IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 8, AUGUST 2011 1113

Scalable High-Performance Phase-ChangeMemory Employing CVD GeBiTe

Jinil Lee, Member, IEEE, Sunglae Cho, Dongho Ahn, Mansug Kang,Seokwoo Nam, Ho-Kyu Kang, and Chilhee Chung

Abstract—We first present chemical-vapor-deposited GeBiTe(CVD GBT) in a confined cell for high-performance phase-changerandom access memory (PRAM). Due to the fast crystallizationof GBT, we were able to reduce the speed to less than 26 nswhile maintaining endurance characteristics up to 109 cycles. Ourresults indicate that the scalable PRAM device enabling the use ofPRAM in dynamic RAM and storage class memory applicationscan be realized using CVD GBT.

Index Terms—Crystallization speed, endurance, GeBiTe (GBT),phase-change random access memory (PRAM).

I. INTRODUCTION

PHASE-CHANGE random access memory (PRAM) hasbeen extensively investigated as one of the candidates for

next-generation nonvolatile memory. Nevertheless, for extend-ing the application of PRAM into dynamic RAM (DRAM) andstorage class memory, increasing memory speed and endurancewhile maintaining data retention is required [1], [2]. Amongphase-change materials (PCMs), Ge2Sb2Te5 (GST) has beenwidely studied due to its large sensing margin, but GST showsslow crystallization above hundreds of nanoseconds. Mostly,the speed of PRAM is dependent on the crystallization of thePCM. Therefore, the exploration of a new PCM aside fromthe GST is required to reduce the crystallization time below thehundreds of nanoseconds for high-performance PRAM. Amongvarious PCMs, much effort has been devoted to the reductionof the crystallization time by Bi doping in the PCM such asGST, GeTe, and SbTe because Bi is considered as a dopant thatcan be used to tailor the crystallization speed of the PCM andreduce the transition temperatures without affecting the crystalstructures [3]–[7].

In addition to speed, a confined-cell structure is requiredfor the next-generation PRAM device due to lower powerconsumption and thermal disturbance than planar cell structure,and chemical vapor deposition (CVD) should be used to scalethe confined-cell structure because the poor filling of the PCM

Manuscript received April 14, 2011; revised May 3, 2011; accepted May10, 2011. Date of publication June 30, 2011; date of current version July 27,2011. This work was supported by the Information Technology Research andDevelopment (R&D) program of the Ministry of Knowledge Economy/KoreaEvaluation Institute of Industry Technology under Project 10030694 of theInternational Collaborative R&D Project for Semiconductor. The review of thisletter was arranged by Editor C.-P. Chang.

The authors are with the Semiconductor Research and Development Center,Samsung Electronics Company, Ltd., Gyeonggi-do 445-701, Korea (e-mail:jinil8.lee@ samsung.com).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2011.2157075

Fig. 1. Transmission electron microscopy image of CVD GBT on a 50-nmcontact device.

within the small contact using conventional sputtering limitshigh-aspect-ratio confined-cell structure [8].

In this letter, CVD GeBiTe (GBT) was suggested in a con-fined cell for a high-speed and low-power-consumption devicedue to its higher crystallization speed and lower melting tem-perature than those of GST [9]. The most previous work relatedto GBT has been studied by sputtering, and up to present, noone reported CVD GBT in a confined cell.

II. EXPERIMENT

Fig. 1 shows the vertical structure of a confined cell. Afterthe metal-0 line, the recessed metal bottom electrode contactwas formed in the contact, in which the CVD GBT film wassuccessfully filled in a contact having an aspect ratio of 3. Forthe recessed bottom electrode in the contact, TiN was filledin the contact and recessed by etch-back process. Metalliza-tion was directly formed on GBT after chemical mechanicalpolishing. CVD was used to deposit a thermally grown GBTfilm using liquid precursors and H2 as a reactant gas at 350 ◦Cand 6.5 × 102 Pa. Argon gas flowing at 50 cm3/min was usedto carry vapors into the chamber. The Bi content was controlledby the Bi pulse time, and three GBT films of variable Bifraction were prepared, as listed in Table I. X-ray diffraction(XRD) and X-ray Fluorescence Spectrometry (XRF) were usedfor structural and composition analysis, and the impurity andthe depth profile were confirmed by Auger electron spectros-copy (AES).

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1114 IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 8, AUGUST 2011

TABLE IGBT FILMS ANALYZED BY XRF

Fig. 2. XRD patterns of CVD GBT1, GBT2, and GBT3 films.

Fig. 3. AES profiles of the CVD GBT film.

III. RESULTS AND DISCUSSION

XRD results in Fig. 2 indicate that the as-deposited GBTfilm consisted of face-centered cubic (FCC) and the phase wasstable up to 15 at.% of Bi. Fig. 3 shows the AES analysis of theGBT film, and the AES result shows that Ge, Te, and Bi wereuniform, along with the depth profile, and other impurities suchas oxygen and nitride were not detected in the GBT film.

Fig. 4 shows the transition behavior of the PRAM cell withGBT1 and GBT2 (GBT1 and GBT2 cells). The GBT2 cellshowed the transition, although the sensing margin was onlyabout a factor of 3. The sensing margin indicates a ratio ofthe resistance of a GBT cell having an amorphous state to theresistance of that GBT cell having a crystalline state. On theother hand, the transition graph of the GBT1 cell exhibitedan improved sensing margin up to two times in comparison

Fig. 4. R–I curve of the (a) GBT1 and (b) GBT2 cells.

Fig. 5. Test results of the CVD GST and GBT cells at an identical confinedcell. (a) SET resistance as a function of SET falling time and (b) R–I transitioncurve.

with that of the GBT2 cell. Therefore, considering the sensingmargin of the GBT cell, the Bi doping concentration is desiredto be controlled to less than 5 at.%.

Fig. 5(a) shows the SET resistance of CVD GST (atomiccomposition: 23% Ge, 22% Sb, and 55% Te) and the GBT1cell as a function of SET falling time in same confined-cellgeometry. The RESET pulse width for GST and GBT1 werekept at 100 and 20 ns, respectively. In the case of the SET, thepulse width/rise time for GST was 200/100 ns, whereas thatof GBT1 was 20/3 ns, and the falling time in each case wasvaried from 3 ns to 1 μs. The SET falling time of the GSTcell was observed to be 500 ns for the fully crystalline state,whereas the GBT1 cell was fully crystallized within 3 ns. Thesignificant improvement of the crystallization speed by CVDGBT is explained by the metallic interatomic binding inducedby the unpinning of the Fermi level induced by the smallamount of Bi in GBT, and the reduced sensing margin withthe increased Bi concentration originates from the increasedamorphous conductivity of GBT, along with the increased Biconcentration [6], [10]–[12]. Fig. 5(b) shows the transitionbehavior of the GST and GBT1 cells in the same cell geometry,which shows that the RESET current of the GBT1 cell wasreduced up to 15% in comparison with that of the GST cell.The reduction of the RESET current will allow more devices tobe programmed in parallel, increasing the WRITE bandwidth,which is critical for enabling the use of PRAM in DRAM andstorage class memory applications [13]. With identical devicestructure and dimensions, the RESET current is governed by the

LEE et al.: SCALABLE HIGH-PERFORMANCE PHASE-CHANGE MEMORY EMPLOYING CVD GeBiTe 1115

Fig. 6. Extrapolation of the ten-year data-retention time.

Fig. 7. Endurance characteristics of a GBT1 cell.

PCM. Since the Bi addition to the PCM is reported to reducemelting temperature, the GBT cell is expected to exhibit thereduced RESET current in comparison with the GST cell [3],[9], [14].

Fig. 6 shows the Arrhenius plot of the data-retention failuretime based on the first retention failure out of 21 cells withincreasing temperature. The estimated activation energy Ea

of the GBT1 cell was calculated as 2.5 eV, and extrapolatingretention results exhibited ten-year data retention at 106 ◦C.

The endurance for the GBT1 cell is shown in Fig. 7. RepeatedSET and RESET were maintained more than 109 cycles, andfurther improvement of endurance characteristics is expectedtoward the sub-20-nm node because endurance cycles are in-versely proportional to program energy (power × time) [15].According to the present results, as previously described, ahigh-performance PRAM device can be realized using CVDGBT, enabling the use of PRAM for DRAM and the storageclass memory applications.

IV. CONCLUSION

The confined-cell structure with CVD GBT has been suc-cessfully integrated in a 50-nm contact device. As-deposited

CVD GBT of the FCC phase has been uniformly filled into acontact having an aspect ratio of 3. In comparison with CVDGST, the SET speed of the PRAM device has been significantlyimproved to less than 26 ns by introducing CVD GBT with15% RESET current reduction. Endurance results of the GBTcell has been maintained up to 109 cycles without failure,and the data retention has been extrapolated to ten years at106 ◦C. By employing CVD GBT to the PRAM confinedcell, we have successfully demonstrated a scalable high-performance PRAM device for DRAM and storage class mem-ory applications.

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