We have measured C-V characteristics and temperature dependence of J-V characteristics of undoped hydrogenated amorphous silicon (a-Si:H) heterojunctions formed on p-type crystalline silicon (p c-Si) substrates with different resistivities. It has been found that an abrupt heterojunction model is valid for a-Si:Hlp c-Si heterojunctions, and the electron affinity of a-Si:H has been estimated as 3.93 ± 0.07 e V from C-V characteristics. The forward current of all the junctions studied shows voltage and temperature dependence expressed as exp( -ilEafl kT) exp(A V), where ilEa! and A are constants independent of voltage and temperature, being successfully explained by a multitunneling capture-emission model. The reverse current is proportional to exp( -ilEarl kT)(V D -V )112, where V D is the diffusion voltage andilE ar is a constant. This current is probably limited by generation-recombination process.

The temperature-dependent hole concentration p͑T͒ and hole mobility p ͑T͒ are obtained in p-type 4H-SiC epilayers with several Al-doping densities. From p͑T͒, 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 p͑T͒ 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.

Dependence of acceptor levels and hole mobility on acceptor density and temperature in Al-doped p -type 4H-SiC epilayersIn order to obtain some of the parameters required to simulate the electric characteristics of silicon carbide (SiC) power electronic devices in a wide temperature range from startup temperatures ͑ഛ30°C͒ to steady-operation temperatures ͑ജ200°C͒, we discuss the dependence of the two donor levels on the total donor density ͑N D ͒ as well as the dependence of the electron mobility on the total impurity density ͑N imp ͒ and operating temperature ͑T͒ in the n-type 4H-SiC. The temperature-dependent electron concentration n͑T͒ and electron mobility n ͑T͒ in the n-type 4H-SiC epilayers with several nitrogen-doping densities are obtained from the Hall-effect measurements. By the graphical peak analysis method (free carrier concentration spectroscopy: FCCS) without any assumptions regarding the donor species, the two types of donor species are detected from n͑T͒. Moreover, the energy level and density of each donor species are determined by the FCCS. Using these results, we obtain the parameters with which the dependence of each donor level on N D can be simulated. Using n ͑T͒ at T Ͼ 250 K, moreover, we obtain the parameters with which the dependence of the electron mobility on N imp and T can be simulated.

Al-implanted p-type 4H–SiC layers with different implantation and annealing temperatures are formed, and the temperature dependence of the hole concentration p(T) is obtained by Hall-effect measurements. The Al acceptor level in SiC is deep (∼180 meV), and its first excited state level calculated by the hydrogenic model is still deep (∼35 meV), which is close to the acceptor level of B in Si. Therefore, in order to determine the reliable acceptor density (NA) from p(T), the Fermi–Dirac distribution function is not appropriate for Al acceptors in SiC, and a distribution function including the influence of the excited states of the Al acceptor is required. It is demonstrated that the proposed distribution function is suitable for obtaining the actual relationship between NA and p(T) in p-type 4H–SiC.

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