Raman Study of Lattice Anharmonicity and Substrate Induced Effects in AlN Heteroepitaxial Layers

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1. Study Background of Lattice Anharmonicity and Substrate Induced Effects in AlN Thin Film

AlN (Aluminum Nitride) is an ultra wideband gap semiconductor with unique physical properties, which is attractive for many strategic and forward-looking applications. Especially, the high thermal and chemical stability, high thermal conductivity, ultra-high sound velocity, and direct bandgap of AlN make it ideal for deep ultraviolet optoelectronic devices and high-power electronics. The AlN heteroepitaxial layer is the most commonly used device configuration. However, highly integrated devices are susceptible to phonon behavior and heat transfer, and the strong influence of substrate induced strain on AlN phonon behavior has not been fully studied. Therefore, studying the interaction between AlN epi layers and substrate is of great significance.

Phonons, known as quasi particles, have complex self energy caused by phonon anharmonicity (∑ (ω)=Δ (ω)+iΓ(ω), where the real and imaginary parts represent frequency shift and linewidth broadening, respectively). They quantify lattice vibrations and play an important role in thermal transport. The anharmonicity of phonons reflects the intrinsic phonon scattering related to thermal conductivity. Due to the fact that self heating and heat transfer are fundamental processes for device operation at different temperatures, a deep understanding of the temperature dependence of quasi particle interactions in AlN, especially phonon-phonon and electron-phonon interactions, is crucial for designing devices and achieving optimal performance.

Raman spectroscopy is a non-destructive and powerful characterization method that can reveal the crystal quality, internal strain, phonon dynamics, and thermal properties of crystal materials. Especially, temperature dependent Raman scattering spectroscopy is very suitable for detecting the anharmonicity of optical phonons and the intrinsic phonon attenuation mechanism near the center of the Brillouin zone. By utilizing the temperature dependence of novel two-dimensional van der Waals layered materials and typical wurtzite epitaxial layers, Raman spectroscopy has successfully studied the anharmonic effect. However, so far, there have been few systematic studies on the temperature dependent phonon behavior of high-quality single crystal AlN epitaxial layers. The research on temperature dependent phonon behavior of AlN crystals both domestically and internationally has not yet provided a detailed analysis of non harmonic phonon scattering, and further in-depth research on AlN crystals is urgently needed temperature dependent study of phonon behavior in AlN heteroepitaxial layers.

2. Raman Study Process and Conclusions for Lattice Anharmonicity and Substrate Induced Effects in AlN

The AlN crystals were prepared using metal organic chemical vapor deposition (MOCVD) and physical vapor transport (PVT) methods. The optical responses of the two main optical phonons of high-quality single crystal AlN epitaxial layers and bulk AlN over a wide temperature range (4K-870K) were studied using various condensed matter spectroscopy characterization methods. Compared to bulk AlN, the AlN epitaxial layer exhibits more pronounced phonon softening, and this stronger temperature dependence can serve as a non-invasive thermal probe to determine local temperature changes in AlN based devices.

Furthermore, by combining first principles calculations and a classical physical model involving thermal expansion, phonon anharmonic effects, and substrate effects, the temperature dependent phonon behavior of the E2 (high) and A1 (LO) phonon modes in AlN heteroepitaxial layers was systematically studied. This provides guidance for a deeper understanding of phonon phonon interactions in AlN films and their device applications under extreme conditions. Moreover, this analysis process can be fundamentally applicable to other emerging epitaxial thin films and two-dimensional materials.

Fig. 1 Temperature dependent Raman spectroscopic characterization of AlN epitaxial layer and bulk AlN

Fig. 2 (a) Linear first-order temperature coefficient of AlN crystal; (b) derivation of biaxial stress coefficient and pure temperature coefficient of E2 (high) mode

Fig. 3 Decoupling analysis of temperature dependent phonon frequency, linewidth, and lifetime of AlN epitaxial layer based on classical physical model

Low temperature Raman scattering experiments have observed weak anomalous phonon behavior in AlN crystals below 100 K, which may be attributed to the negative thermal expansion characteristics of AlN crystals. More importantly, the article proposes a “multi substrate” induced strain control strategy from an application perspective, and based on the AlN phonon distortion potential theory, derives the biaxial stress coefficient and pure temperature coefficient of the E2 (high) phonon mode in AlN crystals as -6.63 ± 0.27 cm^-1/GPa and -0.0217 cm^-1/K, respectively. These coefficients will provide key information for evaluating the impact of temperature and stress on the reliability of AlN based devices.

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