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.