Atomic-Scale Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) is one of the central technologies of THz science. By measuring the oscillating THz electric field after it has interacted with a sample and comparing it to a reference field, the complex dielectric function at THz frequencies may be determined. Based on the same concept, in THz scattering-type scanning near-field optical microscopy (s-SNOM), THz pulses focused onto a scanning probe tip may be used to spatially map the local complex dielectric function on the 10-100 nm scale [1]. However, many open questions in surface science require these properties to be determined at yet smaller length scales. For example, THz-TDS of individual atomic sites would allow the role of defects, dopants, and interfaces on charge transport to be studied in unprecedented detail. Here, we introduce an experimental method for atomic-scale THz-TDS in a THz scanning tunneling microscope (THz-STM) junction. Using our technique, we demonstrate atomically resolved THz-TDS of a silicon-doped GaAs(110) sample, revealing a resonator defect with the hallmarks of the elusive DX center [2]. In our THz-STM setup, a strong-field THz pulse is coupled to the STM tip and acts as an ultrafast bias voltage in the tip-sample junction, inducing a current pulse with a rectified component that is measured electronically [3]. To perform atomic-scale THz-TDS, we use this induced current pulse to sample a second, weaker THz pulse through a cross-correlation (THz-CC) measurement that captures the near-field waveform. First, a THz-CC waveform is recorded on an Au(111) surface, which is used as a reference sample because of its flat spectral response. Then, we can perform STM topography of our GaAs(110) sample surface; where atomic rows are visible, as are multiple types of atomic defects (e.g., gallium vacancies and silicon substitutional dopants). We find a region, where a particular defect exhibits a strong THz-STM signal. Near-field THz-CC waveforms were measured 200 pm away from the defect and in the center of the bright feature in the THz-STM scan already observing a high contrast between the two measurements. We further divide the amplitude spectra of the GaAs(110) near-field waveforms by the Au(111) reference spectrum. This reveals distinct responses for the two sample locations due to dielectric contrast. Spectroscopically, the most prominent contrast occurs at 0.96 THz, where the defect exhibits a strong resonance that is absent at the location 200 pm away. From this signature resonance and other STM characteristics, we identify the defect as a silicon-vacancy complex stabilized in a DX center configuration and the resonance as the vibrational motion of the silicon dopant atom. Although DX centers are of significant interest in semiconductor research due to their prominent role in carrier scattering [4], this is the first time one has been observed directly. With atomic-scale THz-TDS we can now study open questions that previously could not be addressed experimentally, such as the simultaneous spatial and spectral description of defect complexes in semiconductors. As a next step, we envision that THz-TDS within a THz-STM junction will enable time-resolved THz spectroscopy of the transient THz dielectric response on the atomic scale.<br/><br/><b>References</b><br/>[1] T. L. Cocker et al., “Nanoscale terahertz scanning probe microscopy,” Nat. Photon. 15, 558–569 (2021).<br/>[2] V. Jelic et al., “Atomic-scale terahertz time-domain spectroscopy,” in press.<br/>[3] T. L. Cocker et al., “An ultrafast terahertz scanning tunnelling microscope,” Nat. Photon.7, 620–625 (2013).<br/>[4] A. Kundu et al., “Effect of local chemistry and structure on thermal transport in doped GaAs,” Phys. Rev. Mater. 3, 094602 (2019).