6.1 - Conclusion
In this work, we investigated terahertz time-domain spectroscopy of c-plane InN and InN:Mg films. We studied the terahertz wave transmittance through undoped InN and Mg-doped InN film and calculated the complex of conductivity and refractive index. The transmittance of undoped InN film is less than 20%, whereas that of doping magnesium acceptors into InN films enhances the transmittance of terahertz around three times (>60%). Our results reveal that the optical conductivity of Mg-doped InN films is almost frequency independent and does not closely depend on the carrier concentration. We use the simple Drude model to describe the InN and InN:Mg films that can be obtained a best fitting. The calculated carrier mobility and concentration of undoped InN and Mg-doped InN film which are consistent with those from the Hall effect measurement. The refractive index and conductivity of InN:Mg are much smaller than those of undoped InN and the reduction of conductivity is dominated by the carrier scattering time. We use electron effective mass to calculate the carrier concentration and mobility are similar Hall effect measurement that may indicate our result is n-type semiconductor.
For a-plane InN, we found the anisotropy of refractive index and electrical conductivity along parallel and perpendicular to c-axis direction. The THz-TDS and Hall effect measurement experimental results are opposite. The carrier mobility is independence on crystal crystalline quality. It’s indicate that the InN structure induce the internal electrical field affect the carrier mobility.
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6.2 - Future Work
InN is a prospect of material for high-frequency electronic devices. But the crucial for growing p-InN is an advanced challenge. So it is important to successful develop a direct method to identify the doping type of InN. It is inadequate to rely on the terahertz time-domain spectroscopy to identify the type of InN. Currently, we need trying to find other experimental methods distinguishing that is p-type or n-type InN.
Although the InN film growth direction is non-polar face (a-axis or m-axis), the lattice mismatch still due to substrate and InN film and results a built-in electric field. So we need to improve the lattice defect problems and increase the electronic devices efficiency.
We want to obtain more information of difference growth direction and material (GaN) samples by the THz-TDS. Now we are establishing the terahertz imaging system which can help us to understand the corresponding of material informational.
We need to understanding the transient carrier dynamics in the InN:Mg, so it is important to investigate the InN:Mg films by the Optical Pumb-Terahertz Probe (OPTP) system.
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