(002)-oriented ZnO film grown by ultrasonic spray pyrolysis ... - NTU.edu

ARTICLES Highly (002)-oriented ZnO film grown by ultrasonic spray pyrolysis on ZnO-seeded Si (100) substrate Jun-Liang Zhao State Key Laboratory of High-Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China; and Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Xiao-Min Lia) State Key Laboratory of High-Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

Sam Zhangb) School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798

Chang Yang State Key Laboratory of High-Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China; and Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Xiang-Dong Gao and Wei-Dong Yu State Key Laboratory of High-Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China (Received 12 December 2005; accepted 23 January 2006)

ZnO films are grown by the ultrasonic spray pyrolysis method on ZnO seeding layer deposited on Si (100) by pulsed laser deposition. The resultant film possesses a columnar microstructure perpendicular to the substrate and exhibits smooth, dense, and uniform morphology. The preferred orientation along the c-axis of the film is significantly enhanced compared to that without the seeding layer. ZnO film grown on ZnO-seeded silicon exhibits higher hall mobility, lower resisitivity, and higher photoluminescence intensity. I. INTRODUCTION

ZnO film has received extensive attention because of its notable properties1–3 such as a direct wide band gap of 3.37eV and a high exciton bonding energy of 60 meV at room temperature (which is much higher than the 20 meV of ZnSe or 21–25 meV of GaN). Furthermore, ZnO can grow at lower temperatures than GaN and ZnSe—a preferred property in realizing integration of ZnO-based optoelectronic devices into a silicon-based process. As such, ZnO is expected to be a promising candidate for replacing GaN in blue and ultraviolet (UV) optoelectronic applications, such as UV laser diodes, blue-to-UV light emitting diodes, and UV detectors.4,5

a)

Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/ publications/jmr/policy.html. DOI: 10.1557/JMR.2006.0291 J. Mater. Res., Vol. 21, No. 9, Sep 2006

Many techniques have been used to deposit ZnO films, including pulsed laser deposition (PLD),5–7 metalorganic vapor-phase epitaxy,8 magnetron sputtering,9 chemical vapor deposition,10 sol-gel,11 and ultrasonic spray pyrolysis (USP).12–15 USP is a simple and inexpensive method for large-area deposition. The atmospheric growth environment of USP also improves stoichiometry, thus reducing intrinsic defects such as oxygen vacancies that, in turn, improve luminescence properties. This has been demonstrated in preparation of p-type ZnO films.13–15 However, ZnO films of high crystallinity are difficult to obtain via the USP technique owing to its insufficient atomic kinetic energy for proper crystal growth. PLD is an effective method for depositing high-quality films with complex composition at relatively low substrate temperatures.7 To circumvent the USP technology inherited low-energy problem, this article deposited a high-quality ZnO seed layer first with PLD followed by USP growth. The induction mechanism of the seeding layer was studied. The relationships between the electrical and luminescence properties of films and their crystal structure were also investigated. © 2006 Materials Research Society

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J-L. Zhao et al.: Highly (002)-oriented ZnO film grown by ultrasonic spray pyrolysis on ZnO-seeded Si (100) substrate

II. EXPERIMENTAL

A ZnO seeding layer was deposited by PLD. For this purpose, a sintered ZnO (99.99% purity) pellet was used as the ablation target for a KrF excimer laser (wavelength of 248 nm, energy density of 5 J/cm2, and repetition rate of 5 Hz; COMpex; Lambda Physik, Acton, MA). Si (100) was used as the substrate, which was etched with diluted hydrofluoric acid (HF) (10%) for 3 min before loading into the deposition chamber. The substrate temperature was 500 °C and the oxygen partial pressure was 1 × 10−2 Pa. The seed layer deposition was carried out for 15 min to form a layer of about 20–30 nm thick. In USP deposition, an aqueous solution of zinc acetate [Zn(CH3COO)2·2H2O, AR, 0.5 mol/L] was selected as the precursor, and the aerosol of the precursor solution was generated by a commercial ultrasonic nebulizer (with a frequency of 1.65 MHz) and was transported to the heated substrate at temperatures ranging from 400 °C–500 °C. The growth rate was controlled at 2–20 nm/min for total thickness of about 150 nm. For comparison, the ZnO film without a seeding layer was also directly grown on Si (100) substrate by USP. The crystallinity and morphology of ZnO films were characterized by x-ray diffraction (XRD; D/MAX2550V, CuK␣), atomic force microscopy, field emission scanning electron microscopy (FESEM, JSM-6700F), and reflective high-energy electron diffraction (RHEED). The electrical properties, including resistivity, carrier concentration, and hall mobility, were measured by the van der Pauw method using a Hall effect measurement system (HL5500PC) with magnetic field strength of 0.326 T. Silver spot electrodes were made on ZnO films and the Ohmic contact between electrodes and films was confirmed before the electrical measurements. Photoluminescence measurements were performed at room temperature using a 325-nm line of a He-Cd laser as an excitation source. The illuminated area on the sample surface was about 1 mm2 and the maximum power density of the laser used was 2.5 W/cm2.

III. RESULTS AND DISCUSSION A. Structural analysis

Figure 1 shows the XRD patterns of the ZnO seeding layer and the ZnO films grown with and without a seeding layer. The film grown without the seeding layer exhibits a polycrystalline structure with random orientation [Fig. 1(a)]. The ZnO seeding layer by PLD is highly textured in a c-axis orientation (002) [Fig. 1(b)]. The ZnO film grown on the seeded wafer exhibits the same single orientation (002) [Fig. 1(c)]. To evaluate the induction effect of the seed layer, we introduced a factor of (I(002)ZS−I(002)S)/I(002)Z, where I(002)Z, I(002)S, and I(002)ZS are the strength of the (002) peak for the ZnO film 2186

FIG. 1. XRD spectra of (a) ZnO films grown by USP on bare Si (100), (b) ZnO seed layer grown by PLD, and (c) ZnO films grown by USP on PLD seed layer at substrate temperatures of 500 °C.

without a seed layer, the ZnO seed layer, and the film with the seed layer, respectively. The factor calculated from Fig. 1 is up to 200, which means that the strength of (002) peak for the film by USP has been enhanced 200 times because of the induction effect of the seed layer. The degree of preferred orientation change can be quantitatively represented through a coefficient of texture, T(hkl), defined as T共hkl兲 =

Im共hkl兲 1 ⳰ I0共hkl兲 n

n

Im共hkl兲

冱 I 共hkl兲 1

,

(1)

0

where Im(hkl) is the measured relative intensity of the reflection from the (hkl) plane, I0(hkl) is that from the same plane in a standard reference sample (JCPDS 361451), and n is the total number of reflection peaks from the film. In the present analysis, n ⳱ 4 because four major directions are involved (002, 101, 102, and 103). The calculated coefficient of (002) texture T(002) for ZnO film without a seeding layer is 2.5, whereas the T(002) for the film with seeding layer is 4, indicating that the (002) preferred orientation of ZnO film is significantly enhanced by the seeding layer. The RHEED patterns from the surface of ZnO films with and without a seed layer are illustrated in Fig. 2. RHEED for the film without a seed layer presents a ring pattern, indicating a polycrystalline structure with no preferred orientation. The pattern for the film with a seed layer shows a well-aligned spotty pattern, revealing a high (002) textured structure, which is in good agreement with XRD analysis. The growth conditions in the USP process, such as the substrate temperature and film growth rate, have an important effect on the crystallinity of ZnO film on the seeded layer. Figure 3 shows XRD patterns for ZnO films grown on the seed layer with different substrate temperatures. Eq. (1) is used to calculate the coefficient

J. Mater. Res., Vol. 21, No. 9, Sep 2006


FIG. 3. XRD spectra of ZnO films grown by USP with PLD seed layer at the deposition rate of 2 nm/min with different substrate temperatures of (a) 400 °C, (b) 450 °C, and (c) 500 °C. Inset is the calculated coefficient of texture as function of substrate temperature.

FIG. 2. RHEED from the surface of ZnO films grown by USP at the substrate temperature of 500 °C and the growth rate of 2 nm/min: (a) without ZnO seed layer deposited by PLD and (b) with ZnO seed layer deposited by PLD.

of (002) texture as function of deposition temperature, and n ⳱ 2 is assumed here because only two reflection peaks are involved (002 and 101). It can be seen that the degree of (002) preferred orientation increases with the substrate temperature, and the film presents single (002) orientation at 500 °C. Figure 4 shows the XRD patterns for ZnO films grown with different deposition rates. The film grown at a higher rate exhibits (002) and (101) peaks, whereas only the (002) peak is observed in the film grown at a lower rate. Therefore, a higher substrate temperature and a lower deposition rate favors the growth of textural growth of ZnO films in USP deposition on a seeded layer. In our experiments, the

FIG. 4. XRD spectra of ZnO films grown by USP with PLD seed layer at the substrate temperature of 500 °C with various deposition rates of (a) 20 nm/min and (b) 2 nm/min. Inset is the calculated coefficient of texture as function of film deposition rate.

optimum conditions for film growth are given at the substrate temperature of 500 °C and the deposition rate of 2 nm/min. The microstructure of ZnO film grown at optimum conditions is investigated with FESEM. Figure 5 shows SEM images of the ZnO films grown with and without the seeding layer. Without the seeding layer [Fig. 5(a)], the crystalline grains of the ZnO film are irregular aggregates with a characteristic dimension of approximately 100 nm. With the seeding layer [Fig. 5(b)], the film exhibits smoother and more uniformly oriented grains with larger size of approximately 200 nm. From the cross-sectional morphologies [Figs. 5(c) and 5(d)], it can be seen that the film without the seeding layer [Fig. 5(c)] consists of loosely packed grains with random orientation, whereas the film with the seeding layer


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FIG. 5. FESEM morphologies for ZnO films grown by USP at the substrate temperature of 500 °C and the growth rate of 2 nm/min. (a) Surface morphology without PLD seed layer, (b) surface morphology with seed layer, (c) cross-sectional morphology without seed layer, and (d) cross-sectional morphology with seed layer.

[Fig. 5(d)] grows as a densely packed columnar structure perpendicular to the substrate, showing highly preferred orientation, which is in good agreement with the XRD results. B. Induction mechanism of the seeding layer

It is well known that the physical and chemical properties of substrate have a significant influence on nucleation and grain evolution, especially in the initial stages of growth, which determines the further evolution of film morphology and texture.16 When a film is grown on bare Si (100) substrate in an ambient atmosphere, the substrate is always covered with a thin amorphous SiOx layer of about 2–3 nm in thickness. When ZnO film is deposited on a bare silicon wafer, ZnO crystals are effectively forced to self-nucleate on this amorphous SiOx layer, and, naturally, the first few atomic layers of ZnO film are randomly oriented because of the amorphous nature of the nucleating surface. When enough energy is offered for further growth, thermodynamics prevail and a preferred orientation along [002] direction becomes dominant. That is also why (002)-oriented ZnO film can be easily grown on Si (100) substrate by PLD, sputtering, 2188

etc., in which high kinetic energy of the plasma facilitates the atoms to transport to the positions with the lowest free energy, resulting in the (002) textured structure. However, in the USP process, because of the low kinetic energy of atoms and the relatively low substrate temperature, there is not enough energy for such atomic transport during film growth, and as a result, a randomly oriented polycrystalline structure is produced. By introducing a ZnO seeding layer, the substrate surface states change significantly. The seeding layer is well crystallized with highly preferred (002) orientation and it shows an atomic scale smooth surface (with rms of only 0.3 nm as determined by atomic force microscopy analysis). Therefore, the surface of the ZnO-seeded Si substrate exhibits a hexagonal array of atoms well aligned according to the (002) plane of wurtzite ZnO. When growing ZnO film on the seed layer by USP, the first few atomic layers are induced to nucleate along the [002] direction. This is known as the seed layer, or substrateinduced nucleation texture, which has been discussed in detail in Ref. 17. Based on the seed layer-induced nucleation mechanism, the ZnO film can evolve into a (002) texture by USP even at a relatively low film growth energy.



In seeding layer-induced growth of ZnO film by USP deposition, the substrate temperature and the film growth rate are the key factors affecting development of the film structure. A higher substrate temperature offers higher energy for film growth, and a lower growth rate provides enough time for atomic diffusion to their dynamically favorable positions. Therefore, in a reasonable range, a high temperature and low growth rate favor the evolution of preferred (002) orientation. C. Electrical properties

The electrical properties of the ZnO films with and without the seeding layer are summarized in Table I. Both films exhibit n-type conductivity, which originates from the intrinsic donor defects, such as zinc interstitials and oxygen vacancies,18,19 or unintentionally doped defects, such as hydrogen.20,21 However, there exists a difference for the electrical properties between the two types of films. The film with a seeding layer exhibits higher carrier concentration, higher hall mobility, and lower resistivity than that without. With a seeding layer, the crystal structure of the film is significantly improved and the grain size is increased, leading to a reduced concentration of structural defects such as dislocations and grain boundaries. These structural defects act as the carrier recombination center, carrier transportation barrier, or carrier scattering center, and traps for free carriers. Thus, the decrease of the concentration of crystal defects releases trapped carriers, resulting in the increase of free carrier concentration. The improvement of crystal quality reduces the carrier scattering from structural defects, leading to higher hall mobility. D. Photoluminescence

To investigate the optical properties of ZnO films, photoluminescence (PL) measurements were performed on the ZnO films as shown in Fig. 6. A strong near-bandedge UV emission peak at 380 nm and a weak deep-level emission centered at about 500 nm can be observed for both samples. The green emission band is attributed to oxygen-related defects,22,23 which form a deep donor level in the band gap. The relatively weak deep-level emission confirms that the films obtained by USP are well close to stoichiometric ZnO and of optically high TABLE I. Electrical properties for undoped ZnO films grown by USP at the substrate temperature of 500 °C and the growth rate of 2 nm/min with and without ZnO seed layer deposited by PLD. Hall Carrier Hall Resistivity mobility concentration coefficient ⍀⭈cm cm2⭈V−1⭈s−1 cm−3 m2⭈C−1 Without seed layer 8.4 × 10−1 With seed layer 8.3 × 10−2

3.38 12.1

−2.2 × 1018 −6.22 × 1018

−14.2 −5.02

FIG. 6. Photoluminescence spectra of ZnO films grown by USP at the substrate temperature of 500 °C and the growth rate of 2 nm/min: (a) without ZnO seed layer deposited by PLD and (b) with ZnO seed layer deposited by PLD.

quality. With the seed layer, UV and green emission become noticeably stronger. Structural defects, such as dislocations and grain boundaries, can trap photogenerated carriers into a nonradiative recombination process before the near-band-edge and deep-level radiative recombination occur. This nonradiative relaxation process decreases the PL intensity.24 Because the ZnO films with the seeding layer have a much-improved crystallinity and a reduced concentration of nonradiative recombination centers, UV and deep-level emission are noticeably enhanced. IV. CONCLUSION

Highly textured ZnO films (along the c-axis or 002 orientation) were deposited via the USP method on ZnOseeded Si (100) substrate. By introducing the seeding layer, the nucleation mechanism changes from selfnucleation to seeding layer-induced nucleation. As a result, the crystallinity is markedly enhanced and the coefficient of texture is significantly improved in a c-axis orientation. The ZnO films grown at a high temperature and low deposition rate exhibit a smooth, uniform, and dense columnar structure perpendicular to the substrate. Owing to the improved crystalline quality and reduced concentration of structural defects, ZnO film with a seeding layer demonstrates lower resisitivity, higher carrier concentration, and higher hall mobility than that without. PL spectrum for the ZnO film with a seeding layer also gives stronger near-band-edge and deep-level emission. These results provide a good starting point for the growth of high-quality, p-type, doped ZnO film by USP. ACKNOWLEDGMENTS

This work was supported by the Ministry of Science and Technology of China through the 973-Project under


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