Scholarly article on topic 'Low EOT GeO2/Al2O3/HfO2 on Ge substrate using ultrathin Al deposition'

Low EOT GeO2/Al2O3/HfO2 on Ge substrate using ultrathin Al deposition Academic research paper on "Materials engineering"

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Microelectronic Engineering
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{"MOS device" / "Ge MOS" / HfO2 / ALD}

Abstract of research paper on Materials engineering, author of scientific article — S. Mather, N. Sedghi, M. Althobaiti, I.Z. Mitrovic, V. Dhanak, et al.

Abstract High-κ dielectric gate stacks comprising HfO2 were fabricated on Ge with alumina as the barrier level. This was achieved by thermal annealing in an ultra high vacuum to remove the native oxide followed by deposition of aluminium by molecular beam epitaxy. After in situ oxidation at ambient temperature, HfO2 was deposited by atomic layer deposition. The devices underwent physical and electrical characterisation and show low EOT down to 1.3nm, low leakage current of less than 10−7 Acm−2 at ±1V, and CV hysteresis of ∼10mV.

Academic research paper on topic "Low EOT GeO2/Al2O3/HfO2 on Ge substrate using ultrathin Al deposition"

Microelectronic Engineering 109 (2013) 126-128

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Microelectronic Engineering

journal homepage: www.elsevier.com/locate/mee

Low EOT GeO2/Al2O3/HfO2 on Ge substrate using ultrathin Al deposition

S. Mather**, N. Sedghib, M. Althobaitic, I.Z. Mitrovicb, V. Dhanakc, P.R. Chalkera, S. Hallb

a School of Engineering, University of Liverpool, Brownlow Hill, Liverpool L69 3GH, UK

b Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool L69 3GJ, UK c Department of Physics, University of Liverpool, Oxford Street, Liverpool L69 7ZE, UK

ARTICLE INFO

Article history:

Available online 16 March 2013

Keywords: MOS device Ge MOS HfO2 ALD

ABSTRACT

High-j dielectric gate stacks comprising HfO2 were fabricated on Ge with alumina as the barrier level. This was achieved by thermal annealing in an ultra high vacuum to remove the native oxide followed by deposition of aluminium by molecular beam epitaxy. After in situ oxidation at ambient temperature, HfO2 was deposited by atomic layer deposition. The devices underwent physical and electrical characterisation and show low EOT down to 1.3 nm, low leakage current of less than 10 7 Acm 2 at ±1 V, and CV hysteresis of ~10 mV.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Germanium (Ge) is a candidate channel material for sub-16 nm CMOS technology node pMOST devices due to its higher hole (x4) mobility compared with silicon. High-j dielectrics, such as hafnia (HfO2), are needed to achieve the equivalent oxide thicknesses (EOT) required at this scale, whilst maintaining sufficiently low leakage current density. Direct deposition of HfO2 onto Ge leads to high interface state density (Dit) [1] and a thin native oxide, GeO2 interfacial layer (IL) has been shown to be effective for reducing Dit. However, Ge suffers from desorption of volatile GeO to the surface, which causes device instability, high Dit and mobility degradation [2]. A variety of methods have been used for capping the IL to prevent desorption of GeO. Recently alumina (Al2O3) has been proposed as an interlayer between Ge and HfO2 to act as a diffusion barrier [3,4] and stabilize a very thin GeO2 layer on the Ge channel to achieve a low Dit. One approach to form the Al2O3 interlayer is to deposit a thin layer of Al metal by molecular beam epitaxy (MBE) and subsequently oxidize it to form an Al2O3/GeO2/Ge structure [5]. We report here the use of such a passivation scheme, combined with HfO2 as a high-j layer, to achieve an EOT as low as 1.3 nm with an acceptable leakage current of less than 10~7 Acm~2 at ±1 V.

2. Sample preparation

Ge (100) wafers (n- and p-type) were cleaned in ultra high vacuum (<10~6 mbar) at 500 °C and 600 °C for 10 min to evaporate any native oxide and so achieve an oxide free surface. Subsequently, wafers were exposed to an Al flux for a range of times

* Corresponding author. Tel.: +44 151 7952273.

E-mail address: smather@liv.ac.uk (S. Mather).

to deposit ultrathin Al layers. The samples were then oxidized at ambient temperatures in the MBE load lock to produce Al2O3 layers. The samples were transferred within 1 min to an Oxford Instruments OpAL reactor and thin films of HfO2 were deposited on the Al2O3 using atomic layer deposition (ALD). The HfO2 depositions used a [(CpMe)2HfOMeMe] precursor coupled with an O2 plasma as the oxidizing species. Between 30 and 130 ALD cycles were used to grow HfO2 thicknesses from 1.6 to 7 nm at 250 °C. For electrical measurements, circular gold contacts of area 1.96 x 10~3 cm2 were deposited onto the films to form MOS gate electrodes and Al was deposited on the back of the Ge wafers to provide an ohmic contact. After preliminary measurements, the samples were annealed in forming gas (FGA) at 350 °C for 30 min. The oxide leakage current was measured using a Keithley 230B voltage source and Keithley 617B electrometer. The HP 4192A low frequency (LF) impedance analyzer at small signal frequencies between 100 Hz to 1 MHz was used to perform high frequency capacitance-voltage (HF CV) measurements.

3. Results and discussion

Fig. 1 shows an HRTEM image of a sample with 10 s exposure to the MBE Al source with 130 ALD cycles to deposit HfO2, obtained with aJEOL 2100F TEM operating in STEM mode with an operating voltage of 200 kV.

The image indicates a 2 nm thick layer of GeO2/Al2O3 with a 7 nm layer of HfO2 on top. X-ray Photoelectron spectroscopy (XPS) was carried out to investigate the chemical bonding present in the films. Fig. 2 shows XPS Al(2p) spectra confirming that Al2O3 is formed when compared to a reference Al foil. The small peak at 73 eV is attributed to differential charging across the thin alumina

0167-9317/$ - see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.032

Fig. 4. Current Voltage characteristics of sample with 5 s exposure to Al MBE source followed by 65 ALD cycles of HfO2 deposition.

Fig. 1. TEM image showing a 2 nm thick GeO2/Al2O3 layer with HfO2 on top.

i,. Al2°3 .........AL0„ reference

if vtv^ ,-k

78 76 74

Binding energy (eV)

■ 100 Hz

» 500 Hz

A 1kHz

T 2kHz

♦ 5kHz

» 50kHz

* 100kHz

» 200kHz

—a— 500kHz

Fig. 2. XPS spectra for Al 2p from 10 s Al MBE exposure and 30 ALD cycles of HfO2.

Voltage (V)

Fig. 5. HF CV characteristics of sample with 5 s exposure to Al MBE source followed by 65 ALD cycles of HfO2 deposition.

- Experimental Ge3d

----Ge3d

---GeOx

----Ge02

---Fitting

34 32 30

Binding energy (eV)

Fig. 3. XPS spectra for Ge 3d from 10 s Al MBE exposure and 30 ALD cycles of HfO2.

layer. The XPS Ge(3d) data of Fig. 3, shows that a layer of GeO2 is present at the Ge surface.

The leakage current density shown in Fig. 4 was less than 1 x 10~7 Acm~2 at ±1 V for the sample thermally cleaned at 500 °C with 3.5 nm of HfO2. Typical CV plots measured in the range 100 Hz - 500 kHz are shown in Fig. 5. The plots indicate very low

frequency dispersion in the accumulation region and well-behaved variation of inversion capacitance with frequency at negative voltages. The CV plots show very small hysteresis of ca. 10 mV, as shown in Fig. 6.

To estimate the IL thickness, samples with various thickness of HfO2 were fabricated. Fig. 7 shows the relationship between the

Voltage (V)

Fig. 6. CV plots showing the hysteresis of ca. 10 mV measured at 1 MHz.

5.0 4.5 4.0

? 3.0 c

- 2.5 2.0 1.5 1.0 0.5 0.0

■ 5 s Al MBE • 10 s Al MBE

----Fit to 5 s

-Fit to 10 s

50 100 150 200 Number of cycles

Fig. 7. CET against number of ALD cycles for different MBE exposure times of Al for as-deposited samples.

-•— As Deposited -▼— After FGA

0 1 Voltage (V)

Fig. 8. CV plots before and after FGA.

-1 0 1 Voltage (V)

capacitance equivalent thickness (CET) and the physical thickness of high-j layers calculated from the number of ALD cycles on a sample cleaned at 500 °C for two different Al deposition times. The CET of GeO2/Al2O3 layer can be found from the linear interpolation of these data, from which the thickness of the alumina layer was estimated by extrapolating the CET at zero HfO2 thickness for 5 s and 10 s Al deposition with the difference being attributed to increasing alumina thickness. The alumina thickness was found to be 0.6 nm per 5 s and the thickness of GeO2 was calculated to be ~1.6 nm by using k values of 9 and 5 for alumina and GeO2 respectively. For the sample cleaned at 600 °C the thickness of GeO2 was reduced to 1 nm (EOT of 0.65 nm).

The CV plots of a sample with 10 s Al MBE exposure and 30 ALD cycles of HfO2 before and after FGA are shown in Fig. 8, which shows a steeper slope and larger accumulation after FGA. The steeper slope is assumed to be due to a reduction of interface states by the FGA. In the presence of interface states, the CV plot is broadened and cannot saturate to the oxide thickness in the accumulation region in the swept voltage range. This can explain the difference in oxide capacitance before and after FGA, corresponding to reduction of EOT of the gate stack from 1.7 nm to 1.3 nm by FGA. The hysteresis was slightly improved on some samples with FGA but degraded slightly on others. This observation is under investigation.

The CV plots shown in Fig. 9 are for samples with thermal clean at 500 °C and 600 °C with 130 cycles of HfO2 at small frequency of 1 kHz. The sample cleaned at 600 °C shows about 30% higher oxide capacitance which gives the EOT value of 2.3 nm, compared to the value of 3 nm for the sample cleaned at 500 °C. The reduction of EOT for the sample cleaned at higher temperature is an indication of lower thickness of GeO2.

4. Conclusions

ALD hafnia high-j dielectric gate stack was fabricated on Ge with alumina as the barrier level using combined MBE and ALD technique and have been characterised by physical and electrical techniques. The devices show low EOT down to 1.3 nm, low leakage current of less than 10~7 Acm~2 at ±1 V, and CV hysteresis of ca. 10 mV. The thicknesses of GeO2 interfacial layer and alumina barrier layer were estimated by comparing samples with different high-j thickness. The forming gas anneal indicates an improvement in the shape of CV plots due to reduction of interface states. Thermal cleaning at higher temperature reduces the thickness of GeO2 resulting in an improved EOT.

Acknowledgement

This work was funded by EPSRC, project number EP/1012907/1. References

[1] A. Dimoulas et al., Thin Solid Films 515 (2007) 6337-6343.

[2] S. Wang et al., J. Appl. Phys. 108 (2010) 054104.

[3] R. Zhang et al., Appl. Phys. Lett. 98 (2011) 112902.

[4] H. Li, Phys. Lett. 101 (2012) 052903.

[5] I. Hideshima et al., Curr. Appl. Phys. 12 (2012) S75-S78.

Fig. 9. CV plots of samples cleaned at 500 "C and 600 "C.