Short-period Strained-layer Superlattices Research Articles

Short-period InAsSb-based strained layer superlattices for high quantum efficiency long-wave infrared detectors

Infrared detector barrier heterostructures with strained layer superlattice (SLS) absorbers with different periods were compared. The first was a reference using a conventional barrier heterostructure with a low temperature energy gap corresponding to a wavelength of 10 μm in a 2-μm-thick undoped absorber using a 10.9 nm period with InAs/InAsSb0.36 compositions grown directly on a GaSb substrate. The second structure, in contrast, used a significantly shorter 4.3 nm period absorber with InAsSb0.3/InAsSb0.55 compositions, similar energy gap, and absorber thickness, which were grown on a 6.2 Å lattice constant GaIn0.3Sb virtual substrate on GaSb. It was found that in the short period SLS, the vertical hole mobility and minority carrier lifetime in the temperature range of 80–150 K were a factor on 2–3 greater than in the reference structure. The improvement of the vertical hole mobility was attributed to the effect of hole delocalization. The latter results in an increase in the optical absorption coefficient and the quantum efficiency.

Behaviors of Al xGa 1− xN (0.5⩽ x⩽1.0 )/GaN short period strained-layer superlattices on the threading dislocation density reduction in GaN films

GaN films were grown on c-Al 2O 3 substrates with the insertion of Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN short period strained-layer superlattices (SPSLSs) at elevated temperatures. It appears that the insertion of an Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN SPSLS having certain thickness and pair combinations is helpful to reduce the etching pit density (EPD) in GaN film for more than one order of magnitude. Cross-sectional transmission electron microscopic (XTEM) observations confirm the efficiency of Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN intermediate SPSLSs on blocking threading dislocation (TD) propagation in GaN films. The presence of intermediate Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN SPSLS in a GaN film is believed to encourage TD density reduction in the film through TD annihilation and de-multiplication processes involving interactions between two edge-type TDs.

The Nature and Origin of Lateral Composition Modulations in Short-Period Strained-Layer Superlattices

AbstractThe nature and origin of lateral composition modulations in (AlAs)m(InAs)nshort-period strained-layer superlattices grown by molecular beam epitaxy on InP substrates have been investigated by x-ray diffraction, atomic force microscopy, and transmission electron microscopy. Strong modulations were observed for growth temperatures between ≈ 540 and 560° C. The maximum strength of modulations was found for SPS samples with InAs mole fraction x (= n/(n+m)) close to ≈ 0.50 and when n≈m≈2. The modulations were suppressed at both high and low values of x. For x > 0.52 (global compression), the modulations were along the <100> directions in the (001) growth plane. For x < 0.52 (global tension), the modulations were along the two <310> directions rotated ≈ ±27° from [110] in the growth plane. The remarkably constant wavelength of the modulations, between ≈ 20–30 nm, and the different modulation directions observed, suggest that the origin of the modulations is due to surface roughening associated with the high misfit between the individual SPS layers and the InP substrate. Highly uniform unidirectional modulations have been grown by control of the InAs mole fraction and growth on suitably offcut substrates, which show great promise for application in device structures.

Novel type II strained layer superlattices for long wavelength infrared detectors

Novel type II (InAs) Na/(In xGa 1−xSb) Nb short period strained layer superlattices (SLs) have shown considerable promise as candidates for applications in infrared imaging in the 10–12 μm wavelength regions and beyond. Absorption α( ℏω) calculations are difficult, however, because of the strong misalignment of the band edges of the two host materials at the interface and because of a large lattice mismatch. Theoretical studies of the energy band gaps and cut-off wavelengths in (InAs) Na/(In xGa 1−xSb) Nb SLs grown on GaSb are reported as a function of composition and layer thickness using a modified second neighbor empirical tight-binding (ETBM), effective bond-orbital (EBOM) and 8×8 k.p models. The strain in the ETBM is included by scaling the matrix elements according to the Harrison's universal 1/ d 2 rule and by appropriately modifying the angular dependence. The EBOM and k.p calculations include the strain via the deformation theory. By appropriate choice of the In composition and layer thickness, cut-off wavelengths ( λ c) in the 10–12 μm range are achievable. The study of α( ℏω) for thin layer SLs in the k.p scheme suggest enhancement of absorption with increaing x at a fixed energy due to a large overlap of the electron–hole wavefunctions.

InAs/GaAs short-period strained-layer superlattices grown on GaAs as spatial light modulators: uniformity measurements

Optical properties of two-dimensional transmission-type spatial light modulators using InAs/GaAs short-period strained-layer superlattices (SPSLSs) in quantum wells, which operate near 960 nm, are investigated. The absorption characteristics of the individual pixels across a ∼200 pixellated wafer, i.e. the uniformity of the devices, are analyzed. The limits to performance of this spatial light modulator due to nonuniformity are also discussed.

Modelling intermixing of short period strained layer superlattices by means of X-ray diffraction analysis

Intermixing of an (InAs) n /(GaAs) m short period strained layer superlattice (SPSSL) is analyzed. Lattice matched ( n ≈ mm) and indium rich ( n > m) superlattices have been prepared by molecular beam epitaxy in order to provide a ternary In xGa 1 − x As channel in InP-HFET with a reduced alloy scattering as theoretically predicted. For the optimization the interface intermixing of highly strained heterostructures was investigated using high-resolution double-crystal X-ray diffractometry combined with computer simulations. A method of modelling imperfections in semiconductor heterostructures was introduced to perform the fit between the simulated and measured data in an objective manner. The key steps of this method are (i) the sensitivity analysis of experimental diffraction curve details with respect to imperfections, and (ii) the development of objective error criteria. The result of the modelling process enables a quantitative and objective determination of the intermixing at InAs/GaAs interfaces in relation to the molecular beam epitaxy (MBE) growth parameters.

Growth and characterization of [formula omitted] short-period strained-layer superlattices grown by almbe

We report for the first time the successful growth of short period strained-layer superlattices of InSb InP by atomic layer molecular beam epitaxy. This sample combination is especially interesting because the equivalent ternary alloy has a very wide miscibility gap in which it cannot be grown. The samples were characterized by reflection high-energy electron diffraction, X-ray diffraction, Hall measurements and Raman spectroscopy. The X-ray measurements show clearly satellite peaks that indicate the formation of the superlattice. Raman spectra show the confined phonon peaks of InSb (194 cm −1) and InP (300 cm −1), respectively. Exciting in resonance with the E 1 transition of the SPSL the peaks corresponding to the LO 1 and LO 2 phonons are clearly identified.

InAs/GaAs short-period strained-layer superlattice modulators grown using advanced digital reflection high-energy electron diffraction techniques

We report the growth optimization of multiple quantum well and modulator samples containing InAs/GaAs short-period strained-layer superlattice (SPSLS) well using our advanced digitized reflection high-energy electron diffraction (RHEED) system. Modulator samples for both 980 nm and 1.06 μm wavelengths have been investigated. Samples with narrow room temperature absorbance linewidths and rapidly decaying absorbance tails are necessary for state-of-the-art modulators. We have employed various conventional (only As shutter open) and total growth interrupt times (all shutters closed), as well as different InAs and GaAs growth rates in order to optimize the SPSLSs.

Broadening of Exciton Luminescence Line in Modified CdTe/ZnTe Multi-Quantum Wells

Photoluminescence in modified CdTe/ZnTe superlattice multi-quantum wells (MQW's) was investigated. In order to reduce the number of misfit dislocations, the well region is formed from a short-period strained-layer superlattice composed of CdTe and ZnTe. The observed peak energies of the free exciton line are higher than those calculated. The full width at half-maximum of the observed line has a minimum around 45 K in the temperature range between 1.8 and 210 K. The broadening above 45 K is attributed to exciton scattering with phonons and ionized impurities, while the broadening below 45 K is due to localized states.

Electronic states and binding energies in ZnS-ZnSe superlattices.

We present a detailed study of the optical properties of short-period ZnS-ZnSe strained-layer superlattices. These superlattices have been grown by metalorganic vapor-phase epitaxy. We show that the photoluminescence exhibits a low-energy tail due to localization of the exciton to interfacial potential fluctuations. A detailed analysis of the electronic structure has been performed using the envelope-function approach to obtain the valence-band and conduction-band envelope functions and band lineups. This was completed by a self-consistent calculation of the exciton binding energies. In this calculation the marginal conduction-band offset deduced from the standard envelope-function calculation is corrected by the electrostatic deformation produced by the presence of a localized hole wave function.

Low-pressure metalorganic vapour phase epitaxy growth of InAs/GaAs short period superlattices on InP substrates

Short-period strained-layer superlattices (SPSLSs) could potentially find applications as replacements for the corresponding alloys combining superlattice effects with those associated to strain. For more fundamental research, SPSLS have attracted considerable interest due to their capability of exhibiting the physical behaviour of binary in horizontal and of ternary alloy in vertical direction. The characterization of interface qualities in dependence of growth parameters like growth temperature and layer growth time is of particular interest in such highly strained heterostructure systems. In this work we will show the capability of low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) growth for InAs/GaAs ultra-short period superlattices on InP substrates. A detailed study of growth characteristics of highly strained extremely thin InAs and GaAs layers on InP substrates was made. Single strained layer heterostructures as well as SPSLS were grown. InAs/In 0.53Ga 0.47As, GaAs/In 0.53Ga 0.47As and InAs/GaAs/In 0.53Ga 0.47As superlattice structures were grown with respect to the special requirements of high resolution X-ray diffraction (HRXRD). Experimental rocking curves were compared with computer aided simulations which confirmed our method for individual layer thickness evaluation on the base of experimental data solely.

Pulsed-jet epitaxy: Application to device processes

We report recent activities in the application of pulsed-jet epitaxy (PJE), our new type of atomic layer epitaxy, to device processes. We discuss three major factors of PJE; excellent uniformity of thickness and interface controllability, low-temperature growth and heavy doping, and selective epitaxy. To study the degree of thickness control, we compare the thickness profile of the GaAs epitaxial layer between PJE and MOVPE. The inherent ability of PJE to make a uniform layer is clarified. We find good thickness and interface control of PJE also in the successful fabrication of InAs/InP and GaAs/GaP short-period strained-layer superlattices. Making use of the low-temperature growth and high-concentration Be doping into PJE-grown InGaAs, we fabricate the InGaAs/InP heterojunction bipolar transistor (HBT) structures using a novel PJE/MOVPE hybrid growth process. We show that PJE is a key technology used for making a high-performance HBT. The selective growth of PJE-GaAs shows ideal features in both morphology and thickness uniformity. As an application, we report the formation of a nonalloyed contact layer using selective regrowth of GaAs on a processed substrate.

All-binary InAs/GaAs optical waveguide phase modulator at 1.06 /spl mu/m

We demonstrate the use of multiple [(InAs) (GaAs)]/GaAs short-period strained-layer superlattice quantum wells for optical waveguide modulation at 1.06 /spl mu/m. We achieve /spl pi/ phase modulation with 2.3 V applied (V/sub /spl pi///spl times/L=4.6 V mm, or 39/spl deg//V mm) in the presence of negligible absorption change using this all-binary modulator. >

The influence of tellurium interdiffusion on the optical properties of MOVPE-grown ZnSe-ZnTe superlattices

ABSTRACTWe have used low pressure MOVPE to grow a series of short-period ZnSe-ZnTe strained-layer superlattices onto either ZnSe or ZnTe buffer layers. Typical superlattice periods range from 2 to 6.5 nm with constant ZnSe thickness of 2 monolayers. X-Ray diffraction analyses have shown that the superlattices are free-standing if grown on a ZnSe buffer but lattice matched to ZnTe if grown on ZnTe buffer layers. The luminescence emitted by the free standing samples has been studied using both the photoluminescence technique at low temperature and cathodoluminescence at higher temperature, for improved pumping. Our samples exhibit a complicated temperature dependence similar to the behaviour of ZnSeTe alloys where exciton self-trapping to tellurium gives strong photoluminescence bands labelled SI and S2. A high temperature, high energy feature is observed and is associated with the fundamental superlattice transition. The energy position of this structure suggest a high value of the ZnSe/ZnTe valence band offset, in the range 1200-1400 meV.

Ambipolar diffusion and carrier lifetime measurements in all-binary (InAs)2(GaAs)5 strained quantum wells grown on GaAs

The in-well ambipolar diffusion coefficients and carrier lifetimes in ordered all-binary quantum wells, composed of (InAs)2(GaAs)5 short-period strained-layer superlattices (SPSLSs) grown on [001]-oriented GaAs substrates, are measured using picosecond optically induced transient grating and pump-probe techniques. Quantum wells containing SPSLSs may be expected to exhibit higher in-plane hole mobilities compared to InGaAs ternary alloy quantum wells because of a larger average indium content and reduced disorder scattering in the SPSLS structures. The results obtained in the SPSLSs are compared to those obtained in InGaAs alloy quantum wells of comparable well width and confinement energies. This indicates that, for a given SPSLS and its equivalent alloy, the ambipolar diffusion coefficients are comparable while the carrier lifetimes in the SPSLSs are longer. The longer carrier lifetimes in the SPSLSs suggest that these binary structures possess fewer nonradiative recombination centers than the alloys. The absence of a dramatic improvement in the transport properties, however, may indicate that imperfect interfaces in the SPSLSs may be offsetting their possible advantages, in spite of the fact that optical measurements in the SPSLSs indicate high quality and x-ray and transmission electron microscopic measurements demonstrate the binary nature of these structures.

Multiple quantum wells consisting of InAs/GaAs short-period strained-layer superlattice wells for 1.3–1.55 μm photonic applications

Solid source molecular-beam epitaxy has been employed to grow InAs/GaAs short period strained-layer superlattices (SPSLS) on InP substrates, and multiple quantum well samples incorporating these SPSLS as the well regions. Excitons are observed in the room-temperature transmission data at 1.3 μm wavelength, without the use of phosphorous. We discuss our novel shutter sequence which involves a total growth interrupt (all shutters closed), and the growth conditions necessary to obtain high quality SPSLS on InP substrates. Furthermore, data including room-temperature absorbance spectra, low temperature photoluminescence, and x-ray diffraction on various samples with band edges in the 1.3–1.6 μm wavelength range are presented.

Microstructure in molecular-beam-epitaxy-grown Si/Ge short-period strained-layer superlattices

The microstructures of Si 12 /Ge 4 and Si 18 /Ge 6 short-period strained-layer superlattices (SLS's) on a Si substrate were observed using electron microscopes. These SLS's were fabricated by the phase-locked-epitaxy technique in molecular-beam epitaxy, and were reported to show strong optical transitions [Okumara et al., Jpn. J. Appl. Phys. 28, L1893 (1989); Mater. Sci. Eng. B 9, 245 (1991)]. Flat SLS layers were observed in the region near the Si substrate. After the growth of about ten layers, the cross-sectional image showed gradual wavy layers in the Si 18 /Ge 6 SLS

Lattice mismatch adjustment due to internal strain in CdTe/ZnTe superlattices

Photoluminescence from the [(CdTe) m/(ZnTe) n] ℓ/ZnTe (ℓ=2∼10) MQW system has been observed at various temperatures. The well region of the MQW system is composed of short-period strained layer superlattices (m,n=2∼4) and misfit dislocations are reduced by widely spreading the internal strain. Actual experimental data show that the full width at half maximum of the exciton line is very narrow and prove that this sample has very high quality. The PL line arising from valence band splitting due to internal strain is observed. The line has a tail on its low energy side. We make the suggestion that its origin is the disorder of atomic alignment at interfaces.

Photoluminescence from short-period strained-layer superlattices of (Si 6Ge 4) p after hydrogen passivation

Low temperature photoluminescence from different short-period (Si m Ge n ) p superlattices ( m = 6, n = 4, p = 60, 100) is determined before and after various hydrogen plasma treatments. The original samples exhibit well-known broad luminescence features with maxima at about 0.8 eV and display no excitonic recombinations from the superlattice. After hydrogen plasma treatment, the total luminescence intensity increase, and new luminescence bands appear. In particular, in samples with minor quality, luminescence from transition metals is identified. In our best samples, however, we find evidence for an excitonic recombination in the superlattice.

InAs/GaAs short-period strained-layer superlattices grown on GaAs as quantum confined Stark effect modulators

We report here the first use of InAs/GaAs short-period strained-layer superlattices as Stark effect modulators. We have observed differential transmission changes as large as 38% with a corresponding change in quantum-well absorption of 0.28 μm−1 at an applied field of 43 kV/cm. The Kramers–Kronig relation predicts a maximum change in the quantum-well index of refraction of 0.019 for a field of 43 kV/cm. There is a unique light-hole feature in the absorption and electroabsorption spectra that is attributed to resonant tunneling in the presence of high internal strain in the InAs layers.