Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-26

Characterization of near-interface border-traps in GeO2/Ge gate stacks grownby thermal oxidation using deep-level transient spectroscopy
Wei-Chen Wen,1 Keisuke Yamamoto,1 Dong Wang1 and Hiroshi Nakashima2
1Interdisciplinary Graduate School of Engineering, Kyushu University
2Global Innovation Center, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan,Abstract
We developed a method for characterizing the near-interface border-traps (BTs) in GeO2/Gegate stacks using deep-level transient spectroscopy. The BT density (NBT) in p-MOS grown bylow temperature oxidation is smaller than that by high temperature oxidation. By contrast, theNBT in n-MOS is almost the same regardless of the oxidation temperature. In addition, the NBT
in p-MOS is drastically decreased by Al post metallization annealing (Al-PMA), but the NBT
in n-MOS is not decreased. These results suggest that the species of BT in n-MOS aredifferent from that of p-MOS.
optional Al-PMA was carried out at 300°C
for 30 min in N2. The equivalent oxide
thicknesses (EOTs) of the MOSCAPs with
PTO at 550 and 425°C were ~19.5 and 16.8
nm, corresponding to GeO2 thicknesses of6.6 and 2.6 nm, respectively.
DLTS measurements were performed
using a lock-in integrator. Pulse frequency
f=10 Hz were used, which corresponds toBT position z0=1.4 nm from the interface for
p-MOS and z0=2.0 nm for n-MOS.

1. Introduction
A high-quality gate-stack formation on
Ge is essential for high-performance GeMOSFETs. Although a GeO2 is effective inreducing the interface-traps (ITs), a large
amount of border-traps (BTs) causes serious
problems. In this study, we established amethod of BT characterization by deep-leveltransient spectroscopy (DLTS), andinvestigated BTs in two kinds of MOS
capacitors (CAPs) with a structure ofSiO2/GeO2/Ge. One was a relatively thick
GeO2 by thermal oxidation at 550°C, atwhich GeO2 volatilization may occur at theGeO2/Ge interface.[1] The other was a thin
GeO2 by the oxidation at 425°C, at which
the volatilization may not occur. The Al post
metallization annealing (PMA) effect on BTpassivation was also studied.

3. Results and disscussion

IDLTS (1/pF2)

2.0x10-5 p-MOS with 550oC-PTO w/o PMA

2. Experimental
After substrate cleaning, 1 nm-SiO2/1
nm-GeO2 bilayer passivation was performed[2], followed by a post thermal oxidation(PTO) at 550°C for 15 min or 425°C for 9 hin O2 ambient. Next, 14-nm-thick SiO2 wasdeposited on the both samples, followed bya post deposition annealing at 400°C for 30
min in N2. Then, an Al gate film wasdeposited on the SiO2 surface by thermalevaporation. Before electrodes formation, an

1.5x10-580 K
100 K
120 K

1.0x10-5
5.0x10-6
0.0012345|VP| (V)Fig. 1 IDLTS dependence on VP for a p-MOS
with PTO at 550°C without Al-PMA.

Figure 1 shows DLTS signal intensity(IDLTS) dependence on pulse height (VP) witha maximum pulse width (tw) of 5 ms at
different temperatures (Ts) for a MOSCAP
with PTO at 550°C and without Al-PMA.

71

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-26
5. Conclusion
Near-interface BT in GeO2/Gestructures were characterized by DLTS. The
Nbt in p-MOS were drastically decreased byAl-PMA, but the Nbt in n-MOS was notdecreased. This is a reason for difficulty ofthe electron mobility enhancement.
oNbt (cm-3)

p-MOS with 550 C-PTO
1019 (a)o(b) p-MOS with 425 C-PTO

w/o Al-PMA

w/o Al-PMA

1018
w/ Al-PMA
1017

w/ Al-PMA
EAP= 2 MV/cm

EAP= 2 MV/cm

1019 (c)Nbt (cm-3)

Let us pick up the result at 80 K as anexample. The IDLTS drastically increased
when the VP increased from 0 to 0.5 V. Note
0.5 V is approximately the VFB at 80 K for
this sample. Therefore, this increase isdominated by the capture process of IT.[3]We believe that the weakest injection
condition (tw=2 µs) under pulse intensity
EAP=0 MV/cm is enough to compose the ITsignals. (EAP =VAP/EOT, where VAP is theapplied voltage above flat band voltage.)
Therefore, we use these data for IT density(Dit) calculation. In the case of VP > 0.5 V,
namely after the accumulation wasestablished, the increase in IDLTS with VP >0.5 V was governed by the capture processof BT because of an increase in holenumbers at Ge side of the GeO2/Ge interface.
After subtracting the IT signal from IDLTS,
we calculate the BT density (Nbt). The
details of which will be presented in theforum.
Dit characteristics (not shown) arealmost identical for both the MOSCAPs,
implying that the interface property is not
influenced by GeO2 volatilization. The
Al-PMA is effective in decreasing Dit near
the valence band edge but not near theconduction band edge (even worse), whichis consistent with our previous experimental
results of Al-PMA effects on Ge-MOSFET
performance.[3, 4]
Figures 2(a) and 2(b) show dependenceof Nbt on T for p-MOSCAPs with PTO at
550 and 425°C, respectively. Figures 2(c)and 2(d) represent the similar results forn-MOSCAPs with PTO at 550 and 425°C,respectively. It was confirmed from theseresults that 1) the Nbt for p-MOS with
425°C-PTO is a half of that with550°C-PTO
under
EAP=2MV/cm,suggesting BT in p-MOS is associated withGeO2 volatilization; 2) the Nbt for p-MOS isdrastically decreased by Al-PMA. The Nbts
for n-MOS with PTO at 425 and 550°C (notshown) are almost the same and notdecreased by Al-PMA, suggesting thespecies of BT in n-MOS are different fromthat of p-MOS.

n-MOS with 550oC-PTO
w/o Al-PMAo(d) n-MOS with 425 C-PTO
w/o Al-PMA

w/ Al-PMA

w/ Al-PMA
1018

1017

EAP= 2 MV/cm

EAP= 2 MV/cm

8080

90 100 110 120 130 140

Temperature (K)90 100 110 120 130 140

Temperature (K)Fig. 2 Dependence of Nbt on T for p-MOSwith(a)550°C-PTO

and(b)

425°C-PTO. Dependence of Nbt on T
for n-MOS with (c) 550°C-PTO and(d) 425°C-PTO. The results withAl-PMA are also shown.

Acknowledgment
This study was partially supported by(JSPS) KAKENHI (Grant No. 26289090).
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Email: 3ES17010E@s.kyushu-u.ac.jp72