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porous silicon
Materials Research Bulletin 41 (2006) 253–259 www.elsevier.com/locate/matresbu
Physical and electronic properties of ZnO:Al/porous silicon Choongmo Kim, Anna Park, K. Prabakar, Chongmu Lee * Department of Materials Science and Engineering, Inha University, 253 Younghyun-dong, Nam-Ku, Incheon 402-751, South Korea Received 13 October 2004; received in revised form 8 March 2005; accepted 26 August 2005 Available online 19 September 2005
Abstract UV, violet and blue-green photoluminescence has been achieved at room temperature (RT) from ZnO:Al (AZO) films deposited by radio frequency (rf) co-sputtering. As the ZnO target power increases from 100 W, the violet luminescence vanishes and the blue and green-blue luminescences appear. The most intense UV and blue-green luminescence is obtained for the films deposited at higher sputtering powers depending upon the stoichiometry of the films as well as the crystalline quality. The as-prepared porous silicon (PS) emission band lies in the blue-green spectral region and is blue shifted due to the AZO deposition. The current–voltage characteristics of AZO/PS heterostructures have been studied. The ideality factor is found to be 19 and the series resistance as determined from the forward characteristics is 36 MV. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Thin films; B. Sputtering; C. Atomic force microscopy; D. Luminescence
1. Introduction Discovery of efficient room temperature luminescence from porous silicon (PS) has evoked a lot of scientific and technological interest [1,2]. Much effort has been directed to obtain efficient electroluminescence devices based on PS [3–5]. Special attempts have been made first by Richiter et al. to realize solid state light emitting diodes (LED) based on metal–PS heterostructures [6]. Since then other groups have reported on such devices [7–9]. In most of them low efficiency is found, and light emission is achieved only for quite high voltages. These structures showed rectifying behaviour and electroluminescence (EL) under forward bias. The large ideality factor of the current–voltage characteristics was interpreted with an additional voltage drop in the PS layer due to the charging effect of interface states. The EL efficiency could be increased strongly by filling pores with indium or aluminium zinc oxide in a certain special region of the PS. This tendency led to various attempts to improve electroluminescence efficiency by replacing the gold contacts used by Richiter et al. with other materials, including indium tin oxide [10], copper films [11], ndoped microcrystalline silicon carbide [12] and conducting polymers [13]. Obviously additional investigations have still have to be performed in order to optimize the main features of the fabricated devices. Al:ZnO (AZO) is a wide direct band gap II–VI semiconductor with such potential applications in transparent conductive films, varistors and optoelectronic devices [14–16]. Although deposition of ZnO films on PS substrates have been carried out [17,18], the transport mechanism in PS is still under debate. The growth of AZO films on various substrates has been extensively studied, but there were no reports on the growth of AZO on the PS substrate to the best of our knowledge. In this work, * Corresponding author. Tel.: +82 32 860 7536; fax: +82 32 862 5546. E-mail address: [email protected] (C. Lee). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.08.018
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the properties of AZO, PS and AZO/PS structures were characterized to find out the suitability of these structures as applied in optoelectronic devices. 2. Experimental Porous silicon layers were electrochemically anodized in dark on polished p-type (1 0 0) Si wafers with a resistivity of 1–30 V cm and a thickness of 500 mm, using 1:1 mixture of HF (48 wt.%) and ethanol (98 wt.%) as an electrolyte. Initially, the Si wafers were cleaned successively in a sonicating bath with CCl4, toluene, acetone, ethyl alcohol and 18.5 MV cm deionized water. Al films with a thickness of 500 nm were deposited by radio frequency (rf)-sputtering on the backside of the Si wafers and then annealed at 450 8C for half an hour in vacuum to form good ohmic contacts. Anodization was carried out using Pt as a counter electrode at a current density of 30 mA cm2. After anodization, the PS layers were dried to reduce the capillary stress using pentane, which has very low surface tension and no chemical reactivity with the PS layer. The samples were then rinsed with 98% methanol followed by deionized water (18.5 MV). Finally the samples were dried at about 50 8C on a hot plate rather than drying in the N2 nozzle in order to avoid cracking and peeling of the PS layer. The porosity (gravimetric) of the PS layer was found to be 65%. The ZnO and Al (99.99% purity) targets were supplied by Kojundo Chemical Lab Co. Ltd., Japan. ZnO:Al films were codeposited by rf-sputtering at a pressure of 0.05 Torr in an argon atmosphere. The rf-power of the ZnO target was in the range of 100–200 W while keeping the Al target power as 100 W throughout the experiment. The distance between the target and the substrate was kept fixed at 15 cm. The microstructures of the AZO films on PS were investigated using scanning electron microscopy (SEM; Hitachi S 4200) and the surface profiles were analysed using atomic force microscopy (AFM; Topometrix-Accurex II). The films were characterized by X-ray diffraction (XRD; Rigaku 2500PC) at a scanning rate of 3 s1 in the 2u range (30–708) using the Cu Ka characteristic radiation of wavelength ˚ . The PL spectroscopic analyses were performed using 488 nm line of an Ar laser as an excitation light source. 1.54 A 3. Results and discussion Fig. 1(i) and (ii) represents the XRD spectra of AZO film of thickness 300 nm deposited at various rf-powers on silicon and on PS substrates, etched for 90 s, respectively. XRD analysis reveals that all the deposited films are polycrystalline in nature with a hexagonal close packed lattice and c-axis oriented perpendicular to the substrate surface. With increasing rf-power the locations of the measured diffraction angle do not change significantly and the dominant (0 0 2) peak at 34.678 becomes sharper indicating the well-established c-axis orientation of AZO films. This suggests that the crystallinity of the resulting film increases and the grain size becomes larger with increasing rf-power. In addition to the c-axis orientation, a little peak shift (0.128) towards the higher diffraction angle is observed compared with the bulk ZnO powder (34.458). This shift may be attributed to the incorporation of the added Al into the ZnO lattice resulting in the reduced lattice constant [19]. However, the peak position not only depends on the substitution of Al3+ for Zn2+ in ZnO films, but also is strongly related to other parameters, such as sputtering conditions
Fig. 1. XRD patterns of (i) AZO films deposited on silicon and (ii) AZO films deposited on PS at different rf-powers: (a) 100 W, (b) 150 W and (c) 200 W.
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Fig. 2. Plan-view SEM images of (i) AZO films deposited on PS substrates at different rf-powers: (a) 100 W, (b) 150 W and (c) 200 W, and (ii) the corresponding vertical cross sectional SEM images.
and electric fields [20]. The AZO films deposited on PS shows that the growth depends on the substrate as well as the sputtering condition. We have calculated the particle size (D) from the full width at half maximum (b) value for the prominent (0 0 2) peaks using the Scherrer formula, D = 0.94l/b cos u, where l is the wavelength of the Ka characteristic X-ray and u is the diffraction (Bragg) angle. The estimated average particle sizes are 39, 44 and 41 nm for the AZO films sputtered at the rf-powers of 100, 150 and 200 W, respectively. This indicates that the films deposited at 150 W are expected to have the best crystallinity. This may be because as the rf-power increases higher than 150 W, the collision probability between the sputtered atoms and Ar ions increases, so that the energy of the atoms arriving at the substrate surface is reduced and the surface migration is limited [19]. It is found that there is not much difference in the crystallinity between the films deposited on silicon and those deposited on PS. Fig. 2 shows the SEM images of AZO films deposited at different rf-powers on PS substrates etched at 90 s. It shows a columnar structure with no void and preferred c-axis orientation and as the rf-power increases, the crystallinity of the films is improved
Fig. 3. The AFM images of (a) the AZO films deposited at 150 W on silicon and (b) PS etched for 90 s.
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Fig. 4. The room temperature PL spectra of the AZO film deposited on the single crystal Si substrate at different rf-powers.
and the columnar film growth becomes more dominant for the films deposited at 150 W which is in agreement with our XRD measurement. Fig. 3 shows the AFM images of the AZO films deposited at 150 W on silicon and PS, respectively. The average roughness (Ra) and the root mean square (RMS) roughness values were measured using a high pass filter to eject the factor for smoother substrates. The RMS roughness value increases with increasing the rfpower (not shown) and reaches maximum at 150 W and then decreases, This increase may be because columnar growth is dominant, because its grain size becomes much larger and also because its surface turns much harder at 150 W. The films deposited on PS are smoother than those deposited on silicon substrates, which may be because of the partial filling of the pores in PS by the deposited AZO film. Fig. 4 shows the room temperature PL spectra of the AZO film deposited on single crystal silicon substrate at different rf-powers. The spectrum consists of sharp UV emission peaks at about 369, 366 and 373 nm for the films deposited at 100, 150 and 200 W, respectively. As the rf-power increases, the optical absorption edge shifts towards a shorter wavelength region for the films deposited at 150 W and then shifts to a longer wavelength region for those deposited at 200 W. The growth rate increases with increasing the sputtering power. This increase indicates that the number of atoms sputtered from the target is nearly proportional to the rf-power and reaches maximum for the films deposited at 200 W. Hence, the Al concentration will be less than that of the films deposited at 150 W. Consequently the peak shifts to shorter wavelength for the films deposited at 150 W. In addition to the strong UV emission, the films deposited at 100 W also show emission peaks in violet and green spectral regions. The violet luminescence (418 nm) is probably due to radiative defects related to the interface traps existing at the grain boundaries and emitted from the radiative transition between this level and the valence band [21]. The films may have more grain boundary defects emitting the violet luminescence of higher intensity because they have smaller grain sizes and larger grain boundary area. However, if the grains are not preferentially oriented, the emitted light may not be effectively detected, or the grain boundary may produce different kinds of defects, such as non-radiative defects. The films deposited at higher sputtering powers (200 W) results in much higher intensities of luminescence than those deposited at lower sputtering power (150 W). It is thus expected that the film grown at 200 W will probably have improved stoichiometry with less oxygen vacancies. These phenomena may indicate that the ZnO grown at 100 W is far inferior in both stoichiometry and the crystalline quality, which is also very true as shown in XRD. The green emission (519 nm) originates from the oxygen vacancies according to the result of Vanheusden et al. [22]. All the prepared films are n-type and the majority donors are oxygen vacancies (Vo) and zinc interstitials (Zni). It is proved that the singly ionized oxygen vacancy is responsible for the green emission and this emission results from the recombination of a photogenerated hole with a singly ionized charge state of this defect. The more are singly ionized oxygen vacancies are, the stronger the luminescence intensity is. As the ZnO target power increases, the oxygen concentration related to Zn–O bonding increases. In this O-rich condition, the amount of oxygen that diffuses into the sample increases and the concentration of the oxygen vacancies decreases. Also, antisite oxide (OZn) in the films at an O-rich condition easily formed from oxygen interstitials (Oi) and zinc vacancies (VZn), because the antisite oxide has relatively low formation energy.
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Fig. 5. The room temperature PL spectra of the AZO film deposited at the ZnO target power of 150 W and the PS etched for 90 s.
Egelhaaf et al. [23] reported that those defect related luminescence are caused by radiative transitions between shallow donors (related to oxygen vacancies) and deep acceptors (Zn vacancies). As the oxygen vacancy concentration decreases, the zinc vacancy concentration may increase resulting in the pronounced emission of the blue-green luminescence, which may be owing to the donor (oxygen vacancy)–acceptor (zinc vacancy) transition. Therefore, the peaks positions are shifted to blue and blue-green spectral regions for the films deposited at 150 and 200 W, respectively. It is well observed in the PL spectra that both the UV and the blue-green emission increase with the ZnO target power. On the contrary, oxygen deficient or non-stoichiometric films become to show absence or the decreased intensities of those emissions which are characteristic of the films deposited at 100 W. Fig. 5 shows the room temperature PL spectra of the AZO film deposited at the ZnO target power of 150 W on PS and the PS etched for 90 s, respectively. The PL spectrum of freshly prepared PS shows a broad band in the green spectral region with a peak intensity at 514 nm. The large FWHM of 69.5 nm may be associated with the inhomogeneous distribution of the nanopores or wires, and the complex nature of the surface states present at the large internal surface of the PS. Several possible models for the green and blue emission from PS have been proposed: bandto-band recombination in silicon nanocrystallites, emission due to silicon oxide, and emission due to surface states [24–26]. The emission from silicon oxide widely accepted, suggests that different types of defect in silicon oxide can be responsible. This idea gains support from the correlation between the intensity of the PL band and the intensity of the Si–O infrared absorption. It is suggested that the green luminescence of oxidized PS comes from creation of
Fig. 6. The I–V characteristic curve of a AZO-PS-p-Si structure.
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oxygen, carbon or halogen related defects in silicon dioxide. Deposition of the AZO films on PS is found to increase the PL intensity and shifts the peak position to 474 nm. This blue shift of 40 nm indicates that localized states in PS or the nanocrystallites are subjected to the variation with the deposition of AZO. We know that PS is highly surface sensitive and even a few minutes of exposure to ambient air will lead to surface related defects. Therefore, we believe that deposition of AZO on PS reduces the concentration of the recombination centers in the films and consequently, decreases the non-radiation recombination and increases the radiation recombination for non-equilibrium photogenerated carriers. The structure and the density variation of the defects in the interfacial variation might be responsible for the appearance of two small PL peaks and the intensity variation. Radiative recombination of the photoexcited carriers should occur via relaxed electronic states, possibly oxygen related defect states at the interface between PS and AZO thin films. Hence, we suggest that the non-radiative centers on PS surface can be passivated by the deposition of AZO thin films. I–V measurement was performed in a two terminal AZO-PS-p-Si configuration and is shown in Fig. 6. The I–V characteristics of the sandwich structures seem to be rectifying, however, the rectifying ratio is quite high (IF/IR 19 at 30 V) even for this high voltage. The characteristics of such a device are usually represented as a serial combination of a diode and a resistor. In can be represented [27] Vtotal ¼ VD þ IR;
(1)
where I ¼ Is ðV; TÞðeqVD =kT 1Þ;
(2)
VD is the voltage drop on the diode, IS the saturation current in the reverse bias and R is the resistance usually assumed to be independent of voltage. For a forward bias, in the limit I Is the applied voltage mainly drop on the diode and the current tends to saturate. For a forward bias (larger than kT/q) it is assumed that the current through the diode is given by I ¼ Is ðTÞeqVD =nkT
(3)
where Is is the saturation current at zero bias and n the ideality factor, which takes the voltage dependence of the saturation current into account. The value of Is is found by extrapolation of the I–V curve to zero bias. Putting this value into Eq. (1) we obtain
Vtotal ¼ IR þ
nkT I ln q Is
(4)
When V is large compared with nkT/q, a linear I–V relation with a slope R is expected. Our sample shows such a regime up to 25 V. Assuming that the second term in Eq. (4) is dominating, the plot of ln I versus V is found to be non-linear and the ideality factor determined from the plot less than 25 V is 19 and the linear resistance is 36 MV. The fact that the ideality factor is very large suggests that most of the applied voltage does not drop on the barrier, but rather drops on the PS layer. Therefore, the conductivity dependence on the applied voltage should be considered. A 36 MV resistance is reasonable taking the high resistivity of the PS layer into account, and the high ideality factor can be explained on the basis of the theory of PS layer acting as a series high layer resistance and a diode reported by other researchers [27]. The conductance of the PS layer extracted from the forward bias characteristics IF/V as a function of p ffiffiffiffi V is found to be exponentially proportional to the square root of the voltage. For thin layers the PS resistance is small and the current might exceed the saturation current of the diode. Therefore, in the reverse bias the current will be limited by the diode, resulting in rectifying characteristics. Since the applied voltage is much larger than any reasonable barrier height, the diode term in Eq. (4) is negligible. On the other hand, thick PS samples (in our case 1 mm) will have much higher resistance, so that for all the range of the applied voltage the current is limited by this resistance and the forward bias characteristics are controlled by the PS resistance. 4. Conclusion We have deposited Al:ZnO thin film on silicon and porous silicon by rf co-sputtering. UV, violet and green-blue photoluminescence are observed for the AZO films deposited at 100 W. As the rf-power increases, the violet
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luminescence vanishes and the intensity of blue-green increases. It is concluded that the intensity and the emission colour depends on the stoichiometry of the film as well as the crystal quality. It is suggested that the green luminescence of PS comes from creation of oxygen, carbon or halogen related defects in silicon dioxide. Deposition of the AZO films on PS is found to increase the PL intensity and shifts the peak position to 474 nm. Radiative recombination of the photoexcited carriers should occur via relaxed electronic states, possibly oxygen related defect states at the interface between the PS and the AZO thin films. Hence, we suggest that the concentration of nonradiative centers on PS surface can be passivated by the deposition of AZO thin films. Acknowledgment This work was supported by Korea Research Foundation Grant (KRF-2001-005-E00008). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. N. Koshida, H. Koyama, Appl. Phys. Lett. 60 (1992) 347. L.G. Jacobsohn, B.L. Bennett, D.W. Cooke, R.E. Muenchausen, M. Nastasi, J. Appl. Phys. 97 (2005) 33528. S.K. Lazarouk, A.A. Leshok, V.A. Labunov, V.E. Borisenko, Semiconductors 39 (2005) 136. A.E. Pap, K. Kordas, G. Toth, J. Levoska, A. Uusimaki, J. Vahakangas, S. Leppavuori, T.E. George, Appl. Phys. Lett. 86 (2005) 41501. A. Richiter, P. Steiner, F. Kozlowski, W. Lang, IEEE Electron Device Lett. 12 (1991) 691. S. Chattopadhyay, P.W. Bohn, J. Appl. Phys. 96 (2004) 6888. Z.H. Mkhitaryan, A.A. Shatveryan, A.Z. Adamyan, V.M. Aroutiounian, Opt. Mater. 27 (2005) 962. Z. Chen, G. Bosman, R. Ochoa, Appl. Phys. Lett. 62 (1993) 708. S. Gosh, H. Kim, K. Hong, C. Lee, Mater. Sci. Eng. B 95 (2002) 171. S. Gosh, K. Hong, C. Lee, Mater. Sci. Eng. B 96 (2002) 53. T. Futagi, T. Mastsumoto, M. Katsuno, Y. Ohta, H. Mimura, K. Kitamura, Appl. Phys. Lett. 63 (2003) 1209. N. Koshida, H. Koyama, Y. Yamamoto, G.J. Collins, Appl. Phys. Lett. 63 (1993) 2655. J.F. Cordaro, Y. Shim, J.E. May, J. Appl. Phys. 60 (1986) 4186. B. Sang, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 37 (1998) L206. C.X. Xu, X.W. Sun, B.J. Chen, P. Shum, S. Li, X. Hu, J. Appl. Phys. 95 (2004) 661. Y.L. Liu, Y.C. Liu, W.B. Wang, J.Y. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, J. Phys. D: Appl. Phys. 36 (2003) 2705. D.D. Malinovska, M. Nikolaeva, Vacuum 69 (2003) 227. K.C. Park, D.Y. Ma, K.H. Kim, Thin Solid Films 305 (1997) 201. M. Chen, Z.L. Pei, X. Wang, C. Sun, L.S. Wen, Mater. Lett. 48 (2001) 137. B.J. Jin, S. Im, S.Y. Lee, Thin Solid Films 366 (2000) 107. K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. H.J. Egelhaaf, D. Oelkrug, J. Cryst. Growth 161 (1996) 190. Q.W. Chen, D.L. Zhu, C. Zhu, J. Wang, Y.G. Zhang, App. Phys. Lett. 82 (2003) 1018. L. Tsybeskov, J.V. Vandyshev, P.M. Fauchet, Phys. Rev. B 49 (1994) 7821. R.E. Hummel, M.H. Ludwig, S.S. Chang, P.M. Fauchet, J.V. Vandyshev, L. Tsybeskov, Solid State Commun. 95 (1995) 553. M. Ben-Chorin, F. Moller, F. Koch, J. Appl. Phys. 77 (1995) 4482.
Physical and electronic properties of ZnO:Al/porous silicon Choongmo Kim, Anna Park, K. Prabakar, Chongmu Lee * Department of Materials Science and Engineering, Inha University, 253 Younghyun-dong, Nam-Ku, Incheon 402-751, South Korea Received 13 October 2004; received in revised form 8 March 2005; accepted 26 August 2005 Available online 19 September 2005
Abstract UV, violet and blue-green photoluminescence has been achieved at room temperature (RT) from ZnO:Al (AZO) films deposited by radio frequency (rf) co-sputtering. As the ZnO target power increases from 100 W, the violet luminescence vanishes and the blue and green-blue luminescences appear. The most intense UV and blue-green luminescence is obtained for the films deposited at higher sputtering powers depending upon the stoichiometry of the films as well as the crystalline quality. The as-prepared porous silicon (PS) emission band lies in the blue-green spectral region and is blue shifted due to the AZO deposition. The current–voltage characteristics of AZO/PS heterostructures have been studied. The ideality factor is found to be 19 and the series resistance as determined from the forward characteristics is 36 MV. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Thin films; B. Sputtering; C. Atomic force microscopy; D. Luminescence
1. Introduction Discovery of efficient room temperature luminescence from porous silicon (PS) has evoked a lot of scientific and technological interest [1,2]. Much effort has been directed to obtain efficient electroluminescence devices based on PS [3–5]. Special attempts have been made first by Richiter et al. to realize solid state light emitting diodes (LED) based on metal–PS heterostructures [6]. Since then other groups have reported on such devices [7–9]. In most of them low efficiency is found, and light emission is achieved only for quite high voltages. These structures showed rectifying behaviour and electroluminescence (EL) under forward bias. The large ideality factor of the current–voltage characteristics was interpreted with an additional voltage drop in the PS layer due to the charging effect of interface states. The EL efficiency could be increased strongly by filling pores with indium or aluminium zinc oxide in a certain special region of the PS. This tendency led to various attempts to improve electroluminescence efficiency by replacing the gold contacts used by Richiter et al. with other materials, including indium tin oxide [10], copper films [11], ndoped microcrystalline silicon carbide [12] and conducting polymers [13]. Obviously additional investigations have still have to be performed in order to optimize the main features of the fabricated devices. Al:ZnO (AZO) is a wide direct band gap II–VI semiconductor with such potential applications in transparent conductive films, varistors and optoelectronic devices [14–16]. Although deposition of ZnO films on PS substrates have been carried out [17,18], the transport mechanism in PS is still under debate. The growth of AZO films on various substrates has been extensively studied, but there were no reports on the growth of AZO on the PS substrate to the best of our knowledge. In this work, * Corresponding author. Tel.: +82 32 860 7536; fax: +82 32 862 5546. E-mail address: [email protected] (C. Lee). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.08.018
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the properties of AZO, PS and AZO/PS structures were characterized to find out the suitability of these structures as applied in optoelectronic devices. 2. Experimental Porous silicon layers were electrochemically anodized in dark on polished p-type (1 0 0) Si wafers with a resistivity of 1–30 V cm and a thickness of 500 mm, using 1:1 mixture of HF (48 wt.%) and ethanol (98 wt.%) as an electrolyte. Initially, the Si wafers were cleaned successively in a sonicating bath with CCl4, toluene, acetone, ethyl alcohol and 18.5 MV cm deionized water. Al films with a thickness of 500 nm were deposited by radio frequency (rf)-sputtering on the backside of the Si wafers and then annealed at 450 8C for half an hour in vacuum to form good ohmic contacts. Anodization was carried out using Pt as a counter electrode at a current density of 30 mA cm2. After anodization, the PS layers were dried to reduce the capillary stress using pentane, which has very low surface tension and no chemical reactivity with the PS layer. The samples were then rinsed with 98% methanol followed by deionized water (18.5 MV). Finally the samples were dried at about 50 8C on a hot plate rather than drying in the N2 nozzle in order to avoid cracking and peeling of the PS layer. The porosity (gravimetric) of the PS layer was found to be 65%. The ZnO and Al (99.99% purity) targets were supplied by Kojundo Chemical Lab Co. Ltd., Japan. ZnO:Al films were codeposited by rf-sputtering at a pressure of 0.05 Torr in an argon atmosphere. The rf-power of the ZnO target was in the range of 100–200 W while keeping the Al target power as 100 W throughout the experiment. The distance between the target and the substrate was kept fixed at 15 cm. The microstructures of the AZO films on PS were investigated using scanning electron microscopy (SEM; Hitachi S 4200) and the surface profiles were analysed using atomic force microscopy (AFM; Topometrix-Accurex II). The films were characterized by X-ray diffraction (XRD; Rigaku 2500PC) at a scanning rate of 3 s1 in the 2u range (30–708) using the Cu Ka characteristic radiation of wavelength ˚ . The PL spectroscopic analyses were performed using 488 nm line of an Ar laser as an excitation light source. 1.54 A 3. Results and discussion Fig. 1(i) and (ii) represents the XRD spectra of AZO film of thickness 300 nm deposited at various rf-powers on silicon and on PS substrates, etched for 90 s, respectively. XRD analysis reveals that all the deposited films are polycrystalline in nature with a hexagonal close packed lattice and c-axis oriented perpendicular to the substrate surface. With increasing rf-power the locations of the measured diffraction angle do not change significantly and the dominant (0 0 2) peak at 34.678 becomes sharper indicating the well-established c-axis orientation of AZO films. This suggests that the crystallinity of the resulting film increases and the grain size becomes larger with increasing rf-power. In addition to the c-axis orientation, a little peak shift (0.128) towards the higher diffraction angle is observed compared with the bulk ZnO powder (34.458). This shift may be attributed to the incorporation of the added Al into the ZnO lattice resulting in the reduced lattice constant [19]. However, the peak position not only depends on the substitution of Al3+ for Zn2+ in ZnO films, but also is strongly related to other parameters, such as sputtering conditions
Fig. 1. XRD patterns of (i) AZO films deposited on silicon and (ii) AZO films deposited on PS at different rf-powers: (a) 100 W, (b) 150 W and (c) 200 W.
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Fig. 2. Plan-view SEM images of (i) AZO films deposited on PS substrates at different rf-powers: (a) 100 W, (b) 150 W and (c) 200 W, and (ii) the corresponding vertical cross sectional SEM images.
and electric fields [20]. The AZO films deposited on PS shows that the growth depends on the substrate as well as the sputtering condition. We have calculated the particle size (D) from the full width at half maximum (b) value for the prominent (0 0 2) peaks using the Scherrer formula, D = 0.94l/b cos u, where l is the wavelength of the Ka characteristic X-ray and u is the diffraction (Bragg) angle. The estimated average particle sizes are 39, 44 and 41 nm for the AZO films sputtered at the rf-powers of 100, 150 and 200 W, respectively. This indicates that the films deposited at 150 W are expected to have the best crystallinity. This may be because as the rf-power increases higher than 150 W, the collision probability between the sputtered atoms and Ar ions increases, so that the energy of the atoms arriving at the substrate surface is reduced and the surface migration is limited [19]. It is found that there is not much difference in the crystallinity between the films deposited on silicon and those deposited on PS. Fig. 2 shows the SEM images of AZO films deposited at different rf-powers on PS substrates etched at 90 s. It shows a columnar structure with no void and preferred c-axis orientation and as the rf-power increases, the crystallinity of the films is improved
Fig. 3. The AFM images of (a) the AZO films deposited at 150 W on silicon and (b) PS etched for 90 s.
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Fig. 4. The room temperature PL spectra of the AZO film deposited on the single crystal Si substrate at different rf-powers.
and the columnar film growth becomes more dominant for the films deposited at 150 W which is in agreement with our XRD measurement. Fig. 3 shows the AFM images of the AZO films deposited at 150 W on silicon and PS, respectively. The average roughness (Ra) and the root mean square (RMS) roughness values were measured using a high pass filter to eject the factor for smoother substrates. The RMS roughness value increases with increasing the rfpower (not shown) and reaches maximum at 150 W and then decreases, This increase may be because columnar growth is dominant, because its grain size becomes much larger and also because its surface turns much harder at 150 W. The films deposited on PS are smoother than those deposited on silicon substrates, which may be because of the partial filling of the pores in PS by the deposited AZO film. Fig. 4 shows the room temperature PL spectra of the AZO film deposited on single crystal silicon substrate at different rf-powers. The spectrum consists of sharp UV emission peaks at about 369, 366 and 373 nm for the films deposited at 100, 150 and 200 W, respectively. As the rf-power increases, the optical absorption edge shifts towards a shorter wavelength region for the films deposited at 150 W and then shifts to a longer wavelength region for those deposited at 200 W. The growth rate increases with increasing the sputtering power. This increase indicates that the number of atoms sputtered from the target is nearly proportional to the rf-power and reaches maximum for the films deposited at 200 W. Hence, the Al concentration will be less than that of the films deposited at 150 W. Consequently the peak shifts to shorter wavelength for the films deposited at 150 W. In addition to the strong UV emission, the films deposited at 100 W also show emission peaks in violet and green spectral regions. The violet luminescence (418 nm) is probably due to radiative defects related to the interface traps existing at the grain boundaries and emitted from the radiative transition between this level and the valence band [21]. The films may have more grain boundary defects emitting the violet luminescence of higher intensity because they have smaller grain sizes and larger grain boundary area. However, if the grains are not preferentially oriented, the emitted light may not be effectively detected, or the grain boundary may produce different kinds of defects, such as non-radiative defects. The films deposited at higher sputtering powers (200 W) results in much higher intensities of luminescence than those deposited at lower sputtering power (150 W). It is thus expected that the film grown at 200 W will probably have improved stoichiometry with less oxygen vacancies. These phenomena may indicate that the ZnO grown at 100 W is far inferior in both stoichiometry and the crystalline quality, which is also very true as shown in XRD. The green emission (519 nm) originates from the oxygen vacancies according to the result of Vanheusden et al. [22]. All the prepared films are n-type and the majority donors are oxygen vacancies (Vo) and zinc interstitials (Zni). It is proved that the singly ionized oxygen vacancy is responsible for the green emission and this emission results from the recombination of a photogenerated hole with a singly ionized charge state of this defect. The more are singly ionized oxygen vacancies are, the stronger the luminescence intensity is. As the ZnO target power increases, the oxygen concentration related to Zn–O bonding increases. In this O-rich condition, the amount of oxygen that diffuses into the sample increases and the concentration of the oxygen vacancies decreases. Also, antisite oxide (OZn) in the films at an O-rich condition easily formed from oxygen interstitials (Oi) and zinc vacancies (VZn), because the antisite oxide has relatively low formation energy.
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Fig. 5. The room temperature PL spectra of the AZO film deposited at the ZnO target power of 150 W and the PS etched for 90 s.
Egelhaaf et al. [23] reported that those defect related luminescence are caused by radiative transitions between shallow donors (related to oxygen vacancies) and deep acceptors (Zn vacancies). As the oxygen vacancy concentration decreases, the zinc vacancy concentration may increase resulting in the pronounced emission of the blue-green luminescence, which may be owing to the donor (oxygen vacancy)–acceptor (zinc vacancy) transition. Therefore, the peaks positions are shifted to blue and blue-green spectral regions for the films deposited at 150 and 200 W, respectively. It is well observed in the PL spectra that both the UV and the blue-green emission increase with the ZnO target power. On the contrary, oxygen deficient or non-stoichiometric films become to show absence or the decreased intensities of those emissions which are characteristic of the films deposited at 100 W. Fig. 5 shows the room temperature PL spectra of the AZO film deposited at the ZnO target power of 150 W on PS and the PS etched for 90 s, respectively. The PL spectrum of freshly prepared PS shows a broad band in the green spectral region with a peak intensity at 514 nm. The large FWHM of 69.5 nm may be associated with the inhomogeneous distribution of the nanopores or wires, and the complex nature of the surface states present at the large internal surface of the PS. Several possible models for the green and blue emission from PS have been proposed: bandto-band recombination in silicon nanocrystallites, emission due to silicon oxide, and emission due to surface states [24–26]. The emission from silicon oxide widely accepted, suggests that different types of defect in silicon oxide can be responsible. This idea gains support from the correlation between the intensity of the PL band and the intensity of the Si–O infrared absorption. It is suggested that the green luminescence of oxidized PS comes from creation of
Fig. 6. The I–V characteristic curve of a AZO-PS-p-Si structure.
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oxygen, carbon or halogen related defects in silicon dioxide. Deposition of the AZO films on PS is found to increase the PL intensity and shifts the peak position to 474 nm. This blue shift of 40 nm indicates that localized states in PS or the nanocrystallites are subjected to the variation with the deposition of AZO. We know that PS is highly surface sensitive and even a few minutes of exposure to ambient air will lead to surface related defects. Therefore, we believe that deposition of AZO on PS reduces the concentration of the recombination centers in the films and consequently, decreases the non-radiation recombination and increases the radiation recombination for non-equilibrium photogenerated carriers. The structure and the density variation of the defects in the interfacial variation might be responsible for the appearance of two small PL peaks and the intensity variation. Radiative recombination of the photoexcited carriers should occur via relaxed electronic states, possibly oxygen related defect states at the interface between PS and AZO thin films. Hence, we suggest that the non-radiative centers on PS surface can be passivated by the deposition of AZO thin films. I–V measurement was performed in a two terminal AZO-PS-p-Si configuration and is shown in Fig. 6. The I–V characteristics of the sandwich structures seem to be rectifying, however, the rectifying ratio is quite high (IF/IR 19 at 30 V) even for this high voltage. The characteristics of such a device are usually represented as a serial combination of a diode and a resistor. In can be represented [27] Vtotal ¼ VD þ IR;
(1)
where I ¼ Is ðV; TÞðeqVD =kT 1Þ;
(2)
VD is the voltage drop on the diode, IS the saturation current in the reverse bias and R is the resistance usually assumed to be independent of voltage. For a forward bias, in the limit I Is the applied voltage mainly drop on the diode and the current tends to saturate. For a forward bias (larger than kT/q) it is assumed that the current through the diode is given by I ¼ Is ðTÞeqVD =nkT
(3)
where Is is the saturation current at zero bias and n the ideality factor, which takes the voltage dependence of the saturation current into account. The value of Is is found by extrapolation of the I–V curve to zero bias. Putting this value into Eq. (1) we obtain
Vtotal ¼ IR þ
nkT I ln q Is
(4)
When V is large compared with nkT/q, a linear I–V relation with a slope R is expected. Our sample shows such a regime up to 25 V. Assuming that the second term in Eq. (4) is dominating, the plot of ln I versus V is found to be non-linear and the ideality factor determined from the plot less than 25 V is 19 and the linear resistance is 36 MV. The fact that the ideality factor is very large suggests that most of the applied voltage does not drop on the barrier, but rather drops on the PS layer. Therefore, the conductivity dependence on the applied voltage should be considered. A 36 MV resistance is reasonable taking the high resistivity of the PS layer into account, and the high ideality factor can be explained on the basis of the theory of PS layer acting as a series high layer resistance and a diode reported by other researchers [27]. The conductance of the PS layer extracted from the forward bias characteristics IF/V as a function of p ffiffiffiffi V is found to be exponentially proportional to the square root of the voltage. For thin layers the PS resistance is small and the current might exceed the saturation current of the diode. Therefore, in the reverse bias the current will be limited by the diode, resulting in rectifying characteristics. Since the applied voltage is much larger than any reasonable barrier height, the diode term in Eq. (4) is negligible. On the other hand, thick PS samples (in our case 1 mm) will have much higher resistance, so that for all the range of the applied voltage the current is limited by this resistance and the forward bias characteristics are controlled by the PS resistance. 4. Conclusion We have deposited Al:ZnO thin film on silicon and porous silicon by rf co-sputtering. UV, violet and green-blue photoluminescence are observed for the AZO films deposited at 100 W. As the rf-power increases, the violet
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luminescence vanishes and the intensity of blue-green increases. It is concluded that the intensity and the emission colour depends on the stoichiometry of the film as well as the crystal quality. It is suggested that the green luminescence of PS comes from creation of oxygen, carbon or halogen related defects in silicon dioxide. Deposition of the AZO films on PS is found to increase the PL intensity and shifts the peak position to 474 nm. Radiative recombination of the photoexcited carriers should occur via relaxed electronic states, possibly oxygen related defect states at the interface between the PS and the AZO thin films. Hence, we suggest that the concentration of nonradiative centers on PS surface can be passivated by the deposition of AZO thin films. Acknowledgment This work was supported by Korea Research Foundation Grant (KRF-2001-005-E00008). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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