Xingfang Liu

Fracture Properties of Silicon Carbide Thin Films by Bulge Test of Long Rectangular Membrane

This paper reports the mechanical properties and fracture behavior of silicon carbide (3C-SiC) thin films grown on silicon substrates. Using bulge testing combined with a refined load-deflection model of long rectangular membranes, which takes into account the bending stiffness and prestress of the membrane material, the Youngs modulus, prestress, and fracture strength for the 3C-SiC thin films with thicknesses of 0.40 and 1.42 mum were extracted. The stress distribution in the membranes under a load was calculated analytically. The prestresses for the two films were 322 plusmn 47 and 201 plusmn 34 MPa, respectively. The thinner 3C-SiC film with a strong (111) orientation has a plane-gstrain moduli of 415 plusmn 61 GPa, whereas the thicker film with a mixture of both (111) and (110) orientations exhibited a plane-strain moduli of 329 plusmn 49 GPa. The corresponding fracture strengths for the two kinds of SiC films were 6.49 plusmn 0.88 and 3.16 plusmn 0.38 GPa, respectively. The reference stresses were computed by integrating the local stress of the membrane at the fracture over edge, surface, and volume of the specimens and were fitted with Weibull distribution function. For the 0.40-mum-thick membranes, the surface integration has a better agreement between the data and the model, implying that the surface flaws are the dominant fracture origin. For the 1.42-mum-thick membranes, the surface integration presented only a slightly better fitting quality than the other two, and therefore, it is difficult to rule out unambiguously the effects of the volume and edge flaws. [2007-0191].

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.

ITO-free and air stable organic light-emitting diodes using MoO3:PTCDA modified Al as semitransparent anode

Ultrathin Al combined with a buffer layer of MoO3 doped perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) has been demonstrated as an efficient semitransparent anode in OLED. The ITO-free OLED utilizing this composite anode exhibits a low turn on voltage of 2.6 V and a high current efficiency of 5.6 cd A−1. Based on accelerated degradation tests, a device half-life of about 945 h was obtained at an initial device luminance of 100 cd m−2. The charge transfer (CT) complexes formed between MoO3 and PTCDA played a critical role in promoting hole injection. The buffer layer also kept a smooth and thermal stable morphology, which benefits the stability of OLED greatly.

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.

Dependence of the electrical and optical properties on growth interruption in AlAs/In0.53Ga0.47As/InAs resonant tunneling diodes

The dependence of interface roughness of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant tunneling diodes [RTDs] grown by molecular beam epitaxy on interruption time was studied by current-voltage [I-V] characteristics, photoluminescence [PL] spectroscopy, and transmission electron microscopy [TEM]. We have observed that a splitting in the quantum-well PL due to island formation in the quantum well is sensitive to growth interruption at the AlAs/In0.53Ga0.47As interfaces. TEM images also show flatter interfaces with a few islands which only occur by applying an optimum value of interruption time. The symmetry of I-V characteristics of RTDs with PL and TEM results is consistent because tunneling current is highly dependent on barrier thickness and interface roughness.

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.

Influences of organic–inorganic interfacial properties on the performance of a hybrid near-infrared optical upconverter

In this article, we explored in detail the influences of organic–inorganic interfacial properties on the performance of a hybrid near-infrared optical upconverter, mainly the leakage current and brightness. The upconverter that can convert input near-infrared to visible light was fabricated by directly integrating an organic light emitting diode (OLED) with an In0.12Ga0.88As/GaAs multi-quantum wells (MQWs) photodetector (PD). MoO3 doped perylene-3,4,9,10-tetra carboxylic dianhydride (PTCDA) was inserted between the PD and OLED as an interfacial connection layer. The possible interaction mechanism of the PTCDA molecule on the GaAs surface such as π electron cloud spreading or Ga–O–C bonds model was suggested and verified to explain the excellent hole injection property at the GaAs/MoO3:PTCDA interface. In addition, choosing an effective interface passivation layer was proven to retain leakage current markedly.

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.

Fracture properties of Silicon carbide thin films characterized by bulge test of long membranes

The mechanical properties and fracture behavior of silicon carbide (3C-SiC) thin films grown on silicon substrates were characterized using bulge testing combined with a refined load-deflection model for long rectangular membranes. Plane-strain modulus Eps, prestress s0, and fracture strength smax for 3C-SiC thin films with thickness of 0.40 mum and 1.42 mum were extracted. The Eps values of SiC are strongly dependent on grain orientation. The thicker SiC film presents lower s0 than the thinner film due to stress relaxation. The smax values decrease with increasing film thickness. The statistical analysis of the fracture strength data were achieved by Weibull distribution function and the fracture origins were predicted.