Electron emission properties of relaxation-induced traps in InAs/GaAs quantum dots and the effect of electronic band structure

Abstract

The electron-emission properties of relaxation-induced traps in InAs/GaAs quantum dots ͑QDs͒ are studied in detail using capacitance-voltage ͑C-V͒ profiling and bias-dependent deep-level transient spectroscopy. Strain relaxation is shown to induce a threading-dislocation-related trap in the top GaAs layer and a misfit-dislocation-related trap near the QD. The threading trap decreases its electron-emission energy from 0.63 to 0.36 eV from sample surface toward the QD, whereas the misfit trap gradually increases its electron-emission energy from 0.28 to 0.42 eV from near the QD toward the GaAs bottom layer, indicating that both traps near the QD have lower electron-emission energies. Hence, the emission-energy change is attributed to the related traps across the QD interface where a band offset exists. The C-V profiling at 300 K shows extended carrier depletion near the QD. As temperature is increased, an electron-emission peak emerges at the QD followed by a prominent peak, suggesting that the trap responsible for the prominent peak lies in energy below the QD electron ground state. From a simulation, this trap is identified to be the misfit trap located at the QD and at the observed emission energy below the GaAs conduction band. Based on the energy location of this trap, we deduce a possible mode of strain relaxation.

Compensation effect and differential capacitance analysis of electronic energy band structure in relaxed InAs quantum dots

“…This defect state behaves like a point defect state whose properties were previously studied in details. 19 It is similar as the one ͑0.395 eV, =1 ϫ 10 −16 cm 2 ͒ observed by Uchida et al 23 in strain relaxed InGaAs/GaAs quantum well structures. The fact that the tail of the QD ͑at Ϫ3.4 V͒ is immediately followed by the defect state stipulates that the tail of the EGS shall have a confinement energy of ϳ0.37 eV.…”

“…This plateau is attributed to the relaxation-induced defect state previously identified at 0.37 eV below the GaAs CB by deep-level transient spectroscopy. 19 The nearly voltage-independent capacitance suggests a pinning of Fermi level, implying a density of states being comparable to the background ionized impurity. Figure 5͑b͒ shows drastic carrier depletion after the QD, suggesting a lower background concentration due to the carrier compensation of this defect state.…”

“…Strain relaxation can enhance the fluctuation of the QD states and form a long low-energy tail, suggesting the process is not very homogenous. 19 Note that the carrier compensation effect caused by the relaxation-induced defect state can increase L 1 from 0.074 to 0.132 m at the tail of the EGS. This, along with the reduction in N D from 8 ϫ 10 16 to 2.9ϫ 10 16 cm −3 , would reduce the electric field at the QD from 8.2ϫ 10 6 to 5.3ϫ 10 6 V / m from the expression E = ͑q / ͒N D Ј L 1 .…”

“…18 Strain relaxation in the QDs can lead to the generation of threading dislocations in the top GaAs layer and lattice misfits near the QDs. 19 Capping the QDs with an InGaAs strain-relieving layer, strain relaxation in the QDs can be accommodated by the lattice misfits near the QDs and the top GaAs layer is dislocation-free. 18 Most of the lattice misfits were observed in the bottom GaAs layer under the QDs by transmission electron microscopy, 18 suggesting strain relaxation through the bottom GaAs layer.…”

“…A relaxation-induced defect state at 0.37 eV has been observed in the bottom GaAs layer. 19 Understanding the energy band structure in the relaxed QDs is believed to shed light on the strain effect in the coherently strained QDs. Thus, in this paper, we have derived an expression of capacitance at different frequency and voltage to analyze the electronic band structure in the relaxed QDs.…”