Dielectric carbon films were grown on n- and p-type GaAs and InP substrates using plasmas generated at 30 KHz from gaseous hydrocarbons. The effect of gas source, flow rate, and power on film growth were investigated. Methane and n-butane gases were utilized. The flow rate and power ranged from 30 to 50 sccm and 25 to 300 W, respectively. AES measurements show only carbon to be present in the films. The relative Ar ion sputtering rate (3 KeV) of carbon depends on the ratio power/pressure. In addition, the degree of asymmetry associated with the carbon-semiconductor interface is approximately power-independent. SIMS spectra indicate different H-C bonding configurations to be present in the films. Band gaps as high as 3.05 eV are obtained from optical absorption studies

Liu, D. C.

Pouch, J. J.

Warner, J. D.

English

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" 
Carbon Films Grown from Plasma 
on ill-V Semiconductors 
811~O) CAnHON flI.rtS GROiN FHO" 
{NAS.i-Trt- SElHC ONDUCIURS (N ASA) 12 p 
PLASt1A UN I II -V CSCL 0 9C 
HL A.J 2/M f AJI 
J.J. Pouch, J.D. Warner. 
and D.C. Liu 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
G3/7b 
N8b-12135 
Unclas 
2761~ 
One hundred and sixty-eighth Meeting of the Electrochemical Society 
Las Vegas, Nevada. October 11-17, 1985 
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CARBON FILMS GROWN FROM PLASMA ON III-V SEMICONDUCTORS 
J.J. Pouch, J.D. Warner, and D.C. L1u 
Nat10nal Aeronaut1cs and Space Admin1strat10n 
Lew1s Research Center 
Cleveland, Oh10 44135 
ABSTRACT 
D1electr1c carbon f1lms were grown on n- and p-type 
GaAs and lnP substra~es us1ng ~lasmas generated at 30 KHz 
from gaseous hydrocarbons. The effect of gas source, flow 
rate, and power on f1lm growth were 1nvest1gated. Methane 
and n-butane gases were ut1l1zed. The flow rate and power 
ranged from 30 to 50 scem and 25 to 300 W, respect1vely. 
AES measurements show only carbon to be present 1n 
the f1lms. The relat1ve Ar 10n sputter1ng rate (3 KeV) of 
carbon depends on the rat10 power/pressure. In add1t10n, 
the degree of asyrrrnetry assoc1ated wHh the carbon-
sem1conductor 1nterface is approx1mately power-1ndependent. 
SIMS spectra 1nd1cate d1fferent H-C bond1ng configurat10ns 
to be present 1n the films. Band gaps as h1gh as 3.05 eV 
are obta1ned from optical absorpt10n stud1es. 
Introduct10n 
Carbon f1lms exh1b1ting unique properties can be formed on d1f-
ferent substrates by 10n-beam deposition (1), ion-beam sputter1ng 
(2,3,5,6) and plasma decompos~tion of gaseous hydrocarbons (4-7). 
These f1lms are generally hard and semi- transparent. The electr1cal 
res1st1vity can be greater than 1013 Q- cm8 . The films may be 
useful as opt1cal coatings . In add1tion, they may 1mpact process 
areas 1n the semiconductor technology, e.g., metal - insulator-metal 
(HIM) fabrication (9), and device encapsulation and pass1vation. 
There 1s presently an 1ncreasing need to grow dielectr1c films 
on III-V semiconductors for metal - insulator-semiconductor field -effect 
transistor (MISFET) applications (10). The d1electr1c must be a h1gh 
quality insulator, and the depos1tion process must not 1ntroduce a 
high interface state dens1ty which would degrade the device 
performance. 
In this invest1gation, carbon films were grown on GaAs and InP 
substrates by means of plasma chemical vapor deposition (CVD) at 
30 KHz. The 1nfluence of growth conditions on film properties were 
studied by Auger electron spectroscopy (AES), x-ray photoelectron 
spectr'oscoPY (XPS), secondary ion mass spectrometry (SIMS) and 
UV/VIS/NIR sp~ctrophotometry. Breakdown fields were obtained from 
... , 
electr1cal mea~urements performed on metal-1nsulator-sem1conductor 
(MIS) structures (carbon was used as the 1nsulator). 
Exper1mental 
A planar plasma reactor operat1ng at 30 KHz was ut111zed for the 
carbon depos1t10ns. The power was capac1t1vely coupled to the upper 
electrode, and the bottom electrode and chamber wall were grounded. 
The chamber pressure was controlled by the 1nput gas flow rate and 
pump1ng speed. The gas sources were CH4 and C4H10' The substrates 
were <100> GaAs and InP crystals grown by the 11qu1d encapsulated 
Czochra1sk1 (LEC) method. Dopant concentrat10ns were -1017 cm-3; 
room temperature mob1l1t1es were -2700 cm2/(V-sec). After standard 
clean1ng (4), the samples were placed on the grounded anode 1ns1de 
the chamber. The background pressure was -20 mTorr. The depos1t10n 
gas (CH4 or C4H10) was used to flush the system three t1mes pr10r 
to each run. Amorphous carbon f11ms were produced by subject1ng the 
substrates to the ac glow d1scharge. The power and flow rate sett1ngs 
covered the ranges 25 to 300 Wand 30 to 90 sccm, respect1vely. The 
1n1t1al substrate temperature was 25 °C, and 1t 1ncreased a few 
degrees dur1ng each depos1t1on. 
In order to study the electr1cal propert1es of the MIS struc-
tures, an ohm1c contact was made to the rear surface of an-type GaAs 
wafer by thermal evaporat1on of Au-Ge, N1, and Au. Annea11ng was 
performed 1n a N2 amb1ent at 425 °C for 5 m1n, followed by thermal 
evaporat10n of Al onto the carbon f11m surface. 
~esults and D1scuss1ons 
Auger depth prof1les of the f1lms were obta1ned us1ng a PHI 
AES/XPS/SIMS system 1nterfaced w1th a mult1ple techn1que analyt1cal 
computer system (MACS). Sputter1ng was done w1th 3 KeV Ar 10ns at 
25 rnA. The sens1t1v1ty of the AES measurements 1nd1cated that tne 
f1lms conta1ned only carbon 1n the bulk, and a few percent of oxygen 
at the f1lm-sem1conductor 1nterface. These results were 1ndependent 
of the source gas used for the depos1t10ns. A representat1ve AES 
prof1le of a carbon f11m depos1ted from C4H10 on InP 1s shown 1n 
F1g. 1. The flow rate 1s 30 sccm (corre~oond1ng to a steady-state 
pressure of 175 mTorr), and the power 1s 25 W. 
The relat1ve Ar 10n sputter1ng rate, G, at f1xed power, 1s 
def1ned by the re1at1on G = tp/td' where tp 1s the AES pro-
f1l1ng t1me for the carbon f1lm and td 1s the depos1t10n t1me 
(11). F1gure 2 shows the dependence of G (for 3 KeV Ar 10ns) on 
the rat10 power/pressure (proport1onal to averagp. energy per part1cle) 
for two C4H10 pressures: 175 and 245 mTorr. We note that G 
becomes sens1t1ve to the chamber pressure for power/pressure rat10s 
ZO.85. 
2 
,~ 
, 
The degree of asymmetry associated with the carbon-semiconductor 
interface is proportional to 0 = Bx(TR - 2TM + TL) where B 
is a constant, and TR, TM, and TL are the AES sputtering 
t\mes corresponding to the carbon peak - to-peak s1gnals at 10, 50, and 
90 percent of the mox~mum atta1ned 1n a depth prof1le, respect1vely. 
The 1nfluence of CH4 flow rate on 0 1s presented 1n Fig. 3 for 
several powers. The ~ :: ~'ts 1nd1cate that 0 1s approxlmately power-
1ndependent 1n the 50 to 300 W range (flxed pressure). It appears 
that 0 (flxed power) approaches zero (symmetrlc 1nterface) as the 
chamber pressure 1s 1ncreased. 
A 10 KeV Al KQ x- ray source was used for XPS stud1es . The 
deposition gas was CH4 and the power varied from 25 to 300 W. It 
was determ1ned that the asymmetry 1n the C ls 11neshape decreased 
with power for a total chamber pressure of 245 mTorr . Th1s trend 1s 
presumably influenced by the amount of hydrogen conta1ned in the 
f 11 ms . 
Relative counts of hydrocarbon ions sputtered from carbon 
(substrate-InP) were de t ermined from SIMS depth profillng studles 
(performed wHh 3 KeV Ar ions). In F\g. 4(a), the d1str1bution of 
ion counts is plotted as a fun ction of mass-to- charge ratio fo r var-
10us deposltion cond1t i on s (C4H10 plasma). The predominant ion 
is (CH)+, and contributions from (CH2)+' (CH3)+' (C2H)+, (C2H2)+' 
and (C2H3)+ are evident . 
Figure 4(b) shuws the ion distr i bution obta1ned from carbo' f1lms 
prepared by a methane discharge . We note that (CH)+ has a higher 
probability of being sputter ed fr om each film. (In comparison, 
F1gs. 4(a) and (b) indicate that the lowest populations are associated 
w1th (CH3)+') Th ese r es ult s de pict some of the H- C bonding 
arrangements whi ch ar e preval ent in t he plasma-d eposited f11ms (12) . 
A SIMS depth profile (3 KeV Ar i o s ) of carbon deposited on GaAs 
is presented in Fi g. 5 . Mas s windows for Ga +, As+ . 0+, and CHx+ 
(x = 0,1,2,3) were uti l ized . Th e CHx + di strib uti on s ar e uniform in 
the bulk of the fi l m, and they drop to l ower levels in the substrate 
reg1on. In addition, Fig . 5 sh ows oxygen to be present throughout 
the carbon film. (Thi s determination wa s not possible with the less-
sensitive AE S techniqu e. ) Finally, it 1s appar ent tha t Ga+ and As+ 
are read\ly dete cte d as the carbon film is sputtered . 
Figure 6 shows a SIMS depth profil e of car bon on InP . In+, P+, 
0+, and CHx+ (x = 0,1,2,3) were monHored. The CHx+ (x = 0,1,2) 
distributions are unif orm in the film r egion, and progress1vely lower 
level s are attained in t he vi ci nity of the film- InP interface. We 
note that the 0+ and CH~ profil es are "noi sy. " Figure 6 illustrates 
that In+ and P+ conce nt rati on s are being detected during the film 
sputtering proce ss . 
3 
1 
1 
~ . ~----------------------------~~~--- ---------------~~--------------~.~ .. 
Measurements of opt1cal absorpt10n as a funct10n of photon energy 
for carbon f1lms depos1ted on fused s1l1ca plates were made w1th a 
UV/VIS/NIR spectrophotometer (Perk1n-Elmer, model Lamda 9). Pr10r to 
the f11m measurements, the sample compartment was cont1nuously purged 
w1th dry N2, and a scan of the background for data correct1on was 
made over the photon energy range. A s11t w1dth of 2 nm was used for 
the f1lm measurements. In add1t10n, a blank fused s1l1ca plate was 
placed 1n the reference beam path. The d1fferences 1n f1lm and s1l1ca 
plate reflect1v1t1es were not taken 1nto account . 
F1gure 7 1llustrates the dependence of (AE)1/2 as a funct10n 
of E for a carbon f1lm depos1ted on the s1l1ca plate. The absorb-
ance 1s A and the photon energy 1s E. (The C4H10 flow rate and 
power were 50 sccm and 150 W, respect1vely.) In order to determ1ne 
the opt1cal band gap, Eg, we referred to a theoret1cal model descr1b-
1ng 1nterband opt1cal absorpt10n 1n a noncrystal11ne so11d (13). We 
est1mate E9 - 3.05 eV. In compar1son, we note that Eg for carbon 
f1lms on InP obta1ned from plasma decompos1t1on of CH4 1s typ1cally 
<2.35 eV (11). 
Capac Hance-voltage measurements (1 MHz) of t \.e MIS structures 
on GaAs y1elded a flat C-V curve. Th1s results suggests Ferm1 level 
p1nn1ng of the carbon-GaAs 1nterface. The phenomenon 1s probably 
attr1butable to the ex1stence of a large 1nterface state dens1ty. 
Breakdown f1e1ds (V/cm) and d1electr1c constants are summar1zed 
1n Table T for var10us depos1t10n cond1t'ons. The power was f1xed at 
100 W. The breakdown strength of the carLon f1lms can therefore be 
enhanced by subst1tut1ng C4H10 for CH4 1n the depos1t10n process. 
In add1t10n, we note that the d1electr1c constants for as-prepared 
f1lms are in the range 2.5 to 4.3. 
Conclus10ns 
We have demonstrated that carbon f1lms can be depos1ted on GaAs 
and InP by means of plasma riecompos1t10n of gaseous hydrocarbons 
(CH4 and C4H10)' AES measurements show only carbon to be present 
in the f11ms. The relat1ve Ar 10n sputtering rate (3 KeV) of carbon 
depends on the ratio power/pressure. In addition, the degree of 
asymmetry associated with the carbon-semiccnductor interface is 
approximately power-independent. SIMS spect-a 1nd1cate d1fferent H-C 
bo~~ing configurations to be present in the films. Band gaps as h1gh 
as 3.05 eV are obtained from optical absorpt1on stud1es. 
Acknowledgment 
We thank Dr. Samuel Alterovitz for review1ng this manuscr1pt and 
for stimulating discussions . 
4 
I 
I 
" I 
.. , 
, 
REFERENCES 
1. S. A1senberg and R. Chabot, J. Appl. Phys. 42, 2953 (11). 
2. B.A. Banks and S.K. Rutledge, J. Vac. Sci. Technol. fl, 801 (82). 
3. A.A. Khan, J.A. Woollam, Y. Chung, and B.A. Banks, IEEE Elect. 
Oev. Letts. EOL-4, 146 (83). 
4. J.n. Warner, J.J. Pouch, S.A. Alterov1tz, D. C. L1u, and 
W.A. Lanford, J. Vac. Sc1. Technol. A3, 900 (85). 
5. C.W. We1ssmantel, G. Re'sse, H.J. Erler, f. Henny, K. Bew1logua, 
U. Eoersback, and C. Shurer, Th1n Sol1d films 63, 315 (19). 
6. O. Math1ne, R.O. 01110n, A.A. Khan, G. Bu-Abbud, J.A. Woollam, 
D.C. L'u, B.A. Banks, and S. 00m1tz, J. Vac. Sc1. Technol. A2, 
365 (84). 
1. R.O. 01llon, J.A. Woollam, and V. Katkanant, Phys. Rev. B29, 3482 
(84) . 
8. K. fuz11, N. Shohata, M. M1kam1, and M. Yonezawa, Appl. Phys. 
Lett. 41, 370 (85). 
9. J. Lamb and J.A. Woollam, J. AlJpl. Phys. ~, 5420 (85). 
10. D.C. L1u, G.J. Valco, G.G. Skebe, and V.J. Kapoor, E1ectrochem. 
Soc. Proc. 83-8, 161 (83). 
11. J.J. Pouch, S.A. Alterov1tz, J.~. Warner, D.C . L'u, and 
W.A. Lanford, Mat. Res. Soc. Symp. Proc. San Franc1sco, CA, 
Apr11, 1985 (accepted for publ1cat10n). 
12. A. Benn1nghoven, Surf. ScL 53,596 (75). 
13. N.F. Mott and E.A. Dav's, ELectron'c Processes 1n Noncrystall1ne 
Mater1als (Clarendon Press, Oxford) 291 (79). 
5 
TABLE I. - DIELECTRIC CONSTANTS AND 
BREAKDOWN fIELDS fOR CARBON f I LMS 
DEPOSITED ON n- GaAs 
Depo ~ \tion Dielectric BreaKdown 
parameters constant field . 
V/cm 
CH •• 1 DO W 3.4 1 . ax1 06 
30 sccm 
CH •• 100 W 4.3 6. Sx10S 
SO seem 
C4H10. 100 ft 3. 2 b. Ox10b 
30 sccm 
C41i10. 100 W 2. S S. bx10b 
50 seem 
/ 
____________ ...L-- __ _ ,J; _ _ _ __ _ • _ • _. __ _______________ _ _ _____ _ 
V> 
~ 
Z 
:;:J 
>-
'" ... 
eo 
co 
'" 5 
<..:> 
• ,1 
: . 
80 
60 
40 
20 
a 
~(\ ppor IL( 0111 8/ 85 v57L1 U 
Figure 1. - AE5 depth profile ot carbon on InP. Deposition parameters : 
t::4HIO. 30 seem. 25 W. 
0 175 mTo rr 
6 245 mTorr 
6 0 0 
::. 
0 
6 
0 
60 6 I I 
. 4 .6 .8 1.0 1.2 
POWER IPRESSURE. watts ImTorr 
Figure 2. - Relative Ar ion sputtering rate as a tunetion ot the ratio power I 
pressl .re tor carbon on InP. 
( , 
... , 
1.4 
12 <> g 
t:,. 0 25W 
t:,. 50W 
;;, 8 0 lOOW 
~ '\l 150W 
~ 
::l <> 200W 
>- 0 0 300 ~ ' 
coo: 
c: 4 ~ 0 
... 
coo: 0 t:,. g 
~ 
00 ~ 0 0 0 
-4 I I I I I 
'~ 30 40 50 60 70 80 Q() 
FLOW RATE Iseeml 
Figure 3, - b versus CH4 110w rate for carbon on InP , 
, 3 I <> 30 seem, 38 W 1:::. 30 seem, 150W 1:::. 30 seem , 150 W o 50 seem, 50 W 0 50 seem, 50 W o 50 seem, 150 W 0 0 5Osccm, I50W 
0 
, 2 
.... I CH ;z 0 ::l CH 
0 CzH38 1:::.\ u 
;z 0 
Q .. 
.., 
" ~ S ~\C2 cz): .., <> 
'" 
.1 
0 
\ 
CHZ Cii)~ jCzHz I 
o lal 8 CH 3 <> I fl CzH3 Ibl CHZ B CH3 I C2 ~ I 
10 B 20 o 30 10 20 3n 
m/e 
Figure.l , - Relat ive ion count as a function of mass-to-charge ratio for carbon deposited on inP 
uSing C4HI0 lal and CH4 plasmas Ibl . 
~( 
pPor IL£ 
: La;) 
' f' 
OR; ' ,I • ,_ r .,: ...  IS 
OF POOR QUALITY 
, I 
"-. , 
~L-____ -L~ __ ~ 
- t '1 ' ",' ~ ~U[ ~' ________ ~.~-'~ _______ ~
, .Z l l. ~ 2: .\1 32 .• r' <a. 
: ' U p 11 "[ '"IH, ' 
Figure 5, - SIMS depth profile of ca rbon on GaAs, 
,< .8 
.:1 : ppor IL£ 88 e 8, JPI " 
'\ 
i , i 
:=l t 
r, 
, " .a .. 
_~I , ~: . e t ~ 1;, 
. I ~ . , 
...J lf< ~ 1 ~ t· , , , ' I 
;c~ "r W a: 
· -~'~ :I t ..... 
• t., ;"': 
· -!.-. ... q 
· -......... ---:..-----.--...., 
,. . .a .. ' 
./ --------------
: , 
Figure O. - SIMS depth profile of ca rbo n on InP, 
1 
--------------------------====~====~~- -----------~-------------~--~ 
... 
2. 5 
2 
1.5 
. 5 
---~--'" O L-------~ ____ _L __ _L ________ ~ ________ ~ ______ ~ 
1.5 4. 5 6.5 
[ (t Il) 
Figure 7. - (A[11/2 versus [ for carbon on fused silica. Tht straight line is a least squares 
fit to the data. Deposlt l,n parameters : C4HID. 50 seem. 150 W. 


Carbon films grown from plasma on III-V semiconductors

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