Thin film quantum materials

Abstract:

Self organized etching techniques for fabricating a quasiregular array of MnAs nanoislands were discussed. The strain balance in the MnAs layer grown on GaAs substrates collapsed when the heterostructure was immersed in a wet-chemical etch solution and regular row of cracks and submicron-wide strips were carved from it. It was also shown that MnAs islands could serve as a nearly ideal etch mask to create GaAs columns by dry etching.

Abstract:

Thin epitaxial films of MnAs--promising candidates for the spin injection into semiconductors--are well known to undergo simultaneously a first-order structural and magnetic phase transition at 10-40 degrees C. The evolution of stress and magnetization of MnAs/GaAs(001), both measured quantitatively with our cantilever beam magnetometer at the coexistence region of alpha-MnAs and beta-MnAs, reveal an orthorhombically distorted unit cell of the ferromagnetic phase, which provides important clues on the origin of ferromagnetism in MnAs.

Abstract:

The epitaxially grown MnAs films on GaAs(001) were studied using variable-temperature magnetic force microscopy (VT-MFM). The MnAs film underwent a first order structural phase transition around a critical temperature of 40°C. It was found that, due to the strain involved, the ferromagnetic α-MnAs and paramagnetic β-MnAs phase coexisted as stripes along MnAs[0001].

Abstract:

Optical lithography is the method of choice for mass production of electronic as well as acoustic devices. Cost issues, in particular, make it superior over slow but high-resolution methods, such as electron beam lithography. Also, its applicability for nonconductive substrates is an important feature for acoustic device fabrication on ceramics. In order to be able to continue the use of diffraction-limited optical lithography, new schemes have been developed that enhance the resolution. Rather complex phase-shifting masks, for instance, alter both the amplitude and the phase of the exposing light and lead to higher resolution. However, by using an elastomeric phase mask derived from a photoresist master (made by conventional photolithography), features as small as 90 nm have been demonstrated. We report on the application of the near-field phase shift technique for the fabrication of surface acoustic wave (SAW) devices. This technique is best suited for the fabrication of narrow electrode gap SAW devices that are designed for the efficient SAW excitation at higher harmonics. The combination of near-field phase shift lithography with narrow-gap SAW designs thus opens up a way for simple and low-cost SAW devices operating above 5 GHz.

Abstract:

MnAs films were grown by MBE on GaAs(0 0 1) and GaAs(1 1 3)A. We performed a systematic study about the correlation between their structural and magnetic order, which is intimately connected with the phase transition between β-MnAs and α-MnAs during sample cooling after growth. A phase separation process leads to a self-organized structure on the sub-micron scale with alternating ferromagnetic and nonferromagnetic stripes. The well-ordered structure contains defects which correspond to typical crystal imperfections on the atomic level. © 2002 Elsevier Science B.V. All rights reserved.