Damage accumulation and implanted Gd and Au position in a- and c-plane GaN

Highlights

• Defect depth profiles differ for c-plane and a-plane GaN as-implanted samples.

• a-plane GaN shown lower accumulated disorder shifted into the deeper layer.

• Annealing caused ion channelling recovery in a-plane compared to c-plane GaN.

• Different defect complexes were created depending on GaN orientation.

Abstract

(0001) c-plane and (11−20) a-plane GaN epitaxial layers were implanted with 400 keV Au⁺ and Gd⁺ ions using ion implantation fluences of 5 × 10¹⁴, 1 × 10¹⁵ and 5 × 10¹⁵ cm⁻². Rutherford Back-Scattering spectrometry in channelling mode (RBS/C) was used to follow the dopant depth profiles and the introduced disorder; the angular dependence of the backscattered ions (angular scans) in c- and a-plane GaN was measured to get insight into structural modification and dopant position in various crystallographic orientations. Defect-accumulation depth profiles exhibited differences for a- and c-plane GaN, with a-plane showing significantly lower accumulated disorder in the buried layer, accompanied by the shift of the maximum damage accumulation into the deeper layer with respect to the theoretical prediction, than c-plane GaN. Angular scans showed channelling preservation in as-implanted samples and better channelling recovery in the annealed a-plane GaN compared to c-plane GaN. The angular scan widths were simulated by FLUX code as well as the half-width modifications of angular scans were discussed in connection to the damage accumulation. Photoluminescence measurement followed in detail yellow band and band edge luminescence decline after the implantation and the recovery of luminescence spectra features after annealing.

Introduction

Ion implantation doping of GaN has attracted increasing interest in the last decades due to its potential for the precise control of the distribution and concentration of dopants [[1], [2], [3], [4]]. It was found in the previous research that in rare earth doped GaN the lattice site occupied by dopant can significantly influence the luminescence properties of the doped layers in GaN [1]. The drawback of implantation is introduction of defects and damage which can deteriorate optical properties, thus the damage recovery after the annealing is real issue. Ion-beam channelling allows the identification an impurity site of relatively high concentration by direct comparison between matrix and impurity signals [1,4]. Dopant position is closely connected to damage accumulation in the implanted GaN lattice as vacancies and larger extended defects created during ion implantation directly influencing the dopant position in the modified crystalline matrix.

Rutherford Back-Scattering spectrometry in channelling mode (RBS/C) makes it possible to determine defect concentrations with depth resolution, dopant depth profiles and dopant position in a host crystalline matrix via angle scans of the backscattered ion yield close to the channelling axis. RBS/C has been applied in the past by many groups to investigate implantation damage build-up in GaN in various crystallographic orientations (e.g. [[5], [6], [7], [8]]), where dynamic annealing (the recombination of interstitials and vacancies) was identified as a main reason for radiation hardness of GaN which strongly depends on the implantation conditions (see e.g. [9,10]). Damage accumulation was studied in polar and non-polar GaN layers, but various dopant positions in different crystallographic orientations are still not fully explored. The lattice-site location of rare-earth ions in polar GaN (c-plane) (0001) was studied in [11,13] and Ga-substitutional site was found as a preferential. However, non-polar GaN (a-plane) (11–20) was not investigated in details from this point of view.

Lorenz et al. [8] have performed RBS/C experiments including angular scans in (0001) GaN implanted with 300 keV Eu ions at fluences of about 10¹⁵ cm⁻² and subsequently annealed at 1000 °C and it was shown that Eu is located on substitutional lattice sites in a host matrix and annealing does not induce the further incorporation of Eu into substitutional sites. However, in this work was used RBS/C in the tilted 〈10−11〉 plane enabling evidence of a small displacement along the c-axis from near-substitutional Ga sites in GaN. Our intention is to use the similar procedure to follow the dopant position in crystalline host in details. A similar study performed in another associated semiconductor crystal, ZnO, at higher ion implantation fluences up to 10¹⁶ cm⁻² has shown that from a certain damage limit, the annealing causes a massive decline in rare-earth dopant in substitutional positions; this analysis was done only for polar ZnO (0001) [7,13].

Implantation into various crystallographic orientations exhibits differences, which have been studied by several groups with the conclusion that defect accumulation is different for c-, a- and m-plane GaN [6,14] and the references therein. The work [6] has identified several defect-accumulation regimes for Ar 300-keV implantation in GaN and at ion implantation fluences ranging between 1 × 10¹⁵ and 4 × 10¹⁵ cm⁻², where defect profiles exhibit distinct variation of the shape compared with those simulated by the Stopping and Range of Ions into Matter (SRIM) code.

A steepest increase in damage was observed for c-plane GaN [6]. The authors claim that beside primary defects, the influence of the Ar ions themselves cannot be excluded [6].

The crystallographic orientation used for implantation influences the optical response of the implanted structure and we have found such differences for various crystallographic orientations in other crystalline structures such as sapphire and lithium niobate (see [15,16]). The annealing procedure influences dynamic recovery and point-defect diffusion, possibly including large defect stabilisation, which are closely connected to the dopant lattice position and depend on the ion-implantation parameters. This issue is still important to study.

Comprehensive information on the dopant position after ion implantation into specific crystallographic orientations can be obtained using a combination of normal and non-normal ion channelling. Either the RBS channelling (RBS/C) procedure is used along the normal axis at a given GaN orientation (the axis is very close to the normal axis to the sample surface), or non-normal channelling can be implemented using additional axial channels at appropriate angles found based on higher crystal symmetry [17]. Non-normal channelling is mostly used for compressive or tensile strain determination from an angle shift of non-normal axes, but it can also be used for an angular scan to study the dopant position.

For the reasons mentioned above, we decided to conduct an experiment using Au and Gd ion implantation at 400 keV at ion fluences up to 5 × 10¹⁵ cm⁻² to avoid complete amorphisation. Dopant positioning was done simultaneously using non-normal channelling to investigate the specific position in GaN and its modification after annealing. The structure modification and vacancies introduced by ion implantation were experimentally determined by angular scans with respect to normal and non-normal channelling directions and simulation of the angular scans was provided using FLUX [18]. Finally, the results were discussed in connection with photoluminescence properties.