Interfacial Grading Research Articles

Effect of interface grading on the optical performance of distributed Bragg reflector multilayers in Fabry-Pérot optical filters

The design, fabrication, and characterization of abrupt interface and graded interface distributed Bragg reflector (DBR) multilayers is reported. Electron-beam deposited zinc selenide (ZnSe) and Barium Fluoride (BaF2) thin films were chosen as the high and low refractive index dielectric materials respectively, for the DBRs in the mid-wavelength infrared (MWIR) region. In this region, DBRs comprised of 3 stacks of abrupt interface ZnSe and BaF2 thin films with central wavelength of 4 µm were found to suffer from defects and absorption loss that later affect the optical transmission and spectral resolution of assembled Fabry-Perot optical filters. Interfacial grading of the DBRs with a thin layer of amorphous silicon monoxide (SiO) ~ 80 nm, reduced the absorption loss and the interdiffusion of defect atoms at the interface between alternating layers. Fabry-Perot filters assembled from abrupt interface DBRs showed 22% optical transmission, whereas those assembled from graded interface DBRs acheived 26.5% optical transmission. The Full-width at half maximum (FWHM) of the transmission peak for graded interface Fabry-Perot optical filter decreased significantly compared to the abrupt interface optical filter, improving its spectral resolution.

Domino Effect of Thickness Fluctuation on Subband Structure and Electron Transport within Semiconductor Cascade Structures.

Effectively restraining random fluctuation of layer thickness (RFT) during the thin-film epitaxy plays an essential part in improving the quality of low-dimensional materials for device application. While it is already challenging to obtain an ideal growth condition for thickness control, the tangle of RFT with interfacial problems makes it even more difficult to guarantee the properties of heterostructures and the performance of devices. In our research, the RFT of potential barriers and wells within a semiconductor multilayer is demonstrated to correlate with the interfacial grading effect (IFG) and to affect the band offset strongly. Then, the synergetic effect of RFT and IFG that serves as the first domino is shown to impact the subband structure and the electron transport successively. On the basis of an investigation of a quantum cascade structure, statistical results indicate a normal distribution of RFT with a standard deviation of about 1 Å and an extreme value of 3 Å (about one monolayer) for all the layers within 38 cascade periods. The "seemingly negligible" RFT could actually reduce the conduction band offset for tens to hundreds of meV and alter the subband gaps at a rate of 40 meV/monolayer at most. Furthermore, the dependence of different subband gaps on the barrier/well thickness differs from one another. In addition, the distribution of wave function could also be regulated dramatically by RFT to change the type of electron transition and thus the carrier lifetime. Further impacts of RFT and the RFT-modulated subband alignment on electron transport result in two different mechanisms (injection-dominant and extraction-dominant) of electron population inversion (PI), which is manifested by comparatively discussing the results of in situ electron holography and macro performances.

Effects of Gas switching sequences on GaAs/GaAs 1−ySb y superlattices

The interfacial structure of GaAs/GaAs 1− y Sb y superlattices (SL) was modified through changes in the interlayer gas switching sequence. Trimethyl gallium, trimethyl antimony, and arsine were used as the precursors in the growth of thin-layer SL structures. Sufficient antimony introduction during the growth pause prior to GaAs 1− y Sb y growth was required for obtaining abrupt interfaces with a minimal amount of compositional grading as determined by X-ray diffraction, as was the arsine introduction after alloy layer growth. In both cases, the specific amount of Sb or As deposition was found to produce an optimal structure with minimal interfacial grading. The antimony composition of the pseudomorphic alloy layers appear to be constrained by thermodynamic and kinetic limitations.

Ultrathin type-II GaSb/GaAs quantum wells grown by OMVPE

Heterostructures containing monolayer (ML) and submonolayer GaSb insertions in GaAs were grown using organometallic vapour phase epitaxy. At the GaAs-on-GaSb interface, strong intermixing occurs due to the surface segregation of Sb. To form structures with relatively abrupt interfaces, a flashoff growth sequence, in which growth interruptions are employed to desorb Sb from the surface, was introduced. Reflectance-difference spectroscopy and high-resolution X-ray diffraction data demonstrate that interfacial grading is strongly reduced by this procedure. For layer structures grown with the flashoff sequence, a GaSb coverage up to 1 ML can be obtained in the two-dimensional (2D) growth mode. For uncapped GaSb layers, on the other hand, atomic force microscope images show that the 2D–3D growth mode transition occurs at a submonolayer coverage between 0.3 and 0.5 ML. Low-temperature photoluminescence spectra of multiple quantum well samples grown using the flashoff sequence show a strong quantum well-related peak which shifts to lower energies as the amount of Sb incorporated increases. The PL peak energies are consistent with a type-II band lineup at the GaAs/GaSb interface.

MOVPE grown quantum cascade lasers

We report the metal-organic vapour phase epitaxy growth of quantum cascade lasers (QCLs) in both the GaAs–AlGaAs and InGaAs–AlInAs–InP materials systems. The GaAs based sample shows an increased emission wavelength relative to an equivalent design grown by molecular beam epitaxy, which we attribute to interfacial grading in the metal-organic vapour phase epitaxy device. The InP-based QCL (λ≈8.5 μm) operates in pulsed mode at room temperature, with low temperature threshold current density ∼1500 A/ cm 2 . This performance is comparable with that previously reported for similar structures grown by molecular beam epitaxy. The high quality interfaces necessary for successful QCL operation are achieved by using individually purged vent/run valves in the gas-handling section of the reactor, as well as a low growth rate for the active region of the structure.

Ion Implantation Effect on Dislocation Propagation in Pseudomorphically Strained P/P+ Silicon

AbstractWe studied damage evolution and the influence on defect interactions as a function of Si self implantation dose in p/p+ silicon wafers. Highly boron doped 150 mm diameter silicon substrate wafers with a 2.5 μm thick nominally un-doped epitaxial layer (p/p+) were employed. Due to the misfit strain, misfit dislocations formed during the epitaxial growth process around the wafer edges. This localized dislocation distribution was utilized to study the role of the implant on both the nucleation and growth of the misfit dislocation segments. Triple axis x-ray diffraction was used to determine changes to the strain in the layer due to both the implant and to subsequent annealing. Double axis x-ray topography combined with rapid thermal annealing was used to measure the nucleation and extension of the misfit dislocation segments after annealing. For the lower implantation dose (1012 cm−2) samples, the velocity of dislocations was reduced measurably and the density of newly formed misfit dislocations also decreased significantly relative to regions that received no ion implantation. A higher implantation dose reduced the nucleation and glide velocity further such that neither glide nor nucleation of new segments was detected. SIMS measurements confirmed that the interfacial lattice parameter grading was not appreciably different after an anneal step for the lower implant dose sample, but was measurable for the higher dose sample. In the latter case, transient point defect enhanced diffusion of boron was responsible for the grading. However, the lack of grading in the low dose sample indicates that interfacial grading did not significantly impede dislocation kinetics. This comparison indicates that it is the excess point defects that are produced during implantation that retard both the nucleation and extension of misfit dislocation segments.

The effect of interfacial mixing on Ag/Al2O3 by N+ implantation

The Ag film/Al2O3 implanted with an N ion energy of 110 keV. The ion-induced interfacial mixing were examined using AES, STD and XPS. The frictional coefficient of implanted Ag/Al2O3 in air was determined by Drive friction precise measuring apparatus (DFPM). The influence of N+ implantation on the interfacial chemistry and adhesion of Ag films on Al2O3 substrates was examined, compounds formed by introducing a thin layer between the Ag and the Al2O3 at 1 × 1017 N · cm−2. This investigation resulted in extensive interfacial grading, and new chemical bonding across the Ag/Al2O3 interfaces. The case for the Ag/Al2O3 reaction lead to the metal ceramic of 13Al2O3 · AIN and α-AgAlO2 formation though non equilibrium processes of implantation, and friction decreased in initial cycle where ion implantation of lower vacuum lead to surface carbon film. The combination of these effects provided an adhesion increase that was approximately 3 times that obtained in unimplanted Ag/Al2O3 specimens.

Effect of interfacial mixing by N+ implantation on the adhesion and friction of Ti film on Si3N4

A thin ion-plated Ti film on sintered Si3N4 was implanted with 110 keV N+ at doses of 1 x 1016 to 3 x 1017 ions cm-2 and a substrate temperature of 60°C. The adhesion of the ion-plated Ti film and the N+-implanted films on Si3N4 was examined using a scratch tester and a scanning electron microscope. The interfacial chemistry of the N+-implanted Ti/Sl3N4 specimens was examined using sample tilting diffraction (STD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). It was found that at a dose of 3 x 1017 N+ cm-2, the critical load for the scratch test on the N+-implanted Ti film/Si3N4 substrate increased from 10.8 to 50.0 N; consequently, the implanted films were not easily removed by interfacial failure as in the case of the unimplanted one. AES indicated that N+ implantation produced substantial interfacial mixing and interfacial grading in the Ti/Si3N4 system. The STD and XPS analyses revealed that a TixN (x = 1, 2) compound was formed by N+ implantation. The combination of these effects in the N+implanted Ti/Si3N4 specimen provided an increased adhesion that was approximately 4.6 times that of the unimplanted system. In addition, the hydrocarbon adsorbed in the near-surface region of Ti films reduced the adhesion component of friction between the metal and the N+-implanted specimen.

Fabrication and characterization of selectively grown Si 1- xGe x/Si p +/n heterojunctions using pulsed laser induced epitaxy and gas immersion laser doping

A pulsed XeCl excimer laser is used to grow ideally strained heteroepitaxial Si 1- x Ge x /Si layers with Ge fractions up to 21% by intermixing a structure of electron beam evaporated a-Ge on Si(100). The rapid regrowth process induces an interfacial grading of the Ge fraction, which results in unusual stability of the layer strain upon heat treatment, as confirmed by MeV-ion channeling along <100> and <110>. Boron dopant is incorporated during the meltprocess by using a BF 3 gas ambient. Hall/van der Pauw and SIMS analysis reveal that the incorporated dopant dose scales with the number of laser pulses. The junction depth is controlled by the incident laser fluence. The melt time is monitored in-situ utilizing the transient reflectance of the sample during the phase transformations. A patterned reflective aluminum mask is used to obtain spatially selective melting. In-plane Hall mobilities are found to be lower for the heteroepitaxial junctions than for Si homojunctions. We believe this is due to different transport behavior for holes in the observed doping regime of 10 18–10 20 cm -3. Quasiplanar p +/n heterojunction diodes are fabricated and exhibit near-ideal forward I-V characteristics. Heterojunction diodes exhibit lower turn-on voltages than equivalent Si homojunction diodes, indicative of a lowered bandgap. The turn-on voltages also depend on the B junction depth with respect to the Si 1- x Ge x /Si interface. Both quantities are controlled independently by separating the epitaxy from the doping step.

Crystallinity, strain, and thermal stability of heteroepitaxial Si1−xGex/Si (100) layers created using pulsed laser induced epitaxy

Heteroepitaxy of Si1−xGex/Si alloy layers on Si (100) substrates has been achieved using pulsed laser induced epitaxy (PLIE). The energy of 1 to 20 pulses from a spatially homogenized XeCl excimer laser beam is used to melt a structure consisting of electron-beam evaporated Ge on Si (100) substrates. Alloy films with different Ge fractions are investigated and films with up to 21% Ge content are found to exhibit excellent crystallinity, as confirmed by MeV-ion channeling along the 〈100〉 direction. MeV-ion channeling is also used to determine the level of strain in the layers. This is done by comparing angular yield curves around the 〈110〉 direction for the substrate and alloy layer. The strain values obtained match with calculations for an ideally strained layer state. The strain is also measured for layers that have been subjected to different thermal cycles. A high level of strain is preserved in the alloy layer even after heating to 950 °C for 1 h. This unusual thermal stability is believed to be due to two process features, the ultrarapid nature of the process and the interfacial grading of the Ge fraction created during the melt-regrowth process.

Ion-induced Ni/ferrite adhesion enhancement: 28Si + versus 40Ar + ion mixing

The adhesion between 30 nm Ni films and ferrite substrates was examined as a function of the interfacial chemistry and mixing induced by low temperature 40Ar + and 28Si + ion mixing. The temperature of the Ni/ferrite specimens was kept between 50 and 100°C while the specimens were implanted with either 65 keV 40Ar + or 50 keV 28Si +. The doses of both types of implants were varied between 1 and 10×10 16 atom/cm 2. The interfacial mixing, interfacial chemistry and the Ni adhesion produced by the ion mixing processes was evaluated using XPS depth profiling, high resolution XPS and scratch testing, respectively. Both types of implants were observed to induce substantial interfacial grading, although this grading was greater for the 40r + than the 28Si + ion mixing. XPS analyses suggested that NiOM (where M is either Fe, Mn, or Zn) interfacial complexes were present in the as-deposited specimens, and that they were responsible for the relatively good adhesion (8 N) of the as-deposited films. These intrinsic interfacial complexes were destroyed by both the 40Ar + and the 28Si + ion mixing. In the case of 40Ar + ion mixing, no new interfacial complexes were produced. This resulted in the reduction of Ni/ferrite adhesion (to a critical force of 2 N). In the case of 28Si + ion mixing, new interfacial chemistry (NiSi, FeSi, and SiO) was produced, suggesting the formation of adhesive interfacial complexes (such as NiSiFeO and NiSiOFe). This new interfacial chemistry resulted in significant adhesion enhancements (up to 26 N).

Ni/quartz adhesion enhancement: Comparison of Ar+ and Si+ ion mixing

The adhesion between 30 nm Ni films and quartz substrates was examined as a function of the interfacial chemistry and mixing induced by low temperature (&amp;lt;100 °C) 40Ar+ and 28Si+ ion mixing. The Ni/quartz specimens were implanted with either 65 keV 40Ar+ or 55 keV 28Si+ to doses between 1 and 10×1016 atom/cm2. Both types of implants were observed to induce extensive interfacial grading. X-ray photoelectron spectroscopy analyses indicated that Ni–O–Si types of interfacial complexes were present in the as-deposited specimens, but that these complexes were eliminated by the 40Ar+ ion mixing. In contrast, high dose 28Si+ ion mixing was shown to induce the formation of new interfacial complexes (Ni–Si–O and new Ni–O–Si complexes). Adhesion measurements, performed using a scratch tester, indicated that the adhesion of the as-deposited specimens was good (20 N), but that this adhesion got progressively worse (to less than 1 N) with 40Ar+ implantation. In the case of 28Si+ ion mixing, adhesion was reduced (to 5 N) in the low dose specimens, but was substantially increased (to 45 N) in the high dose specimens. Overall, Ni/quartz adhesion was shown to correlate with the concentration of interfacial complexes that could chemically attach the Ni films and quartz substrates.

Adhesion enhancement of Ni films on polyimide using ion processing. I. 28Si+ implantation

The influence of 28Si+ ion implantation on the interfacial chemistry and adhesion of 30 nm Ni films on po1y[N,N-(p,p′-oxydiphenylene) pyromellitimide] (PI) substrates was examined. The Ni/PI specimens were held at temperatures below 100 °C, and implanted with 55 keV 28Si+ at doses of either 1×1016 , 5×1016 or 1×1017 Si/cm2 . The 1×1016 Si/cm2 implants had no influence on the surface topography, but the higher dose implants introduced some submicron void formation. Cross-sectional transmission electron microscopy and Auger electron spectroscopy depth profile analyses showed that the as-deposited Ni films formed very sharp interfaces with the polyimide substrates. However, after implantation the interfaces became graded by up to a factor of 2 compared to the as-deposited specimens. Large quantities of C were transferred into the Ni films, resulting in the formation of a C overlayer. The concentration of implanted Si reached a maximum (at approximately 16 at. % for the 1×1017 Si/cm2 implant) in the Ni films and extended deeply into the PI substrates. X-ray photoelectron spectroscopy analysis of the interfaces of the specimens implanted with 1×1017 Si/cm2 showed a mixture of Ni–Si, Si–O, and C–O chemical bonding, consistent with Ni–Si–O–C complex formation. Adhesion testing showed substantial adhesion increases (up to a factor of 20) with 28Si+ dose. The Ni films implanted with 1×1017 Si/cm2 were only removed upon substrate failure. The adhesion increases induced by 28 Si+ implantation are attributed to a combination of substrate toughening, mechanical interlocking, interfacial grading, and chemical bonding across the interfaces.

Ion-induced adhesion enhancement of Ni films on polyester: 28Si + implantation

The adhesion between 30 nm Ni films and poly[ethylene terephthalate] (PET) substrates was examined as a function of the morphological and chemical modifications induced by 28Si + ion mixing through the Ni/PET interfaces. The Ni/PET specimens were held below 100°C and implanted with 55 keV 28Si + at doses ranging from 1 × 10 16 to 1 × 10 17 Si/cm 2. The low dose implantation had no observed influence on the surface topography, but the 1 × 10 17 Si/cm 2 implant did result in increased submicron void formation in the films. Auger electron spectroscopy (AES) and cross-sectional transmission electron microscopy (XTEM) analysis showed that the as-deposited films formed rather discrete interfaces with the PET substrates. After 28Si + ion mixing, those interfaces were graded substantially (up to five times the as-deposited interfacial distance). The extent of interfacial grading increased with the ion dose. The implanted Si reached a maximum concentration of approximately 15 at.% (for the 1 × 10 17 Si/cm 2 implant) in the Ni film and then extended deeply into the PET substrate. X-ray photoelectron spectroscopy (XPS) and electron diffraction analysis showed no detectable interaction between the Ni films and the PET substrates in the as-deposited specimens. However, after 28Si + implantation, the presence of NiSi and SiO bonding was observed at the Ni/PET interface. This bonding was shown to gradually change from primarily NiSi bonding in the Ni-rich regions to primarily Si-0 bonding in the C-rich regions of the substrate. In addition, the concentration of ether-type (COC) bonding in the interfacial region increased after 28Si + implantat compared to the as-received PET and the as-deposited Ni/PET interface. This ion-induced chemical bonding is consistent with NiSiOC complex formation throughout the interfacial region. Adhesion testing, performed using a scratch test in conjunction with AES analysis, showed substantial adhesion increases with the increasing dose of the 28Si + implantation. The as-deposited films were removed at forces less than 1 N. In contrast, the Ni films implanted with 1 × 10 17 Si/cm 2 were only removed at forces of approximately 19 N. This film removal occurred as a result of catastrophic substrate failure. The adhesion increases induced by 28Si + implantation were attributed to a combination of substrate toughening, mechanical interlocking, interfacial grading, and chemical bonding across the interfaces.

Characterization of (Al,Ga)As/GaAs Multilayer Structures by X-Ray Interference

ABSTRACTAn experimental study of x-ray interference effects from GaAs/AlxGa1−xAs multilayer structures was conducted. The presence of a GaAs layer, whose thickness ranged from 50 Å to 10 μm, between two Al0.30Ga0.70As layers was found to significantly affect the x-ray rocking curves from these structures. We determined both the thickness and the composition of the AlxGa1−xAs layers and the thickness of the GaAs layers by comparing the rocking curves to simulated ones using a dynamical x-ray diffraction model. A sensitivity analysis showed that the thickness predicted for the GaAs buried layer was precise to within 90 Å for a ≃ 2000 Å GaAs layer and that the barrier layer Al content and thickness could be determined to within 1% and 80 Å, respectively for ≃ 5000Å layers. This analysis also showed that, within experimental resolution limits, the structure determined from comparing simulated and experimental rocking curves was unique. Both the limits of this technique in determining interfacial grading and applications to specific device structures and other materials systems are discussed.

Achievement of high gain in a planar heterojunction bipolar transistor with lateral current flow

The authors have demonstrated transistor action in an L-HBT (lateral heterojunction bipolar transistor) device with multiple, quantum sized base channels. The small size of the individual channels has made it possible to increase the steepness of the alloy composition grading at the emitter-base junction of the transistor when the emitter is formed by Si diffusion disordering. It is well known that this interfacial grading must be optimized in order to achieve the maximum transistor device performance. As a result of these and other design modifications, transistor action with a maximum gain beta well in excess of 200 was achieved. These gain values demonstrate the practicality of transistor devices of the L-HBT design.

Ion-enhanced adhesion of Ni films on glassy carbon: I. 28Si + implantation

The adhesion of 30 nm Ni films on glassy carbon (GC) was examined as a function of the interface modification produced by 28Si + implantation through the interface. The Ni/GC specimens were held at 100°C and implanted with 55 keV 28Si + at doses ranging from 1 × 10 16 to 5 × 10 17 Si/cm 2. The ion implantation had very little influence on the surface topography, but had a tremendous influence on the depth distribution of the elements. Auger electron spectroscopy (AES) depth profiling revealed that the as-deposited films formed rather sharp interfaces with the GC substrates. Following ion implantation, the Ni/GC interface was substantially graded. This interfacial grading increased with the ion dose. The implanted Si reached a maximum concentration (15 at% for the 1 × 10 17 Si/cm 2 implant) in the Ni layer and then extended deep into the GC substrate. X-ray photoelectron spectroscopy (XPS) analysis of the interfaces of the specimens implanted with high doses of 28Si + (> 5 × 10 16 Si/cm 2) showed that it was composed of a mixture of Ni-Si, Si-C, Si-O and C-O chemical bonding. The XPS results showed no evidence of chemical bonding between Ni and C at the Ni/GC interfaces. Adhesion testing, using a scratch tester in conjunction with SEM/AES analysis, showed a dramatic increase in adhesion for the 28Si + implanted specimens. The as-deposited specimens were readily removed at forces less than 1 N. In contrast, the films implanted with doses of 5 × 10 16 Si/cm 2 or greater could not be removed short of catastrophic substrate failure (at approximately 45 N). This adhesion increase was attributed to a combination of enhanced interfacial bonding, mechanical interlocking, and stress relief through interfacial grading.