Electrode effects on amorphous germanium

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ELECTRODE EFFECTS ON AMORPHOUS GERMANIUM

M. MORGAN AND A. K. JONSCHER

Chelsea College, University of London, P&ton Place, London, S. W.6 (Gt. Britain)

(Received April 21, 1971 ; in revised form June 10, 1971)

67

SUMMARY

Experiments are described which were designed to show the influence of the presence or absence of aluminium electrodes on the electrical properties of evaporated amorphous germanium (a-Ge) films. Both sandwich and planar structures with different electrode spacings were employed and the residual pressure during evaporation was varied separately for a-Ge and Al. Measure- ments were made of the d.c. voltage-current characteristics and of the dispersion of a.c. parameters with frequency, as functions of ageing and of heat treatment of the specimens. The smallest changes with ageing and heat treatment were found in specimens in which both a-Ge and Al were evaporated under high vacuum conditions. Samples in which &Ge was evaporated under poor vacuum but Al electrodes under good vacuum showed generally similar behaviour, while those samples in which both a-Ge and Al were evaporated under poor vacuum conditions exhibited strong drift in time, dispersion in frequency and also were more prone to irreversible annealing. Comparison of annealing effects with and without aluminium electrodes shows that specimens with electrodes are much more prone to drift and irreversible changes on annealing. Evidence of recrystallization and diffusion of electrodes into a-Ge is also reported.

INTRODUCTION

There has been a considerable interest recently in the electrical properties of amorphous germanium (a-Ge). This may be explained partly by the general interest in amorphous materials and, in particular, by the expectation that amorphous germanium may represent an ideal amorphous system, both from the point of view of its elemental simplicity and because crystalline germanium is very well understood.

Thin Solid Films, 9 (1971) 67-82

68 M. MORGAN, A. K. JONSCHER

The published experimental evidence may be divided into electrical, optical

and structural studies. Electrical measurements are normally carried out as a

function of the temperature and of the electric field, and sometimes also they

include the effects of the past thermal history (annealing) and of the deposition

conditions” ‘. Optica14* ‘9 “9 ’ 3 and structura11”16 studies are usually less

extensive in terms of past history and deposition conditions, and they differ from

the electrical measurements in the important respect that they use specimens with-

out electrodes and sometimes specimens of much smaller thickness, e.g. 100 A as

against several hundred to thousands ofAngstrom units. We will not at this stage

attempt to review the present state of the theoretical interpretation of the available

experimental evidence which is reviewed adequately elsewhere43 7* ’ ‘* 17-19. It will

be sufficient to point out that, in general, the electric current is a function of the

electric field in the specimen, of temperature and also of time, although the drift

of properties with time has seldom been expressly acknowledged as an important

experimental consideration’. In addition, the dependence of the properties on

the evaporation rate and the background pressure has been discussed6.

It is a well-known fact that an elemental amorphous material such as a-Ge

would be expected to represent a metastable structure in which severe disorder

is “frozen in” by deposition at relatively low temperatures and in which there

would always be tendencies to progressive recrystallization”, 18, ‘O. One would

therefore expect both instability with time and inhomogeneity of the micro-

scopic structure of the material, both factors depending critically on the past

history of the specimen and both influencing in a significant manner the measured

properties and their theoretical interpretation.

In addition to all these considerations, one further important factor seems

to have escaped notice in most of the reported work on a-Ge and this is the

influence of the metal electrodes on the electrical and structural properties of the

material being studied’. This is surprising since the tendency of metals to cause

devitrification in glasses is generally recognized. It is also to be expected that metal

electrodes would diffuse rapidly in relatively strongly disordered materials,

with resulting consequences for the electrical properties.

Different investigators have used different electrodes and this point makes

detailed comparison difficult. Au, In and Al were used by Chopra and Bahl’ ’

and Ag paste by Brodsky7. Bosnell and Voisey* conclude that the presence of

Au, Ag and Cu gives rise to incipient crystallization which they have not found

with Ta, MO and W. In particular, one feature of the published data is the irre-

versible changes of physical properties when films are annealed at temperatures

of the order of 450”-500 “C sometimes with metallic electrodes as in conductivity measurements3* 6, ‘9 ” and sometimes without as in E.S.R.’ and optical’, l2

measurements.

The object of the present paper is to report some experiments specifically

designed to study the effect of the presence of metallic electrodes-mainly

Thin Solid Films, 9 (197 1) 67-82

ELECTRODE EFFECTS ON AMORPHOUS GERMANIUM 69

aluminium--on the properties of a-Ge as function of the thermal history and of the manner of preparation of the specimens. This work was stimulated by the observation of such phenomena as rapid diffusion of metal electrodes Au, Cu and Al to the extent of their complete disappearance into the specimens after heat treatment, or even at room temperature, and also visible discolouration and formation of small crystallites near the edge of the metal electrode on a-Ge.

EXPERIMENTAL

Both sandwich and planar structures were prepared by successive evaporation of a-Ge and Al in a conventional vacuum system using an oil diffusion pump with a liquid nitrogen cold trap, with a background pressure of 10m6 torr. Germanium, aluminium and Cr-Au lands were evaporated from an electron bombardment source at relatively high evaporation rates in excess of 20 A/set and usually around 30 A/set. The source-to-substrate distance was approximately 15 cm. The substrates were nominally at room temperature, without deliberate cooling and with the only heating being due to indirect radiation from the electron beam source which should be less severe than in the case of evaporation from a heated strip. The geometry and dimensions of specimens are shown in Fig. 1, the areas of the sandwich specimens being lop3 and lop2 cm2, the spacing between the electrodes in planar specimens being 1.0 and 0.1 cm.

LaKl 4 Cr ,+Au

__- w : : ;-.i I _

I I

‘___I ‘-1 i.:*

(cl Cr+

Fig. 1. Configurations of specimens used in this investigation; the evaporants are indicated. (a) Planar long gap (1 cm). (b) Planar short gap (0.1 cm). (c) Sandwich structures of various areas.


70 M. MORGAN, A. K. JONSCHBR

The first part of the experiment consisted in evaporating a-Ge and Al under varying conditions of background pressure, either air or argon bled into the system, separately for the semiconductor and the metal. These are set out in Table I which also identifies the code letters used later in the text. Measurements were made as a function of time after deposition.

TABLE I

CONDITIONS OF EVAPORATION OF a-Ge AND Al IN

SANDWICH AND PLANAR STRUCTURES UNDER

VARIABLE BACKGROUND PRESSURE

Type Code Conditions

a both a_Ge and Al evaporated under the same

pressure of residual air.

b as above, but with high background pressure of argon at 6 x 10m5 torr bled into the

system, with the partial pressure of air at

10m6 torr level (one experimental point only).

c a-Ge deposition at 2 x 10e6 torr. Al at an air

pressure of 10m4 torr (one experimental

point only).

d a-Ge deposition at variable air pressure,

Al at 10m6 torr.

The second part of the investigation was concerned with the effects of ageing and of annealing at temperatures higher than the deposition temperature, on planar a-Ge layers evaporated under high vacuum conditions. A direct comparison was attempted between the behaviour of planar structures in which the a-Ge and the Al electrodes were evaporated before the ageing and annealing treatment, and the behaviour of u-Ge layers which were annealed and aged before the application of electrodes, the latter being applied just prior to the electrical measurements.

Both d.c. and a.c. measurements were made, the latter covering a range of frequencies, since the a.c. properties are known to be very sensitive to the presence of any structural inhomogeneities, the so-called Maxwell-Wagner effect. This phenomenon should be distinguished very clearly from the frequency-dependent conductivity expected in systems in which conduction is due to hopping of localized carriers which is known in many amorphous solids’8-20. This type of frequency dependence does not arise in a-Ge specimens which are homogeneous, because of the relatively high level of d.c. conductivity, so that any dispersion effects that may be present are a clear indication of inhomogeneity.



3 I (a 1

I

Planar structures at 300.K

2- 0

l A e

II a I 1

(b)

Sandwich structures at 77’K

7-

6-

0 a

Cl b

0 c

A d

-6 -5 -4 -3

Log P, (torr)

Fig. 2. The dependence of the resistivity p of a-Ge in planar structures (measured at 300°K) and in

sandwich structures (measured at 77 “K), as function of the residual pressure during evaporations. Symbols a-d refer to the method of preparation as set out in Table I.



INFLUENCE OF THE PREPARATION CONDITIONS

Figure 2 shows the resistivity of a-Ge prepared under the conditions defined

in Table I, respectively for planar structures at 3OO”K, diagram (a), and for

sandwich structures at 77 “K, diagram (b). Measurements are given at these

two temperatures since the resistance of planar layers is too high at 77 “K and

that of sandwich layers is rather low at 300°K. In those cases in which a direct

comparison was made at both temperatures it was ascertained that the resistivity

was the same for sandwich and planar structures of type a prepared at low pres-

sures, in agreement with the results reported by Walley6. This shows that there

is no anisotropy in evaporated a-Ge. All data shown in Fig. 2 refer to measure-

ments taken one hour after the deposition, with the specimen in air. Exposure

to air makes no measurable difference to the electrical resistivity compared with

measurement in ucIcu0.

Samples of type a and d cover a wide range of pressures and show clearly

that the former are very pressure-sensitive, while the latter are not. There is only

one measurement for type b and this agrees well with type d at high pressure.

The point corresponding to type c represents several specimens.

More detailed voltage-current measurements at different times after

deposition are shown in Fig. 3 for type a samples and in Fig. 4 for type d. It is

immediately clear that the samples of type a show much more dramatic drift

with time at the higher air pressures than the samples of type d. As in the case of

Fig. 2, this clearly shows the great importance of the conditions under which the

Al electrode was prepared and the relatively smaller significance of the deposition

TABLE II

Residual Rate Thickness Area Graph in Film type pressure (tmr) (A set- ‘) (A) (crV2) Figs. 3 and 5

2 x 10-5 30 2,700 1o-3 (i) a 3.8 x 1O-5 18 1,600 1o-3 (ii)

4.3 x 1o-5 47 4,200 1O-3 (iii)

b 6.3 x 1O-5 38 4,500 lo-’ (iv)

TABLE III

Residual Rate Thickness Area Graph in Film rype pressure (torr) (A see-‘) (A) (cm-‘) Figs. 4 and 6

d 1.1 x 10-5 21 2,540 1o-3 (i) 3.4 x lo-’ 29 3,450 1o-3 (ii)

4.3 x 1O-5 25 3,000 1o-3 (iii)

6.3 x 1O-5 35 4,160 1o-3 (iv) 7.4 x 10-S 25 3,000 10-j (v)

Thin Solid Films. 9 (1971) 67-82


-61

/; , , -1 0 1 I , pr = 6.3~16 5 torr

I

: iv) j

/:I

/I/

./:/

L’

60 y$

/‘,’ 1.2~10~ min

/‘/’ , I -1 0 1

! ’ i -6_

I I -2

/’ j log V(volt)

/’ ‘3xlO3min -11 x102mic I

-2 -1 0

log v (volt)

1 ,

i-

-6 - /

-4-

-5-

1

z+* 3.6x165 tort

(ii)

Fig. 3. The d.c. voltage-current characteristics of a-Ge films of types a and b, prepared under different

pressures, with the period of ageing at 300 “K as parameter. Graphs (iHiv) relate to conditions set out

in Table II.



p, = I.1 x 165torr 1

c (i) 1

-3

-4-

-2 -1 0

-4-

-1 0 1

log v (volt)

Fig. 4. The d.c. voltage-current characteristics of a-Ge films of type d, prepared under different pressures, with the period of ageing at 300 “K as parameter. Graphs (iHv) relate to conditions set out

in Table III.



conditions of the a-Ge layer itself. The result for sample b prepared at a high argon pressure shows its relative freedom from drift.

Further light is thrown on this dependence by a.c. measurements given in Figs. 5 and 6, which correspond directly to Figs. 3 and 4, respectively. Here we conclude that electrodes prepared under background air pressure give rise to considerable temporal drift leading to strong frequency dispersion of both resistance and capacitance, regardless of the manner in which the a-Ge itself is prepared. Conversely, good electrodes mean negligible dispersion with frequency, even though there is some ageing effect present.

We observe that the results for samples of type a in Fig. 2(a) correspond essentially to Walley’s earlier findings 6, but the results for samples of type d

~l(i)~~~

2 3 4 5 2 3 4 5

3

I_ 2: D

g 2 4 5 2

[L 4-9,_- 5 1 (iii)

3 4 5 u

4 I ,

0”

$ _ - __ --__ -. : _. *\ -4

‘\ ’ \ ’

\ \ \ \

’ ‘\ \ xx\-\

-3 3 4.3~16~ torr I -m-e _-_____ _

2 3 4 5 2 3 4 5

log F(Hz) log F (Hz)

Fig. 5. The dispersion of a.c. resistance R and capacitance C with frequency, for a-Ge films of types

a and b as set out in Table II, except that all areas are lo-’ cm’. Ageing times as set out in Fig. 3.



(i I I

p,= 1.1 x165 torr

I I

(ii) I I

p,. 3.4~16~ torr

3- -3

_- _______------______ _________------_______--‘I-,‘----

--__ I 1

(iii)

P K

F 3

p,= 4.3x165 torr

-3

~_=-------_-_---_===------- -------___ = -_-=_ = == ==--&----_=-_=== = = ---------_z_=====___

(iv) p, = 6.3 x1d5 toh I

(VI I

p =7.4x1G5 torr

1

c ______-__--------_____________ 3 -_ -_-_ = -_= -_-_----_ = = -_=-=_-_-~_-z= = ‘= = -_ -_ -_ = -_ -I 3

2 3 4 5

log F (Hz)

Fig. 6. The dispersion of a.c. resistance R (continuous lines) and capacitance C (dotted lines) with frequency for films of type d, as set out in Table III, except that all areas are lo-’ cm’. Ageing times

as set out in Fig. 4.



represent new results which cast doubt on the validity of some of the published experimental data for a-Ge.

EFFECT OF THE PRESENCE OF CONTACTS

We now proceed to present the experimental results relating to the effect of the presence of contacts on the ageing and annealing characteristics of a-Ge evaporated under high vacuum conditions. The experimental points in Fig. 7 correspond to planar samples annealed for 4 hours at the specified temperatures, measurements being taken in each case at 300°K. Specimens marked “with electrodes” had electrodes evaporated immediately after the deposition of a-Ge

@ with electrode

0 without

0

, 1, I I 51

50 100 150 200 250 300 350 400 450 500

TVC)

Fig. 7. The resistance R, after four hours’ annealing at the temperatures indicated, measured at

3OO”K, with and without electrodes. Samples annealed without electrodes had electrodes applied after annealing.



and were therefore heat treated in the presence of electrodes. Samples marked “without electrodes” had electrodes evaporated after the heat treatment at the temperatures indicated. Each point in this category corresponds, therefore, to a separate specimen. Electrode spacing on the planar samples was 0.1 cm in all cases.

Allowing for some scatter of experimental points, it is clear that the initial stages of annealing, up to some 150 “C, do not depend significantly on the presence or absence of electrodes, the resistance increasing by approximately one order of magnitude in this interval. In the range 150-3OO”C, the resistance of samples annealed with electrodes increases more rapidly but this is followed by a dramatic drop by several orders of magnitude between 350” and 390°C. Severe damage to the films is visible near the edge of the electrodes. In contrast with this, the samples annealed without electrodes show a continuous slow rise which appears to saturate in the range 390”470 “C.

This result shows very convincingly that what was often taken for a “recrystallization temperature”, corresponding to the rapid drop of resistance, may have been affected seriously by the presence of electrodes, if these were present during heat treatment. This critical temperature can only be described as the temperature of electrode-induced recrystallization, it has nothing to do with the intrinsic properties of a-Ge.

A different phenomenon, ageing in the presence of electrodes at a steady temperature, is shown in Fig. 8(a) and (b) relating, respectively, to planar specimens with long (1 cm) and short (0.1 cm) gaps between electrodes. The resistance of the samples was monitored as a function of time at the temperatures stated and the ratio of this resistance R, to the initial resistance R, is plotted. It is seen that ageing is much more rapid in short gap specimens which also show a reversal of the trend at higher temperatures, where diffusion of electrodes becomes visible and makes the interpretation difficult.

Figure 9 shows a different aspect of the properties of planar a-Ge structures with “long” (1.0 cm) and “short” (0.1 cm) gaps between electrodes. The specimens were evaporated under high vacuum conditions, complete with their electrodes, and were then annealed below their respective temperatures of irreversible resistance change. The resistivity was then plotted against the reciprocal temperature and was quite reversible at temperatures below the annealing temperature. The point shown in Fig. 9 is that the temperature of irreversible change is significantly higher in long samples450 “C than in short samples-350 “C.

The ageing characteristics at 300 “K of the planar and sandwich samples prepared under different evaporation conditions are shown in Fig. 10. The pressures indicated refer to types a and d. Only sandwich structures of type a evaporated under relatively poor vacuum conditions show rapid drift of resistance by some orders of magnitude. All other specimens give relatively slow drift of resistance with time.



1.6

:a) 1 cm gap

1.5-

(b) 0.1 cm gap

0 60 140

Time (mid

Fig. 8. Time dependence of the ratio of the resistance R, after ageing for a time t at the specified

temperature, over the initial resistance R,, all measured at the temperatures indicated. Planar

structures with 1 cm gap (a) and 0.1 cm gap(b).

CONCLUSIONS

The results described in the present paper show clearly the influence which

the presence of aluminium contacts may have on the properties of amorphous

germanium films. Two aspects are brought out. One concerns the effect of the

evaporating conditions of the a-Ge films and of the electrodes on the properties

of the specimens. The conclusion is that the critical factor is the background

pressure during the evaporation of aluminium and that the pressure during the

evaporation of germanium appears to be relatively less critical, provided the

electrodes were deposited under good vacuum.

Specimens with electrodes deposited under a high background pressure

show marked dispersion of a.c. characteristics with frequency and also a high


80

5

4

3

2

1

0

-1

0 1.0 cm gap

M. MORGAN, A. K. JONSCHER

1.0 1.5 2.0 2.5 3-o 3.5

103/T (OK-'1

Fig. 9. Temperature dependence of the resistivity p of long and short planar samples, showing the

irreversible transition at high temperatures.

drift in time. One could interpret this as being due to diffusion of oxygen from the

electrodes into germanium, an effect which would be far more pronounced in

sandwich than in planar structures, which implies inhomogeneity and barrier

effects.

The second aspect is the effect of the presence of electrodes on the ageing and

annealing characteristics of a-Ge specimens. In the presence of electrodes, heat

treatment leads to a more rapid change of resistance and to final collapse of

resistance due to “recrystallization” at a lower temperature than with a-Ge

in the absence of electrodes. Similarly, drift of resistance with time at various

temperatures in planar structures with electrodes is much more pronounced in

specimens with “short” gaps between electrodes than in those with “long” gaps.

While the results reported here relate specifically to aluminium electrodes

on a-Ge, very similar effects may be expected in other amorphous solids in the

presence of other metallic electrodes. The common feature is the relative ease



4

3

$’

4”

z

1

0

Planal

0

Sandwich

a

0

@

0

cl

A

3.8 Y lo5 torr

1 2 3 4

log time (min)

Fig. 10. Ageing characteristics at 300°K of different structures, both planar and sandwich, of a-Ge,

expressed as the ratio ofresistivity after time t, p,, over the initial resistivity, p,,. The pressures indicated

refer to the evaporation conditions of specimens of type a, both planar and sandwich, of which only

the latter show a pressure dependence. The results for planar samples of type a are effectively pressure-

independent.

of diffusion of metallic atoms in strongly disordered systems and the tendency

to nucleation induced by the presence of the metal.

ACKNOWLEDGEMENT

One of us (M.M.) wishes to acknowledge the tenure of a Chelsea College

Research Studentship.

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Electrode effects on amorphous germanium

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