FisMat2017 - Submission - View

Abstract's title: Avalanche photodiodes based on GaAs/AlGaAs- the detectors for 4th generation light sources
Submitting author: Tereza Steinhartova
Affiliation: IOM CNR, Laboratorio TASC;Department of Physics, University of Trieste
Affiliation Address: Edificio MM, 34149 Trieste TS Piazzale Europa, 1, 34127 Trieste TS
Country: Italy
Oral presentation/Poster (Author's request): Poster
Other authors and affiliations: T. Steinhartov√°a,b, M. Antonellic, G. Cauteroc,d, K. Koshmaka, A. Gigliaa, S. Nannaronea, R.H. Menkc,d,e, F. Arfellib,c and G. Biasiola a IOM CNR, Laboratorio TASC, Area Science Park Basovizza, 34149 Trieste, Italy b Department of Physics, University of Trieste, 34128 Trieste, Italy c Elettra-Sincrotrone Trieste S.C.p.A, Area Science Park Basovizza, 34149 Trieste, Italy d Istituto Nazionale di Fisica Nucleare, INFN Sezione di Trieste, Trieste, 34100, Italy e Department of Medical Imaging, University of Saskatchewan, Saskatoon, SK S7N 5A2, Canada

Avalanche photodiodes (APDs) for X-ray detection have been traditionally based on silicon, as it is the most mature and prevalent technology [1]. With the advent of 4th generation light sources, more stringent requirements are put on APD detectors, which would benefit from a new choice of materials. GaAs-based compounds have been suggested [2]. One of the key points of AlxGa1-xAs -based materials is the higher atomic number compared to Si. Another important advantage is the material properties by changing the composition x in alloys AlxGa1-xAs. We are developing APDs with separated absorption and multiplication regions, as described in [3] utilizing molecular beam epitaxy. The absorption region of 4.5 µm of i-GaAs is separated from multiplication region by a highly p-GaAs layer, which ensures that after applying a reverse bias, the vast majority of the potential drop occurs in the multiplication region. This region profits from the mentioned possibility to tailor the band gap: thin layers of AlGaAs and GaAs alternate periodically in a so-called stair case structure [2] to create a periodic modulation of the band gap, which under bias enables well defined multiplication resulting in a lower noise of device. We tested several devices utilizing visible (2.2 eV laser), soft X-ray (500eV at BEAR beam-line of  Elettra synchrotron) and hard X-ray sources (8-12keV at SYRMEP beam-line and X-ray tube of synchrotron Elettra). With increasing reverse bias the devices are asymptotically reaching the full charge collection regime. With further increase and subsequent of bias electrons generated in absorption region are attaining sufficient energy within the multiplication region to trigger the charge multiplication process resulting in a signal increase. The gain is estimated by normalizing the signal to its asymptotic value in the full charge collection region. Depicted in fig. 1. are the measured gain curves versus the reverse bias for one of our APDs exposed to polychromatic radiation of a Cu anode X-ray tube (characteristic lines ~ 8 keV)  and monochromatic soft X-rays of 500 eV.  In the insert is a photo current map acquired by a mesh scan of an X-ray beam. In this contribution, device simulation, production steps and preliminary device characterization will be discussed.