Electric Field In GaN Research Articles

The influence of the photovoltaic effect on the surface electric field in GaN

The surface potential barrier is an important issue for design and development of semiconductors devices, particularly based on heterostructures. It can be investigated with the electromodulation (EM) spectroscopy, as this technique allows the study of a surface electric field. However, the latter may be altered significantly by the photovoltaic (PV) effect. In this paper we present comprehensive studies of the influence of the PV effect on the surface electric field established using contactless electroreflectance (CER) and photoreflectance in dark and bright experimental configuration. Materials under investigation are the so called ‘van Hoof’ GaN structures with upward and downward band bending. This kind of sample architecture enables the probing of the built-in electric field through the investigation of Franz-Keldysh oscillations observed in the measured spectrum. It is shown that results obtained in bright configuration are more susceptible to PV effect than in dark configuration, however even for dark configuration this effect should not be neglected. What is more, it is highlighted that due to reduction of PV effect, CER enables more precise investigation of surface electric field than PR experiment. Relative changes in the surface electric field obtained for different lighting power densities are determined and discussed in this article.

Impact ionization coefficients and critical electric field in GaN

Avalanche multiplication characteristics in a reverse-biased homoepitaxial GaN p–n junction diode are experimentally investigated at 223–373 K by novel photomultiplication measurements utilizing above- and below-bandgap illumination. The device has a non-punch-through one-side abrupt p–-n+ junction structure, in which the depletion layer mainly extends to the p-type region. For above-bandgap illumination, the light is absorbed at the surface p+-layer, and the generated electrons diffuse and reach the depletion layer, resulting in an electron-injected photocurrent. On the other hand, for below-bandgap illumination, the light penetrates a GaN layer and is absorbed owing to the Franz–Keldysh effect in the high electric field region (near the p–n junction interface), resulting in a hole-induced photocurrent. The theoretical (non-multiplicated) photocurrents are calculated elaborately, and the electron- and hole-initiated multiplication factors are extracted as ratios of the experimental data to the calculated values. Through the mathematical analyses of the multiplication factors, the temperature dependences of the impact ionization coefficients of electrons and holes in GaN are extracted and formulated by the Okuto–Crowell model. The ideal breakdown voltage and the critical electric field for GaN p–n junctions of varying doping concentration are simulated using the obtained impact ionization coefficients, and their temperature dependence and conduction-type dependence were discussed. The simulated breakdown characteristics show good agreement with data reported previously, suggesting the high accuracy of the impact ionization coefficients obtained in this study.

Carrier Transport Properties of GAN in High Electric Field

The Monte Carlo (MC) simulation of the carrier transport mechanisms including impact ionization at high electric field in GaN is presented. Two non-parabolic conduction and valence bands were considered for the simulation of transport properties of electron and hole respectively. The carriers’ drift velocity and energy are simulated as a function of applied electric field at room temperature. The maximum velocity of electron is 2.85 × 107 cm/s at 140 kV/cm. The velocity of electron is saturated at 2 × 107 cm/s at electric field greater than 300 kV/cm. In our work, the velocity of hole is 5 × 106 cm/s at 500 kV/cm. Electron energy increases as the electric field increase and fluctuated at electric field greater than 600 kV/cm when impact ionization occurred. The impact ionization rates are obtained by using modified Keldysh equation. The hole impact ionization rate is higher than that of electron. This work also shows higher electron impact ionization coefficient than that of hole at electric field greater than 4.04 MV/cm

Toward Quantitative Electrochemical Nanomachining of III-Nitrides

In this paper, a comprehensive quantitative investigation is conducted on III-N electrochemical nanomachining to explore the underlying mechanisms. An electric circuit model is proposed for the etching system to interpret the various distinct etching dynamics. Factors such as the geometry of patterning, charge transfer and mass transport properties are taken into consideration as nonlinear lumped elements, which compete for voltage with the GaN/electrolyte interface during etching, thus change the etching behaviors. Quantitative study of nanoporous morphologies suggests that the anodic etching is driven by net voltage bias on the GaN/electrolyte interface and the high electric field in GaN, both related to the generation of charge carrier holes. And by three-dimensional analysis of the electrical field, the etching phase diagram is reconstructed. This study not only offers a mechanistic understanding of GaN anodic etching, but also provides a systematical prospective in constructing nanostructures on a broad range of semiconductor materials using electrochemical method.

Design of high breakdown voltage vertical GaN p-n diodes with high-K/low-K compound dielectric structure for power electronics applications

In this work, a vertical GaN p-n diode with a high-K/low-K compound dielectric structure (GaN CD-VGD) is proposed and designed to achieve a record high breakdown voltage (BV) with a low specific on-resistance (Ron,sp). By introducing compound dielectric structure, the electric field near the p-n junction interface is suppressed due to the effects of high-K passivation layer, and a new electric field peak is induced into the n-type drift region, because of a discontinuity of electrical field at the interface of high-K and low-K layer. Therefore the distribution of electric field in GaN p-n diode becomes more uniform and an enhancement of breakdown voltage can be achieved. Numerical simulations demonstrate that GaN CD-VGD with a BV of 10650 V and a Ron,sp of 14.3 mΩ cm2, resulting in a record high figure-of-merit of 8 GW/cm2.

Theoretical and experimental studies of electric field distribution in N-polar GaN/AlGaN/GaN heterostructures

Electric field distribution in N-polar GaN(channel)/AlGaN/GaN(buffer) heterostructures was studied theoretically by solving Schrodinger and Poisson equations in a self-consistent manner for various boundary conditions and comparing results of these calculations with experimental data, i.e., measurements of electric field in GaN(channel) and AlGaN layers by electromodulation spectroscopy. A very good agreement between theoretical calculations and experimental data has been found for the Fermi-level located at ∼0.3 eV below the conduction band at N-polar GaN surface. With this surface boundary condition, the electric field distribution and two dimensional electron gas concentration are determined for GaN(channel)/AlGaN/GaN(buffer) heterostructures of various thicknesses of GaN(channel) and AlGaN layers.

Bipolar characteristics of AlGaN/AlN/GaN/AlGaN double heterojunction structure with AlGaN as buffer layer

Abstract This is a theoretical study of AlGaN/AlN/GaN/AlGaN double heterojunction (DH) structure with AlGaN as buffer layer. Our calculation shows that as the buffer Al content increases, though two-dimensional electron gas (2DEG) sheet density decreases, the channel back-barrier caused by polarization-induced electric field in GaN provides better electron confinement. And under certain conditions the DH-structure shows bipolar characteristics, with an additional two-dimensional hole gas (2DHG) formed at GaN/AlGaN interface. The influence of the buffer Al content and GaN channel thickness on the 2DEG and 2DHG sheet densities are investigated, and the conditions for the disappearance of 2DHG are discussed. Also, the mobility inhibited by dislocation scattering is enhanced in DH-structure due to the enhancement of screening effect of the 2DEG.

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)

The surface boundary conditions in GaN/AlGaN/GaN transistor heterostructures

The distribution of electric field in GaN(cap)/AlGaN/GaN(buffer) transistor heterostructures with various AlGaN layer thicknesses (10, 20, and 30 nm) has been studied by contactless electroreflectance and compared with theoretical calculations performed for various positions of the Fermi-level on GaN surface. For the three samples the best agreement between experimental data and theoretical calculations has been found at the same position of the Fermi-level on GaN surface (i.e., 0.55±0.05 eV below the conduction band). It means that the Fermi-level is pinned on GaN surface and this pinning can be treated as the boundary condition for the distribution of polarization-related fields in this heterostructure.

Built-in electric field and large Stokes shift in near-lattice-matched GaN∕AlInN quantum wells

Near-lattice-matched GaN∕AlInN quantum wells are investigated by means of contactless electroreflectance (CER) and temperature-dependent photoluminescence (PL). Large Stokes shifts, up to 400meV, between PL peak energies and CER resonances are identified. This Stokes shift is attributed to large potential profile fluctuations (PPFs) in the AlInN barriers. Further evidence for such PPFs and for the additional influence of QW width fluctuations is provided by temperature-dependent PL measurements, demonstrating large PL halfwidths and clear “S-shape” behavior. The influence of a large Stokes shift on the correct determination of the value of the built-in electric field is discussed, and it is shown that PL measurements may lead to a significant overestimation of the built-in electric field in GaN∕AlInN QWs.

An analytical model for GaN MESFET's using new velocity‐field dependence

AbstractA physical analytical model is developed for the output current voltage characteristics and for describing the behavior of electrons in sub‐micron GaN MESFET's. A semi‐empirical formula has been proposed to approximate the electron drift velocity versus electric field in GaN by fitting it with the Monte Carlo simulation data. It gives a true physical description and an excellent approximation of the velocity field dependence in GaN revealing the presence of a velocity peak which is substantially higher than the saturation velocity at temperature close to 300 K and at typical doping concentration of 1017/cm3. The resulting I‐V curves are in excellent agreement with the experimental data. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A modified transferred-electron high-field mobility model for GaN devices simulation

An approximation formula for electron high-field mobility in GaN is proposed. One is tested in wide range of temperatures and doping concentrations and is able to replicate the specific electron drift velocity dependence on electric field in GaN more accurately than conventionally used Canali and transferred-electron models. The simulations of the current–voltage characteristics of GaN metal–semiconductor–metal structure are performed for considered electron high-field mobility models. Simulations are compared with available experimental data.

Impact ionization of excitons in an electric field in GaN

The impact ionization of excitonic states is studied in epitaxial GaN filmsand GaN/AlGaN quantum well structures. The investigation is carried out byoptical methods involving the observation of quenching of the excitonphotoluminescence under an applied electric field. It is found that impurityscattering rules the momentum and energy relaxation, rather than the acousticphonon scattering. The effective mean free path of hot electrons is estimated.In GaN/AlGaN quantum wells the mean free paths of hot electrons appear to bean order of magnitude larger than those of GaN films due to the decrease inscattering probability of the electron in the two-dimensional case.

Impact Ionization of Excitons in an Electric Field in GaN

Quenching of the exciton luminescence of GaN epitaxial films and GaN/AlGaN quantum wells in an electric field is observed and attributed to impact ionization of excitonic states by hot electrons. It is found that impurity scattering rules the momentum relaxation of electrons rather than the acoustic phonon scattering. The effective mean free path of hot electrons l(eff) is estimated. The value of l(eff) for quantum wells appears to be one order of magnitude larger than that for epitaxial films.

Time-resolved photoluminescence as a probe of internal electric fields in GaN-(GaAl)N quantum wells

Very strong coefficients for spontaneous and piezoelectric polarizations have recently been predicted for III-V nitride semiconductors with natural wurtzite symmetry. Such polarizations influence significantly the mechanisms of radiative emissions in quantum-confinement heterostructures based on these materials. The photoluminescence decay time of excitons is used as a probe of internal electric fields in GaN-(Ga, Al)N quantum wells in various configurations of strain, well widths, and barrier widths. The measured decays are not only controlled by radiative lifetimes, which depend on the fields inside GaN wells but also on the nonradiative escape of carriers through ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{y}\mathrm{N}$ barriers, which depends on their widths and on the electric field in these layers. It is shown in particular that the magnitude of the field in the wells is not a simple function of the strain of these layers via the only piezoelectric effect, but rather the result of the interplay of spontaneous and piezoelectric polarizations in both well and barrier materials.