Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA13 / P25
Formation of conductive layers on singlecrystalline diamond by Excimer laser
irradiations
Eslam Abubakr1,2*, Abdelrahman Zkria1,3, Hiroshi Ikenoue4, and Tsuyoshi Yoshitake1
Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka816-8580, Japan2
Department of Electrical Engineering, Faculty of Engineering, Aswan University, Aswan 81542, Egypt
3
Department of Physics, Faculty of Science, Aswan University, Aswan 81528, Egypt
4
Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka 819-0395, Japan
E-mail: abubakr_eslam@kyudai.jp
1

Abstract: For formation of conductive layers on singlecrystalline diamond, excimer-laser-induced surface doping
technique was employed to singlecrystalline diamond substrates. Nano-second pulsed laser with a wavelength of 193 nm
was applied and irradiated upon immersed substrate in boric acid which acts as a dopant source. Surface conductivity
progressively enhanced with increasing laser fluence and number of laser shots. Furthermore, resistivity decreased withincreasing temperature, which implies that the surface layers generated by laser irradiations are semiconducting. Depth
profile from secondary ion mass spectroscopy showed successful incorporation of dopant species up to 40 nm depths
below the surface. Results are promising as a new method for surface doping of single crystalline diamond.
Keywords: ArF excimer Laser; singlecrystalline diamond; Boron doping.
1. INTRODUCTION
Diamond is a promising wide bandgap semiconductor
with outstanding electrical and optical properties,
applicable to devices that handle more power with higher
efficiencies than those of conventional semiconductors[1-3]. Owing to its favorable inherently physical andelectrical properties such as a high breakdown voltage
combined with high mobility, negative electron affinity,
small dielectric constant, and prime radiation hardness.
Thus, diamond is an ideal candidate for electronic andoptical applications such as ultraviolet light emitting
diodes (UV-LED), cold cathode electron emitters, andhigh-power and high-frequency devices [1].
The fabrication of diamond devices has serious
problems in doping. It is extremely difficult to form
ohmic contacts on doped diamond as well. In addition, it
requires relatively high temperature treatments, which isharmful for devices that cannot undergo hightemperatures. for diamond, the doping must be achieved
during the crystalline growth by chemical vapor
deposition (CVD) [4], which is completely different fromother existing semiconductors that can be treatedthermally after deposition.
Recently laser-induced doping in acid liquids havereceived attention as a new doping method [5]. Since it
can provide a low thermal-damage owing to its local andvery short heating mechanism. In this work, we appliedlaser-induced doping method to singlecrystalline
diamond, for the first time to our knowledge. Doping ofboron into singlecrystalline diamond is discussed fromthe chemical compositionally and electrically viewpoints.

to ensure full coverage of irradiated area. The shape ofthe laser beam was rectangular with a size of 200 µm ´
400 µm. The laser beam irradiation was carried out atdifferent frequencies and different laser fluences.

CCD
ArF excimer laser(193 nm)
SlitMonitor
Illumination light

DUV
MirrorLens
Dopant liquid

SCD

XYZ stage

Figure 1. Schematic diagram of experimental setup
for laser-induced doping.
Current-voltage characteristics of irradiated areas wereperformed on the range of (-10 to 10 V at different
number of shots and temperatures using a two probe
source meter (Keithley 2400) after the cleaning of thesurface twice with a boiling acid (H2SO4:HNO3[3:1]) at
220° C for 50 min to ensure graphitic layer removal fromthe surface. Probe heads were directly contacted with thesample surface. The incorporation of boron atoms into
diamond was investigated by Secondary Ion Mass
Spectroscopy (SIMS).

2. EXPERIMENTAL PROCEDURES
Singlecrystalline diamond (100) plates (Ib) (Sumitomo
Electric Industries, high-pressure and high-temperature
conditions of 5 GPa and 1,300 ºC or higher (HPHT))
were immersed in the boric acid solution (H3BO3 (2%).
and ArF excimer laser (Gigaphoton Inc. wavelength: 193nm) beams were irradiated on the immersed diamondsubstrates as schematically shown in Figure 1. Dopantliquid was only few millimeters above the sample surface

3. RESULTS AND DISCUSSION
Figure 2 illustrates image of singlecrystalline diamondafter irradiation with different frequency and number ofshots. Obviously, the laser-irradiated areas have different
colors as compared with non-irradiated areas, and the- 33 -color gradually become dark with increasing number ofshots. Black debris on the irradiated areas, which might
be graphitic, were peeled off from the surface by theboiling acid treatment. Results shows thatsinglecrystalline diamond hardly damaged even at amaximum fluence of 5 J/cm2.

Figure 2. Optical images of areas laser-irradiated at 5
J/cm2 and different irradiation conditions.

Figure 4. SIMS depth profile of Boron doped
Singlecrystalline diamond.

I-V curve measurements indicated that the electricalconductivity increases with increasing fluence andnumber of laser shots. Activation energy was estimatedfrom the temperature dependence of the electricalresistance. The activation energy decreases withincreasing shot number, which might be attributed to anincrease in the B/C ratio as shown in Figure 3.

The process here is different from thermal doping method;
it is most likely photochemical doping. the process starts
with the absorption of the laser pulse by the substrate, due
to fast electron-phonon energy transfer, we assume thatthe photons energy is locally and instantaneously
converted into heat then a plasma plume expands into thesurrounding liquid, accompanied by the emission of ashockwave through the substrate itself. During this timeand before ultra-fast quenching, the surface remains
ablated for a short time which was proved to be same aslaser pulse duration (20 ns) during which reaction withdopant source and incorporation occurs.
4. CONCLUSION
Laser-induce doping in liquids containing dopant
elements is applied to singlecrystalline diamond. It
certainly increased the electrical conductivity of thesurface. Although the evidence of substitutional doping
is not obtained, it should be applicable to facilitating theformation of ohmic contacts between n-type/p-type
diamond and metallic electrodes. This method has apotential for exploring new dopant elements that have notexperimentally investigated due to difficulties in thedoping during the deposition so far.

Figure 3. Arrhenius plot of electrical conductivity for
laser-irradiated substrate at different shot numbers.
This evidently indicates that laser-irradiated surface issemiconducting. Which means that the laser-induced
doping might be beneficial for facilitating formation ofohmic contacts between diamond and electrodes.
Boron incorporation depth profile was investigated by
SIMS. The measurements were carried out near thecenter of the irradiated area. Curves evidently reveals theincorporation of boron to singlecrystalline-diamond bylaser irradiation in a depth range from 0 to 40 nm. It is
important to consider that the sample used for SIMS
measurements was irradiated at a lower laser focusing
compared with all other samples used for electricalmeasurements. The sample prepared at higher laser
focusing has conductivity that is one order of magnitude
higher than that prepared at lower focusing (used forSIMS) under same irradiation conditions. With focus
enhancement, a higher concentration and slightly deeper
incorporation is expected. This demonstrates that laserinduced doping is applicable to boron doping of
singlecrystalline diamond.

5. ACKNOWLEDGEMENT
This study was partially supported by JSPS KAKENHI
Grant Numbers JP15H04127, JP16K14391, JP17F17380.
6. REFERENCES[1] S. Shikata, Single crystal diamond wafers for highpower electronics, Diamond Relat. Mater 65 (2016)
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[5] A. Ikeda, K. Nishi, H. Ikenoue, and T. Asano,
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