P Nanostructure Fabricated by a Chemical

J. Phys. Chem. C 2009, 113, 18527–18530

18527

Optical Properties of a ZnO/P Nanostructure Fabricated by a Chemical Vapor Deposition Method C. H. Zang,* D. M. Zhang, C. J. Tang, S. J. Fang, Z. J. Zong, Y. X. Yang, C. H. Zhao, and Y. S. Zhang Luoyang Institute of Science and Technology, Luoyang 471023, People’s Republic of China ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: September 17, 2009

A diamond-shaped P-doped ZnO nanostructure was fabricated on a Si (100) substrate by a chemical vapor deposition method. The photoluminescence properties of the ZnO nanostructure were studied with a temperature range from 81 to 306 K. At 81 K, a series of transitions of donor-acceptor pairs and their phonon replicas were observed in the PL spectrum. These results revealed that shallow-donor and deep-acceptor impurity bands existed in the P-doped ZnO nanostructure. From 81 to 111 K, the abnormal UV emission intensity was observed. The multiphonon scattering spectra were attributed to the interaction of electrons and phonons. Introduction Low-dimensional ZnO nanostructure materials have been paid great attention because ZnO is with a direct wide band gap (3.37 eV) and large exciton binding energy (∼60 meV) at room temperature, which makes ZnO nanostructure materials have potential applications on nanosensors, ultraviolet (UV) nanolasers, and photodetectors.1 By now, most efforts to grow P-ZnO materials have been focused on the fabrication of ZnO thin films. P-ZnO films have been fabricated by dopants, such as N, P, Sb, P-In, and so on.2-4 The optical properties of ZnO/P have been studied widely, but mostly in film material with few studies focused on nanostructures. The researched ZnO/P film materials were fabricated by MBE MOCVD and magnetron sputtering techniques. In our experiment, ZnO dopant P nanostructure materials have been fabricated by thermal evaporation of a Zn and P2O5 mixture. The special diamond-shaped ZnO/P nanostructure has been obtained. The optical properties have been studied concerning this special morphology and fabrication method. In P-doped ZnO nanostructures, a lot of lattice defects can be formed, such as Zni, PZn, VZn, Oi, and so on. A broad emission band results from the shallow-donor to the deep-acceptor transitions in the PL spectra. The different deep acceptors can have their own vibronic states by adsorption or emit local phonon.5 Thus, a complicated DAP transition with its local phonon and lattice phonon replica can be observed. The multiphonon Raman scattering is also observed due to the interaction of electrons and phonons. The potential of the displacement is proportional to the amplitude of the phonon.6 Experiment and Details

Figure 1. SEM images (a-c) of ZnO/P nanostructures. (b) The sideby-side configuration in the rectangular shape and the end-to-end configuration in the ellipsoid shape.

Figure 2. EDX spectrum of the ZnO/P nanostructure.

X-ray spectroscopy (EDS, GENE SIS 2000 XMS 60S, EDAX, Inc.) attached to an SEM The structural characterization was analyzed by X-ray diffraction (XRD, Rigaku D/max-γA) spectroscopy with a Cu KR line of 0.154 nm. The micro-PL and Raman were excited with a He-Cd laser at 325 and 488 nm, respectively. Results and Discussion

In this paper, the diamond-shaped ZnO nanostructure was prepared by a vapor-solid method on a silicon substrate. 4 N Zn power and 5% 3 N P2O5 were used as source materials. The sample was grown at 580 °C in ambient Ar with a rate of 50 sccm for 1 h. It was then cooled to room temperature. The sample was investigated by field emission scanning electron microscopy (FESEM, Hitachis-4800) and energy-dispersive * To whom correspondence should be addressed. Tel: +86-37965928281. Fax: +86-379-65928281. E-mail: [email protected].

Figure 1 shows the FESEM images of the as-grown sample. The uniform ZnO nanostructure is observed in Figure 1a, the low-magnification image. Figure 1b,c shows the high-magnification images. The diamond-shaped ZnO nanostructure is observed. The diamond-shaped ZnO nanostructure consists of 14 faces: a hexagonal top, bottom, and 12 sides. Figure 2 shows the results of the EDS analysis that there are only zinc, oxygen, and phosphorus elements in the diamond-shaped ZnO nanostructure with an unstochiometric content (Zn/O ) 61.62:35.67

10.1021/jp905648m CCC: $40.75  2009 American Chemical Society Published on Web 10/05/2009

18528

J. Phys. Chem. C, Vol. 113, No. 43, 2009

Figure 3. XRD spectrum of the ZnO/P nanostructure.

Figure 4. Sketch of the ZnO/P nanostructure growth process.

atom %). The P concentration is about 2.71%. Figure 3 shows the XRD spectrum of the sample. The peaks at 31.76, 34.4, 36.2, 47.45, 56.6, 62.8, and 67.9° are ZnO with (100), (002), (101), (102), (110), (103), and (112) orientation, respectively. The peak at 69.3° is attributed to the (400) orientation of the Si substrates. The XRD pattern illustrates that the ZnO was with a wurtzite structure. Figure 4 depicts the diamond-shaped ZnO nanostructure growth process. The growth process is depicted as a vapor-solid (VS) process. The three major steps involved in diamond-shaped ZnO nanostructure are illustrated in Figure 4. As is well-known, the degree of supersaturation determines the prevailing growth morphology. A low supersaturation is required for whisker growth, a medium supersaturation supports bulk crystal growth, and a high supersaturation supports powder formation by homogeneous nucleation in the vapor phase.7 At first, ZnO nucleated homogenously at a medium supersaturation, when the temperature was raised rapidly to 580 °C. The diameter of the ZnO nucleation is about several hundred nanometers; the hexagonal ZnO column has been formed when the temperature was kept at this point for several minutes, as shown in Figure 4a. The oxygen partial pressure was then increased gradually due to P2O5 thermal decomposition. The ratio of zinc to oxygen partial pressure changed gradually. The uneven width change along the length of the diamond-shaped ZnO nanostructure is related to the anisotropy of ZnO materials; the tower-like ZnO structure is formed, as shown in Figure 4b. At last, when the temperature was cooled gradually to room temperature, the supersaturation changed from high to low degree; the 1D structure was the preferred growth mechanism. The diameter of the tip turned slim, as shown in Figure 4c. The diamondshaped ZnO nanostructure has been formed by changing the degree of supersaturation. K. J. M. Bishop and his co-workers reported the nanoscale force and their uses in self-assembly.8 They discussed that vdW (van der Waals) had influence on the anisotropic nanoparticles’ (NPs) assembly by the highly directional interactions. In our experiment, the obtained NPs’ self-assembly mainly includes side-by-side and end-to-end configurations, which are determined by vdW potentials (Figure 1b). One, which has more vdW potentials between side-by-side and end-to-end configurations, is easier to be formed than another. The other disorder

Zang et al.

Figure 5. Raman spectrum of the ZnO/P nanostructure.

assembly arrangements are caused by competing side-by-side and end-to-end arrangements. Besides vdW forces, electrostatic forces play an important role in the NPs’ assembly. Electrostatic interactions can influence the NPs’ structure and their arrangement. In the Figure 3 XRD pattern, a strong diffractive peak of ZnO (0002) orientation is observed; it suggests that the NPs with a zinc- and oxygen-terminated surface existed.9 The zincand oxygen-terminated (0001) surface is with the positive and negative charges, respectively; NPs can be either attractive or repulsive by electrostatic interactions to form end-to-end or sideby-side configuration assembly arrangements. Moreover, a lot of disorder arrangements are also observed; electrostatic interactions can form the stable NP structure and their assembly arrangements and lower directly the system energy. The UV resonant Raman scattering at room temperature was performed to investigate the vibrational properties of the P-doped ZnO nanostructure, as shown in Figure 5. The Raman peaks are located at 553, 1130, and 1707 cm-1. The space separation is 577 cm-1. They are assigned to longitude optical (LO) multiphonon Raman scattering mode. As is well-known, group theory analysis predicts that the zone-center optical modes have the symmetries A1 + 2B1 + E1 + 2E2. The A1 and E1 modes are both Raman and infra active, the two E2 modes are Raman active, whereas B1 modes are forbidden modes and the A1 and E1 modes are polar.10 The frequency of the A1(LO) and E1(LO) modes of ZnO bulk are at 568 and 586 cm-1, respectively. The A1(LO) mode is acquired from the c face of the wurtzite structure, whereas E1(LO) is a forbidden mode in a backscattering geometry from both a and c faces. In our experiment, the crystal orientation of the diamond-shaped ZnO nanostructures is random and Raman scattering is not from a or c axes; the acquired Raman scattering mode is known as quasimodes.11 According to Loudon’s model, the quasimodes are mixed symmetry modes and 1LO phonon frequency should be at between 568 and 586 cm-1. In our experiment, 1LO phonon peak is at 553 cm-1. The considerable red shift is attributed to the remained stress. The multiphonon Raman scattering is attributed to the interaction of electrons and phonons. Figure 6 shows the temperature dependence of the PL spectra of the ZnO nanostructure in the range from 81 to 111 K. The abnormal UV PL feature can be found that the emission intensity increases with the temperature increasing from 81 to 111 K. Generally, the anomalous enhancement of the PL intensity with temperature can be explained as carrier population increase. The extra carriers are supplied by spatially localized excitons at surface states.12 In our experiment, the UV near-band-edge (NBE) emission intensity ratio of 111 to 96 K (I111 K/I96 K) and 96 to 81 K (I96 K/I81 K) is about 1.48 and 2.61, respectively. It suggests that a number of extra electrons are thermally elevated

Optical Properties of a ZnO/P Nanostructure

Figure 6. Temperature-dependence PL spectra in the range of 81-111 K of the ZnO/P nanostructure.

into the conduction. In P-doped ZnO nanostructure, the thermally activated electrons come from two facts: the shallow donor ionization and trapped effect. On the one hand, Zni, VO, PZn, and Pi may be as the shallow donor; the discrete donor energy states can split into a band of energy under the conductive band. On the other hand, the lattice imperfection of ZnO/P can form the trapped states energy level. The level of Zni is about 0.03, 0.05, and 0.06 eV, and the VO level is about 0.05 eV below the conduction band by the different reports.5,13 Theoretical investigation shows that the level of PZn is lying near or above the conduction band edge.14 In our PL measurement, the blue shift of the UV NBE emission is not observed for the doped P at a low concentration; we infer that the PZn level is near the conduction band and it may be overlapped with the Zni or VO level. Moreover, Pi is difficult to be formed for the large atomic radius of P. The transition from the trapped energy level to the deep-acceptor energy level may be forbidden for the selected rules. The electron is easily thermally ionized at the shallow-donor energy band. Thus, the anomalous PL intensity is predominantly attributed to the trapped effect. The electron is excited into the conduction band; then, it decays to the trapped states. The electron from the trapped states is gradually thermally elevated into the conduction band with the temperature increase. The UV NBE emission is enhanced. Simultaneously, the escaping trapped electron decays to the lower energy by emitting one or more phonons from the conductive band and it recombines with the acceptor, such as VZn or Oi, to emit a photon. Therefore, the DAP recombination is also enhanced. Moreover, the abnormal UV NBE emission may be attributed to the enhanced phonon-exciton coupling. For the acceptor states, vibration intensified with the temperature increase; the enhanced DAP emission can be attributed to the spatial wave function overlap extending between the shallowdonor states and the deep-acceptor states. The deep level emission intensity ratio of 111 to 96 K (I111 K/I96 K) and 96 to 81 K (I96 K/I81 K) is about 0.8 and 1.25, respectively. The green light band emission intensity is not enhanced obviously with the temperature increase. It suggests that the abnormal PL feature should not be attributed to the surface states. Figure 7 shows the temperature dependence of the PL spectra of the sample in the range from 111 to 276 K. The ZnO PL intensity decreases with the temperature increase. Figure 8 shows that the PL peaks are at 3.358, 3.317, 3.239, 3.224, 3.171, 3.106, 3.003, 2.930, 2.903, and 2.836 eV. The peaks at 3.358 and 3.317 eV are assigned to the neutral-acceptor-bound exciton (A0X) and free electron to the acceptor transition (FA), respectively. The peaks at 3.239 and 3.171 eV are identified as the first and

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18529

Figure 7. Temperature-dependence PL spectra in the range of 111-276 K of the ZnO/P nanostructure.

Figure 8. Low-temperature PL spectra at 81 K of the ZnO/P nanostructure.

Figure 9. Low-temperature PL spectra at 111 K of the ZnO/P nanostructure.

second order LO phonon replicas of the FA; the separation of the FA and 2LO (FA) is 146 meV. The peak at 3.106 eV is identified as a local phonon replica of the FA; the separation of 2LO (FA) and the local phonon replica of the FA is about 65 meV.5 The well-resolved emissions located at 3.224 and 3.003 eV, which could be attributed to the radiant recombination from donor-acceptor pair 1 (DAP1) and DAP2, respectively. The peak at 3.003 eV blues hifts about 35 meV to 3.038 eV as the temperature increases from 81 to 96 K, and it blue shifts about 17 meV to 3.055 eV for the temperature increase from 96 to 111 K, as shown in Figures 6, 8, and 9. The blue shift of the PL peak is as an evidence of DAP transition. The peak at 2.930 eV is assigned to 1LO replica of the DAP2 transition. Furthermore, the peaks at 2.903 and 2833 eV are ascribed to DAP3 and its phonon replica, respectively. The transition from the conductive band and a shallow-donor level D to a deep-acceptor level A can often be described in

18530

J. Phys. Chem. C, Vol. 113, No. 43, 2009

Zang et al.

terms of the so-called “vibronic”, or “configuration-coordinate” model.5,15 The acceptor can have its own set of vibrational states, EA + ηELOC, where EA is the ground state energy of the deep acceptor, ELOC is a local phonon energy of about 64 meV, and η is an integer. The vibrational states can vibrate up and down by release or absorption of one or more local phonons around the ground state energy of the deep acceptor. The zero-phonon line (ZPL) will have an energy, EZPL ) ED(or EC) - EA, where EC is the conduction band energy level. The transition can involve not only local phonons but also LO phonons. Thus, the transition energy of the DAP and FA can be explained as

EDAP(or EFA) ) ED(or EC) - (EA + ηELOC) - η’ELO

(1) where η′is an integer. D. C. Look and D. C. Reynolds constructed this model and use it to explain the fine structure on the green band in ZnO. Here, we use this model to explain the transition of the DAP and FA. In our experiment, the acceptor may be VZn or Oi. The research shows that VZn and Oi as acceptor level are at 3.06 and 2.96 eV below the conduction band, respectively; the similar VZn is also observed at 3.05 eV in the Sb-doped p-type ZnO film.13,16,17 In Figure 8, the EZPL of EDAP1, EDAP2, and EDAP3 is at 3.224, 3.003, and 2.903 eV, respectively. The binding energy of the shallow acceptor EA1 can be calculated with the equation

EA ) Egap - ED + EDAP

e2 + 4πεε0r

(2)

where the donor binding energy ED is reported to be 60 meV,18 the intrinsic band gap Egap ) 3.437 eV at 4.2 K,19-21 r is the separation, and ε is the dielectric constant of ZnO (8.6). The last term of eq 2 is ∼30-60 meV.22 In our experiment, the value of EA1 was calculated to be 183-213 meV, which is similar to the reported value of 195 meV.2 The binding energy of the acceptor is about 195 meV; the shallow acceptor is explained as the PO or PZn - 2VZn by some reporters.4,23,24 The peaks at 3.171 and 3.106 eV are 1LO phonon replica and a local phonon replica; the peak position is in good agreement with the value calculated using formula 1. The peaks at 3.003 and 2.903 eV are original from the transition of ED to VZn and Oi, respectively. The observed transition of DAP2 and DAP3 is also in good agreement with the reported value. The energy of the deep acceptor of VZn and Oi is about 3.06 and 2.96 eV below the conduction band.13 The 1LO phonon replica of DAP2 and DAP3 is also observed. In Figure 9, when the temperature is increased to 111 K, the transition of the DAP2 blue shifts to 3.055 eV. It suggests that the acceptor VZn may be at the excited states or the vibrational states. The peaks at 2.988, 2.918, and 2.847 eV are the one local phonon, 2LO, and 3LO phonon replica of the DAP2, respectively. Conclusion In summary, a diamond-shaped ZnO/P nanostructure was fabricated by a chemical vapor deposition method. In the Raman spectrum, the considerable red shift of the 1LO phonon peak was mostly attributed to the remained stress caused by the big lattice mismatch from P doped in ZnO. The PL properties of the P-doped ZnO nanostructure were characterized by low-

temperature and temperature-dependent PL spectra. The abnormal UV NBE emission was attributed to a number of trapped carriers by thermal elevation into the conduction band at a temperature range from 81 to 111 K. The electron in the conduction band decayed to the lower energy level by emitting one or more phonons, and from there, it emitted a photon and recombined with an acceptor. Thus, the DAP emission intensity was also enhanced. The local phonon and LO phonon replicas of the DAP transition were observed and explained by using the DAP transition model. The enhanced UV NBE emission can be also attributed to the enhanced phonon-exciton coupling. As the temperature increased, the deep acceptor vibration intensified. With the local and lattice phonons assistant, the overlap of the spatial wave function may be extended between the shallow-donor and deep-accept states, so the DAP transition was enhanced. Acknowledgment. This work is supported by the National Natural Science Foundation of China under Grant Nos. 60876014 and 60976014, the Nature Science Foundation of Henan Province under No. 072300410180. The project is also sponsored by the Program for Science & Technology Innovation Talent in University of Henan Province under No. 2008HASTIT029. The project is also supported by the Science & Technology Development Plan of Henan Province in 2009 under Nos. 092300410156 and 092300410131. References and Notes (1) Luo, L.; Sosnowchik, B. D.; Lin, L. W. Appl. Phys. Lett. 2007, 90, 093101. (2) Ye, J. D.; Gu, S. L.; Li, F.; Zhu, S. M.; Zhang, R.; Shi, Y.; Zheng, Y. D.; Sun, X. W.; Lo, G. Q.; Kwong, D. L. Appl. Phys. Lett. 2007, 90, 152108. (3) Kwon, B. J.; Kwack, H. S.; Lee, S. K.; Cho, Y. H.; Hwang, D. K.; Park, S. J. Appl. Phys. Lett. 2007, 91, 061903. (4) Limpijumnong, S.; Zhang, S. B.; Wei, S. H.; Park, C. H. Phys. ReV. Lett. 2003, 92, 155504. (5) Reynold, D. C.; Look, D. C.; Jogai, B. J. Appl. Phys. 2001, 89, 6189. (6) Wang, R. P.; Xu, G.; Jin, P. Phys. ReV. B 2004, 69, 113303. (7) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (8) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Small 2009, 5, 1600. (9) Kong, X. Y.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 975. (10) Bergman, L.; Chen, X. B.; Huso, J.; Morrison, J. L.; Hoeck, H. J. Appl. Phys. 2005, 98, 093507. (11) Bergman, L.; Duttta, M.; Balkas, C.; Davis, R. F.; Alexson, D.; Nemanich, R. J. J. Appl. Phys. 1999, 85, 3535. (12) Tong, Y. H.; Liu, Y. C.; Dong, L.; Zhao, D. X.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. J. Phys. Chem. B 2006, 110, 20263. (13) Djuris˘ic´, A. B.; Leung, Y. H. Small 2006, 2, 944. (14) Lee, W.; Kang, J.; Chang, K. J. Physica B 2006, 376-377, 699. (15) Reshchikov, M. A.; Shahedipour, F.; Korotkov, R. Y.; Wessels, B. W.; Ulmer, M. P. J. Appl. Phys. 2000, 87, 3351. (16) Lin, B. X.; Fu, Z. X.; Jia, Y. B. Appl. Phys. Lett. 2001, 79, 943. (17) Xiu, F. X.; Yang, Z.; Mandalapu, L. J.; Zhao, D. T.; Liu, J. L. Appl. Phys. Lett. 2005, 87, 252102. (18) Reynolds, D. C.; Look, D. C.; Jogai, B.; Litton, C. W.; Collins, T. C.; Harsch, W.; Cantwell, G. Phys. ReV. B 1998, 57, 12151. (19) Hwang, D. K.; Kim, H. S.; Lim, J. H.; Oh, J. Y.; Yang, J. H.; Park, S. J.; Kim, K. K.; Look, D. C.; Park, Y. S. Appl. Phys. Lett. 2005, 86, 151917. (20) Ryu, Y. R.; Lee, T. S.; White, H. W. Appl. Phys. Lett. 2003, 83, 87. (21) Wang, L. J.; Giles, N. C. J. Appl. Phys. 2003, 94, 973. (22) Xiu, F. X.; Yang, Z.; Mandalapu, L. J.; Zhao, D. T.; Liu, J. L.; Beyermann, W. P. Appl. Phys. Lett. 2005, 87, 152101. (23) Lee, W. J.; Kang, J. G.; Chang, K. J. Phys. ReV. B 2006, 73, 024117. (24) Kim, K. K.; Kim, H. S.; Hwang, D. K.; Lim, J. H.; Park, S. J. Appl. Phys. Lett. 2003, 83, 63.

JP905648M