Runyu Liu

Doped semiconductors with band-edge plasma frequencies

In this work, the authors demonstrate the potential of epitaxially grown highly doped InSb as an engineered, wavelength-flexible mid-IR plasmonic material. The authors achieve doping concentrations over an order of magnitude larger than previously published results and show that such materials have plasma frequencies corresponding to energies larger than the materials band-gap. These semiconductor-based plasmonic metals open the door to homoepitaxial integration of plasmonic or epsilon-near-zero materials with optoelectronic devices at mid-infrared wavelengths. The materials are characterized by Hall measurements, mid-infrared transmission and reflection spectroscopy, and near-infrared transmission spectroscopy. The opportunities offered and the limitations presented by this material system are discussed and analyzed.

Direct minority carrier transport characterization of InAs/InAsSb superlattice nBn photodetectors

We present an extensive characterization of the minority carrier transport properties in an nBn mid-wave infrared detector incorporating a Ga-free InAs/InAsSb type-II superlattice as the absorbing region. Using a modified electron beam induced current technique in conjunction with time-resolved photoluminescence, we were able to determine several important transport parameters of the absorber region in the device, which uses a barrier layer to reduce dark current. For a device at liquid He temperatures, we report a minority carrier diffusion length of 750 nm and a minority carrier lifetime of 200 ns, with a vertical diffusivity of 3 × 10−2 cm2/s. We also report on the devices optical response characteristics at 78 K.

Maximizing cubic phase gallium nitride surface coverage on nano-patterned silicon (100)

Here we investigate the hexagonal-to-cubic phase transition in metalorganic-chemical-vapor-deposition-grown gallium nitride enabled via silicon (100) nano-patterning. Electron backscatter diffraction and depth-resolved cathodoluminescence experiments show complete cubic phase GaN surface coverage when GaN deposition thickness ( hc), etch depth ( td), and opening width ( p) obey hc≈1.06p−0.75td; in line with a geometrical model based on crystallography. Cubic GaN uniformity is studied via electron backscatter diffraction and cathodoluminescence measurements. Atomic force microscopy reveals a smooth cubic GaN surface. Phase-transition cubic GaN shows promising optical and structural quality for integrated photonic devices.

Flat mid-infrared composite plasmonic materials using lateral doping-patterned semiconductors

We demonstrate lateral control of carrier concentration in doped Si for mid-infrared plasmonic applications. Using commercially available spin-dopants, we show that doped silicon can act as a plasmonic material at mid-infrared wavelengths, and that control of the doping pattern allows for the development of flat, single-material plasmonic composites. Our materials are characterized by infrared spectroscopy and microscopy, surface profilometry and infrared emissivity measurements. We demonstrate the ability to fabricate subwavelength doped features and show distinct diffraction from one-dimensional arrays of ‘metal’ lines patterned in our material system.

Cathodoluminescence study of luminescence centers in hexagonal and cubic phase GaN hetero-integrated on Si(100)

Hexagonal and cubic GaN—integrated on on-axis Si(100) substrate by metalorganic chemical vapor deposition via selective epitaxy and hexagonal-to-cubic-phase transition, respectively—are studied by temperature- and injection-intensity-dependent cathodoluminescence to explore the origins of their respective luminescence centers. In hexagonal (cubic) GaN integrated on Si, we identify at room temperature the near band edge luminescence at 3.43 eV (3.22 eV), and a defect peak at 2.21 eV (2.72 eV). At low temperature, we report additional hexagonal (cubic) GaN bound exciton transition at 3.49 eV (3.28 eV), and a donor-to-acceptor transition at 3.31 eV (3.18 eV and 2.95 eV). In cubic GaN, two defect-related acceptor energies are identified as 110 and 360 meV. For hexagonal (cubic) GaN (using Debye Temperature ( β) of 600 K), Varshni coefficients of α=7.37±0.13×10−4 (6.83±0.22×10−4)eV/K and E0=3.51±0.01 (3.31±0.01) eV are extracted. Hexagonal and cubic GaN integrated on CMOS compatible on-axis Si(100) are shown t...

Mid-infrared emission from In(Ga)Sb layers on InAs(Sb).

We demonstrate infrared light emission from thin epitaxially-grown In(Ga)Sb layers in InAs(Sb) matrices across a wide range (3-8 µm) of the mid-infrared spectral range. Our structures are characterized by x-ray diffraction, photoelectron spectroscopy, atomic force microscopy and transmission electron microscopy. Emission is characterized by temperature- and power-dependent infrared step-scan photoluminescence spectroscopy. The epitaxial In(Ga)Sb layers are observed to form either quantum wells, quantum dots, or disordered quantum wells, depending on the insertion layer and substrate material composition. The observed optical properties of the monolayer-scale insertions are correlated to their structural properties, as determined by transmission electron and atomic force microscopy.

Multiplexed infrared photodetection using resonant radio-frequency circuits

We demonstrate a room-temperature semiconductor-based photodetector where readout is achieved using a resonant radio-frequency (RF) circuit consisting of a microstrip split-ring resonator coupled to a microstrip busline, fabricated on a semiconductor substrate. The RF resonant circuits are characterized at RF frequencies as function of resonator geometry, as well as for their response to incident IR radiation. The detectors are modeled analytically and using commercial simulation software, with good agreement to our experimental results. Though the detector sensitivity is weak, the detector architecture offers the potential for multiplexing arrays of detectors on a single read-out line, in addition to high speed response for either direct coupling of optical signals to RF circuitry, or alternatively, carrier dynamics characterization of semiconductor, or other, material systems.

Cubic phase light emitters hetero-integrated on silicon

GaN emitters have historically been of hexagonal phase due to natural crystallization. Here we introduce a cubic phase GaN emitter technology that is polarization-free via cointegration on cheap and scalable CMOS-compatible Si(100) substrate.

Polarization-free integrated gallium-nitride photonics

Gallium Nitride (GaN) materials are the backbone of emerging solid state lighting. To date, GaN research has been primarily focused on hexagonal phase devices due to the natural crystallization. This approach limits the output power and efficiency of LEDs, particularly in the green spectrum. However, GaN can also be engineered to be in cubic phase. Cubic GaN has a lower bandgap (~200 meV) than hexagonal GaN that enables green LEDs much easily. Besides, cubic GaN has more isotropic properties (smaller effective masses, higher carrier mobility, higher doping efficiency, and higher optical gain than hexagonal GaN), and cleavage planes. Due to phase instability, however, cubic phase materials and devices have remained mostly unexplored. Here we review a new method of cubic phase GaN generation: Hexagonal-to-cubic phase transition, based on novel nano-patterning. We report a new crystallographic modelling of this hexagonal-to-cubic phase transition and systematically study the effects of nano-patterning on the GaN phase transition via transmission electron microscopy and electron backscatter diffraction experiments. In summary, silicon-integrated cubic phase GaN light emitters offer a unique opportunity for exploration in next generation photonics.

Enhanced responsivity resonant RF photodetectors

The responsivity of room-temperature, semiconductor-based photodetectors consisting of resonant RF circuits coupled to microstrip buslines is investigated. The dependence of the photodetector response on the semiconductor material and RF circuit geometry is presented, as is the detector response as a function of the spatial position of the incident light. We demonstrate significant improvement in detector response by choice of photoconductive material, and for a given material, by positioning our optical signal to overlap with positions of RF field enhancement. Design of RF circuits with strong field enhancement are demonstrated to further improve detector response. The improved detector response demonstrated offers opportunities for applications in RF photonics, materials metrology, or single read-out multiplexed detector arrays.