HAL Id: jpa-00226207
https://hal.archives-ouvertes.fr/jpa-00226207
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
LOCAL STRUCTURE IN InGaAsP QUATERNARY ALLOYS
H. Oyanagi, Y. Takeda, T. Matsushita, T. Ishiguro, A. Sasaki
To cite this version:
H. Oyanagi, Y. Takeda, T. Matsushita, T. Ishiguro, A. Sasaki. LOCAL STRUCTURE IN In- GaAsP QUATERNARY ALLOYS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-423-C8-426.
�10.1051/jphyscol:1986885�. �jpa-00226207�
LOCAL STRUCTURE IN InGaAsP QUATERNARY ALLOYS
H. OYANAGI, Y, TAKEDA*, T. MATSUSHITA*", T. ISHIGURO and A. SASAKI*
E l e c t r o t e c h n i c a l L a b o r a t o r y , Umezono, Sakuramura, N i i h a r i g u n , I b a r a k i 3 0 5 , Japan
' ~ e p a r t m e n t o f E l e c t r i c a l E n g i n e e r i n g , Kyoto U n i v e r s i t y , K y o t o 6 0 6 , Japan . f N a t i o n a l L a b o r a t o r y f o r High Energy P h y s i c s , Ohornachi.
Tsukubagun, I b a r a k i 3 0 5 , Japan
ABSTRACT
The local s t r u c t u r e of InGaAsP quaternary alloys epitaxially grown on InP substrates has been studied by EXAFS and near-edge structure. The bimodal distribution of bondlength a t both cation (In,Ga) and anion (As,P) sites w e r e confirmed. The bondlengths w e r e found t o deviate from t h e VCA average by 2.4- 5.5 % whereas they deviate from those in pure binary compounds by only 0.9-1.6
%. These deviations w e r e found t o be constant within 0.4 % over a wide range of composition when t h e VCA average i s kept constant. Two cation species (Ga,In) and anion species (As,P) a r e expected t o be randomly distributed on e a c h sublattices. The average interatomic distance calculated from t h e weighted sum of bondlengths agrees the VCA average over a wide range of alloy composition. These results imply that l a t t i c e relaxation of lattice-matched quaternary alloys c a n b e realized by only bond-bending.
INTRODUCTION
InGaAsP alloy semiconductors lattice-matched to InP have been extensively studied and developed for their use in the optical communication system in t h e
I-pm wavelength region. The alloy composition dependence of t h e fundamental properties have been experimentally studied and estimated using a linear interpolation from t h e parameters of t h e constituent binary compounds o r t h e virtual crystal approximation (VCA). To understand t h e characteristic properties of these alloys such as a bowing of optical gap, t h e atomic-scale structure within a few A around each constituent species of atom is essential.
The bimodal distribution o f bondlength in pseudobinary semiconductor alloys has been predicted by Fong. e t al. [ I ] and experimentally confirmed by Mikkelsen e t al. [2] using EXAFS. Previous EXAFS studies of ternary alloys such as InGaAs revealed t h a t t h e bondlength is r a t h e r close t o that in constituent binary compound deviating from the average interatomic distance obtained from X-ray d a t a and the VCA. In this study, t h e local s t r u c t u r e and l a t t i c e relaxation in quaternary alloys have been investigated, for which t h e VCA is expected t o b e more valid than for t h e ternary system because of more random distribution of chemical bonds.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986885
JOURNAL DE PHYSIQUE
EXPERIMENTAL
InGaAsP quaternary alloys and InGaAs ternary alloys lattice-matched t o InP were grgwn by liquid phase epitaxy (LPE) a t 6 2 5 4 3 5 ° C with a typical thickness of 5000 A. For t h e EXAFS measurements t h e InP substrate was thinned down t o 50 u m and a t t a c h e d t o a silicon-holder with a hole t o pass t h e transmitted X- rays through it. G a and As K-edge X-ray absorption s p e c t r a were obtained utilizing synchrotron radiation from t h e 2.5 GeV s t o r a g e ring a t t h e Photon Factory and a fluorescence EXAFS apparatus [3J. X-rays from the s t o r a g e ring were monochromatized and focused horizontally a t the sample by a sagittally bent crystal monochromator 141. Fluorescence X-rays w e r e collected by a scintillation d e t e c t o r a r r a y which covers 18 % of t h e total solid angle. The ,,
energy resolution was 2 eV a t 9 keV and a typical incident photon flux of 10 photons/sec was obtained.
RESULTS AND DISCUSSION
The equi-energy gap contour for InGaAsP quaternary alloys is illustrated in Fig. 1. If t h e alloy composition is chosen along t h e dashed lines, t h e l a t t i c e constant c a n be matched t o those of InP or GaAs. In this study, t h e composition was carefully chosen s o t h a t t h e l a t t i c e constant matches that of InP. (a), (b), and (c) denote t h e compositions for In0.53Ga0.47As, Ino.60Ga0.4oAs0.89P0.11, and Ino.74Gao.26Aso.58P0.42, respectively. Figure 2 snows t h e results o t F o u r i e ~ transform of As K-edge EXAFS oscillations. The first peak located a t 2.3 A consists of two shells. These two distances correspond t o As-Ga bond and As-In bond, respectively.
The nearest neighbor contribution is back-Fourier transformed into a k-space and f i t t e d by a single scattering formula [5]. The phase shift functions for Ga-As, Ga-P, As-Ga, and As-In pair w e r e determined by fitting t h e G a and As K- edge EXAFS d a t a for GaAs, Gap, and InAs with t h e s a m e formula using t h e theoretical backscattering amplitude and known l a t t i c e parameters. Utilizing the experimental phase shift functions and theoretical amplitude functions, t h e bondlengths and Debye-Waller factors were obtained by a least-squares fit.
Fig. 1 Equi-energy gap contour of InGaAsP alloys.
R ( A ) ENERGY ( e V )
Fig. 2 Fourier transform of As K-edge EXAFS. ~ i 3 AS ~ . K-XANES spectra.
Various ratios of cation species (Ga,In) and anion species (As,P) w e r e tested in t h e least squares fit fixing t h e t o t a l coordination number of four.
The best fit was obtained when the ratio proportional t o t h e alloy composition was used, indicating t h a t e a c h sublattice is randomly distributed by t h e two species.
In Table I, t h e results of this least-squares analysis a r e summarized. Thg Ga-As bondlength in ternary o r quaternary alloys is conGderably (ca. 0.08 A) shorter than t h e average igteratomic distance of 2.54 A but r a t h e r close t o t h a t of pure GaAs (2.448 A) indicating t h e existence of bond alternation in quaternary alloys. The Ga-As, bondlength in InGaAsP alloys lattice-matched t o InP is unchanged within 0.001 A.
T h e Ga-P bondlength in In10,7q,Ga 0,26As0,58P0.42 is shorter than t h e average i n t e r a t o ~ i c distance by 0.14 A while i t IS longer than that of pure G a p by 0.035 A. The As-In bondlengthoin InGaAsP alloys is longer t h a n t h e average i ~ t e r a t o m i c distance by 0.07-0.08 A but shorter than t h a t of pure InAs by 0.23 A. The In-P bondlength in InGaAsP alloys is not available but i t is estimated
t o b e close t o t h e In-P distance in pure InP, o r t h e average interatomic distance (2.5412 A) since t h e quaternary alloys a r e lattice-matched t o InP.
Inspecting t h e values given in Table I, i t i s clear t h a t t h e bondlengths a r e insensitive t o t h e average coordination number of t w o cation o r anion species.
The Debye-Waller factor of e a c h bond in InGaAsP alloys was found t o be close t o that in pure binary compounds.
Figure 3 shows t h e As K near-edge s p e c t r a of InGaAsP alloys. Solid and
dotted lines indicate t h e raw spectra and their first derivatives. Although no
appreciable edge shift has been observed between alloys and pure binary
compounds, a new doublet s t r u c t u r e is observed in t h e derivative s p e c t r a a t t h e
absorption threshold, which is not observed in pure InAs o r t h e 1:l mixture of
InAs and GaAs. The peak located a t t h e lower energy increases i t s strength on
going from (a) t o (b) indicating that this s t r u c t u r e is caused by alloying with
indium. The Td symmetry around As atom in InGaAsP alloys is expected t o b e
distorted because of In atom, which might split t h e degenerated conduction band
by a crystal field e f f e c t o r enhance t h e s-d transition by mixing with p-
states.
JOURNAL DE PHYSIQUE
"Obtained by x-ray diffraction.
2.60
Table I. Bondlengths in Inl-xGaxAsl- P -
'U
Specimen Bondlengths (A) -- 2.58
I Ga-As Ga-P As-Ga As-In I-
(3
(a)x=0.47 2.56
y=O 2.47 - 2.47 2.60 J
(b)x=0.40 n
y=O.11 2.47 - 2.47 2.60 jsj 2.54
m (c)x=0.26
y=O.42 2.47 2.40 2.47 2.59 W 2 2.52
VCA" 2.54 2.54 2.54 2.54 C3Z
w
GaAs* 2.448 2.448 2.50
Fig. 4 Average bondlengths in InGaAsP quaternary alloys.
In,-, Ga, Py
- -
-
V C A (2.5412A
o u u
-
I
