Optical and Mechanical Properties of Several Coating Materials in UV Range
Silicon carbide (SiC) has shown considerable potential for ultraviolet (UV) photodetectors due to its properties such as wide band gap (3.26 eV for 4H-SiC), high break down electric field and high thermal stability. 4H-SiC-based UV photodetectors such as Schottky, metal-semiconductor-metal (MSM), metal-insulator-semiconductor (MIS) and avalanche have been presenting excellent performance for UV detection application in flame detection, ozone-hole sensing, short-range communication, etc. Generally, the most widely used antireflection coating and passivation layer for 4H-SiC-based photodetectors are native SiO2 grown by heating 4H-SiC in O2 in order to improve the absorption and passivation of photodetectors. Nevertheless, the thermally grown SiO2 single layer suffers from high reflection, large absorption and inaccurate thickness. Therefore, in this chapter, UV antireflection coatings were designed, fabricated and applied in order to reduce optical losses and improve the quantum efficiency (QE) of 4H-SiC-based photodetectors. The important results will be introduced as follows:
- Antireflection coatings
- Refraction index
- Band gap
- Transparent range
Silicon carbide (SiC) has been considered to be a new generation semiconductor material for optoelectronic and power devices due to its wide band gap (3.26 eV for 4H-SiC), high break down electric field (3.0 MV/cm for 4H-SiC) and high thermal stability (4.9 W/cm K for 4H-SiC). In the recent decade, SiC-based ultraviolet (UV) photodetectors have been improved and developed greatly with all sorts of structures including Schottky diodes[1-3], metal-semiconductor-metal (MSM)[4-6], p-i-n[7-9], metal-insulator-semiconductor (MIS) and avalanche photodiodes (APD)[11-13] for application in flame detection, ozone-hole sensing, short-range communication, etc. Generally, the photons can be absorbed in 4H-SiC overall below 360 nm except that more than 20% of them are reflected, as shown in Figure 1(a). Meanwhile, the absorption coefficient of the 4H-SiC is larger than 1000 below a wavelength of 300 nm and the penetration length of the previously mentioned UV light in 4H-SiC is below 4 μm, as depicted in Figure 1(b). Thus, high-quality antireflection (AR) coatings and clean interface between 4H-SiC and coatings are needed for reducing optical loss on the surfaces and interfaces in these SiC-based photodetectors. The recent results showed that the internal quantum efficiency of SiC photodetectors had reached 100%. Thus, improvement of external quantum efficiency of SiC photodetectors is particularly important and significant, which can be approached by depositing AR coatings.
The most widely used AR coatings for SiC-based photodetectors is silicon dioxide (SiO2), which is thermally grown by oxidizing the surface of SiC in a furnace at 1100°C–1300°C. The reflection can be reduced to below 8% in the range of 220–380 nm. Although the SiO2 layer usually has good passivation quality, its optical properties such as large absorption, uncertain refractive index, inaccurate film thickness and high reflection cannot be controlled ideally. Therefore, it is necessary to redesign and grow the AR coatings for 4H-SiC UV photodetectors.
2. Selection of AR coatings
There are many factors that need to be considered for the selection of UV AR coatings for 4H-SiC photodetectors including transparent range, refraction index, mechanical and chemical properties, etc. Although there are hundreds of materials that can be prepared for AR coatings, few of them are suitable for 4H-SiC in UV range.
2.1. Transparent range
Transparent range is the first and most important factor for 4H-SiC UV photodetectors, which means that AR coatings need to be transparent in the UV range especially at 200–400 nm. Only several oxides and fluorides can satisfy the condition such as Al2O3, SiO2, HfO2, CaF2, BaF2, MgF2, etc., as shown in Table 1. These materials have a large band gap so that the UV photons do not have adequate energy to achieve transition. Extinction coefficient is another factor that can characterize the transparent level of an optical film. The lower the extinction coefficient is, the better the transmittance of the film. The extinction coefficient is related to the crystal structure of the AR coatings. Generally, polycrystalline films have the largest extinction coefficients, amorphous films have lower extinction coefficients and single crystal lowest extinction coefficients. UV AR coatings have higher requirements on transmittance and extinction coefficients than visible AR coatings, thus only a few materials can meet the requirements, as shown in Table 1.
2.2. Refraction index
Refraction index is another important parameter for an optical material to be applied in photodetectors, which is required to match that of substrates so that AR effect can be achieved. Generally, refraction index increases with the decrease of the wavelength. The relationship between refraction index (
2.3. Mechanical property and stability
SiC is a superstable material with ultrahigh hardness, which is only lower than that of diamond. Thus, its AR coatings require suited mechanical property and high stability. Generally, fluorides such as CaF2, BaF2 and MgF2 are very soft materials, although their transparent ranges are adequately wide, as shown in Table 1, which are not suitable for SiC photodetectors. Oxides are probably the most suitable materials because they not only have high hardness but also high stability. It is necessary to notice that some oxides such as MgO have suitable transparent range and hardness but it is not stable and reacts with CO2 in the air, which cannot be used for AR coatings. Oxides such as Al2O3 and HfO2 have ultrahigh hardness, as shown in Table 1, which are good choices for SiC photodetectors.
2.4. Electronic property
In photodetectors, AR coatings are also used as passivation layers to restrain the leakage current. Thus, electronic properties of the AR coatings are also significant for 4H-SiC UV photodetectors. Excellent coatings have wide band gap, high dielectric constant, high critical electric field and high conduction band gap. As shown in Table 2, some materials such as HfO2 have high dielectric constant but its critical electric field and conduction band gap are low. Meanwhile, other materials such as AlN and Si3N4 have ideal critical electric field and conduction band gap but narrow band gap and low dielectric constant.
Under comprehensive evaluation, though dielectric constants are not adequate high, Al2O3 and SiO2 are materials that not only have wide band gap and ideal transparent range but also suitable refractive index, high critical electric field and conduction band gap, which is the most suitable optical material as AR coatings for application in SiC UV photodetectors.
3. Design of AR coatings
Design of AR coatings is the procedure to determine the thickness and refractive index of the optical materials. Admittance matching method was used to design AR films for 4H-SiC photodetectors. The reflection will happen when the incidence of UV light is from one material to another due to the mismatch of the refraction indices. The relationship between admittance (
3.1. Single-layer AR coating
Single-layer AR coating is considered at first due to it is the simple system, as shown in Figure 2. The reflection coefficient of single-layer AR coating is
where and are reflection coefficients of surfaces of substrate and AR film, which can be presented as and . is the phase thickness of single-layer AR coating and can be presented as . Then the reflectance R is
Thus, when the light beam performs vertical incidence, the sufficient condition of
Optical thickness of the coating is , that is .
The refraction index of the coating must satisfy .
If satisfying these two conditions, the coating is matching to the substrate and incident medium.
3.2. Double-layer AR coatings
Refraction indices of a substrate and an incident medium are
If , the , the two following equations can be obtained:
The equations can be transformed into
Generally, there are three cases for the solutions:
Set , if the light beam performs vertical incidence, the optic thicknesses of the two films are both , that is, , one of the and can be fixed at first, another can be solved by Equations (2) and (3).
Set and , according to Equations (2) and (3), and can be solved. The refraction indices are fixed to solve the thickness of the two films. For the system of air, Al2O3, SiO2 films and 4H-SiC substrate under the vertical incidence of light with the wavelength of = 280 nm, the refraction indices are = 1.000, = 1.685, = 1.495, = 2.995, according to (2) and (3), , ( = 1, 2, 3...), = 1 is selected to reduce the optical absorption of the films.
Set , , that is, and . In this case, the reflectance of the light beam is not influenced by the second layer. However, this layer has an achromatic effect for the light beam with a wavelength of . Therefore, for the system of air, Al2O3, SiO2 films and 4H-SiC substrate, the third case is the same as the second. The second layer SiO2 film has no contribution to the reflectance of the light beam. The Al2O3 film is the layer that reduces the reflectance.
Comparison of reflectance has been made between Al2O3/SiO2 double layer and SiO2 single layer, as shown in Figure 4. (1) The reflectance of the Al2O3/SiO2 film (0.071%) has lower value than that of SiO2 film (2.1%). (2) The reflectance spectrum of the Al2O3/SiO2 film is narrower than that of SiO2 film, which indicates that there is a better selection of the wavelength for the Al2O3/SiO2 film. Therefore, the Al2O3/SiO2 film is more suitable as the AR coating for the 4H-SiC-based photodetectors.
4. Growth of AR coatings
4.1. Thermal oxidation
SiO2 formed by thermal oxidation of SiC is most widely used as AR coatings in SiC UV photodetectors. As shown in Figure 5, the SiC wafers are put into the middle of the furnace with permanent temperature. Dry and wet oxygen (with H2O) are used to oxidize the SiC at 1100°C–1300°C. The oxidation rate is nonlinear and very low that 40-nm-thick SiO2 layer usually takes 4 h. With the increase of thickness, the oxidation rate is greatly reduced. Although the oxidized SiO2 is the densest state compared to the other growth methods and the leakage current of SiC devices with the layer is the lowest, absorption is also the largest especially in Si suboxides, which are usually formed at the interface between SiO2 and SiC. People added NO, N2O[18, 19], POCl3, etc., during the oxidation, which can reduce the sub-oxides and interface states. However, the absorption of the films still cannot be ignored. Thus, the oxidized SiO2 is suitable for passivation layer not for AR coatings on the window of SiC UV photodetectors.
4.2. Electron beam evaporation
Electron beam evaporation is a physical vapor deposition that is applied to deposit oxides, fluorides, metals, etc. The electron beam is used to heat the surface of bulk materials to be vapor state and then the vapor deposit on the dome with lots of substrates, as illustrated in Figure 6. The AR coatings prepared by electron beam evaporation usually have high transmittance and low absorption, which perfectly meet the requirement of the AR coatings so that optical films are widely deposited and grown by using this technique. A shortcoming of the technique is that the deposited films are not as dense as that prepared by other techniques such as oxidation and sputtering, which may induce large leakage current in the 4H-SiC photodetectors. Therefore, the electron beam evaporation can be applied to grow AR coatings on the windows of the 4H-SiC photodetectors but is not suitable for deposition of passivation layer to reduce the leakage current.
4.3. Atomic layer deposition
Atomic layer deposition (ALD) is a special chemical vapor deposition that is widely applied in microelectronic and optoelectronic fields to deposit oxides, nitrides, metals, etc[21-23]. As illustrated in Figure 7, the precursors and oxidants are pulsed into the chamber successively to deposit on the surface of the substrates monolayer by monolayer and cleaned by inert gases such as nitrogen during the deposition. A deposition sequence is usually that oxidants nitrogen precursors nitrogen.
The precursors and oxidants do not meet and react with each other directly, otherwise, that would be a typical chemical vapor deposition. Al2O3, SiO2 and HfO2 films are usually grown by using ALD as gate dielectrics in microelectronic fields, which are so uniform that they can be deposited on a rough surface with almost the same thickness by using this technique. The leakage current can also be restrained well through the deposition. Optical properties of ALD films[27, 28] were examined and demonstrated that ALD can be an advanced technique to balance the optical and electrical properties of thin films. In order to get the best optical and electrical properties, thermal oxidation and electron beam evaporation were both applied to deposit Al2O3 and SiO2 AR films. SiC UV photodetectors with Al2O3 and SiO2 AR films are studied and demonstrated in the following sections.
5. SiC Photodetectors with Al2O3 and SiO2 films
5.1. 4H-SiC MSM photodetectors with Al2O3 and SiO2 films
Two 4H-SiC MSM photodetectors were separately fabricated with electron beam-evaporated Al2O3/SiO2 double-layer films and thermal SiO2 single-layer films for comparison, which were prepared on two identical n-type 4H-SiC wafers with epilayers of 3.4 μm and doping level of 3.0 × 1015 cm–3. One of them was oxidized at 1150°C in an O2 atmosphere for 4 h. The final thickness of the SiO2 layer was approximately 40 nm, as measured by an ellipsometer. SiO2 and A12O3 layers were successively deposited on the other wafer by electron beam evaporation. The final thicknesses of the A12O3 and SiO2 films were 42 nm and 96 nm, respectively, which are quarter-wave and half-wave of the UV wavelength of 280 nm corresponding to calculated peak responsivity. Then, the reflectance spectra of these samples were measured by a commercial spectrophotometer. After confirmation of the spectra, the device fabrication process was immediately performed on two samples. Lithography and wet etching were done on the samples, then interdigitated electrodes were deposited by sputtering Au and Ni with a width and spacing of 2 and 2 μm, as shown in Figure 8. Finally, Au bonding pads were deposited on the ends of the electrodes. The spectral response measurements were performed on the devices by using a light source of LAX 1450 M Xe lamp and an Actron SpectraPro-2500i monochromator. The output power of the monochromatic light was measured and calibrated by a Si 222 photodetector. Then the light was illuminated on these two MSM photodetectors. The current-voltage (I–V) characteristics of these devices were measured by using an electrometer and sourcemeter.
These two Al2O3/SiO2/4H-SiC and SiO2/4H-SiC MSM devices both exhibit excellent optoelectronic properties, as presented in Figure 9(a). The photocurrent of Al2O3/SiO2/4H-SiC devices are approximately double that of SiO2/4H-SiC due to the lower reflection and absorption of Al2O3/SiO2 films. Meanwhile, the photocurrent of SiO2/4H-SiC devices increased with increasing voltage, which may be attributed to the charge traps at the interface between SiO2 and 4H-SiC. The dark current of the SiO2/4H-SiC (around 0.50 pA) was lower than that of the Al2O3/SiO2/4H-SiC devices (7.5 pA at 10 V), as shown in Figure 9(b), which is attributed to the fact that the thermally grown SiO2 layer on 4H-SiC substrates was denser than the electron beam evaporated Al2O3/SiO2 films to restrain the leakage current of the devices.
Spectral responses of Al2O3/SiO2/4H-SiC and SiO2/4H-SiC MSM photodetectors were measured under reverse voltage from 5 to 20 V, as shown in Figure 10(a). The peak responsivity of Al2O3/SiO2/4H-SiC photodetectors is 0.12 A/W at 20 V, which is more than twice that of SiO2/4H-SiC (0.055 A/W at 20 V) due to the lower reflection on Al2O3/SiO2/4H-SiC surface, as shown in Figure 10(b). Both devices achieved high UV to visible rejection ratio of >103. The minimum reflectance of SiO2/4H-SiC was 3.2% at 262 nm, more than twelve times higher than that of Al2O3/SiO2/4H-SiC (0.25% at 284 nm close to the reference wavelength 280 nm). However, the peak responsivity of the SiO2/4H-SiC detectors was kept at a wavelength of 290 nm with the increasing voltage, which mainly relied on the thickness of the active epilayer (3.4 μm). The penetration depths of 4H-SiC were approximately 2.8 and 3.6 μm at the wavelengths of 290 and 300 nm, respectively, which indicates that photons can penetrate the active layer at the wavelength of 300 nm. Meanwhile, the shorter-wavelength light was also absorbed by the thermally grown SiO2 layer and the difference of reflectance between 260 nm and 290 nm was very small. Therefore, the wavelength corresponding to peak responsivity is 290 nm in SiO2/4H-SiC devices. The same phenomenon and analysis can also be observed and used in the Al2O3/SiO2/4H-SiC detectors.
The external quantum efficiency of photodetectors can be obtained from , where
The absorption of Al2O3/SiO2 films was ignored in the above expression because of the low extinction coefficients (10–5) of the coatings prepared by electron beam evaporation and inexistence of Si suboxides, as shown in Figure 11(a). While the absorption of the thermally grown SiO2 layer cannot be ignored due to the larger density and Si sub-oxides such as Si+ at the SiO2/4H-SiC interface, as shown in Figure 11(b), which are the reasons for the low quantum efficiency of SiO2/4H-SiC photodetectors.
Comparisons between reflectance and response spectra were made in the Al2O3/SiO2/4H-SiC photodetectors to examine the spectral restriction effect of the Al2O3/SiO2 films, as shown in Figure 12. The spectral response is not exactly consistent with the reflectance from 200 to 240 nm, which may be due to the influence of the surface recombination. Then the variation of the spectral response matches the reflectance from 240 to 300 nm, which indicates that reflection is the dominant factor when the photons can be absorbed by the active layer completely as discussed above. Spectral response is not consistent with the reflectance from 300 to 380 nm, which is attributed to the fact that the photons can only be absorbed partly by the epilayer so that the photon absorption became dominant and the restriction effect of Al2O3/SiO2 films weakened.
The 4H-SiC MSM photodetectors with Al2O3/SiO2 AR coatings and SiO2 layer were fabricated and demonstrated. The highest responsivity and maximum external quantum efficiency were obtained with the Al2O3/SiO2 films, which proved that the design and application of Al2O3/SiO2 films on the 4H-SiC MSM photodetectors are successful. The optical and electrical properties of the Al2O3/SiO2 films prepared by electron beam evaporation will be further studied in 4H-SiC MIS photodetectors.
5.2. 4H-SiC MIS Photodiodes with Al2O3 and SiO2 films
Two 4H-SiC MIS UV photodetectors with Al2O3/SiO2 and SiO2 films were fabricated on 4H-SiC wafers with a structure of 250 nm p type (
Photocurrents of SiO2/4H-SiC and A12O3/SiO2/4H-SiC MIS detectors are shown in Figure 14(a). The photocurrents of A12O3/SiO2/4H-SiC device was almost triple as much as that of SiO2/4H-SiC at 10 V, which is attributed to the fact that more photons can be absorbed by the devices with lower reflectance of A12O3/SiO2 films and generate more electron-hole pairs than the device with thermally grown SiO2 layer.
The MIS photodetectors are actually a combination of MIS and p-i-n devices. Thus, the photocurrent was determined by both MIS and p-i-n structures in the devices. When the SiO2 layer in MIS was tunneled at a certain voltage, p-i-n junction restrained the tunneling currents so that the photocurrent was constant, as illustrated in Figure 14(a). However, the photocurrent of A12O3/SiO2/4H-SiC saturated form 0 V while that of S/4H-SiC saturated from 2.5 V, which is attributed to the fact that lots of electrons were trapped in the evaporated A12O3/SiO2 films as described above, and the charges made holes accumulate on the surface of 4H-SiC substrate and tunneled through the SiO2 layer. Therefore, built-in field and depletion regions were formed in the p-i-n structure and photocurrent can be generated without voltage applied, which is called the normally-on mode. However, the charges in SiO2/4H-SiC photodetectors were few so that the carriers could not tunnel through the SiO2 layer. A certain voltage needs to be applied on the device to achieve this purpose, which can be recognized as normally-off mode.
Leakage (dark) currents of SiO2/4H-SiC and A12O3/SiO2/4H-SiC MIS photodetectors were measured from 0 to 15 V to examine the electrical property of these films, as shown in Figure 14(b). Good passivation property of SiO2/4H-SiC was achieved for that the leakage current was lower than 0.13 pA at 10 V. However, the leakage current of the A12O3/SiO2/4H-SiC MIS device was 3.9 pA at 10 V, nearly 30 times higher than that of SiO2/4H-SiC device, which is due to the fact that the electron beam-evaporated A12O3/SiO2 films were not as dense as the thermally grown SiO2 layers and had trapped charges when the films were prepared. Thus, the electrical properties of the electron beam evaporated A12O3/SiO2 films need to be further improved.
The spectral response of the 4H-SiC UV detectors with A12O3/SiO2 and SiO2 films were measured and studied in the wavelength range from 200 to 400 nm. The peak responsivities of these devices were 30 mA/W at 260 nm with a single SiO2 layer and 50 mA/W at 270 nm with A12O3/SiO2 double-layer, respectively, as shown in Figure 15(a). The surface reflectances of the two coatings on 4H-SiC substrate are shown in Figure 15(b). The minimum reflectance of A12O3/SiO2 films is 0.34% nm and only 1/10 of single SiO2 layer, which agrees well with the AR coatings designed above. It is interesting that the peak responsivity of the A12O3/SiO2/4H-SiC detectors is not at the wavelength of 280 nm, which corresponds to the minimum reflectance of the A12O3/SiO2 films. The following reasons can be considered. First, the active region length in the device is 2.25 µm, including the thickness of depletion region and hole diffusion length. The penetration depths of the wavelength at 270 and 280 nm are 1.83 and 2.30 µm, respectively, so that the 270 nm UV light can be absorbed completely. Thus, the peak responsivity of the A12O3/SiO2/4H-SiC MIS photodetectors is at the wavelength of 270 nm. Meanwhile, the diffusion length of the electron is approximately 21 µm, which is longer than the nonintentionally doped n-type epilayer so that the electrons can approach the bottom electrode easily. The responsivity of these two devices was not as much as above MSM photodetectors because the carriers need to tunnel through the 6 nm SiO2 insulator in the MIS detectors, which reduced the quantum efficiency and responsivity. Nevertheless, the leakage current of the MIS device was improved greatly, which is only 1/5 of the MSM device. The UV-to-visible rejection ratios in MIS detectors also achieved 2 × 103 by using the A12O3/SiO2 double-layer AR coatings, which is the highest in 4H-SiC-based MIS photodetectors and is attributed to good passivation property of thermally grown SiO2 layer and A12O3/SiO2 double-layer and low leakage current.
In the wavelength from 250 to 380 nm, the responsivity of 4H-SiC MIS detectors with A12O3/SiO2 film is higher than that of with SiO2 film, as shown in Figure 15(a). The wavelength range is even wider than the lower reflectance region (260 to 300 nm) of the A12O3/SiO2 films, as shown in Figure 15(b). I-V characteristics of these two devices were performed without UV exposure to study the wideband higher response. Ohmic conduction mechanism is dominant according to the current curve-fitting in the low electric field, as illustrated in Figure 16(a). Fowler–Nordheim (FN) tunneling is converted to the main conduction mechanism in both devices with increasing electric field (
4H-SiC-based MIS UV photodetectors with thermally grown SiO2 layer and evaporated A12O3/SiO2 films were fabricated and demonstrated. Low leakage current and high UV-to-visible rejection ratios >2 × 103 had been achieved for these devices. The 4H-SiC MIS photodetectors with A12O3/SiO2 AR coatings presented higher responsivity and quantum efficiency, which demonstrate that the design and deposition of Al2O3/SiO2 films are effective and significant for the 4H-SiC UV photodetectors.
The author acknowledges support from the National Basic Research Program of China (grant no. 2015CB759600), National Natural Science Foundation of China (grant no. 61474113) and Beijing Natural Science Foundation (grant no. 4132076).
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