influence of deposition conditions and substrate

Influence of deposition conditions and substrate morphology on the electrical properties

of sputtered ZnO:Al grown on texture-etched glass

Nicolas Sommer (1,*), Stefan Götzendörfer (2), Florian Köhler (1), Mirko Ziegner (3), and

Jürgen Hüpkes (1) 5

(1) IEK5 - Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, GERMANY

(2) Berliner Glas Surface Technology, 89428 Syrgenstein, GERMANY

(3) IEK2 - Werkstoffstruktur und -eigenschaften, Forschungszentrum Jülich GmbH,

52425 Jülich, GERMANY 10

e-mail: [email protected], tel.: +49 2461 61 1550, fax: +49 2461 61 3735

ABSTRACT: 15

The focus of this work is the growth of aluminum-doped zinc oxide (ZnO:Al) on texture-

etched glass substrates. We investigated the influence of sputter parameters, pressure and

temperature, on the charge carrier mobility of ZnO:Al films grown on different substrate

textures. An optimized sputtering process was developed which led to charge carrier

mobilities on textured substrates that are close to those on flat substrates. Based on x-ray 20

diffraction measurements, we qualitatively explain the effect of different sputtering

conditions. Furthermore, the ZnO:Al charge carrier mobility was related to the substrate

morphology. ZnO:Al films on U-shaped surface morphologies showed significantly higher

charge carrier mobilities than on V-shaped structures. ZnO:Al damp heat stability and etching

behavior provided evidence that the number of ZnO:Al growth disturbances on textured 25

substrates can be reduced by adequate substrate morphology and sputtering conditions.

Keywords: TCO, ZnO:Al, sputtering, silicon thin film solar cells, texture-etched

glass, rough substrates

1. Introduction

Textured interfaces improve the light trapping in silicon thin film solar cells [1, 2, 3, 4, 5, 6]. 30

Thereby the short circuit current density increases and thus higher solar cell efficiencies are

achieved. Commonly, the transparent conductive oxide (TCO) is textured [2, 3, 4]. Sputter

deposition of aluminum-doped zinc oxide (ZnO:Al) layers onto flat substrates and subsequent

mailto:[email protected]

etching in HCl, HF or via electrochemical methods leads to textured ZnO:Al surfaces [7, 8,

9]. Doped zinc oxide grown via low pressure chemical vapor deposition (LPCVD) can also 35

produce self-textured surface morphologies which induce light trapping in solar cells [3, 5].

The texture and thus the quality of light trapping, however, depends on the specific growth

conditions, such as deposition pressure, temperature [10] or layer thickness [5, 6]. Hence, the

deposition parameters have to be carefully adjusted. Additionally, there is a tradeoff between

optical, electrical and texture properties, e.g. a thicker layer may enhance the light trapping 40

capability of the textured TCO, but it increases at the same time the parasitic absorption in the

TCO layer [5, 11].

Here, we present texture-etched glass substrates to overcome the above mentioned obstacles

of textured zinc oxide. The etched glass provides the texture. The subsequently sputter-

deposited ZnO:Al layer must be optimized regarding electrical and optical properties only. 45

Hence, textured glass substrates allow the decoupling of texture on the one hand and electrical

as well as optical properties on the other hand. Specifically, the layer thickness can be

adjusted with regards to the layer resistance, thereby reducing the parasitic absorption in the

ZnO:Al layer. However, optical and electrical properties are not decoupled completely,

because the light-incoupling on textured substrates remains a function of the TCO layer 50

thickness [12].

Although various etching methods [8] or plasma treatments [13] of textured zinc oxide can

modify the resulting surface morphology to a certain extent, textured glass substrates offer a

larger variety of surface morphologies.

Besides wet-chemically etched glass presented in this paper, further methods to obtain rough 55

glass have been presented in literature. Nanoimprint lithography is a sophisticated method to

produce textured substrates [14, 15, 16]. Furthermore, reactive ion etching leads to suitable

surface morphologies [17, 18]. Additionally, rough glass is obtained by ion beam treatment of

a sputter-etched ZnO:Al mask, thereby transferring the ZnO:Al texture into the glass [19].

Textured glass was also obtained by aluminum induced texturization [20]. 60

Despite the high number of differently textured glass substrates, studies about the ZnO:Al

growth on these substrates are very limited. The growth of sputtered ZnO:Al on rough

substrates with regard to its damp heat stability has been investigated for the application in

chalcopyrite-based solar modules [21, 22, 23]. The challenge of ZnO:Al growth on textured

substrates is the decrease of charge carrier mobility in comparison to flat substrates. 65

Moreover, damp heat treatment of ZnO:Al on rough substrates leads to a strong resistivity

increase. ZnO:Al growth disturbances, also called extended grain boundaries, were given as a

reason for this behavior.

Nevertheless, a comprehensive investigation regarding the influence of different sputtering

conditions and substrate morphologies on the charge carrier mobility is not available. 70

In the following, we characterize four differently etched glass substrates. We present an

extensive growth study and reveal optimized growth conditions for ZnO:Al on textured glass

leading to high charge carrier mobilities. On the basis of x-ray diffraction pole figures, we

explain in a simple, qualitative model the impact of different deposition conditions on the

ZnO:Al growth on textured substrates. Furthermore, we show that the extent of mobility 75

decrease depends on the specific glass surface morphology. Damp heat and etching

experiments suggest different amounts of ZnO:Al growth disturbances to occur on the various

texture-etched glass substrates.

2. Experimental details

Sputtering with a circular magnetron was used to deposit aluminum-doped zinc oxide films in 80

a high-vacuum system (Lesker Inc., USA) in radio-frequency mode from a ceramic target with

0.5 wt% Al2O3 at a base pressure of ~2x10-7

mbar. The target diameter was 15.2 cm, the

target-to-substrate distance was 7.8 cm and pure argon was used as sputtering gas.

All ZnO:Al thin films were deposited on low-iron solar glass (EuroWhite, Euroglas,

Germany). After initial basic etching, the morphology of the glass surface was modified in a 85

second etching step. The substrates were treated in an acid mixture consisting of 45 wt% of

sulfuric acid and 0.5 wt% of hydrofluoric acid. For glass substrates I to IV, the etching time of

the second step was increased from 0 s to 120 s in 40 s steps (figure 1). Subsequently, the

glass substrates were coated with a 140 nm SiOxNy barrier layer. For details about this

process, see ref. [24]. This layer slightly smoothed the glass texture, but preserved the general 90

morphology. The substrate size was 3 x 10 cm². We always coated two substrates within one

deposition: a textured glass and a flat reference. The resulting 6 x 10 cm² glass area was

positioned over the center of the target. Therefore, the outer parts of the substrates were above

the race-track which had a diameter of roughly 10 cm. Measurements characterizing the

layers were performed in the center of each substrate. 95

The ZnO:Al layer thickness was measured on the flat reference substrates to be between 600

nm and 700 nm using a surface profiler (Dektak 3030, Veeco, USA). We assumed the layer

thickness on the textured substrates to be the same as on the flat reference substrate. Hall

measurements were conducted by van der Pauw method (RH2030, PhysTech, Germany). The

surface morphology was investigated by atomic force microscopy (AFM) in a Nanostation 100

300 (SIS, Germany) using non-contact mode and by scanning electron microscopy (SEM) in a

LEO 1550 VP GEMINI (Zeiss, Germany). Damp heat degradation was carried out in a

climatic chamber (Nema NCC4020) at 85 °C and 85 % humidity. X-ray diffraction pole

figures of the ZnO:Al (002)-reflex were recorded by a Philips X’Pert Pro MRD with an

Eulerian cradle using CuK radiation in order to investigate the texture. 105

3. Results

3.1. Substrate textures

AFM images of four investigated etched glass substrates are shown in figure 1. In a first

etching step, we obtained substrate morphology I that is characterized by pyramids with

sharp valleys and high plateaus (figure 1a). In a second etching step, the sharp features are 110

predominantly attacked by the acid, therefore widening and rounding the valleys.

Applying this second etching step for different duration, we modified the etched glass

type I towards round, smooth, crater-like surface morphologies II, III, IV (figure 1b, c, d).

Following the description of e.g. Python et al. [13], we will call substrate I “V-shaped”

and substrate IV “U-shaped”. Substrate II and III present gradual steps between I and IV. 115

It has to be emphasized that the root mean square (rms) roughness does not show a clear

trend as a function of the second etching time and statistically varies between 98 and

114 nm (see caption, figure 1).

3.2. Influence of ZnO:Al deposition conditions on charge carrier mobility

We investigated the influence of the deposition parameters, substrate temperature and 120

sputter pressure, on the ZnO:Al charge carrier mobility. A flat reference and a textured

substrate with surface morphology I (figure 1a) were co-deposited. Figure 2 shows the

mobility as a function of growth temperature for two different deposition pressures. We

found a significant offset between the flat reference and the textured substrate for the

whole temperature range using a deposition pressure of 0.67 Pa. However, for a deposition 125

pressure of 0.13 Pa, we observed a decreasing offset with decreasing deposition

temperature. For a deposition temperature of 200 °C, ZnO:Al on the flat reference

substrate exhibited a mobility of 35.8 cm²/Vs. The mobility of the layer deposited on the

textured substrate was 32.1 cm²/Vs. Hence, it was only slightly lower than the value of the

flat reference. In the following, we will use these deposition conditions (200 °C, 0.13 Pa) 130

for further experiments and call them “optimized deposition conditions”.

In contrast, for the same deposition temperature of 200 °C, but a higher pressure of

0.67 Pa, we found a similarly high ZnO:Al mobility on the flat reference substrate of

33.7 cm²/Vs, but a much lower mobility of 20.7 cm²/Vs for ZnO:Al layers deposited on

the textured substrate (“non-optimized deposition conditions”). 135

We also checked the mobility for deposition pressures lower than 0.13 Pa and higher than

0.67 Pa using a deposition temperature of 200 °. For lower deposition pressures than

0.13 Pa, the sputtering plasma was instable and the results were not reproducible. For

deposition pressures exceeding 0.67 Pa, the mobility of the flat reference substrate was

lower than the mobility of the layer deposited at 0.13 Pa, e.g. the mobility dropped to 140

23.1 cm²/Vs using a deposition pressure of 1.34 Pa.

Besides, charge carrier densities were comparable for the flat and the textured substrates.

They increased with increasing temperature from 2 x 1020

cm-3

to 3 x 1020

cm-3

. Thus, the

mobility is the main factor determining the actual resistance on the flat and textured

substrates. Hence, for the optimized deposition conditions the resistance on the textured 145

glass substrates nearly equals the value on the flat reference substrate.

Note that we reproduced the severe mobility drop between 250 °C and 325 °C on flat as

well as on textured substrates. Other authors presented similar data [4, 25]. However, the

reason for this effect is unclear and shall not be discussed here.

3.3. Influence of substrate morphology on ZnO:Al charge carrier mobility 150

The optimized and non-optimized deposition conditions were used to coat 4 differently

textured substrates I-IV (figure 1) and a flat reference substrate. The modification of the

substrate morphology from V- to U-shaped structures via a second etching step led to an

increase of ZnO:Al charge carrier mobility for both growth conditions (figure 3). More

importantly, the trend was less pronounced for the optimized deposition conditions 155

because the initial offset between the mobility on the V-shaped morphologies and the flat

reference substrate was smaller. Consequently, ZnO:Al layers grown on the U-shaped

morphologies exhibited charge carrier mobilities close to the values of the flat reference

substrate independent of the deposition conditions.

3.4. Stability and film structure 160

The damp heat degradation of ZnO:Al grown on the V- and U-shaped surface textures I

and IV with optimized and non-optimized deposition conditions is shown in figure 4. All

ZnO:Al films degraded upon treatment. The ZnO:Al layers deposited on the V-shaped

surface textures showed the strongest increase in resistivity as a function of damp heat

time. After 1000 h of damp heat treatment, the resistivity increased by a factor of 40 for 165

optimized deposition conditions and it exceeded the measurement tool’s limit of 1 Ωcm

for non-optimized deposition conditions. In comparison, ZnO:Al on the U-shaped

structures and the flat reference substrate degraded to a lesser extent. Both, a lower

charge carrier concentration and mobility, contributed to the higher resistivity. ZnO:Al

layers deposited with optimized growth conditions showed lower degradation on all three 170

substrates.

Figure 5 shows SEM surface images of ZnO:Al films after 5 s etching in dilute HCl. The

dark spots in the images indicate deep holes in the ZnO:Al layer. We found a lower

number of holes for optimized (figure 5a, c) than for non-optimized deposition conditions

(figure 5b, d). The comparison between V- and U-shaped morphologies I and IV revealed 175

a higher amount of holes in the layer grown on the V-shaped substrate.

X-ray diffraction pole figures of the [002] direction were recorded to determine the

ZnO:Al grain orientations on the V-shaped structures I for optimized (figure 6 (a)) and

non-optimized (figure 6 (b)) deposition conditions. The orientation distribution of the

crystallites was narrowed for optimized, low pressure conditions (pdep=0.13 Pa) as 180

compared to non-optimized, high-pressure (pdep=0.67 Pa) deposition conditions. The full

width at half maximum value of the pole figures was 19° and 27.5°, respectively.

4. Discussion

In this paragraph, we want to expound our current qualitative understanding of the

difference between optimized and non-optimized deposition conditions. 185

The deposition pressure impacts the film formation on textured but also on flat substrates.

Two effects seem to be important: The deposition pressure influences the energy and the

angular distribution of the incoming particles. Taking both effects into account, Kluth et

al. have developed a modified Thornton model for the growth of ZnO:Al on flat substrates

describing the effects of varying deposition pressures [10]. At low pressure, high energy 190

and narrow angular distribution of the incoming particles increase adatom surface

mobility and reduce intercrystallite shadowing effects. Thus, the layers become more

compact. As a consequence, the mobility and the damp heat stability increase [10, 26, 27].

The two representative pressure values of the optimized, low pressure and non-optimized,

high pressure deposition conditions were carefully chosen to exhibit similar charge carrier 195

mobility and damp heat stability on the flat reference substrates (figure 2 and 4). On

textured substrates however, there is a clear difference between the two deposition

pressures regarding the layers’ mobility and damp heat stability.

We assume the influence of the particle energy on the growth to be identical for flat and

textured substrates. However, the angle of incidence is not only determined by the 200

pressure, but also by the local substrate tilt. We thus think that the difference of the

angular distribution is the main factor determining the film properties on textured

substrates. It will follow quite naturally that the growth on flat substrates is hardly

influenced whereas the film formation on textured substrates is changed. In the following,

we discuss our hypothesis in more detail: 205

Sputtered ZnO:Al thin films consist of crystalline columns. Generally, the orientation of

crystalline columns in a sputtering process is determined by the angle between the particle

flux and the substrate normal [28, 29, 30, 31, 32, 33, 34]. Several analytical expressions

have been derived connecting the substrate orientation and the angle of the incident

particle beam with the column orientation [28, 29, 30]. All expressions have in common 210

that the column axis is inclined from the substrate normal to the direction of the particle

flux. For oblique sputtered ZnO, however, the results are somewhat contradictory

regarding the relation of column orientation and particle flux [31, 32, 33, 34]. This seems

to reflect the various deposition conditions and system geometries used in these studies.

If we neglect shadowing effects, ZnO:Al growth on textured substrates resembles oblique 215

sputtering with locally very different substrate angles. Therefore, columns on textured

substrates are to some extent oriented according to the local substrate angle [35, 36]. As

outlined above, the column orientation is furthermore influenced by the incident particle

flux. Amongst others, the direction of this particle flux is determined by the deposition

pressure. Lower deposition pressures induce less particle collisions in the plasma. 220

Consequently, the particle flux for low deposition pressures is more direct in comparison

to higher deposition pressures with rather diffuse particle flux [37, 38]. Thus, for lower

deposition pressures we expect a more parallel orientation of the ZnO:Al columns rather

than perpendicular growth on the local substrate facets. Indeed, if we assume grain growth

along the c-axis [33], this effect would explain x-ray diffraction pole figures (figure 6). 225

We suggest that a more vertical column orientation results in fewer or less harmful growth

disturbances (shadowed area in figure 7). As ZnO:Al growth disturbances on textured

substrates reduce the mobility [36], a diminution of these growth disturbances using lower

deposition pressures increases the mobility. More importantly, the proposed growth

mechanism should hardly influence the film formation on flat substrates. Furthermore, 230

experimentally determined ZnO:Al damp heat stability (a) and etching behavior (b) are in

line with this model:

(a) Optimized ZnO:Al on textured substrates was more stable under damp heat conditions

than non-optimized ZnO:Al (figure 4). Admittedly, a part of the stronger degradation

for non-optimized deposition conditions on textured substrates resulted from the 235

already lower ZnO:Al stability on flat reference substrates. As shown before, the

higher stability on flat substrates using optimized, low pressure deposition conditions

can be explained by a denser layer structure at lower deposition pressures [26, 27].

However, ZnO:Al stability differences on textured substrates regarding the two

deposition conditions were too pronounced to be explained simply by the same reason. 240

Greiner et al. [21, 22] suggest the damp heat instability of ZnO:Al on rough substrates

to be governed by extended grain boundaries. Therefore, we conclude to have a lower

number of growth disturbances when depositing with optimized, low pressures

deposition conditions.

(b) ZnO:Al layers on texture-etched glass showed deep holes after etching in dilute HCl 245

(figure 5). Growth disturbances exhibit a higher etching rate than the bulk layer [39].

Hence, we attribute the holes in the ZnO:Al to result from preferential etching of

extended grain boundaries. As optimized layers showed fewer holes than non-

optimized films, we may state again to have a lower number of growth disturbances

when depositing with optimized, low pressures deposition conditions. 250

We conclude our discussion about the impact of different deposition conditions by noting

three points that are still unclear and need further research:

Contradictory to the above outlined argumentation, the mobility difference between layers

on textured and flat substrates increased for deposition temperatures above 300 °C and

low deposition pressures (figure 2). We suspect enhanced adatom surface mobility due to 255

higher deposition temperatures to influence the grain orientation [29].

We are aware of the fact that geometrical effects such as target-to-substrate distance are

also important factors determining the angular distribution of incoming particles [37] and,

therefore, the column orientation.

Besides the number of growth disturbances, their resistance and distribution might also 260

differ as a function of deposition conditions.

Besides the deposition conditions, the substrate morphology is an important factor

determining the ZnO:Al mobility and stability on textured substrates [22]. We

demonstrated the dependency of the ZnO:Al resistivity to the shape of the substrate 265

features. The mobility increased from V-shaped to U-shaped textures (figure 3).

Furthermore, ZnO:Al grown on U-shaped structures was more stable under damp heat

conditions than on V-shaped morphologies (figure 4). Hence, we have a higher number of

extended grain boundaries on V-shaped than on U-shaped surface textures. This

interpretation is supported by etching experiments (figure 5). Consequently, the higher 270

ZnO:Al mobility on U-shaped structures compared to V-shaped morphologies could be

explained by a lower number of growth disturbances constricting the current in the

ZnO:Al layer.

5. Summary

ZnO:Al deposited on textured glass as substrate for silicon thin film solar cells is a 275

promising alternative to textured ZnO:Al grown on flat glass.

We presented optimized ZnO:Al growth conditions that overcome the drawback of

decreased ZnO:Al charge carrier mobilities on textured substrates. Supported by x-ray

diffraction measurements, a simple, qualitative model was introduced that could explain

the effects of different growth conditions. Furthermore, the substrate morphology 280

influenced the mobility. More precisely, we observed higher mobilities on U-shaped than

on V-shaped structures. Damp heat stability and etching behavior suggested that

optimized growth conditions and U-shape morphologies induce less ZnO:Al growth

disturbances. This could explain the higher ZnO:Al mobility and stability for optimized

growth conditions and films deposited on U-shaped surface textures. 285

Acknowledgement

We thank W. Appenzeller, H. Siekmann, H.W. Bochem and H. Täuber for technical

assistance and our project partners in the joint project LIST for fruitful discussions.

Financial support by the German Ministry BMWi (contract no. 0325299) is gratefully

acknowledged. 290

Figure 1 AFM measurements of the etched glass substrates I-IV. The glass substrates

underwent the second etching step for different times: (a) 0 s, (b) 40 s, (c) 80 s, (d) 295

120 s. The corresponding rms values are: (a) 99 nm, (b) 114 nm, (c) 98 nm, (d)

107 nm. Schematic diagrams show the valley shaping effect of the second etching

step.

300

Figure 2 ZnO:Al charge carrier mobility as a function of deposition temperature for

two different deposition pressures, 0.13 Pa (black squares) and 0.67 Pa (red circles).

A flat reference substrate (closed symbols, solid lines) and a textured substrate I 305

(open symbols, dashed lines) were co-deposited.

310

Figure 3 ZnO:Al charge carrier mobility for different substrate morphologies. The

textured substrates are characterized by the time of the second etching step (see figure

1). The ZnO:Al films were grown at optimized (pdep=0.13 Pa, black squares) and non-

optimized (pdep=0.67 Pa, red circles) sputter conditions. 315

320

Figure 4 Dependence of ZnO:Al resistivity on the damp heat time t. ZnO:Al was

deposited on a flat reference substrate (black squares) and the substrates I (V-shape,

red circles) and IV (U-shape, blue triangles). ZnO:Al layers were grown with

optimized (pdep=0.13 Pa, solid lines) and non-optimized (pdep=0.67 Pa, dashed lines) 325

deposition conditions.

330

Figure 5 SEM top-view images of ZnO:Al films deposited on textured substrates

subsequently etched 5 s in 0.5% HCl. Holes in the ZnO:Al layer are exemplarily

marked with red rings. The brightness was increased for parts of the images in order 335

to accentuate the holes in the layer. Optimized (pdep=0.13 Pa, left) and non-optimized

deposition (pdep=0.67 Pa, right) conditions were used to coat the substrates. Different

texture-etched substrates were used: (a) & (b) substrate I (V-shape), (c) & (d)

substrate IV (U-shape).

340

Figure 6 X-ray diffraction pole figures of the ZnO:Al (002)-reflex on textured

substrate I (V-shape): (a) optimized deposition conditions (pdep=0.13 Pa), (b) non-

optimized deposition conditions (pdep=0.67 Pa). 345

Figure 7 Qualitative sketch of the grain orientation on textured substrates:

(a) optimized deposition conditions (pdep=0.13 Pa), (b) non-optimized deposition

conditions (pdep=0.67 Pa). Growth disturbances are marked in red. Because of the 350

more vertical orientation of the grains, the growth disturbance in the layer is less

pronounced for optimized growth conditions than for non-optimized growth

conditions.

355

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