Ming-Hua Mao

2023 International Conference on Solid State Devices and Materials ,Photonic Inter-Layer Vias for Three-Dimensional Optical Signal Transmission and Effect of Increased Surface Recombination Velocity

In this study, we proposed and demonstrated a brand new type of monolithic photonic devices which realizes the three-dimensional (3D) all-optical switching for inter-layer signal transmission. This device is composed of a vertical Si microrod which serves as optical absorption material within a SiN waveguide in one layer and as an index modulation structure within a SiN microdisk resonator lying in the other layer. The ambipolar photo-carrier transport property in the Si microrod was studied by measuring the resonant wavelength shifts under continuous-wave laser pumping. The ambipolar diffusion length can be extracted to be 0.88 µm. Based on the ambipolar photo-carrier transport in a Si microrod through different layers, we presented a fully-integrated all-optical switching operation using this Si microrod and a SiN microdisk with a pump-probe technique through the on-chip SiN waveguides. The switching time windows for the on-resonance operation mode and the off-resonance operation mode can be extracted to be 439 ps and 87 ps, respectively. This device shows potential applications for the future all-optical computing and communication with more practical and flexible configurations in monolithic 3D photonic integrated circuits (3D-PICs).

Carrier transport was studied both numerically and experimentally using scanning photocurrent microscopy (SPCM) in two-dimensional (2D) transport structures, where the structure size in the third dimension is much smaller than the diffusion length and electrodes cover the whole terminal on both sides. Originally, one would expect that with increasing width in 2D transport structures, scanning photocurrent profiles will gradually deviate from those of the ideal one-dimensional (1D) transport structure. However, the scanning photocurrent simulation results surprisingly showed almost identical profiles from structures with different widths. In order to clarify this phenomenon, we observed the spatial distribution of carriers. The simulation results indicate that the integrated carrier distribution in the 2D transport structures with finite width can be well described by a simple-exponential-decay function with the carrier decay length as the fitting parameter, just like in the 1D transport structures. For ohmic-contact 2D transport structures, the feasibility of the fitting formula from our previous 1D analytical model was confirmed. On the other hand, the application of a simple-exponential-decay function in scanning photocurrent profiles for the diffusion length extraction in Schottky-contact 2D transport structures was also justified. Furthermore, our simulation results demonstrate that the scanning photocurrent profiles in the ohmic- or Schottky-contact three-dimensional (3D) transport structures with electrodes covering the whole terminal on both sides will reduce to those described by the corresponding 1D fitting formulae. Finally, experimental SPCM on a p-type InGaAs air-bridge two-terminal thin-film device was carried out. The measured photocurrent profiles can be well fitted by the specific fitting formula derived from our previous 1D analytical model and the extracted electron mobility-lifetime product of this thin-film device is 6.6 × 10^–7 cm²·V⁻¹. This study allows us to extract the minority carrier decay length and to obtain the mobility-lifetime product which can be used to evaluate the performance of 2D carrier transport devices.

A new simple method is proposed to extract the ambipolar diffusion length for two-dimensional (2D) electronic transport in thin film structures using a scanning photoluminescence microscopy (SPLM) setup. No spatially-resolved photoluminescence detection methods are required. By measuring the excitation-position-dependent PL intensity across the edge of a semiconductor, ambipolar diffusion length can be extracted from the SPLM profile through a simple analytic fitting function. Numerical simulation was first used to verify the fitting method. Then the fitting method was applied to extract the ambipolar diffusion length from the measured SPLM profile of a GaAs thin film structure. Carrier lifetime was obtained in an accompanying time-resolved photoluminescence measurement under the same excitation condition, and thus the ambipolar diffusion coefficient can be determined simultaneously. The new fitting method provides a simple way to evaluate carrier transport properties in 2D electronic transport structures such as thin films or quantum wells.

Spatially resolved current measurements such as scanning photocurrent microscopy (SPCM) have been extensively applied to investigate carrier transport properties in semiconductor nanowires. A traditional simple-exponential-decay formula based on the assumption of carrier diffusion dominance in the scanning photocurrent profiles can be applied for carrier diffusion length extraction using SPCM in Schottky-contact-based or p-n junction-based devices where large built-in electric fields exist. However, it is also important to study the electric-field dependent transport properties in widely used ohmic-contact nanowire devices where the assumption of carrier diffusion dominance is invalid. Here we derive an analytic formula for scanning photocurrent profiles in such ohmic-contact nanowire devices under uniform applied electric fields and weak optical excitation. Under these operation conditions and the influence of photo-carrier-induced electric field, the scanning photocurrent profile and the carrier spatial distribution strikingly do not share the same functional form. Instead, a surprising new analytic relation between the scanning photocurrent profile and the minority carrier decay length was established. Then the derived analytic formula was validated numerically and experimentally. This analytic formula provides a new fitting method for SPCM profiles to correctly determine the minority carrier decay length, which allows us to quantitatively evaluate the performance of nanowire-based devices.

We report continuous wave operation of SiO ₂ sandwiched colloidal CdSe/ZnS quantum-dot microdisk lasers at room temperature. The SiO ₂ sandwiched quantum dot structure is formed first and then the disk patterns are defined by e-beam lithography. The benefits of introducing the sandwich structure are two folds: one to increase the confinement factor and the other to reduce the photo-oxidation of quantum dots. The luminescence stability of CdSe/ZnS quantum dots is much improved after they are sandwiched in silicon dioxide. We observe whispering gallery modes spatially and spectrally in the microdisks. We also fabricated Si ₃ N ₄ microdisks for comparison. The result shows that Si ₃ N ₄ microdisks are severely damaged when operating under continuous wave excitation while SiO ₂ microdisks demonstrate stable lasing behavior at the same excitation condition. Therefore, SiO ₂ microdisks are more suitable for high excitation and/or continuous wave applications such as lasers. This device structure shows great potentials to be applied to other wavelength in the visible range.

We report photo-stability enhancement of colloidal CdSe/ZnS quantum dots (QDs) passivated in Al₂O₃ thin film using the atomic layer deposition (ALD) technique. 62% of the original peak photoluminescence (PL) intensity remained after ALD. The photo-oxidation and photo-induced fluorescence enhancement effects of both the unpassivated and passivated QDs were studied under various conditions, including different excitation sources, power densities, and environment. The unpassivated QDs showed rapid PL degradation under high excitation due to strong photo-oxidation in air while the PL intensity of Al₂O₃ passivated QDs was found to remain stable. Furthermore, recombination dynamics of the unpassivated and passivated QDs were investigated by time-resolved measurements. The average lifetime of the unpassivated QDs decreases with laser irradiation time due to photo-oxidation. Photo-oxidation creates surface defects which reduces the QD emission intensity and enhances the non-radiative recombination rate. From the comparison of PL decay profiles of the unpassivated and passivated QDs, photo-oxidation-induced surface defects unexpectedly also reduce the radiative recombination rate. The ALD passivation of Al₂O₃ protects QDs from photo-oxidation and therefore avoids the reduction of radiative recombination rate. Our experimental results demonstrated that passivation of colloidal QDs by ALD is a promising method to well encapsulate QDs to prevent gas permeation and to enhance photo-stability, including the PL intensity and carrier lifetime in air. This is essential for the applications of colloidal QDs in light-emitting devices.

We report room-temperature operation of current-injection InAs quantum-dot microdisk lasers with quality factor >23,000. The lasing wavelength is at 1136nm with threshold current of 0.39mA.

We demonstrated high-speed all-optical switching in a GaAs microdisk resonator using the tapered fiber pump probe technique. The optical transmission of the resonator was modulated with a time response of 43 ps.