Journal of Crystal Growth 226 (2001) 123–129 Nitrogen doped ZnO film grown by the plasma-assisted metal-organic chemical vapor deposition Xinqiang Wang a, *, Shuren Yang a , Jinzhong Wang a , Mingtao Li a , Xiuying Jiang a , Guotong Du a , Xiang Liu b , R.P.H. Chang b a Department of Electronic Engineering, State Key Lab on Integrated Optoelectronics, Jilin University, Changchun 130023, People’s Republic of China b Materials Research Center, Northwestern University, Evanston, Il, USA Received 28 December 2000; accepted 5 March 2001 Communicated by M. Schieber Abstract Nitrogen doped and non-doped ZnO films are grown by the plasma-assisted metal-organic chemical vapor deposition (MOCVD) on sapphire. X-ray diffraction spectra show that they are both strongly c-oriented while the N-doped sample is of better crystal quality. A strong emission coming from A-exciton is observed at 10 and 77 K photoluminescence (PL) spectra in both samples while deep level transition is hardly observed. More emission peaks are found in the PL spectrum of the N-doped sample relative to that of a non-doped one. Raman scattering is also performed in back scattering configuration. E 2 mode is observed in both samples while A 1(LO) mode can only be found in the N-doped sample, which indicates that high quality of the N-doped ZnO film. A high resistive ZnO film is obtained by nitrogen doping. # 2001 Published by Elsevier Science B.V. Keywords: A1. Doping; A3. Metalorganic chemical vapor deposition; B2. Semiconducting materials 1. Introduction ZnO, a wide gap semiconductor with band-gap of 3.36 eV at room temperature, is attracting more attention because of its good optical, electrical and piezo-electrical properties. It has potential uses in optoelectronical systems such as light emitting diodes (LEDs) [1], photodetectors [2], electrolumi- nescence devices and solar cells. Recently, opti- cally pumped ultraviolet (UV) lasing of ZnO film by molecular beam epitaxy (MBE) and pulsed laser deposition has been reported by several authors [3–5]. GaN is known as a good material for the fabrication of optical devices such as LEDs and laser diodes (LDs). ZnO has not only the same crystal structure as GaN, but also a larger exciton binding energy of 60 meV, which is 2.4 times that of GaN. Furthermore, Yu et al. have shown that textured ZnO films might have higher quantum efficiency than GaN [6]. This indicates that ZnO should be the most potential material to realize the next generation UV semiconductor laser. ZnO is also a promising material for surface acoustic wave (SAW) devices, which is very important in this information age. However, high resistive or *Corresponding author. Tel. +86-431-8922331. E-mail address: ysr@mail.jlu.edu.cn (X. Wang). 0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0022-0248(01)01367-7 p-type ZnO film is needed to realize the devices mentioned above. Nitrogen doping, which has been successful in fabricating p-type ZnSe [7], is considered as an effective method to realize high resistive or p-type ZnO film. In this paper, a non- doped ZnO film and a nitrogen doped one are deposited by plasma-assisted metal-organic che- mical vapor deposition (MOCVD). We have investigated their structural and optical qualities and found that the nitrogen doped (N-doped) ZnO film shows better crystal quality and higher resistivity relative to the non-doped one. 2. Experiment ZnO film was grown by MOCVD on (0 0 0 6) sapphire substrate. Di-ethyl zinc (DEZn) and O 2 were used as sources. High purity Ar was passed through the DEZn bubbler and saturated with DEZn vapor to the reactor. N 2 was used as the carrier gas. The substrate was cleaned in acetone, methanol, deionized water, and then chemical etched in H 2 SO 4 :H 3 PO 4 =3 : 1 for 10 min at 1608C followed by deionized water rinse. The reaction temperature was 6008C and the power of the plasma source was 900 W. The growth rate was 1 mm/h. The non-doped sample (sample A) was grown with O 2 flow of 10 SCCM while the N-doped one (sample B) was deposited with the flow ratio of O 2 :N 2 =1 : 1. The N-doped sample looked slight red while the non-doped one was transparent. We used SIEMENS D 5005 X-ray diffract- ometer and Rigaku DMAX 2400 X-ray diffraction (XRD) to investigate crystal quality. The absorp- tion measurement was performed by UV-3100 SHIMADZU UV-VIS-NIR Recording spectro- photometer with Xe lamp as optical source. Raman measurement was taken by RE- NISHAW-Ramascope at room temperature in back-scattering configuration by using 514.5 nm Ar + laser line excitation with an arriving power of about 70 mW. The scattered light was detected by a water-cooled charge coupled device (CCD) detector. The diameter of the laser beam was about 1 mm. PL spectrum was measured by a 325 nm He–Cd laser. The PL signal from the sample was filtered by a monochromator and picked up by a CCD detector. The power arriving at the sample was about 3 mW with a beam diameter of 200 mm. For low temperature mea- surement, the sample was mounted on a closed- cycle refrigerator. 3. Results and discussion XRD y22y scan spectra of N-doped (sample B) and non-doped ZnO film (sample A) are shown in Fig. 1(a) and (b), respectively. In both the two spectra, we find a dominant peak at around 2y of Fig. 1. X-ray diffraction spectra of ZnO film. The o-rocking curve is shown in the inset. (a) Non-doped ZnO film; (b) N-doped ZnO film. X. Wang et al. / Journal of Crystal Growth 226 (2001) 123–129124 34.68 due to (0 0 0 2) ZnO. This shows that the films are both strongly c-oriented. The full-width at half-maximum (FWHM) of (0 0 0 2) peak of sample B is 0.1488, which is narrower than that of sample A, 0.1978. The narrower FWHM implies that the N-doped film is of better crystal quality. From the statistical result, we inferred that the length of c-axis was 5.166 ( A in sample A while it is 5.181 ( A in sample B. They are both slightly smaller than that of bulk ZnO whose c-axis length is 5.2071 ( A, 2y=34.428. This can be ascribed to tensile stress induced by the deposition process. The o-rocking curves are shown in the inset of Fig. 1. It shows that ZnO grows in single c-axis orientation with the c-axis normal to the sapphire basal plane, indicating a heteroepitaxial relation- ship of (0 0 0 1) ZnO k(0 0 0 1) sapphire . The FWHM values of o-rocking curve of samples A and B are 0.568 and 0.348, which indicates that the N-doped sample has smaller mosaicity. Room temperature photoluminescence (RT-PL) spectra are performed as shown in Fig. 2. Ultra- violet (UV) emission, with peak energy positions of 3.30 and 3.289 eV, is dominantly observed in samples A and B respectively. The FWHM was 87 meV for sample A, which is narrower than the value 97 meV of sample B. The FWHM values of both samples A and B are higher than that of ZnO films reported by others using MBE and MOCVD [8,9]. A deep level emission at around 2.513 eV can be weakly observed in sample A. Its enlarged figure is shown inset. This deep visible transition is believed to come from oxygen vacancies, inter- stitial zinc or zinc vacancies [10–12]. In sample B, the peak position of deep level transition is weakly observed around 2.229 eV. In comparison with a previous study [13], the intensity of the deep level transition is much lower. The deep level transition shifts to the lower energy side relative to that of sample A, which may be related to nitrogen doping. The ratios of the intensity of UV emission (I UV ) to that of deep level emission (I DLE ) are 193 (sample A) and 136 (sample B), at room tempera- ture, respectively. The values are both rather high in comparison with another reported ratio value of 60 observed in ZnO film deposited by molecular beam epitaxy (MBE) [14] and of 1 by MOCVD [15]. This high ratio implies that our sample is of high optical quality. The relatively small I UV =I DLE value of sample B may be due to a slight increase in intrinsic defects in ZnO due to nitrogen doping. PL spectra under different excitation powers were performed. We do not find any shift of the position of the UV emission peak. The integrated PL intensity as a function of the excitation power on the logarithmic scale is plotted as shown in Fig. 3. From the figure, it is obvious that the solid line fits well the data shown by the black square in both samples. The PL intensity is linearly dependent on the excitation power. This indicates that the Fig. 2. Room temperature PL spectrum of ZnO film. The enlarged deep level transition is shown in the inset. (a) Non-doped ZnO film; (b) N-doped ZnO film. X. Wang et al. / Journal of Crystal Growth 226 (2001) 123–129 125 dominant photoluminescence of our sample should be the excitonic radiative recombination at room temperature. Low temperature measurement is performed at 10 and 77 K for further study of the optical properties. Fig. 4(a) and (b) correspond to samples A and B, respectively. As shown in Fig. 4(a), four peaks appear at 3.377, 3.370, 3.333 and 3.241 eV, respectively. The dominant peak at 3.377 eV is ascribed to the A-exciton emission while the peak at 3.370 eV corresponds to D8X bound exciton transition. The peaks at 3.333 and 3.241 eV are due to the donor–acceptor pair transition and LO phonon replica, respectively. In the PL spectrum of 77 K shown in the inset of Fig. 4, the peak due to D8X bound exciton transition is hardly observed. The energy positions of other peaks are at 3.373, 3.314 and 3.238 eV, respectively. They all shift to the low energy side compared to that at 10 K due to thermal effect. From Fig. 4(b), we can find two dominant peaks at 3.386 and 3.372 eV and four other peaks with low intensity at 3.324, 3.248, 3.196 and 3.125 eV, respectively. The peak at 3.386 eV is ascribed to A-exciton emission while the peak at 3.372 eV is due to D8X bound exciton transition. In comparison with that of sample A, the two peaks shift to high energy level. The peak at 3.324 eV corresponds to donor–acceptor transi- Fig. 3. The room temperature dependence of integrated output intensity on excitation intensity on the logarithmic scale. (a) Non- doped ZnO film; (b) N-doped ZnO film. Fig. 4. Low temperature PL spectra of ZnO film. (a) Non-doped ZnO film; (b) N-doped ZnO film. X. Wang et al. / Journal of Crystal Growth 226 (2001) 123–129126 tion while the peaks at 3.248, 3.196 and 3.125 eV are due to the LO phonon replicas, respectively. From the 77 K PL spectrum of sample B as shown in the inset, we find that all the peak positions shift to the low energy side. Six peaks can also be found at 3.372, 3.360, 3.3137, 3.239, 3.189 and 3.106 eV. In comparison with PL spectrum of sample A, we find that more peaks are observed and the dominant peak shifts to the higher energy side. The possible reason is the doping of nitrogen. We find that sample B shows better optical quality than sample A at low temperature. By the way, deep level emission is hardly observed in the PL spectra of both samples at low temperature. Raman scattering is performed on both samples at room temperature in back-scattering configura- tion as shown in Fig. 5(a) and (b). ZnO has a hexagonal wurtzite structure and belongs to the C 6n symmetry group. In our back-scattering configuration, A 1(LO) and E 2 are Raman active. The peaks at 437.6 cm À1 in Fig. 5(a) and 437.9 cm À1 in Fig. 5(b) are ascribed to high frequency E 2 mode. Since the measure range of our micro-Raman system was from 100 to 4000 cm À1 , we cannot observe the low frequency E 2 mode. Raman spectrum of ZnO powder is also performed as shown in Fig. 5(c), in which we observe the peak position of E 2 modes lies at 437.4 cm À1 . The slight discrepancy of the position of the E 2 mode of ZnO films and powder shows that our samples are almost free of stress. We find the second order Raman spectrum arising from zone-boundary phonons 2-E 2 (M) at 338 cm À1 [16]. As shown in Fig. 5(a), the peaks at 380, 417.4, 447.7, 575.9 and 749.4 cm À1 are ascribed to sapphire substrate. Since our ZnO film is relatively Fig. 5. Raman spectra of ZnO films and ZnO powder. (a) Raman spectrum of non-doped ZnO film; (b) Raman spectrum of N-doped ZnO film; (c) Raman spectrum of ZnO powder. X. Wang et al. / Journal of Crystal Growth 226 (2001) 123–129 127 thin and non-doped ZnO and sapphire are both nearly transparent to visible laser light as shown in Fig. 6, we can clearly observe the Raman peaks of sapphire. In Fig. 5(b), the second order Raman spectrum arising from zone-boundary phonons 2- E 2 (M) is also observed at 331 cm À1 . The Raman peaks from sapphire have a low intensity. This is due to the higher absorption ratio of nitrogen doped ZnO film relative to non-doped one as shown in Fig. 6. We can find a dominant peak at 581 cm À1 in Fig. 5(b) which is ascribed to A 1(LO) mode. We did not find this A 1(LO) mode clearly in non-doped ZnO film. In the Raman study of GaN, the intensity of A 1(LO) mode increased with the decrease of carrier concentration [17]. We think that ZnO may have the same characteristics as GaN. Since the carrier concentration of nitrogen doped ZnO is more than 1000 times smaller than that of the non-doped one, which is about 4.0 Â 10 17 cm À3 , we expect that the intensity of A 1(LO) should be stronger. This may be the reason why we find A 1(LO) mode in N-doped sample but not in the non-doped sample. Both the higher intensity of A 1(LO) mode and the observation of E 2 mode and A 1(LO) mode coinciding with the prediction of group theory imply the better crystal quality of nitrogen doped ZnO film. In the Raman spectrum of nitrogen doped ZnO film, other peaks at 274, 508, 641.9, and 857 cm À1 can be found. The possible reason may be related to nitrogen doping and further study about this should be going on. The resistivity of non-doped ZnO film by four- point probe measurements is 0.65 O cm. Due to the high resistivity and low carrier concentration of N- doped film, four-point probe measurements are not reliable. However, ohmic contacts are formed by Al metal on high resistive ZnO film, which indicates that the N-doped ZnO film is still n-type. The resistivity of N-doped ZnO film is estimated to be 5 Â 10 4 O cm by I–V measurement. 4. Conclusions High quality N-doped and non-doped ZnO films are successfully deposited. XRD spectra show that ZnO films are both c-oriented while the N-doped films show better crystal quality with smaller mosaicity relative to non-doped samples. UV emission and deep level transition are ob- served from PL spectrum at room temperature. The high value of I UV =I DLE indicates high optical quality. We find A band free exciton emission and D8X bound exciton transition from PL spectrum at 10 K in both samples. In Raman spectrum of ZnO film, E 2 and A 1(LO) modes are observed in N- doped sample while A 1(LO) is hardly observed at non-doped one. In comparison with Raman spectrum of ZnO powder, we find that the peak positions of E 2 mode are almost the same indicating that ZnO films are almost free of stress. High resistive but not p-type ZnO film is realized by nitrogen doping. Acknowledgements This work was supported by NSFC-RGC (No. 59910161983) and Jilin Province Science Fund (No. 19990518-1). References [1] Toru Soki, Yoshinori Hatanaka, David C. Look, Appl. Phys. Lett. 76 (2) (2000) 3257. [2] Y. Liu, C.R. Gorla, S. Liang et al., J. Electron. Mater. 29 (1) (2000) 60. [3] H. Cao, Y.G. Zhao, H.C. Ong et al., Appl. Phys. Lett. 73 (25) (1998) 3656. Fig. 6. Absorption spectrum of ZnO film. (Solid line}non- doped ZnO film; Dash line}N-doped ZnO film.) X. Wang et al. / Journal of Crystal Growth 226 (2001) 123–129128 [4] Z.K. Tang, G.K.L. Wong, P. Yu et al., Appl. Phys. Lett. 72 (25) (1998) 3270. [5] D.M. Bagnall, Y.F. Chen, Z. Zhu et al., Appl. Phys. Lett. 70 (17) (1997) 2230. [6] P. Yu, Z.K. Tang, G.K.L. Wong, M. Kawaski, A. 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Wang et al. / Journal of Crystal Growth 226 (2001) 123–129 129 . 2001 Communicated by M. Schieber Abstract Nitrogen doped and non -doped ZnO films are grown by the plasma-assisted metal-organic chemical vapor deposition (MOCVD). of Crystal Growth 226 (2001) 123–129 Nitrogen doped ZnO film grown by the plasma-assisted metal-organic chemical vapor deposition Xinqiang Wang a, *, Shuren