Semi-insulating 4H Research Articles

Influence of optical grating on the triggering efficiency of VCSI silicon carbide photoconductive semiconductor switch

In this article, we present a new configuration of opposed-contact vanadium-compensation semi-insulating (VCSI) 4H–SiC photoconductive semiconductor switch (PCSS) to reduce the input laser energy for operation. The configuration is made up of three parts: the PCSS with optical grating, the convex lens in front of PCSS, and the additional reflector behind PCSS. Periodic surface structures were fabricated on the VCSI 4H–SiC substrate by ultrafine femtosecond laser direct writing to etch optical gratings. The microstructure on the lower surface plays the role of reflection grating, so that the laser forms a specific distribution pattern and enhances the laser absorption. Experiments and simulations show that the triggering efficiency increased 3.7 times compared to PCSS without grating at 532 nm laser. And output voltage can reach 90 % of the input bias voltage. This SiC PCSS optical system significantly reduces the laser energy without reducing the electrodes spacing.

Effect of hydrogen on the unintentional doping of 4H silicon carbide

High-purity semi-insulating (HPSI) 4H silicon carbide (4H-SiC) single crystals are critical semiconductor materials for fabricating GaN-based high-frequency devices. One of the major challenges for the growth of HPSI 4H-SiC single crystals is the unintentional doping of nitrogen (N) and boron (B). The addition of hydrogen has been supposed to mitigate unintentional doping. However, the underlying mechanism has not been well understood. In this work, the role of hydrogen in the growth of HPSI 4H-SiC single crystals is investigated by first-principles formation-energy calculations. We find that the addition of hydrogen significantly mitigates N doping while hardly affecting B doping. Once hydrogen is added, hydrogen may adsorb at the growing surface of 4H-SiC, leading to surface passivation. Since N can react with hydrogen to form stable NH3 (g), the chemical potential of N is reduced, so that the formation energy of N in 4H-SiC increases. Hence, the critical partial pressure of nitrogen required for the growth of HPSI 4H-SiC single crystals increases by two orders of magnitude. Moreover, we reveal that the adjustment of relative B and N doping concentrations has a substantial impact on the Fermi energy of HPSI 4H-SiC. When the doping concentration of N is higher than that of B, N interacts with carbon vacancies (VC) to pin the Fermi energy at Z1/2. When the doping concentration of B is higher than that of N, the Fermi energy is pinned at EH6/7. This explains that the resistivity of unintentionally doped HPSI 4H-SiC may vary.

Investigation on the photocurrent tailof vanadium-compensated 4H–SiC for microwave application

Vanadium-compensated semi-insulating 4H–SiC photoconductive semiconductor switch (PCSS) has been a promising candidate for frequency-agile microwave generation. This application usually requires the PCSS to operate in linear mode so that a short carrier lifetime is required. However, in our experiment, some samples showed a long tail of photocurrent when illuminated with 532 nm light. To investigate the cause of the tail, we performed photocurrent tests at 532 and 1064 nm for two 4H–SiC samples with different doping. From the experimental results, we deduce that the cause is a hole trap that was not investigated previously. To verify it, we constructed a versatile transient simulation model of 4H–SiC triggered at sub-bandgap light. The model can deal with amphoteric V in steady state, the extrinsic light absorption and recombination process with more than one trap level. The simulation results agree well with the experiments. By characterizing the properties of the trap, we deduce that the unintentional doping of aluminum’s shallow acceptor level functions as the hole trap.

P-Type carrier concentration enhancement analysis of 4H–SiC by wet chemical laser doping

Al was laser doped in semi-insulating 4H–SiC showing highly enhanced impurity diffusion via 660 μm AlCl3 aqueous layer by optimization of maximum power delivered by KrF-Excimer Laser at substrate. Due to optimized laser fluence, modifications in surface stoichiometry and residual bands formation were expected to occur. Irradiation induces p-type carrier concentration up to 1015 cm-2 having low surface loss with no damage to substrate. Analysis shows c-Si out diffusion at surface and some residual a-Si without any carbonization accompanying variety of complexes formation followed via irradiation thermal environment. Processing did not show any negative impact on the substrate quality and defect formation altering carrier concentration at a large scale. Minor change in Raman mode has been observed due to increasing Al doping concentration and irradiation.

Enhancement of graphene-related and substrate-related Raman modes through dielectric layer deposition

In this report, we demonstrate a method for the enhancement of Raman active modes of hydrogen-intercalated quasi-free-standing epitaxial chemical vapor deposition graphene and the underlying semi-insulating 6H–SiC(0001) substrate through constructive signal interference within atomic-layer-deposited amorphous Al2O3 passivation. We find that an optimum Al2O3 thickness of 85 nm for the graphene 2D mode and one of 82 nm for the SiC longitudinal optical A1 mode at 964 cm–1 enable a 60% increase in their spectra intensities. We demonstrate the method's efficiency in Raman-based determination of the dielectric thickness and high-resolution topographic imaging of a graphene surface.

Space charge formation in the high purity semi-insulating bulk 4H–silicon carbide

We investigated the carrier transport and space charge formation in the high purity semi-insulating 4H-SiC bulk single crystal. Using the Laser–induced Transient Current Technique we observed anomalous short current waveform oscillations that are caused by very short carrier lifetime ≤ 1 ns. We conclude from a detailed inspection of current waveform shape completed by the analysis of collected charge that the electron mobility decreases in an increasing electric field. The saturation value of electron drift velocity 8.7 × 106 cm2s-1 was evaluated. The Hall effect measurement revealed the n-type electrical conductivity and dominant donor energy of ED = 0.85 eV. We also observed the blocking character of prepared Au contacts, which induce the electron depletion and positive space charge formation in biased sample. The continuous increase of the space charge in a wide time interval of 10-1 s – 104 s, attested to testified on a presence of a dispersed impurity band localized below the Fermi energy. Based on these findings, we proposed a theoretical model describing the observed phenomena very well.

Correlation between the response performance of epitaxial graphene/SiC UV-photodetectors and the number of carriers in graphene

Metal-Graphene-Metal (MGM) UV photodetectors have been fabricated by using an Epitaxial Graphene (EG)/SiC junction on a semi-insulating 4H–SiC. The migration of carriers in the EG/SiC photodetectors under 365 nm light irradiation is clarified by introducing a top gate electrode. Besides, it is found that the response performance of EG/SiC photodetectors is affected by the geometric parameters of the graphene channel. When the graphene channel of the detector is reduced from multi-layer to single-layer and its aspect ratio (AR) is increased from 15 to 100, the detector shows a faster response under illumination that its rise time (tr) is reduced from about 20 s to 5 m s, and its decay time (td) reduced from far more than 20 s to 15 m s at a bias of 10 V. At the same time, the responsivity of the detector has been increased to 189.7 mA/V, its External Quantum Efficiency (EQE) has been increased to 66.6 % and its response signal becomes more stable. After systematic analysis, the geometric parameters of the graphene channel are proved to determine the number of carriers in it, the correlation between the response performance of EG/SiC photodetectors and the number of carriers in graphene is found out.

Lateral spreads of ion-implanted Al and P atoms in silicon carbide

Lateral spreads of Al and P atoms implanted (10 ∼ 700 keV) into a high-purity semi-insulating 4H silicon carbide (SiC) substrate have been experimentally investigated. Scanning capacitance microscopy observation revealed that lateral distributions are highly dependent on the implantation energies of ions. Asymmetric lateral distributions were observed along the direction, which may be attributed to the 4° off-angle of the substrate. The lateral spreads are determined to be 0.33 μm for Al atoms, 0.38 ± 0.02 μm for P atoms along the direction, and 0.28 μm/0.38 μm for Al atoms, 0.25 μm/0.40 μm for P atoms along the direction (upstream/downstream side of step flow). Comparison between the lateral spreads of Al and P atoms is discussed with Al and P stopping powers and ranges in SiC.

Enhanced selective solar absorption of surface nanotextured semi-insulating 6H–SiC

Surface treatments were performed on single-crystal semi-insulating 6H–SiC by femtosecond pulsed laser irradiation, aimed at analyzing the effect of the laser-induced periodic surface structures (LIPSSs) on the films’ optical properties. The surface morphology study of the laser-induced nanostructures allows determining the modification threshold fluence of about 0.7 J cm−2 as well as detecting fine (160 nm) and coarse (450–550 nm) ripples according to different values of the laser pulse fluence (ΦP) released to the material. Micro-Raman spectroscopy allows determining the presence of undesired amorphous structural phases when ΦP exceeds 2.15 J cm−2, whereas no compositional variations occur for lower values of ΦP. Samples treated on the entire surface with the pulse fluence conditions to obtain fine ripples were optically tested. Although the long-range order is progressively lost as the accumulated laser fluence increases, the heaviest treated samples show solar absorptance values > 75% and spectral selectivity up to 1.7 projected at the operating temperature of 1000 K, thus pointing out the suitability of fs-laser surface textured 6H–SiC to act as a selective solar absorber for energy conversion devices operating at high temperature.

Growth of thin AlN nucleation layer and its impact on GaN-on-SiC heteroepitaxy

Abstract AlN nucleation layer (NL) with thickness of 35 nm was grown on semi-insulating 4H–SiC substrate by metal organic chemical vapor deposition (MOCVD). Focused on the evolutions of the surface microstructural morphology and crystalline quality, the growth mechanism of the thin NL was studied by series of growth procedures. As interval supply of ammonia (NH3) and alternative supply of Trimethylaluminum (TMA)/NH3 are applied to NL growth, complete coalescence of AlN nucleate islands with a smooth surface and low density of defects is obtained. In addition, the impact of the thin NL on the subsequent GaN buffer layer (BL) was also discussed. Overgrown on the well-coalesced NL, surface roughening and three-dimensional mode get predominant at the initial GaN growth, which facilitates high crystalline quality of GaN BL with low wafer bow.

Wafer-scale 4H-silicon carbide-on-insulator (4H–SiCOI) platform for nonlinear integrated optical devices

4H-silicon carbide-on-insulator (4H–SiCOI) serves as a novel and high efficient integration platform for nonlinear optics and quantum photonics. The realization of wafer-scale fabrication of single-crystalline semi-insulating 4H–SiC film on Si (100) substrate using the ion-cutting and layer transferring technique was demonstrated in this work. The thermodynamics of 4H–SiC surface blistering is investigated via observing the blistering phenomenon with a series of implanted fluences and annealing temperatures. Surface tomography and the depth dependent film quality of the 4H–SiC have been extensively studied by employing scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Moreover, X-ray diffraction (XRD) was carried out and the diffraction spectrum reveals a narrow peak with a full width at half maximum (FWHM) of 75.6 arcsec, indicating a good maintenance of the single-crystalline phase for the prepared thin film of 4H–SiC as compared to its bulk counterpart. With the single-crystalline 4H–SiCOI, we have successfully fabricated a micro-ring resonator with a quality factor as high as 6.6 × 104. The reported 4H–SiCOI wafer provides a feasible monolithic platform for integrated photonic applications.

Tuning electronic properties of epitaxial multilayer-graphene/4H–SiC(0001) by Joule heating decomposition in hydrogen

On the semi-insulating 4H–SiC (0001) surface, hydrogenated multilayers graphene (MLG) were epitaxially prepared by the method of Joule heating decomposition in the hydrogen atmosphere. The structural and chemical characteristics of multilayers graphene have been elaborately analyzed by the X-ray photoelectron and Raman spectroscopies, showing the level of hydrogenation being promoted with the increase of hydrogen pressure. Then, diodes with MLG/4H–SiC contact were fabricated and studied, proving that the Schottky barrier height (SBH) of MLG/4H–SiC junction was enhanced by the hydrogenation. By studying the typical current-voltage characteristics, the SBH was observed to be heightened from 0.84 eV to 1.0 eV along with the hydrogen pressure increasing from 10−2 mbar to 102 mbar. Finally, graphene-semiconductor-graphene photodetectors were fabricated, showing peak responsivity as high as~ 0.9 A/W and external quantum efficiency of 345%, under the 324 nm illumination and biased at 3V.

The Test of a High-Power, Semi-Insulating, Linear-Mode, Vertical 6H-SiC PCSS

A high-power photoconductive semiconductor switch (PCSS) working in linear mode can be used for RF generation by modulating the illuminating light. Such a PCSS constructed of a semi-insulating 6H silicon carbide (6H-SiC) substrate and its characterization under a high electric field is presented. The PCSS is of a vertical structure, with one hollow/transparent electrode to let through the laser light and the other highly reflective to increase light absorption. First, to optimize the high-voltage withstanding, the factors affecting the field enhancement, such as substrate thickness, electrode profiles, and encapsulating materials, are discussed. Then, the PCSS is assembled and tested, and the 0.8-mm-thick 6H-SiC PCSS is able to work with electric fields up to 225 kV/cm and the power capacity up to 10.6 MW without failure when triggered by a Nd:YAG laser (532-nm, 17-ns full wavelength at half maximum). By varying the optical energy from 3 to 31.9 mJ, the minimum conducting resistance and the peak photocurrent of the PCSS are obtained. The PCSS fails with further increasing voltage, and the cause of the failure was identified to be a bulk breakdown. The result shows that the PCSS can work linearly and it is possible to be used in high-power compact RF generation.

Electron-Diffraction Study of the Structure of Epitaxial Graphene Grown by the Method of Thermal Destruction of 6H- and 4H-SiC (0001) in Vacuum

The method of reflection high-energy electron diffraction (RHEED) is used for studying the structure of graphene layers formed on the surface of the Si-face of conductive and semi-insulating 6H- and 4H-SiC(0001) substrates by thermal desorption of Si atoms in high vacuum, depending on the temperature and time of sublimating Si atoms as well as depending on the method of preprocessing the substrate surface. Diffraction patterns are recorded in the $$[\bar 12\bar 10]$$ and $$[1\bar 100]$$ crystallographic directions of the substrates. It is found that in all experiments the formation of graphene layers occurs with a rotation of the graphene crystal lattice by 30° relative to the SiC lattice.

The Effect of Interfacial Charge on the Development of Wafer Bonded Silicon-on-Silicon-Carbide Power Devices

A new generation of power electronic semiconductor devices are being developed for the benefit of space and terrestrial harsh-environment applications. 200-600 V lateral transistors and diodes are being fabricated in a thin layer of silicon (Si) wafer bonded to semi-insulating 4H silicon carbide (SiC) leading to a Si/SiC substrate solution that promises to combine the benefits of silicon-on-insulator (SOI) technology with that of SiC. Here, details of a process are given to produce thin films of silicon 1 and 2 μm thick on the SiC. Simple metal-oxide-semiconductor capacitors (MOS-Cs) and Schottky diodes in these layers revealed that the Si device layer that had been expected to be n-type, was now behaving as a p-type semiconductor. Transmission electron microscopy (TEM) of the interface revealed that the high temperature process employed to transfer the Si device layer from the SOI to the SiC substrate caused lateral inhomogeneity and damage at the interface. This is expected to have increased the amount of trapped charge at the interface, leading to Fermi pinning at the interface, and band bending throughout the Si layer.

Current status of modelling the semi-insulating 4H–SiC transient photoconductivity for application to photoconductive switches

In this paper we present the current status of modelling the time evolution of the transient conductivity of photoexcited semi-insulating (SI) 4H–SiC taking into account the properties of defect centres. A comprehensive model that includes the presence of six, the most significant, point defects occurring in SI 4H–SiC crystals is presented. The defect centres are attributed to the two kinds of nitrogen-related shallow donors, a boron-related shallow acceptor, deep electron and hole traps, and the Z 1/2 recombination centre. We present the results of the state-of-the-art numerical simulations showing how the photoconductivity transients change in time and how these changes are affected by the properties of defect centres. The properties of defect centres assumed for modelling are compared with the results of experimental studies of deep-level defects in high purity (HP) SI 4H–SiC wafers performed by the high-resolution photoinduced transient spectroscopy (HRPITS). The simulated photoconductivity transients are also compared with the experimental photocurrent transients for the HP SI 4H–SiC wafers with different deep-level defects. It is shown that a high-temperature annealing producing the C-rich material enables the fast photocurrent transients to be achieved. The presented analysis can be useful for technology of SI 4H–SiC high-power photoconductive switches with suitable characteristics.

Analysis of intensity dependent near-bandedge absorption in semi-insulating 4H–SiC for photoconductive switch applications

We report on the intensity-dependent behavior of the absorption coefficient (α) in semi-insulating 4H–SiC material. Data from as-received samples show a monotonic decrease in α with incident energy density, with a pronounced change in slope at around 10 mJ cm−2. Annealed samples, on the other hand, exhibit an experimental trend of increasing α with intensity. Qualitative explanation of the observed behavior is presented that probes the possible role of spontaneous and stimulated emission for as-received samples. With annealing, trap related recombination is strongly reduced leading to higher carrier densities and increased free-carrier absorption with incident intensity. The role of band-filling and permittivity changes are shown to be inconsequential, while changes in internal fields could contribute to decreases in absorption.

Spectral broadening induced by intense ultra-short pulse in 4H–SiC crystals**Project supported by the National High Technology Research and Development Program of China (Grant No. 2014AA041402) and the National Natural Science Foundation of China (Grant Nos. 51272276 and 51322211).

We report the observation of spectral broadening induced by 200 femtosecond laser pulses with the repetition rate of 1 kHz at the wavelength of 532 nm in semi-insulating 4H–SiC single crystals. It is demonstrated that the full width at half maximum of output spectrum increases linearly with the light propagation length and the peak power density, reaching a maximum 870 cm−1 on a crystal of 19 mm long under an incident laser with a peak power density of 60.1 GW/cm2. Such spectral broadening can be well explained by the self-phase modulation model which correlates time-dependent phase change of pulses to intensity-dependent refractive index. The nonlinear refractive index n2 is estimated to be 1.88×10−15 cm2/W. The intensity-dependent refractive index is probably due to both the nonlinear optical polarizability of the bound electrons and the increase of free electrons induced by the two-photon absorption process. Super continuum spectra could arise as crystals are long enough to induce the self-focusing effect. The results show that SiC crystals may find applications in spectral broadening of high power lasers.

High temperature annealing effects on deep-level defects in a high purity semi-insulating 4H-SiC substrate

Effects of high-temperature annealing on deep-level defects in a high-purity semi-insulating 4H silicon carbide substrate have been studied by employing current-voltage, capacitance-voltage, junction spectroscopy, and chemical impurity analysis measurements. Secondary ion mass spectrometry data reveal that the substrate contains boron with concentration in the mid 1015 cm−3 range, while other impurities including nitrogen, aluminum, titanium, vanadium and chromium are below their detection limits (typically ∼1014 cm−3). Schottky barrier diodes fabricated on substrates annealed at 1400–1700 °C exhibit metal/p-type semiconductor behavior with a current rectification of up to 8 orders of magnitude at bias voltages of ±3 V. With increasing annealing temperature, the series resistance of the Schottky barrier diodes decreases, and the net acceptor concentration in the substrates increases approaching the chemical boron content. Admittance spectroscopy results unveil the presence of shallow boron acceptors and deep-level defects with levels in lower half of the bandgap. After the 1400 °C annealing, the boron acceptor still remains strongly compensated at room temperature by deep donor-like levels located close to mid-gap. However, the latter decrease in concentration with increasing annealing temperature and after 1700 °C, the boron acceptor is essentially uncompensated. Hence, the deep donors are decisive for the semi-insulating properties of the substrates, and their thermal evolution limits the thermal budget for device processing. The origin of the deep donors is not well-established, but substantial evidence supporting an assignment to carbon vacancies is presented.

Simulation of a new hybrid Si/SiC power device for harsh environment applications

A new power device structure is proposed, conceived to operate in a high temperature, harsh environment, for example within a motor drive application down hole, as an inverter in the engine bay of an electric car, or as a solar inverter in space. The lateral silicon power device resembles a laterally diffused MOSFET (LDMOS), such as those implemented within silicon on insulator (SOI) substrates. However, unlike SOI, the Si thin film has been transferred directly onto a semi-insulating 6H silicon carbide (6H-SiC) substrate via a wafer bonding process. Thermal simulations of the hybrid Si/SiC substrate have shown that the high thermal conductivity of the SiC will have a junction-to-case temperature approximately 4 times less that an equivalent SOI device, reducing the effects of self-heating. Electrical simulations of a 600 V power device, implemented entirely with the silicon thin film, suggest that it will retain the ability of SOI to minimise leakage at high temperature, but does so with 50% less conduction losses.