The electrical properties of Al/Ni/Ge/n-GaAs interfaces

L. Da vid a, *, B. Kova cs b, I. Mojzes c, Zs. Kincses b, L. Dobosd

aInstitute of Microelectronics and Technology, Kando KaÂlmaÂn Polytechnic, TavaszmezoÄ u. 17, 1084 Budapest, HungarybDepartment of Electronic Technology, Technical University of Budapest, 1521 Budapest, Hungary

cDepartment of Electronic Technology, Tu of Budapest-Z. Bay Foundation for Applied Research, FeheÂrvaÂri uÂt 130,

1116 Budapest, HungarydResearch Institute for Technical Physics of the Hungarian Academy of Sciences, 1521 Budapest, Hungary

Received 27 March 1997; in revised form 28 October 1997

Abstract

An Al(130 nm)/Ni(30 nm)/Ge(40 nm) layer deposited onto n-type GaAs by thermal evaporation was electricallystudied. The electrical properties of these contacts were characterized by current±voltage curves and contact noisemeasurements. The samples have been annealed for di�erent times at di�erent temperatures in ¯owing forming gas,

H2:N2 (5%:95%), in a tube furnace. The I±V characteristics of the AlNiGe samples annealed at di�erenttemperatures show Schottky character. The I±V plots of the samples that had high noise indexes show a doubleslope structure. # 1998 Elsevier Science Ltd. All rights reserved.

1. Introduction

High quality and reliable ohmic and Schottky contacts

to GaAs play an important role in the fabrication of

high speed integrated circuits. Nowadays many metalli-

zation systems are used to form thermally stable, low

resistant, reproducible and reliable ohmic and Schottky

contacts.

A new candidate of contact material for n-GaAs

originally proposed in Refs [1, 2] is the Al/Ni/Ge

multilayer [3±7]. The electrical characterization of the

Al/Ni/Ge/n-GaAs depends on the doping concen-

tration of the contact layer of the GaAs, the heat treat-

ment temperature, the time of annealing and the

layering sequence of the Al, Ge and Ni metals. The

low frequency noise measurement is a very sensitive in-

vestigation tool of quality and reliability aspects of

device technology [8±11, 15]. We investigated the in¯u-

ences of the annealing temperatures on the I±V charac-

teristic and on the characteristic noise spectra of the

devices in the case of AlNiGe metallization.

2. Experimental procedure

The investigated samples were diode-like structures

prepared on n±n++ GaAs wafer. The epitaxial n-

GaAs layer having sulphur dopant concentration of

Nd=8 � 1015 cmÿ3 and 6 mm thickness was grown by

E�er±Nozaki type VPE reactor. The samples were

carefully degreased, then the lift-o� patterns were pre-

pared using Shipley AZ-1450J resin. Prior to the metal

deposition the surface was etched by the mixture of

NH4OH:H2O2:H2O (1:1:100) at 08C for 10 s followed

by a rinse with 18 MO water and dried with ®ltered

dry nitrogen. Ge(40 nm), Ni(30 nm) and Al(150 nm)

layers were deposited by thermal evaporation at room

temperature in an oil-free vacuum system. The evapor-

ation rates were 0.4, 0.2 and 1 nm/s, respectively. The

pressure was kept below 10ÿ4 Pa at the beginning of

the Ge deposition and did not exceed 10ÿ3 Pa during

the whole process. The metallization was patterned by

a lift-o� technique; the contacts had a circular shape

with a diameter of 250±110 mm size. The samples have

been annealed in ¯owing forming gas, H2 (5%):N2

(95%), at three di�erent temperatures (400, 450 and

5008C) for di�erent times.

Microelectronics Reliability 38 (1998) 787±793

0026-2714/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved.

PII: S0026-2714(97 )00218-7

PERGAMON

* To whom correspondence should be addressed.

The patterned samples were encapsulated using

AuGe eutectic on their back side and Al wire ultra-

sonic bonding on the studied contacts. Special care

was taken to reduce and minimize the thermal shock

originating from the encasing process. The surface was

studied by scanning electron microscopy (SEM). The

detailed results of the structural investigations were

published elsewhere [12]. The temperature dependent

current±voltages were measured applying Keithley 230

Programmable Voltage Source, Keithley 236 Source

and Measure. The device noise was measured by a

modi®ed setup of IEC recommendation [13]. Two

pieces of a low-noise series separator resistor (10 kOboth) were used to make a quasi-current source mode

measurement and adjust the impedance to the input of

the ampli®er described in [14] (Fig. 1). Applying this

setup the diode current was kept in 0.01±1 mA range.

The voltage noise spectral density was measured in the

frequency range of 1.6 Hz±20 kHz in 42 third-octave

band steps by a real-time digital frequency analyzertype BruÈ el and Kjaer 2131 controlled by PC 486's.

When we checked the noise contribution of RS2 it wasfound to be 6 dBm lower in any frequency band thanthe smallest noise of any studied D.U.T. in series with

RS2.

3. Results and discussion

We investigated the low frequency noise spectra of

as-deposited and annealed samples. To obtain the biasvoltage (Udc) dependence of the voltage noise spectraldensity, therefore noise spectra (dB, mV2/Hz) measure-ments were carried out at four di�erent forward biases

(Udc) (4.5, 9, 13.5 and 18 V). The bias voltage depen-dence of the noise spectra was found to be similar ineach sample but the sample heat treated at 4008C for 5

min had the highest noise spectra. Figs. 2 and 3 showthe noise spectra of two samples. The ®rst sample wasnot heat treated (as-deposited) and it had a lowest

noise spectra at every bias voltage. The second sampleheat treated at 4008C for 5 min had a much highernoise spectra than the ®rst sample. All samples had

similar diameters (170 mm). It can be seen that thenoise spectra of the samples depend on the annealingtemperature. As discussed later we assume that theincreasing noise can be associated with the increasing

inhomogeneities at the metal±semiconductor interface.Although the noise spectral density is much better forpresentation than noise index [15], to simplify the com-

parison the noise index was introduced as

noise index � 20*log Un �mV�=Ue �V� �mV=V=decad �,

Fig. 1. Noise measurement setup (D.U.T. = Device under

test).

Fig. 2. Voltage noise spectral density of as-deposited AlNiGe contact with 170 mm diameter as function of frequency.

L. DaÂvid et al. / Microelectronics Reliability 38 (1998) 787±793788

where Un is the measured noise voltage/decad and Ue

is dc voltage obtained between A and C points in

Fig. 1.

The noise index at 1 kHz was calculated from each

spectra and it is shown in Fig. 4. As can be seen, the

obtained noise index curves are strongly dependent on

the heat treatment processes. The exact explanation is

yet unclear.

We measured the temperature dependent current±

voltage characteristics of these contacts (Fig. 5). The

contacts with high noise indexes had a log I±V plot

with a double slope structure measured at room tem-

perature (samples heat treated at 4008C for 5 min and

4258C for 5 min). The log I±V curves of the samples

heat treated at 4508C for 20 min did not show double

slope character measured at room temperature. Their

Fig. 3. Voltage noise spectral density of AlNiGe contact with 170 mm diameter heat treated at 4008C for 5 min as function of fre-

quency.

Fig. 4. Noise index at 1 kHz of the AlNiGe contact at various temperatures and times as function of forward bias voltage.

L. DaÂvid et al. / Microelectronics Reliability 38 (1998) 787±793 789

noise indexes, however, were higher than those of the

samples heat treated at 5008C for 20 min. Therefore

we measured the temperature dependent I±V. These

curves are shown in Fig. 6. It can be seen that when

the measurement temperature of the samples decreased

then the double slope character of the samples became

more apparent. The Schottky barriers and the ideality

factors of these samples were evaluated using both

slopes. FB/lb indicates FB as calculated from the slope

belonging to low bias voltages. FB/hb indicates FB as

calculated from the slope belonging to high bias vol-

tages. The same symbols were used in the case of n

ideality factor, as well. The values calculated from

these curves are shown in Table 1. Increasing the heat

treatment temperature the FB/lb is increased and FB/hb

is decreased. Both curves approach the same value.This supports the assumption that a new crystalline

phase forms at the highest (4508C) temperature.

Since the double slope character is attributed to thelateral inhomogeneity [16], the connection between the

existence of a double slope and a high noise index sup-

ports the assumption that the high noise indexes of the

contacts are due to lateral inhomogeneity at the semi-

conductor±metal interface and the segregation of Ge.The cross-section transmission microscopy (XTEM) in-

vestigation of the as-deposited samples shows that the

metal±semiconductor interface was very sharp (Fig. 7).

The noise indexes of these samples were very low. The

scanning electron microscopy (SEM) investigationsshow that Ge appears on the surface of Al after heat

treatment at 4008C for 2 min (Fig. 8). At the same

time small pits appeared at the metal±semiconductor

interface. At these annealing temperatures and short

time values the noise of the samples was high. Weattributed this to the segregation of Ge and to the for-

mation of the pits. In the case of samples heat treated

at 5008C the noise indexes of the samples decreased.

Fig. 5. I±V characteristics of AlNiGe samples heat treated at

various temperatures and times.

Fig. 6. Temperature dependent current±voltage characteristics

of AlNiGe heat treated at 4508C for 20 min.

Table 1

Schottky-barrier height and n ideality factor calculated from double slope I±V characteristics

FB/lb (eV) nlb FB/hb (eV) nhb

4008C/5 min 0.53 2.80 0.67 1.50

4258C/5 min 0.53 2.82 0.61 1.78

4508C/20 min 0.60 1.95 0.62 1.95

L. DaÂvid et al. / Microelectronics Reliability 38 (1998) 787±793790

Fig. 7. XTEM image of as-deposited AlNiGe contact. The sample was heat treated at 1808C during the XTEM preparation.

Fig. 8. SEM image of AlNiGe contact heat treated at 4008C for 2 min.

The XTEM images showed that an Al/Ga/As ternarycompound was formed at the interface. (Fig. 9). Theappearance of the ternary compound indicated an

ordered interface and the noise index of these sampleswas lower than for other samples.

4. Conclusion

It is assumed that the higher noise indexes of thesamples were due to the inhomogeneity of the inter-

faces and the segregation of Ge. When the metal±semi-conductor interface was sharp (as-deposited samples)and Ge, Ni, Al metallization had a bilayer structure (a

Ni±Ge mixed layer directly above the semiconductorand a pure Al upper layer), then the lowest noise indexwas observed and the log I±V curves of these samplesdid not show double slope character. When the inter-

face was rough (in the samples annealed at 5008C),large grains grew into GaAs as pyramidal pits butbetween the metal grains and GaAs an interface layer

was found. The detected components and the presenceof a second crystalline phase suggest the formation ofan Al/Ga/As ternary compound. This ternary com-

pound formed between the metal and GaAs means amore ordered metal±semiconductor interface and thenoise index of this interface was lower than in other

cases and the log I±V curves of these samples did notshow a double slope character. When the segregationof Ge was so high that Ge appeared at the top of the

Al layer (¯orets formation on the sample surface heattreated at 400 and 4258C for 5 min) and at the metal±semiconductor interface randomly distributed protru-

sions grew into the semiconductor due to the longannealing time (4508C for 20 min) the noise index ofthese samples was the highest and the samples had alog I±V plot with a double slope structure.

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