Optimizing Silicon‑Doped Quantum Barriers Boosts Brightness and Efficiency of Eight‑Period InGaN/GaN Blue LEDs

Abstract

We investigated eight‑period In_0.2Ga_0.8N/GaN quantum wells (QWs) in blue light‑emitting diodes (LEDs) with silicon (Si) doping introduced in the first two to five quantum barriers (QBs). Epilayers were grown on 20‑pair In_0.02Ga_0.98N/GaN superlattices, serving as strain‑relief layers (SRLs), on patterned sapphire substrates (PSSs) via low‑pressure metal‑organic chemical vapor deposition (LP‑MOCVD). Temperature‑dependent photoluminescence (PL), current‑voltage (I‑V), light‑output (L‑I), and high‑resolution transmission electron microscopy (HRTEM) measurements revealed that the sample with four Si‑doped QBs exhibited the highest carrier‑localization energy (41 meV), the lowest turn‑on voltage (3.27 V), the most favorable breakdown voltage (−6.77 V), and the strongest light‑output at high injection currents. The reduced barrier height creates a soft confinement potential, leading to better carrier distribution, suppressed Auger recombination, and higher radiative efficiency. These findings demonstrate that a four‑QB Si‑doping scheme effectively mitigates the quantum‑confined Stark effect (QCSE) and enhances the optical performance of InGaN/GaN blue LEDs.

Background

Silicon doping in GaN QBs screens polarization fields, thereby suppressing the quantum‑confined Stark effect (QCSE) in InGaN/GaN QWs. This suppression markedly enhances exciton radiative recombination, as reported in several studies [1], [2]. In ternary InGaN alloys, compositional fluctuations and spinodal phase separation create indium‑rich clusters that act as deep carrier traps, but also as powerful localization sites that reduce non‑radiative recombination. Accounting for indium inhomogeneity is essential for accurate modeling of internal quantum efficiency (IQE), external quantum efficiency (EQE), and current‑voltage characteristics.

Si‑doped QBs have been shown to modify the nanostructure of QWs, promote thermal stability, improve light‑output power, and enhance electro‑static discharge performance as the doping level increases. They also block hole transport, forcing recombination to occur in the well region between p‑GaN and the doped barriers. Recent reports indicate that 9–12 QW periods on PSSs yield significant improvements in brightness and EQE droop. Therefore, optimizing the number and thickness of Si‑doped QBs is critical for maximizing high‑current blue‑LED performance.

Experimental Methods

Figure 1 shows the epitaxial structure of the fabricated blue‑LED chips. Epilayers were deposited on c‑plane PSSs (pyramid height 1.5 µm, interval 1 µm) using a horizontal LP‑MOCVD reactor. Precursors included TMGa, TMIn, TMAl, NH₃, SiH₄ (for Si), and Cp₂Mg (for Mg). Carrier gases were a 1:1 mixture of H₂ and N₂. A 3‑µm undoped GaN buffer was followed by a 3.3‑µm n‑GaN layer doped to 10¹⁹ cm⁻³. Twenty 2/2 nm In_0.02Ga_0.98N/GaN superlattice pairs served as SRLs. The active region comprised eight 2.5‑nm In_0.2Ga_0.8N QWs separated by 8‑nm GaN QBs, grown at 750 °C (QWs) and 900 °C (QBs). Si doping (≈3×10¹⁷ cm⁻³) was applied to the first two (sample A), three (sample B), four (sample C), or five (sample D) QBs. A 20‑nm Al_0.16Ga_0.84N electron‑blocking layer (EBL) and a 100‑nm p‑GaN window (10¹⁹ cm⁻³) were grown at 950 °C. Mesa‑type devices (1 mm²) were defined by ICP etching, with ITO transparent contacts and Cr/Pt/Au ohmic layers.

Electrical characteristics (I‑V, L‑I) were measured from 20 mA to 300 mA at room temperature. Temperature‑dependent PL (10–300 K) used a 325‑nm He‑Cd laser (45 mW). HRTEM imaging employed a 300 kV FE‑TEM.

Results and Discussion

Figure 2 displays the temperature‑dependent PL spectra. All samples exhibit the expected quenching with rising temperature due to increased non‑radiative processes. Gaussian fitting of the PL peaks (Figure 3) reveals that Si‑doped QBs induce a blue shift relative to the undoped reference, confirming effective QCSE suppression. The Varshni‑type model fits the S‑shaped temperature dependence, yielding localization energies of 24 meV (A), 28 meV (B), 41 meV (C), and 13 meV (D). Sample C, with four Si‑doped QBs, shows the strongest localization, correlating with superior radiative recombination.

I‑V curves (Figure 4) show that sample C achieves the lowest turn‑on voltage (3.27 V) and the highest breakdown voltage (−6.77 V). The reduced QB barrier height in this sample creates a smoother confinement potential, reducing Auger recombination and carrier leakage. L‑I measurements (Figure 5) confirm that sample C delivers the highest light output at 300 mA, with minimal saturation, whereas sample D suffers from the weakest confinement and lowest output.

HRTEM images (Figures 6–8) illustrate the layer structure and compositional variations. Sample C exhibits more diffusive QW/QB interfaces, indicative of interdiffusion that softens the potential barrier and improves carrier spread. Indium‑rich clusters and associated strain relaxation are observed in all samples, but the most pronounced localization occurs in sample C. Threading dislocations are absent across the samples, underscoring the high crystalline quality afforded by PSSs and SRLs.

Conclusions

Our systematic study demonstrates that introducing Si doping into the first four QBs of an eight‑period In_0.2Ga_0.8N/GaN structure yields the most favorable optical and electrical performance. The resulting soft confinement potential, enhanced carrier localization, and reduced QCSE collectively suppress Auger processes and leakage, thereby boosting light output under high injection. These insights provide a clear pathway for optimizing Si‑doped QBs in high‑efficiency blue LEDs.

Abbreviations

Al

Aluminum

Cp₂Mg

Bis‑cyclopentadienyl magnesium

EBL

Electron blocking layer

EQE

External quantum efficiency

FE‑TEM

Field emission transmission electron microscope

Ga

Gallium

HRTEM

High‑resolution transmission electron microscopy

ICP

Inductively coupled plasma

In

Indium

IQE

Internal quantum efficiency

ITO

Indium tin oxide

I‑V

Current‑voltage

L‑I

Light‑output versus injection current

LP‑MOCVD

Low‑pressure metal‑organic chemical vapor deposition

Mg

Magnesium

N

Nitrogen

n‑GaN

n‑type GaN

NRCs

Non‑radiative recombination centers

PL

Photoluminescence

PSSs

Patterned sapphire substrates

PZ

Piezoelectric

QBs

Quantum barriers

QCSE

Quantum‑confined Stark effect

QWs

Quantum wells

RT

Room temperature

Si

Silicon

SiH₄

Silane

SRLs

Strain relief layers

TCL

Transparent contact layer

TMAl

Trimethylaluminum

TMGa

Trimethylgallium

TMIn

Trimethylindium

u‑GaN

Undoped GaN