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Preparation and characterization of aluminum-incorporated cadmium oxide films
Materials Science in Semiconductor Processing 13 (2010) 109–114
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Preparation and characterization of aluminum-incorporated cadmium oxide films I. Akyuz a,, S. Kose a, F. Atay a, V. Bilgin b a b
Eskisehir Osmangazi University, Physics Department, Eskisehir, Turkey Canakkale Onsekiz Mart University, Physics Department, Canakkale, Turkey
a r t i c l e in fo
abstract
Available online 2 July 2010
Opto-electronic and photovoltaic solar cell technologies which are developing day by day need novel and alternative materials. Also, economic cost is the other important parameter when producing and using these materials. With this purpose, we have prepared aluminum-incorporated CdO films by ultrasonic spray pyrolysis (USP) technique. Firstly, elemental analyses were performed to observe the distribution rate of Al in the structure. We have attempted to explain the structural and electrical properties of these films in detail. The crystalline structure was studied by X-ray diffraction (XRD). Besides, some structural parameters such as texture coefficient, grain size and dislocation density were calculated. Van der Pauw and Hall measurements were used to investigate the electrical properties. Electrical conductivity, carrier concentration and mobility values were determined for all films. Finally, we conclude that Al-incorporated CdO films with low Al concentrations will be promising materials for future works because of their high conductivity and mobility values as compared to others. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors X-ray diffraction Electrical properties
1. Introduction Transparent conducting oxides (TCOs) have become popular in semiconductor technology because of their low cost and high performance. But the present TCO technology has been limited to a few materials. Thin films of binary oxides such as SnO2, In2O3, ZnO and CdO show a metal-like conductivity and high transparency in the visible and near-infrared regions of the spectrum. These materials have a large application area in technology. Among these, CdO is a promising material for future works because of its suitable physical properties and can be used as an alternative material for opto-electronic devices and solar cells. CdO films have high conductivity and high transmittance in the visible region which makes
Corresponding author.
E-mail addresses: [email protected] (I. Akyuz), [email protected] (S. Kose), [email protected] (F. Atay), [email protected] (V. Bilgin). 1369-8001/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2010.05.006
them good candidates for solar cell applications as transparent electrodes. Also CdO films find applications as photo transistors, photo diodes, gas sensors [1,2] and displays [3]. CdO is an n-type semiconductor with a NaCl structure. Nonstoichiometric unincorporated CdO thin films usually exhibit low resistivity due to native defects of oxygen vacancies and cadmium interstitials. Hence, low resistivity films can be obtained by controlling these native defects [4]. Doping with different elements will change the properties of the films. Although CdO films have previously been produced using spray pyrolysis, studies on developing their physical characteristics by doping have not been carried out intensively. There are works on In [5], Sn [4,6], F [7,8] and Ti [9,10] incorporated CdO films. Besides, a literature survey showed that there are limited numbers of reports on Al-incorporated CdO films obtained by different techniques [11–15]. CdO thin films have been grown using many different deposition techniques such as chemical bath deposition [16], sol–gel [17], spray pyrolysis [2,8,18–21], thermal evaporation
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I. Akyuz et al. / Materials Science in Semiconductor Processing 13 (2010) 109–114
[22], atmospheric metal organic chemical vapor deposition [4] and pulsed laser deposition [23,24]. Today, available materials have become insufficient for devices that need higher performance, and technology needs studies to produce novel materials. With this aim, we have produced unincorporated and Al-incorporated CdO films by USP which is a suitable technique for research/development studies and made a detailed investigation on their physical characteristics for optoelectronic and solar cell applications. We conclude that Al-incorporated CdO films are promising materials for technological applications. Al as an incorporation element can be used to change electrical and structural properties and to raise the chemical stability. 2. Experimental Films were produced with different Al incorporation percentages (1, 2 and 3 at%) using the USP technique. Details of the USP technique were given in our previous works [25]. In total 0.1 M of Cd(CH3COO)2 2H2O and AlCl3 6H2O solutions were prepared. Deionized water was used as a solvent. The spraying solution was prepared by mixing the two solutions at required volumes. Totally 200 ml of solution was sprayed onto pyrex glass substrates, which were previously heated to 300 1C, during the 40 min growth. Flow rate was controlled with a flowmeter and kept at 5 ml min 1. The spraying solution was mixed by a magnetic mixer before and during the deposition process. Compressed air was used as carrier gas ( 1 bar). The distance between the nozzle and substrate was about 30 cm. The thicknesses of the films were measured by a computer controlled Leitz PMM 12106 thickness gauge. The thicknesses and volumes of used solutions are given in Table 1. The produced films are named as A-0 (unincorporated CdO film), A-1, A-2 and A-3 (Al-incorporated CdO films at 1%, 2% and 3%, respectively). The structural properties of all films were studied by an X-ray diffractometer (Rigaku ˚ with CuKa radiation. The electrical Model, l = 1.5418 A) properties of the films were investigated by Van der Pauw technique, and carrier concentration and mobility values were obtained by Hall experiment. Also, energy dispersive X-ray spectroscopy was used to analyze elemental distribution of the films.
Fig. 1. (a) % Elemental weights of Al and Cd elements and (b) Al/Cd atomic ratios in the films.
that of the solid film. Fig. 1(a) and (b) shows the % elemental weights of Al and Cd elements and Al/Cd atomic ratios in the films, respectively. This figure and Al/O ratio values, which are not given here, show that low Al incorporations make the structure have more Cd atoms. But, when the amount of Al element increased, less Cd elements are present in the film. This must be related to the high probability of Al making covalent bond with oxygen. A-2 and A-3 samples are far from stoichiometry. A-1 is the most stoichiometric one among others.
3. Results and discussion 3.1. Structural properties
2.1. Elemental analyses It was determined from EDS analyses that the amount of Al element in the spraying solution is different from Table 1 The production parameters of the films. Material
Cd(CH3OO)2 2H2O A0 A1 A2 A3
Thickness (mm)
Solution amount (ml)
200 198 196 194
AlCl3 6H2O – 2 4 6
XRD patterns of all films are given in Fig. 2. Using these patterns, the crystallinity levels and some structural properties of the films were investigated. Fig. 2 shows the polycrystalline nature of them. The peak at 231 and other a few small peaks (for sample A-0 with lowest thickness) belong to a Si-based phase, which probably result from the substrate itself. The effect of these peaks gets weak or disappears as the thicknesses of the films increase with Al doping. Three most intensive peaks on each pattern belong to reflections from (1 1 1) CdO2, (1 1 1) CdO and (2 0 0) CdO planes. Haris analysis was performed to determine the texture coefficient using the equation below [26]
Total 200 200 200 200
2.00 2.58 3.70 3.88
Pðhi ki li Þ ¼
" #1 n Iðhi ki li Þ 1 X Iðhi ki li Þ I0 ðhi ki li Þ n i ¼ 1 I0 ðhi ki li Þ
ð1Þ
where I0 represents the standard intensity (ASTM, American Society of Testing Materials), I the measured intensity of ðhi ki li Þ plane and n the reflection number. For a preferential orientation, Pðhi ki li Þ has to be
I. Akyuz et al. / Materials Science in Semiconductor Processing 13 (2010) 109–114
111
Fig. 2. XRD patterns of the films. larger than one [27]. Grain sizes were calculated using the Scherrer formula [28] D¼
0:9l b cos y
ð2Þ
where b is the half-peak width as radian of the peak which has maximum intensity, D the grain size, y the Bragg angle and l the wavelength of light used. The dislocation density (d) is defined as the length of dislocation lines per unit volume of the crystal. It can be calculated using the formula [8]
d¼
n D2
ð3Þ
The d-value is criterion of crystallization level. Lower d-values indicate higher crystallinity levels for the films since these values represent the amount of defects in the film. The calculated structural parameters for three most intensive peaks are given in Tables 2–4. XRD patterns show that peak intensities became lower, half-peak widths and background intensity rise especially for samples A-2 and A-3. We can say that Al incorporation at 2% and 3% deteriorates the film structure. But unstable CdO2 phase became recessive with Al incorporation; therefore, the film goes to a more stable phase. The bonding of Al with Cd and unstable oxygen in the structure could be the reason of this. The peaks with low intensity (CdAl4O7 and CdAl2O4) in Al-incorporated films support this idea.
P-values for each three peaks are given in Fig. 3. There is not a linear change in P-values with respect to Al doping. This is probably due to the unstable character of Al doping at high rates (at 2% and 3%). One percent Al incorporation caused the films to go to a more stable structure with decreased CdO2 peaks. Another reason of the random variation of preferential orientation by Al incorporation is probably the dominant existence of interstitial Al atoms besides substituted (Al-Cd) atoms which differs from sample to sample. Bonding of Al with oxygen also affected the sizes of the grains through (1 1 1) CdO2 orientation, as shown in Fig. 4.
3.2. Electrical properties Data taken from Hall measurements and resistivity values are given in Table 5. The variation of conductivity with Al incorporation is given in Fig. 5. Table 5 and Fig. 5 show that the conductivity decreases with increasing Al incorporation. This could be attributed to the Al atoms located at interstitial positions and formed crystal defects and/or low carrier concentration. Also, metallic Al could be located at grain boundaries and became electrically inactive. In addition, Al2O3 is a defect which can easily be formed. We think that there are Al2O3 phases which can not be observed by XRD since amount of them is low. This would deteriorate the electrical characteristics of the films as seen by Tsuji and Hirohashi [29].
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Table 2 Structural parameters for (1 1 1) CdO2 phase. Material
A0 A1 A2 A3
d 10 6
P
2y (1)
˚ d (A)
˚ a (A)
(ASTM:29.141)
˚ (ASTM:3.062 A)
˚ (ASTM:5.313 A)
˚ D (A)
29.36 29.40 29.32 29.46
3.040 3.036 3.044 3.030
5.265 5.258 5.272 5.247
523 449 442 383
1.399 0.748 1.395 1.237
˚ D (A)
P
(line/nm2) 3.65 4.95 5.11 6.81
Table 3 Structural parameters for (1 1 1) CdO phase. Material
A0 A1 A2 A3
2y (1)
˚ d (A)
˚ a (A)
(ASTM:33.001)
˚ (ASTM:2.712 A)
˚ (ASTM:4.695 A)
32.94 33.04 32.90 33.22
2.717 2.709 2.720 2.695
4.706 4.692 4.712 4.667
244 232 189 243
0.653 1.049 0.851 1.289
˚ d (A)
˚ a (A)
˚ D (A)
P
d 10 6 (line/nm2) 16.82 18.56 28.12 16.98
Table 4 Structural parameters for (2 0 0) CdO phase. Material
A0 A1 A2 A3
2y (1) (ASTM:38.291)
˚ (ASTM:2.349 A)
˚ (ASTM:4.695 A)
38.24 38.36 38.14 38.40
2.352 2.345 2.358 2.342
4.703 4.689 4.715 4.685
234 199 212 189
0.601 1.252 0.605 0.763
18.30 25.29 22.33 28.11
Fig. 4. Grain size (D) values of the films.
Fig. 3. Texture coefficient (P) values of the films.
The variation of carrier concentration for Al-incorporated CdO films is given in Fig. 6. There is a decrease in carrier concentration with increasing Al incorporation. This could be explained by two ways: (i) Al atoms which have three electrons desire to make covalent bond with oxygen. Al atom takes the fourth electron from its environment and decreases the carrier concentration in the sample. (ii) It is known that Al element has a property (namely Lewis acidity) of locating electron couples on its empty valance orbitals. So, Al atoms would catch electron couples from valance band and decrease the carrier concentration. Also, every broken bond in especially poor crystallized A-2 and A-3 samples
d 10 6 (line/nm2)
Table 5 Electrical parameters of the films. Material r (O cm) s (O cm) 1 n 1019 (cm 3) m (cm2 V 1 s 1) A0 A1 A2 A3
0.52 1.32 16.45 14.23
1.92 0.76 0.0607 0.0702
4.89 1.89 0.68 0.52
0.245 0.25 0.056 0.085
I. Akyuz et al. / Materials Science in Semiconductor Processing 13 (2010) 109–114
113
Fig. 7. The variation of mobility with Al incorporation. ˚ Since the It was found that the values are between 0.14 and 81.7 A. grain size values determined from XRD analyses are higher, there is a low probability of grain-boundary scattering.
Fig. 5. The variation of conductivity with Al incorporation.
4. Conclusions
Fig. 6. The variation of carrier concentration for Al-incorporated cadmium oxide films. would decrease one electron in the crystal and cause the carrier concentration become lower. The variation of mobility with Al incorporation is given in Fig. 7. Samples A-2 and A-3 have low mobility values. But, in sample A-1 mobility became higher. The low mobility of A-2 and A-3 could probably due to the extended defects. We know from XRD analyses that grain sizes were decreased by Al incorporation. This means that the number of grain boundaries increased. High defect densities could locate at boundaries and decrease the mobility [30]. Mobility and electrical conductivity values are lower as compared to other works. This is probably due to the unstable CdO2 phase which presents in the films, especially for samples having high Al amount (A-2 and A-3). However, these values are promising for such a simple and economic technique (USP) which needs no vacuum. Post-deposition processes such as thermal annealing would probably cause the unstable CdO2 phase to disappear and an increase both in mobility and electrical conductivity. Grain-boundary scattering could also affect the mobility. We have made a simple calculation to see this. We determine the value of mean free path as [8] ‘ ¼ ‘ ð3p2 nÞ1=3
m e
ð4Þ
In this work, Al-incorporated CdO films which have a great importance in opto-electronics and photovoltaic solar cells were produced by a low-cost Ultrasonic Spray Pyrolysis Technique. Elemental analyses were performed, and structural and electrical properties were investigated. Al-incorporated samples except A-1 have a poor crystallinity and contain deformations with broken bonds. It was seen that high Al concentration deteriorated the lattice and lower the crystalline level. But, Al incorporation at 1% showed a hope for future works. Especially, the high mobility of this film is an advantage among other TCOs like SnO2, ZnO and In2O3. We predict that post-deposition treatments and alternative experimental parameters will lead to higher quality material. Al incorporation at high rates (samples A-2 and A-3) limited the electrical conductivity. Low mobility and high resistivity values were obtained for samples A-2 and A-3. It was seen that the carrier concentration decreased at high Al incorporation rates. We think that Al element formed a hole effect in the structure. Maybe, the idea of producing a p-type material by rising up the amount of Al could be thought (an alloy like CdxOAl1 x). More effective heterojunctions can be obtained by using these n- and p-type layers whose atomic radii are close to each other. Also, Al-incorporated films have an advantage of their low carrier concentrations as they would show a high transparency in the infrared part of the spectrum. References [1] Subramanyam TK, Naidu BS, Uthana S. Appl Surf Sci 2001;169– 170:529. [2] Reddy KTR, Shanthini GM, Johnston D, Miles RW. Thin Solid Films 2003;427:397. [3] Kim B, Ok YW, Seong TY, Ashrafi ABMA, Kumano H, Suemune I. J Cryst Growth 2003;252:219. [4] Zhao Z, Morel DL, Ferekides CS. Thin Solid Films 2002;413:203. [5] Freeman AJ, Poeppelmeier KR, Mason TO, Chang RPH, Marks TJ. Mater Res Soc Bull 2000;25:45.
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[6] Yan M, Lane M, Kannewurf CR, Chang RPH. Appl Phys Lett 2001;78:2342. [7] Ghosh PK, Maity R, Chattopadhyay KK. Sol Energy Mater Sol Cells 2004;81:279. [8] Ferro R, Rodriguez JA, Vigil O, Morales-Acevedo A. Mater Sci Eng B 2001;87:83. [9] Gupta RK, Ghosh K, Patel R, Kahol PK. Appl Surf Sci 2009;255:6252. [10] Gupta RK, Ghosh K, Patel R, Kahol PK. Appl Surf Sci 2008;255:2414. [11] Maity R, Chattopadhyay KK. Sol Energy Mater Sol Cells 2006;90:597. [12] Khan MKR, Rahman MA, Shahjahan M, Rahman MM, Hakim MA, Saha DK, et al., Curr Appl Phys, in press. [13] Gupta RK, Ghosh K, Patel R, Mishra SR, Kahol PK. Curr Appl Phys 2009;9:673–7. [14] Gupta RK, Ghosh K, Patel R, Mishra SR, Kahol PK. Appl Surf Sci 2008;254:5868–73. [15] Saha B, Das S, Chattopadhyay KK. Sol Energy Mater Sol Cells 2007;91:1692–7. [16] Ocampo M, Sabastian PJ, Campos J. Phys Status Solidi A 1994;143:K29. [17] Carbelleda-Galicia DM, Casanedo-Perez R, Jimemez-Sandoval O, Jimenez-Sandoval S, Torres-Delgado G, Zuniga-Romero CI. Thin Solid Films 2000;317:105.
[18] Ferro R, Rodriguez JA, Vigil O, Morales-Acevedo A, Contreras-Puente G. Phys Status Solidi A 2000;177:477. [19] Lokhande BJ, Uplane MD. Mater Res Bull 2001;36:439. [20] Vigil O, Cruz F, Morales-Acevedo A, Contreras-Puente G, Vaillant L, Santana G. Mater Chem Phys 2001;38:249. [21] Uplane MD, Kshirsagan PN, Lokhande BJ, Bhosale CH. Mater Chem Phys 2000;64:75. [22] Dakhel AA, Henari FZ. Cryst Res Technol 2003;38:979. [23] Gupta RK, Ghosh K, Patel R, Kahol PK. Appl Surf Sci 2009;255:4466. [24] Gupta RK, Ghosh K, Patel R, Mishra SR, Kahol PK. Appl Surf Sci 2008;254:5868. [25] Atay F, Bilgin V, Akyuz I, Kose S. Mater Sci Semicond Process 2003;6:197. [26] Barrett CS, Massalski TB. Structure of metals. Oxford: Pergamon; 1980 (p. 204). [27] Nair JP, Jayakrishnan R, Chaure NB, Pandey RK. Letter to the editor. Semicond Sci Technol 1998;13:340. [28] Mamazza Jr R, Morel DL, Ferekides CS. Thin Solid Films 2005;484:26. [29] Tsuji T, Hirohashi M. Appl Surf Sci 2000;157:47. [30] Mondragon-Suarez H, Maldonado A, Olvera M, Reyes A, CastanedoPerez R, Torres-Delgado G, et al. Appl Surf Sci 2002;7794:1.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Preparation and characterization of aluminum-incorporated cadmium oxide films I. Akyuz a,, S. Kose a, F. Atay a, V. Bilgin b a b
Eskisehir Osmangazi University, Physics Department, Eskisehir, Turkey Canakkale Onsekiz Mart University, Physics Department, Canakkale, Turkey
a r t i c l e in fo
abstract
Available online 2 July 2010
Opto-electronic and photovoltaic solar cell technologies which are developing day by day need novel and alternative materials. Also, economic cost is the other important parameter when producing and using these materials. With this purpose, we have prepared aluminum-incorporated CdO films by ultrasonic spray pyrolysis (USP) technique. Firstly, elemental analyses were performed to observe the distribution rate of Al in the structure. We have attempted to explain the structural and electrical properties of these films in detail. The crystalline structure was studied by X-ray diffraction (XRD). Besides, some structural parameters such as texture coefficient, grain size and dislocation density were calculated. Van der Pauw and Hall measurements were used to investigate the electrical properties. Electrical conductivity, carrier concentration and mobility values were determined for all films. Finally, we conclude that Al-incorporated CdO films with low Al concentrations will be promising materials for future works because of their high conductivity and mobility values as compared to others. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors X-ray diffraction Electrical properties
1. Introduction Transparent conducting oxides (TCOs) have become popular in semiconductor technology because of their low cost and high performance. But the present TCO technology has been limited to a few materials. Thin films of binary oxides such as SnO2, In2O3, ZnO and CdO show a metal-like conductivity and high transparency in the visible and near-infrared regions of the spectrum. These materials have a large application area in technology. Among these, CdO is a promising material for future works because of its suitable physical properties and can be used as an alternative material for opto-electronic devices and solar cells. CdO films have high conductivity and high transmittance in the visible region which makes
Corresponding author.
E-mail addresses: [email protected] (I. Akyuz), [email protected] (S. Kose), [email protected] (F. Atay), [email protected] (V. Bilgin). 1369-8001/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2010.05.006
them good candidates for solar cell applications as transparent electrodes. Also CdO films find applications as photo transistors, photo diodes, gas sensors [1,2] and displays [3]. CdO is an n-type semiconductor with a NaCl structure. Nonstoichiometric unincorporated CdO thin films usually exhibit low resistivity due to native defects of oxygen vacancies and cadmium interstitials. Hence, low resistivity films can be obtained by controlling these native defects [4]. Doping with different elements will change the properties of the films. Although CdO films have previously been produced using spray pyrolysis, studies on developing their physical characteristics by doping have not been carried out intensively. There are works on In [5], Sn [4,6], F [7,8] and Ti [9,10] incorporated CdO films. Besides, a literature survey showed that there are limited numbers of reports on Al-incorporated CdO films obtained by different techniques [11–15]. CdO thin films have been grown using many different deposition techniques such as chemical bath deposition [16], sol–gel [17], spray pyrolysis [2,8,18–21], thermal evaporation
110
I. Akyuz et al. / Materials Science in Semiconductor Processing 13 (2010) 109–114
[22], atmospheric metal organic chemical vapor deposition [4] and pulsed laser deposition [23,24]. Today, available materials have become insufficient for devices that need higher performance, and technology needs studies to produce novel materials. With this aim, we have produced unincorporated and Al-incorporated CdO films by USP which is a suitable technique for research/development studies and made a detailed investigation on their physical characteristics for optoelectronic and solar cell applications. We conclude that Al-incorporated CdO films are promising materials for technological applications. Al as an incorporation element can be used to change electrical and structural properties and to raise the chemical stability. 2. Experimental Films were produced with different Al incorporation percentages (1, 2 and 3 at%) using the USP technique. Details of the USP technique were given in our previous works [25]. In total 0.1 M of Cd(CH3COO)2 2H2O and AlCl3 6H2O solutions were prepared. Deionized water was used as a solvent. The spraying solution was prepared by mixing the two solutions at required volumes. Totally 200 ml of solution was sprayed onto pyrex glass substrates, which were previously heated to 300 1C, during the 40 min growth. Flow rate was controlled with a flowmeter and kept at 5 ml min 1. The spraying solution was mixed by a magnetic mixer before and during the deposition process. Compressed air was used as carrier gas ( 1 bar). The distance between the nozzle and substrate was about 30 cm. The thicknesses of the films were measured by a computer controlled Leitz PMM 12106 thickness gauge. The thicknesses and volumes of used solutions are given in Table 1. The produced films are named as A-0 (unincorporated CdO film), A-1, A-2 and A-3 (Al-incorporated CdO films at 1%, 2% and 3%, respectively). The structural properties of all films were studied by an X-ray diffractometer (Rigaku ˚ with CuKa radiation. The electrical Model, l = 1.5418 A) properties of the films were investigated by Van der Pauw technique, and carrier concentration and mobility values were obtained by Hall experiment. Also, energy dispersive X-ray spectroscopy was used to analyze elemental distribution of the films.
Fig. 1. (a) % Elemental weights of Al and Cd elements and (b) Al/Cd atomic ratios in the films.
that of the solid film. Fig. 1(a) and (b) shows the % elemental weights of Al and Cd elements and Al/Cd atomic ratios in the films, respectively. This figure and Al/O ratio values, which are not given here, show that low Al incorporations make the structure have more Cd atoms. But, when the amount of Al element increased, less Cd elements are present in the film. This must be related to the high probability of Al making covalent bond with oxygen. A-2 and A-3 samples are far from stoichiometry. A-1 is the most stoichiometric one among others.
3. Results and discussion 3.1. Structural properties
2.1. Elemental analyses It was determined from EDS analyses that the amount of Al element in the spraying solution is different from Table 1 The production parameters of the films. Material
Cd(CH3OO)2 2H2O A0 A1 A2 A3
Thickness (mm)
Solution amount (ml)
200 198 196 194
AlCl3 6H2O – 2 4 6
XRD patterns of all films are given in Fig. 2. Using these patterns, the crystallinity levels and some structural properties of the films were investigated. Fig. 2 shows the polycrystalline nature of them. The peak at 231 and other a few small peaks (for sample A-0 with lowest thickness) belong to a Si-based phase, which probably result from the substrate itself. The effect of these peaks gets weak or disappears as the thicknesses of the films increase with Al doping. Three most intensive peaks on each pattern belong to reflections from (1 1 1) CdO2, (1 1 1) CdO and (2 0 0) CdO planes. Haris analysis was performed to determine the texture coefficient using the equation below [26]
Total 200 200 200 200
2.00 2.58 3.70 3.88
Pðhi ki li Þ ¼
" #1 n Iðhi ki li Þ 1 X Iðhi ki li Þ I0 ðhi ki li Þ n i ¼ 1 I0 ðhi ki li Þ
ð1Þ
where I0 represents the standard intensity (ASTM, American Society of Testing Materials), I the measured intensity of ðhi ki li Þ plane and n the reflection number. For a preferential orientation, Pðhi ki li Þ has to be
I. Akyuz et al. / Materials Science in Semiconductor Processing 13 (2010) 109–114
111
Fig. 2. XRD patterns of the films. larger than one [27]. Grain sizes were calculated using the Scherrer formula [28] D¼
0:9l b cos y
ð2Þ
where b is the half-peak width as radian of the peak which has maximum intensity, D the grain size, y the Bragg angle and l the wavelength of light used. The dislocation density (d) is defined as the length of dislocation lines per unit volume of the crystal. It can be calculated using the formula [8]
d¼
n D2
ð3Þ
The d-value is criterion of crystallization level. Lower d-values indicate higher crystallinity levels for the films since these values represent the amount of defects in the film. The calculated structural parameters for three most intensive peaks are given in Tables 2–4. XRD patterns show that peak intensities became lower, half-peak widths and background intensity rise especially for samples A-2 and A-3. We can say that Al incorporation at 2% and 3% deteriorates the film structure. But unstable CdO2 phase became recessive with Al incorporation; therefore, the film goes to a more stable phase. The bonding of Al with Cd and unstable oxygen in the structure could be the reason of this. The peaks with low intensity (CdAl4O7 and CdAl2O4) in Al-incorporated films support this idea.
P-values for each three peaks are given in Fig. 3. There is not a linear change in P-values with respect to Al doping. This is probably due to the unstable character of Al doping at high rates (at 2% and 3%). One percent Al incorporation caused the films to go to a more stable structure with decreased CdO2 peaks. Another reason of the random variation of preferential orientation by Al incorporation is probably the dominant existence of interstitial Al atoms besides substituted (Al-Cd) atoms which differs from sample to sample. Bonding of Al with oxygen also affected the sizes of the grains through (1 1 1) CdO2 orientation, as shown in Fig. 4.
3.2. Electrical properties Data taken from Hall measurements and resistivity values are given in Table 5. The variation of conductivity with Al incorporation is given in Fig. 5. Table 5 and Fig. 5 show that the conductivity decreases with increasing Al incorporation. This could be attributed to the Al atoms located at interstitial positions and formed crystal defects and/or low carrier concentration. Also, metallic Al could be located at grain boundaries and became electrically inactive. In addition, Al2O3 is a defect which can easily be formed. We think that there are Al2O3 phases which can not be observed by XRD since amount of them is low. This would deteriorate the electrical characteristics of the films as seen by Tsuji and Hirohashi [29].
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Table 2 Structural parameters for (1 1 1) CdO2 phase. Material
A0 A1 A2 A3
d 10 6
P
2y (1)
˚ d (A)
˚ a (A)
(ASTM:29.141)
˚ (ASTM:3.062 A)
˚ (ASTM:5.313 A)
˚ D (A)
29.36 29.40 29.32 29.46
3.040 3.036 3.044 3.030
5.265 5.258 5.272 5.247
523 449 442 383
1.399 0.748 1.395 1.237
˚ D (A)
P
(line/nm2) 3.65 4.95 5.11 6.81
Table 3 Structural parameters for (1 1 1) CdO phase. Material
A0 A1 A2 A3
2y (1)
˚ d (A)
˚ a (A)
(ASTM:33.001)
˚ (ASTM:2.712 A)
˚ (ASTM:4.695 A)
32.94 33.04 32.90 33.22
2.717 2.709 2.720 2.695
4.706 4.692 4.712 4.667
244 232 189 243
0.653 1.049 0.851 1.289
˚ d (A)
˚ a (A)
˚ D (A)
P
d 10 6 (line/nm2) 16.82 18.56 28.12 16.98
Table 4 Structural parameters for (2 0 0) CdO phase. Material
A0 A1 A2 A3
2y (1) (ASTM:38.291)
˚ (ASTM:2.349 A)
˚ (ASTM:4.695 A)
38.24 38.36 38.14 38.40
2.352 2.345 2.358 2.342
4.703 4.689 4.715 4.685
234 199 212 189
0.601 1.252 0.605 0.763
18.30 25.29 22.33 28.11
Fig. 4. Grain size (D) values of the films.
Fig. 3. Texture coefficient (P) values of the films.
The variation of carrier concentration for Al-incorporated CdO films is given in Fig. 6. There is a decrease in carrier concentration with increasing Al incorporation. This could be explained by two ways: (i) Al atoms which have three electrons desire to make covalent bond with oxygen. Al atom takes the fourth electron from its environment and decreases the carrier concentration in the sample. (ii) It is known that Al element has a property (namely Lewis acidity) of locating electron couples on its empty valance orbitals. So, Al atoms would catch electron couples from valance band and decrease the carrier concentration. Also, every broken bond in especially poor crystallized A-2 and A-3 samples
d 10 6 (line/nm2)
Table 5 Electrical parameters of the films. Material r (O cm) s (O cm) 1 n 1019 (cm 3) m (cm2 V 1 s 1) A0 A1 A2 A3
0.52 1.32 16.45 14.23
1.92 0.76 0.0607 0.0702
4.89 1.89 0.68 0.52
0.245 0.25 0.056 0.085
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Fig. 7. The variation of mobility with Al incorporation. ˚ Since the It was found that the values are between 0.14 and 81.7 A. grain size values determined from XRD analyses are higher, there is a low probability of grain-boundary scattering.
Fig. 5. The variation of conductivity with Al incorporation.
4. Conclusions
Fig. 6. The variation of carrier concentration for Al-incorporated cadmium oxide films. would decrease one electron in the crystal and cause the carrier concentration become lower. The variation of mobility with Al incorporation is given in Fig. 7. Samples A-2 and A-3 have low mobility values. But, in sample A-1 mobility became higher. The low mobility of A-2 and A-3 could probably due to the extended defects. We know from XRD analyses that grain sizes were decreased by Al incorporation. This means that the number of grain boundaries increased. High defect densities could locate at boundaries and decrease the mobility [30]. Mobility and electrical conductivity values are lower as compared to other works. This is probably due to the unstable CdO2 phase which presents in the films, especially for samples having high Al amount (A-2 and A-3). However, these values are promising for such a simple and economic technique (USP) which needs no vacuum. Post-deposition processes such as thermal annealing would probably cause the unstable CdO2 phase to disappear and an increase both in mobility and electrical conductivity. Grain-boundary scattering could also affect the mobility. We have made a simple calculation to see this. We determine the value of mean free path as [8] ‘ ¼ ‘ ð3p2 nÞ1=3
m e
ð4Þ
In this work, Al-incorporated CdO films which have a great importance in opto-electronics and photovoltaic solar cells were produced by a low-cost Ultrasonic Spray Pyrolysis Technique. Elemental analyses were performed, and structural and electrical properties were investigated. Al-incorporated samples except A-1 have a poor crystallinity and contain deformations with broken bonds. It was seen that high Al concentration deteriorated the lattice and lower the crystalline level. But, Al incorporation at 1% showed a hope for future works. Especially, the high mobility of this film is an advantage among other TCOs like SnO2, ZnO and In2O3. We predict that post-deposition treatments and alternative experimental parameters will lead to higher quality material. Al incorporation at high rates (samples A-2 and A-3) limited the electrical conductivity. Low mobility and high resistivity values were obtained for samples A-2 and A-3. It was seen that the carrier concentration decreased at high Al incorporation rates. We think that Al element formed a hole effect in the structure. Maybe, the idea of producing a p-type material by rising up the amount of Al could be thought (an alloy like CdxOAl1 x). More effective heterojunctions can be obtained by using these n- and p-type layers whose atomic radii are close to each other. Also, Al-incorporated films have an advantage of their low carrier concentrations as they would show a high transparency in the infrared part of the spectrum. References [1] Subramanyam TK, Naidu BS, Uthana S. Appl Surf Sci 2001;169– 170:529. [2] Reddy KTR, Shanthini GM, Johnston D, Miles RW. Thin Solid Films 2003;427:397. [3] Kim B, Ok YW, Seong TY, Ashrafi ABMA, Kumano H, Suemune I. J Cryst Growth 2003;252:219. [4] Zhao Z, Morel DL, Ferekides CS. Thin Solid Films 2002;413:203. [5] Freeman AJ, Poeppelmeier KR, Mason TO, Chang RPH, Marks TJ. Mater Res Soc Bull 2000;25:45.
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