K. H. Baik

Pressure-induced changes in the conductivity of AlGaN∕GaN high-electron mobility-transistor membranes

AlGaN∕GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source∕drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN∕GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of −(+)6.4×10−2mS∕bar for application of compressive (tensile) strain. The AlGaN∕GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.

Temperature-dependent characteristics of Pt Schottky contacts on n-type ZnO

The Schottky barrier height of Pt contacts on n-type (n∼1016u2009cm−3) thin film ZnO deposited by pulsed laser deposition was obtained from current–voltage measurements as a function of temperature. The resulting values ranged from 0.61±0.04u2009eV at 25u200a°C to 0.46±0.06u2009eV at 100u200a°C with saturation current densities of 1.5×10−4u2009Au200acm−2 (25u200a°C) to 6.0×10−2u2009Au200acm−2 (100u200a°C), respectively. The reverse current magnitude was larger than predicted by thermionic emission alone. The measured barrier height for Pt on ZnO is similar to the value reported for both Au and Ag rectifying contacts on this material.

Carrier concentration dependence of Ti/Al/Pt/Au contact resistance on n-type ZnO

Ti/Al/Pt/Au ohmic contacts on n-type ZnO with a range of carrier concentrations (7.5×1015–1.5×1020 cm−3) show as-deposited specific contact resistances in the range from 3×10−4 to 8×10−7u2002Ωu200acm2. Temperature-dependent measurements showed that the dominant transport mechanisms were tunneling in the contacts in the most highly doped films and thermionic emission in the more lightly doped films. After annealing at 200u200a°C, the lowest specific contact resistance achieved was 2.2×10−8u2002Ωu200acm2. However, the contacts show evidence of reactions between the Ti and the ZnO film even for this low annealing temperature, suggesting that applications requiring good thermal stability will need metallurgy with better thermal stability.

Effect of external strain on the conductivity of AlGaN/GaN high-electron-mobility transistors

The changes in conductance of the channel of AlGaN/GaN high electron mobility transistor structures during application of both tensile and compressive strain were measured. For fixed Al mole fraction, the changes in conductance were roughly linear over the range up to 2.7x 10 8 N.cm -2 , with coefficients for planar devices of -6.0 +/- 2.5 x 10 -10 S.N -1 .m -2 for tensile strain and +9.5+/-3.5 x10 -10 S.N -1 .m -2 for compressive strain .For mesa-isolated structures, the coefficients were smaller due to the reduced effect of the AlGaN strain, with values of 5.5 +/- 1.1 x10 -13 S.N -1 .m -2 for tensile strain and 4.8 x10 -13 S.N -1 .m -2 for compressive strain. The large changes in conductance demonstrate that simple AlGaN/GaN heterostructures are promising for pressure and strain sensor applications.

Improved Pt/Au and W/Pt/Au Schottky contacts on n-type ZnO using ozone cleaning

UV-ozone cleaning prior to metal deposition of either e-beam Pt contacts or sputtered W contacts on n-type single-crystal ZnO is found to significantly improve their rectifying characteristics. Pt contacts deposited directly on the as-received ZnO surface are Ohmic but show rectifying behavior with ozone cleaning. The Schottky barrier height of these Pt contacts was 0.70eV, with ideality factor of 1.5 and a saturation current density of 6.2×10−6Acm−2. In contrast, the as-deposited W contacts are Ohmic, independent of the use of ozone cleaning. Postdeposition annealing at 700°C produces rectifying behavior with Schottky barrier heights of 0.45eV for control samples and 0.49eV for those cleaned with ozone exposure. The improvement in rectifying properties of both the Pt and W contacts is related to removal of surface carbon contamination from the ZnO.

Effect of high-density plasma etching on the optical properties and surface stoichiometry of ZnO

Bulk, single-crystal ZnO was etched in Cl2/Ar and CH4/H2/Ar inductively coupled plasmas as a function of ion impact energy. For CH4/H2/Ar, the etch rate (R) increases with ion energy (E) as predicted from a model of ion enhanced sputtering by a collision–cascade process, R∝(E0.5−ETH0.5), where the threshold energy, ETH, is ∼96u2009eV. Band edge photoluminescence intensity decreases with incident ion energy in both chemistries, with a 70% decrease even for low energies (∼116u2009eV). Surface roughness is also a function of ion energy with a minimum at ∼250u2009eV, where Auger electron spectroscopy shows there is no measurable change in near-surface stoichiometry from that of unetched control samples.

Etch characteristics of HfO2 films on Si substrates

Abstract The etch rates and mechanisms for HfO2 thin films in Cl2-, SF6- or CH4/H2-based plasmas were measured as a function of source power, r.f. chuck power and discharge composition. Both Cl2- and SF6-based plasmas produced some degree of chemical enhancement in the etch mechanism. Selectivities between 0.2 and 5 were obtained for Si over HfO2 in these two plasma chemistries. High fidelity pattern transfer was achieved for photoresist-masked HfO2/Si structures etched with Cl2/Ar over a broad range of pressures or with SF6/Ar at low pressures. The surface morphologies of both HfO2 and Si were smooth over a wide range of etching conditions.

Fabrication of Hybrid n-ZnMgO/n-ZnO/p-AlGaN/p-GaN Light-Emitting Diodes

We report on the fabrication of UV light-emitting diodes (LEDs) based on a p–n junction n-ZnMgO/n-ZnO/ p-AlGaN/ p-GaN semiconductor triple-heterostructure. Radio-frequency plasma-assisted molecular beam epitaxy was used to grow the complete heterostructure on p-AlGaN/ p-GaN c-plane sapphire templates. Cross-sectional transmission electron microscopy showed single-crystal quality of the pseudomorphically grown ZnO active region of the device. The LEDs were fabricated by a process involving both wet and dry etching. Electroluminescence emission most likely associated with ZnO excitonic transition was observed up to 370°C. The results show the potential of ZnO-based materials for UV emitters of potentially lower cost and with comparable or higher emission intensity than comparable AlGaN/GaN devices.

Capacitance pressure sensor based on GaN high-electron-mobility transistor-on-Si membrane

The changes in the capacitance of the channel of an AlGaN∕GaN high-electron-mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The capacitance of the channel displays a change of 7.19±0.45×10−3pF∕μm as a function of the radius of the membrane at a fixed pressure of +9.5bar and exhibits a linear characteristic response between −0.5 and +1bar with a sensitivity of 0.86pF∕bar for a 600μm radius membrane. The hysteresis was 0.4% in the linear range. These AlGaN∕GaN HEMT membrane-based sensors appear to be promising for both room-temperature and elevated-temperature pressure-sensing applications.

Temperature dependence of forward current characteristics of GaN junction and Schottky rectifiers

Abstract A comparison was made of the forward current–voltage characteristics of bulk GaN Schottky and p–n junction rectifiers using a quasi-three-dimensional simulator. The model includes incomplete ionization of the deep Mg acceptor (175 meV) in the p+-GaN side of the junction and the temperature dependence of mobility and GaN band gap. The forward turn-on voltages, VF, decrease with increasing temperature for both types of rectifier and are ∼2.5 V at 100 Axa0cm−2 at 573 K for the junction diodes and ⩽1 V under similar conditions for the Schottky diodes. The effect of p-layer thickness and doping in the p–n junction was also investigated.