Hydrogen‑Assisted Atomic Rearrangement in GaN‑Based Multiple Quantum Wells: Enhancing Structural Uniformity and Optical Efficiency at 750 °C

Abstract

We report three GaN‑based multiple quantum well (MQW) samples grown at a low temperature of 750 °C. Instead of the conventional temperature ramp‑up, we introduced a hydrogen/ammonia (H₂/NH₃) gas mixture during the interruption after depositing the InGaN well layers. By varying the hydrogen flux, we observed a pronounced atomic rearrangement within the MQWs. Transmission electron microscopy (TEM) reveals that an optimal hydrogen flow produces sharp, flat interfaces and a homogeneous indium distribution. Consequently, luminescence efficiency is markedly improved through suppression of non‑radiative recombination. The atomic rearrangement is attributed to the enhanced diffusion of gallium and indium adatoms in the H₂/NH₃ environment, lowering the potential barrier for thermodynamic equilibration. Excessive hydrogen, however, partially damages the MQWs and degrades optical performance.

Introduction

InGaN/GaN multiple quantum wells are central to high‑efficiency optoelectronic devices across the visible spectrum. Growing high‑indium‑content MQWs for blue and green LEDs and laser diodes remains challenging, primarily because indium incorporation is suppressed at elevated temperatures, while low temperatures hinder gallium surface diffusion, leading to three‑dimensional growth and rough barrier layers. Additionally, the large lattice mismatch between InN and GaN promotes phase segregation and indium compositional grading, further compromising uniformity.

Numerous strategies—higher‑temperature barrier growth, post‑growth temperature ramp‑up, growth interruptions, and hydrogen‑assisted barrier deposition—have been explored to sharpen interfaces and homogenize indium distribution. Yet, most approaches still rely on temperature ramp‑up, which limits indium incorporation and can degrade high‑indium MQWs. Hydrogen incorporation during InGaN growth is generally avoided due to its detrimental effect on indium retention. In this study, we introduce a controlled H₂/NH₃ mixture during a brief interruption following InGaN well deposition, without any temperature ramp‑up, and investigate its influence on MQW structure and optical properties.

Experimental Process

Samples A, B, and C were fabricated on c‑plane sapphire using a Thomas Swan 3×2 in. close‑coupled showerhead reactor. Triethylgallium (TEGa), trimethylindium (TMIn), and ammonia (NH₃) served as Ga, In, and N precursors, respectively. Each structure comprises a 2‑µm Si‑doped GaN buffer, a two‑period unintentionally doped InGaN/(In)GaN MQW active region, and a 150‑nm Mg‑doped GaN cap. All layers were grown at 750 °C. A thin GaN cap was deposited over each InGaN well to protect it during hydrogen exposure.

Sample A (reference) continued directly with barrier growth after the cap. Sample B received 100 sccm H₂ for 100 s, followed by barrier deposition. Sample C received 200 sccm H₂ for the same duration. In all cases, NH₃ flow was maintained at 3 slm, ensuring a mixed H₂/NH₃ atmosphere during hydrogen treatment. The remaining growth parameters were identical across the three samples.

Structural analysis employed a JEOL JEM‑F200 TEM. High‑resolution X‑ray diffraction (HRXRD) was performed on a Rigaku SmartLab. Temperature‑dependent photoluminescence (TD‑PL) spectra were recorded from 30 to 300 K using a 325 nm He‑Cd laser. Microscopic PL (μ‑PL) imaging utilized a Nikon A1 confocal microscope excited with a 405 nm laser.

Results and Discussion

Photoluminescence at Room Temperature

Room‑temperature PL spectra show that sample B, treated with 100 sccm H₂, exhibits the strongest emission, while sample C’s intensity is intermediate, and sample A’s is the lowest. All samples peak near 455 nm, corresponding to InGaN/GaN interband transitions, with a secondary peak around 365 nm attributed to near‑band‑gap GaN emission. These results indicate that a moderate hydrogen flux enhances luminescence, whereas excessive hydrogen degrades it.

TEM Analysis of MQW Interfaces

TEM images reveal that sample A has undulated QW/QB interfaces and significant indium clustering at the well surfaces. In contrast, samples B and C display steep, flat interfaces with minimal thickness fluctuations. The indium distribution in sample B is uniform, whereas sample C shows interface disruptions, suggesting partial etching at the higher hydrogen flux. These observations confirm that an optimal H₂ flow (100 sccm) induces an atomic rearrangement that smooths interfaces and homogenizes indium.

Mechanism of Hydrogen‑Induced Rearrangement

At 750 °C, gallium adatoms possess limited surface mobility, favoring three‑dimensional barrier growth and undulated interfaces. Indium atoms, due to the large miscibility gap with GaN, tend to cluster. Introducing H₂ in the presence of NH₃ increases surface coverage of NH₂ radicals, reducing the binding energy of Ga and In adatoms and thereby enhancing their diffusion and desorption. During the 100‑s hydrogen exposure, gallium and indium can migrate longer distances, achieving thermodynamic equilibrium and flattening interfaces. Indium‑rich clusters desorb preferentially, leading to a more uniform composition. Excessive hydrogen (200 sccm) further increases indium desorption and introduces etching, causing interface damage observed in sample C.

Structural Parameters from HRXRD

ω–2θ scans around the (0002) reflection were fitted using Global Fit. Sample B shows a slight reduction in indium content relative to sample A, attributed to hydrogen‑induced etching of the wells. Sample C exhibits further reductions in well thickness and indium content, while the barrier layers become thicker and more indium‑rich due to re‑incorporation of desorbed indium. These trends align with the TEM findings.

Low‑Temperature PL and Luminescence Uniformity

At 30 K, all samples display a phonon replica ~90 meV above the main peak, indicating GaN phonon coupling. Sample A’s peak energy is lower than that of B and C, reflecting its higher indium content. Full‑width at half maximum (FWHM) values are 12.3 nm (A), 10.1 nm (B), and 12.6 nm (C). The narrowest FWHM in sample B confirms superior uniformity of indium distribution and well thickness.

Temperature‑Dependent PL Shift

All samples exhibit an initial blue shift followed by a red shift as temperature increases, characteristic of localized state dynamics in GaN‑based MQWs. The magnitude of the blue shift is largest for sample A and smallest for sample B, indicating the most homogeneous localization landscape in sample B. Sample C shows a distinct red‑shift turning temperature (~160 K) compared to ~200 K for A and B, likely due to interface damage altering carrier localization.

Micro‑PL Mapping

μ‑PL images reveal that sample A contains numerous non‑luminescent spots, reflecting indium clustering and associated non‑radiative centers. Sample B shows a significant reduction in such spots, consistent with improved indium homogeneity. Sample C re‑introduces a few small non‑luminescent regions, attributable to the partially damaged MQWs.

Internal Quantum Efficiency

Using the integrated PL intensity ratio I₃₀₀K/I₃₀K as a proxy for IQE, sample B achieves an IQE of 30.21 %, a dramatic increase from 1.61 % (A) and 18.48 % (C). The improvement stems from flattened interfaces, uniform indium distribution, and suppressed non‑radiative recombination. Excessive hydrogen reduces IQE due to interface damage.

Conclusion

We demonstrate that a brief H₂/NH₃ treatment after InGaN well deposition, at 750 °C without temperature ramp‑up, induces an atomic rearrangement that produces flat, sharp MQW interfaces and a homogeneous indium profile. This process yields a three‑fold increase in luminescence intensity and a 30 % IQE. However, hydrogen flux must be carefully controlled; excess flux damages the wells and degrades performance.