Photo-assisted Chemical Etching Research Articles

Photoassisted chemical smoothing of AlGaN surface after laser lift-off

To fabricate AlGaN-based ultraviolet (UV) vertical cavity surface-emitting laser diodes, a pair of distributed Bragg reflectors (DBRs) having a smooth surface is desired to have a high quality factor (Q). In this work, photoassisted chemical etching (PCE) was attempted to smoothen the -c(000-2) AlN surface after removing the sapphire substrate by means of the laser lift-off process. First, the conditions for PCE were optimized using an Xe lamp and KOH solution. The root-mean-square roughness of the -c(000-2) AlN surface was reduced from 30.5 to 5.6 nm, which enables us to fabricate an improved HfO2/SiO2 DBR on the AlN surface. Then, using the optimized PCE technique, improved UV (Al,Ga)N planar microcavities were fabricated and distinct cavity-mode-related emissions were observed using the photoluminescence (PL) technique. By comparing the PL spectra between the samples with and without PCE treatment, the Q value at 303 nm for the case of the improved DBR was determined to increase from 174 to 270. The increase in the Q value is mainly attributed to the reduction of scattering losses in optical cavities. Furthermore, the discussion on the mechanism of improved surface during the PCE treatment is given. Consequently, PCE is demonstrated to be a feasible approach to refine the quality of ultraviolet nitride microcavities.

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.

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.

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.

Optical properties of n-type porous silicon obtained by photoelectrochemical etching

The optical studies of n-type porous silicon prepared by the photo-assisted chemical etching are reported here. The optical properties of samples obtained under different conditions have been investigated by photoluminescence and Fourier transform infrared absorption measurements, and they are compared with that of p-type porous silicon. Our results clearly demonstrate that the blue emission in porous silicon originates from surface compounds. From the infrared absorption measurement, we point out that the surface compounds are Si–OH complexes. This conclusion is further supported by a recent calculation which shows that Si–OH complexes can emit the photon energy in the range observed here. We show that the optical properties of the n-type porous silicon are more stable than that of the p-type porous silicon. The result provides the evidence to support the fact that the n-type porous silicon is a better candidate for the application in optoelectronics.

Characterization of bond and etch-back silicon-on-insulator wafers by photoluminescence under ultraviolet excitation

Photoluminescence (PL) spectroscopy under the ultraviolet (UV) light excitation has been used for the characterization of bond and etch-back silicon-on-insulator (BESOI) wafers. Since the UV light can excite only the silicon-on-insulator (SOI) layer and the SOI structure prevents the photoexcited carriers from diffusing into the base wafer, the PL light is emitted only from the SOI layer. We have detected defects induced during the photoassisted chemical etching process in BESOI wafers. The defect-related emission disappears and the electron-hole droplet signal appears after annealing at 800 °C for 30 min. This allows us to suggest that the annealed SOI layer is comparable in crystalline quality with that of conventional bulk Si wafers and that the recombination velocity at the interfaces is low.