Nanocomposites and nanomaterials

Deep Level Influence on Photoconductivity of Metamorphic InAs/InGaAs Quantum Dot Structures 
Golovynskyi S.1,2, Datsenko O.I.3, Seravalli L.4, Trevisi G.4, Frigeri P.4, Babichuk I.S.1,2, Golovynska I.1, Li Baikui1, Qu Junle1
1 Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, People’s Republic of China
E-mail: serge@szu.edu.cn
2 Institute of Semiconductor Physics, NAS of Ukraine, Kyiv 03028, Ukraine.
3 Department of Physics, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine.
4 Institute of Materials for Electronics and Magnetism, CNR-IMEM, I-43124 Parma, Italy.

Dynamics of near-IR-excited photocurrent in metamorphic InAs/InxGa1–xAs quantum dot (QD) structures with x = 0.15 and 0.31 were studied. After a fast increase in photoconductivity (PC) at the excitation start, the sample with x = 0.15 revealed a monotonous degradation of PC under stable excitation of QD ground states and buffer band-to-band. The degradation dynamics was found to be well fitted by allometric exponent ~ exp[–(t/)] with different fitting parameters depending on the excitation energy. We attribute the PC degradation to electron traps found near QDs by thermally stimulated current (TSC) spectroscopy. These traps, when being filled by electrons due to the sample illumination, are assumed to cause Coulomb screening of the Wetting Layer (WL) conductivity channel against the electrons coming from both the QDs and embedding InGaAs layers. On the other hand, the PC degradation effect in the structure with x = 0.31 is found to be less pronounced, showing a long-term increase kinetics. This might be due to a higher density of traps near the QD layer, which was revealed by TSC study. A significant fraction of these traps is suggested to be located not only around QDs and, therefore, these traps can lead to the usual deceleration of increase kinetics. With such a distribution of defects, WL Coulomb screening apparently is not effective.
Herein, this study of current dynamics shows the critical involvement of deep levels to the mechanism of PC.