Deep level defects in unintentionally doped 4H-SiC homoepitaxial layer

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition (HWCVD) have been studied using photoluminescence (PL) technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon (Vc) are revealed by electron spin resonance (ESR) technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broad-band green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.     

非刻意掺杂4H-SiC同质外延中的深能级缺陷

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition （HWCVD） have been studied using photoluminescence （PL） technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon （Vc） are revealed by electron spin resonance （ESR） technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broad-band green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.     

4H-SiC中本征缺陷浓度的第一性原理计算

Based on the first-principles pseudopotentials and the plane wave energy band method,the supercells of perfect crystal 4H-SiC and those with intrinsic defects VC,VSi,VC-C and VC-Si were calculated.Ignoring the atomic relaxations,the results show that the formation energy of intrinsic defects is ranked,from low to high,as VC,VC-C,VSi to VSi-Si at 0 K.The equilibrium concentration of each intrinsic defect can be deduced from the formation energy of each intrinsic defect.The concentration ranks,from high to low,as VC,VC-C,VSi,VSi-Si,which is in accordance with the ESR and PL results.The stabilizing process of metastable defects VSi converting to VC-C was explained by formation energy.     

氙光作用后半绝缘4H-SiC外延片本征缺陷的ESR谱特性

The intrinsic defects in epitaxial semi-insulating 4H-SiC prepared by low pressure chemical vapor deposition （LPCVD） are studied by electron spin resonance （ESR） with different illumination times. The results show that the intrinsic defects in as-grown 4H-SiC consist of carbon vacancy （Vc） and complex-compounds-related Vc. There are two other apexes presented in the ESR spectra after illumination by Xe light, which are likely to be Vsi and VcCsi. Illumination time changes the relative density of intrinsic defects in 4H-SiC; the relative density of intrinsic defects reaches a maximum when the illumination time is 2.5 min, and the ratio of Vc to complex compounds is minimized simultaneously. It can be deduced that some Vsi may be transformed to the complex-compounds-related Vc because of the illumination.     

ESR characters of intrinsic defects in epitaxial semi-insulating 4H-SiC illuminated by Xe light

The intrinsic defects in epitaxial semi-insulating 4H-SiC prepared by low pressure chemical vapor de-position (LPCVD) are studied by electron spin resonance (ESR) with different illumination times. The results show that the intrinsic defects in as-grown 4H-SiC consist of carbon vacancy (V_C) and complex-compounds-related V_C.There are two other apexes presented in the ESR spectra after illumination by Xe light, which are likely to be V_(Si) and V_CC_(Si). Illumination time changes the relative density of intrinsic defects in 4H-SiC; the relative density of intrinsic defects reaches a maximum when the illumination time is 2.5 min, and the ratio of V_C to complex compounds is minimized simultaneously. It can be deduced that some V_(Si) may be transformed to the complex-compounds-related V_C because of the illumination.     

Effect of rapid thermal annealing on the current-voltage characteristics of 4H-SiC Schottky diodes

The effect of rapid thermal annealing (temperature 600–800°C; duration 2 min) on the forward current-voltage characteristics of 4H-SiC Schottky diodes is studied. Tungsten, nickel, chromium, and molybdenum deposited by electron-beam sputtering in vacuum are used as the Schottky-contact metals. Dissimilar types of influence exerted by the thermal treatment on the barrier height and scatter of the contact parameters, which characterize the degree of their uniformity, are found for different metals.     

Effects of rapid thermal annealing on nitrided gate oxide grown on 4H-SiC

The electrical characteristics of thermally nitrided gate oxides on n-type 4H-SiC, with and without rapid thermal annealing processes, have been investigated and compared in this paper. The effects of annealing time (isothermal annealing) and annealing temperature (isochronal annealing) on the gate oxide quality have also been systematically investigated. After rapid isothermal and isochronal annealings, there has been a significant increase in positive oxide-charge density and in oxide-breakdown time. A correlation between the density of the positive oxide charge and the oxide breakdown reliability has been established. We proposed that the improvement in the oxide-breakdown reliability, tested at electric field of 11 MV/cm, is attributed to trapping of injected electron by the positive oxide charge and not solely due to reduction of SiC-SiO₂ interface-trap density.     

Planar defects in 4H-SiC PiN diodes

Using plan-view transmission electron microscopy (PVTEM), we have identified stacking faults (SFs) and planar defects in 4H-SiC PiN diodes subjected to electrical bias. Our observations suggest that not all planar defects seen in the PiN diodes are SFs. By performing diffraction-contrast imaging experiments using TEM, we can distinguish SFs from other planar defects. In addition, high-resolution TEM (HRTEM) imaging and analytical TEM have revealed that some planar defects consist of a 3-nm-wide SiC amorphous layer. Many of these planar defects are orientated parallel to {1 00} planes, whereas others are roughly parallel to the (0001) plane. The appearance of these planar defects suggests that they are grain boundaries.     

4H-SiC表面热氧化生长SiOx薄膜特性的研究

采用扫描电子显微镜（SEM）、原子力显微镜（AFM）和X射线光电子能谱（XPS）测试方法对4H-SiC上热氧化生长的氧化硅（SiOx）薄膜表面形貌进行观测，并分析研究SiOx薄膜和SiOx/4H-SiC界面的相关性质，包括拟合Si2p、O1s和C1s的XPS谱线和分析其相应的结合能，以及分析SiOx层中各主要元素随不同深度的组分变化情况，从而获得该热氧化SiOx薄膜的化学组成和化学态结构，并更好地了解其构成情况以及SiOx/4H-SiC的界面性质。     

Rapid thermal annealing of neutron transmutation doped silicon

Rapid thermal annealing (RTA) of neutron transmutation doped Si wafers is shown to be an alternative to conventional furnace annealing. Measurements of resistivity and deep level transient spectroscopy (DLTS), demonstrated annealing on wafers with diameters up to 75 mm. A 4.5 kW incoherent-light RTA furnace was used. Evidence for crystalline slip was found but this did not appear to affect the results. The slip was more severe for the larger diameter wafers. Some results from a DLTS examination of a partially rapid-thermal-annealed wafer are presented.     

Rapid thermal annealing of ion implanted 6H-SiC by microwave processing

Rapid thermal processing utilizing microwave energy has been used to anneal N, P, and Al ion-implanted 6H-SiC. The microwaves raise the temperature of the sample at a rate of 200°C/min vs 10°C/min for conventional ceramic furnace annealing. Samples were annealed in the temperature range of 1400-1700°C for 2-10 min. The implanted/annealed samples were characterized using van der Pauw Hall, Rutherford backscattering, and secondary ion mass spectrometry. For a given annealing temperature, the characteristics of the microwave-annealed material are similar to those of conventional furnace anneals despite the difference in cycle time.     

MOS Characteristics of C-Face 4H-SiC

Metal–oxide–semiconductor (MOS) capacitors and MOS field-effect transistors (MOSFETs) fabricated on the carbon face of 4H-SiC were characterized following different postoxidation annealing methods used to passivate the oxide–semiconductor (O–S) interface. Of the various processes studied, sequential postoxidation annealing in NO followed by atomic hydrogen gave the lowest interface trap density (D _it). Direct oxidation/passivation in NO yielded somewhat better I–V characteristics, though all passivation ambients produced approximately the same breakdown field strength. n-Channel MOSFETs showed high channel mobility at low field, which is likely caused by the presence of mobile ions at the O–S interface. Comparisons with the silicon face are presented for interface trap density, oxide breakdown field, and channel mobility. These comparisons suggest that the carbon face does not offer significant performance advantages.     

Surface morphology of 6H-SiC after thermal diffusion

Diffusion of aluminum into 6H-SiC has been carried out in the temperature range of 1800–2100°C. Aluminum carbide (Al₄C₃) was used for a p-type impurity source; the diffused surface exhibited good stoichiometry and surface morphology. A thin-layer graphite film was developed to protect the wafer surface from deterioration during the high-temperature diffusion process. A high-resolution optical microscope (HROM) and atomic force microscopy (AFM) were employed to evaluate the surface morphology of the diffused samples. The protective graphite layer significantly decreased the surface roughness. X-ray photoelectron spectroscopy (XPS) was used to identify the Si/C ratio near the surface regions. Very little surface graphitization occurred during diffusion. In addition, secondary ion-mass spectroscopy (SIMS) was used to investigate the influence of the thin graphite film on the diffusion properties in SiC. There were no significant differences in doping profiles in the samples with and without the graphite film.     

Effect of annealing process on the surface roughness in multiple Al implanted 4H-SiC

A P-layer can be formed on a SiC wafer surface by using multiple Al ion implantations and post-implantation annealing in a low pressure CVD reactor.The Al depth profile was almost box shaped with a height of 1×10¹⁹cm^-3 and a depth of 550 nm.Three different annealing processes were developed to protect the wafer surface.Variations in RMS roughness have been measured and compared with each other.The implanted SiC, annealed with a carbon cap,maintains a high-quality surface with an RMS roughness as low as 3.8 nm.Macrosteps and terraces were found in the SiC surface,which annealed by the other two processes（protect in Ar/protect with SiC capped wafer in Ar）.The RMS roughness is 12.2 nm and 6.6 nm,respectively.     

退火工艺对Al离子注入的4H-SiC表面形貌的影响

通过应用多次Al离子注入和CVD中的高温退火技术,在SiC片表面形成了p型层。p型层中各深度下Al的浓度均为110¹⁹cm^-3,层厚为550nm。本文应用三种不同的退火工艺对注入后的SiC进行退火。通过测量和比较表面粗糙度,发现通过石墨层覆盖来保护表面的退火工艺可以很好的阻止SiC表面在高温退火下的粗化,粗糙度保持在3.8nm。通过其他两种(在氩气保护下、在SiC保护片的覆盖下)退火工艺退火所得到的表面有明显的台阶,粗糙度分别为12.2nm和6.6nm。     

Cross-polarization imaging and micro-raman detection of defects in the epitaxy of 4H-SiC

Cross-polarized imaging with an optical scanner and confocal μ-Raman spectroscopy have been used to reveal and to study structural defects in n-4H-SiC. We introduce a new method of polarized imaging that uses a combination of linear and circular polarization, which enhances contrast in images of defect regions. Regions that show intensity contrast have been observed to have characteristics of either low-angle grain boundaries or sharp lines of delineation between low and high strain areas. Different types of polytype inclusions have also been observed, and the material within the inclusions has been shown to be either 3C-SiC or 6H-SiC. Polytype inclusions occur as isolated features or form cores of holes, and they can form at any point of the growth.     

Effects of thermal annealing on In-induced metastable defects in InGaN films

We investigated the effects of thermal annealing on the properties of InGaN layers. From secondary ion mass spectroscopy results, it was found that severe In desorption occurred after annealing. Photoluminescence and X-ray diffraction results indicate that significant amounts of In vacancy-related defects exist in the annealing samples. It was also found that persistent photoconductivity decay time constants were 211, 893 and 1040 s, while the decay exponents were 0.153, 0.120 and 0.213 for the as-grown, 800 °C-annealed and 1000 °C-annealed InGaN epitaxial layers, respectively.     

Effect of rapid thermal annealing on deep level defects in the Si-doped GaN

Rapid thermal annealing effects on deep level defects in the n-type GaN layer grown by metalorganic chemical vapor deposition (MOCVD) have been characterized using deep level transient spectroscopy (DLTS) technique. The samples were first characterized by current-voltage (I-V) and capacitance-voltage (C-V) measurements. The measurements showed that the barrier height of the as-grown sample to be 0.74 eV (I-V) and 0.95 eV (C-V) respectively. However, the Schottky barrier height of the sample annealed at 800 °C increased to 0.84 eV (I-V) and 0.99 eV (C-V) respectively in nitrogen atmosphere for 1 min. Further, it was observed that the Schottky barrier height slightly decreased after annealing at 900 °C. DLTS results showed that the two deep levels are identified in as-grown sample (E1 and E3), which have activation energies of 0.19 ± 0.01 eV and 0.80 ± 0.01 eV with capture cross-sections 2.06 × 10⁻¹⁷ cm² and 7.68 × 10⁻¹⁸ cm², which can be related to point defects. After annealing at 700 °C, the appearance of one new peak (E2) at activation energy of 0.49 ± 0.02 eV with capture-cross section σ_n = 5.43 × 10⁻¹⁷ cm², suggest that E2 level is most probably associated with the nitrogen antisites. Thermal annealing at 800 °C caused the E1 and E3 levels to be annealed out, which suggest that they are most probably associated with the point defects. After annealing at 900 °C the same (E1 and E3) deep levels are identified, which were identified in as-grown n-GaN layer.     

Radiation defects in n-4H-SiC irradiated with 8-MeV protons

Capacitance-related methods and electron spin resonance were used to study the deep-level centers formed in n-4H-SiC as a result of irradiation with 8-MeV protons. For the samples, Schottky diodes and p-n structures formed on the layers either obtained by sublimational epitaxy or produced commercially by CREE Inc. (United States) were used. It was found that the type of centers introduced by irradiation is independent of the technology of the material growth and the type of charged particles. On the basis of the results of annealing the defects and the data of electron spin resonance, the possible structure of the centers is suggested.     

Characteristics of Dislocation Half-Loop Arrays in 4H-SiC Homo-Epilayer

Dislocation “half-loop arrays” (HLAs) in 4H-SiC homo-epilayers are studied by molten KOH etching and atomic force microscopy (AFM). It is found that the dislocation half-loops in an array exist at different depths in the epilayer, and they are aligned roughly but not exactly perpendicular to the off-cut direction. These results indicate that the dislocation half-loops in an array are not formed simultaneously, but the array extends by generation of new half-loops during growth. It is also demonstrated that the HLAs can be artificially induced by creating strain in the material, followed by annealing.