Periodic Reporting for period 2 - SPRInG (Short Period Superlattices for Rational (In,Ga)N)
Reporting period: 2017-01-01 to 2018-12-31
Our data showed that the sub-ML (In,Ga)N under investigation act electronically as two-dimensional random alloys rather than ordered InGaN or InN ML islands. For studying the interwell coupling, we have grown superlattices with different barrier widths.Within the project “SPRInG”, we proposed to utilize strain engineering by choosing or creating a substrate with a lattice constant more favorable than the one of GaN for higher In incorporation in order to overcome the In content limitation. Two different approaches are pursued:
1. The growth of InN on relaxed (In,Ga)N buffer on GaN substrates. Heteroepitaxial deposition of In-rich (In,Ga)N layers directly on GaN is complicated by the presence of a large lattice mismatch. The latter resulted in very rough surfaces and defected layers. However in particular growth conditions we could achieve almost full relaxation of the (In,Ga)N buffer layer that had very high In contents and in addition, still exhibited PL emission.
2. The growth of InN on ZnO substrates of both polarities. ZnO is attractive as it is isomorphic to GaN and lattice-matched to In0.2Ga0.8N. However, we had to circumvent the high chemical reactivity of Ga with ZnO by growing a very thin coherent InN buffer layer between (In,Ga)N and ZnO.
Both strain engineering attempts do not enable to overcome the limitation of In incorporation either. However in that case, smooth (In,Ga)N SPSL structures with In content of 0.25 – 0.3 in single monolayer-thick QWs are obtained, which luminesce down to 2.95 eV (420 nm).
We implemented SPSLs as MQWs and studied their characteristics to allow a new design of the active structure of light emitting devices (LEDs). All devices successfully exhibit a room temperature electroluminescence around 420nm.
Within the SPRInG project the ESRs students share their work between two institutions, this enables them to gain exposure to both the academic and industrial sectors while growing and expanding their skill sets. For the ESRs the advantages turned out to be that they retain more talent because they foster different learning opportunities. They have got the chance to explore their interests and hidden potential. ESRs gain a fuller picture of how the business in both environments works. They are used to different attitudes, more business or more research driven, which benefits all involved ERSs for their future job seek.
Not only ESRs’ skill sets have been broadened, but also employee networks have been established, further preparing the ESRs for future job. ESRs learn the different working styles and cultures, as well, which encourages their collaboration spirit. They also provide back the institutions with novel capabilities.
Secondments of ESRs leaded at the beginning to the effect that the ESRs, in the need of learning more than in a single place, felt less productive in the short term as they changed. However, on the long terms they showed a very flexible working style and they adapted nicely to the steep learning curve.
The concept of student mobility is now better accepted as students understand that knowledge gained at any point of time never goes waste. Examples like the ESRs trained in such projects like SPRInG will strongly contribute to enhance the impact of such funding programs. Important is however the structure of support, such as training and support by the supervisors. They actually guide and help ESRs at every step, correct ESRs whenever they are going wrong and getting deviated from the actual purpose of the SPRInG project.