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Energy losses in nitride light-emitting diodes

Periodic Reporting for period 1 - NITRIDE-SRH (Energy losses in nitride light-emitting diodes)

Reporting period: 2015-05-01 to 2017-01-31

Broadly, the project deals with the new generation of lighting sources, mainly light-emitting diodes (LEDs) based on the nitride family of semiconductors. More specifically, the focus of the project is non-radiative recombination in these devices. Despite huge recent breakthroughts, modern LEDs still do no reach their theoretical efficiencies, i.e. the fraction of electrical energy that is converted to light. A big part of this under-performance is due to energy losses in the material itself. The main purpose of Marie Sklodowska-Curie project is the theoretical investigation of defect-related nonradiative recombination (also called Shockley-Read-Hall recombination) in nitride (InGaN) LEDs.

Theoretical investigations are crucial to understand defect-related nonradiative recombination in nitride light-emitting diodes, as experimental unambiguous identification of defects is notoriously difficult. During the project we were able to identify microscopic mechanisms by which non-radiative recombination occurs in these materials. Specifically, we found out that gallium vacancy complexes and transition metal impurities act as very detrimental defects. The results will help guiding experimental efforts for improving device efficiencies and achieving better solid state lighting solutions.
The core methodology of theoretical investigations are first-principles calculations based on density functional theory (DFT). These calculations yield accurate defect formation energies and the position of defect levels within the band gap of the material. The information about formation energies and defect levels is then used to narrow-down the list of candidate defects that could contribute to Shockley-Read-Hall recombination.

Regarding the formation energy the requirement is that this formation energy is sufficiently low so that the considered defects occur in appreciable concentrations. If the formation energy is high, such defects are not expected to form, and thus they cannot be at the origin of the experimentally observed Shockley-Read-Hall recombination. Regarding the defect levels, the conventional wisdom tells that these levels be position at approximately the mid-gap region so that both electrons and holes are captured efficiently. This makes the defect a detrimental center at which Shockley-Read-Hall recombination is very efficient. However, for defects in pure GaN (band-gap 3.5 eV) there are no obvious candidates that fulfil both of these requirements. However, there were strong experimental proof that non-radiative recombination can be very efficient both in InGaN alloys with smaller band gaps as well as GaN itself. This made this project very challenging and prompted us to search for alternative mechanisms that could explain Shockley-Read-Hall recombination.

In this project we identified two different mechanisms that makes specific defects detrimental non-radiative recombination centers in nitrides. Importantly, it was discovered that the behaviour of point defects in InGaN alloys cannot be understood solely from their behaviour in pure GaN. There is a class of defects that in GaN have their defect levels in the lower part of the band gap, enabling efficient hole capture, but inefficient electron capture. Thus, these defects do not contribute to Shockley-Read-Hall recombination in gallium nitride. However, when the band gap of the nitride material decreases due to alloying with indium (as required for light emission in the visible range of the spectrum), defect levels moves towards mid-gap and turn into efficient non-radiative centers. This behaviour was found for certain gallium vacancy complexes and some more unexpected defects such as calcium. We have also found out that some defects posses electronically excited states, enabling efficient carrier capture even if the position of defect levels would indicate otherwise. This discovery is the most important fundamental result of the entire project. In InGaN specific gallium vacancy complexes turn into efficient recombination centers because of this mechanism. Even more unexpectedly, we have discovered that this is precisely the mechanism that involves excited states that turns Fe impurities in GaN into extremely detrimental non-radiative centers. The same mechanism is proposed to be operational for many other transition metal impurities in wide band-gap materials.

As of April 2017 the results have been published in six peer-review articles (two more articles reporting results of the Marie Sklodowska-Curie project are in preparation). The results of the project, especially the work on gallium vacancy complexes and iron in GaN, received numerous press releases worldwide (most importantly, in a monthly magazines Compound Semiconductors and Photonics Spectra, as well as news web-sites such as Phys.Org EurekAlert, Newswise, etc). The results have been also presented in international scientific conferences, such as the March meeting of the American Physical Society, the International Conference on Defects in Semiconductors, the Gordon Research Conference on Defects in Semiconductors, the International Workshop on Nitride Semiconductors, the Psi-k 2015 conference.
Progress beyond state-of-the-art achieved in this project falls into two categories: (i) new understanding achieved for nitride materials specifically and (ii) new discovered mechanisms of Shockley-Read-Hall recombinations that are applicable to a wider range of materials.

(I) New understanding of nitride materials. In the course of the project we have discovered a few classes of defects that are effective Shockley-Read-Hall recombination centers. They include gallium vacancies, gallium vacancy complexes with oxygen and carbon, as well as unexpected impurities such as iron calcium and iron. This the first time for nitrides that microscopic identification of nonradiative defect centers has been made. Nitride materials are at the core of the multi-billion LED market (around 30 billion US dollars in 2016), and therefore this microscopic identification of culprit defects is expected to have a very big impact in future improvements of solid state light emitters.

(ii) New mechanisms of Shockley-Read-Hall recombination. In retrospect, it is surprising that Shockley-Read-Hall recombination is wide-band-gap material such as GaN or InGaN alloys is present at all. For GaN with a band-gap of 3.5 eV and optical phonon frequency of 0.09 eV, about 38 phonons should be emitter during a non-radiative recombination cycle. We have found out that no simple mechanism could explain defect-assisted nonradiative recombination in GaN. Instead, we have discovered a new mechanism by which this recombination is predicted to happen not only in GaN, but in many other wide-band-gap materials as well. The mechanism involves electronically excited states of defects. Such mechanisms were identified for certain gallium vacancy complexes and iron impurities in GaN. In particular, transition metals that are known to possess a rich spectrum of electronic states are predicted to be extremely harmful nonradiative centers. This discovered mechanism is an important outcome of the fundamental research performed in this project.
Shockley-Read-Hall recombination in nitride light-emitting diodes. Courtesy of Peter Allen.