Showing papers by "Stacia Keller published in 2010"

Ultralow nonalloyed Ohmic contact resistance to self aligned N-polar GaN high electron mobility transistors by In(Ga)N regrowth

Abstract: Ultralow Ohmic contact resistance and a self-aligned device structure are necessary to reduce the effect of parasitic elements and obtain higher ft and fmax in high electron mobility transistors (HEMTs). N-polar (0001¯) GaN HEMTs, offer a natural advantage over Ga-polar HEMTs, in terms of contact resistance since the contact is not made through a high band gap material [Al(Ga)N]. In this work, we extend the advantage by making use of polarization induced three-dimensional electron-gas through regrowth of graded InGaN and thin InN cap in the contact regions by plasma (molecular beam epitaxy), to obtain an ultralow Ohmic contact resistance of 27 Ω μm to a GaN 2DEG.

Ion versus pH sensitivity of ungated AlGaN/GaN heterostructure-based devices

Abstract: We have investigated the pH and ion sensitivity of AlGaN/GaN heterostructure devices; these devices are sensitive to the ion concentration rather than to the pH of the solution. Sheet resistance as a function of pH for calibrated pH solutions and dilute NaOH, HCl, KOH, and NaCl showed an increase as a function of ionic concentration, regardless of whether the pH was acidic, basic, or neutral. An increase in resistance corresponds to accumulation of negative ions at the AlGaN surface, indicating device selectivity toward the negative ions. We attribute this to the formation of a double layer at the liquid/semiconductor interface.

Growth and characterization of In-polar and N-polar InAlN by metal organic chemical vapor deposition

Abstract: InxAl1−xN layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward ⟨101¯0⟩GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor.

Influence of AlN interlayer on the anisotropic electron mobility and the device characteristics of N-polar AlGaN/GaN metal-insulator-semiconductor-high electron mobility transistors grown on vicinal substrates

Abstract: This paper presents an experimental investigation of the influence of an AlN interlayer on the electron mobility and the device characteristics of N-polar AlGaN/GaN metal-insulator-semiconductor-high electron mobility transistors grown by metal-organic chemical vapor deposition on miscut sapphire substrates. Use of miscut substrates leads to the formation of multiatomic steps at the AlGaN/GaN interface and anisotropy in electron transport properties. A combination of van der Pauw Hall, gated transfer length measurements, and capacitance-voltage measurements has been used to study the desired properties in directions parallel and perpendicular to the multiatomic steps and qualitative explanations were provided for the observed trends. Similar to the Ga-polar devices, the introduction of AlN interlayer improved the device performance by increasing both the electron mobility and the two-dimensional electron gas charge density in the devices. Orienting the devices such that the conduction occurred parallel to the multiatomic steps was beneficial for better electron transport and device performance.

N-Polar InAlN/AlN/GaN MIS-HEMTs

Abstract: N-polar metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) were fabricated from a GaN/AlN/InAlN/GaN heterostructure grown by metalorganic chemical vapor deposition on a vicinal sapphire substrate, using Si3N4 as the gate insulator. Hall measurements in van der Pauw geometry on the heterostructure showed a sheet charge density and a mobility of 2.15 × 1013 cm-2 and 1135 cm2·V-1·s-1, respectively. Resistance measurements revealed anisotropic conductivity with respect to the surface steps induced by the substrate misorientation, and the sheet resistance of the 2-D electron gas was as low as 226 Ω/□ in the parallel direction. MIS-HEMTs with a gate length of 0.7 μm and a source-drain spacing of 2.2 μm had a peak drain current of 1.47 A/mm and an on-resistance of 1.45 Ω·mm. At a drain bias of 8 V, the current- and power-gain cutoff frequencies were 14 and 25 GHz, respectively.

Low Ohmic Contact Resistance m-Plane AlGaN/GaN Heterojunction Field-Effect Transistors with Enhancement-Mode Operations

Abstract: Low ohmic contact resistance m-plane AlGaN/GaN heterojunction field-effect transistors were demonstrated using a regrown n+-GaN contact layer. An ohmic contact resistance of 0.25 Ωmm was obtained with an 80-nm-thick Si-doped regrown GaN contact layer deposited by metal organic chemical vapor deposition. Enhancement-mode m-plane AlGaN/GaN heterojunction field-effect transistors were demonstrated with a threshold voltage of +1.4 V and 2.0 mΩcm2 on-state resistances at +5 V of gate–source voltage.

Lateral confinement of electrons in vicinal N-polar AlGaN/GaN heterostructure

Abstract: We studied orientation dependent transport in vicinal N-polar AlGaN/GaN heterostructures. We observed significant anisotropy in the current carrying charge parallel and perpendicular to the miscut direction. A quantitative estimate of the charge anisotropy was made based on gated transmission line measurement and Hall measurements. The formation of electrostatically confined one-dimensional channels is hypothesized to explain charge anisotropy. A mathematical model was used to verify that polarization charges distributed on miscut structure can create lateral one-dimensional confinement in vicinal substrates. This polarization-engineered electrostatic confinement observed is promising for new research on low-dimensional physics and devices besides providing a template for manufacturable one-dimensional devices.

N-Polar GaN/AlN MIS-HEMT for Ka-Band Power Applications

Abstract: In this letter, we demonstrate the millimeter-wave power performance from N-polar GaN-based metal-insulator-semiconductor high-electron-mobility transistors. The device consists of a GaN spacer structure with an AlN barrier to reduce the alloy scattering. High Si doping in GaN without excessive surface roughening has been achieved using a digital doping scheme with a low ohmic contact resistance of 0.16 Ω·mm. An fT and an fMAX of 56 and 130 GHz, respectively, were obtained for a 150-nm gate length. A peak output power of 1.9 W/mm with a power-added efficiency (PAE) of 14% was achieved for VDS = 20 V, and a peak output power of 2.2 W/mm with a 12% efficiency and a linear transducer power gain of 5.7 dB was achieved for VDS = 30 V at 30 GHz. The cause of the low PAE was determined to be due to the current collapse from the RF-IV measurements, and remedies have been suggested to minimize the dc-RF dispersion.

Very Low Ohmic Contact Resistance through an AlGaN Etch-Stop in Nitrogen-Polar GaN-Based High Electron Mobility Transistors

Abstract: Ultra-low ohmic contact resistance of 01 Ω mm has been obtained as a step towards a deep-recess structure using N-polar GaN-based high electron mobility transistors (HEMTs) An AlGaN etchstop layer was investigated to obtain smooth and reliable gate recess However due to reverse polarization, AlGaN results in a polarization-induced Schottky barrier which prevents ohmic contact to the channel through the etchstop In this work, we have proposed a novel methodology to contact the two-dimensional electron gas (2DEG) by etching through the GaN cap and the AlGaN etchstop to eliminate the barrier and angular-evaporation of metals to achieve side-alloying resulting in very low ohmic contact resistance of 01 Ω mm achieved to N-polar GaN 2DEG This result is state-of-the-art for alloyed contacts achieved to GaN-based 2DEG

T-gate technology for N-polar GaN-based self-aligned MIS-HEMTs with state-of-the-art f MAX of 127 GHz: Pathway towards scaling to 30nm GaN HEMTs

Abstract: N-polar GaN/AlGaN HEMTs have been of interest to the nitride community recently due to their several advantages over Ga-polar GaN-based HEMTs such as lower contact resistance [1], better electron confinement [2], and enhancement mode operation. For very high frequency performance, it is necessary to scale the parasitic elements in the device along with the gate length. First reports on scaled self-aligned HEMTs on N-face with regrown access regions [3] demonstrated minimization of access resistances which resulted in linear scaling of total delay up to 120 nm gate length (Fig. 1). However, even though excellent f T .L g products of 16.8 GHz-µm were achieved, very low f MAX of 17 GHz was achieved due to high resistivity of the W-gates in the gate-first process.