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Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films
Materials Science in Semiconductor Processing 13 (2010) 288–294
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films G. Hema Chandra a,n, C. Udayakumar a, N. Padhy b, S. Uthanna c a
Thin Film Laboratory, Materials Science Division, VIT University, Vellore 632 014, India Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India c Department of Physics, Sri Venkateswara University, Tirupati 517 502, India b
a r t i c l e in fo
abstract
Available online 24 November 2010
CuIn0.75Al0.25Se2 thin films prepared onto glass substrates at TS = 573 K were single phase, nearly stoichiometric and polycrystalline with a strong (1 1 1) preferred orientation showing sphalerite structure. The results of X-ray diffraction and electron diffraction studies are compared, interpreted and correlated with micro-Raman spectra. The optical absorption studies indicated a direct band gap of 1.16 eV with high absorption coefficient ( 4 104 cm 1) near the fundamental absorption edge. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Semiconductor thin films X-ray Diffraction Electron Diffraction Raman spectra Optical properties
1. Introduction The wide range of optical band gaps and carrier mobilities offered by ternary and quaternary I–III–VI2 chalcopyrites, as well as their ability to form various solid solutions and to accommodate different dopants, has recently led to their emergence as technologically significant materials, including applications in photovoltaic solar cells, light-emitting diodes and in various non-linear optical devices [1]. Among these, CuInSe2 thin films have emerged as promising candidate for low cost solar cells. Significant effort has been made on increase of band gap by substituting Ga for In and S for Se to enhance the photovoltaic conversion efficiency in CuInSe2 based solar cells. Thin films of CuInxGa1 xSe2 showed an efficiency of 19.5% [2] after decades of research. However, gallium is a scarce and expensive material. CuIn1 xAlxSe2 is a viable alternative material for the fabrication of low cost heterojunction solar cells and tandem cells. By gradually substituting indium with aluminium, the optical band gap can be varied from 1.04 to 2.67 eV. CuIn1 xAlxSe2 system can be suitable to act as an absorber material for both top and bottom cell n Corresponding author. Present address: INESC Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal. Tel.: +351 934 514 055; fax: + 351 220 402 437. E-mail address: [email protected] (G. Hema Chandra).
1369-8001/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2010.10.017
in making two thin film tandem cell by varying the value of ‘x’. Considerable attention has been devoted by various authors in the preparation of CuIn1 xAlxSe2 thin films by different methods like sequential deposition method followed by annealing/selenization [3–9], co-evaporation [10–13], flash evaporation [14,15], rf magnetron sputtering of mixed binary selenides [16], chemical bath deposition [17] and molecular beam epitaxy [18]. CuIn1 xAlxSe2 (x =0.13) single junction solar cell with co-evaporation method exhibited an efficiency of 16.9% [11]. The decisive requirements for the efficient performance of the devices are compositional uniformity and crystallinity. Further, the technique adopted should be simple and cost effective. In the case of four source evaporation, which is believed to be a reliable technique, there are difficult processing conditions like controlling evaporation rates of individual elements, which in turn governs the composition of the deposited films. From an industrial point of view the dosage of the effusion from two or more sources in large areas is difficult, whereas the ‘flash’ evaporation from a single source presents an alternative granting homogeneity of composition and larger areas at lower temperatures. In the present study, the impact of substrate temperature on structural, morphological and optical properties of CuIn0.75Al0.25Se2 (CIAS) films deposited by flash evaporation are investigated and discussed systematically. An effort is made to resolve the structure of flash evaporated
G. Hema Chandra et al. / Materials Science in Semiconductor Processing 13 (2010) 288–294
CIAS thin films by XRD, selected area electron diffraction and Raman spectroscopy. The results obtained were analyzed to evaluate the potentiality of the flash evaporated copper indium aluminium dieseline films as an absorber layer in heterojunction solar cells.
289
500–2500 nm using Hitachi U3400 UV–vis-NIR double beam spectrophotometer. 3. Results and discussion 3.1. Characterization of source material
2. Experimental details The CuIn0.75Al0.25Se2 compound was examined with EDAX for compositional analysis and the calculated wt% of copper, indium, aluminium and selenium were found to be 20.22, 27.40, 2.14 and 50.24, respectively, for starting material. The powder was then characterized with X-ray diffractometer. Fig. 1 shows the X-ray diffraction spectra of CuIn0.75Al0.25Se2 powder. The X-ray diffraction pattern indicates a strong preferred orientation along (1 1 2) showing chalcopyrite structure with lattice parameters a=0.576 nm and c=1.162 nm and these evaluated values are in good agreement with the reported data of CuIn0.75Al0.25Se2 alloy [19]. 3.2. Characterization of thin films CuIn0.75Al0.25Se2 films (100–300 nm) were prepared by the flash evaporation technique onto glass substrates kept in the range of 303–603 K at a pressure of 4 10 6 Torr. CuIn0.75Al0.25Se2 films were grown under identical conditions by keeping all the deposition parameters constant, except the substrate temperature (TS). The deposited films were uniform, pinhole free and strongly adherent to the substrate.
(112)
3.2.1. Compositional analysis Energy dispersive X-ray spectra of CuIn0.75Al0.25Se2 thin films revealed that the Cu, In, Al and Se contents depend critically on substrate temperature (TS). Fig. 2 shows elemental weight percentages of Cu, In, Al and Se with TS for CuIn0.75Al0.25Se2 films of 300 nm thickness. It is seen that CuIn0.75Al0.25Se2 thin films formed at TS o573 K contain excess selenium and indium, and were deficient in aluminium and copper. This may be due to slight decomposition of compound and/or due to the difference in vapour pressures [20] of the elements in the quaternary
600
(204)/(220)
500
100
(211)
200
(103)
300
(400)
(116)/(312)
400
(101)
Intensity (counts/sec)
Ingots of CuIn0.75Al0.25Se2 have been prepared using constituent elements of 99.999% pure copper, indium, aluminium and selenium (obtained from M/S Aldrich, USA) weighed in stoichiometric ratio. Quartz ampoule was coated inside with boron nitride to avoid the reaction of Al with quartz tube and mixture was sealed in a quartz ampoule under a pressure of less than 10 5 Torr. The sealed ampoule was placed in a vertical furnace and the temperature was slowly increased at the rate of 30 K h 1 up to 473 K. It was maintained at this temperature for a period of 24 h in order to minimize the pressure build-up and to avoid possible strong exothermic reactions. The temperature was then increased at the rate of 60 K h 1 up to 973 K, beyond which a slow heating process was used to minimize the risk of cracking. The ampoule was kept at 1423 K for a period of 48 h, and rotated intermittently for 2–3 h to ensure complete mixing and reaction of the constituents. It was then cooled slowly to room temperature. In order to ensure better homogeneity, the mixture was heated two times and each time it was kept at 1423 K for 12 h and slowly cooled to room temperature. The compound was then ground and reduced to 200–300 mesh powders. CuIn0.75Al0.25Se2 films were prepared by flash evaporation technique onto ultrasonically cleaned glass substrates kept in the temperature range of 303–603 K at a pressure of 4 10 6 Torr using 12A4D Hind Hivac coating unit. The substrate temperature was monitored using chromel– alumel thermocouple in direct contact with the substrate. The powder was gradually dropped onto a molybdenum boat heated to 1773 K through a vibratory spiral feeder, resulting in the flash evaporation of the material. The evaporation rate was about 2 nm s 1. The thickness of the films varied from 100 to 300 nm and was monitored using a quartz crystal thickness monitor during deposition. The elemental composition of the films was determined using energy dispersive X-ray analysis (EDAX) attached to the JEOL scanning electron microscope (SEM) model JSM-35CF. The excitation voltages were kept low and the spectrum was analysed carefully, keeping in mind that EDAX peaks of aluminium and selenium are quite close. X-ray diffraction patterns of the films were recorded with Philips X-ray diffractometer using CuKa radiation (l =0.15406 nm). Jeol 200CX Transmission electron microscope was used to obtain the selected area diffraction (SAED) patterns of CuIn0.75Al0.25Se2 thin films (100 nm). A Dilor–Jobin–Yvon–Spex integrated Raman spectrometer (Model Labram) was used for the present experiments. He–Ne (l =632.81 nm) 20 mW laser beam was used as the excitation source. The Raman studies were carried out at the room temperature. The surface morphology of the films has been observed by atomic force microscope (AFM; Solver PRO EC (NT-MDT)). The optical transmittance and reflectance of the films were recorded at room temperature in the wavelength range
0 15 20 25 30 35 40 45 50 55 60 65 70 2θ (degrees) Fig. 1. XRD pattern of CuIn0.75Al0.25Se2 powder.
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formed at TS 4 573 K indicates the loss of selenium and indium in elemental form. This is due to re-evaporation of mobile selenium and indium ad-atoms, before they were bound into the growing crystalline structure.
compound (PSe 4PIn 4PAl 4PCu). The films deposited at TS =573 K were nearly stoichiometric and the calculated weight percentages of copper, indium, aluminium and selenium were 20.64, 27.15, 2.63 and 49.58, respectively, which are comparable to the starting material composition (Cu =20.22 wt%, In = 27.40 wt%, Al=2.14 wt% and Se=50.24 wt%) within the limits of the experimental error 70.5 wt%. The films deposited at TS 4573 K contained excess copper and aluminium and are deficient in indium and selenium. The decrease in wt% of selenium and indium in films
Wt %
54 52 50 48
3.2.2. Structural properties The substrate temperature (TS) was found to have significant influence on structure of the films. CuIn0.75Al0.25Se2 films grown at TS =303–393 K were all amorphous with a mean grain size of about 80 nm. CuIn0.75Al0.25Se2 films grown at TS =423–573 K were polycrystalline and non-stoichiometric. The films formed at TS =573 K were single phase, polycrystalline and diffraction lines became sharp with strong (1 1 1) orientation. fig. 3 depicts the XRD spectra of CuIn0.75Al0.25Se2 films grown at five typical substrate temperatures 423, 483, 543, 573 and 603 K. The films deposited between 423 and 573 K were oriented with (1 1 2) planes of chalcopyrite structure or (1 1 1) planes of the sphalerite structure. The sphalerite phase may be regarded as a disordered chalcopyrite structure and all its diffraction peaks are also shared by the chalcopyrite CIAS structure. A straight forward structural characterization of the film is therefore difficult. However, the chalcopyrite structure has some characteristic peaks, which are due to the (1 0 1), (1 0 3), (2 1 1) and (2 1 3) planes. One can therefore use these peaks to distinguish the chalcopyrite from the sphalerite phase. The absence of these peaks in the CuIn0.75Al0.25Se2 samples prepared probably indicates that they have a sphalerite phase. A similar type of growth has been observed and was reported on CuInSe2 and CuInxGa1 xSe2 films [21–22] by various deposition methods.
Se
30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0
In
Cu
Al
300
350
400
450 500 TS (K)
550
600
650
Cu2-XSe (220)
TS = 603 K
1000 750 500 250 0
TS = 573 K
800 600 400 200 0
TS = 543 K
TS = 483 K
600 450 300 150 0 (111)
Intensity (counts/sec)
750 500 250 0
(111)
Cu2-XSe (111)
Fig. 2. Variation in elemental weight percentages of Cu, In, Al, Se with TS for CuIn0.75Al0.25Se2 thin films of 300 nm thickness.
Cu2-XSe (311)
290
300 200 100 0 20
TS = 423 K
30
40
50
60
70
2θ (degrees) Fig. 3. X-ray diffraction spectra of CuIn0.75Al0.25Se2 films (300 nm) grown at five typical substrate temperatures 423, 483, 543, 573 and 603 K.
G. Hema Chandra et al. / Materials Science in Semiconductor Processing 13 (2010) 288–294
It is found that the peak position for (1 1 1) orientation shifts from 26.731 to 26.751 with increase in substrate temperature due to slight variation in Cu/(In+Al) and or due to Al/(In+Al) ratio leading to slight decrease of interplanar spacing. The lattice parameter for the CuIn0.75Al0.25Se2 films deposited at TS =573 K was found to be 0.5767 nm. The spectra clearly show the enhancement in the (1 1 1) peak height and decrease in FWHM with increase in substrate temperature from 423 to 573 K due to an increase
291
in the crystallinity of the films. The films grown at substrate temperature 603 K exhibited two phases, namely CuIn0.75Al0.25Se2 and Cu2 xSe. The peaks observed at 2y =26.751 is due to the CIAS phase corresponding to the (1 1 1) and the peaks observed at 2y =26.681, 44.621 and 52.941 are due to the Cu2 xSe phase corresponding to (1 1 1), (2 2 0) and (3 1 1), respectively, which match with the JCPDS data (card no. 06-0680). Fig. 4(a) and (b) shows the selected area electron diffraction patterns of the CuIn0.75Al0.25Se2 films (100 nm) grown at substrate temperatures of 573 and 603 K, respectively. The SAED pattern for the CIAS film deposited at TS =573 K contains sharp ring expected for polycrystalline films with highly preferred orientation. This confirms the single phase nature of the polycrystalline films grown at TS =573 K. A very prominent bright ring seen in SAED patterns for the films grown at TS =573 K was due to the (1 1 1) plane, indicating the preferred orientation along that direction. SAED pattern for the films grown at TS =603 K revealed the existence of binary secondary phase. It can be noticed that there are three characteristic major intensity peaks (1 1 1), (2 2 0) and (3 1 1) of Cu2 xSe along with CIAS phase. Due to the structural similarity of Cu2 xSe and chalcopyrite/sphalerite CIAS, a broad band (overlapping) was observed, which may represent either the Cu2 xSe (1 1 1) reflection or the CIAS (1 1 1). These reflections indicate that the secondary phase must be of compatible crystal structure and is in excellent register with the CIAS host lattice. An attempt has been made to identify the structure of CuIn0.75Al0.25Se2 thin film samples by SAED on the large grains. The SAED pattern of CuIn0.75Al0.25Se2 thin films deposited at TS =573 K at larger grains is shown in Fig. 4(c), which clearly indicates sphalerite structure.
176 cm-1
3.2.3. Micro-Raman analysis Raman spectroscopy was identified as a tool for finding the phase homogeneity and crystalline quality of the thin films. It is used to monitor not only the chemical composition but also the presence of additional phases and regions with
1200
CIAS: TS = 603 K
600 500 400 300
261 cm-1
700
184 cm-1
800
183 cm-1
Intensity (counts/sec)
900
242 cm-1
1000 216 cm-1 224 cm-1
CIAS Powder CIAS: TS = 573 K
128 cm-1
1100
200 100 0 100
125
150
175
200
225
250
275
300
Raman shift (cm-1) Fig. 4. Selected area electron diffraction patterns of CuIn0.75Al0.25Se2 films formed at (a) TS = 573 K, (b) TS = 603 K and (c) SAED pattern of large crystallite in CuIn0.75Al0.25Se2 thin film deposited at TS = 573 K.
Fig. 5. Raman spectra of CuIn0.75Al0.25Se2 powder and thin films deposited at substrate temperatures 573 and 603 K, respectively.
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local lattice order. The Raman spectra of CuIn0.75Al0.25Se2 powder is shown in Fig. 5. Five peaks are observed in the spectrum, with most intense line at 176 cm 1 due to A1 mode generally observed for I–III–VI2 compounds having chalcopyrite structure. This arises due to the in-plane motion of selenium atoms with the cations at rest. The A1 phonon frequency obtained for CuIn0.75Al0.25Se2 powder in the present investigation is in good agreement with nanocrystalline CuIn1 xAlxSe2 with x=0.25 [23]. The remaining four peaks at 128 and 242 cm 1 are due to B1 modes and E2 mode at 224 cm 1, B2 mode at 216 cm 1, respectively. A shift in the A1 mode to 184 cm 1 was observed for the CIAS film deposited at TS =573 K, indicating a sphalerite phase. A similar trend has been observed during the incorporation of zinc
(exceeding 20 at%) in Zn2 2xCuxInxSe2 films [24]. An additional peak at 261 cm 1 observed for the films deposited at substrate temperature of 603 K was due to Cu2 xSe phase. So, it is evident from the XRD analysis, electron diffraction measurements and micro-Raman studies that the CuIn0.75Al0.25Se2 films deposited at TS =573 K exhibit sphalerite structure unlike the prepared bulk CuIn0.75Al0.25Se2 compound with chalcopyrite structure. 3.2.4. Surface morphology The AFM images of CuIn0.75Al0.25Se2 films formed at different substrate temperatures were shown in Fig. 6(a)–(c). The films formed at 423 K showed a large number of particles uniformly distributed throughout the
Fig. 6. AFM images of CuIn0.75Al0.25Se2 films deposited at different substrate temperatures (a) TS = 423 K, (b) TS =573 K and (c) TS = 603 K.
G. Hema Chandra et al. / Materials Science in Semiconductor Processing 13 (2010) 288–294
surface. Crystallites show columnar growth with mean grain size of around 110 nm. As the substrate temperature increased to 573 K, the grain size of the films increases due to the improvement in the crystallinity. The films formed at 573 K showed an enhancement in the particle size with mean grain size of around 390 nm. A decrease in grain size ( 280 nm) was observed for the films deposited at 603 K. This might be due to re-evaporation of selenium and indium ad-atoms from the surface of the sample and or due to incorporation of aluminium into the lattice sites and or might be due to growth of Cu2 xSe phase at the expense of suppressing CIAS phase. Usually copper-rich I–III–VI2 films, leading to the growth of Cu2 xSe phase, should have larger crystallite size, but a decrease in the grain size has been observed for the films deposited at 603 K. The reason might be due to insufficient substrate temperature for the exaggerated grain growth of Cu2 xSe ( o656 K) [25].
where t is the thickness of the film, T is the transmittance and R is the reflectance. It is considered as a direct transition between the top of the valence band and bottom of the conduction band in order to estimate the optical band gap (Eg) of the films using
ahn ¼ Aðhn-Eg Þ1=2
ð2Þ
where A is the edge width parameter and hn is the photon energy. The optical band gap of the films was determined from the extrapolation of the linear plot of (ahn)2 versus hn at a = 0 shown in Fig. 7(b). The energy gap of CuIn0.75Al0.25Se2 film (300 nm) deposited at TS = 573 K was found to be 1.16 eV. This is in good agreement with the reported value for CuIn0.75Al0.25Se2 thin films deposited by other methods [3,4,12]. An additional absorption hump observed with an energy gap of 1.8 eV along with 1.22 eV (fundamental gap) for the CuIn0.75Al0.25Se2 film (300 nm) deposited at TS =603 K was due to the formation of Cu2 xSe phase. This is consistent with results obtained from XRD, SAED and Raman spectra. The increase in the fundamental energy gap with increase in substrate temperature may be attributed to the decrease in particle size, leading to an increase in strain and/or due to slight increase in Al/(In+ Al) ratio (observed from compositional analysis). 4. Conclusion CuIn0.75Al0.25Se2 thin films were grown by flash evaporation method onto glass substrates held at substrate temperatures in the range 303–603 K. The impact of substrate temperature on growth of the films was studied. The highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films were observed at TS =573 K, exhibiting sphalerite structure. These nearly
TS = 573 K TS = 603 K
100 90 80
T
70
Cu2-xSe
60 50 40 30
R
20 10 0 600
800 1000 1200 1400 1600 1800 2000 2200 2400 Wavelength (nm)
1.6x109 1.4x109 1.2x109 (αhν)2 (eV cm-1)2
3.2.5. Optical properties The optical transmittance and reflectance of the CuIn0.75 Al0.25Se2 film (300 nm) deposited at TS =573 and 603 K are shown in the Fig. 7(a). The optical absorption coefficient (a) was calculated from the optical transmittance (T) and reflectance (R) data by 1 T ln a¼ ð1Þ 2 t 1R
293
1.0x10
9
8.0x108 6.0x108 4.0x108 2.0x10
8
0.0 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 hν (eV)
Fig. 7. (a) Optical transmittance and reflectance spectra of the CuIn0.75 Al0.25Se2. films (300 nm) deposited at TS =573 and 603 K and (b) Plot of (ahn)2 versus hn of CuIn0.75Al0.25Se2 film deposited at 573 K.
stoichiometric and single phase CuIn0.75Al0.25Se2 films with optical band gap of 1.16 eV could be used as an absorber material in the fabrication of thin film heterojunction solar cells. XRD, SAED and micro-Raman measurements indicated the presence of Cu2 xSe binary phase for the CuIn0.75Al0.25Se2 films deposited at TS =603 K.
Acknowledgements The authors are thankful to the Defence Research and Development Organization, New Delhi, India, for providing financial support to carry out the present research work. The authors are grateful to Prof. S.B. Krupanidhi, MRC, IISc, Bangalore, for valuable technical discussions and for his help in SAED characterization. We would like to thank Dr. U. Kamachi Mudali for providing AFM facility at IGCAR, Kalpakkam. The authors are thankful to Dr. Harish C. Barshilia, NAL, Bangalore, for extending his co-operation for microRaman facility. We are thankful to Prof. V. Sundara Raja, S.V. University, Tirupati, for fruitful discussions.
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films G. Hema Chandra a,n, C. Udayakumar a, N. Padhy b, S. Uthanna c a
Thin Film Laboratory, Materials Science Division, VIT University, Vellore 632 014, India Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India c Department of Physics, Sri Venkateswara University, Tirupati 517 502, India b
a r t i c l e in fo
abstract
Available online 24 November 2010
CuIn0.75Al0.25Se2 thin films prepared onto glass substrates at TS = 573 K were single phase, nearly stoichiometric and polycrystalline with a strong (1 1 1) preferred orientation showing sphalerite structure. The results of X-ray diffraction and electron diffraction studies are compared, interpreted and correlated with micro-Raman spectra. The optical absorption studies indicated a direct band gap of 1.16 eV with high absorption coefficient ( 4 104 cm 1) near the fundamental absorption edge. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Semiconductor thin films X-ray Diffraction Electron Diffraction Raman spectra Optical properties
1. Introduction The wide range of optical band gaps and carrier mobilities offered by ternary and quaternary I–III–VI2 chalcopyrites, as well as their ability to form various solid solutions and to accommodate different dopants, has recently led to their emergence as technologically significant materials, including applications in photovoltaic solar cells, light-emitting diodes and in various non-linear optical devices [1]. Among these, CuInSe2 thin films have emerged as promising candidate for low cost solar cells. Significant effort has been made on increase of band gap by substituting Ga for In and S for Se to enhance the photovoltaic conversion efficiency in CuInSe2 based solar cells. Thin films of CuInxGa1 xSe2 showed an efficiency of 19.5% [2] after decades of research. However, gallium is a scarce and expensive material. CuIn1 xAlxSe2 is a viable alternative material for the fabrication of low cost heterojunction solar cells and tandem cells. By gradually substituting indium with aluminium, the optical band gap can be varied from 1.04 to 2.67 eV. CuIn1 xAlxSe2 system can be suitable to act as an absorber material for both top and bottom cell n Corresponding author. Present address: INESC Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal. Tel.: +351 934 514 055; fax: + 351 220 402 437. E-mail address: [email protected] (G. Hema Chandra).
1369-8001/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2010.10.017
in making two thin film tandem cell by varying the value of ‘x’. Considerable attention has been devoted by various authors in the preparation of CuIn1 xAlxSe2 thin films by different methods like sequential deposition method followed by annealing/selenization [3–9], co-evaporation [10–13], flash evaporation [14,15], rf magnetron sputtering of mixed binary selenides [16], chemical bath deposition [17] and molecular beam epitaxy [18]. CuIn1 xAlxSe2 (x =0.13) single junction solar cell with co-evaporation method exhibited an efficiency of 16.9% [11]. The decisive requirements for the efficient performance of the devices are compositional uniformity and crystallinity. Further, the technique adopted should be simple and cost effective. In the case of four source evaporation, which is believed to be a reliable technique, there are difficult processing conditions like controlling evaporation rates of individual elements, which in turn governs the composition of the deposited films. From an industrial point of view the dosage of the effusion from two or more sources in large areas is difficult, whereas the ‘flash’ evaporation from a single source presents an alternative granting homogeneity of composition and larger areas at lower temperatures. In the present study, the impact of substrate temperature on structural, morphological and optical properties of CuIn0.75Al0.25Se2 (CIAS) films deposited by flash evaporation are investigated and discussed systematically. An effort is made to resolve the structure of flash evaporated
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CIAS thin films by XRD, selected area electron diffraction and Raman spectroscopy. The results obtained were analyzed to evaluate the potentiality of the flash evaporated copper indium aluminium dieseline films as an absorber layer in heterojunction solar cells.
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500–2500 nm using Hitachi U3400 UV–vis-NIR double beam spectrophotometer. 3. Results and discussion 3.1. Characterization of source material
2. Experimental details The CuIn0.75Al0.25Se2 compound was examined with EDAX for compositional analysis and the calculated wt% of copper, indium, aluminium and selenium were found to be 20.22, 27.40, 2.14 and 50.24, respectively, for starting material. The powder was then characterized with X-ray diffractometer. Fig. 1 shows the X-ray diffraction spectra of CuIn0.75Al0.25Se2 powder. The X-ray diffraction pattern indicates a strong preferred orientation along (1 1 2) showing chalcopyrite structure with lattice parameters a=0.576 nm and c=1.162 nm and these evaluated values are in good agreement with the reported data of CuIn0.75Al0.25Se2 alloy [19]. 3.2. Characterization of thin films CuIn0.75Al0.25Se2 films (100–300 nm) were prepared by the flash evaporation technique onto glass substrates kept in the range of 303–603 K at a pressure of 4 10 6 Torr. CuIn0.75Al0.25Se2 films were grown under identical conditions by keeping all the deposition parameters constant, except the substrate temperature (TS). The deposited films were uniform, pinhole free and strongly adherent to the substrate.
(112)
3.2.1. Compositional analysis Energy dispersive X-ray spectra of CuIn0.75Al0.25Se2 thin films revealed that the Cu, In, Al and Se contents depend critically on substrate temperature (TS). Fig. 2 shows elemental weight percentages of Cu, In, Al and Se with TS for CuIn0.75Al0.25Se2 films of 300 nm thickness. It is seen that CuIn0.75Al0.25Se2 thin films formed at TS o573 K contain excess selenium and indium, and were deficient in aluminium and copper. This may be due to slight decomposition of compound and/or due to the difference in vapour pressures [20] of the elements in the quaternary
600
(204)/(220)
500
100
(211)
200
(103)
300
(400)
(116)/(312)
400
(101)
Intensity (counts/sec)
Ingots of CuIn0.75Al0.25Se2 have been prepared using constituent elements of 99.999% pure copper, indium, aluminium and selenium (obtained from M/S Aldrich, USA) weighed in stoichiometric ratio. Quartz ampoule was coated inside with boron nitride to avoid the reaction of Al with quartz tube and mixture was sealed in a quartz ampoule under a pressure of less than 10 5 Torr. The sealed ampoule was placed in a vertical furnace and the temperature was slowly increased at the rate of 30 K h 1 up to 473 K. It was maintained at this temperature for a period of 24 h in order to minimize the pressure build-up and to avoid possible strong exothermic reactions. The temperature was then increased at the rate of 60 K h 1 up to 973 K, beyond which a slow heating process was used to minimize the risk of cracking. The ampoule was kept at 1423 K for a period of 48 h, and rotated intermittently for 2–3 h to ensure complete mixing and reaction of the constituents. It was then cooled slowly to room temperature. In order to ensure better homogeneity, the mixture was heated two times and each time it was kept at 1423 K for 12 h and slowly cooled to room temperature. The compound was then ground and reduced to 200–300 mesh powders. CuIn0.75Al0.25Se2 films were prepared by flash evaporation technique onto ultrasonically cleaned glass substrates kept in the temperature range of 303–603 K at a pressure of 4 10 6 Torr using 12A4D Hind Hivac coating unit. The substrate temperature was monitored using chromel– alumel thermocouple in direct contact with the substrate. The powder was gradually dropped onto a molybdenum boat heated to 1773 K through a vibratory spiral feeder, resulting in the flash evaporation of the material. The evaporation rate was about 2 nm s 1. The thickness of the films varied from 100 to 300 nm and was monitored using a quartz crystal thickness monitor during deposition. The elemental composition of the films was determined using energy dispersive X-ray analysis (EDAX) attached to the JEOL scanning electron microscope (SEM) model JSM-35CF. The excitation voltages were kept low and the spectrum was analysed carefully, keeping in mind that EDAX peaks of aluminium and selenium are quite close. X-ray diffraction patterns of the films were recorded with Philips X-ray diffractometer using CuKa radiation (l =0.15406 nm). Jeol 200CX Transmission electron microscope was used to obtain the selected area diffraction (SAED) patterns of CuIn0.75Al0.25Se2 thin films (100 nm). A Dilor–Jobin–Yvon–Spex integrated Raman spectrometer (Model Labram) was used for the present experiments. He–Ne (l =632.81 nm) 20 mW laser beam was used as the excitation source. The Raman studies were carried out at the room temperature. The surface morphology of the films has been observed by atomic force microscope (AFM; Solver PRO EC (NT-MDT)). The optical transmittance and reflectance of the films were recorded at room temperature in the wavelength range
0 15 20 25 30 35 40 45 50 55 60 65 70 2θ (degrees) Fig. 1. XRD pattern of CuIn0.75Al0.25Se2 powder.
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formed at TS 4 573 K indicates the loss of selenium and indium in elemental form. This is due to re-evaporation of mobile selenium and indium ad-atoms, before they were bound into the growing crystalline structure.
compound (PSe 4PIn 4PAl 4PCu). The films deposited at TS =573 K were nearly stoichiometric and the calculated weight percentages of copper, indium, aluminium and selenium were 20.64, 27.15, 2.63 and 49.58, respectively, which are comparable to the starting material composition (Cu =20.22 wt%, In = 27.40 wt%, Al=2.14 wt% and Se=50.24 wt%) within the limits of the experimental error 70.5 wt%. The films deposited at TS 4573 K contained excess copper and aluminium and are deficient in indium and selenium. The decrease in wt% of selenium and indium in films
Wt %
54 52 50 48
3.2.2. Structural properties The substrate temperature (TS) was found to have significant influence on structure of the films. CuIn0.75Al0.25Se2 films grown at TS =303–393 K were all amorphous with a mean grain size of about 80 nm. CuIn0.75Al0.25Se2 films grown at TS =423–573 K were polycrystalline and non-stoichiometric. The films formed at TS =573 K were single phase, polycrystalline and diffraction lines became sharp with strong (1 1 1) orientation. fig. 3 depicts the XRD spectra of CuIn0.75Al0.25Se2 films grown at five typical substrate temperatures 423, 483, 543, 573 and 603 K. The films deposited between 423 and 573 K were oriented with (1 1 2) planes of chalcopyrite structure or (1 1 1) planes of the sphalerite structure. The sphalerite phase may be regarded as a disordered chalcopyrite structure and all its diffraction peaks are also shared by the chalcopyrite CIAS structure. A straight forward structural characterization of the film is therefore difficult. However, the chalcopyrite structure has some characteristic peaks, which are due to the (1 0 1), (1 0 3), (2 1 1) and (2 1 3) planes. One can therefore use these peaks to distinguish the chalcopyrite from the sphalerite phase. The absence of these peaks in the CuIn0.75Al0.25Se2 samples prepared probably indicates that they have a sphalerite phase. A similar type of growth has been observed and was reported on CuInSe2 and CuInxGa1 xSe2 films [21–22] by various deposition methods.
Se
30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0
In
Cu
Al
300
350
400
450 500 TS (K)
550
600
650
Cu2-XSe (220)
TS = 603 K
1000 750 500 250 0
TS = 573 K
800 600 400 200 0
TS = 543 K
TS = 483 K
600 450 300 150 0 (111)
Intensity (counts/sec)
750 500 250 0
(111)
Cu2-XSe (111)
Fig. 2. Variation in elemental weight percentages of Cu, In, Al, Se with TS for CuIn0.75Al0.25Se2 thin films of 300 nm thickness.
Cu2-XSe (311)
290
300 200 100 0 20
TS = 423 K
30
40
50
60
70
2θ (degrees) Fig. 3. X-ray diffraction spectra of CuIn0.75Al0.25Se2 films (300 nm) grown at five typical substrate temperatures 423, 483, 543, 573 and 603 K.
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It is found that the peak position for (1 1 1) orientation shifts from 26.731 to 26.751 with increase in substrate temperature due to slight variation in Cu/(In+Al) and or due to Al/(In+Al) ratio leading to slight decrease of interplanar spacing. The lattice parameter for the CuIn0.75Al0.25Se2 films deposited at TS =573 K was found to be 0.5767 nm. The spectra clearly show the enhancement in the (1 1 1) peak height and decrease in FWHM with increase in substrate temperature from 423 to 573 K due to an increase
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in the crystallinity of the films. The films grown at substrate temperature 603 K exhibited two phases, namely CuIn0.75Al0.25Se2 and Cu2 xSe. The peaks observed at 2y =26.751 is due to the CIAS phase corresponding to the (1 1 1) and the peaks observed at 2y =26.681, 44.621 and 52.941 are due to the Cu2 xSe phase corresponding to (1 1 1), (2 2 0) and (3 1 1), respectively, which match with the JCPDS data (card no. 06-0680). Fig. 4(a) and (b) shows the selected area electron diffraction patterns of the CuIn0.75Al0.25Se2 films (100 nm) grown at substrate temperatures of 573 and 603 K, respectively. The SAED pattern for the CIAS film deposited at TS =573 K contains sharp ring expected for polycrystalline films with highly preferred orientation. This confirms the single phase nature of the polycrystalline films grown at TS =573 K. A very prominent bright ring seen in SAED patterns for the films grown at TS =573 K was due to the (1 1 1) plane, indicating the preferred orientation along that direction. SAED pattern for the films grown at TS =603 K revealed the existence of binary secondary phase. It can be noticed that there are three characteristic major intensity peaks (1 1 1), (2 2 0) and (3 1 1) of Cu2 xSe along with CIAS phase. Due to the structural similarity of Cu2 xSe and chalcopyrite/sphalerite CIAS, a broad band (overlapping) was observed, which may represent either the Cu2 xSe (1 1 1) reflection or the CIAS (1 1 1). These reflections indicate that the secondary phase must be of compatible crystal structure and is in excellent register with the CIAS host lattice. An attempt has been made to identify the structure of CuIn0.75Al0.25Se2 thin film samples by SAED on the large grains. The SAED pattern of CuIn0.75Al0.25Se2 thin films deposited at TS =573 K at larger grains is shown in Fig. 4(c), which clearly indicates sphalerite structure.
176 cm-1
3.2.3. Micro-Raman analysis Raman spectroscopy was identified as a tool for finding the phase homogeneity and crystalline quality of the thin films. It is used to monitor not only the chemical composition but also the presence of additional phases and regions with
1200
CIAS: TS = 603 K
600 500 400 300
261 cm-1
700
184 cm-1
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183 cm-1
Intensity (counts/sec)
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242 cm-1
1000 216 cm-1 224 cm-1
CIAS Powder CIAS: TS = 573 K
128 cm-1
1100
200 100 0 100
125
150
175
200
225
250
275
300
Raman shift (cm-1) Fig. 4. Selected area electron diffraction patterns of CuIn0.75Al0.25Se2 films formed at (a) TS = 573 K, (b) TS = 603 K and (c) SAED pattern of large crystallite in CuIn0.75Al0.25Se2 thin film deposited at TS = 573 K.
Fig. 5. Raman spectra of CuIn0.75Al0.25Se2 powder and thin films deposited at substrate temperatures 573 and 603 K, respectively.
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local lattice order. The Raman spectra of CuIn0.75Al0.25Se2 powder is shown in Fig. 5. Five peaks are observed in the spectrum, with most intense line at 176 cm 1 due to A1 mode generally observed for I–III–VI2 compounds having chalcopyrite structure. This arises due to the in-plane motion of selenium atoms with the cations at rest. The A1 phonon frequency obtained for CuIn0.75Al0.25Se2 powder in the present investigation is in good agreement with nanocrystalline CuIn1 xAlxSe2 with x=0.25 [23]. The remaining four peaks at 128 and 242 cm 1 are due to B1 modes and E2 mode at 224 cm 1, B2 mode at 216 cm 1, respectively. A shift in the A1 mode to 184 cm 1 was observed for the CIAS film deposited at TS =573 K, indicating a sphalerite phase. A similar trend has been observed during the incorporation of zinc
(exceeding 20 at%) in Zn2 2xCuxInxSe2 films [24]. An additional peak at 261 cm 1 observed for the films deposited at substrate temperature of 603 K was due to Cu2 xSe phase. So, it is evident from the XRD analysis, electron diffraction measurements and micro-Raman studies that the CuIn0.75Al0.25Se2 films deposited at TS =573 K exhibit sphalerite structure unlike the prepared bulk CuIn0.75Al0.25Se2 compound with chalcopyrite structure. 3.2.4. Surface morphology The AFM images of CuIn0.75Al0.25Se2 films formed at different substrate temperatures were shown in Fig. 6(a)–(c). The films formed at 423 K showed a large number of particles uniformly distributed throughout the
Fig. 6. AFM images of CuIn0.75Al0.25Se2 films deposited at different substrate temperatures (a) TS = 423 K, (b) TS =573 K and (c) TS = 603 K.
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surface. Crystallites show columnar growth with mean grain size of around 110 nm. As the substrate temperature increased to 573 K, the grain size of the films increases due to the improvement in the crystallinity. The films formed at 573 K showed an enhancement in the particle size with mean grain size of around 390 nm. A decrease in grain size ( 280 nm) was observed for the films deposited at 603 K. This might be due to re-evaporation of selenium and indium ad-atoms from the surface of the sample and or due to incorporation of aluminium into the lattice sites and or might be due to growth of Cu2 xSe phase at the expense of suppressing CIAS phase. Usually copper-rich I–III–VI2 films, leading to the growth of Cu2 xSe phase, should have larger crystallite size, but a decrease in the grain size has been observed for the films deposited at 603 K. The reason might be due to insufficient substrate temperature for the exaggerated grain growth of Cu2 xSe ( o656 K) [25].
where t is the thickness of the film, T is the transmittance and R is the reflectance. It is considered as a direct transition between the top of the valence band and bottom of the conduction band in order to estimate the optical band gap (Eg) of the films using
ahn ¼ Aðhn-Eg Þ1=2
ð2Þ
where A is the edge width parameter and hn is the photon energy. The optical band gap of the films was determined from the extrapolation of the linear plot of (ahn)2 versus hn at a = 0 shown in Fig. 7(b). The energy gap of CuIn0.75Al0.25Se2 film (300 nm) deposited at TS = 573 K was found to be 1.16 eV. This is in good agreement with the reported value for CuIn0.75Al0.25Se2 thin films deposited by other methods [3,4,12]. An additional absorption hump observed with an energy gap of 1.8 eV along with 1.22 eV (fundamental gap) for the CuIn0.75Al0.25Se2 film (300 nm) deposited at TS =603 K was due to the formation of Cu2 xSe phase. This is consistent with results obtained from XRD, SAED and Raman spectra. The increase in the fundamental energy gap with increase in substrate temperature may be attributed to the decrease in particle size, leading to an increase in strain and/or due to slight increase in Al/(In+ Al) ratio (observed from compositional analysis). 4. Conclusion CuIn0.75Al0.25Se2 thin films were grown by flash evaporation method onto glass substrates held at substrate temperatures in the range 303–603 K. The impact of substrate temperature on growth of the films was studied. The highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films were observed at TS =573 K, exhibiting sphalerite structure. These nearly
TS = 573 K TS = 603 K
100 90 80
T
70
Cu2-xSe
60 50 40 30
R
20 10 0 600
800 1000 1200 1400 1600 1800 2000 2200 2400 Wavelength (nm)
1.6x109 1.4x109 1.2x109 (αhν)2 (eV cm-1)2
3.2.5. Optical properties The optical transmittance and reflectance of the CuIn0.75 Al0.25Se2 film (300 nm) deposited at TS =573 and 603 K are shown in the Fig. 7(a). The optical absorption coefficient (a) was calculated from the optical transmittance (T) and reflectance (R) data by 1 T ln a¼ ð1Þ 2 t 1R
293
1.0x10
9
8.0x108 6.0x108 4.0x108 2.0x10
8
0.0 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 hν (eV)
Fig. 7. (a) Optical transmittance and reflectance spectra of the CuIn0.75 Al0.25Se2. films (300 nm) deposited at TS =573 and 603 K and (b) Plot of (ahn)2 versus hn of CuIn0.75Al0.25Se2 film deposited at 573 K.
stoichiometric and single phase CuIn0.75Al0.25Se2 films with optical band gap of 1.16 eV could be used as an absorber material in the fabrication of thin film heterojunction solar cells. XRD, SAED and micro-Raman measurements indicated the presence of Cu2 xSe binary phase for the CuIn0.75Al0.25Se2 films deposited at TS =603 K.
Acknowledgements The authors are thankful to the Defence Research and Development Organization, New Delhi, India, for providing financial support to carry out the present research work. The authors are grateful to Prof. S.B. Krupanidhi, MRC, IISc, Bangalore, for valuable technical discussions and for his help in SAED characterization. We would like to thank Dr. U. Kamachi Mudali for providing AFM facility at IGCAR, Kalpakkam. The authors are thankful to Dr. Harish C. Barshilia, NAL, Bangalore, for extending his co-operation for microRaman facility. We are thankful to Prof. V. Sundara Raja, S.V. University, Tirupati, for fruitful discussions.
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