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Review
. 2018 Sep 22;11(10):1800.
doi: 10.3390/ma11101800.

Hysteretic Photochromic Switching (HPS) in Doubly Doped GaN(Mg):Eu-A Summary of Recent Results

Paul R Edwards  1 , Kevin P O'Donnell  2 , Akhilesh K Singh  3   4 , Douglas Cameron  5 , Katharina Lorenz  6 , Mitsuo Yamaga  7 , Jacob H Leach  8 , Menno J Kappers  9 , Michal Boćkowski  10
Affiliations
Review

Hysteretic Photochromic Switching (HPS) in Doubly Doped GaN(Mg):Eu-A Summary of Recent Results

Paul R Edwards et al. Materials (Basel). .

Abstract

Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu⁺ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and Unipress Warsaw, discovered hysteretic photochromic switching (HPS) in the photoluminescence spectrum of doubly doped GaN(Mg):Eu. Our recent work, summarised in this contribution, has used time-, temperature- and light-induced changes in the Eu intra-4f shell emission spectrum to deduce the microscopic nature of the Mg-Eu defects that form in this material. As well as shedding light on the Mg acceptor in GaN, we propose a possible role for these emission centres in quantum information and computing.

Keywords: europium; gallium nitride; photochromism; photoluminescence; qubit; rare earth ions.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure 1
Figure 1
Ion implantation using multiple beam energies produces a top-hat dopant profile as shown in the above example, determined by Monte Carlo simulations using the SRIM code for Eu ions in GaN.
Figure 2
Figure 2
When excited with above-bandgap light at RT, spectrally pure GaN(Mg):Eu samples show only the Eu0 spectrum (reproduced from Ref. [9]).
Figure 3
Figure 3
Showing the two very different 5D0 to 7F1 spectral patterns for the photochromic pair Eu0 and Eu1(Mg), dominant at 100 K and 25 K respectively (reproduced from Ref. [9]).
Figure 4
Figure 4
Showing various hysteresis loops associated with the HPS of Eu0-Eu1(Mg) during a cooling-warming cycle. The mean signal intensities are plotted against temperature for cooling and warming runs, as indicated by arrows, undertaken at a fixed rate of 6 K per minute. Data are recorded at 1 K intervals (reproduced from Ref. [3]).
Figure 5
Figure 5
(a) Temperature dependence of switching at fixed excitation density and wavelength, the inset showing temperature independence below 20 K; (b) anti-Arrhenius fits to the temperature dependence above ~20 K show an independence of the activation energy on the power density (after Ref. [3]).
Figure 6
Figure 6
The transitions from the 5D1 states of both Eu0 and Eu1(Mg) configurations of the Eu-Mg defects can be observed at the appropriate temperatures, allowing for photochromic switching (reproduced from Ref. [6]). For an explanation, see text.
Figure 7
Figure 7
Temperature dependences of intensities of transitions as indicated (cooling run). Note the anti-correlation of Eu0 PL signals from the 5D1 and 5D0 states in the temperature range 200 K to 100 K (reproduced from Ref. [6]).
Figure 8
Figure 8
After prolonged exposure to UV light at low temperature, this ‘half-cooked’ sample displays the characteristic 620.9 nm line of the prime Eu2 centre, produced by unassociated substitutional Eu3+ ions [11].
Figure 9
Figure 9
Proposed microscopic model showing Eu sharing a nitrogen atom axially bonded to the Mg. The distortion of this arrangement due to the localization of a hole at this bond is also depicted (reproduced from Ref. [3]).
See this image and copyright information in PMC

References

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