H. McD. Hobgood

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 ...

Status of 4H-SiC Substrate and Epitaxial Materials for Commercial Power Applications

The performance enhancements offered by the next generation of SiC high power devices offer potential for enormous growth in SiC power device markets in the next few years. For this growth to occur, it is imperative that substrate and epitaxial material quality increases to meet the needs of the targeted applications. We will discuss the status and requirements for SiC substrates and epitaxial material for power devices such as Schottky and PiN diodes. For the SiC Schottky device where current production is approaching 50 amp devices, there are several material aspects that are key. These include; wafer diameter (3-inch and 100-mm), micropipe density ( −2 for 3-inch substrates and 16 cm −2 for 100-mm substrates), epitaxial defect densities (total electrically active defects −2 ), epitaxial doping and epitaxial thickness uniformity. For the PiN diodes the major challenge is the degradation of the Vf characteristics due to the introduction of stacking faults during the device operation. We have demonstrated that the stacking faults are often generated from basal plane dislocations in the active region of the device. Additionally we have demonstrated that by reducing the basal plane dislocation density, stable PiN diodes can be produced. At present typical basal plane dislocation densities in our epitaxial layers are 100 to 500 cm −2 ; however, we have achieved basal plane dislocation densities as low as 4 cm −2 in epitaxial layers grown on 8° off-axis 4H-SiC substrates.

Electron paramagnetic resonance studies of a carbon vacancy-related defect in as-grown 4H–SiC

Abstract An intrinsic defect (ID) has been identified in as-grown 4H–SiC by electron paramagnetic resonance (EPR). The EPR parameters of an ID measured in our nominally semi-insulating material are similar to the literature data of the EI5 defect produced in p-type 4H and 6H–SiC by 2.5xa0MeV electrons and assigned to the carbon vacancy (N.T. Son, P.N. Hai, E. Jansen, Phys. Rev. B 63 (2000) 201201). However, comparison of the ID and EI5 centers reveals that the as-grown and radiation-induced centers exhibit different annealing behavior. Photo-induced EPR locates the ID level 0.9xa0eV above the valence band edge.

Nitrogen doping and multiplicity of stacking faults in SiC

This paper reports on the strong enhancement of stacking fault (SF) formation in 4H–SiC by heavy nitrogen doping. The paper consists of two separate observations. The first part reports on localized but severe deformation bands observed in certain regions of 4H–SiC wafers that had undergone high temperature processing during device fabrication. Using a combination of dynamic secondary ion mass spectroscopy (SIMS) and conventional, weak-beam (WB) and high-resolution (HR) transmission electron microscopy (TEM), the affected regions of the wafers were found to have a much higher concentration of nitrogen and to contain a high density of stacking faults. In contrast, in the non-affected regions of the wafers, the nitrogen concentration was lower and no lattice defects could be observed by TEM, indicating that the severely deformed morphology of the affected regions was due to the high stacking fault content. Moreover, the stacking faults in the affected regions were found to be invariably double and not single-layered, formed by the glide of two leading partial dislocations on adjacent (0001) planes. The second part of the paper reports on the occurrence of stacking faults during deformation tests on heavily nitrogen-doped 4H–SiC. Combining optical microscopy, HR and weak-beam (WB) TEM, the generated faults were found to be double-layered as well. It is interesting that in neither type of experiment, trailing partials were observed: it appears that the SFs were not in the form of ribbons bound by leading and trailing partials but rather in the form of faulted loops on two adjacent planes, each loop bound by a leading Shockley partial of the same Burgers vector. The results of the two observations are explained by the stabilization of double-layer stacking faults (DSFs) when the Fermi level of the faulted crystal is pushed up by nitrogen doping to above the stacking fault energy level.

High Carrier Lifetime Bulk-Grown 4H-SiC Substrates for Power Applications

A thermal anneal process has been developed that significantly enhances minority carrier lifetime (MCL) in bulk-grown substrates. Microwave photoconductivity decay (MPCD) measurements on bulk grown substrates subjected to this process have exhibited decay times in excess of 35 μs. Electron Beam Induced Current (EBIC) measurements indicated a minority carrier diffusion length (MCDL) of 65 μm resulting in a calculated MCL of 15 μs, well within the range of that measured by MPCD. Deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z1/2, 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. PiN diodes were fabricated employing 350 μm thick bulk-grown substrates as the intrinsic drift region and thin p- and n-type epitaxial layers on either face of the substrate to act as the anode and cathode, respectively. Conductivity modulation was achieved in these diodes with a 10x effective carrier concentration increase over the background doping as extracted from the differential on-resistance. Significant stacking fault generation observed during forward operation served as additional evidence of conductivity modulation and underscores the importance of reducing dislocation densities in substrates in order to produce a viable bulk-grown drift layer.

Growth and Characterization of Semiconductor Silicon Carbide for Electronic and Optoelectronic Applications: An Industrial Perspective

Publisher Summary nDuring the past decade, silicon carbide (SiC) semiconductor device technology for electronic and optoelectronic applications has made tremendous progress resulting primarily from the commercial availability of SiC substrates of ever increasing diameter and quality. Throughout the technical evolution of semiconductor SiC, the fabrication of SiC crystals exhibiting the desired electrical and crystalline properties has played a central role in the realization of the full potential of this important semiconductor material. The aim of this chapter is to discuss, from an industrial viewpoint, the current state of SiC crystal growth technology and to present empirical results that reflect the recent advances in SiC crystal growth. Recent progress in the development of the physical vapor transport (PVT) technique for SiC bulk growth has led to substrate diameters up to 100-mm, residual impurities in the lO15 cm-3 range, thermal conductivity approaching 5.0 W/cmK in bulk crystals, transparent 6H and 4H-SiC at crystal diameters up to 75-mm, and micropipe densities as low as 0.9 cm-2 over a 50-mm diameter 4H-SiC wafer. These advances help to position SiC for an exciting future and provide a sound foundation for the realization of the full potential of SiC for high power density electronic devices, optoelectronic devices of high brightness, and SiC materials applications requiring low optical absorption.