Nanoscale process integration demands novel nanopatterning techniques in compliance with the requirements of next
generation devices. Conventionally, top-down subtractive (etch) or additive (deposition/lift-off) processes in conjunction
with various lithography techniques is employed to achieve film patterning, which become increasingly challenging due
to the ever-shrinking alignment requirements. To reduce the complexity burden of lithographic alignment in critical
fabrication steps, self-aligned processes such as selective deposition and selective etching might provide attractive
solutions. Selective atomic layer deposition (SALD) has attracted immense attention in recent years for self-aligned
accurate pattern placement with sub-nanometer thickness control. During the atomic layer deposition (ALD) process,
film nucleation is critically dependent on the surface chemistry of the substrate which makes it possible to achieve
selective-ALD (SALD) by chemically modifying the substrate surface. Local modification of substrate surface opens up
possibilities to achieve lateral control over film growth in addition to robust thickness control during ALD process.
SALD offers numerous advantages in nanoscale device fabrication such as reduction of the lithography steps required,
elimination of complicated etching processes, and minimization of expensive reagent use. In this work, we review our
recent SALD efforts using various inhibition layers resulting in promising self-aligned deposition solutions for metaloxide,
metal, and III-nitride thin films. We report a comprehensive investigation to select the most compatible inhibition
layer among poly(methylmethacrylate) (PMMA), polyvinylpyrrolidone (PVP), and ICP-polymerized fluorocarbon layers
for SALD of metal-oxide and metallic thin films. In addition, single-layer and multi-layered graphene layers are
explored as plasma-compatible inhibition layers for selective deposition of III-nitride materials. Extensive materials
characterization efforts are carried out to correlate the ALD recipe parameters with the selective deposition performance.
The materials and deposition recipes developed in this work overcome various challenges associated with previous
methods of SALD and provide alternative routes towards nano-patterning particularly for the sub-10 nm CMOS
technology nodes as well as for sensors, photovoltaics, materials for energy storage, catalysis, etc.
Recent experimental research efforts on developing functional nanostructured III-nitride and metal-oxide materials via low-temperature atomic layer deposition (ALD) will be reviewed. Ultimate conformality, a unique propoerty of ALD process, is utilized to fabricate core-shell and hollow tubular nanostructures on various nano-templates including electrospun nanofibrous polymers, self-assembled peptide nanofibers, metallic nanowires, and multi-wall carbon nanotubes (MWCNTs). III-nitride and metal-oxide coatings were deposited on these nano-templates via thermal and plasma-enhanced ALD processes with thickness values ranging from a few mono-layers to 40 nm.
Metal-oxide materials studied include ZnO, TiO2, HfO2, ZrO2, and Al2O3. Standard ALD growth recipes were modified so that precursor molecules have enough time to diffuse and penetrate within the layers/pores of the nano-template material. As a result, uniform and conformal coatings on high-surface area nano-templates were demonstrated. Substrate temperatures were kept below 200C and within the self-limiting ALD window, so that temperature-sensitive template materials preserved their integrity III-nitride coatings were applied to similar nano-templates via plasma-enhanced ALD (PEALD) technique. AlN, GaN, and InN thin-film coating recipes were optimized to achieve self-limiting growth with deposition temperatures as low as 100C. BN growth took place only for >350C, in which precursor decomposition occured and therefore growth proceeded in CVD regime. III-nitride core-shell and hollow tubular single and multi-layered nanostructures were fabricated.
The resulting metal-oxide and III-nitride core-shell and hollow nano-tubular structures were used for photocatalysis, dye sensitized solar cell (DSSC), energy storage and chemical sensing applications. Significantly enhanced catalysis, solar efficiency, charge capacity and sensitivity performance are reported. Moreover, core-shell metal-oxide and III-nitride materials showed promise to be used in applications where flexibility is critical like functional membranes, textile and flexible electronic applications.
The potential use of optical forces in microfluidic environment enables highly selective bio-particle manipulation. Manipulation could be accomplished via trapping or pushing a particle due to optical field. Empirical determination of optical force is often needed to ensure efficient operation of manipulation. The external force applied to a trapped particle in a microfluidic channel is a combination of optical and drag forces. The optical force can be found by measuring the particle velocity for a certain laser power level and a multiplicative correction factor is applied for the proximity of the particle to the channel surface. This method is not accurate especially for small microfluidic geometries where the particle size is in Mie regime and is comparable to channel cross section. In this work, we propose to use Boundary Element Method (BEM) to simulate fluid flow within the micro-channel with the presence of the particle to predict drag force. Pushing experiments were performed in a dual-beam optical trap and particle’s position information was extracted. The drag force acting on the particle was then obtained using BEM and other analytical expressions, and was compared to the calculated optical force. BEM was able to predict the behavior of the optical force due to the inclusion of all the channel walls.
Proof-of-concept, first metal-semiconductor-metal ultraviolet photodetectors based on nanocrystalline gallium nitride (GaN) layers grown by low-temperature hollow-cathode plasma-assisted atomic layer deposition are demonstrated. Electrical and optical characteristics of the fabricated devices are investigated. Dark current values as low as 14 pA at a 30 V reverse bias are obtained. Fabricated devices exhibit a 15× UV/VIS rejection ratio based on photoresponsivity values at 200 nm (UV) and 390 nm (VIS) wavelengths. These devices can offer a promising alternative for flexible optoelectronics and the complementary metal oxide semiconductor integration of such devices.
Spin-orbit coupling is investigated by magnetoconductivity measurements in wurtzite AlxGa1-xN/AlN/GaN
heterostructures with a polarization induced two-dimensional electron gas with different Al concentrations ranging from
x = 0.1 to 0.35. By employing the persistent photoconductivity effect and by gating we are able to vary the carrier
density of the samples in a controllable manner from 0.8
×1012 cm-2 to 10.6 ×1012 cm-2. The samples are characterized
using magnetoresistance measurements. To characterize the spin-orbit interaction we measured quantum corrections to
conductance at low magnetic fields. All the samples we studied exhibit a weak antilocalization feature at liquid He
temperatures. The zero-field electron spin-splitting energies extracted from the weak antilocalization measurements are
found to scale with the Fermi wavevector kF as 2( ακF + γκF3) with effective linear and cubic spin-orbit parameters of
-α= 5.01×10−13 eV • m and γ= 1.6 ×10−31 eV •m3, respectively. The linear spin-orbit coupling arises from both the bulk
inversion asymmetry of the crystal and the structural inversion asymmetry of the heterostructure whereas the cubic spinorbit
coupling parameter is purely due to the bulk inversion asymmetry of the wurtzite crystal. We also extracted phase
coherence times from the amplitude of the weak antilocalization feature. The measured phase coherence times ranged
from 3-40 ps and were in agreement with the theory of decoherence based on electron-electron interactions.
We study AlxGa1-xN/AlN/GaN heterostructures with a two-dimensional-electron-gas (2DEG) grown on different GaN
templates using low-temperature magneto-transport measurements. Heterostructures with different Al compositions are
grown by metal-organic vapor phase epitaxy (MOVPE) on three different templates; conventional undoped GaN (u-
GaN), epitaxial lateral overgrown GaN (ELO-GaN), and in situ ELO-GaN using a SixNy nanomask layer (SiN-GaN).
Field-dependent magneto-resistance and Hall measurements indicated that in addition to 2DEG, the overgrown
heterostructures had a parallel conducting layer. The contact resistance for the parallel channels was large so that it
introduced errors in the quantitative mobility spectrum analysis (QMSA) of the data. Notwithstanding complexities
introduced by parallel conducting channels in mobility analysis in SiN-GaN and ELO-GaN samples, we were able to
observe Shubnikov-de Haas (SdH) oscillations in all samples, which confirmed the existence of 2DEGs. To characterize
the parallel channel, we repeated the transport measurements after the removal of the 2DEG by etching the
heterostructure. The 2DEG carrier density values were extracted from the SdH data, whereas the zero-field 2DEG
conductivity was determined by subtracting the parallel channel conductivity from the total conductivity. The resulting
2DEG mobility was significantly higher (about a factor of 2) in the ELO-GaN and SiN-GaN samples as compared to the
standard control sample. The mobility enhancement is attributed to the threading dislocation reduction by both ELO
techniques.
Preliminary results on nanoheteroepitaxy of GaN on silicon face (Si-face) and carbon face (C-face) nano-columnar
SiC (CSC) by metalorganic chemical vapor deposition (MOCVD) are reported. The CSC
substrates are fabricated from standard SiC wafers by photo-enhanced electrochemical etching, with typical
diameter of pores around 20nm. Noticeable reduction of threading dislocations (TDs) in GaN is realized on
the CSC substrates. On the C-face CSC, GaN nuclei have an inverted pyramidal shape which contains high
density of stacking faults (SFs). These SFs block possible extension of TDs into upper portion of the layer.
On the Si-face CSC, TDs are annihilated by forming nanoscale TD half-loops over the surface pores. These
nanoscale TD loops confine the defective layer in GaN to within ~50 nm thickness from the GaN/CSC
interface. High density (~5x108 cm-2) of remnant TDs still presents in GaN grown on CSC, chiefly because
the surface damages on CSC were not properly removed before growth.
Design, fabrication, and characterization of high-performance AlxGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA leakage currents at 6 V and 12 V reverse bias were measured on p-i-n and Schottky photodiode samples respectively. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9x1014 cmHz1/2/W-1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.15 A/W and 0.11 A/W peak responsivity values at 267 nm and 261 nm respectively. All samples displayed true solar-blind response with cut-off wavelengths smaller than 280 nm. A visible rejection of 4x104 was achieved with Schottky detector samples. High speed measurements at 267 nm resulted in fast pulse responses with >GHz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.
In this work, AlGaN layers were grown on sapphire by metal-organic vapor phase epitaxy (MOVPE) on (0001)-oriented sapphire substrates, with the intention to investigate the effect of varying Al/MO and V/III ratios on the Al incorporation into the AlGaN layers. The parameters Al/MO and V/III describe the proportions of source material inside the reactor. With the help of optical transmission measurements, characteristic cut-off wavelengths of the AlGaN layers were determined. These wavelengths were used to calculate the Al content x of the layers, leading to values between 26.6% and 52.1%. Using the two process parameters Al/MO and V/III as input and the Al content of the AlGaN layers as a response variable, the experimental results were further investigated with the help of the software STATGRAPHICS. An estimated response surface for the variable x was generated. It was found that the Al incorporation is only tunable within a wide range for high V/III ratios of about 900. For constant Al/MO ratios and varying V/III ratios, two different growth characteristics were observed at high and low Al/MO values. This behavior is ascribed to the superposition of two oppositional effects.
In this talk, we will review our research efforts on resonant cavity enhanced (RCE) high-speed high-efficiency photodiodes (PDs) operating in the 1st and 3rd optical communication windows. Using a microwave compatible planar fabrication process, we have designed and fabricated GaAs and InGaAs based RCE photodiodes. For RCE GaAs Schottky type photodiodes, we have achieved peak quantum efficiencies of 50% and 75% with semi-transparent (Au) and transparent (indium-tin-oxide) Schottky layers respectively. Along with 3-dB bandwidths of 50 and 60 GHz, these devices exhibit bandwidth-efficiency (BWE) products of 25 GHz and 45 GHz respectively. By using a postprocess recess etch, we tuned the resonance wavelength of an RCE InGaAs PD from 1605 to 1558 nm while keeping the peak efficiencies above 60%. The maximum quantum efficiency was 66% at 1572 nm which was in good agreement with our theoretical calculations. The photodiode had a linear response up to 6 mW optical power, where we obtained 5 mA photocurrent at 3 V reverse bias. The photodetector had a temporal response of 16 psec at 7 V bias. After system response deconvolution, the 3-dB bandwidth of the device was 31 GHz, which corresponds to a bandwidth-efficiency product of 20 GHz.
In this paper we present our efforts on the design, fabrication and characterization of high-speed, visible-blind, GaN-based ultra-violet (UV) photodiodes using indium-tin-oxide (ITO) Schottky contacts. ITO is known as a transparent conducting material for the visible and near infrared part of the electromagnetic spectrum. We have investigated the optical properties of thin ITO films in the ultraviolet spectrum. The transmission and reflection measurements showed that thin ITO films had better transparencies than thin Au films for wavelengths greater than 280 nm. Using a microwave compatible fabrication process, we have fabricated Au and ITO based Schottky photodiodes on n-/n+ GaN epitaxial layers. We have made current-voltage (I-V), spectral quantum efficiency, and high-speed characterization of the fabricated devices. I-V characterization showed us that the Au-Schottky samples had better electrical characteristics mainly due to the larger Schottky barrier. However, due to the better optical transparency, ITO-Schottky devices exhibited higher quantum efficiencies compared to Au-Schottky devices. ITO-Schottky photodiodes with ~80 nm thick ITO films resulted in a maximum quantum efficiency of 47%, whereas Au-Schottky photodiode samples with ~10 nm thick Au films displayed a maximum efficiency of 27% in the visible-blind spectrum. UV/visible rejection ratios over three orders of magnitude were obtained for both samples. High-frequency characterization of the devices was performed via pulse-response measurements at 360 nm. ITO-Schottky photodiodes showed excellent high-speed characteristics with rise times as small as 12 psec and RC-time constant limited pulse-widths of 60 psec.
High-speed photodetectors operating at 1.3 and 1.55 micrometers are important for long distance fiber optic based telecommunication applications. We fabricated GaAs based photodetectors operating at 1.3 micrometers that depend on internal photoemission as the absorption mechanism. Detectors using internal photoemission have usually very low quantum efficiency. We increased the quantum efficiency using resonant cavity enhancement effect. Resonant cavity enhancement effect also introduced wavelength selectivity which is very important for wavelength division multiplexing based communication systems. The top-illuminated Schottky photodiodes were fabricated by a microwave-compatible monolithic microfabrication process. The top metal layer serves as the top mirror of the Fabry-Perot cavity. Bottom mirror is composed of 15 pair AlAs/GaAs distributed Bragg reflector. We have used transfer matrix method to simulate the optical properties of the photodiodes. Our room temperature quantum efficiency measurement and simulation of our photodiodes at zero bias show that, we have achieved 9 fold enhancement in the quantum efficiency, with respect to a similar photodetector without a cavity. We also investigated the effect of reverse bias on quantum efficiency. Our devices are RC time constant limited with a predicted 3-dB bandwidth of 70 GHz.
We designed, fabricated and characterized AlxGa1- xAs/GaAs p-i-n resonant cavity enhanced (RCE) photodetectors with near-unity quantum efficiency. The peak wavelength is in the 780 - 830 nm region and post-process adjustable by recessing the top surface. Transit time limited bandwidth for these devices is in excess of 50 GHz. Possible applications of these detectors include conventional measurements of low light levels, quantum optical experiments that use pulsed sources and short-haul high speed communications.
In this paper, we review our research efforts on RCE high- speed high-efficiency p-i-n and Schottky photodiodes. Using a microwave compatible planar fabrication process, we have designed and fabricated GaAs based RCE photodiodes. For RCE Schottky photodiodes, we have achieved a peak quantum efficiency of 50% along with a 3-dB bandwidth of 100 GHz. The tunability of the detectors via a recess etch is also demonstrated. For p-i-n type photodiodes, we have fabricated and tested widely tunable devices with near 100% quantum efficiencies, along with a 3-dB bandwidth of 50 GHz. Both of these results correspond to the fastest RCE photodetectors published in scientific literature.
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