Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia StrategyP-27

Development of Cleaning Solution for Germanium (Ge) Wafer
Sampad Ghosh1, Keisuke Yamamoto1, Hiroshi Nakashima2 and Kiichi Hamamoto1
1

Interdisciplinary Graduate School of Engineering, Kyushu University
2
Global Innovation Center, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

Abstract
In this paper, we worked on development of cleaning solution for Ge surface and characterizationfrom the electrical point of view. Samples supplied from two companies (Umicore and TDYCorporation) were cleaned by three different cleaning solutions (HF, HCl, and mixed HF-HCl), andsubsequently heated by an electric furnace. Carrier mobility and bulk carrier concentration weremeasured by Hall system and compared these values for different samples. Cleaning by HCl gives
better performance than HF and mixed HF-HCl solutions.
Corporation (four from each company). All
samples are degreased by acetone withultrasonic for 3min, and then washed fivetimes in running deionized (DI) water
followed by DI water with ultrasonic for
2min, and washed in running DI water. Three
concentrations of HF (45% HF: DI
water=1:5), HCl (35% HCl: DI water=1:1),
and mixed HF-HCl (45% HF: 35% HCl: DIwater=1:1:5) are prepared. Two samples fromdifferent vendor are immersed into HF
solution for 1min and then washed in running
DI water. Again these samples are dipped into
HF for another 1min and washed. This
procedure is repeated for five times. Other
two pairs of sample are immersed into HCl
and mixed HF-HCl solution respectively for1min. Subsequently washed in running DI
water for five times. Finally, all samples are
dried by N2 gas and placed into an electricfurnace. Heat treatment is performed at
6000C for 30min in presence of N2 gas insideof the chamber.
2.2. Electrode Formation
Electrode is formed on cleaned wafer
for further measurement and Titanium nitride(TiN) is suitable for making Ohmic contact in

1. Introduction
Semiconductor materials has many
applications in modern devices because theirelectrical properties can be easily changed byadding impurities and silicon (Si) substrate iscommonly used for these applications.
However, the scaling of advanced MOSdevices is approaching its technological andfundamental limits. [1] Because continuous
reduction in the process technology and gate
length causes leakage current flows throughthe gate and mobility deration problems inthe substrate. Therefore, new high-k material
and high mobility substrates are mandatory.
Many researchers have found several
candidates [2-4] of high mobility channelmaterial including germanium (Ge). Ge
offers higher hole and electron mobility thanSi [4], and has good affinity with Si. In orderto fabricate the devices on Ge, surface
contaminants should be in minimum levelswith minimal increase in surface roughness.
There is well established cleaning
method [5] for Si surface. For Ge surface
cleaning, scientists have applied various
chemicals [6] to remove contaminants as wellas used different equipment [7-9] to measuresurface roughness. Till now there is no widely
accepted cleaning method for Ge surface.

Running DI water

2. Methods
2.1. Cleaning Procedures
Eight 1x1 cm2 n-type Ge wafers arecollected from Umicore and TDY

N2 blow

3min

3min

1min

GeGeGeAcetone

DI water

30min
600C
600C
Ge
600C

Solutions
HeatingcetoneFig 1. Flow of Ge surface cleaning
n-type Ge, and is formed by sputtering. Ar73

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

P-27
and N2 gases of 30 and 0.1sccm respectively
are introduced when the chamber waspumped down to vacuum level. After
addition of these gases, set pressure to 6Pa
with RF power 200W for 1min 30sec to grow
27nm of TiN on Ge samples. TiN wasdeveloped close to the four corners by thehelp of a metal mask that had four holes (Fig.
2a). For better probe connection andmeasurement, aluminum (Al) is deposited onTiN position because the grown TiN is hardand is done by evaporation technique after
placing small Al pieces into the evaporator.
The machine is turned OFF, once 200nm
thick Al is deposited on TiN (Fig.2b).
3. Results and Discussion
Samples that experienced chemical cleaningand heating are placed into a Hall system, andobtained data from electrical point of view.
The measured data contained different
parameters. Among them, mobility and bulk
carrier concentration are important becauseothers can be calculated by putting thesevalues into numerical equations andcomparison of these values between samples
gives the impact of different cleaning
solutions. Data are measured for one pair ofsamples without cleaning and referred to as‘As received’, and is used to compare withthe data that obtained after cleaning. Fig.3
and 4 shows the majority carrier mobility andbulk carrier concentration for Umicore andTDY samples respectively.
From Fig.3, it can be clearly seen that
HCl cleaning offers comparatively higher
values of mobility than HF and mixed HFHCl cleaning solutions for both companies.
However, this values is smaller than samples
value when no cleaning solution and heating
are employed. In Fig.4, both HF and mixed
HF-HCl cleaning solution exhibits higher

Electrode

(a)

(b)

Fig 2. (a) Metal mask, (b) Top view of a sample

Fig 3. Carrier mobility of samples after cleaningFig 4. Carrier concentrations after cleaning

values of carrier concentration than HCl.
Higher values of carrier concentration is dueto the effect of contaminants i.e. less removalof contaminants from the surface. Mixed HFHCl solution gives worst results in both casesof mobility and carrier concentration.
4. Conclusion
Three different chemical solution wasemployed for Ge surface cleaning and foundHCl solution brought better impact for Ge
cleaning in terms of mobility and carrier
concentration regardless of the samples
provided by two companies.
Acknowledgment
Thanks to Green Asia to give chance todo this work in Nakashima laboratory,
Kyushu University as a part of lab rotation.

[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]

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Email: sampad04cuet@yahoo.co.in

after electrode formation74