Direct synthesis and characterization of optically transparent conformal zinc oxide nanocrystalline thin films by rapid thermal plasma CVD
© Pedersen et al; licensee Springer. 2011
Received: 14 September 2011
Accepted: 31 October 2011
Published: 31 October 2011
We report a rapid, self-catalyzed, solid precursor-based thermal plasma chemical vapor deposition process for depositing a conformal, nonporous, and optically transparent nanocrystalline ZnO thin film at 130 Torr (0.17 atm). Pure solid zinc is inductively heated and melted, followed by ionization by thermal induction argon/oxygen plasma to produce conformal, nonporous nanocrystalline ZnO films at a growth rate of up to 50 nm/min on amorphous and crystalline substrates including Si (100), fused quartz, glass, muscovite, c- and a-plane sapphire (Al2O3), gold, titanium, and polyimide. X-ray diffraction indicates the grains of as-deposited ZnO to be highly textured, with the fastest growth occurring along the c-axis. The individual grains are observed to be faceted by (103) planes which are the slowest growth planes. ZnO nanocrystalline films of nominal thicknesses of 200 nm are deposited at substrate temperatures of 330°C and 160°C on metal/ceramic substrates and polymer substrates, respectively. In addition, 20-nm- and 200-nm-thick films are also deposited on quartz substrates for optical characterization. At optical spectra above 375 nm, the measured optical transmittance of a 200-nm-thick ZnO film is greater than 80%, while that of a 20-nm-thick film is close to 100%. For a 200-nm-thick ZnO film with an average grain size of 100 nm, a four-point probe measurement shows electrical conductivity of up to 910 S/m. Annealing of 200-nm-thick ZnO films in 300 sccm pure argon at temperatures ranging from 750°C to 950°C (at homologous temperatures between 0.46 and 0.54) alters the textures and morphologies of the thin film. Based on scanning electron microscope images, higher annealing temperatures appear to restructure the ZnO nanocrystalline films to form nanorods of ZnO due to a combination of grain boundary diffusion and bulk diffusion.
PACS: films and coatings, 81.15.-z; nanocrystalline materials, 81.07.Bc; II-VI semiconductors, 81.05.Dz.
Keywordszinc oxide transparent nanocrystalline film thermal plasma chemical vapor deposition annealing nanorods
Zinc oxide [ZnO] is a direct, wide bandgap (Eg = 3.37 eV at room temperature) semiconductor which has a high exciton binding energy (60 meV) [1–5]. The large bandgap renders pure ZnO to be colorless in appearance and non-absorbing in the visible to infrared wavelengths (optical spectra at and above 375 nm). The high exciton binding energy of ZnO allows excitonic laser action at or above room temperature, in addition to making ZnO the brightest emitter among GaN (26 meV) and ZnSe (20 meV). From an electronic standpoint, ZnO has one of the best conductivities among the transparent conducting oxides [TCO] due to its high charge carrier mobility - ZnO has high experimentally derived electron Hall mobility of up to 200 cm2/V-s [6, 7] and hole mobilities ranging from 2 to 8 cm2/V-s [8, 9]. These desirable attributes make ZnO suitable for optoelectronic applications such as transparent thin transistor [10, 11], TCO and buffer layers in photovoltaic cells [12, 13], light-emitting diode [8, 9], UV laser , optical waveguide , and biochemical sensors . In spite of these desirable attributes, most current methods of synthesizing ZnO thin films - including plasma enhanced chemical vapor deposition [CVD] , thermal CVD , radio frequency [RF] or DC magnetron sputtering [19–21], metal organic chemical vapor deposition [MOCVD] , spray pyrolysis , pulsed laser deposition , thermal evaporation , hydrothermal , and sol-gel processes  - often require substantial vacuum, expensive consumables (e.g., diethyl zinc, dimethyl zinc, ZnO sputter target), catalyst (e.g., gold), and lengthy synthesis time. While solution-based methods - such as hydrothermal and sol-gel - can produce good quality films  at a much lower processing temperature (approximately 100°C) that are favorable to mass production, vapor phase methods such as thermal evaporation and MOCVD provide important alternative routes to produce high quality films. Nevertheless, in addition to the high vacuum (10-4 to approximately 10-5 Torrs) required, the high temperature at which these vapor phase methods are performed (800°C and above) also makes the process not CMOS-compatible. Therefore, a direct, rapid, close-to-ambient pressure vapor phase synthesis method using inexpensive precursors is highly desirable from a synthesis and process development standpoint.
To address such challenges, this paper reports a rapid, direct, self-catalyzed thermal plasma chemical CVD process for depositing a conformal, nonporous nanocrystalline ZnO thin film on various crystalline and amorphous substrates using solid zinc as the precursor material at 130 Torr. Thermal plasmas - high power discharges - can be produced at or near ambient pressure using high-power sources, such as RF induction plasma system . Previous research has shown that inductive heating can provide a useful and efficient means to rapidly introduce a large amount of heat for nanomaterial synthesis [30–32]. This is attributed to the high enthalpy of RF induction plasma and its being capable of high-frequency (13.56 MHz) switching, making it well suited for applications where high-temperature and high-heating rate heat treatments are needed . In particular, RF induction plasma systems have shown an industry-scale utility for synthesis of high-quality nanoparticles . In thermal induction plasma nanoparticle synthesis methods, concurrent introduction of complex liquid, gas, or powder precursors enables a one-step, cost-effective, and time-efficient synthesis. During synthesis, the reagents are introduced into a plasma-entrained flow, become fully ionized, and condense as droplets as they leave the plasma region. In addition to nanoparticle synthesis, thermal plasma CVD has also found success in ZnO thin film synthesis at a subatmospheric pressure using gaseous precursors such as diethyl zinc or dimethyl zinc [34–36]. While diethyl zinc has been the gaseous precursor of choice, it is expensive, toxic, and pyrophoric and requires special care in handling. Using an environmentally benign precursor is therefore highly desirable. To date, little has been done using solid zinc as the precursor in thermal induction CVD due to the higher temperature typically required in creating Zn vapor. In this paper, we introduce a thermal plasma CVD process using only solid zinc as the source material, thereby simplifying the design of the synthesis system. We demonstrate the deposition of conformal, nanocrystalline ZnO films that are electrically conductive and optically transmissive.
ZnO thin film synthesis
We deposit ZnO on crystalline and amorphous growth substrates including p-type silicon (100), mica (muscovite), fused quartz, c- and a-plane sapphire, borosilicate glass, tin-doped indium oxide [ITO], and polyimide (Kapton®, DuPont, Wilmington, DE, USA). The deposition rate (10 to 50 nm/min) is tightly controlled by a closed-loop temperature control algorithm where the output RF power is modulated by the source temperature. Based on experience, for a high rate of deposition, the RF power and plasma intensity - which is proportional to the rate of change of the temperature of the nickel heating chamber - must be relatively high, yet the nickel heating chamber temperature is to be maintained well below the boiling point of Zn, so that Zn droplets do not form and deposit on the substrate as metallic zinc. This is achieved by maintaining the temperature of the nickel heating chamber using a saw-toothed temperature profile to attenuate the power periodically. The controller RF output is pulsed to high power to maintain the appropriate nickel heating chamber temperature rate increase. As an upper temperature limit is reached, RF power is automatically reduced allowing the nickel heating chamber to cool to a predetermined temperature. Further pulses can be programmed until the zinc source is completely depleted. At the end of the deposition run, oxygen gas is switched off, and the system is allowed to cool down to room temperature under only Ar gas flow at the original flow rate.
Post-process film treatment and characterization
The surface morphology, film thickness, and crystal dimensions of the synthesized ZnO nanocrystalline films are characterized by scanning electron microscopy [SEM] on a Zeiss Ultra 55 (Carl Zeiss Microscopy, Peabody, MA, USA) that is equipped with a Schottky field emission gun. Elemental analysis is conducted using an Oxford energy dispersive X-ray probe. Film crystallinity is investigated using an X-ray diffractometer (Bruker D8 ADVANCE, Bruker AXS Inc., Madison, WI, USA) with Cu-Kα radiation (λ = 1.54178 Å) and a scanning range of 2θ between 24° and 100°. Electrical conductivity measurement is conducted using a four-point probe, and transmittance of the as-deposited film is measured using a Lambda UV-Vis spectrophotometer (PerkinElmer, Inc., Waltham, MA, USA) with an integrating sphere. The spectra are collected in the 200- to 800-nm spectral range. Thermal annealing of samples is performed in a tube furnace (MTI GSL-1100X, MTI Corporation, Richmond, CA, USA) at 300 sccm of argon flow at temperatures ranging from 750°C to 950°C for 1 h.
Results and discussion
Properties of as-deposited ZnO film
Influence of source temperature profile and growth duration
As the number of saw-tooth ('pulse') and the total synthesis duration above 420°C increase, the nominal thicknesses of the films increase proportionately from 25 nm (1 pulse, 135 s, Figure 3a) to 70 nm (3 pulses, 290 s, Figure 3b), and 110 nm (5 pulses, 445s, Figure 3c) at a growth rate of 16.7 nm/min. We also investigated the influence of the resident time that the source temperature stays at the peak temperature (570°C) on film thickness. We compare two samples, Figure 3a, d, where each has an identical saw-toothed heating profile. The sample in Figure 3a has 1 s of resident time at 570°C, while that of Figure 3d has 5 s of resident time at 570°C. Our results show that there is no significant difference in the thicknesses between these two samples, and the differences are within the range of errors - the sample in Figure 3a has a nominal thickness of 25 nm, while the sample in Figure 3d has a nominal thickness of 22 nm. As shown, there are no noticeable differences in the thicknesses despite the fact that the sample in Figure 3d has four more seconds at the peak temperature. This shows that the total duration the source temperature stays above 420°C, instead of at the peak temperature of 570°C, plays a more critical role in influencing the thicknesses of the films. Furthermore, when comparing Figures 3 d and e, the effect of the duration the source stays above 420°C becomes more obvious - longer synthesis time above 420°C leads to thickening of the film (to 57 nm) as shown in Figure 3e, the sample which is exposed to 70 s longer than the sample in Figure 3d. It is evident that film thickness is predominantly influenced by the heating duration at or above the melting point of zinc and, to a minimal extent, by changes in the resident time at the peak temperature. The evidence indicates that heating to just above 420°C is sufficient to allow deposition to occur in the argon-plasma environment. The grain sizes also appear to increase from an average grain diameter of 44 nm to 75 nm as synthesis duration increases from 135 s to 445 s.
Influence of substrate type and substrate temperature
Influence of thermal annealing
where E is the optical bandgap, ħω is the photon energy in eV, α(ω) is the absorption coefficient, ω is the angular frequency, B is a constant between 105 and 106 cm-1, T is the transmittance, d is the film thickness, and n is an exponential value = 1/2 . The optical bandgap energy for our films is closest to films deposited by spray pyrolysis (3.26 eV)  and close to films deposited by other methods such as CVD (3.19 to 3.23 eV)  and pulsed laser deposition (3.26 eV)  and (3.1 eV) . As shown in Figure 9b and the inset, ZnO films at 20 nm and 200 nm exhibit high transmittance in the visible range; however, the transmittance below 375 nm depends largely on the film thickness - thinner films appear to be more transmissive, while thicker films are less. Figure 9c, d shows the grain morphologies of the 20-nm- and 200-nm-thick ZnO films, respectively. The average grain sizes correspond to 25 nm and 100 nm, respectively, for the 20-nm and 200-nm films. Finally, for the 200-nm-thick ZnO film, the four-point probe measurement shows a conductivity of up to 910 S/m, indicating the ZnO film to be highly conductive.
We have successfully demonstrated a direct, catalyst-free synthesis method of depositing conformal nanocrystalline ZnO films on crystalline and amorphous substrates using a fast thermal plasma CVD process that is preceded by inductive heating. SEM indicates that the ZnO films deposited on ceramic and metal substrates at 330°C are highly conformal with evenly distributed grain sizes and a preferential growth direction along the c-axis. ZnO films deposited at 160°C on polyimide are conformal; however, due to a large thermal mismatch between the film and the substrate, stress-induced porosity is observed. Such porosity is expected to be reduced with a lower substrate temperature. Optical measurements of 20-nm- and 200-nm-thick ZnO films show a high optical transmittance at spectra 375 nm and above, which corresponds to the optical bandgap of ZnO. Thermal annealing of ZnO films at temperatures ranging from 750°C to 950°C causes restructuring of the grain. As annealing temperature increases, grain boundary diffusion and bulk diffusion cause restructuring of ZnO grains into ZnO nanorods.
chemical vapor deposition
energy dispersive X-ray
tin-doped indium oxide
scanning electron microscopy
transparent conducting oxides
The authors thank the following people: Dr. Andrew Ichimura for his assistance with X-ray diffraction and UV-Vis spectrophotometry, and for his fruitful discussions; Tom Franco and Richard Moore for the help in designing the machine mechanical fixtures used in the experiments; Curtis Hilger for his work on the implementation and tuning of the control system; and Mark Brunson for a portion of the data collection work. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research under grant no. 49524-UNI 10.
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