Tzu-Ming Lu

Very low-refractive-index optical thin films consisting of an array of SiO 2 nanorods

The refractive-index contrast in dielectric multilayer structures, optical resonators, and photonic crystals is an important figure of merit that creates a strong demand for high-quality thin films with a low refractive index. A SiO2 nanorod layer with low refractive index of n = 1.08, to our knowledge the lowest ever reported in thin-film materials, is grown by oblique-angle electron-beam deposition of SiO2. A single-pair distributed Bragg reflector employing a SiO2 nanorod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-refractive-index films for applications in photonic structures and devices.

Pressure dependent Parylene-N pore sealant penetration in porous low-κ dielectrics

The introduction of porosity in dielectrics is desirable to reduce the dielectric constant; but it causes integration problems such as CVD∕ALD precursor penetration for barrier layer∕seed layer deposition. CVD Parylene-N has been shown to work as a pore sealant for porous low-κ materials but penetrates itself slightly into porous dielectric. The depth profile of Parylene-N in porous MSQ can be obtained using the Nuclear Reaction Analysis (NRA) of C12. The penetration of Parylene-N can be controlled by deposition at higher pressure where the deposition rate is also high. High deposition rate can also be attained by adding a carrier gas which also shows low Parylene-N penetration. The experimentally measured dielectric constants, after pore sealing, are compared to those calculated using the NRA data of Parylene-N penetration.

Enhancement-mode buried strained silicon channel quantum dot with tunable lateral geometry

We propose and demonstrate a relaxed-SiGe/strained-Si enhancement-mode gate stack for quantum dots. A mobility of 1.6 × 105 cm2/Vs at 5.8 × 1011/cm2 is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade measured in a double-top-gated lateral quantum dot nanostructure terminates with open diamonds up to ±10 mV of dc voltage across the device. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics. A modified implant, polycrystalline silicon formation and annealing conditions were utilized to minimize the thermal budget that potentially leads to Ge/Si interdiffusion.

Electron spin lifetime of a single antimony donor in silicon

We present measurements of the electron spin lifetime (T1) of a single Sb donor in Si. For a magnetic field (B) oriented along the [100] Si crystal direction and low temperature (T) such that kT≪gμB, we find T1−1=KB5, where K=1.7×10−3 Hz T−5. The T1−1∝B5 dependence is expected for donor electron spin relaxation due to g-factor dependence on crystal strain. The magnitude of T1 is within a factor of two of theoretical estimates and is in close agreement with values obtained for bulk donor ensembles.

Charge sensed Pauli blockade in a metal-oxide-semiconductor lateral double quantum dot.

We report Pauli blockade in a multielectron silicon metal-oxide-semiconductor double quantum dot with an integrated charge sensor. The current is rectified up to a blockade energy of 0.18 ± 0.03 meV. The blockade energy is analogous to singlet-triplet splitting in a two electron double quantum dot. Built-in imbalances of tunnel rates in the MOS DQD obfuscate some edges of the bias triangles. A method to extract the bias triangles is described, and a numeric rate-equation simulation is used to understand the effect of tunneling imbalances and finite temperature on charge stability (honeycomb) diagram, in particular the identification of missing and shifting edges. A bound on relaxation time of the triplet-like state is also obtained from this measurement.

Fabrication of quantum dots in undoped Si/Si0.8Ge0.2 heterostructures using a single metal-gate layer

Enhancement-mode Si/SiGe electron quantum dots have been pursued extensively by many groups for their potential in quantum computing. Most of the reported dot designs utilize multiple metal-gate layers and use Si/SiGe heterostructures with Ge concentration close to 30%. Here, we report the fabrication and low-temperature characterization of quantum dots in the Si/Si0.8Ge0.2 heterostructures using only one metal-gate layer. We find that the threshold voltage of a channel narrower than 1 μm increases as the width decreases. The higher threshold can be attributed to the combination of quantum confinement and disorder. We also find that the lower Ge ratio used here leads to a narrower operational gate bias range. The higher threshold combined with the limited gate bias range constrains the device design of lithographic quantum dots. We incorporate such considerations in our device design and demonstrate a quantum dot that can be tuned from a single dot to a double dot. The device uses only a single metal-gate...

All-optical lithography process for contacting nanometer precision donor devices

We describe an all-optical lithography process that can make electrical contact to nanometer-precision donor devices fabricated in silicon using scanning tunneling microscopy (STM). This is accomplished by implementing a cleaning procedure in the STM that allows the integration of metal alignment marks and ion-implanted contacts at the wafer level. Low-temperature transport measurements of a patterned device establish the viability of the process.

Process Development toward Enhancement-Mode Strained-Si/SiGe Double Quantum Dot

In this talk, we present our recent efforts in process development toward enhancement-mode strained Si/SiGe double quantum dots, utilizing either a 150mm Si foundry at wafer-level or a modern cleanroom setting at die-level. We focus on the following aspects: choice of gate insulator, device stability, and thermal budget.

Electrically Injected UV-Visible Nanowire Lasers

There is strong interest in minimizing the volume of lasers to enable ultracompact, low-power, coherent light sources. Nanowires represent an ideal candidate for such nanolasers as stand-alone optical cavities and gain media, and optically pumped nanowire lasing has been demonstrated in several semiconductor systems. Electrically injected nanowire lasers are needed to realize actual working devices but have been elusive due to limitations of current methods to address the requirement for nanowire device heterostructures with high material quality, controlled doping and geometry, low optical loss, and efficient carrier injection. In this project we proposed to demonstrate electrically injected single nanowire lasers emitting in the important UV to visible wavelengths. Our approach to simultaneously address these challenges is based on high quality III-nitride nanowire device heterostructures with precisely controlled geometries and strong gain and mode confinement to minimize lasing thresholds, enabled by a unique top-down nanowire fabrication technique.

Optical Thin Films with Extremely Low Refractive Index

An optical thin film consisting of SiO2 nano-rods is demonstrated to have an extremely low refractive index of n = 1.08. This is the lowest refractive index ever reported for viable optical thin film.