Synthesis and Growth of Gallium Nitride by the Chemical Vapor Reaction Process (CVRP)
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Internet Journal Nitride Semiconductor Research
Synthesis and Growth of Gallium Nitride by the Chemical Vapor Reaction Process (CVRP) M. Callahan1, M. Harris1, M. Suscavage1, D. Bliss1 and J. Bailey2 1Air
Force Research Laboratory, Sensors Directorate, Electromagnetic Materials/SNHX, State Scientific Corp.,
2Solid
(Received Tuesday, August 10, 1999; accepted Thursday, September 16, 1999)
A new process for synthesis and bulk crystal growth of GaN is described. GaN single crystal c-plane platelets up to 9mm by 2mm by 100µm thick have been grown by the Chemical Vapor Reaction Process (CVRP). The reaction between gallium and a nitrogen precursor is produced by sublimation of solid ammonium chloride in a carrier gas, which passes over gallium at a temperature of approximately 900°C at near atmospheric pressures. Growth rates for the platelets were 25-100 µm/ hr in the hexagonal plane. Seeded growth in the c-direction was also accomplished by re-growth on previously grown c-plane platelets. The crystals were characterized by X-ray diffractometry, atomic force microscopy, secondary ion mass spectrometry, inert gas fusion, and room temperature Hall effect and resistivity measurements.
1 Introduction Because truly large single crystals of GaN have not yet been achieved for use as substrates, most GaN epitaxial growth and device research and development has relied upon hetero-epitaxial growth. The most common substrate currently used is sapphire, but LiGaO2, ZnO, and SiC have also been tried [1] [2] [3]. In order to obtain very low defect density GaN on sapphire and SiC, growth of thick films of GaN by HVPE [4], and innovations such as pendeo-epitaxy [5] [6] and Epitaxial Layer Overgrowth (ELO) [7], have been employed. Even with these additional processing steps, many defects remain which will reduce device yield and degrade device performance. Device cost will also be driven higher due to the larger number of processing steps required for device manufacture. Growth of free-standing GaN bulk crystals has also been attempted. Square centimeter single crystal pieces have been obtained with ultra high-pressure growth [8]. Another approach has been to grow a thick gallium nitride film on a heterogeneous substrate such as SiC and then to remove the film from the substrate. This GaN film has less stress when the foreign substrate is removed at an elevated temperature and is thick enough to be used as substrate for epitaxial growth [9]. However, these growth methods may be of limited commercial value when producing large quantities of material. A third approach is to react gallium with gaseous ammonia
to form GaN [10]. This has always suffered from the problem of massive nucleation of very small crystals, usually smaller than 0.5 mm in diameter. Several groups have used GaN substrates for homoepitaxy; the resulting films showed great improvement in quality [11] [12]. Thus, nitride devices made using films grown on single crystal GaN substrates should be superior to devices made using hetroepitaxial substrates. In this articl
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