Characterization of ohmic contacts to InP

Characterization of ohmic contacts to InP

Thin Solid Films, 64 (1979) 421-426 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands 421 C H A R A C T E R I Z A T I O N O F O H M I C ...

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Thin Solid Films, 64 (1979) 421-426 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

421

C H A R A C T E R I Z A T I O N O F O H M I C C O N T A C T S TO InP* L. P. ERICKSON, A. WASEEM AND G. Y. ROBINSON

Department of Electrical Engineering, University of Minnesota, Minneapolis, Minn. 55455 (U.S.A.) (Received April 12, 1979; accepted April 25, 1979)

The results of a study of the electrical and metallurgical properties of thin metallic layers deposited on InP for use as ohmic contacts are presented. The layers were heat treated at temperatures up to 550 °C and were examined with Auger electron spectroscopy. For contact to n-type InP three thin film systems were investigated: gold, nickel and a composite Ni/Au/Ge layer. Nickel was found to produce ohmic behavior in the Ni/Au/Ge/InP system with a minimum specific contact resistance r c of 3 x 10 -5 f~ c m 2 for a net doping of 3 x 1016 c m - 3 . For contact to p-type InP a film consisting of Au/Mg was investigated. For heat treatment of the Au/Mg/InP system above 350 °C, rc decreased as the temperature of the heat treatment increased and the surface morphology exhibited increasing signs of alloying at higher temperatures. The smoothest surface was obtained at 446 °C for 50 min with re ~ 1 x 10 -4 f~ c m 2 for a net doping of 6 x 1017 c m -3.

1. INTRODUCTION

The use of the compound semiconductor InP is becoming increasingly widespread for applications in microwave and optoelectronic devices. However, practical methods for forming low resistance ohmic contacts, which are essential for proper device operation, have yet to be developed for both p-type and n-type InP. Multilayered metal films formed by vacuum deposition and subsequent heat treatment have found application as ohmic contacts to other III-V semiconductors 1. Hence we examined ohmic contacts formed by similar techniques on InP. We report here the preliminary results of a study of the electrical and metallurgical properties of two thin film systems: a multilayer consisting of nickel, gold and germanium on n-type InP; a multilayer of gold and magnesium on p-type InP. 2. EXPERIMENTAL PROCEDURE

The n-type and p-type wafers were of (100) and (111) orientation with doping 1018 cm-3. After degreasing, the wafers were etched for 5 min in 3:1:1

f r o m 1016 t o

* Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 23-27, 1979.

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L.P. ERICKSON,A. WASEEM,G. Y. ROBINSON

H 2 S O 4 : H 2 0 2 : H 2 0 . An SiO 2 masking layer 3000 A thick was deposited using pyrolytic chemical vapor deposition at 300 °C. Subsequent steps to etch holes in the oxide layer and to define the metal contacts utilized conventional photolithographic techniques. Each metal layer of the multilayered Ni/Au/Ge and Au/Mg structures was sequentially deposited using electron beam evaporation in an ion-pumped vacuum system during the same pumpdown. The wafers were maintained below 50 °C during deposition. The metal-semiconductor contact systems will be designated "as deposited" for samples used immediately after deposition of metal layers or "as fabricated" for samples where processing subsequent to metal deposition (i.e. a postphotolithography bake at 150 °C for 5 rain in air) may have altered the as-deposited structure. Samples that are referred to as "heat treated" were placed in an open-tube furnace with flowing N 2 gas after completion of all fabrication steps. Characterization of the samples before and after heat treatment, including the correlation of electrical and metallurgical data, involved a number of techniques previously used for the study of ohmic contacts to GaAs 2. Current-voltage measurements were used to obtain the Schottky barrier energy ~bBand the specific contact resistance re, and capacitance-voltage measurements were used to find the carrier c o n c e n t r a t i o n [N o - N A [ and ~bB. Auger electron spectroscopy (AES) was used for surface chemical analysis and, when combined with ion beam etching, was utilized for depth-composition profiling of the multilayered thin film structures 2. The surface morphology was characterized with scanning electron microscopy and optical microscopy. X-ray diffraction analysis was also conducted on specially prepared Ni-Ge films.

3. RESULTSAND DISCUSSION 3.1. Contacts to n-type I n P

For contact to n-type InP the primary metallization system chosen for study was Ni/Au/Ge; supporting work on separately prepared control samples of nickel and gold was also performed. The Ni/Au/Ge system has been successfully used as an ohmic contact to n-type GaAs 3 and thus it is of interest to determine the behavior of the Ni/Au/Ge system on n-type InP. The specific contact resistance as a function of heat treatment temperature for the Ni/Au/Ge/n-InP, Ni/n-InP and Au/n-InP systems is given in Fig. 1. Figure 2 shows typical AES profiles for both as-fabricated and heat-treated Ni/Au/Ge/n-InP samples. Auger analysis revealed that for samples heat treated up to about 250 °C germanium moves away from the InP interface and through the intervening gold layer, to react with the top nickel layer (Fig. 2(a)). Changes in the germanium Auger energy spectra and independent X-ray diffraction studies indicated that essentially all the germanium had reacted to form a layer of NiGe after 5 min at 300 °C. Simultaneously excess nickel diffused to the InP interface (Fig. 2(b)). The thin interfacial layer of nickel appears to control the contact resistance in the heat treatment range 250-350 °C, especially since the sharp drop in r c for both the Ni/Au/Ge/n-InP and the Ni/n-InP systems occurred at the same temperature, 300°C, resulting in a minimum in r c at 325 °C. At higher heat treatment temperatures considerable intermixing of all the element occurred. Contact surfaces

423

CHARACTERIZATION OF OHMIC CONTACTS TO I n P

became increasingly non-uniform and more scatter in the ro data was found. No abrupt change in electrical behavior was observed at 360 °C, the melting point for the Au-Ge eutectic composition used. A S - FABRICATED

t061 0

I

I 200

I

I 400

I

600

HEAT TREATMENT TEMPERATURE (°C)

Fig. l. The specific contact resistance as a function of the heat treatment temperature (treatment lasted 5 min) for gold, nickel and Ni/Au/Ge films on n-type InP with N D - N A = 3 x 1016 c m - 3. The insets depict the composition of the Ni/Au/Ge contacts after various heat treatment conditions.

IOO

I00 Au

8O

8 5 In

§ 4°

40-

2O

.L

0

(a)

I O N DOSE

(b)

ION DOSE

Fig. 2. Auger profiles of Ni/Au/Ge/n-InP diodes: (a) an as-fabricated sample with film thicknesses of 150 A nickel, 520 A gold and 250 A germanium (much of the germanium, originally at the interface, had already reacted with the surface nickel layer); (b) the sample after heat treatment for 5 min at 275 °C (the nickel now appears at the metal-semiconductor interface).

424

L.P.

ERICKSON,

A. WASEEM,

G. Y. ROBINSON

The lowest r c value observed for the Ni/n-InP system was 4 x 10- 5 f~ cm 2 (heat treated at 325 °C for 5 min). Specific contact resistances for the Ni/Au/Ge/n-InP system were as low as 3 x 10- 5 f~ cm 2 (325 °C, 5 min) and 5 x 10- 5 f~ cm 2 (400 °C, 2 min) for N o - N A = 3 x 1016 c m - 3. The Au/n-InP system exhibited an as-fabricated barrier energy ~bB, of 0.50 eV and did not produce acceptable ohmic behavior after heat treatment. 3.2. Contacts to p-type l n P For contact to p-type InP a composite layer of Au/Mg was studied since magnesium is an acceptor in InP and can be readily vacuum deposited. The electrical characteristics of the as-deposited Au/Mg/p-InP samples were typical of Schottky diodes with large barrier energies (i.e. ~bBp= 0.7-1.0 eV), and in all cases the contact resistance on p-type InP was much higher than on the n-type material. As shown in Fig. 3, the contact resistance for the Au/Mg/p-InP system decreased rapidly with increasing heat treatment above 300 °C. The lowest contact resistance measured was approximately 1 x 10 -4 f~ cm 2 for N A - N D = 6 x 1017 cm -3 after heat treatment at 446 °C for 50 min, which also resulted in the smoothest heattreated surface. A S - DEPO SI TED

E ~

4

z t-" O3

I'--

~o4

i0 z

1.0

FZ 8 KJa la-

co 10-4

o

I

2~o

I

4~o'

HEAT TREATMENT T E M P E R A T U R E (*C)

Fig. 3. The specific contact resistance of Au/Mg/p-InP diodes as a function of heat treatment temperature for a fixed time of 5 min, with NA- N D= 6 x 1017cm- 3. The as-depositedfilm consisted of 400 A of magnesiumfollowedby a deposition of 1600A of gold. Figure 4 illustrates typical AES profiles of the Au/Mg/p-InP system. For the asdeposited samples magnesium was found on the surface of the gold as well as at the InP interface, indicating that considerable migration of the magnesium took place during the gold deposition. In addition, most of the magnesium on the surface, and to a lesser extent at the InP interface, was present as the oxide MgO. The profile for the heat-treated sample of Fig. 4(b) shows a thin M g O layer covering a A u - I n layer

CHARACTERIZATION OF OHMIC CONTACTS TO

InP

425

which covers a region of mixed composition, all covering the InP substrate. The ratio of gold to indium in the Au-In layer corresponds to the solubility of indium in gold at the temperature of heat treatment 4. Extensive loss of indium and phosphorus from the substrate was found for samples heat treated above 400 °C.

I00 I00A

8O

~

8o

In

I'-- 60 o

S 4o

40

\

~ IP n In 2O

20 ION DOSE

Mg

0 ION DOSE

(a) (b) Fig. 4. Depth-composition profiles of the A u / M g / p - I n P diodes of Fig. 3: (a) an as-deposited sample; (b) the sample after heat treatment at 446 °C for 50 min.

The surface morphology of the Au/Mg/p-InP system was found to be strongly dependent on the heat treatment conditions. At or above the Au-In eutectic temperature 4 of 457 °C, melting took place during heat treatment, and at 500 °C evidence of extensive lateral melting of the Au/Mg film on top of the SiO2 adjacent to the contact area was found. A significant amount of indium was found by Auger analysis in the metal film on top of the SiO 2, indicating appreciable loss of indium from the InP substrate at temperatures above 457 °C. 4. CONCLUSIONS On n-type material a film of pure nickel exhibited a low contact resistance after heat treatment at 325 °C. The composite film Ni/Au/Ge exhibited similar behavior, primarily because nickel diffused to the InP interface. Since low resistance contacts were found below the A u - G e eutectic temperature, the alloy regrowth mechanism responsible for ohmic contact formation in the Ni/Au/Ge/n-GaAs system does not apply to Ni/Au/Ge/n-InP. Before heat treatment, contact resistance and Schottky barrier energies were always higher on p-type InP than on n-type InP, unlike Schottky barriers on GaAs and silicon. After heat treatment, the Au/Mg/p-InP was found to produce ohmic behavior with a specific contact resistance as low as 1 x 10 -4 ~ cm 2 for N A - N D --- 6 x 1017 c m - 3. However, poor surface uniformity resulted from the high solubility of indium in gold at the heat treatment temperatures used. The substitution of magnesium for indium in the InP immediately under the metal film would produce a p + layer and thus may be responsible for the ohmic behavior of the heat-treated Au/Mg/p-InP contact.

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L . P . ERICKSON, A. WASEEM, G. Y. ROBINSON

ACKNOWLEDGMENTS

The authors would like to acknowledge the laboratory assistance of W. Smith and D. Amundson. This work was supported by the Air Force, Rome Air Development Center, Deputy for Electronic Technology. REFERENCES 1 2 3 4

V.L. Rideout, Solid-State Electron., 18 (1975) 541. G.Y. Robinson, Solid-State Electron., 18 (1975) 331. N. Braslau, J. B. Gunn and J. L. Staples, Solid-State Electron., 10 (1967) 381. M. Hanson, Constitution of Binary Alloys, McGraw-Hill, New York, 1958.