Hexagonal Pits Research Articles

The roles of buffer layer thickness on the properties of the ZnO epitaxial films

In this article, the authors have investigated the optimization of the buffer thickness for obtaining high-quality ZnO epi-films on sapphire substrates. The growth mechanism of the buffers with different thickness has been clearly revealed, including the initial nucleation and vertical growth, the subsequent lateral growth with small grain coalescence, and the final vertical growth along the existing larger grains. Overall, the quality of the buffer improves with increasing thickness except the deformed surface morphology. However, by a full-scale evaluation of the properties for the epi-layers, the quality of the epi-film is briefly determined by the surface morphology of the buffer, rather than the structural, optical, or electrical properties of it. The best quality epi-layer has been grown on the buffer with a smooth surface and well-coalescent grains. Meanwhile, due to the huge lattice mismatch between sapphire and ZnO, dislocations are inevitably formed during the growth of buffers. More importantly, as the film grows thicker, the dislocations may attracting other smaller dislocations and defects to reduce the total line energy and thus result in the formation of V-shape defects, which are connected with the bottom of the threading dislocations in the buffers. The V-defects appear as deep and large hexagonal pits from top view and they may act as electron traps which would affect the free carrier concentration of the epi-layers.

Real-time observation of graphene oxidation on Pt(111) by low-energy electron microscopy

A monolayer of graphene was prepared by thermal decomposition of ethylene gas on Pt(111). The graphene can be readily removed by dosing O2 at pressures in 10−8mbar range and surface temperatures (Ts) near 1000K. Residual gas analysis during the oxygen treatment of graphene layer detected CO to be the only formed product. The oxidation process has been continuously imaged by Low-energy Electron Microscope (LEEM) operated in mirror-electron mode. LEEM observations revealed that the oxidation of graphene on Pt(111) occurs simultaneously at the outer island perimeter and in the interior of the graphene island. Symmetric hexagonal pits were observed to form continuously within graphene sheets, the pits proceeded isotropically. The etch rate was determined to be equal for both modes and independent of the surface environment with the exception of areas above Pt step edges. The pit growth rate at constant oxygen pressure was found to increase exponentially with respect to temperature over the investigated Ts range of 927–1014K, yielding an apparent activation energy of 479kJ/mol.

Surface state of GaN after rapid-thermal-annealing using AlN cap-layer

Critical issues need to be overcome to produce high performance Schottky diodes on gallium nitride (GaN). To activate dopant, high temperature thermal treatments are required but damage GaN surface where hexagonal pits appear and prevent any device processing. In this paper, we investigated the efficiency of cap-layers on GaN during thermal treatments to avoid degradation. Aluminum nitride (AlN) and silicon oxide (SiOx) were grown on GaN by direct current reactive magnetron sputtering and plasma-enhanced chemical vapor deposition, respectively. AlN growth parameters were studied to understand their effect on the grown layers and their protection efficiency. Focused ion beam was used to measure AlN layer thickness. Crystalline quality and exact composition were verified using X-ray diffraction and energy dispersive X-ray spectroscopy. Two types of rapid thermal annealing at high temperatures were investigated. Surface roughness and pits density were evaluated using atomic force microscopy and scanning electron microscopy. Cap-layers wet etching was processed in H3PO4 at 120°C for AlN and in HF (10%) for SiOx. This work reveals effective protection of GaN during thermal treatments at temperatures as high as 1150°C. Low surface roughness was obtained. Furthermore, no hexagonal pit was observed on the surface.

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.

Pit assisted oxygen chemisorption on GaN surfaces.

A comprehensive analysis of oxygen chemisorption on epitaxial gallium nitride (GaN) films grown at different substrate temperatures via RF-molecular beam epitaxy was carried out. Photoemission (XPS and UPS) measurements were performed to investigate the nature of the surface oxide and corresponding changes in the electronic structure. It was observed that the growth of GaN films at lower temperatures leads to a lower amount of surface oxide and vice versa was observed for a higher temperature growth. The XPS core level (CL) and valence band maximum (VBM) positions shifted towards higher binding energies (BE) with oxide coverage and revealed a downward band bending. XPS valence band spectra were de-convoluted to understand the nature of the hybridization states. UPS analysis divulged higher values of electronic affinity and ionization energy for GaN films grown at a higher substrate temperature. The surface morphology and pit structure were probed via microscopic measurements (FESEM and AFM). FESEM and AFM analysis revealed that the film surface was covered with hexagonal pits, which played a significant role in oxygen chemisorption. The favourable energetics of the pits offered an ideal site for oxygen adsorption. Pit density and pit depth were observed to be important parameters that governed the surface oxide coverage. The contribution of surface oxide was increased with an increase in average pit density as well as pit depth.

화학적 습식 에칭을 통한 AlN와 GaN의 결함 및 표면 특성 분석

화학적 습식 에칭을 통해 AlN와 GaN의 결함 및 표면 특성을 분석했다. 화학적 습식 에칭은 단결정의 결함을 선택적으로 에칭하기 때문에 결정의 품질을 평가하는 좋은 방법으로 주목 받고 있다. AlN와 GaN의 단결정은 NaOH/KOH 용융액을 이용하여 에칭을 했으며, 에칭 후 표면 특성을 알아보기 위해 주사전자현미경(SEM)과 원자힘 현미경(AFM)을 촬영했다. 에치 핏의 깊이를 측정하여 표면에 따른 에칭 속도를 계산했다. 그 결과 AlN와 GaN 표면에는 두 개의 다른 형태에 에치 핏이 형성 되었다. (0001)면의 metal-face(Al, Ga)는 육각 추를 뒤집어 놓은 형태를 갖는 반면 N-face는 육각형 형태의 소구 모양(hillock structure)을 하고 있었다. 에칭 속도는 N-face가 metal-face(Al, Ga)보다 각 각 약 109배(AlN)와 5배 정도 빨랐다. 에칭이 진행되는 동안 에치 핏은 일정한 크기로 증가하다 서로 이웃한 에치 핏들과 합쳐지는 것으로 보여졌다. 또한 AlN와 GaN의 에칭 공정을 화학적 메커니즘을 통해 알아 보았는데, 수산화 이온(<TEX>$OH^-$</TEX>)과 질소의 dangling bond에 영향을 받아 metal-face(Al, Ga)와 N-face가 선택적으로 에칭되는 것으로 추론되었다. We investigated defects and surface polarity in AlN and GaN by using wet chemical etching. Therefore, the effectiveness and reliability of estimating the single crystals by defect selective etching in NaOH/KOH eutectic alloy have been successfully demonstrated. High-quality AlN and GaN single crystals were etched in molten NaOH/KOH eutectic alloy. The etching characteristics and surface morphologies were carried out by scanning electron microscope (SEM) and atomic force microscope (AFM). The etch rates of AlN and GaN surface were calculated by etching depth as a function of etching time. As a result, two-types of etch pits with different sizes were revealed on AlN and GaN surface, respectively. Etching produced hexagonal pits on the metal-face (Al, Ga) (0001) plane, while hexagonal hillocks formed on the N-face. On etching rate calibration, it was found that N-face had approximately 109 and 15 times higher etch rate than the metal-face of AlN and GaN, respectively. The size of etch pits increased with an increase of the etching time and they tend to merge together with a neighbouring etch pits. Also, the chemical mechanism of each etching process was discussed. It was found that hydroxide ion (<TEX>$OH^-$</TEX>) and the dangling bond of nitrogen play an important role in the selective etching of the metal-face and N-face.

Improved light extraction efficiency and leakage characteristics in light-emitting diodes by nanorod arrays formed on hexagonal pits

We report the enhancement of light extraction efficiency (LEE) and electrical performance in GaN-based green light-emitting diodes (LEDs) using ZnO nanorods formed on the etched surface of p-GaN. Green LEDs with hybrid ZnO nanorod structures grown on the hexagonally etched topmost layer of the LEDs, show an improvement in electroluminescence intensity that is 3.5 times higher than LEDs without any other surface treatments. The improvement in LEE in LEDs with nanohybrid structures was confirmed by finite-difference time-domain simulation analysis. Besides LEE enhancement, the surface etching effects on the reduction of leakage current of fabricated LEDs were also investigated.

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.

Integrating Surface Textures on ZnO Substrate for High Light Extraction Efficiency Light-Emitting Diode

Due to the large refractive index of ZnO material, light extraction efficiency (LEE) of ZnO based light-emitting diode (LED) is limited by the total reflection. We present a combined experimental and computational approach to study the effects of integrated surface textures on the efficiency of light extraction from the ZnO material. Hexagonal pyramids and pits textures were obtained by selective and anisotropic etching of O-polarity ZnO single crystal in HCl and KOH solutions, respectively. Dense hexagonal pyramids were integrated into the hexagonal pits via a two-step wet etching process. Ray-tracing simulations indicate that the substrate texturing can improve the performance of the ZnO based LEDs by enhancing the LEE and improving the light field uniformity. The integrated photoluminescence extraction was improved by a factor of 117% with respect to that of a planar ZnO single crystal through integrating the pyramids textures into the hexagonal pits.

Comparison of electrostatic and localized plasmon induced light enhancement in hybrid InGaN/GaN quantum wells

The light enhancement phenomena in InGaN/GaN multi-quantum wells (MQWs) infiltrated with metal nanoparticles (NPs) are studied using resonant and off-resonant localized plasmon interactions. The emission and recombination characteristics of carriers in InGaN/GaN MQW structures with inverted hexagonal pits (IHPs) are modified distinctly depending on the nature of their interaction with the metal NPs and with the pumping and emitted photons. It is observed that the emission intensity of light is significantly enhanced when the emission energy is off-resonant to the localized plasmon frequency of the metal nanoparticles. This results in enhanced emission from MQW due to Au nanoparticles and from IHPs due to Ag nanoparticles. At resonant-plasmon frequency of the Ag NPs, the emission from MQWs is quenched due to the re-absorption of the emitted photons, or due to the drift carriers from c-plane MQWs towards the NPs because of the Coulomb forces induced by the image charge effect.

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.

Effect of growth temperature on defects in epitaxial GaN film grown by plasma assisted molecular beam epitaxy

We report the effect of growth temperature on defect states of GaN epitaxial layers grown on 3.5 μm thick GaN epi-layer on sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The GaN samples grown at three different substrate temperatures at 730, 740 and 750 °C were characterized using atomic force microscopy and photoluminescence spectroscopy. The atomic force microscopy images of these samples show the presence of small surface and large hexagonal pits on the GaN film surfaces. The surface defect density of high temperature grown sample is smaller (4.0 × 108 cm−2 at 750 °C) than that of the low temperature grown sample (1.1 × 109 cm−2 at 730 °C). A correlation between growth temperature and concentration of deep centre defect states from photoluminescence spectra is also presented. The GaN film grown at 750 °C exhibits the lowest defect concentration which confirms that the growth temperature strongly influences the surface morphology and affects the optical properties of the GaN epitaxial films.

Controlled nucleation and growth of nanostructures by employing surface modified GaN based layers/heterostructures as bottom layer

Controlled nucleation and growth of Zinc oxide nanorods is achieved on GaN, etched GaN and AlGaN/GaN heterostructure bottom layers grown by a metal organic chemical vapour deposition technique. The effects of the bottom crystalline layers on the structural, morphological and optical properties of the as grown ZnO nanorods have been investigated by high-resolution X-ray diffraction, scanning electron microscopy, photoluminescence and Raman measurements. HRXRD (0002) reciprocal-space mapping (RSMs) studies were performed on GaN and AlGaN/GaN layers before and after the growth of ZnO nanostructures to investigate the impact of strain upon the ZnO layer grown on GaN layers and AlGaN/GaN heterostructures. Raman intensity mapping shows the densely nucleated hexagonal pit like structures for the etched GaN layer, providing an enhanced surface area for primary nucleation suggesting that the growth species prefer to condense on locations with maximum binding energy. The increase in nucleation density for etched GaN layers also result in dense nanorods which exhibit better excitonic emission. Our studies suggest that ZnO nanostructures with improved optical and structural properties can be grown on etched-GaN as well as AlGaN/GaN heterostructures as the bottom layer. It is interesting to observe that the bottom GaN layer can be easily employed to determine the optical quality of ZnO layer.

Effect of V-Shaped Pit Size on the Reverse Leakage Current of InGaN/GaN Light-Emitting Diodes

We report the effect of V-shaped pit size, inverted hexagonal pits embedded in InGaN multiple quantum well, on the reverse leakage current of InGaN light-emitting diodes (LEDs). It was found that the reverse leakage current of InGaN LEDs with larger V-shaped pits is significantly reduced from 1.80 mA down to 3.84 nA at -30 V by several orders of magnitude. We claim that this improvement is accounted for increased Poole-Frenkel barrier height of carrier trapped at the deep centers via enlarged V-pit formation, consequently resulting in lower reverse current of InGaN LEDs.

Fabrication of GaN nanodots via GaN thermal decomposition in H2 atmosphere

GaN nanodots were fabricated by thermal decomposition of GaN in H2 atmosphere at high temperatures. By varying annealing time and temperature, it was found that dot size and density were highly dependent on annealing conditions. Surface morphology of the sample indicates that the dot formation mechanism is chemical erosion under H2 flow. The etching proceeds in lateral directions, and the dots are formed in hexagonal pit areas. The fitted activation energy of GaN decomposition agrees well with the chemical properties of GaN in the experimental conditions.

Step-flow growth mode instability of N-polar GaN under N-excess

GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards ⟨101¯0⟩, interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions.

Evidence of deep traps in overgrown v-shaped defects in epitaxial GaN layers

The geometric and electronic structure of overgrown v-shaped defects in GaN epitaxial layers are investigated by cross-sectional scanning tunneling microscopy and spectroscopy. The v-defects are found to be hexagonal pit structures delimited by six {112¯2} planes. The electronic properties are inhomogeneous. In some areas the center of the v-defects exhibits a strongly inhibited tunneling current, indicating the presence of deep traps.

Hydrogen- and corrosion-induced defect characterization of ZnO single crystal: A slow positron beam study

Hydrogen- and corrosion-induced microstructure and defects of ZnO single crystal under acidic conditions have been studied by using slow positron beam spectroscopy and microscopy techniques. Slow positron beam measurements revealed that a near-surface damaged region with a high concentration of defects was formed after hydrogen charging, but not after corrosion. The formation of hexagonal pyramids was found after electrochemical charging with hydrogen, while the formation of stepped hexagonal pits was observed after direct immersion in the solution. The incorporation of hydrogen by electrochemical charging enhanced the near-band-edge (NBE) emission intensity.

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.