Abstract
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.
Highlights
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]
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]
Reshchikov et al reported that deep donor energy levels that are approximately 0.4 eV below the conduction-band minimum may be due to defects related to the hydrogen in Mg-doped GaN grown by M OCVDc26
Summary
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]. 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]. THz emission from semiconductor surfaces is caused by the acceleration of photocarriers by a built-in field near the surface, as illustrated, and/or photocarrier diffusion from the surface to the bulk In the former case, electrons and holes drift in the opposite direction from each other, and in the latter case, diffusion is in the same direction but at different speeds, a phenomenon known as the Photo-Dember e ffect[13,16,17,18,19], the model of which is often used to explain the THz emission mechanism from narrow bandgap semiconductors. The THz emission waveforms from the samples under a variety of conditions were examined, and the THz emission mechanisms are discussed using the short-time approximation model from R ef[20], together with the PL and Ultraviolet Visible Absorption Spectroscopy (UV–vis) measurements
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