# III-nitride semiconductors are more flexible than other compound semiconductors for integrated photonics and efficient nonlinear interactions

Maksym Gromovyi, Nagesh Bhat, Hervé Tronche, Pascal Baldi, Moustafa El Kurdi, Xavier Checoury, Antoine Reserbat-Plantey, Minh Tuan Dau, Mohamed Al Khalfioui, Blandine Alloing, et al.

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Maksym Gromovyi, Nagesh Bhat, Hervé Tronche, Pascal Baldi, Moustafa El Kurdi, et al. III-nitride semiconductors are more flexible than other compound semiconductors for integrated photonics and efficient nonlinear interactions. Gallium Nitride Materials and Devices XX, Jan 2025, San Francisco, United States. pp.13, 10.1117/12.3042971 . hal-05357904

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III-Nitride Semiconductors are more flexible than other compound semiconductors for integrated photonics and efficient nonlinear interactions

M. Gromovyiª, N. Bhatª, H. Troncheᵇ, P. Baldiᵇ, M. El Kurdiᶜ, X. Checouryᶜ, A. Reserbat-Planteyª, M. T. Dauª, M. Al Khalfiouiª, B. Alloingª, F. Semondª, B. Damilanoª, and P. Boucaud*ª

ªUniversité Côte d'Azur, CNRS, CRHEA, Rue Bernard Grégory, 06905 Sophia-Antipolis, France ;

ᵇUniversité Côte d'Azur, CNRS, INPHYNI, 06200 Nice, France ;
ᶜUniversité Paris-Saclay, CNRS, C2N, Rue Thomas Gobert, 91120 Palaiseau, France

ABSTRACT

III-nitride semiconductors like GaN and AlN have very specific properties that can be useful for integrated photonic circuits. We discuss some recent results associated with single-arm Mach-Zehnder interferometers, sign reversal of second-order nonlinear susceptibility and integration of two-dimensional materials.

**Keywords:** III-Nitride semiconductors, photonic integrated circuits, Mach-Zehnder interferometers, nonlinear conversion, 2D materials

1. INTRODUCTION

The III-nitride semiconductors offer every building block needed for a universal photonic platform. Materials like gallium nitride and aluminum nitride have a very large transparency window that is well suited for photonics circuits operating from the near-infrared to the visible and ultra-violet spectral range. Lasers, LEDs, modulators, transistors can be easily fabricated with III-nitride semiconductors. The wide band gap enables room temperature or even high temperature operation of these devices.

Consequently, the III-nitride semiconductors bring many advantages in terms of monolithic integration as compared to other solutions that rely on heterogeneous integration [1]. The silicon nitride (SiN) is a well-established platform but lacks an integrated light source. Operation at very short wavelength is prohibited. Lithium niobate exhibits a very large second-order nonlinear coefficient (d33 = 25 pm/V). Its processing in forms of thin film layers has open multiple possibilities for nanophotonic circuits. Lithium niobate still face significant challenges when considering the integration of electrically-driven optical sources and the platform development in a complementary metal oxide semiconductor silicon fab environment is a tough issue. Other materials like silicon carbide and diamond also offer interesting alternatives but without the full capabilities brought by the III-nitride semiconductors.

2. RESULTS

Over the last decade, photonic circuits with high quality factor resonators have been obtained either on silicon or sapphire substrates [2, 3]. The first demonstrations were reported for the near-infrared spectral range and later extended to the visible and ultra-violet spectral ranges [4]. Controlling the phase for light propagation in photonic circuits is a major challenge. A standard approach is to engineer Mach-Zehnder interferometers in which light propagation is split into two arms. Single-arm Mach-Zehnder interferometers can as well be implemented by taking advantage of multimode waveguides. Such approach has been demonstrated in Ref. [5]. The structure did consist of a GaN and AlN bilayer epitaxially-grown by metal-organic chemical vapor deposition on a sapphire substrate. Two optical modes in TM polarizations (TM00 and TM01) could be excited in the 1.5 µm wide waveguides. The interferences of these two modes with different wavevectors lead to a modulation of the transmission equivalent to the one observed in two-arms Mach-Zehnder interferometers. When combined with microring resonators, one can control the phase of the microring resonances, as done with double arm
interferometers [6]. The associated spectral shapes can point either downwards, upwards and any intermediate signature.
Quality factors up to 400 000 were achieved in these compact interference circuits [5].

One very peculiar property of the III-nitride semiconductors is linked to the crystalline tetrahedral bonding in these wurtzite-type structures and to their band structure leading to opposite signs for the second-order nonlinear susceptibility between GaN and AlN [7]. This property is quite unique for compound semiconductors. It is well known that modulating the sign of the nonlinear susceptibility is the most standard approach for achieving quasi-phase matching for second-order nonlinear interactions [8]. Different approaches were implemented so far to reach this goal with III-nitrides, as for example by a succession of etching steps and epitaxial regrowth or by wafer bonding. The direct epitaxial growth of GaN and AlN bilayers can drastically simplify the approach. By finely adjusting the thicknesses of the layers, one can achieve modal phase matching with fundamental and first-order modes and enhance significantly the second-order nonlinear conversion efficiency. Continuous-wave conversion efficiencies of 4%/W/cm² were demonstrated in straight waveguides using this very simple monolithic approach [9]. We can note that GaN and AlN are not the only III-nitride material combination with sign reversal of nonlinear susceptibility. The variation of scandium composition in scandium aluminum nitride alloys leads as well to a sign change in the electro-optic coefficient [10]. III-nitrides offer other approaches to control the polarity of the materials. Niobium nitride can be grown on III-nitride. This material offers unprecedented options to combine metal/superconductor materials with semiconductors [11]. An ultra-thin layer of niobium nitride can invert the polarity of the III-nitride, from metal-polarity to nitrogen polarity. It increases the options available in the III-nitride toolbox for photonic circuits engineering.

The spectral ranges covered by the III-nitrides can be enriched by the integration of two-dimensional materials. Beyond layer transfer, transition metal dichalcogenides like $MoS_2$, $WS_2$, $MoSe_2$ can be epitaxially grown using molecular beam epitaxy on gallium nitride or other III-nitride compounds [11]. Wafer-scale surface coverage is achievable when combined with thermal annealing. Other 2D materials, including boron nitride, can be integrated thus paving the way for complex van der Waals heterostructures. The prospects offered by combining 2D materials with III-nitrides for photonics are huge.

## ACKNOWLEDGEMENTS

This work was supported by the French RENATECH network, the French National Research Agency (Agence Nationale de la Recherche, ANR) through funding of the OPOINt project (ANR-19-CE24-0015). Nagesh Bhat was partly funded by Labex GANEX (ANR-11-LABX-0014). GANEX belongs to the public funded "Investissements d'Avenir" program managed by the French ANR agency. This work did benefit from a France 2030 government grant managed by the French National Research Agency (ANR-22-PEEL-0011) - PEPR Electronique - ADICT.

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*philippe.boucaud@crhea.cnrs.fr; www.crhea.cnrs.fr