School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, ChinaSchool of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
Abstract: The GaAs MOS capacitor was fabricated with HfTiON as high-k gate dielectric and NH3-plasma-treated ZnON as interfacial passivation layer (IPL), and its interfacial and electrical properties are investigated compared to its counterparts with ZnON IPL but no NH3-plasma treatment and without ZnON IPL and no plasma treatment. Experimental results show that low interface-state density near midgap (1.17×1012 cm-2eV-1) and small gate leakage current density have been achieved for the GaAs MOS device with the stacked gate dielectric of HfTiON/ZnON plus NH3-plasma treatment. These improvements could be ascribed to the fact that the ZnON IPL can effectively block in-diffusion of oxygen atoms and out-diffusion of Ga and As atoms, and the NH3-plasma treatment can provide not only N atoms but also H atoms and NH radicals, which is greatly beneficial to removal of defective Ga/As oxides and As-As band, giving a high-quality ZnON/GaAs interface.
As Si-based MOSFET is approaching its physical limits, lots of research has been carried out to find alternative channel material to Si[1-3]. The Ⅲ-V compound materials, e.g. GaAs, InGaAs, have been widely studied as a new type of channel material due to their high mobility and wide band gap. However, the main problem for preparing high-quality GaAs or InGaAs MOSFETs is lack of suitable high-k dielectric material. A direct deposition of high-k dielectric on GaAs yields poor electrical characteristics due to easy formation of the native oxides on the GaAs surface, which results in an extremely high interface-state density and the pinning of Fermi-level at the GaAs/high-k interface[4]. Through a lot of studies, it was found that the stacked gate dielectric structure with high k dielectric layer, e.g. HfO2[5], Ga2O3[6] and HfTiON[7], plus interfacial passivation layer (IPL) like Ge[8], TaON[7] and so on, can effectively improve the interfacial quality and electrical properties of the MOS devices. Also, some studies showed that the MOS device with ZnO IPL can achieve better performances in terms of hysteresis, interface-state density and leakage current than other IPL[9, 10]. In addition, the oxygen vacancies and interstitials that lay in the lower bandgap region of ZnO had been covered up when the nitrogen is incorporated into ZnO to generate ZnON[11]. Moreover, the incorporation of nitrogen into ZnO can not only increase the k value of the dielectric, but also enhance the thermal stability and reliability of the device[11, 12]. So in this work, a stacked gate dielectric of HfTiON/ZnON is proposed to fabricate GaAs MOS capacitors. On the other hand, the plasma-nitridation treatment of the GaAs surface has been considered to be an effective way to improve the electrical and thermal properties of the device by providing the N atoms, H atoms and NH radicals to reduce Ga-/As-oxides on the surface of GaAs substrate[13]. However, a direct plasma treatment on the surface of the GaAs substrate may damage the chemical bonds of the surface to generate the traps[14, 15]. So in this work, after deposition of the ZnON IPL, the NH3 plasma is employed to treat ZnON IPL instead of GaAs surface[13, 16]. As a result, better interfacial and electrical properties have been obtained for the stacked gate dielectric GaAs MOS capacitor with ZnON IPL plus NH3-plasma treatment compared to its counterparts without IPL or without plasma treatment on the IPL.
2.
Experiments
GaAs MOS capacitors were fabricated on Si-doped n-type (100) substrate with a doping concentration of 5 × 1017cm-3. The wafers were cleaned in turn with deionized water, acetone and isopropanol, ethanol for five minutes respectively. Then the wafers were dipped in diluted HCl to remove the native oxides, followed by (NH4)2S dipping for 40 min at room temperature for sulfur passivation of the GaAs surface. After drying by N2, a thin ZnON film of~2 nm was deposited on the GaAs surface as IPL by reactive sputtering of ZnO in an Ar/N2(15 sccm/6 sccm) atmosphere at room temperature. Then a part of the samples were subjected to NH3-plasma treatment in PECVD chamber at 350 ℃ for 10 min, with a gas flow rate of 4 sccm and a radio frequency (RF) power of 120 W. Next, an~8-nm HfTiN gate dielectric was deposited by co-sputtering Hf and Ti targets in the same atmosphere for all the samples. The two groups of samples are denoted as NH3-ZnON (with NH3-plasma treatment) and ZnON (without NH3-plasma treatment) samples, respectively. For comparison, a sample with only 10-nm HfTiON as gate dielectric and without ZnON IPL and NH3-plasma treatment was prepared as control sample. Then all the samples were annealed in O2 (50 sccm) + N2 (500 sccm) atmosphere at 600℃ for 60 s to convert HfTiN to HfTiON. Finally, Al was thermally evaporated and patterned by photolithography as gate electrode with an area of 7.85 × 10−5 cm2, followed by N2 annealing at 300℃ for 20 min to reduce their contact resistance.
High-frequency (HF, 1 MHz) capacitance-voltage (C-V) curves and gate leakage current density versus gate voltage (Jg-Vg) curves of the samples were measured using HP4284A precision LCR meter and Keithley 4200-SCS semiconductor characterization system, respectively. All electrical measurements were carried out under a light-tight and electrically-shielded condition at room temperature.
3.
Results and discussion
Fig. 1 shows normalized HF C-V curve of all samples. For the control sample without ZnON IPL and NH3-plasma treatment, a large stretch out of the C-V curve is obviously observed, indicating a high density of defective states at the conduction-band edge of GaAs caused by a considerable amount of As-O and Ga-O bonds at the GaAs/HfTiON interface[17-19]. However, the stretch out is obviously decreased for the samples with ZnON IPL and almost disappears for the sample with NH3-plasma treated ZnON IPL. This is attributed to the blocking role of ZnON IPL on the in-diffusion of oxygen from HfTiON to GaAs, thus reducing the weak As-O and Ga-O bands at the interface of GaAs substrate[16]. The large slope in the depletion regime for the NH3-ZnON sample also indicates its good interface properties.
Figure
1.
Normalized HF (1 MHz) C-V curve of the three samples
Fig. 2 is the C-V curves of the three samples measured from 50 kHz to 1 MHz. Obviously, the NH3-ZnON sample exhibits the smallest frequency dispersion, followed by the ZnON sample, indicating its less slow states and border traps at/near the ZnON/GaAs interface[20], attributed to the roles of ZnON IPL and NH3-plasma treatment.
Figure
2.
Frequency dispersion of C-V curves for all the samples, measured from 50 kHz to 1 MHz
The electrical parameters of the devices extracted from the HF C-V curves are listed in Table 1, where the Ditis extracted by Terman's method through comparing the ideal C-V curve (Dit=0) with the measured C-V curve (Dit≠ 0)[21]. Thickness of the gate dielectric of three samples is measured by ellipsometer after annealing, as listed in Table 1. The Vfb extracted from the C-V curve is 0.92, 0.95, and 1.25 V for the NH3-ZnON sample, ZnON sample and control sample respectively, with obvious improvement for the devices with ZnON IPL. The smallest Vfb for NH3-ZnON sample could be due to the fact that the NH3-plasma treatment for ZnON IPL can incorporate extra nitrogen atoms in the ZnON IPL to fill the oxygen vacancies produced during the previous deposition[22, 23]and block the out-diffusions of Ga and As atoms from GaAs to the HfTiON[22]. More significantly, lower Dit is achieved for NH3-ZnON (1.17 × 1012 cm-2eV-1and ZnON (2.58 × 1012 cm-2eV-1 samples than the control sample (2.48 × 1013 cm-2eV-1, indicating an excellent passivation role of ZnON IPL on the interface, especially for the NH3-plasma treated ZnON IPL.
Table
1.
Electrical and physical parameters of three samples extracted from their HF C-V curves
Fig. 3 shows the gate leakage characteristics of NH3-ZnON, ZnON and control samples. A much larger Jg is observed for the control sample than the other two samples with ZnON IPL, which could be mainly attributed to the interface-trap-assisted tunneling due to high density of interface states at the high-k/GaAs interface[24, 25]. The leakage current density has been greatly reduced for the NH3-ZnON and ZnON samples.The lower gate leakage current of NH3-ZnON sample than ZnON sample further supports the fact that the interface quality of ZnON IPL can be improved by NH3-plasma treatment[26].
Figure
3.Jg versus Eg characteristics of three GaAs MOS capacitors
To identify the composition and chemical states of the stacked gate dielectric and further analyze the effects of ZnON IPL and NH3-plasma-treated ZnON IPL on the ZnON/GaAs interface quality, the HfTiON film is etched to a depth of 3 nm from the GaAs surface using an in-situ Ar+ ion beam in the XPS chamber.
Firstly, the existence of ZnON can be confirmed by the Zn 2p and Zn 3d spectra, as shown in Fig. 4 and its inset, respectively. The two peaks located at 1022.1 eV and 1044.9 eV[27] correspond to Zn 2p3/2 and Zn 2p1/2, and a peak occurring at 10.1 eV in the inset of Fig. 4 corresponds to the Zn 3d peak[27].
Figure
4.
XPS spectra of Zn 2p and Zn 3d (inset) for NH3-ZnON sample and ZnON sample
Fig. 5 is the Ti 2p spectra of the three samples. The two main peaks occurring at 454.8 and 464.7 eV correspond to Ti 2p3/2 and Ti 2p1/2[28] respectively. As can be seen, the sample with ZnON IPL has weaker intensities of peaks than control sample, implying that the ZnON IPL can effectively prevent the Ti-atom diffusing from high-k dielectric to the surface of substrate, especially for the NH3-plasma treated ZnON IPL.
The XPS spectrum of the Ga 3d for three samples is shown in Fig. 6, where the Ga-O, Ga-S and Ga-N peaks appear at 21.14[29], 20.19[30], and 19.33 eV[18] respectively, and the peaks at 15.6 and 16.7 eV are from Hf-N[31] and Hf-O[32] bonds respectively. According to the peak-area ratio of Ga-O/Ga 3d in Fig. 6, the proportion of the Ga-O bond at the interface is calculated to be 1.98%, 3.03% and 4.57% for the NH3-ZnON sample [Fig. 6(a)], the ZnON sample [Fig. 6(b)] and the control sample [Fig. 6(c)], respectively, indicating that the ZnON IPL can effectively reduce the weak Ga-O bonds, especially for the NH3-plasma treated ZnON IPL[33]. The content of Ga-N for NH3-ZnON sample is 11.8%, which is higher than that of ZnON sample (10.47%) and control sample (10.54%), implying that the NH3-plasma treatment can penetrate the ZnON IPL to nitridize GaAs surface and thus improve the interface properties. The content of Ga-As bond is 81.31%, 81.5% and 79.63% for the NH3-ZnON, ZnON and control samples respectively. A large proportion of Ga-As bands for the NH3-ZnON and ZnON samples indicate that less oxidization happened on the GaAs surface, further confirming the role of ZnON IPL on suppressing growth of As/Ga oxides.
Figure
6.
XPS spectrum of Ga 3d for (a) NH3-ZnON sample, (b) ZnON sample, and (c) control sample
The XPS spectrum of the As 3d for three samples is shown in Fig. 7. In Figs. 7(b) and 7(c) for the ZnON and control samples, the As-O and As-As peaks appear at 43.03 eV[21, 34] and 41.51 eV[34], respectively. The content of the As-O bond is calculated to be 2.2% for the ZnON sample and 3.12% for the control sample using a similar method to the above. Fortunately, in Fig. 7(a) for the NH3-ZnON sample, the As-O peak has disappeared, indicating that the NH3-plasma-treated ZnON IPL can greatly reduce the weak As-O bonds. This is why the NH3-ZnON sample has better interface quality and electrical properties than the other two samples. Besides, the As-S peak occurs at 42.19 eV[34, 35], from the passivation of sulfur on the GaAs surface.
Figure
7.
XPS spectrum of As 3d for (a) NH3-ZnON sample, (b) ZnON sample, and (c) control sample
It is well known that the As from the decomposition of weak As2O3 at the surface of the substrate can produce near-midgap states, which can cause the pinning of the Fermi level and the nonradiative recombination[34]. The NH3 plasma can provide H atoms and NH radicals except for N atoms, which can passivate the substrate and thus decrease As/Ga oxides by the reactions of 12H+As2O3→3H2O+2AsH3[36], Ga2O3+ 4H→Ga2O+2H2O and Ga2O+2H→2Ga +H2O[37].This is why the NH3-plasma treatment can effectively reduce the Ga-/As-oxides and improve the GaAs/high-k interface quality.
4.
Conclusion
The stacked HfTiON/ZnON gate dielectric GaAs MOS capacitors with or without NH3-plasma treatment are fabricated, and their interfacial and electrical properties are investigated and compared with their counterpart without ZnON IPL and NH3-plasma treatment. The experimental results show that the samples with ZnON IPL exhibit the good interfacial properties, and the NH3-plasma treatment on the ZnON IPL can further improve the interfacial and electrical performances of the devices to achieve low gate leakage current density and interface-state density (1.17 × 1012 cm−2eV−1). The XPS analyses show that the reactive species like H atoms, N atoms and NH radicals produced by NH3-plasma can effectively reduce the interfacial Ga-O, As-O and As-As bonds, and the NH3-plasma treatment on ZnON IPL can incorporate more nitrogen into the ZnON IPL, thus more effectively suppressing growth of the native oxides. Therefore the NH3-plasma-treated ZnON IPL is a promising way for preparing high-performance GaAs MOS devices.
References
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Fig. 1.
Normalized HF (1 MHz) C-V curve of the three samples
Table 1.
Electrical and physical parameters of three samples extracted from their HF C-V curves
[1]
He G, Chen X, Sun Z. Interface engineering and chemistry of Hf-based high-k dielectrics on Ⅲ-V substrates. Surf Sci Rep, 2013, 68(1):68 doi: 10.1016/j.surfrep.2013.01.002
[2]
Suzuki R, Taoka N, Yokoyama M, et al. 1-nm-capacitance-equivalent-thickness HfO2/Al2O3/InGaAs metal-oxide-semiconductor structure with low interface trap density and low gate leakage current density. Appl Phys Lett, 2012, 100(13):132906 doi: 10.1063/1.3698095
[3]
Das P S, Biswas A. Investigations on electrical characteristics and reliability properties of MOS capacitors using HfAlOx on n-GaAs substrates. Microelectron Reliab, 2012, 52(1):112 doi: 10.1016/j.microrel.2011.08.005
[4]
Kundu S, Roy S, Banerji P, et al. Studies on Al/ZrO2/GaAs metal-oxide-semiconductor capacitors and determination of its electrical parameters in the frequency range of 10 kHz-1 MHz. J Vac Sci Technol B, 2011, 29(3):1203 http://independent.academia.edu/souvikkundu1
[5]
Ji F, Xu J P, Liu J G, et al. Improved interface properties of Ge metal-oxide-semiconductor capacitor with TaTiO gate dielectric by using in situ TaON passivation interlayer. Appl Phys Lett, 2011, 98(18):182901 doi: 10.1063/1.3581891
[6]
Passlack M, Medendorp N, Gregory R, et al. Role of Ga2O3 template thickness and gadolinium mole fraction in GdxGa0.4-xO0.6/Ga2O3 gate dielectric stacks on GaAs. Appl Phys Lett, 2003, 83(25):5262 doi: 10.1063/1.1635068
[7]
Wang L S, Xu J P, Zhu S Y, et al. Improved interfacial and electrical properties of GaAs metal-oxide-semiconductor capacitors with HfTiON as gate dielectric and TaON as passivation interlayer. Appl Phys Lett, 2013, 103(9):112901 http://hub.hku.hk/bitstream/10722/202921/1/Content.pdf?accept=1
[8]
Shahrjerdi D, Oye M M, Holmes A L Jr, et al. Unpinned metal gate/high-k GaAs capacitors:fabrication and characterization. Appl Phys Lett, 2006, 89(4):49 doi: 10.1063/1.2234837?journalCode=apl
Kundu S, Shripathi T, Banerji P. Interface engineering with an MOCVD grown ZnO interface passivation layer for ZrO2-GaAs metal-oxide-semiconductor devices. Solid State Commun, 2011, 151(24):1881 doi: 10.1016/j.ssc.2011.09.033
[11]
Lee S, Nathan A, Ye Y, et al. Localized tail states and electron mobility in amorphous ZnON thin film transistors. Sci Rep, 2015, 5:13467 doi: 10.1038/srep13467
Lu H, Xu J P, Liu L, et al. Improved interfacial quality of GaAs metal-oxide-semiconductor device with NH3-plasma treated yittrium-oxynitride as interfacial passivation layer. Microelectron Reliab, 2016, 56:17 https://dialnet.unirioja.es/servlet/articulo?codigo=5305745
[14]
Yasui K, Arayama T, Okutani S, et al. Generation of ammonia plasma using a helical antenna and nitridation of GaAs surface. Appl Surf Sci, 2003, 212(2):619 http://cat.inist.fr/?aModele=exportN&cpsidt=14887882
Losurdo M, Capezzuto P, Bruno G, et al. N2-H2 remote plasma nitridation for GaAs surface passivation. Appl Phys Lett, 2002, 81(1):16 doi: 10.1063/1.1490414
Lu H L, Sun L, Ding S J, et al. Characterization of atomic-layer-deposited Al2O3/GaAs interface improved by NH3 plasma pretreatment. Appl Phys Lett, 2006, 89(15):1897 doi: 10.1063/1.2363145?journalCode=apl
[19]
Yang T, Xuan Y, Zemlyanov D, et al. Interface studies of GaAs metal-oxide-semiconductor structures using atomic-layer-deposited HfO2/Al2O3 nanolaminate gate dielectric. Appl Phys Lett, 2007, 91(14):1099 http://ieeexplore.ieee.org/document/4829101/
[20]
Wang L S, Xu J P, Liu L, et al. Plasma-nitrided Ga2O3(Gd2O3) as interfacial passivation layer for InGaAs metal-oxide-semiconductor capacitor with HfTiON gate dielectric. IEEE Trans Electron Devices, 2015, 62(4):1235 doi: 10.1109/TED.2015.2396972
[21]
Terman L M. An investigation of surface states at a silicon/silicon oxide interface employing metal-oxide-silicon diodes. Solid State Electron, 1962, 5:285 doi: 10.1016/0038-1101(62)90111-9
[22]
Li C C, Chang-Liao K S, Fu C H, et al. Improved electrical characteristics high-k gated MOS devices with in-situ remote plasma treatment in atomic layer deposition. Microelectron Eng, 2013, 109(6):64 http://www.sciencedirect.com/science/article/pii/S0167931713002773
Crupi F, Ciofi C, Germanó A, et al. On the role of interface states in low-voltage leakage currents of metal-oxide-semiconductor structures. Appl Phys Lett, 2002, 80(24):4597 doi: 10.1063/1.1487450
[25]
Xu H X, Xu J P. Electrical properties of Ge metal-oxide-semiconductor capacitors with high-k La2O3 gate dielectric incorporated by N or/and Ti. J Semicond, 2016, 37(6):77 http://d.wanfangdata.com.cn/Periodical/bdtxb201606011
[26]
Wang L S, Liu L, Xu J P, et al. Electrical properties of HfTiON gate-dielectric GaAs metal-oxide-semiconductor capacitor with AlON as interlayer. IEEE Trans Electron Devices, 2014, 61:1 doi: 10.1109/TED.2013.2292251
[27]
Lee S, Nathan A, Ye Y, et al. Localized tail states and electron mobility in amorphous ZnON thin film transistors. Sci Rep, 2015, 5:13467 doi: 10.1038/srep13467
[28]
Gonbeau D, Guimon C, Pfister-Guillouzo G, et al. XPS study of thin films of titanium oxysulfides. Surf Sci, 1991, 254(1-3):81 doi: 10.1016/0039-6028(91)90640-E
[29]
Flina B J McIntyre N S. Studies of the UV/ozone oxidation of GaAs using angle-resolved X-ray photoelectron spectroscopy. Surf Interface Anal, 1990, 15:19 doi: 10.1002/(ISSN)1096-9918
[30]
Sugahara H, Oshima M, Oigawa H, et al. Synchrotron radiation photoemission analysis for (NH4)2Sx-treated GaAs. J Appl Phys, 1991, 69(8):4349 doi: 10.1063/1.348380
[31]
Liu C W, Xu J P, Liu L, et al. High-gate dielectric GaAs MOS device with LaON as interlayer and NH-plasma surface pretreatment. Chin Phys B, 2015, 24:494 http://cpb.iphy.ac.cn/fileup/PDF/2015-12-127304.pdf
Zhu S Y, Xu J P, Wang L S, et al. Comparison of interfacial and electrical properties between Al2O3 and ZnO as interface passivation layer of GaAs MOS device with HfTiO gate dielectric. J Semicond, 2015, 36(3):74 doi: 10.1021/am506351u?src=recsys
[34]
Wu Y C, Chang E Y, Lin Y C, et al. Study of the inversion behaviors of Al2O3/InxGa1-xAs metal-oxide-semiconductor capacitors with different In contents. Solid State Electron, 2010, 54(1):37 doi: 10.1016/j.sse.2009.09.033
[35]
Yan T, Zhang Y M, Lü H L, et al. Temperature dependent interfacial and electrical characteristics during atomic layer deposition and annealing of HfO2 films in p-GaAs metal-oxide-semiconductor capacitors. J Semicond, 2015, 36(12):124003 doi: 10.1088/1674-4926/36/12/124003
[36]
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Jingkang Gong, Jingping Xu, Lu Liu, Hanhan Lu, Xiaoyu Liu, Yaoyao Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. Journal of Semiconductors, 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004
J K Gong, J P Xu, L Liu, H H Lu, X Y Liu, Y Y Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. J. Semicond., 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004.
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Received: 20 October 2016Revised: 17 March 2017Online:Published: 01 September 2017
Jingkang Gong, Jingping Xu, Lu Liu, Hanhan Lu, Xiaoyu Liu, Yaoyao Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. Journal of Semiconductors, 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004 ****J K Gong, J P Xu, L Liu, H H Lu, X Y Liu, Y Y Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. J. Semicond., 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004.
Citation:
Jingkang Gong, Jingping Xu, Lu Liu, Hanhan Lu, Xiaoyu Liu, Yaoyao Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. Journal of Semiconductors, 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004
****
J K Gong, J P Xu, L Liu, H H Lu, X Y Liu, Y Y Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. J. Semicond., 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004.
Jingkang Gong, Jingping Xu, Lu Liu, Hanhan Lu, Xiaoyu Liu, Yaoyao Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. Journal of Semiconductors, 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004 ****J K Gong, J P Xu, L Liu, H H Lu, X Y Liu, Y Y Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. J. Semicond., 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004.
Citation:
Jingkang Gong, Jingping Xu, Lu Liu, Hanhan Lu, Xiaoyu Liu, Yaoyao Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. Journal of Semiconductors, 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004
****
J K Gong, J P Xu, L Liu, H H Lu, X Y Liu, Y Y Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. J. Semicond., 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004.
The GaAs MOS capacitor was fabricated with HfTiON as high-k gate dielectric and NH3-plasma-treated ZnON as interfacial passivation layer (IPL), and its interfacial and electrical properties are investigated compared to its counterparts with ZnON IPL but no NH3-plasma treatment and without ZnON IPL and no plasma treatment. Experimental results show that low interface-state density near midgap (1.17×1012 cm-2eV-1) and small gate leakage current density have been achieved for the GaAs MOS device with the stacked gate dielectric of HfTiON/ZnON plus NH3-plasma treatment. These improvements could be ascribed to the fact that the ZnON IPL can effectively block in-diffusion of oxygen atoms and out-diffusion of Ga and As atoms, and the NH3-plasma treatment can provide not only N atoms but also H atoms and NH radicals, which is greatly beneficial to removal of defective Ga/As oxides and As-As band, giving a high-quality ZnON/GaAs interface.
As Si-based MOSFET is approaching its physical limits, lots of research has been carried out to find alternative channel material to Si[1-3]. The Ⅲ-V compound materials, e.g. GaAs, InGaAs, have been widely studied as a new type of channel material due to their high mobility and wide band gap. However, the main problem for preparing high-quality GaAs or InGaAs MOSFETs is lack of suitable high-k dielectric material. A direct deposition of high-k dielectric on GaAs yields poor electrical characteristics due to easy formation of the native oxides on the GaAs surface, which results in an extremely high interface-state density and the pinning of Fermi-level at the GaAs/high-k interface[4]. Through a lot of studies, it was found that the stacked gate dielectric structure with high k dielectric layer, e.g. HfO2[5], Ga2O3[6] and HfTiON[7], plus interfacial passivation layer (IPL) like Ge[8], TaON[7] and so on, can effectively improve the interfacial quality and electrical properties of the MOS devices. Also, some studies showed that the MOS device with ZnO IPL can achieve better performances in terms of hysteresis, interface-state density and leakage current than other IPL[9, 10]. In addition, the oxygen vacancies and interstitials that lay in the lower bandgap region of ZnO had been covered up when the nitrogen is incorporated into ZnO to generate ZnON[11]. Moreover, the incorporation of nitrogen into ZnO can not only increase the k value of the dielectric, but also enhance the thermal stability and reliability of the device[11, 12]. So in this work, a stacked gate dielectric of HfTiON/ZnON is proposed to fabricate GaAs MOS capacitors. On the other hand, the plasma-nitridation treatment of the GaAs surface has been considered to be an effective way to improve the electrical and thermal properties of the device by providing the N atoms, H atoms and NH radicals to reduce Ga-/As-oxides on the surface of GaAs substrate[13]. However, a direct plasma treatment on the surface of the GaAs substrate may damage the chemical bonds of the surface to generate the traps[14, 15]. So in this work, after deposition of the ZnON IPL, the NH3 plasma is employed to treat ZnON IPL instead of GaAs surface[13, 16]. As a result, better interfacial and electrical properties have been obtained for the stacked gate dielectric GaAs MOS capacitor with ZnON IPL plus NH3-plasma treatment compared to its counterparts without IPL or without plasma treatment on the IPL.
2.
Experiments
GaAs MOS capacitors were fabricated on Si-doped n-type (100) substrate with a doping concentration of 5 × 1017cm-3. The wafers were cleaned in turn with deionized water, acetone and isopropanol, ethanol for five minutes respectively. Then the wafers were dipped in diluted HCl to remove the native oxides, followed by (NH4)2S dipping for 40 min at room temperature for sulfur passivation of the GaAs surface. After drying by N2, a thin ZnON film of~2 nm was deposited on the GaAs surface as IPL by reactive sputtering of ZnO in an Ar/N2(15 sccm/6 sccm) atmosphere at room temperature. Then a part of the samples were subjected to NH3-plasma treatment in PECVD chamber at 350 ℃ for 10 min, with a gas flow rate of 4 sccm and a radio frequency (RF) power of 120 W. Next, an~8-nm HfTiN gate dielectric was deposited by co-sputtering Hf and Ti targets in the same atmosphere for all the samples. The two groups of samples are denoted as NH3-ZnON (with NH3-plasma treatment) and ZnON (without NH3-plasma treatment) samples, respectively. For comparison, a sample with only 10-nm HfTiON as gate dielectric and without ZnON IPL and NH3-plasma treatment was prepared as control sample. Then all the samples were annealed in O2 (50 sccm) + N2 (500 sccm) atmosphere at 600℃ for 60 s to convert HfTiN to HfTiON. Finally, Al was thermally evaporated and patterned by photolithography as gate electrode with an area of 7.85 × 10−5 cm2, followed by N2 annealing at 300℃ for 20 min to reduce their contact resistance.
High-frequency (HF, 1 MHz) capacitance-voltage (C-V) curves and gate leakage current density versus gate voltage (Jg-Vg) curves of the samples were measured using HP4284A precision LCR meter and Keithley 4200-SCS semiconductor characterization system, respectively. All electrical measurements were carried out under a light-tight and electrically-shielded condition at room temperature.
3.
Results and discussion
Fig. 1 shows normalized HF C-V curve of all samples. For the control sample without ZnON IPL and NH3-plasma treatment, a large stretch out of the C-V curve is obviously observed, indicating a high density of defective states at the conduction-band edge of GaAs caused by a considerable amount of As-O and Ga-O bonds at the GaAs/HfTiON interface[17-19]. However, the stretch out is obviously decreased for the samples with ZnON IPL and almost disappears for the sample with NH3-plasma treated ZnON IPL. This is attributed to the blocking role of ZnON IPL on the in-diffusion of oxygen from HfTiON to GaAs, thus reducing the weak As-O and Ga-O bands at the interface of GaAs substrate[16]. The large slope in the depletion regime for the NH3-ZnON sample also indicates its good interface properties.
Figure
1.
Normalized HF (1 MHz) C-V curve of the three samples
Fig. 2 is the C-V curves of the three samples measured from 50 kHz to 1 MHz. Obviously, the NH3-ZnON sample exhibits the smallest frequency dispersion, followed by the ZnON sample, indicating its less slow states and border traps at/near the ZnON/GaAs interface[20], attributed to the roles of ZnON IPL and NH3-plasma treatment.
Figure
2.
Frequency dispersion of C-V curves for all the samples, measured from 50 kHz to 1 MHz
The electrical parameters of the devices extracted from the HF C-V curves are listed in Table 1, where the Ditis extracted by Terman's method through comparing the ideal C-V curve (Dit=0) with the measured C-V curve (Dit≠ 0)[21]. Thickness of the gate dielectric of three samples is measured by ellipsometer after annealing, as listed in Table 1. The Vfb extracted from the C-V curve is 0.92, 0.95, and 1.25 V for the NH3-ZnON sample, ZnON sample and control sample respectively, with obvious improvement for the devices with ZnON IPL. The smallest Vfb for NH3-ZnON sample could be due to the fact that the NH3-plasma treatment for ZnON IPL can incorporate extra nitrogen atoms in the ZnON IPL to fill the oxygen vacancies produced during the previous deposition[22, 23]and block the out-diffusions of Ga and As atoms from GaAs to the HfTiON[22]. More significantly, lower Dit is achieved for NH3-ZnON (1.17 × 1012 cm-2eV-1and ZnON (2.58 × 1012 cm-2eV-1 samples than the control sample (2.48 × 1013 cm-2eV-1, indicating an excellent passivation role of ZnON IPL on the interface, especially for the NH3-plasma treated ZnON IPL.
Table
1.
Electrical and physical parameters of three samples extracted from their HF C-V curves
Fig. 3 shows the gate leakage characteristics of NH3-ZnON, ZnON and control samples. A much larger Jg is observed for the control sample than the other two samples with ZnON IPL, which could be mainly attributed to the interface-trap-assisted tunneling due to high density of interface states at the high-k/GaAs interface[24, 25]. The leakage current density has been greatly reduced for the NH3-ZnON and ZnON samples.The lower gate leakage current of NH3-ZnON sample than ZnON sample further supports the fact that the interface quality of ZnON IPL can be improved by NH3-plasma treatment[26].
Figure
3.Jg versus Eg characteristics of three GaAs MOS capacitors
To identify the composition and chemical states of the stacked gate dielectric and further analyze the effects of ZnON IPL and NH3-plasma-treated ZnON IPL on the ZnON/GaAs interface quality, the HfTiON film is etched to a depth of 3 nm from the GaAs surface using an in-situ Ar+ ion beam in the XPS chamber.
Firstly, the existence of ZnON can be confirmed by the Zn 2p and Zn 3d spectra, as shown in Fig. 4 and its inset, respectively. The two peaks located at 1022.1 eV and 1044.9 eV[27] correspond to Zn 2p3/2 and Zn 2p1/2, and a peak occurring at 10.1 eV in the inset of Fig. 4 corresponds to the Zn 3d peak[27].
Figure
4.
XPS spectra of Zn 2p and Zn 3d (inset) for NH3-ZnON sample and ZnON sample
Fig. 5 is the Ti 2p spectra of the three samples. The two main peaks occurring at 454.8 and 464.7 eV correspond to Ti 2p3/2 and Ti 2p1/2[28] respectively. As can be seen, the sample with ZnON IPL has weaker intensities of peaks than control sample, implying that the ZnON IPL can effectively prevent the Ti-atom diffusing from high-k dielectric to the surface of substrate, especially for the NH3-plasma treated ZnON IPL.
The XPS spectrum of the Ga 3d for three samples is shown in Fig. 6, where the Ga-O, Ga-S and Ga-N peaks appear at 21.14[29], 20.19[30], and 19.33 eV[18] respectively, and the peaks at 15.6 and 16.7 eV are from Hf-N[31] and Hf-O[32] bonds respectively. According to the peak-area ratio of Ga-O/Ga 3d in Fig. 6, the proportion of the Ga-O bond at the interface is calculated to be 1.98%, 3.03% and 4.57% for the NH3-ZnON sample [Fig. 6(a)], the ZnON sample [Fig. 6(b)] and the control sample [Fig. 6(c)], respectively, indicating that the ZnON IPL can effectively reduce the weak Ga-O bonds, especially for the NH3-plasma treated ZnON IPL[33]. The content of Ga-N for NH3-ZnON sample is 11.8%, which is higher than that of ZnON sample (10.47%) and control sample (10.54%), implying that the NH3-plasma treatment can penetrate the ZnON IPL to nitridize GaAs surface and thus improve the interface properties. The content of Ga-As bond is 81.31%, 81.5% and 79.63% for the NH3-ZnON, ZnON and control samples respectively. A large proportion of Ga-As bands for the NH3-ZnON and ZnON samples indicate that less oxidization happened on the GaAs surface, further confirming the role of ZnON IPL on suppressing growth of As/Ga oxides.
Figure
6.
XPS spectrum of Ga 3d for (a) NH3-ZnON sample, (b) ZnON sample, and (c) control sample
The XPS spectrum of the As 3d for three samples is shown in Fig. 7. In Figs. 7(b) and 7(c) for the ZnON and control samples, the As-O and As-As peaks appear at 43.03 eV[21, 34] and 41.51 eV[34], respectively. The content of the As-O bond is calculated to be 2.2% for the ZnON sample and 3.12% for the control sample using a similar method to the above. Fortunately, in Fig. 7(a) for the NH3-ZnON sample, the As-O peak has disappeared, indicating that the NH3-plasma-treated ZnON IPL can greatly reduce the weak As-O bonds. This is why the NH3-ZnON sample has better interface quality and electrical properties than the other two samples. Besides, the As-S peak occurs at 42.19 eV[34, 35], from the passivation of sulfur on the GaAs surface.
Figure
7.
XPS spectrum of As 3d for (a) NH3-ZnON sample, (b) ZnON sample, and (c) control sample
It is well known that the As from the decomposition of weak As2O3 at the surface of the substrate can produce near-midgap states, which can cause the pinning of the Fermi level and the nonradiative recombination[34]. The NH3 plasma can provide H atoms and NH radicals except for N atoms, which can passivate the substrate and thus decrease As/Ga oxides by the reactions of 12H+As2O3→3H2O+2AsH3[36], Ga2O3+ 4H→Ga2O+2H2O and Ga2O+2H→2Ga +H2O[37].This is why the NH3-plasma treatment can effectively reduce the Ga-/As-oxides and improve the GaAs/high-k interface quality.
4.
Conclusion
The stacked HfTiON/ZnON gate dielectric GaAs MOS capacitors with or without NH3-plasma treatment are fabricated, and their interfacial and electrical properties are investigated and compared with their counterpart without ZnON IPL and NH3-plasma treatment. The experimental results show that the samples with ZnON IPL exhibit the good interfacial properties, and the NH3-plasma treatment on the ZnON IPL can further improve the interfacial and electrical performances of the devices to achieve low gate leakage current density and interface-state density (1.17 × 1012 cm−2eV−1). The XPS analyses show that the reactive species like H atoms, N atoms and NH radicals produced by NH3-plasma can effectively reduce the interfacial Ga-O, As-O and As-As bonds, and the NH3-plasma treatment on ZnON IPL can incorporate more nitrogen into the ZnON IPL, thus more effectively suppressing growth of the native oxides. Therefore the NH3-plasma-treated ZnON IPL is a promising way for preparing high-performance GaAs MOS devices.
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Marchiori C, Webb D J, Rossel C, et al. H plasma cleaning and a-Si passivation of GaAs for surface channel device applications. J Appl Phys, 2009, 106(11):1245 doi: 10.1063/1.3260251?journalCode=jap
Jingkang Gong, Jingping Xu, Lu Liu, Hanhan Lu, Xiaoyu Liu, Yaoyao Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. Journal of Semiconductors, 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004 ****J K Gong, J P Xu, L Liu, H H Lu, X Y Liu, Y Y Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. J. Semicond., 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004.
Jingkang Gong, Jingping Xu, Lu Liu, Hanhan Lu, Xiaoyu Liu, Yaoyao Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. Journal of Semiconductors, 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004
****
J K Gong, J P Xu, L Liu, H H Lu, X Y Liu, Y Y Feng. Improved interfacial properties of GaAs MOS capacitor with NH3-plasma-treated ZnON as interfacial passivation layer[J]. J. Semicond., 2017, 38(9): 094004. doi: 10.1088/1674-4926/38/9/094004.
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