2004-2005 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Choy, CL

Dai, J

Feng, SL

Lee, PF

Liu, WL

Song, ZT

Wang, J

Applied Physics Letters

2004-2005 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

Self-organized Ge nanocrystals embedded in HfAlO fabricated bypulsed-laser deposition and application to floating gate memoryW. L. LiuDepartment of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China and TheResearch Center of Semiconductor Functional Film Engineering Technology, State Key Laboratory ofFunctional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences, 865 Changning Road, Shanghai, 200050, ChinaP. F. Lee, J. Y. Dai,a) J. Wang, H. L. W. Chan, and C. L. ChoyDepartment of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, ChinaZ. T. Song and S. L. FengThe Research Center of Semiconductor Functional Film Engineering Technology, State Key Laboratory ofFunctional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences, 865 Changning Road, Shanghai, 200050, China(Received 27 July 2004; accepted 5 November 2004; published online 28 December 2004)A trilayer metal-oxide-semiconductor structure containing a HfAlO tunnel layer, isolated Genanocrystals, and a HfAlO control layer, was obtained using pulsed-laser deposition (PLD).Self-organized Ge nanocrystals were formed by PLD at 600 °C, suggesting a usefullow-temperature process for fabricating Ge nanocrystals embedded in dielectric materials. Theself-organized Ge nanocrystals so formed were uniform in size and distribution with a densityapproaching 1012 cm−2. The effects of deposition temperature and growth rate in forming Genanocrystals were investigated and it was revealed that a relatively low temperature and growth rateare favorable for the formation of Ge nanocrystals. The memory effect of the Ge nanocrystals withstorage charge density of up to 1012 cm−2 has been demonstrated by the presence of hysteresis in thecapacitance-voltage curves. © 2005 American Institute of Physics. [DOI: 10.1063/1.1846154]The nanocrystal floating gate embedded in dielectricshas attracted a great deal of attention recently because it canpotentially be applied in nonvolatile, high-speed, high-density, and low-power consuming memory.1–15 Compared toconventional floating gate memories such as flash, a devicecomposed of nanocrystals isolated by dielectrics benefitsfrom a relatively low operating voltage, high endurance, fastwrite-erase speeds, and better immunity to soft errors.1 Forthe first nano-floating-gate memory, the materials of nanoc-rystals and dielectrics were Si and SiO2, respectively.1 Inrecent years, several new memory nodes such as Ge, SiGe,Au, TiN, and Ag have been investigated, and some of themwere incorporated with high-k dielectrics.3–14 The Ge nano-crystal is considered to be an ideal memory node due to itsrelatively small band gap compared to Si and compatibilitywith current complementary metal-oxide-semiconductor(CMOS) technology. However, it is still difficult to fabricateuniform and self-organized Ge nanocrystals. Most of themethods, including the thermal annealing of Ge and dielec-tric mixture layer,13–15 Ge ion implantation,5 and the oxida-tion of SiGe,6,7 require annealing at high temperatures. Baronet al.16 has fabricated Ge nanodots on the SiO2 matrix bylow-pressure chemical vapor deposition. However, siliconnuclei are needed in this method. Pulsed-laser deposition(PLD) is a good method for growing high-quality structuresat low temperatures. Hassan et al.17 employed it to fabricateGe nanocrystals embedded in an AlN matrix for optical ap-plication. In this Letter, we report on the formation of self-organized Ge nanocrystals at relatively low substrate tem-peratures using PLD, and the memory effect of the metal-oxide-semiconductor (MOS) structure composed of Ptelectrode, a HfAlO control gate, Ge nanocrystals, and aHfAlO tunnel layer on Si substrate. The motivation for inte-grating the Ge nanocrystals with HfAlO is its potential forimplementation in the new generation CMOS technology. Infact, recent studies have shown that high-k dielectrics, in-stead of SiO2, for the tunnel layer in nanocrystal floating gatememory can also improve the tradeoff between the data re-tention and program efficiency due to the unique band asym-metry of high-k dielectrics in the programming and retentionmodes.9 In addition, our earlier works have shown thatHfAlO is a promising high-k material because of its thermalstability.18A pulsed excimer laser with multiple targets was used todeposit HfAlO films and Ge nanocrystals, where the atomicratio of HfO2 to Al2O3 in the composite target was 1:2. Thelaser fluence and frequency used were 5 J /cm2 and 2 Hz,respectively. A 7-nm-thick tunnel HfAlO layer was firstgrown on a P-type (100) Si substrate at 550 °C. Subse-quently, the Ge nanocrystals were deposited on the surface ofthe HfAlO at 600 °C. Different deposition time and substratetemperatures were implemented in order to study their ef-fects on the formation of Ge nanocrystals. The memorystructure used in this study is a MOS capacitor with a dielec-tric stack made up of Ge nanocrystals sandwiched betweenthe tunnel and control HfAlO layers. The control HfAlOlayer, with a thickness of 10 nm, was deposited at 550 °C.The samples were finally annealed at 800 °C for 30 min inN2 ambient, and Pt dot electrodes with a diameter of 0.2 mmwere subsequently deposited by PLD.Ge nanocrystals were characterized by atomic force mi-croscopy (AFM) and scanning electron microscopy (SEM)before the deposition of the control layer. Trilayer structuresa)Author to whom correspondence should be addressed; electronic mail:apdaijy@inet.polyu.edu.hkAPPLIED PHYSICS LETTERS 86, 013110 (2005)0003-6951/2005/86(1)/013110/3/$22.50 © 2005 American Institute of Physics86, 013110-1were investigated by high-resolution transmission electronmicroscopy (TEM). The memory effect in Ge nanocrystal-embedded capacitors fabricated by PLD was characterizedby measurements of capacitance voltage sC-Vd and currentvoltage sI-Vd.Figure 1(a) shows a typical SEM image of Ge nanocrys-tals grown at 600 °C. It can be seen that Ge nanocrystals arewell self-organized and distribute uniformly. The density andmean size of the dots are about 831011 cm−2 and 25 nm,respectively. The island structure of the Ge suggests that thegrowth is based on the Volmer-Webber mode, where Ge at-oms can easily combine with each other but not with HfAlOon Si in our experimental conditions. The most remarkabledifference between PLD and other vapor deposition methodsis the high energy of the plasma generated by the laser abla-tion of the desired target material. Therefore, during PLD theGe plasma has a relatively high forward velocity, whichleads to the relatively uniform distribution of Ge nanocrys-tals. According to the observation by cross-sectional TEM(not shown here), the height of these nanocrystals is about25 nm, resulting in an increased surface roughness on the topdielectric that hinders the application of nanocrystal floatinggate memory. Therefore, the deposition time of Ge has beenreduced accordingly in the following experiments.The substrate temperature was found to be an importantparameter for the growth of Ge nanocrystals. Figures 1(b)and 1(c) show AFM images of the Ge nanocrystals grown atthe temperatures of 600 and 700 °C, respectively. From Fig.1(b), it can be seen that the Ge nanocrystals are well self-organized and their diameter is about 10 nm. By contrast, thediameter of the Ge nanocrystals deposited at 700 °C, asshown in Fig. 1(c), is larger and their height becomessmaller. This suggests that when the substrate temperatureincreases, the diffusion coefficient of Ge atoms also in-creases, and the Ge atoms are relatively easier to combinewith the HfAlO atoms. Therefore, the substrate temperatureof 600 °C for the growth of Ge nanocrystals has been usedto fabricate memory capacitors.The structure of the memory capacitor was investigatedby cross-sectional TEM. Figure 2(a) shows the structure ofthe as-grown samples containing three obvious layers on sili-con substrate, i.e., the HfAlO control layer, the Ge nanocrys-tals layer, and the HfAlO tunnel layer. The Ge nanocrystalsindicated by white circles are uniformly distributed betweenthe HfAlO control layer and the tunnel layer. The thicknessesof the tunnel layer and the control layer are about 7 and14 nm, respectively. From the high-resolution TEM image,as shown in Fig. 2(a), it can be seen that the HfAlO layershave an amorphous structure and the Ge nanocrystals aresingle crystalline. The lattice fringes that were observed areidentified as {111} planes of Ge. Our experimental resultsshowed that the growth rate can also affect the crystallinityof the Ge nanocrystals. When the growth rate is higher, theFIG. 1. SEM picture of high-density self-organized Ge nanocrystals depos-ited on (a) HfAlO matrix by PLD for 1 min at 600 °C, and AFM images ofGe nanocrystals on HfAlO matrix by PLD for 30 s at (b) 600 °C and (c)700 °C.FIG. 2. High-resolution TEM images of samples composed of HfAlO con-trol gate, Ge nanocrystals, and HfAlO tunnel layer on Si substrate: (a) as-grown and (b) annealed at 800 °C.013110-2 Liu et al. Appl. Phys. Lett. 86, 013110 (2005)Ge nanocrystals are amorphous. It can also be seen from Fig.2(a) that the Ge nanocrystals are spherical in shape and theirdiameter is about 10 nm, which is consistent with the AFMresult shown in Fig. 1(b). The interface between the HfAlOtunnel layer and the silicon substrate is almost free from anyinterfacial layer. Figure 2(b) is the high-resolution TEM im-age of the MOS structure annealed at 800 °C. It can beclearly seen that even after annealing at 800 °C, the interfacebetween the HfAlO tunnel layer and the silicon substrate isstill sharp and is free from any interfacial layer. It can also beseen that after annealing at 800 °C, the Ge nanocrystals in-creased in size. Some crystal lattices in the control layer maybe due to the diffusion of Ge. It is also worth noting that theexistence of Ge and Ge diffusion may decrease the crystalli-zation temperature of HfAlO and lead to localized crystalli-zation. The detailed mechanism should be studied further.The C-V measurement of the as-grown and 800 °C an-nealed MOS capacitors incorporating the Ge nanocrystals re-vealed that the as-grown sample exhibits abnormal C-V char-acteristics (not shown) and the sample annealed at 800 °Cpresents a significant hysteresis, indicating successful chargestorage. Figure 3(a) shows the high-frequency s1-MHzdC-V curve of the sample annealed at 800 °C. Anti-clockwisehysteresis indicates electron trapping in the capacitor. Theeffect of the mobile ions on the charge trapping can be ruledout, since no hysteresis was observed in the control samplewithout Ge nanocrystals under our experimental conditionsand no obvious deformation can be observed in the C-Vcurves as shown in Fig. 3(b). The flatband voltage shift isabout 0.6 V. Calculated by the formula in Refs. 1 and 19, thestored charge density is up to 1012 cm−2.The different C-V characteristics between the as-grownand the annealed samples can be understood by measuringtheir leakage currents. Figure 3(b) shows the leakage cur-rents of the two samples with and without annealing. It canbe seen that the leakage current of the memory structureannealed at 800 °C is very small and it is much smaller thanthat of the sample without annealing. The relatively largeleakage current of the sample without annealing is due to thepresence of a large amount of oxygen vacancies in the as-grown HfAlO films. Annealing at 800 °C in a quartz tubereduced the oxygen vacancies and the leakage current.In summary, we have fabricated at relatively low tem-perature by PLD method a memory structure containing aHfAlO control gate, well self-organized Ge nanocrystals, anda HfAlO tunnel layer. The substrate temperature and growthrate were found to be the key factors influencing the size anddistribution of the Ge nanocrystals. The obvious memoryeffect of the trilayer structure was demonstrated by the pres-ence of hysteresis in the C-V curves with a storage chargedensity of up to 1012 cm−2.This work was supported by the Hong Kong PolytechnicUniversity Central Grant (G-YD55), the Chinese NaturalScience Foundation (Grant No. 60201004), and the ShanghaiScience Committee Foundation (Grants No. 03DE11009 and04QMX1463).1S. Tiwari, F. Rana, H. Hanafi, A. 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Phys. Lett. 82, 2527 (2003).FIG. 3. (a) High-frequency s1-MHzd C-V curves and (b) I-V curves ofas-grown and 800 °C annealed MOS capacitors.013110-3 Liu et al. Appl. Phys. Lett. 86, 013110 (2005)

Self-organized Ge nanocrystals embedded in HfAlO fabricated by pulsed-laser deposition and application to floating gate memory

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