WSe 2 layers synthesized by annealing tungsten foils and r,[\-sputtered tungsten thin films under selenium pressure have been investigated by scanning ...
Thin Solid k)hns, 2(t8(19921 252 259
252
Preparation and characterization of tungsten diselenide thin films J. P o u z e t
a n d J. C. B e r n e d e
Laboratoh'e de Ph),sique des Mat~;rhutx pour I'E/eetronique, Faculty; des Sciences et des Techniques, 2 rue de la fIoussini~;re. F-44072 Nantes (i'dex 03 (France)
A. Khellil Laboratoire de Micro-optoOleetronique, Unicersitb d'Oran-h,~-Senia, BP 1642, Oran (Algeria)
H . E s s a i d i a n d S. B e n h i d a Laborutoire de Physique des Matbri¢lux pour l'E/eclronique, k)tcullO des Sciences el des Techniques, 2 rue de la HoussiniOre, F-44072 Nattles Cede.~: 03 (France)
(Received July 23, 1991: accepted September 9, 19911
Abstract WSe 2 layers synthesized by annealing tungsten foils and r,[\-sputtered tungsten thin films under selenium pressure have been investigated by scanning electron microscopy, X-ray analysis, X-ray photoelectron spectroscopy (XPS). optical absorption and electrical resistance measurements. It has been found that stoichiometric layers are obtained after appropriate processing at a temperature lower than the glass melting temperature. The films crystallize in the hexagonal structure. The crystallites develop preferentially along the c axis. The binding energies deduced from the XPS lines were found to be in good agreement with those of the reference powder. The electrical resistance is governed by hopping conduction in the low temperature range (80 250 K) and by grain-boundary-scattering mechanisms at higher temperature.
1. Introduction
2. Thin film preparation and characterization
Transition metal dichalcogerlides (MoTez, MoSe2, WSez) are semiconductors which can act as efficient electrodes in the realization of photoelectrochemical solar cells [1-3]. In 1988 we described a process for obtaining MoSe2 thin films [4]. MoSe, (x < 2) layers prepared by d.c. diode sputtering were annealed under selenium pressure in order to obtain stoichiometric, well-crystallized MoSe2 thin films. Later we proposed a simplification of the thin film process [5]. Thin films of MoSe~ were obtained by heating molybdenum foils under selenium pressure. Jager-Waldau and coworkers have called the process soft selenization [6]. They report in two recent papers the properties of MoSe2 [6] and WSe, [7] thin fihns obtained by soft selenization of r.f.-sputtered metal films. Here we compare the properties of WSe2 obtained by soft selenization of tungsten foils and r.f.-sputtered tungsten films in a low temperature process, The preparation and the structural, morphological and chemical characterization of these films will be described. The electrical properties will also be presented.
The films were synthesized by soft selenization of the surface of tungsten substrates (10 x 13 x 0.25 mm 3) and r.f.-sputtered tungsten films. Tungsten films of thicknesses between 50 and 600 nm were deposited on chemically cleaned glass substrates ( 10 x 20 x 1 m m 3) by r.f. sputtering using a diode system manufactured by A T E A (Nantes). The system has been modified by the manufacturer in order to locate the target below the substrate holder and not opposite it as in the initial system. The substrate holder was grounded. The dimensions of the tungsten target foil were: diameter ~b = 75 mm, thickness t = 0.25 mm. The sputtering argon pressure was 5 Pa; the target-to-substrate distance and the sputtering power were 1.5cm and 100W respectively. Prior to film deposition the chamber was evacuated to a pressure of 10 4 Pa. Before sputter deposition the target was cleaned by presputtering until the pressure was stabilized; then the shutter was removed and the deposition started. The deposition rate was 5 10 n m m i n ~. Before breaking the vacuum, a 50 nm amorphous selenium capping layer was deposited by evaporation (evaporation rate 2 nm s- ~) to protect the tungsten films from oxidation
0040-6090/92/S5~00
1992 - - Elsevier Sequoia. All rights reserved
J. Pouzet et al. / Preparation and characterization o f WSe2 thin films
during transfer from the deposition apparatus to the Pyrex tube. Because the surface of the tungsten foils was oxidized, one side of the samples was mechanically cleaned before introduction into the Pyrex tube. Oxidation of the tungsten samples before soft selenization has been studied by X-ray photoelectron spectroscopy and will be discussed below. The tungsten foil or r.f.-sputtered tungsten layer was placed in a vacuum-sealed Pyrex tube with a small amount of selenium and heated to temperatures between 723 and 833 K for 3-72 h. Early works [4, 5] have shown that some selenium condensation takes place on the surface of the layers during the cooling of the Pyrex tube. Therefore this selenium excess is sublimated by annealing the samples under dynamic vacuum for 24h at T - - 7 2 3 K . The structure of the films was examined using an X-ray goniometer. The grain size d was estimated from the full width at half-maximum ( F W H M ) of the diffraction peaks [8, 9]. The degrees of preferential orientation F(0, 0, l) and F(h, k, 0), i.e. crystallites with the c axis perpendicular and parallel to the plane of the substrate respectively, were estimated from the formula given by Janda and Kubovy [10]. X-ray photoelectron spectroscopy (XPS) measurements were performed with a magnesium X-ray source (1253 eV) operating at 10 kV and 10 mA at the University of Nantes. Data acquisition and treatment are realized through a computer and a standard programme. The quantitative studies were based on the determination of the W 4f and Se 3d peak areas with 2.14 and 0.57 respectively as sensitivity factors (the sensitivity factors of the spectrometer are given by Leybold, the manufacturer). In the case of insulating substrates (10 × 11 × 1 mm 3) the samples were grounded with silver paste. The depth profiling was traced by recording successive X-ray photoelectron spectra obtained after argon ion sputtering for short periods. Sputtering was accomplished at pressures of less than 5 × 10 -3 Pa, a 10 mA emission current and a 3 kV beam energy using an ion gun. The Ar + beam could be rastered over the entire sample surface. Before sputtering, the pressure was better than 5 × 10 - 6 Pa. Electrical measurements were performed on sandwich samples for films obtained from tungsten foils and on planar samples for films obtained from r.f.-sputtered thin tungsten layers. It has been checked by XPS analysis that after synthesis of WSe2, the oxidized face of the tungsten foil has not chemically reacted with selenium. Therefore, after mechanical etching, the tungsten foil substrate works as the lower electrode in the sandwich structure. The upper ohmic contact was a small dot (diameter 2 mm) of evaporated gold on the WSe2 film. Evaporated gold was also used in order to deposit
253
electrodes on the planar structures. Copper wire was attached to the gold with silver paste. The d.c. conductance of the sandwich structures was measured by the conventional four-wire method, while for the more resistive planar samples an electrometer was used. During measurements the currents generated by the electrometer were between 1 nA (2 GO) and 1 p.A (2 Mr2). Electrical measurements were carried out in the dark between 80 and 500 K. Gold gives a good ohmic contact, the layers obtained being p-type semiconducting as shown by the classical hot probe technique. The optical measurements were carried out at room temperature using a carry spectrophotometer. The optical density was measured at wavelengths from 2 to 0.4 p.m. The absorption coefficient ~ has been calculated from the transmission T and from the single-crystal surface reflectivity R0 ~ 0.35 [1 I]. Since in the spectral regions of interest in this work, i.e. at energies below the A exciton, the optical dispersion is small and the surface reflectivity has been taken to be effectively constant. Any errors incurred in the values of ~ cause uncertainties in R0 that are much less than the errors in the thickness measurements which dominate the experimental errors [12]: T ~ ( 1 - R0) 2 e x p ( - ~ d ) , where d is the thin film thickness. The indirect energy gap is determined by extrapolating different plots of = A(hv - Eo) ~ to zero absorption. A being a constant, h the Planck constant and v the frequency, n can be anywhere in the range I - 2 , WSe2 being a layered indirect band gap semiconductor [13]. The thicknesses are measured by interferometry.
3. Results 3.1. Physicochemical characterization o f the WSee films In order to obtain the best crystallization (grain size and orientation), the annealing time (fi) and temperature (T~) were varied from 3 to 168 h and from 723 to 833 K respectively. With r.f.-sputtered tungsten layers, W S e 2 growth requires an annealing temperature of at least 773 K (Table 1). Irrespective of the annealing condition, the XPS analysis (Table 2) reveals that there is often an excess of selenium as described earlier [4, 5]. This excess, which appears to be caused by selenium condensation during the cooling of the tube, is sublimated by heating the sample under vacuum (10 -4 Pa). Experiment has shown that it is necessary to anneal the layer at a temperature T 2 >~ 723 K for 24 h to obtain reproducibly stoichiometric WSe2 layers (Table 2) irrespective of the type of sample. At the end of the process the X-ray spectra (Fig. 1) show that layers of MoSe2 in the hexagonal structure have been synthesized. From diffraction peaks which
254
J. Pouzet et al. / Preparation and characterization q f WSe 3 thin fihns
TABLE 1. Preferential orientations and crystallite sizes for different synthesis durations, temperatures and tungsten substrates W sample
Synthesis conditions
Foil r.f\ sputtered
Duration
Temperature
(h)
(K)
3 - 12 3- 12 72 !2-72 12 72
723 773 723 773 833
F( O, O, 1)
F(h, k, O)
dr, (nm)
(t~ ,.(ran)
[).7 0.65 No WSe2 hexagonal structure 0.5 0.55
0.2 0.2
10 10
100
0.4 0.3
30 20
150 200
100
TABLE 2. Results of WSe~ XPS analysis Sample
Binding energy (eV) before any sputtering Sc 3d
W 4f5/:
W 4fv,.2
Energy difference bE(eV) between the Se 3d and W 3d5/= peaks
Composition (at.%) Mo
Se
W foil before sputtering (oxidized) W foil after sputtering WSe 2 powder
54.5
33.35 29.35 30.05
35.5 31.5 32.20
24.45
32
68
Synthesized WSe2 not annealed under dynamic vacuum from W foil from W r.f. sputtered
54.7 54.5
29.85 30. I0
32 32.25
24.75 24.4
28 20
72 80
54.5 54.5 54.5 54.5 54.2
30.05 30 29.95 30.05 29.85
32.20 32.15 32.1 32.3 32
24.45 24.5 24.55 24.45 24.35
32 32
68 68
35
65
Synthesized WSe2 annealed under dynamic vacuum W sample Synthesis conditions
Foil r,L sputtered
Temperature (K)
Duration (h)
723 773 773 783 823
3 6 168 24 168
so
40
3o
2b
*'-- E(ev)
(a)
/ 50
40
30
20
.,-- ECev)
(b)
Fig. I. X-ray diffraction patterns (Cu K~ radiation) of WSe2 layers synthesized from (a) tungsten foil (t =24h, T = 770K) and (b) r.f,-sputtered tungsten layer (t = 24 h, T = 800 K).
c o r r e s p o n d to p l a n e s parallel a n d p e r p e n d i c u l a r to the m a i n c axis, the d e t e r m i n a t i o n o f the g r a i n size s h o w s that the crystallites d e v e l o p b e t t e r a l o n g the c axis ( T a b l e 1). T a b l e 1 also s h o w s F ( h , k, 0) a n d F ( 0 , 0, l) for the different a n n e a l i n g times. T h e p e r c e n t a g e o f reflections with the c axis p e r p e n d i c u l a r to the p l a n e o f the s u b s t r a t e is h i g h e r t h a n t h a t with the c axis parallel to the p l a n e o f the s u b s t r a t e irrespective o f the a n n e a l i n g time. N e a r l y all crystallites are o r i e n t e d with their c axis either p e r p e n d i c u l a r o r parallel to the p l a n e o f the s u b s t r a t e . W e c a n see also f r o m T a b l e s 1 a n d 2 that WSe2 is m o r e easily synthesized o n t u n g s t e n foil t h a n o n r . f . - s p u t t e r e d t u n g s t e n t h i n films. T a b l e 2 p r e s e n t s the results o f the X P S analysis. R e f e r e n c e a n a l y s i s v a l u e s were o b t a i n e d f r o m MoSe2 p o w d e r . T h e h e x a g o n a l s t r u c t u r e a n d the g o o d stoic h i o m e t r y (33 a t . % W, 67 a t . % Se) have b e e n c h e c k e d by X - r a y d i f f r a c t i o n a n d e l e c t r o n m i c r o p r o b e analysis. It c a n be seen f r o m T a b l e 2 t h a t all the films are n e a r l y
255
J. Pouzet et al. / Preparation and characterization of WSe2 thin films [J
Se
B.U.
:(at
W
Se
50
10 55 I
1
I
I 5
I
I
I
I
[ 10
45
35
""E~v}
t
(ran)
Fig. 2. XPS elemental profiles through samples: - - , WS% reference powder; I , WSe2 film synthesized for 24 h at 770 K from tungsten foil; ©, WSe2 film synthesized for 24 h at 800 K from r.f.-sputtered tungsten layer. stoichiometric at the end o f the process. After ion sputtering, there was a tungsten excess, since the speed o f etching o f selenium was higher than that o f tungsten. Therefore, to discuss the depth profiling o f the layers, we have used the composition profile o f the powder as reference (Fig. 2). As we can see in Fig. 2, the depth profiles o f the layers were identical to that o f the p o w d e r reference. We can conclude that the films are stoichiometric not only at the surface but also at depth. We n o w discuss the XPS chemical analysis. The binding energies o f the W 4f and Se 3d peaks are given in Table 2 along with the binding energies o f the elements, the oxides and the reference WSe2 powder. We can see that before mechanical etching, the tungsten foil is oxidized (Fig. 3). After this etching, the tungsten is nearly free o f oxide at the surface (Fig. 3). In the case
Fig. 4. X-ray photoelectron spectra of W 4f and Se 3d after 3 rain sputtering of (1) WSe2 reference powder, (2) WSe2 layer synthesized from tungsten foil and (3) WSe2 layer synthesized from r.f.-sputtered tungsten foil.
o f r,f.-sputtered tungsten films we have shown earlier [ 14] that an a m o r p h o u s selenium thin film is an efficient protection against oxygen contamination. Table 2 shows that the W 4f5/2 peak o f the WS% reference p o w d e r is situated at 30.05 eV while the Se 3d peak is situated at 54.5 eV. Therefore there is an energy difference between W 4t"5/2and Se 3d o f 24.45 eV when tungsten and selenium are combined to give WSe2. F o r the thin films we notice that identical results are obtained irrespective o f the type o f sample (Fig. 4). The binding energies are very close to those obtained with WSe2 powder, which confirms that we have synthesized WSe 2 layers. The oxygen pollution of the layers was studied by recording successive X - r a y photoelectron spectra obtained before and after argon ion etching (Fig. 5). We see that after 3 rain o f etching the oxygen is not quantifiable.
W
I
I
a.Ll
a.u
~--Etev~ Fig. 3. X-ray photoelectron spectra of W 4f of tungsten foil (I) before mechanical etching, (2) before mechanical etching and after 3 min sputtering, (3) after mechanical etching and (4) after mechanical etching and 3 min sputtering.
2
538
5~
5)0
4
IrCt,d
Fig. 5. X-ray photoelectron spectra of 0 I s of (1) WSe2 synthesized layer before etching and (2) WSe2 synthesized layer after 3 min etching.
256
J. Pouzet et al. / Preparation and characteri--ation a[ WSe2 thin .fih~ls
(a)
(b)
(c)
(d)
Fig. 6. Surface morphology of W S e 2 layers synthesized from (a), (b) tungsten foil (t = 2.4 h, T = 770 K) and (c), (d) r.f.-sputtered tungsten foil (t = 24 h, T = 800 K).
Photographs showing the surface morphology after synthesis of the layers are shown in Fig. 6. Although the previous results were quite similar, the photographs show striking differences in the surface morphology. At low magnifications (Figs. 6(a) and 6(c)) we can see that the surfaces of the films grown on tungsten foils appear comparatively rough with small heaps randomly arranged. Moreover, these films were poorly adhesive to the tungsten substrate. Higher magnifications show that the surfaces of all the films appear granular with grains irregularly shaped. However, in the case of sputtered tungsten the synthesized films present a lamellar microstructure, which is the structure expected in these transition metal dichalcogenide compounds. Moreover, the photographs confirm that m a n y crystallites are preferentially oriented with the c axis parallel to the plane of the substrate.
3.2. O p t i c a l a n d e l e c t r i c a l c h a r a c t e r i z a t i o n
Optical characterization has been performed only on samples synthesized on glass substrates, i.e. samples obtained from r.f.-sputtered tungsten films. In order to estimate the indirect band gap of the layers, the variation in the optical absorption with the photon energy hv is shown in Fig. 7. The optical indirect band gap of the WSe2 films was determined to be about 1.12 and 1.25 eV by extrapolating the straight lines of ~ ~'~ vs. E and ~ vs. E respectively, We can see from Fig. 7 that the absorption below the threshold energy value is still important, which may induce large uncertainty in the gap determination. Therefore we can only conclude that the values obtained for the band gaps of the WSe2 films are of the same order as those of WSea single crystals [1] and other thin films [7, 15]. The high absorption coefficient below the threshold energy value is related to the roughness of the surface
J. Pouzet et al.
E"
/
Preparation and characterization
of"
257
kVSe 2 thin films
~-.
f
-"
IE
t
-~ ' 6 0 0 -t~.
)
# A
-8"
2. /
/
/
I
[]
[]
[]
/
/
I
/
/ / /
-12.
d
/ []
'400
~ t/ t
/
/
"
a/ /
•
t
/
/
200
-16 ¸
I
/ /m /
/
/
0
,'
/
IO~/T
I
/
o
/
'
'
~
113
ECev;
1;5
0
Fig. 7. Variation in optical absorption • with photon energy for a WSe2 layer synthesized from r.f.-sputtered tungsten foil: S , n = 1; i,n=2.
15
10'
5
I03/T (K "~) Fig. 8. Temperature dependence of electrical resistance of WSe 2 thin films synthesized from tungsten foil.
i
6
8
(K'4J
16
Fig. 9, Temperature dependence of electrical conductivity of WSe 2 thin films synthesized from r.f.-sputtered tungsten layer.
of the films as shown in Fig. 6. The scattering of the incident light increases as the roughness of the sample increases [ 16]. Therefore we can deduce that the absorption below the threshold energy value is related to the morphological inhomogeneity of the films and corresponds to high scattering (low absorption) of the light by disoriented microcrystals. The room temperature resistivity of the films synthesized from r.f.-sputtered tungsten foil varied from 10 to 100 f2 cm. The temperature dependences of the electrical resistivity and conductivity between 80 and 500 K are shown in Figs. 8 and 9 respectively for typical samples. The samples do not follow an Arrhenius dependence but exhibit marked variations in (3(ln a)/~T (Fig. 8) and 0(ln R)/OT (Fig. 9) with temperature. The observed slopes are always increasing with temperature. As shown in previous papers [5, 12], these results could be well interpreted in terms of grain boundary theory. The conduction in the low temperature region is due to hopping of the charge carriers in the trap states at the grain boundaries [17-22]. In the temperature region above 250 K, since the studied films are polycrystalline, grain boundary theories may have to be taken into account [23, 24]. We have estimated the barrier height at the grain boundary to be about 0.2 eV.
4. Discussion and conclusions
In Table 3 the characterization parameters of our WSe2 thin films are compared with those of Jager-
258
J. Pouzet el al. / Preparation and characterization q f WSe 2 thin films
T A B L E 3. Comparison between the present results and those of Jager-Waldau and Bucher [7] Present results
Reference 7
Substrate
Tungsten foil
Glass
Quartz
Tungsten layer
Tungsten foil
r.f. sputtered
r.f. magnetron sputtered
Synthesis temperature (K) time (h)
723 3 12
823 12 72
I 123 18 24
Chemical composition
Stoichiometric
Stoichiometric
Stoichiometric
Crystalline structure
Hexagonal
Hexagonal
Hexagonal
Preferred orientation
c axis predominantly perpendicular to the plane of the substrale
c axis either perpendicular or parallel to the plane of the substrate, with predominance of c axis perpendicular
A: c axis predominantly parallel to the substrate B: c axis predominantly perpendicular to the substrale
Grain size {nm)
10 100
20 200
10 35
Scanning electron microscopy studies
The surface is rough and the films appear porous
Homogeneous films with lamellar microstructure
Type A surface is more homogeneous than that of type B
Indirect optical gap ~' (eV)
1.12
