| AVS 59th Annual International Symposium and Exhibition | |
| Electronic Materials and Processing | Thursday Sessions |
| Session EM+TF+AS-ThA |
| Session: | Growth and Characterization of Group III-Nitride Materials |
| Presenter: | A. Yoshikawa, Chiba University, Japan |
| Authors: | A. Yoshikawa, Chiba University, Japan K. Kusakabe, Chiba University, Japan N. Hashimoto, Chiba University, Japan T. Okuda, Chiba University, Japan T. Itoi, Chiba University, Japan |
| Correspondent: | Click to Email |
We have recently proposed “SMART” III-N tandem solar cells in which all sub-cells could be coherent-structure high-quality pn junctions with low leakage current, resulting in high performance solar cells. SMART means “Superstructure Magic Alloys fabricated at Raised Temperature”. The most important feature in the proposed SMART solar cell is a novel idea for realizing ordered and/or quasi InGaN-ternary alloys with InN/GaN Short-Period Superlattices (SPS) enabling coherent-structure band engineering for the (InN)n/(GaN)m SPSs with simple integer pairs of (n, m)≤4. In this symposium, detailed epitaxy processes, structural and physical properties of SPSs, and also the idea and features of proposed “SMART” III-N tandem solar cells are reported.
We have ever reported successful growth of fine and coherent-structure 1-ML InN/GaN matrix QWs, and they can be fabricated so under self-limiting and self-ordering growth processes at remarkably higher and/or “raised” temperatures (~650 °C) than the critical one (~500 °C) for growing thick InN layer under +c growth regime in MBE. We are now underway to extend this understanding and the corresponding epitaxy technology to realize the proposed (InN)n/(GaN)m SPSs, and we have started to achieve (InN)1/(GaN)m (m=1-20) SPSs. When fabricating high structural quality those SPSs, very careful surface stoichiometry control such as (In+Ga)/N and In/Ga composition in adlayers, and also periodical complete surface dry-up of In and Ga for each one-cycle growth of SPSs are necessary and quite important.
In brief, 50-100 periods of (InN)1/(GaN)m SPSs were grown on MOCVD-grown +c-GaN template at 650 °C by a conventional plasma-assisted MBE. Surface stoichiometry and surface dry up were quite carefully monitored and controlled by in-situ Spectroscopic-Ellipsometry. First, structural properties of 50 periods of (InN)1/(GaN)m SPSs were characterized with XRD diffraction patterns taking the m as a parameter. It was found that coherent structure SPSs could be fairly easily fabricated even when the m was decreased down to 4. Generally, much more careful surface stoichiometry control was necessary with decreasing the m, though it was confirmed coherent structure (InN)1/(GaN)4 SPSs could be grown finally after quite careful control, such as selective re-evaporation between In and Ga consuming a long time. This leads to complete In re-evaporation leaving only some Ga metals on the surface. Of course those Ga metals must be completely dried up with irradiating plasma-excited nitrogen just before the following deposition of 1ML InN on it. It is still difficult at present, however, to grow fine structure InN/GaN SPSs with the m below 3.