電気通信大学大学院電子工学専攻光エレクトロニクス講座
博士課程前期修士論文資料
Investigation of carrier dynamics in CVD diamond thin filmwith ultra-short pulse laser
光エレクトロニクス講座

0330057

主指導教官：米田助教授

Wijaya Hennywati

副指導教員：植田教授

1. Introduction
Chemical vapor deposited (CVD) diamond is expected to be used for ultra-fastelectric-optical devices. Because of its high breakdown threshold, high carrier saturated
velocity and low dielectric constant properties; attractive features are achieved for highelectrical field application, such as emitter of electromagnetic wave radiation (THz) [1]. For
these applications, it is important to understand the carrier dynamics of CVD diamond.
Specially, it has the grain boundary that is considered to affect largely on the dynamics.
Therefore, the purpose of this study is to clarify the details of carrier dynamics in thegrain boundary of CVD diamond under high electric field. There are also diagnostics tools
are developed, such as micro Raman system having an ultra fast laser pump-probe ability,and deep ultraviolet radiation with several other photoconductive monitors.
CCD camera

170mm

Beam
splitter

lens

40mm

150mm
microscopereflective
objective lensLED

90mm

Beamsplitter

Raman
Spectrometer

microscope
objective lens

95mm

51mm
24.5mmz6mm

yx

Fig. 1 Image and schematic of pump-probe micro Raman measurement system

1photomultiplier2. Experimental setup
A. Micro Raman measurement system
For investigation of grain boundary, with ultra-fast temporal resolution, the micro
Raman measurement system using various pump-probe lasers are constructed.
In this system, there are two set of micro-optics for UV pump and visible –IR probebeam. They have 2.6 µm spatial resolution for visible and IR with refractive objective and 4µm x 10 µm resolution for UV with reflective objectives. This resolution is high enough todistinguish the difference between grain boundary and the grain bulk material for highmobility CVD diamond. The resolution of microscope objective is 2.6 µm and 4 µm x 10µm resolution for reflective objective was achieved. In order to detect small signal of Ramanspectrum, double monochromator as Raman spectrometer, photo multiplier and dual phaselock-in amplifier were used. The sizes of Raman spectrometer are 90 cm x 40 cm x 55 cm
and its resolution and reproducibility are 1 cm-1 and 0.3 cm-1, respectively. About 30 – 50
kV/cm of electric field was applied to the sample. Fig. 1 shows the image and schematic ofpump-probe micro Raman measurement system.
Diamonds having different grain boundary features were investigated with this system.
There are diamonds that have different size of grain (100, 5 µm) and single natural crystals.
In addition, in order to change the grain boundary conditions, Li-ions also doped into CVD
diamond with thermal diffusion method (LiO2, T=500oC) for 4, 10, 20, 40 hours. Fig. 2
shows the CVD diamond samples that used in this experiment. In large grain size CVD
diamond, 21 detection points on the sample with 8 µm distances, started from position neargrain boundary that is clearly seen, were detected. Only 1 point, which is located between
electrodes, was detected in 5 µm grain size CVD diamond film. The distance between theelectrodes is 10 µm. There were 2 detection angles of Raman signal, as seen in Fig. 3. One,detected by microscope objective, has 70o angle from pump laser and another one having
115o angle was detected by lens.

(a)

10µmelectrode

undoped diamond

40h–doped diamond(b)(c)
Fig. 2 Diamond samples (a) large grain size of CVD diamond (100 µm grain size), (b) smallgrain size of CVD diamond (5 µm grain size), (c) undoped and 40 hours Li-doped CVDdiamond2θ=70oθ =115o

65o

20ks kso

70 k
i
20o+E
sample+E-E-E(a)(b)
Fig. 3 The image (a) and schematic (b) of detection angle of Raman signal

B. Ultra fast pulse laser
Ultra-fast Ti-Sapphire laser system was also been set up. Fig. 4 is ultra fast system
image and its schematic. Argon-ion laser with 2-3 W power as pump laser and 2 mm ofTi-Sapphire crystal were used.

150

Prism forGVD
mirror270 dispersion
470

compensation
130
130

GVD
mirrorconcavemirror
Ti:Al2O3
crystalconcavemirrorOutputCoupler

PrismsSlitHighreflection
mirror230

GVD
mirrorConcavemirror80 R=100d=2Concavemirror
R=100

Ti:Al2O3
f=-50

Output

f=250

f=76

z=102〜107

GVD
mirror

CouplerPump
3W
Ar-ionlaser

outputHighreflectorslit
3806201000(a)(b)
Fig.4 The image (a) and schematic feature (b) of ultra fast Ti-Sapphire laser system

3. Experiment and Result
The typical Raman spectrum of single crystal natural diamond without electric field
located at 1332 cm-1 wave number. The Full Width Half Maximum of this spectrum is 4.5
cm-1. In natural single crystal diamond, we applied 2 directions of electrical field, positive3

direction that has same direction with pump laser and negative field that has the opposite
direction. Fig. 5a-d is the shifted of Raman peak due to applied electrical field. If weassumed 0 position is the position without electric field condition and red bars are errors due
to spectrometer reproducibility, we made the schematic result of Fig. 5, as seen in Fig. 6.
(a)
1(b)

10 .1 6 70 .9 5c m

- 10 . 1 1 7c m

- 10 . 9 50 .90 . 90 .8 50 .8
1 3 2 91 3 3 2w a v e n u m

b e r

( c m

-10 . 8 5
1 3 3 01 3 3 5)w

a1 3 3 2v en um

1 3 3 4b er

1 .1

1 .1

0 .2 4 5c m

- 10 .1 3 3←(c)

1c m- 1( c m)-11(d)→
0 .9

0 .9

0 .8
1 3 2 91 3 3 2w a v e n u m

b e r

( c m

-10 .81 3 2 91 3 3 5w)

1 3 3 2a v e n u mb e r( c m- 11 3 3 5)

Fig. 5 Comparison of Raman spectrum with (red) and without (blue) electrical field innatural diamond sample from 70o angle detection with +E (a), with –E (b), from 115o angle
detection with +E (c), with –E (d)detection angle (θ=1150)
detection angle (θ=700)-0.167-0.245

0.117
0.139

0.139

-0.03 00.053
Spectrometererrors

positive E-0.133
0.053
-0.20

negative E(a)(b)
Fig. 6 Schematic result of spectrum shifted in natural diamond with +E (r) and –E (b)from 70o (a) and 115o (b) detection angle

According to Fig. 5 and 6, two types of origin for shifting the Raman peak wereobserved with applied electrical field. One has E-field directional dependence and the other
only has field strength dependence. The possible explanations to analyze these shifted are
the interaction between the carriers and phonon induce phonon shift and the stress that isinduced by applied electrical field.
Generally, Raman shift near the grain boundary are considered due to the residual stresschanged in the polycrystalline crystals [2], also seen in Fig. 7b in without electrical fieldcondition. Effect of electrical field was also observed in CVD diamond samples, as seen inFig 7 and 8. This measurement data (Fig. 7a) shows the grain boundary at position 0 has
smaller peak intensity and it is also observed at position 48 µm and 104 µm. We assumed
that these positions are also grain boundary areas. Therefore, the grains having 48 µm and4

56 µm were detected. Full Raman spectrum at specific positions (Fig. 7b) shows that Raman
peak position near grain boundary at position 48 µm changed by applied electrical field. To
clarify the physical meaning of this effect, need farther investigation such as UV
pump-probe experiment or direct measurements of carriers.
Num ber of scan (a.u.)

N umbe r of s can (a.u)
2.2-4-202468101214161332.8

02016468

101232486480

9621332.6
1.8
1.6

1332.41.4

1332.2

1.2
11332
0.8
0.6-3 2 -1 601632

48

64

80

96

112 128

1331.8

Pos ition (m icrom eter)

Po sition (micro meter)

(a)

(b)

Fig. 7 (a) Scanning Raman peak measurement of large grain CVD diamond sample with 8µm distance (b) Raman peak position with (green) and without (pink) E from 70o detection
angle
0 .2 80 .5 8c m-10 .2 2 5 c m0 .1 8-10 .2 60 .2 40 .1 40 .2 20 .2
1 3 2 80 .1
1 3 3 21 3 3 6133013341338-1w a v e n u m b e r ( c m) (b)(a)Fig. 7 Raman peak position of small grain CVD diamond sample with (red) and without(blue) E from 70o (a) and 115o (b) detection angle

In addition, photocurrent experiment for micro spot illumination of 450 nm light atdifferent positions is shown in Fig. 8a. This signal is proportional to the carrier density andmobility. According to Yoneda, et.al. [3, 4], lifetime and mobility are smaller near grainboundary. If we assumed E is uniform, so the increasing of photocurrent at grain boundary
may be contributed by generation efficiency
Li-doped samples (Fig. 8b) have another evidence for carrier mobility and energychange of the grain boundary. From microscopic view of doped samples, the Li doped
density is maximized at the grain boundary. With help of previous experiments about
lifetime feature in the grain [3, 4], this result denoted carrier generation efficiency under the5nominal band gap photon increased with Li-doping. Therefore, band edge energy structure
will be changed.
P h o to c u rre n t m e a su re m e n t
w it h p o s iti o n d ep e n d e n c e

Absorbance of diamond sample
514

10

undoped diamondabsorb-dia1-G20
4 hours Li-doped
absorb-no-norm-4h
10 hours Li-doped
absorb-no-normalize-10h
20 hours Li-doped
absorb-20h-1
40 hours Li-doped
absorb-doped-40h

12410101000

8

6100
42-4 8 -3 2 -1 60163248648010100

9 6 11 2 1 2 8P o s it io n (m ic ro m e te r)

200300400500600

700

800

900

wavelength (nm)(a)(b)
Fig. 8 (a) Photocurrent measurement of large CVD diamond sample at different positions(b) Absorption spectrum of Li doped sample
In other hand, 30 mW ultra fast Ti-Sapphire with 19 ns (530 MHz) could be achievedby 2 W pump power. This smaller power might be caused by cavity losses, mode
mismatching position inside the crystal, losses in prisms, air disturbance and vibration. In
order to have stable and higher output power, the using of Brewster angle prisms, instead of
60o apex angle, might have an effect. Reducing air disturbance by covering the laser is alsosuggested.

4. Conclusions
New types of shifts were observed under electrical field; E-field directional dependence
that connects to carriers and E-field strength dependence that induced stress. Raman shift issignificant near grain boundary in CVD diamond by applied electrical field. To clarify thephysical meaning of this, investigation such as UV pump-probe experiment or direct
measurements of carrier is needed. Li doping results denoted carrier generation efficiencychange. It is considered that the energy band structure can be controlled selectively near thegrain boundary.

5. References[1] H.Yoneda et. al., Appl. Opt., 40-36, 6733 (2001)[2] M. C. Rossi, Appl. Phys. Lett. 73-9 (1998)[3] H. Yoneda et.al., Appl. Phys. Let. 77, 10 (2000)[4] H. Yoneda et.al., J. Appl. Phys. 83, 3 (1998)
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