Surface Passivation Techniques Research Articles

Crystalline Si Surface Passivation with Nafion for Bulk Defects Detection with Electron Paramagnetic Resonance.

In monocrystalline Si (c-Si) solar cells, identification and mitigation of bulk defects are crucial to achieving a high photoconversion efficiency. To spectroscopically detect defects in the c-Si bulk, it is desirable to passivate the surface defects. Passivation of the c-Si surface with dielectrics such as Al2O3 and SiNx requires deposition at elevated temperatures, which can influence defects in the bulk. Herein, we report on the passivation of different Czochralski (Cz) Si wafer surfaces by an organic copolymer, Nafion. We test the efficacy of the surface passivation at temperatures ranging from 6 to 473 K to detect bulk defects using electron paramagnetic resonance (EPR) spectroscopy. By comparing with state-of-the-art passivation layers, including Al2O3 and liquid HF/HCl, we found that at room temperature, Nafion can provide comparable passivation of n-type Cz Si with an implied open-circuit voltage (iVoc) of 713 mV and a recombination current prefactor J0 of 5 fA/cm2. For p-type Cz Si, we obtained an iVoc of 682 mV with a J0 of 22.4 fA/cm2. Scanning electron microscopy and photoluminescence reveal that Nafion can also be used to passivate the surface of c-Si solar cell fragments scribed from a solar cell module by using a laser. Consistent with previous studies, analysis of the EPR spectroscopy data confirms that the H-terminated surface is necessary, and fixed negative charge in Nafion is responsible for the field-effect passivation. While the surface passivation quality was maintained for almost 24 h, which is sufficient for spectroscopic measurements, the passivation degraded over longer durations, which can be attributed to surface SiOx growth. These results show that Nafion is a promising room-temperature surface passivation technique to study bulk defects in c-Si.

The surface chemistry of colloidal lead halide perovskite nanowires.

This study explored the interplay between the ligand-surface chemistry of colloidal CsPbBr3 nanowires (NWs) and their optical properties. The ligand equilibrium was probed using nuclear magnetic resonance spectroscopy, and by perturbing the equilibrium via dilution, the gradual removal of ligands from the CsPbBr3 surface was observed. This removal was correlated with an increase in the surface defect density, as suggested by a broadening of the photoluminescence (PL) spectrum, a decrease in the PL quantum yield (PLQY), and quenching of the PL decay. These results highlight similar surface binding between the traditional CsPbBr3 quantum dots and our NWs, thereby expanding the scope of well-established ligand chemistry to a relatively unexplored nanocrystal morphology. By controlling the dilution factor, it was revealed that CsPbBr3 NWs achieve a PLQY of 72% ± 2% and a relatively long average PL lifetime of 400 ± 10ns, without relying on additional surface passivation techniques, such as ligand exchange.

Adequate method to study the surface passivation effectiveness in HEM multicristallin silicon wafers

In this work we have examined the effectiveness of surface passivation on ascut multicrystalline silicon (mc-Si) wafers using different techniques. The study is based on minority carrier lifetime measurements with quasi steady state photo-conductance, ‘QSSPC’. Effective minority carrier lifetime ()measured values of 12.4, 8.9, 4.9 and 3.1 are obtained respectively with four silicon surface passivation techniques: 1- Shallow phosphorous diffusion emitter (n+p), 2- Iodine-Ethanol (I-E), 3-Hydrofluoric acid (HF) emersion and 4-SiNx layer deposition. These results suggest that the shallow n+p emitter gives the eff close to the bulk lifetime due to the better surface passivation quality. Simulations made with Hornbeck-Haynes model indicate that the improvement can be correlated with the decrease of the surface recombination velocity (SRV) and the increment of bulk lifetime.

Aberrant Putamen Connectivity in Adolescent Bipolar Disorder Type II Revealed by fMRI Analysis

This chapter discusses the fundamentals of porous silicon (PSi) based DNA biosensors. Various surface functionalization approaches are introduced, including surface passivation techniques that can be used to prevent corrosion of the PSi matrix upon DNA hybridization. Several PSi DNA sensor configurations are presented, ranging from single-layer interferometers to on-chip ring resonators, with a focus on performance in terms of sensitivity and limit of detection. Challenges to real-world implementation of PSi DNA biosensors are addressed and important areas of progress in on-chip integration and incorporation of microfluidic channels are highlighted. The chapter ends with an outlook on future point-of-care diagnostic applications.

Comprehensively Improved Performance of β-Ga2O3 Solar-Blind Photodetector Enabled by a Homojunction with Unique Passivation Mechanisms.

Ga2O3-based solar-blind photodetectors have been extensively investigated for a wide range of applications. However, to date, a lot of research has focused on optimizing the epitaxial technique or constructing a heterojunction, and studies concerning surface passivation, a key technique in electronic and optoelectronic devices, are severely lacking. Here, we report an ultrasensitive metal-semiconductor-metal photodetector employing a β-Ga2O3 homojunction structure realized by low-energy surface fluorine plasma treatment, in which an ultrathin fluorine-doped layer served for surface passivation. Without inserting/capping a foreign layer, this strategy utilized fluorine dopants to both passivate local oxygen vacancies and suppress surface chemisorption. The dual effects have opposite impacts on device current magnitude (by suppressing metal/semiconductor junction leakage and inhibiting surface-chemisorption-induced carrier consumption) but dominate in dark and under illumination, respectively. By means of such unique mechanisms, the simultaneous improvement on dark and photo current characteristics was achieved, leading to the sensitivity enhanced by nearly 1 order of magnitude. Accordingly, the 15 min treated sample exhibited striking competitiveness in terms of comprehensive properties, including a dark current as low as 6 pA, a responsivity of 18.43 A/W, an external quantum efficiency approaching 1 × 104%, a specific detectivity of 2.48 × 1014 Jones, and a solar-blind/UV rejection ratio close to 1 × 105. Furthermore, the response speed was effectively accelerated because of the reduction on metal/semiconductor interface trap states. Our findings provide a facile, economical, and contamination-free surface passivation technique, which unlocks the potential for comprehensively improving the performance of β-Ga2O3 solar-blind metal-semiconductor-metal photodetectors.

Molecular Perturbation of 2D Organic Modifiers on Porous Carbon Interlayer: Promoted Redox Kinetics of Polysulfides in Lithium-Sulfur Batteries

The rate performance of Li-S batteries still remains poor due to the limited redox kinetics of polysulfides (PSs). Herein, by selectively coating a set of 2-demensional (2D) organic molecules, chenodeoxycholic acid (CA), 4,4′,4′′,4′′′-(porphine-5,10,15,20-tetrayl) tetrakis (benzoic acid) (TTBA), berberine chloride hydrate (BCH) and poly(diallyl dimethyl ammonium chloride) (PDDA) on honey comb-like porous carbon (HC) interlayers, respectively, the redox kinetics of PSs are remarkably enhanced in the interspace between the 2D molecules and carbon matrix, in the order of CA<TTBA<BCH<PDDA, with the most efficient molecules featured with positively charged amino groups. The capacity of Li-S batteries with PDDA on HC interlayer goes up to 637.6 mAh g–1 at 10 C and maintains 601.7 and 483.5 mAh g–1 after 1200 cycles at 2 C and 4 C, corresponding to a decay as low as 0.024% and 0.031% per cycle. Moreover, a reversible high areal capacity of 8.4 mAh cm−2 at 0.1 C and superior stability was achieved at a sulfur loading of 8.9 mg cm−2. This surface passivation technique is demonstrated to be a general strategy for any other carbon materials in promoting the redox kinetics of PSs as interlayers in Li-S batteries.

A solvent-based surface cleaning and passivation technique for suppressing ionic defects in high-mobility perovskite field-effect transistors

Organometal halide perovskite semiconductors could potentially be used to create field-effect transistors (FETs) with high carrier mobilities. However, the performance of these transistors is currently limited by the migration of ionic surface defects. Here, we show that a surface cleaning and passivation technique, which is based on a sequence of three solution-based steps, can reduce the concentration of ionic surface defects in halide-based perovskites without perturbing the crystal lattice. The approach consists of an initial cleaning step using a polar/nonpolar solvent, a healing step to remove surface organic halide vacancies and a second cleaning step. The surface treatment is shown to restore clean, near hysteresis-free transistor operation, even if the perovskite films are formed under non-optimized conditions, and can improve room-temperature FET mobility by two to three orders of magnitude compared to untreated films. Our methylammonium lead iodide (MAPbI3) FETs exhibit high n- and p-type mobilities of 3.0 cm2 V−1 s−1 and 1.8 cm2 V−1 s−1, respectively, at 300 K, and higher values (9.2 cm2 V−1 s−1; n-type) at 80 K. We also show that the approach can be used to transform PbI2 single crystals into high-quality, two-dimensional perovskite single crystals. A three-stage solution-based cleaning technique can increase the room-temperature mobility and reduce the hysteresis of organometal halide perovskite transistors by decreasing the surface defects in the perovskite films.

Surface Crystal Growth of Perovskite Nanocrystals via Postsynthetic Lead(II) Bromide Treatment to Increase the Colloidal Stability and Efficiency of Light-Emitting Devices.

The surface modification of metal halide perovskite nanocrystals (NCs) significantly impacts their optical properties and colloidal stability. This subsequently affects the performance of light-emitting devices (LEDs). Therefore, numerous surface passivation techniques like ligand exchange and metal halide doping have been explored to passivate the surface defects of perovskite NCs and obtain highly efficient LEDs. In this study, we demonstrated the postsynthetic metal halide doping treatment using lead(II) bromide (PbBr2) to passivate the surface defects of the CsPbBr3 NCs at a moderate reaction temperature of 80 °C. The alkyl quaternary ammonium salt, didodecyldimethylammonium bromide (DC12AB), enabled the complete dissolution of PbBr2 in a nonpolar solvent, toluene. Because of surface crystal growth, the particle sizes of the PbBr2-doped CsPbBr3 NCs were higher than those of the as-synthesized CsPbBr3 NCs. The photoluminescence quantum yield of the CsPbBr3 NCs drastically increased from 26.8 to 83.9% after the PbBr2 doping treatment. Moreover, the PbBr2-doped CsPbBr3 NCs possessed long-term colloidal stability of more than 2 months that indicates the strong bonding between the NCs and ligands. We observed that the alkyl chain length of the quaternary alkyl ammonium salts affected the luminance and device stability during operations. In this study, a promising strategy was devised to achieve highly luminescent perovskite NCs with excellent colloidal stability that can enhance the performance of LEDs.

(Invited) Doping Considerations for Finfet, Gate-All-Around, and Nanosheet Based Devices

The semiconductor industry has been based on Si device scaling for many decades. The motivation for scaling Si is well known, as the reduced dimensions can lead to increased field-effect transistor (FET) device (logic) performance with lower power consumption, thereby enabling a vast array of consumer electronic products. While other materials such as Ge, III-Vs, and 2D materials have threatened to replace Si in FET device channels, it remains at the core of modern digital logic transistors and circuits.Si has been scaled below 10 nm in multi-gate and silicon-on-insulator (SOI) device technologies, but its thickness cannot be reduced indefinitely and larger surface-to-bulk ratios will impact the behaviour of the Si. In general, surface physics cannot be discounted as thickness approaches 5 nm. It is known that effects must be considered at these scales such as carrier surface scattering, dielectric screening, interface states, as well as decreased doping efficiency, increased dopant trapping, and an increase in Si bandgap. If not properly controlled, or at least understood, these factors may inhibit proper semiconductor operation.Damage-free doping processes are desirable as crystal defects or surface roughness can, not only, negatively impact individual transistor device performance, but can also introduce an uncontrolled variability among devices. A conformal process implies access to all surfaces equally in order to, in this case, uniformly and reliably modify the material. Uniformity is necessary as it reduces local resistance variation which again can lead to poor electrical behaviour. As we transition to Gate-All-Around (GAA) device architectures, where all sides of the semiconductor are accessible in theory, having processes that can actually demonstrate access to the bottom surface are becoming increasingly sought after. When dealing with 3D structures the impact of surface effects becomes crucial in determining the active dopant levels. These surface effects lead to problems for dopant incorporation/activation or as gettering points where dopants pile up. Another problem with the greater exposure to the perimeter of the 3D devices is when the doping process etches the available surface, severely damaging or destroying these devices.In this presentation we will consider doping techniques based on ion implant, solid-source in-diffusion, liquid-source in-diffusion, and gas-source in-diffusion. As Si devices scale, the effect of beamline induced crystal damage compounds as a greater percentage of the device area will consist of damaged crystal. Beamline induced surface damage will become a more prominent issue with increasing surface to volume ratios of Si devices. Sputtering of the Si surface by the ion beam is likely to become a more limiting factor once device pitch becomes scaled limiting the acceptable implantation angles. Limitation of the beamline angles will lead to shadowing of the Si devices, and therefore asymmetrical dopant levels. With the advent of GAA devices, line-of-sight implantation methodologies may be unsuitable for the doping of 3D stacked devices.On the other hand in-diffusion techniques suffer due to a surface barrier that must be overcome in order to enter the Si, while ions can always be implanted beyond the surface into the target. Pioneering solutions may exist in the fields of proximity doping and novel surface passivation techniques. Gas-phase doping has a greater potential than liquid or solid phase processes to dope the increasingly complex 3D geometries of Si devices as liquid coverage can be limited by wetting, and solid sources can be limited when fin and nanowire pitches are scaled.

Functionally Graded Materials for Long-Cycle Lithium-Ion Batteries

Transition metal (TM) dissolution and volume expansion of high-capacity TM-based materials are always the the Achilles Heel of lithium-ion batteries (LIBs), leading to poor safety and reliability of LIBs-powered devices and instruments. Even though some surface passivation materials and techniques have been employeed to address these issues, more parasitic effects occur at the heterogeneous coating-electrode interfaces, such as internal resistance, lattice strains, and capacitive internal electric field, which negatively affect LIBs performance. Hence, it is of vital importance but very challenging to develop an effective strategy for the TM-based electrode protection. In this work,we used functionally graded materials (FGM) as an ideal solution to developing novel electrodes for long-cycle LIBs. In such materials, a homologous passivation layer (HPL) with a similar elemental composition but different stoichiometry is rationally designed to passivate "conversion-type" cathodes by forming a structure/composition gradient in order to resolve these fundamental challenges and technical problems for the TM-based electrode surface passivation. The structure and composition reconstruction of HPL help to buffer the volume change and trap the dissolved transition metals. As a result, a significantly improved cyclability of LIBs is achieved, which opens a new avenue for the electrode stabilization.

Surface-passivated high-Q GaAs photonic crystal nanocavity with quantum dots

Photonic crystal (PhC) nanocavities with high quality (Q) factors have attracted much attention because of their strong spatial and temporal light confinement capability. The resulting enhanced light–matter interactions are beneficial for diverse photonic applications, ranging from on-chip optical communications to sensing. However, currently achievable Q factors for active PhC nanocavities, which embed active emitters inside, are much lower than those of the passive structures because of large optical loss, presumably originating from light scattering by structural imperfections and/or optical absorptions. Here, we demonstrate a significant improvement of Q factors up to ∼160 000 in GaAs active PhC nanocavities using a sulfur-based surface passivation technique. This value is the highest ever reported for any active PhC nanocavities with semiconductor quantum dots. The surface-passivated cavities also exhibit reduced variation in both Q factors and cavity resonant wavelengths. We find that the improvement in the cavity performance presumably arises from suppressed light absorption at the surface of the PhC’s host material by performing a set of PL measurements in spectral and time domains. With the surface passivation technique, we also demonstrate a strongly coupled single quantum dot-cavity system based on a PhC nanocavity with a high Q factor of ∼100 000. These results will pave the way for advanced quantum dot-based cavity quantum electrodynamics and GaAs micro/nanophotonic applications containing active emitters.

Toward ultrafast, ultra-stable imaging arrays: Superlattice doping to enhance the performance of backside-illuminated 3D-hybridized silicon photodetectors

In this paper, the authors report the latest results on their development of superlattice-doped, thinned, backside-illuminated (BSI), 3D-integrated photodiode detectors—a step toward their ultimate goal of demonstrating ultrafast, ultrastable CMOS imaging arrays. As with most silicon-based photodetectors, backside-illumination and backside surface passivation are keys to achieve the highest performance capability. The two-dimensional (2D) doping technique developed at the Jet Propulsion Laboratory (JPL) has proved to result in a highly efficient, highly stable detector response when combined with a variety of detectors. Here, JPL's 2D-doping has been combined with Sandia's BSI photodetectors hybridized with custom fanout wafer via copper Direct Bond Interconnect (DBI®), a technology that is rapidly becoming industry standard for BSI CMOS imaging arrays. The prototype detectors were packaged and evaluated with respect to their response to low energy electrons. The authors find that the responsivity of 2D-doped BSI detectors is higher than devices prepared using other surface passivation techniques (i.e., ion implantation). The success of the work described herein verifies that the 2D-doping processes previously developed for Sandia's frontside-illuminated photodetectors are generally applicable to BSI detectors and demonstrates for the first time that JPL's 2D-doping process is compatible with the Cu-DBI® technology.

Overcoming Zinc Oxide Interface Instability with a Methylammonium‐Free Perovskite for High‐Performance Solar Cells

AbstractPerovskite solar cells have achieved the highest power conversion efficiencies on metal oxide n‐type layers, including SnO2 and TiO2. Despite ZnO having superior optoelectronic properties to these metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI3, ZnO is largely overlooked due to a chemical instability when in contact with metal halide perovskites, which leads to rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether all metal halide perovskites exhibit this instability with ZnO are yet to be undertaken. Experimental methods to elucidate the degradation mechanisms at ZnO–MAPbI3 interfaces are developed. By substituting MA with formamidinium (FA) and cesium (Cs), the stability of the perovskite–ZnO interface is greatly enhanced and it is found that stability compares favorably with SnO2‐based devices after high‐intensity UV irradiation and 85 °C thermal stressing. For devices comprising FA‐ and Cs‐based metal halide perovskite absorber layers on ZnO, a 21.1% scanned power conversion efficiency and 18% steady‐state power output are achieved. This work demonstrates that ZnO appears to be as feasible an n‐type charge extraction layer as SnO2, with many foreseeable advantages, provided that MA cations are avoided.

Rear contact silicon solar cells with a-SiCX:H based front surface passivation for near-ultraviolet radiation stability

Surface recombination (due to dangling bonds) and lower absorption (due to the low absorption coefficient of silicon (Si)) are the major hindrances in silicon-based photovoltaic (PV) devices. To overcome this, numerous complex texturing schemes are projected to enhance the light trapping. However nanostructured cells are not efficient due to the large surface to volume ratio which enhances surface recombination. Thus, the nanostructured cells require additional passivation scheme to mitigate the recombination losses. Here, we have designed a nontextured, 15% efficient, amorphous silicon carbide hydrogenated (a-SiCX:H) passivated, 10-μm thick rear contact Si solar cell device. Considerable reduction in photo reflectance is obtained in the near ultraviolet (UV)/visible spectral region together with near UV stability at higher surface recombination velocity (SRV). External quantum efficiency (EQE) > 90% is achieved by the a-SiCX:H based device (within the wavelength spectrum of 480–620 nm). Improvement in spectrum response give rise to 28.1 mA cm−2 short circuit current density (JSC). Further, the performance of a-SiCX:H passivated device is compared with a conventional dielectric anti-reflective coating (ARC) and high-low junction-based surface passivation techniques. Results indicate that the presence of a-SiCX:H reduces the hole concentration near the front surface which eventually decreases the surface recombination. Highly efficient and reliable solar cells can be achieved by the design schemes reported in this paper, which balance both the photonic and electronic effects together.

Antireflection and passivation property of aluminium oxide thin film on silicon nanowire by liquid phase deposition

To improve the silicon nanowires solar cell of conversation efficiency is one of the most important and challenging problems, the antireflection coatings and surface passivation technique were very important. In this investigation, aluminium oxide coatings were deposited on silicon nanowires by using liquid phase deposition. The deposition solution of aluminium sulfate and sodium bicarbonate were used for aluminium oxide deposition. The Al2O3 film can cause different surface passivation at different annealing temperatures, allowing the Al2O3/SiNWs interface to form different amounts of negatively charged AlO4 and positively charged SiOx, that the mutually suppressing action of the two oppositely charged structure. Under the optimal condition, the reflectance and effective minority carrier lifetime of liquid phase deposited aluminium oxide film were 0.97% and 31 μs, respectively. The aluminium oxide films were used herein to fabricate antireflection coating and passivation film to ensure low cost, good uniformity, favorable adhesion, mass producibility, and the formation of large-area thin film; thus, the liquid phase deposition-antireflection coating film was highly favorable for silicon-based solar cells.

Surface passivation and aging of InGaAs/InP heterojunction phototransistors

We report the effect of different surface treatment and passivation techniques on the stability of InGaAs/InP heterojunction phototransistors (HPTs). An In0.53Ga0.47As surface passivated with aqueous ammonium sulfide ((NH4)2S), aluminum oxide (Al2O3) grown by atomic layer deposition (ALD), and their combination is evaluated by using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). All samples were kept in the air ambient, and their performances were periodically measured to investigate their long-term stability. Raman spectroscopy revealed that the peak intensity of the GaAs-like longitudinal optical phonon of all passivated samples is decreased compared with that of the control sample. This is attributable to the diminution of the carriers near the passivated surfaces, which was proven by extracted surface potential (Vs). The Vs of all passivated samples was decreased to less than half of that for the control sample. XPS evaluation of As3d spectra showed that arsenic oxides (As2O3 and As2O5) on the surfaces of the samples can be removed by passivation. However, both Raman and XPS spectra show that the (NH4)2S passivated sample reverts back over time and will resemble the untreated control sample. When capped with ALD-grown Al2O3, passivated samples irrespective of the pretreatment show no degradation over the measured time of 4 weeks. Similar conclusions are made from our experimental measurement of the performance of differently passivated HPTs. The ALD-grown Al2O3 passivated devices show an improved optical gain at low optical powers and long-term stability.

An Investigation into the Impact of Surface Passivation Techniques Using Metal-Semiconductor Interfaces

Schottky barrier diodes (SBD) were fabricated on SiC surfaces that had been treated with different surface passivation techniques, so that metal-semiconductor analysis could be used to evaluate the quality of this surface. In this paper, we discuss the results of this study that used Current-Voltage (I-V), Capacitance-Voltage (C-V) and Current-Voltage-Temperature (I-V-T) analysis to look at the impact of untreated oxidation, Nitrous oxide (N2O) and Phosphorus Silicate Glass (PSG) treatments prior to oxide removal, and the formation of Mo, Ni and Ti diodes. While the results of this study did not reveal any consistent patterns between the different treatments, a Mo diode formed on a surface after PSG treatment, displays exceptionally low leakage (4.44×10-5 A/cm2 at 19°C; 7.26×10-4 A/cm2 at 300°C) given a low barrier height (1.27 eV). Moreover, the barrier heights extracted from C-V analysis before contact annealing show a variation across all the diodes, suggesting that the interface is greatly suffering from Fermi-Level pinning, the result of significant interface traps.

Antireflection and passivation property of titanium oxide thin film on silicon nanowire by liquid phase deposition

To improve a high-efficiency silicon nanowires solar cell, using the antireflection coatings and surface passivation technique were very important. In this investigation, titanium oxide antireflection coatings were deposited on silicon nanowires by using liquid phase deposition. The deposition solution of (NH4)2TiF6 and H3BO3 were used for titanium oxide deposition. The concentration of the H3BO3 in the deposition solution play important roles in the formation of Ti–Si1−xOy interface layer between the titanium oxide/silicon nanowires interface and to control the trace amount of hydrofluoric acid in the solution. The titanium oxide films modification decreases and increases the reflectance and effective minority carrier lifetime of the silicon nanowires arrays. Under the optimal condition, the reflectance and effective minority carrier lifetime of liquid phase deposited titanium oxide film were 3.6% and 1.29μs, respectively. The titanium oxide films were used herein to fabricate antireflection coating and passivation film to ensure low cost, good uniformity, favorable adhesion, mass producibility, and the formation of large-area thin film; thus, the liquid phase deposition-antireflection coating film was highly favorable for silicon-based solar cells.

Performance enhancement of AlGaN/AlN/GaN high electron mobility transistors by thermally evaporated SiO passivation

A surface passivation technique has been developed for AlGaN/AlN/GaN high electron mobility transistors (HEMTs) by simple thermal evaporation of silicon monoxide (SiO) at room temperature. Detailed device characteristics were studied and compared with the most commonly used SiNx passivation grown by plasma enhanced chemical vapor deposition at elevated temperatures. Both passivation techniques lead to a similar enhancement in the on-state drain current and transconductance as compared with the unpassivated HEMTs. However, we discovered that the gate leakage current in the SiO passivated devices was more than two orders of magnitude lower than the devices passivated by SiNx. Furthermore, while the SiNx passivated HEMTs exhibited a two orders of magnitude increase in off-state drain current, SiO passivation substantially reduced it, resulting in an overall improvement by a factor of 1429. The extent of the device surface damage caused by passivation was also investigated by characterizing other parameters. The subthreshold slope of the SiO passivated HEMTs was 95 mV dec−1, nearly 5 times better than the SiNx passivated devices. The extracted interface trap density was 1.16 × 1012 cm−2 eV−1, about ten times lower than that in the SiNx passivated HEMTs. Moreover, SiO passivation was found to enhance the gate Schottky barrier height by 60 meV whereas SiNx passivation reduced it, which could partially explain the differences in the gate leakage current. Finally, SiO passivation enabled twice high breakdown voltage than SiNx passivation. The relevant physical mechanisms were discussed.

Ionic liquid gating on atomic layer deposition passivated GaN: Ultra-high electron density induced high drain current and low contact resistance

Herein, we report on achieving ultra-high electron density (exceeding 1014 cm−2) in a GaN bulk material device by ionic liquid gating, through the application of atomic layer deposition (ALD) of Al2O3 to passivate the GaN surface. Output characteristics demonstrate a maximum drain current of 1.47 A/mm, the highest reported among all bulk GaN field-effect transistors, with an on/off ratio of 105 at room temperature. An ultra-high electron density exceeding 1014 cm−2 accumulated at the surface is confirmed via Hall-effect measurement and transfer length measurement. In addition to the ultra-high electron density, we also observe a reduction of the contact resistance due to the narrowing of the Schottky barrier width on the contacts. Taking advantage of the ALD surface passivation and ionic liquid gating technique, this work provides a route to study the field-effect and carrier transport properties of conventional semiconductors in unprecedented ultra-high charge density regions.