Near infrared to UV dielectric functions of Al doped ZnO films deposited on c-plane sapphire substrate using pulsed laser deposition

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Near infrared to UV dielectric functions of Al doped ZnO

ﬁlms

deposited on c-plane sapphire substrate using pulsed laser deposition

R. Thangavel

a,b,c,n

, Mohammad Tariq Yaseen

b,d

, Yia Chung Chang

b,c

, Chia-Hao Hsu

e

,

Kuo-Wei Yeh

e

_{, Maw Kuen Wu}

e

a

Department of Applied Physics, Indian School of Mines, Dhanbad 826004, India b

Research Centre for Applied Sciences, Academia Sinica, Taipei, Taiwan 115, Republic of China c

Department of Photonics, College of Electrical and Computer Engineering, National Chiao Tung University, Hsinchu, Taiwan 300, Republic of China d

Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan, Republic of China e_{Institute of Physics, Academia Sinica, Taipei, Taiwan 115, Republic of China}

a r t i c l e i n f o

Article history:

Received 16 January 2013 Received in revised form 2 April 2013

Accepted 15 May 2013 Available online 4 June 2013 Keywords: A. Thinﬁlms C. Raman spectroscopy C. X-ray diffraction D. Electrical properties D. Optical properties

a b s t r a c t

Transparent conducting polycrystalline Al-doped ZnO (AZO)films were deposited on sapphire substrates at substrate temperatures ranging from 200 to 3001C by pulsed laser deposition (PLD). X-ray diffraction measurement shows that the crystalline quality of AZOfilms was improved with increased substrate temperature. The electrical and optical properties of the AZOfilms have been systematically studied via various experimental tools. The room-temperature micro-photoluminescence (m-PL) spectra show a strong ultraviolet (UV) excitonic emission and weak deep-level emission, which indicate low structural defects in thefilms. A Raman shift of about 11 cm−1_{is observed for the}_{first-order longitudinal-optical} (LO) phonon peak for AZOfilms when compared to the LO phonon peak of bulk ZnO. The Raman spectra obtained with UV resonant excitation at room temperature show multi-phonon LO modes up to third order. Optical response due to free electrons of the AZOfilms was characterized in the photon energy range from 0.6 to 6.5 eV by spectroscopic ellipsometry (SE). The free electron response was expressed by a simple Drude model combined with the Cauchy model are reported.

1. Introduction

Transparent conducting oxide (TCO) ﬁlms have been widely

used for mobile display applications, such as organic light-emitting diodes, liquid crystal displays (LCDs), micro displays, and solar cells[1]. In most cases, indium tin oxide (ITO) has been widely employed as a TCO material because of its superb electrical and optical properties. However, ITO has low stability, high toxicity, and high cost and is a rare material, which is a motivating factor to develop alternatives[2,3]. In particular, ZnOﬁlm doped with Al, an n-type dopant, has attracted attention as TCO because of its low resistance and high transparency to visible lights. ZnO-based TCOs are relatively inexpensive and they also have desirable properties such as nontoxicity, long-term environmental stability and excellent IR shielding[4,5]. Recent research reveals that Al, B

and Ga doped ZnOﬁlms show low resistivity and high

transpar-ency[6–8]. There are several deposition techniques which have

been used to grow AZO thin ﬁlms, including chemical vapor

deposition (CVD), [9,10] magnetron sputtering, [11–13] spray

pyrolysis, [14,15] and pulsed-laser deposition (PLD) [16,17]. In comparison with other techniques, PLD provides several advan-tages. The composition of theﬁlms grown by PLD is quite close to

that of the target, even for multicomponent targets. PLD ﬁlms

crystallize at lower substrate temperatures due to the high kinetic energies (41 eV) of the atoms and ionized species in the laser-produced plasma[18]. Also, the surface of theﬁlms grown by PLD

can be very smooth[19]. For some product applications, the TCO

must be synthesized at a temperature under 3001C[20].

Optical measurements have been carried out to measure refractive indices and absorption coefficients for AZO films in the past [21–25]. The spectroscopic ellipsometry (SE) can provide a precise and more informative measurement. By measuring the ratio of light reflected off the surface of the film for two polariza-tion states (i.e., TM and TE modes) and subsequently utilizing an appropriate dispersion model, one can extract both the real and imaginary parts of the dielectric function directly without invol-ving Kramers_{–Kronig (K–K) analysis. In addition, the film thickness} can also be determined precisely. Spectroscopic ellipsometry has been recently used to determine the optical functions of ZnOfilms [26_–31]. But up to now, very few studies have been reported to Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/jpcs

Journal of Physics and Chemistry of Solids

n_{Corresponding author at: Indian School of Mines Dhanbad 826004, Department} of Applied Physics, Dhanabd 826004, India. Tel.:+91 326 223 5916;

fax:+91 326 229 6563. E-mail address:

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extract the optical constants of AZO ﬁlms by spectroscopic ellipsometry. Here, we have used a sub-molecular doping

techni-que for preparing the Al-doped ZnOﬁlms. The main focus is to

determine the optical constants and band gaps of the AZOﬁlms

using spectroscopic ellipsometry at room temperature.

In this paper, the Raman spectra were studied in detail, and a

new vibrational mode at 498 cm−1 was observed in the Raman

spectra of the AZO ﬁlms. To the best of our knowledge, the

vibrational mode assigned at 498 cm−1, which results from the

Al doping, has not been reported anywhere. The optical response

due to free electrons of the AZO ﬁlms was characterized by

spectroscopic ellipsometry (SE). From the results of the SE analysis and the Hall measurements, electron effective mass (mn) andμopt of the AZOﬁlms were estimated. By comparing μoptandμHall, we have studied the variation in the scattering mechanism causing thickness dependence of μHall. The variation in the scattering mechanism was correlated with the development of the crystal structure with different substrate temperatures.

2. Experiments

AZO thinﬁlms were fabricated on c-sapphire substrates via PLD. The chamber was evacuated to a base pressure of 5.9 10−6_{Torr. An} Al-doped ZnO target (98 wt% ZnO+2 wt% Al2O3) with diameter of 1 in. and thickness of 1/8 of an inch was used for the study. A 248 nm-wavelength KrF laser (Lambda Physik LPX Pro) was focused onto the target at a repetition rate of 2 Hz. The laser energy density was kept at 1.0 J/cm2_{. The oxygen pressure was}_{ﬁxed at 50 mTorr. The} substrate was placed at a distance of 60 mm from the target. The target was rotated continuously during the laser ablation. In this

study, AZO thin ﬁlms were fabricated at a substrate temperature

between 200 and 3001C.

After deposition, the structural and optical properties of the

AZO thin ﬁlms were characterized by X-Ray diffraction

(XRD-PANalytical), micro-PL -Raman setup (HR UV 800) and spectro-scopic ellipsometry (VUV and UV to NIR-VASE, J.A. Wollam Co., Inc., USA). The electrical properties were measured by van der Pauw Hall measurement system (Ecopia-HMS 5000).

2.1. Modeling of dielectric function

Spectroscopic ellipsometry (SE) is a nondestructive tool for analyzing thinfilms. Measuring at several angles of incidence over a wide spectral range produces a wealth of information about the sample. The measured ellipsometric values areΨ and Δ, which are determined from the ratio of the amplitude reflection coefficients rpand rsrespectively for p- and s-polarizations with the following relationship[32]:

ρ ¼ tan Ψ expð−iΔÞ ¼rp

rs; Δ ¼ δp−δs ð1Þ

We ﬁt ellipsometric and transmission spectra using suitable

dielectric function models for ZnO: Al and by adjusting the model-parameters it is possible to minimize deviations between the calculated and experimental data. During theﬁtting, the VWASE32 software determines the mean square error (MSE) and

minimiza-tion of MSE has been done using a Levenberg–Marquardt

algo-rithm [33,34]. MSE is a sum of the squares of the differences

between the measured and calculated data weighted by the standard deviation of the data. In addition, it is necessary to weight the transmission modeling to avoid the overestimation of Ψ- and Δ-data with respect to the transmission data. In our model,

we divide the ZnO:Alﬁlm into two layers: the surface roughness

layer and ZnO:Al bulk layer. The most commonly used models for ﬁtting the ellipsometric spectra in UV-visible range for TCOs are

Lorentz,[35–37] Cauchy,[38]and Sellmeier[39]models. The latter

two are simpliﬁed forms of the Lorentz representation and are

suitable for any material in its transparent region [32]. Tauc–

Lorentz[40] and Forouhi–Bloomer [41] models are more rarely

applied. For the free carrier absorption in the near infrared region (NIR), the Drude model is commonly employed for TCOs and metals. It is used to analyze either the SE or transmission data [42,43]. Cauchy and Drude models for ﬁtting the in situ ellipso-metry data have already been reported in the literature for Al

doped ZnO _ﬁlms [44]. In this work also we use a combined

Cauchy–Drude models for the photon energy ranges from 0.6 to

6.5 eV.

3. Result and discussion

Fig. 1 shows XRD pattern of Al doped ZnO (AZO) ﬁlms. It exhibits two diffraction peaks corresponding to (002) and (004) planes. The Al peak cannot be seen, because only 2 wt% Al2O3was doped. They show the characteristic hexagonal ZnO wurtzite structure with the c-axis being perpendicular to the substrate plane[45]. The FWHM of the (002) diffraction peaks are 0.231 for 2001C and 0.148 for 300 1C. For evaluating the mean grain size (D)

Fig. 1. X-ray diffraction pattern of AZOﬁlms.

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

Normalised PL Intensity (arb.units)

Photon Energy(eV) AZO@2000_C

AZO@3000_C

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of theﬁlm based on the XRD results, we have applied the Scherrer formula[46]

Grain SizeðDÞ ¼ 0:9λ

β cos θ ð2Þ

where λ, β, θ represent the X-ray wavelength (1.5406 Å), the

FWHM of the (002) diffraction peak for AZO, and the Bragg diffraction angle, respectively. Generally the FWHM of the (002) diffraction peak is inversely proportional to the grain size of the ﬁlm. We have calculated the mean grain size of the AZO thin ﬁlm

which are 36.027 nm for 2001C and 56.247 nm for 300 1C.

Fig. 2shows the micro-photoluminescence (μ-PL) spectrum of

AZOﬁlm excited by 325 nm UV light from a He–Cd laser at room

temperature. The room temperatureμ-PL spectra has been taken

for two different substrate temperatures (200 and 3001C) which

contain a strong UV emission (peaked at 3.409 and 3.379 eV) and a very weak, broad band spread from 2.1 to 2.8 eV. The UV emission

originates from the excitonic recombination, corresponding to the

near-band-edge emission of theﬁlm.

Liu et al (2007 and 2009) reported [47,48] that it is very

interesting to note that all the AZOﬁlms with different substrate temperatures observe only ultraviolet emission without noticeable deep level emissions (DLE). As a result, the deep level emission

centered around 2.30 eV is expected for the AZOﬁlm. Because Al

ions exists in Al3+and Zn ions in Zn2+, when Al element is doped in ZnO, Al ions will consume residual O ions and decrease the concentration of Oidefects (yellow luminescence) in AZOﬁlms. A strong UV emission with very slight or without deep-level

emis-sion were observed from the AZOﬁlms with different substrate

temperatures[49].

The effect of Al doping in ZnO ﬁlm has been examined by

micro-Raman spectroscopy using a 325 nm laser. For all the

samples, multiphonon peaks range from 200 to 3000 cm−1 are

observed. The multi-phonon scattering processes have been observed in doped ZnO thinﬁlms[50]andFig. 3shows the typical

resonant Raman spectra. The Raman spectrum of AZOﬁlm consists

of three sharp lines at 565.5, 1130.98 and 1711 cm−1, which arise

from the emission of 1, 2 and 3 longitudinal optical (A1LO)

phonons of the AZO wurtzite structure. This result is consistent with the previously reported Raman shift of ZnO bulk crystal and thinfilms[50–53]. For AZO thinfilms, a typical Raman spectrum has been observed up to 3rd order. The second order E2(low) peak observed at around 206 cm−1is due to the substitution of the Al atom on the zinc site of the lattice. The strong peaks at about 443 and 435 cm−1are assigned to the E2(high) mode of AZOfilms with substrate temperatures at 200 and 3001C, respectively, which is a Raman active mode in the wurtzite crystal structure. The strong E2(high) mode indicates very good crystallanity[50]. A very small

Raman active peak appear near 315 cm−1can be assigned to the

E2(high)–E2(low), where the E2(high) mode is emitted and E2(low) mode is absorbed. The incorporation of impurities in the host lattice generally can introduce forbidden vibrational modes (FVMs) which are observed in the Raman spectra. A possible physical mechanism for explaining FVMs is that defects induced by impurities break the translational symmetry of the crystal, thus Fig. 3. Micro-Raman spectra of AZOﬁlms.

Wavelength (nm) 200 400 600 800 1000 Ψ in degrees 0 5 10 15 20 25 Model Fit Exp E 55 Exp E 65 Exp E 75 Wavelength (nm) 200 400 600 800 1000 Δ in degrees -100 0 100 200 300 Model Fit Exp E 55 Exp E 65 Exp E 75 Wavelength (nm) 1000 1200 1400 1600 1800 2000 Ψ in degrees 0 5 10 15 20 25 30 Model Fit Exp E 55 Exp E 65 Exp E 75 Wavelength (nm) 1000 1200 1400 1600 1800 2000 Δ in degrees -50 0 50 100 150 200 Model Fit Exp E 55 Exp E 65 Exp E 75

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relaxing the momentum conservation and hence leading to the scattering of phonons with the wave vectors far from the Brillouin zone center [54]. The new FVM assigned as E1(TO)+E2(low) at about 498 cm−1has been clearly observed in the Raman spectra of

AZOﬁlms.

Fig. 4(a, b, c and d) shows typical_{Ψ, Δ spectrum measured at} three different angles (55, 65 and 751) of incidence for the AZO film with thickness of∼300 nm. The best-fit curves to the spectral data using Cauchy model and Drude model analysis, are also shown as solid lines infigures. Here the fitting is performed for the Drude model where the photon energy (wavelength) range is from 0.62 eV (2000 nm) to 1.24 eV (1000 nm) and for the Cauchy model it is between 1.24 eV (1000 nm) and 4.2 eV (300 nm) there is a good agreement between the experimental data and the calculated results. The result indicates that there is definite absorption which arises due to the free electrons over the entire visible range.

The surface roughness, thickness of the AZO layer dAZO, A, B, N, mopt, and mnare used toﬁt in the Cauchy and Drude model of the

SE analysis. The best-ﬁt parameters obtained are summarized in

Table 1.

Fig. 5shows the optical dielectric constants (ε¼ε1+iε2) of AZO films determined by fitting the ellipsometric parameters. A strong peak in the spectra near the band gap is attributed to the free exciton absorption. In the longer wavelength region, we see an evidence for the free carrier absorption in AZOfilms. The changes of optical dielectric constants of thefilms with different substrate

temperatures in the wavelength range of 190 nm–1000 nm are

noticeable. The imaginary part of optical dielectric constant reduces to nearly zero for wavelengths between 400 nm and 1000 nm but it increases above 1000 nm due to the free-carrier absorption[44].

The electrical properties are investigated for Al doped ZnO thin films. Here aluminum acts as an effective donor, which substitute for zinc increasing significantly the concentration of free carriers in ZnO. With the increase of the substrate temperature, the resistivity of the AZOfilm decreases while mobility increases from 11.3 to 13.69 cm2_V−1_s−1 _[47_–_{49]. Comparison of the calculated} values of Noptandmoptfrom SE with NHallandmHalldetermined by

Hall measurements are available in Table 1. It can be seen that

NHallis slightly higher than Nopt, butmHallis lower thanmopt,ﬁnd a fruitful validity of our analyses agreeing with the previous reported values[44].

4. Conclusion

Structural, electrical and optical properties of Al doped ZnO

(AZO)ﬁlms deposited at different substrate temperatures on

c-plane sapphire substrates by pulser laser deposition were inves-tigated to explore the possibility of producing transparent

con-ductive oxide ﬁlms through a simple low-cost process. It was

observed that the AZO ﬁlms were grown with c-axis preferred

orientations without any degradation of ZnO wurtzite structure. Micro-PL spectra clearly indicate a strong UV emission of the

samples. The new FVM assigned as E1(TO)+E2(low) at about

498 cm−1was clearly observed in the Raman spectra of AZOﬁlms. We further investigated the optical carrier concentration and mobility from the SE analyses using the Drude model. The results of the SE analysis have an excellent agreement with Hall measure-ments. This is an important result for the future, potential applications.

Acknowledgments

This research was supported by Academia Sinica, National Chiao Tung University, and National Science Council, Taiwan,

Republic of China under the Grant number NSC-101

–2112-M-001–024-MY3.

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