TECHNOLOGY SUBSTRATES


enables greater electron-hole overlap and more efficient devices. If device developers want to turn to a non-polar plane, they have to choose between two orientations; but if they want to employ a semi-polar plane, they have a vast number of variants to consider, and it is still debatable which is the best of them.


What they are looking for is a plane that combines ease of high-quality crystal growth with straightforward device processing and favourable bandgap engineering. One orientation with much merit is the {2021} plane, which was used by a team from Sumitomo to make the world’s first green laser diode.


GaN substrates Today, hydride vapour phase epitaxy (HVPE) is viewed as a highly practical approach to obtaining thick layers of GaN, which can serve as quasi-bulk substrates. GaN is first grown on a foreign substrate, such as GaAs or sapphire, before the two are separated. The biggest advantage of this technique is its ability to produce high-quality material at high growth rates, thanks to high surface migration of the halide species. For this reason, the vast majority of GaN substrates shipped today are produced with a HVPE process.


The thick polar GaN material produced by HVPE is also used to make semi-polar and non-polar GaN substrates, which are formed by cutting along appropriate planes (see Figure 2). However, the size of nonpolar and semi-polar GaN substrates is typically restricted to just a rectangular area of a few square millimetres, due to the dimensions of the polar GaN.


Due to the small size of semi-polar substrates produced by HVPE, there has been much interest in developing alternative approaches that could yield large-sized, semi-polar and non-polar GaN platforms. Our team at Yamaguchi University has been one of the groups attempting to pioneer new technologies


Figure 2. The conventional fabrication process for semi-polar GaN substrates: A GaN layer more than a few millimetres thick is grown, by HVPE, on c-plane GaN template and sliced in intended semi-polar directions


to produce such orientations. Since 2004, we have been using MOCVD to develop epitaxial lateral overgrowth via selective area growth on a patterned sapphire substrate.


Our approach begins by forming trenches in sapphire with a width and depth of a few micrometres (see


Figure 3). The orientation of these grooves is carefully chosen to ensure that one sidewall of the trench acts as a nucleation plane for c-plane GaN growth. Under optimised conditions, the epitaxial process starts on this sidewall and continues laterally over the terraces to yield a continuous semi-polar GaN layer. With this approach, we have formed a


Figure 3. Growth process of a {2021} GaN layer on patterned sapphire. (a) Formation of c-plane sapphire sidewall by dry etching, which is 74.6° inclined from the {2243} plane of the sapphire. (b) Nucleation of GaN stripes from the c-plane sapphire side-walls by MOVPE. (c) Formation of the {2021} GaN film by the coalescence of neighbouring GaN stripes. (d) Cross-sectional scanning electron microscopy of a {2021} GaN layer


March 2014 www.compoundsemiconductor.net 49


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