Engineering topics
Vadiee, Ehsan
Publications and source records attributed to Vadiee, Ehsan.
The role of Mg bulk hyper-doping and delta-doping in low-resistance GaN homojunction tunnel diodes with negative differential resistance
GaN p ++ /n ++ tunnel junctions (TJs) with heavy bulk or delta Mg doping at the junction were grown via molecular beam epitaxy with a hysteresis-free and repeatable negative differential resistance (NDR). The TJ with Mg doping of 5.5 × 10 20 cm -3 shows NDR at~1.8 V and a large current density of 3.4 KA/cm 2 at -1.0 V. Atomic resolution scanning transmission electron microscopy imaging showed no additional defects despite the doping exceeding the solubility limit in GaN allowing subsequent epitaxy of series-connected layers and devices. GaN homojunction TJs grown on bulk GaN showed an improved current density and NDR stability. In addition, the effect of Mg delta doping at the junction was investigated for the first time showing a dramatic improvement in the tunneling characteristics. A metal-organic chemical vapor deposition (MOCVD) grown InGaN light-emitting diode (LED) with an MBE grown GaN homojunction tunnel contact to the MOCVD grown p-GaN layer shows superior lateral conductivity and improved luminescence uniformity, but suffers an added voltage penalty, assumed to be due to interface impurities, compared to control LED with indium-tin-oxide.
Ultra-wide-bandgap AlGaN homojunction tunnel diodes with negative differential resistance
The power efficiencies of state-of-the-art Al x Ga 1-x N deep-ultraviolet (UV) emitters operating in the <300 nm wavelength region are currently limited to a few percent in part due to limitations in the series and contact resistance which result in excessive drive voltages. Al x Ga 1-x N tunnel contacts and tunnel junctions in deep-UV devices are a promising route toward increasing these efficiencies by improving the contact resistances, hole injection, and reducing optical absorption by removing undesirable p-GaN contact layers. However, due to doping inefficiencies, standalone tunnel diodes have not been realized in the form of homojunction Al x Ga 1-x N. In this work, Al x Ga 1-x N (0.19 ≤ x ≤ 0.58) homojunction tunnel diodes are fabricated with high reverse bias current densities, and one device with x = 0.19 demonstrates a negative differential resistance at ~2.4 V. Al x Ga 1-x N p++/n++/n tunnel diodes are compared to reference p++/i/n diodes to provide clarity about the role of tunneling conduction vs leakage conduction. Transmission electron microscopy verifies that heavy doping does not result in visible defects such as Mg precipitates and allows for subsequent epitaxy, critical for buried tunnel junction structures. Increasing the bandgap energy of Al x Ga 1-x N for higher Al content tunnel junctions decreases the tunnel current, but still allows sufficient conduction necessary for future improvements in deep UV emitter efficiencies.
Observation and mitigation of RF-plasma-induced damage to III-nitrides grown by molecular beam epitaxy
In this work, radio-frequency (RF) plasma-induced damage to III-nitride surfaces and bulk defects is observed and mitigated. It is shown that for InN films, the surface is more sensitive to plasma-induced damage than GaN films, as observed via atomic force microscopy and reflection high energy electron diffraction. In order to isolate any possible plasma-induced damage, a growth window for InN is established, and temperature ranges are determined for other damaging effects which include roughening due to low adatom mobility, InN decomposition, and indium desorption. In situ plasma monitoring and optimization are accomplished with a combination of optical emission spectroscopy as well as a remote Langmuir probe. It is shown that by increasing the plasma nitrogen flow, the positive ion content increases; however, the ion acceleration potential reduces. Additionally, a reduced RF plasma power results in a reduction of atomic nitrogen species. These plasma species and energetic variations result in variations in the bulk unintentional background electron concentrations observed by room temperature Hall effect measurements of ~1 um thick InN films. By increasing the nitrogen flow from 2.5 to 7.5 sccm for a constant RF power of 350 W, the background electron concentration decreases by 74% from 1.36 x 1019 cm-3 to 3.54 x 1018 cm-3, while maintaining a smooth surface morphology. Additionally, photoluminescence spectra indicate optical emission energies shift from ~0.81 to 0.71 eV (closer to the fundamental bandgap of InN) by limiting the damaging plasma species. Finally, conditions are presented to further minimize plasma-induced damage in III-nitride devices.
Negative differential resistance in GaN homojunction tunnel diodes and low voltage loss tunnel contacts
The current-voltage characteristics and metastability in GaN p++/n++ homojunction tunnel diodes and n++/p++/i/n tunnel-contacted diodes grown via metal modulated epitaxy have been reported on. The room temperature negative differential resistance (NDR) beginning at ~1.35 V is discussed for GaN homojunction devices grown on sapphire. The NDR vanishes, and the conductivity increases as multiple I-V sweeps are performed, thus implying that charge trapping states with long trap lifetimes exist at defect sites, and these traps play a crucial role in the tunneling mechanism. Additionally, the use of extremely high n-type (N D ~ 4.6×10 20 cm –3 ) and p-type (N A ~ 7.7×10 20 cm –3 ) doping results in a near linear characteristic with minimal rectification at current densities less than 200 A/cm 2 and soft rectification above this current density. Forward-bias tunneling and NDR are still present at 77 K. The highest silicon-doped n++/p++/i/n tunnel-contacted pin diode demonstrates a turn-on voltage of 3.12 V, only 0.14 V higher than that of the pin control diode, and an improved specific on-resistance of 3.24 ×10 –4 Ω cm 2 , which is 13% lower than that of the control pin diode.
Carrier-selective contact GaP/Si solar cells grown by molecular beam epitaxy
Abstract
Developing High Performance GaP/Si Heterojunction Solar Cells
Not provided.