Amir M. Dabiran

MgZnO/AlGaN heterostructure light-emitting diodes

We report on p–n junction light-emitting diodes fabricated from MgZnO∕ZnO∕AlGaN∕GaN triple heterostructures. Energy band diagrams of the light-emitting diode structure incorporating piezoelectric and spontaneous polarization fields were simulated, revealing a strong hole confinement near the n‐ZnO∕p‐AlGaN interface with a hole sheet density as large as 1.82×1013cm−2 for strained structures. The measured current–voltage (IV) characteristics of the triple heterostructure p–n junctions have rectifying characteristics with a turn-on voltage of ∼3.2V. Electron-beam-induced current measurements confirmed the presence of a p–n junction located at the n‐ZnO∕p‐AlGaN interface. Strong optical emission was observed at ∼390nm as expected for excitonic optical transitions in these structures. Experimental spectral dependence of the photocurrent confirmed the excitonic origin of the optical transition at 390nm. Light emission was measured up to 650K, providing additional confirmation of the excitonic nature of the opti...

Electrical detection of immobilized proteins with ungated AlGaN∕GaN high-electron-mobility Transistors

Ungated AlGaN∕GaN high-electron-mobility transistor (HEMT) structures were functionalized in the gate region with aminopropyl silane. This serves as a binding layer to the AlGaN surface for attachment of fluorescent biological probes. Fluorescence microscopy shows that the chemical treatment creates sites for specific absorption of probes. Biotin was then added to the functionalized surface to bind with high affinity to streptavidin proteins. The HEMT drain-source current showed a clear decrease of 4μA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing.

dc and rf performance of proton-irradiated AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) with a range of gate lengths (0.8–1.2 μm) and widths (100–200 μm) were exposed to 40 MeV protons at fluences of 5×109 or 5×1010 cm−2. The drain–source currents in the devices decreased by 15%–20% at the higher fluence, while the extrinsic transconductance decreased by ∼30% under the same conditions. Based on the increases in the reverse breakdown voltage and the channel resistance, the main degradation mechanism is believed to be creation of deep trap states in the band gap which remove electrons from the channel. The maximum frequency of oscillation, fMAX, also decreased as a result of the proton-induced damage, with a change of −20% at the shorter gate widths and −50% at the largest widths. The reverse recovery switching time was essentially unaffected by the irradiation, remaining at ∼1.6×10−8 s. Postradiation annealing at 800 °C was successful in restoring the dc and rf performance parameters to ⩾90% of their original values. The AlGaN/GaN HEMTs are...

Very high channel conductivity in low-defect AlN/GaN high electron mobility transistor structures

Low defect AlN/GaN high electron mobility transistor (HEMT) structures, with very high values of electron mobility (>1800 cm2/V s) and sheet charge density (>3×1013 cm−2), were grown by rf plasma-assisted molecular beam epitaxy (MBE) on sapphire and SiC, resulting in sheet resistivity values down to ∼100 Ω/◻ at room temperature. Fabricated 1.2 μm gate devices showed excellent current-voltage characteristics, including a zero gate saturation current density of ∼1.3 A/mm and a peak transconductance of ∼260 mS/mm. Here, an all MBE growth of optimized AlN/GaN HEMT structures plus the results of thin-film characterizations and device measurements are presented.

Band gap properties of Zn1- xCdxO alloys grown by molecular-beam epitaxy

Optical absorption and reflectance measurements are performed to evaluate compositional and temperature dependences of band gap energies of Zn1−xCdxO alloys grown by molecular-beam epitaxy. The compositional dependence of the band gap energy, determined by taking into account excitonic contributions, is shown to follow the trend Eg(x) = 3.37−2.82x+0.95x2. Incorporation of Cd was also shown to somewhat slow down thermal variation of the band gap energies, beneficial for future device applications.

Influence of 60Co γ-rays on dc performance of AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) with different gate length and widths were irradiated with 60Co γ-rays to doses up to 600 Mrad. Little measurable change in dc performance of the devices was observed for doses lower than 300 Mrad. At the maximum dose employed, the forward gate current was significantly decreased, with an accompanying increase in reverse breakdown voltage. This is consistent with a decrease in effective carrier density in the channel as a result of the introduction of deep electron trapping states. The threshold voltage shifted to more negative voltages as a result of the irradiation, while the magnitude of the drain–source current was relatively unaffected. This is consistent with a strong increase of trap density in the material. The magnitude of the decrease in transconductance of the AlGaN/GaN HEMTs is roughly comparable to the decrease in dc current gain observed in InGaP/GaAs heterojunction bipolar transistors irradiated under similar conditions.

Development of enhancement mode AlN/GaN high electron mobility transistors

Enhancement mode AlN/GaN high electron mobility transistors (HEMTs) were fabricated from originally depletion-mode structures using oxygen plasma treatment on the gate area prior to the gate metallization. Starting with a depletion mode AlN/GaN HEMT, the threshold voltage of the HEMT could be shifted from −3.2 to 1 V depending on the oxygen plasma treatment time to partially convert the AlN barrier layer into Al oxide. The gate current was reduced and the current-voltage curves show metal-oxide semiconductor diodelike characteristics after oxygen plasma treatment.

Robust detection of hydrogen using differential AlGaN∕GaN high electron mobility transistor sensing diodes

The use of AlGaN∕GaN high electron mobility transistor (HEMT) differential sensing diodes is shown to provide robust detection of 1% H2 in air at 25°C. The active device in the differential pair is coated with 10nm of Pt to enhance catalytic dissociation of molecular hydrogen, while the reference diode is coated with Ti∕Au. The active diode in the pair shows an increase in forward current of several milliamperes at a bias voltage of 2.5V when exposed to 1% H2 in air. The HEMT diodes show a response approximately twice that of GaN Schottky diodes, due to the presence of piezoelectric and spontaneous polarization in the heterostructure. The use of the differential pair removes false alarms due to ambient temperature variations.

Deep traps responsible for hysteresis in capacitance-voltage characteristics of AlGaN∕GaN heterostructure transistors

The origin of hysteresis in capacitance-voltage (C-V) characteristics was studied for Schottky diodes prepared on AlGaN∕GaN transistor structures with GaN (Fe) buffers. The application of reverse bias leads to a shift of C-V curves toward higher positive voltages. The magnitude of the effect is shown to increase for lower temperatures. The phenomenon is attributed to tunneling of electrons from the Schottky gate to localized states in the structure. A technique labeled “reverse” deep level transient spectroscopy was used to show that the deep traps responsible for the hysteresis have activation energies of 0.25, 0.6, and 0.9eV. Comparison with deep trap spectra of GaN buffers and Si doped n-GaN films prepared on GaN buffers suggests that the traps in question are located in the buffer layer.

Hybrid CdZnO/GaN quantum-well light emitting diodes

We report on the demonstration of light emission from hybrid CdZnO quantum-well light emitting diodes. A one-dimensional drift-diffusion method was used to model the expected band structure and carrier injection in the device, demonstrating the potential for 90% internal quantum efficiency when a CdZnO quantum well is used. Fabricated devices produced visible electroluminescence that was found to redshift from 3.32 to 3.15 eV as the forward current was increased from 20 to 40 mA. A further increase in the forward current to 50 mA resulted in a saturation of the redshift.