Developing Precursor Chemistry for Atomic Layer Deposition of High-Density, Conformal GeTe Films for Phase-Change Memory

Abstract

Atomic layer deposition (ALD) of phase-change materials has been suggested as the most feasible technique for the construction of high-aspect-ratio architectures required for ultrahigh-density phase-change random access memory (PcRAM). The recent advances in the ALD technique have established the foundations for the formation of conformal Ge–Te or Ge–Sb–Te films, but their electrical performance as a phase-change memory device has been rarely reported, especially with prolonged cycles. This study introduced Ge­(II)–amido guanidinate (Ge­(guan)­NMe2 (guan = ( i PrN)2CNMe2, Me = CH3)) as a new ALD Ge precursor that was compatible with the high ALD temperature of up to 170 °C, which was necessary for achieving the high-density and stoichiometric as-deposited GeTe thin films. The films were deposited in an amorphous state. Coinjection of NH3 gas with the Te precursor (Te­(SiMe3)2) was essential to initiate the feasible ALD reaction with the new Ge­(II) precursor. Ab initio calculation proposed plausible exergonic chemical reaction pathways where NH3 actively participated in the dissociation of both −SiMe3 and guanidinate ligands from Te and Ge precursors, respectively. The ALD process showed self-limiting growth behavior and produced highly uniform and conformal morphologies. Low impurity levels (<5%) and a low crystallization temperature (180 °C) were observed for the samples deposited at 170 °C. The prototypical memory device showed a current–voltage curve with a voltage snapback region followed by switching to a low resistance state. Over 104 cycling endurance was achieved for the 170 °C grown GeTe film, whereas inferior endurance (<103) was observed for the low-temperature-grown GeTe.

Atomic Layer Deposition of Nanocrystalline-As-Deposited (GeTe)x(Sb2Te3)1–x Films for Endurable Phase Change Memory

“…A faster growth rate of ∼97 ng·cm –2 ·cy –1 was achieved on the ST surface . All of the GT films grown on the three types of substrate were amorphous, which was consistent with the high crystallization temperature (180–190 °C) of the GT layer . The amorphous films on the three different types of the substrate had low RMS roughness (∼0.5 nm); thus, there should be no difference in the growth rate induced by the roughness effect.…”

“…The Ge­(II) precursor, Ge­(guan)­NMe 2 , contains two anionic ligands: the dimethylamino and bidentate guanidinate groups [(i) in Figure a]. The previous study has shown that the high thermal stability of the Ge precursors enabled the ALD of the GT film at high temperatures up to 170 °C . Te­(SiMe 3 ) 2 was coinjected with NH 3 to form H 2 Te [(iii) in Figure a] and to facilitate the dissociation of the guanidinate ligand via protonation, as confirmed in the previous density functional theory study …”

“…Repeating the supercycles results in the deposition of ternary GST films with variations in compositions, depending on the subcycle ratio. The Ge and Te precursor injection/purge times for the GT subcycle were 3/15 and 2/15 s, following the previously reported optimized condition . The injection times for Sb and Te for the ST subcycle were slightly modified from the previously settled condition for high-temperature deposition and were determined to be 2/15 s (Sb injection/purge) and 1/15 s (Te/NH 3 coinjection/purge).…”

“…It appeared that the bulky nature and higher bond strength between the Ge­(II) ion and the ligands suppressed both thermal desorption and decomposition at such a high temperature. Nonetheless, as the strong binding between Ge­(II) and the ligands also led to a low reactivity problem with Te­(SiMe 3 ) 2 , NH 3 was coinjected as an effective reaction enhancer . When the pulse/purge of Ge­(II)-guanidinate and Te­(SiMe 3 ) 2 /NH 3 was attempted, the formation of the intermediate H 2 Te allowed the facile ALD reaction for GT film growth.…”

In-Memory Logic Operations and Neuromorphic Computing in Non-Volatile Random Access Memory

“…As a storage medium, Phase-Change RAM (PCRAM) is mainly based upon chalcogenide (Group VI) and pnictide (Group V) elements, in the form of Germanium-antimony-tellurium (GST) alloys such as Ge 2 Sb 2 Te 5 (usually abbreviated as GST) and compositional variations of GeSb [ 101 ], GeTe [ 102 ], InSbTe [ 103 ], InGeTe [ 104 ], InSbGe, AgInSbTe [ 105 ], GeSbSeTe, GeSbReBi, SiSbTe [ 106 ], and SbTe [ 107 ]. The particularly relevant feature of these compositions, here, is that their electrical/optical properties, such as their electrical resistivity and optical reflectivity, differ sharply between their crystalline state (with a long-range atomic order) and their amorphous state (with a short-range atomic order).…”