GaAs(110) -Al INTERFACES FORMED AT LOW TEMPERATURE

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GaAs(110) -Al INTERFACES FORMED AT LOW TEMPERATURE

C. Bonapace, K. Li, A. Kahn

To cite this version:

C. Bonapace, K. Li, A. Kahn. GaAs(110) -Al INTERFACES FORMED AT LOW TEMPERATURE.

Journal de Physique Colloques, 1984, 45 (C5), pp.C5-409-C5-418. �10.1051/jphyscol:1984562�. �jpa-

00224181�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au n04, Tome 45, avril 1984 page C5-409

G a A s ( 1 1 0 ) - ~ l I N T E R F A C E S FORMED A T LOW TEMPERATURE

C.R. Bonapace, K. L i and A. Kahn

Department of EZectricaZ Engineering and Computer Science, Princeton University, Princeton, New Jersey 08544, U.S.A.

Rgsumk - Nous avons 5tudi8 la structure de l'aluminium 6vapor6 sur la face clivee (110) de GaAs, refroidie 5 100°K. La mobiliti de 1'Al sur la surface est fortenlent rCduite B basse temperature, en accord avec les estimations thkoriques, e t l'homogFin6itB des couches d'Al est accrue. ,La structure atomique de l'interface est differente de la structure obtenue a t e m p k a t u r e ambiante. Une analyse pr'eliminaire des donnges 1,EED suggere que la relaxa- tion de GaAs(ll0) est 6liminde par 1'Al. La variation du travail de sortie, mesurEe par la mgthode de diff&rence de potentiel de contact, indique la for- mation d'un dipole Al-surface beaucoup plus fort qu'5 t e m p k a t u r e ambiante.

Abstract - We report the first characterization of A1 layers deposited on low temperature (100°K) cleaved GaAs(ll0). The surface mobility of A1 is consider- ably reduced as the temperature is lowered, in agreement with estimates of A1 hopping frequency a t the surface of GaAs(1 lo). Reduction in mobility leads to more homogeneous A1 layers. Changes in the atomic geometry of the interface are detected with LEED. A preliminary analysis of the LEED data suggests that the GaAs(ll0) surface 1s unrelaxed by Al. Contact potential difference meas- urements of the evolution of the work function vs. A1 coverage indicate the for- mation of an Al-surface dipole which is much larger than a t room temperature.

I. Introduction

The GaAs(l10)-A1 system has been investigated for several years and has served as a prototypical interface for the study of Schottky barrier formation [I-7J. The atomic structure and composition of this interface are difficult to determine quanti- tatively because three phases of A1 might coexist: A1 chemisorbed as single atom and dispersed on t h e surface, A1 in metallic clusters and A1 in a reacted phase (ALAS). The last two phases have been identified in several experiments [1,3,5,6]. The first has been observed in only one experiment involving ultra-low coverages of the order of 0.05 monolayer (1 monolayer (ML) = 1 A1 / surface atom = 0.89 x 1015 A1 / cm2) [7].

The effect of chemisorbed A1 on the substrate atomic geometry, however, cannot be studied with these small coverages and is therefore still unknown.

A1 does not form a n ordered chemisorbed phase a t room temperature because its surface mobility is high, the Al-substrate bond is weak and the A1-A1 interaction is strong. Hence, the structure of the overlayer is dominated by the A1-A1 interaction

[a].

The growth of 41 follows the Volmer-Weber mode, whereby A1 forms metallic clus- t e r s separated by large areas of clean GaAs. Low energy electron diffraction (LEED) [5,6,8] and photoemission spectroscopy (PES) [7] suggest that clustering occurs even a t sub-monolayer coverage. There is no indication t h a t A1 is present between the clusters. When t h e A1 coverage increases, the clusters grow, coalesce and form islands which have a well-defined epitaxial relationship with the substrate

[a].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984562

C5-410 JOURNAL DE PHYSIQUE

F r o m a chemical point of view, this interface is reactive, i.e. t h e r e p l a c e m e n t of Ga by A1 is thermodynamically favorable [ 9 ] . The A1-Ga exchange h a s bee11 observed a t r o o m [1,3] a s well as high t e m p e r a t u r e [5,10,11]. The reaction js very limited a t 300°K. Its r a t e increases dramatically when t h e t e m p e r a t u r e is raised above 400°K and several layers of AlAs c a n t h e n be grown on top of t h e GaAs s u b s t r a t e [5,10,11,12]. What a r e t h e mechanisms responsible for t h e activation of this r e a c - tion? The energy required t o b r e a k t h e Ga-As surface bonds is g r e a t e r t h a n t h e t h e r - rnal energy available a t 400°K. Specific m e c h a n i s m s m u s t , therefore, c o m e into play.

To d a t e , only one has been proposed, namely t h e formation of A1 c1ust1-rs which release enough energy (2-3 eV/atom) to activate t h e exchange reaction [ I 31. S t e p s , point defects a n d o t h e r imperfections, p r e s e n t o n even t h e b e s t cleaved surfaces, probably also play a significant role in t h e process.

First principle e n e r g y minirnization calculations of t h e s t r u c t u r a l energies of A1 on GaAs(ll0) have provided very useful information o n t h e possible s t a t e s of A1 on t h e surface [14]. The t h r e e phases of Al, i.e. chemisorbed Al, A1 clusters and r e a c t e d Al, were investigated. The energies of A1 chemisorbed o n several sites were com- puted. According t o t h e s e calculations, t h e m o s t energetically favorable chernisorp- tion is in a two-fold bridge posit.ion between a n As and t h e n e a r e s t Ga of t h e n e x t Ga- As chain. Tfre calculations also predic.1 t h a t GaAs(ll0) should unrel.ax, a s A1 satura.tes t h e surface dangling bonds (The relaxation of clean GaAs(ll0) has b e e n d e t e r m i n e d by L I E D analysis [ I s ] . ) . 'The u n i t cell of GaAs(ll0) and t h e position of A1 chenlisorbed c111 t h e unrelaxed to]:, layer a r e shown in Fig. 1.

T O P VIEW SIDE V I E W

---

- ---

>? _{--- ---}

^{I I I I I I I}

^f

----

[oo i]

[110]

@---+

Figure 1 --Top and side view of the (110) surface unit cell with A1 chemisorbed in t h e bridge position [14]. The s u b s t r a t e g e o m e t r y is unrelaxed by A1 ( t h e r e - laxed g e o m e t r y [15] is s k e t c h e d in dashed lines o n t h e side view only)

This corlfiguration, however, was found to b e unstable a t room t e m p e r a t u r e v:ith r e s p e c t t o t h e formation of A1-A1 bonds. It was therefore suggested t h a t ill exists rnainly in metallic c l u s t e r s on the surface, in full a g r e e m e n t with t h e e x p e r i r n e r ~ t a l observation:; [5,8]. 'To quantify t h e s e predictions, a n evaluation of t h e surface mobil- ity o € Al, which is a n essential p a r a m e t e r f o r t h e formation of clusters, was done by calculating t h e migration energy b a r r i e r which s e p a r a t e s t h e m e t a l a t o m , chem- isorbed in one unit cell, f r o m t h e equivalent site i n t h e n e x t unit cell. Two b a r r i e r s

were calculated, 0.3 eV and 0.6 eV, for two different migration paths. These barriers were used t o compute the frequency with which A1 hops from one unit cell t o the next. 'This frequency is Fh =

v

exp(-@,/ kT) where

v

is the substrate atomic vibra- tion frequency and 9, is the migration energy barrier. For @, = 0.3 eV or 0.6 eV and v

=

10" HZ, Fh is much larger than unity a t room temperature and is consistent with fast migration. In agreement with experimental observations, these calculations predict t h e formation of clusters. Finally, a computation of the energy gained when A1 replaces Ga in the first or second atomic layer of t h e substrate was also performed to study the probability of obtaining t h e reacted A1 phase. In agreement with the LEED analysis of A1 reacted a t high temperature on GaAs(ll0) [6], it was shown t h a t A1 occupies its lowest energy position when it replaces Ga in t h e four-fold coordinated site of the second layer. Also in agreement with all experimental observations, it was noted that the reacted A1 phase should not be t h e prevailing one a t room tempera- t u r e because energy is needed to activate the reaction.

Can we modify the experimental conditions in order to enhance the chemisorp- tion of isoltited Al atoms and decrease the formation of clusters? The expression for t h e hopping frequency suggests t h a t , by lowering the substrate temperature before evaporation, one should be able to reduce Fh and drastically decrease t h e surface migration of Al. Hence, we have investigated the evaporatjon of A1 on 100°K CiaAs(ll0). As expected, -we have obtained a more homogeneous distribution of A1 on t h e surface. We have observed changes in the atomic geometry of t h e substrate sur- face induced by the chemisorption of 1ML-2ML of Al. We find t h a t the atomic geometry of t h e GaAs substrate is partially (or totally) unrelaxed by the A1 overlayer.

The variations of t h e surface work function also differ from what can be measured for t h e room temperature deposition and indicate t h e presence of a Low t e m p e r a t u r e stable s t a t e of Al.

Wc now t u r n t o a description of t h e experiment. Results and discussions follow in section 111.

11. Experimental Considerations

The experiments were performed in a Varian UHV chamber equipped with a four-grid LEED optics, a 10 keV cylindrical mirror analyzer for AES analysis, a n A1 evaporation source and a Kelvin probe for CPD and

SPV

measurements. The base pressure in t h e UHV chamber was 8 x lo-'' t o r r , rising t o 2 x 10-lo torr during eva- poration. The GaAs bars (Te doped, 0.4 X 10" ~ m - ~ ) were purchased from Laser Diode Laboratories. These bars were mounted on a manipulator capable of full rnotion in t h e UHV chamber for LFED, AES and CPD measurements as well a s for cleaving in-situ and A1 evaporation on the room or low temperature (100°K) sub- s t r a t e . The temperature of t h e crystal was monitored with a Chromel-Alumel ther- rnocouple held in contact t o t h e bar with conducting UHV epoxy. Al was evaporated from a resistively heated tungsten wire, typically a t a r a t e of 1 ML/minute . The CPD measurements were performed with a homemade Kelvin probe. The probe consisted of a thin Au electrode (diameter = 1 rnm) coupled through. a bellows to a loudspeaker membrane vibrating a t a frequency of 200 Hz. SPV measurements were performed by illuminating the surface with a 150 watt Xenon lamp to obtain t h e direction of the band bending in the semiconductor. 'The C P D between t h e Au electrode and t h e sur- face could be determined with an accuracy of 5 meV, b u t repositioning of t h e surface in front of t h e eleclrode, after each evaporation, limited the reproducibility of the rneasuremei~t t o about 50 meV. To avoid the effects of electron impact on the eiec- tronic properties of the semiconductor surface, all CPD or SPV measurements were performed on surfaces w-hich had not been previously irradiated with the LEED or AES beam. Precautions were also taken t o switch off the ionization gauge before each cleave. The measurements of L4EEI) intensity profiles (I-V) were performed with ei Gamma Scientific spot photometer, following a procedure described elsewhere [16].

Sets of 14 difIracted beams were recorded.

JOURNAL DE PHYSIQUE

111. Results and Discussion

1. Homogeneity of the Al Overlayer

The variations of t h e A1 (66 eV), Ga (49 eV and 53 eV) and As (31 eV) AES peak amplitudes, as a function of A1 coverage, a r e shown in Fig. 2. Panel (a) and (b) correspond to A1 evaporations on room and low temperature substrates, respectively.

We assumed a sticking coefficient equal t o unity for A1 on GaAs throughout the exper- iment. In panel ( a ) , t h e Ga and As peaks exhibit a slow decrease which either reflects t h e formation of a non-uniform overlayer or a strong interdiffusion between t h e rnetal and the semiconductor. The possibility of substantial interdiffusion was dis- carded in earlier publications because of (1) t h e persistence of t h e substrate LEED pattern beyond a coverage of several t e n s of monolayers of A1 and (2) the unlikeli- flood of As diffusion through t h e A1 layer

[a].

The LEED I-V profiles measured with 10 - 20 ML of A1 remain identical to those measured from the clean GaAs(ll0) surface.

LEED therelore "sees" large uncovered areas of t h e substra-te. The background increases, however, because of incoherent diffraction from the A1 clusters. Hence, the sub-exponential decrease of t h e substrate AES-peaks represents t h e growth of A1 clusters which only cover a fraction of the GaAs substrate. Beyond 10 - 15 ML, t h e Al, G a and As AES peak amplitudes almost level off. The contribution of added A1 to t h e signal is small because A1 forms islands which have a larger thicltness t h a n t h e short inelastic mean free path of t h e Auger electrons (3 - 4 a t 65 eV) and t h e additional A1 only slowly extends t h e area of t h e islands covering t h e substrate.

. -.

Al COVERAGE (ML)

Figure 2 --Coverage dependence of t h e Ga, As and A1 low energy AES peaks for A1 evaporated on (a) room temperature and (b) low temperature cleaved GaAs(ll0). The dashed line represents t h e decrease of the substrate peaks for an ideal interface and a 4 ^I% electron escape depth.

When A1 is evaporated on t h e low temperature substrate, the amplitudes of t h e Ga and As AES peaks decrease almost exponentially (Fig. 2 , panel (b)) and fall below

the detection limit between 15 ML and 20 ML of Al. The homogeneity of t h e overlayer is estimated by computing an effective escape depth of the Auger electrons from the decrease of the substrate peaks. Assuming that a monolayer of A1 has a thickness of 1.41 [3], we find a value of 10W. This is about twice t h e accepted value (4

-

5 1 ) and clearly shows t h a t A1 does not follow a layer by layer, Frank-van der Merwe type growth. Nevertheless, i t is more homogeneously distributed on the surface than a t room temperature, in agreement with t h e simple calculation of the surface mobillty of A1 given in the introduction. The hopping frequency Fh is much larger than unity a t room temperature but decreases by many orders of magnitude as T is lowered to 100°K: for 9, = 0.6 ev, Fh

>

lo2 HZ a t 300°K and Fh

<

5x10-l8 Hz a t 100°K. According t o this trend, A1 is expected t o form a more homogeneous overlayer a t low tempera- ture.

The diffraction experiment brings important additional information concerning the continuity of the overlayer. With a coverage of IML, the pattern remains ( 1 x 1) and retains its symmetry, although the background intensity is higher than a t room temperature. Yet, changes in the shape of the I-V profiles, which have never been observed during t h e room temperature experiment, can be detected. They suggest a modification of the interface atomic geometry and demonstrate t h a t most of the sub- s t r a t e is now covered with Al. These changes will be considered in detail in the next section. After the evaporation of XML, the diffraction spots in the (1 x 1) pattern are still sharp (Fig.3, (b)) and the changes in the I-V profiles are complete. The (1 x 1) pattern remains visible up to 10 - 15 ML. With a coverage of 25 ML of Al, only the pat- t e r n due to the large A1 islands is visible (Fig.3 (c)). The epitaxial orientation of these islands is such t h a t this pattern and the GaAs(ll0) pattern exhibit mirror sym- metries across the same axis (Fig.3 (a) and (c)).

Figure 3 --LEED patterns recorded from (a) clean 100°K GaAs (140ev), (b) 100°K EaAs

+

2ML A1 (140ev) and (c) 100°K GaAs

+

25ML A1 (53ev).

The coverage range through which t h e (1 X 1) pattern remains visible is in good agreement with the range of detection of the low energy Auger As and Ga peaks. It corresponds to a nominal A1 thickness of 15

-

²⁰

^{A ,}

a t least three times larger than the LEED inelastic mean free path ( 4

-

5 1 ) . I t confirms t h a t A1 does not grow layer by layer and t h a t some clustering takes place, even a t 100°K. A1 migration and the for- rnation of clusters compete, although less effectively than at room temperature, with the formation of the initial overlayer (The evaporation of 2ML is required to complete the changes in the LEED I-V profiles.). Most of the clustering takes place on top of this overlayer, following a Stranski-Krastanov growth mode. T.his mode should, in principle, lead to a change of slope corresponding to the completion of the overlayer ( a t 2ML) in the AES curve of Fig.1, panel (b). Such a break is not observed, probably because clustering already plays a role during the initial phase of A1 chemisorption.

In summary, both AES and LEED demonstrate t h a t the A1 layer is considerably more homogeneous when i t is evaporated on the low temperature substrate, a conse- quence of the decrease in the mobility of Al. The first 1-2ML of A1 induce changes in the atomic geometry of the interface, a subject we now consider in more detail.

C5-414 JOURNAL DE PHYSIQUE

2. Atomic Structure of the Interface

In Fig. 4 we show the I-V profiles of four beams, i.e. (Oi), (IT), (02) and (20), measured from (a) the clean surface and the surface covered with (b) 1 ML and (c) 2 ML of A1 a t 100°K. The top I-V profiles (d) correspond to the warm-up experiment described later in this section.

0 4 0 80 120 160200240 0

40

8 0 120 160 200 240 ENERGY ( e V )

Figure 4 --LEED I-V profiles of four beams recorded for (a) clean 100°K G a ~ s ( l l O ) , (b) 100°K GaAs(ll0)

+ ^1ML

Al, (c) 100°K GaAs(ll0)

+

2ML A1 and (d) GaAs(ll0)

+ ^2ML

^Al, a f t e r warm-up. Arrows indicate the partial reversal of the I-V profiles toward the clean GaAs profiles.

The changes in t h e I-V profiles and the conservation of t h e ( 1 x 1 ) pattern result from the formation of a new surface geometry which has the same space group sym- metry as GaAs(ll0). A1 changes the structure factor of the GaAs(ll0) unit cell (by chemisorption or reaction in the unit cell and/or modification of the GaAs geometry) but does not change its dimensions. The new interface geometry is complete with the

2ML

coverage and additional A1 does not induce further changes in the I-V profiles. The high background observed in Fig.3 (b) is probably due to the diffraction from A1 clusters.

Some preliminary remarks can be made about the I-V profiles of Fig. 4. Only four are shown here, but they are representative of the full s e t of 14 profiles we have

measured. First, most of the intensity features associated with the multi-layer GaAs relaxation [25-171 are attenuated with 1 ML of A1 and disappear with 2 ML. In Fig. 4, these a r e t h e 90 eV and 144 eV peaks of t h e (01) beam, the 162 eV peak of the (IT) beam, and the 178 eV peak of the (02) beam. Second, several intensity features appear in the I-V profiles: the double peak between 120 eV and 155 eV on the (IT) beam and the small peak developing a t 160 eV on the (20) beam. Although no sys- tematic LEED structure analysis (including R-factor comparison of theory and experi- ment) has yet been performed, the I-V profiles seem to be compatible with the LEED intensities calculated for the unrelaxed GaAs(ll0) surface [18]. Hence, the I-V profiles (Fig. 4) and t h e high intensity background (Fig. 3 (b)) suggest t h a t A1 forms an overlayer which suppresses the GaAs relaxation but which does not have a long range atomic order. As an example, the overlayer might consist of A1 atoms chem- isorbed on various sites of the unit cell which do not have sufficient energy to reach t h e minimum energy position (Fig. 1) and form an ordered array. Also, the A1-A1 interaction might still play a dominant role, a t least locally, and produce very small c l i ~ s t e r s homogeneously dispersed across the surface. The existence of an ordered, or partially ordered, A1 layer cannot be entirely ruled out until a complete explora- tion, via LEED analysis, of the various chemisorption sites on un~eLa.xed GaAs(ll0) is performed. Several chemisorption configurations have been studied in early LEED work [lo], but all the geometries involved a relaxed GaAs substrate.

Could the interface geometry result from an Al-substrate reaction? AES is ambi- guous on this point because we do not know the energy position of the A1 peak for atoms chemisorbed on GaAs. The peak is a t 65 ev or 66 ev for clusters, depending on the size of the clusters, and 63 ev for the reacted phase [5,10]. When the first mono- layer is evaporated on low temperature GaAsjllO), t h e AES peak is a t 63 ev and shifts to 65 ev as the coverage is increased to 2ML. Hence, some questions should be raised about the nature of the A1 which corresponds to this 63 ev peak. Preliminary PES

~lxperiments show that the Al-2p core level, measured for 0.5-1ML of A1 on low tem- perature GaAs(llO), is a t an intermediate energy position between reacted A1 (higher binding energy) [ I 11 and A1 in clusters (lower binding energy). It indicates t h a t at least part of the A1 is not reacted with t h e substrate. A second indication is provided by the "warn-up" experiment. The changes in the I-V profiles are partially reversed (Fig. 4, curves (d)) when the interface formed with 1ML - 2ML of A1 is warmed up to room temperature. This could not take place if the interface layer was totally reacted. O n the other hand, if the interface structure consists of A1 chemisorbed on the substrate, clustering might resume upon warming the surface and lead t o the reversal of the substrate atomic geometry. This reversal is more limited than expected from the high surface mobility of A1 evaporated on 300°K GaAs. A possible reason is t h a t the thermal energy of the A1 atoms impinging on the surface during evaporation and t h e radiation coming from the hot A1 source increase the GaAs sur- face temperature by 20-30". This extra thermal energy, absent when the substrate is warmed up from 100°K to 300°K, can increase the hopping frequency of the incoming A1 atoms by an order of magnitude with respect to the hopping frequency of the over- layer atoms warming up with the surface,

El. Variations of the Interface Work Function

CPD and SPV measurements were performed to follow t h e variations of the work function, AW, when A1 is evaporated on the surface and to determine the direction oE the band bending induced by the metal. GaAs(ll0) does not have intrinsic surface s t ; ~ ~ e s in the band gap. When the surface is well cleaved, i.e. the density of defects is low, the Fermi level is not pinned between the conduction and valence bands. Hence, phenomena such as the influence of adsorbates on t h e surface electronic structure can be studied in great detail.

With t h e roorn temperature experiment, we reproduced the data obtained by Elrillson e t al. [ I ] (Fig.5 (a)). These data have been interpreted in the following way.

For small coverages of Al, AW is the sum of the variation in band bending AqVg and the change in electron affinity AX, i.e. AX = AW - AqVB. AX, in the present case, results from the formation of a metal-semiconductor dipole. Brillson e t al. measured AW by

C5-416 JOURNAL DE PHYSIQUE

CPD and used SPV to obtain AqVB, t h e light-sensitive part of W. They obtained a nega- tive surface dipole, i.e. the A1 layer is more negative than the layers below. This dipole, added to the band bending, increases the work function which goes through a maximum a t 1/2 ML. We believe, however, that the Value of AqVB was substantially underestimated in the SPV measurements. Lassabatere e t al. [19] have recently noted t h a t a n a t t e m p t to flatten t h e bands in GaAs by illuminating the surface always results in values of the surface photovoltage smaller or equal to one-half t h a t of the band bending. This could imply that the magnitude and the sign of the calculated dipole voltage mentioned above might be incorrect. The low temperature experi- ment helps resolve this issue.

Figure 5 --Variation of t h e work function, AW, vs. A1 coverage on (a) the room temperature and (b) t h e 100°K GaAs(ll0) surface. Insets show the surface band diagram and the interface dipole AX. In (b), arrows 1, 2 and 3 indicate the change in W upon cooling the clean GaAs surface to 100°K ( I ) , warming-up to room temperature the surface covered with

1ML

or 2ML (2), and cooling the surface back to 100°K after warm-up (3).

When GaAs is cooled to 10O0K, we first observe a 100

*

20 meV increase in the work function of the clean surface (Fig.5 (b),arrow I ) , in agreement with the values obtained by Monch e t al. [2O]. When A1 is evaporated on the cold surface, W first increases, then decreases rapidly and goes through a minimum a t 2 ML (Fig.5 (b)).

Beyond 8 ML, W approaches the room temperature value when metallic Al begins to dominate the interface work function. The most important part of the curve is between 1/2 ML and 4 ML. SPV measured in this range indicates that the direction and the magnitude of the band bending are equivalent to their room temperature counterparts. The peculiar shape of AW must therefore be explained in terms of A x , the change in electron affinity or, in the present case, the formation of a strong sur- face dipole. A t low temperature, A1 is homogeneously dispersed on the surface.

Therefore, the dipole is expected t o be considerably stronger than in t h e room tem- perature case where A1 is inhomogeneously distributed in clusters with a small effective area of contact with the substrate. In the present experiment, the data

must be explained with apositive dipole which Lowers the surface work function (i.e.

the A1 layer is more positive than the layers below). Its direction is indicated in Fig.5 (b). The magnitude of the dipole is maximum a t 2 ML, confirming the LEED observa- tions which show that the completion of the structural changes occurs a t the same coverage. Between 1 ML and 4 ML, the dipole voltage is strong enough t o lower W below its original value.

We believe t h a t the dipole is also positive a t room temperature, in contradiction with the interpretation [I]. Its magnitude is too small, however, t o substantially modify AW (Fig.5 (a)). The room temperature variations of AW should therefore be interpreted in terms of a rapid increase in band bending (between 0 ML and 1/2 ML), i n accordance with the PES data on Fermi level pinning, followed by a decrease due to the initial formation of metallic clusters. Some Al-Ga exchange might come into play but t h e reaction is sufficiently limited a t room temperature t h a t its effect on AW should be minimal.

We studied t h e effects on AW of warming the surface covered with 1 ML and 2 ML of A1 to room temperature. In both cases, AW increased to the value expected from the room temperature experiment (Fig.5 (b),arrow 2). This evolution is consistent with A1 retrieving mobility, as T increases, and starting to form clusters. Upon cool- ing the same surface again (Fig.5 (b),arrow 3), we only obtained the temperature induced shift of the work function. This is to be expected because the transition from the chemisorbed layer to the clusters is not reversible. This experiment is con- sistent with the LEED experiment described in section 2 , although it demonstrates a much larger sensitivity of the CPD to the warm-up process. A t this time, we can only speculate t h a t the 2ML overlayer consists of small clusters as well as isolated A1 atoms. Both contribute to suppress the Ga4s relaxation but the isolated A1 atoms are responsible for most of the dipole voltage (charge transfer between the adatom and t h e substrate). When t h e surface warms up, isolated atoms move to form clusters and most of the dipole component vanishes.

N.

Synopsis

We have demonstrated t h a t Al, evaporated on low temperature GaAs, has a low surface mobility and, consequently, forms an overlayer which is more homogeneous than a t room temperature. We believe that part of this overlayer consists of isolated atoms or very small clusters which modify the surface relaxation of GaAs. The con- t a c t potential measurements suggest t h a t A1 creates a strong surface dipole which lowers the work function. The magnitude of the dipole is much larger than a t room temperature because A1 is dispersed and chemisorbed more evenly on the surface.

Yi7e believe that these experiments have allowed us t o probe t h e structural and electrical interactions between i s o l a t e d A1 atoms and the surface.

Acknowledgement

This material is based upon work supported by t h e National Science Foundation under Grant No. DMR-8205132.

Kef erences

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2 . L.J. Brillson, Surf. Sci. Rep. 2 (1982) 123.

3. P. Skeath, I. Lindau, P. Pianetta, P.W. Chye, C.Y. Su and W.E. Spicer, J. Elec.

Spect. and Related Phenomena, 17 (1979) 259.

4. D.J. Chadi and R.Z. Bachrach, J. Vac. Sci. Technol. 16 (1979) 1159.

5. A Kahn, J. Carelli, D. Kanani, C.B. Duke, A. Paton and L.J. Brillson, J. Vac. Sci.

Technol. 19 (1981) 331.

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