Impurity-induced Layer Disordering Research Articles

Impurity-Induced Layer Disordering: Current Understanding and Areas for Future Investigation

AbstractThe purpose of this work is to give an overview of the current phenomenological understanding of impurity-induced layer disordering (IILD). First, we identify key experimental findings such as the influence of the crystal surface-ambient interaction, the Fermi-level effect, and the impurity concentration on Al-Ga interdiffusion. Second, we review the strengths and weaknesses of existing IILD models in consideration of the above mentioned experimental data. Finally, we discuss the pitfalls involved in generalizing the results of individual Al-Ga interdiffusion experiments in order to explain a broader collection of IILD data.

Column III and V ordering in InGaAsP and GaAsP grown on GaAs by metalorganic chemical vapor deposition

Data are presented showing that GaAs1−y Py grown on GaAs by metalorganic chemical vapor deposition (MOCVD) at relatively low temperature (∼640 °C) exhibits ordering on the column V sublattice. These data, with electron diffraction data and impurity-induced layer disordering data, show that column III site and column V site ordering is possible for the quaternary InGaAsP grown on GaAs by MOCVD at relatively low temperature (∼640 °C). Ordered InGaAsP grown on GaAs shifts in photoluminescence wavelength ∼130 meV higher in energy with disordering by annealing or by impurity-induced intermixing.

Atom diffusion and impurity-induced layer disordering in quantum well III-V semiconductor heterostructures

The process of impurity-induced layer disordering (IILD) or layer intermixing, in AlxGa1−xAs-GaAs quantum well heterostructures (QWHs) and superlattices (SLs), and in related III-V quantum well heterostructures, has developed extensively and is reviewed. A large variety of experimental data on IILD are discussed and provide newer information and further perspective on crystal self-diffusion, impurity diffusion, and also the important defect mechanisms that control diffusion in AlxGa1−xAs-GaAs, and in related III-V semiconductors. Based on the behavior of Column III vacancies and Column III interstitials, models for the crystal self-diffusion and impurity diffusion that describe IILD are reviewed and discussed. Because impurity-induced layer disordering has proved to be an important method for III-V quantum well heterostructure device fabrication, we also review the application of IILD to several different laser diode structures, as well as to passive waveguides. We mention that it may be possible to realize even more advanced device structures using IILD, for example, quantum well wires or quantum well boxes. These will require an even greater understanding of the mechanisms (crystal processes) that control IILD, as well as require more refined methods of pattern definition, masking procedures, and crystal processing.

Impurity-induced layer disordering of In0.53Ga0.47As/In0.52Al0.48As heterostructures

Impurity-induced layer disordering of In0.53 Ga0.47 As/In0.52 Al0.48 As heterostructures grown by molecular beam epitaxy has been observed by Auger electron spectroscopy depth profiling. We find that Si+ ion implantation to concentrations greater than 2×1019 atoms cm−3 enhances the intermixing of Ga and Al in these heterostructures at an annealing temperature of 1075 K. However, the relatively high temperature which is required to activate the interdiffusion of Ga and Al in the region of high Si concentration is sufficient to induce In diffusion in regions of lower Si concentration. Zinc diffusion is found to completely intermix the Ga and Al in the heterolayers at temperatures as low as 825 K, which is below the temperature at which significant In diffusion occurs in undoped regions.

Losses in semiconductor waveguide S bends fabricated by impurity-induced layer disordering

An experimental comparison is made between the losses of S bends fabricated by impurity-induced layer disordering in GaAs-AlAs superlattices. For parallel waveguides offset by 100 microm, transition lengths corresponding to 3-dB excess loss were found to be approximately 290 microm for approximately single-mode S bends in the form of a raised cosine and approximately 335 microm for bends in the form of two constant-radius-of-curvature sections. These buried-channel bends appear to have losses that compare favorably with those formed from rib waveguides.

Dislocation reduction by impurity diffusion in epitaxial GaAs grown on Si

Data are presented showing that low-temperature Zn diffusion (680 °C) is effective in reducing the dislocation density in epitaxial GaAs grown on Si. The GaAs-on-Si is analyzed using both cross-sectional and plan-view transmission electron microscopy. For comparison, simple thermal annealing of the GaAs-on-Si at higher temperature (850 °C) is also performed and analyzed. The reduction in the dislocation density that occurs with Zn diffusion is suggested to be due to the increased concentration of point defects generated during the Zn diffusion, resulting in enhanced dislocation climb. This mechanism is consistent with impurity-induced layer disordering, via Zn diffusion, in AlxGa1−xAs-GaAs heterostructures.

Impurity-induced layer disordering of high gap Iny(AlxGa1−x)1−yP heterostructures

Data are presented showing the impurity-induced layer disordering (IILD), via low-temperature (600–675 °C) Zn diffusion, of In0.5(AlxGa1−x)0.5P quantum well heterostructures and an In0.5Al0.2Ga0.3P-GaAs heterojunction grown using metalorganic chemical vapor deposition. Secondary ion mass spectroscopy, transmission electron microscopy, and photoluminescence are used to confirm IILD, which occurs via atom intermixing on the column III site aided by column-III-atom interstitials. In addition, high-temperature anneals (800–850 °C) are performed on the same crystals to confirm the thermal stability of the heterointerfaces.

Buried heterostructure AlxGa1−xAs-GaAs quantum well lasers by Ge diffusion from the vapor

Data are presented on a method to diffuse Ge into quantum well AlxGa1−xAs-GaAs crystals from a vapor source, thus effecting impurity-induced layer disordering, and shift from lower to higher gap. The Ge diffusion is characterized on undoped GaAs by using secondary ion mass spectroscopy and capacitance-voltage electrochemical profiling. The layer disordering with Ge is used to fabricate 5-μm-wide buried heterostructure quantum well lasers (250 μm long) with continuous wave thresholds as low as 7 mA and output powers of greater than 90 mW (both facets).

Carbon-doped AlxGa1−xAs-GaAs quantum well lasers

Data are presented demonstrating that carbon (C) can be used as the active p-type dopant in high-quality AlxGa1−xAs-GaAs quantum well laser crystals. We show, by fabricating three different types of stripe geometry laser diodes (oxide stripe, hydrogenated stripe, and impurity-induced layer-disordered stripe), that C is a stable dopant and compatible in behavior with typical integrated-circuit style of device processing. The data suggest that more complicated laser geometries are possible on C-doped material because of minimal pattern ‘‘undercutting’’ after processing by, for example, hydrogenation or impurity-induced layer disordering.

Effect of surface encapsulation and As4 overpressure on Si diffusion and impurity-induced layer disordering in GaAs, AlxGa1-xAs, and AlxGa1-xAs-GaAs quantum well heterostructures

Data are presented demonstrating that the surface encapsulant and the As4 overpressure strongly affect Si diffusion in GaAs and AlxGa1-xAs, and thus are important parameters in impurity-induced layer disordering. Increasing As4 overpressure results in anincrease in diffusion depth in the case of GaAs, and adecrease in diffusion depth for AlxGa1-xAs. In addition, the band-edge exciton is observed in absorption on an AlxGa1-xAs-GaAs superlattice that is diffused with Si and is converted to bulk crystal AlyGa1-yAs via impurity-induced layer disordering. In contrast, the exciton is not observed in absorption on GaAs diffused with Si in spite of the high degree of compensation. These data indicate that the Si diffusion process, and the properties of the diffused material, are different for GaAs and for AlxGa1-xAs-GaAs superlattices converted into uniform AlyGa1-yAs (0 ≤y ≤x ≤ 1) via impurity-induced layer disordering with the amphoteric dopant Si.

Impurity-Induced Layer Disordering in AlxGa1−xAs-GaAs Quantum well Heterostructures -

ABSTRACTSelective interdiffusion of Al and Ga at AlxGa1−x As-GaAs heterointerfaces can be carried out by conventional masking procedures and diffusion of acceptor impurities (e.g., Zn), or donor impurities (e.g., Si), or also by ion implantation. This process, impurity-induced layer disordering (IILD), makes it possible to convert quantum well heterostructures (QWHs) such as AlxGa1−xAs-GaAs superlattices (SLs) into bulk homogeneous AlyGa1−yAs where y is the average Al composition of the QWH or SL. Since th IILY process is maskable and thus selective, heterojunctions can be formed in directions perpendicular to the crystal growth direction, i.e., between as-grown “ordered” and IILD “disordered” regions. To date this process has been used most effectively in the fabrication of buriedheterostructure QW lasers, single and multiple stripe, where the disordered regions provide both optical and electrical confinement. The IILD process has also been used to advantage in the fabrication of high power laser diodes with non-absorbing “windows” at the laser facets and thus with better immunity from facet damage. In this paper we present data on the application of the IILD process to the fabrication of buried-heterostructure QW laser diodes. We also describe possible mechanisms by which the impurity-induced layer disordering proceeds based on Column III “Frenkel” defects and the influence of the crystal Fermi level on the defect solubility. These mechanisms are supported by experimental data.

Anomalous threshold current and time delays in index-guided AlxGa1−xAs-GaAs quantum-well lasers

Anomalous threshold current (Ith) variation with temperature and with pulse length, and large delays (up to 6 μs) between excitation and the turn-on of stimulated emission are observed in index-guided AlxGa1−xAs-GaAs quantum-well heterostructure (QWH) lasers. These effects are found in laser diodes incorporating ‘‘spike’’ doping layers (δ-Mg and δ-Se) within the QWH active region and that are fabricated via laser-assisted Si impurity-induced layer disordering. The introduction of contaminants during the localized melting and regrowth of the laser-induced layer disordering, and the effect of these impurities (or defects) with the active region ‘‘spike’’ doping create traps. The traps cause Ith to increase (not decrease) at lower temperature and at shorter current pulses, and cause time delay in turn-on of the operation.

Coupled-stripe AlxGa1−xAs-GaAs quantum well lasers defined by vacancy-enhanced impurity-induced layer disordering from (Si2)y(GaAs)1−y barriers

Data are presented showing that high-performance (>100 mW) AlxGa1−xAs-GaAs quantum well heterostructure (QWH) coupled-stripe laser arrays can be realized by vacancy-enhanced impurity-induced layer disordering (IILD) employing an internal (Si2)y(GaAs)1−y barrier. This form of Si IILD (i.e., layer disordering from a ‘‘finite’’ internal Si source) results in coupled-emitter laser operation, even for relatively large stripe (7–10 μm) and spacing dimensions (2 μm), at currents just above threshold and higher. As a separate issue, the Si IILD QWH laser cross sections show that Si diffusion from a surface source is ‘‘slower’’ than the simultaneous vacancy diffusion from an SiO2 surface encapsulant (‘‘cap’’).

Coupled stripe AlxGa1−xAs-GaAs quantum well lasers defined by impurity-induced (Si) layer disordering

A high-performance index-guided ten-stripe AlxGa1−xAs-GaAs quantum well heterostructure laser array fabricated using Si diffusion to effect impurity-induced layer disordering between the active region stripes is described. The fine spacing (1 μm) between (3 μm) emitters allows coupled mode laser operation at thresholds (Ith) as low as 3–4 mA per stripe and with stable near- and far-field patterns in spite of band filling (single quantum well). This form of coupled stripe laser is capable of high efficiency and high power output (250 mW at 300 mA) as well as a large excitation range extending from Ith to 9Ith.

Index-guided AlxGa1−xAs-GaAs quantum well heterostructure lasers fabricated by vacancy-enhanced impurity-induced layer disordering from an internal (Si2)y(GaAs)1−y source

A unique form of Si impurity-induced layer disordering (Si IILD) is described that utilizes a ‘‘buried’’ Si source, a (Si2)y(GaAs)1−y barrier, and a patterned external source of column III vacancies, an SiO2 cap, to define the layer disordering. This form of Si IILD is used to fabricate stripe-geometry index-guided laser diodes that are capable of kink-free single-mode operation.

Impurity-induced layer-disordered buried heterostructure AlxGa1−xAs-GaAs quantum well edge-injection laser array

A laser array is described that makes use of edge injection into two sides (two ‘‘edges’’) of a stack of three AlxGa1−xAs-GaAs multiple quantum well active regions. The edge-injection array is realized by impurity-induced layer disordering, which forms a higher gap Si-doped n-type emitter that edge injects electrons into either side of a stack of three lower gap multiple quantum well p-type active regions. The far-field beam divergence in the vertical direction (θ⊥) of the array diode is reduced from 45° to 15° as determined by the laser operation, for comparison, of one of the quantum well active regions (oxide-defined stripe geometry diode).

Impurity induced layer disordering of Si implanted AlxGa1−xAs-GaAs quantum-well heterostructures: Layer disordering via diffusion from extrinsic dislocation loops

Extensive data are presented on impurity-induced layer disordering (IILD) of AlxGa1−xAs-GaAs quantum-well heterostructures and superlattices that are Si implanted and annealed (Si+-IILD) at three different implant doses. We show that impurity activation is not critical to the layer disordering process and that Si diffusion from the implanted profile initiates Si+-IILD. When the implant dose is as high as φ≥5×1013/cm2 (nSi ≥2×1018/cm3), Si interstitial loops (Si-ILs) form by diffusion and agglomeration of the implanted Si atoms during the initial stages of annealing. If a source of Ga vacancies is provided (e.g., via an As overpressure or SiO2 encapsulation), the Si-ILs dissociate and supply Si atoms for diffusion and hence Si+-IILD during the latter stages of annealing. If a Si3N4 encapsulant is employed, however, fewer Si-ILs form and Si diffusion is inhibited. For an implantation dose as low as φ=1×1012/cm2 (nSi =3×1016/cm3), extensive Si+-IILD is realized via capless annealing and Si-ILs are not observed. It is significant for device applications that the layer-disordered material operates as a cw 77 K photopumped laser, which indicates that the layer averaging (IILD) does not damage the crystal.

Impurity-induced disordering of AlxGa1−xAs-GaAs quantum well heterostructures with (Si2)x(GaAs)1−x barriers

Recent work indicates that the alloy (Si2)x(GaAs)1−x can be formed within the GaAs quantum well of an AlxGa1−xAs-GaAs quantum well heterostructure (QWH) and results in a shift of laser operation to higher energy. In this paper we show, by SIMS and EDS measurements, that the Si concentration in the (Si2)x(GaAs)1−x layer far exceeds typical “doping” levels. The stability of these QWHs has been investigated with respect to thermal annealing and Zn impurity-induced layer disordering (Zn-IILD). Data are presented showing that the (Si2)x(GaAs)1−x alloy is stable against thermal annealing unless a rich source of Ga vacancies is provided, and that relatively low temperature Zn diffusion greatly enhances the disordering process of the alloy layer.

Single-mode single-lobe operation of broad area AlxGa1−xAs-GaAs quantum well lasers

A buried heterostructure broad area quantum well heterostructure AlxGa1−xAs-GaAs laser diode modified with a small multiple stripe pattern near its center is described that operates single mode and narrow lobe (<2 °) from threshold (Ith) to 2Ith or 3Ith. The broad area (wide stripe) buried heterostructure laser is made by impurity-induced layer disordering.

Stacking and layer disordering of AlxGa1−xAs-GaAs quantum well heterostructures

Data are presented showing that an AlxGa1−x As-GaAs quantum well heterostructure (QWH) or superlattice (SL) can be selectively disordered into higher gap bulk crystal by impurity (Si) diffusion and then have grown epitaxially on it (‘‘stacked’’ on it) another SL (or QWH), which can be subjected to further impurity-induced layer disordering in a patterned form. The resulting three-dimensional array can be operated as a photopumped laser.