Emission Barrier Height Research Articles

Development of p-i-n radiation detectors based on semi-insulating 4H-SiC substrate via dual-face ion implantation

To explore the potential of semi-insulating (SI) SiC substrate for radiation detector applications, p-i-n diodes with thick depletion layers are fabricated by using a dual-face ion implantation technique on SI 4H-SiC substrate. The Poole-Frenkel (P-F) emission is found responsible for the reverse leakage current transport of the p-i-n diode at elevated temperatures. The derived emission barrier height is likely induced by the residual boron-related shallow-level states, which are located ∼ 0.3 eV above the top of the valence band. Detection tests of α-particles are performed on each side of the p-i-n detector to determine the location of the depleted region, which reveals weak p-type conduction of the SI 4H-SiC substrate after high temperature post-implantation annealing. The energy resolution of the detector operating in photovoltaic mode is ∼ 4.3%, which would degrade at higher biases. The saturated charge collection efficiency (CCE) of ∼ 99% for detection of 5.48 MeV α-particles is obtained as reverse bias exceeds 80 V.

Effect of the post-gate annealing on the gate reliability of AlGaN/GaN HEMTs

In this paper, we investigated the effect of post-gate annealing (PGA) on reverse gate leakage and the reverse bias reliability of Al0.23Ga0.77N/GaN high electron mobility transistors (HEMTs). We found that the Poole–Frenkel (PF) emission is dominant in the reverse gate leakage current at the low reverse bias region (V th < V G < 0 V) for the unannealed and annealed HEMTs. The emission barrier height of HEMT is increased from 0.139 to 0.256 eV after the PGA process, which results in a reduction of the reverse leakage current by more than one order. Besides, the reverse step stress was conducted to study the gate reliability of both HEMTs. After the stress, the unannealed HEMT shows a higher reverse leakage current due to the permanent damage of the Schottky gate. In contrast, the annealed HEMT shows a little change in reverse leakage current. This indicates that the PGA can reduce the reverse gate leakage and improve the gate reliability.

Semiconductor-less vertical transistor with ION/IOFF of 106

Semiconductors have long been perceived as a prerequisite for solid-state transistors. Although switching principles for nanometer-scale devices have emerged based on the deployment of two-dimensional (2D) van der Waals heterostructures, tunneling and ballistic currents through short channels are difficult to control, and semiconducting channel materials remain indispensable for practical switching. In this study, we report a semiconductor-less solid-state electronic device that exhibits an industry-applicable switching of the ballistic current. This device modulates the field emission barrier height across the graphene-hexagonal boron nitride interface with ION/IOFF of 106 obtained from the transfer curves and adjustable intrinsic gain up to 4, and exhibits unprecedented current stability in temperature range of 15–400 K. The vertical device operation can be optimized with the capacitive coupling in the device geometry. The semiconductor-less switching resolves the long-standing issue of temperature-dependent device performance, thereby extending the potential of 2D van der Waals devices to applications in extreme environments.

The role of substrate temperature on defects, electronic transitions, and dark current behavior of ZnO films fabricated by spray technique

We have studied structural, electrical, and optical properties of ZnO thin films deposited by a spray technique. The samples were deposited by varying the deposition temperature from 200 °C (S200) up to 500 °C (S500). As the results, using X-ray diffraction and Raman scattering experiments, improvement in crystalline is observed by increasing the deposition temperature as indicated by the intensity enhancement of (002) plane and the E2-high vibration mode. From I–V curves, the photosensitivity of the films also increases up to 37 times (S500) with increasing the temperature. We reveal that strong UV absorption in S500 plays a significant role in the enhancement of sensitivity. On the other hand, the low dark current in S300 is due to the high density of oxygen vacancies, leading to higher mid-gap transitions, green emission, and Schottky barrier height. In the same time, the narrowing effect of the band gap from 3.28 to 3.20 eV is observed with the decrease of the temperature, which is related to the defect states from oxygen vacancies. Our study is essential to design and massively fabricate ZnO-based optoelectronic devices using a simple technique.

Fabrication and characterization of Au/n-ZnO/p-Si/Al sandwich device with Sol–gel spin coating method

ZnO thin films with the n-type properties were obtained on the p-Si substrates for multi-layered sandwich device depositing by sol–gel method. Au and Al were conducted on two side of hetero-junction by PVD technique. From X-ray diffraction pattern, a strong (002) direction is observed, which means the film is hexagonal texture. Cross-section image from the scanning electron microscope demonstrated that the coated ZnO films had ~ 427 nm thickness. Electrical behavior of the device was characterized by applying voltage versus current. High purity Au and Al metals were vacuum evaporated to make Schottky and Ohmic contacts onto the device. Au/n-ZnO/p-Si/Al architecture exhibited a rectification behavior. According to Thermionic emission mechanism theory, barrier height and ideality factor of device were found as 1.4 and 0.54 eV, respectively. The series resistance values were obtained from dV/d(lnI) versus I and H(I) versus I graphs as 43.6 Ω and 229.9 Ω, respectively.

Fluorine plasma treatment induced deep level traps and their effect on current transportation in Al0.83In0.17N/AlN/GaN Schottky barrier diodes

The deep level traps and the electrical properties of fluorine plasma treated (F-treated) and non-treated Al0.83In0.17N/AlN/GaN Schottky barrier diodes (SBDs) were investigated by the temperature-dependent current–voltage (I–V) and deep level transient spectroscopy (DLTS) measurements. Three deep level traps were detected in the SBD after F-treatment at ~Ec − 0.17 eV, ~Ec − 0.27 eV and ~Ec − 1.14 eV. One of the deep level traps at ~Ec − 1.14 eV is mainly located in the Al0.83In0.17N barrier layer with a captured cross section (σ) of ~6.50 × 10−18 cm2. This F-related deep level trap has 3–4 orders of magnitude of the larger σ and ~0.46 eV greater active energy than that of the dislocation-related one at ~Ec − 0.68 eV with σ of ~1.92 × 10−21 cm2. Meanwhile, the leakage current of F-treated SBD at −5 V is reduced by ~2 orders of magnitude compared with that of the non-treated one. This leakage current reduction is mainly attributed to the increase of the Poole–Frenkel emission barrier height from ~0.09 eV in non-treated SBD to ~0.46 eV in the F-treated one. It is believed that the main reverse current transportation is the Poole–Frenkel emission from the F-related deep level trap states into the continuum states of the dislocations in F-treated Al0.83In0.17N/AlN/GaN SBD.

Electrical and Optical Analysis on Green Organic Light Emitting Device Using Finite Element Method.

PSS); Bis[(1-naphthyl)-N-phenyl]benzidine (α-NPD); 4,4'-N,N'-dicarbazole-biphenyl (CBP) doped with Ir(ppy)3; a 2-(4-Biphenylyl)-5-(4-tert-butylphenyl-1,3,4-oxadiazole) (Bu-PBD). We demonstrate that the refractive indexes of each material affect the emission property and barrier height of the interface influences the behavior of charges and the generation of exciton.

Leakage current reduction by thermal oxidation in Ni/Au Schottky contacts on lattice-matched In0.18Al0.82N/GaN heterostructures

By using temperature-dependent current—voltage, variable-frequency capacitance-voltage, and Hall measurements, the effects of the thermal oxidation on the electrical properties of Ni/Au Schottky contacts on lattice-matched In0.18Al0.82N/GaN heterostructures are investigated. Decrease of the reverse leakage current down to six orders of magnitude is observed after the thermal oxidation of the In0.8Al0.82N/GaN heterostructures at 700 °C. It is confirmed that the reverse leakage current is dominated by the Frenkel—Poole emission, and the main origin of the leakage current is the emission of electrons from a trap state near the metal/semiconductor interface into a continuum of electronic states associated with the conductive dislocations in the InxAl1−xN barrier. It is believed that the thermal oxidation results in the formation of a thin oxide layer on the InxAl1−xN surface, which increases the electron emission barrier height.

Electrical Characteristics of Pt Schottky Contacts on AlInN:Mg/GaN Heterostructures

The electrical characteristics of Pt Schottky contacts on AlInN:Mg/GaN heterostructures were investigated. A barrier inhomogeneity model applied to temperature-dependent forward current–voltage (I–V) curves revealed the mean barrier height of 1.67 eV and the standard deviation of 0.22 eV. The reverse leakage current could be understood in terms of the Poole–Frenkel emission model, yielding the emission barrier height of 0.26 eV and the high-frequency relative dielectric permittivity of 5.32. On the basis of these obtained values, the continuum of electronic states could be predicted to locate 1.41 eV below the valence band.

Study of the transport properties of La0.8Ba0.2MnO3/SrTiO3:Nd heterojunctions under reverse bias

Heterojunctions composed of P-type La0.8Ba0.2MnO3 and N-type Nd doped SrTiO3 are successfully fabricated via chemical solution deposition. Good rectifying property is obtained. Detailed studies of the transport properties under reverse bias show that above the metal-insulator transition temperature TMI, the Frenkel–Poole emission dominates the leakage current and the emission barrier height was extracted. When placed in magnetic field, the junctions show different transport behaviors below and above TMI. It shows negative magnetoresistive effect below TMI and positive one above TMI. This is explained by the phase separation mechanism and the band structure of manganites.

Analysis of electrical properties and deep level defects in undoped GaN Schottky barrier diode

Electrical and deep level defects have been investigated in GaN Schottky barrier diode (SBD) in the temperature ranging from 125K to 425K. The study was carried out by combined current density–voltage (J–V), capacitance–voltage (C–V) and deep level transient spectroscopy (DLTS) characterization techniques. It is found that the ideality factor n of the diode decreases and the corresponding Schottky barrier height (SBH) increases with increasing temperature, which indicates the barrier in-homogeneity at metal/semiconductor interface. Thermionic emission with Gaussian distribution of SBHs is thought to be responsible for the electrical behavior of the diode over the temperature region. The possible explanation for this discrepancy in the estimated SBHs from J–V and C–V is presented. The DLTS measurement has revealed two deep level traps in GaN with activation energies Ec−0.23eV and Ec−0.45eV having different capture cross-sections. In addition, we observed that the reverse leakage current in GaN SBD above 275K is due to Frenkel–Poole emission (FPE). The estimated emission barrier height by FPE model is about ~0.25eV. Hence, the reverse leakage current is due to the emission of electrons from the trap state near the metal–semiconductor interface into a continuum of states, associated with conductive dislocations.

Analysis of the reverse leakage current in AlGaN/GaN Schottky barrier diodes treated with fluorine plasma

The carrier transport mechanism of CF4 plasma-treated AlGaN/GaN Schottky barrier diodes (SBDs) under reverse bias is investigated. The reverse leakage current is reduced by ∼2 orders of magnitude after the CF4 plasma treatment, but increases exponentially with increasing temperature, indicating that a thermally activated transport mechanism is involved. Based on the activation energy estimated from temperature-dependent current-voltage characteristics and the emission barrier height extracted from Frenkel-Poole emission model, it is suggested that the dominant carrier transport mechanism in the CF4 plasma treated SBDs is the Frenkel-Poole emission from fluorine-related deep-level states into the continuum states of dislocations.

Temperature dependence of current density and admittance in metal-insulator-semiconductor junctions with molecular insulator

Electrical transport in ultrathin Metal-insulator-semiconductor (MIS) tunnel junctions is analyzed using the temperature dependence of current density and admittance characteristics, as illustrated by Hg//C12H25 – n Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to Si(111). The voltage partition is obtained from J(V, T) characteristics, over eight decades in current. In the low forward bias regime (0.2–0.4 V) governed by thermionic emission, the observed linear T-dependence of the effective barrier height, qΦEFF(T)=qΦB+(kT)β0dT, provides the tunnel barrier attenuation, exp(-β0dT), with β0= 0.93 Å−1 and the thermionic emission barrier height, ΦB=0.53 eV. In the high-forward-bias regime (0.5–2.0 V), the bias dependence of the tunnel barrier transparency, approximated by a modified Simmons model for a rectangular tunnel barrier, provides the tunnel barrier height, ΦT=0.5 eV; the fitted prefactor value, G0 = 10−10 Ω−1, is four decades smaller than the theoretical Simmons prefactor for MIM structures. The density distribution of defects localized at the C12H25 – n Si interface is deduced from admittance data (low-high frequency method) and from a simulation of the response time τR(V) using Gomila’s model for a non equilibrium tunnel junction. The low density of electrically active defects near mid-gap (DS &amp;lt; 2 × 1011 eV−1.cm−2) indicates a good passivation of dangling bonds at the dodecyl – n Si (111) interface.

Analysis of leakage current mechanisms in Pt/Au Schottky contact on Ga-polarity GaN by Frenkel-Poole emission and deep level studies

We report the Frenkel-Poole emission in Pt/Au Schottky contact on Ga-polarity GaN grown by molecular beam epitaxy using current-voltage-temperature (I-V-T) characteristics in the temperature ranging from 200 K to 375 K. Using thermionic emission model, the estimated Schottky barrier height is 0.49 eV at 200 K and 0.83 eV at 375 K, respectively, and it is observed that the barrier height increases with increase in temperature. The extracted emission barrier height (ϕt) for Ga-polarity GaN Schottky diode by Frenkel-Poole theory is about 0.15 eV. Deep level transient spectroscopy study shows a deep level with activation energy of 0.44 eV, having capture cross-section 6.09 × 10−14 cm2, which is located between the metal and semiconductor interface, and trap nature is most probably associated with dislocations in Ga-polarity GaN. The analysis of I-V-T characteristics represents that the leakage current is due to effects of electrical field and temperature on the emission of electron from a trap state near the metal-semiconductor interface into continuum states associated with conductive dislocations in Ga-polarity GaN Schottky diode.

Effective electron emitters by molybdenum oxide-coated carbon nanotubes core–shell nanostructures

In this article, we showed that simple metal oxide coatings such as MoO3 can be an effective enhancer for carbon nanotubes (CNTs) in field emission (FE) performance. For comparison, the FE properties of the pristine vertically aligned multi-walled CNTs with the metal oxide-coated CNTs were investigated. The metal oxide coating of the pristine CNTs was carried out by metal–organic chemical vapor deposition (MOCVD) method at 400 °C using Mo(CO)6 as the precursor. The core–shell structure of the nanocomposite was studied by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) results showed that the surface of the coating material was mainly MoO3. FE test indicated that the MoO3-coated CNTs film exhibited an enhanced performance than the pristine CNTs with a turn-on field of 1.33 V μm−1 and a field enhancement factor β estimated to be ~7000. Ultraviolet photoelectron spectroscopy (UPS) results confirmed a lower electron emission barrier height for MoO3-coated CNTs than for the pristine CNTs. The mechanism of the enhanced FE performance is discussed based on Schottky barrier effect.

Schottky barriers of rare-earth transition-metal intermetallics on silicon

The ferromagnetic transition metals Fe, Co and Ni have high work functions (4.5 – 5.0 eV) and they invariably form Schottky barriers on Si, often detrimental to efficient spin injection and detection at the interfaces. The rare earths have lower work functions (2.5 – 3.2 eV) but they order magnetically below room temperature. We therefore investigate a series of 12 rare earth metals and alloys of Gd, Ce or Y with Fe or Co, which are evaporated onto (001) Si wafers. The lowest thermionic emission barrier height is found for CeCo5 (48 meV), much less than those found for the 3d ferromagnets or their silicides (~ 500 meV). Y is found to form an ohmic contact after a diffusion anneal at 1300 K. Further alloy additions are needed to lower the barrier height below 25 meV; a target for room temperature operation with significantly increased reverse bias current.

Effect of Tunneling Current on Schottky Barrier Height in ZnO Varistors at Low Temperature

On the basis of the Schottky barrier and thermionic emission models, the temperature dependence of barrier height in ZnO varistors is investigated by the I – V characteristics in a wide temperature range from 93 K to 373 K. The obtained barrier height decreases with reducing temperature, which is ascribed to the contribution of tunneling current in measured current. From the proposed equivalent circuit, it is suggested that two current components coexist. One is thermionic emission current, which reflects the thermionic emission barrier height. The other is tunneling current, which appears even at low voltage, especially in low temperature ranges, and thus makes the barrier height obtained from measured current vary with temperature.

Study of leakage current in metal–insulator–metal Mg–Al co-doped barium strontium titanate thin films as a function of temperature and applied field

This study investigated the electrical properties of Mg–Al co-doped Ba 0.5Sr 0.5TiO 3 (BSTMA) films, fabricated on a P-type Si (100) substrate by a sol–gel spin coating process. Pt electrodes were deposited by radio frequency magnetron sputtering to form metal–insulator–metal capacitors. The current–voltage characteristics of the BSTMA films were also studied for various external electric fields and temperatures. Experimental results indicate that the leakage currents of Mg–Al co-doped BST films are lower than those of Mg-doped specimens. The leakage currents of 5 mol% Mg-doped and 5 mol% Mg–5 mol% Al- co-doped BST films are 1.14 × 10 − 3 and 1.56 × 10 − 5 A/cm 2, respectively, at 805.5 kV/cm and 298 K. The current–voltage characteristics are ohmic behavior in the lower field region ( E ≤ 32 kV/cm) followed by Schottky emission transport behavior in the higher field region (32 ≤ E ≤ 604 kV/cm). While at high field ( E ≥ 604 kV/cm), both the Schottky emission and Poole–Frenkel transport behavior are observed. The Schottky emission barrier height and Poole–Frenkel trap depth level depend on the applied electric field and the temperature at which the measurements are made.

Emission of scission neutrons in the sudden approximation

At a certain finite neck radius during the descent of a fissioning nucleus from the saddle to the scission point, the attractive nuclear forces can no more withstand the repulsive Coulomb forces producing the neck rupture and the sudden absorption of the neck stubs by the fragments. At that moment, the neutrons, although still characterized by their pre-scission wave functions, find themselves in the newly created potential of their interaction with the separated fragments. Their wave functions become wave packets with components in the continuum. The probability to populate such states gives evidently the emission probability of neutrons at scission. In this way, we have studied scission neutrons for the fissioning nucleus 236U, using two-dimensional realistic nuclear shapes. Both the emission probability and the distribution of the emission points relative to the fission fragments strongly depend on the quantum numbers of the pre-scission state from which the neutron is emitted. In particular, it was found that states with Ω π = 1 / 2 + dominate the emission. Depending on the assumed pre- and post-scission configurations and on the emission-barrier height, 30 to 50% of the total scission neutrons are emitted from 1 / 2 + states. Their emission points are concentrated in the region between the newly separated fragments. The upper limit for the total number of neutrons per scission event is predicted to lie between 0.16 and 1.73 (depending on the computational assumptions).

Field emission from H- and O-terminated heavily P-doped homoepitaxial diamond

Field emission properties of phosphorus (P)-doped homoepitaxial diamonds are measured after hydrogen plasma treatment and wet chemical oxidization. We also measure field emission properties of H-terminated B-doped diamond for comparison. P-doped diamonds are grown on type Ib single crystalline diamond by microwave plasma chemical vapor deposition. The resistivity of the films is in the range of (5–8)×102Ωcm. Phosphorus concentration measured by secondary ion mass spectroscopy is in the range of (5–7)×1019cm−3. The threshold voltage for field emission of O-terminated P-doped diamond with positive electron affinity is lower than that of H-terminated P-doped diamond with negative electron affinity. Upward band bending due to ionized phosphorus donors exists at the H-terminated surface. This upward bend bending prevents electrons from reaching to the emitting surface. An effective emission barrier height of O-terminated P-doped diamond surface is estimated to be 1.3eV using slopes of Fowler-Nordheim plots. This value is in good agreement with the reported electron affinities in literature.