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Oxidation of silicon-germanium alloys
Journal of Crystal Growth 17 (1972) 288—297 © North-Ho/land Publishing Co.
OXIDATION OF SILICON-GERMANIUM ALLOYS S. MARGALIT, A. BAR~LEV*,A. B. KUPER**, H. AHARONE and A. NEUGROSCI-IEL Faculty of Electrical Enqineering, Technion, Haifa, israel
Oxidation of Si/Ge alloys, of up to 370 Ge, were investigated in view of the material utilization for MOS devices. Both single crystals and homogeneous polycrystalline Si/Ge, mainly in [Ill] direction, were used. The oxide formed after first oxidation was found to be Si0 2 only, substantiated by several types of measurements. The Ge piled up at the interface. Higher order oxidations included an increasing amount of Ge. MOS capacitors and transistors were produced and showed higher surface State density and negatively charged oxides.
1. Introduction Silicon and germanium are the two most useful semiconductor materials today. The Si—Ge phase diagram1) shows that they can form solid solutions at any desired composition. To utilise the Si/Ge alloy for device purposes, it must usually be in a single crystal from. This can be realised by Czochralski pulling form the melt method, using very slow pulling rates and only up to a few atomic ~ Ge content. Higher Ge concentration, up to 30 at%, can be made by epitaxial growth from the vapour, Silicon transistor and integrated circuit technology are based today on using the thermally grown silicon dioxide for diffusion masking and passivation purposes. Thermal oxide is preferable, being denser, and usually cleaner of ionic contamination than deposited oxide, and is a natural by-product of the previous diffusion step. Finally, it is associated with a relatively low Si surface state density, which is an essential requirement for MOS circuit production. From the technological point of view, it is therefore of paramount interest to investigate the oxidation properties of Si/Ge alloys if devices are to be made of them. For the purpose of the oxidation studies reported in this paper, use was made of both single and polycrystalline materials. The first was made by the authors On sabbatical leave in the University College of Swansea, United Kingdom. . 0 . ** Now in Case Western Reserve University, Cleveland, Ohio, *
2) or by epitaxy3). The either bywhere pulling from the melt second uniform composition polycrystals onginally made by RCA for thermoelectric device purposes. All the single crystals and also the polycrystals were mainly (111) oriented. The investigation included the composition and properties of the oxide, thermally grown on the Si/Ge polished wafers and their dependence on the composition and previous oxidation history of the wafer. Segregation effects of the Ge during oxidation were also encountered and studied. Finally, the feasibility of using the oxide for diffusion masking and for MOS device fabrication was checked, and some interesting properties of the resulting devices were measured and the device potentialities discussed. 2. Methods used for evaluation of thermally oxidized Si/Ge alloys The oxides grown on various samples of Si/Ge alloys were compared to thermally grown Si0 2 on silicon, processed at the same run and of approximately the same conductivity. The purpose of the comparison was to determine the growth rate, the amount if any of Ge, GeO, or Ge02 included in the oxide, the possible loss of Ge from the sample via Ge02 formation (which is volatile at the oxidising temperature of above 1000 °C),the etching properties, and the dielectric and refractive constants of these oxides. The following five independent methods of evaluation were used: (a) Optical interference method for thickness (or growth —
rate) and refractive index measurements.
U.S.A.
288
—
OXIDATION OF SILICON—GERMANIUM ALLOYS
289
as only insignificant difference was found in thickness and refractiveThe index between the Si/Ge and (with the pure Si samples. results for wet oxidation the
7000
6000
oxygen bubbling through 95 °C water) carried on at 1063 °Cfor 60 mm, are shown in fig. 1. There is a small
5000
increase in oxide thickness when the Ge content goes up from 0 to l0°~thereafter it is fixed. This increase O
10
20
At
0/0
30
40
Ge in Si - Ge
I
a
A 000 o~
~°°
~
800 ~00 600
same figure. Refractive index measurements also yielded the same value for Si/Ge first oxide and pure Si0 2, but remained unchanged for second and higher order
Fj—
F-
W
0
is similar to that encountered in the parabolic rate constant for wet oxidation of highly doped silicon6). Second oxidation, on the other hand, appears to proceed at a much faster rate and is shown, for comparison, in the
10
At
20 0/
Ge
30 fl
~
Si-Ge
C b) Fig. I. (a) Oxide thickness after 60 mm wet oxidation at 1063 C for 1st and 2nd oxides. (b) Etch rate in buff. HF at 24 CC for 1st and 2nd oxides: (.)first oxidation; (0) second oxidation,
(b) Controlled etching and comparison with pure Si02 etch rate (Ge inclusion will reduce it). (c) Weight measurements for detection of Ge loss from sample and/or its inclusion in the oxide. (d) An electron probe micro-analyser for detecting Ge segregation and pile-up during oxidation, (e) infra-red absorption for detection of Ge—oxygen bonds in the oxide. Those measurements will now be described in some detail, together with the necessary background theory. 2. 1. OPTICAL INTERFERENCE METHOD These are standard methods4’for 5), oxide thickness and refractive index measurements It was discovered that different results were obtained depending on whether the oxide involved was the first one after the polishing/cleaning steps (this will be refenred to as first oxidation from now on), or if it was re-oxidised after one or more previous oxidation and etching steps (such oxidations will be referred to as second and higher order oxidations from now on). The results for first oxidation were startling, insofar
oxidations. This resulted in the first oxide and pure Si02 having about the same colour, while the second oxidation, though identical in all conditions, yielded a film of completely different colour. These results were repeatedly obtained for various oxidation periods, dry oxidations and utilising samples of widely different conductivittes. 2.2.
ETCHING METHOD
in order to obtain information as to the quality of the oxide in terms of density, unusual bond straining between oxygen and silicon and/or Ge atoms, stoichiometry and amount of dopants included in it, use was made of rate49% measurements. We used Pliskin’s 7) etching (15 parts HF, 10 parts 70°/bHNO P-Etch 3, 300 parts H20). The P-Etch rate is very sensitive to all the above mentioned parameters and was originally used to evaluate Si02 obtained by various techniques. Comparison was made between 36.5 at~ Ge wafer of 0.001 ~ cm resistivity, and Si control wafer of 0.008 Q cm. Both wafers were oxidised together by wet oxygen at 1050 °C.The oxide thickness on each was measured by multiple beam interferometry and they were then subjected to P-Etch 20 °C.The oxide thickness was monitored withattime by comparing the Si0 change 2 colour to a standard colour chart and recording the time delay at which the Si/Ge alloy oxide reached the same colour. Based on the previous result that the two have the same refractive index, the same colour means equal thicknesses. The results are summed up in fig. 2, from which an etch rate of 1.52 A/sec was found for first Si/Ge oxide, as compared with 1.58 A/sec for Si02. These results are very near, in spite of the very high Ge con-
290
S. MARGALIT,
A. BAR-LEV,
A. B. KUPER,
0
44] ~
H. AHARONT AND A. NEUGROSCHEL
M~,initial sample weight; M2, weight after oxidation; M3, remaining sample weight after all the oxide was etched off.
5000 ~
4000
Let us assume that on the average the oxide of the alloy includes both 510,,, and GeO~where the values of x and y are to be found. Also let us designate by N5 and in5 the number of moles of Si in the oxide and its atomic weight respectively, and by N0, /110, N0, m0
3000
2000
——--
~-~--~
-
0
define the oxide theincluded respectively. same quantities total foristhe weight Ge and of given oxygen oxygen in which is in theThe oxide therefore by:W0
1000 ~0~0
20
60
W0
=
(1)
Ino(XNs+YNG).
t(Time) Fig. 2. Comparison of rate of etching by P-Etch (20 C) of Si/Ge 1st oxide and thermal Si02 grown on pure Si.
tent in the original wafer. Etching experiments on the second oxide, on the other hand, have shown a much slower etch rate which was dependent on previous oxidation history. This is shown in fig. lb. One can, therefore, also conclude from the etching experiment that during 1st oxidation of Si/Ge the oxide formed is practically pure Si02, independent of the Ge content up to at least 36 at~. 2. 3.
WEIGHT MEASUREMENTS
The fact that the 1st oxide is almost like pure thermal Si02 immediately raises the question as to what became of the Ge that originally was in the sample surface layer that was converted to oxide. Three possibilities exist: (i) The Ge segregated completely from the oxide interface and piled up on the Si/Ge alloy side of it. (ii) GeO2 was formed during oxidation, and since it is volatile at the oxidation temperatures, it evaporated and was lost. (iii) The Ge diffused through the oxide and again was lost to the ambient. in this case, some Ge should have been left in the oxide at the end of the oxidation period. The second and third possibilities involve weight loss, due to the escaping Ge, which is balanced to some extent by oxygen weight gain. The first possibility involves only oxygen weight gain, which can be calculated as for the case of pure Si oxidation. We therefore employed a micro-balance (accuracy 5 x l0~ g) and performed three weight measurements per sample:
Hence M1—M3
=
mSNS+mGNG+WL,
(2)
M2—iW1
=
inO(xNS+yNG)—WL,
(3)
where WL represents the Ge weight loss. Defining an oxidation factor land Ge-to-Si molar ratio r as M1
f~
M3
~
N0
(4)
~‘
and substituting the appropriate atomic weights for Ge, Si and 0, i.e. in0 = 72.6, ins = 28.06, 1110 = 16, we get 28.06+72.6r+ WL/Ns
~ For pure Si oxidation WL fore ~ = 0.8775.
(5) =
r
=
0 and x
=
2, there-
We shall therefore try to decide the values of WL, and j’, by comparing the measuredfvalues for the alloy oxide according to the definition (4) with values calculated from eq. (5) for a range of those parameters. Fig. 3 shows the behaviour of f as function of r for the followingchoices of the parameters x,y, and WL/NS: x = 2 assumes Si is included in oxide in S1O2 form only; y = 0, 1, 2 assuming that Ge, GeO and Ge02 may all be found in the oxide; WL/Ns = 0, 2 assuming the possibilities of no Ge loss and 5 % of all the Ge in the highest Ge content i”,
OXIDATION
OF SILICON—GERMANIUM
first oxide, nor is it lost to the surroundings during the oxidation process. This result fully agrees with that of
2
f
~S
/ / / / /
0
3 0
the former two experimental approaches. The weighing method was also used to determine the kinetics of Si/Ge oxidation. By drawingthe oxide weight per unit area (M2—M3)/A, measured for a variety of
0
,/ 7’
/,,/
0
20
~
7/
~
— — — ~—‘
/~f._~
/
15
oxidation times and sample compositions, as against the square root of the time, fig. 4 was obtained. The solid line in the figure represents results calculated for pure8). Si It oxidation in theupsame conditionstheusing Deal’s is clear that to 20.4~Ge reaction is data parabolic and practically identical to Si. A slightly faster oxidation rate is found for samples with higher Ge content. Colour comparison with pure Si control wafer yielded the same result.
/7’
-
291
ALLOYS
2
~
~
2.4. into which all experimental results for first oxidation fell
ELECTRON MICROPROBE ANALYSIS
Range 0-5 -
0
025
050
075
The results quoted in the preceding sections indicate that the germanium originally in the oxidized layer of 1-00
600
r
Fig. 3. Dependence of oxidation factorfon Ge/Si molar ratio r in the oxide according to eq. (5). Average of experimental results on 1st oxide is given by point A.
sample of 36.5°~,being lost (strictly speaking, the value of WL/Ns should then be about 2.1). The first assumption is based on the fact that S10 2 bond formation at 1050 °Cinvolves very large change in free energy of 149 kcal/mole, while formation of Ge02 involves a change of only —73 kcal/mole. Therefore practically all the oxidised Si will be in the form of SiO2 with x 2, while for y the three possibilities should be considered. A series of 14 first oxidation experiments were then performed on samples with the Ge content varying between 4 at~ to 36.5 at~’,and oxidation durations varying between 10 mm to 16 hr (all oxidations were wet at 1063 °C).fwas calculated in each case from the weight measurements using its definition (4). All the results fell into the indicated range of fig. 3, and their average value was 0.890, which is shown by point A in the figure. This is so close to the theoretical value of 0.8775 pure Si as or to very forcenear us to conclusion that r must for be either zero it the in every first oxidation, independent of its duration and Ge content. r 0 means that Ge is neither included in any form in the —
=
550
I
-
-
±— 365 (°/~ Gel -
-
0 o
-
20-4 /.0
— -
0
0 E ~El
-
~ ~
-
0
300
-
-
250
-
-
200
-
0
~
0
~ ~
f ~
-
100
0
¶0
-
-
/
,/‘ 14
~
-
separate results)
c
-
0
1
2
~1T,IIHOURS)
2)
I 3
area as function of square root oflime for various wafer compositions. The solid straight line is for pure Si oxidation according to Deal8). Fig. 4.
Weight of 1st oxide per unit
292
S. MARGALIT, A. BAR-LEV, A. B. KUPER, H. AHARONI AND A. NEUGROSCHEL
the wafer must have segregated during the 1st oxidation into the substrate. It was therefore decided to use the electron microprobe to evaluate changes and expected pile-up of Ge at the alloy—oxide interface. The pile-up depth under the interface should be of the order of 2 \/(Dt), with D taken as the diffusion coefficient of Ge in Si, and t as the oxidation time. This yields about 0.2 J.im for 1 hr oxidation at 1050 °C. The electron microprobe primary beam voltage should therefore be adjusted so that the depth of penetration would be shallow enough to detect concentration changes in such a thin layer. For pure Ge the penetration depth, which approximately equals the depth of the layer from which 95 ~ of the secondary radiation emanates, is 1.7 l.tm for 20 keV primary electrons energy. For pure Si the corresponding number is 3.3 ~trn. Our beam voltage had therefore to be reduced as much as possible, cornpatible with focusing requirements, which put the lower limit at 5.5 kV. This primary electron energy is still enough to excite the L8 spectral line (10.4 A) used for the Ge concentration measurements (1.4 keV excitation energy) and the K~line 7.1 A, 1.8 keV excitation energy) used for the Si concentration measurements. A 36.5 at ~ Ge wafer was carefully polished, cleaned, and then cut in two. One half was subjected to wet oxidation at 1050 C for oxide thickness of 5200 A ~.
=
which was then removed by etching. The other half, used only for comparison, was mounted side by side with the first half in the sample holder of the microprobe vacuum chamber. They were then bombarded with primary electrons at three energy levels, and the resulting L0 radiation detected. The detector count reading would be proportional to the integrated amount of Ge in a depth determined by the primary beam voltage, Ify0 indicates the L~count obtained from the control half and emanating from a cylinder having the primary beam area and a penetration depth X(V), then
Ge) I.) 8t
-
~o
-
40
20 ~
2 i
0
Fig. 5.
I
~
X(,urn)
Ge pile-up after 1st oxidation as determined by electron
microprobe analysis and eq. (9). Also marked are depths of penetration for various beam voltages. Cx(v)
Y
~~4~?( 17)
=
J
F(~)d~,
(7)
where F(~)is the Ge concentration function after redistribution took place. From eqs. (6) and (7) 0.365
=
y0
—~-
X(l’)
.
.
5
X(V)
F(x) dy.
(8)
0 .
.
Taking the derivative of both sides with respect to X(V) and re-arranging, we finally get F(X)
d / y\ X ~ (~0.365
=
)
y +
0.365
_.
(9)
C
(6)
The approximate results for F, using the three accelerating voltages 25 kV, 10 kV, and 5.5 kV, are shown in fig. 5 together with the corresponding penetration depths. This is a rough approximation only, since the penetration depth was taken to be linearly dependent upon the composition and this was taken as the original one, independent of the segregation effects of the Ge, which probably gave shorter penetration depths in the oxidised half. Nevertheless, the findings verify the conclusion that Ge indeed piled up into the substrate,
where A is the proportionality constant, ~(V) is the efficiency of L~excitation by primary electrons of energy V, and 0.365 is the relative Ge concentration in the control piece. Withand theetched same half primary dized wouldbeam, yield the previously, oxi-
away from the oxide—alloy interface, during first oxidation. One may also calculate the Ge redistribution and surface concentration after first oxidation, 9) andusing Dealthe et equations developed by Grove et al. al.10). However, this would be of limited value since
y
=
Aq( V)X( 17) x 0.365
.
.
293
OXIDATION OF SILICON—GERMANIUM ALLOYS TABLE
Sample
Oxidation
(at% Ge)
(mm) time
36.5 36.2 20.7 20.4 4.0
10 60 40 60 50
Oxide weight (M2) 2—Ji’13) 1st )~)sg/cm 2nd 57 144 122 140 126.2
75.4 188.4 134.0 163.2 130.5
at the Ge concentrations in question, the linearity assumptions involved would hardly apply. 2. 5. INFRA-RED SPECTROSCOPY The absorption spectrum of Si0 2 indicates three modes of vibrations: anti-symmetric oxygen atom stretching vibration around a wave number of 1100 cm symmetric Si atom stretching vibration around 800 cm which has a relatively weak absorption maximum and is sensitive to oxide type; the third is an Si—O bond deformation vibration around 450 cm The exact absorption maxima depend on the method of oxidation and oxide structure. GeO2, on the other hand, is known to have anti-symmetric stretching vibration mode around 895 cm_i of about the same magnitude as Si02, but no symmetric modern). We checked the absorption spectrum of the 1st oxide, I p.m thick, grown on 14 at°,,/0Ge alloy at 1050 °C,using an unoxidised wafer of the same composition and thickness as reference and for substrate absorption cornpensation. A Perkin—Elmer Model 21 spectrometer was used. This yielded essentially the same absorption spectrum as SiO2 grown on pure Si of the same conductivity with only minute shifts1towards higher for and 1065 cm frequencies as compared the Si/Ge case (795 cm to and 1082 Si; the the 450 former cm’ line was813 notcm’ checked). Thiscm’ againfor confirms results obtained by other methods. 3. Second and higher order oxidations If the 1st oxide on Si/Ge alloy is removed by etching, and a new oxide grown, using the same conditions, we get what was named “the 2nd oxide”. Further etching and re-oxidising cycles would give a 3rd, 4th, and other higher order oxides consequtively. We have found those oxides to be markedly different in their properties from the first one. A comparison of thickness and etch rate between 1st and 2nd oxides all grown at 1063 °Cunder
1
Weight)~)ig/cm2) loss (M1—M3) 1st 2nd 26.75 65.9 55.0 65.0 57.8
51.15 127.0 78.0 101.5 62.1
Ifactor
________
_______
1st
2nd
0.884 0.843 0.82 0.87 0.845
2.09 2.07 1.39 1.65 0.907
—
wet oxygen for 1 hr is included in figs. 1 and 2, and shown as function of the Ge content. It can be seen that 2nd oxide thicknesses are increased (higher oxidation rates), and etching rates are reduced with increased Ge. This points to a different oxide composition. It was also noticed that after repeated oxidations the surface of the wafer gradually lost its shiny, polished appearance, and became matt. This was especially noticeable on high Ge content polycrystalline alloys, and indicated roughening of the surface due to possible Ge loss at the beginning of each high order oxidation cycle. It was therefore decided to use the weighing experiment for comparison of 1st and 2nd oxides. The results, together with the oxidation factorf calculated from the weights according to (4), are shown in table I. It can be seen that in each case the total loss in the sample weight after removal of the oxide, M5 M3, is larger for the 2nd oxide, which proved either Ge inclusion in the 2nd oxide, or Ge loss to the ambient, or possibly both. To evaluate the results for theffactor one must first decide in what form the Ge is included in the oxide [the values of y in eq. (1), i.e. y = 0, 1, 2, correspond to Ge, GeO, GeO2 respectively]. JR absorption measurements similar picture to Si0 on 2nd oxide yielded again a 2 but withand the reduced absorption peakNo at 1 shifted to 770 cm’ in size. 800 cm typical Ge0 2 or any other absorption peaks were found. GeO can probably be ruled out too, because it is unstable and therefore the Ge in the oxide is probably in the form of Ge impurity (y = 0). As already seen from fig. 3, the loss of Ge to the oxidising atmosphere factor WL/Ns, does not appreciably affect f. Furthermore, comparisons of 2nd oxidations on the basis of their duration have shown this factor to tend to zero with increased oxidation time. We therefore replottedffrom eq. (5) as function of r as defined by (4), for two values of the loss factor 0 and 2 and with y = 0, x = 2. The results are presented in fig. 6, in which the experi—
294
5. MARGALIT, A. BAR-LEV, A. B. KUPER, H. AHARONI AND A. NEUGROSCHEL
f
1. 0/mmAt.wet Ge oxidation cycles 50
2nd 1•1
: .: 0
/7365
2/
/ I
‘/oGe
/
-
1-0 0-9
-
I
1
2
3
4
I
I
I
5
6
7
I
8
~ Oxidation—etch cycle no
Fig. 7.
_______________________
4
I
0 02 04 06 r Fig. 6. Oxidation factor f as calculated from eq. (5) together with the experimentally determined values for 2nd oxidation for several wafer compositions,
mentally determined f factors for 2nd oxidation taken from table 1 are also included. It is obvious that in general r, the ratio of Ge to Si that enter the oxide, goes up with the composition during 2nd oxidation and changes from near zero for 4°~Ge to about 0.5 for 35.6~Ge. The etching rate experiments of fig. 2 support this result since they indicate that the higher the Ge content in the oxide, the lower becomes its etch rate, The f values experimentally obtained from weight measurements of repeated oxidation—etch cycles of4°~ Ge sample are shown in fig. 7. A gradual increase is obtained for f (and therefore for the amount of Ge entering the oxide according to fig. 6) at each successive cycle, which levels off after the 6th cycle to an approximate constant value of about 1. From fig. 6, assuming negligible Ge evaporation loss, this means that r is increased from 0 to 0.05. If a fixed, small, Ge loss is assumed to take place at the start of each high order oxidation cycle, then the maximum value of r will be slightly around 0.04.ratio Thisinvalue corresponds exactly to lower, the Ge/Si atomic the original 4°~ substrate. Generally, similar results but with high experimental spread and higher Ge loss to the atmosphere were found for the 20.7 and 36.5~Ge samples. The increased spread may be due to those samples being
Measuredf values for nine consecutive oxidation—etch cycles of 4~ Ge single crystal wafer.
polycrystalline to begin with, and therefore with larger fluctuations in their compositions and oxidation rates. It is therefore possible to reach the conclusion that in the 4 ~ Ge case at least, the 1st oxidation is wholly SiO2 the 2nd would already include some Ge and its amount would increase with successive cycles to reach approximately the Ge/Si ratio in the substrate after the 6th cycle. 4. Summary and discussion of oxidation results All the various experimental approaches yielded the same result, namely that 1st oxidation of Si/Ge alloy is practically pure SiO2 with the Ge being piled up on the alloy side of the alloy—oxide interface. This can therefore be accepted as conclusive. 1st oxidation can, in consequence, be used for masking and subsequent etching of windows for diffusion purposes in much the same way as Si02 is used in pure Si. This is of prime importance for the technology ofjunction devices built in Si/Ge. One can attribute this to two causes: the preference of SiO2 formation as against GeO or GeO2 on the basis of the much larger free energy change involved in SiO2 and the negligible diffusion constant of Ge in SiO2 as compared to both the oxidation rate and the diffusion constant of Ge in the alloy. The segregation coefficient of the Ge is very large. It is interesting to note that there are other cases of oxidation of binary non-dilute alloys where12), onlywhile the major component oxithe other component discs, like Cu/Ni alloy does not participate and has no effect on the oxidation rate until its concentration reaches a certain critical value. In our case we can assume that at the start of the 1st wet oxidation a thin SiO 2 layer immediately forms through which the Ge atom cannot diffuse out-
295
OXIDATION OF SILICON—GERMANIUM ALLOYS
22~NO~ -
-
-
-
-
ntype
-
-
X
-
0o0’22~
-
I
-15
-
-
1-10
I
Freq.olMHz T o300°K -
-
0
Ii~l~I~lt~ -5 0 5
10
15
-
~~1VOLTS)
Fig. 8. Comparison between two MOS capacitors C—Vcharacteristics constructed on Si/Ge and on Si substrates (both n-type about 0.1 Q cm).
side. Theforoxidation however, is high enough the Ge totemperature, have relatively high diffusion constant inside the substrate alloy. This diffusion constant is enhanced by the originally high Ge content. No data on Ge diffusion in Si/Ge alloys are available, but it is generally known that the diffusion coefficients of substitutional diffusants are exponentially dependent on the activation energy of vacancy formation, which is three to four times the20°~~ energy gapvalue of the semicon3).At Ge the of Eg would ductor in question’ be reduced by about 0.1 eV with a consequent increase in the diffusion constant by about exp (0.4/kT) 33 at 1050 ~C. The piled up Ge would therefore diffuse rather fast inside. Still the 2nd oxidation would start with a surface enriched in Ge and with more disturbed crystal structure. Some Ge atoms would now oxidise to GeO 2 and evaporate; others will be trapped by SiO2 bonds formed by oxygen diffusing from all sides in the disturbed surface layer. The value of J’ would therefore grow, and some weight loss would be found. For short oxidation—etch cycles, one may find values of J in cxcess of what might be expected from the original sample composition because of the first two or three cycles pile-up period. For low Ge content monocrystal samples (around I ) which, as we shall see in the next section, are the only ones that now look promising from device properties viewpoint, the change off is expected to be rather slow with succeeding oxidation cycles as can already —
11g. 7). The oxide of such be seen in the 4 °~ sample (1 samples would probably not differ too much from SiO 2 even after three or four oxidising cycles, which is all that is necessary for device production. 5. Oxide properties obtained from MOS devices constructed in Si/Ge monocrystals The devices constructed were MOS capacitors and MOS made on made Si substrates transistors. of the sameIdentical type anddevices, resistivity, were in the same run to facilitate comparison. Only 1st oxide, obtained by either dry or wet oxidation, was used as the dielectric. The capacitors were made on two kinds of substrates. The first was 2.5 at°
296
S. MARGALIT, A. BAR-LEV, A. B. KUPER, H. AHARONI AND A. NEUGROSCUEL
tions’ 6) for the ideal (no surface states) case and assuming the same dielectric constant and band gap for this low Ge content sample and Si. The important conclusion to be drawn is that the FB voltage was shifted in the Si/Ge case from the negative side (normal to n-type Si) to the positive side. In this case it was shifted from 13.37 V for Si to + 8.69 V for Si/Ge. Shifts in the same direction, but much larger, were found for the cases of wet oxidation and for the p-type 4% Ge/Si substrate. The reason for the shifts is not completely clear. They cannot be accounted for by the small differences in work function of the different materials to Al [about 0.12 V difference between Ge and Si for l0~~ cm3 n-type doping14)]. A more reasonable assumption is that the Si/Ge oxide is negatively charged. This was checked and verified by an MOS transistor structure. The surface state density in the forbidden energy band was estimated, using Brown’s methodi7) of C—V measurements at varying temperatures (77 °K to 300 °K).The value of 2.4 x 1012 cm2 was obtained for the 4 % Ge case, which is an order of magnitude higher than in Si. Annealing caused the FB voltage ofa typical 2.50/ Ge capacitor to shift back from 9.3 V to 3.9 V, which indicated that the surface state density can be materially reduced and that they are of the acceptor type. The latter can be expected since the charge neutrality level is 0.3 eV above the valence band level for Si, but is down inside the valence level for Ge’5). For Si/Ge it is somewhere inbetween, and for n-type material, with Fermi level above the middle of the band gap, there would be more acceptor like filled states below it and above the charge neutrality level in Si/Ge than in pure Si. Thechargedistributioninsidetheoxidewasmeasured by the change in channel conductance of a depletion
Qs5x10~m2 1-2 .~ (k’” ~
*
J
±
*_*_+_
06
4 at
0/,
Ge
—
type MOS transistor structure while the gate oxide was etched away by stages. A closed geometry MOS structure was fabricated on 3 ~ cm n-type 100/ Ge/Si epitaxial layer. Because of the aforementioned FB shift, a p-channel depletion type transistor resulted, whose channel conductivity can be measured without gate electrode. The results are shown in fig. 9. No charges on the bare oxide surface were found, which is contrary to Deal’s assumptions’ 8) It can be seen that most of the negative oxide charge is located within about 400 A from the surface. This negative charge may be attri-
02 0
Fig.
on
9. 10%
iobo
1OIJO 0
3Oii
4000 0
~
d l~
Surface charge measurement on MOS transistor made Ge/Si single crystal wafer as function of the gate oxide
thickness.
buted to dangling bonds of Ge atoms piled up near the interface and penetrating the oxide to some extent. An MOS transistor with an Al gate was also fabricated on the same 100/ Ge/Si epitaxial layer. The hole mobility was found to be low, about 15 cm2/V see, otherwise it had the normal characteristics of a depletion device. The numerical values obtained from the devices point to a very strong dependence of FB and VT on the Ge content. 0.5 at0/ of Ge will probably be more than enough to shift the VT of a normal MOS transistor by about 2 V in the positive direction, which is attractive from the circuit point of view. Ge content of below 1 0/ would also enormously simplify the problem of growing single crystals of good quality with much higher mobilities, probably near the silicon vatues. Multiple oxidation processes would also be simplifiedforsuchdevices,asalreadymentionedinsection4. 6. Conclusion Si/Ge alloys were shown to possess some very interesting oxidation properties. 1st oxide is found to be almost equivalent to SiO 2 independent of Ge content. This is not so for higher order oxidation which proceeds at a faster rate to a greater thickness and includes Ge as an impurity. In the case of 40/ Ge, which was the lowest used in the oxidation experiments, the oxide properties changed gradually to include more and more Ge between the 2nd and the 6th oxidation cycle. It is therefore hoped that wafers with below I 0/ Ge could be oxidised several times before the oxide properties would materially change. The 1st oxide was used in production of MOS capacitors and transistors and it was found that the Ge
OXIDATION
OF SILICON—GERMANIUM
introduced large negative charges into the oxide located within 400 A of the interface. These charges caused large shifts in flat band and threshhold voltages of the devices in the positive direction, which is attractive from device utilisation viewpoint, and opens up the possibilities of enhancement type n-channel devices. For shifts of about 2 V it is believed that Ge content of around 0.50/ will be suffIcient. Such low concentration would hopefully not degrade the effective channel mobility or increase surface state densities to the values encountered in our first experimental MOS transistors, built on 100/ Ge substrate, which suffered from very low mobility and high surface state density. References 1) H. Stohr and W. Klemm, Z. Anorg. Chem. 241 (1939) 305. 2) S. Margalit, D. Sc. Thesis, Faculty of Electrical Engineering, Technion, Haifa, Israel, (1971). 3) H. Aharoni, A. Bar-Lev, I. A. Blech and S. Margalit, Thin Solid Films 11 (1972) 313.
ALLOYS
297
4) S. Tolansky, Surface Microtopography (Interscience, New
5) York, G. R. 1960). Booker and C. E. Benjamin, Westinghouse Res. Lab. Scientific Paper 62-126-453-PI (April 1062). 6) H. Wolf, Semiconductors (Wiley—interscience, New York, 1971) p. 352. 7) W. A. Pliskin and H. S. Lehman, J. Electrochem. Soc. 112 (1965) 1013. 8) B. E. Deal, J. Electrochem. Soc. 110 (1963) 527. 9) A. S. Grove, 0. Leistiko Jr. and C. T. Sah, J. AppI. Phys. 35 (1964) 2695. 10) B. E. Deal, A. S. Grove, E. H. Snow and C. T. Sah, J. Electrochem. Soc. 112 (1965) 308. 11) I. Simon, in: Modern Aspects of the Vitreous State, Vol. I, Ed. J. D. Mackenzie (Butterworths, 1966) p. 120. 12) P. Hofstad, High Temperature Oxidation Metals (Wiley, New York, 1966). 13) Ref. 6, p. 148. 14) Ref. 6, p. 426.
15) Ref. 6, p. 429. 16) A. Goetzberger, Bell Syst. Tech. J. 45 (1966) 1097. 17) D. M. Brown and P. V. Gra, J. Electrochem. Soc. 1i5 (1968) 760. 18) B. E. Deal, M. Sklar, A. S. Grove and F. H. Snow, J. Electrochem. Soc. 114 (1967) 266.
OXIDATION OF SILICON-GERMANIUM ALLOYS S. MARGALIT, A. BAR~LEV*,A. B. KUPER**, H. AHARONE and A. NEUGROSCI-IEL Faculty of Electrical Enqineering, Technion, Haifa, israel
Oxidation of Si/Ge alloys, of up to 370 Ge, were investigated in view of the material utilization for MOS devices. Both single crystals and homogeneous polycrystalline Si/Ge, mainly in [Ill] direction, were used. The oxide formed after first oxidation was found to be Si0 2 only, substantiated by several types of measurements. The Ge piled up at the interface. Higher order oxidations included an increasing amount of Ge. MOS capacitors and transistors were produced and showed higher surface State density and negatively charged oxides.
1. Introduction Silicon and germanium are the two most useful semiconductor materials today. The Si—Ge phase diagram1) shows that they can form solid solutions at any desired composition. To utilise the Si/Ge alloy for device purposes, it must usually be in a single crystal from. This can be realised by Czochralski pulling form the melt method, using very slow pulling rates and only up to a few atomic ~ Ge content. Higher Ge concentration, up to 30 at%, can be made by epitaxial growth from the vapour, Silicon transistor and integrated circuit technology are based today on using the thermally grown silicon dioxide for diffusion masking and passivation purposes. Thermal oxide is preferable, being denser, and usually cleaner of ionic contamination than deposited oxide, and is a natural by-product of the previous diffusion step. Finally, it is associated with a relatively low Si surface state density, which is an essential requirement for MOS circuit production. From the technological point of view, it is therefore of paramount interest to investigate the oxidation properties of Si/Ge alloys if devices are to be made of them. For the purpose of the oxidation studies reported in this paper, use was made of both single and polycrystalline materials. The first was made by the authors On sabbatical leave in the University College of Swansea, United Kingdom. . 0 . ** Now in Case Western Reserve University, Cleveland, Ohio, *
2) or by epitaxy3). The either bywhere pulling from the melt second uniform composition polycrystals onginally made by RCA for thermoelectric device purposes. All the single crystals and also the polycrystals were mainly (111) oriented. The investigation included the composition and properties of the oxide, thermally grown on the Si/Ge polished wafers and their dependence on the composition and previous oxidation history of the wafer. Segregation effects of the Ge during oxidation were also encountered and studied. Finally, the feasibility of using the oxide for diffusion masking and for MOS device fabrication was checked, and some interesting properties of the resulting devices were measured and the device potentialities discussed. 2. Methods used for evaluation of thermally oxidized Si/Ge alloys The oxides grown on various samples of Si/Ge alloys were compared to thermally grown Si0 2 on silicon, processed at the same run and of approximately the same conductivity. The purpose of the comparison was to determine the growth rate, the amount if any of Ge, GeO, or Ge02 included in the oxide, the possible loss of Ge from the sample via Ge02 formation (which is volatile at the oxidising temperature of above 1000 °C),the etching properties, and the dielectric and refractive constants of these oxides. The following five independent methods of evaluation were used: (a) Optical interference method for thickness (or growth —
rate) and refractive index measurements.
U.S.A.
288
—
OXIDATION OF SILICON—GERMANIUM ALLOYS
289
as only insignificant difference was found in thickness and refractiveThe index between the Si/Ge and (with the pure Si samples. results for wet oxidation the
7000
6000
oxygen bubbling through 95 °C water) carried on at 1063 °Cfor 60 mm, are shown in fig. 1. There is a small
5000
increase in oxide thickness when the Ge content goes up from 0 to l0°~thereafter it is fixed. This increase O
10
20
At
0/0
30
40
Ge in Si - Ge
I
a
A 000 o~
~°°
~
800 ~00 600
same figure. Refractive index measurements also yielded the same value for Si/Ge first oxide and pure Si0 2, but remained unchanged for second and higher order
Fj—
F-
W
0
is similar to that encountered in the parabolic rate constant for wet oxidation of highly doped silicon6). Second oxidation, on the other hand, appears to proceed at a much faster rate and is shown, for comparison, in the
10
At
20 0/
Ge
30 fl
~
Si-Ge
C b) Fig. I. (a) Oxide thickness after 60 mm wet oxidation at 1063 C for 1st and 2nd oxides. (b) Etch rate in buff. HF at 24 CC for 1st and 2nd oxides: (.)first oxidation; (0) second oxidation,
(b) Controlled etching and comparison with pure Si02 etch rate (Ge inclusion will reduce it). (c) Weight measurements for detection of Ge loss from sample and/or its inclusion in the oxide. (d) An electron probe micro-analyser for detecting Ge segregation and pile-up during oxidation, (e) infra-red absorption for detection of Ge—oxygen bonds in the oxide. Those measurements will now be described in some detail, together with the necessary background theory. 2. 1. OPTICAL INTERFERENCE METHOD These are standard methods4’for 5), oxide thickness and refractive index measurements It was discovered that different results were obtained depending on whether the oxide involved was the first one after the polishing/cleaning steps (this will be refenred to as first oxidation from now on), or if it was re-oxidised after one or more previous oxidation and etching steps (such oxidations will be referred to as second and higher order oxidations from now on). The results for first oxidation were startling, insofar
oxidations. This resulted in the first oxide and pure Si02 having about the same colour, while the second oxidation, though identical in all conditions, yielded a film of completely different colour. These results were repeatedly obtained for various oxidation periods, dry oxidations and utilising samples of widely different conductivittes. 2.2.
ETCHING METHOD
in order to obtain information as to the quality of the oxide in terms of density, unusual bond straining between oxygen and silicon and/or Ge atoms, stoichiometry and amount of dopants included in it, use was made of rate49% measurements. We used Pliskin’s 7) etching (15 parts HF, 10 parts 70°/bHNO P-Etch 3, 300 parts H20). The P-Etch rate is very sensitive to all the above mentioned parameters and was originally used to evaluate Si02 obtained by various techniques. Comparison was made between 36.5 at~ Ge wafer of 0.001 ~ cm resistivity, and Si control wafer of 0.008 Q cm. Both wafers were oxidised together by wet oxygen at 1050 °C.The oxide thickness on each was measured by multiple beam interferometry and they were then subjected to P-Etch 20 °C.The oxide thickness was monitored withattime by comparing the Si0 change 2 colour to a standard colour chart and recording the time delay at which the Si/Ge alloy oxide reached the same colour. Based on the previous result that the two have the same refractive index, the same colour means equal thicknesses. The results are summed up in fig. 2, from which an etch rate of 1.52 A/sec was found for first Si/Ge oxide, as compared with 1.58 A/sec for Si02. These results are very near, in spite of the very high Ge con-
290
S. MARGALIT,
A. BAR-LEV,
A. B. KUPER,
0
44] ~
H. AHARONT AND A. NEUGROSCHEL
M~,initial sample weight; M2, weight after oxidation; M3, remaining sample weight after all the oxide was etched off.
5000 ~
4000
Let us assume that on the average the oxide of the alloy includes both 510,,, and GeO~where the values of x and y are to be found. Also let us designate by N5 and in5 the number of moles of Si in the oxide and its atomic weight respectively, and by N0, /110, N0, m0
3000
2000
——--
~-~--~
-
0
define the oxide theincluded respectively. same quantities total foristhe weight Ge and of given oxygen oxygen in which is in theThe oxide therefore by:W0
1000 ~0~0
20
60
W0
=
(1)
Ino(XNs+YNG).
t(Time) Fig. 2. Comparison of rate of etching by P-Etch (20 C) of Si/Ge 1st oxide and thermal Si02 grown on pure Si.
tent in the original wafer. Etching experiments on the second oxide, on the other hand, have shown a much slower etch rate which was dependent on previous oxidation history. This is shown in fig. lb. One can, therefore, also conclude from the etching experiment that during 1st oxidation of Si/Ge the oxide formed is practically pure Si02, independent of the Ge content up to at least 36 at~. 2. 3.
WEIGHT MEASUREMENTS
The fact that the 1st oxide is almost like pure thermal Si02 immediately raises the question as to what became of the Ge that originally was in the sample surface layer that was converted to oxide. Three possibilities exist: (i) The Ge segregated completely from the oxide interface and piled up on the Si/Ge alloy side of it. (ii) GeO2 was formed during oxidation, and since it is volatile at the oxidation temperatures, it evaporated and was lost. (iii) The Ge diffused through the oxide and again was lost to the ambient. in this case, some Ge should have been left in the oxide at the end of the oxidation period. The second and third possibilities involve weight loss, due to the escaping Ge, which is balanced to some extent by oxygen weight gain. The first possibility involves only oxygen weight gain, which can be calculated as for the case of pure Si oxidation. We therefore employed a micro-balance (accuracy 5 x l0~ g) and performed three weight measurements per sample:
Hence M1—M3
=
mSNS+mGNG+WL,
(2)
M2—iW1
=
inO(xNS+yNG)—WL,
(3)
where WL represents the Ge weight loss. Defining an oxidation factor land Ge-to-Si molar ratio r as M1
f~
M3
~
N0
(4)
~‘
and substituting the appropriate atomic weights for Ge, Si and 0, i.e. in0 = 72.6, ins = 28.06, 1110 = 16, we get 28.06+72.6r+ WL/Ns
~ For pure Si oxidation WL fore ~ = 0.8775.
(5) =
r
=
0 and x
=
2, there-
We shall therefore try to decide the values of WL, and j’, by comparing the measuredfvalues for the alloy oxide according to the definition (4) with values calculated from eq. (5) for a range of those parameters. Fig. 3 shows the behaviour of f as function of r for the followingchoices of the parameters x,y, and WL/NS: x = 2 assumes Si is included in oxide in S1O2 form only; y = 0, 1, 2 assuming that Ge, GeO and Ge02 may all be found in the oxide; WL/Ns = 0, 2 assuming the possibilities of no Ge loss and 5 % of all the Ge in the highest Ge content i”,
OXIDATION
OF SILICON—GERMANIUM
first oxide, nor is it lost to the surroundings during the oxidation process. This result fully agrees with that of
2
f
~S
/ / / / /
0
3 0
the former two experimental approaches. The weighing method was also used to determine the kinetics of Si/Ge oxidation. By drawingthe oxide weight per unit area (M2—M3)/A, measured for a variety of
0
,/ 7’
/,,/
0
20
~
7/
~
— — — ~—‘
/~f._~
/
15
oxidation times and sample compositions, as against the square root of the time, fig. 4 was obtained. The solid line in the figure represents results calculated for pure8). Si It oxidation in theupsame conditionstheusing Deal’s is clear that to 20.4~Ge reaction is data parabolic and practically identical to Si. A slightly faster oxidation rate is found for samples with higher Ge content. Colour comparison with pure Si control wafer yielded the same result.
/7’
-
291
ALLOYS
2
~
~
2.4. into which all experimental results for first oxidation fell
ELECTRON MICROPROBE ANALYSIS
Range 0-5 -
0
025
050
075
The results quoted in the preceding sections indicate that the germanium originally in the oxidized layer of 1-00
600
r
Fig. 3. Dependence of oxidation factorfon Ge/Si molar ratio r in the oxide according to eq. (5). Average of experimental results on 1st oxide is given by point A.
sample of 36.5°~,being lost (strictly speaking, the value of WL/Ns should then be about 2.1). The first assumption is based on the fact that S10 2 bond formation at 1050 °Cinvolves very large change in free energy of 149 kcal/mole, while formation of Ge02 involves a change of only —73 kcal/mole. Therefore practically all the oxidised Si will be in the form of SiO2 with x 2, while for y the three possibilities should be considered. A series of 14 first oxidation experiments were then performed on samples with the Ge content varying between 4 at~ to 36.5 at~’,and oxidation durations varying between 10 mm to 16 hr (all oxidations were wet at 1063 °C).fwas calculated in each case from the weight measurements using its definition (4). All the results fell into the indicated range of fig. 3, and their average value was 0.890, which is shown by point A in the figure. This is so close to the theoretical value of 0.8775 pure Si as or to very forcenear us to conclusion that r must for be either zero it the in every first oxidation, independent of its duration and Ge content. r 0 means that Ge is neither included in any form in the —
=
550
I
-
-
±— 365 (°/~ Gel -
-
0 o
-
20-4 /.0
— -
0
0 E ~El
-
~ ~
-
0
300
-
-
250
-
-
200
-
0
~
0
~ ~
f ~
-
100
0
¶0
-
-
/
,/‘ 14
~
-
separate results)
c
-
0
1
2
~1T,IIHOURS)
2)
I 3
area as function of square root oflime for various wafer compositions. The solid straight line is for pure Si oxidation according to Deal8). Fig. 4.
Weight of 1st oxide per unit
292
S. MARGALIT, A. BAR-LEV, A. B. KUPER, H. AHARONI AND A. NEUGROSCHEL
the wafer must have segregated during the 1st oxidation into the substrate. It was therefore decided to use the electron microprobe to evaluate changes and expected pile-up of Ge at the alloy—oxide interface. The pile-up depth under the interface should be of the order of 2 \/(Dt), with D taken as the diffusion coefficient of Ge in Si, and t as the oxidation time. This yields about 0.2 J.im for 1 hr oxidation at 1050 °C. The electron microprobe primary beam voltage should therefore be adjusted so that the depth of penetration would be shallow enough to detect concentration changes in such a thin layer. For pure Ge the penetration depth, which approximately equals the depth of the layer from which 95 ~ of the secondary radiation emanates, is 1.7 l.tm for 20 keV primary electrons energy. For pure Si the corresponding number is 3.3 ~trn. Our beam voltage had therefore to be reduced as much as possible, cornpatible with focusing requirements, which put the lower limit at 5.5 kV. This primary electron energy is still enough to excite the L8 spectral line (10.4 A) used for the Ge concentration measurements (1.4 keV excitation energy) and the K~line 7.1 A, 1.8 keV excitation energy) used for the Si concentration measurements. A 36.5 at ~ Ge wafer was carefully polished, cleaned, and then cut in two. One half was subjected to wet oxidation at 1050 C for oxide thickness of 5200 A ~.
=
which was then removed by etching. The other half, used only for comparison, was mounted side by side with the first half in the sample holder of the microprobe vacuum chamber. They were then bombarded with primary electrons at three energy levels, and the resulting L0 radiation detected. The detector count reading would be proportional to the integrated amount of Ge in a depth determined by the primary beam voltage, Ify0 indicates the L~count obtained from the control half and emanating from a cylinder having the primary beam area and a penetration depth X(V), then
Ge) I.) 8t
-
~o
-
40
20 ~
2 i
0
Fig. 5.
I
~
X(,urn)
Ge pile-up after 1st oxidation as determined by electron
microprobe analysis and eq. (9). Also marked are depths of penetration for various beam voltages. Cx(v)
Y
~~4~?( 17)
=
J
F(~)d~,
(7)
where F(~)is the Ge concentration function after redistribution took place. From eqs. (6) and (7) 0.365
=
y0
—~-
X(l’)
.
.
5
X(V)
F(x) dy.
(8)
0 .
.
Taking the derivative of both sides with respect to X(V) and re-arranging, we finally get F(X)
d / y\ X ~ (~0.365
=
)
y +
0.365
_.
(9)
C
(6)
The approximate results for F, using the three accelerating voltages 25 kV, 10 kV, and 5.5 kV, are shown in fig. 5 together with the corresponding penetration depths. This is a rough approximation only, since the penetration depth was taken to be linearly dependent upon the composition and this was taken as the original one, independent of the segregation effects of the Ge, which probably gave shorter penetration depths in the oxidised half. Nevertheless, the findings verify the conclusion that Ge indeed piled up into the substrate,
where A is the proportionality constant, ~(V) is the efficiency of L~excitation by primary electrons of energy V, and 0.365 is the relative Ge concentration in the control piece. Withand theetched same half primary dized wouldbeam, yield the previously, oxi-
away from the oxide—alloy interface, during first oxidation. One may also calculate the Ge redistribution and surface concentration after first oxidation, 9) andusing Dealthe et equations developed by Grove et al. al.10). However, this would be of limited value since
y
=
Aq( V)X( 17) x 0.365
.
.
293
OXIDATION OF SILICON—GERMANIUM ALLOYS TABLE
Sample
Oxidation
(at% Ge)
(mm) time
36.5 36.2 20.7 20.4 4.0
10 60 40 60 50
Oxide weight (M2) 2—Ji’13) 1st )~)sg/cm 2nd 57 144 122 140 126.2
75.4 188.4 134.0 163.2 130.5
at the Ge concentrations in question, the linearity assumptions involved would hardly apply. 2. 5. INFRA-RED SPECTROSCOPY The absorption spectrum of Si0 2 indicates three modes of vibrations: anti-symmetric oxygen atom stretching vibration around a wave number of 1100 cm symmetric Si atom stretching vibration around 800 cm which has a relatively weak absorption maximum and is sensitive to oxide type; the third is an Si—O bond deformation vibration around 450 cm The exact absorption maxima depend on the method of oxidation and oxide structure. GeO2, on the other hand, is known to have anti-symmetric stretching vibration mode around 895 cm_i of about the same magnitude as Si02, but no symmetric modern). We checked the absorption spectrum of the 1st oxide, I p.m thick, grown on 14 at°,,/0Ge alloy at 1050 °C,using an unoxidised wafer of the same composition and thickness as reference and for substrate absorption cornpensation. A Perkin—Elmer Model 21 spectrometer was used. This yielded essentially the same absorption spectrum as SiO2 grown on pure Si of the same conductivity with only minute shifts1towards higher for and 1065 cm frequencies as compared the Si/Ge case (795 cm to and 1082 Si; the the 450 former cm’ line was813 notcm’ checked). Thiscm’ againfor confirms results obtained by other methods. 3. Second and higher order oxidations If the 1st oxide on Si/Ge alloy is removed by etching, and a new oxide grown, using the same conditions, we get what was named “the 2nd oxide”. Further etching and re-oxidising cycles would give a 3rd, 4th, and other higher order oxides consequtively. We have found those oxides to be markedly different in their properties from the first one. A comparison of thickness and etch rate between 1st and 2nd oxides all grown at 1063 °Cunder
1
Weight)~)ig/cm2) loss (M1—M3) 1st 2nd 26.75 65.9 55.0 65.0 57.8
51.15 127.0 78.0 101.5 62.1
Ifactor
________
_______
1st
2nd
0.884 0.843 0.82 0.87 0.845
2.09 2.07 1.39 1.65 0.907
—
wet oxygen for 1 hr is included in figs. 1 and 2, and shown as function of the Ge content. It can be seen that 2nd oxide thicknesses are increased (higher oxidation rates), and etching rates are reduced with increased Ge. This points to a different oxide composition. It was also noticed that after repeated oxidations the surface of the wafer gradually lost its shiny, polished appearance, and became matt. This was especially noticeable on high Ge content polycrystalline alloys, and indicated roughening of the surface due to possible Ge loss at the beginning of each high order oxidation cycle. It was therefore decided to use the weighing experiment for comparison of 1st and 2nd oxides. The results, together with the oxidation factorf calculated from the weights according to (4), are shown in table I. It can be seen that in each case the total loss in the sample weight after removal of the oxide, M5 M3, is larger for the 2nd oxide, which proved either Ge inclusion in the 2nd oxide, or Ge loss to the ambient, or possibly both. To evaluate the results for theffactor one must first decide in what form the Ge is included in the oxide [the values of y in eq. (1), i.e. y = 0, 1, 2, correspond to Ge, GeO, GeO2 respectively]. JR absorption measurements similar picture to Si0 on 2nd oxide yielded again a 2 but withand the reduced absorption peakNo at 1 shifted to 770 cm’ in size. 800 cm typical Ge0 2 or any other absorption peaks were found. GeO can probably be ruled out too, because it is unstable and therefore the Ge in the oxide is probably in the form of Ge impurity (y = 0). As already seen from fig. 3, the loss of Ge to the oxidising atmosphere factor WL/Ns, does not appreciably affect f. Furthermore, comparisons of 2nd oxidations on the basis of their duration have shown this factor to tend to zero with increased oxidation time. We therefore replottedffrom eq. (5) as function of r as defined by (4), for two values of the loss factor 0 and 2 and with y = 0, x = 2. The results are presented in fig. 6, in which the experi—
294
5. MARGALIT, A. BAR-LEV, A. B. KUPER, H. AHARONI AND A. NEUGROSCHEL
f
1. 0/mmAt.wet Ge oxidation cycles 50
2nd 1•1
: .: 0
/7365
2/
/ I
‘/oGe
/
-
1-0 0-9
-
I
1
2
3
4
I
I
I
5
6
7
I
8
~ Oxidation—etch cycle no
Fig. 7.
_______________________
4
I
0 02 04 06 r Fig. 6. Oxidation factor f as calculated from eq. (5) together with the experimentally determined values for 2nd oxidation for several wafer compositions,
mentally determined f factors for 2nd oxidation taken from table 1 are also included. It is obvious that in general r, the ratio of Ge to Si that enter the oxide, goes up with the composition during 2nd oxidation and changes from near zero for 4°~Ge to about 0.5 for 35.6~Ge. The etching rate experiments of fig. 2 support this result since they indicate that the higher the Ge content in the oxide, the lower becomes its etch rate, The f values experimentally obtained from weight measurements of repeated oxidation—etch cycles of4°~ Ge sample are shown in fig. 7. A gradual increase is obtained for f (and therefore for the amount of Ge entering the oxide according to fig. 6) at each successive cycle, which levels off after the 6th cycle to an approximate constant value of about 1. From fig. 6, assuming negligible Ge evaporation loss, this means that r is increased from 0 to 0.05. If a fixed, small, Ge loss is assumed to take place at the start of each high order oxidation cycle, then the maximum value of r will be slightly around 0.04.ratio Thisinvalue corresponds exactly to lower, the Ge/Si atomic the original 4°~ substrate. Generally, similar results but with high experimental spread and higher Ge loss to the atmosphere were found for the 20.7 and 36.5~Ge samples. The increased spread may be due to those samples being
Measuredf values for nine consecutive oxidation—etch cycles of 4~ Ge single crystal wafer.
polycrystalline to begin with, and therefore with larger fluctuations in their compositions and oxidation rates. It is therefore possible to reach the conclusion that in the 4 ~ Ge case at least, the 1st oxidation is wholly SiO2 the 2nd would already include some Ge and its amount would increase with successive cycles to reach approximately the Ge/Si ratio in the substrate after the 6th cycle. 4. Summary and discussion of oxidation results All the various experimental approaches yielded the same result, namely that 1st oxidation of Si/Ge alloy is practically pure SiO2 with the Ge being piled up on the alloy side of the alloy—oxide interface. This can therefore be accepted as conclusive. 1st oxidation can, in consequence, be used for masking and subsequent etching of windows for diffusion purposes in much the same way as Si02 is used in pure Si. This is of prime importance for the technology ofjunction devices built in Si/Ge. One can attribute this to two causes: the preference of SiO2 formation as against GeO or GeO2 on the basis of the much larger free energy change involved in SiO2 and the negligible diffusion constant of Ge in SiO2 as compared to both the oxidation rate and the diffusion constant of Ge in the alloy. The segregation coefficient of the Ge is very large. It is interesting to note that there are other cases of oxidation of binary non-dilute alloys where12), onlywhile the major component oxithe other component discs, like Cu/Ni alloy does not participate and has no effect on the oxidation rate until its concentration reaches a certain critical value. In our case we can assume that at the start of the 1st wet oxidation a thin SiO 2 layer immediately forms through which the Ge atom cannot diffuse out-
295
OXIDATION OF SILICON—GERMANIUM ALLOYS
22~NO~ -
-
-
-
-
ntype
-
-
X
-
0o0’22~
-
I
-15
-
-
1-10
I
Freq.olMHz T o300°K -
-
0
Ii~l~I~lt~ -5 0 5
10
15
-
~~1VOLTS)
Fig. 8. Comparison between two MOS capacitors C—Vcharacteristics constructed on Si/Ge and on Si substrates (both n-type about 0.1 Q cm).
side. Theforoxidation however, is high enough the Ge totemperature, have relatively high diffusion constant inside the substrate alloy. This diffusion constant is enhanced by the originally high Ge content. No data on Ge diffusion in Si/Ge alloys are available, but it is generally known that the diffusion coefficients of substitutional diffusants are exponentially dependent on the activation energy of vacancy formation, which is three to four times the20°~~ energy gapvalue of the semicon3).At Ge the of Eg would ductor in question’ be reduced by about 0.1 eV with a consequent increase in the diffusion constant by about exp (0.4/kT) 33 at 1050 ~C. The piled up Ge would therefore diffuse rather fast inside. Still the 2nd oxidation would start with a surface enriched in Ge and with more disturbed crystal structure. Some Ge atoms would now oxidise to GeO 2 and evaporate; others will be trapped by SiO2 bonds formed by oxygen diffusing from all sides in the disturbed surface layer. The value of J’ would therefore grow, and some weight loss would be found. For short oxidation—etch cycles, one may find values of J in cxcess of what might be expected from the original sample composition because of the first two or three cycles pile-up period. For low Ge content monocrystal samples (around I ) which, as we shall see in the next section, are the only ones that now look promising from device properties viewpoint, the change off is expected to be rather slow with succeeding oxidation cycles as can already —
11g. 7). The oxide of such be seen in the 4 °~ sample (1 samples would probably not differ too much from SiO 2 even after three or four oxidising cycles, which is all that is necessary for device production. 5. Oxide properties obtained from MOS devices constructed in Si/Ge monocrystals The devices constructed were MOS capacitors and MOS made on made Si substrates transistors. of the sameIdentical type anddevices, resistivity, were in the same run to facilitate comparison. Only 1st oxide, obtained by either dry or wet oxidation, was used as the dielectric. The capacitors were made on two kinds of substrates. The first was 2.5 at°
296
S. MARGALIT, A. BAR-LEV, A. B. KUPER, H. AHARONI AND A. NEUGROSCUEL
tions’ 6) for the ideal (no surface states) case and assuming the same dielectric constant and band gap for this low Ge content sample and Si. The important conclusion to be drawn is that the FB voltage was shifted in the Si/Ge case from the negative side (normal to n-type Si) to the positive side. In this case it was shifted from 13.37 V for Si to + 8.69 V for Si/Ge. Shifts in the same direction, but much larger, were found for the cases of wet oxidation and for the p-type 4% Ge/Si substrate. The reason for the shifts is not completely clear. They cannot be accounted for by the small differences in work function of the different materials to Al [about 0.12 V difference between Ge and Si for l0~~ cm3 n-type doping14)]. A more reasonable assumption is that the Si/Ge oxide is negatively charged. This was checked and verified by an MOS transistor structure. The surface state density in the forbidden energy band was estimated, using Brown’s methodi7) of C—V measurements at varying temperatures (77 °K to 300 °K).The value of 2.4 x 1012 cm2 was obtained for the 4 % Ge case, which is an order of magnitude higher than in Si. Annealing caused the FB voltage ofa typical 2.50/ Ge capacitor to shift back from 9.3 V to 3.9 V, which indicated that the surface state density can be materially reduced and that they are of the acceptor type. The latter can be expected since the charge neutrality level is 0.3 eV above the valence band level for Si, but is down inside the valence level for Ge’5). For Si/Ge it is somewhere inbetween, and for n-type material, with Fermi level above the middle of the band gap, there would be more acceptor like filled states below it and above the charge neutrality level in Si/Ge than in pure Si. Thechargedistributioninsidetheoxidewasmeasured by the change in channel conductance of a depletion
Qs5x10~m2 1-2 .~ (k’” ~
*
J
±
*_*_+_
06
4 at
0/,
Ge
—
type MOS transistor structure while the gate oxide was etched away by stages. A closed geometry MOS structure was fabricated on 3 ~ cm n-type 100/ Ge/Si epitaxial layer. Because of the aforementioned FB shift, a p-channel depletion type transistor resulted, whose channel conductivity can be measured without gate electrode. The results are shown in fig. 9. No charges on the bare oxide surface were found, which is contrary to Deal’s assumptions’ 8) It can be seen that most of the negative oxide charge is located within about 400 A from the surface. This negative charge may be attri-
02 0
Fig.
on
9. 10%
iobo
1OIJO 0
3Oii
4000 0
~
d l~
Surface charge measurement on MOS transistor made Ge/Si single crystal wafer as function of the gate oxide
thickness.
buted to dangling bonds of Ge atoms piled up near the interface and penetrating the oxide to some extent. An MOS transistor with an Al gate was also fabricated on the same 100/ Ge/Si epitaxial layer. The hole mobility was found to be low, about 15 cm2/V see, otherwise it had the normal characteristics of a depletion device. The numerical values obtained from the devices point to a very strong dependence of FB and VT on the Ge content. 0.5 at0/ of Ge will probably be more than enough to shift the VT of a normal MOS transistor by about 2 V in the positive direction, which is attractive from the circuit point of view. Ge content of below 1 0/ would also enormously simplify the problem of growing single crystals of good quality with much higher mobilities, probably near the silicon vatues. Multiple oxidation processes would also be simplifiedforsuchdevices,asalreadymentionedinsection4. 6. Conclusion Si/Ge alloys were shown to possess some very interesting oxidation properties. 1st oxide is found to be almost equivalent to SiO 2 independent of Ge content. This is not so for higher order oxidation which proceeds at a faster rate to a greater thickness and includes Ge as an impurity. In the case of 40/ Ge, which was the lowest used in the oxidation experiments, the oxide properties changed gradually to include more and more Ge between the 2nd and the 6th oxidation cycle. It is therefore hoped that wafers with below I 0/ Ge could be oxidised several times before the oxide properties would materially change. The 1st oxide was used in production of MOS capacitors and transistors and it was found that the Ge
OXIDATION
OF SILICON—GERMANIUM
introduced large negative charges into the oxide located within 400 A of the interface. These charges caused large shifts in flat band and threshhold voltages of the devices in the positive direction, which is attractive from device utilisation viewpoint, and opens up the possibilities of enhancement type n-channel devices. For shifts of about 2 V it is believed that Ge content of around 0.50/ will be suffIcient. Such low concentration would hopefully not degrade the effective channel mobility or increase surface state densities to the values encountered in our first experimental MOS transistors, built on 100/ Ge substrate, which suffered from very low mobility and high surface state density. References 1) H. Stohr and W. Klemm, Z. Anorg. Chem. 241 (1939) 305. 2) S. Margalit, D. Sc. Thesis, Faculty of Electrical Engineering, Technion, Haifa, Israel, (1971). 3) H. Aharoni, A. Bar-Lev, I. A. Blech and S. Margalit, Thin Solid Films 11 (1972) 313.
ALLOYS
297
4) S. Tolansky, Surface Microtopography (Interscience, New
5) York, G. R. 1960). Booker and C. E. Benjamin, Westinghouse Res. Lab. Scientific Paper 62-126-453-PI (April 1062). 6) H. Wolf, Semiconductors (Wiley—interscience, New York, 1971) p. 352. 7) W. A. Pliskin and H. S. Lehman, J. Electrochem. Soc. 112 (1965) 1013. 8) B. E. Deal, J. Electrochem. Soc. 110 (1963) 527. 9) A. S. Grove, 0. Leistiko Jr. and C. T. Sah, J. AppI. Phys. 35 (1964) 2695. 10) B. E. Deal, A. S. Grove, E. H. Snow and C. T. Sah, J. Electrochem. Soc. 112 (1965) 308. 11) I. Simon, in: Modern Aspects of the Vitreous State, Vol. I, Ed. J. D. Mackenzie (Butterworths, 1966) p. 120. 12) P. Hofstad, High Temperature Oxidation Metals (Wiley, New York, 1966). 13) Ref. 6, p. 148. 14) Ref. 6, p. 426.
15) Ref. 6, p. 429. 16) A. Goetzberger, Bell Syst. Tech. J. 45 (1966) 1097. 17) D. M. Brown and P. V. Gra, J. Electrochem. Soc. 1i5 (1968) 760. 18) B. E. Deal, M. Sklar, A. S. Grove and F. H. Snow, J. Electrochem. Soc. 114 (1967) 266.