Chad S. Gallinat

Solar-blind AlxGa1−xN/AlN/SiC photodiodes with a polarization-induced electron filter

Heterogeneous n-III-nitride/i-p silicon carbide (SiC) photodetectors have been demonstrated that enable the tailoring of the spectral response in the solar blind region below 280 nm. The negative polarization induced charge at the aluminum gallium nitride (AlxGa1−xN)/aluminum nitride (AlN) interface in conjunction with the positive polarization charge at the AlN/SiC interface creates a large barrier to carrier transport across the interface that results in the selective collection of electrons photoexcited to the Γ and L valleys of SiC while blocking the transport of electrons generated in the M valley. In addition, the AlxGa1−xN alloys act as transparent windows that enhance the collection of carriers generated by high energy photons in the fully depleted SiC absorption regions. These two factors combine to create a peak external quantum efficiency of 76% at 242 nm, along with a strong suppression of the long-wavelength response from 260 nm to 380 nm.Heterogeneous n-III-nitride/i-p silicon carbide (SiC) photodetectors have been demonstrated that enable the tailoring of the spectral response in the solar blind region below 280 nm. The negative polarization induced charge at the aluminum gallium nitride (AlxGa1−xN)/aluminum nitride (AlN) interface in conjunction with the positive polarization charge at the AlN/SiC interface creates a large barrier to carrier transport across the interface that results in the selective collection of electrons photoexcited to the Γ and L valleys of SiC while blocking the transport of electrons generated in the M valley. In addition, the AlxGa1−xN alloys act as transparent windows that enhance the collection of carriers generated by high energy photons in the fully depleted SiC absorption regions. These two factors combine to create a peak external quantum efficiency of 76% at 242 nm, along with a strong suppression of the long-wavelength response from 260 nm to 380 nm.

THz emission from a-plane InGaN

Terahertz emission from high stacking fault density a-plane InGaN utilizing in-plane drift fields is shown to produce considerable improvement over c-plane InGaN under identical excitation conditions.

Enhanced THz emission from c-plane InxGa1−xN due to piezoelectric field-induced electron transport

Enhanced terahertz emission from coherently strained InxGa1−xN epilayers on GaN is observed, which exceeds or is comparable to bulk InAs emission at pump wavelengths of 400 nm or 800 nm, respectively. The inverted terahertz waveform from the InxGa1−xN/GaN heterostructure indicates that the dominant terahertz generation mechanism is electron acceleration toward the InxGa1−xN surface in an internal electric field primarily associated with piezoelectric polarization charge at the heterointerface, rather than diffusive transport away from the surface typically observed in bulk semiconductors. The persistence of the inverted waveform for 266 nm excitation provides evidence of ultrafast electron relaxation via LO phonon emission.

Impact of hetero-interface on the photoresponse of GAN/SIC separate absorption and multiplication avalanche photodiodes

III-Nitride/SIC separate absorption and multiplication avalanche photodiodes (SAM APDs) provide a new approach for realizing high sensitivity, high gain and low dark current detectors with a response that is tunable over a wide spectral range. However, the heterojunction interface plays a significant role in the performance of these detectors due to presence of 1) interface charge arising from the difference in polarization between III-Nitride and SiC as well as 2) defects at the interface resulting from lattice mismatch. In this paper we discuss the growth and fabrication of GaN/SiC SAM APDs and analyze the behavior in the context of these two factors.

Effects of strain relaxation on the photoluminescence of semipolar InGaN

We present the effects of partial strain relaxation on the optical properties in lattice mismatched semipolar (1122) InGaN using polarization-dependent photoluminescence measurements to probe the strain dependent band mixing of the valence bands.

Polarization enhanced carrier transport in a p-down n-GaN/i-InGaN/p-GaN solar cell structure

Evidence of a strong electric field aiding carrier collection is observed in an n-GaN/i-InGaN/p-GaN inverted polarity solar cell structure, detected by pump-probe electroabsorption and THz spectroscopy.

III-nitride/SiC avalanche photodetectors for enabling compact biological agent identification and detection

The development of low cost and compact biological agent identification and detection systems, which can be employed in place-and-forget applications or on unmanned vehicles, is constrained by the photodetector currently available. The commonly used photomultiplier tube has significant disadvantages that include high cost, fragility, high voltage operation and poor quantum efficiency in the deep ultraviolet (240-260nm) necessary for methods such as fluorescence-free Raman spectroscopy. A III-Nitride/ SiC separate absorption and multiplication avalanche photodiode (SAM-APD) offers a novel approach for fabricating high gain photodetectors with tunable absorption over a wide spectrum from the visible to deep ultraviolet. However, unlike conventional heterojunction SAM APDs, the performance of these devices are affected by the presence of defects and polarization induced charge at the heterointerface arising from the lattice mismatch and difference in spontaneous polarization between the GaN absorption and the SiC multiplication regions. In this paper we report on the role of defect density and interface charge on the performance of GaN/SiC SAM APDs through simulations of the electric field profile within this device structure and experimental results on fabricated APDs. These devices exhibit a low dark current below 0.1 nA before avalanche breakdown and high avalanche gain in excess of 1000 with active areas 25x larger than that of state of the art GaN APDs. A responsivity of 4 A/W was measured at 365 nm when biased near avalanche breakdown.

III-V nitride semiconductors for solar hydrogen production

Photoelectrochemical cells are devices that can convert solar radiation to hydrogen gas through a water decomposition process. In this process, energy is converted from incident photons to the bonds of the generated H2 molecules. The solar radiation absorption, electron-hole pair splitting, and photoelectrolysis half reactions all occur in the vicinity of the electrode-electrolyte interface. As a result, engineering the electrode material and its interaction with the electrolyte is important in investigating and improving the energy conversion process in these devices. III-V nitride materials are promising candidates for photoelectrochemical energy applications. We demonstrate solar-to-hydrogen conversion in these cells using p-type GaN and n-type InGaN as a photocathode and photoanode material, respectively. Additionally, we demonstrate heteroepitaxial MOCVD growth of GaP on Si, enabling future work in developing GaPN as a photocathode material.

Improved THz emission in c-plane InGaN due to polarization charges at the InGaN/GaN interface

There has been great progress recently in the intensity of broadband Terahertz (THz) signals utilizing surface and field effects from semiconductor materials. Typically, the Photo-Dember effect combined with optical rectification in such materials as GaAs or InAs generates the most intense broadband THz radiation from contactless THz emitters [1]. There still exists room for improvement in terms of THz emitter intensities, and several novel structures have been proposed and fabricated which include the use of large internal electric fields in polar and non-polar semiconductors as well as multiple quantum well structures, all of which show enhanced THz emission over prior materials [2–4]. Nitride semiconductors, which have both a piezoelectric and spontaneous polarization due to its wurtzite crystal structure, can have an order of magnitude larger internal electric field in e.g. InGaN/GaN MQWs than previous THz emitters utilizing piezoelectric fields to generate drift currents such as in InGaAs/GaAs MQWs [4]. It has been predicted that InGaN/GaN heterostructures with much thicker InGaN layers could be used to further enhance THz emission through large internal electric fields and high electron mobility [5]. In this paper we demonstrate that the polarization field in an InGaN/GaN surface heterostructure can be exploited for the enhancement of THz generation, and additionally opens up the possibility of tuning the band gap to wavelengths compatible with excitation by femtosecond fiber lasers.

Polarization enhanced tunnel junctions in tandem solar cells

By utilizing the spontaneous and piezoelectric polarization inherent in wurtzite III-V nitride semiconductors, the insertion of a strained layer of InGaN, AlN or InN along the [0001] crystal orientation of GaN will result in large sheets of fixed charges. The permanent dipole resulting from these charges contribute directly to the high electric field in these layers that bends the conduction band on the n-side below the valence band of the p-side to enable interband (Zener) tunneling. At low reverse bias, this tunneling current can achieve higher magnitudes than its equally biased forward regime. This is essentially backward conduction, which is useful mechanism in providing a low resistance pathway for current in tandem solar cells.