# Spatially-resolved UV-C emission in epitaxial monolayer boron nitride

Adrien Rousseau, Juliette Plo, Pierre Valvin, Tin S. Cheng, Jonathan
Bradford, Tyler James, James Wrigley, Christopher Mellor, Peter Beton,
Sergei Novikov, et al.

► To cite this version:

Adrien Rousseau, Juliette Plo, Pierre Valvin, Tin S. Cheng, Jonathan Bradford, et al. Spatially-resolved UV-C emission in epitaxial monolayer boron nitride. 2D Materials, 2024, 11 (2), pp.025026. 10.1088/2053-1583/ad2f45. hal-04791801

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Spatially-resolved UV-C emission in epitaxial monolayer boron nitride

A. Rousseau,¹ J. Plo,¹ P. Valvin,¹ T. S. Cheng,² J. Bradford,² T. S. S. James,² J. Wrigley,²

C. J. Mellor,² P. H. Beton,² S. V. Novikov,²,\* V. Jacques,¹ B. Gil,¹ G. Cassabois¹,³,\*

¹Laboratoire Charles Coulomb UMR 5221 CNRS-Université de Montpellier, 34095 Montpellier, France

²School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK

³Institut Universitaire de France, 75231 Paris, France

Authors to whom any correspondence should be addressed*

(Dated: January 31, 2024)

We report hyperspectral imaging in the UV-C spectral domain in epitaxial monolayers of hexagonal boron nitride (hBN). Under quasi-resonant laser excitation, the UV-C emission of monolayer hBN consists in resonant Raman scattering and photoluminescence, which appear to be spatially uncorrelated. Systematic measurements as a function of the excitation energy bring evidence of a photoluminescence singlet at ~ 6.045 eV. The spatial variations of the photoluminescence energy are found to be around ~ 10 meV, revealing that the inhomogeneous broadening is lower than the average photoluminescence linewidth of ~ 25 meV, a value close to the radiative limit in monolayer hBN. Our methodology provides an accurate framework for assessing the opto-electronic properties of hBN in the prospect of scalable hBN-based devices fabricated by epitaxy.

Hexagonal boron nitride (hBN) is a pivotal crystal in the family of 2D materials [1, 2]. In its bulk form, the electronic bandgap of hBN is calculated to be indirect at around 6 eV [3–5]. Experimental evidence for indirect-gap bulk hBN was brought by optical spectroscopy in the UV-C spectral range [6–10], and independently confirmed by electron energy loss spectroscopy [11]. The suppression of any interlayer coupling in the limit of atomically-thin hBN induces a subtle shift of the conduction band minimum, that makes monolayer hBN a direct-gap semiconductor [3–5]. The indirect-direct crossover of the bandgap was first experimentally demonstrated in hBN, not by microscopy in samples fabricated by exfoliation of bulk lamellar crystals as in transition metal dichalcogenides [12, 13], but by reflectivity and photoluminescence (PL) spectroscopy in large-scale monolayer hBN epitaxially grown on graphite by high-temperature molecular beam epitaxy (HT-MBE) [14]. This disparity between transition metal dichalcogenides and hBN stemmed from the technical difficulties related to performing optical microscopy in the UV-C spectral range.

Recently, we reported hyperspectral imaging in ex-
foliated hBN flakes at low temperature [15], by means
of a scanning confocal cryomicroscope operating at the
diffraction limit at wavelengths around 200 nm [16].
Emission slightly above 6 eV was specifically detected in
atomically-thin hBN but not in multilayer hBN [15], thus
confirming, in exfoliated hBN flakes, the direct-bandgap
crossover initially observed in epitaxial monolayer hBN
[14]. Beyond the reproducible evidence of direct-gap
monolayer hBN in epitaxial and exfoliated samples, PL
spectroscopy emerges as the preferential characteriza-
tion tool to resolve the emission of atomically-thin hBN,
cathodoluminescence being of limited use because of

intrinsic limitations in the limit of ultrathin crystals
[17, 18].

A specific issue with PL in monolayer hBN arises from
the huge excitonic binding energy, which is calculated to
be ~ 2 eV in suspended monolayer hBN [19]. Although
this value is reduced by ~ 1 eV for monolayer hBN on
graphite because of the screening of the Coulomb inter-
action induced by the substrate [20], the onset of the
continuum absorption still lies at ~ 7 eV (~175 nm),
an energy corresponding to the vacuum UV spectral do-
main, beyond the reach of any tabletop laser. This pre-
vents the photogeneration of free electron-hole pair states
in the absorption continuum of monolayer hBN, so that a
quasi-resonant excitation via phonon-assisted processes is
required [14, 21]. Such configuration leads to the coexis-
tence of resonant Raman scattering and PL [14, 15, 22],
which in turn complicates the interpretation of optical
spectroscopy when characterizing atomically-thin hBN in
the UV-C range.

Here we report hyperspectral imaging in the UV-
C spectral domain in epitaxial monolayer hBN, under
quasi-resonant laser excitation. Our sample consists
of atomically-thin hBN expitaxially grown by HT-MBE
on highly oriented pyrolytic graphite (HOPG) in condi-
tions where hBN partially covers the graphite substrate.
Thanks to our spatially-resolved measurements of the
UV-C emission, we spatially isolate the distinctive con-
tributions of resonant Raman scattering and PL. We pro-
vide a quantitative interpretation of our experiments by
means of a weighted average of resonant Raman scat-
tering and PL, from which we extract the maps of their
respective amplitudes in our epitaxial monolayer hBN
sample. No spatial correlation of the resonant Raman
scattering and PL amplitudes is observed. The PL signal
consists in a singlet at ~ 6.045 eV, with spatial variations
of the PL energy around ~ 10 meV, thus revealing that
the inhomogeneous broadening is lower than the average
PL linewidth of ~ 25 meV, a value close to the radiative
limit determined by reflectivity experiments [23]. Our

<footnote>* E-mail: sergei.novikov@nottingham.ac.uk and guil-laume.cassabois@umontpellier.fr</footnote>
methodology provides an accurate framework for assess-
ing the opto-electronic properties of hBN in the prospect
of scalable hBN-based devices fabricated by epitaxy.

We have developed HT-MBE for the growth of hBN
layers on HOPG at temperatures from 1250 °C to 1700
°C [14, 23–25]. We have used a high-temperature effu-
sion Knudsen cell to provide a boron flux and RF plasma
source to produce a flux of active nitrogen. We have
demonstrated that by growing hBN on HOPG substrates
at these extremely high temperatures it is possible to
produce monolayer thick hBN with atomically-flat sur-
faces. The hBN coverage can be reproducibly controlled
by the growth time, substrate temperature and boron to
nitrogen flux ratios. The majority of the hBN mono-
layers is nucleated at HOPG step edges, but there are
also some hBN islands nucleated on the HOPG terraces.
With increase of the growth time the hBN monolayers
gradually increase in size in a step-flow growth mode.
The decrease of aggregates at higher growth tempera-
tures reduces the available nucleation sites for a poten-
tial second hBN monolayer, therefore improving the vi-
ability of HT-MBE for the formation of atomically-flat
single hBN monolayers without multilayer hBN regions.
hBN coverage, hBN monolayer island shape and the pres-
ence of hBN aggregates can be controlled in HT-MBE,
with the highest quality hBN monolayers grown at a sub-
strate temperature of about 1390 °C [23, 25]. Therefore,
the hBN sample investigated in this paper was grown at
about 1390 °C using a fixed RF power of 550 W and a
nitrogen flow rate of 2 sccm.

<figure><img src="image_2.png" /><figcaption>FIG. 1. Atomic force microscopy (AFM) of monolayer boron nitride grown epitaxially on highly oriented pyrolytic graphite (HOPG), for a growth temperature $T_g=1390$ °C, a boron cell temperature $T_B=1925$ °C and a growth time of 2 hours. Scale bar = 2 µm.</figcaption></figure>

After HT-MBE growth the samples are removed from

the system to allow the acquisition of atomic force mi-
croscopy (AFM) images of the surface [Fig.1]. Tap-
ping mode AFM images were acquired using an Asylum
Research Cypher-S instrument with NuNano Scout 70
probes. Distinguishing between materials is primarily
achieved using the phase channel. All images were then
processed with the Gwyddion software package [26].

The UV-C emission of monolayer hBN epitaxially grown on HOPG is analyzed by hyperspectral imaging with the scanning confocal cryomicroscope described in Ref.[16]. Briefly, the sample held at 6K is excited by the fourth harmonic of a cw mode-locked Ti:Sa oscillator, tuned at a wavelength of 196 nm ($E_{ex}=6.325$ eV) for the measurements displayed in Fig.2. At each point of the 8×12 µm² studied area [which is different from the one characterized in Fig.1], the emission spectrum is recorded over a 30 nm-wide wavelength range centered at 220 nm, with an acquisition time of 60 s at an excitation power of 90 µW. Fig.2(a) displays the spatial variations of the emission intensity integrated between 5.3 and 5.7 eV, as indicated by the orange shaded area at the bottom of panel (a), and in panel (c) of Fig.2. This spectral region corresponds to the recombination of excitons localized at hBN extended defects and stacking faults [15, 27, 28]. In contrast, Fig.2(b) is the map of the intrinsic emission integrated between 6 and 6.1 eV [red shaded area at the bottom of panel (b), and in panel (c)], following previous studies pointing out the optical response of direct-gap monolayer hBN in this specific energy range [14, 15]. Representative emission spectra detected at three different points marked in panels (a,b) are plotted in Fig.2(c), with a zoom around 6 eV for spectra (I) and (II) in Fig.2(d).

The intensity maps in Fig.2(a,b) are non-uniform with
a signal globally distributed along diagonal lines, that
may follow HOPG step-edges. No signal is detected in a
~ 20 µm² region located between markers (I) and (II),
neither for the defect-related PL [Fig.2(a)], nor for the in-
trinsic emission [Fig.2(b)]. Given the ~ 75% surface cov-
erage of atomically-thin hBN determined by AFM in this
sample, we thus attribute this dark regions to uncovered
HOPG in Fig.2. In Fig.2(a) the spatial distribution of
the defect-related luminescence is particularly inhomoge-
neous, with hot spots along lines parallel to the borders of
uncovered HOPG. A typical spectrum recorded at posi-
tion (III) illustrates the carrier relaxation in the presence
of extended defects, with the usual broad luminescence
band centered at ~ 5.5 eV that completely dominates
the emission spectrum at the hBN band-edge [27, 28]. A
comparison of the spatially-resolved map of photogener-
ated excitons and our typical AFM images indicates that
emission occurs from regions close to the edges of the
atomically-thin hBN domains of epitaxially-grown mate-
rial. Specifically we associate the spectra with a dom-
inant peak at ~ 6 eV as arising from monolayer hBN
<figure><img src="image_3.png" /></figure>

FIG. 2. Hyperspectral imaging of epitaxial monolayer hBN in a 8×12 µm² area, at 6K, for an excitation energy of 6.325 eV (λ ~ 196 nm). (a,b) Emission intensity maps integrated in the ranges 5.3-5.7 eV (a) and 6-6.1 eV (b), represented by the orange and red shaded areas in panel (c), respectively. Scale bar = 1 µm. Units of color map = counts/s. (c) Emission spectra at the three locations (I, II, and III) indicated in (a,b). (d) Zoom of spectra (I) and (II) around 6 eV. Spectrum (I) is dominated by PL, while spectrum (II) by Raman scattering.

(consistent with the hyperspectral map discussed below), while the more disordered, multilayer hBN (corresponding to the bright linear structure in our AFM images) accounts for the more complex emission shown in Fig.2 for regions II and III.

Excluding the region attributed to uncovered HOPG, the signal at ~ 6 eV has a more uniform spatial distribution [Fig.2(b)] than the defect-related PL [Fig.2(a)], as could be expected from the intrinsic optical response of free excitons arising from monolayer hBN domains with a morphology similar to that in Fig.1 [15, 27]. Nevertheless, one still observes some hot spots in Fig.2(b). Interestingly, there are significant modifications of the emission spectrum within the recorded map, with two limit cases illustrated by spectra (I) and (II) recorded at the positions indicated in panels (a,b) of Fig.2. The latter corresponds to a hot spot of Fig.2(b), and spectrum (II) displays a complex multiplet structure. In contrast, a single line centered at ~ 6.04 eV is detected at position (I). We will demonstrate below that spectrum (II) stems from resonant Raman scattering, while spectrum (I) is the genuine PL response of monolayer hBN.

A straightforward test to discriminate Raman scattering and PL consists in tuning the laser excitation energy. Under quasi-resonant excitation conditions, both Raman scattering and PL increase in intensity but the PL spectrum remains unchanged while the coherent Raman signal continuously shifts with the excitation energy, in other words, the Raman scattering spectrum is identical when plotted as a function of the detuning with the

laser energy, the so-called Raman shift. This standard phenomenology in semiconductor optics [29] was recently reported in transition metal dichalcogenides monolayers [21]. The interplay between resonant Raman scattering and PL governs the UV-C emission of monolayer hBN as well. As explained in the introduction, this comes from the required quasi-resonant excitation to photo-generate excitons with a binding energy of ~ 1 eV in monolayer hBN on graphite [20]. Fig.3 displays two type-II spectra [as defined in Fig.2], that are recorded for two different excitation energies, and plotted versus the Raman shift. The complex multiplet structure noted in Fig.2 appears to be the same as a function of the Raman shift, thus demonstrating that the limit case of spectrum (II) in Fig.2 is dominated by inelastic light scattering of the exciting laser. Importantly, these spectra exactly match the Raman scattering spectrum reported by Reich *et al.* in Ref.[30] [dashed line, top spectrum in Fig.3]. In the present case of Raman shifts of the order of 300 meV (2500 cm⁻¹), the inelastic scattering signal corresponds to second-order Raman processes. The E₂g phonon mode of hBN at 170 meV (1370 cm⁻¹) gives rise to an intense Lorentzian line in the Raman scattering spectrum, that is accompanied by weak satellites at higher Raman shifts due to second-order processes mostly involving overtones of two identical phonons of opposite momenta [30]. In particular, the so-called α and β lines in Fig.3 were attributed to the emission of two TO and two LA phonons, respectively [30].
<figure><img src="image_4.png" /><figcaption>FIG. 3. Comparison of the second-order Raman scattering spectrum recorded in hBN by Reich et al. [top spectrum, adapted from Ref.[30] and convoluted with the line profile of our femtosecond exciting laser] with two type-II emission spectra [as defined in Fig.2(d)] recorded in epitaxial monolayer hBN for two excitation energies differing by 33 meV ($E_{ex}$=6.325 eV), and plotted as a function of the energy detuning with the laser energy, the so-called Raman shift.</figcaption></figure>

The nature of spectrum (II) being identified, we perform a quantitative analysis of the data at each point $(x, y)$ of the hyperspectral image, with a model based on a weighted average of Raman scattering and PL, following the equation:

$$A_R(x, y)F(E - E_{ex}) + A_{PL}(x, y)L_{\Gamma(x, y)}(E - E_0(x, y)) \quad (1)$$

where $F(E - E_{ex})$ is a normalized ad hoc function mimicking the second-order Raman spectrum of Ref.[30] [Fig.3, top spectrum], and $L_{\Gamma}(E - E_0)$ a normalized Lorentzian function centered at the energy $E_0$, with a full-width at half-maximum $\Gamma$. Therefore, spectra of type-I (type-II) correspond to $A_{PL} \gg A_R$ ($A_{PL} \ll A_R$, respectively). As seen in Fig.4(d) showing an intermediate situation with comparable Raman and PL amplitudes ($A_R/A_{PL} \sim 5.7$), a fair agreement with the experimental data can be reached at any point of the sample with our model. The maps of the Raman ($A_R$) and PL ($A_{PL}$) amplitudes are displayed in Fig.4(b) and (c), respectively, together with the raw data of the UV-C emission integrated between 6 and 6.1 eV in Fig.4(a). There are hot spots in the $A_R$ map, where a roughly fivefold enhancement occurs at some points on the HOPG step-edges, with respect to the average Raman amplitude within monolayer hBN domains. Because these hot spots do not correlate with the ones of the defect-related PL in Fig.2(a), they do not seem to be related to 3D is-

lands preferentially nucleated at the HOPG step-edges. Moreover, despite similarities with the phenomenology of surface-enhanced Raman scattering where the electromagnetic field can be locally enhanced in surface irregularities acting as cavities, such an effect would augment both the Raman and PL amplitudes, which is not the case here when comparing Fig.4(b) and (c). At the present stage, it is thus difficult to conclude on the origin of the Raman hot spots.

The PL amplitude $A_{PL}$ appears more homogeneously distributed, without any hot spot as for the Raman amplitude $A_R$ [Fig.4(c)]. Remarkably, there is almost no spatial correlation between the maps of the PL and Raman amplitudes, with a value of only 13% for the spatial covariance of $A_{PL}$ and $A_R$ normalized by the product of their standard deviations. Our quantitative analysis further resolves the spatial variations of the emission energy $E_0$ and linewidth $\Gamma$. The emission energy $E_0$ is peaked at $\sim 6.045$ eV with a standard deviation of 10 meV [Fig.5]. Importantly, this statistical distribution is unchanged when varying the excitation energy [Fig.5], as expected for PL, thus validating a posteriori our framework of analysis based on a weighted average of Raman and PL. We highlight that the tuning range of the excitation energy in Fig.5 is comparable to the spectral extension of the second-order Raman scattering signal [Fig.3], meaning that the quasi-resonant excitation of the PL is performed via various two-phonon assisted processes in Fig.5. Within this spectral range of excitation, the $A_{PL}/A_R$ ratio does not display any variations within our experimental uncertainties, with a constant value of $\sim 0.2$. The $\sim 10$ meV- variations of the PL energy in Fig.5 reveal that the inhomogeneous broadening is lower than the average PL linewidth $\Gamma \sim 25$ meV [see for instance spectrum (I), Fig.2(d)], a value close to the radiative limit in monolayer hBN [23]. This means that the PL signal can vary in amplitude from one point to another, but with little inhomogeneous broadening.

A major difference between the data of Fig.5 and the ones of Ref.[14] recorded by macro-PL experiments in similar epitaxial monolayer hBN is the absence of a PL doublet in the present case. Previous PL measurements have indeed shown the existence of two lines at 6.05 and 6.08 eV [14]. This discrepancy may be related to the quasi-resonant excitation of the PL. More precisely, depending on the exact laser energy, one changes the overlap between the fixed PL line and the multiplet of second-order Raman scattering, that shifts with the laser energy, thus leading to an overall UV-C emission with a complex and non-universal spectrum. Although one can not exclude the existence of a PL doublet in the specific epitaxial monolayer hBN studied in Ref.[14], it is most likely that the reported doublet in Ref.[14] is not the intrinsic PL response of monolayer hBN, which was hidden by the superposition of the PL and Raman signals, with a spectrum of the latter more intricate than expected. The present systematic study by means of spatially-resolved measurements with different excitation energies allows
<figure><img src="image_5.png" /><figcaption>FIG. 4. (a) Emission intensity map integrated between 6-6.1 eV. (b,c) Maps of the amplitudes $A_R$ and $A_{PL}$ of the Raman (b) and PL (c) components used for fitting the hyperspectral image to the model, as defined by Eq.(1) in the text. (d) Intermediate situation between spectra of types I and II, fitted (dashed line) to Eq.(1) with $A_R/A_{PL} \sim 5.7$, with the corresponding PL and Raman contributions below.</figcaption></figure>

to disentangle the contributions of Raman and PL, and brings the reproducible evidence of PL of monolayer hBN consisting in a singlet at ~ 6.045 eV for hBN on graphite.

Eventually, we comment on the UV-C emission of exfoliated monolayer hBN reported in Ref.[15]. Following the same analysis procedure relying on the weighted average of Raman and PL defined in Eq.(1), a quantitative interpretation of the data in Ref.[15] is reached with $A_R \sim 2.5 A_{PL}$, and again with a PL singlet but with a 35 meV-blueshift ($E_0 \sim 6.08$ eV). Such a difference is consistent with the bandgap variations of monolayer hBN depending on the underlying substrate [31]. The ratio of the Raman and PL amplitudes is of the same order as the one estimated in Fig.4 ($A_R/A_{PL} \sim 6$). This means that the optical response of exfoliated monolayer hBN in Ref.[15] is also determined by the interplay between resonant Raman scattering and PL. This conclusion may be surprising considering the full suppression of the signal above 6 eV in the multilayer region of the exfoliated flake in Ref.[15]. While such phenomenology is straightforward for the PL component because of the direct-indirect crossover of the bandgap, one would expect to still detect some second-order Raman scattering in multilayer hBN. This is not the case because the resonance conditions enhancing the efficiency of the Raman processes are no longer satisfied when moving from monolayer to multilayer hBN. This effect is a second consequence of the drastic modification of the excitonic properties as soon as at least two monolayers are stacked in the hBN crystal [3-5]. First, the interlayer coupling slightly modifies the

electronic bandstructure and the nature of the bandgap, in particular with the conduction band minimum located at the M point of the Brillouin zone for multilayer hBN instead of the K point for monolayer hBN [3-5]. Second, the absorption of the excited excitonic states experiences a shift as large as ~ 200 meV from one to two monolayers [5], which affects the resonant conditions of Raman scattering. This is why the resonant Raman scattering component of the UV-C emission vanishes together with the PL at ~ 6.08 eV as soon as the hBN crystal consists of more than one monolayer in Ref.[15].

We have reported hyperspectral imaging in the UV-C spectral domain in epitaxial monolayers of hBN grown by HT-MBE on a HOPG substrate. We have shown that the UV-C emission of monolayer hBN consists in resonant Raman scattering and photoluminescence, which appear to be spatially uncorrelated. Systematic measurements as a function of the quasi-resonant laser energy unveil a PL singlet at ~ 6.045 eV with spatial variations of the PL energy of ~ 10 meV. This inhomogeneous broadening is lower than the average photoluminescence linewidth of ~ 25 meV, a value close to the radiative limit in monolayer hBN. Our methodology provides an accurate framework for assessing the opto-electronic properties of hBN in the prospect of scalable hBN-based devices fabricated by epitaxy.

**Acknowledgments** We gratefully acknowledge C. L’Henoret and T. Cohen for their technical support at the mechanics workshop. This work was financially supported by the BONASPES (ANR-19-CE30-0007), ZEOLIGHT (ANR-19-CE08-0016) and HETERO-BNC (ANR-20-CE09-0014-02) projects. This work at Notting-
<figure><img src="image_6.png" /><figcaption>FIG. 5. Statistical distribution of the PL energy $E_0$ extracted from spatially-resolved measurements performed at different excitation energies ($E_{ex}=6.325$ eV). Each event in the histograms is weighted by the corresponding PL amplitude $A_{PL}$.</figcaption></figure>

ham was supported by the Engineering and Physical Sciences Research Council UK (Grants No. EP/P019080/1, No. EP/V05323X/1, and No. EP/W035510/1).

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