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

这笔赠款支持南卡罗来纳大学努力了解碳化硅(SIC)和外延石墨烯之间形成的电子结的光学响应。特别是，PI在n型碳化硅衬底上生长的p型碳化硅衬底上生长了透明的外延石墨烯(EG)发射体，在器件中展示了高增益的紫外光检测。紫外线(UV)探测是军事、工业、化学和生物应用的一项重要能力。然而，紫外光只占日光光谱的一小部分，而且可见光吸收很容易淹没典型的紫外光信号，使得宽带隙半导体(如碳化硅)固有的可见光盲性成为紫外光探测器的理想品质，如果能够识别出具有高探测率和紫外光透明接触的结构(如外延石墨烯)。因此，这笔赠款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。这些器件依赖于在基于肖特基的器件中高效注入少数载流子，这是一种非常规工艺，可能会改变许多电子领域，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，提供优于传统硅器件或被视为电力电子黄金标准的最新GaN或SiC器件的性能。肖特基少数载流子注入的物理学将在开发发光二极管(LED)的空穴注入器方面产生革命性的影响，目前这一技术在GaN等材料中面临挑战，能够实现低成本的固态照明，或者在太阳能电池中。PIS还致力于通过研究生培训和K-12外联培养多样化的科学和工程劳动力。该项目将直接支持南加州哥伦比亚市历史悠久的黑人学院和大学每年为10名学生举办的太阳能研究研讨会和津贴，最终将在由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，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，但仍不足以满足高性能器件的要求。在PI的透明外延石墨烯(EG)/p-SiC肖特基界面上的实验结果表明，在365 nm的紫外光照射下，双极光电晶体管电流增益β100，表明高效率的少量注入(伽马95%)。这种现象的产生是由于：1)EG/p-SiC的大肖特基势垒(2.7 eV)，大于许多材料的禁带宽度；2)碳化硅中少数载流子(电子)和多数载流子(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接近或超过UV雪崩光电二极管，在更低的电压(~10S V对300 V)下表现出~102-3A/W(365 Nm)的性能，从而导致更低的暗电流和噪声。PI将使用南卡罗来纳大学形成的SEPT设备，通过频率、时间、空间分辨率光学测量以及与温度相关的直流测量来询问肖特基结上少数载流子的传输。使用唯一自然生长的肖特基界面EG/SIC，可以通过H插层和暴露在极性气体环境中(如H2O、NO2和NH3)来系统地调节界面属性，PI将使用这些环境来控制少数载流子注入。他们还将研究双极注入下堆叠故障形成的作用，这是一个关键的老化过程，以及堆叠故障如何决定器件的响应性、速度和可见拒绝。最后，该项目将允许继续与海军研究实验室合作。

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.