Implantation angle dependence of ion irradiation damage in GaN

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Abstract

We use molecular dynamics (MD) simulations to study the effect of the implantation angle on the damage produced during ion beam irradiation of GaN. We bombard 5 keV Er ions at perfect wurtzite GaN with incident angles of 0°–22° angle against the [0 0 0 1] crystal axis. The simulations reproduce the angular dependence of the damage observed in the experiments. Two main reasons for the experimentally observed suppression in damage production are found. One is the decreased total damage production for small angles and another is the smaller fraction of clustered defects formed during channeling implantations. Large damage clusters are found to form near the surface. The surface damage peak is stronger for large angle bombardment.

Introduction

GaN is a promising candidate for many optoelectronic, high-power and -temperature applications [1]. As with other semiconductor materials, ion implantation has been demonstrated to be a powerful method for modifying the physical properties of GaN [1]. However, ion beam implantation of electrically and/or optically active dopants leads to damage in the crystalline lattice, and controlling the amount and type of the damage is important for obtaining the desired properties. Although GaN is very difficult to amorphize [2], [3], [4], annealing of defects at temperatures as high as 1100 °C leads only to a partial recovery of the damage [5]. Therefore, it is important to control the damage produced during the process by other means.

It has been observed that the implantation angle has a large effect in the damage produced by 80 keV Er implantation of GaN [5], [6]. To gain atomic level insight on the reasons for this suppression of damage, we have simulated the process using molecular dynamics methods. This method has been previously widely applied to the modeling of ion beam induced damage processes in semiconductors [7].

Section snippets

Method

We used classical molecular dynamics (MD) methods and an analytical potential model [8] to study the implantation angle dependence of damage in Er doped GaN.

The simulation cells were created using the wurtzite (WZ) crystal structure with an orthogonal unit cell corresponding to two conventional hexagonal unit cells. Periodic boundary conditions were used for the two directions perpendicular to the c-axis (i.e. the [0 0 0 1] crystal direction). Along the c-axis open boundary conditions were used.

Results and discussion

Fig. 1 shows the angular dependence of the experimental defect density along with the number of interstitials and number of interstitials in clusters produced in the simulations with different angles. Qualitatively all the measures of the amount of damage behave in the same way; the amount of damage is increased when the implantation angle is increased from 5° to 10°, and is almost constant for smaller angles. However, the increase in the total number of interstitials in the simulations (a

Conclusions

We have used molecular dynamics methods to study the effect of implantation angle on the damage production in Er bombardment of (0 0 0 1) wurtzite GaN.

We find two reasons to the suppression of ion beam induced damage during implantation along c-axis. One is the smaller total damage production, and another is the more scattered damage produced by channeled ions.

For both small and large angle implantation a surface peak of large damage clusters is formed, but the effect is much stronger (by about a

Acknowledgements

This research was supported by the Academy of Finland under projects nos. 50578 and 51585, and by the Fund for Scientific Research, Flanders. Grants of computer time from the Center for Scientific Computing in Espoo, Finland are gratefully acknowledged.

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