Hideharu Matsuura

Electrical properties of n‐amorphous/p‐crystalline silicon heterojunctions

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:H/p c‐Si heterojunctions, and the electron affinity of a‐Si:H has been estimated as 3.93±0.07 eV from C‐V characteristics. The forward current of all the junctions studied shows voltage and temperature dependence expressed as exp(−ΔEa f/kT) exp(AV), where ΔEa f 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(−ΔEar/kT)(VD−V)1/2, where VD is the diffusion voltage and ΔEar is a constant. This current is probably limited by generation‐recombination process.

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 typ...

A novel method for determining the gap‐state profile and its application to amorphous Si1−xGex:H films

A novel technique has been proposed for determining the density‐of‐state (DOS) distribution in the energy gap of highly resistive amorphous semiconductors, using amorphous/crystalline heterojunction structures. The technique has been tested and applied to undoped hydrogenated amorphous silicon (a‐Si:H) films and silicon‐germanium (a‐Si1−xGex: H ) alloy films, covering the optical gap (E0) range of 1.3–1.7 eV. For undoped a‐Si:H with E0=1.7 eV, the peak of the midgap DOS distribution has been located at 0.84 eV below the conduction‐band mobility edge, EC, with a value of 2×1015 cm−3 eV−1. For undoped a‐Si1−xGex: H (E0=1.44 eV), the same has been obtained at 0.70 eV below EC, with a magnitude of 7×1016 cm−3 eV−1. Those midgap DOS have been found to be correlated with singly occupied dangling bonds, representative of a homogeneous bulk property of the material, unaffected by interface states.

Effect of Ion Bombardment on the Growth and Properties of Hydrogenated Amorphous Silicon-Germanium Alloys

We report a systematic investigation of the effect of ion bombardment during the growth of amorphous silicon-germanium alloy films from silane and germane rf-glow discharge. Independent control of the plasma and the ion flux and energy is obtained by using a triode configuration. The ion contribution to the total deposition rate can reach 20% on negatively biased substrates. Although the Si and Ge composition of the film does not depend on the ion flux and energy, the optical, structural and electronic properties are drastically modified at low deposition temperatures when the maximum ion energy increases up to 50 eV, and remain constant above 50 eV. For a Ge atomic concentration of 37% and a temperature of 135°C, the optical gap decreases from 1.67 to 1.45 eV. This is correlated with a modification of hydrogen bonding configurations. Silicon dihydride sites disappear and preferential attachment of hydrogen to silicon is reduced in favour of germanium. Moreover the photoconductivity increases which shows that ion bombardment is a key parameter to optimize the quality of low band gap amorphous silicon-germanium alloys.

Dark current transport mechanism of p-i-n hydrogenated amorphous silicon diodes

The dark current‐voltage characteristics of p‐i‐n hydrogenated amorphous silicon diodes with various thicknesses of the intrinsic layer (i‐layer) (770–9300 A) are systematically investigated. The magnitude of the forward current is found to be independent of thickness of the i layer, which is obviously against the simple conventional junction theory. It has been demonstrated through various experiments that the forward current of amorphous p‐i‐n diodes is limited by a layer thinner than 770 A, possibly being the p/i interface or a narrow zone of the i layer.

Parameters required to simulate electric characteristics of SiC devices for n-type 4H-SiC

In 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 (⩽30°C) to steady-operation temperatures (⩾200°C), we discuss the dependence of the two donor levels on the total donor density (ND) as well as the dependence of the electron mobility on the total impurity density (Nimp) and operating temperature (T) in the n-type 4H–SiC. The temperature-dependent electron concentration n(T) and electron mobility μn(T) 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 n(T). 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 depende...

Ohmic Contact Properties of Magnesium Evaporated onto Undoped and P-doped a-Si: H

We have investigated the current-voltage characteristics of metal/undoped a-Si: H/n+ (or p+) c-Si structures using Mg, Au, Pt and Al. It has been indicated that the energy band of undoped a-Si: H shows a downward bending against Mg, being analogous to n+a-Si: H/undoped a-Si: H contact, while the band shows an upward bending against Au, Pt and Al. Mg/undoped a-Si: H contact is found to allow injection of electrons into the conduction band of a-Si: H, providing us a good ohmic contact property. This contact does not exhibit the thermal-degradation at least up to 100°C.

Decrease in Al acceptor density in Al-doped 4H-SiC by irradiation with 4.6 MeV electrons

From the temperature dependence of the hole concentration p(T) in a lightly Al-doped 4H-SiC epilayer, an Al acceptor with ∼200 meV and an unknown defect with ∼370 meV are found. By irradiation with 4.6 MeV electrons, the Al acceptor density is reduced, while the unknown defect density is almost unchanged. This indicates that the substitutional Al atoms in 4H-SiC are displaced by irradiation or that the bonds between the substitutional Al atom and the nearest neighbor atom are broken by irradiation. Moreover, the reduction in p(T) with irradiation arises from the decrease of the Al acceptors, not from the formation of hole traps or donor-like defects.

Determination of Donor Densities and Donor Levels in 3C-SiC Grown from Si2(CH3)6 Using Hall-Effect Measurements

Without any assumption of the number of types of impurities, the densities and energy levels of donors in undoped 3C-SiC grown from Si2(CH3)6 are precisely determined by the simple graphical method proposed here, using the temperature dependence of the majority-carrier concentration obtained by Hall-effect measurements. We detect at least three types of donors whose energy levels (ΔED) are 7–14 meV, 46–54 meV and 97–120 meV as measured from the conduction band, although it was reported that ΔED for nitrogen atoms decreased with an increase in the donor density from ~50 meV to ~15 meV. In addition to the ~15 meV donor that was reported in undoped 3C-SiC grown from a mixture of SiH4 and C3H8, at least two donor levels are detected in undoped epilayers grown from Si2(CH3)6. From the viewpoints of donor density and compensation ratio, the quality of undoped 3C-SiC grown from Si2(CH3)6 is better than that of undoped 3C-SiC grown from a mixture of SiH4 and C3H8.

Nitrogen Donor Concentrations and Its Energy Levels in 4H-SiC Uniquely Determined by a New Graphical Method Based on Hall-Effect Measurement

This is the first report on the experimental results using the precise determination proposed in our previous papers. By means of this analysis, the concentrations and energy levels of dopants in a semiconductor can be uniquely determined using the temperature dependence n(T) of the majority-carrier concentration obtained experimentally from the Hall-effect measurement. In other words, the concentration and energy level of each dopant can be evaluated using the corresponding peak value and temperature of n(T)/kT. In nitrogen (N)-doped 4H-SiC, the concentration and energy level of the shallow donor are 6.45 ×1015 cm-3 and EC-0.0653 eV, respectively, and those of the deep donor are 3.04 ×1016 cm-3 and EC-0.124 eV, respectively, where EC is the bottom of the conduction band. The acceptor concentration is 6.14 ×1013 cm-3. These obtained values are found to be quite reliable.