2DEG Confinement Research Articles

Analytical Optimization of AlGaN/GaN/AlGaN DH-HEMT Device Performance Based on Buffer Characteristics

In this paper, conventional DH-HEMT has been optimized extensively to achieve improved breakdown characteristics while retaining efficient confinement of 2DEG in the buffer and channel. Critical Figures of Merit (FOMs) including DC and stress was analyzed in addition using Synopsys’ Sentaurus TCAD tool, to address recent challenges with single heterojunction HFET. Four different epitaxial designs were tactically manipulated by considering different materials for substrate and buffer. Investigation was executed from multiple aspects. Significant observation includes enhancement of breakdown voltage to 580V with extremely low gate leakage current and adequate carrier mobility in the channel. This is due to suppression of parasitic conduction path usually observed in conventional device. However, low DC output was recorded. The proposed structure was further optimized through important geometric consideration rendering its application in power switching, next generation communication system and radar application. Extensive TCAD modelling was implemented to precisely ensure complexity of heterostructure devices.

The role of boundary conditions in tuning the electronic properties of the (0 0 1) LaAlO3/SrTiO3 interface

First-principles calculations were performed to investigate the influence of different boundary conditions on the two-dimensional electron gas (2-DEG) at the (0 0 1) LaAlO3/SrTiO3 (LAO/STO) interface. It was found that the LAO film can be polar or nonpolar, depending on the boundary conditions. When the LAO film is nonpolar, there always exist metallic states at the interface. A better confinement of 2-DEG and higher carrier density were revealed at the nonpolar LAO/STO interface. Moreover, the effect of surface reconstructions on the redistribution of 2-DEG was also discussed. Our findings provide an in-depth understanding of the origin of 2-DEG and also offer new opportunities to tailor the low-dimensional charge states by the atomic manipulation of the boundary conditions as well as the film thickness.

Self‐consistent simulation of two‐dimensional electron gas characteristics of a novel (InxAl1–xN/AlN) MQWs/InN/GaN heterostructure

In this paper, (InxAl1–xN/AlN) MQWs/InN/GaN heterostructure with a (InxAl1–xN/AlN) MQWs barrier layer and an InN channel layer is presented. The numerical calculations of 2DEG characteristics are made using a self‐consistent solution of the Schrödinger and Poisson equations. The influence of (InxAl1–xN/AlN) MQWs and InN layer on band diagrams, 2DEG sheet density and carrier confinement are studied. The results show that an InN insert layer can improve the carrier‐confinement ability without degrading the 2DEG sheet density in the heterostructure. The (InxAl1–xN/AlN) MQWs/InN/GaN heterostructure has both a larger 2DEG sheet density and better confinement of 2DEG than conventional InAlN/GaN and AlGaN/GaN heterojunctions. The 2DEG sheet density is as dramatically large as 3.85 × 1013 cm−2 with an AlN thickness of 1.5 nm, an In0.18Al0.82N thickness of 1.5 nm, an InN thickness of 0.5 nm, and pair number of 5. In addition, the influence of AlN thickness, InAlN thickness, indium content, and pair number on the 2DEG sheet density of the (InxAl1–xN/AlN) MQWs/InN/GaN heterostructure has been studied.

Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity current-gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure.

Simulation study of two-dimensional electron gas in N-polar GaN/AlGaN heterostructure

By the self-consistent solution of the Schrödinger and poisson equations, the effects of GaN channel layer, AlGaN back barrier layer with and without Si doping and AlN interlayer on two-dimensional electron gas in N-polar GaN/AlGaN heterostructure are systematically studied. The results indicate that the increases of the thickness values of GaN channel layer and AlGaN back barrier layer and Al content value can improve the density of 2DEG to a certain degree, and the influences of different Si doping forms on 2DEG sheet density are not the same, also the confinement of 2DEG could be strengthened by increasing Al content value and thickness value of the AlGaN barrier layer. The AlN interlayer is a comparatively outstanding one in improving the performance of the 2DEG such as the 2DEG sheet density and confinement. When GaN channel layer thickness is less than 5 nm, there is no 2DEG in the simulation, when it exceeds 20 nm the 2DEG sheet density tends to be saturated. 2DEG has a tendency to be saturated when the thickness value of AlGaN back barrier is more than 40 nm. 2DEG sheet densities with uniform doping and delta doping in AlGaN back barrier are saturated when the doping concentration is more than 5×1019 cm-3. The 2DEG sheet density could be increased from 0.93×1013 cm-2 without AlN interlayer to 1.17×1013 cm-2 with 2 nm AlN interlayer.

Density functional study of GaN(0001)/AlN(0001) high electron mobility transistor structures

AlN/GaN high electron mobility transistor (HEMT) structures were theoretically investigated using the density functional theory (DFT). An existence of two-dimensional electron gas (2DEG), generated by strong polarization fields at the AlN/GaN interface, was confirmed by ab initio calculations. The potential profiles were analyzed for the HEMTs with different number of AlN layers, indicating that very thin AlN barrier could not provide a high density of carriers and good confinement of 2DEG. It was shown that for six AlN layers (thickness about 13Å), no electron gas is present as shown by the potential profile. In the investigated electronic structure, the band states associated with the two-dimensional electron gas were identified. It was shown that for a thicker AlN barrier, the occupancy of this states is higher, that is consistent with the experimental data (Dabiran et al., Appl. Phys. Lett. 93 (2008) 082111).

The influence of the 1st AlN and the 2nd GaN layers on properties of AlGaN/2nd AlN/2nd GaN/1st AlN/1st GaN structure

This is a theoretical study of the 1st AlN interlayer and the 2nd GaN layer on properties of the Al0.3Ga0.7N/2nd AlN/2nd GaN/1st AlN/1st GaN HEMT structure by self-consistently solving coupled Schrodinger and Poisson equations. Our calculation shows that by increasing the 1st AlN thickness from 1.0 nm to 3.0 nm, the 2DEG, which is originally confined totally in the 2nd channel, gradually decreases, begins to turn up and eventually concentrates in the 1st one. The total 2DEG (2DEG in both channels) sheet density increases nearly linearly with the increasing 1st AlN thickness. And the slope of the potential profile of the AlGaN changes with the 1st AlN thickness, causing the unusual dependence of the total 2DEG sheet density on the thickness of the AlGaN barrier. The variations of 2DEG distribution, the total 2DEG sheet density and the conduction band profiles as a function of the 2nd GaN thickness also have been discussed. Their physical mechanisms have been investigated on the basis of the surface state theory. And the confinement of 2DEG can be further enhanced by the double-AlN interlayer, compared with the InGaN back-barrier.