P. Daniel Dapkus

Electrical and Optical Characterization of Surface Passivation in GaAs Nanowires

We report a systematic study of carrier dynamics in Al(x)Ga(1-x)As-passivated GaAs nanowires. With passivation, the minority carrier diffusion length (L(diff)) increases from 30 to 180 nm, as measured by electron beam induced current (EBIC) mapping, and the photoluminescence (PL) lifetime increases from sub-60 ps to 1.3 ns. A 48-fold enhancement in the continuous-wave PL intensity is observed on the same individual nanowire with and without the Al(x)Ga(1-x)As passivation layer, indicating a significant reduction in surface recombination. These results indicate that, in passivated nanowires, the minority carrier lifetime is not limited by twin stacking faults. From the PL lifetime and minority carrier diffusion length, we estimate the surface recombination velocity (SRV) to range from 1.7 × 10(3) to 1.1 × 10(4) cm·s(-1), and the minority carrier mobility μ is estimated to lie in the range from 10.3 to 67.5 cm(2) V(-1) s(-1) for the passivated nanowires.

InGaN/GaN Multiple Quantum Wells Grown on Nonpolar Facets of Vertical GaN Nanorod Arrays

Uniform GaN nanorod arrays are grown vertically by selective area growth on (left angle bracket 0001 right angle bracket) substrates. The GaN nanorods present six nonpolar {1⁻100} facets, which serve as growth surfaces for InGaN-based light-emitting diode quantum well active regions. Compared to growth on the polar {0001} plane, the piezoelectric fields in the multiple quantum wells (MQWs) can be eliminated when they are grown on nonpolar planes. The capability of growing ordered GaN nanorod arrays with different rod densities is demonstrated. Light emission from InGaN/GaN MQWs grown on the nonpolar facets is investigated by photoluminescence. Local emission from MQWs grown on different regions of GaN nanorods is studied by cathodoluminescence (CL). The core-shell structure of MQWs grown on GaN nanorods is investigated by cross-sectional transmission electron microscopy in both axial and radial directions. The results show that the active MQWs are predominantly grown on nonpolar planes of GaN nanorods, consistent with the observations from CL. The results suggest that GaN nanorod arrays are suitable growth templates for efficient light-emitting diodes.

GaAs Nanowire Array Solar Cells with Axial p–i–n Junctions

Because of unique structural, optical, and electrical properties, solar cells based on semiconductor nanowires are a rapidly evolving scientific enterprise. Various approaches employing III-V nanowires have emerged, among which GaAs, especially, is under intense research and development. Most reported GaAs nanowire solar cells form p-n junctions in the radial direction; however, nanowires using axial junction may enable the attainment of high open circuit voltage (Voc) and integration into multijunction solar cells. Here, we report GaAs nanowire solar cells with axial p-i-n junctions that achieve 7.58% efficiency. Simulations show that axial junctions are more tolerant to doping variation than radial junctions and lead to higher Voc under certain conditions. We further study the effect of wire diameter and junction depth using electrical characterization and cathodoluminescence. The results show that large diameter and shallow junctions are essential for a high extraction efficiency. Our approach opens up great opportunity for future low-cost, high-efficiency photovoltaics.

Polarization insensitive strained quantum well gain medium for lasers and optical amplifiers

1.3 μm In1−xGaxAsyP1−y/InP lasers with compressive and tensile strained quantum wells have been found to lase in both transverse electric (TE) and transverse magnetic (TM) polarizations. Amplified spontaneous emission spectra indicate gain in both TE and TM modes. The relative magnitudes depend on device length and injection current. This structure can be used as a polarization insensitive optical amplifier.

Optical, electrical, and solar energy-conversion properties of gallium arsenide nanowire-array photoanodes

Periodic arrays of n-GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition on Si and GaAs substrates. The optical absorption characteristics of the nanowire-arrays were investigated experimentally and theoretically, and the photoelectrochemical energy-conversion properties of GaAs nanowire arrays were evaluated in contact with one-electron, reversible, redox species in non-aqueous solvents. The radial semiconductor/liquid junction in the nanowires produced near-unity external carrier-collection efficiencies for nanowire-array photoanodes in contact with nonaqueous electrolytes. These anodes exhibited overall inherent photoelectrode energy-conversion efficiencies of � 8.1% under 100 mW cm � 2 simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590 � 15 mV and short-circuit current densities of 24.6 � 2.0 mA cm � 2 . The high optical absorption, and minimal reflection, at both normal and off-normal incidence of the GaAs nanowire arrays that occupy <5% of the fractional area of the electrode can be attributed to efficient incoupling into radial nanowire guided and leaky waveguide modes. Broader context Due to the voltage requirements to produce fuels from sunlight, water, and CO2 as the inputs, two light-absorbing materials, with band gaps of 1.7 eV and 1.1 eV, respectively, are attractive as the foundation for high-efficiency articial photosynthesis. The integration of materials with 1.7 and 1.1 eV band gaps is, however, very challenging. Accordingly, a nanowire-growth strategy has been developed to integrate single crystal III–V nanowires (e.g. GaAs) with highly mismatched Si substrates. In this work, GaAs nanowire arrays grown on Si were studied using a non-destructive contact method involving non-aqueous photoelectrochemistry. The approach has allowed us to understand the interplay of nanowire growth with the optical absorption and electrical properties of such systems, and will aid in the design and optimization of nanowire-based systems for solar energy-conversion applications. Photoelectrolysis of water for the production of renewable H2 from sunlight faces a constraint in that a potential difference of 1.23 V is required thermodynamically to sustain the watersplitting reaction under standard conditions. In an integrated photoelectrochemical system for water splitting, the operating voltage produced by the light absorber should exceed the sum of

Epitaxial growth and optical characterization of InAs/InGaAsP/InP self-assembled quantum dots

Five stacks of InAs quantum dots (QDs) with InGaAsP barriers were grown on (100) InP and luminescence characteristics were analyzed. Cross-sectional transmission electron microscopy shows that small dots with a lateral size of ∼30 nm and a height of ∼3 nm are formed with an areal density of ∼5×1010 cm−2. The QDs emit strong photoluminescence (PL) peaks in the range of 1.4–1.6 μm that can be controlled by nominal InAs thickness. The integrated PL intensity from QDs stays very high at room temperature as much as 20% of that at 10 K. At weak excitation, the carrier lifetimes are measured to be almost the same across the whole PL band at low temperature with a value of ∼4 ns and they remain at that value at room temperature. These characteristics strongly evidence that individual QDs are well isolated and have a strong carrier confinement at room temperature.

Coupled-ring-resonator-based silicon modulator for enhanced performance.

A compact silicon coupled-ring modulator structure is proposed. Two rings are coupled to each other, and only one of these rings is actively driven and over-coupled to a waveguide, which enables high modulation speed. The resultant notch filter profile is steeper than that of the single ring and has exhibited a smaller resonance shift and lower driving electrical power. Simulation results include: (i) potentially 60-Gb/s non-return-to-zero (NRZ) data modulation and over 20-dB extinction ratio can be achieved by shifting the active ring by a 20 GHz resonance shift, (ii) the frequency chirp of the modulated signals can be adjusted by varying the coupling coefficient between the two rings, and (iii) dispersion tolerance at 0.5-dB power penalty is extended from 18 to 85 ps/nm, for a 40-Gb/s NRZ signal.

CH4-based dry etching of high Q InP microdisks

CH4-based InP dry etching techniques have been investigated by using electron cyclotron resonance etching and reactive ion etching (RIE) methods to obtain microdisk and ring structures having smooth, vertical sidewalls, and specular etched surfaces. The RIE method is chosen for the device etch process owing to the higher perfection of the surfaces generated by this process. Excess CH4 introduced in the InP RIE process was found to generate excessive polymers and resulted in sloped, rough sidewalls. A multistep RIE process involving a high-pressure (75 mTorr) condition, followed by a lower pressure (15 mTorr) etching to the completion of the structure was developed that leads to very smooth sidewalls. This process was successfully utilized in the fabrication of vertically coupled microdisk resonators.

Operation of photonic crystal membrane lasers above room temperature

Operation of photonic crystal lasers for substrate temperatures as high as 50 °C is reported. The temperature dependence of the lasing wavelength and the threshold pump power is also investigated. The characteristic temperature To is 37.7 K.

Atmospheric pressure atomic layer epitaxy : mechanisms and applications

Abstract Atomic layer epitaxy (ALE) has been adapted for hybrid use with conventional atmospheric pressure metalorganic chemical vapor deposition (MOCVD) by using organometallic precursors for the column III elements. Saturation of the growth rate to one monolayer per cycle is observed over a range of growth parameters by thermally driven and laser (or light) driven reactions. The mechanisms causing this saturation are investigated using empirical growth studies and surface reaction studies in UHV conditions. It is concluded that the useful range of ALE growth temperatures for GaAs using metal alkyls is limited by gas phase reactions which frustrate the basic saturation process. Strategies for overcoming this limitation to achieve general purpose epitaxial growth by ALE are described.