ORCID

https://orcid.org/0009-0006-1009-5869

Date of Award

Summer 2025

Language

English

Embargo Period

7-23-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Nanoscale Science and Engineering

Program

Nanoscale Engineering

First Advisor

Fatemeh (Shadi) Shahedipour-Sandvik

Committee Members

Woongje Sung, Vincent Labella, Winston K. Chan, L. Douglas Bell

Keywords

MOCVD, GaN, Semiconductor, Nitrides, Detector, Infrared

Subject Categories

Nanoscience and Nanotechnology | Other Physics

Abstract

III-Nitride material system has been widely used in electronic, sensing, optoelectronic, and power device applications. Owing to the wide and tunable bandgap of this material system and the realization of relatively high conductivity p-type Mg-doped GaN films, considerable research over the past few decades has focused on applications based on wide bandgaps, such as blue and ultraviolet (UV) light-emitting diodes (LEDs), laser diodes, visible-blind and solar-blind photodetectors. The other side of the spectrum, infrared (IR) has remained considerably less explored.

To exploit III-Nitrides for IR detection applications, the quantum well infrared photodetector (QWIP) design is employed. In the QWIP design, an artificially small bandgap region using wide bandgap materials is created. In this type of detector, a wide bandgap material (e.g., GaN or GaAs) is surrounded by a larger bandgap material (e.g., AlxGa1−xN or AlxGa1−xAs) to create a quantum well (QW) where transitions happen between confined (bound) energy states in the QWs in the conduction or the valence band, also known as intersubband transitions or ISBTs. The III-Nitride system is particularly well-suited for QWIP type structures/devices due to the large conduction and valence band offsets in Al(Ga)N/GaN heterostructures, allowing for a broad range of spectral detection. The system’s strong polarization offers additional design flexibility, permitting further tuning of band offset and therefore, of ISBT energies.

One of the challenges impeding the use of III-nitride-based QWIPs in imaging applications is the need for wavelength up-conversion to permit emission in the visible range. Without integrated up-conversion, the QWIP must have an additional readout circuit, which increases device complexity, size, and weight, making it undesirable for many handheld device applications. This challenge was overcome for IR detection-IR emission in GaAs/AlGaAs by monolithic integration of a QWIP and a multi quantum well (MQW) emitter. This thesis discusses an AlxInyGa1−x−yN-based monolithically integrated IR detector-Visible emitter device grown by metal organic chemical vapor deposition (MOCVD). For detection, a Mg-doped p-QWIP is utilized, which allows normal incident absorption based on quantum mechanics polarization selection rule.

A realistic device design and energy band diagram simulation for an integrated AlxGa1−xN/GaN QWIP with an InGaN/GaN MQW emitter for IR detection and emission in the visible range was developed and further optimized throughout the device development process. The integrated device’s layer thicknesses, dopings, and compositions were selected and developed based on feedback provided from material growth and characterization, device fabrication, and electrical characterization.

Targeting 1.55 μm absorption peak, energy band diagram simulations were developed and the valence band offset energy between GaN (QW) and different Al compositions (x) of AlxGa1−xN barrier layers was calculated. As a result, x = 0.45 (in AlxGa1−xN) barrier was selected, which creates ~0.8 eV valence band offset (for a target 1.55 μm absorption peak). MOCVD material growth space was developed to achieve nm-thick layers with high control over the growth rate, abrupt interfaces, and smooth surface morphology. Extensive material characterizations such as atomic force microscopy (AFM), X-ray diffraction (XRD), and scanning transmission electron microscopy (STEM) were used which demonstrated development of smooth and uniform surface morphology with low root-mean-square (RMS) roughness (< 0.5 nm), high crystalline quality, and abrupt interfaces. QWIP devices were fabricated to measure the dark current density and photoresponsivity on a range of devices with varying mesa sizes, from ~100 μm to ~600 μm in diameter. I-V measurements revealed low dark current density in the range of ~10−4 to 10−6 A/cm2 on these photodetectors. Photoresponsivity with a broadband light source in the 700–2200 nm wavelength range showed < 5 μA/W photoresponsivity, corresponding to ISBTs in the valence band. This low photoresponsivity is in part due to insufficient and/or inefficient acceptor dopants (Mg) in QWIP, which in turn, limits the photocurrent generation and overall device performance in the integrated detector-emitter.

Studying dopant concentration and distribution in nm-thick layers by conventional techniques such as secondary ion mass spectrometry (SIMS) is not accurately possible due to SIMS limited analyte volume. Atom probe tomography (APT) was employed to study Mg concentration in layers and interfaces, as well as inside and outside of clusters was analyzed by APT. In addition to information on concentration of Mg and distribution in these nm-thick layers, APT also revealed Mg segregation or clustering in AlGaN/GaN QWIP layers.

Finally, after the design and simulation, growth, fabrication, and characterization of the Mg-doped AlGaN/GaN p-QWIP, the integrated detector-emitter structure was grown and extensive material characterizations using AFM, photoluminescence (PL), XRD, and STEM were performed. The integrated detector-emitter structure utilizes a p-n-p-n thyristor design where QWIP is located as the “gate” or photosensitive part of the npn phototransistor (PT) and InGaN/GaN MQW acts as the “emitter” of the pnp transistor. IR illumination generates photocurrent in the base (QWIP) of the phototransistor. By increasing this current (illumination) at the base of the phototransistor, the voltage on the emitter of the pnp transistor or the MQW emitter increases, leading to light emission in the visible range. I-V characterizations showed the expected thyristor-like behavior on a few devices where there is a knee and by increasing the current, it transitions to a lower resistance state (switching behavior). Additionally, a large number of devices did not show a measurable photocurrent with a diode-like I-V behavior. Our investigations revealed that this is in large part due to carrier capture in the QWIP’s well layers and/or insufficient layer thickness and doping of the p-AlGaN layer, necessitating further design and growth developments in the future to improve device performance.

The research conducted and detailed in this thesis provides insights on multiple fronts, from fundamental design consideration for a realistic AlGaN-based ISBT IR detector, full design consideration and challenges associated with an integrated IR detector-visible emitter based on AlInGaN with their inherent and strong internal polarization field(s), to growth of strained nm scale AlGaN/GaN SL structures and complexity of full integrated device growths-all while facing the challenges of dopant activation in a buried p-doped structure and Mg dopant segregation. The knowledge developed in the thesis is certain to impact not just the specific device detailed here but impactful for development of any AlInGaN/AlInGaN-based SL (MQW) specially in the presence of dopants. These findings on the AlxGa1−xN/GaN superlattice structures not only can be utilized in novel structures based on III-Nitrides such as integrated detector-emitter devices, but also as a platform for any application utilizing these structures such as polarization-enhanced doping, Bragg reflectors, strain management structures, and high electron mobility transistors (HEMTs).

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Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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