4,367 research outputs found
Ballistic electron emission microscopy spectroscopy study of AlSb and InAs/AlSb superlattice barriers
Due to its large band gap, AlSb is often used as a barrier in antimonide heterostructure devices. However, its transport characteristics are not totally clear. We have employed ballistic electron emission microscopy (BEEM) to directly probe AlSb barriers as well as more complicated structures such as selectively doped n-type InAs/AlSb superlattices. The aforementioned structures were grown by molecular beam epitaxy on GaSb substrates. A 100 Ă
InAs or 50 Ă
GaSb capping layer was used to prevent surface oxidation from ex situ processing. Different substrate and capping layer combinations were explored to suppress background current and maximize transport of BEEM current. The samples were finished with a sputter deposited 100 Ă
metal layer so that the final BEEM structure was of the form of a metal/capping layer/semiconductor. Of note is that we have found that hole current contributed significantly to BEEM noise due to type II band alignment in the antimonide system. BEEM data revealed that the electron barrier height of Al/AlSb centered around 1.17 eV, which was attributed to transport through the conduction band minimum near the AlSb X point. Variation in the BEEM threshold indicated unevenness at the Al/AlSb interface. The metal on semiconductor barrier height was too low for the superlattice to allow consistent probing by BEEM spectroscopy. However, the superlattice BEEM signal was elevated above the background noise after repeated stressing of the metal surface. A BEEM threshold of 0.8 eV was observed for the Au/24 Ă
period superlattice system after the stress treatment
Energy gaps in amorphous covalent semiconductors
A calculation of approximate density of states for a disordered covalent semiconductor shows that the energy gap is due to the presence of short range order
Critical analysis of the 'generalized coherent wave approximation'
The formalism developed by Fletcher (1967) to take account of the presence of short range order in the calculation of the electronic energy spectrum of amorphous covalent semiconductors is examined critically and found to have fundamental difficulties
Ideal CdTe/HgTe superlattices
In this paper we consider a new superlattice system consisting of alternating layers of CdTe and HgTe constructed parallel to the (001) zincblende plane. The tightâbinding method is used to calculate the electronic properties of this system, in particular, band edge and interface properties. The energy gap as a function of layer thickness is determined. It is found to decrease monotonically with increasing HgTe layer thickness for a fixed ratio of CdTe to HgTe layer thicknesses. The symmetry of the valence band maximum state is found to change at certain HgTe layer thicknesses. This is explained by relating the superlattice states to bulk CdTe and HgTe states. The existence of interface states is investigated for the superlattice with 12 layers of CdTe alternating with 12 of HgTe. Interface states are found near the boundaries of the Brillouin zone, but none are found in the band gap
Structural perfection in poorly lattice matched heterostructures
Continuum elastic theory is applied to the formation of misfit dislocations and point defects in strained layer structures. Explicit calculations of the energies of misfit dislocations in the doubleâ and singleâkink geometries yield line tensions below which strained films are stable with respect to defect formation. Our results yield a mismatchâdependent stability limit which, in the double kink case, differs from the MatthewsâBlakeslee model by a geometrical factor and by the addition of a stress term associated with climb of the misfit dislocation. While our calculations yield equilibrium stability limits which may not correspond to observed critical thicknesses, the calculated stresses may be applied to descriptions of the kinetics of strain relief in films grown beyond these limits. Last, calculations of strainârelated contributions to the free energy of formation of point defects suggest a contribution âÎG_(strain)â â0.25 eV for a 5% lattice mismatch. This suggests a means of suppressing or enhancing the formation of vacancies or interstitials in semiconductors favoring these defects
Semiempirical calculation of deep levels: divacancy in Si
A study of the electronic levels associated with the divacancy in silicon is reported. The extended Huckel theory is shown to reproduce the band structure of silicon. The electronic levels of the divacancy are calculated by considering a periodic array of large unit cells each containing 62 atoms; a 64 atom perfect cell with two atoms removed to form the divacancy. The results are found to be in qualitative agreement with the results of EPR and infrared absorption measurements
Localization of superlattice electronic states and complex bulk band structures
The relative lineup of the band structures of the two constituents of a semiconductor superlattice can cause charge carriers to be confined. This occurs when the energy of a superlattice state is located in an allowed energy region of one of the constituents (the "well" semiconductor), but in the band gap of the other (the "barrier" semiconductor). A charge carrier will tend to be confined in the layers made from the semiconductor with the allowed region at that energy. It will have an exponentially decaying amplitude to be found in the
semiconductor with a band gap at that energy
Tightâbinding calculation for the AlAsâGaAs (100) interface
We report the results of a study of the electronic properties of the AlAsâGaAs interface using the tightâbinding method. The tightâbinding matrix for the superlattice system is used in the limit in which the thickness of the repeated superlattice slab becomes large. This system is studied in detail with special emphasis placed on the determination of interface states. No interface states with energies within the GaAs forbidden gap are found. The densities of states per layer are calculated and compared with bulk densities of states. They resemble the bulk densities of states except for layers adjacent to the interface
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