Influences of Substrate and Annealing on the Structural and Optical

Oct 9, 2009 - and the annealing process has different influences for the films deposited on different substrates. 1. Introduction. Zinc oxide (ZnO) ha...
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J. Phys. Chem. C 2009, 113, 19139–19144

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Influences of Substrate and Annealing on the Structural and Optical Properties and Photoluminescence of Nanocrystalline ZnO Films Prepared by Plasma Assisted Pulsed Laser Deposition K. Gao, W. Zhang, J. Sun, N. Xu, Z. F. Ying, Q. Li, J. Gan, and J. D. Wu* Key Laboratory for AdVanced Photonic Materials and DeVices, Department of Optical Science and Engineering, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: September 7, 2009; ReVised Manuscript ReceiVed: September 24, 2009

Single-crystalline silicon, amorphous quartz, and single-crystalline sapphire were used as the substrates for the deposition of nanocrystalline zinc oxide (nc-ZnO) thin films by plasma assisted pulsed laser deposition from metallic zinc at low temperature below 80 °C, and a postdeposition annealing process was performed for the deposited nc-ZnO thin films. The influences of substrate and annealing on the structural and optical properties as well as the photoluminescence of the prepared nc-ZnO thin films were studied. Raman spectroscopy and Fourier-transform infrared spectroscopy measurements reveal the wurtzite ZnO phase in the films. X-ray diffraction results indicate the nanocrystalline structure of the films with c axis perpendicular to substrates. Optical characterization shows that the prepared films have an ultraviolet (UV) absorption edge near 380 nm with a direct band gap of about 3.25 eV. The prepared films emit predominantly UV photoluminescence at light excitation. The structure and properties of the films are dependent on substrates and the annealing process has different influences for the films deposited on different substrates. 1. Introduction Zinc oxide (ZnO) has been studied for several decades because of its various applications in a variety of fields including surface acoustic wave devices, thin film transistors, piezoelectric transducers, and gas sensors.1-5 As a wide band gap (∼3.4 eV) semiconductor material with extremely large exciton binding energy (60 mV) at room temperature, ZnO is especially attractive for short-wavelength optics and optoelectronics and has been considered as a promising material for solid state shortwavelength optoelectronic devices such as daylight-blind detectors, blue-ultraviolet light-emitting diodes, and ultraviolet (UV) lasers.6-10 While significant progress has been made in the experimental and theoretical investigations on ZnO properties and many methods have been used to prepare ZnO thin films,3,5,9,11 there has been a growing interest in obtaining singlecrystalline ZnO films on various substrates. The selection of substrate is very important for the growth of thin films because matching in lattice parameters and crystal structure between the film and the substrate strongly affects the growth behavior of the films. Among various substrate materials for ZnO, silicon is a particularly promising candidate. ZnO films on silicon substrates have attracted great attention because of the effective integration of ZnO-based optoelectronic devices with silicon IC technology.12,13 Silicon oxide is also a substrate of choice because of the potential for integration with silicon-based electronic circuitry.14,15 Because of the lack of structurally and thermally matched substrate materials for III-V nitride films, in addition, high quality single-crystalline ZnO is an interesting buffer layer material for nitride films on various substrates for the fabrication of high intensity nitride-based blue light-emitting devices.16-20 Usually, as-deposited films require a thermal annealing process which is widely used to improve crystalline quality, activate dopants, and alloy ohmic contacts. It has been * Corresponding author. Fax: 86-21-65641344; E-mail: jdwu@ fudan.edu.cn.

found that the structure and properties of ZnO thin films are strongly influenced by the annealing process; however, there are several factors such as annealing temperature and annealing ambient which determine the annealing behaviors, and the reported results are diverse.21-24 Hence, the annealing influences need to be further investigated for ZnO. In our previous report, c-axis oriented nanocrystalline ZnO (nc-ZnO) thin films have been successfully deposited on Si (100) substrates by plasma assisted pulsed laser deposition (PAPLD) at low temperature.25 In the present work, in addition to crystalline silicon, amorphous quartz and crystalline sapphire were also used as the substrate materials. Thermal annealing influences were studied for nc-ZnO films deposited on silicon, quartz, and sapphire through structure characterization, optical property study, and photoluminescence (PL) measurement. The results show that c-axis oriented growth of nc-ZnO thin films can be achieved not only on silicon but also on quartz and sapphire by PAPLD, and the influences of postdeposition thermal annealing on structure and properties are not the same for the films deposited on different substrates. An interesting finding is that the UV PL emission can be greatly enhanced by annealing nc-ZnO thin films on silicon substrates, while for films deposited on quartz substrates, the same annealing process results in a decrease of the UV PL intensity. 2. Experimental Section Deposition of nc-ZnO thin films by means of PAPLD has been described elsewhere.25 In brief, the films were deposited by pulsed laser ablation of a metallic zinc target in a reactive environment of oxygen plasma generated from electron cyclotron resonance (ECR) microwave discharge and with the assistance of the ECR oxygen plasma. In the present work, three kinds of widely used wafers, single-crystalline silicon [polished Si (100)], amorphous quartz (double-side polished R-SiO2), and single-crystalline sapphire [double-side polished Al2O3 (001)]

10.1021/jp908616e CCC: $40.75  2009 American Chemical Society Published on Web 10/09/2009

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were used as the substrates. For larger film thickness (∼1 µm), the deposition time was prolonged. No auxiliary heater was applied to the substrate. But, the action of the plasma slightly heated the substrate whose temperature rose to no more than 80 °C during film deposition. After deposition, some of the deposited films were exposed to thermal annealing at 600 °C in air for 60 min. The surface morphology of the prepared nc-ZnO thin films was examined by atomic force microscopy (AFM) with a scanning probe microscope (SPM; PSIA, XE-100) working in the AFM noncontact mode. Raman scattering spectroscopy and Fourier-transform infrared (FTIR) spectroscopy were used for oxide phase analysis and chemical structure characterization. Raman spectra were recorded by collecting the backscattering radiation from the samples at 632.8 nm excitation with a microRaman instrument (Dilor Labram 00). FTIR measurements were carried out using a Nicolet Magna-IR 550-II spectrometer. The crystal structures of the prepared nc-ZnO films were characterized by X-ray diffraction (XRD) using a Rigaku D/max-γB X-ray diffractometer with a rotating anode operating at 40 kV and 300 mA and Ni-filtered Cu KR radiation (λ ) 0.15406 nm). With a Shimutsu UV3101PC photospectrometer, the optical properties were studied by measuring the transmission spectra of the films deposited on transparent quartz and sapphire substrates. Room temperature photoluminescence (PL) measurements were performed for the as-deposited and annealed ncZnO films by normally exciting the samples with 325 nm laser light from a CW He-Cd laser. The PL spectra were recorded by collecting the emitted luminescence at normal direction with a 0.5 m spectrometer (Acton Research, Spectra Pro 500i) and intensified charge-coupled device (ICCD; Andor Technology, iStar DH720) attached on the exit port of the spectrometer. All of the ex situ measurements were done in air at room temperature. 3. Results and Discussion 3.1. Surface Morphology. The prepared nc-ZnO films have good adhesion to silicon, quartz, and sapphire substrates. The examination of surface morphology by AFM reveals that the prepared films exhibit a smooth surface appearance with an average root mean square (rms) value of roughness of about 1.0 nm over 1 µm × 1 µm surface area. AFM images of the as-deposited nc-ZnO films on Si (100) substrates have been presented in ref 25. In this paper, Figure 1a,b shows representative planar and three-dimensional AFM images, respectively, both obtained from an annealed film on a Si (100) substrate. It seems that postdeposition annealing has no significant influence on the surface morphology. 3.2. Oxide Phase. Raman measurements were performed to analyze the vibrational modes and characterize the phase structures of the prepared nc-ZnO films, which confirms the presence of wurtzite ZnO phase. Figure 2 shows a micro-Raman backscattering spectrum taken from an as-deposited nc-ZnO thin film on a Si (100) substrate. Besides the strong backscattering peaks at 303 and 520 cm-1 corresponding to the optical phonon modes excited from the Si (100) substrate, two sharp peaks attributed to the ZnO nonpolar optical phonons (E2) modes are observed. Of them, the peak at about 102 cm-1 is assigned to the E2 low-frequency branch [E2 (low)] and that at about 437 cm-1 to the E2 high-frequency branch [E2 (high)], associated with oxygen and Zn sublattices, respectively. They are the characteristic modes of ZnO as expected from the Raman selection rules in wurtzite crystal structure.26-28 The Raman band corresponding to the longitudinal optical (LO) mode with A1

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Figure 1. Planar AFM image (a) and three-dimensional AFM image (b) of a nc-ZnO film deposited on a Si (100) substrate.

Figure 2. Raman spectrum of a nc-ZnO film deposited on a Si (100) substrate.

symmetry usually appears near 574 cm-1.27,29 The LO mode is, however, actually composed of A1 LO (574 cm-1) mode and E1 LO (583 cm-1) mode from polarization fields parallel and perpendicular to the c axis, respectively.30,31 For our nc-ZnO thin films, the Raman band corresponding to the E1 LO mode centers at 583 cm-1. The appearance of the above characteristic Raman peaks indicates the wurtzite phase of the prepared ncZnO films. However, it is found that the E1 LO peak is greatly suppressed in comparison with that reported in the previous paper for a thinner nc-ZnO film.25 It is widely believed that the appearance of the E1 LO mode in the Raman scattering is because of impurities and structural defects (oxygen vacancies and zinc interstitials). The appearance of the sharp E2 peaks and suppressed E1 peak suggests that the as-deposited nc-ZnO films are good in crystallinity. Furthermore, at the low-frequency shoulder of the Si 303 cm-1 peak appears an additional peak near 275 cm-1. This Raman peak has been reported in literature. Bundesman et al. suggested the lattice defects of ZnO as the origin of the Raman mode at 275 cm-1.31 Kaschner et al. observed this peak in Raman scattering of N-doped ZnO thin

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Figure 3. FTIR reflectance spectrum of a nc-ZnO film deposited on a Si (100) substrate.

films grown by chemical vapor deposition and explained its occurrence in terms of nitrogen-related local vibrational modes.32 FTIR measurements were carried out in the wavenumber range from 400 to 4000 cm-1, which gives further evidence of the formation of wurtzite ZnO phase in the prepared films. Figure 3 displays FTIR reflectance spectra in the 500-1500 cm-1 region taken from the as-deposited and the annealed films on Si substrate. A distinct band appears with center around 586 cm-1. This absorption band is attributed to the infrared (IR) vibrationscorrespondingtotheLOphononswithE1 symmetry,30,31,33 an IR characteristic of wurtzite ZnO phase. Except that the IR band of the annealed film becomes somewhat narrower in comparison with that of the as-deposited film, as shown in Figure 3, no significant changes were observed from FTIR analysis. Both the as-deposited and annealed nc-ZnO films show the characteristics of wurtzite ZnO. 3.3. Crystal Structure. XRD characterization for crystal structure reveals the nearly oriented crystallographic growth of nc-ZnO films with crystallite size of tens of nanometers and the c axis perpendicular to substrates. XRD patterns measured in the range 2θ ) 20-80° for the as-deposited and annealed nc-ZnO films on silicon, quartz, and sapphire substrates are displayed in Figure 4a,b. It can be seen from Figure 4 that the (002) diffraction peak of wurtzite ZnO dominates the XRD patterns for all of the films, while almost no other diffraction peaks are discernible except the films deposited on sapphire substrates. We also notice that, for the films deposited on Si (100) with thinner thickness, weak diffraction peaks other than the (002) one appeared.25 They become indiscernible as the films grow thicker, together with the intensity increasing and width narrowing of the (002) peak. Of particular interest for the films deposited on sapphire substrates, the XRD patterns evolve from (101)-peak dominance25 to (002)-peak dominance as the film growth continues, and the diffraction intensity is remarkably stronger than that for the films deposited on silicon and quartz substrates. This observation reveals the highly oriented crystallographic growth of nc-ZnO films with c axis perpendicular to substrates and suggests that, when depositing on a sapphire substrate, the film grows first with (101) orientation preference at the initial stages and then turns to c axis growth with (002) orientation preference as deposition continues. The preferred oriented growth of ZnO films with c axis perpendicular to substrates is due to the minimal surface energy of the (002) plane that corresponds to the densely packed plane of the ZnO hexagonal structure.34 However, elevated growth temperatures are usually necessary for oriented crystallographic growth of ZnO films with c axis perpendicular to substrates.24,15,19 In the PATPLD process, the growing films were bombarded by the

Figure 4. (a) XRD patterns of the as-deposited nc-ZnO films deposited on Si (100), R-SiO2, and Al2O3 (001) substrates; (b) XRD patterns of the annealed nc-ZnO films deposited on Si (100), R-SiO2, and Al2O3 (001) substrates.

low-energy oxygen plasma stream during film deposition, by which the precursors acquired energy from the energetic plasma stream, enhancing reaction rates at the surface and adding surface mobility to the arriving precursors to occupy energyfavored locations in the ZnO lattice for oriented crystallographic growth. In addition to providing reactant oxygen to react with zinc ablated from the zinc target for the formation of ZnO precursors, the assistance of the ECR oxygen plasma also makes it possible for oriented crystallographic growth of ZnO films with c axis perpendicular to substrates at lower substrate temperature. When depositing directly on a sapphire substrate, however, it seems that ZnO film grows not with (002) orientation at the beginning because of large mismatch, but the film grown at the initial stages serves as a buffer layer for (002)-oriented growth of ZnO. It is worth while noting that the XRD patterns of the films deposited on silicon and quartz substrates are almost identical, especially after annealing, while for the films deposited on sapphire substrates, the (002) peak is remarkably strong. In addition to the strong (002) peak, there appears a weak peak in the XRD patterns taken from the as-deposited and annealing films on sapphire substrates. This peak is indexed to (004) diffraction of hexagonal wurtzite structured ZnO. It is also noticed that the full width at half-maximum (fwhm) of the (002) peak for the as-deposited films is rather large (∼0.5°), indicating that the crystallite size is small. Moreover, the 2θ values corresponding to (002) diffraction from the as-deposited films are all less than 34.422°, the standard value for bulk ZnO (JCPDS: 36-1451), as clearly shown in Figure 5. For all of the films deposited on various substrates, however, the (002) peaks are observed to shift toward 34.4° after annealing at 600 °C in air for 60 min, suggesting that the annealed films are almost under stress-free state whether they were deposited on silicon, quartz, or sapphire substrates. In addition, the (002) peaks become narrower after annealing. The peak position and fwhm of the (002) diffraction peaks of the as-deposited and annealed nc-ZnO films on various substrates are listed in Table 1. The peak shifting and width narrowing of diffraction peaks

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Figure 5. Detailed XRD patterns of the as-deposited and annealed nc-ZnO films deposited on Si (100), R-SiO2, and Al2O3 (001) substrates showing the (002) diffraction peaks before and after annealing.

can be explained by the improvement of crystallinity and the growth of crystallites by postdeposition annealing. From the XRD data of the (002) diffraction peaks, the mean size of the prepared ZnO nano crystallites was calculated using the well-known Scherrer’s formula:

d)

Kλ β cos θ

where d is the mean diameter of crystallites, K, λ, β, and θ are the Scherrer parameter ()0.9 generally), X-ray wavelength (0.15405 nm for Cu KR line here), fwhm of the diffraction peak, and Bragg diffraction angle, respectively. The mean diameters of the c axis-orientated crystallites calculated for the as-deposited and annealed films are listed in Table 1. The size of the crystallites in the as-deposited films in the present work is larger than that in a thinner film reported in the previous paper,25 and the crystallites grow larger during postdeposition annealing. It also seems that the difference of the substrates used here has no apparent influence on the size of the grown crystallites. 3.4. Optical Properties. Optical transmittance of the films deposited on transparent quartz and sapphire substrates was measured for optical characterization in the UV and near IR regions. The recorded transmission spectra ranging from 350 to 1000 nm for the nc-ZnO films on quartz and sapphire substrates are illustrated in Figure 6a,b, where those taken from the as-deposited samples and the samples after annealing are plotted together for comparison. It can be seen that the as-deposited films show poor transparency. The optical transparency of the films is greatly enhanced by postdeposition annealing, especially for the films deposited on quartz. The annealed films on quartz exhibit excellent transparency in the visible and near IR regions with transmittance of about 80% and a sharp ultraviolet absorption edge, although all of the films deposited on quartz and sapphire, as-deposited and annealed, have similar absorption edge near 380 nm. It is found that the relation of (Rhν)2 versus (hν) near the absorption edge for these films show a good linearity, which reveals the direct band gaps of these films.35 By using a linear fit of the (Rhν)2 versus (hν) plots near the band gap and extrapolating it to Rhν ) 0, the

Figure 6. (a) Optical transmission spectra of the as-deposited and annealed nc-ZnO films deposited on quartz substrates; (b) Optical transmission spectra of the as-deposited and annealed nc-ZnO films deposited on sapphire substrates.

band gap energy Eg was determined to be 3.25 eV for the as-deposited and annealed nc-ZnO films on quartz substrates and 3.26 eV for those on sapphire substrates, respectively. All of the determined band gaps are somewhat smaller than the standard value of 3.4 eV for bulk wurtzite ZnO, but within the range reported for ZnO thin films prepared by various methods.11 No obvious changes in the band gap by annealing are observed. 3.5. Photoluminescence. Figure 7a shows the PL spectra of the as-deposited nc-ZnO films on silicon, quartz, and sapphire substrates taken at room temperature. The PL spectra of the as-deposited nc-ZnO films on various substrates are similar in their spectral range and intensity. All of the spectra are dominated by the emission peaking at UV (∼380 nm). The UV PL emission peaks are rather broad and asymmetric tailing to the visible (∼440 nm) at long wavelength side. However, almost no green emission is observed, as shown in the inset of Figure 7a. The UV emission could be related to the near band-edge transitions of ZnO, namely, the recombination of free excitons through an exciton-exciton collision process,11,36 while the green emission is generally explained by the radial recombination of a photogenerated hole with the electron in a singly ionized oxygen vacancy.37 The absence of green emission in the PL spectra indicates that the nc-ZnO films deposited at low temperature have few oxygen vacancies. PL measurements were also made to examine the annealing influence. The annealed

TABLE 1: Measured XRD Data and Calculated Crystallite Diameters substrate before or after annealing (002) peak position (2θ) fwhm d (nm)

Si (100) before annealing 33.94° 0.51° 16

after annealing 34.42° 0.34° 24

R-SiO2 before annealing 33.98° 0.49° 17

after annealing 34.48° 0.33° 25

Al2O3 (001) before annealing 33.97° 0.49° 17

after annealing 34.42° 0.40° 21

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Figure 7. (a) Room temperature PL spectra of the as-deposited ncZnO films deposited on Si (100) (black), R-SiO2 (red), and Al2O3 (001) (blue) substrates; (b) Room temperature PL spectra of the annealed nc-ZnO films deposited on Si (100) (black), R-SiO2 (red), and Al2O3 (001) (blue) substrates. The insets show the green PL emission from the samples before and after annealing.

Nanocrystalline ZnO thin films were prepared on singlecrystalline silicon, amorphous quartz and single-crystalline sapphire substrates from metallic zinc by reactive pulsed laser deposition with assistance of ECR oxygen plasma at low temperature, and the influences of substrate and postdeposition annealing were studied. The structural and optical properties vary depending on the substrates. The deposited films have a wurtzite structure and high crystallographic orientation with c axis perpendicular to substrates. The crystallites in the asdeposited nc-ZnO films have an average diameter of about 17 nm. The as-deposited nc-ZnO films show poor transparency in the region from UV to near IR and have a UV absorption edge near 380 nm with a direct band gap of about 3.25 eV. At the excitation by 325 nm light, the as-deposited nc-ZnO films on various substrates emit similar UV PL extending to blue. The crystallinity of the nc-ZnO films improves, and the crystallites in the films grow larger during annealing in air, together with the enhancement in the transparency of the films. The influences of annealing process are found to be not the same for the films deposited on different substrates. The near band-edge UV PL from the films on silicon and sapphire substrates is enhanced after annealing, especially for the films on silicon, while the UV PL from the films on quartz substrates appears even weaker after the same postdeposition annealing. Acknowledgment. The authors would like to acknowledge the support from National Natural Science Foundation of China under the Contract No. 10875029 and National Basic Research Program of China under the Contract No. 2010CB933703. Acknowledgment is also made to Shanghai AM Research and Development Foundation (No. 07SA11). References and Notes

Figure 8. Room temperature PL spectra of the as-deposited and annealed nc-ZnO films deposited on Si (100) substrates.

films emit predominately UV emission with its fwhm less than 20 nm at the same light excitation, as illustrated in Figure 7b, and the UV emission peaks appear symmetric. Compared with the as-deposited samples, it is found that postdeposition annealing has different influence on the PL intensity for the films deposited on different substrates. The PL of the films on silicon and sapphire substrates appears enhanced after annealing, especially for the annealed films on silicon; the UV PL intensity is greatly increased by more than 3 times compared with the as-deposited one. For comparison, the PL spectra of the asdeposited and annealed nc-ZnO films on Si (100) substrates are displayed in Figure 8. However, no such enhancement was observed for the films deposited on quartz substrates. Quite the reverse, the UV PL of the films on quartz appears even weaker after annealing. In addition to the near band-edge UV emission, the defect-related deep-level green emission is observed from PL measurement taken for the annealed films, as shown in the

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