Eliminating Bimodal Size in InAs/GaAs Quantum Dots for 1.3‑µm Quantum Dot Lasers

Introduction

While the first 1.3‑µm QD laser appeared a decade ago, substantial progress in growth technology has only recently unlocked its full potential. A high density of well‑controlled QDs is essential for low‑power, high‑temperature, and high‑speed operation, making the InAs/GaAs system a prime candidate for next‑generation optical interconnects. However, the large lattice mismatch (~ 7 %) between InAs and GaAs often produces a bimodal size distribution, which broadens the emission spectrum and degrades laser performance. Eliminating this size bimodality is therefore critical for achieving high‑quality, low‑threshold devices.

Growth of InAs on GaAs(001) via molecular beam epitaxy (MBE) follows the Stranski–Krastanov (SK) mechanism, forming three‑dimensional islands that can be tuned by deposition rate and growth temperature. Conventional strategies—high InAs flux and low substrate temperature—boost dot density but introduce lattice defects, reducing PL intensity. In contrast, a post‑growth, single‑layer anneal can heal defects, homogenize dot size, and enhance the crystalline quality of the cap layer, as demonstrated in this work.

Material Optimization

All samples were grown on GaAs(001) (N+) substrates in a Veeco Gen 930 MBE system. Four test wafers (N170813, N170824A–C) were annealed at 630, 680, 730, and 780 °C, respectively, with identical growth parameters (Table 1).

Photoluminescence measurements revealed the strongest PL intensity at 680 °C (Fig. 1). The increase in annealing temperature promotes desorption of excess As and Ga, reducing surface defects while preserving QD lattice integrity.

Comparison of photoluminescence spectra under different annealing temperatures.

Key growth parameters were optimized: arsenic pressure was set to 4 × 10⁻⁷ Torr, and the InAs deposition rate to 0.025 ML s⁻¹. A 2.5 ML InAs layer was grown at 520 °C, capped by 5 nm In_0.15Ga_0.85As, followed by 15 nm GaAs at 520 °C and a final 20 nm GaAs layer at 630 °C (Fig. 2a).

The active‑region structure and its room‑temperature PL spectrum: peak at 1305 nm with a 31 nm FWHM.

AFM imaging of the annealed sample (Fig. 3a) shows a uniform QD density of 3.2 × 10¹⁰ cm⁻² and an average height of 8 nm. In contrast, the non‑annealed sample (Fig. 3b) exhibits a bimodal distribution (5–7 nm) and a lower density of 2.9 × 10¹⁰ cm⁻².

AFM images: (a) annealed, (b) non‑annealed, (c) size distribution with annealing, (d) size distribution without annealing.

The anneal not only homogenizes dot size but also repairs the GaAs cap layer, reducing dislocations that would otherwise propagate into the QD layer. The resulting 1.3‑µm emission spectrum is narrowed, with a PL peak at 1308 nm and a 31 nm FWHM.

Device Design and Preparation

The laser stack consists of five InAs QD layers sandwiched between 1.8 µm Al_0.45Ga_0.55As waveguide layers (n‑type Si: 2 × 10¹⁸ cm⁻³, p‑type Be: 4 × 10¹⁸ cm⁻³). A 200‑nm p⁺ GaAs (Be: 3 × 10¹⁹ cm⁻³) layer provides a low‑resistance contact (Fig. 4a).

Device structure: (a) epitaxial layers; (b) room‑temperature PL with a 1294 nm peak.

After epitaxy, the upper cladding was chemically thinned (H₃PO₄–H₂O₂–H₂O 1:1:4). This step shifts the PL peak to 1294 nm due to a higher growth temperature (650 °C) and indium drift in the In_0.15Ga_0.85As cap.

Photolithography defined a 100 µm ridge, etched to 2 µm via ICP, and insulated with 260 nm SiO₂ (PECVD). Contact windows (90 µm) were opened for current injection, and Ti/Pt/Au (51/94.7/1122 nm) electrodes were sputtered on the front side. The wafer was thinned to 120 µm; a 50 nm AuGeNi (80:10:10 wt%) alloy followed by 300 nm Au was evaporated on the back for the n‑type electrode. A 460 °C, 10 s anneal formed ohmic contacts.

SEM cross‑section: 2 µm GaAs/AlGaAs etch depth, 260 nm PECVD SiO₂.

Electrical and optical performance were measured in CW at room temperature. The device exhibits a threshold current density of 110 A cm⁻² and a lasing wavelength centered at 1.3 µm (Fig. 6). The red‑shift of the lasing peak relative to the PL is attributed to self‑heating during operation. No facet coating or active‑region doping was required, underscoring the high crystalline quality achieved through single‑layer annealing.

Device measurements: (a) P‑I‑V curves; (b) lasing spectrum at 1.3 µm.

Conclusions

By systematically optimizing growth and annealing conditions, we successfully eliminated the bimodal size distribution in InAs/GaAs QDs. The optimal single‑layer anneal at 680 °C, performed 20 nm above the QD layer, yields a uniform dot ensemble and a high‑density of 3.2 × 10¹⁰ cm⁻². The resulting 1.3‑µm Fabry–Perot laser achieves a low threshold current density of 110 A cm⁻² in continuous‑wave, room‑temperature operation, demonstrating the viability of this approach for high‑performance QD laser applications.

Abbreviations

AFM:

Atomic Force Microscope

Annealing T:

Annealing temperature

CW:

Continuous wave

F-P:

Fabry–Perot

FWHM:

Full width at half maximum

Growth T:

Growth temperature

HT:

High temperature

LT:

Low temperature

MBE:

Molecular beam epitaxy

PL:

Photoluminescence

QD:

Quantum dot

RT:

Room temperature

SEM:

Scanning electron microscope

WPE:

Wall plug efficiency