Impurity Compensation Effect Induced by Tin Valence Change in Î±

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Article

Impurity compensation effect induced by tin valence change in #-Ga Sn O thin films 1.4

0.6

3

Xiaolong Zhao, Zhenping Wu, Wei Cui, Yusong Zhi, Daoyou Guo, Linghong Li, and Weihua Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09380 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 12, 2016

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ACS Applied Materials & Interfaces

Impurity compensation effect induced by tin valence change in α-Ga1.4Sn0.6O3 thin films Xiaolong Zhao,† , ‡ Zhenping Wu,*, †, ‡ Wei Cui,† Yusong Zhi,† Daoyou Guo,† Linghong Li,§ and Weihua Tang**, †, ‡

†

Laboratory of Optoelectronics Materials and Devices, School of Science, Beijing University of Posts

and Telecommunications, Beijing 100876, China. ‡

State Key Laboratory of Information Photonics and Optical Communications, Beijing University of

Posts and Telecommunications, Beijing 100876, China. §

Department of Physics, The State University of New York at Potsdam, Potsdam, New York

13676-2294, USA.

1

ACS Paragon Plus Environment


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ABSTRACT Corundum-structured α-phase Ga1.4Sn0.6O3 thin films have been deposited on m-plane Al2O3 (300) substrates using laser molecular beam epitaxy technology. With increasing of the oxygen partial pressure, the crystal lattice of Ga1.4Sn0.6O3 films expands due to tin ions valence changes from Sn4+ to Sn2+. The resistivity of the film deposited under 3×10-5 Pa is 3.54×104 Ω·cm, which decreases by about two orders of magnitude than that fabricated under 3×10-1 Pa. The mixture valence of Sn2+ and Sn4+ ions leads to the impurity altitude compensation effect. The deep ultraviolet photodetector based on α-phase Ga1.4Sn0.6O3 thin films was fabricated. With the oxygen partial pressure reducing gradually, the dark current and the photo current increase, and the relaxation time constants diminish, respectively. KEYWORDS: corundum-structured, α-phase, oxygen partial pressure, valence, resistivity, photodetector.

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1. INTRODUCTION As the development of the epitaxy technology and the processing method, gallium oxide (Ga2O3) has attracted increasing interest as a kind of wide band-gap (4.9 - 5.3 eV) semiconductor.1, 2 Ga2O3 occurs in five polymorphous structures depending on ambient conditions, like α-, β-, γ-, δ-, and ε-phase.3-5 Researchers have paid much attention to β-phase Ga2O3 due to a wide range of applications, such as gas sensing,6 transparent electronic devices,7,

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spintronic devices,9 deep ultraviolet (DUV)

photodetector,10 and in the field of catalysis.11, 12 β-Ga2O3 is a dielectric material13-16 with a dielectric constant of 10.2-14.2 and breakdown field strength of 3.5 MV/cm.17 N-type conduction in Ga2O3 can be obtained by the generation of electrons from oxygen vacancies ionization, but the conductivity is too low. Thus, it is momentous to produce semiconducting properties by doping with appropriate metallic ions for different practical applications like transparent conducting oxide (TCO), etc.18, 19 Tin is an extrinsic dopant to Ga2O3 for producing n-type conduction.7, 20, 21 Recently, Mi et al. reports an increase in the conductivity in a certain range of tin doped Ga2O3 thin films deposited by metal organic chemical vapor deposition (MOCVD) on MgO (110) substrates.22 However, it is significant to control the conduction type and conduction ability for the various practical applications. The corundum-structured α-Ga2O3 is attractive for the researchers to fabricate the alloys with other corundum-structured α-M2O3-type materials. This type alloys materials usually have many unique properties. α-Al2O3 is an optical gain material for planar waveguide laser; α-Fe2O3 is a kind of spintronic materials possessing the feeble 3



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ferromagnetism.23, 24 α-Cr2O3 is the surface coating material on stainless steels with a higher thermal stability;25α-Ti2O3 is a sort of materials with semiconductor-metal transition inducing by temperature;26 At present, α-Ga2O3 has been demonstrated in the field of DUV photodetector as its band-gap is about 5.3 eV.1, 27 The conductivity of α-Ga2O3 thin films28 and the dark current of DUV photodetector29 could be increased by doping Sn4+ ions in our study. In this study, we prepared α-Ga1.4Sn0.6O3 epitaxial films on (300) Al2O3 substrates under different oxygen partial pressure by laser molecular beam epitaxy (L-MBE) technology. The effects of oxygen partial pressure on tin valence states in the thin films were investigated. Meanwhile, the resistivity of films and the performance characteristics of DUV photodetector were explored.

2. EXPERIMENTAL SECTION The Ga1.4Sn0.6O3 target was prepared by mixing analytical grade Ga2O3 and 30 mol % of SnO2 powders, pressed into a disk, and then sintered using standard solid state reaction. The Al2O3 (300) single-crystal substrates were put in the chamber at a base pressure of 1×10-6 Pa. The laser ablation was carried out using a KrF excimer laser (248 nm) with a laser fluence of ~4 J/cm2. The distance between the substrate and the target was 40 mm. During the growth, in-situ reflection high energy electron diffraction (RHEED) was employed to monitor the growth mode. The total number of pulse laser was fixed at 5000 and the laser frequency was 1 Hz. The thickness of the films was estimated to be 100 nm by scanning electron microscope (SEM). The composition in the final samples is almost equal to Sn-doped Ga2O3 target (30 mol %) 4


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measured by the energy dispersive x-ray (EDX) spectrum. The crystal structure of the films was measured through X-ray diffraction (XRD) using a PANalytical X'pert PRO diffractometer with Cu Ka (λ = 1.5405 Å) radiation. The valences of tin ions were analyzed by X-ray photoelectron spectroscopy (XPS). The Hitachi U-3900 ultraviolet-visible (UV-vis) spectrophotometer was adopted to measure UV-vis absorption spectrum of the samples. In order to measure both resistivity and photoelectric response of DUV photodetector, the point and interdigital Au/Ti electrodes were deposited on the surface using shadow mask and RF magnetron sputtering system. The deposition time for point and interdigital electrodes was Ti: 1 min/Au: 5 min, Ti: 20 s/Au: 2 min, followed by thermally annealing at 200 ℃ for 10 min in Ar ambient. The resistivity and photo-response were measured using a Keithley 2450 system. A low-pressure mercury lamp (254 nm) was served as the light source. The characteristic spectral line of mercury vapor under low-pressure is 254 nm, thus, it can provide a single and stable light source. All the measurements were carried out at 300 K.

3. RESULTS AND DISCUSSION Figure 1(a) exhibits the XRD patterns of Ga1.4Sn0.6O3 thin films deposited on m-plane Al2O3 (300) substrates with different oxygen partial pressure at 850 ℃. It is observed that only one diffraction peak is located at around 64° except for the diffraction peak of Al2O3 substrate. All of them belong to Ga1.4Sn0.6O3 and there is no other peaks related to tin mental clusters, tin oxide, etc. The peaks position is located at 64.80°, 64.55°, 64.31°, 63.93° and 63.52° as the oxygen partial pressure is equal to 3×10-5 Pa, 5



3×10-4 Pa, 3×10-3 Pa, 3×10-2 Pa and 3×10-1 Pa, respectively. Based on the powder diffraction file, the peaks position of about 64° is corresponding to (300) of trigonal α-phase Ga2O3 (PDF# 06-0503). The peaks position moves to smaller 2θ gradually with the oxygen partial pressure increase, indicating that an increase in the lattice constants as some Sn4+ ions gradual change into Sn2+ ions [see Figure 2] (the ionic radius of Ga3+, Sn2+, and Sn4+ is equal to 0.62 Å, 1.12 Å, and 0.69 Å, respectively).30, 31

The RHEED image of α-Ga1.4Sn0.6O3 thin film [3×10-5 Pa] is streaky and distinct as

shown by the inset to Figure 1(a), it can be concluded the film possesses single phase, besides, the surface of film is flat and smooth. The relationship between plane distance (300) and average radius of tin ions is shown in Figure 1(b), R is equal to 0.69 Å and 0.86 Å for 3×10-5 Pa and 3×10-1 Pa, respectively, calculated by XPS. Due to the spacing d linearly increases with ionic radius R ,31 the average radius of tin ions is estimated to be 0.73 Å, 0.77 Å and 0.80 Å for the oxygen partial pressure of 3×10-4 Pa, 3×10-3 Pa and 3×10-2 Pa, respectively. (b) (300)

Al 2 O 3(300)

(a)

55

Experiment Fitted line

1.455 。

-5

3×10 Pa -4 3×10 Pa -3 3×10 Pa -2 3×10 Pa -1 3×10 Pa

50

1.460

d (30 0) ( A )

Intensity(a.u.)

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1.450 1.445

60 65 2Θ(degree)

70

1.440

0.70

0.75 0.80 。 R(A)

0.85

Figure 1. (a) XRD patterns of α-Ga1.4Sn0.6O3 films with different oxygen partial pressure, the inset shows RHEED pattern of α-Ga1.4Sn0.6O3 thin film (3×10-5 Pa). (b) Relationship between (300) 6


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plane distance and average radius of tin ions.

To understand the underlying mechanism for the lattice expansion with the tin doping. The element bonding of α-phase Ga1.4Sn0.6O3 films [3×10-5 Pa & 3×10-1 Pa] is examined by XPS as shown in Figure 2. The Sn 3d XPS spectra of ~3×10-5 Pa [see Figure 2(a)] shows two chief peaks centered at 486.4 eV and 495 eV, which can be attributed to the Sn 3d5/2 and 3d3/2 levels in the tetravalent Sn4+ oxidation state.32 In addition, two feeble peaks centered at484.5 eV and 492 eV at the lower energy side of the Sn4+ 3d5/2 and 3d3/2 peaks are also observed corresponding to the 3d levels of metallic Sn0.32-34 Thus, tin dopants in α-phase Ga2O3 mostly occur in the tetravalent Sn4+ oxidation state, and rarely exist in the metallic Sn0 state. On the other hand, the standard Gibbs formation energy for Ga2O3 ( ∆ G fO ∼ 332.8 kJ/mol oxygen) is lower than that of SnO2 ( ∆GfO ∼ 257.9 kJ/mol oxygen).35 Thus, oxygen tends to combine with gallium to form Ga2O3. Therefore, the tin dopants may not be fully oxidized in the oxygen-poor environment. There is no crystalline tin phase in the XRD patterns, implying a tin cluster form in the film.34, 36 The coexisting Sn2+ and Sn4+ ions in the film [3×10-1 Pa] are evidenced by XPS as shown in Figure 2(b). Two peaks in the binding energy range from 478 eV to 500 eV is assignable to be Sn 3d5/2 and Sn 3d3/2. The asymmetric Sn 3d5/2 peak is fitted by two peaks with binding energy at 486.4 eV and 487.3 eV, assigned to Sn2+ and Sn4+, respectively.37-39 Similarly, Sn 3d3/2 peak structure is also fitted by two peaks at 494.8 eV and 495.7 eV. Due to Sn4+ ions and Sn0 ions in the sample [3×10-5 Pa], while Sn2+ ions and Sn4+ ions in the sample [3×10-1 Pa]. It can be concluded that the disproportionation reaction [ SnO2 + Sn ⇔ 2 SnO ] 7



may occur under higher oxygen partial pressure during the preparation process. (a)

(b) 4+

3d5/2

Sn 3d5/2

In ten sity(a.u.)

4+

Intensity(a.u.)

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Sn 3d3/2

0

Sn 3d3/2

0

Sn 3d5/2

480

484 488 492 496 Binding Energy(eV)

500

4+

Sn

3d3/2

2+

Sn

480

484 488 492 496 Binding Energy(eV)

500

Figure 2. XPS spectra of α-Ga1.4Sn0.6O3 films with the oxygen partial pressure of (a) 3×10-5 Pa and (b) 3×10-1 Pa.

Figure 3(a) shows current-voltage characteristic curves in logarithmic form of α-Ga1.4Sn0.6O3 films under different oxygen partial pressure. The evenly spaced Au/Ti point-electrodes were deposited on the films’ surface and the schematic illustration of point-electrodes is shown by the inset in Figure 3(a). The fine ohmic contacts have formed between the electrodes and the films because of a linear increase in current with the increase of applied bias. It can be seen that the current obvious enhances with the decrease of oxygen partial pressure. Figure 3(b) exhibits the resistivity of α-Ga1.4Sn0.6O3 films as a function of various oxygen partial pressure at room temperature. The thicknesses of the as-grown thin films measured by cross sectional SEM are all about 100 nm in our study. It almost linearly increases in the square resistivity of the films with the increase of oxygen partial pressure.

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(b) 1×107

(a) 101 0

Resistivity(Ωcm )

10 Current(nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

10

-2

10

-3

10

-4

10

-1

3×10 Pa -2 3×10 Pa -3 3×10 Pa -4 3×10 Pa -5 3×10 Pa

6

1×10

5

1×10

4

-30 -20 -10 0 10 Voltage(V)

20

30

1×10

-5

-4

-3

-2

-1

3×10 3×10 3×10 3×10 3×10 Oxygen Partial Pressure

Figure 3. (a) Current-voltage characteristic curves in logarithmic form of α-Ga1.4Sn0.6O3 films under various oxygen partial pressure, the inset shows the schematic illustration of point-electrodes. (b) Relationship between resistivity and oxygen partial pressure.

Figure 4(a) shows the band-gap (Eg) determined from the absorption spectrum of α-Ga1.4Sn0.6O3 film (3×10-5 Pa). The Eg of film can be calculated from the formula:40

αhν = B（hν − E g )n

(1)

Where α is the absorption coefficient; h is Planck constant; v is frequency of incident photon; B is the material constant; n is equal to 0.5 due to the direct transition. Eg is obtained through plotting the linear component of the (αhν)2 versus hν plot.29, 41, 42

The Eg of α-Ga1.4Sn0.6O3 film is 4.89 eV (~253.58 nm), which is smaller than that of

α-Ga2O3 reported by others.1, 27 The reason for narrower Eg can be ascribed to the Eg of SnO2 is 3.6 eV,43 which is lower than that of α-Ga2O3. The Eg of α-Ga1.4Sn0.6O3 thin film (3×10-5 Pa) is situated at the DUV region, allowing design and preparation of DUV photodetector. In order to investigate UV photo-response of α-Ga1.4Sn0.6O3 thin film, the shadow mask is put on the surface of film to deposit the four-pair interdigital electrode (the inset to Figure 4(a)). The length, breadth, and finger spacing of interdigitated electrode are 2.8 mm, 0.2 mm, 0.2 mm, respectively. Figure 4(b) shows 9



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the measured I-V characteristic curves of photodetectors under different oxygen partial pressure in logarithmic form. Both the dark current and the photo current increase obviously to photodetectors with the increase of oxygen partial pressure. Figure

4(c) exhibits

the

time-dependent

photo-response

of

α-Ga1.4Sn0.6O3

photodetectors under different oxygen partial pressure to 254 nm illumination by on/off switching at 50 V. All the photodetectors possess high steadiness and good reproducibility as the devices exhibit nearly uniform UV photoresponse under four photoperiods. The current of around 15 nA in dark increases to 18 nA under 254 nm illumination for the photodetector (3×10-1 Pa). However, it increases in the current intensity both under dark and 254 nm illumination with the decrease of oxygen partial pressure. Figure 4(d) shows the fitting photo-response process of photodetectors under 254 nm illumination, the exponential relaxation equation [27] is as follows:

I = I 0 + Ce-t/τ

(2)

Where I0 is stable current; t is time; C is constant; τ is relaxation time constant, respectively. τr represents the relaxation time constant for rising border and τd represents the relaxation time constant for falling border. The τr and τd of photodetectors become longer with the increase of oxygen partial pressure.

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(a)

(b)

30

3

25 2

10

20

Current(nA)

(αhν)2( 10 11eV 2cm -2)

10

15 10

1

10

-5

0

10

5 -1

10

0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Photon Energy(eV)

3×10 Pa Dark -5 3×10 Pa 254nm -4 3×10 Pa Dark -4 3×10 Pa 254nm -3

3×10 Pa Dark

-3

3×10 Pa 254nm -2 3×10 Pa Dark -2 3×10 Pa 254nm -1 3×10 Pa Dark -1 3×10 Pa 254nm

-50 -40 -30 -20 -10 0 10 20 30 40 50 Voltage(V)

(c)

(d)

600 500 400 300 200 300 250 200 150 100 150 120 90 60 60 50 40 30 20 18 17 16 15

UV OFF

-5

3×10 Pa

UV ON -4

UV OFF

3×10 Pa

UV OFF

3×10 Pa

UV ON -3

UV ON -2

3×10 Pa

UV OFF

UV ON -1

3×10 Pa

UV OFF

UV ON

60 120 180 240 300 360 420 480 540 Time(s)

C u rre n t(n A )

C u rre n t(n A )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 -5 3×10 Pa 500 400 τd=8.73 S 300 τr=3.88 S 200 300 -4 3×10 Pa 250 200 τd=10.02 S 150 τr=4.98 S 100 150 -3 3×10 Pa 120 90 τ =5.01 S τd=13.10 S r 60 60 -2 3×10 Pa 50 40 τd=13.57 S 30 τr=8.84 S 20 -1 18 3×10 Pa 17 τd=28.77 S 16 τr=12.85 S 15 300 315 330 345 360 375 390 405 420 Time(s)

Figure 4. (a) Eg of α-Ga1.4Sn0.6O3 thin film (3×10-5 Pa), the schematic illustration of photodetector is shown by the inset. (b) I-V characteristic curves in logarithmic form of photodetectors both in dark and under 254 nm illumination. (c) Time-dependent photo-response of photodetectors under 254nm UV illumination. (d) Experimental and fitted curve of the photo current rising and decay process to 254 nm UV illuminations.

Table 1 shows the characteristic parameters of α-Ga1.4Sn0.6O3 films under different 11



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oxygen partial pressure. It increases in resistivity with the increase of oxygen partial pressure - 3.54×104, 5.85×104, 1.52×105, 3.53×105, 2.17×106 Ω·cm at 3×10-5 Pa, 3×10-4 Pa, 3×10-3 Pa, 3×10-2 Pa, 3×10-1 Pa, respectively. Although an enhancement in the mobility with the decrease of oxygen partial pressure, it is generally lower than the reports by others due to many dopants in the films like Sn0 and Sn2+. The photodetector exhibits more obvious DUV characteristic with higher current intensity and shorter relaxation time constants for the photodetector under lower oxygen partial pressure. Table 1. The characteristic parameters of α-Ga1.4Sn0.6O3 thin films under different oxygen partial pressure. Items

3×10-5 Pa

3×10-4 Pa

3×10-3 Pa

3×10-2 Pa

3×10-1 Pa

Resistivity (Ω·cm)

3.54×104

5.85×104

1.52×105

3.53×105

2.17×106

Mobility (cm2V-1s-1)

0.12

0.093

0.076

0.064

0.022

Dark Current (nA)[50V]

141.30

89.93

45.41

20.49

14.76

Photo Current (nA)[50V]

540.39

282.95

125.83

51.11

17.92

τr (s)

3.88

4.98

5.01

8.84

12.85

τd (s)

8.73

10.02

13.10

13.57

28.77

In order to explain the carrier transport mechanisms in α-Ga1.4Sn0.6O3 thin films, an expected band diagram is proposed in Figure 5. All the electrons can excitated from donor level (ED) to conduction band for the α-Ga1.4Sn0.6O3 thin film (3×10-5 Pa). With the oxygen partial pressure raising gradually, some holes form in acceptor level (EA) 12


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with because of Sn2+ ions exist in the films. The electrons neutralize with holes in the first place, then ionize. The Hall Effect measurements also reveal a decline in the mobility with the increase of oxygen partial pressure. It will result in an increase in the resistivity of α-Ga1.4Sn0.6O3 thin films. The solid red arrow represents the electron-hole pairs generation under 254 nm UV illumination and the virtual arrows show two pathways for the carriers’ recombination as removing UV illumination. A decline in the mobility also will lead to a decrease in dark current and photo current for α-Ga1.4Sn0.6O3 photodetectors. Due to the carrier concentration in α-Ga1.4Sn0.6O3 photodetector (3×10-5 Pa) is more than the others. The carrier concentration rapid change as soon as the light is turned on/off. However, many dopants and low carrier mobility in the photodetector (3×10-1 Pa) will delay the relaxation time

Figure 5. Schematic diagram illustrating the carrier transport mechanisms in α-Ga1.4Sn0.6O3 thin films.

4. CONCLUSIONS (300) oriented α-Ga1.4Sn0.6O3 thin films were deposited on m-plane (300) Al2O3 substrates under different oxygen partial pressure using L-MBE technology. The 13



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variation of tin ions valence state with different oxygen partial pressure was verified by the crystal lattice expansion and XPS spectra results. An increase in resistivity of α-Ga1.4Sn0.6O3 thin films is caused by a decline in the mobility with impurity compensation effect. Thus, the performance of DUV photodetector based on α-Ga1.4Sn0.6O3 thin films can be improved for the stronger current intensity and the shorter relaxation time constants under poor-oxygen atmosphere.

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AUTHOR INFORMATION Corresponding author *Z. Wu. E-mail: [email protected]. **W. Tang. E-mail: [email protected].

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51572033,

11404029,

51172208),

Natural

Science

Foundation

of

Beijing

Municipality (No. 2154055), China Postdoctoral Science Foundation (Grant No. 2014M550661).

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Table of contents (TOC)

23


Impurity Compensation Effect Induced by Tin Valence Change in Î±

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