Degree Of Intermixing Research Articles

Optical properties of annealed, single GaAs quantum wells: Cap doping and mask width dependence

Band edge absorption measurements are used to characterize the degree of Al-Ga intermixing (blue-shifting) and the linear optical loss of waveguides formed in five annealed and encapsulated single GaAs quantum well laser heterostructure wafers which differed only by the amount of Zn doping in the GaAs cap layer. In addition to the transmission measurements, secondary ion mass spectroscopy data was used to verify the diffusion of Zn before and after annealing. High zinc doping in the cap is observed to cause quantum well disordering below the encapsulant (Si3N4) and is attributed to impurity induced layer disordering. Moderate doping in the cap results in selective area intermixing via controlled gallium vacancy production. A stripe width dependence is also observed, which suggests a role of lateral diffusion of species which affect the intermixing. For an undoped (n−) cap, the degree of intermixing is heavily dependent on the arsenic overpressure used during the anneal and is independent of the nitride stripe width suggestive of a volumetric Fermi level dependent production of vacancies within the cap.

Modeling intercomponent mixing effects in rubber‐modified glassy polymers

AbstractA phenomenological model is presented to assess the effects of phase mixing on storage modulus vs. temperature data in polymer blends comprising a continuous glassy matrix and discrete rubbery inclusions. An extension of the Kerner–Dickie model, it is developed for systems of low‐dispersed phase content with a moderate degree of intermixing, and compared with experimental data for six blends of polystyrene or lightly hydroxylated polystyrene with poly(ethyl acrylate) at 90/10 and 70/30 w/w compositions. Phase mixing is induced in this system through intercomponent hydrogen bonding. The model successfully represents data for the four blends of lowest interphase content, but fails for the other two. The model can evidently provide a good description of blends where the bulk of each component resides in a pure phase, but fails when the mixed interphase constitutes the majority of the material. © 1995 John Wiley & Sons, Inc.

Application of intermixing to p-type GaAs/AlAs distributed Bragg reflectors for series resistance reduction in vertical cavity devices

In this paper we experimentally investigate the application of selective interdiffusion to p-type (Zn doped) distributed Bragg reflectors, as employed within a range of vertical cavity devices, as a means of lowering the series resistance. Various rapid thermal anneal temperatures and times are studied, both with and without silica encapsulants. The degree of intermixing, and hence series resistance reduction, is found to be cap dependent and this is verified both by secondary ion mass spectrometry and electrical resistance measurements. Both these techniques suggest that the intermixing, due primarily to Zn enhanced interdiffusion, is increased when no encapsulant is used. In this case a series resistance reduction approaching 50% is achieved within the 14 period GaAs/AlAs Bragg reflector. In the silica encapsulated case Ga vacancies are injected into the structure and these suppress the movement of Zn. The resulting Ga diffusion coefficient in this case is found to decrease by a factor of 3 relative to the uncapped case. The effects of interdiffusion on the reflection properties of the reflector, under various anneal conditions, are also described. This has previously been studied theoretically [Floyd et al., J. Appl. Phys. 75, 7666 (1994)] but here we offer experimental evidence on the optical effects of intermixing. We show that the peak reflectivity is found to decrease only slightly with temperature and time, again depending on capping conditions. The major effect contributing to the reflectivity decrease however is surface degradation due to As out-diffusion. The reflectivity does decrease appreciably (∼10%) when the interdiffusion length of the group III atoms becomes a significant fraction of the quarter-wave thickness of the GaAs/AlAs layers, as demonstrated in samples annealed at high temperatures and for long times (960 °C/360 s). These issues, along with that of planar integration of vertical cavity lasers and other devices such as modulators and detectors, are of importance for future applications.

Uniform intermixing of quantum wells in p-i-n modulator structures by impurity free vacancy diffusion

We present the results of an investigation of impurity free vacancy diffusion (IFVD) post-growth treatments of p-i-n modulator structures. The investigation is in two parts. We first establish that gallium vacancies (VGa) are produced during IFVD (by measuring the intensity of the low temperature 1.2 eV signal from Si-VGa complexes) in a thick Si-doped GaAs sample. The second part of this work investigates the degree of intermixing of three 80A GaAs quantum wells embedded in the intrinsic region of a p-i-n modulator at depths between 1–2 μm from the surface. Photoluminescence studies on etched samples and cathodoluminescence showed that no significant depth dependence occurs as a result of IFVD.

Irradiation-Induced Damage and Intermixing of GaAs-Algaas Quantum Wells

AbstractA comparison of ion irradiation-induced intermixing in GaAs-Al0.54Gao46As quantum well structures with H, O and As ions is investigated by low temperature photoluminescence. Very large energy shifts are observed together with good recovery of the photoluminescence intensities after annealing in samples irradiated with protons. No saturation in the energy shifts is observed in samples irradiated even up to a dose of 4.3 x 1016 cm-2. Similar large shifts with low absorption are also observed in O and As implanted samples but at a significantly lower ion dose. However, both the heavier ions show a saturation effect in the degree of intermixng at higher doses. The degree of intermixing is believed to be a delicate balance among multiple competing processes that occurs across the interface.

Effects of Interface Intermixing on the Magnetoresistance of Spin Valves with Uncoupled Co-Layers

ABSTRACTWe have studied the effect of an artificially intermixed region grown at the interfaces of Co/Cu spin valves with uncoupled layers. Two different structures are used: exchange-biased spin valves and engineered spin valves in which two layers are antiferromagnetically coupled and a third layer, on top of this system, is not coupled to the other two. It is shown that structural effects, induced by variation of the deposition parameters and by the intermixing can play an important role. Since the present study uses sputtered layers an intrinsic initial intermixing of 4-5Å is already present. For both types of spin valves Gp, ΔG and ΔR/R all show a gradual decrease when the nominal thickness of the total intermixed region is enlarged from 0 to 36Å. Also when the initial degree of intermixing is decreased by sputtering at higher Arpressure, Gp, ΔG and ΔR/R still show a gradual decrease as a function of intermixed layer thickness. Combined with the fact that there is no difference between an intermixed region of thickness t at one Co/Cu interface or intermixed regions of thickness t/2 at two interfaces, this indicates that the electron scattering in the intermixed region is predominantly spin independent, although this region preserves a magnetic moment.

Thermally induced change in the profile of GaAs/AlGaAs quantum wells

We investigated the effect of thermal annealing on the low temperature excitonic reflectivity (LTER) spectra of a high quality GaAs/ AlGaAs single quantum well (QW) grown by molecular beam epitaxy. Collecting LTER spectra at 2 K, on different samples annealed at a constant temperature of 950 °C for different time durations, we could resolve three different series of confined transitions up to a final value of the diffusion length L D = 66 A ̊ . These transitions correspond with the (E 1-HH 1), (E 10LH 1) and (E 2-HH 2) electron-hole states respectively. Depending on the transition investigated and the degree of intermixing, these transitions exhibit different confinement energies which allow the QW profile to be probed in great detail. This establishes, on a purely experimental basis, LTER as one of the most powerful (and non-destructive) ways to monitor the effect of QW intermixing (QWI) when processing GaAs/AlGaAs photonic integrated circuits. Next, solving the Schrödinger equation to determine the position of all confined states in the OW as a function of the annealing conditions, we could predict the change in the transition energies expected for the localized states as a function of L D . Comparing with our experimental values, we find that, provided the nominal QW thickness (100 Å) has been adjusted slightly to a final value of about 110 Å, there is very satisfactory agreement (for the five annealed samples) between the theoretical predictions and the experimental results. To the best of our knowledge, this is the only all-optical evidence that, in this range of investigation, Fick's second law can be applied satisfactorily and that it is not necessary to call for any concentration-dependent mechanism to explain the effect of QWI on both the fundamental and excited states.

High quality wavelength tuned multiquantum well GaInAs/GaInAsP lasers fabricated using photoabsorption induced disordering

Broad area oxide strip lasers have been fabricated from GaInAs/GaInAsP multiquantum well laser material which has undergone various degrees of intermixing by photoabsorption induced disordering. This process provides an effective way of altering the emission wavelength of lasers fabricated from a single epitaxial wafer, and we have demonstrated blue shifts of up to 160 nm in lasing spectra. The band gap tuned lasers are also assessed in terms of threshold current density, internal quantum efficiency, and internal loss and it is shown that good device performance is maintained.

Structural characterization of Si/Ge superlattices grown on an Si(001) surface by molecular beam epitaxy

Structural quality of (Si m Ge n ) N superlattices grown on an Si(001)−2×1 surface at different temperatures (300–500 °C) by molecular beam epitaxy was characterized by reflection high energy electron diffraction, X-ray diffraction, Raman scattering and high-resolution transmission electron microscopy. Asymmetric intermixing is observed, in which the Si-on-Ge interface is less abrupt than the Ge-on-Si interface due to Ge segregation into the growing Si overlayer. It is found that the degree of intermixing and crystalline quality depends on the growth temperature. Experimental evidence shows that significant intermixing due to thermal diffusion of Ge atoms into the growing Si layers occurs 500 °C.

Intermixing of MeV ion-implanted and annealed AlGaAs/GaAs superlattices

MeV energy ion implantation with various ions has been adapted to induce layer intermixing in AlGaAs/GaAs superlattices (SLs). Samples are implanted with 1.2 MeV C, 1.3 MeV O, 1.4 MeV F, 1.7 MeV Si, and 2.7 MeV Zn and Ga at a dose of 1 × 10 15/cm 2. Cross-sectional transmission electron microscopy and secondary ion mass spectrometry are used to investigate defect structures and the degree of intermixing. Total intermixing is observed in only Si-implanted SLs after 800°C-8 h annealing over a depth range between 1.3 and 1.8 μm which corresponds to a region having Si concentrations above 6 × 10 18/cm 2. In implanted layers with all the other ions, no complete intermixing is realized, although a slight and gradual damping of Al oscillations is observed with an increase in the depth. Extended defects, almost prismatic dislocation loops, are formed in all of the implanted SL layers beneath the substrate surface with their maximum densities neat depths where each implanted ion shows the maximum peak concentration. It is also found that the distribution and annealing behavior of defects in SLs strongly depend on the species of the implanted ions.

Lattice damage and atomic mixing induced by As++ implantation and thermal annealing in AlAs/GaAs multiple quantum-well structures

The lattice damage and the nature of the atomic intermixing of Al and Ga induced by As++ implantation and thermal annealing in AlAs/GaAs multiple quantum-well structures were investigated. The photoluminescence spectra, which show multiple peaks after implantation and annealing, were analyzed based on the shifts of the excitonic peaks arising from quantum wells located at different depths. The depth profiles of intermixing were obtained using a procedure of successive layer-by-layer chemical etching followed by photoluminescence measurements. It is found that the atomic mixing is maximum near the sample surface and decreases monotonically with depth, suggesting that the profiles follow more closely the ion induced damage than the ion density. It is also observed that the radiation damage extends beyond 1 μm. Within 0.3 μm from the surface, the damage is relatively heavy and the atomic intermixing increases rapidly with ion dose. Beyond 0.3 μm, the degree of intermixing is only sensitive to the anneal temperature but not to the implantation dose. The results show that both direct collisions and interdiffusion are responsible for the atomic mixing. For the samples implanted with ion doses below 1014 cm−2 and annealed at 650 °C, the optical activation from radiation damage is appreciable. However, the interdiffusion becomes important only at temperatures near and above 800 °C.

Processing parameters for selective intermixing of GaAs/AIGaAs quantum wells

Some of the parameters which determine the amount of intermixing of GaAs/AIGaAs quantum wells (QWs) using SiO2 capping and rapid thermal annealing (RTA) have been studied using photoluminescence (PL) techniques. The degree of intermixing of QWs was found to be larger for thicker SiO2 capping layers and for shorter distances between the QWs and the oxide-wafer interface. A maximum PL energy difference of 90 meV was observed between the region covered by a 1.3 μm thick oxide layer and the non-oxide region in a wafer that was annealed at 1100° C for 15 s.

Hydrogen Passivation of Interfacial Defects in MOCVD grown GaAs/InP

AbstractThe results of a low temperature (5K) photoluminescence study of hydrogenation of GaAs on InP grown by metal organic chemical vapor deposition are presented. An emission band at ~ 1.4 eV originating from the GaAs/InP interracial region shows a 30 fold increase in intensity relative to the GaAs band edge emission after exposure to hydrogen plasma for 30 min at 250°C. This improvement in intensity is attributed to hydrogen passivation of defects at the heterointerface caused by the large (≈4%) lattice mismatch between GaAs and InP. The passivation effect recovers on annealing the hydrogenated sample at 350°C. Excitation dependence of the ~1.4 eV band suggests that the interfacial region consists of a compositionally graded layer. Further, this band shifts to higher energy on annealing the sample in the temperature range 150-450°C with the hydrogenated sample exhibiting a larger shift than the untreated sample. It is suggested that the annealing induced peak shift arises due to intermixing of the compositionally graded interface and that the degree of intermixing is greater in the hydrogenated sample compared to the untreated sample.

Epitaxial Growth and Band Structure Effects at the Si/Ge(111) Interface

ABSTRACTX-ray absorption fine structure, Auger electron spectroscopy and low energy electron diffraction have been used to study the evolution of the lattice constant and the degree of intermixing of thin (∼5 Å) silicon films grown on the germanium(111) surface by solid phase epitaxy. Our results indicate that as the system is annealed, the silicon intermixes with the germanium at elevated temperatures to form a pseudomorphic Si-Ge alloy with the germanium bulk lattice constant. We further observe a splitting of the ‘white line’ Is absorption edge in crystalling Si, SiGe and dilute (<10% silicon) SiGe. The measured splittings agree with band structure calculations for Si, SiGe, and Ge, respectively, indicating that the density of conduction band states of SiGe behaves as the interpolation between the states of Si and Ge. Qualitatively, our spectra may be interpreted in terms of a one electron picture without invoking many body excitonic effects. The absorption edge of amorphous silicon is also presented and the features of the density of states due to long versus short range order are extracted.

INTERMIXING OF ION-IMLANTED AlGaAs/GaAs SUPERLATTICES

AbstractCross-sectional Transmission Electron Microscopy, Sputter-Auger spectroscopy, and Raman spectroscopy have been used to study intermixing and residual damage in annealed ion-implanted Al0.3Ga0.7As/GaAs superlattices. Several implant species were studied Nse, Si, Mg, Be). Three different regions can be distinguished in the annealed ionimplanted superlattice samples. The topmost region contains a dense network of stacking faults and microtwins, residual damage from an implantation-amorphized region which has recrystallized during annealing. In the second region, which is relatively defect-free, either total, or at least appreciable intermixing of the GaAs and Al0.3Ga0.7As layers occurs. For fixed annealing conditions, the degree of intermixing varies with the mass of the implanted species. The third region contains many small dislocation loops which form by the agglomeration of point defects during implantation or the subsequent annealing process. Raman spectroscopy is used to compare the degree of intermixing and residual damage between AlGaAs alloys generated by superlattice disordering and uniform “as-grown” alloys of the same composition which have undergone identical implant and anneal treatments.

Review and Classification of Quantitative Mapping Techniques

Facies maps show quantitatively the areal variation of either lithologic composition or other properties of a rock unit. They provide a three-dimensional picture of a stratigraphic unit and can form the framework for a paleogeographic and paleotectonic interpretation. The patterns shown on some maps have a relationship to oil- and gas-producing trends or mineralized areas. Quantitative stratigraphic maps are classified in five categories according to the information they convey: (1) structure and topographic maps, (2) isopach maps, (3) facies maps, (4) vertical variability maps, and (5) maps based on either a statistical analysis or other kind of mathematical treatment of a contour map. Compositional information can be presented in various ways by different types of facies maps. Ratiotype facies maps express the contrast between end members. Two sets of ratio contours describe a three-component system and subdivide the map into areas of different lithological composition. Facies maps based on a classifying function express the lithological composition in terms of classes defined by the relative amount of three components selected for end members. Class boundaries subdivide the map into areas of different lithological composition. Entropy function facies maps express the degree of intermixing of the end members and quantitatively define facies boundaries with one set of contours. The variation in composition can be shown by combining the entropy function with an overl y pattern to differentiate end members. Entropy-ratio facies maps provide both a contrast between the end members and information on the degree of intermixing of the end members. Facies departure maps express the degree of similarity to a pre-selected composition and are very effective for outlining trends related to a definite facies. Two classes of maps show vertical variability. One class shows the degree of differentiation of a section into discrete units of different lithological types, and includes the following kinds of maps: average bed thickness, number of discrete beds of a given lithologic type, average interval entropy. The other class shows the relative position or vertical distribution of one lithological type within the section and includes the following kinds of maps: multi-partite, center of gravity, standard deviation or dispersion, and slice.

The properties of dielectrics. I. Electric moment and molecular structure

This paper reports on experimental and theoretical investigation of atyical temperature-dependent photoluminescence properties of InAs quantum dots in close proximity to InGaAs strain-relief underlying quantum well. The impact of a post-growth intermixing process on these properties has been studied. For the as-grown sample, the maximum of the emission band follows a sigmoidal function while the photoluminescence linewidth mimics a V-shape function as the temperature increases, from 11 to 300 K. These behaviors are attributed to thermally activated carrier transfer mechanisms within the inhomogenious distribution of quantum dots. These atypical behaviors are found to disappear progressively with the degree of intermixing and consequent narrowing of the dot size dispersion. The experimental results have been interpreted in the frame of the localized states ensemble model revealing that the large dots size distribution is the main origin of the observed anomalies. Furthermore, the calculations show that the quantum well continuum states act as a transit channel for the redistribution of thermally activated carriers.