紫外单光子探测器中的超晶格单极雪崩机理研究

Semiconductor avalanche photodiode (APD) is an ideal device to detect the single-photon-level extremely-weak ultra-violet (UV) light. UV APD based on wide-gap semiconductors, such as SiC and GaN are promising in high gain, low dark current, high responsibility and high reliability, compared to those based on Si. However, the current wide-gap APDs are mostly based on the conventional p-i-n structure. Limited by the ionization coefficients, these APDs cannot be used to detect the single photon under linear-mode since the grain is small. When the gain is increased, both electron and hole can induce the avalanche, resulting in high excess noise. So the APD can only works on Geiger mode to detect the single photon, where the detection performance has limits. In this project, we propose a novel avalanche structure based on III-nitride semiconductor superlattices. Utilizing the large energy band offset between GaN and AlN, the ionization coefficients of electron and hole can be well controlled. In this case, it is possible to realize the high gain and the unipolar avalanche simultaneously, meeting to detect the UV single photon under linear mode. Through 4-years study, it is expected to know well the physical mechanism of carrier unipolar avalanche in superlattices with large band offset, and also demonstrate the real APD devices based on GaN/AlN superlattices, which has the gain of 1000000, ionization coefficient ratio less than 0.05, and single photon directivity of 30% under linear mode.

半导体雪崩探测器，是单光子级别微弱紫外光探测的理想器件。基于SiC、 GaN等第三代宽禁带半导体材料制作的紫外雪崩探测器相比传统Si雪崩探测器，具有增益高、暗电流低、紫外响应高、器件稳定性高等优势。但是目前的宽禁带半导体雪崩探测器大都基于传统 p-i-n 同质结结构，受限于载流子离化系数，线性模式工作时增益不够高，无法探测单光子；而当增益提高时，器件中电子和空穴双极雪崩严重，过剩噪声很大，只能工作于盖革模式，探测性能受限。针对此瓶颈，本项目提出基于氮化物超晶格的新型雪崩区结构，利用氮化物异质结带阶大的特点，调控电子和空穴的离化系数，同时实现电子单极雪崩和高增益，满足线性模式紫外单光子探测的要求。拟通过四年的研究，掌握高场强下大带阶超晶格中载流子单极雪崩倍增的物理规律，研制出GaN基超晶格的雪崩探测器原型器件，增益1000000，离化系数之比小于0.05，实现线性模式下单光子探测率30%。

传统的工作在盖革模式下的雪崩探测器，需要较为复杂的淬灭电路，这限制了阵列的设计与集成，实现高增益的线性雪崩探测器可以解决这一技术瓶颈。本文建立载流子在超晶格雪崩区中的输运模型，阐明各种散射机制所占的比重以及和材料结构、晶体质量、电场强度、环境温度的关系。从理论上建立载流子碰撞离化过程的物理模型，并考虑声子、晶格缺陷对碰撞过程的影响。给出优化结构参数的超晶格设计，获得较高的电子离化系数和较大的电子空穴离化系数之比。掌握高质量氮化物超晶格电子型分离吸收倍增单光子探测器件的制备与测试方法，实现线性模式氮化物超晶格单光子探测器，，实现1000000的高增益。线性高增益工作电场不高于3 MV/cm，探测器暗电流密度不高于0.2 A/cm2，温度相关系数低于0.1 V/K @ gain=1000，响应度不低于5 kA/W