N. Sylvain Charbonneau

Comparative study of laser- and ion implantation-induced quantum well intermixing in GaInAsP/InP microstructures

Laser-induced quantum well intermixing (laser-QWI) and ion implantation-induced quantum well intermixing (II-QWI) techniques have been studied to selectively modify the optical properties of GaInAsP/InP laser microstructures. Following the annealing with a cw Nd:YAG laser, a blue shift in the quantum well photoluminescence of up to 124 nm was observed for samples annealed up to 4 min. A comparison of the laser annealing results with those of II-QWI, which were obtained for the same GaInAsP/InP microstructure, indicates that laser- QWI yields material with comparable, or better optical properties. The one-step processing used in the laser-QWI approach makes it an attractive alternative in fabricating photonic integrated circuits at low cost.

Transparent waveguides for WDM transmitter arrays using quantum well shape modification

A technique for fabricating transparent waveguides on the same wafer as a quantum well (QW) DBR laser array has been developed. High [MeV] energy ion implantation is used to create a large number of vacancies and interstitials throughout the active region of the device. Upon annealing, these entities enhance the intermixing of the QW and barrier materials resulting in a blue shift of the QW bandgap. Energy shifts (measured using low temperature photoluminescence spectroscopy) of greater than 60 meV can be achieved. Room temperature waveguide absorption measurements verify the shift in the bandgap energy and confirm that the waveguide is now effectively transparent in the wavelength range of the QW lasers. This technique is being used in an eight wavelength WDM transmitter array in which the waveguiding region is selectively implanted and blue shifted.

Polarization-insensitive quantum well optoelectronic devices using quantum well shape modification

Polarization insensitive 1.5 micrometer QW optical amplifiers, modulators, and detectors were fabricated using a novel, simple, post-growth, integratable technique. The process utilizes ion-implantation-induced, spatially selective, quantum well (QW) shape modification. A simple model shows that if the interdiffusion rate of the anions is larger than that of the cations, the blue shift in the ground state heavy hole transition energy after implantation and annealing is greater than the light hole state blue shift, merging the two bands and thus eliminating the difference between the TE (transverse electric) and TM (transverse magnetic) waveguide propagation modes. Current- voltage measurements indicate that junction characteristics are well maintained after processing. This simple technique for fabricating discrete polarization insensitive optoelectronic devices is readily extended to the monolithic integration of such devices along with other passive and active optoelectronic devices and provides a pathway to practical photonic integrated circuits.

Quantum well intermixing for the realization of photonic integrated circuits

A technique, based on quantum well (QW) intermixing, has been developed for the post growth, spatially selective tuning of the QW bandgap in a laser structure. High energy (MeV) ion implantation is used to create a large number of vacancies and interstitials in the device. During high temperature processing, these defects simultaneously enhance the intermixing of the QW and the barrier materials, producing a blue shift of the QW bandgap, and are annealed out. Increases in bandgap energy (measured using low temperature photoluminescence spectroscopy) of greater than 60 meV can be achieved. Absorption spectroscopy in the waveguide direction is also used to quantify any excess loss in the structure. Using a simple masking scheme to spatially modify the defect concentration, different regions of a wafer can be blue shifted by different amounts. This allows the integration of many different devices such as lasers, detectors, modulators, waveguides etc. on a single wafer using only a single, post-growth processing step. The performance of both passive (waveguide) and active (laser) devices produced using this technique is described, as well as the practicality of this technique in the production of photonic integrated circuits.

Photonic-integrated circuits and components using quantum well intermixing

The monolithic integration of optical components with different functionalities on a single semiconductor wafer requires spatially selective control of bandgap energies. We have developed a simple, post-growth technique based on quantum well intermixing using ion implantation and rapid thermal annealing, which allows multiple selective area bandgap tailoring. Waveguide absorption spectra demonstrate that the bandgap energy can be shifted as much as 90 nm without any excess loss. By depositing a SiO2 layer of different thicknesses in different regions as the implantation mask, quantum wells in different sections of a wafer can be intermixed to different degrees in a single implantation and annealing process. It has also been shown that the heavy-hole and light hole states in the quantum wells can become degenerate at a certain degree of intermixing, which allows the fabrication of polarization insensitive optical amplifiers and electro-absorptive switches. The performance of both active (laser, amplifier, modulator) and passive (waveguide) components produced using this technique will be presented.

Ultrafast electron tunneling times in reverse-biased quantum-well laser structures

We report extremely efficient and fast (approximately 25 pS FWHM) escape times of optically generated carriers in a reverse biased GaAs/AlGaAs graded index separate confined heterostructure single quantum well (GRINSCH-SQW) laser. Room temperature photoconductivity (PC) measurements in a high speed ridge waveguide detector are compared with time resolved photoluminescence (PL) measurements at T equals 20 K, 70 K, and 150 K. By comparing the experimental PL and PC response times and efficiencies as a function of bias voltage and temperature with theory, we show that the results are consistent with a simple model based on electron recombination and escape out of the quantum well. Electron escape occurs by either direct tunneling out of the lower electronic level, by thermally assisted tunneling out of the upper weakly bound state, or by thermionic emission over the barrier, depending on the bias voltage and temperature.

Photonic integrated circuits fabricated using quantum well intermixing

A technique, base don quantum well (QW) intermixing, has been developed for the post growth, spatially selective tuning of the QW bandgap in a semiconductor laser structure. High energy ion implantation is used to create a large number of vacancies and interstitials in the device. During high temperature processing, these defects enhance the intermixing of the QW and the barrier materials while being annealed out, producing a blue shift of the QW bandgap. Increases in bandgap energy of greater than 10 nm at 1.55 micrometers in InGaAs/InGaAsP/InP structures can be achieved. Absorption spectroscopy in the waveguide geometry is used to quantify the losses in the structure. Using a simple masking scheme to spatially modify the defect concentration, different regions of a wafer can be blue shifted by different amounts. This allows the integration of many different devices such as lasers, detectors, modulators, amplifiers and waveguides on a single wafer using only a single, post-growth processing step. The performance of both passive and active devices produced using this technique will be described, as well as the practicality of this technique in the production of photonic integrated circuits.

Enhanced quantum well intermixing using multiple ion implantation

Quantum well intermixing has been performed using ion implantation techniques to increase the optical bandgap in a spatially selective manner. We show that there is a maximum single dose beyond which further intermixing of the QWs is impeded by damage to the semiconductor surface. We overcome this problem by using a series of implants and rapid thermal anneals, with each rapid thermal anneal repairing the crystal surface. Using this technique we have demonstrated shifts in optical bandgap for multiple implants greater than seven times that observed for a single implant.

Evidence of hole tunneling in a double-barrier resonant tunneling structure obtained by time-resolved photoluminescence

Photoluminescence (PL), PL excitation (PLE), and time-resolved PL are employed to study the tunneling of photoexcited holes through a GaAs/A1GaAs double-barrier resonant tunneling structure. Lifetime measurements of the n= 1 heavy-hole (hh) exciton transition from the well were obtained as a function of the applied voltage. For voltages biasing the structure in the non-resonant tunneling regime, beyond the region of negative differential resistance (NDR), the exciton decays with two time constants. The fast component, which was observed at all voltages, is attributed to the decay of the exciton population originating from holes photoexcited directly in the well. The slower time constant is associated with excitons that are created from hOles which are photoexcited in the GaAs contact region, and which subsequently tunnel into the well. This picture for hole tunneling is further supported by the observation of the n= 1 hh exciton emission using exciting photon energies lower than the quantum well bandgap but larger than the GaAs bandgap, when the structure is biased beyond the region of NDR.

Picosecond imaging of hot electron emission from CMOS circuitry

This paper describes the spatially and temporally resolved images from submicron NFETS and a CMOS ring-oscillator circuit. The spatial and temporal information is supplemented by spectral measurements obtained using a set of optical band-pass filters. The intensity of luminescence has been observed on individual transistors with gate-length downs to 0.2 microns. The time-resolution of ~ 100 ps is sufficient to observe the response of individual invertors for gate lengths of 0.8 microns and even lower. Preliminary work on spectral distributions on emission from both the ring oscillator and NFET indicated a peak around 850nm. This may be limited on the long-wavelength side by the response of the photomultiplier photocathode, indicating that better sensitivity could be achieved with extended infra-red sensitivity. The spectral distribution is explained with reference to current theories.