Effect of carrier capture by deep levels on lateral photoconductivity of InGaAs/GaAs quantum dot structures

Abstract

Having used thermally stimulated conductivity (TSC) technique, we identified deep electron traps that produce strong effects on charge carrier transport and photoconductivity in InGaAs/GaAs quantum dot (QD) structures. The values of deep levels below the conduction band of GaAs at 0.16, 0.22, and 0.35 eV are obtained from the analysis of the shapes of TSC curves after the excitation with the quanta energy hv = 0.9, 1.2, and 1.6 eV. The level 0.16 eV in depth is an effective electron trap that provides crossing of lateral conductivity with a high-resistance mode and, therefore, causes a high photocurrent sensitivity of about 3 A/W at 77 K with excitation by interband transitions in QDs. We determined the charge density of electrons captured by the (Ec – 0.16 eV) level to be 2 × 10−6 C/cm2 at 77 K that induces electric field ∼ 105 V/cm in a vicinity of QDs. The state at Ec – 0.22 eV is shown to be related to the recombination center that can hold non-equilibrium holes over a long time under the condition that the non-equilibrium holes are localized by the quantum states of QDs. In the course of long-term electron storage in a vicinity of QDs, an electron trapped at the (Ec – 0.16) eV level can be recaptured by a deeper spatially remote (Ec – 0.22 eV) level that allows the TSC peak observation at 106 K.

Deep level centers and their role in photoconductivity transients of InGaAs/GaAs quantum dot chains

“…As a result, in order for us to estimate the trap energy, we use the simple formula e a ¼ 23kT m . 37,38 These values of e a were found to be 0.16 6 0.02 eV, 0.18 6 0.02 eV, 0.22 6 0.02 eV, and 0.34 6 0.02 eV, which are in good agreement with the previous values observed for InGaAs/GaAs heterostructures. 17,37 Finally, if the sample is exposed to lower energy light in the "quenching" range, hv ¼ 0.83-1.0 eV, after cool down, we find that thermal stimulation results in the conductivity slowly recovering to the room temperature value throughout the temperature range.…”

Photoelectric properties of the metamorphic InAs/InGaAs quantum dot structure at room temperature

“…Considering the data about intrinsic and crystal defects in In(Ga)As/GaAs heterostructures obtained from deep level transient spectroscopy, 23,24 PC spectroscopy, and thermally stimulated current, 16,17,25 we attribute the onset of the absorption spectrum at 0.68 eV to a transition from a EL2 defect center in the InGaAs/GaAs layer where defect concentration is high. 26,27 It is known that in semi-insulating GaAs, the level of EL2 center is located near 0.75 eV below the conduction band, 28 but in strained InGaAs the energy of transition involving EL2 defect states is reduced.…”

“…As for the InGaAs defects, they are usually not photoactive but affect the photoelectrical properties of such heterostructures by trapping the nonequilibrium charge carriers 15,16,25 and, hence, reducing the radiative recombination efficiency: for this reason, additional experiments are necessary to study InGaAs defects.…”

“…4,6,13 Photocurrent or photoconductivity (PC) is the ideal tool to study the electrical response as a function of photon energy for information about interband transitions, carrier transport, and other mechanisms of nonequilibrium processes. 15,16 Metamorphic QDs are complex systems, as their energy system is composed by levels associated to InAs QDs, InAs wetting layer (WL), InGaAs MB, and GaAs substrate. Depending on occupation of these levels in the equilibrium state, i.e., the localization of the Fermi level, electron transitions at different energies are possible.…”

Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures

“…Thereby, the photoelectrons with energy are higher than mobility edge move in GaAs surroundings of QWRs or WL, with inhomogeneous, local electric fields having effect on thermalization and recombination processes [35]. Besides centers of electron-hole recombination in the InGaAs, WL, or GaAs due to Shockley-Read-Hall process, there are slow traps with variety of activation energies and emission/capture rates, which exchange electrons only with conduction band and increase their lifetimes [36,37]. The electrons, whose local environment permits their motion, have the possibility to find themselves localized at some traps until they escape, thus lengthening their contribution to the photocurrent.…”