vertfj the formation of PbS nanocrystallites in the SiOz host. These nanocrystallites show an intensive photoluminescence emission band at about 435 nm, with ...
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Matmink.Vol. 10.No. 2. pp. 131-149.1998 Ekevia ScienceLid (0 1998ActaIWahugica Inc. PfinIedinthcusA. Allrightsrrsaval 0965-9773/98 $19.00+ .OO
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QUANTUM-SIZE EFFECTS OF PbS NANOCRYSTALLITES IN EVAPORATED COMPOSITE FILMS R. Thielsch’, T. Biihme*, R. Reiche3 , D. SchlSfer3, H.-D. Baues and H. B6ttcher4 ‘Fraunhofer Institut ftir Angewandte Opt& und Feinmechanik, Schillerstr. 1, D-07745 Jena, Germany %entronic GbR, Gostritzer Str. 61- 63, D - 01217 Dresden, Germany 31nstitut fur Festkorper- und Werkstofforschung, Helmholtzstr. 20, D - 01069 Dresden, Germany 4Feinchemie GmbH. Sebnitz, Hohenweg 9, D - 01855 Sebnitz, Germany (Accepted March 6,1998)
Abstract- Semiconductor nanocrystallites exhibit electronic, optical and photochemical properties greatly differing from those observed in the related bulk material due to quantum size effects. In view offuture applications, nanoclusters based on sulphur compounds willfind great potential as ,photocatalysts or nonlinear optical materials. The method of interrupted island growth was applied to form PbS nanocrystallites in a dielectric SiO2 host by a two sourceevaporation technology. Depending on the deposition conditions, PbS nanocrysials can beformed with grain size down to about 1 nm, giving a significant increase in the optical band gapfrom 0.41 eVfor the bulk material to about 5.2 eV XPS, XRD and TEh4 investigations were performed to vertfj the formation of PbS nanocrystallites in the SiOz host. These nanocrystallites show an intensive photoluminescence emission band at about 435 nm, with the intensity decreasing with increasing PbS concentration. 01998 Acta Metallurgica Inc. I. INTRODUCTION Semiconductor nanocrystalhtes exhibit interesting electronic, optical and photochemical properties differing significantly from those observed for the related bulk material. A reduction in particle size to the nanometer scale results in quantum size effects at dimensions comparable to the Bohr-diameter of the exciton. At such crystallite dimensions the photogenerated electron-holepairs are spatially confined if the diameter of the crystallites is smaller than the Bohr-diameter of the exciton. ‘Theband gap energy increases as the size of the crystallites decreases. Also, surface effects become significant because of the large surface to volume ratio of very small particles ( l3). It is known that strong confinement is most readily achieved in narrow bandgap semiconductors, in which the Bohr radius of the excitons is large (1). Many studies have been reported either on the preparation of different types of semiconductor nanocrystals or on various precipi131
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R THIELSCH,T B&ME, R REKHE, IJ SCHAEFER,H-D BAUERANOl-i BOTTCHER
tation methods. In view of future applications, nanoclusters based on sulphur compounds will find a great potential as photocatalysts (4) or as nonlinear optical materials due to their large nonlinear optical coefficients (5,6). Among these, PbS is,a promising material, having a bulk bandgap of 0.4 1 eV caused by a direct electron transition and a large Bohr radius of the exciton of about 18 nm. In the literature results on the successful preparation of PbS nanocrystallites by different methods have been reported: PbS in polymers (7), colloidal solutions of PbS (8,9), sol-gel matrices doped with PbS (6,10,11), PbS in inverse micelles (4,12), and PbS in zeolites (13). Most of the preparation methods usedare not compatible with solid state device technologies, restricting their useability to basic research purposes or chemical related applications. Only recently, the preparation of PbS microcrystal-doped Si& glass thin films by &puttering has been reported (14). In these investigations, a significant bandgap shift from the IR region to the near UV region depending on the small size of the PbS crystallites has been found. For the theoretical interpretation of this phenomenon, the correlation between the optical absorption and the dimension of the PbS nanocrystallites is frequently discussed in terms of the finite depth spherical well model by Nosaka (15) or the the hyperbolic band model by Wang et al. (7) . In this work, the growth of PbS nanocrystallites in a dielectric SiOz host is reported, applying the method of interrupted island growth by the use of a _multi-source evaporation technology under normal high vacuum conditions and low substrate temperatures. The influence of the deposition conditions on the formation of the PbS nanocrystallites has been studied as well as the dependence of the optical and optoelectronical properties on the composition of the composite films and on the size of the nanocrystallites by different methods. In some cases, postdeposition annealing of selected samples in vacuum has been performed to improve the crystallinity of the incorporated PbS cluster and to stimulate the grain growth. II. EXPERIMENTAL
PROCEDURE
II.1. Deposition Composite films of PbS and SiOz of various PbS volume concentrations were deposited by thermal co-evaporation from different evaporation sources (tungsten boat for PbS, electron beam crucible for Si@) in a standard Leybold evaporation device (UNIVBX~300). The base pressure before starting the deposition was usually about 1-10m3 Pa, while during the deposition a pressure of about 5.10-3 Pa was maintained due to the partial dissociation of the Si@ evaporation material. The evaporation rate of each material was controlled by a quartz crystal oscillators thickness monitor, carefully shielded against cross-evaporation. Fused silica plates SQl. Coming 7059 glass, and silicon plates were used as substrates, held at,near room temperature. The substrates were placed on a plane substrate holder which rotated with 20 turns per minute. To utilize the method of interrupted island growth to form PbS nanocrystallites in a dielectric host (16,17), the two evaporation sources were well separated from each other by mechanical shields. In this way, during one rotation only a small quantity of PbS was allowed to condense at the surface and to form nanoclusters by the nucleation process. The so-formed nanoclusters were immediately buried by a Si@ overcoat. Therefore, the film composition, i.e. the PbS volume fraction, but also the size of the PbS islands were determined by the evaporation rate of PbS and the geometrical conditions in the vacuum recipient.
EVAPORATED PbS NANOCRYSTALLITES
thickness me,3surement
133
substrate holdel
substrates
thickness
evaporator 2
mechanical shields
evap’orator
1
a) expertmental arrangement
1Onm
Pbz (nanoorystalline)
m
3) Mm structure
Figure 1. Schematical experimental arrangementin the used vacuum device (a) and expected microstructureof the PbS - SiO2 composite films deposited by the method of interruptedcrystallite growth (b).
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R THIELSCH,T BBHME,R REK;HE,D SCHLAFER,H-D BAUERANDH BOTKHER
Two different sets of deposition experiments have been carried out to get samples with PbS volume fractions up to 0.35. For films with PbS volume fractions up to 0.2, the Si@ rate and the deposited Sia thickness measured at the quartz monitor were held constant, while different PbS evaporation rates were used in subsequent deposition runs. For composite films of higher PbS content, the evaporation rates of both materials were adjusted to get the desired rate ratio. During this set of experiments, the PbS evaporation rate was only slightly increased to higher values compared to the rate used for the deposition of the film with the PbS volume fraction of 0.2. In Figure 1, theexperimental anangement in the vacuum plant and the expected microstructure of the films are schematically shown. To improve the crystallinity and to increase the grain size of the formedPbS nanocrystallites, some of the as-deposited samples were stepwise annealed at a temperature of 300°C similar to the procedure given in (18). 11.2. Film Composition The film compositions were calculated from quartz oscillator thickness readings using experimental determined calibration coefficients and by XPS. Additionally, XPS was used to study the chemical composition as well as the bonding state of the lead atoms to sulphur or oxygen. The XPS investigations using Mg& excitation and a Perkin Elmer model PI-R 5600 electron spectrometer are described in detail elsewhere (19). 11.3. Estimationof Optical,Opto-electronicalProperfies, and Film Thickness To determine the refractive index n(h), the absorption constant a(h), and the thickness d of the PbS-Sia-composite films, optical reflection R(h) and transmission T(h) spectra were measuredinthe wavelengthrangefrom2OOnmto 1lOOnmwitbadoublebeamspectralphotometer Perkin Elmer Lambda-2. While n(h) and d were deduced from reflection interference pattern, the absorption constant a(i) was determined from (20)
ill The optical band gap energy Esap of a film was deduced from the Taut relation
a.E-A.(E-Egap)m
121
with m = 1/2,2,3/2or 3 foralloweddirect,allowedindirect, forbiddendirectandforbiddenindirect electronic transitions (21,22). To be able to compare films of different compositions, the absorption constants a(L) wed in formula [2] were normalized by dividing the experimental values with the actual PbS volume traction of the film under consideration. For some samples, the film thickness determined from spectralphotometric reflection measurements was checked by mechanical stylus measurements using a Sloan Dektak 3030 ST profdometer. In the thickness range between about 350 nm to 450 nm, the results obtained by both methods deviate less than about 30 nm.
135
EVAFQRATED PbS NANOCRYSTALLITES
=. \ Y. Wang et al. J. Chem. Phys. 87.7315 (1987)
5-
F
2.
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Y.Naaeka J. Phys.Chem. 95.5054 (1991)
2-
l-
o\ 0
2
4
6
8
10
12
Crystallitesize [nml Figure 2. Theoretical dependence of the optical bandgap energy of PbS nanocrystallites on the mean crystallite diameter.
Photoluminescence measurements were performed with a Spex Fluoromax spectralphotometer system at room temperature. The excitation wavelength was varied between 250 nm and 514 nm. The light emitted from the specimen was detected by a photomultiplier R928 (Hamamtsu). Further details on the luminescence investigations on nanocrystalline PbS-SiO2 composite films are given in (23). 11.4.Determinationof Film Structureand CrystalliteSize In the works of Nosaka (15) and Wang et al. (7), theoreticalrelations, which are shown in Figure 2, ae given for the dependence of the optical bandgap of PbS nanocrystallites on the crystallite siize. Both relations have been proven to agree well with experimental results (7,ll). Therefore, it seems possible to use these relations to determine the size of the PbS nanocrystallites from the experimental estimated bandgap values of the composite films. Due to their great similarity, both relations result in very similar dimensions for a given bandgap value. In this work, the relation of Wang et al. (7) was used to determine the dependence of the crystallite diameter on the preparation conditions because of slighly larger crystallite dimensions. Additionally, the structure of the composite films was also investigated by XRD, TEM and SEM to check the results of the method described above. The crystalline structure and the mean crystallite diameter were determined by X-ray diffraction in Bragg-Brentano geometry using Co-&-radiation. The structural parameters of the
136
R THIELSCH, T B&ME, R F&HE, D SGLAFER.
H-D
BAUER AND
H
BCWFCHER.
films were estimated from difference diffractograms of the coated and uncoated substrates. The mean crystallite size was calculated by applying Scherrer‘s equation (24) with prefactor b = 1.0. Selected films were also investigated by TEM micrographs and microarea electron diffraction in a Philips CM 20-FEG analytical transmission electron microscope. To prepare the samples for electron microscopy, pieces of a film were scraped from the substrate with a steel knife and transferred to an amorphous carbon film support. Most of these pieces were found to be transparent for 200 keV electrons. Before taking the micrographs, the composition was checked by energy dispersive X-ray spectroscopy. All investigated specimens were found to be more or less sensitive to electron irradiation. Forthisreason,attention waspaidtominimizethedepositedenergyduring theTBMinvestigat.ion. III. RESULTS AND DISCUSSION III.1. Chemical Compositionof the Films The composition of the deposited films has been determined using the in situquartz monitor evaporation rate and thickness readings taking into account experimental estimated calibration factors. These calibration factors have been obtained from single material deposition runs. By this procedure, the calculated concentration is given in volume parts or volume percent of each constituent. To check these results, XPS measurements of selected composite films were performed including the volume concentration range from 0 to about 20 vol.% PbS. Additionally, 0,20 I-
0,15 z k .o iii L .o E ,o m 8 z
0,lO
0,05
0,oo L
0,oo
0,05
0,lO
0.15
0.20
1
0,25
Pb:Si atomic ratio [volume fraction]
Figure 3. Comparison of the XPS Pb:Si atomic ratio of the analyzed composite films with the similar ratio calculated from the evaporation rate data using experimental calibration factors without density correction.
EVAPORATED PbS NANOCRYSTALLITES
137
a single evaporated PbS film was analyzed for comparison. The XPS investigations were also performed to evaluate in which bonding state the incorporated Pb ions exist after evaporation and condensation in an oxygen rich environment. In the as-deposited state, the films were found to be covered with a carbon containing surface contamination layer and some PbSO4. For analysis of the film composition this surface layer had to be removed. To our knowledge, though noble gas ion bombardment may alter the composition in the analysis region, it is the only method for contamination removal from thin composite films. Therefore, ion beam sputter cleaning was applied using Ar+ ions of 3.5 keV energy and 30” incidence to the surface normal. After an ion fluence of (4...8) - 1015Ar+/cm*, the surfaces were free of carbon residuals and neither sulphate nor sulphite could be detected. Now, the only constituents were found to be lead, silicon, oxygen and sulphur. As was already mentioned, ion beam bombardment may alter the surface composition due to preferential sputtering and recoil implantation, mainly of the lighter atoms (25). It is assumed, that the Pb tc Si atomic ratio is not seriously affected by this process. In Figure 3, the Pb:Si atomic ratio taken from XPS analysis is shown versus the similar ratio calculated from the evaporation rate data of the films. As it is evident from this figure, an expected linear relationship was found but the slope is somewhat smaller than one. This lower slope may be explained by the fact that most of the evaporated dielectric or semiconducting films suffer from a reduced density in comparison to the bulk material, if they are deposited at low substrate temperatures (26,27). This density effect was not taken into account by the determination of the calibration factors. So, the Si& content may be overestimated, because the density of pure evaporated SQ films can be lower than 80% of the bulk (28). This effect would result in a lower Pb:Si ratio calculated from evaporation data and so in a slope smaller than 1. To avoid confusion, all concentrations below in this paper are given in volume fractions (or volume parts) of PbS calculated from the evaporation data, with no attempt to correct for the density effects. Surfacecharging wasobservedin thecompositefihnsdecreasingopposite totbePbS content of the investigated samples. Therefore, the measured peak energies of all composite samples were corrected using the Si2p peak energy of SiO2 as an internal standard, which is located at a binding energy of 103.3 eV. This way a surface potential of 3.3 V was deduced from the Si2p peak shift for the film with a PbS volume fraction of 0.03. In the case of the pure PbS film, negligible electrostatic charging as small as 0.1 V occurred due to the high conductivity of PbS. In relation to the chemilcal bonding of the lead atoms in the composite films, it can be deduced from the Pb4f7n peak structure that most of the lead atoms arc bonded to sulphur to form PbS, but also some PbO or Pb-S-O complex compounds exist in the films beside a small amount of non-bonded metallic lead, which results from the ion bombardment induced decomposition of the lead compounds. The latter was used to conclude the small charge value for the pure PbS film because the binding energy position of the curve-fitted elemental lead was 0.1 eV higher than that of a lead standard. The existence of some PbO and possibly Pb-S-O is also indicated by the double peak structure of the 01s peak with one peak at a binding energy of (532.7 &0.1) eV, which is assigned to the Si-0 bonds in Si@, and the second one shifting from 530.7 eV in the 20 vol.% sample to 530.3 eV in the 3 vol.% sample. The second 01s peak may be composed of the two binding states that were found in the PbS film, the weaker one of those has its binding energy position at 530.75
138
R THIELSCH,T B&ME, Fi RECHE,
D
SCHLAFER,
237
H-D BAUERANDH ~IXHER
234
228
231
225
Binding Energy /eV
IO 5
0
30
27
24
21
18
6 15 12 9 Binding Energy /eV
3
0
Figure 4. Photoelectron spectra of S2s, 02s, PbSd core level spectra and the valence band region. Electrostatic charging of the Si@(PbS) samples is corrected against Si2p at 103.3 eV in Si@ shifting the oxygen line 02s to 25.1 eV. Satellite structures (mainly Pb5d) are due to non-monochromatic Mg Ka excitation eV, while the other one is typical for sputter cleaned PbO being at a binding energy of 529.5 eV (19). The former binding state definitely belongs to a lead oxide with a stoichiometry different from PbO because depth profiling revealed no other sulphur component but the sulphide one except for the surface and the interface. Thus, it follows that there is no Pb-S-O compound that resists sputtering in a PbS environment. At the Si@ interface the 01s peak structure is supplemented by the 01s (Sioz) peak, i.e. for PbS sputtered in a Si& environment a Pb-S-O compound can be excluded, too. The silicon atoms were found to be bonded to oxygen in all cases, the Si2p line-shape is not influenced by the PbS content. As it is obvious from Figure 4, shifts of lead as well as sulphurelectronic states are observed depending on the PbS content. As it will be shown in the following sections, these changes are related to the size of the PbS clusters. Comparing the photoelectron spectraof the PbS film to those of a sample with 5 vol.% PbS, size related shifts are indicated, for example the valence band edge is changed from 0.2 eV to 1.9eV due to a reduced density of Pb6p states, the Pb5Qn and S2s core level peaks are shifted from 19.1eV to 20.0 eV and from 225.4 eV to 227.2 eV, respectively. As depth profiling of the PbS film comes to the Sio2 interface, an additional sulphur component appears, broadening and shifting the S2s peak as is observed in the Sioz-pbs composite samples. More details on the sixe related changes of electronic states in PbS found by photoelectron spectroscopy are given in (19).
EVAFORATEDPbS NANOCRYSTAUITES
2,25
1
A
g
I
1
I
I
139
experimentaldata
2,oo
5: s C ii 0 .c
al .z
.’
.’
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._.-. _._.-----
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I,50
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0,20
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Figure 5. Refractive index n(550 nm) of the composite films at h = 550 nm. Also shown are calculations according to different effective medium models and to the upper and lower bonds of the dielectric constant of a binary mixture. 111.2.OpticalProperties 111.2.1,Refractive index of the compositefilms
In Figure 5, the experimental determined refractive indices of the as-deposited PbS- Si@ composite films at a wavelength of 550 nm are shown in dependence on the PbS volume concentration. It is known that SiOz is a low index material with a refractive index of about 1.46 ata wavelenl~thof550nm,whilebullrPbSisahighlyabsorbingmaterialwithacomplexrefraction index R= n - i*k = 4.3 - i-1.23 in the visible spectral region (29). Aheady with a small quantity of PbS incorporated into the Si@ host, there is a drastic increase in the effective refraction index of the composite film. To describe the refractive index of a composite material,different effective medium models areavailable from literature(30,31). Among these,calculationsaccording to the Maxwell-Gamett model (32), the simple linear mixing model (30) and the upper and lower bonds of the dielectric constant of a two component material have been performed. The upper bond is due to a microstructure consisting of an array of parallel cylinders of both components, while the lower bond is due to the parallel slab case (31). The results of these calculationsare also shown in Figure 5 for comparison with the experimental data to give a basic idea of a useful model. As it can be seen, the experimental data matchboth the linear model as well as the Maxwell-Gamettmodel with similar good agreement.
140
R THIELSCH,T B&ME, R REICHE,D SHLAFER, H-D BAUERAND H
200
300
400
500
Wavelength
600
E%TKHEFI
700
[nm]
Figure 6, Measured transmission spectra of PbS-SiOz composite films, which contain different PbS volume concentrations. Film thicknesses were in the range from of 300 nm to 400 nm.
III.2.2. Opticalabsorptionana’the bandgap In Figure 6, the measured transmission spectraof some PbS - SiOzcomposite films of similar thicknesses but of different PbS content are shown. All films were grown on fused silica substrates. As expected, the range of transparency of the films strongly depends on the actual PbS content of the films and is shifted to longer wavelengths, i.e. lower photon energies with increasing PbS volume fraction. Because bulk lead sulfide is known to be a highly absorbing material in the visible spectral region, the very high transmission of these films in that spectral region cannotbe explained on the basis of a simple mixture of the involved two materials without any changes of the electronical properties in relation to the bulk. From the XPS analysis it is obvious that most of the PbS and of the Si@ still exist in their own chemical state also in the composite films. The absorption spectra estimated from the measured reflection and transmission spectra at room temperature showed smooth and featureless absorption edges without any indications of exciton absorption. According to (7), the absence of excitonic absorption structures may be attributed to two main reasons: first, the weak exciton binding energy due to the strong Coulomb screening in narrowgap semiconductors, and second, the existing size distribution of the nanocrystallites. The electronic properties, especially the width of the optical band gap and the nature of the transition occurring in the composite films, were investigated. PbS is known to be a narrowgap direct semiconductor with an optical gap energy of 0.41 eV.
EWOMED Pb!3NAWXYSTALLITES
3
4
6
5 Photon energy
141
[ev]
Figure 7. Taut plots for the direct transition (a l E)2 versus E of nanocrystalline PbS-Si& composite films of two different compositions. The linear parts of the plots were extrapolated to zero absorption to give the direct bandgap energies.
PbS volume fraction
Figure 8. Bandgap energy in dependence on the PbS volume concentration of as-deposited and some annealed films. The annealing was performed at 300°C for 3 hours.
142
R THIELSCH, T B&ME, R REICHE, D SCHLAFER, H-D BAUER AND Ii
BOTCHER
From the Taut plots (wE)~ - E, which are shown in Figure 7, for example, for two films of PbS volume fractions of 0.03 and 0.08 respectively, the electron transition at the fundamental absorption edge is found to obey the direct transition law also in the composite films in agreement with ( 11,14). The bandgap energies found from the extrapolation of the linear part of the plot to zero absorption were significantly higher than the corresponding bulk value of PbS. In Figure 8, the bandgap energies are given in dependence on the PbS volume concentration in the composite film for the as-deposited and in some cases for the annealed state. The bandgap was as high as 5.2 eV for the lowest PbS content and still about 3 eV for the film with a volume fraction of 0.2. For higher concentrations, the gap energies remained nearly constant due to the fact, that the PbS evaporation rate was only slightly larger during these deposition runs compared to the deposition of the film of 0.2 volume fraction PbS. As expected, annealing of the films results in a growth of the existing PbS crystallites, documented by the decrease of the bandgap energy. 1112.3. Photoluminescenceproperties Photoluminescence (PL) and photoluminescence-excitation (PLE) investigations have been performed with films in the composition range of PbS volume fraction from 0.02 to 0.35. For example, the PL spectra of two composite films with PbS volume fractions of 0.03 and 0.08 are
2x1 o6
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250
300
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350
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400
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450
500
550
600
Wavelength [m-n] Figure 9. The PLE - and PL-spectra of two composite films with PbS volume fractions of 0.03 and 0.08. The excitation wavelength was 3 11 run, which corresponds to a photon energy of 4 eV.
EVAPORATED PbS NANOCRYSTAUITES
143
shown in Figure 9. The excitation wavelength was 311 nm, which corresponds to a photon energy of 4 eV. All obtained spectra exhibit an intensive luminescence band located at a wavelength of about 435 nm (photon energy 2.85 eV). The intensity of this emission band depends on the the PbS volume fraction. As it is seen from Figure 9 , the intensity markedly decreases with increasing PbS concentration in the films. For composite films with PbS volume fractions larger than 0.2 no emission was measured. For lower concentrations, the wavelength position of the emission maximum was found to shift to higher wavelengths, i.e. lower photon energies with increasing PbS concentration: PbS concentration PL wavelength (nm) Photonenergy (ev)
0.02 432 2.87
* *
0.05 436 2.84
j *
0.08 446 2.78
Furthermore, a weak shoulder at 360 nm was observed for the films with the lowest PbS concentrations (0.02 ; 0.03 and 0.05). It should be noted that the wavelength of the light necessary for optimal excitation of the PL emission (i.e. highest PL peak intensity) was also dependent on the PbS volume concentration and therefore on lthe actual size of the PbS nanocrystallites (23). IH.3. Structural Properties and PbS CrystalliteSize In Figure 10, a typical SEM micrograph of the surface morphology of a PbS-Sio;? composite film deposited onto a silicon substrate is shown. The actual PbS volume content of this particular film was 0.08. Apart from only a few nodular artifacts (not shown in this micrograph), the surfaces of all films a~ppezmdsmooth and featureless.
Figure 10. SEM - micrograph of the surface morphology of a PbS-SiOr composite film deposited onto a silicon substrate. The actual PbS volume content of this particular film was 0.08.
144
R THIELSCH, T B~HME, R REICHE, D SCHLAFER, H-D BAUER AND H BOTCHER
~0,38
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.
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0,2
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Figure 11. Mean crystallite diameter of the PbS grains determined from the opticak absorption data and PbS volume concentrations of the films under consideration in dependence on the PbS:Si@ evaporation rate ratio. Based on the correlation between the optical band gap energy and the crystallite size of PbS nanocrystals given by the hyperbolic bond model of Wang et al. (7), the mean crystallite size of the PbS grains was determined from the optical absorption data. The results of this investigation are shown in Figure 11 in dependence on the PbS:Sio;! evaporation rate ratio. Also, the corresponding PbS volume concentrations of the films are shown. From these data it is evident that PbS nanocrystallites with mean dimensions as low asabout 1 nm can be grown by the applied method of interrupted island growth. Because the PbS:Si@ rate ratio is proportional to the deposited amount of PbS per single rotation, the PbS grain size increases with the deposited PbS layer thickness per rotation by a square root functional dependence. Similar growth laws were already reported to be valid for instance in the case of the growth of PbF2 crystallites in thicker evaporated films (33.34). To confirm the results of the size determination by the optical method, some attempts have been made to characterize the film structure of selected samples and to determine the mean grain size by X-ray diffraction and electron microscopy. From XRD, an as-deposited pure PbS tihn with a thickness of about 100 nm was found to exhibit a polycrystalline face centered cubic structure with the typical PbS lattice constant of 0.594 nm and a mean grain dimension of about 30 run. The most intensive peaks in the diffractogram resulted from reflections at the (11 l)-, (200)-, and (220)planes of the fee lattice. The asdeposited composite films were found to be X-ray amorphous if their PbS volume concentration was below 0.1. For a higher PbS content, PbS nanocrystallites of small size have been identified in the films by measuring of difference diffiactograms of coated versus uncoated glass substrates.
145
EVAPOMTEDP~S NANCCRYSTAUIES
500
T
pure P&-film
400
E z .z 300
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50 2 8
60
70
80
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600
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400
.z 2
200
0
W
I
3
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33
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70
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80
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Figure 12.The XIUI diffractogram of a pure PbS f&n of a thickness of about 100 nm (a) compared with the measured d%ractograms of an annealed composite film (c, (PbS) = 0.2), an uncoated glass substrate, and the caiculated difference diffractogram of the annealed composite film and the glass (b).
In Figure 12, the XFUIdiffractogmm of the pure PbS film (Figure 12a) and the measured diffractogram of an annealed composite film (cv (PbS) = 0.2) , the diffractogram of the uncoated glass substrate and the calculated differencediffsactagram(Figure 12b)are shown as an example.
146
R THIELSCH,T B&IME, R F&HE, D SWIAFEFI, H-D BAUERANDH BCITTCHER
TABLE 1 Comparison of the Size of PbS Nanocrystallites in Composite Films Determined by the Optical Method and from the XRD Diffraction Data of the As-deposited State and after Annealing at 300°C for 3 Hours PbS crystallite size as - deposited (nm)
PbS crystallite size annealedat 300°C. 3 hours (m)
PbS:SiqZ rate ratio
optical method
XRD
optical method
XRJI
0.58 0.95 1.1 1.4
1.5 1.8 1.7 1.8
1.0 1.2 1.7 1.7
1.6 2.0 1.8 1.8
1.5 2.8 2.0 3.1
The annealing was performed at a temperature of 300°C for a duration of 3 hours. Similar difference diffractograms were also obtained for unanneakl samples of sufftciently high PbS volume content. The most intensive diffraction peak in the difference spectra of the composite films was indexed to originate from the (200) planes of PbS. While annealing of the samples at 150°C for several hours did not give any measurcable changes in the microstructure, annealing at a temperature of 300°C for about 3 hours was found to improve the crystallinity of the films and to increase the dimension of the grains by thermal activated diffusion. In Table 1, results on the size of PbS nanocrystallites of composite films, determinedbytheopticalmethodandestimatedfrom theXBDd.iffmctiondataapplying Scherrer’s equation in the as-deposited state and after annealing at 300°C for 3 hours, are given. Investigations of composite films with PbS volume concentration of 0.13 and 0.2, respectively, by transmission electron microscopy verified the results on the microstructure received by XBD and optical analysis. In Figure 13, a bright field TEM photograph of the sample with cv(pbS) = 0.2 is shown. Due to the electron diffraction contrast, the dark spots in the picture are assumed to show PbS nanocrystals which are oriented in the so-called Bragg position. The upper part of the photograph shows the film in its nearly virgin state. The crystallite diameter was determined to be 2 to 3 nm. This sample area was only exposed by a low electron dose of about 106 e/nm2. The sample area shown in the lower part of Figure 13 was irradiated by a electron dose of about 10’ e/nm2, which was ten times larger than in the upper part The diameters of the PbS crystallites in this sample area were found to be about 10 nm and the number of crystal&s was decreased. To roughly explain thkexposure to this higher electron irradiation may increase the local temperature by more than about 100 K (35). In this picture, the micro areadiffraction pattern is inserted, which originates from a film area of about 30 nm in diameter. The electron scattering at the limited number of nanocrystallites results in different diffraction spots , which were arranged in a diffraction ring pattern. By computer simulation using the code ELDISCA (36) this diffraction pattern was identified to
EVAMATED PbS NAJKMYSTAUITES
147
Figure 13. TEM bright field photograph from a composite film with PbS volume concentration of 0.2. Upper part: nearly virgin state due to an only low exposure to the electron beam (the crystallite size is 2 to 3 nm). Lower part: grain coarsening to about 10 nm in diameter due to stronger electron irradiation. The micro area electron diffraction pattern taken from an area of about 30 nm lateral diameter is inserted.
originate from the lattice planes of cubic lead sulfide with a lattice constant of 0.594 nm. In Figure 14, the calculated and measured diffraction pattern are shown for comparison.
IV. CONCLUSIONS
The method of interrupted island growth was utilized to form PbS nanocrystallites in a dielectric SiQ! host using a two-source evaporation technology. From the correlation between the optical band gap energy and the crystallite size of PbS nanocrystals, the mean crystallite size of the PbS grains was determined from tbe optical absorption data. Depending on the deposited lead sulfide lalyer per rotation, nanocrystallites with mean dimensions as low as 1 nm in diameter have been formed, which show a significant increase in the bandgap from the bulk value of 0.41 eV up to about 5.2 eV. The crystallite size obeys a square root growth law with the deposited lead sulfide layer per rotation. XPS, XRD and TEM investigations were performed to verify the
148
R THIELSCH. T B&ME. R REICHE,
( 4 c 2 13 c 2
D SCHLAFER, H-DBAUER AND H ~TTCI-IEFI
0 0) 2 2) 11) 0 0)
Figure 14. Comparison of the calculated (above) and measured (below) diffraction fringe pattern. The Miller indices of the lattice planes of cubic lead sulfide, which contribute to this pattern, are given.
formation of PbS nano-crystal&s in the Si& host. From the analysis of the chemical bonding of the lead atoms in the composite films by XPS, it was obvious that most of the Pb atoms were bonded to sulphur to form PbS, but also minor quantities of PbG exist in the films together with a small amount of non-bonded metallic lead, which result from the ion bombardment induced decomposition of the lead compounds. Size related shifts of lead and sulphur electronic states were experimentally proved. From XRLI and micro area electron diffraction pattern the lattice constant was found to agree with that of the bulk PbS. By photoluminescence investigations, all obtained spectra exhibit an intensive luminescence band located at a wavelength of about 435 nm (photon energy 2.85 eV). The intensity of this emission band depends on the the PbS volume fraction. No emission was obtained for samples with PbS volume concentration higher than 0.2.
ACKNOWLEDGMENT This work was partially supported by the Sachsisches Staatsministerium fur Wirtschaft und Arbeit under contract no. 796/l%. T. Boehme also wishes to acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG Fr 1097/l-2). REFERENCES 1. 2. 3.
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EVAPORATED PbS NANOCRYSTALLITES
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