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Structural, electrical and optical properties of N-doped ZnO films synthesized by SS-CVD
Materials Science in Semiconductor Processing 5 (2003) 491–496
Structural, electrical and optical properties of N-doped ZnO films synthesized by SS-CVD Jianguo Lu*, Zhizhen Ye, Lei Wang, Jingyun Huang, Binghui Zhao State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China
Abstract N-doped p-type ZnO films have been synthesized on a-Al2 O3 (0 0 0 1) substrate by solid-source chemical vapor deposition using ZnðCH3 COOÞ2 2H2 O as the precursor and CH3 COONH4 as the nitrogen source. The properties for ZnO films are dependent greatly on the growth conditions. Results show that the best electrical properties of the p-type film, such as carrier density N ¼ 9:8 1017 cm3 ; resistivity r ¼ 20 O cm and Hall mobility m ¼ 0:97 cm2 =V s; were induced at the substrate temperature of 5001C with a precursor temperature of 2501C and a nitrogen source of 1501C; under which the highest mixed orientation for (1 0 0) and (1 1 0) planes of films was also achieved. The p-type ZnO films possess a transmittance of about 90% in visible region and a band gap of about 3:20 eV at room temperature. r 2003 Elsevier Science Ltd. All rights reserved. PACS: 61.72.Vv; 81.05.Dz; 81.15.Gh Keywords: N-dope ZnO films; p-type conduction; SS-CVD; Properties
1. Introduction Zinc oxide is a II–VI compound semiconductor with wurtzite structure (space group P63 mc with cell parameters a ¼ 0:3249 nm; c ¼ 0:5206 nmÞ [1]. The films are commonly (0 0 2) textured (c-axis orientation) due to the lowest surface free energy for (0 0 2) plane [2]. Another preferred orientation is the few reported in [3]. ZnO occurs naturally as n-type conduction due to its various intrinsic donor defects (e.g., zinc interstitials or oxygen vacancies), which can be enhanced by B, Al, Ga, In or F donor dopants [4–8]. Theoretical calculations has predicted that among the acceptor dopants, such as group-I elements (Ag, Cu, Li) and group-V elements (N, P, As), nitrogen is the best candidate for producing a shallow acceptor lever about 100 meV above the valance band in ZnO [9,10], but in practice it is very difficult for *Corresponding author. Tel: +86-571-87952124; fax: +86571-87952625. E-mail addresses: [email protected] (J. Lu), yezz@ zju.edu.cn (Z. Ye), [email protected] (L. Wang), huangjy@ zju.edu.cn (J. Huang).
ZnO to achieve p-type conduction because of its low solubility of the dopant and high self-compensating process on doping. N-doped ZnO films deposited in N2 2O2 or N2 2Ar ambient had been studied before [11,12], but p-type conduction has not been realized until recently using ZnO/Zn as source material by chemical vapor deposition (CVD) in NH3 ambient [13] or through codoping approach (simultaneous doping of Ga donor and nitrogen acceptor) by pulsed laser deposition (PLD) in N2 O ambient [14–16]. CVD is a useful method for the formation of films, offering the advantage of producing high-quality films and availability of different ambient in situ doping process. Diethylzinc (DEZn) [17], basic zinc acetate (BZA) [18], zinc acethylacetonate [19] and zinc acetate dihydrate (ZA) [20] can be used as precursors, and they are usually prepared in various solutions. However, no other studies about p-type ZnO films grown by CVD have been reported since realizing p-type conduction by Minegishi et al. [13], using ZnO/ Zn as source material and NH3 as nitrogen source (loaded by H2 carrier gas) with the resistivity of typically 100 O cm and the carrier density about 1016 cm3 :
1369-8001/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1369-8001(02)00114-2
J. Lu et al. / Materials Science in Semiconductor Processing 5 (2003) 491–496
492
In this paper, we report the N-doped p-type ZnO films with mixed orientation for (1 0 0) and (1 1 0) planes synthesized by solid-source chemical vapor deposition (SS-CVD) using zinc acetate dihydrate (solid) as the precursor and ammonia acetate (solid) as the nitrogen source.
2. Experiment N-doped ZnO films were prepared on a-Al2 O3 (0 0 0 1) substrate by SS-CVD. The substrate temperature ðTs Þ varied in the range of 300–6001C: Zinc acetate dihydrate (ZA, solid) was used as the precursor and ammonium acetate (solid) as the nitrogen source. The temperature is kept as 2501C for ZA and controlled to be 1201C; 1351C and 1501C; respectively, for ammonia acetate. Oxygen (99.99%) was introduced to the reaction chamber through mass flow controllers, and held constant at 30 ml=min: The atmosphere pressure was about 102 Pa during the growth process of films. The deposition time was 20 min: The structural properties of the as-grown ZnO films were investigated by X-ray diffraction (Philips X’Pert) with Cu Ka radiation ðl ¼ 0:1542 nmÞ: Electrical resistivity, carrier density and mobility were measured in a magnetic field of 0:320 T by a four-point probe van der Pauw method using a HL5500 system at room temperature. The optical transmission spectra were examined with a Lambda20 spectrometer.
3. Results and discussion 3.1. Electrical properties The electrical properties for the as-grown ZnO films deposited under different conditions were examined by Hall measurement at room temperature, and summarized in Table 1. The sample Nos. 1–4 correspond ZnO films deposited at different substrate temperature with ammonia acetate temperature ðTN Þ kept constant at 1201C: It can be seen from the results that the film deposited at the substrate temperature of 3001C
(S. No. 1) still shows n-type conduction. But at 4001C and 5001C (S. Nos. 2–3), the carrier type flips from n- to p-type. The better properties of p-type ZnO, such as carrier density 3:7 1017 cm3 ; resistivity 42 O cm and Hall mobility 1:26 cm2 =V s; are achieved at 5001C: However, at a higher substrate temperature, ZnO (S. No. 4) again shows n-type conduction. In addition, when the substrate temperature is kept as 5001C (S. No. 2 and Nos. 5–6), the electrical properties of p-type ZnO films are improved with increasing ammonia acetate temperature, and the best results ðN ¼ 9:8 1017 cm3 ; r ¼ 20 O cm and m ¼ 0:97 cm2 =V sÞ are obtained at TN ¼ 1501C (S. No. 6). The changes mentioned above can be explained by the following nitrogen doping mechanism. According to Kamata et al. if ZnSe:N films are deposited using ammonia as nitrogen source, nitrogen and hydrogen will be incorporated into the ZnSe layer on a one-to-one basis, forming a strong N–H bond in film [21]. It has been also observed in nitrogen doped ZnO films by Minegishi using ammonia as the nitrogen source [13]. As for our experiment, it is anticipated to occur in the same way in the doping process using ammonia acetate as the nitrogen source. Zinc acetate dihydrate is sublimated and decomposed to form ZnO: ZnðCH3 COOÞ2 2H2 OðvÞ -ZnðCH3 COOÞ2 ðvÞ þ 2H2 OðvÞ;
ð1Þ
ZnðCH3 COOÞ2 ðvÞ þ H2 OðvÞ -ZnOðsÞ þ 2CH3 COOHðvÞ:
ð2Þ
Here, v and s represent vapor and solid, respectively. Fig. 1(a) illustrates the regular tetrahedron structure of ZnO. Simultaneously, Zinc acetate dihydrate reacts with ammonia resulted from the decomposition of ammonia acetate producing ZnNH: CH3 COONH4 ðvÞ-CH3 COOHðvÞ þ NH3 ðvÞ;
ð3Þ
ZnðCH3 COOÞ2 ðvÞ þ NH3 ðvÞ -ZnNHðsÞ þ 2CH3 COOHðvÞ:
ð4Þ
Fig. 1(b) shows the ZnNH structure. Nitrogen substitutes to oxygen site in the lattice. Hydrogen neutralizes
Table 1 Summary of the electrical properties obtained for N-doped ZnO films S. No.
Ts ð1CÞ
TN ð1CÞ
Resistivity ðO cmÞ
Mobility ðcm2 =V sÞ
Carrier density ðcm3 Þ
Carrier type
1 2 3 4 5 6
300 400 500 600 500 500
120 120 120 120 135 150
1e+4 2.3e+3 4.2e+1 4.0e+4 3.2e+1 2.0e+1
4.8 3.2 1.26 5.0 1.12 0.97
5.1e+14 +1.3e+15 +3.7e+17 7.1e+14 +6.5e+17 +9.8e+17
n p p n p p
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Fig. 1. The tetrahedron structure for ZnO (a), ZnNH (b) and ZnN (c).
CH3 COOHðvÞ þ 2O2 ðvÞ-2CO2 ðvÞ þ 2H2 OðvÞ:
ð5Þ
Oxygen also facilitates the dissociation of N–H bonds, and this will be discussed soon. The presence of hydrogen can suppress Zn interstitials due to the interstitial sites having been occupied by hydrogen atoms, which enhances greatly the effect of nitrogen dopants. It can be used to explain the failure to obtain p-type ZnO in N2 2O2 or N2 2Ar ambient due to the large density of zinc interstitials existing in films [11,12]. Furthermore, the nitrogen incorporation is also very low in ZnO films grown in N2 2O2 or N2 2Ar ambient because of more difficulty encountered in combining Zn to combine with N2 compared to combining NH3 : But now nitrogen is electrically inactive because of hydrogen passivation. Only when the N–H bond is dissociated under appropriate conditions (e.g., with a certain temperature and/or ambient) in the subsequent growth process, can it be activated, and act as an effective acceptor [22]. Oxygen existing in the growth ambient will facilitate the dissociation of N–H bond on the surface generating hydrogen atoms, and subsequently bind with the absorbed oxygen atoms to form the surface hydroxyl-groups of water [23] ZnNHðsÞ þ ½O -ZnNðsÞ þ ½OH :
ð6Þ
Fig. 1(c) shows the tetrahedron structure of ZnN. Nitrogen acts as an effective acceptor. In addition, an appropriate temperature is also indispensable in the process of dissociation of N–H bond. At a lower growth temperature, there will not be enough energy for nitrogen to be activated, so no p-type condition will be obtained. At a higher growth temperature, however, because the N–H bond has been broken before N is incorporated into the film, ZnO also shows n-type, which is very similar to the N-doped ZnO films in
002 100
No.1
110
101
Intensity (a.u.)
the free hold created by the nitrogen acceptor, and lies in the interstitial site next to nitrogen due to its small atom size. The role of O2 in the ambient during film growth is threefold: it is required as an additional oxygen source for the minimization of oxygen vacancies in the resulting ZnO films and for the carbonization avoidance
No.2
No.3
No.4 20
30
40
50
60
70
2-theta (deg.)
Fig. 2. XRD profiles of N-doped ZnO films as a function of substrate temperature (No. 1: 6001C; No. 2: 5001C; No. 3: 4001C; No. 4: 3001C).
N2 2O2 or N2 2Ar ambient [11,12]. Only in an intermediate temperature region, can nitrogen have a higher incorporation and be activated more easily to achieve p-type ZnO films. In this region, with increasing ammonia acetate temperature, the ammonia concentration in the growth ambient also increases, so the nitrogen incorporation is enhanced, and the electrical properties of p-type ZnO are improved.
3.2. Structural properties Fig. 2 shows the typical XRD profiles of the as-grown N-doped ZnO films at different substrate temperatures, such as 6001C; 5001C; 4001C and 3001C; corresponding to S. Nos. 1–4, respectively. Four diffraction patterns for (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes of ZnO are observed in the figure. ZnO powder (1996 JCPDS, 36-1451) has the relative intensities 57%, 44%, 100%, and 32%, respectively, for the (1 0 0), (0 0 2), (1 0 1) and (1 1 0) reflections. It can be concluded that the as-grown ZnO films deposited by SS-CVD possess mixed orientation for (1 0 0) and (1 1 0) planes, that is to say, with
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c-axis parallel to substrate. With the substrate temperature down to 3001C; the intensities of (1 0 0) and (1 1 0) peaks increase evidently, while the diffraction traces for (0 0 2) and (1 0 1) planes become very weak, indicating that a low growth temperature will facilitate the mixed orientation. Fig. 3 illustrates the XRD profiles of S. No. 2 and Nos. 5–6, corresponding to the ammonia acetate temperatures 1201C; 1351C and 1501C; respectively. Only diffraction patterns for (1 0 0) and (1 1 0) planes are observed at 1351C and 1501C; indicating that nitrogen concentration in films plays an important role in enhancing the mixed orientation for (1 0 0) and (1 1 0) planes. It is very interesting that the diffraction intensity of (1 1 0) plane decreases evidently with ammonia acetate temperature up to 1501C: The effect of preparation conditions on the mixed orientation was investigated by evaluating the mixed texture coefficient Tc : This factor can be estimated using the following formula: !1=n Y Tcðh k lÞ ; ð7Þ Tc ¼ n
where, Tcðh k lÞ is the texture coefficient of the ðh k lÞ plane, n is the number of mixed orientation planes. Tcðh k lÞ can be calculated by the following equation: Tcðh k lÞ
Iðh k lÞ =I0ðh k lÞ P ¼ : 1=N½ N Iðh k lÞ =I0ðh k lÞ
ð8Þ
Here, I is the measured intensity, I0 the corresponding recorded ASTM intensity, and N is the number of preferred directions of growth. Fig. 4 gives the Tc values of the mixed orientation for ð1 0 0Þ and ð1 1 0Þ planes for the as-grown ZnO films. The data increases with
decreasing substrate temperature or increasing nitrogen concentration in films, and a maximum value about 2.19 is reached for the ZnO film (S. No. 6) synthesized at Ts ¼ 5001C and TN ¼ 1501C: Zinc oxide has tetrahedral coordinates caused by sp3 hybridized orbits with the direction of each apex parallel to c-axis, and the three lowest density of the surface free energy is 9.9, 12.3, and 20:9 eV=nm2 for (0 0 2), (1 1 0) and (1 0 0) planes, respectively [2]. Therefore, the films are commonly (0 0 2) textured due to its low surface free energy. But, studies indicate that the surface free energy of the (0 0 2) plane is not necessarily always smaller than those of other planes [24]. If a large density of defects (e.g. vacancies and/or interstitials) and/or impurities exist in films, and/or the deposition is performed at nonequilibrium state (e.g., low temperature and/or high deposition rate), another orientation can be achieved. But ZnO film with another preferred orientation is the few reported due to the difficulty to form it [3]. Our results show that ZnO films with highly mixed orientation for (1 0 0) and (1 1 0) planes have been synthesized successfully by SS-CVD. A low growth temperature, especially a high nitrogen concentration in films, will facilitate evidently the forming of the mixed orientation, by which the theory mentioned above is tested. 3.3. Optical properties Fig. 5 illustrates the typical transmission spectra for the p-type ZnO films (S. No. 2 and Nos. 5–6). The transmittances are about 90% in visible region for all of the films. This value is almost the same as that reported for B, Al, Ga or In doped n-type ZnO [4–7]. The slight absorption at 4302580 nm could be due to interference effect. In the Fig. 5, a sharp absorption is observed. The 2.20
100
002
101
2.15
No.2
110
Texture coefficient
Intensity (a.u.)
2.10
No.5
2.05
2.00
1.95
No.6 1.90
20
30
40
50
60
70
2-theta (deg.)
Fig. 3. XRD profiles of N-doped ZnO films as a function of ammonia acetate temperature (No. 2: 1201C; No. 5: 1351C; No. 6: 1501C).
300
350
400
450
500
550
600
Substrate temperature (°C)
Fig. 4. Texture coefficient of mixed orientation for (1 0 0) and (1 1 0) planes for the as-grown ZnO films (\: Nos. 1–4; m: No. 5; .: No. 6).
J. Lu et al. / Materials Science in Semiconductor Processing 5 (2003) 491–496 100
495
25
No.2 80
No.5
60
No.2 No.5 No.6
40
a2 (×1012 cm-2)
Transmittance (%)
20
20
0 300
400
500
600
700
800
No.6
15
10
5
3.20
0
3.19
Wavelength (nm)
Fig. 5. Transmittance spectra for the p-type ZnO films (S. No. 2 and Nos. 5–6).
3.0
blue shift of the fundamental absorption edge is due to a higher carrier density (from 3.7 to 9:8 1017 cm3 ) in accordance with the Burstein-Moss shift [25]. ZnO film has an absorption coefficient ðaÞ obeying the following direct transition equation for high photon energy ðhgÞ: a2 ¼ Aðhg Eg Þ:
ð9Þ
Here, Eg is the optical band gap, and A is constant. The a value can be evaluated from the transmission spectra with film thickness T ¼ ð1 RÞ2 expðadÞ;
ð10Þ
where T is the transmittance of thin film, R its reflectivity and d its thickness. The variations of the squared adsorption coefficient a2 versus the photon energy hg in the fundamental adsorption region are plotted in Fig. 6. Extrapolation of linear portion to the energy axis at a2 ¼ 0 gives the Eg value. The band gap is determined to be about 3:20 eV for the as-grown N-doped p-type ZnO. The decrease in Eg is probably attributed to the decrease in ionicity due to the formation of Zn–N bands in films. According to the Pauling theory, ionicity in a single bond increases with the difference in values of electron negativity between two elements forming the single bond. The electron negativity of Zn is 1.6, and the electron negativity of O (3.5) is larger than that of N (3.0), so the Zn–O band has larger ionicity than the Zn–N band.
4. Conclusion In summary, N-doped p-type ZnO films with mixed orientation for (1 0 0) and (1 1 0) planes have been
3.1
3.21 3.2 3.3 3.4 Photon energy (eV)
3.5
Fig. 6. Plot of squares absorption coefficient versus photon energy for the p-type ZnO films (S. No. 2 and Nos. 5–6).
synthesized on a-Al2 O3 (0 0 0 1) substrate by SS-CVD. The properties of the as-grown ZnO films are dependent greatly on the growth conditions. Results show that hydrogen plays an important role in the process of doping, and that a larger density of nitrogen in films not only results in better electrical properties but also enhances the mixed orientation. The carrier density, resistivity and Hall mobility of the p-type film were typically 9:8 1017 cm3 ; 20 O cm and 0:97 cm2 =V s respectively, which are the best results for p-type ZnO films obtained by CVD. The p-type ZnO films possess a high transmittance about 90% in visible region and a band gap around 3:20 eV at room temperature.
Acknowledgements This research is supported by the Special Funds for Major State Basic Research Project of China, No. G20000683-06.
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[4] Lokhande BJ, Patil PS, Uplane MD. Physica B 2001; 302–303:59. [5] Kim H, Gilmore CM, Horwitz JS, Pique A, Murata H, Kushto GP, Schlaf R, Kafafi ZH, Chrisey DB. Appl Phys Lett 2000;76:259. [6] Ramakrishna KT, Gopalaswamy H, Reddy PJ. J Cryst Growth 2000;210:516. [7] Nunes P, Fortunato E, Martins R. Thin Solid Films 2001;383:277. [8] Olvera M de la L, Maldonado A, Asomoza R, Solorza O, Acosta DR. Thin Solid Films 2001;394:242. [9] Kobayashi A, Sankey OF, Dow JD. Phys Review B 1983;28:946. [10] Hauksson IS, Simpson J, Wang SY. Appl Phys Lett 1992;61:2208. [11] Sato Y, Sato S. Thin Solid Films 1996;281–282:445. [12] Futsuhara M, Yoshioka K, Takai O. Thin Solid Films 1998;317:322. [13] Minegishi K, Koiwai Y, Kikuchi Y, Yano K, Kasuga M, Shimizu A. Jpn J Appl Phys 1997;36:L1453. [14] Guo XL, Tabata H, Kawai T. J Cryst Growth 2001;223:135.
[15] Joseph M, Tabata H, Saeki H, Ueda K, Kawai T. Physica B 2001;302–303:140. [16] Tabata H, Saeki H, Guo SL, Choi JH, Kawai T. Physica B 2001;308–310:993. [17] Muthukumar S, Gorla CR, Emanetoglu NW, Liang S, Lu Y. J Cryst Growth 2001;225:197. [18] Koch MH, Hartmann AJ, Lamb RN, Neuber M, Grunze M. J Phys Chem B 1997;101:8231. [19] Purica M, Budianu E, Rusu E, Danila M, Gavrila R. Thin Solid Films 2002;403–404:485. [20] Heerden JL vab, Swanepoel R. Thin Solid Films 1997;299:72. [21] Kamato A, Mitsuhashi H, Fujita H. Appl Phys Lett 1993;63:3353. [22] Ogata KI, Kawaguchi D, kera T, Fujita S, Fujita S. J Cryst Growth 1996;159:312. [23] Van De Kerkhof GJCS, Biemolt W, Jansen APJ, Van Santen RA. Surf Sci 1993;284:361. [24] Nguyen H Tran, Andreas J Hartmann, Robert N Lamb. J Phys Chem B 1999;103:4264. [25] Roth AR, Webb JB, Williams DF. Solid State Commun 1981;39:1269.
Structural, electrical and optical properties of N-doped ZnO films synthesized by SS-CVD Jianguo Lu*, Zhizhen Ye, Lei Wang, Jingyun Huang, Binghui Zhao State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China
Abstract N-doped p-type ZnO films have been synthesized on a-Al2 O3 (0 0 0 1) substrate by solid-source chemical vapor deposition using ZnðCH3 COOÞ2 2H2 O as the precursor and CH3 COONH4 as the nitrogen source. The properties for ZnO films are dependent greatly on the growth conditions. Results show that the best electrical properties of the p-type film, such as carrier density N ¼ 9:8 1017 cm3 ; resistivity r ¼ 20 O cm and Hall mobility m ¼ 0:97 cm2 =V s; were induced at the substrate temperature of 5001C with a precursor temperature of 2501C and a nitrogen source of 1501C; under which the highest mixed orientation for (1 0 0) and (1 1 0) planes of films was also achieved. The p-type ZnO films possess a transmittance of about 90% in visible region and a band gap of about 3:20 eV at room temperature. r 2003 Elsevier Science Ltd. All rights reserved. PACS: 61.72.Vv; 81.05.Dz; 81.15.Gh Keywords: N-dope ZnO films; p-type conduction; SS-CVD; Properties
1. Introduction Zinc oxide is a II–VI compound semiconductor with wurtzite structure (space group P63 mc with cell parameters a ¼ 0:3249 nm; c ¼ 0:5206 nmÞ [1]. The films are commonly (0 0 2) textured (c-axis orientation) due to the lowest surface free energy for (0 0 2) plane [2]. Another preferred orientation is the few reported in [3]. ZnO occurs naturally as n-type conduction due to its various intrinsic donor defects (e.g., zinc interstitials or oxygen vacancies), which can be enhanced by B, Al, Ga, In or F donor dopants [4–8]. Theoretical calculations has predicted that among the acceptor dopants, such as group-I elements (Ag, Cu, Li) and group-V elements (N, P, As), nitrogen is the best candidate for producing a shallow acceptor lever about 100 meV above the valance band in ZnO [9,10], but in practice it is very difficult for *Corresponding author. Tel: +86-571-87952124; fax: +86571-87952625. E-mail addresses: [email protected] (J. Lu), yezz@ zju.edu.cn (Z. Ye), [email protected] (L. Wang), huangjy@ zju.edu.cn (J. Huang).
ZnO to achieve p-type conduction because of its low solubility of the dopant and high self-compensating process on doping. N-doped ZnO films deposited in N2 2O2 or N2 2Ar ambient had been studied before [11,12], but p-type conduction has not been realized until recently using ZnO/Zn as source material by chemical vapor deposition (CVD) in NH3 ambient [13] or through codoping approach (simultaneous doping of Ga donor and nitrogen acceptor) by pulsed laser deposition (PLD) in N2 O ambient [14–16]. CVD is a useful method for the formation of films, offering the advantage of producing high-quality films and availability of different ambient in situ doping process. Diethylzinc (DEZn) [17], basic zinc acetate (BZA) [18], zinc acethylacetonate [19] and zinc acetate dihydrate (ZA) [20] can be used as precursors, and they are usually prepared in various solutions. However, no other studies about p-type ZnO films grown by CVD have been reported since realizing p-type conduction by Minegishi et al. [13], using ZnO/ Zn as source material and NH3 as nitrogen source (loaded by H2 carrier gas) with the resistivity of typically 100 O cm and the carrier density about 1016 cm3 :
1369-8001/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1369-8001(02)00114-2
J. Lu et al. / Materials Science in Semiconductor Processing 5 (2003) 491–496
492
In this paper, we report the N-doped p-type ZnO films with mixed orientation for (1 0 0) and (1 1 0) planes synthesized by solid-source chemical vapor deposition (SS-CVD) using zinc acetate dihydrate (solid) as the precursor and ammonia acetate (solid) as the nitrogen source.
2. Experiment N-doped ZnO films were prepared on a-Al2 O3 (0 0 0 1) substrate by SS-CVD. The substrate temperature ðTs Þ varied in the range of 300–6001C: Zinc acetate dihydrate (ZA, solid) was used as the precursor and ammonium acetate (solid) as the nitrogen source. The temperature is kept as 2501C for ZA and controlled to be 1201C; 1351C and 1501C; respectively, for ammonia acetate. Oxygen (99.99%) was introduced to the reaction chamber through mass flow controllers, and held constant at 30 ml=min: The atmosphere pressure was about 102 Pa during the growth process of films. The deposition time was 20 min: The structural properties of the as-grown ZnO films were investigated by X-ray diffraction (Philips X’Pert) with Cu Ka radiation ðl ¼ 0:1542 nmÞ: Electrical resistivity, carrier density and mobility were measured in a magnetic field of 0:320 T by a four-point probe van der Pauw method using a HL5500 system at room temperature. The optical transmission spectra were examined with a Lambda20 spectrometer.
3. Results and discussion 3.1. Electrical properties The electrical properties for the as-grown ZnO films deposited under different conditions were examined by Hall measurement at room temperature, and summarized in Table 1. The sample Nos. 1–4 correspond ZnO films deposited at different substrate temperature with ammonia acetate temperature ðTN Þ kept constant at 1201C: It can be seen from the results that the film deposited at the substrate temperature of 3001C
(S. No. 1) still shows n-type conduction. But at 4001C and 5001C (S. Nos. 2–3), the carrier type flips from n- to p-type. The better properties of p-type ZnO, such as carrier density 3:7 1017 cm3 ; resistivity 42 O cm and Hall mobility 1:26 cm2 =V s; are achieved at 5001C: However, at a higher substrate temperature, ZnO (S. No. 4) again shows n-type conduction. In addition, when the substrate temperature is kept as 5001C (S. No. 2 and Nos. 5–6), the electrical properties of p-type ZnO films are improved with increasing ammonia acetate temperature, and the best results ðN ¼ 9:8 1017 cm3 ; r ¼ 20 O cm and m ¼ 0:97 cm2 =V sÞ are obtained at TN ¼ 1501C (S. No. 6). The changes mentioned above can be explained by the following nitrogen doping mechanism. According to Kamata et al. if ZnSe:N films are deposited using ammonia as nitrogen source, nitrogen and hydrogen will be incorporated into the ZnSe layer on a one-to-one basis, forming a strong N–H bond in film [21]. It has been also observed in nitrogen doped ZnO films by Minegishi using ammonia as the nitrogen source [13]. As for our experiment, it is anticipated to occur in the same way in the doping process using ammonia acetate as the nitrogen source. Zinc acetate dihydrate is sublimated and decomposed to form ZnO: ZnðCH3 COOÞ2 2H2 OðvÞ -ZnðCH3 COOÞ2 ðvÞ þ 2H2 OðvÞ;
ð1Þ
ZnðCH3 COOÞ2 ðvÞ þ H2 OðvÞ -ZnOðsÞ þ 2CH3 COOHðvÞ:
ð2Þ
Here, v and s represent vapor and solid, respectively. Fig. 1(a) illustrates the regular tetrahedron structure of ZnO. Simultaneously, Zinc acetate dihydrate reacts with ammonia resulted from the decomposition of ammonia acetate producing ZnNH: CH3 COONH4 ðvÞ-CH3 COOHðvÞ þ NH3 ðvÞ;
ð3Þ
ZnðCH3 COOÞ2 ðvÞ þ NH3 ðvÞ -ZnNHðsÞ þ 2CH3 COOHðvÞ:
ð4Þ
Fig. 1(b) shows the ZnNH structure. Nitrogen substitutes to oxygen site in the lattice. Hydrogen neutralizes
Table 1 Summary of the electrical properties obtained for N-doped ZnO films S. No.
Ts ð1CÞ
TN ð1CÞ
Resistivity ðO cmÞ
Mobility ðcm2 =V sÞ
Carrier density ðcm3 Þ
Carrier type
1 2 3 4 5 6
300 400 500 600 500 500
120 120 120 120 135 150
1e+4 2.3e+3 4.2e+1 4.0e+4 3.2e+1 2.0e+1
4.8 3.2 1.26 5.0 1.12 0.97
5.1e+14 +1.3e+15 +3.7e+17 7.1e+14 +6.5e+17 +9.8e+17
n p p n p p
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Fig. 1. The tetrahedron structure for ZnO (a), ZnNH (b) and ZnN (c).
CH3 COOHðvÞ þ 2O2 ðvÞ-2CO2 ðvÞ þ 2H2 OðvÞ:
ð5Þ
Oxygen also facilitates the dissociation of N–H bonds, and this will be discussed soon. The presence of hydrogen can suppress Zn interstitials due to the interstitial sites having been occupied by hydrogen atoms, which enhances greatly the effect of nitrogen dopants. It can be used to explain the failure to obtain p-type ZnO in N2 2O2 or N2 2Ar ambient due to the large density of zinc interstitials existing in films [11,12]. Furthermore, the nitrogen incorporation is also very low in ZnO films grown in N2 2O2 or N2 2Ar ambient because of more difficulty encountered in combining Zn to combine with N2 compared to combining NH3 : But now nitrogen is electrically inactive because of hydrogen passivation. Only when the N–H bond is dissociated under appropriate conditions (e.g., with a certain temperature and/or ambient) in the subsequent growth process, can it be activated, and act as an effective acceptor [22]. Oxygen existing in the growth ambient will facilitate the dissociation of N–H bond on the surface generating hydrogen atoms, and subsequently bind with the absorbed oxygen atoms to form the surface hydroxyl-groups of water [23] ZnNHðsÞ þ ½O -ZnNðsÞ þ ½OH :
ð6Þ
Fig. 1(c) shows the tetrahedron structure of ZnN. Nitrogen acts as an effective acceptor. In addition, an appropriate temperature is also indispensable in the process of dissociation of N–H bond. At a lower growth temperature, there will not be enough energy for nitrogen to be activated, so no p-type condition will be obtained. At a higher growth temperature, however, because the N–H bond has been broken before N is incorporated into the film, ZnO also shows n-type, which is very similar to the N-doped ZnO films in
002 100
No.1
110
101
Intensity (a.u.)
the free hold created by the nitrogen acceptor, and lies in the interstitial site next to nitrogen due to its small atom size. The role of O2 in the ambient during film growth is threefold: it is required as an additional oxygen source for the minimization of oxygen vacancies in the resulting ZnO films and for the carbonization avoidance
No.2
No.3
No.4 20
30
40
50
60
70
2-theta (deg.)
Fig. 2. XRD profiles of N-doped ZnO films as a function of substrate temperature (No. 1: 6001C; No. 2: 5001C; No. 3: 4001C; No. 4: 3001C).
N2 2O2 or N2 2Ar ambient [11,12]. Only in an intermediate temperature region, can nitrogen have a higher incorporation and be activated more easily to achieve p-type ZnO films. In this region, with increasing ammonia acetate temperature, the ammonia concentration in the growth ambient also increases, so the nitrogen incorporation is enhanced, and the electrical properties of p-type ZnO are improved.
3.2. Structural properties Fig. 2 shows the typical XRD profiles of the as-grown N-doped ZnO films at different substrate temperatures, such as 6001C; 5001C; 4001C and 3001C; corresponding to S. Nos. 1–4, respectively. Four diffraction patterns for (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes of ZnO are observed in the figure. ZnO powder (1996 JCPDS, 36-1451) has the relative intensities 57%, 44%, 100%, and 32%, respectively, for the (1 0 0), (0 0 2), (1 0 1) and (1 1 0) reflections. It can be concluded that the as-grown ZnO films deposited by SS-CVD possess mixed orientation for (1 0 0) and (1 1 0) planes, that is to say, with
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c-axis parallel to substrate. With the substrate temperature down to 3001C; the intensities of (1 0 0) and (1 1 0) peaks increase evidently, while the diffraction traces for (0 0 2) and (1 0 1) planes become very weak, indicating that a low growth temperature will facilitate the mixed orientation. Fig. 3 illustrates the XRD profiles of S. No. 2 and Nos. 5–6, corresponding to the ammonia acetate temperatures 1201C; 1351C and 1501C; respectively. Only diffraction patterns for (1 0 0) and (1 1 0) planes are observed at 1351C and 1501C; indicating that nitrogen concentration in films plays an important role in enhancing the mixed orientation for (1 0 0) and (1 1 0) planes. It is very interesting that the diffraction intensity of (1 1 0) plane decreases evidently with ammonia acetate temperature up to 1501C: The effect of preparation conditions on the mixed orientation was investigated by evaluating the mixed texture coefficient Tc : This factor can be estimated using the following formula: !1=n Y Tcðh k lÞ ; ð7Þ Tc ¼ n
where, Tcðh k lÞ is the texture coefficient of the ðh k lÞ plane, n is the number of mixed orientation planes. Tcðh k lÞ can be calculated by the following equation: Tcðh k lÞ
Iðh k lÞ =I0ðh k lÞ P ¼ : 1=N½ N Iðh k lÞ =I0ðh k lÞ
ð8Þ
Here, I is the measured intensity, I0 the corresponding recorded ASTM intensity, and N is the number of preferred directions of growth. Fig. 4 gives the Tc values of the mixed orientation for ð1 0 0Þ and ð1 1 0Þ planes for the as-grown ZnO films. The data increases with
decreasing substrate temperature or increasing nitrogen concentration in films, and a maximum value about 2.19 is reached for the ZnO film (S. No. 6) synthesized at Ts ¼ 5001C and TN ¼ 1501C: Zinc oxide has tetrahedral coordinates caused by sp3 hybridized orbits with the direction of each apex parallel to c-axis, and the three lowest density of the surface free energy is 9.9, 12.3, and 20:9 eV=nm2 for (0 0 2), (1 1 0) and (1 0 0) planes, respectively [2]. Therefore, the films are commonly (0 0 2) textured due to its low surface free energy. But, studies indicate that the surface free energy of the (0 0 2) plane is not necessarily always smaller than those of other planes [24]. If a large density of defects (e.g. vacancies and/or interstitials) and/or impurities exist in films, and/or the deposition is performed at nonequilibrium state (e.g., low temperature and/or high deposition rate), another orientation can be achieved. But ZnO film with another preferred orientation is the few reported due to the difficulty to form it [3]. Our results show that ZnO films with highly mixed orientation for (1 0 0) and (1 1 0) planes have been synthesized successfully by SS-CVD. A low growth temperature, especially a high nitrogen concentration in films, will facilitate evidently the forming of the mixed orientation, by which the theory mentioned above is tested. 3.3. Optical properties Fig. 5 illustrates the typical transmission spectra for the p-type ZnO films (S. No. 2 and Nos. 5–6). The transmittances are about 90% in visible region for all of the films. This value is almost the same as that reported for B, Al, Ga or In doped n-type ZnO [4–7]. The slight absorption at 4302580 nm could be due to interference effect. In the Fig. 5, a sharp absorption is observed. The 2.20
100
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2.15
No.2
110
Texture coefficient
Intensity (a.u.)
2.10
No.5
2.05
2.00
1.95
No.6 1.90
20
30
40
50
60
70
2-theta (deg.)
Fig. 3. XRD profiles of N-doped ZnO films as a function of ammonia acetate temperature (No. 2: 1201C; No. 5: 1351C; No. 6: 1501C).
300
350
400
450
500
550
600
Substrate temperature (°C)
Fig. 4. Texture coefficient of mixed orientation for (1 0 0) and (1 1 0) planes for the as-grown ZnO films (\: Nos. 1–4; m: No. 5; .: No. 6).
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25
No.2 80
No.5
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No.2 No.5 No.6
40
a2 (×1012 cm-2)
Transmittance (%)
20
20
0 300
400
500
600
700
800
No.6
15
10
5
3.20
0
3.19
Wavelength (nm)
Fig. 5. Transmittance spectra for the p-type ZnO films (S. No. 2 and Nos. 5–6).
3.0
blue shift of the fundamental absorption edge is due to a higher carrier density (from 3.7 to 9:8 1017 cm3 ) in accordance with the Burstein-Moss shift [25]. ZnO film has an absorption coefficient ðaÞ obeying the following direct transition equation for high photon energy ðhgÞ: a2 ¼ Aðhg Eg Þ:
ð9Þ
Here, Eg is the optical band gap, and A is constant. The a value can be evaluated from the transmission spectra with film thickness T ¼ ð1 RÞ2 expðadÞ;
ð10Þ
where T is the transmittance of thin film, R its reflectivity and d its thickness. The variations of the squared adsorption coefficient a2 versus the photon energy hg in the fundamental adsorption region are plotted in Fig. 6. Extrapolation of linear portion to the energy axis at a2 ¼ 0 gives the Eg value. The band gap is determined to be about 3:20 eV for the as-grown N-doped p-type ZnO. The decrease in Eg is probably attributed to the decrease in ionicity due to the formation of Zn–N bands in films. According to the Pauling theory, ionicity in a single bond increases with the difference in values of electron negativity between two elements forming the single bond. The electron negativity of Zn is 1.6, and the electron negativity of O (3.5) is larger than that of N (3.0), so the Zn–O band has larger ionicity than the Zn–N band.
4. Conclusion In summary, N-doped p-type ZnO films with mixed orientation for (1 0 0) and (1 1 0) planes have been
3.1
3.21 3.2 3.3 3.4 Photon energy (eV)
3.5
Fig. 6. Plot of squares absorption coefficient versus photon energy for the p-type ZnO films (S. No. 2 and Nos. 5–6).
synthesized on a-Al2 O3 (0 0 0 1) substrate by SS-CVD. The properties of the as-grown ZnO films are dependent greatly on the growth conditions. Results show that hydrogen plays an important role in the process of doping, and that a larger density of nitrogen in films not only results in better electrical properties but also enhances the mixed orientation. The carrier density, resistivity and Hall mobility of the p-type film were typically 9:8 1017 cm3 ; 20 O cm and 0:97 cm2 =V s respectively, which are the best results for p-type ZnO films obtained by CVD. The p-type ZnO films possess a high transmittance about 90% in visible region and a band gap around 3:20 eV at room temperature.
Acknowledgements This research is supported by the Special Funds for Major State Basic Research Project of China, No. G20000683-06.
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