Appl. Phys. A (2017)123:448 DOI 10.1007/s00339-017-1057-4

Electrical and optical properties of lanthanum oxide-based films prepared by electron beam evaporation Arsen Igityan1 • Natella Aghamalyan1 • Silva Petrosyan1 • Irina Gambaryan1 Georgi Badalyan1 • Ruben Hovsepyan1 • Yevgenia Kafadaryan1

•

Received: 18 April 2017 / Accepted: 22 May 2017  Springer-Verlag Berlin Heidelberg 2017

Abstract Lanthanum oxide-based films were deposited on n-Si and quartz substrates using e-beam evaporation method. The XRD patterns demonstrated mixed structure consisting of La2O3 and La(OH)3 phases (referred as La2O3–OH). Optical and electrical properties of La2O3– OH films, as well as the effects of the annealing and storage conditions on these properties are described here. It is observed that conductance–voltage characteristics of Al/ La2O3–OH/n-Si devices along with current rectification show negative differential conductance as a result of water molecule redox reaction on the film surface.

1 Introduction Hexagonal lanthanum oxide (h-La2O3) is one of the promising oxides for integration into complementary metal-oxide semiconductor (CMOS) devices [1], because it acts as a key element for suppressing the gate leakage current in CMOS devices due to the high dielectric constant (20–27) [2], large band gap (4.3–6.4 eV) [3], high breakdown electric field (13 MV/cm) [4] and large conduction-band offset at La2O3/Si interface compared to widely used oxides such as SiO2, HfO2 and Al2O3 [5]. In addition, the La2O3 demonstrates optical transparency over a wide range of wavelength from ultraviolet to infrared [6]. The La2O3 films have been prepared using various deposition techniques, such as chemical vapor deposition [7], radio frequency magnetron sputtering [3], thermal & Yevgenia Kafadaryan [email protected]; [email protected] 1

Institute for Physical Research of NAS of Armenia, 0203 Ashtarak-2, Aragatsotn, Armenia

oxidation [8], ultrasonic spray pyrolysis [9] and electron beam evaporation with the ion beam-assisted technique [6, 10]. In this work, La2O3-based films are deposited on n-Si and quartz (SiO2) substrates by e-beam evaporation method. However, some problems such as moisture absorption from environment, air stability and formation of interfacial layer have arisen. We report the structural, optical and electrical properties of films consisted of mixed phases of La2O3 and La(OH)3 (referred as La2O3–OH) in connection with preparation and storage conditions. The studied Al/La2O3–OH/n-Si devices are diodes whose current–voltage characteristics show the presence of a negative differential conductance (NDC) region under forward bias. Unlike conventional two-terminal NDC devices that rely on resonant tunneling and inter-valley transferring [11], the NDC effect in the La2O3–OH films can be explained by the protonic conduction as a result of water molecule redox reaction at the top region of the La2O3–OH films. With growing demands for non-volatile solid-state memories, the data presented here can be potentially used for transparent redox-based resistive switching memory devices.

2 Experimental procedure The La2O3 (with a purity of 99.99%) starting material prepared by sintering at 1400 C in the air ambient for 2 h was used to deposit on the HF-dipped n-Si(100) substrates by an e-beam evaporation technique using BE-1A (Belarus) equipment. The base vacuum was 1.5 9 10-5 Torr and the working pressure was about 3.7 9 10-5 Torr. During the deposition process, the substrate temperature was automatically controlled at 200 C by a heating sys˚ /s. The thicknesses of tem. The rate of deposition was 2 A

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the prepared films were 40, 140 and 545 nm. As-deposited films have been stored in chamber under low vacuum of 5 9 10-2 Torr (rarefied air). After storing in vacuum the films were annealed in a resistance furnace at 650 C in vacuum ambient of 10-3 Torr for 40 min. The rising rate of the temperature was 0.6 C/s. When the annealing process had been completed, the furnace was naturally cooled down to room temperature in the same atmosphere. The 40 nm thick film was annealed immediately after deposition process without the storage in vacuum. Film phase composition was studied using X-ray diffraction (XRD) in a Bragg–Brentano geometry with ˚ ) radiation (Dron-2 instrument). The CuKa (k = 1.5418A surface morphology and elemental composition of the samples were examined by scanning electron microscope (SEM VEGA TS-5130MM) equipped with X-ray energy dispersive microanalysis (EDS) system INCA Energy 300 (Si/Li detector with take-off angle of 458 and work distance 23 mm) operating at 14, 18 and 20 kV electron probe energy. Infrared Specord M-80 spectrophotometer (Carl Ziess Jena) was used to check the water in films. The refractive index and energy band gap of the films were determined using spectrophotometers Specord M-40 and SF-8 (LOMO). The film thickness was determined by optical interference method, SEM and Ambios XP-1 profiler. Conductance–voltage (r - V) measurements of the samples were carried out at 1 kHz under bias voltages from -10.0 to ?10.0 V using digital RLC-meter E7-8 (Russia). The top Al electrodes of 3.4 9 10-3 cm2 areas were patterned by a thermal evaporation method.

3 Results and discussion

Intensity (a.u.)

100 222 101

The phases and crystalline structures of the 40, 140 and 545 nm thick films were investigated by XRD. A polycrystalline structure is identified for all as-deposited films

×-unknown phase ∗ c-La2O3 • h-La2O3 ♦ h-La(OH)3

♦

3 201

101

* • •

2

×♦ ×

1 20

30

40

50

60

(after extraction from evaporation chamber). Figure 1 shows the XRD patterns of the as-deposited, vacuum-stored and annealed 545 nm thick film. As seen in Fig. 1, the as-deposited film (pattern 1) is indexed as a composition of the hexagonal lanthanum hydroxide h-La(OH)3 (XRD card 83-2034) and cubic La2O3 (c-La2O3) phases (card 04-0856). After keeping the films in low vacuum chamber (5 9 10-2 Torr) for 10 and 120 days the XRD results show no change (pattern 2). After annealing, the (101) and (201) reflections related to the h-La(OH)3 phase disappear (pattern 3) due to the dehydration process. Taking into account that the decomposition processes La(OH)3 ? LaO(OH) and LaO(OH) ? La2O3 occur at 360 and 500 C [12], the (222) and (101) reflections in the pattern 3 can be assigned to c-La2O3 and h-La2O3, respectively [13]. As determined by EDS analysis of the films, the atomic concentration of the as-deposited films indicates O/La ratio of 2.7 7 3.3. After annealing, the O/La ratio becomes 1.8 7 2 instead of 1.5. Figure 2 displays the transmittance infrared (IR) spectra of the as-deposited (spectrum 1), stored in vacuum (2) and annealed (3) 140 nm thick film on transparent quartz (4) substrates. It should be noted that the frequencies of surface hydroxyl modes strongly depend on the morphology of the films, preparation and storage conditions [14]. According to several authors [14, 15], the absorption band at 3450 cm-1 observed in all spectra in Fig. 2 is associated with the hydroxyl groups attributed to stretching vibration of O–H bond and bending vibration of H–O–H from water molecules on the surface of the samples as a consequence of moisture absorption from ambience [15]. The width (40–60 cm-1) of the absorption band is related to the large energy spread of such vibrations due to the formation of both intra- and intermolecular hydrogen bonds. The wavenumber and intensity of the band do not change after keeping the films in low vacuum for 10 days.

100 4 Intensity (%)

448

3 1 2

80 3000 3200 3400 3600 3800 4000 Wavenumbers (cm-1)

2θ°(degree) Fig. 1 The XRD patterns of as-deposited (1) vacuum-stored, (2) and annealed (3) 545 nm thick La2O3–OH film

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Fig. 2 IR transmittance spectra of as-deposited (1), vacuum-stored (2) and annealed (3) 140 nm thick La2O3–OH film on quartz (SiO2) substrate (4)

100

80 1 60 2 40 3 20

b

80

1 2 60 3 4 40 5 20

4 0

50000 40000 30000 20000 10000 Wavenumber (cm -1)

Fig. 4 Plots of (ahm)2 versus hm for as-deposited (a), vacuumstored (b) and annealed (c) 140 nm thick film

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100

a

Transmittance (%)

Fig. 3 Transmittance UV–Vis spectra of as-deposited 40 (2), 140 (3) and 545 (4) nm thick La2O3–OH films on SiO2 (1) substrates (a); transmittance spectra of as-deposited (3) vacuum-stored (4) and annealed (5) 140 nm thick film (curves 1 and 2 are spectra of SiO2 substrate after and before annealing) (b)

Transmittance (%)

Electrical and optical properties of lanthanum oxide-based films prepared by electron beam…

50000 40000 Wavenumber (cm-1)

2.5x1010

a

1.5x1012

c

b 11

1.0x10

2.0x1010

(αhν)2, (eVcm-1)2

30000

1.5x1010

1.0x1012

5.0x1010

1.0x1010 5.0x1011 5.0x109

0.0 6.0

6.2

hν (eV) The sharp peak at 3586 cm-1 arises after being kept in low vacuum for 120 days. The La2O3 is well known to be a hygroscopic material [16] and forms a hydroxide even in trace amounts of water that can be present in vacuum chamber. The reason that various surface hydroxyl modes exist lies in the fact that the oxygen atom of the OH group can coordinate with several neighboring metal atoms [14, 15]. The coordinate bonds lead to the lower wavenumber (3450 cm-1), while formation of isolated hydroxyl groups accounts for increased wavenumber value (3586 cm-1). As can be seen in Fig. 2, the band at 3586 cm-1 disappears as the film is heated, while the band at 3450 cm-1 remains almost unchanged. Transmittance UV–Vis spectra of the as-deposited, vacuum-stored and annealed films on quartz substrates are shown in Fig. 3a, b. The 140 and 545 nm thick films exhibit highly reproducible optical edge, which is sensitive to the film thickness (Fig. 3a) and processing conditions (Fig. 3b). The energy gap (Eg) can be determined by assuming a direct transition between valence and conduction bands. The absorption coefficient a as a function of photon energy hm can be expressed as

0.0 5.8

0.0 6.0

hν (eV)

6.2

5.8

6.0

6.2

hν (eV)

ðahmÞ2 ¼ A ðhmEg Þ; where A is constant. The dependences (ahm)2 versus of energy hm for as-deposited, vacuum-stored and annealed 140 nm thick film are shown in Fig. 4. The bandgap energy was obtained by extrapolating the linear part of the curves (ahm)2 as a function hm of the incident radiation to intercept the energy axis (at a = 0). The Eg value of 6.12 eV for 140 nm thick film decreases to 6.06 eV for 545 nm thick film. The changes in bandgap energy and thickness of the asdeposited 140 and 545 nm thick films are presented in Table 1. The results indicate Eg narrowing with respect to the Eg of the as-deposited films. The water bonds are responsible for the formation of some types of defects in the films [17–19]. As the number of water bonds and defects increases, the concentration of localized states in the band structure increases. Hence, the water causes an increase in the energy width of the localized states thereby reducing the optical energy gap. The band gap of 40 nm thick film was not determined due to the limited spectral range of available spectrophotometers. Literature values

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Table 1 The band gap energy and thickness for the as-deposited, vacuum-stored and annealed La2O3–OH films (d is film thickness, Eg bandgap, fOH wavelength of hydroxyl group) d, nm

Eg, eV

d, nm

Eg, eV

fOH, cm-1

140

6.12

545

6.06

3450

Vacuum-storage film

160

5.92

635

5.903

3450, 3586

Annealed film

150

6.04

560

5.958

3450

Refractive Index

As-deposited film

1.88 1.84 1.80 1.76

1

1.72 1.68 200

2 300

400 500 600 Wavelength (nm)

Fig. 5 Wavelength dependence of refractive index for La2O3–OH film. Black dots represent refractive index values for the as-deposited film; the white dots are for the film after annealing

for the band gap of La2O3 vary from 5.4 eV for c-La2O3 to 6.4 eV for h-La2O3 [17]. No band gap values are available for La(OH)3. The band structures obtained from density functional theory calculations indicate 3.69 and 4.04 eV for La(OH)3 [17–19]. The refractive index for as-deposited and annealed film (Fig. 5) was evaluated as 1.69 and 1.735 (at 600 nm), respectively, from the interference fringes of the optical transmittance spectra for thick film using the Swanepoel

envelope method [20]. With annealing, the film might be densified and the defects could be removed. SEM micrographs of the surface of the La2O3–OH films show dense structure without visible pores (Fig. 6). The devices with Al/La2O3–OH/n-Si stack have been fabricated for electrical tests. Figure 7a shows the conductance–voltage (r - V) measurements for the as-deposited, vacuum-stored and annealed La2O3–OH films of different thicknesses. The stabilization time of each measurement was 1 min. The r - V curves of the Al/La2O3–OH/n-Si devices display rectification behavior in negative voltage sweep scans. When the voltage is ramped up from zero towards positive values, an ‘‘N-shaped’’ r - V curve is obtained (Fig. 7a) indicative of resistive switching, i.e. there is a local maximum (the device is in the low resistance, on state) in the current at voltage followed by a region of negative differential conductance (NDC) and a local current minimum (the device is in the high resistance, off state). Experimental observations may be summarized: (1) location of the voltage for maximum or minimum conductivity depends on the film thickness; (2) conductance decreases in the NDC region by a power-law form with decay exponent *1.46; (3) r - V curves are direction-dependent and the peak-to-valley ratio of the curves is about 2:1. The NDC disappears in vacuum-stored films (Fig. 7b) and reappears after annealing (Fig. 7c). As can be seen in Fig. 7c, the NDC location of the annealed thick film is well-defined in the 4–5.5 V region, whereas in the case of the vacuum-stored 140 nm thick film, the annealing leads to a minor NDC effect detected near 9 V. For comparison the r - V plot of the annealed 40 nm thick film without preliminary storage in low vacuum chamber (curve 3) is included in Fig. 7c. NDC also depends on the voltage sweep direction implying that resonant tunneling is not responsible for this phenomenon. It can be assumed, that NDC properties depend on surface

Fig. 6 SEM micrographs of 40 (a) and 545 (b) nm thick La2O3–OH films after annealing; (c) SEM micrograph of as-deposited 140 nm thick films

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Electrical and optical properties of lanthanum oxide-based films prepared by electron beam…

0.4

0.2

3

100

1

200

100

0 -5

Conductance (μS)

100

0 -10

-5 0 5 Voltage (V)

-5

10

0 5 Voltage (V)

10

2

c

80

1

60 40

1

3

20 2 -10

-5

0

Voltage (V)

b

5

2.8

5

10

0

c

Ln I ( μ A)

Log I (μ A)

e

e6

Ln I ( μ A)

1

-10

0

a

2

0

0.0 10

0 5 Voltage (V)

3

Conductance (μS)

-10

Conductance ( μS)

2

Conductance (μS)

200

b Conductance (μS)

a Conductance (μS)

Fig. 7 Conductance–voltage (r–V) characteristics of Al/ La2O3–OH/n-Si devices for asdeposited 140 (2) and 545 nm (3) thick films, curve 1 corresponds to annealed 40 nm thick film without storage in vacuum (a); r–V characteristics of vacuum-stored 140 and 545 nm thick films (b); r– V characteristics of annealed 545 (1), 140 (2) and 40 nm (3) thick La2O3–OH films (c)

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e3

e4

1

e

e2

e-1 e

2

0.5

Voltage (V)

3

n=1.5 2.0 1.6

0

1

2.4

0

1

2

0.5

Voltage (V)

3

-0.4 -0.2

0.0

0.2

0.4

0.6

Log V

Fig. 8 The Schottky emission plot of ln I versus V1/2 (positive branch) for vacuum-stored (a) and annealed (b) 140 nm thick La2O3–OH film; log I–log V plot for as-deposited La2O3–OH film (c)

hydroxyl groups formed on the film during preparation and storage. According to the literature data [21–25], the bistable interfacial contact, redox reaction and proton-mediated mechanisms can be associated with the origin of NDR. In these mechanisms a chemical reaction is involved to create bistable conducting states. For instance, in bistable interfacial contact NDC originates from the transition from strong to weak chemical bonding at film-contact interface [21–23], whereas in redox-mediated mechanism the NDC occurs due to oxidation and reduction of active molecules at a defined voltage [24–26]. In our case, the change in conductance is accompanied by proton adsorption/desorption reactions on surface hydroxyl groups [25]. Water is dissociated to oxygen and

proton at the anode through electrolysis. The fast drift of protons to cathode and slow diffusion of water at the dissociation site, where the water depletion is, cause a slow production of proton and reduction of conductance that initiates NDC. For better understanding of the conductance mechanism, the r - V curves are plotted using the mechanisms of space-charge-limited current (SCLC) with a linear current relation of I  Vn, Schottky emission (ln I  V1/2), Poole–Frenkel emission [ln (I/V)  V1/2] and Fowler– Nordheim tunneling [ln (I/V2) (1/V)] [27]. According to the fittings, the as-deposited films obey the Ohm’s law (I * Vn, n = 1.5 for V \ 3 V) (Fig. 8c), whereas the Schottky emission is the dominant conduction mechanism for vacuum-stored (Fig. 8a) and annealed (Fig. 8b) films.

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4 Conclusions The La2O3-based films were deposited on n-Si and quartz substrates by e-beam evaporation method. XRD pattern of the films showed polycrystalline mixed structure that consisted of La2O3 and La(OH)3 phases (La2O3–OH). The La2O3–OH films were optically and electrically characterized. The annealing and storage conditions significantly influence the optical constants (index of refraction, absorption coefficient a, band gap Eg), thickness and conductance–voltage (r–V) characteristics of Al/La2O3–OH/nSi devices. The r–V curves of the as-deposited La2O3–OH films demonstrate NDC behavior that is lost after storage in low vacuum and recovered after annealing. NDC effect in La2O3–OH films can be explained by the decrease in protonic conduction as a result of limited diffusion of water on the film surface that leads to bistability and switching effect that can potentially be used for RRAM device. Acknowledgements The authors gratefully acknowledge support for this work by Armenian National Science and Education fund grant program, ANSEF (Grant No. 3913).

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