Harnessing Atomic Layer Deposition for Next‑Generation Micro‑LEDs and VCSELs

Introduction

Since the first patent filed by Dr. Tuomo Suntola in 1977, ALD has evolved from a niche research tool into a cornerstone of advanced semiconductor processing. By 2007, Intel demonstrated the first 45 nm HfO₂ gate passivation on a metal‑oxide transistor, underscoring ALD’s precision and scalability. ALD relies on two temporally separated precursor pulses that react only at the surface, yielding self‑limiting monolayer growth. This mechanism ensures growth‑per‑cycle (GPC) values of 0.05–0.1 nm, allowing films to conform to high‑aspect‑ratio structures with nanometer‑scale uniformity. Unlike conventional CVD or PVD, ALD’s surface‑limited chemistry eliminates bulk reactions, yielding dense, pinhole‑free coatings essential for high‑reliability optoelectronic devices. The semiconductor community has embraced ALD for high‑k dielectrics, copper barriers, etch stops, and, crucially, for protective layers on LEDs and VCSELs. In OLED displays, for instance, water‑vapor transmission rates (WVTR) below 10⁻⁴ g m⁻² day⁻¹ are required, a benchmark routinely met by ALD films. For high‑power VCSELs, WVTR <10⁻³ g m⁻² day⁻¹ is essential to maintain performance in humid environments. This review focuses on ALD’s impact on micro‑LEDs and VCSELs, detailing passivation strategies, DBR enhancements, quantum‑well optimization, and transparent electrode deposition that collectively elevate device efficiency and longevity.

ALD Technologies for Micro‑LEDs

InGaN micro‑LEDs, the intrinsic piezoelectric fields give rise to the quantum‑confined Stark effect (QCSE), which shifts emission wavelength and degrades carrier recombination. Recent work has leveraged ring‑shaped nanostructures to relax strain and modulate the QCSE, but the benefits are curtailed by sidewall damage when device dimensions shrink below 20 µm.

While PECVD can deposit high‑k dielectrics rapidly, its loading effects and limited conformality make it suboptimal for sub‑10 µm devices. ALD, in contrast, delivers nanometer‑thick, dense films with superior step coverage and uniformity, even at low deposition temperatures (< 200 °C). Studies by Koehler et al. have shown that ALD‑grown Al₂O₃ and HfO₂ layers reduce leakage currents by up to 80 % compared to PECVD, while simultaneously increasing external quantum efficiency (EQE) by 20–30 % for devices as small as 5 µm. Figures 3 and 4 illustrate the dramatic improvement in reverse‑bias leakage and integrated optical power when ALD is used for sidewall passivation.

Reverse‑bias J‑V curves of micro‑LEDs with PECVD (a) and ALD (b) sidewall passivation [68]

Integrated spectral intensity comparison between ALD and PECVD passivation across current densities [68]

EQE measurements further confirm ALD’s advantage: for 20 × 20 µm² micro‑LEDs, ALD‑passivated devices reach 33 % EQE versus 24 % for PECVD, a 37 % relative improvement. In larger 100 × 100 µm² devices, the sidewall ratio diminishes, and EQE converges across methods, underscoring ALD’s critical role at the microscale.

In summary, ALD‑based sidewall passivation mitigates defect‑induced leakage, preserves carrier recombination, and sustains high luminous efficacy as micro‑LEDs scale down—an essential capability for next‑generation display and lighting technologies.

ALD Technologies for VCSEL

Oxide‑confined VCSELs offer low jitter and high modal stability, but their high‑power operation makes them vulnerable to moisture ingress and thermal stress. The VCSEL architecture—p‑DBR, active cavity, n‑DBR—requires layers with reflectivity > 99 % and minimal interfacial roughness to maintain low threshold currents.

Passivation Layers

Traditional PECVD deposition demands thick Al₂O₃ layers to achieve sufficient barrier performance, which introduces stress and degrades reliability. ALD, by depositing 5–10 nm of Al₂O₃ with atomic precision, provides a dense, pinhole‑free barrier that encapsulates the mesa sidewalls without excess stress. In an 85 °C/85 % RH aging study, devices coated with ALD‑grown Al₂O₃ + PECVD SiN_x survived 960 h, compared to 500 h for SiN_x alone, demonstrating a 92 % improvement in wet high‑temperature operation life (WHTOL).

WHTOL (85 °C/85 % RH) of VCSELs: (a) SiN_x only, (b) Al₂O₃ + SiN_x passivation

DBR Enhancements

ALD enables precise control of the quarter‑wave thicknesses of high‑index/low‑index pairs such as HfO₂/Al₂O₃. Transmission electron microscopy confirms sub‑nanometer interface roughness and sharp compositional boundaries (Fig. 12). Compared to MOCVD, ALD‑grown DBRs exhibit higher reflectivity and lower insertion loss, as shown by transfer‑matrix calculations (Fig. 13).

TEM image of ALD‑deposited HfO₂/Al₂O₃ DBR [84]

Multiple Quantum Wells

For VCSELs operating in the 850‑nm band, ALD‑based anti‑reflection (AR) coatings and surface passivation of InGaAs/GaAs MQWs reduce non‑radiative recombination by > 15 %. Surface‑treated MQWs (e.g., HCl or NH₄OH) followed by Al₂O₃ ALD preserve indium content while introducing a thin oxide that mitigates surface states (Fig. 15).

Effect of surface treatments on InGaN before Al₂O₃ ALD [86]

Transparent Electrodes

Conventional Ti/Au contacts absorb emitted photons and limit current spreading. ALD‑grown indium‑tin‑oxide (ITO) films provide > 96 % transmittance and excellent sheet resistance, yielding a 27 % boost in output power for 850‑nm VCSELs compared to metal contacts. Recent work demonstrates that ALD‑deposited ZnO and Al₂O₃ layers can further improve current uniformity and reduce optical loss.

SEM images of ZnO films deposited by ALD and CVD for transparent electrodes

Conclusions

ALD has proven indispensable for the next wave of optoelectronic devices. Its monolayer‑level control enables sidewall passivation that suppresses leakage in micro‑LEDs and protects oxide‑confined VCSELs from moisture and thermal stress. Al₂O₃/SiN_x stacked dielectrics, ALD‑grown DBRs with precise thickness, and high‑transparency TCO electrodes collectively raise device efficiency, reliability, and scalability. Continued integration of ALD in semiconductor fabs promises further gains in power handling, data‑rate capacity, and energy efficiency for future display, lighting, and communication systems.

Availability of data and materials

The data supporting this review are available from the corresponding authors upon reasonable request.

Abbreviations

PECVD:

Plasma‑enhanced chemical vapor deposition

ALD:

Atomic layer deposition

LED:

Light‑emitting diode

VCSEL:

Vertical cavity surface‑emitting laser

CVD:

Chemical vapor deposition

PVD:

Physical vapor deposition

GPC:

Growth per cycle

HKMG:

High‑K metal gate

FinFET:

Fin field‑effect transistor

WVTR:

Water vapor transmission rate

OLED:

Organic light‑emitting diode

MEMS:

Micro‑electro‑mechanical systems

μLED:

Micro‑LED

WHTOL:

Wet high‑temperature operation life

OOK:

On–off keying

PAM4:

Amplitude modulation 4‑level