AlN Barrier Thickness Research Articles

Record peak current density of over 1500 kA/cm2 in highly scaled AlN/GaN double-barrier resonant tunneling diodes on free-standing GaN substrates

This work demonstrates high-performance AlN/GaN double-barrier resonant tunneling diodes (RTDs) with high peak current density grown by plasma-assisted molecular beam epitaxy on c-plane free-standing GaN substrates, featuring stable and repeatable negative differential resistance (NDR) characteristics at room temperature. By scaling down the barrier thickness of AlN barrier and the lateral mesa size of collector, the record peak current density of 1551 kA/cm2 is achieved along with a peak-to-valley current ratio (PVCR) of 1.24, which is attributed to the reduced resonant tunneling time under thinner AlN barrier and the suppressed external incoherent valley current by reducing the dislocation number contained in the RTD device with the smaller size of collector. By statistically analyzing the NDR performance of RTD devices with different thicknesses of AlN barrier, the average peak current density increases from 145.7 to 1215.1 kA/cm2, while the average PVCR decreases from 1.45 to 1.1, correspondingly, accompanying with a decreased peak voltage from 6.89 to 5.49 V, with downscaling the AlN barrier thickness from 1.5 to 1.25 nm. The peak current density obtained in this work is the highest value among all the reported nitride-based RTDs up until now while maintaining high PVCR value simultaneously, which illustrates that ultra-scaled RTD based on vertical quantum-well structure and lateral collector size is a valuable approach for the development of nitride-based RTDs with excellent NDR characteristics and reveals their great potential applications in high-frequency oscillation sources and high-speed switch circuits.

Investigation on effect of AlN barrier thickness and lateral scalability of Fe-doped recessed T-gate AlN/GaN/SiC HEMT with polarization-graded back barrier for future RF electronic applications

In this study, the influence of AlN barrier thickness (tb) on the RF & DC performances of 50 nm recessed T-gate Fe-doped AlN/GaN/SiC HEMT is investigated. The proposed HEMT incorporates a graded back-barrier (GBB) feature that raises the conduction band (CB) discontinuity at GaN/AlGaN junction to increase carrier confinement. With excellent electron confinement, the electrical characteristics for a 6 nm AlN barrier thickness show the highest transconductance of 468.9 mS/mm, a maximum ID of 1.546 A/mm, and a peak fT of 355 GHz at VDS = 2.5 V due to better epitaxial quality, and a reduction in parasitic channel generation due to GBB. When tb increases, the Vth decreases or becomes more negative i.e., moves left, which causes the ID to fall and degrades performance. This study also examines the influence of gate metals on the electrical performance of the proposed HEMT. According to the TCAD results, Pt-gate HEMTs had better RF performance. Furthermore, the simulation performed to examine the effects of scaling source-drain spacing (LSD) showed that the key to improving RF/DC performance is to reduce gate-to-drain and gate-to-source distances. It has been observed that the electron velocity of the HEMT is significantly influenced by the barrier thickness, LGS, and LGD. It comes as no surprise that this technology will be at the frontlines of future RF power applications.

Monolayer GaN excitonic deep ultraviolet light emitting diodes

We report on the molecular beam epitaxy and characterization of monolayer GaN embedded in N-polar AlN nanowire structures. Deep ultraviolet emission from 4.85 to 5.25 eV is measured by varying the AlN barrier thickness. Detailed optical measurements and direct correlation with first-principles calculations based on density functional and many-body perturbation theory suggest that charge carrier recombination occurs predominantly via excitons in the extremely confined monolayer GaN/AlN heterostructures, with exciton binding energy exceeding 200 meV. We have further demonstrated deep ultraviolet light-emitting diodes (LEDs) with the incorporation of single and double monolayer GaN, which operate at 238 and 270 nm, respectively. These unique deep ultraviolet LEDs exhibit highly stable emission and a small turn-on voltage around 5 V.

Tailoring Fast Directional Mass Transport of Nano-Confined Ag-Cu Alloys upon Heating: Effect of the AlN Barrier Thickness.

This study addresses the phase stability and atomic mobility of Ag-Cu40at.% nano-alloys confined by AlN in a nanomultilayered configuration during thermal treatment. To this end, nanomultilayers (NMLs) with a fixed Ag-Cu40at.% nanolayer thickness of 8 nm and a AlN barrier nanolayer with variable thickness of 4, 8, or 10 nm were deposited by magnetron sputtering on sapphire substrates and subsequently isothermally annealed for 5 or 20 min in air in the range of 200-500 °C. The microstructure of the as-deposited and heat-treated NMLs was analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Annealing of the thicker AlN barrier layers at T > 300 °C leads to the formation of an interconnected network of line-shaped Cu(O) protrusions on the annealed NML surface. The well-defined outflow pattern of Cu(O) originates from the thermally induced surface cracking of the top AlN barriers with subsequent fast mass transport of Cu along the Cu/AlN interfaces toward the surface cracks. The thinnest (i.e., 4 nm thick) AlN barrier layers exhibit a relatively open grain boundary structure and act as nanoporous membranes upon heating, resulting in the formation of a dense and homogenous distribution of Cu(O) and Ag droplets on the NML surface. These findings demonstrate that the microstructure (i.e., layer thicknesses, interface coherency, and texture) of hybrid nanolaminates can be tuned to provide defined pathways for fast, directional transport of the confined metal to the surface at relatively low temperatures, which might open new routes for low-temperature bonding of micro- and nano-scaled systems.

Fabrication and characterization of high-quality all-NbN Josephson tunnel junctions for superconductive quantum interference devices

We have developed high-quality NbN/AlN/NbN Josephson tunnel junctions for direct current-superconducting quantum interference devices (DC-SQUIDs). The junctions were fabricated with epitaxial NbN/AlN/NbN trilayers on single crystal MgO (100) substrates and showed excellent tunneling properties with a quality factor Rsg/Rn above 20, a large gap voltage about 5.7mV, and a large IcRn product above 3.8mV. The critical current density Jc of the NbN/AlN/NbN junctions is controlled by the AlN barrier thickness dAlN in the junctions. The performance of DC-SQUIDs comprising NbN junctions was measured at 4.2K, and the white noise in a flux-locked-loop mode was 5µΦ0/Hz1/2, thus demonstrating the feasibility of all-NbN junctions for SQUID development.

Interface DOS dependent analytical model development for DC characteristics of normally-off AlN/GaN MOSHEMT

In this paper a charge controlled analytical model is developed to predict quantum capacitance, threshold voltage and drain current for normally-off AlN/GaN Metal Oxide Semiconductor High Electron Mobility Transistor (MOSHEMT). The oxide/semiconductor interface Density of State (DOS) dependent model for two dimensional electron gas (2DEG) density is obtained by demonstrating necessary energy band diagrams. Quantum capacitance in the channel and threshold voltage is obtained by using different boundary conditions. By using these expressions the drain currents in both linear and saturation mode are derived. It is interesting to note that a positive threshold voltage necessary for normally-off operation can be obtained by decreasing the AlN barrier thickness below a critical value. The predicted DC characteristics are in good agreement with the experimental results, thus it confirms the validity of the proposed model.

Electron density and currents of AlN/GaN high electron mobility transistors with thin GaN/AlN buffer layer

AlN/GaN high electron mobility transistor (HEMT) structures with thin GaN/AlN buffer layer have been analyzed theoretically and experimentally, and the effects of the AlN barrier and GaN buffer layer thicknesses on two-dimensional electron gas (2DEG) density and transport properties have been evaluated. HEMT structures consisting of [300 nm GaN/ 200 nm AlN] buffer layer on sapphire were grown by plasma-assisted molecular beam epitaxy and exhibited a remarkable agreement with the theoretical calculations, suggesting a negligible influence of the crystalline defects that increase near the heteroepitaxial interface. The 2DEG density varied from 6.8 × 1012 to 2.1 × 1013 cm−2 as the AlN barrier thickness increased from 2.2 to 4.5 nm, while a 4.5 nm AlN barrier would result to 3.1 × 1013 cm−2 on a GaN buffer layer. The 3.0 nm AlN barrier structure exhibited the highest 2DEG mobility of 900 cm2/Vs for a density of 1.3 × 1013 cm−2. The results were also confirmed by the performance of 1 μm gate-length transistors. The scaling of AlN barrier thickness from 1.5 nm to 4.5 nm could modify the drain-source saturation current, for zero gate-source voltage, from zero (normally off condition) to 0.63 A/mm. The maximum drain-source current was 1.1 A/mm for AlN barrier thickness of 3.0 nm and 3.7 nm, and the maximum extrinsic transconductance was 320 mS/mm for 3.0 nm AlN barrier.

Impact of barrier thickness on transistor performance in AlN/GaN high electron mobility transistors grown on free-standing GaN substrates

A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–6 nm have been grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. High electron mobility transistors (HEMTs) were fabricated from the set in order to assess the impact of barrier thickness and homo-epitaxial growth on transistor performance. Room temperature Hall characteristics revealed mobility of 1700 cm2/V s and sheet resistance of 130 Ω/□ for a 3 nm thick barrier, ranking amongst the lowest room-temperature sheet resistance values reported for a polarization-doped single heterostructure in the III-Nitride family. DC and small signal HEMT electrical characteristics from submicron gate length HEMTs further elucidated the effect of the AlN barrier thickness on device performance.

The influence of the AlN barrier thickness on the polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

The electron mobility scattering mechanisms in AlN/GaN heterostuctures with 3 nm and 6 nm AlN barrier thicknesses were investigated by temperature-dependent Hall measurements. The effect of interface roughness (IFR) scattering on the electron mobility was found to be enhanced by increasing AlN barrier thickness. Moreover, using the measured capacitance-voltage and current-voltage characteristics of the fabricated heterostructure field-effect transistors (HFETs) with different Schottky areas on the two heterostuctures, the variations of electron mobility with different gate biases were investigated. Due to enhanced IFR scattering, the influence of polarization Coulomb field (PCF) scattering on electron mobility was found to decrease with increasing AlN barrier layer thickness. However, the PCF scattering remained an important scattering mechanism in the AlN/GaN HFETs.

Tunnel conductance in GaN:Mn/AlN/GaN:Mn (0001) junction from first-principles calculations

The tunnel conductance of diluted ferromagnetic semiconductor tunnel junction GaN:Mn/AlN/GaN:Mn is calculated based on the first-principles calculations. The dependence of the conductance on AlN barrier thickness, doping site, and doping concentration of Mn is studied. The contributions from coherent tunneling and resonant tunneling are discussed by combining the complex band structure of AlN barrier layer and the interface states on both sides of the barrier.

Analyses of 2-DEG characteristics in GaN HEMT with AlN/GaN super-lattice as barrier layer grown by MOCVD

GaN-based high-electron mobility transistors (HEMTs) with AlN/GaN super-lattices (SLs) (4 to 10 periods) as barriers were prepared on (0001) sapphire substrates. An innovative method of calculating the concentration of two-dimensional electron gas (2-DEG) was brought up when AlN/GaN SLs were used as barriers. With this method, the energy band structure of AlN/GaN SLs was analyzed, and it was found that the concentration of 2-DEG is related to the thickness of AlN barrier and the thickness of the period; however, it is independent of the total thickness of the AlN/GaN SLs. In addition, we consider that the sheet carrier concentration in every SL period is equivalent and the 2-DEG concentration measured by Hall effect is the average value in one SL period. The calculation result fitted well with the experimental data. So, we proposed that our method can be conveniently applied to calculate the 2-DEG concentration of HEMT with the AlN/GaN SL barrier.

Electronic transport through GaN/AlN single barriers: Effect of polarisation and dislocations

In this work, we study electronic and conduction properties of a single AlN barrier (thickness from 0.5 to 5nm) embedded in GaN. The electronic structure was simulated using a self-consistent Schrödinger–Poisson solver, and verified by photoluminescence (PL) measurements. For 0.5- and 1-nm-thick barriers, we can identify the PL line related to recombination from the first electron confined level at the bottom interface of the barrier to the first hole confined level at the top interface. The PL evolution with bias reveals that most of the applied voltage drops in the depleted cap layer. Carrier transport was studied using conductive atomic force microscopy (C-AFM). Reference GaN layers display a rectifying behaviour between the C-AFM tip and the GaN surface. In samples with a barrier thickness from 1 to 3nm, blockage by the barrier results in negligible current under direct bias, except for hot spots at dislocations (density ∼1×107cm−2). Finally, in samples with 5-nm-thick AlN barriers, a high-current density appears at reverse bias, which assigned to defects induced by AlN relaxation, as verified by transmission electron microscopy.

Bi‐stable behaviour in GaN‐based resonant tunnelling diode structures

AbstractWe have investigated the vertical electronic transport in AlN/GaN/AlN double barrier structures grown by plasma‐assisted molecular‐beam epitaxy, with AlN barrier thickness of 0.5 nm and a GaN well thickness varying from 0.5 nm to 2 nm. Two different current levels were observed in different devices in all the samples, the highest one being attributed to leakage through dislocations, and the lower one to tunnelling transport through the barriers. In the sample with a well thickness of 0.5 nm, we observed a bi‐stable behaviour, with an apparent negative differential resistance in the current‐voltage characteristic when scanning the voltage from negative to positive bias. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-dimensional electron gases induced by polarization charges in AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy

We discuss the growth and transport properties of two-dimensional electron gas confined at the AlN/GaN heterointerface grown by plasma-assisted molecular-beam epitaxy. The sheet carrier density was found to be highly dependent on the barrier thickness of AlN grown on a doped or undoped GaN layer. The carrier sheet density monotonously increased from 0.8×1012 to 1.1×1013 cm−2 as the AlN barrier thickness on a semi-insulating GaN layer increased from 15 to 25 Å due to spontaneous and piezoelectric polarization. An AlN barrier of 35 Å in thickness grown on n-GaN gave the highest sheet carrier density, which was 4.3×1013 cm−2. In thin AlN barrier layers, the sheet carrier density was low due to surface depletion.

Fabrication of Josephson junctions with as-grown MgB/sub 2/ thin films

We report on the fabrication of as-grown MgB/sub 2/ thin films and MgB/sub 2//AlN/NbN thin film heterostructures having as-grown MgB/sub 2/ thin films. The MgB/sub 2/ thin films deposited by using a carousel-type sputtering system at substrate temperatures T/sub s/ of 252/spl deg/C typically showed a critical temperature of 28 K. The results of XRD measurements of the thin films indicated a c-axis orientation. The surface morphology of the films fabricated at T/sub s/=252/spl deg/C was determined to be very flat and smooth from SEM images. The MgB/sub 2//AlN/NbN trilayer was continuously deposited on sapphire [001] substrates in a single vacuum run. The AlN-barrier and NbN-counter layers were deposited by using DC-reactive magnetron sputtering at ambient substrate temperatures. The junctions were fabricated by using conventional photolithography, reactive ion etching, and electron cyclotron resonance etching techniques. The junctions demonstrated excellent quasiparticle tunneling characteristics having ideal dependence of the normal resistance on the junction area and on the AlN-barrier thickness. Also, the DC magnetic field and temperature dependences of the supercurrent I/sub c/ were measured to investigate the Josephson tunneling behavior in the MgB/sub 2//AlN/NbN junctions. The junction I/sub c/-H curve showed the ideal Fraunhofer pattern.

Josephson tunneling properties in MgB2/AlN/NbN tunnel junctions

We report on the fabrication of Josephson tunnel junctions with as-grown MgB2 thin films and on the Josephson tunneling properties of the junctions. MgB2/AlN/NbN trilayers were continuously deposited on sapphire (0001) substrates in the same vacuum run. The as-grown MgB2 thin films were fabricated using the conventional sputtering method at the low-substrate temperature of 250 °C, and the AlN tunnel barrier and the NbN counter electrode were deposited by reactive sputtering at the ambient substrate temperature. The Tc of the MgB2 thin films was 28 K. The junctions demonstrated clear Josephson tunneling characteristics with evident supercurrents, gap voltages, and small subgap leakage currents. The normal resistance of these junctions had an ideal dependence on the junction areas and on the AlN-barrier thickness. The temperature dependence of the gap voltages and the external dc magnetic-field dependence of the supercurrents were measured to investigate the Josephson tunneling behaviors in the MgB2/AlN/NbN junctions. The junctions showed BCS-like temperature dependence of the gap voltage and ideal Fraunhofer patterns.

Intersubband Absorption at? ? 1.2-1.6 ?m in GaN/AlN Multiple Quantum Wells Grown by rf-Plasma Molecular Beam Epitaxy

In order to investigate the intersubband transition (ISBT), GaN/AIN multiple quantum wells with 90 GaN wells were prepared directly on (0001) sapphire substrates by rf-plasma molecular beam epitaxy. The thickness of GaN well and AlN barrier were 4-8 monolayers (ML) and ∼11 ML, respectively. One ML corresponds to around 2.6 A. ISBT absorptions at 1.15-1.55 μm in wavelength, which covers the whole communication wavelength, were measured, and 1.15 μm is the shortest record in ISBT-wavelength.

2DEG Characteristics of AlN/GaN Heterointerface on Sapphire Substrates Grown by Plasma-Assisted MBE

We report on the growth and transport properties of a two-dimensional electron gas (2DEG) confined at the AlN/GaN heterointerface grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrate nucleated with an AlN buffer. The sheet carrier density of the 2DEG formed at the interface was characterized with respect to the AlN barrier thickness on doped and undoped GaN channels. The strong variation in the sheet carrier density was observed from the two types of grown structures. As the AlN barrier thickness grown on semi-insulating GaN increases from 15 to 25 A, the carrier sheet density monotonously increased from 0.8 x 10 12 to 1.1 × 10 13 cm -2 due to the existence of spontaneous and piezoelectric polarization. Further, the channel conductivity was increased by inserting a thin n-GaN layer on the SI-GaN template, which favoured to enhance the sheet carrier concentration to 4.3 × 10 13 cm -2 , a 35 A thick AlN barrier layer was grown. The surface depletion could be responsible for the lowest sheet carrier density in the case of thin AlN barrier.

High-resolution electron microscopy study of tunnelling junctions with AlN and AlON barriers

Spin-dependent tunnel junctions with AlON and AlN barriers have been investigated using high-resolution electron microscopy. Plasma gas composition, nitridation time, and deposited Al thickness were studied independently. Plasma-nitrided Al produces thinner tunnel barriers than those obtained with an oxygen or oxygen+nitrogen plasma, for the same plasma exposure time. Experiments to study an increase in plasma nitridation time for a constant Al deposition thickness showed that overexposure to the plasma resulted in a thinner barrier—possibly due to the competing processes of nitride formation and slight etching of the barrier. The AlN barrier thickness increases monotonically with deposited Al thickness for a constant nitridation time. Electron energy loss spectroscopy compositional mapping has confirmed that nitrogen is present in the barrier layer. The roughness between the barrier layer and the two ferromagnetic layers has also been estimated.

1/f noise in high current density NbN/AlN/NbN tunnel junctions

Low frequency noise characteristics of epitaxial NbN/AlN/NbN tunnel junctions have been investigated. For all of our junctions with different current densities, the voltage noise power spectrum S/sub v/(f) showed a frequency dependence of 1/f type. The magnitude of the S/sub v/(f) exhibited two distinct types of dependency on the current density and AlN barrier thickness. We consider that this may result from a difference in the crystal structures of the AlN barrier by consulting with the barrier thickness dependence of tunnel barrier heights. We also estimated the 1/f noise parameter /spl eta/ using the Rogers and Buhrman's empirical theory for the S/sub v/(f), and investigated the relationship between the /spl eta/ and the current density J/sub c/ and the tunnel barrier thickness d/sub AlN/. The tunnel barrier was characterized by investigating the /spl eta/-d/sub AlN/ relation. It was found that the /spl eta/-J/sub c/ and /spl eta/-d/sub AlN/ relation in our high current density junctions, i.e, in the epitaxial tunnel junctions, are different from nonepitaxial tunnel junctions.