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Effect of thickness on structural, optical, electrical and morphological properties of nanocrystalline CdSe thin films for optoelectronic applications
Optical Materials xxx (2015) xxx–xxx
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Effect of thickness on structural, optical, electrical and morphological properties of nanocrystalline CdSe thin films for optoelectronic applications Anuradha Purohit a, S. Chander a, S.P. Nehra b, C. Lal c, M.S. Dhaka a,⇑ a b c
Department of Physics, Mohanlal Sukhadia University, Udaipur 313001, India Centre of Excellence for Energy and Environmental Studies, Deenbandhu Chhotu Ram University of Science and Technology, Murthal, Sonepat 131039, India Department of Physics, University of Rajasthan, Jaipur 302004, India
a r t i c l e
i n f o
Article history: Received 24 January 2015 Received in revised form 29 May 2015 Accepted 30 May 2015 Available online xxxx Keywords: CdSe thin films Physical properties Thickness Annealing Thermal evaporation
a b s t r a c t This paper presents effect of thickness on the physical properties of thermally evaporated cadmium selenide thin films. The films of thickness 445 nm, 631 nm and 810 nm were deposited employing thermal evaporation technique on glass and ITO coated glass substrates followed by thermal annealing in air atmosphere at temperature 300 °C. The as-deposited and annealed films were subjected to the XRD, UV–Vis spectrophotometer, source meter, SEM and EDS to find the structural, optical, electrical, morphological and compositional analysis respectively. The structural analysis shows that the films have cubic phase with preferred orientation (1 1 1) and nanocrystalline nature. The structural parameters like inter-planner spacing, lattice constant, grain size, number of crystallites per unit area, internal strain, dislocation density and texture coefficient are calculated. The optical band gap is found in the range 1.69– 1.84 eV and observed to decrease with thickness. The electrical resistivity is found to increase with thickness for as-deposited films and decrease for annealed films. The morphological studies show that the as-deposited and annealed films are homogeneous, smooth, fully covered and free from crystal defects like pin holes and voids. The grains in the as-deposited films are densely packed, well defined and found to be increased with thickness. Ó 2015 Published by Elsevier B.V.
1. Introduction The thin films of II–VI compound semiconductors group attract more attention due to their wide range of applications in the field of optoelectronics like photo-detectors [1,2], gas sensors [3], light emitting diodes (LEDs) [4,5], thin film transistors [6], optical wave guides and solar cells [7–9]. Among this group, cadmium selenide (CdSe) is a well-known compound semiconductor having direct band gap 1.74 eV at room temperature, high absorption coefficient in the visible range and often possess n-type conductivity [10]. The discrete electronic energy levels in the microscopic structure of nanocrystalline CdSe films are found due to quantum confinement effect [11]. These films have larger carrier mobility as compared to the most commonly used silicon films [12]. Recently, more attention has been paid to the development of low cost and high quality thin films for optoelectronic device applications. The CdSe thin ⇑ Corresponding author. E-mail address: [email protected] (M.S. Dhaka).
films are also considered as important material for photovoltaic applications because these may be used as absorber layer on the top of the tandem solar cells [13]. The CdSe thin films may be deposited employing a number of techniques like thermal evaporation [3,10], chemical bath deposition [14,15], electro-deposition [16–19], hot wall deposition [20], spray pyrolysis [21], SILAR [22], etc. These techniques have their own merits and demerits. Among these techniques, the thermal evaporation technique is found more suitable and useful due to high deposition rate, low consumption of material and economical way of deposition process. This technique has been used for many years to fabricate thin films of compound semiconductors, chalcogenide semiconductors and other kind of materials. The optimization of physical properties plays an important role for effective use of CdSe thin films in micro- and optoelectronic applications. For optoelectronic applications, a transparent substrate is required to enhance emitted light by the active area of thin films. Yadav et al. [23] reported the photo-electrochemical properties of CdSe thin films using spray pyrolysis at different substrate
http://dx.doi.org/10.1016/j.optmat.2015.05.053 0925-3467/Ó 2015 Published by Elsevier B.V.
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temperature. They found that the as-deposited films showed n-type conductivity and had a spectral sharp peak at wavelength 725 nm. The hexagonal structure of CdSe films with preferred orientation (0 0 2) was investigated by Kissinger et al. [24] employing electron beam evaporation. The polycrystalline nature with hexagonal phase of cadmium selenide thin films was observed by Pandiyan et al. [25]. They found that the crystallinity was increased with substrate temperature while the band gap energy was decreased. A study of electrical properties of thermally evaporated CdSe thin films was undertaken by Aneva et al. [26]. They observed that the dark and photo conductivity were increased with film thickness. Recently, Purohit et al. [27] reported the effect of air annealing on the physical properties of CdSe thin films using thermal evaporation deposition and found that the crystallinity and band gap energy were varied with annealing temperature. The CdSe and Al:CdSe semiconductor thin films were synthesized on amorphous and FTO glass substrates by Gawali and Bhosale [28] employing spray pyrolysis technique. They observed that the films were polycrystalline in nature with hexagonal phase and optical band gap was varied in the range 1.67–1.87 eV. Hence, the physical and chemical properties of CdSe thin films to find new applications are strongly depended upon preparation conditions, deposition techniques, heat treatment, substrate, doping, film thickness and substrate temperature. The heat treatment may be performed in air and gaseous medium like N2, H2, Ar as well as in vacuum. The crystal structure quality is an important factor which affects the physical properties of thin films. Thorough literature survey reveals that there is a need to investigate the effect of thickness on the physical properties of CdSe thin films. Therefore, a study on effect of thickness on the physical properties of CdSe thin films is undertaken in this paper. The CdSe thin films were deposited employing thermal evaporation technique on glass and ITO coated glass substrates followed by thermal annealing in air atmosphere at temperature 300 °C. The films were subjected to the XRD, UV–Vis spectrophotometer, source meter, SEM and EDS. The crystallographic and optical parameters are calculated and discussed. Generally, the crystallinity is increased with film thickness but the present study warrants that the crystallinity of CdSe thin films may be reduced with thickness owing to stacking of smaller grains on larger grains. Stringent relative results are observed for as-deposited and annealed films which seems absent in the earlier work.
2.2. Characterization The structural properties of as-deposited and annealed CdSe thin films were carried out employing X-ray diffractometer (Bruker AXS D8 Advance) of CuKa radiation in the 2h range from 20° to 60°. The UV–Vis spectrophotometer (SHIMADZU UV-2450) was used to undertake the optical properties within the wavelength range 200–800 nm. The electrical properties were carried out using source meter (Agilent B2901A). The surface morphological and compositional studies of as-deposited films were performed by scanning electron microscope (Nova Nano FE-SEM 450) coupled with energy dispersive spectroscopy (EDS). The surface morphology of annealed films was taken using SEM (Zeiss EVO 18).
3. Results and discussion 3.1. Structural analysis The X-ray diffraction patterns of as-deposited CdSe films of thickness 445 nm, 631 nm and 810 nm are shown in Fig. 1. The XRD pattern of the as-deposited films of thickness 445 nm shows a sharp diffraction peak at angular position 25.43° corresponding to the prominent reflection (1 1 1). The error and trial method was used to identify the reflections. The prominent reflection (1 1 1) is found to be well indexed with the standard JCPDS data file 19-0191 [29] and films have zinc-blende cubic structure with nanocrystalline nature. The formation of nanocrystalline films may be attributed to decrease of grain growth region and increase of nucleation centers [30]. The angular position of prominent peak is observed to shift toward higher and lower sides for films of thickness 631 nm and 810 nm respectively owing to decrease and increase in the corresponding lattice constants. It is also observed that the sharpness and intensity of the prominent reflection (1 1 1) is found to be increased with film thickness which revealed an improvement in the crystallinity owing to decrement in disorderness. Despite the fact, the crystallinity of film of thickness 631 nm is found to decrease as compared to the film 445 nm which may be attributed to the stacking of smaller grains on the larger grains. The results are well agreed with the earlier reported work [14,24,31–33]. The structural parameters like
2. Experimental details 2.1. Deposition of CdSe films The CdSe thin films were deposited on chemically cleaned glass and ITO coated glass substrates employing thermal evaporation technique. The glass substrates were used to undertake the structural, optical, morphological and compositional analysis while the ITO coated glass substrates for electrical analysis. The CdSe powder of purity 99.99% was procured from Alfa Aesar. The substrates of dimension 75 mm 75 mm 1 mm were cut by diamond cutter in the size of 10 mm 10 mm 1 mm. The substrates were cleaned using distilled water, acetone followed by ethyl alcohol. The CdSe powder was pelletized with the help of dye and a molybdenum boat was used to keep the pellet. The rotary and diffusion pumps were used to evacuate the vacuum coating unit up to a pressure 106 torr. The films of thickness 445 nm, 631 nm and 810 nm were prepared and the thickness was also verified by ellipsometry (Woolam M2000). The as-deposited thin films were subjected to air annealing at temperature 300 °C in a furnace (Metrex Muffle) for a period of one hour.
Fig. 1. The X-ray diffraction patterns of as-deposited CdSe thin films.
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inter-planner spacing (d) and lattice constant (a) were calculated using Bragg’s diffraction law and relation concerned [34,35].
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 a¼d h þk þl
ð1Þ
Here, h, k and l are Miller indices. Table 1 depicts the structural parameters like inter-planner spacing (d), lattice constant (a), grain size (D) and number of crystallites per unit area (N) for as-deposited films. It is seen from Table 1 that the lattice constant is varied in the 0
range 6.046–6.065 Å A corresponding to prominent peak (1 1 1) and found to be increased with thickness owing to change in density as well as the nature of native imperfection of the films. The calculated inter-planner spacing is found to be slightly varied from 0
0
3.495 Å A to 3.506 Å A with thickness. The grain size was calculated using Debye–Scherrer formula [36].
Dhkl ¼
kk bCosh
ð2Þ
Here, k is Scherrer constant having value 0.94, h is Bragg’s angle, k is the wavelength of the X-ray used, b is the full width at half maxima (FWHM) of the peak intensity. 0
The grain size is found in the range 306.03–331.79 Å A and observed to decrease for thickness 631 nm owing to the decrease in crystallinity which may be attributed to the formation of smaller grains on the surface of larger grains [37]. It is found to increase for thickness 810 nm which revealed the coalescence or aggregation of small nanocrystalline particles and variation in corresponding FWHM of the prominent reflection [10]. The large grain size indicated the high smoothness of the films deposited on the surface of substrate. The number of crystallites per unit area (N) was calculated [36].
N¼
t
Fig. 2. The X-ray diffraction patterns of CdSe thin films annealed at temperature 300 °C.
Table 2 The structural parameters of annealed CdSe thin films of different thickness. Samples annealed at 300 °C
2h (°)
445 nm 631 nm 810 nm
25.43 25.43 25.44
(h k l)
111 111 111
a (Å A)
D (Å A)
N 1015 m2
6.051 6.051 6.048
302.66 297.57 271.13
16.0 23.0 40.6
0
0
d (Å A) Obs.
Stand.
3.498 3.498 3.496
3.51 3.51 3.51
0
ð3Þ
D3
Here, t is thickness of deposited thin films. The number of crystallites per unit area is found in the range (14.0–22.1) 1015 m2 and observed to increase with thickness owing to variation in corresponding grain size and film thickness. The X-ray diffraction patterns of the annealed films are shown in Fig. 2. Similar to the as-deposited thin films, sharp and high intense prominent reflection is observed at angular position 25.43° corresponding to the plane (1 1 1) for thickness 445 nm which is well agreed with the standard data [29] and the films have zinc-blende cubic structure with nanocrystalline nature. The angular position of prominent reflection is observed at 25.43° and 25.44° for films of thickness 631 nm and 810 nm respectively. It is slightly increased with thickness due to decrease in corresponding lattice constant. The sharpness and intensity of the prominent peak is observed to decrease with thickness while the broadening of the peak increase which revealed the reduction in crystallinity due to increment in the corresponding FWHM and increase in the internal strain of the material. The results are well supported by the earlier reported work of Kale and Lokhande [32]. The
structural parameters for annealed films are calculated and tabulated in Table 2. The lattice constant of annealed films is varied in the range 0
6.048–6.051 Å A corresponding to (1 1 1) plane and found to slightly decrease with film thickness which may be attributed to the shift of the angular position toward higher side. The inter-planner spac0
0
A and observed to decrease ing is found between 3.496 Å A and 3.498 Å with thickness. The grain size is found in the range 271.13– 0
302.66 Å A and decrease with thickness owing to increase in corresponding FWHM and disorder. The number of crystallites per unit area is found in the range (16.0–40.6) 1015 m2 and observed to increase with thickness. The dislocation in a film is an imperfection in the crystal created during growth. The strain (e) in thin films is defined as the disarrangement of lattice and was calculated using relation concerned [38].
e¼
b 4tanh
ð4Þ
Table 1 The structural parameters of as-deposited CdSe thin films of different thickness. Samples as-deposited
2h (°)
(h k l)
445 nm 631 nm 810 nm
25.43 25.46 25.38
111 111 111
0
0
d (Å A)
D (Å A)
N 1015 m2
316.75 306.03 331.79
14.0 22.0 22.1
0
a (Å A)
Obs.
Stand.
Obs.
Stand.
3.498 3.495 3.506
3.51 3.51 3.51
6.051 6.046 6.065
6.05 – –
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Fig. 3. The strain and dislocation density of (a) as-deposited and (b) annealed CdSe thin films.
The dislocation density (d) was calculated using Williamson Smallman’s relation [36].
d¼
1
ð5Þ
D2
The strain and dislocation density of as-deposited and annealed films corresponding to prominent reflection (1 1 1) as a function of film thickness are presented in Fig. 3. It is visible in Fig. 3(a) that the dislocation density and internal strain are found in the range (0.90–1.06) 1011 cm2 and 0.284– 0.307 respectively. Both these parameters are found in the range (1.09–1.36) 1011 cm2 and 0.3115–0.3472 respectively for the annealed films (Fig. 3b). The internal strain is found to increase with thickness for annealed films and decrease for the as-deposited film excluding the thickness 631 nm. A similar behavior of internal strain and dislocation density was also observed by Velumani et al. [39]. The increasing strain shows compressive nature and indicates the formation of smaller grains on the surface of larger grains and decreasing strain shows tensile nature and may be attributed to the improvement in crystallinity which is visible in Fig. 3a for thickness 810 nm while found compressive for thickness 631 nm. The dislocation density of the as-deposited films is varied with thickness and observed to increase for annealed films owing to decrement in grain size. The texture is a non-random distribution of crystal orientation and texture coefficient (TC) presents the texture of a particular plane which implies the preferred growth due to deviation from unity [40]. It was calculated using relation concerned [41] and shown in Fig. 4. Iðh k lÞ I ðh k lÞ Iðh k lÞ N I0 ðh k lÞ
TCðh k lÞ ¼ P0 1 N
ð6Þ
Here, I(h k l) is the measured intensity, I0(h k l) is the JCPDS intensity and N is the reflection number. The texture coefficient is varied in the range (2.26–2.45) and (1.31–2.43) for as-deposited and annealed films respectively corresponding to prominent reflection (1 1 1). The texture is found higher than one which revealed that the films become more texture and all the crystallite are oriented along preferred reflection (1 1 1). It is observed to decrease with film thickness for annealed samples due to decrement in peak intensity.
3.2. Optical analysis The optical transmission spectra of as-deposited and annealed CdSe thin films were recorded in the wavelength range 200– 800 nm using UV–Vis spectrophotometer and presented in Fig. 5. The optical transmittance of as-deposited films is found to increase with wavelength and observed to be more than 58% for thickness 445 nm and 810 nm. It is found maximum 100% for thickness 631 nm in the visible range. It is also observed that the optical transmittance is observed to increase for 631 nm and slightly decrease for 810 nm which may be attributed to the variation in crystallinity of the as-deposited films. The variation in transmittance with thickness is might be due to the surface roughness and scattering [30]. The transmittance spectra for annealed films show that the optical transmittance is found to be more than 80% for thickness 810 nm. It is found maximum 100% in the visible range for thickness 445 nm. Generally, the optical transmittance of thin films is affected by the film thickness, surface morphology and defects at the grain boundaries [42]. It is also sensitive to the variation in height on the surface of the layers and distribution of grains. The transmittance of annealed films is found to be decreased with film thickness due to the stoichiometric deviation of the films [43]. The optical energy band gap (Eg) was determined by classical relation for near edge absorption of compound semiconductors [31].
ahm ¼ Aðhm Eg Þn=2
ð7Þ
Here, A is a constant, n is integer having value one for a direct band gap semiconductor material and four for indirect band gap semiconductor material. The optical energy band gap was evaluated by extrapolating the straight line corresponding to zero absorption coefficients. The (ahm)2 v/s hm plot of the as-deposited and annealed films are shown in Fig. 6. The Tauc plot of as-deposited and annealed films is observed to be approximately linear which show that the transition between conduction band and valance band is found to be direct. The optical band gap of as-deposited films is varied from 1.80 eV to 1.83 eV and found to decrease with thickness due to the nanocrystalline nature and possibility of structural defects in the as-deposited CdSe films. The film of thickness 810 nm has larger band gap as compared to thickness 631 nm owing to the variation in FWHM and improvement in crystallinity. Similar to the as-deposited films,
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Fig. 4. Variation in texture coefficient with thickness for (a) as-deposited and (b) annealed films.
Fig. 5. The transmittance spectra of (a) as-deposited and (b) annealed CdSe thin films.
the optical band gap of the annealed films is found in the range 1.69–1.84 eV and observed to be lowest for thickness 631 nm. It is found to be varied with film thickness which may be attributed to the allowed states in the forbidden region revealed an improvement in the uniformity of the films [44]. Generally, band gap has tendency toward lower side as thickness increases owing to the optical band gap narrowness between the defect levels and the conduction band [45]. The results are in agreement with the earlier reported work [46,47]. The refractive index is a measure of film density which provides information about voids present in the film. It was calculated using Herve–Vandamme formula [48].
n2 ¼ 1 þ
2 A Eg þ B
ð8Þ
Here, A and B are constants having values of 13.6 eV and 3.4 eV respectively. The calculated refractive index of as-deposited and
annealed thin films is found in the range 3.88–3.92 and 3.88–4.06 respectively. It is observed to increase for thickness 631 nm owing to corresponding lowest optical band gap and found minimum for 810 nm which may be attributed to the change in the strain, packing density and improvement in crystallinity [30]. The results are well supported by earlier reported work of Velumani et al. [39]. The extinction coefficient (k) was calculated using relation concerned [10].
k¼
ak 4p
ð9Þ
Here, k is the wavelength of incident radiation. The extinction coefficient of as-deposited and annealed CdSe thin films is varied in order of 102 with wavelength which revealed the dominance of packing density of bound atoms [30]. It is also observed to increase with thickness which may be attributed to the higher absorbance.
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Fig. 6. The Tauc plot of (a) as-deposited and (b) annealed CdSe thin films.
Fig. 7. The current–voltage characteristics of (a) as-deposited and (b) annealed thin films.
3.3. Electrical analysis The current–voltage characteristics of as-deposited and annealed CdSe thin films are presented in Fig. 7. The variation in current with voltage for as-deposited and annealed CdSe thin films is found to be linear. The electrical resistivity (q) of the thin films was calculated employing the relation concerned [14].
q ¼ 2p s
V I
ð10Þ
Here, s is the distance between probes and having value 1 mm. The electrical resistivity for as-deposited and annealed films is found in the order 10 X cm and 102 X cm. It is varied in the range
(5.30–7.99) X cm and (4.81–11.16) X cm for as-deposited and annealed films respectively. The increase in resistivity of the as-deposited films with thickness is due to the variation in grain size and stoichiometry nature of the films. It is decreased for annealed films with film thickness due to decrease in grain boundary and presence of lattice imperfection like interstitials, vacancies in the initial growth stage of films [11,30]. A similar behavior of resistivity is also reported by Erat et al. [14] and Kale and Lokhande [32]. 3.4. Surface morphological and compositional analysis The scanning electron microscopy is a convenient technique for microstructure study of thin films and microstructure image (SEM image) shows the particles deposited on the surface of the glass
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(a)
(c)
(b)
Fig. 8. The SEM images of as-deposited CdSe thin films of thickness (a) 445 nm, (b) 631 nm and (c) 810 nm.
(a)
(b)
(c)
Fig. 9. The SEM images of annealed CdSe thin films of thickness (a) 445 nm, (b) 631 nm and (c) 810 nm.
cps/eV
cps/eV
cps/eV 7
5
(a)
Se
6
Se
5
(b)
4
(c)
4 5
3
Se
3
4
3
2
2
Cd
2
Cd
Cd
1
1 1 Cd
Cd Se
0 0
2
4
6
8
10
Cd
0
0
2
4
keV
6
8
10
Se
0
Se
12
0
2
4
6
8
10
keV
keV
Fig. 10. The typical EDS pattern of as-deposited CdSe thin films of thickness (a) 445 nm, (b) 631 nm and (c) 810 nm.
substrate. The SEM images of as-deposited CdSe thin films are presented in Fig. 8. The SEM images show that the as-deposited CdSe thin films are homogeneous, smooth, fully covered and free from crystal defects like pin holes and cracks. The grains are distributed in similar size, densely packed and well defined. The SEM images of annealed CdSe thin films are presented in Fig. 9. The SEM images of annealed thin films show smooth and uniform surface without pinholes and cracks and well covered to the substrate. The small spherical shape grains are observed to
distribute over the substrate surface and the size of these grains are found to increase with thickness. The quantitative analysis of the as-deposited CdSe thin films was carried out employing energy dispersive spectroscopy and the EDS patterns of as-deposited CdSe thin films are presented in Fig. 10. The relative elemental contents of as-deposited CdSe films are tabulated in Table 3. It is observed from Fig. 10(a) and Table 3 that the average atomic percentage ratio of Cd and Se is found to be 51.96:48.04 for thickness 445 nm which showed that the film is slightly Cd excess. The chemical composition is found to be varied with film
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Table 3 The relative elemental analysis of as-deposited CdSe thin films of different thickness. Samples/ elements
445 nm Weight%
Atomic%
Weight%
Atomic%
Weight%
Atomic%
Cd (L) Se (L)
60.93 39.37
51.96 48.04
58.03 41.97
49.27 50.73
56.66 43.44
47.87 52.13
Total
100
100
100
100
100
100
631 nm
810 nm
thickness owing to the variation in interaction of the deposited elements with the substrate. The EDS pattern for 631 nm thickness shows that the average atomic percentage ratio of Cd and Se is found to be 49.27:50.73 which indicates that the film is slightly Cd deficient and shows good stoichiometric nature of the film. It is also observed that the Cd and Se ratio is found to be decreased with film thickness which may be attributed to the reduction of thermal gradient between the deposited material and substrate surface.
4. Conclusion In this paper, a study on effect of thickness on the physical properties of CdSe thin films is reported. The thin films of thickness 445 nm, 631 nm and 810 nm were deposited on glass and ITO coated glass substrates employing thermal evaporation technique followed by annealing in air atmosphere at temperature 300 °C. The XRD patterns reveal that the films have zinc-blende cubic structure with preferred reflection (1 1 1) and nanocrystalline nature. The structural parameters are calculated and discussed in detail. The grain size of annealed films is found to be decreased with thickness which may be attributed to the stacking of smaller grains on the surface of larger grains. The optical band gap was found in the range 1.80–1.83 eV and 1.69–1.84 eV for as-deposited and annealed films respectively and found to minimum for films of thickness 631 nm. The refractive index and extinction coefficient are also calculated and found to be varied with film thickness. In the as-deposited thin films, the crystallinity of thickness 631 nm is observed to decrease and consequently all the structural and optical parameters show opposite behavior as compared to the other thickness. The electrical studies show that the variation in current with voltage is found to be linear. The resistivity of as-deposited films is observed to increase with thickness while decrease for annealed films. The SEM analysis reveals that the as-deposited and annealed films are homogeneous, smooth, fully covered and free from crystal defects. The compositional analysis showed that the Cd and Se ratio is decreased with film thickness owing to the reduction of thermal gradient between the deposited material and substrate surface. Acknowledgements The authors are thankful to Dr. Pawan K. Kulriya, Scientist, Inter University Acceleration Centre, New Delhi for provide X-ray diffraction facility. S.P.N. and A.P. are grateful to the University Grants Commission, New Delhi, India for financial support under startup grant No. F.20-1/2012(BSR)/20-3(5)/2012(BSR) and UGC-BSR fellowship No. F.25-1/2013-14(BSR)/7-123/2007(BSR) respectively. References [1] P. Servati, A. Colli, S. Hofmann, Y. Fu, P. Beecher, Z.A.K. Durrani, A.C. Ferrari, A. Flewitt, J. Robertson, W.I. Milne, Scalable silicon nanowire photodetectors, Physica E 38 (2007) 64–66.
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Effect of thickness on structural, optical, electrical and morphological properties of nanocrystalline CdSe thin films for optoelectronic applications Anuradha Purohit a, S. Chander a, S.P. Nehra b, C. Lal c, M.S. Dhaka a,⇑ a b c
Department of Physics, Mohanlal Sukhadia University, Udaipur 313001, India Centre of Excellence for Energy and Environmental Studies, Deenbandhu Chhotu Ram University of Science and Technology, Murthal, Sonepat 131039, India Department of Physics, University of Rajasthan, Jaipur 302004, India
a r t i c l e
i n f o
Article history: Received 24 January 2015 Received in revised form 29 May 2015 Accepted 30 May 2015 Available online xxxx Keywords: CdSe thin films Physical properties Thickness Annealing Thermal evaporation
a b s t r a c t This paper presents effect of thickness on the physical properties of thermally evaporated cadmium selenide thin films. The films of thickness 445 nm, 631 nm and 810 nm were deposited employing thermal evaporation technique on glass and ITO coated glass substrates followed by thermal annealing in air atmosphere at temperature 300 °C. The as-deposited and annealed films were subjected to the XRD, UV–Vis spectrophotometer, source meter, SEM and EDS to find the structural, optical, electrical, morphological and compositional analysis respectively. The structural analysis shows that the films have cubic phase with preferred orientation (1 1 1) and nanocrystalline nature. The structural parameters like inter-planner spacing, lattice constant, grain size, number of crystallites per unit area, internal strain, dislocation density and texture coefficient are calculated. The optical band gap is found in the range 1.69– 1.84 eV and observed to decrease with thickness. The electrical resistivity is found to increase with thickness for as-deposited films and decrease for annealed films. The morphological studies show that the as-deposited and annealed films are homogeneous, smooth, fully covered and free from crystal defects like pin holes and voids. The grains in the as-deposited films are densely packed, well defined and found to be increased with thickness. Ó 2015 Published by Elsevier B.V.
1. Introduction The thin films of II–VI compound semiconductors group attract more attention due to their wide range of applications in the field of optoelectronics like photo-detectors [1,2], gas sensors [3], light emitting diodes (LEDs) [4,5], thin film transistors [6], optical wave guides and solar cells [7–9]. Among this group, cadmium selenide (CdSe) is a well-known compound semiconductor having direct band gap 1.74 eV at room temperature, high absorption coefficient in the visible range and often possess n-type conductivity [10]. The discrete electronic energy levels in the microscopic structure of nanocrystalline CdSe films are found due to quantum confinement effect [11]. These films have larger carrier mobility as compared to the most commonly used silicon films [12]. Recently, more attention has been paid to the development of low cost and high quality thin films for optoelectronic device applications. The CdSe thin ⇑ Corresponding author. E-mail address: [email protected] (M.S. Dhaka).
films are also considered as important material for photovoltaic applications because these may be used as absorber layer on the top of the tandem solar cells [13]. The CdSe thin films may be deposited employing a number of techniques like thermal evaporation [3,10], chemical bath deposition [14,15], electro-deposition [16–19], hot wall deposition [20], spray pyrolysis [21], SILAR [22], etc. These techniques have their own merits and demerits. Among these techniques, the thermal evaporation technique is found more suitable and useful due to high deposition rate, low consumption of material and economical way of deposition process. This technique has been used for many years to fabricate thin films of compound semiconductors, chalcogenide semiconductors and other kind of materials. The optimization of physical properties plays an important role for effective use of CdSe thin films in micro- and optoelectronic applications. For optoelectronic applications, a transparent substrate is required to enhance emitted light by the active area of thin films. Yadav et al. [23] reported the photo-electrochemical properties of CdSe thin films using spray pyrolysis at different substrate
http://dx.doi.org/10.1016/j.optmat.2015.05.053 0925-3467/Ó 2015 Published by Elsevier B.V.
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temperature. They found that the as-deposited films showed n-type conductivity and had a spectral sharp peak at wavelength 725 nm. The hexagonal structure of CdSe films with preferred orientation (0 0 2) was investigated by Kissinger et al. [24] employing electron beam evaporation. The polycrystalline nature with hexagonal phase of cadmium selenide thin films was observed by Pandiyan et al. [25]. They found that the crystallinity was increased with substrate temperature while the band gap energy was decreased. A study of electrical properties of thermally evaporated CdSe thin films was undertaken by Aneva et al. [26]. They observed that the dark and photo conductivity were increased with film thickness. Recently, Purohit et al. [27] reported the effect of air annealing on the physical properties of CdSe thin films using thermal evaporation deposition and found that the crystallinity and band gap energy were varied with annealing temperature. The CdSe and Al:CdSe semiconductor thin films were synthesized on amorphous and FTO glass substrates by Gawali and Bhosale [28] employing spray pyrolysis technique. They observed that the films were polycrystalline in nature with hexagonal phase and optical band gap was varied in the range 1.67–1.87 eV. Hence, the physical and chemical properties of CdSe thin films to find new applications are strongly depended upon preparation conditions, deposition techniques, heat treatment, substrate, doping, film thickness and substrate temperature. The heat treatment may be performed in air and gaseous medium like N2, H2, Ar as well as in vacuum. The crystal structure quality is an important factor which affects the physical properties of thin films. Thorough literature survey reveals that there is a need to investigate the effect of thickness on the physical properties of CdSe thin films. Therefore, a study on effect of thickness on the physical properties of CdSe thin films is undertaken in this paper. The CdSe thin films were deposited employing thermal evaporation technique on glass and ITO coated glass substrates followed by thermal annealing in air atmosphere at temperature 300 °C. The films were subjected to the XRD, UV–Vis spectrophotometer, source meter, SEM and EDS. The crystallographic and optical parameters are calculated and discussed. Generally, the crystallinity is increased with film thickness but the present study warrants that the crystallinity of CdSe thin films may be reduced with thickness owing to stacking of smaller grains on larger grains. Stringent relative results are observed for as-deposited and annealed films which seems absent in the earlier work.
2.2. Characterization The structural properties of as-deposited and annealed CdSe thin films were carried out employing X-ray diffractometer (Bruker AXS D8 Advance) of CuKa radiation in the 2h range from 20° to 60°. The UV–Vis spectrophotometer (SHIMADZU UV-2450) was used to undertake the optical properties within the wavelength range 200–800 nm. The electrical properties were carried out using source meter (Agilent B2901A). The surface morphological and compositional studies of as-deposited films were performed by scanning electron microscope (Nova Nano FE-SEM 450) coupled with energy dispersive spectroscopy (EDS). The surface morphology of annealed films was taken using SEM (Zeiss EVO 18).
3. Results and discussion 3.1. Structural analysis The X-ray diffraction patterns of as-deposited CdSe films of thickness 445 nm, 631 nm and 810 nm are shown in Fig. 1. The XRD pattern of the as-deposited films of thickness 445 nm shows a sharp diffraction peak at angular position 25.43° corresponding to the prominent reflection (1 1 1). The error and trial method was used to identify the reflections. The prominent reflection (1 1 1) is found to be well indexed with the standard JCPDS data file 19-0191 [29] and films have zinc-blende cubic structure with nanocrystalline nature. The formation of nanocrystalline films may be attributed to decrease of grain growth region and increase of nucleation centers [30]. The angular position of prominent peak is observed to shift toward higher and lower sides for films of thickness 631 nm and 810 nm respectively owing to decrease and increase in the corresponding lattice constants. It is also observed that the sharpness and intensity of the prominent reflection (1 1 1) is found to be increased with film thickness which revealed an improvement in the crystallinity owing to decrement in disorderness. Despite the fact, the crystallinity of film of thickness 631 nm is found to decrease as compared to the film 445 nm which may be attributed to the stacking of smaller grains on the larger grains. The results are well agreed with the earlier reported work [14,24,31–33]. The structural parameters like
2. Experimental details 2.1. Deposition of CdSe films The CdSe thin films were deposited on chemically cleaned glass and ITO coated glass substrates employing thermal evaporation technique. The glass substrates were used to undertake the structural, optical, morphological and compositional analysis while the ITO coated glass substrates for electrical analysis. The CdSe powder of purity 99.99% was procured from Alfa Aesar. The substrates of dimension 75 mm 75 mm 1 mm were cut by diamond cutter in the size of 10 mm 10 mm 1 mm. The substrates were cleaned using distilled water, acetone followed by ethyl alcohol. The CdSe powder was pelletized with the help of dye and a molybdenum boat was used to keep the pellet. The rotary and diffusion pumps were used to evacuate the vacuum coating unit up to a pressure 106 torr. The films of thickness 445 nm, 631 nm and 810 nm were prepared and the thickness was also verified by ellipsometry (Woolam M2000). The as-deposited thin films were subjected to air annealing at temperature 300 °C in a furnace (Metrex Muffle) for a period of one hour.
Fig. 1. The X-ray diffraction patterns of as-deposited CdSe thin films.
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inter-planner spacing (d) and lattice constant (a) were calculated using Bragg’s diffraction law and relation concerned [34,35].
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 a¼d h þk þl
ð1Þ
Here, h, k and l are Miller indices. Table 1 depicts the structural parameters like inter-planner spacing (d), lattice constant (a), grain size (D) and number of crystallites per unit area (N) for as-deposited films. It is seen from Table 1 that the lattice constant is varied in the 0
range 6.046–6.065 Å A corresponding to prominent peak (1 1 1) and found to be increased with thickness owing to change in density as well as the nature of native imperfection of the films. The calculated inter-planner spacing is found to be slightly varied from 0
0
3.495 Å A to 3.506 Å A with thickness. The grain size was calculated using Debye–Scherrer formula [36].
Dhkl ¼
kk bCosh
ð2Þ
Here, k is Scherrer constant having value 0.94, h is Bragg’s angle, k is the wavelength of the X-ray used, b is the full width at half maxima (FWHM) of the peak intensity. 0
The grain size is found in the range 306.03–331.79 Å A and observed to decrease for thickness 631 nm owing to the decrease in crystallinity which may be attributed to the formation of smaller grains on the surface of larger grains [37]. It is found to increase for thickness 810 nm which revealed the coalescence or aggregation of small nanocrystalline particles and variation in corresponding FWHM of the prominent reflection [10]. The large grain size indicated the high smoothness of the films deposited on the surface of substrate. The number of crystallites per unit area (N) was calculated [36].
N¼
t
Fig. 2. The X-ray diffraction patterns of CdSe thin films annealed at temperature 300 °C.
Table 2 The structural parameters of annealed CdSe thin films of different thickness. Samples annealed at 300 °C
2h (°)
445 nm 631 nm 810 nm
25.43 25.43 25.44
(h k l)
111 111 111
a (Å A)
D (Å A)
N 1015 m2
6.051 6.051 6.048
302.66 297.57 271.13
16.0 23.0 40.6
0
0
d (Å A) Obs.
Stand.
3.498 3.498 3.496
3.51 3.51 3.51
0
ð3Þ
D3
Here, t is thickness of deposited thin films. The number of crystallites per unit area is found in the range (14.0–22.1) 1015 m2 and observed to increase with thickness owing to variation in corresponding grain size and film thickness. The X-ray diffraction patterns of the annealed films are shown in Fig. 2. Similar to the as-deposited thin films, sharp and high intense prominent reflection is observed at angular position 25.43° corresponding to the plane (1 1 1) for thickness 445 nm which is well agreed with the standard data [29] and the films have zinc-blende cubic structure with nanocrystalline nature. The angular position of prominent reflection is observed at 25.43° and 25.44° for films of thickness 631 nm and 810 nm respectively. It is slightly increased with thickness due to decrease in corresponding lattice constant. The sharpness and intensity of the prominent peak is observed to decrease with thickness while the broadening of the peak increase which revealed the reduction in crystallinity due to increment in the corresponding FWHM and increase in the internal strain of the material. The results are well supported by the earlier reported work of Kale and Lokhande [32]. The
structural parameters for annealed films are calculated and tabulated in Table 2. The lattice constant of annealed films is varied in the range 0
6.048–6.051 Å A corresponding to (1 1 1) plane and found to slightly decrease with film thickness which may be attributed to the shift of the angular position toward higher side. The inter-planner spac0
0
A and observed to decrease ing is found between 3.496 Å A and 3.498 Å with thickness. The grain size is found in the range 271.13– 0
302.66 Å A and decrease with thickness owing to increase in corresponding FWHM and disorder. The number of crystallites per unit area is found in the range (16.0–40.6) 1015 m2 and observed to increase with thickness. The dislocation in a film is an imperfection in the crystal created during growth. The strain (e) in thin films is defined as the disarrangement of lattice and was calculated using relation concerned [38].
e¼
b 4tanh
ð4Þ
Table 1 The structural parameters of as-deposited CdSe thin films of different thickness. Samples as-deposited
2h (°)
(h k l)
445 nm 631 nm 810 nm
25.43 25.46 25.38
111 111 111
0
0
d (Å A)
D (Å A)
N 1015 m2
316.75 306.03 331.79
14.0 22.0 22.1
0
a (Å A)
Obs.
Stand.
Obs.
Stand.
3.498 3.495 3.506
3.51 3.51 3.51
6.051 6.046 6.065
6.05 – –
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Fig. 3. The strain and dislocation density of (a) as-deposited and (b) annealed CdSe thin films.
The dislocation density (d) was calculated using Williamson Smallman’s relation [36].
d¼
1
ð5Þ
D2
The strain and dislocation density of as-deposited and annealed films corresponding to prominent reflection (1 1 1) as a function of film thickness are presented in Fig. 3. It is visible in Fig. 3(a) that the dislocation density and internal strain are found in the range (0.90–1.06) 1011 cm2 and 0.284– 0.307 respectively. Both these parameters are found in the range (1.09–1.36) 1011 cm2 and 0.3115–0.3472 respectively for the annealed films (Fig. 3b). The internal strain is found to increase with thickness for annealed films and decrease for the as-deposited film excluding the thickness 631 nm. A similar behavior of internal strain and dislocation density was also observed by Velumani et al. [39]. The increasing strain shows compressive nature and indicates the formation of smaller grains on the surface of larger grains and decreasing strain shows tensile nature and may be attributed to the improvement in crystallinity which is visible in Fig. 3a for thickness 810 nm while found compressive for thickness 631 nm. The dislocation density of the as-deposited films is varied with thickness and observed to increase for annealed films owing to decrement in grain size. The texture is a non-random distribution of crystal orientation and texture coefficient (TC) presents the texture of a particular plane which implies the preferred growth due to deviation from unity [40]. It was calculated using relation concerned [41] and shown in Fig. 4. Iðh k lÞ I ðh k lÞ Iðh k lÞ N I0 ðh k lÞ
TCðh k lÞ ¼ P0 1 N
ð6Þ
Here, I(h k l) is the measured intensity, I0(h k l) is the JCPDS intensity and N is the reflection number. The texture coefficient is varied in the range (2.26–2.45) and (1.31–2.43) for as-deposited and annealed films respectively corresponding to prominent reflection (1 1 1). The texture is found higher than one which revealed that the films become more texture and all the crystallite are oriented along preferred reflection (1 1 1). It is observed to decrease with film thickness for annealed samples due to decrement in peak intensity.
3.2. Optical analysis The optical transmission spectra of as-deposited and annealed CdSe thin films were recorded in the wavelength range 200– 800 nm using UV–Vis spectrophotometer and presented in Fig. 5. The optical transmittance of as-deposited films is found to increase with wavelength and observed to be more than 58% for thickness 445 nm and 810 nm. It is found maximum 100% for thickness 631 nm in the visible range. It is also observed that the optical transmittance is observed to increase for 631 nm and slightly decrease for 810 nm which may be attributed to the variation in crystallinity of the as-deposited films. The variation in transmittance with thickness is might be due to the surface roughness and scattering [30]. The transmittance spectra for annealed films show that the optical transmittance is found to be more than 80% for thickness 810 nm. It is found maximum 100% in the visible range for thickness 445 nm. Generally, the optical transmittance of thin films is affected by the film thickness, surface morphology and defects at the grain boundaries [42]. It is also sensitive to the variation in height on the surface of the layers and distribution of grains. The transmittance of annealed films is found to be decreased with film thickness due to the stoichiometric deviation of the films [43]. The optical energy band gap (Eg) was determined by classical relation for near edge absorption of compound semiconductors [31].
ahm ¼ Aðhm Eg Þn=2
ð7Þ
Here, A is a constant, n is integer having value one for a direct band gap semiconductor material and four for indirect band gap semiconductor material. The optical energy band gap was evaluated by extrapolating the straight line corresponding to zero absorption coefficients. The (ahm)2 v/s hm plot of the as-deposited and annealed films are shown in Fig. 6. The Tauc plot of as-deposited and annealed films is observed to be approximately linear which show that the transition between conduction band and valance band is found to be direct. The optical band gap of as-deposited films is varied from 1.80 eV to 1.83 eV and found to decrease with thickness due to the nanocrystalline nature and possibility of structural defects in the as-deposited CdSe films. The film of thickness 810 nm has larger band gap as compared to thickness 631 nm owing to the variation in FWHM and improvement in crystallinity. Similar to the as-deposited films,
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Fig. 4. Variation in texture coefficient with thickness for (a) as-deposited and (b) annealed films.
Fig. 5. The transmittance spectra of (a) as-deposited and (b) annealed CdSe thin films.
the optical band gap of the annealed films is found in the range 1.69–1.84 eV and observed to be lowest for thickness 631 nm. It is found to be varied with film thickness which may be attributed to the allowed states in the forbidden region revealed an improvement in the uniformity of the films [44]. Generally, band gap has tendency toward lower side as thickness increases owing to the optical band gap narrowness between the defect levels and the conduction band [45]. The results are in agreement with the earlier reported work [46,47]. The refractive index is a measure of film density which provides information about voids present in the film. It was calculated using Herve–Vandamme formula [48].
n2 ¼ 1 þ
2 A Eg þ B
ð8Þ
Here, A and B are constants having values of 13.6 eV and 3.4 eV respectively. The calculated refractive index of as-deposited and
annealed thin films is found in the range 3.88–3.92 and 3.88–4.06 respectively. It is observed to increase for thickness 631 nm owing to corresponding lowest optical band gap and found minimum for 810 nm which may be attributed to the change in the strain, packing density and improvement in crystallinity [30]. The results are well supported by earlier reported work of Velumani et al. [39]. The extinction coefficient (k) was calculated using relation concerned [10].
k¼
ak 4p
ð9Þ
Here, k is the wavelength of incident radiation. The extinction coefficient of as-deposited and annealed CdSe thin films is varied in order of 102 with wavelength which revealed the dominance of packing density of bound atoms [30]. It is also observed to increase with thickness which may be attributed to the higher absorbance.
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A. Purohit et al. / Optical Materials xxx (2015) xxx–xxx
Fig. 6. The Tauc plot of (a) as-deposited and (b) annealed CdSe thin films.
Fig. 7. The current–voltage characteristics of (a) as-deposited and (b) annealed thin films.
3.3. Electrical analysis The current–voltage characteristics of as-deposited and annealed CdSe thin films are presented in Fig. 7. The variation in current with voltage for as-deposited and annealed CdSe thin films is found to be linear. The electrical resistivity (q) of the thin films was calculated employing the relation concerned [14].
q ¼ 2p s
V I
ð10Þ
Here, s is the distance between probes and having value 1 mm. The electrical resistivity for as-deposited and annealed films is found in the order 10 X cm and 102 X cm. It is varied in the range
(5.30–7.99) X cm and (4.81–11.16) X cm for as-deposited and annealed films respectively. The increase in resistivity of the as-deposited films with thickness is due to the variation in grain size and stoichiometry nature of the films. It is decreased for annealed films with film thickness due to decrease in grain boundary and presence of lattice imperfection like interstitials, vacancies in the initial growth stage of films [11,30]. A similar behavior of resistivity is also reported by Erat et al. [14] and Kale and Lokhande [32]. 3.4. Surface morphological and compositional analysis The scanning electron microscopy is a convenient technique for microstructure study of thin films and microstructure image (SEM image) shows the particles deposited on the surface of the glass
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A. Purohit et al. / Optical Materials xxx (2015) xxx–xxx
(a)
(c)
(b)
Fig. 8. The SEM images of as-deposited CdSe thin films of thickness (a) 445 nm, (b) 631 nm and (c) 810 nm.
(a)
(b)
(c)
Fig. 9. The SEM images of annealed CdSe thin films of thickness (a) 445 nm, (b) 631 nm and (c) 810 nm.
cps/eV
cps/eV
cps/eV 7
5
(a)
Se
6
Se
5
(b)
4
(c)
4 5
3
Se
3
4
3
2
2
Cd
2
Cd
Cd
1
1 1 Cd
Cd Se
0 0
2
4
6
8
10
Cd
0
0
2
4
keV
6
8
10
Se
0
Se
12
0
2
4
6
8
10
keV
keV
Fig. 10. The typical EDS pattern of as-deposited CdSe thin films of thickness (a) 445 nm, (b) 631 nm and (c) 810 nm.
substrate. The SEM images of as-deposited CdSe thin films are presented in Fig. 8. The SEM images show that the as-deposited CdSe thin films are homogeneous, smooth, fully covered and free from crystal defects like pin holes and cracks. The grains are distributed in similar size, densely packed and well defined. The SEM images of annealed CdSe thin films are presented in Fig. 9. The SEM images of annealed thin films show smooth and uniform surface without pinholes and cracks and well covered to the substrate. The small spherical shape grains are observed to
distribute over the substrate surface and the size of these grains are found to increase with thickness. The quantitative analysis of the as-deposited CdSe thin films was carried out employing energy dispersive spectroscopy and the EDS patterns of as-deposited CdSe thin films are presented in Fig. 10. The relative elemental contents of as-deposited CdSe films are tabulated in Table 3. It is observed from Fig. 10(a) and Table 3 that the average atomic percentage ratio of Cd and Se is found to be 51.96:48.04 for thickness 445 nm which showed that the film is slightly Cd excess. The chemical composition is found to be varied with film
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A. Purohit et al. / Optical Materials xxx (2015) xxx–xxx
Table 3 The relative elemental analysis of as-deposited CdSe thin films of different thickness. Samples/ elements
445 nm Weight%
Atomic%
Weight%
Atomic%
Weight%
Atomic%
Cd (L) Se (L)
60.93 39.37
51.96 48.04
58.03 41.97
49.27 50.73
56.66 43.44
47.87 52.13
Total
100
100
100
100
100
100
631 nm
810 nm
thickness owing to the variation in interaction of the deposited elements with the substrate. The EDS pattern for 631 nm thickness shows that the average atomic percentage ratio of Cd and Se is found to be 49.27:50.73 which indicates that the film is slightly Cd deficient and shows good stoichiometric nature of the film. It is also observed that the Cd and Se ratio is found to be decreased with film thickness which may be attributed to the reduction of thermal gradient between the deposited material and substrate surface.
4. Conclusion In this paper, a study on effect of thickness on the physical properties of CdSe thin films is reported. The thin films of thickness 445 nm, 631 nm and 810 nm were deposited on glass and ITO coated glass substrates employing thermal evaporation technique followed by annealing in air atmosphere at temperature 300 °C. The XRD patterns reveal that the films have zinc-blende cubic structure with preferred reflection (1 1 1) and nanocrystalline nature. The structural parameters are calculated and discussed in detail. The grain size of annealed films is found to be decreased with thickness which may be attributed to the stacking of smaller grains on the surface of larger grains. The optical band gap was found in the range 1.80–1.83 eV and 1.69–1.84 eV for as-deposited and annealed films respectively and found to minimum for films of thickness 631 nm. The refractive index and extinction coefficient are also calculated and found to be varied with film thickness. In the as-deposited thin films, the crystallinity of thickness 631 nm is observed to decrease and consequently all the structural and optical parameters show opposite behavior as compared to the other thickness. The electrical studies show that the variation in current with voltage is found to be linear. The resistivity of as-deposited films is observed to increase with thickness while decrease for annealed films. The SEM analysis reveals that the as-deposited and annealed films are homogeneous, smooth, fully covered and free from crystal defects. The compositional analysis showed that the Cd and Se ratio is decreased with film thickness owing to the reduction of thermal gradient between the deposited material and substrate surface. Acknowledgements The authors are thankful to Dr. Pawan K. Kulriya, Scientist, Inter University Acceleration Centre, New Delhi for provide X-ray diffraction facility. S.P.N. and A.P. are grateful to the University Grants Commission, New Delhi, India for financial support under startup grant No. F.20-1/2012(BSR)/20-3(5)/2012(BSR) and UGC-BSR fellowship No. F.25-1/2013-14(BSR)/7-123/2007(BSR) respectively. References [1] P. Servati, A. Colli, S. Hofmann, Y. Fu, P. Beecher, Z.A.K. Durrani, A.C. Ferrari, A. Flewitt, J. Robertson, W.I. Milne, Scalable silicon nanowire photodetectors, Physica E 38 (2007) 64–66.
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