Search

InAs quantum dots (QDs) embedded into a waveguiding GaAs semiconductor matrix may produce scintillation detectors with exceptional speed and yield, making them valuable for nuclear security, medical imaging, and high-energy physics applications. In this work, we developed thick (~ 25 μm) epitaxial heterostructures with high luminescence efficiency composed of self-assembled nano-engineered InAs QDs grown by molecular beam epitaxy. The bulk GaAs acts as a stopping material for incident particles and as a waveguide when layer-transferred onto a low-index substrate. Waveguiding and self-absorption (

Integration of body-diode-based back-gate control in AlGaN/GaN high-electron mobility transistors (HEMTs) was recently demonstrated by the authors to enable dynamic control of the device characteristics. Here, we present an experimental study of drain-voltage-induced secondary effects in AlGaN/GaN HEMTs with integrated body-diode. Both the three-terminal and four-terminal device characteristics are studied to understand the secondary effects in this configuration. The results show that the voltage at the drain terminal plays a crucial role in biasing the body-diode under dynamic operation. Drain-voltage-induced dynamic biasing of the body-diode is observed both under the biased and floating back-gate conditions. The kink-effect in the saturation channel current and light emission at the source terminal are observed at higher drain voltages. Our results show that a combination of high resistivity of the p-GaN layer and the direct bandgap nature of III-nitride (III-N) material prevents the parasitic bipolar junction transistor (BJT)-triggered latch-up mechanism in this device structure.

Among various phase-change memory materials (PCMs), Ge 2 Sb 2 Te 5 (GST) is an outstanding representative, widely used in both optical storage and electronic memories [1] . The intrinsic drawbacks of this GST, however, such as tellurium volatility and low amorphous phase stability (resistance retention), hinder it from being a perfect candidate. Tellurium-free antimony-based PCMs have been the subject of considerable recent interest, due to their excellent resistance contrast, rapid crystallization, and high amorphous phase stability [2] . Moreover, with increasing demand for high-capacity memory in consumer electronics, the multibit high-density storage capability of various memory devices has become very attractive [3] – [5] . Here, a bilayer Ga-Sb stack structure is demonstrated, showing multilevel switching properties. The origin of multilevel states and their stability are also studied.

Material properties of Ga–Sb binary alloy thin films deposited under ultra-high vacuum conditions were studied for analog phase change memory (PCM) applications. Crystallization of this alloy was shown to occur in the temperature range of 180–264 °C, with activation energy >2.5 eV depending on the composition. X-ray diffraction (XRD) studies showed phase separation upon crystallization into two phases, Ga-doped A7 antimony and cubic zinc-blende GaSb. Synchrotron in situ XRD analysis revealed that crystallization into the A7 phase is accompanied by Ga out-diffusion from the grains. X-ray absorption fine structure studies of the local structure of these alloys demonstrated a bond length decrease with a stable coordination number of 4 upon amorphous-to-crystalline phase transformation. Mushroom cell structures built with Ga–Sb alloys on ø110 nm TiN heater show a phase change material resistance switching behavior with resistance ratio >100 under electrical pulse measurements. TEM and Energy Dispersive Spectroscopy (EDS) studies of the Ga–Sb cells after ∼100 switching cycles revealed that partial SET or intermediate resistance states are attained by the variation of the grain size of the material as well as the Ga content in the A7 phase. A mechanism for a reversible composition control is proposed for analog cell performance. These results indicate that Te-free Ga–Sb binary alloys are potential candidates for analog PCM applications.

Monolithic integration of a laser diode with a modulator usually requires complicated and potentially defect-generating processing steps such as epitaxial regrowth or bonding. Alternatively, the same medium grown in a single epitaxial run can be used in both gain and electro-absorption section. We demonstrate a universal active medium with tunnel coupled InGaAs short period superlattice (SPSL) quantum well grown on InAs quantum dots (QW-on-QDs) in an AlGaAs waveguide. In this configuration, the QW serves as a tunnel injector of electrons into the QDs in the forward-biased gain section, while in the reverse-biased modulator section the InGaAs QW acts as a wavelength matched quantum Stark effect electro-absorber. The optimum QW and QD ground state separation is close to 30meV for efficient tunnel injection into QDs to increase the gain. The 3x(QW-on-QDs) double heterostructure with separate 0.3μm waveguide was grown by molecular beam epitaxy on n-type GaAs substrate. The device was processed into two-section dies with an intermediate distributed Bragg reflector (DBR) between the sections. Focused ion beam air-gap DBR fabrication with low electrical leakage and required (~30%) optical mode reflection was employed. The laser-modulator performance with slope efficiency over 50%, 6dB power modulation, linear dynamic range>24dB at 2GHz was shown with a 200μm long modulator section and low optical feedback from the modulator to laser.

We investigated structural and emission properties of self-organized InP/GaInP quantum dots (QD) grown by metal organic chemical vapor deposition using an amount of deposited In from 7 to 2 monolayers (ML). In the uncapped samples, using atomic force microscopy (AFM), we observed lateral sizes of 100–200 nm, together with a bimodal height distribution having maxima at ∼5 and ∼15 nm, which we denoted as QDs of type A and B, respectively; and reduction of the density of the type-B dots from 4.4 to 1.6 μm–2. The reduction of the density of B-type dots were observed also using transmission electron microscopy of the capped samples. Using single dot low-temperature photoluminescence (PL) spectroscopy we demonstrated effects of Wigner localization for the electrons accumulated in these dots.

Material properties of Al-Sb binary alloy thin films deposited under ultra-high vacuum conditions were studied for multi-level phase change memory applications. Crystallization of this alloy was shown to occur in the temperature range of 180 °C–280 °C, with activation energy >2 eV. X-ray diffraction (XRD) from annealed alloy films indicates the formation of two crystalline phases, (i) an Al-doped A7 antimony phase, and (ii) a stable cubic AlSb phase. In-situ XRD analysis of these films show the AlSb phase crystalizes at a much higher temperature as compared to the A7 phase after annealing of the film to 650 °C. Mushroom cell structures formed with Al-Sb alloys on 120 nm TiN heater show a phase change material resistance switching behavior with reset/set resistance ratio >1000 under pulse measurements. TEM and in situ synchrotron XRD studies indicate fine nucleation grain sizes of ∼8–10 nm, and low elemental redistribution that is useful for improving reliability of the devices. These results indicate that Te-free Al-Sb binary alloys are possible candidates for analog PCM applications.

Novel approach to optimize quantum dot (QD) materials for specific optoelectronic applications is based on engineering of nanoscale potential profile, which is created by charged QDs. The nanoscale barriers prevent capture of photocarriers and drastically increase the photoelectron lifetime, which in turn strongly improves the photoconductive gain, responsivity, and sensitivity of photodetectors and decreases the nonradiative recombination losses of photovoltaic devices. QD charging may be created by various types of selective doping. To investigate effects of selective doping, we model, fabricated, and characterized AlGaAs/InAs QD structures with n-doping of QD layers, doping of interdot layers, and bipolar doping, which combines p-doping of QD layers with strong n-doping of the interdot space. We have measured spectral characteristics of photoresponse, photocurrent and dark current. The experimental data show that providing the same electron population of QDs, the bipolar doping creates the most contrasting nanoscale profile with the highest barriers around dots.

In this letter, we use gated Hall method for direct measurement of free carrier density and electron mobility in inversion InGaAs MOSFET channels. At room temperature, the highest Hall mobility of 1800 cm2/Vs is observed at electron density in the channel $\approx 1\times 10^{12}$ cm $^{-2}$ . A comparison with mobility values estimated from transistor characteristics reveals a significant underestimation of mobility, which arises from overestimation of channel density obtained from $C$ – $V$ measurements. Temperature dependence of the electron mobility provides the evidence that remote Coulomb scattering dominates at electron density $ cm $^{-2}$ . Contrary to the capacitance-based methods commonly used for extraction of interface trap density, a gated Hall method can separate the contributions of fast-trapped charges and free carriers in the channel to the total charge of a MOS capacitor. This allows for reliable estimation of trap density at the III–V/high- $k$ interface including border traps. The results illustrate that even high-quality interfaces providing high-mobility transport suffer from fast border traps above the conduction band. In contrast with Si where the effect of border traps is negligible, in low density-of-state InGaAs channel material as much as half of the channel electrons can be trapped and excluded from transport, increasing switching energy and dissipated power.

A picosecond-range timing of charged particles and photons is a long-standing challenge for many high-energy physics, biophysics, medical and security applications. We present a design, technological pathway and challenges, and some properties important for realization of an ultrafast high-efficient room-temperature semiconductor scintillator based on self-assembled InAs quantum dots (QD) embedded in a GaAs matrix. Low QD density ( $ ), fast ( $\sim 5~\hbox{ps}$ ) electron capture, luminescence peak redshifted by 0.2-0.3 eV from GaAs absorption edge with fast decay time (0.5-1 ns) along with the efficient energy transfer in the GaAs matrix (4.2 eV/pair) allows for fabrication of a semiconductor scintillator with the unsurpassed performance parameters. The major technological challenge is fabrication of a large volume ( $ > 1~\hbox{cm}^{3}$ ) of epitaxial QD medium. This requires multiple film separation and bonding, likely using separate epitaxial films as waveguides for improved light coupling. Compared to traditional inorganic scintillators, the semiconductor-QD based scintillators could have about 5x higher light yield and 20x faster decay time, opening a way to gamma detectors with the energy resolution better than 1% and sustaining counting rates $ > 100~\hbox{MHz}$ . Picosecond-scale timing requires segmented low-capacitance photodiodes integrated with the scintillator. For photons, the proposed detector inherently provides the depth-of-interaction information.

p-type GaSb metal–oxide–semiconductor capacitors with thin InAs surface capping layers were prepared on Si(001) substrates. Epitaxial structures with superlattice metamorphic buffer layers were grown by molecular beam epitaxy. Chemical surface treatment and atomic layer deposition methods were employed for a semiconductor surface passivation and Al2O3 high-k oxide fabrication, respectively. Capacitance-voltage measurements and scanning and transmission electron microscopies were used to correlate electrical properties with the oxide-semiconductor interface structure of the capacitors. Unexpectedly, fast minority carrier response present down to liquid nitrogen temperature was observed in the capacitors passivated by an ammonium sulfide solution. This fast response was found to be related to etch pitlike surface morphology developed upon chemical passivation at the surface steps formed by microtwins and antiphase domain boundaries. Preferential InAs etching by ammonium sulfide at the surface defects was confirmed by analytical TEM studies. Very low activation energy of minority carrier response suggests the presence of electron sources under the gate; they result from growth-related surface defects that give rise to potential fluctuations of as high as half the GaSb bandgap.

Picosecond timing of energetic charged particles and photons is a challenge for many high-energy physics and medical applications. InAs Quantum Dots (QDs) embedded in GaAs matrix are expected to have unsurpassed scintillation properties. The advantages come from highly efficient energy conversion, as well as from fast electron capture and radiative recombination in QDs. We present design considerations and demonstration of an ultrafast, high photon yield room-temperature semiconductor scintillator. Due to the high refractive index of GaAs, the scintillator is fabricated in the form of a 20 μ m thick planar waveguide with an integrated InGaAs photodiode. QD luminescence of about 60% efficiency at room temperature has been shown, and modal attenuation stabilized at 1 cm−1. Scintillation responses from 5.5 MeV alpha particles show an extremely fast decay time of 280 ps, a collection efficiency of 11% and a time resolution of 60 ps. This data confirms the unique potential properties of this scintillation detector.

Structural and emission properties of few-electron In(Ga)P/GaInP quantum dots (QDs) representing natural Wigner molecules (WM) and whispering gallery mode (WGM) electron ($e$) cavities have been investigated. QD structures were grown using self-organized metal-organic vapor phase epitaxy and deposition from $\ensuremath{\sim}3$ to 7 monolayers of InP at $700{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Using atomic force microscopy, transmission electron microscopy, near-field scanning optical microscopy (NSOM), and $\ensuremath{\mu}$-photoluminescence ($\ensuremath{\mu}$-PL) spectra we obtained In(Ga)P/GaInP QDs having lateral size 80--180 nm, height 5--30 nm, Ga content 0.0--0.4, density $2\ensuremath{-}10\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}$, and electron population up to 20 and demonstrated control of their density and size distribution. Using high-spatial-resolution low-temperature PL spectra, NSOM imaging, and calculations of charge density distributions we observed Wigner localization and $e$-cavity effects for a series of dots having quantum confinement $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=0.5\ensuremath{-}6\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. We used these data together with time-resolved PL measurements to clarify the effect of Coulomb interaction and WM formation on emission spectra of few-electron QDs. We present direct observation of $2e$, $6e$, and $9e$ WMs; $2e$ and $4e$ WGMs; and Fabry-Perot $e$ modes and establish conditions of $e$-WGM-cavity formation in these QDs.

Group III-Sb compound semiconductors are promising materials for future CMOS circuits. Especially, In1-xGaxSb is considered as a complimentary p-type channel material to n-type In1-xGaxAs MOSFET due to the superior hole transport properties and similar chemical properties in III-Sb’s to those of InGaAs. The heteroepitaxial growth of In1-xGaxSb on Si substrate has significant advantage for volume fabrication of III-V ICs. However large lattice mismatch between InGaSb and Si results in many growth-related defects (micro twins, threading dislocations and antiphase domain boundaries); these defects also act as deep acceptor levels. Accordingly, unintentional doping in InGaSb films causes additional scattering, increase junction leakages and affects the interface properties. In this paper, we studied the correlations between of defects and hole carrier densities in GaSb and strained In1-xGaxSb quantum well layers by using various designs of metamorphic superlattice buffers.

Quantum dot infrared photodetectors (QDIPs) have attracted significant attention due to selective photoresponse, high photoconductive gain, and numerous possibilities for nanoscale engineering of photoelectron processes, which control the detector characteristics. Our approach to improving QDIP performance is based on optimization of three dimensional nanoscale potential profile created by charged quantum dots (QDs). Nanoscale profile around QDs allows us to control photoelectron capture processes, which determines the photoelectron lifetime, detector operating speed, responsivity, the spectral density of noise, noise bandwidth, and the detector dynamic range. The nanoscale potential profile is determined by doping of QDs and inter-dot space. In this work, we study various ways of selective doping and its effects on characteristics of photodetectors. We investigate and compare intra-dot doping, inter-dot doping, and complex bipolar doping. To investigate effects of selective doping, we fabricated AlGaAs/InAs QD structures with n-doping of QD layers, structures with n-doping of barriers, and structures with p-doping of QD layers and n-doping of interdot space. We measured dark current, spectral photoresponse, voltage dependence of responsivity, and noise characteristic. The photoresponse is improved due to photon-electron coupling, which increases with QD filling by electrons. However, the noise current also increases due to increase in QD filling. Therefore, possibilities for improvement of QDIP structures with unipolar doping are very limited. Our results show that spectral photoresponse, responsivity, and detector sensitivity are substantially improved due to bipolar doping, which provides decoupled control of electron filling of QDs and the potential barriers around QDs.

The paper reports on the growth of group III-Sb’s on silicon, substrate preparation, optimization of AlGaSb metamorphic buffer, formation of defects (threading dislocations, microtwins and anti-phase boundaries) and their effect on the surface morphology and electrical properties of these high hole mobility materials for future III-V CMOS technology. Defect density was found to be 2-3x higher than in similar structures grown on GaAs, resulting in 2x higher roughness. Defects also result in background p-type doping well above 1017 cm-3 causing inversion of polarity from n-type to p-type in thin n-type doped GaSb. MOS Capacitors fabricated on these buffers demonstrate similar characteristics to higher quality GaSb-on-GaAs. The highest hole mobility obtained in a strained InGaSb QW MOS channel grown on silicon is ∼630 cm2/V-s which is ∼30% lower than similar channels grown on GaAs substrates.

High-efficiency, picosecond scintillation detectors are critically needed for many applications, particularly in the fields of medical imaging, high energy physics and nuclear security. Recently, we proposed a design and considerations for a semiconductor scintillator composed of InAs Quantum Dots (QDs) acting as luminescence centers in a GaAs stopping matrix/waveguide, integrated with a photodetector grown epitaxially on top of the waveguide (WG) [1]. This material has appealing potential photonic properties including high light yield (~240,000 photons/MeV) due to the narrow semiconductor bandgap, fast capture of electrons in QDs (2-5ps) and high radiation hardness (>104 Gy). The high refractive index of GaAs (n=3.4) ensures light emitted by the QDs is waveguided within the GaAs matrix, which can then be collected by an integrated InGaAs p-i-n photodiode (PD). A proof-of-concept integrated scintillation detector (Fig. 1) was grown using molecular beam epitaxy on a GaAs substrate. It has a 20 μm thick GaAs layer with embedded sheets of modulation p-type doped InAs QDs. A generic excitation source is shown interacting with the GaAs matrix, resulting in QD luminescence, which is then waveguided towards an InGaAs PD, grown on top of the WG and tuned to the QD luminescence band (~1150 nm). Employing QD structure engineering improves the internal efficiency of the QD luminescence to ~60% at room temperature and reduces overall waveguide attenuation to about 3-4 cm-1. A sample, as shown in Fig. 1, was used to evaluate the timing characteristics of 5.5 MeV alpha particle responses. The source had a radioactivity of 1 µCi, suspended in air approximately 1 cm above the QD waveguide. The scintillation response (Fig. 2) had a decay time of 0.3 ns, corresponding to QD recombination time, and a noise-limited time resolution of 54 ps (Fig. 3). With a 2D waveguide and an integrated PD covering only 16% of the WG width, the collected charge averaged 1.5x105 electrons (Fig. 4), corresponding to a collection efficiency of about 14%. This can be estimated as 34,000 photoelectrons per 1 MeV of incident energy, given about 1 MeV of alpha particle energy loss in air. This data confirms the unique photonic properties of this scintillation detector, which has the potential to be much faster than any currently used inorganic scintillator. Additionally, technologies for separating an epitaxial film from a substrate and bonding multiple waveguides using organic adhesives are tested and evaluated. [1] S. Oktyabrsky, et al. "Integrated Semiconductor Quantum Dot Scintillation Detector: Ultimate Limit for Speed and Light Yield," IEEE TNS, 63, 656 (2016). Figure 1

The GaSb-on-GaAs growth was optimized for the fabrication of metal-oxide-semiconductor (MOS) capacitors (Caps) with low interface state trap density ( D it ) using in-situ deposited amorphous silicon (a-Si) interface passivation layer (IPL) and high-k oxides. The best top surface with the average roughness R a =0.37 nm and with spiral type “step-flow” growth mode was observed in the GaSb structure with the initial 0.5 μm grown at 410 °C and the top 0.5 μm grown at 485 °C. N- and p-type GaSb MOSCaps with reasonable capacitance–voltage(C–V) characteristics at room temperature (RT) were demonstrated using all in-situ 0.5 nm a-Si IPL and 10 nm Al 2 O 3 +HfO 2 or Al 2 O 3 . A-Si IPL was found essential for n-MOSCaps but not in the case of p-MOSCaps where comparable C–V characteristics with a similarly low D it =1–2×10 12 cm −2 eV −1 were demonstrated without IPL.

Group III–V compound semiconduc-tors due to their better transport properties (mobility, effec-tive mass, injection velocity) have significant advantages over Si-based metal–oxide–semiconductor field effect transistors (MOSFETs) for high-speed and low-power digital applications. Substantial progress has been made in n-type InGaAs MOSFETs over the last 5–7 years [1–3] which also requires parallel development of p-type MOS-FETs for future high performance CMOS applications. The group III–Sb semiconductors have the highest bulk hole mobility among compound semiconductors, and in a strained two-dimensional channel a lower “in-plane” effec-tive mass corresponding to the heavy hole subband, as compared to their major p-type competing material, Ge [4]. The potential superior transport in inversion layers of 2% strained InGaSb semiconductors was projected by Monte-Carlo simulations [5] and more recently demonstrated in long-channel p-MOSFETs by a few groups [6, 7]. Superior transport properties are a major reason why III–V’s are considered for advanced CMOS circuits. How-ever, placing the semiconductor channel in close proximity to the high-k gate oxide reduces mobility significantly in a way similar to the Si or n-type InGaAs channels due to in-terface roughness, soft phonon, and remote Coulomb scat-tering (RCS) [8–11]. Understanding of the transport degra-dation mechanisms due to interface and oxide scattering is an important issue, and the corresponding baseline data will constitute a basis for comparison of prospective p-type materials for novel CMOS circuits.

The highly reactive GaSb surface was passivated with a thin InAs layer to limit interface trap state density ( D it ) at the III–V/high-k oxide interface. This InAs surface was subjected to various cleaning processes to effectively reduce native oxides before atomic layer deposition (ALD). Ammonium sulfide pre-cleaning and trimethylaluminum/water ALD were used in conjunction to provide a clean interface and annealing in forming gas (FG) at 350 °C resulted in an optimized fabrication for n -GaSb/InAs/high-k gate stacks. Interface trap density, D it ≈ 2–3 × 10 12 cm −2 eV −1 resided near the n -GaSb conductance band which was extracted and compared with three different methods. Conductance–voltage–frequency plots showed efficient Fermi level movement and a sub-threshold slope of 200 mV/dec. A composite high-k oxide process was also developed using ALD of Al 2 O 3 and HfO 2 resulting in a D it ≈ 6–7 × 10 12 cm −2 eV −1 . Subjecting these samples to a higher (450 °C) processing temperature results in increased oxidation and a thermally unstable interface. p-GaSb displayed very fast minority carrier generation/recombination likely due to a high density of bulk traps in GaSb.

Electrical and structural properties of GaSb metal-oxide-semiconductor capacitors with in-situ deposited HfO2 gate dielectric and in-situ deposited amorphous silicon interface passivation layer are presented. Capacitance-voltage characteristics with low C-V stretch out, reduced hysteresis, lower EOT is obtained in a temperature range of 3500C-5500C post deposition annealing in forming gas. The gate leakage current of the gate stack was ≤ 1μA/cm2. p-MOSFETs fabricated on epitaxially grown strained (In)GaSb quantum channel (ε=1.6%) channels using this all in-situ high-k gate stack showed a maximum ION=23mA/mm for a 3μm gate length device

Metamorphic Al 0.7 Ga 0.3 As y Sb 1− y buffers on GaAs substrates to reduce defect density in the strained GaSb QW p-channels were developed by employing superlattice (SL) consisting of alternating 10 nm-thick layers with different As-contents. The maximum hole mobility of 1070 cm 2 /V s was obtained in a sample with As-, Sb-valves toggled, and Al-, Ga-shutters constantly open. The p-channel hole mobility strongly depended on the As average composition in AlGaAsSb SL and the resulting strain in GaSb QW. The integral room temperature photoluminescence (PL) intensity was found to decrease monotonically with increasing of biaxial strain in the GaSb QW p-channel.

Hall electron mobility in buried QW InGaAs channels, grown on InP substrates with HfO 2 gate oxide, is analyzed experimentally and theoretically as a function of top barrier thickness and composition, carrier density, and temperature. Temperature slope α in μ ~Tα dependence is changing from α=-1.1 to +1 with the reduction of the top barrier thickness indicating the dominant role of remote Coulomb scattering (RCS) in interface-related contribution to mobility degradation. Insertion of low-k SiO x interface layer formed by oxidation of thin in-situ MBE grown amorphous Si passivation layer has been found to improve the channel mobility, but at the expense of increased EOT. This mobility improvement is also consistent with dominant role of RCS. We were able to a obtain a reasonable match between experiment and simple theory of the RCS assuming the density of charges at the high-k/barrier interface to be in the range of (2-4)×1013 cm-2.

However, when the emitters are not only periodically arranged as in refs 2 to 4, but are also placed in a periodically modulated dielectric environment, the interaction between them is carried by the electromagnetic Bloch waves of the photonic crystal 7,8 . Here we report the first observation of this effect using a periodic arrangement of GaAs/AlGaAs quantum wells. The formation of coherently coupled photonic-crystal excitonic-lattice polaritons manifests in our experiments through enhanced reflectivity and reconstruction of the photonic bandgap in the vicinity of the excitons. Experimental evidence of hybrid light hole–heavy hole excitonic-lattice polaritons is also presented. Coherent coupling between excitons and Bloch waves is established through comparisons of the experimental results with theory. Finally, we demonstrate the tuning of these polariton states by an electric field. Modification of the optical properties of emitters by confining their electromagnetic field in structures such as microcavities and photonic crystals have been studied extensively. Another method of manipulation of optical properties is based on the possibility of coherent radiative coupling between a collection of emissive species. R. H. Dicke showed 1 that if the spacing between the emitters was much smaller than their emission wavelength, the emitters became coherently coupled by the common radiative field. As a result, new collective states are formed, with one of them exhibiting the effect of superradiance. A similar phenomenon can also be realized when the emitters are arranged periodically in a uniform dielectric background or vacuum with the period equal to half of the emission wavelength 2 .

With the goal to demonstrate feasibility of high-mobility buried channel, we used in-situ high-k deposition approach and show for the first time operational MOSFET with buried HfO 2 /In 0.52 Al 0.48 As/In 0.53 Ga 0.47 As/InP channel with mobility 1800 cm 2 /V-s at 3 × 10 12 cm -2 and e-mode operation. Interface properties are compared with a similar gate stack with 2 monolayer thick InGaAs "passivation" layer between InAlAs and the oxide. The latter gate stack has shown significantly improved gate control and ON/OFF ratio due to reduction of the interface state density.

An overview of III-V MOSFET technological challenges in comparison to well-established heterostructure-based FET technologies is presented with an emphasis on required properties and possible solutions. Possible approaches to achieve thermodynamically stable high- k gate stack with low interface trap density are reviewed, followed with our results on amorphous Si interface passivation layer (IPL) in-situ deposited on top of GaAs or strained InGaAs MOSFET channels grown by molecular beam epitaxy. Main issues of Si IPL, namely increased equivalent oxide thickness due to IPL oxidation and Si diffusion into the semiconductor channel, are addressed using an in-situ deposited HfO 2 with ultrathin (down to 0.25 nm) Si IPL and controlling its bonding state at the interface. Enhancement mode inversion-type MOSFET with HfO 2 high- k oxide is demonstrated. The device employs amorphous Si interface passivation layer, sputter-deposited high- k oxide and metal TaN gate and modulation p-doped GaAs / AlGaAs heterostructure with inversion n -channel formed at the interface with the oxide. The MOSFET with equivalent oxide thickness of 3.7 nm and long 100 μm channel have maximum DC transonductance of 0.9 mS/mm, Ion/Ioff = 2×104 (at low Ioff of 30 nA) and effective channel mobility exceeding 1000 cm2/V-s at sheet electron density 12 cm-2.

To reduce density of interface states and avoid Fermi level pinning at the III–V-high-k interface we employed an amorphous Si interface passivation layer ( a -Si IPL) in situ deposited on top of GaAs or InGaAs MOSFET channels grown by molecular beam epitaxy. The high-k gate stack was further fabricated ex situ on top of the IPL with HfO 2 dielectric and TaN metal gate. Combination of transmission electron microscopy, X-ray photoelectron spectroscopy and capacitance–voltage methods was applied to the samples with various IPL thicknesses to study correlations of the interface structure and its chemistry with the formation/passivation of interface states. An unpinned Fermi level is demonstrated on both GaAs and InGaAs wafers when Si IPL is partially oxidized, corresponding to the minimum thickness of the a -Si IPL of 1.5 nm. Thermal stability of the gate stack up to 750 °C was demonstrated, making it appropriate for Si implant activation within MOSFET technology. Both depletion mode and enhancement mode n-channel MOSFETs were demonstrated with transconductance 0.27 mS/mm for 100 μm—long channel and channel electron mobility as high as 1100 cm 2 /V s.

A number of nano-engineering methods are proposed and tested to improve optical properties of a laser gain medium using the self-assembled InAs quantum dot (QD) ensemble. The laser characteristics of concern include higher gain, larger modulation bandwidth, higher efficiency at elevated temperatures, higher thermal stability, and enhanced reliability. The focus of this paper is on the management of QD properties through design and molecular beam epitaxial growth and modification of QD heterostructures. This includes digital alloys as high-quality wide-bandgap barrier; under- and overlayers with various compositions to control the dynamics of QD formation and evolution on the surface; shape engineering of QDs to improve electron-hole overlap and reduce inhomogeneous broadening; band engineering of QD heterostructures to enhance the carrier localization by reduction of thermal escape from dots; as well as tunnel injection from quantum wells (QWs) to accelerate carrier transfer to the lasing state. Beneficial properties of the developed QD media are demonstrated at room temperature in laser diodes with unsurpassed thermal stability with a characteristic temperature of 380 K, high waveguide modal gain >50 cm−1, unsurpassed defect tolerance over two orders of magnitude higher than that of QWs typically used in lasers, and efficient emission from a two-dimensional (2-D) photonic crystal nanocavity.

Alternative channel materials with superior transport properties over conventional silicon based systems are required for supply voltage scaling in CMOS circuits. Group III-Sb's are a candidate for high mobility p-channel applications due to a low hole effective mass, large injection velocity in scaled devices and the ability to achieve enhanced hole mobility in strained quantum wells (QW). Multiple challenges in antimonide MOSFET development are assessed and developed technologies were implemented into p-channel MOSFET fabrication with a low thermal processing budget of 350°C. These challenges include growth of “bulk” GaSb and bi-axial compressively strained InxGa1-xSb QW channels on lattice mismatched GaAs substrates, reduction of interface trap state density (Dit) at the III-Sb/high-k oxide interface and avoiding ion implanted source and drain contacts with high temperature activation annealing. A “self-aligned” single mask p-channel MOSFET fabrication process was developed on buried In0.36Ga0.64Sb QW channels using intermetallic source and drain contacts. The first “gate-last” MOSFET process on In0.36Ga0.64Sb QW channels with pre-grown epitaxial p++-GaSb contacts is demonstrated. InAs has been proven to be an excellent etch stop layer when using an optimized tetramethylammonium hydroxide (TMAH) etch of p++-GaSb to prevent InGaSb QW damage.

State University of New York, Buffalo, New York, USA U.S. Army Research Laboratory, Adelphi, Maryland 20783, USA SUNY Polytechnic Institute, Albany, New York, USA Memory R&D Division, SK Hynix, Icheon-si, Gyeonggi-do, 467-701, Korea Center for Micro- and Nanostructures, TU Vienna, Vienna 1040, Austria

The 6.1 A III–V “high-mobility” semiconductor family includes materials with beneficial transport properties of both electrons and holes (InAs, GaSb), which are appealing for fast and low-power complementary metal–oxide–semiconductor applications. Yet their large lattice mismatch with Si (∼12%) results in three dimensional island nucleation and therefore growth defects. The solution for deposition of this high mismatch material is the growth of the entire device from a single nucleus, such as in vertical nanowires. Two types of GaSb nanowires (NWs) are demonstrated on a Si(111) substrate: vertically stacked InAs/GaSb NWs and coaxial core/shell NWs. This paper summarizes surface preparation, growth conditions, and postprocessing steps which can be used to create nanowires with small enough diameters for use as logic devices.

This paper reports on correlation of growth-related defects and electrical properties in GaSb films grown on different Si substrates using metamorphic buffers. Large lattice mismatch between GaSb and Si (∼11%) results in the formation of threading dislocations (TDs) and microtwins (MTs) along with antiphase domains due to the lack of inversion symmetry in III-V's. The defect density profiles were analyzed using transmission electron microscopy and atomic force microscopy. The TD density of just below 108 cm−2 and MT density below 104 cm−1 were found in 2.1 μm thick structures, and were found to be four times higher than in similar GaSb structures on GaAs substrates. Hole density and mobility profiles were obtained using differential Hall method and show that dislocations (TDs or MT partials) generate about 25 acceptors/nm. Minimum midgap interface trap density values are similar in the metal-oxide-semiconductor structures prepared on GaAs and Si, ∼2 × 1012 cm2 eV−1.

Group III–V semiconductor materials are being considered as a Si replacement for advanced logic devices for quite some time. Advances in III–V processing technologies, such as interface and surface passivation, large area deep submicron lithography with high-aspect ratio etching primarily driven by the metal-oxide-semiconductor field-effect transistor development can also be used for other applications. In this paper we will focus on photodetectors with the drift field parallel to the surface. We compare the proposed concept to the state-of-the-art Si-based technology and discuss requirements which need to be satisfied for such detectors to be used in a single photon counting mode in blue and ultraviolet spectral region with about 10 ps photon timing resolution essential for numerous applications ranging from high-energy physics to medical imaging.

We have proposed and demonstrated the principle of optical decoupling in a vertical-cavity surface-emitting laser (VCSEL)-electroabsorption modulator device by use of a duo-cavity device architecture. This technique promises ultrahigh-frequency modulation of the VCSEL output due to the elimination of the resonance influences on the intrinsic modulation transfer function. By adjusting spectral detuning between the resonances of the duo-cavity configuration, optical decoupling of output modulation by an electroabsorption modulator can be achieved. A flat ( 3 dB) response up to 20 GHz has been measured by proper detuning of the resonances.

Low density of states (DOS) and typically high interface and border trap densities (D it ) in high mobility group III-V semiconductors provide difficulties in quantification of D it near the conduction band edge. The trap response above the threshold voltage can be very fast, and conventional D it extraction methods, based on capacitance/conductance response (CV methods) of MOS capacitors at frequencies ox value. Another implication of these properties of III-V interfaces is an ambiguity of determination of electron density in the MOSFET channel. Traditional evaluation of carrier density by integration of the C-V curve, gives significantly overestimated results even if corrected by D it . It happens because the CV methods can distinguish free and trap carriers exclusively by their response kinetics, and therefore all trapped electrons responding faster than ~1μs are treated as free electrons. In this work, we are using a gated Hall method [1,2] which allows for direct measurement of free carrier density, and therefore, when combined with CV measurements or electrostatic modeling allows for accurate quantification of Dit spectrum. In addition, the former approach does not need knowledge of Cox.

A method for layer transfer via attachment and release of GaAs-based components onto silicon platform in a single-step process (oxidation lift-off method) is proposed. The method involves moderate temperature (∼400°C) alloy bonding of GaAs devices with simultaneous removal of the GaAs substrate by lateral oxidation of sacrificial AlAs layer. Selectivity with respect to Al content is high enough to release the vertical cavity laser structures containing layers of AlxGa1−xAs with x=0.9. This characteristic of the oxidation process allows for the release of components and form oxide apertures during a single step. The technology can be employed for heterogeneous integration of various compound semiconductor devices with Si or other substrates.

Optical properties of the quantum dots (QDs) were optimized by shape engineering through the adjustment of the thickness of the GaAs overlayer prior to an additional heating step leading to QD truncation. QDs with a 6-nm-thick overlayer with the subsequent heating step were found to have the highest photoluminescence intensity at room temperature and the lowest luminescence bandwidth, 29 meV. 1.22 μm edge-emitting laser with a triple-layer truncated QD gain medium demonstrated room temperature threshold current density, 56 A/cm2, and saturated modal gain, 16 cm−1. An extremely high characteristic temperature for lasing threshold, T0=380 K up to 55 °C, and a maximum ground state lasing temperature of 219 °C were measured for these laser diodes.

Resistance to temperature and ionizing radiation of space optoelectronic devices can be improved through control of carrier kinetics in nanoscale systems. Recent results in the science and technology of self-assembled heteroepitaxial InAs quantum dot (QD) medium related to photonic applications are discussed. Focus is placed on management of carrier kinetics via nanoengineering of electronic spectrum and potential profiles in the QD ensemble using modeling and controlled fabrication of QDs with molecular beam epitaxy. Shape-engineered QD sheets embedded into GaAs quantum wells were found to withstand two orders of magnitude higher proton dose than QWs and to account for high luminescence efficiency and thermally stable laser diodes. Built-in charge in QDs is responsible for improvement of both near and mid-IR optical absorption, but also control photoelectron lifetime in the structures. The negatively charged QD medium was the first QD material that has recently shown credible improvement of solar cell efficiency. It has resulted from IR energy harvesting and suppressed fast electron capture processes. It is thus expected that QD InAs/GaAs photovoltaics will overcome the efficiency and lifespan of multi-junction solar cells. Potentials due to QD built-in charge are also responsible for improved photoelectron lifetime in QD infrared photodetectors. QD correlated clusters provide even higher collective potential barriers around clusters and constitute the novel approach to the optoelectronic materials combining manageable photoelectron lifetime, high mobility, and tunable localized and conducting states.

InGaSb material family with its higher hole transport properties are potential candidates for group III–V CMOS circuits. Understanding of the dominant scattering mechanisms is crucial for the development of future high speed, low power device applications. We present Hall mobility data of p-type InGaSb quantum well (QW) channels and derive the dominant scattering mechanisms related to the interface and trapped charges that degrade mobility in these structures.

A major challenge in fabricating a high mobility (In)GaSb based transistor is reducing the density of trap states (D it ) at the III-V/high-k interface [1]. Unoccupied surface bonds react with ambient oxygen and create energy levels within the GaSb bandgap which limit Fermi level movement and degrade sub-threshold slope. InGaAs and other As-rich surfaces have been thoroughly investigated and exhibit significant trap state reduction with Ammonium Sulfide passivation and further native oxide reduction when exposed to TMA precursor used in ALD [2,3]. Interface engineering of GaSb is accomplished using an epitaxially grown 2nm InAs top layer. This GaSb/InAs gate stack has shown immense improvement in C-V characteristics and a significant reduction of D it at the III-V/high-k interface.

Here we report the observation of Bloch-polaritons in a GaAs/AlGaAs multiple-quantum-well structure. We observe three polariton states formed by the coherent interaction between light- and heavy-hole excitonic-lattices, and Bloch waves of the background photonic crystal.

III–V compound semiconductors with extremely high mobilities have recently emerged as a potential technology of choice for future beyond-Si CMOS logic applications [1]. InAs PHEMT (on InP) containing an InAs strained layer inserted in an In 0.53 Ga 0.47 As channel layer has been demonstrated with improved scalability and performance [2], albeit wi th Schottky gate and non sel f aligned source d rain contacts. This i s unsuitable for future VLS I logic applications. Here, we integrate, for the first time, an in-situ deposited high-k dielectric (HfO 2 ) and metal gate (TaN) stack with the InAs quantum-well device layers incorporating Pd/Gesource-drain contacts fully self-aligned with the gate edge and contacting the quantum-well directly. A bias and temperature dependent experimental study of the off-state leakage and the carrier transport identifies the various physical mechanisms limiting the device performance.

Future CMOS logic applications call for replacement of silicon channels with higher mobility III–V compound semiconductors, whereas the need for power reduction requires utilization of gate oxides with higher dielectric constant [1]. However, as compared to HEMTs, high-k gated surface inversion channel N-MOSFETs demonstrate significant mobility degradation due to close proximity of the channel to the interface. In this work we present results of detailed investigation of major scattering mechanisms leading to degradation of mobility in HfO 2 -gated, modulation doped, strained In 0.53 Ga 0.47 As/In 0.77 Ga 0.23 As QW structures grown on semi-insulating InP substrates. By varying In 0.53 Ga 0.47 As/InAlAs barrier and HfO 2 thickness along with controlling interface state density (D it ) and using temperature dependent Hall mobility measurements we have de-convoluted the contribution of various scattering mechanisms to channel mobility degradation.

Structures with tunnel-coupled pairs consisting of InGaAs quantum wells (QWs) grown on top of self-assembled InAs quantum dots (QDs) were used previously as a gain medium for vertical cavity surface emitting lasers (VCSELs) to eliminate problems with QD-limited maximum saturated gain. Conventional molecular beam epitaxy of tunnel-coupled QDs with slow InAs growth rate and InGaAs solid solution QW injector with high InAs growth rate required a long delay in growth process for changing indium source temperature/flux. This leads to non-intentional doping of tunnel barrier and reproducibility issues. To overcome these problems, structures of tunnel-coupled QDs-QW pairs consisting of InAs/InGaAs short period superlattice (SPSL) QW injector with compatible slow InAs growth rate (QDs-SPSL) were developed and compared with traditional InAs-InGaAs (QDs-InGaAs). Photoluminescence (PL) and electroluminescence were used to study the properties of the "well-on-dots" active medium with InAs/InGaAs SPSL QW and with InGaAs QW. The optimized tunnel triple pair QDs-SPSL structure with 2x reduction of growth time has demonstrated a 2x enhanced PL efficiency as compared with traditional QDs-InGaAs structures. A novel tunnel-coupled triple QDs InAs-SPSL was successfully employed as a gain medium of VCSELs with doped all-epitaxial distributed Bragg reflectors (DBRs). Room temperature CW lasing wavelengths in the range from 1100 nm to 1150 nm were measured in VCSELs with attuned DBRs. These QDs-SPSL VCSELs demonstrated minimum threshold current value Ith = 0.85 mA and maximum differential efficiency of 0.16 W/A.