Selective Wet Etching Technique Research Articles

High-temperature annealing of ( ) β-Ga2O3 substrates for reducing structural defects after diamond sawing

A commercial epi-ready ( ) β-Ga2O3 wafer was investigated upon diamond sawing into pieces measuring 2.5 × 3 mm2. The defect structure and crystallinity in the cut samples has been studied by X-ray diffraction and a selective wet etching technique. The density of defects was estimated from the average value of etch pits calculated, including near-edge regions, and was obtained close to 109 cm−2. Blocks with lattice orientation deviated by angles of 1−3 arcmin, as well as non-stoichiometric fractions with a relative strain about (1.0−1.5) × 10−4 in the [ ] direction, were found. Crystal perfection was shown to decrease significantly towards the cutting lines of the samples. To reduce the number of structural defects and increase the crystal perfection of the samples via increasing defect motion mobility, the thermal annealing was employed. Polygonization and formation of a mosaic structure coupled with dislocation wall appearance upon 3 h of annealing at 1100 °C was observed. The fractions characterized by non-stoichiometry phases and the block deviation disappeared. The annealing for 11 h improved the homogeneity and perfection in the crystals. The average density of the etch pits dropped down significantly to 8 × 106 cm−2.

High-temperature-capable ALD-based inorganic lift-off process

Lift-off is a useful method to pattern metals or other materials in semiconductor device fabrication. Polymer photoresists are commonly used for lift-off; however, these resists cannot be used when the device fabrication requires high temperature with the lift-off materials in place. Here, we have used bilayer stacks of metal oxides deposited by plasma-enhanced atomic layer deposition (PEALD) to provide a lift-off process compatible with high temperatures (>500 °C). By using selective wet-etching techniques, we successfully created reentrant or undercut structures and achieved lift-off for micro- and nano-scale devices. We also demonstrated that the metal-oxide based inorganic lift-off process allows high-temperature material deposition and/or annealing as a part of the fabrication process. The inorganic lift-off technique overcomes temperature limits of polymer resists and provides a photoresist-residue-free and high-temperature capable process.

Defect evaluation in strain-relaxed Ge0.947Sn0.053 grown on (001) Si

Defects in Ge0.947Sn0.053 layers grown using molecular beam epitaxy on (001) Si substrates with 4.9% mismatch are investigated using optical, scanning, and transmission electron and atomic force microscopies. It is shown that the strain relaxation occurs via the introduction of 90° misfit dislocations of short length, at the Ge0.947Sn0.053/Si interface. An irregular morphology in the form of mounds is observed on the surface of epitaxial Ge1−xSnx (0.031 ≤ x ≤ 0.093) and is found to be associated with carbon impurities at the hetero-interface. A low-cost and fast defect selective wet etching technique is described to determine the etch pit density in epitaxial Ge1−xSnx with a low Sn content (≤5.3%). On the basis of etch pit morphology, different defects, e.g., dislocations, stacking faults, and crystal originated particles, are distinguished.

Abnormal strain in suspended GeSn microstructures

A suspended GeSn microstructure is realized by a two-step selective wet etching technique with the aim of fully relaxing the compressive strain, promoting the indirect to direct bandgap transition and improving the optical property of GeSn thin films grown on Ge. The compressive strain was found efficiently relaxed, and even unexpected large tensile strain was displayed on regions of the microstructure by micro-Raman spectroscopy. Residual Ge patches under the suspended GeSn microstructure were found by scanning electron microscopy and proved to be the origin of the tensile strain by finite element method simulations. The tensile strain on the surface is beneficial for direct bandgap conversion and carrier accumulation. Significant enhancement of photoluminescence was obtained in the GeSn microstructures than the original thin film.

Two-step gate-recess process combining selective wet-etching and digital wet-etching for InAlAs/InGaAs InP-based HEMTs

A two-step gate-recess process combining high selective wet-etching and non-selective digital wet-etching techniques has been proposed for InAlAs/InGaAs InP-based high electron mobility transistors (HEMTs). High etching-selectivity ratio of InGaAs to InAlAs material larger than 100 is achieved by using mixture solution of succinic acid and hydrogen peroxide (H2O2). Selective wet-etching is validated in the gate-recess process of InAlAs/InGaAs InP-based HEMTs, which proceeds and automatically stops at the InAlAs barrier layer. The non-selective digital wet-etching process is developed using a separately controlled oxidation/de-oxidation technique, and during each digital etching cycle 1.2 nm InAlAs material is removed. The two-step gate-recess etching technique has been successfully incorporated into device fabrication. Digital wet-etching is repeated for two cycles with about 3 nm InAlAs barrier layer being etched off. InP-based HEMTs have demonstrated superior extrinsic transconductance and RF characteristics to devices fabricated during only the selective gate-recess etching process because of the smaller gate to channel distance.

Fabrication of Thick Free-Standing Lightly-Doped n-Type 4H-SiC Wafers

In this work, we have developed a selective wet etching technique for n+-SiC substrate using electrochemical etch process. A mixture of hydrofluoric acid and hydrogen peroxide was used as an electrolyte and the etch rates exceeding 200 μm per hour at the current density of 50 mAcm-2 was achieved. This process is highly selective and the etching process stops at the interface of n+ SiC substrate and n-SiC epi layer. Using this process, we have successfully fabricated 180 to 250 μm thick 4’’ n-SiC epitaxial free standing wafers by separating them from a 350 μm thick n+ SiC substrate. After the substrate is completely removed, free standing wafer is characterized for wafer bow and minority carrier lifetime, using both Si-face and C-face. The wafer bow was reduced from 40 μm to 20 μm after the substrate removal. It was found that the process of removing the substrate does not introduce any extra damage to the wafer as far as the lifetime is concerned. The hole lifetime measured by microwave photoconductivity decay technique was unchanged at around 2 μsec, measured from both Si-face and C-face. These results are very promising and open up many avenues for many device applications where lightly doped free standing epitaxial semiconductor thin film is needed.

Towards nanometer-spaced silicon contacts to proteins

A vertical nanogap device (VND) structure comprising all-silicon contacts as electrodes for the investigation of electronic transport processes in bioelectronic systems is reported. Devices were fabricated from silicon-on-insulator substrates whose buried oxide (SiO2) layer of a few nanometers in thickness is embedded within two highly doped single crystalline silicon layers. Individual VNDs were fabricated by standard photolithography and a combination of anisotropic and selective wet etching techniques, resulting in p+ silicon contacts, vertically separated by 4 or 8 nm, depending on the chosen buried oxide thickness. The buried oxide was selectively recess-etched with buffered hydrofluoric acid, exposing a nanogap. For verification of the devices’ electrical functionality, gold nanoparticles were successfully trapped onto the nanogap electrodes’ edges using AC dielectrophoresis. Subsequently, the suitability of the VND structures for transport measurements on proteins was investigated by functionalizing the devices with cytochrome c protein from solution, thereby providing non-destructive, permanent semiconducting contacts to the proteins. Current–voltage measurements performed after protein deposition exhibited an increase in the junctions’ conductance of up to several orders of magnitude relative to that measured prior to cytochrome c immobilization. This increase in conductance was lost upon heating the functionalized device to above the protein’s denaturation temperature (80 °C). Thus, the VND junctions allow conductance measurements which reflect the averaged electronic transport through a large number of protein molecules, contacted in parallel with permanent contacts and, for the first time, in a symmetrical Si–protein–Si configuration.

InAsSb/InAsPSb multiple quantum well disk cavities with pedestal structures on a GaSb substrate for mid-infrared whispering-gallery-mode emission beyond 4 μm

The mid-infrared whispering-gallery-mode disk cavities with InAs0.85Sb0.15/InAs0.53P0.23Sb0.24 multiple quantum wells active medium on a GaSb substrate were fabricated. For this material system in the mid-infrared range, fabrication techniques were developed to form the disk cavity structure. The smooth sidewalls of the disk cavities were achieved by appropriate gas mixture flow ratio of BCl3/Ar in the inductively coupled plasma-reactive ion etching. In addition, selective wet etching technique was used to form the pedestal of the disk cavity using dilute hydrofluoric acid with good selectivity. For efficient confinement of the whispering gallery modes along the radial direction, the extent of the lateral etching was carefully controlled. The processed 30-μm-diameter disk cavities were optically pumped, and the whispering gallery modes with wavelengths around 4.1 μm can be observed up to 90 K.

GaSb-Based Mid-Infrared Single Lateral Mode Lasers Fabricated by Selective Wet Etching Technique with an Etch Stop Layer

We have demonstrated a wet etching technique for fabrication of narrow ridge lasers. Precise control over etching depth and ridge width was realized by introducing an etch stop layer into a laser structure and by using two etchants with different selectivity. The 6-μm-wide ridge laser emitting at 2 μm generated continuous-wave power of 70 mW at 20°C. Single lateral mode operation was observed up to 400 mA, corresponding to 8 × Ith.

High-Q optomechanical GaAs nanomembranes

We present a simple fabrication method for the realization of suspended GaAs nanomembranes for cavity quantum optomechanics experiments. GaAs nanomembranes with an area of 1.36 mm by 1.91 mm and a thickness of 160 nm are obtained by using a two-step selective wet-etching technique. The frequency noise spectrum reveals several mechanical modes in the kilohertz regime with mechanical Q-factors up to 2,300,000 at room temperature. The measured mechanical mode profiles agree well with a taut rectangular drumhead model. Our results show that GaAs nanomembranes provide a promising path towards quantum optical control of massive nanomechanical systems.

Single spatial mode 3 [micro sign]m diode lasers with continuous-wave output power of 15 mW at room temperature

Single spatial mode GaSb-based type-I quantum well diode lasers with 3 μm emission and continuous-wave output power above 15 mW at room temperature are reported. The devices with 5.5 μm-wide ridge waveguides were fabricated by selective wet etching technique.

습식식각 방법으로 제작한 패턴 형성 사파이어 기판을 가지는 GaN계 청색 LED

Sapphire substrate was patterned by a selective chemical wet etching technique, and GaN/InGaN structures were grown on this substrate by MOVPE (Metal Organic Vapor Phase Epitaxy). The surface of grown GaN on patterned sapphire substrate (PSS) has good morphology and uniformity. The patterned sapphire substrate LED showed better light output than conventional LED that improvement 50%. We think these results come from enhancement of internal quantum efficiency by decrease of threading dislocation and increase of light extraction efficiency. Also these LED showed more uniform emission distribution in angle than conventional LED.

Single crystal Fe elements patterned by one-step selective chemical wet etching

A technique has been developed to pattern single crystal ultrathin Fe films by selective chemical wet etching of the Au capping layer and then simultaneous oxidization of the ferromagnetic Fe layer underneath. The focused magneto-optical Kerr effect and ferromagnetic resonance measurements demonstrate that the intrinsic magnetic anisotropy has not been changed in the patterned elements, showing that the chemical bonding at the metal–semiconductor interface remains the same. Further x-ray energy dispersive spectroscopy measurements show that this selective wet-etching technique is suitable for the patterning of thin Fe films with thicknesses less than around 25 ML.

Enhanced photonic bandgap in ion-implanted lithium niobate waveguides by improving index contrast

We report on utilizing focused ion beam milling and selective wet-etching techniques to enlarge the optical barrier in helium ion-implanted lithium niobate single crystals. We realized a high-efficiency narrow-bandwidth photonic band filter by introducing an air cavity as the optical barrier on a waveguide portion. The air cavity was created beneath the Bragg gratings (1-D photonic crystals) to strengthen the light confinement and optical response. The experimental performance of photonic crystals with and without air cavity is presented and compared with the theoretical modeling. These photonic crystal devices with high refractive-index contrast make a further step towards ultracompact optical circuit integration and applications based on single crystals.

Suspended slab and photonic crystal waveguides in lithium niobate

Suspended waveguides have been widely applied to silicon-on-insulator structures because they are easily fabricated with processing techniques similar to those of integrated circuit design. However, it is difficult to fabricate such structures in lithium niobate, which is also a very important material for optoelectronics. One main challenge is the difficulty of etching lithium niobate. In this work, the authors show a method to fabricate suspended slab waveguides in lithium niobate by combining ion implantation, focused ion beam milling, and selective wet etching techniques. The method does not involve wafer bonding or crystal ion slicing and is entirely monolithic. Lattice damage can be introduced to a buried thin layer of a certain depth beneath the sample surface by ion implantation, resulting in a considerable wet etching selectivity to bulk material. The etching rate has been investigated to control the size of the suspended membrane. Fabrication of suspended photonic crystal waveguides has also been demonstrated. The results show an effective method of fabricating suspended devices in lithium niobate, which enables new applications such as waveguides, modulators, and infrared detectors.

Sapphire-etched vertical-electrode nitride-based semiconductor light-emitting diode (SEVENS LED) fabricated by the wet etching technique

A sapphire-etched vertical-electrode nitride-based semiconductor (SEVENS) light-emitting diode (LED) has been fabricated by a selective wet etching technique. The light output power of the SEVENS LED is substantially enhanced compared with a conventional NiAu lateral-electrode (LE) GaN-based LED formed on a sapphire substrate. The light output power of the SEVENS LED is not saturated up to as high as 300 mA junction current, being a notable improvement over that (250 mA junction current) of the LE LED. The 250 mA light output power of the SEVENS LED with an entire sapphire-removed surface is 4.3 times larger than that of the LE LED. The variations of peak wavelength and full width at half maximum are less for the SEVENS LED compared to the LE LED. This performance improvement is attributed to the transfer of the sapphire substrate to a silicon substrate and the microroughened surface self-formed by wet etching of the sapphire substrate. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN-Based Light-Emitting Diode with Indium–Tin–Oxide and Vertical Electrode

A sapphire-etched vertical-electrode nitride semiconductor (SEVENS) light-emitting diode (LED) is fabricated by a selective wet-etching technique. The SEVENS-LED has an indium–tin–oxide (ITO) transparent metal electrode (TME) and a sapphire via hole. The SEVENS-LED exhibits excellent device performances compared with a lateral-electrode (LE) GaN-based LED formed on a sapphire substrate. The light-output power of the SEVENS-LED is ∼7 mW at a 20 mA junction current, which means 13% external quantum efficiency (EQE). The electrostatic discharge (ESD) test shows excellent ESD characteristics of the SEVENS-LED compared with those of the ITO LE-LED. The reverse ESD amplitude of SEVENS-LED is approximately 200 V, which is two times higher than that (∼100 V) of the ITO LE-LED. These improvements of ESD properties are attributed to the change of the lateral electrode to a vertical electrode, without requiring the transfer of the sapphire substrate to a conducting substrate.

Improvement of GaN-based light-emitting diode by indium-tin-oxide transparent electrode and vertical electrode

A sapphire-etched vertical-electrode nitride semiconductor (SEVENS) light-emitting diode (LED) is fabricated by means of a selective chemical wet-etching technique. The SEVENS-LED formed on sapphire substrate exhibits excellent device performance compared to a conventional NiAu lateral-electrode (LE) GaN-based LED formed on the same sapphire substrate. The integral light-output power of SEVENS-LED is /spl sim/7 mW, which is 1.75 times stronger than that of the conventional NiAu LE-LED (/spl sim/4 mW). The external quantum efficiency of SEVENS-LED is estimated to be approximately 13%.

Vertical Electrode GaN-Based Light-Emitting Diode Fabricated by Selective Wet Etching Technique

In order to improve the device performance and productivity, a sapphire-etched vertical-electrode nitride semiconductor (SEVENS) light-emitting diode (LED) is fabricated by a selective wet-etching technique. The sapphire substrate is removed by chemical wet etching and transferred to a conducting Si substrate with a reflector metal. The SEVENS-LED exhibits excellent device performance. Integral light-output power is approximately 3.5 mW at a 20 mA junction current, which indicates a 6.6% external quantum efficiency (EQE). The light-output power was linearly increased with increasing junction current, and the peak wavelength was saturated even with a higher junction current. The enhanced performance of the SEVENS-LED is attributed to changing the lateral electrode to a vertical electrode and transferring a sapphire substrate to a Si receptor with a reflector metal. The SEVENS-LED technique is anticipated to be useful for improving the performance of GaN-based LEDs for future solid-state general illumination applications.

GSMBE grown In 0.49Ga 0.51P/GaAs heterojunction bipolar transistors with heavily beryllium doped base and undoped Spacer

Lattice-matched N–p +–n In 0.49Ga 0.51P/GaAs heterojunction bipolar transistors (HBT's) with heavily beryllium doped base and undoped spacers were grown by gas source molecular beam epitaxy (GSMBE). The epitaxial structure consists of, from the bottom to the top, a 5000 Å n +-GaAs sub-collector with Si doping level of 3×10 18 cm −3, a 7000 Å n-GaAs collector doped at concentration of 3×10 16 cm −3, a 60 nm p +-GaAs base with Be doping level up to 3×10 19 cm −3 inserted between two undoped GaAs spacers, a 100 nm N-In 0.49Ga 0.51P emitter doped at 3×10 17 cm −3. The structure was completed with a 150 nm n +-GaAs cap layer with Si doping level of 3×10 18 cm −3. Devices with emitter area of 100×100 μm were fabricated on the grown wafer by using selective wet chemical etching technique and photolithographic process. AuGeNi/Au was used for the emitter and collector contacts, and Cr/Au was used for the base contact. The turn-on voltages of p +-GaAs/N-In 0.49Ga 0.51P heterojunction and p +-GaAs/n-GaAs junction are 1.0 and 0.65 V. The reverse breakdown voltages of the B–E and B–C junctions are 10 and 12 V. The common-emitter current-voltage characteristics show that the maximum current gain reaches 320 at the collector current of 90 mA.