Thickness-Dependent Optical Constants and Annealed Phase

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Thickness-Dependent Optical Constants and Annealed Phase Transitions of Ultrathin ZnO Films Hua Zheng, Rong-Jun Zhang, Ji-Ping Xu, Shuxia Wang, Tianning Zhang, Yan Sun, Yuxiang Zheng, Songyou Wang, Xin Chen, Liangyao Chen, and Ning Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06173 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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The Journal of Physical Chemistry

Thickness-Dependent Optical Constants and Annealed Phase Transitions of Ultrathin ZnO Films Hua Zheng, †, ‡ Rong-Jun Zhang, †,* Ji-Ping Xu, † Shu-Xia Wang, ‡ Tian-Ning Zhang, ‡ Yan Sun, ‡ Yu-Xiang Zheng, † Song-You Wang, † Xin Chen, ‡,* Liang-Yao Chen, † Ning Dai‡

†

Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Department of

Optical Science and Engineering, Fudan University, Shanghai 200433, China ‡

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese

Academy of Sciences, Shanghai 200083, China

ACS Paragon Plus Environment


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ABSTRACT：： The

thickness-dependent

optical

constants

and

annealed

phase

transitions

of

atomic-layer-deposited ZnO ultrathin films with a thickness of less than 50 nm have been demonstrated by spectroscopic ellipsometry. The thickness-dependence of refractive index and extinction coefficient were discussed and the mechanisms were given in the molecule level based on previous reports. Furthermore, the optical properties of ZnO ultrathin films varied with annealing temperatures, and the phase transition was found at high annealing temperature. The thickness of the ultrathin films decreased obviously and the refractive index of the ultrathin films changed a lot after annealed at high temperature while Zn2SiO4 formed at a temperature above 800 °C. The low phase transition temperature of Zn2SiO4 may be due to the ultrathin scale effect. What’s more, photoluminescence spectra showed the annealing effect on ultrathin films and the enhanced defects luminescence were observed. We believe that these investigations will help improved understanding of essential physical chemistry and optoelectronic devices based on ultrathin oxide films for optical and photoelectric applications.


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INTRODUCTION：： Ultrathin films have recently drawn much attention with the extension of nanoscale interface or surface in the construction of thinner and smaller devices. ZnO thin films have been widely used in piezoelectric transducers, photodetectors, gas sensors, luminescent phosphors, thin film solar cells.1-4 In addition, ZnO composite films doped with Al, Mg, Ti, N or other elements have shown high carrier mobility and been applied to manipulate the optical and optoelectronic behaviors. 5-11 It is well known that both thickness and interface are critical to manipulate optical and photoelectric properties when the thin films are with less than 50 nm in thickness. 7, 12-16 The conditional ZnO thin films with more than 100 nm thick seems to be polycrystalline.17 However, when the thickness of ZnO ultrathin films increased from 25.0 nm to 49.8 nm, a phase transition from amorphous to polycrystalline had been found.17 For the investigations on ultrathin films, spectroscopic ellipsometry (SE) is a wide-used, non-destructive method for determining thickness and optical constants of ultrathin films.13,

18-21

The refractive index n, extinction

coefficient k and thickness d can be determined based on precise simulations22-23 by the use of Forouhi-Bloomer dispersion model

13, 24

and point-by-point analysis.25-27 Here, we present the

thickness-dependent optical constants and phase transitions of annealed ZnO ultrathin films with a thickness of less than 50 nm. ZnO ultrathin films were obtained and adjusted by atomic layer deposition (ALD). SE measurements, X-ray diffraction (XRD) patterns, photoluminescence (PL) spectra and transmission electron microscopy (TEM) suggest that transitions occurred when the ZnO ultrathin films were annealed at high temperature. These investigations will bring a choice to better understand structure and optical properties of ZnO ultrathin films for optical and



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photoelectric applications. EXPERIMENTAL SECTION: ZnO thin films were deposited on Si (100) (ρ = 1-5 Ω·cm, d=500±10 µm) by alternating diethylzinc (Zn(CH2CH3)2, DEZ) and deionized water (H2O) in an ALD reactor (Picosun) at 150°C. A typical ALD cycle for ZnO thin films consisted of 0.1 s DEZ pulse, 10 s N2 purge, 0.1 s H2O pulse and 10 s N2 purge according to our previous reports.28 The mechanism of ZnO ALD is the chemical vapor deposition reaction. Zn(CH2CH3)2+ H2O→ZnO + 2C2H6

(1)

There are two reactions in an ALD cycle. ZnOH* + Zn(CH2CH)2→ZnOZn(CH2CH3)*+ C2H6 Zn(CH2CH3)* +H2O→ZnOH* + C2H6

(2) (3)

Where * indicates a surface species. Here, the thickness of ZnO thin films was varied by controlling ALD cycles. The thicknesses and optical properties of ZnO ultrathin films were obtained by spectroscopic ellipsometry (J.A. Woollam Co. M2000X-FB-300XTF, Lincoln, NE, USA) and scanning electron microscopy (SEM, FEI Siron200). The incident angle was fixed at 65° and the wavelength ranged from 300 to 800 nm. The surface morphologies of thin films were investigated by atomic force microscopy (AFM; Bruker Dimension Icon VT-1000, Santa Barbara, CA, USA). The ZnO ultrathin films with 300 ALD cycles were rapidly thermally annealed in an oven (RTP, AS-ONE, Montpellier, France) under N2 atmosphere for five minutes at 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 850°C and 900°C. Then, all measurements were performed by XRD, PL, SE, SEM, AFM and


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TEM (FEI Tecnai G2 F20, Hillsboro, OR, USA). RESULTS AND DISCUSSION：： For the growth of ZnO thin films with a thickness of less than 50 nm,

29-31

existing ALD is a

powerful method for precisely controlling thickness with a low interfacial roughness.

32-33

ZnO

thin films were obtained with different thickness by performing 200, 300, 400 ALD cycles. These ultrathin films are polycrystalline and with the thickness decreasing, the crystallinity decreases, which is proved by SEM and XRD data shown in Figure S1 and S2. And the thickness of 200 ALD cycles ZnO is the critical thickness for crystal films in this study. Then, the refractive index n, extinction coefficient k and thickness d of the resulting ZnO ultrathin films were obtained by the SE measurement, as shown in Figure 1a. During the SE measurement, a linearly polarized light will turn into elliptically polarized light after reflection on the ZnO ultrathin films, and the wavelength of the light was within the wavelength of 300-800 nm. There are two measurement parameters acquired from the polarized light, i.e. amplitude ratio (Ψ) and phase shift (∆). The root mean square error (RMSE) is minimized to get an accuracy fitting:

RMSE =

M 1 [(Ψi cal −Ψi exp )2 + (∆ical − ∆iexp )2 ] ∑ 2 N − M − 1 i =1

(4)

Here N is the number of data points in the spectra, M is the number of variable parameters in the model, and ‘exp’ and ‘cal’ represent the experimental and the calculated data, respectively.13 Then, Forouhi-Bloomer dispersion model was used to fit Ψ and ∆ for describing the refractive index n and extinction coefficient k, and estimating the band gaps and thicknesses of the resulting ZnO ultrathin films.13, 19, 24 Furthermore, point-by-point analysis based on the thickness



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simulation was also used to calculate n and k of the resulting ZnO ultrathin films.25-26 When the roughness layer and the interfacial layer were ignored, a monolayer model was used as the optical model and the F-B dispersion model was used to simulate the thickness. Figure 1 indicates the similar regularity of refractive index for the resulting ZnO ultrathin films with different thickness. The fitting results using F-B model (Figure 1a) display the nearly same band gap at about 3.2 eV. However, extinction coefficient k does not depend on thickness as shown in Figure 1b. For F-B dispersion model, the k is assumed to be zero before the band gap. Therefore, Figure 1c and d further present the calculated n and k results using point-by-point analysis. Both the n and k are disciplinary. According to the SE measurements and analyses, the average growth rate is about 0.16 nm per ALD cycle (Table 1). The n of ZnO ultrathin films is consisted with that reported elsewhere.5 All these results demonstrate that the optical properties of ZnO thin films depend on the film thickness. For example, Table 1 shows the value of n at 450 nm increasing from 2.0344 to 2.0788, gets close to that of bulk ZnO with the thickness increasing.


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Figure 1. Refractive index (n) and extinction coefficient (k) derived from the SE measurements of the thin films with different thickness. (a, b) were obtained and simulated by F-B model, and (c, d) by point-by-point analysis. Table 1. The different values of n at 450 nm 200 300 400 Bulk cycles cycles cycles ZnO Thickness 32.66 49.00 65.10 (nm) n

2.0344 2.0647 2.0788 2.1054

For the thickness-dependent optical constants, there have been two theories to explain the evolution of the n and k. First, defects may exist in the thin films, including the imperfect crystal or the stress and strain left in the films.20, 27, 34 And the thickness of the film have effect on the ratio of defects. Thinner films may have non-crystalline phase and thicker films may exist stress and strain. As an evidence, thermal processes have been used to improve the crystal and to



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release the stress, which changes the n and k. In another way, some researchers considered the effects from interface and surface.35-38 Normally, the roughness of the surface always has low influence on the n and k where the interfacial layer plays an important role.35 If the thickness of the material layer increases, the values of n and k tend to be as bulk.14 In conclusion, both of these theories are appropriated for our ultrathin films. Generally, two theories have the same mechanism in the molecule level and are integrated as follow. The n and k are the parameters in optics while they can be transformed to relative dielectric constant in electrics. = − + 2 = 1 + = 1 +

= ∑

(5) (6) (7)

Where is dielectric constant of vacuum, E is the electric field, is the electric susceptibility and P is electric polarization. (Nonlinear susceptibility is not considered here) The and are electric charge of electric dipole and distance between the charge pair, respectively. The values of are different for different molecules or the same molecule in the different circumstance, which are corresponding to interfacial layer and defects. For our ultrathin films, stress and strain exist after the ALD growth and defects always exist in the n-type ZnO. Also, the influences from interface are not negligible. As a result, defects may be filled but the influences from the interfacial layer are inevitable at the ultrathin scale.

Next, in order to improve the crystal properties followed by optical properties of the ultrathin


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films, we annealed the ZnO ultrathin films, and measured the thicknesses at different RTP temperature as shown in Figure 2. There are three temperature zones during the annealing processes. Zone I is below 400 °C, the thickness of the ZnO ultrathin films slightly increases with the temperature increasing. The thickness decreases from 50.1 to 45.2 nm in Zone II between 400°C and 700°C. When the temperature is more than 700°C, the thicknesses obviously change. Actually, rapid thermal processing will change the characterizations of ZnO ultrathin films, surface and interface.

9-10, 30, 39

The thickness analyses of SE are generally consistent with

SEM measurements of the ZnO ultrathin films annealed at different temperature, as illustrated in Figure 2a. SEM images also suggest that the film becomes abnormally rough and rugged in Zone III, which are shown in Figure S3 in the Supporting Information. The result was further confirmed by the RMS roughness (Rq) data in AFM measurements. The Rq is about 0.82 nm for the films without RTP, and then turns to 0.83 nm, 1.72 nm and 2.43 nm after an RTP annealing process at 400°C, 600°C and 800°C, respectively. AFM images are shown in the Supporting Information. All these results indicate there are some changes for ZnO ultrathin films after annealing.

It is reported that evaporation does effects at high temperature. 40 The thickness and roughness will change with the annealing time and annealing temperature. In this study, the evaporation contributed to the crystallinity and reduced thickness of ZnO thin films below 700°C. When the temperature raised up to 800 °C, evaporation is not enough to explain the transition in optical properties shown in Figure 2b. The great change takes place at 800 °C RTP for 300 cycles ZnO.



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Not only the thickness, but also n and k of the ultrathin films changes a lot. And it is obvious that there remains some ZnO properties at 800 °C but little upon 850 °C.

Figure 2. (a) The temperature dependent thicknesses of annealed ZnO ultrathin films, which were derived from SE and SEM measurements. Here, the ZnO ultrathin films were obtained by an ALD process including 300 cycles (~49.5 nm). (b) The curves of refractive index n of ZnO ultrathin films after RTP.

According to SE analysis in Figure 3a, we found that the extinction coefficient k has a blue shift corresponding to the increased temperature from 300°C to 700°C. Furthermore, to obtain the band gap of ZnO films, a linear extrapolation to (αE) 2 = 0 was used at the absorption edge in Figure 3b where α is the absorption coefficient (α=4πk/λ) and E is the photon energy.27 Detailed information is collected in Table 2, showing the blue shift of band gap from 3.270 to 3.296 eV, which might result from the changed crystalline structures of the annealed ZnO ultrathin films. For further study, XRD, SEM and PL spectra were then used to explore these annealed ZnO ultrathin films at various temperatures.


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Figure 3. (a) Extinction coefficient k of the ZnO ultrathin films annealed at different temperature by SE; (b) Plotting of (αE) 2 vs. E from the data in (a). Table 2. The band gaps of ZnO films in Figure 3(b) RTP 300 400 500 600 700 temperature (°C) Band gap 3.270 3.282 3.290 3.292 3.296 （eV））

Figure 4 shows the XRD patterns and SEM images of the annealed ZnO films at different temperature. For the as-grown ZnO film, there are two peaks at 2θ= 31.7°, 34.3°, which correspond to (100), (002) reflections in hexagonal wurtzite phase of polycrystalline ZnO.17 (002) orientation is the preferred orientation and (101) orientation at 2θ= 36.1° is invisible with comparatively lower intensity.11 With the annealing temperature rising from 300 °C to 700 °C, the intensity of the (002) peak enhanced. The mechanism of the enhanced crystallinity of ZnO has been discussed by Kim, et al, 40 which is to fit with lower surface energy. Furthermore, SEM images in Figure 4b-f display the improved crystalline of the annealed ZnO films with the temperature increasing from 300°C to 700°C. Especially, the crystalline and the morphologies



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take changes gradually with the temperature increasing as the SEM images and XRD patterns indicated in Figure 4. The grains of the ZnO grows up corresponding to the increase of the full width half maximum (FWHM). We noted that RTP hardly affected the properties of ZnO ultrathin films at a temperature below 400°C while a better crystallization occurred at 700°C. However, the films become abnormally rough and rugged at an annealing temperature higher than 800 °C while the phase transition appears. Figure 4a indicates that the peaks for crystalline ZnO in XRD patterns nearly disappeared whereas the (220) peak of Zn2SiO4 present. Although the Peak (220) is not obvious at a large scale in Figure 4a, it does exist and XRD patterns of 800 °C, 850 °C, and 900 °C are shown in the Supporting Information at an appropriate scale. The chemical reactions between ZnO and the interfacial layer (SiO2) are responsible for these changes. It is reported that the peaks of crystal ZnO still existed although a phase transition from the hexagonal (ZnO) to the tetragonal (Zn2SiO4) structure for the annealed ZnO films of ~200 nm thick. 41


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Figure 4. XRD patterns (a) and top-view SEM images (b-g) of the ZnO ultrathin films annealed at different temperature for five minutes. Here, the ZnO ultrathin films were obtained by an ALD process including 300 cycles. For the right part (b) 300 ºC; (c) 400 ºC; (d) 500 ºC; (e) 600 ºC; (f) 700 ºC; (g) 800 ºC.

In summary, Figure 2, 3, and 4 show the RTP effects on ZnO ultrathin films including thickness, refractive index, extinction coefficient, band gap, crystallinity, and surface morphology. Three zones in temperature were divided by the properties of ultrathin films. The thickness of the ultrathin film increases in Zone I but decreases in Zone II. In Zone I and II, the band gap, crystallinity and the surface roughness increase with the temperature, and the n and k take a little change during the RTP. In Zone III, the past crystallization is broken and the n of ultrathin films has a very greatly change. Not only evaporation but also transition effects in Zone III. SE simulations suggest the loss of ZnO and there are none crystalline ZnO but crystal Zn2SiO4 shown in XRD patterns. Similar phenomena were also discovered for 200 cycles ZnO. The only distinction is the critical temperature due to lower thickness. Detailed data are supplied in the Supporting Information.

In our cases, for ZnO ultrathin films with

Thickness-Dependent Optical Constants and Annealed Phase

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