Annealing effects on SiO
xN
ythin fi lms: Optical and morphological properties
M. Perani
a,⁎ , N. Brinkmann
b, M.A. Fazio
a, A. Hammud
b, B. Terheiden
b, D. Cavalcoli
aaDepartment of Physics and Astronomy, University of Bologna, v.le B. Pichat 6/2, 40127 Bologna, Italy
bDepartment of Physics, University of Konstanz, Universitätstraβe 10, 78464 Konstanz, Germany
a b s t r a c t
The annealing effect on the properties of silicon oxynitride (SiOxNy) thinfilms has been investigated. The present contribution aims to study the structural and optical properties of SiOxNythinfilms deposited by plasma en- hanced chemical vapor deposition in view of their application in thefield of photovoltaics. Evolution of the sur- face morphology and increase of the optical band gap with the thermal treatment have been determined and discussed in view of the application of thefilm as an emitter layer in heterojunction solar cells.
Keywords:
Silicon oxy-nitride Atomic force microscopy Tauc gap
Optical properties
Height–height correlation function Image segmentation
1. Introduction
During the last years, silicon oxynitride (SiOxNy) has been deep- ly investigated, due to its possible application in differentfields, as well as its low-cost fabrication technologies. In fact, its relatively high refractive index ranging from 1.45 (SiO2, where no N is pres- ent) to 4.08 (a-Si:H, total absence of both N and O)[1,2], together with low optical losses and large uniformity, make SiOxNya very good candidate in photovoltaic devices, as emitter layer in Si based heterojunction solar cells and in optoelectronic devices, both as core and buffer layer in planar optical waveguides[1,3–7].
In addition, low density of surface states, high dielectric permittiv- ity and a band gap that can range up to 9 eV varying the O/N ratio, promoted SiOxNyas a better oxide layer with respect to SiO2even in ultra thin-films transistors and nonvolatile memory devices[8–14].
Silicon-based oxynitride has received great attention also in LED appli- cation, thanks to its encouraging luminescence properties and in partic- ular its ability in light emission in a very wide range[15].
The present manuscript aims to present a detailed characterization of silicon oxynitride thinfilms in view of their photovoltaic (PV) appli- cations. Indeed, this material has been used as window layer and antire- flective coating in thinfilm solar cells[16–18]and both SiOxand SiOxNy
layers might substitute nc-Si and a-Si emitter layers in silicon heterojunction solar cells, as they suffer less from parasitic absorption
due to a larger band gap, while keeping high conductivity[19]. The in- vestigation of the optical properties of such materials is therefore of great interest.
Due to coexistence of different phases and compositions upon annealing, the structure and the formation of SiOxNytogether with the role of crystalline fraction and oxygen content in changing optical and electrical properties are still unclear. The dependence of conductivity and optical band gap from oxygen content has been investigated only recently in amorphous and annealed SiOxNy[2,16,20–22].
This manuscript focuses on the study of structural, morphological and optical properties of different samples of SiOxNy grown by Plasma Enhanced Chemical Vapor Deposition (PECVD) and annealed to increase the crystalline fraction for application in silicon heterojunction solar cells.
It has been found that the concentration of N2O as precursor gas and differ- ent annealing times affect both morphological and optical properties. The SiOxNyfilms containing nanocrystals have been obtained by thermal annealing process, which is known to be a process not fully compatible with PV technology; however high crystallinity can be also obtained by a proper choice of the deposition parameters in the PECVD growth chamber, becoming in this way fully compatible with solar cell technology.
2. Experimental details
SiOxNythinfilms have been deposited by PECVD on FZ-Si and glass substrates. The system used is a PlasmaLab 100 from Oxford Instrument in a parallel plate configuration and the radio frequency is set at 13.56 MHz. The precursor gases used are silane (SiH4), hydrogen (H2),
⁎ Corresponding author.
E-mail address:martina.perani2@unibo.it(M. Perani).
Konstanzer Online-Publikations-System (KOPS)
URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2-1pbxtu3vhdcy31 https://dx.doi.org/10.1016/j.tsf.2016.03.067
nitrous oxide (N2O) and diborane (B2H6). The latter is diluted in hydro- gen (0.5%) and in the following discussion the N2O gasflow is referred to its dilution in silane:
RN2O¼ ½N2O N2O
½ þ½SiH4 ð1Þ
where the square brackets indicate the gas concentrations. All the layers have been deposited at 300 °C and then subsequently annealed at 800 °C in nitrogen atmosphere to increase the crystallized fraction.
Hydrogen is present in the as-deposited layers, while it desorbs during the annealing[23].Fig. 1shows the Fourier Transform Infra-Red (FTIR) spectra of a sample deposited at RN2O= 16% in the region of Si\\Hx
bond (1900–2200 cm−1), showing that the signal related to this bond is not present after annealing. Moreover, no peak related to N\\H and O\\H bonds is observed in the region 3300–3900 cm−1(not shown).
Please note that all the layers have been named SiOxNyfor sake of clarity.
The silicon crystallized fractionχis defined as:
χ¼ IC
ICþIA ð2Þ
whereICis the integrated area of the Raman peak at 520 cm−1related to the crystallized Si content of the layer, whileIAis the integrated area of the Raman peak at 480 cm−1 related to the amorphous Si phase [24–26]. Eq.(2)is used to compare the Si crystallized fraction of differ- ent samples. The excitation wavelength of the laser used is 488 nm.
The surface morphology of the thinfilms has been measured using Atomic Force Microscopy (AFM) with a Park NX10 system in non- contact mode. The tips used feature a super-sharp apex, with a curva- ture radius less than 5 nm (Nanosensors SSS-NCHR). The map size used for the analysis is 1 × 1μm2with a resolution of 512 × 512 px.
The investigation of surface morphology is of great interest, as it affects the growth of subsequent layers and the performances of thefinal de- vice, as evidenced by studies on thinfilm silicon solar cells[27].
The height–height correlation function (HHCF) has been evaluated from the AFM maps andfitted in the hypothesis of self-affine surfaces using the following expression:
HHCF rð Þ ¼2R2HHCF 1exp r ξ
2α
" #
( )
ð3Þ
withrthe lateral distance between two points,RHHCFthe surface rough- ness,αthe Hurst exponent andξthe lateral correlation length.ξrepre- sents the length scale beyond which the heights of two points of the surface are no more correlated and the Hurst exponent is a measure of the local roughness of the structures present on the surface[28,29].
The Equivalent Disk Radius (EDR) of the structures present on the layer surface has been obtained from AFM maps superimposing a seg- mentation mask obtained using the software Gwyddion[30]. The fea- tures present on the sample surfaces are grain-like and will be called grains in the following discussion. Afiltering of the obtained mask is necessary to remove incorrectly identified grains[31], so that grains with area less than 4 px and minimum height value less than the mini- mum height + 1 nm are removed from the analysis.
The optical band gap ETauc has been evaluated using reflection (R(hν)) and transmission (T(hν)) spectra measured in the range 300– 1500 nm obtained using a Cary spectrometer from Varian. The absorp- tion coefficientα(hν) has been evaluated by:
αð Þ ¼hν 1
dln 1R hð Þν T hð Þν
ð4Þ
wheredis the thickness of thefilm obtained from ellipsometry mea- surements. Eq.(4)has been used to reduce the contribution of interfer- ence fringes, which are observed in the measured spectra[32,33]. The value ofETauchas been deduced in the hypothesis of a disordered mate- rial from a linearfit of the Tauc plot obtained following[34]:
a hð Þ∙ν hν
ð Þ1=2∝ðhνETaucÞ: ð5Þ
The optical analysis has been performed on the layers deposited on glass.
Fig. 2.SEM image (a) and Raman spectrum (b) of a SiOxNylayer deposited with RN2O= 16%, 3 h annealed.
Fig. 1.FTIR spectra of the sample with RN2O= 16% as-deposited (squares) and annealed for 3 h in the region 1900–220 cm−1.
3. Results and discussion
Fig. 2a shows a cross section view of a SiOxNylayer deposited on FZ‐ Si substrate and annealed for 3 h. The image has been acquired by Scan- ning Electron Microscopy (SEM) and the sample is 224 nm thick. Re- gions with different contrasts are visible and they can be ascribed to the nanocrystals (NCs) presence in the layers after the annealing pro- cess. High resolution transmission electron microscopy measurements (reported in reference[22]) reveal that, after the annealing, the NCs are randomly distributed with no preferential orientation.
The thermal treatment deeply affects the structure of the SiOxNy
films, with an increase of the Si crystallized fractionχfrom 67% to 88%, with no additional increment after 3 h of annealing (seeTable 1).
Fig. 2b shows the Raman spectrum of a sample with RN2O= 16%
annealed for 3 h. The two peak areas, corresponding to the crystallized and amorphous fractions respectively, are high-lighted.
The variations induced by the annealing are reflected in an evolution of the surface morphology. Non-contact AFM maps have been analyzed following Eq.(3)and the result for the sample with RN2O= 9%, as- deposited, is reported inFig. 3a, while a typical AFM map of this sample is shown inFig. 3b. The HHCF averaged from 5 maps (dots) as well as thefit performed following Eq.(3)(solid line) are reported. It is worth noting that the HHCF function increases for smallrvalues with a slope equal to 2α, and then it shows constant values equal to the surface roughnessRHHCFfor largervalues. The transition between these two be- haviors is placed at values ofrclose to the lateral correlation lengthξ. No fluctuation of the HHCF is visible forrNξafter averaging several maps, revealing that the surface shows a self-affine behavior[27]. The values of the parametersξ,α, andRHHCFare obtained from thefit of the HHCF and are discussed in the following paragraphs.
The evolution of the surface morphology with the thermal treatment is reflected by an increase of the lateral correlation length from 18.5 to 26.6 nm (seeTable 1). This observation reveals that larger structures form on the surface of the SiOxNylayers with increasing annealing time. The surface roughnessRHHCFincreases with annealing time as well, ranging from 1.204 to 1.476 nm (seeTable 1).
Fig. 3b shows the AFM morphology of the sample with RN2O= 9%, as-deposited. The mask obtained after the image segmentation process has been inverted to facilitate the reading and has been superimposed to the AFM map. The average EDR values have been obtained for sam- ples with RN2O= 9% and different annealing times starting from the segmentation masks; the results are reported inTable 1. An increment of EDR is observed for increasing annealing times, revealing with an in- dependent analysis technique that larger structures are forming on the layers surface after the thermal treatment. The Equivalent Disk Diame- ter (EDD), twice the EDR, is of the same order of the lateral correlation length. The fact that EDD is always smaller thanξcan be accounted for considering thatξis sensitive to clustering of grains, while EDD re- solves individual grains[31].
The optical Tauc gap ETauchas been obtained from reflection and transmission spectra using Eqs.(4)and(5).Fig. 4a reports the measured R(hν) andT(hν), as well as the values ofT(hν)/(1-R(hν)) as a function of the wavelength of the incident light. The measurements reported are relative to the sample with RN2O= 17% as-deposited state on glass. It is worth noting that the function T(hν)/(1-R(hν)) shows a reduced os- cillation due to thinfilm interference effects with respect toT(hν) [33]. This effect is more pronounced for as-deposited samples, which have lower crystalline fraction values. In the case of annealed samples (not reported), the functionT(hν)/(1-R(hν)) still shows some oscilla- tion and this observation could be ascribed to the reduced crystalline disorder of these layers (seeTable 1).
Fig. 4b shows the Tauc plot of the sample with RN2O= 17% for differ- ent annealing times. The Tauc gap has been obtained from a linearfit in the region of high absorption following Eq.(5)for all the analyzed sam- ples.Fig. 4b depicts (αhν)1/2as a function ofhνand the lines represent thefits (the solid, dashed and dot lines are relative to as-deposited, 3 h and 6 h annealed samples, respectively). According to Eq.(5), the inter- section of the linearfit with thex-axis gives the value of the Tauc gap.
Table 2reports the Tauc gap measured for different RN2Oand anneal- ing times. ETaucincreases after the annealing for all the RN2O. This fact is strongly related to the structural changes that the thermal treatment in- duces in the layers, such as the increased crystalline fraction and NCs formation, which could lead to quantum confinement effects[35]. In ad- dition an increasing trend of ETaucwith RN2Ois observed in the as- deposited state, due to an increased oxygen content of the layers[2], while no defined trend of ETaucis visible after the annealing.
The thermal treatment deeply affects the properties of the SiOxNy
thinfilms. It has already been shown that annealing promotes oxygen relocation within the layers, resulting in a non-homogenous oxygen dis- tribution[22]. Nanocrystal formation upon annealing also contributes to the variations observed in the structure and optical properties of the layers.
Fig. 3.a) HHCF of the sample with RN2O= 9% (circles) andfit of the experimental data (solid line). b) AFM map and segmentation mask of the sample with RN2O= 9%, as-deposited.
Table 1
Crystalline fractionχ, lateral correlation lengthξ, roughnessRand equivalent disk radius EDR for samples with RN2O= 9% for different annealing times. The errors reported are the statistical ones.
Annealing time (h) χ% ξ(nm) RHHCF(nm) EDR (nm)
0 67 ± 2 18.5 ± 0.2 1.204 ± 0.001 8.21 ± 0.03
3 88 ± 2 23.8 ± 0.2 1.369 ± 0.001 8.56 ± 0.03
6 86 ± 2 26.6 ± 0.4 1.476 ± 0.002 9.66 ± 0.04
It is well known that thermal annealing affects hydrogenated nano- crystalline silicon, promoting a recrystallization of the layers that strongly depends on PECVD deposition conditions[36]. We have shown that a significant effect of the thermal annealing on the structure and the optoelectronic properties of the layers is present in SiOxNy, which is a multiphase and complex material with N and O present in the as-deposited layers together with Si and H.
The surface morphology exhibits grain like structures and evolves during the thermal treatment: the lateral correlation length increases upon annealing, ranging from 18.5 to 26.6 nm. An increase of the di- mension of the structures present on the surface is revealed by the in- crement of equivalent disk radius as well.
The Tauc gap determination of the SiOxNylayers has revealed the ef- fect of the thermal treatment as well as of the oxygen incorporation on the optical properties of the layers. The reduced optical absorption of the SiOxNyfilms after the annealing is a suitable property for the appli- cation in the photovoltaicfield, as the absorption within the emitter layer is undesirable as the carriers recombine before the extraction[37].
4. Conclusions
In the present contribution, we studied the effects of thermal anneal- ing on the structure and optical properties of SiOxNythinfilms. The ther- mal treatment is effective in the enhancement of the crystalline content of the thinfilms, reaching a high value of 88% for RN2O= 9.09%. The morphology of the layers is affected by annealing: an increase of the lat- eral correlation length and of the equivalent disk radius is observed with respect to the as‐deposited samples. Both HHCF and grain size analyses indicate the presence of larger structures on the sample surfaces.
The band gap of the SiOxNylayers increases up to 2.5 eV after 3 h an- nealing for all RN2Ovalues and this observation can be due to both quan- tum confinement effects and the formation of oxygen-rich areas, with a shift of the gap towards SiO2values. After additional 3 h annealing the energy band gap slightly decreases (as reported in ref.[22]), possibly due to the formation of larger nanocrystals.
The knowledge of the properties of this complex and multiphase material is essential for its practical applications. High values of the
optical band gap, in particular, are very promising in view of the em- ployment of SiOxNyfilms as emitter layers in Si based heterojunction solar cells for obtaining a reduced parasitic light absorption. Moreover, the surface morphology affects the growth of a layered structure and the quantitative AFM studies reported in this paper evidenced an an- nealing induced morphology evolution.
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