Microscopic investigations of morphology and thermal properties of ZnO thin films grown by atomic layer deposition method

Highlights

• The manuscript entitled: ”Microscopic investigations of morphology and thermal properties of ZnO thin films grown by atomic layer deposition method” presents the application of Scanning Probe Microscopy methods, among them the atomic force microscopy (AFM) and scanning thermal microscopy (SThM), for characterization of morphological and thermal properties of thin ZnO films. The ALD deposited samples were polycrystalline layers with average grain size varying from 18 to 44 nm. The thermal conductivity changes followed changes in the morphological parameters and the best result equal to 4.3 Wm⁻¹K⁻¹ was determined for sample doped with Al. Analysis of results showed strong dependence of the thermal conductivity on average crystallite size and domination of phonon contribution to total thermal conductivity over electron part.

Abstract

This work presents the study of morphology and thermal properties of thin ZnO films fabricated by atomic layer deposition. The layers were deposited on n-Si(100) wafers at 200 °C. X-ray diffraction measurements showed the polycrystalline structure of the thin films with preferred (100) orientation. The thinner ZnO layers were fine grained, while the thicker films were formed with larger, elongated grains. Surface morphology parameters and the thermal conductivities were obtained from microscopic measurements. Thermal properties correlated with surface roughness of the ZnO thin films. Variations in thermal conductivity followed the changes in morphology of the layers. The mean surface roughness depended on the number of deposition cycles and varied from 1.1–2.6 nm. Thermal conductivity varied from 0.28 to 4.29 Wm⁻¹K⁻¹ and increased also with an increase of average crystallite size. The possible correlations between electrical conductivity and thermal conductivity were also analyzed. The phonon contribution to total thermal conductivity dominates over the electron thermal conductivity.

Introduction

Zinc oxide is a multifunctional material and found numerous applications in electronics, optoelectronics, corrosion protection coatings as well as chemical and pharmaceutical industry. ZnO is applied as a transparent conductive oxide (TCO), a channel layer in thin film transistors (TFTs), a sensitive element in gas and vapor sensors [1], [2], [3], a UV photon sensor [4] and solar cells [5, 6]. This wide band gap semiconductor is non-toxic, chemically stable, has a long electron excited state lifetime and high charge mobility [7]. The electrical and optical properties of ZnO can be modified by doping it with Group III elements like Ga, Al and In. ZnO typically possesses a hexagonal wurtzite crystal structure. The lattice constants are equal to a = 0.325 nm and c = 0.520 nm [8].

Among many thin films deposition techniques like molecular beam epitaxy (MBE) [9, 10], sputtering [11, 12], pulse laser deposition (PLD) [13], chemical vapor deposition (CVD) [14], the atomic layer deposition (ALD) allows for precise thickness control on a nanometer scale over very large deposition area [15], [16], [17], [18], [19]. This method has found numerous applications [20], [21], [22], [23], [24], [25], [26].

Thermal conductivity of thin films is an important material property due to specific applications in high power and high temperature electronic devices. Moreover, progressive miniaturization, increasing integration and higher operating frequencies of electronic devices cause problems with efficient heat dissipation [27]. Knowledge about thin film thermal conductivities allow for engineering effective heat transfer which improves reliability of optoelectronic structures. Thin film thermal conductivities are usually lower than bulk samples [28, 29] and correlate with film thickness and grain size [30]. The thermal conductivity of a ZnO bulk single crystal was reported to be 100 Wm⁻¹K⁻¹; about 50 Wm⁻¹K⁻¹ for ZnO ceramics and that value was typically a few Wm⁻¹K⁻¹ for ZnO thin films. These lower values for thin film thermal conductivities can be ascribed to different phonon scattering processes. The termal conductivity can be theoretically predicted applying the Debye-Callaway model [31]:

$$k=\frac{k_B}{2{\pi }^2\upsilon }{\left(\frac{k_{B}T}{\hslash }\right)}^3\underset{0}{\overset{\Theta /T}{\int }}\frac{\tau \left(x\right)x^4{e}^x}{{\left(e^x-1\right)}^2}dx$$

where: ℏ; k_B and ℏ are Boltzmann's and Planck's constants, υ is an average sound velocity, Θ is the Debye temperature, τ is the relaxation time at temperature T for a phonon of frequency ω and τ is the relaxation time describing different scattering mechanisms. The model assumes that these mechanisms are independent, the scattering rates are additive and an effective relaxation time for N processes can be calculated using the following:

$${\tau }_{ef}^{-1}=\sum _i^N{\tau }_i^{-1}$$

The effective relaxation time describes different scattering mechanisms. Among them, phonon-phonon scattering, point defects, phonon scattering on extended defects, dislocations and scattering on grain boundaries are distinguished. The native defects in ZnO are intrinsic defects like oxygen vacancies and interstitial Zn atoms. Other defects are hydrogen impurities introduced during the deposition process. Heat transport mechanisms investigation in polycrystalline thin films requires careful structural analysis including determination of crystalline grain size, shape, distribution and film quality. A grain size dependent thermal conductivity investigations for ZnO thin films produced by reactive sputtering with different amounts of oxygen in the sputtered gas has been reported in [32]. The out-plane thermal conductivity changed from 5.4 to 4.0 Wm⁻¹K⁻¹; the in-plane thermal conductivity decreased from 4.86 to 2.66 Wm⁻¹K⁻¹ for oxygen contents of 30%, 60%, and 90% during film deposition. Similar values for thermal conductivities of ZnO:Al thin films deposited on sapphire substrates of 4.5 Wm⁻¹K⁻¹ have been reported in [33]. In that study, the authors determined the Seebeck coefficient and reported that Al doped ZnO thin films can be applied as thermoelectric materials. Higher thermal conductivity values of hetero-epitaxial ZnO thin films have been reported in [34]. The authors investigated c- and a-axis oriented thin films with thicknesses of 100, 200 and 300 nm. ZnO was synthesized by radio frequency magnetron sputtering on a sapphire substrate. Thermal conductivities for the c-axis-oriented ZnO thin films ranged from 18–24 Wm⁻¹ K⁻¹; for the a-axis-oriented ZnO thin films, the range was 24–29 Wm⁻¹ K⁻¹.

The correlation between the thermal conductivity and electrical conductivity has been reported in [35]. The thermal diffusivity of magnetron sputtered 100 nm Al-doped ZnO thin films was determined from nanosecond thermoreflectance. Thermal conductivities determined on the basis of thermal diffusivity results varied from 5.0–6.8 Wm⁻¹K⁻¹ and followed variations in electrical conductivity. Thermal conductivity was an order of magnitude lower than ceramics prepared from Al₂O₃ and ZnO powders and larger than for In₂O₃. Thermal properties of ZnMnO and ZnCoO nanostructures have been previously reported [36]. Two different methods were used for ZnO fabrication, chemical deposition and ultrasonic sputtering. Chemical deposition was used to deposit pure ZnO and ultrasonic sputtering was applied for fabrication of magnesia and cobalt doped ZnO nanostructures. The thermal conductivity of the thinnest layer was 1.4 Wm⁻¹K⁻¹ while of the thickest one was 16.4 Wm⁻¹K⁻¹. The ZnO samples were nanocrystalline films with an average grain size varying from 40–100 nm. Polycrystalline ZnO thin films deposited onto glass substrates by chemical spray pyrolysis featured a thermal conductivity minimum of 0.6 Wm⁻¹K⁻¹ [37]. This value changed slightly as the solution concentration increased and reached maximum value of 0.85 Wm⁻¹K⁻¹. The in-plane thermal conductivity of ALD ZnO thin films deposited onto a 20 μm thick polycarbonate membrane determined by laser flash method has been reported in [38]. The ZnO thermal conductivity varied from 1.9 to 2.1 Wm⁻¹K⁻¹. Transient thermoreflectance was used to determine the thermal conductivity of sol-gel ZnO thin films [39]. The layers ranged in thickness from 80–276 nm and the thermal conductivity varied from 1.4–6.5 Wm⁻¹K⁻¹. Thermal properties of the aforementioned ZnO thin films are gathered in Table 1. Literature values of thermal conductivities as a function of ZnO film thickness are shown in Fig. 1. Results from this work were compared with literature as black dots in Fig. 1. A literature search showed that thermal conductivities of ZnO thin films varied from 0.4–20 Wm⁻¹K⁻¹ with changes in layer thickness.

This paper reports results of thermal conductivity measurements on a set of polycrystalline ZnO thin films and an analysis of possible correlations between their thermal/electrical properties and morphologies. This paper contains four sections: An introduction to thermal properties of ZnO thin films (Section I) is followed by experimental methods with measurement details described in Section II. Section III presents results from structural and microscopic measurements with analyses of structural and thermal properties of ZnO samples. The last section IV contains conclusions and final remarks.