Jon M. Pikal

CdSe quantum dots synthesized by laser ablation in water and their photovoltaic applications

CdSe quantum dots (QDs) have been prepared by a facile and clean synthesis method––laser ablation in water. The structural and luminescent properties of the CdSe QDs have been investigated. The CdSe QDs of wurtzite crystal structure have an average particle size of about 5 nm. The QDs can be attached to ZnO nanowires making them ideal for applications in QD-sensitized nanowire solar cells. A uniqueness of the QDs attached to the ZnO nanowires by this laser ablation method is that they do not contain ligands, and the preparation avoids the complicated process of ligand exchange.

Controlled synthesis of Eu2+ and Eu3+ doped ZnS quantum dots and their photovoltaic and magnetic properties

Eu-doped ZnS quantum dots (QDs) have been synthesized by wet-chemical method and found to form in zinc blende (cubic) structure. Both Eu2+ and Eu3+ doped ZnS can be controllably synthesized. The Eu2+ doped ZnS QDs show broad photoluminescence emission peak around 512 nm, which is from the Eu2+ intra-ion transition of 4f6d1 – 4f7, while the Eu3+ doped samples exhibit narrow emission lines characteristic of transitions between the 4f levels. The investigation of the magnetic properties shows that the Eu3+ doped samples exhibit signs of ferromagnetism, on the other hand, Eu2+ doped samples are paramagnetic of Curie-Weiss type. The incident photon to electron conversion efficiency is increased with the Eu doping, which suggests the QD solar cell efficiency can be enhanced by Eu doping due to widened absorption windows. This is an attractive approach to utilize benign and environmentally friendly wide band gap ZnS QDs in solar cell technology.

Frequency response of strain-compensated InGaAsN-GaAsP-GaAs SQW lasers

We investigate the below and above threshold frequency response of InGaAsN lasers with different nitrogen content. This is accomplished through detailed analysis of the small signal modulation response of the laser diodes using a comprehensive model based on rate equations and that incorporates the effect of parasitics. For below threshold conditions the model is instrumental in separating the contributions from the parasitics (more severe at low bias) and carrier recombination (predominant at higher bias) to the measured carrier lifetime. It is found that the addition of nitrogen reduces the recombination lifetime, mainly as a result of a four-fold increase in monomolecular recombination which predominates even near threshold. For bias above threshold the analysis compares electrical versus optical modulation frequency responses and concludes that resonance frequency and damping extracted from the electrical modulation responses are significantly influenced by the device parasitics. Instead, it is shown that optical modulation traces allow extraction of a relaxation frequency that is shaped only by the stimulated processes in the laser active region. Even in this case, the damping is found to be affected by the parasitics. When compared with nitrogen free lasers, a reduction in the resonance frequency is observed, while the damping is not altered. The former arises from a factor of /spl sim/2.5 decrease in the combined effect of the differential gain and carrier transport parameters. The latter reflects the more significant contribution of the parasitics to the damping.

Small-signal response of 1.3-/spl mu/m InAsP-InGaAsP quantum-well laser diodes obtained with a terahertz-bandwidth frequency comb

We describe an optical method to carry out small-signal frequency response measurements in laser diodes. This method uses a midinfrared tunable femtosecond optical parametric oscillator (OPO), whose frequency spectrum consists of a comb composed by the multiple harmonics of the 81-MHz repetition rate of the same intensity and extending over 2 THz. Tuning of the OPO allows selective generation of carriers in the different regions of the test laser active area without compromising the frequency bandwidth. The small-signal frequency response of the test laser is retrieved from the intensity changes in the frequency comb spectrum. We apply this method to investigate the small-signal frequency response of 1.3-/spl mu/m InAsP quantum-well (QW) lasers. The results of these experiments show that the intrinsic frequency bandwidth of these lasers is limited to less than 10 GHz as a result of state filling and related carrier escape out of the well. An analysis of the frequency response traces through a solution of a system of rate equations allows us to estimate the magnitude of the gain compression associated with this process.

Steady state and time resolved optical characterization studies of Zn2SnO4 nanowires for solar cell applications

Nanowires are a promising option for sensitized solar cells, sensors, and display technology. Most of the work thus far has focused on binary oxides for these nanowires, but ternary oxides have advantages in additional control of optical and electronic properties. Here, we report on the diffuse reflectance, Low Temperature and Room Temperature Photoluminescence (PL), PL excitation spectrum, and Time Resolved PL (TRPL) of Zinc Tin Oxide (ZTO) nanowires grown by Chemical Vapor Deposition. The PL from the ZTO nanowires does not exhibit any band gap or near gap emission, and the diffuse reflectance measurement confirms that these ZTO nanowires have a direct forbidden transition. The broad PL spectrum reveals two Gaussian peaks centered at 1.86 eV (red) and 2.81 eV (blue), representing two distinct defect states or complexes. The PL spectra were further studied by the Time Resolved Emission Spectrum and intensity dependent PL and TRPL. The time resolved measurements show complex non-exponential decays at all w...

Carrier Recombination Dynamics Investigations of Strain-Compensated InGaAsN Quantum Wells

The time evolution of the photoluminescence (PL) of 1300-nm emitting InGaAsN/ GaAs/GaAsP strain-compensated single quantum well (QW) in the temperature range of T 1/4 10 K - 300 K is investigated. The PL spectra observed at the early stages of carrier recombination is dominated by two transitions. These two transitions are identified as the first quantized electron state to heavy-hole state (e1-hh1) and electron to light-hole state (e1-lh1) from the analysis of polarized photocurrent measurements in combination with k · p simulation of the band structure. At longer time delays, the dilute-nitride QW exhibits carrier localization at low temperatures and faster recombination time at higher temperatures. The PL dynamics characteristics observed in the InGaAsN QW are different from those measured from the InGaAs QW.

Small signal frequency response of laser diodes using a femtosectond frequency comb

A novel method to measure the optical modulation response of laser diodes that uses as the modulation source the output of a femtosecond optical parametric oscillator (OPO) is described. The femtosecond OPO generates a train of ~ 150 fs pulses tunable between 1.03 and 1.35 μm with an average power of 12 mW at a repetition rate of 81 MHz. With such a narrow pulse a rich frequency spectrum of flat intensity distribution that easily surpasses the 2000 GHz 3 dB-bandwidth is obtained. To perform modulation response measurements the OPO is selectively tuned to modulate the carrier population in either the well or separate confinement region of the laser diode. Modulation traces obtained with this method in 1.3 μm InAsP lasers are presented and compared with those obtained from electrical modulation at the same operating conditions.

Rate equation modeling of current injection efficiency in 1.3-μm InAs-InGaAs quantum dot lasers

Cavity length vs. inverse of slope efficiency technique is most widely used to extract the injection efficiency in semiconductor lasers which assumes that all the carriers occupy single energy level in the laser active region. However, QD lasers contain multiple higher lying energy levels in addition to the ground level and have significant carrier capture times which results in the occupation of these higher energy levels. In addition to the multiple energy levels, the density of states of each energy level is inhomogeneously broadened, which leads to the broadening of the gain spectrum as a whole. Inhomogeneous broadening is a result of the random size distribution of QDs grown by the self-assembled growth technique. In this work, we present the results of an above threshold multi-level rate equation model developed to understand the effect of inhomogeneous broadening on the measured low injection efficiencies of InAs-InGaAs based quantum-dot (QD) lasers operating at 1.3 μm.

High Bandwidth Constant Current Modulation Circuit for Carrier Lifetime Measurements in Semiconductor Lasers

In this paper we present a novel implementation of high bandwidth constant modulation current circuit to the traditional small signal optical response technique used to determine the differential carrier lifetime of a semiconductor laser. This circuit is designed for the voltage to current conversion and to deliver a constant modulation current to the laser diode. The circuit rectifies parasitic effects of high value surface mount resistor at high frequencies used in the impedance independent optical technique and also has lower crosstalk. The application of this circuit can be generalized where the requirement arises for a high bandwidth constant modulation current circuit.

Effect of free carriers and excitons on the gain and temperature characteristics of InAs/InGaAs quantum dot lasers

In this work we study the role of free carriers and excitons on the characteristics of 1.3 μm InAs/InGaAs quantum dot lasers. The study is carried out theoretically by building a mathematical model to calculate the threshold current in the laser and the charateristic temperature, T0. In order to determine the role of free carrier and excitons on the laser characteristics the model allows for different carrier distribution assumptions to be used, and we look at three cases; all free carriers, all excitons, and both free carriers and excitons in the dots. Our model results show that if we allow either free carriers or excitons to exist but not both, the calculated threshold current and T0 do not match with the experimental values. Thus we conclude that both free and bound carriers must exist and develop a method for modeling this case. We use a modified form of the Saha equation to calculate the ratio of free carriers to excitons and modify the material gain to account for this ratio. This model results in a threshold current density of approximately 39 A/cm2 and a T0 of 83 K, both of which are in excellent agreement with experimental results.