Improvement of the Electrical Properties of Layered Semiconductor Thin Films by Iodine Treatment

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Improvement of the Electrical Properties of Layered Semiconductor Thin Films by Iodine Treatment

J. Bernede, H. Hadouda, S. Li, H. Essaidi, J. Pouzet, A. Khelil

To cite this version:

J. Bernede, H. Hadouda, S. Li, H. Essaidi, J. Pouzet, et al.. Improvement of the Electrical Properties of Layered Semiconductor Thin Films by Iodine Treatment. Journal de Physique III, EDP Sciences, 1996, 6 (12), pp.1697-1704. �10.1051/jp3:1996208�. �jpa-00249552�

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Improvenlent of the Electrical Properties of Layered

Semiconductor Thin Films by Iodine Treatment

J-C- Bernede (~), H. Hadouda (~), S-J- Li (~), ^H. ^Essaidi (~), ^J. ^Pouzet _(~>*) ^and

A. Khelil (~)

(~) Laboratoire de Physique des Mat6riaux _pour l'fllectronique,

kquipe _Couches _Minces _et _Mat6riaux _Nouveaux, Facult6 des Sciences et des Techniques, Universit6 de Nantes, 2

rue de la Houssinibre, 44072 Nantes Cedex 03, France (~) Laboratoire de Physique des Mat6riaux et Composants de I'(lectronique,

Universit6 d'oran As S6nia, BP 1642, Oran, Algeria

(Received18 March 1996, revised 4 June 1996, accepted 13 September1996)

PACS.73. Electronic structure and electrical properties of surfaces, interfaces and thin films

Abstract. The conductivity of WSe2, MoS2, WS2 thin films, obtained by solid state reaction between the metal and the chalcogen sequentially deposited, is controlled by grain boundary scattering mechanisms. A mild annealing treatment at 373 K for 1/2 h to 6 h under iodine

atmosphere improves the conductivity of the films. In the _case of WSe2, which gives the better results, the _room temperature conductivity _can be multiplied by 25. If the conductivity is improved, its dependence with the temperature demonstrates that the conductivity _process is still controlled by grain boundary scattering mechanisms. All the conductivity results _are

interpreted with the help of _a grain boundary model. Barrier height fluctuations at the grain boundary _are introduced to have experimental and theoretical data fit together. After iodine

treatment the _mean barrier height has decreased and the homogeneity of the films is improved.

These improvements _are _very promising for photovoltaic application since it appears that iodine

passivates defects at the grain boundaries.

Introduction

The transition metal semiconductors such _as WSe2, MoS2, WS2 belong to the family of

dichalcogenides with _a layer-type structure. They _can act _as efficient absorber films in photovoltaic cells because of their band _gaps which _are well matched with the solar spectrum ^and their high absorption coefficients for the wavelength _range of the solar spectrum [1j.

MoS2, WS2 and WSe2 thin films _are extremely promising because only _a _very small amount of materia1is needed to construct an absorber being able to harvest most of the solar light.

They have been intensively investigated ^for photovoltaic applications. It has been shown that

van der Waals planes perpendicular to the c-axis _are not chemically active while the surface perpendicular to these _van der Waals planes _are _very active and _are mainly responsible of poor performance ^of solar cells having large density of steps at the surface [2-4j. Therefore

a method _to reduce these disadvantgeous effects _on the electrical properties of these layered (*) Author for correspondence

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1698 JOURNAL DE PHYSIQUE III N°12

semiconductors is needed. There is two possibilities to decrease the density ^of these defaults, either increase the size of the crystallites which have their c-axis perpendicular to the plane of the substrate, _or passivate the active planes.

Fortunately _some authors have succeeded in the first _way [5-9] but the _process used im-

plies _a temperature higher than the melting temperature of glass which _means that expensive substrates _are needed. In this paper we describe _a low temperature process which allows to

improve the conductivity of the films by iodine passivation ^of the defaults.

Experimental

All the films studied in this work (WSe2, MoS2, WS2) have been obtained by the _same tech-

nique called "solid state reaction induced by annealing between the constituents sequentially deposited"

This technique has been described in detail in earlier _papers [10-13j, therefore it will be

only shortly recalled here. Layers of M(M

= W _or Mo) and XIX

= Se _or S) _were deposited sequentially in order to obtain (M/X/M/. MIX) _sequences. After deposition, the multi-

layer structures _were annealed in _a _vacuum sealed Pyrex tube. Chalcogen have _a high _vapour

pressure. Therefore during the annealing, the last chalcogen layer _was evaporated and the thicknesses of the other layers _were calculated _to achieve the desired composition. The annealing temperature ^and ^time ^varied ^between ^{823 K} to 853 K and 4 h to 145 h respectively [10-13j.

The chalcogen atmosphere, during the annealing, improves strongly the crystalline quality of the films. During the cooling of the Pyrex tube, there is _some chalcogen condensation at the

surface of the films. Therefore the samples _were annealed for 4 h at 723 K under _vacuum in order to sublimate this _excess of chalcogen at the surface of the samples.

At the end of _the _process, the quality of the films has been checked by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and microprobe analysis. All the films _are cry-stallized in the 2H &fX2 structure with _a texturation factor Foot higher than 0.7 which _means that the majority of the crystallites ^have their c-axis perpendicular to the

plane of the substrate. All these films _are nearly stoichiometric (32 < M at.% _< 35).

The films _are systematically polycrystalline with _a surface _more _or less homogeneous. How- ever, there _are no large cracks visible in scanning electron microscopy. The influence of iodine

treatment on the electrical properties of the films has been studied. Before and after iodine

treatment, the variation of the conductivity of the MX2 films with the temperature was mea-

sured. Electrical measurements _were performed _on planar samples. Evaporated gold _was used in order _to deposit electrodes _on the samples. Cu wire _was attached _to the Au electrode with

Ag paste.

The d-c- conductance of the structures _was measured with _an electrometer Keithley 617.

During measurements, the currents generated by the electrometer _were between I nA and 1 ~IA. Electrical measurements _were carried out in the dark at temperatures ^between 100 to

500 K. Au gives a good ohmic contact.

In order to proceed to iodine treatment, ^the samples have been introduced with _a small

amount of iodine in Pyrex tubes. These tubes _were sealed under _vacuum and annealed for

1/2 h to 6 h at 363 _or 373 K. For smaller temperature, there _was not any modification of the conductivity of the films. For _a higher temperature, there _was _some transport ^efsect and _a high density of pinholes appeared in the films.

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Experimental Results and Discussion

All the films have been physicochemically characterized before _any conductivity study [10,12, 13j. The thickness of the films varies between 100 _nm to 500 _nm. All the films _are nearly

stoichiometric (32 ^at.% < M _< 35 at.%) and crystallize in the 2H MX2 hexagonal structure.

The surface homogeneity of the films has been checked by SEM. Only the films without cracks and without _a large density of pinholes have been studied.

In order _to make _sure that the modifications of the electrical properties of the samples _were

not due _to the heating cycles during the electrical measurements, _we ^have applied systematically

two measurements cycles: before and after iodine treatment. Before iodine treatment, the

conductivity properties of all the films _are stable. The _room temperature conductivity is

nearly the _same at the beginning and _at the end of _a heating cycle (100 K 500 K). After

annealing under iodine atmosphere, the _room temperature conductivity of the samples has increased (Tab. I). The most significant result is obtained with the WSe2 samples. In that

case the conductivity is increased by _a factor of 25.

It has been shown earlier [14j ^that ^the ^carrier mobility _(~1) and the conductivity la) of the films studied here is far smaller than the _one expected in single crystals which demonstrates

that their conductivity is controlled by grain boundary scattering mechanisms.

We _can _see in Figure that the conductivity does not follow _an Arrhenius dependence,

as expected by using ^the ^classical grain boundary ^model ^of ^Seto [15j, ^the samples exhibiting

marked variations in idIn aT~~)/bT with temperature. ^The ^observed slopes _are always in-

creasing with the temperature which is compatible with modified grain boundary scattering

mechanisms [16j.

The classical thermal emission _across grain boundary _[17] is given by:

J = A*T~e~Q"~~'e~~~~~/~~'(1 e~~~d/~~) ii)

with:

A* efsective Richardson constant, kT/q thermal energy,

q( Fermi level position within the grain: q( = EC EF

" kTln(Nc In),

Nc effective density of state, n carriers density,

Vgb barrier potential at the grain boundary, Vd bias voltage.

As shown by Werner [16], ^if we introduce potential variations among different boundaries and model the fluctuating barrier # by _a Gaussian distribution:

pj~) =

e-(T-4)~/(2a~) _j~~

aj@

with T _mean barrier and aj standard deviation, _we obtain:

,2

~~~~~~ ~~~~ 2k'/q ~~~

It _can be _seen that #e~ decreases _upon cooling. The slopes of Arrhenius plots of conductivities

are therefore curved upwards. Werner has shown that the temperature dependent activation energy Eact is given by:

~

l,2

~~~~~~~ ~dT-1 ^~~~'~~~ ^~ ^~~~ ^~~ k/q~ ^~~~

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~ l-

o

~~ -14

o

2 3 4 5 6 7

~) ^T"~ (K"~)

~d~

S

~ _~~

C~

# -24

~

°

§ -25 -26

-27 ~~q

'~~

2 3 4 5 6

b) ^T"~ (K~)

Fig. 1. Temperature dependence of the electrical conductivity of WSe2 and MoS2 films before and after iodine treatment. a) (D) WSe2 before iodine treatment, (o) WSe2 after iodine treatment (373 K

-1/2 h). b) (D) MoS2 before iodine treatment, (o) MoS2 after iodine _treatment (373 K h).

As _can be _seen in Figure 1, there is _a good agreement ^between the experimental points and

parabola. The corresponding values obtained for 4 _and

a~ are reported in Table I. We have

reported in Table I the homogeneity factor, H

= T(T

= 0)la~, proposed by Werner [16]. ^It can

be _seen that, if H is able to estimate the homogeneity of the films, _we need _a _more restricting quality factor when these films should also have their electrical and optical properties _as _near

as possible to those of _a single crystal. Therefore the estimation of the quality of the films

need to take into account their conductivity and their band gap. In _a previous _paper [18j we

have used this model in the _case of MoSe2 thin films and _we have proposed _a quality factor Q such that:

Q

=

'~~ H(i jE~tf E~~~j/E~~~) is)

asc

with Eg ^for optical _gap, subscript "tf' for thin film and subscript "sc" for the single crystal.

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Table I. Electrical parameters ^and quality factor ^deduced from electrical measurements before

and after iodine treatment.

Sample Single crystal reference D* Foot Annealing conditions _a300

x ~ a~ H Q

asc 300 K Egsc (nm) (itCm) (mV)

WSe2 853 K 4 h 5.2 _x 10-3 ₂₈₀ 34 8.3 0 02

500 _nm [24( ^1.16 (1] ^> 200 +373 K 1/2 h 130 _x 10~~ ₂₂₅ 29 7.9 0.47

under iodine pressure

iVS2 823 II 145 h 70 x 10-3 ₂₇₅ ₅₅ ₅ o-IS

loo _nm 6 1.29 lo _am 0.7 +363 K 6 h 170 _x 10-~ ₂₆₀ 50 5.2 0.42

MoS2 823 K 24 h lo _x 10-~ 155 42 4 0.08

500 _nm 0.5 [26] ^1.14 ii] 20 _JJm 0.9 +373 K I h 65 _x 10~~ ₁₂₉ ₃₆ ₃ _0.38

MoS2 823 1( 24 h 40 _x 10~~ ₁₅₀ ₄₀ _3.8 _0.3

loo _am 0.5 1.14 20 _am 0.9 +373 K I h 130 _x 10~~ _lls 35 3.3 0 85

mean measurements

The higher ^Q, the higher the quality of the films. For each compound, to have meaning, the

quality factors should be calculated by using always the _same value for _a~c and Eg~c.

As _we _can _see in Table I, the quality factor Q of the films is strongly improved by iodine treatment. The _room temperature conductivity has increased, but also the barrier height and the barrier fluctuation have decreased.

It _can be seen in Figure 1, that, if the iodine treatment improves the conductivity of the films,

the evolution of _a with 1IT follows the _same variation which _means that probably it follows the _same law. Since before iodine treatment the small value of _a and

~1 have been attributed to grain boundary scattering mechanisms, we can imagine that after _iodine _treatment the

conductivity is still controlled by grain boundary scattering mechanisms and that the iodine influence _concerns mainly the grain boundaries.

Since, it has been shown that Ii in photo electrochemical cells _can passivate dangling bonds [19]. We _can imagine that the _same _process _can work in the experimental conditions described

above. It has been shown that after treatment of WSe2 in polyiodide electrolyte, photovoltaic performance of _a solid _state WSe2/Au junction is improved _[19]. The polyiodide electrolyte

is believed to passivate surface recombination centers due to dangling bonds. It is presumed that this passivating effect remains _even when the WSe2 is removed from the electrolyte and is

effective in solid state cells prepared from the passivated WSe2 We _can imagine that annealing WSe2, WS2 and MoS2 films under iodine _pressure induces the _same process.

This proposition is sustained by another work of Hodes et al. [20] ^where they have shown that iodine _vapor treatment (closing of _n WSe2/Au sample in _a bottle with elemental I at room temperature ^for 5 min) improves the device performance. They have attributed this

improvement to neutralization of defects mainly situated _on the surface perpendicular to the c-axis of the crystal.

They have shown that, even in single crystal, iodine is not just absorbed _on the surface but is present along tens of _nm. In the _case of MoS2 _we have shown in _an earlier work [21] ^that while the thin films (< 100 _nm thick) _are highly homogeneous and textured with the c-axis perpendicular to the plane of the substrate, the thicker films _are _more _porous. It _can be _seen in Table I that the iodine effect is higher in the _case of the thicker MoS2 films.

Therefore _we _can conclude that this efsect is not only _a surface effect and that the iodine influence is higher in the _case of thicker MoS2 films because it diffuses easily in these _porous thick films and not in the _more compact ^thinner ^films.

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' '~

- W

#U O

~

~~

i

~

bind. energy (eil) Fig. 2. XPS spectrum of I3d5/2 ⁱⁿ a WS2 film.

The iodine depth profile of iodine in the thin films studied here should be quite ^different from the _one found by Hodes et al. [20]. ^In ^the present case we are not working on single crystals

but _on _more _or less porous thin films. Moreover the iodine treatment has been carried out at 373 K for at least half _on hour while Hodes _et al. used only room temperature treatment for five minutes.

However the iodine present ⁱⁿ these films is very difficult to study because there is only _a

very small quantity. No iodine has been detected by microprobe analysis which is _a technique

which analyse all the thickness of the sample. By XPS, ^which is _a surface analysis, _some iodine has been detected. The sensitivity of these _two techniques being nearly the _same, the above results demonstrates that iodine is present at the surface of the crystallites and probably at the grain boundaries but not in the bulk of the crystallites.

The iodine species peak detected by XPS being _very small (Fig. 2), it is not possible to

decompose this peak, but the full width at half maximum of this peak is too large to be attributed to only _one iodine. Covalent and ionic (Ii iodine _are probably _present in the films.

If the Ii has passivated the dangling bonds, there is also probably _some segregation ^of neutral iodine at the grain boundaries.

It _can be _seen in Figure 3 that, when the temperature ^used during the conductivity _mea-

surements is higher than 470 K the conductivity of _a iodine treated film evolves slowly towards its value before treatment. However if _we proceed to conductivity measurements below 350 K

there is not any evolution of the conductivity from _one measurement to another _one. When

the measurement temperature ^exceeds ⁴⁰⁰ K, _a _new cycle shows that the evolution of _a with

the reciprocal temperature nearly follows _an Arrhenius law.

Therefore, _we _can imagine that there is _some iodine loss during the _measures at high tem-

perature. Probably, _some Ii _escape from the samples which explain the small decrease of conductivity. But mainly the iodine loss (no XPS signals) corresponds to the segregated (not bonded) covalent iodine, which explain the improvement of the homogeneity factor of the films

(Fig. 3, Tab. I). In the _case of polycrystalline selenium thin films doped ^with iodine, _we have shown that the iodine segregated at the grain boundaries induces large strains which modify

the properties of the grain ^boundaries [22, 23).

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-2

+

-4 ~~

~i +

f +

~ ^~

~' ~+

~

~5 +

p ^-6

+

o +

~' +

~ +

-7

+

.8

"~

l 2 3 4 5 (*10"~)

T"~ (K'~)

Fig. 3. Temperature dependence of the electrical conductivity of _a WS2 film (thickness

= 100 nm)

annealed _at 823 K for 145 h and annealed under iodine atmosphere (363 K 6 h). (.) Before iodine treatment, (+) After iodine treatment: first cycle, (D) After iodine treatment: second cycle.

Conclusion

A _very simple method to improve the electrical properties of MX2 films has been described.

The results _are _very promising ^for photovoltaic applications, since it has been shown that the

conductivity _can be multiplied by 25 and therefore the quality factor is strongly improved.

Hall effect measurements are under _way in the laboratory to check the influence of the iodine treatment on the carrier concentration and mobility.

Acknowledgments

This work _was supported by _an ECC _contract (JOU II CT 930352) and partly by the CMEP 95 (95 DRU 125).

References

[1] Jager-Waldau A., Lux-Steiner M.Ch. and Bucher E., MoS2, MoSe2, WS2 and WSe2 thin films for photovoltaic, Polycrystalline Semiconductors III, Physics and Technology, Solid

State Phenomena, Vol. 37-38, ²¹⁴ (Scitec Publications, Switzerland, 1994).

[2) Tenne R. and Wold A., Passivation of recombination centers in _n WSe2 yields high

efficiency (> 14%) photoelectro-chemical cell, Appt. Phys. Lett. 47 (1985) 707.

[3] Tenne R., ^Selective electrochemical etching ^of _p CdTe (for photovoltaic cells), Appt.

Phys. Lett. 2 (1983) 43.