(Invited) Neutron Irradiation Effect on GaN-Based Materials

For the predominant property, GaN becomes a promising material for ultraviolet/ blue light-emitting diode, solar blind ultraviolet photodetector, radiation resistant particle detectors, and high power high temperature electronic devices. Because of the radiation hardness of GaN, those devices have the potential to be used in outer space and other radiation environment. Irradiation induced defects play the main role in the reliability of GaN based devices. Recently, a strong effort by several groups was devoted to the analyses of electrical and optical properties of defects and their interaction by irradiating GaN with electrons, ions, neutrons and protons [1–4], which is very important for the devices operating at extreme harsh environment.

Positron annihilation spectroscopy (PAS) with a slow-positron beam is an efficient tool for the investigation of open-volume defects such as vacancies in semiconductors, at which they get trapped. In a vacancy the electron density is decreased which makes the positron lifetime longer and narrows the positron-electron momentum distribution. The latter can be observed by recording the Doppler broadening of the energy spectrum of the annihilation radiation. Especially the momentum distribution of the core electrons is sensitive to the chemical environment of an annihilation site. However, in the case of light elements such as O or N the annihilation probability with the core electrons is negligible.

The samples used in the present work were grown on C-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD) with unintentionally doped and Si doped. A 20 nm low temperature GaN nucleation layer was initially grown, and 4 μm GaN film grown at 1050°C was followed. The sample was cut into small pieces with an area of about 10×10 mm², and irradiated with 2MeV proton from heavy ion accelerator. The incident fluence is 10¹⁴, 10¹⁵, or 10¹⁶ cm^-2, respectively.  Positron annihilation spectroscopy, XRD, and Raman spectra were measured.

Irradiation induced defect is vacancies which increased with incident proton. XRD result shows that the crystal-order was kept after proton irradiation. Raman spectra of the Si doped sample shows a “carrier removal” effect but there is not any change in the other spectra.

References:

1. Look D C, Reynolds D C, Hemsky J W, et al. Defect Donor and Acceptor in GaN, J. Phys. Rev. Lett., 1997, 79: 2273-2276.

2. Fang Z Q, Hemsky J W, Look D C, et al. Electron-irradiation-induced deep level in n-type GaN, Appl. Phys. Lett., 1998, 72: 448-449.

3. Auret F D, Goodmann S A, Koschnick F K, et al. Effect of heating rate on positive-temperature-coefficient-of-resistivity behavior of conductive composite thin films, Appl. Phys. Lett., 1998, 73: 3742-3744.

4. Auret F D, Goodmann S A, Koschnick F K, et al. Proton bombardment-induced electron traps in epitaxially grown n-GaN, Appl. Phys. Lett., 1999, 74: 407-409.