Oxide Confinement Research Articles

High-efficiency edge-emitting laser diodes with oxidation confinement stripe structure

High-efficiency, low-cost, high-power laser diodes (LDs) are in the spotlight due to the growing market demand. This study proposes a new edge-emitting LD structure that utilizes the facile oxidization properties of the high aluminum (Al) component AlGaAs material and simplifies LD fabrication to only one-step lithography. The key innovation is incorporating a ∼200 nm thick high-Al AlGaAs layer in the vertical epitaxial structure. This layer serves two purposes: 1) The low refractive index improves vertical mode confinement, enabling a thinner overall epitaxial structure. 2) Oxidation of this layer forms a dielectric isolator and a current aperture. Experimental LDs exhibited a peak efficiency of 77.8 %, peak power of 33.6 W, vertical divergence angle of 19.4°, and horizontal divergence angle of 8.8°. Compared to conventional edge-emitting LD, this LD demonstrates a 39 % lower thermal resistance, better optoelectronic performance, a simpler process, and a lower production cost.

Fast rate lithium metal batteries with long lifespan enabled by graphene oxide confinement

Dendritic growth of lithium (Li) is hindering potential applications of Li-metal batteries, and new approaches are needed to address this challenge.

Chemical imaging of oxide confinement layers in VCSELs

The authors have studied the lateral oxidation of layers buried in vertical-cavity surface-emitting lasers using cross-sectional scanning transmission electron microscopy coupled with electron energy loss spectroscopy. Chemical maps and composition profiles of the oxidized AlO x layers have been produced. The sensitivity is such that trace compositions of a few % As and Ga can be detected in the AlO x with a spatial resolution of a few nanometers on the recorded chemical maps. To demonstrate the performance of the mapping technique, we compare results from an area in the vertical-cavity surface-emitting laser (VCSEL) which is pure to an oxidized area. These measurements are performed on a thin sample prepared by the focused ion beam technique within an actual VCSEL, which makes the mapping technique applicable for degradation investigations in devices at different stages of their lifetime. More generally, this measurement method is effective for detailed evaluation of AlO x layers and their fabrication process.

Preparation of magnesium oxide confined in activated carbon synthesized from palm kernel shell and its application for hydrogen sulfide removal

In this study, magnesium oxide (MgO) confined in porous carbon was synthesized and the efficacy of this material was analyzed for hydrogen sulfide removal. The palm kernel shell was carbonated to obtain the porous carbon material (labelled as AC-TKS). The confinement of magnesium oxide was carried out by the incipient wetness impregnation process of a metal oxide precursor followed by calcination. The MgO content in porous carbon was optimized to maintain a 5 wt. % total content of oxide (MgO-5%/AC-TKS). Materials were characterized using N2-sorption analysis, X-ray diffraction (XRD) and scanning electron microscopy-EDX (SEM-EDX). AC-TKS has a high specific surface area (SSA) of 708 m2/g. Meanwhile, the specific surface area decreased by up to 50% (ca. 248 m2/g) for MgO-5%/AC-TKS caused by the blocking of the MgO content. The successful confinement process was proven by X-ray diffraction analysis which revealed that MgO was formed by the appearance of MgO peaks. This is supported by the results of SEM-EDX mapping, which showed the presence of Mg and O elements on the carbon surface. The higher H2S removal was shown by MgO-5%/AC-TKS which could reduce H2S up to 98.6% and showed a better performance compared to only porous carbon.

Studying the light current characteristics of VCSEL by changing oxide confinement aperture

GaAs-based VCSELs with oxide-confined (OC) are performed by using fine selective radial “oxidation of AlAs-rich (AlGaAs) layers” from outside to the device axis leaving the central part unaffected. Such oxide apertures produced inside the cavities enhance the radial confinements related to current spreading and to a field that is optical altogether. A Modern design with a single fundamental mode is proposed to reduce threshold current operation to the extent of higher output by using the two OC VCSELs. Both of the oxide apertures were added to the node position of the resonator standing wave. Then, the diameters of together apertures can be adjusted independently giving VCSEL design an extra degree of freedom that allows optimization. It found that when both apertures had a radius of 4.2 µm, the low threshold current operation at a higher extent output is achieved with a single fundamental mode.

Giant Optical Anisotropy of Perovskite Nanowire Array Films

AbstractPolarizers play a key role in generating polarized light for display, imaging, and data communication, but adoption often suffers from high optical loss. Recently, due to superior optoelectronic properties, halide perovskites have been widely developed for lighting applications; however, highly polarized emission (polarization degree >0.8) has not yet been realized with perovskites. Herein, by incorporating inkjet printing and an anodic aluminum oxide (AAO) confinement strategy, highly ordered perovskite nanowire (NW) arrays are demonstrated for anisotropic optical applications. The optical device based on perovskite NW arrays reveals a high photoluminescence external quantum efficiency of 21.6% and emits highly polarized light with polarization degree up to 0.84. The highly polarized emission from perovskite NW arrays has potential to considerably reduce the optical loss of polarizers, which may attract great interest in developing polarized light sources for next‐generation optoelectronic applications.

Multimode VCSEL Enables 42-GBaud PAM-4 and 35-GBaud 16-QAM OFDM for 100-m OM5 MMF Data Link

By enhancing the differential gain and reducing the capacitance of the 850-nm multi-mode vertical cavity surface emitting laser (VCSEL) with an analog bandwidth beyond 25 GHz via the use of InGaAs/AlGaAs quantum wells and multiple oxide confinement layers, the transmission of directly encoded 4-level pulse amplitude modulation (PAM-4) data at 42 GBaud and 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM OFDM) data at 35 GBaud are demonstrated. After passing through 100-m OM5 multimode fiber (MMF), the detailed comparison between the VCSELs designed with different aperture sizes of 5.5/7.5 μm is performed. The 7.5-μm-aperture VCSEL provides the higher power with larger quantum efficiency but exhibits the narrower 3-dB bandwidth and higher noise level than those of the 5.5-μm-aperture VCSEL. Shrinking the oxide-confined aperture to 5.5 μm assists the VCSEL to expand its 3-dB bandwidth to 25.2 GHz and suppresses its relative intensity noise to -135 dBc/Hz, which contributes to support the highest data rate up to 84 and 140 Gbit/s, respectively, for PAM-4 and 16-QAM OFDM data under forward error correction criterion at optical back-to-back condition. Even after transmitting through 100-m-long OM5-MMF, the allowable data transmission rate still remains at 80 Gbit/s for PAM-4 and 120 Gbit/s for 16-QAM OFDM with their receiving power penalty of 3.24 and 3.1 dB, respectively, when the data is carried by the 5.5-μm-aperture VCSEL. Such a newly designed VCSEL structure promotes its allowable bandwidth to manifest the feasibility toward 50-GBaud per channel capacity for future data center applications.

Optical Characterizations of VCSEL for Emission at 850 nm with Al Oxide Confinement Layers

In-plane micro-photoluminescence (μ-PL) and micro-reflectivity measurements have been performed at room temperature by optical excitation perpendicular to the surface of two different structures: a complete vertical surface-emitting laser (VCSEL) structure and a VCSEL without the upper p-type distributed Bragg reflector (P-DBR). The two structures were both laterally oxidized and measurements were made on the top of oxidized and unoxidized regions. We show that, since the photoluminescence (PL) spectra consist of the cumulative effect of InGaAs/AlGaAs multi-quantum wells (MQWs) luminescence and interferences in the DBR, the presence or not of the P-DBR and oxide layers can significantly modify the spectrum. μ-PL mapping performed on full VCSEL structures clearly shows oxidized and unoxidized regions that are not resolved with visible light optical microscopy. Finally, preliminary measurements of the degree of polarization (DOP) of the PL have been made on a complete VCSEL structure before and after an oxidation process. We obtain an image of DOP measured by polarization-resolved μ-PL. These measurements allow us to evaluate the main components of strain.

Assembly of multicomponent nanoframes via the synergistic actions of graphene oxide space confinement effect and oriented cation exchange

Multicomponent nanoframes (NFs) with a hollow structural character have shown the potential to be applied in many fields. Here we report a novel strategy to synthesize ZnxCd1−xS NFs via the synergistic actions of the graphene oxide (GO) confinement effect and oriented cation exchange. The obtained samples have been systematically characterized by x-ray diffractometry (XRD), field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray photospectroscopy (XPS) and Raman spectrometry. The results show that the two dimensional space confinement effect induced by GO and the oriented cation exchange reaction are responsible for the formation of the multicomponent NFs. The high photoelectrochemical activity and the low cost of the starting materials will make the multicomponent NFs applicable in photoelectronic and photoelectrocatalytic fields.

Oxide confinement and high contrast grating mirrors for Mid-infrared VCSELs

A new mid-infrared (MIR) Vertical Cavity Surface Emitting Laser (VCSEL) structure is proposed. We have integrated to the VCSEL structure both an oxide aperture for lateral confinement, and a sub-wavelength high-contrast-grating top mirror. Upon the GaSb-based half-VCSEL, we have grown a metamorphic AlGaAs heterostructure to enable thermal oxidation and grating mirror fabrication steps. A methodology based on optimization and anti-optimization methods has been used to design the optical grating, with improved parameter tolerances regarding processing errors. Finally, we show the complete fabrication of an electrically-pumped MIR monolithic VCSEL structure implementing both oxide confinement and a subwavelength grating top mirror.

Numerical Self-Consistent Analysis of VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) yield single-longitudinal-mode operation, low-divergence circular output beam, and low threshold current. This paper gives an overview on theoretical, self-consistent modelling of physical phenomena occurring in a VCSEL. The model has been experimentally confirmed. We present versatile numerical methods for nitride, arsenide, and phosphide VCSELs emitting light at wavelengths varying from violet to near infrared. We also discuss different designs with respect to optical confinement: gain guidance using tunnel junctions and index guidance using oxide confinement or photonic crystal and we focus on the problem of single-transverse-mode operation.

Fabrication and Characterization of Bragg Reflector Slow-Light Waveguide with Oxide Lateral Confinement

Slowing light in Bragg reflector waveguides enables us to miniaturize optical devices. A platform of low-loss slow-light waveguides with lateral optical confinement is required. We propose and fabricate a slow-light waveguide with lateral oxide confinement using an equivalent index difference in a lateral direction. The coupling and propagation losses are evaluated from the waveguide length dependence of insertion losses. The propagation loss is in the range of 0.1 dB/µm for transverse magnetic (TM) modes. Also, it is predicted that the propagation loss can be reduced if we replace a top Au/distributed Bragg reflector (DBR) hybrid mirror with a Si/SiO2 DBR.

Single Mode Photonic Crystal Vertical Cavity Surface Emitting Lasers

We review the design, fabrication, and performance of photonic crystal vertical cavity surface emitting lasers (VCSELs). Using a periodic pattern of etched holes in the top facet of the VCSEL, the optical cavity can be designed to support the fundamental mode only. The electrical confinement is independently defined by proton implantation or oxide confinement. By control of the refractive index and loss created by the photonic crystal, operation in the Gaussian mode can be insured, independent of the lasing wavelength.

All-Optical Flip-Flop Operation at 1-mA Bias Current in Polarization Bistable Vertical-Cavity Surface-Emitting Lasers With an Oxide Confinement Structure

Polarization bistable vertical-cavity surface-emitting lasers (VCSELs) with an oxidation confinement structure were fabricated for low power consumption operation. Threshold current of the VCSEL was reduced to 0.22 mA. The VCSEL operated in a single longitudinal and transverse mode and in one of two orthogonal linear polarization modes. All-optical flip-flop operation based on polarization bistability was achieved at a record low bias current of 1.15 mA, which is about one tenth of our previously developed polarization bistable VCSEL without a current confinement structure.

Oxide-Confined Formation of Germanium Nanowire Heterostructures for High-Performance Transistors

Over the past several years, the formation of nanowire heterostructures via a solid-state reaction between a semiconductor nanowire and metal contact pads has attracted great interest. This is owing to its ready application in nanowire field-effect transistors (FETs) with a well-controlled channel length using a facile rapid thermal annealing process. We report the effect of oxide confinement on the formation of Ge nanowire heterostructures via a controlled reaction between a vapor-liquid-solid-grown, single-crystalline Ge nanowire and Ni pads. In contrast to the previous formation of Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni(2)Ge and Ge with the confinement of Al(2)O(3) during annealing. Significantly, back-gate FETs based on this Ni(2)Ge/NiGe/Ge/NiGe/Ni(2)Ge heterostructure demonstrated a high-performance p-type transistor behavior, showing a large on/off ratio of more than 10(5) and a high normalized transconductance of 2.4 μS/μm. The field-effect hole mobility was extracted to be 210 cm(2)/(V s). Temperature-dependent I-V measurements further confirmed that NiGe has an ideal ohmic contact to p-type Ge with a small Schottky barrier height of 0.11 eV. Moreover, the hysteresis during gate bias sweeping was significantly reduced after Al(2)O(3) passivation, and our Ω-gate Ge nanowire FETs using Al(2)O(3) as the top-gate dielectric showed an enhanced subthreshold swing and transconductance. Therefore, we conclude that the Al(2)O(3) layer can effectively passivate the Ge surface and also serve as a good gate dielectric in Ge top-gate FETs. Our innovative approach provides another freedom to control the growth of nanowire heterostructure and to further achieve high-performance nanowire transistors.

Current spreading modification to enhance single-fundamental-mode VCSEL operation at higher temperatures

AbstractContinuous-wave (CW) performance of modern oxide-confined (OC) vertical-cavity surface-emitting diode lasers (VCSELs) at room and elevated temperatures is investigated with the aid of the comprehensive fully self-consistent optical-electrical-thermal-gain model. A standard OC GaInNAs/GaAs double-quantum-well VCSEL emitting the 1.3-μm radiation is used as a typical modern VCSEL structure. The oxide aperture is placed at the anti-node position of an optical standing wave within a VCSEL cavity. The desired single-fundamental-mode (SFM) operation has been found to be expected only in VCSELs equipped with relatively small active regions of diameters equal or smaller than 10 μm. Therefore a proton implantation used as an radial additional confinement of the current spreading from the upper annular contact towards the centrally located active region is proposed and its impact on the VCSEL performance is investigated. The above structure modification has been found to enable a radical improvement in the VCSEL performance. In particular, in this case, the SFM VCSEL operation is possible even in VCSELs with quite large active regions and for much wider ambient-temperature range than in the standard OC VCSELs.

Formation and Device Application of Ge Nanowire Heterostructures via Rapid Thermal Annealing

We reviewed the formation of Ge nanowire heterostructure and its field-effect characteristics by a controlled reaction between a single-crystalline Ge nanowire and Ni contact pads using a facile rapid thermal annealing process. Scanning electron microscopy and transmission electron microscopy demonstrated a wide temperature range of 400~500°C to convert the Ge nanowire to a single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure with atomically sharp interfaces. More importantly, we studied the effect of oxide confinement during the formation of nickel germanides in a Ge nanowire. In contrast to the formation of Ni2Ge/Ge/Ni2Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni2Ge with the confinement of Al2O3during annealing. A twisted epitaxial growth mode was observed in both two Ge nanowire heterostructures to accommodate the large lattice mismatch in the NixGe/Ge interface. Moreover, we have demonstrated field-effect transistors using the nickel germanide regions as source/drain contacts to the Ge nanowire channel. Our Ge nanowire transistors have shown a high-performancep-type behavior with a high on/off ratio of 105and a field-effect hole mobility of 210 cm2/Vs, which showed a significant improvement compared with that from unreacted Ge nanowire transistors.

Coherent polarization stabilization in large-aperture rectangular post bottom-emitting vertical-cavity surface-emitting lasers

The output characteristics of large-aperture rectangular post bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) were investigated. It was shown that the output power of the rectangle VCSELs can be up to 660mW at a current of 5A. Both H-polarization (horizontal) and V-polarization (vertical) demonstrated a coherent stabilization over the entire range of operation current, and coherent spectrum blue-shift of H-polarization light occurred with respect to V-polarization light at three different injected currents. The polarization states of output light were stabilized in the two orthogonal directions and H-polarization was the most principal polarization which was parallel to the longer side of the rectangular aperture. From the relationship between polarization ratio and aspect ratio of the oxidation confinement aperture (OCA), it was found that the highest polarization ratio (about 2:1) took place when the appropriate aspect ratio was 5:3, which meant better polarization stabilization in large-aperture VCSELs.

Growth of Nickel Silicides in Si and Si/SiOx Core/Shell Nanowires

We exploited the oxide shell structure to explore the structure confinement effect on the nickel silicide growth in one-dimensional nanowire template. The oxide confinement structure is similar to the contact structure (via hole) in the thin film system or nanodevices passivated by oxide or nitride film. Silicon nanowires in direct contact with nickel pads transform into two phases of nickel silicides, Ni31Si12 and NiSi2, after one-step annealing at 550 °C. In a bare Si nanowire during the annealing process, NiSi2 grows initially through the nanowire, followed by the transformation of NiSi2 into the nickel-rich phase, Ni31Si12 starting from near the nickel pad. Ni31Si12 is also observed under the nickel pads. Although the same phase transformations of Si to nickel silicides are observed in nanowires with oxide confinement structure, the growth rate of nickel silicides, Ni31Si12 and NiSi2, is retarded dramatically. With increasing oxide thickness from 5 to 50 nm, the retarding effect of the Ni31Si12 growth and the annihilation of Ni2Si into the oxide confined-Si is clearly observed. Ni31Si12 and Ni2Si phases are limited to grow into the Si/SiOx core-shell nanowire as the shell thickness reaches 50 nm. It is experimental evidence that phase transformation is influenced by the stressed structure at nanoscale.

Comparative analysis of lasing performance of oxide-confined and proton-implanted vertical-cavity surface-emitting diode lasers

The comprehensive optical-electrical-thermal-recombination self-consistent VCSEL model is used to compare the performance of oxide-confined (OC) and proton-implanted (PI) VCSELs and to optimise their structures. Generally index-guided (IG) OC VCSELs demonstrate lower lasing thresholds whereas both gain-guided (GG) OC and PI ones manifest much better mode selectivity. Therefore, their either low-threshold IG or mode-selective GG versions may be intentionally used for different VCSEL applications. Lasing thresholds of OC IG VCSELs have been found to be very sensitive to the exact localisation of their thin oxide apertures, which should be shifted as close as possible towards the anti-node position. PI VCSELs, on the other hand, are simpler and cheaper in their manufacturing than OC ones. Although lower threshold currents are manifested by PI VCSELs with very thick implanted regions, lower threshold powers are achieved in these devices with much thicker upper unaffected layer used for the radial current flow from the ring contact towards the laser axis. Paradoxically poor thermal properties of PI VCSELs enable lower lasing thresholds of slightly detuned devices. To conclude, cheaper and mode-selective PI VCSELs may be used instead of OC ones in many of their applications provided ambient temperatures and laser outputs are not too high.