Miao-Gen Chen

Strain driven enhancement of ferroelectricity and magnetoelectric effect in multiferroic tunnel junction

The strain effect on the ferroelectric and magnetoelectric coupling in multiferroic tunnel junction (MFTJ) Co/BaTiO3/Co has been investigated systematically by using first-principles calculations within density functional theory. It is found that both in-plane compressive strain and uniaxial tensile strain lead to the enhancement of ferroelectric polarization stability and intensity of magnetoelectric coupling in the MFTJ. There is a transition from the paraelectric phase to the ferroelectric phase for the BaTiO3 layer in MFTJ when the loaded in-plane compressive strain increases up to -2.8% and the corresponding average ferroelectric polarization is about 0.13 C m(-2). Meanwhile, the calculated surface magnetoelectric coefficients increase with increasing in-plane compressive strain. Similar phenomena have been also observed in the case of uniaxial tensile strain implemented in MFTJ. The results suggest that the ferroelectric polarization and magnetoelectric coupling in multiferroic tunnel junctions can be controlled by strain and we expect that this study can provide a theoretical basis for the design of spintronic devices.

In situ Probe of GaN Film Surfaces under Plasma Conditions by Photoluminescence Technique

We have developed a method that enables us to in situ observe full photoluminescence (PL) spectra of n-type GaN film under plasma conditions. By subtracting background plasma noise from the total one, respectable luminescence of GaN film can be successfully obtained. After the plasma exposure, the near-band-edge emissions and yellow luminescence decrease but the blue luminescence intensity increases mainly due to the top surface damage of the GaN film. The results give us evidence that this in situ PL probe method can be potentially used for real-time damage monitoring of GaN film surfaces in plasma processes.

In-situ photoluminescence monitoring of GaN in plasma exposure

We have investigated in-situ photoluminescence properties of GaN surfaces exposed to Ar plasma. With increasing plasma exposure time, both intensities of near-band-edge (NBE) peak and yellow band luminescence (YL) significantly decrease, whereas that of blue band luminescence (BL) is seen to gradually increase. Additionally, the YL/NBE intensity ratio is almost invariant, but the BL/NBE intensity ratio increases dramatically with increasing the plasma exposure time. The results suggest that the increase in the BL intensity is probably due to the plasma-induced damage accumulation and the in-situ monitoring of the BL/NBE intensity ratio can be used to observe the plasma-induced damage.

THICKNESS DEPENDENCE OF BUCKLING PATTERNS OF Ta FILMS SPUTTERED ON GLASS SUBSTRATES

Tantalum (Ta) films deposited on glass substrates have been prepared by a direct current magnetron sputtering method, and buckling patterns induced by residual compressive stress are investigated in detail. When the film thickness increases, the buckling morphologies evolve from straight-sided buckle network to wavy or wormlike wrinkles gradually, and finally change into telephone cord buckles. The geometrical parameters of the buckling patterns are found to increase linearly with the film thickness. Based on the geometrical parameters of the buckling patterns, the mechanical properties of the Ta films are also discussed in the frame of continuum elastic theory.

COMPARISON OF STRESS RELIEF MECHANISMS OF METAL FILMS DEPOSITED ON LIQUID SUBSTRATES BY THERMAL EVAPORATING AND SPUTTERING

Various metal film systems, deposited on liquid (silicone oil) substrates by thermal evaporating and DC-magnetron sputtering methods, have been successfully fabricated and the stress relief mechanisms are systematically studied by analyzing the characteristic surface morphologies. The experiment shows that the evaporating metal films can move on silicone oil surfaces freely due to the nearly zero adhesion of solid–liquid interface, which results in spontaneous formation of ordered surface patterns with a characteristic sandwiched structure driven by the internal stress. For the sputtering metal film system, however, the top surface of silicone oil can be modified to form an elastomeric polymer layer on the liquid substrate during deposition. Subsequent cooling of the system creates a higher compressive stress in the film, which is relieved by buckling of the film to form periodic wavy structures because the adhesion of solid–elastomer interface is quite strong.

Impurity induced wrinkling patterns in metal films deposited on soft elastic substrates

Metal (iron and nickel) films have been deposited on soft elastic polydimethylsiloxane (PDMS) substrates by direct current sputtering technique and the impurity induced wrinkling patterns are investigated by using optical microscopy and atomic force microscopy. It is found that the metal films can spontaneously form disordered wrinkles due to the isotropic compressive stress. In the vicinity of film impurities such as extraneous particles, linear defects, cracks and thickness-gradient film edges, the stress field becomes anisotropic owing to symmetry breaking and thus complex wrinkling patterns including straight stripes, herringbones, crossings, labyrinths and their transitions can be observed. The morphological evolutions, structural characteristics and physical mechanisms of the impurity induced wrinkles have been discussed and analyzed based on the continuum elastic theory.

Photoluminescence of n-type GaN film in an argon plasma

The photoluminescence of n-type GaN thin films in an argon plasma condition has been studied by in situ measurement. The characteristic near-band-edge (NBE) peak and yellow luminescence (YL) of GaN films are observed in the spectra. However, compared to the luminescence of GaN in air and in a vacuum, in the plasma of 80 W power, the NBE peak and YL intensity first increase dramatically and then decrease with extended time. The effects are attributed mainly to an increasing temperature induced by the plasma, but also to a lesser extent by damage to the film caused by the plasma.

Optical and electrical investigation of Ar+-irradiated GaN

The generation behavior of deep-level defects in Ar+-irradiated GaN has been investigated by photoluminescence, capacitance–voltage, and photocapacitance techniques. Ar+ irradiation induces a significant increase in near-band edge emission peak intensity due to donor-bound excitons, in agreement with the largely increased effective carrier concentration in a shallow-lying region near the surface. More interestingly, with increasing irradiation time, the carrier concentration tends to decrease in a deep-lying region, compared with that in the inner bulk region, which is considered to stem from carrier trapping and/or carrier compensation via the diffusion and formation of acceptor-type deep-level defects, such as Ga vacancies and residual C.

THE INFLUENCE OF EXTERNAL DISTURBANCE ON BUCKLING PATTERNS IN WEDGE-SHAPED Fe FILMS

A wedge-shaped iron (Fe) film system, quenched by silicone oil during deposition, has been prepared on glass substrates by direct current magnetron sputtering, and the influence of external disturbance on buckling patterns is investigated. The experiment shows that the wedge-shaped Fe film possesses a high compressive stress, which is relieved by spontaneous formation of telephone cord buckles in the atmosphere condition. The growth process can last several days to more than a month. When the film is disturbed by an external force, the stored stress energy will release drastically to form straight-sided buckles within several seconds. The morphological characteristics, edge effect and formation mechanism of the straight-sided buckles are discussed in detail.

In situ electric properties of Ag films deposited on rough substrates

Silver (Ag) films have been deposited on rough substrates (including frosted glass and silicone grease), and for comparison on flat glass, by DC-magnetron sputtering, and their sheet resistances measured in situ during deposition. It is found that the growth of Ag films proceeds through three distinct stages: discontinuous, semi-continuous, and continuous regimes. The sheet resistance on rough substrates jumps in the vicinity of the percolation threshold, whereas the resistance on flat substrates decreases monotonically during deposition. The abnormal in situ electric properties on rough substrates are well explained based on the differences of the growth mechanism and microstructure of Ag films on different substrates.