Víctor Hugo Méndez-García

Study of InAlAs/InGaAs self-switching diodes for energy harvesting applications

In order to improve the rectification efficiency and current–voltage characteristics of self-switching diodes (SSD) the DC response is analyzed using technology computer aided design (TCAD). It is demonstrated that by varying geometrical parameters of L- and V-shaped SSDs or changing the dielectric permittivity of the trenches, a near zero threshold voltage can obtained, which is essential for energy harvesting applications. The carrier distribution inside the nanochannel is successfully simulated in two-dimensional mode for zero-, reverse-, and forward-bias conditions. This process allows for the evaluation of the effect of the lateral surface-charge on the formation and spatial distribution of the depletion region, in addition to, obtaining information on the physics of the SSD through the propose optimized geometries that were designed for tailoring and matching the desired frequencies of operation. The numerical results showed some insights for the improvement of the rectification efficiency and integration density using parallel SSD arrays.

Effect of surface states on the electrical properties of MBE grown modulation doped AlGaAs/GaAs

The influence of near surface structure termination and surface treatments on the surface electric fields and mobility of modulation doped AlGaAs/GaAs heterostructures (MDH) were investigated. The built-in and surface electric fields were evaluated by photoreflectance spectroscopy, and these values were utilized to simulate the conduction band bending of the MDH. When the capping layer of the MDH was changed, both the built in internal electric field and the surface electric field are decreased, while the electron mobility of the samples is increased. After passivated the surface samples with Si, the surface electric fields were also reduced. Finally, a (NH4)2Sx-based treatment of the surface was applied, the surface electric field is annulled, and the conduction band modeling showed an important redistribution of carriers in the films. The electron mobility of the passivated samples does not show any change, neither the internal electric fields, corroborating the close relationship that exist between the...

Determination of the depletion layer width and effects on the formation of double-2DEG in AlGaAs/GaAs heterostructures

In this work, the influence of the surface depletion layer on the formation of a two-dimensional electron gas in AlGaAs/GaAs modulated doped heterostructures is studied. The authors explore a method for estimating the depletion region inside of the GaAs-based heterostructures by using the longitudinal optical and L- amplitude modes observed in Raman spectra, which are supported by the modeling results. The authors found that the position of the topmost doping layer changes the electron distribution in the heterostructure and decreases the influence of the depletion layer. Similar effects are perceived when an optimized solution of (NH4)2SX and isopropanol is employed. The authors present a method to evaluate the formation of a double two-dimensional electron gas in a heterostructure by the adequate use of modulation line in the photoreflectance spectroscopy.

Terahertz harvesting with shape-optimized InAlAs/InGaAs self-switching nanodiodes

In this letter, self-switching nanochannels have been proposed as an enabling technology for energy gathering in the terahertz (THz) regime. Such devices combine their diode-like behavior and high-speed of operation in order to generate DC electrical power from high-frequency signals. By using finite-element simulations, we have improved the sensitivity of L-shaped and V-shaped nanochannels based on InAlAs/InGaAs samples. Since those devices combine geometrical effects with their rectifying properties at zero-bias, we have improved their performance by optimizing their shape. Results show nominal sensitivities at zero-bias in the order of 40 V−1 and 20 V−1, attractive values for harvesting applications with square-law rectifiers.

Structural characterization of AlGaAs:Si/GaAs (631) heterostructures as a function of As pressure

AlGaAs:Si/GaAs heterostructures were grown on (631) and (100) GaAs substrates and studied as a function of the As cell beam equivalent pressure. High-resolution x-ray diffraction patterns showed that the highest quality AlGaAs epitaxial layers were grown at PAs = 1.9 × 10−5 for (100)- and PAs = 4 × 10−5 mbar for (631)-oriented substrates. Raman spectroscopy revealed higher crystalline quality for films grown on (631) oriented substrates. The GaAs- and AlAs-like modes of the AlGaAs(631) films exhibited increased intensity ratios between the transverse optical phonons and longitudinal optical phonons with increasing PAs, whereas the ratios were decreased for the (100) plane. This is in agreement with the selection rules for (631) and high-resolution x-ray diffraction observations. Anisotropy and surface corrugation of the AlGaAs(631) films also were characterized using atomic force microscopy and Raman spectroscopy.

Optical characterization of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under As2 and As4 fluxes

The authors have investigated the optical properties of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under different experimental conditions, mainly by changing the molecular species of the As beam stemming from a cracker cell at different temperatures. By low-temperature photoluminescence (PL) spectroscopy, they observed that using As2 instead of As4, the crystal quality of the ternary alloy improves significantly since narrower lines in the near-band-gap energy of AlGaAs were obtained when the temperature of the As-cracker zone increased. The experimental position of the energy of the ternary band gap transition as a function of temperature fits the Varshni and Vina functional forms satisfactorily, while it is difficult to assess the data for those samples grown employing As4. It was found that the As2 molecular beam reduces the creation of donor acceptor levels in AlGaAs, as compared with As4. The PL spectra line shape in the GaAs band gap region shows lines associated with band-to-band tran...

Efficiency of broadband terahertz rectennas based on self-switching nanodiodes

Abstract. The authors investigate the efficiency of a series of broadband rectennas designed to harvest the free-propagating electromagnetic energy at terahertz frequencies. We analyze by simulations the case of self-complementary square- and Archimedean-spiral antennas coupled to L-shaped self-switching diodes (L-SSDs). First, the geometry (i.e., the width and length of the channel) of the L-SSD was optimized to obtain a remarkable diode-like I–V response. Subsequently, the optimized L-SSD geometry was coupled to both types of spiral antennas and their characteristic impedance was studied. Finally, the energy conversion efficiency was evaluated for both rectenna architectures.

Photoreflectance and Raman Study of Surface Electric States on AlGaAs/GaAs Heterostructures

Photoreflectance (PR) and Raman are two very useful spectroscopy techniques that usually are used to know the surface electronic states in GaAs-based semiconductor devices. However, although they are exceptional tools there are few reports where both techniques were used in these kinds of devices. In this work, the surface electronic states on AlGaAs/GaAs heterostructures were studied in order to identify the effect of factors like laser penetration depth, cap layer thickness, and surface passivation over PR and Raman spectra. PR measurements were performed alternately with two lasers (532 nm and 375 nm wavelength) as the modulation sources in order to identify internal and surface features. The surface electric field calculated by PR analysis decreased whereas the GaAs cap layer thickness increased, in good agreement with a similar behavior observed in Raman measurements ( ratio). When the heterostructures were treated by Si-flux, these techniques showed contrary behaviors. PR analysis revealed a diminution in the surface electric field due to a passivation process whereas the ratio did not present the same behavior because it was dominated by the depletion layers width (cap layer thickness) and the laser penetration depth.

Effects of growth temperature on the incorporation of nitrogen in GaNAs layers

Ternary III-N-V semiconductor alloys are interesting and complex materials. GaNAs is one such material that has been studied extensively; however, the accurate determination of the N content within this material in which the growth conditions significantly increases the amount of interstitial N has not yet been reported. To address this problem, GaNAs layers (100 nm) were prepared using molecular beam epitaxy at temperatures between 400 and 600 °C with a high nominal N concentration (3%). The N content was determined using high resolution x-ray diffraction (HRXRD), secondary ion mass spectrometry (SIMS), and low-temperature photoluminescence (PL). The N concentration determined using these techniques was compared. Additionally, the relationship between the growth temperature and N concentration is discussed. The incorporation of N into interstitial sites resulted in significant variations in the N content as estimated by SIMS, HRXRD, and PL.

Polarization and Excitation Dependence of Photoluminescence of InAs Quantum Wires and Dots Grown on GaAs(631)

We studied the optical properties of self-assembled InAs nanostructures grown by molecular beam epitaxy on GaAs substrates with (631) crystallographic orientation by photoluminescence (PL) spectroscopy. The growth of InAs proceeded on GaAs(631) buffer layers conformed by corrugated surfaces with a high degree of lateral periodicity (uniformly spaced nano grooves). The grooved surface is sustained even after the growth of InAs with thicknesses below 2 monolayers (ML), indicating the formation of InAs quantum wires. The one-dimensional confinement is corroborated by PL polarized along the [113] and [8,19,9] orthogonal directions. The calculated polarization degree, Πd, was 0.13 and 0.125 for InAs thicknesses of 1 and 1.5 ML, respectively. As the InAs thickness is increased, the InAs film reaches the critical thickness and quantum dots aligned along the grooved surface are formed, as observed by atomic force microscopy. Excitation-power-dependent luminescence is observed to a larger extent for low-energy PL lines in addition to a blue shift related to the band-filling effect.