Al-doped Epilayers Research Articles

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.

Basal Plane Dislocations in 4H-SiC Epilayers with Different Dopings

Morphologies of basal plane dislocations (BPDs) in 4H-SiC epilayers doped with nitrogen or aluminum are explained in detail. While BPDs in low N-doped or Al-doped epilayers show the morphology of gliding dislocations responding to stresses, BPDs in highly N-doped (≥1.0×1018 cm-3) epilayers appear different. Some of them are parallel to [11-20] while others are straight and tilt from [11-20]. Tilt BPDs were also studied by TEM. Factors that relate to such morphology are discussed.

Doping induced lattice misfit in 4H–SiC homoepitaxy

The impact of doping on the lattice constants of 4H–silicon carbide (4H–SiC) is an important material aspect influencing several steps of material and device production. Dopant incorporation in 4H–SiC causes misfit between the highly N-doped substrate and differently doped epilayers and hence, wafer bowing and the existence of a critical epilayer thickness. In this paper, the wafer bow is determined by geometrical measurements of the substrate prior to and after the epitaxial growth of single epilayers with different epilayer thicknesses and doping states, i.e. with nitrogen (N) or aluminum (Al) doping and different dopant concentrations. The misfit between substrate and epilayer is deduced from these bow measurements based on a model by Stoney. For highly Al-doped epilayers grown on highly N-doped substrates, the misfit is determined additionally by HRXRD measurements. The doping-dependent misfit, obtained from bow and HRXRD measurements, is compared to theoretical values calculated based on a model by Jacobson. For Al-doped epilayers, the experimental and theoretical values of misfit agree well. For N-doped epilayers, an effective covalent radius of N in 4H–SiC of 66 pm has to be introduced to match the theoretical misfit to the experimental one. The critical epilayer thickness is calculated based on the models by Matthews and Blakeslee and by People and Bean. The comparison of calculated values and experimental findings with respect to dislocation behavior at the substrate–epilayer interface and the dislocation densities of epilayers proves that the model by Matthews and Blakeslee underestimates the critical epilayer thickness for 4H–SiC homoepitaxy.

Dependence of acceptor levels and hole mobility on acceptor density and temperature in Al-doped p-type 4H-SiC epilayers

The temperature-dependent hole concentration p(T) and hole mobility μp(T) are obtained in p-type 4H-SiC epilayers with several Al-doping densities. From p(T), the densities and energy levels of acceptors are determined by the graphical peak analysis method (free carrier concentration spectroscopy: FCCS) without any assumptions regarding the acceptor species. In the heavily Al-doped case, the excited states of acceptors affect p(T) because the Fermi level is located between the valence band maximum and the acceptor level (i.e., the ground state level of the acceptor), indicating that a distribution function for acceptors, which includes the influence of excited states of acceptors, should be required. Here, FCCS can determine acceptor densities and acceptor levels using any distribution function (e.g., the Fermi-Dirac distributing function or the distribution function including the influence of excited states). Two types of acceptor species are detected in the lightly Al-doped epilayers, while only one type of acceptor species is found in the heavily Al-doped epilayer. Some of the parameters required to simulate electric characteristics of 4H-SiC power electronic devices are obtained; (1) the dependence of each acceptor level on a total acceptor density and (2) the dependence of the hole mobility on temperature and total impurity density.