Search

This work investigates the surfactant effect on exposed and buried InAs quantum dots (QDs) by incorporating Sb into the QD layers with various Sb beam equivalent pressures (BEPs). Secondary ion mass spectroscopy shows the presence of Sb in the exposed and buried QD layers with the Sb intensity in the exposed layer substantially exceeding that in the buried layer. Incorporating Sb can reduce the density of the exposed QDs by more than two orders of magnitude. However, a high Sb BEP yields a surface morphology with a regular periodic structure of ellipsoid terraces. A good room-temperature photoluminescence (PL) at ∼1600 nm from the exposed QDs is observed, suggesting that the Sb incorporation probably improves the emission efficiency by reducing the surface recombination velocity at the surface of the exposed QDs. Increasing Sb BEP causes a blueshift of the emission from the exposed QDs due to a reduction in the dot height as suggested by atomic force microscopy. Increasing Sb BEP can also blueshift the ∼1300 nm emission from the buried QDs by decreasing the dot height. However, a high Sb BEP yields a quantum well-like PL feature formed by the clustering of the buried QDs into an undulated planar layer. These results indicate a marked Sb surfactant effect that can be used to control the density, shape, and luminescence of the exposed and buried QDs. [ABSTRACT FROM AUTHOR]

The electron-emission properties of relaxation-induced traps in InAs/GaAs quantum dots (QDs) are studied in detail using capacitance-voltage (C-V) 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. [ABSTRACT FROM AUTHOR]

We perform capacitance-voltage and admittance spectroscopy to investigate the effect of incorporating an InAlAs layer before an InGaAs cap layer on electron emission in self-assembled InAs quantum dots (QDs). We show that this incorporation of a high potential barrier increases the emission time of the electrons thermally activated from the QD ground to the first excited state. The energy separation between the ground and first excited states in the conduction band increases from 57.2 to 79.1, to 89.2, and to 95.6 meV with increasing the thickness of the InAlAs layer from 0 to 10, to 14, and to 20 Å. Combining with photoluminescence (PL) data, the ratios of the energy separation between the ground and first excited states in the conduction band and valence band are determined to be 7.3:2.7 and 7.8:2.2 for 0 and 10 Å InAlAs, respectively. In addition, this incorporation is shown to blueshift the PL first excited state much larger than the ground state. [ABSTRACT FROM AUTHOR]

This study presents the results of incorporating N into self-assembled InAs quantum dots (QDs) capped with an InGaAs cap layer. Experimental results indicate that such incorporation can redshift the QD ground state and decrease the energy spacing between the QD ground and first excited states. However, this incorporation reduces the potential barrier of the cap layer, increasing the electron escape from the QDs. Capacitance-voltage profiling shows that a broad shoulder corresponding to the electron emission from the QD ground to first-excited state cannot be resolved from the peak related to the electron emission from the excited states upon this incorporation. This finding implies that this incorporation reduces the energy spacing between the QD ground and first-excited states in the conduction band, thus correlating well with the photoluminescence data. In contrast, incorporating N directly into the InAs QD produces no redshift of the emission wavelength but introduces a deep trap at ∼0.21 eV that depletes the electrons in the QDs. [ABSTRACT FROM AUTHOR]

The effect of strain relaxation in a relaxed InAs quantum dot (QD) capped with InGaAs is investigated by admittance and deep-level transient spectroscopy (DLTS). Strain relaxation markedly increases the emission time in the QD region and extends carrier depletion to the bottom GaAs layer. The experimental data show the presence of relaxation traps in the QD region and the neighboring bottom GaAs layer. The electron emission from the QD region is governed by a trap located at 0.17–0.21 eV below the QD ground state. The electron-escape process is identified as thermal activation at high temperatures and direct tunneling at low temperatures from the trap. In the bottom GaAs layer near the QD, DLTS reveals a relaxation trap at 0.37–0.41 eV relative to the GaAs conduction band. The energy difference between these two traps is comparable to the QD ground-state energy relative to the GaAs conduction-band edge, suggesting that the two traps may be the same trap which is pinned to the GaAs conduction band. The considerable difference between their properties may result from different atoms surrounding the trap. [ABSTRACT FROM AUTHOR]

The optoelectronic characteristics of self-assembled InAs quantum dots (QDs) with strain-reduced layers (SRLs) were investigated using photoluminescence (PL) spectroscopy. Various SRLs that combine In0.14Al0.86As and In0.14Ga0.86As with the same total thickness were examined to ascertain their confining effect on carriers in InAs QDs. The emission wavelength is blueshifted as the thickness of InAlAs is increased. The energy separation between the ground state and the first excited state of QDs with InAlAs SRLs greatly exceeds that of QDs with InGaAs SRLs. Atomic force microscopic images and PL spectra of the QD samples demonstrated that high-quality InAs QDs with long emission wavelengths and a large energy separation can be generated by growing a low-temperature, thin InAlAs SRL onto self-assembled QDs. [ABSTRACT FROM AUTHOR]

The onset of strain relaxation and induced defects in InAs0.94Sb0.06 quantum dots are investigated. We show that the relaxation causes partial carrier depletion in the dots and drastic carrier depletion in the top GaAs layer due to the introduction of two defect traps at 0.35 and 0.64 eV. This result is consistent with transmission electron microscopy data which show misfit dislocations on the edges of the dot upper boundary and threading dislocations in the top GaAs layer. The bottom GaAs layer is dislocation-free, and thus the strain relaxation may initially occur on the edges of the dots. [ABSTRACT FROM AUTHOR]

We present detailed studies of the onset of strain relaxation in InAs/InGaAs quantum dots. We show that the ground-state photoluminescence (PL) emission redshifts with increasing the InAs coverage before relaxation and blueshifts when relaxation occurs. PL spectra of the relaxed samples show two predominant families of dots with very different temperature-dependent efficiency. By comparison we show that the dots emitting at long wavelength are degraded by relaxation while the dots emitting at short wavelength remain coherently strained. Consequently, the PL spectra are dominated by the dots emitting at short wavelength, leading to the observed blueshift. This result suggests that the relaxation does not occur uniformly. In addition, we show that the relaxation occurs in the dot bottom interface. [ABSTRACT FROM AUTHOR]