Differential scanning calorimetry (DSC) is widely used to study the stability of amorphous solids, characterizing the kinetics of crystallization close to the glass-transition temperature Tg. We apply ultrafast DSC to the phase-change material Ge2Sb2Te5 (GST) and show that if the range of heating rates is extended to more than 104 K s-1, the analysis can cover a wider temperature range, up to the point where the crystal growth rate approaches its maximum. The growth rates that can be characterized are some four orders of magnitude higher than in conventional DSC, reaching values relevant for the application of GST as a data-storage medium. The kinetic coefficient for crystal growth has a strongly non-Arrhenius temperature dependence, revealing that supercooled liquid GST has a high fragility. Near Tg there is evidence for decoupling of the crystal-growth kinetics from viscous flow, matching the behaviour for a fragile liquid suggested by studies on oxide and organic systems.

Characterization of supercooled liquid Ge2Sb2Te5 and its crystallization by ultrafast-heating calorimetry

The density of sputtered Ge2Sb2.04Te4.74 thin films upon annealing has been precisely determined by x-ray reflection and compared to the values determined from x-ray diffraction (XRD) data. The film density increases in two steps around 130 and 280 °C upon annealing up to 400 °C. These increases are consequences of phase transitions from amorphous to NaCl type and from NaCl type to hexagonal structure, respectively, as revealed by XRD. Average density values of 5.87±0.02, 6.27±0.02, and 6.39±0.02 g/cm3 were measured for the amorphous, NaCl-type, and hexagonal phases, respectively. This corresponds to density changes upon crystallization of 6.8±0.2% and 8.8±0.2% for NaCl-type and hexagonal phases, respectively. The accompanying film thickness reductions were determined to be 6.5±0.2% and 8.2±0.2%, which compares very well with the density changes. The corresponding XRD values are determined to be 6.43–6.48 and 6.48 g/cm3 for NaCl-type and the hexagonal phases, respectively. This shows that nearly void-free...

Density changes upon crystallization of Ge2Sb2.04Te4.74 films

The crystallization behavior of Ge2Sb2Te5 thin films has been analyzed by atomic force microscopy and optical reflection measurements on various time scales in order to determine the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier, and the Avrami coefficient. On the minute time scale, thin amorphous films were isothermally crystallized in a furnace under a protective Ar atmosphere. From these measurements the activation energy for crystallization was determined to be (2.0±0.2) eV, in close agreement with previous studies using different techniques. The isothermal measurements also revealed a temperature dependent incubation time for the formation of critical nuclei, which is compared with recent theories. On the nanosecond time scale, Ge2Sb2Te5 was locally crystallized with a focused laser. Either crystalline spots of submicron size were generated in an as deposited amorphous film or amorphous bits in an otherwise crystalline film were recrystallized. For the formation of crystalline spots in an as deposited amorphous film a minimum time of (100±10) ns was found, which is identified as the minimum incubation time for the formation of critical nuclei. In contrast, the complete crystallization of melt-quenched amorphous bits in a crystalline matrix was possible in 10 ns. This is attributed to the presence of quenched-in nuclei inside the amorphous bits. The combination of optical measurements with atomic force microscopy reveals the formation and growth of crystalline bits and shows that the crystal growth in vertical direction strongly affects the reflectivity changes.

Laser induced crystallization of amorphous Ge2Sb2Te5 films

Chalcogenide alloys are materials of interest for optical recording and non-volatile memories. We perform ab-initio molecular dynamics simulations aiming at shading light onto the structure of amorphous Ge2Sb2Te5 (GST), the prototypical material in this class. First principles simulations show that amorphous GST obtained by quenching from the liquid phase displays two types of short range order. One third of Ge atoms are in a tetrahedral environment while the remaining Ge, Sb and Te atoms display a defective octahedral environment, reminiscent of cubic crystalline GST.

/pdf/coexistence-of-tetrahedral-and-octahedral-like-sites-in-83u398q3br.pdf

Coexistence of tetrahedral and octahedral-like sites in amorphous phase change materials

Chalcogenide alloys are materials of interest for optical recording and nonvolatile memories. We perform ab initio molecular dynamics simulations aiming at shading light onto the structure of amorphous Ge2Sb2Te5 (GST), the prototypical material in this class. First principles simulations show that amorphous GST obtained by quenching from the liquid phase displays two types of short range order. One third of Ge atoms are in a tetrahedral environment while the remaining Ge, Sb, and Te atoms display a defective octahedral environment, reminiscent of cubic crystalline GST.

/pdf/coexistence-of-tetrahedral-and-octahedral-like-sites-in-1qxoid7qhi.pdf

Coexistence of tetrahedral- and octahedral-like sites in amorphous phase change materials

The difference in crystallization behavior between the as-deposited amorphous state and the melt-quenched amorphous state of Ge2Sb2Te5 alloy has been studied using a dynamic tester. The melt-quenched amorphous alloy showed much shorter crystallization time than did the as-deposited one. We laser-annealed the as-deposited amorphous alloy under a certain condition so as to make it exhibit the same crystallization property as that of the melt-quenched one. The crystallization kinetics of this laser-annealed amorphous alloy and the as-deposited amorphous alloy has been explained by applying transmission electron microscopy (TEM) and continuously heating the specimens in a differential scanning calorimeter (DSC). From the TEM results, it has been shown that preexisting atomic clusters accelerate the crystallization speed of the amorphous alloy. The two states have been found to have different crystallization temperatures and activation energies.

https://iopscience.iop.org/article/10.1143/JJAP.38.4775/pdf

Characterization of Amorphous Phases of Ge2Sb2Te5 Phase-Change Optical Recording Material on Their Crystallization Behavior

Frequency lock-in-induced deadband phenomena are major problems of ring laser gyroscopes (RLGs), which deteriorate linear responses to changes in the applied rotation rate. In this work, the frequency lock-in phenomenon occurring in the RLG was successfully investigated by compensating for the Sagnac effect through frequency analysis using a newly defined error function. Integrative and generalized viewpoints from the analyzed results provide new possibilities for relevant performance improvements of optical gyroscopes, as well as a deeper understanding of locked states in principle aspects.

/pdf/sagnac-effect-compensations-and-locked-states-in-a-ring-2qq19bmk.pdf

Sagnac Effect Compensations and Locked States in a Ring Laser Gyroscope

Frequency lock-in is the most principal phenomenological feature to be characterized and eliminated for the development of high precision ring laser gyroscope (RLG). As an effective method to solve the lock-in induced problem, sinusoidal dithering by mechanical manner has been proposed and has widely used successfully to date. However, in real applications, pure-sinusoidal dithering is nearly impossible, so additional nonlinear factors should be considered for modeling of dithered RLG. In this work, output variations of RLG in the presence of non-ideal sinusoidal dithering are investigated by performing numerical experiments.

Investigation on output variations of ring laser gyroscope in the presence of non-ideal sinusoidal dithering

The frequency lock-in phenomenon is the most important error factor that degrades the performance of a ring laser gyroscope by causing a nonlinear output to the rotational angular velocity input. To solve the problem of frequency lock-in, a mechanical dithering technique has been proposed and most successfully and widely applied to date. In addition, in the case of dithering with a sinusoidal signal of a single-frequency component, the theoretical background is well established. However, in actual applications, it is almost impossible to implement a perfect single sinusoidal signal, owing to various electrical and mechanical constraints. In general, although the occupied portion is smaller than the fundamental frequency component, the dithering signal inevitably includes harmonics. Therefore, it is essential to analyze the influence of the corresponding harmonic component to accurately predict the frequency lock-in characteristics of a ring laser gyroscope. In this study, it is assumed that the dithering signal has odd harmonics, and through numerical experiments, the frequency lock-in widths of the ring laser gyroscope according to the dithering amplitude change and the effectiveness of improving the related characteristics according to the application of random dithering are analyzed. The analysis confirmed that the resonance condition, including the higher harmonics of mechanical dithering, can be a major variable that determines the frequency lock-in characteristics of the ring laser gyroscope. To improve the relevant performance of the ring laser gyroscope, in-depth research from that point of view is necessary.

Effective Frequency Lock-In Changes of a Ring Laser Gyroscope Due to Harmonic Components of Dithering Signal

This study proposes a combination of fast and slow dithering in a ring laser gyroscope (RLG) and analyzes the related characteristics through numerical experiments. The results of this study are significant compared to the existing methods that use sinusoidal dithering with a single frequency component. The results show that the proposed method can contribute to developing new alternatives for improving RLG performance when high static lock-in frequencies are considered.

Woo S. Choi

Papers

Characterization of Amorphous Phases of Ge2Sb2Te5 Phase-Change Optical Recording Material on Their Crystallization Behavior

Sagnac Effect Compensations and Locked States in a Ring Laser Gyroscope

Investigation on output variations of ring laser gyroscope in the presence of non-ideal sinusoidal dithering

Effective Frequency Lock-In Changes of a Ring Laser Gyroscope Due to Harmonic Components of Dithering Signal

Dynamical Encircling of a Lock-In Area with a Combination of Fast and Slow Dithering in a Ring Laser Gyroscope