We studied on the growth of N-polarity InN/AlInN MQWs by radio frequency plasma-assisted MBE. We found that the growth temperature greatly affected the structural properties of InN/AlInN MQWs. The growth temperature for achieving fine periodic MQWs was in the range from 550 to 580 o C. By optimizing the growth conditions, we for the first time successfully fabricated InN-based MQWs with Al x In 1-x N (~0.2 < x < ~0.3) barrier layers. Clear satellite peaks up to the 3 rd order in high resolution X-ray diffraction were observed, indicating that being fine periodic MQWs-structures with fairly flat and sharp interfaces. Photoluminescence peaks ranging from 0.68 to 0.99 eV were observed depending on the well thickness and they were in good agreement with those estimated by simple theoretical calculation.1 Introduction It is expected that InN-based MQWs structures are applicable for photonic devices operating in optical communication wavelengths [1][2][3]. One of them is the ultra-high speed optical modulator with utilizing the ultra high-speed inter-subbands electron transition (ISBT). The energy relaxation rates for excited electrons in the InN-based MQWs are expected to be about 10 times faster than those for GaAs-based systems [4,5]. For the formation of higher energy levels in the conduction band of MQWs, the presence of large conduction band offset between the well and barrier is necessary. If we consider using GaN as the barrier layer, it would be able to get such large conduction band offset as 1.8 eV [6]. However, it would be difficult to construct high structural quality InN/GaN MQWs, because the lattice mismatch between InN and GaN is as large as 11% [7].On the other hand, AlInN ternary alloys are essentially more promising materials as the barrier layer than both GaN and InGaN, because the lattice mismatch can be remarkably decreased with keeping the conduction band offset larger than those when using GaN and InGaN as the barrier [6].We already studied on the epitaxy of AlInN ternary alloys by RF-MBE and succeeded in growing AlInN ternary alloys without any apparent phase separation in the whole composition range by optimizing the growth conditions. Further it was found that the bowing parameter of the energy bandgap E g for AlInN ternary alloys was about 4.78 ± 0.3 eV, where E g 's of InN and AlN were 0.64 and 6.14 eV, respectively [8]. Then we have expected that the Al composition x in Al x In 1-x N barrier for the InN well with keeping the same conduction band offset as that of InN/GaN MQWs is about 0.68. In this case the lattice mismatch in InN/Al 0.68 In 0.32 N MQWs is about 9%, i.e., the degree of the lattice mismatch is reduced to be about 80% compared with that of InN/GaN.
