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Abstract
Low-temperature metal-induced-crystallized germanium is a promising alternative to silicon in complementary metal-oxide-semiconductor (CMOS) technology. Palladium (Pd) is one of the metals suitable for inducing the low-temperature crystallization. However, it is not certain how residual Pd atoms are integrated into the germanium lattice and what associated defect complexes may be formed.
In this present study, therefore, the time-differential γ-γ perturbed angular correlation (TDPAC) technique using a 100Pd(â 100Rh) nuclear probe has been applied to study the hyperfine interactions in terms of the electric field gradient (EFG) created at the probe in both undoped and doped crystalline germanium samples. Interpreting TDPAC data and assigning a particular configuration and direction (sign) to the EFG of an observed defect is complex. Therefore, the experimental work has been extended and complemented with theoretical studies. In particular, density functional theory (DFT) calculations have been performed to better understand the electronic and structural properties of likely and observed Pd-defect complexes formed in germanium. Similar calculations have also been performed for defects in silicon to allow for comparisons.
The experimental results identify a Pd-vacancy (V) complex with a unique interaction frequency Ï 0 = 8.4(5) Mrad/s in undoped, as well as in gallium (Ga)- and antimony (Sb)-doped germanium samples. For an asymmetry parameter η = 0, this corresponds to a quadrupole coupling constant of νQ = 10.7(5) MHz. Orientation measurements with the detectors aligned along <100> and <110> sample crystallographic directions confirm that the EFG of the Pd-V complex was orientated along the <111> direction in the undoped, Ga- and Sb-doped germanium. This means the Pd pairs with a nearest-neighbour vacancy in the three germanium samples.
Furthermore, this Pd-V complex was measured to have a maximum relative fraction following annealing at 350 oC. Further annealing at higher temperatures reduces this fraction, possibly via dissociation of the complex. Calculations suggest an average dissociation energy of 2.00(7) eV for the Pd-V complex in the three germanium samples.
The superconducting solenoidal separator of evaporation residues SOLITAIRE, developed at the Australian National University, has been successfully applied in the implantation of clean 100Pd(â 100Rh) probes into germanium by suppressing the unwanted intense flux of scattered projectiles, the elastics. Comparison of the 100Pd(â 100Rh) TDPAC spectroscopy results of the suppressed and unsuppressed elastics in doped germanium confirms that the elastics do not interfere with the measurements. This is also corroborated by SRIM calculations, which show that elastics do not create vacancies, the only defect that can pair with the probe, in the three germanium samples but only pass via it by displacing the host atoms slightly from their lattice sites.
The DFT calculations performed in this work are in good agreement with the experimental values for the EFG for the defect complex in germanium. The DFT calculations predict a split-vacancy configuration with the Pd on a bond-centred interstitial site having a nearest-neighbour semi-vacancy on both sides (V-PdBI-V). More specifically, a double-negative charge state for the undoped, Ga- and Sb-doped germanium, and a single-negative charge state for the Ga-, phosphorous (P) - and Sb-doped silicon are predicted by the calculations. Charge analysis shows that the Pd atom is covalently bonded to the host nearest-neighbour atoms. In contrast, due to the smaller size of boron (B) and P relative to germanium and silicon, a Pd-dopant (D) configuration with Pd on a bond-centred interstitial (BI) site between a germanium (or silicon) atom and a substitutional (SS) D atom (PdBI-DSS) has been predicted for B-doped silicon; a PdBI-DSS configuration has been predicted for B- and P-doped germanium with a neutral charge state.
This work demonstrates that the TDPAC technique can provide information about the nature and orientation of defect complexes in semiconductors that may be readily interpreted and complemented with detailed density functional theory calculations.