Chun-Yung Chi

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.

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

Effects of Surface Passivation on Twin-Free GaAs Nanosheets

Unlike nanowires, GaAs nanosheets exhibit no twin defects, stacking faults, or dislocations even when grown on lattice mismatched substrates. As such, they are excellent candidates for optoelectronic applications, including LEDs and solar cells. We report substantial enhancements in the photoluminescence efficiency and the lifetime of passivated GaAs nanosheets produced using the selected area growth (SAG) method with metal organic chemical vapor deposition (MOCVD). Measurements are performed on individual GaAs nanosheets with and without an AlGaAs passivation layer. Both steady-state photoluminescence and time-resolved photoluminescence spectroscopy are performed to study the optoelectronic performance of these nanostructures. Our results show that AlGaAs passivation of GaAs nanosheets leads to a 30- to 40-fold enhancement in the photoluminescence intensity. The photoluminescence lifetime increases from less than 30 to 300 ps with passivation, indicating an order of magnitude improvement in the minority carrier lifetime. We attribute these enhancements to the reduction of nonradiative recombination due to the compensation of surface states after passivation. The surface recombination velocity decreases from an initial value of 2.5 × 10(5) to 2.7 × 10(4) cm/s with passivation.

Carrier dynamics and doping profiles in GaAs nanosheets

We have recently demonstrated that GaAs nanosheets can be grown by metal-organic chemical vapor deposition (MOCVD). Here, we investigate these nanosheets by secondary electron scanning electron microscopy (SE-SEM) and electron beam induced current (EBIC) imaging. An abrupt boundary is observed between an initial growth region and an overgrowth region in the nanosheets. The SE-SEM contrast between these two regions is attributed to the inversion of doping at the boundary. EBIC mapping reveals a p-n junction formed along the boundary between these two regions. Rectifying I–V behavior is observed across the boundary further indicating the formation of a p-n junction. The electron concentration (ND) of the initial growth region is around 1 × 1018 cm−3, as determined by both Hall effect measurements and low temperature photoluminescence (PL) spectroscopy. Based on the EBIC data, the minority carrier diffusion length of the nanosheets is 177 nm, which is substantially longer than the corresponding length in unpassivated GaAs nanowires measured previously.

Facile Five-Step Heteroepitaxial Growth of GaAs Nanowires on Silicon Substrates and the Twin Formation Mechanism.

Monolithic integration of III-V semiconductors with Si has been pursued for some time in the semiconductor industry. However, the mismatch of lattice constants and thermal expansion coefficients represents a large technological challenge for the heteroepitaxial growth. Nanowires, due to their small lateral dimension, can relieve strain and mitigate dislocation formation to allow single-crystal III-V materials to be grown on Si. Here, we report a facile five-step heteroepitaxial growth of GaAs nanowires on Si using selective area growth (SAG) in metalorganic chemical vapor deposition, and we further report an in-depth study on the twin formation mechanism. Rotational twin defects were observed in the nanowire structures and showed strong dependence on the growth condition and nanowire size. We adopt a model of faceted growth to demonstrate the formation of twins during growth, which is well supported by both a transmission electron microscopy study and simulation based on nucleation energetics. Our study has led to twin-free segments in the length up to 80 nm, a significant improvement compared to previous work using SAG. The achievements may open up opportunities for future functional III-V-on-Si heterostructure devices.

Formation of Fabry-Perot cavity in one-dimensional and two-dimensional GaAs nanostructures

We report formation of an optical cavity and observation of Fabry-Perot resonance in GaAs nanowires and nanosheets grown by metal organic chemical vapor deposition (MOCVD) with selective area growth (SAG). These nanostructures are grown along the (111)B direction. The formation of an optical cavity in the nanowires and nanosheets are fundamentally different from each other. In nanowires the optical cavity is formed along the length of the nanowire with ends of the nanowire behaving as two parallel mirrors. In nanosheets, however, the three non-parallel edges of the GaAs nanosheets are involved in trapping of the light through total internal reflection, thus forming a 2D cavity. We show that through surface passivation and local field enhancement, both the photoluminescence intensity and hence Fabry-Perot peak intensity increases significantly. Transferring the GaAs nanowires and nanosheets to the gold substrate (instead of Si/SiO2 substrate) leads to substantial enhancement in the photoluminescence intensity by 5X (for nanowires) and 3.7X (for nanosheets) to infinite enhancement of the FP peaks intensities. In order to reduce the non-radiative recombination in these nanowires the surface states in the nanowires can be passivated by either an ionic liquid (EMIM-TFSI) or an AlGaAs surface layer. Both passivations methods lead to an enhancement of the optical response by up to 12X.