Nanocomposites and nanomaterials

Metamorphic InAs/InGaAs Quantum Dot Structure with Additional GaAs Confining Layers: Photocurrent and Defect Level Study
Datsenko O.I.1, Golovynskyi S.2,3, Seravalli L.4, Trevisi G.4, Frigeri P.4, Li Baikui2, Qu Junle2
1 Department of Physics, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine.
E-mail: lesoto8g@gmail.com
2 Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, People’s Republic of China
3 Institute of Semiconductor Physics, NAS of Ukraine, Kyiv 03028, Ukraine.
4 Institute of Materials for Electronics and Magnetism, CNR-IMEM, I-43124 Parma, Italy.

Defect levels and their effect on the photoelectric properties were studied by in-plane photocurrent (PC), photoluminescence (PL) and thermally stimulated conductivity (TSC) spectroscopies in a metamorphic InAs/In0.15Ga0.85As quantum dot (QD) structure with GaAs additional barriers used to enhance the carrier confinement into QDs and, thus, luminescence efficiency, in order to optimize the performance of QD-based optoelectronic devices for long-wavelength emission.
The PC and TSC spectra of such structure and reference barrier-free sample show that point defects EL2, EL6, EL7, EL9 and EL10 and extended defects with activation energies of 0.43, 0.48 and 0.52-0.54 eV inherent in GaAs material and found in InAs/InGaAs nanostructures. Furthermore, a hole trap at (Ev + 0.55 eV) is found in the TSC spectra and identified as HL3 trap, observed earlier in GaAs and InAs/GaAs nanostructures, but not found yet in metamorphic ones. 
In comparison with the reference sample, the structure with the GaAs barriers, though having a lower PC signal, reveals lower dark current at room temperature, higher PL intensity and a lower response of the electron traps in the TSC spectra. 
The PC kinetics after the start of illumination is found to have a feature: a slow decay is observed after the expectable fast increase. We explain the effect by Coulomb screening of charges in the wetting layer (WL) by the charged electron traps located near QDs. As the WL acts as an in-plane conductivity channel, this screening inhibits the inflow of charge carriers from outside.