Ričardas Norkus

GeSn alloys have garnered significant interest for electronic and optoelectronic applications over the past decade, primarily because of their monolithic compatibility with complementary metal–oxide–semiconductor fabrication platforms, tunable band structure, extended optical absorption into the short-wave infrared range, and excellent charge-transport properties. Recently, ion-implanted GeSn has been explored for the fabrication of photoconductive terahertz (THz) devices as a competitive alternative to low-temperature-grown III–V compound semiconductors. Since their ultrafast photoconductivity response governs the THz performance of such devices, this work provides a comprehensive characterization of the transient optoelectronic properties of ion-implanted GeSn alloys under different implantation doses and energies. In particular, optical pump–THz probe measurements were performed to assess the photoconductivity dynamics more accurately, while the Drude–Smith model was employed to extract the transient optoelectronic parameters of the samples under ultrafast excitation. Complementary characterization techniques, such as x-ray diffraction (XRD), photoluminescence (PL), and scanning electron microscopy (SEM), were also used to gain deeper insight into the properties of the ion-implanted GeSn samples. The results show that GeSn treated with moderate ion-implantation dose and energy exhibits an optimized balance between short carrier lifetime, good crystallinity, and high mobility, highlighting the importance of defect engineering for tailoring the transient optoelectronic properties of GeSn for ultra-broadband THz sensors. These findings are expected to support the development of next-generation GeSn-based functional devices.

THz excitation spectroscopy was used to investigate doped (100) GaSb surfaces. THz pulses were generated using excitation photon energies ranging from 0.6 eV to 1.2 eV. Distinct THz generation mechanisms were identified for p- and n-doped samples: the photo-Dember effect and drift current in the surface field, respectively. Moreover, the THz pulse amplitude exhibited azimuthal angle dependence, which arises from non-parabolicity in the Γ valley at low excitation photon energies. When the excitation energy exceeds 0.82 eV, a polarity shift in the azimuthal angle dependence is attributed to carrier scattering into the higher-lying subsidiary L valley.

This paper presents the fabrication and characterization of photoconductive terahertz switches based on GaAs epitaxial layers grown directly on centrosymmetric germanium substrates. The study aims to overcome the bandwidth limitations of conventional GaAs-based switches by leveraging the broader THz transmission of Ge. The authors investigate the influence of GaAs layer thickness on device performance, including breakdown fields, dark currents, and emitted THz spectra. The results demonstrate the potential of GaAs/Ge devices for broadband THz generation, despite certain challenges. The paper also discusses the unforeseen charge transport phenomena observed in these devices, which may provide insights for future optimizations.

In this work, heterojunctions consisting of thin monoelemental bismuth (Bi) layers have been grown on crystalline GaAs substrates with various doping types and investigated using THz pulse generation excited via femtosecond optical pulses of different wavelengths. It was determined that the influence of the layers of Bi can be approached in two distinct ways: first, through the additional absorption of the optical pulse and second, through changes in the electron energy band line-ups at the heterojunction. The strongest influence of the latter effect was observed at the interface between Bi and p-GaAs when the internal electric field exceeded the saturation field of the drift velocity of the p-GaAs holes. Such high fields can lead to an overshooting effect of the holes when the holes move at speeds above 2 × 107 cm/s and emit a much stronger THz signal than in the case of p-GaAs. This finding is promising for the development of efficient surface THz pulse emitters based on p-GaAs and other p-type semiconductors.

Two-dimensional or other thin materials have high potential for use in next-generation electronic and optoelectronic devices. Recently, tellurium has gained much interest due to its broad applicability prospects. In this work, the physical properties of thin tellurium layers fabricated using two relatively simple and inexpensive technologies based on the deposition from a chemical solution and by thermal evaporation were compared. The morphology of the grown surfaces was analyzed using atomic force microscopy. The chemically deposited tellurium surface consists of nanometer-sized flakes, while polycrystalline layers are formed in the case of deposition by thermal evaporation. Additionally, the characteristics of the fabricated samples varied depending on their thickness, as observed in both Raman spectroscopy and THz spectroscopy measurements. Furthermore, the non-contact optical pump THz probe technique revealed that the layers had different carrier lifetimes and mobilities. The carrier lifetime of samples deposited by thermal evaporation is short, less than 40 ps, with mobility up to a few hundred cm2/V s. In contrast, chemically deposited samples have a longer carrier lifetime, ranging more than 500 ps, and superior mobility up to 1000 cm2/V s.

Characterizing subcells in two-terminal multi-junction (M-J) solar cells is challenging due to the lack of direct electrical access. This work presents a novel contactless spectral characterization technique for analysing individual subcells. The technique involves probing terahertz (THz) radiation generated by femtosecond laser pulse excitation and varying the exciting wavelength to selectively absorb light in the desired subcell. The registered THz pulse integral is then proportional to the induced photocurrent in that subcell. The THz photocurrent spectroscopy technique is demonstrated on GaAs and AlGaAs single-junction solar cells, as well as on the triple-junction AlGaAs/GaAs/GaAsBi solar cell. The results show that the recently developed GaAsBi-based subcell, with a nominal energy bandgap of 1.0 eV, exhibits improved electron–hole separation efficiency and can enhance energy harvesting by M-J solar cells.

Surfaces of semiconducting materials excited by femtosecond laser pulses emit electromagnetic waves in the terahertz (THz) frequency range, which by definition is the 0.1–10 THz region. The nature of terahertz radiation pulses is, in the majority of cases, explained by the appearance of ultrafast photocurrents. THz pulse duration is comparable with the photocarrier momentum relaxation time, thus such hot-carrier effects as the velocity overshoot, ballistic carrier motion, and optical carrier alignment must be taken into consideration when explaining experimental observations of terahertz emission. Novel commercially available tools such as optical parametric amplifiers that are capable of generating femtosecond optical pulses within a wide spectral range allow performing new unique experiments. By exciting semiconductor surfaces with various photon energies, it is possible to look into the ultrafast processes taking place at different electron energy levels of the investigated materials. The experimental technique known as the THz excitation spectroscopy (TES) can be used as a contactless method to study the band structure and investigate the ultrafast processes of various technologically important materials. A recent decade of investigations with the THz excitation spectroscopy method is reviewed in this article. TES experiments performed on the common bulk A3B5 compounds such as the wide-gap GaAs, and narrow-gap InAs and InSb, as well as Ge, Te, GaSe and other bulk semiconductors are reviewed. Finally, the results obtained by this non-contact technique on low-dimensional materials such as ultrathin mono-elemental Bi films, InAs, InGaAs, and GaAs nanowires are also presented.

Puslaidininkinių medžiagų ir darinių tyrimas terahercinių impulsų žadinimo spektroskopijos metodu

Thinner than 10 nm layers of bismuth (Bi) were grown on (111) Si substrates by molecular beam epitaxy. Terahertz (THz) radiation pulses from these layers excited by tunable wavelength femtosecond optical pulses were measured. THz emission sets on when the photon energy exceeds 0.45 eV, which was explained by the semimetal-to-semiconductor transition at this Bi layer thickness. A THz signal has both isotropic and anisotropic components that could be caused by the lack of balance of lateral photocurrent components and the shift currents, respectively.

Terahertz radiation pulses emitted after exciting semiconductor heterostructures by femtosecond optical pulses were used to determine the electron energy band offsets between different constituent materials. It has been shown that when the photon energy is sufficient enough to excite electrons in the narrower bandgap layer with an energy greater than the conduction band offset, the terahertz pulse changes its polarity. Theoretical analysis performed both analytically and by numerical Monte Carlo simulation has shown that the polarity inversion is caused by the electrons that are excited in the narrow bandgap layer with energies sufficient to surmount the band offset with the wide bandgap substrate. This effect is used to evaluate the energy band offsets in GaInAs/InP and GaInAsBi/InP heterostructures.