# Controlled Growth of Rare-Earth-Doped TiO2 Thin Films on III–V Semiconductors for Hybrid Quantum Photonic Interfaces

## TL;DR

Researchers developed a method to grow smooth, rare-earth-doped TiO2 thin films on semiconductor materials, enabling integration of quantum photon sources and memories for scalable quantum chips.

## Contribution

A low-temperature growth strategy for Er3+-doped TiO2 thin films on III–V semiconductors is demonstrated, preserving quantum dot functionality and enabling hybrid quantum photonic integration.

## Key findings

- Low-temperature pulsed laser deposition enables epitaxial TiO2 growth on GaAs and GaSb with sub-300 pm surface roughness.
- High-temperature conditions lead to polycrystalline rutile films, confirmed by transmission electron microscopy.
- Raman and cryogenic photoluminescence spectroscopy confirm crystal phase and Er3+ optical activation.

## Abstract

Quantum photonic
networks require two distinct functionalities:
bright single-photon sources and long-lived quantum memories. III–V
semiconductor quantum dots (QDs) excel as deterministic and coherent
photon emitters, while rare-earth ions such as erbium (Er3+) in crystalline oxides offer exceptional spin and optical coherence
at telecom wavelengths. Combining these systems and their functionalities
via direct epitaxy is challenging due to lattice mismatch and incompatible
growth conditions. Here, we demonstrate the low-temperature pulsed
laser deposition of Er3+-doped TiO2 thin films
directly on GaAs and GaSb substrates. Controlled surface preparation
with an arsenic cap and an oxygen-deficient buffer layer enables the
growth of epitaxial anatase TiO2 (001) at ∼390 °C
with sub-300 pm surface roughness, while avoiding interface degradation.
In contrast, high-temperature oxide desorption or growth temperatures
drive the transition to a rough, polycrystalline rutile film, as confirmed
by transmission electron microscopy. Minimal coincident interface
area (MCIA) modeling explains the orientation-selective growth on
GaAs and GaSb. Raman and cryogenic photoluminescence excitation spectroscopy
verify the crystal phase and optical activation of Er3+ ions. This multiparameter growth strategy helps preserve III–V
quantum dot functionality and yields smooth surfaces suitable for
low-loss nanophotonic structures. Our results establish a materials
platform for monolithically integrating rare-earth quantum memories
with semiconductor photon sources, paving the way toward scalable
hybrid quantum photonic chips.