Solar Energy Materials 10 (1984) 151-170 151 North-Holland, Amsterdam

PHOTOELECTRONIC PROPERTIES OF AMORPHOUS SILICON NITRIDE COMPOUNDS

F. ALVAREZ and I. CHAMBOULEYRON

Institute of Physics, UNICAMP, CP 6165, Campinas, SP, Brazil

Received 12 January 1984

We present results on the photoconductivity and optical transmission of hydrogenated silicon nitride compounds prepared by the glow discharge decomposition of silane and nitrogen. Samples having different aitrogen content and dopant impurity concentrations were analyzed. Photocon- ductivity measurements permitted the extension of absorption coefficient data down to nearly 1 c m - 1 The photoconductivity onset was related to the dark activation energy. It is shown that for undoped samples, the electrons are the majority carrier. For undoped and lightly boron doped samples, a supralinear dependence of the photocurrent on light intensity is found within the illumination range = 101°-1013 photons c m - 2 s - 1. Light boron doping unsensitizes the material, while heavy doping sensitizes it again. The opposite behaviour is found with phosphorus doping. The supralinear behaviour is interpreted assuming two types of defect centers, having energies that are located above and below the dark quasi-Fermi level. These two defect centres possess a large difference in their electron capture cross section. For a monochromatic light illumination level greater than --1013 photons c m - 2 s -1, all the samples show a linear dependence of photocurrent on light intensity. Undoped samples having an optical gap of nearly 2 eV possess a trap controlled drift mobility of - 0.83 × 10 -~ cm2/V s,

1. Introduction

Recent work has shown that high conversion efficiencies can be obtained in a-Si:H based solar cells using amorphous silicon carbide compounds as window material [1]. Other attempts to develop new window materials led to a-SiOx:H and a-SiNx:H compounds [2,3]. Kurata et al. have produced amorphous silicon nitride compounds having good optical and electronic properties. They prepared their samples by the simultaneous glow discharge plasma decomposition of silane and ammonia [3]. The similar dissociation energy of these molecules permits control of the nitrogen content in the samples, by varying the NHa/SiH 4 gaseous ratio. They also found that minute amounts of BEH 6 in the reaction chamber produced large conductivity variations. Less impressive conductivity changes are obtained when PH 3 is used as a dopant. Our research group at UNICAMP was able to produce variable band gap amorphous silicon nitride compounds by the plasma decomposi- tion of nitrogen and silane gaseous mixtures; the nitrogen content of the material being controlled by the rf power delivered into the reactor chamber [4]. Recently, Alvarez et al. studied the doping effects on samples prepared by the above men- tioned method [5]. Their results were in fair agreement with those of Kurata et al. [3]

0165-1633/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

152 F. A h~arez, I. Chambouleyron / Photoelectric properties of amorphous" silicon nitride compounds

and a tentative explanation of the differences between boron and phosphorus doping was presented.

The purpose of the present work is to study the optical and electrical properties of doped and undoped amorphous silicon nitride compounds prepared by the glow discharge decomposition of N 2 and Sill 4 gaseous mixtures. The use of nitrogen instead of ammonia has several advantages, among which we may mention the simplicity of purification and handling. We studied the conductivity and photocon- ductivity behaviour of our samples. From the photoconductivity dependence on light intensity, we could study the nature of the electronic gap states introduced by the nitrogen atoms. Mobility carrier lifetime products could be estimated and by the use of Schottky barrier type devices, the nature of the limiting carrier determined.

2. Sample preparation and experimental techniques

The samples were prepared in a capacitively coupled glow discharge system similar to the one reported by Knights [6]. The combined Sill4 and N 2 flow was varied between 280 and 360 sccm and the substrate temperature was kept constant at 280°C. The deposition pressure was maintained between 0.6 and 0.8 Torr. The peak to peak voltage was continuously monitored and a dc bias of - 1 0 0 V was applied to the substrate holder, all the samples being of the anodic type. The power delivered by the 10 MHz generator was measured by an rf power meter positioned as close as possible to the input electrode. Argon diluted phosphine and diborane, as well as silane and nitrogen of electronic grade were used. For photoconductivity and conductivity measurements, Corning 7059 glass was used as the substrate. For infrared studies, layers of a-SiNx:H were grown onto wafers of intrinsic crystalline silicon. Optical measurements were performed with a Karl Zeiss DM 25 Spectropho- tometer and sample thicknesses were determined by a profilometer. For coplanar conductivity measurements, aluminium strips of 10 mm length and 0.5 mm separa- tion were thermally evaporated onto the samples. Photoconductivity measurements were performed on coplanar as well as on sandwich type structures. Excellent ohmic behaviour was found in all cases. Dark conductivities for undoped and doped samples (-- 1 ~m thick) were measured on as grown material in a nitrogen atmo- sphere. In all cases, the dark conductivity was measured both on heating the samples up to 200°C and on cooling them down. No differences in o(T) were found in either case for the same temperatures. Photon fluxes ranging from 10 ~° to 10 ~5 cm-2s -1 were used together with a standard phase detection system. In all cases, the modulation frequency was 3.3 Hz. An E G G calibrated silicon photodiode was used as a standard quantum detector. To extend the absorption coefficient measurements by the photoconductivity method, electric fields of up to 5 kV c m - l were applied. Schottky barrier devices were fabricated onto Ti coated Coming 7059 glasses by the sucessive deposition of (n + ) a-SiNx:H, (i) a-SiNx:H and a semitransparent pal- ladium layer. All samples were prepared from a fixed S iH4 /N 2 = 0.33 gaseous ratio; the only difference between them being the rf power and the dopant gas concentra- tions.

F, A Ivarez, I. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds 153

,,,z°X> 6 t ~/'/°'5400~-/~° o-

, -4 o < ~ " 3 • U. W ¢w

2 , 2.5 5.0

RF POWER ( w / c m 2)

3 A >

0.

2 ~ _1

¢.3 i- a. o

I

Fig. 1. Refractive index for X = 5400 A and extrapolated optical gap vs. rf power. The S i H 4 / N 2 = 0.33 gaseous ratio, substrate temperature --- 300 o C and reactor chamber pressure --- 0.4-0.6 Torr, were kept constant.

3. E x p e r i m e n t a l r e su l t s

Fig. 1 shows the refract ive index n for h = 5400 A and the opt ica l gap E o of our samples as a funct ion of the rf power del ivered to the reactor . The op t ica l gap E 0 is

r I [ Power Density (W/cm 2)

5.0 1.5 o 1.0 (a 400 0.8 •

A

300 S3g $41 $3

200 II

,oo

/ , , o ~ I / / I I

1.5 z.o 2.5 3.o PHOTON ENERGY (eV)

Fig. 2. (ah~) 1/2 vs. photon energy for different a-SiN x layers obtained from the gd of a Sill 4 and N 2 mixture of fixed composition (S i H 4 / N 2 = 0.33). RF power density dissipated in the plasma is indicated for each curve.

154 k~ A h~arez, 1+ Chamboul£vron / Photoelectric properties of amorphous stilton nitrlde compoumts

d e f i n e d by assuming (~hv) t/2 = B ( h v - Eo) and determined by extrapolating a plot o f (ahv) ]/2 versus hv to zero ct (see fig. 2). In this high photon energy region, usually described as the intrinsic part of a, a good straight line fit is obtained. For a relatively low rf power density (0.6 W ca -2 ) , the material has an optical gap that is similar to a-Si:H. As can be seen in fig. 3, at intermediate and low photon energies, the absorption coefficient was deduced from photoconductivity measurements. T h e r e we have plotted a vs. hr for a few samples, following a procedure which will be explained later on. We see that the intermediate region shows an exponential decrease• This behaviour is normally associated with optical transitions involving tails of electronic states in the band gap in the neighbourhood of band extrema [7]. Fig. 2 shows that for a fixed SiH4/N 2 --- 0.33 gaseous ratio, the optical gap increases as the power delivered to the plasma increases. A decreasing growth rate with rf power was also observed• Fig. 4 indicates the measured values and the corresponding peak to peak voltages applied to the electrodes• These data suggest that a competi- tive phenomenom between film formation and film etching is occurring,

Doping effects were systematically investigated in a material having a fixed Si /N ratio• The chosen material had an extrapolated optical gap of nearly 2 eV (curve $36, fig. 2). If, during film growth, minute amounts of B2H 6 or PH 3 are introduced into the reactor, important changes in the dark conductivity are measured. Fig. 5a and b

! I 1 I f I I I I I

• , ,~o T R A N S M I S S I O N

5 10 D < - o , P H O T O C O N D u C T I jy~ l T Y

. j r . • o o .+ ,+." o

10 - ° -

E 1 0 - - ÷ ' • 5 - U + ~ . I

'~ ++++ s3, I O +++ "/' -

~,p~ ~ ..... • - .~ , hvo ...'"'"

• • , .."

• I

oo s ~ .. o ~ , , . ~n.(~v), .5

1.0 2.0 3.0

PHOTON ENERGY [oV ' I

Fig. 3. Absorption coefficient a vs. photon energy. The upper part of the curves was obtained by transmission measurements and the lower one by photoconductivity; see text. Star and daisy characters: undoped sample $39. Smaller dots and open circles: undoped sample $36. Crosses and triangles: boron doped sample $43 (B2H6/SiH 4 = 0.4%). Dark dots: undoped sample $37; for the sake of clarity only sub-gap absorption is plotted for this sample. Inset: (ahu) 1/2 vs. hv for low energy absorption region.

F. A lvarez, I. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds 155

:s[*

g :SJ

4000

3000

200

I I

~ - B O R O N O O P E D - I ~

O - i N T R ~ N S i C - O Q.-'"

... '~

•

: % • -" ~

%

o \

2 t~

I o 1.

0 2.5 S

P o w e r (W/cm2j Fig. 4. Anode peak to peak voltage (Vpp) and growth rate deposition vs. power density into the reactor.

show such behaviour represented in a conductivity versus inverse temperature type plot. The extrapolated room temperature conductivity of those samples and the corresponding activation energies as a function of doping are shown in fig. 6a and b. It can be seen that, in this particular silicon-nitrogen compound, boron doping produces at first a decrease in the conductivity and an increase in the activation energy. This suggests that boron is actually compensating the material in a way similar to what has already been observed in a-Si:H. In other words, the material as grown, or defect controlled is n-type, having its Fermi level located 0.1 eV above the optical mid-gap. Here we have associated the dark Fermi level position with the measured activation energy. If the diborane to silane gaseous ratio is increased above --10 -3, a transition from n-type to p-type occurs and the conductivity abruptly increases. The material then becomes p-type and the Fermi level approaches the valence band. For (B2Hr)/(SiH4) --- 0.012, the activation energy is reduced to 0.53 eV and several orders of magnitude change in conductivity are measured. At the same time, the optical gap shrinks with increasing boron content. Phosphorus doped samples show less important conductivity changes than boron doped material. Fig. 5b shows that, when phosphorus doped, the material possesses an activated type conductivity. However, introducing large amounts of phosphine into the reactor will produce less impressive conductivity changes. Preliminary results indicate that phosphorus doping does not cause optical gap shrinking. The conductivity of the samples on illumination (ELH, 100 mW cm -2) as a function of doping has also been represented in fig. 6a.

As is expected, the nitrogen content in the samples influences their transport

156 F. A loarez, L Chamboule.vron / Photoelectric properties of amorphous silicon nitride compounds

a % B2H+ Si H, b

~,o • 1,2 ".~ . ~ , o.e +

0 0,4 ,it= o.o

- - 4 e ~ "i" 0,08 ~ PH3 . 8 .oz %

e ~ O 0.I Sill4 o

, Ea=0.53 eV ~ 0.4 --li '-- . o.; -'~

" \ / 1 " * o oo+,

, , . //%

o,,, -~ , eV

le 0.88 e V

--IO N ~ 0,91 eV ~ ~ - -

\ O 1,0 eV

I I I I I t I I I ~ I 2.1 2,5 29 2.1 2.5 2.9

103/T ( ~ ' ) 103/T (K--l)

Fig. 5. (a) Log conductivity vs. 103/T for several diborane doping conditions. All other growing parameters identical to those of sample $36 (T~ ~ 300 o C, ISiH41/INI = 0.33; pressure 0.5 Torr; (b) log conductivity vs. 103/T for several phosphlne doping conditions. All other parameters identical to those of sample $36.

properties. In fig. 7, we have plotted the conductivity under dark and illuminated conditions as a function of rf power density. In the top part of the figure, we show the ratio between photo (ELH, 100 mW cm-2) and dark conductivity.

Photoconductivity measurements were performed, in order to obtain some in- formation on the low absorption region of the spectra. The photocurrent Ip induced by monochromatic light excitation can be expressed as [8]:

lp - eF( ' r /T t ) , (1)

where e is the electronic charge, F the carrier generation rate, T t the transit time and r the carrier lifetime. If the corresponding absorption coefficient is a and ~1 is the generation quantum efficiency, then the photocurrent can be written as:

Ip = eNo(1 - R)A(1 - e-~a)71(r /T t ) , (2)

F.. A loarez, I. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds 157

>- I,--

- 4 z .s I - u

,-, - 6 2 . 3 ~ Z ~ o7.... ,~

0.. o _1 o " -12 1.7

>-

"" 1.0 ,,, 1.4 z w z - 0 .e 12. F o > r- ~ i.o ~ 0.6

0.8 (..) <c

0 . 0 0 - 2 - 4 - 6 - 6 - 4 - 2 0

- t s, . . j L O G . G A S E O U S

IMPURITY RATIO

Fig. 6. (a) Open circles: extrapolated room temperature conductivity vs. gaseous doping ratio for samples grown under conditions identical to those of sample $36. Open squares: the effect of optical gap decrease. Daisy characters: room temperature photoconductivity on ELH illumination (100 mW cm-2); (b) open circles: dark conductivity activation energies vs. volume gaseous doping ratio for samples grown under conditions identical to those of samples $36. Filled circles: photoeurrent-light intensity exponent dependence on doping. The excitation light energy was hr = 2.2 eV and the photon flux varied between =101° and 1013 cm-2s -1.

where N O is the incident photon flux intensity, A the interelectrode area, (1 - R) the transmitted fight (experimentally determined) and d the sample thickness. Fig. 8 shows the photocurrent measured in different samples as a function of photon energy. At low energies, light interference patterns modulate the photocurrent signal. The observed fall-off for low photon energies is due to weak absorption, while the high photon energy fall-off is related to surface recombination. Eq. (2) assumes that only one type of carrier contributes to the photoconductivity. In order to confirm this assumption and the validity of using eq. (2) in our calculations, we performed some measurements on Schottky type devices made out of our material. Dalai and Alvarez [9] proposed that collection efficiencies in forward and reverse biased solar cells provided useful information on the nature of the limiting carrier. If collection efficiencies at different wavelengths are bias independent, almost equal #'r products

158 t:~ A hmrez, L Chamboulevron / Photoelectric properties of amorphous silicon nitride compoundv

5

@ 3 O) 0

- 4

t., -6

b O) - 8 2

- I0

I I

I

\ \ r'l

/ \ \ • _

I I 2.5 5

POWE n (W c m "2)

Fig. 7. Filled squares: room temperature dark conductivity as a function of rf power density for undoped samples. Open squares: room temperature photoconductivity on ELH illumination (100 mW cm-2) . Open triangles: ratio of room temperature photoconductivity to dark conductivity as a function of rf power density.

.~}."~-... .~... . ,

/ ~ . S 3 9

10-3 i

': / ~" "'e. 36 / ..%./,~.-" "..... ~li " / "'"e

.,..~1~" - / , ,

~ ' I05 ~,~ .... ~ 4 4 , _ (,(..' : ~ . . I r " ' l l . . . . 46 _

' ~ ,.:/ ..l' ..... • / ~.~ ..

,, ...: / ." . . . . . ' m .l:::l" ~ "'" 'D. 43 "

-'" i" .... .~" ..' t~ ,-,-. | i, ." ..' ." .." "'a

/ ~ . v .n ..ri

l ; : | I I I 2

P H O T O N E N E R G Y ( e V ]

Fig. 8. Normalized photocurrent as a function of photon energy for different samples. An interference pattern is modulating the photocurrent.

F. AIvarez, I. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds 159

for electrons and holes exist within the active layer. On the contrary, collection efficiency variations are indicative of different/~" products [10]. With the material having an optical gap of 2 eV, we fabricated Schottky type solar cells having the structure shown in fig. 9b. We show in fig. 9a the collection efficiency variations under a +0.5 V bias with respect to zero bias at different photon wavelengths. Carriers generated deeper in the active layer (low energy photons) experience greater or smaller difficulties in being collected, depending on the bias sign. The analysis of these data indicate that in our material, the limiting carriers are the holes. Thus /z p~-p << # nTn, where p and n stand for holes and electrons and eq. (2) can be applied to our calculations. In the spectral region where otd <_ 0.4 eq. (2) can be approxi- mated by:

Ip = eN0(1 - R ) A a d ~ ( r / T t ) . (3)

The transit time can be written as 1 / T t = ~tV/L 2, where L is the interelectrode distance in the coplanar structures used to measure Ip,/~ is the mobility and V the applied voltage. Substituting in eq. (3) gives:

Ip = [ eNo(1 - R ) / L 2] AV~l#rad. (4a)

! . !

~. 1 . 0

0.9

! I I I I I

® ~ 0 S

0 ~ 0 ~ O"

I I I i I I

400 500 600

WAVELENGTH (nm)

| I

0 . 5 V -

-0.5 V -

I I

7OO

®

Fig. 9. (a) Relative quantum efficiency as a function of excitation wavelength for a Schottky type structure for forward and reverse polarization; (b) Schottky structure used in the above experiment. The intrinsic layer was grown under conditions identical to those of sample $36. The back contact was made by doping the first 300-500 ,~ with PH3/SiH 4 ---0.012. The barrier was obtained by thermal evaporation of a semi-transparent palladium dot.

160 F. Alvarez, 1. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds

We define the normalized photocurrent as:

I p / e N o (1 - R ) = ( A V~ll"rd/ L 2 ) a. (4b)

Assuming that ~/#~- is a function that varies slowly with photon energy and keeping the photon flux constant, the normalized photocurrent is proportional to the absorption coefficient. To circumvent calibration problems, Moustakas [11] and Moddel et al. [12] proposed a scaling technique using the lowest absorption coefficients determined by transmission measurements. Matching these values and the t~ values determined by photoconductivity at corresponding photon energies, the absorption coefficient plot can be extended to very low a values. When applied to our samples, this method gives the results shown in fig. 3.

Combined measurements of photocurrent vs. light intensity and doping are useful

mm

7,

== o -

ill .6

ld 8

16 l °

I I I / S

r a

[] ~ ' : 1.15

I I i 11 1013

!o

F EPHOTON/em 2 $]

Fig. 10. Photocurrent as a function of photon flux for monochromatic light excitation of = 2.2 eV. See table 2 for samples reference. The exponents are also indicated.

F. Alvarez, I. Chambouleyron / Photoelectric properties of amorphous silicon nitride compound~ 161

A

Z

P.

7 I

7

162 F. A h~arec, I. Chamhoulevron / Photoelectric properties of amorphous xilicon nitride compound~

tools to gain a deeper understanding of the nature and possible location of defect centers. In fig. 10, we show the photocurrent versus light intensity for undoped samples grown at different rf power densities. In the same figure, we also show results for the boron doped sample $42, which was grown under the same conditions as sample $36, except for doping. Table 1 contains the measured data for all samples investigated. For convenience of discussion, we consider two ranges of photon flux excitation; a low range with light intensities going from 101° to 1013 photons cm-2s -I, and a high range for intensities greater than 1013 photons cm :s 5. The photocurrent dependence on light intensity was studied using monochromatic light having a wavelength of 5500 A. Undoped samples show a supralinear behaviour in the low excitation range, while for high excitation, the behaviour becomes linear or sublinear. Within the studied photon flux, samples prepared with a B2H6/SiH 4 ~> 10 2 gaseous ratio also show a linear or sublinear behaviour. As can be seen from fig. 6a, slight boron doping reduces the photoconductivity.

4. Discussion

In previous contributions, our group has discussed the valency controlability of variable band gap glow discharge produced amorphous silicon nitride compounds [4,5]. For a fixed nitrogen to silane gaseous ratio, the optical gap varies with the rf power being delivered to the plasma. This behaviour was interpreted in terms of the different dissociation energies of the nitrogen and silane molecules. At low rf power, only a small amount of N 2 molecules are dissociated and consequently the material has a composition not far from a-Si:H. When rf power is increased, more and more nitrogen molecules dissociate and the samples become richer in nitrogen. The optical gap widens continuously (see fig. 1). The differences which appear when a-SiN~:H is doped with boron or phosphorus (fig. 6) were explained by Alvarez et al. [5] in terms of different bonding configurations for the impurities. In stoichiometric Si3N 4, nitrogen atoms are three fold coordinated. As the optical gap of a-SiNx:H widens with increasing nitrogen content, we assume that, in our material, most of the nitrogen atoms are three fold coordinated. In silicon-rich material, boron will produce doping effects similar to those found in a-Si:H. Phosphorus will prefer to coordinate according to its own valence configuration, going into three fold coordi- nated nitrogen sites and thus producing no states in the gap. Consequently, much less impressive conductivity changes are measured for P doping.

4.1. Density of states and absorption coefficient

The absorption coefficient vs. photon energy plot for amorphous semiconductors usually shows three distinct regions: 1) a high energy region, also called intrinsic absorption, that roughly corresponds to band to band transition and where a > 104 cm-1; 2) an intermediate energy region having an exponential shape normally associated with transitions involving exponential tails of states beyond the valence and conduction band extrema and 3) a low energy region associated with transitions

F. Alvarez, I. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds 163

between defect and impurity states and the valence or conduction band. In the intrinsic region, the a dependence on energy can be represented by ( a h v ) 1/2 = B(hJ,

- E0), where B contains an average matrix element constant with energy and the joint density of states of the valence and conduction bands. In fig. 11, we have plotted B as a function of rf power density. It can be seen that in the range studied (except one experimental point), the B value remains essentially constant with increasing 'nitrogen content. This might indicate that nitrogen incorporation in- creases the separation of the valence and conduction bands without altering the shape of either. Boron doping produces similar effects.

A small decrease in B is measured as more dopant gas is allowed to go into the reactor (see table 1). However, a concomitant decrease of the optical gap suggests that new states are being created with energies in the pseudo gap. The analysis of the exponential part of a ( h p ) gives support to this interpretation. In fig. 3, we show log a vs. hp for four different samples. Curves $36, $37 and $43 correspond to undoped and boron doped a-SiNx:H respectively, having the same N content, while curve $39 was grown at lower rf power and is Si richer. Assuming that in the exponential region of a the average optical transition matrix element is constant with energy and supposing, as in a-Si:H [13], that the valence band tail characteristic energy is larger than the one associated with the conduction band, the absorption coefficient is given by

a = C e x p ( h l , / E v * ) , (5)

where E* is the valence band tail characteristic energy and C is a constant. The undoped sample ($36 fig. 3) has E* = 0.27 eV and curve $39 shows almost the same value. The doped sample $43, on the contrary, has a much larger E* value. This is consistent with an increased density of states coming from the boron impurities and occurring at the energy of the valence band tail. On the other hand, similar E* values between undoped samples $36 and $39 indicate that the density of states near the valence band is not strongly affected by nitrogen incorporation. Sample $39 shows a shoulder near 1.2 eV that has often been found in a-Si:H samples and that strongly depends on deposition parameters. This structure is not so evident in curve

U

1000

500

I I I I I

O

e - " ~ " * . . . . . . . . . . . . . . . . . . - , - -

0 1 I I I I 0 ! 2 3 4 5

R F P O W E R ( W c m ~ )

Fig. 11. Coefficient B in the equation ( a h t ' ) 1/2 = B(E - Eo) plotted for undoped samples as a function of power density.

164 F: A lvarez. I. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds

$36, but as the growing conditions were different, we cannot draw any conclusions. However, as can be seen in fig. 3, sample $36 shows a smaller sub-gap absorption strength than sample $39, which was grown at a lower rf power density.

One possible cause of the lower density of states in sample $36 could be a higher hydrogen content. Our infrared studies show an apparent increasing hydrogen content with rf power (see fig. 12). However, the H content might be overestimated as a result of an increased oscillator strength produced by ion bombardment damage at high rf powers.

4.2. Photoconductivi ty onset

We observed a linear dependence of (aht,) ]/2 on photon energy for the lowest absorption coefficient (a < 10 cm-1) as determined by photoconductivity measure- ments. An example is shown in fig. 3, inset. The photoconductivity onset hv o is defined by extrapolating this linear region to zero absorption [14]. The best fit was obtained by the least squares method; the results are shown in table 1. For undoped and slightly boron doped samples, fairly good agreement is obtained with the corresponding activation energies. The large differences between E a and hv o in heavily boron doped samples may be accounted for by the contribution of both kinds of carrier to the photocurrent.

Let us now return to the assumption of the constancy with photon energy of the product ~#~'. In our photocurrent experiments, the excitation light was chopped at 3.3 Hz, i.e. sufficiently slowly for a steady state condition to be reached. At energies near 2 eV, the absorption coefficient can be directly determined from transmission measurements. Substituting these values of a (hv) into eq. (2) and neglecting surface recombination effects, 7/#d~" 0 can be obtained, where #d is the trap dominated mobility and ~0 the photoconductor response time. The results are shown in table 2 and approximately constant values are obtained. We have no direct a measurements below 1.92 eV, but if the current is sustained by electrons near the mobility edge as photocurrent onset and activation energies suggest, we have no reason to suspect tha ~#d~0 products will strongly depend on photon energy.

|o ~

0

| o ~

I I I

I J I I 2 3

RF POWER IWcnT21

Fig. 12. Integrated area of the Si-H stretching mode (2000 cm -1) vs. rf power density for undoped samples.

F. AIvarez, I. Chambouleyron / Photoelectric properties of amorphous sificon nitride compounds 165

Recent experiments on a-Si:H samples show that ~#dr0 [15] products are essen- tially constant in the 0.9-2.2 eV range. The similarities existing between our Si rich a-SiNx:H material and a-Si:H lead us to expect similar behaviour in our case.

4. 3. Recombination and its dependence on excitation intensity

The dependence of the photocurrent on light intensity is a powerful tool in the understanding of recombination mechanisms. In fig. 10, we show photocurrent vs. light intensity for a few samples. The behaviour can be well described by

Ip - k F ~, (6)

where F is the photon flux and 7 is a power dependence. Table 1 gives the values of y corresponding to doped and undoped samples at

different excitation levels. The undoped samples show two values of 7 depending on the photon flux. The photocurrent dependence on light intensity is supralinear in the low intensity range and sublinear for higher photon fluxes. Following Rose [8], the supralinearity can be explained assuming two types (I and II) of deep defect states located above and below the dark Fermi level respectively. This picture does not include, at this point, any consideration about the origin of the defects. This will be discussed in section 4.5. We show in fig. 13 a sketch of the postulated gap density of states. According to this model, the defect states below the dark Fermi level are hole traps, which have a relatively small electron capture cross section. The states lying above the Fermi level have a much larger electron capture cross section. On illumination, there is an electron transfer via the conduction band to the type I centres. The holes continue to be trapped in type II centres. As the type I centres become occupied with electrons, the electron lifetime in the conduction band increases and a current supralinear on illumination intensity will result. Eventually, as the light intensity is further increased, the probability of capture of electrons into type I centres will become smaller than that of electrons into type II centres and this tendency will be accentuated if the hole quasi-Fermi level sweeps through the type II levels. There may be a region of photocurrent linear in light intensity. At the highest intensities, bimolecular recombination will be expected to dominate these effects, which depend on the gap recombination centres.

For light boron doping, the photoconducitivity decreases. This may be under- stood as caused by electron transfer to the acceptor levels and a lowering of the

Table 2 • q / t d ~ o products deduced from photoconductivity and optical absorption

hu T/pd~" o (eV) (cm2/V)

1.92 3.5 x 10- s 2.00 3.3 x 10- s 2.10 3 . 3 x 1 0 - s 2.20 3.0 x 10 - s

166 F. A lvarez, 1. Chambouleyron / Photoelectric properties of amorphous sificon mtride compounds"

DARK

FERMI LEVEL

E ( e V )

.,"'"'" '"'I J, i" h'v,= 0.85

~z j

type ~ defects

/ / Ec

2.03

Fig. 13. Sketch of the density of states as a function of energy for sample $36, The photocurrent onset h% for sample $36 is also indicated. All values are in eV.

Fermi level. More type I (and possibly type II) centres are available for electron capture and the photoconductivity will decrease. However, as long as the photocur- rent is dominated by electrons, a supralinear dependence of current on light intensity will result. At higher boron dopings, the effects of increased hole current must be considered.

Experimentally, we find that 3' depends on the doping gaseous ratio. In fig. 6, we show that (B2H6) / (S iH4)= 10 -4 is necessary to compensate for the effects of the type II centres. This analysis remains valid as long as the photocurrent is mainly due to electrons. Increasing boron doping above the mentioned value enhances the hole contribution to photocurrent. A quasi-linear behaviour is found and the material becomes more photoconductive for heavy doping. Phosphorus, on the contrary, sensitizes the material for light doping and decreases the photoconductivity for heavy doping. Light doping effects can be understood on the basis of the simple model being discussed. Heavy doping analysis, however, should include considera- tions on the nature and energy of the states introduced into the network by the corresponding impurities and the type of majority carrier as well. At present, we are starting a study on photoconductivity behaviour as a function of temperature. These data might give more detailed information concerning the heavy doping effects.

Within our boron doping levels, we did not find negative photoconductivity as other researchers did [3].

4.4. Response t ime

A rough estimate of the density of states acting as traps around the quasi-Fermi level N ( E F , ) can be made following Wronski and Daniel [16]. If n t is the density of trapped electrons in the vicinity of the quasi-Fermi level E v , , n t ~ k T N ( E F , ). In order to find n t, we proceed as follows.

From the 7//~d~- 0 values listed in table 2, we can estimate the electron drift mobility through the photocurrent decay time analysis. Following Moustakas [11], we define

F. A loarez, 1. Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds 167

the response time by the relation:

Iss (7) ,r o - ( d i ( t ) / d t ) t = ° ,

where Iss is the steady state photocurrent and I ( t ) is the photocurrent after the light has been switched off. For sample $36, the measured decay time is % -- 4 ms, when the material is illuminated with photons having an energy of 2 eV. From table 2 and assuming r /= 1 for the quantum efficiency generation, one obtains/~ d -- 0.83 x 10-5 cm2V-ls -1. Taking sample $36 as an example, one can estimate the free carrier density contributing to the photocurrent. The transition from supralinear into the 7 - - 1 regime occcurs at an illumination level o f = 4 x 1012 cm -2 s -1. Here the sample conductivity is o =ql~nn --- 2.7 x 10 -9 (~ cm) -1, ~n is the microscopic mobility of electrons and n the free carrier density. Assuming/~ -- 1 cm 2 V- 1 s - 1, n --- 1.7 x 10 l° cm-3 is obtained. The electron quasi-Fermi level can be estimated by n - - N c e x p [ - ( E c - E F n ) / k T 1, where N c is an effective density of states at the conduction mobility edge and EFn is the electron quasi-Fermi level. Supposing N c - 1020 c m -3 , a value that is consistent with a F, = 1 cm z V -1 s -a, E c - E v n -- 0.58 eV, indicating that the electron quasi-Fermi level has shifted 0.32 eV upwards on illumination. The density of trapped electrons n t can be estimated, recalling that the drift and microscopic mobilities are related by btd = n / ( n + n t ) / x n [8,17]. If n t >> n, we can approximate [8] by /~d--" ( n / n t ) l - t , . Substituting /~d and /~,, one obtains n t / n = 1.2 x 105. For the highest photon flux used in our experiments (-- 1014 cm -2 s - l ) , the free carrier density is n = 1.25 x 1011 cm -3 and then n t -- 1.5 X 1016 c m -3 .

From k T N ( E F n ) -- n t, the density of states acting at traps at room temperature is estimated to be N ( E c - 0.58 eV)-- 6 x 1017 cm -3.

4.5. Defects

Let us now speculate about the origin of the defect centres. In the analysis of the supralinearity, it was assumed that the type I centres have a large capture cross section for the first electron and a much lower capture cross section for the second. Two models might conceivably give such centres; one involving nitrogen atoms directly, the other silicon dangling bonds. Let us consider the first possibility. Because nitrogen inclusion into the network widens the optical gap, we assume that nitrogen atoms are mostly incorporated in a planar three fold coordination config- uration, in a way similar to that in stoichiometric silicon nitride. Our results on non-intentionally doped a-SiN x suggest, however, that nitrogen atoms could be four fold coordinated. When tetrahedrally coordinated, N would have a level analogous to the hydrogenic state of a classical donor, except that its large electronegativity and its presence in a disordered network would yield a deep level in the upper half of the pseudogap. With a Fermi level near the midgap, the spN~- would be positively charged. The type II centres might be due to an undercoordinated pN 2 defect. Such a defect, to be negatively charged for a Fermi level near the midgap, would have to have both non-bonding p-state electrons at energies below the midgap.

168 F. A lvarez, L Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds'

The second possibility might be the charged states associated with silicon dan- gling bonds. The density of dangling bonds in a-Si:H might well be increased by three fold coordinated N atoms, which distort the network in their vicinity.

In this picture, the first electron in the dangling bond has an energy just below the gap centre and the second electron an energy level just above. The type I centres would then correspond to the neutral dangling bond and their electron capture cross section is altered if they gain a second electron when the electron quasi-Fermi level sweeps through them. If the hole quasi-Fermi level sweeps through the dangling bond band, it will produce D + centers with energies for electron reception analo- gous to those of the type II centres.

We observed experimentally that nitrogen is responsible for the changes in the recombination kinetics. Consistent with this, both models include the participation of N.

5. Conclusions and summary

Glow discharge decomposition of nitrogen and silane gaseous mixtures of fixed composition gives a material having an optical band gap that varies with the rf power delivered to the plasma. This behaviour was interpreted in terms of the different dissociation energies of nitrogen and silane molecules. It was shown that off-stoichiometric silicon nitride can be doped by adding minute amounts of diborane or phosphine into the reactor chamber. For the chosen composition (E 0 -- 2.0 eV), the minimum dark conductivity (o d -~ 10 -13 fi-1 cm-1) is obtained with a diborane gaseous ratio of (B 2 H6)/(SiH 4) ~ 10-3. Phosphorus doping causes a less impressive conductivity change than boron doping. Photoconductivity appears to be trap controlled in all samples. Undoped materials show conductivity changes on illumination (ELH, 100 mW cm -2) of up to 3 to 5 orders of magnitude, depending on preparation parameters. Doped samples show a photoconductive behaviour that depends on the nature and concentration of impurities. Light phosphorus doping improves the photoconductivity, but heavy doping unsensitizes the samples. A diborane to silane gaseous ratio of less than 10 -3 unsensitizes the material and higher boron concentrations sensitize it again. Sensitization and unsensitization for light phosphorus and boron doping respectively have been explained in terms of a simple model having two types of defect centres, possessing different capture cross sections for electrons. Heavy doping produces opposite effects. Sensitization due to heavy boron doping might be explained by taking into account the contribution of holes to photoconductivity. Unsensitization produced by high phosphorus content appears more difficult to explain in terms of our simple model. Other defect centres have to be considered, together with the position of the Fermi level in the pseudo gap.

Photoconductivity measurements allowed us to identify exponential tails of states in the bandgap. It was found that the rf power delivered to the plasma affects the subgap absorption. Boron doping decreases the optical band gap, suggesting that defects and/or new electronic states are created. Electrons are the majority carriers

F. Alvarez, L Chambouleyron / Photoelectric properties of amorphous silicon nitride compounds 169

for undoped samples and the photocurrent is sustained by electrons being promoted from the Fermi level to the conduction band extended states. This interpretation is supported by the good agreement between photocurrent onset and dark conductivity activation energies. The supralinear photocurrent dependence with light intensity was interpreted assuming two types of defects near the Fermi level. The ones lying below the Fermi energy act as hole traps and possess a small electron capture cross section. Above the Fermi energy, we postulate defect centres having a larger electron cross section. On illumination, there is a transfer of electrons from the former to the latter and the material is sensitized. We speculated about the possible origin of these defects that we associate to nitrogen content. Under and over coordinated nitrogen atoms as well as charged states of silicon dangling bands might produce the kind of proposed defects. We could estimate the mobility lifetime product of photocarriers. A value of---3.5 × 10 -8 cm2V -1 was obtained for ~td~- 0 and from photocurrent response, we got a trap controlled drift mobility o f = 10 -5 cm2V-ls -1. A rough calculation indicates that, at approximately 0.6 eV below the conduction band, there is a density of electronic states acting as traps of = 6 × 1017 cm -3, Experiments of electron spin resonance, photocurrent infrared quenching and luminescence are necessary to gain a deeper understanding of the origin, nature, density and energy levels of the defects that influence the overall process.

Finally, the high dark resistivity (> 1013 fl cm) obtained for slightly boron doped samples and changes over 5 orders of magnitude on illumination (ELH, 100 mw cm-2) make this material potentially useful for xerographic applications, as well as for photosensor devices.

Acknowledgements

The authors are indebted to Prof. W. Paul of Harvard University, Cambridge for critical reading of the manuscript and for many fruitful discussions on the defect center model we propose here. We thank I. Pereyra of the University of Sho Paulo and G. Storti, Solarex, Rockville for helpful comments. E. Bustarret made the IR measurements and A.M. Andrade (USP) provided the equipment for photoconduc- tivity measurements.

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