Uniform Indium Quantum Dots Formed via Two‑Step Cooling on InGaN Surfaces: A Novel MOCVD Approach

Introduction

Indium gallium nitride (InGaN) has become the cornerstone of high‑efficiency light‑emitting diodes (LEDs) and laser diodes (LDs) due to its high absorption, wide spectral tunability, and radiation hardness [1‑5]. However, fabricating high‑quality InGaN remains challenging. Lattice mismatch between InN and GaN produces a solid‑phase miscibility gap [6,7], while the higher vapor pressure of InN compared to GaN suppresses indium incorporation [8]. Moreover, differing formation enthalpies drive pronounced indium surface segregation during growth [9]. During InGaN layer deposition, an indium‑rich surface layer inevitably forms because of the pulling effect, which can contaminate subsequent multiple quantum well (MQW) structures [10]. Overcoming these obstacles is essential for pushing the performance of InGaN‑based optoelectronics. While optimizing growth parameters for a single InGaN layer, we observed that a two‑step cooling regimen, rather than the conventional one‑step cooldown, consistently produced uniform‑size In QDs on the surface. This observation prompted a systematic investigation into the underlying mechanism and its implications for surface‑layer characterization.

Experiment

Samples were grown on c‑plane sapphire using an AIXTRON 6 × 2 in close‑coupled showerhead MOCVD reactor under a nitrogen atmosphere. Trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH₃) served as Ga, In, and N precursors, respectively. Single InGaN layers (<60 nm thick, <15 % In) were deposited atop a 2‑µm unintentionally doped GaN buffer, which itself rested on a 25‑nm GaN seed layer (Fig. 1). Hydrogen (H₂) was employed as the carrier gas during GaN growth to improve crystal quality, whereas nitrogen (N₂) was used for the InGaN layer to prevent indium corrosion [11,12]. Conventional cooling involves a direct temperature drop to room temperature in N₂ (“one‑step cooling”). Our novel “two‑step cooling” protocol first reduces the temperature to 400 °C in N₂, followed by a final drop to room temperature in H₂. Structural and morphological characterization employed high‑resolution X‑ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy‑dispersive spectrometry (EDS). The schematic of the sample structure is shown in Fig. 1.

The structure schematic of a single InGaN layer grown on a GaN template on sapphire.

Result and Discussion

AFM images reveal a distinct surface morphology for samples cooled with the two‑step protocol compared to the one‑step method (Fig. 2a,b). While both display the characteristic step‑flow 2D growth of InGaN and 3D islands associated with screw dislocations, the two‑step cooled surface also exhibits numerous uniform‑size quantum dots (average 100 × 100 nm, ~20 nm tall, density ~1.6 × 10⁸ cm⁻²). The key variable is the use of H₂ during the low‑temperature stage; thus the dot formation is linked to hydrogen’s interaction with the surface indium layer. To isolate the effect of the cooling gas, we applied the same two‑step cooling to GaN layers grown under identical conditions. AFM shows no dots on GaN (Fig. 3a), confirming that the phenomenon is specific to InGaN. When the In content is reduced to 1 %, the dot density drops sharply (Fig. 3b), indicating a direct correlation with surface indium concentration. XRD scans further distinguish the dots’ composition. Two‑step cooled samples display an additional peak at 2θ = 32.8°, absent in one‑step samples. This peak aligns with known diffraction from indium droplets [16,17], confirming that the dots are indium‑rich. SEM and EDS mapping (Fig. 5) corroborate that the dots are enriched in indium while surrounding Ga and N remain uniformly distributed. An intriguing observation is that annealing the InGaN surface at 800 °C for 60 s removes the indium‑rich layer. Subsequent two‑step cooling fails to produce dots (Fig. 6), establishing that the dots originate from the indium‑rich surface layer rather than the bulk InGaN lattice. We propose that hydrogen acts as a corrosive agent at low temperatures (<400 °C), converting surface indium nitride to metallic indium and ammonia (3 H₂ + 2 InN → 2 In + 2 NH₃). Unlike high‑temperature growth where liberated indium diffuses away, the low thermal energy prevents desorption, allowing indium atoms to coalesce into droplets. Further experiments varying In content (7.65 %–9.6 %) and layer thickness (≈13 nm vs. 41 nm) demonstrate that dot density increases with both parameters (Fig. 7). This trend mirrors theoretical predictions that surface indium concentration rises with bulk InGaN composition and thickness [21], reinforcing the surface‑origin hypothesis.

AFM topography of InGaN with (a) one‑step cooling and (b) two‑step cooling; inset: 3D view of surface.

AFM topography: (a) GaN with two‑step cooling; (b) InGaN (1 % In) with two‑step cooling.

XRD ω/2θ spectra: two‑step cooling (red) shows an extra peak at 32.8°, confirming indium droplets.

SEM image (a) and EDS maps (b–d) of In, N, Ga on two‑step cooled surface.

AFM topography after 60‑s annealing at 800 °C before two‑step cooling: no dots appear.

AFM topography of InGaN layers (a–d) with varying In content and thickness; higher In and thicker layers yield more dots.

Conclusion

We have demonstrated a reproducible method to fabricate uniform‑size indium quantum dots on InGaN surfaces via a two‑step cooling scheme. The dots arise from the reaction between the indium‑rich surface layer and hydrogen gas at temperatures below 400 °C. This process not only offers a new route to nanostructure fabrication but also provides a direct, non‑destructive probe of the elusive indium‑rich surface layer that impacts subsequent device performance.

Method

Samples were fabricated using an AIXTRON 6 × 2 in close‑coupled showerhead MOCVD reactor. Characterization employed XRD, AFM, SEM, and EDS. The study was conducted by researchers from the University of Chinese Academy of Sciences.

Availability of Data and Materials

The datasets used and/or analyzed during this study are available from the corresponding author upon reasonable request.

Abbreviations

AFM

Atomic force microscopy

EDS

Energy dispersive spectrometer

GaN

Gallium nitride

In QDs

Indium quantum dots

InGaN

Indium gallium nitride

InN

Indium nitride

LD

Laser diode

LED

Light‑emitting device

MOCVD

Metalorganic chemical deposition

MQW

Multiple quantum well

NH₃

Ammonia

QDs

Quantum dots

SEM

Scanning electron microscope

TMGa

Trimethylgallium

TMIn

Trimethylindium

XRD

X‑ray diffraction