S. E. Mohney

Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n-type GaN

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7×1017 cm−3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8×10−6 Ω cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5×1017 cm−3) had resistivities of 7×10−6 and 5×10−6 Ω cm2 after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN.

X-ray photoelectron spectroscopy and x-ray diffraction study of the thermal oxide on gallium nitride

The oxidation of single crystal gallium nitride in dry air has been investigated. X-ray photoelectron spectroscopy (XPS) revealed minimal oxide growth at 450 and 750 °C for up to 25 h. However, at 900 °C the growth of an oxide approximately 5000 A thick was observed after 25 h. This oxide was determined to be the monoclinic β-Ga2O3 using glancing angle x-ray diffraction. XPS spectra of the Ga 3d and Ga 2p core levels indicated peak shifts of 1.2 and 1.3 eV, respectively, from Ga–O to Ga–N bonding. The Ga L3M45M45 core level binding energy was also investigated and β-Ga2O3 and GaN each presented a characteristic peak shape.

Fabrication of micromechanical devices from polysilicon films with smooth surfaces

Abstract Micromechanical devices such as bearings require smooth surfaces. Fine-grained polysilicon can be produced with a surface roughness near 8Arms. The ability to anneal films of this type into tension eliminates size restrictions due to compressive buckling. The use of these films in micromechanical devices has been restricted because hydrogen fluoride-etched structures are covered by an etch residue that leads to contact welding. Contact between opposing surfaces is induced mainly by surface tension effects. This problem may be avoided by removing the deflection mechanism. Thus, freezing of a water-methanol rinse after sacrifical ethcing all but eliminates surface tension. Removal of the ice mixture via sublimation at 0.15 millibar occurs readily. Free-standing structures with smooth surfaces and small gaps are next passivated by silicon deposition or other techniques.

High temperature Pt Schottky diode gas sensors on n-type GaN

The characteristics of Pt Schottky diodes on n-type GaN in hydrogen and propane are reported for the first time. This response from 200–400°C has been characterized by current–voltage measurements, revealing that the diodes are able to detect hydrogen from 200–400°C and propane from 300–400°C. The high temperature stability of Pt diodes on GaN has been investigated by long term annealing at 400°C in Ar or 20% O2 in Ar. The diodes have been held at 400°C for 500 h without degradation of their electrical characteristics or response to hydrogen-containing gases.

Near‐Band‐Edge Electroluminescence from Heavy‐Metal‐Free Colloidal Quantum Dots

The band-edge electroluminescence (EL) of colloidal QDs of cadmium compounds, i.e., Cd(S,Se,Te), exhibits sizetunable spectral emission (450–760 nm) and narrow bandwidth (FWHM ∼ 30–40 nm), allowing for the design and fabrication of color-saturated red, green and blue (RGB) QD-LEDs with simple device confi gurations and high spectral purities that outperform those of liquid crystal displays and organic light emitting diodes. The maximum brightness of RGB QD-LEDs has improved by three orders of magnitude in less than one decade and exceeds 15 000 cd/m 2 for red emitters, corresponding to a current effi ciency of ∼ 2.3 cd/A. [ 15 ] With mercury and lead saltsbased colloidal nanocrystals (HgTe and Pb(S,Se)), the emission

Analysis of a thin AlN interfacial layer in Ti/Al and Pd/Al ohmic contacts to n-type GaN

Using high resolution transmission electron microscopy, a thin pseudomorphic AlN layer (2–3 nm) has been observed at the metal/GaN interfaces of Ti/Al (35/115 nm) and Pd/Al (25/125 nm) ohmic contacts to n-type GaN annealed in Ar at 600 °C for 15 and 30 s, respectively. The interfacial layer has a c-plane (002) lattice spacing of 2.48±.03 A and an a-plane (100) spacing matching that of GaN (2.76 A), and the identification of this layer as AlN is consistent with chemical analysis by energy dispersive x-ray spectroscopy. The formation of this interfacial AlN layer coincides with the onset of ohmic behavior in Ti/Al and Pd/Al contacts annealed at 600 °C and may be the cause.

Titanium and aluminum-titanium ohmic contacts to p-type SiC

Abstract Very low resistance ohmic contacts to p-type SiC were fabricated by depositing a 90-10 wt.% alloy of Al and Ti followed by a high temperature anneal of approximately 1000°C for 2 min. Specific contact resistances ranged from approximately 5 × 10 −6 to 3 × 10 −5 Ω cm 2 on material with a doping of 1.3 × 1019 cm−3. The initial AlTi thickness before annealing was found to be critical to controlling the AlTi sheet resistance during the anneal. In addition, chemically etching the AlTi layer after annealing revealed pitting indicative of severe reaction between the AlTi and SiC surface, as confirmed by Rutherford Backscattering. In contrast, ohmic contacts to the same SiC material were fabricated by depositing pure Ti and annealing at 800°C for 1 min. These contacts were ohmic with a specific contact resistance between 2 × 10 −5 and 4 × 10 −5 Ω cm 2 . Examination of the SiC surface after chemically etching away the annealed contact revealed a smooth surface, suggesting a much more planar Ti/SiC interface.

An investigation into the early stages of oxide growth on gallium nitride

Abstract The early stages of thermal oxidation of gallium nitride epilayers in dry O 2 have been studied using surface sensitive analytical techniques and transmission electron microscopy. Deconvolution of the Ga 3d core level spectra from X-ray photoelectron spectroscopy revealed peak positions representing gallium nitride, gallium oxide, and an intermediate phase. Transmission electron microscopy revealed an overlayer approximately 1.5–3.0 nm thick with registry to the (0001) GaN after dry oxidation at 800°C for 1 h. Based on the data from X-ray photoelectron spectroscopy, this layer is believed to be a Ga ( x +2) N 3 x O (3−3 x ) compound. Atomic force microscopy and transmission electron microscopy revealed the formation of discrete oxide crystallites on top of the oxynitride layer. The crystallites become more numerous and grow with continued oxidation.

Mist fabrication of light emitting diodes with colloidal nanocrystal quantum dots

In this letter, we report a mist-deposition process for the assembly and patterning of nanocrystal quantum dots (NQDs) during the fabrication of quantum dot light emitting diodes (QD-LEDs), which allows for tight controls over the thickness, surface morphology, composition, and resolution of NQD emissive layers. A defect-free featuring uniform brightness QD-LED containing a mist-deposited emissive CdSe(ZnS) NQD layer was demonstrated. Additionally, the technique of successive mist deposition of multicolor NQDs through a set of registered shallow masks was employed to create a 6×6 matrix of alternating pixels composed of 5nm diameter CdSe(ZnS) NQDs (green) and 8nm diameter CdSe(ZnS) NQDs (red) on the same substrate. The results obtained demonstrate the potential of mist-deposition technology in the future development of full-color QD-LED displays.

Influence of oxygen on the activation of p-type GaN

The presence of oxygen in the annealing environment can exhibit a strong influence on the activation of p-GaN, as demonstrated by experiments described in this letter. We activated p-GaN at 600–900 °C in four environments: ultrahigh purity (UHP) N2 gettered to remove residual O2, UHP N2 without gettering, 99.5% UHP N2/0.5% UHP O2, and 90% UHP N2/10% UHP O2. The resistivity of the p-GaN was lowest when O2 was intentionally introduced during activation and was highest when extra care was taken to getter residual O2 from the annealing gas. The experiments also demonstrate that unintentionally incorporated O2 can be at high enough levels to influence the activation process.