AVS2011 Session NS+EM-MoM: Nanowires and Nanoparticles I: Assembly and Devices

Recently, zinc oxide nanowires (ZnO NWs) have attracted much attention for high mobility and sensing properties. These advantageous give us strong possibility to use nanostructures as nanoelectronic device application; such as field effect transistors (FETs), diodes, and logic circuit devices.[1] In this work, we fabricated the logic circuit inverter using difference of gate electrodes which have the different work function.[2]

In order to fabricate the inverter devices, grown ZnO NWs were dispersed to the SiO₂/Si substrate by using a drop-and-dry method. The Ni/Ti source and drain electrodes were deposited by e-beam evaporator with a combination of photo-lithography and lift-off process. To make 30nm-thick Al₂O₃ gate insulator layer, we used Atomic Layer Deposition (ALD) system. And then, Pd and Ni/Ti top gate electrodes were deposited and these two devices were connected by wire bonding technique.

The threshold voltage of the Pd top gate ZnO NWs FET shows more positive value (~ 0 V) than that of the other FET (~ -1 V) with Ni/Ti top gate, and these transistors are able to be used as a driver and a load, respectively. The linear mobility of the driver shows about 119 cm²/Vs at V_D = 0.6 V and the inverter device has high gain value of ~15 at V_DD = 5 V. Furthermore, the dynamic property of the logic inverter was measured under the 5 V square input voltages.

More details will be discussed in the meeting.

References

1. G.J, W. K. Hong, J.S. Maeng, M.H. Choe, W.J. Park, and T. K. Lee, Appl. Phys. Lett. 94 173118 (2009)

2. K.M. Lee, J.H. Kim and S.I. Im, Appl. Phys. Lett. 88, 023504 (2006)

III-V Nanowire transistors are cosidered possible candidates to extend the transistor scaling roadmap. The improved electrostatic control in the cylindrical geometry provides benefits for scaling and the advantageous transport properties of the III-V materials may be used to increase the drive current. Besides heterostructure design may be used to tailor the properties in the transistor channel.

In this talk, we will review some of the efforts made in Lund to realize high-perfromance III-V nanowire transistors using vertical nanowires grown by MOVPE. We will show how bottom-up technologies can be combined with top-down processing to realize nanowire-based RF-devices on Si 2" wafers. We use CV techniques to characterize the properties of the high-k material in vertical nanowire capacitors and compare the data to the 1/f-noise characteristics of scaled transistors to evaluate the influence of the high-k material on the transistor performance. We also show that the transistor channel may be reduced down to a diameter of 15 nm without degradation of the transport properties. Finally, we explore the use of novel materials in the transistor structures as we developed GaSb/InAs heterostructures with excellent Esaki diode characteristics to be used for TFET implementations.

In our previous study, we were able to fabricate full-color, 4-inch display made of colloidal quantum dot (QD). Despite such a demonstration of QD light emitting device which is one of candidates for next-generation display, understanding the interface characteristics between QD layer and electron (or hole) accumulation layer is still lacking and further study for improvement of quantum efficiency is essential. Here, we report on a study of scanning tunneling microscopy (STM), spectroscopy (STS) and cathode luminescence induced by tunneling current, performed on individually manipulated QD. We control the distance between two QDs using STM to reveal the mechanism of interaction between QDs. STS measurement showed shift of energy levels as manipulating the distance between two QDs. This result suggests that there exists the optimal distance between QDs for efficient light emission. Besides by making contacts between separated QDs and organic molecules, we simulated contacts between QD layer and electron (or hole) accumulation layer. From these experiments, we could understand excitonic behavior and carrier hopping from QD to QD or surrounding materials. Our findings thus suggest optimal configuration for QD application in display.

One-dimensional electron gas (nanowire) based devices are of great interest due to their promise in high-performance electronics and other future device applications. However, synthesis and patterning of arrays of nanowires is a challenge in all material systems since both bottom-up and top-down approaches have their own merits and demerits.

Here we report on the demonstration of pure 1-dimensional arrays of electrons with current density up to 130 mA/mm and carrier confinement greater than 100 meV using lateral polarization engineering in N-polar vicinal AlGaN/GaN heterostructures. The width of the atomic terraces characteristic of vicinal surfaces defines the dimensions of the nanowires which are found to exhibit sharp and clear signatures of 1-dimensionality at room temperature making them promising for novel device applications.

We report on devices fabricated on MOCVD grown N-polar AlGaN/GaN HEMT structures on vicinal sapphire substrate (4⁰ miscut towards a-plane) with anisotropy in current and channel pinch-off voltages. Channels parallel to the miscut direction pinched off at higher negative gate biases than those perpendicular to the steps and carried more charge as measured by direction-dependent C-V profiling. An electrostatic model which predicts a saw-tooth energy band profile in the lateral direction has been proposed to explain the charge anisotropy. Each atomic terrace characteristic of the surface morphology of vicinal GaN with its corresponding saw-tooth energy profile is proposed to exhibit quasi-1D confinement. We will discuss the heterostructure/polarization design of structures demonstrating pure 1-D transport in direction parallel to steps.

Gated structures were fabricated to investigate the physics of the system as the Fermi occupation function is varied by varying gate bias. To confirm that the carriers are indeed 1-dimensional, we used direction-dependent small-signal capacitance voltage measurements to probe the density of state function and hence dimensionality of electrons as a function of gate bias. We developed a 2-band model consisting of one 1-D and one 2-D subband to describe the behavior of these wires at room temperatures. The variation of capacitance as well as charge density for a pure 1-D and a pure 2-D system as a function of applied gate bias as predicted by our 2-band model based on density of states matches very well with the data measured experimentally for 1-D and 2DEG respectively. This confirms that the channels created are indeed 1-dimensional in nature. Since 1-D channels are atomic terrace defined, they are promising for eliminating the disadvantages of both bottom-up and top-down approaches.

Heat applied to thin films below a critical thickness will generally cause transformation of the film into isolated particles. This process is known as dewetting. Solid state dewetting occurs below the melting temperature of the film and is governed by diffusive mass transport. Currently two mechanisms of dewetting are distinguished: hole nucleation and growth, and spinodal dewetting. Spinodal dewetting proceeds via film surface undulations that have characteristic wavelengths related to the thickness of the film. Lithographic patterning of thin films has been utilized to direct the dewetting instability development toward designed nanostructured geometry of nanoparticle arrays. Tailoring the geometry of thin film edge have been shown to affect both heterogeneous nucleation and spinodal dewetting regimes. Nanoimprint lithography, conventionally used for definition of the edges of thin films, is a fabrication method where a stamp is pressed into a thin normally monomer or polymer film at elevated temperatures. Nanoimprinting can also be conducted in direct mode where the stamp is pressed into a metallic film. Surface undulations characteristic for spinodal dewetting will be used to direct the stamp design. Such imprinting allows setting initial conditions, programming instability, in the thin metallic film that is linked to the spinodal surface instability. In this work we are presenting the results of the investigation into behavior of thin films in which a 3D structure has been imprinted. We present observations on the effect of direct nanoimprint lithography on nanoscale Au and Ni films using periodic arrays of cylinders. Our focus is on the spatial distribution of the particles produced from dewetting of the nanoimprinted films. Particles in patterned regions are characterized in terms of their spacing, periodicity and size, and shape. The geometry of the dewetted patterns is compared to that of the 3D features created after direct nanoimprinting of the films. Analysis of spatial correlation of the final dewetted patterns to stamp patterns is presented.