New Type Of Superlattice Research Articles

Structure and optoelectronic properties of Si/O superlattice

We have carried out structural study of the Si/O semiconductor atomic superlattices (SAS) with up to 18 Si/O layers fabricated by molecular beam epitaxy and in situ oxygen exposure on both Sb-doped and undoped Si buffer layers, and correlated the results with our photoluminescence, electroluminescence (EL) and I– V data. The Si/O SAS is a new type of superlattice, where monolayers of oxygen are sandwiched between the Si layers. High-resolution cross-sectional transmission electron microscopy (TEM) study has confirmed the presence of the superlattice and shown epitaxy in the Si/O superlattices. The high structural quality of the layers grown on the undoped Si buffer layers with low density of stacking faults—less than 10 7/cm 2—was established by TEM. Although structure perfection is very important allowing this new class of superlattices to be extended to other systems, it is important to point out that a 9-period SAS-based EL device with emission of light in green has been life-tested with stable output for over 1 year of continuous operation. The Si/O superlattice also serves as an epitaxially grown insulating layer as possible replacement of silicon-on-insulator. Together with the tailor-made effective band gap, this epitaxially grown superlattice may serve as future silicon-based three-dimensional integrated circuits.

Structure of MBE grown semiconductor–atomic superlattices

A new type of superlattice, semiconductor–atomic superlattice, consisting of multiple periods of semiconductor layers separated by adsorbed atomic/molecular monolayers, requires that the growth remains epitaxial. Specifically, the Si/O superlattices have been fabricated with epitaxially grown thin silicon layers (1–2 nm thick) separated by periodically introduced monolayers of oxygen. High-resolution TEM shows defect density at the Si/O interface is below 10 9/cm 2. Electroluminescence diodes show a peak in the yellow–green of the visible spectrum, and have been life-tested for over a year without signs of degradation. Measured effective barrier height is between 0.5 and 1 eV. Models of the structures, showing how epitaxy can be continued with a monolayer of oxygen, are presented. Preliminary calculation shows that the strain is relatively low. The door is opened for device applications in silicon optoelectronics.

Phenomena in silicon nanostructure devices

In nanostructures, whenever the electron mean-free-path exceeds the appropriate dimensions of the device structure, quantum natures may dictate the physical properties of devices. Among many important issues, some are selected in this work, whereas others, such as the reduction of dielectric constant, the increased binding energy of dopants, etc., are discussed briefly with references for further considerations. In the past several years, resonant tunneling via nanoscale silicon particles imbedded in an oxide matrix has shown striking similarity to the so-called soft breakdown (SBD), an important current subject in devices with ultrathin oxide gates. The relevance in applying results discussed here to SBD is discussed. A Si/O superlattice, a particular form of a new type of superlattice, semiconductor-atomic superlattice (SAS), is fully discussed. This Si/O superlattice can be used in silicon quantum and light-emitting devices. A diode structure with green electroluminescence has been life-tested for more than one year without degradation. High-resolution TEM shows defect density below 109/cm2. Preliminary calculation shows that the Si/O complexes result in a barrier height of 0.9 eV for silicon, sufficient for an epitaxially grown SOI, which is potentially far better than the SOI using buried oxide implantation followed by high temperature anneal.

Si Based Green ELD: Si-Oxygen Superlattice

A new type of superlattice, semiconductor-atomic superlattice (SAS), is discussed. Specifically, a superlattice with multiple periods of Si/O shows green electroluminescence. Oxygen is introduced via absorption followed by thin Si deposition, usually between 1 and 2 nm thick. The MBE deposited structure shows epitaxy. The electroluminescence diode (ELD) has been life-tested for more than one year without degradation. High resolution TEM shows that defect density at the Si/O interface is below 109 cm–2. A preliminary calculation shows that the Si/O complex results in a barrier height of 0.9 eV for silicon. The door is opened for device applications in silicon optoelectronics.

New Type of Superlattice: An Epitaxial Semiconductor-Atomic Superlattice, SAS

ABSTRACTConventional superlattices are formed with repeating a basic period consisting of a heterojunction between two materials. A new type of superlattice are formed by replacing the heterojunction between adjacent semiconductors with semiconductor layers separated by adsorbed species such as oxygen atoms; and CO, molecules, etc. This new type of superlattice, SAS, semiconductor-atomic-superlattice, fabricated epitaxially, enriches the present class of heterojunction superlattices and quantum wells for quantum devices. The Si growth beyond the adsorbed monolayer of oxygen is epitaxial with fairly low defect density. At present, such a structure shows stable electroluminescence and insulating behavior, useful for optoelectronic and SOI (silicon-on-insulator) applications. SAS may form the basis of future all silicon ‘superchip’ with both electrons and photons.

Electronic and Optical Properties of Orientational Superlattices in Gainp Alloys

AbstractWe demonstrate the formation, and the electronic and optical properties of a novel type of semiconductor superlattice in spontaneously ordered GaInP alloys. The most frequently observed ordered structure in MOCVD grown GaInP has CuPt symmetry where the ordering directions occur in the two [111]B directions, corresponding to two distinct ordered variants. A new type of superlattice, termed an orientational superlattice, emerges as the ordered domains are stacked in a sequence whereby the ordering direction switches alternatively from the [111] direction in one domain to the [111] direction in the next domain. The novelty of this type of superlattice lies in that there is neither a band-gap nor an effective mass discontinuity along the superlattice axis. When the GaInP epilayer is grown on an exact (001) or [111]A tilt GaAs substrate, the two ordered variants are equally favorable. Thus, ordered domain twins appear in ordered GaInP epilayers. We present a comparitive study between the single-variant ordered structure and the double-variant ordered superlattice structure, using TEM and time-resolved differential absorption. We show that for a same order parameter, the band-gap of an orientational superlattice is higher than that of a single-variant ordered structure, and the in-plane optical anisotropy between the [110] and [110]B directions is greatly enhanced due to the superlattice effect. The experimental results are explained in terms of the band structure of the orientational superlattice.

Interband absorption of long-wavelength radiation in δ-doped superlattices based on single-crystal wide-gap semiconductors

A new type of superlattice formed in a single-crystal nondegenerate, wide-gap semiconductor by a sequence of pairs of closely spaced, δ-doped donor and acceptor layers is proposed. It is shown that because of the superstrong electric fields generated between these δ-doped layers, the electroabsorption of long-wavelength radiation is determined by tunneling optical transitions of electrons from the heavy-hole band (in contrast to the case of moderately strong fields when the electroabsorption is determined by light holes). The magnitude of the electroabsorption is close to the interband absorption for light and is virtually independent of the photon energy up to the far-infrared region. It was found that in the proposed InSb-based superlattice the absorption in superstrong fields can exceed 103 cm−1 up to radiation wavelengths approximately equal to 50–100 µm. It is noted that because of the spatial separation of the photogenerated electrons and holes, their lifetime and the long-wavelength sensitivity of such a superlattice have giant values.

Twinning superlattices

A new type of superlattice based on a periodic array of twinned stacking faults is proposed. Such superlattices based on Si and Ge are predicted to be characterised by zero miniband energy gaps, evanescent state derived Bloch minibands and minibands with anomalous dispersion. They can be direct band gap materials and offer the significant benefits of simple chemistry, lack of stress and a favoured technological environment.

Ultrasonic spectrum in Fibonacci acoustic superlattices.

A new type of superlattice, a Fibonacci acoustic superlattice, is presented. Its ultrasonic spectrum, which is excited by the piezoelectric effect, has been studied theoretically and experimentally. The results are in good agreement with each other.

Negative resistance in a finite superlattice owing to quantum mechanical reflection

Sasaki [Jpn. Inst. Electron. Commun. Eng. Tech. Rep. ED-84, 27 (1984)] has proposed a new type of superlattice, a modulation mass superlattice where the effective mass of electrons is changed periodically and the conduction-band edge is aligned so as to eliminate potential discontinuity. We investigate the quantum mechanical transmission of electrons for the case where a voltage is applied perpendicularly to the layers. We find that the modulation mass superlattice exhibits a new type of negative resistance owing to quantum mechanical reflection of electrons. The threshold voltage of negative resistance is much lower (around 10 mV than that (above 0.1 V) of compositional superlattice. The current density of the modulation mass superlattice is two orders of magnitude greater than that of a compositional superlattice. Such properties are very desirable for achieving a low-power ultrafast logic device.

Effective-mass superlattice

A new type of superlattice which is constructed with a periodic modulation of effective mass is proposed. Candidate materials for this superlattice are presented. The analysis shows that there exist continuously electronic subbands in real space.

Observation of Tunable Band Gap and Two-Dimensional Subbands in a Novel GaAs Superlattice

Luminescence and Raman measurements on a new type of superlattice consisting of $n$- and $p$-type doped GaAs layers grown by molecular-beam epitaxy confirm crucial predictions of theory. A strongly tunable energy gap is found in luminescence. Raman experiments provide the first observation of electronic subbands in purely space-charge-induced quantum wells. A combined analysis of the luminescence and Raman data yields excellent agreement with self-consistent subband calculations based only on the design parameters of the sample.

The Use of Si and Be Impurities for Novel Periodic Doping Structures in GaAs Grown by Molecular Beam Epitaxy

A new type of superlattice in consisting of a periodic sequence of ultrathin p‐ and n‐doped layers (also called “nipi” crystal) has been achieved by molecular beam epitaxy (MBE). The individual p‐ and n‐type layers of which the structure is composed were doped with beryllium as acceptor and silicon as donor impurities, respectively. Low temperature photoluminescence studies and transport measurements were performed first to attest a superior quality of the intentionally p‐ and n‐doped constituent layers and of the characteristics of the individual p‐n junctions prepared by MBE. The existence of a novel superlattice effect in the periodic p‐n doping multilayer structures in was then established by studying the optical absorption tails which extend far into the gap of the unmodulated semiconductor depending on the constituent doping levels and on the artificially introduced periodicity of the crystal.

Periodic doping structure in GaAs

A new type of superlattice consisting of a periodic sequence of ultrathin p- and n- doped semiconductor layers, possibly with intrinsic (i-) layers in between, is considered. In addition to the features related to the subband formation, as known from the study of the familiar heterostructure superlattices of the type GaxAl1−xAs, crystals with doping superstructure also show other intriguing pecularities. A semiconductor miconductor with doping superstructure, in fact, represents a crystal whose band gap and carrier concentration can be tuned within wide limits. In the first part of this review, some of the unique features of this class of materials are discussed. In the second part, approaches for the realization of doping structures and nipi superlattices in GaAs by the technique of molecular beam epitaxy are outlined.

Two-dimensional electronic structure in InAs-GaSb superlattices

We have achieved by molecular-beam-epitaxy the new type of superlattice of InAs and GaSb whose energy gaps do not overlap. The observed Shubnikov-de Haas oscillations manifest the two-dimensional electronic subband structure, corroborating theoretical calculations. The deduced electron mass is enhanced primarily as a result of the strong nonparabolicity in the conduction-band of InAs.