Tae-Yon Lee

Separate domain formation in Ge2Sb2Te5–SiOx mixed layer

We report separate domain formation in cosputtered Ge2Sb2Te5–SiOx mixed layer, with SiOx amount less than 10mol%. As-prepared Ge2Sb2Te5–SiOx layer exhibits amorphous phase with separate domains smaller than 20nm. The separation maintains after thermal annealing, which results in crystallization into fcc phase. The crystallization activation energies of Ge2Sb2Te5–SiOx are obtained as 4.99 and 6.44eV for mixed layers containing 5.3 and 8.4mol% SiOx, respectively. Those are larger than 2.75eV of pure Ge2Sb2Te5. Furthermore, the mixed layer exhibits sublimation at increased temperature. These are interpreted as formation of Ge2Sb2Te5-rich domains separated from each other by SiOx-rich domains.

Thin film alloy mixtures for high speed phase change optical storage: A study on (Ge1Sb2Te4)1−x(Sn1Bi2Te4)x

An approach is proposed to develop recording materials for high speed phase change optical data storage. It utilizes a thin film alloy mixture between a stoichiometric GeSbTe alloy and an additive ternary telluride alloy. Selection rules for an additive alloy are suggested. For a test, (Ge1Sb2Te4)1−x(Sn1Bi2Te4)x thin films are deposited by co-sputtering and their structural and thermal properties are studied. Ge1Sb2Te4 and Sn1Bi2Te4 are found to form a completely soluble pseudo-binary system, whose crystalline lattice parameters obey Vegard’s rule over the entire range of x (0<x<1). Furthermore, the alloy mixtures display an increasing tendency for crystallization with Sn1Bi2Te4 content. Dynamic tests of disk samples are made to show the effectiveness of the approach for high speed erasure.

Phase separation behavior of Ge2Sb2Te5 line structure during electrical stress biasing

We report the breakdown behavior of a patterned Ge2Sb2Te5 multiline structure during the voltage-driven electric stress biasing. Scanning Auger microscope analysis shows that the breakdown process accompanies with a phase separation of Ge2Sb2Te5 into an Sb, Te-rich phase and a Ge-rich phase. The phase separation is explained by the incongruent melting of Ge2Sb2Te5 based on the pseudobinary phase diagram between Sb2Te3 and GeTe. It is claimed that this phase separation behavior by incongruent melting provides one of the plausible mechanisms of the device failure in a phase change memory.

Optical properties of (GeTe, Sb2Te3) pseudobinary thin films studied with spectroscopic ellipsometry

The authors measure the dielectric functions of (GeTe, Sb2Te3) pseudobinary thin films by using spectroscopic ellipsometry. By using standard critical point model, they obtained the optical transition (critical point) energies of the amorphous (crystalline) thin films. The optical (indirect band) gap energies of the amorphous (crystalline) phase are estimated from the linear extrapolation of the absorption coefficients. The band structure calculations show that GeTe, Ge2Sb2Te5, and Ge1Sb2Te4 have indirect gap whereas Ge1Sb4Te7 and Sb2Te3 have direct gap. The measured indirect band gap energies match well with electronic band structure calculations.

Effect of indium on phase-change characteristics and local chemical states of In–Ge–Sb–Te alloys

We introduce single-phase In–Ge–Sb–Te (IGST) quaternary thin film (fcc structure when crystallized) deposited by cosputtering from Ge2Sb2Te5(GST) and In3Sb1Te2 targets. This film, compared with the GST ternary system, provides a significant increase of amorphous-to-crystalline transformation temperature. High-resolution x-ray photoelectron spectroscopy (HRXPS) revealed that, with increasing In amounts, the Sbu20094d and Geu20093d core peaks shift toward lower binding energies (BEs), with negligible changes in spectral linewidths, whereas the Inu20094d and Teu20094d core peaks show insignificant changes in BEs. HRXPS interpretation suggests that the Na site in IGST can be occupied by Te, Sb, In, and vacancy, whereas in GST it is occupied only by Te.

Structural characterization of amorphized InP: Evidence for chemical disorder

Extended x-ray absorption fine-structure measurements at the In K edge of amorphous InP are presented. The presence of chemical disorder in the form of like-atom bonding has been unambiguously demonstrated in stoichiometric InP amorphized by ion implantation. In–In bonding comprised 14%±4% of the In–atom constituent bonds. Also, relative to the crystalline value of four P atoms, an increase in the total In coordination number to 4.16±0.32 atoms was observed for the amorphous phase, as composed of 3.56±0.19u200aP and 0.60±0.13u200aIn atoms. Experimental results were consistent with recent ab initio structural calculations and, furthermore, demonstrated that amorphous InP is best described by a Polk-like continuous random network, containing both even- and odd-membered rings.

Electron-beam lithography of Co∕Pd multilayer with hydrogen silsesquioxane and amorphous Si intermediate layer

We propose a patterning method to form nanostructures of a Co∕Pd multilayer by using electron-beam lithography with an amorphous silicon (a-Si) layer and two-step etching process. On the Co∕Pd multilayer, a-Si is sputter deposited and hydrogen silsesquioxane (HSQ), the electron-beam resist, is spin coated sequentially. We found that an a-Si intermediate layer between the Co∕Pd underlayer and HSQ overlayer improves adhesion of HSQ on the metallic underlayer after electron-beam dosing and chemical development; it also increases etch selectivity between the Co∕Pd multilayer and its overlayers. We demonstrate that a Co∕Pd multilayer can be patterned successfully as a nanowire array using the suggested process.

Electron-Beam Lithography Patterning of Ge2Sb2Te5 Nanostructures Using Hydrogen Silsesquioxane and Amorphous Si Intermediate Layer

We report the successful electron-beam patterning of Ge 2 Sb 2 Te 5 nanostructures, in the form of both lines and dots, using hydrogen silsesquioxane (HSQ) resist. Although HSQ has proven to be a good resolution electron-beam resist, the adhesion between Ge 2 Sb 2 Te 5 and the HSQ resist is problematic when trying to form fine patterns. To promote their adhesion, we introduced an amorphous Si (a-Si) layer between the Ge 2 Sb 2 Te 5 and HSQ layers, and the layers were then sequentially removed using Cl 2 reactive ion etching and Ar etching with good selectivity of each layer. The selectivities for HSQ:a-Si etched by Cl 2 reactive gas and a-Si:Ge 2 Sb 2 Te 5 etched by Ar gas were 1:2.7 and 1:17, respectively. On the basis of this multilayer structure and a two-step dry etching process, various Ge 2 Sb 2 Te 5 nanostructures as small as 40 nm were successfully fabricated.

Amorphous Semiconductor Sample Preparation For Transmission Exafs Measurements

A novel methodology has been developed for the preparation of amorphous semiconductor samples for use in transmission extended x-ray absorption fine structure (EXAFS) measurements. Epitaxial heterostructures were fabricated by metal organic chemical vapor deposition (group III-Vs) or molecular beam epitaxy (group IVs). An epitaxial layer of {approximately} 2 {micro}m thickness was separated from the underlying substrate by selective chemical etching of an intermediate sacrificial layer. Ion implantation was utilized to amorphize the epitaxial layer either before or after selective chemical etching. The resulting samples were both stoichiometric and homogeneous in contrast to those produced by conventional techniques. The fabrication of amorphous GaAs, InP, In{sub 0.53}Ga{sub 0.47}As and Si{sub x}Ge{sub 1{minus}x} samples is described. Furthermore, EXAFS measurements comparing both fluorescence and transmission detection, and crystalline and amorphized GaAs, are shown.

High Speed Phase Change Random Access Memory with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 Complete Solid Solution

We investigated structures and phase transformation kinetics of (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 alloy mixture and its application for the phase change random access memory device. As-sputtered (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 thin film forms crystalline fcc phase. Meanwhile, we could obtain amorphous RESET state and crystalline SET state reproducibly by using appropriate voltage pulse conditions in device structure. We demonstrate that the minimum time for SET operation of phase change random access memory device with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 goes down to 20 ns, which is much smaller than 100 ns for device with Ge1Sb2Te4. The accelerated SET operation of the device with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 is interpreted to originate from reduced bond strength in comparison to pure Ge1Sb2Te4.