Optical Performance of Two Dimensional Electron Gas and GaN:C Buffer Layers in AlGaN/AlN/GaN Heterostructures on SiC Substrate

Abstract

Terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy were developed as the method for the investigation of high-frequency characteristics of two-dimensional electron gas and GaN:С buffer layers in AlGaN/AlN/GaN heterostructures grown on a semi-insulating SiC substrate. The reflectance and transmittance spectra of the selected heterostructure layers were studied after the top layers were removed by a reactive ion etching. Results were numerically analyzed using the transfer matrix method taking into account the high-frequency electron conductivity via a Drude model and complex dielectric permittivity of each epitaxial layer via a one-phonon-resonance approximation. Good agreement between the experiment and theory was achieved revealing the temperature dependent electron effective mass in AlGaN/AlN/GaN high electron mobility transistor structures and the small damping factors of optical phonons due to high crystal quality of the epitaxial layers fabricated on the SiC substrate.

High-Frequency and High-Power Performance of n-Type GaN Epilayers with Low Electron Density Grown on Native Substrate

“…Frequency of the longitudinal, ω LO , and transverse, ω TO , optical phonons with corresponding damping factors, γ LO , γ TO , free-carrier density, n , and mobility, μ , and the total thickness of the substrate with epilayer, d , were obtained from the fitting procedure, revealing the values to be ω LO = 743 cm −1 , ω TO = 558 cm −1 , γ LO = 6.9 cm −1 , γ TO = 3.2 cm −1 , n = 1.19 × 10 16 cm −3 , μ = 952 cm 2 /V·s, and d = 375 μm, respectively. Measured phonon frequencies and damping factors agree well with other findings reported in the literature [ 24 , 37 ]. It is worth noting that IR-active impurity mode with a characteristic frequency of about ω IM = 736 cm −1 was included in the spectral analysis [ 38 ].…”

“…Measured transmission spectrum of the GaN epilayer in the THz region is shown in Figure 10 with a red solid line. The contribution of free electrons was calculated using the high-frequency Drude conductivity model by adding the respective component to the dielectric function of the GaN epilayer [ 24 ]. The transfer matrix approach modified for conducting interfaces [ 39 ] was applied with electron density, mobility, and structure thickness as the fitting parameters.…”

“…Transmission was obtained in the zone of the GaN epilayer that was not affected during the fabrication of test devices with removed 200 nm-thick subcontact layers. Modelling of THz transmission was conducted using the transfer matrix method with dielectric function derived from the Drude conductivity model, a detailed description of which can be found elsewhere [ 24 , 25 ].…”

Investigation of Electron Effective Mass in AlGaN/GaN Heterostructures by THz Spectroscopy of Drude Conductivity

“…Usually Drude model of electron conductivity is employed for study of THz wave interaction with optically thin conductive layers [11]- [13]. A sophisticated transfer matrix method is widely used to describe spectra of multilayer samples [14], [15]. As an alternative, analytical equations of the transmission coefficient were derived by solving Maxwell equations for a two-component system composed of a thin conductive layer on a thick insulating substrate [16].…”

“…The electron effective mass, m * , is an important basic parameter that defines the mobility of electrons and influences the speed of electronic device. Thermal renormalization of m * was recently found in the AlGaN/GaN heterostructures using advanced experimental methods [15], [18]- [21]. Usually, the AlN spacer between the barrier and channel layers is used to localize 2DEG in a quantum well by minimizing the penetration of wave function into the barrier layer [22].…”