HfO2 gate dielectrics on Ge substrate

Applied Surface Science 321 (2014) 214–218

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Atomic layer deposition of CeO2 /HfO2 gate dielectrics on Ge substrate Wan Joo Maeng a , Il-Kwon Oh b , Woo-Hee Kim c , Min-Kyu Kim b , Chang-Wan Lee b , Clement Lansalot-Matras d , David Thompson e , Schubert Chu e , Hyungjun Kim b,∗ a

Department of Material Science & Engineering, University of Wisconsin Madison, Madison, WI 53706, United States School of Electrical and Electronics Engineering, Yonsei University, 50 Yonsei Ro, Seodaemun-gu, Seoul, Republic of Korea Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, CA 94305, United States d Air Liquide Korea Co., Yonsei University, 50 Yonsei-Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea e Applied Materials, 974 E. Arques Avenue, M/S 81280, Sunnyvale, CA 94085, United States b c

a r t i c l e

i n f o

Article history: Received 2 August 2014 Received in revised form 23 September 2014 Accepted 3 October 2014 Available online 13 October 2014 Keywords: CeO2 /HfO2 Atomic layer deposition Ge substrate Gate dielectric

a b s t r a c t We systematically investigated atomic layer deposition (ALD) of HfO2 , CeO2 and Ce-doped HfO2 thin films on Ge substrates by using tetrakis dimethylamino hafnium (TDMAH) and tris(isopropylcyclopentadienyl)cerium [Ce(iPrCp)3 ] precursors with H2 O. The growth characteristics, chemical and electrical properties were comparatively characterized. On the basis of X-ray photoemission spectroscopy analyses, it was confirmed that the ALD CeO2 on Ge can form a stable interfacial layer composed of Ge1+ and Ge3+ , leading to improved interfacial properties. In addition, Ce-doped HfO2 films with various Ce compositions (Ce:Hf = 1:1, 1:2, 1:4 and 1:8) were prepared by an ALD supercycle process on Ge substrates. Thereby, we demonstrated that overall electrical properties including dielectric constant, interface state density, hysteresis and leakage current density are significantly improved. © 2014 Elsevier B.V. All rights reserved.

1. Introduction For the last several decades, the continuous downscaling of metal-oxide-semiconductor field-effect transistor (MOSFET) devices has made the adoption of new channel materials increasingly necessary to overcome fundamental material limitations beyond the 10 nm technology regime [1,2]. Accordingly, in order to achieve further enhancement in transistor performance, high-k gate dielectrics have attracted a great deal of attention in conjunction with high mobility channel materials such as Ge and III-V semiconductors [1,2]. Of the several alternative materials, Ge has become known as one of the most promising channel materials, especially for next-generation p-channel MOSFETs. However, contrary to its behavior in well-established Si-based MOSFETs, a recent publication revealed that HfO2 as a representative high-k gate dielectric material is not a stable gate insulator for Ge substrates because of Ge diffusion through the HfO2 and poor Ge-rich native oxide layer formation [3]. To address this problem, researches devoted to developing a stable interlayer must be carried out to significantly improve the electrical properties of Hf-based dielectrics on Ge. Previously, although various growth techniques on the

∗ Corresponding author. Tel.: +82 2 2123 5773. E-mail address: [email protected] (H. Kim). http://dx.doi.org/10.1016/j.apsusc.2014.10.025 0169-4332/© 2014 Elsevier B.V. All rights reserved.

interfacial passivation layer such as GeOx Ny treatment, SiH4 treatment and thermally grown GeOx have been employed to improve interface quality prior to the deposition of high k dielectrics, further detailed investigation still must be conducted to enable practical implementation of a reliable gate dielectric layer process [4–6]. In addition, rare earth oxides (e.g., La2 O3 and Y2 O3 ) have been the subject of interesting investigations for pursuing promising passivation interlayers to achieve a high-quality, high-k layer on Ge [7,8]. Among various rare earth oxides, cerium oxide (CeO2 ) has shown potential for forming a good passivation layer, because the ceria transforms from CeO2 (Ce4+ ) to Ce2 O3 (Ce3+ ) by releasing oxygen, followed by subsequent reaction with Ge. It thus helps to stabilize Ge, leading to better quality interfacial layers by minimizing Ge inter-diffusion [9,10]. In addition, CeO2 itself is considered to be a good gate dielectric material on Ge, since it has a high dielectric constant (23–52), and reasonable lattice mismatch to Ge (4%) [11,12]. The CeO2 thin films prepared by molecular beam epitaxy (MBE) [10,13,14] and sputtering [11] have been investigated for gate dielectrics in Ge MOS devices. However, high leakage currents in CeO2 dielectric films were found to be a major concern, due to their small band gap [10,12]. CeO2 /HfO2 bilayer structures have been investigated in recent attempts to improve the leakage current and its interfacial state qualities [10,13,14]. They reported improved electrical properties with a CeO2 layer or CeO2 gate insulator. More interestingly, the CeO2 /HfO2 bilayer structure can attain

W.J. Maeng et al. / Applied Surface Science 321 (2014) 214–218

2. Experimental procedures For this study, a commercial ALD chamber equipped with a direct liquid injection (DLI) delivery system and double shower head system was utilized. Ce(iprCp)3 [tris(isopropyl-cyclopentadienyl)cerium] [19] and TDMAH [tetrakis(dimethylamino)hafnium] were used as the Ce and Hf precursors, respectively oxidant, with H2 O oxidant. The Ce(iPrCp)3 precursor dissolved in toluene solution was injected onto a hot vaporizer heated at 160 ◦ C by modulating the opening time of electrically-operated nozzle. TDMAH was contained in a stainless steel bubbler and evaporated at 50 ◦ C to produce sufficient vapor pressure. Both precursor vapors were carried into the reaction chamber by an Ar carrier gas whose flow rate was controlled by mass flow controller (MFC). Ar gas with the same flow rate was also used for the purging of excess gas molecules and byproducts between each precursor and reactant exposure steps. The substrate temperature was maintained at 250 ◦ C throughout the entire ALD process. An ALD super-cycle process was utilized to accomplish CeO2 doping into HfO2 . The super-cycle is composed of one ALD CeO2 cycle, followed by n repetitions of the ALD HfO2 cycles. It enables us to modulate Ce/(Ce + Hf) composition x for the ALD Cex Hf1−x O2 films. For Ge MOS capacitor fabrication, undoped Ge substrates were grown epitaxially to a thickness of 1 ␮m onto a p-type Si substrate. This substrate was dipped into an HF water solution for 10 s to remove native oxide. ALD HfO2 and CeO2 films were then deposited by ALD. After the oxide deposition, post deposition annealing (PDA) was carried out in a rapid thermal annealing (RTA) system in an N2 ambient atmosphere at 400 ◦ C for 10 min. Al was then thermally evaporated as a metal gate through a dot-patterned shadow mask with a radius of 100 ␮m. Finally, Au was thermally evaporated to form the back contact. The thickness of the films was measured by spectroscopic ellipsometry. The chemical composition and the impurity level were analyzed by X-ray photoelectron spectroscopy (XPS) using an Al K␣ monochromatic source of 1486.6 eV. The electrical properties including capacitive–voltage (C–V) and current–voltage (I–V) characteristics were evaluated with a Keithley 590 C-V analyzer and an Agilent 4155C semiconductor parameter analyzer. Herein, the interface state density (Dit ) was determined by conductance method using a following equation: Dit =

G  p

ω

max

[qfD (s )A]

−1

Growth Rate (A/cycle)

1.5

(a) ALD HfO2 or CeO2/Ge 1.2 CeO2 HfO2

0.3

0

0

1

2

3

4

5

6

7

Precursor Exposure Time (s)

Film Thickness (nm)

equivalent oxide thickness (EOT) scaling, although the exact mechanism is still unclear [10,14]. Similarly, for HfO2 gate dielectrics on Si, a proper amount of Ce doping (∼10%) enabled EOT reduction by triggering a phase transformation to a cubic or tetragonal phase with a higher dielectric constant [15,16]. Motivated by these intriguing properties, Ce-doped HfO2 on Ge is of great interest for future Ge MOS applications. Atomic layer deposition (ALD) is a promising deposition method with great advantages including accurate concentration and thickness control, a result of its growth mechanism, which is controlled by a self-limited surface reaction [17,18]. Despite its advantages, very few studies have investigated Ce-doped HfO2 on Si [14,15]. This prompted a systematic investigation of materials and dielectric properties of Ce-doped HfO2 on Ge. We therefore investigated ALD growth characteristics of HfO2 and CeO2 thin films, and then characterized their resulting chemical and electrical properties. Besides, with the objectives of a higher dielectric constant and better interface quality, ALD supercycle processes of Ce-doped HfO2 films with a variety of Ce/(Ce + Hf) compositions were implemented and the resulting electrical properties were further analyzed.

215

20 (b) ALD HfO2 or CeO2/Ge 15 CeO2

10

HfO2

5 0

0

50

100

150

ALD Growth Cycles (#) Fig. 1. (a) Growth rates per cycle of CeO2 and HfO2 as a function of precursor exposure time. (b) Dependence of film thickness of CeO2 and HfO2 on ALD growth cycles.

where Gp /ω is a corrected conductance loss, ω is an angular frequency (ω = 2f, f is the measurement frequency), q is an electronic charge (1.6 × 1019 C), fD is a universal function as a function of standard deviation of band banding  s , and A is an area of metal gate. A more detailed description of the electrical evaluation can be found in our previous publications [16,20]. 3. Results and discussions ALD growth characteristics of HfO2 and CeO2 thin films were investigated. Fig. 1(a) shows the growth rates per cycle of ALD HfO2 and CeO2 films on Ge as a function of precursor exposure time (ts ). In both cases, well-defined saturated growth, a typical ˚ ALD characteristic, is achieved with ts ≥ 3 s (1.2 A/cycle for HfO2 and ˚ 0.2 A/cycle for CeO2 ), indicating that both the precursors are suitable for ALD. Accordingly, the precursor exposure time was fixed at 3 s for both ALD processes. Under these conditions, the film thickness was measured as a function of ALD growth cycles, and a nearly linear relationship was observed without any significant number of incubation cycles on Ge substrates, as shown in Fig. 1(b). Fig. 2(a) shows C–V curves of the Al/4 nm-thick ALD HfO2 or CeO2 /Ge MOS structure. The accumulation capacitances of both gate insulators are nearly the same. However, both the C–V curves showed large hysteresis window and a high level of interface state densities (Dit ). Similar results have been reported for untreated Ge MOS capacitors elsewhere [21,22]. More specifically, the hysteresis value with the CeO2 dielectric layer is 1.5 times higher than the HfO2 . This hysteresis value is comparable to previous reports on CeO2 dielectrics on Ge [10,14]. In addition, the flat band voltage (VFB ) value of the CeO2 was shifted to the positive direction (∼0.6 V), as compared to the HfO2 . The positive shift in the VFB for the CeO2 insulator can be associated either with the formation of negative charge traps or a reduction of positive charge traps at the

W.J. Maeng et al. / Applied Surface Science 321 (2014) 214–218

4

Intensity (A.U.)

3

2

1

Ge

Ge 3d (a) ALD HfO2 film

(a) Al/ALD HfO2 or CeO2/Ge

2

Capacitance (µF/cm )

216

CeO2

1+

Ge

4+

Ge

3+

Ge

2+

Ge

(b) ALD CeO2 film

HfO2

0 -5 -4 -3 -2 -1

0

1

2

3

4

5 34

Applied Voltage (V)

32

30

28

Binding Energy (eV) 1

(b) Al/ALD HfO2 or CeO2/Ge

2

Current Density (A/cm )

10

Fig. 4. XPS spectra of Ge 3d core level: (a) ALD HfO2 /Ge and (b) ALD CeO2 /Ge.

-1

10

-3

10

-5

10

-7

CeO2

10

HfO2 -9

10

-4

-3

-2

-1

0

1

2

3

4

Electric Fields (MV/cm) Fig. 2. Electrical properties of MOS capacitors with HfO2 and CeO2 films: (a) C–V curves and (b) I–V curves.

interface [23]. However, it is interesting to note that the Dit value of the CeO2 (∼1.7 × 1013 cm−2 eV−1 ) is almost 3 times lower than that of the HfO2 (∼4.8 × 1013 cm−2 eV−1 ). The origin for better interface quality is attributed to Ge stabilization via formation of Ce2 O3 [4,5]. Fig. 2(b) shows the leakage current density of HfO2 and CeO2 thin films on Ge. Higher leakage currents were observed for the CeO2 thin film compared with the HfO2 , mainly due to its small band gap of ∼3.5 eV [24]. To examine the chemical composition and binding structures of the CeO2 thin film, XPS analysis was carried out, as shown in Fig. 3. The C 1s core level spectrum was not observed in Fig. 3(a), which indicates negligible carbon contamination (at least lower than the XPS detection limit of ∼0.1%) from the Ce(iPrCp)3 and H2 O precursors. The O 1s spectrum exhibits both Ce3+ and Ce4+ components, as shown in Fig. 3(b). According to a previous publication on epitaxially grown CeO2 films on Ge, the formation of Ce3+ was ascribed

Intensity (A.U.)

(a) C 1s

294

292

(b) O 1s

533

ALD CeO2 film

290

288

286

284

282

Ge-O-Ge 3+ 4+ Ce Ce

532

531

530

529

528

Binding Energy (eV) Fig. 3. XPS spectra of O 1s and C 1s core levels: (a) C 1s and (b) O 1s of ALD CeO2 /Ge.

to the catalytic behavior of Ce for Ge oxidation [10]. This can play a significant role in improving the interface properties by modifying the interfacial Ge O bonding structures. For a more elaborate analysis of the interfacial binding structures, we analyzed the Ge 3d XPS spectra of both HfO2 and CeO2 in Fig. 4. Here, GeOx spectra shifted from the Ge 3d peak were deconvoluted to four distinctive GeOx peaks composed of Ge1+ , Ge2+ , Ge3+ and Ge4+ with energy shifts of 0.8, 1.8, 2.6, and 3.4 eV, respectively, from the binding energy of elemental Ge3d5/2 [25]. Of these sub-oxides, Ge2+ is known to result in detrimental effects on film properties [26]. For the case of ALD CeO2 on Ge, a significant reduction in the content of Ge2+ and Ge4+ was observed, along with an increase in Ge3+ . Based on previous reports, Ge4+ has poor thermal stability compared to Ge3+ . Over a temperature of 400 ◦ C, GeO2 tends to not only decompose to GeO(s) [23,25], but also be desorbed as the gas phase GeO(g) from the dielectric layer [25,26], resulting in a poor film quality. In other words, it suggests that the deposition of CeO2 leads to a more stable interface, resulting from the formation of stable Ge1+ and Ge3+ . Besides, it can prevent interdiffusion of Ge from the stabilized GeOx layer via the catalytic behavior of Ce, which also results in better interface and bulk qualities [9]. In addition to the interface issues, further EOT scaling is of great interest for practical Ge MOS applications. Accordingly, we investigated the dielectric properties of Ce-doped HfO2 thin films with various Ce:Hf composition ratios (1:1, 1:2, 1:4 and 1:3), which were prepared by ALD super cycles as specified in the experimental section. To obtain a thermally stable interface, CeO2 deposition was employed at the beginning of the super cycles. Fig. 5(a) shows C–V curves of Al/HfO2 (as a reference) or Ce-doped HfO2 /Ge MOS capacitors. The HfO2 /Ge MOS capacitor therein shows low accumulation capacitance and large hysteresis. On the other hand, with the Ce doping into the HfO2 dielectrics, increased capacitance and decreased hysteresis were observed. In rare earth-doped HfO2 systems, an increase in dielectric constant is expected, and this is seen in the amorphous structure as well as higher-k structures such as cubic or tetragonal phases stabilized by high temperature annealing [25]. More particularly, Ce can induce local atom ordering in a very limited area, which contributes to further improvement of the dielectric constant [26]. The maximum dielectric constant value, at least under the current experimental conditions, is found to be ∼20.5 for the 1:4 sample, which is ∼20% higher than the HfO2 sample (17.3). The reduction in hysteresis can be explained by the passivation of oxygen vacancies. Unlike the divalent and trivalent dopants, it is believed that Ce4+ is readily incorporated into the HfO2 crystal lattice even when passivating oxygen vacancies are present [27]. In addition, improved Dit values were observed for all the Ce-doped HfO2 samples and this was believed to be the Ce-catalytic effects as above. These results are comparable with previously reported HfO2 /La2 O3 bilayers and HfO2 dielectrics

1.5

HfO2 Ce:Hf (1:1) Ce:Hf (1:2) Ce:Hf (1:4) Ce:Hf (1:8)

1.0

Acknowledgments

0.5

0 -4

-3

-2

-1

0

1

2

3

4

2

Current Density (A/cm )

Applied Voltage (V)

10

0

217

various Ce compositions (Ce:Hf = 1:1, 1:2, 1:4 and 1:8) by ALD supercycles. As a result, significant improvement in the dielectric constant, interface state density, hysteresis and leakage current density was achieved. These interesting features are expected to give potential opportunities in the development of next-generation Ge MOSFETs.

2.0 (a) Al/ALD Ce-doped HfO /Ge 2

2

Capacitance (µF/cm )

W.J. Maeng et al. / Applied Surface Science 321 (2014) 214–218

This work was supported by the Industrial Strategic Technology Development Program (10041926, Development of high density plasma technologies for thin film deposition of nanoscale semiconductor and flexible display processing) funded by the Ministry of Knowledge Economy (MKE, Korea).

(b) Al/ALD Ce-doped HfO2/Ge References

-2

10

-4

10

-6

10

HfO2 Ce:Hf (1:1) Ce:Hf (1:2) Ce:Hf (1:4) Ce:Hf (1:8)

-8

10 10

-10

-5 -4 -3 -2 -1

0

1

2

3

4

5

Electric Fields (MV/cm) Fig. 5. Electrical properties of MOS capacitors with various Cex Hf1−x O2 films (Ce:Hf = 1:1, 1:2, 1:4 and 1:8): (a) C–V curves and (b) I–V-curves. Table 1 The summary of electrical properties of MOS capacitors with various CeX Hf1−x O2 films. Oxides (Ce:Hf)

k-Value

Dit (cm−2 ev−1 )

Hysteresis (mV)

Leakage currents at −1 MV/cm (A/cm−2 )

HfO2 (0:1) CeHfO2 (1:1) CeHfO2 (1:2) CeHfO2 (1:4) CeHfO2 (1:8)

17.3 17.6 19.1 20.5 18

4.8 × 1013 1.7 × 1013 7.1 × 1012 1.6 × 1013 1.8 × 1013

500 300 150 300 300

1.2 × 10−6 1.6 × 10−7 6.0 × 10−8 1.0 × 10−7 3.0 × 10−7

passivated by La2 O3 on Ge substrates [28–30]. The leakage current densities as a function of gate bias (I–V curves) for all the MOS samples are shown in Fig. 5(b). Under the gate injection condition at −1 MV/cm, a significant reduction in leakage currents was observed for all the Ce-doped HfO2 samples. Similar results have been reported for optimized La2 O3 /HfO2 on Ge [28,30]. The key electrical parameters have been summarized in Table 1. It should be noted that controllable Ce doping into HfO2 dielectrics can be advantageous to improve the overall electrical properties of the dielectric, including dielectric constant, Dit , hysteresis and leakage currents. 4. Conclusions In summary, growth characteristics, chemical and dielectric properties of ALD HfO2 , CeO2 and Ce-doped HfO2 thin films were systematically investigated for Ge MOS applications. ALD HfO2 and CeO2 processes using TDMAH and Ce(iPrCp)3 precursors yielded saturated and linear growth characteristics. On the basis of XPS studies, we were able to confirm that Ce played a passivating role at the interface between CeO2 and Ge by forming stable Ge2 O(1+) and Ge2 O3 (3+). To further enhance the dielectric constant along with interface quality, we prepared Ce-doped HfO2 films with

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