Selective area growth of GaN nanocolumns by rf-plasma assisted MBE at low temperature and under nitrogen-rich conditions.
Résumé
Ordered GaN-based nanocolumnar structures are very appealing for optoelectronic applications like LEDs due to their homogenous size and length distribution which facilitates the fabrication of devices. One very important issue for LED applications is the incorporation of InGaN structures in the ordered GaN nanocolumns (NCs) to produce emission in the visible optical region. Due to the lower growth temperature of InN, caused by the much weaker bonding of indium and nitrogen, the growth of InGaN structures has to be performed at a much lower temperature (700ºC) than the one "traditionally" used for the growth of ordered GaN nanocolumns (around 900ºC). This fact poses a big problem at the time of combining these two materials while maintaining the nanocolumnar growth morphology. This work reports on the selective area growth of GaN on a Ti-mask using lower substrate temperatures than normally used for SAG growth of GaN on a Ti-mask and nominally nitrogen rich conditions. For this purpose, a two step growth procedure is followed: (1) growth of ordered GaN NCs under "traditional" conditions, namely at 900 ºC and nominally under Ga-rich conditions and (2) change of the growth conditions to a substrate temperature of 840 ºC and nominally N-rich conditions. It will be shown that when growing at lower temperature it is essential to adjust the III/V ratio in order to preserve the nanocolumnar structure. Additionally we will show that this method presents a way of selective area growth of GaN NCs with negligible lateral growth, which is difficult to obtain using "traditional" ordered growth conditions, while enhancing the vertical growth rate.