# Assessing the Stress Induced by Novel Packaging in GaN HEMT Devices via Raman Spectroscopy
## 3 Results and Discussion  
Figure 5a shows an optical microphotograph of the cross-section of the sample on which the study was conducted. In particular, the 128 spectra were acquired along a vertical line crossing the layers (the red line crossing Figure 5a). This strategy allows us to observe how the Raman peaks shift from one layer to another and thus highlights the presence of stress at the interfaces. Since obtaining a sufficient signal-to-noise ratio required integrating the signal for very long time, we chose to proceed with acquiring the Raman spectra along a line instead of performing a full Raman mapping. Indeed, the latter would have required extremely long measurement times, potentially leading to results affected by thermal drift and unpredictable variations in experimental conditions. Moreover, it would not have provided additional useful information to the measurement. Figure 5b shows the 1D map of the Raman spectra acquired during the experiment along the 10 \(\upmu\)m investigated (i.e., the Raman intensities acquired point by point plotted against the Raman shift). It can roughly be said that the spectral structures shift when moving from silicon through the layers of GaN, indicating the presence of intrinsic stress. In particular, the GaN Raman peaks in the lower part of the map start to redshift as the position of the area under analysis approaches the GaN/Si interface. The frequency center of Raman peaks is intimately correlated to the interatomic potential between atoms [28]. Accordingly, changes in this potential due to intrinsic stress within the crystal lead to Raman shifts of specific and characteristic vibrational modes of the structure, regardless of its origin [28]. Figure 4: Image of the sample holder (**a**) with details highlighting the sample position (**b**). Figure 5: Detailed view of the AlGaN/GaN HEMT-PD heterostructure (**a**) and the resulting Raman map along the 10 \(\upmu\)m scanned path (**b**). Figure 6 displays typical micro-Raman spectra acquired at the GaN epitaxy layer (Figure 6a) and at the GaN/Si interface (Figure 6b). The micro-Raman profile collected at the GaN epitaxy layer (Figure 6a) reveals distinct contributions at 531.7 cm\({}^{-1}\), 558.7 cm\({}^{-1}\), and 567.7 cm\({}^{-1}\), respectively, associated to the A1(TO), E1(TO), and E2(high) modes of the hexagonal structure of wurtzite. At the GaN/Si interface (Figure 6b), as expected, a characteristic Si Raman peak centered at 521.4 cm\({}^{-1}\), arising from the Si substrate, was clearly distinguished. Furthermore, the Raman mode deriving from GaN structures, E2(high) at 569.6 cm\({}^{-1}\), was also observed. The observed frequency shift of the E2(high) mode, occurring from the GaN epitaxy layer to the GaN/Si interface, can be attributed to the different stress conditions in these two regions. Since the main purpose of the work was to identify the presence of stress among the layers using Raman spectroscopy, it was necessary to measure the Raman peak positions with the utmost precision. Despite the rather high spectral resolution of the measurement instrument, obtaining accurate information required fitting the experimental data using an appropriate model. In particular, the used model consists of three Lorentzian lines describing the Raman modes of GaN (A1(TO), E1(TO), and E2(high)), and also takes into account the presence of silicon when it needs. All the 128 spectra were fitted using the model described above to obtain the characteristic frequency \(\omega\) of the modes point by point along the scanned line. Notably, E2(high) mode was found to be particularly sensitive to biaxial stresses compared to both A1(TO) and E1(TO) modes. For this reason, and in agreement with literature, it was used as a marker for quantifying residual stresses [29]. This may be reasonably due to the nature of the E2(high) mode, characterized by atomic oscillations almost perpendicular to the c-axis of wurtzite and hence more sensitive to stress along the chosen axis (see Figure 2).