As quantum computing, communication, and sensing play increasingly important roles in the coming decades, and the demands for high sensitivity imaging grow rapidly, the sensitivity of photodetectors has become a main concern in several key technology sectors. This thesis reports two novel light detection and internal amplification mechanisms that enable photodiodes to achieve desired characteristics such as high detection efficiency, single photon sensitivity, low noise and high speed.
Cycling excitation process (CEP) is an intrinsic signal amplification mechanism that was firstly discovered in a heavily-doped compensated Si p/n junction device. By relaxing the k-selection rule (i.e. conservation of momentum), CEP detectors possess high gain, high efficiency, and ultralow noise. Above all, CEP detectors can be made from disordered materials such as amorphous silicon (a-Si), which allows low cost manufacturing and scalability to large array size for intended applications. An a-Si photodiode is demonstrated with ultra-high gain-bandwidth product of 2.25 THz and low noise, based on a very simple structure.
Dark current is a key challenge for photodiodes to detect extremely low power such as single photon as it can produce shot noise. A proper designed based on band gap engineering is shown to reduce the dark current of a-Si CEP detectors. The result of temperature dependent dark current measurement conveys the message that the dark current mechanism of a-Si CEP detectors is indirect tunneling followed by Poole-Frenkel effect. The key part is how to block the electron tunneling without affecting the device photo response. Cupric oxide (Cu2O) is an intrinsic p-type semiconductor material, with electron affinity of -3.2 eV and band gap of 2.1 eV. By inserting a thin layer of Cu2O between the a-Si and top electrode, the new structure has a much large electron tunneling barrier. On the other hand, the photogenerated holes can move as before without any additional hole blocking barrier. The measured results support that Cu2O based dark current reduction CEP devices have at least 1-2 orders of magnitude lower dark current at reverse bias 5 V compared to the typical a-Si CEP detectors while the photo responsivity remain the same.
Realizing that CEP effect occurs in a thin layer of a-Si, we designed an a-Si CEP detector without any semiconductor substrate. The key challenges are to improve the absorption efficiency and frequency response. Utilizing the localized surface plasmon resonance (LSPR) effect incorporated with CEP effect, a plasmonically enhanced a-Si detector achieves high external quantum efficiency with a record fast impulse response of 170 ps (FWHM). This approach raises the possibility of making detectors out of amorphous materials for high frame rate imaging and optical communications in spite of the low carrier mobilities in the materials.
The second optical signal amplification mechanism was observed in organometallic perovskite based detectors when the input power was down to a few or even single photon. It is a quasi-persistent photo response which takes tens of seconds for the current level to increase and reach to saturation after the absorption of a single photon. Based on the observation, we proposed an internal amplification mechanism, ionic impact ionization (I3), to elucidate the phenomenon, which involves a cascade process of ion migration.
