Wanshun Zhao

Interfacial study and energy-band alignment of annealed Al2O3 films prepared by atomic layer deposition on 4H-SiC

Al2O3 films were prepared by atomic layer deposition using trimethylaluminum and H2O at 250 °C on 4H-SiC substrates and annealed at 1000 °C in N2. The as-deposited and annealed Al2O3 films were measured and analyzed near the Al2O3/SiC interfaces by using an X-ray photoelectron spectroscopy (XPS) with etching processing. The XPS results showed that as-deposited Al2O3 films were O-rich and converted to anhydride Al2O3 films after annealed at 1000 °C in N2. Si suboxides were found both at as-deposited and annealed Al2O3/SiC interfaces. Energy band shift between Al2O3 and 4H-SiC was found after annealing. The conduction band offsets of as-grown and annealed Al2O3/SiC were 1.90 and 1.53 eV, respectively. These results demonstrated that Al2O3 can be a good candidate to be applied in SiC metal-oxide-semiconductor devices.

The thermal stability study and improvement of 4H-SiC ohmic contact

The thermal stability of the standard Ni/SiC and a TiW/Ni/SiC Ohmic contacts was investigated and compared after being aged at 400 °C in the N2 atmosphere. The Ohmic contact was characterized using a combination of I-V measurements, the optical microscopic imaging, X-ray diffraction (XRD), and Auger electron spectroscopy (AES) techniques. It is shown that the standard Ni/SiC Ohmic contact failed after being aged at 400 °C for 20 h in the N2 atmosphere, while the TiW/Ni/SiC Ohmic contact could stand after 100 h. The TiW/Ni/SiC Ohmic contact was found kept a smooth surface morphology during the rapid thermal annealing and aging process, while the standard Ni/SiC Ohmic metal surface was found rougher. Both the Ohmic contact deteriorations after high temperature aging could be attributed to the formation of graphite which is confirmed by the XRD results. The XRD and AES results reveal that the better thermal stability of the TiW/Ni/SiC could be explained by the formation of CW3 and TiC, which deter the C atom diffusion to form graphite.

Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement

The free carrier density and mobility in n-type 4H-SiC substrates and epilayers were determined by accurately analysing the frequency shift and the full-shape of the longitudinal optic phonon—plasmon coupled (LOPC) modes, and compared with those determined by Hall-effect measurement and that provided by the vendors. The transport properties of thick and thin 4H-SiC epilayers grown in both vertical and horizontal reactors were also studied. The free carrier density ranges between 2×1018 cm−3 and 8×1018 cm−3 with a carrier mobility of 30–55 cm2/(V·s) for n-type 4H-SiC substrates and 1× 1016−3×1016 cm−3 with mobility of 290–490 cm2/(V·s) for both thick and thin 4H-SiC epilayers grown in a horizontal reactor, while thick 4H-SiC epilayers grown in vertical reactor have a slightly higher carrier concentration of around 8.1×1016 cm−3 with mobility of 380 cm2/(V·s). It was shown that Raman spectroscopy is a potential technique for determining the transport properties of 4H-SiC wafers with the advantage of being able to probe very small volumes and also being non-destructive. This is especially useful for future mass production of 4H-SiC epi-wafers.

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 10 19 cm 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.

Growth of Hexagonal Columnar Nanograin Structured SiC Thin Films on Silicon Substrates with Graphene-Graphitic Carbon Nanoflakes Templates from Solid Carbon Sources

We report a new method for growing hexagonal columnar nanograin structured silicon carbide (SiC) thin films on silicon substrates by using graphene–graphitic carbon nanoflakes (GGNs) templates from solid carbon sources. The growth was carried out in a conventional low pressure chemical vapor deposition system (LPCVD). The GGNs are small plates with lateral sizes of around 100 nm and overlap each other, and are made up of nanosized multilayer graphene and graphitic carbon matrix (GCM). Long and straight SiC nanograins with hexagonal shapes, and with lateral sizes of around 200–400 nm are synthesized on the GGNs, which form compact SiC thin films.

Visible blind p+-π-n--n+ ultraviolet photodetectors based on 4H-SiC homoepilayers

p^+-@p-n^--n^+ ultraviolet photodetectors based on 4H-SiC homoepilayers have been presented. The growth of the 4H-SiC homoepilayers was carried out in a LPCVD system. The size of the active area of the photodetectors was 300x300@mm^2. The dark and illuminated I-V characteristics had been measured at reverse biases form 0 to 20V at room temperature, and the illuminated current was at least two orders of magnitude than that of dark current below 13V bias. The peak value zones of the photoresponse were located at 280-310nm at different reverse biases, and the peak value located at 300nm was 100 times greater than the cut-off response value in 380nm at a bias of 10V, which showed the device had good visible blind performance. A small red-shift about 5nm on the peak responsivity occurred when reverse bias increased from 5 to 15V.

Influences of annealing on structural and compositional properties of Al2O3 thin films grown on 4H–SiC by atomic layer deposition*

Annealing effects on structural and compositional performances of Al2O3 thin films on 4H?SiC substrates are studied comprehensively. The Al2O3 films are grown by atomic layer deposition through using trimethylaluminum and H2O as precursors at 300 ?C, and annealed at various temperatures in ambient N2 for 1 min. The Al2O3 film transits from amorphous phase to crystalline phase as annealing temperature increases from 750 ?C to 768 ?C. The refractive index increases with annealing temperature rising, which indicates that densification occurs during annealing. The densification and grain formation of the film upon annealing are due to crystallization which is relative with second-nearest-neighbor coordination variation according to the x-ray photoelectron spectroscopy (XPS). Although the binding energies of Al 2p and O 1s increase together during crystallization, separations between Al 2p and O 1s are identical between as-deposited and annealed sample, which suggests that the nearest-neighbour coordination is similar.

Enhancement of Piezoelectric Properties of PMN-PT Ceramics at Low Temperatures

The temperature dependence of dielectric, elastic, electromechanical, and piezoelectric properties of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) ceramics near MPB with x = 0.33, 0.35, 0.36, 0.37, and 0.38 was systematically investigated within the temperature range 120–360 K. Compositions of x = 0.37 and 0.38 were found to exhibit temperature-independent piezoelectric properties over a temperature range from 185 to 360 K. The temperature-stable piezoelectric response was attributed to a counter-balance among the dielectric, elastic, and electromechanical responses in a broad ferroelectric phase transition that was revealed from the dielectric constant, elastic compliance constant, and electromechanical coupling factor.

Multi-wafer 3C-SiC thin films grown on Si (100) in a vertical HWLPCVD reactor

We report the latest results of the 3C-SiC layer growth on Si (100) substrates by employing a novel home-made horizontal hot wall low pressure chemical vapour deposition (HWLPCVD) system with a rotating susceptor that was designed to support up to three 50 mm-diameter wafers. 3C-SiC film properties of the intra-wafer and the wafer-to-wafer, including crystalline morphologies and electronics, are characterized systematically. Intra-wafer layer thickness and sheet resistance uniformity (σ/mean) of ~3.40% and ~5.37% have been achieved in the 3 × 50 mm configuration. Within a run, the deviations of wafer-to-wafer thickness and sheet resistance are less than 4% and 4.24%, respectively.

High-Performance 4H–SiC-Based Metal–Insulator–Semiconductor Ultraviolet Photodetectors With

4H-SiC-based metal-insulator-semiconductor (MIS) ultraviolet (UV) photodetectors with thermally grown SiO₂ and evaporated Al₂O₃/SiO₂ (A/S) films are fabricated and demonstrated as normally-off and normally-on mode devices, respectively. Ultralow dark currents of 3.25 × 10^-10 and 9.75 × 10^-9 A/cm² and high UV-to-visible rejection ratios of >; 2 × 10³ have been achieved at 10 V. The peak responsivities of these devices were separately 30 mA/W at 260 nm and 50 mA/W at 270 nm at 10 V. These results demonstrate that S/4H-SiC and A/S/4H-SiC MIS photodetectors are promising candidates to be applied in optoelectronic integrated circuits.