Spectral Dependence of Optical Absorption of 4H-SiC Doped with Boron and Aluminum

Optical absorption of p-n-4H-SiC structures doped with boron and aluminum by low-temperature diffusion was studied for the first time. Diffusion of impurities was performed from aluminum-silicate and boron-silicate films (sources) fabricated by various methods. In the spectral dependences of optical absorption at room temperature, bands associated with transitions from impurity levels, as well as absorption bands associated with defects of the vacancy nature, were observed. -e level of absorption in the samples was used to estimate concentration of defects. It was shown that the use of sources of impurity atoms created by using boron and aluminum chlorides allows one to reduce the concentration of vacancy defects.


Introduction
Among wide-gap semiconductor materials (GaP, ZnS, ZnSe, ZnTe, CdS, and SiC), silicon carbide is a unique material due to high thermal conductivity and mechanical, chemical, and radiation hardness.Based on silicon carbide, power electronics elements, nuclear radiation detectors, and UV LEDs for special applications are made [1,2].In this regard, there is a significant interest drawn by researchers and technologists to this material.
Impurities of boron and aluminum are used to form p-regions in structures based on 4H-SiC by means of ion implantation or thermal diffusion.Diffusion of these impurities in the silicon carbide is based on rather complex mechanism taking place at temperatures above 2000 °C.Impurities move both through carbon and silicon sublattices of the crystal.Moreover, solubility and diffusion coefficient of impurities in different sublattices differ greatly.
For example, in the silicon sublattice, the boron solubility (∼6-9 × 10 19 cm −3 ) is almost an order of magnitude higher than that in the carbon sublattice (1-10 × 10 18 cm −3 ) [3].Differences in diffusion coefficients lead to formation of the so-called "tails" in the impurity distribution depthwise, which significantly reduces quality of the p-n junctions fabricated by means of thermal diffusion.Tails reduce the breakdown voltage, and traps associated with nitrogen (which are formed at high diffusion temperatures) increase the switching time of the diodes [4].
e ion implantation method allows one to achieve impurity concentration to be close to the maximal solubility.However, in the case of silicon carbide, annealing of defects requires temperatures up to 1800-2000 °C.At such high temperatures, one can observe redistribution of impurities depthwise, formation of defects on the surface, and so on.
at is why instead of thermal diffusion and ion implantation methods [5,6], the method of epitaxial growth of a silicon carbide layer can be only used to manufacture highvoltage p-n junctions, since the epitaxial film does not contain growth defects such as micropipes.
We have developed a new low-temperature method for shallow impurities diffusion in silicon carbide at temperatures of 1150-1300 °C.e method is described in details in publications [7][8][9][10][11] and patented in Uzbekistan and the USA [12,13].A significant decrease in temperature is due to the fact that diffusion occurs in the flow of carbon and silicon vacancies.Below we describe briefly the mechanism of formation of V C and V Si vacancies.
In [14], it is described that, at temperatures 1100-1400 °C, SiC exhibits two types of oxidation behavior, "active" and "passive," depending upon the ambient oxygen potential.At high oxygen pressures, "passive" oxidation occurs wherein a protective lm of SiO 2 (s) is formed on the surface by the reaction: At low oxygen potentials, severe "active" oxidation occurs due to the formation of gaseous products according to the following reactions: Active oxidation of SiC occurs only at oxygen pressures lower than ∼3•10 −4 atm at 1400 °C.
As seen from ( 2) and ( 3), surface oxidation of silicon carbide in this condition leads to production ow of both as carbon and silicon vacancies from the surface into the bulk of the crystal (may be mostly of carbon vacancies).Obviously, the vacancy concentration in this case can be much higher as compared with the introduction of vacancies by irradiation.
Advantages of low-temperature di usion are as follows: shallow impurity concentration of up to 10 20 cm −3 (unreachable for conventional thermal di usion and ion implantation technique), and fast switching time of <10 ns of p-i-n SiC diodes fabricated by this method (>20 ns for diodes fabricated by conventional technology) [10,11].
However, the technology of manufacturing of such p-i-n SiC diodes needs to be improved.In particular, it is necessary to improve the technology of creating a source of impurity atoms on the surface of the crystal for conducting low-temperature di usion.
e source of impurity atoms for di usion is a borosilicate or alumosilicate layer, which is formed on the surface of the sample before di usion.e source-layer is formed at 600 °C by di erent methods (synthesis from a boric acid layer, synthesis from aluminum chloride layer, and by oxidation of aluminum metal layer deposited by thermal evaporation in vacuum).
As the di usion occurs in the ow of defects, di erent defects with deep levels [15] as well as clusters of impurity atoms are formed in the sample [16].It is clear that the concentration of defects depends on the technology of manufacturing aluminum and borosilicate lms.
In this article, the optical absorption spectral dependence data was used to estimate the concentration of defects in crystals and to improve the technology for p-n junctions manufacturing by the low-temperature di usion method.

Experimental
In this paper, we used single-crystal samples of silicon carbide 4H-n-SiC grown by means of the physical vapor transport (PVT) method (Cree Research, Inc, USA) with a relatively low concentration of growth defects: N d dislocations 10 4 cm −2 and N m micropipes ∼10-10 2 cm −2 , thickness ∼300-600 μm, surface ∼0.25 cm 2 , speci c resistance ∼3.6-20 Ω•cm, and nitrogen impurity concentration Prior to low-temperature di usion, the samples were etched in KOH (potassium hydroxide)-water solution upon ultraviolet (UV) stimulation [17].
e source of boron impurity atoms was borosilicate lm which was formed as follows: alcoholic solution of boric acid or boric anhydride was applied to the surface of silicon carbide, which then was dried and annealed at 650 °C in air.
e source of aluminum impurity atoms for di usion was formed in two ways: (1) aluminum lm thermally sputtered onto the surface of the sample that was oxidized at 650 °C in vacuum and (2) formation from aluminum chloride sputtered on the surface of the silicon carbide at 600-700 °C.
Low-temperature di usion was conducted in the air for 30 min at temperatures ranging between 1150 °C and 1300 °C.
As a result of the silicon carbide surface oxidation at 1150-1300 °C, a ow of carbon and silicon vacancies was formed on the surface of the crystal [18,19].
is ow interacting with the impurity atoms signi cantly increased their di usion coe cient and solubility.As a result of the di usion, a layer containing aluminum and silicon oxides was formed on the surface of the samples.is layer was removed by hydro uoric acid to reveal the p-region of the sample.
According to electrophysical measurements, the impurity concentration reached 10 20 -10 21 cm −3 in a thin nearsurface layer [18].According to luminescence data [15], the sample contained defects of the vacancy nature doped during di usion.
To measure the spectral characteristics of the structures, dual-beam spectrophotometer (SPECORD 210 (Germany)) was used. is spectrophotometer had a wave band from 190 to 1100 nm. e Spectrum BXII Fourier spectrometer with spectral range from 7800 to 350 cm −1 was used for measurements in the IR spectral region.

Results and Discussion
e levels of impurities and defects in silicon carbide are studied in details.Based on the literature data [20][21][22], levels of the background nitrogen impurity, the acceptor levels of boron and aluminum, and the vacancy levels and traps in 4H silicon carbide are presented in Figure 1.It is assumed that the 2 acceptor levels with di erent energies correspond to di erent impurity positions (in the carbon and silicon 2 Journal of Spectroscopy sublattices).However, it is still not nally established which level corresponds to which sublattice.
It should be noted that there are also experimental data on carbon vacancy levels obtained by photocapacitance measurements.e vacancy of carbon has a band containing up to 5 levels with ionization energy from F S −1.80 to F S −0.74 eV for various charge vacancy states [23].
e optical absorption coe cient α can be determined by using the following known expression: where I is intensity of light that passes through the sample, I 0 is initial light intensity, R is coe cient of re ection, and d is thickness of sample.e re ection coe cient depends both on the optical properties of the material and on the state of the sample's surface.In case of a mirror surface and normal incidence of rays, the re ection coe cient is related to the refractive index of the material n determined by the Fresnel formula [24]: For long-wave radiation, the refractive index does not practically change, although in a region close to its own absorption the refractive index can change its value.According to the data of [25], in the wavelength region up to 440 nm (band-band transitions and above), the refractive index of silicon carbide varies nonmonotonically in the range of 2.4-3.4.However, in the region of absorption by defects (from the levels within the band gap), n varies insigni cantly from 3.4 to 3.2.Respectively, according to (5), the re ection coe cient also varies insigni cantly within 0.27-0.29.
As it is known, the Fourier spectrometer allows taking absorption spectra of very thin layers on the crystal surface [26].We measured absorption spectra of both doped thin layer and undoped reverse side of the sample.Such spectra of doped and undoped layers of the same sample make it possible to increase con dence of the conclusions.Figure 2 shows the optical absorption spectral dependence in the IR region.
As one can see from Figure 2, a wide structureless band observed in the absorption spectrum at 2500 cm −1 or more after di usion evidences of the presence of relatively closely located defect levels in the crystal with di erent transition energies.
In this part of the spectrum, transitions from the valence band to boron levels (with energies of 0.35 and 0.65 eV) can be also found.Let us also consider the IR absorption spectrum of the initial crystal.e region up to 2000 cm −1 relates to absorption by lattice vibrations.According to [27], phonon energy TA, LA, TO, and LO are 0.045, 0.067, 0.0955, and 0.1055 eV, respectively.Moreover, in samples treated with hydro uoric acid, a surface phonon is observed at 950 cm −1 (energy 0.116 eV) [28].
Figure 3 shows the optical absorption spectral dependence measured at room temperature for the p-n-4H-SiC structures doped with boron.As one can see from the diagram, transitions from the valence band to the conduction band (beginning at 3.23 eV), transitions of electrons through acceptor levels of boron to the conduction band (from 2.6 to 2.9 eV), and transitions from the valence band to boron levels (from 0.35 to 0.65 eV) should be observed.
In addition, the sample has defects of vacancy nature after the low-temperature di usion process.According to the diagram, carbon vacancies also allow absorption bands from 0.74 eV to 2.6 eV.Contribution of each absorption band is determined by concentration of impurities and defects, cross sections for capture of current carriers, and so on.
In Figure 3, a sharp increase in the absorption at 2.7-2.8eV is observed in the absorption spectrum of the sample doped with boron, which is associated with transitions from the boron levels to the conduction band.A sharp yield to saturation indirectly con rms our data obtained by electrophysical measurements on a signi cant boron concentration up to 10 20 cm −3 [18].Absorption in the region of 1.5-2.6 eV is associated with transitions to vacancy-type defects.However, as one can see in the previous gure, at lower energies the structureless broad band tail continuation can be observed.us, in general, the absorption spectrum corresponds to the diagram.
Figure 4 shows the optical absorption spectrum of p-n-4H-SiC structures doped with aluminum (from a source created using aluminum chloride) measured at room temperature and a diagram of optical absorption transitions.
As it can be seen from Figure 4, the absorption spectrum is similar to the previous case.Absorption bands from acceptor levels of aluminum to the conduction band of 2.9-3.0 eV are observed.A wide absorption band at energies below 2.8 eV is associated with vacancy defects in the crystal.Similarly to those observed in Figure 3, one can see a small step at 1.7 eV as well.However, the level of absorption on the defects is signi cantly, to 3 times, lower, which indicates a reduced concentration of defects.Figure 5 shows the absorption spectrum of the sample doped with aluminum (from a source created from a thin aluminum lm sputtered in vacuum).
As it can be seen from Figure 5, along with the absorption band at 3.0 eV (transitions to the conduction band from aluminum levels), an additional absorption band with the energy of 2.5-2.6 eV is observed.A high level of absorption indicates a high concentration of defects.As it can be seen from Figures 4 and 5, relative contribution of absorption to the defects is higher when the impurity source is created from a thin lm of aluminum sputtered in vacuum.Consequently, in this case, additional defects of the vacancy nature are formed in the crystal during di usion.
In all samples (doped with both boron and aluminum), an increase in absorption of up to 1.7 eV (the carbon vacancy level in silicon carbide is around 1.7 eV), then a step, which extends to 2.5 eV, and then again the growth of absorption on the defects are observed.In view of the complexity of the optical absorption processes in the presence of many types of defects, it is di cult to identify which defects are responsible for peaks.
We can note, however, that 1.7 eV is close to the transition energy of an electron from the valence band to the vacancy level of carbon, and 2.3 and 2.5 eV are close to the energies of the transition of electrons from the valence band to other levels of the same carbon vacancy.
Further, with an increase in the quantum energy up to the energy of transition from the levels of boron or aluminum impurities to the conduction band, a sharp increase in absorption is observed, con rming the data on the high impurity concentration.

Conclusions
Optical absorption of p-n-4H-SiC structures doped with boron and aluminum by low-temperature di usion was studied for the rst time.Di usion of impurities was performed from aluminum-silicate and boron-silicate lms (sources) fabricated by various methods.
In the spectral dependences of optical absorption at room temperature, bands associated with transitions from impurity levels as well as absorption bands associated with defects of the vacancy nature were observed.
e level of absorption in the samples was used to estimate the concentration of defects.It was shown that the use of sources of impurity atoms created by using boron and aluminum chlorides allows one to reduce the concentration of vacancy defects.

Figure 2 :
Figure 2: Spectral dependence of optical absorption in the IR region: 1, initial 4H-SiC crystal; 2, spectrum taken with Al-doped side; 3, spectrum taken from the reverse side of the crystal.