## 1 Introduction  
Semiconductor materials play a critical role in modern society and have been made into light emitting diodes (LEDs) [1; 2; 3], surface acoustic wave (SAW) devices [4], sensors [5; 6; 7; 8], solar cells [9], and so on [4; 10; 11; 12], which significantly promote our life quality. ZnO is an enduring star of the third-generation semiconductor materials owing to its unique merits. First of all, ZnO is non-toxic and of perfect biocompatibility, leading to the great potential applications of biodetectors and biomedical restorative materials [13]. Furthermore, the electrical properties of ZnO can be improved by doping with other elements, and ZnO has been widely applied in LEDs [1, 2], laser diodes [4], SAW devices [4], etc. ZnO is of low cost, and Al-doped ZnO is of excellent conductivity as well as cheap, which is considered a superior substitute for the expensive indium tin oxide (ITO) transparent conductive material [14; 15; 16]. In addition, ZnO is found to be very sensitive to ultraviolet (UV) light, humidity, and gas and has been successfully applied in UV sensors [17], graphic imaging sensors [18], humidity sensors, and gas sensors [19; 20]. Last but not least, it has been demonstrated that ZnO with bionic structure level holes is suitable for energy storage and utilization [21]. Therefore, ZnO materials have been favored by researchers.  
In order to further utilize ZnO materials, they are always made into thin films. Of course, cost control is also considered to be a key factor for the applications of the ZnO thin films. In the past decades, ZnO thin films have been synthesized by metal-organic chemical vapor deposition (MOCVD) [22], molecular beam epitaxy (MBE) [23], pulsed laser deposition (PLD) [24], magnetron sputtering [11], thermal evaporation [25], sol-gel method [26], etc. [27; 28]. MOCVD, MBE, and PLD are beneficial for forming single crystalline ZnO thin films, but the cost is usually high. Both thermal evaporation and the sol-gel method are cheap for synthesizing polycrystalline or well-preferred orientation ZnO thin films. Unfortunately, the quality of the ZnO thin films formed by these two methods needs to be further improved, and the process repeatability is still a huge challenge. On the contrary, magnetron sputtering is suitable for growing ZnO thin films with well-preferred orientation and even monocrystalline or single crystalline ZnO thin films. More importantly, the cost of the ZnO thin films grown by magnetron sputtering is relatively low, and magnetron sputtering also exhibits good process repeatability. Thus, magnetron sputtering is one of the common technologies for preparing ZnO thin films in industrial production.  
It should be noted that the choice of the substrate also occupies a large chunk of the cost of ZnO thin film preparation. Both glass and sapphire are common substrates for growing ZnO thin films [17]. It is well known that the glass substrates are much cheaper than sapphire substrates. Hence, it has an important significance for growing ZnO thin films with high quality on the glass substrates. In this work, we report a comparative investigation of ZnO thin films on the quartz glass and sapphire substrates by means of magnetron sputtering and high-temperature annealing in order to optimize the technological parameters for preparing ZnO films of high quality on the quartz glass substrates.