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PHOTOEMISSION STUDY OF ALLOYS AND HETEROSTRUCTURES OF III-V COMPOUND
SEMICONDUCTORS
H. Okumura, I. Yoshida, E. Muneyama, S. Misawa, K. Endo, R. Yoshida
To cite this version:
H. Okumura, I. Yoshida, E. Muneyama, S. Misawa, K. Endo, et al.. PHOTOEMISSION STUDY OF
ALLOYS AND HETEROSTRUCTURES OF III-V COMPOUND SEMICONDUCTORS. Journal de
Physique Colloques, 1987, 48 (C5), pp.C5-45-C5-49. �10.1051/jphyscol:1987507�. �jpa-00226678�
JOURNAL D E PHYSIQUE
Colloque C5, supplbment au n o l l , Tome 48, novembre 1 9 8 7
PHOTOEMISSION STUDY O F ALLOYS AND HETEROSTRUCTURES O F 111-V COMPOUND SEMICONDUCTORS
H. OKUMURA, I. YOSHIDA' , E. MUNEYAMA'
'
, S. MISAWA, K. END0 and S. YOSHIDAElectrotechnical Laboratory 1-1-4, Umezono, Sakura-mura, Niihari-gun, Ibaraki 305, Japan
Abstract
The electronic structures of MBE-grown AlAs:GaAs, A1Sb:GaSb and GaSb:GaAs compound semiconductor alloys and heterostructures were investigated by in-situ photoemission measurements. It was found that the binding energies of inner core levels for the alloys do not change with the compositions within
+
O.leV, and correspond with those of the heterostructures. AlGaAs and AlGaSb alloys show some changes of valence bands in accordance with the compositions, while the variation of GaAsSb alloy is not monotonous. For AlAs-GaAs heterostructures, the minimum layer thickness necessary to show the bulk-like band structure was estimated to be 5-6 monolayers. The valence band offsets of AlAs-GaAs, AISb-GaSb and GaSb-GaAs heterostructures were estimated to be 0.12, 0.19 and 0.35eV9 respectively.Introduction
Recently, the research of superlattices of X - V compound semiconductors has been much advanced from the viewpoint of both the application to electronic devices and the research of solid state physics. It has become possible to control the deposition of 1 monolayer[l]. However, there have been few photoemission studies regarding the electronic structures of clean surfaces of semiconductor alloys and heterostructures[2,3], which are components of superlattices. From photoemission spectra, we can obtain informations about the electronic structure in the vicinity of the surface, such as binding energies of component atoms, the density of states of valence band, and the energy of the valence band edge.
In this paper, we report the results of in-situ photoemission measurements (XPS and UPS) of AlAs:GaAs, A1Sb:GaSb (X-X- V systems) and GaSb:GaAs
(a -
V - V system)alloys and heterostructures for the first time. We discuss the binding energies of inner core levels, the differences of valence bands among these three alloy systems, the formation process of valence bands, and the valence band offsets(A$)
.
Experiment
The samples measured were A1,Gal-, As, A1,Gal-, Sb, GaAs S ~ I - ~ alloys and N monolayers(ML) AlAs on GaAs, N ML GaAs on AlAs, N ML GaAs on ~ a g b , N ML GaSb on GaAs heterostructures. The samples were grown on n+-GaAs(OO1) substrates by MBE under the As(Sb)-stabilized condition. The alloy compositions were determined from the peak intensities of XPS. The heterostructures were prepared monitoring the intensity oscillation of RHEED[l]. After the growth, photoemission measurements were immediately done wi,thout exposing the samples to air, using a GMA type electron energy analyzer. The excitation sources were MgKa-line for inner core levels(XPS) and HeI-line for valence bands(UPS). The resolutions of the analyser were 1 .O, and
(''present address : Sanyo TsukUba Research Center
("present address : UBE Scientific Analysis Laboratory
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987507
C5-16 JOURNAL DE PHYSIQUE
0.3eV, respectively. The base pressures of the growth chamber and the analysis chamber were less than 2x10-l0 Torr.
Results and Discussion
(1) Inner core levels.
---
In Figs. l (a), and (b), the XPS spectra of inner core levels (A12p, G d d , As3d and Sb4d) for AlxGal-xAs and GaAsySbl- alloys are shown.The binding energies of the inner core levels and the energy intervals between them show no change with the alloy compositions within the error of + O.leV. The microscopic environments of the component atoms should be mainly determined by the four nearest neighbor atoms. Therefore, some chemical shifts are expected for non- alloying components, i.e., As for AlGaAs and Ga for GaAsSb, owing to the differences between the electronegativities of alloying components. However, no chemical shift was observed in our measurements. The heterostructures also showed no chemical shifts. These results mean that the differences of the binding energies are less than O.leV for the alloys. The small differences of chemical shifts indicate thu similarity of the chemical bonding for thesea
-
V compound semiconductor systems.(2) Valence b k d structures of alloys.
---
Figure 2 shows the UPS spectra of the valence bands of A1,Gal-,As alloys. The background due to non-elastic scattering was subtracted. The peaks observed at around -0.6eV are due to Ga3d core level excited by extra Hex[-line. All the spectra show two major structures denoted by P1 and P2.The origins of these structures are attributed to the anion (As, Sb) hybrid p- orbitals (PI) and the cation (Al, Ga) S-orbitals (P2). The structures P1 and P2 are considered to inherit the features mainly from anion and cation atoms, respectively.
The peak energies and the widths of the structure P2 against the alloy composition X are shown in Fig. 3. The peak energy decreases by the amount of 0.8eV as the composition X increases from 0 to 1 . The surface segregation of Ga was reported by Massies et a1.[4], However, neither peak separation for P;! which might result from AlAs-GaAs clustering nor indication of elemental Ga segregation for Ga inner core levels was observedfor our samples. We think such segregation is too little, if any. Considering the origin of the structure
4,
the shift of P2 isAI Ga - As
(a) Ga 3 d X 2
2 2
-
m
-
C c-
44 40
Binding Energy/& Binding ~ n e r g y / e v
Fig. 1 levels
XPS spectra of (a) AlxGa1-xAs, (b) G a A ~ ~ s b l - ~ alloys. The outer-most of the component atoms (A12p, Ga3d, As3d and Sb4d) are shown.
-
Binding Energy Widthm U
P
1.0
8 6 4 2 0 0.0 1.0
core
B i n d i n g E n e r g y / e V AI composition
Fig. 2 UPS spectra of the valence Fig. 3 The composition X dependence of bands of Alfial-xAs alloys. the peak energies and widths of the
structure P2 for AlxCal-xAs alloys.
reasonable. According to the band calculations for GaAs[5], AlAs[6,7] and AlGaAs[8,91, structure P 2 corresponds to the critical points of L6 and X6, and the difference of L6 and X6 energy between GaAs and A1As is around 1.3-1.6eV. Although the linear feature of the shift is in agreement with the calculations, the amount of the shift is smaller than the results of the calculations. As regards the width of P2, the alloys show broader widths than those of GaAs and AlAs. The composition where the width takes the maximum is around x=0.5. The broad feature of P2 for the alloys can be interpreted as the band broadening due to the alloy disorder. This experimental result suggests that ambiguousness is contained in the band structures of alloy compound semiconductors compared with those of non-alloy ones. On the other hand, obvious changes of the structure P1 with the composition are not observed. The origins of the structure P1 are the anion p-orbitals, and the anion is common for the alloys between AlAs and GaAs (D - X I - V alloys). Therefore, large change is not expected to occur for the structure PI
,
as seen in Fig. 2. For AlxGa1-,Sb alloys, the similar features were observed, i.e., so small change of the structure P1 and the shift of P2. The amount of the shift of P 2 is 0.8eV, comparable to that for AlGaAs alloy. However, the relative binding energies of P2 is 0.3eV lower than those of AlGaAs. This difference can be understood as the difference of the electronegativity between As and Sb.The UPS spectra of the valence bands of alloys are shown in Fig. 4 . GaAsSb is the alloy between GaAs and GaSb
(a -
V- V alloy). Differing from AlGaAs and AlGaSb alloys, changes may be expected to occur for the structure PI,
considering the origins of the structures. However, some changes are observed for the structure Pp rather than for PI. The peak energies of the structure P2 are plotted against the composition y in Fig. 5. The amount of the shift is around 0.4eV and it is noted that the maximum binding energy is taken at the intermediate composition, which is in contrast with the cases of AlGaAs and AlGaSb alloys. This is the first direct observation of non-monotonous feature of the electronic structure against the composition. For GaAsSb alloys, the band gap energy also takes the minimum at the intermediate composition[l0]. These non-monotonous
GaAs,Sb,-,
Binding ~ n e r g y / e V
Fig. 4 UPS spectra of the valence bands of G ~ A S ~ S ~ T - ~ alloys.
.
N ML GaAs on A I AsD N ML A I As on GaAs
7 . 0 1 - 1
-
4
0 1 2 3 4 5 6m Number of overlayers
Fig. 6 The peak energies of the structure P 2 for N monolayers (ML) GaAs on AlAs and N ML AlAs on GaAs against N.
/ / I I I L I 1 I / l
0.0 0.5 1 .o
Composition y
Fig. 5 The peak energies of the structure P 2 for G ~ A S ~ S ~ ~ - ~ alloys.
N ML GaSb on GaAs
Binding Energy/eV
Fig. 7 The UPS spectra of the valence bands of N monolayers
(m)
GaSb on GaAs.oo means thick GaSb overlayers enough.
C5-48 J O U R N A L DE PHYSIQUE
features are often observed for the electronic structures of Sb-including alloys.
We think that such non-monotonous features are related to the large difference of the size between As and Sb atoms.
( 3 ) Valence band structures of heterostructures.
---
In Fig. 6, the bindingenergies of the structure P 2 are plotted against the number of overlayers for N ML GaAs on AlAs and N ML AlAs on GaAs. The variations in accordance with the number of overlayers N are observed. The escape depth of photoelectrons is around 51 in the UPS measurements, which means that the probed region is within almost 2 ML of the overlayers from the surface. 2 ML GaAs on AlAs or 2 ML AlAs on GaAs do not exhibit the peculiar features of GaAs or AlAs itself. Comparing the spectra of the heterostructures with those of thick GaAs or AlAs, it was shown that the minimum value of N to exhibit the bulk band features of thick GaAs or AlAs layers is 5-6.
The valence band structure of the overlayers with N less than 5 is considered to be different from that of thick layers and bulk materials. To complete the valence band structure peculiar to GaAs or AlAs, at least 5-6 ML are necessary. The similar situation is supposed for the conduction band. The design of superlattices is ordinarily based on Kr6nig-Penney model, where band parameters of bulk materials are used. However, it is unproper to design band structures using bulk band parameters in the case of the superlattices with N less than 5 .
In Fig. 7, the UPS spectra of N ML GaSb on GaAs heterostructures are shown. In this case, even 10 ML of GaSb overlayers does not show the features of thick GaSb layers. There is as much as 7.45% lattice mismatch between GaSb overlayers and GaAs underlayers. This amount of lattice mismatch is much larger than that of AlAs/GaAs heterostructures (0.34%). Therefore, the GaSb overlayers are considered to be much strained or distorted. The strain and distortion may have an influence on the formation of the GaSb band. One reason for the fact that the band of 10 ML GaSb on GaAs is not completed can be attributed to the effects of the strain or distortion.
Much attention should be paid for the design of strained layer superlattices.
(4)
Valence band offsets.---
By photoemission measurements, the density of states of valence bands can be directly observed. The binding energies of the valence band edges are determined from the spectra of valence bands. To date, there have been several studies regarding band offsets of heterointerfaces[ll-151. However, consistent results were not necessarily obtained. From the in-situ measured UPS spectra of the alloys and the heterostructures, We tried to determine the valence band edge energies, because the resolution of UPS is better than that of XPS. The band edges were determined as the intersecting points of the lines showing the density of states of the valence bands and the base lines, as shown in Fig. 8. ' We checked the energy reference for the absolute values of the binding energies using the Ga3d excess peaks in UPS spectra. The shifts of the whole spectra due to the charge-up effect were within 0.leV. For all the cases, the shift of the edge was observed in accordance with the structures(x, y and N ) . .The measured values for the valence band offsets for AlAs/GaAs, AlSb/GaSb and GaAs/GaSb heterointerfac'es were 0.12, 0.19 and 0.35eV, respectively. The experimental error of the determination of the band edge energies is estimated to be 0.23eV, considering the resolution of the analyser and the accuracy of the energy reference. For AlAs/GaAs heterointerface, the experimental error is larger than the obtained AE, value, unfortunately.However, as much as 0.5eV of LIE,, which has been recently proposed for AlAs/GaAs heterointerface[l6], was not observed.
Fig. 8 The valence band ed'ges observed in the UPS spectra for N monolayers (ML) GaAs on AlAs. oa means thick GaAs overlayers enough.
Acknowledgment
We wish to thank Y.Kuronuma and A.Arai for the help in sample preparations. We are also grateful to N.Hashizume and S.Maekawa for their continuing encouragement.
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