Impurity-induced Layer Disordering Research Articles

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest because of applications that span facial recognition and 3D imaging in mobile telephones, Light Detection and Ranging (LiDAR) for autonomous vehicles, and Pulse Amplitude Modulation (PAM-4) for high-speed (>100 Gbps) data transmission within data centers. A key challenge with oxide-confined VCSELs is that the strong refractive index contrast between the oxidized ~AlAs aperture and surrounding AlGaAs layers combined with the oxide aperture size create a photonic cavity that fundamentally supports multiple transverse modes. When the size of the aperture is reduced to create a single-mode cavity, the output power is limited by the electrically pumped gain region within the aperture. There is thus a fundamental trade-off between size and power which manifests itself in a limited capability to simultaneously achieve high power and single mode operation. Various methods have been employed by groups working on VCSELs to overcome this limitation. Neglecting external cavity methods which are fundamentally not scalable, the most promising results have been achieved by groups using etched surface relief where a pattern is etched in the top layer or layers to change the standing wave pattern in the cavity and reduce modal gain by lowering the magnitude of the electric field in regions overlapping with the quantum wells, which provide laser gain. A downside of this approach is that material is removed, impacting the uniformity of current injection across the area of the device. A key question of interest is therefore how to achieve mode suppression in large area devices in a way that does not impact current injection. In this presentation, novel methods for mode control that rely on spatial variation of modal gain across the area of the VCSEL aperture will be discussed, and results on high-power single-mode operations of VCSELs will be presented. Two distinct mode control methods will be reviewed. The first method of mode control uses deposited optical coatings on the top surface of the VCSEL to spatially modify the electric field standing wave in the quantum well heterostructure region. Through patterning the coating in either a pillar or anulus, specific modes can be suppressed by selectively changing the gain available for those modes. These filters have been made using both SiO2/TiO2 stacks and alternatively a single layer of silicon. The capability to pattern these layers in a wafer-scale process makes this method attractive for volume manufacturing. The second method of mode control utilizes impurity-induced layer disordering via patterned diffusion of Zn to selectively intermix Distributed Bragg Reflector (DBR) layers, thereby creating a top laser mirror whose reflectivity varies spatially across the aperture. Designing the diffused region to maximize overlap with high-order modes while minimizing overlap with the fundamental mode provides mode suppression and enhanced single-mode power. An added benefit of this approach is a reduction in resistance through the device top DBR layers brought about by the higher p-doping level in the Zn-diffused regions, minimizing Ohmic losses and heat generation. Through the use of strain control in the diffusion mask, diffusion front shape is also shown to be controllable in a way that can be used to minimize the impact of the diffused region on fundamental mode reflectivity. Tensile and compressive strain of diffusion masks are shown to have differing effects on vertical (into the device) versus lateral (across the area of the device under the mask) diffusion, allowing more precise spatial control of reflectivity. Using these methods, single-mode output powers in excess of 10 mW have been demonstrated, with side-mode suppression ratios in excess of 30 dB. These results will be discussed, as well as further work to enhance VCSEL performance. Figure 1

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest due to performance needs driven by facial recognition and 3D imaging in mobile telephones. Two distinct mode control methods that rely on exploiting the spatial field distribution of optical transverse modes to achieve high-power single-mode operation in VCSELs will be discussed. The first method uses a surface-deposited optical coating of multilayer SiO2/TiO2 or single-layer silicon to achieve single-mode emission. The capability to pattern these layers in a wafer-scale process makes this method attractive for high-volume manufacturing. The second method utilizes impurity-induced layer disordering (IID) to selectively intermix the top distributed Bragg reflector (DBR) in a VCSEL, thereby creating a mirror whose reflectivity spatially varies across the aperture. Using these techniques, single-mode output power in excess of 10 mW has been demonstrated with side-mode suppression ratios in excess of 35 db.

Silicon impurity-induced layer disordering of AlGaN/AlN superlattices

Impurity-induced layer disordering is demonstrated in Al0.1Ga0.9N/AlN superlattices grown by metal-organic vapor phase epitaxy. During growth at temperatures as low as 885 °C and under post growth annealing at 1000 °C in N2 the heterointerfaces of Si-doped (Si concentration >8×1019 cm−3) superlattices exhibit layer disordering (intermixing) while the unintentionally doped superlattices remain stable. Shifts in the intersubband energy transitions and scanning transmission electron microscope images showing changes in the layer abruptness are used to verify layer disordering due to Si diffusion in Al0.1Ga0.9N/AlN superlattices.

Fabrication of GaAs-based photonic band gap materials

A relatively simple technique for fabrication of GaAs-based quasi-three-dimensional photonic crystals has been investigated. Selective impurity-induced layer disordering and wet oxidation techniques are utilized. Fourier-transform infrared spectroscopy measurement reveals a stop band between 15 and 20 μm for a sample with scattering center spacing of 6.3 μm. Another narrow transmittance dip is observable in the wavelength range of 1.1–1.58 μm, with an attenuation of 12 dB at 1.18 μm. The process is reproducible and lends itself to integration with other optoelectronic and electronic devices on the same substrate.

Improvement of catastrophic optical damage (COD) level for high-power 0.98-μm GaInAs-GaInP laser

We investigate improvement of catastrophic optical damage (COD) level for Al-free 0.98-/spl mu/m ridge waveguide laser diodes (LDs) using the impurity induced layer disordering (IILD) process applied near the facets. The IILD is used for the purpose of forming transparent windows near both facets of the LDs utilizing its ability to increase bandgap energy of the GaInAs-GaInAsP strained quantum-well (QW) active layer. Improvement of the cod level by at least 1.65 times compared to the conventional LDs is obtained for the LDs with Si/sup +/ implantation followed by annealing at 900/spl deg/C for 10 min.

Impurity-induced layer disordering of quantum-well heterostructures: discovery and prospects

The circumstances leading to the discovery in 1980 of impurity-induced layer disordering (IILD) of AlAs-GaAs (Al/sub x/Ga/sub 1-x/As) quantum-well heterostructures (QWHs) and superlattices (SLs) are described. In view of the great stability of a QWH or SL (AlAs-GaAs) against ordinary thermal annealing, IILD came as a surprise, i.e., the lower temperature (selective) change from red-gap QW crystal to yellow-gap bulk crystal. Layer disordering can be carried out most effectively, via diffusion or implantation, with two-site dopants such as Zn (acceptor) or Si (donor), but is not restricted to active impurities alone. This maskable planar technology, which (with crystal conservation) transforms a coarser layered III-V alloy to a smoother stochastic alloy, and higher bandgap, is capable of forming, as desired, regions that confine carriers and photons. Accordingly, IILD has broad and growing use in optoelectronics (lasers, waveguides, etc.), particularly for III-V systems employing Al and Ga which easily substitute for one another and are sensitive to IILD. The atomic rearrangement of diffusion, a small scale (microscopic) lattice change, is in essence amplified by IILD into a large scale (macroscopic) layer change (patterned) that provides a method to study III-V diffusion mechanisms. IILD, a planar technology and growing area of work, is useful in optoelectronics applications as well as for basic diffusion studies in III-Vs and potentially other crystal systems.

MQW electroabsorption modulator integrated with a tapered waveguide vertical interconnect

The integration of a GaAs/AlGaAs multi-quantum well electroabsorption modulator and a tapered waveguide vertical direction optical interconnect has been performed without the complicated regrowth process. Zn impurity-induced layer disordering of MQW layer is used to achieve the energy transfer between SQW and MQW regions. Light coupled into a SQW region was transferred to an MQW region and an intensity modulation of 10 dB extinction ratio was demonstrated.

Localised impurity induced layer disordering for lithographic control of the lateral oxidation of AlAs

The lateral oxidation of AlAs is becoming an important processing tool, but has proven difficult to control. A method is presented to achieve lithographic control of the lateral oxidation process. The technique uses impurity induced layer disordering in buried, heavily Si-doped layers to locally change the oxidation rate of an AlAs layer. Several types of capping conditions during the anneal are discussed.

The semiconductor laser: a thirty-five-year perspective

Starting in 1962 with GaAs and the alloy GaAsP, the prototype III-V alloy the semiconductor laser developed slowly from a pulse-operated simple p-n junction to a continuously operated (300-K) double heterojunction in 1970. Then, in 1977, in another metamorphosis, it became a quantum-well (QW) device of reduced dimensionality (two dimensional) and improved density of states and performance. The fact that the semiconductor laser is based on an ideal form of "lamp", a p-n junction of potentially 100% quantum efficiency in conversion of electron-hole pairs to photons, gives it a built-in advantage over all other forms of lasers-all the indirectly excited lasers. Also, being a condensed system, it can be small-smaller than all other forms of lasers-and yet cover a great wavelength and power range, which continue to expand. Because of its third-layer form, a QW heterostructure (the product of more or less easily controlled epitaxial crystal growth) is amenable to modification by relatively simple processing operations such as impurity-induced layer disordering and now "wet" oxidation of Al-based layers, which makes possible a new generation of high-performance vertical-cavity surface emitting lasers (VCSELs). The VCSEL, with its oxide-defined high-Q flat cavity, reduced mode density, and approach now to instant "turn-on" (/spl mu/A currents), indicates that semiconductor lasers may not become much better than VCSELs, even if there is a change (not certain) in form from two to one to zero dimensional (and, if so, to problems in uniformity and reproducibility). If the technologies of the high-brightness transparent-substrate light emitting diode (LED) and the VCSEL converge much more, it is likely that the VCSEL-LED will become an addressable array (a display), a projection television, or a low- or high-power lamp, perhaps eventually on a large "diameter" Si platform. The study of the semiconductor laser, the first practical QW device, is not complete, nor is its development, which is certain to continue.

Planar disorder- and native-oxide-defined photopumped AlAs–GaAs superlattice minidisk lasers

Data are presented on the photopumped laser operation of planar AlAs–GaAs superlattice (SL) minidisk lasers. The SL minidisk (70 Å AlAs, 30 Å GaAs; 100 periods; ∼37 μm diameter) is defined by impurity-induced layer disordering (IILD), followed by wet oxidation (N2+H2O vapor, 400 °C which surrounds the minidisk with a low-refractive-index AlGaAs oxide. The planar minidisks exhibit laser operation at λ∼7540 Å, with wider mode separation (Δλ∼13 Å) than disks defined by only IILD (a smaller refractive index step) and cleaved sample edges. The mode separation of Δλ∼13 Å corresponds to disk modes that utilize the perimeter of the oxide-defined disks. In the fabrication of the SL minidisks, IILD forms a structural and doping difference beyond the disk perimeter that acts, in effect, as a p–n junction during etching or wet oxidation. Etch profiles are shown demonstrating this behavior.

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.

Deep-oxide curved resonator for low-threshold AlGaAs–GaAs quantum well heterostructure ring lasers

Data are presented on curved-resonator AlGaAs–GaAs quantum well heterostructure laser diodes employing a deep-oxide guiding structure formed by a combination of impurity-induced layer disordering and native oxidation. Room-temperature, continuous wave (cw) threshold currents as low as ∼13 mA are measured for a ∼5 μm wide, 100 μm radius half-ring laser employing a deep-oxide structure. For similar 150 μm radius devices, cw threshold currents as low as ∼19 mA are observed (∼7 μm width), with total output powers exceeding 30 mW. Spectral data indicate single-mode operation over a large current range (25–100 mA). The low-threshold characteristics are attributed to the excellent current and optical confinement of the deep-oxide structure, combined with low edge scattering loss.

Deep-oxide planar buried-heterostructure AlGaAs–GaAs quantum well heterostructure laser diodes

Data are presented on deep-oxide planar buried-heterostructure AlGaAs–GaAs quantum well heterostructure laser diodes fabricated using a self-aligned process that combines native oxide and impurity-induced layer disordering (IILD) technologies. Silicon IILD intermixes the waveguide layers on either side of an active area stripe and allows ‘‘wet’’ oxidation to penetrate and create a low-index (n∼1.7) deep-oxide structure for electrical and optical confinement. Continuous-wave (cw) threshold currents of ∼3.4 mA are measured for ∼3.5-μm-wide active regions (L∼250 μm), with maximum cw output powers greater than 29 mW/facet and external differential quantum efficiencies as high as 70% (300 K, uncoated facets).

Low threshold current dual wavelength planar buried heterostructure lasers with close spatial and large spectral separation

A novel technique for achieving closely spaced laser devices with large, but well-controlled, wavelength separation is described. Preferential population of the lowest band-gap active region in a stacked active layer structure is used along with patterned etching of the long wavelength active regions and regrowth over all devices to achieve the desired wavelength. Dual-wavelength dual-stripe buried heterostructure lasers with 5 μm stripes on 20 μm centers formed by impurity-induced layer disordering in a stacked active layer structure have threshold currents of 9.1 and 10.9 mA for laser wavelengths of 846 and 760 nm, respectively.

An MQW-SQW tapered waveguide transition

We demonstrate an adiabatic, tapered transition in a multiquantum-well (MQW) and single-quantum-well (SQW) coupled structure. The refractive index of the MQW is tapered by the disordering of the AlGaAs/GaAs layers using z-variant, Zn diffused, impurity-induced layer disordering (IILD). We analyze the tapered structure using the step transition model and multilayer stack theory. Results are in excellent agreement with the measurements, establishing the adiabatic nature of the transition. >

Epitaxial Growth, Fabrication, and Performance of Ingaas Strained Quantum Well Laser Structures

ABSTRACTThere is considerable interest in InGaAs/GaAs strained quantum well lasers for applications within the 0.9 to 1.1 μm wavelength range, such as high power lasers for Efdoped fiber amplifiers and rare-earth-ion solid state lasers, obtaining blue-green laser emission by frequency doubling, and optoelectronic integrated circuits. Epitaxial growth of these structures by organometallic vapor phase epitaxy, molecular beam epitaxy, and gas source molecular beam epitaxy will be discussed. The relative merits of AlGaAs and InGaP cladding layers will be examined with respect to growth challenges, laser processing and performance, and device reliability. Several device structures which provide transverse and lateral confinement will be reviewed. Reduction of the transverse far-field angle, which improves fiber coupling efficiency, can be accomplished through the use of periodic index separate confinement and depressed index cladding heterostructures. The performance of ridge waveguide lasers, which require accurate control of the ridge height, can be improved through the incorporation of etch-stop layers, either InGaP or AlAs, in the AlGaAs cladding layer. Nonplanar growth over mesas etched into the substrate is a convenient method to obtain buried heterostructure lasers. Carbon-doped planar InGaAs/AlGaAs lasers, using CC14 as an extrinsic dopant, have been fabricated with an impurity-induced layer disordering process. Microcylinder lasers have been fabricated out of InGaAs/InGaP layer structures.

Dopant-induced disordering of annealed AlGaAs heterostructures

Impurity-induced layer disordering, i.e., outdiffusion of dopant and matrix elements, has been observed in superlattices and quantum wells. In this letter, we report on disordering in AlGaAs heterostructures in which only the central region of 650-Å-thick AlGaAs layers is Si doped. Disordering is clearly shown to be dependent on Si concentration and outdiffusion.

Defect Generation and Suppression During the Impurity Induced Layer Disordering of Quantum Sized GaAs/GalnP Layers

Impurity induced layer disordering (JILD) has been demonstrated as an extremely powerful technique for the fabrication of optoelectronic devices within the AlGaAs alloy system. The success of this technique in this material system is due in large part to the similarity of the lattice parameters between the two binary constituents in this system, AlAs and GaAs. There are many alloy systems in which it would be highly desirable to exploit the fabrication technique of JILD, but within which the binary alloy constituents are not so well matched in lattice parameter. Perhaps the most extensively explored of such systems are the AlGaInP alloy system lattice matched to a GaAs substrate and the InGaAsP alloy system lattice matched to an InP substrate.

Effect of as Overpressure on Mn-Induced Layer Disordering in AlGaAs-GaAs Superlattices: An Investigation of the Mn Diffusion Mechanism

ABSTRACTData are presented showing the effect of As overpressure on the diffusion of Mn into an AlGaAs-GaAs superlattice (SL) using MnAs as the diffusion source. Arsenic overpressure has been shown to play a significant role in both impurity-induced layer disordering (IILD) and the microstructure of the Mn-diffused samples. The degree to which layer disordering occurs decreases as As overpressure increases. Furthermore, dislocation loops are observed for the diffusion of Mn under Ga-rich conditions at prolonged diffusion times. Both results, in addition to the effect of As overpressure on the Mn diffusion profile, indicate that Mn diffusion into GaAs or GaAs-AlGaAs heterostructures takes place by an interstitial-substitutional mechanism involving column III vacancies under As-rich conditions and a “kick-out” mechanism involving column III interstitials under Ga-rich conditions.

Low-loss, single-mode In0.53Ga0.47As/In0.52Al0.48As/InP optical waveguides fabricated by Zn-induced impurity-induced layer disordering

We have investigated the properties of Zn diffusion in the In0.53Ga0.47As/In0.52Al0.48As superlattice and have made optical guides delineated by Zn-induced layer disordering in the multilayers. Measurements on the single-mode waveguides show loses as low as 2.3 dB/cm including coupling loss.