Quantum technology is a field of significant interest that will benefit many applications including communications and sensing. SiC is a promising material for quantum applications such as quantum memories, due to point defects, specifically V Si , in the material, which result in long spin coherence times. We have found that no V Si are present in our epitaxially grown unintentionally and nitrogen-doped 4H-SiC with electron concentrations ranging from 10 14 to 10 18 cm -3 . We create these vacancies using electron irradiation, in concentrations from single defects to ensembles. To utilize the defect luminescence for realistic applications, we have fabricated the SiC into photonic crystal arrays. We present the processing steps required to create photonic crystal cavities in SiC and subsequent challenges.

Depth profiling of the ambipolar carrier lifetime was performed in n-type, 140mm thick silicon carbide (SiC) epilayer using excitation by two-photon absorption (TPA) with a pulsed 586nm laser, and confocal measurement of time resolved photoluminescence (TRPL) decay from the excited region. A depth resolution of ≈10mm was obtained. The PL decay curves were analyzed using a recently developed formalism that takes into account the TPA excitation, carrier diffusion and surface/interface recombination. The carrier lifetime decreases near the top surface of the epitaxial layer as well as near its interface with the substrate.

Silicon carbide is a material of interest for high-voltage and high-power switching device applications. Basal plane dislocations (BPDs) are a major concern for SiC bipolar devices as they source Shockley-type stacking faults in the presence of an electron-hole plasma and reduce minority carrier lifetimes [1, 2]. Many researchers have investigated methods to reduce the BPD density by experimenting with pre-growth treatments [3-5], substrate orientation [6], growth parameters [6, 7] and growth interrupts [8]. This work investigates extended defects, morphology and lifetime in 4H-SiC epilayers grown on substrates offcut 2° toward the [11-20]. Epilayers were synthesized on 2° offcut substrates in a horizontal hot-wall reactor using the standard chemistry of silane (2% in H 2 ) and propane. Epilayers were grown at various growth rates, C/Si ratios, and growth temperatures. The pressure was maintained at 100 mbar for all growths. Some samples were grown with a 5 µm highly doped n + buffer layer using ultra high purity nitrogen prior to the low-doped epilayers. Ultraviolet photoluminescence (UVPL) imaging was used to identify BPDs in the low doped epilayers. Time resolved photoluminescence measurements were performed to determine the minority carrier lifetime of the layers and analysis of Raman spectroscopic maps revealed the location of polytype inclusions. Electron trap concentrations were determined using deep level transient spectroscopy (DLTS). Surface roughness was measured by atomic force microscopy and the morphology was also characterized using Nomarski microscopy and white light interferometry. When a 15 µm epilayer was grown without a buffer layer, step bunching was observed and the surface roughness was 6.0 nm RMS. For comparison, a standard 4° offcut sample typically has 3.0 nm RMS for a 20 µm epilayer. Using UVPL, it was found that after 4 µm of epi, 90% of the BPDs had converted in the low doped layer as compared to 70% in a 4° offcut sample, indicating the conversion is faster in the lower offcut material. The conversion results were from an older substrate and vendor A. For newer substrates, vendor B, the density of BPDs at the epilayer/substrate interface was ≤ 0.2 cm -2 . 3C-SiC inclusions were present in the epilayers as verified using Raman spectroscopy for both unintentionally doped (UID) and N+ epilayers. These inclusions were eliminated by increasing the growth temperature and lowering the C/Si ratio for N+ epilayers, but by increasing C/Si ratios for UID films. Changing these growth parameters resulted in specular film morphology and resulted in minority carrier lifetimes of approximately 1 µs. [1] J.P. Bergman, et al. Mater. Sci. Forum Vol. 353-356 , 299 (2001). [2] R.E. Stahlbush, et al., J. Electron. Mater. 31 , 370 (2002). [3] Z. Zhang, et al., Appl. Phys. Lett. 89 , 081910 (2006). [4] J.J. Sumakeris, et al., Mater. Sci. Forum 527-529 , 529 (2006). [5] H. Tsuchida, et al., Mater. Sci. Forum 483-485 , 97 (2005). [6] W. Chen and M.A. Capano J. Appl. Phys. 98 , 114907 (2005). [7] T. Ohno, et al., J. Cryst. Growth 271 , 1 (2004). [8] R. E. Stahlbush, et al., Jr., Appl. Phys. WeLett. 94 , 041916 (2009).

Single silicon vacancies V Si in silicon carbide nanostructures hold great promise for future technological applications in scalable quantum computing and information processing for simulation, sensing, and communication. These defects are typically created by ion implantation or neutron/electron irradiation. Identification of these defects, knowledge of their characteristics, control of their concentrations, isolation of single spin defects and understanding the effects of semiconductor processing on their properties are crucial to the applications of SiC in quantum electronic and integrated photonic devices. These vacancies embedded in photonic crystal cavities (PCC) have the capability of high efficiency emission of single photons which can significantly improve the performance of on-chip photonic networks and long-distance quantum communication systems, as compared to conventional solid-state emitters. Here we investigate the impact of processing on the photoluminescence properties of PCCs fabricated using three approaches: hydrogen implantation to form thin SiC layers on an oxide layer that can be easily etched away to form an air gap under the PCC, wafer bonding and mechanical thinning of the SiC, also on an oxide layer, and selective photo-electrochemical etching of an n-p epitaxial SiC structure to form an air gap. We also assess the impact of electron irradiation for these three fabrication approaches.

Carrier lifetime control in thick silicon carbide (SiC) epilayers is essential for fabricating > 10kV devices. Lifetime depth profiles were investigated in n-type and p-type SiC epilayers using photoluminescence (PL) decay excited by two-photon absorption (TPA), using 586nm laser pulses. TPA limits the excitation to a small volume, and the observed PL decays exhibit nonexponential behavior resulting from the three-dimensional carrier diffusion occurring during the decay. The results were analyzed using a formalism that includes the effects of carrier lifetime, carrier diffusion, and surface recombination on the PL decay. The lifetime depth profiles exhibited a nonuniform lifetimedegrading defect concentration within the epilayers.

Materials with a negative dielectric permittivity (e.g. metals) display high reflectance and can be shaped into nanoscale optical-resonators exhibiting extreme mode confinement, a central theme of nanophotonics. However, the ability to $actively$ tune these effects remains elusive. By photoexciting free carriers in 4H-SiC, we induce dramatic changes in reflectance near the "Reststrahlen band" where the permittivity is negative due to charge oscillations of the polar optical phonons in the mid-infrared. We infer carrier-induced changes in the permittivity required for useful tunability (~ 40 cm$^{-1}$) in nanoscale resonators, providing a direct avenue towards the realization of actively tunable nanophotonic devices in the mid-infrared to terahertz spectral range.

Lifetime maps for two 4H-SiC epi-wafers (samples 1 and 2) were recorded using microwave photoconductive decay (μPCD) measurements and correlated with the type and distribution of structural defects mapped by synchrotron X-ray topography (white beam and monochromatic). Sample 1 showed lower lifetime inside one of its higher doped facet regions and along its edges. The low lifetime in the facet region was associated with the presence of a high density of multi-layered Shockley stacking faults (SFs) and low angle grain boundaries (LAGBs). These stacking faults are likely double Shockley stacking faults (DSSFs) and probably nucleated from scratches present on the substrate surface and LAGBs present in that region, propagating during epilayer growth. In contrast, sample 2 showed a reduced carrier lifetime in the middle region associated with a network of interfacial dislocations (IDs) and half loop arrays (HLAs) originating from 3C inclusions that are generated during epilayer growth. Along the edges of both samples, overlapping triangular defects, microcracks and BPD loops lowered lifetime.

Submitted for the MAR15 Meeting of The American Physical Society Transient Infrared Studies of Carrier Injection Effects on the Reststrahlen Band of SiC BRYAN SPANN1, RYAN COMPTON2, ADAM DUNKELBERGER3, JAMES LONG, Naval Research Laboratory, PAUL KLEIN, Sotera Defense Solutions Inc., JOSH CALDWELL, JEFF OWRUTSKY, Naval Research Laboratory — Sub-diffraction light confinement has led to advances in imaging, metamaterials, and plasmonics among other fields. A phenomenon that can provide sub-diffraction light is the surface phonon polariton (SPhP). SPhPs couple infrared photons with optical phonons. Because SPhPs are coupled directly to phonons, lifetimes can be longer than that of surface plasmon polaritons (SPPs) whose lifetimes are dominated by electron scattering. SiC is one material that exhibits SPhPs. SiC SPhPs are activated by photons with energies near the Reststrahlen band. In this study we investigate aspects of carrier dynamics by photoinjecting free carriers into the SiC conduction band using a pulsed 355 nm pump laser and probe the resulting dynamics near the Reststrahlen band using a tunable CO2 laser. Variable pump fluences provided free carrier densities of 1x1017 to 1x1019. Probing the excited state dynamics near the Reststrahlen band revealed complex transient behavior resulting in positive and negative changes in transient reflectance depending on the photo-injection level and the probe energy. Numerical simulations were carried out to mimic the initial photo-injection level provided by the transient experiment and resulted in qualitative agreement with the experiment. 1National Research Council Postdoctoral Fellow 2National Research Council Postdoctoral Fellow 3National Research Council Postdoctoral Fellow Bryan Spann Naval Research Lab Date submitted: 13 Nov 2014 Electronic form version 1.4

Elimination of basal plane dislocations (BPDs) in epitaxial 4H-SiC is demonstrated via a novel pulsed annealing technique in a moderate N 2 overpressure of 0.55 MPa. BPD removal in 15 µm thick epitaxial 4H-SiC was confirmed using ultraviolet photoluminescence (UVPL) imaging before and after the annealing process. The samples were capped with a carbon cap, introduced into the annealing chamber, and brought up to a base temperature (T BASE ) of around 1550 °C for the pulsed anneal. The multicycle rapid thermal anneal (MRTA) was then performed in the T BASE :T MAX range, where T MAX = 1875 °C was the peak temperature reached by the annealing cycles. Post-anneal surface quality and carrier lifetime were characterized by atomic force microscopy and time-resolved photoluminescence decay.

Sub-diffractional confinement of light has led to advancements in imaging, metamaterials, nano-manufacturing, plasmonics, and other fields. One potential route to sub-diffractional confinement is via stimulated surface phonon polaritons (SPhPs). SPhPs couple infrared photons with optical phonons and consequently their lifetimes can be longer than surface plasmon polaritons (SPPs), whose lifetimes are dominated by electron scattering events. Thus, materials capable of generating SPhPs are of general interest to study. SPhPs are activated by photons with energies near the Reststrahlen band of semiconductors such as SiC. In this work we examine aspects of carrier dynamics by photo-injecting electrons into the SiC conduction band using a pulsed 355 nm laser and probe the resulting dynamics near the Reststrahlen band using a tunable CO2 laser. The fluence of the pump laser was varied to provide photo-injection levels ranging from ~1x10^17 to 1x10^19 free carriers. Probing the excited-state dynamics near the blue-edge of the Reststrahlen band resulted in complex transient behavior, showing both positive and negative changes in transient reflectance depending on the level of photo-injected carriers and probe energy. Numerical calculations of the SiC reflectance spectra with different doping levels were done to simulate the initial photo-injection level provided by the transient experiment. The computed spectra and the experimentally measured excited spectra for different photo-injection levels were compared and resulted in qualitative agreement.

Silicon carbide (SiC) is a material of interest for high-temperature, high-voltage and high-power switching device applications. Key materials challenges inhibiting such devices include elimination of basal plane dislocations (BPDs) and enhancement of minority carrier lifetimes in the drift or blocking regions of the device. Recent progress in addressing both of these issues is presented. BPDs are a major concern for the SiC bipolar devices required for high-voltage applications as they source Shockley-type stacking faults in the presence of an electron-hole plasma and reduce minority carrier lifetimes. Many methods have been investigated to reduce the BPD density including pre-growth treatments, substrate orientation, growth parameters and growth interrupts. It has been shown that the conversion of BPDs to threading edge dislocations (TEDs) continues throughout the epitaxial growth process in 4° off-axis SiC material and that a minimum thickness of ~16 µm is required to convert all BPDs to TEDs. Here we show that optimizing a hydrogen etch of the substrate prior to epitaxial growth significantly enhances conversion efficiency in a thin highly doped n+ buffer layer (BL). IN this work, epitaxial layers were grown on 4° off-axis substrates in an Aixtron/Epigress VP508 horizontal hot-wall reactor using the standard chemistry of silane (2% in H2) and propane. In addition, using various growth approaches, low-doped epitaxial layers of only 20 µm in thickness on a 5 µm highly doped buffer layer have demonstrated minority carrier lifetimes up to 4 µs, as measured by time resolved photoluminescence, without any pre- or post- processing treatment. Interface recombination likely dominates these measurements. We extend this approach by presenting new data investigating the non-uniformity of lifetime found on as-grown material under various conditions. This work is supported by the Office of Naval Research

The widespread adoption of energy storage deployment requires the cost for the power conversion stages to be significantly reduced. The high cost of inversion is driven largely by the performance limitations of Si-IGBTs and SiIGCTs, especially when a higher DC-link voltage (>1 kV) is desired to facilitate the reduction of the balance-of-system. SiC unipolar devices, however, offer great promise for increasing the DC link voltage, while maintaining high system efficiency without increasing system complexity. Such a switch platform could enable small, lightweight, transformerless topologies for industrial medium voltage grid applications operating at 3.3 or 4.16 kV AC. This report discusses the state-of-the-art 1.2kV SiC JFETs as a proven technology platform, and demonstrates the benefits of the JFET through operational comparisons with SiC-MOSFETs and Si-IGBTs. In addition, SiC JFET device reliability is demonstrated up to 200°C and the impact of a 6.5 kV JFET platform on power conversion systems is discussed.

Recent advances in preparing n-type 4H-SiC with long carrier lifetimes have greatly enhanced the possibility of realizing commercially available, very high voltage and high power solid state switching diodes. For the range > several kV, vertical bipolar structures are required with drift layers exhibiting carrier lifetimes ≥ several µsec. Recently, low-doped epilayers with carrier lifetimes in excess of this have been demonstrated, thus approaching a goal that has been pursued for over a decade. Historically, the short lifetimes in early epitaxial layers (a few hundred nsec) were eventually identified with the V c -related Z 1/2 lifetime killer. Current strategies to minimize this defect are an essential ingredient in the procedure for obtaining long-lifetime material. In order to optimize the attainable lifetimes, it has been shown that in addition to low Z 1/2 levels, very thick layers are required to minimize the effects of recombination in the substrate and surface passivation is also necessary to minimize surface recombination (S < 1000 cm/sec).

The indirect band gap of icosahedral B12As2 (IBA) has been determined by variable temperature photoluminescence measurements (8 K-294 K) on solution-grown bulk samples. In addition, evidence of three shallow acceptor levels and one shallow donor level is reported. The low-temperature spectra were characterized by broad and intense deep defect emission, donor-acceptor pair (DAP) bands, and exciton recombination. The appearance of DAP emission verifies the incorporation of a donor in IBA, which has not been reported previously. The temperature dependence of the free exciton (FE) intensity reflected a FE binding energy of 45 meV. The variation of the FE peak position with temperature was fitted with both Varshni and Pässler models to determine an expression for the temperature dependence of the indirect band gap. The resulting low and room temperature band gaps are Eg(0) = 3.470 eV and Eg(294 K) = 3.373 eV, respectively. The latter is not consistent with previous reports of the room temperature band gap, 3.20 eV and 3.47 eV, derived from band structure calculations and optical absorption, respectively. The origin of these discrepancies is discussed. The DAP spectra reveal three relatively shallow acceptors with binding energies of ≈175, 255, and 291 meV, and a shallow donor with binding energy ≈25 meV. Although the identity of the individual acceptors is not known, they appear to be associated with the light-hole band. The small donor binding energy is suggestive of an interstitial donor impurity, which is suspected to be Ni.

Thick, high-purity semi-insulating (SI) homoepitaxial layers on 4H-SiC were demonstrated using a novel compensation scheme controlled by defect-competition epitaxy at C/Si ratios of 1.3–1.5. These showed resistivity of ∼109 Ω cm. Comparison of secondary ion mass spectra between low-doped epilayers grown at C/Si ratio <1.3 and SI epilayers grown at C/Si ratio >1.3 showed little difference in residual impurity concentrations. A reconciliation of impurity concentration with measured resistivity indicated a compensating trap concentration of ∼1015 cm-3 present only in SI epilayers. High-resolution photoinduced transient spectroscopy (HRPITS) identified them as Si vacancy related centers, with no detectable EH6/7 and Z1/2 levels. Recombination lifetimes of ∼5 ns suggest application in fast-switching power devices.

The emission and waveguiding properties of individual GaN microwires as well as devices based on an n-GaN microwire/p-Si (100) junction were studied for relevance in optoelectronics and optical circuits. Pulsed photoluminescence of the GaN microwire excited in the transverse or longitudinal direction demonstrated gain. These n-type GaN microwires were positioned mechanically or by dielectrophoretic force onto pre-patterned electrodes on a p-type Si (100) substrate. Electroluminescence from this p-n point junction was characteristic of a heterostructure light-emitting diode. Additionally, waveguiding was observed along the length of the microwire for light originating from photoluminescence as well as from electroluminescence generated at the p-n junction.

Characteristic 1.54 μm Era3+ emission has been observed from Er-implanted and annealed, low-temperature grown GaAs:Be samples. Cross-sectional transmission electron microscopy (TEM) studies reveal very little structural damage for elevated temperature implants up to an Er total fluence of 1.36 × 1014 Er/cm2. No Er emission was observed from any of the as-implanted samples, while post-implantation annealing optimized the Er emission intensity near 650°C. The Er-emission appears on top of a broad background luminescence peaking near 1500 nm. Significant enhancement of the optically active Er incorporation was achieved when the implantation was carried out at 300TC. The Er emission intensity was found to scale linearly with the Er implantation fluence for samples with an Er concentration up to ~019 Er/cm3. The sample with the highest Er concentration (~020 Er/cm3) began to show a sublinear dependence. The beginning of Er precipitation was observed after 750°C annealing, but it could even be observed after a 650°C annealing for the highest Er concentration sample. These precipitates are likely ErAs.

Here we present optical beam induced current, electroluminescence, time resolved photoluminescence and current-voltage measurements on several 4H-SiC PiN diodes containing in-grown stacking faults (IGSFs). These defects were observed to act as either current shorts, creating a direct electrical contact between the p+ and n+ layers, or as a current barrier. Carrier lifetime measurements verify that the change in behavior is indeed associated with changes in the conductivity of the material in the vicinity of the defect and not due to local changes in the carrier lifetime. The IGSFs discussed here appear to differ from those previously discussed in the literature and may constitute a new, multi-layered IGSF.

Epitaxial growth of Er-doped silicon films has been performed by plasma-enhanced chemical vapor deposition at low temperature (430°C) using an electron cyclotron resonance source. The goal was to incorporate an optically active center, erbium surrounded by nitrogen, through the use of the metal organic compound tris (bis trimethyl silyl amido) erbium. Films were analyzed by Rutherford backscatter spectrometry, secondary ion mass spectroscopy and high resolution x-ray diffraction. The characteristic 1.54 μm emission was observed by photoluminescence spectroscopy. Previous attempts to incorporate the complex (ErO6) using tris (2,2,6,6-tetramethyl–3,5-heptanedionato) erbium (III) indicated that excessive interstitial carbon lowered epitaxial quality and reduced photoluminescent intensity. In this study, much of the carbon was introduced onto substitutional sites maintaining good epitaxial growth. A response surface method was employed to find the plasma growth parameters yielding the highest quality luminescent films. Luminescent intensity increased for anneals up to 600°C but decreased at higher temperatures.