Alternative Routes to the MOVPE Growth of GaN and Aln
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337 Mat. Res. Soc. Symp. Proc. Vol. 395 ©1996 Materials Research Society
It has been shown that the growth of AIN by MOCVD from the combination Me3 Al and Nil 3 arises from an initial formation of the adduct [Me 3AI:NH 3], followed by the sequential elimination of CH4 thus' -CH 4 -CH 4 -CI- 4 Me 3Al + NH 3 -- [Me 3AI:NH3] -ý [Me 2AINH 2]3 -- ' [MeA1NH]M--AIN (1) In this work the initial step of this process is external to the reactor, since the adduct Me 3Al:NH3 has been utilised as a precursor. Any prereactions between Me 3A1 and NH 3, leading to the formation of the adduct, are therefore prevented. Growth on the Si(100) substrates was expected to be polycrystalline as no attempt was made to remove the silicon native oxide. Epitaxial growth of AIN on c-plane sapphire (A120 3 (0001)) using the single source precursor Me 3AI:NH 3 (flow rate = 300 sccm, source temperature = 45 C) was studied over the temperature range 800 C to 1050 C. Prior do deposition on sapphire the substrates were pre-treated in an atmosphere of hydrogen at 1000 C for 10 min. The high vapour pressure of both Me3Al (8.67 Torr @ 20 C) and t-Butylamine (288 Torr @ 20 C) allows the reactor to be operated at atmospheric pressure; the single source precursor Me 3Al:NH 3 requires low pressure growth due to the lower vapour pressure of this source.
Precursor(s) Substrate Me 3A1 + t-BuNH 2 Si/SiO 2 Me 3AI:NH 3 I Me 3Al:NH 3
SiSiO 2 I _ 1cc-A120 3
Growth Temp 550 C, 950 C
Pressure AP
400 C to 800 C LP I_ I 800 C to 1000 CI LP
Comments High carbon content and polycrystalline Growth rate = 33 - 467 nm/hr Iand polycrystalline Epitaxial growth rate 250 nm/hr
Table 1 - Precursors studied for the deposition of AIN Gallium Precursors The range of precursor reactions investigated include t-BuNH 2 /trimethylgallium (Me 3 Ga), t-BuNH 2/ trimethylgallium-ammonia adduct (Me 3Ga:NH 3) as well as several single source precursors based on ammonia adducts with gallium alkyls. These are summarised, together with the growth conditions used, in table 2. Precursor(s) Me 3Ga + t-BuNH 2 Me 3Ga:NH 3 + t-BuNH 2 Me 3Ga:NH 3 Et 3 Ga:NH 3 Me 2GaCI:NH 3
Substrate GaAs
Growth Temp 700 C
Pressure AP
Comments Ga droplets formed
Ut-A12
03
700 C, 950 C
AP
Ga droplets formed
Si/Si0 2 cc-A120 3 cc-A120 3
200 C, 700 C 900 C 700 C
AP LP LP
Ga droplets formed Ga droplets formed Ga droplets formed
Table 2 - Precursors studied for the deposition of GaN
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Film Analysis Film composition was measured by Auger Electron Spectroscopy (AES) using a Varian Scanning Auger spectrometer. The atomic compositions quoted are from the bulk of the film, free from surface contaminants, and were obtained by combining AES with sequential ionbombardment until comparable compositions were obtained for consecutive data points. Layer thickness was determined by ellipsometry combined with a calibrated scanning electron micrograph of cleaved samples; this was used in growth rate calculations. Epitaxial crystalline quality for the AIN/A120 3 samples was determined from the Full Width at Half
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