Quantum confinement effect in multilayer structure of alternate CdSe and SiOx insulator matrix thinfilms

Superlattices and Microstructures 58 (2013) 154–164

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Quantum confinement effect in multilayer structure of alternate CdSe and SiOx insulator matrix thinfilms M. Melvin David Kumar ⇑, Suganthi Devadason Thin Film Laboratory, Department of Physics, Karunya University, Coimbatore 641 114, India

a r t i c l e

i n f o

Article history: Received 16 February 2013 Received in revised form 21 March 2013 Accepted 22 March 2013 Available online 1 April 2013 Keywords: CdSe/SiOx Multilayer structure Quantum confinement Thermal evaporation

a b s t r a c t Multilayer (ML) structure of layer-by-layer deposited CdSe/SiOx thin films and their monolayers were prepared using sequential thermal evaporation technique. X-ray diffraction study confirmed the (002) plane of CdSe with wurtzite structure. It is noticed that the microstrain, developed in ML thin films, increased with decreasing particle size. Experimentally measured band gap energies confirmed the splitting of valence band energy levels which rise due to hole confinement in CdSe. Crystallite sizes (5–7 nm) were calculated using the effective mass approximation model (i.e., Brus model) which shows that the diameter of crystallites was smaller than the Bohr exciton diameter (11.2 nm) of CdSe. The main band in the emission spectra of ML samples gradually shifted to longer wavelength side when particle size was increased from 5 to 7 nm. This is characteristic of quantum size effect. It is inferred that disorderliness in CdSe/SiOx ML thin films would increase when the thickness of CdSe sublayer is greater than that of SiOx matrix layer. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The study of nanocrystals of II–VI semiconductor materials such as CdSe, ZnSe, CdTe embedded in an insulator matrix like SiOx or in a strained system [1–3] is of current importance owing to their potential applications in the field of electronics, nanotechnology, medicine and engineering. Significant work has been done in analyzing the properties of CdSe/SiOx systems due to their applications in ⇑ Corresponding author. Tel.: +91 9994720997; fax: +91 4222615615. E-mail address: [email protected] (M. Melvin David Kumar). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.03.016

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visible and near infrared regions where the insulator matrix is transparent [4–6]. Moreover, as surface valleys are present in SiOx matrix with a length up to several hundreds of nanometers, nanoclusters of semiconductor materials such as CdSe, ZnSe may be formed on the surface [7]. In addition, photoluminescence of CdSe nanocrystals is interesting on account of their changing emission wavelength with changing nanocrystallite size [8,9]. CdSe offers the possibility of studying quantum confinement effects in higher cluster size regimes as it is having large Bohr exciton diameter [10–12]. Mechanism of nanocluster formation, band edge absorption spectroscopy, photoluminescence and photoreflectance properties of CdSe/SiOx multilayer (dSiOx > dCdSe) systems have already been studied by various research groups [13–16]. Preparation of Multilayer (ML) thin films showed that the layers deposited in a step-by-step manner were smoother than those made in one step [17]. This article deals with CdSe/ SiOx ML samples consisting of thin layers of CdSe sandwiched between SiOx matrix layers in alternate manner prepared by varying layer thickness. Sublayer thickness can be controlled precisely during the deposition process using digital thickness monitor (DTM) in the physical vapor deposition method. In this paper, we mainly emphasize the impact of layer thicknesses of CdSe and SiOx on quantum size effect. Also we report the (i) results of CdSe/SiOx ML (dCdSe > dSiOx) thinfilms, (ii) optical properties of CdSe/SiOx ML thinfilms with direct and indirect electronic transitions, (iii) blue shift in emission spectra of ML samples and (iv) the quantum confinement effect of CdSe nanocrystals in connection with the sublayer thicknesses of CdSe and SiOx matrix layers.

2. Experimental details Nanocrystalline ML thin films of CdSe/SiOx were prepared by consecutive thermal evaporation of CdSe and SiOx (99.99% Sigma Aldrich Chem. Co.,) materials from two independent molybdenum crucibles in vacuum at a pressure of 5  105 mbar. The thickness of sublayers and the deposition rate of both the materials were measured during deposition by calibrated DTM. The glass substrates were fixed at the greatest possible distance from the source materials. A step -by- step procedure was applied in the deposition of each sub layer in the multilayer structures. Single layer and multilayer samples of CdSe and SiOx materials were prepared for analysis. Alternate layers of CdSe and SiOx materials (six layers each) were coated with thicknesses of 5 and 50 nm respectively in the first sample S1. Then the thickness of CdSe sublayers was increased to 10 nm and the thickness of matrix layers was maintained at 50 nm in sample S2. In both the samples (S1 and S2), the thicknesses of SiOx matrices are greater than that of CdSe sublayers ie., ten times in S1 and five times in S2. We decided to increase the CdSe layer thickness more than the thickness of SiOx matrix layer in sample S3 so as to analyze the changes that took place in structural and optical properties of CdSe/SiOx multilayer samples with respect to layer thickness could be analyzed. The top layer in samples S1, S2 and S3 is coated with CdSe material to analyze the surface morphology of the samples. Details of layer thicknesses, materials and the number of layers in samples S1, S2 and S3 are given in Table 1. The structural properties of the samples were analyzed by X-ray diffractometer (Shimadzu 6000), Scanning Electron Microscopes (FEI Quanta FEG 200 and JEOL – JSM 6390) and Energy Dispersive analysis of X-rays (EDX). The absorption and emission properties of the samples were studied by using UV–Vis spectrophotometer (JASCO V – 550) and Photo Luminescence spectrometer (FLUOROLOG – 3 HJY) respectively.

Table 1 Details of the preparation of CdSe/SiOx multilayer thin film samples. S. no.

1 2 3 4 5

Name of the sample

Total number of layers in the sample

Order of sublayers

Thickness of sublayers (Å)

Bottom layer

Top layer

SiOx

CdSe

CdSe SiOx S1 S2 S3

1 1 12 12 12

CdSe SiOx SiOx SiOx SiOx

– – CdSe CdSe CdSe

– 1000 500 500 200

500 – 50 100 300

Total thickness of the film (Å) 500 1000 3300 3600 3000

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3. Results and discussion 3.1. Structural properties The average crystallite size and the nanocrystalline phases of CdSe/SiOx multilayer thin films, coated under specific conditions, were calculated from X-ray diffraction spectra (Fig. 1). Prominent peak in the XRD spectra of the samples appear approximately at 25° which refers [JCPDS-02-0330] to (0 0 2) plane of the CdSe with wurtzite structure. From Fig. 1, it is clearly seen that the diffraction peak gets sharpened from samples S1 to S3 when thickness of CdSe sublayer increases from 5 to 30 nm. As mentioned earlier, surface of SiOx matrix layer consists of surface valleys and chains. Depth of these valleys can be raised by increasing the layer thickness of SiOx matrix [13]. When thin layer of CdSe material is coated in between SiOx matrices, CdSe particles may be found buried partially or completely inside the surface wells of SiOx matrix depending upon its depth. The layer thickness of SiOx matrices is ten times greater than that of CdSe sublayer in sample S1 and five times greater in sample S2. Therefore it is expected that thin layers of CdSe nanocrystals may be filled in the surface wells of relatively thick (50 nm) SiOx matrix layer. This will lead to the formations of nano-sized regions on the surface of SiOx matrix [18]. This results in broad peaks in XRD spectra of the samples S1 and S2. But in sample S3, CdSe sublayers of 30 nm thickness cannot be hidden completely in the surface wells of 20 nm thick SiOx matrix layers. As a result, the XRD peak in the sample S3 becomes sharp. In order to calculate the average size of nanocrystallites, the Debye–Scherrer’s equation

D¼

0:94k b cos h

ð1Þ

Fig. 1. X-ray diffraction spectra of CdSe/SiOx multilayer thin film samples S1, S2 and S3 with (0 0 2) and (1 1 0) planes of CdSe with wurtzite structure.

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Sample

2h (deg)

d (Å)

FWHM (deg)

Strain (e)

D (nm)

1 2 3

S1 S2 S3

24.2 25.35 25.4251

3.67477 3.59012 3.50041

4.6 3.5 0.3323

0.04993 0.02574 0.00011

2 2.5 25

was employed. In this equation, h and b are the position and FWHM of the prominent peak. Crystallite sizes were calculated as 2, 2.5 and 25 nm for the samples S1, S2 and S3 respectively as given in Table 2. As inferred from Table 2, broadness of the peaks in the XRD profile of samples S1 and S2 indicates that the size of the particles is very small. One can expect quantum size effects in this range of crystallite sizes [7]. This result agrees well with the results reported in literatures [13,18]. Microstrain is one of

Fig. 2. Scanning Electron Microscope images of CdSe/SiOx ML sample S1 showing layer arrangement of CdSe and SiOx (a) and agglomerated particles on the surface of sample S3 (b).

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Fig. 3. EDX spectra showing composition ratios of CdSe and SiOx in ML sample S1.

the causes for quantum size effects in multilayer system due to heap arrangement [15]. Therefore, approximate level of microstrain, developed in CdSe/SiOx ML thinfilms, were calculated using the following relation [19],

 2  2 b cos h 1 sin h ¼ 2 þ 16e2 k k d

ð2Þ

where e is the microstrain of prepared CdSe/SiOx ML samples. The strain observed in the samples S1, S2 and S3 was obtained as 49  103, 28  103 and 0.11  103 respectively and are given in Table 2 which proves that increasing microstrain in CdSe layers decreases crystallite sizes. The value of microstrain is comparatively small in sample S3 where dCdSe > dSiOx, and hence the size of the particle in sample S3 exceeds the Bohr exciton diameter (11.2 nm) of CdSe [20]. Surface images of ML samples S1 and S3, recorded using Scanning Electron Microscope (SEM), are shown in Fig. 2a and b respectively. The surface of the sample S1 appears smooth due to the presence of concealed particles on the surface. The layer formation of CdSe and SiOx matrix is clearly shown in Fig. 2a. It is seen that SiOx layers are relatively thicker than CdSe sublayer. In Fig. 2b, particles are found agglomerated due to increasing CdSe sublayer thickness, getting thicker than SiOx matrix. The elemental percentage compositions of CdSe and SiOx materials in ML sample were estimated from EDX spectra (Fig. 3). Though the XRD and SEM methods reveal the morphological properties of the multilayer systems with some approximations, the phenomena like spin-orbit coupling, splitting of valence bands, energy level shifting, etc., could not be characterized by these techniques. Therefore, confinement effects of CdSe/SiOx systems have been usually discussed with optical absorption and emission data [21–23]. 3.2. Optical properties CdSe/SiOx multilayer thin film shows interesting and peculiar properties in absorption and emission spectra due to its type II band structure alignment [14,15,24]. As the electron mobility in SiOx matrix layers is very low due to large number of defects in them, the carrier (electrons and holes) transport takes place only through CdSe sublayers exclusively [25,26]. From Fig. 4, it is known that holes are confined inside nanocrystals whereas electrons are tunneling through SiOx matrix. For this reason, electrons make direct and indirect transitions between energy states of CdSe and SiOx which are spatially separated as shown in Fig. 4. Since the excited electrons are probably trapped in the traps of SiOx matrices, size dependent shift in absorption edges of CdSe/SiOx ML thinfilms is caused by hole

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Fig. 4. Real space band diagram of CdSe/SiOx multilayers with direct and indirect electronic transitions [14,15].

Fig. 5. Absorption spectra of reference SiOx and CdSe single layer thin films with layer thickness of 100 and 50 nm respectively.

confinement in CdSe nanocrystals [14]. Absorption spectra of single layer and multilayer samples of CdSe and SiOx materials are shown in Figs. 5 and 6 respectively. Absorption spectra of CdSe/SiOx ML samples are more structured compared to the absorption spectra of individual films. It was expected that there would be a red shift in the absorption edges of ML samples as the thickness of CdSe sublayer increases. Instead of that, absorption edge of sample S2 is slightly shifted towards longer wavelength side compared to the absorption edge of S3. Typically absorption edge gets related to transition that takes place between the highest valence band and the lowest conduction band of CdSe semiconductor [27]. Here, the energy equivalent to the absorption edges of the samples S2 (1.54 eV) and S3 (1.6 eV) is lesser than the band gap of bulk CdSe (1.74 eV) [18]. Thus, these transitions corresponding to absorption edges of S2 and S3 could not have taken place between valence and conduction band

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Fig. 6. Absorption Spectra of CdSe/SiOx ML samples indicating the blue shifts by arrow mark in the absorption onset.

energy levels of CdSe nanocrystals. As these values are close to the transition which is marked by dotted line in Fig. 4, the absorption edges in samples S2 and S3 arise due to the indirect transition of electrons from the valence band of CdSe to the conduction band of SiOx. Furthermore, next to the absorption edge, towards lower wavelength side, there is a shift or shoulder (marked by arrow mark in Fig. 6) observed in the absorption onset of all the ML samples. This might be due to the electron transitions between valence and conduction bands of CdSe nanocrystals and due to the splitting of valence band in CdSe [14,23]. The shift in the absorption onset of ML samples correlate well with thickness of CdSe sublayers, i.e., gradual red shift in this region is observed from 550 to 630 nm in ML samples S1, S2 and S3 as the sublayer thickness of CdSe increases from 5 to 30 nm. However the quantitative analysis to characterize the band-to-band electronic transitions is carried out with the optical absorption and transmission spectra of the prepared CdSe/SiOx ML films. Absorption co-efficients of the prepared ML samples can be calculated from the experimentally measured optical transmission values (T) as follows,

aðhmÞ ¼ d1 lnðTðhmÞÞ1

ð3Þ

where d is the thickness of the thin films noted from DTM. The band gap energies of CdSe/SiOx ML samples S1, S2 and S3 were calculated by plotting a typical graph between energy (hm) and (ahm)2, as shown in Fig. 7a and b, by using the following relation [27],

ðahv Þ2 ¼ Aðhm  Eg Þn

ð4Þ

In Eq. (4), A is a constant which arises from Fermi’s golden rule for band-to-band electronic transitions. The calculated band gap energy (Eg) values for ML samples S1, S2 and S3 were 2.24, 2.11 and 1.96 eV respectively. Absorption coefficients could not be measured for other transitions observed in Fig. 7a and b as they are not in between valence and conduction bands of CdSe. Calculated band gap values (Eg) are higher than the optical band gap (Eg(bulk)) of bulk CdSe which is 1.74 eV at room temperature [18]. For precise predictions, the size of the particles were calculated from the shift in energy gap using Brus effective mass approximation as given, 2

Eg ¼ EgðbulkÞ þ

2h

p2

ld2

ð5Þ

where Eg(bulk) is the band gap of bulk CdSe, d is the diameter of the crystallites and l is the effective mass of electron-hole pair given by,

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1

l

¼

1 1 þ me mh

161

ð6Þ

where me and mh are effective mass of electron and hole respectively. The average sizes of the nanocrystallites calculated using Eq. (5) were 5, 6 and 7 nm for samples S1, S2 and S3 respectively. It is

Fig. 7. The plots of (hm) Vs (ahm)2 graphs for the samples S1 (a) and S2 and S3 (b) showing the electronic transition between valence and conduction bands of CdSe nanocrystals.

Table 3 Size of the crystallites calculated from UV–Vis data by Brus model. S. no.

Sample

Band gap (Eg) eV

Shift in band gap (Eg  Eg(bulk)) eV

d (nm)

d/dB

1 2 3

S1 S2 S3

2.24 2.11 1.96

0.50 0.37 0.22

5 6 7

0.45 0.54 0.63

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obviously seen from Table 3 that the band gap energies (Eg) increase with decreasing layer thickness of CdSe. More over it is evident that the nanocrystallite sizes (d) of the CdSe/SiOx ML samples are much smaller than the Bohr exciton diameter (dB) of CdSe (11.2 nm). If d/dB  2, the film exhibits a single particle confinement behavior in which electrons and holes are independently confined [28]. Hence, the prepared CdSe/SiOx ML samples confirm the strong quantum confinement effect. The discrepancy between crystallite sizes calculated using X-ray diffraction data and effective mass approximation is less in ML samples S1 and S2. This less discrepancy is caused by existing microstrain in multilayer systems as explained in Section 3.1 and deformations in the nanocrystal arrangement [19]. But the discrepancy in size calculations in sample S3 is comparatively larger, i.e., particle size calculated using Bragg’s equation is 25 nm and using effective mass approximation is 7 nm (see Tables 2 and 3). Previously, 50–60% discrepancies in the particle size calculation have been reported in CdSe/ SiOx ML system [29]. In order to explain this observed discrepancy one should consider that X-ray diffraction peak reflects the properties of only few layers from top in the ML system. In sample S3, the top layer is coated with relatively thick CdSe (30 nm) material. As this top layer particles could not be hidden completely in 20 nm thick SiOx layer, XRD peak becomes sharp and as a result, the diameter of the particle becomes large. But in UV–Vis spectrophotometer, all the sublayers are analyzed from top to bottom surface. Hence top layer alone cannot influence much in the optical properties CdSe/SiOx ML system. Moreover, in sample S3, all the CdSe sublayers are sandwiched in between SiOx matrix layers of 20 nm thick each, except the top CdSe layer. This may lead to confinement effect which increases the band gap energy. Photoluminescence of CdSe nanocrystals embedded over SiOx matrix can be studied only if the matrix layer is transparent in the region of band and sub-band absorption of CdSe nanocrystals [13]. On comparing the absorption spectra of SiOx single layer thinfilm and CdSe/SiOx ML thinfilms (Figs. 5 and 6), it is noted that SiOx matrix film is completely transparent in the region of band absorption (k > 500 nm) of CdSe nanocrystals and it does not have any peculiarities at lower wavelength sides.

Fig. 8. Photoluminescence spectra of CdSe/SiOx ML samples.

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Therefore, peculiarities observed in Fig. 6 at energies greater than the band gap of bulk CdSe may be related to the electronic transitions between different energy levels of valence and conductions bands of CdSe. Fig. 8 shows the photoluminescence spectra of CdSe/SiOx ML samples S1, S2 and S3. Apparent differences in structure, intensity and FWHM are seen in the emission spectrum of sample S3 compared with other two spectra. There is a gradual blue shift observed in the emission band of ML samples with decreasing crystallite size from 7 to 5 nm. Due to low level surface defects and low surface state concentration in ML samples S1 and S2, FWHM and intensity of photoluminescence spectra are large [30,31]. Moreover, FWHM of samples S1 and S2 are larger than 200 meV, which is far larger than that of ordinary bulk semiconductor sample. The main reason of this phenomenon is the size fluctuation of the quantum dots [32]. There is a splitting of the emission maximum observed in sample S3 and this may be due to the recombination through surface defect states. Here, we conclude that the increased thickness of CdSe more than SiOx leads to increased surface defects and thereby enhanced particle size. 4. Conclusion Alternate layers of CdSe and SiOx were coated by varying sublayer thicknesses. Complexities in the structural and optical properties of ML samples where the layer thickness of CdSe is greater than that of SiOx are explained. Spin-orbit splitting of valence band (identified from the absorption spectra of the samples) and electronic transitions (calculated from (ahm)2 Vs (hm) relation) are related with the sublayer thicknesses of both the materials. Size of the crystallite in sample S3 is comparably high and thus the emission peak is red shifted. We conclude that the sublayer thickness and the size of the particle play an important role in quantum confinement effect and energy gap increases with increasing sub layer thickness of SiOx. Further studies will be carried out in order to explain the interdot coupling between the quantum dots through tunneling effect in the multilayer strained systems. Acknowledgments The authors are thankful to the Management and the Central Research Facility of Karunya University, Pondicherry University and Sophisticated Analytical Instrumentation Facility, Indian Institute technology for their support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

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