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Reaction at a platinum-gallium arsenide interface
Surface Science 104 (1981) 341-353 North-Holland Publishing Company
REACTION AT A PLATINUM-GALLIUM
D.L. BEGLEY **, R.W. ALEXANDER, Electrical Engineering and Physics Departments, Missouri, Rolla, Missouri 65401, USA Received
13 September
1980; accepted
ARSENIDE INTERFACE
*
R.J. BELL and C.A. GOBEN ** 100 Physics Building, University of
for publication
3 November
1980
The reaction at a Pt/nGaAs interface as a result of annealing at 4OO’C was investigated. Both electron beam evaporated and electro-plated Pt films were studied. Optical reflection and transmission spectroscopy, secondary ion mass spectroscopy, X-ray diffractometry, scanning electron and ion microscopy, and X-ray energy spectroscopy were employed to examine the metal-semiconductor interface. An increase in the optical reflectivity was observed early in the annealing sequence. As has been observed by others, a complex layered structure formed at the interface, but the electron and ion micrographs revealed that gallium diffused rapidly through the platinum film at apparent imperfections in the Pt film. These imperfections were not visible in the micrographs prior to annealing and their nature has not been determined. These areas are believed to be primary sites for premature device failure. It appears that techniques such as SIMS which average over micron-sized or larger areas which indicate gallium diffusion to the Pt surface are correct, but may be misleading as to the mechanism.
1. Introduction
The electrical properties of metal-semiconductor interfaces, such as Schottky barrier height and contact resistance, depend upon the quality of the interface due to the metallization process and any deterioration which may occur during operation of the device. For Schottky barrier IMPATT devices in particular, the position of the metal-semiconductor interface and the metallurgical composition of the materials are critical to device operation. There are currently many high-power and high-frequency solid state devices which employ a metal-semiconductor contact and which operate at temperatures estimated to exceed 250-300°C because of power dissipation heating of the device. Degradation of the device due to a solid state metallurgical reaction at high operating temperatures is therefore a major concern. There have been numerous studies made of the reactions at various metal-semi-
* This work was supported in part by AFOSR grant No. 76-2938. ** Present address: Department of Electrical Sciences and Systems Illinois University at Carbondale, Carbondale, Illinois 62901, USA.
0039-6028/81/0000-OOOO/$
02.50 0 1981 North-Holland
Engineering,
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Company
342
D.L. Begley et al. /Reaction
at Ptln-GaAs interface
conductor interfaces [l-7] and Schottky barrier formation [8-121. (A good review can be found in ref. [7] .) The Pt/n-GaAs system was chosen for investigation because of its wide use in a large number of solid state devices. Various analytical techniques were employed to gain an understanding of the reaction mechanism and the degradation of the metal-semiconductor system during annealing. Optical reflection and transmission spectroscopy, secondary ion mass spectroscopy, X-ray diffractometry, scanning electron and ion microscopy, and X-ray energy spectroscopy analysis were conducted on Pt/n-GaAs samples at various stages of annealing at 400°C. This temperature was chosen so that the annealing times necessary to achieve the completed reaction could be shortened.
2. Sample preparation Single crystal semi-insulating GaAs slices and high carrier concentration &As slices were obtained from the Monsanto Company. The semi-insulating GaAs slices were n-type, Cr doped, with (100) orientation (off lo-0.3” toward [l lo]) with a resistivity of -2.4 X 10s fi cm. The high carrier concentration (1.5 X 10s cmm3) slices were n-type, Si doped with (100) orientation (off lo-3” toward [ 1001) with a resistivity of -2 X 1Om3R cm. Slices were approximately 2.5 cm X 5.5 cm and 0.038 cm thick and were electro-polished on one surface resulting in an etch pit density of -1 .O X 10’ ep/cm2 for the semi-insulating and -5.0 X lo3 ep/cm’ for the high carrier concentration slices. Samples were prepared by electron beam vacuum deposition of platinum onto the GaAs substrates. Prior to insertion into the vacuum chamber, the substrates were chemically etched for 5 min in a solution of 50% HCl and 50% methyl alcohol to remove the native oxide and rinsed in methyl alcohol and acetone [3,4]. A small amount of oxide (approximately 25 A thick) is expected to have grown on the substrate surface before deposition; however, it has been shown that this oxide does not affect the reaction rate of Pt with GaAs [3]. It has been shown that thick oxide intentionally grown before Pt deposition can affect the reaction at the metal-GaAs interface [3]. The surface of an etched GaAs sample was examined with Auger spectroscopy to determine the effect of the etchant and the general condition of the surface before deposition. A relatively small amount of Cl, 0, and C were found to be present originating from the HCl etch and native oxide at the sample surface. The platinum was deposited at a pressure of 1 .O X 10v6 Torr, and the thickness of the deposited film was monitored during deposition by a quartz crystal thickness monitor. The substrate temperature, which was also monitored during deposition, was observed to be below 100°C. Additional samples obtained from Texas Instruments consisted of evaporation deposited and electro-plated films of Pt on n-type GaAs substrates. Segments of each of the samples were subjected to a series of heat treatments. Each sample segment was annealed at 400°C in an evacuated stainless steel tube placed in a resistive
D.L. Begley et al. /Reaction
heater furnace. A diffusion pump connected -1 .O X lo-’ Torr during the heat treatment.
at Ptfn-GaAs interface
to the tube maintained
343
a pressure of
3. Analysis 3.1. Optical reflection
and transmission
Optical reflection and transmission studies were made on semi-insulating substrates with a 150 A Pt film. The measurements were made utilizing a Perkin-Elmer 599 spectrophotometer (400 to 3000 cm-‘) with the beam incident upon the metal side of the sample. The reflectivity (transmission) versus anneal time at a given wavelength was a slightly increasing (decreasing) curve with a minor peak (minimum) occurring at one hour of annealing time. The reflectivity curve is shown in fig. 1. This increase is believed to be due to the formation of a highly reflecting intermetallic compound early in the anneal sequence. It is interesting to note that the increase in reflectivity is similar to an increase in resistivity of Pt films reported previously by Murarka [3] and Kumar [4]. The absorption data indicated broadband frequency dependent absorption occurring due to the formation of the intermetallic compounds. Thicker Pt films were investigated in the hope that a larger amount of the various compounds would be formed, resulting in larger absorption and the appearance of some structure. Platinum films 500 and 2000 A thick were examined. The films were annealed in stages, and spectra were taken after each anneal until the films were reacted to completion [4] as indicated by X-ray diffraction. No structure was observed.
0.7 0.6 0.5 0.4 0.3 0.2 0. I
ANNEAL
TIME
(hrl
Fig. 1. Reflectivity versus anneal time at various wave numbers for 150 A of Pt on Cr-doped GaAs: (A) 3000 cm-l , (f) 1500 cm-‘, (=) 800 cm-‘, (0) 400 cm-l.
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D.L. Begley et al. /Reaction
at Pt/n-tiaAs
interface
Fig. 2. SEM images of 2000 A of Pt on GaAs: (a) annealed for l/2 h at 400°C, the width of the figure corresponds to 24 vm; (b) annealed for 3 h at 4000°C, the width of the figure corresponds to 8 pm.
Reflectivity measurements were also made on samples with higher carrier concentration substrates. These were also reacted to completion, with no structure observed in the spectra. 3.2. Electron and ion microscopy A Jeolco JSM-2 scanning electron microscope (SEM) was employed to investigate the surface appearance of samples after various stages of anneal. SEM photographs of an as-deposited sample revealed that the Pt film was not continuous, with
Fig. 3. SEM images of 2000 iI of Pt on GaAs: (a) annealed for 5 h at 400°C; 16 h at 4OO’C. The width of the figures corresponds to 8 pm.
(b) annealed
for
D.L. Begley et al. /Reaction
at Ptln-GaAs interface
Fig .4. Ion microscope image of Ga on 2000 A of Pt on GaAs annealed wid th of the figure corresponds to 250 Wm.
for 3 h at 400°C.
Fig. 5. Ion microscqpe image of Ga on 2000 A of Pt on GaAs annealed width of the figure corresponds to 250 pm.
for 5 h at 4OO’C. The
The
346
D.L. Begley et al. /Reaction
at Pt/n-GaAs
interface
Fig. 6. SEM images of Texas Instruments sample with 200 A of Pt: (a) evaporated on GaAs and annealed for l/2 h at 400°C; (b) electroplated on GaAs and annealed for l/2 h at 4OO’C. The width of the figures corresponds to 8 pm
holes on the order of 0.5 to 1.5 pm in These regions of pin holes are believed the GaAs prior to metallization. Figs. 1 X lo4 magnification) of the surface
diameter observed in some areas of the film. to be caused by particle contamination of 2 and 3 are SEM photographs (3 X lo3 and of Pt (2000 A) on n-type GaAs (n = 1.5 X
1018 cm-“) after annealing at 400°C. At l/2 h of annealing time, fig. 2a, craters are observed on the sample surface with protrusions appearing near the center. These craters are composed of a protrusion forming a nodule at the center and a depressed outer region. These craters have not been reported before. After 3 h of anneal the surface has become very irregular with a large number of protrusions appearing through the Pt film, as illustrated in fig. 2b. X-ray energy spectroscopy (XES) analysis of the protrusions reveal that the protruding nodules had a much higher Ga content than the surrounding surface area. It is therefore believed that the protrusions are caused by the initial diffusion of gallium through a thin area in the Pt film in the form of gallium or a gallium-rich compound. The sample surface is also beginning to take on a granular appearance. After 5 h of anneal at 4OO”C, the entire surface of the film has been altered as illustrated by the photograph in fig. 3a. The surface has now been transformed from relatively smooth to becoming extremely granular in appearance. XES analysis of the sample revealed a high gallium concentration at the surface. In fig. 3b the surface of the sample after 16 h of anneal at 400°C appears much the same as at 5 h except for some increase in the size of the individual granules. To investigate the extent of the gallium diffusion to the sample surface, the ion optics of a Cameca IMS 200 Microscope-Microprobe were adjusted to detect the spatial distribution of gallium at the sample surface. Figs. 4 and 5 are gallium ion microscope photographs of samples annealed for 3 and 5 h respectively. These
D.L. Begley et al. /Reaction
at Pt/n-GaAs interface
347
photographs represent a distance of approximately 2.50 pm across the sample surface. The advancement of the gallium formation at the sample surface is observed, as well as the formation of surface islands of gallium. The lateral resolution of the ion microscope (-1 ym) is considerably lower than that of the electron microscope (-200 a). This lower resolution did allow Ga to be detected for the one-half and one-hour annealed samples. Samples obtained from Texas Instruments were also examined with the SEM. Fig. 6 contains photographs of evaporated and electro-plated samples annealed for l/2 h. The protrusion of nodules near the center of a hole in the Pt film is clearly illustrated in fig. 6a. Similar nodule protrusions are observed for the electro-plated sample in fig. 6b. 3.3. Secondary ion mass spectroscopy (SIMS) and X-ray diffracrometry (XRD) In-depth compositional profiling of the Pt-AgAs samples was performed utilizing the Cameca IMS 300 Ion Microscope-Microprobe. A primary beam of O’, was used to sputter material from the sample surface. A 2000 A Pt fnm was deposited on n-GaAs in an e-beam evaporator. SIMS depth
1.0 0.9
0.8 :
v,
E
0.7
2-
0.6
d
0.5
ZJ v,
0.4
9 N
0.3
7 Z
0.2
E
0.
I
200
300
400
SPUTTERING Fig. 7. SIMS intensity line, platinum; double
profile for as-deposited dashed line, arsenic.
500
TIME
600
700
600
(set)
2000 A of Pt on GaAs:
solid line, gallium;
dashed
D.L. Begley et al. /Reaction
348
at Pt/n-GaAs interface
I.0 0.9
z cn z
0.8
W
0.7
!S -
0.6
G
0.5
z G
0.4
n w N
0.3
I
0
100
Fig. 8. SIMS intensity gallium;
dashed
profile line, platinum;
200
300
400
SPUTTERING
TIME
500
600
700
(set)
for 2000 ii of Pt on GaAs annealed double dashed line, arsenic.
l/2 h at 4OO’C:
solid line,
profiles were made on samples after various stages of anneal. Fig. 7 represents a profile for a typical sample prior to anneal. A “bump” such as seen at about 500 s on the As profile typically occurs upon sputtering through an interface and can be used as a signature of an interface. A definite interface between Pt and GaAs is seen within the 40 A depth resolution of the instrument. Fig. 8 illustrates the profiles for a sample annealed l/2 h. Considerable interdiffusion is clear even for this short anneal. Both Ga and As have diffused into the Pt with Ga having moved further and faster. The bump in the Pt profile at 500 s indicates the interface with the bulk GaAs. Peaks in the Ga profne (and As and Pt profiles) at about 280,350 and 425 s indicate interfaces between layers of apparent intermetallic compounds which have been formed. It can be seen that the outmost compound is of Pt and Ga with no As, the next (second compound) layer contains more Ga and some As, and the third layer a little more Ga and considerably more As. Because the sputtering rate is a function of the material being sputtered, the concentrations of the elements cannot be determined from these SIMS profiles. After 1 h of annealing, the layers have become less distinct as can be seen in fig. 9. After 3 h at 4OO”C, the layer structure is further washed out, although it is clear that Ga is diffusing faster than As (fig. 10). Finally, after 5 h, Ga is completely through the film (fig. 11).
1.0
,
0.9
_
z Gi
0.8
_
5
0.7
L / / I
5-
I
I\ ’
Id\
pg
b
,J’“,/\/ v 1
AC
I
Ga /
lJ
/
Pi
1
I
I
I
too
0
:’’T---!
“\
I
A--J4$ -A/
200
300
400
500
SPUTTERING Fig. 9. SIMS intensity profile gallium; dashed line, platinum;
TIME
for 2000 A of Pt on GaAs double dashed line, arsenic.
600
700
000
(set) annealed
1 h at 4OO’C:
solid line,
1.0
0.9 > l-
0.8
cn El
0.7
5 -
0.6
0
100
200
300
400
SPUTTERING
500
TIME
600
700
800
kec 1
Fig. 10. SIMS intensity profile fox 2000 A of Pt on GaAs annealed gallium; dashed line, platinum; double dashed line, arsenic.
3 h at 4OO’C:
solid line,
D.L. Begley et al. /Reaction
3.50
0
100
200
at Ptln-GaAs interface
300
SPUTTERING
400 TIME
500
600
(set)
Fig. 11. SIMS intensity profile for 2000 A of Pt on GaAs annealed gallium; dashed line, platinum; double dashed line, arsenic.
5 h at 400°C:
solid line,
From the XRD data discussed below and from the work of Chang et al. [6] using Auger electron spectroscopy, it is tempting to identify the outmost compound layer seen in fig. 8 as PtaGa, the next layer as PtGa, and finally PtAs, with a considerable amount of Ga included. It should be recalled that these SIMS profiles are averages over a 250 pm diameter spot and the SEM measurements discussed above indicate considerable inhomogeneity over a region of this size. Evaporated and electro-plated samples with varied film thicknesses obtained from Texas Instruments were also examined. Fig. 12 represents a SIMS profile of 2000 A Pt film electro-plated on a n-type GaAs sample after a l/2 h anneal at 400°C. The diffusion of Ga and As has proceeded at a much greater rate than for our samples. Similar results were found for evaporated samples obtained from Texas Instruments. No reason was found for the considerably more rapid diffusion through the TI produced Pt films. A General Electric XRD 700 X-ray diffractometer was used to irradiate the metal side of the sample with Ni filtered Cu K, radiation. Diffractometry data revealed no indication of compound formation for l/2 and 1 h annealed samples. After 3 h of annealing, XRD revealed diffraction lines for the compounds PtaGa, PtGa, and PtAsa. These lines were stronger for the 5 h annealed sample.
D.L. Begley et al. 1 Reaction at Ptln-GaAs interface
0.9 0.8
I
I
0
100
I 200
-i_+ 300
400
1
I
1
500
600
700
SPUTTERING
TIME
‘\I 800
I
1
900
1000
kec)
Fig. 12. SIMS intensity profile for Texas Instruments sample, 2000 A of Pt electro-plated GaAs, annealed l/2 h at 4OO’C: solid line, gallium; dashed line, arsenic.
on
The electron micrographs show that the layers observed in SIMS depth profiles are not homogeneous and that gallium penetration to the surface is primarily through defects in the platinum film. It is important to note that the sites of Ga penetration are not just pin holes in the Pt film observable with the electron microscope. SEM observable pin holes were found to have a much lower density than the sites of Ga penetration (SEM with X-ray fluorescence). Caution must be exercised interpreting depth profiles from SIMS and AES which average over large areas compared to the few hundred angstrom diameters of the gallium protrusions.
4. Conclusions The Pt/n-GaAs interface reaction annealing time. Auger spectroscopy tion indicated that the surface was C, and 0, which should not greatly [31. Optical reflectivity
measurements
has been investigated at 400°C as a function of analysis of the GaAs surface before Pt evaporaclean except for relatively small amounts of Cl, affect the metallization of the sample surface indicated
that there is an increase in the reflec-
352
D.L. Begley et al. /Reaction
at Pt/n-GaAs interface
tivity as a function of annealing time followed by a decrease and a leveling off trend. Transmission spectra indicated possible broad-band frequency-dependent absorption caused by annealing. However, no structure was observed in the reflection or transmission spectra. The nature of the compound formation was found to be even more complex than has been observed by others. SIMS depth profiles showed that there is definite layer structure formed at the original Pt-GaAs interface. This layer structure has a tendency to wash out as the anneal time is increased. SEM analysis of the sample surface at various stages of the annealing sequence revealed an early protrusion of gallium or a gallium-rich compound through the Pt film. During the annealing procedure the protrusions became more pronounced with the surface taking on a granular structure. The effect appears to occur independently of the method of film preparation, with both evaporated and electro-plated samples yielding similar results. The diffusion and subsequent reaction are not uniform across the surface of the sample. Ion microscope photographs confirmed the appearance of the gallium through the Pt film at the sample surface. After further anneal, the entire Pt film appears to be consumed by gallium diffusion, resulting in a complete deterioration of the Pt surface and interface region. It has been observed that premature breakdown of planar Schottky barrier diodes occurs at the edge of the metallization and at sites of missing Pt (pin holes) [13]. It is believed that the observed initial metallurgical interaction at sites of imperfections in the metal films are the main areas for early device failure. Areas of metallization defects can produce high electric field gradients under bias conditions which may cause premature breakdown. Within these areas the Schottky barrier is no longer one of Pt-CaAs, but of various intermetallic compounds. The movement of the metal-semiconductor interface by interdiffusion would also be a significant contributing factor for device degradation in devices with parameters dependent upon the position of the interface. The interdiffusion and reaction at the platinum-gallium arsenide interface presents complex and difficult problems in the understanding of device failure and degradation during actual operation. However, through qualitative analysis of the interface reaction, utilizing a number of complementary techniques, an insight into possible device failure mechanisms has been made.
Acknowledgements The authors wish to thank H. Rice for the SEM photcgraphs; J. Stubbs for X-ray analysis; J. Kleefeld for Auger analysis; D. Shaw for the Texas Instruments samples; J. Boone for the use of the electron beam evaporator; Professor G. Morrison and D. Leta of Cornell University for use of the SIMS; and C. Ward for helpful comments.
D.L. Begley et al. /Reaction
at Ptln-GaAs interface
353
References [ 1] [2] [ 31 [4] [5] [6] [7] [8] [9]
[lo] [ 111 [12] [13]
A.K. Sinha and J.M. Poate, Appl. Phys. Letters 23 (1976) 666. D.J. Coleman Jr., W.R. Wiseman and D.W. Shaw, Appl. Phys. Letters 24 (1974) 355. S.P. Murarka, Solid State Electron. 17 (1974) 869. V. Kumar, J. Phys. Chem. Solids 36 (1975) 535. J.J. Berenz, G.J. Scilla, V.L. Wrick, LG. Eastmen and G.J. Morrison, J. Vacuum Sci. Technol. 13 (1976) 1152. C.C. Chang, S.P. Murarka, V. Kumar and G. Quintana, J. Appl. Phys. 46 (1975) 4237. J.M. Poate, K.N. Tu and J.W. Mayer, Thin Films - Interdiffusion and Reactions (Wiley, New York, 1978). G. Margaritondo, J.E. Rowe and S.B. Christman, Phys. Rev. B14 (1976) 5396. P.W. Chye, I. Lindau, P. Pianetta, C.M. Garner, C.Y. Su and W.E. Spicer, Phys. Rev. B18 (1978) 5545. I. Lindau, P.W. Chye, C.M. Garner, P. Pianetta, C.Y. Su and W.E. Spicer, J. Vacuum Sci. Technol. 15 (1978) 1332. J.M. Andrews and J.C. Phillips, Phys. Rev. Letters 35 (1975) 56. L.J. Brillson, Phys. Rev. Letters 40 (1978) 260. R.B. Marcus, S.E. Haszko, S.P. Murarka and J.C. Irvin, J. Electrochem. Sot. 121 (1974) 693.
REACTION AT A PLATINUM-GALLIUM
D.L. BEGLEY **, R.W. ALEXANDER, Electrical Engineering and Physics Departments, Missouri, Rolla, Missouri 65401, USA Received
13 September
1980; accepted
ARSENIDE INTERFACE
*
R.J. BELL and C.A. GOBEN ** 100 Physics Building, University of
for publication
3 November
1980
The reaction at a Pt/nGaAs interface as a result of annealing at 4OO’C was investigated. Both electron beam evaporated and electro-plated Pt films were studied. Optical reflection and transmission spectroscopy, secondary ion mass spectroscopy, X-ray diffractometry, scanning electron and ion microscopy, and X-ray energy spectroscopy were employed to examine the metal-semiconductor interface. An increase in the optical reflectivity was observed early in the annealing sequence. As has been observed by others, a complex layered structure formed at the interface, but the electron and ion micrographs revealed that gallium diffused rapidly through the platinum film at apparent imperfections in the Pt film. These imperfections were not visible in the micrographs prior to annealing and their nature has not been determined. These areas are believed to be primary sites for premature device failure. It appears that techniques such as SIMS which average over micron-sized or larger areas which indicate gallium diffusion to the Pt surface are correct, but may be misleading as to the mechanism.
1. Introduction
The electrical properties of metal-semiconductor interfaces, such as Schottky barrier height and contact resistance, depend upon the quality of the interface due to the metallization process and any deterioration which may occur during operation of the device. For Schottky barrier IMPATT devices in particular, the position of the metal-semiconductor interface and the metallurgical composition of the materials are critical to device operation. There are currently many high-power and high-frequency solid state devices which employ a metal-semiconductor contact and which operate at temperatures estimated to exceed 250-300°C because of power dissipation heating of the device. Degradation of the device due to a solid state metallurgical reaction at high operating temperatures is therefore a major concern. There have been numerous studies made of the reactions at various metal-semi-
* This work was supported in part by AFOSR grant No. 76-2938. ** Present address: Department of Electrical Sciences and Systems Illinois University at Carbondale, Carbondale, Illinois 62901, USA.
0039-6028/81/0000-OOOO/$
02.50 0 1981 North-Holland
Engineering,
Publishing
Southern
Company
342
D.L. Begley et al. /Reaction
at Ptln-GaAs interface
conductor interfaces [l-7] and Schottky barrier formation [8-121. (A good review can be found in ref. [7] .) The Pt/n-GaAs system was chosen for investigation because of its wide use in a large number of solid state devices. Various analytical techniques were employed to gain an understanding of the reaction mechanism and the degradation of the metal-semiconductor system during annealing. Optical reflection and transmission spectroscopy, secondary ion mass spectroscopy, X-ray diffractometry, scanning electron and ion microscopy, and X-ray energy spectroscopy analysis were conducted on Pt/n-GaAs samples at various stages of annealing at 400°C. This temperature was chosen so that the annealing times necessary to achieve the completed reaction could be shortened.
2. Sample preparation Single crystal semi-insulating GaAs slices and high carrier concentration &As slices were obtained from the Monsanto Company. The semi-insulating GaAs slices were n-type, Cr doped, with (100) orientation (off lo-0.3” toward [l lo]) with a resistivity of -2.4 X 10s fi cm. The high carrier concentration (1.5 X 10s cmm3) slices were n-type, Si doped with (100) orientation (off lo-3” toward [ 1001) with a resistivity of -2 X 1Om3R cm. Slices were approximately 2.5 cm X 5.5 cm and 0.038 cm thick and were electro-polished on one surface resulting in an etch pit density of -1 .O X 10’ ep/cm2 for the semi-insulating and -5.0 X lo3 ep/cm’ for the high carrier concentration slices. Samples were prepared by electron beam vacuum deposition of platinum onto the GaAs substrates. Prior to insertion into the vacuum chamber, the substrates were chemically etched for 5 min in a solution of 50% HCl and 50% methyl alcohol to remove the native oxide and rinsed in methyl alcohol and acetone [3,4]. A small amount of oxide (approximately 25 A thick) is expected to have grown on the substrate surface before deposition; however, it has been shown that this oxide does not affect the reaction rate of Pt with GaAs [3]. It has been shown that thick oxide intentionally grown before Pt deposition can affect the reaction at the metal-GaAs interface [3]. The surface of an etched GaAs sample was examined with Auger spectroscopy to determine the effect of the etchant and the general condition of the surface before deposition. A relatively small amount of Cl, 0, and C were found to be present originating from the HCl etch and native oxide at the sample surface. The platinum was deposited at a pressure of 1 .O X 10v6 Torr, and the thickness of the deposited film was monitored during deposition by a quartz crystal thickness monitor. The substrate temperature, which was also monitored during deposition, was observed to be below 100°C. Additional samples obtained from Texas Instruments consisted of evaporation deposited and electro-plated films of Pt on n-type GaAs substrates. Segments of each of the samples were subjected to a series of heat treatments. Each sample segment was annealed at 400°C in an evacuated stainless steel tube placed in a resistive
D.L. Begley et al. /Reaction
heater furnace. A diffusion pump connected -1 .O X lo-’ Torr during the heat treatment.
at Ptfn-GaAs interface
to the tube maintained
343
a pressure of
3. Analysis 3.1. Optical reflection
and transmission
Optical reflection and transmission studies were made on semi-insulating substrates with a 150 A Pt film. The measurements were made utilizing a Perkin-Elmer 599 spectrophotometer (400 to 3000 cm-‘) with the beam incident upon the metal side of the sample. The reflectivity (transmission) versus anneal time at a given wavelength was a slightly increasing (decreasing) curve with a minor peak (minimum) occurring at one hour of annealing time. The reflectivity curve is shown in fig. 1. This increase is believed to be due to the formation of a highly reflecting intermetallic compound early in the anneal sequence. It is interesting to note that the increase in reflectivity is similar to an increase in resistivity of Pt films reported previously by Murarka [3] and Kumar [4]. The absorption data indicated broadband frequency dependent absorption occurring due to the formation of the intermetallic compounds. Thicker Pt films were investigated in the hope that a larger amount of the various compounds would be formed, resulting in larger absorption and the appearance of some structure. Platinum films 500 and 2000 A thick were examined. The films were annealed in stages, and spectra were taken after each anneal until the films were reacted to completion [4] as indicated by X-ray diffraction. No structure was observed.
0.7 0.6 0.5 0.4 0.3 0.2 0. I
ANNEAL
TIME
(hrl
Fig. 1. Reflectivity versus anneal time at various wave numbers for 150 A of Pt on Cr-doped GaAs: (A) 3000 cm-l , (f) 1500 cm-‘, (=) 800 cm-‘, (0) 400 cm-l.
344
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at Pt/n-tiaAs
interface
Fig. 2. SEM images of 2000 A of Pt on GaAs: (a) annealed for l/2 h at 400°C, the width of the figure corresponds to 24 vm; (b) annealed for 3 h at 4000°C, the width of the figure corresponds to 8 pm.
Reflectivity measurements were also made on samples with higher carrier concentration substrates. These were also reacted to completion, with no structure observed in the spectra. 3.2. Electron and ion microscopy A Jeolco JSM-2 scanning electron microscope (SEM) was employed to investigate the surface appearance of samples after various stages of anneal. SEM photographs of an as-deposited sample revealed that the Pt film was not continuous, with
Fig. 3. SEM images of 2000 iI of Pt on GaAs: (a) annealed for 5 h at 400°C; 16 h at 4OO’C. The width of the figures corresponds to 8 pm.
(b) annealed
for
D.L. Begley et al. /Reaction
at Ptln-GaAs interface
Fig .4. Ion microscope image of Ga on 2000 A of Pt on GaAs annealed wid th of the figure corresponds to 250 Wm.
for 3 h at 400°C.
Fig. 5. Ion microscqpe image of Ga on 2000 A of Pt on GaAs annealed width of the figure corresponds to 250 pm.
for 5 h at 4OO’C. The
The
346
D.L. Begley et al. /Reaction
at Pt/n-GaAs
interface
Fig. 6. SEM images of Texas Instruments sample with 200 A of Pt: (a) evaporated on GaAs and annealed for l/2 h at 400°C; (b) electroplated on GaAs and annealed for l/2 h at 4OO’C. The width of the figures corresponds to 8 pm
holes on the order of 0.5 to 1.5 pm in These regions of pin holes are believed the GaAs prior to metallization. Figs. 1 X lo4 magnification) of the surface
diameter observed in some areas of the film. to be caused by particle contamination of 2 and 3 are SEM photographs (3 X lo3 and of Pt (2000 A) on n-type GaAs (n = 1.5 X
1018 cm-“) after annealing at 400°C. At l/2 h of annealing time, fig. 2a, craters are observed on the sample surface with protrusions appearing near the center. These craters are composed of a protrusion forming a nodule at the center and a depressed outer region. These craters have not been reported before. After 3 h of anneal the surface has become very irregular with a large number of protrusions appearing through the Pt film, as illustrated in fig. 2b. X-ray energy spectroscopy (XES) analysis of the protrusions reveal that the protruding nodules had a much higher Ga content than the surrounding surface area. It is therefore believed that the protrusions are caused by the initial diffusion of gallium through a thin area in the Pt film in the form of gallium or a gallium-rich compound. The sample surface is also beginning to take on a granular appearance. After 5 h of anneal at 4OO”C, the entire surface of the film has been altered as illustrated by the photograph in fig. 3a. The surface has now been transformed from relatively smooth to becoming extremely granular in appearance. XES analysis of the sample revealed a high gallium concentration at the surface. In fig. 3b the surface of the sample after 16 h of anneal at 400°C appears much the same as at 5 h except for some increase in the size of the individual granules. To investigate the extent of the gallium diffusion to the sample surface, the ion optics of a Cameca IMS 200 Microscope-Microprobe were adjusted to detect the spatial distribution of gallium at the sample surface. Figs. 4 and 5 are gallium ion microscope photographs of samples annealed for 3 and 5 h respectively. These
D.L. Begley et al. /Reaction
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347
photographs represent a distance of approximately 2.50 pm across the sample surface. The advancement of the gallium formation at the sample surface is observed, as well as the formation of surface islands of gallium. The lateral resolution of the ion microscope (-1 ym) is considerably lower than that of the electron microscope (-200 a). This lower resolution did allow Ga to be detected for the one-half and one-hour annealed samples. Samples obtained from Texas Instruments were also examined with the SEM. Fig. 6 contains photographs of evaporated and electro-plated samples annealed for l/2 h. The protrusion of nodules near the center of a hole in the Pt film is clearly illustrated in fig. 6a. Similar nodule protrusions are observed for the electro-plated sample in fig. 6b. 3.3. Secondary ion mass spectroscopy (SIMS) and X-ray diffracrometry (XRD) In-depth compositional profiling of the Pt-AgAs samples was performed utilizing the Cameca IMS 300 Ion Microscope-Microprobe. A primary beam of O’, was used to sputter material from the sample surface. A 2000 A Pt fnm was deposited on n-GaAs in an e-beam evaporator. SIMS depth
1.0 0.9
0.8 :
v,
E
0.7
2-
0.6
d
0.5
ZJ v,
0.4
9 N
0.3
7 Z
0.2
E
0.
I
200
300
400
SPUTTERING Fig. 7. SIMS intensity line, platinum; double
profile for as-deposited dashed line, arsenic.
500
TIME
600
700
600
(set)
2000 A of Pt on GaAs:
solid line, gallium;
dashed
D.L. Begley et al. /Reaction
348
at Pt/n-GaAs interface
I.0 0.9
z cn z
0.8
W
0.7
!S -
0.6
G
0.5
z G
0.4
n w N
0.3
I
0
100
Fig. 8. SIMS intensity gallium;
dashed
profile line, platinum;
200
300
400
SPUTTERING
TIME
500
600
700
(set)
for 2000 ii of Pt on GaAs annealed double dashed line, arsenic.
l/2 h at 4OO’C:
solid line,
profiles were made on samples after various stages of anneal. Fig. 7 represents a profile for a typical sample prior to anneal. A “bump” such as seen at about 500 s on the As profile typically occurs upon sputtering through an interface and can be used as a signature of an interface. A definite interface between Pt and GaAs is seen within the 40 A depth resolution of the instrument. Fig. 8 illustrates the profiles for a sample annealed l/2 h. Considerable interdiffusion is clear even for this short anneal. Both Ga and As have diffused into the Pt with Ga having moved further and faster. The bump in the Pt profile at 500 s indicates the interface with the bulk GaAs. Peaks in the Ga profne (and As and Pt profiles) at about 280,350 and 425 s indicate interfaces between layers of apparent intermetallic compounds which have been formed. It can be seen that the outmost compound is of Pt and Ga with no As, the next (second compound) layer contains more Ga and some As, and the third layer a little more Ga and considerably more As. Because the sputtering rate is a function of the material being sputtered, the concentrations of the elements cannot be determined from these SIMS profiles. After 1 h of annealing, the layers have become less distinct as can be seen in fig. 9. After 3 h at 4OO”C, the layer structure is further washed out, although it is clear that Ga is diffusing faster than As (fig. 10). Finally, after 5 h, Ga is completely through the film (fig. 11).
1.0
,
0.9
_
z Gi
0.8
_
5
0.7
L / / I
5-
I
I\ ’
Id\
pg
b
,J’“,/\/ v 1
AC
I
Ga /
lJ
/
Pi
1
I
I
I
too
0
:’’T---!
“\
I
A--J4$ -A/
200
300
400
500
SPUTTERING Fig. 9. SIMS intensity profile gallium; dashed line, platinum;
TIME
for 2000 A of Pt on GaAs double dashed line, arsenic.
600
700
000
(set) annealed
1 h at 4OO’C:
solid line,
1.0
0.9 > l-
0.8
cn El
0.7
5 -
0.6
0
100
200
300
400
SPUTTERING
500
TIME
600
700
800
kec 1
Fig. 10. SIMS intensity profile fox 2000 A of Pt on GaAs annealed gallium; dashed line, platinum; double dashed line, arsenic.
3 h at 4OO’C:
solid line,
D.L. Begley et al. /Reaction
3.50
0
100
200
at Ptln-GaAs interface
300
SPUTTERING
400 TIME
500
600
(set)
Fig. 11. SIMS intensity profile for 2000 A of Pt on GaAs annealed gallium; dashed line, platinum; double dashed line, arsenic.
5 h at 400°C:
solid line,
From the XRD data discussed below and from the work of Chang et al. [6] using Auger electron spectroscopy, it is tempting to identify the outmost compound layer seen in fig. 8 as PtaGa, the next layer as PtGa, and finally PtAs, with a considerable amount of Ga included. It should be recalled that these SIMS profiles are averages over a 250 pm diameter spot and the SEM measurements discussed above indicate considerable inhomogeneity over a region of this size. Evaporated and electro-plated samples with varied film thicknesses obtained from Texas Instruments were also examined. Fig. 12 represents a SIMS profile of 2000 A Pt film electro-plated on a n-type GaAs sample after a l/2 h anneal at 400°C. The diffusion of Ga and As has proceeded at a much greater rate than for our samples. Similar results were found for evaporated samples obtained from Texas Instruments. No reason was found for the considerably more rapid diffusion through the TI produced Pt films. A General Electric XRD 700 X-ray diffractometer was used to irradiate the metal side of the sample with Ni filtered Cu K, radiation. Diffractometry data revealed no indication of compound formation for l/2 and 1 h annealed samples. After 3 h of annealing, XRD revealed diffraction lines for the compounds PtaGa, PtGa, and PtAsa. These lines were stronger for the 5 h annealed sample.
D.L. Begley et al. 1 Reaction at Ptln-GaAs interface
0.9 0.8
I
I
0
100
I 200
-i_+ 300
400
1
I
1
500
600
700
SPUTTERING
TIME
‘\I 800
I
1
900
1000
kec)
Fig. 12. SIMS intensity profile for Texas Instruments sample, 2000 A of Pt electro-plated GaAs, annealed l/2 h at 4OO’C: solid line, gallium; dashed line, arsenic.
on
The electron micrographs show that the layers observed in SIMS depth profiles are not homogeneous and that gallium penetration to the surface is primarily through defects in the platinum film. It is important to note that the sites of Ga penetration are not just pin holes in the Pt film observable with the electron microscope. SEM observable pin holes were found to have a much lower density than the sites of Ga penetration (SEM with X-ray fluorescence). Caution must be exercised interpreting depth profiles from SIMS and AES which average over large areas compared to the few hundred angstrom diameters of the gallium protrusions.
4. Conclusions The Pt/n-GaAs interface reaction annealing time. Auger spectroscopy tion indicated that the surface was C, and 0, which should not greatly [31. Optical reflectivity
measurements
has been investigated at 400°C as a function of analysis of the GaAs surface before Pt evaporaclean except for relatively small amounts of Cl, affect the metallization of the sample surface indicated
that there is an increase in the reflec-
352
D.L. Begley et al. /Reaction
at Pt/n-GaAs interface
tivity as a function of annealing time followed by a decrease and a leveling off trend. Transmission spectra indicated possible broad-band frequency-dependent absorption caused by annealing. However, no structure was observed in the reflection or transmission spectra. The nature of the compound formation was found to be even more complex than has been observed by others. SIMS depth profiles showed that there is definite layer structure formed at the original Pt-GaAs interface. This layer structure has a tendency to wash out as the anneal time is increased. SEM analysis of the sample surface at various stages of the annealing sequence revealed an early protrusion of gallium or a gallium-rich compound through the Pt film. During the annealing procedure the protrusions became more pronounced with the surface taking on a granular structure. The effect appears to occur independently of the method of film preparation, with both evaporated and electro-plated samples yielding similar results. The diffusion and subsequent reaction are not uniform across the surface of the sample. Ion microscope photographs confirmed the appearance of the gallium through the Pt film at the sample surface. After further anneal, the entire Pt film appears to be consumed by gallium diffusion, resulting in a complete deterioration of the Pt surface and interface region. It has been observed that premature breakdown of planar Schottky barrier diodes occurs at the edge of the metallization and at sites of missing Pt (pin holes) [13]. It is believed that the observed initial metallurgical interaction at sites of imperfections in the metal films are the main areas for early device failure. Areas of metallization defects can produce high electric field gradients under bias conditions which may cause premature breakdown. Within these areas the Schottky barrier is no longer one of Pt-CaAs, but of various intermetallic compounds. The movement of the metal-semiconductor interface by interdiffusion would also be a significant contributing factor for device degradation in devices with parameters dependent upon the position of the interface. The interdiffusion and reaction at the platinum-gallium arsenide interface presents complex and difficult problems in the understanding of device failure and degradation during actual operation. However, through qualitative analysis of the interface reaction, utilizing a number of complementary techniques, an insight into possible device failure mechanisms has been made.
Acknowledgements The authors wish to thank H. Rice for the SEM photcgraphs; J. Stubbs for X-ray analysis; J. Kleefeld for Auger analysis; D. Shaw for the Texas Instruments samples; J. Boone for the use of the electron beam evaporator; Professor G. Morrison and D. Leta of Cornell University for use of the SIMS; and C. Ward for helpful comments.
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353
References [ 1] [2] [ 31 [4] [5] [6] [7] [8] [9]
[lo] [ 111 [12] [13]
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