Two-dimensional Hole Gas Research Articles

Electrical property measurement of two-dimensional hole-gas layer on hydrogen-terminated diamond surface in vacuum-gap-gate structure

Hydrogen-terminated diamond metal-insulator-semiconductor field-effect transistors are candidates for power devices that require a high breakdown field and stable, high-frequency operation. A two-dimensional hole-gas layer can form on H-terminated diamond surfaces. To understand the electrical properties of bare H-terminated diamond surfaces, we investigate the surface impurities on a H-terminated diamond surface in a vacuum-gap gate structure, which uses a H-terminated diamond channel and a vacuum gap as gate dielectrics. To obtain a bare surface without surface adsorbate, the device is annealed in a vacuum. The transconductance is increased by removing adsorbates. The mobility and interface-state density at the H-terminated diamond surface with no adsorbates are 25 cm2 V−1 s−1 and 1 × 1012 cm−2 eV−1, respectively.

Tunable carrier density and high mobility of two-dimensional hole gases on diamond: The role of oxygen adsorption and surface roughness

The transport properties of two-dimensional hole gases (2DHGs) on chemical-vapor-deposition (CVD)-grown diamond are investigated. A hydrogen plasma treatment and exposure to ambient atmosphere are used to establish and tailor the properties of the 2DHG. The transport parameters of the 2DHGs (namely carrier density and mobility) are characterized by temperature-dependent Hall measurements. The importance of the surface oxygen adsorption, determined by X-ray photoelectron spectroscopy (XPS), on the carrier density and mobility is shown. Hall measurements reveal that for oxygen concentrations below 2.2% (relative XPS signal) the carrier density is increasing from 1.4 ∙ 1010 cm−2 to 1.5 ∙ 1013 cm−2 with increasing oxygen adsorption. For oxygen concentrations above 2.2%, the charge carrier density decreases again. The carrier density remains constant over a temperature range between 4.2 K and 325 K. At room temperature, the mobility increases with decreasing carrier concentration. The opposite behavior is observed for 4.2 K.By decreasing the surface roughness to 8.2 nm, we were able to increase the mobility to above 250 cm2/V s at room temperature for a carrier density of 1.2 ∙ 1013 cm−2. This is among the highest values reported for 2DHGs on diamond.

Electric-field-induced two-dimensional hole gas in undoped GaSb quantum wells

We have measured hole transport in electrically induced two-dimensional hole gases in undoped GaSb/AlSb quantum wells. In order to access the electrically induced two-dimensional hole gas in GaSb quantum wells, recessed ohmic contacts were formed and the low-temperature magnetoresistance was measured for a gate-defined Hall bar geometry. The mobility of the sample increases with increasing hole density and reaches 20 000 cm2/V s at a hole density of 5.3 × 1011 cm−2 for an 8-nm-thick GaSb quantum well. The longitudinal and Hall resistivities show Shubnikov–de Haas oscillations and integer quantum Hall plateaus, respectively. These results establish a platform for realizing spin-based electronics using the strong spin–orbit interaction of this material and are also useful for understanding the transport properties of the two-dimensional topological insulator realized in InAs/GaSb double quantum well structures.

Formation of two-dimensional electron and hole gases at the interface of half-Heusler semiconductors

Heuslers are a prominent family of multi-functional materials that includes semiconductors, half metals, topological semimetals, and magnetic superconductors. Owing to their same crystalline structure, yet quite different electronic properties and flexibility in chemical composition, Heusler-based heterostructures can be designed to show intriguing properties at the interface. Using electronic structure calculations, we show that two dimensional electron or hole gases (2DEG or 2DHG) form at the interface of half-Heusler (HH) semiconductors without any chemical doping. We use CoTiSb/NiTiSn as an example, and show that the 2DEG at the TiSb/Ni(001) termination and the 2DHG at the Co/TiSn(001) termination are intrinsic to the interface, and hold rather high charge densities of 3x10^14 carriers/cm^2. These excess charge carriers are tightly bound to the interface plane and are fully accommodated in transition-metal d sub-bands. The formation of 2DEG and 2DHG are not specific to the CoTiSb/NiTiSn system; a list of combinations of HH semiconductors that are predicted to form 2DEG or 2DHG is provided based on band alignment, interface termination, and lattice mismatch.

Normally-OFF Two-Dimensional Hole Gas Diamond MOSFETs Through Nitrogen-Ion Implantation

Diamond is a promising material for power applications owing to its excellent physical properties. Two-dimensional hole gas (2DHG) diamond metal–oxide–semiconductor field-effect transistors (MOSFETs) with hydrogen-terminated (C-H) channel have high current densities and high breakdown fields but often show normally- ON operation. From the viewpoint of safety, normally- OFF operation is required for power applications. In this letter, we used ion implantation to form a shallow and thin nitrogen-doped layer below the C-H channel region, which realized normally- OFF operation. Nitrogen-ion implanted length is fixed at 5 or 10 $\mu \text{m}$ . Nitrogen is a deep donor (1.7 eV) and the nitrogen-doped layer prevents hole accumulation near the surface. The threshold voltage was as high as −2.5 V and no obvious dependence on the threshold voltage of nitrogen-ion implanted length is observed. The breakdown field was 2.7 MV/cm at room temperature. Of 64 devices with a common gate length, 75% showed normally- OFF operation. We confirmed the threshold voltage shift by a thin and shallow nitrogen-doped layer formed by ion implantation.

Analysis of back-gate effect on threshold voltage of p-channel GaN MOSFETs on polarization-junction substrates

GaN-based p-channel devices using a two-dimensional hole gas are of interest for the realization of GaN CMOS circuits, and threshold voltage control using a back-gate is useful for the circuit design of these devices. In this paper, the back-gate effect on p-channel GaN MOSFETs on polarization-junction substrates is studied. The results show that the obtained dependence of the threshold voltage on the back-gate voltage can be derived by a model equation considering the back surface channel. Also, by comparing the measured characteristics with the simulation result under ideal conditions, it is found that the amount of interfacial charges can be quantitatively evaluated for the fabricated device.

Strain induced mobility enhancement in p-type silicon structures: Bulk and quantum well (quantum kinetic approach)

Analytic expressions for low field mobility have been obtained in the high strained p-type silicon structures with three- and two-dimensional hole gases. Much attention is paid to study how confinement in one spatial dimension changes the strain mobility enhancement in comparison with bulk material. The mobility enhancement factor has been calculated when applying both the uniaxial and biaxial strains. Acoustic and optic phonons, charged impurities, and surface roughness have been accepted as a scattering system. Our theoretical consideration is based on the quantum kinetic equation and a special form of the non-equilibrium distribution function (shifted Fermi distribution). Results of the calculation are compared with known experimental data.

Tuning the topological insulator states of artificial graphene

We develop a robust, non-perturbative approach to study the band structure of artificial graphene. Artificial graphene, as considered here, is generated by imposing a superlattice structure on top of a two dimensional hole gas in a semiconductor heterostructure, where the hole gas naturally possesses large spin-orbit coupling. Via tuning of the system parameters we demonstrate how best to exploit the spin-orbit coupling to generate time reversal symmetry-protected topological insulator phases. Our major conclusion is the identification of a second set of topological Dirac bands in the band structure (with spin Chern number $C=3$), which were not reliably obtainable in previous perturbative approaches to artificial graphene. Importantly, the second Dirac bands host more desirable features than the previously studied first set of Dirac bands (with $C=1$). Moreover, we find that upon tuning of the system parameters, we can drive the system to the highly desirable regime of the topological flat band. We discuss the possibilities this opens up for exotic, strongly correlated phases.

3.8 W/mm RF Power Density for ALD Al2O3-Based Two-Dimensional Hole Gas Diamond MOSFET Operating at Saturation Velocity

This letter reports the small-signal and large-signal performances at high drain voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> ) ranging up to 60 V for a 0.5 μm gate length two-dimensional hole gas diamond metal-oxide-semiconductor field-effect transistor with a 100-nm-thick atomic-layer-deposited Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> film on a IIa-type polycrystalline diamond substrate with (110) preferential surfaces. This diamond FET demonstrated a cutoff frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of 31 GHz, indicating that its carrier velocity was reaching 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s for the first time in diamond. In addition, a f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> of 24 GHz was obtained at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = -60 V, thus giving a f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> × V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> product of 1.44 THz·V. This diamond FET is promising for use as a high-frequency transistor under high voltage conditions. Under application of a high voltage, a maximum output power density of 3.8 W/mm (the highest in diamond) with an associated gain and power added efficiency were 11.6 dB and 23.1% was obtained when biased at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = -50 V using a load-pull system at 1 GHz.

G -factor and well-width fluctuations as a function of carrier density in the two-dimensional hole accumulation layer of transfer-doped diamond

The two-dimensional (2D) hole gas at the surface of transfer-doped diamond shows quantum-mechanical interference effects in magnetoresistance in the form of weak localization and weak antilocalization (WAL) at temperatures below about 5 K. Here we use the quenching of the WAL by an additional magnetic field applied parallel to the 2D plane to extract the magnitude of the in-plane $g$-factor of the holes and fluctuations in the well width as a function of carrier density. Carrier densities are varied between 1.71 and $4.35\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ by gating a Hall bar device with an ionic liquid. Over this range, calculated values of $|g|$ vary between 1.6 and 2.3 and the extracted well-width variation drops from 3 to 1.3 nm rms over the phase coherence length of 33 nm for a fixed geometrical surface roughness of about 1 nm as measured by atomic force microscopy. Possible mechanisms for the extracted variations in the presence of the ionic liquid are discussed.

A Large-Signal Model for Two-Dimensional Hole Gas Diamond MOSFET Based on the QPZD

A compact large-signal model for hydrogen-terminated (C-H) diamond metal-oxide field effect transistors (MOSFETs) is presented based on an improved quasi-physical zone division (QPZD) model. Unlike the conventional QPZD model for the AlGaN/GaN high-electron-mobility transistors (HEMTs), the linear-mode current-voltage (I-V) model of the diamond FET is analytically deduced with an improved velocity-electric field relation and temperature-dependent effective hole mobility μeff(T ). The I-V model can directly demonstrate the relation between the negative Vds and the negative Ids of the p-type diamond FETs, which cannot be achieved by the conventional QPZD model. Finally, the proposed model has been verified by the on-wafer measurements of an in-house 2 x 500 μm diamond FET. The good consistency shows that the presented compact large-signal model can accurately predict the DC I-V, multibias scattering-parameters (S-parameters), and large-signal performances. The results of this paper will be useful for the microwave diamond-based transistor and circuit designs.

First-principles studies of a two-dimensional electron gas at the interface of polar/polar LaAlO3/KNbO3 superlattices.

We explored the possibility of producing a two-dimensional electron gas (2DEG) in polar/polar (LaAlO3)m/(KNbO3)n perovskite superlattices that have N type and P type interfaces using the first-principles electronic structure calculations. Two different kinds of LaAlO3/KNbO3 superlattices were constructed, namely stoichiometric NP superlattice (NP-SL) and non-stoichiometric NN superlattice (NN-SL). We discovered that the NP-SL undergoes a transition from an insulating to a metallic state when LaAlO3 has more than 3 unit cells. This reveals the completely spin-polarized two-dimensional hole gas (2DHG), as well as 2DEG with an interfacial charge carrier density of n ∼ 1013 cm-2 and an electron effective mass of 0.240me (for 5 unit cells of LaAlO3). In comparison, the NN-SL is intrinsically metallic, and when LaAlO3 has 4.5 unit cells, the structure shows a 2DEG with a larger density (n ∼ 1014 cm-2) and a smaller electron effective mass (0.185me). In addition, the charge carrier properties are highly sensitive to the number of LaAlO3 unit cells in the NP-SL model, while the size effect of LaAlO3 is negligible for the NN-SL one. Our results demonstrate that electronic reconstruction at the interfaces of the stoichiometric structure can produce both the 2DHG and 2DEG, whereas extra electrons are introduced to form solely the 2DEG at the non-stoichiometric structure interfaces. This research provides fundamental insights into the different interfacial electronic properties and the primary mechanism responsible for the formation of polar/polar heterojunction LaAlO3/KNbO3 superlattices.

Photon Noise Suppression by a Built-in Feedback Loop.

Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50% (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization.

P-Diamond as candidate for plasmonic terahertz and far infrared applications

High values of the hole mobility, low contact resistance, and high hole sheet densities in diamond two-dimensional hole gas make p-diamond field effect transistors superb candidates for implementing high temperature plasmonic sub-terahertz, terahertz, and far infrared devices. Our calculations show that p-diamond sub-THz transistors are viable contenders, especially for applications in the 200 to 600 GHz atmospheric window which are of special interest for the beyond 5 G sub-THz communications.

Enhancement of electrons confinement in AlGaN/AlN/GaN heterostructure using BGaN buffer with a small B-content

Since the small lattice constant a of wurtzite BGaN alloy and the good insulating property of its epitaxial layer, the use of the BGaN with a small B-content (0.005–0.02) as a buffer has potential to be a back-barrier for GaN high-electron mobility transistors (HEMTs). This paper presents an improved BGaN back-barrier HEMT structure (AlGaN/AlN/GaN/BGaN buffer) that does not create parasitic electron channel. The energy-band profile and carrier distribution of the BGaN buffer structure are studied by one-dimensional self-consistent simulation. The results show that the BGaN buffer with a very small B-content can provide a sufficient back-barrier to enhance electrons confinement, the principle of which is similar to the formation of back-barrier of AlGaN buffer. When the channel increases to a certain thickness, the low-density two-dimensional hole gas (2DHG) can even be induced at the GaN/BGaN interface by increasing the B-content from 0 to 0.02. Once the 2DHG is formed, continued increase in B-content would result in higher density 2DHG, while the effect on the energy-band profile, electrons confinement, and two-dimensional electron gas (2DEG) density becomes less pronounced.

Modeling charge density in AlGaN/AlN/InGaN/GaN-based double heterostructures including InGaN layer strain relaxation

An analytical model is presented to calculate the two-dimensional electron gas (2DEG) density and barrier height of bare surface AlGaN/AlN/InGaN/GaN double heterostructures, which use InGaN as the conducting layer. The basic model is derived from electrostatic analysis of the different material interfaces. The effect of strain relaxation in the InGaN layer is also incorporated here. Further, the impact of a two-dimensional hole gas at the InGaN/GaN interface, formed when the InGaN layer thickness is high, has been considered. The presented results are seen to agree with the available experimental results. Thus, this model can be a useful tool in the design and modeling of InGaN-based III-nitride heterostructures.

Generation of hole gas in non-inverted InAl(Ga)N/GaN heterostructures

InAlN/GaN structures are grown using organometallic chemical vapor deposition at 730 °C. The sample for which the chamber cleaning step was applied after GaN growth shows a sharp In0.3Al0.7N/GaN transition, free electron density of ∼2 × 1011 cm−2 and mobility of 44 cm2 V−1 s−1. On the other hand, the sample prepared without growth interruption demonstrated In0.4Al0.15Ga0.45N at the interface and compositional grading towards the In0.4Al0.6N surface. In this case a two-dimensional hole gas (2DHG) is created with a density of ∼2 × 1012 cm−2 and mobility of ∼0.6 cm2 V−1 s−1. Ga incorporation in the InAlN barrier is crucial for designing non-inverted 2DHG transistors.

Polarity Control within One Monolayer at ZnO/GaN Heterointerface: (0001) Plane Inversion Domain Boundary.

In semiconductor heterojunction, polarity critically governs the physical properties, with an impact on electronic or optoelectronic devices through the presence of pyroelectric and piezoelectric fields at the active heteropolar interface. In the present work, the abrupt O-polar ZnO/Ga-polar GaN heterointerface was successfully achieved by using high O/Zn ratio flux during the ZnO nucleation growth. Atomic-resolution high-angle annular dark-field and bright-field transmission electron microscopy observation revealed that this polarity inversion confines within one monolayer by forming the (0001) plane inversion domain boundary (IDB) at the ZnO/GaN heterointerface. Through theoretical calculation and topology analysis, the geometry of this IDB was determined to possess an octahedral Ga atomic layer in the interface, with one O/N layer symmetrically bonded at the tetrahedral site. The computed electronic structure of all considered IDBs revealed a metallic character at the heterointerface. More interestingly, the presence of two-dimensional (2D) hole gas (2DHG) or 2D electron gas (2DEG) is uncovered by investigating the chemical bonding and charge transfer at the heterointerface. This work not only clarifies the polarity control and interfacial configuration of the O-polar ZnO/Ga-polar GaN heterojunction but, more importantly, also gives insight into their further application on heterojunction field-effect transistors as well as hybrid ZnO/GaN optoelectronic devices. Moreover, such polarity control at the monolayer scale might have practical implications for heterojunction devices based on other polar semiconductors.

Anisotropic polarization-induced conductance at a ferroelectric-insulator interface.

Coupling between different degrees of freedom, that is, charge, spin, orbital and lattice, is responsible for emergent phenomena in complex oxide heterostrutures1,2. One example is the formation of a two-dimensional electron gas (2DEG) at the polar/non-polar LaAlO3/SrTiO3 (LAO/STO)3-7 interface. This is caused by the polar discontinuity and counteracts the electrostatic potential build-up across the LAO film3. The ferroelectric polarization at a ferroelectric/insulator interface can also give rise to a polar discontinuity8-10. Depending on the polarization orientation, either electrons or holes are transferred to the interface, to form either a 2DEG or two-dimensional hole gas (2DHG)11-13. While recent first-principles modelling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces9,10,12-14, experimental evidence of a ferroelectrically induced interfacial 2DEG remains elusive. Here, we report the emergence of strongly anisotropic polarization-induced conductivity at a ferroelectric/insulator interface, which shows a strong dependence on the polarization orientation. By probing the local conductance and ferroelectric polarization over a cross-section of a BiFeO3-TbScO3 (BFO/TSO) (001) heterostructure, we demonstrate that this interface is conducting along the 109° domain stripes in BFO, whereas it is insulating in the direction perpendicular to these domain stripes. Electron energy-loss spectroscopy and theoretical modelling suggest that the anisotropy of the interfacial conduction is caused by an alternating polarization associated with the ferroelectric domains, producing either electron or hole doping of the BFO/TSO interface.

Spatial mapping of band bending in semiconductor devices using in situ quantum sensors

Band bending is a central concept in solid-state physics that arises from local variations in charge distribution especially near semiconductor interfaces and surfaces. Its precision measurement is vital in a variety of contexts from the optimisation of field effect transistors to the engineering of qubit devices with enhanced stability and coherence. Existing methods are surface sensitive and are unable to probe band bending at depth from surface or bulk charges related to crystal defects. Here we propose an in-situ method for probing band bending in a semiconductor device by imaging an array of atomic-sized quantum sensing defects to report on the local electric field. We implement the concept using the nitrogen-vacancy centre in diamond, and map the electric field at different depths under various surface terminations. We then fabricate a two-terminal device based on the conductive two-dimensional hole gas formed at a hydrogen-terminated diamond surface, and observe an unexpected spatial modulation of the electric field attributed to a complex interplay between charge injection and photo-ionisation effects. Our method opens the way to three-dimensional mapping of band bending in diamond and other semiconductors hosting suitable quantum sensors, combined with simultaneous imaging of charge transport in complex operating devices.