Vincent Meyers

Characterization of the optical properties of GaN:Fe for high voltage photoconductive switch applications

The optical properties of bulk semi-insulating GaN:Fe are obtained to assess its future suitability as a high voltage photoconductive semiconductor switch (PCSS). The material properties of GaN:Fe hold significant promise to improve devices for pulsed power and other applications. Growth techniques of bulk GaN:Fe, which have hitherto been largely insufficient for commercial applications, are nearing the point that anticipatory characterization research is warranted. In this paper, the optical constants of bulk GaN:Fe (refractive index, absorption coefficient, and off-state dielectric function) were determined by optical reflection/transmission analysis. The results of this analysis are compared with a similar treatment of bulk 4H-SiC as well as possible elements of PCSS housing: Sylgard 184 elastomer, and EFI 20003/50013 electrical potting epoxy. The data presented provide foundational material characterization to enable assessment of the feasibility of GaN:Fe as a practical high voltage PCSS material. Beyond basic materials research, these properties inform design optimization in PCSS construction and implementation.

Nonlinear UV absorption properties of bulk 4H-SiC

The intensity-dependent light absorption in bulk high-purity semi-insulating 4H-SiC at above band gap photon energies has been studied. In particular, 3.49 eV (355 nm) UV absorption of 160 μm-thick samples of varying recombination lifetimes in the intensity range of 1 mJ/cm2–30 mJ/cm2 is addressed. The effective absorption coefficient was found to vary up to 30% within this range. Assuming deep level trapping, interband absorption, and free carrier absorption as dominant processes, a four energy level model reproduces the experimentally observed absorption behavior. While nonlinearities in the optical absorption behavior of SiC have been studied previously as function of wavelength α(λ), temperature α(T) and, to a very limited extent, at below bandgap optical intensities, the presented elucidates the UV intensity-dependent nonlinear absorption behavior, α(I), of SiC at above bandgap photon energies.

Evaluation of GaN:Fe as a high voltage photoconductive semiconductor switch for pulsed power applications

Semi-insulating Gallium Nitride is evaluated as a candidate material for use as a high voltage photoconductive semiconductor switch (PCSS) for pulsed power applications. The GaN:Fe samples used for this investigation were commercially available, bulk, semi-insulating samples measuring 10 mm × 10 mm × 475 μm. Their optical and crystallographic properties were determined utilizing cathodoluminesence, photoluminescence, RHEED, as well as microwave reflection techniques for carrier lifetime studies. Experimental results are presented elucidating the potential of GaN:Fe sustaining high potential differences in both lateral and vertical geometry devices. For instance, electric field hold-off exceeding 100 kV/cm was observed in lateral geometry with mm sized gaps. In addition, a process for the homo-epitaxial growth of GaN:Si was developed in order to facilitate the fabrication of high quality ohmic contacts. Lastly, experimental results evaluating the on-state performance and photo-current efficiency of a GaN:Fe based PCSS are presented.

Pulsed characterization of a UV LED for pulsed power applications on a silicon carbide photoconductive semiconductor switch

Summary form only given. The electrical and optical characteristics of a high power UV LED (365 nm wavelength) were evaluated under pulsed operating conditions for pulsed power applications. Measurements were made over varying pulse width (30 ns-100 μs), current (0 A-250 A), repetition rate (single shot -1 MHz), and temperature (23° C-80° C). Diagnostics used included a calibrated photodiode operating in the linear regime for transient optical power measurements, a grating / high speed ICCD based spectrograph for transient spectral analysis, and multiple 10:1 standard oscilloscope probes configured differentially for electrical measurements. A red shift was observed in the output spectrum of the LED with increasing temperature and increasing pulse-width. LED forward voltage was observed to increase linearly with increasing current (≈ 3.5 V-5.2 V) and decrease with increasing pulse-width. The peak optical power observed was > 13 W and a maximum efficiency of 22 % was observed. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a SiC photoconductive semiconductor switch (PCSS) under high impedance conditions.

Laser ablation on lithium-ion battery electrode solid electrolyte interface removal

The formation of a solid electrolyte interface (SEI) on a Li-ion battery electrode during usage cycling is a critical reason for battery capacity loss. In this paper, laser ablation technology is applied to remove SEI from a graphite electrode surface. Characterization methods including scanning electron microscopy, Raman microscopy, and Fourier transform infrared spectroscopy are used to study the structure and morphology changes of the SEI on the electrode surface. The results show that laser ablation can successfully remove the SEI, indicating a feasible method to clean the electrode surface.

Optical nonlinear absorption characterization of bulk semi-insulating 4H-SIC at and above the band edge

Nonlinearity of optical absorption in semi-insulating bulk 4H-SiC has been investigated. Of interest was the optical bleaching behavior of 4H-SiC at and just above the band edge in the range 3.11-3.33 eV (wavelength 380-355 nm). Results of experiments on 200 μm and 490 μm thickness samples indicate partial bleaching in the optical fluence range from 70 W/cm2 to 1.8 kW/cm2, and the absorption coefficient was found to vary by approximately 10% within this range. These experimental findings are supported by simulation results obtained from a first order semi-empirical rate based model linking excitation-induced change in density of states with the absorption coefficient over the range of tested power densities. As expected, this effect scales with photon energy. Characterization of 4H-SiC absorption behavior under varying fluence will aid in design optimization of a Photoconductive Semiconductor Switch (PCSS).

The path to a transportable ionospheric heater

A transportable ionospheric heater, TIH, research design is presented that will enable plasma studies of the ionosphere in latitudes that are presently inaccessible by fixed installations such as HAARP (High Frequency Active Auroral Research Program). The equatorial latitude with close to zero vertical magnetic fields is especially of interest for basic plasma physics studies as well as rf communication enhancement. To achieve a power level in the ionosphere of at least 70 dBW ERP in a footprint significantly smaller than HAARP the radiated power needs to be substantially increased. This minimum ERP is achievable in a 4 × 4 antenna array with 370 kW input power per element with about 25 m by 25 m footprint vs. HAARPs equivalent 365 m by 365 m (360 antenna elements total, 10 kW maximum per antenna). Maximum ERP, up to 95 dBW, may be achieved with the TIH on a 115 m by 70 m platform, a factor 17 reduced size from HAARP. Tunable, Electrically Small Antennas, ESAs are employed to overcome the maximum power limitations of the HAARP dipole based antennas. This demands a step-up from 10 kW to several 100 kW cw power in the 3 to 10 MHz band, which is required to effectively heat the ionosphere. Driving the ESAs necessitates a tunable rf source in the same power and frequency regime, where a more traditional rf tube or all solid state approach may be pursued. The focus of the driver related research has been on photoconductive solid state switching, PCSS, in a direct drive mode that incorporates the driver into the antenna itself. A full size ESA operating at 9.5 to 10 MHz has been demonstrated at 500 W cw power levels and ~ 90% efficiency, driven by a single SiC switch mimicking the full power PCSS operation. The challenges and physics limitations of scaling the switch, the tunable ESA antenna design, as well as their coupling are presented. The significant progress made towards a transportable ionospheric heater as it relates to the physics of the PCSS switching efficiency, electrical breakdown in the MHz regime in large gaps, lower power experiments, and numerical simulations is discussed.

Limitations of bulk SiC photoconductive semiconductor switches, and evaluation of PIN SiC photoconductive semiconductor switches

A comprehensive picture of the relationship between optical fluence, optical wavelength, system load, and photocurrent efficiency (PE) in SiC photoconductive semiconductor switches (PCSSs) is presented. Variation of the optical wavelength (300-380 nm) and optical fluence (0.2-200 J m-2) was accomplished with a Nd:YAG pumped optical parametric oscillator (7 ns FWHM) and a broadband variable attenuator. The PE was found to typically be in the range of 1-2 %, depending on wavelength, and the bulk PCSS on-state voltage driven by external circuit parameters. Features of the high electric field stress behavior (> 200 kV/cm) of the bulk PCSS were captured with high fidelity in a 1D drift-diffusion model with a self-consistent Poisson solver including trap assisted tunneling, Poole-Frenkel, and barrier lowering with enhanced tunneling effects. In addition, trap to band impact ionization as well as Coulombic and repulsive trapping potentials were included.