PREPARATION AND PROPERTIES OF ION BEAM DEPOSITED a-SiHx

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PREPARATION AND PROPERTIES OF ION BEAM DEPOSITED a-SiHx

G. Ceasar, K. Okumura, S. Grimshaw

To cite this version:

G. Ceasar, K. Okumura, S. Grimshaw. PREPARATION AND PROPERTIES OF ION BEAM DEPOSITED a-SiHx. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-627-C4-630.

�10.1051/jphyscol:19814137�. �jpa-00220757�

J O U R N A L DE PHYSIQUE

ColZoque C4, suppZ6nent uu nOIO, 'i'ome 4 2 , octobre 1301

PREPARATION A N D PROPERTIES OF ION BEAM DEPOSITED a-SiH,

G.P. Ceasar, K . Okumura a n d S . I . G r i m s h a w

Xerox kiebster Hesearch Center, kk./c::ter, i'lew York i4580, 1j.S.A.

Abstract.- lon beam deposition (IRD) has been used to prepare a-SiH, thin films over a --

range of deposition conditions. Concentration of hydrogen in the ion beam is found to be a major factor in determining the properties of these materials. Increasing the hydrcgcn concentration of the ion beam from 0 to 75% produces a monotonic decrease in room temperature dark conductivity from to 10-lo (Rcm)-l. This is accompanied by an increase in the optical Band gap from 1.2 to 1.8 eV and a decrease in the esr spin density 10 <

l ~ l ~ c r n - ~ . Monohydride SiH bonding predominates in the ir spectra with no eiidence for polysilane coordination. Densities are greater than 2.2 g/cni3 for all films made in this study suggesting an absence of voids. Phase contrast

TEM

gives no indication of columnar morphology.

Several thin film deposition methods have proven useful for preparing a-SiH, alloy.

l h e s e include plasma deposition1, rf sputtering2, high temperature cvd3 and reactive evaporation4. Recently we described a new route for preparing a-SiH, alloys using ion beam deposition5. This technique may offer improved control over the deposition process. In this paper we report on the properties of a-SiH, prepared by this technique.

Ion Beam De~osition (IBD)

T h e experimental arrangement used in this study is shown schematically in Fig. 1. A collimated monoenergetic beam of positive ions is extracted from a uniform plasma contained in a hot cathode Kaufrnan type ion source. This beam is directed at a target of the material to be deposited. Atoms are sputtered from the target and collected onto heatable substrates which are well removed from the ion beam path. Control of beam energy from 0-1.5 KeV with a spread of a few el1 is possible by the voltage applied to the grid assembly. The positively biased screen grid, consisting o f a number of small holes (0.lmm) in a pyrolytic carbon electrode, serves to bunch positive ions into beamlets. These beamlets are extracted and accelerated by the negative accelerator grid and emerge from the ion gun as a collimated beam o f positive ions.

The grids also serve as a radiation and mass

barrier screening the substrate from plasma contained in the ion source and maintaining low background pressures ( < 1 0 - 4 ~ o r r ) in the deposition chamber.

Unlike glow discharge o r if q~uttering, thin film fonnation in ion beam dtposition occurs in a field free region essentially independent from the plasma +eneration and primary ion beam processes. I'he use of an ion gun makes it possible to control beam species, their energy ant1 flux and obviates problems associated with controlling floating plasma potentials common to glow discharge techniques. Collimation of the beam by the

ION SOURCE -

A

MAGNET " --?-SUBSTRATE ANODE

HEATED

CATHODE SCREEN & ACCELERATOR GiilDS

Fig. 1. Ion beam deposition apparatus.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814137

C6-628 J O U R S A I , LIE PHYS I Q U K

grids isolates the substrate ninimi.!ing the probabi!ity of process ~nduced defect$. Since ion beam dcposition is dcne at loner working gas pressures (10'~-10-' rorr), higher purity films with less incorpatior! of background gas result. Because ion generation, sputtering and film formation are decoupled, the angle of dcposition can be readil) varied.

The sputtering beam used in this work was a 500eV beam of argon and hydrogen of high flux 0 3 ma/cm2) produced from a discharge of both gases in a 6" Kaufrnan type ion source. ? h i s was used to sputter a high purir), undopcd silicon target employing the dcposition conditions described previously5.

Materials were prepared as a function of h!:drogen ion beam concentration and substrate temperature.

Properties of IRD a-Si:Ii

Hydrogen incorporation and bonding to the silicon network in the IBD thin films is mident from SlMS and Fourier transform infrared spectra. The SIMS data show a strong peak at m/e = 1 indicative of H for samples prepared with a hydrogen ion beam whereas this feature is absent in unhydrogenated films. Fig. 2 shows a Fourier transform infrared spectrum of a film prepared at Ts = 200°C with an ion beam conraining 7.5% hydrogen and deposited onto a polished, high resistivity silicon wafer. Excitations of vibrations characteristic of silicon hydrogen coordination are evident in the 2000 cm-I and 630 cm-I pcaks which are usually assigned to Si-H stretching and bending modes. This spectrum is similar to that reported by Lucovskp et a16 for glow discharge a-Si:H with low band gap defects in which monohydride type coordination prevails. The spectra of films prepared at 10, 25 and 50% hydrogen ion beam concentration were similar to Fig. 2 with only lower intensities of absorption peaks.

Xone of the spectra showed a discrete peak at d40 cm-I characteristic of polysilane (SiHZ), wag modes or a discernible 2100cm-I peak from multihpdride stretches, thus indicating that monohydride type bonding predominates in IBD a-Si:H.

Fig. 2 Fourier transform infrared specuum o f IBD a-Si:H prepared with 75% H2 ion beam and Ts = 200°C.

Phase contrast transmission electron microscopy was used to check for columnar morpholog~ in hydrogenated IBD films. As shown in Fig. 3, no void network or any other structure was seen down to the resolution of these studies (-4A). This quasi grain boundary type defect is usually absent from the best glow discharge material but is characteristic of conventionally evaporated and poor g.d. a - ~ i : ~ . ~ Besides columnar morphology, a-Si:H with micro- inhomogenr:ities show a large density of dangling bond defects (high esr spin count) and the presence of a significant amounts of polysilane as detected by the 845 cm-I wagging mode7. Prttliminary ESR measurements done on the IBD films give a resonance at g = 2.0055. Spin densities decrease with increasing hydrogen icln beam concentration and show a spin density Ns

<

lo1' c m - j for material produced with 75% hydrogen. More exact determination of

Ns

using a dual cavity are in progress. ~ e n s i t y - measurements using the flotation method are consistent with the conclusions of the IR, TE.V, and ESR studies.

Values for the density for a number of films prepared with hydrogen concentration from O- 75% ranged between 2.2

-

2.3 g/cm3.

Densities as large as this are typically seen in films in which columnar morphology is absent8.

Fig. 3 Phase contrast transmission electron micrograph of a thin IBD a-Si:H film. Inset is a selected area diffractogram showing amorphous nature of film.

Dark conductivities of the ion beam deposited a-Si:H films were measured to further probe band gap defects. Films were deposited onto quartz substrates which contained a photo-lithographically printed pattern of 150k thick chromium electrodes arranged for precise four-probe resistivity measurements. Figure 4 summarizes the results of troom temperature dark conductivity a d measurements. Increasing substrate deposition temperature from ambient to 200°C has a relativey small effect on a d decreasing it by about an order of magnitude. A more important variable is the hydrogen concentration of the ion beam which shows a strong correlation with ud. T h e e results show that a d can be tuned from 10-I -10-lo (S2cm)- consistent with hydrogen titration of dangling bond defects. At high hydrogen

concentration, thc ion beam deposition process produces a-Si:H wiU1 a resistivit!. >_10~!2cm.

Tnis is cornpsrable to the glow discharge or rf sputtered materials an3 is indicative of a low densi~!. of gap states8.

SUBSTRATE o SUBSTRATE A SUBSTRATE

u

UNHEATED AT 200° C AT 3OO0C

I-1.

0 20 40 60 80 100

% H2 in ION BEAM Fig. 4

Fig. 5 shows that the optical band gap Eo, deduced from the x axis intercept of a (ahv)lI2 vs hv plot, changes systematically with hydrogen concentration. Gaps span a range from 1.2-2.0eV and are similar to those observed in g.d. and rf sputtered materials8.

These data point to hydrogen removal of dangling bond gap states and.reconstruction of the tail band edges.

In summary, ion beam deposition has used to prepare a-SiH, thin films over a wide range of deposition conditions. Materials made by this technique exhibit good chemical purity, high density (>2.2g/cm3) and an

1.1 0 2 0 40 60 80

% H2 in ION BEAM

REFERENCES

[ 61 ^' L ~ ~ O V S K Y , G., KEMANICH, R.J.

and KNIGHTS, J.C., Phys.

Rev..

B19, 2064 (1979).

[ 71 ' KNIGHTS, J.C. Non-Crysr. Solids 35 S: 36, 159 (1980).

[ 81 KKIGHTS, J.C and LUCOVSKY, G., CRC Critical Reviews in Solid Srare and Marerids Science 9, 211 (1980).

Fig. 5

absence of columnar morpholog)

.

Infrared spectra show predominantly monohydride type Si:H coordination with absorption peaks at 2000 and 630cm-l that are independent of hydrogen concentration. ESR spin densities, dark conductivities and optical band gaps depend systematically on hydrogen ion beam concentration. All of these data point to a reduced density of band gap defects.

[ 11 SPEAR, W.E., and LE COMBER, P.B., Solid Stare Commun. 17. 1193 (1975).

[ 2 ] PAUL, W., LEWIS, A.J., CONNELL, G.A.N., and MOUSTAKAS, T.D., Solid Srafe Commun. -02 969 (1976).

[ 31 NAKASHITA, T., HIROSE. M. and OSAKA, Y., Jap. J. Appl. Phja. 18,405 (1979).

[ 41 MILLER, D.L., LUTZ, H.

WEISMAN, H., ROCK, E., GHOSH, A.K., RA_VAMOORTHY, S. and STRONGIN, M.

J., Appl. Phys. 49, 6192 (1978).

[ 51 CEASAR, G.P., GRIMSHAW, S.F., and OKUMURA, K., Solids State Commun.

38, 89 (1981).