A wafer-scale CMOS-compatible process for heterogeneous integration of III-V epitaxial material onto silicon for photonic device fabrication is presented. Transfer of AlGaAs-GaAs Vertical-Cavity Surface-Emitting Laser (VCSEL) epitaxial material onto silicon using a carrier wafer process and metallic bonding is used to form III-V islands which are subsequently processed into VCSELs. The transfer process begins with the bonding of III-V wafer pieces epitaxy-down on a carrier wafer using a temporary bonding material. Following substrate removal, precisely-located islands of material are formed using photolithography and dry etching. These islands are bonded onto a silicon host wafer using a thin-film non-gold metal bonding process and the transfer wafer is removed. Following the bonding of the epitaxial islands onto the silicon wafer, standard processing methods are used to form VCSELs with non-gold contacts. The removal of the GaAs substrate prior to bonding provides an improved thermal pathway which leads to a reduction in wavelength shift with output power under continuous-wave (CW) excitation. Unlike prior work in which fullyfabricated VCSELs are flip-chip bonded to silicon, all photonic device processing takes place after the epitaxial transfer process. The electrical and optical performance of heterogeneously integrated 850nm GaAs VCSELs on silicon is compared to their as-grown counterparts. The demonstrated method creates the potential for the integration of III-V photonic devices with silicon CMOS, including CMOS imaging arrays. Such devices could have use in applications ranging from 3D imaging to LiDAR.
An array of active photonic devices is fabricated in unison after a heterogeneous integration process first metal-eutectically bonds these distinct materials as a distribution onto a silicon host wafer. The patterning out of heterogeneous materials followed by the formation of all photonic devices allows for wide-area fine-alignment without the need for discrete die alignment or placement. The integration process is designed as a CMOS-compatible, scalable method for bringing together distinct III-V epitaxial structures and optical-waveguiding epitaxial structures, demonstrating the capabilities of forming a multi-chip layer of photonic materials. Integrated GaAs-based vertical light-emitting transistors (LET) are designed and fabricated as the active devices whose third electrical terminal provides an electrical interconnect and thermal dissipation path to the silicon host wafer. The performance of these devices as both electrical transistors and spontaneous-emission optical devices is compared to their monolithically-integrated counterparts to investigate improvements in device characteristics when integrated onto silicon. The fabrication methods are modified and optimized for thin-film transferred materials and are then extended to transistor laser (TL) fabrication. Passive waveguiding structures are designed and simulated for coupling light from the active devices, and their fabrication scheme is presented such that it can be similarly performed with transferred materials. Work toward the demonstration of integrated transistor lasers is shown to represent progress toward an electronic-photonic circuit network. The combination of heterogeneous integration with three-terminal photonic structures enables an elegant solution to both packaging and signal interconnect constraints for the implementation of photonic logic in silicon photonics systems.
Impurity-induced disordering in vertical-cavity surface-emitting lasers (VCSELs) has demonstrated enhanced performance such as higher modulation speeds, reduced series resistance, and higher-order mode suppression for singlemode operation. Initiated by the diffusion of Zn, impurity-induced disordering intermixes discrete AlGaAs-based distributed Bragg reflectors (DBR) pairs which leads to lower mirror power reflectivity and increased optical loss. When formed into an aperture where the center is non-disordered, suppression of higher-order transverse modes for high-power single-mode operation can be achieved. For maximal mode suppression, deep disordering apertures are desirable. However, due to the isotropic nature of diffusion, these apertures are limited to the lateral diffusion encroaching onto the fundamental mode. By tailoring the film stress of the SiNx diffusion mask, the capability to modify the diffusion front of the disordering aperture is demonstrated. Defined by their lateral-to-vertical (L/V) diffusion ratios, an L/V ratio of 3.7 to 0.90 is measured for corresponding SiNx diffusion mask strains ranging from a compressive -797 MPa to a tensile +347 MPa. This demonstrates that tensile strained diffusion masks limit the amount of lateral diffusion. To further reduce the lateral encroachment, increasingly tensile diffusion masks are deposited by modifying the SiH4/NH3 flow ratios. This diffusion mask is employed to fabricate high-power single-mode VCSELs designed for 850 nm emission. Compared to VCSELs fabricated with non-optimized disordering apertures, enhanced transverse-mode control is achieved and singlemode output power in excess of 3.8 mW with a side mode suppression ratio greater than 30 dB is measured.
Approaches are demonstrated that enable mobile devices, such as smartphones, to function as spectrophotometers with equivalent performance to laboratory instruments for measuring any diagnostic test that generates a colored liquid, fluorescent liquid, or colored solid surface. We envision mobile health diagnostic applications in which smartphone integrated measurement of point-of-care assays enables smart service systems for efficiently connecting patients with health care providers and other health services. A key to this capability is to offer valid tests that are equivalent to those performed in the laboratory by utilizing the same reagents, experimental controls, and calibration standards as conventional assays.
Progress on the modeling, fabrication, and characterization of the transistor-injected quantum-cascade laser (TI-QCL) is presented. As a novel variant of the quantum cascade laser, the TI-QCL has been projected to have advantages over conventional QCLs in certain applications because of its 3-terminal nature. The separation of field and current is expected to allow separate amplitude and frequency modulation, and the location of the cascade structure in a p-n junction depletion region is expected to reduce free carrier absorption and improve efficiency. At the same time, the added complexity of the structure creates challenges in the realization of working devices. An overview of the basic operating principles of the TI-QCL is first given, and projected advantages discussed. Next, work on modeling GaAsbased TI-QCLs is presented, and a design for devices in this system is presented. Finally, work on fabrication and characterization of devices is examined and ongoing challenges are discussed. The role of quantum state alignment in the QCL region on electron-hole recombination in the base is also examined, showing the capability of using basecollector voltage to modulate the optical output from the direct-bandgap transistor base.
Top emission 850-nm vertical-cavity surface-emitting lasers (VCSELs) demonstrating transverse mode selection via impurity-induced disordering (IID) are presented. The IID apertures are fabricated via closed ampoule zinc diffusion. A simple 1-D plane wave model based on the intermixing of Group III atoms during IID is presented to optimize the mirror loss of higher-order modes as a function of IID strength and depth. In addition, the impact of impurity diffusion into the cap layer of the lasers is shown to improve contact resistance. Further investigation of the mode-dependent characteristics of the device imply an increase in the thermal impedance associated with the fraction of IID contained within the oxide aperture. The optimization of the ratio of the IID aperture to oxide aperture is experimentally determined. Single fundamental mode output of 1.6 mW with 30 dBm side mode suppression ratio is achieved by a 3.0 μm oxide-confined device with an IID aperture of 1.3 μm indicating an optimal IID aperture size of 43% of the oxide aperture.
A novel method for controlling the transverse lasing modes in both proton implanted and oxide-confined vertical- cavity surface-emitting lasers (VCSELs) with a multi-layer, patterned, dielectric anti-phase (DAP) filter is pre- sented. Using a simple photolithographic liftoff process, dielectric layers are deposited and patterned on individual VCSELs to modify (increase or decrease) the mirror reflectivity across the emission aperture via anti-phase reflections, creating spatially-dependent threshold material gain. The shape of the dielectric pattern can be tailored to overlap with specific transverse VCSEL modes or subsets of transverse modes to either facilitate or inhibit lasing by decreasing or increasing, respectively, the threshold modal gain. A silicon dioxide (SiO2) and titanium dioxide (TiO2) anti-phase filter is used to achieve a single-fundamental-mode, continuous-wave output power greater than 4.0 mW in an oxide-confined VCSEL at a lasing wavelength of 850 nm. A filter consisting of SiO2 and TiO2 is used to facilitate injection-current-insensitive fundamental mode and lower order mode lasing in proton implanted VCSELs at a lasing wavelength of 850 nm. Higher refractive index dielectric materials such as amorphous silicon (a-Si) can be used to increase the effectiveness of the anti-phase filter on proton implanted devices by reducing the threshold modal gain of any spatially overlapping modes. This additive, non-destructive method allows for mode selection at any lasing wavelength and for any VCSEL layer structure without the need for semiconductor etching or epitaxial regrowth. It also offers the capability of designing a filter based upon available optical coating materials.
Recombination of carriers in the direct-bandgap base of a transistor-injected quantum cascade laser (TI-QCL) is shown to be controllable through the field applied across the quantum cascade region located in the transistor’s base-collector junction. The influence of the electric field on the quantum states in the cascade region’s superlattice allows free flow of electrons out of the transistor base only for field values near the design field that provides optimal QCL gain. Quantum modulation of base recombination in the light-emitting transistor is therefore observed. In a GaAs-based light-emitting transistor, a periodic superlattice is grown between the p-type base and the n-type collector. Under different base-collector biasing conditions the distribution of quantum states, and as a consequence transition probabilities through the wells and barriers forming the cascade region, leads to strong field-dependent mobility for electrons in transit through the base-collector junction. The radiative base recombination, which is influenced by minority carrier transition lifetime, can be modulated through the quantum states alignment in the superlattice. A GaAs-based transistor-injected quantum cascade laser with AlGaAs/GaAs superlattice is designed and fabricated. Radiative base recombination is measured under both common-emitter and common-base configuration. In both configurations the optical output from the base is proportional to the emitter injection. When the quantum states in the superlattice are aligned the optical output in the base is reduced as electrons encounter less impedance entering the collector; when the quantum states are misaligned electrons have longer lifetime in the base and the radiative base recombination process is enhanced.
Bandwidth requirements continue to drive the need for low-power, high speed interconnects. Harnessing the mature CMOS technology for high volume manufacturing, Silicon Photonics is a top candidate for providing a viable solution for high bandwidth, low cost, low power, and high packing density, optical interconnects. The major drawback of silicon, however, is that it is an indirect bandgap material, and thus cannot produce coherent light. Consequently, different integration schemes of III/V materials on silicon are being explored. An integrated CMOS tunable laser is demonstrated as part of a composite-CMOS integration platform that enables high bandwidth optical interconnects. The integration platform embeds III-V into silicon chips using a metal bonding technique that provides low thermal resistance and avoids lattice mismatch problems. The performance of the laser including side mode suppression ratio, relative intensity noise, and linewidth is summarized.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.