Junaid Muhammad, Hsiao Ching-Lien, Chen Yen-Ting, Lu Jun, Palisaitis Justinas, Persson Per Ola Åke, Hultman Lars, Birch Jens
Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.
Nanomaterials (Basel). 2018 Apr 7;8(4):223. doi: 10.3390/nano8040223.
GaN nanorods, essentially free from crystal defects and exhibiting very sharp band-edge luminescence, have been grown by reactive direct-current magnetron sputter epitaxy onto Si (111) substrates at a low working pressure of 5 mTorr. Upon diluting the reactive N₂ working gas with a small amount of Ar (0.5 mTorr), we observed an increase in the nanorod aspect ratio from 8 to ~35, a decrease in the average diameter from 74 to 35 nm, and a two-fold increase in nanorod density. With further dilution (Ar = 2.5 mTorr), the aspect ratio decreased to 14, while the diameter increased to 60 nm and the nanorod density increased to a maximum of 2.4 × 10⁸ cm. Yet, lower N₂ partial pressures eventually led to the growth of continuous GaN films. The observed morphological dependence on N₂ partial pressure is explained by a change from N-rich to Ga-rich growth conditions, combined with reduced GaN-poisoning of the Ga-target as the N₂ gas pressure is reduced. Nanorods grown at 2.5 mTorr N₂ partial pressure exhibited a high intensity 4 K photoluminescence neutral donor bound exciton transitions (D⁰X) peak at ~3.479 eV with a full-width-at-half-maximum of 1.7 meV. High-resolution transmission electron microscopy corroborated the excellent crystalline quality of the nanorods.
氮化镓纳米棒基本没有晶体缺陷，呈现出非常尖锐的带边发光，通过反应性直流磁控溅射外延在5毫托的低工作压力下生长在硅（111）衬底上。在用少量氩气（0.5毫托）稀释反应性氮气工作气体后，我们观察到纳米棒的长径比从8增加到约35，平均直径从74纳米减小到35纳米，纳米棒密度增加了两倍。随着进一步稀释（氩气 = 2.5毫托），长径比降至14，而直径增加到60纳米，纳米棒密度增加到最大值2.4×10⁸厘米⁻²。然而，较低的氮气分压最终导致连续氮化镓薄膜的生长。观察到的形态对氮气分压的依赖性可以通过从富氮生长条件到富镓生长条件的变化来解释，同时随着氮气气压降低，镓靶的氮化镓中毒减少。在2.5毫托氮气分压下生长的纳米棒在约3.479电子伏特处表现出高强度的4K光致发光中性施主束缚激子跃迁（D⁰X）峰，半高宽为1.7毫电子伏特。高分辨率透射电子显微镜证实了纳米棒优异的晶体质量。
