In semiconductor industry, it always requires active nanoscale device materials with high carrier (i.e. electron and hole) mobility, which is similar to the fact that railway transport needs efficient high-speed trains. However, typical semiconductor materials, such as InP nanowires, usually have unavoidable crystal defects slowing down their electron mobility.  

This low electron mobility constrains the successful application of InP nanowires in high-performance field-effect transistors and photo-detectors, etc. Therefore, it is a great challenge to minimize the crystal defects in order to "pave a flat and straight railway" for high-speed electron transport. 

Recently, Prof. HAN Ning from the Institute of Process Engineering (IPE) of the Chinese Academy of Sciences and his collaborators demonstrated that single-crystalline nonpolar-oriented InP nanowires could be successfully obtained with the minimized crystal defects and high electron mobility. 

InP nanowires usually grow in the thermodynamically favorable orientations, such as <111> in cubic phase and <0001> in hexagonal phase. Since these two growth planes have similar atom alignments, a rotation of {111} plane atoms would easily change to {0001} plane atoms. Therefore, these nanowires usually have the inter-stacking atoms forming crystal faults, which limits their electron mobility < 1000 cm²/Vs.  

In the study, the obtained InP nanowires were uniquely stacked along the {2—110} crystal planes. In this plane, the atoms were strongly inter-bonded to the three nearest planes, prohibiting the possible rotation of the plane atoms.  

Importantly, there were limited stacking faults so that the electrons could exhibit a high mobility of 2000 cm²/Vs, approaching the theoretical limit. Due to the high electron mobility, a low subthreshold slope of down to 91 mV/dec was obtained for the fabricated transistors and a high photoresponsivity of 10⁴ A/W was obtained for the constructed photodetectors. 

Why do these InP nanowires grow in nonpolar orientation uniquely different from the others reported in the literature? The adopted Pd catalyst plays the key role here. The high melting point Pd will form a solid PdIn alloy for the InP nanowire growth.  

The preferred orientation of PdIn<210> will dictate the InP stacking planes of {2(—)110}, which would be called the catalyst epitaxy. Their findings were published online in ACS Nano. 

Catalyst epitaxy enables single-crystalline and high-electron-mobility InP nanowires (Image by HAN Ning)