We have used x-ray photoelectron spectroscopy to measure the valence-band offsets for the lattice matched MgSe/Cd,,&!&.&e and MgTe/Cd,,ssZQ.,,Te heterojunctions grown by molecular beam epitaxy. By measuring core level to valence-band maxima and core level to core level binding energy separations, we obtain values of 0.5610.07 eV and 0.43IfrO.11 eV for the valence-band offsets of MgSe/Cdo,54Zno,,Se and MgTe/Cd,,ssZn O.lzTe, respectively. Both of these values deviate from the common anion rule, as may be expected given the unoccupied cation d orbitals in Mg. Application of our results to the design of current II-VI wide band-gap light emitters is discussed. For the ZnSe-based devices, knowledge of the valenceband offset (m,) arising from the addition of Mg in the Zn,-,M@,,Sel-, cladding layers is needed to design LDs with adequate carrier confinement, and to predict general trends in the dopability of the cladding layers. For the graded electron injector devices, AE, is needed to design for efficient electron injection. Currently, both types of devices are designed using band lineups based primarily on the common anion rule. Since the common anion rule' was originally observed not to apply to the one cation, Al, then studied from the third row of the periodic table, we should not expect it to necessarily apply to Mg, another third row cation. Similarly, Wei and Zunger" predict that the common anion rule should only apply when the d orbitals of the cations on both sides of a heterojunction, e.g., MgSe/Cd0.54Zn0,4$e, interact with their respective valence bands in a comparable manner. heterojunctions by x-ray photoelectron spectroscopy @PS)* Cdo.54Zno.4, Se and Cdos8Zno.1,Te were chosen to avoid difficulties associated with measuring band offsets in lattice mismatched systems, and to test the validity of the common anion rule for Mg-based compounds.The structures studied were grown in two Perk&Elmer 430P MBE systems, one devoted to III-V and the other to II-VI semiconductor growth. After GaSb buffer layers were grown on GaSb (100) substrates to provide a smooth growth surface, the samples were transferred via an ultrahigh vacuum (UHV) transfer tube to the II-VI growth chamber. Thick, relaxed layers of Cdo.54Zn,,46Se (Cdo~88Zno~1,Te), followed by the band offset measurement structures, were then grown at a substrate temperature of 270 "C (300 "C). The ternary compositions were calibrated using XPS and x-ray diffraction to keep the lattice mismatch to less than 0.5%. The reflection high-energy electron diffraction patterns showed no indications of deviation from the cubic zincblende structure. Following the II-VI growth, the samples were transferred via an UHV transfer tube to a Perkin-Elmer model 5500 analysis system with a monochromatic Al Ka x-ray source for the ?LpS measurements. The base pressure in the XPS chamber was typically -3X1O-1o Torr.To determine bE, using XPS, we used the following relation:
We have used x-ray photoelectron spectroscopy (XPS) to measure the dependence of the InAs/GaSb valence band offset on both interface composition and growth order. Molecular beam epitaxy was used to grow InAs-on-GaSb and GaSb-on-InAs interfaces with both InSb-like and GaAs-like interface compositions. Analysis of XPS core level separations showed no dependence of the valence band offset on interface composition; however, a 90 meV increase in the valence band offset was observed for InAs grown on GaSb compared to GaSb grown on InAs. This difference is attributed to the extended nature of the InAs-on-GaSb interface. Results from analysis of an intentionally extended GaSb-on-InAs interface were consistent with this conclusion.
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