Electron-spin polarization in photoemission from strained GaAs grown on GaAs1-xPx.

Spin-polarized electron photoemission has been investigated for strained GaAs epitaxially grown on a GaAsr-,P, buffer. The lattice mismatched heterostructure results in a highly strained epitaxial layer and significant enhancement of electronspin polarization is observed. The effect of epitaxial layer strain is studied for a variety of samples with epitaxial layer thicknesses varying from 0.1 pm to 0.3 pm and the phosphorous concentration x varying from 0.21 to 0.28. Electron spin polarization as high as 90% has been observed. The 0.3 pm-thick sample, well in excess of theoretical estimates for the critical thickness for pseudomorphic growth, reaches an electron-spin polarization of SO%, demonstrating a significant persistence of lattice strain. ,PACS numbers: 29.25.Bx, 29.75.+x, 73.60.Br, 79.60.Eq Recently electron spin polarization in excess of 70% has been observed in photoemission from a thin strained epitaxial InGaAs layer grown on a GaAs substrate!‘] The observed polarization enhancement is a result of strain in the InGaAs layer due to a small (0.9%) lattice mismatch of the epitaxial layer relative to the GaAs substrate. Strain induces a valence band splitting which permits optical excitation of a single band transition, thus leading potentially to 100% polarization of the photoemitted electrons. A similar polarization enhancement was subsequently observed in photoemission from epitaxial GaAs grown on a thick GaAsP buffer layer!’ Although these pioneering experiments have demonstrated strain enhanced electron spin polarization, there has been no systematic investigation of the conditions required to optimize the heterostructure parameters for maximum polarization and high quantum efficiency. This letter reports the first systematic study of the effects of strain on electron polarization using samples with a heterojunction of GaAs epitaxially grown on a GaAsP buffer layer. Samples with varying epitaxial layer thickness and varying buffer layer phosphorous concentration were used to cover a range of strains, and the electron spin polarization and quantum efficiency were measured for each sample as a function of excitation photon energy. The theory relating strain to band structure has been discussed extensively in the literature!’ The band structure of the strained layer is altered such that the heavy-hole and light-hole valance bands are no longer degenerate in energy at the I’ point, and the energy splitting is then proportional to the strain. A suitably thin epitaxial layer of GaAs grown on a GaAsP substrate incorporates a biaxial compressive strain in the plane of the interface and a tensile strain along ..v 3 the growth direction. The strain dependent energy levels of the heavy-hole (HH) and light-hole (LH) bands relative to the conduction band (C) are given by:[4’ EC,HH = EQ + SEH SEs, ECyLH = Eo + SEH + SEs (6E~)~/2Ao +. . . where Eo is the direct band gap of fully relaxed GaAs and A0 is the spin orbit splitting. The quantities SEH and SEs represent the hydrostatic shift of the center of gravity of the P312 multiplet and the linear splitting of the Psi2 multiplet respectively, and are given in terms of the biaxial strain c parallel to the interface by: sEH = 2a[(‘% CK~)/C~~]E Ws = b[(Gl + 2G2)/C11]~ where the parameters a and b are the interband hydrostatic pressure and uniaxial deformation potentials, respectively, and the C;j are the elastic-stiffness constants appropriate to the GaAs crystal structure. Since the biaxial strain E is compressive for the present structures, the effect of strain is to increase the band gap energy of GaAs and to remove the degeneracy of the heavy-hole and light-hole levels such that EC,HH < EC~LH. In th e present experiment the lattice mismatch, and hence the strain, is determined by the phosphorous concentration of the buffer layer on which the epitaxial layer is grown. -. . . When a lattice mismatched layer is grown on a substrate, the misfit between the layers is accommodated by elastic strain in the epitaxial layer and pseudomorphic growth takes place. However, if the epitaxial layer exceeds a characteristic critical thickness, the stored elastic strain in the epitaxial layer is relieved by producing misfit dislocations. Theoretical considerations based on thermodynamic .equilibrium arguments at the heterostructure interface have been used to calculate the equilibrium critical thicknes$’ and it has been experimentally confirmed that the dislocation density in the structure increases rapidly when the critical thickness is exceeded!’ However, there is experimental evidence indicating that the lattice strain is preserved to thicknesses beyond the critical thickness, although with a signific&nt epitaxial layer dislocation density. Significant strain relief can be observed only above a second critical thickness (the non-equilibrium critical thickness) which I71 is roughly an order of magnitude greater than the equilibrium critical thickness. bl Several models have been proposed to explain the persistence of lattice strain. The samples for the present experiment were grown by the Spire Corporation”’ using Metal-Organic-Chemical-Vapor-Deposition (MOCVD). A 0.25 pm thick ptype GaAs buffer layer was grown on a (100) p-type GaAs substrate oriented two degrees towards (110) d’ nection. In order to produce a strain relieved GaAsr-,P, layer on GaAs, a 2.5 pm-thick GaAsr-,P, layer was grown with an increasing phosphorous fraction from 0 to x to accommodate the lattice mismatch, followed by an additional 2.5 pm-thick GaAsr-,P, layer with a fixed phosphorous fraction. The lattice mismatched GaAs epitaxial surface layer was then grown on this buffer. The epitaxial surface layers were p-type doped with zinc to a value of 4-6~lO”crn-~. In order to preserve an atomically clean surface the samples were anodized to form