Al Acceptor Research Articles

Carrier compensation near tail region in aluminum- or boron-implanted 4H–SiC (0001)

Electrical behavior of implanted Al and B near implant-tail region in 4H–SiC (0001) after high-temperature annealing has been investigated. Depth profiles of Al and B acceptors determined by capacitance-voltage characteristics are compared with those of Al and B atoms measured by secondary-ion-mass spectrometry. For Al+ (aluminum-ion) implantation, slight in-diffusion of Al implants occurred in the initial stage of annealing at 1700°C. The profile of the Al-acceptor concentration in a “box-profile” region as well as an “implant-tail” region is in good agreement with that of the Al-atom concentration, indicating that nearly all of the implanted Al atoms, including the in-diffused Al atoms, work as Al acceptors. Several electrically deep centers were formed by Al+ implantation. For B+ (boron-ion) implantation, significant out- and in-diffusion of B implants occurred in the initial stage of annealing at 1700°C. A high density of B-related D centers exists near the tail region. In the tail region, the sum of B-acceptor concentration and D-center concentration corresponds to the B-atom concentration. C+ (carbon-ion) coimplantation with a ten times higher dose than B+ effectively suppressed the B diffusion, but additional deep centers were introduced by C+ coimplantation.

Ionization energies of phosphorus and nitrogen donors and aluminum acceptors in4Hsilicon carbide from the donor-acceptor pair emission

This paper deals with fitting the donor-acceptor pair luminescence due to P-Al pairs in 4H-SiC. It was possible to identify P at the Si cubic site as the shallower donor with ionization energy of 60.7meV, as well as to distinguish the contribution in the spectrum from pairs involving this donor and Al acceptors from both the cubic and hexagonal lattice sites, leading to justification of their ionization energies. The case of N-Al pair luminescence was revisited and the ionization energy of the deeper Nc donor at the cubic site was determined, 125.5meV. © 2005 The American Physical Society.

Complexes, clustering, and native-defect-assisted diffusion of aluminum in silicon

We study from first principles the energetics, complex formation, and diffusion of Al in crystalline Si. We first consider the interaction of Al with self-interstitials ${\mathrm{Si}}_{i}$: at variance with $\mathrm{B}$ and In, the complex formed by substitutional ${\mathrm{Al}}_{\mathrm{Si}}$ and a self-interstitial ${\mathrm{Si}}_{i}$ is metastable and trasforms into interstitial ${\mathrm{Al}}_{i}$ in the tetrahedral site. Therefore in the presence of excess self-interstitials ${\mathrm{Si}}_{i}$, Al will go off-site and cease to be an acceptor. ${\mathrm{Al}}_{i}$ can then bind to ${\mathrm{Al}}_{\mathrm{Si}}$, forming a trigonal complex (also not an acceptor) compatible with the properties of the Si-G19 center. A further sizable energy gain is obtained forming the deep-donor Al-Al-self-interstitial complex. We thus predict that a sufficiently high externally generated ${\mathrm{Si}}_{i}$ population will cause the deactivation, and presumably the clustering and disordering, of the Al acceptor in Si. As to Al-vacancy interaction, ${\mathrm{Al}}_{\mathrm{Si}}$ and a first-neighbor Si vacancy spontaneously form a divacancy-interstitial complex, similarly to In. We evaluate the activation energy for diffusion for the various mechanisms investigated, and find it to be predominantly interstitial-assisted; calculated activation energies are in good general agreement with experiment.

Effects of high-temperature anneals on 4H–SiC Implanted with Al or Al and Si

Co-implanting Si into 4H–SiC with Al hinders the ability of the Al acceptors to activate electrically at the lower annealing temperatures, but for annealing temperatures>1600°C, the effect is much less, suggesting that the activation energy for incorporating Al as an acceptor no longer controls the rate-determining step in this process. The cathodoluminescence data indicate that Al acceptors are trapped out by the DI defect, and this effect is more pronounced for the higher annealing temperatures. The increase in χmin with the annealing temperature at the higher temperatures can be explained by the nucleation and growth of structural defects, and the transmission electron miscroscopy results show that these defects are stacking faults. The stacking faults can be described as being quantum dots of different polytypes or domain walls with a point-defect periodic structure between the faulted and unfaulted regions, and they could be the source for the peaks associated with the DI defect. Also, we observed that the higher-energy peak in the DI doublet increases in intensity relative to the lower-energy peak as the annealing temperature and the initial implant damage increase.

Electrical activation of high-concentration aluminum implanted in 4H-SiC

High-dose aluminum-ion (Al+) implantation into 4H-SiC (0001) and (112¯0) has been investigated. The dependences of the electrical properties on the implanted Al+ dose and on the annealing time were examined by Hall-effect measurements. A low sheet resistance of 2.3kΩ∕◻ (0.2μm deep) was obtained in a (0001) sample by implantation of Al+ with a dose of 3.0×1016cm−2 at 500°C and a subsequent high-temperature anneal at 1800°C for a short time of 1min. In the case of (112¯0) samples, even room-temperature implantation resulted in a low sheet resistance of 2.3kΩ∕◻ (0.2μm-deep) after anneal at 1800°C. The Hall data are compared with the calculated values determined by using the doping-concentration dependent ionization energy of Al acceptors. The experimentally obtained free-hole concentrations agree well with the theoretically expected values. Hole mobilities are not as high as the empirical mobilities obtained in Al-doped epitaxial layers. The differences in the electrical properties between the experimental data and expected values are discussed.

Reduction in Al Acceptor Density by Electron Irradiation in Al-Doped 4H-SiC

The influence of electron irradiation on the hole concentration in Al-doped 4H-SiC epilayers is investigated with free carrier concentration spectroscopy (FCCS) using the temperature dependent hole concentration . By 4.6-MeV electron irradiation, is reduced over the whole temperature range. Using FCCS, the densities and energy levels of acceptors or hole traps are determined. In the unirradiated and irradiated samples, ~200 meV and ~370 meV acceptor levels or hole-trap levels are detected. By irradiation, only the density of Al acceptors whose energy level is ~200 meV is reduced from 6.2×10 ) (T p ) (T p

Activation of Implanted Al and Co-Implanted Al/C or Al/Si in 4H-SiC

Rsh, CL, EPR, RBS, and TEM measurements were made on samples implanted with Al, Al and C, or Al and Si to a depth of 0.3 μm with a concentration of 10 20 cm -3 and annealed with an AlN cap. Rsh measurements suggest that at the lower annealing temperatures the implanted C facilitates the chemical reactions for the incorporation of Al into a Si site and Si impedes it, but for annealing temperatures >1600°C the co-implants have little effect because the activation energy for these reactions is no longer the rate determining step. The CL measurements show that the peak near 3.0 eV associated with a free electron recombining with a hole bound to an Al acceptor decreases as the annealing temperature increases suggesting defects are trapping out the holes. This can explain the lower mobilities, lower electrical activation, and our inability to detect the EPR peak associated with AlSi in these heavily implanted samples. Our RBS and TEM measurements suggest that these defects are extended residual defects that nucleate and grow, as opposed to being annealed out, and that they could be stacking faults.

Formation of surface states during Schottky barrier fabrication on Al-doped p-type 6H–SiC

I– V measurements of an as-deposited Ru Schottky barrier diode (SBD) revealed a shift of −0.6 V in the minimum current. This shift is attributed to the presence of localized interface states that modifies the barrier height. Annealing as-deposited and plasma exposed Ru SBDs revealed that the off-set was temperature dependent, suggesting that appropriate selection of anneal temperature may result in reconstruction of the surface, consequently removing the surface states responsible for the distorted I– V response. The effect of hydrogen on the Schottky contact was investigated by exposing the metalized front surface of the SiC to a d.c. hydrogen plasma for 120 min. Capacitance–Voltage ( C– V) depth profiles of the H-plasma exposed SBDs revealed a reduction in the near-surface free carrier concentration, suggesting passivation of the Al acceptors by hydrogen. This was confirmed by subsequent reactivation of the electrical activity of the Schottky contact through a series of isothermal reverse bias annealing experiments.

Electrical, CL, EPR and RBS study of annealed SiC implanted with Al or Al and C

Lightly n-doped, 4H-SiC films implanted with 1020 cm−3 Al or co-implanted with 1020 cm−3 Al and 1020 cm−3 C were examined in the unannealed state or annealed at 1300, 1400, 1500, 1600 or 1650 °C after they had been coated with an AlN cap. For a given annealing temperature, TA, the sheet resistance, Rsh, is smaller in the co-implanted samples at the lower TA's, but not at the higher ones because the C catalyzes the chemical reactions for the incorporation of Al into a Si site, and at the higher TA's the activation energies for these reactions no longer control the reaction kinetics. The CL peak created by a free electron recombining with a hole bound to an Al acceptor decreases in intensity as TA increases because the Al acceptors can be trapped by the deep donors related to the DI defect that increase in number as TA increases. However, the process must be more complicated because the temperature dependence of Rsh implies that the activation energy for conduction is much less than that of the 0.35 eV hole trap. The excellent temperature stability of the DI defect suggests it is likely to be associated with defects that nucleate and grow such as dislocation loops and/or stacking faults. This is consistent with decreased ion beam channeling at the higher TA's, and the small anisotropy of the EPR peak associated with implant damage. The higher energy peak in the DI doublet increases in intensity relative to the lower energy peak as TA increases, and for the same TA the relative intensity of the higher energy peak is larger in the co-implanted samples. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ionization mechanisms of aluminum acceptor impurity in silicon

Processes of ionization of shallow acceptor centers (ACs) in silicon are studied. In crystalline silicon samples with phosphorus (1.6×1013, 2.7×1013, and 2.3×1015cm−3) and boron (1.3×1015cm−3) impurities, μAl impurity atoms were produced by implantation of negative muons. It is found that thermal ionization is the main mechanism for ionizing the Al acceptor impurity in both p-type and n-type silicon with an impurity concentration of ≲1015cm−3 at T>45 K. The thermal ionization rate of Al ACs in Si varies from ∼105 to ∼106s−1 in the temperature range 45–55 K.

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.

Real Relationship between Acceptor Density and Hole Concentration in Al-Implanted 4H-SiC

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

Occupation probability for acceptor in Al-implanted p-type 4H–SiC

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.

Optically induced formation of the hydrogen complex responsible for the 4B luminescence in 4H-SiC

Formation of a boron-related defect responsible for the 4B0 emission line in the low-temperature photoluminescence spectrum of 4H SiC has been investigated. The 4B0 luminescence was absent in as-grown epitaxial layers. This line appeared after hydrogenation along with other hydrogen-related lines and disappeared after high-temperature annealing. This is an indication of involvement of hydrogen in formation of the corresponding radiative recombination center. Prolonged excitation of the hydrogenated epitaxial layers with above-band gap light caused not only previously reported enhanced passivation of Al acceptors but also additional strong nonmetastable increase of the 4B0 luminescence due to recombination-enhanced formation of the corresponding defect complex.

Hydrogen interaction with defects and impurities in 6H-SiC

The dynamics of hydrogen capture and release from trapping centers in 6H-SiC after plasma hydrogenation and annealing was investigated by low-temperature photoluminescence (PL). Indications of competing processes of hydrogen capture by different trapping centers were observed. Passivation of Al acceptors with hydrogen is the dominating process during plasma hydrogenation or plasma etching. Irreversible release of hydrogen from Al-trapping centers and additional trapping of hydrogen by Si vacancy (VSi) to form a VSi+H complex occurred during annealing at temperatures above 300°C. It is suggested that the VSi+H complex may be playing an important role in keeping hydrogen in SiC after higher temperature treatment.

Investigation of acceptor centers in semiconductors by means of negative muons

The behavior (in the temperature range of 4– 300 K ) of the polarization of the negative muon spin for more than 20 n- and p-type crystalline silicon samples with impurity concentrations from ∼10 12 to ∼10 20 cm −3 made it possible (a) to infer the mechanisms for the relaxation of the magnetic moment of the Al acceptor in Si at different impurity concentrations, including the concentration region above the critical concentration corresponding to the Mott (insulator-to-metal) transition; (b) to find the value of the hyperfine interaction constant for the acceptor and to estimate the density of the hole wave function on the nucleus of the impurity atom.

Nanocrystalline SiC layers produced by ion-beam-induced crystallization—morphology and resistivity

Ion-beam-induced crystallization was used to transform amorphized, heavily Al doped SiC layers to nanocrystalline material. The morphology of the as-implanted and the annealed layers was studied by XTEM. The electrical properties were analyzed by sheet resistance and Hall measurements and compared with crystalline reference samples. A high-temperature annealing step is necessary to activate the implanted Al acceptor atoms. During annealing the mean grain size of the nanocrystals grow from 3 to 35 nm. The Al doped, nanocrystalline SiC has a much lower sheet resistance than the crystalline reference samples. It was found that this is due to the enhanced hole concentration which could be explained by a higher solid solubility of Al in the nanocrystalline SiC.

Changes in Al-related photoluminescence in 4H-SiC caused by hydrogenation

Both reduction of the intensity of aluminum-related photoluminescence after hydrogenation and the phenomenon of the optical quenching of the Al bound exciton (Al–BE) previously reported for hydrogenated 6H-SiC was observed now in the 4H-SiC polytype. Hydrogenation caused also a reduction of the nitrogen–aluminum donor-acceptor pair emission. Prolonged excitation of hydrogenated samples with above-band gap light resulted in additional quenching of the residual Al–BE photoluminescence. The quenching kinetics and some differences between the 6H and 4H polytypes are investigated. The observed phenomena are attributed to optically enhanced passivation of Al acceptors with hydrogen.

Selective doping of 4H–SiC by codiffusion of aluminum and boron

A graphite mask was used to realize selective doping of aluminum/boron in 4H–SiC by thermal diffusion at a temperature range of 1800–2100 °C. The doping profiles investigated by secondary ion mass spectrometry show that a high aluminum concentration of 5×1019 cm−3 near the surface and linearly graded boron profile up to several micrometers in depth can be obtained. Hall effect measurement was also employed to obtain the electrical characteristics of the diffused region, from which the carrier concentration (1×1019 cm−3) and hole mobility (7 cm2/V s) at room temperature were extracted. Room temperature photoluminescence indicates that the dominant luminescence is attributed to the donor acceptor pair recombination, in which boron D complex is the prevailing center rather than Al and boron shallow acceptors. Cathodoluminescence micrographs clearly illustrate a pattern with the locally diffused regions. To confirm the viability of the diffusion process, planar p-n diodes with a fairly low forward voltage drop (3.3 V at 100 A/cm2) and high reverse blocking capability (more than 1100 V) were fabricated. Built-in voltage of 2.9 V, which is typical for 4H–SiC p-n diodes, was obtained by capacitance–voltage measurement.

Spreading resistance measurements on nanocrystalline SiC produced by ion beam induced crystallisation

Ion beam induced crystallisation (IBIC) of preamorphised surface layers of 6H–SiC has been stimulated by high dose Al implantation (0.3−3×10 17 cm −2) at elevated temperatures (300–500 °C) Randomly oriented 3C–SiC nanocrystals with diameters between 2 and 25 nm are formed depending on the implantation parameters as proved by XRD and XTEM. Spreading resistance measurements have been performed at bevelled as-implanted and annealed (1500 °C, 10 min) samples in order to study the electrical behaviour of the Al acceptors in the nanocrystalline layer. Reference experiments have been carried out on single crystalline 6H–SiC wafers implanted at the similar conditions. It has been found that in the as-implanted state Al doped fine granular SiC has much lower spreading resistance than the corresponding single crystalline SiC. Only minor differences have been observed between the nano- and single crystalline samples after annealing.