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

掺杂是掺入选定的原子杂质，使材料性能更好或不同。中村修二（Shuji Nakamura）发明了一种通过激活镁（Mg）原子来生产导电GaN的方法，他延续了所有现代电子设备的基本传统。Mg掺杂氮化镓使得如今无处不在的“白色”发光二极管（LED）的p-n结得以生产，并为中村赢得了2014年的诺贝尔物理学奖。以同样的方式，铕（Eu）氧化物荧光粉的掺杂为之前的照明革命中的“荧光灯”提供了必要的红色光学发射。我们现在建议将eu掺杂GaN的科学超越改进红色iii -氮化led的有限目标。我们的目标是探索我们最近在与Eu和Mg共掺杂的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年代末进行的稀土（RE-）掺杂半导体的首次试验表明，带隙更宽的材料具有更好的高温性能，因此比硅等传统半导体更有利于II-VI材料和iii -氮化物。然而，直到本世纪，在蓝宝石上生长成高质量外延薄膜的III-N半导体才足够好地验证这一猜想；又过了十年，藤原在生长过程中展示了基于掺有Eu的GaN的LED（2010）。作者及其同事在2001-2011年间对Eu掺杂方法进行了广泛的比较研究，结果表明，尽管如此厚的GaN:Eu样品可以产生更亮的总发光，但通过离子注入然后退火产生的材料实际上在低温下每个掺杂离子的效率更高，最高可达400倍。我们还发现，导致GaN:Eu红色LED发光的缺陷是“素数”缺陷Eu2，由Ga晶格位点上孤立的Eu离子组成。普通的Eu1缺陷具有更复杂的发射光谱，表明Eu原子受到晶格缺陷（如空位或间隙原子）的扰动。欧盟文献中报道的这种复杂中心的总数超过10个。在试图通过在p型或n型GaN模板中植入Eu进一步提高发光优势的同时，我们发现了在GaN(Mg):Eu: p型、Mg掺杂GaN样品中植入Eu离子并进行退火的滞后光致变色开关（HPS）。HPS在光致发光光谱的温度依赖性中表现出来。在室温下，由于中心的Eu0，主发射在619 nm处显示出一条尖锐的线。相比之下，Eu1在622 nm处有峰，Eu2在621 nm处有峰。在冷却样品时，Eu0强度增加，正如预期的那样，直到大约230k，当它看起来饱和时。在30 K以下，随着温度向冷却系统的基础温度下降，我们观察到Eu0惊人的快速下降。同时，在11k处出现了类似eu1的光谱，有效地取代了Eu0。我们推断在一个狭窄的温度范围内冷却时，Eu0以某种方式转变为Eu1。如果温度从11 K增加到30 K，这种开关不会逆转。事实上，Eu1的衰减相当缓慢，只允许Eu0在~ 100k以上重新出现；这就是滞后。样品发射在200 K左右达到最大值，然后在230 K和室温之间可逆地减弱。在冷却时在20k附近发生的光致变色开关以及升温时的发光滞后现象被称为HPS （hysteretic photochromic switching）。令人惊讶的是：对于在黑暗中冷却的样品，可以在时域上看到从Eu0到Eu1的转换；在Eu0和Eu1之间的中间波长处出现共振线。提出的项目旨在确定共振是否是Eu0和Eu1的实际叠加，有望基于Mg受体缺陷的新颖而简单的固态量子比特。

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.