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

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

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 Å InAs or 50 Å 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 Å 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 Å period superlattice system after the stress treatment

Near infrared avalanche photodiodes with bulk Al0.04Ga0.96Sb and GaSb/AlSb superlattice gain layers

We demonstrate the use of bulk Al0.04Ga0.96Sb and GaSb/AlSb superlattice as the gain material in a separate absorption/multiplication avalanche photodiode with sensitivity up to 1.74 µm. Both gain schemes were implemented in a molecular-beam epitaxy grown structure with a selectively doped InAs/AlSb superlattice as the n-type layer. Hole impact ionization enhancement was observed in Al0.04Ga0.96Sb by using a two wavelength injection scheme. The superlattice gain layer device exhibited multiplication factors in excess of 300, and surface limited dark current at a level comparable to InGaAs/InAlAs devices of similar design. The superlattice gain layer was found to be more promising than its bulk counterpart due to its inherent lower dark current

Electronic properties of deep levels in p‐type CdTe

DLTS and associated electrical measurements were made on unintentionally doped CdTe crystals obtained from several vendors, on Cu‐doped CdTe, and on Te‐annealed CdTe. All of the crystals were p‐type. Four majority carrier deep levels were observed in the temperature range from 100–300 K with activation energies relative to the valence band of 0.2, 0.41, 0.45, and 0.65 eV. Two of these levels were specific to certain crystals while the other two were seen in every sample and are attributed to common impurities or native defects. Fluctuations in the concentrations of levels across samples and as a result of modest sample heating (400 K) were also observed

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 tight‐binding 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

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

Tight‐binding calculation for the AlAs–GaAs (100) interface

We report the results of a study of the electronic properties of the AlAs–GaAs interface using the tight‐binding method. The tight‐binding 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