Nano Express | Open | Published:
Effect of Composition, Interface, and Deposition Sequence on Electrical Properties of Nanolayered Ta2O5-Al2O3 Films Grown on Silicon by Atomic Layer Deposition
Nanoscale Research Lettersvolume 14, Article number: 75 (2019)
Nanolayered Ta2O5-Al2O3 composite films were grown on n-type silicon by atomic layer deposition (ALD) within the overlapped ALD window of 220–270 °C. Moreover, post-annealing treatment was carried out to eliminate defects and improve film quality. Nanolayered Ta2O5-Al2O3 composite films remain amorphous after 700 °C annealing. The effects of composition, interface, and deposition sequence on electrical properties of Ta2O5-Al2O3 composite films were investigated in detail utilizing MIS devices. The results demonstrate that the formation of Ta2O5-Al2O3 composite films by mixing Al2O3 into Ta2O5 can decrease the leakage current effectively, but it leads to the decrease of the dielectric constant and the enhancement of the hysteresis effect. The interfaces in composite films are not conducive to prevent the leakage current. The deposition sequence of Si/(Al2O3/Ta2O5)n, Al2O3 as the first covering layer, reduces the leakage current and the hysteresis effect effectively. Therefore, the electrical properties of Ta2O5-Al2O3 composite films could be regulated by adjusting components and structures via ALD to acquire relatively great dielectric constants and acceptable leakage currents.
With the shrinking of the sizes, the limitations of silicon oxide (SiO2) gate dielectric for ultra large-scale integration (ULSI) devices have been reached, hence developing new gate dielectrics for next generation of microelectronic devices has become an urgent task in semiconductor industry . It is required that the leakage current of new gate dielectrics has to be lower than that of the conventional SiO2 under the same equivalent oxide thickness. Therefore, various high-k dielectric materials have been recommended to replace SiO2 [2, 3].
Recently, alternative metal oxide films have been extensively investigated such as Ta2O5, Al2O3, ZrO2, HfO2, Nb2O5, and TiO2. Among them, tantalum pentoxide (Ta2O5) has been considered as one of the most promising candidates to replace SiO2 due to its relatively high dielectric constant of about 20~60 [4,5,6,7,8]. However, Ta2O5 has noticeable high-field conductivity and cannot prevent carriers leakage due to its small band gap of 4.4 eV, which means this metal oxide cannot be independently used as a dielectric film. Hence, it is necessary to introduce an excellent insulating material to block leakage current . Al2O3 is one of the most investigated materials with large band gap (8.7 eV) and high breakdown electric field [10,11,12,13]. To optimize the electrical property of Ta2O5 as gate dielectric, ultrathin Al2O3 can be mixed into Ta2O5 thin films for its current-blocking capability [14,15,16]. This composite structure is believed to provide a high dielectric constant and an acceptable leakage current by controlling the composition and structure [17,18,19,20,21,22,23].
As for film deposition methods, atomic layer deposition (ALD) based on saturated self-limiting surface reactions has become an important film deposition technique in the semiconductor industry. It exhibits many advantages over other deposition routes, such as precise thickness control at atomic layer level, high uniformity over a large area, excellent conformity in many complex nanostructures, and controllable film structure and composition [24,25,26,27,28]. Min-Kyu et al.  reported the film deposition of Ta2O5 via thermal and ozone (O3) ALD using pentaethoxytantlum as Ta precursor. Hyunchol et al.  reported the growth of the ZrO2/Ta2O5 multi-laminate films by ALD and the relation between their dielectric and chemical properties. Partida-Manzanera et al.  reported (Ta2O5)x(Al2O3)1−x thin films deposited by ALD using pentakis(dimethylamino)tantalum as Ta precursor and DI water as oxidizer, and the effects of tantalum doping and annealing on dielectric performance. Nevertheless, the effect of composition, interface, and the deposition sequence in composite thin films on electrical properties of Ta2O5-Al2O3 film deposited by ALD still need to be further illustrated.
In this work, we deposited nanolayered Ta2O5-Al2O3 composite thin films on n-type silicon wafers by ALD technology using pentakis(dimethylamino)tantalum (PDMATa) and trimethylaluminum (TAM) as metal precursors, as well as O3 as an oxidizer. Moreover, post-annealing treatments were carried out to eliminate defects and improve film quality . The electrical properties of films were studied utilizing the MIS device with Ta2O5-Al2O3 as dielectric layer . The effects of film composition, interface, and the deposition sequence on electrical properties of film were investigated in detail by capacitance-voltage and current-voltage measurement.
Nanolayered Ta2O5-Al2O3 composite films were grown onto oriented n-type silicon wafers using an ALD reactor (MNT Ltd.). Trimethylaluminium was held at room temperature and pentakis(dimethylamino)tantalum was heated to 80 °C. Ozone as an oxidant was generated from oxygen (99.999% purity) by an ozone generator (Newland Ltd.). High purity nitrogen gas (99.999%) was used as the carrying and purging gas. Moreover, the temperature of the reactor chamber and the delivery lines was remained at 230 °C and 120 °C, respectively. All the samples were annealed at 700 °C for 2 h under nitrogen ambient. The Al electrodes on both sides of the samples were deposited by physical vapor deposition. The samples were annealed at 250 °C for 0.5 h to assure reliable ohmic contacts. The samples with varying ratios and varying interface number were prepared by controlling the ALD cycles or sub-layer thickness of Ta2O5 and Al2O3.
The thicknesses and refractive indexes of all samples were measured by an ellipsometer. The crystal structure of the Ta2O5-Al2O3 films was characterized by glancing angle X-ray diffraction (GAXRD) with Cu Kα radiation. Current-voltage (I-V) measurements were carried out by a Keithley 2410 1100 V source measurement unit (Keithley Instruments Inc.) and capacitance-voltage (C-V) measurements were carried out by TH2828S LCR meter (Tonghui Electronics). All the measurements were completed at room temperature.
Results and Discussion
Figure 1a shows the change of deposition rate as a function of deposition temperature. There is an overlap for ALD temperature windows of Ta2O5 and Al2O3. Therefore, Ta2O5-Al2O3 composite films can be deposited within the temperature range of 220~270 °C, in which it is controllable to grow uniform and high-quality dielectric films by ALD manner. Moreover, the deposition rates of Ta2O5 and Al2O3 are constant 0.52 Å/cycle and 1.01 Å cycle in ALD temperature windows, respectively. The deposition rates can be used to design the thickness and component contents of the composite film. Annealing treatment is regarded as a necessary process to eliminate defects and improve film quality . Figure 1b shows the GAXRD patterns of Ta2O5, Al2O3, and Ta2O5-Al2O3 films annealed at 700 °C. Pure Al2O3 film remained amorphous state after 700 °C annealing. In the pattern of Ta2O5, the strong peaks at 22.8° and 56.8° are indexed to the orthorhombic Ta2O5 (PDF Card 25-0922), and the peaks at 28.5°, 36.9°, and 46.8° are indexed to the hexagonal Ta2O5 (PDF Card 18-1304). However, no diffraction peak was detected in the pattern of Ta2O5-Al2O3 composite films with various composition and interfaces. One possible explanation is that crystallization is inhibited in the ultrathin Ta2O5 sub-layers. The other is that amorphous Al2O3 mixed in the composite film increases the crystallization temperature of Ta2O5 film.
Three series of experiments, as shown in Table 1, were carried out to investigate the effects of component ratio, the number of interface, and deposition sequence on electrical properties. The nanolayered Ta2O5-Al2O3 composite films have a periodic structure consisted of several sub-layered Ta2O5-Al2O3. The electrical properties of composite films were studied utilizing the metal-insulator-semiconductor (MIS) devices, as shown in Fig. 2.
To study the effect of the component ratio in composite films on the electrical properties, in experiment I, the thickness ratios of Ta2O5 to Al2O3 in films varied from 1:0 to 0:1. Figure 3a shows that the curves of current density versus electric field intensity. For pure Al2O3 film, it is difficult to inject current due to its strong insulativity. For pure Ta2O5, it shows obvious leakage current and low breakdown field strength. In Fig. 3b, the current density of pure Ta2O5 (Ta2O5:Al2O3 = 1:0) film at 2 MV/cm is 0.329 A/cm2 due to high-field conductivity and abundant grain boundary as the leakage paths . Then, the current density decreases correspondingly with decreasing the thickness ratios of Ta2O5 to Al2O3 from 1:0 to 0:1, and it finally declines down to 2.62 × 10−8 A/cm2. The results demonstrate that the mixing Al2O3 into Ta2O5 thin film can decrease the leakage current effectively. One reason is Al2O3 with wide band gap has strong insulativity and can act as a barrier layer to prevent leakage current. The other is that the amorphous phase of composite film blocks leakage current path. To calculate the dielectric constants of Ta2O5-Al2O3 composite films, the C-V measurement was carried out at 100 kHz at a ramp rate of 100 mV/s, as shown in Fig. 3c. A low capacitance state is a depletion region in the negative voltage range and a high capacitance state is an accumulation region in the positive voltage range for MIS capacitors. The capacitances decrease with reducing the thickness ratio of Ta2O5 to Al2O3. Moreover, the C-V data of Ta2O5-Al2O3 composite films display significant flat band shifts to more positive voltages and additionally significant hysteresis with increasing Al2O3 content ratios. The positive shifts of flat band voltage can be attributed to the negative charges from trapping of electrons as well as fixed charges at the interface or in the film. Hysteresis effect in C-V measurements is normally attributed to charge trapping in the oxide or at the interface, mobile charge, and remnant polarization . In Fig. 3d, the dielectric constant of pure Ta2O5 (Ta2O5:Al2O3 = 0:1) and pure Al2O3 film was calculated at 24.6 and 6.28, respectively. For Ta2O5-Al2O3 composite films, as is expected, the dielectric constants decrease continuously with the increase of Al2O3 content correspondingly.
To explore the effect of interface in composite films on the electrical properties, in experiment II, the number of the interfaces varied from 14 to 100. Figure 4a shows the leakage current behaviors of Ta2O5-Al2O3 composite films with various number of interfaces. It can be found that the interface has smaller effects on leakage current compared to the film component. In Fig. 4b, the current density of Ta2O5-Al2O3 composite films is 7.81 × 10−7 A/cm2 when the number of interfaces is 14, and then it increases continuously with increasing the number of interfaces from 14 to 100 at the electric field of 2 MV/cm. These results demonstrate that interfaces in Ta2O5-Al2O3 films are not conducive to prevent the leakage current. These defects trend to generate at interfaces due to the different ionic radius and valence states for Ta5+ and Al3+. Moreover, more interfaces mean thinner Ta2O5-Al2O3 sub-layers in fixed-thickness film. The interface defect density will increase with the reduction of film thickness , which may cause an increase of leakage current. In addition, the effect of SiO2 interface on the electrical properties of the nanolayered film is relatively minor after 700 °C annealing under N2 ambient. Before ALD processes, the native oxide has been removed by an HF last cleaning step immediately before the deposition. The HF step gives rise to a hydrogen-passivated surface, which becomes the initial state for the ALD process. After the film deposition, the samples were annealed at 700 °C under N2 ambient. The inert gas can prevent the oxidation of Si and the further growth of SiO2 interface. Moreover, the Al2O3 films are not permeable for oxygen diffusion . Al2O3 as a barrier layer in nanolayered Ta2O5-Al2O3 film can suppress oxygen diffusion toward the interface between Si and nanolayered film. Therefore, the effect of the SiO2 interface on the electrical properties of the nanolayered film is limited below 900 °C annealing. However, the SiO2 interface has an effect on the electrical properties of nanolayered Ta2O5-Al2O3 film when the annealing temperature is above 1000 °C. As shown in Fig. 5, the reduction of leakage current and dielectric constant can be attributed to the growth of the SiO2 interface during the annealing processes.
The effect of the deposition sequence on the electrical properties was compared in experiment III. The deposition sequence of composite films on silicon was first Ta2O5 and then Al2O3, which was defined as Si/(Ta2O5/Al2O3)n. Otherwise, it was defined as Si/(Al2O3/Ta2O5)n. Figure 6a, b depicts the leakage current behaviors and the curves of C-V. The current density of Si/(Ta2O5/Al2O3) film is higher than that of Si/(Al2O3/Ta2O5) film at the electric field of 4 MV/cm, and the breakdown field of Si/(Ta2O5/Al2O3) film is obviously weaker than that of Si/(Al2O3/Ta2O5) film. In addition, the hysteresis of the C-V curve for Si/(Ta2O5/Al2O3) film is obviously greater. It is reported that Al2O3 thin film has a low interface trap density [38, 39] and can improve interfacial properties . It can be seen that there are lesser defects at the Si/Al2O3 interface compared to the Si/Ta2O5 interface. Moreover, the Al2O3 films are not permeable for oxygen diffusion. It can act as a barrier layer to cover Si in order to prevent the diffusion of oxygen in film toward Si/Al2O3 interface.
The above results illustrate that film composition, structure, and interface state density act as the key factors to affect the electrical properties. A compromise property was obtained by mixing Al2O3 into Ta2O5 film. The increase of film crystallinity can not only increase the dielectric constant, but also increase the leakage current due to abundant grain boundary as a leakage path. Moreover, high interface state density should be avoided for the laminated or doped film on account of the negative influence on leakage current. Therefore, the amorphous dielectric film with high dielectric constant, relatively large band gap energy, and low interface state density may be a promising gate dielectric to replace SiO2. In addition, deposition technology also as a key factor has an important effect on electrical properties of gate dielectric.
Nanolayered Ta2O5-Al2O3 composite films were grown on n-type silicon by ALD. The overlapped temperature window for Ta2O5 and Al2O3 is 220~270 °C using pentakis(dimethylamino)tantalum as the Ta precursor and O3 as the oxidant. Nanolayered Ta2O5-Al2O3 composite films remain amorphous after annealing treatment at 700 °C. The formation of Ta2O5-Al2O3 composite films by introducing Al2O3 into Ta2O5 can decrease the leakage current effectively due to the excellent insulator for amorphous Al2O3, but lead to the decrease of the dielectric constant. Moreover, the interfaces in composite films are not conducive to prevent the leakage current. In addition, the deposition sequence of Si/(Al2O3/Ta2O5)n, Al2O3 as the first covering layer, reduces effectively the leakage current and the hysteresis effect due to its thermostability and barrier effect. Therefore, the electrical properties of Ta2O5-Al2O3 composite films could be regulated by adjusting components and structures via ALD to acquire relatively great dielectric constants and acceptable leakage currents.
Atomic layer deposition
- C-V :
Glancing angle X-ray diffraction
- I-V :
- O3 :
Ultra large-scale integration
Chen W, Ren W, Zhang Y, Liu M, Ye ZG (2015) Preparation and properties of ZrO2 and TiO2 films and their nanolaminates by atomic layer deposition. Ceram Int 41:S278–S282
Young CD, Heh D, Nadkarni SV, Choi R, Peterson JJ, Barnett J, Lee BH, Bersuker G (2006) Electron trap generation in high-k gate stacks by constant voltage stress. IEEE T Reliab 6:123–131
Gusev EP, Narayanan V, Frank MM (2006) Advanced high-k dielectric stacks with polySi and metal gates: recent progress and current challenges. IBM J Res Dev 50:387–410
Partida-Manzanera T, Roberts JW, Bhat TN, Zhang Z, Tan HR, Dolmanan SB, Sedghi N, Tripathy S, Potter RJ (2016) Comparative analysis of the effects of tantalum doping and annealing on atomic layer deposited (Ta2O5)x(Al2O3)1−x as potential gate dielectrics for GaN/AlxGa1−xN/GaN high electron mobility transistors. J Appl Phys 119:1059–1052
Kolkovsky V, Kurth E, Kunath C (2016) Enhanced dielectric properties of thin Ta2O5 films grown on 65 nm SiO2/Si. Phys Status Solidi 13:786–789
Cheng S, Sang L, Liao M, Liu J, Imura M, Li H, Koide Y (2012) Integration of high-dielectric constant Ta2O5 oxides on diamond for power devices. Appl Phys Lett 101:331–359
Zhang L, Li J, Zhang XW, Jiang XY, Zhang ZL (2010) High-performance ZnO thin film transistors with sputtering SiO2/Ta2O5/SiO2 multilayer gate dielectric. Thin Solid Films 518:6130–6133
Kolkovsky V, Lukat K, Kurth E, Kunath C (2015) Reactively sputtered hafnium oxide on silicon dioxide: structural and electrical properties. Solid State Electron 106:63–67
Atanassova E, Georgieva M (2010) High-k HfO2-Ta2O5 mixed layers: electrical characteristics and mechanisms of conductivity. Microelectron Eng 87:668–676
Wei D, Edgar JH, Briggs DP, Retterer ST (2014) Atomic layer deposition TiO2-Al2O3 stack: an improved gate dielectric on Ga-polar GaN metal oxide semiconductor capacitors. J Vac Sci Technol, B 32:060602–060604
Chang S, Song YW, Lee S, Sang YL, Ju BK (2008) Efficient suppression of charge trapping in ZnO-based transparent thin film transistors with novel Al2O3/HfO2/Al2O3 structure. Appl Phys Lett 92:113505
Kukli K, Ritala M, Leskelä M (2001) Development of dielectric properties of niobium oxide, tantalum oxide, and aluminum oxide based nanolayered materials. J Electrochem Soc 148:156–162
Chun BS, Wu HC, Abid M, Chu IC, Serrano-Guisan S, Shvets IV, Choi DS (2010) The effect of deposition power on the electrical properties of Al-doped zinc oxide thin films. Appl Phys Lett 97:1245
Nakamura R, Toda T, Tsukui S, Tane M, Ishimaru M, Suzuki T, Nakajima H (2014) Diffusion of oxygen in amorphous Al2O3, Ta2O5, and Nb2O5. J Appl Phys 116:222904
Atanassova E, Spassov D, Novkovski N, Paskaleva A (2012) Constant current stress of lightly Al-doped Ta2O5. Mater Sci Semicond Process 15:98–107
Spassov D, Atanassova E, Paskaleva A (2011) Lightly Al-doped Ta2O5: electrical properties and mechanisms of conductivity. Microelectron Reliab 51:2102–2109
HorngHwa LU (2011) Effects of the Ta content on the microstructure and electrical property of reactively sputtered TaxZr1−xN thin films. Thin Solid Films 519:4987–4991
Zhang H, Solanki R, Roberds B, Bai G, Banerjee I (2000) High permittivity thin film nanolaminates. J Appl Phys 87:1921–1924
Nam M, Kim A, Kang K, Choi E, Kwon SH, Lee SJ, Pyo SG (2016) Characterization of atomic layer deposited Al2O3/HfO2 and Ta2O5/Al2O3 combination stacks. Sci Adv Mater 8:1958–1962
Kukli K, Kemell M, Vehkamäki M, Heikkilä MJ, Mizohata K, Kalam K, Ritala M, Leskelä M, Kundrata I, Fröhlich K (2017) Atomic layer deposition and properties of mixed Ta2O5 and ZrO2 films. AIP Adv 7:025001
Jõgi I, Tamm A, Kukli K, Kemell M, Lu J, Sajavaara T, Ritala M, Leskelä M (2010) Investigation of ZrO2-Gd2O3 based high-k materials as capacitor dielectrics. J Electrochem Soc 157:G202-10
Ding SJ, Zhu C, Li MF, Zhang DW (2005) Atomic-layer-deposited Al2O3-HfO2-Al2O3 dielectrics for metal-insulator-metal capacitor applications. Appl Phys Lett 87:886
Ding SJ, Xu J, Huang Y, Sun QQ, Zhang DW, Li MF (2008) Electrical characteristics and conduction mechanisms of metal-insulator-metal capacitors with nanolaminated Al2O3–HfO2 dielectrics. Appl Phys Lett 93:79
Lee S, Kim H, Lee J, Yu IH, Lee JH, Hwang C (2014) Effects of O3 and H2O as oxygen sources on the atomic layer deposition of HfO2 gate dielectrics at different deposition temperatures. J Mater Chem C 2:2558–2568
Smith SW, Mcauliffe KG, Conley JF (2010) Atomic layer deposited high-k nanolaminate capacitors. Solid State Electron 54:1076–1082
Sang WL, Kwon OS, Hwan Han J, Seong Hwang C (2008) Enhanced electrical properties of SrTiO3 thin films grown by atomic layer deposition at high temperature for dynamic random access memory applications. Appl Phys Lett 92:G127
Zhu MW, Gong J, Sun C, Xia JH, Jiang X (2008) Investigation of correlation between the microstructure and electrical properties of sol-gel derived ZnO based thin films. J Appl Phys 104:247
Jun JH, Choi DJ, Kim KH, Oh KY, Hwang CJ (2014) Effect of structural properties on electrical properties of lanthanum oxide thin film as a gate dielectric. Japn J Appl Phys 42:3519–3522
Kim MK, Kim WH, Lee T, Kim H (2013) Growth characteristics and electrical properties of Ta2O5 grown by thermal and O3-based atomic layer deposition on TiN substrates for metal-insulator-metal capacitor applications. Thin Solid Films 542:71–75
Cho H, Park KW, Park CH, Cho HJ, Yeom SJ, Hong K, Kwak NJ, Ahn JH (2015) Abnormally enhanced dielectric constant in ZrO2/Ta2O5 multi-laminate structures by metallic Ta formation. Mater Lett 154:148–151
Hao T, Deng Z, Liu Z, Huang C, Huang J, Hai L, Chong W, Cao Y (2011) Effects of post-annealing on structural, optical and electrical properties of Al-doped ZnO thin films. Appl Surf Sci 257:4906–4911
Roy Chaudhuri A, Fissel A, Osten HJ (2014) Superior dielectric properties for template assisted grown (100) oriented Gd2O3 thin films on Si(100). Appl Phys Lett 104:18
Wang X, Ishiwara H (2014) Improvement of electrical property of sol-gel-derived lead zirconate titanate thin films by multiple rapid thermal annealing. Japn J Appl Phys 40:7002–7006
Nguyen NV, Richter CA, Yong JC, Alers GB, Stirling LA (2000) Effects of high-temperature annealing on the dielectric function of Ta2O5 films observed by spectroscopic ellipsometry. Appl Phys Lett 77:3012–3014
Johnson RS, Hong JG, Lucovsky G (2001) Electron traps at interfaces between Si(100) and noncrystalline Al2O3, Ta2O5, and (Ta2O5)x(Al2O3)1-x alloys. J Vac Sci Technol B 19:1606–1610
Saint-Cast P, Heo YH, Billot E, Olwal P, Hofmann M, Rentsch J, Glunz SW, Preu R (2011) Variation of the layer thickness to study the electrical property of PECVD Al2O3/c-Si interface. Energy Procedia 8:642–647
Lebedev M S, Ayupov B M (2008) Investigation of thin-film nanocomposite materials by monochromatic null ellipsometry. 9th Interational workshop and tutorials EDM’2008, session I, 30-33
Geng GZ, Liu GX, Shan FK, Liu A, Zhang Q, Lee WJ, Shin BC, Wu HZ (2014) Improved performance of InGaZnO thin-film transistors with Ta2O5/Al2O3 stack deposited using pulsed laser deposition. Curr Appl Phys 14:S2–S6
Werner F, Cosceev A, Schmidt J (2012) Interface recombination parameters of atomic-layer-deposited Al2O3 on crystalline silicon. J Appl Phys 111:073710
This work is supported by the National Natural Science Foundation of China (no. 61674085).
National Natural Science Foundation of China (no. 61674085).
Availability of Data and Materials
All data are fully available without restriction.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.