Polarization-induced Sheet Charges Research Articles

Improved hole injection for AlGaN-based DUV LEDs with graded-composition multiple quantum barrier insertion layers

The primary impediments to achieving high external quantum efficiency (EQE) and light output power (LOP) in AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) are inadequate hole injection efficiency and pronounced electron leakage. The significant polarization-induced positive charges, originating from the discontinuity in the Al composition between the last quantum barrier (LQB) and electron-blocking layer (EBL), attract electrons, consume holes, and reflect holes back into the p-type region, leading to severe electron leakage and low hole injection efficiency. In this paper, we introduce a graded-composition multiple quantum barrier (GQB) structure at the LQB/EBL interface. We adjust the Al composition in the insertion layer to alter the electrical polarity of the polarization-induced sheet charges, thereby modifying the electric field distribution in the EBL, LQB, and inserted GQB structure. Consequently, holes acquire boosted energy during their migration towards the active region. In addition, we enhance the hole injection and electron confinement ability via reducing the effective valence band barrier height and increasing the effective conduction band barrier height, thus diminishing the possibility of electron leakage from the active region into the p-type region. Therefore, the GQB structure proposed in this study provides a promising approach to improving the optical and electrical performance of DUV LEDs.

Electron accumulation and distribution at interfaces of hexagonal ScxAl1−xN/GaN- and ScxAl1−xN/InN-heterostructures

Electron charges and distribution profiles induced by polarization gradients at the interfaces of pseudomorphic, hexagonal ScxAl1−xN/GaN- and ScxAl1−xN/InN-heterostructures are simulated by using a Schrödinger–Poisson solver across the entire range of random and metal-face ScxAl1−xN-alloys, considering the transition from wurtzite to hexagonal layered crystal structure. In contrast to previous calculations of polarization-induced sheet charges, we use Dryer’s modern theory of polarization, which allows for consideration of the spontaneous polarization measured on ferroelectric ScxAl1−xN-layers. Because the sheet density of the electrons accumulating at the heterostructure interfaces can strongly depend both on the data set of the piezoelectric and structural coefficients and on the alloying region of the ScxAl1−xN-layers in which the transition from the wurtzite to the hexagonal layered crystal structure occurs, we have calculated the charge carrier sheet densities and profiles for three representative data sets and evaluated their relevance for devices. We predict electron sheet densities of (2.26±0.20)×1014cm−2 and (6.25±0.20)×1014cm−2 for all three sets of data for Ni/AlN/InN- and Ni/ScN/InN-heterostructures, respectively. We demonstrate that the polarization-induced interface charges of Ni/ScxAl1−xN/InN-heterostructures are always positive, tend to increase with increasing Sc-content, and can cause electron accumulations that lead to flooding of the triangular quantum wells at the semiconductor interface. We identify Ni/ScxAl1−xN/GaN-heterostructures with 0.13≤x≤0.19 as particularly promising candidates for the processing of energy-efficient high electron mobility transistors due to their missing or low mechanical strain and their large electron sheet densities between (4.11±0.20)×1013cm−2 and (6.37±0.20)×1013cm−2. Furthermore, we present simulation results of highly strained Ni/ScxAl1−xN/GaN-heterostructures for 0.81≤x≤1.0, which point to electron accumulations of up to (8.02±0.40)×1014cm−2. These heterostructures are not suitable for transistor devices, but they may be of great interest for the implementation of low impedance contacts.

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures

Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.0074% at 18.8 mA. In contrast to traditional GaN light emitters, p-type doping and p-contacts are completely avoided, and instead, holes are created in the GaN on the emitter side of the tunneling structure by direct interband (that is, Zener) tunneling from the valence band to the conduction band on the collector side. The Zener tunneling is enhanced by the high electric fields (~5 × 106 V cm−1) created by the notably large polarization-induced sheet charge at the interfaces between the AlN and GaN.

Polarization-Engineered High-Efficiency GaInN Light-Emitting Diodes Optimized by Genetic Algorithm

A genetic algorithm is employed to find an optimum epitaxial structure of multiple quantum wells (MQWs) and electron-blocking layer (EBL) for a GaInN-based light-emitting diode (LED). The optimized LED is composed of locally Si-doped quantum barriers (QBs) in the MQWs and a quaternary heterostructured AlGaInN EBL having a polarization-induced electric field directed oppositely to that of a conventional AlGaN EBL. The optimized LED shows 15.6% higher internal quantum efficiency, 24.6% smaller efficiency droop, and 0.21 V lower forward voltage at 200 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> comparing to the reference LED, which has fully Si-doped QB and 20-nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> N EBL. We find that local Si doping near the QB/QW interface compensates the negative polarization-induced sheet charge at the interface and reduces electric field in the QWs, thereby enhancing electron-hole wave function overlap. In addition, the inverted polarization field in the quaternary EBL provides a high barrier for electrons but a low barrier for holes, resulting in enhanced electron-blocking and hole-injection characteristics.

GaN Heterostructure Barrier Diodes Exploiting Polarization-Induced &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$\delta$ &lt;/tex-math&gt;&lt;/inline-formula&gt;-Doping

A GaN-based heterostructure barrier diode (HBD) similar to GaAs planar-doped barrier diodes is demonstrated. Instead of doping with impurities, the polarization-induced sheet charge at the III-nitride heterojunction behaves as an effective δ -doping. An AlGaN/GaN heterostructure is used for the demonstration. The rectifying characteristics of the polarization-induced GaN HBDs can be tuned by controlling the graded AlGaN thickness and composition. Such polarization-engineered HBDs can find applications in high-voltage and high-frequency electronics.

A study of BGaN back-barriers for AlGaN/GaN HEMTs

We study the use of a BGaN back-barrier layer in the GaN buffer of Al y Ga1 −y N/GaN highelectron mobility transistors to improve confinement of carriers in the 2D electron gas region. Unlike InGaN back-barrier designs, whose polarization-induced sheet charges form an electrostatic barrier at the backbarrier/ buffer interface, BGaN back-barrier designs create an electrostatic barrier at the channel/backbarrier interface. This can result in carrier confinement to sub-15 nm thickness, even when the channel is 30 nm wide. Although polarization sheet charges due to the BGaN back-barrier form a secondary well at the back-barrier/buffer interface, increasing the thickness of the back-barrier may move the secondary well so that it no longer interacts with the primary channel.

Interfacial Band Bendings in Al Ohmic Contacts to Laser-Irradiated Ga-Face and N-Face n-GaN

We investigate interfacial band bendings in Al ohmic contacts to laser-irradiated Ga-face and N-face n-GaN using synchrotron radiation photoemission spectroscopy. Both samples show increased band bendings after annealing at , but annealing at leads to completely opposite band bendings. These two different behaviors with annealing temperature are consistent with electrical properties of the contacts and are well explained by the annihilation of N vacancies at the interface and the formation of polarization-induced sheet charges at AlN/GaN heterointerfaces, respectively.

Group-III Nitride P-Ttype Nanowire Heterostructure Field Effect Transistors

A two-step deposition process was developed in a metal organic chemical vapor deposition system to create a defect-free p-type structure consisting of a GaN nanowire (NW) core surrounded by a distinct AlGaN radial shell. The m-plane growth direction yields a nanowire with an isosceles triangular cross section with two identical {-110-1} facets and a third (0001) facet. The AlGaN shell introduces a polarization-induced sheet charge at the AlGaN/GaN interface and may shield the NW from surface-state induced depletion. Transmission electron microscopy confirmed that single-component NWs and sufficiently thin NW heterostructures are free of extended defects. High hole currents and current modulation were demonstrated with back-gated NW transistor structures.

Two-dimensional electron gas density in Al1−xInxN/AlN/GaN heterostructures (0.03≤x≤0.23)

Compared to the AlGaN alloy, which can only be grown under tensile strain on GaN, the AlInN alloy is predicted by Vegard’s law to be lattice-matched (LM) on fully relaxed GaN templates for an indium content of ∼17.5%, i.e., it can be grown either tensely or compressively on GaN. The effect of strain on the polarization induced sheet charge density at the Al1−xInxN/AlN/GaN heterointerfaces is carefully investigated for 6 and 14 nm thick AlInN barriers including a 1 nm thick AlN interlayer. The barrier indium content ranges at 0.03≤x≤0.23 for 6 nm thick barriers and 0.07≤x≤0.21 for 14 nm thick barriers. It is found that the two-dimensional electron gas (2DEG) density varies between (3.5±0.1)×1013cm−2 and (2.2±0.1)×1013 cm−2 for 14 nm thick barriers. Finally, a 2DEG density up to (1.7±0.1)×1013 cm−2 is obtained for a nearly LM AlInN barrier with ∼14.5% indium on GaN as thin as 6 nm.

Two dimensional electron gas in InN-based heterostructures: Effects of spontaneous and piezoelectric polarization

This paper predicts and describes the formation of two dimensional electron gas (2DEG) in InN-based heterostructures, as a promising candidate for future high performance high electron mobility transistor (HEMT). The effects of spontaneous and piezoelectric polarization on carrier confinement and localization of 2DEGs have been studied. These include the theoretical analysis and calculation of spontaneous and piezoelectric polarization, and polarization induced sheet charge bound at the interfaces of InN/InGa(Al)N/InN heterostructures as a function of lattice mismatch induced strain. Significant effects of polarity are found on carrier confinement and localization of 2DEGs. A high sheet carrier concentration and strong electron confinement at specific interfaces of the InN-based heterostructures are predicted as a consequence of piezoelectric and spontaneous polarization effects. The calculated sheet carrier concentration reaches as high as 2.12 × 10 14 and 7.08 × 10 13 cm −2 for AlN and GaN barriers, respectively. These sheet carriers generated in InN-based heterostructures are higher than the reported values for the conventional GaN-based AlGaN/GaN HEMTs. The accumulation of electrons can be confirmed for a barrier thickness of above 4 nm.

EXTRACTION OF POLARIZATION-INDUCED CHARGE DENSITY INMODULATION-DOPED AlxGa1-xN/GaN HETEROSTRUCTURETHROUGH THE SIMULATION OF THE SCHOTTKY CAPACITANCE-VOLTAGE CHARACTERISTICS

Polarization-induced charge density in modulation-doped AlxGa1-xN/GaN heterostructures is extracted through the simulation of the Schottky Capacitance-Voltage (C-V) characteristics. C-V measurements were made on Pt Schottky contacts on modulation-doped Al0.22Ga0.78N/GaN heterostructures. The C-V characteristics were simulated numerically using the three-dimensional Fermi model.Influence of sample parameters on C-V characteristics is analized by the simulation.Polarization-induced charge density,n-AlGaN doping level and Schottky barrier height have different influences on the C-V characteristics,and thus the polarization-induced charge density can be extracted accurately.The polarization-induced sheet charge density at the heterointerface is extracted to be 6.78×1012cm-2 in the Al0.22Ga0.78N/GaN heterostructure with the Al0.22Ga0.78 N thickness of 45nm.This work provides a method for quantitative analysis of the polarization-indeced charge in modulation-doped AlxGa1-xN/GaN heterostructures.

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

Two dimensional electron gases in AlxGa1−xN/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance–voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined self-consistently from a coupled Schrödinger and Poisson equation solver for pseudomorphically and partially relaxed barriers with different alloy compositions. By comparison of theoretical and experimental results, we demonstrate that the formation of two dimensional electron gases in undoped and doped AlGaN/GaN structures rely both on piezoelectric and spontaneous polarization induced effects. In addition, mechanisms reducing the sheet carrier concentrations like nonabrupt interfaces, dislocations, and the possible influence of surface states on the two dimensional electron gases will be discussed briefly.