Structural and Optical Characteristics of InGaN/GaN Thin Films Prepared by 12MeV Silicon Ion Irradiation

Structural and Optical Characteristics of InGaN/GaN Thin Films Prepared by 12MeV Silicon Ion Irradiation *

1. Abstract

This paper examines the impact of silicon (Si) ion irradiation, at a high energy of 12 MeV, on the structural and optical properties of InGaN/GaN thin films. Irradiation was performed at varying Si ion fluxes ranging from 1×10^13 to 1×10^15 ions/cm^2. X-ray diffraction (XRD) patterns of the pristine films showcased only the (0 0 2) orientation of InGaN grains, while post-irradiation films also exhibited additional phases (InN and GaN). Ion irradiation at different dosages did not significantly affect the grain size of InGaN or was negligible, aside from shifts in peak positions, indicative of the development of tensile stress. The presence of additional phases in the irradiated film patterns signified phase separation within InGaN. Furthermore, shifts in peaks at 669 cm^-1 and the emergence of new peaks in the Raman spectra underscored defects induced by irradiation. This study reports the phenomenon of decreasing optical bandgaps of InGaN/GaN multilayer thin films with increasing ion irradiation dosage.

2. Si Ion Irradiation Preparation for InGaN/GaN Thin Film

Experiments were conducted on InGaN/GaN films grown on sapphire substrates provided by PAM-XIAMEN. The grown films underwent exposure to a collimated beam of Si ions at an energy of 12.0 MeV, provided by the 5UDH-2 Pelletron tandem accelerator. During irradiation, vacuum levels were maintained at 10^-6 Torr. Utilizing the ion stopping/range material codes revealed that the projected range of 12 MeV Si ions in InGaN is approximately 4.08µm—significantly greater than the film thickness, ensuring uniform damage distribution. InGaN/GaN wafer was mounted on a high-precision goniometer to accurately control the sample’s orientation relative to the silicon ion beam. Experiments proceeded at varying ion fluxes of 1×10^13, 1×10^14, and 1×10^15 ions/cm^-2. A InGaN/GaN epitaxial wafer remained unirradiated, serving as a reference.

Structural analysis of both the original and irradiated films was conducted through X-ray diffraction (XRD) and Raman spectroscopy, employing a Bruker X-ray diffractometer, at conditions of 40kV, 40mA, and 25°C, utilizing Cu-Kα radiation (λ=1.54191Å), with data collected from 20° to 80° in 2θ, at a step width of 0.02°. Rutherford backscattering spectroscopy (RBS) offered a quantitative and qualitative analysis of elemental composition. Raman spectra were gathered using LabRAM III with a 514nm wavelength, 150mW argon-ion laser. For optical properties, a UV-visible spectrophotometer was utilized.

Fig. 1 XRD spectra of original InGaNGaN epitaxial layers and Si+ irradiated layers

Fig. 1 XRD spectra of original InGaN/GaN epitaxial layers and Si+ irradiated layers under different fluxes

Fig. 2 (a) Variation of InN peak intensity with dose; (b) Peak shift of InGaN under different Si ion fluxes

Fig. 3 (a) Raman spectra of InGaNGaN in its grown state and post-Si beam irradiation; (b) Variations in the FWHM of the Raman E2h band

Fig. 3 (a) Raman spectra of InGaN/GaN in its grown state and post-Si beam irradiation; (b) Variations in the FWHM of the Raman E2h band

Fig. 4 RBS spectra (a) Spectrum of the unirradiated InGaNGaN wafer

Fig. 4 RBS spectra: (a) Spectrum of the unirradiated InGaN/GaN wafer; (b)–(d) Spectra irradiated InGaN/GaN wafers at 1×10^13, 1×10^14, and 1×10^15 ions/cm^2 respectively

Fig. 5 Relationship between the transmission rate (%T) and wavelength at different dosages

Fig. 6 (a) Calculated bandgap energies of InGaN films; (b) Variation of the bandgap with dosage rates

3. Conclusions

The study underscores that Si+ irradiation (12MeV high energy) not only causes structural changes in the InGaN/GaN epitaxial layer, but also affects its optical properties.

In XRD spectra, the InGaN diffraction peaks shifted from their original positions towards higher angles with increasing ion flux, suggesting inflaterated modulation of structural aspects and reinforcing the presence of enhanced stress within the lattice. This analysis paves the way for understanding the nuanced effects of high-energy ion irradiation on semiconductor materials, specifically elucidating its role in modifying the intrinsic properties of InGaN/GaN thin films.

In the XRD spectrum, the InGaN diffraction peak shifts from its original position to a higher angle under higher ion flux. It also exhibits peak splitting of InGaN phase, indicating segregation of InGaN phase in the growing film. The Rutherford backscatter spectroscopy technique indicates that the thickness and composition of the irradiated film remain unchanged, indicating the radiation tolerance of the InGaN film. The Raman spectra of sapphire based GaN wafers show two peaks at 567 cm^-1 and 736 cm^-1, corresponding to E2h and A1 (LO) phonon modes. Due to lattice damage caused by ion beams at high throughput, ion irradiation also caused a new peak at 669 cm^-1 in Raman spectroscopy, which is consistent with XRD results.

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