GaN Cap UV Spectroscopy Assessment in AlGaN/GaN HEMT

. This work discusses the use of gallium nitride (GaN)-based solid-state devices for high-power, high-frequency, and high-temperature technology. The article presents the results of an investigation into the Al fraction of AlGaN as a function of GaN cap growth time through µ -Raman and µ -Photoluminescence ( µ -PL) spectroscopy under λ =325 and 266 nm laser source. The data exhibit that the detected Al fraction decreases as the GaN cap layer size increases, consistently with the surface quantum well effect in the layer stack. The study confirms that the GaN cap layer is acting as a potential well and enables the design of a non-destructive and quantitative assessment of the grown thickness of the GaN cap layer through UV laser spectroscopy. The interpretation of the data also rules out the possibility of thermal migration of Al in the adjacent GaN layers during MOCVD growth.


Introduction
Recently gallium nitride (GaN)-based solid-state devices have demonstrated extraordinary effectiveness for high-power, high-frequency, and high-temperature technology.The basic properties of the AlGaN/GaN material combination make it an excellent choice for microwave devices.SiC and sapphire are the most often utilized substrates for the formation of AlxGa1-xN/GaN heterostructures.However, these substrates have limitations such as high cost, limited wafer diameter, and inferior thermal performance to SiC in the case of sapphire [1].As a result of these statements, Si is the most adopted substrate for large-scale fabrication of GaN electrical devices.However, interfacial stress, dislocation density and growth parameter control remain open questions to achieve smooth step flow surface morphology [2].The progress of GaN devices must take into account the development of non-destructive investigations techniques that enable reliable control and characterization of the structural and electrical properties of the HEMT heterostructure.In this regard, this work presents an in-depth UV spectroscopic investigation of epilayers developed by metal organic chemical vapor deposition (MOCVD) and disclose AlGaN/GaN hetero structures grown on 150 mm diameter (111)oriented Si substrates together with assessment of GaN cap layer thickness and its interface interaction with underlaying AlGaN layer.

Materials and Methods
Trimethylgallium (TMGa), Trimethylaluminum (TMAl) and ammonia (NH3) were used as the precursors for Gallium (Ga), Aluminium (Al) and Nitrogen (N) sources respectively.HEMT growth was carried out on (111) oriented 150 µm Si wafer substrates where the deposition of a 2 µm GaN buffer layer at 1020 °C was followed by the growth of 16 nm thick AlGaN layer.Afterwards three of the four wafers underwent deposition of a thin GaN cap layer for 21, 40 and 60 s.The resulting HEMT layer structure GaN (cap)/ AlGaN (barrier)/ GaN (buffer layer) were optically characterized by means of µ-Raman spectroscopy and µ-PL through λ=325 nm He-Cd and λ=266 nm Nd:YAG fourth harmonic UV laser sources in order to characterize interface phonon modes arising from 2DEG AlGaN/GaN junctions and Al composition within AlGaN.

Results and Discussion
Raman spectra acquired from the AlGaN/GaN epiwafer under z(x,_ )z polarization geometry are shown in Figure 1(a-b).To prevent thermal red shift, the Raman spectra were stimulated with a P=15 mW low laser intensity.The Raman spectra are dominated by peaks at 588-595, 725-727, and 770-772 cm -1 , in addition to the normal GaN E2(TO)-high mode [3].In Fig. 1a, starting with the design without the GaN cap, Raman spectra exhibits an E2(TO) mode related to the GaN buffer layer at 567.2 cm -1 and an interface mode (strongly linked to the Al content in the AlGaN layer) at frequencies greater than 588.1 cm -1 .When a very thin GaN cap layer is inspected, the upper layers of GaN/AlGaN/GaN trigger peak position variation and softening of the high GaN E2 phonon mode compared to the strain-free peak value of 568.1 cm -1 (recorded by an independent GaN substrate) and reveal the onset of a biaxial compressive stress [4].

Fig. 1 Raman analysis of GaN HEMT wafers with different thicknesses of GaN cap layer grown within the region of a) TO and b) LO phonon modes; c) vibrational frequency of the E2(TO) modes as a function of the grown GaN cap thickness
In particular, the E2(TO) mode peak broadens when the 1 nm GaN cap layer is grown.Indeed, as determined by Lorentzian fit deconvolution, E2(TO) mode is influenced by both GaN buffer layer and GaN cap layer signal, which suffers compressive stress persisting along the various grown GaN cap thicknesses.Within the domain of the longitudinal optical modes (Fig. 1b), in presence of a bare GaN AlGaN junction, the weak feature at 693.2 cm -1 arises by a disorder-activated mode (indicated as DA) promoted by lattice defects at the AlGaN/GaN interface, as suggested by the Lü and Cao transfer matrix model [5].A major variance is observed with the appearance of AlGaN/GaN interface modes at 722.0 cm -1 (IF2 phonon mode) that emerge flanked by GaN A1-longitudinal optical (LO) modes and suggest the heterointerface 2DEG nature.Since A1(LO) modes display an anti-symmetric

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Growing and Forming of Semiconductor Layers potential and interact more strongly with free carriers, the surface charge redistributes with the GaN cap grown on AlGaN, causing the development of the LO peak associated with the resonance effects of 2DEG.Furthermore, biaxial stress influences the shift of A1(LO) modes at higher frequencies, defining GaN and GaN cap peaks at 752.7 and 770.1 cm -1 respectively, as observed by deconvoluted modes.Through the increase of the GaN cap thickness to 3 nm the deconvolutions of the Raman peaks report a shift to lower frequencies at 738.3 cm -1 and 760.3 cm -1 .The above resettlement towards the nominal value of the A1(LO) of GaN at 733.6 cm -1 for UV investigations is associated with the relaxation of the crystalline lattice of GaN cap, which, through onset of dislocations, relaxes from compressive stress conditions addressed to adaptation of the GaN lattice parameter to that of the AlGaN seed on which the GaN cap is deposited.Fig. 1c shows the vibrational frequency of the E2(TO) modes as well as the neighboring interface peak.The E2(TO) of the GaN cap has a vibrational frequency of 576.2 cm -1 at a thickness of 1 nm, revealing a compressive stress of 2.8 GPa which settles down to 1.1 GPa and 1.9 GPa as the thickness of the GaN cap increases.The PL investigation under λ=325 nm showed on the samples both the intense bandgap signal localized at 363 nm and the weak intrabangap signal studded by Fabry-Perot oscillations including blue and yellow band related to GaN defects at ~ 420 and 560 nm respectevely (see Fig. 2a).The sources of yellow band could be dislocations, surface defects, Ga vacancies (VGa) or VGa complexes, such as VGaON or VGaSiN as well as C related point defects [6], while blue band is prevalently caused by transition from conduction band to relatively deep acceptor (ZnGa), from Zn contamination by impure gases adopted in MOCVD [7].As shown in the inset of Fig. 2a, the investigation of the layer after 1 nm GaN cap deposition exhibits a weak signal at 332 nm significant of the aforementioned GaN crystal distortion.The extraction of the Al fraction from the PL conducted with λ=266 nm allows to excite also the AlGaN underlying the GaN cap since the bandgap energy is dependent to the incorporated Al fraction.
Fig. 2 a) PL spectra at λ=325 nm of the GaN HEMT wafers and the above bandgap region with GaN cap emission reported within the inset; b) Al fraction % extracted from the PL analysis performed with λ=266 nm and, in the inset, the spectrum of an acquisition of the wafer without GaN cap in the above bandgap region.
The graph in Fig. 2b shows the Al fraction of the AlGaN extracted from this investigation as a function of the GaN cap growth time.The AlGaN bandgap signal located at 313 nm implies an Al fraction of 31.33%.The overlapping of the GaN cap layer from 1 nm to 3nm implies the reduction of the detected Al fraction from 18.25% to 10.46%.This data is consistent with the value extracted from the interface modes detected by the µ-Raman survey in correspondence with the A1(LO) resonant enhanced mode and with its shift.The peak is due to the GaN cap layer surface quantum well (SQW) effect in the layer stack.According to SQW system carriers are confined in the well (GaN cap layer), therefore PL from the well is enhanced as compared to AlGaN emission in sample confirming that Solid State Phenomena Vol.362 the cap layer is acting as potential well.The carriers from the AlGaN interface fall into the GaN well (SQW) [8], and since fewer carriers recombine for AlGaN, the AlGaN intensity drops while the SQW intensity increases.In accordance, the well width and difference between the conduction band and valence band energy level of the potential well are roughly comparable to the thickness of the GaN cap layer.[9].The relevance of this interpretation enables to rule out the possibility that the extracted Al concentration data reflect thermal migration of Al in the adjacent GaN layers during MOCVD growth and to identify a correlation between quantum well energy and the GaN cap dimension, therefore allowing the design of a non-destructive and quantitative assessment of the grown thickness of the GaN cap layer through UV laser spectroscopy.

Summary
The article presents an in-depth UV spectroscopic investigation of epilayers developed by metal organic chemical vapor deposition (MOCVD) and discloses AlGaN/GaN heterostructures grown on 150 mm diameter (111)-oriented Si substrates.The study also assesses GaN cap layer thickness and its interface interaction with underlying AlGaN layer.
This article presents the results of a µ−Raman spectroscopy investigation of an AlGaN/GaN epiwafer.
The study reveals the onset of a biaxial compressive stress when a very thin GaN cap layer is grown, causing the E2(TO) mode peak to broaden.The surface charge redistributes with the GaN cap grown on AlGaN, causing the development of the LO peak associated with the resonance effects of 2DEG.Biaxial stress influences the shift of A1(LO) modes at higher frequencies, defining GaN and GaN cap peaks at 752.7 and 770.1 cm -1 , respectively.The increase of the GaN cap thickness to 3 nm results in a resettlement towards the nominal value of the A1(LO) of GaN at 733.6 cm-1, due to the relaxation of the crystalline lattice of GaN cap.The study shows that the detected Al fraction decreases as the GaN cap layer thickness increases, consistent with the surface quantum well effect in the layer stack.
The study confirms that the GaN cap layer is acting as a potential well, and the carriers from the AlGaN interface fall into the GaN well.The interpretation of the data rules out the possibility of thermal migration of Al in the adjacent GaN layers during MOCVD growth and allows for the design of a non-destructive and quantitative assessment of the grown thickness of the GaN cap layer through UV laser spectroscopy.which can improve the design and fabrication of GaN-based devices.