Photo-enhanced Chemical Etching Research Articles

Tuning the Threshold Voltage of Exfoliated β-Ga2O3 Flake-Based Field-Effect Transistors by Photo-Enhanced H3PO4 Wet Etching

Monoclinic β-Ga2O3 with an ultra-wide energy bandgap of ∼4.9 eV has facile cleavage planes because of the high anisotropy in the lattice structure. An exfoliated nanolayer flake of β-Ga2O3 can be used as an active channel layer in nanoelectronics. However, the mechanical exfoliation method used poses a fundamental issue in that the separated individual layer has a random thickness, which makes it challenging to obtain reproducible device performance. In our study, we demonstrated a photo-enhanced chemical etching technique to control the thickness of exfoliated β-Ga2O3 nanolayers, enabling us to adjust the threshold voltage of field-effect transistors (FETs) based on these nanolayers. Thickness-dependent device characteristics (threshold voltage, subthreshold swing, on/off ratio, and field-effect mobility) of the back-gated FETs were investigated as a function of the active channel layer (β-Ga2O3) thickness achieved by the photo-enhanced H3PO4 etching. Our study paves the way for the implementation of exfoliated β-Ga2O3 nanolayers in nanoelectronics.

Efficient Wet Etching of GaN (0001) Substrate with Subsurface Damage Layer

Photoenhanced chemical (PEC) etching is applicable for processing an n-GaN (0001) surface rapidly. In this process, the surface oxidation is enhanced by photo-generated holes and the resulting oxide can dissolve into solutions. In current work, we conduct bias-assisted PEC etching in a KOH solution with a positively biased wafer, to remove the crystallographically highly damaged layer. The employed substrate was mechanically polished with diamond slurry of sub-micrometer particle size. Without the positive bias, the rate of PEC etching was quite low because the photogenerated holes were quickly depleted by the recombination process at the crystallographic defects and they could not contribute to the oxidation. On the other hand, in the case where the bias was applied, the photogenerated holes and electrons are separated forcibly in the band-bended surface, which effectively contributed to surface oxidation. As a result, a high removal rate was realized even on the damaged surface.

Surface treatment on the growth surface of semi‐insulating GaN bulk substrate for III‐nitride heterostructure field‐effect transistors

AbstractAl0.25Ga0.75N/GaN heterostructure field‐effect transistor (HFET) structures are grown on semi‐insulating GaN substrates by metalorganic chemical vapor deposition with Fe‐doped GaN buffer layer and various surface treatments prior to the epitaxial growth of the structure. The surface treatments are intended to remove suspected surface‐charge‐containing layer and they include wet chemical etching, in‐situ thermal etching, plasma dry etching, and photo‐enhanced chemical etching. Photo‐enhanced chemical (PEC) etching was effective in achieving microscopically smooth surface and in eliminating surface charge layer on the surface of semi‐insulating GaN substrates. RMS surface roughness of PEC etched surface of the semi‐insulating GaN substrate is 0.1∼0.2 nm for 1×1 μm2 scan. C ‐V measurement shows well‐defined 2‐dimensional‐gas‐related charge without any growth‐interface‐related charge for a HFET grown on a semi‐insulating GaN substrate with optimized photo‐enhanced chemical etching of the surface. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

III-nitride heterostructure field-effect transistors grown on semi-insulating GaN substrate without regrowth interface charge

Charge is observed at the regrowth interface for heterostructure field-effect transistors (HFETs) grown on semi-insulating (SI) bulk GaN substrates, even with Fe doping in the regrown buffer layer for reduction of the interface charge. Ultraviolet photoenhanced chemical (PEC) etching is used to treat the surface of SI bulk GaN substrates. Employing optimized etching conditions, a very smooth surface is achieved for the bulk GaN substrate after the etching. The charge at the regrowth interface is eliminated for HFETs grown on etched SI GaN substrates. Secondary ion mass spectrometry measurements show that the Si impurity concentration at the regrowth interface for HFETs grown on etched SI GaN substrates is much lower than that for HFETs grown on unetched SI GaN substrates, which suggests that the charge-containing layer on the SI substrate is removed by PEC etching and that the effects of the reduced charge layer near the regrowth interface can be eliminated by Fe doping for HFETs grown on etched SI substrates.

Studies of improving light extraction efficiency of high power blue light-emitting diode by photo-enhanced chemical etching

The n-type GaN layers of high power blue light-emitting diodes (LEDs) were roughened successfully and formed a “cone-like” surface by photo-enhanced chemical (PEC) etching method. As a result, the extraction efficiency of the surface roughened LED increased. A simple model has been constructed to explain the relationship between the hexagonal cones and the extraction efficiency of roughened surface. A promising effect as high as 2.5 folds of surface roughing was predicted by the model.

High reliability GaN-based light-emitting diodes with photo-enhanced wet etching

This study concerns the generation of GaN-based light-emitting diodes (LEDs), whose n-GaN surfaces were processed using photo-enhanced chemical etching (PEC) by illumination using an unfiltered Hg arc lamp. Etching rates of 29.8 Å/min were obtained using KOH solution at a volume concentration of 45% and an illumination intensity of 50 mW/cm 2. The GaN light-emitting diodes with PEC process using KOH solution for 10 min, exhibit reverse currents of 4.83 × 10 −10 and 4.06 × 10 −8 A at reverse biases of 5 and 10 V, respectively. The GaN-based LEDs have an ideality factor n of 1.17 at a forward bias of 1.1 V. The degradation rate of the output power of light did not exceed 0.3% after burning-in for 500 h in a reliability test at 50 mA and room temperature.

Photo-enhanced chemical wet etching of GaN

In this paper, we report a photo-enhanced chemical etch rate study on two GaN samples of differing structural and electrical quality as a function of the KOH or H 3PO 4 etch solution molarity. The etch rate of KOH was observed to be higher than that of H 3PO 4. This was found to be result from the effects of surface band bending, and surface pinning on the chemical etching and photo-assisted etching of the layers. It was also found that the optimal etch rate occurred at different values of molarity for the two samples and that very different morphologies were observed after etching.

Investigation of Optical and Structural Properties of GaN Grown by Hydride Vapor-Phase Epitaxy

Photoluminescence and reflectance experiments of high quality GaN layers grown by hydride vapor-phase epitaxy reveal narrow (2-3 meV) free and bound exciton transitions. Quenching of three acceptor-bound exciton peaks is observed with increasing film thickness (3.5-400 μm). Changes in the PL features appear to be independent of the impurity concentrations, as measured by secondary ion mass spectrometry, and the dislocation densities, obtained by photo-enhanced chemical etching. Therefore, the observed acceptor levels are assigned to intrinsic defects originating from the substrate/layer interface and decreasing in density with the thickness of the film.

Investigation of Optical and Structural Properties of GaN Grown by Hydride Vapor-Phase Epitaxy

Photoluminescence and reflectance experiments of high quality GaN layers grown by hydride vapor-phase epitaxy reveal narrow (2-3 meV) free and bound exciton transitions. Quenching of three acceptor-bound exciton peaks is observed with increasing film thickness (3.5-400 μm). Changes in the PL features appear to be independent of the impurity concentrations, as measured by secondary ion mass spectrometry, and the dislocation densities, obtained by photo-enhanced chemical etching. Therefore, the observed acceptor levels are assigned to intrinsic defects originating from the substrate/layer interface and decreasing in density with the thickness of the film.