Hexagonal Etch Pits Research Articles

Dry and wet etching of single-crystal AlN

The dry etching of high crystal quality c-plane AlN grown by metal organic chemical vapor deposition was examined as a function of source and chuck power in inductively coupled plasmas of Cl2/Ar or Cl2/Ar/CHF3. Maximum etch rates of ∼1500 Å min−1 were obtained at high powers, with selectivity over SiO2 up to 3. The as-etched surfaces in Cl2/Ar/CHF3 have F-related residues, which can be removed in NH4OH solutions. The Al-polar basal plane was found to etch slowly in either KOH or H3PO4 liquid formulations with extensive formation of hexagonal etch pits related to dislocations. The activation energies for KOH- or H3PO4-based wet etching rates within these pits were 124 and 183 kJ/mol, respectively, which are indicative of reaction-limited etching.

Spatial correlation of defect-selective etching and dark luminescence spots in Al x Ga1−x N

Defect-selective etching with molten Ba(OH)2/MgO etch drops was performed on c-plane AlGaN layers covering the entire composition range between GaN and AlN. Regardless of the aluminum content, the etchant produced shallow, hexagonal etch pits with depth-to-diameter ratios of 1/10–1/100. Two predominant types of etch pits were observed, which differed in size. In addition, the etch rate decreased from the center to the edge of the area exposed to the etch drops, providing a radially symmetric variation in etch pit size. For all AlGaN compositions, the positions of the etch pits correlate perfectly with the positions of the dark luminescence spots in cathodoluminescence measurements. Areas on the AlGaN samples that were not exposed to the etching procedure showed identical dark spots with the same size and density as those in the etched regions. Additionally, the density of etch pits and dark spots corresponded to the density of threading dislocations in the AlGaN layers. These observations demonstrate that the density of threading dislocations in c-plane AlGaN layers can be determined by destructive defect-selective etching with Ba(OH)2/MgO and etch pit counting, as well as by nondestructive counting of the dark spots in cathodoluminescence images.

Micro-Raman analysis of HVPE grown etched GaN epilayer with porous formation

The GaN epilayer grown by hydride vapor phase epitaxy was wet etched by phosphoric acid as the etchant. X-ray diffraction confirms that the GaN has a wurtzite structure. Scanning electron microscopy shows various sizes of hexagonal pits for different times of etchant reactions. Atomic force microscopy shows increase in surface roughness with different etchant rate. The photoluminescence gives a 3.4 eV luminescence for the pristine GaN epilayer. In the etched films, the deep-level defect belonging to yellow and green luminescence was found. The deconvoluted Ga 3d peaks of etched samples show Ga-rich epilayers. Micro-Raman spectroscopy is a non-destructive method for measuring carrier concentration, phonon lifetime and strain using A 1 (LO) spectra of Raman vibration mode was utilized via the Lorentz fitting method. The carrier concentration increases while the phonon lifetime decreases with etching rate. Overall, in the 9 min reaction, the epilayer was etched heavily with a perfect hexagonal etch pit structure.

Photoelectrochemical Stability of NiO Thin Film As a Protective Layer Formed on GaN-Based Photoanode for Water Splitting Reaction Under Light Irradiation for 300 h

Gallium nitride (GaN) is expected as a photoanode to generate oxygen and hydrogen through a water splitting reaction. This is because the band-gap energy of GaN is 3.4 eV (λ ≤ 365 nm) and the top of the valence band is lower than the oxidation potential of water and the bottom of the conduction band is higher than the reduction potential of protons. However, some holes generated in the light-absorbing layer of a GaN-based photoanode are used not in the water oxidation reaction but rather in the etching reaction due to self-oxidation of the photoanode. As a result, the solar-to-hydrogen conversion efficiency η STH decreases over time. To improve η STH and prevent degradation of the GaN-based photoanode surface, it has been reported that supporting NiO on a GaN-based photoanode surface by spin coating using a metal organic decomposition solution is effective. The NiO/GaN-based photoanode has been evaluated under intermittent light irradiation for over 100 h [1]. To prevent the etching reaction from progressing, we have proposed to use NiO thin film as a protective layer for the photoanode on a sapphire substrate [2-4]. When we used this NiO thin film/GaN-based photoanode, the photocurrent density was 76% of the initial value after 100 h of continuous light irradiation [3]. Since some etch pits were observed after this time, we speculate that the decrease in photocurrent is related to the occurrence of etch pits. A transmission electron microscope (TEM) observation indicated that the etch pits are caused by dislocations that grew from the GaN-based layer to the surface. In this study, we investigated the photoelectrochemical stability to light irradiation for several hundred hours when using a GaN-based layer that was more crystalline than those in the previous studies as a photoanode.Al0.1Ga0.9N (AlGaN)/n-GaN heterostructures were grown by metal organic chemical vapor deposition on n-GaN substrate. Ni thin film (thickness: 1 nm) was fabricated on the AlGaN surface by vacuum evaporation. The sample was then heat-treated at 563 K for 60 min in air to form NiO thin film (thickness: 2.5 nm). The NiO thin film/AlGaN/n-GaN and Pt wire were immersed in 1 mol/L NaOH as the photoanode and cathode, respectively. The photocurrent was measured with a potentio-galvanostat (Solartron Analytical, 1287A) under UV light irradiation at 2.2 mW/cm2 (λ ≤ 365 nm). A Xe lamp (Asahi Spectra, MAX-303 with SHX450 filter) was used as the light source. The irradiated sample area was 1 cm × 1 cm.The photocurrent density 1 min after irradiation was 0.195 mA/cm2. The photocurrent density decreased with irradiation time and was 0.126 mA/cm2 at 300 h. However, in this experiment, the light intensity decreased by 30% after 300 h because the Xe lamp deteriorated with operating time. In order to correct the light intensity effect on the photocurrent density, the light intensity was adjusted to the initial value and the photocurrent was measured after continuous operation. The resulting photocurrent density at 300 h was 0.174 mA/cm2, which was 89% of the initial value. The surface and sectional structure of the photoanode before and after 300 h of irradiation were observed with a scanning electron microscope in order to investigate the structural change caused by the etching reaction. Most of the surface of the photoanode maintained its initial structure, though some hexagonal etch pits were observed. The etch pits occupied 8% of the surface area. The elemental analysis for pit depth direction was performed with TEM and an energy dispersive X-ray spectrometer in order to conform the layer structure at area with no pits and bottom of pit. The no-pit areas consisted of NiO, AlGaN, and n-GaN layers, while the AlGaN layer was not observed at the bottom of the pit. Therefore, the NiO and AlGaN layers occupied 92% of the photanode surface area. From these results, we consider that the loss of the effective reaction area due to the increase in etch pit density is the reason for the decrease in the photocurrent.[1] K. Ohkawa et al., Jpn. J. Appl. Phys., 52, 08JH04 (2013).[2] Y. Ono et al., Book of , ICARP, p4-17 (2017).[3] Y. Ono et al., The 85th ECSJ Spring Meeting, 1Q21 (2018) in Japanese.[4] K. Kumakura et al., The 79th JSAP Autumn Meeting, 19p-PA4-22 (2018) in Japanese. Figure 1

(Invited) Effects of Annealing Capped (0001) Oriented GaN Samples

Changes in the topological, structural, electrical and optical properties produced by annealing (0001) oriented GaN homo- and hetero- epitaxial films that were protected by an AlN cap and annealed at a temperature, TA, ranging from 1150 to 1300 ºC for a time, t A, ranging from 1 to 8 min are discussed. The films were as-grown or Mg implanted unintentionally doped n-type, or they were Mg doped. For as-grown hetero-epitaxial films the step-flow growth patterns on the as-grown surface survive up to TA ≤ 1300 ºC and t A ≤ 2 min. Small angle grain boundaries parallel to the {1100} M-planes become more defined on the surface of the hetero-structure films as TA and/or t A increases so long as it retains its smooth step-flow growth pattern. Small hexagonal etch pits associated with threading edge dislocations decorate these boundaries suggesting that the grain boundaries are created by polygonization. These boundaries are present in the as-grown film, but they do evolve over time at different rates at different temperatures as is illustrated in the (00.2), (10.2), (30.2) x-ray rocking curves, (10.5) ω - 2θ scans, and reciprocal space maps. Similar studies of films grown on ammonothermal bulk single crystals show similar trends, but they are much more muted. All of these studies require an annealing cap to minimize the rate of preferential evaporation of nitrogen. The GaN surface must be clean and smooth, the cap must adhere well, and one must be able to preferentially etch it off after the anneal. A thin AlN film grown epitaxially by MOCVD at low temperatures combined with a thicker sputtered AlN film meets all of these requirements, but it only does so up TA = 1300 ºC for t A < 8 min. It fails at higher TA because the N vapor pressure at 1300 ºC is about equal to the yield stress of AlN. Our caps fail when t A > 8 min at TA 1300 ºC because even some N escapes with a good seal. When the cap does not seal as well, the GaN film decomposes first at the small angle grain boundaries. Even when the surface of the film is smooth, the electrical properties show that the anneal has altered the internal structure. The conductance above the voltage knee of Schotty diodes fabricated on them initially increases with TA and t A, but it begins to drop off at TA = 1200 or 1250 ºC suggesting that at lower TA some of the structural defects are annealed out thereby increasing the mobility, but at higher TA other types of defects, possibly nitrogen vacancy, NV, complexes are created. This explanation is consistent with the observation of the conductance below the voltage knee being larger at the larger TA because the tunneling current through the defects associated with the NV is larger. The reverse leakage current follows the same trend – it being larger at the larger TA. The CV characteristics of the diode suggests that this defect layer is mostly confined to the region near the surface as the net carrier concentration as measured at the end of the depletion layer does not change with TA until TA ≥ 1250 ºC when the defected area is at the end of the depletion layer at zero bias. Defects of a different nature are created by the implant process as determined by the increased breadth of the XRD curves and the creation of another peak. But much of that damage is annealed out as the peak widths return to almost their original breadth and the second peak disappears. However, they do not return to their original breadth. It is not clear whether this is due to not removing all of the implant damage or the damage created by the annealing process contributing to the increased breadth. We are now in the process of deciphering the more complicated electrical measurements made on the Mg implanted or doped samples, and expect to report on them at the meeting.

Surface morphology and dislocation characteristics near the surface of 4H-SiC wafer using multi-directional scanning transmission electron microscopy.

To improve the reliability of silicon carbide (SiC) electronic power devices, the characteristics of various kinds of crystal defects should be precisely understood. Of particular importance is understanding the correlation between the surface morphology and the near surface dislocations. In order to analyze the dislocations near the surface of 4H-SiC wafers, a dislocation analysis protocol has been developed. This protocol consists of the following process: (1) inspection of surface defects using low energy scanning electron microscopy (LESEM), (2) identification of small and shallow etch pits using KOH low temperature etching, (3) classification of etch pits using LESEM, (4) specimen preparation of several hundred nanometer thick sample using the in-situ focused ion beam micro-sampling® technique, (5) crystallographic analysis using the selected diffraction mode of the scanning transmission electron microscope (STEM), and (6) determination of the Burgers vector using multi-directional STEM (MD-STEM). The results show a correlation between the triangular terrace shaped surface defects and an hexagonal etch pit arising from threading dislocations, linear shaped surface defects and elliptical shaped etch pits arising from basal plane dislocations. Through the observation of the sample from two orthogonal directions via the MD-STEM technique, a basal plane dislocation is found to dissociate into an extended dislocation bound by two partial dislocations. A protocol developed and presented in this paper enables one to correlate near surface defects of a 4H-SiC wafer with the root cause dislocations giving rise to those surface defects.

Superluminescent light emitting diodes on naturally survived InGaN/GaN lateral nanowires

We demonstrate a method for nanowire formation by natural selection during wet anisotropic chemical etching in boiling phosphoric acid. Nanowires of sub-10 nm lateral dimensions and lengths of 700 nm or more are naturally formed during the wet etching due to the convergence of the nearby crystallographic hexagonal etch pits. These nanowires are site controlled when formed in augmentation with dry etching. Temperature and power dependent photoluminescence characterizations confirm excitonic transitions up to room temperature. The exciton confinement is enhanced by using two-dimensional confinement whereby enforcing greater overlap of the electron-hole wave-functions. The surviving nanowires have less defects and a small temperature variation of the output electroluminescent light. We have observed superluminescent behaviour of the light emitting diodes formed on these nanowires. There is no observable efficiency roll off for current densities up to 400 A/cm2.

Effects of phosphoric acid on the surface morphology and reflectance of AlN grown by MBE under Al-rich conditions

Aluminium nitride (AlN) films grown by molecular beam epitaxy (MBE) were exposed to phosphoric acid (H3PO4) heated to 70°C for 10min. The H3PO4 treatment removed excess aluminium (Al) from the surface of AlN grown under Al-rich conditions. A side effect of the H3PO4 process was the formation of hexagonal etch pits up to 180nm in size. Reflectance measurements were performed before and after the removal of excess Al from the film surface. Excess surface Al in the form of droplets was found to strongly influence the reflectance of the AlN films. Consequently the Al surface droplets (AlSD) introduced an error into film thickness values derived from Lorentz oscillator model fitting of the specular reflectance. The film thickness measurement error due to excess surface Al was quantified as a function of the AlSD surface coverage percentage. The effect of AlSD on reflectance-based film thickness measurements must be taken into account when using AlN grown in the Al droplet regime to characterize the nitrogen (N) growth rate.

Defect‐enabled electrical current leakage in ultraviolet light‐emitting diodes

Electrical current leakage paths in AlGaN‐based ultraviolet (UV) light‐emitting diodes (LEDs) are identified using conductive atomic force microscopy. Open‐core threading dislocations are found to conduct current through insulating Al0.7Ga0.3N layers. A defect‐sensitive H3PO4 etch reveals these open‐core threading dislocations as 1–2 µm wide hexagonal etch pits visible with optical microscopy. Additionally, closed‐core threading dislocations are decorated with smaller and more numerous nanometer‐scale pits, which are quantifiable by atomic‐force microscopy. The performances of UV‐LEDs fabricated on similar Si‐doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these electrically conductive open‐core dislocations, while the total threading dislocation densities of the UV‐LEDs remain relatively unchanged.

Electrical current leakage and open-core threading dislocations in AlGaN-based deep ultraviolet light-emitting diodes

Electrical current transport through leakage paths in AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) and their effect on LED performance are investigated. Open-core threading dislocations, or nanopipes, are found to conduct current through nominally insulating Al0.7Ga0.3N layers and limit the performance of DUV-LEDs. A defect-sensitive phosphoric acid etch reveals these open-core threading dislocations in the form of large, micron-scale hexagonal etch pits visible with optical microscopy, while closed-core screw-, edge-, and mixed-type threading dislocations are represented by smaller and more numerous nanometer-scale pits visible by atomic-force microscopy. The electrical and optical performances of DUV-LEDs fabricated on similar Si-doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these nanopipes, despite their small fraction (&amp;lt;0.1% in this study) of the total density of threading dislocations.

Dislocation Revelation for 4H-SiC by Using Vaporized NaOH: A Possible Way to Distinguish Edge, Screw and Mixed Threading Dislocations by Etch Pit Method

In this paper, we report a newly developed dislocation-revealing etch pit method for 4H-SiC single crystal, which can distinguish edge (TED, Burgers vector b=a), elementary screw (TSD, b=1c) and mixed (TMD, b=c+a) threading dislocations. In this method, vaporized NaOH gas was used to etch the Si-face of a SiC wafer at substrate temperature around 950 °C. By a side-by-side comparison between the optical images of the etch pits and the X-ray topographic (XRT) images, it has been found that threading dislocations (TDs) in SiC could be revealed as hexagonal etch pits with distinct geometrical features (shape, size and facet orientation) depending on their Burgers vectors. Based on these results, we consider this etch pit method as an easily-operated and inexpensive technique to categorize TDs, and it may help to promote our understanding on the different roles that these types of TDs have played in the performance degradation of SiC power devices.

Extreme Monolayer-Selectivity of Hydrogen-Plasma Reactions with Graphene

We study the effect of remote hydrogen plasma on graphene deposited on SiO₂. We observe strong monolayer selectivity for reactions with plasma species, characterized by isotropic hole formation in the basal plane of monolayers and etching from the sheet edges. The areal density of etch pits on monolayers is 2 orders of magnitude higher than on bilayers or thicker sheets. For bilayer or thicker sheets, the etch pit morphology is also quite different: hexagonal etch pits of uniform size, indicating that etching is highly anisotropic and proceeds from pre-existing defects rather than nucleating continuously as on monolayers. The etch rate displays a pronounced dependence on sample temperature for monolayer and multilayer graphene alike: very slow at room temperature, peaking at 400 °C and suppressed entirely at 700 °C. Applying the same hydrogen plasma treatment to graphene deposited on the much smoother substrate mica leads to very similar phenomenology as on the rougher SiO₂, suggesting that a factor other than substrate roughness controls the reactivity of monolayer graphene with hydrogen plasma species.

HVPE법으로 성장시킨 GaN 단결정의 wet etching에 의한 표면 변화

In this paper, we investigated characteristics of etching induced surface morphology variation by wet etching of GaN epilayer were grown on sapphire (0001) substrate by hydride vapor phase epitaxy (HVPE). As a results of scanning electron microscope (SEM) observation, three types of hexagonal etch pits (Edge, Screw, Mixed) were formed by the GaN epilayer thickness variations. A lot of etch pits, attributed to screw and mixed type TD, were observed at thinner epilayer, leading to high etch pit density. On the other hand, the thickness of GaN epilayer increased with the number of etch pits corresponding to edge and mixed dislocations, which are the majority of TDs are observed.

Variation of Etch Pit Size by Screw Dislocation Tilt in 4H-SiC Wafer

The wide size distribution of the hexagonal etch pit of screw dislocations (SD) in 4H-SiC wafer was found in spite of the narrow size distribution of the SD pit in epitaxial film. Calculation on the basis of the strain energy equation indicated that etch pit size depends on the Burgers vector and dislocation tilt. Size variation of SD etch pits in 4H-SiC wafer fabricated by sublimation method is explained to be caused by the dislocation tilt by observing the sizes and the positions of etch pits from the surface of the epitaxial film to the inside of 4H-SiC wafer. The SDs in 4H-SiC wafer fabricated by sublimation method propagate to c-axis direction in macroscopic but changing tilt in microscopic.

Dislocations in Gan Grown on Silicon

In order to understand dislocations in GaN grown on silicon using metal organic chemical vapor deposition (MOCVD), two samples with different film thickness were grown. The distribution of dislocations and its influence on epitaxial layer were studied. Wet etching can effectively reveal the dislocations in GaN, and the hexagonal etch pits are the emergence of dislocations. After etching in the aqueous KOH: H2O =3:20 (m:m), there are more pits with longer etch time. SEM shows that there are more hexagonal etch pits near the micro-cracks because many dislocations are induced by micro-cracks. In the experiment, ribbon slopes in the two sides of the pits are observed, and they are paralleled to each other. If the growth time prolonged, the ribbon slopes would disappear and the surface become flat, and this mechanism is discussed in this paper.

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.

Molten KOH Etching with Na2O2 Additive for Dislocation Revelation in 4H-SiC Epilayers and Substrates

A novel etching solution using molten KOH with Na2O2 additive (KN etching) for dislocation revelation in 4H-SiC epilayers and substrates has been proposed. Threading screw and edge dislocations (TSDs and TEDs) have been clearly revealed as hexagonal etch pits differing in pit size, and basal plane dislocations (BPDs) as seashell-shaped pits. KN etching has provided a solution to the problem that KOH etching is not effective for dislocation identification in n+-4H-SiC. The influences of SiC off-axis angles, carrier concentrations, and growth techniques on the effectiveness of KN etching have also been investigated. It has been shown that KN etching is applicable to SiC epilayers and substrates with any off-axis angle from 0 to 8° and electron concentrations from 1015 to 1019 cm-3.

Microscopic Study of Hydroxyapatite Dissolution As Affected by Fluoride Ions

Fluoride ions play a critical role in preventing tooth decay. We investigated the microscopic effects of fluoride ions on hydroxyapatite (100) surface dissolution using in situ atomic force microscopy. In the presence of 10 mM NaF, individual surface step retraction velocities decreased by about a factor of 5 as compared to NaF-free conditions. Importantly, elongated hexagonal etch pits, which are characteristic of (100) surface dissolution, were no longer observed when NaF was present. The alteration of pit shape is more distinct at a higher NaF concentration (50 mM) where triangular etch pits evolved during dissolution. Furthermore, in a fluoride concentration typical for tap water (10 μM), we observed roughening of individual step lines, resulting in the formation of scalloped morphologies. Morphological changes to individual steps across a wide range of fluoride concentrations suggest that the cariostatic capabilities of fluoride ions originate from their strong interactions with molecular steps.

Effects of Surface and Crystalline Defects on Reverse Characteristics of 4H-SiC Junction Barrier Schottky Diodes

To clarify the relationship between the dispersed reverse characteristics of 4H-SiC junction barrier Schottky (JBS) diodes and defects, we investigated the sensitivity of the reverse characteristics to surface and crystalline defects in 4H-SiC epitaxial layers. Strong correlations were obtained between the reverse characteristics of 4H-SiC JBS diodes and surface defects. Micropipes or particles reduced blocking voltage and carrot defects increased leakage current. Furthermore, the leakage current of 4H-SiC JBS diodes depends on the etch pit density of threading dislocations (TDs). Etch pits formed from TDs increased leakage current by about 10-9 A in samples without surface defects. In addition, hexagonal etch pits formed from unusual crystalline defects were observed; they also increased leakage current.

Effects of Surface and Crystalline Defects on Reverse Characteristics of 4H-SiC Junction Barrier Schottky Diodes

To clarify the relationship between the dispersed reverse characteristics of 4H-SiC junction barrier Schottky (JBS) diodes and defects, we investigated the sensitivity of the reverse characteristics to surface and crystalline defects in 4H-SiC epitaxial layers. Strong correlations were obtained between the reverse characteristics of 4H-SiC JBS diodes and surface defects. Micropipes or particles reduced blocking voltage and carrot defects increased leakage current. Furthermore, the leakage current of 4H-SiC JBS diodes depends on the etch pit density of threading dislocations (TDs). Etch pits formed from TDs increased leakage current by about 10-9 A in samples without surface defects. In addition, hexagonal etch pits formed from unusual crystalline defects were observed; they also increased leakage current.