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Structure and optical properties of amorphous silicon oxide thin films with different porosities
Journal of Non-Crystalline Solids 320 (2003) 64–75 www.elsevier.com/locate/jnoncrysol
Structure and optical properties of amorphous silicon oxide thin films with different porosities H. Rinnert *, M. Vergnat Laboratoire de Physique des Mat eriaux (U.M.R. au C.N.R.S. No. 7556), Universit e Henri Poincar e Nancy 1, B.P. 239, 54506 Vandœuvre-l es-Nancy cedex, France Received 9 May 2002; received in revised form 20 November 2002
Abstract Amorphous silicon oxide thin films were prepared by evaporation of a silicon oxide powder. Samples were prepared under ultrahigh vacuum, under a flow of hydrogen ions or under a molecular hydrogen atmosphere. Two others sets of samples were prepared using deuterium instead of hydrogen. These five groups of samples were then annealed to different temperatures up to 950 °C and were exposed to the ambient air. The samples present different densities and microstructures. The sample prepared under ultrahigh vacuum is dense, hydrogen free and OH-bond free. Samples prepared under atomic hydrogen and deuterium flows contain Si–H and Si–D bonds, respectively, and are OH-bond free. The sample prepared under a molecular hydrogen atmosphere is very similar to that prepared under a molecular deuterium atmosphere. Both samples are porous and contain Si–H bonds and OH-groups coming from the exposure to the air. All the samples show visible photoluminescence attributed to isolated silicon clusters. The photoluminescence intensity increases with thermal annealing post-treatments up to an optimal annealing temperature. This maximum value is equal to 650 °C for the unhydrogenated sample and the sample prepared under an atomic hydrogen flow and to 800 °C for the sample prepared under a molecular hydrogen atmosphere. This difference is correlated to the different microstructures of the samples. Moreover the strongest photoluminescence intensity is obtained for the porous sample. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk; 78.66.Jg; 81.15.Jj
1. Introduction The silicon oxide alloys are among the most studied materials in the form of thin films, especially because of their interesting dielectric properties. A new field of interest has emerged since the
*
Corresponding author. Tel.: +33-3 83 68 48 19; fax: +33-3 83 68 48 01. E-mail address: [email protected] (H. Rinnert).
discovery of the photoluminescence (PL) in silicon nanostructures [1]. The potential use of silicon nanometer-size clusters embedded in a dielectric matrix in novel quantum devices is very attractive due to the low cost of manufacturing. The usual way to obtain more efficient silicon-based optoelectronic devices is the preparation of materials with lower dimensions in which the carrier motion is reduced involving an increase of the gap of the semiconductor and of the radiative recombination yield. A great number of techniques was used to
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00078-4
H. Rinnert, M. Vergnat / Journal of Non-Crystalline Solids 320 (2003) 64–75
obtain such structures and photoluminescence was observed in Siþ implanted SiO2 films [2,3], in Sirich SiO2 samples grown by chemical vapour deposition [4,5] or by sputtering [6] and in Si/SiO2 multilayers [7]. Interestingly, solid SiO1:0 is thermodynamically unstable below 1173 °C and the reaction: 2SiO ! SiO2 + Si appears when the material is annealed, leading to the formation of silicon nanoclusters embedded in a silicon oxide matrix [8–10]. Using this dissociation effect is probably one of the simplest method to obtain silicon-based light-emitting materials. Several hypotheses were proposed to explain the PL origin like quantum confinement [11,12], silicon-based chemical compounds [13,14], interface states [15] or defects states [16]. The detailed origin of the PL is still a matter of debate. A lot of experimental and theoretical works seem to show evidence that confinement and passivation of nonradiative centers are the two main conditions to get visible light from low-dimension silicon-based structures. In a previous study [17], we have shown that it was possible to obtain visible photoluminescence in silicon oxide thin films prepared by evaporation of SiO powder under ultrahigh vacuum. In another work [18], we have correlated the photoluminescence phenomenon to the formation of silicon clusters embedded in the silicon oxide matrix. By varying the silicon concentration and the silicon-cluster size, it was possible to change the PL energy. The samples were hydrogen free and dense. In the case of such photoluminescent silicon-based materials obtained by the phase separation of the SiO films into Si and SiO2 , it is clear that the structure of the as-deposited film is of prime importance on the photoluminescence properties. The aim of this work is to study the effect of hydrogen introduced in the evaporation chamber during the process on the microstructure and on the photoluminescence properties of the films. Three sets of samples were prepared: the first is prepared under ultrahigh vacuum, the second under a flow of atomic hydrogen with a pressure in the chamber equal to 4 105 Torr and the third under a 4 105 Torr molecular hydrogen pressure. In order to obtain a better understanding of the chemical bonding, two others sets of samples
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were prepared with deuterium instead of hydrogen. It is shown that these different preparation conditions lead to materials with different chemical bonding but also with different densities. Moreover, it is demonstrated that the structure of the silicon oxide thin films, especially the density of the material can affect the evolution of the photoluminescence properties with thermal annealing treatments. The structure is then correlated to the photoluminescence properties and finally the photoluminescence origin is discussed.
2. Experiment Five sets of samples have been prepared in a growth chamber evacuated by a cryogenerator with a pumping rate of 1000 l s1 . The background pressure was 108 Torr. Each sample was prepared by evaporation of SiO powder using a tantalum thermal cell. The substrates were maintained at 100 °C. The deposition rate, equal to 0.1 nm s1 was controlled by a quartz microbalance system. The unhydrogenated SiOx alloys (set A) were prepared under a pressure equal to 107 Torr. The samples of the groups B and D were prepared by evaporation of SiO powder under a flow of hydrogen and deuterium ions, respectively. The ions were created by an electron cyclotron resonance plasma source. The 2.45 GHz microwave energy was 200 W. The flow in the ion source was regulated by maintaining the total pressure in the evaporation chamber at 4 105 Torr. The ion beam was directed on the substrate with an angle of 30° from the substrate normal and the distance between the silicon oxide source and the substrate was 15 cm. For the samples of the group C and E, SiO was evaporated under a molecular hydrogen and deuterium atmosphere, respectively. The pressure was equal to 4 105 Torr. Silicon substrates were used for photoluminescence, IR absorption spectrometry and thermal desorption spectrometry experiments. Fused silica glass substrates were used for ultraviolet, visible and near infrared (UV–Vis–NIR) transmission experiments. By using a SiO density equal to 2.13, the thicknesses were 200 and 500 nm for silicon and glass substrates, respectively. The average chemical
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composition was analyzed by energy dispersive Xray spectroscopy (EDXS). It was demonstrated that each film has the composition SiO0:95 . Each film was annealed at different temperatures Ta ¼ 350, 500, 650, 800 and 950 °C in a quartz tube evacuated by an ionic pump. The background pressure was 107 Torr and the heating rate was equal to 10 K min1 . The samples were cooled down immediately after the annealed temperature was reached. The oxygen and hydrogen (deuterium) bonding configurations were obtained from Fourier transform infrared (FTIR) transmission measurements with a resolution of 4 cm1 . The samples were transferred to the spectrometer at the ambient atmosphere. For each sample, a reference spectrum of an uncoated silicon substrate was subtracted from the experimental spectra. Photoluminescence experiments were performed with a 64 cm Raman spectrometer equipped with a multichannel charge coupled device camera cooled at 140 K. The excitation light source at 488 nm was emitted from an argon laser with an incident power of around 10 mW/mm2 . A 300 grooves/mm grating was used to disperse emitted light for the photoluminescence experiments. Optical measurements were performed with a dispersive wavelength spectrometer. Absorption spectra were obtained from 190 to 3000 nm, using two different light sources and two different detectors. An UV deuterium lamp and a wire tungsten lamp were used as sources, and a PbS photodetector and a photomultiplier as detectors. The films were deposited onto fused silica glasses so as to increase transmission in the near UV and to avoid absorption by the Si–O–H groups in the near infrared range. As the refractive indices of the deposited thin film and the substrate are different, interference fringes appear in the transmission curve. By simulating the transmission curves, the exact value of the thickness was compared to the calculated thickness to get information on the density of the deposited samples. The thermal stability of hydrogen-doped films was studied by thermal desorption spectrometry. The films were inserted into a silica glass tube evacuated by an ionic pump allowing a base pressure of 5 109 Torr in the chamber and were heated at a constant rate of 10 K/min to 950 °C. The gaseous
components desorbing from the surface were ionized and detected by a quadrupole mass analyzer.
3. Results 3.1. Infrared absorption spectrometry The IR absorption spectra are very complicated because of a number of chemical bonds. Indeed Si–O, Si–H and O–H bonds can appear in silicon oxide films. Let us present the different expected absorption bands in a-SiOx :H thin films. Un-hydrogenated silicon oxide thin films are characterized by absorption bands in the domains 650–810 and 940–1000 cm1 , attributed to the Si– O–Si bending and asymmetric stretching modes, respectively. Due to the strong electronegativity of the oxygen atom, their frequencies are increasing functions of the oxygen content. The absorption bands at 650 and 940 cm1 appear in oxygen doped a-Si films [19,20]. Those at 810 and 1080 cm1 appear in SiO2 [21]. In hydrogenated materials, the Si–H stretching mode is empirically related to the electronegativity/inductive effect of the neighbouring atoms [22,23]. In pure a-Si:H films the SiH bond-bending and bond-stretching frequencies are equal to 630 and 1990 cm1 , respectively [24]. The absorption bands at 780, 840 and 880 cm1 are attributed to the Si–H bending motion with one, two and three oxygen atoms backbonded to the silicon one, respectively [25,26]. The stretching mode frequencies of the Si–H bond for these three configurations are equal to 2115, 2200 and 2260 cm1 , respectively [25,26]. The O–H bonds are characterized by a strong absorption in the 3000 cm1 range and a weak absorption at 1630 cm1 . These modes are attributed to O–H stretching and bending modes, respectively. Infrared absorption spectra of the as-deposited samples A, B, C, D and E are represented in Figs. 1 and 2 for the 500–1400 and 1400–4000 cm1 range, respectively. The IR absorption spectra are characterized by an intense band at 1000 cm1 . The frequency of this peak is equal to 995 cm1 for the sample A. By using a linear relation between the oxygen content and the frequency of the Si–O–Si asymmetric stretching mode [27], it can be
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0.8 Si-H Si-O-Si
Si-D 0.7
Absorbance
0.6
sample E
0.5 0.4
sample D
0.3 sample C 0.2 sample B
0.1
sample A 0.0 600
800 1000 1200 -1 Wavenumber (cm )
1400
Fig. 1. Infrared spectra of the as-deposited samples A, B, C, D and E in the 500–1400 cm1 range.
0.14
Si-D
O-H
Si-H
0.12
sample E
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which is attributed to the presence of hydrogen backbonded to the Si–O–Si groups. The presence of absorption bands located at the above-mentioned frequencies show that (Oy Si3y )SiH (with y ¼ 1, 2 or 3) configurations are present. Whereas sample B is OH free, sample C contains a lot of hydroxyl groups, as demonstrated by the presence of bands at 1630 and 3000 cm1 . The sample A contains neither Si–H bonds nor O–H bonds. The deuterium–hydrogen isotopic substitution is a powerful way to identify some Si–H bonds. Indeed, as the atomic mass is multiplied by a factor 2, the Si–D frequencies are pffiffiffi in good approximation divided by a factor 2. The samples D contain Si–D bonds as demonstrated by the bands at 1500 and 620 cm1 , but do not contain any O–H groups. This sample is very similar to the sample B where deuterium has taken place instead of hydrogen. The samples E, prepared under a molecular deuterium pressure, do not present any Si–D bond. On the contrary it presents the same IR bands than those of sample C whereas no hydrogen was present in the preparation chamber during the growth of the film. Such a result suggests that the SiH and SiOH groups have been formed after the preparation, during the exposure to the air. 3.2. Thermal desorption spectrometry
Absorbance
0.10 sample D 0.08 0.06 sample C 0.04 sample B 0.02 sample A
(x2)
0.00
1500 2000 2500 3000 3500 4000 Wavenumber (cm-1) Fig. 2. Infrared spectra of the as-deposited samples A, B, C, D and E in the 1400–4000 cm1 range.
deduced that x is equal to 1. The frequency of this peak is 35 cm1 higher for the other samples,
Thermal desorption spectrometry results are represented in Fig. 3 for the samples B, C, D and E. The sample A was also studied and neither hydrogen nor water was detected. Samples B and D show very similar spectra for hydrogen and deuterium desorption, respectively. They are characterized by a strong peak with a maximum centered at 500 °C. Such a peak is characteristic of hydrogen desorption from hydrogenated silicon oxide groups. In the work of Beyer [28] on OH-free silicon oxide thin films, it is demonstrated that a transition from a compact material to a material with an interconnected void structure is observed for an oxygen concentration near 20%. While the compact material exhibits an hydrogen effusion temperature in the 600–700 °C range, the void-rich material shows an effusion peak centered at 500 °C, which is explained by the surface desorption of hydrogen molecules through the void-rich
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40 Sample E
35
950
Hydrogen Deuterium Water
900 850 800
25 Sample D 20 15
Thickness (nm)
Effusion (arb. units)
30
750 700 650
sample A sample B sample C
600 550 500
10
Sample C
450 400
5 Sample B 0 0
150 300 450 600 750 900 Temperature (˚C)
Fig. 3. Thermal desorption spectra of the samples B, C, D and E.
structure. In oxygen-free amorphous silicon, hydrogen desorbs at 400 °C [29] but, due to the electronegativity of oxygen atoms, the desorption energy is shifted to higher energies. It must be noted that no hydrogen has effused from the deuterated sample. Furthermore no water is detected in the case of samples B and D. Samples C and E have exactly identical thermal desorption spectra. In both cases, no deuterium is detected. A strong hydrogen desorption peak containing several contributions can be observed. Two first contributions at very low temperatures appear between 100 and 300 °C. A tentative explanation will be given in the next. The peak at 500 °C is attributed to the surface desorption of hydrogen, and has probably the same origin as for the samples B and D. Furthermore the desorption of water is very similar in both cases and is clearly attributed to adsorbed water. 3.3. Optical measurements The thicknesses of the as-deposited and annealed films were obtained by simulation of the
150 300 450 600 750 900 Annealing temperature (˚C)
Fig. 4. Thickness of the samples A, B and C as a function of the annealing temperature.
UV–Vis–NIR transmission spectra. They are represented in Fig. 4 as a function of the annealing temperature Ta . Whereas the same SiO weight was deposited for each sample, the thicknesses are different. The sample deposited in ultrahigh vacuum has a thickness equal to 500 nm, which is the value calculated with a SiO density equal to 2.13. The measured thicknesses of the samples deposited in atomic and molecular hydrogen atmospheres are equal to 650 and 900 nm, respectively. The error bars are due to the fact that the reactor geometry could involve in a thickness variation of 5% depending on the position of the sample on the substrate holder. The error involved by the optical spectra simulation is estimated to around 5 nm. Such a thickness difference for the samples A, B and C can clearly be correlated to a density difference. The as-deposited samples B and C are respectively 30% and 80% more porous than the sample A. Moreover, with annealing treatments, the thickness of the sample A remains constant. On the contrary, the samples B and C become denser. This evolution is correlated to hydrogen effusion from these samples which presumably involves in a microstructure modification. The evolution of the index of refraction n, shown in Fig. 5 as a function of the annealing temperature Ta ,
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1.95 1.90
Refraction index
1.85
sample A sample B sample C
1.80 1.75 1.70
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domains in a more oxidized silicon oxide matrix. The appearance of pure silicon domains can explain the increase of n, as shown by the compositemedium Lorentz theory. The compared results of the thickness and the index of refraction suggest that both the porosity and the phase separation have an influence on the evolution of n. However, the determination of their respective contribution remains a very complicated question which is not the subject of this present work.
1.65
3.4. Photoluminescence
1.60 1.55 0
200 400 600 800 1000 Annealing temperature (˚C)
Fig. 5. Refraction index of the samples A, B and C as a function of the annealing temperature.
is similar for the three sets of samples. For Ta < 500 °C, n is nearly constant. For greater Ta , n is an increasing function of Ta . It is well known that the index of refraction is dependent on the oxygen concentration in SiOx films. In our case, the EDXS results show that the as-deposited materials contain the same oxygen concentration. The different values of n can therefore be explained by two other contributions. Firstly, it is well established that the density of a material can affect the optical properties of a film. The index of refraction is an increasing function of the density. The strong difference between the refraction indices of the samples A and B on the one hand and C on the other hand can be explained by the density. In particular, for as-deposited materials and for Ta 6 500 °C, the indices of refraction and the thicknesses are correlated. Moreover, for Ta > 500 °C, the indices of refraction of the samples A and B are practically equal, which is also the case for the thicknesses. However the increase of the indices of refraction of the samples A, B and C for Ta > 500 °C cannot be explained only by the increase of the density, in particular for the sample A for which the thickness remains constant. In this temperature range, as shown in a previous work [18], a phase separation appears in the film, which involves in the creation of pure amorphous silicon
Photoluminescence experiments were performed for all the samples. The spectra of the samples A, B and C are shown in Fig. 6 for the asdeposited samples and for the samples annealed at 500, 650, 800 and 950 °C. It has been verified that the PL spectra are identical for the samples B and D on the one hand and for the samples C and E on the other hand. For all the samples, the evolution of the PL with the temperature is similar. In a first step, the PL intensity increases till the annealing temperature Tmax is reached and then it decreases. Moreover, the PL energy continuously decreases with increasing annealing temperature. Such an evolution was discussed in detail in a previous study [18] and PL was attributed to a quantum confinement effect in silicon clusters embedded in the silicon oxide matrix. For as-deposited materials, the silicon clusters are supposed to be very small and the PL energy is large. In the first stage of annealing, not only the silicon clusters grow, which induces a decrease of the PL energy, but also new silicon clusters are created, which improves the PL intensity. When the annealing temperature is greater than Tmax , there is a coalescence phenomenon due to the clusters growth. The size of the silicon clusters becomes too large to present the quantum confinement effect and the PL signal disappears. For the as-deposited samples, the most efficient sample is the sample prepared with a hydrogen plasma. For Ta equal to 500 °C, the PL of the samples A and B are very similar while the PL of the sample C is very small. Finally for Ta equal to 800 °C, the sample C is two times more luminescent than the others.
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Fig. 6. Photoluminescence spectra of the samples A, B and C for the as-deposited samples and for the annealing temperature Ta equal to 500, 650, 800 and 950 °C.
4. Discussion 4.1. Microstructure of the as-deposited samples The thickness measurements show that the samples have very different densities, depending on the preparation conditions. The FTIR spectrometry and TDS experiments give supplementary
information about the microstructure. They show that the sample A, prepared under ultrahigh vacuum, contains very little hydrogen and OH groups, which is in agreement with the fact that it is very dense. The samples B and D are prepared under hydrogen and deuterium plasmas. As sample D presents Si–D bonds, it is clear that the deuterium
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atoms come from the plasma. The same conclusion can be made for the hydrogen atoms in the sample B because the only difference between these two samples is the isotopic change. Moreover, TDS experiments show a very large peak at 500 °C with a full width at half maximum (FWHM) equal to 200 °C. In comparison, the effusion peak of a hydrogenated amorphous silicon film prepared with the same experimental conditions presents a maximum at 400 °C with a FWHM equal to 80 °C. This peak is usually associated with desorption from internal surfaces in a void-rich material [29,30]. The shift of the a-SiO:H effusion spectrum towards the higher temperatures and the increase of the width can be explained by the fact that hydrogen effuses from different Si3n On –Si–H (n ¼ 1, 2, 3) groups in which they are one, two or three oxygen atoms backbonded to the silicon atom. These groups have desorption energies higher than that of the Si3 –Si–H configuration and the TDS spectrum represents the convolution of the desorption peaks corresponding to the different oxidized groups. In conclusion, the lower density of the samples B and D compared to that of the sample A can be explained by the formation of hydrogen or deuterium-saturated internal surfaces during the evaporation. Moreover these samples do not contain any O–H groups. It is possible that the porosity is too low to allow water absorption during exposure to ambient air. It can be also supposed that the passivation of the dangling bonds by the hydrogen and deuterium atoms strongly reduces the chemical reactivity of the sample. The samples C and E are prepared under molecular hydrogen and deuterium atmospheres, respectively. The sample C shows infrared absorption in the 2100 cm1 domain, which could indicate the formation of Si–H bonds during the evaporation. Compared to the samples B and D, the intensity of these bands is a little lower and the vibrations at 780 and 850 cm1 are very weak. However, the sample E exhibits exactly the same infrared spectrum. The band at 1630 cm1 is ambiguous because it can be due to the SiD stretching motion as well as to the bending motion of the O–H bonds. As this band is also present for the deuterium-free sample C, with the same ratio
71
compared with the O–H stretching mode, this band cannot be attributed to the Si–D group motion. No deuterium is incorporated in the sample E and the presence of Si–H bonds in the samples C and E must be attributed to the exposure to ambient air and not to the gas present in the evaporation chamber. There have been a lot of studies concerning the adsorption of water in silica glass samples. The formation of Si–H bonds in the films can be explained by the dissociative adsorption of a water molecule, as proposed by Burneau et al. [31]. In this case, the adsorption of water is favoured by silicon dangling bonds which are supposed to be present in the silicon sub-oxide. As shown in Fig. 7(a) and (b), adsorption of water molecules leads to the formation of Si–H bonds and Si–OH groups with IR absorption bands at 2266 and 3735 cm1 , respectively. The infrared band at 2266 cm1 , which corresponds to fully oxidized silicon atoms, i.e. to O3 SiH groups, is observed in both samples C and E. The occurrence of vibrations at lower frequencies than 2266 cm1 is explained by the existence of weakly oxidized hydrogenated silicon atoms. The contribution at 3735 cm1 is very weak in samples C and E but the SiOH groups do exist with a signature at lower frequencies, as explained in the next. Another adsorption process of a water molecule can occur by breaking of a Si–O–Si bond which leads to the formation of two Si–OH groups, as explained by Davis et al. [32]. The reaction between water and the silicon oxide films can be described by a fourstage process as shown in Fig. 7(c)–(f). The water vapor firstly diffuses into the film as free molecular water (Fig. 7(c)). The IR spectrum is characterized by the symmetric stretching vibration of the H2 O molecule with a frequency equal to 3425 cm1 . A vibration at 3225 cm1 is also characteristic of free water molecules in a void-rich structure. This mode is generally accompanied with the bending mode at 1630 cm1 . The initial interaction between water and silicon oxide is the appearance of bound molecular water allowed by the creation of a fivecoordinated silicon atom (Fig. 7(d)). This configuration is characterized by the vibration at around 3425 cm1 . By breaking of the SiO and the OH bonds, two hydrogen-bonded Si–OH groups appear (Fig. 7(e)). The frequency of the OH
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H. Rinnert, M. Vergnat / Journal of Non-Crystalline Solids 320 (2003) 64–75 H2O
a)
+
b)
H
H Si O
Si
Si O
O
O
O
O
c) O
O
H Si O
O O
O
f) O
O
H O
Si
O
O Si
O
O
O Si
O
O
H O
O
O Si
O
e)
H
O
Si
O
O O
O
O
O
Si
+
Si O
H
d) O
H2O
O
O
O O
O H
Si O O
Fig. 7. Schematic diagram of the water-glass reaction. (a,b) represent the dissociative adsorption of a water molecule leading to the formation of a Si–H and a Si–OH bonds. The four other diagrams represent an other possible adsorption process leading to the formation of two Si–OH groups.
stretching mode is equal to about 3550 cm1 . The disappearance of the hydrogen bonds leads to the formation of two independent Si–OH groups (Fig. 7(f)). The frequency of the OH stretching mode is then equal to 3676 cm1 . As shown by the strong absorption band located at 3420 cm1 with a shoulder at lower frequencies, it can be concluded that the samples C and E contain mostly hydrogen-bonded water molecules. These configurations are likely to occur in porous silicon oxide films. Moreover, the low absorption intensity at higher frequencies indicates the low quantity of isolated Si–OH groups, which appear in dense silica glass. This result also explains why the dissociative adsorption of water leads to an absorption frequency smaller than 3737 cm1 as proposed by Burneau et al. [31]. Therefore, the samples prepared under a molecular hydrogen atmosphere have a void-rich structure which contains hydrogen-bonded Si–OH groups rather than isolated ones. These results are confirmed by the TDS experiments. For the samples C and E, the hydrogen effusion spectra are identical and no deuterium signal is detected. Therefore, the hydrogenation appears after exposure to ambient air. The water thermal desorption shows two components. The first peak, at low temperatures, is attributed to free
molecular water or to hydrogen-bonded water located in the voids of the structure. The weak peak at higher temperatures is attributed to the desorption of water involving the breaking of one SiOH group. The low-temperature contribution of the hydrogen desorption can be attributed to the strong quantity of adsorbed water. To give more insight on the origin of this peak, the following experiment has been done: the as-deposited sample was annealed to 500 °C a first time. The infrared spectrum shows that the molecular water has almost disappeared. The sample was then annealed a second time at 650 °C. It was shown that the first peak of the water desorption and the two lowtemperature components of the hydrogen desorption have disappeared. This experiment does not give the exact process of the hydrogen formation but shows the correlation between the low-temperature hydrogen effusion and the water content. 4.2. Evolution of the microstructure with annealing During exposure to ambient air, the samples C and E adsorb a lot of water molecules. As it has been said before, the annealing treatment leads to the desorption of water and the formation of Si–O–Si bonds. In order to compare the evolution
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0.12 Sample C 0.11
Ta=950˚C
Ta=500˚C
Absorbance
0.10 0.09 as-deposited 0.08
Sample B
Ta=950˚C
0.07
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which induces an enormous reactivity to air exposure. After annealing treatments at 950 °C, the samples A and B are very similar. Indeed the infrared spectra and the thicknesses are identical. After the hydrogen desorption, the sample B becomes denser, the microstructure is presumably very similar to that of the sample A. However the sample C annealed at 950 °C shows the same infrared spectrum but is still almost two times thicker than the sample A. The annealed sample C remains probably very porous even after annealing at 950 °C. However the number of silicon dangling bonds is reduced and the sample is no more reactive to water.
Ta=500˚C
4.3. Photoluminescence
0.06 as-deposited 0.05 1800 2100 2400 3000 3500 4000 -1
Wavenumber (cm ) Fig. 8. Evolution with the temperature of the infrared spectra for the samples B and C.
of the microstructure of samples prepared with plasma and molecular atmosphere, the infrared spectra of the samples B and C are represented in Fig. 8 for the as-deposited samples and for the samples annealed at 500 and 950 °C. After an annealing at 500 °C, the physisorbed water has effused from the sample C and the O–H stretching mode is very low. No more water is adsorbed after the thermal treatment, because the number of dangling bonds, which are the water-adsorption sites, is extremely reduced. Only the stretching modes of the Si–H vibrations are visible, with similar absorption bands for both the samples. The hydrogen atoms come from the plasma for the sample B and from the adsorption of water for the sample C. These atoms desorb between 500 and 800 °C. In conclusion, the infrared and TDS results show the very different microstructures of the hydrogenated silicon oxide films B and C. Both contain voids but the former is denser than the latter even after annealing at 950 °C. The sample C is porous and contains a large internal surface
The three sets of samples A, B, and C have clearly different microstructures which can explain the PL behaviours of the samples A and B on the one hand and the sample C on the other hand. For as-deposited samples, the PL is very weak. The more intense PL for the sample B can be related to a better passivation of the silicon clusters by hydrogen atoms. For annealing temperatures greater than or equal to 500 °C, the behaviour of the PL of the samples A and B is similar, which is explained by the desorption of almost all the hydrogen atoms from the sample B. The PL of the samples A and B annealed at 500 °C is very intense and reaches its maximum at 650 °C while the maximum of the PL intensity of the sample C appears at 800 °C. The important result is the increase of Tmax for the more porous sample (sample C). This increase is correlated to the fact that the coalescence phenomenon appears at higher temperature for the sample C than for the samples A and B. In such a porous structure, the diffusion of atoms and the induced phase demixion is not favoured. The PL intensity maximum is significantly more intense for the sample C which can be explained by the presence of a greater number of silicon clusters because the porous structure allows us to diminish the number of interconnections between the silicon clusters. Moreover for the annealing temperature equal to 800 °C the PL energy of the sample C is greater than that of the samples A and B. This difference is explained by the smaller size of the
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clusters in the sample C than that of the clusters in the samples A and B, which is in agreement with the appearance of the coalescence phenomenon at higher temperature.
modifying the porosity of the films and/or by obtaining an efficient passivation of the silicon clusters.
References 5. Conclusion Silicon oxide thin films have been prepared by evaporation of SiO powder. The effect of a molecular or an atomic hydrogen atmosphere during the preparation on the microstructure and on the optical properties has been studied. It is demonstrated that it is necessary to use atomic hydrogen in order to introduce hydrogen in the film during the evaporation. Nevertheless the sample prepared under molecular hydrogen does contain hydrogen which is incorporated after the exposure to the air. The structural study shows the films have very different porosities. The samples prepared under an atomic and a molecular hydrogen atmosphere are 30% and 80% more porous, respectively, than the sample prepared under ultrahigh vacuum. The reactivity to water vapor is very high for the sample prepared under a molecular hydrogen atmosphere. This strong reactivity leads to the formation of hydrogenated silicon oxide groups. The optical properties are also very different. The index of refraction is higher for the pure silicon oxide and for the plasma hydrogenated films than for the sample prepared under the molecular hydrogen atmosphere. Moreover the pure silicon oxide film and the plasma hydrogenated film present almost the same photoluminescence properties. The PL is due to silicon clusters embedded in the silicon oxide matrix and the PL intensity reaches its maximum for an anneal at 650 °C. For the porous sample, the PL appears at higher annealing temperatures with an optimal temperature equal to 800 °C. The formation of isolated silicon clusters is more difficult in the porous sample presumably because the diffusion of atoms is stopped by the void-rich structure. In summary, this study shows that the evaporation technique allows us to obtain very different materials by incorporating molecular or atomic hydrogen during the deposition process. The results suggest that an improvement of the photoluminescence properties can be obtained by
[1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [3] S. Guha, M.D. Pace, D.N. Dunn, I.L. Singer, Appl. Phys. Lett. 70 (1997) 1207. [4] A.J. Kenyon, P.F. Trwoga, C.W. Pitt, G. Rehm, J. Appl. Phys. 79 (1996) 9291. [5] E. Edelberg, S. Bergh, R. Naone, M. Hall, S. Aydil, Appl. Phys. Lett. 68 (1996) 1415. [6] H.Z. Song, X.M. Bao, N.S. Li, X.L. Wu, Appl. Phys. Lett. 72 (1998) 356. [7] D.J. Lockwood, Z.H. Lu, J.M. Baribeau, Phys. Rev. Lett. 76 (1995) 539. [8] B.J. Hinds, F. Wang, D.M. Wolfe, C.L. Hinkle, G. Lucovsky, J. Non-Cryst. Solids 227 (1998) 507. [9] F. Rochet, G. Dufour, H. Roulet, B. Pelloie, J. Perriere, E. Fogarassy, A. Slaoui, M. Froment, Phys. Rev. B 37 (1988) 6468. [10] K. Furukawa, Y. Liu, H. Nakashima, D. Gao, K. Uchino, K. Muraoka, H. Tsuzuki, Appl. Phys. Lett. 72 (1998) 725. [11] M. Ehbrecht, B. Hohn, F. Huisken, M.A. Laguna, V. Paillard, Phys. Rev. B 56 (1997) 6958. [12] S. Schuppler, S.L. Friedman, M.A. Marcus, D.L. Adler, Y.H. Xie, F.M. Ross, Y.J. Chabal, T.D. Harris, L.E. Brus, W.L. Brown, E.E. Chaban, P.F. Szajowski, S.B. Christman, P.H. Citrin, Phys. Rev. B 52 (1995) 4910. [13] M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber, M. Cardona, Solid State Commun. 81 (1992) 307. [14] Y.D. Glinka, S.H. Lin, Y.T. Chen, Appl. Phys. Lett. 75 (1999) 778. [15] J. Wang, L. Song, B. Zou, M.A. El-Sayed, Phys. Rev. B 59 (1999) 5026. [16] S.M. Prokes, W.E. Carlos, S. Veprek, Ch. Ossadnik, Phys. Rev. B 58 (1998) 15632. [17] H. Rinnert, M. Vergnat, G. Marchal, A. Burneau, Appl. Phys. Lett. 72 (1998) 3157. [18] H. Rinnert, M. Vergnat, A. Burneau, J. Appl. Phys. 89 (2001) 237. [19] G. Lucovsky, J. Yang, S.S. Chao, J.E. Tyler, W. Czubatyj, Phys. Rev. B 29 (1984) 2302. [20] G. Lucovsky, J.D. Jaonnopoulos (Eds.), The Physics of Hydrogenated Amorphous Silicon II, Springer, Berlin, 1984, p. 235. [21] C.T. Kirk, Phys. Rev. B 38 (1988) 1255. [22] G. Lucovsky, J. Yang, S.S. Chao, J.E. Tyler, W. Czubatyj, Phys. Rev. B 28 (1983) 3225. [23] D.V. Tsu, G. Lucovsky, B.N. Davidson, Phys. Rev. B 40 (1989) 1795. [24] G. Lucovsky, J. Non-Cryst. Solids 141 (1992) 241.
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[29] H. Rinnert, M. Vergnat, G. Marchal, A. Burneau, Appl. Phys. Lett. 69 (1996) 1582. [30] W. Beyer, Physica B 170 (1991) 105. [31] A. Burneau, J. Lalevee, C. Carteret, J. Phys. Chem. B 104 (2000) 990. [32] K.M. Davis, M. Tomozawa, J. Non-Cryst. Solids 201 (1996) 177.
Structure and optical properties of amorphous silicon oxide thin films with different porosities H. Rinnert *, M. Vergnat Laboratoire de Physique des Mat eriaux (U.M.R. au C.N.R.S. No. 7556), Universit e Henri Poincar e Nancy 1, B.P. 239, 54506 Vandœuvre-l es-Nancy cedex, France Received 9 May 2002; received in revised form 20 November 2002
Abstract Amorphous silicon oxide thin films were prepared by evaporation of a silicon oxide powder. Samples were prepared under ultrahigh vacuum, under a flow of hydrogen ions or under a molecular hydrogen atmosphere. Two others sets of samples were prepared using deuterium instead of hydrogen. These five groups of samples were then annealed to different temperatures up to 950 °C and were exposed to the ambient air. The samples present different densities and microstructures. The sample prepared under ultrahigh vacuum is dense, hydrogen free and OH-bond free. Samples prepared under atomic hydrogen and deuterium flows contain Si–H and Si–D bonds, respectively, and are OH-bond free. The sample prepared under a molecular hydrogen atmosphere is very similar to that prepared under a molecular deuterium atmosphere. Both samples are porous and contain Si–H bonds and OH-groups coming from the exposure to the air. All the samples show visible photoluminescence attributed to isolated silicon clusters. The photoluminescence intensity increases with thermal annealing post-treatments up to an optimal annealing temperature. This maximum value is equal to 650 °C for the unhydrogenated sample and the sample prepared under an atomic hydrogen flow and to 800 °C for the sample prepared under a molecular hydrogen atmosphere. This difference is correlated to the different microstructures of the samples. Moreover the strongest photoluminescence intensity is obtained for the porous sample. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk; 78.66.Jg; 81.15.Jj
1. Introduction The silicon oxide alloys are among the most studied materials in the form of thin films, especially because of their interesting dielectric properties. A new field of interest has emerged since the
*
Corresponding author. Tel.: +33-3 83 68 48 19; fax: +33-3 83 68 48 01. E-mail address: [email protected] (H. Rinnert).
discovery of the photoluminescence (PL) in silicon nanostructures [1]. The potential use of silicon nanometer-size clusters embedded in a dielectric matrix in novel quantum devices is very attractive due to the low cost of manufacturing. The usual way to obtain more efficient silicon-based optoelectronic devices is the preparation of materials with lower dimensions in which the carrier motion is reduced involving an increase of the gap of the semiconductor and of the radiative recombination yield. A great number of techniques was used to
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00078-4
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obtain such structures and photoluminescence was observed in Siþ implanted SiO2 films [2,3], in Sirich SiO2 samples grown by chemical vapour deposition [4,5] or by sputtering [6] and in Si/SiO2 multilayers [7]. Interestingly, solid SiO1:0 is thermodynamically unstable below 1173 °C and the reaction: 2SiO ! SiO2 + Si appears when the material is annealed, leading to the formation of silicon nanoclusters embedded in a silicon oxide matrix [8–10]. Using this dissociation effect is probably one of the simplest method to obtain silicon-based light-emitting materials. Several hypotheses were proposed to explain the PL origin like quantum confinement [11,12], silicon-based chemical compounds [13,14], interface states [15] or defects states [16]. The detailed origin of the PL is still a matter of debate. A lot of experimental and theoretical works seem to show evidence that confinement and passivation of nonradiative centers are the two main conditions to get visible light from low-dimension silicon-based structures. In a previous study [17], we have shown that it was possible to obtain visible photoluminescence in silicon oxide thin films prepared by evaporation of SiO powder under ultrahigh vacuum. In another work [18], we have correlated the photoluminescence phenomenon to the formation of silicon clusters embedded in the silicon oxide matrix. By varying the silicon concentration and the silicon-cluster size, it was possible to change the PL energy. The samples were hydrogen free and dense. In the case of such photoluminescent silicon-based materials obtained by the phase separation of the SiO films into Si and SiO2 , it is clear that the structure of the as-deposited film is of prime importance on the photoluminescence properties. The aim of this work is to study the effect of hydrogen introduced in the evaporation chamber during the process on the microstructure and on the photoluminescence properties of the films. Three sets of samples were prepared: the first is prepared under ultrahigh vacuum, the second under a flow of atomic hydrogen with a pressure in the chamber equal to 4 105 Torr and the third under a 4 105 Torr molecular hydrogen pressure. In order to obtain a better understanding of the chemical bonding, two others sets of samples
65
were prepared with deuterium instead of hydrogen. It is shown that these different preparation conditions lead to materials with different chemical bonding but also with different densities. Moreover, it is demonstrated that the structure of the silicon oxide thin films, especially the density of the material can affect the evolution of the photoluminescence properties with thermal annealing treatments. The structure is then correlated to the photoluminescence properties and finally the photoluminescence origin is discussed.
2. Experiment Five sets of samples have been prepared in a growth chamber evacuated by a cryogenerator with a pumping rate of 1000 l s1 . The background pressure was 108 Torr. Each sample was prepared by evaporation of SiO powder using a tantalum thermal cell. The substrates were maintained at 100 °C. The deposition rate, equal to 0.1 nm s1 was controlled by a quartz microbalance system. The unhydrogenated SiOx alloys (set A) were prepared under a pressure equal to 107 Torr. The samples of the groups B and D were prepared by evaporation of SiO powder under a flow of hydrogen and deuterium ions, respectively. The ions were created by an electron cyclotron resonance plasma source. The 2.45 GHz microwave energy was 200 W. The flow in the ion source was regulated by maintaining the total pressure in the evaporation chamber at 4 105 Torr. The ion beam was directed on the substrate with an angle of 30° from the substrate normal and the distance between the silicon oxide source and the substrate was 15 cm. For the samples of the group C and E, SiO was evaporated under a molecular hydrogen and deuterium atmosphere, respectively. The pressure was equal to 4 105 Torr. Silicon substrates were used for photoluminescence, IR absorption spectrometry and thermal desorption spectrometry experiments. Fused silica glass substrates were used for ultraviolet, visible and near infrared (UV–Vis–NIR) transmission experiments. By using a SiO density equal to 2.13, the thicknesses were 200 and 500 nm for silicon and glass substrates, respectively. The average chemical
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composition was analyzed by energy dispersive Xray spectroscopy (EDXS). It was demonstrated that each film has the composition SiO0:95 . Each film was annealed at different temperatures Ta ¼ 350, 500, 650, 800 and 950 °C in a quartz tube evacuated by an ionic pump. The background pressure was 107 Torr and the heating rate was equal to 10 K min1 . The samples were cooled down immediately after the annealed temperature was reached. The oxygen and hydrogen (deuterium) bonding configurations were obtained from Fourier transform infrared (FTIR) transmission measurements with a resolution of 4 cm1 . The samples were transferred to the spectrometer at the ambient atmosphere. For each sample, a reference spectrum of an uncoated silicon substrate was subtracted from the experimental spectra. Photoluminescence experiments were performed with a 64 cm Raman spectrometer equipped with a multichannel charge coupled device camera cooled at 140 K. The excitation light source at 488 nm was emitted from an argon laser with an incident power of around 10 mW/mm2 . A 300 grooves/mm grating was used to disperse emitted light for the photoluminescence experiments. Optical measurements were performed with a dispersive wavelength spectrometer. Absorption spectra were obtained from 190 to 3000 nm, using two different light sources and two different detectors. An UV deuterium lamp and a wire tungsten lamp were used as sources, and a PbS photodetector and a photomultiplier as detectors. The films were deposited onto fused silica glasses so as to increase transmission in the near UV and to avoid absorption by the Si–O–H groups in the near infrared range. As the refractive indices of the deposited thin film and the substrate are different, interference fringes appear in the transmission curve. By simulating the transmission curves, the exact value of the thickness was compared to the calculated thickness to get information on the density of the deposited samples. The thermal stability of hydrogen-doped films was studied by thermal desorption spectrometry. The films were inserted into a silica glass tube evacuated by an ionic pump allowing a base pressure of 5 109 Torr in the chamber and were heated at a constant rate of 10 K/min to 950 °C. The gaseous
components desorbing from the surface were ionized and detected by a quadrupole mass analyzer.
3. Results 3.1. Infrared absorption spectrometry The IR absorption spectra are very complicated because of a number of chemical bonds. Indeed Si–O, Si–H and O–H bonds can appear in silicon oxide films. Let us present the different expected absorption bands in a-SiOx :H thin films. Un-hydrogenated silicon oxide thin films are characterized by absorption bands in the domains 650–810 and 940–1000 cm1 , attributed to the Si– O–Si bending and asymmetric stretching modes, respectively. Due to the strong electronegativity of the oxygen atom, their frequencies are increasing functions of the oxygen content. The absorption bands at 650 and 940 cm1 appear in oxygen doped a-Si films [19,20]. Those at 810 and 1080 cm1 appear in SiO2 [21]. In hydrogenated materials, the Si–H stretching mode is empirically related to the electronegativity/inductive effect of the neighbouring atoms [22,23]. In pure a-Si:H films the SiH bond-bending and bond-stretching frequencies are equal to 630 and 1990 cm1 , respectively [24]. The absorption bands at 780, 840 and 880 cm1 are attributed to the Si–H bending motion with one, two and three oxygen atoms backbonded to the silicon one, respectively [25,26]. The stretching mode frequencies of the Si–H bond for these three configurations are equal to 2115, 2200 and 2260 cm1 , respectively [25,26]. The O–H bonds are characterized by a strong absorption in the 3000 cm1 range and a weak absorption at 1630 cm1 . These modes are attributed to O–H stretching and bending modes, respectively. Infrared absorption spectra of the as-deposited samples A, B, C, D and E are represented in Figs. 1 and 2 for the 500–1400 and 1400–4000 cm1 range, respectively. The IR absorption spectra are characterized by an intense band at 1000 cm1 . The frequency of this peak is equal to 995 cm1 for the sample A. By using a linear relation between the oxygen content and the frequency of the Si–O–Si asymmetric stretching mode [27], it can be
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0.8 Si-H Si-O-Si
Si-D 0.7
Absorbance
0.6
sample E
0.5 0.4
sample D
0.3 sample C 0.2 sample B
0.1
sample A 0.0 600
800 1000 1200 -1 Wavenumber (cm )
1400
Fig. 1. Infrared spectra of the as-deposited samples A, B, C, D and E in the 500–1400 cm1 range.
0.14
Si-D
O-H
Si-H
0.12
sample E
67
which is attributed to the presence of hydrogen backbonded to the Si–O–Si groups. The presence of absorption bands located at the above-mentioned frequencies show that (Oy Si3y )SiH (with y ¼ 1, 2 or 3) configurations are present. Whereas sample B is OH free, sample C contains a lot of hydroxyl groups, as demonstrated by the presence of bands at 1630 and 3000 cm1 . The sample A contains neither Si–H bonds nor O–H bonds. The deuterium–hydrogen isotopic substitution is a powerful way to identify some Si–H bonds. Indeed, as the atomic mass is multiplied by a factor 2, the Si–D frequencies are pffiffiffi in good approximation divided by a factor 2. The samples D contain Si–D bonds as demonstrated by the bands at 1500 and 620 cm1 , but do not contain any O–H groups. This sample is very similar to the sample B where deuterium has taken place instead of hydrogen. The samples E, prepared under a molecular deuterium pressure, do not present any Si–D bond. On the contrary it presents the same IR bands than those of sample C whereas no hydrogen was present in the preparation chamber during the growth of the film. Such a result suggests that the SiH and SiOH groups have been formed after the preparation, during the exposure to the air. 3.2. Thermal desorption spectrometry
Absorbance
0.10 sample D 0.08 0.06 sample C 0.04 sample B 0.02 sample A
(x2)
0.00
1500 2000 2500 3000 3500 4000 Wavenumber (cm-1) Fig. 2. Infrared spectra of the as-deposited samples A, B, C, D and E in the 1400–4000 cm1 range.
deduced that x is equal to 1. The frequency of this peak is 35 cm1 higher for the other samples,
Thermal desorption spectrometry results are represented in Fig. 3 for the samples B, C, D and E. The sample A was also studied and neither hydrogen nor water was detected. Samples B and D show very similar spectra for hydrogen and deuterium desorption, respectively. They are characterized by a strong peak with a maximum centered at 500 °C. Such a peak is characteristic of hydrogen desorption from hydrogenated silicon oxide groups. In the work of Beyer [28] on OH-free silicon oxide thin films, it is demonstrated that a transition from a compact material to a material with an interconnected void structure is observed for an oxygen concentration near 20%. While the compact material exhibits an hydrogen effusion temperature in the 600–700 °C range, the void-rich material shows an effusion peak centered at 500 °C, which is explained by the surface desorption of hydrogen molecules through the void-rich
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40 Sample E
35
950
Hydrogen Deuterium Water
900 850 800
25 Sample D 20 15
Thickness (nm)
Effusion (arb. units)
30
750 700 650
sample A sample B sample C
600 550 500
10
Sample C
450 400
5 Sample B 0 0
150 300 450 600 750 900 Temperature (˚C)
Fig. 3. Thermal desorption spectra of the samples B, C, D and E.
structure. In oxygen-free amorphous silicon, hydrogen desorbs at 400 °C [29] but, due to the electronegativity of oxygen atoms, the desorption energy is shifted to higher energies. It must be noted that no hydrogen has effused from the deuterated sample. Furthermore no water is detected in the case of samples B and D. Samples C and E have exactly identical thermal desorption spectra. In both cases, no deuterium is detected. A strong hydrogen desorption peak containing several contributions can be observed. Two first contributions at very low temperatures appear between 100 and 300 °C. A tentative explanation will be given in the next. The peak at 500 °C is attributed to the surface desorption of hydrogen, and has probably the same origin as for the samples B and D. Furthermore the desorption of water is very similar in both cases and is clearly attributed to adsorbed water. 3.3. Optical measurements The thicknesses of the as-deposited and annealed films were obtained by simulation of the
150 300 450 600 750 900 Annealing temperature (˚C)
Fig. 4. Thickness of the samples A, B and C as a function of the annealing temperature.
UV–Vis–NIR transmission spectra. They are represented in Fig. 4 as a function of the annealing temperature Ta . Whereas the same SiO weight was deposited for each sample, the thicknesses are different. The sample deposited in ultrahigh vacuum has a thickness equal to 500 nm, which is the value calculated with a SiO density equal to 2.13. The measured thicknesses of the samples deposited in atomic and molecular hydrogen atmospheres are equal to 650 and 900 nm, respectively. The error bars are due to the fact that the reactor geometry could involve in a thickness variation of 5% depending on the position of the sample on the substrate holder. The error involved by the optical spectra simulation is estimated to around 5 nm. Such a thickness difference for the samples A, B and C can clearly be correlated to a density difference. The as-deposited samples B and C are respectively 30% and 80% more porous than the sample A. Moreover, with annealing treatments, the thickness of the sample A remains constant. On the contrary, the samples B and C become denser. This evolution is correlated to hydrogen effusion from these samples which presumably involves in a microstructure modification. The evolution of the index of refraction n, shown in Fig. 5 as a function of the annealing temperature Ta ,
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1.95 1.90
Refraction index
1.85
sample A sample B sample C
1.80 1.75 1.70
69
domains in a more oxidized silicon oxide matrix. The appearance of pure silicon domains can explain the increase of n, as shown by the compositemedium Lorentz theory. The compared results of the thickness and the index of refraction suggest that both the porosity and the phase separation have an influence on the evolution of n. However, the determination of their respective contribution remains a very complicated question which is not the subject of this present work.
1.65
3.4. Photoluminescence
1.60 1.55 0
200 400 600 800 1000 Annealing temperature (˚C)
Fig. 5. Refraction index of the samples A, B and C as a function of the annealing temperature.
is similar for the three sets of samples. For Ta < 500 °C, n is nearly constant. For greater Ta , n is an increasing function of Ta . It is well known that the index of refraction is dependent on the oxygen concentration in SiOx films. In our case, the EDXS results show that the as-deposited materials contain the same oxygen concentration. The different values of n can therefore be explained by two other contributions. Firstly, it is well established that the density of a material can affect the optical properties of a film. The index of refraction is an increasing function of the density. The strong difference between the refraction indices of the samples A and B on the one hand and C on the other hand can be explained by the density. In particular, for as-deposited materials and for Ta 6 500 °C, the indices of refraction and the thicknesses are correlated. Moreover, for Ta > 500 °C, the indices of refraction of the samples A and B are practically equal, which is also the case for the thicknesses. However the increase of the indices of refraction of the samples A, B and C for Ta > 500 °C cannot be explained only by the increase of the density, in particular for the sample A for which the thickness remains constant. In this temperature range, as shown in a previous work [18], a phase separation appears in the film, which involves in the creation of pure amorphous silicon
Photoluminescence experiments were performed for all the samples. The spectra of the samples A, B and C are shown in Fig. 6 for the asdeposited samples and for the samples annealed at 500, 650, 800 and 950 °C. It has been verified that the PL spectra are identical for the samples B and D on the one hand and for the samples C and E on the other hand. For all the samples, the evolution of the PL with the temperature is similar. In a first step, the PL intensity increases till the annealing temperature Tmax is reached and then it decreases. Moreover, the PL energy continuously decreases with increasing annealing temperature. Such an evolution was discussed in detail in a previous study [18] and PL was attributed to a quantum confinement effect in silicon clusters embedded in the silicon oxide matrix. For as-deposited materials, the silicon clusters are supposed to be very small and the PL energy is large. In the first stage of annealing, not only the silicon clusters grow, which induces a decrease of the PL energy, but also new silicon clusters are created, which improves the PL intensity. When the annealing temperature is greater than Tmax , there is a coalescence phenomenon due to the clusters growth. The size of the silicon clusters becomes too large to present the quantum confinement effect and the PL signal disappears. For the as-deposited samples, the most efficient sample is the sample prepared with a hydrogen plasma. For Ta equal to 500 °C, the PL of the samples A and B are very similar while the PL of the sample C is very small. Finally for Ta equal to 800 °C, the sample C is two times more luminescent than the others.
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Fig. 6. Photoluminescence spectra of the samples A, B and C for the as-deposited samples and for the annealing temperature Ta equal to 500, 650, 800 and 950 °C.
4. Discussion 4.1. Microstructure of the as-deposited samples The thickness measurements show that the samples have very different densities, depending on the preparation conditions. The FTIR spectrometry and TDS experiments give supplementary
information about the microstructure. They show that the sample A, prepared under ultrahigh vacuum, contains very little hydrogen and OH groups, which is in agreement with the fact that it is very dense. The samples B and D are prepared under hydrogen and deuterium plasmas. As sample D presents Si–D bonds, it is clear that the deuterium
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atoms come from the plasma. The same conclusion can be made for the hydrogen atoms in the sample B because the only difference between these two samples is the isotopic change. Moreover, TDS experiments show a very large peak at 500 °C with a full width at half maximum (FWHM) equal to 200 °C. In comparison, the effusion peak of a hydrogenated amorphous silicon film prepared with the same experimental conditions presents a maximum at 400 °C with a FWHM equal to 80 °C. This peak is usually associated with desorption from internal surfaces in a void-rich material [29,30]. The shift of the a-SiO:H effusion spectrum towards the higher temperatures and the increase of the width can be explained by the fact that hydrogen effuses from different Si3n On –Si–H (n ¼ 1, 2, 3) groups in which they are one, two or three oxygen atoms backbonded to the silicon atom. These groups have desorption energies higher than that of the Si3 –Si–H configuration and the TDS spectrum represents the convolution of the desorption peaks corresponding to the different oxidized groups. In conclusion, the lower density of the samples B and D compared to that of the sample A can be explained by the formation of hydrogen or deuterium-saturated internal surfaces during the evaporation. Moreover these samples do not contain any O–H groups. It is possible that the porosity is too low to allow water absorption during exposure to ambient air. It can be also supposed that the passivation of the dangling bonds by the hydrogen and deuterium atoms strongly reduces the chemical reactivity of the sample. The samples C and E are prepared under molecular hydrogen and deuterium atmospheres, respectively. The sample C shows infrared absorption in the 2100 cm1 domain, which could indicate the formation of Si–H bonds during the evaporation. Compared to the samples B and D, the intensity of these bands is a little lower and the vibrations at 780 and 850 cm1 are very weak. However, the sample E exhibits exactly the same infrared spectrum. The band at 1630 cm1 is ambiguous because it can be due to the SiD stretching motion as well as to the bending motion of the O–H bonds. As this band is also present for the deuterium-free sample C, with the same ratio
71
compared with the O–H stretching mode, this band cannot be attributed to the Si–D group motion. No deuterium is incorporated in the sample E and the presence of Si–H bonds in the samples C and E must be attributed to the exposure to ambient air and not to the gas present in the evaporation chamber. There have been a lot of studies concerning the adsorption of water in silica glass samples. The formation of Si–H bonds in the films can be explained by the dissociative adsorption of a water molecule, as proposed by Burneau et al. [31]. In this case, the adsorption of water is favoured by silicon dangling bonds which are supposed to be present in the silicon sub-oxide. As shown in Fig. 7(a) and (b), adsorption of water molecules leads to the formation of Si–H bonds and Si–OH groups with IR absorption bands at 2266 and 3735 cm1 , respectively. The infrared band at 2266 cm1 , which corresponds to fully oxidized silicon atoms, i.e. to O3 SiH groups, is observed in both samples C and E. The occurrence of vibrations at lower frequencies than 2266 cm1 is explained by the existence of weakly oxidized hydrogenated silicon atoms. The contribution at 3735 cm1 is very weak in samples C and E but the SiOH groups do exist with a signature at lower frequencies, as explained in the next. Another adsorption process of a water molecule can occur by breaking of a Si–O–Si bond which leads to the formation of two Si–OH groups, as explained by Davis et al. [32]. The reaction between water and the silicon oxide films can be described by a fourstage process as shown in Fig. 7(c)–(f). The water vapor firstly diffuses into the film as free molecular water (Fig. 7(c)). The IR spectrum is characterized by the symmetric stretching vibration of the H2 O molecule with a frequency equal to 3425 cm1 . A vibration at 3225 cm1 is also characteristic of free water molecules in a void-rich structure. This mode is generally accompanied with the bending mode at 1630 cm1 . The initial interaction between water and silicon oxide is the appearance of bound molecular water allowed by the creation of a fivecoordinated silicon atom (Fig. 7(d)). This configuration is characterized by the vibration at around 3425 cm1 . By breaking of the SiO and the OH bonds, two hydrogen-bonded Si–OH groups appear (Fig. 7(e)). The frequency of the OH
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H. Rinnert, M. Vergnat / Journal of Non-Crystalline Solids 320 (2003) 64–75 H2O
a)
+
b)
H
H Si O
Si
Si O
O
O
O
O
c) O
O
H Si O
O O
O
f) O
O
H O
Si
O
O Si
O
O
O Si
O
O
H O
O
O Si
O
e)
H
O
Si
O
O O
O
O
O
Si
+
Si O
H
d) O
H2O
O
O
O O
O H
Si O O
Fig. 7. Schematic diagram of the water-glass reaction. (a,b) represent the dissociative adsorption of a water molecule leading to the formation of a Si–H and a Si–OH bonds. The four other diagrams represent an other possible adsorption process leading to the formation of two Si–OH groups.
stretching mode is equal to about 3550 cm1 . The disappearance of the hydrogen bonds leads to the formation of two independent Si–OH groups (Fig. 7(f)). The frequency of the OH stretching mode is then equal to 3676 cm1 . As shown by the strong absorption band located at 3420 cm1 with a shoulder at lower frequencies, it can be concluded that the samples C and E contain mostly hydrogen-bonded water molecules. These configurations are likely to occur in porous silicon oxide films. Moreover, the low absorption intensity at higher frequencies indicates the low quantity of isolated Si–OH groups, which appear in dense silica glass. This result also explains why the dissociative adsorption of water leads to an absorption frequency smaller than 3737 cm1 as proposed by Burneau et al. [31]. Therefore, the samples prepared under a molecular hydrogen atmosphere have a void-rich structure which contains hydrogen-bonded Si–OH groups rather than isolated ones. These results are confirmed by the TDS experiments. For the samples C and E, the hydrogen effusion spectra are identical and no deuterium signal is detected. Therefore, the hydrogenation appears after exposure to ambient air. The water thermal desorption shows two components. The first peak, at low temperatures, is attributed to free
molecular water or to hydrogen-bonded water located in the voids of the structure. The weak peak at higher temperatures is attributed to the desorption of water involving the breaking of one SiOH group. The low-temperature contribution of the hydrogen desorption can be attributed to the strong quantity of adsorbed water. To give more insight on the origin of this peak, the following experiment has been done: the as-deposited sample was annealed to 500 °C a first time. The infrared spectrum shows that the molecular water has almost disappeared. The sample was then annealed a second time at 650 °C. It was shown that the first peak of the water desorption and the two lowtemperature components of the hydrogen desorption have disappeared. This experiment does not give the exact process of the hydrogen formation but shows the correlation between the low-temperature hydrogen effusion and the water content. 4.2. Evolution of the microstructure with annealing During exposure to ambient air, the samples C and E adsorb a lot of water molecules. As it has been said before, the annealing treatment leads to the desorption of water and the formation of Si–O–Si bonds. In order to compare the evolution
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0.12 Sample C 0.11
Ta=950˚C
Ta=500˚C
Absorbance
0.10 0.09 as-deposited 0.08
Sample B
Ta=950˚C
0.07
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which induces an enormous reactivity to air exposure. After annealing treatments at 950 °C, the samples A and B are very similar. Indeed the infrared spectra and the thicknesses are identical. After the hydrogen desorption, the sample B becomes denser, the microstructure is presumably very similar to that of the sample A. However the sample C annealed at 950 °C shows the same infrared spectrum but is still almost two times thicker than the sample A. The annealed sample C remains probably very porous even after annealing at 950 °C. However the number of silicon dangling bonds is reduced and the sample is no more reactive to water.
Ta=500˚C
4.3. Photoluminescence
0.06 as-deposited 0.05 1800 2100 2400 3000 3500 4000 -1
Wavenumber (cm ) Fig. 8. Evolution with the temperature of the infrared spectra for the samples B and C.
of the microstructure of samples prepared with plasma and molecular atmosphere, the infrared spectra of the samples B and C are represented in Fig. 8 for the as-deposited samples and for the samples annealed at 500 and 950 °C. After an annealing at 500 °C, the physisorbed water has effused from the sample C and the O–H stretching mode is very low. No more water is adsorbed after the thermal treatment, because the number of dangling bonds, which are the water-adsorption sites, is extremely reduced. Only the stretching modes of the Si–H vibrations are visible, with similar absorption bands for both the samples. The hydrogen atoms come from the plasma for the sample B and from the adsorption of water for the sample C. These atoms desorb between 500 and 800 °C. In conclusion, the infrared and TDS results show the very different microstructures of the hydrogenated silicon oxide films B and C. Both contain voids but the former is denser than the latter even after annealing at 950 °C. The sample C is porous and contains a large internal surface
The three sets of samples A, B, and C have clearly different microstructures which can explain the PL behaviours of the samples A and B on the one hand and the sample C on the other hand. For as-deposited samples, the PL is very weak. The more intense PL for the sample B can be related to a better passivation of the silicon clusters by hydrogen atoms. For annealing temperatures greater than or equal to 500 °C, the behaviour of the PL of the samples A and B is similar, which is explained by the desorption of almost all the hydrogen atoms from the sample B. The PL of the samples A and B annealed at 500 °C is very intense and reaches its maximum at 650 °C while the maximum of the PL intensity of the sample C appears at 800 °C. The important result is the increase of Tmax for the more porous sample (sample C). This increase is correlated to the fact that the coalescence phenomenon appears at higher temperature for the sample C than for the samples A and B. In such a porous structure, the diffusion of atoms and the induced phase demixion is not favoured. The PL intensity maximum is significantly more intense for the sample C which can be explained by the presence of a greater number of silicon clusters because the porous structure allows us to diminish the number of interconnections between the silicon clusters. Moreover for the annealing temperature equal to 800 °C the PL energy of the sample C is greater than that of the samples A and B. This difference is explained by the smaller size of the
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clusters in the sample C than that of the clusters in the samples A and B, which is in agreement with the appearance of the coalescence phenomenon at higher temperature.
modifying the porosity of the films and/or by obtaining an efficient passivation of the silicon clusters.
References 5. Conclusion Silicon oxide thin films have been prepared by evaporation of SiO powder. The effect of a molecular or an atomic hydrogen atmosphere during the preparation on the microstructure and on the optical properties has been studied. It is demonstrated that it is necessary to use atomic hydrogen in order to introduce hydrogen in the film during the evaporation. Nevertheless the sample prepared under molecular hydrogen does contain hydrogen which is incorporated after the exposure to the air. The structural study shows the films have very different porosities. The samples prepared under an atomic and a molecular hydrogen atmosphere are 30% and 80% more porous, respectively, than the sample prepared under ultrahigh vacuum. The reactivity to water vapor is very high for the sample prepared under a molecular hydrogen atmosphere. This strong reactivity leads to the formation of hydrogenated silicon oxide groups. The optical properties are also very different. The index of refraction is higher for the pure silicon oxide and for the plasma hydrogenated films than for the sample prepared under the molecular hydrogen atmosphere. Moreover the pure silicon oxide film and the plasma hydrogenated film present almost the same photoluminescence properties. The PL is due to silicon clusters embedded in the silicon oxide matrix and the PL intensity reaches its maximum for an anneal at 650 °C. For the porous sample, the PL appears at higher annealing temperatures with an optimal temperature equal to 800 °C. The formation of isolated silicon clusters is more difficult in the porous sample presumably because the diffusion of atoms is stopped by the void-rich structure. In summary, this study shows that the evaporation technique allows us to obtain very different materials by incorporating molecular or atomic hydrogen during the deposition process. The results suggest that an improvement of the photoluminescence properties can be obtained by
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