Diefeng Gu

Synthesis of Nested Coaxial Multiple-Walled Nanotubes by Atomic Layer Deposition

Nested multiple-walled coaxial nanotube structures of transition metal oxides, semiconductors, and metals were successfully synthesized by atomic layer deposition (ALD) techniques utilizing nanoporous anodic aluminum oxide (AAO) as templates. In order to fabricate free-standing tube-in-tube nanostructures, successive ALD nanotubes were grown on the interior template walls of the AAO nanochannels. The coaxial nanotubes were alternated by sacrificial spacers of ALD Al(2)O(3), to be chemically removed to release the nanotubes from the AAO template. In this study, we synthesized a novel nanostructure with up to five nested coaxial nanotubes within AAO templates. This synthesis can be extended to fabricate n-times tube-in-tube nanostructures of different materials with applications in multisensors, broadband detectors, nanocapacitors, and photovoltaic cells.

A low-voltage nano-porous electroosmotic pump

A low-voltage electroosmotic (EO) micropump based on an anodic aluminum oxide (AAO) nano-porous membrane with platinum electrodes coated on both sides has been designed, fabricated, tested, and analyzed. The maximum flow rate of 0.074 ml min(-1) V(-1) cm(-2) for a membrane with porosity of 0.65 was obtained. A theoretical model, considering the head loss along the entire EO micropump system and the finite electrical double layer (EDL) effect on the flow rate, is developed for the first time to analyze the performance of the EO micropump. The theoretical and experimental results are in good agreement. It is revealed that the major head loss could remarkably decrease the flow rate, which thus should be taken into account for the applications of the EO micropump in various Lab-on-a-chip (LOC) devices. However, the effect of the minor head loss on the flow rate is negligible. The resulting flow rate increases with increasing porosity of the porous membrane and kappaa, the ratio of the radius of the nanopore to the Debye length.

Fabrication, characterization and simulation of high performance Si nanowire-based non-volatile memory cells

We report the fabrication, characterization and simulation of Si nanowire SONOS-like non-volatile memory with HfO(2) charge trapping layers of varying thicknesses. The memory cells, which are fabricated by self-aligning in situ grown Si nanowires, exhibit high performance, i.e. fast program/erase operations, long retention time and good endurance. The effect of the trapping layer thickness of the nanowire memory cells has been experimentally measured and studied by simulation. As the thickness of HfO(2) increases from 5 to 30 nm, the charge trap density increases as expected, while the program/erase speed and retention remain the same. These data indicate that the electric field across the tunneling oxide is not affected by HfO(2) thickness, which is in good agreement with simulation results. Our work also shows that the Omega gate structure improves the program speed and retention time for memory applications.

Mechanical and structural characterization of atomic layer deposition-based ZnO films

Zinc oxide thin films were deposited by atomic layer deposition (ALD). The structural and mechanical properties of the thin films were investigated by x-ray diffraction, transmission electron microscopy, atomic force microscopy, and nanoindentation. Diethyl zinc was used as the chemical precursor for zinc and water vapor was used as the oxidation agent. The samples were deposited at 150 °C and at a pressure of 2.1 × 10−1 Torr in the ALD reactor. A growth rate of 2 A per cycle was calculated in the ALD process window. The Nano Indenter XP was used in conjunction with the continuous stiffness method in depth control mode in order to measure and to analyze the mechanical properties of hardness and modulus of ALD ZnO thin film samples. For comparison, we benchmarked the mechanical properties of single crystal bulk ZnO samples against those of our ALD ZnO thin films.

Atomic Layer Deposition of ZrO2 and HfO2 Nanotubes by Template Replication

Highly ordered zirconia and hafnia nanotubes were fabricated by atomic layer deposition (ALD) within the anodic alumina oxide (AAO) template. Scanning electron microscopy and energy-dispersive spectroscopy were used to characterize the morphology and elemental compositions of the different ALD coatings. The diameters of the AAO pores are in the range of 200-300 nm with a thickness of 60 μm. The results indicated that the freestanding nanotubes were uniformly grown through the entire template thickness. The ALD process conformally replicated the AAO template dimensions. The number of ALD cycles controlled the resultant nanotube wall thickness.

Self-assembled nanowire array capacitors: capacitance and interface state profile

Direct characterization of the capacitance and interface states is very important for understanding the electronic properties of a nanowire transistor. However, the capacitance of a single nanowire is too small to precisely measure. In this work we have fabricated metal-oxide-semiconductor capacitors based on a large array of self-assembled Si nanowires. The capacitance and conductance of the nanowire array capacitors are directly measured and the interface state profile is determined by using the conductance method. We demonstrate that the nanowire array capacitor is an effective platform for studying the electronic properties of nanoscale interfaces. This approach provides a useful and efficient metrology for the study of the physics and device properties of nanoscale metal-oxide-semiconductor structures.

Nanomechanical study of amorphous and polycrystalline ALD HfO2 thin films

Thin films of hafnium oxide (HfO2) were deposited by atomic layer deposition (ALD). The structural properties of the deposited films were characterised by transmission electron microscopy (TEM) and X-ray diffraction (XRD). We investigated the effect of phase transformations induced by thermal treatments on the mechanical properties of ALD HfO2 using nanoindentation. The elastic modulus of the amorphous low temperature deposited ALD HfO2 films was measured to be 370 ± 20 GPa. Subsequent to crystallisation by annealing in a rapid thermal annealing (RTA) chamber, the elastic modulus dropped to 240 ± 20 GPa. Similarly, the Meyer hardness decreased from a value of 18 ± 1 GPa for amorphous HfO2 to 15 ± 1 GPa following the transition temperature from amorphous to polycrystalline HfO2.

Nanomechanical Response of the Si Lattice to Hydrogen Implantation and Annealing for Layer Splitting

We studied the effect of hydrogen implantation into Si and the nanomechanical response to defect interaction, which is responsible for wafer splitting during the Smart CutTM layer transfer of (001) oriented Si wafers. Hardness and modulus of H implanted Si samples were measured by nanoindentation technique before and after thermal annealing. A significant weakening of the hardness and elastic modulus of the single crystalline Si lattice in the H implantation-induced damage zone following annealing was observed. Cross-sectional transmission electron microscopy revealed that the majority of extended defects consist of platelets, which developed parallel to the (001) Si surface during annealing.

Thermal Behavior of the Mechanical Properties of GaN throughout Hydrogen-Induced Thin Layer Transfer

commonly known as the ion-cut or Smart-Cut TM . The control and the application of this process to cleave thin layers from bulk or freestanding GaN (fs-GaN) wafers is technologically highly relevant. Indeed, fs-GaN is currently mostly used in the fabrication of blue laser diodes providing a wide spectrum of applications in optoelectronic data storage, visual information, medical devices, and biophotonics. Several industries (e.g., power devices, ultra-high brightness LEDs, etc) can also benefit from the availability of fs-GaN. The current cost of fsGaN wafers is, however, very high. Therefore, large scale production of these new devices would require an important decline in fs-GaN price to compete with the alternative technologies (e.g., SiC). One of the possible strategies to reduce the cost would be to cleave several thin layers from a single fs-GaN wafer (donor wafer) and transfer them onto different handle wafers. In principle, this can be achieved using the ion-cut process. This work elucidates some subtle changes in the mechanical properties of fs-GaN leading to sub-surface microcracking and ultimately to thin layer transfer. Understanding these fundamental scientific aspects is vital for a better control of the ion-cut technology. Here the effect of H implantation-induced exfoliation was investigated by nanoindentation analysis. In case of GaN, the minimum H-fluence required for the occurrence of blistering following post-implantation annealing was found to be 2.6×10 17 cm -2 2 . In this experiment 300μm thick GaN samples were implanted with hydrogen ions at 50 keV with a Hydrogen dose of 2.6×10 17 cm -2 . The peak of the implanted hydrogen in GaN was found to be ~320 nm 3

Raman Spectroscopy of ZnO Thin Films by Atomic Layer Deposition

During recent years, ZnO has attracted attention because of its large binding energy of ~60 meV and a bandgap of around 3.39 eV at room temperature. ZnO is an ideal candidate for transparent electronics such as transparent thin film transistors, optoelectronic, piezoelectric, ferroelectric, and ferromagnetic devices, solar cells, and sensor applications. ZnO can be synthesized by various techniques such as sputtering , pulsed laser deposition 3 , electrochemical decomposition 4 , thermal evaporation 5 , vapor liquid phase 6 , metal organic chemical vapor deposition 7 , molecular beam epitaxy 8 and recently atomic layer deposition (ALD) 9 , 10 . Among these thin film growth techniques, ALD provides unique features such as precise thickness control of ZnO thin films with atomic resolution, high uniformity, good conformity. ALD is capable of coating complex surface morphologies with high aspect ratios. This article presents the Raman properties of ZnO ALD thin films and investigates the effect of annealing in different ambient atmospheres. Raman spectroscopy provides vibrational and rotational properties of materials. Like most wurtzite crystal structure materials, ZnO belongs to the hexagonal system with space group C6v (P63mc) with two formula units per primitive cell where all atoms occupy C3v sites 11 . According to the group theory, ZnO has active phonons of A1, E1, and two E2, and an inactive phonon of B1. The ZnO films with ALD were deposited on p-type Si (100) wafers. The native oxide on the Si wafers was etched prior to deposition by dipping them in 2% of diluted hydrofluoric acid for 4 min. Then, the wafers were rinsed with de-ionized water and dried with N2 gas. The ZnO films were deposited using the Savannah ALD reactor from Cambridge Nanotech. Diethyl zinc (DEZ) was used as the precursor of zinc and H2O was used as an oxidation agent. The ZnO films with different thicknesses were prepared at 150C and 2.1x10 Torr in N2 carrier gas. After ZnO film deposition a portion of the samples were cleaved for post deposition anneal (PDA) in room ambient, nitrogen ambient, and oxygen ambient in a rapid thermal annealing system (Solaris 150 by SSI). The vibrational modes of ZnO films on Si were characterized with a Raman spectrometer (Inspector, DeltaNu) with excitation wavelength at ~785 nm with ~40-W pump power. The signal was integrated for 2 sec and averaged for 5 times for each measurement. X-ray diffraction (XRD) was also done on the ALD ZnO to determine the crystalline nature of the films. Asdeposited polycrystalline ALD ZnO thin films show a preferential growth in the (002) plane or c-axis. Figure 1 shows the XRD plot for the ALD ZnO thin film with 400nm thickness. The crystallite size was calculated to be 28 nm using the Debye Scherrer’s formula. For the highest ALD cycles (1900), there is the apparition of the E2 (high) optical phonon at frequency of 446.33 cm. The peak at 384.75 cm corresponds to the A1 (TO) mode. There was no indication of any LO phonons. However, several phonon peaks at 280.93, 1134.6, 1343.6 and 1422.8 cm were observed. Figure 2 shows the Raman spectra from a ZnO film deposited with 1900 ALD cycles and a bulk ZnO sample as benchmarking. The bulk single crystal ZnO sample exhibits a sharp E2 (high) optical phonon mode at 442 cm. In contrast to the polycrystalline ALD ZnO thin films, the ZnO single crystal did not display the A1(TO) optical phonon. Both bulk single crystal and the ALD thin film of ZnO exhibited the Raman peaks at 270 and 280 cm, respectively. However, neither sample presented E1 optical phonon mode.