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Surface properties of n-type gallium arsenide
SURFACE
SCIENCE
SURFACE
10 (1968) 32--57 0 North-Holland
PROPERTIES
OF n-TYPE
Publishing
GALLIUM
Co., Amsterdam
ARSENIDE
Received 7 August 1967; revised manuscript received 13 September 1967 This paper describes an investigation of GaAs surfaces by means of field effect, surface photovoltage and photoconductance measurements. A wide variety of different etch treatments and also the effect of depositing SiOz on the surface have been investigated. It is found that the surface barrier always bends towards depletion or inversion. On some etched surfaces the total effective surface trap density is at least IOra cm-a but some treatments were found to reduce this to about 3 x IOr cm-e, The effective surface trap density of SiOz covered surfaces was 7 x lOI1 cm-2 and was not sensitive to the etch treatment of the underlying surface. Under ac fields the mean surface barrier height becomes biased in the direction of further depletion due to an increase in the mean occupancy of the traps. The ac field effect mobility is of the order of 2000 cm2 V-r see-r despite the large surface trap densities. It is also deduced from the ac field effect that fast bulk traps with densities of about 10’5 cm-3 are active in the surface space charge layer. A model of the surface is given whith explains the observed phenomena.
1. Introduction The main aim of the work described in this paper has been to investigate the surface field effect on gallium arsenide. The interpretation’) of the field effect is not so simple with gallium arsenide as with silicon or germanium, primarily because of the larger energy band gap and the consequent very low intrinsic carrier density. In particular it does not seem to be possible to deduce the surface potential from field effect measurements. But for any detailed understanding of surface behaviour it is necessary to know the surface potential with some precision. For this reason the surface photovoltage has also been studied to gain additional information. To explore surface behaviour fully one needs to vary the surface potential between wide limits. On the first gallium arsenide surfaces examined, high densities of surface traps were found which tend to lock the surface potential. A wide range of surface treatments has therefore been studied with the aim of minimising trap densities or achieving a range of fixed surface potentials. In general there were only three types of behaviour that may be clearly distinguished. These appear to be associated with the presence of weak depfetion layers, strong depletion layers and inversion layers, and trap densities are always rather high. Chemically etched surfaces 32
SURFACE
tend to have strong depletion
PROPERTIES
OF n-TYPE
GRAS
layers, the surface being nearly intrinsic.
33
If an
SiO, film is deposited on such an etched surface by decomposition of tetraethoxysilane then the barrier height is reduced giving a weaker depletion layer. The main part of this paper will discuss a typical etched surface and a typical SiO, covered surface. An etched surface which appeared to have an inversion layer will also be discussed and some mention will be made of the effects of various surface treatments.
2. Experimental
technique
The measurements described here were made on n-type gallium arsenide with a mobile electron density of 1.8-3.5 x 1Or’ cmW3 and an electron mobility of 4300 cm’/V sec. Test filaments took the form of thin plates 11 mm x 3 mm cut parallel to the 111 plane with alloyed ohmic contacts at the ends. At least 100 pm of material was removed by etching from each surface after the initial preparation, to eliminate mechanical damage. The filaments were reduced to a final thickness of 40-80 pm. For measurement, the filaments were mounted in a holder which pressed transparent electrodes of conducting glass and 25 pm polyethylene film dielectric spacers against the two major faces. These electrodes could be used either for applying fields of up to IO6 V/cm to the surface, or as capacitive surface voltage probes when connected to a high impedance electrometer. Measurements were made in a vacuum of 10m4 Torr at a controlled temperature of 30 “C +O.O5”C For the photovoltage measurements, single pulses of light one millisecond long from a high intensity xenon arc were shone on the surface through the transparent electrode. The maximum number of useful photons arriving at the gallium arsenide surface during the pulse was estimated to be about 3 x 1019 set-l cm- ‘. The photoconductance was used as a measure of injection level, and the intensity of the light was reduced over about seven orders of magnitude by introducing neutral density filters in the beam.
3. Model of gallium arsenide surface It is thought that a short discussion of the model of the surface at this stage may help in understanding the photovoltage and field effect data which will be presented later. The model which seems to fit the experimental facts best is that there is normally a depletion layer at the surface and that there are densities of active surface traps in the range 10”-10’2 cm-’ situated near the Fermi level. The surface behaviour seems to be fairly similar for both n and p-type material ‘),
34
I. FLINN
but only n-type will be considered in what follows, as nearly all the measurements were made on this. In cases where the surface is not inverted there can be no significant number of minority carriers at equilibrium in the whole filament. Furthermore the generation rate must be very small. Thus we assume that minority carriers do not play any part in field effect processes under these conditions (see Q5.5.1 for justification. The case when an inversion layer is present will be discussed in 5 5.5.2). Also, in the absence of minority carrier injection, only majority carriers are involved in the kinetics of the surface traps. The net rate of capture K, by such a set of traps2) is given by the difference between the total rate of capture UC and the total rate of emission U, K,=IJ,--IJ,,
(1)
i.e. I(, =
45”%(1
-fJ
n,
-
~~~~~~~~~
1
(21
where G, is the capture cross section of a trap, ot is the mean thermal velocity of the electronsf, is the fraction of the traps which are occupied, n, is the carrier density at the surface. n, = NCexp [(E,-&)/!cI~“], where E, and EC are the energy levels of the trap and the conduction band edge, and NC is the effective density of states in the conduction band. If the surface is heavily depleted as in the present cases, n, is small and near equilibrium, when K”zO, both the capture and emission of electrons must be relatively slow. This gives a simple picture of the surface model. The analysis of the experimental data will allow a certain amount of detail to be filled in. 4. Surface photovoltage There appear to be at least three causes for the photovoltagea,J) which is picked up by a capacitive probe when light is shone on a semiconductor surface thereby injecting hole electron pairs. (a) Even if there is no change in the occupancy of traps and therefore no change in the net charge contained in the space charge layer, the barrier height falls owing to a reduction in the effective thickness of the barrier region. It can be shown that to achieve flattening of the surface barrier to within 1 kT by this process the injection level An at the boundary between the surface layer and the bulk is given by:
where 11,and pS are the equilibrium surface carrier densities, ~1~and pO are
SURFACE
PROPERTIES
OF II-TYPE
35
GaAS
the bulk densities and Ye is the equilibrium surface barrier height in units of kT/q. It is presumed in eq. (3) that the quasi Fermi levels are substantially flat across the barrier. (b) If there are active traps at the surface, there will in general be a change in their occupancy and the surface barrier may change for this reason also. This could in principle cause either an increase or a flattening of the surface barrier. An increase of barrier height under illumination is very unlikely with the large surface barriers found on GaAs. Williams5) has observed a flattening of the barrier by this mechanism in CdS. (c) There is also another mechanism which may bring about a change of potential with injection. This is the Dember effects), and it is due to the difference in diffusion coefficients of holes and electrons which causes a voltage to be set up across a region where there is a gradient in the nonequilibrium carrier density. The magnitude of the Dember potential Vn may be expressed in the form: I/n = kT b-’ q b+l
log
1 + F+! (
.;2 0
)
(4)
>
where b is the ratio of the diffusion coefficients. This result is strictly only valid in a semiconductor which is free of space charge and has no traps. It would be difficult if not impossible to assess the relative importance of processes (a) and (b) at a given injection level without prior knowledge of the relevant surface parameters. But it does seem that at sufficiently high injection the surface barrier must always eventually become virtually flat. Thus the highest possible injection has been aimed at in the present work. Owing to the very short diffusion length in gallium arsenide, non-penetrating radiation must be used. (Suitably intense sources of penetrating radiation would cause excessive heating of the specimen.) It is then difficult to determine an effective injection level because both surface and bulk recombination are themselves functions of injection level. Since injection is not uniform near the surface the quasi Fermi levels cannot be assumed flat. Thus eq. (3) is not valid and, even if it were, it would be difficult to apply as a criterion for barrier flattening. The Dember potential complicates matters further. Its magnitude is expected to become really important at injection levels where the surface barrier would otherwise be effectively flattened. The relevant injection level for calculating I’, is then the level at the surface, An,. The diffusion length is small and unknown and the surface recombination velocity may be high. So V, is not easily deduced.
1. FLINN
36
(0)
PhotovoltagQ Light Intansity-5x10’2
(b)
photons soc-‘~m-~
Photovoltage Light intansity1013 photons
sac-‘cm -2
0.1
CC) Photovoltage
0.2
Light intensity s 9
_
10’8 photons SC-‘cm
0.3
-2
0.4 0.5
z c 0 Z5 5
300 (d)
200
Photoconductance Light intoosity=
25 o,e
‘oJ8 photons Eo-
100
I
1
0
’
I
I
-2
3
1
4
I
1
1
5 6 7 Time ~millisocs)
w Dumtion of light pulse n-type
Fig. 1.
GaAs(otchod
Photovoltage
surface)
and photoconductance
response.
sac-1cm12
SURFACE
PROPERTIES
OF n-TYPE
GRAS
37
In view of the complexity of the situation it is not possible at present to make a definitive estimate of the surface barrier height from the surface photovoltage. But an interpretation can be put forward, and the estimate of barrier height so deduced seems the more reasonable because it also is compatible with the field effect data. 4.1. EXPERIMENTAL RESULTS Typical photovoltage and photoconductance response is shown in fig. 1. The photovoltage is positive on this n-type material indicating that the potential of the surface goes more positive on illumination. Figs. la, b and c show how the photovoltage varies as light intensity increases and fig. Id shows the photoconductance for the highest intensity. At low intensities the steady state is not reached within the 1 msec that the light is on. There is also a long relaxation time after the light pulse, greater than 10 msec, which must be associated with trapping of photocarriers. This slow process becomes ineffective at higher intensities, when the relaxation time is too rapid to be measured. Typical logarithmic plots of photoconductance versus light intensity are given in fig. 2 for a free etched surface and an SiO, covered surface. Photoconductance is calculated as a surface component in micromhos per square because nearly all the photocarriers are assumed to be in a thin layer near the surface. For high injection levels, the relation Gee (Zr is usually obeyed, where G is the peak photoconductance and Z is the intensity of illumination. There is usually departure from this relation at low intensities where trapping is strong. The injection level rises only slowly with light intensity, the exponent y1being usually in the range 0.3-0.4. When the surface photovoltage is plotted against log of photoconductance, fig. 3, it usually turns out that there is an inflexion in the curves at low injection levels. But above a photoconductance of about 100 pmho/sq. the curve rises linearly with no sign of saturation. The surface photovoltage usually increases by between 0.09 and 0.18 V per decade of photoconductance in this linear range. 4.2. INTERPRETATIONOF PHOTOVOLTAGE At least for the lower injection levels a change in the occupancy of the surface traps (process (b) of 0 4) is thought to account for most of the photovoltage observed. This is deduced from the slow relaxation of the photovoltage after the light pulse, which is due to the slow return of the traps to equilibrium. When an electron-hole pair is created near the surface by light, the hole is swept towards the surface by the field in the space charge layer and the electron is swept towards the bulk. The surface traps which have
38
1. FLINN
been detected by the field effect (9: 5) are thought to be negatively charged when occupied by an electron. They thus have a large capture probability for holes. So when the surface is illuminated the electrons tend to empty out of the surface traps into the valence band. Under weak illumination the capture rate for electrons from the conduction band remains low because mobile electrons are swept away from the surface. Thus the net charge in the traps falls and consequently the charge in the space charge layer also falls. It is the resulting fall in surface barrier height which gives the observed photovoltage at low light intensities. The slow relaxation of the traps when the light pulse goes off, figs. 1a and 1b, is to be expected since they can only be refilled from the conduction band and this is depleted of electrons at the surface. (The re-emission of a hole from the trap to the valence band requires a large increase in its energy and is very improbable.) But as the light intensity increases, the emptying of the traps is also increased with the result that the surface potential barrier falls further, and the density offree electrons at the surface increases. Thus the electron capture rate will increase and at
10
000 :
I
I
n-type GaAs Curve I. Etched w-face Curve 2. Si 0, covered surface
I
1 -6
I
I -5
-4 Loglo
Fig. 2.
relative
Photoconductance
-3 -2 light intensity
vs. light intensity.
-1
SURFACE
PROPERTIES
OF R-TYPE
GRAS
39
higher light intensities the relaxation of the photovoltage becomes much more rapid, fig. lc. In this range the total hole and electron density near the surface must also increase significantly due to the light and this gives an additional flattening of the surface barrier by process (a), 0 4. The increased rate of capture of electrons is equivalent to an increase in the efficiency of the traps as surface recombination centres, as the injection level rises. So it is to be expected that the injection level will be less than proportional to light intensity. The maximum photoconductance normaily observed is about 1500 pmhojsq. This corresponds to at least 2 x lOi cm-’ injected carriers. If it is assumed that these nearly all exist in a layer about one micron deep, the mean injection level in this layer is 2 x lOi cmW3. Since processes (a) and (b) are both operating in the sense of flattening the barrier, the injection is almost certainly adequate for complete flattening of the surface barrier. But a Dember type voltage, process (c), must also appear. In GaAs the mobility ratio is IO: 1 and so the Dember potential is expected to be significant. The Dember potential should increase by 40-50 mV per decade of injection (eq. (4)). The relevant injection level is however not some mean value but that at the surface. It is difficult to determine this. The slow increase of photoconductance with light intensity implies that the effective lifetime and 0.6
I
n-typQ GoAs Curve Curve
1. Etched surface 2.Si02 covered surface
0.5
2 0 0.4 2
__j____-.-
E r g 0.3 5 a
-L--_--
$j 0.2 t 2 0.1
ri-
h
CI_
1c
Surface Fig.
3.
Surface
photoconductance
photovoltage
CpMholsq)
vs. photoconductance.
4.0
diffusion
I. FLINN
length
are decreasing
as the injection
level rises. Thus
even the
mean injection level is not proportional to the photoconductance but must in effect increase more rapidly. The presence of surface recombination as well as bulk recombination makes matters even more complicated. However, it seems that the injection level at the surface is unlikely to be rising more rapidly than the light intensity; but it must on the other hand be rising more rapidly than the total injection which is proportional to the photoconductante. At high intensities the photovoltage increase by 25-43 mV per decade of light intensity and by 90-I 80 mV per decade of photoconductance. Thus it is reasonable to ascribe this steady increase of photovoltage at high intensities to a Dember potential. The part of the curves in fig. 3 below a photoconductance of N 100 pmho/sq. could then be interpreted as showing signs of saturation in the barrier flattening. In this case, the barrier height of the surface depletion layers could be deduced as about 0.45 V for the etched surface and 0.2 V for the SiOZ covered surface. This is indicated on the figure by the dotted lines. Dmitruk and Lyashenko?) have measured barrier heights on gallium arsenide by a contact potential method and their values are mostly in good agreement with ours on similarly prepared surfaces. 5. The field effect The response of the gallium arsenide surface to electric fields can be explained in terms of the model of g 3. With a positive field, electrons are drawn to the surface. The carrier density n, increases rapidly and UC, the rate of capture of electrons by the traps, increases in proportion. But if a negative field is applied, the electron density n,, which was already small, is reduced effectively to zero. So the surface traps do not capture electrons and there is only a low rate of emission by them into a deep non-equilibrium depletion layer. This latter situation was first exploited by Rupprechta) in his pulsed field effect technique. Primachenko et alg) have also studied such depletion layers on silicon and have managed to completeIy deplete their specimens
of carriers
with high fields.
5.1. dc STEP FIELDS The response of an etched surface to step fields is shown in fig. 4. For a positive field, fig. 4a, there is a small increase in conductance. The peak value, which may be expressed as a held effect mobility*, is equivalent to * The field effect mobility ,CLFE is defined as AG/AQ, where dG is the change in conductance per square brought about by a corresponding charge of AQ. In the case of ac fields AC and #Q have been taken as the peak to peak values.
SURFACE
about
PROPERTIES
one tenth of the bulk electron
decay of conductance,
OF II-TYPE
mobility
which is complete
41
GBAS
ps. This is followed
in about
ten seconds,
by a rapid
and a slower
decay which takes some minutes. The final conductance does not differ sensibly from the initial value. It is concluded that most of the induced charge is trapped too rapidly to be observed. One only sees the small residual conductance change equivalent to a small increase in n, when the rate of capture by the traps has become modest. For a negative step field (fig. 4b) the initial fall of conductance is much larger. It corresponds to the formation t v4
5
L-
field
on
I
I
I
0
I
I
IO limo
-va
field
I
20
I
30
---k--
(~4~s.)
on
I
I
I
I
0
I
20 limo
WtypfZ
GaAs.
Fig. 4.
Peak
1
I
10
field
I
30 (sacs) 4.6
x 105V/cm
dc Step effect on etched surface.
I
I
40
-
42
I. FUNN
of a large depletion region when electrons are repelled from the surface. The peak value is equivalent to about half the bulk mobility (on the simple model one would expect equality with @a, this will be discussed in $5.3). The surface traps can only respond to the field by a slow emission of electrons and again there is a decay back to the equilibrium conductance. As with the positive field there is a “fast” process taking about 20 set and the slow process, which takes about 10 min, does not show at all in fig. 4b. In this case the density of fast traps is estimated to be about 5 x 10” cm-’ and there must be at least an equal density of active slow traps. On surfaces covered with SiOz which had a lower surface barrier height as indicated by the photovoltage, the fast traps were present in densities of the order of 7 x 10” cme2 and the slow traps apparently absent. The decay of the field effect conductance was faster on the oxide covered surfaces. This is due to the larger value of n, (eq. (2)) associated with the smaller barrier height. 5.2. ac
FIELDS
It is of interest to know how these surfaces will react to large sinusoidal fields. Measurements at frequencies above 50 Hz show that the ac field effect mobility pFE may be as much as 2000 cm -2 V-’ set-‘, that is, about half its the bulk electron mobility. There is a significant difference between the behaviour of the etched surfaces with large barrier height and the SiO, covered surfaces with smaller barrier height, and these will be discussed separately.
5.2.1, Etched surfaces On etched surfaces ,uFEis constant over the entire range of field strenghts tested 5 x 10-3-106 V cm-r. One gets a pure sinusoidal modulation of the conductance in phase with the field. But the mean conductance has shifted to a value lower than the eq~librium conductance of the surface layer. This may be seen in fig. 5 where the steady response to a field at 570 Wz is shown. The sinusoidal conductance modulation takes place almost completely below the equilibrium level. How this situation is established is seen in fig, 6 which shows the first few cycles of the process. During the first positive half cycle, fig. 6a, the conductance at first rises with the field. This is while depletion conditions still prevail at the surface and II, is small, so the trapping of carriers is slow. But ~1,eventually becomes large enough to cause strong surface trapping and the conductance saturates although the field is still rising. When the field falls these electrons mostly remain trapped because of the slow rate of emission from the states. Thus the ac field causes a bias in the mean surface barrier towards further depletion. The greater part of the effect takes place in the first positive half cycle and the conductance
SURFACE
modulation
is then reasonably
PROPERTIES
OF n-TYPE
sinusoidal.
43
GaAS
If the field starts
on a negative
half cycle, fig. 6b, the first half cycle of conductance is perfectly sinusoidal about the equilibrium value as zero, and biassing does not take place until the field changes over to positive. This is because the negative field can only cause a very slow emission of electrons from the surface traps and nearly Equilibrium luvel
-2oot 0.5
n-typo
Fig. 5.
GaAs.
millisecsldiv
570Hz.
Peak field
6.2 x105V/cm
ac Field effect on etched surface. Steady state.
all the induced charge is in the space charge layer. In figs. 7a and 7b the envelope of the sinusoidal conductance train and the subsequent decay of conductance back to equilibrium with a time constant of about 4 set are shown. It is seen in fig. 7a that the true steady state is only reached after many cycles of the field. Although very slow traps are also present on this surface as mentioned in Q 5.1 they do not play any part in the processes illustrated in figs. 6 and 7. This is because the total duration of departure from equilibrium is only 5 set, which is very short compared with the relaxation time of the slow traps. 5.2.2.
SiO, covered surfaces
Typical response of an SiO, covered surface to an ac field is shown in figs. 8a and 8b. Although self biassing still takes place, the steady state conductance is no longer purely sinusoidal, but shows distortion in the region of positive peak field. This is because of the much faster relaxation of the
1. FLINN
-80
F-iaId
-
012345677 Time (mlllisecs) (a)
First
I
I
Ycyclo
positive
-80 -160
.iald
-
I
I
01
(b)
n-typo
F:ig. 6.
I
I
I
I
I
2345678
GaAs.
Tima
(millisacs)
First
T cyc 10 nagat
570Hz.
PQak
flQld
iva
6.2x105V/cm
ac Field effect. Transient behaviour of etched surface.
_
SURFACE
PROPERTIES
OF II-TYPE
45
GRAS
-80 -160
F iold-
I
I
0
50
I
I
100 Time(millisocs)
I
150
200
(a)
0 -80 - 160
r t
F iold -
012345678 Time (sets)
( b) n-type
Fig. 7.
GaAs.
570Hz.
Peak field
6.2x105V/cm
ac Field effect. Transient behaviour of etched surface.
c
1. FLiNN
46
0 100
200 ~
I
t
I
I
I
1
I
I
t
0
1
2
3
4
5
6
7
8
-
TimQ (m!liisQcs) (a)
I
I
First
I
+cyclQ
I
I
pOS!tivQ
I
I
I
I
012345678 Time
( b) n-typo
Fig, 8,
GaAs.
First
tmiliisocs) t
cyclQ
570Hz.
Paak
negative
field
9.2 x lO’v/Cm
ac Field effect. Transient behaviour OFSiOa covered surface.
_
SURFACE
PROPERTIES
OF II-TYPE
GRAS
47
surface traps, which is also seen when the ac field is switched
off, fig. 9. The
mean steady state ac field effect mobility is however still high, usually being greater than 1000 cm’/V sec. The first negative half cycle, fig. 8b, is still equivalent, at this frequency, to a very similar pFE to the values for free etched surfaces. The somewhat lower mean steady state value of pFE is mainly associated with the distorted positive half cycles.
I
I
0
50
n-typa
Fig. 9.
GaAs.
I
I
100 T i ma ( millrsacs) 570Hz.
Paak
150
fiald 9.2 x105V/cm
ac Field effect. Transient behaviour of SiOz covered surface.
To summarise, the behaviour of the SiO, covered surface may be qualitatively explained by assuming that the emission rate U, of electrons from surface traps is much higher than on etched surfaces although still small compared with average rate at which charge is induced by the field.
The increase in U, is due to n, being several orders of magnitude the oxide covered surface which has a lower barrier height. 5.3. MAGNITUDE
higher on
OF THE ac FIELD EFFECT MOBILITY
The ac field effect mobility is considered to be a “depletion” mobility. The conductance is observed between the ohmic contacts of the filament and it varies because the depth of the highly resistive depletion layer is varied by the field. Since the surface traps are apparently very inactive once the initial biassing has taken place, it might be thought that the observed steady state
1.FLINN
48
ac field effect mobility should equal the bulk electron mobility pg. In fact it is rarely greater than half ps. It is very unlikely that this is due to another set of much faster surface traps. The effect is quite constant up to the highest field strengths that were applied and one would have to postulate a very special set of surface traps to achieve this. It seems that the discrepancy must be attributed to a uniform distribution of trapping levels throughout the bulk of the crystal. If these are situated far enough below the bulk Fermi level to be fairly fully occupied in the bulk, but high enough to be empty in the major part of the depletion layer, and if their relaxation time is shorter than about one microsecond, they explain the behaviour very well. The extension of the depletion layer then involves not only the rejection of mobile majority carriers but also the emptying of bulk traps. This is illustrated diagrammatically in fig. 10. Thus a comparison of the field effect mobility and the bulk mobility indicates that there are active deep lying bulk electron traps present with a density of the order of the bulk density of free carriers (2-3 x lOi cmU3). G(1As crystal Bulk traps empty
\
‘\
traps contain electrons
/Bulk
.--_--_---_
Ei
-7 Movement
of depletion layer boundary causes filling or emptying of traps
Surface Fig. 10.
Energy level diagram showing behaviour of bulk traps in surface space charge layer.
5.4. VARIATION OF ac FIELD EFFECT WITH FREQUENCY The ac measurements described so far were carried out at 570 Hz. It is of interest to see how the field effect mobility varies with frequency. So measurements were made over the range 100 Hz-100 kHz, with small peak fields of about lo4 V cm-‘, using the technique first described by Aigrain et a1.10~lr). Again the main distinction appears to be between the etched surface and the SiO, covered surface. Typical curves are shown in fig. 11. On etched surfaces the field effect mobility isessentially constant throughout
SURFACE
PROPERTIES
OF II-TYPE
49
GaAS
the whole range. This again shows how the effect of the ac field is to render the surface traps almost completely inactive already at 100 Hz. On Si02 covered surfaces, the mobility tends to rise slowly with frequency. This result is in accord with the much faster relaxation in conductance observed with step fields. It is thought that this effect is saturating so that the conductance modulation must be very close to sinusoidal above about 40 kHz. That is, the effective relaxation time of the surface traps is greater than about 4 ,usec at this field amplitude.
Curvo 1. Etched sur
10
1 Field
n-typo Fig. 11.
fraquoncy
100
f kHz)
GaAs. Paak fiald lO%/cm
ac Field effect mobility vs. frequency.
f%L%h’bNORITY CARRIER EFFECTS
So far it has been assumed that minority carriers do not play a significant role. There are two aspects to this assumption: (1) Even if the electric field is of sufficient strength to induce an inversion layer, the generation time for minority carriers is assumed to be so long that they do not appear in the duration of the particular experiment. (2) It is assumed that inversion of the surface does not occur at equilibrium. These assumptions will be examined below. 5.5.1. Possibility of inducing an inversion layer During the initial period before the surface traps have time to react, the peak negative dc step fields, usually at least 6 x 10’ V/cm, would be more
50
I. FLINN
than adequate to induce an inversion layer. However the changes of conductance observed indicate that one is not formed. As described earlier the conductance changes are due to an extension of the depletion region into the bulk and the initial change corresponds to an extension of at least 0.5 pm. If this is assumed to be due to a large non-equilibrium depletion layer then the charge involved corresponds closely to that induced. But if an inversion layer is present the charge involved is impossibly high. On all the surfaces examined these non-equilibrium depletion layers could be maintained for at least a second. Thus it appears, that for experimental situations which are of shorter duration than this, significant numbers of holes are not generated. The decay of the non-equilibrium depletion layer after the application of a negative dc step field has a time constant much longer than one second (fig. 4). This decay is probably almost entirely due to emission of electrons into the conduction band from surface traps. The generation rate for minority carriers is thought to be very low indeed. Berglundi2) has shown that there may be significant generation of minority carriers by tunnelling between the energy bands in the space charge region. But the electric field in the surface is not high enough in the present case for this to be a very significant process. With the present experimental method, it would be impossible to detect conductance in an inversion layer if one were induced on the surface by a dc field. A M.O.S.T. type structure would be necessary for this. It is of interest that other workersis) who have tried this, have not obtained significant conductance between source and drain without building in a channel by diffusion or epitaxy. 5.5.2. Possibility A phenomenon,
of an inversion layer existing which could
at equilibrium
be called “excess self biassing”,
was some-
times observed in the ac field effect. This seems to indicate that some of the etched surfaces examined did have inversion layers on them at equilibrium. Consider the simple model of a very high density of surface traps and, normally, a depletion layer at the surface. On the simple one carrier model, the surface should bias itself under the ac field, until the positive peaks of field just took the surface above the equilibrium conductance, as in fig. 5. It is difficult to conceive surface traps which would cause the biassing to exceed this, if the model is correct. However, on a number of the etched surfaces, the biassing was indeed so large that the conductance never rose to the equilibrium value, even at the positive peaks of the field. This is seen in its most extreme form in fig. 12. In fig. 12a it is seen that after about three cycles, the biassing is already excessive. Fig. 12b shows the envelope of the same sinusoidal train. In the steady state under the ac field the conductance is always at least 230 pmho/sq.
SURFACE
PROPERTIES
OF n-TYPE
GRAS
-1
-_ 2
I 1 012345678
I
I
I
I
I
I
I
I
I
c
Tima (milhsacs) (a)
t
6
-100
-
t; 4 $ -$
-2oo-
t;P E $. ‘0 z L
-3oo-
-400
I I 012345678
I
I
I
I
Tima
(sacs)
I
(b) n-typa
Fig.
12.
GoAs.
570Hz.
Paak
flald
0.72
x 106V/cm
ac Field effect. Transient behaviour showing excess biassing.
)
52
I.
FLlNN
less than the equilibrium level. This surface had been etched in a manner involving silver nitrate and potassium dichromate l4). It also gave the highest peak photovoltage recorded and the shape of the photovoltage-photoconductance curve indicated that the total barrier height may be 0.7 V (28 KZ”),which would mean that an inversion layer is present. The excess biassing might then be explained in terms of the annihilation of the holes in the space charge layer. On positive half cycles of field, their numbers would be excessive and they would tend to recombine. There would not be time for them to regenerate on the negative half cycles, so they would be progressively pumped away. This process might extend outside the region of the field electrode, which covers less than one third of the effective surface of the GaAs filament. Holes would tend to flow due to fringe fields, along the surface and under the field electrode, when there is a large depletion layer; and they would then recombine, when the depletion region collapses, under the influence of the positive field. 5.6.
COMPARISON
OF
ac
FIELD EFFECT
WITH
THEORY
The very simple model of the surface which is used above seems to account qualitatively for the way the gallium arsenide surface responds to ac fields. But it is of interest to probe it more quantitatively. Berz has developed the theoryl5) for the one carrier case. The computed solution of the differential equation has been used to test the model for the two types of behaviour typified by the etched and SiO, covered surfaces described above. For computation, values of the various surface parameters have to be taken. Also the presence of bulk traps must be taken into account as these influence the surface space charge layer in just the same way as donor atoms. These parameters have all been deduced from the surface photovoltage and the dc field effect. The dc field effect measurements at various field amplitudes can only be explained by assuming that the surface traps are in fact distributed in energy near the Fermi level. But the theory is already complicated enough if a set of traps all situated at one discrete energy is taken. So an approximate fit to the experimental data is obtained with such a discrete set. The actual amount of induced charge that is trapped may be determined precisely from the field effect relaxation. To estimate the effective trap density and energy level, a value of the surface barrier height must be taken. The rate of emission U, of the traps at equilibrium (eq. (5)) may also be determined precisely. It is given by the initial rate of decay of conductance when a field is applied so as to increase depletion at the surface. But to deduce the capture cross section CT,one again needs to know the surface barrier. This is not known with great precision (9 4.3, but the most Iikely values are used.
SURFACE
The behaviour,
computed
PROPERTIES
from
OF ll-TYPE
53
GaAS
the full equationsi5),
is shown
etched surface in fig. 13, and for the silicon oxide covered
for the
surface in fig. 14.
The capture rate is normalised as a fraction of the peak field trapping rate if all the charge were trapped. The values of the various surface parameters are indicated on the figures. In fig. 13c, which should be compared with fig. 6, the truncation of the first positive half cycle is seen and this corresponds with heavy trapping. At one point (fig. 13d) the trapping rate is 0.94 of maximum. But when the field goes negative there is negligible emission from the traps. Thereafter the conductance modulation is reasonably sinusoidal and the trapping rate reaches a maximum of only - 0.02 on the second cycle and is progressively reduced until it is only 0.003 in the steady state which can be compared with fig. 5. The constant low rate of emission by the traps FSteady
state
4
+ (a) 0
Field E$m
wt %
Field effect con&c tance’O0 AG (@4ho/sq) -200
I
_-_ 0
I 11
3n
2n
4lT
5Il
J 6n
(12 0
wt
Peak
Surface chmxtemtlcs
NB(bulk
Fig.13.
field
trap
ac Field effect. Computed
E,=
5.6
density)
x 105V/cm
t =570Hz
= 2~4 X10’5c~-3
behaviour of etched surface.
Tr
2n
54
I.
FLlNN
over more than 800/, of the cycle is also seen in fig. 13d. It is to be noted in fig. 13b, that at no point is the field sufficient quite to flatten the surface barrier. As the barrier tends to flatten, ~1,approaches the bulk carrier density -3 x 1015 cmF3 at which level the surface traps are very efficient in taking up induced charge. In fig. 14 the behaviour of the SiOz covered surface is simulated and fig. 14~ should be compared with fig. 8. The second cycle which is identical with the steady state shows distortion in the positive peaks of field where the trapping rate is 0.48. This is because the emission rate at -0.05 is an order higher over the rest of the cycle than in the case of the etched surface. If it were not for the presence of bulk traps, the barrier heights and field effect condu~tances achieved would be approximately twice as high as the values seen in figs. 13 and 14.
t (a) Frld E,sn wt
Surface chamcterMcs
OI
-v
.$ ETb
Fig.14.
-.2lV EC E,
Peak held E, = 5.6 x105V/cm
f = 570t-tY no=3.5x10’5cm‘3
0,=1.0x10+%m2 N =6.6x101’cm+ T NB(bulktmp density)= 3.8xfO’5cm-3
ac Field effect. Computed
behaviour of SiOz covered surface.
SURFACE
PROPERTIES
OF II-TYPE
GaAS
55
The computed behaviour shows quite good agreement with experiment but there are minor differences. For instance it takes many more cycles to achieve the steady state in practice as may be seen in figs. 7a and 9. Also the distortion of the positive peaks is not quite the same and nor is the degree of self biassing. Three possible reasons for these discrepancies are inaccuracy in the parameters used for computation, assumption that the traps are at one energy whereas in fact they are distributed over at least a small range of energy, and also the presence of another set of much slower traps, has been ignored (4 5.1). 6. Surface treatments It is not thought worthwhile to give details of all the surface treatments which have been studied but a short account of some of them may be of interest. The etched surface referred to in this paper was etched in 3 : 2: 1 - HNO, : H,O:HF. All etched surfaces examined had barrier heights of 0.45 V or greater with the bands bending in the sense of depletion or inversion. The total effective surface trap density was usually at least lOI2 cm-‘. But this appears to include a relatively fast species of trap at about 5 x 10” cmm2 plus another set with at least the same density. Some treatments were found to reduce to trap density. For instance washing in dilute aqueous sodium sulphide solution after the above etch reduced the total trap density to about 3 x 10” cmm2 and apparently eliminated the slower traps. This treatment has been found to work for indium antimonidelc) and is thought to act by precipitating metal ions from solution and so avoiding contamination of the surface. A sulphuric hydrogen peroxide etch 5 : 1: 1 - H,SO,: H,O, : H,O 17) also reduced surface trap density to about the same level. SiO, was deposited on a wide range of etched surfaces by decomposition of tetraethoxysilane at 400°C. It was found that the underlying etch treatment made little difference. These surfaces had depletion layers with surface barriers in the range 0.2-0.3 V and the surface trap density appeared to be about 7 x 1O’l cmP2. It is thought that the reduced barrier height may be due to fixed charge in the SiO, layer. No significant difference was detected in the behaviour of the (111) A (gallium) and B (arsenic) faces for any of the surface treatments studied. 7. Conclusions The experimental work described in this paper has allowed a model of the surface of n-type gallium arsenide to be derived. This model explains qualitatively all the phenomena which have been observed. An attempt has been
56
made to derive values
I. FLINN
for the parameters
of active surface
states,
but no
great accuracy can be claimed because of the difficulty in determining the equilibrium surface barrier height accurately. An equally good, or possibly a better fit could be obtained between theory and experiment by small adjustments of the various parameters. But this is not a very useful exercise in the absence of further experimental evidence. However the values of the various surface trap parameters found fall within the range found by other workersl*,lg). It would be difficult to improve the precision of the photovoltage technique for measuring barrier heights in gallium arsenide which always has a short minority carrier diffusion length and a large carrier mobility ratio. These two factors conspire to make the Dember potential at once significant in magnitude and difficult to estimate. It is to be noted that the traps on gallium arsenide surfaces which account for most of the phenomena observed are what are normally termed “fast” surface traps on germanium. That is to say, their cross sections are similar to those of fast traps. Their slow reaction under certain conditions, when they may take seconds to relax, is entirely due to the large band-gap of gallium arsenide. Thus the interchange of either majority or minority carriers between the traps and the bands may be very restricted under depletion conditions. Self biassing of a semiconductor surface under the ac field effect has also been found on germanium by Dordaaa), and Many and Goldstein al) have observed the same phenomenon with pulsed fields. They attribute their effect to tunneling of electrons from oxide traps into the conduction band. This has the effect of causing a net emptying of the traps under ac fields and hence the biassing is such as to reduce the surface barrier. In gallium arsenide the effect is quite the opposite, the traps are filled. No sign of the effect of tunnelling between surface traps and the conduction band has been observed on gallium arsenide although the electric fields employed are of similar strength to those used by the above workers. Possibly there are no oxide traps with appropriate parameters present on gallium arsenide surfaces. Becke and White2a) have made n-channel insulated gate field effect transistors on gallium arsenide which work in the depletion mode. They have concluded that surface traps are present at the semiconductor interface with densities of the order of 10” cm- ‘. It seems that the field effect phenomena which have been described above must also occur in their devices. In particular, it is to be expected from the present model that the transconductance would be small under dc conditions and rise at some low frequency to a useful level as the surface traps are made relatively inactive by self biassing. At a still higher frequency the bulk traps will become inactive and a further rise in trans-conductance should be observed. Becke and White observe such variations in trans-conductance. Under transient con-
SURFACE
PROP5RTIES
OF II-TYPE
GaAS
57
ditions the trans-conductance will exhibit “memory” effects, in that its instantaneous value will depend on the nature of the signal which has previously been applied to the gate. Owing to the ac biassing effect the device should also work for high frequency ac signals without any dc bias being necessary on the gate. Acknowledgements I should like to thank Dr. F. Berz for many helpful discussions. I am also indebted to Mr. M. J. Briggs who designed some of the experimental equipment and to Mrs. 8. J. Rudd for assistance with the computations. This work was supported by the Ministry of Defence. References 1) 2) 3) 4) 5) 6) 7) 8) 9)
10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)
I. Flinn and M. J. Briggs, Surface Sci. 2 (1964) 136. W. Shockley and W. T. Read, Phys. Rev. 87 (19.52) 835. C. G. B. Garrett and W. H. Brattain, Phys. Rev. 99 (I 955) 376. E. 0. Johnson, Phys. Rev. lll(1958) 153. R. Williams, J. Phys. Chem. Solids 23 (1962) 1057. T. S. Moss et al., Proc. Phys. Sot. (London) B 66 (1953) 743. N. L. Dmitruk and V. I. Lyashenko, Fiz. Tverd. Tela 8 (1966) 578. G. Rupprecht, Ann. N.Y. Acad. Sci. lOl(i963) 960. V. E. Primachenko et al., Phys. Status Solidi ll(l965) 711. P. Aigrain et al., J. Phys. Radium 13 (1952) 587. H. C. Montgomery, Phys. Rev. 106 (1952) 441. C. N. Berglund, Appl. Phys. Letters 9 (1966) 441. H. Becke and J. White, Institute of Physics and Physical Society Conference No. 3 (1966) 219. C. Hilsum, private communi~tion. F. Berz, Surface Sci. 10 (1968) 58. J. L. Davis, Surface Sci. 2 (1964) 33. R. Hall and J. P. White, Solid State Electron. 8 (1965) 211. S. Kawaji and H. C. Gatos, Surface Sci. l(l964) 407. G. Rupprecht et al., Report No. RADC-TDR-64-17(USAF), (1964). G. Dorda, Phys. Status Solidi 3 (1963) 1318. A. Many and Y. Goldstein, Surface Sci. 2 (1964) 114. H. W. Becke and J. P. White, Eiectronics 40 (1967) 82.
Series,
SCIENCE
SURFACE
10 (1968) 32--57 0 North-Holland
PROPERTIES
OF n-TYPE
Publishing
GALLIUM
Co., Amsterdam
ARSENIDE
Received 7 August 1967; revised manuscript received 13 September 1967 This paper describes an investigation of GaAs surfaces by means of field effect, surface photovoltage and photoconductance measurements. A wide variety of different etch treatments and also the effect of depositing SiOz on the surface have been investigated. It is found that the surface barrier always bends towards depletion or inversion. On some etched surfaces the total effective surface trap density is at least IOra cm-a but some treatments were found to reduce this to about 3 x IOr cm-e, The effective surface trap density of SiOz covered surfaces was 7 x lOI1 cm-2 and was not sensitive to the etch treatment of the underlying surface. Under ac fields the mean surface barrier height becomes biased in the direction of further depletion due to an increase in the mean occupancy of the traps. The ac field effect mobility is of the order of 2000 cm2 V-r see-r despite the large surface trap densities. It is also deduced from the ac field effect that fast bulk traps with densities of about 10’5 cm-3 are active in the surface space charge layer. A model of the surface is given whith explains the observed phenomena.
1. Introduction The main aim of the work described in this paper has been to investigate the surface field effect on gallium arsenide. The interpretation’) of the field effect is not so simple with gallium arsenide as with silicon or germanium, primarily because of the larger energy band gap and the consequent very low intrinsic carrier density. In particular it does not seem to be possible to deduce the surface potential from field effect measurements. But for any detailed understanding of surface behaviour it is necessary to know the surface potential with some precision. For this reason the surface photovoltage has also been studied to gain additional information. To explore surface behaviour fully one needs to vary the surface potential between wide limits. On the first gallium arsenide surfaces examined, high densities of surface traps were found which tend to lock the surface potential. A wide range of surface treatments has therefore been studied with the aim of minimising trap densities or achieving a range of fixed surface potentials. In general there were only three types of behaviour that may be clearly distinguished. These appear to be associated with the presence of weak depfetion layers, strong depletion layers and inversion layers, and trap densities are always rather high. Chemically etched surfaces 32
SURFACE
tend to have strong depletion
PROPERTIES
OF n-TYPE
GRAS
layers, the surface being nearly intrinsic.
33
If an
SiO, film is deposited on such an etched surface by decomposition of tetraethoxysilane then the barrier height is reduced giving a weaker depletion layer. The main part of this paper will discuss a typical etched surface and a typical SiO, covered surface. An etched surface which appeared to have an inversion layer will also be discussed and some mention will be made of the effects of various surface treatments.
2. Experimental
technique
The measurements described here were made on n-type gallium arsenide with a mobile electron density of 1.8-3.5 x 1Or’ cmW3 and an electron mobility of 4300 cm’/V sec. Test filaments took the form of thin plates 11 mm x 3 mm cut parallel to the 111 plane with alloyed ohmic contacts at the ends. At least 100 pm of material was removed by etching from each surface after the initial preparation, to eliminate mechanical damage. The filaments were reduced to a final thickness of 40-80 pm. For measurement, the filaments were mounted in a holder which pressed transparent electrodes of conducting glass and 25 pm polyethylene film dielectric spacers against the two major faces. These electrodes could be used either for applying fields of up to IO6 V/cm to the surface, or as capacitive surface voltage probes when connected to a high impedance electrometer. Measurements were made in a vacuum of 10m4 Torr at a controlled temperature of 30 “C +O.O5”C For the photovoltage measurements, single pulses of light one millisecond long from a high intensity xenon arc were shone on the surface through the transparent electrode. The maximum number of useful photons arriving at the gallium arsenide surface during the pulse was estimated to be about 3 x 1019 set-l cm- ‘. The photoconductance was used as a measure of injection level, and the intensity of the light was reduced over about seven orders of magnitude by introducing neutral density filters in the beam.
3. Model of gallium arsenide surface It is thought that a short discussion of the model of the surface at this stage may help in understanding the photovoltage and field effect data which will be presented later. The model which seems to fit the experimental facts best is that there is normally a depletion layer at the surface and that there are densities of active surface traps in the range 10”-10’2 cm-’ situated near the Fermi level. The surface behaviour seems to be fairly similar for both n and p-type material ‘),
34
I. FLINN
but only n-type will be considered in what follows, as nearly all the measurements were made on this. In cases where the surface is not inverted there can be no significant number of minority carriers at equilibrium in the whole filament. Furthermore the generation rate must be very small. Thus we assume that minority carriers do not play any part in field effect processes under these conditions (see Q5.5.1 for justification. The case when an inversion layer is present will be discussed in 5 5.5.2). Also, in the absence of minority carrier injection, only majority carriers are involved in the kinetics of the surface traps. The net rate of capture K, by such a set of traps2) is given by the difference between the total rate of capture UC and the total rate of emission U, K,=IJ,--IJ,,
(1)
i.e. I(, =
45”%(1
-fJ
n,
-
~~~~~~~~~
1
(21
where G, is the capture cross section of a trap, ot is the mean thermal velocity of the electronsf, is the fraction of the traps which are occupied, n, is the carrier density at the surface. n, = NCexp [(E,-&)/!cI~“], where E, and EC are the energy levels of the trap and the conduction band edge, and NC is the effective density of states in the conduction band. If the surface is heavily depleted as in the present cases, n, is small and near equilibrium, when K”zO, both the capture and emission of electrons must be relatively slow. This gives a simple picture of the surface model. The analysis of the experimental data will allow a certain amount of detail to be filled in. 4. Surface photovoltage There appear to be at least three causes for the photovoltagea,J) which is picked up by a capacitive probe when light is shone on a semiconductor surface thereby injecting hole electron pairs. (a) Even if there is no change in the occupancy of traps and therefore no change in the net charge contained in the space charge layer, the barrier height falls owing to a reduction in the effective thickness of the barrier region. It can be shown that to achieve flattening of the surface barrier to within 1 kT by this process the injection level An at the boundary between the surface layer and the bulk is given by:
where 11,and pS are the equilibrium surface carrier densities, ~1~and pO are
SURFACE
PROPERTIES
OF II-TYPE
35
GaAS
the bulk densities and Ye is the equilibrium surface barrier height in units of kT/q. It is presumed in eq. (3) that the quasi Fermi levels are substantially flat across the barrier. (b) If there are active traps at the surface, there will in general be a change in their occupancy and the surface barrier may change for this reason also. This could in principle cause either an increase or a flattening of the surface barrier. An increase of barrier height under illumination is very unlikely with the large surface barriers found on GaAs. Williams5) has observed a flattening of the barrier by this mechanism in CdS. (c) There is also another mechanism which may bring about a change of potential with injection. This is the Dember effects), and it is due to the difference in diffusion coefficients of holes and electrons which causes a voltage to be set up across a region where there is a gradient in the nonequilibrium carrier density. The magnitude of the Dember potential Vn may be expressed in the form: I/n = kT b-’ q b+l
log
1 + F+! (
.;2 0
)
(4)
>
where b is the ratio of the diffusion coefficients. This result is strictly only valid in a semiconductor which is free of space charge and has no traps. It would be difficult if not impossible to assess the relative importance of processes (a) and (b) at a given injection level without prior knowledge of the relevant surface parameters. But it does seem that at sufficiently high injection the surface barrier must always eventually become virtually flat. Thus the highest possible injection has been aimed at in the present work. Owing to the very short diffusion length in gallium arsenide, non-penetrating radiation must be used. (Suitably intense sources of penetrating radiation would cause excessive heating of the specimen.) It is then difficult to determine an effective injection level because both surface and bulk recombination are themselves functions of injection level. Since injection is not uniform near the surface the quasi Fermi levels cannot be assumed flat. Thus eq. (3) is not valid and, even if it were, it would be difficult to apply as a criterion for barrier flattening. The Dember potential complicates matters further. Its magnitude is expected to become really important at injection levels where the surface barrier would otherwise be effectively flattened. The relevant injection level for calculating I’, is then the level at the surface, An,. The diffusion length is small and unknown and the surface recombination velocity may be high. So V, is not easily deduced.
1. FLINN
36
(0)
PhotovoltagQ Light Intansity-5x10’2
(b)
photons soc-‘~m-~
Photovoltage Light intansity1013 photons
sac-‘cm -2
0.1
CC) Photovoltage
0.2
Light intensity s 9
_
10’8 photons SC-‘cm
0.3
-2
0.4 0.5
z c 0 Z5 5
300 (d)
200
Photoconductance Light intoosity=
25 o,e
‘oJ8 photons Eo-
100
I
1
0
’
I
I
-2
3
1
4
I
1
1
5 6 7 Time ~millisocs)
w Dumtion of light pulse n-type
Fig. 1.
GaAs(otchod
Photovoltage
surface)
and photoconductance
response.
sac-1cm12
SURFACE
PROPERTIES
OF n-TYPE
GRAS
37
In view of the complexity of the situation it is not possible at present to make a definitive estimate of the surface barrier height from the surface photovoltage. But an interpretation can be put forward, and the estimate of barrier height so deduced seems the more reasonable because it also is compatible with the field effect data. 4.1. EXPERIMENTAL RESULTS Typical photovoltage and photoconductance response is shown in fig. 1. The photovoltage is positive on this n-type material indicating that the potential of the surface goes more positive on illumination. Figs. la, b and c show how the photovoltage varies as light intensity increases and fig. Id shows the photoconductance for the highest intensity. At low intensities the steady state is not reached within the 1 msec that the light is on. There is also a long relaxation time after the light pulse, greater than 10 msec, which must be associated with trapping of photocarriers. This slow process becomes ineffective at higher intensities, when the relaxation time is too rapid to be measured. Typical logarithmic plots of photoconductance versus light intensity are given in fig. 2 for a free etched surface and an SiO, covered surface. Photoconductance is calculated as a surface component in micromhos per square because nearly all the photocarriers are assumed to be in a thin layer near the surface. For high injection levels, the relation Gee (Zr is usually obeyed, where G is the peak photoconductance and Z is the intensity of illumination. There is usually departure from this relation at low intensities where trapping is strong. The injection level rises only slowly with light intensity, the exponent y1being usually in the range 0.3-0.4. When the surface photovoltage is plotted against log of photoconductance, fig. 3, it usually turns out that there is an inflexion in the curves at low injection levels. But above a photoconductance of about 100 pmho/sq. the curve rises linearly with no sign of saturation. The surface photovoltage usually increases by between 0.09 and 0.18 V per decade of photoconductance in this linear range. 4.2. INTERPRETATIONOF PHOTOVOLTAGE At least for the lower injection levels a change in the occupancy of the surface traps (process (b) of 0 4) is thought to account for most of the photovoltage observed. This is deduced from the slow relaxation of the photovoltage after the light pulse, which is due to the slow return of the traps to equilibrium. When an electron-hole pair is created near the surface by light, the hole is swept towards the surface by the field in the space charge layer and the electron is swept towards the bulk. The surface traps which have
38
1. FLINN
been detected by the field effect (9: 5) are thought to be negatively charged when occupied by an electron. They thus have a large capture probability for holes. So when the surface is illuminated the electrons tend to empty out of the surface traps into the valence band. Under weak illumination the capture rate for electrons from the conduction band remains low because mobile electrons are swept away from the surface. Thus the net charge in the traps falls and consequently the charge in the space charge layer also falls. It is the resulting fall in surface barrier height which gives the observed photovoltage at low light intensities. The slow relaxation of the traps when the light pulse goes off, figs. 1a and 1b, is to be expected since they can only be refilled from the conduction band and this is depleted of electrons at the surface. (The re-emission of a hole from the trap to the valence band requires a large increase in its energy and is very improbable.) But as the light intensity increases, the emptying of the traps is also increased with the result that the surface potential barrier falls further, and the density offree electrons at the surface increases. Thus the electron capture rate will increase and at
10
000 :
I
I
n-type GaAs Curve I. Etched w-face Curve 2. Si 0, covered surface
I
1 -6
I
I -5
-4 Loglo
Fig. 2.
relative
Photoconductance
-3 -2 light intensity
vs. light intensity.
-1
SURFACE
PROPERTIES
OF R-TYPE
GRAS
39
higher light intensities the relaxation of the photovoltage becomes much more rapid, fig. lc. In this range the total hole and electron density near the surface must also increase significantly due to the light and this gives an additional flattening of the surface barrier by process (a), 0 4. The increased rate of capture of electrons is equivalent to an increase in the efficiency of the traps as surface recombination centres, as the injection level rises. So it is to be expected that the injection level will be less than proportional to light intensity. The maximum photoconductance normaily observed is about 1500 pmhojsq. This corresponds to at least 2 x lOi cm-’ injected carriers. If it is assumed that these nearly all exist in a layer about one micron deep, the mean injection level in this layer is 2 x lOi cmW3. Since processes (a) and (b) are both operating in the sense of flattening the barrier, the injection is almost certainly adequate for complete flattening of the surface barrier. But a Dember type voltage, process (c), must also appear. In GaAs the mobility ratio is IO: 1 and so the Dember potential is expected to be significant. The Dember potential should increase by 40-50 mV per decade of injection (eq. (4)). The relevant injection level is however not some mean value but that at the surface. It is difficult to determine this. The slow increase of photoconductance with light intensity implies that the effective lifetime and 0.6
I
n-typQ GoAs Curve Curve
1. Etched surface 2.Si02 covered surface
0.5
2 0 0.4 2
__j____-.-
E r g 0.3 5 a
-L--_--
$j 0.2 t 2 0.1
ri-
h
CI_
1c
Surface Fig.
3.
Surface
photoconductance
photovoltage
CpMholsq)
vs. photoconductance.
4.0
diffusion
I. FLINN
length
are decreasing
as the injection
level rises. Thus
even the
mean injection level is not proportional to the photoconductance but must in effect increase more rapidly. The presence of surface recombination as well as bulk recombination makes matters even more complicated. However, it seems that the injection level at the surface is unlikely to be rising more rapidly than the light intensity; but it must on the other hand be rising more rapidly than the total injection which is proportional to the photoconductante. At high intensities the photovoltage increase by 25-43 mV per decade of light intensity and by 90-I 80 mV per decade of photoconductance. Thus it is reasonable to ascribe this steady increase of photovoltage at high intensities to a Dember potential. The part of the curves in fig. 3 below a photoconductance of N 100 pmho/sq. could then be interpreted as showing signs of saturation in the barrier flattening. In this case, the barrier height of the surface depletion layers could be deduced as about 0.45 V for the etched surface and 0.2 V for the SiOZ covered surface. This is indicated on the figure by the dotted lines. Dmitruk and Lyashenko?) have measured barrier heights on gallium arsenide by a contact potential method and their values are mostly in good agreement with ours on similarly prepared surfaces. 5. The field effect The response of the gallium arsenide surface to electric fields can be explained in terms of the model of g 3. With a positive field, electrons are drawn to the surface. The carrier density n, increases rapidly and UC, the rate of capture of electrons by the traps, increases in proportion. But if a negative field is applied, the electron density n,, which was already small, is reduced effectively to zero. So the surface traps do not capture electrons and there is only a low rate of emission by them into a deep non-equilibrium depletion layer. This latter situation was first exploited by Rupprechta) in his pulsed field effect technique. Primachenko et alg) have also studied such depletion layers on silicon and have managed to completeIy deplete their specimens
of carriers
with high fields.
5.1. dc STEP FIELDS The response of an etched surface to step fields is shown in fig. 4. For a positive field, fig. 4a, there is a small increase in conductance. The peak value, which may be expressed as a held effect mobility*, is equivalent to * The field effect mobility ,CLFE is defined as AG/AQ, where dG is the change in conductance per square brought about by a corresponding charge of AQ. In the case of ac fields AC and #Q have been taken as the peak to peak values.
SURFACE
about
PROPERTIES
one tenth of the bulk electron
decay of conductance,
OF II-TYPE
mobility
which is complete
41
GBAS
ps. This is followed
in about
ten seconds,
by a rapid
and a slower
decay which takes some minutes. The final conductance does not differ sensibly from the initial value. It is concluded that most of the induced charge is trapped too rapidly to be observed. One only sees the small residual conductance change equivalent to a small increase in n, when the rate of capture by the traps has become modest. For a negative step field (fig. 4b) the initial fall of conductance is much larger. It corresponds to the formation t v4
5
L-
field
on
I
I
I
0
I
I
IO limo
-va
field
I
20
I
30
---k--
(~4~s.)
on
I
I
I
I
0
I
20 limo
WtypfZ
GaAs.
Fig. 4.
Peak
1
I
10
field
I
30 (sacs) 4.6
x 105V/cm
dc Step effect on etched surface.
I
I
40
-
42
I. FUNN
of a large depletion region when electrons are repelled from the surface. The peak value is equivalent to about half the bulk mobility (on the simple model one would expect equality with @a, this will be discussed in $5.3). The surface traps can only respond to the field by a slow emission of electrons and again there is a decay back to the equilibrium conductance. As with the positive field there is a “fast” process taking about 20 set and the slow process, which takes about 10 min, does not show at all in fig. 4b. In this case the density of fast traps is estimated to be about 5 x 10” cm-’ and there must be at least an equal density of active slow traps. On surfaces covered with SiOz which had a lower surface barrier height as indicated by the photovoltage, the fast traps were present in densities of the order of 7 x 10” cme2 and the slow traps apparently absent. The decay of the field effect conductance was faster on the oxide covered surfaces. This is due to the larger value of n, (eq. (2)) associated with the smaller barrier height. 5.2. ac
FIELDS
It is of interest to know how these surfaces will react to large sinusoidal fields. Measurements at frequencies above 50 Hz show that the ac field effect mobility pFE may be as much as 2000 cm -2 V-’ set-‘, that is, about half its the bulk electron mobility. There is a significant difference between the behaviour of the etched surfaces with large barrier height and the SiO, covered surfaces with smaller barrier height, and these will be discussed separately.
5.2.1, Etched surfaces On etched surfaces ,uFEis constant over the entire range of field strenghts tested 5 x 10-3-106 V cm-r. One gets a pure sinusoidal modulation of the conductance in phase with the field. But the mean conductance has shifted to a value lower than the eq~librium conductance of the surface layer. This may be seen in fig. 5 where the steady response to a field at 570 Wz is shown. The sinusoidal conductance modulation takes place almost completely below the equilibrium level. How this situation is established is seen in fig, 6 which shows the first few cycles of the process. During the first positive half cycle, fig. 6a, the conductance at first rises with the field. This is while depletion conditions still prevail at the surface and II, is small, so the trapping of carriers is slow. But ~1,eventually becomes large enough to cause strong surface trapping and the conductance saturates although the field is still rising. When the field falls these electrons mostly remain trapped because of the slow rate of emission from the states. Thus the ac field causes a bias in the mean surface barrier towards further depletion. The greater part of the effect takes place in the first positive half cycle and the conductance
SURFACE
modulation
is then reasonably
PROPERTIES
OF n-TYPE
sinusoidal.
43
GaAS
If the field starts
on a negative
half cycle, fig. 6b, the first half cycle of conductance is perfectly sinusoidal about the equilibrium value as zero, and biassing does not take place until the field changes over to positive. This is because the negative field can only cause a very slow emission of electrons from the surface traps and nearly Equilibrium luvel
-2oot 0.5
n-typo
Fig. 5.
GaAs.
millisecsldiv
570Hz.
Peak field
6.2 x105V/cm
ac Field effect on etched surface. Steady state.
all the induced charge is in the space charge layer. In figs. 7a and 7b the envelope of the sinusoidal conductance train and the subsequent decay of conductance back to equilibrium with a time constant of about 4 set are shown. It is seen in fig. 7a that the true steady state is only reached after many cycles of the field. Although very slow traps are also present on this surface as mentioned in Q 5.1 they do not play any part in the processes illustrated in figs. 6 and 7. This is because the total duration of departure from equilibrium is only 5 set, which is very short compared with the relaxation time of the slow traps. 5.2.2.
SiO, covered surfaces
Typical response of an SiO, covered surface to an ac field is shown in figs. 8a and 8b. Although self biassing still takes place, the steady state conductance is no longer purely sinusoidal, but shows distortion in the region of positive peak field. This is because of the much faster relaxation of the
1. FLINN
-80
F-iaId
-
012345677 Time (mlllisecs) (a)
First
I
I
Ycyclo
positive
-80 -160
.iald
-
I
I
01
(b)
n-typo
F:ig. 6.
I
I
I
I
I
2345678
GaAs.
Tima
(millisacs)
First
T cyc 10 nagat
570Hz.
PQak
flQld
iva
6.2x105V/cm
ac Field effect. Transient behaviour of etched surface.
_
SURFACE
PROPERTIES
OF II-TYPE
45
GRAS
-80 -160
F iold-
I
I
0
50
I
I
100 Time(millisocs)
I
150
200
(a)
0 -80 - 160
r t
F iold -
012345678 Time (sets)
( b) n-type
Fig. 7.
GaAs.
570Hz.
Peak field
6.2x105V/cm
ac Field effect. Transient behaviour of etched surface.
c
1. FLiNN
46
0 100
200 ~
I
t
I
I
I
1
I
I
t
0
1
2
3
4
5
6
7
8
-
TimQ (m!liisQcs) (a)
I
I
First
I
+cyclQ
I
I
pOS!tivQ
I
I
I
I
012345678 Time
( b) n-typo
Fig, 8,
GaAs.
First
tmiliisocs) t
cyclQ
570Hz.
Paak
negative
field
9.2 x lO’v/Cm
ac Field effect. Transient behaviour OFSiOa covered surface.
_
SURFACE
PROPERTIES
OF II-TYPE
GRAS
47
surface traps, which is also seen when the ac field is switched
off, fig. 9. The
mean steady state ac field effect mobility is however still high, usually being greater than 1000 cm’/V sec. The first negative half cycle, fig. 8b, is still equivalent, at this frequency, to a very similar pFE to the values for free etched surfaces. The somewhat lower mean steady state value of pFE is mainly associated with the distorted positive half cycles.
I
I
0
50
n-typa
Fig. 9.
GaAs.
I
I
100 T i ma ( millrsacs) 570Hz.
Paak
150
fiald 9.2 x105V/cm
ac Field effect. Transient behaviour of SiOz covered surface.
To summarise, the behaviour of the SiO, covered surface may be qualitatively explained by assuming that the emission rate U, of electrons from surface traps is much higher than on etched surfaces although still small compared with average rate at which charge is induced by the field.
The increase in U, is due to n, being several orders of magnitude the oxide covered surface which has a lower barrier height. 5.3. MAGNITUDE
higher on
OF THE ac FIELD EFFECT MOBILITY
The ac field effect mobility is considered to be a “depletion” mobility. The conductance is observed between the ohmic contacts of the filament and it varies because the depth of the highly resistive depletion layer is varied by the field. Since the surface traps are apparently very inactive once the initial biassing has taken place, it might be thought that the observed steady state
1.FLINN
48
ac field effect mobility should equal the bulk electron mobility pg. In fact it is rarely greater than half ps. It is very unlikely that this is due to another set of much faster surface traps. The effect is quite constant up to the highest field strengths that were applied and one would have to postulate a very special set of surface traps to achieve this. It seems that the discrepancy must be attributed to a uniform distribution of trapping levels throughout the bulk of the crystal. If these are situated far enough below the bulk Fermi level to be fairly fully occupied in the bulk, but high enough to be empty in the major part of the depletion layer, and if their relaxation time is shorter than about one microsecond, they explain the behaviour very well. The extension of the depletion layer then involves not only the rejection of mobile majority carriers but also the emptying of bulk traps. This is illustrated diagrammatically in fig. 10. Thus a comparison of the field effect mobility and the bulk mobility indicates that there are active deep lying bulk electron traps present with a density of the order of the bulk density of free carriers (2-3 x lOi cmU3). G(1As crystal Bulk traps empty
\
‘\
traps contain electrons
/Bulk
.--_--_---_
Ei
-7 Movement
of depletion layer boundary causes filling or emptying of traps
Surface Fig. 10.
Energy level diagram showing behaviour of bulk traps in surface space charge layer.
5.4. VARIATION OF ac FIELD EFFECT WITH FREQUENCY The ac measurements described so far were carried out at 570 Hz. It is of interest to see how the field effect mobility varies with frequency. So measurements were made over the range 100 Hz-100 kHz, with small peak fields of about lo4 V cm-‘, using the technique first described by Aigrain et a1.10~lr). Again the main distinction appears to be between the etched surface and the SiO, covered surface. Typical curves are shown in fig. 11. On etched surfaces the field effect mobility isessentially constant throughout
SURFACE
PROPERTIES
OF II-TYPE
49
GaAS
the whole range. This again shows how the effect of the ac field is to render the surface traps almost completely inactive already at 100 Hz. On Si02 covered surfaces, the mobility tends to rise slowly with frequency. This result is in accord with the much faster relaxation in conductance observed with step fields. It is thought that this effect is saturating so that the conductance modulation must be very close to sinusoidal above about 40 kHz. That is, the effective relaxation time of the surface traps is greater than about 4 ,usec at this field amplitude.
Curvo 1. Etched sur
10
1 Field
n-typo Fig. 11.
fraquoncy
100
f kHz)
GaAs. Paak fiald lO%/cm
ac Field effect mobility vs. frequency.
f%L%h’bNORITY CARRIER EFFECTS
So far it has been assumed that minority carriers do not play a significant role. There are two aspects to this assumption: (1) Even if the electric field is of sufficient strength to induce an inversion layer, the generation time for minority carriers is assumed to be so long that they do not appear in the duration of the particular experiment. (2) It is assumed that inversion of the surface does not occur at equilibrium. These assumptions will be examined below. 5.5.1. Possibility of inducing an inversion layer During the initial period before the surface traps have time to react, the peak negative dc step fields, usually at least 6 x 10’ V/cm, would be more
50
I. FLINN
than adequate to induce an inversion layer. However the changes of conductance observed indicate that one is not formed. As described earlier the conductance changes are due to an extension of the depletion region into the bulk and the initial change corresponds to an extension of at least 0.5 pm. If this is assumed to be due to a large non-equilibrium depletion layer then the charge involved corresponds closely to that induced. But if an inversion layer is present the charge involved is impossibly high. On all the surfaces examined these non-equilibrium depletion layers could be maintained for at least a second. Thus it appears, that for experimental situations which are of shorter duration than this, significant numbers of holes are not generated. The decay of the non-equilibrium depletion layer after the application of a negative dc step field has a time constant much longer than one second (fig. 4). This decay is probably almost entirely due to emission of electrons into the conduction band from surface traps. The generation rate for minority carriers is thought to be very low indeed. Berglundi2) has shown that there may be significant generation of minority carriers by tunnelling between the energy bands in the space charge region. But the electric field in the surface is not high enough in the present case for this to be a very significant process. With the present experimental method, it would be impossible to detect conductance in an inversion layer if one were induced on the surface by a dc field. A M.O.S.T. type structure would be necessary for this. It is of interest that other workersis) who have tried this, have not obtained significant conductance between source and drain without building in a channel by diffusion or epitaxy. 5.5.2. Possibility A phenomenon,
of an inversion layer existing which could
at equilibrium
be called “excess self biassing”,
was some-
times observed in the ac field effect. This seems to indicate that some of the etched surfaces examined did have inversion layers on them at equilibrium. Consider the simple model of a very high density of surface traps and, normally, a depletion layer at the surface. On the simple one carrier model, the surface should bias itself under the ac field, until the positive peaks of field just took the surface above the equilibrium conductance, as in fig. 5. It is difficult to conceive surface traps which would cause the biassing to exceed this, if the model is correct. However, on a number of the etched surfaces, the biassing was indeed so large that the conductance never rose to the equilibrium value, even at the positive peaks of the field. This is seen in its most extreme form in fig. 12. In fig. 12a it is seen that after about three cycles, the biassing is already excessive. Fig. 12b shows the envelope of the same sinusoidal train. In the steady state under the ac field the conductance is always at least 230 pmho/sq.
SURFACE
PROPERTIES
OF n-TYPE
GRAS
-1
-_ 2
I 1 012345678
I
I
I
I
I
I
I
I
I
c
Tima (milhsacs) (a)
t
6
-100
-
t; 4 $ -$
-2oo-
t;P E $. ‘0 z L
-3oo-
-400
I I 012345678
I
I
I
I
Tima
(sacs)
I
(b) n-typa
Fig.
12.
GoAs.
570Hz.
Paak
flald
0.72
x 106V/cm
ac Field effect. Transient behaviour showing excess biassing.
)
52
I.
FLlNN
less than the equilibrium level. This surface had been etched in a manner involving silver nitrate and potassium dichromate l4). It also gave the highest peak photovoltage recorded and the shape of the photovoltage-photoconductance curve indicated that the total barrier height may be 0.7 V (28 KZ”),which would mean that an inversion layer is present. The excess biassing might then be explained in terms of the annihilation of the holes in the space charge layer. On positive half cycles of field, their numbers would be excessive and they would tend to recombine. There would not be time for them to regenerate on the negative half cycles, so they would be progressively pumped away. This process might extend outside the region of the field electrode, which covers less than one third of the effective surface of the GaAs filament. Holes would tend to flow due to fringe fields, along the surface and under the field electrode, when there is a large depletion layer; and they would then recombine, when the depletion region collapses, under the influence of the positive field. 5.6.
COMPARISON
OF
ac
FIELD EFFECT
WITH
THEORY
The very simple model of the surface which is used above seems to account qualitatively for the way the gallium arsenide surface responds to ac fields. But it is of interest to probe it more quantitatively. Berz has developed the theoryl5) for the one carrier case. The computed solution of the differential equation has been used to test the model for the two types of behaviour typified by the etched and SiO, covered surfaces described above. For computation, values of the various surface parameters have to be taken. Also the presence of bulk traps must be taken into account as these influence the surface space charge layer in just the same way as donor atoms. These parameters have all been deduced from the surface photovoltage and the dc field effect. The dc field effect measurements at various field amplitudes can only be explained by assuming that the surface traps are in fact distributed in energy near the Fermi level. But the theory is already complicated enough if a set of traps all situated at one discrete energy is taken. So an approximate fit to the experimental data is obtained with such a discrete set. The actual amount of induced charge that is trapped may be determined precisely from the field effect relaxation. To estimate the effective trap density and energy level, a value of the surface barrier height must be taken. The rate of emission U, of the traps at equilibrium (eq. (5)) may also be determined precisely. It is given by the initial rate of decay of conductance when a field is applied so as to increase depletion at the surface. But to deduce the capture cross section CT,one again needs to know the surface barrier. This is not known with great precision (9 4.3, but the most Iikely values are used.
SURFACE
The behaviour,
computed
PROPERTIES
from
OF ll-TYPE
53
GaAS
the full equationsi5),
is shown
etched surface in fig. 13, and for the silicon oxide covered
for the
surface in fig. 14.
The capture rate is normalised as a fraction of the peak field trapping rate if all the charge were trapped. The values of the various surface parameters are indicated on the figures. In fig. 13c, which should be compared with fig. 6, the truncation of the first positive half cycle is seen and this corresponds with heavy trapping. At one point (fig. 13d) the trapping rate is 0.94 of maximum. But when the field goes negative there is negligible emission from the traps. Thereafter the conductance modulation is reasonably sinusoidal and the trapping rate reaches a maximum of only - 0.02 on the second cycle and is progressively reduced until it is only 0.003 in the steady state which can be compared with fig. 5. The constant low rate of emission by the traps FSteady
state
4
+ (a) 0
Field E$m
wt %
Field effect con&c tance’O0 AG (@4ho/sq) -200
I
_-_ 0
I 11
3n
2n
4lT
5Il
J 6n
(12 0
wt
Peak
Surface chmxtemtlcs
NB(bulk
Fig.13.
field
trap
ac Field effect. Computed
E,=
5.6
density)
x 105V/cm
t =570Hz
= 2~4 X10’5c~-3
behaviour of etched surface.
Tr
2n
54
I.
FLlNN
over more than 800/, of the cycle is also seen in fig. 13d. It is to be noted in fig. 13b, that at no point is the field sufficient quite to flatten the surface barrier. As the barrier tends to flatten, ~1,approaches the bulk carrier density -3 x 1015 cmF3 at which level the surface traps are very efficient in taking up induced charge. In fig. 14 the behaviour of the SiOz covered surface is simulated and fig. 14~ should be compared with fig. 8. The second cycle which is identical with the steady state shows distortion in the positive peaks of field where the trapping rate is 0.48. This is because the emission rate at -0.05 is an order higher over the rest of the cycle than in the case of the etched surface. If it were not for the presence of bulk traps, the barrier heights and field effect condu~tances achieved would be approximately twice as high as the values seen in figs. 13 and 14.
t (a) Frld E,sn wt
Surface chamcterMcs
OI
-v
.$ ETb
Fig.14.
-.2lV EC E,
Peak held E, = 5.6 x105V/cm
f = 570t-tY no=3.5x10’5cm‘3
0,=1.0x10+%m2 N =6.6x101’cm+ T NB(bulktmp density)= 3.8xfO’5cm-3
ac Field effect. Computed
behaviour of SiOz covered surface.
SURFACE
PROPERTIES
OF II-TYPE
GaAS
55
The computed behaviour shows quite good agreement with experiment but there are minor differences. For instance it takes many more cycles to achieve the steady state in practice as may be seen in figs. 7a and 9. Also the distortion of the positive peaks is not quite the same and nor is the degree of self biassing. Three possible reasons for these discrepancies are inaccuracy in the parameters used for computation, assumption that the traps are at one energy whereas in fact they are distributed over at least a small range of energy, and also the presence of another set of much slower traps, has been ignored (4 5.1). 6. Surface treatments It is not thought worthwhile to give details of all the surface treatments which have been studied but a short account of some of them may be of interest. The etched surface referred to in this paper was etched in 3 : 2: 1 - HNO, : H,O:HF. All etched surfaces examined had barrier heights of 0.45 V or greater with the bands bending in the sense of depletion or inversion. The total effective surface trap density was usually at least lOI2 cm-‘. But this appears to include a relatively fast species of trap at about 5 x 10” cmm2 plus another set with at least the same density. Some treatments were found to reduce to trap density. For instance washing in dilute aqueous sodium sulphide solution after the above etch reduced the total trap density to about 3 x 10” cmm2 and apparently eliminated the slower traps. This treatment has been found to work for indium antimonidelc) and is thought to act by precipitating metal ions from solution and so avoiding contamination of the surface. A sulphuric hydrogen peroxide etch 5 : 1: 1 - H,SO,: H,O, : H,O 17) also reduced surface trap density to about the same level. SiO, was deposited on a wide range of etched surfaces by decomposition of tetraethoxysilane at 400°C. It was found that the underlying etch treatment made little difference. These surfaces had depletion layers with surface barriers in the range 0.2-0.3 V and the surface trap density appeared to be about 7 x 1O’l cmP2. It is thought that the reduced barrier height may be due to fixed charge in the SiO, layer. No significant difference was detected in the behaviour of the (111) A (gallium) and B (arsenic) faces for any of the surface treatments studied. 7. Conclusions The experimental work described in this paper has allowed a model of the surface of n-type gallium arsenide to be derived. This model explains qualitatively all the phenomena which have been observed. An attempt has been
56
made to derive values
I. FLINN
for the parameters
of active surface
states,
but no
great accuracy can be claimed because of the difficulty in determining the equilibrium surface barrier height accurately. An equally good, or possibly a better fit could be obtained between theory and experiment by small adjustments of the various parameters. But this is not a very useful exercise in the absence of further experimental evidence. However the values of the various surface trap parameters found fall within the range found by other workersl*,lg). It would be difficult to improve the precision of the photovoltage technique for measuring barrier heights in gallium arsenide which always has a short minority carrier diffusion length and a large carrier mobility ratio. These two factors conspire to make the Dember potential at once significant in magnitude and difficult to estimate. It is to be noted that the traps on gallium arsenide surfaces which account for most of the phenomena observed are what are normally termed “fast” surface traps on germanium. That is to say, their cross sections are similar to those of fast traps. Their slow reaction under certain conditions, when they may take seconds to relax, is entirely due to the large band-gap of gallium arsenide. Thus the interchange of either majority or minority carriers between the traps and the bands may be very restricted under depletion conditions. Self biassing of a semiconductor surface under the ac field effect has also been found on germanium by Dordaaa), and Many and Goldstein al) have observed the same phenomenon with pulsed fields. They attribute their effect to tunneling of electrons from oxide traps into the conduction band. This has the effect of causing a net emptying of the traps under ac fields and hence the biassing is such as to reduce the surface barrier. In gallium arsenide the effect is quite the opposite, the traps are filled. No sign of the effect of tunnelling between surface traps and the conduction band has been observed on gallium arsenide although the electric fields employed are of similar strength to those used by the above workers. Possibly there are no oxide traps with appropriate parameters present on gallium arsenide surfaces. Becke and White2a) have made n-channel insulated gate field effect transistors on gallium arsenide which work in the depletion mode. They have concluded that surface traps are present at the semiconductor interface with densities of the order of 10” cm- ‘. It seems that the field effect phenomena which have been described above must also occur in their devices. In particular, it is to be expected from the present model that the transconductance would be small under dc conditions and rise at some low frequency to a useful level as the surface traps are made relatively inactive by self biassing. At a still higher frequency the bulk traps will become inactive and a further rise in trans-conductance should be observed. Becke and White observe such variations in trans-conductance. Under transient con-
SURFACE
PROP5RTIES
OF II-TYPE
GaAS
57
ditions the trans-conductance will exhibit “memory” effects, in that its instantaneous value will depend on the nature of the signal which has previously been applied to the gate. Owing to the ac biassing effect the device should also work for high frequency ac signals without any dc bias being necessary on the gate. Acknowledgements I should like to thank Dr. F. Berz for many helpful discussions. I am also indebted to Mr. M. J. Briggs who designed some of the experimental equipment and to Mrs. 8. J. Rudd for assistance with the computations. This work was supported by the Ministry of Defence. References 1) 2) 3) 4) 5) 6) 7) 8) 9)
10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)
I. Flinn and M. J. Briggs, Surface Sci. 2 (1964) 136. W. Shockley and W. T. Read, Phys. Rev. 87 (19.52) 835. C. G. B. Garrett and W. H. Brattain, Phys. Rev. 99 (I 955) 376. E. 0. Johnson, Phys. Rev. lll(1958) 153. R. Williams, J. Phys. Chem. Solids 23 (1962) 1057. T. S. Moss et al., Proc. Phys. Sot. (London) B 66 (1953) 743. N. L. Dmitruk and V. I. Lyashenko, Fiz. Tverd. Tela 8 (1966) 578. G. Rupprecht, Ann. N.Y. Acad. Sci. lOl(i963) 960. V. E. Primachenko et al., Phys. Status Solidi ll(l965) 711. P. Aigrain et al., J. Phys. Radium 13 (1952) 587. H. C. Montgomery, Phys. Rev. 106 (1952) 441. C. N. Berglund, Appl. Phys. Letters 9 (1966) 441. H. Becke and J. White, Institute of Physics and Physical Society Conference No. 3 (1966) 219. C. Hilsum, private communi~tion. F. Berz, Surface Sci. 10 (1968) 58. J. L. Davis, Surface Sci. 2 (1964) 33. R. Hall and J. P. White, Solid State Electron. 8 (1965) 211. S. Kawaji and H. C. Gatos, Surface Sci. l(l964) 407. G. Rupprecht et al., Report No. RADC-TDR-64-17(USAF), (1964). G. Dorda, Phys. Status Solidi 3 (1963) 1318. A. Many and Y. Goldstein, Surface Sci. 2 (1964) 114. H. W. Becke and J. P. White, Eiectronics 40 (1967) 82.
Series,