Isotope effect on electron paramagnetic resonance of boron acceptors in silicon

Abstract

The fourfold degeneracy of the boron acceptor ground state in silicon, which is easily lifted by any symmetry breaking perturbation, allows for a strong inhomogeneous broadening of the boron-related electron paramagnetic resonance (EPR) lines, e.g. by a random distribution of local strains. However, since EPR of boron acceptors in externally unstrained silicon was reported for the first time, neither the line shape nor the magnitude of the residual broadening observed in samples with high crystalline purity were compatible with the low concentrations of carbon and oxygen point defects, being the predominant source of random local strain. Adapting a theoretical model which has been applied to understand the acceptor ground state splitting in the absence of a magnetic field as an effect due to the presence of different silicon isotopes, we show that local fluctuations of the valence band edge due to different isotopic configurations in the vicinity of the boron acceptors can quantitatively account for all inhomogeneous broadening effects in high purity Si with a natural isotope composition. Our calculations show that such an isotopic perturbation also leads to a shift in the g-value of different boron-related resonances, which we could verify in our experiments. Further, our results provide an independent test and verification of the valence band offsets between the different Si isotopes determined in previous works.

Time-Resolved Dynamics of Shallow Acceptor Transitions in Silicon

“…For a simple two-level exponential population decay with decay constant T 1 , the Fourier transform produces a Lorentzian line with full width at half maximum of 2Á FWHM ¼ 1=T 1 . Natural silicon samples exhibit strong inhomogeneous broadening due to impurities, defects, alloying, and strain so that their linewidths are much greater than this intrinsic minimum [12,26,28], though isotopically pure silicon has been shown to produce much sharper lines [16,17,20] that are close to ideal. The linewidths from the absorption spectrum (FTIR) of isotopically pure material [17] for Si:P and Si:B, and inverse lifetimes from the pump-probe experiments (this work and Ref.…”

“…The silicon-acceptor relaxation lifetime is unknown but bounded from below by the inverse linewidth of absorption spectra [16,17], as was the case for shallow donors until recent time-resolved experiments with the pump-probe technique [15]. It has been shown that very careful sample preparation with isotopic purification and elimination of strain can produce long acceptor spin lifetimes measurable by magnetic resonance spectroscopy [20] and very sharp orbital transitions [16,17], but the linewidths of the lowest energy transitions remain an order of magnitude wider than for donors [17]. The main effect of random strain (or applied stress) is to remove the degeneracy of the light-and heavyhole bands, which splits the acceptor bound states and introduces inhomogeneous broadening.…”

“…The main effect of random strain (or applied stress) is to remove the degeneracy of the light-and heavyhole bands, which splits the acceptor bound states and introduces inhomogeneous broadening. This effect is very important in frequency-domain spin resonance [20] and spectroscopy of orbital transitions [16,17], but, in the time domain, we eliminate all contributions to the inhomogeneous broadening (except those derived from different lifetimes associated with different local defect environments).…”

Isotope engineering of silicon and diamond for quantum computing and sensing applications

“…They observed significant narrowing of the boron ESR lines in 28 Si. [88,89] However, the coherence time of the holes bound to the boron acceptors were limited by the short-hole relaxation time (T 1 ), even at 3 K. Further cooling is needed to extend the T 1 and T 2 for the application of boron acceptors in quantum information processing. Additionally, the properties of the donors have been studied in isotopically enriched, 28 Si device structures.…”

Power-law versus exponential relaxation of 29Si nucleus spins in Si:B crystals

“…At the same time, one can expect more effective entanglement generation in the 29 Si:B crystals accordingly with estimations proposed in [5]. Early experiments focused on 29 Si:B crystals are known for high metal-like concentration of acceptors 410 18 cm À 3 as well as low concentrations [8][9][10][11][12][13]. We have studied light doped (2 Â 10 14 cm À 3 boron concentration) 29 Si crystals.…”