Session A67: Characterization and Imaging of Defects

Understanding recombination-enhanced dislocation motion for reliable III-V on Si lasers

Dislocations are an important class of defects in semiconductors that are key to the epitaxial integration of heterostructures of dissimilar materials in consideration for new types of electronic and photonic circuits. Fundamentally, dislocations in semiconductors affect both mechanical and electronic properties and this leads to well-known coupled phenomena: equilibrium processes such as electronic doping impacting mechanical properties and out-of-equilibrium processes such as non-radiative minority carrier recombination enhancing dislocation motion under otherwise brittle conditions. The latter has been implicated in the accelerated degradation of GaAs-based lasers on Si, where even record low dislocation densities through substrate engineering fail in yielding commercially viable devices for the burgeoning field of silicon photonics as the dislocations increase in length over time during device operation.

In this work, we use new microscopy tools to describe the salient features and driving forces behind recombination-enhanced dislocation glide and climb in GaAs on Si lasers, with an emphasis on InAs quantum dot devices. Our recent experiments harness the mechanical properties of indium to counter III-V and Si thermal expansion mismatch strain and pin the dislocations against glide, yielding record-lifetime lasers on silicon that are now comparable to devices on native substrates. Although the driving force behind dislocation climb remains under investigation, we describe the nature of interaction of point defects with dislocations and the benefits of quantum dots in mediating this. An updated understanding of recombination-enhanced dislocation glide and climb may eventually result in viable integrated lasers in a range of materials systems.      

Nanoscale imaging and spectroscopy of charge carrier distribution in doped Si nanowires with terahertz and mid infrared near-field nanoscopy

We perform terahertz (THz) and mid infrared nanoscopy to probe and quantify charge carriers in doped Si nanowire surfaces at the nanoscale. We implement hyperspectral THz nano-imaging by combining scattering-type scanning near-field optical microscopy (s-SNOM) with THz time-domain spectroscopy (TDS). Combination of nanoscale spectroscopy and Drude model allows for measuring—noninvasively and without the need for Ohmic contacts—the local mobile carrier concentration of the differently doped semiconductor areas.  "   

Frequency-Modulated Charge Pumping of Single-Defects in MOSFETs with Ultra-Thin Gate Dielectrics

The detection of single defects has previously been achieved utilizing charge pumping (CP) measurements in sub-micron MOSFETs.^1,2 However, these prior efforts utilized MOSFETs with thick gate dielectrics, thereby reducing the effect of quantum tunneling currents through the gate oxide, which would obscure the CP response in devices with ultra-thin gate dielectrics. Frequency-modulated CP (FMCP) has been recently developed to overcome gate tunneling currents so that CP in ultra-scaled MOSFET devices may be achieved.³ This technique relies on the frequency independent and dependent nature of tunneling currents and the CP response, respectively. FMCP effectively and easily modulates the CP response such that gate tunneling currents are omitted. In this work, we extend FMCP to detect single traps in ultra-scaled sub-micron MOSFETs, thus extending single trap CP measurements to more modern MOSFET technologies. Our results suggest that single-trap FMCP could serve as a foundation for the development of novel quantum devices and quantum metrology.

¹G. V. Groeseneken et al., IEEE Transactions on Electron Devices, 43, 940-945 June (1996)

²N.S. Saks et al., Appl. Phys. Lett. 68, 1383 (1996)

³J. T. Ryan et al., IEEE Electron Device Letters, 1-1 (2013)   

Frequency Multiplexed simultaneous Hall effect and resistivity transient measurements for van der Pauw samples

Simultaneous measurement of different contact configurations using frequency multiplexing enables continuous full transport characterization of samples under time-varying conditions.

Non-switching van der Pauw technique that was demonstrated in 1998 enabled simultaneous acquisition of both van der Pauw configurations and hence measuring of the sheet resistance and the level of anisotropy in an arbitrarily shaped sample. The method introduced here goes one step further and makes use of two different frequencies and three lock-in amplifiers to measure not only the sheet resistance, but also two conjugate transverse configurations. Due to Onsager – Casimir reciprocity relations, conjugate transverse information allows accurate calculation of Hall slope at any fixed magnetic field. Since mobility and the carrier concentration can be derived from these measurements each quantity can be analyzed separately and to distinguish carrier density transients from mobility transients. This method could also be utilized to examine transport transients due to any other time resolvable variables such as but not limited to gate biasing, temperature, and pressure variations.   

Analyzing Buried Defects in Vanadium Oxide with Bragg Coherent Diffractive Imaging

Metal-insulator transitions accompanied by structural phase transitions in vanadium oxides have attracted considerable scientific interest for applications in photonics, sensing, and IR light modulation. Many stoichiometries have been explored, including V₂O₃, VO₂, V₂O₅, and V₃O₅¹. While the precise mechanism of the transition is controversial, it is influenced by a number of properties like crystal strain, morphology, and electron correlations². Therefore, further study of nanoscale strain and crystal defects in these crystals is pivotal to advance understanding of these transitions.

Past studies of defects in vanadium oxides have been limited to surface techniques or destructive characterization. These fail to give volumetric information, masking the presence of buried defects in as-grown crystals. In this work we use Bragg Coherent Diffractive Imaging (BCDI) to analyze in 3 dimensions a V₂O₃ particle with nanoscale resolution and picometer strain sensitivity. We identify buried defects in the crystal and analyze the role these defects play in particle morphology. Our results emphasize the role defects play in the growth of 2D flake-like V₂O₃ particles.

¹ C. Piccirillo et al, Chemical Vapor Deposition 13, 145 (2007)

² A. D’Elia et al, Applied Surface Science 540, 148341 (2021)   

Probing N- and Bi-related states in GaAsNBi/GaAs heterostructures

Due to the complementary impacts of N and Bi on the GaAs band structure, N-Bi co-alloying to form GaAsNBi provides significant opportunities for band structure engineering. It has been predicted that N (Bi) incorporation primarily impacts the conduction (valence) band of GaAs, while introducing tensile (compressive) in-plane lattice strain with respect to a GaAs substrate. To explore the N- and Bi-related states near the conduction and valence band edges, we are examining the electronic properties of diodes containing  n-GaAs/GaAs(N)(Bi)/n-GaAs quantum wells (QWs) with various N and Bi fractions. Using capacitance-voltage (C-V) measurements interpreted within the depletion approximation, we profile the free carrier concentrations across the QWs.  To quantify the conduction band offsets (CBO), we compare the C-V-determined carrier concentration profiles with those computed using 1D  Schrödinger-Poisson simulations in the effective mass approximation.  Given the mole fractions of N and Bi from x-ray rocking curves and Rutherford backscattering spectrometry, we then associate CBO values with specific alloy compositions. In addition to discussing the composition dependence of the band offsets, we will discuss our progress towards probing Bi-related states.   

Probing Non-Stoichiometry in GaAsBi and GaAsNBi Alloys Using Local-Electrode Atom Probe Tomography

Due to the significant bandgap narrowing induced by incorporation of dilute fractions of N and Bi, dilute nitride-bismide alloys are of interest for optoelectronic devices operating in the near- to mid-infrared range. We recently reported that Bi incorporation into GaAsNBi alloys during molecular-beam epitaxy requires low substrate temperatures which also promote the incorporation of excess As. To minimize excess As incorporation, we determined the substrate temperature needed to eliminate the non-stoichiometric GaAs diffraction peak in x-ray rocking curves (XRC). The resulting [excess As] is below the XRC-detectable limit of ~10¹⁹ cm^-3. To explore excess As incorporation and alloy non-stoichiometry below this limit, we are utilizing local-electrode atom-probe (LEAP) tomography, with LEAP field evaporation conditions optimized for the nearest-stoichiometry of GaAs. Our LEAP data analysis reveals similar V/III ratios for GaAs, GaAsBi, and GaAsNBi, in spite of differences in their effective permittivities that yield variations in field evaporation during LEAP tomography. We discuss the role of arsenic ion species assignments in the LEAP data analysis on apparent stoichiometries reported for a variety of mixed anion and mixed cation III-V semiconductor alloys.   

Imaging reconfigurable molecular concentration on a graphene field-effect transistor

The spatial arrangement of adsorbates deposited onto a clean surface in vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors exhibit reversible, electrically-tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. The gate-tunable molecular concentration can be explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. Molecular surface concentration in this case is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.    

An all optical approach for comprehensive in-operando analysis of radiative and nonradiative recombination processes - deep level optical spectroscopy

We have developed an all optical approach, combining PL and Raman, that can yield practically all of the quantitative information that is pertinent to the carrier recombination dynamics via both radiative and nonradiative processes when deep defect levels are present, including internal quantum efficiency (IQE), minority and majority carrier density, inter-band radiative recombination rate, minority carrier nonradiative recombination rate, defect center occupation fraction, defect center density, and minority and majority carrier capture cross-sections. While some of this information is thought to be obtainable optically, such as IQE, most of the other parameters are generally considered to be attainable only through electrical techniques, such as I-V characteristics and DLTS. This approach uses a band-defect state coupling model that explicitly treats the inter-band radiative recombination and SRH recombination via deep defect states on an equal footing for any defect center occupation fraction. It has been successfully applied to GaAs double-heterostructures that exhibit distinctly different nonradiative recombination characteristics. The all-optical method facilitates comprehensive material and device characterization without the need for any device processing.   

Atomically Defined Wires on P-Type Silicon

Dangling bonds (DBs) on a hydrogen terminated Si(100)-2x1 surface are silicon atoms unbound to hydrogen atoms. They are point defects with electronic states in the band gap. Placing DBs at strategic locations on the surface enables them to be used as fundamental components for atom-defined electronics. In this work, we use low temperature scanning tunneling microscopy techniques to create patterns on boron-doped silicon. We find that wire structures consisting of DB pairs exhibit 1D quantum wells within the band gap. Combining these wires with local point structures make them candidates for electronic control at the atomic scale. The wires exhibit relative immunity to environmental degradation. The natural reactive and thermal robustness of the dangling bond lines combined with vacuum encapsulation is expected to result in very long lived circuitry. Synthesis and characterization of wires and wires adjacent to other structures will be shown.   

Measurement of Si Surface Conduction by Two-probe Scanning Tunneling Microscopy with Ohmic Contact

The miniaturization of electronic devices requires the characterization of nano- and atomic-scale structures. Imaging, fabricating, and measuring of local electronic properties of these structures can be performed with a one-probe scanning tunneling microscope (1P-STM). Yet, assessing the electrical conduction properties lateral to the surface demands two-probe (2P-) and four-probe (4P-) STM. The 4P-STM conductance measurement is generally preferred over the 2P-STM one as it can eliminate tip-to-sample contact resistance and Schottky barrier in semiconductor samples by separating two current-injecting tips from two voltage-measuring tips. However, placing multiple probes into a nano-scale region is an arduous task. Here, we describe a refinement of a 2P-STM by eliminating the issue of the Schottky barrier. To achieve that, we employed field evaporation to clean tip apices and made Ohmic probes that produce linear IV curves on Si surfaces. This Ohmic 2P-STM method allows surface conductance measurement at very low bias, limiting conduction through the space-charge layer and bulk states, with the minimum number of movable probes. Using STM, we also fabricated nano-scale Si structures with insulating trench lines and confirmed the two-dimensional conduction on the surface by measuring resistance.   

Charge state characterization of dangling bond circuitry on hydrogen passivated silicon

With current CMOS technologies approaching their performance limits, nanoscale atomic electronics are poised to provide the next-generation of devices and a continuation of Moore's law. Several promising beyond-CMOS platforms, such as dangling bond (DB) circuitry on hydrogen-passivated silicon require precise knowledge of the location of charges within fabricated atomic structures. To achieve this, atomic force microscopy (AFM) measurements are used to determine the electron population of specific dangling bonds. A key consideration for improving this ability is to ensure that the proximity of the AFM probe does not perturb the ground state charge occupation of circuit elements under investigation. Here, we directly compare AFM measurements with a minimally-perturbative scanning tunneling microscope charge sensing scheme to better establish the optimal parameter space for ground state charge measurements. To achieve this, a DB wire was sequentially legthened near a sensor DB, which allowed for the electronic detection of nearby changes of charge with single electron sensitivity. The same structures were also then investigated and compared with standard AFM characterization techniques and these results will be used to improve the charge characterization of DB structures.   

Atomic-scale study of Si-doped AlAs by cross-sectional scanning tunneling microscopy and density functional theory

Silicon (Si) donors in GaAs have been the topic of extensive studies since Si is the most common and

well understood n-type dopant in III-V semiconductor devices and substrates. The indirect band gap of AlAs

compared to the direct one of GaAs leads to interesting effects when introducing Si dopants. Here we present

a study of cross-sectional scanning tunneling microscopy and density functional theory (DFT) calculations to

study Si donors in AlAs at the atomic scale. Based on their crystal symmetry and contrast strengths, we identify

Si donors up to four layers below the (110) surface of AlAs. Interestingly, their short-range local density of states

(LDOS) is very similar to Si atoms in the (110) surface of GaAs. Additionally, we show high-resolution images

of Si donors in all these layers. For empty state imaging, the experimental and simulated STM images based on

DFT show excellent agreement for Si donors up to two layers below the surface.