We use cross-sectional transmission electron microscopy to study the damage induced below the surface of indium phosphide (InP) samples by single and multiple femtosecond laser pulses with a photon energy lower than the InP band gap. Single-pulse irradiation creates a ∼100 nm deep crater with a resolidified surface layer consisting of quasiamorphous indium phosphide. The resolidified layer has a thickness of ∼60 nm at the center and extends laterally beyond the edge of the crater rim. Exposure to multiple femtosecond pulses of 2050 nm center wavelength results in the formation of laser-induced periodic surface structures (LIPSS) with two different periods, one (∼1730 nm) less than but close to the laser wavelength and one (∼470 nm) four times smaller. Segregation beneath both types of ripples leads to the formation of In-rich particles embedded in the resolidified surface layer. Extended defects are detected only below the center of the multiple-pulse crater and their distribution appears to be correlated with the LIPSS modulation. Finally, LIPSS formation is discussed in terms of the observed subsurface microstructures.
The crystallization kinetics of the pyrochlore to perovskite phase transformation in lead zirconate titanate (PZT) thin films have been analyzed using rapid thermal processing (RTP). Sol-gel PZT thin films, fabricated on platinum electrodes, were annealed at 550 °C to 650 °C with hold times ranging from 1 s to 5 min. Glancing angle x-ray diffraction (XRD) was used for depth profiling to identify the location of phases in the films. Transmission electron microscopy (TEM) provided information on grain structure, nucleation, and growth. The phase information was correlated to the ferroelectric and dielectric properties. The perovskite phase nucleated in the pyrochlore phase throughout the film thickness, and at 650 °C the transformation was complete in 15 s. Fast growing (100) PZT nucleated at the platinum and consumed a small-grained matrix until a columnar structure was obtained. A ramp rate of 100 °C/s was sufficiently fast to prevent transformation during heating and allowed the direct application of an Avrami model for transformation kinetics. An activation energy of 610 kJ/mol was determined.
Two-dimensional carrier profiling using scanning spreading resistance microscopy (SSRM) has recently been reported for Si- and InP-based structures. In this article, we report SSRM measurements solely on III–V material-based structures. We have studied GaAs and InP doping staircase structures, prepared using molecular-beam epitaxy. These structures were then used as calibration standards for the profiling of carrier density in state-of-the-art III–V-based optoelectronic devices. We discovered that SSRM data on GaAs can be obtained with either polarity; however, only one polarity (positive or negative sample bias for n- or p-GaAs, respectively) produces SSRM results that show quantitative correlation with dopant concentration as determined by secondary ion mass spectrometry (SIMS). In comparison, SSRM measurements using both bias polarities on n-InP correlates well with SIMS, while p-InP exhibits a similar polarity dependence to p-type GaAs. A physical model based on a Schottky junction is proposed to explain these results. We also report calibrated SSRM measurements on GaAs and InP heterojunction bipolar transistor structures.
Single and multiple layers of self-assembled InAs quantum dots (QDs) produced by the indium-flush technique have been studied by transmission electron microscopy (TEM) in an effort to develop techniques to reproducibly grow QDs of uniform size and shape. To monitor the changes in QD dimensions, plan-view samples of capped single layers were studied as well as cross-sectional samples of QDs in multiple layers and stacks. The changes in the observed round- and square-shaped QD images under various plan-view TEM imaging conditions, as well as the contrast reversal in the center of QD images viewed in cross-section are modeled using the many-beam Bloch-wave approach, including strain. The sizes and shapes of the QDs are determined through the interpretation of the observed (primarily strain) contrast in plan-view and the observed (primarily atomic number) contrast in cross-sectional TEM.
Self-assembled quantum dots (QDs) of highly strained InAlAs have been grown by molecular beam epitaxy in separate-confinement p–i–n heterostructures on (001) GaAs substrates. Results from a systematic study of samples with varying amounts of deposited material relates the observed emission peaks with QD levels, wetting layer states, or barrier materials. For samples with high-QD concentration, lasing is observed in the upper-QD shells. A sample with contact layers improving carrier and optical confinement operates up to room temperature and displays lowered threshold current densities. A threshold current density of ∼4 A/cm2 is measured for this structure at T=5 K and continuous-wave operation is obtained up to T∼77 K. A material gain larger than 1.7×104 cm−1 is measured for this single-layer structure. Lasing is observed in the upper-QD shells for small gain media, and progresses towards the QD lower states for longer cavity lengths representing an emission shift of 45 meV. A minor dependence of the threshold on QD density is found for samples having densities between 20 and hundreds of QDs per micron squared. For samples with multiple QD layers displaying vertical self-assembling, an increase in the emission linewidth is observed compared with single-layer samples and multilayer samples with uncorrelated growth.
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