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GaAs Schottky barrier diodes by the fluxed wetting of tin and indium
Solid-State
Electronics
Pergamon Press 1970. Vol. 13, pp. 485-489.
GaAs SCHOTTKY
Printed in Great Britain
BARRIER DIODES BY THE FLUXED
WETTING G. 0.
OF TIN AND INDIUM LADD,
Dept. of Electrical Engineering,
and D. L. FEUCHT
JR.*
Carnegie-Mellon
(Received
Univ., Pittsburgh,
Pa., U.S.A.
11 July 1969)
Abstract-A procedure is described for making Schottky barrier diodes on n and p type GaAs by melting metal spheres onto the surface near their melting points. The metals used are tin, indium, and an In-l per cent Zn alloy. Wetting is facilitated by the use of an HCl flux. The electrical characteristics of the diodes are presented and discussed. R&urn&-On decrit un procede pour fabriquer des diodes de barrike Schottky sur 1’AsGa de type n et p par la fonte de spheres metalliques sur la surface aux approches de leur point de fusion. Les metaux employ& sont l’etain, l’indium et un alliage de Zn a In-l pour cent. Le mouillage est facilite par un flux HCl. Les caracteristiques des diodes sont presentees et discutees. Zusammenfassung-Fiir die Herstellung von Schottky-Dioden durch Aufschmelzen van Metallkiigzlchen nahe ihrem Schmelzpunkt auf die Oberfllche von n- und p-Typ GaAs wird beschrieben. Die verwendeten Metalle waren Zinn, Indium und eine Indiumlegierung mit 1 prozent Zink. Die Benetzung wurde erleichtert mittels eines HCl. Flussmittels. Die elektrische Charakteristik der Dioden wird angegeben und diskutiert.
1. INTRODUCTION
SCHOTTKY barrier diodes can be made by introducing a metal onto the ‘clean’ surface of the semiconductor. In practice this is usually done by evaporation of the metal onto a semiconductor which may be cleaved in VQCUOor chemically etched prior to deposition.(l*” GaAs Schotty diodes have also been made by electro-chemical deposition of the metal.(3s4) Schottky barrier diodes on GaAs are frequently used in the semiconductor industry to measure the carrier concentration profiles of epitaxial layers. Where time and facilities permit, good results have been obtained by evaporation of gold, gold-tin alloys, and aluminum, etc. This paper reports on a method for fabricating Schottky barrier diodes of tin, indium, and indium-1 per cent zinc alloy on n and p type GaAs by means of the low temperature wetting of metal spheres to the GaAs * Presently with United Aircraft Corporation, search Laboratory, East Hartford, Conn., U.S.A.
Re485
in an anhydrous HCl gas atmosphere. In many cases this method is more convenient than evaporation. It is also unique in that the tin diodes can be made ohmic to n type GaAs by simply remelting and alloying at a high temperature. Onp-GaAs the In-l per cent Zn may be made ohmic by alloying at a high temperature. The process is apparently made possible by the fluxing action of the HCl atmosphere at temperatures for which the metals involved have a low solubility for GaAs. 2. EXPERIMENTAL The diodes were made by melting the metals onto GaAs chips in an alloy stage which has been previously described.(5) The stage is made of nickel and gold plated brass to prevent corrosion by the HCl gas and uses a thin graphite strip heater. The temperature of the strip is monitored by a chromel-alumel thermocouple. A glass cover, which is sealed to the base plate by an O-ring gasket, permits visual inspection of the process. Provisions are made for introducing high purity
486
G.
0.
LADD,
JR. and
anhydrous HCl gas and a tank mixture of 85 per cent argon-15 per cent hydrogen to the stage and for controlling their flow rates. The argon-hydrogen mixture passes through a calcium sulphate drying tube before entering the stage. The metal spheres (usually 5-10 mils in diameter) are cleaned in dilute hydrochloric acid and stored in methanol until use. The GaAs specimen is polished flat if necessary and etched to remove surface damage. Where epitaxial material is used it is only necessary to clean the surface with solvents provided it is relatively free of gross defects such as pits or hillocks. Where tin spheres are used a heating cycle consists of the following: The stage is first flushed with about 10 alloy stage volumes of the argonhydrogen mixture. Failure to strictly observe this precaution usually produces poor results. Then the argon-hydrogen flow is set to 1.75 volumes/min, the HCI flow is set to give a concentration of 4-5 per cent, and three minutes is allowed for the gas mixture to come to equilibrium in the stage. Next, the heater is turned on and brought up to the tin melting point (232 “C) within about 30 set and then raised to about 250 “C for 5 sec. The heater strip is then cooled to the melting point in 5-10 set and turned off, the HCl is turned off, and the argon-hydrogen flow is turned up to purge the stage as it cools. Cooling to below 100 “C takes less than 10 sec. The entire heating cycle lasts about one minute. Although the temperature is usually reduced slowly as described above from 250 to 232 “C, there appears to be no difference in the results if the heater is shut off after 5 set at 250 “C. The procedure for the indium and indium1 per cent zinc spheres is significantly different: The stage is purged as for the tin cycle but now the argon-hydrogen flow is set at 4 vol/min and the HCl flow to give a concentration of 0.4-0.8 per cent. After waiting l-2 min the heater temperature is raised to about 200 “C within 30 sec. Then the heater is turned off and cools to below 100°C in about 5-10 sec. The approximate 200°C point is determined visually by noting the commencement of rapid evaporation of a ‘scum’ which forms on the surface of the molten indium. At this temperature greatly enhanced wetting of the GaAs by the indium is noted. The scum which forms during the alloying is probably indium trichloride produced by reaction with the HCI gas.
D. L. FEUCHT
The critical features of the process are the use of a low temperature and the HCl flux. The use of the HCl flux has been previously discussed by JADUS et aZ.,(5) and good wetting of lead, zinc and tin on GaAs has also been obtained by SCHWARTZ and SARACE@)using solutions of the metal halides. We have also had some success with ammonium chloride grains laid on the strip heater during heating in the argon-hydrogen ambient. It will be shown below that the use of the HCl gas ambient, at least, provides a very clean metal-semiconductor interface. JADUSet aLc5) have also shown that at somewhat higher alloying temperatures good ohmic contacts can be made to n andp type GaAs using, respectively, Sn and Zn or In-l per cent Zn spheres. Since the In or In-l per cent Zn diodes can be formed below the tin melting point, it may be convenient to use a tin alloy ohmic contact together with an In or In-l per cent Zn diode to evaluate carrier concentrations. The maximum temperature used is the principal difference between the heating cycles for ohmic or rectifying contacts when tin or In-l per cent Zn is used. Ohmic tin contacts which exhibit a contact resistance within 50 per cent of the calculated spreading resistance for the area of contact and the resistivity of the material can be obtained by heating to between 325 and 350°C and then cooling quickly. In the range between 250 and 325°C the contact characteristic varies from rectifying to ohmic. Thus, remelting a previously alloyed contact can raise the contact resistance by a factor of 100. In-l per cent Zn ohmic contacts to p-GaAs may also be made at 350°C. Ohmic contacts to more lightly doped material of the order of 1015 cme3 carrier concentration tend to require a higher temperature alloy cycle. 3. ELECTRICAL AND PHYSICAL CHARACTERISTICS Some information on the penetration of the metal into a smooth GaAs surface may be obtained by etching off the metal in concentrated hydrochloric acid and examining the wetted area under the microscope. In the case of pure In or In-l per cent Zn alloy it is sometimes found that the wetted area cannot be distinguished from the surrounding surface. Usually, however, the wetted area is surrounded by a very shallow ring etched into the
GaAs SCHOTTKY
BARRIER
DIODES
BY THE
FLUXED
surface. The wetted area inside the ring is relatively free of visible attack. Sometimes small rings are found within the wetted area indicating incomplete wetting of the surface by the metal. The wetted areas of many of these diodes have been examined in the course of measuring the carrier concentrations of epitaxial material. This was done in order to get an accurate value of the area of the diode. Since the contact angle of the metal to the GaAs is sometimes greater than go”, it is not always possible to measure the area without etching off the metal. When the contact area is less than 90” good agreement is found between measurements of the wetted area made before and after etching of the metal. Figure 1 is a photograph of the wetted area under a tin diode. The depth of the circular ring has been determined by interference microscope measurements to be less than 0.03 pm. The voltage-current characteristics of these diodes exhibits a very low reverse current of less than low6 A. For diodes on n type GaAs the reverse breakdown voltage is usually within a factor of two of the theoretical value for an abrupt junction in GaAs of the same carrier concentration. The breakdown voltage of diodes on p type GaAs is somewhat lower than the theoretical value. The forward characteristic is exponential and may be described at large currents by the familiar equation I
=
Isexp-
OF TIN
0.2
AND
03
Applied
INDIUM
0.4
volioge,
0.5
487
cL
V
FIG. 3. Forward current-voltage characteristics of tin and In-l per cent Zn alloy diodes on 0.13 Q-cm n-GaAs. IO-: 5
2
eVa
10.‘ 5
rlkT where V, is the applied voltage, e is the electron charge, k is Boltzmann’s constant, T is temperature and q describes the degree to which the curve deviates from an ideal diffusion diode. The current voltage characteristics of two Schottky diodes, one on n type and one on p type GaAs, using In-l per cent Zn alloy dots are shown in Fig. 2. Figure 3 shows the forward log-currentvoltage characteristic of two types of diodes on 0.13 !&cm n-GaAs. The deviation from a straight line at higher current may be explained by the series resistance of the diodes. In view of the simplicity of the fabrication process it is surprising to find that the values of q obtained, which are in the range of 1.02-1.07, are similar to those seen in evaporated diodes. Figure 4 presents the currentvoltage characteristics of the diodes onp type GaAs.
WETTING
a
2
E-
to5
E 5 0
5
2 10~6
? .I
5
IOmil In-I%ZI 5mil TIN
2
IO’
”
”
A
-
“.
I
0.2
0.3
Applied voltage,
0.4
0.5
v
FIG. 4. Forward current-voltage characteristics of tin and In-l per cent Zn alloy diodes on 0.33 f&m p-GaAs.
G.
488
Applied
voltage,
0.
LADD,
JR. and
V
FIG. 5. Capacitance-voltage characteristics of In-l percent Zn alloy and tin diodes on 0.13 n-cm n-GaAs.
The capacitance-voltage characteristic of these diodes follows the inverse square law expected for a metal-semiconductor diode. The good linearity of these plots is exemplified by the plots of reciprocal capacitance squared vs. voltage in Fig. 5 and 6
Applied
voltage,
FIG. 6. Capacitance-voltage
V
characteristics of In-l per cent Zn alloy and tin diodes on 0.33 a-cm p-GaAs.
D.
L.
FEUCHT
for diodes on n and p type GaAs. These measurements were made at 1 MHz on a General Radio Twin-T bridge to a precision of of: 0.1 pF. No drift of the capacitance with time was noted at large reverse biases over periods of several minutes. Typically, however, the capacitance-voltage curves bend downward at larger reverse biases. This may be explained by the presence of traps as discussed by GOODMAN.(~) The carrier concentration of the GaAs can be determined from the slope of the capacitancevoltage curve and the area of the diode. It was was found that diodes physically close to each other on the same piece of GaAs yielded carrier concentrations which differed by at most 20 per cent. Errors of this magnitude might be expected from variations in the material itself or from errors in the determination of the wetted area of the diode. It was also determined that these carrier concentrations were very close to the values supplied by the manufacturer of the material. Of some interest is the voltage to which the capacitance-voltage characteristic extrapolates at infinite capacitance. This value should characterize the height of the barrier to the flow of carriers. The barrier height may also be determined by the photo-response of the diode or by the thermal activation energy of the forward current.(*) The errors in the extrapolated barrier height from capacitance measurements, however, are such that thisvaluewillusuallybe higherthanthetruevalue.c7) The extrapolated barrier heights for the In1 per cent Zn diodes on n-GaAs range from 1.1 to 1.2 V while the range for the tin diodes is 0.70.8 V. Diodes on p-GaAs have not been as extensively studied but both the In-l per cent Zn and the tin diodes give values close to 0.7 v. SPITZERand MEAD(~)give a value of 0.68-0.74 eV for the barrier height derived from capacitance measurements on evaporated tin contacts on n-GaAs. For p-GaAs, of approximately the same resistivity as the diodes of Fig. 6, they report a value for the l/C2 intercept of 0.58-0.69 eV. Thus the values obtained for our diodes compare favorably with these evaporated diodes. No comparison of the indium diodes may be made because of the lack of published data. Diodes were also obtained by melting pure indium spheres on n and p type GaAs. Although the electrical characteristics of these diodes were not
GaAs SCHOTTKY
BARRIER
DIODES
BY THE
studied carefully, electronic curve tracer measurements showed them to be quite similar to the tin and In-l per cent Zn diodes in reverse current and breakdown voltage.
FLUXED
WETTING
OF TIN
AND
INDIUM
489
but the In-l per cent Zn alloy on n-GaAs were in the neighbourhood of 0.7 to 0.8 V, a value typical of most metals on GaAs. MEAD and SPITZES have shown that this is due to the presence of surface states which control the surface potential.
4. DISCUSSION
The nature of the metals and alloy used in preparing these diodes argues against the formation of p-n junctions as an explanation for their behaviour. Diodes were obtained by melting tin onto n-GaAs and In-l per cent Zn onto p-GaAs. Tin is a shallow donor and would not be expected to produce a p-n junction in n-GaAs. Likewise, Zn is a shallow acceptor and should prevent any The fact that pure n-type doping of the p-GaAs. indium produces diodes on both n and p type GaAs shows that Zn is not a necessary component of the In-l per cent Zn alloy. The solubility of GaAs in tin(loJ1) is probably high enough near the tin melting point to produce a regrown, heavily doped layer on the GaAs surface. The time period involved in this low temperature heating cycle is so short, however, that any regrown layer would of necessity have to be extremely thin. Diffusion may be ruled out because of the low temperatures and short times. The possibility of the formation of a heterojunction in which one of the materials might be, for example, In,Ga,_,As afIoy(12) or Zn,As2(6) is subject to the same
considerations as for a regrown p-n junction. ZnaAs, might be precipitated from the In-1 per cent Zn alloy in a manner similar to that reported by DALE(I~I where the precipitation of Mn,As from Mn doped bismuth alloyed into GaAs was observed. This reaction occurred between 350 and 4OO”C, a temperature range close to that used in the meltingwetting cycle. The electrical characteristics of these diodes also argues against the formation of p-n junctions as an explanation for the rectification. The very low values of ‘1observed over several decades of current in the forward direction is not typical of GaAs junctions. Also, the barrier heights obtained from C-Vmeasurements are too low for the doping levels of the GaAs substrates. The barrier heights of all
5. CONCLUSIONS
This paper has reported on the preparation of metal-semiconductor diodes on GaAs by the use of an appropriate flux to enhance the wetting of melted metals. The current-voltage and capacitance-voltage characteristics of these diodes demonstrate that an excellent abrupt junction is formed to the bulk of the semiconductor. Consideration of the metallurgical properties of the metals used and their doping properties in GaAs as well as the electrical results leads to the conclusion that these junctions are Schottky barrier diodes. Adnmledgemmis-This work was supported in part by the NASA Electronics Research Center, Cambridge, Mass., under grant NGR 39-087-002. The authors also wish to express their thanks to HERMAN E. REEDY, for fabricating some of the devices. REFERENCES 1. F. A. PADOVANIand G. G. SUMNER, 1. appf. Phss. 36, 3744 (1965). 2. C. A. MEAD and W. G. SPITZER, P/z~s. Rev. 134, A713 (1964). 3. F. H. DOFWXX, Solid-Sf.Electron. 9,113s (1966). 4. R. WILLCAMS,P&s. Rev. Letr. 8, 402 (1962). 5. D. K. JADUS, H. E. REEDY and D. L. FEUCHT, J. electrochem. Sot. 114,408 (1967). 6. B. SCHWARTZ and J. C. SARACE, Solid-St. Electron. 9, 859 (1966). 7. A. M. GOODMAN, 3. appl. Phyr. 34, 329 (1963). 8. C. A. MEAD, Solid-St. Electron. 9, 1023 (1966). 9. W. G. SPCTZERand C. A. MEAD, J. appl. Phxs. 34, 3061 (1963). 10. M. RUBINSTEIN, J. Electrochem. Sot., 113, 752 (1966). 11. L. D. L~BOU and S. S. MESKIN, Problem of fusingin tin in gallium arsenide, Fore& Technology DivisionReportNo. FTD-HT-66-557(AD665705). Original article submitted January 21. 1964. 12. T. L_ TANSLEY and P. c. NE&M.&‘, Solid-St. Electron. 10. 497 (1967). 13. J. R. DALE, Pkys. Siatus’Solidi 16, 351 (1966).
Electronics
Pergamon Press 1970. Vol. 13, pp. 485-489.
GaAs SCHOTTKY
Printed in Great Britain
BARRIER DIODES BY THE FLUXED
WETTING G. 0.
OF TIN AND INDIUM LADD,
Dept. of Electrical Engineering,
and D. L. FEUCHT
JR.*
Carnegie-Mellon
(Received
Univ., Pittsburgh,
Pa., U.S.A.
11 July 1969)
Abstract-A procedure is described for making Schottky barrier diodes on n and p type GaAs by melting metal spheres onto the surface near their melting points. The metals used are tin, indium, and an In-l per cent Zn alloy. Wetting is facilitated by the use of an HCl flux. The electrical characteristics of the diodes are presented and discussed. R&urn&-On decrit un procede pour fabriquer des diodes de barrike Schottky sur 1’AsGa de type n et p par la fonte de spheres metalliques sur la surface aux approches de leur point de fusion. Les metaux employ& sont l’etain, l’indium et un alliage de Zn a In-l pour cent. Le mouillage est facilite par un flux HCl. Les caracteristiques des diodes sont presentees et discutees. Zusammenfassung-Fiir die Herstellung von Schottky-Dioden durch Aufschmelzen van Metallkiigzlchen nahe ihrem Schmelzpunkt auf die Oberfllche von n- und p-Typ GaAs wird beschrieben. Die verwendeten Metalle waren Zinn, Indium und eine Indiumlegierung mit 1 prozent Zink. Die Benetzung wurde erleichtert mittels eines HCl. Flussmittels. Die elektrische Charakteristik der Dioden wird angegeben und diskutiert.
1. INTRODUCTION
SCHOTTKY barrier diodes can be made by introducing a metal onto the ‘clean’ surface of the semiconductor. In practice this is usually done by evaporation of the metal onto a semiconductor which may be cleaved in VQCUOor chemically etched prior to deposition.(l*” GaAs Schotty diodes have also been made by electro-chemical deposition of the metal.(3s4) Schottky barrier diodes on GaAs are frequently used in the semiconductor industry to measure the carrier concentration profiles of epitaxial layers. Where time and facilities permit, good results have been obtained by evaporation of gold, gold-tin alloys, and aluminum, etc. This paper reports on a method for fabricating Schottky barrier diodes of tin, indium, and indium-1 per cent zinc alloy on n and p type GaAs by means of the low temperature wetting of metal spheres to the GaAs * Presently with United Aircraft Corporation, search Laboratory, East Hartford, Conn., U.S.A.
Re485
in an anhydrous HCl gas atmosphere. In many cases this method is more convenient than evaporation. It is also unique in that the tin diodes can be made ohmic to n type GaAs by simply remelting and alloying at a high temperature. Onp-GaAs the In-l per cent Zn may be made ohmic by alloying at a high temperature. The process is apparently made possible by the fluxing action of the HCl atmosphere at temperatures for which the metals involved have a low solubility for GaAs. 2. EXPERIMENTAL The diodes were made by melting the metals onto GaAs chips in an alloy stage which has been previously described.(5) The stage is made of nickel and gold plated brass to prevent corrosion by the HCl gas and uses a thin graphite strip heater. The temperature of the strip is monitored by a chromel-alumel thermocouple. A glass cover, which is sealed to the base plate by an O-ring gasket, permits visual inspection of the process. Provisions are made for introducing high purity
486
G.
0.
LADD,
JR. and
anhydrous HCl gas and a tank mixture of 85 per cent argon-15 per cent hydrogen to the stage and for controlling their flow rates. The argon-hydrogen mixture passes through a calcium sulphate drying tube before entering the stage. The metal spheres (usually 5-10 mils in diameter) are cleaned in dilute hydrochloric acid and stored in methanol until use. The GaAs specimen is polished flat if necessary and etched to remove surface damage. Where epitaxial material is used it is only necessary to clean the surface with solvents provided it is relatively free of gross defects such as pits or hillocks. Where tin spheres are used a heating cycle consists of the following: The stage is first flushed with about 10 alloy stage volumes of the argonhydrogen mixture. Failure to strictly observe this precaution usually produces poor results. Then the argon-hydrogen flow is set to 1.75 volumes/min, the HCI flow is set to give a concentration of 4-5 per cent, and three minutes is allowed for the gas mixture to come to equilibrium in the stage. Next, the heater is turned on and brought up to the tin melting point (232 “C) within about 30 set and then raised to about 250 “C for 5 sec. The heater strip is then cooled to the melting point in 5-10 set and turned off, the HCl is turned off, and the argon-hydrogen flow is turned up to purge the stage as it cools. Cooling to below 100 “C takes less than 10 sec. The entire heating cycle lasts about one minute. Although the temperature is usually reduced slowly as described above from 250 to 232 “C, there appears to be no difference in the results if the heater is shut off after 5 set at 250 “C. The procedure for the indium and indium1 per cent zinc spheres is significantly different: The stage is purged as for the tin cycle but now the argon-hydrogen flow is set at 4 vol/min and the HCl flow to give a concentration of 0.4-0.8 per cent. After waiting l-2 min the heater temperature is raised to about 200 “C within 30 sec. Then the heater is turned off and cools to below 100°C in about 5-10 sec. The approximate 200°C point is determined visually by noting the commencement of rapid evaporation of a ‘scum’ which forms on the surface of the molten indium. At this temperature greatly enhanced wetting of the GaAs by the indium is noted. The scum which forms during the alloying is probably indium trichloride produced by reaction with the HCI gas.
D. L. FEUCHT
The critical features of the process are the use of a low temperature and the HCl flux. The use of the HCl flux has been previously discussed by JADUS et aZ.,(5) and good wetting of lead, zinc and tin on GaAs has also been obtained by SCHWARTZ and SARACE@)using solutions of the metal halides. We have also had some success with ammonium chloride grains laid on the strip heater during heating in the argon-hydrogen ambient. It will be shown below that the use of the HCl gas ambient, at least, provides a very clean metal-semiconductor interface. JADUSet aLc5) have also shown that at somewhat higher alloying temperatures good ohmic contacts can be made to n andp type GaAs using, respectively, Sn and Zn or In-l per cent Zn spheres. Since the In or In-l per cent Zn diodes can be formed below the tin melting point, it may be convenient to use a tin alloy ohmic contact together with an In or In-l per cent Zn diode to evaluate carrier concentrations. The maximum temperature used is the principal difference between the heating cycles for ohmic or rectifying contacts when tin or In-l per cent Zn is used. Ohmic tin contacts which exhibit a contact resistance within 50 per cent of the calculated spreading resistance for the area of contact and the resistivity of the material can be obtained by heating to between 325 and 350°C and then cooling quickly. In the range between 250 and 325°C the contact characteristic varies from rectifying to ohmic. Thus, remelting a previously alloyed contact can raise the contact resistance by a factor of 100. In-l per cent Zn ohmic contacts to p-GaAs may also be made at 350°C. Ohmic contacts to more lightly doped material of the order of 1015 cme3 carrier concentration tend to require a higher temperature alloy cycle. 3. ELECTRICAL AND PHYSICAL CHARACTERISTICS Some information on the penetration of the metal into a smooth GaAs surface may be obtained by etching off the metal in concentrated hydrochloric acid and examining the wetted area under the microscope. In the case of pure In or In-l per cent Zn alloy it is sometimes found that the wetted area cannot be distinguished from the surrounding surface. Usually, however, the wetted area is surrounded by a very shallow ring etched into the
GaAs SCHOTTKY
BARRIER
DIODES
BY THE
FLUXED
surface. The wetted area inside the ring is relatively free of visible attack. Sometimes small rings are found within the wetted area indicating incomplete wetting of the surface by the metal. The wetted areas of many of these diodes have been examined in the course of measuring the carrier concentrations of epitaxial material. This was done in order to get an accurate value of the area of the diode. Since the contact angle of the metal to the GaAs is sometimes greater than go”, it is not always possible to measure the area without etching off the metal. When the contact area is less than 90” good agreement is found between measurements of the wetted area made before and after etching of the metal. Figure 1 is a photograph of the wetted area under a tin diode. The depth of the circular ring has been determined by interference microscope measurements to be less than 0.03 pm. The voltage-current characteristics of these diodes exhibits a very low reverse current of less than low6 A. For diodes on n type GaAs the reverse breakdown voltage is usually within a factor of two of the theoretical value for an abrupt junction in GaAs of the same carrier concentration. The breakdown voltage of diodes on p type GaAs is somewhat lower than the theoretical value. The forward characteristic is exponential and may be described at large currents by the familiar equation I
=
Isexp-
OF TIN
0.2
AND
03
Applied
INDIUM
0.4
volioge,
0.5
487
cL
V
FIG. 3. Forward current-voltage characteristics of tin and In-l per cent Zn alloy diodes on 0.13 Q-cm n-GaAs. IO-: 5
2
eVa
10.‘ 5
rlkT where V, is the applied voltage, e is the electron charge, k is Boltzmann’s constant, T is temperature and q describes the degree to which the curve deviates from an ideal diffusion diode. The current voltage characteristics of two Schottky diodes, one on n type and one on p type GaAs, using In-l per cent Zn alloy dots are shown in Fig. 2. Figure 3 shows the forward log-currentvoltage characteristic of two types of diodes on 0.13 !&cm n-GaAs. The deviation from a straight line at higher current may be explained by the series resistance of the diodes. In view of the simplicity of the fabrication process it is surprising to find that the values of q obtained, which are in the range of 1.02-1.07, are similar to those seen in evaporated diodes. Figure 4 presents the currentvoltage characteristics of the diodes onp type GaAs.
WETTING
a
2
E-
to5
E 5 0
5
2 10~6
? .I
5
IOmil In-I%ZI 5mil TIN
2
IO’
”
”
A
-
“.
I
0.2
0.3
Applied voltage,
0.4
0.5
v
FIG. 4. Forward current-voltage characteristics of tin and In-l per cent Zn alloy diodes on 0.33 f&m p-GaAs.
G.
488
Applied
voltage,
0.
LADD,
JR. and
V
FIG. 5. Capacitance-voltage characteristics of In-l percent Zn alloy and tin diodes on 0.13 n-cm n-GaAs.
The capacitance-voltage characteristic of these diodes follows the inverse square law expected for a metal-semiconductor diode. The good linearity of these plots is exemplified by the plots of reciprocal capacitance squared vs. voltage in Fig. 5 and 6
Applied
voltage,
FIG. 6. Capacitance-voltage
V
characteristics of In-l per cent Zn alloy and tin diodes on 0.33 a-cm p-GaAs.
D.
L.
FEUCHT
for diodes on n and p type GaAs. These measurements were made at 1 MHz on a General Radio Twin-T bridge to a precision of of: 0.1 pF. No drift of the capacitance with time was noted at large reverse biases over periods of several minutes. Typically, however, the capacitance-voltage curves bend downward at larger reverse biases. This may be explained by the presence of traps as discussed by GOODMAN.(~) The carrier concentration of the GaAs can be determined from the slope of the capacitancevoltage curve and the area of the diode. It was was found that diodes physically close to each other on the same piece of GaAs yielded carrier concentrations which differed by at most 20 per cent. Errors of this magnitude might be expected from variations in the material itself or from errors in the determination of the wetted area of the diode. It was also determined that these carrier concentrations were very close to the values supplied by the manufacturer of the material. Of some interest is the voltage to which the capacitance-voltage characteristic extrapolates at infinite capacitance. This value should characterize the height of the barrier to the flow of carriers. The barrier height may also be determined by the photo-response of the diode or by the thermal activation energy of the forward current.(*) The errors in the extrapolated barrier height from capacitance measurements, however, are such that thisvaluewillusuallybe higherthanthetruevalue.c7) The extrapolated barrier heights for the In1 per cent Zn diodes on n-GaAs range from 1.1 to 1.2 V while the range for the tin diodes is 0.70.8 V. Diodes on p-GaAs have not been as extensively studied but both the In-l per cent Zn and the tin diodes give values close to 0.7 v. SPITZERand MEAD(~)give a value of 0.68-0.74 eV for the barrier height derived from capacitance measurements on evaporated tin contacts on n-GaAs. For p-GaAs, of approximately the same resistivity as the diodes of Fig. 6, they report a value for the l/C2 intercept of 0.58-0.69 eV. Thus the values obtained for our diodes compare favorably with these evaporated diodes. No comparison of the indium diodes may be made because of the lack of published data. Diodes were also obtained by melting pure indium spheres on n and p type GaAs. Although the electrical characteristics of these diodes were not
GaAs SCHOTTKY
BARRIER
DIODES
BY THE
studied carefully, electronic curve tracer measurements showed them to be quite similar to the tin and In-l per cent Zn diodes in reverse current and breakdown voltage.
FLUXED
WETTING
OF TIN
AND
INDIUM
489
but the In-l per cent Zn alloy on n-GaAs were in the neighbourhood of 0.7 to 0.8 V, a value typical of most metals on GaAs. MEAD and SPITZES have shown that this is due to the presence of surface states which control the surface potential.
4. DISCUSSION
The nature of the metals and alloy used in preparing these diodes argues against the formation of p-n junctions as an explanation for their behaviour. Diodes were obtained by melting tin onto n-GaAs and In-l per cent Zn onto p-GaAs. Tin is a shallow donor and would not be expected to produce a p-n junction in n-GaAs. Likewise, Zn is a shallow acceptor and should prevent any The fact that pure n-type doping of the p-GaAs. indium produces diodes on both n and p type GaAs shows that Zn is not a necessary component of the In-l per cent Zn alloy. The solubility of GaAs in tin(loJ1) is probably high enough near the tin melting point to produce a regrown, heavily doped layer on the GaAs surface. The time period involved in this low temperature heating cycle is so short, however, that any regrown layer would of necessity have to be extremely thin. Diffusion may be ruled out because of the low temperatures and short times. The possibility of the formation of a heterojunction in which one of the materials might be, for example, In,Ga,_,As afIoy(12) or Zn,As2(6) is subject to the same
considerations as for a regrown p-n junction. ZnaAs, might be precipitated from the In-1 per cent Zn alloy in a manner similar to that reported by DALE(I~I where the precipitation of Mn,As from Mn doped bismuth alloyed into GaAs was observed. This reaction occurred between 350 and 4OO”C, a temperature range close to that used in the meltingwetting cycle. The electrical characteristics of these diodes also argues against the formation of p-n junctions as an explanation for the rectification. The very low values of ‘1observed over several decades of current in the forward direction is not typical of GaAs junctions. Also, the barrier heights obtained from C-Vmeasurements are too low for the doping levels of the GaAs substrates. The barrier heights of all
5. CONCLUSIONS
This paper has reported on the preparation of metal-semiconductor diodes on GaAs by the use of an appropriate flux to enhance the wetting of melted metals. The current-voltage and capacitance-voltage characteristics of these diodes demonstrate that an excellent abrupt junction is formed to the bulk of the semiconductor. Consideration of the metallurgical properties of the metals used and their doping properties in GaAs as well as the electrical results leads to the conclusion that these junctions are Schottky barrier diodes. Adnmledgemmis-This work was supported in part by the NASA Electronics Research Center, Cambridge, Mass., under grant NGR 39-087-002. The authors also wish to express their thanks to HERMAN E. REEDY, for fabricating some of the devices. REFERENCES 1. F. A. PADOVANIand G. G. SUMNER, 1. appf. Phss. 36, 3744 (1965). 2. C. A. MEAD and W. G. SPITZER, P/z~s. Rev. 134, A713 (1964). 3. F. H. DOFWXX, Solid-Sf.Electron. 9,113s (1966). 4. R. WILLCAMS,P&s. Rev. Letr. 8, 402 (1962). 5. D. K. JADUS, H. E. REEDY and D. L. FEUCHT, J. electrochem. Sot. 114,408 (1967). 6. B. SCHWARTZ and J. C. SARACE, Solid-St. Electron. 9, 859 (1966). 7. A. M. GOODMAN, 3. appl. Phyr. 34, 329 (1963). 8. C. A. MEAD, Solid-St. Electron. 9, 1023 (1966). 9. W. G. SPCTZERand C. A. MEAD, J. appl. Phxs. 34, 3061 (1963). 10. M. RUBINSTEIN, J. Electrochem. Sot., 113, 752 (1966). 11. L. D. L~BOU and S. S. MESKIN, Problem of fusingin tin in gallium arsenide, Fore& Technology DivisionReportNo. FTD-HT-66-557(AD665705). Original article submitted January 21. 1964. 12. T. L_ TANSLEY and P. c. NE&M.&‘, Solid-St. Electron. 10. 497 (1967). 13. J. R. DALE, Pkys. Siatus’Solidi 16, 351 (1966).