Showing papers in "Applied Physics Letters in 1991"
TL;DR: In this paper, the authors measured the contact potential difference between different materials using scanning force microscopy (SfM) for the first time, using images of gold, platinum, and palladium surfaces taken in air.
Abstract: Measurements of the contact potential difference between different materials have been performed for the first time using scanning force microscopy. The instrument has a high resolution for both the contact potential difference (better than 0.1 mV) and the lateral dimension (<50 nm) and allows the simultaneous imaging of topography and contact potential difference. Images of gold, platinum, and palladium surfaces, taken in air, show a large contrast in the contact potential difference and demonstrate the basic concept.
2,188 citations
TL;DR: In this article, the authors reported visible light emission from Shottky diodes made from semiconducting polymers, confirming the discovery by the Cambridge group [Nature 347, 539 (1990)].
Abstract: We report visible light emission from Shottky diodes made from semiconducting polymers, confirming the discovery by the Cambridge group [Nature 347, 539 (1990)]. Our results demonstrate that light‐emitting diodes can be fabricated by casting the polymer film from solution with no subsequent processing or heat treatment required. Electrical characterization reveals diode behavior with rectification ratios greater than 104. We propose that tunneling of electrons from the recitifying metal contact into the gap states of the positive polaron majority carriers dominates current flow and provides the mechanism for light emission.
2,038 citations
TL;DR: In this article, it was shown that a two-dimensional quantum confinement (quantum wire) in the very narrow walls between the pores not only explains the change in band gap energy but also may also explain the dissolution mechanism that leads to porous silicon formation.
Abstract: Porous silicon layers grown on nondegenerated p‐type silicon electrodes in hydrofluoric acid electrolytes are translucent for visible light, which is equivalent to an increased band gap compared to bulk silicon. It will be shown that a two‐dimensional quantum confinement (quantum wire) in the very narrow walls between the pores not only explains the change in band‐gap energy but may also be the key to better understanding the dissolution mechanism that leads to porous silicon formation.
1,705 citations
TL;DR: In this article, a new three-parameter fit to the temperature dependence of semiconductor band gaps was proposed, based on the semi-empirical Varshni equation.
Abstract: In this letter we advocate the use of a new three-parameter fit to the temperature dependence of semiconductor band gaps. This fitting improves upon the semi-empirical Varshni equation* both numerically, since it gives better fits to the data, and theoretically, since the parameters of the fit may be related to an intrinsic interaction of semiconductors, namely the electron-phonon coupling. Similar expressions to ours have appeared in the literature2T3 but the practical and theoretical justification of this kind of data fit have not previously been worked out in detail. We emphasize that our approach is empirical: we aim simply to describe the data as well as possible with the minimum number of free parameters. The Varshni relation for the temperature dependence of semiconductor band gaps is Eg(T)=Eo--cYT2/(T+pA (1)
1,010 citations
TL;DR: In this paper, the InAs/AlSb double-barrier resonant-tunneling diodes at room temperature were used to achieve a power density of 90 W cm−2 at 360 GHz.
Abstract: Oscillations have been obtained at frequencies from 100 to 712 GHz in InAs/AlSb double‐barrier resonant‐tunneling diodes at room temperature. The measured power density at 360 GHz was 90 W cm−2, which is 50 times that generated by GaAs/AlAs diodes at essentially the same frequency. The oscillation at 712 GHz represents the highest frequency reported to date from a solid‐state electronic oscillator at room temperature.
771 citations
TL;DR: In this article, the authors have grown compositionally graded GexSi1−x layers on Si at 900 °C with both molecular beam epitaxy and rapid thermal chemical vapor deposition techniques.
Abstract: We have grown compositionally graded GexSi1−x layers on Si at 900 °C with both molecular beam epitaxy and rapid thermal chemical vapor deposition techniques. Triple‐crystal x‐ray diffraction reveals that for 0.10
744 citations
TL;DR: In this paper, the average radius of the Ge microcrystals in SiO2 was determined by means of Raman spectroscopy and high resolution electron microscope, which is consistent with quantum confinement of electrons and holes.
Abstract: Ge microcrystals embedded in SiO2 glassy matrices were formed by a radio‐frequency magnetron cosputtering technique and then annealed at 800 °C for 30 min. The average radius of the Ge microcrystals in SiO2 was determined to be about 3 nm by means of Raman spectroscopy and high resolution electron microscope. The annealed sample showed a strong room temperature luminescence with a peak at 2.18 eV. This is consistent with quantum confinement of electrons and holes.
573 citations
TL;DR: In this paper, a predeposition process of several minutes duration was introduced in which a high methane fraction in the feed gas was used and in which negative bias voltage was applied to the substrate.
Abstract: Generation of diamond nuclei has been realized on a silicon mirror surface in plasma chemical vapor deposition. Prior to the normal diamond growth process, a predeposition process of several minutes duration was introduced in which a high methane fraction in the feed gas was used and in which a negative bias voltage was applied to the substrate. This resulted in an enormous enhancement of the generation of diamond nuclei. For the onset of diamond nucleation the minimum voltage was −70 V and the minimum methane fraction in the methane‐hydrogen feed gas was 5%. Density of a diamond nuclei as high as 1010/cm2 was attained with this method.
542 citations
TL;DR: In this paper, the authors reported giant values of saturation magnetoresistance in sputtered antiferromagnetic Co/Cu multilayers containing thin Co and Cu layers 8-10 A.
Abstract: We report giant values of saturation magnetoresistance in sputtered antiferromagnetic Co/Cu multilayers containing thin Co and Cu layers 8–10 A thick. We discuss the key importance of the buffer layer in controlling the growth of flat Co and Cu layers. As shown by cross‐section transmission electron microscopy high‐quality structures are found for growth on Fe buffer layers. Such structures display saturation magnetoresistance at 300 K of more than 65% with saturation fields of ≂10 kOe. These values are several times larger than previously found for any magnetic material at room temperature.
537 citations
TL;DR: In this article, laser diode action in the blue green has been observed from (Zn,Cd)Se quantum wells within ZnSe/Zn(S,Se) p•n heterojunctions up to 250 K.
Abstract: Laser diode action in the blue‐green has been observed from (Zn,Cd)Se quantum wells within ZnSe/Zn(S,Se) p‐n heterojunctions up to 250 K. Operation is reported for two different configurations for which the GaAs substrate serves either as the n‐ or p‐type injecting contact. In pulsed operation, output powers exceeding 0.6 W have been measured in devices prepared on both n‐type and p‐type GaAs epitaxial buffer layers and substrates.
498 citations
TL;DR: In this article, a three-layer organic solar cell with an interlayer of codeposited pigments of n−type perylene tetracarboxylic derivative (Me−PTC) and p−type metal-free phthalocyanine (H2Pc) was fabricated.
Abstract: Three‐layered organic solar cell with an interlayer of codeposited pigments of n‐type perylene tetracarboxylic derivative (Me‐PTC) and p‐type metal‐free phthalocyanine (H2Pc) in between the respective pigment layers, was fabricated. Two times larger photocurrent compared to the double‐layered cell without an interlayer was obtained due to the efficient carrier photogeneration in a codeposited layer. The power conversion efficiency reached 0.7% under the intense white light of 100 mW cm−2.
AT&T1
TL;DR: In this article, the authors used vacuum scanning tunneling microscopy to investigate the hydrogen-terminated Si(111) surfaces obtained upon dissolution of the native oxide in HF and NH4F solutions.
Abstract: Vacuum scanning tunneling microscopy has been used to investigate the hydrogen‐terminated Si(111) surfaces obtained upon dissolution of the native oxide in HF and NH4F solutions. Whereas etching in aqueous HF acid produces an atomically rough surface, comparable treatment in NH4F results in atomically flat surfaces. These atomically flat surfaces are extremely well ordered and exhibit terraces which extend thousands of angstroms.
TL;DR: In this article, a femtosecond time-resolved reflectance (FRS) technique was used to measure the lifetime of a photoconductive switch with a full width at half-maximum of 0.6 ps.
Abstract: Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time‐resolved‐reflectance techniques, a sub‐picosecond (<0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at ∼200°C and annealed at 600 °C. With the same material as a photoconductive switch we have measured electrical pulses with a full‐width at half‐maximum of 0.6 ps using the technique of electro‐optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of ∼120–150 cm2/V s. GaAs grown by MBE at 200 °C and annealed at 600 °C is also semi‐insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of subpicosecond photoconductive applications.
TL;DR: In this paper, the spectral distribution of the emitted light was measured−in situ−during the anodic oxidation step, and recorded spectra show a maximum which shifts continuously from red‐orange at the beginning of the process towards the yellow range.
Abstract: Porous silicon/silicon structures under anodic oxidation conditions give rise to an electroluminescence phenomenon in the visible range. Using an optical multichannel analyzer the spectral distribution of the emitted light was measured−in situ−during the anodic oxidation step. Recorded spectra show a maximum which shifts continuously from red‐orange at the beginning of the process towards the yellow range. The visible emission well above the band gap of bulk silicon is attributed to a quantum size effect in the very small size (5–20 A) silicon island which constitutes the porous silicon skeleton. The light emission is interrupted when the current flow stops due to the formation of a continuous oxide layer at the porous silicon/silicon interface.
TL;DR: In this paper, the zinc blende and wurtzitic GaN films have been epitaxially grown onto (001)Si by electron cyclotron resonance microwave plasma assisted molecular beam epitaxy, using a two-step growth process.
Abstract: Zinc blende and wurtzitic GaN films have been epitaxially grown onto (001)Si by electron cyclotron resonance microwave plasma‐assisted molecular beam epitaxy, using a two‐step growth process. In this process a thin buffer layer is grown at relatively low temperatures followed by a higher temperature growth of the rest of the film. GaN films grown on a single crystalline GaN buffer have the zinc blende structure, while those grown on a polycrystalline or amorphous buffer have the wurtzitic structure.
TL;DR: In this article, a novel metalorganic chemical vapor deposition (MOCVD) system has been developed, which has two different flows, one flow carries a reactant gas parallel to the substrate, and the other flow perpendicular to substrate for the purpose of changing the direction of the reactive gas flow.
Abstract: A novel metalorganic chemical vapor deposition (MOCVD) system, which has two different flows, has been developed. One flow carries a reactant gas parallel to the substrate, and the other an inactive gas perpendicular to the substrate for the purpose of changing the direction of the reactant gas flow. The growth of a GaN film was attempted using this system, and a high quality, uniform film was obtained over a 2 in. sapphire substrate. The carrier concentration and Hall mobility are 1×1018/cm3 and 200 cm2/V s, respectively, which are the highest for GaN films grown directly on a sapphire substrate by the MOCVD method.
TL;DR: The existence of barrier height nonuniformities is shown to provide a simple explanation of the following abnormal experimental results, routinely observed from various Schottky barriers: greater than unity ideality factors, the T0 effect, the "soft" reverse characteristics, and the dependence of the barrier height on the technique of measurement as discussed by the authors.
Abstract: A novel approach is presented which leads to analytic solutions to the potential and the electron transport through inhomogeneous Schottky barriers The existence of barrier height nonuniformities is shown to provide a simple explanation of the following abnormal experimental results, routinely observed from various Schottky barriers: greater‐than‐unity ideality factors, the T0 effect, the ‘‘soft’’ reverse characteristics, and the dependence of barrier height on the technique of measurement
TL;DR: In this article, the photoluminescence (PL) intensity and Fourier transform infrared spectroscopy were performed concurrently on porous Si and it was shown that SiH2 is essential to the visible luminescence in porous Si.
Abstract: Thermal annealing studies of the photoluminescence (PL) intensity and Fourier‐transform infrared spectroscopy have been performed concurrently on porous Si. A sharp reduction in the PL intensity is observed for annealing temperatures ≳300 °C and this coincides with desorption of hydrogen from the SiH2 surface species. A brief etch in HF can restore the luminescence of the samples annealed below 400 °C. We conclude that SiH2 is essential to the visible luminescence in porous Si.
TL;DR: In this paper, optical and electrical measurements reveal uniform films over the thickness range 200-1000 A. They obtain optical absorption coefficients having values between those of Si and Ge and a relative permittivity having a value close to that of amorphous SiO2.
Abstract: Thermal sublimation of pure C60 and C70 has been used for depositing well‐characterized fullerene films on a variety of substrates. Film purity is determined by infrared absorption spectra and the extent of crystallinity of the face‐centered cubic structure by x rays. Thickness‐dependent optical and electrical measurements reveal uniform films over the thickness range 200–1000 A. We obtain optical absorption coefficients having values between those of Si and Ge and a relative permittivity having a value close to that of amorphous SiO2.
TL;DR: In this article, the grain boundary junctions in YBa2Cu3O7 thin films were constructed by controlling the inplane epitaxy of the deposited film using seed and buffer layers.
Abstract: We have developed a new way of making grain boundary junctions in YBa2Cu3O7 thin films by controlling the in‐plane epitaxy of the deposited film using seed and buffer layers. We produce 45° grain boundaries along photolithographically defined lines. The typical value of the critical current density of the junctions is 103–104 A/cm2 at 4.2 K and 102–103 A/cm2 at 77 K, while the rest of the film has a critical current density of 1–3×106 A/cm2 at 77 K. The current‐voltage characteristics of the junctions show resistively shunted junction behavior and we have used them to fabricate dc superconducting quantum interference devices (SQUIDs) which show modulation at temperatures well above 77 K. This is the first planar high Tc Josephson junction technology that appears readily extendable to high Tc integrated circuits.
TL;DR: A bilayer CoSi2/TiN has been grown on (100)Si, starting from a (100 p) type Si wafer deposited with thin layers of Ti followed by Co metal, through a two-stage annealing in a nitrogen environment and an intervening etch.
Abstract: A bilayer CoSi2/TiN has been grown on (100)Si, starting from a (100) p‐type Si wafer deposited with thin layers of Ti followed by Co metal, through a two‐stage annealing in a nitrogen environment and an intervening etch. In this process, Co and Ti switch places to form CoSi2 covered with TiN on Si. Cross‐section transmission electron microscopy, and Rutherford backscattering/channeling spectrometry were used to characterize the bilayer sample. The CoSi2 was found to be single crystal, fully coherent, and epitaxial with (100)Si, whereas the TiN at the top of CoSi2 was polycrystalline. The stress in the CoSi2/TiN layer was found to be 1.9×1010 dynes/cm2. The planar (100) CoSi2/Si interface was interrupted with {111} ledges which are believed to be structural ledges present to maintain the coherency at the interface.
TL;DR: In this article, a superconducting epitaxial YBa2Cu3O7−δ/BaTiO3 thin film was grown on Si(001) by pulsed laser deposition.
Abstract: Epitaxial MgO thin films were grown on Si(001) by pulsed laser deposition. In spite of a large (−22.5%) lattice mismatch, epitaxy occurs with alignment of all crystallographic axes. Epitaxial quality and deposition rate are both sensitive to temperature and oxygen pressure. We believe this is the first demonstration of epitaxial MgO on Si. We employ MgO intermediate layers for superconducting epitaxial YBa2Cu3O7−δ/BaTiO3 thin films on Si with a critical current density of 6.7×105 A/cm2 at 77 K.
TL;DR: In this article, the presence of nuclear magnetic resonance can be detected noninductively by coupling the nuclear spin to the motion of a mechanical oscillator by applying a large gradient magnetic field, which exerts a mechanical force on the particle by virtue of its magnetic moment.
Abstract: The presence of nuclear magnetic resonance can be detected noninductively by coupling the nuclear spin to the motion of a mechanical oscillator. The coupling is obtained by applying a large‐gradient magnetic field, which exerts a mechanical force on the particle by virtue of its magnetic moment. The coupling increases in strength as the oscillator mass is decreased and the gradient length scale is made shorter. Oscillator‐based detection is thus only marginally effective for macroscopic samples, but can be quite effective for single protons interacting with a micron‐scale oscillator. This letter describes the physics of devices in which single‐nucleon magnetic resonance is detected by monitoring the excitation of a mechanical oscillator.
TL;DR: In this article, the growth of p−ZnSe:N films by molecular beam epitaxy, employing a free radical nitrogen source, has been investigated, using this technique they have obtained p−type ZnSe with net acceptor concentrations up to 1.0×1018 cm−3, as measured by capacitance-voltage(C-V) profiling.
Abstract: The growth of p‐ZnSe:N films by molecular‐beam epitaxy, employing a free radical nitrogen source, has been investigated. Using this technique we have obtained p‐type ZnSe with net acceptor concentrations up to 1.0×1018 cm−3, as measured by capacitance–voltage(C–V) profiling−this is the highest ever reported for p‐type ZnSe. By adjusting the flux of active nitrogen and the substrate temperature, films with net acceptor concentrations from 1.0×1016 to 1.0×1018 cm−3 were grown. Evidence of compensation was found in the low temperature photoluminescence and C–V measurements; the degree of compensation depends on the amount of nitrogen incorporated into the film. The dependencies of nitrogen density, net acceptor concentration, and degree of compensation upon the flux of active nitrogen and the substrate temperature are discussed.
TL;DR: In this paper, a modulation-doped Si/GexSi1−x structure was fabricated in which a thin Si layer was employed as the conduction channel for the two-dimensional electron gas.
Abstract: A modulation‐doped Si/GexSi1−x structure was fabricated in which a thin Si layer is employed as the conduction channel for the two‐dimensional electron gas. The strained heterostructure is fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The mobility of the two‐dimensional electron gas as determined from Hall measurements was 1600 cm2/V s at 300 K and 96 000 cm2/V s at 4.2 K. Recently, a 4.2 K mobility of 125 000 cm2/V s was observed from a similar sample.
TL;DR: In this paper, it was shown that hard DLC formed in an intermediate voltage range (100-400 V) consists of small graphitic clusters linked in a random network which is stiffened by a high density of quaternary carbon.
Abstract: Carbon bonding environments (measured by nuclear magnetic resonance spectroscopy) and compressive stress in plasma‐deposited hydrogenated diamond‐like carbon (DLC) films have been examined systematically as a function of substrate bias voltage. These results are related in terms of random network theory to show that hard DLC formed in an intermediate voltage range (100–400 V) consists of small graphitic clusters linked in a random network which is stiffened by a high density of quaternary carbon.
TL;DR: In this article, the dependence of a near-band edge emission on the indium mole fraction of InGaN has been investigated, and the photoluminescence has been observed for the first time.
Abstract: InGaN single‐crystal films were grown on (0001) plane sapphire substrates at 800 °C by metalorganic vapor phase epitaxy. By using such a high temperature for growth, the crystalline quality has been greatly improved. But a high nitrogen over pressure and high indium source flow rate were necessary to achieve significant indium incorporation during growth. For the first time, photoluminescence has been observed in InGaN, and near‐band edge emission is seen in the photoluminescence at 77 K. From this photoluminescence, the dependence of a near‐band edge emission on the indium mole fraction of InGaN has been investigated.
TL;DR: In this paper, thin carbon films were deposited by ion beam sputtering at temperatures of 77-1073 K. Using Rutherford backscattering spectrometry and electron energy loss spectroscopy, the trends in film density and bonding were examined as a function of deposition conditions.
Abstract: Thin carbon films were deposited by ion beam sputtering at temperatures of 77–1073 K. Using Rutherford backscattering spectrometry and electron energy loss spectroscopy, the trends in film density and bonding were examined as a function of deposition conditions. It has been found that film density and sp3 bonding character unexpectedly increased with increased substrate thermal conductivity and decreasing substrate temperature, reaching values of 2.9 g/cc and 50%, respectively.
TL;DR: In this article, a new and highly efficient laser cleaning method was proposed by choosing a pulsed laser with short pulse duration and a wavelength that is strongly absorbed by the surface; the removal efficiency was further enhanced by depositing a liquid film of thickness on the order of micron on the surface just before the pulsing laser irradiation.
Abstract: Laser cleaning with pulsed ultraviolet and infrared lasers is successfully employed to remove particulate contamination from silicon wafer surfaces and from delicate lithography membrane masks. Particulate material investigated include latex, alumina, silicon, and gold. Gold particles as small as 0.2 μm can be effectively removed. This new and highly efficient laser cleaning is achieved by choosing a pulsed laser with short pulse duration (without causing substrate damage), and a wavelength that is strongly absorbed by the surface; the removal efficiency is further enhanced by depositing a liquid film of thickness on the order of micron on the surface just before the pulsed laser irradiation.
TL;DR: In this article, the authors proposed to grow pseudomorphic and heteroepitaxial structures on thin, free-standing substrates, and showed that pseudomorphic layers of arbitrary thickness can be grown on a substrate thinner than the critical thickness.
Abstract: We propose to grow pseudomorphic and heteroepitaxial structures on thin, free‐standing substrates. From the theoretical analysis, pseudomorphic layers of arbitrary thickness can be grown on a substrate thinner than the critical thickness. Even if the substrate is twice as thick as the critical thickness, very high‐quality heteroepitaxy can still be achieved with all the threading dislocations gettered by the thin substrate. The conclusions derived from the theoretical models can be applied to most of the pseudomorphic and heteroepitaxial material systems of current interest.