Research

 

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CIS - Harris Group Research Catalog
Materials Research


MBE Growth of Rare Earth-Doped Topological Insulators
Students: Sara Harrison
Description: Recently, breaking the time-reversal symmetry (TRS) in topological insulators (TIs) has become the focus of intense experimental efforts. Incorporating magnetic dopants into TIs is the most popular method employed to break the TRS in three-dimensional bismuth chalcogenide topological insulators. Ferromagnetic coupling between magnetic dopants can lead to TRS-breaking in the doped TI, which is a prerequisite for observing the quantum anomalous Hall (QAHE) effect and other novel magnetoelectric phenomena. We are currently investigating the thin film growth of rare earth-doped 3D topological insulators using molecular beam epitaxy. We perform structural, magnetic, and electronic properties characterization of our materials in order to evaluate their potential for observing novel magnetically induced phenomena and for applications in next generation electronic devices.


 
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Optoelectronic Devices

Development of GeSn-based Technologies for Group-IV Optoelectronics
Students:
Robert Chen and Colleen Shang
Description: The focus of this project is to develop the Group-IV family of materials (Si/Ge/Sn) for optoelectronics/photonics applications. The main target is to achieve a direct or near direct bandgap semiconductor which can be used to build CMOS compatible laser. This is done by engineering the bandstructure through the use of strain and compositions of the alloy. For example, bulk Ge is well-known to be indirect bandgap by 136meV, but the addition of only ~7% Sn can make the material direct bandgap. This has favorable properties for many optoelectronic applications.

We are currently investigating both relaxed and strain GeSn-based materials for device development. Relaxed (Si)GeSn films have been grown using low-temperature molecular beam epitaxy (MBE) on lattice-matched InGaAs buffers on GaAs (001). This platform is extremely advantageous in studying the bulk-like properties of relaxed GeSn films, which are difficult to study in a lattice-mismatched system due to strain's powerful effect on the electronic bandstructure. Our work on GeSn films have resulted in the demonstration of photoluminescence from direct-bandgap GeSn films with up to 8.6% Sn and the demonstration of improved quantum efficiency for light emission with increasing Sn content. Such a property has great potential for the development of high-efficiency photonic devices.

Pseudomorphic GeSn/Ge structures are extremely interesting for development of light-emitting devices due to predictions of improved quantum efficiency, carrier confinement, and large net TE gain. Additionally, such structures can be developed on a Silicon substrate for CMOS integration. We are investigating the optical and structural properties of these material stacks. Using a unique etch-stop feature of GeSn, we are also able to develop high material quality GeSn/Ge QW microdisk resonators for laser applications. A process and technology developed here at Stanford has enabled the creative design of suspended structures with high-quality active regions. Microdisk resonators with GeSn QWs show strong luminescence and whispering-gallery-mode resonances in photoluminescence experiments. Such structures are very interesting for developing on-chip lasers at 2 μm.

We use various characterization techniques to assess the quality of our films and to understand structural, electrical, and optical changes as a function of Sn composition and strain. These include XPS, SIMS, AFM, XRD, SEM, Raman, Photoreflectance, Photoluminescence, and Electroluminescence.

 

 

Ge/SiGe Quantum-Well Devices on Silicon for Optical Interconnects
Students:
Ed Fei, Xiaochi Chen, Kai Zang, Ching-Ying Lu, Yijie Huo (research staff)
Description: There is currently an increasing demand to integrate optoelectronics with the dominant silicon-based semiconductors for telecommunications and computer interconnections. Ge has attracted more and more attentions in recent years due to its pseudo-direct band gap behavior and its compatibility with Si processing technology. Our research focuses on fabrication and characterization of high germanium content Ge/SiGe quantum well (QW) devices, such as modulators, LEDs, lasers and photodetectors, on Si substrate. The device fabrication involves epitaxial chemical vapor deposition and standard lithography and is conducted in the Stanford Nanofabrication Facility (SNF). Our goal is to integrate all these electro-optical interconnections on Si substrate and realize inter- and intra-chips optical data communication.

Quantum-confined Stark effect (QCSE) in Ge/SiGe quantum wells makes electro-absorption modulation possible for integrated on-chip optical interconnects. Prior work established a proof of concept vertical p-i-n QCSE modulator as well as a vertical PIN resonant cavity device. Future work will continue to explore metamorphic growth of high Ge content SiGe on Si substrate using CVD to further research QW properties, including QW material quality and wavelength tuning. We have also designed and fabricated horizontal waveguide modulator devices illustrated in the figure 1 for on-chip optical interconnects.

Schematic diagram of a horizontal waveguide modulator device.

Ge/SiGe QW is a potential structure to achieve a low pump power Ge laser for integrated on-chip optical interconnects. Prior work has shown enhanced photoluminescence (PL) signal in Ge/SiGe QWs over bulk Ge (below, left) as well as strong optical resonance in Ge/SiGe QW microdisk structure (below, right). Future work will focus on material growth, strain engineering and device fabrication for Ge/SiGe QW microdisk structure lasers.

 

   

PL signals of bulk Ge and Ge/SiGe QW samples and of a Ge/SiGe QW microdisk structure.

 

Long Wavelength GaInNAsSb Vertical Cavity Lasers
Students:
Tomas Sarmiento (post-doc with Prof. Vuckovic)
Description: Our work has focused on developing GaInNAsSb on GaAs where we have realized the lowest threshold current 1.55μm edge-emitting lasers and the first monolithic 1.55μm VCSELs. GaInNAsSb is a metastable material with many challenges to realize the longer wavelengths, but the tremendous advantages of producing long wavelength devices on GaAs where excellent DBR mirror technology exists and the potential to integrate photonic crystal waveguides and resonators will enable integration of more functional photonic integrated circuits, arrays of much lower cost, 2-D lasers and modulators which can be easily coupled into fiber or utilized in free space architectures and offer great architectural diversity. Edge emitting lasers from these alloys also offer much greater opportunity to realize very high power semiconductor laser pumps for Raman amplifiers and semiconductor optical amplifiers to open up the entire 1.3-1.6μm low loss fiber region as well as provide resonant pumps for very high power, high efficiency solid-state lasers. We achieved very low threshold current density of 373 A/cm2 for 1.55μm edge-emitting lasers and the first GaAs-based monolithic VCSEL at 1.53μm.

 

Development of Crystalline Optical Coatings with Ultra-low Thermal Noise
Students:
Angie Lin (post-doc with Prof. Fejer)
Description:
The aim of this project is to develop crystalline optical coatings from III-V semiconductors such as GaP and AlGaP that have very low thermal noise, low absorption, and high reflectivity for applications in precision interferometry such as gravitational wave (GW) detection and optical atomic clocks. These instruments rely on having extremely stable Fabry-Perot cavities, however, one of the dominant noise sources arises from the Brownian thermal noise in the optical coatings of the cavity's mirrors. Thus, reducing the phase noise that arises from random motion of atoms in the mirror will improve the instrument's sensitivity. We have fabricated test mirrors and demonstrated an order of magnitude reduction in mechanical loss (a measurement directly related to thermal noise) compared to state-of-the-art optical coatings. Further understanding of the optical and mechanical loss mechanisms in the GaP and AlGaP layers will likely lead to continued and systematic improvements in these new crystalline optical coatings.

A consequence of improving the sensitivity for the Laser Interferometer Gravitational Wave Observatory (LIGO) is that it will allow us to greatly expand the volume of the universe from which we can detect gravitational waves and enable an entirely new field of gravitational wave astronomy with the worldwide network of detectors. At this exciting time when several GW detectors around the world are in operation or being upgraded, humankind is on the brink of directly measuring what Einstein predicted almost 100 years ago in his theory of general relativity. More info at: http://ligo.org

 

Monolithic, Integrated Mode-Locked Lasers
Students:
Ken Leedle
Description: Integrated high power semiconductor mode-locked lasers can be used for a variety of applications, including compact laser-driven accelerators, two-photon microscopy, and in-vivo neural imaging in mice. This includes work on the simulation, design and fabrication of large area Photonic Crystal (PC) lasers. The devices are simulated using commercial laser simulators PICWave and CrystalWave by Photon Design Inc. Laser wafers are grown using MBE and MOCVD. Device fabrication involves standard lithography and also extensive Ebeam Lithography and is conducted in the Stanford Nanofabrication Facility (SNF). Laser testing is done is our Harris Optics labs and at the SLAC National Accelerator Lab. The project combines traditional semiconductor laser design with nanophotonics.


Sub-picosecond IR laser pulses can be used to scale accelerators 4-5 orders of magnitude over conventional microwave wavelengths and allow damage thresholds in the GV/m regime for dielectric materials. Slow light photonic crystal waveguides will be used to increase the number of cavity modes that mode-lock while keeping the device itself small, thereby increasing the pulse energy into the mJ range and decreasing the pulse length to the order of 100fs.



Integrated Index-of-Refraction Bio Sensors
Students:
Sage Doshay and Fariah Mahzabeen
Description: This project focuses on the development of miniaturized optical sensors designed for integrated "lab-on-a-chip" biomedical and bio-defense applications. Previous work demonstrated a fluorescence sensor with a monolithically integrated VCSEL, detector, and filter. Current work focuses on the design, simulation, fabrication, and characterization of an index-of-refraction sensor using guided resonances in 2D photonic crystal slabs. Such resonances offer design scalability, light coupling simplicity, and high sensitivity in a low-loss all-dielectric structure. Fabricated sensors demonstrate the potential for label-free monitoring of biochemical events such as virus binding. Ultimately, the fluorescence and index-of-refraction sensors can be combined on one platform with other detectors to provide portable, rapid, and correlated bio-analysis.

Silicon Nitride photonic crystal slabs were fabricated on quartz substrates using optical holography. The design targets operation in the near-infrared transparency window, for low absorption of water and hemoglobin. Initial measurements agree with 3D Finite Difference Time Domain simulations, and demonstrate the detection of index changes on the order of 10^-3 for bulk aqueous solutions. Present work includes the integration of microfluidic controls and the design/simulation of new sensor architectures for increased sensitivity. We ultimately aim to combine multiple sensing mechanisms on one platform to provide rapid, correlated bio-analysis. This is a collaboration with the Center for High Technology Materials (CHTM) at the University of New Mexico.

In addition, integrated photonic systems are under research for in-vivo biosensors analyzing whole blood coagulation processes. The integrated system consists of fiber-based input terminals and photonic crystal platform by which the allocation and transmission of optical signals are processes. The fabricated biosensor performs the real-time analysis on whole blood coagulation as a function of the amount of thromboplastin. It is the fundamental principle that the time-varying amount of the coagulation catalyst results in time-dependent refractive index of the whole blood which is monitored by the input signals from fibers. This enables controlling the coagulation processes by real-time medication eventually. The research is accomplished by fusing photonics, nanofabrication, and electrical engineering in the collaboration with the medical school as well.



Simulation and Fabrication of Nanostructured Solar Cells
Students:
Yangsen Kang and Yusi Chen
Description: III-V nanostructured thin film solar cell, using nanotechnology to achieve low cost, high efficiency photovoltaic deviceas. Current focus is on nanostructured template fabrication, III-V material growth by MOCVD and modeling and simulation of nanostructured solar thin film cells using TCAD-Sentaurus and Lumerical FDTD simulators.



Electronic Retinal Prosthesis for Sight Restoration using Si-based Photodetectors
Students:
Xin Lei
Description: We have designed and fabricated a silicon photodiode array for use as a subretinal prosthesis aimed at restoring sight to patients who lost their photoreceptors due to retinal degeneration. The device operates in photovoltaic mode. Each pixel in the two-dimensional array independently converts pulsed infrared light into electrical current in order to stimulate remaining retinal neurons without a wired power connection. To enhance the maximum voltage and charge injection levels, each pixel contains three photodiodes connected in series. An active and return electrode in each pixel ensure localized current flow and are sputter coated with iridium oxide to provide high charge injection. The fabrication process consists of eight mask layers, and includes deep reactive ion etching, oxidation, and a polysilicon trench refill for in-pixel photodiode separation and isolation of adjacent pixels. Three sizes of pixels (280-m, 140-m, and 70-m) with activeelectrode diameters of 80-m, 40-m and 20-m have been fabricated. The device layer thickness is 30-m to allow subretinal implantation.


Low Relative-Intensity Noise Lasers
Students:
Seonghyun Paik
Description: Low noise CW laser source with 1.55μm wavelength is required for coherent communication. Broad area 2D DFB photonic crystal lasers have been simulated to reduce relative intensity noise (RIN) and increase side mode suppression ratio (SMSR). Active material for laser structure is novel GaInNAsSb for 1.55 μm emission and grown using MBE in Harris Group, and laser simulation has been performed using commercial laser simulator PICWAVE and MATLAB. I am currently focusing on fabrication of high quality photonic crystal laser cavity in Stanford Nanofabrication Facility (SNF).

 

High-Performance Avalanche Photodiodes
Students:
Kai Zang and Yijie Huo (research staff)
Description: This project involves design, fabrication and characterization of novel Silicon based avalanche photo-detector. Silicon has been demonstrated as an ideal material for avalanche photo-detector due to its low ionization ratio. However, the spectrum response of Si APD has been constrained to near infrared region. Patterning nano-structure on surface would be an interesting and effective way to manipulate light absorption. Our goal is to enhance optical absorption in near infrared region while at the same time keeping APD electrical performance high.



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