Tailoring Optical Properties of InAs Quantum Dots with InAlAs Interlayers in GaAsSb Strain‑Reducing Layers

Background

Self‑assembled InAs/GaAs quantum dots, formed by the Stranski–Krastanov mechanism, have attracted extensive research interest for their tunable electronic and optical properties, which underpin a range of optoelectronic devices [1]. During capping, QD size and shape evolve through intermixing, segregation, and strain‑enhanced diffusion [2]. A pure GaAs capping layer limits emission to wavelengths below 1300 nm; adding strain‑reducing layers (SRLs) composed of (Ga, In)(As, Sb, N) extends emission into the C‑band [3–7]. GaAsSb, in particular, offers a tunable band alignment: by adjusting Sb content it can transition from type I to type II, enabling emission beyond 1.5 µm [8–10]. However, with GaAsSb SRLs the fundamental–excited state separation typically remains only 60–75 meV, which allows thermally activated carrier escape [11]. Introducing a thin barrier between InAs QDs and GaAsSb can increase carrier confinement and lifetime, as demonstrated by GaAs interlayers that improved solar‑cell efficiency by 23% [12]. InAlAs layers are promising for engineering radiative recombination; they can raise the fundamental–excited state separation and suppress intermixing, thereby enhancing device performance [13–17]. Prior work with InAlAs/InGaAs composite SRLs achieved separations up to 104 meV [16, 18]. This paper presents the first systematic study of In_0.15Al_0.85As interlayers on InAs/GaAs_0.85Sb_0.15 QDs, focusing on emission wavelength, type of recombination, linewidth, and state separation.

Methods

The samples were grown on epi‑ready p‑type GaAs (001) substrates in a Veeco Gen20A MBE system. As₂ and Sb₂ dimers were supplied via valved crackers. After a GaAs buffer at 590 °C, the substrate was cooled to ~485 °C to grow 2.5 ML InAs QDs. A 5 nm GaAs layer capped the QDs before the temperature was raised to 570 °C for a 38‑nm GaAs barrier. The GaAsSb SRL thickness was fixed at 60 Å (15 % Sb) while the In_0.15Al_0.85As layer varied from 0 to 60 Å across four samples (A–D). The growth rate for all layers was 0.5 ML s^‑1. Band alignment was modeled following Krijn [19] and parameters in [20], and illustrated in Fig. 1.

Schematic of the grown structures and corresponding energy band diagram of InAs QDs capped with a composite In_0.15Al_0.85As/GaAs_0.85Sb_0.15. The In_0.15Al_0.85As thickness t = 0 Å, 20 Å, 40 Å, and 60 Å for samples A, B, C, and D, respectively.

Crystal quality was assessed by HRXRD on a Panalytical diffractometer. PL measurements were performed at 77 K with a 532‑nm CW laser and a Bruker Vertex 80 FT‑IR spectrometer equipped with a thermoelectrically cooled InGaAs detector [21].

Results and Discussion

HRXRD rocking curves (Fig. 2a) reveal clear satellite peaks for all samples, confirming high crystal quality. Simulations yield an average Sb content of 13 % in the reference sample and In contents of 13.5 % in the interlayers, matching nominal values.

a High‑resolution ω/2θ scans for samples A–D. b PL spectrum of sample A at 77 K, 100 mW. c Power‑dependent PL of sample A. d Peak energy vs. P_ex^1/3 for the first two transitions.

The reference sample (A) shows three Gaussian peaks at 1004 meV, 1068 meV, and 1113 meV, corresponding to ground and first excited states. The ground‑state FWHM is 52 meV, while the excited state is 58 meV. Power‑dependent measurements (Fig. 2c) reveal that the fundamental transition energy decreases with decreasing excitation power, characteristic of a type II recombination (electrons in QD, holes in GaAsSb QW). The first excited transition behaves similarly, indicating electron–hole pairs localized in the QDs. The energy separation ΔE remains ~64 meV across powers, confirming a perpendicular electric field from charge buildup.

Figure 3a shows the band profiles of sample A and the modified profiles for samples B–D. The insertion of In_0.15Al_0.85As alters both confinement and strain. For a 20 Å layer (sample B), the ground‑state peak shifts to 1056 meV (52 meV blueshift) and the FWHM reduces to 39 meV. The ΔE increases to 92 meV. Thicker layers (40–60 Å) produce a redshift; sample D shows a ground‑state at 962 meV, FWHM of 35 meV, and ΔE of 106 meV. These changes reflect the competition between enhanced confinement (blue) and strain‑induced band‑edge shifts (red).

PL intensities increase for B and C relative to A, but drop by a factor of five for D. The thicker InAlAs layer hinders carrier tunneling from GaAsSb to the QDs, favoring type I recombination within the QDs and reducing type II emission. The third PL peak, appearing at higher energies, grows with interlayer thickness, confirming a transition to type II recombination (electron in GaAs, hole in GaAsSb). This behavior is consistent with the suppressed overlap of electron and hole wavefunctions due to the barrier.

Overall, the InAlAs interlayer provides a versatile tool: a 20 Å layer yields a short‑lifetime, type I emitter with large ΔE and narrow linewidth; thicker layers enable longer‑lifetime, type II emission suitable for applications such as quantum‑dot lasers or photodetectors.

Conclusion

InAs QDs capped with composite In_0.15Al_0.85As/GaAs_0.85Sb_0.15 SRLs exhibit tunable optical properties driven by the InAlAs interlayer thickness. The interlayer suppresses type II emission, induces a 52 meV blueshift at 20 Å and a redshift for ≥40 Å, and expands the ground‑to‑first‑excited state separation to 106 meV (sample D). The spectral linewidth narrows from 52 meV to 35 meV, highlighting reduced intermixing and improved size uniformity. These findings demonstrate that strategic interlayer engineering can tailor QD emission for high‑performance optoelectronic devices.

Abbreviations

FWHM:

Full width at half maximum

HRXRD:

High‑resolution X‑ray diffraction

PL:

Photoluminescence

QDs:

Quantum dots

QW:

Quantum well

SRLs:

Strain‑reducing layers