Abstract-Atomically-resolved interface imaging and terahertz emission measurements of gallium arsenide epilayers

Journal of Applied Physics

M. H. Balgos,  R. Jaculbia, E. A. Prieto, M. Tani,  E. Estacio, A. Salvador, A. Somintac, N. Hayazawa, Y. Kim

Accessing and locating the interface through cross-sectional STM/S. (a) Mechanical cleaving in UHV is performed by applying a force normal to the sample surface to expose a clean (110) cross section. (b) An STM tip is scanned on the (110) surface to access the substrate, buffer, and LT-GaAs layers. (c) Constant-current STM image of the sample (I$I$ = 10 pA and Vs=−4.0$V_s=-4.0$ V). (d) STS spectra captured at the lateral positions indicated by the dots in Fig. 1(c) (setpoint: I=10$I=10$ pA and Vs=−4.0$V_s=-4.0$ V, Δz=−0.15$\mathrm{\Delta }z=-0.15$ nm).

https://aip.scitation.org/doi/10.1063/1.5118815

Semiconductor interfaces are the backbone of modern optoelectronic devices. In terahertz (THz) science, the narrow region of an interface is crucial in the emission process. However, reports on the direct correlation of THz emission with local interface properties remain scarce owing to the inherent difficulty of using the same sample for nanoscale and macroscale studies. In this study, we combined scanning tunneling microscopy/spectroscopy (STM/STS) and THz emission spectroscopy to study the interface between a highly n+$n^{+}$-doped and undoped gallium arsenide (GaAs). Using STS, we identify a carrier density of 1×1015$1\times {10}^{15}$ cm−3${}^{-3}$ in the low-temperature-grown GaAs (LT-GaAs) layer, which we used to visualize the energy band diagram at the interface and the surface of LT-GaAs. THz emission intensity is higher in the LT-GaAs/n+$n^{+}$-GaAs structures relative to semi-insulating GaAs owing to the high electric field at the interface regardless of the LT-GaAs layer thickness. Pump fluence dependence of THz showed that the thinner LT-GaAs layers saturate at lower pump fluence compared to thicker LT-GaAs and SI-GaAs. This behavior is explained by the built-in field screening by the photogenerated carriers and the free carriers from the n+$n^{+}$-GaAs to the LT-GaAs. Our results demonstrate the utility of STM/STS to the design of semiconductor-based THz emitters.