Film technologies are becoming more and more important for the development of modern sciences and technologies. The development of film technology needs various and high quality single crystalline substrates. Firstly, the cell parameter of the substrate must match with the film to be grown on the substrate and it varies well, so that only special single crystal which has certain lattice constant, correct orientation and high fabrication accuracy can be used for the film. Secondly, the substrate crystal must be structure perfectly, and the crystal structure defect such as twin and inclusion must be as few as possible. The surface of the substrate must be polished to very high flatness and micro roughness (the test result of AFM should be better than a few angstrom in an 5x5um area, for example). The substrate must be able to stand to high, low temperatures and chemical environment during the film growth and use process. Also some physical, mechanical properties are necessary for the substrate. For example, the microwave dielectric loss must be very low for the substrates used in microwave communication.
   Gadolinium Gallium Garnet (GGG, Gd₃Ga₅O₁₂) is a magneto-optical and microwave substrate. It is the best substrate material for infrared optical isolator (1.3 and 1.5um), which is a very important device in optical communication. It is made of YIG or BIG film on the GGG substrate plus birefringence parts. Also GGG is an important substrate for microwave isolator and other devices. Its physical, mechanical and chemical properties are all good for the above applications.
   Zinc Oxide (ZnO) can be grown as a single crystal semiconductor with very interesting properties. The bandgap is in the 3.4 eV range which makes it attractive for many of the blue and violet applications in opto-electronics as well as UV devices. It is available in bulks, in wafers of up to 2 inches, which gives it a major advantage over Gallium Nitride and other group III Nitrides.
   ZnO also has the potential to become the substrate material of choice for GaN. Like GaN it has a wurtzite structure, with lattice constants closely matched to GaN (a=3.249, c=5.205). ZnO is exactly lattice matched to InGaN with a 22% In content. Perhaps most importantly it is a soft compliant material that is believed to probably and may take up the lattice stress in preference to the growing GaN layer. It suffers from the drawback that it dissociates in ammonia at temperatures above 600°C. With its wide bandgap, Zinc Oxide could prove very useful in optical applications as well as many high speed electronics. ZnO may also be used as a substrate for epitaxial growth.