Altering the Optical Properties of GaAsSb-Capped InAs Quantum Dots by Means of InAlAs Interlayers
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In this work, we investigate the optical properties of InAs quantum dots (QDs) capped with composite In0.15Al0.85As/GaAs0.85Sb0.15 strain-reducing layers (SRLs) by means of high-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) spectroscopy at 77 K. Thin In0.15Al0.85As layers with thickness t = 20 Å, 40 Å, and 60 Å were inserted between the QDs and a 60-Å-thick GaAs0.85Sb0.15 layer. The type II emissions observed for GaAs0.85Sb0.15-capped InAs QDs were suppressed by the insertion of the In0.15Al0.85As interlayer. Moreover, the emission wavelength was blueshifted for t = 20 Å and redshifted for t ≥ 40 Å resulting from the increased confinement potential and increased strain, respectively. The ground state and excited state energy separation is increased reaching 106 meV for t = 60 Å compared to 64 meV for the QDs capped with only GaAsSb SRL. In addition, the use of the In0.15Al0.85As layers narrows significantly the QD spectral linewidth from 52 to 35 meV for the samples with 40- and 60-Å-thick In0.15Al0.85As interlayers.
KeywordsQuantum dots Strain InAlAs/GaAsSb III–V semiconductors
Full width at half maximum
High-resolution X-ray diffraction
In the last decades, self-organized quantum dots (QDs) synthesized using the Stranski–Krastanov technique have attracted a great deal of attention. Their optical and electronic properties have been investigated intensively owing to their potential applications in optoelectronic devices . The widely investigated InAs/GaAs QD system has been employed in a range of optoelectronic devices as active material. During the growth of these nanostructures, significant change in the size and the shape of the QDs occurs during the capping process. This process is quite complex and involves intermixing, segregation, or strain-enhanced diffusion . The use of a pure GaAs capping layer limits the QD emission to less than 1300 nm. To alleviate this issue, strain-reducing layers made of (Ga, In)(As, Sb, N) have been used [2, 3, 4, 5, 6, 7]. In particular, the ternary GaAsSb has received particular attention as its resulting band alignment can be tailored to be of type I or type II by changing the Sb content [8, 9] and by its capability in extending the emission wavelength beyond the C-band . However, the difference in energy between the fundamental and excited state is limited to 60–75 meV when GaAsSb is used as a strain-reducing layer (SRL) . This energy separation does not prevent carriers from escaping thermally from the QDs. For applications requiring a long carrier lifetime, the insertion of a thin barrier between the InAs QDs and GaAsSb will be beneficial, as it will increase the carrier separation between the QDs and GaAsSb quantum well (QW). As an example, GaAs interlayers have been used resulting in an enhancement of solar cell power efficiency by a factor of 23% . The use of InAlAs layers may be of interest to engineering the type of radiative recombination. For type II transition, the insertion of InAlAs will increase the carrier lifetime  and the energy separation between the fundamental and first excited states [14, 15, 16]. Moreover, the insertion of an InAlAs layer between InAs QDs and GaAsSb is expected to decrease In segregation and suppress In and Ga atoms intermixing between the InAs QDs and the GaAsSb SRL and reduce further the QD strain . InAlAs/InGaAs composite SRLs have been used to cap InAs QDs resulting in long wavelength emission and a favorable energy separation between the fundamental and excited state as high as 104 meV [16, 18].
In this paper, we report the first investigation of the effect of using an In0.15Al0.85As interlayer on the optical properties of InAs/GaAs0.85Sb0.15 QDs by means of photoluminescence (PL) spectroscopy. In particular, the emission wavelength variation, the type of optical emission, the spectral linewidth, and the energy separation between the fundamental and first excited state were studied in details.
The crystal quality of the samples was characterized by high-resolution X-ray diffraction (HRXRD) using a Panalytical X-ray diffractometer. The optical properties of the grown samples were assessed by means of PL spectroscopy at 77 K using a PL module connected to a Vertex 80 Fourier Transform Infrared instrument (Bruker Optics GmbH) and using a thermoelectrically cooled high-gain InGaAs detector . The samples were excited with a CW 532-nm solid-state laser source.
Results and Discussion
The ground state transition of sample A occurs at 1004 meV with a FWHM of 52 meV and an energy separation ΔE of 64 meV. Inserting 20 Å of In0.15Al0.85As (sample B) induces a blueshift of the ground state transition by 52 meV. The blueshift is consistent with what have been observed when a composite InAlAs/InGaAs was used for QDs grown at nearly the same growth temperature . The ground state transition energy blueshift of the InAs QDs in sample B results from the increased confinement potential . As the barrier for electrons and holes are increased, the energy level separation of electrons and holes should increase leading to the observed emission blueshift. It is well known that capping InAs with GaAs results in a reduction of the QD height as a consequence of In segregation and In-Ga intermixing . The introduction of Sb in the GaAs capping layer reduces the QD decomposition by inhibiting the strain driven In-Ga intermixing . The insertion of the InAlAs interlayer is expected to suppress further the In segregation and In-Ga intermixing resulting from the inactivity of Al adatoms. In fact, Jun et al.  have shown by means of STEM that the use of a InAlAs/InGaAs combination layer as a capping layer strongly suppresses In segregation, and In–Ga intermixing along the growth direction during the capping process of the InAs QDs, leading to the increased height of the nanostructures and a higher In concentration in InAs QDs after capping. Considering the low growth temperature of the QDs, i.e., 485 °C, the indium segregation and interface intermixing between the QDs and InAlAs interlayer are expected to be insignificant as a result of the inactivity of Al adatoms.
Extracted parameters at 77 K for samples A, B, C, and D
Peak energy (meV) 77 K
The PL intensity of samples B and C was increased compared to sample A; however, a strong reduction of the PL intensity was observed for sample D, i.e., a reduction by a factor of 5 compared to sample C. The reduced PL intensity results from the reduction of carrier injection from the GaAsSb layer to the QDs. In fact, during illumination, numerous carriers are photogenerated and the insertion of the In0.15Al0.85As interlayer creates a barrier for carriers and may limit their injection in the QDs. Carriers may transfer to the QDs through a tunneling process, and the PL intensity is higher for the samples with thinner In0.15Al0.85As barriers . Sample D showed the lowest PL intensity as the tunneling through the 60 Å In0.15Al0.85As is greatly reduced, and this is evidenced by the increased PL emission of the GaAsSb QW as shown in Fig. 4a. The reduction of the tunneling process makes favorable and enhances the radiative recombination of electrons and holes in the GaAsSb QW.
The main observation from the power-dependent PL at 77 K for samples B, C, and D shown in Fig. 4a is the fixed energy positions of the first two peaks with increasing excitation power as opposed to what was observed in sample A. This is a characteristic of a type I emission where both electrons and holes are localized within the QDs. The first two emission peaks result from the recombination of electrons and holes in the fundamental and first excited states in the QDs (E0QD-H0QD and E1QD-H1QD). We believe that the third peak originates from a type II emission resulting from the recombination of electrons within GaAs and holes localized in GaAsSb QW. Indeed, the energy corresponding to this transition increases with increasing excitation power as shown in Fig. 4a and Fig. 4c characteristic of a type II transition. Moreover, we see that its intensity increases with increasing In0.15Al0.85As layer thickness. This is in agreement with the reduction of the PL intensity of the fundamental transition as a thicker In0.15Al0.85As layer reduces carriers tunneling from GaAsSb to the QDs and favors the type II emission obtained from the recombination of electrons and holes located in GaAs and GaAsSb, respectively. A schematic of the recombination channels for samples B, C, and D is depicted in Fig. 3b. The suppression of the type II emission can be understood as follows. The insertion of a 20-Å In0.15Al0.85As layer increases the carrier separation between the QDs and GaAsSb QW, and as a result, the electron and hole wavefunction overlap decreases. Moreover, the fact that the Sb content in the GaAsSb content is close to the type I-type II crossover, the In0.15Al0.85As interlayer will bring the confined level in the QW (H0QW) below the first quantized level in the QDs (H0QD) as shown in Fig. 3b and hence holes captured in the QW may tunnel through the InAlAs layer making less probable the type II emission. For a thicker In0.15Al0.85As interlayer (40 Å and 60 Å), the tunneling is further reduced but the electron and hole wavefunction overlap is substantially reduced favoring the recombination of electrons in GaAs with holes in GaAsSb . The optical transition of InAs/GaAsSb QDs can be tailored to the type of application requiring either short or long lifetimes. In our study, we have considered a Sb content of 13% in GaAsSb, which is close to the type I to type II transition. The insertion of an InAlAs interlayer suppressed the type II emission and increased the energy separation between the fundamental and first excited state, which is desirable for applications requiring a short carrier lifetime. The present study can also be tailored for applications requiring a long carrier lifetime. In fact, the combination of using a higher Sb content in the GaAsSb layer and the insertion of an InAlAs interlayer is expected to maintain the type II emission for thin InAlAs interlayers while increasing significantly the carrier lifetime. At the same time, the increased energy separation between the fundamental and first excited state will reduce carrier thermal escape.
InAs QDs capped with composite In0.15Al0.85As/GaAs0.85Sb0.15 SRLs with varying In0.15Al0.85As thicknesses were grown and characterized. Our analysis shows that the insertion of an In0.15Al0.85As layer suppresses the observed type II emission obtained from InAs/GaAs0.85Sb0.15 QDs. Moreover, the emission wavelength is blueshifted for t = 20 Å and redshifted for t ≥ 40 Å resulting from the increased confinement potential and increased strain, respectively. A large energy separation ΔE of 106 meV was obtained for the sample with a 60-Å-thick In0.15Al0.85As interlayer. In addition, the introduction of the In0.15Al0.85As interlayer reduces significantly the FWHM from 52 meV reaching a minimum of 35 meV.
The authors are grateful for King Abdulaziz City for Science and Technology for financial support. The authors would also like to extend their gratitude to EPSRC for their support of this work via EPSRC-EP/P006973/1 “FUTURE COMPOUND SEMICONDUCTOR MANUFACTURING HUB”.
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
AS conceived the idea and designed the growth experiments. AS performed the molecular beam epitaxial growth with the help of SA and YA. SA performed the PL and HRXRD characterization. AS analyzed the data and wrote the manuscript. HB participated in the discussion. AA, AY, and MM supervised the team. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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