AVS2010 Session EM-TuP: Electronic Materials and Processing Poster Session

Indium nitride films grown at various growth temperatures were prepared on GaN buffer layer using self-designed plasma-assisted metal-organic molecule beam epitaxy. The influence of substrate temperature on film crystal structure, surface morphology, optical and electrical properties is studied using x-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction, Hall effect, and photoluminescence (PL) measurements. The results show that all InN films grown on the GaN template have good quality and the full width at half maximum is around 1000 arcsec. At 500^oC, the optical measurements on the films revealed a luminescence feature in the vicinity of 0.7 eV. Also, TEM images of the films exhibit better structural properties indicated by a sharper InN/GaN interface. SEM images determine the growth rate of about 14 nm/min. These results indicate that the optoelectronic properties and crystalline quality can be improved significantly by increasing the growth temperature.

The difficulty of achieving ohmic contacts to p-type GaN is associated with the inherent difficulties involved in acceptor doping with Mg. Poor doping efficiency results in high contact resistance and high semiconductor sheet resistance. In GaN based devices, these issues lead to parasitic voltage drops and associated Joule heating. When grown via CVD methods a significant amount of Mg forms a complex with hydrogen. This complex prevents Mg from participating in active doping. The challenge is to achieve large concentrations of free Mg in the films. In-situ post-growth annealing has been found to enhance p-type conductivity, but there is a large disparity in activated Mg acceptors versus incorporated Mg atoms. Furthermore, we know of no reports in the literature regarding optimization of the annealing conditions. In this study we have conducted I-V, C-V, Hall, and contact resistance measurements of Ni/Au contacts on Mg-doped GaN films grown on AlN/SiC (0001) substrates as a function of annealing temperature and time, and nitrogen or oxygen concentration for in-situ and ex-situ anneals. We found that in-situ cooling in nitrogen after growth is important for initial activation of Mg dopants; however, additional in-situ annealing in nitrogen after growth had little effect on the electrical properties. We also found that the electrical characteristics are particularly sensitive to the presence of oxygen and temperature during ex-situ annealing; > 200% improvements in hole concentration were observed for annealing in 1:1 N₂:O₂ at 800°C. The electrical data will be correlated with SIMS data of the hydrogen concentration as a function of annealing conditions.

Highly pure 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) nanofilms were deposited on a very high quality OTS-SAM surface at two different substrate temperatures (70°C and 90°C) via the vacuum thermal evaporation (VTE) method. X-ray reflectivity (XRR) and grazing angle incidence x-ray diffraction (GID) measurements over a wide temperature range (30°C-284°C) revealed that out-of-plane crystallinity of the film (~10 nm) remains intact but in-plane crystallinity starts to become poor from ~100°C, and to become much worser from 260°C. Atomic force microscope images showed that TIPS-PEN films (~55 nm) prepared at the substrate temperature of 90°C or above commonly have a number of huge cracks between enormous crystal domains (up to 3mm) whereas the films didn’t form such morphology below Ts=90°C. These results clearly suggest that an optimum substrate temperature of TIPS-PEN nanofilms on OTS-SAM surface must be somewhere between 70°C and 90°C, and the process temperature must be kept below 90°C in order to form and maintain a highly crystalline film for an organic thin film transistor device since in-plane crystallinity of a semiconductor channel deeply affects the performance of a transistor.

For highly scaled atomic layer deposition (ALD) of gate oxides (EOT < 1nm) on III-V semiconductors, the general requirements for an unpinned, high mobility oxide/III-V interface are as follows: (a) The metal precursor cannot disrupt the substrate during deposition; (b) The metal precursor must form a monolayer of nucleation sites in order for aggressive scaling of the equivalent oxide thickness (EOT); (c) The oxide has to be resistant to atom donation to/from the substrate; (d) The oxide needs to bond weakly to the interface or to form nearly covalent bonds to the interface to balance polarity. The nucleation/passivation layer has to enable all four requirements. Half cycle room temperature ALD of trimethylaluminum (TMA) and dimethylaluminium ethoxide (DEAE) have been performed on InAs and InGaAs surface to compare two precursors for the same oxide one of which is oxygen-free and one which contains oxygen. The electronic properties of the clean and deposited surfaces are probed via scanning tunneling spectroscopy and microscopy (STS and STM ), and Kelvin probe force microscopy (KPFM). Previous STS and KPFM studies for both clean InAs and InGaAs (4×2) surfaces determined the Fermi level is pinned 0.3eV above the valance band; DFT studies show that the surface are pinned by homodimers in the trough. STM and STS show that TMA forms an ordered monolayer of absorbates which unpin the Fermi level suggesting that an ordered monolayer layer might be a requirement for unpinning. However, STM of DEAE reacted InGaAs(001)-(4x2) shows a nearly amorphous monolayer layer while STS shows even this amorphous layer unpins the interface. The influence of larger ligands on the DEAE might account for more degeneracy in bonding configurations making order structures less probable. The results are consistent with a multitude of bonding configuration being able to unpin the Fermi level as long as the pining sites are removed and the presence of oxygen in the precursor not being an impediment to passivation as long as there is still attractive interactions between the absorbates which promote monolayer formation.

Gallium oxide has a unique combination of properties that hold significant promise for variety applications. Pure beta-Ga2O3 has band gap of 4.8 eV which has potentials as UV-transparant optics material. Recent findings suggest its n-type semiconducting behavior after treating it in reducing atmospheres, which place it in the group of new generation transparent conductive oxides. On the other hand, its conductivity change after gases like CO and NO2 makes it a potential gas sensing material. Despite its unique optic/optoelectronic properties, incomplete mechanistic understanding of the origins of conductivity becomes a barrier to device development. According to recent findings, oxygen vacancy contribution to the conductivity is widely accepted due to inverse correlation of conductivity and oxygen partial pressure during material growth. However, other models were also proposed, e.g, experimental results show trace amount of impurities can enhance the conductivity of Ga2O3 without changing its crystal structure. On the other hand, formation energy calculation suggests possible vacancy alignment along conducting direction can also improve the electron hopping, thus enhance the conductivity. Unfortunately, most of the conclusions were either lack of experimental support, or based on samples prepared from different synthesizing techniques, whose properties can be substantially changed.

In this report, Single crystal beta-Ga2O3 prepared by float zone technique was used to investigate the electrical property change after heating under different atmospheres. It is discovered that by passing direct current through the material in the ultra high vacuum, resistivity of beta-Ga2O3 along [010] direction can have significant increase, which contradicts to the oxygen vacancies model. A detailed investigation using X-ray Photoemission Spectroscopy (XPS), Scanning Tunneling Microscopy (STM), Physical Property Measurement System(PPMS) reveals the change in stoichiometry, work function, surface morphology and polaron hopping dimensions, which bring insight to the origins of conductivity in Ga2O3 along different crystal orientations. Based on the understanding on the single crystal conductivity, epitaxial Ga2O3 thin film with designated resistive switching properties using Pulse laser deposition can also be prepared.

Nanoimprint lithography is a low-cost method of fabricating nanoscale patterns as small as 6 nm. It has been emerged as a key technology for the fabrication of devices with nanoscale patterns, such as polarizer, optical devices, bio devices, and patterned media. The ultraviolet nanoimprint lithography (UV-NIL) process is also a promising alternative to the expensive conventional optical lithographic process for producing non-volatile memory. Resistive random access memory (RRAM), which utilizes the resistance change effect of oxide material, has attracted considerable attention and been widely investigated due to its potential application in memory devices.

In this study, identical patterns and characteristics of sub-100nm TiO₂-based resistive switching systems on 4 inch silicon substrates are demonstrated using Step and flash imprint lithography (SFIL). SFIL is a nanoimprint lithography technique, offering the advantages of a high aspect-ratio, reliable nano-patterns and a transparent stamp that can be used to facilitate overlay techniques. The overlay process from the alignment system in IMPRIO 100 was appropriate for the fabrication of nanoscaled crossbar arrays in this work. Crossbar arrays consisting of TiO₂ resistive switching material sandwiched between Pt and Al electrodes with a width of 80 nm and a half-pitch of 100 nm are in turn replicated through successive imprinting and etching processes. Use of a direct metal etching process enhances the uniformity of the TiO₂/electrodes interface. The electrical property of the crossbar arrays showed the reproducible resistive switching behavior resulting in the application of the nonvolatile resistive memory.