Short Articles

Enhanced Terahertz Emission from
CuxO/Metal Thin Film Deposited on
Columnar-Structured Porous Silicon
Xu Lu,1 Yohei Ishida,1 Tomobumi Mishina,2Mai Thanh Nguyen,1 and Tetsu Yonezawa*1
1Division of Materials Science and Engineering,
Faculty of Engineering, Hokkaido University,
Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628
2

Department of Physics, Graduate School of Science,
Hokkaido University, Kita 10 Nishi 8, Kita-ku,
Sapporo, Hokkaido 060-0810
E-mail: tetsu@eng.hokudai.ac.jp
Received: May 19, 2015; Accepted: July 6, 2015;
Web Released: July 13, 2015

We propose a novel method to enhance the THz emission from CuxO/metal nano thin ﬁlm by using columnarstructured microsized porous silicon (MS-PSi) as the substrate. THz emission was improved several times higher thanthat from the planar substrate due to the decreased opticalreﬂectance and enlarged surface area.

Introduction
Terahertz (THz, frequency range: 0.1­10 © 1012 Hz) radiation is a promising technology in spectroscopy, imaging, andbiological studies.1­3 Thus, THz emitting materials haveattracted much attention in the last two decades. Nanostructured
materials have been reported as promising candidates for THzemitters, owing to their exceptional properties.4­11 In particular,eﬃcient THz emission has been reported for metallic nanostructured thin ﬁlms and nanoarrays.5,7­10 Recent studies havealso reported the use of surface-modiﬁed indirect semiconductors and metal/semiconductor composite as THz emitterswithout an external power supply.12­14 Among them, Jung
et al. observed the THz emission from Si nanowires,12 andRamanandan et al. investigated plasmon enhanced THz emission from Cu2O on nanosized grating Au.14 In their previousstudy, it was found that the optical absorption in the built-inﬁeld is very important for THz emission in their model.
To decrease the optical loss at the greatest extent, in ourcase, a micro-columnar Si substrate for the deposition of metal­
metal oxide layers was used. Reduced reﬂectance and increased
surface area, arising from the columnar structure can improve
the optical absorption, leading to the increased THz emission.
In this study, based on this approach, we report the enhanced
THz emission from CuxO (x ¯ 2)/AuCu thin ﬁlm deposited on
the columnar-structured microsized porous silicon (MS-PSi).
The THz emission amplitude for the ﬁlm grown on this subBull. Chem. Soc. Jpn. 2015, 88, 1385–1387 | doi:10.1246/bcsj.20150173

strate was several times stronger than that on a planar substrate.
The AuCu and CuxO interface will create a space charge region(SCR). The source of the THz emission is the transient current
induced by the motion of photon-generated carriers driven bythe built-in ﬁeld in the SCR. Because Si, Cu, and Au arecommonly applied to microelectronic chips as the basic logical unit and intermediate layer for electronic adjustment, theenhanced THz emission from CuAu composite on MS-PSi
substrate can make THz technology integrate potentially withmicroelectronic devices.
ExperimentalThe columnar-structured microsized porous silicon (MSPSi) substrate was prepared by an electrochemical anodization.15,16 B-doped (doping concentration is 1.0 © 1019 cm3),©111ª-oriented silicon wafer was used as the anodized electrode(Nilaco, Japan), and Pt was used as the cathode. The electrolyte
consisted of ferric nitrate (0.04 M), HF (46%, Stella Chemifa,Japan), and puriﬁed water (HF:H2O = 7:5 (v/v)). Current
density was set to 100 mA cm¹2 and maintained for 50 min atroom temperature. Copper and gold ﬁlms were deposited by themagnetron sputtering method on both the planar and columnarstructured silicon surfaces. The estimated thicknesses of Au
and Cu are 50 (15 min) and 160 nm (50 min), respectively. The
obtained Cu/Au/Si samples were heated at 110 °C for diﬀerent
time periods in order to oxidize the copper layer. Some samples
were oxidized under ambient conditions for at least one month.
A schematic illustration of the THz detection system is shownin Figure S1 followed by a detailed explanation.
Surface morphology of the prepared samples was characterized with a JEOL JSM-6510 scanning electron microscope(SEM). The JEOL JIB-4600F/HDK multi-beam system wasused to process the sample for transmission electron microscope (TEM) observation and elemental mapping was carriedout using a Hitachi HD2000 scanning transmission electron
microscope (STEM). Surface compositions were analyzed
using a JEOL JPS-9200 X-ray photoelectron spectrometer(XPS). The lateral structure, selected area electron diﬀraction(SAED) patterns and the EDS spectra of the prepared samples
were obtained using a JEM-2010 TEM operated at 200 kV.
Results and Discussion
Figure 1a showed the regular microsized columnar surfaceof the MS-PSi before and after metal deposition. The formation/%

Selected Paper/nm

Figure 1. The surface structures of (a) the fresh Cu/Au/
MS-PSi (53° tilted) with the sputtering time of 15 and50 min for Au and Cu respectively (inset: the SEM top
view of the bare MS-PSi); (b) UV­vis diﬀuse reﬂectancespectra of the MS-PSi and planar Si.
© 2015 The Chemical Society of Japan | 1385

/ps/ps/ps

Figure 2. THz emission spectra of the oxidized Cu/Au thinﬁlms under various treatment conditions: (a) using theoxidized Cu/Au thin ﬁlm deposited on the columnar andplanar Si substrates, (b) varying the heating time at 110 °Cfor Cu/Au/MS-PSi samples, and (c) changing the thicknessof Cu layer by varying the sputtering time for the 2.5 hoxidized Cu/Au/MS-PSi samples at 110 °C. GaAs, a conventional THz emission material, was used as the reference.

of regular microsized columns can be observed on the surface.
The average column height, top column diameter, and distance
between two columns are 2.5, 1.0, and 3.5 ¯m, respectively.
Figure 1b exhibited the signiﬁcantly decreased reﬂectance fromMS-PSi compared to that from the planar Si substrate.
A schematic illustration of the THz detection system isshown in Figure S1. The femtosecond laser beam is split into
two: the detection beam and the pump beam. The latter, at 45°
with respect to the sample, illuminates the sample, generating
THz emission, which is then collected by two parabolic mirrors
and converges with the detection beam. These beams passthrough an electro-optical crystal (ZeTe), and the signal amplitude is recorded on a computer. The femtosecond laser has acentral wavelength of 800 nm with an 80-MHz repetition rateand 100-fs pulse width.
It is well-known that THz emission originates from thesemiconductor­metal interface, in this case, CuxO­CuAu interface. The morphology of the substrate, the oxidation degree ofCu in the as-prepared Cu/Au/MS-PSi and the thickness of Cu
and CuxO layer are important parameters controlling the THzemission. Therefore, we have investigated the inﬂuence of thosefactors on the THz emission and the results are collected inFigure 2. In Figure 2a, THz emissions from the oxidized Cu/
Au on the planar and columnar substrates after heating at 110 °C
for 2.5 h is displayed. THz emission from the bare MS-PSisubstrate can be ignored (Figure S2). The sample prepared on
the planar substrate without heat treatment did not show THzemission (blue in Figure 2a). Between the heat treated samples,a stronger THz signal was detected from the Cu/Au on MS-PSi(red in Figure 2a) with amplitude comparable to 25% amplitude
1386 | Bull. Chem. Soc. Jpn. 2015, 88, 1385–1387 | doi:10.1246/bcsj.20150173

of the THz emission of unbiased gallium arsenide (GaAs, blackin Figure 2a). However, the signal to noise ratio was increasedby the inﬂuence of water or CO2 molecules (porous structure
has strong adsorption capacity). The oxidized Cu/Au thin ﬁlm
deposited on the planar substrate yields a relatively weak signal(cyan in Figure 2a). These results indicate that under the sameheat treatment columnar substrate positively contributed to theimprovement of the THz emission. This improvement is mainly
attributed to two reasons. First, the eﬃcient metal oxide­metal
contact area is enlarged due to the increased surface area of thesubstrate. Speciﬁcally, based on the dimensions and the distancebetween two columns, the surface area of the MS-PSi was calculated about ca. 4­6 times higher than that of the planar substrate.
The second reason is the increase of the optical absorption asshown in Figure 1b. In all regions from 200 to 800 nm, the reﬂectance of prepared MS-PSi decreased signiﬁcantly comparedto that of planar Si. The mean reﬂectance value was ca. 5%.
Generally the incident light is expressed by the sum of reﬂection
light, absorption light, and transmission light. Since the transmission light can be ignored for thick solid samples, decreased
reﬂectance indicates the increase of absorption. This phenomenon arose from the multireﬂection among the columnar structures. As a consequence, most of the incident light was absorbed
at the columnar surface. THz emission amplitude is proportional
to the change rate of the photocurrent in the built-in ﬁeld. Thechange rate of the photocurrent increases with higher density
and shorter lifetime of the photon-generated carriers. Moreover,the density of the photon-generated carriers is determined by
the intensity of the built-in ﬁeld and the optical absorption rateof the incident light. So, the enhanced optical absorption is an
important parameter for the THz emission improvement.
The columnar substrate has been demonstrated to be superior
over the conventional planar one in the enhanced THz emission
from oxidized Cu/Au interface, so, MS-PSi was used as thesubstrate to further optimize the THz emission. We found thatthe heating duration inﬂuences the THz emission eﬃciency remarkably, as shown in Figure 2b. As the heating time increased
from 1.5 to 3.0 h, THz emission amplitude gradually increases,reaches its maximum at 2.5 h and then signiﬁcantly decreases
for the heating time of 3.0 h. Changing the heating time isexpected to change the oxidation degree of Cu metal which is an
important factor not only for matching the surface SCR condition but also for the behavior of photon-generated carriers. BothCu2O and CuO are p-type semiconductors with band gaps of
2.1 (direct) and 1.2 eV (indirect), which are higher and lower
than the energy of the femtosecond laser (1.55 eV). Therefore, asuitable composition of the Cu oxide that matches the energy of
the incident light is essential for light absorption and consequent
photon-induced carrier generation.17 The emission from femtosecond laser has a broad band, however, band gap energy ofCu2O (2.1 eV, ca. 590 nm) still higher than that of the shortwave band edge (2.0 eV, ca. 620 nm) of the laser source.
Meanwhile, the laser beam spot was not perfectly focused,which resulted in negligible two or multiphoton absorption
arising from three or higher order nonlinear optics. So, if Cu2O
dominates the surface, its band gap energy higher than that ofthe incident light will inﬂuence the number of the photongenerated carriers. On the other hand, if CuO exists in aconsiderable amount, its indirect band gap will increase the© 2015 The Chemical Society of Japanto crystalline Cu2O at oxide-metal alloy interface, from XPS
results (Figure S7), amorphous CuO was also detected on thesurface. These results emphasized the formation of CuxO-metalalloy interface which well correlated with the THz emission inoptimal oxidized Cu/Au/MS-PSi sample.
Conclusion
This report has demonstrated that using the columnar porous
Si substrate with high surface area and eﬃciently reduced
reﬂectance is an eﬃcient method to enhance THz emission
from CuxO-metal interface. Using MS-PSi substrate, the optimal THz emission has been achieved via feasible adjustment of
the oxidation degree of the Cu layer.

Figure 3. (a) Interface structure of Cu (50-min deposition)/Au (15-min deposition) thin ﬁlm on the PSi heated at
110 °C for 2.5 h; (b) and (c) SAED patterns of the black (Cuarea) and white (Au area) circle areas in (a), respectively.

relaxation time of the photon-generated carriers, resulting in an
ineﬃcient radiation. A comparison of the oxidation degree ofthe samples heated for diﬀerent time is shown in Figure S3. The
results suggested that slight change of the oxidation degree(from Cu1.53O to Cu1.34O for 2.5 to 3.0 h heat treatment respectively) probably due to more CuO formed is in charge for theincrease of the carrier relaxation time, hence reduces THz emission (Figure 2b). Therefore, a suitable composition of the Cu
oxides that matches the energy of the incident light is essential
for light absorption and the consequent photon-induced carrier
generation, which is important for enhancing THz emission. Inour case, such optimal oxidation condition is heating at 110 °C
for 2.5 h. Under the optimal heating conditions, THz emission
from samples with diﬀerent Cu layer thickness by varying thesputtering time from 25 to 75 min was studied and the results aregiven in Figure 2c. The 50-min Cu-deposited sample shows thebest performance which is consistent with the above discussion,as the Cu thickness changed, under the same heating time, theoxidation degree was also varied.
Metallic Cu/Au interface (blue in Figure 2a) did not produce
any THz radiation. In order to conﬁrm the existence of copperoxides-metal interface and understand the relation between
structure and THz enhancement, a detailed analysis of theoxidized Cu/Au/MS-PSi was conducted. Figure 3 shows thestructure of the heated Cu/Au layer (110 °C, 2.5 h) on the MSPSi substrate (see Figure S4 for the 3.0 h heated one). Fourlayers were clearly observed: the bottom one was the silicon
substrate, the top one was the W layer used to protect the Cu/Aulayers during TEM sample preparation by focused ion beam,and the middle layers were Au and Cu layers with thickness ofca. 70 and ca. 130 nm, respectively, which are in good agreement with thickness calculated from sputtering rate of ca. 3.5
nm min¹1 measured for metal deposition on the planar substrate shown in Figure S5 (THz emission spectrum from asimilar sample is shown in Figure S6). SAED patterns shown inFigures 3c and 3d conﬁrm that both Cu2O and CuAu alloys18,19
formed at the Cu­Au interface. On the other hand, EDS
mapping images shown in Figures S5c­S5f suggested that Cu
diﬀused into the Au plate, forming the AuCu alloy. Additional
Bull. Chem. Soc. Jpn. 2015, 88, 1385–1387 | doi:10.1246/bcsj.20150173

This work was partially supported by Grant from Hokkaido
University (to TY) and by Grant-in-Aid for Scientiﬁc Research(A) (No. 24241041, to TY).
Supporting Information
THz detection system; THz emission spectrum of the bareMS-PSi; EDS quantitative analysis of the heat treated Cu/Au/MS-PSi; interface TEM image of the 3.0 h heated Cu/Au/MSPSi; interface structure and EDS mapping result of the Cu/Au/planar Si oxidized in air; additional THz emission spectrum
and XPS analysis, These materials are available electronicallyon J-STAGE.
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© 2015 The Chemical Society of Japan | 1387