Crystal Structure and Microstructure of Nitrogen-Doped Ge₂Sb₂Te₅ Thin Film

Abstract

Ge 2 Sb 2 Te 5 thin film is a promising candidate for recording material of phase-change optical disks, and nitrogen is doped into this film to increase overwrite characteristics. In this study, the crystal structure and the microstructure of nitrogen-doped Ge 2 Sb 2 Te 5 thin film were investigated. In the annealed nitrogen-doped thin film, the characteristic face-centered cubic peaks on the X-ray diffraction pattern were broadened and shifted to a smaller angle with the increase of nitrogen content. In addition, a remarkably reduced grain size and a highly strained structure are seen in the transmission electron microscopy image. Doped nitrogen in Ge 2 Sb 2 Te 5 thin film plays two roles. One is to distort the crystal lattice and induce a strain field in the film. The other is to refine the grain size of the film through precipitation. The crystal lattice is transformed from face-centered cubic to a hexagonal structure in nitrogen content above 20 at.%.

Study on the crystallization by an electrical resistance measurement in Ge2Sb2Te5 and N-doped Ge2Sb2Te5 films

“…5͒, higher than the value of undoped GST as reported by Jeong et al, 7 Privitera et al, 16 and Kim et al 17 The possible reason is that N cannot substitute Ge, Sb, and Te atoms but locates at the tetrahedral interstitial sites or accumulates in the grain boundaries. 17 The N atoms locating inside the cubic structure contribute to the increase of active energy because the volume of a tetrahedral site is not sufficiently large for a N atom to occupy inside, which results in a distortion of the unit cell and a strain field.…”

“…The higher temperature used for N-GST is due to increasing of the crystallization temperature for GST after N doping. 7,16,17 RESULT AND DISCUSSION Figure 1 shows the resistance as a function of time measured at 140°C in an undoped GST film. The as-deposited sample was amorphous.…”

“…Therefore, a considerable number of studies for the kinetics of amorphous-crystalline transformation in phase-change media have been done. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Oshima, 1 Jeong et al, 2 Weidenhof et al, 3 Ruitenberg et al, 4 and Laine et al,5 reported that the crystallization behaviors of the GeSbTe films in isothermal condition satisfy the Johnson-Mehl-Avrami-Kolmogrov ͑JMAK͒ equation f =1 − exp͑−kt n ͒, where f is crystallinity, n is the Avrami coefficient, and k is an effective rate constant describing both the nucleation and growth. The JMAK parameters such as Avrami coefficient, activation energy, etc.…”

“…The pure Ge 2 Sb 2 Te 5 ͑GST͒ and nitrogendoped Ge 2 Sb 2 Te 5 ͑N-GST͒ were investigated, which can enhance the cell endurance and reduce the writing current for PCRAM than GST. 7,16,17 Then, the results for the GST and the N-GST have been compared to obtain how the nitrogen doping affects the crystallization parameter of GST.…”

Compositionally matched nitrogen-doped Ge2Sb2Te5/Ge2Sb2Te5 superlattice-like structures for phase change random access memory

“…15 The advantage of N-GST and GST pairing is evident since the crystal structure for N-GST with low nitrogen concentrations remains nearly the same as the undoped GST FCC structure with some strained bonds that slightly increase the lattice constant. 16 At the same time, the similar composition reduces the drive for intermixing of the layers. The choice of nitrogen concentration is critical in the design of the N-GST/GST SLL.…”