Modulation-doped Heterostructures Research Articles

Normally-off recessed gate β-Ga2O3 MOSHFETs with a modulation-doped heterostructure back-barrier

This study demonstrates enhancement-mode recessed-gate β-Ga2O3 metal–oxide–semiconductor heterojunction field-effect transistors (MOSHFETs) with a combination of the MOS channel and a modulation-doped heterostructure to improve maximum drain current and on-resistance (RON). In this proposed device concept, modulation doping in the heterostructure back-barrier inserted into the MOS channel increases the electron density in the MOS channel while maintaining a normally-off operation. First, 2D simulations of enhancement-mode recessed-gate β-Ga2O3 metal–oxide–semiconductor field-effect transistors (MOSFETs) were performed in a Silvaco ATLAS TCAD environment to calibrate the transfer characteristics with the measured data of the investigated device reported previously. Second, using calibrated physical models and parameters, the transfer and transconductance characteristics, and output and off-state characteristics of the enhancement-mode recessed-gate β-Ga2O3 MOSHFETs were comprehensively investigated. The maximum drain current at VGS = 8 V and VDS = 10 V could be increased up to 32.6 mA mm−1 from 9.1 mA mm−1 with the MOSHFET in comparison with that of the recessed-gate MOSFET. The breakdown voltage increased considerably from 186 V to 226 V for the recessed-gate MOSHFET. The proposed device also showed a lower RON, which decreased from 354 Ω.mm to 214 Ω.mm owing to greater electron accumulation in the channel owing to the introduction of the modulation-doped heterostructure.

Modulation-doping a correlated electron insulator

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2—especially without inducing structural changes—has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard X-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5 × 1021 cm−3 without inducing any measurable structural changes. We find that the MIT temperature (TMIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e−/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.

Growth and characterization of metalorganic vapor-phase epitaxy-grown β-(Al x Ga1−x )2O3/β-Ga2O3 heterostructure channels

We report on the growth and characterization of metalorganic vapor-phase epitaxy-grown β-(Al x Ga1−x )2O3/β-Ga2O3 modulation-doped heterostructures. Electron channel is realized in the heterostructure by utilizing a delta-doped β-(Al x Ga1–x )2O3 barrier. The electron channel characteristics are studied using transfer length method, capacitance–voltage and Hall measurements. A Hall sheet charge density of 1.06 × 1013 cm−2 and a mobility of 111 cm2 V−1 s−1 is measured at room temperature. The fabricated transistor showed a peak current of 22 mA mm−1 and an on–off ratio of 8 × 106. A sheet resistance of 5.3 kΩ/square is measured at room temperature, which includes contribution from a parallel channel in β-(Al x Ga1–x )2O3.

A modulation-doped heterostructure-based terahertz photoconductive antenna emitter with recessed metal contacts

We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We compare the performance of the MDH PCA having recessed contacts with a PCA fabricated on bulk semi-insulating GaAs, on low temperature-grown GaAs, and a MDH PCA with the contacts fabricated on the surface. By recessing the contacts, the applied bias can effectively accelerate the high-mobility carriers within the 2DEG, which increases the THz power emission by at least an order of magnitude compared to those with conventional structures. The dynamic range (62 dB) and bandwidth characteristics (3.2 THz) in the power spectrum are shown to be comparable with the reference samples. Drude-Lorentz simulations corroborate the results that the higher-mobility carriers in the MDH, increase the THz emission. The saturation characteristics were also measured via optical fluence dependence, revealing a lower saturation value compared to the reference samples. The high THz conversion efficiency of the MDH-PCA with recessed contacts at low optical power makes it an attractive candidate for THz-time domain spectroscopy systems powered by low power fiber lasers.

Improving reproducibility of quantum devices with completely undoped architectures

The reproducible operation of quantum electronic devices is a key requirement for future quantum information processing and spintronics applications. Traditionally, quantum devices have been fabricated from modulation-doped heterostructures, where there is an intrinsic lack of reproducibility due to the random potential from ionized donors. Here, we show that we can greatly improve reproducibility over modulation-doped devices by using a completely undoped architecture, with superior uniformity in the confinement potential and more consistent operating voltages for both electron and hole devices. Our results demonstrate that undoped heterostructures have significant advantages over modulation doping for reproducible manufacturing of quantum devices.

Thermal and Quantum Melting Phase Diagrams for a Magnetic-Field-Induced Wigner Solid.

A sufficiently large perpendicular magnetic field quenches the kinetic (Fermi) energy of an interacting two-dimensional (2D) system of fermions, making them susceptible to the formation of a Wigner solid (WS) phase in which the charged carriers organize themselves in a periodic array in order to minimize their Coulomb repulsion energy. In low-disorder 2D electron systems confined to modulation-doped GaAs heterostructures, signatures of a magnetic-field-induced WS appear at low temperatures and very small Landau level filling factors (ν≃1/5). In dilute GaAs 2D hole systems, on the other hand, thanks to the larger hole effective mass and the ensuing Landau level mixing, the WS forms at relatively higher fillings (ν≃1/3). Here we report our measurements of the fundamental temperature vs filling phase diagram for the 2D holes' WS-liquid thermal melting. Moreover, via changing the 2D hole density, we also probe their Landau level mixing vs filling WS-liquid quantum melting phase diagram. We find our data to be in good agreement with the results of very recent calculations, although intriguing subtleties remain.

Efficacy of proposed 2DEG-based photoconductive antenna using magnetic bias-controlled carrier transport

An externally applied magnetic field was used to induce increased photocarrier transport along the high mobility channel in GaAs/AlGaAs modulation-doped heterostructures (MDH). The terahertz (THz) emission from GaAs/AlGaAs MDH increases with increasing magnetic field, applied parallel to the heterojunction. The THz emission enhancement factors due to the magnetic field in MDH are higher than in undoped GaAs/AlGaAs heterojunction and in bulk SI-GaAs. This demonstrates that properly utilizing the high-mobility channel for carrier transport promises to be a viable design consideration for efficient THz photoconductive antenna (PCA) devices. Moreover, it was observed that for MDH, as well as for an undoped GaAs/AlGaAs heterojunction, the enhancement for one magnetic field direction is greater than the enhancement for the opposite direction. This is in contrast to the symmetric enhancement with magnetic field direction observed in a bulk SI-GaAs. An analysis of photocarrier trajectories under an external magnetic field supports the explanation that the enhancement asymmetry with magnetic field direction in MDH is due to the cycloid motion of electrons as affected by the GaAs/AlGaAs interface.

Temperature dependence of THz emission and junction electric field of GaAs–AlGaAs modulation-doped heterostructures with different i-AlGaAs spacer layer thicknesses

Photocarrier dynamics in GaAs/AlGaAs modulation-doped heterostructures (MDH) having i-AlGaAs spacer layers of different thicknesses were investigated using temperature-dependent terahertz time-domain spectroscopy (THz-TDS) and photoreflectance spectroscopy. In particular, results are discussed in the framework of the temperature dependence of the heterojunction electric field and photocarrier velocity for two i-AlGaAs spacer layer thickness values. The junction electric field, THz emission intensity and bandwidth of the MDH samples all decrease as temperature decreases. In contrast, the THz emission intensity and bandwidth of a reference bulk undoped GaAs does not significantly vary with temperature. These results imply that THz emission of MDH’s originates primarily from carrier drift due to the GaAs/AlGaAs junction electric field. A general decrease in the THz emission bandwidth of the MDH’s is attributed to a decrease in carrier velocity at lower temperatures, presumably due to the weaker electric field. Moreover, the MDH sample with thinner spacer layer exhibited a higher junction electric field. This work demonstrates the study of temperature-dependent photocarrier transport and junction electric field measurements. The results may provide useful insights in the design of MDH-based devices.

Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures

The time dependencies of the carrier relaxation in modulation-doped InGaAs-GaAs low-dimensional structures with quantum wires have been studied as functions of temperature and light excitation levels. The photoconductivity (PC) relaxation follows a stretched exponent with decay constant, which depends on the morphology of InGaAs epitaxial layers, presence of deep traps, and energy disorder due to inhomogeneous distribution of size and composition. A hopping model, where electron tunnels between bands of localized states, gives appropriate interpretation for temperature-independent PC decay across the temperature range 150–290 K. At low temperatures (T < 150 K), multiple trapping–retrapping via 1D states of InGaAs quantum wires (QWRs), sub-bands of two-dimensional electron gas of modulation-doped n-GaAs spacers, as well as defect states in the GaAs environment are the dominant relaxation mechanism. The PC and photoluminescence transients for samples with different morphologies of the InGaAs nanostructures are compared. The relaxation rates are found to be largely dependent on energy disorder due to inhomogeneous distribution of strain, nanostructure size and composition, and piezoelectric fields in and around nanostructures, which have a strong impact on efficiency of carrier exchange between bands of the InGaAs QWRs, GaAs spacers, or wetting layers; presence of local electric fields; and deep traps.

Modulation-doped β-(Al0.2Ga0.8)2O3/Ga2O3 field-effect transistor

Modulation-doped heterostructures are a key enabler for realizing high mobility and better scaling properties for high performance transistors. We report the realization of a modulation-doped two-dimensional electron gas (2DEG) at the β-(Al0.2Ga0.8)2O3/Ga2O3 heterojunction by silicon delta doping. The formation of a 2DEG was confirmed using capacitance voltage measurements. A modulation-doped 2DEG channel was used to realize a modulation-doped field-effect transistor. The demonstration of modulation doping in the β-(Al0.2Ga0.8)2O3/Ga2O3 material system could enable heterojunction devices for high performance electronics.

Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures.

The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure–cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron–hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays.

Using light and heat to controllably switch and reset disorder configuration in nanoscale devices

Quantum dots exhibit reproducible conductance fluctuations at low temperatures due to electron quantum interference. These fluctuations are not solely determined by dot geometry; they are also highly sensitive to the underlying disorder potential. Here we exploit this sensitivity to better understand the role that background impurities play in the electronic properties of undoped AlGaAs/GaAs heterostructures, and nanoscale devices based thereon. In particular, we report the remarkable ability to first alter the disorder potential in an undoped AlGaAs/GaAs heterostructure by optical illumination and then reset it back to its initial configuration by room temperature thermal cycling in the dark. We attribute this behavior to a mixture of C background impurities acting as shallow acceptors and deep trapping by Si background donor impurities, i.e., DX centers. This ``alter and reset'' capability is not possible in modulation-doped heterostructures and offers a route to new studies of how background impurities influence transport in nanoscale semiconductor devices.

High-In-content InGaAs quantum point contacts fabricated using focused ion beam system equipped with N2 gas field ion source

Quantum point contacts (QPCs) in high-In-content InGaAs modulation-doped heterostructures fabricated using a focused ion beam (FIB) system equipped with a N2 gas field ion source (GFIS) are demonstrated. The minimum physical size of the fabricated QPCs in this study is ∼30 nm, which is smaller than the typical physical size of QPCs (>50 nm) obtained by electron beam lithography and etching techniques. In addition, the fabricated QPCs are characterized electrically at low temperatures with magnetic fields. Since some of them show conductance quantization behaviors, the results indicate that the GFIS-FIB process is promising for quantum device fabrication.

Nitrogen-related effects on low-temperature electronic properties of two-dimensional electron gas in very dilute nitride GaNxAs1−x/AlGaAs (x = 0 and 0.08%) modulation-doped heterostructures

The low-temperature (4 K) two-dimensional (2D) electron gas mobility data versus carrier concentration in the modulation-doped dilute nitride GaAs1−xNx/Al0.3Ga0.7As (x = 0 and 0.08%) heterostructures are analyzed. Theoretical analysis is based on Fermi–Dirac statistics for the occupation of the quantum confined electronic states in the triangular quantum wells and the width of the quantum well versus 2D concentration. In addition, the mobility analysis is based on Matthiessen's rule for various scattering mechanisms. We found that the N-related neutral cluster alloy scattering together with crystal dislocations created at the interface strongly affects the electrons' mobility in the N-contained channel sample. We also found that as the electron concentration in the well increases from ∼1 × 1011 to 3.5 × 1011 cm−2 the carriers mainly occupy the first subband, tending to remain closer and closer to the hetero-interface.

MBE growth and characterization of (100) and (631)-oriented modulation doped AlGaAs/GaAs heterostructures

In this work, the optical and electrical properties of simultaneously grown modulation-doped heterostructures (MDH) on (100)- and (631)-oriented GaAs substrates are investigated. Due to the amphoteric behavior of Si in AlGaAs doped films two dimensional electron (2DEG) and hole gas (2DHG) structures for the growth on (100) and (631) planes, respectively are obtained. Atomic force microscopy (AFM) revealed atomically flat surface for the (100)-MDH sample. On the contrary, (631)-MDH sustained uniform corrugation along [1̄13] after the growth of the GaAs films, which provoked anisotropic mobility of the carriers at 77K as confirmed by the Hall effect in a double arm bar. By photoluminescence spectroscopy (PL) the band to band transition, carbon and Si-related lines were identified. The concentration of the ternary alloy and impurities were evaluated by secondary ion mass spectrometry.

Electron Distribution in AlGaN/GaN Modulation-doped Heterostructures

We present a calculation of the distribution of two-dimensional electron gas (2DEG)along the quantization direction in an AlGaN/GaN modulation-doped heterostructure (MDHS).The main confinement sources from ionized donors, 2DEG and polarization charges are prop-erly taken into account. We show that the 2DEG distribution near the MDHS interface dependsstrongly on the model of potential barrier in use. Within the ideal model of infinite potential bar-rier, the 2DEG distribution near the interface is increased with a rise of the sheet densities of2DEG and polarization charges. On the contrary, this distribution is decreased within the realisticmodel of finite potential barrier. Since the key mechanisms limiting the 2DEG mobility in MDHSare alloy disorder and surface roughness scatterings that are very sensitive to the near-interface2DEG distribution, with a rise of the sheet densities of 2DEG and polarization charges the 2DEGmobility is decreased within the infinite-barrier model, while increased within the finite one.

BALLISTIC ELECTRON TRANSPORT IN A HYBRID FERROMAGNET/TWO-DIMENSIONAL ELECTRON GAS NANOSTRUCTURE

We have theoretically studied the ballistic electron transport in a ferromagnet/two-dimensional electron gas structure, which could be experimentally realized by depositing a magnetic metallic stripe on the top or bottom of a modulation-doped semiconductor heterostructure. The hybrid structure based on an electrical barrier with inversely proportional function and two magnetic barriers with parallel or antiparallel configurations performs high spin polarization, which strongly depends on the incident electron energy, the incident wave vector parallel to the barrier and the height of the electric potential. In addition, our numerical results show that the magneto-conductance of this structure can be strongly modulated by adjusting the incident electron energy or the electric potential.

Structural and magnetic properties of Ni 2MnIn Heusler thin films grown on modulation-doped InAs heterostructures with metamorphic buffer

This paper reports on the morphological, structural, magnetic, and magneto-optic properties of Ni 2MnIn Heusler films grown on InAs-high electron-mobility transistor structures (HEMT) with metamorphic buffers for spintronic applications. Similar to our previous results on the Ni 2MnIn/InAs (001) system, the Heusler layer is found to have a (110) orientation relative to the (001) InAs-HEMT surface. We observe almost equal spin-polarizations for Heusler films on (001) InAs-HEMT as well as on (001) InAs. In addition, we find further support for interfacial intermixing previously reported for the Ni 2MnIn/InAs (001) system. On the other hand, the Heusler/InAs-HEMT system shows distinct morphologic, structural, and magnetic properties as compared to the Ni 2MnIn/InAs (001) system. In particular, more rapid and complex plastic deformation effects resulting in a high surface density of pin-holes in the Heusler films are found. We report on complex mutual deformation effects between the Heusler films and the underlying InAs-HEMT structure. Furthermore, a hysteresis loop squareness close to 1 for a 50 nm Heusler film on InAs-HEMT is observed. We tentatively associate these phenomena with the higher mismatch strain of the Ni 2MnIn/InAs-HEMT system compared to Ni 2MnIn films grown on (001) InAs.

Scattering due to spacer layer thickness fluctuation on two dimensional electron gas in AlGaAs/GaAs modulation-doped heterostructures

The electron mobility limited by spacer layer thickness fluctuation (SLTF) scattering on the two-dimensional electron gas in AlGaAs/GaAs modulation-doped heterostructure is investigated. Although the SLTF scattering and the interface roughness scattering are both induced by the roughness of the AlGaAs/GaAs interface, they are two different scattering mechanisms. The interface roughness will lead to the fluctuation of the distance from the electrons to the ideal interface and the fluctuation of the spacer layer thickness. The former induces the fluctuation of the electron potential, which works as the interface roughness scattering potential. The latter induces the fluctuation of the sheet carrier density in the channel, which causes a fluctuation in the quantization energy level. The quantization energy level fluctuation works as the SLTF scattering potential. Compared with the interface roughness scattering, the results reveal that the SLTF scattering becomes the dominant scattering mechanism when the doping density in the AlGaAs is high enough.

An Integrated Photodetector Based on a Modulation-Doped Heterostructure

An optically active heterostructure field effect transistor (HFET) structure is described as a three terminal detector. With an in-plane waveguide optical input it demonstrates natural optoelectronic integration with transistors. Both depletion and enhancement threshold devices are realized with photocurrent enhancement of the HFET current of 1.72 for direct coupling. A simple model of photocurrent generation based on bipolar operation provides good agreement. The bidirectional response may be suitable for optical gating functions in balanced detection.