Etched GaN Surface Research Articles

Study of enhancement-mode GaN pFET with H plasma treated gate recess

This letter showcases the successful fabrication of an enhancement-mode (E-mode) buried p-channel GaN field-effect-transistor on a standard p-GaN/AlGaN/GaN-on-Si power HEMT substrate. The transistor exhibits a threshold voltage (V TH) of −3.8 V, a maximum ON-state current (I ON) of 1.12 mA/mm, and an impressive I ON/I OFF ratio of 107. To achieve these remarkable results, an H plasma treatment was strategically applied to the gated p-GaN region, where a relatively thick GaN layer (i.e., 70 nm) was kept intact without aggressive gate recess. Through this treatment, the top portion of the GaN layer was converted to be hole-free, leaving only the bottom portion p-type and spatially separated from the etched GaN surface and gate-oxide/GaN interface. This approach allows for E-mode operation while retaining high-quality p-channel characteristics.

The isolation feature geometry dependence of reverse gate-leakage current of AlGaN/GaN HFETs

Reverse gate-leakage current of AlGaN/GaN heterojunction field-effect transistors (HFETs) realized on array of submicron sized fins and conventional mesa isolation feature geometries is investigated at room temperature and zero drain-source bias. For each of the abovementioned device categories, the significance of leakage from the top surface gate as well as gated etched GaN surfaces, especially sidewalls, is studied for a wide range of gate-source voltages (VGS) (i.e. below and above the threshold voltage). It is proven that in the explored fin-type HFETs, for all values of VGS leakage through the gated GaN surfaces, especially the sidewalls, is more significant than the leakage from the top surface gate. This is while in the mesa category, the sidewall leakage is of importance only at less negative values of VGS, and leakage from the top surface gate substantially takes over at more negative VGS values. The discrepancy in the dominance of the aforementioned leakage paths at more negative VGS values among the explored fin and mesa-type HFETs is demonstrated to be due to the stronger electric field across the barrier in the gated region of the mesa-type HFET for this range of VGS. While in the explored fin-type HFETs Ion/Ioff ratio is as high as 2 × 107, the total amount of reverse gate-leakage at all values of VGS is substantially larger compared to the mesa category sharing an equal value of the overall gate width, which substantiates the significance of leakage through etched GaN surfaces in devices composed of larger number of sidewalls, incorporating larger area of gate-overlapping etched GaN surface.

Smooth, low rate, selective GaN/AlGaN etch

The aluminum content is widely used in III-N semiconductors as a determiner of material etch characters. Applications consisting of thin GaN/AlGaN heterostructures can afford a maximum of 6 nm of a thin AlGaN layer over etch demand complex processes with precise low etch rates and low etched GaN surface roughness. In this paper, the effects of bias power and SF6 flow ratios on the chlorine chemistry etch rate, selectivity, and GaN surface roughness are investigated in a high-frequency bias generator and low power inductively coupled plasma configuration. Bias power and SF6 flow control the etch responses and are used to find the optimal spots for low etch rate, low GaN roughness, and high GaN:AlGaN selectivity for the fabrication of devices consisting of thin GaN/AlGaN heterostructures. The results are compared with the other selective gas chemistries and the more common 13.56 MHz frequency bias generator.

Exploration of gate trench module for vertical GaN devices

The aim of this work is to present the optimization of the gate trench module for use in vertical GaN devices: we considered the impact of cleaning process of the etched surface of the gate trench, thickness of gate dielectric, and magnesium concentration of the p-GaN layer. The analysis was carried out by comparing the main DC parameters of devices that differ in surface cleaning process of the gate trench, gate dielectric thickness, and body layer doping. On the basis of experimental results, we report that: (i) a good cleaning process of the etched GaN surface of the gate trench is a key factor to enhance the device performance, (ii) a gate dielectric >35-nm SiO2 results in a narrow distribution for DC characteristics, (iii) lowering the p-doping in the body layer improves the ON-resistance (RON). Gate capacitance measurements are performed to further confirm the results. Hypotheses on dielectric trapping/de-trapping mechanisms under positive and negative gate bias are reported.

Charge-Transfer Mapping of Nanostructured GaN/Ag Surfaces using Surface Enhanced Raman Spectroscopy

In this work we present a method for mapping the degree of charge transfer (ρCT) on variously etched GaN surfaces covered by nanoclusters of silver using Raman imaging. The goal was to map ρCT across the surface of a substrate in order to discover the surface features which stimulate relatively large charge transfer from molecule to substrate. These substrates were used because they contain a variety of physical features designed to facilitate surface enhanced Raman spectroscopy (SERS). A clear correlation was found between these features and the degree of charge transfer. To our knowledge no previously published report had used degree of charge transfer as a mapping tool. This type of analysis could lend itself to many applications, such as the evaluation of charge injection across the surface of a light harvesting dye-sensitized solar cell or photovoltaic device.

High ${I}_{\text{ON}}$ and ${I}_{\text{ON}}$ /${I}_{\text{OFF}}$ Ratio Enhancement-Mode Buried ${p}$ -Channel GaN MOSFETs on ${p}$ -GaN Gate Power HEMT Platform

Enhancement-mode (E-mode) buried ${p}$ -channel GaN metal-oxide-semiconductor field-effect-transistors ( ${p}$ -GaN-MOSFET’s) with threshold voltage ( ${V}_{\text {TH}}$ ) of −1.7 V, maximum ON-state current ( ${I}_{\text {ON}}$ ) of 6.1 mA/mm and ${I}_{\text {ON}}/{I}_{\text {OFF}}$ ratio of 107 are demonstrated on a standard ${p}$ -GaN/AlGaN/GaN-on-Si power HEMT substrate. An oxygen plasma treatment (OPT) was deployed to the gated ${p}$ -GaN region where a relatively thick (i.e. 31 nm) GaN is retained without aggressive gate recess. The OPT converts the top portion of the GaN layer to be free of holes so that only the bottom portion remains ${p}$ -type while being spatially separated from the etched GaN surface and gate-oxide/GaN interface. As a result, E-mode operation is enabled while a high-quality ${p}$ -channel is retained. Multi-energy fluorine ion implantation was implemented for planar isolation of GaN ${p}$ -channel FETs with mesa edges and sidewalls eliminated. Consequently, high ${I}_{\text {ON}}/{I}_{\text {OFF}}$ ratio is obtained.

Determination of electronic band structure by electron holography of etched-and-regrown interfaces in GaN p-i-n diodes

The electrostatic potential variation across etched-and-regrown GaN p-i-n diodes for power electronics has been studied using electron holography in a transmission electron microscope. The potential profiles have been correlated with the composition profiles of Mg, Si, and O obtained by secondary ion mass spectroscopy. Electronic charges obtained from the potential profiles correlate well with the presence of Si and O impurities at regrown interfaces. The overlap of Mg and Si when Mg doped GaN is grown directly over an etched undoped GaN surface results in the formation of a highly doped p-n junction. The introduction of a thin undoped layer over the etched GaN surface prevents the formation of such a junction as the regrowth interface is moved away from the Mg-doped GaN, and results in diodes with improved reverse leakage currents, close to the best values of continuously grown p-i-n diodes. Potential profiles of continuously grown (not etched) p-i-n diodes are compared to those of etched-and-regrown diodes.

A Study of the Photoelectrochemical Etching of n-GaN in H3PO4 and KOH Electrolytes

We investigated the photoelectrochemical etching of n-GaN in H3PO4 and KOH as a function of electrolyte concentration, potential and light intensity. Etch rates measured by stylus profilometry were compared with coulometric and amperometric values. In both electrolytes, etch rates increased with concentration, reaching a maximum at 3.0 mol dm−3 and decreasing at higher concentrations. The increase in etch rate with concentration of either H3PO4 or KOH reflects the amphoteric nature of gallium and the decrease above 3.0 mol dm−3 is attributed to common-ion effects. Profilometric etch rates were lower than coulometric and amperometric etch rates reflecting formation of a surface film. SEM and profilometry demonstrated that thick surface films are formed at lower concentrations. Etch rates increased linearly with light intensity indicating a carrier-limited etching regime: a quantum efficiency of 57.6% was obtained. At light intensities greater than ∼35 mW cm−2 the etch rates showed evidence of saturation. AFM and SEM images of the etched GaN surfaces showed a distinctive ridge-trench structure with a hexagonal appearance. Photoluminescence spectra of the etched GaN show a significant increase in the defect-related yellow luminescence peak suggesting correlation to the formation of the ridge structures, which may represent dislocations terminating at the surface.

Investigation of GaN-on-GaN vertical p-n diode with regrown p-GaN by metalorganic chemical vapor deposition

To mimic selective-area doping, p-GaN was regrown on an etched GaN surface on GaN substrates by metalorganic chemical vapor deposition. Vertical GaN-on-GaN p-n diodes were fabricated to investigate the effects of the etch-then-regrowth process on device performance. The crystal quality of the sample after each epitaxial step was characterized by X-ray diffraction, where the etch-then-regrowth process led to a very slight increase in edge dislocations. A regrowth interfacial layer was clearly shown by transmission electron microscopy. Strong electroluminescence was observed with three emission peaks at 2.2 eV, 2.8 eV, and 3.0 eV. The forward current density increased slightly with increasing temperature, while the reverse current density was almost temperature independent indicating tunneling as the reverse transport mechanism. This result is very similar to the reported Zener tunnel diode comprising a high doping profile at the junction interface. High levels of silicon and oxygen concentrations were observed at the regrowth interface with a distribution width of ∼100 nm. This work provides valuable information on p-GaN regrowth and regrown GaN p-n diodes, which can serve as an important reference for developing selective doping for advanced GaN power electronics for high voltage and high power applications.

Nonaqueous organic electrolyte for photoelectrochemical etching of gallium nitride surface

Nonaqueous organic electrolyte was utilized to etch GaN via photo-electrochemical etching technique. From the point of electrochemical window, the organic electrolyte has wider electrochemical window than conventional aqueous etchants. SEM images show that the pore density is up to 1.2 × 107 per square centimeter. The difference of XPS spectra about survey, Ga 3d and N 1s between the non-etched and freshly etched GaN surface can be explained the formation of Ga(BF4)3. The proposed dissolution mechanism can be broadly applied for explaining the etching process of nonaqueous etchants under the assist of electric field and strong light.

SF6 plasma treatment for leakage current reduction of AlGaN/GaN heterojunction field-effect transistors

Abstract In this study, we have investigated the effects of SF6 plasma treatment on the isolation leakage current characteristics of AlGaN/GaN heterojunction field-effect transistors (HFETs). The device isolation was performed by Cl2/BCl3-based plasma etching process followed by SF6 plasma treatment for 1 min. It was found that the SF6 plasma post-etching treatment was an effective method to improve the electrical isolation characteristics. X-ray photoelectron spectroscopy analysis was carried out to investigate the chemical bonding states of the etched GaN surfaces.

Controllable process of nanostructured GaN by maskless inductively coupled plasma (ICP) etching

This work improved the anisotropically etching profile of GaN with Cl2 ICP by adjusting etching pressure and gas flow. High etching rate is achieved by lowering pressure and gas flow instead of increasing etching power. High etching power is unfavorable because it may cause physical damages on the surface. In addition, it is noticed that the material of the carrier, used for holding samples during etching, has significant effects on the morphology and profile of the etched GaN surface. A smooth and large-area GaN surface was achieved by proper ICP etching with a big piece of Si carrier; whereas, with other kinds of carriers, various nano-structures were formed on the GaN surfaces after etching. In fact, it is the etching resistance of carrier materials that impacts the surface profile of etched GaN. Needle-like and grass-like nanostructures on etched GaN surfaces were observed with Al and sapphire carriers, of which the process is very similar to RIE-grass or black-silicon technology. This controllable maskless dry-etching process for the GaN nanostructured surface may show more potential applications in GaN devices.

Ohmic Contact Properties of Photo-Chemically Etched Nonpolar A-Plane GaN Films

Nonpolar a-plane GaN (a-GaN) was photo-chemically etched under ultraviolet illumination using alkali KOH solution, and the Ohmic contact properties on the etched a-GaN epitaxial layer were investigated. After photo-chemical etching, the striated surface morphology along the c-axis [0001] direction was obtained having triangular prisms consisting of m-plane facets. The properties of Ti/Al/Ni/Au Ohmic contact on surface-textured a-GaN films were studied with transmission line method patterns aligned along the specific crystal orientations. The minimum specific contact resistance of 9.97 × 10−5 Ω·cm2 was achieved along the c-axis on the etched GaN surface at the annealing temperature of 750°C. The etched a-GaN showed higher electrical conductivity along the c-axis than along m-axis. This anisotropic behavior is in contrast to that of the unetched a-GaN where carrier scattering by basal plane stacking faults in the direction perpendicular to the c-axis is a major factor in determining opposite anisotropic conductivity. The higher conductivity along the c-axis of the etched a-GaN film could be explained by more dominant effect of surface roughness scattering.

Temperature dependence of GaN MOS capacitor characteristics

AbstractGaN MOS capacitors on both as‐grown and dry/wet‐etched GaN surfaces are characterized by C‐V and G‐ω measurements at elevated temperatures. The nature of GaN/SiO2 interface traps are determined in detail by extracting interface trap density, surface potential fluctuation, trap time constant and capture cross‐section. When temperature increases, trap time constant decreases so that more traps become active and contribute to observed higher interface trap density, especially for traps at deeper energy levels. The deeper trap levels activated at higher temperatures also induce corresponding temperature‐dependent surface potential fluctuation variations. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

1/<inline-formula> <tex-math notation="LaTeX">$f$ </tex-math></inline-formula> Noise Characteristics of Surface-Treated Normally-Off Al<sub>2</sub>O<sub>3</sub>/GaN MOSFETs

Normally off Al 2 O 3 /GaN MOSFETs are fabricated with a tetramethylammonium hydroxide (TMAH) treatment as a postgate recess etch. The effects of the surface treatment on the etched GaN surface are investigated using low-frequency (1/f ) noise and capacitance-voltage (C-V) measurements. For a quantitative comparison with conventional devices, the oxide trap density (N ot ) is extracted using the unified 1/f noise model, whereas the interface trap density (D it ) is extracted using the high-low-frequency C-V method. After the TMAH treatment, N ot is found to have decreased from 5.40 × 10 19 to 2.50 × 10 19 eV -1 cm -3 , whereas D it is decreased from 2.8 × 10 12 to 1.1 × 10 11 eV -1 cm -2 , as compared with conventional devices. The surface treatment is thus shown to lower trap density in the Al 2 O 3 /GaN MOSFETs by smoothing the surface and suppressing plasma damage in the recessed GaN surfaces.

Deep UV laser etching of GaN epilayers grown on sapphire substrate

The laser etching of GaN epilayers on sapphire substrate was carried out using a deep ultraviolet pulsed laser (157 nm wavelength, 20 ns pulse width). The quality and morphology of the etched GaN surface were evaluated by scanning electron microscopy, atomic force microscopy and scanning profilometer. Quadrate micro-hole and micro-trenches etched by the 157 nm laser exhibited clean and smooth edges, sharp side walls and very small heat affected zone (HAZ). In order to achieve controllable high-quality etching, the laser and processing parameters, such as laser repetition rate, scan speed, were systematically investigated and optimized. The mechanism analysis shows that, direct photoionization or photo-chemical reaction play predominant role within 157 nm laser etching of GaN epilayers.

Optimized ICP etching process for fabrication of oblique GaN sidewall and its application in LED

Inductively coupled plasma (ICP) etching of GaN is systemically investigated by changing ICP power/RF bias power, operating pressure, and Cl2/BCl3 gas mixing ratio. The hexagonal etch pits related to screw dislocation existing along GaN epitaxial layer were observed on the etched GaN surface after ICP etching. The intensity of band-edge emission is significantly reduced from the etched n-GaN surface, which reveals that plasma-induced damage are generated after ICP etching. The oblique sidewall is transferred into GaN using a combination of Cl2/BCl3 plasma chemistry and hard mask SiO2. By adjusting ICP etching process parameters, oblique sidewalls with various oblique angles can be formed, allowing for conformal metal lines coverage across the mesa structures, which can play an important role in the interconnection of multiple microchips for light emitting diodes (LEDs) fabrication.

Defect-related emission characteristics of nonpolar m-plane GaN revealed by selective etching

A comprehensive study of the morphology and luminescence characteristics of nonpolar m-plane GaN etched in hot acids was presented. It was found that many four-sided pyramidal pits were distributed on the etched GaN surface with the long side perpendicular to the [1 1 2̄ 0] direction, corresponding to the threading dislocations. When compared to the as-grown GaN, DAP emission intensity and its LO-phonon coupling phenomenon in the etched GaN were greatly attenuated, whereas the intensity of BSF-related band almost kept constant due to its immunity to chemical etching. Especially, a new PSF-related emission at 3.32 eV emerged in CL spectra of etched GaN. Simultaneously, partial relaxation of compressive stress happened for the etched GaN epilayer according to the red shift of NBE emission in photoluminescence (PL) and E 2(high) phonon peak in the Raman spectra. Contrary, the DAP peak in etched GaN was blueshifted, likely due to the reduced impurity level fluctuation by etching. In addition, the different behaviors were discussed for NBE and defect-related transitions in the etched GaN, characterized by excitation power- and temperature-dependent PL.

Optimization of inductively coupled plasma deep etching of GaN and etching damage analysis

Inductively coupled plasma (ICP) etching of GaN with an etching depth up to 4μm is systemically studied by varying ICP power, RF power and chamber pressure, respectively, which results in etch rates ranging from ∼370nm/min to 900nm/min. The surface morphology and damages of the etched surface are characterized by optical microscope, scanning electron microscope, atomic force microscopy, cathodoluminescence mapping and photoluminescence (PL) spectroscopy. Sub-micrometer-scale hexagonal pits and pillars originating from part of the structural defects within the original GaN layer are observed on the etched surface. The density of these surface features varies with etching conditions. Considerable reduction of PL band-edge emission from the etched GaN surface indicates that high-density non-radiative recombination centers are created by ICP etching. The density of these non-radiative recombination centers is found largely dependent on the degree of physical bombardments, which is a strong function of the RF power applied. Finally, a low-surface-damage etch recipe with high ICP power, low RF power, high chamber pressure is suggested.

Determination of dislocation density in MOVPE grown GaN layers using KOH defect etching

We report on molten KOH-based defect etching of GaN epitaxial layers for the quantitative determination of the dislocation density. Etching process parameters were established at 450 °C that are suitable to reveal threading edge and threading screw dislocations at the same time and, hence, allow for the quantitative determination of the total dislocation density in the metal organic vapor-phase epitaxy (MOVPE) grown GaN layers. The determined dislocation numbers in the 10 8 cm −2 range are correlated (i) with the full-width half-maximum of rocking curves of the (3 0 2¯) reflection and (ii) dark spots observed by cathodoluminescence of the etched GaN surfaces.