Al Acceptor Research Articles

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.

Effects of Implantation Temperature on Sheet and Contact Resistance of Heavily Al Implanted 4H-SiC

Effects of implantation temperature on electrical properties of heavily-Al-doped 4H-SiC layer formed with Al implantation have been investigated. To form the p++ 4H-SiC with the original 4H-stacking structure, the implantation temperature above 175 °C is needed. A decrease in the implantation temperature below 250 °C leads to an increase in the NA-ND. It is suggested that an increase in the density of vacancies with a decrease in the implantation temperature promotes the Al substitution to lattice sites during activation annealing. The lower-temperature implantation also causes a decrease in activation energy for the p-type electrical conduction and a decrease in p-type ohmic contact resistivity. We presume that the increase in the Al acceptors at low-implantation temperatures gives expansion of the impurity bands and formation of valence band tail-states, causing the decrease in the impurity binding energy. The properties obtained with the lower-temperature implantation are desirable for practical applications especially at low temperatures.

Thermally Stimulated Current Separation of Hole and Acceptor Trap Density in 4H-SiC MOS Devices Using Gamma Ray Irradiation

Thermally stimulated current (TSC) measurements on epitaxial and implanted 4H-SiC MOS capacitors are presented. The effect of gamma ray irradiation on the TSC spectra of epitaxial 4H-SiC MOSCAP devices is discussed. On non-irradiated samples, two TSC peaks are observed near 55 K and 80 K. Due to the generated oxide charge during irradiation, the 80 K emission split into two constituent peaks. These have been attributed to hole traps and Al acceptors.

Shallow Defects Observed in As-Grown and Electron-Irradiated or He<sup>+</sup>-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).

LTPL Investigation of N-Ga and N-Al Donor-Acceptor Pair Spectra in 3C-SiC Layers Grown by VLS on 6H-SiC Substrates

Ga-doped 3C-SiC layers have been grown on on-axis 6H-SiC (0001) substrates by the VLS technique and investigated by low temperature photoluminescence (LTPL) measurements. On these Ga-doped samples, all experimental spectra collected at 5K were found dominated by strong N-Ga donor-acceptor pair (DAP) transitions and phonon replicas. As expected, the N-Ga DAP zero-phonon line (ZPL) was located at lower energy (~ 86 meV) below the N-Al one. Fitting the transition energies for the N-Al close DAP lines gave 251 meV for the Al acceptor binding energy in 3C-SiC and, by comparison, 337 meV for the Ga acceptor one.

Ab initio studies of hydrogen and acceptor defects in rutile TiO2

The behaviour of hydrogen in both defect free and acceptor doped bulk rutile TiO2 is investigated through defect calculations performed within the density functional theory formalism. Both interstitial and substitutional hydrogen defects are shown to behave as shallow donors in the material, thereby existing as effectively positive hydroxide defects, OH*(O), and substitutional hydrogen defects, H*O. Of the investigated isolated hydrogen defects, the OH*(O) defect is shown to have the lowest formation energy over the whole Fermi level range under both oxidising and reducing conditions. However, the formation energy of H*O is only 0.35 eV higher under conditions where TiO2 is in equilibrium with Ti2O3, indicating that it exists as a minority defect in rutile TiO2 under highly reducing conditions such as under growth of TiO2 on a Ti metal surface. The enthalpy of hydration of oxygen vacancies is calculated to be -1.64 and -1.60 eV for the 2 x 2 x 3 and 3 x 3 x 3 supercells, indicating that OH*(O) defects will prevail in wet atmospheres, even at high temperatures. Hydrogen incorporated in the material can furthermore be expected to associate significantly with various acceptor centres (e.g., substitutional N and Al acceptors and Ti vacancies). The binding energies of the substitutional NH(x)O (imide) defect and of the association complexes between H and fully ionised substitutional Al, (Al'Ti x OH*(O)), and Ti vacancies, (V(Ti)x OH*(O)and (V(Ti) x 4OH*(O)) (i.e., Ruetschi type defects), are calculated to be -0.36, -0.12, -0.47 and -0.82 eV, respectively.

Mechanisms of changes of hole concentration in Al-doped 6H-SiC by electron irradiation and annealing

Abstract The temperature dependence of the hole concentration p ( T ) in Al-doped p-type 6H-SiC irradiated by 100 or 200 keV electrons is investigated. Since p ( T ) is unchanged by 100 keV electron irradiation, the threshold displacement energy in SiC is higher than 20 eV. Therefore, 200 keV electrons cannot displace substitutional Si and Al in Al-doped 6H-SiC. Using p ( T ) , two types of acceptor species are detected, and the density and energy level of each acceptor species are determined. By 200 keV electron irradiation, the density ( N Al ) of the shallow acceptor (i.e., Al acceptor) decreases monotonously with increasing fluence of electrons, whereas the density ( N DA ) of the deep acceptor initially increases and then decreases. By irradiation with the 1 × 10 16 cm - 2 fluence of 200 keV electrons, especially, the decrement of N Al is nearly equal to the increment of N DA . By annealing at 500 °C, on the other hand, the increment of N Al is close to the decrement of N DA .

Temperature and doping dependencies of electrical properties in Al-doped 4H-SiC epitaxial layers

The free hole concentration and the low-field transport properties of Al-doped 4H-SiC epilayers with several acceptor concentrations grown on semi-insulating substrates have been investigated in the temperature range from 100to500K by Hall-effect measurements. Samples have been grown by cold-wall chemical vapor deposition (CVD) in the Al acceptor concentration range from 3×1015to5.5×1019cm−3. The dependencies of the acceptor ionization ratio at 300K and the ionization energy on the acceptor concentration were estimated. Numerical calculations of the hole Hall mobility and the Hall scattering factor have been performed based on the low-field transport model using relaxation-time approximation. At the low acceptor concentrations, the acoustic phonon scattering dominates the hole mobility at 300K. At the high acceptor concentrations, on the other hand, the neutral impurity scattering dominates the mobility. A Caughey–Thomas mobility model with temperature dependent parameters is used to describe the dependence of the hole mobilities on the acceptor concentration, and the physical meanings of the parameters are discussed.

C-V and DLTS Analyses of Trap-Induced Graded Junctions: The Case of Al<sup>+</sup> Implanted JTE p<sup>+</sup>n 4H-SiC Diodes

Capacitance versus Voltage (C-V) and Deep Level Transient Spectroscopy (DLTS) measurements of Al+ implanted p+n diodes with Al+ implanted Junction Termination Extension are here studied. These diodes present C-V characteristics like graded junction for low forward bias values, i.e. > 0.4 V , or like abrupt junctions for large reverse bias, i.e. between 0.4V and -10V. The depth range of the graded junction, computed by the capacitance values, is much larger than the simulated tail of the ion implanted Al+ profile. DLTS spectra have been measured both in injection and standard configuration and always show minority carrier traps in the temperature range 0-300K. Three are the minority carrier related peaks, one attributed to the Al acceptor and the others to the D and D1 defects. The depth distribution of these hole traps will be discussed with respect to the apparent carrier concentration, obtained by C-V analysis.

Silver-induced layer exchange for the low-temperature preparation of intrinsic polycrystalline silicon films

The preparation of large grained continuous polycrystalline silicon layers by metal-induced crystallization is reported. The macroscopic layer exchange of an amorphous silicon precursor layer in contact with a silver layer was observed for temperatures below the softening point of glass. This process is quite similar to the well-known aluminum-induced layer exchange. However, due to the use of silver as a catalyst, the recrystallized layers are electrically intrinsic rather than highly doped with Al acceptors. The resulting polycrystalline silicon layers show a good crystalline quality as deduced from Raman scattering, x-ray diffraction, and UV-reflectance measurements.

Mechanisms of unexpected reduction in hole concentration in Al-doped 4H-SiC by 200 keV electron irradiation

The hole concentration in Al-doped p-type 4H-SiC was found to be significantly reduced by electron irradiation when compared to the hole concentration in Al-doped p-type Si; this is an unexpected result. The temperature dependence of the hole concentration p(T) in Al-doped 4H-SiC irradiated with 200 keV electrons at various fluences was measured. Only substitutional C atoms in SiC can be displaced by irradiation with 200 keV electrons. The reduction in p(T) by the electron irradiation was found to be mainly due to a decrease in Al acceptors and not due to an increase in defects (e.g., C vacancies) located around the middle of the bandgap in SiC. Based on the analysis of p(T), two types of acceptor species were detected and the density and energy level of each acceptor species were determined. An equation describing the fluence dependence of each acceptor density is proposed. The results suggest that for the 200 keV electron irradiation, a substitutional C atom bonded to an Al acceptor was more easily displaced than a substitutional C atom bonded to four Si atoms.

Mechanisms of Reduction in Hole Concentration in Al-Implanted p-Type 6H-SiC by 1 MeV Electron Irradiation

The reduction in temperature-dependent hole concentration p(T) in Al-implanted p-type 6H-SiC by 1 MeV electron irradiation is investigated. By analysis of p(T), the density (NA), level position (EA) in the bandgap and nature of the acceptor are determined, and this acceptor is assigned to an Al acceptor. EA is independent of irradiation fluence (Φ), while NA is strongly dependent on Φ. We derived an analytical expression for the fluence dependence of NA and we estimated the removal coefficient (i.e., removal cross-section) of NA to be 6.4×10-18 cm2 for 1 MeV electron irradiation. The reduction in p(T) by electron irradiation is found to be mainly due to the decrease in NA, not to the increase in the density of deep-level defects in the bandgap, because the decrement in NA is much larger than the increment in the density of deep-level defects.

Shuttle Activation Annealing of Implanted Al in 4H-SiC

A shuttle-type annealing process as a new high-temperature activation annealing technique for SiC has been investigated. Wide-range annealing time (1 s to 3 h) and temperature (1300 to 2000 °C) dependences of electrical properties for Al-ion-implanted 4H-SiC are examined using capacitance–voltage (C–V) and Hall measurement. When the annealing temperature is between 1700 and 1900 °C, the acceptor activation ratio of 80% shows no dependence on the annealing time from 1 s to 30 min, while the Hall measurement reveals that both the hole density and the mobility increase with an increase in the annealing time and/or annealing temperature. This indicates that the activation of Al acceptors is a fast process compared with the implantation-damage annealing-out processes. The surface morphology of rapidly annealed samples is fairly smooth. An appropriate annealing condition should be determined with due consideration of the trade-off between electrical quality, surface morphology, and throughput.

Electrical and Structural Properties of Al-Implanted and Annealed 4H-SiC

Aluminum ions (Al+) were implanted at room temperature or at 500°C into n-type 4HSiC. The implantation damage (displaced Si atoms) and the electrical activation of Al+ ions (concentration of Al acceptors) were determined by Rutherford backscattering in channeling mode and Hall effect, respectively, as a function of the annealing temperature.

Impurity Conduction in Silicon Carbide

We report on admittance spectroscopy (AS) investigations taken on aluminum (Al)- doped 6H-SiC crystals at low temperatures. Admittance spectra taken on Schottky contacts of highly doped samples (NA ≥ 7.2×1017 cm-3) reveal two series of conductance peaks, which cause two different slopes of the Arrhenius plot. The steep slope is attributed to the Al acceptor, while the flatter one - obtained from the low temperature peaks - is attributed to the activation energy ε3 of nearest neighbor hopping. We propose a model, which explains the unexpected sharpness of the low temperature conductance peaks and the disappearance of these peaks for low acceptor concentrations. The model is verified by simulation, and the AS results are compared with corresponding results obtained from resistivity measurements taken on 4H- and the identical 6HSiC samples.

A comparative study of the electrical properties of heavily Al implanted, single crystalline and nanocrystalline SiC

The electrical properties of heavily Al doped single and nanocrystalline 4H–SiC layers on semi-insulating 4H–SiC substrate, prepared by multienergy, high-fluence Al implantation and subsequent furnace annealing, are investigated by sheet resistance and Hall effect measurements. Ion beam induced crystallization is used to prepare the nanocrystalline SiC layers. The doping levels are chosen around the solid solubility limit of 2×1020cm−3 in the range from 5×1019to1.5×1021cm−3. The comparison of the results shows that heavily Al doped single crystalline SiC layers have superior conduction properties. The lowest resistivities measured at room temperature are 0.08 and 0.8Ωcm for the single crystalline and nanocrystalline samples, respectively. Recent results on enhanced Al acceptor activation in nanocrystalline SiC cannot be confirmed. There is an upper limit for the hole concentration in the nanocrystalline samples independent of the Al supersaturation level in the as-implanted state due to outdiffusion of Al in excess to the solid solubility limit during annealing. In contrast to the nanocrystalline SiC layers the as-implanted Al profile in single crystalline material remains stable after annealing even for concentrations above the solid solubility limit. Therefore, in single crystalline material efficient impurity band conduction due to strongly interacting acceptors can be achieved in the range of supersaturation. For lower doping levels impurity band conduction is more effective in nanocrystalline SiC.

Characterization of 4H-SiC grown on AlN/Si(100) by CVD

By using chemical vapor deposition method, 4H-SiC films have been grown on AlN/Si(100) complex substrates at the temperature below 1100 °C. Substitutional Al and N are found in the SiC film. Photoluminescence (PL) peaks, related to Al acceptor and N shallow donor, respectively, have been observed at the room-temperature. The higher partial pressure ratio of SiH 4 and C 2H 4 ( P C 2H 4 / P SiH 4 ≧ 1.62) results in more Si-vacancy ( V Si) in the film. The PL peak related to the V Si acceptor level is also observed.

Relationship between defects induced by irradiation and reduction of hole concentration in Al-doped 4H–SiC

From the temperature dependence of the hole concentration in lightly Al-doped 4H–SiC epilayers, a shallow acceptor with E V + 0.2 eV , which is an Al atom ( Al Si ) at a Si sublattice site, and an unknown deep defect with E V + 0.35 eV are found, where E V is the valence band maximum. In unirradiated epilayers, moreover, the density ( N Defect ) of this defect is close to the Al acceptor density ( N Al ) . With irradiation of 0.2 MeV electrons, the N Al is reduced, while the N Defect is increased. Judging from the minimum electron energy required to displace a substitutional C atom ( C s ) or Al Si , the bond between Al Si and its nearest neighbor C s is broken due to the displacement of the C s by this irradiation. Moreover, the displacement of the C s results in the creation of a complex ( Al Si – V C ) of Al Si and its nearest neighbor C vacancy ( V C ) , indicating that the possible origin of the defect with E V + 0.35 eV is Al Si – V C .

Behavior of shallow acceptor impurities in uniaxially stressed silicon and in synthetic diamond studied by [formula omitted

The results of investigating the behavior of the boron impurity in synthetic diamond and the effect of uniaxial static pressure on the aluminum acceptor in silicon are presented. The data on the hyperfine interaction and on the relaxation of the magnetic moment of Al acceptor in silicon were obtained. It was found that the uniaxial stress changed both the absolute value and the temperature dependence of the relaxation rate of the acceptor. Unlike the case in silicon, in CVD diamond the paramagnetic shift in the muon spin precession frequency was not observed within the accuracy of the measurements, and a missing fraction of muon polarization was found at T < 90 K .

Mechanisms of reduction in hole concentration in Al-doped 4H-SiC by electron irradiation

From the temperature dependence of the hole concentration in unirradiated lightly Al-doped 4H-SiC epilayers, an Al acceptor with E V + 0.2 eV, which is an Al atom (Al Si) at a Si sublattice site, and an unknown deep acceptor with E V + 0.35 eV are found, where E V is the top of the valence band. Both the densities are similar. With irradiation of 0.2 MeV electrons the Al acceptor density is reduced, while the unknown deep acceptor density is increased. Judging from the minimum electron energy required to displace a substitutional C atom (C s) or the Al Si, the bond between the Al Si and its nearest neighbor C s is broken due to the displacement of the C s by this irradiation. Moreover, the displacement of the C s results in the creation of a complex (Al Si–V C) of Al Si and a carbon vacancy (V C), indicating that the possible origin of the deep acceptor with E V + 0.35 eV is Al Si–V C.