Calvin H. Carter

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.

Characterization of thick 4H-SiC hot-wall CVD layers

Epitaxial 4H-SiC layers suitable for high power devices have been grown in a hot-wall chemical-vapor deposition (CVD) system. These layers were subsequently characterized for many parameters important in device development and production. The uniformity of both thickness and doping will be presented. Doping trends vs. temperature and growth rate will be shown for the p -type dopant used. The n -type dopant drops in concentration with increasing temperature or increasing growth rate. In contrast, the p -type dopant increases in concentration with decreasing temperature or increasing growth rate. A simple descriptive model for this behavior will be presented. The outcome from capacitance-voltage and SIMS measurements demonstrate that transitions from n to n − , or p to p − , and even n to p levels can be made quickly without adjustment to growth conditions. The ability to produce sharp transitions without process changes avoids degrading the resulting surface morphology or repeatability of the process. Avoiding process changes is particularly important in growth of thick layers since surface roughness tends to increase with layer thickness. Device results from diodes producing two different blocking voltages in excess of 5 kV will also be shown. The higher voltage diodes exhibited a breakdown behavior which was near the theoretical limit for the epitaxial layer thickness and doping level grown.

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 During 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.