# Sputtered AlN Buffer Layer for Low-Loss Crystalline AlN-on-Sapphire Integrated Photonics

## TL;DR

This paper shows how adding a sputtered buffer layer improves the quality of AlN-on-sapphire for photonic applications by reducing void-related losses.

## Contribution

A sputtered AlN buffer layer is introduced to eliminate voids in AlN-on-sapphire, achieving low propagation losses.

## Key findings

- Voids in AlN-on-sapphire layers cause propagation losses exceeding 30 dB cm–1 at 1550 nm.
- Sputtered buffer layers produce void-free AlN with propagation losses below 0.2 dB cm–1 at 1550 nm.
- Void-free layers enable high-quality microring resonators and efficient nonlinear optical processes.

## Abstract

In recent years, aluminum nitride (AlN) has emerged as
an attractive
material for integrated photonics due to its low propagation losses,
wide transparency window, and presence of both second- and third-order
optical nonlinearities. However, most of the research led on this
platform has primarily focused on applications rather than material
optimization, although the latter is equally important to ensure its
technological maturity. In this work, we show that voids, which are
commonly found in crystalline AlN-on-sapphire epilayers, have a detrimental
role in related photonic structures, as they can lead to propagation
losses exceeding 30 dB cm–1 at 1550 nm. Their impact
on light propagation is further quantified through finite-difference
time-domain simulations that reveal void-related scattering losses
are strongly dependent on their size and density in the layer. As
a possible solution, we demonstrate that when introducing a thin sputtered
AlN buffer layer prior to initiating AlN epitaxial growth, void-free
layers are obtained. They exhibit intrinsic quality factors in microring
resonators as high as 2.0 × 106, corresponding to
propagation losses lower than 0.2 dB cm–1 at 1550
nm. These void-free layers are further benchmarked for high-power
applications through second-harmonic and supercontinuum generation
in dispersion-engineered waveguides. Such layers are highly promising
candidates for short-wavelength photonic integrated circuit applications,
particularly given the strong potential of AlN for visible photonics.
Given that volumetric scattering losses scale as λ–4, the platform quality becomes increasingly critical in the visible
and ultraviolet range, where our improved layers are expected to deliver
enhanced performance.