Electronic Band-structure of Mg 1-x Zn x S y Se 1-y semiconductor alloy
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K. L. Teo , Y. P. Feng , M. F. Li , T. C. Chongt t Center for Optoelectronics, Department of Electrical Engineering t Depatment of Physics National University of Singapore, Singapore 0511
Abstract Il-VI semiconductor alloys have recently received considerable attention for their possible use 4 in double heterostructure (DH) blue laser diodes (LDs).'The purpose of this paper is to present the empirical pseudopotential method within virtual crystal approximation for calculating the band structure of MgZnSSe quaternary alloy. The dependence of band gap energies on alloy composition has shown that MgZnSSe can be a direct or an indirect semiconductor. Electron and hole effective masses are calculated for different composition. Camel's back structure for the X valley conduction band has been found for certain composition range. 1 Introduction Research on I1-VI semiconductors has shown that ZnSSe and ZnCdS are regarded as suitable materials for the active layer of a blue laser diode (LD). Thus far, ZnCdS and ZnSTe have been 6 regarded as possible materials for the cladding layer of these materials. Recently, Okuyama el al', have reported a new material, MgZnSSe, whose band-gap energy can be varied from 2.8 eV to near 4 eV, maintaining lattice matching to a (100) GaAs substrate. Their results have shown that ZnMgSSe is a promising material for the cladding layer of blue LDs. To enhance our understanding on future device concepts and applications, w6 have carried out 78 the band structure calculation on MgZnSe using the empirical pseudopotential method , (EPM). 2 Pseudopotential calculation The local EPM through the use of VCA is used to calculate the band structure of MgZnSSe. The form factors for Mgl_,Zn.,SySel_. are obtained for each composition (x,y) from V
11.o
=
[(1 -
X)(1 - Y)SAMgSeVMgSe + (1 - X)YQMgSVMgs
+X(1 - Y)OlznseVznse + ryQz.sVz",s]/Q
(1)
where V.11,y and Q)are the form factors and the volume of the primitive cell of MgZnSSe respectively. 6) ) MgSe, OMgS,irZnSe and QzZs are the primitive cell volumes of the endpoint binary compounds (EBC). The lattice constant of the quaternary alloy is calculated by Vergard's law: a = (1 - x)(1 - y)aMgS + x(1 - y)aznSe + (1 - x)yaMgS + xyaZnS.
(2)
where aMgSe, aznse, aMgs and azns are the lattice constants of MgSe, ZnSe, MgS and ZnS respec8 tively. The calculation described in this study used a modified empty-core model for the atomic pseudopotential based on local approximation: Vi (r) =(-Ai r < R, 3 _Zir-le .... r >R 3
139 Mat. Res. Soc. Symp. Proc. Vol. 326. @1994 Materials Research Society
The subscript i = 1, 2 denotes the cation and anion of a EBC. The fourier transform of Eq.(3) is
Vi(G) =
-Sr
[>ze-R
(cos GRi + a- sinGR,) + -L--(sinA - GR, cosG14)]
(4)
where Qij is the volume of the primitive cell of the EBC formed by elements i and j. The eight parameters A 6 , Zi, ai and R,. (i = 1, 2) of each EBC are adjusted to obtain the best fit to discrete form factors of the EBC by requiring the calculation to reproduce the observed gap as accurate
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