Acoustic and Optical Phonon Frequencies and Acoustic Phonon Velocities in Silicon-Doped Aluminum Nitride Thin Films

We report the results of the study of the acoustic and optical phonons in Si-doped AlN thin films grown by metalorganic chemical vapor deposition on sapphire substrates. The Brillouin - Mandelstam and Raman light scattering spectroscopies were used to measure the acoustic and optical phonon frequencies close to the Brillouin zone center. The optical phonon frequencies reveal non-monotonic changes, reflective of the variations in the thin-film strain and dislocation densities with the addition of Si dopant atoms. The acoustic phonon velocity decreases monotonically with increasing Si dopant concentration, reducing by ~300 m/s at the doping level of 3 x 10^19 1/cm3. Knowledge of the acoustic phonon velocities can be used to optimize ultra-wide bandgap semiconductor heterostructures and minimize the thermal boundary resistance of high-power devices.

💡 Research Summary

In this work the authors investigate how silicon (Si) doping influences both optical and acoustic phonons in aluminum nitride (AlN) thin films grown by metal‑organic chemical vapor deposition (MOCVD) on c‑plane sapphire substrates. A set of films with Si concentrations ranging from the unintentionally doped (UID) level up to 3 × 10¹⁹ cm⁻³ were prepared. Raman spectroscopy (488 nm excitation) was employed to probe the zone‑center optical phonons (A₁(LO), E₂(high), and E₁(TO)). The UID film exhibits slight compressive strain, reflected in modest shifts of the Raman peaks relative to bulk AlN. As Si is introduced, the Raman frequencies display a non‑monotonic behavior: at low Si concentrations (≤5 × 10¹⁸ cm⁻³) the E₂ and A₁ modes shift to lower wavenumbers, indicating relaxation of the initial strain; at higher concentrations (>5 × 10¹⁸ cm⁻³) the peaks shift back to higher wavenumbers because the smaller Si atoms create local compressive strain (Si–N bonds are shorter than Al–N bonds). The E₁ mode follows the opposite trend due to its distinct deformation‑potential coefficients. Simultaneously, the full‑width‑half‑maximum (FWHM) of the Raman lines varies non‑monotonically, mirroring changes in dislocation density. High‑resolution X‑ray rocking‑curve measurements reveal that the dislocation density remains low for UID and low‑doped samples but rises above 10⁹ cm⁻² once the Si concentration exceeds ~1 × 10¹⁸ cm⁻³, establishing a clear correlation between defect density and the observed phonon shifts.

Acoustic phonons were examined using Brillouin‑Mandelstam spectroscopy (BMS) with a 532 nm laser at a fixed 45° incidence angle. The refractive index of each film, measured by spectroscopic ellipsometry, increases sharply at low Si concentrations and saturates at higher doping levels; at 532 nm the index is used to calculate the phonon wavevector q = 4πn/λ. The longitudinal acoustic (LA) mode appears as a clear Lorentzian peak in the BMS spectra. By converting the Brillouin shift f to a group velocity v = 2πf/q, the authors find that the LA velocity decreases monotonically with Si content, dropping by roughly 300 m/s (≈3 %) at the highest doping level compared with the UID reference (≈9.8 km s⁻¹). This softening is attributed to two intertwined mechanisms: (i) increased point‑defect scattering strength (Γ ∝ N, where N is the dopant concentration) and (ii) a reduction in the elastic stiffness of the lattice caused by Si incorporation and the accompanying rise in dislocation density. The modest velocity reduction, while small, has direct implications for thermal transport: acoustic phonons dominate heat conduction in UWBG materials, so a lower phonon speed reduces the phonon mean free path and bulk thermal conductivity, and it also modifies the phonon density of states at interfaces, potentially increasing thermal boundary resistance (TBR) in high‑power devices.

The paper thus demonstrates that Si doping affects optical and acoustic phonons in fundamentally different ways. Optical phonons respond to a complex interplay of strain relaxation, local compressive strain, deformation potentials, and defect generation, leading to non‑monotonic frequency shifts. Acoustic phonons, by contrast, exhibit a systematic velocity reduction driven by overall lattice softening and enhanced scattering. These findings provide quantitative guidance for engineering AlN‑based ultra‑wide‑bandgap (UWBG) devices: designers can balance the need for higher carrier concentrations (via Si doping) against the modest penalty in thermal conductivity and interface resistance. The authors suggest that future work should explore other dopants, temperature‑dependent phonon behavior, and direct measurements of TBR in device structures to refine thermal management strategies for AlN and related UWBG semiconductors.