Yiping Zeng

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.

In Situ Boron and Aluminum Doping and Their Memory Effects in 4H-SiC Homoepitaxial Layers Grown by Hot-Wall LPCVD

The in-situ p-type doping of 4H-SiC grown on off-oriented (0001) 4H-SiC substrates was performed with trimethylaluminum (TMA) and/or diborane (B2H6) as the dopants. The incorporations of Al and B atoms and their memory effects and the electrical properties of p-type 4H-SiC epilayers were characterized by secondary ion mass spectroscopy (SIMS) and Hall effect measurements, respectively. Both Al- and B-doped 4H-SiC epilayers were p-type conduction. It was shown that the profiles of the incorporated boron and aluminum concentration were in agreement with the designed TMA and B2H6 flow rate diagrams. The maximum hole concentration for the Al doped 4H-SiC was 3.52×1020 cm-3 with Hall mobility of about 1 cm2/Vs and resistivity of 1.6~2.2×10-2 Wcm. The heavily boron-doped 4H-SiC samples were also obtained with B2H6 gas flow rate of 5 sccm, yielding values of 0.328 Wcm for resistivity, 5.3×1018 cm-3 for hole carrier concentration, and 7 cm2/Vs for hole mobility. The doping efficiency of Al in SiC is larger than that of B. The memory effects of Al and B were investigated in undoped 4H-SiC by using SIMS measurement after a few run of doped 4H-SiC growth. It was clearly shown that the memory effect of Al is stronger than that of B. It is suggested that p-type 4H-SiC growth should be carried out in a separate reactor, especially for Al doping, in order to avoid the join contamination on the subsequent n-type growth. 4H-SiC PiN diodes were fabricated by using heavily B doped epilayers. Preliminary results of PiN diodes with blocking voltage of 300 V and forward voltage drop of 3.0 V were obtained.

Homoepitaxial Growth and Characterization of 4H-SiC Epilayers by Low-Pressure Hot-Wall Chemical Vapor Deposition

Horizontal air-cooled low-pressure hot-wall CVD (LP-HWCVD) system is developed to get high quality 4H-SiC epilayers. Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates purchased from Cree is performed at a typical temperature of 1500°C with a pressure of 40 Torr by using SiH4+C2H4+H2 gas system. The surface morphologies and structural and optical properties of 4H-SiC epilayers are characterized with Nomarski optical microscope, atomic force microscopy (AFM), x-ray diffraction, Raman scattering, and low temperature photoluminescence (LTPL). The background doping of 32 μm-thick sample has been reduced to 2-5×1015 cm-3. The FWHM of the rocking curve is 9-16 arcsec. Intentional N-doped and B-doped 4H-SiC epilayers are obtained by in-situ doping of NH3 and B2H6, respectively. Schottky barrier diodes with reverse blocking voltage of over 1000 V are achieved preliminarily.

Polarized photoluminescence and temperature-dependent - Photoluminescence study of InAs quantum wires on InP(001)

InAs quantum wires (QWRs) have been fabricated on the InP(001), which has been evidenced by TEM and polarized photoluminescence measurements (PPL). The monlayer-splitting peaks (MSPs) in the PL spectrum of InAs QWRs can be clearly observed at low temperature measurements. Supposing a peak-shift of MSP identical to that of bulk material, we obtain the thermal activation energies of up to 5 MSPs. The smaller thermal activation energies for the MSPs of higher energy lead to the fast red-shift of PL peak as a whole.

Raman scattering investigation of defects in thick homoepitaxial 4H-SiC films

Three types of defects, namely comet I, comet II and carrot, in thick 4H-SiC homoepilayer were investigated by the micro-Raman scattering measurements. Comet defects were originated from certain cores which caused by the point defects or the inclusions on the surface of the substrate. 3C-SiC inclusions, which were not contained in carrot, were found in comet defects. The different distribution of 3C-SiC in comet I and comet II was investigated by using micro-Raman scattering measurement. The formation mechanisms of these three defects were discussed.

SiC Warm-wall LPCVD growth on multiple 50-mm diameter wafers

Cubic silicon carbide (3C-SiC) has received a great deal of attention since it is a suitable material for electronic and MEMS devices operating in harsh environments. But difficulties still exist in realizing high throughput of high quality material. In the present paper, 3C-SiC layers have been grown on Si(111) in a vertical multi-wafer WCVD (Warm-wall Chemical Vapor Deposition) reactor with a rotating susceptor that can support up to six 50 mm-diameter wafers. Results from the initial growth runs are presented. The results have shown that all the films exhibited a SiC(111) texture at 35.6°. Also, atomic force microscopy (AFM) scans indicated an atomically smooth surface with a roughness (RMS) of 5.74 nm (5×5µm2).

A high-G silicon carbide vertical capacitive micromachined accelerometer

A micromachined acceleration sensor for high-g detection is presented, which introduces silicon carbide as a mechanical material. The sensor structure itself consists of a fixed capacitor and a changeable one whose top plate moves perpendicular to the chip surface. The external acceleration can be obtained by measuring the change of the capacitance. Conventional surface micro-machining technologies are enough for the fabrication of the sensor. A basic theoretical model is given out, which is then used to calculate the vital performance parameters of the sensor. The theoretical measuring range is about 0–110kG. In addition, two theoretical results are compared with the simulation ones obtained by ANSYS.

Interface-related in-plane optical anisotropy of quantum wells studied by reflectance-difference spectroscopy

The in-plane optical anisotropy of three groups of GaAs/AlGaAs quantum well structures has been studied by reflectance-difference spectroscopy (RDS). For GaAs/Al0.36Ga0.64As single QW structures, it is found that the optical anisotropy increases quickly as the well width is decreased. For an Al0.02Ga0.98As/AlAs multiple QW with a well width of 20nm, the optical anisotropy is observed not only for the transitions between ground states but also for those between the excited states with transition index n up to 5. An increase of the anisotropy with the transition energy, or equivalently the transition index n, is clearly observed. The detailed analysis shows that the observed anisotropy arises from the interface asymmetry of QWs, which is introduced by atomic segregation or anisotropic interface roughness formed during the growth of the structures. More, when the 1 ML InAs is inserted at one interface of GaAs/AlGaAs QW, the optical anisotropy of the QW can be increased by a factor of 8 due to the enhanced asymmetry of the QW. These results demonstrate clearly that the RDS is a sensitive and powerful tool for the characterization of semiconductor interfaces.

Edge dislocation of b= [001]/2 in the InAs nanostructure on InP(001)

The self-organized InAs/In/sub 0.52/Al/sub 0.48/As nanostructure were grown on InP(001) using molecular beam epitaxy (MBE). The nanostructure has been studied using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The edge dislocations with the Burgers vector b =[001]/2 and extending along the [11~0] direction are observed. The results show that in the region near an edge dislocation, no InAs wires were formed, while in the regions free of dislocation, wire-like nanostructures were formed. The mechanisms for the formation of the [001]/2 edge dislocations were discussed.

Growth and characterization of 4H-SiC by horizontal hot-wall CVD

4H-SiC layers have been homoepitaxially grown at 1500/spl deg/C with the use of a horizontal hot-wall chemical vapor deposition (CVD) system, which was built in the authors group. The typical growth rate was 2 /spl mu/m/h at a pressure of 40 Torr. The background donor concentration has been reduced to 2.3/spl times/10 cm/sup -3/ during a prolonged growth run. It confirmed the idea that the high background concentration of thin films was caused by the impurities inside the susceptor and thermal insulator. The FWHM of X-ray /spl omega/-rocking curves show 9-15 arcsecs in five different areas of a 32 /spl mu/m-thick 4H-SiC epilayer. The free exciton peaks dominated in the near-band-edge low-temperature photoluminescence spectrum (LTPL), indicating high crystal quality.