ECS Transactions [ECS 218th ECS Meeting - Las Vegas, NV (October 10 - October 15, 2010)] -...

Achievement of Excellent C-V Characteristics in GeO2/Ge System

Using Post Metal Deposition Annealing

H. Koumo, Y. Suzuki, Y. Oniki, Y. Iwazaki, and T. Ueno.

Department of Electrical and Electronic Engineering,

Tokyo University of Agriculture and Technology

2-24-16 Nakacho, Koganei, Tokyo 1840012 Japan

Impacts of structural transformations of top and bottom interface

of metal/GeO2/Ge structure on electrical properties were

investigated. The GeO2/Ge interface that was formed by low

temperature oxidation achieved negligible hysteresis because of

suppressing GeO volatilization. On the other hand, the metal/GeO2

interface with post metal deposition annealing attained

disappearance of VFB shift. XPS and TDS measurements show that

GeOx at the interface of the as-deposited metal/GeO2 vanished

after annealing. It suggests that the improvement of the VFB shift

was achieved by volatilization of GeOx including defects at the

metal/GeO2 interface.

Introduction

Germanium (Ge) is expected to be one of the candidates of channel materials for

future metal-oxide-semiconductor field -effect -transistors (MOSFETs) because it has

higher intrinsic carrier mobilities and smaller band gap than those of silicon (Si) (1).

However, compared with Si dioxide which has good chemical stability, Ge dioxide

(GeO2) has thermal instability and water solubility (2-4). Since deposition of high-

dielectrics on Ge channel is highly expected, GeO2 formation would be inevitable at the

interface because almost all kinds of high- dielectrics contain oxygen (5-6). Hence,

formation of high quality GeO2/Ge structure is an important challenge. One of the

deterioration factors of GeO2/Ge system is thermally GeO volatilization due to inter-

reaction (2),

)(2)()(2 gGeOsGesGeO [1]

GeO desorption leads to a critical deterioration of the interface properties such as an

increase in a negative flat-band voltage (VFB) shift and capacitance-voltage (C-V)

hysteresis. It has been reported that the improvement on electrical properties of the

GeO2/Ge structure is achieved by an annealing with a cap layer (7). Since this

improvement is not obtained by a post oxidation anneal (8), we must consider the

structure variations of not only the interface of GeO2/Ge but also that of metal/GeO2. In

this work, effects of structural transformations of both top and bottom interface of the

metal/GeO2/Ge structure on electrical properties have been investigated.

ECS Transactions, 33 (6) 111-119 (2010)10.1149/1.3487539 © The Electrochemical Society

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Experimental

p-Ge(100) substrates with a resistivity of 0.1 ~ 1 cm were cleaned by conventional

chemical process followed by diluted HF dipping. After each cleaning step, the wafers

were rinsed by de-ionized water and treated blow drying. Then, the samples were

immediately loaded into a furnace tube and thermally oxidized in O2 ambient at 500C

for 30 min to form GeO2/Ge stacks. In order to form good quality interface, some

samples was taken additional low temperature oxidation at 400C for 30min. After the

additional oxidation, Al and Hf metal thin films (~ 1.0 nm) were deposited on the

GeO2/Ge structures by using dc sputtering. Post metal deposition anneal of the

metal/GeO2/Ge stacks was carried out in ultrahigh vacuum (UHV) chamber. GeO

desorption from the fabricated structures were measured using thermal desorption

spectroscopy (TDS). The samples were also analyzed using X-ray photoelectron

spectroscopy (XPS). 1 MHz C-V characteristics of the GeO2 /Ge and the high-k/Ge

stacks were measured after Al electrode evaporation.

Results and Discussions

Since GeO desorption starts at around 420C at the bottom interface of GeO2/Ge (2),

formation of the GeO2/Ge interface using low temperature oxidation has been proposed

to suppress the GeO volatilization and to form a high quality interface. To investigate the

effect of low temperature oxidation, GeO2/Ge stacks were fabricated by thermal

oxidation of Ge at higher temperature of 500C with and without an additional

oxidization at 400C [Fig. 1(a)]. Figure 1(b) shows C-V curves of the Al/GeO2/Ge

capacitors measured at 1 MHz. Although the as-oxidized sample shows C-V hysteresis,

there are virtually no hysteresis in the sample with additional low temperature oxidation.

However, the VFB still shows a large negative shift from an ideal value in Al/GeO2/p-Ge

system (-0.34 V). These results indicate that the high temperature oxidation process

would mainly affect C-V hysteresis.

Fig. 1. (a) GeO2/Ge structures that were fabricated using thermal oxidation at 500C for

30 min with and without additional low temperature oxidation at 400C for 30 min. (b)

C-V characteristic of the GeO2/Ge structures were fabricated thermal oxidation. The

GeO2 film thicknesses of (1) and (2) were approximately 18 nm and 22 nm. These

thicknesses were determined by capacitance equivalent thickness (CET) calculated using

a -value of 6.0 for GeO2.

(a) (b)

ECS Transactions, 33 (6) 111-119 (2010)



To clear the origin of negative VFB shift, the impact of structural changes at

metal/GeO2 interface on C-V characteristics using annealing with a cap layer was

investigated. Here, Al and Hf films (~ 1 nm) were deposited on GeO2/Ge after the

additional low temperature oxidation. Then, the samples were annealed at low

temperature at 350C for 30 min in UHV condition. [Fig. 2(a)]. Figure 2(b) shows 1 MHz

C-V curves of Al/GeO2/Ge with and without metal deposition annealing process. The

results show that the negative VFB shift of as-oxidized sample is drastically improved

after the metal deposition annealing. Moreover, the increase of accumulation capacitance

is observed after the annealing. In this case, it is expected that the metals and GeO2 at the

metal/GeO2 interface interact with each other during the annealing process.

Fig. 2. (a) Stacks of GeO2/Ge and metal/GeO2/Ge. (b) C-V characteristics of GeO2/Ge

with and without thin metal deposition annealing at 350C for 30 min.

Chemical states of the top interface of metal/GeO2/Ge structures before and after the

metal deposition annealing were investigated by using angle-resolved XPS. Figure 3

shows that the comparison of Ge 3d spectra of Al/GeO2/Ge and Hf/GeO2/Ge structures

with and without annealing at 350C for 30 min in UHV condition. From no-annealed

metal/GeO2 structures, it clearly shows that GeOx peaks whose binding energies are from

29 to 31.5 eV appear at surface regions of GeO2 films. On the other hand, it is obvious

that sub GeOx peaks decreased except for that attributed to metal(M)GeOx bondings in

annealed metal/GeO2 structures. These results indicate that once the metals deposited on

GeO2, the surface layer including GeOx is formed at the metal/GeO2 interface. Then, the

GeOx changes to MGeOx after the annealing.

(a) (b)




Fig. 3. Ge 3d spectra of (a) as-deposited Al/GeO2/Ge structure, (b) with annealing, (c) as-

deposited Hf/GeO2/Ge structure and (d) with annealing. The insets also show that GeOx

peaks which was formed by metal deposition was disappeared and peaks based on metal

(M)-Ge-O bonding were increased by additional annealing.

These results suggest that the GeOx peaks originated from deoxidization of GeO2

surface induced by activated metals as indicated Fig. 4. It is considered that although this

surface layer had much defects due to GeOx for the origin of the VFB shift, the metal

deposition annealing process affected disappearance of the defects.

Fig. 4. Formation of surface layer on GeO2 by metal deposition. The surface layer

contains GeOx.




In general, the increase of accumulation capacitance is achieved by the increase of

dielectric constant of the film or thinning of the gate insulator film thickness. According

to the results of XPS analysis that GeOx peaks disappeared in metal/GeO2/Ge structure

with annealing, a possibility that GeO2 became thinner due to GeO volatilization is

expected. Therefore, we examined GeO volatilization from metal(1 nm)/GeO2/Ge

structures using TDS measurements. To investigate the GeO desorption, TDS

measurements were performed for the three samples; (1) GeO2/Ge, (2) Al/GeO2/Ge, and

(3) Hf/GeO2/Ge [Fig. 5(a) and (c)].

Fig. 5 (a), and (c) The structures of (1) GeO2/Ge, (2) Al/GeO2/Ge and (3) Hf/GeO2/Ge.

(b), and (d) GeO volatilizations from the structures. The mass numbers of 86, 88 and 90

were considered as the signals for GeO. Although the GeO signal was mainly observed

above 400C from GeO2/Ge system, metal/GeO2/Ge structures had two-stage GeO

volatilizations.

Figure 5 shows the TDS results corresponding to GeO, where the mass numbers of 86,

88 and 90 were taken into account as the GeO signals. The results shows that GeO

desorbed from GeO2/Ge interface above 400C, while GeO desorbed from

metal/GeO2/Ge in two stages from 300C. In metal/GeO2/Ge structure, there is a strong

possibility that the additional GeO volatilization at around 300C is not from GeO2/Ge

interface. Next, to find the origin of the additional GeO volatilization in metal/GeO2/Ge

systems, we measured GeO desorption from metal/GeO2/SiO2 and GeO2/SiO2 systems

that contains no GeO2/Ge interface.

(a) (b)

(c) (d)




Fig. 6. (a), and (c) The structures of (1) Al/GeO2/Ge, (2) Hf/GeO2/Ge, (3) Al/GeO2/SiO2,

(4) Hf/GeO2/SiO2 and (5) GeO2/SiO2. (b) and (d) GeO volatilizations from the structures.

The lines of metal/GeO2/SiO2 structures laps over first step of GeO desorption for

metal/GeO2/Ge structures.

Figure 6 shows comparisons of GeO volatilization from metal/GeO2/Ge,

metal/GeO2/SiO2 and GeO2/SiO2 structures. The results indicate that the additional GeO

volatilization occurred at the metal/GeO2 interface. Therefore, it is expected that the

metals operated as catalyst for GeO volatilization at the interface of metal/GeO2.

Considering the results of C-V characteristics, XPS and TDS measurements, a

following model for VFB shift of metal/GeO2/Ge system are proposed.

(a) (b)

(c) (d)




Fig. 7. Models for structural transformation of surface region in GeO2/Ge from metal

deposition to annealing. (a) Metal deposition, (b) Appearance of positive charge due to

charged elemental Ge, (c) GeO including positive charge desorption due to annealing and

(d) Disappearance of charge because of urging combination of M-O-Ge.

Fig. 8. Comparison of band diagrams between (a) Ideal metal/GeO2/Ge structure and (b)

The defects-containing structure. The positive charge due to defects induced negative VFB

shift for MOS capacitors.




Figure 7 shows the schematic models of structural transformation at top interface of

metal/GeO2/Ge by a metal deposition annealing. When the metal films deposited on

GeO2, activated metals deprives oxygen of GeO2. In this case, elemental Ge has positive

charge. Figure 8 shows that comparison of band diagrams between ideal metal/GeO2/Ge

structure and that containing defects. The negative VFB shift should attribute to the

positive charges. However, once the samples were annealed, the metals could be a

catalyst for occurrence of GeO volatilization at the metal/GeO2 interface. Then, the GeO

including positive charge were volatilized with formation of MGeOx. As a result, the

accumulation capacitance increased due to the increase of dielectric constant that was

based on metal-O bonding and thinning GeO2 film.

Finally, we tried application of scaling for the GeO2 film thickness to the metal

deposition annealing process. HF-last p-type Ge (100) substrates were thermally oxidized

at 400C for 3 hours in a furnace to form GeO2/Ge. The CET value of the structure was

6.5 nm. Hf (1 nm) deposited on some samples, and metal deposition annealing at 350C

for 30 min was operated in UHV chamber (in situ). And gate electrode and back surface

contact of all samples were formed by conventional vacuum evaporation of Al. Then, C-

V measurements were performed.

Fig. 9. (1) C-V characteristic of GeO2/Ge structure that was thermally oxidized at 400C

for 3 hours. (2) C-V characteristic of GeO2/Ge structure added post Hf (1 nm) deposition

annealing at 350C for 30 min. The C-V curve of (b) is very close to ideal curve, and

EOT is 2.9 nm.

Figure 9 shows 1 MHz C-V curves of GeO2/Ge with and without Hf deposition

annealing. Compared with as-oxidized sample that has little hysteresis but large negative

VFB shift, the sample with Hf deposition annealing achieves ideal VFB value. Hence, these

techniques may be useful for formation of the high-/Ge gate stack.




Summary

We investigated the C-V characteristics of metal/GeO2/Ge structures whose both top

and bottom interface were controlled. The low temperature oxidation was effective in

decreasing C-V hysteresis because of suppression GeO volatilization at the interface of

GeO2/Ge. On the other hand, VFB shift was vanished by metal deposition annealing.

According to XPS and TDS measurements, it is expected that although the defects based

on GeOx at the interface of metal/GeO2 that were formed by activated metal, the metal

deposition annealing urged MGeOx formation and GeO including defects volatilization.

This metal deposition annealing technique may be useful for formation of High-/Ge

structure.

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