Ohmic contacts for GaAs devices

Ohmic contacts for GaAs devices

Solid-State EZectronks Pergamon Press 1967. Vol. 10, pp. 1213-1218. OHMIC CONTACTS Printed in Great Britain FOR GaAs DEVICES* R. H. COX and H. ...

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Solid-State

EZectronks

Pergamon

Press 1967. Vol. 10, pp. 1213-1218.

OHMIC CONTACTS

Printed in Great Britain

FOR GaAs DEVICES*

R. H. COX and H. STRACK Texas Instruments (Received

20 February

Inc., Dallas, Texas, 1967; in

revisedform

U.S.A. 27 July 1967)

Abstract-Contact alloys were developed for use on a wide variety of GaAs devices such as high temperature transistors and Gunn oscillators. The alloys are composed of silver, indium and germanium for n-type GaAs and of silver, indium and zinc for p-type GaAs. Fabrication steps that require temperatures of up to 770°K for already contacted devices can be performed. GaAs transistors can be operated over a range from 20 to 770°K using Ag-In-Ge contacts for emitter and collector and Ag-In-Zn contacts for the base. Gunn oscillators have been built for the frequency range between 13 and 26 GHz with efficiencies as high as 3 percent at 15.8 GHz and as high as 1 percent at 25 GHz in continuous wave operation. A simple technique was developed to evaluate the specific contact resistance on thin epitaxial layers. Specific contact resistance is well below 10e4 R-cm2 on 0.1 Q-cm or lower resistivity p- or n-type GaAs. The highest value was 1 x 10 -3 n-cm2 measured on 0.6-2.6 a-cm n-type GaAs. R&urn&Des alliages de contact ont et6 d&elopp& pour Ctre employ& dans une grande variCt6 de dispositifs AsGa tels que les transistors a haute temperature et les oscillateurs Gunn. Ces alliages sont composCs d’argent, d’indium et de germanium pour 1’AsGa de type 1z et d’argent, indium et zinc pour 1’AsGa de type p. Les &apes de la fabrication qui nCcessitent des temperatures atteignant 770°K pour des dispositifs a contact peuvent &tre obtenues. Les transistors AsGa peuvent &tre op&& le long d’une gamme de 20 g 770°K employant des contacts Ag-In-Ge pour l’emetteur et le collecteur et des contacts Ag-In-Zn pour la base. Des oscillateurs Gunn ont t%e construits pour la gamme de frCquence entre 13 et 26 GHz ayant des rendements aussi Clevts que 3% B 15,8 GHz et 1 y0 g 25GHz g regime d’ondes continues. Une simple technique a CtB dCveloppCe pour Cvaluer la &istance spbcifique de contact sur de fines couches CpitaxiCes. La rCsistance sptcifique de contact est bien au-dessous de 10-452-cm2 sur 1’AsGa de type P et N ayant une r&istivitC de 0,l Q-cm au moins. La valeur maximum etait 1 X 10m3 Q-cm2 mesurte sur 1’AsGa de type Ng 0,2-2,6 Q-cm. Zusammenfassung-Es wurden Legierungen zur Kontaktierung verschiedener GaAs-Bauelemente entwickelt, wie z.B. fiir Hochtemperatur GaAs-Transistoren und Gunn-Oszillatoren. Die Legierung besteht aus Silber, Indium und Germanium fiir n-leitendes GaAs und aus Silber, Indium und Zink fiir p-leitendes GaAs. Herstellungsprozesse, die Temperaturen bis 770°K erfordern, kannen an den bereits kontaktierten Elementen durchgefiihrt werden. GaAs-Transistoren kiinnen iiber einen Temperaturbereich von 20 bis 770°K betrieben werden, wenn Ag-In-Ge Kontakte fiir den Emitter und Ag-In-& Kontakte fiir die Basis verwendet werden. Es wurden Gunn-Oszillatoren mit einem Wirkungsgrad bis zu 3 Prozent bei einer Frequenz von 15,8 GHz und bis zu 1 Prozent bei einer Frequenz von 25 GHz in Dauerstrich-Betrieb gebaut. Eine einfache Methode, den spezifischen Kontaktwiderstand auf diinnen Epitaxie-Schichten zu ermitteln wurde entwickelt. Der spezifische Kontaktwiderstand ist niedriger als 10m4 Q-cm2 auf 0,l R-cm und hiiher dotiertem p- oder n-leitendem GaAs. Der hijchste Wert betrug 1 x 10m3 n-cm2 auf 0,6-2,6 Q-cm n-leitendem GaAs.

INTRODUCTION

METHODS for making ohmic contacts to specific GaAs devices such as Gunn oscillators(l) or tran*This

work was supported

Avionics Laboratory. 7

by the U.S.

Air Force 1213

have been described in the literature. For most applications it is not sufficient to specify contacts to be ohmic. One also has to require a low specific contact resistance. No data on the specific contact resistance have been found in the [email protected]

1214

R. H. COX and H. STRACK

literature. It is also desirable that the same contacting technique be applicable to a variety of GaAs devices including transistors with an operating temperature up to 500°C. This paper consists of three parts. First, a simple method is given to separate the various components of the total contact resistance. Then the development and electrical properties of silver-base contacts for n- and p-type GaAs are described. Finally, fabrication techniques for GaAs devices employing silver-base alloys are mentioned. CONTACT EVALUATION The electrical quality of ohmic contacts is conveniently characterized by the specific contact resistance. One problem in evaluating ohmic contacts on thin epitaxial layers is the separation of the total resistance into spreading, contact, and residual resistances. A separation can be achieved when the contact resistance of contacts of different diameters is measured and use is made of the dependence of the total resistance on contact diameter. Let us consider an array of circular contacts like those of Fig. 1. These contacts are fabricated on top of the epitaxial layer while the back side contact to the heavily doped substrate may be a large area contact common to all circular contacts. The spreading resistance of a circular disc type contact can be written as R,=&B

(1)

the correct namely

value

for a cylindrical

lim R, = -. tqd

conductor,

Pt

7442)

(3) 2

In Fig. 2 equation (2) is plotted vs. the ratio of contact diameter to layer thickness. The solid line

. o

0. I

EXPERIMENTAL EQlJATlON(2)

IO

I

100

d/t

FIG. 2. Correction factor of contact resistance vs. ratio of contact diameter to layer thickness.

connects experimental points obtained from electrolytic tank experiments, designed to simulate the spreading resistance of a circular contact on a finite layer of uniform resistivity. Maximum deviation of calculated and measured values is 8 percent. The contact resistance is taken into account by the exnression A

where p is the resistivity of the epitaxial material, d is the contact diameter and B is a factor which corrects for the finite thickness of the epitaxial layer. The correction factor can be calculated only for the resistance between a circular disc and another equipotential surface which has the shape of a rotational semi-ellipsoid.(3) In reality, contact is made to an epitaxial layer of uniform thickness, t, deposited on a low-resistivity susbtrate in which the equipotential lines are parallel to the surface. We will show by comparison with experimental data obtained from electrolytic tank experiments that the correction factor can be approximated by 2 B - -arc 5T

4 tan-.

For thin layers, the spreading

dlt resistance

assumes

R=-

RC 442) 2

where R, is the specific contact resistance. Residual resistances due to the substrate or the contact resistance of the back side contact are denoted by Ro, independent of the contact diameter. The total resistance can now be written as 4 417, R, = !- arc tan--f---+R,. d/t n-d2 dir

(5)

The contact resistance can be obtained by curve fitting methods, when the resistivity and the layer thickness are known. A parallel shift along the resistance axis is observed when the back side contact has an appreciable contact resistance. If the resistivity is not known, the curve can be fitted with a value for the resistivity in the range

FIG. 1. Four-dot

FIG. 4. 90 wt.:/, Ag-5

array for measuring contact resistance ( x 100).

wt.% In-5

wt.% Ge on (100) GaAs, alloyed 1 min at 600°C (4.5” lap, x 300).

[facing p. 1214

OHMIC

CONTACTS

FOR

GaAs

DEVICES

1215

(a)

PO.15 ohm-cm

R, = 3 x 1Ci4 ohm cm2 t =0.9 mll /

20 c E d ct!IO

50 1/2d (cm-‘) FIG. 3. Contact resistance vs. reciprocal contact diameter.

of large diameters where the contact resistance contribution becomes negligible. Figure 3 shows a plot of equation (5) for the case of a thin epitaxial layer with and without contact resistance (curves 2 and 3), assuming values for R,, t and p, For comparison the spreading resistance into a semiinfinite slab of material with no contact resistance present is shown (curve 1). Soecific contact resistances described in the next section were evaluated using the 4-circle contact pattern shown in Fig. 1. The areas were 32, 16, 8 and 4 mi12. The pattern was repeated on 30 mil centers. Chips w&e scribed and mounted on TO-18 headers. The results agreed well with those obtained by directly probing the contacts on the slice and using a curve tracer to display the I-V characteristic. DEVELOPMENT PROPERTIES

AN’D ELECTRICAL

OF CONTACT

ALLOYS

Silver-base alloys were developed for use as ohmic contacts to GaAs transistors and Gunn oscillators. The following requirements were set for the contacts.

The contact-semiconductor interface must be planar. (b) The contacts must be solid below 500°C. (c) The contacts must be tin free. (d) The contacts must have a low specific contact resistance for both n- and p-type GaAs.

Planar contacts are necessary to establish a uniform electric field and current density in the GaAs. This is an important factor in high frequency Gunn oscillators where nonuniform fields produce incoherent oscillations(l) and the active region is very thin. High melting contacts, solid to 5OO”C, possess several advantages. High-temperature fabrication steps, such as the deposition of SiO, layers by the oxidative decomposition of tetraethylorthosilicate at 475”C, are possible. Furthermore, devices can be stored and operated at SOO”C, thus utilizing the high temperature capability of GaAs. Tin-free contacts are desired because tin has a rapidly-diffusing component at 5OO”C.(*)This can produce unwanted current paths in the GaAs, greatly degrading device performance. Finally, low specific contact resistance is necessary for efficient, high power output devices. The alloying elements for the contacts were selected for the following metallurgical properties. Silver was chosen as the major component because the Ag-GaAs eutectic occurs at 650”C,(5) a temperature well above that used for device fabrication and operation. The addition of minor amounts of alloying elements was not expected to significantly affect the eutectic temperature. Indium was added to lower the liquid alloy surface tension and, hence, promote good wetting of the GaAs. Zinc was used as the p-type dopant, and germanium was used as the n-type dopant. Germanium is to be preferred to tin, as it is not known to have a fast diffusing component in GaAs. Metallurgical techniques were employed to evaluate the contacts. The hot-stage microscope was used to observe alloying behavior and hightemperature stability of contacts on GaAs. Ag-Indopant alloys did not begin to melt until 540°C or higher. Alloying was rapid and uniform, producing clearly defined homogeneous contacts. Anglelapped sections showed that GaAs-alloy interfaces were planar for (m) GaAs and that metal pyramids formed in (100) GaAs could be minimized

1216

R.

H.

COX

and H.

N 0.1 p in depth, as illustrated in Fig. 4. In contrast, the Au-GaAs eutectic at 45Cl”C and the low melting point of tin, 232X, preclude the use of these metals for high-temperature contacts. Furthermore, many gold- and silver-base alloys melt well below the alloy-GaAs eutectic temperature. This means that there is a range of temperature over which the liquid metal must be heated until the alloy melts into the GaAs. High surface tension causes the metal to ball up, and an irregular contact-GaAs interface is formed. Therefore, it is quite advantageous to employ a contacting alloy which is solid up to the alloy-GaAs eutectic temperature. An 80 wt.% Ag-10 wt.% In-10 wt.% Zn alloy was used as a p-type contact. Figure 5 is a plot of

STRACK

t0

0 ALLOY: N-TYPE

9OAg-5 OaAG

In-5

60

150

f =I.2 ohm-cm

20

ALLOY: 80 A~.10 In.IOZn P - TYPE GoAs

P- 0. I ohm -Cm

P %

Pmeor=0.08 ohm -.cm

IO

l/2

a?

FIG.

wt.%

I

0 0

1

100

50 l/.?d km-‘)

FIG.

In-10

I 100

30

5. Contact resistance of 80 wt.% Ag-10 wt.% wt% Zn contacts to p-type GaAs vs. reciprocal contact diameter.

total resistance vs. reciprocal contact diameter for this alloy on 0.06-0.1 Q-cm, bulk p-type GaAs. The GaAs resistivities calculated from this curve agreed well with independent measurements of resistivity on Hall bars. Only spreading resistance was seen for these samples. One can estimate an upper limit of the specific contact resistance by using equation (4). For R, = 10V4 Q-cm2 the specific contact resistance for the smallest contact diameter would contribute about 3 Q to Ii, and the measured points would fall on a parabolic shaped curve rather than on a straight line. The specific contact resistance, therefore, is well below

d km-l)

6. Contact resistance of 90 wt.% Ag-5 wt.% In-5 Ge contacts to n-type GaAs vs. reciprocal contact diameter.

10v4 Q-cm2.

A 90 wt.% Ag-5 wt.% In-5 wt.% Ge alloy was used as an n-type contact. A plot of total resistance vs. reciprocal contact diameter is shown in Fig. 6 for contacts to 0.3-2.6 Q-cm, epitaxial GaAs. The specific contact resistance was about 1 x 10e3 Q-cm2 for 0.6-2.6 Q-cm layers, and it was 6 x 10e4 n-cm2 for 0.3 S&cm layers. Data from other experiments showed that layers which were equal to or less than 0.15 Q-cm in resistivity had a specific contact resistance less than 1 x 10e4 !&cm2. CONTACT

APPLICATION

Ag-In-Zn base contacts and Ag-In-Ge emitter contacts have been applied in the fabrication of double-diffused GaAs transistors. Typical contact fabrication procedure is illustrated in Fig. 7. The slice after emitter diffusion is shown in Fig. 7(a). A photoresist is applied and the emitter contact opening, which is slightly smaller than the

OHMIC

CONTACTS

FOR

(a)

GaAs

DEVICES

1217

(d)

PHOTORESIST

(b)

(cl FIG. 7. Transistor

(f) contact fabrication process.

opening in the emitter diffusion mask, is formed, The exposed GaAs is cleaned with a sulfuric acid-hydrogen peroxide etch then with a solution of ethylenediaminetetracetic acid in sodium hydroxide. Each cleaning etch is followed with a deionized water rinse. The emitter alloy (90 Ag-5 In-5 Ge) is evaporated onto a 180°C substrate [Fig. 7(b)]. The.excess of metal is floated off by using an appropriate solvent for the photoresist, the slice is then recoated with a SiOa layer produced by decomposition of tetraethylorthosilicate at 475°C [Fig. 7(c)]. The base contact opening is made by applying a photoresist mask and using buffered HF to etch the SiOa. The cleaning procedure is repeated, and the base contact alloy (80 Ag-10 In-10 Zn) is evaporated [Fig. 7(d)]. The excess metal is again removed by dissolving the photoresist, and all SiOs layers are removed [Fig. 7(e)]. Alloying is performed at 600°C in a forming gas atmosphere. Figure 7(f) shows the finished mesa-etched device after alloying contacts and applying a backing for the collector. Transistors using these contacts have been operated from 20 to 770°K. Gunn oscillators were built from vapor-phase epitaxial GaAs using the 90 Ag-5 In-5 Ge alloy

to contact both the 0.3 Q-cm n- layer and the n+ substrate. The same contacting procedure used for forming the emitter contacts of transistors is employed to contact the n- layer. The resulting device is then mounted in a microwave package; the smaller contact to the n- layer limits the input power and allows continuous wave operation. Continuous wave oscillators have been operated at 15.8 GHz with 26 mW output and at 25 GHz with 8 mW output. The efficiencies were 3 and 1 percent, respectively. Outputs of 120 and 160 mW at 13 GHz were obtained under pulsed operation (5 percent duty cycle). The efficiencies were 1.3 and 1.0 percent, respectively. Device frequency was controlled primarily by the resonance of the package used. Bandwidths were 10 kHz at 3 dB down in power and 50 kHz at 20 dB. The sharpness of the bandwidth is indicative of the planar alloy-semiconductor interface formed on (m) GaAs. Stability of the oscillations was good, being of the order of one part in lo5 for short-term operation and one part in lo4 for long-term operation. A continuous wave device has been operating continuously at 13 GHz with 3 mW output for longer than 3000 hr with little change in properties. Solution-grown GaAs in which the n-

1218

R. H. COX and H.

layer had N 1 x 1Ol6 electrons/cm3 was also employed for fabricating Gunn oscillators. These devices were operated pulsed (5 percent duty cycle) at 10 and 18 GHz. SUMMARY

Silver-base alloys were developed as ohmic contacts to GaAs transistors and Gunn oscillators. The specific contact resistance was determined by measuring the total contact resistance of circular contacts of different diameters and separating the components contributing to the resistance according to their dependence on the reciprocal diameter. The contact-GaAs interface was planar for (m) GaAs, and the metal pyramids into (100) GaAs could be minimized to N 0.1 CL. Contacts were solid, to at least 54O”C, so that high temperature fabrication steps and high temperature device operation were possible. The n-type contacts

STRACK

contained germanium rather than tin as the dopant to prevent spiking. The GaAs transistors were operated from 20 to 700°K. Gunn oscillators were operated continuous wave at 15.8 GHz with 26 mW output (3 percent efficiency) and at 25 GHz with 8 mW output (1 percent efficiency). Acknowledgments-The authors would like to thank E. HARP for electrical measurements and Dr. J. R. BIARD for supplying the data of the electrolytic tank measurements.

REFERENCES 1. B. W. HAKKI and S. KNIGHT, IEEE Trans. electron Devices 13, 94 (1966). 2. T. WUSTENHAGEN,2. Nuturf. 19, 1433 (1964). 3. R. HOW, Electric Contacts Handbook, p. 16, Springer, Berlin (1958). 4. G. B. LARABE~and J. F. OSBORNE(to be published). 5. L. BERNSTEIN, J. electrochem. Sot. 109,270 (1962).