Keye Sun

Temperature dependent energy levels of methylammonium lead iodide perovskite

Temperature dependent energy levels of methylammonium lead iodide are investigated using a combination of ultraviolet photoemission spectroscopy and optical spectroscopy. Our results show that the valence band maximum and conduction band minimum shift down in energy by 110 meV and 77 meV as temperature increases from 28 °C to 85 °C. Density functional theory calculations using slab structures show that the decreased orbital splitting due to thermal expansion is a major contribution to the experimentally observed shift in energy levels. Our results have implications for solar cell performance under operating conditions with continued sunlight exposure and increased temperature.

Design of emitter structures based on resonant perfect absorption for thermophotovoltaic applications.

We report a class of thermophotovoltaic emitter structures built upon planar films that support resonant modes, known as perfectly-absorbing modes, that facilitate an exceptional optical response for selective emission. These planar structures have several key advantages over previously-proposed designs for TPV applications: they are simple to fabricate, are stable across a range of temperatures and conditions, and are capable of achieving some of the highest spectral efficiencies reported of any class of emitter structure. Utilization of these emitters leads to exceptionally high device efficiencies under low operating temperature conditions, which should open new opportunities for waste heat management. We present a theoretical framework for understanding this performance, and show that this framework can be leveraged as a search algorithm for promising candidate structures. In addition to providing an efficient theoretical methodology for identifying high-performance emitter structures, our methodology provides new insight into underlying design principles and should pave way for future design of structures that are simple to fabricate, temperature stable, and possess exceptional optical properties.

Germanium p-n Junctions by Laser Doping for Photonics/Microelectronic Devices

A method of forming germanium p-n junction by laser doping is demonstrated. Low bulk and surface leakage current density of 5.4 mA/cm2 and 2.0 μA/cm, respectively, were obtained. The leakage current density was comparable with the diodes formed by rapid thermal diffusion, but approximately two orders of magnitude lower than the diodes formed by ion implantation. The performance of the laser doped junction in photonic devices was demonstrated through the fabrication of 130-μm diameter photodetector, which showed a responsivity of 0.46 A/W at 1.55-μm wavelength at 0 V bias and a -3-dB bandwidth of 190 MHz.

Self-organized 2D periodic arrays of nanostructures in silicon by nanosecond laser irradiation

We report a phenomenon of spontaneous formation of self-organized 2D periodic arrays of nanostructures (protrusions) by directly exposing a silicon surface to multiple nanosecond laser pulses. These self-organized 2D periodic nanostructures are produced toward the edge as an annular region around the circular laser spot. The heights of these nanostructures are around 500 nm with tip diameter ~100 nm. The period of the nanostructures is about 1064 nm, the wavelength of the incident radiation. In the central region of the laser spot, nanostructures are destroyed because of the higher laser intensity (due to the Gaussian shape of the laser beam) and accumulation of large number of laser pulses. Optical diffraction from these nanostructures indicates a threefold symmetry, which is in accordance with the observed morphological symmetries of these nanostructures.

Segmented waveguide photodetector with 90% quantum efficiency

We demonstrate a novel InGaAsP/InP segmented waveguide photodetector based on directional couplers. By matching the imaginary parts of the propagation constants of the even and odd modes, we designed a photodetector with 6 elements, each with an absorber volume of only 19 μm3 and a bandwidth of 15 GHz, that has an internal quantum efficiency (QE) of 90% at 1550 nm wavelength corresponding to 1.13 A/W.

High-power InGaAs/InP MUTC photodetector modules for RF photonics links and ROF

High-performance photodetectors (HPPDs), with high output power and bandwidth, are needed for RF photonics links. Applications for these HPPDs range from high-power remote antennas, low-duty-cycle RF pulse generation, linear photonic links, high dynamic range optical systems, and radio-over-fiber (ROF). Freedom Photonics is a manufacturer of high-power photodetectors (HPPD) for the 1480 to 1620nm wavelength range, now being offered commercially. In 2016, Freedom has developed a HPPD for similar applications extending into the V-band. The basic device structure used for these photodetectors can achieve over 100-GHz bandwidths with slight variations. This work shows data for RF power and bandwidth performance for various size photodiodes, between 10 μm and 28 μm in diameter. Measurement data will be presented, which were collected at both assembly level and for fully packaged detectors. For detector devices with bandwidth performance over 50 GHz, the generated RF power achieved is expected to be over 15 dBm. This performance is exceptional considering the photodiode is fully integrated into a hermetic package designed for 65 GHz. Improvements in the coplanar waveguide (CPW) transmission line and flip-chip bonding design were integral in achieving the higher saturation at the higher bandwidth performance. Further development is required to achieve a >100 GHz packaged photodetector module.

Pulse Laser Sulfur-Hyperdoping of Germanium and High Quantum Efficiency Photodiodes

Germanium was successfully hyperdoped with sulfur by means of nanosecond pulse laser and SF6 gas. A photodiode was fabricated based on sulfur-hyperdoped germanium. Rectifying current-voltage characteristics were observed for the device. The sulfur-hyperdoped germanium photodiode showed an internal quantum efficiency close to 150% from visible to near infrared and increased to 275% for the ultraviolet region. The -3 dB bandwidth was measured to be 20 MHz. Sulfur was demonstrated to be an effective deep level dopant for realizing high-performance germanium photodiodes.

Laser doping of germanium for photodetector applications

A method of doping germanium using 1064 nm pulsed fiber laser was demonstrated. The secondary ion mass spectrometry showed a p-n junction of 800 nm deep with a peak phosphorus concentration of 2×1019 cm-3. Germanium photodiodes were fabricated on the laser-doped p-n junctions. Low bulk and surface leakage current values were obtained which were comparable to diodes fabricated by rapid thermal diffusion. Laser doping allows low thermal budget, minimization of surface desorption and selective doping without requiring photolithography. Laser doping was shown to be an effective method for fabrication of electronic and optoelectronic devices.