Superlattice Thickness Research Articles

Theoretical investigation of InAs/GaInSb type-II superlattice infrared detectors for long wavelength and very long wavelength infrared applications

Type-II superlattices (SLs) based on InAs/GaInSb material systems have been theoretically investigated and optimized to achieve large valence band splitting and thin constituent layers, which are desirable for high detectivity and large absorption coefficient for the long-wavelength infrared (LWIR) and very long-wavelength infrared (VLWIR) detection covering the waveband from 8 to 14 μm and longer. Two approaches have been employed to optimize the SL: the first approach is to increase the InSb mole fraction in GaInSb alloy, keeping the balance between the compression and tension in the SL, and the second approach is to increase the SL thickness ratio, allowing small misfit in the SL. In the first approach, the valence band splitting was found to increase with increasing InSb composition and reached a maximum at 25% of InSb composition for 9 μm and at 30% of InSb composition for 11, 13, 15, 17, and 19 μm peak wavelength, respectively. When the thickness ratio is increased with fixed InSb composition, the valence band splitting has its maximum around a thickness ratio of 2–3, but the advantage of increasing the thickness ratio is obtained only for SL with less than 15% of InSb composition. It was also found in the first approach that higher InSb composition in GaInSb alloy was very efficient in reducing the total thickness of one period of SL up to 35% of InSb mole fraction. In the second approach, the minimum thickness of SL is obtained at the point where the valence band splitting is maximized. However, since the second approach is useful only for InSb composition less than 15%, the reduction of total thickness of SL is less effective than the first approach. Consequently, we can design high performance type-II SL IR detectors by both approaches for LWIR and VLWIR applications.

Direct correlation betweenTcandCuO2bilayer spacing inYBa2Cu3O7−x

We report the effects of epitaxial strain and deoxygenation on high quality [YBa2Cu3O7-x (YBCO)_N / PrBa2Cu3O7 (PBCO)_5] (1000 A thick) superlattices, with 1 <= N <= 12 unit cells. High spatial resolution electron energy loss spectroscopy (EELS) shows that strained, fully oxygenated YBCO layers are underdoped. Irrespective of whether underdoping is induced by strain or deoxygenation, X-ray diffraction analysis shows that Tc correlates directly with the separation of the CuO2 bilayers.

Low temperature synthesis of chromium tellurides using superlattice reactants: crystallisation of layered CrTe 3 at 100°C and the decomposition into Cr 2Te 3

Thin chromium and tellurium superlattice reactants with individual layer thickness ranging from a few Angstrom to thousands of Angstrom were deposited on (100)-Si substrate and the reactions were investigated with ex-situ and in-situ X-ray powder diffraction and X-ray reflectometry. The number of layers was adjusted between 3 and 653 and the compositions of the films ranged from 71 to 86 at% Te. Samples with thick Cr/Te double-layers (300–3000 Å) react at about 300°C to form crystalline and highly textured (h00)-CrTe 3. The temperature of formation is dramatically reduced to 100°C using Cr–Te multi-layers exhibiting Cr/Te double-layers with a thickness below 25 Å. At about 275°C (h00)-CrTe 3 is decomposed followed by the crystallisation of Cr 2Te 3 which also crystallises as a highly oriented material with a (00l)-texture. In a narrow temperature range both compounds coexist. The decomposition of the monochromium tritelluride followed by the crystallisation of Cr 2Te 3 could only be observed using the thin film samples whereas for the thick superlattices no such decomposition could be observed. We also show the very detailed analysis of a sample containing 6 Cr/Te double-layers. The results obtained with different analytical techniques like energy dispersive X-ray spectroscopy, atomic force microscopy, transmission electron microscopy, and X-ray reflectometry lead to a very comprehensive picture of the multi-layer structure of this sample.

Artificially ordered Bi/Sb superlattice alloys: Fabrication and transport properties

We have fabricated Bi/Sb superlattice alloys that are artificially ordered on the atomic scale using molecular-beam epitaxy. We observe that by changing the superlattice period thickness, the electronic structure can be ``tuned'' from a semimetal, through zero gap, to a narrow-gap semiconductor. These unique properties, which are distinct from those in random alloys, are believed to be a consequence of ordered atomic configurations.

Standing Optical Phonons in Finite Semiconductor Superlattices Studied by Resonant Raman Scattering in a Double Microcavity

We report optical double resonant enhancement of Raman scattering in a new double microcavity geometry. The design allows almost backscattering geometries, providing easy access to the excitations' in-plane dispersion. The cavity is used to study the phonon spectra of a finite GaAs/AlAs superlattice. A new type of "standing optical vibration" is demonstrated involving the GaAs confined phonons with a standing wave envelope determined by the superlattice thickness. A strong dispersion of the first order standing wave mode is observed, as well as its anticrossing with higher order confined modes of the same symmetry.

Investigation of InP/InGaAs superlattice-emitter resonant tunneling bipolar transistors (RTBTs)

An InP/InGaAs superlattice-emitter resonant tunneling bipolar transistor (SE-RTBT) has been fabricated and demonstrated. The influence of the superlattice and emitter thickness on the device characteristics is studied. The insertion of the superlattice and a well-designed emitter improve the characteristics of the SE-RTBT. Common-emitter current gains up to 170 and 54 are obtained for the studied devices with emitter thicknesses of 800 Å and 150 Å, respectively. Based on the specified structure, the lower offset voltage and saturation voltage ( ≤1.5 V) are obtained. Experimentally, the device with a 5-period superlattice and an emitter thickness of 800 Å provides higher dc performance and stable temperature-dependent characteristics.

89 Y NMR Studies of MBE Grown DyFe 2 /YFe 2 Multilayer Films

89Y (spin I=1/2) NMR studies of DyFe2/YFe2 multilayer films at 4.2 K are reported and discussed. Results are presented for a 1000 A thick film of YFe2, and a 24,000 A thick superlattice film [300 A DyFe2/300 A YFe2]×40. For comparative purposes, measurements were also made on bulk powdered YFe2 samples. In practice, there are significant differences between the 89Y NMR in thin film and powder form. In the MBE films the NMR frequency (45.85±0.2 MHz) is shifted down in frequency by ∼0.1 MHz, with respect to bulk (powdered) YFe2 (45.94±0.09 MHz). The origin of this shift is attributed to the expanded nature of the MBE films (∼0.8%), relative to bulk YFe2. In addition, the NMR line width is broader in the MBE films by about a factor of two. This indicates either the presence of strain within the MBE films, and/or differing dipolar fields associated with the DyFe2/YFe2 interfaces. This feature is also confirmed by the spin–spin lattice relaxation time T 2, which is almost ten times longer in the superlattice film (T 2=5.1 ms), than in bulk YFe2 (T 2=0.6 ms). It is argued that this is due to the increased inhomogeneous line-width, which inhibits energy-conserving mutual spin–spin-flops.

Structural properties and relaxation behavior of short-period BeTe/ZnSe superlattices

Transmission and scanning electron microscopy, Nomarski interference light microscopy and atomic force microscopy were applied to study the structural properties of short-period BeTe/ZnSe superlattices (SLs). The relaxation behavior was investigated as a function of the SL thickness for SLs under compressive and tensile stress. The strain state of the SLs is determined by the layer thickness ratio and the chemical bonds (BeSe or ZnTe) at the interfaces. The relaxation of SLs under tensile stress occurs exclusively by cracks. Misfit dislocations are generated in the SLs under compressive stress at thicknesses which significantly exceed the calculated critical thickness.

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.

Experimental Investigation of Thin Film InGaAsP Coolers

ABSTRACTMost optoelectronic devices for long haul optical communications are based on the InP/InGaAsP family of materials. Thin film coolers based on the same material system can be monolithically integrated with optoelectronic devices such as lasers, switches, and photodetectors to control precisely the device characteristics such as wavelength and optical power. Superlattice structures of InGaAs/InP and InGaAs/InGaAsP are used to optimize the thermionic emission resulting in a cooling behavior beyond what is possible with only the Peltier effect. A careful experimental study of these coolers is undertaken. Mesa sizes, superlattice thickness, and ambient temperature are all varied to determine their effect on cooling performance. A three-dimensional, self-consistent thermal-electric simulation and an effective one-dimensional model are used to understand the experimental observations and to predict what will occur for other untested parameters. The packaging of the coolers is also determined to have consequences in the overall device performance. Cooling on the order of 1 to 2.3 degrees over 1-micron thick barriers is reported.

Artificially Atomic-scale Ordered Superlattice Alloys for Thermoelectric Applications

ABSTRACTWe report artificially atomic-scale ordered superlattice alloy systems, new scheme to pursue high-ZT materials. We have fabricated Bi/Sb superlattice alloys that are artificially ordered on the atomic scale using MBE, confirmed by the presence of XRD superlattice satellites. We have observed that the electronic structure can be modified from semimetal, through zero-gap, to semiconductor by changing the superlattice period and sublayer thicknesses using electrical resistivity, thermopower, and magneto-transport measurements. InSb/Bi superlattice alloys have also been prepared and studied using XRD and thermopower measurements, which shows that their thermoelectric transport properties can be modified in accordance with structural modification. This superlattice alloy scheme gives us one more tool to control and tune the electronic structure and consequently the thermoelectric properties.

Phonon Wave Heat Conduction in Thin Films and Superlattices

Heat conduction in thin films and superlattices is important for many engineering applications such as thin-film based microelectronic, photonic, thermoelectric, and thermionic devices. Past modeling efforts on the thermal conductivity of thin films were based on solving the Boltzmann transport equation that treats phonons as particles. The effects of phonon interference and tunneling on the heat conduction and the thermal conductivity of thin films and superlattices remain to be explored. In this work, the wave effects on the heat conduction in thin films and superlattices are studied based on the consideration of the acoustic wave propagation in thin film structures and neglecting the internal scattering. A transfer matrix method is used to calculate the phonon transmission and heat conduction through these structures. The effects considered in this work include the phonon interference, tunneling, and confinement. The phonon dispersion is considered by introducing frequency-dependent Lamb constants. A ray-tracing method that treats phonons as particles is also developed for comparison. Sample calculations are performed on double heterojunction structures resembling Ge/Si/Ge and n-period superlattices similar to Ge/Si/n(Si/Ge)/Ge, It is found that phonon confinements caused by the phonon spectra mismatch and by the total internal reflection create a dramatic decrease of the overall thermal conductance of thin films. The phonon interference in a single layer does not have a strong effect on its thermal conductance but for superlattice structures, the stop bands created by the interference effects can further reduce the thermal conductance. Tunneling of phonon waves occurs when the constituent layers are 1–3 monolayer thick and causes a slight recovery in the thermal conductance when compared to thicker layers. The thermal conductance obtained from the ray tracing and the wave methods approaches the same results for a single layer. For superlattices, however, the wave method leads to a finite thermal conductance even for infinitely thick superlattices while the ray tracing method gives a thermal conductance that decreases with increasing number of layers. Implications of these results on explaining the recent thermal conductivity data of superlattices are explored.

Structure and stress of TiAlN/CrN superlattice coatings as a function of CrN layer thickness

TiAlN/CrN superlattice coatings have been grown in an industrial sized multiple target PVD coater by a combined steered arc/unbalanced magnetron process. The coatings were deposited using three Ti 0.5 Al 0.5 alloy targets and one Cr target. The power to each of the Ti 0.5 Al 0.5 targets was maintained at 8 kW in all the processes, whilst the target power to the Cr target was varied between 1 kW and 12 kW to produce different superlattice period thickness. The structural characteristics were determined by X-ray diffraction using both Bragg–Brentano and glancing angle parallel beam (sin2ψ method) geometries. Superlattice period thickness was determined using low angle X-ray diffraction and was found to increase from 2.2 nm at 1 kW Cr power to 4.8 nm at 12 kW Cr target power. An approximately linear dependence of Cr power on superlattice period thickness was also observed.A maximum Knoop hardness of HK0.025 3500 was observed at 8 kW Cr power when the thickness of the individual layers was equal. The residual state of stress was found to be compressive in all cases rising from 3.25 GPa at 2 kW Cr power to 6.5 GPa at 10 kW Cr power. Lattice parameters were determined using the Cohen–Wagner method and were observed to increase from 0.4183 nm for coating deposited at 2 kW Cr power to a maximum of 0.4197 nm at 10 kW Cr power.

Characteristics of InAlGaAs/InAlAs Superlattice Avalanche Photodiodes for Ultra-low Optical Power Detection in the Near Infrared

Static characteristics of two different structured InAlGaAs/InAlAs superlattice avalanche photodiodes (SLAPDs) cooled by liquid nitrogen were evaluated at a wavelength of 1.5 μm. The dark current of the SLAPD having a thick superlattice layer of 0.504 μm was 5 × 10−13 A. This was successively reduced by four orders of magnitude compared to that of the thin layer SLAPD of 0.231 μm at a breakdown voltage of around 20 V. The thickened layer was effective in suppressing tunneling dark current. An output current of 1.7×l0−12 A at a bias voltage of 15 V was measured for an optical input with a wavelength of 1.5 μm and a signal power of 1 × 10−12 W. This showed a sharp distinction from the dark current.

ON THE CHARACTERIZATION OF METALLIC SUPERLATTICE STRUCTURES BY X-RAY DIFFRACTION

To solve the problem on the microstructural characterization of metallic superlattices, taking the NiFe/Cu superlattices as example, we show that the structures of metallic superlattices can be characterized exactly by combining low-angle X-ray diffraction with high-angle X-ray diffraction. First, we determine exactly the total film thickness by a straightforward and precise method based on a modified Bragg law from the subsidiary maxima around the low-angle X-ray diffraction peak. Then, by combining with the simulation of high-angle X-ray diffraction, we obtain the structural parameters such as the superlattice period, the sublayer and buffer thickness. This characterization procedure is also applicable to other types of metallic superlattices.

Elastic constants and surface acoustic waves of superlattices

Elastic constants of superlattices and their surface acoustic waves are theoretically studied. An exact expression to calculate the surface acoustic wave velocity dispersions is derived for superlattices of finite thickness consisting of two kinds of layers rigidly stacked upon each other. The formulations are performed for both free superlattice plates and substrated superlattices. Numerical calculations for Cu/Al and Cu/Ag systems are compared with those obtained using the effective elastic constant (EEC) model (Grimsditch M 1985 Phys. Rev. B 31 6818). A comparison clarifies the applicability of the EEC model.

Multiple-angle-of-incidence ellipsometry for GaAs\\GaPAs superlattices

In this paper we present results of multiple-angle-of-incidence (MAI) ellipsometry for strained GaAs\\GaPxAs1−x superlattices (SL) with the composition x ≈ 0.4 and with different layer thickness in the range of 8–80 nm. SLs have been grown on the GaAs (100) substrates by chemical vapour deposition. The geometrical parameters of SLs have been determined by X-ray diffraction patterns. From MAI measurements the thickness and optical constants of SL films have been calculated by solving inverse ellipsometric problem. The latter was solved by using Tichonov’s regularization algorithm. The anisotropic film model of SL and capping isotropic layer were taken into account at solving. The optical constants obtained together with the full thickness of SL have been related to the SL microstructure using an effective medium approximation. These results are compared with ones obtained by far-infrared reflection spectroscopy in the ‘‘reststrahlen’’ band region. The quality of SLs in connection with technology of thin films growth are discussed.

Dislocation imaging of an InAlGaAs opto-electronic modulator using IBICC

This paper presents Ion Beam Induced Charge Collection (IBICC) contrast images showing regions of differing charge collection efficiency within optoelectronic modulator devices. The experiments were carried out at the Sandia Nuclear Microprobe using 18 MeV carbon and 2 MeV helium ions. Lines of varying densities are observed to run along the different {110} directions which correlate with misfit dislocations within the 392 nm thick strained-layer superlattice quantum well of the modulator structure. Independent cross-sectional TEM studies and the electrical properties of the devices under investigation suggest the presence of threading dislocations in the active device region at a density of ≲ 10 6 cm −2. However, no clear evidence of threading dislocations was observed in the IBICC images as they are possibly masked by the strong contrast of the misfit dislocations. Charge carrier transport within the modulator is used to explain the observed contrast. The different signal to noise levels and rates of damage of the incident ions are assessed.

Inter-layer subb and mixing in MBE-grown HgTe/CdTe superlattices

We report transport results for molecular beam epitaxially grown HgTe/CdTe superlattices (SLs) with CdTe barrier thickness of Lb systematically varied from 100A down to 20A, and demonstrate the increasing intersubband mixing between adjacent layers with decreasing Lb. Magnetotransport data measured at 1.5K and for B up to 10T show monotonously varying features with Lb, implying a dimensional change from 2D to quasi-3D. As Lb decreases, the quantum Hall plateaus become weaker and the number of effective 2D channels are decreased, which indicates increasing growth-direction energy dispersion. This enhanced interlayer subband coupling with decreasing Lb is further confirmed by the angle-dependence of SdH oscillations. For Lb = 20A, all of 2D-related transport behavior are washed out. A quantitative analysis with calculated subband energy dispersion relations demonstrates the close interplay between superlattice barrier thickness and the strength of the intersubband mixing. These transport results are directly related to successful control of the growth-axis carrier effective mass and will contribute to the development of high-performance HgTe/CdTe SL-based IR photo-voltaic devices and lasers, in which tunneling noise due to diffusion currents can be considerably reduced.

Organized growth of lateral superlattices on vicinal surfaces: where are the limits?

Abstract We report on three issues concerning the growth of GaAs AlAs lateral superlattices on vicinal surfaces to obtain quantum wires with negligible inter-wire tunnel coupling. We show that the ledge roughness increases with increasing terrace length and decreases for slower growth rates. The tilt angle of lateral superlattices is not the relevant parameter as long as electronic properties are concerned: one has to consider the product of the coverage error by the lateral superlattice thickness. The Ga flux stability is a critical parameter to be controlled. The use of dual-filament cells improves it by a factor two compared to single-filament ones.