Tyler A. Growden

Selective deuteron production using target normal sheath acceleration

We report on the first successful demonstration of selective deuteron acceleration by the target normal sheath acceleration mechanism in which the normally overwhelming proton and carbon ion contaminant signals are suppressed by orders of magnitude relative to the deuteron signal. The deuterium ions originated from a layer of heavy ice that was deposited on to the rear surface of a 500 nm thick membrane of Si3N4 and Al. Our data show that the measured spectrum of ions produced by heavy ice targets is comprised of ∼99% deuterium ions. With a laser pulse of approximately 0.5 J, 120 fs duration, and ∼5×1018Wcm-2 mean intensity, the maximum recorded deuterium ion energy and yield normal to the target rear surface were 3.5 MeV and 1.2×1012sr−1, respectively.

Boron Delta-Doping Dependence on Si/SiGe Resonant Interband Tunneling Diodes Grown by Chemical Vapor Deposition

Si/SiGe resonant interband tunnel diodes (RITD) were fabricated using CVD on 200-mm silicon wafers. The RITD devices consist of a p+-i-n+ structure with δ-doped quantum wells providing resonant interband tunneling through a nominally intrinsic Si/SiGe region. The vapor-phase doping technique was used to obtain abrupt degenerate doping profiles. The boron doping in the δ-doped region was varied, and its effect on peak current density Jp and peak-to-valley current ratio (PVCR) was studied. As the flow rate is reduced, Jp was found to reduce while the PVCR initially increases and then decreases. Device simulations were performed using the ATLAS simulator developed by SILVACO to interpret the results. A maximum PVCR of 2.95 was obtained, and the highest Jp recorded was 600 A/cm2. This is the highest reported PVCR for any CVD-grown Si/SiGe RITD.

Highly repeatable room temperature negative differential resistance in AlN/GaN resonant tunneling diodes grown by molecular beam epitaxy

AlN/GaN resonant tunneling diodes grown on low dislocation density semi-insulating bulk GaN substrates via plasma-assisted molecular-beam epitaxy are reported. The devices were fabricated using a six mask level, fully isolated process. Stable room temperature negative differential resistance (NDR) was observed across the entire sample. The NDR exhibited no hysteresis, background light sensitivity, or degradation of any kind after more than 1000 continuous up-and-down voltage sweeps. The sample exhibited a ∼90% yield of operational devices which routinely displayed an average peak current density of 2.7 kA/cm2 and a peak-to-valley current ratio of ≈1.15 across different sizes.

Methods for attaining high interband tunneling current in III-Nitrides

In conclusion, the authors have reported an increase in forward interband tunneling current density from 17.7 A/cm2 [2] to 39.8 kA/cm2 by applying outside AIGaN confmement barriers and 6-doping to a common structure. Some of the devices still exhibit hysteresis effects caused by traps, but some seem to display less of an effect, which needs to be studied further to provide stability. Optimization of the barrier thickness and Indium composition must also be performed to continue to push the peak current density up in value

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures

Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.0074% at 18.8 mA. In contrast to traditional GaN light emitters, p-type doping and p-contacts are completely avoided, and instead, holes are created in the GaN on the emitter side of the tunneling structure by direct interband (that is, Zener) tunneling from the valence band to the conduction band on the collector side. The Zener tunneling is enhanced by the high electric fields (~5 × 106 V cm−1) created by the notably large polarization-induced sheet charge at the interfaces between the AlN and GaN.

431 kA/cm2 peak tunneling current density in GaN/AlN resonant tunneling diodes

We report on the design and fabrication of high current density GaN/AlN double barrier resonant tunneling diodes grown via plasma assisted molecular-beam epitaxy on bulk GaN substrates. A quantum-transport solver was used to model and optimize designs with high levels of doping and ultra-thin AlN barriers. The devices displayed repeatable room temperature negative differential resistance with peak-to-valley current ratios ranging from 1.20 to 1.60. A maximum peak tunneling current density (Jp) of 431 kA/cm2 was observed. Cross-gap near-UV (370–385 nm) electroluminescence (EL) was observed above +6 V when holes, generated from a polarization induced Zener tunneling effect, recombine with electrons in the emitter region. Analysis of temperature dependent measurements, thermal resistance, and the measured EL spectra revealed the presence of severe self-heating effects.

AlN/GaN/AlN resonant tunneling diodes grown by rf-plasma assisted molecular beam epitaxy on freestanding GaN

The authors report the growth by rf-plasma assisted molecular beam epitaxy of AlN/GaN/AlN resonant tunneling diodes which exhibit stable, repeatable, and hysteresis-free negative differential resistance (NDR) at room temperature for more than 1000 bias sweeps between −2.5 and +5.5 V. The device layers were grown on freestanding, Ga-polar GaN substrates grown by hydride vapor phase epitaxy and having a density of threading dislocations between 106 and 107 cm−2. The authors speculate that the repeatable NDR is facilitated by the low-dislocation density substrates.

A Nonlinear Circuit Simulation of Switching Process in Resonant-Tunneling Diodes

A large-signal circuit model is used to compute the switching time for double-barrier resonant-tunneling diodes. The model consists of linear circuit elements plus a nonlinear I-V characteristic. The linear elements include a series resistor, a capacitor, and an inductor. The capacitance considers the charge accumulation, depletion in spacer layers, as well as charging-discharging of the quantum-well (QW) region. The inductance accounts for the delay of the current with respect to the voltage across the QW during the abrupt switching transition through the negative differential resistance region. A second-order Runge-Kutta method is used to solve for the switching transient, and then fit to experimental data for a high-quality InGaAs/AlAs resonant tunneling diode (RTD) using the QW inductance as a fitting parameter. Excellent agreement is found for the 10%-90% switching time with a calculated capacitance of 98 fF and a fitted inductance of 1300 pH. This large-signal inductance is approximately 7× greater than the small-signal inductance that has been successfully used to predict the fmax of RTDs such as the one tested here.

Experimental determination of quantum-well lifetime effect on large-signal resonant tunneling diode switching time

An experimental determination is presented of the effect the quantum-well lifetime has on a large-signal resonant tunneling diode (RTD) switching time. Traditional vertical In0.53Ga0.47As/AlAs RTDs were grown, fabricated, and characterized. The switching time was measured with a high-speed oscilloscope and found to be close to the sum of the calculated RC-limited 10%–90% switching time and the quantum-well quasibound-state lifetime. This method displays experimental evidence that the two intrinsic resonant-tunneling characteristic times act independently, and that the quasibound-state lifetime then serves as a quantum-limit on the large-signal speed of RTDs.