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Phys. Status Solidi A 206, No. 2, 238–242 (2009) / DOI 10.1002/pssa.200824228 p s sapplications and materials science

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Amorphous and microcrystalline GeC:H films prepared by magnetron sputtering

N. Saito*, 1, H. Iwata2, I. Nakaaki3, and K. Nishioka1

1 Miyazaki University, Miyazaki 889-2192, Japan 2 Takamatsu National College of Technology, Takamatsu 761-8058, Japan 3 Shizuoka Industrial Research Institute of Shizuoka Prefecture, Shizuoka 421-1298, Japan

Received 6 June 2008, revised 15 December 2008, accepted 15 December 2008

Published online 26 January 2009

PACS 61.05.cp, 73.61.Jc, 78.30.Fs, 78.66.Jg, 81.05.Gc, 81.15.Cd

* Corresponding author: e-mail saito@phys.miyazaki-u.ac.jp

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The research on crystallization of films with tetrahedrally coordinated bonding structure has been actively done recently as the effectivity of the micro-crystalline film has attracted attention as the intrinsic layer of pin-type solar cells [1], from the viewpoint of the im-provement of the conversion efficiency of hydrogenated amorphous silicon (a-Si :H) solar cell. Likewise, the control of the bandgap (optical gap Eo) plays an important role in these materials. Research on the tetrahedrally bonded amorphous semiconductor with rela-tively narrow Eo < 1.5 eV has been aggressively performed. The formation of such binary material is possible either by the reduction of Eo for a-Si:H (Eo: typically 1.7 eV) or by the expansion of Eo for a-Ge:H (Eo ~ 0.7 eV). We have re-ported basic data on the optical and electrical properties of a-GeC:H films with narrow bandgap prepared by the reac-tive magnetron sputtering method [2], and also on the result of comparing these data with a-SiC:H and a-Ge:H [3, 4]. In recent years, research on the polycrystalline or microcrystalline tetrahedrally bonded semiconductor mate-rial is developing, such as microcrystalline SiC, polycrys-talline GeC and microcrystalline GeC [5–9]. In these stud-ies, experimental methods of deposition have been mainly focused on the chemical vapor deposition such as the

plasma-enhanced CVD method, reflecting a potential for the industrial applications. On the contrary, the reports on microcrystalline GeC films prepared by sputtering methods have remained unexplored until now. In the present study, we tried to deposit microcrystalline GeC:H film by intro-ducing He during the sputtering process. The influence of He addition during deposition upon the optical, structural and electrical properties of GeC:H films has been investi-gated. 2 Experimental details A radio-frequency (rf; 13.56 MHz) magnetron sputtering apparatus with an elec-trode 80 mm in diameter was used. The target material was a germanium disk, 5 mm in thickness, with a purity of 99.999%. The sputtering gas was helium (99.99995%) and methane (99.999% diluted to 50% by argon (99.9999%)). The partial pressure ratio (R) of helium to the total gas pressure was varied from 0 to 90%. The rf power, the sput-tering pressure, and the substrate temperature were kept at 100 W, 3.7 Pa and 573 K, respectively, during the deposi-tion of all the films investigated. Substrates were glass plate (Corning 7059) for all measurements except the in-frared spectrum (FTIR) measurements, for which silicon wafers were used.

Hydrogenated germanium-carbon alloy (GeC:H) films have

been deposited by a reactive rf magnetron sputtering of Ge in

methane–argon gas mixtures. As the second sputtering gas,

helium is utilized in order to control the film properties. The

effect of helium partial pressure ratio R to the total sputtering

gas on the structural, optical and electrical properties of the

films was investigated. The films show the amorphous nature

in the low-R range up to 60%. In the high-R range above

60%, the bonding configuration in IR spectra changes, the

diffraction lines of XRD appear, the optical bandgap de-

creases, and the dc conductivity increases clearly. It is found

from these data that the microcrystallization of GeC:H films

could be achieved by introducing helium during the sputter-

ing process.

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The thickness of films was measured by a stylus in-strument (TOKYO SEIMITSU: Surfcom 480). The com-position of films was measured by an electron probe mi-croanalyzer (EPMA; Shimadzu EPMA-1600) using an electron beam of 0.1 mm diameter, an acceleration voltage of 15 kV and sample current of 10 nA. The measurement of the X-ray diffraction (Rigaku: RINT2000) was made us-ing Cu K

α radiation. The optical absorption coefficient α

from the visible to near-infrared range was obtained from the reflectance and transmittance data measured by a dou-ble-beam spectrophotometer (Jacso: V570) in the range from 190 nm to 2500 nm. The infrared (IR) transmittance spectra were measured by a Fourier transform infrared spectrophotometer (Nicolet: Impact 400D) from 400 cm–1 to 4000 cm–1. The electrode for the measurement of conductivity was formed in a photocell-type configuration by evaporating aluminum onto GeC:H films. The temperature dependence of the dc conductivity was measured in the temperature range up to 523 K under a vacuum of 10–1 Pa. 3 Results and discussion Figure 1 shows the depo-sition rate and the film composition excluding hydrogen in Ge

xC1–x :H films as a function of the partial pressure ratio

(R) of helium to the total gas pressure. Although the depo-sition rate increases slightly at R = 30%, it tends to de-crease as a whole, reflecting that the ratio of helium with low sputtering yield increases. The R dependence of the film composition is weak. We can, however, appreciate a small shift in detail; the germanium content x tends to de-crease slightly with increasing R up to 50%, after which its restoration is observed, despite the decrease in the partial pressure ratio of argon with high sputtering yield. The de-crease of carbon content 1 – x in this range of R could be due to the decrease in the partial pressure ratio of methane. Figure 2 shows the R dependence of the X-ray diffrac-tion. The deposited films show the amorphous nature for R <70%. On the contrary, for high R, it is found that a micro-crystalline structure is formed; the peak of the diffraction

Figure 1 Composition of Ge and C (filled symbols) and deposi-

tion rate (empty circles) as a function of the partial pressure ratio

R of helium to the total sputtering gas.

Figure 2 Variation of X-ray diffraction pattern with R.

line such as Ge(111) and Ge(220) appears and becomes marked with increasing R. Moreover, a weak diffraction line of GeC(004) is observed for R ≥ 80%. Since the marked shift of film composition is not observed as shown in Fig. 1, Ge–C microcrystalline cluster as well as Ge clus-ters could be formed. In the part outside these clusters, it is suggested that the amorphous structure is maintained in which the complex bonding between C and H as the major constituent becomes apparent from the IR spectra de-scribed below. Figure 3a shows the IR absorption spectra in the range from 400 cm–1 to 1400 cm–1. For R = 0%, two strong bands are observed around 630 cm–1 and 760 cm–1. The latter band is attributed to the Ge–CH3 wagging band [10] and tends to decrease with increasing R. The former band is ascribed to the Ge–C stretching vibration at 630 cm–1 [7] and is predominant for R below 30%. With increasing R, the intensity of the Ge–H wagging-bending vibration at 550 cm–1 [10] increases and may be superimposed on the Ge–C stretching band. For R > 50%, the following modes of oscillation are observed as the absorption bands: C–C–Ge stretching; 660 cm–1, Ge–H

n (n = 1–3) scissors bending; 830 cm–1,

C–Hn wagging or rocking; 930 cm–1, and C–C stretching;

1030 cm–1 [10–12]. The appearance of these carbon-containing bands is clear for R > 70%, which implies that the mechanism of deposition under the reactive sputtering changes considerably; the chemical reaction process of methane becomes predominant compared with the physical sputtering process of germanium. This change results in the increase of hydrocarbon species in the plasma and as a consequence in the films. These results show that the film structure changes drastically for R > 70%, which is re-flected in the optical and electrical properties described later. The absorption bands due to stretching modes concern-ing germanium and hydrogen are generally observed around 1900 cm–1 and 2000 cm–1, which have been as-signed to Ge–H and Ge–H2 groups, respectively, in

240 N. Saito et al.: Amorphous and microcrystalline GeC:H films prepared by magnetron sputtering

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a-Ge:H films [11]. In the present study, we could observe in Fig. 3b that the 2000 cm–1 band is predominant for R = 0% and its peak shifts toward lower wave number with in-creasing R. It has been generally recognized for hydrogen-ated amorphous films, e.g. a-Si :H, that the absorption peaks of stretching modes concerning hydrogen shift to-ward higher wave number owing to the increase in the sum of the electronegativity around Si atoms bonding to the hy-drogen atom [13]. As can be seen in Fig. 3a, the intensity of the Ge–C band at 630 cm–1 decreases with increasing R. This implies a decrease of the carbon atoms bonded to the Ge atoms vibrating with hydrogen. Thus, the observed peak shift in Fig. 3b could be interpreted as a decrease of the electronegativity (Ge: 1.35, C: 2.5 [14]) of the nearest-neighbor of Ge atoms bonded to hydrogen atoms. The concentration of hydrogen bonded to germanium (NGe–H) is shown in Fig. 4. NGe–H was estimated by using the relation N = AIa, where Ia is the integrated intensity of the absorption coefficient obtained from the IR data on the Ge–H

n stretching band (1800–2000 cm–1). The propor-

tionality constant A used was 1.4 × 1020 cm–2 [15].

Figure 3 Variation of IR absorption spectra in the range (a)

400–1400 cm–1 and (b) 1900–2100 cm–1 with R.

The measurement of the concentration of hydrogen bonded to carbon was also carried out with great care using the C–H

n stretching band (2800–3000 cm–1). However,

unfortunately, the spectra could not be distinguished clearly from the background signal, owing mainly to the weak oscillator strength of the C–H

n stretching modes.

In a previous paper on a similar material [4], we found that the extent of hydrogen bonding to germanium during the reactive sputtering was enhanced by using methane, in-stead of merely hydrogen. In the present data, it is also recognized that the chemical reactions between the meth-ane-derived hydrogen and the sputtered germanium in the plasma result in the formation of Ge–H bonds during film growth. NGe–H increases gradually with increasing R and shows a maximum at R = 30%. This implies that the chemical reaction in the plasma between sputtered Ge and dissociated hydrogen from methane could be enhanced by the addition of an appropriate amount of helium to the sputtering atmosphere of methane. Although NGe–H decreases for R ≥ 30%, it becomes ap-proximately constant for R ≥ 50%, in spite of the decrease of the partial pressure ratio of methane. This indicates that under high-R conditions, dissociated hydrogen species in the plasma could be effectively incorporated into the grow-ing film as Ge–H bonds, which suggests that the plasma decomposition and reaction process of methane could be enhanced. Accordingly, it is found that the decrease of the methane concentration does not influence significantly the decrease of NGe–H. The optical bandgap Eo is shown in Fig. 5 and the band-edge width factor B is shown in Fig. 4; these data are deduced from the Tauc’s relation in amorphous semicon-ductors: αhν = B(αhν – Eo)

2, by applying to the optical absorption spectra, where α is the absorption coefficient and B is a constant representing the degree of disorder of the amorphous structure. Strictly, the application of the Tauc’s formula is unsuitable for R > 70%, since the film has microcrystallized structure. However, because the de-gree of crystallization is not strong, we applied this for-mula to all samples for convenience of discussion.

Figure 4 Concentration of hydrogen bonded to Ge: NGe–H (open

circles) and the value B (closed circles) defined by αhν =

B(αhν – Eo)2 (Tauc’s formula).

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With increasing R up to 60%, Eo tends to increase weakly as a whole. However, Eo decreases rapidly for R > 70 % by about 0.4 eV. Generally, as for the hydrogen-ated binary amorphous semiconductors with the tetrahe-drally bonded structure, Eo is affected by the shift of each concentration of the constituent elements. However, as mentioned above, the content of germanium or carbon hardly varies. As the other factor affecting the value of Eo, NGe–H is important; Eo decreases generally with the de-crease of NGe–H in hydrogenated amorphous Ge. However, it is noteworthy that NGe–H does not show a significant re-duction for R > 70% in Fig. 4. Therefore, we can suggest that the decrease of Eo is due to the microcrystallization [1]. The B-value, as a whole, shows the tendency broadly similar to Eo. It decreases rapidly with increasing R above 60%. In amorphous semiconductors, a lower value of B in-dicates generally the larger degree of structural disorder that induces the localized band tail states in the gap; B is inversely proportional to dE, where dE is the width of the band tail extending into the gap [16]. Unfortunately, how-ever, in this range of R, the film is weakly microcrystal-lized. Therefore, discussion on the structural disorder seems to be difficult only by the result of the B-value, which is effective for amorphous structure. Figure 6 shows the variation of the temperature de-pendence of the dc conductivity σD with R. It has been re-cognized that the temperature dependence of σD of the tet-rahedrally bonded amorphous semiconductor over a wide temperature range consists of several components corre-sponding to each transport mechanism [17]. Around room temperature (RT) and above, σD generally consists of two components: one is that due to the activated-type conduc-tion through the extended states above the mobility edge, which is proportional to exp (–E/kT) observed markedly in the high-temperature region, and the other is the hopping conduction around RT that is mainly attributed to the car-rier transport through the localized states below the mobil-ity edge. As can be seen from Fig. 6, the measured data of

Figure 5 Optical bandgap Eo (closed squares) and the activation

energy Ea of conductivity (open circles).

Figure 6 Variation of the temperature dependence of dc conduc-

tivity σD with R.

σD at RT deviate from the exponential formula. This im-plies that the former component of conduction is much lower compared with the latter component; we could de-duce the value of the former at RT from the extrapolation of the measured σD at high temperature to RT according to the activated-type conduction formula. Figure 7 shows measured σD (open circles) at RT (294 K) as a function of R. With increasing R up to 60%, σD decreases gradually; it decreases more than one order of magnitude compared with R = 0%. With further increasing R above 60%, σD increases suddenly by more than two or-ders of magnitude. This abrupt increase of σD could be at-tributed to the microcrystallization [18] in which the effec-tivity of depositing crystallized films by using light inert gases as sputtering ions has been investigated. We have in-terpreted that the rearrangement of the adatom at the sub-strate could be enhanced on decreasing the energy of the sputtering ion, which could facilitate the microcrystalliza-tion. In this figure, the estimated values of the activated-type conductivity “σa” (filled triangles) at RT are also shown. As described above, σa can be obtained by the

Figure 7 Measured conductivity σD, estimated activated-type

conductivity σa and the hopping part of conduction at room tem-

perature (294 K).

242 N. Saito et al.: Amorphous and microcrystalline GeC:H films prepared by magnetron sputtering

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extrapolation from a straight line (e.g., a line “A” for R = 60%) fitting in the high-temperature activated-region of the measured data σD to RT (1000/T = 3.4) in Fig. 6. Thus, for example, we could estimate σa at RT ~ 4.5 × 10–8 for R = 60%. From Fig. 7, we could recognize that the component of hopping conduction at RT is obtained as the difference between σD and σa. The part of the hopping conduction (σD–σa)/σD is plotted in Fig. 7 (empty squares). It is found from this plot that the hopping part of the con-duction increases with increasing R up to 60%, which sug-gests that the density of localized states increases. For R above 60%, on the contrary, it decreases rapidly, suggest-ing the decrease of the localized states by microcrystalliza-tion. The linear fitting to the measured σD in the high-temperature region in Fig. 6 represents the activated-type conduction equation: σD = σ0 exp (–Ea/kT). The activation energy Ea is obtained from the slope of the fitted line (e.g., the line “A” for R = 60%). The results are shown in Fig. 5 as a function of R. In the range of R up to 60%, Ea tends to increase gradually, which is almost correlated with Eo. In the high range of R > 60%, Ea shows the rapid decrease by about 0.15 eV. Assuming that these films are n-type semi-conductors, similar to a previous result for the same mate-rial [2, 3], Ea is the energy from the Fermi level EF to EC (mobility edge of the extended conduction states). From a linear fitting between Ea and Eo data, we can tentatively obtain the relation: Ea = (EC – EF) = 0.08Eo + 0.26 for R > 60%. This relation indicates that EF locates near EC rather than the midgap (0.5Eo). 4 Conclusion We reviewed the optical and electrical properties of GeC:H films prepared by reactive sputtering of a Ge target in CH4. The effects of the addition of He on these properties were investigated. In the case of He partial pressure ratio R < 70%, the films show the amorphous properties. In contrast to this range of R, it is found that the microcrystallization of films could be achieved with the increase of R ≥ 70%. In this range of R, (1) several X-ray diffraction lines are observed, (2) the optical gap Eo as well as the B value decreases, (3) the conductivity σD increases

clearly and the activation energy Ea decreases, owing to the microcrystallization.

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