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We have designed and built a low noise, broadband transimpedance amplifier that allows for sensing both the amplitude and phase of an oscillating quartz tuning fork for a cryogenic atomic force microscope. The circuit uses a global feedback scheme, where the input stage, located next to the quartz tuning fork at low temperature, is followed by an operational amplifier stage operated at room temperature. At 4.2 K, with a 1 MΩ metal film feedback resistor, the amplifier yields an output noise floor of 2×10−7 V/Hz and a bandwidth of 200 kHz. When it is used with a commercial 32 kHz quartz tuning fork, the calibrated sensitivity at 4.2 K is determined to be 30.8 μV/pm.

We have designed and fabricated a cryogenic scanning tunneling microscope for probing lithography defined nanometer-scale devices. The piezoelectric double tube is capable of scanning an area up to 22 μm×22 μm, while maintaining atomic resolution. In addition, the sample mount has a 5 mm×4 mm traveling range. Most importantly, the system is compact and, as a result, it can be inserted into the bore of a superconducting magnet. In this work, we demonstrate a unique application of scanning tunneling system, i.e., the scanning tip is in direct contact with the sample. The spectroscopic information therefore reflects the true characteristics of the devices under test, unlike the typical case where the tunneling barrier through vacuum imposes a large series resistance, on the order of 109 Ω. The design as well as the operation of this compact scanning tunneling microscope is described.

We report a fabrication method for laterally confining the two-dimensional electrons in InAs/AlSb single quantum wells into artificially patterned conducting wires. The minimum wire width is demonstrated to be ∼30 nm, among the smallest reported to date. The confining potential is approximately square and abrupt, and that makes the electron’s spatial distribution in the transverse direction the same as the physical width of the wire. The conducting electrons have close proximity to the surface charges, thus there is always a reduction in the elastic mean free path when the wire width decreases. Despite the reduction in mean free path, we find that the phase coherence length is approximately 1 μm at 2.2 K, a factor of 30 larger than the minimum feature size.

Transmission spectra of GaSb have been obtained over a temperature range from 10 to 470 °C. Using this information, transmission thermometry is applied to obtain accurate measurements of sample temperature during molecular beam epitaxy growth on GaSb substrates. A GaSb surface reconstruction transition is determined as a function of Sb flux and substrate temperature, establishing a laboratory-independent temperature standard.

C-ion irradiation was performed in CoFe/PtMn and CoFe/IrMn systems with an applied field during irradiation to investigate the effects of the energy transfer between C ions and antiferromagnetic atoms (AF) on the exchange field Hex. The irradiated CoFe/PtMn samples showed reduced exchange fields and coercivity, which was attributed to the lattice distortion of PtMn. When samples were irradiated with an applied field antiparallel to the original exchange-bias direction, a partially reversed Hex was found in CoFe/PtMn, while a fully reversed Hex in CoFe/IrMn was observed at the dose of 2×1014 ions/cm2. Rapid energy transfer and temperature increase, originating from the interaction between C ions and AF atoms, resulted in spin reversal and formation of reversed AF domains when samples were irradiated in antiparallel fields. In addition, the thermal relaxation of Hex in CoFe/IrMn was suppressed by ion irradiation.

We report a technique for nanofabrication in the InAs/GaSb/AlSb 6.1 A material system that utilizes the large difference in the surface Fermi level pinning position for InAs [Efs(InAs)] compared with that for AlSb. An InAs/AlSb single quantum well is capped with a 3 nm, intentionally p-doped InAs layer. As a result of its construction and a relatively low Efs(InAs) there are no free carriers in the InAs/AlSb single quantum well making the quantum well insulating as-grown. Simply by selectively removing the thin p-doped InAs cap layer with a wet etch, the surface Fermi level becomes pinned on AlSb and shifted upward by half an electron volt. This results in a drastic change in band bending and creates a conducting electron channel in the buried InAs quantum well. We demonstrate with experiment and the support of a self-consistent band bending calculation that this scheme is highly effective for nanofabrication.

To improve beam quality, an injection‐seeded unstable resonator was applied to a discharge‐pumped F2 laser, which is normally dominated by amplified spontaneous emission with a large beam divergence when using a normal stable cavity, due to the laser’s extremely high gain (a small‐signal gain‐length product ∼29). The time‐dependent injection‐seeding efficiency was obtained for various injection timings by measuring a ratio of the output laser intensity with the same polarization as the seed laser. Correlation of the injection‐seeding efficiency with the temporal evolution of the laser‐beam divergence was also studied. Both the time‐dependent beam divergence and the injection‐seeding efficiency showed degradation at a later part of the laser pulse.

We have investigated magneto‐transport and cyclotron resonance (CR) of two‐dimensional electron gas in silicon δ‐doped p‐InSb under a magnetic field of up to 12 T at 4.2 K. Because there are multiple subbands occupied, Shubnikov–de Haas oscillations show a beating behavior. The CR spectra also display several peaks originating from different subbands. Effective masses of electrons associated with the lowest three subbands can therefore be directly determined, and they are in excellent agreement with a self‐consistent calculation, which takes into account the electrostatic Poisson equation, the Schrodinger equation, and realistic sample parameters. Furthermore, we observed an absorption peak, whose resonance position has anomalous angle dependence. It is attributed to impurity CR where donors are in the vicinity of the δ‐doped sheet.

An approach to the lateral confinement of electrons in an InAs quantum well has been developed. Conducting quantum wires, rings, and dots with lateral dimension ⩾50 nm have been fabricated by using electron-beam lithography and reactive ion etching. Due to the Fermi level pinning at InAs surface, the lateral confinement is abrupt and approximately square. Magnetotransport characterization on a series of fabricated hallbars demonstrate that, despite damage by energetic ions, the electron transport in these quantum wires is in the quasiballistic regime at 4.2 K.

Infrared photoluminescence has been used to study the band-gap energy of InAs1−xSbx digital superlattices and band alignment of InAs1−xSbx/AlSb quantum wells at 5 K. It is found that the InAs1−xSbx digital alloys have a smaller effective band gap than InAs1−xSbx random alloys. In addition, the valence band offset between type-II InAs/AlSb is determined to be 130 meV. This number reduces as the Sb mole fraction in InAs1−xSbx is increased, and the alignment between InAs1−xSbx/AlSb becomes type I when x>0.15.

We report a calculation on the current‐voltage characteristics of double‐barrier resonant tunneling diodes. The results can well‐explain experimental observations, in particular, the temperature dependence of both the absolute current amplitude and the resonant voltage positions. Our calculation takes into account the self‐consistent Poisson equation, quantum mechanical tunneling, and band‐gap offset at the heterojunctions. In addition, the calculation includes a realistic scattering‐induced broadening effect, with which a quantitative fit throughout the entire negative differential resistance (NDR) region is demonstrated. Finally, we show that the differential resistance in the NDR region can be tuned, when asymmetrical tunneling barriers are used.

A tunneling field-effect transistor with an ultrashort channel-length of 25 nm has been experimentally realized using InAs/AlSb heterostructures. The conduction between the source and the drain is through a sequential process, including tunneling and drift-diffusion mechanisms. According to its operating principle, the transistor is inherently free of the conventional short-channel effects. The results demonstrate a new scheme of building nanometer-scale transistors.

We have investigated the transport properties of a field‐effect transistor (FET) with a composite quantum well, which consists of two adjacent semiconductor quantum wells, GaSb and InAs, sandwiched by AlSb barriers. This FET shows a novel V‐shaped transfer characteristic, which is a direct result of the switching between electron and hole channels.

The transverse‐optical (TO) phonon energy in strained InAs quantum wells has been investigated by using far‐infrared absorption. We observe that the TO phonon energy decreases when the misfit‐induced biaxial tension in the InAs single quantum well is increased. Our result shows a stronger phonon energy dependence on the strain than the one reported by Cerdeira et al. [Phys. Rev. B 5, 580 (1972)]. The discrepancy may be explained by stress relaxation near the surface in their experiment. The application of our result will be discussed.

An experimental realization and application of a new quantum transistor is reported herein. The conduction mechanism is a resonant‐tunneling process between regions of two‐dimensional density of states. As a result, the transistor displays new current‐voltage characteristics. A static memory cell using a single transistor is demonstrated.

We reported the observation of steplike drain current and the negative transconductance, in a field‐effect transistor. Our transistor has a double‐quantum‐well channel, and the quantum well close to the surface is depleted. We demonstrate that the novel transistor characteristics are from tunneling of electrons in between two‐dimensional quantum wells. The observed current‐voltage characteristic in gate tunneling current has a large peak‐to‐valley ratio (380–1), which is close to an idealized delta functionlike behavior.

We report the first observation of negative persistent photoconductivity at 4.2 K in an n‐channel modulation doped GaAs/Al0.33Ga0.67As single heterostructure, where two‐dimensional electrons have a mobility of ∼550 000 cm2/V s when density is ∼3.0×1011 cm−2. Based on extensive magnetotransport measurements, we conclude that the negative persistent photoconductivity effect comes from the time dependence of (1) the annihilation of two‐dimensional electrons by photoexcited holes, and (2) the trapping and de‐trapping of photoexcited electrons by shallow donors in doped Al0.33Ga0.67As. A model that quantitatively explains the nonexponential recovery time is presented.

Room‐temperature operation of a quantum transistor demonstrating negative transconductance and power gain is reported. The operating principle is based on resonant tunneling: the resonant tunneling probability between two terminals is strongly modulated by a third terminal. Criteria for this observation are discussed.

We present quantum mechanical self‐consistent calculations on the transfer characteristics of a new resonant tunneling transistor. The model structure consists of a source, a tunneling barrier, a quantum‐well drain, a thick insulator, and a backgate. The tunneling barrier consists of a double‐barrier structure. We demonstrate that based on energy and momentum conservations, the transistors display oscillatory negative transconductance, as the gate can control the resonant tunneling probability between source and drain. With the inclusion of a realistic energy relaxation time of ∼0.1 ps, the double‐barrier resonant tunneling transistor shows an enhancement of the tunneling current density and the negative transconductance feature is only slightly changed. We also find that the quantum‐well drain is not able to completely screen the electric field imposed by the backgate bias as a result of limited density of states of two‐dimensional systems.

We show that for narrow band‐gap semiconductors, the doping concentration deduced from capacitance‐voltage measurements can be substantially underestimated due to surface quantization effect. While our derivation can be applied to all materials, detailed accounts on p‐type Hg0.788Cd0.212Te are given as an example. We found that for a practical doping range, 1×1015 cm−3 to 1×1017 cm−3, the doping level can be underestimated by 53% to 173%, respectively.

We report experimental observation on the area and temperature dependence of the hysteresis in the current‐voltage characteristics of double‐barrier resonant tunneling diodes. Our observation shows that the hysteresis can simply result from a load line effect, since (1) the hysteresis will disappear when the the device area is reduced, and (2) the hysteresis becomes wider at lower temperatures. We compare the data with theoretical predictions from the intrinsic bistability picture, and obtain the criterion for observing current bistability.

We investigated the current‐voltage characteristics of double‐barrier resonant tunneling diodes with areas ranged from ∼400 μm2 to ∼10 μm2 at a temperature of 300 and 4.2 K. For diodes of large areas, the characteristic negative differential resistance is always accompanied by a hysteresis and oscillation at several tens of MHz. However, for smaller diodes, the hysteresis and oscillation disappear, and the tunneling current shows a smooth transition between peak and valley. Our observation shows that the hysteresis can result from a load line effect. Were the hysteresis an intrinsic behavior, it should persist as the device area is reduced. We therefore conclude that the intrinsic bistability is yet to be experimentally confirmed.

Our micromagnetic calculation demonstrates that the dominant coercivity mechanism is self-pinning in most exchange-coupled permanent and composite magnets. Such a pinning is attributed to the change of the intrinsic parameters associated with the phase change at the interface. From this self-pinning some more specific formulas on pinning field can be derived. In particular, for sufficiently large soft grains/defects, the pinning field can be expressed as HP=αHK, where HK=2k∕MS is the anisotropy field and α depends on the material parameters and micromagnetic structures. For an exchange-coupled Nd2Fe14B–αFe system with abrupt change of parameters in the interface, α≈0.1. Reducing the size of the soft grain will increase the coercivity, while the smooth change of the parameters in the interface will lead to a reduction in the coercivity. Comparison with experimental data justifies our calculation.

Exchange bias may be used to couple a normal ferromagnet to a ferroelectric antiferromagnet and thus create a multiferroic system with nonzero magnetization. In implementing this idea the authors developed a synthesis method for composite films of nanoscale clusters of ferromagnetic MnFe2O4 embedded in multiferroic Mn-doped BiFeO3. The method utilizes the Bi volatility to obtain the composite films via thermal annealing of multilayers composed of BiFeO3 and BiMnO3. The cluster size varies from 20to80nm depending on the film thickness. The composite films possess both ferroelectric and ferromagnetic properties; the magnetic hysteresis curves, in particular, manifest exchange bias.

An efficient near-infrared (NIR) quantum-cutting (QC), involving the emission of two low-energy NIR photons from an absorbed visible photon via a cooperative downconversion mechanism in GdBO3:Tb3+,Yb3+ nanophosphors, has been demonstrated. Upon excitation of Tb3+ with a visible photon at 486nm, two NIR photons could be emitted by Yb3+ through cooperative energy transfer from Tb3+ to two Yb3+ ions. The dependence of Yb3+ doping concentration on the visible and NIR emissions, decay lifetime, and quantum efficiencies from the QC phosphors has been investigated. Calculations indicate that the optimal NIR quantum efficiency approaches 182% before reaching concentration quenching threshold. Application of the QC nanophosphors in silicon-based solar cells might greatly enhance its response.

Near-infrared (NIR) quantum cutting (QC) involving the emission of two NIR photons per absorbed photon via a cooperative downconversion mechanism in one-dimensional (1D) (YbxGd1−x)Al3(BO3)4:Tb3+ nanorods has been demonstrated. The authors have analyzed the measured luminescence spectra and decay lifetimes and proposed a mechanism to rationalize the QC effect. Upon excitation of Tb3+ with a blue-visible photon at 485nm, two NIR photons could be emitted by Yb3+ through an efficient cooperative energy transfer from Tb3+ to two Yb3+ with optimal quantum efficiency as great as 196%. The development of 1D Tb3+–Yb3+ QC nanomaterials could open up a possibility to realize high efficiency silicon-based solar cells by means of downconversion of the green-to-ultraviolet part of the solar spectrum to ∼1000nm photons with a twofold increase in the photon number.

The onset of antiferromagnetic transition, spin reorientation, and spin compensation of TmFeO3 single crystals were investigated by the magnetic and heat capacity measurements. Control of spin reorientation by magnetic field and anomalous hysteretic behavior in domain switching were clarified. No appreciable magnetodielectric effect was observed in spin reorientation temperature region. On the other hand, below the spin compensation temperature both a dielectric anomaly along the c axis and a concomitant magnetodielectric effect up to ∼4% at 80kOe were observed. This suggests that rare-earth orthoferrites can be another candidate for magnetodielectric system through the mediation of spin compensation phenomena.

The authors demonstrate a silicon-based single-electron transistor (SET) in the few-electron regime. Our structure is similar to a metal-oxide-semiconductor field-effect transistor. The substrate, however, is undoped and could be isotope enriched so that any nonuniformity and spin decoherence due to impurity and nuclear spins can be minimized. A bilayer-gated configuration provides flexibility in manipulating single electrons. The stability chart measured at 4.2K shows diamondlike domains with a charging energy of 18meV, indicating a quantum dot of 20nm in diameter. The benefits of using this enhancement-mode SET in silicon and its potential application for scalable quantum computing are discussed.

We report on the demonstration of a silicon-based quantum cellular automata (QCA) unit cell incorporating two pairs of metallically doped (n+) phosphorus-implanted nanoscale dots, separated from source and drain reservoirs by nominally undoped tunnel barriers. Metallic cell control gates, together with Al–AlOx single electron transistors for noninvasive cell-state readout, are located on the device surface and capacitively coupled to the buried QCA cell. Operation at subkelvin temperatures was demonstrated by switching of a single electron between output dots, induced by a driven single electron transfer in the input dots. The stability limits of the QCA cell operation were also determined.

We propose and demonstrate an enhancement-mode AlxGa1−xAs∕GaAs single heterojunction field-effect transistor, in which the gate dielectric consists of an epitaxial AlxGa1−xAs layer and spin-on benzocyclobutene. The as-grown sample is undoped and, as a result, nonconductive at 4.2K. However, under a positive gate bias, high mobility two-dimensional electrons at the heterointerface are induced, as evidenced by magnetotransport characteristics. Numerical simulations of a model structure provide insight into applications in mesoscopic devices.

The dispersion and intensity of coupled plasma excitation in an electron-hole bilayer with Rashba spin-orbit coupling is calculated. We propose to use the spin-orbit coupling in individual layers to tune the intensity of two plasmons. The mechanism can be used to develop a two color terahertz source with tunable intensities.

We report a design procedure and performance of a voltage amplifier using commercially available GaAs metal-semiconductor field-effect transistors. There are two stages: a common-source amplifier that provides a moderate gain, followed by a common-drain stage for driving the 50Ω coaxial cable. The overall voltage gain measured at 4.2K with a 50Ω load is 3.3, with a bandwidth of 36MHz. The input-referred voltage noise is measured to be approximately 3.2nV∕√Hz in a wide frequency range.

C-ion irradiation was performed on PtMn-based spin valves and the effects of ion irradiation on magnetoresistance (MR) and on the exchange field were investigated. When ion irradiation was performed prior to post-annealing, the exchange field was substantially enhanced after annealing, especially for an annealing temperature below 240 °C. The enhancement could be related to the formation of PtMn(C) phase, which might accelerate the formation of ordered PtMn phase and might increase the anisotropy of PtMn. Although the majority of C ions located in capping layers Ta and PtMn, small amounts of C ions may go into the interface of CoFe/Cu/CoFe, leading to a small MR.

The stress relaxation behavior during thermal cycling of metal/quartz and metal/polyimide/quartz layered structures has been investigated using a cantilever bending beam technique. The metals chosen for this study include Cu and Cr, two materials with contrasting mechanical and interfacial bonding properties. A finite element analysis, as well as an analytical calculation, has been carried out to deduce the stress distribution in the layered structures. Results indicate that the extent of stress relaxation strongly depends on the intrinsic mechanical property of the metal film with significantly higher stresses residing in the Cr film than in the Cu film. The interfacial polyimide layer has been found to serve as an effective buffering layer to reduce the residual stress in metal films. Observations from transmission electron microscopy suggest that the stress is partially released through the deformation of the polyimide near the metal/polyimide interface.

Adhesion and interfacial failure of Cr, Cu, and Cu/Cr films on a thin pyromellitic dianhydride‐oxydianiline polyimide substrate have been investigated using a recently developed stretch deformation technique. This method directly measures the energy required to separate the interface from the difference in the load versus elongation curves between film/substrate and substrate structures. The failure mode was observed in combination with morphological studies by in situ optical microscopy and scanning and transmission electron microscopies. The stress distribution in the sample upon stretching has been computed by a finite element analysis. The difference in the stress relief modes, film fracture, and delamination behavior for Cu versus Cr films is discussed. The important role of crack formation in the metal overlayer for initiating failure is demonstrated. The effects due to the addition of a brittle Cr interface layer between polyimide and Cu on the buildup of stress and the onset of failure have also b...

A simple function with two undetermined parameters has been used in place of the Thomson-Gibbs relation to relate the activation energy of the vaporization reaction to cluster size. The parameters are iterated to assume optimum values in numerical computation so experimental data may be correlated. Calculations show this approach closely predicts and correlates available data for water, benzene, and ethanol. The nucleation formulism is redeveloped with an emphasis on the chemical kinetic view. Surface tension of the liquid and free energy of droplet formation are not used in its derivation.

The conductance method has been used to measure the density of interface states of the ZnS/Hg0.775Cd0.225Te metal‐insulator‐semiconductor (MIS) system with three different HgCdTe surface treatments. It is found that the density of fast interface states increases from ∼1011 eV−1 cm−2 at the conduction‐band minimum to ∼1013 eV−1 cm−2 near the valence‐band maximum. In addition, the interface states located in the lower part of the band gap communicate with the valence band so efficiently that the effective band gap is reduced. Our observations explain why the p‐type MIS photodiode is superior to the n‐type version in terms of breakdown voltage and storage time.

We have studied photoluminescence from GaAs/Al0.3Ga0.7 As modulation‐doped single heterojunctions, using excitation sources from infrared to ultraviolet near‐liquid‐helium temperature. The spectra have a strong interface component, accompanied by bulk GaAs and AlGaAs band‐gap luminescence. Using ultraviolet instead of infrared as the excitation, the interface signal is greatly enhanced relative to the bulk GaAs luminescence. The interface signal can be shifted to the higher energies when a semitransparent front gate is positively biased. Our observations indicate that the interface luminescence comes from the recombination of the two‐dimensional electrons with holes trapped at or near the interface. The peaks and the shoulders in the interface spectra appear to correspond to subbands of the two‐dimensional electron gas at the interface. Our results demonstrate the feasibility of doing spectroscopic studies on high‐mobility electrons at single heterojunctions.

We demonstrate for the first time that ion implantation and implant activation annealing, combined with a heavily doped InGaAs surface layer, can be used to make nonalloying shallow ohmic contact to an n‐type InGaAs (or GaAs) quantum well. Quantum Hall effect and Shubnikov–de Haas oscillations are clearly observed, which indicates that electrons in the quantum well remain two dimensional despite the post‐implantation high‐temperature annealing. This technique can be applied to devices that would need to make shallow ohmic contact to a thin (∼100 A or less) quantum well, where existing selective etching approaches fail to work.

We present device characteristics of a field‐effect, unipolar, resonant tunneling transistor. An oscillatory tunneling current in the transfer characteristics is observed for the first time. Our observation confirms a recent hypothesis that a mere three‐to‐two dimensional resonant tunneling can occur when scattering rate is less than the attempt frequency of tunneling electrons in the quantum well.