Hysteretic photochromic switching (HPS) of europium-magnesium defects in gallium nitride: a potential route to a new solid-state qubit

掺杂是指掺入选定的原子杂质，使材料表现得更好或不同。当中村开发出一种通过激活镁（Mg）原子来生产导电GaN的方法时，他延续了所有现代电子设备的基础传统。GaN的Mg掺杂允许为今天无处不在的“白色”发光二极管（LED）生产p-n结，并为中村赢得了2014年诺贝尔物理学奖。以同样的方式，氧化物磷光体的铕（Eu）掺杂在先前照明革命的“荧光”灯中提供了必要的红光发射。我们现在建议将Eu掺杂GaN的科学超越改善红色III族氮化物LED的有限目标。我们的目标是探索我们最近在共掺杂Eu和Mg的GaN中发现的滞后光致变色开关（HPS）的潜力，以形成新的固态量子比特或量子比特的基础。20世纪80年代后期进行的稀土（RE-）掺杂半导体的首次试验表明，具有更宽带隙的材料将显示出更好的高温性能，因此比传统的半导体如硅更有利于II-VI族材料和III族氮化物。然而，直到本世纪，在蓝宝石上生长的高质量外延薄膜III-N半导体才足以验证这一猜想;又过了十年，Fujiwara展示了基于GaN的LED，在生长过程中掺杂Eu（2010年）。在2001年至2011年的十年中，提出者及其同事对Eu掺杂方法进行了广泛的比较研究，结果表明，虽然如此厚的GaN：Eu样品可以产生更明亮的整体发射，但通过离子注入然后退火产生的材料实际上每个掺杂剂离子的效率更高，在低温下高达400倍。我们还表明，负责GaN：Eu红色LED发射的缺陷是“主要”缺陷，Eu 2，由Ga晶格位置上的孤立Eu离子组成。较常见的Eu 1缺陷具有更复杂的发射光谱，表明Eu原子受到晶格缺陷（例如空位或间隙原子）的扰动。在GaN：Eu文献中报道的这种复合中心的总数大于10。在试图通过在p型或n型GaN模板中注入Eu来进一步改善发光优势的同时，我们发现了GaN（Mg）：Eu：p型、Mg掺杂的GaN样品中的滞后光致变色开关（HPS），该样品用Eu离子注入并退火。HPS表现在光致发光光谱的温度依赖性上。在室温下，由于中心Eu 0，主要发射在619 nm处显示出尖锐的线。作为比较，Eu 1在622 nm处具有峰，Eu 2在621 nm处具有峰。在冷却样品时，Eu 0的强度增加，正如预期的那样，直到约230 K，当它出现饱和。低于30 K，我们观察到一个令人惊讶的快速下降的Eu 0的温度下降对基础温度的冷却系统。同时，在11 K下出现了类似Eu 1的光谱，并有效地取代了Eu 0。我们推断，Eu 0不知何故切换到Eu 1冷却在一个狭窄的温度范围内。如果温度随后从11 K升高到30 K，则这种切换不会反转。事实上，Eu 1的衰减相当缓慢，只有在~ 100 K以上才能重新出现Eu 0，这就是滞后现象。样品发射在约200 K时最大，然后在230 K和室温之间可逆地衰减。光致变色开关的发生在冷却时接近20 K，随后在升温时发光滞后，被称为首字母缩写HPS（滞后光致变色开关）。惊喜还在继续：对于在黑暗中冷却的样品，在时域中可以看到从Eu 0到Eu 1的切换;并且在Eu 0和Eu 1之间的中间波长处出现共振线。该项目的目的是确定共振是否是Eu 0和Eu 1的实际叠加，从而有望基于Mg受体缺陷的新型简单固态量子比特。

Doping is the incorporation of chosen atomic impurities to make a material behave better or differently. When Shuji Nakamura developed a method of producing electrically conducting GaN by activating magnesium (Mg) atoms, he continued a tradition fundamental to all modern electronic devices. Mg doping of GaN allowed production of p-n junctions for today's ubiquitous 'white' light-emitting diodes (LED) and won Nakamura a share in the 2014 Physics Nobel prize. In the same way, europium (Eu) doping of oxide phosphors provided the necessary red optical emission in the 'fluorescent' lamps of a previous lighting revolution. We now propose to take the science of Eu-doped GaN beyond the limited goal of improving red III-nitride LEDs. We aim to explore the potential of hysteretic photochromic switching (HPS), recently discovered by us in GaN co-doped with Eu and Mg, to form the basis of a new solid state qubit or quantum bit.First trials of rare earth (RE-) doped semiconductors, carried out in the late 1980's, suggested that materials with a wider band gap would show better high-temperature performance, thus favouring II-VI materials and III-nitrides over conventional semiconductors like silicon. However it was not until the present century that III-N semiconductors, grown as high-quality epitaxial thin films on sapphire, were good enough to test this conjecture; another decade passed before Fujiwara demonstrated an LED based on GaN doped with Eu during growth (2010). Extensive comparative studies of Eu doping methods by the proposer and coworkers in the decade 2001-2011 established that, while such thick GaN:Eu samples could produce brighter overall emission, material produced by ion implantation, followed by annealing, was actually more efficient per dopant ion, by up to 400 times at low temperatures. We also showed that the defect responsible for the GaN:Eu red LED emission was the 'prime' defect, Eu2, consisting of an isolated Eu ion on a Ga lattice site. The commoner Eu1 defect has a more complex emission spectrum, suggesting a Eu atom perturbed by a lattice defect, such as a vacancy or interstitial atom. The total number of such complex centres reported in the GaN:Eu literature is larger than 10. While attempting to improve the light emission advantage further by implanting Eu in p-type or n-type GaN templates, we discovered hysteretic photochromic switching (HPS) in GaN(Mg):Eu: p-type, Mg-doped GaN samples implanted with Eu ions and annealed. The HPS shows itself in the temperature dependence of the photoluminescence spectrum. At room temperature, the dominant emission, due to the centre Eu0, shows a sharp line at 619 nm. For comparison, Eu1 has a peak at 622 nm and Eu2 at 621 nm. On cooling the sample, the Eu0 intensity increases, as expected, until about 230 K, when it appears to saturate. Below 30 K, we observe a surprising rapid decline of Eu0 as the temperature decreases towards the base temperature of the cooling system. At the same time, an Eu1-like spectrum emerges and effectively replaces Eu0 at 11 K. We deduce that Eu0 somehow switches to Eu1 on cooling over a narrow temperature range. This switching does not reverse if the temperature is then increased from 11 K through 30 K. In fact, Eu1 fades rather slowly, allowing Eu0 to reappear only above ~ 100 K; this is hysteresis. Sample emission is maximum at about 200 K and then fades, reversibly, between 230 K and room temperature. The occurrence of photochromic switching near 20 K on cooldown followed by luminescence hysteresis on warming is given the acronym HPS (hysteretic photochromic switching). The surprises continue: for samples cooled in the dark, switching from Eu0 to Eu1 can be seen in the time domain; and a resonance line appears at an intermediate wavelength between Eu0 and Eu1. The proposed project aims to determine if the resonance is an actual superposition of Eu0 and Eu1, promising a novel and simple solid state qubit based on Mg acceptor defects.