Author: jdallesa@illinois.edu

Negative differential resistance, self-oscillations, and light emission in transistor-injected quantum cascade structures

Posted on by jdallesa@illinois.edu

We experimentally demonstrate negative differential resistance, collector current oscillations, and light emission in a transistor-injected quantum cascade structure. Distinct transport regimes are accessed by varying the reverse bias across the base-collector junction while independently controlling emitter injection. The collector current shows pronounced oscillations and alternating high–low levels as the reverse bias changes, indicating quantum transport through the superlattice. Oscillations at a peak frequency of 3.2 MHz occur within high-bias negative differential resistance regions. Spontaneous short-wave infrared (SWIR) emission from the base region is characterized using Fourier transform infrared spectroscopy, revealing a peak wavelength of 1.58 m. Coupling between the emitted light and superlattice modes is confirmed by SWIR intensity oscillations synchronized with collector current oscillations. Under forward bias of the base-collector junction, the device emits coherent SWIR light at 1.58 m, aided by carrier confinement from a quantum impedance matching region. Additionally, spontaneous mid-wave infrared emission is observed from the quantum cascade structure. This three-terminal, transistor-injected platform integrates electronic modulation with tunable optical output, showing promise for a versatile alternative to conventional two-terminal quantum cascade devices that could enable new opportunities in long-wavelength photonics for sensing, communication, and spectroscopy applications.

https://www.doi.org/10.1007/s00340-026-08676-3

Title of research paper published in Applied Physics

Utilization of silicon optical coatings for transverse-mode suppression in high-power oxide-confined vertical-cavity surface-emitting lasers

Posted on by jdallesa@illinois.edu

Engineering of the electric-field standing-wave pattern of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) via a scalable, high-refractive-index silicon optical coating is demonstrated to achieve high-power, single-transverse-mode emission. The electron-beam deposition of a thin silicon film atop a standard VCSEL structure reduces the amplitude of the electric-field standing wave over the quantum well gain region, raising the threshold modal gain. Patterning the coating into the shape of an annulus increases the threshold modal gain in the periphery of the VCSEL aperture overlapping with the higher-order transverse modes while leaving the central region unaffected. This patterning creates a radially dependent threshold modal gain profile, suppressing higher-order modes and encouraging operation in a single-fundamental mode. High-power continuous-wave single-mode emission with an output power of 7.43 mW in 850 nm AlGaAs-based VCSELs with silicon coatings is demonstrated, as well as submillamp threshold currents, thermal rollover delay, and a side-mode suppression ratio exceeding 30 dB for single-fundamental-mode operation at room temperature.

https://www.doi.org/10.1063/5.0260091

Ultra-sensitive current bistability and light switching in a resonant tunneling superlattice transistor

Posted on by jdallesa@illinois.edu

Bistability in the current–voltage characteristics of semiconductor superlattices and quantum cascade laser structures has the potential for wide-ranging applications, particularly in sensing systems. However, the interdependency of applied bias and current injection in conventional two-terminal structures has led to complications in analysis and rendered the bistability phenomenon difficult to implement in practical applications. Here, we report a new kind of electronic bistability coupled to optical switching in a resonant tunneling bipolar superlattice transistor. This bistability manifests as sharp discontinuities in the collector current with extremely small variations of the applied voltage, which arise from unstable tunneling transmission across the hetero-barrier between the two-dimensional electron gas (2DEG) at the edge of the transistor base and the collector superlattice structure. The electronic transitions between high and low quantum mechanical transmissions are demonstrated to be caused by self-consistent variations of the internal electric field at the heterointerface between the 2DEG and the superlattice. They are also present in the base current of the three-terminal device and result in sharp switching of near-infrared spontaneous light emission output from an interband radiative recombination process with a peak emission wavelength of 1.58  m. A comprehensive quantum mechanical theoretical model accounting for the self-consistent bistable tunneling transmission is in quantitative agreement with the experimental data. The measured peak transconductance sensitivity value of 6000 mS can be used in the highly sensitive detector and non-linear device applications.

https://doi.org/10.1063/5.0190385

Achieving high-power single-mode operation in vertical-cavity surface-emitting lasers via scalable, higher-order mode suppression techniques

Posted on by jdallesa@illinois.edu

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.

ECS Transactions Article Link

Impact of Diffusion Profile on the Modulation Response of Single-Mode Disorder-Defined VCSELs

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The impact of diffusion profile shaping through the use of tensilely and compressively strained diffusion masks on the modulation response of single-mode vertical-cavity surface-emitting lasers (VCSELs) using disorder-defined apertures is investigated. VCSELs designed for high-power single-fundamental-mode emission through the use apertures defined via impurity-induced disordering (IID) in conjunction with a standard oxide-confinement process are characterized to extract high-frequency optical cavity parameters across oxide aperture and diffusion mask strain variations. The 3-dB small-signal bandwidth is maintained for a 7.68 mW single-mode 850 nm VCSEL with an oxide aperture of 13 μm using a tensilely strained diffusion mask relative to a non-disordered multimode device of the same oxide aperture. A large K-factor reduction is also observed, from 0.248 ns to 0.045 ns, indicating that damping and photon lifetimes within the cavity of VCSELs employing disorder-defined apertures are substantially reduced. Performance implications to data communication applications are discussed.

IEEE Photonics Technology Letters Article Link

Impact of Diffusion Mask Strain on Impurity-Induced Disordered VCSELs Designed for Single-Fundamental-Mode Operation

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The strain of diffusion masks utilized for impurity-induced disordering is demonstrated to control the curvature of the diffusion front, and therefore disordering front, of the disordered distributed Bragg reflector (DBR) aperture. As a result, the disordered apertures formed under strain conditions varying from compressive to tensile are shown to significantly impact the electro-optical performance and spectral characteristics of impurity-induced disordered (IID) VCSELs designed for single-fundamental-mode operation. An investigation and analysis of the electro-optical performance and spectral characteristics of IID-VCSELs as a result of varying diffusion mask strain is presented.

CS Mantech Article Link

Heterogeneously integrated VCSELs on silicon

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A wafer-scale CMOS-compatible process for heterogeneous integration of III-V epitaxial material onto silicon for photonic device fabrication is presented. Transfer of AlGaAs-GaAs Vertical-Cavity Surface-Emitting Laser (VCSEL) epitaxial material onto silicon using a carrier wafer process and metallic bonding is used to form III-V islands which are subsequently processed into VCSELs. The transfer process begins with the bonding of III-V wafer pieces epitaxy-down on a carrier wafer using a temporary bonding material. Following substrate removal, precisely-located islands of material are formed using photolithography and dry etching. These islands are bonded onto a silicon host wafer using a thin-film non-gold metal bonding process and the transfer wafer is removed. Following the bonding of the epitaxial islands onto the silicon wafer, standard processing methods are used to form VCSELs with non-gold contacts. The removal of the GaAs substrate prior to bonding provides an improved thermal pathway which leads to a reduction in wavelength shift with output power under continuous-wave (CW) excitation. Unlike prior work in which fullyfabricated VCSELs are flip-chip bonded to silicon, all photonic device processing takes place after the epitaxial transfer process. The electrical and optical performance of heterogeneously integrated 850nm GaAs VCSELs on silicon is compared to their as-grown counterparts. The demonstrated method creates the potential for the integration of III-V photonic devices with silicon CMOS, including CMOS imaging arrays. Such devices could have use in applications ranging from 3D imaging to LiDAR.

SPIE Article Link

Record High-Power Single-Mode VCSELs using Strain-Controlled Impurity-Induced Disordering Apertures

Posted on by jdallesa@illinois.edu

Patrick Su demonstrates record high-power single-mode performance in VCSELs using strain-controlled impurity-induced disordering apertures. Oxide-VCSELs of aperture sizes from 9-13 um are shown to achieve record single-mode output powers of 8.52 mW, 9.57 mW, 10.20 mW, 10.57 mW, and 10.95 mW respectively.

HMNTL News Article

Applied Physics Letter Article Link

Optical Transitions and Magnetism in Mn-Implanted Gallium Nitride for Three-Level Magnetooptic Devices

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A method of preparing passive materials suitable for magnetooptic interactions is shown using manganese implantation into gallium nitride (GaN) epitaxial layers, which establishes both dilute ferromagnetism and a three-level optical system with persistence to over 300 K. A sweep of thermal anneal parameters for a high implant dose into Mg-doped p-type GaN films is presented, and the materials are tested for both their magnetization and photoluminescence (PL). The optimal anneal process at 825 °C for 5 min maintains ferromagnetism with TC>305 K, confirming magnetic alignment at room temperature with a coercivity of ~100 Oe. PL and spectrophotometry of the optimally prepared materials show the effects of the mid-gap defect state on the material’s optical characteristics. The anneal process returns the real part index to its baseline dispersion while retaining an onset of absorption starting at the defect level EA=1.8 eV, signifying a stable mid-gap energy transition with a measured state lifetime of τPL=2.7 ns. The scalability of this process for producing three-level transition magnetic materials suggests passive optical or magnetooptic devices can be constructed that interconnect photonic and spintronic effects for emerging system designs and potential applications in quantum information.

IEEE Transactions on Electron Device Article Link

Modeling Photocurrent Spectra of High-Indium-Content InN/InGaN Disk-in-Wire Photodiodes

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This work reports comprehensive modeling of photocurrent spectra generated by an InGaN/InGaN disk-in-wire photodiode based on the effective bond-orbital model. The photocurrent spectra contributed by both single-photon absorption and two-photon absorption are calculated. The physical mechanisms for the observed prominent peaks are identified and investigated.

IEEE IPC Article Link