(a) XTEM image of the GaN nanorod/Si (111) interface region after growth of nominally undoped nanorods without intentional nitridation. (b) XTEM image after 15 min additional nitridation in the initial stage of growth. Extended nitridation does not lead to a thicker silicon nitride intermediate layer but increases the homogeneity. (c) SAED pattern obtained for the sample without intentional nitridation. (d) SAED pattern of the 15 min nitridated sample. Arrows indicate the growth direction ( axis).
(a) CBED pattern obtained in sample with a 2 min nitridated sample. (b) Simulated CBED pattern confirming that the GaN growth occurs along the (0001) direction (Ga-face polarity).
SEM images of GaN nanorod samples grown on a Si (111) substrate after different growth durations, taken under a declination angle of . Nucleated nanorods were observed for growth times above 30 min. Nucleation proceeded up to a growth time of 60 min. After that, the density of nanorods decreased due to merging and coalescence.
(a) Nanorod density as a function of growth time. For a substrate temperature of (solid triangles) nucleation is finished after approximately 60 min. A decrease in substrate temperature leads to a decreased nucleation time (open triangles), whereas an increase leads to no apparent nucleation even after 45 min of growth (open circle). For a substrate temperature of , two different additional nitridation times (0, 45 min) besides the standard nitridation time of 15 min were evaluated after 30 min growth, yielding almost identical values. (b) Bright-field scanning TEM image showing the merging of tilted and adjacent straight nanorods, leading to an apparent decrease of the nanorod density.
Spatially resolved EELS measurement of the GaN/Si interface region. Besides the formation of a silicon nitride interfacial layer, diffusion of silicon into the GaN nanorod is observed.
(a) Dependence of the nanorod height on the growth duration. After nucleation, a constant growth rate of is obtained. (b) Dependence of the nanorod diameter on the growth duration. A real radial growth rate cannot be extracted as the apparent increase of the diameter at is due to the merging of adjacent nanorods [cf. Fig. 4(b)].
XTEM image of the GaN nanorod/Si (111) interface region of a highly silicon doped GaN nanorod with . The formation of an additional silicon nitride layer after nucleation of the nanorod is observed between the nanorods. The amorphous material on the sidewalls of the NR is part of the glue used for XTEM sample preparation.
Dependence of the nanorod diameter (left axis) and length (right axis) on the . An increase in diameter due to tapering of nanorods is accompanied by a decrease in height.
High-resolution TEM image of a silicon-doped GaN nanorod. The tapering effect is caused by a gradual increase of the nanorod diameter due to subsequent groups of monoatomic steps on the sidewall (indicated by the arrows). High silicon doping leads to strong tapering, a decreased nanorod density, and suppressed nanorod merging.
Dependence of the nanorod diameter (left axis) and length (right axis) on the . The increasing nanorod diameter is due to an increase of the lateral growth rate. Solid and dashed line are guides to the eye only.
Test structures to study the effect of Mg on nanorod growth. Schematic drawing of the samples summarized in Table II.
Indexation of the SAED patterns for the nanorod samples shown in Figs. 1(c) and 1(d). Zone axis is Si, which is parallel to GaN.
Growth conditions and dimensions of reference samples and compound nanorods to analyze the effect of Mg doping on the growth mechanism (cf. Fig. 11).
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