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High quality Material and intrinsic Properties of InN and indium rich Nitride Alloys - (The RAINBOW ITN)

Final Report Summary - RAINBOW (High quality Material and intrinsic Properties of InN and indium rich Nitride Alloys - The RAINBOW ITN)

Project context and objectives

The originality of the RAINBOW ITN is the combination of a concerted industrial and academic research effort to improve the quality of InN(Indium nitride)-based materials, for which many basic properties are still to be determined and understood. We rely on the planned strong interaction between the epitaxial growth teams (UPM, UW, TUB, AIXTRON, III-V Lab, EPFL), the experimental groups carrying out the layers characterisation (CIMAP, UNIBO, USTRA, TUIL, IFPAN, AALTO) and the theoretical groups (FSU, CIMAP, USTRA). The RAINBOW research programme contains several original approaches including in-situ monitoring of epitaxial growth for dislocation density reduction, the development of HVPE (hydride vapour phase epitaxy), the growth of InN-containing heterostructures, studies of surfaces and defects, and investigation of controlled p-type doping.

The key scientific and technological objective of the project is to address fundamental materials issues in order to accelerate the development of InN-based devices, exploiting areas where the partners are making significant and world-leading contributions. The research necessary to achieve device-quality material requires a combined experimental and theoretical effort with the following objectives:

a) growth of high-quality InN materials and its alloys by PAMBE (Plasma assisted molecular beam epitaxy), MOVPE (Metalorganic vapour phase epitaxy) and HVPE;% b) understanding the surface and interface properties of InN and its alloys;
c) investigate the defects and develop doping of InN and its alloys;
d) growth and properties of InN-based quantum wells and heterostructures;
e) training Eligible researchers (ER)/Early-stage researchers (ESR) in the new technologies.

In this large network made of 14 first level partners, with Lawrence Berkeley National Laboratory, USA, which has wished to join as an associated partner, there has been four years of intense exchanges and cooperation. An important ingredient if this success was that each partner has focused the training programme of their ESR/ER on the main topic of their research. This has been an important advantage of the RAINBOW collaboration, indeed the growers (TUB, AIXTRON, III-VLab, UPM, EPFL) were relying on the results obtained by characterisation (CIMAP, UNIBO, USTRA, UW, TUIL, IFPAN, AALTO,OVGU) and modelling (CIMAP, USTRA and FSU) in order to improve their processes. This has allowed the most efficient exchange of samples and results in this fast moving field of nitrides, with important results as shown below by the number of contributions at international conferences, published articles and invited talks.

In particular:

1. Optimised growth of InN layers was attained by molecular beam epitaxy (MBE) at UPM.
2. The HVPE technique was shown to reach high growth rates also for InN (>4 µm/h) around 500 °C in comparison to MBE, with reasonably flat surfaces although the layers quality need more work for optimization.
3. X-ray photoemission spectroscopy and secondary ion mass spectrometry have been used to show that the surface Fermi level decreases as the Mg concentration increases, with the sheet electron density falling to below 10exp8/cm-2.Moreover surface space-charge calculations indicate that the lowering of the surface Fermi level increases the density of unoccupied donor-type surface states and that these are largely compensated by Mg acceptors in the near-surface hole depletion region rather than by accumulated electrons. This is a significant step towards the realisation of InN-based optoelectronic devices (Linhart et al., Phys. Rev. Lett. published in Jan. 2013).
4. The extensive collaboration on indium rich alloys has been fruitful in the view of the common results published. In this instance a new understanding for the optimisation of thick InAlN (Indium aluminium nitride) and InGaN (Indium gallium nitride) layers revealed the need of much more fundamental research. The RAINBOW project has clearly shown that the conventional notions of critical thickness and strain relaxation may not be strictly followed.
5. In the thinnest layers of these alloys, it has been possible to attain optimised devices and in case of High-electron-mobility transistors to push further the state of the art.
6. For InGaN/GaN nanostructures, white light emission has been attained on patterned substrates.
7. One important point is that, in RAINBOW, the private and public institutions cooperation has operated at the most efficient level, with visits, fluent exchange of samples and results, leading to rapid publication of common results.
8. Interestingly, a patent application came in from one of the public institutions (Naresh Kumar et al., Phys. Rev. Lett. 108 135503 (2012), needless to mention that the knowledge gained by the two private companies will certainly lead to the production of further intellectual property.
9. It also worth noting that the two companies which have been partners have either already hired one of the ESRs (E. Sakalauskas at AIXTRON since March 2012), or have manifested a strong interest to hire their fellow (III-VLab: P. Gamarra).