protection from ion beam damage).
significant improvement in the electron mobility compared with the poly-Si films after SPC only. The poly-Si films after DLA processing have higher Hall mobility than those after RTA. It can be explained by further intragrain defect elimination at a higher temperature provided by DLA processing. Although reducing intragrain defect density improves the electron mobility, the free carrier concentration caused by better dopant activation can also result in lowering the electron mobility due to carrier scattering by the ionised dopant atoms [28]. Consequently, the two opposing effects can balance each other leading to an insignificant electron mobility improvement after DLA processing.
Fig. 7. Effects of SPC, RTA, and DLA on the FWHM of Raman peak for 80 nm, 120 nm and 160 nm thick SPC poly-Si films.
W. Li et al. / Thin Solid Films 576 (2015) 42–49
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Fig. 8. (a) Dark field image and (b) diffraction pattern of a 80 nm poly-Si film after SPC only. (The Pt layer was coated against the ion beam damage during TEM sample preparation).
Fig. 10(c) shows a decrease in the resistivity of poly-Si films after RTA and particularly after DLA processing. Lower resistivity can be interpreted in terms of activation of P atoms and electron mobility improvement. Because the DLA treated poly-Si films have higher free carrier concentration and higher electron mobility, their resistivity is much lower than those after SPC or RTA. The film thickness effects on the carrier concentration, the electron mobility and the resistivity are also shown in Fig. 10. The thicker polySi films have higher carrier concentrations, higher Hall mobility and lower resistivity. It is possible that thinner films are more likely to be influenced by defects at the interface between SiNx and the poly-Si film. Moreover, due to the limited absorption of the 808 nm laser radiation, lower maximum temperatures are reached causing incomplete defect elimination in the thinner poly-Si films after DLA as shown in Fig. 9. Therefore, as the film thickness increases from 80 nm to 160 nm, better electronic properties are obtained for the DLA treated poly-Si films. Such low-defect laser annealed poly-Si films can be used as high quality seed layers for epitaxial thickening for solar cell and thin-film transistor applications.
Fig. 9. Dark-field cross-section TEM micrograph of the DLA SPC poly-Si films with different film thicknesses, (a) 160 nm, (b) 120 nm and (c) 80 nm. (The diffraction pattern of each grain is shown in the inset and SiOx capping layer was removed by HF dip so it is not seen in these figures).
4. Conclusion SPC poly-Si contains a high density of intragrain defects which limit its electronic properties. In this paper, effects of DLA processing on
electrical and structural properties of ultra-thin SPC films are presented. The films obtained by SPC are always under tensile stress which leads to cracking during DLA. By RTA at 800 °C for 60 s, the residual stress in SPC
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not accepted by the Australian Government. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Electron Microscope Unit, The University of New South Wales (UNSW). Wei Li would like to appreciate the Ph.D. scholarship from UNSW and China Scholarship Council (CSC). References
Fig. 10. Effects of SPC, RTA, and DLA on (a) the carrier concentration, (b) electron mobility and (c) resistivity of the 80 nm, 120 nm and 160 nm thick P-doped SPC poly-Si films.
poly-Si thin films can be effectively relieved thus providing a stable precursor material for laser annealing. TEM, Raman, and Hall measurements showed that DLA significantly improves the structural and electronic properties of poly-Si, by reducing the intragrain defect density. AFM proves that a DLA-treated SPC film retains a smooth planar surface as before DLA processing. Film thickness plays an important role in diode laser annealing. Due to the limited absorption of 808 nm laser radiation, the quality of thinner SPC poly-Si films improves less than that for the thicker films. Acknowledgement This programme has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) (LP0883548) and Australian Research Council (ARC) (2A014A). Responsibility for the views, information or advice expressed herein is
[1] S. Varlamov, J. Dore, R. Evans, D. Ong, B. Eggleston, O. Kunz, U. Schubert, T. Young, J. Huang, T. Soderstrom, K. Omaki, K. Kim, A. Teal, M. Jung, J. Yun, Z.M. Pakhuruddin, R. Egan, M.A. Green, Polycrystalline silicon on glass thin-film solar cells: a transition from solid-phase to liquid-phase crystallised silicon, Sol. Energy Mater. Sol. Cells 119 (2013) 246. [2] S. Inoue, S. Utsunomiya, T. Saeki, T. Shimoda, Surface-free technology by laser annealing (SUFTLA) and its application to poly-Si TFT-LCDs on plastic film with integrated drivers, IEEE Trans. Electron. Devices 49 (2002) 1353. [3] T.L.Y.M. Keevers, U. Schubert, R. Evans, R.J. Egan, M.A. Green, 10% efficient CSG minimodules, Proceedings of the 22nd EUPVSEC, Milan, Italy, 2007, p. 1783. [4] L. Haji, P. Joubert, J. Stoemenos, N.A. Economou, Mode of growth and microstructure of polycrystalline silicon obtained by solid-phase crystallization of an amorphous silicon film, J. Appl. Phys. 75 (1994) 3944. [5] Y. Masayuki, K. Shin-Ichiro, S. Tadashi, K. Koji, High-performance poly-Si thin film transistors with highly biaxially oriented poly-Si thin films using double line beam continuous-wave laser lateral crystallization, Jpn. J. Appl. Phys. 53 (2014) 03CC02. [6] W. Li, S. Varlamov, M. Jung, J. Huang, Vapour-phase and solid-phase epitaxy of silicon on solid-phase crystallised seed layers for solar cells application, Int. J. Photoenergy 2014 (2014) 234602. [7] J.P. Colinge, E. Demoulin, F. Delannay, M. Lobet, J.M. Temerson, Grain size and resistivity of LPCVD polycrystalline silicon films, J. Electrochem. Soc. 128 (1981) 2009. [8] K. Hartman, M. Bertoni, J. Serdy, T. Buonassisi, Dislocation density reduction in multicrystalline silicon solar cell material by high temperature annealing, Appl. Phys. Lett. 93 (2008) 122108. [9] W. Li, S. Varlamov, J. Dore, M. Green, Defect annealing in ultra-thin polycrystalline silicon films on glass: rapid thermal versus laser processing, Mater. Lett. 107 (2013) 1. [10] R. Carluccio, J. Stoemenos, G. Fortunato, D.B. Meakin, M. Bianconi, Microstructure of polycrystalline silicon films obtained by combined furnace and laser annealing, Appl. Phys. Lett. 66 (1995) 1394. [11] S. Friligkos, V. Papaioannou, J. Stoemenos, R. Carluccio, S. Cina, G. Fortunato, Structural characterization of α-Si films crystallized by combined furnace and laser annealing, J. Cryst. Growth 182 (1997) 341. [12] M. Miyasaka, J. Stoemenos, Excimer laser annealing of amorphous and solid-phasecrystallized silicon films, J. Appl. Phys. 86 (1999) 5556. [13] K. Kitahara, Y. Ohashi, Y. Katoh, A. Hara, N. Sasaki, Submicron-scale characterization of poly-Si thin films crystallized by excimer laser and continuous-wave laser, J. Appl. Phys. 95 (2004) 7850. [14] G. Andrä, J. Bergmann, F. Falk, Laser crystallized multicrystalline silicon thin films on glass, Thin Solid Films 487 (2005) 77. [15] I. DeWolf, Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits, Semicond. Sci. Technol. 11 (1996) 139. [16] W. Li, J. Chen, C. Yan, F. Liu, X. Hao, Transmission electron microscopy analysis for the process of crystallization of Cu2ZnSnS4 film from sputtered Zn/CuSn precursor, Nanotechnology 25 (2014) 195701. [17] F. Cerdeira, C.J. Buchenauer, F.H. Pollak, M. Cardona, Stress-induced shifts of first-order Raman frequencies of diamond- and zinc-blende-type semiconductors, Phys. Rev. B 5 (1972) 580. [18] A.K. Kalkan, S.J. Fonash, Stress and dopant activation in solid phase crystalized Si films, in: B.R. Chalamala, R.H. Friend, T.N. Jackson, F.R. Libsch (Eds.), Flat-Panel Displays and Sensors: Principles, Materials and Processes, Materials Research Society, Warrendale, 2000, p. 225. [19] X. Zhang, T.-Y. Zhang, M. Wong, Y. Zohar, Residual-stress relaxation in polysilicon thin films by high-temperature rapid thermal annealing, Sensors Actuators A Phys. 64 (1998) 109. [20] K. Kitahara, Y. Ohashi, K. Yamamoto, N. Sasaki, Electrochemical and Raman-scattering characterizations of defects in polycrystalline silicon thin films formed by excimerlaser annealing, solid-phase crystallization, and continuous-wave laser lateral crystallization, Jpn. J. Appl. Phys. 48 (2009) 021205. [21] D. Maierschneider, J. Maibach, E. Obermeier, D. Schneider, Variations in Young's modulus and intrinsic stress of LPCVD-polysilicon due to high-temperature annealing, J. Micromech. Microeng. 5 (1995) 121. [22] Z. Xin, Z. Tong-Yi, W. Man, Z. Yitshak, Rapid thermal annealing of polysilicon thin films, J. Microelectromech. Syst. 7 (1998) 356. [23] V. Paillard, P. Puech, M.A. Laguna, P. Temple-Boyer, B. Caussat, J.P. Couderc, B.D. Mauduit, Resonant Raman scattering in polycrystalline silicon thin films, Appl. Phys. Lett. 73 (1998) 1718. [24] N.H. Nickel, P. Lengsfeld, I. Sieber, Raman spectroscopy of heavily doped polycrystalline silicon thin films, Phys. Rev. B 61 (2000) 15558. [25] R. Saleh, N.H. Nickel, Raman spectroscopy of doped and compensated laser crystallized polycrystalline silicon thin films, Surf. Coat. Technol. 198 (2005) 143. [26] M. Chandrasekhar, J.B. Renucci, M. Cardona, Effects of interband excitations on Raman phonons in heavily doped n-Si, Phys. Rev. B 17 (1978) 1623. [27] U. Fano, Effects of configuration interaction on intensities and phase shifts, Phys. Rev. 124 (1961) 1866.
W. Li et al. / Thin Solid Films 576 (2015) 42–49 [28] S.M. Sze, M.-K. Lee, Semiconductor Devices: Physics and Technology, 3rd ed. John Wiley & Sons, Inc., 2012.. [29] A. Hara, M. Takei, F. Takeuchi, K. Suga, K. Yoshino, M. Chida, T. Kakehi, Y. Ebiko, Y. Sano, N. Sasaki, High performance low temperature polycrystalline silicon thin film transistors on non-alkaline glass produced using diode pumped solid state continuous wave laser lateral crystallization, Jpn. J. Appl. Phys. 1 Regul. Pap. Short Notes Rev. Pap. 43 (2004) 1269. [30] S. Fujii, S.I. Kuroki, M. Numata, K. Kotani, T. Ito, Roughness reduction in polycrystalline silicon thin films formed by continuous-wave laser lateral crystallization with cap SiO2 thin films, Jpn. J. Appl. Phys. 48 (2009) 04C129. [31] D. Song, D. Inns, A. Straub, M.L. Terry, P. Campbell, A.G. Aberle, Solid phase crystallized polycrystalline thin-films on glass from evaporated silicon for photovoltaic applications, Thin Solid Films 513 (2006) 356. [32] M. Chandrasekhar, M. Cardona, E.O. Kane, Intraband Raman scattering by free carriers in heavily doped n-Si, Phys. Rev. B 16 (1977) 3579.
49
[33] X.-Z. Bo, N. Yao, S.R. Shieh, T.S. Duffy, J.C. Sturm, Large-grain polycrystalline silicon films with low intragranular defect density by low-temperature solid-phase crystallization without underlying oxide, J. Appl. Phys. 91 (2002) 2910. [34] G. Andrä, C. Lehmann, J. Plentz, A. Gawlik, E. Ose, F. Falk, Varying the layer structure in multicrystalline LLC-silicon thin-film solar cells, 33rd IEEE Photovoltaic Specialists Conference, IEEE, San Diego, California, vol. 1–4, 2008, p. 1830. [35] J. Dore, D. Ong, S. Varlamov, R. Egan, M.A. Green, Progress in laser-crystallized thin-film polycrystalline silicon solar cells: intermediate layers, light trapping, and metallization, IEEE J. Photovolt. 4 (2014) 33. [36] J. Chen, D. Yang, Z. Xi, T. Sekiguchi, Recombination activity of Sigma 3 boundaries in boron-doped multicrystalline silicon: influence of iron contamination, J. Appl. Phys. 97 (2005) 033701. [37] S.E. Thompson, S. Guangyu, C. Youn Sung, T. Nishida, Uniaxial-process-induced strained-Si: extending the CMOS roadmap, IEEE Trans. Electron. Devices 53 (2006) 1010.