Controlled growth of rare-earth-doped TiO$_{2}$ thin films on III-V semiconductors for hybrid quantum photonic interfaces

TL;DR

This work tackles the integration of bright III-V quantum-dot photon sources with long-lived rare-earth quantum memories by directly growing Er3+:TiO2 thin films on GaAs and GaSb via low-temperature pulsed laser deposition. A surface-chemistry–driven approach uses arsenic-capped substrates and an oxygen-deficient buffer to achieve epitaxial anatase TiO2 at ~390 °C with sub-300 pm roughness, while buffer thickness and growth temperature steer phase toward anatase or rutile, as predicted by MCIA modeling and confirmed by XRD. Optical activity of Er3+ is verified through Raman and cryogenic PLE, revealing phase- and substrate-dependent linewidths and lifetimes, and linking microstructure and interface chemistry to dephasing and nonradiative losses. Collectively, these results establish a scalable materials platform for monolithically integrating Er3+ quantum memories with III-V quantum light sources, advancing the development of hybrid quantum photonic chips for telecom wavelengths.

Abstract

Quantum photonic networks require two distinct functionalities: bright single-photon sources and long-lived quantum memories. III-V semiconductor quantum dots excel as deterministic and coherent photon emitters, while rare-earth ions such as erbium (Er$^{3+}$) 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 low-temperature pulsed laser deposition of Er$^{3+}$-doped TiO$_{2}$ 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 TiO$_{2}$ (001) at 390$^{o}$C with sub-300 pm surface roughness, while avoiding interface degradation. In contrast, high-temperature oxide desorption or growth temperatures drive the transition to 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 Er$^{3+}$ ions. This multi-parameter 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.

Figures (10)

• Figure 1: Growth process for bulk-doped Er$^{3+}$:TiO$_2$-(III-V) samples synthesized using PLD.
• Figure 2: Images of the epi-ready GaAs substrate (sample GaAs-HT-1) after native oxide layer desorption are shown in (a) and (b) for the [010] and [011] directions, respectively. RHEED patterns for a separate sample (GaAs-HT-2) after $\sim90nm$ of TiO$_2$ growth utilizing a similar epi-ready GaAs substrate is shown in (c) and (d) for the [010] and [011] directions, respectively. Images of the GaAs substrate (sample GaAs-LT-1) post-amorphous arsenic cap desorption are shown in (e) and (f) for the [010] and [011] directions, respectively. RHEED patterns for a separate sample (GaAs-LT-2) after $\sim60nm$ of TiO$_2$ growth also utilizing an amorphous arsenic capped GaAs substrate is shown in (g) and (h) for the [010] and [011] directions, respectively.
• Figure 3: Example AFM scan after post-processing for a (a) low-temperature (A-TiO$_2$)-GaAs sample (GaAs-LT-3) utilizing a protective amorphous arsenic cap and (b) high-temperature (R-TiO$_2$)-GaAs sample (GaAs-HT-1) synthesized after desorbing the native oxide layer present on the GaAs substrate. (c) Box-and-whisker plot of mean RMS roughness values extracted across multiple scans organized by substrate. (d) Scatter plot comparing ending growth temperature (T$_{grow}$) to mean RMS roughness value extracted across multiple scans.
• Figure 4: (a) Raman spectroscopy results for two high temperature (R-TiO2)-III-V samples. The dark red trace is sample GaSb-HT-1 (bulk-doped; GaSb substrate), and the light red trace is sample GaAs-HT-3 (sandwich-doped; GaAs substrate). Sample buffer shots and buffer growth temperatures are included within the figure. (b) $Z_1 \rightarrow Y_1$ Er$^{3+}$ PLE results for samples GaAs-HT-4 (purple) and GaSb-HT-1 (green). (c) Raman spectroscopy results for two low temperature (A-TiO2)-III-V samples. The dark blue trace is sample GaSb-LT-1 (bulk-doped; GaSb substrate), and the light blue trace is sample ST2417 (sandwich-doped; GaAs substrate). Sample buffer shots and buffer growth temperatures are included within the figure. (d) $Z_1 \rightarrow Y_1$ Er$^{3+}$ PLE results for samples GaAs-LT-1 (purple) and GaSb-LT-1 (green).
• Figure 5: (a) Phase diagram for all synthesized TiO$_2$ thin films on GaAs and GaSb substrates. (b) Raman spectra for the open purple triangle R-TiO$_2$ film on GaAs substrate (red trace; sample GaAs-LT-6) data point in (a) along with a bulk-doped A-TiO$_2$ film on GaAs substrate (blue trace; sample GaAs-LT-1) for comparison. Relevant phonon modes with drop lines to guide the eye are labeled for GaAs, A-TiO$_2$ (blue "A" prefix), and R-TiO$_2$ (red "R" prefix).