Thermionic Field Emission Mechanism Research Articles

Ni Schottky barrier on heavily doped phosphorous implanted 4H-SiC

The electrical behavior of Ni Schottky barrier formed onto heavily doped (N D > 1019 cm−3) n-type phosphorous implanted silicon carbide (4H-SiC) was investigated, with a focus on the current transport mechanisms in both forward and reverse bias. The forward current–voltage characterization of Schottky diodes showed that the predominant current transport is a thermionic-field emission mechanism. On the other hand, the reverse bias characteristics could not be described by a unique mechanism. In fact, under moderate reverse bias, implantation-induced damage is responsible for the temperature increase in the leakage current, while a pure field emission mechanism is approached with bias increasing. The potential application of metal/4H-SiC contacts on heavily doped layers in real devices is discussed.

Active dopant profiling and Ohmic contacts behavior in degenerate n-type implanted silicon carbide

This Letter reports on the active dopant profiling and Ohmic contact behavior in degenerate P-implanted silicon carbide (4H-SiC) layers. Hall measurements showed a nearly temperature-independent electron density, corresponding to an electrical activation of about 80% of the total implanted dose. Using the Hall result as calibration, the depth resolved active P-profile was extracted by scanning capacitance microscopy (SCM). Such information on the active P-profile permitted to elucidate the current injection mechanism at the interface of annealed Ni Ohmic contacts with the degenerate n-type 4H-SiC layer. Modeling the temperature dependence of the specific contact resistance with the thermionic field emission mechanism allowed extracting a doping concentration of 8.5 × 1019 cm−3 below the metal/4H-SiC interface, in excellent agreement with the value independently obtained by the SCM depth profiling. The demonstrated active dopant profiling methodology can have important implications in the 4H-SiC device technology.

Metal/Semiconductor Barrier Properties of Non-Recessed Ti/Al/Ti and Ta/Al/Ta Ohmic Contacts on AlGaN/GaN Heterostructures

This paper compares the metal/semiconductor barrier height properties of non-recessed Ti/Al/Ti and Ta/Al/Ta contacts on AlGaN/GaN heterostructures. Both contacts exhibited a rectifying behavior after deposition and after annealing at temperatures up to 550 °C. The ohmic behavior was reached after annealing at 600 °C. High-resolution morphological and electrical mapping by conductive atomic force microscopy showed a flat surface for both contacts, with the presence of isolated hillocks, which had no significant impact on the contact resistance. Structural analyses indicated the formation of the Al3Ti and Al3Ta phases upon annealing. Furthermore, a thin interfacial TiN layer was observed in the Ti/Al/Ti samples, which is likely responsible for a lower barrier and a better specific contact resistance (c = 1.6 10−4 Ωcm2) with respect to the Ta/Al/Ta samples (c = 4.0 10−4 Ωcm2). The temperature dependence of the specific contact resistance was described by a thermionic field emission mechanism, determining barrier height values in the range of 0.58–0.63 eV. These results were discussed in terms of the different microstructures of the interfaces in the two systems.

Simulation and analysis of the current–voltage–temperature characteristics of Al/Ti/4H-SiC Schottky barrier diodes

The current–voltage characteristics of Al/Ti/4H-SiC Schottky barrier diodes have been investigated in the 85–445 K temperature range by means of a combined numerical and analytical simulation study. Simulation results showed a good agreement with measurements in the whole explored current range from 10 μA to 10 mA. The main device electrical parameters, namely the barrier height (BH) and ideality factor, were found to be strongly temperature-dependent. In particular, the ideality factor decreases while the BH increases with increasing temperature. The observed behaviours have been successfully interpreted by using the thermionic emission theory with a triple Gaussian distribution of the BH in three different temperature ranges, i.e. 85 ≤ ΔT1 ≤ 135 K, 180 ≤ ΔT2 ≤ 270 K, and 315 ≤ ΔT3 ≤ 445 K. The corresponding Richardson constants are A1* = 149.26 A cm−2 K−2, A2* = 138.19 A cm−2 K−2, and A3* = 173.21 A cm−2 K−2, respectively. These values are close to the theoretical result of 146 A cm−2 K−2 expected for n-type 4H-SiC. Finally, it has been highlighted that the current flowing through the Schottky junction is also determined by the thermionic-field emission mechanism.

Electrical and structural properties of surfaces and interfaces in Ti/Al/Ni Ohmic contacts to p-type implanted 4H-SiC

In this work, the electrical and structural properties of Ti/Al/Ni Ohmic contacts to p-type implanted silicon carbide (4H-SiC) were studied employing different techniques. With increasing the annealing temperature, an improvement of the electrical properties of the contacts is highlighted, until an Ohmic behavior is obtained at 950°C, with a specific contact resistance ρc=2.3×10−4Ωcm2. A considerable intermixing of the metal layers occurred upon annealing, as a consequence of the formation of different phases, both in the uppermost part of the stack (mainly Al3Ni2) and at the interface with SiC, where the formation of preferentially aligned TiC is observed. The formation of an Ohmic contact was associated with the occurrence of the reaction and the disorder at the interface, where the current transport is dominated by the thermionic field emission mechanism with a barrier height of 0.56eV.

Barrier inhomogeneities of platinum contacts to 4H-SiC

The barrier properties of platinum contacts to lightly and highly doped (1 × 1016 cm−3 and 1 × 1018 cm−3) 4H-SiC are investigated. The values of barrier height (BH) and ideality factor deduced from the experimental current-voltage data on the basis of thermionic emission (TE) theory for both samples are abnormally temperature dependent. The anomalies could be commented by a single Gaussian distribution (GD) of inhomogeneous BHs for the lightly doped sample, however a double Gaussianly distributed BHs for the highly doped sample. A theoretical model based on GD of inhomogeneous BHs is proposed to analyze the electrical characteristics of metal/SiC contact. The theoretical simulated characteristics are well agreement with the experimental data for the lightly doped sample. However, obvious discrepancies between the theoretical and experimental properties are observed for the highly doped sample in the low temperature range, suggesting that TE is not the dominant transport mechanism. The double GD of inhomogeneous BHs could be attributed to thermionic field emission mechanism at low temperatures.

Microstructure and Temperature Dependent Electrical Characteristics of Ohmic Contacts to AlGaN/GaN Heterostructures

AlGaN/GaN heterostructures are important materials for the fabrication of high power and high frequency devices. However, the mechanisms of Ohmic contacts formation on these systems are continuously under scientific debate. In this paper, a structural and electrical investigation of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures is reported. In particular, the behavior of Ti/Al/Ni/Au multilayers was monitored at different annealing temperatures. The contacts became Ohmic after annealing at 750°C and showed a decreasing temperature behavior of the specific contact resistance RC, described by a thermionic field emission mechanism. On the other hand, annealing at 850°C led to a further reduction of RC , with a slightly increasing dependence of RC on the measurement temperature (here regarded as a “metal-like” behavior). The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.

Thermal stability of Ni/Ti/Al ohmic contacts to p-type 4H-SiC

Low resistivity Ni/Ti/Al ohmic contacts on p-type 4H-SiC epilayer were developed, and their thermal stabilities were also experimentally investigated through high temperature storage at 600 °C for 100 h. The contact resistance of the Al/Ti/Ni/SiC contacts degraded in different degrees, and the contact morphology deteriorated with the increases of the average surface roughness and interface voids. X-ray spectra showed that Ni2Si and Ti3SiC2, which were formed during ohmic contact annealing and contributed to low contact resistivity, were stable under high temperature storage. The existence of the TiAl3 and NiAl3 intermetallic phases was helpful to prevent Al agglomeration on the interface and make the contacts thermally stable. Auger electron spectroscopy indicated that the incorporation of oxygen at the surface and interface led to the oxidation of Al or Ti resulting in increased contact resistance. Also, the formation of these oxides roughened the surface and interface. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism dominates the current transport for contacts before and after the thermal treatment. It suggests that the Ni/Ti/Al composite ohmic contacts are promising for SiC devices to be used in high temperature applications.

Thermal stability of the current transport mechanisms in Ni-based Ohmic contacts on n- and p-implanted 4H-SiC

Studying the temperature dependence of the electrical properties of Ohmic contacts formed on ion-implanted SiC layers is fundamental to understand and to predict the behaviour of practical devices. This paper reports the electrical characterization, as a function of temperature, of Ni-based Ohmic contacts, simultaneously formed on both n- or p-type implanted 4H-SiC. A structural analysis showed the formation of the Ni2Si phase after an annealing leading to Ohmic behaviour. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism (TFE) dominates the current transport for contacts formed on p-type material, while a field emission (FE) is likely occurring in the contacts formed on n-type implanted SiC. The values of the barrier height were 0.75 eV on p-type material and 0.45 eV on n-type material. The thermal stability of the current transport mechanisms and related physical parameters has been demonstrated upon a long-term (up to 95 h) cycling in the temperature range 200–400 °C.

Comparative Study of the Current Transport Mechanisms in Ni<sub>2</sub>Si Ohmic Contacts on n- and p-Type Implanted 4H-SiC

The knowledge of the temperature behavior of Ohmic contacts is an important issue to understand the device operation. This work reports an electrical characterization as a function of the temperature carried out on nickel silicide (Ni2Si) Ohmic contacts, used both for n-type and p-type implanted 4H-SiC layers. The temperature dependence of the specific contact resistance suggested that a thermionic field emission mechanism dominates the current transport for contacts on p-type material, whereas a current transport by tunneling is likely occurring in the contacts on n-type implanted SiC. Furthermore, from the temperature dependence of the electrical characteristics, the activation energies for Al and P dopants were determined, resulting of 145 meV and 35 meV, respectively. The thermal stability of the electrical parameters has been demonstrated upon a long-term (up to ~100 hours) cycling in the temperature range 200-400°C.

Correlation between microstructure and temperature dependent electrical behavior of annealed Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures

This letter reports on the temperature behavior of the structural and electrical properties of Ti/Al/Ni/Au contacts to AlGaN/GaN heterostructures. While Ohmic contacts formed at 750 °C showed a decreasing temperature behavior of the specific contact resistance ρC, which was explained by a thermionic field emission mechanism, an increasing trend is observed in the contacts formed at 850 °C. In this case, ρC exhibits a “metal-like” behavior, i.e., describable by a T1.8 dependence. The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.

Study of ohmic contacts to n-type Ge: Snowplow and laser activation

Typical contacts to n-Ge result in high specific contact resistivity (ρC) or rectifying behaviour due to Fermi level pinning (resulting in high barrier heights) and low dopant activation generally observed for n-Ge. In this work, we fabricated and studied ohmic contacts to n-type Ge prepared by Ni germanidation of samples with As ion implantation for two different approaches: (1) dopant activation by sub-melt laser anneal (LA) prior to germanidation and (2) Snowplow, i.e., dopant segregation during Ni germanidation without previous thermal activation. Low resistivity ohmic contacts of 8 × 10−7 Ω cm2 for LA and 2 × 10−5 Ω cm2 for Snowplow were obtained using circular transfer length method to determine ρC. We show contacts to be thermally stable to 350 °C. Furthermore, by studying the temperature dependence of ρC, we clarify that the underlying cause of the low contact resistivity for both approaches is the high active dopant concentration obtained. We find the results in both cases to be consistent with the thermionic field-emission mechanism for a barrier height of 0.75 eV and with active dopant concentrations of 3 × 1019 cm−3 for LA and 1.5 × 1019 cm−3 for Snowplow samples.

High-temperature characteristics of GaN nano-Schottky diodes

We report on the current transport mechanism in Au/GaN nanowire–Schottky diodes at high temperature using current–voltage characterization in the temperature range of 300–573 K. We observed that the ideality factor for the GaN nano-Schottky diodes decreased from 11.2 at 323 K and reached its lowest value of 7.9 at 500 K. Then, it started to increase with increasing temperatures up to 573 K. On the other hand, the Schottky barrier height increased throughout all of the temperature ranges up to 573 K. We found that the abnormal behaviors of the barrier heights and the ideality factors in the current transport by varying the temperature of 300–500 K can be well explained by a combination of the thermionic field emission (TFE) mechanism and a model with a Gaussian distribution of the barrier heights that were predominant at the interface between the Schottky metal (Au) and the semiconductor (GaN nanowire). We also suggested that the increase of the ideality factor at a temperature higher than 500 K could be attributed to a trap-assisted tunneling current in a TFE mechanism.

Temperature dependence of current–voltage characteristics of Sn/p-Si Schottky contacts

The current–voltage ( I–V) characteristics of Sn/p-Si Schottky barrier diode have been measured over a wide range of temperature (80–300 K) and interpreted on the basis of thermionic emission mechanism by merging the concept of barrier inhomogeneities through a Gaussian distribution function. The analysis has revealed an anomalous decrease of apparent barrier height Φ b0, increase of ideality factor n, and nonlinearity of the activation energy plot at lower temperatures. A Φ b0 versus 1/ T plot has been drawn to obtain evidence of a Gaussian distribution of barrier heights, and values of 0.97 eV and 0.084 V for the mean barrier height Φ ¯ b 0 and standard deviation σ 0 have been obtained, respectively, from this plot. A modified ln( I 0 /T 2)−( q 2 σ 0 2/2 k 2 T 2) versus 1/ T plot gives Φ ¯ b 0 and Richardson constant A** as 0.95 eV and 15.6 A cm −2 K −2, respectively. It can be concluded that the temperature dependent I–V characteristics of the Sn/p-Si Schottky barrier diode can be successfully explained on the basis of a thermionic emission mechanism with Gaussian distribution of the barrier heights. We have also discussed whether or not the junction current has been connected with thermionic field-emission mechanism.

Electrical properties of titanium-HgCdTe contacts

Transfer length method (TLM) structures were fabricated to characterize the Ti-HgCdTe contacts. Low-temperature measurement of contact resistance was found to be affected by the background-generated carriers in long wavelength infrared HgCdTe material. Measurements carried out by keeping the TLM structures behind a cold shield showed low contact resistance indicative of the formation of a good “Ohmic” contact. Low specific contact resistance of the order of 10−4 Ω-cm2 makes this contact scheme suitable for the fabrication of photoconductive as well as photovoltaic HgCdTe detectors. Annealing the contacts in air at 60°C for 15 days yielded the specific contact resistance values of the same order of magnitude at room temperature; however, low-temperature measurements show a minor change in the specific contact resistance. The current-voltage measurements show that current transport is dominated by the thermionic field emission mechanism.

Current Transport Study of Schottky and P-N Junction Solar Cells Using Metal-Induced Growth Poly-Si Thin Films

AbstractPoly-Si thin films deposited at low temperature by using the metal-induced growth (MIG) method have the advantage of less metal impurity contaminations and relative large grains with preferred crystal orientation of (220). In recent research, the Schottky solar diode made of MIG poly-Si shows Jsc of 12 mA/cm2 and Voc of 0.214V. In this paper, current transport mechanisms were studied by current-voltage-temperature (I-V-T) testing from 100 K to 400K. For the samples deposited by a one-step sputtering process, the large value of ideality factor (n) and abnormal increase of barrier height with the temperature implies that the current transport mechanism does not follow the pure thermionic-emission theory, which was proven to be thermionic-field emission due to the highly doped Si film. By using a two-step sputtering process, the ideality factor and Au-Schottky barrier height at room temperature were about 1.5 and 0.7 eV, which was improved from one-step sputtering. Hydrogenation by electron cyclotron resonance (ECR) plasma can further improve the Schottky diode ideality factor and barrier height. Although a low-level Phosphors-doped Si target was used for poly-Si thin film deposition, a thermionic-field emission mechanism was still found by plotting the activation energy (E0) versus the testing temperature range. Capacitance-voltage (C-V) analysis revealed an unexpected carrier density of 1017 cm−3 level, which is 1000 times higher than the doping density level in the Si film. “Oxygen thermal donor” effect was assumed due to high oxygen level (1020 cm−3) detected by SIMS and processing at ∼ 600 °C. Increasing of the total carrier density due to the oxygen donor may cause the transport mechanism change from pure thermionic emission to thermionic-field emission. Reducing oxygen in the Si film by filtering the sputtering gas to 50 ppb oxygen level was proven to be effective. C-V results gave ∼1016 cm−3 level of carrier density after using oxygen filtering. P-N junction solar cells were made by B-ion implantation into n-type Si film and dopant activation at 700 °C. I-V-T study showed similar curves for P-N junction as for Schottky junction devices. This implies that the current transport was dominated by the Si films instead of the junctions for both Schottky and P-N junction devices.

Bulk- or interface-limited electrical conductions in IrO2/(Ba,Sr)TiO3/IrO2 thin film capacitors

The electrical conduction behavior of sputter-grown (Ba,Sr)TiO3 thin films having IrO2 electrodes were studied under the assumption of a fully accumulated film having a negative space charge density of 1 × 1019 cm−3 at 25 °C. The negative space charge decreased the actual field strength in the film and resulted in a decreasing leakage current with increasing film thickness at a given applied field. The current conduction in a very low field, roughly less than 150 KV/cm, showed a linear current density–voltage (J–V) behavior at 25 °C. From that field to about 420 KV/cm, the bulk-limited Poole–Frenkel mechanism controlled the overall conduction property at room temperature. Under high field strength, from 420 KV/cm to 1 MV/cm, the interface-limited thermionic field emission mechanism was dominant. The dielectric constant obtained from Poole–Frenkel fitting was approximately 300 ± 50 at 25 °C, which was in qualitative agreement with the value obtained from low-frequency capacitance measurements. The detailed mechanisms of the linear and nonlinear field-dependent emission conductions were discussed with reference to the direction of band bending, not to the carrier concentration.

Mechanisms of interface trap-induced drain leakage current in off-state n-MOSFET's

An interface trap-assisted tunneling and thermionic emission model has been developed to study an increased drain leakage current in off-state n-MOSFET's after hot carrier stress. In the model, a complete band-trap-band leakage path is formed at the Si/SiO/sub 2/ interface by hole emission from interface traps to a valence band and electron emission from interface traps to a conduction band. Both hole and electron emissions are carried out via quantum tunneling or thermal excitation. In this experiment, a 0.5 /spl mu/m n-MOSFET was subjected to a dc voltage stress to generate interface traps. The drain leakage current was characterized to compare with the model. Our study reveals that the interface trap-assisted two-step tunneling, hole tunneling followed by electron tunneling, holds responsibility for the leakage current at a large drain-to-gate bias (V/sub dg/). The lateral field plays a major role in the two-step tunneling process. The additional drain leakage current due to band-trap-band tunneling is adequately described by an analytical expression /spl Delta/I/sub d/=Aexp(B/sub it//F). The value of B/sub it/ about 13 mV/cm was obtained in a stressed MOSFET, which is significantly lower than in the GIDL current attributed to direct band-to-band tunneling. As V/sub dg/ decreases, a thermionic-field emission mechanism, hole thermionic emission and electron tunneling, becomes a primary leakage path. At a sufficiently low V/sub dg/, our model reduces to the Shockley-Read-Hall theory and thermal generation of electron-hole pairs through traps is dominant.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>