Impurity-induced Layer Disordering Research Articles

Layer intermixing and related long-term instability in heavily carbon-doped AlGaAs/GaAs superlattices

Superlattices (SLs) of Alo.3Gao.7As/GaAs grown by metalorganic chemical vapor deposition and heavily doped with carbon using CClp4 were annealed for 24 h at 825° C under a variety of ambient and surface encapsulation conditions. Photoluminescence atT = 1.7 K has been employed to determine approximate Al-Ga interdiffusion coefficients (D Ai-Ga) for different annealing conditions. For all encapsulants studiedD Al-Ga increases with increasing As4 pressure in the annealing ampoule. This result disagrees with trends reported for Mg-doped crystals, and with predictions of the charged point-defect (Fermilevel) model. The Si3N4 cap provides the most effecitve surface sealing against ambientstimulated layer interdiffusion (D Al-Ga ≈ 1.5-3.9 x 10-19 cm2/sec). The most extensive layer intermixing has occurred for an uncapped SL annealed under As-rich ambient (D Al-Ga ≈ 3.3 x 10-18 cm2/sec). These values are up to ~40 times greater than those previously reported for nominally undoped AIGaAs/GaAs SLs, implying that theC As doping slightly enhances layer intermixing, but significantly less than predicted by the Fermi-level effect. The discrepancies between the experimental data and the model are discussed. Pronounced changes in the optical properties of the annealed SLs with storage time at room temperature are also reported. These changes may indicate a degraded thermal stability of the annealed crystals due to high-temperature-induced lattice defects. A possibly related effect of the systematic failure to fabricate buried heterostructure quantum well lasers via impurity-induced layer disordering in similarly doped AIGaAs/GaAs crystals is discussed.

Low-threshold disorder-defined native-oxide delineated buried-heterostructure AlxGa1−xAs-GaAs quantum well lasers

Impurity-induced layer disordering (IILD) along with oxidation (native oxide) of high-gap AlxGa1−xAs confining layers is employed to fabricate low-threshold stripe-geometry buried-heterostructure AlxGa1−xAs-GaAs quantum well heterostructure (QWH) lasers. Silicon IILD is used to intermix the quantum well and waveguide regions with the surrounding confining layers (beyond the laser stripe) to provide optical and current confinement in the QW region of the stripe. The high-gap AlxGa1−xAs upper confining layer is oxidized in a self-aligned configuration defined by the contact stripe and reduces IILD leakage currents at the crystal surface and diffused shunt junctions. AlxGa1−xAs-GaAs QWH lasers fabricated by this method have continuous 300 K threshold currents as low as 5 mA and powers ≳ 3l mW/facet for ∼ 3-μm-wide active regions.

Native-oxide masked impurity-induced layer disordering of AlxGa1−xAs quantum well heterostructures

Data are presented showing that the native oxide that can be formed on high Al composition AlxGa1−xAs (x≳0.7) confining layers on AlyGa1−yAs-AlzGa1−zAs (y≳z) superlattices or quantum well heterostructures serves as an effective mask against impurity diffusion (Zn or Si), and thus against impurity-induced layer disordering. The high quality native oxide is produced by the conversion of high Al composition AlxGa1−xAs (x≳0.7) confining layers, which can be grown on a variety of heterostructures, via H2O vapor oxidation (≳400 °C) in an N2 carrier gas.

InGaAs(P)/InP MQW mixing by Zn diffusion, Ge and S implantation for optoelectronic applications

There is a growing interest in impurity-induced layer disordering for the technologically important InGaAs(P)/InP system. More complicated than in the AlGaAs/GaAs ternary system, which concerns only interdiffusion of group III atoms, interdiffusion in this quaternary system can occur for both group III (Ga,In) and group V (P,As) atoms, which may or may not result in a strain-free alloy lattice-matched to InP, a major concern for device applications. After a brief review on the thermal stability of InP/ InGaAs quantum well structures, we show that Zn diffusion at moderate temperature leads to intermixing on the group III sublattice, only, with subsequent lattice mismatch. On the other hand, either Ge or S implantation of InGaAs/InP quantum wells results in intermixing involving both the group III and the group V sublattice and approximating the lattice-matched condition.

Instability of partially disordered carbon-doped AlGaAs/GaAs superlattices

Superlattices of Al0.3Ga0.7As/GaAs grown by metalorganic chemical vapor deposition and heavily doped with carbon using CCl4 were annealed for 24 h at 825 °C under a variety of ambient and surface encapsulation conditions. Pronounced changes in photoluminescence from the annealed superlattices with storage time at room temperature, as opposed to an excellent reproducibility of that from the as-grown, not annealed samples, are reported. These changes may be indicative of degraded thermal stability of the annealed superlattice crystals due to high-temperature-induced lattice defects. The systematic failure to fabricate buried-heterostructure quantum well lasers via impurity-induced layer disordering in similarly doped AlGaAs/GaAs crystals, which may be related to the same effect, is also discussed.

Low-threshold disorder-defined buried-heterostructure AlxGa1−xAs-GaAs quantum well lasers by open-tube rapid thermal annealing

AlxGa1−xAs-GaAs single stripe quantum well heterostructure (QWH) lasers fabricated via Si impurity-induced layer disordering (IILD) in an As-free open tube rapid thermal annealing furnace are reported. The Si IILD, with good surface morphology, is obtained using a Si/Si3N4 source layer with the QWH wafer in face-to-face contact with a GaAs substrate during the anneal (13 min, 1000 °C). Continuous wave (cw) 300 K operation of the lasers with uncoated facets has produced output powers as high as 25 mW/facet with threshold currents as low as 7 mA. The devices operate single mode at a wavelength of 812 nm and have high differential quantum efficiencies of ∼44%, with some as high as 57%.

Si incorporation in laser-melted AlxGa1−xAs-GaAs quantum well heterostructures from a dielectric source

Data are presented describing the incorporation of Si in locally laser-melted AlxGa1−xAs-GaAs quantum well heterostructures from a thin-film dielectric source. The composition of the melted material, the effects of the Si source (SiO2 or Si3N4) on impurity incorporation, and the doping behavior are examined via secondary-ion mass spectroscopy, electron dispersion x-ray spectroscopy, transmission electron microscopy, and scanning electron microscopy. The data indicate that upon melting, a (Si)y(AlxGa1−xAs)1−y alloy is formed from which impurity-induced layer disordering may be effected. After annealing the melt region is found to contain crystalline segregates, which are attributed to rapid thermal quenching of the melt. Applications of these results to the fabrication of buried-heterostructure lasers by laser-assisted disordering are discussed.

Si impurity-induced layer disordering of Alx Ga1−x -GaAs quantum-well heterostructures by As-free open-tube rapid thermal annealing

Data are presented describing Si impurity-induced layer disordering (IILD) and Al-Ga interdiffusion in AlxGa1−x As-GaAs quantum-well heterostructures (QWHs) using open-tube rapid thermal annealing (900–1000 °C) in a flowing N2/H2 ambient. The data show that Al-Ga interdiffusion is enhanced by n-type crystal doping and suppressed by p-type doping. By surrounding the active layers of the heterostructure with layers of opposite doping, we show that the surrounding layers modify the interdiffusion by controlling the diffusion of point defects to the active layers of the heterostructure. Data are presented showing the effects of dielectric encapsulation (SiO2, Si3N4 ) on Al-Ga interdiffusion. The data show that regardless of doping SiO2 enhances interdiffusion as compared to Si3N4. To achieve more thorough layer intermixing of AlxGa1−x As-GaAs QWHs, Si IILD is also investigated in the high-temperature As-poor regime. The experimental data show that in a high-temperature As-poor annealing ambient, little or no Si diffusion occurs from an elemental Si source in contact with a p-type GaAs QWH cap layer. To achieve Si diffusion under these conditions requires removal of the GaAs cap and the use of Al-reduced SiO2 or Si3N4 as a Si diffusion source. Based on secondary-ion mass spectroscopic measurements, direct comparison of Si diffusion from closed-tube (825 °C, 48 h) and open tube (1000 °C, 10 min) anneals shows increased Si incorporation and layer disordering at higher temperatures.

Low-threshold high-efficiency high-yield impurity-induced layer disordering laser by self-aligned Si-Zn diffusion

Stripe geometry lasers defined by impurity-induced layer disordering (IILD) have been fabricated utilizing a novel technology of self-aligned Si-Zn diffusion from which both optical and electrical confinements are obtained simultaneously. The fabrication process is considerably simpler than that for the conventional IILD lasers and the parasitic p-n junction area in the laser structures is minimized. Typical lasers with threshold current Ith=5.2 mA and differential quantum efficiency ηd=81% at room-temperature continuous operation as well as highly uniform yield ≳80% have been obtained.

S-bend loss in disorder-delineated GaAs heterostructure laser waveguides with native and blue shifted active regions

The routing ability of waveguide S-bend structures patterned by SiO2 impurity induced layer disordering of a single GaAs quantum well graded barrier laser structure is investigated. For a raised-cosine bend with 100 μm offset guides, the measured transition length for 3 dB loss was less than 300 μm for near single mode guides of 1 μm width. In addition, vacancy-induced disordering of the native quantum well region is investigated and is shown to increase the band gap to a point where the material is low loss for radiation generated by the laser. The 3 dB length for these blue shifted cores actually decreased to about 230 μm, a fact attributed to reduced mode conversion.

Characterization of III-V Semiconductor Materials by Surface Sensitive Analytical Techniques A Review

Applications of surface-sensitive analytical techniques to research and development of III-V semiconductor materials are illustrated. The studies reviewed are related to the understanding of various aspects of material processing such as epitaxial growth by molcular beam epitaxy, diffusion of group II elements in InP, ion implantation of n- and p-type dopant species, and dry etching in chlorine-containing discharges. In addition, impurity-induced layer disordering of quantum well heterostructures and superlattices will be discussed. Examples are drawn from a number of analytical techniques sensitive to the surface and to the interface. Special emphasis is given to GaAs and InP which are currently under study most extensively.

Column III-column V sublattice interaction via Zn and Si impurity-induced layer disordering of 13C-doped AlxGa1−xAs-GaAs superlattices

Experiments are described employing secondary-ion mass spectroscopy (SIMS) to study the stability of 13C-doped Al0.5Ga0.5As-GaAs superlattices against Zn and Si impurity-induced layer disordering (IILD). The modulation depth of the SIMS 27Al and 13C signals is used as a sensitive probe of column III and column V sublattice interdiffusion. The data show that CAs is much more stable against Zn and Si IILD than the column III superlattice host crystal itself. The minor enhancement of CAs diffusion via the column III disordering agents, which is present to a significant extent for Si IILD but almost nonexistent for Zn IILD, suggests that there is no direct interchange of column III and column V sublattice atoms. The Zn and Si enhancement of carbon diffusion is probably caused by local Coulombic interaction between the diffusing Zn+i and Si+III species and the C−As acceptor.

High performance AlyGa1−yAs-GaAs-lrxGa1−xAs quantum well lasers defined by silicon-oxygen impurity-induced layer disordering

In these experiments impurity-induced layer disordering (IILD) utilizing chemical reduction of SiO2 by Al (from Al0.8Ga0.2As) is employed to generate Si and O to effect layer disordering. The SiO2-Al0.8Ga0.2As reaction is studied with respect to annealing ambient. By controlling the extent of disordering via As4 overpressure, closely spaced (∼1μm) Si-O IILD buried heterostructure lasers can be optically coupled or uncoupled. Direct observation of O incorporation into the buried layers is shown using secondary ion mass spectroscopy (SIMS). The thermal stability of separate-confinement AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure (QWH) laser crystals is investigated using SIMS, transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The data show that the thermal stability of a strained-layer In0.1Ga0.9As quantum well (QW) is strongly dependent upon: (1) the layer thickness and heterointerfaces of the AlyGa1−yAs-GaAs waveguide layers located directly above and below the QW, (2) the type of surface encapsulant employed, and (3) the annealing ambient. Narrow single-stripe (<2μm) lasers fabricated via Si-O diffusion and layer disordering exhibit low threshold currents (Ith ∼ 4 mA) and differential quantum efficiencies,η, of 22% per facet under continuous (cw) room-temperature operation.

Defect-accelerated donor diffusion and layer intermixing of GaAs/AlAs superlattices on laser-patterned substrates

Data are presented showing that donor diffusion and Alx Ga1−x As-GaAs layer intermixing are greatly enhanced in the presence of defects created by crystal overgrowth on locally laser-melted substrates. Accelerated defect and impurity-induced layer disordering, and donor diffusion from a solid source (SiO2 ), a vapor source (Ge), and from a grown-in source (Se) are observed in regions of high defect density. The enhanced donor diffusion and crystal self-diffusion are attributed to an increased density of column-III defects and dislocations in the crystal.

Dislocation-accelerated impurity-induced layer disordering of AlxGa1−xAs-GaAs quantum well heterostructures grown on GaAs-on-Si

Data are presented showing that dislocations and Si autodiffusion promote accelerated layer disordering of AlxGa1−xAs-GaAs quantum well heterostructures grown on GaAs-on-Si ‘‘substrates’’ via metalorganic chemical vapor deposition. The accelerated impurity-induced layer disordering is more extreme at higher temperatures (&amp;gt;800 °C) and virtually nonexistent at lower temperatures (≲775 °C).

High-power disorder-defined coupled stripe AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure lasers

Data are presented describing continuous (cw) room-temperature laser operation of AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure (QWH) phase-locked arrays. The ten-stripe arrays have 3 μm emitters, with emitter to emitter spacing of 4 μm, and are patterned onto the QWH crystal using a self-aligned Si-O impurity-induced layer disordering (IILD) procedure. The IILD process is devised to provide limited layer intermixing to ensure optical coupling (across ∼1 μm). The coupled stripe QWH lasers exhibit narrow twin-lobed far-field patterns that show unambiguously phase locking in the highest order supermode. The cw output power of the lasers (differential quantum efficiency 52%) is shown from threshold (∼75 mA) to over 280 mW (both facets, no optical coatings).

Impurity-induced layer disordering in In0.5(Alx Ga1−x)0.5P-InGaP quantum-well heterostructures: Visible-spectrum-buried heterostructure lasers

Diffusion of Si into quantum-well heterostructures and superlattices employing the high gap III-V quaternary system Iny (AlxGa1−x )1−yP is shown to result in impurity-induced layer disordering. Secondary ion mass spectroscopy, transmission electron microscopy, and photoluminescence data indicate that the diffusion of Si into an InAlP-InGaP superlattice grown lattice matched on GaAs (y≊0.5) results in the intermixing of the layers, thus forming an alloy of average composition. Buried-heterostructure lasers are fabricated using Si layer disordering of In0.5 (Alx Ga1−x )0.5 P p-n quantum-well heterostructures. The disorder-defined stripe-geometry diode lasers operate pulsed at 300 K near 6400 Å. Continuous wave operation at λ∼6255 Å is achieved at −47 °C.

Disorder-defined buried-heterostructure AlxGa1−xAs-GaAs quantum well lasers by diffusion of silicon and oxygen from Al-reduced SiO2

We describe a convenient method utilizing chemical reduction of SiO2 by Al (from AlxGa1−xAs) to generate Si and O for impurity-induced layer disordering (IILD) of AlxGa1−xAs-GaAs quantum well heterostructures (QWHs). Experimental data show that Si-O diffusion (from SiO2) is an effective source of Si for Si-IILD and of O that compensates the Si donor, thus resulting in higher resistivity layer-disordered crystal. The usefulness of the Si-O IILD source for fabricating low-threshold disorder-defined buried-heterostructure AlxGa1−xAs-GaAs QWH lasers is demonstrated.

A proposed model to explain impurity-induced layer disordering in AlAs-GaAs heterostructures

Impurity-induced disordering of III-V compound semiconductor quantum well heterostructures and superlattices has been extensively studied. However, the theories proposed to explain this phenomenon have dealt primarily with vacancy diffusion mechanisms and have not succeeded in explaining all the experimental observations satisfactorily. In this communication, we propose a two-atom ring mechanism of diffusion based on a simplified model of covalent bonding in III-V compound semiconductors and suggest that the phenomenon of intermixing is a consequence of a lowering in the activation energy due to coulombic interactions between the substitutional impurity atoms and the host atoms. This mechanism is consistent with experimental observations and is able to predict the behavior of other dopants. The additive effect of the simultaneous presence of different dopants can also be explained by this theory which could possibly be used to achieve controlled layer disordering for use in device technology.

Low-threshold disorder-defined buried heterostructure strained-layer AlyGa1−yAs-GaAs-InxGa1−xAs quantum well lasers (λ∼910 nm)

The stability of strained-layer Aly Ga1−yAs-GaAs-InxGa1−x As single quantum well heterostructures against thermal processing is examined using transmission and scanning electron microscopy. A self-aligned impurity-induced layer disordering process employing Si-O diffusion is used to produce buried heterostructure stripe geometry lasers with a pseudomorphic InxGa1−x As quantum well active region. The 2-μm-wide stripe laser diodes exhibit high efficiency (η∼41%/facet), low threshold (Ith =7 mA), and high output power (Pout &amp;gt;20 mW/facet).