M. S. Feng

Schottky contact and the thermal stability of Ni on n‐type GaN

The Schottky barrier height of Ni on n‐GaN has been measured to be 0.56 and 0.66 eV by capacitance–voltage (C–V) and current–density–temperature (J–T) methods, respectively. Gallium nickel (Ga4Ni3) is formed as Ni is deposited on the GaN film, which affects the barrier height markedly. The thermal stability of Ni on GaN is also investigated by annealing these specimens at various temperatures. Specimen annealing at temperatures above 200u2009°C leads to the formation of nickel nitrides Ni3N and Ni4N at the interface of Ni and GaN. These interfacial compounds change the measured barrier height to 1.0 and 0.8 eV by C–V and J–T methods, respectively. Comparisons of Schottky characteristics of Ni with those of Pt, Pd, Au, and Ti are also discussed.

STUDY OF SCHOTTKY BARRIERS ON N-TYPE GAN GROWN BY LOW-PRESSURE METALORGANIC CHEMICAL VAPOR DEPOSITION

Schottky barriers on n‐type GaN films grown by low‐pressure metalorganic chemical vapor deposition are characterized and derived. A thin Pt or a Pd layer is deposited by electron‐gun evaporation to form Schottky contacts in a vacuum below 1×10−6 Torr. The Schottky barrier heights of Pt on the n‐GaN film are determined to be 1.04 and 1.03 eV by current–voltage (C–V) and current density–temperature (J–T) measurements, respectively. Also based on C–V and J–T measurements, the measured barrier height of Pd on n‐GaN is 0.94 and 0.91 eV, respectively. Schottky characteristics of Pt and Pd observed in the experiment are compared with those of Au and Ti in previous reports.

EFFECTS OF COLUMN-III ALKYL SOURCES ON DEEP LEVELS IN GAN GROWN BY ORGANOMETALLIC VAPOR-PHASE EPITAXY

Two different kinds of n‐type GaN films were prepared by organometallic vapor phase epitaxy, one by using trimethylgallium (TMGa) and another by using triethylgallium (TEGa) as the alkyl source. Schottky diodes with well‐behaved current–voltage and capacitance–voltage characteristics were fabricated. Deep‐level transient spectroscopy studies were performed on these samples. Three distinct deep levels, labeled E1, E2, and E3, were measured in the film grown with TMGa, with an activation energy of 0.14, 0.49, and 1.63±0.3 eV, respectively. Only one level, E3, was observed in the film prepared with TEGa.

Persistent photoconductivity in n-type GaN

The persistent photoconductivity（PPC） phenomena in n-type GaN Films grown by metalorganic chemical vapor deposition（MOCVD） have been studied. After using some testing and analysis methods, such as the double crystal X-ray diffraction（DCXRD）, the photolumineseence（PL） spectra, etc, it is found that the issue which influences PPC in n-type GaN is not relative to the dislocations and yellow band （YB）, and is caused by the doping level of Si most likely.

Cathodoluminescent characteristics of ZnGa2O4 phosphor grown by radio frequency magnetron sputtering

Low voltage cathodoluminescent characteristics of ZnGa2O4 phosphor grown by rf magnetron sputtering have been investigated from 300 to 700 nm. The effects of substrate heating and annealing treatment on the luminescent characteristics are also studied. A blue cathodoluminescent emission peaked at 470 nm is observed. Better luminescent properties are achieved on the films which have crystal structure with a standard powder x‐ray diffraction pattern of ZnGa2O4. The effect of the strength of the ligand field on the resultant energy levels for the ZnGa2O4 phosphor is investigated. Low‐voltage phosphor films with excellent cathodoluminescent characteristics have been successfully developed in this research.

A bilayer Ti/Ag ohmic contact for highly doped n-type GaN films

Ohmic contacts with low resistance are fabricated on n‐type GaN films using Ti/Ag bilayer metallization. The GaN films are grown by low pressure metalorganic chemical vapor deposition (LP‐MOCVD) with Si as the dopant. Ohmic characteristics are studied for films with carrier concentration range from 1.5×1017 to 1.7×1019 cm−3. The lowest value for the specific contact resistivity of 6.5×10−5 Ωu2009cm2 is obtained without annealing. The barrier height of Ti on GaN is calculated to be 0.067 eV.

The dependence of the electrical characteristics of the GaN epitaxial layer on the thermal treatment of the GaN buffer layer

The GaN buffer layer was grown on the sapphire substrate by low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD) at 525u2009°C. The following 1.3 μm epitaxial GaN growth was carried out at 1025u2009°C. We varied the ramping rate from 12.5 to 100u2009°C/min to study the quality of the epitaxial GaN. It has been found that the x‐ray peak width, photoluminescence (PL) linewidth, Hall mobilities, and carrier concentrations of GaN epitaxial layer strongly depend on the in situ thermal ramping rate. An optimum thermal ramping rate was found to be of 20u2009°C/min. The maximum mobility is 435 cm2/Vu2009s at carrier concentration of 1.7×1017 cm−3. The minimum full width at half maximum (FWHM) of x ray and PL were 5.5 min and 12 meV occur at a ramping rate of 20u2009°C/min. The decrease of the mobility at high and low ramping rate can be attributed to the thermal stress and the reevaporation of the GaN buffer layer.

Nitric acid-based slurry with citric acid as an inhibitor for copper chemical mechanical polishing

Abstract A novel inhibitor, citric acid, was introduced in the HNO3-based slurry for copper chemical mechanical polishing. It was anticipated that a passivation layer could be formed on the copper surface in the presence of citric acid. In a 3xa0vol.% HNO3 solution, the passivation effect derived from citric acid saturated as the citric acid concentration reached ca. 0.01xa0M. The polishing rate was found to decrease with the addition of citric acid. The results showed that the HNO3-based slurry with citric acid as an inhibitor could achieve good surface planarity for copper chemical mechanical polishing.

X‐ray crystallographic study of GaN epitaxial films on Al2O3(0001) substrates with GaN buffer layers

Samples of GaN(0001) epitaxial films on sapphire Al2O3(0001) with different thicknesses of GaN buffer layers were characterized by x‐ray diffraction method in both in‐plane and plane‐normal directions. The results show that all the epitaxial films are of good quality with the GaN[1010] ∥Al2O3[1120] and GaN[1210]∥Al2O3[1100]. This arrangement of crystal orientation can be attributed to the chemical potential overriding the lattice spacing mismatch. The x‐ray results also indicate that the crystal coherence lengths in the in‐plane direction are smaller than those measured in the plane‐normal direction, i.e., a columnar‐like structure normal to the film is observed. The rocking curve widths in the in‐plane direction are also larger than those measured in the plane‐normal direction. In‐plane measurement of rocking curve and coherence length are essential physical quantities directly related to the electron mobility which was measured predominantly in the in‐plane direction. The best epitaxial structure is...

Growth and characterizations of GaN on SiC substrates with buffer layers

High quality GaN epitaxial layers were grown on 6H–SiC substrates by using low-pressure metalorganic chemical vapor deposition method. Samples employing a three-period GaN/Al0.08Ga0.92N (100u2009A/100u2009A) as a buffer layer produce a good quality GaN epitaxial layer, with mobility and carrier concentration of 612u2009cm2/V⋅s and 1.3×1017u2009cm−3 (at 300 K), respectively. The enhanced electron mobility in the Al0.08Ga0.92N/GaN heterostructures is also observed. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility at 4.2 K for the Al0.08Ga0.92N/GaN heterostructure are 5.8×1012u2009cm−2 and 5300u2009cm2/V⋅s, respectively. Strong SdH (Shubnikov–de Haas) oscillations were observed to confirm the two-dimensional electron gas (2DEG) phenomenon at the AlGaN/GaN top heterointerface. In addition, an extra SdH oscillation also resulted from the high-quality 2DEG channel of the GaN/AlGaN bottom heterointerface.