# Assessing the Stress Induced by Novel Packaging in GaN HEMT Devices via Raman Spectroscopy
## 2 Materials and Methods  
### Material  
In this study, we explore the impact of packaging on local stress through a comparative analysis of the Raman spectral characteristics of the packaged (PD) AlGaN/GaN HEMT and its unpackaged counterpart, the bare (B) AlGaN/GaN HEMT, both provided by STMi-croelectronics S.r.l. (Catania, Italy). Figure 1 displays images of both the bare AlGaN/GaN HEMT (a) and its packaged counterpart (b). \begin{table}
\begin{tabular}{l l} \hline \hline
**Symbols and Acronyms** & **Description** \\ \hline MOSFET & Metal-Oxide–Semiconductor Field Effect Transistor \\ HEMT & High-electron-mobility transistor \\ HEMT-B & Bare HEMT \\ HEMT-PD & Packaged HEMT \\ WBS & Wide-Bandgap Semiconductors \\
2-DEG & Two-dimensional electron gas \\ RT & Room temperature \\ MOCVD & Metal-Organic Chemical Vapor Deposition \\ SEM & Scanning Electron Microscopy \\ DPSS & Diode-Pumped Solid State \\ CCD & Charge-Coupled Device \\ \(\omega\) & Measured frequency center of the E2(high) Raman peak \\ \(\omega_{0}\) & Stress-free frequency center of the E2(high) Raman peak \\ \(\Delta\omega\) & Frequency shift of the E2(high) Raman peak \\ \(\varphi\) & Residual stress \\ K & Stress coefficient \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of symbols and notations used in this paper. AlGaN/GaN HEMTs are manufactured by STMicroelectronics S.r.l. by means of Metal-Organic Chemical Vapor Deposition (MOCVD). The process begins with the growth of a 5 \(\upmu\)m thick GaN(0001) layer on an n-Si(111) substrate, followed by a 16 nm Al\({}_{0.25}\)Ga\({}_{0.75}\)N barrier. Afterwards, a p-GaN capping layer is added on the gate region and then the metallic contacts are deposited. Subsequently, a SiO\({}_{2}\) passivation layer is deposited to ensure electrical isolation for the AlGaN/GaN high-electron-mobility transistors and, ultimately, the packaging. Figure 2 illustrates a cross-sectional view of the AlGaN/GaN HEMT structure. As displayed in Figure 2, the GaN layer can be split in two layers. The first one, with a thickness of roughly 2 \(\upmu\)m, acts as a strain modulation layer at the GaN/Si interface, minimizing the lattice mismatch between the two semiconductors. In this way, the GaN epitaxial layer will experience a lower built-in mechanical stress. The employment of the p-GaN cap gate and the equilibrium between the aluminum molar fraction and the thickness of the AlGaN layer achieve normally off HEMTs with a reasonably positive threshold voltage [22]. Moreover, the two-dimensional electron gas (2-DEG) sheet at the GaN epitaxy/AlGaN interface results in efficient charge depletion processes. Two identical devices were prepared; one device was at wafer level (HEMT-B) while the second was encapsulated (HEMT-PD). The purpose was to analyze their Raman spectra and highlight the impact of encapsulation on residual stress. To acquire the Raman spectra of the device layers, it was necessary to cut it transversely. However, a simple cut and subsequent mechanical polishing are not suitable experimental strategies as the resulting surface roughness would be very high, adding noticeable additional stress to the layers and leading to imprecise measurements. For this reason, an ion beam milling technique was employed on both samples under the same milling conditions. The ion milling procedure involves amorphization of the semiconduc  
Figure 1: Photographs of the AlGaN/GaN HEMT-B (**a**) and AlGaN/GaN HEMT-PD (**b**). Figure 2: Schematic representation of the internal structure of AlGaN/GaN HEMT-B (**a**) and AlGaN/GaN HEMT-PD (**b**) investigated in this study (not to scale). The vertical dashed line indicates the uniaxial direction along which micro-Raman measurements were performed. tor's surface layers; thus, in principle, the Raman spectrum could be influenced by the cleaning treatment.