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A wet-etching technique based on a mixture of hydrochloric (HCl) and nitric (HNO3) acids is introduced, demonstrating exceptional 42:1 selectivity for etching N-polar GaN over Al0.24Ga0.76N. In the absence of an AlGaN etch stop layer, the etchant primarily targets N-polar unintentionally doped (UID) GaN, indicating its potential as a suitable replacement for selective dry etches in the fabrication of GaN high-electron-mobility transistors (HEMTs). The efficacy and selectivity of this etchant were confirmed through its application to a gate recess module of a deep-recess HEMT, where, despite a 228% over-etch, the 2.6 nm AlGaN etch stop layer remained intact. We also evaluated the proposed method for the selective etching of the GaN cap in the n+ regrowth process, achieving a contact resistance matching that of a BCl3/SF6 ICP process. These findings underscore the applicability and versatility of the etchant in both the electronic and photonic domains and are particularly applicable to the development of N-polar deep-recess HEMTs. [ABSTRACT FROM AUTHOR]

The authors report a novel structure to improve the high‐frequency performance of nitrogen‐polar GaN deep‐recess high electron mobility transistors (HEMTs) wherein a timed plasma etch was used to tailor the profile of the gate dielectric. A favourable trade‐off between gate fringing capacitance and transconductance was established, resulting in an improved current gain cutoff frequency times gate length (ft·LG) figure of merit compared to standard Schottky‐gate and metal‐insulator‐semiconductor (MIS) HEMTs. The etched gate dielectric MISHEMT demonstrated a maximum ft·LG of 15.0 GHz·µm for an LG of 100 nm. [ABSTRACT FROM AUTHOR]

We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 1019 cm−3 as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 1019 cm−3 was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.

The bulk and interfacial properties of aluminum silicon oxide (AlSiO) on N-polar GaN were investigated systematically employing capacitance-voltage (C-V) methods on metal-oxide-semiconductor capacitors using a thickness series of the AlSiO dielectric. The fixed charge density, electron slow trap density, and electron fast trap density located near the interface were extracted to be -1.5 × 1012 cm-2, 3.7 × 1011 cm-2, and 1.9 × 1011 cm-2, respectively. Using ultraviolet (UV) assisted C-V methods, an average interface state density of ~4.4 × 1011 cm-2 eV-1 and a hole trap concentration in bulk AlSiO of ~8.4 × 1018 cm-3 were measured. The negative fixed interface charge makes it favorable to achieve a normally off GaN transistor. The analysis presented in this paper provides a systematic and quantitative model to study the properties of oxide dielectrics on wide bandgap (WBG) semiconductors, which can promote the development of metal-oxide-semiconductor-based WBG semiconductor devices. [ABSTRACT FROM AUTHOR]

The electrical properties of InN give it potential for applications in III-nitride electronic devices, and the use of lower-dimensional epitaxial structures could mitigate issues with the high lattice mismatch of InN to GaN (10%). N-polar MOCVD growth of InN was performed to explore the growth parameter space of the horizontal one-dimensional InN quantum wire-like structures on miscut substrates. The InN growth temperature, InN thickness, and NH3 flow during growth were varied to determine optimal quantum wire segment growth conditions. Quantum wire segment formation was observed through AFM images for N-polar InN samples with a low growth temperature of 540 °C and 1–2 nm of InN. Below 1 nm of InN, quantum dashes formed, and 2-D layers were formed above 2 nm of InN. One-dimensional anisotropy of the electrical conduction of N-polar InN wire-like samples was observed through TLM measurements. The sheet resistances of wire-like samples varied from 10–26 kΩ/□ in the longitudinal direction of the wire segments. The high sheet resistances were attributed to the close proximity of the treading dislocations at the InN/GaN interface and might be lowered by reducing the lattice mismatch of InN wire-like structures with the substrate using high lattice constant base layers such as relaxed InGaN. [ABSTRACT FROM AUTHOR]

The electrical performances of III-nitride blue micro-light-emitting diodes (µLEDs) with different tunnel junction (TJ) epitaxial architectures grown by metalorganic chemical vapor deposition are investigated. A new TJ structure that employs AlGaN is introduced. The current density–voltage characteristic is improved by incorporating AlGaN layer above the n-side of the TJ layer, and the effects of the AlGaN/GaN superlattices is examined. Based upon the data from band diagram simulation, net positive polarization charge is formed at the AlGaN/GaN interface, which results in a reduction in tunneling distance and increase in tunneling probability. Moreover, similar electrical improvement is observed in various device dimensions and is independent of operating current density, suggesting that AlGaN/GaN biaxial tensile strain or current spreading is not the main contribution for the improvement. Finally, the effects on the efficiency performances are determined. While the maximum external quantum efficiency of the TJ devices remains identical, the wall-plug efficiency of µLEDs is enhanced significantly by the proposed AlGaN-enhanced TJ design. This work reveals the possibility of energy efficient TJ contact with high transparency in the visible wavelength range. [ABSTRACT FROM AUTHOR]

Nitride based light emitting diodes (LEDs) have demonstrated very high efficiency in the blue and green wavelength ranges. For full color LED based displays, efficient and small sized devices emitting in the three primary colors - red, green and blue are required. While phosphide based red LEDs are highly efficient, high surface recombination velocities limit their scaling to small dimensions. Nitride emitters can be scaled to smaller sizes, however for longer emission wavelength such as yellow, orange, and red, the indium content in the active region of the devices needs to be >30%. Due to the large lattice mismatch between GaN and InN, the needed high indium content introduces a large strain in the structure, often resulting in poor material quality. Recently, a novel technique to mitigate this lattice mismatch induced strain was demonstrated with the use of compliant porous GaN underlayer based substrates. In our previous reports, we have discussed the surface morphology, strain state evolution and device demonstrations using these substrates, however a detailed discussion regarding the pore cross-sections and dislocation propagation for each generation of these porous GaN substrates was yet to be reported. In this work, we will discuss the cross-sectional transmission electron microscopy (TEM) results of these substrates across multiple stages of the substrate technology development. [ABSTRACT FROM AUTHOR]

Fully relaxed, crack free, smooth AlxGa1−xN layers with up to 50% Al composition were demonstrated on pseudo-substrates composed of dense arrays of 10 × 10 µm2 compliant porous GaN-on-porous-GaN tiles. The AlGaN layers were grown in steps for a total of 1.3 µm. The growth conditions necessary to demonstrate high quality films at higher Al compositions also suppressed any sidewall growth. [ABSTRACT FROM AUTHOR]

The shortcomings with acceptors in p-type III-nitride semiconductors have resulted in not many efforts being presented on III-nitride based p-channel electronic devices (here, field effect transistors (FETs)). The polarization effects in III-nitride superlattices (SLs) lead to the periodic oscillation of the energy bands, exhibiting enhanced ionization of the deep acceptors (Mg in this study), and hence their use in III-nitride semiconductor-based light-emitting diodes (LEDs) and p-channel FETs is beneficial. This study experimentally demonstrates the presence of acceptor-like traps at the positive polarization interfaces acting as the primary source of holes in Ga-polar p-type uniformly doped (AlGaN/AlN)/GaN SLs with limited Mg doping. The observed concentration of holes exceeding that of the dopants incorporated into the samples during growth can be attributed to the ionization of acceptor-like traps, located at 0.8 eV above the valence band of GaN, at positive polarization interfaces. All samples were grown using the metal organic vapor phase epitaxy (MOVPE) technique, and the materials' characterization was carried out using X-ray diffraction and Hall effect measurements. The hole concentrations experimentally measured are juxtaposed with the calculated value of hole concentrations from FETIS®, and the measured trends in mobility are explained using the amplitude of separation of the two-dimensional hole gas in the systems from the positive polarization interfaces. [ABSTRACT FROM AUTHOR]

The strain dependence of the free-exciton resonance energies in AlN epilayers and the values are analyzed by using an appropriated Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. The strain dependence of the valence band ordering, optical transition probability and free-exciton binding energy are analyzed.

A novel mask design consisting of interdigitated multipixel arrays for the fabrication of high power light-emitting diodes (LED) with very low series resistance, reduced current crowding, and improving heat sinking is reported. Results suggest that the optical performance of high-power LEDs can be significantly improved by minimizing the series resistance and self-heating by adopting an interdigitated multipixel geometry.

The electrical and structural characterization of Mg-doped Al-rich p-type [Al.sub.0.69][Ga.sub.0.31]N alloys on SiC substrate grown by metal organic chemical vapor deposition is presented. Transmission electron microscopy has shown the generation of vertical spike-like defects with increasing Mg doping and the increase in resistivity with the increase in Mg doping.

A study to reduce the access resistance in AlGaN/GaN high electron mobility transistors using double-channel structures was conducted. From the results the carrier densities as high as 2.9 x 10(super 13) cm(super -2) and mobilities in the 1300 cm(super 2)/ V.s range were obtained in the access region.

This paper investigates the effects of the oxygen precursor flow supplied during metalorganic chemical vapor deposition (MOCVD) of Al2O3 films on the forward bias behavior of Al2O3/GaN metal-oxide-semiconductor capacitors. The low oxygen flow (100 sccm) delivered during the in situ growth of Al2O3 on GaN resulted in films that exhibited a stable capacitance under forward stress, a lower density of stress-generated negative fixed charges, and a higher dielectric breakdown strength compared to Al2O3 films grown under high oxygen flow (480 sccm). The low oxygen grown Al2O3 dielectrics exhibited lower gate current transients in stress/recovery measurements, providing evidence of a reduced density of trap states near the GaN conduction band and an enhanced robustness under accumulated gate stress. This work reveals oxygen flow variance in MOCVD to be a strategy for controlling the dielectric properties and performance. [ABSTRACT FROM AUTHOR]

InAlN dipole diodes were developed and fabricated on both (0001) Ga-Face and (000...) N-face oriented GaN on sapphire templates by molecular beam epitaxy. The orientation and direction of the InAlN polarization dipole are functions of the substrate orientation and composition, respectively. Special consideration was taken to minimize growth differences and impurity uptake during growth on these orientations of opposite polarity. Comparison of devices on similarly grown structures with In compositions in excess of 50% reveals that dipole diodes shows poorer forward bias performance and exhibited an increase in reverse bias leakage, regardless of orientation. Similarly, (0001) Ga-face oriented InAlN at a lowered 40% In composition had poor device characteristics, namely, the absence of expected exponential turn on in forward bias. By contrast, at In compositions close to 40%, (000...) N-face oriented InAlN devices had excellent performance, with over five orders of magnitude of rectification and extracted barrier heights of 0.53- 0.62 eV; these values are in close agreement with simulation. Extracted ideality factors ranging from 1.08 to 1.38 on these devices are further evidence of their optimal performance. Further discussion focuses on the growth and orientation conditions that may lead to this discrepancy yet emphasizes that with proper design and growth strategy, the rectifying dipole diodes can be achieved with InAlN nitride dipole layers. [ABSTRACT FROM AUTHOR]

Proton irradiation decreases the doping concentration and increases the ideality factor and series resistance, but has very little effect on the Schottky barrier height in n-Gallium nitride Schottky diodes. 1.0-MeV protons cause greater degradation than 1.8-MeV protons because of their higher nonionizing energy loss. The displacement damage recovers during annealing. Comparison between Schottky diodes and high electron-mobility transistors suggests that the degradation in both types of devices is predominantly due to carrier removal and mobility degradation caused by radiation-induced defect centers in the crystal lattice, with interface disorder playing a relatively insignificant part in overall device degradation. Index Terms--Gallium (Ga) alloys, Schottky diodes, proton radiation effects.

The decrease of transconductance g(sub m) and current gain cutoff frequency f(sub T) at high drain current levels in AlGaN/GaN high-electron mobility transistors (HEMTs) severely limit the linearity and power performance of the devices at high frequencies. The increase of differential source access resistance r(sub s), with drain current is shown to play an important role in the fall of g(sub m) and f(sub T).

The effect of noise figure of different AlGaN/GaN high electron-mobility transistor (HEMT) epitaxy structures is reported. The addition of a thin AIN layer between the barrier and channel gives better performance at biasings other than the best for minimum noise figure. However, varying Al composition in the HEMT barrier does not change the noise performance, contrary to a 2003 study by Lu et al. The measurements are checked with both the Pospieszalski and van der Ziel (Pucel) models. The models are used on six different samples, helping to reinforce the measurements and showing the strengths and weaknesses of each. Index Terms--AlGaN, GaN, high electron-mobility transistor (HEMT), noise figure, Pospieszalski, Pucel, van der Ziei.

1.8 MeV proton radiation-induced degradation in high electron mobility transistors with Si-doped [Al.sub.x][Ga.sub.1] - x N and thick GaN cap layers is studied up to a fluence of 1 x [10.sup.15] protons/[cm.sup.2]. The thick GaN cap layer reduces sheet charge modulation induced by the surface states, as it electrostatically separates the active device layers from the surface, thereby enhancing the device performance. The devices exhibit good tolerance up to [10.sup.14] protons/[cm.sup.2], with displacement damage being the primary degradation mechanism. Charged defect centers introduced by proton radiation in the active device layers degrade carrier mobility and sheet carrier density. Proton radiation alters the barrier height at the Schottky gate and increases the resistance of the thin film structure. Index Terms--Gallium alloys, modulation-doped field effect transistors (MODFETs), proton radiation effects.

The investigation of drain current dispersion effects in AlGaN-GaN HEMTs by means of pulsed, transient, and small signal measurements is done. The simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge.

The effects of proton irradiation at various energies are reported for AlGaN/GaN high electron mobility transistors (HEMTs). The devices exhibit little degradation when irradiated with 15-, 40-, and 105-MeV protons at fluences up to [10.sup.13] [cm.sup.-2], and the damage completely recovers after annealing at room temperature. For 1.8-MeV proton irradiation, the drain saturation current decreases 10.6% and the maximum transconductance decreases 6.1% at a fluence of [10.sup.12] [cm.sup.-2]. The greater degradation measured at the lowest proton energy considered here is caused by the much larger nonionizing energy loss of the 1.8-MeV protons. Index Terms--Displacement damage, GaN, high electron mobility transistors, nonioizing energy loss (NIEL), proton radiation effects.

The spontaneous emission mechanisms in stain-free Al (sub x) Gad (sub 1-x) N films were studied in this article. The results showed that they exhibited a band-gap bowing with a parameter of approximately minus 0.82 eV.

The degradation of AlGaN/AlN/GaN high electron mobility transistors due to 1.8-MeV proton irradiation was measured at fluences up to 3 x [10.sup.15] [cm.sup.-2]. The devices have much higher mobility than AlGaN/GaN devices, but they possess similarly high radiation tolerance, exhibiting little degradation at fluences up to 1 x [10.sup.14] [cm.sup.-2]. Decreased sheet carrier mobility due to increased carrier scattering and decreased sheet carrier density due to carrier removal are the primary damage mechanisms. The device degradation is observed as a decrease in the maximum transconductance, an increase in the threshold voltage, and a decrease in the drain saturation current. Index Terms--Gallium alloys, MODFETs, proton radiation effects.

A 36-dBm high-linearity single-ended common-source class-B monolithic-microwave integrated-circuit power amplifier is reported in GaN high electron-mobility transistor technology. We also describe the design and simulation of highly linear and highly efficient common-source and common-drain class-B power amplifiers. Single-ended class-B amplifiers with bandpass filtering have equivalent efficiency and linearity to push-pull configurations. The common-source class-B circuit demonstrates high linearity, greater than 35 dBc of third-order intermodulation (IM3) suppression and high power-added efficiency (PAE) of 34%. Simulations of common-drain class-B designs predict a PAE of 54% with a superior IM3 suppression of more than 45 dBc over a wider range of bias due to the strong series-series negative feedback offered by the load resistance. Index Terms--GaN high electron-mobility transistor (HEMT), high linearity, intermodulation suppression, monolithic-microwave integrated-circuit (MMIC) power amplifiers.

DC current-switching and power-switching transients of various GaN-based FET structures are investigated. Two different characteristics are compared, namely, thermal and electronic transients. While the thermal transients are mainly reflected in changes in channel carrier mobility, the electronic transients are dominated by charge instabilities caused by the polar nature of the material. The discussion of the electronic transients focuses, therefore, on instabilities caused by polarization-induced image charges. Three structures are discussed, which are: 1) a conventional AlGaN/GaN heterostructure FET; 2) an InGaN-channel FET; and 3) an AlGaN/GaN double-barrier structure. In structures 2) and 3), field-induced image charges are substituted by doping impurities, eliminating this source of related instability. This is indeed observed. Index Terms--GaN, heterostructure field-effect transistors (HFETs), switching experiments, transient analysis.

This study experimentally shows the existence of acceptor traps at positive polarization interfaces (PPIs) acting as the source of holes in N-polar p-type modulation doped GaN/(AlN/AlGaN) superlattices (SLs) with low Mg doping. The observation of measured hole concentration higher than the dopants (here, Mg) put in the samples during growth can be explained by the ionization of acceptor traps, which are placed 0.8 eV above the valence band of GaN at the PPI. All samples were epitaxially grown using metal organic chemical vapor deposition and were characterized using x-ray diffraction and room-temperature Hall measurements. The measured hole concentrations are compared against calculated values from STR FETIS®, and the measured mobility trends are explained using the separation of the positive polarization interfaces from the two-dimensional hole gas in the systems, strengthening the hypothesis. A second study is also presented where acceptor traps were ionized in a p-type modulation doped GaN/AlGaN SL without an AlN layer—by keeping the superlattice period thickness constant and increasing the AlGaN composition. Following the hypothesis of the existence of an acceptor trap, these experimental studies demonstrate the charge-balance in systems that show p-type behavior without sufficient doping. [ABSTRACT FROM AUTHOR]

A two-dimensional sheet charge and mobility in GaN/AlGaN heterostructure field effect transistors is investigated. Experimental results on samples grown by metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are reported, whereas, theoretical studies are performed to examine the spontaneous polarization and piezoelectric effect to control the sheet charge density.

There are three possible ways of reducing the charge density (ns) in the N-polar high electron mobility transistors (HEMT) structures, by decreasing the channel thickness, applying reverse gate bias, or modifying the back-barrier. Understanding the behavior of 2DEG mobility as a function of ns is essential to design high performance HEMT devices. Experimental data show that in the N-polar HEMT structures, the 2DEG mobility reduces as the ns decreases by applying reverse gate bias or decreasing channel thickness, whereas in the Ga-polar HEMT structures, the 2DEG mobility increases as the ns in the channel decreases by applying reverse gate bias. In this paper, the 2DEG mobility as a function of ns is calculated in N-polar HEMTs for three different aforementioned cases, and is compared to that in the Ga-polar HEMT structures. It is shown that the conventional scattering mechanisms cannot explain these different behaviors. Two new scattering mechanisms, such as scattering from charged interface states and surface state dipoles (SSD), are introduced. It is revealed that in N-polar HEMT structures, reducing ns by applying reverse gate bias or decreasing channel thickness moves the charge centroid closer to the AlGaN-GaN interface. A combination of lower charge density (less screening of the scattering potential) and smaller distance between charge centroid and charged states at the interface leads to a severe mobility degradation in these cases. In contrast, reducing ns by modifying the back-barrier (decreasing back-barrier doping and/or decreasing AlGaN composition) in N-polar HEMT structures moves the charge centroid away from the interface. This behavior is similar to that in the Ga-polar HEMT structures. Therefore, in the last two mentioned cases, the 2DEG mobility first increases slightly as the ns decreases, and decreases slightly at very low charge densities. It is also shown that SSDs have large impact on the 2DEG mobility only in the N-polar (Ga-polar) HEMTs with thin channels (barriers). [ABSTRACT FROM AUTHOR]