A Fabrication of Germanium Nanocrystal Embedded in ... - CiteSeerX

Electrochemical and Solid-State Letters, 9 共12兲 G358-G360 共2006兲

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1099-0062/2006/9共12兲/G358/3/$20.00 © The Electrochemical Society

A Fabrication of Germanium Nanocrystal Embedded in Silicon-Oxygen-Nitride Layer Chun-Hao Tu,a Ting-Chang Chang,b,*,z Po-Tsun Liu,c Hsin-Chou Liu,a Chi-Feng Weng,d Jang-Hung Shy,e Bae-Heng Tseng,e Tseung-Yuan Tseng,a Simon M. Sze,a and Chun-Yen Changa a

Institute of Electronics, National Chiao Tung University, Hsin-Chu, Taiwan Department of Physics and Institute of Electro-Optical Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan c Department of Photonics and Display Institute, National Chiao Tung University, Hsin-Chu, Taiwan d Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan e Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan b

The formation of germanium nanocrystals embedded in silicon-oxygen-nitride 共SiON兲 layer acting as distributed charge storage elements is proposed in this work. A large memory window is observed due to the isolated Ge nanocrystals in the SiON gate stack layer. The Ge nanocrystals were nucleated after the high-temperature oxidation of SiGeN layer. The nonvolatile memory device with the Ge nanocrystals embedded in SiON stack layer exhibits 4 V threshold voltage shift under 7 V write operation. Also, the sequent high-temperature oxidation of the SiGeN layer acting as the blocking oxide is proposed to enhance the performance of nonvolatile memory devices. © 2006 The Electrochemical Society. 关DOI: 10.1149/1.2357983兴 All rights reserved. Manuscript submitted May 30, 2006; revised manuscript received July 7, 2006. Available electronically October 12, 2006.

In the past few years, portable electronic devices have significantly impacted the market of consumer electronics. Because of the low working voltage and nonvolatility, the selection of storage media for most portable electronic devices is the Flash memory, which is almost based on the structure of the continuous floating gate 共FG兲.1,2 The commercially available Flash memory contains the structure of poly-Si FG, which is severed as a charge-trapping layer. The invention of FG memory impacts more than the replacement of magnetic-core memory, and creates a moment of portable electronic systems. Despite huge success in commercialization, conventional FG devices have some drawbacks.2 The most prominent one is that once there is a charge leakage path 共resulting from P/E-cycle degradation兲 in gate oxide, all the charges stored in the FG will leak away from this one single path because charges are stored in continuous energy level 共conduction band兲 in FG. To overcome the scaling limits of the conventional FG structure, Tiwari et al.3 for the first time demonstrated the Si nanocrystal FG memory device in the early 1990s. The local leaky path will not cause the fatal loss of information for the nanocrystal nonvolatile memory. Also, the nanocrystal memory device can lower the power consumption when tunnel oxide is thinner.3-5 The self-assembling of silicon or germanium nanocrystals embedded in SiO2 layers has been widely studied, and strong memory effects were reported.3,6,7 Recently, different charge storage elements instead of silicon nitride have been studied to achieve the robust distributed charge storage.8-13 In this contribution, germanium-incorporated silicon nitride 共SiGeN兲 was investigated to be a self-assembling layer. The SiGeN layer were in situ deposited by plasma enhanced chemical vapor deposition 共PECVD兲 system. The sequent thermal oxidation step was performed in a thermal furnace at 900°C to form a blocking oxide layer 共SiON兲 and nucleate Ge nanocrystals. The structure with Ge nanocrystals embedded in SiON layer exhibits obvious charge-trapping memory effects under electrical measurements. Also, material analysis techniques such as Raman spectroscopy and transmission electron microscopy 共TEM兲 were utilized to observe the Ge nanocrystal nucleation in the oxidized SiGeN film.

substrate by dry oxidation in an atmospheric pressure chemical vapor deposition 共APCVD兲 furnace. Subsequently, a 50 nm amorphous silicon germanium nitride layer was deposited by PECVD on the tunnel oxide. The oxidation process was performed to fabricate the oxygen-incorporated SiON serving as blocking oxide, and the oxidation temperature was 900°C. Furthermore, the Ge nanocrystals were nucleated during the blocking oxide formation. The deposition of the SiGeN film was kept at 200°C at a low pressure of 0.6 mTorr with precursors of SiH4 共20 sccm兲, GeH4 共5 sccm兲, NH3 共30 sccm兲, and N2 共500 sccm兲 and plasma power of 20 W. The low pressure of 0.6 mTorr during deposition leads the mean free path of electrons to be increased and to improve the uniformity of the thin film. Next, the high-temperature thermal oxidation was performed in the thermal furnace under oxygen ambient. Finally, the Al gate was patterned and sintered to form a metal-oxide-insulator-oxide-silicon 共MOIOS兲 structure with the SiON insulator embedded with the Ge nanocrystals. Results and Discussion The Ge nanocrystal embedded SiON layer of a MOIOS memory device is utilized to capture the injected carriers from the channel, which causes a variation in the threshold voltage of the memory device. Figure 2a shows the capacitance-voltage 共C-V兲 hysteresis of the MOIOS structure. The electrical C-V measurements were performed by bidirectional voltage sweeping. The sweeping condition was operated from 共i兲 3 to −3 V and reversely, 共ii兲 from 5 to −5 V and reversely 共iii兲 from 7 to −7 V and reversely. It is clearly shown in Fig. 2a that the threshold-voltage shift 共memory window, ⌬VTH兲 of the MOIOS structure is prominent for 900°C oxidation. When the MOIOS structure is programmed, the electrons directly tunnel from the Si substrate through the tunnel oxide, and are transferred to the conduction band of the Ge nanocryatal in the SiON layer. For the

Experimental Figure 1 exhibits the process flow in this work. First, a 5 nm thick thermal oxide was grown as the tunnel oxide on p-type Si

* Electrochemical Society Active Member. z

E-mail: [email protected]

Figure 1. The proposed process flow to form Ge nanocrystals embedded in SiON layer.


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Figure 2. 共a兲 The C-V hysteresis of the MOIOS structure. The electrical C-V measurements are performed by bidirectional voltage sweeping from 共i兲 3–共−3兲 V and 共−3兲–3 V, 共ii兲 5–共−5兲 V and 共−5兲–5 V, 共iii兲 7–共−7兲 V and 共−7兲–7 V. 共b兲 The measurement of current density for the MOIOS structure by voltage sweeping from 共i兲 0–10 V and 共ii兲 0–共−10兲 V.

erasure process, the holes may tunnel from the valence band of the Si substrate and recombine with the electrons trapped in the Ge nanocrystals in SiON layer. The blocking oxide 共SiON兲is utilized to prevent the carriers of gate electrode from injecting into the chargetrapping layer by Fowler-Nordheim 共F-N兲 tunneling. The thresholdvoltage shift after the 7 V programming operation is 4 V for the Ge nanocrystal embedded in SiON memory device. The large threshold voltage shift of memory device with oxidized SiGeN film as blocking oxide layer is attributed to the presence of Ge nanocrystal in the SiON film, which is larger than reported silicon oxynitride as charge trapping layer.13 When electrons are captured in the Ge nanocrystals of the SiON film, a large threshold voltage shift is shown for a memory device. The leakage current in the MOIOS structure is shown in Fig. 2b. It is tolerated for MOIOS structure with Ge nanocrystal embedded SiON layer to avoid the charges leaking to the gate. In addition, the density of Ge dots can be electrically calculated through the threshold-voltage shift, ⌬Vt.14 The number of electrons trapped in the Ge nanocrystals was calculated to be 1.08 ⫻ 1012 cm−2. Raman spectra of the SiGeN stack film followed by an oxidation process are schematically shown in Fig. 3. The crystalline phase for Ge-Ge is not found for the as-deposited SiGeN layer. As a result of the Si easily combining with oxygen first to form SiO2,14 the Ge will be nucleated after the thermal oxidation. The signal of Ge-Ge bonding is clearly found after the thermal oxidation, as shown in Fig. 3. The SiGeN layer is completely oxidized after the thermal oxidation in thermal furnace in O2 ambient. The TEM of nucleated Ge nano-

Figure 4. TEM analysis of thermally oxidized SiGeN film.

crystals is also shown in Fig. 4. Therefore, the memory windows were formed after the programming operation in the proposed technology. The electrons trapped near the channel will dominate the threshold voltage more significantly than those far from the channel. The proposed technology of Ge nanocrystals embedded in SiON stack layer, therefore, contributes larger memory window and provides the additional blocking oxide deposition for the nonvolatile memory application. Conclusion The fabrication of Ge nanocrystal embedded in SiON stack film has been demonstrated successfully for memory application. The proposed technology can form both the distributed storage elements and upside blocking oxide layer in the nonvolatile memory device. The exhibition of memory windows after programming resulted from the formation of Ge nanocrystals in SiON layer. A significant C-V hysteresis of VTH shift of 4 V is observed during 7 V operation. Acknowledgments This work was performed at National Nano Device Laboratory and was supported by the National Science Council of the Republic of China under contract no. NSC94-2215-E009-063, NSC94-2215E009-031, and NSC94-2120-M-110-005. Also, the authors acknowledge the support of the Plasma Enhanced Chemical Vapor Deposition 共PECVD兲 system at National Chiao Tung University 共NCTU兲 in HsinChu. Furthermore, this work was partially supported by MOEA Technology Development for Academia project no. 94-EC-17-A-07S1-046. National Sun Yat-Sen University assisted in meeting the publication costs of this article.

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Figure 3. Raman spectra of the SiGeN with a top capping a-Si layer, before and after thermal oxidation process.

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