Field emission mechanism from a single-layer ultra-thin semiconductor film cathode

Field emission (FE) from a single-layer ultra-thin semiconductor film cathode (SUSC) on a metal substrate has been investigated theoretically. The self-consistent quantum FE model is developed by synthetically considering the energy band bending and electron scattering. As a typical example, we calculate the FE properties of ultra-thin A1N film with an adjustable film thickness from 1 to 10 nm. The calculated results show that the FE characteristic is evidently modulated by varying the film thickness, and there is an optimum thickness of about 3 nm. Furthermore, a four-step FE mechanism is suggested such that the distinct FE current of a SUSC is rooted in the thickness sensitivity of its quantum structure, and the optimum FE properties of the SUSC should be attributed to the change in the effective potential combined with the attenuation of electron scattering.

Cathodoluminescence and Raman research of V-shape inverted pyramid in HVPE grown GaN film

Thick GaN films with high quality have been grown on (0001) sapphire substrate in a home-made vertical HVPE reactor. Micron-size hexagonal pits with inverted pyramid shape appear on the film surface, which have six triangular {10-11} facets. These I {10-11} facets show strong luminescence emission and are characteristic of doped n-type materials. Broad red emission is suppressed in {10-11} facets and is only found at the flat region out of the pit, which is related with the decreasing defects on {10-11} facets. Low CL emission intensity is observed at the apex of V-shape pits due to the enhanced nonradiative recombination. Raman spectra show that there are higher carrier concentration and low strain in the pit in comparison to the flat region out of the pit. The strain relaxation may be the main mechanism of the V-shape pits formation on the GaN film surface. (c) 2006 Elsevier B.V. All rights reserved.

Improved performance in organic light emitting diodes with a mixed electron donor-acceptor film involved in hole injection

Organic light emitting diodes using a mixed layer of electron acceptor 3, 4, 9, 10 perylenetetracarboxylic dianhydride and electron donor copper phthalocyanine (PTCDA:CuPc) on indium tin oxide (ITO) anodes were fabricated. The device properties were found to be strongly dependent on the thickness of the PTCDA:CuPc film: both the power efficiency and the driving voltage of the device were optimized with a thickness of PTCDA:CuPc ranging from 10 to 20 nm. As compared to the conventional ITO/CuPc hole injection structure, the ITO/PTCDA:CuPc hole injection structure could remarkably enhance both the luminance and the power efficiencies of devices. A mechanism of static-induced, very efficient hole-electron pairs generation in mixed PTCDA:CuPc films was proposed to explain the experimental phenomena. The structural and optical properties of PTCDA:CuPc film were examined as well. (c) 2007 American Institute of Physics.

Effect of growth conditions on the GaN thin film by sputtering deposition

The phase transition between thermodynamically stable hexagonal wurtzite (h-WZ) gallium nitride (GaN) and metastable cubic zinc-blende (c-ZB) GaN during growth by radio-frequency planar magnetron sputtering is studied. GaN films grown on substrates with lower mismatches tend to have a h-WZ structure, but when grown on substrates with higher mismatches, a c-ZB structure is preferred. GaN films grown under high nitrogen pressure also tend to have a h-WZ structure, whereas a c-ZB structure is preferred when grown under low nitrogen pressure. In addition, low target-power growth not only helps to improve hexagonal GaN (h-GaN) crystalline quality at high nitrogen pressure on low-mismatch substrates, but also enhances cubic GaN (c-GaN) quality at low nitrogen pressure on high-mismatch substrates. (c) 2007 Elsevier B.V. All rights reserved.

Net-like ferromagnetic MnSb film deposited on porous silicon substrates

MnSb films were deposited on porous silicon substrates by physical vapor deposition (PVD) technique. Modulation effects due to the substrate on microstructure and magnetic properties of the MnSb film's were studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and measurements of hysteresis loops. SEM images of the MnSb films indicate that net-like structures were obtained because of the special morphology of the substrates. The net-like MnSb films exhibit some novel magnetic properties different from the unpatterned referenced samples. For example, in the case of net-like morphology, the coercive field is as low as 60 Oe. (c) 2006 Elsevier B.V. All rights reserved.

Growth of AlGaN epitaxial film with high Al content by metalorganic chemical vapour deposition

A high-Al-content AlGaN epilayer is grown on a low-temperature-deposited AlN buffer on (0001) sapphire by low pressure metalorganic chemical vapour deposition. The dependence of surface roughness, tilted mosaicity, and twisted mosaicity on the conditions of the AlGaN epilayer deposition is evaluated. An AlGaN epilayer with favourable surface morphology and crystal quality is deposited on a 20 nm low-temperature-deposited AlN buffer at a low V/III flow ratio of 783 and at a low reactor pressure of 100 Torr, and the adduct reaction between trimethylaluminium and NH3 is considered.

Temperature dependence of the Raman-active modes in the nonpolar a-plane GaN film

Temperature dependences of the polarized Raman scattering spectra in the backscattering configuration of the nonpolar a-plane (or [11 (2) over bar0]-oriented) GaN thin film are analyzed in the range from 100 to 570 K. The nonpolar a-plane GaN film is grown on an r-plane [or (1 (1) over bar 02)-oriented] sapphire substrate by metal organic chemical vapor deposition. The spectral features of the Raman shifts, intensities, and linewidths of the active phonons modes A(1)(TO), E-1(TO), and E-2(high) are significantly revealed, and corresponding temperature coefficients are well deduced by the empirical relationships. With increasing the measurement temperature the Raman frequencies are substantially redshifted and the linewidths gradually broaden. The compressive-strain-free temperature for the nonpolar a-plane GaN film is found to be at about 400 K. Our studies will lead to a better understanding of the fundamental physical characteristics of the nonpolar GaN film. (c) 2007 American Institute of Physics.

PIXE analysis of Fe content in Fe-implanted GaN film

The diluted magnetic semiconductors (DMSs) were achieved by the ion implantation. Fe+ ions (250 keV) were implanted into n-type GaN at room temperature with doses ranging from 8 X 10(15) cm(-2) to 8 X 10(16) cm(-2) and subsequently rapidly annealed at 800 degrees C for 5 m in N-2 ambient. PIXE was employed to determine the Fe-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters are detected within the sensitivity of XRD. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed. (c) 2006 Elsevier B.V. All rights reserved.

Anisotropic strain relaxation of thin Fe film on c(4 x 4) reconstructed GaAs (001) surface

National Natural Science Foundation of China 60836002 10674130 60521001;Major State Basic Research of China 2007CB924903;Chinese Academy of Sciences KJCX2.YW.W09-1

Effects of ZnO interlayers on thick GaN/Si film prepared by RF magnetron sputtering

We have successfully prepared a high-quality 2 mu m-thick GaN film with three inserted 30 nm-thick ZnO interlayers on Si (111) substrate without cracks by magnetron sputtering. The effects of the thickness and number of ZnO interlayers on the crystal quality of the GaN films were studied. It was found that the GaN crystal quality initially improved with the increase of the thickness of ZnO interlayers, but deteriorated quickly when the thickness exceeded 30 nm. Multiple ZnO interlayers were used as an effective means to further improve the crystal quality of the GaN film. By increasing the number of interlayers up to three, the cracks can be constrained to a certain extent, and the crystal quality of the GaN film greatly improved. (c) 2006 Elsevier B.V. All rights reserved.

Influence of indium-tin-oxide thin-film quality on reverse leakage current of indium-tin-oxide/n-GaN Schottky contacts

Indium-tin-oxide (ITO)/n-GaN Schottky contacts were prepared by e-beam evaporation at 200 degrees C under various partial pressures of oxygen. X-ray photoemission spectroscopy and positron beam measurements were employed to obtain chemical and structural information of the deposited ITO films. The results indicated that the observed variation in the reverse leakage current of the Schottky contact and the optical transmittance of the ITO films were strongly dependent on the quality of the ITO film. The high concentration of point defects at the ITO-GaN interface is suggested to be responsible for the large observed leakage current of the ITO/n-GaN Schottky contacts. (c) 2006 American Institute of Physics.

Biaxial stress dependence of the electrostimulated near-band-gap spectrum of GaN epitaxial film grown on (0001) sapphire substrate

The biaxial piezospectroscopic coefficient (i.e., the rate of spectral shift with stress) of the electrostimulated near-band-gap luminescence of gallium nitride (GaN) was determined as Pi=-25.8 +/- 0.2 meV/GPa. A controlled biaxial stress field was applied on a hexagonal GaN film, epitaxially grown on (0001) sapphire using a ball-on-ring biaxial bending jig, and the spectral shift of the electrostimulated near-band-gap was measured in situ in the scanning electron microscope. This calibration method can be useful to overcome the lack of a bulk crystal of relatively large size for more conventional uniaxial bending calibrations, which has so far hampered the precise determination of the piezospectroscopic coefficient of GaN. The main source of error involved with the present calibration method is represented by the selection of appropriate values for the elastic stiffness constants of both film and substrate. The ball-on-ring calibration method can be generally applied to directly determine the biaxial-stress dependence of selected cathodoluminescence bands of epilayer/substrate materials without requiring separation of the film from the substrate. (c) 2006 American Institute of Physics.

Enhanced photoresponse from the ordered microstructure of naphthalocyanine-carbon nanotube composite film

Naphthalocyanine-sensitized multi-walled carbon nanotube (NaPc-MWNT) composites have been synthesized through the pi-stacking between naphthalocyanine (NaPc) and carbon nanotubes. The resultant nanocomposites were characterized with a scanning electron microscope (SEM), a transmission electron microscope (TEM), and by UV - vis absorption and photocurrent spectra. The long-range ordering was observed in the NaPc - MWNT composites by using a TEM. The enhancement in the absorption intensity and the broadening of the absorption wavelength observed in the composite films, which were due to the attachment of NaPc on the MWNT surface, is discussed based on the measured UV - vis absorption spectra. Furthermore, the photoconductivity of the poly( 3-hexylthiophene)(PAT6) - NaPc - MWNT composite film was found to increase remarkably in the visible region and broaden towards the red regions. These new phenomena were ascribed to the larger donor/acceptor (D/A) interface and the formation of a biconsecutive D/A network structure, as discussed in consideration of the photoinduced charge transfer between PAT6 and NaPc - MWNT.

Metal film effect on performance of very-small-aperture lasers

Special characteristics of very-small-aperture lasers are observed, including threshold current change, red shift of the spectral position, and short lifetime at low drive current. Physical mechanisms that underlie these special characteristics are analyzed: we find that optical feedback caused by a metal film and heat accumulation inside the laser diode lead to the special characteristics of VSALs, such as threshold current change, red shift of the spectral position, and short lifetime at low drive current, etc. Theoretical simulation is in good agreement with the experimental results.

Magnetron sputtering growth of InAs0.3Sb0.7 films on (100) GaAs substrates: Strong effect of growth conditions on film structure

The growth of highly lattice-mismatched InAs0.3Sb0.7 films on (100) GaAs Substrates by magnetron Sputtering has been investigated and even epitaxial lnAs(0.3)Sb(0.7) films have been successfully obtained. A strong effect of the growth conditions on the film structure was observed, revealing that there was a growth mechanism transition from three-dimensional nucleation growth to epitaxial layer-by-layer growth mode when increasing the substrate temperature. A qualitative explanation for that transition was proposed and the critical conditions for the epitaxial layer-by-layer growth mode were also discussed. (c) 2005 Elsevier B.V. All rights reserved.