Benefited from the series of excellent electrical and optical properties, graphene demonstrates attractive applications in high-performance photodetectors with the excellent broadband operation and ultra-fast response.
Especially, the graphene/silicon heterojunction Schottky photodiode exhibits the most promising applications to the graphene integrated silicon photonics, due to its prominent rectification behavior, low dark current, good stability and high photo-responsivity. Currently, graphene/silicon heterojunction photodetectors have achieved the comparable detecting ability to that of state-of-the-art silicon based devices, besides their superior advantages of low-cost and easy-integration with silicon technologies. However, in order to gain its practicality beyond the traditional silicon-based detectors, additional photo-gian mechanism, typically such as the tunneling induced impact ionization, is worth studying in depth for the further enhancement in responsivity and detectivity.
In a new paper published in Light Science & Application, a team of scientists from Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Pen-Tung Sah Institute of Micro-Nano Science and Technology, Department of Physics, Xiamen University, China, and co-workers have developed a novel engineered tunneling layer with enhanced impact ionization to realize the further photo-gain for detection improvement in the graphene/ insulator/silicon heterostructure photodetectors. Theoretical and experimental results indicate that the tunneling process generated novel impact ionization within the engineered AlN-insulating layer both for hot carries from graphene and minority carriers from silicon contribute to the obvious photo-gain. Based on this, a champion responsivity for the photodetectors reached a relatively stable value of ~1.03 A W-1 at a reverse bias of -10 V under the typical UV detection wavelength (365 nm), showing the great potential applications in sensing with 4.20 times enhancement comparing with the conventional graphene/silicon photodetectors and 7.16-times increment comparing with the typical commercial silicon PIN photodetectors. The significance of the present work is the development of a novel strategy achieve the synergistic effect for both obvious enhancements in responsivity and detectivity for graphene heterojunction photodetectors. Considering the low-cost, high performance and silicon integrability, of this kind of graphene/silicon tunneling heterojunction photodetectors shows great potential applications in communication and sensing.
"From the perspective of the band structure for the heterojunction, a wide band gap semiconductor material as a tunneling layer can achieve the effective suppression of dark current. However, the paradox is that a thicker tunneling layer thickness will greatly reduce the tunneling probability, while a too thin tunneling layer will inhibit the effective multiplication from impact ionization, which is not conducive to the improvement of responsivity. Thus, how to select a feasible tunneling layer, taking into account of the impact ionization efficiency and photo-carriers' transport, so as to achieve the above-mentioned application of killing two birds with one stone is still a big challenge." These scientists summarize the key issues to realize tunneling process enhanced multiplication in graphene heterojunction photodetectors.
"With considering the suitable band structure of the insulation material and the special defect states, the atomic layer deposition (ALD) prepared wide-bandgap insulating layer of AlN was introduced into the interface of graphene/silicon heterojunction. Thus, the promoted tunneling process benefited from the trap-assisted tunneling have well help the impact ionization with photogain not only for the regular minority carriers from silicon, but also for the novel hot carries from graphene." they added.
"This presented technique provide a new strategy of interface engineering which could realize the synergistic enhancement of responsivity and detectivity of graphene heterojunction photodetectors. Considering the simple process, significant performance improvement and silicon-based integration, this graphene/silicon tunnel heterojunction photodetector has shown great application potential in communications and smart sensing." the scientists forecast.