# Effects of disorder on the optical properties of deep UV quantum well emitters as assessed by photonic atom probe

S. Ndiaye, E. Coccato, N. Al Helou, S. Kumar Gautam, J. Cañas, A.

Jannaud, Sergi Cuesta, J. Houard, A. Grenier, N. Rochat, et al.

► To cite this version:

S. Ndiaye, E. Coccato, N. Al Helou, S. Kumar Gautam, J. Cañas, et al., Effects of disorder on the optical properties of deep UV quantum well emitters as assessed by photonic atom probe. Applied Physics Letters, 2026, 128 (9), pp.093302. 10.1063/5.0301054. hal-05533284

HAL Id: hal-05533284

https://hal.science/hal-05533284v1

Submitted on 2 Mar 2026

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Effects of disorder on the optical properties of deep UV quantum well
emitters as assessed by photonic atom probe

S. Ndiaye¹, E. Coccato², N. Al Helou¹, S. Kumar Gautam¹, J. Cañas², A. Jannaud³, S. Cuesta², J.

Houard¹, A. Grenier³, N. Rochat³, E. Monroy², L. Rigutti¹*

¹University of Rouen Normandie, INSA Rouen Normandie, CNRS, Groupe de Physique des Matériaux, 76000 Rouen, France

²Univ. Grenoble-Alpes, CEA, Grenoble INP, IRIG, PHELIQS, 38000 Grenoble, France

³Univ. Grenoble Alpes, CEA, LETI, 38000 Grenoble, France

⁴Univ. Grenoble Alpes, CEA, IRIG, MEM, 38000 Grenoble, France

* Corresponding Author : lorenzo.rigutti@univ-rouen.fr

ABSTRACT

The Photonic Atom Probe, a technique allowing the simultaneous, *in-situ* acquisition of photoluminescence within a tomographic atom probe, is applied to the study of AlGaN quantum wells within deep-UV separate confinement heterostructures. The analyzed sample exhibits a high degree of compositional and morphological disorder, translating into spatial inhomogeneities and a significant dispersion of photoluminescence emission energy. We show how the proposed *in-situ* correlative microscopy approach can explain different optical emission features in these structures, particularly the distinction between the optical signatures of non-defective QWs and of QWs located within defective regions. The results, including time-resolved photoluminescence, are interpreted through 8-band k.p calculations which take as an input the statistical properties extracted from the APT composition maps.
Disorder-related effects in semiconductors play a key role in the determination of their optical properties¹ and represent an ideal field for the application of correlative microscopy protocols ²⁻⁵. In the case of AlGaN-based quantum structures, disorder may arise from the presence of point⁶ or extended defects⁷, but can also affect the morphology of quantum well⁸,⁹ (QW) or quantum dot¹⁰ (QD) interfaces, as well as the microscopic distribution of the alloy atoms¹¹,¹². Structural or point defects act as transport-impairing or non-radiative recombination centers, lowering internal quantum efficiency in quantum confining systems; alloy disorder and interface roughness in multi-QW (MQW) structures broaden emission spectra through local variations¹³. Extended regions with different orientations or compositions may even cause bimodal luminescence¹⁴. Disorder can be interpreted by correlative microscopy ¹⁵. Its impact in terms of efficiency, reliability and lifetime needs investigation in deep-UV (DUV) structures, suffering from lower efficiency and specific material issues¹⁶. The Photonic Atom Probe (PAP), combining Atom Probe Tomography (APT) and photoluminescence (PL) spectroscopy¹⁷, has emerged as a powerful tool for wide bandgap semiconductors¹⁸, enabling optical spectra to be linked with morphology, composition¹⁹, and point defects ²⁰,²¹. Here, PAP is applied to probe the relation between structural and optical properties in AlGaN MQW DUV emitters.

The structures addressed in this work are asymmetric graded-index separate confinement heterostructures (GRINSCH) designed for electron beam-pumped laser emission at 280 nm. The laser structures were fabricated on bulk AlN(0001) substrates using plasma-assisted molecular-beam epitaxy. The structure is schematically presented in Fig. 1(a) and described in Table S1 and in in Fig. S1 and S2 of the Supporting Information, with growth details and
related references (23). A previous correlative microscopy study on similar structures involving
cathodoluminescence (CL) and scanning transmission electron microscopy (STEM) has
enabled direct mapping of carrier diffusion and recombination in AlGaN/GaN heterostructures,
as well as their first design optimization²².

The STEM-HAADF micrograph reported in Fig. 1-(b) provides qualitative evidence of disorder
phenomena. The MQW structure exhibits clear roughness effects, producing either a sort of
serrated interface profile of nearly-planar QW structures, as in the lateral parts of the image or
more pronounced V-like profiles, as in the central part. This disordered QW morphology is first
assessed by HRXRD θ-2θ scans around the (0002) reflection of AlN, as reported in Fig. 1-(c).
The experimental profile is compared with a theoretical calculation assuming the nominal
growth parameters and the structure pseudomorphic on the bottom outer cladding layer (graded
AlGaN with an Al III-site fraction x=70→100 %). The more intense peak of the diffractogram
is attributed to the AlN substrate. The reflection of the MQW presents several satellites, which
allow a precise determination of the MQW period. The optical properties of the structure have
been preliminarily investigated by CL at room temperature in a planar configuration, i.e. the
electron beam impinges on the thin film structure at normal incidence on the upper surface, and
the light emitted through the upper surface is collected by the CL parabolic mirror of the
microscope. All spectra display a broad emission at an energy of approximately E=4.35 eV
(wavelength λ=285 nm), and the highest intensity is reached for a beam energy of E<sub>beam</sub>~7 keV,
in agreement with the design of the structure.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Investigated sample and preliminary characterization. (a) Scheme of the thin film structure; (b) HAADF-STEM micrograph from a lamella; (c) XRD spectra, compared with a calculation based on the nominal growth parameters as reported in (a); (d) Cathodoluminescence spectra produced with the sample in a planar configuration (excitation perpendicular to the upper surface, collection of light transmitted through the upper surface).</figcaption></figure>

The structure was analyzed within the PAP<sup>17</sup> after standard preparation by focused ion beam
of needle-shaped field emission specimens<sup>24</sup>. Three specimens were successfully analyzed. The
data obtained from Specimen 1 are displayed in Fig. 2. This specimen was analyzed at a
temperature of T=50 K. Laser pulses at λ=285 nm, frequency f=400 kHz and energy E<sub>laser</sub>=20
pJ/pulse, focused with a beam waist w~2µm were used in order to excite both ion evaporation
and PL emission. Details on mass spectra, the 3D reconstruction protocol and other parameters
of the measurement are provided in the supporting information (Figs. S3 and S4). Figure 2-(a)
and (b) display the 2D compositional maps calculated over 2-nm thick slices extracted from the
3D reconstructed volume. These two slices contain the axis of the volume and are mutually
perpendicular. The set of PL spectra in the energy interval from 4.15 eV to 4.55 eV is reported
in Fig. 2-(c) (see the supporting Fig. S5, for spectra in the full spectral range). Minor distortions
occur in the upper part of the reconstructed volume of Fig. 2-(a,b): this is due to the adoption
of one single set of reconstruction parameters in order to best match the inter-QW distance as
given by the STEM and XRD characterization. These distortions do not influence the
composition measurement and are negligible in the central part of the reconstruction. The MQW
structure exhibits two different types of regions. A set of nearly planar QWs can be found close
to the specimen axis in both maps. The statistical properties of the AlGaN alloy have been
extracted by sampling the region of interest (ROI) A as indicated in Fig. 2-(a). The composition
histogram (Fig. 3-(a)) indicates an average Al III-site fraction of 65%. On the other hand, a
region appearing as richer in Al content and displaying slightly misoriented QW interfaces can
be seen on the left-hand side of the map in Fig. 2-(a) and on both lateral sides of the map in Fig;
2-(b). A subvolume of this region, most likely associated with the V-shaped domain visible in
Fig. 1-(b), named ROI B, displays different statistical properties (Fig. 3-(a)). The average Al
III-site fraction is here around 75%. The two regions A and B also differ in the QW composition,
measured by tracing around 20 one-dimensional composition profiles within boxes of 2x2 nm²
cross section. The histograms of the Al III-site fraction in the QWs are reported in Fig. 3-(b),
yielding an average fraction of 35% in ROI A and 45% in ROI B. The one-dimensional profiles
also allowed extraction of the QW thickness, which weakly fluctuates around 2 nm in both
regions (these data are reported in the supporting Fig. S5).

As reported in Fig. 2-(c), each PL spectrum is ordered by the spectral index iPL. A subset of the
volumes evaporated during the acquisition of selected PL spectra is highlighted in Fig. 2-(a,b)
as a white area. The band on the low-energy side is peaked at 3.95 eV (see supporting Fig. S6)
and corresponds to defects in the AlN substrate²⁵. The spectral features labeled as Band 1
(EPL=4.2-4.35 eV) and Band 2 (EPL=4.35-4.5 eV) disappear from the dataset approximately at
iPL=150, i.e. when the evaporation front is approaching the first QW from the bottom. These
bands are a set of close narrow line emissions, which evolve similarly during specimen evaporation.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Photonic Atom Probe analysis of the thin film structure. (a,b) Al III-site fraction maps as measured on 2 nm-thick slices extracted on the XZ and on the YZ planes, respectively, of the reconstructed volume issued from the APT analysis. The regions highlighted in white on the maps correspond to the volumes evaporated during the acquisition of the PL spectra whose spectrum index $i_{PL}$ is specified between the two maps. ROI A (red rectangle) corresponds to planar structures, while ROI B (blue rectangle) includes QWs within a defective region only. (c) Set of PL spectra measured during the evaporation of the specimen; the ordinate axis corresponds to the spectrum index $i_{PL}$. The main emission bands from the MQW system, labeled as Band 1 and Band 2, are visible for $i_{PL}<150$ only. [Animated version available as Supporting Information at LINK].</figcaption></figure>

Bands 1 and 2 are thus attributed to the emission of the MQW system. Band 1 undergoes a significant quenching with $i_{PL}$ between 80 and 100, while Band 2 undergoes a spectral red shift with a partial quenching up to $i_{PL}=110$, then a slow quenching without spectral shift. Notice that the spectral shift may be analyzed in terms of field-induced stress²⁶, which is not within the scope of this work. That this bimodal emission could reflect the presence of two different regions within the analyzed volume, but this hypothesis needs confirmation because both the planar and the tilted QWs are present in the volume until the graded AlGaN bottom superlattice
is reached. The attribution should rely on a sound quantitative analysis considering not only the average statistical properties of the different regions of the system, but also their fluctuations.

For this reason, we conducted a set of one-dimensional 8-band k.p Schrödinger-Poisson simulations using the software Nextnano²⁷, with the material parameters described by Kandaswamy et al.²⁸, with all the bowing parameters were set to zero. This 1D approach is less precise than 3D continuum models solving strain and Schrödinger equations ²⁷,²⁹, localization landscape theory³⁰, or atomistic methods³¹, because all specific 3D effects such as volume composition fluctuations, interface roughness, microscopic strain and field distribution cannot be accounted for³². However, we may justify a posteriori that 1D calculations represent a viable compromise that can quite accurately account for the main experimental observations reported.

Furthermore, 1D calculations straightforwardly yield trends as a function of a single varying calculation parameter (for instance, the QW composition). The effect of disorder can be considered as a fluctuation of this parameter around an average value.

The simulation models a representative region of the MQW, consisting of three Al<sub>x<sub>w</sub></sub>Ga<sub>1-x<sub>w</sub></sub>N QWs separated by four Al<sub>x<sub>b</sub></sub>Ga<sub>1-x<sub>b</sub></sub>N barriers, grown along the [0001] wurtzite axis, as described in supporting Figure S5. A homogeneous strain approximation is applied, assuming pseudomorphic growth, with the in-plane lattice parameter a calculated as the relaxed weighted average of alloy composition and lattice thickness:

$$a = \frac{t_b a_b + t_w a_w}{t_b + t_w} \quad (1)$$

where t<sub>b</sub> and t<sub>w</sub> are the thickness of the barrier and well, respectively, and a<sub>b</sub> and a<sub>w</sub> are the in-plane lattice parameter of the alloy of barriers and wells, respectively, assuming them fully relaxed. For the Poisson solver, we applied periodic boundary conditions along the z direction.

The quantum-confined electronic levels are calculated in the pink shadowed region in Figure S5, using Dirichlet boundary conditions.
Three sets of calculations were performed for each ROI. In the first set, we assumed constant $t_w = 2$ nm and constant Al III-site fraction within the QW, i.e. the average $x_w$ measured for the ROI, and we vary the Al III-fraction in the barriers $x_b$. The predicted PL emission energies issued from this set of calculations are displayed in Fig. 3-(a). The symbols correspond to the calculated energy distance between the electron and hole ground states, while the line corresponds to the subtraction of the exciton binding energy calculated in each case using the Nextnano software, which is in the range of 45-47 meV, consistently with previous experimental and theoretical works performed on similar GaN/AlGaN QWs$^{33,34}$. In the second set, we assumed $t_w = 2$ nm and a constant $x_b$ that corresponds to the average from the statistical analysis, and we vary the $x_w$. The results of this set of calculations are shown in Fig. 3-(b). The results in Fig. 3-(a) show that for both ROI A and B, the energies slightly decrease with increasing $x_b$. This occurs because the compositional contrast between well and barrier increases the quantum confined Stark effect (QCSE). These results can be directly compared with the PL spectrum acquired at the beginning of the evaporation ($i_{PL}=1$, Fig. 3-(c)): the calculated PL energies for ROI A including the effect of exciton binding approximately lie within the energy interval of Band 1, while those of ROI B well match with the energy interval of Band 2. The results displayed in Fig. 3-(b) also point to the attribution of the emission of ROI A to band 1 and of ROI B to band 2. This dataset also shows that, as expected, the PL energy increases with $x_b$, as a consequence of both QW bandgap increase and the reduction of the QCSE.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Correlation between the statistical properties of the MQW system and its optical properties via effective mass simulation performed on identically thick QWs (t<sub>QW</sub>=2 nm). (a) Composition histogram of the ROI A (QWs within the defect r planar QWs, red bars) and of the ROI B (QWs within the defect, blue bars); PL energies calculated as a function of the Al III-site fraction of the barriers with fixed Al III-site fraction in the QWs (x<sub>w</sub>=35%, ROI A, red filled squares and x<sub>w</sub>=45%, QWs in the ROI B, blue empty squares). (b) Histogram of the Al III-site fraction within the QWs of the ROI A (red bars) and of the ROI B (blue bars); PL energies calculated as a function of the Al III-site fraction within the QWs at fixed Al III-site fraction in the barriers (x<sub>b</sub>=65%, barriers in ROI A, red filled circles and x<sub>b</sub>=75%, barriers in ROI B, blue empty circles). (c) PL spectrum with spectral index i<sub>PL</sub>=1, highlighting the bimodal emission over two bands labeled as Band 1 and Band 2 as in Fig.2.</figcaption></figure>

The performed set of calculations also makes it possible to investigate further effects of disorder
on the optical properties of the system. Time-resolved PL spectroscopy (TRPL) was performed
on a different field emission specimen at temperature T=30 K, a laser frequency f=4MHz and
an energy per pulse E<sub>pulse</sub>=25 pJ. The collected spectrum is displayed in Fig. 4-(a). This
specimen does not exhibit the same pronounced bimodal emission but the MQW PL emission
is distributed within the same spectral interval, most likely due to a different disorder
configuration within the system. Fig. 4-(b) presents the normalized PL decay curves of the
MQW emission in different energy intervals. The PL decay curves are fitted with biexponential-
decay fit to determine the average PL lifetime using I(t) = A<sub>1</sub> exp(-t/τ<sub>1</sub>) + A<sub>2</sub>exp(-t/τ<sub>1</sub>),
where A<sub>1</sub> and A<sub>2</sub> are the amplitudes, and τ<sub>1</sub> and τ<sub>2</sub> are the lifetime constants<sup>35</sup>. The averaged
lifetime was calculated using: τ<sub>avg</sub> = (A<sub>1</sub>τ<sub>1</sub><sup>2</sup> + A<sub>2</sub>τ<sub>2</sub><sup>2</sup>)/(A<sub>1</sub>τ<sub>1</sub> + A<sub>2</sub>τ<sub>2</sub>). The inset of Fig. 4-(b)
illustrates the variation of the QW peak decay lifetime with the PL energy interval. It appears
quite clearly that the PL lifetime decreases with increasing energy. This behavior has already
been reported in different III-N QW systems³⁶,³⁷ and can be seen as a manifestation of the QCSE. The PL lifetime $\tau$ is indeed related to the radiative lifetime $\tau_{rad}$ and to the non-radiative lifetime $\tau_{nr}$ through the relationship

$$ \tau^{-1} = \tau_{rad}^{-1} + \tau_{nr}^{-1}, \qquad (2) $$

while the radiative lifetime is related through the squared overlap $\langle\psi_e|\psi_h\rangle^2$ of the envelope ground state wavefunctions through

$$ \tau_{rad} = \frac{4\epsilon_0 m_0 c^3 \hbar^2}{e^2 n_r E_{PL} E_p} \cdot \frac{1}{\langle\psi_e|\psi_h\rangle^2}, \qquad (3) $$

where in our case we adopt an average PL energy $E_{PL}=4.3$ eV, a Kane energy $E_p=20$ eV³⁸ and a refractive index $n_r=2.3$³⁹, the rest of the terms being universal constants. Thus, for a given composition of barriers and wells, a QW thickness decrease yields both a larger wavefunction overlap and a higher PL energy. The same effects are produced for a given QW thickness and barrier composition, when an increase in the QW bandgap yields both an increase of the PL energy and of the wavefunction overlap. The trend is the same when, for a given QW thickness and composition, the barrier bandgap decreases. These two latter trends are well reproduced by the calculations performed considering the statistical features of ROI A and B, as displayed in Fig. 4-(c). However, it should be underlined that the complexity of the studied system translates into a net decrease of wavefunction overlap when passing from ROI A to ROI B, which means that the trend found experimentally by PL (Fig. 4-(b)) is not univocally reproducible by the calculations. Another observation is that the interval of calculated radiative lifetimes does not match with the observed PL decay lifetimes (shaded region in Fig. 4-(c)). This offers the possibility to estimate $\tau_{nr}$ according to eq. (2) and on the interpolation of the measured PL decay lifetime as a function of PL energy (Inset of Fig. 4-(b)). The estimate of $\tau_{nr}$ is reported in Fig. 4-(c) and the main conclusion it allows to draw is that $\tau_{nr}$ is of the same order of
magnitude of the radiative lifetime, or slightly higher. These results are reasonable for the
measurement temperature of T=30 K, and are consistent with results previously reported in the
literature of DUV emitters<sup>40</sup>, although no direct quantitative comparison can be drawn between
this work and other correlative microscopy and optical spectroscopy studies at the nanoscale
for AlGaN/AlGaN QWs.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** (a) Time-resolved PL spectra from a field-emission specimen containing the AlGaN MQW system. (b) Normalized PL time decay profiles extracted from (a) at different emission energies; in the inset, the measured PL lifetime as a function of the PL energy. (c) Squared overlap of the ground state electron and hole envelope wavefunctions (left ordinates) and radiative lifetime (right ordinates) as a function of the PL energy calculated by the 1D 8 band k.p effective mass for ROI A and B (same system parameters displayed in Fig. 3). (d) Estimated non-radiative lifetime based on the calculated overlap integrals and on the trend of the PL decay lifetime with energy.</figcaption></figure>
In summary, correlative methods based on the PAP allow for the discrimination of optical signatures of different regions within the nanoscale volume of a MQW system affected by the presence of disorder and extended defects, which are quite common problems in the domain of DUV light emitters. The application of a 1D k.p model and the consideration of the statistical properties of different regions of the emitter as characterized by APT reliably yield a quantitative interpretation of PAP data in terms of PL emission energies. Furthermore, TRPL on nanoscale sample yields a time decay behavior which is compatible with the PAP data, with lifetimes showing a correlation with the QW morphologies and compositions, compatibly with what is reported in the literature. In perspective, 3D modeling on alloy maps or on simplified alloy distributions would be welcome in order to directly interpret APT data, while the role of strain/stress can be quantitatively elucidated.

## Acknowledgements

Financial support was provided by the French National Research Agency (ANR) via the ASCESE-3D and FUSSL projects (ANR-21-CE50-0016 and ANR-22-CE09-0024) as well as by the France 20230 - ANR-22-PEEL-0014 program. APT data have been analyzed with the Gpm_3dSAT software (IDDN.FR.001.430017.000.S.P.2020.000.10000).

## Author contributions

S. N., Investigation (lead), Data curation (equal), Formal analysis (equal); E. C. Investigation (supporting), Data curation (equal), Formal analysis (equal) Writing - review and editing (equal); N. A. H. Investigation (supporting), Data curation (supporting), Validation (equal); S. K. G. Investigation (supporting), Data curation (supporting), Formal Analysis (equal); J. C., Validation (equal); A. J., Investigation (supporting); S. C. Investigation (supporting), Data curation (supporting); J. H, Supervision (equal), Resources (lead); A. G., Project administration (supporting), Investigation (supporting), Resources (supporting), Validation (equal); N. R. Investigation (supporting), Resources
(supporting), Validation (equal); E. M. Project administration (supporting), Validation (equal), Writing
– review and editing (equal), L. R. Project administration (lead), Formal Analysis (equal), Validation
(equal), Writing – original draft, Writing - review and editing (equal).

Data availability

The data that support the findings of this study are available from the corresponding author upon
reasonable request.

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