QUATERNARY ALGAASSB CAPPING LAYER

Longer recombination lifetime makes the type-II InAs/GaAsSb QDs promising for memory and photovoltaic devices, but the degraded recombination efficiency is however detrimental for light emitting devices. Several works have been devoted to the tailoring of the optical properties of GaAsSb-capped InAs QDs. However, since the effects of strain reduction and decomposition suppression are proportional to the Sb content in the CL, it seems unlikely to take the advantages of GaAsSb CL while retaining a type-I QD band alignment. Replacing the GaAsSb by an AlGaAsSb CL appears to be a promising alternative.

Figure 2.18(a) shows a contour map of the unstrained VB offset (VBO) between Al_xGa_1-xAs_1-ySb_y and InAs [i.e., E_V (Al_xGa1−xAs1−ySb_y) − EV (InAs) , where E_V is the valence band maximum] as functions of the Al (x) and the Sb (y) contents according to the material parameters in Ref. 70. If the quaternary AlGaAsSb alloy is used for capping the InAs/GaAs

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QDs, the band alignment can be separated into the type-I and type-II regions by the boundary of zero VBO (solid line). Although the boundary line would be changed by the inhomogeneous stain distribution and the quantum confinement of the QDs, it is evident that the band alignment can be restored to type-I by adding Al into the CL when the Sb content exceeds 0.14. Furthermore, InAs/AlGaAsSb QDs can offer stronger electron confinement when the VBO is zero for certain Al and Sb contents in the CL, which is also preferable for the development of QD-based intermediate-band solar cells.

In this section, we demonstrate the tuning of band alignment and optical properties of InAs/GaAs QDs using a thin quaternary AlGaAsSb CL. As evidenced from power dependent PL and TRPL measurements, the GaAsSb-capped QDs with type-II band alignment can be changed to type-I by adding Al into the GaAsSb CL. The evolution of band alignment with the Al content in the CL is also compared with theoretical calculations based on 8-band k ⋅p model.

2.4.1 Experimental details

The samples were grown on GaAs substrates by MBE. After the growth of a 200 nm thick GaAs buffer layer on the substrate, a layer of self-assembled InAs QDs (2.7 ML) was deposited at 500 ^oC and subsequently capped with a 5 nm thick AlxGa1-xAs1-ySby CL. Four samples with nominal Al contents of x=0, 0.1, 0.2, and 0.3 have been grown. The nominal Sb content is y= 0.2 for all samples. It is worth to mention that the growth rate for the AlGaAsSb layer in all samples was kept the same (500 nm/hr) in order to minimize variations in the Sb incorporation rate and to mitigate Sb segregations. A sample with GaAs capped InAs QDs was also grown as a reference sample of type-I QDs. Finally, all samples were capped by a 50 nm GaAs layer. Atomic force microscopy revealed that uncapped surface QDs are lens shaped, with an average height of 8 nm, an average diameter of 20 nm, and an areal density of about 3×10¹⁰ cm^-2.

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Fig. 2.19: The PL spectra measured at T = 12 K for the GaAs-capped and AlGaAsSb-capped InAs QDs with different Al contents (x).

2.4.2 The evolution of band alignments

Figure 2.19 shows the PL spectra for the samples measured at T=12 K under a low excitation power of Pex= 10 μW. The QD PL peak blueshifts systematically with increasing Al content x. For the GaAsSb-capped sample (x= 0), the InAs-GaAsSb interface is expected to exhibit a type-II band alignment. Since the lattice constant of AlxGa1-xAsSb is similar to that of GaAsSb, strain redistribution caused by adding Al into the CL can be excluded. The blueshift in PL peak can thus be attributed to the reduction in the VBO at the InAs-Al_xGa_1-xAsSb interface.

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Fig. 2.20: (a) The ground state PL peak energies as a function of Pex1/3

. (b) TRPL spectra for the investigated samples. (c) The ground state PL peak energies as a function of Al contents (x). (d) The estimated wave function overlaps according to the measured decay lifetimes.

In order to examine the band alignments, we have performed power dependent PL measurements. Figure 2.20(a) shows the PL peak energy of QD ground state as a function of the cube root of excitation power . The PL peak of GaAsSb-capped QDs shows a large energy blueshift with increasing Pex and exhibits a nearly linear dependence on , signifying its type-II character. Such an energy blueshift is not observed in the type-I GaAs-capped QDs. For the AlGaAsSb-capped QDs, the PL peak blueshifts at low excitation powers but becomes nearly unchanged under higher excitation conditions. As the Al content x in the CL is increased, the energy blueshift becomes less significant. This behavior can be

explained by the gradual evolution from type-II to type-I recombination with the increasing

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Al content x in the CL. Increasing Al content in the CL tends to reduce the VBO at the QD-CL interface, which becomes unable to confine holes in the CL, resulting in type-I like behaviors. It is remarkable for the AlGaAsSb-capped QDs with x= 0.3, where the PL peak energy is nearly independent of excitation power, indicating that QD-CL interface has changed to a type-I band alignment.

The QD band alignments can be further investigated by TRPL measurements. Figure 2.20(b) shows the TRPL decay traces of the investigated samples measured at T= 12 K. The measured decay lifetime for GaAs-capped QDs in the reference sample is 0.8 ns, comparable to the typical reported value of ~1 ns. In contrast, the GaAsSb-capped sample exhibits a much longer lifetime of 15.7 ns due to its type-II band alignment. Increasing Al content in the CL leads to a significant shortening in decay lifetimes, indicating that more hole wave function penetrate into the QDs. The deduced decay lifetimes are 14.5, 6.5, and 2.2 ns for samples with x= 0.1, 0.2, and 0.3, respectively. Because the radiative recombination lifetime is inversely proportional to the square of the overlap integral of the electron and hole wave functions and proportional to the emission energy, the measured lifetimes can thus be a measure of the electron-hole overlap in the QDs. If we assume that the overlap in the GaAs-capped type-I QDs is 100%, we estimated that the electron-hole overlaps in the AlxGa1-xAsSb-capped QDs are 27, 27, 40, and 70% for samples with x= 0, 0.1, 0.2, and 0.3, respectively.

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2.4.3 Simulations

Theoretical calculations based on eight-band k·p model [69] have been carried out in order to understand the evolution of hole wave function distribution with the Al content x in the CL. We consider the InAs QD as a truncated pyramid with {101} facets and having a base length b=14 nm, a height h=3.5 nm and a 5 nm AlxGa1-xAs0.8Sb0.2 CL covering thereon in a conformal way. The inhomogeneous strain distribution and the strain-induced piezoelectric polarization have also been included. The calculated wave function distributions for the hole ground state on the (110) plane for different Al contents in the CL are depicted in Fig. 2.21(a) to 2.21(d). For the GaAsSb-capped QDs, the hole wave function is localized in the CL and close to the QD base along the [1-10] direction, where the potential is a minimum for the hole. With increasing x, the hole wave function penetrates gradually into the QD and eventually well-localized in the QD for x = 0.3. This can be attributed to the change of VBO by introducing Al into the CL, as can be seen from the calculated band structures along the growth direction shown in Fig. 2.21(e) to 2.21(h), where the potential minimum for the hole has been moved from the CL to the QD for x ≥ 0.2. The calculated transition energy and wave function overlaps as function of Al content x are displayed in Figs. 2.21(i) and 2.21(j).

They fit very well with the measured PL energy shift and the estimated wave function overlaps. For x< 0.1, the energy shift rate follows very well with the VBO change rate (4.1 meV/Al%) due to its type-II character. When x exceeds 0.2, increasing x only leads to higher confined potentials for both the electron and hole, which have less effect on the transition energy of type-I QDs.

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Fig. 2.21: (a)-(d) The calculated wave function distributions for the hole ground state on the (1-10) plane for different Al contents (x=0, 0.1, 0.2, 0.3) in the CL. (e)-(h) The calculated band structures along the growth direction through the center of the QD (solid line, A) and through the CL near the QD base (dotted line, B). (i) The ground state PL peak energies as a function of Al contents and (j) the estimated wave function overlaps according to the measured decay lifetimes, where the curves are the calculated transition energy and wave function overlaps as function of Al content.

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Fig. 2.22: (a) Arrhenius plot of the integrated PL intensity for GaAs-capped and AlGaAsSb-capped InAs QDs with x = 0.3. (b) The PL spectra measured at RT.

We have also performed temperature-dependent PL measurements in order to understand the thermal stability of QD emission property after AlGaAsSb capping. Figure 2.22(a) shows the Arrhenius plot of the integrated PL intensity for GaAs-capped and Al0.3Ga0.7AsSb-capped InAs QDs. For the conventional GaAs-capped QDs, the PL intensity started to drop at T > 120 K. In contrast, the PL intensity for the Al_0.3Ga_0.7AsSb-capped sample can persist up to T = 200 K. The thermal activation energy for PL quenching also increases from 371 to 505 meV, implicating the improved thermal stability of QD PL after AlGaAsSb capping. We would like to mention that the Al0.3Ga0.7AsSb-capped sample exhibits a large enhancement in the RT PL intensity (×7) as compared with that of the GaAs-capped reference sample [see Fig. 2.22(b)]. Such an improvement in the optical properties makes the AlGaAsSb capped InAs QDs very promising for long wavelength applications.

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In summary, the optical properties of AlGaAsSb-capped InAs QDs have been investigated by PL and TRPL measurements. The original type-II band alignment in GaAsSb-capped InAs QDs can be restored to type-I by adding Al into the CL. Furthermore, the AlGaAsSb CL also improves the PL thermal stability and the RT PL efficiency. We demonstrate that using a quaternary AlGaAsSb CL can take the advantages of GaAsSb CL (i.e., strain reduction and decomposition suppression) on the InAs QDs while retaining their type-I QD characters.

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