The Effect of the Buffer Layer on the Structure, Mobility and Photoluminescence of MBE grown GaN
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structure, mobility and photoluminescence of the epilayer. This improvement corresponds to a smooth and flat buffer layer surface. Thus the epilayer has a decrease in the dislocation and stacking fault density, hence increasing the recombinant DBE efficiency of GaN. GROWTH AND ELECTRICAL PROPERTIES GaN was grown on a (0001) sapphire substrate using a modified VG-semicom V80 research grade MBE machine; using an ammonia injection source with a high flux rate for the nitrogen and solid source for the Gallium to give a high III/V ratio (100:1) [3]. A low temperature (500 °C) GaN buffer layer was first deposited on the substrate to allow some of the large lattice mismatch (13.4% between Al2O3 and GaN) to be taken up by dislocations. The buffer layer was subsequently annealed (1050 °C) for 5-80 minutes in order to improve the growth surface morphology, which promotes 2-D growth of the homoepitaxial GaN epilayer. The epilayers were subsequently grown at 1000 0 C (showing no decomposition of GaN) until they were 4µm thick. Seven samples were grown, with varying buffer layer thicknesses and annealing times. Table I shows the effect of these variables on the electron mobility, as measured by the Hall experiment. Increased buffer layer thickness improves the mobility of the epilayer by up to 50%. However, increasing the annealing time of the buffer layer has little effect on the mobility. When the buffer layer was annealed for an extended amount of time (>46mins) then the electrical properties of the bulk GaN was found to deteriorate. Table I The effect of buffer layer thickness and anneal time on the mobility of MBE grown GaN Sample N119 N125 N126 N129 N135 N139 N141
Buffer layer thickness (nm) 40 95 210 20 20 20 20
Annealing time of buffer (mins) 5 15 5 15 5 46 80
Mobility (cm2/Vs) 170 254 229 155 154 223 186
PHOTOLUMINESCENCE PL was measured at 10K using a He-Cd (325 nm) laser to excite the sample and a mercury arc lamp to calibrate the spectrometer. All samples show a blue donor band exciton (DBE) transition at around 3.48eV [3] (value varies with strain in epilayer). At 3.27eV a "structural peak" appears [3], this unwanted transition is understood to be related to defects in GaN. A broad 2.2eV peak occurs at the yellow end of the spectrum, this may arise from Ga-O complexes, which are attracted by the strain field of edge dislocations in GaN [4]. High quality PL was categorised by a high DBE to structural peak intensity (figure 1).
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PL at 10K
1.0 0E +0 6 C o u n ts p e r s e con d
DB E 1.0 0E +0 5
Yellow lum ines c enc e
1.0 0E +0 4
S truc tural P eak
1.0 0E +0 3 1.0 0E +0 2 1.0 0E +0 1 1 .7 5
2
2 .2 5
2 .5
2 .7 5
3
3.2 5
3.5
3.7 5
E ne rg y (eV)
Figure 1 Example of PL spectrum measured at 10Kof MBE grown GaN, highlighting the important electron- hole transitions Table II highlights the effect of the buffer layer thickness and anneal time on the DBE/structural peak ratio. Increased buffer layer thickness and annealing time has a marked improvement on this ratio, and a small decre
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