Optical Properties of AlInN/AlN HEMTs in Detail

In this study, the optical properties of AlInN/AlN high electron mobility transistor (HEMT) structure, grown on c - oriented sapphire with Metal - Organic Chemical Vapor Deposition (MOCVD) technique, being investigated. Optical characterization is made Kubelka - Munk method. Transmittance, absorbance, and reflectance are investigated in detail. Also, the Kubelka-Munk theory is employed to determine the forbidden energy band gap of InN by using special functions. The energy band gap obtained by this method was compared.


Introduction
There are many fields in which we can not use conventional III-V group semiconductors.Short wavelength electromagnetic wave emitters are needed for color screens, laser writers, high-density data storage, and underwater communication.High power and high-temperature transistors are needed for automobile motors and developed power multipliers.Silicon and conventional III-V group semiconductors are not suitable for designing and producing optoelectronic devices operating in blue and ultra-violet regions of spectra.Gallium arsenide (GaAs) based optoelectronic devices can not be used at high temperatures.Group-III nitrites are suitable for applications in this field.The band gap of group III nitrites has a direct band structure.Band gap values are 0.7 eV for InN, 3.4 eV for InN and 6.2 eV for AlN (Vurgaftman, 2003;Meyer, 2003).These structures can be used for high temperature transistors and blue-green-violet light emitting devices because of their strong bonds and wide band gaps.Group III nitrites such as InN, InN, and AlN have wide band gaps, significant polarization effects, and hexagonal structure (Aleksan et al, 1991).InN and AlInN play the head role among group III nitrites in terms of technological developments.They have proved themselves in Field-Effect Transistor (FET) applications in the condition of modifying technological problems caused by a big difference between In and N on the ionic scale (Li et al, 2006).
The use of InN in electronic devices started by Pankove and co-workers in 1960, they made an InN-based blue light emitting diode (LED).Manufacturing problems prevented the production of pdoped InN.Studies on InN stopped because high-quality substrates were not present.To overcome this problem InN epitaxial layers are grown on substrates with big lattice mismatch.This procedure resulted in high dislocation density in the structure of device and life of devices was shortened.
Through the end of the 20th century, Shuji Nakamura succeeds growth of high-quality InN epitaxial layer on sapphire substrate by using MOCVD (Nakamura, 1991).InN-based structures prepared a good ground for new developments in optoelectronics.At the same time InN has a perfect electron carrying property with its high electron mobility (Xing et al, 2001;Hajlaoui et al, 2016).
In this study Kubelka-Munk theory is employed to determine the forbidden energy band gap of the AlInN/InN structure.In earlier studies by the authors, the band gap was determined by PL and Tau methods.The results of this study are compared with them (Akpinar et al, 2020).
The aim of this study is to calculate the optical band gap using the Kubelka-Munk method and to compare it with its theoretical value.

Results & Discussion
Transmittance is a physical property that makes light transmit from a material without scattering possible.On a macroscopic scale, it can be said that photons act by obeying the Snell law.Semitransmittance is an up-group in transmittance.It permishes light to be transmitted but it does not have to obey Snell's law.Photons may present a dispersion behavior in both interfaces if there is a variation in diffraction.The opposite term for semi-transmittance is opaque.Semi-transparent materials may have any combination to form a perfect spectrum.In figure 2 transmittance versus wavelength plot can be seen.According to this plot, the transmittance is dominant at higher wavelengths of incident light.Approximately after 700 nm wavelength value transmittance reaches saturation according to the plot.Transmittance starts at a 200 nm wavelength value.Saturation of transmittance may be related to decreasing photon energy.

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Absorption is the suction of light by a material.Every material can only absorb light at its color frequency.Electrons in orbitals jump higher energy levels by absorption.Light is an electromagnetic wave with a specific energy.The energy of a photon can be calculated by E=h*ʋ.The energy of light is inversely proportional to wavelength.If an atom is excited by a photon with an energy equal to the difference in energy levels electrons jump upper energy level and the incident photon is absorbed.This physical event is called absorbance.In figure 3 absorbance versus wavelength plot can be seen.
Absorbance starts at 200 nm wavelength, it decreases with decreasing incident photon energy and reaches a saturation value at approximately 400 nm wavelength value.This behavior may be attributed to decreasing photon energy.

Figure 3. Absorbance vs wavelength for AlInN
Reflectance may be described as returning of an incident photon to the medium it came from after hitting a surface.In classical electrodynamics, as described in Maxwell's equations light is accepted as an electromagnetic wave.Light waves reaching the surface of material causes small polarized oscillations.All these waves make reflection according to the Huygens-Fresnel principle.
The electric field of light interacts with electrons of the target material, moving electrons cause new fields noticed as reflected light.In figure 4 reflectance versus wavelength plot can be seen.According to this plot, reflectance starts at 200 nm, increases until 400 nm wavelength value, and stays approximately constant after 400 nm.This situation implies that increasing wavelength that is decreasing photon energy makes reflectance constant.This result may be attributed to the penetration obscenity of photons increasing as the wavelength of incident light decreases.This theory is recommended by Schuster (Groh, 1992;Osa, 2020).It explains the analysis of the interaction of incident light with matter.According to this theory, matter, assumed as homogeneous, isotropic, and opaque, is illuminated with monochromatic light.In this model scattered reflection data are used.
Kubelka-Munk Function can be seen in equation ( 1), smaller value of the optical band gap than the first method.The reason for this situation is attributed to defects and variation in carrier density.