Δ-doped Layer Research Articles

Retention time in multiple-tunnel junction memory device

A computationally inexpensive approximation is obtained for the retention time of charges stored on a memory node of a multiple-tunnel junction (MTJ) memory device, based on previous simplifying assumptions by Jensen and Martinis. The approximation takes into account both thermally assisted single electron tunneling and higher order processes, or cotunneling and is in good agreement with a full master equation simulation of the device up to a temperature T≈T0/10, where T0=e2/kBC. For the case of a memory device formed within a δ-doped layer in GaAs, it is predicted that leakage due to single tunneling starts to dominate over cotunneling at temperatures above T≈T0/60, and that a sharp reduction in retention time occurs above T≈T0/100. Our analysis also shows that with the typical dimensions of present devices, a memory lifetime of a year requires the stringent condition of an 11-junction MTJ operated at below 1 K.

Post-annealing-induced free-carrier compensation in shallow-buried δ layers of GaAs(100)

The influence of post-annealing on the spatial redistribution of silicon dopants in δ-doped GaAs(100) was studied by high-resolution electron energy-loss spectroscopy (HREELS). Samples with different δ-layer depths (100, 200 and 300 Å) were investigated. As follows from the model calculations, plasmons of the two-dimensional electron gas in the δ layer couple to phonons, giving rise to the plasmaron modes ω − and ω +. The lower-energy plasmaron ω − causes an appreciable broadening of the quasi-elastic peak in HREEL spectra, compared with that of undoped GaAs(100). However, the post-annealing affects the energy-loss spectra of the samples differently. For the samples with the δ-doping plane at a depth of 200 or 300 Å, the quasi-elastic peak broadening still persists even after annealing at up to 580°C. In contrast, after annealing of the sample with the δ-doping plane at a depth of 100 Å, the broadening of the quasi-elastic peak is no longer indicated and the energy-loss spectrum appears to be very similar to that of the undoped sample. This effect manifests the full free-electron compensation in this sample, contrary to the samples with the deeper position of the δ layer beneath the surface. The free-electron compensation is explained by the trapping of free electrons by acceptor defects, which are effectively produced at the surface during annealing, and also by defects formed within the δ-doped layer. The irregular variation of the areal free-electron density in the δ layers during annealing to successively higher temperatures is interpreted as a result of a complex redistribution of the silicon dopants.

Effect of ionized impurities on electron tunneling in GaAs/AlGaAs triple quantum wells

Photoluminescence and electroreflectance measurements in Si δ-doped GaAs/Al0.35Ga0.65-A triple quantum wells are performed in order to study the effect of ionized impurities on electron tunneling. It is theoretically shown that a thin quantum well with a δ-doping layer inserted between narrow and wide quantum wells of asymmetric double quantum wells enhances impurity scattering rate significantly. Photoluminescence decay time is found to decrease 30% at maximum compared with a sample without a δ-doping layer.

Optical Emission Related to Holes Confined in p-Type δ-Doped Layers in GaAs

AbstractWe present an optical investigation of thin Zn-doped GaAs layers embedded in bulk GaAs, by means of stationary optical spectroscopy. The samples were grown by metalorganic vapor phase epitaxy (MOVPE) The concentration of doped Zn acceptors were aimed at 2×1020/cm3 in 4 nm wide doping regions. We observed a novel optical radiative transition (denoted as F-emission) appearing in photoluminescence (PL) spectra below the energy position of the transition between the free electrons and holes bound to acceptors in bulk GaAs. The F emission shows a strong dependence on excitation intensity and temperature. The energy position varies from 1.46 eV to 1.49 eV when the excitation density increases from about 40 mW/cm2 to 23 W/cm2. Our results indicate that this emission is related to the transition between spatially separated electrons and holes. The holes are located in the p-type Zn δ-doped region, while the electrons are located in the undoped GaAs region.

Weak localization and intersubband transitions in δ-doped GaAs

The negative magnetoresistance in δ-doped GaAs is investigated experimentally. It is shown for highly perfect structures that the value of the prefactor in the expression for the negative magnetoresistance significantly exceeds the theoretical value for a two-dimensional film with a single filled size-quantized subband. The role of a large number of filled subbands and intersubband transitions is discussed. It is shown that the symmetry of the wave functions and the scattering potential in δ-doped layers can cause the times of interband transitions between subbands with different parity (τ i,j ) to be greater than the phase-relaxation time of the wave function (τ ϕ ). Such a relation between τ i,j and τ ϕ should be manifested as a significant increase in the prefactor.

Trapezoidal Delta-Doped Superlattice for Far-Infrared Detection

A new superlattice is proposed that is based on non-degenerate single-crystal conventional semiconductors of InSb, InAs, GaAs, and Ge type. The superlattice consists of alternating pairs of δ-doped ultra-thin layers of p and n type. The distance between the identical layers of p (or n) type, that constitute a pair, is chosen in such a way that the energy spectrum of holes (or electrons) in quasi-continuous, The distance between two adjacent pairs of layers of p and n type is sufficiently small, so that an extra-large built-in electric field may be generated in the regions between the pairs. In such a superlattice, potential has a trapezoidal shape and can be viewed as a periodic set of alternating non-quantized wells for electrons and holes that are separated by very thin regions of extra-large field. We found that the light absorption in the regions of extra-large field is significant up to 3 μm for the GaAs superlattice and up to 4.5 μm for the Ge superlattice. For the InSb and InAs superstructures, the interband absorption coefficient is close to its fundamental-band edge value and slightly depends on the wavelength up to 50 to 100 μm. In contrast to the quantum-well superlattices, the proposed superstructure a) does not have heterojunctions and b) absorbs IR radiation of any polarization in a very wide spectral region. Effective spatial separation of photo-generated carriers ensures their lifetimes to be gigantic thus achieving a high level of photo-response.

DX centers in Al0.3Ga0.7As/GaAs analyzed by point contact measurements

The influence of DX centers on the low-dimensional transport through point contacts in silicon δ-doped Al0.3Ga0.7As/GaAs heterostructures is investigated. The charge state of the DX centers is changed by temperature as well as optical and infrared irradiation. The experimental results reveal that the point contact resistance is strongly influenced by carriers in a quantum well formed at the δ-doping layer. The carriers screen the split gate voltage of the point contact and the point contact resistance is reduced or even suppressed. During irradiation with visible light a part of the electrons ionized from the DX centers remains in the quantum well at the silicon dopants and hinders formation of a point contact. After irradiation a strongly temperature dependent relaxation of these carriers was observed. Infrared radiation with wavelengths up to 11.5 μm reduces the screening effect. Infrared and temperature dependent measurements suggest a logarithmic dependence of the screening effect on the carrier density at the silicon doping layer.

Interface quality and electron transfer at the GaInP on GaAs heterojunction

Hall measurements performed on Ga0.50In0.50P/In0.20Ga0.80As structures show abnormally low mobility both at room temperature and at 77 K, and too high electron densities which cannot be attributed to a normal two-dimensional electron gas in the channel. On the other hand, low temperature photoluminescence on asymmetrical AlGaAs/GaAs/GaInP quantum wells and x-ray photoemission spectroscopy measurements reveal the presence of arsenic atoms in the GaInP barrier. Using a one-dimensional Schrödinger–Poisson simulation with a nonabrupt interface model, we show that the presence of arsenic in GaInP leads to the formation of a parasitic GaInAsP well between the δ-doped layer and the channel, trapping the main part of transferred electrons. We experimentally show that the electron transfer can be drastically improved by inserting a thin AlInP layer at the interface. Insertion of at least six monolayers of AlInP is needed to recover a normal electron transfer as high as 2.1×1012 cm−2.

Substrate orientation effect on Zn δ-doping in GaAs grown by metal organic vapour-phase epitaxy

The substrate orientation dependence of Zn incorporation on the nongrowing surface in the AsH 3 containing ambient was investigated using Zn δ-doped layers in GaAs grown by metal organic vapour-phase epitaxy (MOVPE). We found that the Zn δ-doping concentration significantly decreases with an increase of the (1 0 0) off-angle towards (1 1 1)B along the [0 1 1 ̄ ] direction. The Zn incorporation on the nongrowing As-terminated (1 1 1)B surface is negligible. A notable increase of the Zn incorporation was observed with increasing the (1 0 0) off-angle towards (1 1 1)A, while the maximum Zn δ-doping concentration was obtained on the (3 1 1)A surface. The Zn density decreases with a further increase in the (1 0 0) off-angle towards (1 1 1)A after (3 1 1)A. Based on the bonding configurations of the nongrowing surface, the possible mechanism of orientation effect on Zn incorporation is discussed.

SIMS depth profile correction for the study of the first step of the diffusion of boron in silicon

The analysis by secondary ions mass spectrometry (SIMS) is a powerful tool for the study of the diffusion of boron in silicon. It has been extensively used in order to determine diffusion coefficients. Most of the time, the SIMS profiles used to calculate the diffusion coefficients are sufficiently large so that the SIMS analysis do not modify the real concentration distribution in a significant manner. But more and more, the first steps of the diffusion are to be considered in order to obtain very thin diffused structures. In that case, where very thin layers are analyzed by SIMS, it is no more possible to measure directly the real concentration distribution because the SIMS analysis modify it rather significantly. When the real width or the real shape of the analyzed layers is needed, the measured profile has to be corrected in some way. We show that in the case of Gaussian original profiles, the SIMS profiles can be efficiently corrected, and the exact second order moment of the profile determined from the measured profile until the original second order moment is as low as 20 Å. This can be done by the study of the properties of the convolution of the profiles by the SIMS analysis response function. In the case of non-Gaussian profiles, the real shape and width of the profiles can be determined by a deconvolution procedure that we have previously described. (B. Gautier, J.C. Dupuy, G. Prudon, J.P. Vallard, C. Dubois, Proceedings of SIMS X, The International Conference on Secondary Ion Mass Spectroscopy and Related Techniques, Wiley, Chichester, 1996, pp. 443; B. Gautier, R. Prost, J.C. Dupuy, G. Prudon, Surface and Interface Analysis 24 (11) (1996) 733). This procedure is applied to the case of the SIMS measurement of δ-doped layers of boron in silicon before and after rapid thermal annealing (RTA).

Hybrid superconductor/semiconductor step junctions with three terminals

We report on experimental results with three-terminal superconductor/semiconductor hybrid junctions, which are based on the two-dimensional electron gas at the surface of p-type InAs. A short distance (≈150 nm) between superconducting Nb contacts is obtained using a step geometry. The step geometry allows the realization of different heterostructure potential profiles along the two-dimensional channel. The critical current of the step junctions can be controlled by applying a voltage to highly doped (δ-doped) layers embedded in the heterostructure. With p-δ-doped layers, a p-n junction is introduced in the two-dimensional channel and an asymmetric change of the critical current with respect to the gate voltage or gate current is observed. With n-δ-doped layers, the change is symmetrical.

Strain as driving force for interface roughening of δ-doping layers

The growth of smooth and abrupt heteroepitaxial semiconductor interfaces is limited by strain originating from the lattice mismatch of the different materials. Employing measurements of crystal truncation rods, we have performed an extensive study of the impact of intra-layer strain on structural properties of ultra-thin ( δ) doping layers. The use of molecular beam epitaxy and related methods, i.e., the use of surfactants and low growth temperatures with subsequent annealing (solid phase epitaxy) allows the preparation of Bi, Sb, Ge and Si 1− x C x δ layers with doping profiles of monolayer width and high peak layer concentration (>10%). We find a linear dependence of the interface roughness on the intra-layer strain.

Cyclotron resonance of interacting quantum Hall droplets

The line shape and position of cyclotron resonance in gated GaAs/GaAlAs heterojunctions with δ-doped layers of negatively charged beryllium acceptors, that provide strong potential fluctuations in the channels of the quasi-two-dimensional electron systems, are examined. Specifically, the magnetic quantum limit is considered when the electrons are localized in separate quantum Hall droplets in the valleys of the disorder potential. A model treating disorder and electron–electron interaction on an equal footing accounts for all of the principal experimental findings: blue shifts from the unperturbed cyclotron frequency that decrease when the electron density is reduced, surprisingly narrow lines in the magnetic quantum limit, and asymmetric lines due to additional oscillator strength on their high-frequency sides.

Microscopic behavior of silicon in silicon delta-doped layer in GaAs

Silicon δ-doped layers in GaAs have been studied using scanning tunneling microscopy and scanning tunneling spectroscopy on cleaved (110) surfaces. The samples were grown using molecular beam epitaxy with two growth temperatures: 480 and 580 °C. The concentration of the δ-doped layer was 0.03 monolayer. We show that, as in the case of bulk doping, the silicon has an amphoteric character: both SiGa and SiAs are observed at 480 °C and donors are formed before acceptors. At 580 °C, the spatial repartition of silicon evidences the segregation of silicon.

Electron transfer efficiency of Si δ-modulation-doped pseudomorphic GaAs/In0.2Ga0.8As/AlxGa1−xAs quantum wells

In Si δ-modulation-doped GaAs/In0.2Ga0.8As/AlxGa1−xAs quantum well structures (QWs), the electrons from the ionized Si donors are initially confined in the V-shaped potential well (V-PW) formed at the position of a Si δ-doped layer. The efficiency of electrons transferring from the V-PW to the QW was investigated as a function of Si δ-doping concentration in the symmetric GaAs/In0.2Ga0.8As/GaAs QW at 1.7 K. The electron density in the QW increases linearly with an increase of Si δ-doping concentration, while the electron transfer efficiency remains unchanged either in the dark or under the illumination. The asymmetric GaAs/In0.2Ga0.8As/Al0.2Ga0.8As QW has a relatively higher electron transfer efficiency. The effect of grading the Al mole fraction over the AlxGa1−xAs spacer layer on the electron transfer efficiency was also reported.

Quasi-Medium Energy Ion Scattering Spectroscopy Observation of a Ge δ-doped Layer Fabricated by Hydrogen Mediated Epitaxy

We have observed the behavior of Ge δ-doped layers in Si(001) by Quasi-medium energy ion scattering spectroscopy (Q-MEIS). The δ-doped layers were fabricated by hydrogen mediated epitaxy (HME) of Si. We found that, in the δ-doped layers fabricated by HME, the surface segregation of Ge atoms was reduced compared with that by conventional molecular beam epitaxy (MBE). The Ge atoms, however, were widely spread in the growing film. We assume that in the HME sample, hydrogen atoms are segregated to the top-most layer of the growth surface. Moreover, the Si buffer layer has a comparatively good crystalline quality in the HME sample.

Magneto-transport studies of Si/SiGe and Si/SiGeC quantum well structures grown by molecular beam epitaxy at low temperatures

The magneto-transport properties of SiGe and SiGeC quantum well structures were studied in relation to their dependence on the growth temperature, Ge and C concentration, well width, and spacer width. It is found that interface roughness and charged impurities are the main origins for scattering in SiGe and SiGeC two-dimensional hole gas (2DHG) structures. Rapid thermal annealing subsequent to growth improves the mobility in SiGeC 2DHG by a factor of 2, whereas only a 20% increase is observed for SiGe 2DHG. At 1.6 K a mobility of 1930 cm2/V s for Si0.81Ge0.185C0.05 and 6900 cm2/V s for Si0.85Ge0.15 channels was deduced from Shubnikov–de Haas oscillations measured up to 8 T. The effective mass determined for holes in the SiGeC alloy is 0.21±0.02. B δ-doped Si layers were used to determine the B diffusion in the temperature range from 700 to 850 °C by intersubband absorption spectroscopy.

δ-doping InGaP/GaAs heterojunction bipolar transistor

An InGaP/GaAs heterojunction bipolar transistor with a 50 undoped spacer and δ-doping sheet at base-emitter heterointerface is fabricated and studied. A common-emitter current gain of 280 and an offset voltage as small as 55 mV are obtained, respectively. These results shows that high current gain and low offset voltage can be attained simultaneously without the passivation of the emitter-base junction. The experimental results show that the potential spike is reduced by the employment of a δ-doped layer simultaneously. On the contrary, theoretical consideration also shows that the potential spike vanishes by inserting a δ-doping sheet between the emitter-base heterointerface.

Effects of growth interruption in in situ process for buried quantum structures

In order to reduce the carrier depletion at the growth-interrupted interface during the fabrication of buried quantum structures using the in situ system which consists of the low-energy focused ion beam and the molecular beam epitaxy, several methods are investigated. It is shown that the passivation using the As layer during the growth interruption is effective. It is also shown that the distance between the growth-interrupted interface and the δ-doped layer should be separated by more than 10 nm. The level of the interface state at the growth-interrupted interface is discussed using the capacitance-voltage method. The obtained results suggest that the interface state locates at shallow level from the conduction band edge.

Observation of behavior of Ge δ-doped layer in Si(001)

We have observed the behavior of Ge δ-doped layers in Si(001) by low energy ion scattering spectroscopy (LEIS). The stopping power of He in a-Si has been estimated to be about 9.5 eV/A, which is larger than the theoretically calculated stopping power. We found that, in the focusing direction, the yield due to the lattice atoms buried beneath ∼50 A can be regarded as similar to those in “random” directions without perturbation due to the focusing effect, as for Rutherford Backscattering Spectrometry (RBS). The Ge δ-doped layer fabricated by solid phase epitaxy (SPE) was found to be located at the interface. At an annealing temperature of 600°C, Ge diffused around the interface and alloyed with the Si atoms.