Physics Seminar
III-nitride based nanowire photodetectors
Speaker: Dr Eva Monroy (Univ. Grenoble-Alpes, CEA-IRIG, PHELIQS)
Date: Wednesday 24 March 2021
Time: 15:00 in UK
Venue: Zoom
Nanowire photodetectors constitute a promising approach to circumventing the efficiency–speed trade-off. Their low electrical cross section implies low electrical capacitance, and this occurs without degradation of total optical absorption due to antenna effects. Another interesting feature of nanowire photodetectors is their compatibility with silicon, either as substrate for self-assembled growth or carrier wafer with simple transfer procedures, which should enable the monolithic integration of detector and readout. Particularly attractive is the possibility of growth or transfer to flexible materials, which opens up prospects for the development of wearable devices.
Although various designs have been considered as nanowire photodetectors, most works focus on the metal-semiconductor-metal architecture, with ohmic or Schottky contacts. Such devices present high gain, but they are strongly sublinear and their response time is generally in the millisecond range. Improvement is possible by generating an internal electric field in the structure, e.g. by inserting a heterostructure. In a single GaN nanowire UV photodetector, the insertion of a GaN/AlN heterostructure can lead to an increase of the responsivity by two orders of magnitude, improved linearity, and the possibility to select the detected wavelength, while maintaining a UV/visible contrast larger than six orders of magnitude. The linearity can be fully recovered if the nanowires are thin enough to be depleted due to the effect of surface states. On the other hand, the insertion of quantum dots in nanowires is also interesting for infrared photodetection using intraband transitions. Intraband transitions in GaN/AlGaN nanowire heterostructures have been investigated, varying the geometry and doping level of the GaN insertions to cover from near-infrared to mid-infrared (1.3 to 6 µm). Based on this research, we have presented the first single-nanowire quantum well infrared photodetector (NW-QWIP), operating at the 1.55 µm telecom band.
We have also explored the performance of single-GaN-nanowire photodetectors containing an axial p-n junction. To allow the use of the same metal scheme to contact both edges of the wires, the structure incorporates a p+/n+ tunnel junction to contact the p region of the diode. Single nanowire devices present the rectifying current-voltage characteristic of a p-n diode, but their photovoltaic response to ultraviolet radiation scales sublinearly with the incident optical power. This behavior is attributed to the dominant role of surface states. When the junction is reverse biased, the role of the surface becomes negligible in comparison to the drift of photogenerated carriers in the depletion region. Therefore, the responsivity increases by about three orders of magnitude and the photocurrent scales linearly with the excitation. Reverse-biased nanowires display decay times in the range of 10 µs. Their ultraviolet/visible contrast of several orders of magnitude is suitable for applications requiring high spectral selectivity.
Bio: Dr. Eva Monroy received her Master (1996) and Doctorate (2000) in Telecommunication Engineering at the Polytechnic University of Madrid (Spain). From 1999 to 2001 she worked as assistant professor at the Polytechnic University of Madrid. Since 2001 she is a senior researcher at the Interdisciplinary Research Institute of Grenoble (IRIG) of the French Commission for Atomic Energy and Alternative Energies (CEA). In 2004, she was Invited Professor during the spring semester at the University of Göttingen (Germany), and in 2005 she obtained her Habilitation as research supervisor (HDR) from the University of Grenoble. Her main research topic is the study of III-nitride materials and nanostructures, and their incorporation in novel optoelectronic devices. She obtained an ERC Starting Grant in 2011 and she received the Emmy Noether Distinction for Women in Physics by the European Physical Society in 2016. She is author or co-author of more than 300 publications in international journals.