Gallium nitride (Gan) is a promising wide-bandgap semiconductor, and new characterization tools are needed to study its local crystallinity, carrier dynamics, and doping effects. Terahertz (THz) emission spectroscopy (teS) is an emerging experimental technique that can probe the ultrafast carrier dynamics in optically excited semiconductors. In this work, the carrier dynamics and THz emission mechanisms of Gan were examined in unintentionally doped n-type, Si-doped n-type, and Mg-doped p-type GaN films. The photocarriers excited near the surface travel from the excited-area in an ultrafast manner and generate THz radiation in accordance with the time derivative of the surge drift current. The polarity of the THz amplitude can be used to determine the majority carrier type in GaN films through a non-contact and non-destructive method. Unique THz emission excited by photon energies less than the bandgap was also observed in the p-type GaN film. Gallium nitride (GaN) is one of the most important wide-bandgap semiconductors, which attracts a remarkable interest for light-emitting, high power, and high-frequency devices 1,2. Despite great efforts, there are still many quality problems such as inefficient doping, defects, surface states, and defects in passivation, which occur in both crystals and films 3,4. For instance, there is naturally a strong spontaneous polarization on the Ga face of GaN along the c-axis direction 5. Due to this polarization and the defects in GaN, a typical AlGaN/GaN highelectron-mobility transistor (HEMT) functions as a normally-on (depletion mode) device or it must be operated with a back-gate 6,7. There are still many challenges that need to be overcome to achieve better GaN devices and materials, and new material characterization tools are vital to advancing research in this field. Terahertz (THz) emission spectroscopy (TES) and an imaging system known as a laser THz emission microscopy (LTEM) are emerging tools used to study the ultrafast dynamic carrier motion and displacement in optically excited materials 8,9. Recently, we have demonstrated the application of LTEM on various semiconductors 10-12. LTEM uses a femtosecond (fs) laser to excite the carriers in the materials. The carriers are accelerated by an electric field and they diffuse from the excited area, which induces a transient current, and THz waves are emitted in accordance with the time derivative of the photocurrent. The temporal THz waveforms reflect the ultrafast carrier dynamics, which are on the timescale of a few tens of femtoseconds 13. In wide-bandgap semiconductors, the penetration depth of light is shallow, and most photocarriers are generated near the semiconductor surface when the photon energy is larger than the semiconductor bandgap. Photoluminescence (PL) and electroluminescence can be used to characterize carrier recombination leading to photon emission for recombination times on the order of picoseconds to nanoseconds. Leitenstofer et al. has proven that, in GaAs, TES provides information on the instantaneous...

The concept of the tricolor superstructure (TCS), which is a triple-layer stack structure containing two types of high dielectric constant (high-k) layers (designated HK1 and HK2) and a SiO2 layer, is proposed to control the moment and the polarity of the interface dipole layer that are induced at the high-k/SiO2 interfaces. The interface dipole layer is formed by oxygen ion migration from the layer with higher oxygen areal density (σ) to that with lower σ. When the two high-k materials are selected with the order of σHK1 > σSiO2 > σHK2 in a SiO2/HK2/HK1/SiO2 TCS, the dipole directions of the interface dipole layers at the SiO2/HK2 and the HK1/SiO2 interfaces are aligned. Additionally, in the transposed SiO2/HK1/HK2/SiO2 TCS, the total polarity is reversed. The concept is demonstrated using Al2O3 and Y2O3 layers because they offer the order of σAl2O3 > σSiO2 > σY2O3. The two stacking sequence samples composed of SiO2/Y2O3/Al2O3/SiO2 and SiO2/Al2O3/Y2O3/SiO2 that were fabricated using superlattice technique by pulsed laser deposition obviously show opposite dipole polarities. Increasing repetition of the deposited TCS unit also causes the dipole moments to increase systematically. The TCS technique enables control of the properties of the interface dipole layer at high-k/SiO2 interfaces in amorphous systems.

Terahertz (THz) emission spectroscopy and microscopy are applied to investigate the electron and lattice dynamics of Ga0.8In0.2N/GaN multiple quantum wells (MQWs). The THz emission consists of three distinct, differently timed signals, whose physical mechanisms are attributed to i) laser‐induced ultrafast dynamical screening of built‐in bias electric field in MQWs followed by ii) capacitive charge oscillation of the excited carriers and iii) the coherent acoustic phonon (CAP)‐driven polarization surge at the discontinuity between the GaN capping layer and air. These multifunctional optical responses show strong dependence on the quantum well width and photon energies. The temporal separation between the first and third THz pulses corresponds to the propagation of the CAP across the GaN capping layer of the MQW structure, whose thickness can thus be determined with 10 nm precision.

This study investigates the band bending profile of gallium-nitride-based heterostructures with respect to the change in the direction of the induced current in the out-of-plane plane direction, which gives the phase of the terahertz emission peak and can be detected by laser THz emission microscopy. We use a wavelength-tunable laser THz emission spectroscope to observe the transport of charges within the band bending regions, excitation of nearby photocarriers, and carrier scattering. We observe flip-flop peak-to-peak THz emission waveforms for a GaN heterostructure field-effect transistor (FET) compared to a strong THz emission radiated from an Al0.3Ga0.7N/GaN high-electron-mobility-transistor (HEMT). The flip-flop THz emission indicates the emission from various interfaces inside the FET structure, and intense THz emission from the HEMT structure indicates band-edge excitation. Our results provide a valuable perspective for characterizing complex heterostructures that provide insight into possible defects, carrier mobilities, and band bending of multilayer interface electronic devices.