Molecular beam epitaxy of wafer-scale O-band InAs/InGaAs quantum dots on GaAs for quantum photonics

Abstract: We report a scalable molecular beam epitaxy strategy to achieve a low density of O-band electrically tunable InAs/InGaAs quantum dots (QDs) on GaAs(001) substrates. Our approach is based on a gradient deposition of InAs in the sub-ML regime and subsequent capping with an InGaA strain-reducing layer to redshift the emission wavelength. For different growth conditions, we investigate the optical properties of the dots using photoluminescence mapping and correlate with structural properties determined by scanning transmission electron microscopy. Using a surface roughness modulation technique and synchronizing InAs sub-monolayer deposition cycles with substrate rotation, we control the dot density and position low-density regions (< 1 QD per um^2) on the substrate. Hyperspectral imaging is used to map the spatial and spectral characteristics of many individual dots in the low-density region, confirming that our approach is universally applicable to conventional MBE growth on (001) surfaces. Finally, we tune the QD emission wavelength within the O-band using electric fields and demonstrate single-photon emission with g(2)(0) = 0.020(14).

• The paper demonstrates a novel MBE method using sub-monolayer gradient deposition and PDL-induced roughness to achieve QD densities below 10⁸ cm⁻² for telecom applications.
• It reports robust O-band emission at ~1310 nm with narrow excitonic transitions and strong single-photon characteristics confirmed by PL and micro-PL spectroscopy.
• Electrical tuning via n-i-Schottky photodiode structures reveals QCSE effects with dipole moments up to 0.4 nm and polarizabilities up to 0.7 µeV·kV⁻²·cm² for integrated quantum devices.

Scalable MBE Growth of Telecom O-band InAs/InGaAs QDs on GaAs for Quantum Photonics

Introduction and Motivation

The fabrication of high-quality single-photon sources (SPSs) compatible with optical fiber transmission is critical for quantum photonics, quantum communications, and scalable quantum networks. While GaAs-based photonic heterostructures provide mature and scalable platforms, achieving emission in the telecom O-band (~1.3 μm), with low quantum dot (QD) densities and tunable properties, remains non-trivial due to limitations in dot size, composition, and strain management in conventional InAs/GaAs QDs. Previous strategies (strain reducing layers, quantum wells, metamorphic buffers) have incrementally advanced emission wavelength control, but wafer-scale, reproducible, and spatially controllable growth for O-band SPSs with low dot densities is not fully realized.

MBE Growth Strategy and Wafer-scale Density Control

The paper presents a molecular beam epitaxy (MBE) approach utilizing:

• Sub-monolayer (sub-ML) gradient deposition of InAs with synchronized substrate rotation
• Surface roughness modulation via GaAs pattern-defining layer (PDL)
• Subsequent capping with In₀.₂₉Ga₀.₇₁As strain-reducing layer (SRL) to achieve desired redshifts

Precise gradient-control of InAs coverage across the wafer, enabled by synchronization of shutter sequences with substrate rotation (step/continuous modes), allows deterministic spatial positioning and modulation of low-density QD regions (<10⁸ cm⁻²). The roughness modulation inherent to the PDL preferentially enhances QD nucleation in rough regions due to reduced nucleation barrier ($\Delta G_{\text{rough}} < \Delta G_{\text{flat}}$), enabling lateral ordering and density control.

Structural and Optical Characterization

Atomic force microscopy (AFM), aberration-corrected scanning transmission electron microscopy (STEM), and energy-dispersive X-ray (EDX) spectroscopy elucidate QD morphologies, indium content, and strain profiles. PL mapping and hyperspectral imaging provide spatial and spectral ensemble properties.

• InAs/InGaAs QDs exhibit lens-like morphology (aspect ratio $H/L\approx 0.2$ post-capping), with STEM-derived indium content $x_{\text{min}}\approx 0.41$, $x_{\text{max}}\approx 0.49$.
• PL results demonstrate robust emission in the O-band (peak near 1310 nm, ground-state redshift of ~80 meV relative to InAs/GaAs QDs), with high ensemble uniformity in density-controlled regions.

Notable findings include two distinct QD populations (small and large) as a function of local coverage, consistent with kinetic nucleation and ripening models. Larger QDs responsible for PL emission are tuned in size and density by the gradient coverage and roughness modulation.

Single-photon Emission and Electrical Tuning

Micro-PL spectroscopy confirms the quantum optical performance:

• Single-photon emission with measured $g^{(2)}(0) = 0.020 \pm 0.014$ (CX transition) under CW excitation, indicative of strong antibunching and suitability for SPS applications.
• Excitonic transitions (X, XX, CX) characterized by FWHM as narrow as 20-30 μeV (instrument-limited), and power-law dependences typical of QD systems.

Electrical tuning via embedding QDs in n-i-Schottky photodiode structures demonstrates robust quantum confined Stark effect (QCSE) across the O-band. Parabolic Stark shifts yield extracted dipole moments $p/e$ in the range 0.3–0.4 nm and polarizabilities up to 0.7 μeV·kV⁻²·cm², significantly exceeding those in conventional InAs/GaAs dots due to increased dot height and compositional gradients. The tuning range and deep carrier confinement enable fine detuning for integration with nanophotonic resonators and multiplexed photonic circuits.

Implications and Future Directions

Practical implications include:

• Wafer-scale spatial control allows deterministic placement of emission centers for lithography, integration with cavity structures, and scalable device fabrication.
• The robust single-photon emission and electrical tunability are pivotal for on-demand, wavelength-matched sources in quantum networks.
• Potential extension of technique to other material systems (III-V, II-VI, group-IV) is plausible, given universal applicability of the roughness and gradient nucleation mechanisms.

Theoretically, the approach enables exploration of advanced QD-molecule heterostructures and vertically correlated arrays, with implications for entangled-photon generation and multiplexing. The fine control of indium distribution and strain in SRL provides avenues for further band structure engineering.

Looking forward, integration with photonic crystal structures, on-chip quantum gates, and low-loss fiber couplers, combined with site-selective lithography, could significantly advance scalable quantum photonic technologies. Further optimization of growth kinetics, thermal gradients, and in situ monitoring is expected to yield even greater uniformity and reproducibility.

Conclusion

This work establishes a universal, scalable MBE strategy for O-band InAs/InGaAs quantum dots on GaAs(001), with controllable density below 10⁸ cm⁻², robust single-photon emission, and electrical tunability. The synergy of gradient sub-ML deposition, PDL-induced roughness modulation, and InGaAs SRL enables wafer-scale quantum light source fabrication optimized for telecom applications. The approach is instrumental in advancing quantum photonic devices scalable for integrated quantum networks and offers a versatile platform for future materials engineering and device innovation (2603.29934).