ISOTHERMAL CAPACITANCE TRANSIENT SPECTROSCOPY : ITS APPLICATION TO THE STUDY OF GAP STATES OF a-Si:H

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ISOTHERMAL CAPACITANCE TRANSIENT SPECTROSCOPY : ITS APPLICATION TO THE

STUDY OF GAP STATES OF a-Si:H

H. Okushi, Y. Tokumaru, S. Yamasaki, H. Oheda, K. Tanaka

To cite this version:

H. Okushi, Y. Tokumaru, S. Yamasaki, H. Oheda, K. Tanaka. ISOTHERMAL CAPACITANCE TRANSIENT SPECTROSCOPY : ITS APPLICATION TO THE STUDY OF GAP STATES OF a- Si:H. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-613-C4-616. �10.1051/jphyscol:19814134�.

�jpa-00220753�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppZdment au nOIO, Tome 42, octobre 1981 page C 4 - 6 1 3

ISOTHERMAL CAPACITANCE TRANSIENT SPECTROSCOPY : ITS APPLICATION TO THE STUDY OF GAP STATES OF a-Si:H

H. Okushi, Y. Tokumaru, S. Yamasaki, H. Oheda and K. Tanaka EZectrotechnicaZ Laboratory, Sakuramura, Ibaraki 305, Japan

Abstract.- A basic ideaof ICTS (Isothermal Capacitance Transient Spectroscopy) fora system of continuously-distributed gap state and experimental data by the ICTS applied to P-doped a-Si:H Schottky barrier diode are presented.. It is shown that the ICTS is a useful tool for the study of gap state of a-Si:H whose material parameters are strongly temperature-dependent.

Introduction.- The natureof gap states hasbeenoneofthemain subjects for understanding the electronic properties of hydrogenated amorphous silicon (a-Si:H). Several techniaues (1)-(3) for obtaining information on the gap states have been proposed so far, but many cases it is difficult to avoid the influence of the surface properties on the ex- perimental results. Inorder to overcome this problem the DLTS method has been applied to a-Si:H for studying the bulk d-ensityofgap states (4). However, the DLTS measurement essentially involves a temperature scanning process (51, by which a critical problem arises because the material parameters of a-Si:H are strongly temperature-dependent.

In our recent works, we have proposed a new measurement method for deep levels in semiconductors controlled by a programmable cal- culator ( 6 ) , ( 7 ) . This is based on the measurement of the transient junction capacitance in a time domain under an isothermal condition (ICTS, Isothermal Capacitance Transient Spectroscopy). The informa- tion obtained fromtheICTS signal is euuivalent to that from the DLTS signal(6), but the measurement under the isothermal condition is more suitable for a-Si:H compared with the conventional DLTS.

In this article, we present a basic idea of ICTS for the system of continuously-distributed gap states, andalso present experimental resultsonthe gap-state Cistribution of P-doped a-Si:H using the ICTS.

ICTS for a system of continuously-distributed trap levels.- The ICTS method presented here isavariation of capacitance transient spectro- scopy ( 8 ) . For a p+n junction or a ~ c h o t t k ~ barrier diode which contains trap levels distributed continuously in energy within the band gap, the depletion layer capacitance C(t) of the junction at a time t after applying a voltage pulse under a steady reverse voltage VR is related with the density of state-distribution g(E) through the following equation;

e, (E)

f (t) = C (tP

-

C (-1' = B C C g(E' en (E) tep (E) exp(- (en (E)+ep(E))tId~t (1)

v

where C(-) is the steady capacitance, B = q k ~ c , ~ ~ / [ Z(VD+VR 1 3 , q the electronic charge, k s ~ o the dielectria constant, A the junction

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814134

JOURNAL DE PHYSIQUE

area VD the built-in potential, en ( E )

,

(ep(E)) the thermal emission rate for electron (holes) and Ec and Ev the conduction and the valence band edge, respectively. Equation (1) is based on the assumption that the majority carrier is electron and the voltage pulse introduces the electrons into the depletion region (the majority-carrier intro- duction). If we assume en(E) >> ep(E) for electron trap levels, we find the ICTS signal def ind as tdf (t)/dt (6) as follows,

tdf (t)/dt = -B

I.":

g(E)en(E)t exp(-en(E) t)dE.

The function en (E) t exp (-en (E) t) = D (E, t) takes a maximum value at en(E)t = 1 and can be approximated 'using a delta function (10),

where Em is the energy at which D(E,t) takes a maximuxi? value when en(Em)tm = 1. The relation of Em and tm is derived from the relation en (Em) = V (Em) exp (Ern-Ec) / k ~ ,

where v(Em) is the attempt-to-escape frequency of the electron in the level Em.

From eqs. ( 2 ) , (3) and (4), we obtain the relationship between the ICTS signal and g (E)

,

Equation (5) implies that g(E) is determined directly from the ICTS signal if the energy dependence of v(E) is known. It should be noted that v(E) can be determined by the capture rate measurement (11) or the temperature dependence measurement of ICTS signal (7).

The specimens used in

the experiment were prepared Fig.1: A b l o c k diagram o f the ICTS system.

Experiment and results.- Figure 1 shows a block diagram of the ICTS system. The transient capacitance meter (Sanwa MI-401,2MHz), the transie6t convertor (Riken-

denshi TCD-4000), the digital sample

voltmeter (HP3455A) and

Tectronix CP4165 mini-computer

pp--:

^{C R T}

are used. By this system, the L--- ---- ^d transient change of the capa-

citance C(t) afterthemajority-

-

carrier introduction was measured under the isothermal

capacitance meter ( 2 M H z )

*El

condition (297 K) in the time

range of lo9to 10' sec and signals were converted to

- 11 ^-

digital ones and memorized in the mini-computer. Detailed calculation of the relation between g (E) and the ICTS

mini- computer Tektronix [cp4f6(65]

signal was performed by the ⁴ +

computer. ^{A .}

A-D convertor

(wave r n e m o r y ) c

1 6 ~ - 2 sec digital voltmeter

16'- lo4sec

G

p I

'

= a

by glow-discharge technique (11). The deposition conditions are summarized in Table 1. The optical gap Eoof the specimens 1.7 ev has been determined using empirical relation Jahw vs (hw-Eo)

.

The Schottky barrier diode was of sandwich, configuration with n+ crystalline Si substrate (p b 0.01 ohm-cm) and sen?i-transparent Au film with an area of 4.5~10-%mZ (12).

The diodes show a good rectifying I-V characteristic and C-V characteristic satisfies eq.(l). Figure 2 shows a typical transient- capacitance signal taken at 297K on the diode by superimposing a

voltage pulse (height (V ) = 1 V, width (Wp) = 10 ms) on a steady reverse voltage VR = - 1

8.

As shown in the figure, C (t) shows a long decaytlme anddoesnotreturn tothe steady valueC(m) even after 400 sec.

The ICTS signal is obtained from the C(t) curve. The signal depends on Vp and I*7p of the voltage pulse and VR. Figure 3 shows the ICTS signals for the different Hp's when Vp = 1 V and VR = - l V in the p-doped (0.1%) a-Si:H. As shown in the figure, the ICTS signal increases slightly with the increase of V p and saturates for Wp 2 5 ms except for the time range of 102 to 103 sec. This result indicates that the present method has a high-sensitivity and both of the experi- mental and analytical errors are smaller than 5 10%.

Flgure 4 shows the ICTS signals for the different p-doped a-Si:H specimens when VR = -1 V, Vp = l V and W ^- It is clearly observed that the ICTS signal, 1. e., g (ET inf:ezf ;S with an increase in the concentration of P atoms in the a-Si:E. The value of g(E) was determined from eq. (5) using B = 4.6~10-l3 [PF) 2cm3, which was estimated from the steady C-V characteristics of the diodes and the analyses for VR, Vp and Wp dependence of the ICTS signal. It should be noted that

g(E) even in the hlghly-doped (1% P-doped) specimen is smaller than the minimum value of g(E) ( ~ l ~ 1 0 ' ~ c n 3 / e v ) for the undoped specimen determined by the field effect experiment (1). For this difference, we can consider that the present Sata are obtained fromthebnlkstates, which are separated from the surface at least by a depletion-layer width at thermal eauilibrium because of Vp+VR = 0 V.

Table 1. Deposition conditions

Gas I Substrate I Ts

I

RF power I Gas pressure PH3/SiH4(l%-0.001%)

1

c-Si(n ,-0.01 Qcm)

1

^{300 "C}

I

108

-

¹⁰⁶

LL a

-

104 P-doped (0.01 %) a-Si:H

-electron tra

T I M E ( s )

I C I S signal at 297K

W :

A t y p i c a l t r a n s i e n t - c a p a c i t a n c e Fig.3: ICTS s i g n a l f o r d i f f e r e n t v o l t a g e s i g n a l on t h e P-doped (0.01%) a-Si:H p u l s e widths(O.l , l ,TO ms) i n t h e P-doped Schottky b a r r i e r diode. (0.1 %) S c h o t t k y b a r r i e r diode.

C4-6 16 JOURNAL DE PHYSIQUE

P-doped a-51 H The energy scale in the

g

flgure was determined from eq.(4) using the assumption of constant

- 2 x 1 ~ 1 7 u E v (E) = 3xl0'Tsec. This value was

-

obtained by Cohen et al. uslng the

- x l o 7 DLTS method ( 4

.

^{In the ICTS method,}

F however v(E) can be determined as

2

v, a function of E from the measure-

L 1 a ment of the W dependence of the

2 x ~ 1 6 s signal at eacf: t. According to

our rough estimation, thevalues of

0

1,1,j6 v (E) of the present specimens are

much smaller than 3xl0~ysec. In

g

this sense, it shoulc? be cared 16' 8 that the energy scale in the figure

20

10 ^{1 1 1 1 , 1} 1x1d5 energy scale wlll be published

1 0 3 107 16' 10' 10' 102 16 104 elsewhere.

T I M E ( s

I I

.6 .7 .8 .9

ENERGY BELOW Ec ( e V )

F i g . 4 : ICTS s i g n a l s and t h e d e n s i t y o f

.

s t a t e d i s t r i b u t i o n g(E) f o r e l e c t r o n trapping i n t h e v a r i o u s P-doped a-Si:H S c h o t t k y b a r r i e r d i o d e s .

Conclusion.- We have presented a basic idea of ICTS for the systen of continuously-distributed gap states, and also presented experimental results on the gap-state distribution of P-doped a-Si:H. Ee observed clearly that the density of gap states increases with an increase in the doping level of phosphorus. The experimental results indicate that the ICTS method is a useful tool for the study of gap states of a-S~:H.

Acknowledgements.- We acknowledge our colleagues of the amorphous materials section of ETL for their fruitful discussions. We are also indebted to M. Hidezima and A. Motoki for their assistance in the experiment and to Y. Miyake for her assistance in the preparing the manuscript.

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-

I C T S s~gnal at 297 K

involves some ambiguities. The

-2x1dS detailed treatment of the transfor-

mation from the time scale to the