Vol:.(1234567890)

Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478https://doi.org/10.1007/s10854-018-0521-z

1 3

Nanotube structures: material characterization and structural analysis of Ge–Se thin films

Muhammad R. Latif1 · Dmitri A. Tenne2 · Maria Mitkova1

Received: 16 October 2018 / Accepted: 5 December 2018 / Published online: 11 December 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

AbstractNanotube structures, formed in thin films of chalcogenide glasses open avenues for new applications, since they offer direc-tional options for many effects in these otherwise completely disordered systems. One way to grow nanotubes is through formation of nano-columnar structures by confining them. In this work nano-columnar growth of thermally evaporated GexSe100−x (x = 20, 30, 40) films is achieved through oblique films deposition. The columnar structural organization of the films and its dependence upon the deposition angle and films composition are established by imaging the cross-sectional areas of the films through scanning electron microscopy. Atomic force microscopy, energy dispersive X-ray spectroscopy and Raman Spectroscopy studies reveal respectively variation in the surface porosity, composition changes and structural reorganizations occurring in the films as a function of obliqueness angles and material’s composition. These results are dis-cussed in respect to the structural organization of films deposited under normal flux incidence and deformations and other structural effects caused by films’ deposition under variable angles. Based on the experimental results an empirical formula for the tangent rule is suggested which links the incident flux angle α, and the nanotube inclination angle β.

1 Introduction

Chalcogenide glasses (ChG), are widely explored material for phase change [1] and conductive bridge random access memory (CBRAM) [2] devices. Each one of them has dif-ferent mode of operation and there are different require-ments towards the chalcogenide material building these memory solutions. While for the former low crystalliza-tion temperature and enthalpy are of primary importance [3], the later rely on collective transport of ions within the glass matrix [4]. The operation of the CBRAM devices is based on formation and dissolution of a conductive bridge between two electrodes, one of which is electrochemically active (for example Ag) and the other is electrochemically inert (for example W). Unlike electrons, ions cannot be delocalized in solid. They are confined to lattice sites and move by random hopping in the crystal lattice or through

a network of channels, resulting from the lack of long range order, as usually happens in disordered systems. This requires small ions to be used in order to have high mobility in the disordered systems. In that aspect Ag is a good solution for application in the CBRAM devices because of its chemical stability, high conductivity and high electrochemical activity (low voltage requirements for occurrence of oxy-reduction process). Its ionic size is quite small (between 89 pm and 129 pm depending upon its coordination) compared to this of the small ions like Li+ (90 pm), Na+ (116 pm) or K+ (152 pm). When bias is applied on CBRAM devices such that the silver electrode is the anode, it acts as metal ions supplier for the formation of the conductive bridge, which extends between the two electrodes after silver ions undergo reduction at the coun-ter electrode (cathode), forming the “on” or low resistance state of the device [4]. At application of an opposite bias, oxidation of Ag occurs and Ag ions deposit back on the Ag electrode by which the device turns to the high resistance state—the “off” state. It is clear that the diffusion of Ag ion is crucial for the device performance and the ChG is an ideal medium for this type of devices since it supplies electrons for the bridge growth to occur and Ag has very high mobility in it (in the order of 10−2 cm2 V−1 s−1) [4]. However, since the chalcogenide glasses are amorphous

* Maria Mitkova mariamitkova@boisesatet.edu

1 Department of Electrical and Computer Engineering, Boise State University, 1910 University Dr, Boise, ID 83725-2075, USA

2 Department of Physics, Boise State University, 1910 University Dr, Boise, ID 83725-1560, USA

2471Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

material, the bridge growth is stochastic and its length can be longer than the distance between the two electrodes due to many deviations following the channels in the dis-ordered system. As it grows, because of the channels dis-tribution, many branches can develop. They indeed do not contribute to the bridging between the two electrodes, but can take Ag ions for saturation of the existing free valence states and use energy for their development. This all said, shows the importance of finding a way to organize the growth of Ag bridge, which simultaneously can improve the speed of the devices and their reliability. In the same time this solution should not be reached through crystalli-zation of the material—it has to be in amorphous condition which keeps it in its high resistive state. One technological way to solve this problem could be formation of a nano-tube structure within the ChG where the Ag atoms build-ing the conductive bridge could be confined. This will help to organize the bridge growth and stability. Such structure can be achieved through different methods, for example self-assembly [5], electropolimerization [6], by treating of solutions from which films are grown [7], or different vari-ations of physical vapor deposition (PVD). The focus of this work is to use oblique angle films’ deposition by PVD method and specifically thermal vacuum evaporation. This method has advantages of high deposition rate, simplic-ity, easy application, strict control, compatibility with the CMOS technology and has been of interest for decades.

Directional nano-columnar growth, which is of interest for this research, is a result of atomic shadowing mechanism [8] at the substrate surface. Self-shadowing mechanism in thin films is predominantly observed when the material flux impinges on the substrate surface under some angle. During the initial stages of film growth, adatoms condense onto the substrate and form individual separated islands, or nuclei [9, 10]. When the substrate is tilted so that the incident vapor flux arrives at an oblique angle, the adatoms nuclei topog-raphy results in geometrical shadowing over regions of the substrate, preventing the coalescence of nuclei into a con-tinuous thin film layer. Instead, the nuclei capture the vapor flux that should have landed in the shadowed regions, result-ing in the formation of column growing in the direction of the vapor source as illustrated in Fig. 1. Naturally the space between the columns forms a nanotube voids in which the growth of the conductive bridge will be confined.

Formation of column structure in ChG has been achieved first by Chopra and co-workers [11–13]. Spence and Elliott [14] studied the photo-induced effects in obliquely deposited GeSe2, GeSe3 and GeS2 films and found the difference in the band gap—for example, films deposited under normal incidence towards the substrate (α = 90°) had wider band gap − 2.6 eV, compared to films deposited at high oblique-ness angle (α = 70°) − 1.84(4) eV. They also found that the films deposited at normal incidence undergo photobleaching under irradiation with IR light, while at the same irradia-tion conditions the films deposited under high obliqueness (α = 80°) undergo photodarkening. Marquez et al. [15] con-firmed the results of Spence and Elliott and gave evidences for the surface oxidation when these experiments are car-ried out in air. Starbova et al. [16] studied the morphology, micro hardness and optical properties of obliquely deposited As2S3, GeS2 and AgBr films. The work published by Jin et al. meticulously describes the structural and properties details of the obliquely deposited films from Ge–Se glasses. It  also focuses on the photo-contraction data [17]. The authors clearly indicate the role of the intermediate phase in this system and view the columns formed in the films as composites or bundles of nanometric-sized atomic filaments and suggest that the giant photocontraction effect charac-teristic for the interemediate phase compositions is due to photomelting of the filaments of Ge25Se75 which collapse and form films with much denser structure.

In this work, the nanotubes formed by columns growth in thin films obtained from GexSe100−x(x = 20, 30, 40) depos-ited by thermal evaporation technique under various depo-sition angles are studied. They are characterized by their structure via Raman spectroscopy, inclination of the col-umns building during the deposition process and confining the nanotubes forming between them, studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), as well as their surface roughness, elucidated by AFM. Based on the experimental results empirical tangent rule for the studied materials is suggested. The number of variables influencing the films formation has been reduced by focusing the experiments on thermal evaporation, room temperature of the substrate, constant deposition rate and thickness, constant rotation of the substrate during the depo-sition, and application of < 100 > p-type Si substrate with a deposited 1 µm amorphous SiO2 film, which are standard

Vapor Flux Vapor FluxAdatom Geometrical shadowing

Column inclination angleβα

(a) (b) (c)

Fig. 1 Columns growth at glancing deposition

2472 Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

conditions for production of thin films and devices for the research group performing the experiments. This gave a good opportunity for a rich variation of the collimation of the sample holder and the studied compositions.

2 Experimental details

Thin films of Ge–Se ChG were thermally evaporated on a p-type Si < 100 > substrate covered with a µm thin film of SiO2. The evaporation process was conducted using Cress-ington 308R Desktop coating system. A crucible resembling a semi-Knudsen cell for equilibrating the vapor pressure of the source ChG material throughout the chamber was used. The film thickness was monitored using 6 MHz quartz crys-tal resonator. The deposition pressure was 10−3 mbars with deposition rate of 2 nm s−1 and the final film thickness was 500 nm. The substrate temperature was monitored during the deposition process and was kept at room temperature. The Ge–Se films were deposited under various deposition angles (α = 90°, 80°, 70°, 60°, 45°, 30°). This was achieved by tilt-ing the wafer holder to the required angle, measured with a goniometer, since the wafer holder had a 360° angle of rotation—Fig. 2. Three source compositions for films depo-sition were used: Ge20Se80, Ge30Se70, and Ge40Se60. These glasses were freshly synthesized from high purity Ge and Se (99,999%) trace metals basis (Aldrich) by melt quench-ing method.

Hitachi field emission scanning electron microscope (FESEM) with backscattering detector and Quartz imaging capture software was used to analyze the thin film morphol-ogy. The samples were cleaved to have the cross sectional area parallel to the incident vapor. The cross sectional area was coated with gold to avoid charging of the samples which are high resistive.

The compositional analysis of the films was performed by energy dispersive spectroscopy (EDS). The results were acquired by averaging data over five points on each sample using a Hitachi S-3400N EDS system.

The porous morphology and surface roughness of Ge–Se films were studied by using OTESPA probe on a Veeco Dimensions 3100 AFM system equipped with Nanoscope IV controller in tapping mode.

Structural alterations under different depositions angles were investigated by Raman spectroscopy collected using a Horiba Jobin Yvon T64000 Raman spectroscopic system in back scattering mode supplied with a triple monochromator with a liquid-nitrogen-cooled multichannel coupled charge (CCD) device detector. The films were placed inside a cham-ber at temperature 100 K and 1 × 10−5 Torr pressure and were excited using a 441.6 nm He–Cd laser focused on an area with 0.1 mm diameter and laser intensity of 30 mW. These experimental conditions (low temperature and laser power, vacuum and short period of data collection) prevent occurrence of photoinduced changes in the films although the laser wavelength is within the absorption edge of the films.

3 Results

The first step of the study was to check the composition of the synthesized glasses used as sources for film deposition. The EDS analysis of their composition confirmed Ge content of 20 ± 0.3 at.%; 30 ± 0.4 at.% and 40 ± 0.2 at.% respectively. The films evaporated under normal angle (90°) were used as a standard for further evaluation of the films composi-tion. While the films deposited from Ge30Se70 and Ge40Se60 sources had 0.1–0.4  at.% difference in Ge composition compared to the source material, the films deposited from

Fig. 2 a Schematics of thermal evaporation chamber illustrat-ing incident vapor measuring method where α1 is the nor-mally deposited film (α1 = 90°) and α2 is some angle less than 90° (α1 < 90°) b semi Knudsen cell structure used for thermally evaporated films

QCRSample Holder

Shu�er

Crucibels

Pump

360˚Substrate/Stage Axis

Vapor Flux

α1

Crucible Source

Openings made in the crucible cover to collimate the vapor flux

α2

(a)(b)

2473Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

a Ge20Se80 source were 5 ± 0.2 at.% richer in Ge. The prop-erties of these films are discussed by considering this fact. The EDS analysis of the obliquely deposited films revealed that for Ge25Se75 and Ge30Se70 compositions the Se con-centration increases with decreasing the incident vapor flux angle, whereas for the Ge rich composition Ge40Se60, Se

concentration decreases as the incident vapors flux hits the substrate under smaller angles as represented in Fig. 3.

Raman spectroscopy analysis of the obliquely deposited films, presented in Fig. 4, provides data of the structural changes occurring in the films as a result of deposition under different angles of incidence. Three regions are distinguish-able in these spectra: (i) a relatively high intensity band around 197 cm−1 corresponding to symmetric stretching of Se atoms in Ge–Se–Ge linkages between corner shared (CS) GeSe4 tetrahedra, along with a shoulder lobe around 214 cm−1 corresponding to the breathing mode of a pair of Se atoms that are positioned at the edges shared (ES) between two neighboring GeSe4 tetrahedra [18–22]. (ii) a high intensity and broad spanning 225–300 cm−1 peak related to Se–Se stretching in Se chains and rings [23]. (iii) a low intensity band ranging from 305 to 330 cm−1 due to an asymmetric vibration in the GeSe4 edge shared tetrahedra [24]. In addition, the spectra of Ge40Se60 reveal a low inten-sity band centered at ~ 178 cm−1 originating from Ge–Ge homopolar bonds building the ethane like (ETH) structural units [25]. Also, in this composition the band in the region at 263 cm−1 shifts to higher wave numbers with increas-ing the deposition angle and in the Ge40Se60 film both ini-tially resolved bands in the 2nd and 3rd regions (~ 263 and ~ 310 cm−1) merge into a broad band at normal deposition

90 80 70 60 50 40 3050

55

60

65

70

75

80

Ato

mic

% o

f Se

Vapor Incident angle (α) (Degrees)

Se75

Se70

Se60

Fig. 3 GexSe100−x compositions of obliquely deposited films

Fig. 4 Raman Spectra for obliquely deposited a Ge20Se80 b Ge30Se70 c Ge40Se60 films under different angles

150 200 250 300 350 400

80 degree

70 degree

60 degree

45 degree

30 degree

90 degreeESSe-SeES

Cou

nts

(arb

.)

Wavenumber (cm-1)

CS

(a) (b)

(c)

Ge20Se80

150 200 250 300 350 400

ESSe-SeESCS

Cou

nts

(arb

.)

Wavenumber (cm-1)

ETH90 degree

30 degree

45 degree

60 degree

70 degree

80 degree

Ge30Se70

150 200 250 300 350 400

Ge40Se60

Cou

nts

(arb

.)

Wavenumber (cm-1)

ESSe-SeESCS

90 degree

30 degree

45 degree

60 degree

70 degree

80 degree

ETH

2474 Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

angle. There is a remarkable resemblance between the EDS and Raman data. For Ge25Se75 and Ge30Se70 films the ratio of CS/ES structural units and the area of the peaks denoting the presence of Se–Se chains increases with decreasing the deposition angle, thus demonstrating enrichment of the films in Se concentration. Ge rich films show more complicated structural development which is discussed later in this work.

SEM imaging was used to demonstrate the growth pro-file of the column structure, although this method is not very suitable for study of ChG due to their high sensitivity towards interaction with the e-beam, which affects the stud-ied structure and prevents formation of clear SEM images. The sensitivity of the ChG towards the electron beam is so high that it has been used for example, for formation of relief image on them [26]. However, in the context of this work SEM has been chosen because of its high resolution. Illustration of the growth profile of the column structures in Ge25Se75 film under 45°, Ge30Se70 film under 80° and Ge40Se60 film under 30° obtained by SEM, are presented in Fig. 5. SEM images demonstrated no clear column formation at normal deposition angle while at smaller incident angles the columns run through the entire film volume from the substrate to the free surface.

Owing to the method of columns formation due to shad-owing mechanism, the individual columns are divided by inter-columnar separation and free inter-columnar volume, forming nanotubes. The angle of incidence controls the tilt of the columns and affects the degree of shadowing and thus the porosity of the film. The vapor incidence angle (α) and columnar growth angle (β) are measured with respect to the vapor source and the stage normal. The existence of correla-tion between the incidence angle and the columnar growth angle, was formulated by Nieuwenhuizen and Haanstra [27] as:

known as “Tangent Rule” which is valid for a variety of amorphous and crystalline thin films [28].

(1)tan � = 2 tan �,

AFM image of columnar structure by scanning on the sample cross-section of Ge30Se70 under angle of inci-dence 45° is presented in Fig. 6a. Surface morphologies of the angular deposited films, imaged by the AFM in tap-ping mode, illustrated the porous nature of the obliquely deposited films as shown in Fig. 6b. It is observed that the films deposited under normal incidence have relatively smooth surface. As the films are grown under smaller angles, voids form on the surface. A close observation of the AFM height bar scale, z-scale, indicates that the films containing higher concentration of Se (Ge20Se80 and Ge30Se70) have lower surface roughness, compared to the Ge rich films (Ge40Se60), which have more structured surface even at nor-mal angle of deposition. Besides, the voids diameter grows as the deposition angle decreases i.e. inter-columnar spacing increases, meaning that the pores formed in this manner have bigger dimensions at steeper angles.

4 Discussion

The films evaporated under normal angle, which were used as a standard for evaluation of the evaporation technique, support the choice of the crucible and the evaporation rate for Ge30Se70 and Ge40Se60 since their compositions corre-spond to the source material. The films deposited from a source material Ge20Se80 result in Ge richer films (Ge25Se75) which is related to the Se high partial pressure [29, 30]. This causes repulsion of some Se atoms reaching the substrate with high energy. Evaporation under oblique angles con-tributes to formation of films with higher Se content, com-pared to the films deposited at normal incidence, since the Se atoms arrive at the surface under a glancing angle which reduces the energy of the interaction with the substrate and so diminishes the repulsion of Se atoms. The slight decrease of the Se content for the Ge rich films in the process of oblique deposition could be related to the full structural reor-ganization of these films, discussed later.

α = 45ͦα = 80ͦ

α = 30ͦ

β

ββ

(a) (b) (c)

Fig. 5 SEM Images of a Ge25Se75 under 45° b Ge30Se70 under 80° and c Ge40Se60 under 30°

2475Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

The obtained compositional results can be linked to the structure demonstrated by Raman Spectroscopy stud-ies. They proof that although the obliquely deposited films have well expressed morphology, they are amorphous in their nature. Raman spectra of Ge20(25)Se80(75) films show a clear red shift in CS frequency mode, characteristic for increased Se concentration [31], illustrated in Fig. 7a. The Raman data also proved an increase of the areal intensity of the Se–Se band vibration as shown in Fig. 8a. The increased areal intensity of the CS vibrations compared to the ES areal intensity as presented in Fig. 8b suggests that the structure relaxes by enhancement of its floppiness due to the forma-tion of three dimensional network built predominantly by CS Ge–Se tetrahedra. This indeed is a structure which closely resembles the structure of the Ge20Se80 bulk glass [31], i.e.

demonstrates enrichment of the films with Se as a function of decreasing deposition angle.

The structural data for the Ge30Se70films are ambiguous since the frequency mode of the CS building blocks under-goes variations which are not in a clear dependence of the deposition angle. At the same time, the Se–Se areal intensity, as well as the amount of Se in these films increases as a function of decreasing deposition angle. There is not a clear dependence of these compositional changes associated to changes of the areal intensity of the ES/CS structural units, Fig. 8b. This suggests that for this particular case, a phase separation occurs which keeps the equilibrium between the three dimensional Ge containing structural units with the Se–Se chains and rings. Phase separation in this material has also been reported in [17].

Fig. 6 AFM images for obliquely deposited Ge30Se70 films: a cross section of a film, deposited under an angle of 45 degrees b surface morphol-ogy under different deposition angles

SiO

2

W

Ge 3

0Se 7

0

Agα=45˚

1 2 3 4µ

1 2 3 4µ

1

2

3

1

2

3

4µ

90 ͦ

i)

vi)

30 ͦ

v)

45 ͦ

iv)

60 ͦ

iii)

70 ͦ 80 ͦ

Angular deposi�on of Ge30Se70 films

ii)100

200300

400

100200

300400

100200

300400

100200

300400

100200

300400nm

100200

300400

0.2nm

3.0nm

100200

300400nm

100200

300400

0.5nm

100200

300400nm

100200

300400nm

0.1nm4.9nm

200

400

100200

300400

100200

300400

500nm0.3nm6.5nm

100200

300400

500

100

200300

400

500nm

100200

300400

500nm

5.7nm0.2nm

500nm

0.0nm1.9nm

500nm

5.0nm

200300

400

(b)

(a)

2476 Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

At normal angle deposition the structure of Ge40Se60 resembles the one of the bulk material. However, the structure degrades by decreasing the deposition angle and cannot be fitted with the peaks characteristic for this stoi-chiometry. This composition is the representative of the Ge–Se glasses with the highest packing fraction [32]. The packing stress is obviously released by destruction of the ETH structural units, which contributes to dense structural organization that is relieved by formation of tetrahedral structure with much bigger molecular volume. Indeed the ETH structural units contain Ge–Ge bond which is the bond with the lowest energy among the structures form-ing in the Ge–Se system. This bond eventually breaks in response to the structural requirements of the obliquely deposited films. At the lowest angle, the ES structural units degrade as well and although there is no clear evidence for dominance of formation of a distorted rock salt structure, its presence is possible since it is expected to exist in this compositional region [33]. This type of structure is com-bined by threefold coordinated Ge and Se atoms bonded by two covalent and one dative (coordination) bond, which considerably reduces the steric rigidity of the structure. The so formed structure consumes almost all available Se atoms participating in Se–Se bonding formation as shown in Fig. 8a. and its general organization resembles rather CS

as found by the Raman study shown in Fig. 8b. Therefore, we suggest that stress release is the main reason for the structural reorganization at angle decrease for this origi-nally very rigid structure.

Table 1 compares the angle of experimentally grown columnar structures with the predicted angle by Tangent Rule. A deviation of approximately 30% is observed in the experimentally grown column angles and the angle esti-mated by Tangent Rule. There are several models [34–36] which try to find alternative dependences of the incident flux angle, α, and the column inclination angle, β, but each of these models are valid for particular materials. Due to the sensitivity of the columnar structure on deposition condi-tions and material dependent properties, all materials behave differently and accurate predictions are difficult. For ChG, in this work, a modified empirical formula is suggested, in which a coefficient A is introduced:

where A is a parameter that depends on the material and deposition rate and is found to be 0.625 for the ChG studied in this work. This value for the constant A has been obtained after modeling the curve representing the tangent rule to closely resemble the measured angles positions.

(2)tan � =2

Atan (�) = 3.2 tan (�)

90 80 70 60 50 40 30

195.2

195.6

196.0

196.4

196.8C

S m

ode

freq

(cm

-1)

Deposition Angle (Degree)90 80 70 60 50 40 30

199.00

199.25

199.50

199.75

200.00

CS

mod

e fr

eq ( c

m-1

)

Deposition Angle(Degree)90 80 70 60 50 40 30

197.0

197.2

197.4

197.6

197.8

198.0

198.2

CS

mod

e fr

eq (c

m-1

)

Deposition Angle(Degree)

Ge20Se80 Ge30Se70 Ge40Se60

(a) (b) (c)

Fig. 7 Variations in CS-mode as a function of obliqueness angle in a Ge20Se80 b Ge30Se70 c Ge40Se60

Fig. 8 Area plot of the a Se–Se peak for Ge–Se system Raman spectra in Fig. 4 b area plot for the ES/CS area ratio, calculated from the Raman spectra in Fig. 4

(a) (b)

2477Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

A comparison of the measured angles for the deposited Ge–Se films, tangent rule and the suggested correction is presented in Fig. 9. The suggested correction predicts the growth angle within 10% of the experimentally found angle. Since with decreasing the deposition angle the nano-tubes become bigger, it will be expected that surface roughness will be the lowest for the normally deposited films and will

gradually increase as the deposition angle turns out to be smaller. This result is confirmed by analyzing the surface roughness of the obliquely deposited samples with AFM scan on over a 5 µm × 5 µm area. Data in this aspect are presented in Fig. 10a, b.

We would like to propose our answer to the tempt-ing question as what is building the columns and what is between them, forming the nano-tubes? The observed results let us conclude that the columns are built by three-dimen-sional structural units containing Ge–Se tetrahedra con-nected in the inter-column space with Se–Se bonds. This suggestion needs more detailed experimental validation which is subject of further studies.

5 Conclusion

Formation of thin films with nanotube structure based on Ge–Se chalcogenides has been demonstrated at various vapor incident angles. The Se rich glasses and the Ge rich glass react differently to the obliquely deposition. The com-position of initial material deposited at normal incidence changes with decrease of the deposition angle to form Se richer films in the Se rich compositions and vice versa for the Ge rich system. The Raman results showed changes in the structure occurring in the obliquely deposited films due to formation of a phase separated material for Se-rich glasses

Table 1 Deposition and column angle growth for Ge–Se films (all units in degrees)

Incident vapor flux (α)

Angle calculated by tangent rule

Ge20Se80 Ge30Se70 Ge40Se60 Angle calculated by suggested correctionMeasured columnar angle (β)

80 70.575 72 66 70 75.86270 53.948 61 63 63 67.98160 40.893 70 64 69 57.32045 26.565 46 49 48 41.98730 16.102 35 32 34 27.457

90 80 70 60 50 40 30 20

010

2030

4050607080

90

Tangent Rule Correction Ge20Se80 Measured angle Ge30Se70Measured angle Ge40Se60Measured angle

Col

umn

Gro

wth

Ang

le (β

) (de

gree

)

Vapor Flux Incident Angle (α) (degree)

Fig. 9 Comparison of measured angles versus tangent rule and sug-gested correction

Fig. 10 Average surface rough-ness’s observed in a Ge30Se70 b Ge40Se60 films under different depositions angles

90 80 70 60 50 40 300.0

0.3

0.6

0.9

1.2

1.5

1.8 Ge30Se70

Ave

rage

Sur

face

Rou

ghne

ss (n

m)

Incident Angle (Degree)90 80 70 60 50 40 30

0

1

2

3

4

5

Incident Angle (Degree)

Ge40Se60

Ave

rage

Sur

face

Rou

ghne

ss (n

m)

(a) (b)

2478 Journal of Materials Science: Materials in Electronics (2019) 30:2470–2478

1 3

or for the relief of the packaging stress in the Ge-rich compo-sitions. The experimental results demonstrate an increase in the surface porosity with decreasing of the deposition angle. Since the nanotube structure within the thin film strongly influences its electrical, optical and mechanical properties, a functional layer having nanotube structures can be devel-oped with desired properties for a wide practical application without involving any additional cost in the fabrication line. Data in this aspect regarding tremendous improvement of the CBRAM devices performance, based on thin films with nanotube structure are reported in [37].

Acknowledgements This work has been supported by funding through Idaho State Board of Education under Grant No. IF 14-004. The authors would also like to acknowledge the Surface Science Lab at Boise State University for AFM use and Dr. Paul Davis for assistance in performing the AFM studies.

References

1. N. Raeis-Hosseini, J. Rho, Materials 10, 1046 (2017). https ://doi.org/10.3390/ma100 91046

2. M.N. Kozicki, H.J. Barnaby, Semicond. Sci. Technol. 31, 11301 (2016)

3. S. Hudgens, B. Johnson, MRS Bull. 29, 829 (2004) 4. M.N. Kozicki, M. Mitkova, I. Valov, in Resistive Switching:

From Fundamentals of Nanoionic Redox Processes to Memris-tive Device Applications, ed. by D. Ielmini, R. Waser (Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2016), p. 483

5. M. Yoshio, T. Kagata, K. Hoshino, T. Mukai, H. Ohno, T. Kato, J. Am. Chem. Soc. 128, 5570 (2006)

6. K. Naoi, Y. Oura, M. Maeda, S. Nakamura, J. Electrochem. Soc. 142, 417 (1995)

7. A. Tracz, J.K. Jeszka, M.D. Watson, W. Pisula, K. Müllen, T. Pakula, J. Am. Chem. Soc. 125, 1682 (2003)

8. A.G. Dirks, H.J. Leamy, Thin Solid Films 47, 219 (1977) 9. N. Kaiser, H.K. Pulker, in Optical Interference Coatings, Springer

Series in Optical Sciences, vol. 1, ed. B.N. Kaiser, H.K. Pulker (Springer, Berlin, 2003), p. 88

10. M. Ohring, A. Milton, Mater. Sci. Thin Films 12, 718 (2002) 11. B. Singh, S. Rajagopal, P.K. Bhat, D.K. Pandya, K.L. Chopra, J.

Non-Cryst. Solids 35–36, 1053 (1980)

12. B. Singh, S. Rajagopalan, K.L. Chopra, J. Appl. Phys. 51, 1768 (1980)

13. B. Singh, S. Rajagopalan, P.K. Bhat, D.K. Pandya, K.L. Chopra, Solid State Commun. 29, 167 (1979)

14. C.A. Spence, S.R. Elliott, Phys. Rev. B 39, 5452 (1989) 15. E. Marquez, A.M. Bernal-Oliva, J.M. Gonzalez-Leal, R. Prieto,

Alcón, R. Jiménez-Garay, J. Non-Cryst. Sol. 222, 250 (1997) 16. K. Starbova, J. Dikova, N. Starbov, J. Non-Cryst. Sol. 210, 261

(1997) 17. M. Jin, P. Chen, P. Boolchand, T. Rajagopalan, K.L. Chopra, K.

Starbova, N. Starbov, Phys. Rev. B 78, 214201 (2008) 18. S. Sugai, Phys. Rev. B 35, 1345 (1987) 19. Y. Wang, O. Matsuda, K. Inoue, O. Yamamuro, T. Matsuo, K.

Murase, J. Non-Cryst. Sol. 232, 702 (1998) 20. Y. Wang, K. Murase, J. Non-Cryst. Sol. 326, 379 (2003) 21. T. Edwards, S. Sen, J. Phys. Chem. B 115, 4307 (2011) 22. G. Lucovsky, A. Mooradian, W. Taylor, G. Wright, R. Keezer,

Solid State Commun. 5, 113, (1967) 23. M. Cobb, D. Drabold, R. Cappelletti, Phys. Rev. B 54, 12162

(1996) 24. P. Boolchand, W. Bresser, Philos. Mag. B 80, 1757 (2000) 25. P. Boolchand, P. Chen, M. Jin, B. Goodman, W.J. Bresser, Phys.

B 389, 18 (2007) 26. M. Trunov, V. Takats, I. Csarnovics, C. Cserhati, A. Csik, S.

Kokenyesi, in Proceedings v. 2 TMS 139th Annual Meeting and Exhibition, Seattle, 14–18 Feb 2010, p. 641

27. J.M. Nieuwenhuizen, H.B. Haanstra, Philips Tech. Rev. 27, 87 (1966)

28. E. Krumov, V. Mankov, K. Starbova, Vacuum 76, 211 (2004) 29. D.R. Lide, CRC Handbook of Chemistry and Physics 2004–2005:

A Ready-Reference Book of Chemical and Physical Data (CRC press, Boca Raton, 2004)

30. R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser, K.K. Kelley, Selected values of the thermodynamic properties of the elements, DTIC Document (1973)

31. X. Feng, W. Bresser, P. Boolchand, Phys. Rev. Lett. 78, 4422 (1997)

32. V. Georgieva, M. Mitkova, P. Chen, D. Tenne, K. Wolf, V. Gad-janova, Mat. Chem. Phys. 137, 552 (2012)

33. P. Boolchand, J. Grothaus, M. Tenhover, M.A. Hazle, R.K. Gras-selli, Phys. Rev. B 33, 5421 (1986)

34. R. Tait, T. Smy, M. Brett, J. Vac. Sci. Technol. A 10, 1518 (1992) 35. S. Lichter, J. Chen, Phys. Rev. Lett. 56, 1396 (1996) 36. J. Krug, Materialwiss. Werkstofftech. 26, 22 (1995) (In German) 37. M.R. Latif, P.H. Davis, W.B. Knowton, M. Mitkova, J. Mater. Sci.

(2018). https ://doi.org/10.1007/s1085 4-018-0512-0