OBSERVATION OF ACOUSTIC EMISSION FROM a-Si : H PIN JUNCTIONS

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OBSERVATION OF ACOUSTIC EMISSION FROM a-Si : H PIN JUNCTIONS

Y. Mishima, M. Hirose, I. Suemune, M. Yamanishi, Y. Osaka

To cite this version:

Y. Mishima, M. Hirose, I. Suemune, M. Yamanishi, Y. Osaka. OBSERVATION OF ACOUSTIC

EMISSION FROM a-Si : H PIN JUNCTIONS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-

447-C4-450. �10.1051/jphyscol:1981493�. �jpa-00220950�

OBSERVATION O F ACOUSTIC EMISSION FROM a-Si:H

PIN

JUNCTIONS

Y. Mishima, M. Hirose, I. Suemune, M. Yamanishi and Y. Osaka

Department of EZectricaZ Engineering, Hiroshima University, Hiroshima 730, Japan

Abstract.- We have observed, for the first time, acoustic signal originating from the nonradiative recombination of photocarriers generated by ~ r + ion laser in biased a-Si:H pin diodes. The photoacoustic signal increases with laser power as well as with forward bias. It is found that the ratio of the photoacoustic signal at a given forward bias to the signal at zero bias in- creases with a decrease of gap state density in the i layer of the diode and exhibits correlation with the n-factor.

Introduction.- Recently photoacoustic spectroscopy(PAS) has received considerable attention as a useful technique for investigating nonradiative recombination centers in solids [l]. PAS offers a direct and sensitive probe to reveal nonradiative proc- esses and provides information about extrinsic optical-absorption spectra or non- radiative states in semiconductors [1,2]. In order to clarify relationship between device performance and nonradiative recombination process, a new technique to mea- sure acoustic signal from a semiconductor laser diode under forward bias has also been developed as current-injection-acoustic spectroscopy (CIAS)[3]. Nonradiative pathway of carrier recombination in amorphous hydrogenated silicon has so far been studied by means of photoluminescence or photoconductivity [4,51. A more direct way to identify the nonradiative process in a-Si:H is needed especially in the field of photovoltaic application.

In this paper, we report the first observation of acoustic emission from for- ward- and reverse-biased a-Si:H pin diodes excited by ~ r + ion laser irradiation. I t is found that the photoacoustic signal is influenced not only by the properties of an undoped layer in a pin junction but also by the diode characteristics. Possi- bility of acoustic emission from a diode excited by current injection is also briefly discussed.

Experimental.- An a-Si:H pin diode was prepared by the glow discharge decomposition of 11% silane diluted with Hp at 9 substrate temperature of 300°C. .,The sample structure was composed gf Pd(200 A, 2mm in diameter)lp+ a-Si:H(2OO A)/undoped a-Si:H

(3000 fi)/nf a-Si:H(300 A)/stainless steel. The preparation conditions for undoped a-Si:H were as follows; an rf power of 5 watts (13.56MHz), a total pressure of 0.3 Torr, a magnetic field of 0 or 0.8 KG in the direction perpendicular to the sub- strate surface, and a gas flow rate of 30 SCCM. The specimen was excited either by laser irradiation (PAS) or carrier injection (CIAS) as schematically shown in Fig. 1. A pin diode was mounted on a 1 mm-thick brass platg. In the case of PAS, a biased pin diode was irradiated by ~ r + laser light (4880 A) in the power range 1.6 % 14.7 mW at a chopped frequency of 25 Hz. The effective power density passing through a Pd gate is estimated to be in the range 10 % 100 mw/cm2, if the spot size of the laser light (2 mm in diameter) and the transmittance of the Pd gate (about 20%) are taken into account. In the case of CIAS, the injection current of a pin diode is modulated by a small AC bias (40 mV) superimposed with DC bias. The acoustic signal was detected by a piezoelectric transducer cemented on the other

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981493

JOURNAL DE PHYSIQUE

s u r f a c e of t h e b r a s s p l a t e and t h e t r a n s d u c e r c o n s i s t s of a 1 mm-thick l e a d z i r c o - n a t e t i t a n a t e (PZT) p l a t e w i t h t h e p o l a r i z a t i o n a x i s normal t o s u r f a c e . The photo- c u r r e n t measurement of a diode was c a r r i e d o u t under t h e same c o n d i t i o n s a s t h e PA measurement. The CIA s i g n a l was s o s m a l l t h a t i t was d i f f i c u l t t o d i s t i n g u i s h t h e s i g n a l from t h e l e a k a g e n o i s e of AC modulation b i a s . Therefore we s h a l l focus our d i s c u s s i o n mainly on t h e r e s u l t of PAS.

'

DETECTION SYSTEMS

EXCITATION

SOURCE

O F

PA

OR CIA SIGNAL

l^'%@-i

OSCILLATOR

Fig. 1 : Experimental s e t up f o r p h o t o a c o u s t i c spectroscopy(PAS) and c u r r e n t - i n j e c t i o n - a c o u s t i c spectroscopy (CIAS)

.

St OFF AND S2 ON FOR CIAS

Results.- Two t y p e s of diode, A and B, w i t h d i f f e r e n t p r o p e r t i e s of t h e i l a y e r s were used i n t h e p r e s e n t experiment. Table 1 summarizes t h e d e n s i t y of gap s t a t e s n e a r midgap N(Ei) determined by t h e C-V method [ 6 ] , d a r k c o n d u c t i v i t y and i t s a c t i v a t i o n energy of t h e i l a y e r s t o g e t h e r w i t h t h e n - f a c t o r of t h e d i o d e s . F i g u r e 2 shows t h e a p p l i e d b i a s dependence of t h e r a t i o of t h e p h o t o a c o u s t i c s i g n a l i n t e n - s i t y a t V t o t h e i n t e n s i t y a t zero b i a s f o r t y p e A and B diodes. The PA s i g n a l l e v e l a p p r e c i a b l y i n c r e a s e s a t b i a s e s about 1 v o l t . I n t h i s b i a s r e g i o n , t h e c u r r e n t t r a n s p o r t i s no l o n g e r l i m i t e d by t h e j u n c t i o n b a r r i e r , b u t i t is predominated by t h e space-charge-limited conduction through t h e i l a y e r s i n c e t h e d a r k c u r r e n t i s p r o p o r t i o n a l t o (V-VD) where VD i s t h e b u i l t - i n v o l t a g e e s t i m a t e d by t h e open- c i r c u i t v o l t a g e of t h e diode. Note t h a t a high d e n s i t y of gap s t a t e s i n t h e i l a y e r of a t y p e B diode r e s u l t s i n t h e weak b i a s dependence of t h e PA s i g n a l i n t e n s i t y r a t i o . The i n s e t of F i g . 2 shows t h e PA s i g n a l i n t e n s i t y p l o t t e d a s a f u n c t i o n of l a s e r power P f o r a t y p e A diode a t v a r i o u s b i a s e s . The PA s i g n a l i n c r e a s e s w i t h pm and t h e v a l u e of m changes from 1.0 t o 0.5 w i t h an i n c r e a s e of forward b i a s , w h i l e m f o r a type B diode remains u n i t y i r r e s p e c t i v e of b i a s . Such d i f f e r e n c e i n t h e behavior of m-value may i n v o l v e a corresponding d i f f e r e n c e of recombination k i n e t i c s of p h o t o c a r r i e r s i n t h e i l a y e r . The PA s i g n a l l e v e l is s l i g h t l y i n f l u - enced by c o n d i t i o n s of sample mounting because t h e a c o u s t i c coupling between a sample and b r a s s p l a t e i s achieved by mechanical compression. However t h e r e l a t i v e changes i n t h e PA s i g n a l measured a s f u n c t i o n s of l a s e r power and a p p l i e d v o l t a g e a r e i n s e n s i t i v e t o t h e mounting c o n d i t i o n s .

Table 1 : P r o p e r t i e s of i - l a y e r f o r type A and B diodes.

SAMPLE ,

A B

N(Ei) (~m-~ev-')

2 1016

2 1 0 1 8

( E ~ - E ~ ) ~ ( e V ) 0.7

0.8 d a r k

c o n d u c t i v i t y (G cm)-' 2 ^X

4 ^X 10-l'

n-f a c t o r 5 1.26

1.29

1 10

LASER POWER (mW )

/-

to the signal at V = 0 volt plotted as a function of applied bias at a laser power of 14.7 mW. The in- set shows the laser power dependence of PA signal intensity for a type

A

diode at various biases.

Figure 3-represents a plot of photocurrent versus laser power for a type A diode. At reverse biases or the short-circuit condition, the photocurrent Ip in- creases with pY, where

y =

1.0. At forward biases below the built-in voltage VD or the open-circuit voltage Voc, Ip decreases remarkably and the sign of Ip changes around a bias of Voc = 0.7 volt. When the bias exceeds Voc, again the relationship Ip a

PY

is satisfied and the values of y increase towards 0.5 with increasing for- ward bias. This feature compares to the change in the current transport mechanism of a pin diode, from the barrier-limited region to the bulk-limited. In the bulk-

r=

^0.37

BIAS

r=

0.34 2.3

2 0 1.7

1.0

Fig.

4

: Plots of n-factor vs ratio of photoacoustic signal at V = 2.0 volts to V = 0 volt for type A and B diodes.

h

>

R

Y

a

z

h

9

CV

3

2 : ^1.0-

LASER

POWER

( mW)

LASER POWER

-c

8.5 mW + 14.7 mW

' - O -

\

-

⁰

Fig. 3 : Photocurrent of a type A diode as a function of laser power.

$, ; U

d ^{1.2 1.3}

n-factor

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l i m i t e d r e g i o n , t h e v a l u e s of m ( t h e i n s e t of F i g . 2) and

Y

( F i g . 3) which, r e - s p e c t i v e l y , c h a r a c t e r i z e l a s e r power dependence of t h e PA s i g n a l and p h o t o c u r r e n t , e x h i b i t o p p o s i t e t r e n d f o r forward b i a s e s above 1 v o l t ; i . e . , t h e m-value becomes s m a l l w i t h b i a s , w h i l e t h e y-value becomes l a r g e . It i s c l e a r t h a t t h e r e d u c t i o n o f t h e PA s i g n a l l e a d s t o t h e enhancement of t h e p h o t o c u r r e n t b e c a u s e t h e PA s i g n a l a r i s e s from t h e n o n r a d i a t i v e r e c o m b i n a t i o n i n t h e i l a y e r .

The n - f a c t o r f o r a p i n d i o d e c o u l d q u a l i t a t i v e l y b e a s s o c i a t e d w i t h t h e amount o f r e c o m b i n a t i o n c e n t e r s l o c a t e d i n t h e d e p l e t i o n l a y e r ; a h i g h e r v a l u e o f n - f a c t o r , i n g e n e r a l , i m p l i e s a h i g h e r r a t e o f c a r r i e r r e c o m b i n a t i o n . F i g u r e 4 shows t h a t t h e r a t i o of t h e PA s i g n a l a t V = 2.0 v o l t s t o t h e o n e a t z e r o b i a s i s l a r g e r f o r a d i o d e w i t h a smaller n - f a c t o r , i n c o n s i s t e n c e w i t h t h e r e s u l t o f Fig. 2.

F i n a l l y we would d e s c r i b e t h e r e s u l t o f CIAS; t h e CIA s i g n a l from a forward- b i a s e d p i n d i o d e d e c r e a s e s w i t h i n c r e a s i n g f r e q u e n c y , p o s s i b l y b e c a u s e t h e d i s - p e r s i v e r e c o m b i n a t i o n of i n j e c t e d c a r r i e r s i n t h e i l a y e r becomes l e s s i m p o r t a n t a t h i g h f r e q u e n c i e s . Also C I A s i g n a l l e v e l i s found t o b e p r o p o r t i o n a l t o t h e AC in- c r e m e n t a l change i n t h e i n j e c t e d power, i n agreement w i t h t h e t h e o r e t i c a l p r e d i c - t i o n . However, i t a p p e a r s d i f f i c u l t t o o b t a i n a d e f i n i t e c o n c l u s i o n a b o u t t h e CIA s i g n a l b e c a u s e of v e r y poor s i g n a l t o n o i s e r a t i o .

D i s c u s s i o n and Summary.- I n t h e b u l k - l i m i t e d t r a n s p o r t r e g i o n f o r a t y p e A d i o d e , t h e p h o t o c u r r e n t i s p r o p o r t i o n a l t o pY, where y i s 0.37 a t 2.3 v o l t s ( F i g . 3) and w i l l i n c r e a s e t o a v a l u e n e a r 0 . 5 b e c a u s e t h e i l a y e r o f a p i n d i o d e i s n o t com- p l e t e l y accumulated a t 2.3 v o l t s . According t o t h e f a c t t h a t b o t h m and y a r e n e a r 0.5 a t a h i g h forward b i a s , t h e o r i g i n of t h e PA s i g n a l c o u l d b e r e l a t e d t o t h e b i - m o l e c u l a r r e c o m b i n a t i o n between h o l e s and e l e c t r o n s t r a p p e d i n t h e t a i l s t a t e s . At t h e s h o r t - c i r c u i t c o n d i t i o n and r e v e r s e b i a s e s , b o t h t h e p h o t o c u r r e n t and PA s i g n a l a r e p r o p o r t i o n a l t o l a s e r power, and t h e v a l u e s o f m and y a r e c l o s e t o u n i t y . T h i s i s e x p l a i n e d by t h e i d e a t h a t t h e PA s i g n a l i s p r o p o r t i o n a l t o p h o t o c a r r i e r s i n t h e b a r r i e r - l i m i t e d t r a n s p o r t r e g i o n . T h i s i n t e r p r e t a t i o n would b e c o m p a t i b l e w i t h a n earlier work t o d e t e r m i n e t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t o f a-Si:H u s i n g PAS [ 7 ] . On t h e o t h e r hand, i n t h e c a s e of a t y p e B d i o d e t h e v a l u e o f m f o r t h e PA s i g n a l i s i n d e p e n d e n t o f b i a s and m = 1 even i n t h e b u l k - l i m i t e d t r a n s p o r t r e g i o n , b e c a u s e t h e PA s i g n a l from t h e undoped l a y e r w i t h a l a r g e d e n s i t y o f gap s t a t e s might b e dominated by t h e monomolecular r e c o m b i n a t i o n p r o c e s s even under s t r o n g f o r - ward b i a s e s . O t h e r w i s e , a h i g h d e n s i t y of gap s t a t e s p r e v e n t s t h e a c c u m u l a t i o n l a y e r f o r m a t i o n even a t h i g h f o r w a r d b i a s e s .

I n c o n c l u s i o n , we h a v e d e m o n s t r a t e d t h a t t h e a c o u s t i c s i g n a l s o r i g i n a t e from t h e n o n r a d i a t i v e r e c o m b i n a t i o n p r o c e s s of p h o t o c a r r i e r s e x c i t e d i n a n a-Si:H p i n diode. It i s a l s o found t h a t t h e PA s i g n a l i s c o n n e c t e d n o t o n l y w i t h t h e p r o p e r t y o f t h e i l a y e r d e f i n e d w i t h gap s t a t e d e n s i t y b u t a l s o w i t h t h e d i o d e c h a r a c t e r i s t i c s u c h as n - f a c t o r .

P a r t o f t h i s work was s u p p o r t e d by t h e Grand-in-Aid f o r Research Program "Sun- s h i n e P r o j e c t " g i v e n by t h e X i n i s t r y of I n t e r n a t i o n a l Trade and I n d u s t r y , t h e J a p a n e s e Government.

R e f e r e n c e

1. ROSENCWAIG A., and GERSHO A., 3. Appl. Phys.

67

(1976) 64.

2. WASA K., TSUBOUCHI., and MIKOSHIBA N., J p n . J.App1. Phys.

19

(1980) L475.

3 . SUEMUNE I., YAMANISHI M., MIKOSHIBA N., and KAWANO T., Jpn. J. Appl. Phys.

2

(1981) L9.

4. STREET R. A., KNIGHTS J.C., and BIEGELSEN D . K., Phys. Rev.

B18

(1978) 1880.

5. SPEAR W. E , , LOVELAND R. J., and AL-SHARBATY A., J . Non-Cryst. S o l i d s

15

(1974) 410.

6. HIROSE M., SUZUKI T., and DOEHLER G. H., Appl. Phys. L e t t . 34 (1979) 234.

7. YAMASAKI S., NAKAGAWA K., YAElAMOTO H., MATSmlA A., OKUSHI : H and TANAKA K., P r o c . o f t h e T o p i c a l Conf. o n T e t r a h e d r a l l y Bonded Amorphous Semiconductors

( C a r e f r e e , 1981) t o b e p u b l i s h e d .