Semi-insulating Silicon Carbide Substrates Research Articles

30 GHz ferroelectric phase shifter on silicon carbide

Thin ferroelectric barium-strontium titanate (BST) films of high structure quality have been grown on semi-insulating silicon carbide substrates. The crystal structure and elemental composition of the obtained films were studied by the X-ray diffraction and Medium Energy Ions Scattering. Planar capacitors based on these BST films reveal the combination of high tunability and low losses at microwaves. 30 GHz ferroelectric phase shifter demonstrating a figure of merit of 22 deg/dB with a phase shift of Δϕ = 220 deg is implemented based on BST films on silicon carbide substrate.

Terahertz monolithic integrated narrow-band filter based on the silicon carbide substrate

This paper presented a narrow-band terahertz integrated cavity filter based on the silicon carbide (SiC) substrate. By introducing two metalized via-holes in a rectangular integrated cavity, an integrated coupling cavity was built, on the one hand, to operate as a coupling structure; on the other hand, to control the direct coupling between input and output feeding lines, further to control the frequency of the transmission zero. This novel approach was thoroughly investigated with attention paid to the position of the via-holes. Based on this approach, a terahertz narrow-band filter was realized on the semi-insulating SiC substrate, demonstrated good filtering performance, especially the stopband rejection characteristic, and established the groundwork for the production of the terahertz monolithic integrated circuit.

Development of a World Class Silicon Carbide Substrate Manufacturing Capability

Silicon carbide (SiC) semiconductor substrates provide the foundation for revolutionary improvements in the cost, size, weight and performance of a broad range of military and commercial radio frequency (RF) and power switching devices. Due to the lack of a viable, native gallium nitride (GaN) substrate, semi-insulating (SI) SiC substrates are the substrate of choice for high power AlGaN/GaN High Electron Mobility Transistors (HEMTs) due to their near lattice-match to GaN, superior thermal conductivity and commercial availability. GaN has emerged as the technology of choice for RF power because of its superior output power capability compared to other semiconductors. Similarly, semi-conducting (N+) SiC substrates are required for fabrication of high voltage Schottky Diodes and Metal Oxide Semiconductor Field Effect Transistor (MOSFET) power switching devices. Critical to this realization is the availability of affordable, high quality, large diameter SI and N+ SiC substrates for production of GaN and SiC power semiconductors. SiC is unique in that bulk single crystals cannot be grown via traditional melt-based manufacturing processes such as Czochralski. Rather, a high temperature sublimation process is required. In the late 1980’s, pioneering physical vapor transport research taking place at North Carolina State University ultimately led to the formation of Cree Research and subsequently the wide bandgap semiconductor industry. U.S. Department of Defense investment in wide bandgap semiconductor development has easily exceeded $1B spawning the creation of an entirely new industry. During the early 1990’s, SiC physical vapor transport growth development was fraught with perceived insurmountable technical challenges associated with micropipes, polytype conversion, diameter expansion and crystalline defects. Despite these monumental crystal growth technology hurdles, SiC substrates are now manufactured at a cost and quality never thought possible. This article highlights more than 20 years of Air Force Research Laboratory (AFRL) sponsored development with II-VI aimed at positioning itself as a merchant, world-class manufacturer of SiC substrates.

Normally-off 400 °C Operation of n- and p-JFETs With a Side-Gate Structure Fabricated by Ion Implantation Into a High-Purity Semi-Insulating SiC Substrate

We demonstrate normally-off 400 °C operation of n-channel and p-channel junction field-effect transistors (JFETs) fabricated by an ion implantation into a common high-purity semi-insulating silicon carbide (SiC) substrate. The side-gate structure proposed in this letter has good controllability of threshold voltage. The present results assure the potential of JFET-based complementary logic integrated circuits (ICs) operational at high temperature.

Structural and microwave characterization of BaSrTiO3 thin films deposited on semi-insulating silicon carbide

Thin ferroelectric barium strontium titanate (BST) layers of high structure quality have been grown for the first time directly on semi-insulating silicon carbide substrates by RF magnetron sputtering of a ceramic target. The structural and microwave properties of the films were substantially improved by an intermediate annealing of the layers during the growing process. Prepared under this approach, BST films have a well-formed crystal structure, which has a positive effect on their electrical properties, specifically on nonlinearity and dielectric losses. Planar capacitors based on these BST films demonstrate the best combination of high tunability and low losses for BST/SiC structures at microwaves.

High-Temperature Operation of n- and p-Channel JFETs Fabricated by Ion Implantation Into a High-Purity Semi-Insulating SiC Substrate

Silicon carbide (SiC) n-channel and p-channel junction field-effect transistors (JFETs) were fabricated by direct ion implantation into a high-purity semi-insulating 4H-SiC substrate toward complementary-JFET integrated circuit applications under a harsh environment. The fabricated n-JFET and p-JFET on a common substrate without an epitaxial layer show normal transistor operations up to 400 °C. Their electrical characteristics are comparable with theoretical estimations obtained from material parameters of n- and p-type SiC epitaxial layers. The present results assure that not only JFETs but also a variety of devices can be made by direct ion implantation while keeping material properties of epitaxially grown SiC.

Substrate Integrated Waveguide Structural Transmission Line and Filter on Silicon Carbide Substrate

Substrate integrated waveguide (SIW) structural transmission line (TL) and filter are designed and fabricated on semi-insulating silicon carbide (SiC) substrates for operation at the low end of $H$ -band. The via-holes as the sidewall of SIW are achieved by using a fine dry-etching technique, where the radius and the via–via distance of the via-hole row are determined according to the transmission characteristics and the fabrication process. Measurements show that SIW-based TLs have the attenuation constant of as low as 0.45 dB/mm at 220 GHz, a third of that for TLs on grounded coplanar waveguide, and SIW-structural filters have the minimum insertion loss of <1 dB at the center frequency of 183 GHz with a bandwidth of 5.3 %. It illustrates the availability of passive device fabrications on SiC substrate, and paves the way for the forthcoming terahertz monolithic integrated circuits based on wide bandgap semiconductors.

Aspect-ratio driven evolution of high-order resonant modes and near-field distributions in localized surface phonon polariton nanostructures.

Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators. In the present work, we investigate the excitation of 16 distinct high-order, multipolar, localized surface phonon polariton resonances that are optically excited in rectangular pillars etched into a semi-insulating silicon carbide substrate. By elongating a single pillar axis we are able to significantly modify the far- and near-field properties of localized SPhP resonances, opening the door to realizing narrow-band infrared sources with tailored radiation patterns. Such control of the near-field behavior of resonances can also impact surface enhanced infrared optical sensing, which is mediated by polarization selection rules, as well as the morphology and strength of resonator hot spots. Furthermore, through the careful choice of polar dielectric material, these results can also serve as the guiding principles for the generalized design of optical devices that operate from the mid- to far-infrared.

Carbon-Doped GaN on SiC Materials for Low-Memory-Effect Devices

I. INTRODUCTION Gallium nitride (GaN) high electron mobility transistors (HEMT) are today used in military radar and telecom applications. However, there are still several issues that remain to be solved primarily on the material side. The best performing HEMT devices today are those produced on GaN on silicon carbide (SiC). SiC is used on account of its high thermal conductivity and relatively close lattice match with AlN and GaN. Thermal management of the devices is an issue that needs to be improved and, especially for telecom applications, the memory effect must be reduced in order to simplify linearization of the systems. We present epitaxial GaN on SiC of extraordinary high quality where the thermal barrier to the SiC substrate is minimized and where devices made on this material exhibit a substantially lower memory effect than commercially available devices today. II. RESULTS The epitaxial growth of GaN on SiC substrates starts with the deposition of an aluminium nitride (AlN) wetting layer in order to ensure that the GaN grows in a two-dimensional fashion. The structural quality of the AlN is often rather poor and gives rise to a thermal barrier resistance (TBR) that accumulates heat in the device. This results in a decrease in the device efficiency but may result in a catastrophic thermal breakdown of the device in the worst case. We have optimized the in-situ pre-treatment of the substrate surface and also the initial growth procedure of the AlN so that the AlN layer is grown in a layer-by-layer mode, resulting in a very high structural quality of the AlN and also the subsequent GaN that is grown on top of the AlN. The typical values of the full width of half maximum of XRD rocking curves for the C-doped GaN buffer and the AlN (102) planes are around 200 and 100 arcsec, respectively. The absence of pores in the AlN layer also allows this layer to be made very thin which further reduces the TBR of the structure. Figure 1 shows atomic force microscopy images of a non-optimized (left) and an optimized (right) growth of the 35 nm-thick AlN nucleation layers. The entire growth has been successfully scaled up to 100 mm semi-insulating SiC substrates. The non-uniformity over a 100 mm epiwafer of the total thickness and the sheet resistance are typically less than 1% and 2%, respectively. High electron mobility of ~2100 and ~2300 cm2/V-s was achieved from the 2 nm-thick GaN cap/14 nm-thick Al0.29Ga0.71N/GaN structures with and without a 1 nm-thick AlN exclusion layer, respectively, which is as a result of the low dislocation density and sharp channel interface. On the device level, the distortion of amplitude modulated (AM) signals due to memory effects induced by deep electron traps with long time constants, imposes the practice of linearization schemes using digital pre-distortion which are impractical if the time constants are long, requiring large computational capacity and memory. Reducing the amplitude and time constant of the memory effects will greatly improve the AM performance and reduce the cost to produce the systems. The origin of the trapping is not fully understood however it can be reduced by exchanging the dopant in the buffer layer from Fe, which is the standard dopant, to C. In figure 2, the memory effect is illustrated in a commercial HEMT device with Fe doping and a device produced using C doping in the buffer. The measurement is done by applying a 1 µs long voltage pulse between the drain and source from a quiescent working point. Unfortunately the change of dopant also carries the price of a reduced output power from the device. This issue is currently addressed and a solution is close. III CONCLUSIONS We present material enabling high-power and high-frequency GaN devices to be made, which have substantially better performance and suits the use of GaN HEMT devices in mobile base stations. We highlight here in particular the low TBR, the high mobility, and the 50 times lower memory effect. Figure 1

Deep Trap Levels Responsible for Current Collapse in AlGaN/GaN MISHFET

GaN-based heterostructure field-effect transistors (HFETs) have been attracted considerably as candidates for next-generation devices for both high-power and high frequency applications, because of its excellent material properties such as high breakdown field, high polarization-induced two dimensional electron gas (2DEG) and high saturated electron velocity [1-3]. Despite the advantages, current collapse has been concerned for reliability of AlGaN/GaN HFETs. In this work, we investigated origins of the current collapse in AlGaN/GaN MISHFETs after negative gate bias by observing the spectral photo-responsive drain current and gate-drain spacing dependent effect. Schematic structure of the fabricated MISHFETs is shown in Fig. 1(a), in which the epitaxial layers were grown on the sapphire (0001) substrate by MOCVD. After mesa isolation by TCP-RIE, a 5 nm-thick Al2O3 layer was deposited using PAALD. Si/Ti/Al/Ni/Au (1/25/10/40/100 nm) were deposited for ohmic metal stacks by e-beam evaporator and rapid thermal annealing (RTA) was performed at 850 ˚C for 30 s in N2flow. Subsequently, Ni/Au (40/50 nm) was deposited as the schottky gate contacts. The defined gate length and width of the fabricated devices were 3 μm and 50 μm, respectively. Fig 1(b) shows typical output characteristics of the fabricated MISHFET. Fig. 2 shows the drain currents measured before and after the bias stress when the monochrome lights of different energy were illuminated. Under the illumination condition, it showed only slight collapses of drain current for 500 ~ 600 nm light after the negative bias stress, while showed serious collapse for the other wavelengths. It means that the photons with energy of 2.25 eV was effective to excite the electrons trapped by the deep level which was attributable to the current collapse due to the negative gate bias. This trap energy might be related to the Ga vacancy in GaN bulk layer as widely known as a culprit in previous researches in AlGaN/GaN HFET [4]. Fig. 3 shows a schematic band diagram for the current collapse caused by a negative bias stress. When a high negative bias stress is applied to the gate, the energy band near AlGaN/GaN junction will be bended and the electrons be released from crystal interface which have been trapped in acceptor like states such as Ga vacancy levels as shown in Fig. 3 (a). Fig. 4 shows the drain current suppression at a bias condition of Vgs=1 V and Vds=7V after negative bias stress of -20 V, in which the current decreases as the gate-drain spacing increases. For wide gate-drain spacing, even the electric field between gate and drain decreases, more states seemed to be exposed and contributed to the current collapse at the AlGaN or GaN layer. In conclusion, the current collapse in AlGaN/GaN MISHFET was significant for the wide gate-drain spacing. The photons with energy of 2.25 eV was effective to excite the electrons trapped by the deep levels which were attributed to the current collapse induced by the negative gate bias. This trap energy might be related to the Ga vacancy in GaN bulk layer. Reference [1] P. Waltereit, W. Bronner, R. Quay, M. Dammann, R. Kiefer, S. Müller, M. Musser, J. Kühn, F. van Raay, M. Seelmann, M. Mikulla, O. Ambacher, F. van Rijs, T. Rödle, and K. Riepe, “GaN HEMT and MMIC development at Fraunhofer IAF: performance and reliability,” phys. Status Solidi A, vol. 206, pp. 1215-1220, 2009. [2] D. S. Kim, K. S. Im, K. W. Kim, H. S. Kang, D. K. Kim, S. J. Chang, Y. Bae, S. H. Hahm, S. Cristoloveanu, and J. H. Lee, “Normally-off GaN MOSFETs on insulating substrate,” Solid-State Electron., vol. 90, pp. 79-85, 2013. [3] S. T. Sheppard, K. Doverspike, W. L. Probble, S. T. Allen, J. W. Palmour, L. T. Kehias, and T. J. Jenkins, “High-power microwave GaN/AlGaN HEMTs on semi-insulating silicon carbide substrates,” IEEE Electron Device Lett., vol. 20, pp. 161-163, 1999. [4] J. Neugebauer and C. G. Van de Walle, “Gallium vacancies and the yellow luminescence in GaN,” Appl. Phys. Lett., vol. 69, pp. 503-505, 1996. Figure 1

Ferroelectric films of barium strontium titanate on semi-insulating silicon carbide substrates

Thin ferroelectric BaxSr1–xTiO3 (BST) layers have been grown for the first time on semi-insulating silicon carbide substrates by RF magnetron sputtering of a ceramic target without using buffer sublayers. Results of investigation of the structure of obtained BST films and the electrical properties of related planar capacitors are presented. The obtained structures are characterized by high nonlinearity and low dielectric losses at microwave frequencies.

Silicon Carbide (SiC) Antennas for High-Temperature and High-Power Applications

The main objective of this letter is to evaluate semi-insulating silicon carbide (SiC) material as a candidate for dielectric substrate for patch antennas, with a long-term potential for monolithic antenna integration on a SiC semiconductor chip, operation in extreme environments, and other applications. First, computer-aided design of microstrip patch antennas operating at 10 GHz was conducted. The antenna designs were implemented using semi-insulating SiC substrates and gold ground planes and patches. A good agreement between the experimental results and simulation was obtained at the band of operation. Return loss and radiation patterns were investigated. As future work, a possibility of utilizing highly conductive (heavily doped) SiC epitaxial layers as the ground planes and radiating patches were investigated using computer simulations.

107-GHz (Al,Ga)N/GaN HEMTs on Silicon With Improved Maximum Oscillation Frequencies

We report high-speed fully passivated deep submicrometer (Al,Ga)N/GaN high-electron mobility transistors (HEMTs) grown on (111) high-resistivity silicon with current gain cutoff frequencies of as high as fT = 107 GHz and maximum oscillation frequencies reaching fMAX = 150 GHz. Together, these are the highest fT and fMAX values achieved for GaN-based HEMTs implemented on silicon substrates to date. The performance reported here is competitive with recently published results for similar geometry high-performance passivated HEMTs on semi-insulating silicon-carbide substrates.

GaN Schottky Diodes for Microwave Power Rectification

A GaN Schottky diode with a lateral structure for microwave power rectification was developed on a semi-insulating silicon carbide substrate. Device evaluation showed that the turn-on voltage was around 0.8 V. The on-resistance of the diode with one finger was 25.6 Ω, the breakdown voltages for those with the field plate reached 93 V, for the wafer with a doping level of 4.0 ×1016 cm-3. The forward and reverse characteristics became stabilized after surface etching. RF measurement at 2.45 GHz showed that the capacitance of the diode was about 0.29 pF at a bias of 0 V. The value is satisfactorily small for microwave rectification. If the superior material characteristics of GaN are fully utilized, GaN Schottky diodes will play key roles in microwave power transmission applications.