Picosecond kinetics of the electron‐hole layers formation in wide‐bandgap II‐VI type‐II heterostructures

Abstract

Considerable slowdown of luminescence kinetics of the direct optical transition was discovered in ZnSe/BeTe type‐II heterostructures under high levels of optical pumping. The effect is attributed to forming of a potential barrier for holes in the ZnSe layer due to band bending at high densities of spatially separated carriers. That results in a longer time of the photoexcited holes energy relaxation to their ground state in the BeTe layer. The decrease of overlapping of electron and hole wavefunctions in the ZnSe layer in thick ZnSe/BeTe structures at high levels of optical excitation reveals an additional important effect, that leads to sufficient retardation of radiative recombination time for photoexcited carriers (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the external electric field on the kinetics of recombination of photoexcited carriers in a ZnSe/BeTe type II heterostructure

“…The optical pumping density cor responded to the photoexcited carrier density n ≈ 7 × 10 9 cm -2 in one ZnSe layer, which is close to the flat band case [11]. Figure 3 shows the measured lifetime of the direct transition in ZnSe/BeTe as a function of the field strength inside the structure.…”

“…The time of the escape of the hole to the BeTe layer depends on the thicknesses of the layers of the heterostructure and varies from τ rel < 2 ps to τ rel Ӎ 20 ps for ZnSe/BeTe structures with the thicknesses of the ZnSe layer from 10 to 20 nm, respectively [2,11]. Thus, in ZnSe/BeTe structures with the thicknesses of the ZnSe layer up to 20 nm, the lifetime τ of the hole in the above barrier state at a low optical pumping density is primarily determined by the time τ rel of the escape of the hole to the BeTe layer [2].…”

“…This leads to an increase in the radiative time according to the formula (2) Here, τ rad is the time of the radiative recombination of the electron in the ground state and the hole in the lowest above barrier state, τ 0 = τ rad in the case of flat bands, is the wavefunction of the electron in the ground state, and is the wavefunction of the hole in the lowest above barrier state. The time τ rel of the escape of the hole from the low est above barrier state to the ground state in the BeTe layer is determined by the integral of the square of the absolute value of the wavefunction of the above barrier hole in the BeTe layer [10,11]:…”

“…High electric fields appearing inside the structure at a high density of sep arated carriers also lead to a significant increase in the duration of the relaxation of the spatially direct transi tion. Longer times in the direct transition are associ ated with the formation of a metastable hole state and with an increase in the time of radiative recombination of carriers because of a decrease in the overlapping of the wavefunctions of carriers involved in the direct transition [10,11].The modification of the band structure can be caused not only by high internal electric fields at a high density of spatially separated electrons and holes, but also by high external fields. As was shown in [12], the application of an external electric field to a type I het erostructure increases the lifetime of photoexcited carriers, because the electron and hole in the ground state are localized near different interfaces of the quantum well and the overlapping of their wavefunc tions is smaller.…”

“…High electric fields appearing inside the structure at a high density of sep arated carriers also lead to a significant increase in the duration of the relaxation of the spatially direct transi tion. Longer times in the direct transition are associ ated with the formation of a metastable hole state and with an increase in the time of radiative recombination of carriers because of a decrease in the overlapping of the wavefunctions of carriers involved in the direct transition [10,11].…”