Focal plane arrays for submillimeter waves using two-dimensional electron gas elements: A grant under the Innovative Research Program

This final report describes a three-year research effort, aimed at developing new types of THz low noise receivers, based on bulk effect ('hot electron') nonlinearities in the Two-Dimensional Electron Gas (2DEG) Medium, and the inclusion of such receivers in focal plane arrays. 2DEG hot electron mixers have been demonstrated at 35 and 94 GHz with three orders of magnitude wider bandwidth than previous hot electron mixers, which use bulk InSb. The 2DEG mixers employ a new mode of operation, which was invented during this program. Only moderate cooling is required for this mode, to temperatures in the range 20-77 K. Based on the results of this research, it is now possible to design a hot electron mixer focal plane array for the THz range, which is anticipated to have a DSB receiver noise temperature of 500-1000K. In our work on this grant, we have found similar results the the Cronin group (resident at the University of Bath, UK). Neither group has so far demonstrated heterodyne detection in this mode, however. We discovered and explored some new effects in the magnetic field mode, and these are described in the report. In particular, detection of 94 GHz and 238 GHz, respectively, by a new effect, 'Shubnikov de Haas detection', was found to be considerably stronger in our materials than the cyclotron resonance detection. All experiments utilized devices with an active 2DEG region of size of the order of 10-40 micrometers long, and 20-200 micrometers wide, formed at the heterojunction between AlGaAs and GaAs. All device fabrication was performed in-house. The materials for the devices were also grown in-house, utilizing OMCVD (Organo Metallic Chemical Vapor Deposition). In the course of this grant, we developed new techniques for growing AlGaAs/GaAs with mobilities equalling the highest values published by any laboratory. We believe that the field of hot electron mixers and detectors will grow substantially in importance in the next few years, partly as a result of the opportunity given us through this grant, which represents the major effort in the US so far. We note, however, that parallel research on hot electron mixers in thin film superconductors in Russia, and recently in Sweden, have demonstrated mixing up to 1 THz, with the potential for low-noise receivers for frequencies up to many THz. The three groups recently assessed the relative adtantages of 2DEG and superconducting film mixers in a joint paper (Kollberg et al., 1992; see Appendix II)

Epitaxial growth of highly mismatched III-V materials on (001) silicon for electronics and optoelectronics

Monolithic integration of III-V on silicon has been a scientifically appealing concept for decades. Notable progress has recently been made in this research area, fueled by significant interests of the electronics industry in high-mobility channel transistors and the booming development of silicon photonics technology. In this review article, we outline the fundamental roadblocks for the epitaxial growth of highly mismatched III-V materials, including arsenides, phosphides, and antimonides, on (001) oriented silicon substrates. Advances in hetero-epitaxy and selective-area hetero-epitaxy from micro to nano length scales are discussed. Opportunities in emerging electronics and integrated photonics are also presented

Monolithic integration of tunnel diode based inverters on exact (001) Si substrates

Monolithic integration of tunnel diode-based inverters on exact (001) Si substrates for the future high-speed, low-power, and compact digital circuits is demonstrated. A two-state inverter was fabricated using a forward biased fin-array tunnel diode as drive and a reverse-biased counterpart as load. On-chip operation and reduced fabrication complexity were achieved by exploiting the resistive characteristic of the reverse-biased tunnel diodes and the pre-defined patterns on the Si substrat

Coalescence of planar GaAs nanowires into strain-free three-dimensional crystals on exact (001) silicon

We report three dimensional (3D) disk-shaped GaAs crystals on V-groove patterned (001) Si substrates by metalorganic chemical vapor deposition. Planar GaAs nanowires with triangular cross-sections were grown inside Si V-grooves by nano-scale selective heteroepitaxy. These nanowires were then partially confined in micro-sized SiO2 cavities and coalesced into uniform arrays of 3D crystals. Scanning electron microscope and atomic force microscopy inspection showed the absence of antiphase-domains and smooth top surface morphology. Superior structural and optical properties over GaAs thin films on planar Si were also demonstrated. More remarkably, by growing the 3D crystals on V-grooved Si, we were able to overcome the residual tensile stress induced by the thermal mismatch between GaAs and Si. Strain-free GaAs was uncovered in the crystals with a dimension of 3×3 µm2

Epitaxial growth of GaSb on V-grooved Si (001) substrates with an ultrathin GaAs stress relaxing layer

We report epitaxial growth of GaSb nano-ridge structures and planar thin films on V-groove patterned Si (001) substrates by leveraging the aspect ratio trapping technique. GaSb was deposited on {111} Si facets of the V-shaped trenches using metal-organic chemical vapor deposition with a 7 nm GaAs growth initiation layer. Transmission electron microscopy analysis reveals the critical role of the GaAs layer in providing a U-shaped surface for subsequent GaSb epitaxy. A network of misfit dislocations was uncovered at the GaSb/GaAs hetero-interface. We studied the evolution of the lattice relaxation as the growth progresses from closely pitched GaSb ridges to coalesced thin films using x-ray diffraction. The omega rocking curve full-width-at-half-maximum of the resultant GaSb thin film is among the lowest values reported by molecular beam epitaxy, substantiating the effectiveness of the defect necking mechanism. These results thus present promising opportunities for the heterogeneous integration of devices based on 6.1A ° family compound semiconductor

Fin-array tunneling trigger with tunable hysteresis on (001) silicon substrate

We report the fabrication and characterization of a GaAs fin-array tunneling trigger monolithically integrated on an exact (001) silicon substrate. A Schmitt-trigger-like behavior was observed under double sweep condition by connecting the tunnel diode with an on-chip load resistor. The tunneling trigger circuit was studied using load line analysis. Critical parameters of the circuit were extracted. We found that the circuit hysteresis can be tuned by tailoring of the diode dimensions and load resistor value

Growing antiphase-domain-free GaAs thin films out of highly ordered planar nanowire arrays on exact (001) silicon

We report the use of highly ordered, dense, and regular arrays of in-plane GaAs nanowires as building blocks to produce antiphase-domain-free GaAs thin films on exact (001) silicon. High quality GaAs nanowires were grown on V-grooved Si (001) substrates using the selective aspect ratio trapping concept. The 4.1% lattice mismatch has been accommodated by the initial GaAs, a few nanometer-thick with high density stacking faults. The bulk of the GaAs wires exhibited smooth facets and a low defect density. An unusual defect trapping mechanism by a “tiara”-like structure formed by Si undercuts was discovered. As a result, we were able to grow large-area antiphase-domain- free GaAs thin films out of the nanowires without using SiO2 sidewalls for defect termination. Analysis from XRD x-rocking curves yielded full-width-at-half-maximum values of 238 and 154 arc sec from 900 to 2000 nm GaAs thin films, respectively, indicating high crystalline quality. The growth scheme in this work offers a promising path towards integrated III-V electronic, photonic, or photovoltaic devices on large scale silicon platfor

GaAs-InGaAs-GaAs fin-array tunnel diodes on (001) Si substrates with room-temperature peak-to-valley current ratio of 5.4

In this letter, we report the selective area growth of GaAs, In0.2Ga0.8As, and GaAs/In0.2Ga0.8As/GaAs quantum-well fins of 65-nm width on exactly orientated (001) Si substrates. By exploiting high aspect ratio trenches formed by patterned SiO2 on Si and a V-grooved Si (111) surface in the aspect ratio trapping process, we are able to achieve good material quality and structural properties, as evidenced by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The fabricated GaAs-In0.2Ga0.8As-GaAs fin-array tunnel diodes exhibit a maximum room-temperature peak-to-valley current ratio of 5.4, and negative differential resistance characteristics up to 200 °C

Continuous-wave lasing from InP/InGaAs nanoridges at telecommunication wavelengths

We report continuous-wave lasing from InP/InGaAs nanoridges grown on a patterned (001) Si substrate by aspect ratio trapping. Multi-InGaAs ridge quantum wells inside InP nanoridges are designed as active gain materials for emission in the 1500 nm band. The good crystalline quality and optical property of the InGaAs quantum wells are attested by transmission electron microscopy and microphotoluminescence measurements. After transfer of the InP/InGaAs nanoridges onto a SiO2/Si substrate, amplified Fabry-Perot resonant modes at room temperature and multi-mode lasing behavior in the 1400 nm band under continuous-wave optical pumping at 4.5 K are observed. This result thus marks an important step towards integrating InP/InGaAs nanolasers directly grown on microelectronic standard (001) Si substrates. Semiconductor nanowires are emerging as ideal building blocks for ultra-compact optoelectronic devices with low-energy dissipation.1 As a result of axially guided optical modes and feedback provided by end-facets, lasing behaviors have been observed in various II-VI and III-V compound semiconductor nanostructures.2–16 In particular, indium phosphide (InP) and indium gallium arsenide (InGaAs) nanolasers, emitting at silicon(Si)-transparent wavelengths, show great promise to fill a key missing on-chip component in Si photonic-based optical interconnects.17–21 However, most of the previously demonstrated InP/InGaAs nanolasers operate under pulsed-conditions.22–24 Continuous-wave (CW) lasing at telecom wavelengths has only been achieved in InP/InGaAs nanopillars grown on (111) Si substrates25 and InAsP/InP nanowires (inside Si photonic crystal cavity) grown on (111)B InP substrates, with lasing wavelengths situated at the 1200 and 1300 nm bands.26 Extending the lasing wavelengths to the 1400 nm and 1500 nm bands is desirable for high density inter/intra-chip data transmission. In this letter, we utilized InP/InGaAs nanoridges grown on a (001) Si substrate to demonstrate CW lasing behavior at the 1400 nm band. Compared with other hetero-epitaxial growth techniques, selective area growth combined with the aspect ratio trapping (ART) method provides a viable route to form well-aligned, millimeter-long horizontal in-plane nanowires on CMOS-standard (001) Si substrates.27–34 Previously, we have leveraged this approach to grow InP nanoridges with embedded InGaAs quantum wells (QWs) and quasi-quantum wires (QWRs) with strong photolumiescence.35,36 Here, we observe CW lasing at the telecommunication band from high quality multi-InGaAs ridge QWs inside the InP nanoridges directly grown on nanopatterned silicon. To explore the potential of the InP/InGaAs nanoridges as nanoscale light sources, we separated the InP/InGaAs nanoridges from the initial patterned Si substrate and transferred them onto a SiO2/Si substrate for optical characterization. We observed CW lasing at 4.5 K under optical excitation and strong optical mode modulation at room temperature. The InP/InGaAs nanoridges used in this experiment were grown on (001) Si substrates using a metal-organic chemical vapor deposition (MOCVD) system with a horizontal reactor (AIXTRON 200/4). [110] direction oriented SiO2 stripe patterns with a line pitch of 1 μm and a trench opening width of 450 nm were used to define the growth regions. Detailed sample preparation and the growth procedure have been reported elsewhere.35,36 Figure 1(a) presents the top-view scanning electron microscopy (SEM) image of the as-grown sample, showing a uniform morphology across a large area. The 70° tilted-view SEM image in Fig. 1(b) reveals symmetrical {111} faceting. A zoomed-in SEM image in Fig. 1(c) highlights the multi-QW active region. Notably, to enhance contrast, the InGaAs layers were selectively etched in a H2PO4:H2O2:H2O (3:1:50) solution. Five uniform InGaAs ridge QWs and the GaAs nucleation buffer are clearly identified