††thanks: These authors contributed equally.††thanks: These authors
contributed equally.

# Quasi-deterministic Localization of Er Emitters in Thin Film TiO2 through
Submicron-scale Crystalline Phase Control

Sean E. Sullivan<EMAIL_ADDRESS>memQ, Inc. Chicago, IL 60615, United States
Jonghoon Ahn<EMAIL_ADDRESS>Center for Molecular Engineering, Argonne
National Laboratory, Lemont, IL 60439, United States  Materials Science
Division, Argonne National Laboratory, Lemont, IL 60439, United States  Tao
Zhou  Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL
60439, United States  Preetha Saha  memQ, Inc. Chicago, IL 60615, United
States  Martin V. Holt  Center for Nanoscale Materials, Argonne National
Laboratory, Lemont, IL 60439, United States  Supratik Guha  Pritzker School of
Molecular Engineering, University of Chicago, Chicago, IL 60637, United States
Materials Science Division, Argonne National Laboratory, Lemont, IL 60439,
United States  F. J. Heremans  Center for Molecular Engineering, Argonne
National Laboratory, Lemont, IL 60439, United States  Materials Science
Division, Argonne National Laboratory, Lemont, IL 60439, United States
Pritzker School of Molecular Engineering, University of Chicago, Chicago, IL
60637, United States  Manish Kumar Singh  memQ, Inc. Chicago, IL 60615, United
States

###### Abstract

With their shielded 4f orbitals, rare-earth ions (REIs) offer optical and
electron spin transitions with good coherence properties even when embedded in
a host crystal matrix, highlighting their utility as promising quantum
emitters and memories for quantum information processing. Among REIs,
trivalent erbium (Er3+) uniquely has an optical transition in the telecom
C-band, ideal for transmission over optical fibers, and making it well-suited
for applications in quantum communication. The deployment of Er3+ emitters
into a thin film TiO2 platform has been a promising step towards scalable
integration; however, like many solid-state systems, the deterministic spatial
placement of quantum emitters remains an open challenge. We investigate laser
annealing as a means to locally tune the optical resonance of Er3+ emitters in
TiO2 thin films on Si. Using both nanoscale X-ray diffraction measurements and
cryogenic photoluminescence spectroscopy, we show that tightly focused below-
gap laser annealing can induce anatase to rutile phase transitions in a nearly
diffraction-limited area of the films and improve local crystallinity through
grain growth. As a percentage of the Er:TiO2 is converted to rutile, the Er3+
optical transition blueshifts by 13 nm. We explore the effects of changing
laser annealing time and show that the amount of optically active Er:rutile
increases linearly with laser power. We additionally demonstrate local phase
conversion on microfabricated Si structures, which holds significance for
quantum photonics.

††preprint: APS/123-QED

##  I Introduction

Solid-state quantum emitters in wide-bandgap semiconductors offer atom-like
level structures, making them candidate optically addressable spin qubits. [1]
Among these quantum emitters, rare-earth ions (REIs), are promising for
applications in quantum information processing owing to their long optical[2]
and spin coherence times.[3, 4, 5] Trivalent erbium (Er3+), in particular,
offers an optical transition in the telecom C-band (near 1.5 $\mu$m), which is
well suited for optical fiber-based quantum communication.[6] In addition,
integration of Er-doped crystals with nanophotonic cavities has enabled
emission of both single[7] and indistinguishable[8] photons, and single-shot
optical readout of hyperfine spin states.[9, 10] In contrast to trapped
neutral atoms or ions, the properties of solid-state spin qubits are
intimately tied to their environment, i.e., the host crystal in which they are
embedded. While the local crystal environment can be a source of noise or
relaxation from charge carriers or phonons, it also offers a potential phase
space for tuning the properties of the quantum emitter through materials
engineering. Although the 4f-4f optical transitions of REIs are somewhat
shielded from nearby sources of noise, the optical lifetimes, optical
coherence, and transition energies are nevertheless influenced by the local
crystal environment.[11] For Er3+, several suitable oxide host materials have
been identified that minimize the REI’s first-order sensitivity to local
electric fields by virtue of inversion-symmetric crystal sites. [12]
Furthermore, the spin coherence properties of these REIs can be enhanced by
selecting host crystals with a naturally low abundance of isotopes that have
nuclear spins, which can introduce phase noise through magnetic dipolar
interactions. [13, 4, 14] One such promising host material on both accounts is
TiO2, which has been explored as a host material with very low inhomogeneous
broadening ($<$ 460 MHz) in its bulk crystalline form. [15]

Importantly, high quality TiO2 films can readily be synthesized and integrated
into standard CMOS process flows, [16] enabling a pathway towards scalable
fabrication. Given its standard thin film integration, several recent works
have investigated TiO2 thin films as a host for Er quantum emitters in rutile,
anatase, and mixed-phase polycrystalline forms. [17, 18, 19] These works
showed that optical properties of the TiO2 films could be tuned through
engineering of the thin film stack by delta doping, epitaxial growth, or by
adjusting the growth temperature to select either the lower temperature
anatase crystal phase or the higher temperature rutile phase in aggregate. As
one of the most heavily studied metal oxide materials for
photoelectrochemistry, the optical properties of TiO2 nanoparticles and films
doped with Er have also been investigated for applications in photocatalysis
and photodetection, where high concentrations ($\lesssim$1 at%) of Er act as
up-conversion sensitizers in the visible range. [20, 21, 22]

In the anatase phase, the optical transition between the lowest crystal field-
split levels in the ground and first excited states (Z1-Y1) emits at a
wavelength near 1533 nm. [23, 19] Meanwhile, in the rutile phase, the Z1-Y1
transition is near 1520 nm. [15] This considerable difference in the
transition energies between the two phases yields opportunities for spectrally
isolating quantum emitters of interest. In parallel, prior efforts have
demonstrated the ability to locally adjust the crystalline phase of TiO2 films
and nanoparticles by laser annealing, showing conversion from amorphous films
to a crystalline phase or from anatase to rutile as detected by micro-Raman
spectroscopy[24, 25, 26] or transmission electron microscopy.[27] As with
other solid-state quantum emitters, such as those based on crystalline
defects, a key challenge for scalable quantum technologies is the
deterministic spatial placement of the emitter within a desired crystal
volume. Laser writing has been used as a localization technique for generating
vacancy complex-based quantum emitters in diamond[28] and silicon carbide.[29]
In a similar vein, we utilize micro-focused laser annealing as a means toward
localization of Er3+ emitters in TiO2 thin films. Here, we demonstrate
localized anatase to rutile conversion of moderately Er-doped TiO2 films with
near diffraction-limited spatial resolution. We use cryogenic
photoluminescence mapping and nanoprobe synchrotron-based X-ray diffraction
measurements to confirm the phase transition and to measure rutile grain size.

##  II Main

Polycrystalline thin films of TiO2 were grown on silicon substrates in a
molecular beam deposition chamber. The growth method and phase control via
growth temperature are described in more detail elsewhere.[17] In this work,
all measurements were performed on a 55 nm-thick polycrystalline anatase film
grown on 3-inch Si(100), which comprised three layers (40 nm, 10 nm, and 5 nm
from bottom to top) with the middle 10 nm doped with erbium at a $\sim$500
parts per million (ppm) concentration (Fig. 1(a) inset). This wafer was
subsequently diced into chips for different annealing experiments. The
particular film stack was chosen to maximize the observed brightness of the
Er3+ photoluminescence in this experiment while maintaining a thin profile
with an eye toward integrated quantum photonic applications. [18]

Figure 1: (a) A simplified schematic of the experimental setup (DMLP: dichroic
mirror long pass, LPF: long pass filter, BPF: band pass filter, SNSPD:
superconducting nanowire single-photon detector). A blue laser ($\lambda=449$
nm) is used for the laser anneal of the Er-doped TiO2 films. An off-resonant
laser ($\lambda=908$ nm) is used for the optical excitation for the cryogenic
photoluminescence measurements. The Er emission collected by the objective
passes through dichroic mirrors and filters and is detected by the
spectrometer or the SNSPD. The inset shows a schematic of the Er-doped TiO2
film stack. (b) Representative photoluminescence spectra of Er:TiO2 before
laser annealing (orange) and after annealing (blue). Measurements taken at
T=3.6 K.

The films were loaded into an optical cryostat integrated with a custom
confocal microscope setup as shown in Fig. 1(a). During the laser annealing
process, the films were kept at room temperature and under ambient atmospheric
conditions while a variable power $\lambda=449$ nm continuous wave (CW) laser
was tightly focused through an air objective lens with an N.A. of 0.67. The
objective lens was mounted on a three-dimensional periscope stage, which
allowed the control of the position and focus of the laser beam with respect
to the sample films. The laser anneal was performed in sets of arrays of 12
spots with 5 $\mu$m spacings, wherein among the different sets, the effective
$\lambda=449$ nm laser power was varied from below 100 mW to approximately 185
mW (at sample position) and the duration of the laser anneal was varied from 1
to 100 seconds. After the laser annealing step, the films were quickly
evaluated for phase change as-is, or they underwent a post-laser-annealing
process in a tube furnace in a 20% O2/balance N2 environment heated to either
400 ∘C or 500 ∘C for one hour. A quick verification of phase change could be
confirmed by measuring the photoluminescence (PL) spectra of the films (Fig.
1(b)).

We observe that laser annealing at high powers causes a morphological change
in the films. Figure 2(a) shows a scanning electron micrograph of a laser-
annealed spot (1 second, 185 mW) after a 400∘C post-anneal. We see that for
these conditions, the visibly small grains of the surrounding polycrystalline
film undergo grain growth within the irradiated spot, which is on the order of
450 nm in diameter and is only slightly larger than the expected diffraction-
limited $1/e$ Gaussian beam diameter of $0.62\lambda/$NA$\approx 415$ nm for
an overfilled objective lens.

To investigate local changes in the crystal phase, we performed scanning X-ray
diffraction microscopy measurements on the Hard X-ray Nanoprobe Beamline
operated by the Center for Nanoscale Materials at the Advanced Photon Source
at Argonne National Laboratory. A 10 keV X-ray beam was focused to a 20 nm
spot and scanned across the surface of a laser-annealed film. In contrast to
the chips with the arrays of 12 identical laser annealed spots, this chip was
diced from the same wafer and patterned with Cr fiducial markers. Subsequent
laser annealing was performed in an array between the markers, varying the
laser power in each row and increasing the laser annealing time in each column
of the array. This chip was then annealed for 1 hour at 400∘C in 20% O2 prior
to the XRD measurements. As shown in Fig. 2, rutile X-ray diffraction was only
observed for the highest laser annealing power (185 mW) used in this array.
Figure 2(b) shows the summed diffraction pattern from a raster scan covering
an area of $3\times 3~{}\mu$m2. The anatase 101 reflection exhibits a
continuous powder ring due to the random crystal orientation of the nanosized
crystalline grains within the probed region. A donut-shaped diffraction spot
was observed at the 2$\theta$ value for the rutile 110 reflection. The donut
is an image of the focusing optics, the appearance of which indicates good
crystallinity. We note that the crystal orientation of the rutile phase is
also random, but the diffraction spots of the rutile phase do not form a
continuous ring because statistically only the diffraction from one or two
grains can be observed on the detector for a given incident angle. Diffraction
from other rutile grains were observed after tilting the sample to different
incident angles. Figure 2(c) shows a dark field image of 29 rutile grains
observed for an incident angle range of 6.0∘ to 12.8∘. The rutile phase was
only found at a few locations scattered between the two Cr fiducial markers.
These locations are consistent with the position of the laser annealed spots
at a power of 185 mW. In this particular laser-annealed sample, the next
lowest laser power used was $<100$ mW and no rutile signal was observed. The
size of individual rutile grains was determined by performing refined line
scans across individual grains and fitting the rutile diffraction signal with
a Gaussian profile. The average size of the 29 grains is about 88 nm as shown
in Fig. 2(d).

Figure 2:  (a) Scanning electron micrograph of a single laser-annealed spot on
an anatase film (1 second at $\sim$185 mW, post-anneal 400 ∘C for 1 hour in
20% O2) showing grain growth in the center. (b) X-ray diffraction pattern
produced by summing up all the detector images acquired during a 3x3 $\mu$m2
raster scan on a film patterned with Cr alignment markers. The angular
$2\theta$ range covers both the anatase 101 and the rutile 110 reflections.
The continuous ring along the $\gamma$ direction is formed by diffraction from
crystals with random orientations. (c) Composite image showing the locations
of the rutile phase. The real-space image was produced by summing up dark
field images of the rutile 110 reflection acquired over a range of incidence
angles between 6.0 and 12.8∘. The spots are wider in the X direction due to
the projection of the incident X-ray beam at shallow incident angles. Red
circles highlight three laser-exposed spots ($\sim$185 mW, 1 sec, 10 sec, 100
sec left to right). Also shown is X-ray fluorescence signal from the nearby Cr
alignment structures. (d) Statistics of measured individual rutile grains
extracted from spatial scans. The teal dashed line is a Gaussian fit to the
data, indicating an average grain size of $88\pm 30$ nm.

We additionally investigated the effect of laser annealing power and duration
on the Er:rutile transition through cryogenic PL measurements. After the laser
annealing and a subsequent tube furnace annealing process, the films were
cooled to T=3.6 K and excited with an off-resonant $\lambda=908$ nm laser. The
PL spectra collected from the laser annealed areas showed an enhanced PL
signal at about 1520 nm along with the peak at about 1533 nm, attributed to
Er:rutile and Er:anatase, respectively (Fig. 1(b)). The rutile PL signal was
selectively collected using a tunable bandpass filter set to a center
wavelength of 1520 nm and a FWHM of 950 pm positioned before the
superconducting nanowire single photon detector (SNSPD). As the excitation
laser was scanned across the sample, clear and bright micron-scale spots were
observed as in Fig. 3(a). We note that laser annealing powers below about 169
mW incident on the sample with 1 second duration yielded no detectable phase
change while a weak rutile signal appeared when increasing the duration to 10
seconds. As shown in Fig. 3(c), post-annealing at 500 ∘C seemed to have a
similar effect on the PL scans compared to 400 ∘C. While the post-anneal in
the tube furnace was not critical to observe the anatase$\rightarrow$rutile
transition, we ultimately found that the intensity of the principle rutile PL
line was dramatically increased after the furnace annealing. We additionally
attempted laser annealing with the same conditions in a partial vacuum
environment (few mTorr) at room temperature and at 3.6 K; however, no rutile
PL signal was observed. This is in contrast to a prior work, which observed
anatase$\rightarrow$rutile transition in thin films after annealing in vacuum
with high-power 532 nm excitation focused to a spot size of about 100
$\mu$m.[26] Meanwhile, the presence of oxygen vacancies and their enhanced
ionic mobility in the presence of an oxygen-rich atmosphere has been proposed
as a mechanism by which the anatase$\rightarrow$rutile transition can occur
during photoexcitation at below-gap energies ($<$3.2 eV).[25] We attribute the
apparent lack of phase conversion to the reduced oxygen concentration during
the laser annealing process. This further suggests a phase change mechanism
that is not based on heating alone from the below-gap excitation.

Next, we remove the tunable bandpass filter and directly collect the PL signal
with a spectrometer at the locations that showed the bright spots in the
rutile-filtered spatial maps. Fitting each of these peaks to Lorentzian
profiles, we examine the rutile/anatase integrated intensity ratio$-$in order
to account for any drifts in focus$-$as the annealing laser power is
increased. Figures 3(b) and (d) show the intensity ratio $I_{R}/I_{A}$ for
films that underwent 400 ∘C and 500 ∘C post-anneals, respectively, while the
blue circles correspond to 1 second laser anneals and the red squares
correspond to 10 second laser anneals. We find that the intensity ratio
increases linearly with laser power; however, for the 169 mW annealing case,
phase conversion was only observed for exposure times $\geq$10 seconds.
Additionally, as the post-anneal temperature increases, we observe an increase
in the relative anatase intensity and a decrease in the intensity of the
background PL signal. This increase in anatase signal with increasing post-
anneal temperature could be potentially attributed to further crystallization
and grain growth of nearby anatase grains while staying below the point of the
rutile transition. It should be additionally noted that the diffraction-
limited probe laser spot (at $\lambda=908$ nm) used for PL measurements would
be considerably larger than the $\lambda=449$ nm annealing laser. Thus, the PL
signal will include anatase phase TiO2 from the periphery of the annealed
spot. The decrease in the background intensity, meanwhile, is likely
attributable to healing of other crystalline defects and desorption/combustion
of carbon compounds on the surface that may contribute to fluorescence.

Figure 3:  (a) Spatial map of the laser annealing dose array showing rutile-
filtered PL. Blocks of 12 points share identical laser annealing conditions,
as indicated by the labels. Regions that were annealed at lower powers that
did not result in visible phase change are indicated by red boxes. (b)
Rutile/anatase integrated intensity ratio as a function of laser annealing
power. The points represent the average measured value for the 12 annealed
spots in each array block in (a). Blue circles correspond to 1 second laser
annealing times and red squares represent 10 second laser anneals. The dashed
lines are linear fits to the measurement data. The inset shows the averaged
spectrum for 1 second laser anneals at the maximum laser power in each case.
Error bars are calculated from the standard deviation of the 12 measurements
for each condition at the 95% confidence level. Both (a) and (b) show results
for the T=400 ∘C post-annealed film. (c) The PL map and (d) integrated
rutile/anatase intensity ratio for the T=500 ∘C post-annealed film. The bright
spot visible in (a) is an artifact from a scratch on the film surface.

Lastly, we demonstrate laser-induced anatase$\rightarrow$rutile phase change
on microstructures etched into the Si substrate. We first patterned ridge-like
structures into the Si through standard photolithography and plasma etching.
These structures range from approximately 500 nm to a couple of microns in
width and are approximately 3 $\mu$m tall (Fig. 4(a)). After fabrication, we
laser annealed the TiO2 films on top of the ridges; however, instead of spot
annealing, we scanned the laser across the ridges at a rate of 5 mm/sec. For
each vertical cross-cut, we set a different laser power. After a 400 ∘C post-
anneal, rutile PL is visible at low temperatures for regions exposed to the
three highest annealing powers, as shown in Fig. 4(b). Examining these
annealed spots under SEM, we observe similar morphological changes in the TiO2
films, often concentrated near the edges of ridge (Fig. 4(c-d)). Additionally,
we observe occasional straggle along the direction of the ridge, which is
transverse to the direction of laser scanning. We attribute this straggle to
vibrations in the assembly holding the objective lens. Future improvements to
the laser scanning assembly design can mitigate unwanted vibrations or
straggle of the focused laser spot, which could, for example, ultimately
facilitate the placement of Er:rutile at the anti-node of a photonic crystal
cavity. Beyond the anatase$\rightarrow$rutile transition, sub-bandgap laser
annealing has been shown to crystallize amorphous TiO2 films,[25] as well as
to effectively transform rutile back to anatase through amorphization of the
rutile phase and subsequent anatase crystallization.[26] Fully reversible
phase control would provide an important tuning knob, especially when
considering Er:TiO2 films that are evanescently coupled to proximal photonic
cavities whose resonant wavelength is deeply affected by the local
environment.

Figure 4: (a) An optical micrograph of the etched ridge structures, depicting
the path of the laser at the three highest powers (blue arrows). (b) The PL
map corresponding to the red outlined rectangle in (a) with white dashed lines
highlighting the ridge structures for clarity. (c) and (d) Scanning electron
micrographs of the annealing conditions highlighted in (b) by red and orange
squares, respectively.

##  III Conclusion

In conclusion, we have demonstrated local, submicron
anatase$\rightarrow$rutile phase conversion of Er:TiO2 thin films grown on Si
using below-gap ($\lambda=$449 nm) laser excitation. This phase change was
found to occur most readily when the laser annealing was performed in standard
atmosphere and at room temperature, and the Er3+ photoluminescence, especially
that of the anatase phase, was improved by post-annealing in an oxygen-rich
environment. While the rutile PL signal increased linearly with laser power,
no phase change was observed by PL for annealing powers below about 169 mW,
except for longer duration anneals (10 seconds). In addition, nanoscale X-ray
diffraction measurements highlight the improved crystallinity of the rutile
grains, vis-à-vis anatase, attributed to the laser annealing. Fine spatial
scans indicated an average grain size of $88\pm 30$ nm. Finally, we
demonstrated that this same phase change process can extended to TiO2 films on
Si microstructures, which are similar in form to waveguide structures in Si
photonics. Through further tuning of the anneal parameters and improvements in
the laser positioning capabilities, the ability to adjust the phase of host
materials with submicron resolution coupled with delta doping, opens up a new
avenue for deterministic placement of quantum emitters and spin qubits.

###### Acknowledgements.

The authors thank Nazar Delegan and Robert Pettit for helpful discussions.
Work at Argonne (M.V.H., D.D.A., S.G., F. J. H.) was supported by the U.S.
Department of Energy, Office of Science, National Quantum Information Science
Research Centers as part of Q-NEXT. We acknowledge additional support from the
U.S. Department of Energy, Office of Basic Energy Sciences, Materials Science
and Engineering Division (J A.). Work by S.E.S. and M.K.S. was carried out at
Argonne National Laboratory with support from the U.S. Department of Energy
Office of Science Advanced Scientific Computing Research program under CRADA
A22112 through the Chain Reaction Innovations program. Work performed at the
Center for Nanoscale Materials and Advanced Photon Source, both U.S.
Department of Energy Office of Science User Facilities, was supported by the
U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-
AC02-06CH11357.

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