Polarization-induced Electron Research Articles

Density Functional Theory Study of Polarization-Induced Electron Confinement by Dilute Boron Alloying in ε-Ga2O3 Nanometer-Thick Film for High Electron Mobility Transistor

Density Functional Theory Study of Polarization-Induced Electron Confinement by Dilute Boron Alloying in ε-Ga₂O₃ Nanometer-Thick Film for High Electron Mobility Transistor

Spontaneous Polarization and Polarization-Induced Electron Sheet Charge of YbAlN on GaN: A First-Principles Study

Spontaneous Polarization and Polarization-Induced Electron Sheet Charge of YbAlN on GaN: A First-Principles Study

Nonvolatile Reconfigurable 2D Schottky Barrier Transistors.

Nonvolatile reconfigurable transistors can be used to implement highly flexible and compact logic circuits with low power consumption in maintaining the configuration. In this paper, we build nonvolatile reconfigurable transistors based on 2D CuInP2S6/MoTe2 heterostructures. The ferroelectric polarization-induced electron and hole doping in the heterostructure are investigated. By introducing the ferroelectric doping into the source/drain contacts, we demonstrate reconfigurable Schottky barrier transistors, whose polarity (n-type or p-type) can be dynamically programmed, where the configuration is nonvolatile in nature. These transistors exhibit a tunable photoresponse, where the n-n doping state leads to negative photocurrent, whereas the p-p doping state gives rise to a positive photocurrent. The transistor with asymmetric (n-p or p-n) contacts exhibits a strong photovoltaic effect. These reconfigurable logic and optoelectronic transistors will enable a new type of device fabric for future computing systems and sensing networks.

Enhanced carrier injection in InGaN/GaN multiple quantum wells LED with polarization-induced electron blocking barrier

In this report, we designed a light emitting diode (LED) structure in which an N-polar p-GaN layer is grown on top of Ga-polar In0.1Ga0.9N/GaN quantum wells (QWs) on an n-GaN layer. Numerical simulation reveals that the large polarization field at the polarity inversion interface induces a potential barrier in the conduction band, which can block electron overflow out of the QWs. Compared with a conventional LED structure with an Al0.2Ga0.8N electron blocking layer (EBL), the proposed LED structure shows much lower electron current leakage, higher hole injection, and a significant improvement in the internal quantum efficiency (IQE). These results suggest that the polarization induced barrier (PIB) is more effective than the AlGaN EBL in suppressing electron overflow and improving hole transport in GaN-based LEDs.

Solar-blind AlxGa1−xN/AlN/SiC photodiodes with a polarization-induced electron filter

Heterogeneous n-III-nitride/i-p silicon carbide (SiC) photodetectors have been demonstrated that enable the tailoring of the spectral response in the solar blind region below 280 nm. The negative polarization induced charge at the aluminum gallium nitride (AlxGa1−xN)/aluminum nitride (AlN) interface in conjunction with the positive polarization charge at the AlN/SiC interface creates a large barrier to carrier transport across the interface that results in the selective collection of electrons photoexcited to the Γ and L valleys of SiC while blocking the transport of electrons generated in the M valley. In addition, the AlxGa1−xN alloys act as transparent windows that enhance the collection of carriers generated by high energy photons in the fully depleted SiC absorption regions. These two factors combine to create a peak external quantum efficiency of 76% at 242 nm, along with a strong suppression of the long-wavelength response from 260 nm to 380 nm.

Temperature mapping of Al0.85In0.15N/AlN/GaN high electron mobility transistors through micro-photoluminescence studies

Crack-free lattice-matched Al 0.85 In 0.15 N/GaN heterostructures were grown on sapphire substrates with barrier thicknesses up to 100 nm which exhibit very high polarization-induced electron sheet density ( cm −2 ) located at the heterointerface. These layers have been further processed as high electron mobility transistors (HEMTs). Optical characterization of these structures was carried out by photoluminescence and microphotoluminescence (PL) for different biased voltages. The insertion of an InGaN back-barrier unambiguously reveals that spatially direct optical recombinations occur within the AlInN alloy. Since the GaN excitonic bandgap is very sensitive to local temperature changes, the PL technique allows mapping very precisely the actual local temperature distribution in biased HEMT devices. For a gate length of 1.5 m temperatures up to 1130 K were found at a drain-source voltage of 20 V thus indicating the presence of a hot phonon bath.

Polarization-induced electron island in semiconductor grain placed into pyroelectric matrix

We consider the electron distribution in an n-type semiconductor grain inserted into a pyroelectric matrix. We show that for typical values of parameters, the electric field induced in the grain by the pyroelectric spontaneous polarization is very strong and cannot be screened by the electrons in the grain. This field removes the electrons from donors and leads to the formation of a two-dimensional electron island on the surface of the grain with the size much smaller than the grain size. This effect can be used for creation of small two-dimensional quantum dots.

Two-dimensional electron gas transport properties in AlGaN/GaN single- and double-heterostructure field effect transistors

Two-dimensional electron gas transport properties have been investigated in both AlGaN/GaN single- and AlGaN/GaN/AlGaN double-heterostructure field effect transistors, by means of the Hall effect measurement under the gate-voltage application. In AlGaN/GaN single-heterostructures, the dependencies of the electron mobility on the electron density have systematically been examined from 30 to 400 K. Below room temperature, the mobility has been shown to assume a maximum value at a critical electron density where electrons begin to overflow into the AlGaN barrier layer. Above room temperature, even at high electron densities that exceed the channel electron capacity, the degradation in the mobility has been found to be small. This is a feature favorable for high-power and high-temperature device operation. In AlGaN/GaN/AlGaN double-heterostructures, a striking effect has been observed that the mobility is drastically enhanced compared with that in the AlGaN/GaN single-heterostructure at low temperatures. The dependencies of the mobility on the electron density measured at 4.2 K in both heterostructures have been analyzed from the viewpoint of the electron distribution in the channel. The observed mobility enhancement in the double-heterostructure has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Transport properties specific to AlGaN/GaN single- and double-heterostructures have thus successfully been revealed through the mobility-density relations that have been determined by the Hall effect measurement under the gate-voltage application.

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrier confinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at /spl sim/3.8 eV or /spl sim/4.0 eV, GaN band edge emission at /spl sim/3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region.

Polarization-induced electron populations

Electron populations induced by spontaneous and piezoelectric polarization in semiconductor heterostructures can be estimated simply by using elementary electrostatic theory. The method is illustrated for the AlGaN/GaN system in which the AlGaN barrier is either undoped, or doped n type, and the effect of a GaN overlayer is described.

Enhanced effect of polarization on electron transport properties in AlGaN/GaN double-heterostructure field-effect transistors

In order to understand the effect of polarization on the electron transport properties in AlGaN/GaN heterostructures, the dependence of the two-dimensional electron gas mobility on the density has been examined both in the GaN single- and 200-Å GaN double-heterostructure field-effect transistors, by means of the Hall effect measurement under the gate-voltage application at 4.2 K. In the single heterostructure, a maximum mobility of 5700 cm2/V s has been obtained at 9.0×1012 cm−2; whereas in the double heterostructure, a higher maximum mobility of 8100 cm2/V s has been obtained at a lower electron density of 6.8×1012 cm−2. The results have been analyzed in terms of potential profiles and electron distributions in both structures where both piezoelectric and spontaneous polarization effects are taken into account. The analysis has shown that the observed mobility enhancement is mainly due to the enhanced polarization-induced electron confinement in the double heterostructure and, additionally, to the improvement of the interface roughness in the structure.

The polarization-induced electron gas in a heterostructure

Models for the magnitude of the electron density induced by spontaneous and piezoelectric polarization in a polar heterostructure are given. In the case of pure material the induced electrons derive from `ambient' doping associated with surface states and surface ions. Where the donor density is large enough depletion will contribute to the electron gas.

Two-Dimensional Electron Gas Transport Properties in AlGaN/(In)GaN/AlGaN Double-Heterostructure Field Effect Transistors

Two-dimensional electron gas transport properties have been investigated in nitride double-heterostructures. A striking effect has been observed that the two-dimensional electron gas mobility has been drastically enhanced in the AlGaN/GaN/AlGaN double-heterostructure, compared with that in the conventional AlGaN/GaN single-heterostructure. The observed mobility enhancement has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Device operation of an AlGaN/GaN/AlGaN double-heterostructure field effect transistor has been demonstrated: a maximum transconductance of 180 mS/mm has been obtained for a 0.4 μm-gate-length device. In the double-heterostructure using InGaN channel, the increased capacity for the two-dimensional electron gas has been observed. The AlGaN/(In)GaN/AlGaN double-heterostructures are effective for improving the electron transport properties.

Enhanced Two-Dimensional Electron Gas Confinement Effect on Transport Properties in AlGaN/InGaN/AlGaN Double-Heterostructures

A striking effect of polarization-induced electron confinement on transport properties has been observed in nitride double-heterostructures. The two-dimensional electron gas mobility has shown to be drastically enhanced in the AlGaN/GaN/AlGaN double-heterostructure, compared with that in the conventional AlGaN/GaN single-heterostructure. The observed mobility enhancement results from the strong polarization-induced electron confinement in the double-heterostructure. Device operation of an AlGaN/GaN/AlGaN double-heterostructure field effect transistor has been demonstrated: a maximum transconductance of 180 mS/mm has been obtained for a 0.4 μm gate length device. In the AlGaN/InGaN/AlGaN double-heterostructure, the increased capacity for the two-dimensional electron gas has been observed in addition to the enhanced electron mobility. The AlGaN/(In)GaN/AlGaN double-heterostructures are promising for field effect transistor applications because of their superior electron transport properties.

Two-Dimensional Electron Gas Transport Properties in AlGaN/(In)GaN/AlGaN Double-Heterostructure Field Effect Transistors

Two-dimensional electron gas transport properties have been investigated in nitride double-heterostructures. A striking effect has been observed that the two-dimensional electron gas mobility has been drastically enhanced in the AlGaN/GaN/AlGaN doubleheterostructure, compared with that in the conventional AlGaN/GaN singleheterostructure. The observed mobility enhancement has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Device operation of an AlGaN/GaN/AlGaN double-heterostructure field effect transistor has been demonstrated: a maximum transconductance of 180 mS/mm has been obtained for a 0.4 mm-gate-length device. In the double-heterostructure using InGaN channel, the increased capacity for the two-dimensional electron gas has been observed. The AlGaN/(In)GaN/AlGaN double-heterostructures are effective for improving the electron transport properties.