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Porous 6H-SiC samples with different thicknesses were fabricated through anodic etching in diluted hydrofluoric acid. Scanning electron microscope images show that the dendritic pore formation in 6H-SiC is anisotropic, which has different lateral and vertical formation rates. Strong photoluminescence was observed and the etching process was optimized in terms of etching time and thickness. Enormous enhancement as well as redshift and broadening of photoluminescence spectra were observed after the passivation by atomic layer deposited Al2O3 and TiO2 films. No obvious luminescence was observed above the 6H-SiC crystal band gap, which suggests that the strong photoluminescence is ascribed to surface state produced during the anodic etching.
© 2016 Optical Society of America
1. Introduction
Silicon carbide (SiC) is a highly chemical resistant semicondutor material at room temperature. Porous SiC is a potentially attractive material for light emitting diodes (LEDs), photodetectors, sensors and substrate for monocrystalline SiC epitaxy [1–4]. Among these applications, porous SiC has potential to realize fluorescent SiC based white LEDs [5–7]. Porous SiC was first fabricated by Shor et al. [8] by using anodic etching method, which anodizing single crystal n-type 6H-SiC in hydrofluoric acid and under UV illumination. Since then, a lot of studies of porous SiC have been reported with specific changes in morphology, optical transmittance and photoluminescence (PL) properties after the formation of porous structures in both 6H and 4H n-type or p-type SiC wafers [4, 5, 9–15].
The wide difference of porous morphology often exists and agreement on the explanation of the PL spectra in porous SiC has not been reached yet. To date, there are mainly two different interpretations: a) quantum confinement effect [5]; b) surface-state upon anodic etching [4, 12, 16–18]. However, the large surface area also leads to an enhanced non-radiative recombination rate due to the surface damage caused by etching process [19]. It is known that the Al2O3 and TiO2 films are widely used as passivation materials with high melting point, excellent mechanical property and chemical stability [20–26]. Furthermore, atomic layer deposition (ALD) is a modernized deposition method for surface passivation, which allows homogeneity to be achieved on complex structures [7]. Thus, coating porous SiC by ALD Al2O3 or TiO2 could be an ideal way to realize surface passivation and increase the PL intensity. To our knowledge, a study of the passivation effect on different thickness of porous 6H-SiC by using atomic layer deposited Al2O3 and TiO2 films has not been reported yet, and the luminescence mechanism is still quite blurring.
In this work, we investigated the passivation effect of ALD Al2O3 and TiO2 on porous 6H-SiC. For comparison, we also present the preliminary result of the effect of anodic etching time on porous 6H-SiC structures. The morphology, reflectance, transmittance and PL properties were investigated first. Then, the porous samples passivated by Al2O3 and TiO2 films, respectively, were fabricated and the passivation effects on different thickness of porous layer were analyzed.
2. Experiments and results
The single-crystal 6H-SiC (n-type) samples used in this study were Si-face polished substrate with a thickness of 250 µm supplied by SiCrystal AG (Germany). The substrate was doped with nitrogen and had a resistivity of around 0.1 Ω cm. Ohmic contacts on the back side of SiC substrate were formed by evaporation of 200 nm thick aluminum (Alcatel SCM 600 E-beam and sputtering deposition system). To obtain pores in 6H-SiC, samples were mounted in a Teflon etch-cell with a platinum (Pt) counter electrode. The etching solution is hydrofluoric acid (HF) diluted solution with a concentration of 5 wt. %.
The anodic etching was done under yellow light environment and all the samples were etched at constant current density of 2 mA/cm2. During the electroetching process, the hydroxide ions arrived at the surface of 6H-SiC and partially oxidized 6H-SiC to form SiO2 and CO2. Simultaneously, SiO2 reacted with HF and formed soluble H2SiF6. The electrochemical etching processes were proposed as follows [5]:
6H-SiC samples a, b, c and d were prepared with a etching time of 60, 150, 260, and 360 minutes, respectively.
2.1 Morphology of porous 6H-SiC
The morphology of porous 6H-SiC was investigated by scanning electron microscopy (SEM). Cross-sectional SEM images taken at different depths of sample d are shown in Fig. 1. The total etching time for this sample is 360 min. It can be obviously seen that there are holes around 63nm on the surface, as shown in Fig. 1(a). The formed thickness of porous layer is 38.88 µm, which consists of a layer of ultra-small triangular pores (top layer), a layer of sparse dendritic pores (middle layer) and a thick layer of dense dendritic pores (bottom layer). As one can see that, not only the porosity changes but also there are obvious boundaries between two different porous layers, as shown in Fig. 1(c) and 1(d). The porosity of the electrochemically etch 6H-SiC crystal is quite non-uniform throughout the porous layer. The pore in the top layer is quite small and the size gradually increases towards the bottom layer. However, the formed channels branch horizontally and also tend to propagate perpendicularly to the basal plane. Finally, the channels are occasionally intersected with each other.
Fig. 1 Planar and cross-sectional view of SEM images of sample d: (a) the hole on the surface is around 63nm and (b) the overview of the porous layer consisting of three layers with a total thickness of 38.88 µm, (c) the boundary between top layer and middle layer, and (d) the boundary between the middle and bottom layer.
Samples a, b and c produced under the same conditions etched for 60, 150 and 260 min respectively also have three different layers with the same structure profiles and pore size as sample d. The thicknesses of the porous layer for sample a, b, c, and d were 2.65 µm, 22.14 µm, 24.74 µm and 38.88 µm, respectively. The photographs and corresponding SEM images of the bottom layer area are shown in Fig. 2. The dark circles in the photographs are the porous area, which becomes uneven with etching time increased.
Fig. 2 Photographs of porous SiC with different etching time (a) 60 min, (b) 150 min, (c) 260 min and (d) 360 min. The corresponding cross-sectional SEM images are also shown underneath, taken near the interface between the porous layer and the substrate.
The relationship between the porous thickness and etching time is shown in Fig. 3. Assuming, the anodic etching process is anisotropic, i.e., there are vertical and lateral etching, which competed with each other. As the anodic etching time increases, the relative thickness of the dendritic layer (compared with the top layer) also increases, as shown in Fig. 3. This indicates that the small triangular pores in the top layer is the initial stage of the dendritic pore structures [27]. As the porous layer became thicker, the formed SiO2 in deep layer could not be dissolved efficiently, which could block the vertical etching.
Fig. 3 Relationship of porous thickness and dendritic thickness with etching time for 6H-SiC: the trend for total porous layer and dendritic layer (middle + bottom layer) was plotted for analyzation.
2.2 Reflectance and transmittance in porous 6H-SiC
In order to investigate the optical properties of the porous 6H-SiC samples, reflectance and transmittance spectra have been measured by using a calibrated integrating sphere system (Gooch & Housego OL 700-71). The results are shown in Fig. 4. Figure 4(a) shows that the reflectance of porous samples has been increased over the wavelength range from 380 to 780 nm, compared with the flat reference sample. The reflectance also increased with the increasing of anodic etching time due to the reflection on sidewall of pores. It can be noticed that the reflectance of sample a is higher than that of sample b, which may be related to the larger dendritic structures, as shown in Fig. 2. The maximum value of reflectance red shifts from 475 to 550 nm as the anodic etching time increased. This phenomenon is in accordance with the photographs in Fig. 2, that is, the longer wavelength could be reflected with the porous thickness increasing.
Fig. 4 Comparison of the measured (a) reflectance and (b) transmittance spectra for the flat 6H-SiC sample and the porous samples a, b, c and d.
As shown in Fig. 4(b), the transmittance of porous samples has been significantly decreased compared to the flat sample, especially for samples c and d. After anodic etching, the porous 6H-SiC layers are still crystalline and they should be transparent for the wavelength above bandgap. However, the observed low transmittance performance of porous 6H-SiC sample is attributable to the enormous surface states [4].
2.3 Atomic layer deposited Al2O3 and TiO2 films
The surface state may also contain non-radiative recombination component, thus surface passivation should be introduced to improve the light emitting properties. In order to passivate the enormous surface of the porous samples, ALD (Model R200, Picosun, Finland) Al2O3 and TiO2 films were used to cover the pore surface. The Al2O3 thin films were deposited on porous samples using H2O and trimethylaluminium (TMA: Al(CH3)3) gases as precursor materials for the oxygen and aluminum, respectively. The reaction took place in cycles. In this work, one deposition cycle included 0.1 s pulse of TMA, 3 s N2 purge, 0.1 s H2O exposure and followed by 3 s N2 purge. The surface chemical reaction of Al2O3 deposition consists of two half reactions [26, 28]:
As the similar deposition process for TiO2 thin films, the reaction is also divided into the following two half-reactions [29]:
The desired Al2O3 or TiO2 thickness can be obtained by controlling the number of ALD deposition cycles. In this paper, a 20 nm (200 cycles) thick Al2O3 layer was deposited on porous samples at 160 °C with a growth rate of 1 Å/cycle. For comparison, a same thickness of TiO2 (410 cycles) was deposited on another batch of samples at 150 °C with a deposition rate of 0.49 Å/cycle. It is well known, at the deposition temperature around 150°C, Al2O3 and TiO2 are definitely in amorphous phase [30–32]. It should be mentioned that the ALD deposited Al2O3 and TiO2 films contain O-H bond, which has been confirmed in [19, 24–26, 29]. A post-deposition annealing was performed at 350 °C for 5 min in N2 (10 sccm) environment by rapid thermal annealing (RTA, Jipelec) for all the samples in order to activate the passivation layer.
The plan and cross-sectional SEM images of sample d with 20 nm thick TiO2 are shown in Fig. 5. The holes on the surface are still clealy visible, which haven’t been blocked by 20nm films. However, the TiO2 films could conformally coat and almost fulfill the pores in deep layers. If the surface porous layer are completely filled by Al2O3 or TiO2, the penetration of precursor molecules inside the pores should decrease, i.e. the pores closed. The morphology of porous sample covered with 20 nm thick Al2O3 is similar to that of TiO2.
Fig. 5 Planar and cross-sectional SEM images of sample d (360 min): (a) the surface holes, (b) top layer, (c) middle layer and (d) bottom layer covered with 20 nm thick TiO2.
2.4 Photoluminescence and passivation effect of ALD Al2O3 and TiO2 films
PL measurements were carried by using a 375 nm laser as excitation source coupled into a microscope. In the present experiments, the excitation light was focused on the sample surface through the microscope with a dichroic mirror and a long working distance 20x objective lens. The luminescence signal was filtered by a long-pass filter with cutoff wavelength at 420nm and collected by an optical spectrometer (CAS 140 B, Compact Array Spectrometer, Instrument System). Figure 6(a) shows the PL spectra for the flat and porous 6H-SiC samples a-d. It is apparent that the original flat sample has very weak light emitting, but all the porous samples have a strong luminescence peak around 535 nm with a full width at half maximum (FWHM) of 160 nm. For 6H-SiC, the exciton Bohr radius is 0.7 nm [33]. The diameter size of the current porous samples are above 6 nm, which is much greater than the exciton Bohr diameter of 6H-SiC, so the PL in porous samples should not be dominated by quantum size effects. And it could be attributed to surface states on the large internal surface of porous structures, such as oxygen vacancies and dangling bonds [13, 16, 34]. Surface states generally formed on the porous samples were associated with the chemical reaction during anodic etching process [16–18]. Surface states also contained non-radiative recombination component which need to be passivated to increase the radiative recombination component. The PL performance of porous samples a, b, c and d passivated by Al2O3 or TiO2 is investigated, and their PL spectra plotted in the visible light spectral range are illustrated in Fig. 6(b) and 6(c), respectively. PL spectra of porous samples show a broadening and red shift with respect to the spectra of porous 6H-SiC samples without surface passivation.
Fig. 6 PL spectra of porous samples (a) with different anodically etching time; (b) passivated by 20 nm thick Al2O3; (c) passivated by 20 nm thick TiO2.
Figure 6(a) shows that the PL intensity increased with the porous layer thickness (anodic etching time) and this trend is directly shown in Fig. 7. The PL spectra with Al2O3 and TiO2 passivation in Fig. (6) show red shifts of approximately 27 nm, 37 nm, 42 nm and 50 nm for samples a, b, c and d, respectively. After Al2O3 passivation, the porous samples a, b, c and d exhibit a 222%, 170%, 104% and 94% enhancement, respectively, as shown in Fig. 6(b). However, the improvement for TiO2 passivation is slightly smaller than that for Al2O3 films due to the less quantity of O-H bonds [24, 28, 30, 35]. After TiO2 passivation, the porous samples a, b, c and d have a PL increase of 154%, 83.4%, 78.6% and 71.5%, respectively, as shown in Fig. 6(c). With longer anodically etching time, the thicker of the porous layer formed. And during ALD deposition process, it is difficult for precursors to penetrate uniformly into the deeper layer. Consequently, excess oxygen atoms may exist in the pores and oxidized the porous surface during the annealing process, leading to red shift in porous SiC. Such red shift can be reasonably attributed to the radiative recombination at some oxygen-related defects [36], which were introduced during post-annealing of Al2O3 and TiO2 films.
Fig. 7 The relationship between PL intensity and anodic etching time before and after passivation (PSV) by 20 nm Al2O3 or TiO2.
In present experiments, the same thickness was also deposited on both flat Si and 6H-SiC samples. No PL enhancement was observed after thermal annealing, which excludes the emission of TiO2 films on passivation effect. Moreover, it is worth mentioning that 20 nm for Al2O3 and TiO2 is an optimized thickness, especially for sample d with a 38.8 µm porous layer. Regarding the thickness, the porous layer couldn’t be fully filled with thinner Al2O3 or TiO2 films, conversely, the pores maybe oxidized by excess films.
3. Conclusion
In summary, the morphology and optical properties of anodic etched porous SiC has been investigated. Although the origin of the luminescence is not clear at the moment, quantum confinement effects can be excluded and the luminescence is attributed to the surface related effect. We have demonstrated for the first time a significant increase in the PL intensity of porous 6H-SiC passivated by ALD Al2O3 and TiO2 films. These results support the interpretation that surface states are responsible for the light emitting. Compared with the uncoated porous SiC, the Al2O3 or TiO2 coated porous samples exhibit a strong enhancement of photoluminescence, which is attributed to the decrease of non-radiative recombination. Although, the passivation process could give rise to red shift in PL spectra, here we report a effective method to improve the luminescence intensity.
Acknowledgment
This work was supported by Innovation Fund Denmark (project No 4106-00018B).
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