Julia I. Deitz

Tunnel junction enhanced nanowire ultraviolet light emitting diodes

Polarization engineered interband tunnel junctions (TJs) are integrated in nanowire ultraviolet (UV) light emitting diodes (LEDs). A ∼6 V reduction in turn-on voltage is achieved by the integration of tunnel junction at the base of polarization doped nanowire UV LEDs. Moreover, efficient hole injection into the nanowire LEDs leads to suppressed efficiency droop in TJ integrated nanowire LEDs. The combination of both reduced bias voltage and increased hole injection increases the wall plug efficiency in these devices. More than 100 μW of UV emission at ∼310 nm is measured with external quantum efficiency in the range of 4–6 m%. The realization of tunnel junction within the nanowire LEDs opens a pathway towards the monolithic integration of cascaded multi-junction nanowire LEDs on silicon.

Rapid misfit dislocation characterization in heteroepitaxial III-V/Si thin films by electron channeling contrast imaging

Electron channeling contrast imaging (ECCI) is used to characterize misfit dislocations in heteroepitaxial layers of GaP grown on Si(100) substrates. Electron channeling patterns serve as a guide to tilt and rotate sample orientation so that imaging can occur under specific diffraction conditions. This leads to the selective contrast of misfit dislocations depending on imaging conditions, confirmed by dynamical simulations, similar to using standard invisibility criteria in transmission electron microscopy (TEM). The onset and evolution of misfit dislocations in GaP films with varying thicknesses (30 to 250 nm) are studied. This application simultaneously reveals interesting information about misfit dislocations in GaP/Si layers and demonstrates a specific measurement for which ECCI is preferable versus traditional plan-view TEM.

Applications of Electron Channeling Contrast Imaging for the Rapid Characterization of Extended Defects in III–V/Si Heterostructures

Electron channeling contrast imaging (ECCI) is a nondestructive diffraction-based scanning electron microscopy (SEM) technique that can provide microstructural analysis similar to transmission electron microscopy (TEM). However, because ECCI is performed within an SEM and requires little to no sample preparation, such analysis can be accomplished in a fraction of the time. Like TEM, ECCI can be used to image a variety of extended defects and enables the use of standard invisibility criteria to provide further defect characterization (e.g., Burgers vector determination). Here, we use ECCI to characterize various extended defects, including threading dislocations, misfit dislocations, and stacking faults, in heteroepitaxial GaP/Si(1 0 0) samples. We also present applications for which ECCI is particularly well suited compared with conventional methods. First, misfit dislocations are surveyed via ECCI across the radius of a 4-in GaP/Si wafer, yielding a proof-of-concept rapid (~3 h) approach to large-area defect characterization. Second, by simply wet etching away a portion of a thick epitaxial GaP-on-Si layer, we use ECCI to image specific targeted interfaces within a heterostructure. Both of these applications are prime examples of how ECCI is a compelling alternative to TEM in circumstances where the required sample preparation would be prohibitively time-consuming or difficult.

Characterization of encapsulated quantum dots via electron channeling contrast imaging

A method for characterization of encapsulated epitaxial quantum dots (QD) in plan-view geometry using electron channeling contrast imaging (ECCI) is presented. The efficacy of the method, which requires minimal sample preparation, is demonstrated with proof-of-concept data from encapsulated (sub-surface) epitaxial InAs QDs within a GaAs matrix. Imaging of the QDs under multiple diffraction conditions is presented, establishing that ECCI can provide effectively identical visualization capabilities as conventional two-beam transmission electron microscopy. This method facilitates rapid, non-destructive characterization of sub-surface QDs giving immediate access to valuable nanostructural information.

Electrochemical Liquid Phase Epitaxy (ec-LPE): A New Methodology for the Synthesis of Crystalline Group IV Semiconductor Epifilms

Deposition of epitaxial germanium (Ge) thin films on silicon (Si) wafers has been achieved over large areas with aqueous feedstock solutions using electrochemical liquid phase epitaxy (ec-LPE) at low temperatures (T ≤ 90 °C). The ec-LPE method uniquely blends the simplicity and control of traditional electrodeposition with the material quality of melt growth. A new electrochemical cell design based on the compression of a liquid metal electrode into a thin cavity that enables ec-LPE is described. The epitaxial nature, low strain character, and crystallographic defect content of the resultant solid Ge films were analyzed by electron backscatter diffraction, scanning transmission electron microscopy, high resolution X-ray diffraction, and electron channeling contrast imaging. The results here show the first step toward a manufacturing infrastructure for traditional crystalline inorganic semiconductor epifilms that does not require high temperature, gaseous precursors, or complex apparatus.

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities - high spatial resolution and richness of microstructural data - with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

Accessing High Spatial Resolution Low-Loss EELS Information without Cerenkov Radiation

Advances in monochromation for scanning transmission electron microscopy (STEM) uniquely position electron energy-loss spectroscopy (EELS) to be the only analytical technique that can provide atomically-resolved chemical and electronic structure information. These advances open the potential for the detailed investigation of bandgaps and defect levels using the low energy-loss region of the spectrum, which is critical for continued progress in the development of advanced semiconductor materials and structures. However, Čerenkov radiation in the low energy-loss region has thus far limited the accuracy of such measurements. In this contribution, we report on EELS investigations carried out using an accelerating voltage of 40 kV in monochromated STEM, allowing for the determination of electronic structure information for semiconductors with high spatial resolution.

Site-Specific TEM Specimen Preparation of Samples with Sub-Surface Features

Characterization of crystalline defects has traditionally been performed in the TEM, as it has the required capabilities in terms of sensitivity and resolution to ensure sufficient accuracy. However, TEM analysis requires electron transparency, which requires preparation of thin specimens, typically achieved using focus ion beam (FIB)-based foil extraction and thinning. For specimens with features that cannot be seen on the surface, samples extracted via FIB are effectively selected “blindly.” That is, the area from where the sample is extracted is, for the most part, randomly selected, and there is little or no prior knowledge as to what features will be included in the small, and extremely thin, region (approximately 10 μm × 5 μm × 100 nm). Such a small sample can provide a poor statistical representation of the material as a whole, and it does not allow for a desired feature to be obtained in an efficient manner. Here, we develop a method to locate sub-surface features using electron channeling contrast imaging (ECCI) in order to extract site-specific TEM specimens using the dual beam focused ion beam (FIB) instrument.

Investigation of digital alloyed AlInSb metamorphic buffers

Al1-xInxSb metamorphic step-graded buffers with Al0.6In0.4Sb terminal layers, designed to serve as a virtual substrate to support integrated InAs0.5Sb0.5 long-wave infrared absorber layers, were grown on GaSb wafers via molecular beam epitaxy. Two different structural profiles were used to define the effective composition of each buffer step: one based on digital alloys (1 nm period, ∼1.6 unit cells) and the other based on short period superlattices (10 nm period, ∼16 unit cells). Characterization via optical Nomarski microscopy, x-ray diffraction reciprocal space mapping, and transmission electron microscopy indicates that the digital alloy based structure behaves similar to that expected for a conventional bulk ternary alloy based structure, while the short period superlattice structure exhibits significantly hindered relaxation within the buffer layers.

Spatial correlation of the EC-0.57 eV trap state with edge dislocations in epitaxial n-type gallium nitride

Defects in semiconductors lead to deleterious effects in electron devices, but identifying their physical sources can be difficult. An example of this in gallium nitride (GaN) high electron mobility transistors is the well-known trap state located at approximately EC-0.57 eV. This trap is strongly correlated with output power degradation and reliability issues, but despite two decades of study, its specific physical source is still unknown. To address this long-standing question, two complementary nm-resolution characterization techniques—scanning probe deep level transient spectroscopy (SP-DLTS) and electron channeling contrast imaging (ECCI)—were used to spatially map the lateral distribution of these traps and to image and characterize their relation to residual threading dislocations within NH3-MBE-grown n-type GaN. Direct comparison of the SP-DLTS and ECCI measurements on the same sample region reveals highly localized concentrations of EC-0.57 eV traps that are spatially correlated with pure edge ty...