Characterization of insulator layers in magnetic tunneling junctions with applications in memory devices

• Main Author: Ren, Ciwei
• Format: Others
• Published: 2007
• Online Access: http://spectrum.library.concordia.ca/975315/1/MR34719.pdf; Ren, Ciwei <http://spectrum.library.concordia.ca/view/creators/Ren=3ACiwei=3A=3A.html> (2007) Characterization of insulator layers in magnetic tunneling junctions with applications in memory devices. Masters thesis, Concordia University.

Magnetic tunnel junctions ( MTJ s) are electrical devices that display a large change in resistance when an external magnetic field applied to the junctions. MTJ s have applications in non-volatile memory element in Magnetic Random Access Memory (MRAM). MTJ is composed of two ferromagnetic metallic electrodes and one insulating layer. The insulating layer is inserted in between the two electrodes. When a voltage is applied between the two electrodes, a tunneling current will flow through the insulating layer. An applied magnetic field can change the current direction due to influences on ferromagnetic material causing a change in resistance. The tunneling magnetoresistance ( TMR ) is a measure of the sensitivity of the device to magnetic fields. It is common that an aluminium oxide ( Al 2 O 3 ) tunneling barrier, which is made by oxidation of a thin layer of aluminium ( Al ), is used in most MTJ s design. Several oxidation processes have been applied to achieve high quality barrier layers. Typical processes are oxidations in atmosphere, in a pure oxygen environment, oxygen reactive sputtering and RF plasma. By applying these techniques, several groups have reported the TMR ratios greater than 30%. The main disadvantage of these processes is that the oxidation rate is very fast, and usually it takes not more than 1{598}40 seconds to complete the oxidation. It is difficult to control the thickness of oxidation layer, which yields an Al 2 O 3 layer either under-oxidized or over-oxidized. Aluminium nitride ( AlN ) is an insulating material which can take the place of Al 2 O 3 in these devices. The nitridation process of Al films is expected to progress much slower than the oxidation process of Al films, due to the lower diffusion coefficient of nitrogen in the insulating layer compare to that of oxygen. The research objective of this work is to investigate the effect of nitridation of the insulator layers in MTJ devices. For this, different methods are used to nitridize the insulator layer which is chosen to be as a thin layer of Al. The effect of post-processing annealing is also studied. The tunneling junction resistances versus external magnetic field, and tunneling junction I{598}V characteristic for different samples were measured, compared and discussed. Finally, conclusions are made to compare these techniques with those already available in practise