High-quality Schottky Contacts Research Articles

Amorphous Ga2O3 Schottky photodiodes with high-responsivity and photo-to-dark current ratio

Schottky photodiodes with sputtered amorphous Ga2O3 (a-Ga2O3) and asymmetric electrodes were fabricated for the first time and achieved excellent performances under 254-nm light illumination with the superhigh responsivity of 1021.8 A W−1, high photo-to-dark current ratio of 2.3 × 106, fast rise/decay response time of 144/208 ms, high detectivity of 1.66 × 1016 Jones, and high external quantum efficiency of 5.0 × 105 %. The excellent solar-blind detection performances are suggested to be attribute to the large concentration of oxygen vacancies, the possible photo released carriers from the deep-level acceptors, and the high film uniformity, while the former two contribute to the high photo carrier concentration, and the latter one contributes to high quality Schottky contact to achieve the low dark current. A 10 × 10 array based on these a-Ga2O3 Schottky photodiodes realized the imaging of a “E” character with high contrast. The results give a feasible way to achieve large area, low cost, high contrast, and high detection sensitivity solar-blind imaging.

Normally-off GaN HEMTs with InGaN p-gate cap layer formed by polarization doping

Narrow gate margin has been the critical limiting factor for the p-gate normally-off GaN HEMTs, imposing significant challenges in both gate drive design and gate reliability. In this work, by developing dopant-free p-type polarization doping technique in composition-graded InGaN layer, high-quality Schottky contact between the gate metal and cap layer was demonstrated, achieving excellent gate current blocking performance (10–6 mA mm−1) after the turning-on of the gate heterojunction structure. Resultantly, normally-off GaN HEMTs with enhanced gate breakdown voltage up to 15.2 V was realized, being especially beneficial for the simplification of gate drive design and the safe operation of gate terminal.

The evaluation of the current–voltage and capacitance–voltage-frequency measurements of Yb/p-Si Schottky diodes with a high zero-bias barrier height

The rare-earth metal Yb (Ytterbium) which has a very low work function of 2.60 eV has been shown to high quality Schottky contacts to p-Si. Two distinct linear regions in the semi-logarithmic current density–voltage plot are observed for Yb/p-Si Schottky diodes. From the linear regions, the very high zero-bias barrier height values are determined as 0.83 and 0.88 eV. The value of rectification ratio is in order of ~ 106 which is close to the commercial Schottky diodes. From Cheung functions, the value of series resistance is found to be 12.60 Ω. Furthermore, the important contact parameters of the diodes are calculated by using modified Norde method. The contact parameters obtained from reverse bias capacitance–voltage-frequency measurements do not show an important dependence on frequency. The calculated values of acceptor concentration are in close agreement with the value of doping concentration of the p-Si wafer used in this study.

Au/GLAD-SnO2 nanowire array-based fast response Schottky UV detector

In this paper, we have demonstrated UV photodetector based on SnO2 nanowire (NW) arrays fabricated using a catalytic free and controlled growth process called glancing angle deposition technique. The fabricated SnO2 NWs were amorphous in nature with highly periodic and perpendicularly oriented structures of length ~ 160 ± 5 nm with ~ 60 ± 5 nm average diameter. The reported Au/SnO2 NW/n-Si device showed a good rectifying behavior with a rectification ratio of ~ 6 due to the formation of high-quality Schottky contact at the Au/SnO2 NW interface. The Au/SnO2 NW/n-Si device exhibited a high responsivity (0.142 A/W) and external quantum efficiency (56.8%) at − 2 V applied bias as compared to the Au/SnO2 thin-film (TF)/n-Si device. Moreover, the Au/SnO2 NW/n-Si device attained a high detectivity of 10.8 × 1010 Jones and noise equivalent power as low as 38.8 × 10−12 W. The high surface to volume ratio and the enormous amount of photogenerated carriers in case of SnO2 NW arrays made the Au/SnO2 NW/n-Si device to exhibit high photosensitivity. Furthermore, on UV illumination, the Au/SnO2 NW/n-Si detector showed fast device response with a rise time of 0.18 s and a fall time of 0.25 s. The current conduction mechanism in case of Au/SnO2 NW/n-Si device is explained with respect to device band diagram.

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Current transport mechanism of Schottky contact of Pt/Au/n-InGaN

The Pt/Au Schottky contacts to InGaN samples with different background carrier concentrations are fabricated. The crystal qualities of InGaN samples are characterized by X-ray diffraction (XRD) and atomic force microscope (AFM), and the correlation between threading dislocation density of InGaN and growth temperature is further clarified. The full width at half maximum (FWHM) values of the InGaN (0002) XRD rocking curves show that the density of threading dislocations in InGaN, which can seriously deteriorate InGaN crystal quality and surface morphology, decreases rapidly with increasing growth temperature. The Hall measurements show that the background carrier concentration of InGaN increases by two orders of magnitude as growth temperature decreases from 750 to 700℃, which is due to a reduced ammonia decomposition efficiency leading to the presence of high-density donor-type nitrogen vacancy (VN) defects at lower temperature. Therefore, combining the studies of XRD, AFM and Hall, it can be concluded that the higher growth temperature is favorable for realizing the InGaN film with low density of VN defects and threading dislocations for fabricating high-quality Schottky contacts, and then the barrier characteristics and current transport mechanism of Pt/Au/n-InGaN Schottky contact are investigated by current-voltage measurements and theory analysis based on the thermionic emission (TE) model and thermionic field emission (TFE) model. The results show that Schottky characteristics for InGaN with different carrier concentrations manifest obvious differences. It is noted that the high carrier concentration leads to the Schottky barrier height and the ideality factor obtained by TE model are quite different from that by TFE model due to the presence of high density of VN defects. This discrepancy suggests that the VN defects lead to the formation of the tunneling current and further reduced Schottky barrier height. Consequently, the presence of tunneling current results in the increasing of total transport current, which means that the defects-assisted tunneling transport and TE constitute the current transport mechanism in the Schottky. However, the fitted results obtained by TE and TFE models are almost identical for the InGaN with lower carrier concentration, indicating that TE is the dominant current transport mechanism. The above studies prove that the Pt/Au/n-InGaN Schottky contact fabricated using low background carrier concentration shows better Schottky characteristics. Thus, the properly designed growth parameters can effectively suppress defects-assisted tunneling transport, which is crucial to fabricating high-quality Schottky devices.

High performance InGaZnO‐based Schottky diodes fabricated at room temperature

AbstractRecently, Indium gallium zinc oxide (InGaZnO or IGZO) has attracted much attention for flexible and transparent electronics, because of its superior electric properties, optical transparency and low processing temperature. In this work, Schottky diodes with a structure of Pd/IGZO/Ti/Au were fabricated by radio‐frequency magnetron sputtering at room temperature without any thermal treatment. The optimised diode with a 66‐nm‐IGZO layer shows an extremely large barrier height (1.02 eV), a very high rectification ratio (> 8.5 × 107), and a close to unit ideality factor (1.23). Our results suggest that Pd and IGZO can form very high quality Schottky contact with a large barrier height, extremely low defects density and high uniformity. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Method of choice for fabrication of high-quality ZnO-based Schottky diodes

We present a comprehensive comparison of electrical properties of differently fabricated high quality Schottky contacts on ZnO thin films grown by pulsed laser deposition. Thermally evaporated Pd/ZnO Schottky contacts exhibit ideality factors as low as 1.06 due to their high lateral homogeneity. The effective Richardson constant determined using these homogeneous contacts is (7.7±4.8)A cm−2 K−2 close to the theoretical value of 32 A cm−2 K−2. However, their rectification ratio is at most five orders of magnitude due to their comparably small barrier height (≈0.7eV). The largest effective barrier height (1.11 eV) and rectification ratio (7×1010) was obtained for reactively sputtered PdOx/ZnO Schottky contacts. Eclipse pulsed laser deposited IrOx/ZnO Schottky contacts were found to combine very good lateral homogeneity (n≈1.1), with a reasonably large barrier height (0.96 eV) and large rectification ratio (≈9 orders of magnitude). Our results for differently fabricated Schottky contacts suggest that the barrier formation is highly dependent on the presence of oxygen vacancies close to the interface and the different compensation mechanisms involved.

Characterization of surface roughness of Pt Schottky contacts on quaternary n-Al0.08In0.08Ga0.84N thin film assessed by atomic force microscopy and fractal analysis

The purpose of this study was to analyze surface topography of Pt Schottky contacts on quaternary n-Al0.08In0.08Ga0.84N thin film. To understand how the effect of temperature changes the layers surface, the surface topography was characterized through atomic force microscopy (AFM) and fractal analysis. Pt Schottky contacts grown on nanostructure Al0.08In0.08Ga0.84N thin film grown by molecular beam epitaxy technique on sapphire substrate at annealing temperatures range of 300–500 °C were used. AFM analysis was performed in contact mode, on square areas of 10 × 10 μm2, by using a Nanosurf Easyscan 2 AFM system. Detailed surface characterization of the surface topography was obtained using statistical parameters of 3D surface roughness, according with ISO 25178-2: 2012, provided by the AFM software. The results revealed that the high quality Schottky contact with the Schottky barrier heights and ideality factor of 0.76 and 1.03 respectively can be obtained under 30 min annealing at 400 °C in N2 ambience. The surface roughness of Pt Schottky contacts on quaternary n-Al0.08In0.08Ga0.84N thin film revealed a fractal structure at nanometer scale. Results obtained by fractal analysis confirm the relationship between the value of the fractal dimension and the statistical surface roughness parameters. AFM and fractal analysis are accurate tools that may assist manufacturers in developing Pt Schottky contacts on quaternary n-Al0.08In0.08Ga0.84N thin film with optimal surface characteristics and provides different yet complementary information to that offered by traditional surface statistical parameters.

Electrical characterization of ensemble of GaN nanowires grown by the molecular beam epitaxy technique

High quality Schottky contacts are formed on GaN nanowires (NWs) structures grown by the molecular beam epitaxy technique on Si(111) substrate. The current-voltage characteristics show the rectification ratio of about 103 and the leakage current of about 10−4 A/cm2 at room temperature. From the capacitance-voltage measurements the free carrier concentration in GaN NWs is determined as about 1016 cm−3. Two deep levels (H200 and E280) are found in the structures containing GaN NWs. H200 is attributed to an extended defect located at the interface between the substrate and SiNx or near the sidewalls at the bottom of the NWs whereas E280 is tentatively assigned to a gallium-vacancy- or nitrogen interstitials-related defect.

Annealing Effects on Electrical Properties and Interfacial Reactions of Ni/Cu Schottky Rectifiers on n-Type InP

We report on the effect of annealing temperature on electrical, interfacial reactions and surface morphological properties of Ni/Cu Schottky contacts on n-type InP. The extracted barrier height of as-deposited Ni/Cu Schottky contact is 0.59 eV (I-V) respectively. The high-quality Schottky contact with barrier height and ideality factor of 0.65 eV (I-V) and 1.15 respectively, can be obtained after annealing at 300℃ for 1 min in a nitrogen atmosphere. However, annealing at 400℃, results the decrease in the barrier height to 0.54 eV (I-V). From the above observations, it is observed that Ni/Cu Schottky contact exhibited excellent electrical properties after annealing at 300℃. Hence, the optimum annealing temperature for the Ni/Cu Schottky contact is 300℃. Furthermore, Cheung’s functions is used to extract the diode parameters including ideality factor, barrier height and series resistance. According to the XRD analysis, the formation of the indium phases at the Ni/Cu/n-InP interface could be the reason for the increase in the barrier height at annealing temperature 300℃. Further, the degradation of the barrier heights after annealing at 400℃ may be due to the formation of phosphide phases at the Ni/Cu/n-InP interface. Scanning electron microscopy (SEM) results show that the overall surface morphology of the Ni/Cu Schottky contact is reasonably smooth.

Structural and electrical properties of rapidly annealed Ni/Mo Schottky barriers on n‐type GaN

AbstractThermal annealing effects on the electrical and structural properties of Ni/Mo Schottky contacts on n‐type GaN have been investigated by current–voltage (I–V), capacitance–voltage (C–V), Secondary ion mass spectrometer (SIMS), and X‐ray diffraction (XRD) techniques. The extracted Schottky barrier height (SBH) of the as‐deposited contacts was found to be 0.66 eV (I–V), 0.74 eV (C–V). However, both measurements indicate that the barrier height slightly increases when the contacts are annealed at 300 and 400 °C. Experimental results indicate that high quality Schottky contact with barrier height and ideality factor of 0.75 eV (I–V), 0.96 eV (C–V), and 1.13, respectively, can be achieved under 1 min annealing at 500 °C in nitrogen atmosphere. Further, it is observed that the barrier height slightly decreases upon annealing at 600 °C. The above observations establish that the Ni/Mo contact exhibited excellent electrical characteristics even after thermal annealing at 600 °C. Based on the SIMS and XRD analysis, the formation of gallide phases at the Ni/Mo/n‐GaN interface could be the reason for the improvement of SBH after annealing at 500 °C. The above results indicate that the Ni/Mo contact can be promising for metallization scheme for high‐temperature device applications.

Thermal annealing behaviour of Al/Ni/Au multilayer on n-GaN Schottky contacts

Recently GaN-based high electron mobility transistors (HEMTs) have revealed the superior properties of a high breakdown field and high electron saturation velocity. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. This paper reports that an Al layer as thin as 3 nm was inserted between the conventional Ni/Au Schottky contact and n-GaN epilayers, and the Schottky behaviour of Al/Ni/Au contact was investigated under various annealing conditions by current-voltage (I–V) measurements. A non-linear fitting method was used to extract the contact parameters from the I–V characteristic curves. Experimental results indicate that reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing. And high quality Schottky contact with a barrier height of 0.875 eV and the lowest reverse-bias leakage current, respectively, can be obtained under 12 min annealing at 450 °C in N2 ambience.

High-quality Schottky contacts to n-InGaN alloys prepared for photovoltaic devices

Large-area Au∕Pt∕n-In0.2Ga0.8N Schottky contacts have been fabricated for photovoltaic devices. The current transport mechanisms of the Schottky contacts to n-In0.2Ga0.8N with different background carrier concentrations are investigated. The thermionic emission is a dominating current transport mechanism at the Pt∕n-InGaN interface in a low background carrier concentration sample, while the defect-assisted tunneling current and trap-related recombination current play important roles in high background carrier concentration samples. The Schottky diode fabricated using the low background carrier concentration sample gives much better Schottky barrier characteristics and exhibits a three to four order of magnitude higher spectral responsivity and a larger rejection ratio in comparison with those fabricated using the high background carrier concentration samples.

Influence of oxygen vacancies on Schottky contacts to ZnO

Ni, Ir, Pd, Pt, and silver oxide Schottky contacts were fabricated on the Zn-polar face of hydrothermally grown, bulk ZnO. A relationship was found between the barrier height of the contact and the free energy of formation of its “metal” oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately 0.7eV below the conduction band minimum. Therefore, a key goal in the fabrication of high quality Schottky contacts should be the minimization of oxygen vacancies near the metal-ZnO interface.

Mechanisms in the Formation of High Quality Schottky Contacts to n-type ZnO

AbstractPt, Ir, Ni, Pd, and silver oxide Schottky contacts were fabricated on the Zn-polar surface of hydrothermally grown bulk ZnO. A relationship was observed between the barrier height of the contact and the free energy of formation of the “metal” oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately EC - 0.7 eV, where EC is the conduction band minimum. Valence band x-ray photoemission spectroscopy and the current - voltage characteristics of planar Schottky diodes, measured on similar Zn-polar surfaces, showed the existence of a vacuum activated surface accumulation layer. This is possibly a consequence of the observed OH termination of the Zn-polar surface. The surface accumulation layer is compensated in atmospheric conditions by the presence of acceptor-like adsorbates, such as O2 and H2O. The formation of high quality Schottky contacts to ZnO should therefore involve the reduction of near surface oxygen vacancies and the removal of H or OH from the surface.

Environmental sensitivity of Au diodes on n-AlGaN

With growing interest in AlGaN for ultraviolet detectors and high-power/high-temperature electronic devices, the problem of forming high-quality Schottky contacts to this semiconductor has become increasingly important. It was shown that wet-chemical surface pretreatments affect the as-deposited diode characteristics for Au/n-AlGaN Schottky diodes. However, these diodes improve over the course of days when exposed to air at room temperature, exhibiting reduced leakage currents, enhanced barrier heights, and reduced ideality factors. Exposure to oxygen, with an enhanced effect in the presence of water vapor, is responsible for the environmental aging. The environmental aging was found to occur regardless of the source of AlGaN, surface preparation, and metal deposition technique. It was determined that high asdeposited reverse currents were due to current transport beneath the contact area, rather than across the semiconductor surface. Two findings further suggested that the change in electrical characteristics was due to a phenomenon occurring at the metal/semiconductor interface. First, metal thickness played a key role in the rate of change of the electrical characteristics, with thicker contacts being more impervious to surrounding gas species at room temperature. Second, a metal that readily forms an oxide, Ni, exhibited little environmental aging, while noble metals, such as Au and Pt, showed dramatic effects. Mild anneals revealed that the environmental change was partially reversible, which suggests the passivation of electrically active defects at the metal/semiconductor interface as the cause of the altered diode behavior. Taken together, the data indicate that oxidizing species diffuse through noble metal contacts to the metal/semiconductor interface and passivate electrically active defects, which may be reactivated upon mild anneals in N2.

Preparation of p-type InP layers for detection of radiation

We have focused on the investigation of the impact of Ce, Tm, Tm 2O 3, and Lu addition in the liquid-phase epitaxial growth process on the structural and electro-optical properties of InP layers in the context of their possible application in detector structures, where detection will be mediated via the depletion layer of high quality Schottky contact. The effect of Tm 2O 3 and Lu is reported for the first time. The grown layers were examined via scanning electron microscopy, low-temperature photoluminescence spectroscopy, capacitance–voltage measurements using the mercury probe and by the temperature-dependent Hall effect. Availing Tm addition with concentration 5.4×10 −2 at%, we have prepared thick (>10 μm) p-type conductivity InP layers with the structural defect density reduced by a half-order of magnitude and reduced electrically active impurity concentration up to ∼7×10 14 cm −3. We point out that Tm appears as a promising candidate for the preparation of very pure p-type InP layers. The mechanism of purification efficiency of different rare earths from donors and acceptors leading to the n→p conductivity type crossover has been discussed.

Thermal annealing behaviour of Ni/Au on n-GaN Schottky contacts*

The Schottky behaviour of Ni/Au contact on n-GaN was investigated under various annealing conditions by current–voltage (I–V) measurements. A non-linear fitting method was used to extract the contact parameters from the I–V characteristic curves. Experimental results indicate that high quality Schottky contact with a barrier height and ideality factor of 0.86±0.02 eV and 1.19±0.02 eV, respectively, can be obtained under 5 min annealing at 600°C in N2 ambience.