Advanced measurement technology for future semiconductor devices– Terahertz time domain spectroscopy for ultra-high frequency
response of gallium oxide –1.

Key points of the work

 An emerging semiconductor for future power devices, beta-gallium oxide (-Ga2O3), wasinvestigated using a technique called transmission terahertz time-domain spectroscopy (THz-TDS)
for the first time The findings on the fundamental properties of -Ga2O3 at THz frequencies are significant to thedevelopment of this semiconductor’s power electronic applications
 THz-TDS can be used as a noninvasive tool for the evaluation of electrical properties instead of
conventional electrical measurements that degrade the semiconductor quality
2.

Overview of the workThe -Ga2O3 ultra-wide bandgap (UWBG) semiconductor [1,2] could revolutionize the powerelectronics industry with its superior material properties for efficient power conversion in comparison tocurrent commercial semiconductors such as silicon carbide and gallium nitride (Figure 1). To develop
electronic applications, material properties in the millimeter and terahertz wave regions must beelucidated. A research group including Associate Professor Makoto Nakajima, Verdad C. Agulto, Valynn
Katrine Mag-usara of Osaka University, Toshiyuki Iwamoto of Nippo Precision Co., Ltd., Professor
Yoshinao Kumagai, Associate Professor Hisashi Murakami, and Assistant Professor Ken Goto of Tokyo
University of Agriculture and Technology employed a noninvasive technique called terahertz timedomain spectroscopy (THz-TDS) [3] to investigate -Ga2O3 materials using THz waves [4] anddemonstrated the viability of this tool for the practical evaluation of -Ga2O3-based devices.

Figure 1. The Ga2O3 semiconductor could pave the way for next-generation semiconductor devices.
The use of terahertz (THz) waves to probe semiconductor properties in replacement of conventional
yet invasive electrical measurements would accelerate the development of future Ga2O3-based
devices and applications.
THz-TDS is used to measure the permittivity of a material – a fundamental property that describes
how much electrical energy the material can store and is an important parameter relevant to power devices.
The researchers determined the permittivity of -Ga2O3 in the THz region using the THz-TDS technique.
From the permittivity, they were also able to extract the electrical properties such as carrier density andresistivity with an accuracy that is comparable to conventional albeit invasive measurement methods usedin the semiconductor industry. The researchers believe that the application of THz-TDS in semiconductor
characterization would be advantageous towards the rapid development of -Ga2O3 materials for future
power devices.
The article, “Anisotropic complex refractive index of -Ga2O3 bulk and epilayer evaluated byterahertz time-domain spectroscopy,” was published online in Applied Physics Letters on January 25,2021, 11:00 AM Eastern Standard Time (January 26, 2021, 1:00 AM Japan Standard Time) at DOI:
https://doi.org/10.1063/5.0031531.
3.

Details of the work
Terahertz frequencies (about 100 GHz to 10 THz) lie between the microwave and infrared regionsof the electromagnetic spectrum, with the frequency band around 100 GHz being considered as an ultrahigh frequency region for the next-generation communication band also known as Beyond 5G / 6G. By
studying the interaction of semiconductors with THz radiation, various information about thesemiconductor properties can be acquired. The THz-TDS investigation of -Ga2O3 materials  semiinsulating bulk crystal and doped homoepitaxial film  was led by Associate Professor Makoto Nakajimaof the Institute of Laser Engineering, Osaka University. The transmission of THz beam through the Ga2O3 materials was measured using the equipment developed by Nippo Precision Co. Ltd. (PNP) called
Tera Prospector [5]. Since -Ga2O3 is an anisotropic crystal (i.e., its properties vary along different crystal

axes), the transmission measurements were done along two different crystal directions that are parallel tothe a and b crystal axes. By evaluating the measured transmission, fundamental material properties suchas the anisotropic complex refractive index [6] in the THz region were determined. The complex refractive
index is in turn related to the material’s permittivity.
Using the THz-TDS technique, the researchers succeeded in obtaining the complex refractive index
spectra in the terahertz band which is an ultra-high frequency region. Figure 2 shows the complex
refractive index of the semi-insulating bulk crystal and the doped homoepitaxial film (or epilayer) in theTHz region measured by THz-TDS. The semi-insulating -Ga2O3 single crystal sample exhibits
increasing refractive index with increasing frequency, which indicates the presence of phonon absorption
in the infrared region. On the other hand, the n-type -Ga2O3 epitaxial film exhibits increasing refractiveindex with decreasing frequency which is different from the semi-insulating sample. This increase inrefractive index in the low-frequency side indicates absorption depending on the amount of carrier densityin -Ga2O3. By evaluating this low-frequency region, it is possible to obtain information on carrier density,carrier scattering time, and mobility. The results are summarized in Table I. It was found that the refractive
index at 0.2 THz is 3.165 along a-axis and 3.231 along b-axis. The researchers used the Drude-Lorentz
model [7] to analyze the experimental data. Using this model, they extracted the static permittivity [8] of
-Ga2O3 as well as the electrical properties of the doped epilayer. The static permittivity (dielectricconstant at 0 Hz) was found to be 10.02 along a-axis and 10.43 along b-axis. A carrier density of ~4.61017
cm-3, a mobility of ~100 cm2 V-1 s-1, and a resistivity of 0.13 Ωcm were obtained for the n-type -Ga2O3
epitaxial film. In device fabrication, the electrical properties of semiconductor films are tuned via doping(intentional addition of impurities). As such, the electrical characterization of doped films is imperative.
Conventional electrical characterization techniques include the Hall method [9], where attachment ofmetal contacts is necessary, and capacitance-voltage (CV) measurements [10], which sometimes use toxic
mercury (Hg) probes. These techniques, however, are becoming impractical for rapid device development
because of their invasive nature and relatively time-consuming measurement. In this regard, THz-TDS is
an attractive alternative to conventional methods because it is nondestructive and more straightforward as
it doesn’t necessitate any type of electrical contacts or electrodes. By modelling the experimental
refractive index derived from the THz-TDS measurements, the researchers were able to extract theelectrical properties of the doped epilayer with good accuracy. The obtained electrical properties areclosely comparable to the results of the conventional Hall measurement (Table I). These findings
demonstrate that THz-TDS is a reliable and viable technique to replace conventional methods for theelectrical characterization of -Ga2O3 semiconductors.
In this work, the researchers reported the fundamental properties of -Ga2O3 in the THz frequency
region. Moreover, they demonstrated the noninvasive evaluation of electrical properties using the THzTDS technique. Their findings have significant contribution to the rapid development of -Ga2O3 devices
and applications.

Figure 2. Frequency-dependent refractive index and extinction coefficient of semi-insulating Ga2O3 (left) and doped -Ga2O3 epilayer (right) along the a crystal axis (E||a) and along the b
crystal axis (E||b). The gradual increase in the bulk’s refractive index and extinction coefficient withincreasing frequency is ascribed to phonon absorption at higher frequencies. The rapid increase in theepilayer’s refractive index and extinction coefficient with decreasing frequency is due to the effect offree carriers. The Drude-Lorentz model is represented by the black dashed curves. Since -Ga2O3 isan anisotropic crystal, its complex refractive index differs along different crystal directions. The
refractive index is higher along b axis than along a axis. In the epilayer’s case, the anisotropic
difference between the two axes in the low-frequency region is relatively small because the refractiveindex is dominated by the effect of free carriers.
Table I. Material properties of -Ga2O3 bulk and epilayer evaluated by THz-TDS along with theelectrical properties measured by Hall measurement using Van der Pauw configuration. Using theDrude-Lorentz model, the static permittivity and the electrical properties were determined. The staticpermittivity and refractive index are higher along b axis than along a axis. The electrical parameters are
similar along the two axes and the results obtained by THz-TDS are closely comparable to conventional
Hall measurements.
THz-TDSHall(Van der Pauw)(𝐸||𝑎)(𝐸||𝑏)

Static permittivity

10.02 ± 0.03

10.43 ± 0.03

Refractive index (0.2 THz)

3.165 ± 0.005

3.231 ± 0.005

Carrier density (cm-3)(4.7 ± 0.3)  1017

(4.5 ± 0.3)  1017

4.6  1017

Mobility (cm2 V-1 s-1 )100 ± 8105 ± 7117

0.132 ± 0.001

0.133 ± 0.001

0.12

BulkEpilayer

Resistivity (Ωcm)

4.

Glossary(1) Gallium oxide (Ga2O3)
Ga2O3 belongs to a group of materials referred to as ultrawide bandgap semiconductors. It exists
in various forms, but the most stable one is the -Ga2O3 crystal structure. Research on -Ga2O3

(2)

(3)

(4)

(5)

(6)

materials is a highly active area nowadays because the -Ga2O3 semiconductor shows significant
potential for high-power and high-voltage devices.
Ultra-wide bandgap (UWBG) semiconductors
Semiconductors are often categorized according to their bandgap – a key property that determines
their uses and applications. The conventional silicon (Si) semiconductor has a bandgap of 1.12
eV and is commonly used in integrated circuits. Wide-bandgap semiconductors have a larger
bandgap, e.g., gallium nitride (GaN) and silicon carbide (SiC) which have bandgaps of 3.4 eV
and 3.26 eV, respectively. Semiconductors with wider bandgaps are used in devices that need tooperate at higher voltages and temperatures. UWBG semiconductors are those that have abandgap larger than that of GaN, like -Ga2O3 which has a bandgap of ~4.8 eV. Such materials
are attractive for high-power and high-voltage applications.
Terahertz time-domain spectroscopy (THz-TDS)
THz-TDS is a technique that probes material properties using terahertz waves. Unlike ultraviolet
light or x-rays, terahertz radiation is nondestructive due to its lower energies. THz-TDS is widelyused to characterize the dielectric and electrical properties of semiconductors and other materials
in the far-infrared region.
Terahertz wave (THz wave)
THz waves refer to electromagnetic waves with frequencies in the order of 10 12 Hz, specificallyin the wavelength region between 3 mm and 30 μm (0.1~10 THz). Aside from their scientificapplications like material analysis, THz waves can also be used in technologies such as medical
imaging, security screening, and telecommunications. With frequencies higher than radio waves,THz waves can pave the way for high-frequency operation devices and high-speed, ultra-highbandwidth communications. As such, THz is considered key to communication technologies
beyond 5G/6G.
Tera Prospector
Tera Prospector is a general-purpose THz time-domain spectrometer developed by Nippo
Precision Co., Ltd. The complex refractive index / permittivity spectra in the terahertz region canbe measured accurately using this equipment. In addition to Ga2O3, it can be used for othersemiconductors such as Si, SiC and GaN.
https://terahertzwave.com/en/
Complex refractive index
The complex refractive index is a material property that describes how light propagates throughthe material. The real part is the refractive index which is related to the speed of propagation,
while the imaginary part is the extinction coefficient which indicates how strongly the light isattenuated as it propagates in the material. Light is attenuated when part of the energy is absorbed

by the crystal atoms (phonon absorption) or by the free electrons in the material (free carrierabsorption). The complex refractive index can also be expressed in terms of permittivity as afunction of frequency.
(7) Drude-Lorentz model
The Drude-Lorentz model quantifies the dependence of the complex refractive index andpermittivity on the frequency of light, taking into account the effects of phonons and free carriers.
This model is commonly used in evaluating semiconductor materials.
(8) Static permittivity
The static permittivity refers to the permittivity at zero frequency or 0 Hz.
(9) Hall measurement
Hall measurement is a conventional characterization technique to measure the electrical propertiesof semiconductors. In the Hall method using Van der Pauw configuration, current is applied andvoltage is measured with the use metal contacts.
(10) Capacitance-voltage (CV) measurements
CV measurements are another conventional technique used to characterize the electricalproperties of semiconductors and devices. In this measurement, voltage is applied then thecapacitance of the material is measured with varying voltage with the use of electrodes.
About Osaka University
Osaka University was founded in 1931 as one of the seven imperial universities of Japan and is now oneof Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupledwith a singular drive for innovation that extends throughout the scientific process, from fundamental
research to the creation of applied technology with positive economic impacts. Its commitment toinnovation has been recognized in Japan and around the world, being named Japan's most innovative
university in 2015 (Reuters 2015 Top 100) and one of the most innovative institutions in the world in2017 (Innovative Universities and the Nature Index Innovation 2017). Now, Osaka University isleveraging its role as a Designated National University Corporation selected by the Ministry of Education,Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable
development of society, and social transformation.
Website: https://resou.osaka-u.ac.jp/en

Primary Keyword: Technology/Engineering/Computer Science
Additional Keywords: Electrical Engineering/Electronics, Superconductors/Semiconductors, Industrial
Engineering/Chemistry, Materials, Optics
Summary: A team of researchers led by Osaka University investigated beta-gallium oxide (-Ga2O3), an
emerging semiconductor for next-generation power devices, using an advanced method involving
terahertz waves – the technology that could replace conventional yet invasive electrical semiconductor
characterizations.
Funded by: Japan Society for the Promotion of Science (JSPS); Ministry of Education, Culture, Sports,Science, and Technology (MEXT)