We report on III-Nitride blue light emitting diodes (LEDs) comprising a GaN-based tunnel junction (TJ) all realized by metalorganic vapor phase epitaxy in a single growth process. The TJ grown atop the LED structures consists of a Mg-doped GaN layer and subsequently grown highly Ge-doped GaN. Long thermal annealing of 60 min at 800 °C is important to reduce the series resistance of the LEDs due to blockage of acceptor-passivating hydrogen diffusion through the n-type doped top layer. Secondary ion mass spectroscopy measurements reveal Mg-incorporation into the topmost GaN:Ge layer, implying a non-abrupt p-n tunnel junction and increased depletion width. Still, significantly improved lateral current spreading as compared to conventional semi-transparent Ni/Au p-contact metallization and consequently a more homogeneous electroluminescence distribution across 1 × 1 mm2 LED structures is achieved. Direct estimation of the depletion width is obtained from electron holography experiments, which allows for a discussion of the possible tunneling mechanism.
In this paper the internal electric fields of nearly lattice matched InAlN/GaN heterostructures were determined. Pin-diodes containing InAlN/GaN heterostructures grown on (0 0 0 1) sapphire substrates by metalorganic vapour phase epitaxy were fabricated by standard lithography and metallization techniques. To determine the polarization fields in the InAlN quantum wells capacitance-voltage-measurements were performed on the pin-diodes. To reduce the measurement error, the heterostructure thicknesses were accurately determined by transmission electron microscopy. Large polarization fields, which correspond mainly to the spontaneous polarizations, for In 0.15 Al 0.85 N (5.9 ± 0.8 MV cm −1 ), In 0.18 Al 0.82 N (5.4 ± 0.9 MV cm −1 ) and In 0.21 Al 0.79 N (5.1 ± 0.8 MV cm −1 ) quantum wells were observed. The results of the internal field strength and field direction are in excellent agreement with values predicted by theory and a CVM-based coupled Poisson/carrier transport simulation approach.
Semi-insulating GaN is a prerequisite for lateral high frequency and high power electronic devices to isolate the device region from parasitic conductive channels. The commonly used dopants for achieving semi-insulating GaN, Fe, and C cause distinct properties of GaN layers since the Fermi-level is located either above (Fe) or below (C) the midgap position. In this study, precursor-based doping of GaN in metalorganic vapor phase epitaxy is used at otherwise identical growth conditions to control the dopant concentrations in the layer. Using electric force microscopy, we have investigated the contact potentials of Fe- and C-doped samples with respect to a cobalt metal probe tip in dependence of on the dopant concentration. While in Fe-doped samples the sign of the contact potential is constant, a change from positive to negative contact potential values is observed at high carbon concentrations, indicating the shift of the Fermi-level below the midgap position. In vertical transport measurements, C-doped GaN layers with a dopant concentration of 4.6 × 1018 cm−3 exhibit up to 5 orders of magnitude lower dark current at room temperature and significantly lower temperature dependence than Fe-doped samples with a similar dopant concentration. Therefore, precursor-based carbon doping is the superior doping technique to achieve semi-insulating GaN.
We report on an over 50% reduction in polarization field strength in c-axis oriented InGaN multi-quantum wells (MQW) by applying quaternary AlGaInN barrier layers with better polarization matching to InGaN than GaN barriers. With the reduction in polarization fields, a strong blue-shift in photoluminescence is observed in agreement with theoretical expectation and simulations. By gracing incidence x-ray diffraction measurements, we demonstrate that partial relaxation already occurs for GaN/InGaN MQWs. As a consequence, the requirement of higher In-content layers for green light emission is in conflict with increasing strain leading to lattice relaxation.
In order to deposit semipolar GaN layers on foreign substrates while still starting the growth along the c-direction, we have etched trenches into r-plane and n-plane sapphire wafers. The GaN MOVPE growth then starts from c-plane-like sidewalls of these trenches, eventually leading to semipolar {1122} and {1011} surfaces with very good properties. GaInN quantum wells grown on such surfaces show very uniform properties on {1011} surfaces, but still reflect the stripe geometry on {1122} surfaces by a slightly larger In uptake at the stripe coalescence regions
Polarization‐field reduction in c‐plane InGaN multi‐quantum well (MQW) structures is achieved by pulsed‐flow growth of quaternary AlInGaN barriers using metalorganic vapor phase epitaxy (MOVPE). The pulsed‐flow growth allows for precise control of the quaternary composition at very low growth rate. In photoluminescence (PL) experiments a blue‐shift of the MQW emission wavelength is observed by successive adjustment of the AlInGaN barriers toward reduced polarization mismatch to the InGaN quantum wells. Accordingly, we find a decreasing radiative lifetime by time resolved cathodoluminescence (TRCL). The application of AlInGaN as barrier material instead of conventional GaN is limited by plastic relaxation. While for InGaN‐QWs emitting in the violet‐blue spectral region, fully polarization‐matched AlInGaN barriers can be realized, for long‐wavelength (green spectral region) severe lattice relaxation is observed leading to inferior optical properties as compared to binary GaN barriers.
Modulation of the electronic band profiles of wide band gap GaN semiconductors is achieved by the macromolecular dipole potentials exerted from ordered monolayers of synthetic, nonbiological aldehyde terminated helical peptides deposited on wet chemically oxidized GaN surfaces functionalized with aminosilanes. The selective coupling of either N‐ or C‐terminal to the amino‐terminated surface enables one to control the direction of the dipole moment, while the number of amino acids determines its magnitude. After confirming the formation of highly ordered peptide monolayers, the impact of macromolecular dipole potentials is quantified by electrochemical impedance spectroscopy. Moreover, the chronoamperometry measurements of ferrocene‐terminated peptides suggest that the transfer of electrons injected from ferrocene follows inelastic hopping, while the current responses of peptides with no ferrocene moieties are purely capacitive. Finally, the same functionalization steps are transferred to GaN/AlGaN/GaN high electron mobility transistor structures. Stable and quantitative modulation of the current–voltage characteristics of the 2D electron gas by the deposition of bioinspired peptides is a promising strategy for the macromolecular dipole engineering of GaN semiconductors.
We report on metalorganic vapor phase epitaxy of highly conductive germanium-doped GaN layers and their application for blue tunnel-junction light emitting diodes (TJ-LEDs). Using Ge as donor, carrier densities of up to 2 × 1020 cm−3 and low bulk resistivities down to 3 × 10−4 Ωcm are achieved. Under optimum growth conditions, no degradation of the crystalline quality is observed and layers exhibit high transparency making GaN:Ge very attractive as current spreading layer in light emitting devices. We have realized GaN-based TJ-LEDs by capping conventional InGaN LED structures with highly doped GaN:Ge. Such TJ-LEDs withstand operation currents up to 20 kA/cm2 in continuous and up to 60 kA/cm2 in pulsed operation conditions. Moreover, TJ-LEDs exhibit homogeneous electroluminescence and light output through the top surface that is increased by more than 60% as compared to conventional LEDs with transparent indium tin oxide contacts. The impact of the doping profile, carrier gas conditions, and acceptor activation by annealing for low-resistive TJ characteristics is discussed. Light output and current voltage characteristics of blue-light emitting devices with GaN-TJ are presented at low and high-current densities.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.