Duanjun Cai

Copper Nanowires as Fully Transparent Conductive Electrodes

In pondering of new promising transparent conductors to replace the cost rising tin-doped indium oxide (ITO), metal nanowires have been widely concerned. Herein, we demonstrate an approach for successful synthesis of long and fine Cu nanowires (NWs) through a novel catalytic scheme involving nickel ions. Such Cu NWs in high aspect ratio (diameter of 16.2 ± 2 nm and length up to 40 μm) provide long distance for electron transport and, meanwhile, large space for light transmission. Transparent electrodes fabricated using the Cu NW ink achieve a low sheet resistance of 1.4 Ohm/sq at 14% transmittance and a high transparency of 93.1% at 51.5 Ohm/sq. The flexibility and stability were tested with 100-timebending by 180°and no resistance change occurred. Ohmic contact was achieved to the p- and n-GaN on blue light emitting diode chip and bright electroluminescence from the front face confirmed the excellent transparency.

Improved p -type conductivity in Al-rich AlGaN using multidimensional Mg-doped superlattices

A novel multidimensional Mg-doped superlattice (SL) is proposed to enhance vertical hole conductivity in conventional Mg-doped AlGaN SL which generally suffers from large potential barrier for holes. Electronic structure calculations within the first-principle theoretical framework indicate that the densities of states (DOS) of the valence band nearby the Fermi level are more delocalized along the c-axis than that in conventional SL, and the potential barrier significantly decreases. Hole concentration is greatly enhanced in the barrier of multidimensional SL. Detailed comparisons of partial charges and decomposed DOS reveal that the improvement of vertical conductance may be ascribed to the stronger pz hybridization between Mg and N. Based on the theoretical analysis, highly conductive p-type multidimensional Al0.63Ga0.37N/Al0.51Ga0.49N SLs are grown with identified steps via metalorganic vapor-phase epitaxy. The hole concentration reaches up to 3.5 × 1018 cm−3, while the corresponding resistivity reduces to 0.7 Ω cm at room temperature, which is tens times improvement in conductivity compared with that of conventional SLs. High hole concentration can be maintained even at 100 K. High p-type conductivity in Al-rich structural material is an important step for the future design of superior AlGaN-based deep ultraviolet devices.

Large-roll growth of 25-inch hexagonal BN monolayer film for self-release buffer layer of free-standing GaN wafer

Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2” tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2” GaN or 4” Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate.

Kinetic behavior of nitrogen penetration into indium double layer improving the smoothness of InN film

The kinetic process of the formation of InN thin film was clarified via the investigation of the layer-by-layer deposition on (0001) surface, by first-principles calculations. Site selection and diffusion behavior of In and N adatoms revealed an extraordinary growth kinetics. The indium bilayer preferably deposits in the initial stage and then the N atoms come up and penetrate vertically through a specific channel into between this double layer, finally forming the tetrahedral coordination of InN. Following this kinetic process, alternative pulse supply of precursors was proposed for the InN film growing and smoothening, which can effectively improve the surface smoothness.

Ohmic contact to n-AlGaN through bonding state transition at TiAl interface

We report the optimized ohmic contact to high Al content n-AlGaN through modification of the interfacial bonding state of TiAl alloy. First-principles calculations demonstrate that the change of interfacial bonding state (N rich to Al rich) at the TiAl/n-AlGaN interface is crucial for the formation of low barrier contact. The significant electron-transfer and strong orbital hybridization between the Ti atoms and the nearest Al atoms plays a key role in lowering the contact barrier. After treatment of the TiAl/n-AlGaN sample via rapid thermal annealing, perfectly linear I-V characteristic is achieved and the elemental profile by Auger electron spectroscopy confirms the N-rich-to-Al-rich local state transition in the interfacial layers.

Modified pulse growth and misfit strain release of an AlN heteroepilayer with a Mg–Si codoping pair by MOCVD

We report the modified pulse growth method together with an alternating introduction of larger-radius impurity (Mg) for the quality improvement and misfit strain release of an AlN epitaxial layer by the metal–organic chemical vapour deposition (MOCVD) method. Various pulse growth methods were employed to control the migration of Al atoms on the substrate surface. The results showed that the pulse time and overlapping of V/III flux is closely related with the enhancement of the 2D and 3D growth mode. In order to reduce the misfit strain between AlN and sapphire, an impurity of larger atomic radius (e.g. Mg) was doped into the AlN lattice to minimize the rigidity of the AlN epilayer. It was found that the codoping of Mg–Si ultrathin layers could significantly minimize the residual strain as well as the density of threading dislocations.

One-Pot Synthesis of Superfine Core–Shell Cu@metal Nanowires for Highly Tenacious Transparent LED Dimmer

We demonstrate a one-pot, low-cost, and scalable method for fast synthesis of superfine and uniform core-shell Cu nanowires (NWs) coated with optional metals and/or alloy. Cu NWs in high aspect ratio (>3000) were synthesized through an oleylamine-mediated solution method, and tunable shell coating was performed by injecting metal-organic precursors at the last stage of reaction. Superfine Cu@metal NWs (Ti, Zn, V, Ni, Ag, NiZn, etc) were achieved in diameter of ∼30 nm and length of ∼50 μm. Transparent conductive films were obtained by imprinting method, showing high optoelectronic performance (51 Ω/sq at 93% transmittance), high mechanical tenacity over bending, twisting, stretching, and compressing, and robust antioxidant ability (high temperature and high humidity). A transparent film dimmer for light-emitting diode (LED) lighting was fabricated with the stretchable Cu@Ti NWs network. The LED luminance could be accurately tuned by the deformation strain of Cu@Ti NWs film.

Band engineering of GaN/AlN quantum wells by Si dopants

The electronic properties of GaN/AlN quantum wells are engineered by Si doped in different positions with the aid of the first-principle calculations. The local potential where the dopant located is dragged down as a result of negative center induced by the Si atom, leading to a different shift of the potential, and further affects the band bending and carrier distribution. The band profiles are depicted by analyzing the projected densities of states, it is found that the different positions of Si doping lead to a different band bending owing to the modified polarization fields. The spatial distributions of electrons and holes plotted by the partial charge densities reveal that electrons and holes experience redistribution by Si dopant in different positions. The above results demonstrate that the effect of polarization on the band bending has been significantly modulated by Si doped in different positions. Such modification of electronic structure is especially valuable for the fabrication of GaN/AlN QWs under desired control.

Highly transparent light emitting diodes on graphene encapsulated Cu nanowires network

The internal quantum efficiency of blue LEDs is almost close to the limit, therefore, advanced transparent electrode has been long explored for gaining high external quantum efficiency. However, work function mismatch at electrode-semiconductor interface remains the fundamental difficulty in obtaining low resistance ohmic contact. Here, we demonstrate the gas phase encapsulation of graphene layer on superfine Cu nanowires network by chemical vapor deposition for highly transparent LEDs. The fast encapsulation of graphene shell layer on Cu nanowires achieves high optoelectronic performance (33 Ω/sq @ 95% T), broad transparency range (200~3000 nm) and strong antioxidant stability. A novel phenomenon of scattered-point contact is revealed at the Cu nanowires/GaN interface. Point discharge effect is found to produce locally high injection current through contact points, which can effectively overcome Schottky barrier and form ohmic contact. The transparent LED on Cu@graphene nanowire network is successfully lighted with bright blue emission.