Photo-assisted Etching Research Articles

High-Efficiency and High-Quality Photogalvanic Etching of the Silicon Doped N-Type Gallium Nitride Using Potassium Peroxomonosulfate Oxidant

Photogalvanic etching, or photo-assisted electroless etching, is a simple wet-etching approach to fabricate n-type gallium nitride (GaN)-based devices without any external power supply. However, the current technology is far from practical because efficient etching can only be realized by a potassium persulfate (PS) oxidant/alkaline electrolyte system that inevitably bends the etching surface. In this study, we proposed and tested a new potassium peroxomonosulfate (PMS) oxidant/weak acid electrolyte system for the photogalvanic etching of platinum (Pt) photo-mask patterned GaN wafers. A novel finding is that Pt can catalyze PMS decomposition in acids, predominantly generating singlet oxygen (1O2), whose onset reduction potential is 0.95 V higher than PS. Under 25.4 mW.cm2 ultraviolet (UV) irradiation, PS-driven photogalvanic etching of inert silicon-doped GaN (si-GaN) wafers is invalid, whereas the new system enables high efficiency and quality etching in 20 mM PMS + 0.1 M K2SO4 electrolyte (pH = 3.0). The etching rate reaches 12.0 nm.min−1, while the as-prepared groove bottoms have nanometer surface flatness, and the surface roughness (Ra) attains 5.25 nm (5 × 5 μm2). In summary, the PMS/weak acid electrolyte system makes photogalvanic etching is a promising practical technique.

In-plasma photo-assisted etching of Si with chlorine aided by an external vacuum ultraviolet source

Photo-assisted etching of p-type Si was previously found to occur in a chlorine-containing, Faraday-shielded, inductively coupled plasma (ICP), and this was attributed to the vacuum ultraviolet (VUV) light generated by the plasma. Other causes for the very high etching rates were ruled out, including ion bombardment. In the present study, the substrate in the main Cl2/Ar ICP was subjected to extra VUV light that was generated in an independently controlled, auxiliary Ar/He ICP in tandem with the main ICP. The ICPs were separated by a tungsten mesh and a bundle of high-aspect-ratio quartz tubes in a honeycomb configuration. There was no measurable perturbation of the main plasma by the auxiliary plasma. The etching rate was found to be enhanced by 11%–51% with the additional VUV light provided by the auxiliary ICP. With absolute measurements of the auxiliary ICP photon flux at the sample surface, as described elsewhere, incredibly large etching yields of 90–240 Si atoms per photon were obtained. It is argued that etching is not a result of electron–hole pair formation but is instead ascribed to a photocatalytic chain reaction.

Wet etching of semi-polar (11–22) GaN on m-sapphire by different methods

Wet etching methods with conventional alkali solution, molten alkali and photo-assisted alkali solution were performed on semi-polar (11–22) GaN separately to study the wet etching characteristics of it. Different corrosion morphologies were compared and the causes were analyzed. The results show that, contrary to the general view of chemical corrosion resistance of GaN, the semi-polar GaN is easy to be corroded even at room temperature. Prism structures with different sizes are formed after etching, and the size diversity increases with prolonging etching time, which could attribute to the extension of dislocations. Molten KOH corrosion could form a layer like structure and strip corrosion pits reflecting dislocations along c-direction. Moreover, a more uniform morphology could be obtained and the density of threading dislocations could be revealed by photo-assisted etching.

(Invited) Photo-Electrochemical Etching and Porosification of III-Nitride Semiconductors

Gallium nitride (GaN), which is a III-nitride semiconductor, has wide direct bandgap (3.4 eV), high breakdown electric field (3.3x106 V/cm) and high saturation electron velocity (2.7 x107 cm/sec). On the basis of excellent optical and electrical properties, significant progress has been achieved in the GaN and III-nitride based devices, such as ultraviolet laser diodes, white light-emitting diodes, and high-frequency power transistors. In addition, GaN and its alloys are getting much attention as building block materials for electrochemical (EC) energy conversion systems such as chemical sensors, water splitting, and artificial photosynthesis due to their superior chemical stability, direct transition, and widely tunable bandgap by alloying. For the fabrication of nitride semiconductor devices, the plasma-assisted dry etching process has commonly been used. However, various types of damage and defects induced by the bombardment of reactive ions and radicals leads to severe operational stability problems in optical and electrical devices.Photo-electrochemical (PEC) etching probably solves these problems on nitride semiconductors. PEC etching possesses some big advantages such as low-damage process conducted at room temperature. In this presentation, we introduce our recent work on the PEC etching and porosification of III-nitride semiconductors. The etching features such as smooth and flat etching and porosification etching have been controlled by changing the PEC conditions [1,2]. The porous structure formed by the present method has attractive features such as a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. In addition, this technique does not require any complicated process such as lithography, indicating higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth.One example is the formation of straight pores along with the electric field induced in the vertical to the GaN substrate. This kind of straight pores was formed in the dark condition under the high-electric filed applied. Here, we found that the pore diameter was enlarged by the irradiation of UV light to the GaN surface during the porosification. In the case of the irradiation of UV light with a photon energy (hv) above the bandgap energy (E g), the pore diameter was enlarged only in the top region with a depth less than 1µm, where the photo-absorption occurred only near the surface. On the other hand, under the illumination with hvbelow E g, the pore diameter was enlarged overall pore from the top to the bottom. This kind of photo-assisted etching can be explained by the sub-bandgap absorption during the porosification. Sub-bandgap light with a photon energy below the bandgap energy was transmitted through bulk GaN, but a certain type of photo-absorption additively occurred at the pore tip where high electric filed was concentrated. To confirm such specific absorption process, we systematically investigated the correlation between the photocurrents and photon energy by changing the light intensity. The photocurrents observed when the high-electric field was applied to n-GaN were well reproduced by calculation taking into account the Frantz-Keldysh effect [3]. The results suggest that the independent control of pore diameter and depth is possible by controlling the PEC conditions.Acknowledgement: This work was supported by JSPS KAKENHI-JP16H06421, 20H02175.[1] Y. Kumazaki et al., J. Appl. Phys., 121, 184501 (2017).[2] Y. Kumazaki et al., J. Electrochem. Soc., 164, H477 (2017).[3] M. Toguchi et al., J. Electrochem. Soc., 166, H510 (2019). Figure 1

Effects of O2 addition on in-plasma photo-assisted etching of Si with chlorine

Addition of oxygen was used to control the in-plasma photo-assisted etching (PAE) of p-type Si(100) and poly-Si in a high density, inductively coupled, Faraday-shielded, Ar/Cl2 (225/25 SCCM), 60 mTorr plasma. After etching, samples were transferred under vacuum to an UHV x-ray photoelectron spectroscopy chamber for surface analysis. Samples etched under PAE conditions (ion energies below the ion-assisted etching, IAE, threshold) had a thicker surface oxide and lower [Cl] surface concentration, when compared to samples etched under IAE conditions (ion energies above the IAE threshold). PAE was found not to be affected by 0.1 or 0.25 SCCM O2 addition, while etching stopped with more than 0.5 SCCM O2 addition. IAE with RF power on the sample stage, resulting in −65 V self-bias, was not affected by up to 2 SCCM of oxygen addition but decreased rapidly when more than 5 SCCM O2 was added to the plasma. These results imply that PAE may be completely suppressed, while IAE occurs unobstructed. The implications of these findings are discussed in view of applications involving continuous wave and pulsed-plasma processes.

(Invited) Photoluminescence Diagnostics of Nanoscale Dissolution of III-V Semiconductor Nanoheterostructures

Etching of semiconducting materials with atomic level resolution is of high interest to technologies addressing the fabrication of low-dimensional devices and the tunability of their optoelectronic properties. Atomic layer etching (ALE) based on cycling between a self-limited surface reaction and selective removal of the reaction products is a promising technique addressing the challenges of controlling the variability necessary in the atomic-scale material removal [1]. However, the requirement to alter the processing hardware while completing two cycles of ALE, as well as the lack of a simple diagnostic technique for monitoring in situ material etching with a sub-monolayer resolution, have restricted the application of this technique to laboratories equipped with relatively expensive hardware infrastructures. To alleviate some of the problems of the ALE technique, we have investigated digital photo-assisted etching of GaAs/AlGaAs nanoheterostructures, which has the potential to offer a simple approach for fabrication of low-dimensional devices. The premise of this approach is based on the expectation that dark corrosion of a semiconductor is negligible in a processed solution. This is the case, for instance, for some of the III-V semiconductors immersed in deionized water or weak aqueous solutions of ammonia. The photo-ALE is an innovative method of a cycled digital photocorrosion (DIP) of semiconductors that we have explored for shallow etching of three-dimensional GaAs/AlGaAs nanoheterostructures [2, 3]. The rate and stability of DIP depends on the energy and intensity of photons employed for optical excitation of the samples, as well as on the chemistry of a liquid environment employed for processing [4]. The excitation of electron-hole pairs allows the convenient application of the photoluminescence (PL) effect for monitoring in situ of the DIP process. The DIP process can also be monitored by measuring the open circuit potential of revealed surfaces [5], which is of a particular interest to materials with negligible PL emission.In the frame of this presentation, I will discuss some fundamental aspects of the DIP process and application of the PL effect for monitoring sub-monolayer removal of GaAs and AlGaAs. The compatibility of the photo-ALE technology with the chemistry of liquids employed for passivation of surfaces and coating with self-assembled monolayers offers the potential for in situ passivation of atomically clean surfaces. Furthermore, the sensitivity of a low-excitation intensity generated PL signal to surface defects allows spatially-resolvable recording of the perturbation of the DIP process. I will also discuss the perspective of an application of the photo-ALE to other III-V materials, as well as to Si and SiGe.______________________[1] K. J. Kanarik et al., "Overview of atomic layer etching in the semiconductor industry," Journal of Vacuum Science& Technology A: Vacuum, Surfaces, and Films, vol. 33, no. 2, p. 020802, 2015, doi: 10.1116/1.4913379.[2] S. Aithal, N. Liu, and J. J. Dubowski, "Photocorrosion metrology of photoluminescence emitting GaAs/AlGaAs heterostructures," Journal of Physics D: Applied Physics, vol. 50, no. 3, p. 035106, 2017.[3] M. R. Aziziyan, H. Sharma, and J. J. Dubowski, "Photo-Atomic Layer Etching of GaAs/AlGaAs Nanoheterostructures," Acs Appl Mater Inter, vol. 11, no. 19, pp. 17968-17978, 2019/05/15 2019, doi: 10.1021/acsami.9b02079.[4] H. Sharma, K. Moumanis, and J. J. Dubowski, "pH-Dependent Photocorrosion of GaAs/AlGaAs Quantum Well Microstructures," The Journal of Physical Chemistry C, vol. 120, no. 45, pp. 26129-26137, 2016/11/17 2016, doi: 10.1021/acs.jpcc.6b08844.[5]. S. Aithal and J. J. Dubowski, "Open circuit potential monitored digital photocorrosion of GaAs/AlGaAs quantum well microstructures," Appl Phys Lett, vol. 112, no. 15, p. 153102, 2018, doi: 10.1063/1.5023134.

Influence of Illumination on Porous Silicon Formed by Photo-Assisted Etching of p-Type Si with a Different Doping Level.

The influence of illumination intensity and p-type silicon doping level on the dissolution rate of Si and total current by photo-assisted etching was studied. The impact of etching duration, illumination intensity, and wafer doping level on the etching process was investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and Ultraviolet-Visible Spectroscopy (UV–Vis–NIR). The silicon dissolution rate was found to be directly proportional to the illumination intensity and inversely proportional to the wafer resistivity. High light intensity during etching treatment led to increased total current on the Si surface. It was shown that porous silicon of different thicknesses, pore diameters, and porosities can be effectively fabricated by photo-assisted etching on a Si surface without external bias or metals.

Evidence for anti-synergism between ion-assisted etching and in-plasma photoassisted etching of silicon in a high-density chlorine plasma

Etching of p-Si in 60 mTorr 10%Cl2/90%Ar Faraday-shielded inductively coupled high density plasmas was investigated under both ion-assisted etching (IAE) and photoassisted etching (PAE) conditions. Real-time etching rates and after-etching Si surface chemical compositions were obtained by laser interferometry and vacuum-transfer x-ray photoelectron spectroscopy (XPS), respectively. Precisely controlled ion energy distributions (IEDs) were generated by applying pulsed negative DC bias on the conductive sample stage. Above a 36 eV threshold at a total flow rate of 250 SCCM, the IAE rate increased with the square root of the ion energy. In contrast to the DC bias, etching under RF bias did not exhibit a threshold ion energy because of the wide IED. XPS spectra revealed that the surface layer under PAE conditions had a significantly lower chlorine content, composed of only SiCl. Under IAE conditions, however, silicon dangling bonds (Si•), SiCl2, and SiCl3 were found on the surface, in addition to SiCl, with a relative abundance of SiCl > SiCl2 > SiCl3. The absence of higher chlorides and Si• under PAE conditions suggested that vacuum ultraviolet photons and above threshold-energy ions interact with the surface very differently. By varying the duty cycle of the pulsed DC bias, it was found that the IAE rate scaled with the energetic ion dose, but only for low duty cycles. For higher duty cycles, the apparent IAE yield fell off with an increasing Cl coverage on the surface, as the duty cycle went up, which pointed to a negative synergy (antisynergism) between PAE and IAE as the explanation. This antisynergism was further supported by the observed decrease of the total etching rate with an increasing period of the pulsed DC bias. A plausible mechanism is that increasing the pulsing period causes more near-surface damage, creating more recombination centers that lead to a higher loss rate of electron-hole pairs through recombination, thereby reducing the PAE rate.

(Invited) Photo-Assisted Etching of Porous Silicon Nanostructures in Hydrofluoric Acid Using Monochromatic Light

Silicon can be made porous by electrochemical (anodization) etching in hydrofluoric acid (HF). This is the most controlled and used technique for the preparation of porous silicon (PSi). The porous structure depends on several factors, such as silicon resistivity and type, as well as HF concentration and anodization current. In the case of lightly-doped p-type silicon, uniform sponge-like nano-structures can be formed. This type of PSi layer is characterized in particular by its porosity and thickness. PSi is interesting for a variety of properties, some related to quantum confinement in its nanostructure, such as luminescence and porosity-dependent refractive index and surface area. It has potential applications in optoelectronics, photovoltaics, medicine and sensing. The post-anodization behavior of PSi in various environments is of interest for stability and degradation evaluation. The evolution of PSi is sometimes made intentional in order to further tailor the PSi properties after anodization. Typically, the effects of PSi oxidation (mainly for stabilization purposes), etching (to increase the porosity) or degradation in various environments (typically for biotechnology) are sometimes studied. Recently, we have made use of a particular behavior of the PSi photoconduction in HF: when PSi is illuminated in HF, only the charge carriers photogenerated in the Si substrate contribute to the measured photocurrent, and thus, the photocurrent is a measure of the optical transmission of PSi. This behavior allowed us to measure the PSi optical constants for various optical wavelengths and PSi porosities in situ and thus for the most pristine form of PSi and avoiding all the problems related to the drying and handling of high-porosity PSi free-standing layers1. We have also used the photoconduction to study the chemical etching of PSi in HF. We could monitor the etching as it proceeded and developed a model that allowed us to derive the HF-dependent etching rates and the absorption coefficient of PSi for a large range of porosities not explored so far, from as-prepared to close to 100% porosity (PSi fully dissolved)2. A small degree of tuning of the photoluminescence (PL) band of PSi may be achieved by adjusting anodization conditions, but fine tuning remains difficult. A few attempts aiming at controlling the PL or understanding PSi PL in HF have been attempted by using post-anodization photo-assisted etching in HF. The photoetching process itself was not studied in much detail, and were focusing on as-formed luminescent PSi. A better understanding of the photoetching process would be welcome to get better control and understanding of the effects. In this paper, we show how the photoetching of PSi (initially luminescent or not) can be monitored and its rate evaluated using the photoconduction technique that we have used previously in other studies1 , 2. The photocurrent was continuously measured during the illumination of PSi by monochromatic light, which was inducing photoetching. As PSi was etched, the PSi absorption was decreasing, more light was transmitted to the substrate and, as a result, the photocurrent was increasing. Figure 1 shows results for an initial porosity of 62%, an illumination wavelength of 450 nm, and different PSi thicknesses. A saturation state is reached when the PSi layer does not absorb anymore at the illumination wavelength. The waves observed for 1 μm and 2 μm-thick layers were attributed to thin film optical interference. The curve for 16 μm-thick PSi was affected by reproducibility problems because of instabilities in the diffusion of species and hydrogen bubbles inside the pores. For clean photoetching study, 4-μm thick PSi layers were used. Figure 2 shows the time evolution of the photocurrent for 4 μm-thick PSi layers, with different illumination powers at 450 nm. As expected, the photocurrent at saturation is proportional to the incident illumination power. Solid lines are fits obtained using a model that will be presented. Two regimes were characterized, one in which the photoetch rate is limited by the supply of photo-generated holes at the Si surface, and another one where it is limited by the photoetching reaction. The maximum photoetch rate was derived. The evolution of the in depth porosity profile can be obtained from the model as well. The effects of a range of parameters will be presented. 1 B. Gelloz, H. Fuwa, and L. H. Jin, Ecs Journal of Solid State Science and Technology 5 (3), P190 (2016). 2 B. Gelloz, K. Ichimura, H. Fuwa, E. Kondoh, and L. Jin, Ecs Journal of Solid State Science and Technology 6 (1), R1 (2017). Figure 1

(Invited) Photo-Assisted Etching of Porous Silicon Nanostructures in Hydrofluoric Acid Using Monochromatic Light

Photo-assisted etching of porous silicon (PSi) in hydrofluoric acid (HF) solution has been, so far, not well controlled and characterized. In this paper, the progress of the photoetching of PSi formed from lightly-doped p-type silicon in ethanoic HF solutions was monitored using an in situ photoconduction technique. A model was proposed to explain the results. Two regimes were characterized, one in which the photoetch rate is limited by the supply of photo-generated holes at the Si surface, and another one where it is limited by the rate R0 of the chemical reactions after initial hole capture, for illumination powers greater than a threshold value. This value was about 1 mW/cm2 when using a wavelength of 450 nm for a porosity of 62%. R0 was evaluated as about 0.06 Å/min. The model was used to calculate porosity profiles during photoetching.

Photoetching of Porous Silicon Nanostructures in Hydrofluoric Acid Using Monochromatic Light

Photo-assisted etching of porous silicon (PSi) in hydrofluoric acid (HF) solution has been, so far, not well controlled and characterized. In this paper, the progress of photoetching of PSi formed from lightly-doped p-type silicon in ethanolic HF solutions was monitored using an in situ photoconduction technique. A model was proposed to explain the results. Two regimes were characterized, one in which the photoetch rate is limited by the supply of photo-generated holes at the Si surface, and another one where it is limited by the rate R0 of the chemical reactions after initial hole capture, for illumination powers greater than a threshold value. This value was about 1 mW/cm2 when using a wavelength of 450 nm for a porosity of 62%. R0 was evaluated as about 0.06 Å/min. The model was used to calculate porosity profiles during photoetching. The effect of illumination wavelengths is discussed. A signature of quantum confinement in high-porosity PSi was observed.

Impact of the crystal orientation of Fe-doped lithium niobate on photo-assisted proton exchange and chemical etching

Photo-assisted proton-exchange (PAPE) is carried out on the +c- and y-surfaces of Fe-doped LiNbO3 crystals and the impact of the crystal orientation on the PAPE and the subsequent photo-assisted chemical etching (PACE) is investigated. The proton distributions and the morphologies of the proton-exchanged surfaces are studied by using Micro-FT-IR, Micro-Raman, optical and scanning electron microscopy. Through the PAPE process the proton-exchange can be confined in a specific region by an incident laser beam with fixed intensity profile. It is found that the y-surface is much more fragile than the +c-surface and that micro-cracks are easily generated on the y-surface during the PAPE process. Moreover, the range and number of these micro-cracks can be controlled by the experimental parameters of the PAPE process. The etching morphology of the y-surface shows apparent directional features along the c-axis of LiNbO3 crystal and the proton spatial distribution is found elongated along the c-axis. Both effects are attributed to the accumulation of photovoltaic charges at the two sides of the illumination area along the c-axis.

Morphological evolution and characterization of GaN pyramid arrays fabricated by photo-assisted chemical etching

GaN pyramid arrays have been successfully synthesized by selective photo-assisted chemical etching in a K2S2O8/KOH solution. A detailed analysis of time evolution of surface morphology has been conducted, which describes an etching process of GaN pyramids. Room temperature cathodoluminescence images indicate that these pyramids are composed of crystalline GaN surrounding dislocations, which is caused by the greater recombination rate of electrons and holes at dislocation than that of crystalline GaN. The Raman results show a stress relaxation in GaN pyramids compared with unetched GaN. The optical property of both unetched GaN and GaN pyramids has been studied by photoluminescence. The formation mechanism and feature of GaN pyramids are also rationally explained.

Insights into the mechanism of in-plasma photo-assisted etching using optical emission spectroscopy

Various mechanisms have been proposed to explain photo-assisted etching (PAE) of Si, including photogenerated carrier-mediated etching, photon-stimulated desorption, and photon-induced damage (breaking of Si–Si bonds) caused mainly by vacuum ultraviolet photons irradiating the substrate. In this study, the authors use optical emission spectroscopy to gain an insight into possible in-plasma PAE mechanisms. Emissions from Cl, Si, SiCl, and Ar were recorded as a function of power while etching p-Si in Cl2/Ar Faraday-shielded inductively coupled plasmas at a pressure of 60 mTorr with no substrate bias. Under these conditions, ion-assisted etching was negligible and PAE was dominant. The Si:Ar optical emission intensity ratio, ISi/IAr (proportional to the etching rate of Si), increased substantially with power. Accounting for the contribution to this signal from the dissociation of SiClx (x = 1–4) etch products, the residual increase in the emission indicated that the PAE rate also increased with power. Time resolved emissions were also recorded in a pulsed plasma where power was modulated between 500 and 300 W. ISi/IAr was found to modulate with the instantaneous power. This rules out the photon-induced damage mechanism since, if this mechanism was dominant, the ISi/IAr signal would not be modulated.

Preparation and characterization of GaN micro/nano- structures by the electrodeless photo-assisted chemical etching

GaN micro/nano-structures of different morphology were successfully preparated by the electrodeless photo-assisted chemical etching in KOH solution, with K2S2O8 as the oxidizing agent. The structure, crystalline quality, stress state and optical property of both normal GaN and etched GaN were provided by the combined results of scanning electron microscopy (SEM) images, cathodoluminescence mapping (CL mapping), high-resolution X-ray diffraction (HRXRD) detection, micro-Raman spectra and photoluminescence (PL), respectively. The results show that pits, micro/ nanopillars and nanowires of high crystalline quality are formed in the high KOH concentrations (1 mol/L) and the low light intensity; GaN pyramid arrays have been fabricated in 0.4 mol/L KOH solution with high UV light intensity, and CL mapping indicates that these pyramids are composed of crystalline GaN surrounding a dislocation; Whiskers can also be obtained in 0.1 mol/L KOH with high UV light intensity after etching. The whiskers are believed to be directly correlated with line-type defects of edge and mixed character. The formation mechanism and the feature of GaN different morphology were rationally explained. And the etching temperature and the polarity of GaN surface have an evident influence on the surface morphology of GaN.

Formation of nanogaps in InAs nanowires by selectively etching embedded InP segments

We present a method to fabricate nanometer scale gaps within InAs nanowires by selectively etching InAs/InP heterostructure nanowires. We used vapor–liquid–solid grown InAs nanowires with embedded InP segments of 10–60 nm length and developed an etching recipe to selectively remove the InP segment. A photo-assisted wet etching process in a mixture of acetic acid and hydrobromic acid gave high selectivity, with accurate removal of InP segments down to 20 nm, leaving the InAs wire largely unattacked, as verified using scanning electron and transmission electron microscopy. The obtained nanogaps in InAs wires have potential as semiconducting electrodes to investigate electronic transport in nanoscale objects. We demonstrate this functionality by dielectrophoretically trapping 30 nm diameter gold nanoparticles into the gap.

Photo-assisted etching of silicon in chlorine- and bromine-containing plasmas

Cl2, Br2, HBr, Br2/Cl2, and HBr/Cl2 feed gases diluted in Ar (50%–50% by volume) were used to study etching of p-type Si(100) in a rf inductively coupled, Faraday-shielded plasma, with a focus on the photo-assisted etching component. Etching rates were measured as a function of ion energy. Etching at ion energies below the threshold for ion-assisted etching was observed in all cases, with Br2/Ar and HBr/Cl2/Ar plasmas having the lowest and highest sub-threshold etching rates, respectively. Sub-threshold etching rates scaled with the product of surface halogen coverage (measured by X-ray photoelectron spectroscopy) and Ar emission intensity (7504 Å). Etching rates measured under MgF2, quartz, and opaque windows showed that sub-threshold etching is due to photon-stimulated processes on the surface, with vacuum ultraviolet photons being much more effective than longer wavelengths. Scanning electron and atomic force microscopy revealed that photo-etched surfaces were very rough, quite likely due to the inability of the photo-assisted process to remove contaminants from the surface. Photo-assisted etching in Cl2/Ar plasmas resulted in the formation of 4-sided pyramidal features with bases that formed an angle of 45° with respect to ⟨110⟩ cleavage planes, suggesting that photo-assisted etching can be sensitive to crystal orientation.

GaN hexagonal pyramids formed by a photo-assisted chemical etching method

A series of experiments were conducted to systematically study the effects of etching conditions on GaN by a convenient photo-assisted chemical (PAC) etching method. The solution concentration has an evident influence on the surface morphology of GaN and the optimal solution concentrations for GaN hexagonal pyramids have been identified. GaN with hexagonal pyramids have higher crystal quality and tensile strain relaxation compared with as-grown GaN. A detailed analysis about evolution of the size, density and optical property of GaN hexagonal pyramids is described as a function of light intensity. The intensity of photoluminescence spectra of GaN etched with hexagonal pyramids significantly increases compared to that of as-grown GaN due to multiple scattering events, high quality GaN with pyramids and the Bragg effect.

Surprising importance of photo-assisted etching of silicon in chlorine-containing plasmas

The authors report a new, important phenomenon: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous dc bias on a “boundary electrode” during the afterglow of a pulsed, inductively coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data. Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons generated by Auger neutralization of low-energy ions, leaving photo-assisted etching as the only likely explanation. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons, predominately at wavelengths < 1700 Å. The photo-assisted etching rate was equal to the ion-assisted etching rate at 36 eV, causing substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. Under these conditions, photo-assisted etching likely plays an important role in profile evolution of features etched in Si with chlorine-containing plasmas, contributing to the commonly observed sloped sidewalls and microtrenches.

Fabrication of High-Aspect-Ratio Electrode Arrays for Three-Dimensional Microbatteries

Silicon-micromachining techniques have been combined with conventional material-synthesis methods to develop microelectrodes for 3-D microbatteries. The resulting electrodes feature an organized array of high-aspect-ratio microscale posts fabricated on the current collector to increase their surface area and volume for a given footprint area of the device. The diameter of the posts ranges from a few micrometers to a few hundred micrometers, with aspect ratios as high as 50. The fabrication approach is based on micromolding of the electrode materials and subsequent etching of the mold to release the electrode structures. Deep reactive-ion-etching or photo-assisted anodic etching has been used to form an array of deep holes in the silicon mold. Electroplating or colloidal-processing method has been used to fill the mold with battery-electrode materials. Measurements on electrochemical half-cells indicated that the 3-D electrode arrays, which are composed of vanadium oxide nanorolls or carbon, exhibited much greater energy densities (per-footprint area) than that of the traditional 2-D electrode geometries. The use of electroplating enabled us to fabricate 3-D interdigitated arrays of nickel and zinc; and battery operation was demonstrated. [2006-0293].