Large Reverse Bias Research Articles

Coexistence of deep levels with optically active InAs quantum dots

We present direct experimental evidence of the coexistence of deep levels with intrinsic quantum confinement states in large, self-assembled InAs quantum dots embedded in a GaAs matrix. The InAs quantum dots show very good optical properties, as evidenced by the strong photoluminescence at room temperature at $\ensuremath{\sim}1.3\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Deep levels 160 and 484 meV below the GaAs conduction band edge have been identified at large reverse biases and high temperatures using deep level transient spectroscopy (DLTS) measurements. The reverse-bias dependence of the DLTS signal together with experimental results from the reference samples, containing thin InAs layers but no quantum dots, confirms that the deep levels coexist in the same layer as the InAs dots, and are most likely caused by the strain field during the lattice mismatched growth process. The densities of the deep levels in the structure are comparable to the density of the optically active quantum dots.

Carbon Nanotube Schottky Diodes Using Ti−Schottky and Pt−Ohmic Contacts for High Frequency Applications

We have demonstrated Schottky diodes using semiconducting single-walled nanotubes (s-SWNTs) with titanium Schottky and platinum Ohmic contacts for high-frequency applications. The diodes are fabricated using angled evaporation of dissimilar metal contacts over an s-SWNT. The devices demonstrate rectifying behavior with large reverse bias breakdown voltages of greater than -15 V. To decrease the series resistance, multiple SWNTs are grown in parallel in a single device, and the metallic tubes are burnt-out selectively. At low biases these diodes showed ideality factors in the range of 1.5 to 1.9. Modeling of these diodes as direct detectors at room temperature at 2.5 terahertz (THz) frequency indicates noise equivalent powers (NEP) potentially comparable to that of the state-of-the-art gallium arsenide solid-state Schottky diodes, in the range of 10(-13) W/ radical Hz.

Comparative study of GaAs corrosion in H2SO4 and NH3·H2O solutions by electrochemical methods and surface analysis

Corrosion of GaAs can readily happen in acidic and alkaline aqueous electrolytes. The instability of GaAs is a major difficulty in implementing GaAs-based photoelectrochemical solar energy-conversion systems. In this paper, the corrosion behavior was investigated by electrochemical methods and surface-analysis approaches, in both H2SO4 and NH3·H2O solutions. Open-circuit testing showed that the GaAs surface was very sensitive to water/air although the process to build a stable interface between GaAs and electrolyte is slow. Comparison of the impedance data for n- and p-GaAs showed that an inductive loop only appeared at a high corrosion rate. This induction loop was due to the Faradaic adsorption process being the rate-determining step. This can only be explained by a mechanism in which a reaction intermediate, instead of holes, after capture of the first hole is the oxidant for further decomposition steps in combination with a chemical reaction with water. The results suggest that the same corrosion/decomposition mechanism occurred on both n- and p-GaAs and also in the two solutions. When the surface hole concentration was low, the hole injection or generation process was the rate-determining step. At large reverse bias on n-GaAs, with high hole concentration or forward biasing of p-GaAs, the corrosion rate was limited by a Faradaic adsorption process, involving an adsorbed intermediate. The different surface compositions after corrosion in the two solutions can be explained by the dissolution of As(0). It is pointed out that the further oxidation of As(0) to As(III) in NH3·H2O solution is still slow, as suggested from X-ray photoelectron spectroscopy (XPS) analysis.

Novel optical probes of InGaN/GaN light‐emitting diodes: 1. Electroreflectance Stark spectroscopy, and 2. Time‐resolved emission

New optical techniques have been implemented to compare LED properties with fundamental models. Voltage-dependent, contacted electroreflectance measurements and step-recovery measurements are reported for an InGaN/GaN single quantum well LED. Electroreflectance spectra failed to exhibit a blue-shift at small reverse bias, as predicted by Schrödinger-Poisson quantum well band solutions, suggesting that excess charge trapped in localized states enhances screening of the polarization electric field. In contrast, at large reverse bias where the QW is depleted of charge, a predicted red-shift was observed. Electrical and optical responses of the LED to forward-to-reverse bias step-recovery measurements were self-consistently fit to a diffusion equation solution, providing values of minority-carrier hole transport parameters: τp ≈ 760 ns, Lp ≈ 590 nm, and μp ≈ 0.2 cm2/Vs. The long lifetime is suggestive of slow emission processes occuring through deep levels, and the small mobility is indicative of trap-modulated transport. Our results demonstrate the highly non-ideal nature of GaN-based LEDs grown on sapphire. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reliability of multistacked tantalum-based structure as the barrier film in ultralarge-scale integrated metallization

Diffusion barrier properties of Ta films with and without plasma treatments have been investigated in the study. The nitrogen-incorporated Ta films were prepared by NH 3 plasma treatment or reactive sputtering. Barrier properties were evaluated by sheet resistance, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and reverse-biased junction leakage current. An amorphous-like TaN x layer was formed on Ta barrier film after plasma treatments. The thickness of the amorphous TaN x layer is about 3 nm and NH 3 plasma-treated Ta films (TaN x /Ta) possess lower resistivity and smaller grain sizes. The Cu/TaN x /Ta(10 nm)/Si remained stable after annealing at 750 °C for 1 h. NH 3 plasma-treated Ta films (TaN x /Ta) possess better thermal stability than Ta and TaN films. It is attributed to the formation of a new amorphous layer on the surface of Ta film after the plasma treatments. For thermal stability of Cu/Ta(-N)/n +-p diodes, Cu/Ta/n +-p and Cu/TaN/n +-p junction diodes resulted in large reverse-bias junction leakage current after annealing at 500 and 525 °C, respectively. On the other hand, TaN x /Ta diffusion barriers will improve the integrity of Cu/Ta(-N)/n +-p junction diodes to 650 °C.

Amorphous Silicon Based p-i-i-n Structure for Color Sensor

AbstractThis work deals with the study of the role of the film thickness and composition on the color selectivity of the collecting spectrum of glass/ZnO:Ga/p-a-Si1-xCx:H/ a-Si1-xCx:H /a-Si:H/n-a-Si:H/Al photoelectronic detectors produced in a single chamber plasma enhanced chemical vapor deposition (PECVD) system. The cross contaminations were minimized by a rotate-cover substrate holder system. The devices can detect the blue illumination at small reverse bias and detect red illumination at large reverse bias. The role of the process parameters, especially the thickness of the p-type and intrinsic a-Si1-xCx:H, and the intrinsic a-Si:H layers on the device performances were studied in detail aiming to achieve a better detectivity.

Electroreflectance studies of Stark shifts and polarization-induced electric fields in InGaN/GaN single quantum wells

To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In0.07Ga0.93N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schrödinger–Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states.

Analysis of reverse-bias leakage current mechanisms in GaN grown by molecular-beam epitaxy

Temperature-dependent current–voltage measurements have been used to determine the reverse-bias leakage current mechanisms in Schottky diodes fabricated on GaN grown by molecular-beam epitaxy, and two dominant mechanisms are clearly identified. The first mechanism is field-emission tunneling from the metal into the semiconductor, which is dominant at low temperatures and which, at higher temperatures, becomes significant for large reverse-bias voltages. The second mechanism, presumed to be associated with dislocation-related leakage current paths, is observed to have an exponential temperature dependence and becomes significant above approximately 275 K. The temperature dependence of the second mechanism is consistent with either one-dimensional variable-range-hopping conduction along the dislocation or trap-assisted tunneling.

Assessment of InSb photodetectors on Si substrates

We present the detailed characteristics of small area (33×33 μm2) InSb photodiodes grown on GaAs coated Si substrates by molecular beam epitaxy. In spite of very large lattice mismatch, 80 K peak detectivity of ∼1×1010 cm Hz1/2 W−1 has been measured under backside illumination without anti-reflection coating. Differential resistance at 80 K is limited by Ohmic leakage under small reverse bias and trap assisted tunnelling (TAT) under moderately large reverse bias. In the temperature range of 80–140 K, the zero-bias resistance is considerably degraded by Ohmic leakage which has a small activation energy of 25 meV. While the excess leakage seems to be related with the dislocations, the peak quantum efficiency is not considerably affected by the presence of dislocations. 80 K 1/f noise is dominated by TAT processes, and the noise current at 1 Hz follows the empirical relation in = αTAT(ITAT)β with αTAT = 7.5×10−7 and β = 0.55. Bias dependence of the generation–recombination noise is discussed and compared with the predictions of Kleinpenning's mobility fluctuation model.

Characteristics of InAs0.8Sb0.2 photodetectors on GaAs substrates

As an alternative to HgCdTe, InAsxSb1−x is an important alloy providing bandgaps suitable for infrared detection in both 3–5 and 8–12 μm atmospheric windows. There has been interest in high-quality growth of this material on GaAs and Si substrates. This paper reports the fabrication and characteristics of mid-infrared InAs0.8Sb0.2 photodetectors grown on GaAs substrates by molecular beam epitaxy. Peak detectivities of 1.4 × 1010 and 7.5 × 108 cm Hz1/2 W−1 were measured at 77 K and 240 K, respectively, showing that the alloy is promising as an alternative to thermal detectors for near room temperature operation. A detailed analysis of the dark current showed that the reverse-bias differential resistance was limited by trap-assisted tunneling in a wide temperature range up to 150 K under low and moderately large reverse bias voltages. Noise measurements have shown that trap assisted tunneling is the source of 1/f noise which is increased with the addition of band to band tunneling under large reverse bias. Our observations on InAs0.8Sb0.2 photodiodes show resemblance to those reported for HgCdTe detectors. The results are encouraging for the development of mid-infrared large area focal plane arrays on GaAs substrates.

Device physics of organic light-emitting diodes based on molecular materials

Abstract Electrical transport in single- and hetero-layer organic light-emitting diodes based on aromatic amines like N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) or N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) and the aluminium chelate complex Alq (tris(8-hydroxyquinolato)aluminium) has been investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current–voltage (I–V) characteristics provides a unique criterion to discriminate between (1) injection limited behaviour, (2) trap-charge limited conduction with an exponential trap distribution and a field-independent mobility, and (3) trap-free space-charge limited conduction (SCLC) with a field and temperature dependent mobility. The I–V characteristics of NPB-based hole-only devices with indium–tin oxide anodes are neither purely injection nor purely space-charge limited, although the current shows a square-law dependence on the applied voltage. In Al/Alq/Ca electron-only devices with Alq thickness in the range 100–350 nm the observed thickness and temperature dependent I–V characteristics can be described by SCLC with a hopping-type charge carrier mobility. Additionally, trapping in energetically distributed trap states is involved at low voltages and for thick layers. The electric field and temperature dependence of the charge carrier mobility in Alq has been independently determined from transient electroluminescence. The obtained values of the electron mobility are consistent with temperature dependent I–V characteristics and can be described by both the phenomenological Poole–Frenkel model with a zero-field activation energy ΔE=0.4–0.5 eV and the Gaussian disorder model with a disorder parameter σ=100 meV. Measurements of the bias-dependent capacitance in NPB/Alq hetero-layer devices give clear evidence for the presence of negative charges with a density of about 6.8×10 11 cm −2 at the organic–organic interface under large reverse bias. This leads to a non-uniform electric field distribution in the hetero-layer device, which has to be considered in device description.

Interfacial charges and electric field distribution in organic hetero-layer light-emitting devices

Abstract The electric field distribution in organic hetero-layer light-emitting devices based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1′-biphenyl-4,4′-diamine (NPB) and 8-tris-hydroxyquinoline aluminium (Alq3) has been investigated under different bias conditions using capacitance–voltage measurements. Although this method yields primarily information on the differential capacitance, the data give clear evidence for the presence of negative interfacial charges with a density of 6.8×10 11 e cm −2 at the NPB/Alq3 interface at large reverse bias. This leads to a jump of the electric field at the interface and a non-uniform field distribution in the hetero-layer device.

Junction Properties of Metal/SrTiO3 Systems

AbstractElectrical properties of Nb-doped SrTiO3 (STO:Nb) Schottky barrier (SB) junctions have been investigated in detail for a comprehensive understanding of metal/oxide interfaces. Using a high-purity ozone surface treatment, rectification ratio over 9th order of magnitude has been successfully obtained, while without the surface treatment, anomalous large reverse bias leak currents were observed in the current-voltage characteristic of the junctions. The X-ray photoelectron spectroscopy (XPS) shows that carbon contamination which adsorbed the STO:Nb surface in air, induces surface states in the band gap of the STO:Nb, which probably originate the large reverse bias leak currents of the metal/STO:Nb junctions. Thus we present importance, of surface treatment for oxides to obtain controllability and reproducibility of the electrical properties of the oxide devices. Photocapacitance spectroscopy has been performed to investigate deep levels due to bulk defects and impurities in the Au/STO:Nb junctions. The photocapacitance spectra clearly indicate existence of the deep levels in the Au/STO:Nb and the concentration of the deep levels were of the order of 1013∼1015 cm−3. These values are too low to affect the Fermi level pinning at the interface if the deep levels exist in the near surface region of the bulk STO:Nb. We have shown some interesting electrical properties, characteristic of the SB junction of the dielectric oxide compared with that of the conventional semiconductor's. The schematic band diagram of the Au/STO:Nb junction with the intrinsic low permittivity layer at the interface has been proposed, which explains all the characteristic electrical properties. Considering the chemical trend of the SB height (SBH) estimated from the J-V results, we have pointed out the importance of the metal reactivity for understanding the formation mechanism of the SBH.

New generation 1200 V power module with trench gate IGBT and super soft recovery diode and its evaluations

A 1200 V IGBT with trench gate and punch through structures is developed on the basis of simulation and experimental analyses. Both results of simulation and measurement of the prototype device have good agreement. Though the chip area of the new IGBT is about 30-50% smaller than that of the conventional IGBT, the saturation voltage is about 30-40% lower, the switching loss is also about 20%, smaller than those of conventional IGBT, respectively, and the new IGBT has a larger reverse bias safe operating area (RBSOA). Capability to withstand short circuits is achieved by a new current limiting circuit. In addition, a fast switching diode with an excellent soft recovery characteristic is practically developed by using local lifetime control processing in the anode side n-layer. As an experimental result, its power dissipation, spike surge voltage and electromagnetic noise can be lowered. The structures and characteristics of the developed IGBT, diode and power module are presented, and their analytical results are discussed and evaluated from a practical and an applications point of view.

Photocurrent spectroscopy and study of subband parameters for heavy holes in nanoscale In0.53Ga0.47As/In0.52Al0.48As multiquantum well structures

Nanoscale In0.53Ga0.47As/In0.52Al0.48As multiquantum wells structures were studied by photocurrent spectroscopy. Photocurrent spectra showed clear steplike structures accentuated by exciton peaks. Many interband transitions were assigned from the spectral structure. As peaks of forbidden transitions, which appeared in large reverse bias voltages, were extrapolated to zero bias voltage on the photocurrent spectra, transition energies were estimated in a square potential well. New estimation methods of valence band parameters, heavy hole effective mass and valence band offset, were derived from a saturation of the heavy hole subband in the valence potential well, using the envelope function model in the effective mass approximation. The heavy hole effective mass in a direction normal to the quantum well plane and the valence band offset were 0.38m0 and 0.22 eV.

Aspects of the Conductivity Modulation Enhancement Effect in a 4500 V Planar Metal Oxide Semiconductor Device and Its Electrical Characteristics

A new high-power (4500 V) planar metal oxide semiconductor (MOS) device is fabricated and its properties are evaluated. It has low forward voltage drop and a large reverse bias safe operating area (RBSOA), and allows easy MOS control. These properties are superior to those of the conventional insulated gate bipolar transistor (IGBT) and the gate turn-off thyristor (GTO). This is due to the conductivity modulation enhancement effect resulting from an extremely wide gate electrode, which increases the hole storage and the electron injection. At the same time, the junction field effect transistor (JFET) resistance effect does not affect the forward voltage drop. The rate of increase of the off-state voltage (dVD/dt) during the turn-off process affects RBSOA markedly, therefore, it is important to set the gate resistance to an appropriate value. The press pack device that includes 20 chips of this device and 10 fast recovery diode chips can turn off a current of 1200 A at 3600 V DC voltage supply without a snubber circuit.

Electrical characteristics of magnesium-doped gallium nitride junction diodes

Electrical characteristics of lateral p+n diodes made from gallium nitride epitaxial layers on sapphire substrates are reported. The current–voltage characteristics are observed to have several distinct regions in which a tunneling current has been identified at low forward bias in addition to the conventional temperature-dependent diffusion current observed at moderate forward bias. A tunneling behavior indicates the presence of deep-level traps at the junction, which alter the electrical behavior of these junctions compared to the conventional behavior. In addition, space-charge-limited currents are found to influence these junctions at large forward and reverse bias.

The Double-surface Transistor

In a series of Letters to the Editor [1] appearing in a recent issue of this journal, there are described the physical construction and proposed theory of operation of a solid state semiconductor triode. This device, which is now called the type A transistor, comprises a block of high back-voltage germanium on one of the faces of which are two contacts, side by side with each other and close together. These contacts are called the emitter and collector, respectively. A large-area contact to the opposite face of the semiconductor block is called the base contact. The present communication describes another semiconductor triode, the double-surface transistor, in which the emitter and collector contacts bear on the two opposite faces of a thin wedge or slab of semiconductor. This slab is prepared from an ingot of high back-voltage germanium of N-type [2]. After being ground approximately to the desired shape, the slab is etched and provided with a suitable large-area base contact. For good gain characteristics it is advantageous that the thickness of the slab be no greater than about 0.01 cm at the place where the contacts bear upon it. These contacts should be within about the same distance of coming exactly opposite each other on the two faces of the slab. Tungsten, copper, and phosphor bronze have been used successfully as contact materials. This device, together with its electrical connections for use as a grounded-base amplifier, is illustrated schematically in Fig. 1. In operation a comparatively large dc reverse bias ( 50 to 100 V) is applied to the collector, while a comparatively small dc forward bias (a few tenths of a volt) is applied to the emitter. Because of positive feedback effects in the base contact and semiconductor body, the emitter bias voltage-to-base may in some cases be zero or even negative. The static characteristics of a double-surface transistor are presented in Fig. 2. Families of collector voltage versus collector current curves are given, with constant emitter current as parameter for the solid lines, and with constant emitter voltage as parameter for the dashed lines. Such a plot allows one to make judicious choice of dc operating point. It furnishes in addition complete information from which can be obtained, almost by inspection, the dynamic

Actively mode-locked strained-InGaAsP multiquantum-well lasers integrated with electroabsorption modulators and distributed Bragg reflectors

This paper describes picosecond pulse generation at 20 Gb/s by monolithic mode-locked lasers integrated with electroabsorption modulators and distributed Bragg reflectors. The electroabsorption modulator using strained-InGaAsP multiquantum wells acts as a pulse shortening gate when a sinusoidal voltage is driven at a large reverse bias voltage. To obtain transform-limited picosecond pulses, the required spectral bandwidth of the distributed Bragg reflector is estimated. Pulse generation around 4 ps with a time-bandwidth product of 0.5 has been performed at a repetition rate of 20 GHz. Driving conditions of the modulator, such as bias voltage and modulation frequency, are investigated. It is shown that an increase in the intensity noise is the main factor limiting performance.

Bias and temperature-dependent photocurrent spectroscopy of a compressively strained GaInP/AlGaInP single quantum well

Photocurrent (PC) spectroscopy, as a function of temperature and applied voltage, of a compressively strained 100 AA Ga0.44In0.56P/(Al0.5Ga0.5)0.52In0.48P single quantum well reveals (a) the quantum-confined Stark effect for heavyand light-hole excitons and a parity-forbidden exciton transition at 200 K and (b) evidence for carrier recombination dominating over carrier escape at moderate to low temperature, i.e. <or=200 K and at small applied bias. However, a PC can be observed at low temperature with a large reverse bias ( approximately 10 V), indicating an inhibition of the recombination process due to spatial separation of electrons and holes.