P-type 4H-SiC Epilayers Research Articles

Characterization of dislocation etch pits by molten KOH etching in n- and p-type 4H–SiC epilayers doped by ion implantation

Dislocation etch pits by molten KOH etching in 4H–SiC epilayers doped by ion implantation have been characterized in this work. The sizes and shapes of dislocation etch pits are studied in an implanted nitrogen concentration range from 2 × 1019 cm−3 to 4 × 1019 cm−3 and an implanted aluminum concentration range from 1 × 1018 cm−3 to 1 × 1019 cm−3. It has been found that the etch rates at dislocations follow a trend of n+ < n− < p− ≈ p + while in all samples the threading dislocation etch pits show the anisotropic etching characteristic of hexagonal shape. Characteristics of dislocation etch pits in n + -4H–SiC samples are analyzed based on the effect of preferential N occupation at dislocations.

Hole capture cross section of the Al acceptor level in 4H-SiC

We performed current deep level transient spectroscopy (I-DLTS) on Al-doped p-type 4H-SiC epilayers. The epilayers exhibited a peak at 100–120 K in the I-DLTS spectra. The correspondence of the deep-level concentrations with the net acceptor concentration implies that the peak originates from the Al acceptor level. The observed activation energy of the peak was low compared to the Al acceptor level reported by Hall measurements owing to the Poole-Frenkel effect. We estimated capture cross section σ for holes of the Al acceptor level at the peak temperature based on the I-DLTS peak height dependence on the injection pulse widths and analyzed the temperature dependence of σ for holes.

Carrier lifetime control by intentional boron doping in aluminum doped p-type 4H-SiC epilayers

The carrier lifetime control in p-type 4H-SiC epilayers with intentional aluminum (Al) and boron (B) doping is demonstrated as part of work to develop a p-type “recombination-enhancing layer” for n-channel insulated gate bipolar devices fabricated on p-type substrates. The (Al + B)-doped epilayers (Al: 5 × 1017, B: 4 × 1016 cm−3) showed a very short minority carrier lifetime of less than 20 ns at 293 K, resembling that of highly Al-doped epilayers (Al: 1 × 1019 cm−3). Besides, the minority carrier lifetimes in (Al + B)-doped epilayers are stable against post-annealing in Ar and H2 ambient, while that of Al-doped epilayers varied considerably. PiN diodes with a 10 μm-thick (Al + B)-doped buffer layer inserted on p-type substrates showed no evident degradation after a stress test under a pulse current density of 2000 A/cm2.

Anomalous Temperature Dependence of the Hall Coefficient of Heavily Al-Doped 4H-SiC Epilayers in the Band Conduction Region

We investigate the temperature dependence of the resistivity and Hall coefficient for heavily Al-doped p-type 4H-SiC epilayers with Al concentrations (C_Al) of &gt; 2E19 cm^−3, which are substrates for the collectors of insulated-gate bipolar transistors. The signs of the measured Hall co- efficients (R_H) changed from positive to negative at low temperatures. For epilayers with C_Al values of &lt; 3E19 cm^−3, a negative R_H was observed in the hopping conduction region. In contrast, for epilayers with C_Al values of &gt; 3E19 cm^−3, a negative R_H was observed in not only the hopping conduction region but also the band conduction region, which is a striking feature because the movement of free holes in the valence band should make R_H positive. For an epilayer with C_Al of 1.8E20 cm^−3, the sign of R_H clearly changed three times in the band conduction region. Moreover, the activation energies of the temperature-dependent R_H values were similar to those of the temperature-dependent resistivity in the corresponding temperature ranges, irrespective of the conduction mechanisms (band and hopping conduction).

Comparison of Conduction Mechanisms in Heavily Al-Doped 4H-SiC and Heavily Al- and N-Codoped 4H-SiC

The conduction mechanism in heavily Al-doped or heavily Al-and N-codoped p-type 4H-SiC epilayers was investigated. In both the singly-doped and codoped samples with an Al concentration (CAl) between 4x1019 and 2x1020 cm-3, band and nearest-neighbor hopping (NNH) conductions appeared in high and low temperature ranges, respectively. The codoping of N donors makes the NNH conduction dominant at temperatures higher than in the singly-doped samples. In both the singly-doped and codoped samples with CAl between 1x1019 and 4x1019 cm-3, an unexpected conduction appeared between the regions of the band and NNH conductions.

Formation of D-Center in p-Type 4H-SiC Epi-Layers during High Temperature Treatments

The current work is devoted to studying the evolution of deep level defects in the lower half of the 4H-SiC bandgap after high temperature processing and ion implantation. Two as-grown and pre-oxidized 4H-SiC sets of samples have been thermally treated at temperatures up to 1950 °C for 10 min duration using RF inductive heating. Another set of as grown samples was implanted by 4.2 MeV Si ions at room temperature (RT) with different doses (1-4×108 cm-2). The so-called “D-center” at EV+0.6 eV dominates and forms after the elevated heat treatments, while it shows no change after the ion implantations (EV denotes the valence band edge). In contrast, the concentration of the so-called HK4 level at EV+1.44 eV increases with the implantation dose, whereas it anneals out after heat treatment at ≥ 1700 °C.

Optical Characterization of p-Type 4H-SiC Epilayers

A series of aluminium doped (from 2×1016 to 8×1019 cm-3) 4H-SiC epitaxial layers were mainly studied by Low Temperature Photoluminescence and time-resolved optical pump-probe techniques to determine the concentration of aluminium, its activation ratio, the doping related carrier lifetime, hole mobility and excess carrier diffusion length.

Determination of carrier lifetime and diffusion length in Al-doped 4H–SiC epilayers by time-resolved optical techniques

A series of p-type 4H–SiC epilayers with aluminium concentration ranging from 2 × 1016 to 8 × 1019 cm−3 were investigated by time-resolved optical techniques in order to determine the effect of aluminium doping on high-injection carrier lifetime at room temperature and the diffusion coefficient at different injections (from ≈3 × 1018 to ≈5 × 1019 cm−3) and temperatures (from 78 to 730 K). We find that the defect limited carrier lifetime τSRH decreases from 20 ns in the low-doped samples down to ≈0.6 ns in the heavily doped epilayers. Accordingly, the ambipolar diffusion coefficient decreases from Da = 3.5 cm2 s−1 down to ≈0.6 cm2 s−1, corresponding to the hole mobility of µh = 70 cm2 Vs−1 and 12 cm2 Vs−1, respectively. In the highly doped epilayers, the injection-induced decrease of the diffusion coefficient, due to the transition from the minority carrier diffusion to the ambipolar diffusion, provided the electron diffusion coefficient of De ≈ 3 cm2 s−1. The Al-doping resulted in the gradual decrease of the ambipolar diffusion length, from LD = 2.7 µm down to LD = 0.25 µm in the epilayers with the lowest and highest aluminium concentrations.

Enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epitaxial layers by combination of thermal oxidation and hydrogen annealing

We investigated the enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epilayers (NA ≃ 2 × 1014 cm−3) by postgrowth processing. A carrier lifetime of 2.8 µs in an as-grown epilayer is increased to 5.1 µs by carbon vacancy elimination, i.e., thermal oxidation at 1400 °C for 48 h. It reaches 10 µs by subsequent hydrogen annealing at 1000 °C for 10 min. The carrier lifetime in the as-grown epilayer is also increased to 4.0 µs by only hydrogen annealing. These results suggest that, in addition to carbon vacancy, there is another lifetime killer in p-type SiC, which cannot be eliminated by thermal oxidation but can be passivated by hydrogen annealing.

Observation of deep levels and their hole capture behavior in p-type 4H-SiC epilayers with and without electron irradiation

To design SiC bipolar devices, information on deep levels acting as recombination centers is essential. In this paper, we report on the observation of deep levels in p-type 4H-SiC epilayers with and without electron irradiation before and after annealing at 1000 °C. We performed current deep level transient spectroscopy (I-DLTS) for the samples, and the observed deep levels were located near the valence band (with the activation energies less than 0.35 eV) in all the samples. Based on the change of I-DLTS spectra by the electron irradiation or the annealing, we discussed the origins of the deep levels. Then we estimated the time constants of hole capture by the deep levels and discussed the possibility that the observed deep levels behave as recombination centers.

Minority Carrier Transient Spectroscopy of As-Grown, Electron Irradiated and Thermally Oxidized p-Type 4H-SiC

A total of nine electrically active levels have been detected in as-grown and electron irradiated p-type 4H-SiC epilayers. These traps are found in the 0.32-2.26 eV energy range, above the valence band edge (EV). Of these, six are majority carrier traps whereas three are minority carrier traps. We found that thermal oxidation affects the concentrations of two midgap levels, the majority carrier trap, labeled HK4 and the minority carrier trap identified as EH6/7. The analysis of the irradiation energy and dose dependence of the concentration of these two traps, rules out the possibility that they may share the same origin.

Alternative method of interface traps passivation by introducing of thin silicon nitride layer at 4H-SiC/SiO2 interface

ABSTRACTAn alternative approach for reduction of interface traps density at 4H-SiC/SiO2 interface is proposed. Silicon nitride / silicon oxide stack was deposited on p-type 4H-SiC (0001) epilayers and subsequently over-oxidized. The electrical characterization of the interface was done by employing metal-oxide semiconductor (MOS) devices, inversion-channel MOS devices and lateral MOS field effect transistors (MOSFETs).

Capacitance spectroscopy study of deep levels in Cl-implanted 4H-SiC

We report on a deep level transient spectroscopy study of Cl-implanted n- and p-type 4H-SiC epilayers. Samples were electrically characterized after each step of an isochronal annealing series, between room temperature and 1800 ∘C, and twelve deep traps were detected. Of these, seven traps were found in n-type material, ranging from 0.15 to 1.6 eV below the conduction band minimum (EC), and five were detected in p-type material, located between 0.5 and 1.7 eV above the valence band maximum (EV). Besides the presence of the well known Z1/2 and EH6/7 levels in n-type 4H-SiC and of the D-center in p-type samples, we found that Cl implantation gives rise to three new traps in n-type material at EC−0.37 eV, EC−1.06 eV, and EC−1.3 eV and one new level in p-type at EV+0.97 eV. These traps are persistent after annealing at 1800 ∘C, and no data were found in the previous experimental studies reported in the literature. The possible involvement of Cl in the microscopic structure of these defects is discussed based on a depth profiling analysis of their concentration.

Enhancement and control of carrier lifetimes in p-type 4H-SiC epilayers

Enhancement and control of carrier lifetimes in p-type 4H-SiC have been investigated. In this study, thermal oxidation and carbon ion implantation methods, both of which are effective for lifetime enhancement in n-type SiC, were attempted on 147-μm thick p-type 4H-SiC epilayers. Effects of surface passivation on carrier lifetimes were also investigated. The carrier lifetimes in p-type SiC could be enhanced from 0.9 μs (as-grown) to 2.6 μs by either thermal oxidation or carbon implantation and subsequent Ar annealing, although the improvement effect for the p-type epilayers was smaller than that for the n-type epilayers. After the lifetime enhancement, electron irradiation was performed to control the carrier lifetime. The distribution of carrier lifetimes in each irradiated region was rather uniform, along with successful lifetime control in the p-type epilayer in the range from 0.1 to 1.6 μs.

Chlorine in SiC: Experiment and Theory

An annealing study, in the 100-1400 C temperature range ,was carried out on Cl-implanted n- or p-type 4H-SiC epilayers. The electrical characterization of the epilayers shows the rise of several deep levels and the role of Cl, on both carrier concentration and defects' microscopic structure, is discussed in the light of theoretical results obtained by density functional calculations performed on a 64-atom cubic SiC supercell.

Excess Carrier Lifetime in p-Type 4H-SiC Epilayers with and without Low-Energy Electron Irradiation

Excess Carrier Lifetime in p-Type 4H-SiC Epilayers with and without Low-Energy Electron Irradiation

Excess Carrier Lifetime in p-Type 4H-SiC Epilayers with and without Low-Energy Electron Irradiation

We measured excess carrier lifetimes in as-grown and low-energy electron-irradiated p-type 4H-SiC epitaxial layers by the microwave photoconductivity decay method. The carrier lifetime increases with the excitation density in the epilayers. This dependence suggests that the dominant recombination center in the epilayers has a larger capture cross section for electrons than for holes. The low-energy electron irradiation reduces the carrier lifetime in the epilayers, and 1000 °C annealing increases the carrier lifetime in the electron-irradiated samples.

Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers

Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers are investigated. The authors reported that the carrier lifetime in n-type epilayers increased by reduction of deep levels through thermal oxidation and thermal annealing. However, the carrier lifetimes in p-type epilayers were not significantly enhanced. In this study, in order to investigate the influence of surface passivation on the carrier lifetimes, the epilayer surface was passivated by different oxidation techniques. While the improvement of the carrier lifetime in n-type epilayers was small, the carrier lifetime in p-type epilayers were remarkably improved by appropriate surface passivation. For instance, the carrier lifetime was improved from 1.4 μs to 2.6 μs by passivation with deposited SiO2 annealed in NO. From these results, it was revealed that surface recombination is a limiting factor of carrier lifetimes in p-type 4H-SiC epilayers.

Reduction in Majority-Carrier Concentration in Lightly-Doped 4H-SiC Epilayers by Electron Irradiation

The mechanisms for the reduction in the hole concentration in lightly Al-doped p-type 4H-SiC epilayers by electron irradiation as well as in the electron concentration in lightly N-doped n-type 4H-SiC epilayers by electron irradiation are investigated. In the p-type 4H-SiC epilayers, the temperature dependence of the hole concentration, , is not changed by 100 keV electron irradiation, while the is decreased by 150 keV electron irradiation. The density of Al acceptors with energy level eV decreases with increasing fluence of 150 keV electrons, whereas the density of deep acceptors with energy level eV increases. In the n-type 4H-SiC epilayers, the temperature dependence of the electron concentration, , is decreased by 200 keV electron irradiation. The density of N donors located at hexagonal C-sublattice sites decreases significantly with increasing fluence of 200 keV electrons, whereas the density of N donors located at cubic C-sublattice site decreases slightly.

Temperature and Injection Level Dependencies of Carrier Lifetimes in p-Type and n-Type 4H-SiC Epilayers

Temperature and injection level dependencies of carrier lifetimes in p-type and n-type 4H-SiC epilayers have been investigated. The carrier lifetimes have been measured by differential microwave photoconductance decay measurements at various injection levels and temperatures. In both p-type and n-type epilayers, the carrier lifetimes gradually increased with increasing the injection level except for the very high injection condition. And the carrier lifetimes exhibited continuous increase with elevating the temperature for both epilayers. The carrier lifetime reached 3.3 µs in p-type and 4.2 µs in n-type epilayers at 250°C and an injection level of 1.8x1016 cm-3.