Effects of electronic and nuclear stopping power on disorder induced in GaN under swift heavy ion irradiation

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Abstract

Wurtzite GaN epilayers, grown on the c-plane of sapphire substrate, have been irradiated with swift heavy ions at different energies and fluences, and thereafter studied by Raman scattering spectroscopy, UV–visible spectroscopy and transmission electron microscopy. Raman spectra show strong structural modifications in the GaN layer. Indeed, in addition to the broadening of the allowed modes, a large continuum and three new modes at approximately 200 cm−1, 300 cm−1 and 670 cm−1 appear after irradiation attributed to disorder-activated Raman scattering. In this case, spectra are driven by the phonon density of states of the material due to the loss of translation symmetry of the lattice induced by defects. It was shown qualitatively that both electronic excitations and elastic collisions play an important role in the disorder induced by irradiation. UV–visible spectra reveal an absorption band at 2.8 eV which is linked to the new mode at 300 cm−1 observed in irradiated Raman spectra and comes from Ga-vacancies. These color centers are produced by elastic collisions (without any visible effect of electronic excitations).

Introduction

III–N semiconductors (AlN, GaN, InN and their corresponding alloys) are interesting materials for the development of electronic and optoelectronic devices due to their excellent properties such as large direct bandgap and high thermal conductivity. Thus, they have lots of applications in high efficiency Light Emitting Diodes (LED), transistors and so on. Particularly, GaN received strong attention for applications in the blue and UV spectral domains [1], [2], [3], [4], [5].

In order to improve functional properties of these semiconductors, ion implantation is a well-used technique for n- or p-doping or rare-earth incorporation [6], [7], [8], [9], [10], [11]. In this case, projectiles have energies of some keV/u, thus radiation induced damages occur and are mainly induced by elastic collisions. Many papers have been published on III–V semiconductors (GaAs, InP, InAs, GaP) in this range of energy projectile [12]. Concerning ion implantation in GaN, a number of studies are published in the literature for example with Mg, Si, O, N, Ce, Ar, C, P and Ca ions [13], [14], [15], [16].

Moreover, in order to integrate these materials in devices working on the outer space, where they can be subjected to cosmic rays and/or solar winds, their behavior under Swift Heavy Ions (SHI) has to be understood. In this case, projectiles have energies of some MeV/u. Thus, an effect of electronic excitations can take place even if direct formation of defects by pure radiolysis is not expected in semiconductors. Many works have already been reported about SHI irradiations in GaN [17], [18], [19], [20].

In addition, an effect of both the elastic collisions and electronic excitations has already been studied in other semiconductors. Indeed, in AlN, we have demonstrated that color center creation comes from a synergy between these two processes [21]. On the contrary, in many other semiconductors, like SiC, recovery of elastic damages can take place by the effect of electronic excitations [22].

In this work, irradiation conditions cover a span of electronic (Se) and nuclear (Sn) stopping powers. This allows studying the contribution of electronic excitations and nuclear collisions on disorder induced in GaN.

Section snippets

Experimental details

Samples used for these experiments are 3.5 μm thick single crystal wurtzite GaN epilayers grown by Metal–Organic Chemical Vapor Deposition (MOCVD) on c-plane sapphire substrate. They are n-doped with Si at a carrier concentration of ≈2 × 1018 cm−3. SHI irradiations were performed at the GANIL accelerator (Caen, France) whereas the C and He ones were done at JANNUS facility (Saclay, France). Eu ion implantation experiments were also carried out at LATR (Instituto Superior Técnico, Portugal). The

Point defects and disorder

Raman scattering spectroscopy is commonly used on GaN to probe information about disorder, stress and doping rate as it is reported in the review [24]. Fig. 1 shows Raman spectra of virgin GaN and after irradiation with 106 MeV U ions irradiation at different fluences until 5 × 1013 ions/cm2 (where many overlaps of the ion tracks take place). For the virgin sample, all the allowed modes (E2 (low) (142.3 cm−1), E2 (high) (568.9 cm−1) and A1 (LO) (735.9 cm−1)) predicted by group theory analysis for

Conclusions

Structural disorder and point defects induced by Swift Heavy Ion irradiation in GaN were investigated and the role of both electronic excitations and elastic collisions was analyzed. New modes observable in Raman spectroscopy are attributed to Disorder-Activated Raman Scattering where spectra are driven by the phonon density of states. We observed a mode at 670 cm−1 coming from N-sublattice related defects and modes at 200 cm−1 and 300 cm−1 attributed to Ga-sublattice related defects.

Acknowledgments

This work has been supported by the EMIR and iPAC platforms. GaN samples were provided by Saint-Gobain Crystals, Vallauris, France. Authors would like to thank staff from Jannus (Saclay, France), from Laboratório de Aceleradores e Tecnologias da Radiação (LATR) at Instituto Superior Técnico, Portugal (Savacém, Portugal) and all staff of CIMAP/GANIL for their help during irradiations and the spectroscopy experiments. Special thanks to K. Lorenz and P. Ruterana.

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