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Short Period Superlattices for Rational (In,Ga)N

Periodic Reporting for period 2 - SPRInG (Short Period Superlattices for Rational (In,Ga)N)

Reporting period: 2017-01-01 to 2018-12-31

The scientific mission of this project is to develop a novel (In,Ga)N alloy that offers completely new opportunities to tune bandgaps and piezoelectric fields in quantum structures for highly efficient optoelectronic devices. This novel “rational” (n InN/m GaN) alloy is based on short period superlattices that stack integer numbers of m (n) monolayers (MLs) of InN (GaN), i.e. heterostructures of pure InN MLs embedded in a GaN matrix. The development of fundamental knowledge on this rational (InN/GaN) alloy as established for other III-V compounds is envisioned as the base for such devices. To achieve this objective, we Forschungsverbund Berlin (FVB), as academic institution, and TopGaN sp.z.o.o. (TopGaN), as non-academic partner, have concerted our action in a bilateral collaboration. Besides, the Humboldt-Universität zu Berlin (HU) and the Institute of High- Pressure Physics – Polish Academy of Sciences (UNIPRESS) are also involved as external partner organizations to provide the academic structure necessary to the promotion of the early-stage researchers (ESRs) hired by the project. As a specific strength, within the project “Short Period Superlattices for Rational (In,Ga)N” (SPRInG) we are unifying a unique portfolio of experimental competencies with high-level resources and infrastructures. These create an ideal research environment that promotes an interdisciplinary collaboration and an international networking of ESRs. Within such a framework, the ESRs are developing professional qualification to to solve challenging and very exciting scientific questions of the potential III-nitride future technology for solid state lighting. A quality monitoring scheme ensures that the ESRs receive an optimized training, which is preparing and qualifying them for the research and development of future technologies in academic and non-academic organizations.
In framework of the present project “SPRInG” [Short Period superlattices (SPSLs) for Rational (In,Ga)N] we fabricated and characterized rational (In,Ga)N in the form of SPSLs. We reported on the study of the In content in ultra-thin QWs using several methods. We demonstrated that even slight modifications of the growth parameters are associated with significant changes in the resulting structures. During the first year we showed that although our samples (prepared using the growth sequences reported in literature) emit in the same range as the one formerly reported they are actually constituted by 3 to 10 MLs of (In,Ga)N with In content of only about 10%, instead of 1 single ML of pure InN. After improvement of the growth conditions, we obtain SPSLs with abrupt Interfaces with (In,Ga)N of 1 ML thickness and an In content of 0.29 and GaN QBs as thin as 6 MLs, that give rise to a PL band at about 3.16 eV coinciding with the theoretically predicted values.
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.
Transparency Market Research, in its report 'GaN Industrial Devices Market - Global Industry Analysis, Size, Share, Growth, Trends and Forecast 2015 – 2021', forecasts the global GaN industrial devices market rising at a compound annual growth rate (CAGR) of 15.1% from $482 millions in 2014 to $1315 millions in 2021. The market’s growth trend expected at the time of SPRInG submission is thus nowadays confirmed. The key drivers of this market are still on the one hand the development of new technologies and on the other the increasing application scope of the products. In this respect the digital (Ga,In)N/GaN alloys could bring a huge improvement in the actual optoelectronic technology, particularly when implemented in LDs as electron coolers. The global semiconductor laser market size is in fact expected to reach $9.52 billions by 2024, according to a new report by Grand View Research, Inc.. Moreover the emergence of new advantageous digital alloys may also be of interest for other electronic application areas.
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.