Bernd Zippelius

High temperature annealing of n-type 4H-SiC: Impact on intrinsic defects and carrier lifetime

In this paper, the impact of high-temperature annealing of 4H silicon carbide (SiC) on the formation of intrinsic defects, such as Z1/2 and EH6/7, and on carrier lifetimes was studied. Four nitrogen-doped epitaxial layers with various initial concentrations of the Z1/2- and EH6/7-centers (1011-1014cm-3) were investigated by means of deep level transient spectroscopy and microwave photoconductance decay. It turned out that the high-temperature annealing leads to a monotone increase of the Z1/2- and EH6/7-concentration starting at temperatures between 1600 °C and 1750 °C, depending on the initial defect concentration. In the case of samples with high initial defect concentration (1014cm-3) a distinct decrease in Z1/2- and EH6/7-concentration in the temperature range from 1600 °C to 1750 °C was observed, being consistent with previous reports. For higher annealing temperatures (Tanneal≥1750 ∘C), the defect concentration is independent of the samples’ initial values. As a consequence, beside the growth conditions, such as C/Si ratio, the thermal post-growth processing has a severe impact on carrier lifetimes, which are strongly reduced for samples annealed at high temperatures.In this paper, the impact of high-temperature annealing of 4H silicon carbide (SiC) on the formation of intrinsic defects, such as Z1/2 and EH6/7, and on carrier lifetimes was studied. Four nitrogen-doped epitaxial layers with various initial concentrations of the Z1/2- and EH6/7-centers (1011-1014cm-3) were investigated by means of deep level transient spectroscopy and microwave photoconductance decay. It turned out that the high-temperature annealing leads to a monotone increase of the Z1/2- and EH6/7-concentration starting at temperatures between 1600 °C and 1750 °C, depending on the initial defect concentration. In the case of samples with high initial defect concentration (1014cm-3) a distinct decrease in Z1/2- and EH6/7-concentration in the temperature range from 1600 °C to 1750 °C was observed, being consistent with previous reports. For higher annealing temperatures (Tanneal≥1750 ∘C), the defect concentration is independent of the samples’ initial values. As a consequence, beside the growth condit...

Lifetime Investigations of 4H-SiC PiN Power Diodes

Lifetime measurements are performed on 4H-SiC pin power diodes (6.5 kV). The lifetime values in the base range from 1.1 s to 2.1 s; these values demonstrate the high quality of the 4H-SiC epilayer and the optimized device processing. The observed lifetimes are correlated with deep defect centers detected by deep level transient spectroscopy. The role of the Z1/2-center as a lifetime killer is discussed.

Detection and Electrical Characterization of Defects at the SiO2/4H-SiC Interface

Two electrical measurement techniques are frequently employed for the characteri- zation of traps at the SiO2/SiC interface: the thermal dielectric relaxation current (TDRC) and the conductance method (CM). When plotting Dit as a function of the energy position Eit in the bandgap both techniques reveal comparable results for deep interface traps (ECEit > 0:3 eV). For shallower traps, CM always shows a strong increase of Dit which originates from near interface traps (NIT). TDRC provides a contradictory result, namely a slight decrease of Dit. In this paper, we show that the position of NITs in the oxide close to the interface is responsible for the invisibility of these traps in TDRC spectra. We further show that NITs become detectable by the TDRC method by using a discharging voltage Vdis close to the accumulation regime. However, due to the Shockley-Ramo-Theorem the contribution of NITs to the Dit in TDRC spectra is strongly suppressed and can be increased by using thin oxides.

Correlation between Leakage Current and Stacking Fault Density of p-n Diodes Fabricated on 3C-SiC

The correlation between leakage current and stacking fault (SF) density in p-n diodes fabricated on 3C-SiC homo-epitaxial layer is investigated. The leakage current density at reverse bias strongly depends on the SF density; an increase of one order of magnitude in the SF density enhances the leakage current by five orders of magnitude at a reverse bias of 400 V. In order to obtain commercially suitable MOSFETs with 10-4Acm-2 at 600V, the SF density has to be reduced below 6×104 cm-2. Photoemission caused by hot electrons, which travel along a leakage path, can be observed at the crossing between a SF and the edge of p-well region; where the maximum electric field is induced. The mechanism of the leakage current is discussed in detail in a separate paper.

Iron-Related Defect Centers in 4H-SiC Detected by Deep Level Transient Spectroscopy

Fe-implanted n-/p-type 4H-SiC samples were investigated by deep level transient spectroscopy (DLTS). In order to be able to separate Fe-related defect centers from defects caused by implantation damage, a corresponding Ar-profile was implanted. No Fe-related defects were observed in n-type 4H-SiC, while two Fe-related centers could be identified in p-type 4H-SiC. The electrical behavior of these centers is donor-like.

Thermal Stability of Defect Centers in n- and p-Type 4H-SiC Epilayers Generated by Irradiation with High-Energy Electrons

This paper comprises a systematic study of the thermal stability of defect centers observed in n- and p-type 4H-SiC by deep level transient spectroscopy (DLTS); the defects are generated by irradiation with high-energy electrons of 170 keV or 1 MeV.

Preannealing Effect on Mobility of N-/Al-Coimplanted and Over-Oxidized 4H-SiC MOSFETs

The authors investigated the effect of preannealing on N-/Al-coimplanted and over-oxidized Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs). The preannealing process causes a decrease of the Hall mobility and the effective mobility, and an increase of the interface state density. Secondary ion mass spectroscopy (SIMS) measurements revealed that the N concentration at the SiO2/SiC interface in preannealed samples is lower than in not-preannealed samples, which might be the reason for in the increase of the interface state density. In MOSFETs without preannealing, more N atoms are piled up at the SiO2/SiC interface, leading to the lower interface state density and higher mobility.

Dependence of the Channel Mobility in 3C-SiC n-MOSFETs on the Crystal Orientation and Channel Length

The channel mobility in 3C-SiC n-MOSFETs is investigated by current-voltage and Hall-effect measurements. For comparison, these techniques are also applied to 3C-SiC bulk rods. It turns out that the channel mobility depends on the orientation of the crystal and channel length. The observed results are traced back to the influence of Si-terminated stacking faults (Si-SFs), to the resistance of the drain/source contact and to the warping of the wafer caused by the special growth technique.

Iron-Related Defect Centers in 3C-SiC

p-type 3C-SiC samples were implanted by iron (Fe) and investigated by means of deep level transient spectroscopy (DLTS). Corresponding argon (Ar) profiles with similar implantation damage were implanted in order to distinguish between iron-related defects and defects caused by implantation damage. Two donor-like iron-related centers were identified in p-type 3C-SiC.

Shallow Defects Observed in As-Grown and Electron-Irradiated or He+-Implanted Al-Doped 4H-SiC Epilayers

Aluminum-doped 4H-SiC samples were either irradiated with high-energy electrons (170 keV or 1 MeV) or implanted with a box-shaped He+-profile. Admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) were employed to search for defect centers. AS spectra of as-grown as well as of electron-irradiated (170 keV or 1 MeV) 4H-SiC epilayers reveal the Al acceptor (ΔE(Al) = 200 meV) and an unknown defect (ΔE(SB) = 177 meV), while AS spectra of the He+-implanted and annealed sample show in addition to the Al-acceptor two energetically deeper acceptor-like defect centers (ΔE(RE3) = 255 meV and ΔE(KR3) = 375 meV). The KR3-center is not directly formed by the He+-implantation, it requires an annealing process. The DLTS spectra of the He+-implanted and annealed sample resolve a double-peak structure of the KR3-defect (ΔE(KR3A) = 380 meV and ΔE(KR3B) = 410 meV).