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Investigation of electrically active defects in
undoped BaSi2 light absorber layers using deeplevel transient spectroscopy
To cite this article: Yudai Yamashita et al 2018 Jpn. J. Appl. Phys. 57 075801

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Japanese Journal of Applied Physics 57, 075801 (2018)https://doi.org/10.7567/JJAP.57.075801

Investigation of electrically active defects in undoped BaSi2 light absorber layers
using deep-level transient spectroscopy
Yudai Yamashita, Takuma Sato, Miftahullatif Emha Bayu, Kaoru Toko, and Takashi Suemasu
Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Received January 30, 2018; accepted April 10, 2018; published online June 12, 2018
We grow 500-nm-thick undoped n-BaSi2 epitaxial ﬁlms on n-Si(111) substrates by molecular beam epitaxy with an optimized Ba-to-Si deposition
rate ratio of approximately 2.2, and investigate defect levels and their densities by deep-level transient spectroscopy (DLTS). A hole trap level (H1)with a density NT = 1 ' 1013 cm%3 is detected at 0.27 eV from the valence band maximum (VBM). It is suggested from a sharp rise in capacitanceduring trap ﬁlling processes that H1 is caused by point defects, which is interpreted to originate from Si vacancies. The leakage current of the diode
under reverse-biased voltages increases signiﬁcantly once the sample temperature exceeds approximately 200 °C during DLTS. Postannealing
performed at 200 °C in N2 for 20 min gives rise to similar results. The measured value of NT is approximately two orders of magnitude smaller thanthose reported for undoped BaSi2 ﬁlms. We also discuss the minimum lifetime of BaSi2 from the viewpoint of the Shockley–Read–Hallrecombination model. © 2018 The Japan Society of Applied Physics

1.

Introduction

III–V multijunction solar cells composed of stacks of materials with diﬀerent bandgaps can eﬃciently utilize the solar
spectra and achieve high conversion eﬃciencies η beyond
40%.1–5) However, III–V solar cells are relatively expensive
and therefore large-scale deployment is not easy. In order tosolve this problem, the development of III–V=Si solar cells inwhich an expensive Ge substrate is replaced with inexpensive
Si is in progress.6) Under these circumstances, we have paid
special attention to barium disilicide (BaSi2), which shows
great promise as a new material for thin-ﬁlm solar cells on a
Si substrate.7) BaSi2 is a rare-earth-free and nontoxic semiconductor consisting of Ba and Si, and can be grown epitaxially on a Si(111) substrate with its a-axis normal to thesubstrate surface.8) It has a suitable bandgap of 1.3 eV, a large
optical absorption coeﬃcient of 3 × 104 cm−1 for a photon
energy of 1.5 eV, which is more than 50 times higher thanthat of crystalline Si,9–12) and a large minority carrier diﬀusion length of about 10 µm.13) We succeeded in increasing theminority carrier lifetime of undoped BaSi2 epitaxial layers
from 0.1 to 10 µs by covering with an amorphous Si (a-Si)
layer,14–16) which is suﬃciently large for thin-ﬁlm solar cells.
With the help of a-Si passivation layers, we achieved ηapproaching 10% in p-BaSi2=n-Si heterojunction solar
cells,17,18) where the p-BaSi2 layer acts as an emitter, and a
conduction-band oﬀset ΔEC = 0.85 eV and a valence-band
oﬀset ΔEV = 0.65 eV at the heterointerface due to the low
electron aﬃnity of BaSi2 (3.2 eV),19) promoting the separation of photogenerated carriers. However, the built-in potential of p-BaSi2=n-Si is as small as 0.1 V. For this reason, theopen-circuit voltage (VOC) of p-BaSi2=n-Si solar cells is
smaller than 0.5 V.17,18) Hence, it is not easy to make a drastic
improvement in the η of p-BaSi2=n-Si solar cells. To improveη much further, we next target BaSi2 pn homojunction solar
cells using an undoped BaSi2 layer as a light absorber layer.
Both VOC > 0.8 V and η > 25% can be expected for a 2-µmthick BaSi2 homojunction solar cell.20) Therefore, the formation of high-quality BaSi2 light absorbers with a low defectdensity is inevitable.
Deep-level transient spectroscopy (DLTS) is one of themost powerful techniques for detecting electrically active
defect levels and their concentrations,21,22) and has been

applied to investigate defect levels in other semiconducting
silicides such as β-FeSi2.23–25) However, there is only one
report on the DLTS study of BaSi2.26) In our previous
study,26) DLTS revealed the presence of majority carrier(electron) traps in undoped n-BaSi2 located at approximately
0.1 and 0.2 eV below the conduction band minimum (CBM).
The densities of these trap levels were approximately 1 ×
1015 cm−3. Such large defect densities degrade the opticalproperties of BaSi2. Very recently, we have found that thecarrier concentration of undoped BaSi2 changes with a slightdeviation of the Ba=Si atomic ratio from stoichiometry.27)
The photoresponsivity drastically changes as a function of thedeposition rate ratio of Ba to Si (RBa=RSi) during molecularbeam epitaxy (MBE); it reaches a maximum at RBa=RSi = 2.2.
First-principles calculation suggests that Si vacancies aremostly likely to occur as point defects in BaSi2, and thatthey give rise to localized states within the band gap, leading
to the degradation of the minority carrier properties of
BaSi2.27,28) Hence, it is of particular interest to know how thedefect levels of BaSi2 and their densities would be improvedby optimizing RBa=RSi.
In this article, we aim to investigate defect levels and theirdensities in undoped BaSi2 by DLTS. We also discuss theminimum lifetime (τmin) of BaSi2 from the viewpoint of the
Shockley–Read–Hall (SRH) recombination model.29) Photovoltaic absorbers have to be conditioned in such a way thatthe lifetime of photogenerated carriers is equal to or longer
than τmin.
2.

Experimental methods

An ion-pumped MBE system equipped with a standard
Knudsen cell for Ba and an electron-beam evaporation source
for Si was used for the growth. We fabricated a 500-nm-thickundoped n-BaSi2 layer on Czochralski (CZ) n-Si(111) substrates with diﬀerent resistivities ρ = 0.1–1 and 1–4 Ω cm
to form a heterojunction diode. The electron concentrationsof such n-Si substrates were approximately n = 5 × 1016 and2 × 1015 cm−3, respectively, while the n of undoped BaSi2
was below 1016 cm−3. Although BaSi2 epitaxial layers(a-axis-oriented) have three epitaxial variants that rotate by
120° from each other around the surface normal,30) the grainboundaries (GBs) between the variants do not act as recombination centers.16,31) As will be described later, the leakage

075801-1© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 075801 (2018)

Y. Yamashita et al.

3.
3.1

Results and discussion

Defects in as-grown sample
Voltages were applied to n-Si (p-BaSi2) with respect ton-BaSi2 (n-Si) for the n-BaSi2=n-Si (p-BaSi2=n-Si) diodes.
Although both as-grown n-BaSi2=n-Si and p-BaSi2=n-Si
diodes exhibited rectifying properties in the J–V characteristics, they degraded as ρ decreased. Figures 1(a) and 1(b)show their representative curves, respectively. For the sample
grown on a lower-ρ n-Si substrate, the leakage current under
the reverse-bias voltages increased signiﬁcantly, as shownby the broken line in Fig. 1(a). According to our previous
work,32) step bunching occurs to a far greater extent when aheavily doped (low resistivity ρ < 0.01 Ω cm) Si substrate isheated at 900 °C for 30 min in an ultrahigh-vacuum chamber
to remove the protective oxide layer on the surface. When ρis low, step bunching, which causes the generation of defects
around the BaSi2=Si heterointerface, cannot be avoided by
TC at a reduced temperature of 800 °C. Such large leakage
currents make it diﬃcult to evaluate the junction capacitanceof the diodes under the reverse-biased condition. On the basisof these results, we chose the sample, i.e., undoped n-BaSi2=
n-Si formed on n-Si substrates with ρ = 1–4 Ω cm for DLTS.
A schematic diagram of DLTS is shown in Fig. 2(a). A
forward ﬁlling pulse voltage (VP) disturbs the steady-state
reverse-bias condition, causing the electric ﬁeld in the deple-

Current density [mA/cm2]

(a)

400
300
200

Undoped n-BaSi2/n-Si
ρ = 1 – 4 Ωcm
ρ = 0.1 – 1.0 Ωcm

100
0
-100
-200
-1.0

(b)

Current density [mA/cm2]

current in the current density versus voltage (J–V ) characteristics under reverse-biased voltages is very large for diodes
formed on the n-Si substrate having a low ρ. To systematically investigate the inﬂuence of ρ on the J–V characteristics,
we formed B-doped p-BaSi2 (300 nm)=n-Si(111) substrates
with varying ρ in the range between 0.01–0.1 and 1–4 Ω cm,with the hole concentration of p-BaSi2 ﬁxed at 1016 cm−3.
Sample preparation was performed as follows: The Si substrate underwent thermal cleaning (TC) at 800 °C underultrahigh vacuum in addition to the simultaneous depositionof a 1-nm-thick Si layer. With this technique, the defectdensity in the n-Si substrate became less than 1011 cm−3.32)
First, a 50-nm-thick i-Si buﬀer layer was deposited at 800 °C
prior to the formation of BaSi2 layers. Next, we prepared a
5-nm-thick BaSi2 template layer by depositing Ba on an i-Si
buﬀer layer at 500 °C by reactive deposition epitaxy,33)
followed by the codeposition of Ba and Si at 580 °C by
MBE to form a 500-nm-thick undoped n-BaSi2 or 300-nmthick B-doped p-BaSi2 epitaxial layer with an optimized
condition of RBa=RSi = 2.2. Finally, a 3-nm-thick a-Si layer
was deposited in situ on the surface for surface passivation.34)
The a-Si capping layer suppresses the surface oxidation andbehaves as a surface passivation layer. Indium–tin oxide(ITO) electrodes with a diameter of 1 mm and a thicknessof 80 nm were sputtered on the surface, and Al electrodes on
the entire back surface. For comparison, postannealing was
conducted at 200 °C for 20 min in N2 using a rapid thermal
annealing (RTA) system (Thermo Riko MILA-5000-UHV).
The postannealing was performed before the electrode deposition. X-ray diﬀraction (XRD; Rigaku Smart Lab) analysis
was used to characterize the crystalline quality of the grownlayers. DLTS was carried out in the temperature range of
80–470 K using a 1 MHz capacitance versus voltage (C–V )meter (HP 4280A). The rate window was varied between
1 and 512 ms.

-0.5

0.0
0.5
Voltage [V]

1.0

800
p-BaSi2/n-Si400

ρ = 0.01 – 0.1 Ωcm
ρ = 0.1 – 1.0 Ωcm
ρ = 1 – 4 Ωcm

0
-400
-800
-1.0

-0.5

0.0
0.5
Voltage [V]1.0

Fig. 1. (Color online) J–V characteristics of (a) undoped n-BaSi2(500 nm)=n-Si and (b) B-doped p-BaSi2 (300 nm)=n-Si diodes on n-Si
substrates having diﬀerent ρ values.

tion region to decrease. This causes the defect levels to berecharged. When the voltage returns to its steady-state value,
the defect levels begin to discharge by emitting trapped
carriers by thermal emission, and the resultant time evolutionof the capacitance change ΔC(T ) is measured for various rate
windows. DLTS allows for the immediate determinationof whether the detected defects act as minority or majority
carrier traps from the sign of the DLTS signals; positive
signals indicate the presence of minority carrier traps, andnegative signals indicate the existence of majority carrier
traps. The measured DLTS proﬁle of the as-grown sample isshown in Fig. 2(b). The forward ﬁlling bias voltage VP wasset at 0.5 V, the pulse width tpw at 100 ms, and the reverse
bias voltage VR at −0.5 V. The electron concentration of
BaSi2 is below 1016 cm−3 and that of the Si substrate is 2 ×
1015 cm−3 at room temperature. Hence, the depletion region
stretches in both sides. However, with a modiﬁed pretreatment of the Si substrate, the density of defects in the n-Si
region (ρ = 1–4 Ω cm) decreases to a value below the detection limit.32) For this reason, it is reasonable to suppose thatthe detected peaks indicate defects in the n-BaSi2 region. In
the ﬁrst experiment denoted by Measurement 1, temperature
was increased from 80 to 470 K. An upward facing peak due
to minority carrier (hole) traps was observed at approximately
126 K; however, a distinctive change in capacitance ΔC wasobserved at temperatures higher than 350 K. That is why we
performed the second DLTS on the same sample from 80 to

075801-2© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 075801 (2018)

Y. Yamashita et al.

400

V

(a)
0

Current density [mA/cm2]

VP
t

VR

Pulse widthC

ΔC=C(t1)-C(t2)Rate window

C(t=∞)

(b)

1.0
0.5

Rate window : 4 mst2 t

Postannealed
After DLTS

As-grown
ρ = 1 – 4 Ωcm

200
100
0
-100
-200
-1.0

C(t=0)
0 t1

300

-0.5

0.0
0.5
Voltage [V]1.0

Fig. 3. (Color online) J–V characteristics of undoped n-BaSi2 (500 nm)=n-Si diodes on an n-Si substrate (ρ = 1–4 Ω cm) measured after DLTS
(Measurement 1) with temperatures swept from 80 to 470 K (broken line),
and after the postannealing at 200 °C in N2 for 20 min (dotted line).

128 msMeasurement 1

ΔC [pF]

0.0
-0.5
-1.0
-1.5

(a)

Measurement 2
V

50 ms

V

100 ms

VP=0.5 V

VP = 0.5 V
t
VR =

VR

t

0.5 V

VR = -0.5 V
VR = -0.2 V
VR = -0.1 V-2.0 100 150 200 250 300 350 400 450

1 pF

DLTS Signal [arb. unit]

Temperature [K]Fig. 2. (Color online) (a) Principle of DLTS and (b) DLTS proﬁles of
as-grown sample obtained under ﬁrst measurement (Measurement 1) andsecond measurement (Measurement 2) at VP = 0.5 V, VR = −0.5 V, and
tpw = 100 ms. The rate window was 4–128 ms.

2 ms – 64 ms
4 ms – 128 ms
8 ms – 256 ms
16 ms – 512 ms

H1
100

150
200
250
Temperature [K]

(b)

16

300

H1: 0.27 eV

14ln(T2/ent)

350 K, which is denoted by Measurement 2. In contrast tothat in Measurement 1, a downward facing peak caused bymajority carrier (electron) traps was observed at 100–250 K.
Once the downward facing peak was observed, the upwardfacing peak was never detected again. This change is interpreted to originate from the increased temperature up to
470 K (193 °C) during DLTS.
Figure 3 shows the representative J–V characteristics ofthe diodes measured after the DLTS Measurement 1 (brokenline) and after the postannealing without DLTS (dotted line).
The degradation of the rectifying properties was also conﬁrmed in the J–V characteristics, showing that the majority
carrier traps obtained in DLTS presented in Fig. 2(b) areresponsible for the large leakage current at negative bias
voltages. To focus on the defect properties of the as-grownsample, we carried out DLTS on another fresh as-grownsample. This time, the measurement temperature was limited
up to 300 K to prevent the postannealing eﬀect during themeasurement.
Figure 4(a) shows the DLTS proﬁle of the as-grownsample. VP was set at 0.5 V, and tpw at 50 ms, VR was varied
from −0.5 to −0.1 V. An upward facing peak caused bya minority carrier (holes) trap level (H1) was observed at
approximately 126 K in Fig. 4(a). The hole trap level was
calculated to be approximately 0.27 eV from the valence band
maximum (VBM) in Fig. 4(b), and the defect density NT was
1 × 1013 cm−3. This value is approximately two orders ofmagnitude smaller than that previously reported in undoped

1 ms – 32 ms

12
104

5

6

7
8
1000/T [K−1]910Fig. 4. (Color online) (a) DLTS proﬁles of as-grown sample at VP = 0.5 V
and tpw = 50 ms. VR was varied as −0.5, −0.2, and −0.1 V. The rate window
was varied as 1–32, 2–64, 4–128, 8–256, and 16–512 ms. (b) Arrhenius plot
for hole trap level (H1).

n-BaSi2 grown with RBa=RSi ∼ 3.26) The DLTS signal intensity observed at approximately 100–125 K decreased as ∣VR∣decreased from 0.5 to 0.1 V, indicating that the defect density
decreased when it approached the interface and therefore thepresence of defects in the bulk region of BaSi2. To investigate
whether the hole trap level H1 was caused by point or
extensive defects, we changed tpw from 0.01 to 800 ms, andmeasured the signals at 126 K, as shown in Fig. 5(a). The

075801-3© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 075801 (2018)

1.5

Rate window : 8 ms – 256 ms
tpw [ms]
800 msVp = 0.5 V 10 ms
t 5 ms
1 ms
VR = 0.5 V
0.5 ms
0.1 ms0.01 msV

ΔC [pF]

1.0
0.5
0.0

10-5

10-16 cm2
10-15 cm2
10-14 cm2
10-13 cm2
10-12 cm2

10-7
τmin = 10 ns

10-111010 1011 1012 1013 1014 1015 1016

300

NT [cm-3]

1.6

Fig. 6. (Color online) Dependence of minimum SRH lifetime on NT
under the assumption of equal capture cross sections for electrons and holes
being in the range between 10−12 and 10−16 cm2. τmin is supposed to be 10 ns
for BaSi2.

1.2

ΔC [pF]vth = 107 cm/s

10-3

10-9

H1
-0.5 100
150
200
250
Temperature [K]
(b)

10-1

τmin [s](a)Y. Yamashita et al.

0.8
0.4
0.0

T = 126 K
10-2

10-1

100

101

102

Pulse width [ms]103

Fig. 5. (Color online) (a) DLTS proﬁles obtained at diﬀerent tpw values
from 0.01 to 800 ms and (b) dependence of the DLTS signal ΔC on pulse
width of VP, measured at 126 K.

DLTS signal ΔC(T) increased sharply as tpw increased from
0.1 to 10 ms in Fig. 5(b), which is caused by the presence ofpoint defects.35) BaSi2 has six types of native point defects:
silicon vacancies (VSi), barium vacancies (VBa), silicon substituted for barium antisites (SiBa), barium substituted forsilicon antisites (BaSi), silicon interstitials (Sii), and barium
interstitials (Bai). First-principles calculation revealed that VSi
is most likely to occur from the viewpoint of formation energy
in these native point defects.28) One of the calculated defectlevels due to VSi is in good agreement with H1. Takabe
et al.32) found from Rutherford backscattering spectrometrythat the Si atomic ratio in the BaSi2 ﬁlms increased around the
BaSi2=Si interface. We therefore suppose that the decrease indefect density around the BaSi2=Si interface presented inFig. 4(a) is ascribed to the reduction in the density of VSi by
the diﬀusion of Si atoms from the Si substrate into BaSi2.
However, we cannot distinguish VSi from other defects by
DLTS alone. Thus, more studies are needed to verify thisassumption.
3.2 Discussion on minimum SRH lifetime
We next discuss how the defect density on the order of1013 cm−3 detected in this work would aﬀect the opticalproperties of BaSi2 from the viewpoint of SRH recombination. The τmin condition follows from the requirement of lightabsorption in relation to charge transport; it is obtained when
the capture rates are maximum. In this situation, the τmin ofelectrons and holes can be described by Eq. (1) under theassumption of equal capture cross sections σ for electrons
and holes:36)
min ¼1:vth NT

ð1Þ

Here, the carrier thermal velocity vth is approximated to be
107 cm=s. The capture cross sections of traps can beunderstood as the extension of a trap perpendicular to thedirection of motion of a free charge carrier. Since it is noteasy to evaluate σ from the present DLTS proﬁles, we
assumed σ to be in the range between 10−12 and 10−16 cm2.
Figure 6 shows the τmin versus NT plot. We estimate the τminfor BaSi2 as follows. In the light-absorbing layer of the solarcell, three times the reciprocal of the light absorption coeﬃcient is necessary to absorb the light suﬃciently. Similarly,
the minority carrier diﬀusion length L needs to be longer than
this. Therefore, τmin is obtained as36)
min ¼

9
:
D2

ð2Þ

Here, D is the diﬀusion coeﬃcient of carriers and ispractically constant for carrier densities below about
1017 cm−3.36) Assuming that 3=α is 1 µm and D is 1 cm2=sfor BaSi2,20) τmin is calculated to be 10 ns. In Fig. 6, τmin
(10 ns) is reached at NT = 1013 cm−3 for undoped BaSi2 at
σ = 10−12 cm2. However, the capture cross sections of gap
states are usually much smaller than this value (10−12 cm2),
which is reported to be 10−18–10−16 cm2 in a-Si:H37) and
10−16–10−15 cm2 in GaAs (EL2).38,39) Hence, the condition ofσ = 10−12 cm−2 is very harsh for BaSi2 and is not likely tooccur. On the basis of these discussions, we conclude thatthe defect density of about 1013 cm−3 present in BaSi2 does
not signiﬁcantly aﬀect the minority carrier properties of
BaSi2.
4.

Conclusions

We grew 500-nm-thick undoped n-BaSi2 epitaxial layers
by MBE with an optimized condition of RBa=RSi = 2.2 andinvestigated defect levels by DLTS. The as-grown sample
had a hole trap level (H1) of 0.27 eV from the VBM, andthe density was approximately 1013 cm−3. This value wasapproximately two orders of magnitude smaller than that
previously reported. This trap level was supposed to becaused by Si vacancies in BaSi2 because its density decreased
close to the BaSi2=Si interface. From the viewpoint of SRH
recombination, a defect density on the order of 1013 cm−3
does not have a detrimental inﬂuence on the minority carrier
properties of BaSi2.

075801-4© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 075801 (2018)

Y. Yamashita et al.

Acknowledgements

This work was ﬁnancially supported by JSPS KAKENHI
Grant Numbers 15H02237 and 17K18865, and JST MIRAI.

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