Minority Carriers in Graphene/SiC Schottky Emitter Bipolar Phototransistors for High Gain Visible Blind UV Detection

这项拨款支持南卡罗来纳大学努力了解碳化硅（SiC）和外延石墨烯层之间形成的电子结的光学响应。特别是，pi已经在具有透明外延石墨烯（EG）发射器的器件中展示了具有高增益的紫外线（UV）光检测，该发射器生长在n型SiC衬底上的p型SiC衬底脱毛层上。紫外线（UV）检测是军事、工业、化学和生物应用的重要能力。然而，紫外线只占日光光谱的一小部分，可见光吸收可以很容易地压倒典型的紫外线信号，使得宽禁带半导体（如SiC）中固有的可见盲区，因此，如果可以识别具有高探测性和紫外线透明接触（如外延石墨烯）的架构，则紫外线探测器的理想质量。因此，该资助支持肖特基发射极双极光电晶体管（SEPT）器件的开发和研究，包括使用扫描光电流显微镜（SPCM）进行详细分析。这种器件依赖于肖特基基器件中少数载流子的高效注入，这是一种非常规的工艺，可以改变许多电子领域，从消费电子的柔性显示器到光电子，再到电力电子。例如，石墨烯-发射极双极晶体管将实现智能电网的高频、高功率、低损耗运行，其性能优于传统硅器件或最新的GaN或SiC器件，这些器件被认为是电力电子领域的黄金标准。肖特基少数载流子注入的物理学原理将对开发用于发光二极管（led）的空穴注入器产生革命性的影响，而发光二极管目前在GaN等材料中是一个挑战，可以实现低成本的固态照明或太阳能电池。此外，pi还致力于通过研究生培训和K-12外展来发展多样化的科学和工程劳动力。该项目将直接支持太阳能研究研讨会，并为来自南卡罗来纳州哥伦比亚地区历史悠久的黑人学院和大学的10名学生提供每年的奖学金，最终在由Co-PI组织的南加州大学可持续发展展示会上发表演讲。在大多数肖特基结中，热离子发射占主导地位，并且观察到少数载流子注入效率为20%，尽管肖特基电极提供了潜在的高速度，但对于高性能器件来说仍然不足。PI实验室在透明外延石墨烯(EG)/p-SiC肖特基界面上的结果表明，双极光电流增益双极光晶体管电流增益β 100响应365nm紫外线辐射，表明高效的少数注入（γ 95%）。假设这种行为的发生是由于i) EG/p-SiC的大肖特基势垒（2.7eV）大于许多材料的带隙，ii) SiC中少数载流子（电子）的迁移率与多数载流子（空穴）的迁移率之比很大。该项目的最终目标是使EG/SiC肖特基发射极光电晶体管在更低的电压（~10s V vs.300 V）下达到或超过UV雪崩光电二极管的性能~102-3A/W (365nm)，从而降低暗电流和噪声。pi将使用南卡罗来纳大学的SEPT设备，通过频率、时间、空间分辨光学测量以及温度相关的直流测量来询问肖特基结的少数载流子的输运。使用唯一的本地生长的肖特基界面EG/SiC，可以通过h插层和暴露于极性气体环境（如H2O， NO2和NH3）来系统地调整界面性质，pi将使用这些气体来控制少数载流子注入。他们还将研究在双极注入下层错形成的作用，这是一个关键的老化过程，以及层错如何决定器件的响应性、速度和可见排斥。最后，该项目将允许与海军研究实验室继续合作。

This grant supports the University of South Carolina in the effort to understand the optical response of electronic junctions formed between silicon carbide (SiC) and epitaxial graphene layers. In particular, the PIs have demonstrated ultraviolet (UV) photodetection with high gain in devices featuring a transparent epitaxial graphene (EG) emitter grown on a p-type SiC base epilayer on n-type SiC substrates. Ultraviolet (UV) detection is an important capability for military, industrial, chemical, and biological applications. However, UV makes up only a small portion of the daylight spectrum and visible light absorption can easily overwhelm the typical UV signal, making the inherent visible blindness found in wide-bandgap semiconductors (such as SiC) therefore a desirable quality for UV detectors if architectures with high detectivity and UV-transparent contacts (such as epitaxial graphene) can be identified. Accordingly this grant supports the development and study of Schottky-emitter bipolar phototransistor (SEPT) devices including detailed analysis using scanning photocurrent microscopy (SPCM). The devices rely on high-efficiency injection of minority carriers in Schottky-based devices, an unconventional process that could transform many fields of electronics from flexible displays for consumer electronics, to optoelectronics, to power electronics. For example, a graphene-emitter bipolar transistor will enable high frequency, high power, low loss operation of smartgrids, offering performance superior to that available from either traditional silicon devices, or the latest GaN or SiC devices, considered the gold-standards in power electronics. The physics of Schottky minority carrier injection would be transformative in developing hole-injectors for light-emitting diodes (LEDs), currently a challenge in materials such as GaN, enabling low-cost solid-state lighting, or in solar cells. The PIs are additionally committed to development of a diverse science and engineering workforce through graduate training and K-12 outreach. This project will directly support research workshops on solar energy and stipends for 10 students/year from historically Black colleges and universities in the Columbia, SC area, culminating in presentations at the USC Sustainability Showcase organized by Co-PI. In most Schottky junctions, thermionic emission dominates, and minority carrier injection efficiency gamma 20% is observed, insufficient for high performance devices despite the potential high speed that Schottky electrodes offer. Results at the PI's labs on the transparent epitaxial graphene (EG)/p-SiC Schottky interface have demonstrated bipolar photocurrent gain bipolar phototransistor current gain beta 100 in response to 365nm UV radiation, indicative of highly efficient minority injection (gamma 95%). This behavior is hypothesized to occur due to i) the large Schottky barrier of EG/p-SiC (2.7eV), larger than the bandgap of many materials and ii) the large ratio of the mobility of the minority carriers (electrons) to that of the majority carriers (holes) in SiC. The ultimate goal of this project is to make EG/SiC Schottky emitter phototransistors that approach or beat UV avalanche photodiode performance ~102-3A/W (365nm) at much lower voltages (~10s V vs.300 V), leading to lower dark current and noise. The PIs will use SEPT devices formed at the University of South Carolina to interrogate the transport of minority carriers at Schottky junctions using frequency, time, spatially resolved optical measurements, as well as temperature dependent DC measurements. The use of the only natively grown Schottky interface EG/SiC enables systematic tuning of the interfacial properties using H-intercalation, and by exposure to polar gas ambients such as H2O, NO2 and NH3, which the PIs will use to control minority carrier injection. They will also investigate the role of stacking fault formation under bipolar injection, a key aging process, as well as how stacking faults determine the responsivity, speed, and visible rejection of the devices. Finally, the project will permit continued collaboration with the Naval Research Laboratory.