THE INTERACTION OF HYDROGEN WITH GaAs SURFACES

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THE INTERACTION OF HYDROGEN WITH GaAs SURFACES

R. Bachrach, R. Bringans

To cite this version:

R. Bachrach, R. Bringans. THE INTERACTION OF HYDROGEN WITH GaAs SURFACES. Journal de Physique Colloques, 1982, 43 (C5), pp.C5-145-C5-151. �10.1051/jphyscol:1982518�. �jpa-00222237�

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CoZZoque C5, suppZdment au n012, Tome 43, ddcembre 1982 page C5-145

T H E INTERACTION OF HYDROGEN W I T H GaAs SURFACES

R.Z. Bachrach and R.D. Bringans

Xerox Pa20 A l t o Research Center, 3333 Coyote Hi22 Road, Pa20 A l t o , Ca 94304, U. S. A.

S t a n f o r d Synchrotron R a d i a t i o n Laboratory, S t a n f o r d , Ca 94305, U.S.A.

ABSTRACT

-

Aspects of the interaction of hydrogen with MBE grown GaAs surfaces have been studied in situ with synchrotron radiation excited photoemission core level spectroscopy. We show how this characterization technique is a sensitive probe of the surface chemical composition. Using this technique, the GaAs(100) and (111) surface phase diagram at room temperature has been obtained as well as aspects of the AlAs surface. The measurements showed that for GaAs(100) the surface reconsuuctions proceed through a series of centered [c(4x4), ~(2x8) c(8x2)I and primitive structures [(1x6) ,(4x6)] and for the (111) surface (2x2). (J3x J 3)R(30°), ( J 19xJ 19)R(23.4O) as the surface As to Ga ratio is decreased. The study of the clean surfaces has been extended with an investigation of the interaction with atomic and molecular hydrogen. This is of interest both because hydrogen is a common growth ambient and because there is some indication that MBE growth in the presence of hydrogen improves crystal electrical quality. In all cases, the experiments showed a saturation hydrogen coverage was achieved at an exposure of 1 0 6 ~ of hydrogen when a cracking filament is used. Core level spectroscopy shows that the surface composition is modified by the presence of hydrogen and we have deduced a mechanism whereby the hydrogen forms a surface gallium hydride which sublimes. The hydrogen converts the clean surface reconstructions to a distinct semiconducting reconstruction with a specific composition independent of the starting reconstruction. These results indicate that hydrogen could modify the MBE growth surface thereby effect impurity incorporation.

1. INTRODUCTION

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Characterisation of the surface phase changes are important for understanding in detail a number of the propedes of semiconductor surfaces and can have a particularly strong influence on epitaxial growth. Dopant and impurity incorporation effects are also influenced by modifications of the reconstrucion during growth. The starting point for the discussion presented here is the clean GaAs(100) and ~ a ~ s ( i i i ) surfaces which exhibit a range of reconstructions depending on the precise composition of the surface 1 a ~ e r . l . ~ We then describe the effect of hydrogen chemisorption on the properties of these surfaces. Although hydrogen is a common growth ambient for epitaxy, very liiile work has been performed in the 111-V's investigating the extent and microscopin of the interaction. Calawa has found3 for MBE owth in

4

his system a significant increase in mobility when he grows in a hydrogen ambient of 10- tom but

F

the origin of the effect has not been explained. We find that molecular hydrogen has a very negligible interaction while the bonding of atomic H to the dangling bonds at semiconductor surfaces is very strong and provides a large modification of the surface structure. We also find that for these polar surfaces that hydrogen causes a composition change at the surface as well as effecting the surface reconstn~ction and electronic states.

The surface electronic structure of clean GaAs(100) and ~ a A s ( i i i ) has been explored to some extent, both t h e o r e t i ~ a l l ~ ~ . ~ and e ~ ~ e r i m e n t a l l ~ ~ - ~ ~ while work on the interaction with hydrogen has not been done. The composition of the surface atomic layer can vary from Ga-rich to As-rich and a variety of stable reconstructions can occur at room and growth temperatures at specific surface layer compositions ranges. In the intervening ranges, the surfaces are disordered. A correlation shown in Figure 1 has been found previously between the reconstructions seen on the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982518

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C5-146 JOURNAL DE PHYSIQUE

(100) surface and the positions and intensities in core level photoemission spectra of the Ga and As 3d core levels8 with photons in the energy range 70-210eV. We will review these results for the determination of the room temperature surface phase diagram for GaAs and for AIAs(100) since these are pertinent to undemanding GaAlAs surfaces. We then discuss the hydrogen chemisorption. The Z

:

^-

photemission showed that the surfacez reconstruction varied in the sequence c(4x4), c(2x8), c(8x2), (1x6), (4x6) as the surface As to ^- Ga ratio is decreased. Auger spectroscopy has also been used to correlate the change in surface composition with the recon~tructions.69~ but in the core level work, a revised order was

obtained and shown to correlate with the ELECTRON KINETIC ENERGY lev)

evolution of other properties. For the ( i i i )

surface a similar effect is seen and the Ga and As 3d core level spectra measured at 130 eV for various reconstruction varies from (2x2) to reconstructions on GaAs(1OO) from ref. 8 ( 4 3x4 3)~(30O) to ( 4 19x4 19)R(23.4°) as the plotted normalized to excitation intensity.

As composition of the surface layer

decrease^.^

EXPERIMENTAL

The GaAs substrates were polished following established MBE procedures.14*15 Si-doped substrates were mounted with indium on molybdenum blocks heated with a separate resistive heater. GaAs(100) and (111) surfaces were prepared by sputter cleaning, annealing in arsenic and then growing a thin epitaxial layer in situ by molecular beam epitaxy.

Specific surface reconstructions were obtained by annealing in arsenic or vacuum at 500-600 OC and then cooling to room temperature. A variety of approaches were taken in repeating the data on the various reconstructions, and consistent results were obtained. Surface cleanliness was monitored by Auger spectroscopy and low energy electron diffraction (LEED) was used to determine the reconstruction of the surfaces. Hydrogen exposure was carried out at a pressure of 3x10-~ tom with a hot (%2000°C) filament near the sample being used to dissociate the H2. The filament was not in line of sight with the sample. Saturation coverage occurred after exposure to about lo6 Langrnuirs but the cracking fraction was not determined.

Photoemission measurements were carried out at the Stanford Synchrotron Radiation Laboratory in the energy range 100 to 150eV. The combined resolution of the Grasshopper monochromator and the spectrometer was set to 0.5eV.

RESULTS AND DISCUSSION for Clean Surfaces Figure 1 shows the Ga and As 3d core line spexra as a function of surface reconstruction measured at 130 eV and normalized to excitation intensity. Core level photoemission measurements were taken as a function of photon energy over the range 70-210 eV to probe effects which could relate to escape depth effects in analyzing the data for surface composition. Valence band spectra were taken at 130 eV since the s to p cross section ratio is about one at this energy as well as the escape depth being short Ga 3d to As 3d core line area ratios were converted to an As coverage scale derived from a model of the photoemission yield. The calibration was established with respect to data obtained from the

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0.0 0.5 1.0 As COVERAGE (MONOLAYERSI

GaAs (100) X=4.5& -

d = 1 . 4 1 A -

-

B ^-

0 -

-

⁽²

X:r\C:"

^4):

'j 0.5 - -

-

Ga 3d/As 3d core level area ratio plotted versus As coverage for the respective reconstruction.

Our dala PreviousdataP) As atoms per As atoms per As Ga atoms in As Ga atoms in R = cover- the unit cover- the unit

Rec. Ga/As age cell age cell

c(4 X 4) 0.65 1.00 818 0.86 718

c ( 2 X 8) 0.72 0.89 0.61 518

c(8 X 2) 0.98 0.52 0.22 218

I X 6 1.07 0.42 2.516 0.52 316 4 X 6 1.17 0.31 7.5/24 0.27 65/24 ')Drathen, Ranke, Jacobi, Surf. Sci. 77, L162 (1978).

GaAs(ll0) face. The non-polar (110) face has a known composition and the structure for its 1x1 relaxation is now accepted. The (110) surface is usually created by cleavage, and one finds a i 10%

variation on a cleave-to-cleave basis for the Ga 3d/As 3d ratio. This probably reflects a variation in the step density. Steps on GaAs(ll0) should expose principally extra Ga edge atoms.

To determine the As scale, a model of the electron yield was derived in terms of the surface arsenic composition x from the core area ratio R=Ga 3d/As 3d. This yield model has several limiting cases and the solution rotates around x = 1/2 as a function of the effective interplanar spacing to escape depth ratio. We find that an escape depth of 4.5 A is most consistent with the data, and provides the scale shown in Fig. 2. This scale has the ~(4x4) as a complete monolayer of arsenic termination. This surface will adsorb additional As from an As4 source which yields a saturation ratio R=0.3 and corresponds to an additional monolayer. The strong contrast achieved in both the core level and valence band data is consistant however with such a small value.

Table I shows the room temperature composition values derived from the analysis. For comparison, the composition derived by Drathen et a@ obtained with Auger spectroscopy is also shown. The principal difference is the assignment for the ~(8x2) which in our case is 0.52 and in theirs 0.22. Most likely, the discrepancy arises from the use of Auger peak-to-peak ratios which can be distorted by changes in background and valence band width.

The chemisorption of excess As was investigated for both the (100) and (110) face. One finds in both cases a very rapid uptake saturating at about 10 Langmuirs when an As4 source is used. The excess arsenic is reflected in a diffuse 1 x 1 LEED pattern. The deposition with As;? does not saturate. In both cases, annealing in vacuum to approximately 300 C will desorb the excesswe have explored driving the surface further gallium-rich by depositing excess monolayers of Ga at room temperature. Typically, the surface shows a fuzzy 1x1 LEED pattern. Annealing the surface at temperatures below 6W°C usually restores the 4x6 reconstruction and Ga/As ratio presumably due to As out diffusion.

Two other aspects correlate with the surface phase diagram. One of these is the change in surface core level binding energy as a function of reconstruction. The data will be presented along with the hydrogen results. Several trends are obtained from the valence band spectra presented elsewhere.

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C5-148 JOURNAL DE PHYSIQUE

The data presented have several aspects which pertain to obtaining structural models for the GaAs(100) and AlAs(100) reconstructions. One sees from Table I that the GaAs reconstructions are ordered through a series of centered structures and then non-centered ones. Within the composition scale established, the centered structures have an integral number of surface As atoms and vacancies per unit cell and the Ga is confined to the second layer. The centered structures are related to dimerization of the surface a r ~ e n i c . . ~ , ~ ~ , ~ ~ The non-centered structures would then appear to have a mixture in the outer layer of Ga, As, and vacancies. From the annealing results, one concludes that the surface does not equilibrate to more Ga rich structures, probably due to As outdiffusion. This type of behaviour has been shown earlier for Ga on ~ a A s ( l l o ) . l ~ This trend also helps to understand the fact that the ~(8x2) is the prominant high temperature phase since the minimum possible surface As depletion may in fact decrease.

A similar set of measurements has been performed on the AlAs(100) surface. The AIAs(ll0) reference surface was grown on a cleaved GaAs(ll0) substrate and very sharp l x l

The AlAs(100) surface intrinsically shows different behaviour from the GaAs(100) in the existance of such a large disordered composition range8. This most likely is an inhomogeneous disorder consistting of a large number of random dimerized microdomains. The ordered 3x2 structure that was found is indicative of a Uimerization which is not found on the GaAs(100) surface. This different tendency may be related to the different surface state spectrum to be expected for AlAs which has an indirect band structure. As discussed recently by chadi19, couplings between critical point charge densities can influence the stable reconstructions. This intrinisic difference may also explain why it is generally more difficult to initiate MBE growth of GaAs on GaAlAs for special structures. One would expect from our results though that greater sucess would be achieved by growing closer to metal rich. To date we have note been able to well order the GaAlAs surfaces.

LEEDpatternwasobtainedwhichwasasgood ^{' 5 -}

as the original cleaved GaAs(ll0) LEED pattern.

4 ' 0

The AlAs(100) surface, however, showed a very 3 t

different behavior from GaAs(100), which is summarized in Fig. 3. Fig. 3 plots the A1 2p/As

3

^0.5

3d core level ratio as a function of the surfaces investigated. An ordered 3x2 reconstruction was

I I I I I I I I At& I 1100) I -

-

- e '

' e

\ e

3x2 WP4RENT 1x1

- -

found at 25% As coverage but, above about 30% Î Î ' ^{0 5}Î Î Î ' ^?.oI I '

&COVERAGE

As coverage, only weak apparent 1x1 patterns could be obtained. This probably reflects a

large amount of surface disorder. As was the A1 2~Zp/As 3d 'Ore level ratio as a case with the GaAs(100) surface, it is difficult to function of As coverage for Ahs(100). The surface exhibits a wide composition range drive the surface excessively Al-rich by where only an apparent lxl LEED pattern is

annealing. observed.

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GaAr (100) (4 X 6 ) hv = 130 eV

I

1.8 -GaAsIlOOI -

--- GaAr (TTT) -

P

-

12 x 2) -

-

0-

Ga and As 3d core level spectra Ô.SO Î Î Î Î measured at 130 eV for GaAs(100) (4x6). ⁵_HYDROGEN¹⁰EXPOSURE ( 1 0 5 ~ ) ¹⁵ ²⁰

Fig. 5 Variation of As/Ga 3d core level intensity ratio as a function of Hydrogen exposure for (100) and ( I l l ) surfaces as a function of reconstruction.

RESULTS AND DISCUSSION for HYDROGEN EXPOSURE

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LEED studies showed that after hydrogen chemisorption the GaAs(ll1) surface reverted to a (1x1) symmetry. This was the case both for the As-rich (2x2) and the Ga-rich ( J 19x J 19)R(23.4O) reconstructions. On GaAs(100) the same symmetry was reached in all cases after hydrogen exposure, however the LEED pattern was not purely (1x1) but contained faint diffuse spots at (&%,+I) but not at (*l,*lh). This pattern is also seen on the clean surface when its composition is intermediate between that of the As-rich ~(4x4) and 42x8) reconstructions. Vacuum annealing of the GaAs(100):H surface yielded a ~(2x8) reconstruction independent of whether the starting surface was As or Ga-rich. On the clean surface however, vacuum annealing changes the reconstruction in the sequence c(4x4), c(2x8), 48x2), (1x6), (4x6), suggesting that GaAs(100):H has a composition between 44x4) and ~(2x8).

The Ga 3d and As 3d core levels were measured for a range of reconstructions on GaAs(100) and ( i l l ) as a function of hydrogen coverage. As an example, Fig. 4 shows the effect of hydrogen chemisorption on the core line spectra for GaAs(100) (4x6). All spectra have been normalized to the same peak height and the levels for the hydrogen covered surface have been shifted to lower binding energies by 0.12 and 0.03eV for Ga 3d and As 3d respectively, to compare spectral shapes.

The As 3d lineshape can be seen to be most strongly effected by the hydrogen chemisorption, with extra spectral weight appearing on the high binding energy side. This result was seen for both (100) and ( i i i ) surfaces and for all reconstructions examined.

The earlier work on clean GaAs(100) showed that both the peak positions and intensities of the core levels could be correlated with the surface reconstructions.20. A similar analysis here enables us to gain further insight into the effect of hydrogen on the surface. The integrated intensity for the core levels was measured after background subtraction, and the intensity ratio As/Ga is plotted in Fig. 5 as a function of hydrogen exposure. For GaAs(100), the intensity ratio for the As-rich

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C5-150 JOURNAL DE PHYSIQUE

44x4) surface and for the Ga-rich (4x6) surface differ considerably, but after H exposure become virtually the same. A similar trend is seen for ~ a ~ s ( I i 1 ) . The occurrence of a common surface after hydrogen exposure is consistent with the LEED results described above.

In the upper half of Fig, 6 we plot the As/Ga intensity results for hydrogen chemisorption with the earlier results8 for clean GaAs(100). The earlier results are shown by the solid circles and the arrows show the changes caused by an exposure to lo6 Langmuirs of hydrogen.

A similar plot for the separation between the As 3d and Ga 3d peaks is shown in the lower half

surface. All of the results described are

upper) Variation of As/Ga 3d core level consistent with the formation of an As-rich intensity ratio data a presented in Fig. and surface layer after H chemisorption and with the nlodification induced by a hydrogen bonding being between the As and H atoms. saturation coverage. The hydrogen saturated of Fig. 6. The As 3d and Ga 3d binding energy, ⁱ ^l ⁱ ^l ^J ^l ^l ^l ^l ^J ^l ^l ^~

3d INTENSITY RATIO

respectively, show changes as a function of ^ArIGa reconstruction versus photon energy over the 1.5 -

range 70-210 eV, but the change in relative separation shown in Fig. 6 results from a surface dipole which is very dependent upon the As surface dimerization. One sees a rapid change

This can be due either to removal of Ga from surface comes to a common ratio independent the surface layer or a migration of As to the of the starting surface composition.

from 44x4) to c(8x2), but relatively little further o 5.-

change for the more Ga rich reconstructions. ^0.0

22,1.-

Most of the shift is in the Arsenic peak. The variation in the surface As 3d binding energy

shifts is

+

0.64 and the Ga 3d -0.42 eV in

,,,-

going from the excess As monolayer to the

2 -

excess Ga monolayer. The major change in the $ Ga 3d occurs, however, in going from the 4x6 to

2

the excess Ga monolayer. One deduces from

this that the clean surfaces show a fundamental 21.8

change in the nature of the reconstrcutions as the surface goes Ga rich.

surface from the bulk. Most likely a volatile As 3d Ga 3d core peak gallium hydride forms and plays a major role in separation as function of As coverage for clean the compostion changes that are seen. and hydrogen saturated surfaces. The Experiments to conclusively establish this are in hydrogen saturated surface comes to a

progress. common separation independent of the starting

surface composition.

Examination of this combined data reveals that ^ASCOVERAGE IMONOLAYERS)

the surface after hydrogen chemisorption is

similar in character to the clean As-rich ~(2x8)

&d

- -

m *

X X X X X

-

^e

- -

^- ^m^U

- -

^N^L2 ^P^‘3 ^-

~ : f : l l : ; l l : :

3d PEAK SEPARATION -

A

/!

-

.

^X^.^-

²

I I I I I I I I I I I I .

0.0 0.5

:

1.0

-

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surfaces of GaAs could modify the growth surface and effect impurity incorporation via modifying the growth interface composition and reconstruction. The results presented here show a mechanism wherby MBE growth could be influenced by the presence of a hydrogen ambient, but the growth system would have to have hot filaments or other means of cracking the input molecular hydrogen.

In the case of Calawa's results3, enough detail is not presented to deduce whether hi system configuration was likely to have induced a significant fraction of cracked hydrogen.

Acknowledgement--The authors acknowledge stimulating conversations with Dr. D.J. Chadi. We thank B. Krusor, Lars Swartz, B. Goldberg, and H. Sang for help with the measurements. Some of the materials incorporated in this work were supported by NSF Grant DMR 81-08343 and developed at the SSRL, which is supported by the NSF (DMR77-27489) in cooperation with SLAC and the DOE.

References

1. A.Y. Cho, J. Appl. Phys., 42, 2074 (1971).

2. J.R. Arthur, Surf. Sci.,

43,

449 (1974).

3. A.R. Calawa, Appl. Phys. Lett., 33, 1020, (1978).

4. J.A. Appelbaum, G.A. Baraff and D.R. Hamann, Phys. Rev. B,

u,

1623 (1976).

5. I. Ivanov, A. Mazur and J. Pollman, Surf. Sci.,

92,

365 (1980).

6. J. Massies, P.Etienne and N.T. Linh, Thomson-CSF Technical Review,

4,

5 (1976).

7. P. Drathen, W. Ranke and K. Jacobi, Surf. Sci., 77, L162 (1978).

8. R.Z. Bachrach, R.S. Bauer, P. Chiaradia, G.V. Hansson, J. Vac. Sci. Tech,

B,

797 (1981).

9. K. Jakobi, C.V. Muschwitz and W. Ranke, Surf. Sci., 82, 270 (1979).

10. R. Ludeke and A. Koma, CRC Critical Rev., Solid State Sci.,

5,

259 (1975).

11. P.K. Larsen, J.H. Neave and B.A. Joyce, J. Phys. C, l4, 167 (1981).

12. J. Massies, P. Etienne, E. Dezaly and N.T. Linh, Surf. Sci.,

8,

121 (1980).

13. R.D. Brjngans and R.Z Bachrach to be published.

14. R.Z. Bachrach, "MBE-Molecular Beam Epitaxial Growth", Chapt 6, Crystal Growrh, 2nd ea!

edited by Brian Pamplin, Pergamon Press, New York, 1980.

15. R.Z. Bachrach and B.S. Krusor, J. Vac. Sci. Technol., @, 756 (1980).

16. D.J. Chadi, J.S. Ihm, and C. Tanner, Surface Science Lett., to be published.

17. A.Y. Cho, J.App1 Phys, 47, 2841, (1976).

18. R.Z. Bachrach and A. Bianconi, J. Vac. Sci. Technol., l5, 756 (1978).

19. D.J. Chadi, J. Vac Sci TechnoL, 17, 989, (1980).

20. RZ. Bachrach, R.S. Bauer, P. Chiaradia, and G.V. Hansson, J. Vac Sci Technol., 19, 335, (1981).