Donor and acceptor energy levels in impurity Sb-, In-, Ag- and Cu-doped semiconducting BaSi2 thin films for device applications

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Energy (2017) 000–000 612–620 EnergyProcedia Procedia124 00 (2017) www.elsevier.com/locate/procedia

7th International Conference on Silicon Photovoltaics, SiliconPV 2017 7th International Conference on Silicon Photovoltaics, SiliconPV 2017

Donor and acceptor energy levels in impurity Sb-, In-, Ag- and CuDonor and acceptor energy levels in impurity Sb-, In-, Ag- and Cufilms forHeating device doped semiconducting BaSi The 15th International Symposium District and applications Cooling 2 thin on doped semiconducting BaSi 2 thin films for device applications a* b Ajmal Khanof Takashi Suemasu a* and Assessing theM. feasibility using the heat demand-outdoor M. Ajmal Khan and Takashi Suemasub Institute of Technology, Fukushima College, Iwaki, Fukushima, 970-8034, Japan temperatureNational function for a long-term district heat demand forecast National of Technology, Fukushima College, Tsukuba, Iwaki, Fukushima, 970-8034, Japan InstituteInstitute of Applied Physics, University of Tsukuba, Ibaraki 305-8573, Japan a

a b b

Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

a,b,c

I. Andrić

*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc

a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Abstract Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes energy Énergétiques Environnement - IMTSb-, Atlantique, 4 rue Alfred Kastler,BaSi 44300 Nantes, France In this article donor and acceptor levelsetdue to the impurity In-, Agand Cu-doped 2 films grown in the ultraIn thisvacuum, article donor acceptorbeam energyepitaxy levels due to the impurity Sb-, In-, The Ag- temperature and Cu-dopeddependence BaSi2 filmsofgrown in the high UHVand molecular (MBE) were investigated. electron or ultrahole high vacuum, UHV molecular epitaxy (MBE) investigated. The Ag-doped temperature dependence of electron or hole concentrations indicated that thebeam acceptor energy levelswere in impurity, In-, and BaSi 2 are 86 meV, and 126 meV, meV, and 126 meV, concentrations indicated thatenergy the acceptor levels impurity, In-, BaSi and 2Ag-doped BaSiand 2 are are 35 meV, 4786 meV respectively. Two respectively, and the donor levels inenergy impurity Cu-,in and Sb-doped Abstract respectively, the donor in impurity and Sb-doped are Cu 35 impurity meV, andatoms 47 meV respectively. Two shallow donorand energy levelsenergy of 47 levels meV and 35 meVCu-, respectively due toBaSi Sb 2and in n-type BaSi2 were shallow donor energy for levels of 47 meV and 35including meV respectively due to Sb and Cu impurity atoms in n-type BaSi2 were successfully identified the device applications photovoltaic. District heating networks commonly addressed in thephotovoltaic. literature as one of the most effective solutions for decreasing the successfully identified for theare device applications including greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat © 2017 The Authors. Published by Elsevier Ltd. ©sales. 2017 Due The Authors. Authors. Published by Elsevier Elsevier Ltd. and building renovation policies, heat demand in the future could decrease, to the changed climate conditions © 2017 The Published by Ltd. Peer review the scientific conference committee Peer review by by the scientificreturn conference committee of of SiliconPV SiliconPV 2017 2017 under under responsibility responsibility of of PSE PSE AG. AG. prolonging the investment period. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand Keywords: Donor energy levels; acceptor energy levels; impurity-doped BaSi2; molecular beam epitaxy (MBE); electronic devices, potovoltaic forecast. Donor The district of Alvalade, located in Lisbon (Portugal), used as a case study. The districtdevices, is consisted of 665 Keywords: energy levels; acceptor energy levels; impurity-doped BaSi2was ; molecular beam epitaxy (MBE); electronic potovoltaic devices buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district devices renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were with results from a dynamic heat demand model, previously developed and validated by the authors. 1.compared Introduction results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1.The Introduction (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation As the population increases the demand for energy also increases and it has been predicted that the global energy scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). As thewill population increases thedouble demandbyfortheenergy increases andaverage it has been predicted thatsolar the global energy demand more than ofalso 2050 [1]. of The of the radiation The value of increase slope coefficient increased on averageend within the range 3.8% up to intensity 8% per decade, that corresponds tothat the 20 by the end of 2050 [1]. The average intensity of the solar radiation that demand will increase more than double could during provide much season energy(depending as we currently consume over the entire strikes theinearth in one ofhour (4.3hours × 1020 decrease the number heating of J) 22-139h theasheating on the combination of weather and 20 in one hour (4.3 × 10 J) could as much energy we currently over the entire strikes the× earth J ~13considered). TW) on the earth [2].provide According to Lewis et.al,asfor covering of 0.16% area(depending on planet earth year (4.1 10 renovation scenarios Onplanet the other hand, function intercept increased 7.8-12.7% perconsume decade on the 20 J ~13 TW) on the planet earth [2]. According to Lewis et.al, covering of 0.16% area on planet earth year (4.1 × 10 with 10%scenarios). efficient solar cell could possibly of power [3], which is sufficient enough for our yearly coupled The values suggested couldprovide be used20 to TW modify the function parameters for the scenarios considered, and with 10%the efficient solar celldemand could possibly provide 20 TW of for power [3], which is sufficient enough forrenewables our yearly improve accuracy of heat estimations. consumption of energy. Year 2014 was said to be a benchmark renewable energy in the Europe, with

consumption of energy. Yearthe 2014 was said to be benchmark forThe renewable energy in the Europe, renewables producing more power than nuclear power fora the first time. PV market total capacity haswith reached to 242 producing more power than the nuclear power for the first time. The PV market total capacity has reached to high 242 © 2017 The Authors. Published by Elsevier Ltd. GW in 2016 globally, which is reported recently by Fraunhofer ISE 2016. Therefore, the demand for low cost, Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and GW in 2016 globally, which is reported recently by Fraunhofer ISE 2016. Therefore, the demand for low cost, high efficiency and safe energy resources imposes strict requirements on the choice of materials for photovoltaic Cooling. and safe energy resources imposes strict requirements on the choice of materials for photovoltaic efficiency

1876-6102 2017demand; The Authors. Published bychange Elsevier Ltd. Keywords:©Heat Forecast; Climate 1876-6102 The Authors. Published by Elsevier Ltd. Peer review©by2017 the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. 10.1016/j.egypro.2017.09.089

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applications. Thus, exploring different materials other than Si, CIGS, and III-V compound semiconductors are very important for thermoelectric device application [4] as well as for photovoltaic device application too [5]. Among such materials, we have been focused on semiconducting BaSi2 [6-9]. BaSi2 is an emerging candidate for an absorber-layer material of thin-film solar cells because its constituent elements in the earth’s crust are abundant and it has suitable band gap around 1.34 eV [10,11]. We have shown the photocurrent limit for different configuration having different band gap under the solar spectrum in Fig.1(a), where the heterojunction type configuration based on BaSi2 could possibly matching the solar spectrum, as shown in Fig. 1(a)-(b). It has high absorption coefficients in spite of having indirect band gap [11]. It has a long minority-carrier lifetime of 10 μs [12-14] and diffusion length of 10 μm [15,16]. The absorption coefficient of BaSi2 reaches 3×104 cm-1 at 1.5 eV [11,17], which is 30 times larger than the single crystal Si. Such high absorption has been attributed to the large dipole values of the matrix elements across the gap, due to a mixture of Ba-pd and Si-spd states [17,18]. The top of the valence band of BaSi2 was proved to be consist of mainly Si p state by hard x-ray photoelectron spectroscopy (XPS) [19]. Recently a high power conversion efficiency solar cell approaching 10% has been reported under the standard AM 1.5 illumination by using a p-type BaSi2/n-type Si heterojunction structure [20-22, 29]. In semiconducting BaSi2, we must introduce impurities into an extremely pure intrinsic BaSi2 semiconductor by intentional doping for the purpose of modulating its electrical properties for the desired homojunction or heterojunction solar cells application as shown in Fig. 1(b). The role of impurities levels are strongly dependent upon the quality of intrinsic BaSi2 semiconductor and the properties that it needs to have for its intended purpose in p-n junction for device applications. The expected band alignment of the p-BaSi2/n-Si heterojunction due to the difference in the carrier concentration between the p-BaSi2 (p=2.2×1019 cm-3) and n-Si (n=2×1015 cm-3) has been shown in Fig. (b). Thus, control of carrier type, ionization energy levels and conductivity due to impurity doping are very important elements for the design and development of any electronic device structure. BaSi2 exhibits intrinsically n-type conductivity, where the electron density is approximately 5×1015 cm-3 and the carrier mobility is 820 cm2/V·s at RT [10]. A rst-principles density functional theory supercell approach revealed that this n-type conductivity arises from Si vacancies [23] in BaSi2. According to Imai and Watanabe, substitution of Si in the BaSi2 lattice is more favourable than substitution of Ba from an energetic perspective [24]. In the past very few researchers, were focused on the research of impurity doping into BaSi2 materials to modulate the electrical properties (donors or acceptors energy levels) for the design and development of photovoltaic device application. Several impurity dopant candidates were attempted either to make a n-type or p-type BaSi2 thin film by using group 11 elements (Cu[25] and Ag[26]), group 13 elements (B[20,27-29], Al[30], Ga[31], and In[31,32]), and group 15 elements (P[33-35], As[36], and Sb[32]). In line with the theoretical expectation [24,37], BaSi2 films doped with group 13 elements except Ga exhibits n-type conductivity, while those with Cu or group 15 elements exhibit p-type conductivity. However there is no experimental report about the energy levels identification due to the impurity Sb-, In-, Ag- and Cu-doped p(n)-type BaSi2.

(a)

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3

(b)

Fig. 1. (a) Photocurrent limits under the solar spectrum for different band gap materials with different device configuration as a top cell, middle cell and bottom cell respectively, and (b) Expected band alignment of the p-BaSi2/n-Si heterojunction due to the difference in the carrier concentration between the p-BaSi2 and n-Si.

Therefore, in this work it was aimed to identify the donor and acceptor energy levels due to the impurity Sb-, In-, Ag- and Cu-doped BaSi2 films on Si(111) substrates by using UHV-MBE growth conditions for the design and development of homojunction or heterojunction solar cells and some other electronic devices too. 2. Experimental procedure An ion-pumped MBE system equipped with standard Knudsen cells (K-cells) for Ba and for other impurity Kcells crucible temperature sources (Sb, Cu, In and Ag), were used respectively. An electron-beam evaporation source was used for Si atoms in the MBE system. We used high resistivity (ρ > 1000 Ω.cm) floating-zone n-type or p-type Si(111) depending on the carrier type of the grown layers for Hall measurement. We used a two-step growth method including reactive deposition epitaxy (RDE; Ba deposition on hot Si) [38] to form epitaxial templates and MBE (co-deposition of Ba, Si, and impurity) [39]. During RDE, the deposition rate of Ba was fixed around 1.0-1.2 nm/min and then 10-20 nm thick BaSi2 epitaxial templates (films) were grown on Si(111) at 550°C. Templates act as a seed crystal for overlayers, owing to which a-axis oriented epitaxial layers of BaSi2 can be grown over a wide temperature range from 450 to 700ºC [39]. During MBE growth, Ba, Si, and impurity were co-evaporated onto the BaSi2 template to form impurity doped n-type or p-type BaSi2 layers. The substrate temperature, TS and K-cell crucible temperature sources, TIn, TSb, TCu, and TAg for each impurity type are summarized in Table 1. The crystalline quality of grown films were characterized by x-ray diffraction (XRD) with Cu Kα radiation (wavelength 1.5418 Å) and reflection of high-energy electron diffraction (RHEED) observed along the [1–10] azimuth of the Si(111) substrate. The carrier type, carrier concentration, and mobility were measured by Hall measurements using van der Pauw method at RT. The carrier transport mechanism were also investigated using Matthiessen’s rule [2]. The ionized impurity scattering in conventional semiconductors is predicted, as given by Matthiessen’s rule in Eq. (1):

1 



1

I



1

L

(1)

Here, μI and μL are the carrier mobility limited by impurity and lattice scattering, respectively. Then the donor or acceptor levels, that is, ED and EA, respectively, were measured from the temperature dependence of electron concentration n(T) or hole concentration p(T) using Eqs. (2) and (3) respectively,

M. Ajmal Khan / Energy 00 (2017)124 000–000 M. Ajmal Khan et al.Procedia / Energy Procedia (2017) 612–620

4

615

 E  n(T )  exp   D   2kT 

(2)

 E  p(T )  exp   A   2kT 

(3)

Here, k is the Boltzmann constant and T is the absolute temperature. A magnetic field of approximately 0.2 T was applied normal to the sample surface during the hall measurement both at RT as well as at low temperature. 3. Results and discussion 3.1. Characterization of the crystalline structure

XRD Intensity [a. u]

(a)

BaSi2(200)

Si(111) BaSi2(400)

o

Sb temp, TSb 250 C

(*) BaSi (600) 2 o

Cu temp, TCu 975 C

20

30

40 50 2deg]

60

70

80

XRD Intensity [a. u]

One sample was chosen from each impurity type Sb-, Cu-, In-, and Ag-doped BaSi2 films respectively, for RHEED patterns as well as for ߠ-2ߠ XRD patterns observation. After cleaning the p-type or n-type Si(111) substrate by RCA method and subsequently by in-situ thermal cleaning at 850°C for 30 min in the UHV environment, then well-developed 7×7 RHEED pattern were confirmed before the growth of template layer and after the growth of each impurity doped BaSi2 samples. After the MBE growth of each impurity doped BaSi2 sample the RHEED patterns were also observed along the [1–10] azimuth of the Si(111) substrate and either spotty or streaky pattern were confirmed in all samples, which indicated that all samples with a-axis oriented BaSi2 were successfully grown. The K-cell crucible temperature sources for each impurity type In-, Sb-, Cu-, and Ag-doped BaSi2 were chosen to be, TSb=250°C, TCu=975°C, TIn=650°C and TAg=700°C respectively for donor and acceptor energy levels measurement. The diffraction peaks of (100)-oriented BaSi2 are dominant in the ߠ-2ߠ XRD patterns in all impurity Sb- and Cu- as shown in Fig. 2(a). Similarly the diffraction peaks of (100)-oriented BaSi2 are also dominant in the ߠ-2ߠ XRD patterns in all impurity Ag- and In-doped BaSi2 respectively as shown Figure 2(b). The peak of Si(222) designated by (*) in the XRD pattern were appeared due to double diffraction phenomena and therefore it is called forbidden diffraction.

(b)

BaSi2(200)

Si(111) BaSi2(400)

o

Ag temp, TAg 700 C

(*) BaSi2(600) o

In temp, TIn 650 C

20

30

40 50 2deg]

60

70

80

Fig. 2. ߠ-2ߠ XRD measurements of the impurity (a) Sb-, and Cu-doped BaSi2, and (b) In-, and Ag-doped BaSi2 films grown for ionization energy levels (donor levels, ED and acceptor levels, EA) measurement, where the K-cell crucible temperature sources are indicated in each impurity sample. Table 1. Impurity, carrier type, substrate temperature, and crucible temperature sources of impurities. Impurity/carrier type

TS (°C)

Crucible temp.(°C)

Sb/n

500-600

250-300

Cu/n

600

800-1200

In/p

600

550-650

Ag/p

600

600-900

616 5

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3.2. Electrical characterization Many semiconductor technologies and devices rely on the ability to fabricate two different types of electrically conducting layers: n-type and p-type for the formation of p-n junction diode or solar cells. Therefore, we attempted to identify the donor and acceptor energy levels in p(n)-type BaSi2. Similarly, we also need to understand the transport mechanism of carrier’s drifts across the p-n junction and therefore first, we will focus on the carriers transport mechanism in p(n)-type BaSi2. In the next subsection, we reviewed the hole and electron mobilities trends as a function of the impurity K-cell crucible temperature sources. 3.2.1. Hole and electron mobilities Figure 3(a)-(b) shows the measured electron mobilities versus the carrier concentrations for each impurity Sb-, and Cu-doped BaSi2 films at RT, which were grown by different impurity K-cell crucible temperature sources, TSb, and TCu, respectively. It was found that the electron mobilities are always larger than the hole mobilities for a given carrier concentration. According to Migas et al., this is attributed to a smaller effective mass for electrons than holes traveling along the BaSi2(100) plane[18]. The electron mobilities decreases with increasing electron concentration for the n-type impurity Sb-, and Cu-doped BaSi2. This trend is usually predicted by ionized impurity scattering in the conventional semiconductors, as given by Matthiessen’s rule in Eq. (1). In the case of Sb-doped BaSi2, the ntype conductivity was reported, where electron concentrations were varied in the range between 1016 and 1020 cm-3 at RT by changing the Sb K-cell crucible temperature, TSb (250-300°C), as shown in Fig. 3(a). In case of Cu-doped BaSi2 the n-type conductivity was confirmed. Figure 3(b) gives the relationship between measured mobility and electron concentrations for Cu-doped n-type BaSi2 at RT by changing the Cu K-cell crucible temperature, TCu (8001000°C) [25]. The electron concentration in Cu-doped BaSi2 remained unchanged, even when the Cu temperature was increased to 950°C, but increased sharply up to more than 1020 cm-3 around 1000°C. Hence, it was found that the control over the electron concentrations in Cu-doped BaSi2 was slightly difficult. In order to understand the scattering phenomenon due to the ionized impurity atoms in the semiconducting p-type BaSi2, then we need to understand the hole mobilities trends due the impurity In- and Ag-doped BaSi2 as a function of the impurity K-cell crucible temperature sources, TIn and TAg at RT. The hole mobilities decreases with increasing electron concentration as shown in Figures 3(c)-3(d), for the p-type impurity In- and Ag-doped BaSi2 respectively. First we investigated In-doped BaSi2, and it was found that some of the Si atoms in the BaSi2 lattice structure were replaced by In atoms thereby generating holes. The hole concentration in the In-doped p-type BaSi2 was controlled in the range between 1016 and 1017 cm-3 at RT by changing the In K-cell crucible temperature, TIn (550-650°C), as shown in Figure. 3(c) and the relationship of measured mobility and hole concentrations for In-doped p-type BaSi2, grown at RT has been shown. According to the energetic principle calculation, the substitution of Si in the BaSi2 lattice by Ga or In is more favorable than that of Ba [40]. The hole concentration in In-doped p-type BaSi2 layer were found lower than the expected desire level and therefore, we attempted to explore some better impurity candidate to make p-type BaSi2 thin film. In this regard, silver (Ag) was chosen, as a next p-type impurity dopant in BaSi2, where Figure 3(d) gives the relationship of measured mobility and electron concentrations in the Ag-doped ptype BaSi2, at RT for the samples grown by changing the Ag K-cell crucible temperature, TAg (600-900°C). In this case the hole concentration increases gradually from 3×1015 to 3×1016 cm-3 with increasing Ag source temperature, showing that the hole concentrations can be controlled as a function of Ag K-cell crucible temperature source. However, the hole concentrations were found to be less than those in Al [30], and In-doped p-type BaSi2 respectively. We speculated that this is attributed to a smaller ionization rate of Ag atoms in BaS2 than Al and Inatoms respectively in p-type BaSi2. In the case of Ag-doped p-type BaSi2 films grown with 700°C, carrier concentration and mobility behaviour are almost the same as those for Ga-doped [32] BaSi2. It is evident from the temperature dependent mobility behavior that the mobility contribution is not coming from the scattering of impurity atoms but comes from the Coulomb’s lattice scattering mechanism, which is supported by low hole concentration in the grown sample of Ag-doped p-type BaSi2. In order to enhance hole concentrations and to suppress Ag impurity-atomic diffusion in p-type BaSi2 grown layer, we fabricated p-type BaSi2 by using boron doping, which has been already reported elsewhere[29]. The hole concentration in the B-doped p-type BaSi2 were well controlled in the range between 1016 and 1020 cm-3 at RT by changing the Boron K-cell crucible temperature, TB

M. Ajmal Khan / Energy Procedia 00 (2017) 000–000 M. Ajmal Khan et al. / Energy Procedia 124 (2017) 612–620

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(a)

e [cm /V.s]

800

2

600

Sb-doped n-BaSi2

400

2

400

100 16

10

17

18

10

10

-3

n [cm ]

(c)

19

10

0

20

10

160

17

10

18

10

-3

n [cm ]

900 (d)

In-doped p-BaSi2

h [cm /V.s]

15

10

120

2

2

300 200

200

h [cm /V.s]

Cu-doped n-BaSi2

500

600

200

(b)

e [cm /V.s]

1000

617

80 40

19

20

10

10

Ag-doped p-BaSi2

750 600 450 300 150

17

10

-3

p [cm ]

18

10

1

16

-3

p [10 cm ]

2

3

Fig. 3. Carrier mobility vs. carrier concentrations measured at RT for n-type (a) Sb-doped BaSi2 films grown at Sb K-cell crucible temperature in the range, TSb=250-300°C, (b) Cu-doped BaSi2, film at Cu K-cell crucible temperature source in the range, TCu= 800-975°C. Similarly carrier mobility vs. carrier concentrations measured at RT for p-type (c) In-doped BaSi2 films grown at In K-cell crucible temperature in the range, TIn= 550-650°C, and (d) Ag-doped BaSi2, film at Ag K-cell crucible temperature in the range, TAg= 600-900°C.

When the TS was increased to 630°C and the K-cell crucible temperature, TB was decreased to 1250°C, then surprisingly, p reached to 6.4×1019 cm-3 [29], which were higher than those in the impurity Ag-, Al-, and In-doped p-type BaSi2. The efficiency were enhanced from 1.6% (Sb-doped n-type BaSi2 heterojunction solar cells) [28] to 10% [20-22] by using the B-doped p-type BaSi2 epilayer in the heterojunction solar cells [29]. 3.2.2. Donor and acceptor energy levels An electrically active dopant atoms provides a free carriers to the conduction or valence band by creating an energy level that is very close to one of the bands (shallow levels) in the host p-n junction of BaSi2 for homojunction or heterojunction solar cells application. In contrast, in some cases the electrically active impurity dopant atoms provide some free carriers by creating an energy level that is very deep and away from either conduction or valance bands (deep levels) respectively, which are not desired for the efficient photovoltaic applications of the desired configuration of the device structure as shown in Fig. 1(b). In this article, we mainly focused on the investigation of donor and acceptor energy levels (ED and EA) due to each impurity Sb-, In-, Cu-, and Ag-doped atoms in BaSi2 epitaxial layers grown by UHV-MBE system. Figure 4(a)-4(b) gives the temperature dependence of electron concentrations, n in the impurity Sb-, and Cu-doped BaSi2 prepared at the K-cell crucible temperature sources, TSb=250°C, and TCu=1000°C respectively. It has been shown in Figure 4(a), that the electron concentrations in Sb-doped n-type BaSi2 is about 3.3×1018 cm-3 at 300K, and then decreases gradually to 2.8×1017 cm-3 with decreasing temperature up to 27K. In this case one shallow donor level, ED=47 meV in the Sb-doped n-type BaSi2 were identified by using Eq. (2). Similarly Figure 4(b) shows the

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temperature dependence of electron concentrations, n in the Cu-doped BaSi2 prepared at K-cell crucible temperature, TCu=1000°C.

(b)

Sb-doped n-BaSi2

10

20

10

ED= 47 meV

4

6

8

10

12

-1

1000/T [K ]

16

16

2x10 -3

-3

17

10

EA2 = 11 meV

0

3.0

2x10

EA1 = 86meV

16

10

18

In-doped p-BaSi2

10

ln p [cm ]

14

16

(c) 18

10

ED = 35 meV 15

2

5

10

15

20

-1

1000/T[K ]

25

4.0

4.5

-1

1000/T [K ]

(d)

5.0

5.5

Ag-doped p-BaSi2

16

15

5x10

30

3.5

1x10

ln p [cm ]

18

16

10

19

10

10

Cu-doped n-BaSi2

17

-3

-3

ln n [cm ]

(a)

ln n [cm ]

21

10

3.0

EA = 126 meV

3.5

4.0

-1

1000/T [K ]

4.5

5.0

Fig. 4. Temperature dependence of electron concentrations, n in the n-type (a) Sb-doped BaSi2 film, prepared at TSb=250°C, and (b) Cu- doped BaSi2 film, prepared at TCu=975°C. Similarly the temperature dependence of hole concentrations, p in the p-type (c) In-doped BaSi2 film, prepared at TIn=650°C, and (d) Ag- doped BaSi2 film, prepared at TAg=700°C.

Temperature dependence of electron concentrations for Cu-doped n-BaS2 has been shown in Figure 4(b) and shallow accepter level, ED=35 meV in Cu-doped n-type BaSi2 were identified by using Eq. (2). In the next paragraph, we will discuss about the acceptor energy levels due to the impurity In-, and Ag-doped BaSi2 thin films. The temperature dependence of hole concentrations, p in the impurity In- and Ag-doped BaSi2 thin films prepared at K-cell crucible temperature sources, TIn=650°C, and TAg=700°C respectively, have been shown in Figure 4(c)-4(d). First, Indium (In) was chosen as an impurity dopant to grow p-type BaSi2 and then to investigate the ionization energy levels. Figure 4(c) gives the temperature dependence of hole concentrations, p in In-doped BaSi2, where the hole concentration reached 1×1018 cm-3 at RT, and then decreased with decreasing temperatures up to 1×1016 cm-3 after following Eq. (3). The novel acceptor levels, EIn1=11, and EIn2=86 meV respectively were identified using Eq. (3) respectively. Due to the issue of low hole concentrations and high diffusion tendency in Aldoped BaSi2 [30], therefore we further attempted to explore some more suitable p-type dopant candidate to fabricate p-type BaSi2 thin film. In this regard silver (Ag) was chosen as a next potential candidate to grow p-type BaSi2. In Figure 4(d), the novel temperature dependence of hole concentrations, p in Ag-doped p-type BaSi2 prepared at Kcell crucible temperature, TAg=700°C has been shown. The mobility is higher when temperature is lower and therefore, we can approximate the hole concentrations, p in the p-type BaSi2, which behave like Eq. (3). Consequently one deep accepter level, EA=120 meV were identified in the Ag-doped p-type BaSi2. In the case of Ag-doped BaSi2 the hole concentrations were to be lower than our desired target (p≃1019 cm-3), which is probably caused by the Ag-atomic diffusion tendency in the grown samples. Thanks to the B-doped p-type BaSi2 epitaxial

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layer in 2012 [29], the efficiency was enhanced from 1.6% (Sb-doped n-type BaSi2 heterojunction solar cells) [28] to 10% (B-doped p-type BaSi2 heterojunction solar cells) [20-22]. 4. Conclusion In this research work, (111)-oriented impurity Sb-, In-, Cu-, and Ag-doped BaSi2 thin films were successfully grown in the UHV environment using solid source MBE system for homojunction or heterojunction solar cells. The temperature dependence of electron and hole concentrations using the Van der Pauw method indicated that the activation energy levels in the impurity In-, and Ag-doped p-type BaSi2 are acceptor like, 86 meV, and 126 meV, and the activation energy levels in the impurity Sb-, and Cu-doped n-type BaSi2 are donor like, 47 meV, and 35 meV respectively. These two shallow donor energy levels, around 47 meV, 35 meV respectively in the Sb- and Cudoped n-type BaSi2 respectively could possibly be useful candidates for the design of BaSi2 based heterojunction or homojunction solar cells and also for some other electronic device applications. Acknowledgements The authors acknowledge Prof. N. Usami of Nagoya University, Dr. K. O. Hara of the University of Yamanashi, and Dr. K. Toko, of the University of Tsukuba for their fruitful discussions. This work was supported in part by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency and by a Grant-in-Aid for Scientific Research (A) (No. 15H02237) from the JSPS. References [1] A. R. Jha, Solar Cell Technology and Applications (CRC Press, Taylor and Francis Group, 2010), p.6. LLC, Boca Raton, Fla, USA. [2] S. M. Sze, Semiconductor Devices (2nd edition, Physics and Technology, 2002), p. 319. Wiley Interscience, New York, USA. [3] Lewis, N. S., Science, 315, 798 (2007). [4] Yidong Xu et al, Funct. Mater. Lett. 09, 1650017 (2016). [5] A. Polman, M. Knight, E. C. Garnett, B. Ehrler, and W. C. Sinke, Science 352, 307 (2016). [6] H. K. Janzon, H. Schafer, and A. Weiss, Z. Anorg. Allg. Chem. 372, 87 (1970). [7] J. Evers, G. Oehlinger, and A. Weiss, Angew. Chem. Int. Ed. Engl. 16, 659 (1977). [8] R. A. Mackee, F. J. Walker, J. R. Conner, R. Raj, Appl. Phys. Lett. 63, 2818 (1993). [9] M. Imai and T. Hirano, Phys. Rev. B 58, 11922 (1998). [10] K. Morita, Y. Inomata, T. Suemasu, Thin Solid Films 508,363 (2006). [11] K. Toh, T. Saito, T. Suemasu, Jpn. J. Appl. Phys. 50, 068001 (2011). [12] K. O. Hara, N. Usami, K. Toh, M. Baba, T. Toko, and T. Suemasu, J. Appl. Phys. 112, 083108 (2012). [13] K. O. Hara, N. Usami, K. Nakamura, R. Takabe, M. Baba, K. Toko, and T. Suemasu, Appl. Phys. Express 6, 112302 (2013). [14] R. Takabe, K. O. Hara, M. Baba, W. Du, N. Shimada, K. Toko, N. Usami, and T. Suemasu, J. Appl. Phys. 115, 193510 (2014). [15] M. Baba et al, J. Cryst. Growth 348, 75 (2012). [16] M. Baba, K. Watanabe, K. O. Hara, K. Toko, T. Sekiguchi, N. Usami, and T. Suemasu, Jpn. J. Appl. Phys. 53, 078004 (2014). [17] M. Kumar, N. Umezawa, and M. Imai, Appl. Phys. Express 7, 071203 (2014). [18] D. B. Migas, V. L. Shaposhnikov, V. E. Borisenko, Phys. Status Solidi B 244, 2611 (2007). [19] M. Baba, K. Ito, W. Du, T. Sanai, K. Okamoto, K. Toko, S. Ueda, A. Kimura, and T. Suemasu, J. Appl. Phys. 114, 123702 (2013). [20] D. Tsukahara et al, Appl. Phys. Lett. 108, 152101 (2016). [21] S. Yachi, R. Takabe, H. Takeuchi, K. Toko, and T. Suemasu, Appl. Phys. Lett. 109, 072103 (2016). [22] R. Takabe, S. Yachi, W. Du, D. Tsukahara, H. Takeuchi, K. Toko, and T. Sumasu, AIP Advances 6, 085107 (2016). [23] M. Kumar, N. Umezawa, and M. Imai, Spring Meeting of the Japan Society of Applied Physic, 21a-S223-2, Tokyo, Japan, March 21 (2016). [24] Y. Imai, A. Watanabe, Thin Solid Films 515, 8219 (2007). [25] M. Ajmal Khan, M. Takeishi, Y. Matsumoto, T. Saito, and T. Suemasu, Physics Procedia 11,11 (2011). [26] M. Ajmal Khan, T. Saito, K. Nakamura, M. Baba, W. Du, K. Toh, K. Toko, and T. Suemasu, Thin Solid Films 522, 95 (2012). [27] K. O. Hara, N. Usami, Y. Hoshi, Y. Shiraki, M. Suzuno, K. Toko, and T. Suemasu, Jpn. J. Appl. Phys. 50, 121202 (2011). [28] T. Suemasu , T. Saito, A. Okada, K. Toh, M. Ajmal Khan, N. Usami, Toward high-efficiency thin-film solar cells using semiconducting BaSi2, IEICE Technical Report: ED2010-53, SDM2010-54 (2010). [29] M. Ajmal Khan, K. Nakamura, W. Du, K. Toko, N. Usami, and T. Suemasu, Appl. Phys. Lett. 104, 252104 (2014). [30] M. Takeishi, Y. Matsumoto, R. Sasaki, T. Saito, and T. Suemasu, Physics Procedia 11, 27 (2011). [31] M. Kobayashi, K. Morita, and T. Suemasu, Thin Solid Films 515, 8242 (2007). [32] M. Kobayashi, Y. Matsumoto, Y. Ichikawa, D. Tsukada, and T. Suemasu, Appl. Phys. Express 1, 051403 (2008).

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