The magnetoresistance effect of magnetic tunnel junctions has attracted significant interest in recent years for magneto-electronic applications such as magnetic read heads and magnetic random access memory elements. The tunneling current between two magnetic layers separated by a very thin insulating barrier depends on the relative orientation of the magnetization in the two adjacent magnetic layers. This is the origin of spin dependent tunneling. The tunnel probability of electrons through the insulating barrier depends on the product of the electron density of state of Fermi surface of both magnetic electrodes. When two magnetic layers have their magnetizations aligned parallel, the tunneling probability is high and the resistance of the system is low whereas when the magnetizations are antiferromagnetically aligned, the tunnel probability is low and the tunnel resistance is high. A current flow is perpendicular to the plane of the layers in magnetic tunnel junctions. This makes these junctions rather promising candidates for use as magnetic read heads and non-volatile memory elements.
In this study, for high TMR ratio and thermal stability of magnetic tunnel junctions, amorphous CoFeB electrodes were used for the junctions. Their properties were investigated and compared with those of magnetic tunnel junctions with CoFe crystalline electrode. Also amorphous Al-oxide and crystalline MgO barrier were studied. The annealing effects on structural and magnetic properties of the magnetic tunnel junctions were investigated.
In the first part, we have fabricated magnetic tunnel junctions, where the three kinds of bottom electrodes; amorphous CoFeB, CoFeB inserted CoFe/CoFeB/CoFe were used and these results were compared with that of crystalline CoFe. The tunnel barriers were formed by depositing 1.1 nm Al layers and oxidizing by a reactive method. The TMR of the plasma oxidized junctions increased with annealing temperature initially and decreased with a further increase of the annealing temperature. The TMR peak appeared upon annealing in range of 260℃ to 280℃ in all the specimens. The maximum TMR ratio of 60 % was observed in the CoFe/CoFeB/CoFe pinned junction after 280℃ annealing and 55 % in the CoFeB pinned junction at 260℃. The TMR ratio of the CoFe pinned junction was lower than the others over the entire temperature range. The RA products increased with annealing up to 280℃ and 300℃ and decreased beyond those temperatures in the tunnel junctions with the CoFe/CoFeB/CoFe and the CoFe pinned layers, respectively. In the CoFeB pinned junctions, the RA products increased with increasing annealing temperature up to 380℃.
From the TEM images, $Al_2O_3$ formed on CoFeB amorphous layer had a smoother surface than that formed on CoFe crystalline layer. This may be the main reason for the high TMR ratio of the junctions with CoFeB electrode at as-deposited and annealed states. The monolithic CoFeB started crystallization at 340℃, but crystallization of the CoFeB layer sandwiched in between the CoFe crystalline layers was completed at a temperature lower than 340℃. This indicates that the crystallization temperature of the CoFeB layer is different depending on the structure and lattice parameters of the adjacent layer. The maximum MR ratio was observed with annealing at approximately 260℃ to 280℃. In this temperature range, the amorphous CoFeB layer in CoFeB pinned layer and the crystalline CoFe layer in CoFe/CoFeB/CoFe pinned layer were neighboring the $Al_2O_3$ insulating layer but their maximum MR ratios were comparable. This means that the surface flatness may have affected the higher MR ratio. When the CoFeB was used the pinned layer, B diffused into adjacent Al-oxide barrier during the annealing and the B-oxide was formed at the interface of the pinned layer and Al-oxide barrier. The B has high chemical affinity to oxygen. The higher MR and RA products in the MTJ with the CoFeB layer were associated with chemically clean interface due to the gathering effects of the excess oxygen from the pinned layer. This may make chemically clean the pinned layer thus increasing polarization. The CoFeB amorphous layer improved the chemical and physical properties of pinned layer/Al-oxide interface, therefore the high TMR ratio was obtained in the magnetic tunnel junctions.
In the second part, we investigated the annealing effect on structural and magnetic properties of magnetic tunnel junctions with MgO barrier. The (001) growth of the MgO barrier without undesirable oxidation of electrode materials is a crucial factor of high TMR ratio. We obtained reproducible resistance and stable sputtering rate by 1hr-pre-sputtering before MgO deposition. The MgO film deposited at deposition power less than 120W produces a relatively highly (100)-oriented film formation. The TMR ratio of the 280℃ annealed junction was a relatively small MR of ∼35%. The TMR ratio increased drastically by higher than 340℃ annealing and a TMR ratio of 110% obtained after 380℃ annealing. The MgO barrier showed (001) texture at as-deposited and annealed junctions in cross-sectional HRTEM images. At as-deposited state, the top and bottom CoFeB electrodes were amorphous structure and the CoFeB electrodes showed amorphous structure up to 280℃ annealing. The crystallization partially appeared after 340℃ annealing at the interface between the MgO barrier and the CoFeB layers and the CoFeB layers had completed crystallization after 380℃ annealing. The (100) plane of MgO makes an angle of 45? with (110) plane of CoFeB, suggesting that the following orientation relation exists on MgO/CoFeB interface; MgO (001) [110] || CoFeB (001) [100]. This good lattice matching must be the cause of high MR ratio as the theory had predicted. Also, we have studied the compositional change of MgO barrier in CoFeB/MgO/CoFeB junctions by XPS depth profile. The Fe-oxide was formed at the interface of the bottom electrode and the MgO barrier in the as-deposited state. However, the Fe-oxide peak did not exist in the MgO barrier after 340℃ annealing. During CoFeB crystallization upon 340℃ annealing, B diffused into MgO barrier side and formed B-oxide by collecting excess oxygen from the bottom CoFeB layer. At the interface between MgO and top electrode, no Fe-oxide was formed and only B-oxide existed in the as-deposited state. Upon annealing there were no appreciable changes besides marginal increase of B-oxide. The increase of polarization at the bottom electrode sides is one of the secondary reasons for the higher TMR ratio in CoFeB/MgO/CoFeB junctions.
최근 비휘발성 자기메모리나 고밀도자기기록용 헤드용 소자로 사용을 목표로 자기터널접합에 대한 관심이 고조되고 있다. 자기터널접합은 기본적으로 두 층의 강자성체와 그 사이의 절연층으로 구성되어 있다. 본 연구에서는 높은 MR 값과 열적 안정성을 위해 비정질 CoFeB을 자기터널 접합의 전극으로 이용하였다. 일반적으로 널리 쓰이는 결정질 CoFe 전극과 비정질 CoFeB 전극의 특성을 비교하고 CoFeB 전극의 결정화가 barrier 계면 특성과 터널 접합의 자기적 특성에 미치는 영향에 대해 연구하였다. 또한 CoFeB 전극을 이용한 터널 접합에서 비정질 Al-oxid barrier 대신 결정질 MgO barrier를 이용하였다.
우선, CoFeB과 CoFe/CoFeB/CoFe 전극을 이용하여 자기 터널 접합을 형성하여 CoFe 전극을 이용한 터널 접합과 특성을 비교하였다. CoFeB 전극을 이용한 터널 접합에서 55%의 높은 MR비를 보였으며, CoFeB을 CoFe 사이에 삽입한 결과 60% 의 MR을 얻었다. CoFe 전극을 이용한 경우에는 열처리 전 온도 구간에서 낮은 MR비를 나타내었다. TEM 분석 결과, 비정질 CoFeB 전극은 340℃ 열처리 후 결정화가 시작되었으며, CoFeB 이 CoFe과 접해있는 경우에는 340℃ 에서 결정화가 완전히 이루어졌다. CoFeB의 결정화 온도는 인접한 층의 구조와 격자상수에 의존한다. 구조가 다른 전극을 사용한 터널 접합에서 열처리 후 최대 MR비를 보인 온도 구간에서 MR 비의 큰 차이를 보이지 않았으므로, CoFeB 전극을 이용한 접합에서 barrier와 전극 사이 계면이 flat하고 sharp 한 것이 높은 MR비를 나타낸 주요 원인으로 생각된다. 또한 CoFeB 전극이 결정화 되면서 B이 Al-oxide로 확산하여, 전극과 절연층 계면에 존재하던 잉여의 O와 결합하여 B-oxide를 형성하는데, 이것이 물리적/화학적으로 clean 한 계면을 형성해주어 높은 MR비를 보이는 주요 원인으로 생각된다.
결정질 MgO 절연층을 이용한 자기 터널 접합에서는 MgO 절연층의 형성이 터널 접합의 특성에 큰 영향을 미친다. MgO 절연층을 형성할 때 전극의 산화 없이 (001) 성장 시키는 것이 높은 MR비를 얻는 중요한 요소이다. 본 연구에서는 rf-스퍼터 방법으로 MgO 절연층을 형성하여 280℃ 열처리 후 ~35%의 MR 비를 얻었으며, 340℃ 열처리 후에는 MR이 급격히 증가하는 경향을 보였다. 또한 380℃ 열처리 후 110%의 높은 MR비를 나타내었다. TEM 분석결과 MgO 절연층은 (001) texture를 가지고 잘 성장했으며, CoFeB이 결정화되면서 MgO (001) [110] || CoFeB (001) [100]의 구조를 가지고 성장 한 것이 높은 MR비를 나타낸 원인으로 판단된다. 또한, XPS 분석 결과 CoFeB이 결정화 되면서, 바닥 전극에 존재하던 B 확산으로 CoFeB 층의 잉여의 O와 결합하여 B-oxide를 형성한 것이 높은 MR비를 나타내는 부차적 요인으로 생각된다.
