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Gold on GaAs: Its crystallographic orientation and control on the orientation of the Au-Ga reaction product
Thin Solid Films, 128 (1985) 299-3 19 PREPARATION
299
AND CHARACTERIZATION
GOLD ON GaAs: ITS CRYSTALLOGRAPHIC ORIENTATION AND CONTROL ON THE ORIENTATION OF THE Au-Ga REACTION PRODUCT D. D. L. CHUNG
AND EDWARD
Department qfMetallurgical PA 15213 i U.S.A.) (Received
May 29,1984;
BEAM III
Engineering and Materials Science, Carnegie-Mellon
accepted
February
University, Pittsburgh,
25,1985)
X-ray diffraction of a 1500 A evaporated gold thin film on GaAs( 100) was used to study the textures of gold and the Au-Ga reaction product (Au7Ga, or P-(Au-Ga)). Post-deposition annealing up to 450°C for 30 min was found to enhance the gold orientation with Au(100) // GaAs(lOO) and Au[OOl] // GaAs[OOl] and weaken the gold orientation with Au( 11 l)// GaAs(lOO) and Au[OlT] // GaAs[Ol l] (and its fourfold rotation twins around GaAs[lOO]). The gold-rich terminal substitutional solid solution rx-(Au-Ga) was observed to begin forming below 350 “C. Dissolution of a-(Au-Ga) at 450-500 “C on heating and precipitation of a-(Au-Ga) on subsequent cooling caused a sharp increase in the orientation with Au( 110) // GaAs( 100) and Au[ liO] // GaAs[Ol 11, which has the smallest lattice misfit compared with the other gold orientations. The dissolution and precipitation of a-(Au-Ga) also caused the appearance of aligned rectangular pits. The crystallographic orientation of Au,Ga, (or P-(Au-Ga)) was controlled by the orientation of cr-(Au-Ga) that had been enhanced by the dissolution and precipitation of cr-(Au-Ga) such that the lattice match between Au,Ga, and gold was nearly perfect on the Au{ ill} interface planes. A dominant orientation relationship in the Au( 111) plane is . Au(lll)//Au,Ga,(llO)//GaAs(21i) Au[ liO] // Au,Ga,[OOl]
1.
//GaAs[Ol
l]
INTRODUCTION
The structural effects of heating gold thin films on single-crystal GaAs have been the subject of numerous papers because of the importance of gold-based metallizations for ohmic contacts in GaAs very-large-scale integration and solar cell technologies. These effects include the interfacial phase formation and the morphological changes. A structural effect which had not previously been investigated is the effect of heating on the texture (crystallographic orientations) of the gold thin film. It is thus the subject of this paper. It should be made clear that this heating refers to post0040-6090/85/$3.30
((3 Elsevier Sequoia/Printed
in The Netherlands
300
D. D. L. CHUNG,
E. BEAM 111
deposition heating rather than to substrate heating during deposition. The effect of the latter form of heating on the gold texture had been previously reported’. By using the pole figure method of X-ray diffraction on Au/GaAs( 100) we found that post-deposition heating caused the gold texture to increase in purity, such that the stable orientation grows at the expense of the less stable orientations. The most stable orientation had Au( 100) // GaAs( 100) and Au[OOl] // GaAs[OOl]. The least stable orientation observed had Au( 111) // GaAs( 100) and exhibits fourfold twinning about the GaAs[ 1001 direction. The a-(Au-Ga) phase (the terminal gold-rich solid solution) should form as a result of the diffusion of gallium from GaAs to gold. Because of the close similarity between Ir-(AuGa) and gold, the distinction between these two phases had not previously been made. In this work, by distinguishing between these two phases through careful X-ray diffraction measurement, we have obtained the first evidence of the formation of c*-(AuGa) in Au/GaAs. Furthermore, the lattice constant of m-(Au-Ga) was determined to be 0.997 of the lattice constant of gold, in agreement with that predicted by substitutional solid solution theory’. Additional evidence for cr-(Au-Ga) was provided by in situ X-ray diffraction observation3 of the complete dissolution of cr-(AuGa) in liquid AuGa at 525 F 25 “C and 1 atm. After the complete dissolution of cc-(AuGa) in liquid Au-Ga, cooling resulted in the precipitation of r-(AuGa) at 525 + 25 “C and 1 atm ‘. In this work, this phase transformation was found to alter the texture of x-(Au-Ga), so that the 110 texture component was greatly enhanced. A similar effect was also observed4 for the melting of the gold-rich solid solution in Au/Ge/Au/GaAs(lOO) and Ni/Au/Ge/Au/GaAs( 100). In Au/GaAs, subsequent to the dissolution and precipitation of cr-(Au-Ga), liquid Au-Ga and x-(Au-Ga) underwent3 a peritectic transformation to Au,Ga, (or lS(AuGa)) at 415 i 5 -C and 1 atm. Previous work on 300 8, Au/GaAs( 111) after heating in vacuum (lOeh Torr) up to 450 “C at 100 “C mini showed that Au,Ga, (or l!L(Au-Ga)) was present with an orientation relationship which corresponded to a perfect lattice match between Au,Ga, and gold, suggesting that the crystallographic orientation of Au,Ga, was controlled by gold’. Consistent with these observations, we found that, in 1500 8, Au/GaAs( 100) the crystallographic orientation of Au,Ga, (or lS(AuGa)) was controlled by the a-(Au-Ga) texture component which had been enhanced by the dissolution and precipitation of cx-(Au-Ga), namely CL-(Au-Ga) with the orientation Au( 110) // GaAs( 100) Au[lTO] //GaAs[Ol
l]
For 600 8, gold deposited onto GaAs( 100) at 280 “C and subsequently annealed at 330 ‘C, pyramidal pits bounded by GaAs{ Ill} planes and agglomerated monocrystalline gold had been observed, together with a monocrystalline hexagonal AuGa phase5. The interface on the GaAs{lll} planes was fully coherent5. Consistent with this observation, we observed that, in 1500 A Au/GaAs(lOO), the lattice match between Au,Ga, (or P-(Au-Ga)) and a-(Au-Ga) of the 110 texture component was almost perfect on the Au{ 111) planes.
ORIENTATIONOF
2. EXPERIMENTAL
301
Au ON GaAs
TECHNIQUES
Au/GaAs(lOO) wafers were kindly provided by P. Lindquist of the HewlettPackard Corporation. The GaAs substrates were tellurium doped ((2-3) x 10” onto the substrate at 3 8, s-r at a cm 3), and the gold was boat evaporated temperature of less than 100 “C. The final gold thickness was 1500 A. The wafers were diced into small rectangular pieces of about 5 mm x 7 mm. They were cleaned ultrasonically with acetone, isopropyl alcohol and deionized water and were then wrapped in similarly cleaned pieces of tantalum foil. The foil was used to reduce oxidation of the gold surface during heat treatment. The wrapped samples were encapsulated in Vycor glass tubing at a pressure of about 1 x lop6 Torr. Heat treatment was carried out by immersing the sealed glass tubes into molten lead baths for a fixed time (30 min). The temperatures used were accurate to approximately + 1 “C. Subsequent to heat treating, samples were made for current-voltage (Z-I/) testing by making a number of contact pads on the substrate. These contact pads (approximately 250 urn x 350 urn) were produced photolithographically, and the unwanted gold was removed with a KII-H,O etchant. The samples were then heat treated as previously described. The I-1/ characteristics were measured using a Tektronics type 576 curve tracer with one probe on a contact pad and the other on the main area of the gold film. Scanning electron microscopy (SEM) was performed at room temperature after the heat treatment, using a Cambridge scanning electron microscope. X-ray diffraction results were obtained by using a Rigaku D/MAX II powder X-ray diffractometer system. A fine-focus copper X-ray tube (Cu Ka radiation) was used. Detection was achieved with a scintillation counter. All results were obtained by using the step scan mode of the diffractometer. Pole figures were obtained by using the Schulz reflection method (Fig. 1). The X-ray source and counter were kept at an appropriate 20 angle while the sample was rotated in its own plane about an axis normal to the GaAs(lOO) substrate (i.e. axis BB’, corresponding to a change in the angle fl, which ranges from 0” to 360”) and a
X-RN SOURCE
COUNTER
Fig. I. The Schulz reflection figure diagram.
‘8.270. (b)
(4
method
of pole figure measurement:
(a) experimental
geometry;
(b) a pole
302
D. D.
L. CHUNG,
E. REAM III
horizontal axis along the intersection of the specimen surface and the diffraction plane (i.e. axis AA’, corresponding to a change in the angle c(,which ranges from 0” to 90). The angle LXwas taken to be 0” when the specimen surface was parallel to the diffraction plane. Pole figure measurements were made by changing a in 2 steps (from 0” to 70”) and p continuously (from 0” to 360”) at either 80” mini ’ or 320” min I. The data were plotted on a chart recorder as intensity uersus /I for each fixed value of a and were subsequently transcribed to a pole figure diagram (Fig. l(b)). 3.
EXPERIMENTAL
RESULTS
3.1. Current-voltage clzuruc.teristics The I-V characteristics obtained with the as-deposited sample and after heating at 350, 450, 500 and 550°C for 30 min are shown in Fig. 2. The characteristics were of a Schottky type in the as-deposited sample and changed to ohmic behavior at 300-350°C. The ohmic behavior persisted up to the highest temperature investigated, i.e. 550 “C.
550°C pT
-500°C
I I
I (mA) I
in, *0
I 0 VP.0
for
RT
Fig, 2. I-V characteristics obtained before heating, i.e. at room temperature 3.50, 450, 500 and 550 “C for 30 min (for these temperatures the horizontal by 20).
(RT), and after heating at axis should be multiplied
ORIENTATION
OF Au
ON
303
GaAs
3.2. Scanning electron microscopy Figure 3 shows SEM photographs obtained after heating at 400,450,500 and 550 “C for 30 min. Aligned rectangular pits were observed in the gold film after heating at 400 “C. At 450 “C the color remained gold; at 500 “C the color became silver-gold; at 550°C the color became completely silver. Concomitant with this color change was the formation of a large number of aligned rectangular protrusions, as previously reported by a large number of workers5-14. The contact was considerably more uniform below 500 “C than above 500 “C.
(4
(b)
(c) Fig. 3. SEM photographs for 30 min.
3.3.
obtained
ex
(4 situ after heating at (a) 400 “C, (b) 450 ‘C,(c) 500 “C and (d) 550 “C
Texture of goldand its temperature dependence 3.3.1. 0-29 results Although the pole figure method is required for texture analysis, the results of 8-28 (powder) X-ray diffraction strongly suggested that the texture of gold changed on heating. Table I lists the intensities of the Au 111, Au 200, Au 220 and Au,Ga, (or p-(Au-Ga)) 030 peaks obtained by the 8-28 method at room temperature in the asdeposited sample and after heating at 350,400,450, 500 and 550 “C for 30 min. The Au,Ga, (or 0) 030 peak was not observed up to 450°C. After heating at 5OO”C, Au,Ga, (or 0) 030 appeared as a weak peak. After heating at 550 “C, it had grown considerably and Au,Ga, (or p) 113 was also observed weakly. Although phase formation was not observed below 500 “C, significant changes in the intensities of the gold peaks were observed in this temperature range. As shown in Table I, the
D. D. L. CHUNG.
304
TABLE
E. BEAM 111
I
INTENSITIESOF PEAKSOBSERVED
BY 62H
HEATING ATVARIOUSTEMPERATURESFOR Peak
Intensity (counts
X-RAY
DIFFRACTION AT ROOM
TEMPERATURE BEFORE ANU AFTER
30Itlitl
sm ‘)jor thefollowing
Room temperoturr
350
c
temperatures 400°C
450
5750 2000 700
4200 2500 800
c
500 ‘C
550 ‘C
3000 3500 1000 100
450
ius deposited)
Au 111 Au 200 Au 220
7500 880 500
8500 2400 400
Au,Ga, (or j3) 030 -.
850
not observed.
intensity of the Au 111 peak increased slightly after heating at 350°C but monotonically decreased for higher heat treatment temperatures. In particular, the drop in intensity between heat treatment temperatures of 500 and 550°C was precipitous. The intensities of the Au 220 and Au 200 peaks both exhibit an increasing trend with increasing heat treatment temperature. However, between heat treatment temperatures of 500 and 550 “C, the Au 200 and Au 220 intensities vanished. These results suggest that the Au(ll1) texture component weakened on heating, while the Au(100) and Au(ll0) texture components strengthened on heating. The precipitous drop of the gold intensities between 500 and 550 “C was apparently due to the significant Au,Ga, (or p) phase formation and the resulting gold consumption in this temperature range. 3.3.2. PoleJigure results To confirm the effect of heating on the gold texture and to identify the azimuthal orientation(s) of each texture component, the pole figure method was used. Firstly, let us consider the azimuthal orientations of each texture component. Figure 4 shows the pole figures obtained for various gold (Figs. 4(a)-4(c)) and GaAs (Fig. 4(d)) peaks. The pole positions shown in Fig. 4 were the same both for the asdeposited sample and for samples heated at 350,400 and 450 “C. However, the pole intensities varied with the heat treatment temperature, as discussed later in this section. Figure 4(a) shows the Au(ll0) and Au(100) pole figures corresponding to the texture component Au( 111) // GaAs( 100). There are four sets of these poles. The four sets, 90” from one another, correspond to the four azimuthal orientations due to fourfold twinning about the Au[lll] direction. In other words, for the epitaxial relationship Au(lll)//GaAs(lOO) the four azimuthal Au[Oli]
orientations
//GaAs[Ol
Au[ 112]// GaAs[Ol
l] l]
are
ORIENTATION
OF
305
Au ON GaAs
(II0
Au(lll) x
000)
@
(a)
Fig. 4. Pole figures of (a) gold with texture Au(1 ll)//GaAs(lOO), GaAs( IOO), (c) gold with texture Au(lOO)//GaAs(lOO) and (d) GaAs.
Au[lOi]
//GaAs[Ol
l]
A~[21 l] //GaAs[Ol
l]
(b) gold
with texture
Au(1 lo)//
These azimuthal orientations were obtained by combining the information in Figs. 4(a) and 4(d). Figure 5 shows the plot of intensity uersus the fi angle at the 20 angle of Au 200 and at u = 55”. This plot shows 12 broad peaks at p values of 30”, 60”, 90”, 120”, 150”, 180”, 210”, 240”, 270”, 300”, 330” and 360”. These peaks correspond to the 12 Au 100 poles shown in the outer ring of Fig. 4(a). The large width of these poles means that the texture component Au( 111) // GaAs( 100) is weak. Figure 4(b) shows the Au( 100) and Au(ll1) pole figures corresponding to the texture component Au( 110) // GaAs( 100). There are two sets of these poles. The two sets, 90” from one another, correspond to the two azimuthal orientations due to twinning about the Au[l lo] direction. In other words, for the epitaxial relationship Au(llO)//GaAs(lOO) the azimuthal
orientations
are
Au[ liO] // GaAs[Ol
l]
Au[OOl] //GaAs[Ol
l]
and
306
D. D. L. CHUNG,
/ 360
,
I
300
240
Fig. 5. Plot of intensity
,
,
If30 p (degrees)
,
,
120
E. REAM
III
,
60
0
t’x the p angle at the 20 angle of Au 200 and at x = 55 (Au(
I 1I) 11111 GaAs( 100)).
These azimuthal orientations were obtained by combining the information in Figs. 4(b) and 4(d). Figure 6(b) shows the plot of intensity versus the p angle at the 28 angle of Au 111 and at c( = 35”. This plot shows four peaks such that two of the peaks are about twice as strong as the other two. These four peaks correspond to the two sets of poles in the inner ring of Fig. 4(b). Hence, the azimuthal orientation with Au[OOl] //GaAs[Ol l] was about twice as abundant as that with Au[liO] // GaAs[Ol 11. The peaks in Fig. 6(b) are much sharper than those in Fig. 5. This means that the texture component with Au(1 lO)//GaAs( 100) is much stronger than that with Au( 111) // GaAs( 100).
I
360
I
I
I
160
r 0
I
360
1
I
I
160 p (degrees)
I
0
(b)
(a) p(degrees)
Fig. 6. Plots of intensity vs. the /I angle (a) at the 20 angle of Au I I I and at IY= 55’ (Au( 100) :/GaAs( 100)) and (b) at the 20 angle of Au 111 and at r = 35” (Au( 1 10) // GaAs( 100)).
Figure 4(c) shows the Au(ll0) and Au(ll1) pole figures corresponding to the texture component Au( 100) // GaAs( 100). There is only one set of these poles. Thus, no twinning was observed about the Au[lOO] direction. For the epitaxial relationship Au( 100) // GaAs( 100) the azimuthal
orientation
Au[OOl] //GaAs[OOl]
is
Au
ORIENTATIONOF
ON
307
GaAs
This azimuthal orientation was obtained by combining the information in Figs. 4(c) and 4(d). Figure 6(a) shows the plot of intensity uersus the p angle at the 28 angle of Au 111 and at c( = 55”. This plot shows four peaks which are 90” apart from one another. They correspond to the poles in the outer ring of Fig. 4(c). The peaks in Fig. 6(a) are also much sharper than those in Fig. 5. Hence, the texture component with Au( 100) // GaAs( 100) is much stronger than that with Au( 111) // GaAs( 100). Although the degree of texturing is quite strong for the texture components with Au( 100) // GaAs( 100) and Au( 1 lo)// GaAs( loo), the degree of texturing is considerably less than that of the single-crystal GaAs substrate. The plot of intensity uersus the fi angle at the 28 angle of GaAs(220) and at c( = 45” is shown in Fig. 7. The sharpness of the poles in Fig. 7 is consistent with the single-crystal character of GaAs. Next, let us consider the effect of heating on the gold texture. Table II lists in column (a) the intensity of a pole representing the texture component Au(1 lo)// GaAs(lOO) relative to the intensity of a pole representing the texture component Au(lOO)//GaAs(lOO) and in column (b) the intensity of a pole representing the texture component Au( 111) // GaAs( 100) relative to the intensity of a pole represent-
I
I
360
270
I 180
I
I
90
0
PCdegrees)
Fig. 7. Plot of intensity
TABLE RELATIVE
us. the b angle at the 20 angle of GaAs 220 and at a = 45”.
II POLE INTENSITIES
Heat treatment
As deposited 350 400 450 500 550
temperature
(“C)
b-4 Au(llO)//GaAs(lC@)
(b) Au(lll)//GaAs(100)
Au(lOO)//GaAs(lOO)
Au(lOO)//GaAs(lOO)
0.75 0.50 0.50 0.11 1.00
0.375 0.200 0.150 0.100 0.066
308
D. D. L. CHUNG,
E. REAM III
ing the texture component Au( lOO)// GaAs( 100). The pole representing the texture component Au( 100) // GaAs( 100) is the Au( 111) pole at E = 54”; the pole representing the texture component Au(ll0) // GaAs( 100) is the Au( 111) pole at E = 36”; the pole representing the texture component Au( 111) // GaAs( 100) is the Au( 100) pole at E = 55”. Because the representatives are different poles, it is meaningless to compare column (a) with column (b) in a horizontal direction in Table II. However, comparison within the same column in the vertical direction in Table II is meaningful and yields the effect of heating on the relative abundance of the texture components. Column (b) shows that the abundance of the texture component with Au(l1 l)//GaAs( 100) relative to that of the component with Au( lOO)//GaAs( 100) decreases monotonically with increasing heat treatment temperature. This trend is consistent with that suggested by the U-20 results in Table I. Column (a) shows that the abundance of the texture component with Au( 110) // GaAs( 100) relative to that of the component with Au(lOO)//GaAs( 100) decreases with increasing temperature up to 450 “C and then increases sharply between 450 and 500°C. This behavior was not revealed by the H-20 results in Table I and shows that the O-20 results alone cannot give definitive texture information. Table III shows the relative pole densities obtained by correcting the relative pole intensities of Table II. The correction was made by compensating for the difference in structure factors and in LYbetween different pole representatives. For column (a), no correction was necessary because both pole representatives are Au 111 and both occur at a < 54”. For column (b), correction was made to compensate for the difference in structure factors between the pole representatives; no additional correction was necessary because both pole representatives occur at the same c( of 54”-55”. TABLE RELATIVE
III POLE DENSITIES
Hear treatment temperature (‘C)
(b)
(4 Au( 110) //GaAs(
100)
Au(111)//GaAs(lOO)
Au(lOO)//GaAs(lOO)
Au(lOO)//GaAs(lOO)
As deposited
0.75
0.721
350
0.50
0.385
400
0.50
0.288
450
0.11
0.192
500
1.00
0.127
Table III shows that the texture component with Au(100) // GaAs(100) was the most abundant (i.e. with the highest volume fraction) in the as-deposited sample and became even more dominating as the heat treatment temperature increased to 450 “C. After heating at 500 “C, the texture component with Au(l1 l)//GaAs(lOO) was negligible in volume fraction compared with the other two texture components, which were comparable in volume fraction. Combining the information in Tables I and III, we can conclude that the main trends in the effect of heating on the texture are the following.
ORIENTATIONOF
Au ON GaAs
309
(1) The fraction of gold with a texture of Au( 11 l)// GaAs(lOO) decreased significantly with increasing heat treatment temperature up to 500 “C. (2) The fraction of gold with a texture of Au(lOO)//GaAs(lOO) increased significantly with increasing heat treatment temperature up to 450 “C. (3) The fraction ofgold with a texture of Au( 110) // GaAs( 100) increased slightly with increasing heat treatment temperature up to 450°C and increased sharply between 450 and 500 “C. 3.4.
Texture
of Au,Ga,
(or p)
The 8-28 (powder) X-ray diffraction method revealed essentially only the Au,Ga, (or p) 030 peak, as shown in Table I. This peak first appeared after heating at 500°C and became much stronger after heating at 550°C. To confirm the identification of the Au,Ga, (or p) phase and to study the texture of this phase, the pole figure method was used. Figure 6 shows plots of intensity versus the bangle at 20 and a angles where the GaAs 220, Au,Ga, (or p) 223 and Au,Ga, (or 0) 113 poles were maximum in intensity. The observation of Au,Ga, (or p) 113 at c( = 58”, Au,Ga, (or p) 223 at tl = 44” and Au,Ga, (or p) 030 at c( = 0” indicates that the epitaxial relationship of Au,Ga, (or p) was Au,Ga,(
100) // GaAs(lOO)
and furthermore confirms the phase identification. A large number of sharp peaks were observed in the plots of intensity uersus the fl angle for Au,Ga, (or p) 113 and 223, as shown in Fig. 8. The peaks are considerably sharper in the plots of intensity versus the /? angle than in the plots of intensity versus the ct angle. The 010 stereographic projection of Au,Ga, is shown in Fig. 9. The planes (113) (i23), (113) and (i23) belong to one family, as can be seen by converting the Miller indices from the three-index system to the four-index system. Similarly, the planes (223), (243), (223) and (243) belong to another family. The peaks labeled 1, l’, 2 and 2’ in Fig. 8 are due to the following four azimuthal orientations of Au,Ga,, as shown by the pole figure plot of these peaks in Fig. 10, which is equivalent to four sets of Fig. 9 poles superimposed together. 1: Au,Ga,[OOl] //GaAs[Oli] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 8” from GaAs[OlO], toward GaAs[OOi]). 1’: Au,Ga,[OOl] //GaAs[Ol l] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 8” from GaAs[OOl], toward GaAs[OlO]). 2: Au,Ga,[30i]//GaAs[OOl] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 17” from GaAs[OlO], toward GaAs[OOl]). 2’: Au,Ga,[30i] // GaAs[OiO] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 17” from GaAs[OOl], toward GaAs[OiO]). The azimuthal orientations 1 and 1’ are 90” from one another; the azimuthal orientations 2 and 2’ are also 90” from one another. Hence, twinning of Au,Ga, occurs. Examination of Fig. 8 indicates that (i) all the peaks due to orientation 1 are intense, (ii) some of the peaks due to orientation 2 are intense and (iii) all the peaks
310
D. D. L. C-HUNG.
E. BEAM 111
(a)
36C -
p (degrees) Fig. 8. Plots of intensity us. the /I angle for (a) GaAs 220 and a = 45‘,(b) Au,Ga, (or P-(Au-Ga)) 223 and t( = 44” and (c) Au,Ga* (or P-(Au-Ga)) 113 and a = 58’. The peaks labeled 1, l’, 2 and 2’ correspond to the poles in Fig. 10.
due to orientations 1’ and 2’ are weak. This means that (i) orientation 1 is the major azimuthal orientation, (ii) orientation 2 is the minor azimuthal orientation and (iii) orientations 1’ and 2’ are almost negligible. Thus, the proportion of twinned regions is low. The peaks labeled X in Fig. S(b) are due to GaAs(220), which is so intense that diffraction using the white radiation part of the X-ray spectrum yields a peak at the
ORIENTATION
OF Au ON GaAs
311
223 .
010 .
2io0
223 . II3 .
ooi Fig. 9. 010 stereographic projection of Au,Ga,
20 angle of Au,Ga, 223. Since the GaAs 220 peak occurs at a = 45” and the Au,Ga, 223 peak occurs at a = 44”, the former was observable in Fig. 8(b). The unlabeled peaks in Figs. 8(b) and 8(c) are all weak. They are due to other very minor azimuthal orientations. 3.5. Formation of a-(Au-Ga) Table IV lists the 28 values of the strongest gold and GaAs diffraction peaks observed at room temperature in the as-deposited sample and after heating at various temperatures from 350 to 500 “C for 30 min. The gold peaks shifted to larger angles between room temperature (as-deposited sample) and 350 “C and stayed at the upshifted positions with heating to higher temperatures. In contrast, the GaAs peak remained at the same position for all temperatures. The amount of upshift of either gold peak corresponds to a decrease in the d spacing of the peak by a factor of 0.997. This means that the lattice constant also decreases by this factor. The decrease in lattice constant is interpreted as being due to the formation of a substitutional Au-Ga solid solution with gold as the host. The atomic radii of gallium and gold are 1.41 8, and 1.46 8, respectively. These values are close enough for substitutional solid solubility. The Au-Ga phase diagram indicates that the maximum solid solubility of gallium in gold is 12 at.% Ga. Vegard’s law indicates that, for a composition with 12 at.% Ga, the lattice constant of the host should decrease by a factor of 0.994 on formation of the solid solution. Furthermore, the lattice constant of an Au-Ga solid solution containing 11 at.% Ga had been reported to be 0.9985 of that of gold l5 . Therefore, the observed decrease in lattice constant can be explained by the formation of cr-(Au-Ga). It should be noted that
312
D. D. L. CHUNG,
E. REAM III
2 2
GoAs’,
oio
\
Fig. 10. Superposition of the pole figures of GaAs 100, Au,Ga, (or B-(Au-Ga)) 223 and Au,Ga, (or B-(Au-Ga)) 113. For the Au,Ga, (or B-(Au-Ga)) pole figures, only the poles corresponding to the peaks labeled 1, l’, 2 and 2’ in Fig. 8 are shown: 0, poles due to 1 and 1’; 0, poles due to 2 and 2’. TABLE IV 20 VALUES OF THE STRONGEST GOLD AND GaAs
DIFFRACTION PEAKS OBSERVED IN THE AS-DEPOSITED
SAMPLE AND AFTER HEATING AT VARIOUS TEMPERATURES FOR 30tllin
Peak
28 (deg)for
Au 111 Au 200 GaAs 400
samples heated a~ the,following temperatures
As deposited
350 “C
400
38.18 44.42 66.04
38.30 44.60 66.04
38.30 44.58 66.04
(c
425 ‘C
450,‘C
5OO‘C
55O’C
38.30 44.62 66.04
38.30 44.60 66.04
38.30 44.62 66.04
38.30 44.60 66.04
thermal expansion or vacancy formation would have caused the lattice constant increase instead and thus cannot explain the observed lattice constant decrease. 4.
to
DISCUSSION
The observation of c+(Au-Ga) after heating between gold and GaAs takes the form Au+Ga
at 350 “C implies that the reaction
-+ cr-(Au-Ga)
and this reaction occurs in the solid state at 350°C or below. An in situ X-ray diffraction study of 1500 A Au/GaAs (the same sample as used in this work) showed3 that further heating causes the dissolution (or melting) of cr-(Au-Ga), such that complete dissolution (or melting) occurs at 525+25 “C and 1 atm (1.01 x lo5 Pa) and at 425_f25 “C in a dynamic vacuum of 56.7 Pa. The observation of the dissolution of cr-(Au-Ga) gives additional evidence for the existence of cr-(Au-Ga).
ORIENTATIONOF Au ON GaAs
313
1n situ X-ray diffraction also shows that cooling subsequent to the dissolution of ol-(Au-Ga) causes the formation of P-(Au-Ga) (or Au,GaJ3. Thus the minimum heat treatment temperature required for the formation of P-(Au-Ga) (or Au,GaJ is 525 &-25 “C at 1 atm (1 .Ol x lo5 Pa) and 425 + 25 “C in a dynamic vacuum of 56.7 Pa. The minimum l3-(Au-Ga) (or Au,Ga,) formation temperature (about 500 “C) found ex situ in this work (Table I) agrees with the temperature for the complete dissolution of cr-(Au-Ga) at 1 atm (1.01 x 10’ Pa) but does not agree with that in a dynamic vacuum of 56.7 Pa. This is attributed to the fact that the ex situ heating was performed in a sealed evacuated tube and the arsenic evolution which occurred from the encapsulated sample on heating caused the arsenic vapor pressure in the tube to build up. Compound formation temperatures below 500°C have been previously reported 1,5*9,16 for gold films thinner than 1500 A. This is due to the effect of the gold film thickness on the form of the reaction3. The sharp increase in the volume fraction of gold with the texture component Au(1 lO)//GaAs(lOO), observed ex situ between 450 and 500 “C (Table III), is attributed to the partial dissolution (or melting) of a-(Au-Ga) on heating to 500 “C and the precipitation (or solidification) of cr-(Au-Ga) on subsequent cooling3. The cr-(Au-Ga) phase that precipitates tends to have an orientation corresponding to a small lattice misfit with respect to GaAs. The orientation with Au( 100) // GaAs( 100) corresponds to misfits which are smaller than those of the other two gold orientations, as shown later in this section. The P-(Au-Ga) (or Au,Ga,) phase forms from cr-(Au-Ga) (which had undergone dissolution and precipitation) through a peritectic transformation on cooling3; so the orientation of @(Au-Ga) (or Au,Ga,) is controlled by that of a-(Au-Ga) after a-(Au-Ga) has undergone dissolution and precipitation. The marked degradation of the contact uniformity observed ex situ by SEM at 550°C (Fig. 2(d)) is also attributed to the dissolution of a-(Au-Ga) at this temperature. The dissolution on heating and precipitation of cl-(Au-Ga) followed by P-(Au-Ga) (or Au,Ga,) on subsequent cooling led to the formation of aligned rectangular protrusions, which were essentially identical with those previously reported (ref. 13, Fig. 4). Consistent with this reaction form is the fact that the rectangular protrusions were much richer in gold than the matrix was13. The change from Schottky to ohmic behavior was observed ex situ at 300-350°C (Fig. 3), although l3-(Au-Ga) formation was not observed by X-ray diffraction below 450 “C (Table I). This effect has recently been shown by Newman et al.” to be caused by leakage currents at the mesa periphery due to the presence of impurities. With a mesa etch, Newman et al. obtained a stable barrier height for temperatures to over 425 “C. In the as-deposited as well as heated samples used in this work, the texture components with Au(100) // GaAs( 100) and with Au( 1 lo)// GaAs( 100) were much stronger (i.e. higher degree of texturing) than that with Au(l1 l)//GaAs(lOO). Moreover, the volume fraction with the texture component Au( 100) // GaAs( 100) was the highest. The texture component with Au(lOO)//GaAs( 100) has the single azimuthal orientation of Au[OOl] // GaAs[OOl] and corresponds to a misfit of 27.9% if aAu is
314
D. D. L. CHUNG,
E. BEAM III
compared with aCaAs. or 8.2’jb if 3aAu is compared with 2aGaAF, where aAu and aGaA, are the lattice constants of gold and GaAs respectively. This epitaxial relationship has been reported previously’s la “. The texture component with Au(1 lO)//GaAs(lOO) has not been reported previously. This texture component involves two symmetrically equivalent azimuthal orientations, with Au[liO]~/GaAs[Oll] and Au[OOl] //GaAs[Ol
l]
Each of these azimuthal orientations corresponds to misfits of 2.0% and 3.80/;,in two perpendicular directions. The weak texture component, with Au( 111) // GaAs(lOO), has not been previously reported either. This texture component exhibited fourfold twinning, which results in the four azimuthal orientations Au[Oli]
//GaAs[Ol
l]
Au[llZ]
//GaAs[Ol
l]
Au[ lOi] // GaAs[Ol
l]
A~[21 l] //GaAs[Ol
l]
These four orientations have identical misfits of 27.9% (or 8.2% in one direction and 25.0% (or 0.07%) in a perpendicular direction). The misfit of 0.027% was obtained by comparing 4($1/2aAu with 5(1/21’2)ao,A,. Fourfold rotation twinning about the Au[ 11 l] direction had been previously observed in gold thin films on GaAs(lll), where the epitaxial relationship’ was Au(lll) // GaAs(lll). In this work, we observed fourfold rotation twinning about Au[ll l] in gold thin films on GaAs(lOO), where Au(l1 l)//GaAs(lOO), although the texture was weaker than for the GaAs( 111) counterpart. Such twinning is thus quite general and its effects on the electrical behavior and contact morphology are still to be explored. The above observations clearly show that the amount of misfit does not govern the preferred epitaxial relationship between as-deposited gold and GaAs. However, it governs that between a-(Au-Ga) and GaAs after the melting and solidification of cl-(AuGa). On heating to 450 C, the volume fraction of the texture component Au( 111) h’ GaAs(lOO) diminished. The thermal instability of this texture component is consistent with its low degree of texturing. This was perhaps why this texture component has not been reported previously. On heating to 450 “C, the volume fraction of the texture component Au( 110) // GaAs( 100) increased slightly, whereas the volume fraction of the texture component Au( 100) // GaAs( 100) increased considerably. Thus, after heating at 450 “C, the the other components, as texture component Au( 100) // GaAs( 100) d ominated shown in Table III. This behavior is consistent with the fact that the texture with Au( 100) // GaAs( 100) predominates in samples deposited at substrate temperatures of 200 “C or above1,18 20.
ORIENTATIONOF
Next, let us consider epitaxial relationship was Au,Ga,(
315
Au ON GaAs the texture
of Au,Ga,
(or b-(Au-Ga)).
The observed
100) // GaAs( 100)
For this epitaxial relationship, a major azimuthal orientation (orientation 1) and a minor azimuthal orientation (orientation 2) were observed. In orientation 1, Au,Ga,[OOl] //GaAs[Oli]. The lattice misfit of Au,Ga, with respect to GaAs is 9.5% in this direction and - 3.4% in a perpendicular direction. For gold with the orientation Au( 100) // GaAs( 100) and Au[OOl] // GaAs[OOl], orientation 1 corresponds to Au,Ga,( lOO)// Au( 100) and Au,Ga,[OOl] // Au[Oli]. The lattice misfit of Au,Ga, with respect to gold of this orientation is 1.1% along this direction on the Au( 100) plane and - 10.8% along a perpendicular direction on this plane. For gold with the orientation Au(1 lo)// GaAs( 100) and Au[liO] // GaAs[Ol 11, orientation 1 corresponds to Au,Ga,( 100) // Au( 110) and Au,Ga,[OOl] // Au[l lo]. The lattice misfit of Au,Ga, with respect to gold of this orientation is 1.1% along this direction on the Au(ll0) plane and - 5.4% along a perpendicular direction on this plane. Hence, the lattice misfits between Au,Ga, and GaAs and between Au,Ga, and gold in the plane parallel to GaAs( 100) are all quite high. Let us consider then the misfits in the plane parallel to GaAs(l1 l), since GaAs(ll1) is parallel to the true interface5. Because the poles are sharper as a function of the /3 angle than as a function of the Mangle, the orientation relationship Au,Ga,( 100) // Au( 100) and Au,Ga,[OOl] // Au[Oli] is nearly equivalent to the relationship Au,Ga,(OlO) // Au( 111) and Au,Ga,[OOl] // Au[Oli], for which the lattice misfit is 1.1% along this direction on the Au( 111) plane and - 7.3% along a perpendicular direction on this plane. Similarly, the orientation relationship Au,Ga,(100)//Au(110) and Au,Ga,[OOl] //Au[l lo] is nearly equivalent to Au,Ga,( 110) // Au( 111) and Au,Ga,[OOl] //Au[l lo], for which the lattice misfit is 1.1% along this direction on the Au(ll1) plane and 0.4% along a perpendicular direction on this plane. The last set of lattice misfits is considerably smaller than all the other sets. In fact, the lattice misfits of the last set are so small that the lattice match can be considered to be perfect, as illustrated in Fig. 11. Therefore, orientation 1 is attributed to a perfect lattice match between Au,Ga, and gold in the Au(ll1) plane for gold with the orientation Au( 110) // GaAs( 100) and Au[ liO] // GaAs[Ol 11. The interfacial orientation relationship is thus Au(lll)//Au,Gaz(l10)//GaAs(21i) Au[ liO] // Au,Ga,[OOl]
// GaAs[Ol
l]
In orientation 2, Au,Ga,[30i]//GaAs[OOl]. Using an argument similar to that used to explain orientation 1, it is found that orientation 2 may be attributed to a moderately good lattice match between Au,Ga, and gold in the Au(ll1) plane for gold with the orientation Au( 110) // GaAs( 100) and Au[ liO] // GaAs[Ol 11. The interfacial orientation relationship is Au(1 ll)//Au,Ga,(221)//GaAs(21i) Au[ liO] // Au,Ga,[Oi2]
//GaAs[Ol
l]
316
D. D. L. CHUNG,
The interfacial orientation method for 300 A Au/GaAs(ll Au( 11 I)// Au,Ga,(OlO) Au[liO] + 10” toward GaAs[Oi 1] Within
experimental
E. BEAM III
relationship observed by the same X-ray diffraction 1) heated to 450 “C in a vacuum of lO-6 Torr was’
//GaAs( 111) Au[Oil]
//Au,Ga,[OOl]
error, this is equivalent
//GaAs[lfO]
+ lo-’ toward
to
Au(l1 l)//Au,Ga,(OlO)//GaAs(l11) A~[5411 //Au,Ga,[OOl]
//GaAs[541]
as illustrated in Fig. 12. The lattice misfit of Au,Ga, with respect to gold is -0.70/, along Au,Ga,[OOl] and 1.2% along Au,Ga,[lOO]. Thus the lattice match may be considered to be perfect between Au,Ga, and gold. The difference between this orientation relationship and those found in this work is attributed to the difference in the GaAs substrate orientations, which affected the texture of gold.
Fig. 1 I. Schematic representation of the orientation 1500 8, Au/GaAs( 100). The circles are gold atoms.
relationship
on Au( 111) and Au,GaJ
1 IO) planes in
Fig. 12. Schematic representation of the orientation 300 8, Au/GaAs( 111). The circles are gold atoms.
relationship
on Au(l11) and Au,Ga,(OlO)
planes in
Although the interfacial orientation relationship depends on the GaAs substrate orientation, the following generalizations can be made, based on the results of this work (on Au/GaAs(lOO)) and of ref. 1 (on Au/GaAs( 111)). (1) The orientation of Au,Ga, is such that the lattice misfit is less than 2% for A+Ga, with respect to gold. (2) If the Au,Ga, forms after the occurrence of the melting (or dissolution) of the gold-rich solid solution, the orientation of Au,Ga, is governed by the gold-rich solid solution that has been reoriented by the melting. (3) Nearly perfect lattice match (a misfit of less than 2%) between Au,Ga, and gold occurs on the Au{ 111) planes. The interfacial orientation relationship observed by electron diffraction for
ORIENTATION
OF
317
Au ON GaAs
600 A gold deposited onto GaAs(lOO) 330 “C had been reported to be5 Au( 111) // Au-Ga(Ol0) Au[ liO] // Au-Ga[OOl]
at 280 “C and subsequently
annealed
at
// GaAs( 111) //GaAs[
liO]
where Au-Ga is a hexagonal phase. For the lattice misfit calculation we assume that the Au-Ga phase is Au,Ga,. The lattice misfit of Au,Gaz with respect to gold is 1.1% along Au,Ga,[OOl] and -7.3% along Au,Ga,[lOO]. The difference between this and the orientation relationships found in this work is attributed to the small gold thickness, the high deposition temperature and the low annealing temperature, which apparently caused the reaction product to form in the solid state without the melting (or dissolution) of a-(Au-Ga). The relatively large misfit between Au,Ga, and gold is in contrast with the small misfits observed in this work and in ref. 1. The origin of this discrepancy is not clear at present. Au,Ga, and P-(Au-Ga) are both hexagonal, with almost identical lattice constants. The misfits given above were calculated for Au,Ga, rather than for P-(Au-Ga). The misfits calculated for P-(Au-Ga) are essentially the same as those calculated for Au,Ga,. Thus the conclusions given above are valid for either Au,Ga, or P-(Au-Ga). The two phases could not be distinguished. Gold and a-(Au-Ga) are both f.c.c., with lattice constants which differ by about 0.3%. The misfits given above were calculated for gold rather than for a-(Au-Ga). Although the misfits would have been slightly different if they were calculated for a-(Au-Ga) instead, the differences are small so that the conclusion given above concerning a nearly perfect lattice match is not affected. 90” rotation twinning was observed in Au,Ga, (or p-(Au-Ga)) of both orientations 1 and 2, although the proportion of twinned regions was low. Since fourfold twinning was not observed for the gold orientation which controls these Au,Ga, orientations, the origin of the twinning is attributed to the GaAs(100) substrate. In contrast, twinning was not observed’ in Au,Ga, (or P-(Au-Ga)) formed on heating gold on GaAs( 111). 5. CONCLUSION
This work has shown that post-deposition annealing of Au/GaAs results in two distinct structural changes: texture change and phase formation. The texture change involves an increase in the purity of the texture so that the texture component with Au( 100) // GaAs( 100) increases in volume fraction compared with other texture components. In particular, the texture component with Au( 100) // GaAs( 100) grows at the expense of that with Au( 111) // GaAs( 100). This tendency indicates that the texture component with Au( 100) // GaAs( 100) is stable, whereas that with Au(lll) // GaAs( 100) is only metastable. The gold-rich terminal solid solution cl-(Au-Ga) was observed to form at 350 “C or below. It is a substitutional solid solution with a lattice constant equal to 0.997 of that of gold. A large increase in the volume fraction with Au( 110) // GaAs( 100) was observed ex situ between 450 and 500 “C. This is attributed to the dissolution of a-(Au-Ga) on
318
D. D. L. CHUNG.
E. BEAM 111
heating and the precipitation of a-(Au-Ga) on subsequent cooling, as revealed by in situ X-ray diffraction3. This orientation was favored because ofits small lattice misfit compared with those of the other two orientations. The dissolution and subsequent precipitation of cc-(Au-Ga) also resulted in aligned rectangular pits which degraded the contact uniformity. This work has demonstrated that the crystallographic orientation of Au,Ga, (or P-(Au-Ga)) is controlled by the crystallographic orientation of cc-(Au-Ga) after the dissolution and precipitation of a-(Au-Ga), as the dissolution and precipitation of a-(Au-Ga) precede the formation of Au,Ga, (or P-(Au-Ga)). The control is such that the lattice match is perfect between Au,Ga, (or b-(Au-Ga)) and gold (or ol-(Au-Ga)) in the Au(ll1) plane, which is the true interface plane. The orientation relationship depends on the GaAs substrate orientation. For 1500 A Au/GaAs( 1’00); it is Au(l1 l)//Au,Ga,(llO)//GaAs(21i) Au[liO]//Au,Ga,[OOl]//GaAs[Ol For 300 A Au/GaAs(
l]
11 l), it is
Au(l1 l)//Au,Ga,(OlO)//GaAs(lll) Au[54i]
//Au,Ga,[OOl]
//GaAs[541]
ACKNOWLEDGMENTS
The X-ray diffraction equipment grant from the Division of Materials Research of the National Science Foundation under Grant DMR-8005380 was essential for this work. Equipment support from the Materials Research Laboratory Section, Division of Materials Research, National Science Foundation, under Grant DMR 76-8 156 1 A01 is also acknowledged. The technical assistance of T. Kim of CarnegieMellon University is greatly appreciated. REFERENCES
1 2 3 4 5 6 7 8 9 10 I1 12 I3 14
S. Leung, D. D. L. Chung and A. G. Mimes, Thin SolidFilms, 104 (1983) 109. Charles Barrett and T. B. Massalski, Structureof’Metuls (International Series on Materials Science and Technology, Vol. 35), Pergamon, Oxford, 1980. E. Beam III and D. D. L. Chung, Thin Solid Films, 128 (1985) 321. T. Kim and D. D. L. Chung, to be published. T. Yoshiie, C. L. Bauer and A. G. Milnes, Thin SolidFilms, II I (1984) 149. .I. Gyulai, .I. W. Mayer, V. Rodriguez, A. Y. C. Y u and H. J. Gopen, J. Appl. fhys., 42 (1971) 3578. C. J. Todd, G. W. B. Ashwell, J. D. Speight and R. Heckingbottom, Merul-Semiconductor Contacts, Inst. Phys. ConJ Ser. 22 (1974) 171. T. J. Magee and J. Peng, Phys. Status Solidi A, 32 (1975) 695. K. Kumar, Jpn. J. Appl. Phys., 18 (1979) 713. A. Christou, Solid-State Electron., 22 (1979) 141. E. Kinsbron, P. K. Gallagher and A. T. English, Solid-State Electron., 22 ( 1979) 5 17. J. M. Vandenberg and E. Kinsbron, Thin Solid Films, 65 (I 980) 259. X.-F. Zeng and D. D. L. Chung, Thin Solid Films, 93 (1982) 207. A. K. Rai, R. S. Bhattacharya and Y. S. Park, Thin Solid Films, 114 (1984) 379.
ORIENTATION
15 16 17 18 19 20
OF
Au ON GaAs
319
E. A. Owen and E. A. O’D. Roberts, J. Inst. Mel., 71(1945) 213. X.-F. Zeng and D. D. L. Chug, Solid-Stare Electron., 27 (1984) 339. N. Newman, W. Petro, T. Kendelewicz, S. H. Pan, S. J. Eglash and W. E. Spicer, LayeredStructures, Epitcr.xy and Intwfirces, Molrrirrls Reseurch Sociely Symp. Proc., 1984. K. Takeda, T. Hanawa and T. Shimojo, Proc. 6th Int. Vucuun~ Congr., Kyoro, 1974. in ~pn. J. Appl. Phys., Suppl. 2. Part I (1974) 589. L. W. Snyman, J. S. Vcrmaak and F. D. Aurct, Verri~tinXs-Ekekrronmikroskopieuereni~ing S. Afi., 7 (1977)45. J. S. Vermaak.
L. W. Snyman
and F. D. Auret, J. Cryst. Grow,th. 42 (1977) 132.
299
AND CHARACTERIZATION
GOLD ON GaAs: ITS CRYSTALLOGRAPHIC ORIENTATION AND CONTROL ON THE ORIENTATION OF THE Au-Ga REACTION PRODUCT D. D. L. CHUNG
AND EDWARD
Department qfMetallurgical PA 15213 i U.S.A.) (Received
May 29,1984;
BEAM III
Engineering and Materials Science, Carnegie-Mellon
accepted
February
University, Pittsburgh,
25,1985)
X-ray diffraction of a 1500 A evaporated gold thin film on GaAs( 100) was used to study the textures of gold and the Au-Ga reaction product (Au7Ga, or P-(Au-Ga)). Post-deposition annealing up to 450°C for 30 min was found to enhance the gold orientation with Au(100) // GaAs(lOO) and Au[OOl] // GaAs[OOl] and weaken the gold orientation with Au( 11 l)// GaAs(lOO) and Au[OlT] // GaAs[Ol l] (and its fourfold rotation twins around GaAs[lOO]). The gold-rich terminal substitutional solid solution rx-(Au-Ga) was observed to begin forming below 350 “C. Dissolution of a-(Au-Ga) at 450-500 “C on heating and precipitation of a-(Au-Ga) on subsequent cooling caused a sharp increase in the orientation with Au( 110) // GaAs( 100) and Au[ liO] // GaAs[Ol 11, which has the smallest lattice misfit compared with the other gold orientations. The dissolution and precipitation of a-(Au-Ga) also caused the appearance of aligned rectangular pits. The crystallographic orientation of Au,Ga, (or P-(Au-Ga)) was controlled by the orientation of cr-(Au-Ga) that had been enhanced by the dissolution and precipitation of cr-(Au-Ga) such that the lattice match between Au,Ga, and gold was nearly perfect on the Au{ ill} interface planes. A dominant orientation relationship in the Au( 111) plane is . Au(lll)//Au,Ga,(llO)//GaAs(21i) Au[ liO] // Au,Ga,[OOl]
1.
//GaAs[Ol
l]
INTRODUCTION
The structural effects of heating gold thin films on single-crystal GaAs have been the subject of numerous papers because of the importance of gold-based metallizations for ohmic contacts in GaAs very-large-scale integration and solar cell technologies. These effects include the interfacial phase formation and the morphological changes. A structural effect which had not previously been investigated is the effect of heating on the texture (crystallographic orientations) of the gold thin film. It is thus the subject of this paper. It should be made clear that this heating refers to post0040-6090/85/$3.30
((3 Elsevier Sequoia/Printed
in The Netherlands
300
D. D. L. CHUNG,
E. BEAM 111
deposition heating rather than to substrate heating during deposition. The effect of the latter form of heating on the gold texture had been previously reported’. By using the pole figure method of X-ray diffraction on Au/GaAs( 100) we found that post-deposition heating caused the gold texture to increase in purity, such that the stable orientation grows at the expense of the less stable orientations. The most stable orientation had Au( 100) // GaAs( 100) and Au[OOl] // GaAs[OOl]. The least stable orientation observed had Au( 111) // GaAs( 100) and exhibits fourfold twinning about the GaAs[ 1001 direction. The a-(Au-Ga) phase (the terminal gold-rich solid solution) should form as a result of the diffusion of gallium from GaAs to gold. Because of the close similarity between Ir-(AuGa) and gold, the distinction between these two phases had not previously been made. In this work, by distinguishing between these two phases through careful X-ray diffraction measurement, we have obtained the first evidence of the formation of c*-(AuGa) in Au/GaAs. Furthermore, the lattice constant of m-(Au-Ga) was determined to be 0.997 of the lattice constant of gold, in agreement with that predicted by substitutional solid solution theory’. Additional evidence for cr-(Au-Ga) was provided by in situ X-ray diffraction observation3 of the complete dissolution of cr-(AuGa) in liquid AuGa at 525 F 25 “C and 1 atm. After the complete dissolution of cc-(AuGa) in liquid Au-Ga, cooling resulted in the precipitation of r-(AuGa) at 525 + 25 “C and 1 atm ‘. In this work, this phase transformation was found to alter the texture of x-(Au-Ga), so that the 110 texture component was greatly enhanced. A similar effect was also observed4 for the melting of the gold-rich solid solution in Au/Ge/Au/GaAs(lOO) and Ni/Au/Ge/Au/GaAs( 100). In Au/GaAs, subsequent to the dissolution and precipitation of cr-(Au-Ga), liquid Au-Ga and x-(Au-Ga) underwent3 a peritectic transformation to Au,Ga, (or lS(AuGa)) at 415 i 5 -C and 1 atm. Previous work on 300 8, Au/GaAs( 111) after heating in vacuum (lOeh Torr) up to 450 “C at 100 “C mini showed that Au,Ga, (or l!L(Au-Ga)) was present with an orientation relationship which corresponded to a perfect lattice match between Au,Ga, and gold, suggesting that the crystallographic orientation of Au,Ga, was controlled by gold’. Consistent with these observations, we found that, in 1500 8, Au/GaAs( 100) the crystallographic orientation of Au,Ga, (or lS(AuGa)) was controlled by the a-(Au-Ga) texture component which had been enhanced by the dissolution and precipitation of cx-(Au-Ga), namely CL-(Au-Ga) with the orientation Au( 110) // GaAs( 100) Au[lTO] //GaAs[Ol
l]
For 600 8, gold deposited onto GaAs( 100) at 280 “C and subsequently annealed at 330 ‘C, pyramidal pits bounded by GaAs{ Ill} planes and agglomerated monocrystalline gold had been observed, together with a monocrystalline hexagonal AuGa phase5. The interface on the GaAs{lll} planes was fully coherent5. Consistent with this observation, we observed that, in 1500 A Au/GaAs(lOO), the lattice match between Au,Ga, (or P-(Au-Ga)) and a-(Au-Ga) of the 110 texture component was almost perfect on the Au{ 111) planes.
ORIENTATIONOF
2. EXPERIMENTAL
301
Au ON GaAs
TECHNIQUES
Au/GaAs(lOO) wafers were kindly provided by P. Lindquist of the HewlettPackard Corporation. The GaAs substrates were tellurium doped ((2-3) x 10” onto the substrate at 3 8, s-r at a cm 3), and the gold was boat evaporated temperature of less than 100 “C. The final gold thickness was 1500 A. The wafers were diced into small rectangular pieces of about 5 mm x 7 mm. They were cleaned ultrasonically with acetone, isopropyl alcohol and deionized water and were then wrapped in similarly cleaned pieces of tantalum foil. The foil was used to reduce oxidation of the gold surface during heat treatment. The wrapped samples were encapsulated in Vycor glass tubing at a pressure of about 1 x lop6 Torr. Heat treatment was carried out by immersing the sealed glass tubes into molten lead baths for a fixed time (30 min). The temperatures used were accurate to approximately + 1 “C. Subsequent to heat treating, samples were made for current-voltage (Z-I/) testing by making a number of contact pads on the substrate. These contact pads (approximately 250 urn x 350 urn) were produced photolithographically, and the unwanted gold was removed with a KII-H,O etchant. The samples were then heat treated as previously described. The I-1/ characteristics were measured using a Tektronics type 576 curve tracer with one probe on a contact pad and the other on the main area of the gold film. Scanning electron microscopy (SEM) was performed at room temperature after the heat treatment, using a Cambridge scanning electron microscope. X-ray diffraction results were obtained by using a Rigaku D/MAX II powder X-ray diffractometer system. A fine-focus copper X-ray tube (Cu Ka radiation) was used. Detection was achieved with a scintillation counter. All results were obtained by using the step scan mode of the diffractometer. Pole figures were obtained by using the Schulz reflection method (Fig. 1). The X-ray source and counter were kept at an appropriate 20 angle while the sample was rotated in its own plane about an axis normal to the GaAs(lOO) substrate (i.e. axis BB’, corresponding to a change in the angle fl, which ranges from 0” to 360”) and a
X-RN SOURCE
COUNTER
Fig. I. The Schulz reflection figure diagram.
‘8.270. (b)
(4
method
of pole figure measurement:
(a) experimental
geometry;
(b) a pole
302
D. D.
L. CHUNG,
E. REAM III
horizontal axis along the intersection of the specimen surface and the diffraction plane (i.e. axis AA’, corresponding to a change in the angle c(,which ranges from 0” to 90). The angle LXwas taken to be 0” when the specimen surface was parallel to the diffraction plane. Pole figure measurements were made by changing a in 2 steps (from 0” to 70”) and p continuously (from 0” to 360”) at either 80” mini ’ or 320” min I. The data were plotted on a chart recorder as intensity uersus /I for each fixed value of a and were subsequently transcribed to a pole figure diagram (Fig. l(b)). 3.
EXPERIMENTAL
RESULTS
3.1. Current-voltage clzuruc.teristics The I-V characteristics obtained with the as-deposited sample and after heating at 350, 450, 500 and 550°C for 30 min are shown in Fig. 2. The characteristics were of a Schottky type in the as-deposited sample and changed to ohmic behavior at 300-350°C. The ohmic behavior persisted up to the highest temperature investigated, i.e. 550 “C.
550°C pT
-500°C
I I
I (mA) I
in, *0
I 0 VP.0
for
RT
Fig, 2. I-V characteristics obtained before heating, i.e. at room temperature 3.50, 450, 500 and 550 “C for 30 min (for these temperatures the horizontal by 20).
(RT), and after heating at axis should be multiplied
ORIENTATION
OF Au
ON
303
GaAs
3.2. Scanning electron microscopy Figure 3 shows SEM photographs obtained after heating at 400,450,500 and 550 “C for 30 min. Aligned rectangular pits were observed in the gold film after heating at 400 “C. At 450 “C the color remained gold; at 500 “C the color became silver-gold; at 550°C the color became completely silver. Concomitant with this color change was the formation of a large number of aligned rectangular protrusions, as previously reported by a large number of workers5-14. The contact was considerably more uniform below 500 “C than above 500 “C.
(4
(b)
(c) Fig. 3. SEM photographs for 30 min.
3.3.
obtained
ex
(4 situ after heating at (a) 400 “C, (b) 450 ‘C,(c) 500 “C and (d) 550 “C
Texture of goldand its temperature dependence 3.3.1. 0-29 results Although the pole figure method is required for texture analysis, the results of 8-28 (powder) X-ray diffraction strongly suggested that the texture of gold changed on heating. Table I lists the intensities of the Au 111, Au 200, Au 220 and Au,Ga, (or p-(Au-Ga)) 030 peaks obtained by the 8-28 method at room temperature in the asdeposited sample and after heating at 350,400,450, 500 and 550 “C for 30 min. The Au,Ga, (or 0) 030 peak was not observed up to 450°C. After heating at 5OO”C, Au,Ga, (or 0) 030 appeared as a weak peak. After heating at 550 “C, it had grown considerably and Au,Ga, (or p) 113 was also observed weakly. Although phase formation was not observed below 500 “C, significant changes in the intensities of the gold peaks were observed in this temperature range. As shown in Table I, the
D. D. L. CHUNG.
304
TABLE
E. BEAM 111
I
INTENSITIESOF PEAKSOBSERVED
BY 62H
HEATING ATVARIOUSTEMPERATURESFOR Peak
Intensity (counts
X-RAY
DIFFRACTION AT ROOM
TEMPERATURE BEFORE ANU AFTER
30Itlitl
sm ‘)jor thefollowing
Room temperoturr
350
c
temperatures 400°C
450
5750 2000 700
4200 2500 800
c
500 ‘C
550 ‘C
3000 3500 1000 100
450
ius deposited)
Au 111 Au 200 Au 220
7500 880 500
8500 2400 400
Au,Ga, (or j3) 030 -.
850
not observed.
intensity of the Au 111 peak increased slightly after heating at 350°C but monotonically decreased for higher heat treatment temperatures. In particular, the drop in intensity between heat treatment temperatures of 500 and 550°C was precipitous. The intensities of the Au 220 and Au 200 peaks both exhibit an increasing trend with increasing heat treatment temperature. However, between heat treatment temperatures of 500 and 550 “C, the Au 200 and Au 220 intensities vanished. These results suggest that the Au(ll1) texture component weakened on heating, while the Au(100) and Au(ll0) texture components strengthened on heating. The precipitous drop of the gold intensities between 500 and 550 “C was apparently due to the significant Au,Ga, (or p) phase formation and the resulting gold consumption in this temperature range. 3.3.2. PoleJigure results To confirm the effect of heating on the gold texture and to identify the azimuthal orientation(s) of each texture component, the pole figure method was used. Firstly, let us consider the azimuthal orientations of each texture component. Figure 4 shows the pole figures obtained for various gold (Figs. 4(a)-4(c)) and GaAs (Fig. 4(d)) peaks. The pole positions shown in Fig. 4 were the same both for the asdeposited sample and for samples heated at 350,400 and 450 “C. However, the pole intensities varied with the heat treatment temperature, as discussed later in this section. Figure 4(a) shows the Au(ll0) and Au(100) pole figures corresponding to the texture component Au( 111) // GaAs( 100). There are four sets of these poles. The four sets, 90” from one another, correspond to the four azimuthal orientations due to fourfold twinning about the Au[lll] direction. In other words, for the epitaxial relationship Au(lll)//GaAs(lOO) the four azimuthal Au[Oli]
orientations
//GaAs[Ol
Au[ 112]// GaAs[Ol
l] l]
are
ORIENTATION
OF
305
Au ON GaAs
(II0
Au(lll) x
000)
@
(a)
Fig. 4. Pole figures of (a) gold with texture Au(1 ll)//GaAs(lOO), GaAs( IOO), (c) gold with texture Au(lOO)//GaAs(lOO) and (d) GaAs.
Au[lOi]
//GaAs[Ol
l]
A~[21 l] //GaAs[Ol
l]
(b) gold
with texture
Au(1 lo)//
These azimuthal orientations were obtained by combining the information in Figs. 4(a) and 4(d). Figure 5 shows the plot of intensity uersus the fi angle at the 20 angle of Au 200 and at u = 55”. This plot shows 12 broad peaks at p values of 30”, 60”, 90”, 120”, 150”, 180”, 210”, 240”, 270”, 300”, 330” and 360”. These peaks correspond to the 12 Au 100 poles shown in the outer ring of Fig. 4(a). The large width of these poles means that the texture component Au( 111) // GaAs( 100) is weak. Figure 4(b) shows the Au( 100) and Au(ll1) pole figures corresponding to the texture component Au( 110) // GaAs( 100). There are two sets of these poles. The two sets, 90” from one another, correspond to the two azimuthal orientations due to twinning about the Au[l lo] direction. In other words, for the epitaxial relationship Au(llO)//GaAs(lOO) the azimuthal
orientations
are
Au[ liO] // GaAs[Ol
l]
Au[OOl] //GaAs[Ol
l]
and
306
D. D. L. CHUNG,
/ 360
,
I
300
240
Fig. 5. Plot of intensity
,
,
If30 p (degrees)
,
,
120
E. REAM
III
,
60
0
t’x the p angle at the 20 angle of Au 200 and at x = 55 (Au(
I 1I) 11111 GaAs( 100)).
These azimuthal orientations were obtained by combining the information in Figs. 4(b) and 4(d). Figure 6(b) shows the plot of intensity versus the p angle at the 28 angle of Au 111 and at c( = 35”. This plot shows four peaks such that two of the peaks are about twice as strong as the other two. These four peaks correspond to the two sets of poles in the inner ring of Fig. 4(b). Hence, the azimuthal orientation with Au[OOl] //GaAs[Ol l] was about twice as abundant as that with Au[liO] // GaAs[Ol 11. The peaks in Fig. 6(b) are much sharper than those in Fig. 5. This means that the texture component with Au(1 lO)//GaAs( 100) is much stronger than that with Au( 111) // GaAs( 100).
I
360
I
I
I
160
r 0
I
360
1
I
I
160 p (degrees)
I
0
(b)
(a) p(degrees)
Fig. 6. Plots of intensity vs. the /I angle (a) at the 20 angle of Au I I I and at IY= 55’ (Au( 100) :/GaAs( 100)) and (b) at the 20 angle of Au 111 and at r = 35” (Au( 1 10) // GaAs( 100)).
Figure 4(c) shows the Au(ll0) and Au(ll1) pole figures corresponding to the texture component Au( 100) // GaAs( 100). There is only one set of these poles. Thus, no twinning was observed about the Au[lOO] direction. For the epitaxial relationship Au( 100) // GaAs( 100) the azimuthal
orientation
Au[OOl] //GaAs[OOl]
is
Au
ORIENTATIONOF
ON
307
GaAs
This azimuthal orientation was obtained by combining the information in Figs. 4(c) and 4(d). Figure 6(a) shows the plot of intensity uersus the p angle at the 28 angle of Au 111 and at c( = 55”. This plot shows four peaks which are 90” apart from one another. They correspond to the poles in the outer ring of Fig. 4(c). The peaks in Fig. 6(a) are also much sharper than those in Fig. 5. Hence, the texture component with Au( 100) // GaAs( 100) is much stronger than that with Au( 111) // GaAs( 100). Although the degree of texturing is quite strong for the texture components with Au( 100) // GaAs( 100) and Au( 1 lo)// GaAs( loo), the degree of texturing is considerably less than that of the single-crystal GaAs substrate. The plot of intensity uersus the fi angle at the 28 angle of GaAs(220) and at c( = 45” is shown in Fig. 7. The sharpness of the poles in Fig. 7 is consistent with the single-crystal character of GaAs. Next, let us consider the effect of heating on the gold texture. Table II lists in column (a) the intensity of a pole representing the texture component Au(1 lo)// GaAs(lOO) relative to the intensity of a pole representing the texture component Au(lOO)//GaAs(lOO) and in column (b) the intensity of a pole representing the texture component Au( 111) // GaAs( 100) relative to the intensity of a pole represent-
I
I
360
270
I 180
I
I
90
0
PCdegrees)
Fig. 7. Plot of intensity
TABLE RELATIVE
us. the b angle at the 20 angle of GaAs 220 and at a = 45”.
II POLE INTENSITIES
Heat treatment
As deposited 350 400 450 500 550
temperature
(“C)
b-4 Au(llO)//GaAs(lC@)
(b) Au(lll)//GaAs(100)
Au(lOO)//GaAs(lOO)
Au(lOO)//GaAs(lOO)
0.75 0.50 0.50 0.11 1.00
0.375 0.200 0.150 0.100 0.066
308
D. D. L. CHUNG,
E. REAM III
ing the texture component Au( lOO)// GaAs( 100). The pole representing the texture component Au( 100) // GaAs( 100) is the Au( 111) pole at E = 54”; the pole representing the texture component Au(ll0) // GaAs( 100) is the Au( 111) pole at E = 36”; the pole representing the texture component Au( 111) // GaAs( 100) is the Au( 100) pole at E = 55”. Because the representatives are different poles, it is meaningless to compare column (a) with column (b) in a horizontal direction in Table II. However, comparison within the same column in the vertical direction in Table II is meaningful and yields the effect of heating on the relative abundance of the texture components. Column (b) shows that the abundance of the texture component with Au(l1 l)//GaAs( 100) relative to that of the component with Au( lOO)//GaAs( 100) decreases monotonically with increasing heat treatment temperature. This trend is consistent with that suggested by the U-20 results in Table I. Column (a) shows that the abundance of the texture component with Au( 110) // GaAs( 100) relative to that of the component with Au(lOO)//GaAs( 100) decreases with increasing temperature up to 450 “C and then increases sharply between 450 and 500°C. This behavior was not revealed by the H-20 results in Table I and shows that the O-20 results alone cannot give definitive texture information. Table III shows the relative pole densities obtained by correcting the relative pole intensities of Table II. The correction was made by compensating for the difference in structure factors and in LYbetween different pole representatives. For column (a), no correction was necessary because both pole representatives are Au 111 and both occur at a < 54”. For column (b), correction was made to compensate for the difference in structure factors between the pole representatives; no additional correction was necessary because both pole representatives occur at the same c( of 54”-55”. TABLE RELATIVE
III POLE DENSITIES
Hear treatment temperature (‘C)
(b)
(4 Au( 110) //GaAs(
100)
Au(111)//GaAs(lOO)
Au(lOO)//GaAs(lOO)
Au(lOO)//GaAs(lOO)
As deposited
0.75
0.721
350
0.50
0.385
400
0.50
0.288
450
0.11
0.192
500
1.00
0.127
Table III shows that the texture component with Au(100) // GaAs(100) was the most abundant (i.e. with the highest volume fraction) in the as-deposited sample and became even more dominating as the heat treatment temperature increased to 450 “C. After heating at 500 “C, the texture component with Au(l1 l)//GaAs(lOO) was negligible in volume fraction compared with the other two texture components, which were comparable in volume fraction. Combining the information in Tables I and III, we can conclude that the main trends in the effect of heating on the texture are the following.
ORIENTATIONOF
Au ON GaAs
309
(1) The fraction of gold with a texture of Au( 11 l)// GaAs(lOO) decreased significantly with increasing heat treatment temperature up to 500 “C. (2) The fraction of gold with a texture of Au(lOO)//GaAs(lOO) increased significantly with increasing heat treatment temperature up to 450 “C. (3) The fraction ofgold with a texture of Au( 110) // GaAs( 100) increased slightly with increasing heat treatment temperature up to 450°C and increased sharply between 450 and 500 “C. 3.4.
Texture
of Au,Ga,
(or p)
The 8-28 (powder) X-ray diffraction method revealed essentially only the Au,Ga, (or p) 030 peak, as shown in Table I. This peak first appeared after heating at 500°C and became much stronger after heating at 550°C. To confirm the identification of the Au,Ga, (or p) phase and to study the texture of this phase, the pole figure method was used. Figure 6 shows plots of intensity versus the bangle at 20 and a angles where the GaAs 220, Au,Ga, (or p) 223 and Au,Ga, (or 0) 113 poles were maximum in intensity. The observation of Au,Ga, (or p) 113 at c( = 58”, Au,Ga, (or p) 223 at tl = 44” and Au,Ga, (or p) 030 at c( = 0” indicates that the epitaxial relationship of Au,Ga, (or p) was Au,Ga,(
100) // GaAs(lOO)
and furthermore confirms the phase identification. A large number of sharp peaks were observed in the plots of intensity uersus the fl angle for Au,Ga, (or p) 113 and 223, as shown in Fig. 8. The peaks are considerably sharper in the plots of intensity versus the /? angle than in the plots of intensity versus the ct angle. The 010 stereographic projection of Au,Ga, is shown in Fig. 9. The planes (113) (i23), (113) and (i23) belong to one family, as can be seen by converting the Miller indices from the three-index system to the four-index system. Similarly, the planes (223), (243), (223) and (243) belong to another family. The peaks labeled 1, l’, 2 and 2’ in Fig. 8 are due to the following four azimuthal orientations of Au,Ga,, as shown by the pole figure plot of these peaks in Fig. 10, which is equivalent to four sets of Fig. 9 poles superimposed together. 1: Au,Ga,[OOl] //GaAs[Oli] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 8” from GaAs[OlO], toward GaAs[OOi]). 1’: Au,Ga,[OOl] //GaAs[Ol l] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 8” from GaAs[OOl], toward GaAs[OlO]). 2: Au,Ga,[30i]//GaAs[OOl] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 17” from GaAs[OlO], toward GaAs[OOl]). 2’: Au,Ga,[30i] // GaAs[OiO] (equivalent to having the direction perpendicular to Au,Ga,(2i3) off by 17” from GaAs[OOl], toward GaAs[OiO]). The azimuthal orientations 1 and 1’ are 90” from one another; the azimuthal orientations 2 and 2’ are also 90” from one another. Hence, twinning of Au,Ga, occurs. Examination of Fig. 8 indicates that (i) all the peaks due to orientation 1 are intense, (ii) some of the peaks due to orientation 2 are intense and (iii) all the peaks
310
D. D. L. C-HUNG.
E. BEAM 111
(a)
36C -
p (degrees) Fig. 8. Plots of intensity us. the /I angle for (a) GaAs 220 and a = 45‘,(b) Au,Ga, (or P-(Au-Ga)) 223 and t( = 44” and (c) Au,Ga* (or P-(Au-Ga)) 113 and a = 58’. The peaks labeled 1, l’, 2 and 2’ correspond to the poles in Fig. 10.
due to orientations 1’ and 2’ are weak. This means that (i) orientation 1 is the major azimuthal orientation, (ii) orientation 2 is the minor azimuthal orientation and (iii) orientations 1’ and 2’ are almost negligible. Thus, the proportion of twinned regions is low. The peaks labeled X in Fig. S(b) are due to GaAs(220), which is so intense that diffraction using the white radiation part of the X-ray spectrum yields a peak at the
ORIENTATION
OF Au ON GaAs
311
223 .
010 .
2io0
223 . II3 .
ooi Fig. 9. 010 stereographic projection of Au,Ga,
20 angle of Au,Ga, 223. Since the GaAs 220 peak occurs at a = 45” and the Au,Ga, 223 peak occurs at a = 44”, the former was observable in Fig. 8(b). The unlabeled peaks in Figs. 8(b) and 8(c) are all weak. They are due to other very minor azimuthal orientations. 3.5. Formation of a-(Au-Ga) Table IV lists the 28 values of the strongest gold and GaAs diffraction peaks observed at room temperature in the as-deposited sample and after heating at various temperatures from 350 to 500 “C for 30 min. The gold peaks shifted to larger angles between room temperature (as-deposited sample) and 350 “C and stayed at the upshifted positions with heating to higher temperatures. In contrast, the GaAs peak remained at the same position for all temperatures. The amount of upshift of either gold peak corresponds to a decrease in the d spacing of the peak by a factor of 0.997. This means that the lattice constant also decreases by this factor. The decrease in lattice constant is interpreted as being due to the formation of a substitutional Au-Ga solid solution with gold as the host. The atomic radii of gallium and gold are 1.41 8, and 1.46 8, respectively. These values are close enough for substitutional solid solubility. The Au-Ga phase diagram indicates that the maximum solid solubility of gallium in gold is 12 at.% Ga. Vegard’s law indicates that, for a composition with 12 at.% Ga, the lattice constant of the host should decrease by a factor of 0.994 on formation of the solid solution. Furthermore, the lattice constant of an Au-Ga solid solution containing 11 at.% Ga had been reported to be 0.9985 of that of gold l5 . Therefore, the observed decrease in lattice constant can be explained by the formation of cr-(Au-Ga). It should be noted that
312
D. D. L. CHUNG,
E. REAM III
2 2
GoAs’,
oio
\
Fig. 10. Superposition of the pole figures of GaAs 100, Au,Ga, (or B-(Au-Ga)) 223 and Au,Ga, (or B-(Au-Ga)) 113. For the Au,Ga, (or B-(Au-Ga)) pole figures, only the poles corresponding to the peaks labeled 1, l’, 2 and 2’ in Fig. 8 are shown: 0, poles due to 1 and 1’; 0, poles due to 2 and 2’. TABLE IV 20 VALUES OF THE STRONGEST GOLD AND GaAs
DIFFRACTION PEAKS OBSERVED IN THE AS-DEPOSITED
SAMPLE AND AFTER HEATING AT VARIOUS TEMPERATURES FOR 30tllin
Peak
28 (deg)for
Au 111 Au 200 GaAs 400
samples heated a~ the,following temperatures
As deposited
350 “C
400
38.18 44.42 66.04
38.30 44.60 66.04
38.30 44.58 66.04
(c
425 ‘C
450,‘C
5OO‘C
55O’C
38.30 44.62 66.04
38.30 44.60 66.04
38.30 44.62 66.04
38.30 44.60 66.04
thermal expansion or vacancy formation would have caused the lattice constant increase instead and thus cannot explain the observed lattice constant decrease. 4.
to
DISCUSSION
The observation of c+(Au-Ga) after heating between gold and GaAs takes the form Au+Ga
at 350 “C implies that the reaction
-+ cr-(Au-Ga)
and this reaction occurs in the solid state at 350°C or below. An in situ X-ray diffraction study of 1500 A Au/GaAs (the same sample as used in this work) showed3 that further heating causes the dissolution (or melting) of cr-(Au-Ga), such that complete dissolution (or melting) occurs at 525+25 “C and 1 atm (1.01 x lo5 Pa) and at 425_f25 “C in a dynamic vacuum of 56.7 Pa. The observation of the dissolution of cr-(Au-Ga) gives additional evidence for the existence of cr-(Au-Ga).
ORIENTATIONOF Au ON GaAs
313
1n situ X-ray diffraction also shows that cooling subsequent to the dissolution of ol-(Au-Ga) causes the formation of P-(Au-Ga) (or Au,GaJ3. Thus the minimum heat treatment temperature required for the formation of P-(Au-Ga) (or Au,GaJ is 525 &-25 “C at 1 atm (1 .Ol x lo5 Pa) and 425 + 25 “C in a dynamic vacuum of 56.7 Pa. The minimum l3-(Au-Ga) (or Au,Ga,) formation temperature (about 500 “C) found ex situ in this work (Table I) agrees with the temperature for the complete dissolution of cr-(Au-Ga) at 1 atm (1.01 x 10’ Pa) but does not agree with that in a dynamic vacuum of 56.7 Pa. This is attributed to the fact that the ex situ heating was performed in a sealed evacuated tube and the arsenic evolution which occurred from the encapsulated sample on heating caused the arsenic vapor pressure in the tube to build up. Compound formation temperatures below 500°C have been previously reported 1,5*9,16 for gold films thinner than 1500 A. This is due to the effect of the gold film thickness on the form of the reaction3. The sharp increase in the volume fraction of gold with the texture component Au(1 lO)//GaAs(lOO), observed ex situ between 450 and 500 “C (Table III), is attributed to the partial dissolution (or melting) of a-(Au-Ga) on heating to 500 “C and the precipitation (or solidification) of cr-(Au-Ga) on subsequent cooling3. The cr-(Au-Ga) phase that precipitates tends to have an orientation corresponding to a small lattice misfit with respect to GaAs. The orientation with Au( 100) // GaAs( 100) corresponds to misfits which are smaller than those of the other two gold orientations, as shown later in this section. The P-(Au-Ga) (or Au,Ga,) phase forms from cr-(Au-Ga) (which had undergone dissolution and precipitation) through a peritectic transformation on cooling3; so the orientation of @(Au-Ga) (or Au,Ga,) is controlled by that of a-(Au-Ga) after a-(Au-Ga) has undergone dissolution and precipitation. The marked degradation of the contact uniformity observed ex situ by SEM at 550°C (Fig. 2(d)) is also attributed to the dissolution of a-(Au-Ga) at this temperature. The dissolution on heating and precipitation of cl-(Au-Ga) followed by P-(Au-Ga) (or Au,Ga,) on subsequent cooling led to the formation of aligned rectangular protrusions, which were essentially identical with those previously reported (ref. 13, Fig. 4). Consistent with this reaction form is the fact that the rectangular protrusions were much richer in gold than the matrix was13. The change from Schottky to ohmic behavior was observed ex situ at 300-350°C (Fig. 3), although l3-(Au-Ga) formation was not observed by X-ray diffraction below 450 “C (Table I). This effect has recently been shown by Newman et al.” to be caused by leakage currents at the mesa periphery due to the presence of impurities. With a mesa etch, Newman et al. obtained a stable barrier height for temperatures to over 425 “C. In the as-deposited as well as heated samples used in this work, the texture components with Au(100) // GaAs( 100) and with Au( 1 lo)// GaAs( 100) were much stronger (i.e. higher degree of texturing) than that with Au(l1 l)//GaAs(lOO). Moreover, the volume fraction with the texture component Au( 100) // GaAs( 100) was the highest. The texture component with Au(lOO)//GaAs( 100) has the single azimuthal orientation of Au[OOl] // GaAs[OOl] and corresponds to a misfit of 27.9% if aAu is
314
D. D. L. CHUNG,
E. BEAM III
compared with aCaAs. or 8.2’jb if 3aAu is compared with 2aGaAF, where aAu and aGaA, are the lattice constants of gold and GaAs respectively. This epitaxial relationship has been reported previously’s la “. The texture component with Au(1 lO)//GaAs(lOO) has not been reported previously. This texture component involves two symmetrically equivalent azimuthal orientations, with Au[liO]~/GaAs[Oll] and Au[OOl] //GaAs[Ol
l]
Each of these azimuthal orientations corresponds to misfits of 2.0% and 3.80/;,in two perpendicular directions. The weak texture component, with Au( 111) // GaAs(lOO), has not been previously reported either. This texture component exhibited fourfold twinning, which results in the four azimuthal orientations Au[Oli]
//GaAs[Ol
l]
Au[llZ]
//GaAs[Ol
l]
Au[ lOi] // GaAs[Ol
l]
A~[21 l] //GaAs[Ol
l]
These four orientations have identical misfits of 27.9% (or 8.2% in one direction and 25.0% (or 0.07%) in a perpendicular direction). The misfit of 0.027% was obtained by comparing 4($1/2aAu with 5(1/21’2)ao,A,. Fourfold rotation twinning about the Au[ 11 l] direction had been previously observed in gold thin films on GaAs(lll), where the epitaxial relationship’ was Au(lll) // GaAs(lll). In this work, we observed fourfold rotation twinning about Au[ll l] in gold thin films on GaAs(lOO), where Au(l1 l)//GaAs(lOO), although the texture was weaker than for the GaAs( 111) counterpart. Such twinning is thus quite general and its effects on the electrical behavior and contact morphology are still to be explored. The above observations clearly show that the amount of misfit does not govern the preferred epitaxial relationship between as-deposited gold and GaAs. However, it governs that between a-(Au-Ga) and GaAs after the melting and solidification of cl-(AuGa). On heating to 450 C, the volume fraction of the texture component Au( 111) h’ GaAs(lOO) diminished. The thermal instability of this texture component is consistent with its low degree of texturing. This was perhaps why this texture component has not been reported previously. On heating to 450 “C, the volume fraction of the texture component Au( 110) // GaAs( 100) increased slightly, whereas the volume fraction of the texture component Au( 100) // GaAs( 100) increased considerably. Thus, after heating at 450 “C, the the other components, as texture component Au( 100) // GaAs( 100) d ominated shown in Table III. This behavior is consistent with the fact that the texture with Au( 100) // GaAs( 100) predominates in samples deposited at substrate temperatures of 200 “C or above1,18 20.
ORIENTATIONOF
Next, let us consider epitaxial relationship was Au,Ga,(
315
Au ON GaAs the texture
of Au,Ga,
(or b-(Au-Ga)).
The observed
100) // GaAs( 100)
For this epitaxial relationship, a major azimuthal orientation (orientation 1) and a minor azimuthal orientation (orientation 2) were observed. In orientation 1, Au,Ga,[OOl] //GaAs[Oli]. The lattice misfit of Au,Ga, with respect to GaAs is 9.5% in this direction and - 3.4% in a perpendicular direction. For gold with the orientation Au( 100) // GaAs( 100) and Au[OOl] // GaAs[OOl], orientation 1 corresponds to Au,Ga,( lOO)// Au( 100) and Au,Ga,[OOl] // Au[Oli]. The lattice misfit of Au,Ga, with respect to gold of this orientation is 1.1% along this direction on the Au( 100) plane and - 10.8% along a perpendicular direction on this plane. For gold with the orientation Au(1 lo)// GaAs( 100) and Au[liO] // GaAs[Ol 11, orientation 1 corresponds to Au,Ga,( 100) // Au( 110) and Au,Ga,[OOl] // Au[l lo]. The lattice misfit of Au,Ga, with respect to gold of this orientation is 1.1% along this direction on the Au(ll0) plane and - 5.4% along a perpendicular direction on this plane. Hence, the lattice misfits between Au,Ga, and GaAs and between Au,Ga, and gold in the plane parallel to GaAs( 100) are all quite high. Let us consider then the misfits in the plane parallel to GaAs(l1 l), since GaAs(ll1) is parallel to the true interface5. Because the poles are sharper as a function of the /3 angle than as a function of the Mangle, the orientation relationship Au,Ga,( 100) // Au( 100) and Au,Ga,[OOl] // Au[Oli] is nearly equivalent to the relationship Au,Ga,(OlO) // Au( 111) and Au,Ga,[OOl] // Au[Oli], for which the lattice misfit is 1.1% along this direction on the Au( 111) plane and - 7.3% along a perpendicular direction on this plane. Similarly, the orientation relationship Au,Ga,(100)//Au(110) and Au,Ga,[OOl] //Au[l lo] is nearly equivalent to Au,Ga,( 110) // Au( 111) and Au,Ga,[OOl] //Au[l lo], for which the lattice misfit is 1.1% along this direction on the Au(ll1) plane and 0.4% along a perpendicular direction on this plane. The last set of lattice misfits is considerably smaller than all the other sets. In fact, the lattice misfits of the last set are so small that the lattice match can be considered to be perfect, as illustrated in Fig. 11. Therefore, orientation 1 is attributed to a perfect lattice match between Au,Ga, and gold in the Au(ll1) plane for gold with the orientation Au( 110) // GaAs( 100) and Au[ liO] // GaAs[Ol 11. The interfacial orientation relationship is thus Au(lll)//Au,Gaz(l10)//GaAs(21i) Au[ liO] // Au,Ga,[OOl]
// GaAs[Ol
l]
In orientation 2, Au,Ga,[30i]//GaAs[OOl]. Using an argument similar to that used to explain orientation 1, it is found that orientation 2 may be attributed to a moderately good lattice match between Au,Ga, and gold in the Au(ll1) plane for gold with the orientation Au( 110) // GaAs( 100) and Au[ liO] // GaAs[Ol 11. The interfacial orientation relationship is Au(1 ll)//Au,Ga,(221)//GaAs(21i) Au[ liO] // Au,Ga,[Oi2]
//GaAs[Ol
l]
316
D. D. L. CHUNG,
The interfacial orientation method for 300 A Au/GaAs(ll Au( 11 I)// Au,Ga,(OlO) Au[liO] + 10” toward GaAs[Oi 1] Within
experimental
E. BEAM III
relationship observed by the same X-ray diffraction 1) heated to 450 “C in a vacuum of lO-6 Torr was’
//GaAs( 111) Au[Oil]
//Au,Ga,[OOl]
error, this is equivalent
//GaAs[lfO]
+ lo-’ toward
to
Au(l1 l)//Au,Ga,(OlO)//GaAs(l11) A~[5411 //Au,Ga,[OOl]
//GaAs[541]
as illustrated in Fig. 12. The lattice misfit of Au,Ga, with respect to gold is -0.70/, along Au,Ga,[OOl] and 1.2% along Au,Ga,[lOO]. Thus the lattice match may be considered to be perfect between Au,Ga, and gold. The difference between this orientation relationship and those found in this work is attributed to the difference in the GaAs substrate orientations, which affected the texture of gold.
Fig. 1 I. Schematic representation of the orientation 1500 8, Au/GaAs( 100). The circles are gold atoms.
relationship
on Au( 111) and Au,GaJ
1 IO) planes in
Fig. 12. Schematic representation of the orientation 300 8, Au/GaAs( 111). The circles are gold atoms.
relationship
on Au(l11) and Au,Ga,(OlO)
planes in
Although the interfacial orientation relationship depends on the GaAs substrate orientation, the following generalizations can be made, based on the results of this work (on Au/GaAs(lOO)) and of ref. 1 (on Au/GaAs( 111)). (1) The orientation of Au,Ga, is such that the lattice misfit is less than 2% for A+Ga, with respect to gold. (2) If the Au,Ga, forms after the occurrence of the melting (or dissolution) of the gold-rich solid solution, the orientation of Au,Ga, is governed by the gold-rich solid solution that has been reoriented by the melting. (3) Nearly perfect lattice match (a misfit of less than 2%) between Au,Ga, and gold occurs on the Au{ 111) planes. The interfacial orientation relationship observed by electron diffraction for
ORIENTATION
OF
317
Au ON GaAs
600 A gold deposited onto GaAs(lOO) 330 “C had been reported to be5 Au( 111) // Au-Ga(Ol0) Au[ liO] // Au-Ga[OOl]
at 280 “C and subsequently
annealed
at
// GaAs( 111) //GaAs[
liO]
where Au-Ga is a hexagonal phase. For the lattice misfit calculation we assume that the Au-Ga phase is Au,Ga,. The lattice misfit of Au,Gaz with respect to gold is 1.1% along Au,Ga,[OOl] and -7.3% along Au,Ga,[lOO]. The difference between this and the orientation relationships found in this work is attributed to the small gold thickness, the high deposition temperature and the low annealing temperature, which apparently caused the reaction product to form in the solid state without the melting (or dissolution) of a-(Au-Ga). The relatively large misfit between Au,Ga, and gold is in contrast with the small misfits observed in this work and in ref. 1. The origin of this discrepancy is not clear at present. Au,Ga, and P-(Au-Ga) are both hexagonal, with almost identical lattice constants. The misfits given above were calculated for Au,Ga, rather than for P-(Au-Ga). The misfits calculated for P-(Au-Ga) are essentially the same as those calculated for Au,Ga,. Thus the conclusions given above are valid for either Au,Ga, or P-(Au-Ga). The two phases could not be distinguished. Gold and a-(Au-Ga) are both f.c.c., with lattice constants which differ by about 0.3%. The misfits given above were calculated for gold rather than for a-(Au-Ga). Although the misfits would have been slightly different if they were calculated for a-(Au-Ga) instead, the differences are small so that the conclusion given above concerning a nearly perfect lattice match is not affected. 90” rotation twinning was observed in Au,Ga, (or p-(Au-Ga)) of both orientations 1 and 2, although the proportion of twinned regions was low. Since fourfold twinning was not observed for the gold orientation which controls these Au,Ga, orientations, the origin of the twinning is attributed to the GaAs(100) substrate. In contrast, twinning was not observed’ in Au,Ga, (or P-(Au-Ga)) formed on heating gold on GaAs( 111). 5. CONCLUSION
This work has shown that post-deposition annealing of Au/GaAs results in two distinct structural changes: texture change and phase formation. The texture change involves an increase in the purity of the texture so that the texture component with Au( 100) // GaAs( 100) increases in volume fraction compared with other texture components. In particular, the texture component with Au( 100) // GaAs( 100) grows at the expense of that with Au( 111) // GaAs( 100). This tendency indicates that the texture component with Au( 100) // GaAs( 100) is stable, whereas that with Au(lll) // GaAs( 100) is only metastable. The gold-rich terminal solid solution cl-(Au-Ga) was observed to form at 350 “C or below. It is a substitutional solid solution with a lattice constant equal to 0.997 of that of gold. A large increase in the volume fraction with Au( 110) // GaAs( 100) was observed ex situ between 450 and 500 “C. This is attributed to the dissolution of a-(Au-Ga) on
318
D. D. L. CHUNG.
E. BEAM 111
heating and the precipitation of a-(Au-Ga) on subsequent cooling, as revealed by in situ X-ray diffraction3. This orientation was favored because ofits small lattice misfit compared with those of the other two orientations. The dissolution and subsequent precipitation of cc-(Au-Ga) also resulted in aligned rectangular pits which degraded the contact uniformity. This work has demonstrated that the crystallographic orientation of Au,Ga, (or P-(Au-Ga)) is controlled by the crystallographic orientation of cc-(Au-Ga) after the dissolution and precipitation of a-(Au-Ga), as the dissolution and precipitation of a-(Au-Ga) precede the formation of Au,Ga, (or P-(Au-Ga)). The control is such that the lattice match is perfect between Au,Ga, (or b-(Au-Ga)) and gold (or ol-(Au-Ga)) in the Au(ll1) plane, which is the true interface plane. The orientation relationship depends on the GaAs substrate orientation. For 1500 A Au/GaAs( 1’00); it is Au(l1 l)//Au,Ga,(llO)//GaAs(21i) Au[liO]//Au,Ga,[OOl]//GaAs[Ol For 300 A Au/GaAs(
l]
11 l), it is
Au(l1 l)//Au,Ga,(OlO)//GaAs(lll) Au[54i]
//Au,Ga,[OOl]
//GaAs[541]
ACKNOWLEDGMENTS
The X-ray diffraction equipment grant from the Division of Materials Research of the National Science Foundation under Grant DMR-8005380 was essential for this work. Equipment support from the Materials Research Laboratory Section, Division of Materials Research, National Science Foundation, under Grant DMR 76-8 156 1 A01 is also acknowledged. The technical assistance of T. Kim of CarnegieMellon University is greatly appreciated. REFERENCES
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