Growth and properties of bulk ALN substrates

Abstract

Bulk crystal growth of aluminum nitride (AlN) comes into focus in order to provide substrates for deep-UV optoelectronics (LEDs, lasers, and sensors) which are typically based on Al-rich AlGaN epitaxial layers and structures. On AlN substrates, pseudomorphic AlGaN layers can be deposited with compressive strain and high structural quality [1-3]. In this context, the growth of AlN crystals by sublimation and recondensation (physical vapor transport method) at temperatures exceeding 2000 °C has proven to be the method of choice, as the boules and substrates show very high structural perfection at reasonable growth rates. Availability of AlN substrates as well as their useable area, structural quality, and electrical/optical properties are directly related to growth technology issues, including selection of set-up materials, seeding strategy, and pre-purification efforts. After a brief overview of history and applications of bulk AlN, the basic principles of AlN bulk growth by physical vapor transport (PVT) are reviewed. The formation of extended defects and the incorporation of impurities during growth as well as their impact on the material’s optical and electrical properties are discussed in detail. The main target of this chapter is to provide readers with enough information about AlN substrate preparation to understand and make informed decisions about employing AlN substrates for deep-UV optoelectronics.