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Effect of Annealing Temperature on Structural and Optical Properties of N-Doped ZnO Films


Vol. 25, No. 7 (2008) 2585

E?ect of Annealing Temperature on Structural and Optical Properties of N-Doped ZnO Films ?
ZHONG Sheng( ), ZHANG Wei-Ying( ), WU Xiao-Peng( FU Zhu-Xi( )?? ), LIN Bi-Xia( ),

Department of Physics, University of Science and Technology of China, Hefei 230026

(Received 27 February 2008)
Nitrogen-doped ZnO (ZnO:N) ?lms are prepared by thermal oxidation of sputtered Zn3 N2 layers on Al2 O3 substrates. The correlation between the structural and optical properties of ZnO:N ?lms and annealing temperatures is investigated. X-ray di?raction result demonstrates that the as-sputtered Zn3 N2 ?lms are transformed into ZnO:N ?lms after annealing above 600? C. X-ray photoelectron spectroscopy reveals that nitrogen has two chemical states in the ZnO:N ?lms: the NO acceptor and the double donor (N2 )O . Due to the No acceptor, the hole concentration in the ?lm annealed at 700? C is predicted to be highest, which is also con?rmed by Hall e?ect measurement. In addition, the temperature dependent photoluminescence spectra allow to calculate the nitrogen acceptor binding energy.

PACS: 61. 72. Vv, 68. 49. Uv, 71. 55. Gs, 81. 05. Dz, 81. 15. Cd Zinc oxide has attracted a great deal of attention in recent decades because of its wide band gap of 3.37 eV and high exciton binding energy of 60 meV, which permits excitonic emission at and above room temperature. Due to these novel properties ZnO can be considered as a prime candidate for UV light emitting diodes and lasers.[1] However, the development of ZnO-based devices is hindered by the di?culty in achieving reliable and reproductive p-type ZnO because this material exhibits natural n-type conductivity. Though p-type ZnO epilayers are not commercially available at present, there have been a number of reports on synthesis of p-type ZnO doped with several dopants, such as Li, N, P, As, etc.[2?5] Among these doping elements, nitrogen is considered as the most promising p-type dopant for its ionic radius is most similar to that of O. As a simple way to fabricate p-type ZnO, thermal oxidation of Zn3 N2 ?lms can overcome the crucial problem concerning the low solubility of the N acceptors in ZnO.[6,7] However, the structural, especially the chemical state of the nitrogen, and optical properties of the ZnO:N ?lm achieved by this means have not yet been investigated. In this Letter, we present a detailed study in these aspects. Zn3 N2 ?lms with a thickness of 300 nm were deposited on Al2 O3 substrates by dc magnetron sputtering of a metallic Zn target (99.99% purity) in a mixture of Ar and N2 . Here, the substrate temperature was kept at 350? C and the sputtering pressure in the vacuum chamber was 2 × 10?2 Torr. After deposition, the samples were thermally oxidized at temperatures from 600? C to 800? C for one hour in forming gas (N2 and O2 ). For comparison, an undoped ZnO thin ?lm was also fabricated using Ar and O2 . The crystal structure was characterized by x-ray di?raction (XRD) performed on a Japan MAC SCI? Supported ?? Email:

ENCE M18X x-ray di?ractometer with Cu Kα radiation (λ = 1.541 ?). X-ray photoelectron spectroscopy A (XPS) data were obtained on a British VG ESCALABMKII photoemission system. Hall e?ect measurement was carried out on a van der Pauw con?guration. In addition, the low-temperature photoluminescence (PL) spectra were also carried out with the excitation of 325 nm line of a He–Cd laser (15 mW).

Fig. 1. XRD patterns of as-sputtered Zn3 N2 ?lm and ZnO:N ?lms annealed at di?erent temperatures.

Figure 1 shows the XRD patterns of the asdeposited Zn3 N2 sample and the ZnO:N samples annealed at di?erent temperatures. For the as-deposited Zn3 N2 ?lm, di?raction peaks corresponding to the Zn3 N2 (321) and (640) di?raction were observed. After thermal oxidation at 600? C, all the di?raction peaks observed can be identi?ed as ZnO phases. As annealing temperature increased to 700? C and 800? C, only the ZnO (002) peak can be observed, which in dicates that the Zn3 N2 ?lm was entirely transformed

by the National Natural Science Foundation of China under Grant No 50532070. fuzx@ustc.edu.cn c 2008 Chinese Physical Society and IOP Publishing Ltd


ZHONG Sheng et al.

Vol. 25

into the ZnO:N ?lm with high crystalline quality. In addition, all the peaks of ZnO (002) di?raction shift to the lower angle, which indicates that partial O sites have been occupied by N atoms after annealing.[8]

Fig. 2. XPS spectra of ZnO:N ?lms annealed at di?erent temperatures.

Zn3 N2 ?lm, the N 1s peaks are found at 396.4 eV and 403.2 eV, as shown in Fig. 2(a). The lower binding energy at 396.4 eV is generally considered to arise from N-Zn bond because it is consistent with the value of N3? in metal nitrides in which the N atom has received substantial charge from the surrounding metal atoms.[9] The higher binding energy at 403.2 eV suggests a higher chemical valence of nitrogen in the sample. Generally, this peak can not be assigned to N–O bond because the N 1s binding energy of N–O bond (in NO2? or NO? ) is higher at the range of 406– 3 2 408 eV.[10,11] Sanghera et al.[12] also observed a similar N 1s peak at 403.3 eV in Cu samples bombarded with N2+ beams, and they attributed it to weakly bound nitrogen. Hereby, we attribute the higher 403.2 eV peak to (N2 )O , denoting a substitution of molecular N for O sublattice, which is considered as an double donor.[13] When annealing temperature rises to 700? C, the 403.2 eV peak disappears while the lower one still remains. This implies that the thermal stability of (N2 )O is considerably lower than that of NO at 700? C and the former is easy to be disassociated. Up to 800? C, signals of the two peaks are both closed to the noise level and can not be distinguished, which means both of the (N2 )O and No were dissolved at this temperature. Assuming the NO acceptor were all ionized, we can calculate the theoretical hole concentration due to nitrogen doping in the ?lms annealed at 600? C and 700? C: I396.6eV /SFN ρ= , (1) ΣIi /SFi where I396.6eV is the integrated intensity of the NO peak, SFN is the sensitivity factor of nitrogen, Ii and SFi are the intensities and SFs for zinc, oxygen, and nitrogen. The results and other properties of the samples are given in Table 1.

In order to identify chemical states of nitrogen in ZnO:N ?lms, x-ray photoelectron spectroscopy (XPS) was carried out. Figure 2 displays the core level spectra covering the N 1s region of ZnO:N ?lms annealed at 600? C, 700? C and 800? C. For the 600? C annealed

Table 1. Properties of ZnO:N ?lms annealed at di?erent temperatures. Sample 1# 2# 3# Annealing temperature (? C) 600 700 800 Visual appearance Deep yellow Yellow Pale yellow XPS-predicted hole concentration (cm?3 ) 7 × 1017 1 × 1018 Carrier concentration (cm?3 ) 1 × 1018 3 × 1017 3 × 1018 Measured conductivity type n n n

Table 1 also shows the results obtained from Hall e?ect measurements. Although a high hole concentration of about 1 × 1018 cm?3 due to the fact that nitrogen acceptor has been predicted by XPS spectra, the p-type ZnO:N ?lm is not obtained. As shown in Table 1, the carrier concentration of samples annealed at 600? C and 800? C is an order of magnitude higher than that of 700? C. It can be interpreted that when the annealing temperature is 600? C, the No acceptors can not compensate for the extrinsic and intrinsic defects, such as (N2 )O , O vacancies (VO ) and Zn interstitial (Zni ), so the samples behave strong n-type conductivity. As rising to 700? C, the concentration of the (N2 )O is reduced markedly and No acceptors com-

pensate most of the donor defects, which induces the reduction of the carrier concentration in the sample. With the increasing temperature to 800? C, NO and (N2 )O are all thermally dissolved, which results in the ?lm exhibited obvious n-type conductivity. The Hall e?ect measurement suggests that the carrier concentration in the ZnO:N ?lms is a?ected by the annealing temperature signi?cantly. Low-temperature photoluminescence (PL) spectra of undoped ZnO and ZnO:N ?lms are shown in Fig. 3. As seen in Fig. 3(a), the PL spectrum of an undoped ZnO ?lm is dominated by a peak centred at 3.363 eV assigned to D0 X,[14] which corresponds to a recombination of donor-bound exciton. Besides the D0 X

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ZHONG Sheng et al.


emission, a shoulder at 3.377 eV can also be observed, which is attributed to free exciton (FX) emission.[15] The peaks at 3.320 eV and 3.333 eV could be suggested as donor acceptor pair (DAP)[16] and two electron satellite (TES),[17] respectively. As another characteristic of neutral donor bound exciton, TES transition involves radiative recombination of an exciton bound to a neutral donor, leaving the donor in the excited state. In the e?ective-mass approximation, the energy di?erence between D0 X and their excited states (TES) can be used to determine the donor binding energy ED : 4 ED = (ED0 X ? ETES ) ≈ 40 meV. (2) 3 Furthermore, according to Haynes’ rule,[18] which shows a linear relation between ED and Eloc (the separation between the FX and D0 X), we can also determine the Haynes coe?cient α as α = (EFX ? ED0 X )/ED ≈ 0.35. (3) This result is in agreement with the value reported in the literature.[14,17]

to acceptor) transition.[19] Moreover, the integrated intensity of the 3.311 eV emission decreases gradually as the temperature increases over the measurement range, and it exhibits the thermal characteristic of the FA transition.[20] Thus the 3.311 and 3.285 eV peaks can be assigned to FA and DAP transition, respectively. The acceptor binding energy EA is given by EFA = Eg ? EA + kB T /2, (4) where Eg is the band gap and kB is Boltzmann’s constant. If we use the value of Eg = 3.437 eV at 10 K,[15] we can obtain an EA value of about 130 meV. Similar conclusions have been derived by Meyer et al.[14] on nitrogen doped ZnO. The acceptor energy of nitrogen can also be given from the DAP transition: EA = Eg ? ED ? EDAP + e2 /4πεε0 r . (5) The last term denotes the Coulomb potential and it can be roughly estimated by βNA 1/3 , where NA is the acceptor concentration of about 1 × 1018 cm?3 and β is constant equal to 2.7 × 10?5 meV·cm.[17] With the knowledge of Eg = 3.437 eV[15] and ED = 40 meV, an acceptor binding energy of 130 ± 10 meV can also be obtained. This result is quite compatible with the one from Eq. (3) within expected accuracy. In conclusion, nitrogen-doped ZnO ?lms have been prepared by thermal oxidation of Zn3 N2 layers on Al2 O3 substrates by dc magnetron sputtering. A detailed study of the in?uence of annealing temperature on the structural and optical properties of ZnO:N ?lms is carried out. XPS measurement reveals that N has two chemical states in ZnO:N ?lms: one is (N2 )O and the other NO , which are considered as donor and acceptor respectively. Furthermore, the XPS result predicts a high hole concentration in the ?lm annealed at 700? C, which is also implied by Hall e?ect measurement. By low-temperature PL spectra, the NO acceptor binding energy of nitrogen is determined.

[1] [2] [3] [4] [5] [6] [7] [8] [9] Tang Z K et al 1998 Appl. Phys. Lett. 72 3270 Zeng Y J et al 2006 Appl. Phys. Lett. 88 062107 Tsukazaki A et al 2005 Nat. Mater. 4 42 Kim K K et al 2003 Appl. Phys. Lett. 83 63 Look D C et al 2004 Appl. Phys. Lett. 85 5269 Yoshitaka N et al 2006 Appl. Phys. Lett. 88 172103 Kaminska E et al 2005 Solid State Commun. 135 11-15 Yao B et al 2006 J. Appl. Phys. 99 123510 Moulder J F et al 1992 Handbook of X-ray Photoelectron Spectroscopy (Eden Prairie: Perkin-Elmer Corporation) p 43 Torres J et al 2003 J. Phys. Chem. B 107 5558 Bian J M et al 2004 Appl. Phys. Lett. 84 541 Sanghera H K et al 1999 Surf. Interface Anal. 27 678 Yan Y F et al 2001 Phys. Rev. Lett. 86 5723 Meyer B K et al 2005 Semicond. Sci. Technol. 20 S62 Look D C 2001 Mater. Sci. Eng. B 80 383 Kang H S et al 2006 Appl. Phys. Lett. 88 202108 Thonke K et al 2001 Physica B 308-310 945 Haynes J R 1960 Phys. Rev. Lett. 4 361-363 Tamura K et al 2003 Solid State Commun. 127 265 Myoung J M et al 1996 Appl. Phys. Lett. 69 2722

Fig. 3. Low temperature PL spectra of undoped ZnO (a) and ZnO:N (b) ?lms annealed at di?erent temperatures.

Figure 3(b) presents the temperature-dependent PL spectra of the ZnO:N ?lms annealed at 700? C. In comparison with undoped ZnO ?lm, the 3.363 peak attributed to D0 X is relatively weaker which demonstrates that most donors were compensated for by NO acceptors. The broad line centred at about 3.29 eV is composed of two emissions: 3.311 eV and 3.285 eV. As the measurement temperature increases from 10 K up to 50 K, the emission at 3.311 eV shows tiny blue shift, which is a typical characteristic of FA (free electron

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]


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