Optical absorption spectra in a-Si:H measured by PDS and CPM in situ

438

Journal of Non-Crystalline Solids 114 (1989) 438-440 North-Holland

OPTICAL ABSORPTION SPECTRA IN A-Si:H MFASURED BY PDS AND CPM in situ

Yang Xi~O and bsxing RAN Institute of Physics, Academia Sinica, Beijing, China Photothermal Deflection Spectroscopy(PDS) and Constant Photoconductivity Methed(CPM) were employed to obtain optical absorption spectra for intrinsic and doped a-Si:H. The difference of subgap absorption deduced from PDS and CPM is attributed not only to bulk or surface effects but also to the photon energy dependence of carrier's lifetime, which is deeply cozmected with the excitation process and gap states occupancy.

The

knowledge of

essential for

band gap

states

is

the research of a-Si:H film and

phosphorus.

A

transverse

PDS

setup with an

available photon energy region 0.5eV
amorphous film devices. Many methods to measure

was used. When DC-(R~ was carried out in

the

system, neutral filters

sub gap states have been

example, Photothermal

employedj

for

Deflection Spectroscopy

the intensity

were

used

of monochromatic

(PD6)1, Constant Photoconductivity Method(CPM) 2,

Kelthley 616 electrometer was used the photccurrant. While in A ~ ,

Among

Transient

Spectroscopy

them~ PE~5 is a

contactless

direct s

(DLTS)3. sensitive,

meamJrement and observes all

the

the the

photocurrent

(2M

CPM and AC-(R~ were

the

transport process in the bulk and to the

occupancy

of

defect

electron

is sensitive states, a(hv)

d~9,=ed from CPM requires the assumption the electron mobility-lifetime product independent of hu. intensity

uT

In order to keep the

occupancy of states, light

that

one way is to change

to keep

constant in the energy

the

region of

is same the

photocurrant measurement,

induced

lock-in amplifier. The electrodes in both coplanar

carbon paint at the sane

and

area on

chopped

sample by

energy, shoulder

and asymptotes to the value related to doping level. For undoped sample, the DC-CPM subgap explained

lower by

than

all

monochromatic beam as a perturbation. In the present study we have tested the reliability

optical transitions including electrons,

of

of PDS. Since hole transitions

PDS~ ACmCPM and DCm(l~M to know under which

of

the three methods have little difference in the

that from PDS. It was

a

the

Urbach region. For lower photon hu
absorption =(hu) is a factor of i0

and

DC-

made

where PDS was carried out. As shown in Fig.l, the spectra obtained

intense

illumination

by

light was detected by a

this is DC-CPM; smoth~-r way, AC-(R~, is using a bias

to record a beam of

alternative

optical transitions including the contribution from bulk and surface4'5. Compared with PDS, with

and a

He-Ne laser was used as the bias light and modulated ~ > n o c ~ t i c

is mainly ~

to change

light

Deep Level

this

the

holes

and surface states contributing to the results may

contribute

condition they can be used to expl~4n certain

about a factor of 2, it was concluded that

the

physical process. The i ~m a-Si:H samples were prepared by glow discharge on quartz substrates. No.l was an u~doped one and No.4 was doped with lOppm

surface effects dominate in PDS.

the

0022-3093/89/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland)

surface effect was not measurement if one ~ e s

However,

so obvious in our with the results of

AC-CPM. One can see from Fig.l,

that

sub-band

Y. Xiao, D. Han / Optical absorption spectra in a-Si:H gap ~(hv) increases with increasing bias level, and it can be even higher than that fru, PDS. Although A C - ~ is a dual-be~n techrdque, neither infrared quenching nor e~uur~ment was Eound at room

temperature.

However,

similar

(hv) spectra for the heavily-doped sample were obtained from PDS, DC-CPM and A C - ~ , as shown in Fig. 2. For the heavily P-doped sample,

by t ~

0~ ~2

'Eu 102

the

quasi-Fermi level is very close to the conduction band and D" is dominant, so any change of incident flux has little influence on the occupation function. Therefore, we obtained almost the sa~e result of ~(hu) methods °

I

439

,o° I

0.6

I

I

I

I.O

I

1.4 hv

I

I

1.8

(eV)

three

FI~0RE 2

We would attribute the difference of a(hv) in DC- and AO-CPM of the undopad staple to the fact that the lifetime • depends on the recombination process, which not only depends on the excitation intensity but also on the photon energy hr. Unfortunately, the last factor has not been taken into account in CPM.

Absorption coefficient measured by PDS and CPM vs. photon energy of lOppm P-doped sample. Curve (1)~6DC~MII(2): PI~; (3): AC-CRq 17 FD=128.x~0" ( c m " s ) ; (4): ACedZPM Fp=l.0xl0 ,

(~" s" ).

One way to checkhow the lifetime • depends on hv is to meesure the d e ~ e r ~ e o f

photocAErent

Cph on incident flux F as follows,

(I)

~ph:C~

where c is a constant. We fotn~d ~-0.7 when, hv>l.5eV and ~-0.8-0.9 when hv
4

,oo 0.6

intensity

bias light and Fm be the intensity of modulated light then we have

(2)

A~ph:C~F~-IF m 1.o

1.4

hv

1.8

(eV)

From ~ . ( 2 ) ~ '

~

be

obtained

of

the

by

alternative photocurrent A~ph when (shown in Fig.3)and the values

% varies of ~ as a

FIGURE 1 Absorption coefficient ~ s u r e d by PDS and CPM vs. photon energy of intrinsic sample. Curve I~ (1)" DC-Cm; (2): PDS; (3): AO~=~ ~-2.1xi0 "~

fmlction of hv are listed also. Since decreases when hv>l.4eV, when one normalizes

(cm'2s'l); (4): AC-<:Rq Fp=l.Oxl017 (cm'2s'l).

spectrum will rise up in low photon energy region and the shoulder could be higher than

~ p h at hv-l.SeV to get the a(hu) spectrum, the

Y. Xiao, D. Han / Optical absorption spectra in a-Si:H

440

more gap states are

occupied,

this decreases

the recombination through D O and increases the

G ¢-

electron life-time. ~(e). • (d)

Therefore, by A C ~

one

could obtain e v ~ higher ~(hu) shoulder than by PDS. In

(c)

/

summary,

the

difference

of

subgap

absorption d~9~ed from PDS and CPM is attributed not only to the bulk or surface effect but also to the photon energy dependence

0 o

I

I

-1

0

of carriers lifetime r, which is affected by the e~=itation process and the occupancy of gap

Ioglo Fp (orb.units)

states. This effect is especially important in undoped samples where there is a large r~nber of ~ u p i e d states. FIGURE 3 Alternative photommrent ~°'ph VS. bias light intensity Fp. ~ for e~]~ p~oton energy are listed below: Curve a b c d e f by (eV) 1.20 1.40 1.42 1.50 1.67 1.80 1.26 1.20 1.14 0.97 0.92 0.93

that of PDS, as shown in Fig.l and Fig.2. For high quality intrinsic a-Si:H with neutral dangling bond density about i016cm-3 and

surface

states density

lOl2cm "2,

the

optical absorption due to surface and bulk defect states contribute to PDS a~st equally when the film thickness is about several ~m or less. The problem is that when the photon energy is smaller than the optical gap, one cannot keep the same situation of recombination centers by keeping the photecurrent constant. For example, the infrared light excites electron from D" to the conduction band, meanwhile, enhances the recombination

through

DO

and decreases

the

electron life-time~ so that one always obtains a lower ~(hu) shoulder from DC-CPM. In AC-CPM, the bias light shifts the guesi-Fermi level and

A

~

We are grateful to Dr. l.l. Pankove for reading and correcting this manuscript. This work was supported by NSF of China. REFERENCES i. W.B. Jackson, N.M. Amer, A.C. Boccara and D. Fournier, Appl. Optics 20 (1981) 1333. 2. J. Kocka, M. Vanecek and F. Schauer, J. of Non-Crystalline Solid 97&98 (1987) 715 3. D.V. Lang, J.D. Cohen and J.P. Harbison, Phys. Rev. B 25 (1982) 5285. 4. H. Curtins and M. Favre, Surface and Bulk States by Phototbmmmal Deflection Spectroscopy, in: Amorphous Silicon and Related Materials, (World Scientific, Singapore, 1989) pp. 329. 5. Z.E. Smith, V. (hu, K. Shepard, S. Aljishi, D. Slobodin, J. Kolozey, ~ S. Wagner, Appl. Phys. Lett. 50 (1986) 2 5