Growth of High Quality (In,Ga,Al)N/GaN Heterostructure Materials and Devices by Atmospheric Pressure MOCVD
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23 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
EXPERIMENTAL Epitaxial films were grown on c-plane sapphire in a two-flow atmospheric pressure MOCVD reactor (Thomas Swan, Ltd.). Trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia were used as precursors. After annealing the substrates at 1050 °C, a 190 A thick GaN nucleation layer was deposited at 600 °C. The temperature was then raised to 1080 °C to grow GaN of 1.4 to 2.4 jim thickness. The growth of InGaN was investigated in the temperature range between 650 and 850 °C. The 0.1 to 0.2 jim thick InGaN layers were grown on top of the thick GaN layers. The input flows of TMGa and TMIn were varied from i to 13 jimol/min and 1.9 to 24 jimol/min, respectively. The ammonia flow was kept constant at 0.12 mol/min.
RESULTS For nominally undoped 4 jim thick GaN films, we obtained 300 K mobilities of 780
cm 2iNs (n300K = 6*1016 cm- 3 ), which is among the highest values reported for this
carrier concentration range. Surface nitridation of the sapphire substrate was found to highly influence the GaN film mobility and double crystal x-ray diffraction linewidth. Double crystal x-ray rocking curves display a minimum FWHM of 40 arcsec or 300 arcsec for 2 micron thick GaN films depending on the nitridation procedure. Surprisingly the best mobilities are routinely obtained from the samples with broader DCXRD linewidths. On these samples dislocation densities as low as 4*"108 cm 2 for GaN on Sapphire substrates were observed in cross-sectional TEM measurements. The films are characterized by a band edge to deep level luminescence intensity ratio of higher than 1300 for photoluminescence measurements (220 mW/cm 2 ) performed at 22 K. Even at excitation levels as low as 2.2 mW/cm 2 , the 300 K PL is dominated by the
near band edge emission . The FWHM of the (002) x-ray reflection was 180 arcsec. A relatively low dislocation density was obtained from cross-sectional electron microscopy analyses and amounted to 4x10~8 cm 2 .5
transmission
TABLE I. FWHM of symmetric and asymmetric XRD GaN reflections and dislocation densities for 1 .2 jim thick films grown on sapphire with different NH 3 pretreatments NH3 Pref low
Dislocations
Symmetric (002)
Asymm(102)
(FWHM/ arcsec)
(FWHM/ arcsec)
60 s
269"
413"
4*108
400 s
40"
740"
2"101°
24
(cm-2)
The growth of InGaN has to be performed at temperatures below 850 °C because of the high volatility of indium at common GaN growth temperatures of above 1000 °C. But even on InGaN layers grown at temperatures below 800 °C, In droplet formation was observed.6 Fig. l'a shows the 300 K PL spectra of three InGaN films of varying composition. Sample (a) was deposited at 800 OC with a TMGa and TMIn flow of 3.8 and 10 jmol/min, respectively. Sample (b) was grown at 760 °C with a TMGa and TMIn flow of 3.8 and 7.2 jimol/min, respectively, and sample (c) at 700 °C with a TMGa flow of 1.9 jimol/min and TMIn flow of 4.8 jimol/min. The InGaN films were Si doped with a Si2H6 flow of 0.7 nmol/min. The spectra are do
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