Speaker
Description
Gallium Arsenide (GaAs) has increasingly become a material of choice for radiation detection, effectively bridging the gap between Silicon and Cadmium Telluride-based detectors. Its rising prominence is attributed to significant advancements in crystal doping and growth techniques, which have directly influenced the detectors' performance and efficiency. To align with the trend of crystal doping via Chromium (Cr) compensation and new annealing approaches, transport properties of the material like charge transport dynamics have to be thoroughly studied.
We have investigated the dynamics of charge carriers within GaAs:Cr planar diodes under the influence of femtosecond red laser pulses. The diodes with sizes $5 \times 5 \times 0.5 \, \text{mm}^3$ were metallized with Cr on both sides with metal thicknesses of 25-50 nm. By applying different polarities of direct current (DC) biases and adjusting the intensity of laser pulses, we were able to simulate various electric field scenarios within the sensors, enabling a detailed examination of charge carrier behavior under conditions that mimic real-world applications. The experimental methodology was also extended to explore the thermal effects on charge carrier mobility, drift times, and lifetimes over a temperature range of 5 - 25$^\circ$C. Using a well-focused, collimated laser beam, we scanned the entire metallized area. This allowed us to reconstruct two-dimensional images of charge distribution, foreshadowing differences in electric field distribution under different operating modes. We found that electron mobility was in the range of 4000-4300 $\text{cm}^2/\text{V$\cdot$s}$, while hole mobility was around 750-900 $\text{cm}^2/\text{V$\cdot$s}$. From the full charge collection, the Hecht equation was utilized to effectively reconstruct the $\mu \tau$ product for both voltage polarities, providing a robust approach for understanding the interaction dynamics of the charge polarity that was drifting through the entire bulk length. With this method, we estimated that the electron mobility lifetime $\mu_e \tau_e$ for each planar detector was in the range of 1-4.5 $ \times 10^{-4}$ $\text{cm}^2/\text{V}$. Different increased photon fluxes were also studied that allowed us to find differences in the charge collection at high and low laser intensities. At lower intensities, the role of surface and bulk recombination centers became more pronounced relative to the overall carrier dynamics. We observed that the carriers were trapped or recombined before they could contribute to the total current, thereby delaying the onset of collection, and as a result, a longer signal rise time was measured.
This study enriches existing research [1]-[3] by providing additional insights into the space charge morphological distribution within GaAs:Cr sensors. Our findings highlight how the varying charge excitation and transport conditions affect space charge behavior, thereby providing information on charge transport dynamics and space charge formation.
References
[1] D. Greiffenberg, M. Andrä, R. Barten, A. Bergamaschi, M. Brückner, P. Busca, S. Chiriotti, I. Chsherbakov, R. Dinapoli, P. Fajardo, E. Fröjdh, S. Hasanaj, P. Kozlowski, C. Lopez Cuenca, A. Lozinskaya, M. Meyer, D. Mezza, A. Mozzanica, S. Redford, M. Ruat, C. Ruder, B. Schmitt, D. Thattil, G. Tinti, O. Tolbanov, A. Tyazhev, S. Vetter, A. Zarubin, J. Zhang, ``Characterization of chromium compensated GaAs sensors with the charge-integrating JUNGFRAU readout chip by means of a highly collimated pencil beam'', \textit{Sensors}, vol. 21, no. 4, 1550-1-22, 2021, doi:10.3390/s21041550.
[2] E. Belas, R. Grill, J. Pipek, P. Praus, J. Bok, A. Musiienko, P. Moravec, O. Tolbanov, A.V. Tyazhev, A. Zarubin, ``Space charge formation in chromium compensated GaAs radiation detectors'', \textit{Journal of Physics D: Applied Physics}, vol. 53, no. 47, 475102-1-8, 2020, doi: 10.1088/1361-6463/aba570.
[3] H.T. Philipp, M.W. Tate, K.S. Shanks, P. Purohit, S.M. Gruner, “Practical considerations for high-speed X-ray pixel array detectors and X-ray sensing materials”, Nuclear Instruments and Methods in Physics Research A, vol. 925, 18-23, 2019, doi: 10.1016/j.nima.2019.01.066.
