Jason Ronald Jenny

Sublimation-Grown Semi-Insulating SiC for High Frequency Devices

In this paper we show the progression in the development of semi-insula ti g SiC grown by the sublimation technique from extrinsically doped material to hig h purity semi-insulating (HPSI) 4H-SiC bulk crystals of 2-inch and 3-inch diameter without re sorting to the intentional introduction of elemental deep level dopants, such as vanadium. Secondary ion m ass spectrometry, optical absorption, deep level transient spectroscopy and electron parama gnetic resonance data suggest that the semi-insulating behavior in HPSI material orig inates from deep levels associated with intrinsic point defects. While high temperature resistivity measurements on different high purity 4H-SiC samples indicate activation energies ranging from 0.9 to 1.6 eV, HPSI wafers with homogeneous activation energies near mid-gap are demonstrated. The roomtemperature thermal conductivity of this material approaches the theoretical maximum of ~ 5 W/cmK. Additionally, HPSI substrates exhibit micropipe densities as low as 8 cm -2 over the full diameter of a 3-inch wafer. MESFETs produced on HPSI wafers are free of backgating effects and have resulted in the best combination of power density and efficiency reported to date for SiC M ESFETs of 5.2 W/mm and 63% power added efficiency (PAE) at 3.5 GHz.

100 mm 4HN-SiC Wafers with Zero Micropipe Density

Recent advances in PVT c-axis growth process have shown a path for eliminating micropipes in 4HN-SiC, leading to the demonstration of zero micropipe density 100 mm 4HN-SiC wafers. Combined techniques of KOH etching and cross-polarizer inspections were used to confirm the absence of micropipes. Crystal growth studies for 3-inch material with similar processes have demonstrated a 1c screw dislocation median density of 175 cm-2, compared to typical densities of 2x103 to 4x103 cm-2 in current production wafers. These values were obtained through optical scanning analyzer methods and verified by x-ray topography.

Development of Large Diameter High-Purity Semi-Insulating 4H-SiC Wafers for Microwave Devices

The next generation of wireless infrastructure will rely heavily upon wide band gap semiconductors owing to their unique materials properties, including: their large bandgap, high thermal conductivity, and high breakdown field. To facilitate implementation of this next generation, a significant effort is required to make SiC MESFET and GaN HEMT microwave devices more suitable for widespread application. Currently, the interest in high-purity semiinsulating (HPSI) 4H-SiC is critically tied to its influence on microwave devices, whether performance or affordability. To address these issues, we have developed high-purity 3-inch and 100 mm 4H-SiC substrates with low micropipe densities (as low as 1.4 cm -2 in 3-inch and <60 cm -2 in 100 mm) and uniform semi-insulating properties (>10 9 Ωcm) over the full wafer diameter. These wafers possess typical residual shallow level contamination less than 1x10 16 cm -3 (5x10 15 nitrogen and 3x10 15 boron) with best nitrogen values of 3x10 14 . In this paper, we will report on the development of our HPSI growth process focusing on the specific areas of the assessment of semiinsulating character and device applicability.

Silicon Carbide Crystal and Substrate Technology: A Survey of Recent Advances

The quest of driving SiC toward the realization of its full potential as a semiconductor material continues in many organizations world-wide. R&D and manufacturing efforts continue to address issues of scale-up of wafer size, improvements in wafer shape and surface characteristics, reduction of background impurities in bulk crystals, controlled uniformity of electrical properties, and reduction and control of crystalline defects. Significant progress has been made in several key areas. Increased manufacturing activity in the production of 3-inch diameter crystals has led to substrates with micropipes densities <30 cm -2 in n-type and <80 cm -2 in semi-insulating material, and R&D demonstrations of substrates exhibiting micropipe densities <0.5 cm -2 in n-type and <5 cm -2 in semi-insulating wafers. Developmental 100-mm diameter substrates exhibiting micropipe densities <60 cm -2 in both n-type and semi-insulating materials have now been demonstrated. Significant improvement in bulk crystal purity has been achieved with reduction of impurity concentrations below 5 x 10 15 cm -3 .

Effects of annealing on carrier lifetime in 4H-SiC

We present results of a thermal anneal process that increases the minority carrier lifetime in SiC substrates to in excess of 3μs, compared to the starting as-grown substrates with lifetimes typically in the <10ns range. Measurement of lifetimes was conducted using microwave-photoconductive decay. Electron beam induced current measurements exhibited minority carrier diffusion lengths of up to 65μm, confirming the enhanced carrier lifetime of the annealed substrate material. Additionally, positron annihilation spectroscopy and deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z1∕Z2, may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. The operational performance of p-i-n diodes employing drift la...

GROWTH OF SiC SUBSTRATES

In recent years SiC has metamorphisized from an R&D based materials system to emerge as a key substrate technology for a significant fraction of the world production of green, blue and ultraviolet LEDs. Emerging markets for SiC homoepitaxy include high-power switching devices and microwave devices. Applications for heteroepitaxial GaN-based structures on SiC substrates include lasers and microwave devices. In this paper we review the properties of SiC, assess the current status of substrate and epitaxial growth, and outline our expectations for SiC in the future.

Microwave photoconductivity decay characterization of high-purity 4H-SiC substrates

A microwave photoconductivity decay (MPCD) technique, which probes conductivity change in wafers in response to either an above-band-gap or below-band-gap laser pulse, has been used to characterize recombination lifetime in high-purity 4H-SiC substrates produced with three different anneal processes. The above-band-gap (266nm) decay times vary from ∼10ns to tens of microseconds in the 4H-SiC substrates depending on the wafer growth parameters. Wafers produced using the three processes A (as-grown), B (annealed at 2000°C), and C (annealed at 2600°C) have decay times of 10–20ns, 50–500ns, and tens of microseconds, respectively. The differences in decay times are attributed to low, medium, and high densities of recombination centers in process C, B, and A wafers, respectively. The MPCD results correlate with other characterization results such as deep level transient spectroscopy, which also showed that the 2600°C anneal process significantly reduces defect densities, resulting in the enhanced recombination ...

RF Performance and Reliability of SiC MESFETs on High Purity Semi-Insulating Substrates

In this paper we report on our efforts to reduce trap effects, increase efficiency, and improve the yield and reliability of SiC MESFETs. By minimizing substrate and surface-related trapping effects that have previously been observed in SiC MESFETs, drain efficiencies as high as 68% have been achieved at 3.5 GHz with associated CW power densities of 3.8 W/mm. MESFETs fabricated with this process have passed 1,000 hour High Temperature Reverse Bias test (HTRB) with negligible change in dc or RF parameters. A sampling of these devices have also been running for over 2,000 hours in an RF high temperature operating life test (HTOL) with negligible change in parameters. This MESFET process has been transferred to 3-inch high purity semiinsulating (HPSI) substrates. The quality of this process is demonstrated by the cross-wafer uniformity of the breakdown voltage and a standard deviation in gate threshold voltage of 0.6 V. Introduction SiC MESFETs have received increased attention in recent years due to their high power density and high operating voltage, which will enable wider bandwidth, higher performance, and lighter weight systems than those using conventional Si or GaAs technology. Significant progress has been achieved in the development and demonstration of high power MESFETs and wide band amplifiers based on this technology [1]. However, undesirable problems related to trapping in the substrate and/or the surface have also been reported [2-5]. These issues need to be fully resolved to improve device performance, reliability, and to make this technology commercially viable. In this paper we present our recent results for SiC MESFETs that exhibit minimal trap-related effects. Extensive data on the reliability of these devices is also presented. MESFET Performance The MESFETs in this work were fabricated on 2-inch diameter high-purity semi-insulating (HPSI) 4H-SiC substrates available from Cree. The MESFETs were fabricated with dry-etched isolation mesas, sintered Ni ohmic contacts, a gate length of 0.45 μm, and air-bridge source interconnects. These devices were fully passivated with 0.5 μm of silicon nitride for environmental protection and reliability. Devices fabricated with this process typically show maximum channel current, Imax, of 360 mA/mm, pinch-off voltage of –10 V, peak transconductance of 42 mS/mm, and gate-drain breakdown voltage in excess of 120 V. These DC characteristics translate to excellent RF performance in both Class A and deep Class AB conditions. Fig. 1 shows an on-wafer CW load pull measurement at 3.5 GHz of a 1-mm device biased at a very low bias current of 3% Imax and VDS=50 V. The power output is 3.8 W with an associated drain efficiency of 68%. The associated PAE under these conditions is typically in the range of 53% 58%. The high drain efficiency under near pinch-off condition clearly demonstrates that substrate and surface trapping effects previously observed in SiC MESFETs have been greatly reduced. Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1205-1208 doi:10.4028/www.scientific.net/MSF.457-460.1205

Enhanced Carrier Lifetime in Bulk-Grown 4H-SiC Substrates

To devise a means of circumventing the cost of thick SiC epitaxy to generate drift layers in PiN diodes for >10kV operation, we have endeavored to enhance the minority carrier lifetimes in bulk-grown substrates. In this paper, we discuss the results of a process that has been developed to enhance minority carrier lifetimes to in excess of 30 μs in bulk-grown 4H-SiC substrates. Measurement of lifetimes was principally conducted using microwave-photoconductive decay (MPCD). Confirmation of the MPCD lifetime result was obtained by electron beam induced current (EBIC) measurements. Additionally, deep level transient spectroscopic analysis of samples subjected to this process suggests that a significant reduction of deep level defects in general and of Z1/Z2, specifically, may account for the significantly enhanced lifetimes. Finally, a study of operational performance in devices employing drift layers fabricated from substrates produced by this process confirmed ambipolar lifetimes in the microsecond range.

Comparison between measurement techniques used for determination of the micropipe density in SiC substrates

Micropipe density (MPD) is a crucial parameter for silicon carbide (SiC) substrates that determines the quality, stability and yield of the semiconductor devices built on these substrates. The importance of MPD is underscored by the fact that all existing specifications for 6H- and 4H-SiC substrates set upper limits for it. Several methods for measuring the MPD are known, however, their reliability and applicability to various types of substrates (e.g. semiinsulating, conducting, etc.) has not been systematically studied. The subject of this paper is a comparative study of various techniques used for the MPD measurement accompanied by statistical analysis of the results. The study was initiated by several organizations working in the immediate field of silicon carbide or in closely related fields and included SiC substrate manufacturers, substrate consumers, equipment manufacturers and universities. The study represented a round robin experiment in which MPD was measured on thirty SiC wafers of various pedigrees. The values of MPD have been determined using both destructive and non-destructive techniques. The repeatability of each technique is analyzed and compared with that of other techniques.