Minority carrier lifetime distributions in silicon web dendrites are measured. Emphasis is placed on measuring areal homogeneity of lifetime, show its dependency on structural defects, and its unique change during hot processing. The internal gettering action of defect layers present in web crystals and their relation to minority carrier lifetime distributions is discussed. Minority carrier lifetime maps of web dendrites obtained before and after high temperature heat treatment are compared to similar maps obtained from 100 mm diameter Czochralski silicon wafers. Such maps indicate similar or superior areal homogeneity of minority carrier lifetime in webs

Dumas, K. A.

Koliwad, K.

Schwuttke, G. H.

English

NASA Technical Reports Server

• E1~~rica1 and Structural qharacterization of Web Dendr1te Crystals 
G. H. Schwuttke 
Arizona State University 
Tempe, AZ 85287 
, 
K. Kol1wad and K.A. Dumas 
Jet Propulsion Laboratory 
California Institute of Technology 
Pasadena, CA 91109 
Abstract 
Minority carrier lifetime distributions in si11con web dendr1tes are 
measured. Emphasis is placed on measuring areal homogeneity of lifetime, show 
its dependency on structural defects, and fts unique change dur1ng hot 
processing. The internal gettering action of defect layers present in web 
crystals and their relation to m1nority car~ier lifetime distribut10ns is 
discussed. Minority carrier lifetime maps of web dendr1tes obta1ned before ana 
after high temperature heat treatment are compared to s1mf1ar maps obta1ned from 
100 mm diameter Czochra1ski silicon wafers. Such maps indfcate sim11ar or 
superior areal homogeneity of m1nority carrier lifetime in webs. 
Introduction 
Recently, calculations have been made wh1ch show that the presence 'of areal 
inhomogeneity of minority carrier lifetime in si11con wafers can devastate solar 
cell efficiency (1). For instance, it was shown that for no more than 5% of the 
solar cell area being inferior compared to the rest of the eel I area, solar cell 
efficiency can be as poor as ft would be if the entire area ~ou1d be of inferfor 
quality (1). Thus areal homogeneity of m1nority carr1er 11tetime is an 
important requirement for high efficiency solar ce1 IS. 
This paper reports measurements of minority 
"distributions" made on silicon web dendr1te sections. 
compared to similar data obtained from 100 mm diameter 
wafers. 
Structural Aspects Of Silicon Web 
carrier 11fetlme 
Such measurements are 
Czochra1sk1 5i11con 
A brief description of the as-grown web geometry and dis1ocat10n structure 
is needed to describe our electrical measurements clearly. For detailed results 
we refer to the original papers on web growth (2) and dis1ocat10n structures (3-
5>' 
The web surface 1 s ·paralle1 to the <111> p1 ane, and the web pu111 ng 
direction is [2iil. A cross-sectfon of web shows its most prom1nent structural 
feature: multiple twin lamellae in the center of the web covered by two thick 
(100 pm) surface layers which are also in twin relation. An example is shown in 
Figs. 1a,b. . 
PRECEDING PAGE BLANt( NOT FILMED 
279 
https://ntrs.nasa.gov/search.jsp?R=19850023320 2020-03-20T17:32:08+00:00Z
• 
Dislocations in web are generated through "melt entrapment" caused by 
"wing" growth along the dendrites (Fig. 2). Propagation of such dislocations 
depends on the curvature of the solid-liquid interface of the web during growth. 
An interface curvature of concave downward concentrates the dislocations in the 
center of the web (Fig. 3). Webs grown with a straight interface concentrate 
the dislocations in the fillet region. Thus the web grows free of any 
dislocations (4). Fig. 4. Most webs_..<>f today contain bundles of very long 
dislocation lines parallel to the [211] direction (Fig. 5). They are piled up 
against the twin planes. In addition. annealed web contains large area stacking 
faults. The electrical activity and the influence of these faults on sol ar cell 
efficiency has not yet been investigated. Such an influence is assumed to be 
relatively small but it may be of importance for high efficiency solar cells. 
These defects are described in more detail in References 3 and 5. examples are 
shown in the x-ray topographs of Figs. 6. 
If excessive stresses are present during web growth (buckling) large 
numbers of slip dislocations and small angle grain boundaries may occur. Such 
defects degrade minority carrier lifetime. An example is shown in the x·-ray 
topographs of Figs. 7. 
Minority Carrier Lifetime Measyrement In Web 
Silicon web dendrite sections of various quality (Figs. 5.6,7) were 
evaluated through minority carrier lifetime "mapping" using 1.5 mm diameter MOS 
capacitors. The total number of MaS capacitors per web section was 300. The 
MaS dots were arranged into 15 rows. The dendrites were i ncl uded f nto the 
evaluation. The dimensions of the web sections were 2 inches long and 32 mm 
wide. The webs were e~aluated after each oxidation cycle <15 min wet-liO min 
dry-15 min wet at 1000 C. Three or four consecutive oxidation cycles were used. 
Minority carrier lifetime maps of the total web section were obtained after each 
oxidation cycle by measuring the lifetime under each capacitor as described by 
Fahrner & Schneider (6). Subsequently. the lifetime was computer plotted versus 
its position on the web surface. Local lifetime degrading defects revealed by 
such lifetime maps were analysed by advanced characterization techniques. A 
result typical for web sections of quality as shown in Figs. 5 and 6 is given in 
F·~g. 8. This figure summarizes the influence of high temperature heat cycles on 
minority carrier lifetime. The lifetime data are displayed in 6 maps. Each map 
represents a matrix of 300 capacitor dots. The maps on the 1 eft si de of the· 
figure carry -the label oxidation 1 to 3 and represent the lifetime data obtained 
after the first. second. and third oxidation cycle. The devices were obviously 
stripped before every successive oxidation step. The z~5cale represents the 
measured lifetime values in microseconds. The range of lifetime data present on 
t~e web section is also included in the label. For instance. the minimum 
lifetime obtained at a spec1fic location (capacitor) after the fi rst oxi dati on 
was 0.274 microseconds. The maximum lifetime is 207 m1croseconds. The lifetime 
map shows a relatively smooth plateau. Noteworthy is, that this plateau extends 
over the total web section surveyed. Accordingly. this web is of ~xcellent 
crystal quality and compares well with the best Czochralski silicon as available 
. today. Interesting is a comparison of the maps obtained after the first. second 
and third oxidation •. The lifetime plateau obtained after the second oxidation 
is well above the one obtained after the first oxidation and tne same is true 
for the third oxidation. The third plateau is a little bit lower than the 
second one. Using Czochralski wafer qu~lity as a standard. the 
280 
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material obtained after the th1rd oxidation shows very uniform l;fetime 
distribution data. 
The maps on the r1ght side of Fig. 8 are labeled DECREASE or INCREASE. The 
DECREASE map shows the degradation of lifetime as a result of the second or 
third oxidation cycle. The INCREASE map shows the area where the 11tetime 
increased after the third oxidation relative to the f1rst ox1dat10n. These maps 
show clearly the improvement of 11fetime as a result of ox1~ation. Th1s 
improvement is fairly un1form and covers pract1cally the total web area. 
Decreases in lifetime occur only along the dendr1tes, the. f111et region or along 
the edges where the web section was cut by the d1cing saw. This result 1s 
unique and rel ates to the spec1 al crystal structure of the "perfect" web. The 
defect pl anes, sandwiched between the perfect web surtace layers, provide for 
"internal" gettering during high temperature heat cycles. Thus a substantial 
improvement of minority carrier lifetime can result. 
Web mater1al of the quality as d1splayed in the topographs of Figs. 7 do 
not g1ve comparable results. It was found that the "s11p" dislocations - wh1ch 
result from buck11ng stresses - degrade m1nor1ty carries 11fet1me substantially. 
These dislocations are of the same type as the "process" induced slip 
dislocat10n encountered during IC processing. It is known that the electr1cal 
activity of these d1s10cations is high. The lifetime map shown in Fig. 9 
corresponds to the sample shown in Fig. 7b. Note the good correlatl0n between 
low lifetime and defect area in the center of this sample. 
Minority Carrier Lifet1me Measurements in CZQChralsk1 Silicon 
Recently, we conducted an evaluat10n of 100 mm oiameter Czochralsk1 si11con 
wafers as availabe from major vendors throughout the world. The evaluat10n 
concentrated on m1nor1ty carr1er lifet1me mapping as descr1bed for the web 
crystal. Four consecutive oxidation cycles were used. The MOS dot ma~r1x 
cons1sted of 25 times 25 MOS capaCitors of 1.5 nm diameter. A result typ1cal 
for the evaluated Czochralsk1 wafers is shown 1n F1g. 10. The 11fetime data 
shown 1n F1g. 10 are d1splayed 1n 4 maps. The maps on the left carry aga1n the 
oxidat10n number. The data obtained after the f1rst ox1dation indicate 
excell ent wafer qua11ty. The map has a pl ateau at approx. 250 to 300 
m1croseconds and extends pract1cally across the total wafer area. This wafer 
represents si11con of the best quality. Nevertheless, success1ve ox1dation 
results 1n steady degradat10n of 11fetime. Th1s is seen by compar1ng the maps 
obtained after the f1rst and fourth oxidat10n. 
Interesting are the DECREASE and INCREASE maps shown on the r1ght of F1g. 
10. The decrease mapconf1rms that degradat10n occurs uniformly across the 
total wafer area. After the fourth oxidation cycle small l1fet1me increases are 
observed only at 9 locations. In all other areas 11fetime degraded 
substantially. 
Discussion and Summary 
The data given in Figs. 8 and 10 aro representative of "perfect" web and 
Czochralski silicon wafers. We note a basic difference botween these two 
materials. Oxidation increases generation lffetime in Web but decreases the 
281 
same lifetime in Czochra1ski material. The decrease in lifetime in Czochra1ski 
wafers correlates with the precipitation phenomena of oxygen in s11 tcon (7). 
The large extent to which areal inhomogeneities exist in todays sil1con. ana 
influence device performance (1) is not well known. However, uniform m1nority 
carrier lifetime distributions are a pre-requisite to high performance solar 
cell technology. Solar cell efficiency in excess of 151 for large area single 
crystal silicon cells (10 cm x 10 cm) will be very difficult to achieve without 
proper control of minority carrier lifetime distributions during large scale 
manufacturing. 
References 
1. F. A. Lindholm et a1., Solid State Electronics, 23, 967, (1960). 
2. S. O'Hara and A. I. Bennett, J. App1. Phys., 35, 409, (1964). 
3. S. O'Hara and G. H. Schwuttke, J. Appl. Phys., 36, 2475, (1965). 
4. T. N. Tucker and G. H. Schwu'ttke, App1. Phys. Letter, 9, 219, (1966). 
5. E. S. Meieran and 1. B1ech, IJ. App1. Phys., 38, 3495, (1967). 
6. W. Fahrner and C. P'. Schneider, J. E1ectrochem. Soc., 123, 100, (1976). 
7. G. H. Schwuttke, Semiconductor Industry Association (SIA) Lecture, Stanford 
Univ., Sept. 1981, Palo Alto, CA. 
282 
F1gure Capt10ns 
Fig. 1 a. Web Crystal Geometry 
b. SEM-micrograph of cleaved web cross-section after preferential 
etching. Note twin lamellae in center. 
Fig. 2 SOT X-ray topograph of 2 cm wide web dendr1te. Note w'ng growth 
(arrows) at dendrites below web leading to liquid entrapment ana 
dislocation generation during growth. 
Fig. 3 SOT X-ray topograph of 3 cm wide web dendr1te. Note disl'ocatl0n 
generation at stress centers and propagation of dislocat10ns during 
growth toward center. 
Fig. 4 SOT X-ray topograph of perfect 1.5 cm wide web. Dislocat10ns are 
trapped in fillet region. 
Fig. 5 SOT X-ray topograph of high performance web 22D-reflection. Note long 
dislocations, 73 degree type; Burgers vectors [lOll, [110l. Low 
electrical activity. 
Fig. 6 SOT X-ray topographs of high performance web. Width 3.2 cm. 
a. (111) reflection: Surface twin lamellae 
b. (iii) reflection: Bottom twin lamellae 
Note difference in crystal perfection: Bottom lamellae contains large 
area stacking faults. Topographs recorded after th1rd oxidation. 
Electrical activity of defects low. Electrical data in Fig. 8. 
Fig. 7 a. SOT X-ray topograph of low quality web. Note slip dislocat10ns. 
Electrical activity high. Lifetime range: 0.01 - 1 psec. 
b. SOT X-ray topograph of medium quality web. Electrfcal data in 
Fig. 9. 
Fig. 8 Minority carrier lifetime maps of h1gh performance web. Lifetime 
range: 0.2 - 207 psec. 
Fig. 9 Minority carrier lifetime map of low performance web. Corresponding 
SOT topograph is shown in Fig. 7b. Note correlatl0n between 
dislocation density and low lifetime. Lifetime range: 0.1 - 50 psec. 
Fig. 10 Minority carrier lifetime map of Czochralski wafer for comparison. 
Lifetime range: 4 - 478 psec. 
283 
Growth Di recti on 
win mella 
Fill t 
(Ill) Face 
of Web 
Cleavage Surface 
Typical Regions of Liquid Inclusion 
Fig. la Web Crystal Geanetry 
284 
ORIGINAL PAGE' 
OF POO QUAL r( 
Fig. Ib SEM-micrograph of cleaved web cross-section after preferential 
etching. Note twin lamellae in center. 
285 
Fig. 2 SOT X-ray topograph of 2 em wide web dendrite . Note wi ng growth 
(arrows) at dendrites below web leading to liquid entrapment ana 
d1s1ocation generation dur1 ng growth. 
286 
ONGiNAL PAoE' 
OF POOR Q ALrrY 
Fig. 3 SOT X-ray topograph of 3 cm wide web dendr1te. Note dislocation 
generation at stress centers and propagation of dislocations during 
growth toward center. 
281 
Fig. 4 SOT X-ray topograph of perfect 1.S em wide w.b. Dislocations are 
trapped in fillet region. 
288 
Fig. 5 S T X-ray topograph of high performance web, 22o-reflect10n. Note long 
dislocations, 73 degree type; Burgers vectors [101], [110]. low 
electrical activity. 
289 
Fig.6a SOT X-ray topographs of high performance web. Width 3.2 cm. 
(111) reflection: Surface twin lamellae. 
Electrical data in Fig. 8. 
290 
Fig. 6b SOT X-ray topographs of high performance web. Width 3.2 cm. (lIT) reflection: Bottan tw i n lamellae 
Note di fference in crystal perfection: Bottan lamellae contains 
large area stacking faults. Topographs recorded after thlrd 
oxidation. Electrical activity of defects low. 
291 
Fig 7a SOT X-ray topograph of low quality web. Note slip dislocations. 
Electrical activity high. Lifetime range: 0.01 - 1 ~sec. 
292 
o L P 
OF OOR QUALITY 
Ffg 7b SOT X-ray topograph of medfum quality web. Electrfcal data fn Fig. 9. 
293 
Oxidation 1 
NO. OF OXIDATIONS: 1 
DATA RANGE: .2740 207 
Z.sCALE: MICROSECONDS 
-
... 
400 
eb Dendrite Silico 
Lifetime Distribut= Data 
D as 1 to 2 
OXIDATION NUMBERS: 1 2 
DATA RANGE: .5000 200 
Z-8CALE: MICROSE 0 0 
-
-
-.. 
::]. A _~~ ~~ ::~. ~@@mI~ 
~I~"""""'" 
------Oxidation 2 
NO. OF OXIDATIONS: 2 
DATA RANGE: .2120 808 
L SCALE: MICROSECOND 
-
... 
o ...... .. ........... 
.. " uu",a, ~~-.--
Decrea$e 1 to 3 
OXIDATION NUMBERS: 1 3 
DATA RANGE: .1000 183 
Z-8CALE: MICROSECO DS 
-
-
-.. 
o ......- ......... 
.. 
:100 
:100 
100 
O~~~ ... 
• 
.. ~~~~~~ii~~~~~~~ ,. o' • 
.. ""I' ..... ··· .. · tL. ..-... ___ _ 
Oxidation 3 
NO. OF OXIDATIONS: 3 
DATA RANGE: .,.,0 452 
Z·SCALE: MICROSECOND 
-... 400 
:100 
:100 
100 . 
0-
• 
.. .,."., ...... .. , .. &.- ----- ...... .. . 'f ... ...... 
Increa e 1 to 3 
OXIDATION NUMBERS: 1 3 
DATA RANGE: .,.,0 305 
Z-8CALE: MICROSECOND 
-
-
-.. 
o 
.. -:.W~~~~~~RX 
o 
Fig. 8 Minority carrier lifetime maps of high performance web. 
lifetime range: 0.2 - 207 ~sec. 
294 
~ ...... ... , ...... 
....... ... ........ 
Fig. 9 Minority carrier lifetime map of low performance web. Corresponding 
SOT topograph is shown in Fig. 7b. ~ote ' correlation between 
dislocation density and low lifetime. Lifetime range: 0.1 - 50 ~sec. 
295 
Lifetime Data Distribution Map 
Vendor B 
Oxidation 1 
NO. OF OXIDATIONS: 1 
DATA RANGE: 4.0000 478 
Z-SCALE: MICROSECO D 
p.er.r.r .001 ... , ..... 
Oxidation 4 
NO. OF OXJDATIONS: 4 
DATA RANGE:.53OO 121 
Z-SCALE: MICROSECONDS 
-.. 
-:III .. ,. 
o 
p ........... ....... ... . 
o -- ... _ ... 
o .. ... ..,. 
Decrease 
OX.IDAnoN NUMBERS: 1 4 
DATA RANGE: 7.0000 442 
Z-SCALE: MICROSECO D 
-
-
-
-
-
, . 
• 
Increase 
OXIDATION NUMBER.S: 1 4 
DATA RANGE: 2.0000 41 
Z-5CALE: MICROSECONDS 
-
-
-
-
-
,. 
• 
,. 
'0 
• 
Phall "'" • • o 
..... -----... 
'1 
'0 
..... _ ... _ ... 
Fig. 10 t4inority carri er:- lifetime map of Czochra1ski wafer for comparison. 
Lifetime range: 478 ~sec . 
29 
1 
DISCUSSION 
BANOKA: Do you have any idea why the grown dislocations seem to be not elec-
trically bothersome, whereas the stress-induced ones are? 
SCHWUTTKE: First of all, the core structure of this grown dislocation is very 
benign. It .akes an angle of 730 , and this type of dislocation has been 
described in the literature. This structure is very benign, while a 600 
dislocation, which is a ~lip dislocation, is very active due to its core 
structure. 
297 


Electrical and Structural Characterization of Web Dendrite Crystals

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