A NEW TYPE OF a-Si PREPARED BY dc SPUTTERING : P, B AND H DOPING EFFECT

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A NEW TYPE OF a-Si PREPARED BY dc SPUTTERING : P, B AND H DOPING EFFECT

Nguyen van Dong, Y. Fournier, J. Le Ny

To cite this version:

Nguyen van Dong, Y. Fournier, J. Le Ny. A NEW TYPE OF a-Si PREPARED BY dc SPUTTERING : P, B AND H DOPING EFFECT. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-647-C4-650.

�10.1051/jphyscol:19814142�. �jpa-00220763�

JOURNAL D E PHYSIQUE

CoZZoque C4, supple'ment au nO1O, Tome 4 2 , octobre 1981 page C4-647

A NEW TYPE OF a-Si PREPARED BY dc SPUTTERING : P, B AND H DOPING EFFECT

Nguyen Van dong, Y. Fournier and J.Y. Le Ny

Dzpartement de Physico-Chimis, Centre drEtudes NucZe'aires de Saclay, 91 291 GiT-cur-Yvette Cedex, 17'ranc.e

A b s t r a c t . -

We have obtained by dc sputtering a new type of amorphous si- licon having apparently a low gap states density without the compensation by hydrogen. Our amorphous films which have been prepared under special condition of heating could be easy doped either n or p-type. Conductivity and photoconductivity measure- ments have shown their extreme sensitivity to a small addition of phosphorus or boron. It is remarkable that hydrogen incorpo- rated in our samples exhibits a donor-like character. H doping experiments have been performed over a large range of partial hydrogen pressures. The room temperature conductivity and photo- conductivity are increased as much as 8 and 4 orders of magni- tude respectively with a considerable shift of the Fermi level from the middle of the gap to within % 0.1 eV from the conduc- tion band.

Introduction.- Amorphous Si prepared under normal conditions contains a large density of localized gap states which act as traps for free carriers. A sharp reduction of these states by hydrogen incorporation in material improves considerably the electrical properties leading to practical applications such as photovoltaic solar energy conversion.

However the large concentration of hydrogen in conventional material may be turned to a disadvantage. Defects such as Si-H-Si three-center bonds can be formed which introduce in the gap deep hole traps (1).

Moreover in a-Si:H, germinate recombination seems to be a dominant processus which reduces the quantum efficiency of photogenerated car- riers (2).

Using the cathodic sputtering technique we have obtained under a special condition of preparation a new type of amorphous silicon which has apparently a low concentration of defects without incorporating hydrogen. The new material can be easily doped n-type or p-type and is highly photoconductive. In addition it is very remarkable that hydro- gen in our films acts as not a compensator but as a donor impurity.

Experimental.- A series of amorphous Si films with thickness of about 1 Dm were prepared by cathodic sputtering technique using a dc triode system. The deposition was achieved at a target voltage of -1000 V by introducing pure Ar gas after pumping to a base pressure of

1.5 x Torr. The pressure of Ar gas was maintained at about

5 x 10-3 Torr. The undoped films were obtained from a pure crystalline Si target, while those doped with phosphorus or boron were prepared from different c-Si targets containing various concentrations of P or B. For hydrogen doped films, different partial pressures of hydrogen ranging from 10-4 to 1.25 x 10-3 Torr were used.

Nichrome electrodes separated by a 2-mm gap were previously de- posited onto aluminium oxide substrate which was held at room

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814142

C4-648 J O U R N A L U E PHYSIQUE

tem~erature before deposition. Durinq the initial period of deposi- tion, the plasma heated the substrate whose temperature increased ra- pidly up to approximately 200°C with a heating rate of 20 to 25OC/min.

The dark conductivity was in situ measured as a function of tem- perature from 293 to 500 K. Subsequent to these experiments, the sam- ples were mounted in a cryostat evacuated to about 10-3 Torr for pho- toconductivity measurements. For all samples, the ohmicity was checked at low and high fields and different levels of illumination. The pho- tocurrent was obtained with light of photon energy 2 e V and intensity 1015 photons/cm2.s.

Results and discussion.- The effect of a special heating condition for the deposition on the electrical conductivity of a-Si grown in a pure argon plasma is illustrated in figure 1. For films deposited under normal heating condition (Ts = 220°C), the log O D - 1/T shows a curvature which is ascribed to a high density of localized gap states at the Fermi level. In contrast, films deposited on substrate heated by the plasma with a relatively high heating rate exhibits an activa- ted conductivity whose value near room temperature is lower by several orders of magnitude. This behaviour which is similar to that of films compensated by hydrogen suggests that the defect concentration is re- latively low. However the activation energy for OD in the high ternpe- rature range is substantially larger compared with the case of hydro- genated films ( 3 1 .

Fig.l:A.Normal heating condition. Fig.2: Plots of conductivity of B.Plasma heating condition. doped a-Si films as a

function of temperature.

Substitutional doping : conductivity and n-p type control.

The effect of phosphorus or boron doping on the electrical con- ductivity is shown in figure 2. Doped films were obtained from diffe- rent c-Si targets containing various atomic concentrations of P or B up to 1019 cm-3. The direct determination of these substitutional impurities in the films are underway. In this figure, it is clearly shown that the conductivity of a-Si prepared without incorporating hydrogen is controlled by varying the amounts of phosphorus or boron sputtered from the targets. The conductivity remains activated and its increase is mostly due to the decrease in the activation energy.

According to thermoelectric measurements, P-doped and B-doped films are n-type and p-type respectively. This implies a shift of the Fermi level towards the conduction band or the valence band. It is worthy to note that the decrease in the activation energy of U D is nearly symme- trical for P and B doping levels used in these experiments. In addi- tion, doping does not affect significantly the pre-exponential factor

00 (ao > 103 cm-l), suggesting that the electronic transport in

the extended states remains the dominant mechanism.

Figure 3 shows the photoconductivity against the position of the dark Fermi level. The feature which appears interesting is that boron doping increases substantially aph although in a less degree than the case of phosphorus doping.

Figure 3

Room temperature photoconducti- vity plotted against the posi- tion of the dark Fermi level for P-doped

(I

IEPE,I,

Effect of hydrogen incorporation.

The experimental fact which is very remarkable is that hydrogen incorporated in the samples gives rise to donor-like states near the conduction band. Figure 4 shows the effect of hydrogen incorporation on the conductivity measured as a function of temperature. The room temperature conductivity is increased up to 8 orders of magnitude and the activation energy for a D is decreased to a value as low as 0.11 eV.

The photoconductivity shown in figure 5 increases rapidly as EF ap- proaches the band tail states but decreases slightly as EF enters this

C4-650 J O U R N A L DE PHYSIQUE

band. Similar results have been reported by Paul et al. (4) for sput- tered a-Si. However the effect they observed occured at hydrogen pres- sures much higher than in our experiments. Thus hydrogen incorporation in our films produces the same effect on the electrical properties as phosphorus doping. However for an identical shift o f EF in two cases, H doping tends to change the transport mechanism since the pre-expo- nential factor a. is strongly decreased.

Fig.4: Effect of H incorpora- tion on the conductivi- ty of a-Si for different H pressures.

(E,- E,l,(.V)

Fig.5: Room temperature pho- toconductivity plotted against (EC

-

I< doped films.

Conclusion.- Using the dc pulverisation triode system, we have obtai- ned under a special condition of heating a new type of a-Si which dis- plays some particular features mentioned above. At present it is not possible to establish whether other deposition parameters would be de- terminant in the fabrication of this new material. One may speculate the likehood of contamination from species in the base vacuum. Chemi- cal analysis of some films is underway to determine the suspect impu- rities.

Work is continuing to study this new type of a-Si using other characterisation methods such as optical measurements.

(1) FISH, R. and LICCIARDELLO, D.C., Phys. Rev. Lett. 41 (1978) 889.

(2) CRANDALL, R.S., WILLIAPTS, R. and TOMPKINS, B.E., B ~ I . Am. Phys.

SOC. 24 (1979) 273.

(3)

NGUYENVAN

88

(4) ANDERSON, D.A., MODDEL, G. and WUIAhT PAUL, J .Non-Crystalline Solids 3 5 (1980) 345

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