Metal-germanium Schottky barriers

A systematic study has been made of the electrical characteristics of Schottky barriers fabricated by evaporating various metal films on n-type chemically cleaned germanium substrates. The diodes, with the exception of Al---Ge contacts, exhibit near-ideal electrical characteristics and age only slightly towards lower barrier height values. Al---Ge contacts exhibit very pronounced ageing towards higher barrier height values, due to formation of an extra aluminium oxide interfacial layer. Because of this, the barrier height values of aged Al---Ge contacts derived from I-V and C-V characteristics differ significantly. The dependence of the barrier height, (φ_b) on the metal work function, φ_m, for different metal-germanium contacts shows that surface states play an important role in the formation of the barrier. The density of germanium surface states is estimated to be D_s = 2 × 10¹³ eV⁻¹ cm⁻².     

Process and device characterization of high voltage gallium arsenide P-i-N layers grown by an improved liquid phase epitaxy method

GaAs P-i-N layers with an i-region net doping of less than 10¹² cm⁻³ were grown on P⁺ and N⁺ substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P⁺-P-i-N-N⁺ diode with a 25 μm-wide i-region exhibits a breakdown voltage of 1000 V, a t_rr of 50 ns, and reverse current densities (at V_R = 800 V) of − 3 × 10⁻⁶ A/cm² at 25°C and 10⁻² A/cm² at 260° C.     

Effect of gate length on DC performance of AlGaN/GaN HEMTs grown by MBE

The DC performance of AlGaN/GaN high electron mobility transistors grown by plasma-assisted molecular beam epitaxy was investigated for gate lengths in the range 0.1–1.2 μm. On 0.25 μm gate length devices we obtained 40 V_DS operation with >50 mA peak I_D. The peak drain current density was 0.44 A/mm for 100 μm gate width devices with 1.2 μm gate lengths. The extrinsic transconductance (g_m) decreased with both gate length and gate width and was 75 mS/mm for all gate widths for 0.25 μm devices. E-beam written gates typically produced a slightly lower Schottky barrier height than optically patterned gates.     

Electrical characteristics of GaAsP Schottky barrier diodes

Schottky barrier diodes of chromium on n-type epitaxial gallium arsenide phosphide (GaAsP) were studied from 25°C to 440°C. The diodes showed significant rectification properties up to a temperature of 440°C. At high temperature the reverse leakage current was 1.15 mA at 25 V with a diode area of 1.14×10⁻³ cm² as compared with 0.25-μA current at room temperature. The n factor derived from the slope of the ln I vs. V curves was 1.1. The barrier height for chromium was found to be 1.25 eV from the capacitance measurements and 1.12 eV from the saturation current vs. temperature measurements. The slope of the C-V curves yielded a carrier concentration of 6.0×10¹⁵ carriers per cm³.     

Novel Delta-Doped InAlGaP/GaAs Heterojunction Bipolar Transistor

The first successful demonstration of a delta-doped InAlGaP/GaAs heterojunction bipolar transistor (HBT) is reported. A comparison to a baseline InAlGaP/GaAs HBT without a delta-doping layer is made. Both of these devices exhibit near-ideal current gain (beta) versus the collector current (I _C) characteristics (i.e., beta independent of I _C) at high currents. The delta-InAlGaP/GaAs HBT exhibits a 40% reduction in offset voltage (V _{CE, offset}) and a 250-mV reduction in knee voltage (V _k) without sacrificing beta compared with the baseline InAlGaP/GaAs HBT. At a higher I _C, the decrease in beta of the InAlGaP/GaAs HBTs with increasing temperature is significantly smaller than the corresponding effect measured in the formerly reported GaAs-based HBTs. The rather temperature-insensitive characteristics of these two InAlGaP/GaAs HBTs originate from their large valence-band discontinuity (DeltaE _V) at the emitter-base (E-B) junction. Furthermore, at intermediate base current I _B levels (0.4-1.6 mA), V _{CE, offset} falls as I _B increases, which is a trend contrary to that of most HBTs in the literature. Finally, the experimental dependence of V _{CE, offset} on temperature, I _B, and the effective barrier height at the E-B junction is explained with reference to an extended large-signal model.     

Electrical properties of W/Si interfaces with embedded Ge/Si islands

In this work, we investigated electrical and morphological properties of W/p-type Si Schottky diodes with intentional inhomogeneities introduced by macroscopic Ge-islands embedded beneath the interface. The Si-cap layer thickness (or the island-distance to the interface) was progressively reduced by successive chemical etching cycles. Electrical characterizations were achieved through reverse current–voltage (I–V) at room temperature and forward I–V measurements as a function of the temperature. In parallel, Rutherford backscattering spectroscopy analyses were performed to follow the Si-cap/Ge islands chemical thinning down with increasing the number of etching cycles. In addition, the comparison of topographical and electrical properties of the etched silicon-cap layer was carried out by conductive atomic force microscopy analyses with a nanometer-scale resolution. Our results indicate that the areas on the top of islands exhibit lower resistance than those which covered the wetting layer. This lateral variation of resistance at the surface of the semiconductor may correspond to Schottky barrier height inhomogeneities observed on broad area I–V characteristics of Schottky contacts.     

Optoelectronic properties of Zn0.52Se0.48/Si Schottky diodes

Zn_0.52Se_0.48/Si Schottky diodes are fabricated by depositing zinc selenide (Zn_0.52Se_0.48) thin films onto Si(1 0 0) substrates by vacuum evaporation technique. Rutherford backscattering spectrometry (RBS) analysis shows that the deposited films are nearly stoichiometric in nature. X-ray diffractogram of the films reveals the preferential orientation of the films along (1 1 1) direction. Structural parameters such as crystallite size (D), dislocation density (δ), strain (ε), and the lattice parameter are calculated as 29.13 nm, 1.187 × 10⁻¹⁵ lin/m², 1.354 × 10⁻³ lin⁻² m⁻⁴ and 5.676 × 10⁻¹⁰ m respectively. From the I–V measurements on the Zn_0.52Se_0.48/p-Si Schottky diodes, ideality and diode rectification factors are evaluated, as 1.749 (305 K) and 1.04 × 10⁴ (305 K) respectively. The built-in potential, effective carrier concentration (N_A) and barrier height were also evaluated from C–V measurement, which are found to be 1.02 V, 5.907 × 10¹⁵ cm⁻³ and 1.359 eV respectively.     

Electrical properties of high permittivity ZrO2 gate dielectrics on strained-Si

Deposition and electrical properties of high dielectric constant (high-k) ultrathin ZrO₂ films on tensilely strained silicon (strained-Si) substrate are reported. ZrO₂ thin films have been deposited using a microwave plasma enhanced chemical vapor deposition technique at a low temperature (150 °C). Metal insulator semiconductor (MIS) structures are used for high frequency capacitance–voltage (C–V), current–voltage (I–V), and conductance–voltage (G–V) characterization. Using MIS capacitor structures, the reliability and the leakage current characteristics have been studied both at room and high temperature. Schottky conduction mechanism is found to dominate the current conduction at a high temperature. Observed good electrical and reliability properties suggest the suitability of deposited ZrO₂ thin films as an alternative as gate dielectrics. Compatibility of ZrO₂ as a gate dielectric on strained-Si is shown.     

Dual metal SiC Schottky rectifiers with low power dissipation

Nickel and titanium are the most commonly used metals for Schottky barrier diodes on silicon carbide (SiC). Ti has a low Schottky barrier height (i.e. 0.8 eV on 6H-SiC), whilst Ni has a higher barrier (i.e. 1.25 eV). Therefore, the first metal allows to achieve a low forward voltage drop V_F but leads to a high leakage current. On the other hand, the second one presents the advantage of a lower reverse leakage current but has also a high value of V_F. In this work, dual-metal-planar (DMP) Schottky diodes on silicon carbide are reported. The rectifying barrier was formed by using an array of micrometric Ti and Ni₂Si (nickel silicide) stripes. This low/high Schottky barrier allowed to combine the advantages of the two metals, i.e. to fabricate diodes with a forward voltage drop close to that of a Ti diode and with a level of reverse current comparable to that of a Ni₂Si diode. Under the application point of view, using this kind of barrier can lead to a reduction of the device power dissipation and an increase of the maximum operating temperature.     

Density of States-Based DC $I$– $V$ Model of Amorphous Gallium–Indium–Zinc-Oxide Thin-Film Transistors

The density of states (DOS)-based DC I-V model of an amorphous gallium-indium-zinc oxide (a-GIZO) thin-film transistor (TFT) is proposed and demonstrated with self-consistent methodologies for extracting parameters. By combining the optical charge-pumping technique and the nonlinear relation between the surface potential (phiS) and gate voltage (V _GS), it is verified that the proposed DC model reproduces well both the measured V _GS-dependent mobility and the I _DS-V _GS characteristics. Finally, the extracted DOS parameters are N _TA = 4.4 times 10¹⁷ cm^-3 middot eV^-1, N _DA = 3 times 10¹⁵ cm^-3 middot eV^-1, kT _TA = 0.023 eV, kT _DGA = 1.5 eV, and EO = 1.8 eV, with the formulas of exponential tail states and Gaussian deep states.     

Schottky barrier height of a new ohmic contact NiSi2 to n-type 6H-SiC

We present a new ohmic contact material NiSi₂ to n-type 6H-SiC with a low specific contact resistance. NiSi₂ films are prepared by annealing the Ni and Si films separately deposited on (0 0 0 1)-oriented 6H-SiC substrates with carrier concentrations (n) ranging from 5.8×10¹⁶ to 2.5×10¹⁹ cm⁻³. The deposited films are annealed at 900 °C for 10 min in a flow of Ar gas containing 5 vol.% H₂ gas. The specific contact resistance of NiSi₂ contact exponentially decreases with increasing carrier concentrations of substrates. NiSi₂ contacts formed on the substrates with n=2.5×10¹⁹ cm⁻³ show a relatively low specific contact resistance with 3.6×10⁻⁶ Ω cm². Schottky barrier height of NiSi₂ to n-type 6H-SiC is estimated to be 0.40±0.02 eV using a theoretical relationship for the carrier concentration dependence of the specific contact resistance.     

Reliability and micro-structural properties of GaAs Schottky diodes for submillimeter-wave applications

Whisker contacted GaAs Schottky barrier diodes are the standard devices for mixing and multiplier applications in the THz frequency range. This is mainly due to their minimum parasitics and mature technology. But with the decreasing size of the anode contact, which is required for operation at high frequencies (up to approx. 3 THz), the reliability and the micro-structural understanding of the Schottky barrier becomes increasingly important. This contribution presents new results concerning the reliability of Schottky diodes and the physical properties of small-area Schottky junctions, especially at low current densities. For these purposes a number of different Schottky diodes have been fabricated with different epilayer doping concentrations and anode diameters. Measured I/V characteristics show that the diode current deviates considerably from the ideal thermionic current behavior with decreasing diode diameter. This deviation shows an exponential dependence on the diode voltage and is a function of the doping concentration of the active layer. For a given doping concentration in the epi-layer and decreasing anode diameter, this phenomenon shifts the minimum of the ideality factor towards higher current densities. An explanation is given in terms of a difference of the cyrstallinity of the polycrystalline platinum films on the GaAs for decreasing SiO₂ aperture size in connection with a reduced Pt mobility in the electrolyte. The reliability of Schottky barrier diodes under thermal and electrical stress has been investigated on different THz Schottky diode structures. The results show that the barrier height and the ideality factor of the fabricated structures are not affected by thermal stress. Electrical stress induced by large forward currents up to a current density of 10 kA/mm² even leads to a slight increase of the barrier height and a reduction of the series resistance.     

Molecular beam epitaxy growth of TmP/GaAs and transistor action inGaP/TmP/GaAs heterostructures

The growth of thulium phosphide (TmP) by molecular beam epitaxy (MBE) on GaAs substrate is reported. Good epilayer quality was demonstrated through X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis. The closely lattice matched TmP layer was n-type with an electron concentration of 1.6×10²¹ cm^-3 and a room temperature mobility of 4.8 cm²V^-1s^-1. The Schottky barrier height determined from 1/capacitance² (1/C²) versus voltage (V) measurements is about 0.81 eV which agrees well with the value obtained through the current-voltage (I-V) measurements. In this work, we also report transistor action in a GaP/TmP/GaAs structure, for which chemical bonding techniques were employed. From I-V measurements, a common base current gain α≈0.55 at V_CB=0 was obtained at room temperature     

Performances of novel Pd/Sn and Pd/Sn/Au ohmic metallizations to n-GaAs

Performance of novel Pd/Sn and Pd/Sn/Au Ohmic metallizations to n-GaAs have been investigated. Metallizations were deposited using a resistance heating evaporator and annealings were performed utilizing a conventional graphite strip annealer (cGSA). Metallization samples were characterized using scanning tunneling microscopy (STM), secondary ion mass spectrometry (SIMS) and current–voltage (I–V) measurements. Contact resistivities, ρ_c, of the metallizations were measured utilizing conventional transmission line model (cTLM) method. Novel Pd/Sn and Pd/Sn/Au Ohmic contacts exhibit better thermal stability compared to non-alloyed Pd/Ge metallization. In order to investigate the effectiveness of novel Pd/Sn and Pd/Sn/Au Ohmic metallizations in device applications, gallium arsenide metal-semiconductor field-effect transistors (GaAs MESFETs) have been fabricated. MESFETs fabricated with Pd/Sn/Au Ohmic contacts show a extrinsic transconductance, g_me, of more than 133 mS/mm for a gate length, L_G, of 2 μm.     

Enhanced quantum efficiency of Pd2Si Schottky infrared diodes on 〈111〉 Si

A two-fold improvement in the infrared emission efficiency has been obtained on Pd2Si/p-Si Schottky diodes through a proper choice of the Si substrate orientation. Photoresponse measurements on thin Pd2Si/p-Si infrared Schottky detectors (lambda_{c} = 3.5µ) yielded Fowler C1 coefficients of 66 percent/eV for Si and 32 percent/eV for Si. Leakage current versus temperature measurements at 6-V reverse bias of these Pd2Si/p-Si diodes with guard ring structures agreed with thermionic-emission leakage-current theory using the photoresponse obtained barrier value of 0.35 eV. These results have impact on SWIR detection applications such as earth resources satellite mapping.     

Electrical characteristics of TiB2 for ULSI applications

The work function of TiB₂ was measured using Fowler-Nordheim tunneling in MOS capacitors, Schottky diode current measurements, capacitance-voltage techniques, and contact resistance. The resulting data place the Fermi level of TiB₂ about 0.9 eV below the silicon conduction band. Given this barrier height, Schottky diodes of TiB₂/p-Si exhibit ohmic characteristics, but the contact resistance of TiB₂ to n⁺ junctions is an order of magnitude higher than the generally desired value. Boron outdiffusion from TiB₂ into underlying silicon was observed at temperatures of 1000°C and greater. Boron diffusion from TiB₂ into silicon above 1000°C is enhanced compared to the conventionally accepted value of the boron diffusivity     

A Novel Pt/In0.52Al0.48As Schottky Diode-Type Hydrogen Sensor

On the basis of a Pt/In_0.52Al_0.48As metal-semiconductor structure, a novel hydrogen sensor is fabricated and demonstrated. The studied Pt/In_0.52Al_0.48As Schottky diode-type hydrogen sensor exhibits significant sensing performance including high relative sensitivity ratio of about 2600% (under the 1% H₂/air gas and V_R=-0.5 V at 30 degC), large current variation of 310 muA (under the 1% H₂/air gas and V_R=-5 V at 200 degC), widespread reverse-voltage regime (0~-5 V), stable hydrogen-sensing current-voltage (I-V) curves, and fast transient response time of 1.5 s. The calculated Schottky barrier-height change and series-resistance variation, from the thermionic-emission model and Norde method, are 87.0 meV and 288 Omega, respectively (under the 1% H₂/air gas at 30 degC). The hydrogen concentrations and operating temperatures tested in this letter are in the range of 15 ppm-1% H₂/air and 30 degC-250 degC, respectively. Based on the excellent integration compatibility with InP-based electronic devices, the studied device provides the potentiality in high-performance sensor-array applications     

Schottky diode characteristics of Ti on strained-Si

The Schottky barrier height and ideality factor of Ti on p-type strained-Si (grown on a graded relaxed Si_0.82Ge_0.18 buffer layer) were investigated in the temperature range 200–300 K using the current-voltage (I-V) characteristics and were found to be temperature dependent. While the ideality factor decreases with an increase in temperature, the barrier height increases.     

Thermal noise of a silicon double-injection plasma diode

Measurements of the thermal noise of a silicon p⁺−π−n⁺ diode operating in the Lampert-insulating regime agree within 6 per cent with the prediction S_i = 4kT Re(Y). The noise measurements were performed in the cube-law regime with d.c.-currents from 100 μA to 4mA at room temperature.     

RF Sputtered gold-amorphous silicon Schottky-barrier diodes

Gold Schottky-barrier diodes formed on reactively sputtered amorphous silicon thin films have been investigated. Device forwardI-Vcharacteristics are well modeled as a Schottky diode in series with a temperature activated series resistor. At 300K, the forward current indicates a diode correction factor of 1.4 and a saturation current of 5.8 × 10^-10A/cm². The metal-semiconductor barrier height is 0.93 eV. Capacitance versus frequency measurements indicate a depletion region thickness of 3000 Å. In the depletion region, the mobility-lifetime products are estimated to be of the order of 5 × 10^-11cm²/V which is substantially less than the value of 10^-7cm²/V in the quasi-neutral region, It is suggested that deep gap states are responsible for this difference. Carrier recombination in the depletion region limits the photovoltaic performance.