On the band-structure lineup at Schottky contacts and semiconductor heterostructures

The band-structure lineup at semiconductor interfaces is explained by the intrinsic interface-induced gap states (IFIGS) that derive from the complex band structures of the semiconductors. The barrier heights of metal–semiconductor or Schottky contacts as well as the band-edge offsets of semiconductor heterostructures are composed of a zero-charge-transfer term plus an electrostatic-dipole contribution which are determined by the IFIGS branch-point energies of the semiconductors and the electronegativity difference of the two materials in contact, respectively. This concept will be illustrated by experimental core-level shifts induced by metal adatoms on group-IV semiconductor surfaces. Choosing Si and SiO₂ Schottky contacts and heterostructures as typical examples, it will be demonstrated that the IFIGS-and-electronegativity concept self-consistently explains the barrier heights of Schottky contacts and the valence-band offsets of heterostructures. The IFIGS-and-electronegativity concept also resolves the alleviation of the Fermi-level pinning by ultra-thin insulator interlayers in Schottky contacts. Finally, the modification of Schottky barriers by atomic interlayers will be discussed.     

GaAs surface plasma treatments for Schottky contacts

The properties of different rectifying metallizations (Al, Ti/Pt, WN_x) on GaAs have been investigated for various surface preparation procedures. In particular, in situ hydrogen plasma treatments were used to remove residual surface contamination (mainly O and C) and a nitrogen plasma to grow a thin mixed nitride layer. Al and Ti/Pt Schottky diodes with an ideality factor very close to 1, but with reduced barrier height, were found after the H₂ plasma as a consequence of H diffusion into GaAs. The Schottky barrier height was further reduced if a H₂ + N₂ plasma was performed. The N content in the sputtering environment during the WN_x deposition affects the diode properties of plasma-treated WN_x contacts. By increasing the N₂ partial pressure, the diode barrier height is reduced, probably due to nitridization of the GaAs surface. Such differences disappear after annealing the diodes in arsine overpressure at 800°C. WN_x contacts deposited under different conditions on H₂ plasma treated substrates also show a similar Schottky barrier height after such annealing.     

InP Schottky contacts with increased barrier height

We present an analysis of Schottky barriers in n-InP made by incorporating a thin native oxide. An oxidation technique using nitric acid under illumination produces an oxide layer with uniform composition distribution within the layer. The growth rate is interpreted as being partially limited by diffusion presumably of oxygen through oxide. The Au Schottky barrier formed on a 40–80 Å thick oxide layer exhibits little degradation of the ideality factor n (1.04 < n < 1.10) and an increase of the barrier height by greater than 0.3 eV, resulting in at least a 10^?4 times smaller reverse leakage current density, compared with conventional Au-InP barriers. The barrier height increase is analysed by a generalised model, and is found to be produced by the existence of fixed negative charges in the oxide layer. From the present analysis, a surface state density of 6.0 × 10¹² cm^?2 eV^?1 and an equivalent surface density of negative charges of 2.8 × 10¹² cm^?2 are determined independently. The origins of these, particularly of the surface states, are considered in relation to the P vacancies at the oxide-InP interface.     

High-performing transparent photodetectors based on Schottky contacts

Transparent UV-photodetectors exhibiting very high responsivity and fast operation are discussed. Schottky contact photoelectric devices utilizing wide band gap TiO₂ absorber layer were evaluated for their performances as UV-photodetectors. Three different work function metals Cu, Mo and Ni were used to realize Schottky barrier with TiO₂. Ni Schottky contacts were found to be most suitable to fabricate high responsivity (2.034 A/W) photodetector with faster rise time (0.14 ms) and wide linear dynamic range (128 dB) operating at small applied reverse bias of −1 V. However, higher barrier height in the case of Mo/TiO₂ interface resulted in lowest dark current density of the value 2.21×10⁻⁸ A/cm² with quick fall time of 0.52 ms. The modulation of the barrier height would provide a route for designing fast and high responsive Schottky photodetector with broad linear dynamic range performance.     

The nature of electrical interaction of Schottky contacts

Electrical interaction between metal-semiconductor contacts combined in a diode matrix with a Schottky barrier manifests itself in an appreciable variation in their surface potentials and static current-volt-characteristics. The necessary condition for appearance of electrical interaction between such contacts consists in the presence of a peripheral electric field (a halo) around them; this field propagates to a fairly large distances (<30 μm). The sufficient condition is the presence of regions where the above halos overlap. It has been shown that variation in the surface potential and the current-voltage characteristics of contacts occurs under the effect of the intrinsic electric field of the contact’s periphery and also under the effect of an electric field at matrix periphery; the latter field is formed as a result of superposition of electric fields of halos which form its contacts. The degree of the corresponding effect is governed by the distance between contacts and by the total charge of the space charge regions for all contacts of the matrix: their number, sizes (diameter D _{i, j} ), concentration of doping impurities in the semiconductor N _D , and physical nature of a metal-semiconductor system with a Schottky barrier (with the barrier height φ _b ). It is established that bringing the contacts closer leads to a relative decrease in the threshold value of the “dead” zone in the forward current-voltage characteristics, an increase in the effective height of the barrier, and an insignificant increase in the nonideality factor. An increase in the total area of contacts (a total electric charge in the space charge region) in the matrix brings about an increase in the threshold value of the “dead” zone, a relative decrease in the effective barrier height, and an insignificant increase in the ideality factor.     

Investigation of Ti-Ge/GaAs Schottky barrier contacts

The effect of germanium content in a titanium film, as well as treatment methods and conditions, on the Schottky barrier height (φ_b) and the ideality factor (n) for Ti-Ge / GaAs contacts were investigated. It is shown that photon treatment provides better contacts than thermal treatment.     

Electrical analysis of organic dye-based MIS Schottky contacts

In this work, we prepared metal/interlayer/semiconductor (MIS) diodes by coating of an organic film on p-Si substrate. Metal(Al)/interlayer(Orange GOG)/semiconductor(p-Si) MIS structure had a good rectifying behavior. By using the forward-bias I-V characteristics, the values of ideality factor (n) and barrier height (BH) for the Al/OG/p-Si MIS diode were obtained as 1.73 and 0.77 eV, respectively. It was seen that the BH value of 0.77 eV calculated for the Al/OG/p-Si MIS diode was significantly larger than the value of 0.50 eV of conventional Al/p-Si Schottky diodes. Modification of the potential barrier of Al/p-Si diode was achieved by using thin interlayer of the OG organic material. This was attributed to the fact that the OG organic interlayer increased the effective barrier height by influencing the space charge region of Si. The interface-state density of the MIS diode was found to vary from 2.79 × 10¹³ to 5.80 × 10¹² eV⁻¹ cm⁻².     

Displacement current of Au/p-diamond Schottky contacts

In this study, the displacement current of Au/p-diamond Schottky contacts was studied by comparing them with low-Mg-doped p-GaN Schottky contacts. In the current–voltage (I–V) characteristics, the current was proportional to the voltage sweep speed at V >−1.5 V. The differential output waveform was obtained through AC operation. Therefore, the displacement current was dominant in the low voltage region where in the true current was extremely small due to the large Schottky barrier height of 1.57 eV. The memory effect, due to the charge and discharge of localized acceptor-type deep-level defects, was negligible in the I–V curve. A clear AC differential signal was confirmed. This suggested that the interface defect density of the p-diamond contacts was very small compared to the p-GaN contacts. Hence, p-diamond Schottky contacts were expected to be a good candidate for a key device such as a phase modulator in a wireless transmitter.     

A unified simulation of Schottky and ohmic contacts

The Schottky contact is an important consideration in the development of semiconductor devices. This paper shows that a practical Schottky contact model is available for a unified device simulation of Schottky and ohmic contacts. The present model includes the thermionic emission at the metal/semiconductor interface and the spatially distributed tunneling calculated at each semiconductor around the interface. Simulation results of rectifying characteristics of Schottky barrier diodes (SBD's) and resistances under high impurity concentration conditions are reasonable, compared with measurements. As examples of application to actual devices, the influence of the contact resistance on salicided MOSFETs with source/drain extension and the immunity of Schottky barrier tunnel transistors (SBTTs) from the short-channel effect (SCE) are demonstrated     

Effect of hydrogenation on the properties of metal-GaAs Schottky barrier contacts

The effect of hydrogenation (incorporation of atomic hydrogen) on the properties of n-GaAs and the characteristics of Au-GaAs Schottky barrier contacts (the ideality factor of the current-voltage characteristic n, the barrier height ϕ _b, and the reverse voltage V _r at a current of 10 μA) has been investigated. The n-GaAs surface was bare (A-type samples) or was protected by an ultrathin (∼50 ?) layer of SiO₂ (B-type samples) during hydrogenation. It was shown that there was an optimal hydrogenation regime for the A-type samples (temperature range 150–250 °C and duration 5 min), for which n and V _r reached their minimum and maximum values, respectively. For the B-type samples, n and V _r improve starting from minimal durations and temperatures of hydrogenation and remain constant or even improve over the entire investigated range of temperatures (100–400 °C) and durations (1–50 min). The donor impurity passivation processes are roughly the same for the A-and B-type samples. Fiz. Tekh. Poluprovodn. 32, 1343–1348 (November 1998)     

Electrical characteristics of Pd Schottky contacts on ZnO films

The electrical characteristics of Pd Schottky contacts on ZnO films have been investigated by current-voltage (I–V) and capacitance–voltage (C–V) measurements at different temperatures. ZnO films of two thicknesses (400 nm and 1000 nm) were grown by DC-magnetron sputtering on n-Si substrates. The basic structural, optical and electrical properties of these films are also reported. We compared the two Schottky diodes by means of characteristic parameters, such as rectification ratio, ideality factor (η), barrier height (Φ_b) and series resistance and obtained better results for the 1000 nm-ZnO Schottky diodes. We also discussed the dependence of I‐V characteristics on temperature and the two distinct linear regions observed at low temperatures are attributed to the existence of two different inhomogeneous barrier heights. From I–V plots in a log-log scale we found that the dominant current-transport mechanism at large forward bias is space-charge limited current (SCLC) controlled by the presence of traps within the ZnO bandgap. The existence of such traps (deep states or interface states) is demonstrated by frequency-dependent capacitance and deep-level transient spectroscopy (DLTS) measurements.     

Schottky barrier height of sputtered TiN contacts on silicon

The Schottky barrier height of sputtered TiN on both p- and n-type silicon was determined by I-V and C-V measurements. The barrier height is found to increase on n-Si and to decrease on p-Si, upon thermal annealing. The experimental results are explained in terms of sputtering damage. This damage is modeled by donor-like traps whose concentration decays exponentially from the silicon surface. A characteristic length equal to 45 Å accounts for the observed characteristics. The trap-free values of the barrier height were obtained by I-V measurements after sequential thermal annealing up to 600°C. These values are φ_Bn = 0.55 V on n-type and φ_Bp = 0.57 V on p-type silicon.     

Effect of surface inhomogeneities on the electrical characteristicsof SiC Schottky contacts

This paper reports analysis of the role of defects on the electrical characteristics of high-voltage 6H-SiC Schottky rectifiers. The measured reverse leakage current of high-voltage Ti and Pt rectifiers was found to be much higher than that predicted by thermionic emission theory and using a barrier height extracted from the C-V measurements. In this paper, a model based upon the presence of defects at the 6H-SiC/metal interface is used to explains this behavior. It is proposed that these defects result in lowering of the barrier height in the localized regions and thus, significantly affect the reverse I-V characteristics of the Schottky contacts. The presence of electrically active defects in the Schottky barrier area has been verified by EBIC studies     

Thermal stability of Re Schottky contacts to 6H-SiC

The thermal stability of a 100-nm thick sputter-deposited Re film as contact to 6H-SiC was studied by backscattering spectrometry and by measurements of the forward current-voltage (I-V) characteristic. The initial Schottky barrier height of 0.71 eV and ideality factor of 1.6 change after 2 h of annealing in vacuum at 700/spl deg/C to 1.04 eV and 1.1, respectively. They remain stable after annealing for additional 2 h at that same temperature. The initial change is attributed to a recovery of sputter damage in the SiC. The observed stability of the Schottky barrier is attributed to the thermodynamic stability of Re with SiC, as confirmed by the unchanging backscattering depth profiles. After annealing at 900/spl deg/C, the Schottky barrier becomes unstable although no interaction between the Re film and the SiC substrate is detectable in the depth profiles.     

Modeling the electrical characteristics of Schottky contacts in low-dimensional heterostructure devices

This paper deals with the modeling of the electronic characteristics of semiconductor devices based on Schottky contacts in low-dimensional systems. For the capacitance-voltage characteristics, a quasi-two-dimensional quantum mechanical model is developed and validated. For the current-voltage characteristics, a unified model is presented, considering both the tunneling as well as the thermionic emission mechanisms. Our theoretical predictions suggest that for photodetection applications the use of these contacts, replacing conventional metal-semiconductor junctions, can reduce the dark current by at least one order of magnitude.     

Determination of contact parameters of Ni/n-GaP Schottky contacts

The electrical analysis of Ni/n-GaP structure has been investigated by means of current–voltage (I–V), capacitance–voltage (C–V) and capacitance–frequency (C–f) measurements in the temperature range of 120–320 K in dark conditions. The forward bias I–V characteristics have been analyzed on the basis of standard thermionic emission (TE) theory and the characteristic parameters of the Schottky contacts (SCs) such as Schottky barrier height (SBH), ideality factor (n) and series resistance (R_s) have been determined from the I–V measurements. The experimental values of SBH and n for the device ranged from 1.01 eV and 1.27 (at 320 K) to 0.38 eV and 5.93 (at 120 K) for Ni/n-GaP diode, respectively. The interface states in the semiconductor bandgap and their relaxation time have been determined from the C–f characteristics. The interface state density N_ss has ranged from 2.08 × 10¹⁵ (eV^?1 m^?2) at 120 K to 2.7 × 10¹⁵ (eV^?1 m^?2) at 320 K. C_ss has increased with increasing temperature. The relaxation time has ranged from 4.7 × 10^?7 s at 120 K to 5.15 × 10^?7 s at 320 K.     

Electrical properties of silver Schottky contacts to ZnO thin films

ZnO thin films are deposited on Al/Si substrates by the pulsed laser deposition （PLD） method. The XRD and SEM images of films are examined. Highly c-axis oriented ZnO thin films which have uniform compact surface morphology are fabricated. The size of surface grains is about 30 nm. The Schottky barrier ultraviolet detectors with silver Schottky contacts are made on ZnO thin films. The current-voltage characteristics are measured. The ideality contact factor between Ag and ZnO film is 1.22, while the barrier height is 0.908 e V. After annealing at 600 ℃ for 2h, the ideality factor is 1.18 and the barrier height is 0.988 eV. With the illumination of 325 nm wavelength UV-light, the photocurrent-to-dark current ratios before and after annealing are 140.4 and 138.4 biased at 5 V, respectively. The photocurrents increase more than two orders of magnitude over the dark currents.     

Influence of the pre-treatment anneal on Co–germanide Schottky contacts

A thin cobalt layer is deposited by electron beam evaporation onto a germanium substrate after an in situ cleaning annealing at 400 or 700 °C. The effect of these pre-treatments on the Co/Ge Schottky barrier properties and on the germanide formation is investigated by using different techniques. A strong influence of the pre-treatment is observed. The pre-treatment at 700 °C removes the native oxide but enhances the diffusion of contaminants. After post-metal deposition annealing, the sample pre-treated at 700 °C shows a double layer structure due to interdiffusion, whereas some large isolated islands are present in the sample pre-treated at 400 °C.