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III-V interface with high-k oxide, especially HfO2, is crucial to the development of high mobility microelectronic devices. In this work, we systematically investigated the wurtzite GaN/cubic HfO2 interface based on the first-principles calculations with density functional theory in terms of the geometric structure and electronic properties. In order to mimic the high-k growth conditions, the interfacial oxygen contents in the slab interface models varied to study the interface stability and the relevant electronic structures. It is found that the oxygen rich interface, i.e., oxygen content of 83.3% (O5), shows the most stability in a large range of the oxygen chemical potential from 0 eV to −4.34 eV. Through the calculations of local density of states and Bader charge analysis, we noticed that increasing the interfacial oxygen content leads to the increase of the valence band offset (VBO) and the decrease of the conduction band offset (CBO), respectively. More importantly, interface O5 displays a promising VBO (0.86 eV) and CBO (1.34 eV), which meets the industrial requirements to confine the carrier in the III-V channel materials. Furthermore, no interfacial gap states are observed in interface O5, indicating that O5 is free of Fermi level pinning. This theoretical exploration suggests that varying oxygen content at the interface could result in the optimal interface for the applications of high mobility electronic devices. [ABSTRACT FROM AUTHOR]

Due to high transmittance and high mobility, graphene is one of the promising candidates for a current spreading layer, which is crucial to light emitting diode (LED) performance. In this paper, improved AlGaInP LED performance was reported after graphene was applied on the GaP surface. Due to its lowered work function difference than with the GaN material, the electrical properties remain the same without additional voltage bias. The light output power is enhanced by about 40% under the current injection of 5 mA at room temperature, which was confirmed by the light emission profile analysis in this study. Such results indicate that raphene is a promising candidate as a current spreading layer under low current injection. [ABSTRACT FROM AUTHOR]

The photoelectrochemical responses of GaN, GaNP, and InGaN nanowire ensembles are investigated by the electrical bias dependent photoluminescence, photocurrent, and spin trapping experiments. The results are explained in the frame of the surface band bending model. The model is sufficient for InGaN nanowires, but for GaN nanowires the electrochemical etching processes in the anodic regime have to be considered additionally. These processes lead to oxygen rich surface (GaxOy) conditions as evident from energy dispersive X-ray fluorescence. For the GaNP nanowires, a bias dependence of the carrier transfer to the electrolyte is not reflected in the photoluminescence response, which is tentatively ascribed to a different origin of radiative recombination in this material as compared to (In)GaN. The corresponding consequences for the applications of the materials for water splitting or pH-sensing will be discussed. [ABSTRACT FROM AUTHOR]

Epitaxial overgrowth of semi-polar III-nitride layers and devices often leads to arrowhead-shaped surface features, referred to as chevrons. We report on a study into the optical, structural, and electrical properties of these features occurring in two very different semi-polar structures, a blue-emitting multiple quantum well structure, and an amber-emitting light-emitting diode. Cathodoluminescence (CL) hyperspectral imaging has highlighted shifts in their emission energy, occurring in the region of the chevron. These variations are due to different semi-polar planes introduced in the chevron arms resulting in a lack of uniformity in the InN incorporation across samples, and the disruption of the structure which could cause a narrowing of the quantum wells (QWs) in this region. Atomic force microscopy has revealed that chevrons can penetrate over 150 nm into the sample and quench light emission from the active layers. The dominance of non-radiative recombination in the chevron region was exposed by simultaneous measurement of CL and the electron beam-induced current. Overall, these results provide an overview of the nature and impact of chevrons on the luminescence of semi-polar devices. [ABSTRACT FROM AUTHOR]

Self-assembled, atomic diffusion controlled growth of InN quantum dots was realized on the side facets of dislocation-free and c-oriented GaN nanowires having a hexagonal cross-section. The nanowires were synthesized by selective area metal organic vapor phase epitaxy. A 3 Å thick InN wetting layer was observed after growth, on top of which the InN quantum dots formed, indicating self-assembly in the Stranski-Krastanow growth mode. We found that the InN quantum dots can be tuned to nucleate either preferentially at the edges between GaN nanowire side facets, or directly on the side facets by tuning the adatom migration by controlling the precursor supersaturation and growth temperature. Structural characterization by transmission electron microscopy and reciprocal space mapping show that the InN quantum dots are close to be fully relaxed (residual strain below 1%) and that the c-planes of the InN quantum dots are tilted with respect to the GaN core. The strain relaxes mainly by the formation of misfit dislocations, observed with a periodicity of 3.2 nm at the InN and GaN hetero-interface. The misfit dislocations introduce I1 type stacking faults (…ABABCBC…) in the InN quantum dots. Photoluminescence investigations of the InN quantum dots show that the emissions shift to higher energy with reduced quantum dot size, which we attribute to increased quantum confinement. [ABSTRACT FROM AUTHOR]

GaN Schottky photodetectors are fabricated on heavily doped n-type GaN epitaxial layers grown by the hydride vapour phase epitaxy technique. The effect of 60Co γ-radiation on the electronic transport in GaN epilayers and Schottky detectors is studied. In contrast to earlier observations, a steady rise in the carrier concentration with increasing irradiation dose is clearly seen. By considering a two layer model, the contribution of interfacial dislocations in carrier transport is isolated from that of the bulk layer for both the pristine and irradiated samples. The bulk carrier concentration is fitted by using the charge balance equation which indicates that no new electrically active defects are generated by γ-radiation even at 500 kGy dose. The irradiation induced rise in the bulk carrier concentration is attributed to the activation of native Si impurities that are already present in an electrically inert form in the pristine sample. Further, the rise in interfacial contribution in the carrier concentration is governed by the enhanced rate of formation of nitrogen vacancies by irradiation, which leads to a larger diffusion of oxygen impurities. A large value of the characteristic tunnelling energy for both the pristine and irradiated Au/Ni/GaN Schottky devices confirms that the dislocation-assisted tunnelling dominates the low temperature current transport even after irradiation. The advantage of higher displacement energy and larger bandgap of GaN as compared to GaAs is evident from the change in leakage current after irradiation. Further, a fast recovery of the photoresponse of GaN photodetectors after irradiation signifies their compatibility to operate in high radiation zones. The results presented here are found to be crucial in understanding the interaction of 60Co γ-irradiation with n+-GaN epilayers. [ABSTRACT FROM AUTHOR]

A systematic investigation of radiation hardness of Schottky barrier diodes and GaN epitaxial layers is carried out by employing in-situ electrical resistivity and cross sectional transmission electron microscopy (XTEM) microstructure measurements. The change in the current transport mechanism of Au/n-GaN Schottky barrier diodes due to irradiation is reported. The role of irradiation temperature and ion type was also investigated. Creation of damage is studied in low and medium electron energy loss regimes by selecting different ions, Ag (200 MeV) and O (100 MeV) at various fluences at two irradiation temperatures (80 K and 300 K). GaN resistivity increases up to 6 orders of magnitude under heavy Ag ions. Light O ion irradiation has a much lower influence on sheet resistance. The presence of isolated defect clusters in irradiated GaN epilayers is evident in XTEM investigation which is explained on the basis of the thermal spike model. [ABSTRACT FROM AUTHOR]

Atomic-layer-deposited (ALD) Al2O3 films are the most promising surface passivation and gate insulation layers in non-Si semiconductor devices. Here, we carried out an extensive study on the time-dependent dielectric breakdown characteristics of ALD-Al2O3 films formed on homo-epitaxial GaN substrates using two different oxidants at two different ALD temperatures. The breakdown times were approximated by Weibull distributions with average shape parameters of 8 or larger. These values are reasonably consistent with percolation theory predictions and are sufficiently large to neglect the wear-out lifetime distribution in assessing the long-term reliability of the Al2O3 films. The 63% lifetime of the Al2O3 films increases exponentially with a decreasing field, as observed in thermally grown SiO2 films at low fields. This exponential relationship disproves the correlation between the lifetime and the leakage current. Additionally, the lifetime decreases with measurement temperature with the most remarkable reduction observed in high-temperature (450 °C) O3-grown films. This result agrees with that from a previous study, thereby ruling out high-temperature O3 ALD as a gate insulation process. When compared at 200 °C under an equivalent SiO2 field of 4MV/cm, which is a design guideline for thermal SiO2 on Si, high-temperature H2O-grown Al2O3 films have the longest lifetimes, uniquely achieving the reliability target of 20 years. However, this target is accomplished by a relatively narrow margin and, therefore, improvements in the lifetime are expected to be made, along with efforts to decrease the density of extrinsic Al2O3 defects, if any, to promote the practical use of ALD Al2O3 films. [ABSTRACT FROM AUTHOR]

Defects in the Al2O3(25 nm)/GaN structure were probed by using monoenergetic positron beams. Al2O3 films were deposited on GaN by atomic layer deposition at 300 °C. Temperature treatment above 800 °C leads to the introduction of vacancy-type defects in GaN due to outdiffusion of atoms from GaN into Al2O3. The width of the damaged region was determined to be 40-50 nm from the Al2O3/GaN interface, and some of the vacancies were identified to act as electron trapping centers. In the Al2O3 film before and after annealing treatment at 300-900 °C, open spaces with three different sizes were found to coexist. The density of medium-sized open spaces started to decrease above 800 °C, which was associated with the interaction between GaN and Al2O3. Effects of the electron trapping/detrapping processes of interface states on the flat band voltage and the defects in GaN were also discussed. [ABSTRACT FROM AUTHOR]

Gallium nitride based high electron mobility transistors are widely known for their operational instabilities regarding interface defects to the dielectric. In this paper, we discuss a III-N surface treatment that results in an electrically more defined interface and hence a narrower distribution of electrically present interface states compared to the original, untreated interface. This surface modification is caused by a remote plasma fluorination of the III-N surface. We show that it is a very distinctive surface processing which cannot be reproduced by other plasma techniques or ion implantation. Applying physical and chemical analyses, the fluorination is found to have a remarkable stability towards temperatures up to 700 °C and is also stable in air for up to 180 h. However, an aqueous clean allows the surface to return to its original state. Even though the exact physical origin of the responsible surface donor cannot be inferred, we suggest that fluorine itself might not directly represent the new surface donor but that it rather activates the III-N surface prior to the dielectric deposition or even substitutes and hence reduces the concentration of surface hydroxides. [ABSTRACT FROM AUTHOR]

Initial oxidation of gallium nitride (GaN) (0001) epilayers and subsequent growth of thermal oxides in dry oxygen ambient were investigated by means of x-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy, and x-ray diffraction measurements. It was found that initial oxide formation tends to saturate at temperatures below 800 °C, whereas the selective growth of small oxide grains proceeds at dislocations in the epilayers, followed by noticeable grain growth, leading to a rough surface morphology at higher oxidation temperatures. This indicates that oxide growth and its morphology are crucially dependent on the defect density in the GaN epilayers. Structural characterizations also reveal that polycrystalline α- and β-phase Ga2O3 grains in an epitaxial relation with the GaN substrate are formed from the initial stage of the oxide growth. We propose a comprehensive model for GaNoxidation mediated by nitrogen removal and mass transport and discuss the model on the basis of experimental findings. [ABSTRACT FROM AUTHOR]

Due to the high dissipated power densities in gallium nitride (GaN) high electron mobility transistors (HEMTs), temperature measurement techniques with high spatial resolution, such as micro-Raman thermography, are critical for ensuring device reliability. However, accurately determining the temperature rise in the ON state of a transistor from shifts in the Raman peak positions requires careful decoupling of the simultaneous effects of temperature, stress, strain, and electric field on the optical phonon frequencies. Although it is well-known that the vertical electric field in the GaN epilayers can shift the Raman peak positions through the strain and/or stress induced by the inverse piezoelectric (IPE) effect, previous studies have not shown quantitative agreement between the strain and/or stress components derived from micro-Raman measurements and those predicted by electro-mechanical models. We attribute this discrepancy to the fact that previous studies have not considered the impact of the electric field on the optical phonon frequencies of wurtzite GaN apart from the IPE effect, which results from changes in the atomic coordinates within the crystal basis and in the electronic configuration. Using density functional theory, we calculated the zone center E2 (high), A1 (LO), and E2 (low) modes to shift by -1.39 cm-1/(MV/cm), 2.16 cm-1/(MV/cm), and -0.36 cm-1/(MV/cm), respectively, due to an electric field component along the c-axis, which are an order of magnitude larger than the shifts associated with the IPE effect. Then, we measured changes in the E2 (high) and A1 (LO) Raman peak positions with ≈1 μm spatial resolution in GaN HEMTs biased in the pinched OFF state and showed good agreement between the strain, stress, and electric field components derived from the measurements and our 3D electro-mechanical model. This study helps to explain the reason the pinched OFF state is a suitable reference for removing the contributions of the electric field and the IPE-induced stress from the temperature rise in the ON state and suggests that the IPE-induced stress in the GaN buffer is an order of magnitude smaller than previously believed. Our analysis and experimental results support previous theoretical studies discussing the electric field dependence of optical phonon frequencies apart from the IPE effect and suggest that this is a general phenomenon occurring in all wurtzite and zincblende crystals. The total electric field dependence of the optical phonon frequencies in piezoelectric crystals is a critical consideration in accurately characterizing the stress, strain, electric field, and temperature distributions in microelectronic devices via micro-Raman spectroscopy. [ABSTRACT FROM AUTHOR]

We develop a theory of energy relaxation in semiconductors and insulators highly excited by the long-acting external irradiation. We derive the equation for the non-equilibrium distribution function of excited electrons. The solution for this function breaks up into the sum of two contributions. The low-energy contribution is concentrated in a narrow range near the bottom of the conduction band. It has the typical form of a Fermi distribution with an effective temperature and chemical potential. The effective temperature and chemical potential in this low-energy term are determined by the intensity of carriers' generation, the speed of electron-phonon relaxation, rates of inter-band recombination, and electron capture on the defects. In addition, there is a substantial high-energy correction. This high-energy "tail" largely covers the conduction band. The shape of the high-energy "tail" strongly depends on the rate of electron-phonon relaxation but does not depend on the rates of recombination and trapping. We apply the theory to the calculation of a non-equilibrium distribution of electrons in an irradiated GaN. Probabilities of optical excitations from the valence to conduction band and electron-phonon coupling probabilities in GaN were calculated by the density functional perturbation theory. Our calculation of both parts of distribution function in gallium nitride shows that when the speed of the electron-phonon scattering is comparable with the rate of recombination and trapping then the contribution of the non-Fermi "tail" is comparable with that of the low-energy Fermi-like component. So the high-energy contribution can essentially affect the charge transport in the irradiated and highly doped semiconductors. [ABSTRACT FROM AUTHOR]

We present spatially and spectrally resolved cathodoluminescence investigations on the cross section of semipolar (1122) gallium nitride epitaxial layers with high background doping level. The locally varying high carrier concentration leads in emission to a free electron recombination band (FERB) governed on the high energy side by conduction band filling. For the basal plane stacking fault (BSF) of type I1, typically emitting at 3≈.41 eV in low doped GaN, we find a blue shift in emission correlated to the FERB high energy tail. This shift can be perfectly modeled and understood in a quantum well model for the BSF, taking also into account the varying doping level in the barrier region. Thus, the carrier concentration can be finally calculated either from the actual position of the I1 BSF or alternatively from the FERB-related near band edge emission. [ABSTRACT FROM AUTHOR]

Asymmetric quantum-well (QW) structures including the asymmetric potential-barrier and the asymmetric potential-well are proposed for AlGaN/GaN/AlGaN resonant tunneling diodes (RTDs). Theoretical investigation gives that an appropriate decrease in Al composition and thickness for emitter barrier as well as an appropriate increase of both for collector barrier can evidently improve the negative-differential-resistance characteristic of RTD. Numerical simulation shows that RTD with a 1.5-nm-thick GaN well sandwiched by a 1.3-nm-thick Al0.15Ga0.85N emitter barrier and a 1.7-nm-thick Al0.25Ga0.75N collector barrier can yield the I-V characteristic having the peak current (Ip) and the peak-to-valley current ratio (PVCR) of 0.39A and 3.6, respectively, about double that of RTD with a 1.5-nm-thick Al0.2Ga0.8N for both barriers. It is also found that an introduction of InGaN sub-QW into the diode can change the tunneling mode and achieve higher transmission coefficient of electron. The simulation demonstrates that RTD with a 2.8-nm-thick In0.03Ga0.97N sub-well in front of a 2.0-nm-thick GaN main-well can exhibit the I-V characteristic having Ip and PVCR of 0.07A and 11.6, about 7 times and double the value of RTD without sub-QW, respectively. The purpose of improving the structure of GaN-based QW is to solve apparent contradiction between the device structure and the device manufacturability of new generation RTDs for submillimeter and terahertz applications. [ABSTRACT FROM AUTHOR]

The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape of luminescence spectra correlated well with their structural quality. Additionally, due to the Quantum-Confined Stark Effect, the emission energy decreased by over 1 eV for quantum well thicknesses increasing from 1 nm up to 6 nm, and this effect was accompanied by the drastic drop of the PL efficiency. The experimental results are consistent with theoretical models. Comparison of experimental data obtained by a number of different characterization techniques with the density functional theory results received on the same geometry structure allowed to prove directly the theoretical models and to determine the polarization and the oscillator strengths in the AlN/GaN nitride systems for the first time. [ABSTRACT FROM AUTHOR]

We investigate electron transport in epitaxially grown nitride-based resonant tunneling diodes (RTDs) and superlattice sequential tunneling devices. A density-matrix model is developed, and shown to reproduce the experimentally measured features of the current–voltage curves, with its dephasing terms calculated from semi-classical scattering rates. Lifetime broadening effects are shown to have a significant influence in the experimental data. Additionally, it is shown that the interface roughness geometry has a large effect on current magnitude, peak-to-valley ratios and misalignment features; in some cases eliminating negative differential resistance entirely in RTDs. Sequential tunneling device characteristics are dominated by a parasitic current that is most likely to be caused by dislocations; however, excellent agreement between the simulated and experimentally measured tunneling current magnitude and alignment bias is demonstrated. This analysis of the effects of scattering lifetimes, contact doping and growth quality on electron transport highlights critical optimization parameters for the development of III–nitride unipolar electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

The electrical, structural, and optical properties of coalescent p-n GaN nanowires (NWs) grown by molecular beam epitaxy on Si (111) substrate are investigated. From photoluminescence measurements the full width at half maximum of bound exciton peaks AX and DA is found as 1.3 and 1.2 meV, respectively. These values are lower than those reported previously in the literature. The current-voltage characteristics show the rectification ratio of about 102 and the leakage current of about 10–4 A/cm2 at room temperature. We demonstrate that the thermionic mechanism is not dominant in these samples and spatial inhomogeneties and tunneling processes through a ~2 nm thick SiNx layer between GaN and Si could be responsible for deviation from the ideal diode behavior. The free carrier concentration in GaN NWs determined by capacitance-voltage measurements is about 4×1015cm–3. Two deep levels (H190 and E250) are found in the structures. We attribute H190 to an extended defect located at the interface between the substrate and the SiNx interlayer or near the sidewalls at the bottom of the NWs, whereas E250 is tentatively assigned to a gallium-vacancy- or nitrogen interstitials-related defect. [ABSTRACT FROM AUTHOR]

We study three carbon defects in GaN, isolated CN and its two complexes with donors CN-ON, and CN-SiGa, as a cause of the yellow luminescence using accurate hybrid density functional calculation, which includes the semi-core Ga 3d electrons as valence electrons and uses a larger 300-atom supercell. We show that the isolated CN defect yields good agreement with experiment on the photoluminescence (PL) peak position, zero-phonon line, and thermodynamic defect transition level. We find that the defect state of the complexes that is involved in the PL process is the same as that of the CN defect. The role of the positively charged donors (ON or SiGa) next to CN is to blue-shift the PL peak. Therefore, the complexes cannot be responsible for the same PL peak as isolated CN. Our detailed balance analysis further suggests that under thermal equilibrium at typical growth temperature, the concentration of isolated CN defect is orders of magnitude higher than the defect complexes, which is a result of the small binding energy in these complexes. [ABSTRACT FROM AUTHOR]

Enhancement of light extraction in GaN light-emitting diode (LED) by addressing an array of nanomaterials is investigated by means of three dimensional (3D) finite-difference time-domain (FDTD) simulation experiments. The array of nanomaterials is placed on top of the GaN LED and is used as a light extraction layer. Depending on its empirically capable features, the refractive index of nanomaterials with perfectly spherical (particle) and hemispherical (plano-convex lens) shapes were decided as 1.47 [Polyethylene glycol (PEG)] and 2.13 [Zirconia (ZrO2)]. As a control experiment, a 3D FDTD simulation experiment of GaN LED with PEG film deposited on top is also carried out. Different light extraction profiles between subwavelength- and over-wavelength-scaled nanomaterials addressed GaN LEDs are observed in distributions of Poynting vector intensity of the light extraction layer-applied GaN LEDs. In addition, our results show that the dielectric effect on light extraction is more efficient in the light extraction layer with over-wavelength scaled features. In the case of a Zirconia particle array (ϕ = 500 nm) with hexagonal closed packed (hcp) structure on top of a GaN LED, light extraction along the normal axis of the LED surface is about six times larger than a GaN LED without the extraction layer. [ABSTRACT FROM AUTHOR]

In this paper, we investigate degradation of InGaN/GaN light emitting diodes (LEDs) under reverse-bias operations in water vapor and dry air. To examine failure origins, electrical characterizations including current-voltage, breakdown current profiles, optical measurement, and multiple material analyses were performed. Our findings indicate that the diffusion of indium atoms in water vapor can expedite degradation. Investigation of reverse-bias stress can help provide insight into the effects of water vapor on LEDs. [ABSTRACT FROM AUTHOR]

The breakdown characteristics for a GaN wrapround field plate diode are compared to those of a planar diode and a mesa diode to determine the improvement due to the field plate geometry. Mesa diodes exhibit a higher breakdown voltage compared to planar diodes, in agreement with simulation models. Wraparound field plate diodes, however, show high leakage current resulting in lower breakdown values than predicted. It is found that the extra leakage is caused by damage from the plasma enhanced chemical vapor deposition of the SiNx used to form the field plate. To mitigate the leakage current, atomic layer deposition was used to put down a protective Al2O3 prior to SiNx deposition. This significantly reduced the leakage current and raised the breakdown voltage of the wraparound Schottky diodes. [ABSTRACT FROM AUTHOR]

p-Si/n-β-Ga2O3:Nb heterojunctions were fabricated by RF magnetron sputtering of β-Ga2O3:Nb layers on the p-Si substrates. The effects of annealing and Nb doping on the properties of heterojunctions were studied. The crystallinity of the β-Ga2O3:Nb film was enhanced by annealing, which also improved the electrical properties of the heterojunctions. The Nb doping greatly affected the I-V characteristics of annealed heterojunctions. The ideality factor was calculated by performing plots from the forward bias I-V characteristics. The charge transport properties of the heterojunction were discussed. The activation energy of β-Ga2O3:Nb films were estimated based on the temperature dependence of resistance. [ABSTRACT FROM AUTHOR]

We report on the selective area epitaxy (SAE) of AlGaN/GaN microstructures in stretchable geometric patterns. We have investigated dependence of Al incorporation, lateral/sidewall growth profile and electrical properties of SAE heterostructures on Al/Ga source ratio and mask material. We observe higher Al incorporation along with improvement in heterointerface between the SAE layers with an increase in Al to Ga ratio. AlGaN composition, crystal quality and electrical properties of SAE AlGaN/GaN microstructures are shown to be affected by the choice of mask material. Low resistivity in the range of 0.12-0.35 mΩ cm was measured in these heterostructures. Two terminal current-voltage measurements showed 3-5× higher current density in SAE AlGaN/GaN microstructure devices compared to conventional planar High electron mobility transistor (HEMT). A detailed secondary ion mass spectroscopy (SIMS) profiling on SAE structures showed dependency of unintentional n-type dopants on the mask material. [ABSTRACT FROM AUTHOR]

In this letter, we propose a highly reliable biosensor based on the disposable gate (DG) AlGaN/GaN high-electron mobility transistor (HEMT) sensor. The greatest advantage of this device is that the HEMT structure and the sensing structure containing the sensitive membrane are separate, overcoming the drawback of being easily contaminated in a biochemical environment. The DG structure HEMT sensor exhibited stable and rapid response to the addition of different concentrations of the prostate-specific antigen (PSA), and the achieved limit of PSA detection was as low as 100 fg/mL. It was also found that the DG AlGaN/GaN HEMT sensor shows good linearity and specificity. These results demonstrate the DG HEMT sensor can be considered to be a promising biosensor platform for practical application in disease diagnosis. [ABSTRACT FROM AUTHOR]

High performance enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs) were achieved by using the TiN-based source contact ledge and two-step fluorine treatment. Due to the effective reduction of the source resistance by TiN-based source contact ledge, the peak extrinsic trans-conductance improved by 21.2% from 340 to 412 mS/mm. In addition, due to the enhancement of the back barrier by the two-step fluorine treatment, the three-terminal OFF-state breakdown voltage (BVDS) improved by 11.5% from 162 to 183 V. The proposed E-mode AlGaN/GaN HEMTs exhibit a maximum drain current of 845 mA/mm (@ ${V} _{\text {GS}} =3$ V), a threshold voltage (${V} _{\text {th}}$) of 0.6 V, a high current-gain cutoff frequency ${f} _{\text {T}}$ of 61 GHz, and a high power-gain cutoff frequency ${f} _{\text {MAX}}$ of 130 GHz. Approximately 5% current collapse occurs at a drain quiescent bias of 30 V. The calculated Johnson’s figure of merit (J-FOM = $\text{BV}_{\text {DS}}\times {f}_{T}$) is 11.2 THz-V, the highest so far for GaN-based HEMTs. The calculated $\text {BV}_{\text {DS}} \times {f}_{\text {MAX}}$ value is 23.8 THz-V, indicating significantly improved the speed and power performance of the E-mode GaN-based HEMT. [ABSTRACT FROM AUTHOR]

Carrier transport in GaN terahertz (THz) quantum cascade laser (QCL) structures is theoretically investigated using a non-equilibrium Green's function method. Although scattering due to polar optical phonons in GaN is greatly enhanced with respect to GaAs/AlGaAs THz QCLs, the phonon-induced broadening of the laser levels is found to remain much smaller than other sources of broadening arising from impurity and electron-electron scattering. The gain is calculated self-consistently accounting for the correlation effects in level broadening. Three-well based design with resonant-phonon scheme shows a peak gain of 88/cm at 10 K, and 34/cm at 280 K, which remains above the calculated loss of a double metal waveguide. The results suggest that lasing at 6.6 THz, which is beyond the traditional GaAs THz QCLs, is possible up to 280 K. [ABSTRACT FROM AUTHOR]

The Zn acceptor level position in GaN:Zn was studied on specially designed GaN(undoped)/GaN:Zn([Zn] = 5 × 1016 cm−3) structures by contactless electroreflectance. A well-defined Franz-Keldysh oscillation is present in optical spectra which results from the built-in electric field present in the near-surface layer. The obtained results show that a significant band bending in the GaN:Zn layer occurs lowering the field in the top layer that is in line with predictions for a semiconductor doped with a high ionization energy acceptor. The acceptor level responsible for the Fermi level position in GaN:Zn is found to be at ∼2.3 eV below the conduction band minimum which coincides with levels introduced by Zn atoms substituting nitrogen in the lattice. At the same time, the surface Fermi level of structures is located in the middle of the bandgap, which is in the lower maximum of (0001) GaN surface density of states. [ABSTRACT FROM AUTHOR]

GaN vertical trench-MOSFETs incorporating molecular beam epitaxy (MBE) regrown channel are developed and investigated. The channel regrowth by MBE prevents repassivation of the p-type GaN body while promising higher channel mobility. Two different designs of the lateral portion of the regrown channel are compared: without or with an n+-GaN buried layer. Without an n+ buried layer, a respectable 600-V breakdown voltage (BV) is measured in the absence of edge termination, indicating a decent critical field strength (>1.6 MV/cm) of the regrown channel. However, the ON-resistance is limited by the highly resistive lateral channel due to Mg incorporation. With an n+ buried layer, the limitation is removed. Excellent ON-current of 130 mA/mm and ON-resistivity of $6.4 ~\rm {m\Omega \cdot cm^{2}}$ are demonstrated. The BV is limited by high source–drain leakage current from the channel due to drain-induced barrier lowering (DIBL) effect. Device analysis together with TCAD simulations points out the major cause for the DIBL effect: the presence of interface charge beyond a critical value ( $\sim 6\times 10^{12}\,\,\rm {cm^{-2}}$ ) at the regrowth interface on etched sidewalls. This paper provides valuable insights into the design of GaN vertical trench-MOSFET with a regrown channel, where simultaneous achievement of low ON-resistivity and high BV is expected in devices with reduced interface charge density and improved channel design to eliminate DIBL. [ABSTRACT FROM AUTHOR]

In this brief, the monolithic integration of enhancement (E)-mode and depletion (D)-mode InAlN/GaN high-electron mobility transistors (HEMTs) is presented. The aim of this brief is to show the results of the designed NAND logic cell, which consists of both HEMTs integrated onto a single die. Large-signal models of dual-gate E-mode and D-mode HEMTs are proposed and calibrated by experimental results. We present well-calibrated electrophysical models for 2-D device simulations employing mixed-mode setup in Synopsys TCAD Sentaurus device. The mixed-mode approach interconnects dual-gate E-HEMT and D-HEMT to NAND logic cell circuit, which allows analysis and characterization of the device as the complex system. Good agreement between simulations and experimental results confirms the validity of the proposed models and simulation methodology. [ABSTRACT FROM AUTHOR]

This paper investigates AlGaN/GaN high-electron mobility transistors (HEMTs) fabricated on epistructures with carbon (C)-doped buffers. Metalorganic chemical vapor deposition is used to grow two C-doped structures with different doping profiles, using growth parameters to change the C incorporation. The C concentration is low enough to result in n-type GaN. Reference devices are also fabricated on a structure using iron (Fe) as dopant, to exclude any process related variations and provide a relevant benchmark. All devices exhibit similar dc performance. However, pulsed ${I}$ – ${V}$ measurements show extensive dispersion in the C-doped devices, with values of dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}~3$ –4 times larger than in the dc case. Due to the extensive trapping, the devices with C-doped buffers can only supply about half the output power of the Fe-doped sample, 2.5 W/mm compared to 4.8 W/mm at 10 GHz. In drain current transient measurements, the trap filling time is varied, finding large prevalence of trapping at dislocations for the C-doped samples. Clusters of C around the dislocations are suggested to be the main cause for the increased dispersion. [ABSTRACT FROM AUTHOR]

Ultralow lasing threshold of $413~\mu \text{J}$ /cm2 in GaN-based vertical-cavity surface-emitting lasers (VCSELs) was observed by reducing the cavity lengths between double-side dielectric distributed Bragg reflectors to $6\lambda $. To the best of our knowledge, the threshold is the lowest value ever reported in nitride optically pumped VCSEL. The spontaneous emission factor ( $\beta $ ) was 0.1, and the degree of polarization was 91%. Effects of short cavity on spontaneous emission factors, gain coefficient enhancement, and absorption reduction were analyzed. In addition, effects of coupled quantum wells on the confinement factor were also discussed. We believe that our results would be meaningful for refining nitride VCSEL structure in the future. [ABSTRACT FROM AUTHOR]

A novel AlGaN/GaN-on-Si lateral current regulating diode (CRD) featuring a hybrid trench-gated cathode is proposed and experimentally demonstrated. By tailoring the recess depth and the length of the gate trench, the device characteristics including the knee voltage, the current density, and the current steadiness can be flexibly modulated. The CRD is capable of delivering an excellent steady current in a large voltage range of 0–200 V and a wide temperature range from −50 °C to 250 °C. Meanwhile, the regulating current exhibits small negative temperature coefficients less than −0.3 %/°C. Fast response capability of the CRD is validated by the pulse I–V measurements. The proposed CRD with such a new functionality together with the high performance is of great potential to expand the applications of AlGaN/GaN electronic devices. [ABSTRACT FROM AUTHOR]

This paper reports on the normally off GaN vertical MOSFETs based on nanowires (NWs) with an inverted p-GaN channel and a wrap-around-gate structure for the first time. Both inductively coupled plasma dry reactive-ion etching and wet-chemical etching were employed to fabricate the vertically aligned GaN NWs from epitaxial thin films with a specified doping profile. During the wet etching, the influence of p-doping on the NW morphology was investigated, and the results could be explained by the proposed model. In comparison with other c-axis NW transistors, an enhancement-mode (E-mode) operation with a superior threshold voltage ( $V_{{\text {th}}}$ ) of 2.5 V has been reached in the fabricated GaN MOSFETs. Furthermore, a high driving-current density of 101 kA/cm2 as well as a high ON-/OFF-current ratio of 109 was obtained in the NWs, predicting a potential approach toward future GaN electronics with vertical and smart architecture. [ABSTRACT FROM AUTHOR]

This paper introduces the anode-via vertical field plate Schottky barrier diode (AVFP-SBD) structure as an enhanced breakdown voltage AlGaN/GaN SBD, using the 2-D ATLAS Silvaco software. The breakdown voltage enhancement is achieved by integration of the lateral field plate (LFP) and vertical field plate (VFP) into the AlGaN/GaN SBD. The simulation results indicate that the VFP not only provide a uniform electric field along the channel, but also distributes the internal electric field on the side of the anode evenly. We show that the proposed structure significantly enhances the breakdown voltage of the conventional AlGaN/GaN diodes, i.e., from −246 to −354 V. Moreover, optimal breakdown voltage of −2301 V is achieved using the SBD with anode-via both LFP and VFP, through an integration of the LFP into the AVFP-SBD. Such breakdown voltage enhancement is achieved without any deterioration in the forward characteristics of the diode. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the electronic properties of (In,Ga)N/GaN coupled wetting layer-quantum dot system using the numerical approach. The finite element method code is used to solve the Schrödinger equation, in the presence of the impurity. In our model, parallelepiped-shape, circular and square based-pyramidal and their wetting layers embedded in GaN matrix were considered. Based on the single band parabolic and the effective mass approximations, the envelop function and its corresponding energy eigenvalue are obtained assuming a finite potential barrier. Our results reveal that: (1) the wetting layer has a great influence on the electronic properties especially for a small quantum dot and acts in the opposite sense of the geometrical confinement, (2) a wetting layer-dependent critical QD-size is obtained limiting two different behaviors and (3) its effect is strongly-dependent on the quantum dot-shape. [ABSTRACT FROM AUTHOR]

We characterized the effects of surface morphology on the electrical properties of n‐GaN drift‐layers by using scanning internal photoemission microscopy (SIPM). We grew 12‐μm‐thick low carrier concentration (approximately 1 × 1016 cm−3) n‐GaN layers with both flat and wavy surface morphologies on freestanding GaN substrates by metal organic chemical vapor deposition. In the SIPM results, the samples with flat surfaces exhibited a uniform photocurrent distribution independently of the carrier concentration. In contrast, wavy patterns in the photocurrent maps were the same as the surface morphology for samples with low carrier concentration. These results indicate that the amount of C incorporated during the growth was affected by the off‐angle of the GaN surface, and the carrier compensation by C atoms was changed. We demonstrated that SIPM is a powerful tool for nondestructively visualizing the inhomogeneity of the carrier compensation over the wafers. [ABSTRACT FROM AUTHOR]

The electrical properties of un‐doped and lightly Si‐doped GaN grown on c‐ and m‐plane GaN substrates by metal organic chemical vapor deposition are investigated. The step‐flow growth modes are realized for c‐ and m‐plane GaN epitaxial layer. The carbon contamination in the c‐plane GaN and m‐plane GaN, grown at 1120 °C and V/III = 1000, are found to be 1.4 × 1016 cm−3 and 5.0 × 1015 cm−3, respectively. The m‐plane GaN follows a linear correlation between the carrier concentration and the Si atomic concentration. The Ni/n‐GaN (Si; 1 × 1016 cm−3) Schottky vertical diodes on the c‐ and m‐plane GaN substrates reveal that the reverse current–voltage (I–V) curves are fitted by using the thermionic field emission model under the measured carrier concentration and the Schottky barrier height. The leakage current of the m‐plane GaN is three orders of magnitude larger than the c‐plane GaN, mostly due to the difference in the Schottky barrier height. [ABSTRACT FROM AUTHOR]

III-Nitride semiconductors face the issue of localized surface states, which causes fermi level pinning and large leakage current at the metal semiconductor interface, thereby degrading the device performance. In this work, we have demonstrated the use of a Self-Assembled Monolayer (SAM) of organic molecules to improve the electrical characteristics of Schottky barrier diodes (SBDs) on n-type Gallium Nitride (n-GaN) epitaxial films. The electrical characteristics of diodes were improved by adsorption of SAM of hydroxyl-phenyl metallated porphyrin organic molecules (Zn-TPPOH) onto the surface of n-GaN. SAM-semiconductor bonding via native oxide on the n-GaN surface was confirmed using X-ray photoelectron spectroscopy measurements. Surface morphology and surface electronic properties were characterized using atomic force microscopy and Kelvin probe force microscopy. Current-voltage characteristics of different metal (Cu, Ni) SBDs on bare n-GaN were compared with those of Cu/Zn-TPPOH/n-GaN and Ni/Zn-TPPOH/n-GaN SBDs. It was found that due to the molecular monolayer, the surface potential of n-GaN was decreased by ∼350 mV. This caused an increase in the Schottky barrier height of Cu and Ni SBDs from 1.13 eV to 1.38 eV and 1.07 eV to 1.22 eV, respectively. In addition to this, the reverse bias leakage current was reduced by 3–4 orders of magnitude for both Cu and Ni SBDs. Such a significant improvement in the electrical performance of the diodes can be very useful for better device functioning. [ABSTRACT FROM AUTHOR]

Due to the high dissipated power densities present in GaN high-electron-mobility transistors (HEMTs) in high-power radio frequency applications, thermal analysis and thermal management of these devices are important in achieving their full potential. In this paper, we present a fundamental study of the transient thermal behavior of GaN HEMTs to aid in understanding the complex contributions of multidimensional thermal spreading, multiple epitaxial layers, multiple gate fingers, and thermal boundary resistance to the temperature rise. This complex behavior cannot be accurately described by one or two thermal time constants. Rather, a broad spectrum of time constants from ~1 ns up to several milliseconds are present in the device due to aggressive multidimensional thermal spreading from the narrow region of power dissipation next to each HEMT gate through the substrate, die attach, and package. In order to accurately model the temperature response, at least one time constant per decade over the timescale of interest is required. These findings are crucial in developing an intuitive understanding of the transient thermal behavior of GaN HEMTs and properly accounting for transient temperature rise in electrothermal modeling of high-power GaN-based amplifiers. [ABSTRACT FROM PUBLISHER]

First principles calculations are performed to investigate the electronic and magnetic properties of hydrogenated and fluorinated zigzag GaN nanoribbons (zGaNNRs). Five kinds of possible different hydrogenated structures and four kinds of possible fluorinated structures are considered, and they show various electronic and magnetic properties. We find that the Ga-edges with two hydrogen atoms terminated or two fluorine atoms terminated are ferromagnetic while the N-edges with two hydrogen atoms terminated or two fluorine atoms terminated are nonmagnetic. Results show the structure is half-metal when the Ga-edges are saturated with two fluorine atoms and N-edges saturated with one fluorine atom. The Gibbs free energy of all the considered structures are calculated here to analyze the stability and their relation with chemical potentials. Moreover, the magnetic and electronic properties can be tailored by external electric field. zGaNNRs transform from half-metal to semiconductor under Ga→N direction electric field; it also can change from half-metal to magnetic metal then to nonmagnetic metal under N→Ga direction electric field. [ABSTRACT FROM AUTHOR]

Electronic structure of the Cs/ n -GaN nano-interface has been studied in situ via synchrotron-based photoelectron spectroscopy by excitation in the energy range of 70–400 eV. The GaN sample was grown by an original method of epitaxy of low-defect unstressed nanoscaled films on AlGaN/SiC/Si substrate. Changes in the surface state spectra and in the Ga 3 d , Cs 4 d , Cs 5 p , N 1 s core level spectra have been revealed under different cesium coverages. The intrinsic surface states for the clean GaN surface at binding energies of ∼5.0 eV and ∼7.0 eV are attenuated during Cs adsorption. Simultaneously three Cs induced surface states are found to arise. Drastic changes in the surface state spectrum were ascertained and shown to be originated from the local interacting Ga dangling bonds and adsorbed Cs atoms initiating the electron redistribution effect with formation of the semiconductor-like Cs/ n -GaN interface. [ABSTRACT FROM AUTHOR]

Thermo Electric (TE) properties of InGaN/GaN heterostructure with different Indium compositions grown by Metal Organic Chemical Vapour Deposition (MOCVD) are investigated. Room temperature thermoelectric studies reveals that increasing indium composition (from 6% to 19%) in the InGaN/GaN heterostructure lead to a decrease in Seebeck coefficient (S) due to sharpening of bandgap which in turn increases TE figure of merit. Seebeck coefficient, Power factor and Figure of Merit of the InGaN/GaN heterostructured thin films shows significant enhancement at higher temperature up to 420 K as compared to room temperature measurement. Promising results on the TE properties of as grown InGaN/GaN heterostructures were observed with Figure of Merit (ZT) value of 0.15 at 420 K for In 0.19 Ga 0.81 N sample. The results indicate that InGaN material system could be potentially imparted for high temperature TE devices. [ABSTRACT FROM AUTHOR]

We present an investigation of vertical leakage in GaN-on-Si epitaxial stack through electrical characterization and device simulations. Different structures of increasing complexity have been fabricated and analyzed in order to achieve a complete understanding of the main transport mechanisms. We have clarified the role of the Si substrate through comparison of identical structures built on p-type and n-type Si substrates. We show that in the case of p-Si substrates the leakage current is sustained by carrier generation in the Si depletion region. We also find that experiments on structures grown on n-doped silicon are consistent with considering electron injection from the substrate through the AlN/Si barrier as the main current limiting mechanism. Our insights are supported by device simulations that consistently reproduce the experimental capacitance–voltage and current–voltage characteristics as a function of temperature for all the considered structures. [ABSTRACT FROM PUBLISHER]

GaN is a great material for making optoelectronic devices in the blue, blue-violet and green bands. Vertical-cavity surface-emitting lasers (VCSELs) have many advantages including small footprint, circular symmetry of output beam, two-dimensional scalability and/or addressability, surface-mount packaging, good price-performance ratio, and simple optics/alignment for output coupling. In this paper, we would like to (1) Review the design and fabrication of GaN-based VCSELs including some technology challenges, (2) Discuss the design and metalorganic chemical vapor deposition (MOCVD) growth of electrically pumped blue VCSELs and (3) Demonstrate world first green VCSEL using quantum dots (QDs) active region to overcome the 'green gap'. [ABSTRACT FROM AUTHOR]

Abstract: Ultrawide‐bandgap (UWBG) semiconductors, with bandgaps significantly wider than the 3.4 eV of GaN, represent an exciting and challenging new area of research in semiconductor materials, physics, devices, and applications. Because many figures‐of‐merit for device performance scale nonlinearly with bandgap, these semiconductors have long been known to have compelling potential advantages over their narrower‐bandgap cousins in high‐power and RF electronics, as well as in deep‐UV optoelectronics, quantum information, and extreme‐environment applications. Only recently, however, have the UWBG semiconductor materials, such as high Al‐content AlGaN, diamond and Ga2O3, advanced in maturity to the point where realizing some of their tantalizing advantages is a relatively near‐term possibility. In this article, the materials, physics, device and application research opportunities and challenges for advancing their state of the art are surveyed. [ABSTRACT FROM AUTHOR]

Ga and N lattice displacements in Li-ion implanted GaN are evaluated by Rutherford backscattering using proton beam. Li-ion implantation into GaN single crystal wafer with 450 μm in thickness is performed with energies of 50 keV (ion fluence; 2 x 10 14 cm -2 ), 80 keV (3 x 10 14 cm -2 ) and 120 keV (5 x 10 14 cm -2 ) at room temperature. The aligned spectra are obtained from scattering along the <0001> channeling direction. The number of displaced Ga and N atoms estimated from χ min (the ratio between aligned and random) were 3.5 × 10 20 cm -3 and 8.7 × 10 20 cm -3 , respectively. The displacement concentration of N atoms is larger than that of Ga atoms. This suggests that N interstitial (N i ) relating to the displacement of N atoms is introduced in Li-ion implanted GaN. This result is similar to that of the neutron irradiated GaN. The sheet resistance is 2.6 x 10 2 Ω/sq. for un-implanted samples and 1.3 x 10 3 Ω/sq. for Li-ion implanted ones. The origin of the high resistivity after the Li-ion implantation is mainly attributed to the carrier compensation effect due to the deep level of N i . The resistance decreases to 1.1 x 10 2 Ω/sq. by annealing at 973 K for 20 min in N 2 flow, suggesting the recovery of the implantation induced defects. [ABSTRACT FROM AUTHOR]

Nanoporous (NP) GaN thin films with low nucleation density were fabricated via UV-assisted electrochemical etching approach. Under different applied biases, two NP samples with different porosities were fabricated. Piezoelectric characteristics of the fabricated samples were investigated using piezoresponse force microscopy. It was found that the nanopores have great influence on its piezoelectric characteristics such as weakening of piezoresponse and change of polarization orientation. During the performance of devices, the weakening of piezoresponse should be propitious to the optoelectronic and electronic stabilities of GaN-based devices. [ABSTRACT FROM AUTHOR]

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Recombination in InGaN upper waveguide in c-plane InGaN-based green laser diodes (LDs) has been investigated by both simulation and experiments. It is found that a potential barrier located at the interface between GaN last barrier and InGaN upper waveguide layer causes strong recombination in InGaN upper waveguide layer and lowers the carrier injection efficiency of InGaN-based green LDs when In content is higher than 4%. A suitable reduction of indium content in InGaN upper waveguide layer can effectively suppress recombination in waveguide and thus reduce the threshold current of green LDs. [ABSTRACT FROM AUTHOR]