Crystallographic Wet Etching Research Articles

Room temperature single-photon emission from InGaN quantum dot ordered arrays in GaN nanoneedles

GaN-based single-photon sources have received immense attention for applications in quantum technologies. An isolated semiconductor quantum dot (QD) is an attractive and proven choice. Most experimental demonstrations involve epitaxial growth or etching of the QD embedded in a vertical nanopillar/nanowire structure. Here, we demonstrate room-temperature single-photon emission from an InGaN QD embedded in a GaN nanoneedle. The nanoneedle is tapered at the bottom and is formed by a succession of inductively coupled plasma reactive ion etching and crystallographic direction-dependent wet etching techniques. The nanofabrication process steps yield reproducible and uniform-sized QDs in the needle structures. Cross-sectional SEM images show needles are centered at the center of a hexagonal base, which confirms a good crystalline property of the QD. Micro-photoluminescence measurements on a single QD manifest a short time constant for radiative decay channels without any prominent non-radiative decay path. Second-order correlation measurements confirm the antibunching of the emitted photons. Higher spectral purity and smaller value of the second-order correlation are maintained up to a good excitation power, indicating the usefulness of the methodology for quantum technologies.

Selective deposition of ZnO nanowires on silicon micro-pyramidal arrays

Growth of ZnO nanowires on various crystallographic facets of silicon was carried out in order to promote spatially-selective growth. Spontaneous growth of these structures at specific locations of a wafer is necessary for many applications, including photovoltaics and field-emission. Silicon micro-pyramidal arrays with a base of 80µm square and a plateau 30µm wide at the top were fabricated using the crystallographic wet chemical etching process. ZnO nanowires were grown onto these structures using the vapor-liquid-solid technique employing Au nanoparticles generated by annealing a sputter deposited layer. Several variations were made in the crystallographic etch process to generate different facet morphologies. Furthermore, the gold annealing step was carried out under different conditions, and its effect on the nature and density of ZnO nanowires was determined. It was observed that even for identical deposition conditions for the ZnO nanowires, modification of these two parameters strongly affected the selective deposition on various silicon facets. For relatively low Au annealing temperatures the entire surface of the micro-pyramidal structures was coated densely and uniformly with ZnO nanowires which were mostly aligned vertical to the facet surfaces. SEM as well as Cathodoluminescence (CL) imaging studies show that annealing of the Au film at higher temperature led to deposition of ZnO nanowires on the plateau at the top of the micro-pyramidal structures, while the side facets were completely bare. The nanowires deposited on the plateau on top the pyramids were as long as 40µm with a diameter of ~200nm. CL spectra obtained from the ZnO nanowires showed clear and sharp band edge luminescence along with emission at ~500nm due to surface states. It is envisioned that further reduction of the plateau dimensions will lead to single nanowire deposition on top of each pyramidal structure, which is appropriate for future field-emission devices.

Novel GaN-based vertical heterostructure field effect transistor structures using crystallographic KOH etching and overgrowth

A novel V-groove vertical heterostructure field effect transistor structure is proposed using semi-polar (11-22) GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable a damage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecular beam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essential requirement for the formation of a high mobility channel.

Terahertz filter integrated with a subwavelength structured antireflection coating

Micro-pyramid shaped subwavelength structures (SWSs) were integrated on both sides of a terahertz (THz) filter by means of stamping methods. Two silicon-based stamping molds fabricated via crystallographic wet etching were utilized to replicate SWSs onto cyclo-olefin copolymer (COC) films coated onto both sides of a THz filter at the same time. The SWSs act as an broadband antireflection coating to reduce the surface reflection loss in a frequency range of 0.2 THz to 1.4 THz. Compared to a THz filter without SWSs, the filter integrated with double-sided SWSs exhibits a low standing wave ratio inside the substrate and THz signal transmission enhancement of up to 10.8%.

Si-Nanowire-Array-Based NOT-Logic Circuits Constructed on Plastic Substrates Using Top–Down Methods

Si-nanowire (NW)-array-based NOT-logic circuits were constructed on plastic substrates. The Si-NW arrays were fabricated on a Si wafer through top-down methods, including conventional photolithography and crystallographic wet etching, and transferred onto the plastic substrates. Two field-effect transistors were fabricated on a single Si-NW array composed of five nanowires aligned in parallel and connected in series to form NOT-logic circuits. The excellent flexibility of the fabricated device was confirmed by bending-cycling tests. The voltage-transfer curve of the NOT-logic circuits showed an inverting operation with a logic swing of -92% and voltage gain of -2.5.

Separating InGaN membranes from GaN/sapphire templates through a crystallographic-etch-limited process

InGaN membranes with light-emitting diode (LED) structures were separated from an undoped GaN nanorod structure on sapphire substrates through a chemical lift-off (CLO) process. The CLO processes consisted of a reducing diameter process on the GaN nanorods structure and a crystallographic wet-etching process on an N-face GaN surface. The N-face crystallographic-etching process was limited by the boundary of the GaN nanorods, where a InGaN active layer can prevent etching damage in a hot potassium hydroxide solution. The light output power of the lift-off LED membrane had a 2.28 times enhancement compared with a non-treated LED. A pyramidal-roughened structure was formed on the lift-off GaN surface to increase the light extraction efficiency. The free-standing InGaN LED membranes were realized through a crystallographic-etch-limited CLO process, which has the potential to replace the traditional laser lift-off process for vertical LEDs and be applied to flexible optoelectronic membrane applications.

Surface relief structures for a flexible broadband terahertz absorber

Terahertz (THz) absorbers with surface relief structures (SRSs) were designed and fabricated on a flexible polydimethylsiloxane(PDMS) substrate by using a stamping method. The silicon mold used for the stamping process was prepared by using a crystallographic wet etching method with 45% KOH solution at 80°C. The flexible THz absorber, consisting of micropyramids with a base width of 240 μm, demonstrated nearly perfect absorbance higher than 99% owing to the dramatically reduced surface reflectance of the SRS. The reflectance of the PDMS with the SRS was less than 1%, which is only 1/100th of that measured from a bare PDMS at frequency higher than 1 THz.

Enhancement of Light Extraction Efficiency of InGaN Light-Emitting Diodes with an Air-Hole-Array Structure

The truncated-conical air-hole (TAH) array structure of an InGaN light-emitting diode (LED) was fabricated on the mesa-edge region to increase the light extraction efficiency. The fabrication consisted of a dry process and a crystallographic wet etching process on the AlN buffer layer to form a truncated-conical air-hole array pattern. The light output power of the TAH-LED structure has a 55% enhancement compared with the conventional LED structure at 20 mA operation current. At 20 mA operation current, the forward voltage and peak electroluminescence wavelength of the TAH-LED were measured to be 3.09 V and 455.6 nm, respectively, similar to those of the conventional LED structure because the truncated-conical air-hole array pattern was fabricated around the mesa-edge region without affecting the current injection area with a top transparent contact layer.

Enhancement of Light Extraction Efficiency of InGaN Light-Emitting Diodes with an Air-Hole-Array Structure

The truncated-conical air-hole (TAH) array structure of an InGaN light-emitting diode (LED) was fabricated on the mesa-edge region to increase the light extraction efficiency. The fabrication consisted of a dry process and a crystallographic wet etching process on the AlN buffer layer to form a truncated-conical air-hole array pattern. The light output power of the TAH-LED structure has a 55% enhancement compared with the conventional LED structure at 20 mA operation current. At 20 mA operation current, the forward voltage and peak electroluminescence wavelength of the TAH-LED were measured to be 3.09 V and 455.6 nm, respectively, similar to those of the conventional LED structure because the truncated-conical air-hole array pattern was fabricated around the mesa-edge region without affecting the current injection area with a top transparent contact layer.

Si-Based Flexible Memristive Systems Constructed Using Top-Down Methods

Si-based memristive systems consisting of Ag, amorphous Si, and heavily doped p-type Si nanowires were successfully constructed on plastic substrates through top-down methods, including the crystallographic wet etching of Si wafers, transfer onto plastic substrates, and thin film patterning. The memristive systems showed excellent memory characteristics and flexibility, such as intrinsic hysteric and rectifying behaviors, on/off resistance ratios of >1 × 10(5), and durability for up to 1000 bending cycles. The correlations between the Ag-filament-related nanostructures formed in amorphous Si and the resistance-switching behaviors were carefully examined with the tunneling current model, transmission electron microscopy, and secondary ion mass spectroscopy to explore the switching mechanism. Our study suggests the promising potential of the Si-based memristive systems for the development of next-generation flexible nonvolatile memory.

Investigation on Light Emission in Light-Emitting Diodes Constructed with <I>n</I>-ZnO and <I>p</I>-Si Nanowires

The light emission was investigated in light-emitting diodes (LEDs) constructed with n-ZnO and p-Si nanowires (NWs). ZnO NWs were synthesized by thermal chemical vapor deposition and Si NWs were formed by crystallographic wet etching of a Si wafer. The LEDs were fabricated using the NWs via dielectrophoresis (DEP) and direct transfer methods. The DEP method enabled to align the ZnO NW at the position that led to p-n heterojunction diodes by crossing with the transferred Si NW. The I-V curve of the p-n heterojunction diode showed the well-defined current-rectifying characteristic, with a turn-on voltage of 3 V. The electroluminescence spectrum in the dark showed the strong emission at approximately 385 nm and the broad emission centered at approximately 510 nm, at a forward bias of 30 V. Under the illumination of 325-nm-wavelength light, the luminescence intensity at 385 nm was dramatically enhanced, compared to that in the dark, probably due to the electric-field-induced enhancement of luminescence.

Electrically driven lasing in light-emitting devices composed of n-ZnO and p-Sinanowires

Electrically driven lasing was demonstrated in light-emitting devices composed of n-ZnOand p-Si nanowires (NWs). The ZnO NWs were synthesized by thermal chemical vapordeposition and the Si NWs were formed by crystallographic wet etching of aSi wafer. The p–n heterojunction devices were constructed using the NWs bythe direct transfer and dielectrophoresis methods. At an excitation current of2 µA, the electroluminescence spectrum showed lasing behavior, and this phenomenon wasexplained by the ZnO-nanostructure-related cavity property.

InGaN-based light-emitting diodes with an embedded conical air-voids structure

The conical air-void structure of an InGaN light-emitting diode (LEDs) was formed at the GaN/sapphire interface to increase the light extraction efficiency. The fabrication process of the conical air-void structure consisted of a dry process and a crystallographic wet etching process on an undoped GaN layer, followed by a re-growth process for the InGaN LED structure. A higher light output power (1.54 times) and a small divergent angle (120°) were observed, at a 20 mA operation current, on the treated LED structure when compared to a standard LED without the conical air-void structure. In this electroluminescence spectrum, the emission intensity and the peak wavelength varied periodically by corresponding to the conical air-void patterns that were measured through a 100 nm-optical-aperture fiber probe. The conical air-void structure reduced the compressed strain at the GaN/sapphire interface by inducing the wavelength blueshift phenomenon and the higher internal quantum efficiency of the photoluminescence spectra for the treated LED structure.

Size-tuning of monodisperse PMMA nanoparticles by micro-continuous-flow polymerization using a silicon micro-nozzle array

The formation of PMMA nanoparticles of different size by continuous-flow emulsion polymerization was investigated by using a microreactor with an incorporated nozzle array. The emulsion was formed by pressing the organic phase containing the monomer MMA, a monomer-dissolved initiator as well as a cross-linking agent through an array of micro-pores into a fast streaming aqueous phase. High shear forces are realized by using low channel depths from 0.05 to 0.1mm for the aqueous phase underneath the nozzle array. The array was prepared by a combination of an anisotropic crystallographic wet etching step and an anisotropic ion beam etching step in a Si (100) wafer. The nozzle array consists on a silicon frame and a window covered by a membrane of monocrystalline silicon with 16×48 micro-pores of equal diameter (20μm) in a rectangular arrangement. The liquids were actuated by a computer-controlled 4-channel syringe pump (Cetoni). Flow rates between 25 and 1000μL/min were applied. The formed emulsion was collected in a tempered reaction vessel (97°C) for completion of the polymerization process. The obtained polymer nanoparticles were characterized by scanning electron microscopy (SEM) and by centrifugal sedimentation spectroscopy (DCS). Homogeneous particle populations with diameters between about 60nm and 500nm were realized in dependence on the composition of reactant mixture and process conditions. Larger particles (size between 260 and 500nm diameter) were obtained in the absence of surfactants. Under optimized conditions, the process was robust against flow rate changes. The addition of a surfactant (sodium dodecyl sulfate, SDS) leads to a drastic reduction of the particle sizes. As result an easy way for the generation of PMMA nanoparticles in a micro-continuous-flow process and a convenient way to tune the particle sizes was developed.

Blue light-emitting diodes with a roughened backside fabricated by wet etching

The InGaN-based light-emitting diodes (LEDs) with a roughened patterned backside on the N-face GaN surface were fabricated through a crystallographic etching process to increase light-extraction efficiency. After laser decomposition, laser scribing, and a lateral crystallographic wet etching process at the GaN/Al2O3 interface, stable crystallographic etching planes were formed as the GaN {1011¯} planes that included an angle with the top GaN (0001) plane measured at 58°. The GaN buffer layer acted as the sacrificial layer for the laser decomposition process and the lateral wet etching process with a 26 μm/min etching rate. The LED with the inverted pyramidal N-face GaN surface close to the GaN/Al2O3 interface has a larger light-scattering process than the conventional LED. The light-output power of the LED with the backside roughened surface had a 47% enhancement when measured in LED chip form.

Triangular Extended Microtunnels in GaN Prepared by Selective Crystallographic Wet Chemical Etching

Extended microtunnels with triangular cross sections in thick GaN films were demonstrated using wet chemical etching on specially designed epitaxial lateral overgrowth structures. For tunnels along the and directions of GaN, the and facets are the etch stop planes with activation energies of 23 kcal/mol determined by wet chemical etching. The axial etching rate of the tunnels in the direction is twice as large than that along the direction. The highest etching rate of the tunnels in the axial direction is .

Enhanced Light Extraction Efficiency in InGaN Light-Emitting Diodes Through Photoelectrochemical Sidewall Shaping Process

The cone-shaped-sidewall InGaN-based light-emitting diodes (LEDs) with higher light extraction efficiency were fabricated through the photoelectrochemical (PEC) process. The process procedures were consisted with the PEC selective wet oxidation on n-type GaN without p-type GaN layer, removing Ga2O3 layer, crystallographic wet etching process, and forming cone-shaped sidewall structure. The stable crystallographic etching planes were formed as the p-type GaN { } planes and n-type GaN { } planes with a 27{degree sign} included angle. The InGaN/GaN multi-quantum-well (MQW) layers were located at the cone's tip nano-structure in mesa-edge region, and the micro-photoluminescence spectrums had the blue shift phenomenon from 457.8nm to 450.8nm caused by the quantum confinement effect and particle compress strain released.

Enhanced Light Output Power in InGaN Light-Emitting Diodes by Fabricating Inclined Undercut Structure

The -based light-emitting diode (LED) with an inclined undercut structure is fabricated through the photoelectrochemical two-step process to increase light extraction efficiency. In the first step the sidewall-undercut structure at the p-type and n-type interface is created by selective wet oxidation on an n-type surface in pure solution. In the second step an inclined undercut structure through a crystallographic wet-etching process is formed by immersion in hot solution. This crystallographic wet-etching process can remove the layer and form a p-type stable plane, n-type stable plane on the mesa sidewall. This inclined p-type plane of LED structure can provide the higher overlap of incident light beam core and extraction core overlap on the mesa sidewall, and the total light output power of the treated LED is 2.10 times higher than the standard LED. Consequently, this inclined undercut LED structure is suitable for high-efficiency nitride-based LED application.

Fabrication and characterization of 3-Dimensional MOS transistor tip integrated micro cantilever

We fabricate and characterize a three-dimensional (3-D) MOS (metal–oxide–semiconductor) transistor tip integrated micro cantilever to measure the surface properties. The 3-D MOS transistor tip is fabricated on the front side end of the cantilever, and the cantilever itself works as a tip. These features make the device possible to investigate hard-detecting parts such as the deep trenches and the sidewalls of the structure. The MOS transistor tip has other advantages such as the high operation speed, the high sensitivity, and the reduction of the required equipments like the lock-in-amplifier. The MOS transistor tip is fabricated three-dimensionally, utilizing the lateral diffusion and the anisotropic wet etching with TMAH solution, since the etch rate of {211} plane is much higher than those of {100} or {111} planes. The gate area is formed by self-aligned technique, using crystallographic dependant wet etching. The well-known convex corner compensation pattern is used for the gate length control during the tip fabrication process. The characteristics of the fabricated device are measured with respect to the various electric signals and the results show the well-established detection properties.

Fabrication of Submicron Y-gate InP Metal Semiconductor Field Effect Transistors Using Crystallographically Defined Contact Technology

Submicron Y-gate InP metal semiconductor field effect transistors (MESFETs), using a new crystallographically defined gate contact technology based on conventional optical lithography, are fabricated and their performances are investigated. In this technology, a new submicron gate patterning process has been developed based on the consistent crystallographic wet etching characteristics of an InP dummy layer, which is grown on top of a conventional MESFET layer structure. The Y-shaped gate electrode, which is formed by the crystallographically etched sidewall profile of the double dummy layer structure, is used to realize InP MESFETs having 0.5 µm gate-foot dimension by controlling only the thickness of the InP dummy layer. The fabricated MESFETs using the proposed crystallographically defined gate contact technology show greatly improved overall device performances compared to conventional MESFETs. A maximum DC-transconductance of 234 mS/mm was measured and the maximum current gain cutoff frequency fT of 20 GHz and maximum oscillation frequency fmax of 33 GHz were obtained from the devices fabricated using the proposed technology.