CHEMICAL AND ELECTRICAL CHARACTERIZATION OF POLYCRYSTALLINE SEMICONDUCTORS

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CHEMICAL AND ELECTRICAL

CHARACTERIZATION OF POLYCRYSTALLINE SEMICONDUCTORS

L. Kazmerski, Prue Russell

To cite this version:

L. Kazmerski, Prue Russell. CHEMICAL AND ELECTRICAL CHARACTERIZATION OF POLY- CRYSTALLINE SEMICONDUCTORS. Journal de Physique Colloques, 1982, 43 (C1), pp.C1-171- C1-185. �10.1051/jphyscol:1982124�. �jpa-00221780�

JOURNAL DE PHYSIQUE

CoZloyue Cl, supplément a u n o 10, Tome 43, o c t o b r e 1982 page C l - 1 7 1

C H E M I C A L A N D E L E C T R I C A L C H A R A C T E R I Z A T I O N OF P O L Y C R Y S T A L L I N E SEM1 CONDUCTORS

L.L. Kazmerski and P.E. Russell

S o l o r Energy Research I n s t i t u t e , Co Zden, CoZoraado 80401, U. S. A.

Béauaé - La composition et la chimie des régions entre et dans les grains des semiconducteurs polycristallins peuvent se rapporter aux et même dominer les caractéristiques électriques de ces matériaux et des dispositifs ob ils sont utilises. Dans cette etude, des techniques complémentaires de haute resolution pour l'analyse des surfaces, y compris la spectrometrie Auger (AES), la spectromètrie de masse par ions secondaires (SIMS), la radiospectrométrie photoélectronique (XPS) et la spectrometrie de perte électronique à basses énergies (EELS), sont employées pour examiner les propriétés de composition des grains et des joints de grains, dans le silicium et dans le GaAs. Les positions des impurités separees aux joints de grains et d'autres défauts sont trouvées par spectromètrie Auger exploratrice ayant une résolution latérale de moins de 500 A par combinaison avec une technique de fracture in-situ. Des comparaisons entre les régions dans les grains et entre les grains sont données. L'activité électrique de ces impuritiès à l'intérieur du plan du joint de grains est évaluée par un mesurage modifié de spectromètrie Auger pour déterminer le potentiel de la surface. Les effets de l'illumination sur le potentiel de barrière et sur les temps de vie des porteurs minoritaires des joints de grains de silicium pures et intentionellement dopés (Ai, Ti, Cu) sont données.

Les effets de la recuisson sur les joints de grains de diverses impurités sont discutés. Des résultats montrant que la ségrégation de l'oxygene aux joints de grains de silicium influe fortement sur l'activité électrique des joints de grains sont offerts. AES, EELS, SIMS et la microscopie ionique sont utilisees pour examiner la chimie des joints de grains ce qui indique que l'oxygène se déplace aux régions entre les grains pendant des traitements de chauffage â hautes températures. Des données des positions des ions (resolution 1 um) sont donnees en fonction de temps et de temperature pour montrer la ségrégation de ltoxyg&ne aux joints. Des données complémentaires des temps de vie de porteurs minoritaires et des mesurages du courant induit par rayons électroniques (EBIC) corroborent le modèle de la ségrégation de l'oxygène et indique que ce mécanisme est la cause probable de l'activation électrique de tels joints de grains.

Abstract - The chemistry and composition of inter- and intragrain regions in polycrystalline semiconductors can be related to, as well as dominate, the electrical characteristics of the materials, and devices fabricated from them. In this paper, high-resolution, complementary surface analysis techniques, including Auger electron spectroscopy (AES),. secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS) and low-energy electron loss spectroscopy (EELS), are used to investigate the compositional properties of grains and grain boundaries in Si and GaAs. Segregated impurities localized at grain boundaries and other defects are mapped using scanning AES with better than 500 A lateral resolution, in conjunction with an in-situ fracture technique. Comparisons between grain and grain boundary regions are presented. The electrical activity of these impurities is evaluated

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982124

JOURNAL DE PHYSIQUE

within the grain boundary plane using a modified AES measurement to determine surface potential. The effects of illumination on the barrier potential and minority-carrier lifetimes of clean and intentionally- doped (Al, Te, Cu) silicon grain boundaries are presented. The effects of annealing on grain boundaries containing various impurities are discussed. Evidence is presented to show that oxygen segregation to silicon grain boundaries strongly influences grain boundary electrical activity. AES, EELS, SIMS and ion microscopy are used to investigate grain boundary chemistry - indicating that oxygen moves to the inter- grain regions during high-temperature heat-treatments. Direct ion map- ping data (- 1 Fim resolution) are presented as functions of time and temperature to show the grain boundary oxygen segregation. Complemen- tary minority carrier lifetime data and electron beam induced current (EBIC) measurements support the oxygen segregation mode1 and identifies this mechanism as the probable source for electrical activation of such grain boundaries.

1. Introduction - Polycrystalline semiconductors have become important entities in the electronics industry. with applications ranging from passive micron-size inter- connects using poly-silicon to active, cm-size solar cells using a variety of ele- mental and compound semiconductors [1,2]. Because of growing interests in their applications and in providing low-cost, device-quality semiconducting layers, re- search activities aimed at controlling and understanding the electrical behavior of grain boundaries have increased substantially in recent years 13-10]. The ability to predict the transport properties of polycrystalline semiconductors and the char- acteristics of polycrystalline devices has been enhanced by the development and refinement of modeling techniques. The control of grain boundary parameters by chemical methods has been demonstrated, including the utilization of H, Li and O in various semiconductor hosts [ll,l5]. These studies have further highlighted the importance of the chemistry of the grain boundaries in determining and controlling the electrical characteristics of these inter-crystalline regions. It is the pur- pose of this paper to focus on the correlation between the inherent chemistry/compo- sition of grain boundaries in semiconductors, with the resulting electrical proper- t ies.

The extent to which grain boundaries affect the properties of a semiconductor depends upon several factors including grain size, grain boundary orientation, material treatment and semiconductor type. The intercrystalline regions can impede the flow of majority carriers, act as recombination centers for minority carriers, or provide electrical shunts across the polycrystalline layer. Which of these processes (or combination thereof) dominate differs, for example, from Si to GaAs, and within any single semiconductor type itself depending upon its history. It is apparent in the literature on polycrystalline semiconductors that al1 observations are not generalizations, but must be evaluated in terms of the processing, material and measurement conditions used [16].

This paper focusses on the properties of cast [17], large-grain Si, with some comparitive examples presented from other Si types (e.g., CVD) and GaAs. In gene- ral, the Si grain boundary studies have a common (211) plane, with approximately a 30' mismatch in the (111) - ^planes. This common boundary type (determined by elec- tron channeling and X-ray diffraction is classified as a medium-angle grain bound- ary, and provides a common basis of comparison.

The chemistry and composition of the grain boundaries are evaluated using surface analysis techniques, including Auger electron spectroscopy (AES), eiectron- energy loss spectroscopy (EELS), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). These methods, compared in Table 1, provide information from the topmost atomic layers of the material being analyzed and their utilization in the study of grain boundaries is emphasized. Results of these inves- tigations are correlated with microelectrical characterizations of the same regions using electron-beam induced current (EBIC), minority-carrier lifetime, andspecial- ized scanning AES techniques. The segregation of impurities to the intergrain

Table 1. Summary of

1 AES

Depth Resolution 5 - ⁵⁰ A

1

Probe

Detected Species Spatial Resolution

Detection Sensitivity ( 0.1 at-%

electron electron -300 A

Selected Surface Analysis Techniques

1 ^EELS 1 ^SlMS 1 ^XPS 1 ^UPS 1

regions is investigated, including the properties of specific, intentionally-added species (e.g., Al, Ti, Cu in Si; Sn in GaAs). Grain boundaries are evaluated using these techniques both perpendicular to the planar defects, and within their plane using in-situ fracturing to expose them 1181. Finally, the effects of heat-treat- ment on the diffusion of O to the Si grain boundaries, and potential electrical activation, are reported.

electron electron -300 A

2. Experhental Details - The compositional and chemical determination were per- formed primarily with surface analysis equipment. These include: (1) Perkin- Elmer/Physical Electronics mode1 595 Multiprobe, having better than 500 A AES reso- lution, complementary SIMS, EBIC and EDS capabilities; (2) Perkin-ElmerjPhysical Electronics model 590 Scanning Auger microprobe, with 1500 A resolution, complemen- tary SIMS and EBIC; (3) Perkin-ElmerlPhysical Electronics model 550 XPSjAES system, with EELS, SIMS and EBIC; and, (4) CAMECA IMF-3f ion microprobe, with Ce and O ion sources. AES measurements were typically taken under low current (- 20 ni) condi- tions, using the pulse counting detection mode. XPS data were taken with a 10 kV (40 mA) X-ray source, configured with a Mg anode, utilizing a double pass cylindri- cal mirror analyzer 10 eV pass energy was used for high resolution. The EELS was done using the double-pass analyzer with an incident energy of 100 eV. A 0.5 eV modulation energy was used in these studies. Electron beam induced current studies were perfonned in-situ, as well as in a JEOL 35C SEM, equipped with digital imaging rapabilities. Minority carrier lifetimes were determined using a spatially resolved surface photovoltage method (16,191. Resolution was typically 15 Pm, using a micro- focussing stage. Measurements for the Si samples were taken over the range

0.88 < X < 1.0 Pm, with better than a 5 % reproducibility. Grain boundary potentials were determined under zero-bias conditions (51, using a four-terminal method and Keithley 616 electrometers. A filtered xenon lamp served as the illumination source for the photogeneration grain boundaries studies. The irradiation level was deter- mined using a blackbody radiometer - with intensities accurate within 5%.

The polycrystalline Si was produced by casting [17]. The substrates were p- type (B-doped) with Na

-

Au was used to form a Schottky barrier to the GaAs for the ERIC studies. The thick- ness of this metal layer was typically 100-200 A .

ion (+, -) Ion (+, -)

1 p

3. Besdta and Mscusaions - The interrelationships between the composition or chemical properties of grain boundaries and their measured electrical properties have been recently reported for polycrystalline silicon [16,18,20]. In these studies, the ability to demonstrate impurity localization at grain boundaries is

x-ray electron 102 - ^{103 pm}

ultraviolet electron

-103 pm

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Silicon (a) Grain

\I

primarily due to the inherent surface sensitivity of the analysis methods and the relatively high spatial resolution of the electron and ion probes used [21,22]. The complementary approach used in these analyses allows not only multiple surface analysis techniques to provide compositional information on the same, specific areas, but also correlated electrical characteristics, and (in some cases), composi- tional properties of the bulk ^material. In this section, the analyses of grain boundary semiconductors are discussed. First, the detection and evaluation of impurities at grain boundaries are presented. The presence of these impurities are correlated with EBIC results, minority-carrier lifetime measurements, and illumina- tion-grain boundary potential barrier studies. Second, the effects of impurity movement are indicated for heat-treated polycrystalline samples. Evidence for the activation of the intergrain regions due to oxygen segregation is presented.

= 1 I

5 s i ~ w ~ "KLL

fn

OKLL

(a) Grain Boundary Impurities: The direct evidence for the segregation of impurities to grain boundaries in cast and directionally - solidified multigrained Si has been reported [18]. Since grain boundaries lie essentially perpendicular to the available wafer surface, they offer a low-profile for analysis, even by now- available surface analysis high resolution probes. In-situ fracturing has been demonstrated to be a successful method to expose these interna1 surfaces, providing larger areas for analysis and minimizing potential contamination from sorbed species [la]. Fig. 1 presents AES survey scans for a grain and grain boundary region, which have been exposed for side-by-side analysis by the fracturing method. Differences in the composition of the two regions are readily apparent. The intragrain is clean within the 0.1 at. -% detection limit of the AES technique. The boundary (Fu. 1 )

is seen to have several impurity species present, including C, O, F, Fe and Al. The chemical nature of the oxygen present is indicated to be primarily SiOx by the shape and position of the low-energy S~(KLL) transition [23]. Although the impurity species are easily detected on the grain boundary surfaces indicating compositions

-

Fig. 1. AES survey scans of (a) adjacent grain, and, (b) grain boundary in fractured polycrystal- line Si. (Ep=5 kV, Ip=30 nA).

The high-spatial resolution of AES can be used effectively to map the impurity concentrations on the surface being examined. This is accomplished by tuning the

O 400 800 1200 1600 2000

Elactron Energy (eV)

Fin. 2. Scanning AES mapping sequence of grain boundary exposed by in-situ fracture showing impurity localizations (Ep = lOkV, Jp = 2nA). SED image shows topography. (From ref. 16).

spectrometer to one of the AES peaks - shown in Pig. 1 - and scanning the incident e-beam across the surface. An AES mapping sequence of a fractured Si boundary is presented in Pig. 2. This particular sample had 0.001 at. -4: A i adding during the growth process in order to study its segregation mechanisms. The secondary electron detection (SED) image shows several contrasted features. The Auger maps of this same region give evidence of the impurities (C, O, P, Fe and Al) localized in this area. The localization on the grain boundary surface is illustrated by the depth- profile shown in Fig. 3 taken at point (a) of Fig. 2. The Si (s~o*), Al and O signals diminish as a function of sputtering time until the grain material is reached (indicated by the emergence of the elemental SiKLL signal. Several aspects of the AES maps of Fig. 2 are noteworthy. The complementary positions of the Si and Si(ÇioX) signals are evident. This may be a result of the fragmentation during fracturing. ûxygen containing grain boundaries are more likely to fracture than clean ones. In the Al maps, the AlKLL signal is more intense and covers a greater area than the aLm. ~ h i s is accounted for by the difference in escape depths and sensitivity factors for these two transitions [21].

Several correlations between grain boundary impurity composition and the elec- trical response of theae regions have been previously reported 116, 2 0 , 24-36]. ^One method examined a given grain boundary first by EBIC to establish its electrical activity [ 2 0 ] . Subsequently, the boundary was exposed for compositional analysis by fracturing. This procedure is difficult, both from the ability to fracture the identical grain boundary region examined by EBIC, and from the certainty of identi- fying the impurities responsible for the electrical activity. Another method has utilized the direct measurement of the electrical properties of the grain boundary

C1-176 JOURNAL DE PHYSIQUE

- _{I " ' I ' " I r}

- -

' 8

Si Grain Boundary (Fractured Surface) - \

- _m

C o l - iij ln

IU - 4

0 2 4 6

Sputtering Time (min)

Fig. 3. Depth-compositional profile taken at point (a) indicated in Fig. 3

Fig. 4. Electrical properties of grain boundary area shown in Fig. 2. (a) SED image; (b) Electrical potential mapping across regions a and b shown in SED.

Impurities corresponding to AES signal changes are indicated. (From ref. 16).

Distance (pm)

along its length (i.e., within the boundary plane), and the direct correlation with the region's composition 1161. The analysis presented in Fig. 4 is for the same region shown in Fig. 2.

In this method, two Ag contacts (whose AES signal does not interfer with the detected impurities) are applied to the extreme edges of the grain boundary surface exposed by fracturing. A carbon film (- 20-40 A ) is formed on the surface. The C(271 eV) signal is then monitored while stepping the incident electron bean across the surface (see insert, Fig. 4a). When the sample is biased via the Ag-contacts, the AES spectrum is shifted by an energy that is proportional to the applied bias and surface potential 1271. The observed shift in the carbon Auger peak position provides information on the relative surface potential. Assuming a uniform carbon layer (verified by AES depth-profiling), a correlation between electrical activity and specific impurities is gained. Two such scans are presented in Fig. 4b.

Further indications of exact impurity effects are provided by using light- generated processes. For polycrystalline solar cells, the effects of light on the grain boundary properties are particularly important. Fig. 5 presents the conse- quence of illuminating a p-type grain boundary in terms of the band diagram. For the dark case, the states are filled (in equilibrium) to the Fermi level, and band- bending results to preserve charge neutrality (double-depletion layer). Under illumination, the interface states adjust their charge by initial capture of minor- ity carriers. This, in turn, reduces to diffusion potential and results in the

maximum recombination rate. Effects of uniform illumination on the barrier heights (q$b ) , recombination velocity. photogeneration rates, and conductivity of grain boundaries in Si have been previously reported 133,291. Seager has provided a thorough theoretical and experimental evaluation of light effects at grain bound- aries [ 3 ] . It was predicted and verified that grain boundaries with large (> 0.4 eV) barrier potentials are very sensitive to even low - level illlumination.

Conversely, barriers with small q $ ~ are relatively light-insensitive, since very small readjustments of the barrier 'heights produce substantial excess hole (elec- tron) currents for recombination with collected electrons (holes). These regions which are relatively conductive remain loss sites for photogenerated carriers.

:b) Illuminated ,_ ^..,I ,

.

E c

1 I I

Ti, unannealed - - - - - - - - -- - - -

---

Fig. 6. Grain boundary barrier height dependence upon illumi- nation (0.001 to 0.1 suns) for various clean and impurity- containing intergrain regions.

Annealed samples were processed

8 nity of grain boundary in p-type qa- E f ~ terial, (a) dark case; (b) illuminated E~ case.

- - - - - - - - - - O -

9

-

a

at 900°C for 20 min.

- - - - - - - ^- - - - -

Ein

Fig. 5. Energy band diagrams in vici-

') Clean, annealed

Silicon

I I

v _{0.1 1.0 10} ^I

Irradiation (mW/cm2)

Cl-178 JOURNAL DE PHYSIQUE

The effects of the impurity content of the grain boundary on the barrier heights and the illumination effects are illustrated in Fig. 6. These data involve a set of Si samples prepared with: (i) no additional impurities added (clean); (ii) with 0.001% Al added: (iii) with 0.001% Ti; and, (iv) with 0.0004% Cu. Compositional analysis showed that these impurities did accumulate in the intercrystalline bound- ary region. The effect of illumination on the barrier heights are presented in Fig.

6. The measurement of q g vas made according to the zero-bias conduction method [ 5 ] . As expected [3], th&! barrier potential of al1 sampled grain boundaries de- creases with increasing illumination, over the 0.001 to 0.1 Sun range. The data on clean grain boundaries (Fig. 6(a) and (a')) follows the predictions and observations of Seager (31. In general, the unannealed grain boundaries had lower measured values of q@ (measured in the dark) than the same region after annealing (typi- cally, 900°c? 20 min). The higher barrier height for the annealed case is also more light sensitive than its lower barrier counterpart. The higher barrier potential provides a larger attractive field, making the depletion layer edge a region of very high minority carrier collection.

I l I I

(b) Al, unannealed ' ^{I ' '} T = ^' 20 ^Ipsec ^I

-

C

5

UResolution

Fig. 7. Minority-carrier lifetime

I I I J I ~ I I 1I , line maps over Si grain boundaries of

-80 -40 O 40 80 Fig. 5.

Distance (pm)

The effect of adding impurities to the grain boundaries is, in general, to increase this carrier collection process [16]. This is observed by comparing the data for segregated Al, Ti, and Cu, in Fig. 6, with the clean boundary case. For each impurity situation, the barrier height increases, as does the field at the space charge layer edge. Although the segregation coefficients of Al, Cu and Ti, do not differ greatly, and the relative concentrations of each species at the grain boundary are approximately the same (- 1 0 ~ ~ / c m ~ ) , the effects of Ti and Cu are more pronounced since they act as more efficient minority-carrier lifetime sinks than Al [16]. This is further illustrated in Fig. 7 which presents a series of minority- carrier lifetime scans across the grain boundaries presented in Fig. 6. It is apparent that the three unannealed samples (Figs. 7(a), (b), (c) and (d) show a lower lifetime for the intergrain regions with the intentionally added impurities.

Again, the lifetimes for the Ti and Cu cases are signficantly lower. These data correlate well with the grain boundary barrier height measurements of Fig. 6 and the predictions of Seager. Note that some effect in the grain regions for the Ti and Cu

c a s e s c a n be o b s e r v e d i n F i g . 7 , i n d i c a t e t h a t some p o r t i o n s o f t h e s e i m p u r i t i e s d o n o t s e g r e g a t e e f f i c i e n t l y d u r i n g t h e c a s t i n g p r o c e s s .

The s e g r e g a t i o n o f i m p u r i t i e s t o g r a i n boundary r e g i o n s c a n b e o b s e r v e d i n d i f f e r e n t growth t e c h n i q u e s and i n d i f f e r e n t s e m i c o n d u c t o r s a s w e l l . F i g u r e 8 shows q$ v s i r r a d i a t i o n d a t a f o r a g r a i n boundary i n GaAs f o r two c a s e s : ( a ) c l e a n ; (8) w i t h Sn added d u r i n g MBE growth. T h e s e l a y e r s a r e b o t h 1.0 urn t h i c k , grown o n a Cr-doped, h i g h r e s i s t i v i t y s u b s t r a t e h a v i n g a s i n g l e low-angle g r a i n boundary. An AES map ( F i g . 9 ) c l e a r l y shows t h e Sn a c c u m u l a t i o n f o r t h e Sn-doped c a s e . The m i n o r i t y c a r r i e r l i f e t i m e s o f t h e s e two g r a i n b o u n d a r i e s were e v a l u a t e d u s i n g a p i c o s e c o n d l a s e r (mode-locked, cavity-dumped o p e r a t i o n , w i t h A = 0.832 vm)

A v ^{I 1}

Gallium Arsenide

1

A I I I

0.01 0.1

Irradiation (mW/cm2)

F i g . 8. Grain-boundary b a r r i e r h e i g h t dependence on i r r a d i a t i o n f o r ( a ) undoped, and (b) Sn-doped r e g i o n s i n GaAs .

Distance ( p m) 2 0 -

F i g . 10. Grain-boundary h e i g h t depen- dence on i r r a d i a t i o n f o r ( a ) undoped, and (b) Sn-doped r e g i o n s i n GaAs.

/ Galllum Arsenide

' ^{= 2.8 ns} _I Grain Boundaries

/ !

F i g . 9. S e c o n d a r y e l e c t r o n image a n d AES Sn-map o f GaAs b o u n d a r i e s m e a s u r e d i n F i g . 8.

and m e a s u r i n g t h e p h o t o l u m i n e s c e n t d e c a y v i a s i n g l e photon c o u n t i n g methods (301.

The e f f e c t o f Sn i n l o w e r i n g t h e l i f e t i m e a t t h e g r a i n boundary i s e v i d e n t i n F i g . 10.

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Unheated

Fig. 11. EBIC maps for grain boundaries in golycrystalline Si for unheated, 600 C and 750'~ anneals showing in- creased electrical response with heat- treatment.

(b) Annealing and Oxygen Segregation: The electrical activity of some grain boundaries in cast polycrystalline Si has been reported to be strongly affected by heat treatments (16, 24, 31-33]. Redfield [ 3 2 ] observed this using liquid crystal detection techniques. Figs. 6 and 7 also indicate such properties. In each case, the heat-treated grain boundary had a higher value of q+ and a corresponding

b '

lower measured value of T. EBIC data also confirm this heat-treatment effect. Fig.

11 presents a series of EBIC maps for a region of several grain boundaries, for unannealed, T = 600" and T 750' heat treatments in Ar. The minority carrier collection can be seen to increase signficantly at the higher annealing tempera- tures. Fig. 1 2 shows individual EBIC line scans across the grain boundaries of Fig.

11 (i.e., (a) unheated; (b) heated to 600°C; and (c) heated to 900°C) [ 2 4 ] . These data contain further information on the electrical properties of the boundar re- gion. The slope of the in1 for the 900°C grain bo ndary anneal (-0.08 urn -3 ) is considerably larger than fog%! 600°C case (- 0.02 Assuming that the diffu- sion coefficient is the same for both samples, the value of the effective surface recombination velocity, Seff for the grain boundary heated to 900°C is approximately four times larger than the lower temperature case. Russell, et al. [ 2 4 ] have used the EBIC linescan technique to determine the grain boundary recombination velocity and effective minority carrier diffusion length as function of these heat treat- ments. Statistically, the data show an increase in Seff and decrease in Leff, both as functions of time and temperature for these Si grain boundaries.

The activation of these grain boundary regions has been attributed to either the movement of impurities to the grain boundary, or away from it. Fig. 13 presents SIMS data taken on fractured grain regions, before and after annealing. A signifi-

-'

t

Slope

(a) Unheated

= 0.02/pm \ / ^{(b) 600" C 30 min} 1

(c) 900" C 30 min

Silicon

Grain Boundai

Distance ( p m)

F i u , EBIC l i n e s c a n s a c r o s s ( a ) u n h e a t e d ( b ) 600°c, 30 min, and ( c ) 900°c,

- 30 min h e a t - t r e a t e d g r a i n b o u n d a r i e s i n d i c a t i n g i n c r e a s e i n e l e c t r i c a l a c t i - v i t y of t h e s e r e g i o n s w i t h h e a t - t r e a t m e n t .

c a n t i n c r e a s e i n t h e SiO+, SIOH+, and s i 2 0 + s i g n a l s i s o b s e r v e d a f t e r h e a t i n g (900°C, 201 m i n . ) , i n d i c a t i n g t h e s e g r e g a t i o n o f oxygen t o t h e s e i n t e r n a 1 s u r f a c e s . Also shown i n SIMS d a t a f o r a n a n n e a l e d A l - c o n t a i n i n g g r a i n boundary. Almost no change i n t h e o x i d e s p e c i e s r e s p o n s e was o b s e r v e d . T h i s i s c o n s i s t e n t w i t h t h e measured e l e c t r i c a l p r o p e r t i e s of s u c h g r a i n b o u n d a r i e s ( s e e F i g s . 6 and 7 ) .

I n a d d i t i o n , t h e g r a i n boundary s u r f a c e s were examined by f r a c t u r i n g and SIMS d e p t h - c o m p o s i t i o n a l p r o f i l i n g . The r e s u l t s a r e shown i n F i g . 1 4 ( a ) . Some o x i d e i s o b s e r v e d on t h e unannealed boundary s u r f a c e . By comparison, however, a more pro- nounced o x i d e l a y e r i s measured on t h e a n n e a l e d s u r f a c e . S i n c e t h e d a t a were t a k e n under i d e n t i c a l c o n d i t i o n s , t h e r e l a t i v e d e p t h s and Si-O c o v e r a g e can be q u a l i t a - t i v e l y compared. I n o r d e r t o e n s u r e t h a t t h o s e d i f f e r e n c e s were n o t due t o a r t i - f a c t s o f t h e f r a c t u r e p r o c e d u r e , b o t h s i d e s of t h e f r a c t u r e d s u r f a c e were examined.

T h i s p a i n s t a k i n g p r o c e d u r e r e s u l t e d i n t h e two s e t s o f d a t a shown on e a c h d e p t h p r o f i l e . F o r t h i s p a r t i c u l a r g r a i n boundary, a m i n o r i t y c a r r i e r l i f e t i m e s c a n was a l s o r e c o r d e d ( b e f o r e f r a c t u r e ) i n F i g . 1 4 ( b ) . The a n n e a l e d g r a i n boundary has a lower T, which complements b o t h t h e EBIC and t h e SIMS d a t a . F u r t h e r e v i d e n c e o f t h e o x i d e p r e s e n t a t t h i s g r a i n boundary i s p r e s e n t e d by t h e XPS and EELS d a t a o f F i g s . 1 5 and 16. F i g . 1 5 p r e s e n t s a n g l e - r e s o l v e XPS d a t a f o r b o t h a f r a c t u r e d , u n h e a t e d g r a i n boundary, and one h e a t e d a t 900°C. The Si-2p s i g n a l s a r e shown a s a f u n c t i o n o f t h e e f f e c t i v e a n a l y z e r a c c e p t a n c e a n g l e , 8. EELS d a t a , shown i n F i g . 16, a l s o i n d i c a t e s t h e d i f f e r e n c e s i n t h e u n h e a t - t r e a t e d and h e a t - t r e a t e d g r a i n b o u n d a r i e s . The l o s s peaks a s s o c i a t e d w i t h t h e o x i d e s p e c i e s a r e e a s i l y o b s e r v e d (shaded re- g i o n s ) i n t h e a n n e a l e d g r a i n boundary c a s e . T h e r e f o r e , t h e o r i g i n o f t h e enhanced g r a i n boundary e l e c t r i c a l a c t i v i t y i n h e a t - t r e a t e d p o l y c r y s t a l l i n e S i i s due t o t h e

JOURNAL DE PHYSIQUE

I I I I I I I

Silicon Grain Boundary (unanneaied) -

- m _{O 2 4 6 0 2 4 6 8}

rn Sputtering Time (min)

a S

- f ^{g . : 16 psec} 7-1.1 psec

C

0

rJJ 7- 0.08 psec

- 20 O 20 - 20 O 20

Distance iumi

F i g . 1 4 . S i g r a i n b o u n d a r y : ( a ) SIMS d e p t h - p r o v i l e ( a n n e a l e d a n d u n a n n e a l e d ) . D a t a f o r b o t h s i d e s o f f r a c t u r e d r e g i o n a r e p r e s e n t e d : ( b ) M i n o r i t y c a r r i e r l i f e t i m e s c a n f o r e a c h r e g i o n . (From r e f . 1 6 ) .

F i g . 13. SIMS d a t a f o r a n n e a l e d a n d u n a n n e a l e d and u n a n n e a l e d b o u n d a r i e s ,

( a ) n o i n t e n t i o n a l i m p u r i t i e s , (b) w i t h A l .

1 ' " '

Grain Boundaries

Si0,-2p (103.4 eV)

I '0

$ P

O 4 8 12 16 20

Electron Energy (eV)

I I I

-95 -100 -105

Binding Energy (eV) F i g . 1 6 . EELS d a t a f o r ( a ) a r ~ n e a l e d

~ i g . 15. A n g l e r e s o l v e d XFS d a t a ( s h o w i n g oxygen p e a k s ) ; and ( b ) unan- s h o w i n g oxygen a t f r a c t u r e d g r a i n n e a l e d S i g r a i n b o u n d a r i e s .

b o u n d a r y .

Oxygen a t

Grain Fig. 17. SIMS oxygen map showing oxygen Boundary segregation a f t e r annealing.

segregation of the oxygen to the intercrystalline boundary regions. A final illus- tration of this effect is shown Ln Fig. 17, which presents a SIMS map of oxygen on a polycrystalline Si sample annealed at 900°C. The accumulation of oxygen at the boundary region (intersection of-the grain boundary with the wafer surface) is readily apparent. It must be stressed that not al1 grain boundaries exhibit the enhancement in electrical activity after annealing. This may well be due to other impurity effects - such as those discussed previously in this paper - or to crystal- lographic properties of the grains and grain boundaries.

4. S u a n a r y - The cornpositional and chernical properties of Si and GaAs grain bound-

aries have been correlated with their microelectrical characteristics. The useful- ness and importance of surface analysis methods (scanning AES, EELS, SIMS, XPS) in determining the composition of these intergrain regions have been demonstrated.

Several micro-area-techniques have also been reported that provide information on the electrical properties of the same regions compositionally analyzed. In general, the composition of the grain boundary has been shown to be a controlling factor in determining both materials and device properties.

Acknovledgement - The authors gratefully acknowledge the discussions and work of C.R. Herrington, P.J. Ireland, R. Matson, J.R. Dick, K. Jones and R.K. Ahrenkiel, of the Solar Energy Research Institute, who participated directly in various portions of this research. This work was supported by DOE under Contract No. EG-77-COL-4042.

Re£ erences

[l] COWTS, T.J., (Ed.), "Active and Passive Thin Film Devices." Academic Press London (1978)

[2] KAZMERSKI, L.L., (Rd.), "Polycrystalline and Amorphous Thin Pilms and Devices."

Academic Press, New York (1980).

[3] SEAGER, C.H., J. Appl. Phys. (1981) 3960.

[4] CARD, H.C., and YANG, E.S., IEEE Trans. Electron Devices ED-24 (1977) 397.

[ 5 ] SEAGER, C.A.. and CASTNER, T.G., J. Appl. Phys. Lett. ~ ( 1 9 7 8 ) 3879.

[6] WANG, W., CARD, H.C., and YANG, E.S., Appl. Phys. Lett. E(1980) 315.

[7] SEAGER, C.H., and PIKE, C.E., Appl. Phys. L e t t x (1980) 747.

[8] POSSüK, J.G., and LINDHOLM, F.A., IEEE Trans. Electron Devices ED-27 (1980) 632.

[9] SEAGER, C.H., and PIKE, G.E., Appl. Phys. Lett. 37 (1980) 747.

(101 LEAMY, H.J., PIKE, G.E., and SEAGER, C.B., (~ds.)~rain Boundaries in Semicon- ductors." North-Holland, New York (1982).

[Il] SEAGER, C.H., GINLEY, D.S., and ZOOK, J.D., Appl. Phys. Lett. z(l980) 831.

[12] SEAGER, C . H . , and GINLEY, D.S., J. Appl. Phys. 52 (1981) 1050; Aiso, SEAGER, C.H. SHARP, D.J., PANITZ, J.K.G., and D'AIELLO, R,.v. J. Vac. Sci. Technol. 20

(1982) 430.

[13] PANKOVE, J.I., LAMPERT, M.A., and TARG, M.L., Appl. Phys. Lett. 2 (1978) 439.

1141 SRIVASTAVA, P.C., and BOURGOIN, J.C., in ref. 10, pp. 137-140.

[15] GHANDHI, S.K., BORREGO, J.M., REEP, D., HSU, Y.S., and PANDE, K.P., Appl. Phys.

Lett. 34 (1979) 699.

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[ 1 6 ] KAZMERSKI, L.L., J. Vac. S c i . T e c h n o l . 2q (1982) 423.

[ 1 7 ] FISHER, H., a n d PSCHUNDER, W., P r o c . 1 2 t h IEEE P h o t o v o l t a i c s S p e c . Conf., B a t o n Rouge, pp 86-92. IEEE, New York ( 1 9 7 6 ) .

118) KAZMERSKI, L.L., IRELAND, P . J . , a n d CISZEK, T.F., Appl. P h y s . L e t t . 36 (1980) 323.

( 1 9 1 CHU, T.L., a n d STOKES. E.D., J . Appl. P h y s . 49 (1975) 2996.

[20] KAZMERSKI, L.L., IRELAND, P.J., a n d CISZEK, T.F., J. Vac. S c i . T e c h n o l . 17

(1980) 34.

1211 KAZMERSKI, L.L., Appl. S u r f . S c i . I ( 1 9 8 1 ) 55.

1221 BUCK, O., TIEN, J.K., and MARCUS, H.L., " E l e c t r o n a n d P o s i t r o n S p e c t r o s c o p i e s i n Materials S c i e n c e a n d E n g i n e e r i n g . " Academic P r e s s , New York (1979).

[ 2 3 ] DAVIS, L.E., MACDONALD, N.C., PALMBERG, P.W. RIACH, G.E., a n d WEBER, R-.E.,

"Handbook o f Auger E l e c t r o n S p e c t r o s c o p y . " P h y s i c a l E l e c t r o n i c s , Eden P r a i r i e , Minn. (1976).

[241 RUSSELL, P.E., HERRINGTON, C.R. BURKE, D.E., a n d HOLLOWAY, P.H., i n ref. 1 0 , pp. 185-191.

(251 RUSSELL, P.E., a n d HERRINGTON, C.R., 'Development a n d A p p l i c a t i o n o f F u l l y Automated EBIC T e c h n i q u e s for S o l a r Ce11 Measutements.' I n "Microbeam Analyeis-1962," K.F.J. H e i n r i c h , Ed., S a n F r a n s i c o P r e s s (1982).

[ 2 6 ] RUSSELL, P.E., a n d HERRINGTON, C.R., ' M i c r o a n a l y s i s o f C a n d i d a t e T h i n a n d T h i c k F i l m P h o t o v o l t a i c M a t e r i a l s , ' S c a n n i n g E l e c t r o n M i c r o s c o p y / l 9 8 2 / ( i n - p r e s s ) . [ 2 7 ] COHEN, M.J., HARRIS, J.S., and WALDROP, J.R., i n ' G a l l i u m A r s e n i d e a n d R e l a t e d

Compounds' (Wolfe, C.M., Ed.), pp. 263-269. I n s t i t u t e o f Phys., London ( 1 9 7 9 ) . [ 2 8 ] CARD, H.C., J. Appl. Phys. z ( 1 9 8 1 ) 3671.

[ 2 9 ] CHENG, L.J., a n d SHW, C.M., i n r e f . 1 0 , pp. 105-110.

[30] CLARK, A.H., BURNHAM, N., SHELDON, P., ( u n p u b l i s h e d ) .

( 3 1 1 JAIN, G.C., CHAKRAVARTI, B.C., and PRASAD, A.. J. Appl. Phys. (1981) 3700.

[ 3 2 ] REDFIELD, D., Appl. Phys. L e t t . x ( 1 9 8 1 ) 174.

[ 3 3 ] HIROSE, M., TANIGUEHI, M., a n d OSAKA, Y., J. Appl. P h y s . 5 0 (1979) 377.

DISCUSSION

H.F. MATA&.- 1 ) The a c t u a l c r y s t a l l o g r a p h i c width of a grain-b. i s of t h e order of 10 t o 100 A. E B I C and o t h e r e l e c t r o n i c t e s t s show width of a f a c t o r 100 t o

1000 g r e a t e r . Also your AUGER, SItiS, ESCA - t e s t s show l a r g e a r e a s . How do you e x p l a i n t h i s d i s c r e p a n c y ?

2) Heat t r e a t m e n t o f t e n a l s o d e c r e a s e s t h e g.b. b a r r i e r , i n acciden- t a l l y grown wacker m a t e r i a l a l 1 t h i s i s l e s s c o n t r o l l a b l e than i n monocrystals with one g.b. ( b i c r y s t a l s ) .

L.L. KAZMERSKI.- 1 ) The AES and SIMS a n a l y s i s i n d i c a t e an impurity-width of 30-60 A,

based upon t h e knowledge of t h e s p u t t e r r i b s of the r n a t e r i a l s . Indeed, t h e s u r f a c e photovoltage ( m i n o r i t y - c a r r i e r l i f e t i m e ) i n S i shows a much g r e a t e r width, b u t due p r i m a r i l y t o t h e i n a b i l i t y t o focus t h e l i g h t s o u r c e t o l e s s than ?. 12 Pm. T h e r e f o r e , an i n t e g r a t i n g e f f e c t a c r o s s t h e boundary i s encountered, making t h e width appear much l a r g e r t h a n i t a c t u a l l y i s . Some s i m i l a r e f f e c t s accompany t h e EBIC measurements,j although t h e probe width and a r e a analyzed a r e much c l o s e r t o what i s expected.

2 ) The a n a l y s i s o f b i c r y s t a l s p r o v i d e s a more c o n t r o l l a b l e

experimental c o n d i t i o n . However, t h i s work was concerned p r i m a r i l y with t h o s e g r a i n boundaries encountered i n a c t u a l s o l a r ce11 d e v i c e s .

R.B. WEIL.- 1 ) I n t h e f i r s t AES s p e c t r a you showed, t h e g r a i n was p u r e S i and a t t h e boundary t h e r e was e s s e n t i a l l y no p u r e S i , a l 1 was o x i d i z e d . Does t h i s mean t h a t oxygen d i f f u s e s from t h e a i r t o t h e g r a i n b o u n d a r i e s ?

2 ) How do you d e t e r m i n e , a - p r i o r i , which r e g i o n o f zone broken s u r f a c e c o r r e s p o n d s t o b u l k m a t e r i a l , and which t o g r a i n b o u n d a r i e s ?

L.L. KAZMERSKI.- 1) These AES d a t a were t a k e n w i t h a background p r e s s u r e % 10-l0 t o r r i n t h e a n a l y s i s chamber d u r i n g and a f t e r f r a c t u r e . A l 1 h o t f i l a m e n t s ( e - g . , t h e i o n i z a t i o n g a u g e s ) were o f f d u r i n g t h e p r o c e s s . Beam c u r r e n t s on t h e AES were low

( > 30 nA, p u l s e - c o u n t i n g ) t o minimize t h e e v e n t o f e-beam induced o x i d a t i o n . The f a c t t h a t t h e a d j a c e n t g r a i n d a t a o f F i g . 1 shows fo oxygen s u p p o r t s t h e c o n d i t i o n t h a t t h e o x i d e was n o t measurement-induced. The s o u r c e o f t h e oxygen i s t h o u g h t t o b e d e f e c t s w i t h i n t h e g r a i n s ( e - g . , d i s l o c a t i o n s , e t c . ) . The h e a t - t r e a t m e n t s t h e m s e l v e s were performed i n non-oxygen c o n t a i n i n g e n v i r o n m e n t s .

2 ) The p r o c e s s i n v o l v e d e x a m i n a t i o n b e f o r e and a f t e r f r a c t u r e i n o p t i c a l and s c a n n i n g e l e c t r o n microscopes. T h i s e x a m i n a t i o n o f t h e topography, e s p e c i a l l y t h e i n t e r s e c t i o n o f t h e g r a i n boundary w i t h t h e s u r f a c e , was t h e b a s i s f o r d e t e r m i n i n g whether t h e f r a c t u r e was t h r o u g h t h e boundary o r t h r o u g h t h e g r a i n .

M. MAUTREF.- Do you s t u d y and n o t i c e any i n f l u e n c e o f t h e h e a t i n g and c o o l i n g r a t e f o r a n n e a l i n g p r o c e s s e s a t a same t e m p e r a t u r e and t i m e d u r a t i o n ?

L.L. KAZMERSKI.- Ce p o i n t n ' a p a s é t é p a r t i c u l i è r e m e n t g t u d i é .