# Layer transferred UV emitting hBN/AlGaN heterostructures

Andre Perepeliuc, Ali Kassem, Rajat Gujrati, Phuong Vuong, Vishnu Ottapilakkal, Thi May Tran, Ashutosh Srivastava, Tarik Moudakir, Paul L Voss, Suresh Sundaram, et al.

► To cite this version:

Andre Perepeliuc, Ali Kassem, Rajat Gujrati, Phuong Vuong, Vishnu Ottapilakkal, et al. Layer transferred UV emitting hBN/AlGaN heterostructures. Applied Physics Letters, 2025, 126 (10), 10.1063/5.0257889. hal-05404810

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Submitted on 8 Dec 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Layer transferred UV emitting hBN/AlGaN heterostructures

Andre Perepeliuc¹,³, Ali Kassem², Rajat Gujrati², Phuong Vuong², Vishnu Ottapilakkal², Thi May Tran², Ashutosh Srivastava¹,³, Tarik Moudakir⁴, Paul L. Voss¹, Suresh Sundaram¹,³, Jean Paul Salvestrini¹,³* and Abdallah Ougazzaden¹*

¹Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, Georgia 30332-0250, USA

²CNRS, Georgia Tech - CNRS International Research Lab, 2958, 2 rue Marconi, 57070 Metz, France

³Georgia Tech Europe, 2 rue Marconi, 57070 Metz, France

⁴Institut Lafayette, 2 rue Marconi, 57070 Metz, France

*Authors to whom correspondence should be addressed: jean-paul.salvestrini@georgiatech-metz.fr and aougazza@georgiatech-metz.fr

ABSTRACT

p-hBN/n-AlGaN heterojunctions were fabricated using a dry-selective lift-off/transfer of Mg-doped hBN layer on top of n-AlGaN. Electrical contacts were used as mechanical stressors to provide structural rigidity to hBN layers as well as enabling selective lift-off. These junctions exhibit a rectifying behavior with a rectification ratio of approximately 3x10⁵ at 3V. When junctions were forward biased, UV emission around 262 nm was measured. This emission corresponds to recombinations in the n-AlGaN layer, demonstrating good hole injection in the structure. Full LED structures were fabricated by integrating UV MQWs into these junctions. Produced UV LEDs emit around 290 nm serving as a proof of concept for future layer transferred p-hBN/MQWs/n-AlGaN structures in which the Al-content is increased to go toward DUV emission. The selective pick and place process used to build these LEDs has multiple advantages. First, it allows independent optimization of the p-side as well as of the n-side which includes the quantum wells. Secondly, UV MQWs are protected from the high temperatures needed for high hBN material quality growth and thus their thermal stability is not affected.

TEXT

Usually employed as an insulator or as a sacrificial layer [1,2], hexagonal boron nitride (hBN) is an ultra-wide bandgap 2D semiconductor with indirect gap of 5.995 eV [3], possessing a significant potential for DUV applications as a possible alternative to Al-rich aluminum gallium nitride (AlGaN). In AlGaN materials, an increase of Al-content to reach shorter wavelength DUV emission also increases the activation energy of magnesium (Mg) dopants in the p-doped region, reaching hundreds of meV [4,5]. This leads to weaker hole injection in multi quantum wells (MQWs), drastically reducing the efficiency of AlGaN based DUV LEDs. In addition, DUV structures usually integrate an electron blocking layer (EBL) used to maximize electron-hole recombination in MQWs, but EBL layers are highly resistive to hole transport. So the engineering of p-doped regions is a major issue for the realization of efficient DUV LEDs. This can be potentially alleviated by use of p-doped hBN. Indeed, hBN can be p-doped using Mg [6] and it is transparent in the UV permitting photons to be extracted. Moreover, a favorable band alignment permitting hole transport from hBN to Al(Ga)N allows h-BN to act as an EBL [7], removing the need for an additional resistive layer. Consequently, Mg-doped hBN has potential for realization
of efficient pn junctions with n-AlGaN, which will serve as the foundation for integration of DUV MQWs. Finally, hBN is not only useful as current injecting layers but also as DUV emitter at 215 nm [8]. In addition, band edge emission in hBN is mainly transverse electric (TE) polarized [9,10] compared to AlN in which transverse magnetic (TM) polarization [11] dominates, allowing for better light extraction.

p-hBN/n-AlGaN heterojunctions grown by metal organic chemical vapor deposition (MOCVD) have displayed rectifying behavior, suggesting that hBN plays the role of a hole injection layer in DUV structures [12,13]. A proof-of-concept evidence of emission from AlGaN MQWs in p-hBN/n-AlGaN heterostructure was recently reported [14]. However, even though UV emission was obtained from MQWs, so far, no electro-emission has been reported from the pn heterostructures themselves. Moreover, instead of being directly grown on the n-AlGaN, p-hBN can be transferred on n-AlGaN from another substrate, which is possible due to the weak van der Waals forces between hBN layers. This approach has been used to achieve pn heterojunctions with Ga₂O₃ leading to high quality junctions [15]. This was done with a wet lift-off/transfer process involving “fishing”. Potential issues include dirty interfaces or poorly controlled alignment.

In the present work we report on the fabrication of hBN/AlGaN pn junctions achieved by dry selective lift-off/transfer of Mg-doped hBN layer atop of n-AlGaN. The junctions emit at 262 nm evidence of good electron-hole recombination in the AlGaN layer. We find that the mechanical transfer method has improved electrical and optical characteristics compared to previously reported MOCVD grown hBN/AlGaN pn junctions. Using these junctions as a starting point, in further epitaxy runs, UV MQWs were then integrated into the heterostructures. These heterostructures formed diodes with emission at 290 nm, demonstrating fully working LEDs. The importance of these results is twofold. First, we provide an alternative method of constructing UV emitting hBN/AlGaN diodes by use of a lift-off/transfer approach. Secondly, since hBN is grown at significantly higher temperatures than the MQWs, the lift-off approach protects the MQWs from high temperatures and enables independent optimization of p-hBN growth and MQWs structures.

A 500nm/230nm thick bi-layer of n-AlGaN/AlN was first grown on 2" (0001) sapphire substrate using trimethylaluminum (TMAI), trimethylgallium (TMGa), ammonia (NH₃) and silane (SiH₄) as Al, Ga, N and Si precursors. AlN layer was grown at 1100 °C in hydrogen ambient at 90 mbar. Silicon doped n-AlGaN (57% Al) was deposited under the same conditions but at a slightly lower temperature of 1050 °C. For contacts, a metal stack of Ti/Al/Ni/Au (20/200/25/25nm) was deposited by e-beam deposition and then annealed at 850 °C for 30 seconds in N₂ environment. For the transferred layer, 50 nm thick p-hBN was grown on another 2" (0001) sapphire substrate using triethylboron (TEB) and NH₃ as precursors, Mg dopants being provided by biscyclopentadienyl-magnesium (MgCp₂). Growth was performed at 1280 °C in hydrogen ambient at 85 mbar. XRD measurements presented in figure 1(a) show the growth of the p-hBN on sapphire wafer. More details about hBN MOCVD growth are discussed elsewhere [16]. Finally, non-annealed 500 µm square contacts consisting of Ni/Au (30/100nm) were then deposited. Not only useful for electrical contact, these pads also act as a stressor allowing controlled lift-off and
transfer of p-hBN on previously grown n-AlGaN/AlN bi-layer. Indeed, p-contacts provide structural rigidity as well as allowing selective lift-off of p-hBN.

<figure><img src="image_2.png" /><figcaption>Figure 1: (a) High resolution 2θ-ω scan (b) SEM image of p-hBN grown on sapphire</figcaption></figure>

Assembly of the different layers was achieved by our lift-off/transfer method of p-hBN layers on top of n-AlGaN. To do so, polydimethylsiloxane (PDMS) was pressed by a transfer printing machine for a few seconds against p-pads with a force of 5 N to ensure good adhesion. Once attached, a stack comprising 50 nm thick p-hBN, p-metal and PDMS was then lifted from the sapphire substrate with a rapid retraction of 5 mm/s allowing clean breaking of the van der Waals bonds. The left side of figure 2(a) illustrates the selective lift-off process used in this work. Pads as big as 500x500 µm² were lifted, but different sizes and shapes of PDMS stamps can be accurately produced using etched silicon as a pattern. After precisely aligning the lifted stack with the host substrate, the p-side can be moved to a position of a few tens of µm from the pre-deposited n-pads. Once transferred layers entered in contact with the desired region, pressure was gradually increased to minimize damage on hBN. Finally, after placing the stack, PDMS was then retracted at a slow retraction velocity of 0.001 mm/s. This minimizes adhesion strength associated with viscoelastic effects, leaving the p-hBN/metal stack on n-AlGaN and leading ultimately to p-hBN/n-AlGaN junctions. The whole lift-off/transfer process has been done at room temperature. Microscope image illustrating realized devices is shown on figure 2(b).
<figure><img src="image_3.png" /><figcaption>Figure 2: (a) Lift-off/transfer process used to realize p-hBN/n-AlGaN junctions (b) Top-view of p-hBN/n-AlGaN junction realized by lift-off/transfer. Inset: device's cross-section (c) Multiple p-hBN/metal stacks transferred on 2" wafer</figcaption></figure>

Typical I-V measurements performed on realized junctions are presented on figure 3(a). The diodes display a rectifying behavior with a rectification ratio of approximately 3x10⁵ at 3V, the potential at which series resistance begins to dominate the I-V curve.

<figure><img src="image_4.png" /><figcaption>Figure 3: (a) I-V measurements in semi-log scale performed on p-hBN/n-AlGaN junction. Inset: I-V in linear scale (b) C-V measurements at 100 kHz. Inset: Mott-Schottky plot</figcaption></figure>

This I-V characteristic was fitted using the classical Shockley diode equation, details of the fitting are given in supplementary materials. A series resistance of 250 Ω and an ideality factor of 4.8 were extracted from the fitting. Moreover, reverse-bias saturation current constant could not be fitted using this simple model. Most of the series resistance came from n-AlGaN layer. As a matter of fact, an estimation based on transport measurements indicated that 60% of the total resistance came from n-AlGaN, indicating
that the 50 µm distance between n contact and the p-side is indeed junction's most
resistive path. In addition, non-annealed p-contacts are more rectifying and contribute
therefore to the high ideality factor [17]. Still, such high ideality factor (η>2) is intriguing,
showing a clear departure from classical pn junctions' theory and pointing into the
direction that additional conduction mechanisms could be involved. Hence, even if the
overall behavior of pn junctions including 2D materials is similar to conventional diodes,
these results show that a more refined model is required to describe the detailed physics
of 2D/3D nitride heterojunctions. Indeed, because of the van der Waals gaps in between
hBN layers acting as very thin insulators, tunneling also needs to be considered to fully
describe the behavior of the junctions [18]. Furthermore, p-contacts were not fully
optimized, also contributing to the high ideality factor [19].

Built-in voltage was determined by C-V measurements performed at 100 kHz, shown in figure 3(b). From Mott-Schottky plot, a built-in voltage of 2.4 V is extracted which is consistent with I-V measurements as well as with the voltage at which depletion capacitance drops abruptly. However, measured built-in voltage is much lower than expected. Indeed, for such heterojunctions a turn-on voltage of 3.5-4 V is expected [8,9]. Presence of defects in the depletion region inducing additional conduction paths triggered at lower voltages, could explain the lower turn-on voltage.

<figure><img src="image_5.png" /><figcaption>Figure 4: Junction's electroluminescence in UV range (curves are smoothed using a moving average filter)</figcaption></figure>

When diodes were forward biased, UV emission around 262 nm was measured as shown in figure 4. Due to the structure of p pads preventing light from being extracted, junctions were mounted as bottom emitters during measurements. This emission corresponds to electron-hole recombination in n-AlGaN layer as the crystal was synthesized with 57% Al content leading to a bandgap matching such emission. These observations are the cornerstone of the present work, as since the report of the first p-hBN/n-AlGaN junctions
[8], no intrinsic emission was reported. These results highlight improved efficiency of the junctions realized in this work and their potential use for p-injection layers in DUV LEDs.

Having demonstrated excellent rectification behavior, UV MQWs were then integrated in these junctions to achieve full LED structures. Layers were grown using the same parameters used for the pn junctions with the addition of 5-period Al<sub>0.57</sub>Ga<sub>0.43</sub>N (13.5 nm)/ Al<sub>0.38</sub>Ga<sub>0.62</sub>N (2.2 nm) MQWs grown on top of n-AlGaN layer. XRD measurements presented in figure 5(a) show the growth of MQWs on n-AlGaN/AlN/Sapphire. Also, even though similar, the fabrication process requires an extra etching step to open the n-side because of the integration of MQWs. This leads to a mesa, illustrated in figure 6(a), onto which the p-hBN/metal stack is precisely transferred using a double-beam splitter integrated into the transfer printing machine, allowing us to see simultaneously the host surface as well as the bottom of the stack. Moreover, for these structures, 200x200 µm<sup>2</sup> square pads were used instead of the 500x500 µm<sup>2</sup> pads used for the junctions described above. A microscope image of a transferred stack atop of a mesa is shown in figure 6(b).

<figure><img src="image_6.png" /><figcaption>Figure 5: (a) High resolution 2θ-ω scan (b) SEM image of AlGaN MQWs grown on n-AlGaN/AlN</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6: (a) Lift-off/transfer process used to realize p-hBN/MQWs/n-AlGaN LEDs (b) Top-view of p-hBN/MQWs/n-AlGaN LED realized by lift-off/transfer</figcaption></figure>

Realized LEDs have been characterized electrically and optically, measurements being shown on figure 7.

<figure><img src="image_8.png" /><figcaption>Figure 7: (a) UV LED I-V measurements in semi log-scale (b) Electroluminescence in UV range (curves are smoothed using a moving average filter)</figcaption></figure>

These structure present similar electrical behavior compared to the pn junctions confirming that the introduction of MQWs does not affect the overall conductivity nor the rectifying behavior. Emission centered at 290 nm has been measured corresponding to the Al-content introduced as well as to the thickness of the wells. Also measured was a small emission peak around 262 nm coming from recombinations in the barriers. This 262 nm emission needs to be differentiated from the one observed in the junctions which came from recombinations directly happening in the n-AlGaN layer. Furthermore, even though emission is expected to be TM polarized due to the Al-content (38%), MQWs were designed in such a way that the strain induced by the lattice mismatch between barriers and wells enhances TE polarized emission [20]. This is different from the pn junction's
emission, in which emission at 262 nm is TM polarized (relaxed layer with 57% Al). Consequently, the realized LEDs serve as a proof of concept for future layer transferred p-hBN/MQWs/n-AlGaN structures in which the Al-content is increased to go toward DUV emission, hBN being suited for heterojunctions with Al-rich AlGaN up to AlN.

In summary, p-hBN/n-AlGaN pn junctions were fabricated using a selective lift-off/transfer of hBN layers atop of AlGaN. These junctions exhibit a rectification ratio > 10⁵ and emit in UV range at 262 nm as holes from p-hBN are injected in the n-AlGaN layer. MQWs were then integrated into these structures to achieve fully working UV LEDs which emitted at 290 nm after current injection. The selective pick and place process used to build these LEDs has multiple advantages. First, growing p-hBN on a different substrate allows the half structure including the n-side as well as the quantum wells to be optimized independently, this being also true for the p-side. Secondly, the high temperature needed for high hBN material quality could affect the thermal stability of the MQWs which are thus protected by growing the p-layers separately, a problem also encountered in InGaN/GaN based LEDs [21]. Therefore, this study shows that AlGaN-based UV LEDs in which p-side has been replaced by p-hBN can be built using a selective lift-off/transfer process, leading to high quality junctions. Consequently, p-hBN growth and processing have reached a sufficient level of maturity allowing its future integration with Al-rich (>80%) AlGaN for next generation DUV emitters.

## SUPPLEMENTARY MATERIAL

See the supplementary material for details about extraction of junction's parameters (ideality factor and series resistance).

## ACKNOWLEDGEMENT

This study was partially funded by the French National Research Agency (ANR), under the GANEXT Laboratory of Excellence (LabEx) Project, under the FlexiGaN (Grant No. ANR-22-CE51-0009) Project, under the INMOST (Grant No. ANR-19-CE08-0025) Project, under the BIRD (Grant No. ANR-24-CE08-3820) Project and French PIA Project Lorraine Université d'Excellence (Grant No. ANR-15-IDEX-04-LUE), as well as by the Region Grand Est in France.

## CONFLICT OF INTEREST

The authors have no conflicts to disclose.

## AUTHOR CONTRIBUTIONS

J.P.S and A.O. contributed towards the study design and supervised the whole project. A.P., A.K. and R.G. performed device fabrication. A.P. and A.K. performed electrical and optical measurements. A.P. and J.P.S wrote the manuscript. S.S., P.V., V.O. and T.M.T. performed the epitaxial growth. P.V., V.O. and T.M.T. performed SEM and HRXRD measurements of the epilayers. The manuscript was written through contribution of all authors. All authors have given approval to the final version of the manuscript.
DATA AVAILABILITY

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

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