Electric Field In AlGaN Layer Research Articles

Influence of AlN layer on electric field distribution in GaN/AlGaN/GaN transistor heterostructures

Distribution of built-in electric field in GaN/AlGaN/GaN transistor heterostructures without and with AlN layer was studied theoretically (solving the Schrodinger and Poisson equation) and experimentally (measuring contactless electroreflectance (CER) spectra and analyzing the AlGaN-related Franz-Keldysh oscillation). It is shown that the AlN layer changes very strongly the distribution of electric field in such heterostructures. This change can be very well predicted if the surface boundary conditions for self-consistent Schrodinger-Poisson calculations in GaN/AlGaN/GaN heterostructures are known. These conditions can be determined/verified by CER measurements of AlGaN-related Franz-Keldysh oscillation, which depends on the built-in electric field in AlGaN layer.

Electromodulation spectroscopy of optical transitions and electric field distribution in GaN/AlGaN/GaN transistor heterostructures with various AlGaN layer thicknesses

AbstractGaN/AlGaN/GaN field effect transistor (FET) heterostructures with various thicknesses of AlGaN layer (10, 20, and 30nm) were investigated by electromodulation spectroscopy [i.e. contactless electroreflectance (CER) and photoreflectance (PR)]. In PR spectra optical transitions in GaN cap, AlGaN and GaN buffer layers were clearly observed whereas in CER spectra the optical transition in GaN buffer layer was not observed due to the screening of band bending modulation in CER spectroscopy by the two dimensional electron gas at the AlGaN/GaN interface. The transition related to GaN‐cap was observed ∼0.1 eV above the energy gap of GaN due to the quantum confinement in the surface GaN quantum well. The built‐in electric field in AlGaN layer was determined from the analysis of AlGaN‐related Franz‐Keldysh oscillation. For the sample with 10, 20, and 30nm thick AlGaN layer the electric field has been determined from CER(PR) measurements to be 0.30(0.27), 0.46(0.39) and 0.78(0.63) MV/cm, respectively. The smaller electric field, which is obtained from PR measurements, results from the photovoltaic effect which is eliminated in CER spectroscopy. The electric field distribution in the full heterostructure (GaN(cap) layer, AlGaN layer, and near the AlGaN/GaN interface in GaN(buffer) layer) has been found comparing theoretical calculations for the full heterostructure with the electric field for AlGaN layer obtained from CER measurements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Distribution of built‐in electric field in GaN(cap)/AlGaN/GaN(buffer) transistor heterostructures with various AlGaN thicknesses

AbstractA theoretical approach supported by experimental data (i.e., measurements of built‐in electric field in AlGaN layer and two dimensional electron gas (2DEG) concentration at the AlGaN/GaN interface) has been proposed to determine the distribution of built‐in electric field in undoped polar GaN(cap)/AlGaN/GaN(buffer) heterostructures. In this method the Schrodinger and Poisson equations are solving self consistently for various boundary conditions. It is clearly shown that the built‐in electric field in AlGaN layer depends mainly on the surface boundary condition whereas the concentration of 2DEG at AlGaN/GaN interface depends on two boundary conditions (i.e., the Fermi‐level position on GaN(cap) surface and inside GaN(buffer) layer). Comparing the measured built‐in electric field in AlGaN layer with this one calculated for various Fermi‐level position on GaN surface it is possible to determine the Fermi‐level pinning on GaN(cap) surface. Next it is possible to determine the Fermi‐level position in GaN(buffer) layer at a given distance from AlGaN/GaN interface. For this purpose the measured 2DEG concentration was compared with this one calculated for the fixed Fermi‐level position on GaN(cap) surface and the varied Fermi‐level position in GaN buffer layer. In this way the distribution of built‐in electric can be determined for real heterostructures for which the built‐in electric field in AlGaN layer is measured by electromodulation spectroscopy and 2DEG concentration is extracted from Hall measurements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Contactless electroreflectance study of band bending for undoped, Si- and Mg-doped GaN layers and AlGaN/GaN transistor heterostructures

The surface band bending in undoped, Si-doped and Mg-doped GaN layers with Ga-face polarity as well as AlGaN/GaN heterostructures with Ga(Al)-face polarity has been investigated by room temperature contactless electroreflectance (CER) spectroscopy. The opposite phase of CER resonance (i.e., opposite band bendings) has been observed for n-type (undoped and Si-doped) and p-type (Mg-doped) GaN layers. It means that for thick GaN layers the surface band bending results not from crystal polarity but from the Fermi-level pinning at the surface and carrier type/concentration inside the layer. The crystal polarity can influence the surface band bending for thin (Al)GaN layers for which the screening phenomena can be neglected or are very weak. Such a situation is typical of AlGaN/GaN transistor structures where the thickness of AlGaN layer is below 40nm. In this case, the strong internal electric field in AlGaN layer is manifested in CER spectra by a resonance with a long period Franz–Keldysh oscillation.

Investigations of AlGaN/GaN field-effect transistor structures by photoreflectance spectroscopy

Undoped AlGaN/GaN heterostructures with different content of AlGaN layer have been investigated by photoreflectance (PR) spectroscopy. PR resonances associated with the absorption in both GaN and AlGaN layers have been observed. The observed AlGaN-related transition has been identified as a band-to-band absorption in the layer being in under a strong built-in electric field. The electric field has been determined from the period of Franz-Keldysh Oscillations (FKOs) typical of layers being in a built-in electric field. It has been shown that the internal electric field in AlGaN layer increases by 21% (from ∼240 to 290kV/cm) with the rise of Al mole fraction from 20 to 27%. This phenomenon has been attributed to an increase in the surface potential for AlGaN layer due to the rise of Al mole fraction. In addition, PR signal related to GaN has been analyzed. It has been shown that the GaN-related part of PR spectrum exhibits FKOs, besides, sharp PR features which could be associated with an excitonic absorption far to the AlGaN/GaN, i.e. in the part of GaN layer being in weak internal electric field. The GaN-related FKOs have been attributed the band-to-band absorption in the GaN layer being in strong internal electric field, i.e. in the part of GaN layer close to the AlGaN/GaN interface. It has been determined that this field is in the range of 220–230kV/cm.