**1. Introduction**

Silicon carbide (SiC) has attracted the attention of many researchers due to its outstanding electrical, mechanical, and thermal properties. SiC has been used in many industries for power devices and optoelectronic applications [1–8]. There are more than 200 polytypes of Si-C, with cubically (3C-SiC) and hexagonally (4H-SiC or 6H-SiC) modified compounds being the most used. The differences come from the stacking sequence of the hexagonal structure bonded in the Si-C bilayers [1–8]. The existence of polytypes implies that many different stable atomic arrangements and symmetries can be obtained, including hexagonal, cubic, and rhombohedral arrangements [6,7].

Si-C absorbs UV light and is transparent to visible light, making it an ideal material for optically based sensors or photodetectors at UV wavelengths [8]. A thorough analysis of the carrier recombination mechanisms in SiC is needed to understand the underlying physics of the luminescence phenomena for industrial applications. Photoluminescence (PL) is a standard method for characterizing the emission properties of semiconductor materials and can provide information about defect-related carrier transport dynamics [8]. Of the few studies reporting the optical properties of SiC, many have looked at the 3C-, 6H-, and 4H-SiC structures [4–8]. The 4H-SiC structure has been shown to have several defects

**Citation:** Kim, S.-K.; Jung, E.Y.; Lee, M.-H. Defect-Induced Luminescence Quenching of 4H-SiC Single Crystal Grown by PVT Method through a Control of Incorporated Impurity Concentration. *Compounds* **2022**, *2*, 68–79. https:// doi.org/10.3390/compounds2010006

Academic Editor: Juan Mejuto

Received: 30 December 2021 Accepted: 21 February 2022 Published: 2 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

69

and PL peaks in the band gap. The PL spectra in 4H-SiC originate from a combination of phonon-instigated electronic transitions caused by defects in SiC [9–11]. This observation of luminescence quenching is not evidence of electronic doping [12,13]. Therefore, it is necessary to determine the additional factors that affect the PL quenching and luminescence properties of 4H-SiC by characterizing it fully.

In this study, we investigated the structural and optical properties of the luminescence quenching of 4H-SiC crystals grown with the physical vapor transport (PVT) method. Control of the structure was achieved by using differently treated starting materials of 3C-SiC and by controlling the boron, aluminum, and nitrogen concentrations. The structural defects of the 4H-SiC crystals were analyzed to correlate them with the impurity concentration and optical properties. The 4H-SiC prepared with the PVT method was characterized using Raman spectroscopy, X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), UV-Vis-NIR spectrophotometry, and PL to compare the changes in impurities and structural properties.

#### **2. Materials and Methods**

#### *2.1. Preparation of Starting Material*

The 3C-SiC structure was synthesized using chemical vapor deposition involving vaporization, pyrolysis, nucleation, oxidation–reduction, and substitution [14]. The precursor gas consisted of commercial methyltrichlorosilane (MTS), ammonia (NH3), and carbon dioxide (CO2). MTS was the silicon precursor, and ethylene (C2H4) and propane (C3H8) were used as the hydrocarbon precursors. A mixture of H2 and Ar was used as a carrier gas. The 3C-SiC synthesized was black, indicating the presence of impurities.

The free carbon and silica present in the synthesized 3C-SiC were removed through pulverization (pristine), acid leaching and decarburization (step A), and denitrification treatment (step B), as shown in Figure 1 and Table 1. In the pristine sample, the synthesized 3C-SiC was pulverized. A mixed acid solution dissolved the metal; nitric acid acted as a powerful oxidizer that dissolved to form metal ions ( M<sup>+</sup> 3 ), which reacted with hydrochloric acid to produce chorine anions in the solution. Then, the addition of HF allowed the free silica and free silicon in the mixed acid solution to react and generate the SiF6 gas and SiCl6 in a soluble solution. To decarbonize the powder, oxygen gas was passed through it to react with the free carbon, thus forming CO2 vapor.

**Figure 1.** Flowchart for the fabrication of 4H-SiC crystals with the PVT process with three differently treated starting materials of 3C-SiC (S1, S2, and S3).


**Table 1.** Experimental conditions and identification for the growth of the 4H-SiC crystal samples.

Volatile chloride compounds and chlorine gas were produced in step A. The residual acid salt was purified through volatilization, thus removing the excess oxygen present in SiC through a denitrification treatment. This was done by heating at 105 ◦C in a reducedpressure evaporation in an argon environment. Other byproduct impurities present in the synthesized SiC were removed through heat treatment at 850 ◦C for 1 h in an oxygen environment. The dried and purified 3C-SiC powder obtained was green in color and had a purity greater than 90%. As shown in Figure 1 and Table 1, these purification processes were performed using the three purification processes of S1, S2, and S3 [15].

#### *2.2. Growth of 4H-SiC Crystals*

The 4H-SiC crystals were fabricated by using the generalized PVT method with three differently treated 3C-SiC powders (S1, S2, and S3) as starting materials. This PVT method has already been explained in detail by other researchers [16,17]. Briefly, the 3C-SiC powder was placed at the bottom of the crucible in a PVT chamber. In this process, the growth process was carried out in an argon environment. The growth of the 4H-SiC crystals took place at a growth temperature in the range of 1900–2100 ◦C and a pressure of 10−<sup>2</sup> to 10−<sup>3</sup> mbar in an argon atmosphere. The grown crystal sample was detached from the crucible and ground to two-inch diameter by slicing and then polishing to prepare the SiC wafers. A detailed flowchart for the fabrication of the 4H-SiC crystals with a commercialized PVT process with three differently treated 3C-SiC starting materials is shown in Figure 1 and Table 1.

#### *2.3. X-ray Diffraction*

The crystalline phases of the 3C-SiC powders were characterized using XRD (D/max-2500V/PC, Rigaku, Tokyo, Japan) with Cu K α radiation at 30 mA and 40 kV. In addition, the crystalline orientation of the 4H-SiC crystal samples was characterized using a multifunction X-ray diffractometer (XRD; PANalytical, Malvern, Worcestershire, UK) and highresolution two-dimensional (2D) XRD (Bruker, D8 Discover, Billerica, MA, USA) at the Korea Basic Science Institute (KBSI, Daegu, Korea). To obtain the oriented diffraction plane of the main and minor XRD peaks, the XRD result was obtained with a θ-2θ scan using multi-function XRD with Cu K α radiation at 30 mA and 40 kV.

#### *2.4. Inductively Coupled Plasma Optical Emission Spectrometry*

The elemental compositions of the 4H-SiC crystal samples were determined using inductively coupled plasma optical emission spectrometry (ICP-OES; Optima 5300DV, Perkin Elmer, Waltham, MA, USA). The operation conditions were used at a radio-frequency power of 1.6 kW and a plasma argon gas flow rate of 14.0 L/min. Before measurement, the 4H-SiC crystal samples were pre-treated. To remove the organic components or contaminations in the 4H-SiC crystal samples, nitric acid (HNO3) in an amount of 2–5 mL was put into the sample, and then the resulting products were dried and concentrated. Next, the resulting products were dissolved in the mixture of HCl and HNO3. Finally, the dissolved products were treated by using the prepared aqueous solution with H2SO4 and HF. To fabricate the microwave-assisted acid digestion of the SiC samples, sample preparations were performed by using a microwave digestion system (Milestone Srl - START D, Sorisole, Italy) with PTFE vessels. The quantification of the elemental concentration was performed by using the certified reference material (ECRM 780-1), which was calibrated with a lower standard deviation of 1% for accuracy. For the preparation of the boron analysis, the

resulting samples were put into a beaker. Then, the prepared solutions of hydrofluoric acid and nitric acid were put into the PTFE bottle. After that, they were maintained for 24 h to be dissolved in the mixed acid solution.

#### *2.5. Elemental Analyzer*

The elemental composition of the 4H-SiC crystal samples was determined using an elemental analyzer (EMGA-920, Horiba, Kyoto, Japan). The 4H-SiC crystal samples were placed in a graphite crucible. The operations were performed under the conditions of a power of 5.5 kW and a high temperature at 2650 ◦C. The 4H-SiC crystal samples were introduced into a graphite crucible that was placed between two electrodes. Then, to achieve complete combustion and transfer the generated gas into the detector, the 4H-SiC crystal samples were heated at a high temperature. The generated gas was directly extracted into a thermal conductivity detector (TCD).

#### *2.6. Raman Spectrometry*

The structural phase identification of all of the samples was performed using a Raman spectrometer (Renishew, Wotton-under-Edge, UK) with a 514 nm laser as the excitation source. The Raman spectra were collected over the wavenumber range of 120–2000 cm<sup>−</sup><sup>1</sup> with a four-stage Peltier cooled CCD detector (UV-Vis-NIR range). The objective lens of the microscope (DM500, LEICA, Wetzlar, Germany) had a magnification of 50×, and the exposure time for accumulation was 5 s. The power intensity of the laser beam was 5.0 ± 0.1 mW.

#### *2.7. UV-Vis-NIR Spectrophotometer*

The optical transmittance and absorbance spectra of the 4H-SiC crystal samples in the wavelength range of 200–1000 nm were measured at room temperature (298 K) using an ultraviolet–visible (UV-vis) spectrophotometer (LAMBDA 950, Perkin Elmer, Waltham, MA, USA).

#### *2.8. Photoluminescence Spectrophotometry*

Photoluminescence (PL) spectra were collected at both room temperature (298 K) and an extremely low temperature of 50 K using a PL spectrometer (HORIBA, LabRAM HR Evolution, Kyoto, Japan) with a He-Cd laser with a wavelength of 325 nm and power ranging from 0.15 to 15 mW as the excitation source. The power density ranged from approximately 0.023 to 23.6 kW/cm2. The laser was focused on the sample using a 50× objective lens.
