Abstract
Laser Thermal Annealing (LTA) at various energy densities was used to recrystallize and activate amorphized germanium doped with phosphorous by ion implantation. The structural modifications induced during the recrystallization and the related dopant diffusion were first investigated. After LTA at low energy densities, the P electrical activation was poor while the dopant distribution was mainly localized in the polycrystalline Ge resulting from the anneal. Conversely, full dopant activation (up to 1 × 1020 cm−3) in a perfectly recrystallized material was observed after annealing at higher energy densities. Measurements of lattice parameters performed on the fully activated structures show that P doping results in a lattice expansion, with a perpendicular lattice strain per atom βPs = +0.7 ± 0.1 Å3. This clearly indicates that, despite the small atomic radius of P compared to Ge, the “electronic contribution” to the lattice parameter modification (due to the increased hydrostatic deformation potential in the conduction band of P doped Ge) is larger than the “size mismatch contribution” associated with the atomic radii. Such behavior, predicted by theory, is observed experimentally for the first time, thanks to the high sensitivity of the measurement techniques used in this work.
Highlights
The evidence of this double contribution has been demonstrated for dopants in Si since several decades,[15,16] only recently a strong experimental proof for the Ge case has been provided by Xu et al.[8]
Despite phosphorus exhibiting a significantly smaller atomic radius compared to Ge, the “double contribution” theory predicts that its inclusion in Ge should generate an expansion of the lattice
A slightly negative deformation was measured in Ref. 8. This was explained by the authors by considering that in the doping interval investigated in that work (4–7 × 1019 cm 3) the magnitude of the expected lattice deformation was too close to the fluctuations of the measured lattice parameter in the reference Ge samples
Summary
Since the early 2000s, there is a renewed interest in the use of germanium in the microelectronics industry, as a possible high-mobility replacement for silicon in the CMOS technology[1,2] and as an interesting candidate for the fabrication of active photonic devices for photon detection, modulation, and generation.[3,4] In most cases, highly doped regions must be combined with tensile strain either to reduce the access resistance or to induce the formation of a direct bandgap in photonic devices.[5,6] the introduction of dopant atoms in crystalline materials often results in the generation of stress (and related strain) which depends on the peculiar configuration within the matrix lattice (substitutional or interstitial).[7,8] For this reason, there is a leading technological interest for quantifying the contribution of dopants to the strain. A CBED pattern obtained in the undoped (and undistorted) substrate at a depth of ∼300 nm was initially simulated to determine the precise acceleration voltage of the e-beam (found equal to 299.1 kV, cf Fig. SI-1 of the supplementary material)[28] and used as a reference for the strain measurements.
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