Ha Sul Kim

Ultrathin compound semiconductor on insulator layers for high-performance nanoscale transistors

Over the past several years, the inherent scaling limitations of silicon (Si) electron devices have fuelled the exploration of alternative semiconductors, with high carrier mobility, to further enhance device performance. In particular, compound semiconductors heterogeneously integrated on Si substrates have been actively studied: such devices combine the high mobility of III–V semiconductors and the well established, low-cost processing of Si technology. This integration, however, presents significant challenges. Conventionally, heteroepitaxial growth of complex multilayers on Si has been explored—but besides complexity, high defect densities and junction leakage currents present limitations in this approach. Motivated by this challenge, here we use an epitaxial transfer method for the integration of ultrathin layers of single-crystal InAs on Si/SiO2 substrates. As a parallel with silicon-on-insulator (SOI) technology, we use ‘XOI’ to represent our compound semiconductor-on-insulator platform. Through experiments and simulation, the electrical properties of InAs XOI transistors are explored, elucidating the critical role of quantum confinement in the transport properties of ultrathin XOI layers. Importantly, a high-quality InAs/dielectric interface is obtained by the use of a novel thermally grown interfacial InAsOx layer (~1xa0nm thick). The fabricated field-effect transistors exhibit a peak transconductance of ~1.6xa0mSxa0µm−1 at a drain–source voltage of 0.5xa0V, with an on/off current ratio of greater than 10,000.

Quantum Confinement Effects in Nanoscale-Thickness InAs Membranes

Nanoscale size effects drastically alter the fundamental properties of semiconductors. Here, we investigate the dominant role of quantum confinement in the field-effect device properties of free-standing InAs nanomembranes with varied thicknesses of 5-50 nm. First, optical absorption studies are performed by transferring InAs quantum membranes (QMs) onto transparent substrates, from which the quantized sub-bands are directly visualized. These sub-bands determine the contact resistance of the system with the experimental values consistent with the expected number of quantum transport modes available for a given thickness. Finally, the effective electron mobility of InAs QMs is shown to exhibit anomalous field and thickness dependences that are in distinct contrast to the conventional MOSFET models, arising from the strong quantum confinement of carriers. The results provide an important advance toward establishing the fundamental device physics of two-dimensional semiconductors.

Ultrathin body InAs tunneling field-effect transistors on Si substrates

An ultrathin body InAs tunneling field-effect transistor on Si substrate is demonstrated by using an epitaxial layer transfer technique. A postgrowth, zinc surface doping approach is used for the formation of a p+ source contact which minimizes lattice damage to the ultrathin body InAs compared to ion implantation. The transistor exhibits gated negative differential resistance behavior under forward bias, confirming the tunneling operation of the device. In this device architecture, the ON current is dominated by vertical band-to-band tunneling and is thereby less sensitive to the junction abruptness. The work presents a device and materials platform for exploring III–V tunnel transistors.

Nanoscale InGaSb heterostructure membranes on Si substrates for high hole mobility transistors.

As of yet, III-V p-type field-effect transistors (p-FETs) on Si have not been reported, due partly to materials and processing challenges, presenting an important bottleneck in the development of complementary III-V electronics. Here, we report the first high-mobility III-V p-FET on Si, enabled by the epitaxial layer transfer of InGaSb heterostructures with nanoscale thicknesses. Importantly, the use of ultrathin (thickness, ~2.5 nm) InAs cladding layers results in drastic performance enhancements arising from (i) surface passivation of the InGaSb channel, (ii) mobility enhancement due to the confinement of holes in InGaSb, and (iii) low-resistance, dopant-free contacts due to the type III band alignment of the heterojunction. The fabricated p-FETs display a peak effective mobility of ~820 cm(2)/(V s) for holes with a subthreshold swing of ~130 mV/decade. The results present an important advance in the field of III-V electronics.

Benchmarking the performance of ultrathin body InAs-on-insulator transistors as a function of body thickness

The effect of body thickness (5-13 nm) on the leakage currents of top-gated, InAs-on-insulator field-effect-transistors with a channel length of ∼200 nm is explored. From a combination of experiments and simulation, it is found that the OFF-state currents are primarily dominated by Shockley Read Hall recombination/generation and trap-assisted tunneling. The OFF currents are shown to decrease with thickness reduction, highlighting the importance of the ultrathin body device configuration. The devices exhibit promising performances, with a peak extrinsic and intrinsic transconductances of ∼1.7 and 2.3 mS/μm, respectively, at a low source/drain voltage of 0.5 V and a body thickness of ∼13 nm.

Strain engineering of epitaxially transferred, ultrathin layers of III-V semiconductor on insulator

Strain state of ultrathin InAs-on-insulator layers obtained from an epitaxial transfer process is studied. The as-grown InAs epilayer (10–20 nm thick) on the GaSb/AlGaSb source wafer has the expected ∼0.62% tensile strain. The strain is found to fully release during the epitaxial transfer of the InAs layer onto a Si/SiO2 substrate. In order to engineer the strain of the transferred InAs layers, a ZrOx cap was used during the transfer process to effectively preserve the strain. The work presents an important advance toward the control of materials properties of III-V on insulator layers.

Ultrathin-Body High-Mobility InAsSb-on-Insulator Field-Effect Transistors

Ultrathin-body InAsSb-on-insulator n-type field-effect transistors (FETs) with ultrahigh electron mobilities are reported. The devices are obtained by the layer transfer of ultrathin InAs_0.7Sb_0.3 layers (thickness of 7-17 nm) onto Si/SiO₂ substrates. InAsSb-on-insulator FETs exhibit an effective mobility of ~ 3400 cm²/V·s for a body thickness of 7 nm, which represents ~ 2× enhancement over InAs devices of similar thickness. The top-gated FETs deliver an intrinsic transconductance of ~ 0.56 mS/μm (gate length of ~ 500 nm) at <i>V</i>_DS = 0.5 V with <i>I</i>_ON/<i>I</i>_OFF of 10²-10³. These results demonstrate the utility of the transfer process for obtaining high-mobility n-FETs on Si substrates by using mixed anion arsenide-antimonide as the active channel material.

Collective acceleration of carbon ions to 170 MeV

The collective acceleration of carbon ions to a peak energy in the range 170–200 MeV has been achieved using a 6‐MeV 190‐kA 100‐ns electron beam pulse generated by the Pulserad 1590 facility at Kirtland Air Force Base. Accelerated ions were detected using nuclear activation techniques.

The effect of absorber doping on electrical and optical properties of nBn based type-II InAs/GaSb strained layer superlattice infrared detectors

We have investigated the electrical and optical properties of a nBn based InAs/GaSb strained layer superlattice detector as a function of absorber region background carrier concentration. Temperature dependent dark current, responsivity, and detectivity were measured. The device with a nonintentionally doped absorption region demonstrated the lowest dark current density with a specific detectivity at zero bias equal to 1.2×1011u2002cmu2009Hz1/2/W at 77 K. This value decreased to 6×1010u2002cmu2009Hz1/2/W at 150 K. This contrasts significantly with p-i-n diodes, in which the D∗ decreases by over two orders of magnitude from 77 to 150 K, making nBn devices promising for higher operating temperatures.

Infrared photodiodes based on Type-II strained layer superlattices

The InAs/GaSb Type II strained layer superlattice (SLS) is promising III-V material system for infrared (IR) devices due to the ability to engineer its bandgap between 3-30 μm and potentially have many advantages over current technologies such as high uniformity smaller leakage current due to reduced Auger recombination which are crucial for large IR focal plane arrays. However, an issue with this material system is that it relies on growth on GaSb substrates. These substrates are significantly more expensive than silicon, used for HgCdTe detectors, lower quality and are only available commercially as 3 diameters. Moreover it has to go through thinning down before it could be hybridized to readout integrated circuits. GaAs substrate is a possible alternative. We report on growth and characterisation of Type-II InAs/GaSb SLS photodiodes grown on GaAs substrates for mid-wave infrared with peak responses of 3.5 μm at 77K and 4.1 μm at 295K. Comparisons with similar structure grown on GaSb substrates show similar structural, optical and electrical characteristics. Broadening of X-ray rocking curves were observed on the structure grown on GaAs substrate. A full width half maximum (FWMH) of 25.2 arc sec. for the superlattice was observed near ~30.4 degree for the structure on GaSb substrate compared to near ~30.4 degree for structure grown on GaAs. However peak responsivity values of ~ 1.9 A/W and ~ 0.7 A/W were measured at 77K and 295K for devices grown on GaAs substrate. Room temperature responsivity suggests that these photodiodes are promising as high temperature IR detectors.