Jody Fronheiser

Silicon nanowire solar cells

Over the past decade, silicon nanowire solar cells have been intensively explored as potential platforms for the next-generation photovoltaic (PV) technologies with high power conversion efficiency and low production cost. This chapter discusses the details of the silicon nanowire solar cells in terms of their device structures, fabrication and characterization, electrical and optical properties benefited from the nanowire geometry. These benefits are not only expected to increase the power conversion efficiency, but also considered to reduce the requirement for the material quantity and quality, allowing for potential efficiency improvements and substantial cost reductions.

Strong broadband optical absorption in silicon nanowire films

The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found to be higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

Demonstration of ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes

We report ultraviolet separate absorption and multiplication 4H-SiC avalanche photodiodes. An external quantum efficiency of 83% (187 mA/W) at 278 nm, corresponding to unity gain after reach-through was achieved. A gain higher than 1000 was demonstrated without edge breakdown.

Solar-Blind 4H-SiC Single-Photon Avalanche Diode Operating in Geiger Mode

A solar blind 4H-SiC single photon avalanche diode (SPAD) with a sharp cutoff at a wavelength of 280 nm is reported. The SPAD with separate absorption and multiplication layers was designed for operation in Geiger mode. A thin film optical filter deposited on a sapphire window of the device package provided sensitivity in the wavelength range between 240 and 280 nm with a very high solar photon rejection ratio. An estimated dark current of 0.4 pA (0.75 nA/cm2) at a gain of 1000 was measured on a device with an effective mesa diameter of 260 mum. A single photon detection efficiency of 9.4% and a dark count probability of 4 times 10-4 were demonstrated at a wavelength of 266 nm for the same device.

Spin dependent charge pumping in SiC metal-oxide-semiconductor field-effect-transistors

We demonstrate a very powerful electrically detected magnetic resonance (EDMR) technique, spin dependent charge pumping (SDCP) and apply it to 4H SiC metal-oxide-semiconductor field-effect-transistors. SDCP combines a widely used electrical characterization tool with the most powerful analytical technique for providing atomic scale structure of point defects in electronic materials. SDCP offers a large improvement in sensitivity over the previously established EDMR technique called spin dependent recombination, offering higher sensitivity and accessing a wider energy range within the bandgap.

Comparison of 4H-SiC MOSFETs on (0001), (000-1) and (11-20) Oriented Substrates

The effect of using different orientations of 4H-SiC substrates on the performance of 4H-SiC MOSFETs has been evaluated. Three sets of samples with (0001), (000-1) and (11-20) oriented SiC substrates were used to fabricate the MOSFETs, with a gate oxide process consisting of a low- temperature deposited oxide followed by NO anneal at 1175°C for 2hrs. Various device parameters, particularly threshold voltage, subthreshold slope, field-effect mobility, inversion sheet carrier concentration and Hall mobility have been extracted. Temperature characterization up to 225°C was also performed.

Solar-Blind 4H-SiC Avalanche Photodiodes

In this work, solar-blind UV 4H-SiC avalanche photodetectors were fabricated and tested in linear and Geiger modes. APDs with both PIN and separate absorption and multiplication (SAM) structures were investigated. PIN structures demonstrated higher quantum efficiencies while the SAM structure exhibit lower leakage currents. Deposition of a thin film optical filter on top of the devices was used to provide a high photon rejection ratio of (Stas add value here). However, an external filter showed a better photon rejection ratio compared to the deposited one by about one order of magnitude.

Influence of Defects in 4H-SiC Avalanche Photodiodes on Geiger-Mode Dark Count Probability

4H-SiC single photon avalanche diodes are reported. A separate absorption and multiplication non-reach through device structure was optimized for operation in Geiger mode. An estimated dark current at a gain of 1000 was ranging between 0.4 pA (0.75 nA/cm2) and 20nA (38 A/cm2) on devices with an effective mesa diameter of 260 m. The electron beam induced current technique was used to image defects in the active region of studied devices. Increased reverse bias leakage current and increased Geiger mode dark count probability were correlated with the presence of large number of defects. Single photon detection efficiencies of up to 11% were measured at a wavelength of 266 nm in Geiger mode.

Frequency-Dependent Charge Pumping on 4H-SiC MOSFETs

The quality of the SiC/SiO2 interface is critical to the stability and performance of MOS-based SiC power devices. Charge pumping is a flexible interface characterization technique. In this work, a significant portion of the total traps are found to be located in the near-interface oxide using frequency-dependent charge pumping. Oxide trap tunneling mechanisms are discussed, and trap profile as a function of depth is calculated. The trap density is shown to increase exponentially as it gets closer to the interface.

Multiscale Modeling and Analysis of the Nitridation Effect of SiC/SiO2 Interface

We explain the role of nitrogen in simultaneously increasing the inversion channel mobility and reducing the threshold voltage of SiC MOSFET. A variety of computational techniques have been used to compute the atomic scale configuration of a nitridated SiC/SiO2 interface, and the corresponding change in Fermi level, inversion channel mobility, and threshold voltage. X-ray photoelectron spectroscopy (XPS) has been used to investigate the SiC/SiO2 interface to determine the nitrogen concentrations and chemical bonding. We elucidate the physics behind improved channel mobility due to NO anneal and demonstrate that the trade-off between threshold voltage and inversion channel mobility can be correlated to the extent of nitridation.