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Supersaturation requirement for good ohmic contacts to semiconductors
Thin Solid Films, 36 (1976) 393-397 0 Elsevier Sequoia S.A., Lausanne-Printed
SUPERSATURATION SEMICONDUCTORS* H. HARTNAGEL,
REQUIREMENT
K. TOMIZAWA,
FOR GOOD OHMIC CONTACTS
L. H. HERRON
August
TO
AND B. L. WEISS
The University of Newcastle upon Tyne, Department of Electrical and Electronic The Merz Laboratories, Newcastle upon Tyne, NE1 7R U (Ct. Britain) (Received
393
in Switzerland
Engineen’ng,
25, 1’975)
1. INTRODUCTION
Ohmic contacts to semiconductors often consist of a combination of materials one of which acts as a dopant for the required n+ or p+ layer which is produced by some heat cycle, such as the thin phase epitaxy (TPE) process’ recently reported. Ottaviani et al. 2 have shown how amorphous Si or Ge saturates Al films upon heating, even if the temperature remains below the melting point, and how the dissolved semiconductor produces islands of epitaxial growth on the Al surface owing to supersaturation upon cooling by precipitation of excess material towards single-crystal nucleating centres formed on the film surface. For heating temperatures above the melting point of some or all of the film metals, the relevant phase diagrams show that, upon cooling, first a solid layer with a certain composition is formed and then the remaining metal parts reduce in size, by continuously changing the composition towards the eutectic composition. As eutectic compositions possibly have a predominance of a metal with a high surface tension of the liquid form, balling up of the final stages takes place which gives again a complex pattern of the ohmic contact. Similar island patterns are produced by the doping material of ohmic contact films on GaAs. Our experimental results indicate that the formation of such an island pattern on the surface of the metal films is beneficial for good ohmic contacts. This obviously means that a sufficiently high doping of the growth layer only results if the concentration of the dopant in the metal layer is so high that strong supersaturation of the dopant occurs in the metal layer upon cooling. For many of the common ohmic contact alloys such as In-Ge-Ag, Sn-Ag and Au-Ge-Ni, a beautifully uniform ohmic contact layer does not therefore necessarily represent a good ohmic contact. 2. OHMIC CONTACT
STUDIES
WITH Ag-Sn
The metals were consecutively evaporated, with a Ag thickness of 2000 A preceded by a Sn layer of varying thickness. The alloying cycle employed was 1 min at 450 “C with relatively fast cooling. When the Sn layer was 1000 a or thicker, a rough pattern of balling and raised curvy lines appeared. This did not occur with thinner layers of Sn, when the resulting metal surfaces became beautifully uniform and shiny. However, * Paper presented at the Third Inrernational Conference on Thin Films, “Basic Problems, tions and Trends”, Budapest, Hungary, August 25-29, 1975; Paper 11-17.
Applica-
394
H. HARTNAGEL
et al.
good ohmic contact properties as determined by current-voltage measurements t‘or various temperatures (down to liquid N, temperature) were only obtained with the patterned metal surfaces. Low Sn thicknesses showed in particular high contact resistances also; these were often well above 10 A4Q cm-*. Therefore it is suggested that good ohmic contacts are produced by the following process. On heating the metal layer, some GaAs is dissolved into the metals in accordance with complex solubility phenomena of the only partly mixed multiple-component metal layer. On cooling, Ga and As are supersaturated, and a regrowth of GaAs takes place. Depending on the metals available and their degree of mixing, this-regrown layer is 01‘ different thickness; additionally, depending on the segregation coefficients involved. the dopant material gets included into this regrowth layer which thus becomes highly conductive. If the dopant is an amphoteric material, the type of doping can be determined either by the degree of As or Ga vacancies or by the recently proposed effect ot‘ the semiconductor surface on amphoteric doping3. In our case of Sn doping, sufficient doping of the regrowth layer can therefore only occur if sufficient Sn is available. 3. In-
GOOD
OHMIC CONTACTS
TO SEMICONDUCTORS
395
was metallized as previously described; a second type had 200 A of Ga deposited after the In metallization. Before metallization the samples had been stepped using HaSO t HaOa + Ha0 (3 : 1 : 1) in an attempt to determine the contact resistance. The TPE cycle was used on both samples with 2 “C min -’ of cooling for 20 min. Either circular patterns (100 pm in diameter) were etched on the sample surface using this etch, or planar patterns were made using the float-off method before alloying. The samples with these two types of Ag-In-Ge ohmic contacts were cleaved to expose the cross section of the ohmic contact-semiconductor interface. They were mounted on an aluminium stub with carbon dag, and the contact surface and cleaved edge were studied in a scanning electron microscope (SEM). To determine the distribution of the elements of the ohmic contact in the GaAs a stationary electron beam was used at specified distances from the top of the ohmic contact along the cleaved edge, and the X-rays emitted from each point were analysed in an energy dispersive X-ray analyser (EDAX). Thus the elemental concentrations at each point along the cleaved edge and the elemental distributions are obtained after using certain corrections. If the alloying process is not applied to the sample, the distribution is as expected (see Fig. 1). The broadening of the elemental profiles is due to the resolution of the microprobe which is of the order of 3000 A. In is not seen as a small amount was used in this case. No particular attention should be paid to the finer details as these might be caused by the measurement process. For example, the small increase in some of the
Penetmo”
I arbWary
unrts1
Fig. 1. The elemental
concentration
of a pre-alloying
Fig. 2. The elemental
concentration
after 5 min alloying
metallized
contact;
no heat cycle.
in Hz gas; heat cycle; no As atmosphere.
396
11. HAKTNAGELef
a/.
elements near the metal surface is surely caused by the edge. Similarly the large variations of the Ga and As concentrations near the metal-semiconductor interface are surprising and might not be real. Therefore a chain curve has been shown giving the distribution to be expected. Samples with a 3 min alloying cycle in Hz b“as have a surface showing a crazed SEM pattern (similar to that of Ag-Sn). The results from the EDAX show that most of the Ge is at the surFdce of GaAs. One might speculate that a very thin Ge layer exists at the interface, tbrtning a Ge-GaAs heterojunction. Similar but more pronounced results were found with a 5 min heat cycle (Fig. 3). With the TPE alloying cycle Ge diffused in further and accumulated more strongly at the metal-GaAs interface; Ga and As diffused out more strongly. If only a very little In was provided, no Ga and As peaks occurred near the rnetal surface. However, when a large amount of In was available on the metal surface, an entirely different distribution resulted (Fig. 3). A large peak of In, Ga and As occurred at the metal surEace. In fact. by careful calibratian it was found that the atomic densities of In plus Ga eyualled that of As; this suggests that a ternary compound of InO,,Gao,,As is grown there. Following the results of Ottaviani et al. ‘, it is possible that this is caused by the production of single-crystal nucleating sites on the metal surface at which precipitation occurs so that this ternary compound can develop there. Because of the differences
GOOD
OHMIC CONTACTS
397
TO SEMICONDUCTORS
in the thermal expansion of In-Ga-As, Ag and GaAs, SEM photographs of the island pattern of the metal surface.
show a cracking
ACKNOWLEDGMENTS
The authors are grateful for the financial assistance provided by the U.K. S.R.C. and to Mr. E. Boult and Miss B. Arnold for the use and operation respectively of the SEM facilities in the School of Chemistry. REFERENCES
1 T. Sebestyen, H. Hartnagel and L. Herron, Electron. Lett., IO (1974) 372. 2 G. Ottaviani, D. Sigurd, V. Marello, J. W. Mayer and J. 0. McCaldin, J. Appl. Phys, 45 (1974) 1730. 3 M. Jaros and H. Hartnagel, Solid-State Electron., I8 (1975) 1029.
SUPERSATURATION SEMICONDUCTORS* H. HARTNAGEL,
REQUIREMENT
K. TOMIZAWA,
FOR GOOD OHMIC CONTACTS
L. H. HERRON
August
TO
AND B. L. WEISS
The University of Newcastle upon Tyne, Department of Electrical and Electronic The Merz Laboratories, Newcastle upon Tyne, NE1 7R U (Ct. Britain) (Received
393
in Switzerland
Engineen’ng,
25, 1’975)
1. INTRODUCTION
Ohmic contacts to semiconductors often consist of a combination of materials one of which acts as a dopant for the required n+ or p+ layer which is produced by some heat cycle, such as the thin phase epitaxy (TPE) process’ recently reported. Ottaviani et al. 2 have shown how amorphous Si or Ge saturates Al films upon heating, even if the temperature remains below the melting point, and how the dissolved semiconductor produces islands of epitaxial growth on the Al surface owing to supersaturation upon cooling by precipitation of excess material towards single-crystal nucleating centres formed on the film surface. For heating temperatures above the melting point of some or all of the film metals, the relevant phase diagrams show that, upon cooling, first a solid layer with a certain composition is formed and then the remaining metal parts reduce in size, by continuously changing the composition towards the eutectic composition. As eutectic compositions possibly have a predominance of a metal with a high surface tension of the liquid form, balling up of the final stages takes place which gives again a complex pattern of the ohmic contact. Similar island patterns are produced by the doping material of ohmic contact films on GaAs. Our experimental results indicate that the formation of such an island pattern on the surface of the metal films is beneficial for good ohmic contacts. This obviously means that a sufficiently high doping of the growth layer only results if the concentration of the dopant in the metal layer is so high that strong supersaturation of the dopant occurs in the metal layer upon cooling. For many of the common ohmic contact alloys such as In-Ge-Ag, Sn-Ag and Au-Ge-Ni, a beautifully uniform ohmic contact layer does not therefore necessarily represent a good ohmic contact. 2. OHMIC CONTACT
STUDIES
WITH Ag-Sn
The metals were consecutively evaporated, with a Ag thickness of 2000 A preceded by a Sn layer of varying thickness. The alloying cycle employed was 1 min at 450 “C with relatively fast cooling. When the Sn layer was 1000 a or thicker, a rough pattern of balling and raised curvy lines appeared. This did not occur with thinner layers of Sn, when the resulting metal surfaces became beautifully uniform and shiny. However, * Paper presented at the Third Inrernational Conference on Thin Films, “Basic Problems, tions and Trends”, Budapest, Hungary, August 25-29, 1975; Paper 11-17.
Applica-
394
H. HARTNAGEL
et al.
good ohmic contact properties as determined by current-voltage measurements t‘or various temperatures (down to liquid N, temperature) were only obtained with the patterned metal surfaces. Low Sn thicknesses showed in particular high contact resistances also; these were often well above 10 A4Q cm-*. Therefore it is suggested that good ohmic contacts are produced by the following process. On heating the metal layer, some GaAs is dissolved into the metals in accordance with complex solubility phenomena of the only partly mixed multiple-component metal layer. On cooling, Ga and As are supersaturated, and a regrowth of GaAs takes place. Depending on the metals available and their degree of mixing, this-regrown layer is 01‘ different thickness; additionally, depending on the segregation coefficients involved. the dopant material gets included into this regrowth layer which thus becomes highly conductive. If the dopant is an amphoteric material, the type of doping can be determined either by the degree of As or Ga vacancies or by the recently proposed effect ot‘ the semiconductor surface on amphoteric doping3. In our case of Sn doping, sufficient doping of the regrowth layer can therefore only occur if sufficient Sn is available. 3. In-
GOOD
OHMIC CONTACTS
TO SEMICONDUCTORS
395
was metallized as previously described; a second type had 200 A of Ga deposited after the In metallization. Before metallization the samples had been stepped using HaSO t HaOa + Ha0 (3 : 1 : 1) in an attempt to determine the contact resistance. The TPE cycle was used on both samples with 2 “C min -’ of cooling for 20 min. Either circular patterns (100 pm in diameter) were etched on the sample surface using this etch, or planar patterns were made using the float-off method before alloying. The samples with these two types of Ag-In-Ge ohmic contacts were cleaved to expose the cross section of the ohmic contact-semiconductor interface. They were mounted on an aluminium stub with carbon dag, and the contact surface and cleaved edge were studied in a scanning electron microscope (SEM). To determine the distribution of the elements of the ohmic contact in the GaAs a stationary electron beam was used at specified distances from the top of the ohmic contact along the cleaved edge, and the X-rays emitted from each point were analysed in an energy dispersive X-ray analyser (EDAX). Thus the elemental concentrations at each point along the cleaved edge and the elemental distributions are obtained after using certain corrections. If the alloying process is not applied to the sample, the distribution is as expected (see Fig. 1). The broadening of the elemental profiles is due to the resolution of the microprobe which is of the order of 3000 A. In is not seen as a small amount was used in this case. No particular attention should be paid to the finer details as these might be caused by the measurement process. For example, the small increase in some of the
Penetmo”
I arbWary
unrts1
Fig. 1. The elemental
concentration
of a pre-alloying
Fig. 2. The elemental
concentration
after 5 min alloying
metallized
contact;
no heat cycle.
in Hz gas; heat cycle; no As atmosphere.
396
11. HAKTNAGELef
a/.
elements near the metal surface is surely caused by the edge. Similarly the large variations of the Ga and As concentrations near the metal-semiconductor interface are surprising and might not be real. Therefore a chain curve has been shown giving the distribution to be expected. Samples with a 3 min alloying cycle in Hz b“as have a surface showing a crazed SEM pattern (similar to that of Ag-Sn). The results from the EDAX show that most of the Ge is at the surFdce of GaAs. One might speculate that a very thin Ge layer exists at the interface, tbrtning a Ge-GaAs heterojunction. Similar but more pronounced results were found with a 5 min heat cycle (Fig. 3). With the TPE alloying cycle Ge diffused in further and accumulated more strongly at the metal-GaAs interface; Ga and As diffused out more strongly. If only a very little In was provided, no Ga and As peaks occurred near the rnetal surface. However, when a large amount of In was available on the metal surface, an entirely different distribution resulted (Fig. 3). A large peak of In, Ga and As occurred at the metal surEace. In fact. by careful calibratian it was found that the atomic densities of In plus Ga eyualled that of As; this suggests that a ternary compound of InO,,Gao,,As is grown there. Following the results of Ottaviani et al. ‘, it is possible that this is caused by the production of single-crystal nucleating sites on the metal surface at which precipitation occurs so that this ternary compound can develop there. Because of the differences
GOOD
OHMIC CONTACTS
397
TO SEMICONDUCTORS
in the thermal expansion of In-Ga-As, Ag and GaAs, SEM photographs of the island pattern of the metal surface.
show a cracking
ACKNOWLEDGMENTS
The authors are grateful for the financial assistance provided by the U.K. S.R.C. and to Mr. E. Boult and Miss B. Arnold for the use and operation respectively of the SEM facilities in the School of Chemistry. REFERENCES
1 T. Sebestyen, H. Hartnagel and L. Herron, Electron. Lett., IO (1974) 372. 2 G. Ottaviani, D. Sigurd, V. Marello, J. W. Mayer and J. 0. McCaldin, J. Appl. Phys, 45 (1974) 1730. 3 M. Jaros and H. Hartnagel, Solid-State Electron., I8 (1975) 1029.