Properties of the contact on ion cleaned n and p type silicon surfaces

Schottky contacts were produced by silver evaporation on Si(100) surfaces cleaned by ion sputtering and partial annealing. The samples work function were measured before and after metal deposition with the Kelvin method, in an experimental set-up which allowed a topografical study and direct comparison between n and p types. Clean surfaces with and without a residual layer of oxide was achieved and controlled by AES. It was found that the Fermi level of all the surfaces was pinned by donor states created by the bombardment and that there was no barrier on n type and an important surface barrier on p type. The diodes we obtained presented no barrier on n type and a rectifying contact on p type. So we deduced that the Schottky barrier is already fully formed before metal contact is achieved. Furthermore study of the electrical properties of the diodes had shown that the bombardment creates donor states responsible for the barrier and a perturbated layer with deep acceptor traps responsible for the current flow mechanism. A residual layer of oxide and a post annealing of the device did not noticeably change the Schottky barrier in the diodes achieved on p type but led to clearly differenciated performances for the diodes achieved on both p and n type substrate. So we therefore concluded that the characteristics of the deep acceptor traps of the superfacial layer are modified by the oxide and annealing, both of which on the other hand having no effect on the surface donor states.     

Barrier height control of Pd2 Si/Si schottky diodes using diffusion from doped Pd

The barrier height of metal-semiconductor contacts can be varied within wide limits by a suitable doping (Sb, Al) of the metal layer itself and application of a temperature treatment to the sandwich structure. As a result the doping elements are weakly diffused into the semiconductor surface. This leads to a change of the band bending and finally to a change of the barrier height. $\text{Pd}{}_2\text{Si}\text{n-}\text{Si}$ diodes with barrier heights q·φ_B between 0.5 and 0.8 eV were fabricated reproducibly by this method. The barrier height of undoped Pd₂Si/Si contacts equals 0.72 eV. The doping elements were introduced into the metal layer by partly covering the Pd-cathode of a DC-sputtering apparatus with Al or Sb, and subsequent sputtering of the composite cathode onto the silicon slices.The concentration of doping elements in the sputtered metal layer is given by the relation of the part of the cathode surface covered with the doping element to the whole cathode surface.     

Inhomogeneous Ni/Ge Schottky barriers due to variation in Fermi-level pinning

To achieve high performance Ge nMOSFETs it is necessary to reduce the metal/semiconductor Schottky barrier heights at the source and drain. Ni/Ge and NiGe/Ge Schottky barriers are fabricated by electrodeposition using n-type Ge substrates. Current (I)–voltage (V) and capacitance (C)–voltage (V) and low temperature I–V measurements are presented. A high-quality Schottky barrier with extremely low reverse leakage current is revealed. The results are shown to fit an inhomogeneous barrier model for thermionic emission over a Schottky barrier. A mean value of 0.57 eV and a standard deviation of 52 meV is obtained for the Schottky barrier height at room temperature. A likely explanation for the distribution of the Schottky barrier height is the spatial variation of the metal induced gap states at the Ge surface due to a variation in interfacial oxide thickness, which de-pins the Fermi level.     

Small-signal field effect in GaAs/InAs quantum-dot heterostructures

InAs quantum dots (QDs), incorporated into the space-charge region of an epitaxial n-GaAs film at different distances (5?C300 nm) from the surface, decrease the potential barrier for the electrons located in n-GaAs. For tunnel-thin coating layers this decrease is related to tunneling through QD energy levels. For thick layers this decrease is caused by negative charging of the QDlevels and defects located near QDs. The decrease in the barrier increases the efficiency of electron capture by surface states and shifts the frequency dispersion of mobility under the field effect, related to this capture, toward higher frequencies. When QDs are incorporated near the barrier??s base, they manifest themselves in the relaxation of the small-signal field effect. Some parameters of the QD levels are determined. Defect formation is revealed in the layers adjacent to QDs.     

Ionic contamination-induced degradation of low current hFE

Ionic contamination in the oxide is the most common cause of low current _FE degradation in both npn and pnp transistors. Practical methods of preventing this type of failure have been developed at Fairchild and elsewhere. This paper is intended to provide a detailed account of the important information related to this subject. After reviewing the mechanism involved in ion-induced degradation of h_FE for both pnp's and npn's, methods of eliminating and preventing ionic contamination are presented. It is shown that topside barrier protection alone does not provide complete device immunity to contamination. Possible lateral migration of ionic contamination dictates protection of oxide cut edges as well as the topside surface if complete immunity to ionic contamination is to be achieved. Finally, the effects of surface charge migration along the dielectric surface and the importance of overlapping the emitter-base junction with emitter or base metallization are demonstrated.     

Effect of non-uniformly doped surface layer on the barrier height of a Schottky contact

The presence of a highly doped p-type surface layer at the interface of a metal n-type semiconductor Schottky contact increases the barrier height. An analysis of such an increase in barrier height is given for different impurity distributions in the surface layer. It has been shown that of the three distributions considered, namely exponential, Gaussian and erfc, the increase of barrier height (Δφ_B) is a maximum for the Gaussian and minimum for the exponential distribution. It is found that the increase of barrier height results in degradation of the diode ideality factor n, which can be minimised when Δφ_B is obtained by changing the doping concentration of the surface layer and keeping the thickness to a minimum.     

Ag/Ai schottky contacts on n-InP

Schottky contacts have been fabricated onn- InP using a Ag/Al/InP configuration where the Ag and Al thicknesses are 1000 and 40-50Å, respectively. Diodes fabricated on InP substrates withn ≈ 7 x 10¹⁶ cm_-3, have effective barrier heights, Ø_beff, of 0.4 eV and reverse bias leakage current densities of >4 A/cm² atV _r = - 3V. Appropriate heat treating at temperatures between 400–500° C raises barrier heights by as much as 0.25 eV, resulting in Ø_beff ≈ 0.65 eV and reverse bias leakage current densities less than 0.002 A/cm². Diode characteristics are found to vary dramatically with different surface preparations prior to metallization; results of x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiling studies indicate that native oxides which are predominantly InPO₄ produce superior contacts and that aluminum first reacts with the native oxide and then migrates through the silver to the free metal surface which results in the dramatic improvements observed upon annealing.     

The effects of surface treatments on the Pt/n-GaAs Schottky interface

The effects of both chemical and laser annealing surface treatment on the electroplated Pt/n-GaAs interface have been studied. Surface studies were carried out using scanning Auger Electron Spectroscopy (SAES) while interface analysis used SAES along with noble ion sputtering. When the Pt/n-GaAs interface is used as a mixer diode, the mixer noise temperature is critically dependent on the chemical nature of that interface, as expressed through the ideality factor, η, and series resistance, R_s. Surface and interface information was obtained in the form of Ga-to-As ratios, (Ga/As), in range of 0.35 to ～2.5. Both chemical treatment and laser annealing showed that excess As at the interface leads to high η values indicating high mixer noise temperatures. Since high η values were accompanied by low barrier heights, φ_B, it was concluded that excess As at the interface lowers the barrier height and decreases the barrier width. This enhanced field emission, an emission mode not acceptable for low mixer noise temperatures. To confine the emission mechanism to thermionic emission requires stoichiometric or Ga rich interfaces and alkaline solutions of H₂O₂ with slow etch rates were found to be best suited for this purpose. Cw laser annealing proved to be a useful technique to control (Ga/As) on GaAs where high vapor pressure of As (uncapped anneal) and high diffusion coefficient of Ga through SiO₂ (capped anneal) were exploited. However, cw annealing was not found to be suitable for mixer diode processing even when Ga rich interfaces are desired, due to sample exposure to oxygen.     

Effect of a Nanodimensional Polyethylenimine Layer on Current–Voltage Characteristics of Hybrid Structures Based on Single-Crystal Silicon

In this paper the study of the tunneling current–voltage (I–V) characteristics of silicon surfaces with n- and p-type conductivity as a function of roughness in the presence of an adsorbed insulating layer of polyethylenimine (PEI) is presented. A new approach is proposed for analysis of the tunnel current–voltage characteristics of a metal–insulator–semiconductor structure based on the combination of two models (Simmons and Schottky). Such joint analysis demonstrates the effect of surface states and evaluates changes in the band bending and electron affinity after the deposition of the polyelectrolyte layer on the semiconductor surface. As a result, we are able to differentiate between the equilibrium tunnel barrier (qφ ₀) and the barrier height (qφ _B). It is shown that the deposition of the polymer leads to an increase of the equilibrium tunnel barrier by more than 250 meV, irrespective of the roughness and the conductivity type of the silicon substrate. The PEI deposition also leads to changes in the barrier height (less than 25 meV) that are smaller than the equilibrium tunnel barrier changes, indicating pinning of the Fermi level by the electron surface states that are energetically close to it. These surface states can trap charge carriers, a process leading to the formation of a depletion region and band bending on the semiconductor surface. Moreover, the change in the barrier height qΔφ _B depends on the conductivity type of the semiconductor, being positive for n-type and negative for p-type, in contrast to qΔφ ₀, which is positive for all substrates. The change is explained by capture of electrons preferably from the semiconductor space-charge region in the presence of a cationic polyelectrolyte, e.g., PEI.     

Studies of n-type GaAs material properties by anodic current behaviour

The carrier concentrations of n-type GaAs material have been measured by a technique using anodisation in the dark. The results are compared with a theoretical model and with other results based on Schottky barrier reverse breakdown. The technique has also been extended to include surfaces containing more than one type of material and n-type materials of different carrier concentrations. Additionally the technique has been used to make an assessment of the surface quality of the material.     

Electronic structures at organic heterojunctions of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB)-based organic light emitting diodes

Interface electronic structures of four-kinds of electron transporting or hole blocking organic materials (n-type) on a widely-used hole transporting material (p-type) in organic light emitting diodes (OLEDs), N,N′-bis (1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB), were investigated by means of photoelectron spectroscopy (PES). 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (OXD-7) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) overlayers show continuous energy shift of each overlayer-derived spectral components and the vacuum level proportional to the thickness. This energy shift is ascribed to a spontaneous building up of the electrostatic potential within the organic layers (giant surface potential; GSP). The energy shift of the overlayers induced by GSP as well as the interface vacuum level shift are adequately taken into account to determine the actual energy barrier heights of the hole conduction levels at the heterojunctions. 4,4′-bis(9-carbazolyl)biphenyl (CBP) and p-bis(triphenylsilyl) benzene (UGH2) induce band bending in the NPB film which presumably results from charge transfer (CT) to the n-type materials from NPB. Despite absence of a practical vacuum level shift and thickness dependent shift of the overlayer-derived electronic states, the CT-derived energy shift of NPB reduces the actual energy barrier height with respect to the nominal barrier height being simply interpreted from PES spectra of a thick overlayer of each material. The energy level diagrams across these ‘n-on-p’ organic–organic heterojunctions were finely determined based on the above interpretation of the PES spectra.     

Electrical and photovoltaic properties of a n-Si/chitosan/Ag photodiode

The electrical and photovoltaic properties of AuSb/n-Si/chitosan/Ag diode have been investigated. The ideality factor, barrier height and Richardson constant values of the diode at room temperature were found to be 1.91, 0.88 eV and 121.4 A/cm² K², respectively. The ideality factor of the diode is higher than unity, suggesting that the diode shows a non-ideal behaviour due to series resistance and barrier height inhomogeneities. The barrier height and ideality factor values of Ag/CHT/n-Si diode at room temperature are significantly larger than that of the conventional Ag/n-Si Schottky diode. The φ_B value obtained from C-V measurement is higher than that of φ_B value obtained from I-V measurement. The discrepancy between φ_B(C-V) and φ_B(I-V) barrier height values can be explained by Schottky barrier height inhomogeneities. AuSb/n-Si/chitosan/Ag diode indicates a photovoltaic behaviour with open circuit voltage (V_oc = 0.23 V) and short-circuit current density (J_sc = 0.10 μA/cm⁻²) values.     

The current-voltage-temperature characteristics of Al/NPB/p-Si contact

The current-voltage (I-V) characteristics of the Al/NPB/p-Si contact shows rectifying behavior with a potential barrier formed at the contact interface. The barrier height and ideality factor values of 0.65 eV and 1.33 are measured at the forward bias of the diode. The barrier height of the Al/NPB/p-Si diode at room temperature is larger that (∼0.58 eV) of conventional Al/p-Si diode. It reveals the NPB organic film control the carrier transport of the diode at the contact interface. The temperature effect on the I-V measurement is also performed to reveal the junction characteristics. The ideality factor of the Al/NPB/p-Si contact increases with decreasing temperature. And the barrier height decreases with decreasing temperature. The effects are due to the existence of the interface states and the inhomogeneous of the barrier at the junction.     

Molecular order,charge injection efficiency and the role of intramolecular polar bonds at organic/organic heterointerfaces

The effect of orientational changes in thin films of the non-crystalline hole transport material α-N-N′-diphenyl N-N″-bis(1 naphthayl)-1,1′-biphenyl-4,4′-diamine (α-NPD) on the energy level alignment and the film electronic structure has been investigated by angle-resolved ultraviolet photoelectron spectroscopy and related to the transport characteristics of hole-only devices. Changes in the anisotropic α-sexithiophene (α-6T) substrate from a “standing” to a “flat” molecular orientation induced by mechanical rubbing lead to molecular order and a preferential orientation in subsequently deposited thin α-NPD films and cause a reduction of the charge injection barrier at the organic/organic interface. The results show that the height of this barrier is determined by the surface dipoles of the individual organic films that relate to the orientation of intramolecular polar bonds at the interface.     

Effect of series resistance on the electrical characteristics and interface state energy distributions of Sn/p-Si (MS) Schottky diodes

In this study, electrical characteristics of the Sn/p-type Si (MS) Schottky diodes have been investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements at room temperature. The barrier height obtained from C-V measurement is higher than obtained from I-V measurement and this discrepancy can be explained by introducing a spatial distribution of barrier heights due to barrier height inhomogeneities, which are available at the nanostructure Sn/p-Si interface. A modified Norde’s function combined with conventional forward I-V method was used to extract the parameters including barrier height (Φ_b) and the series resistance (R_S). The barrier height and series resistance obtained from Norde’s function was compared with those from Cheung functions. In addition, the interface-state density (N_SS) as a function of energy distribution (E_SS-E_V) was extracted from the forward-bias I-V measurements by taking into account the bias dependence of the effective barrier height (Φ_b) and series resistance (R_S) for the Schottky diodes. While the interface-state density (N_SS) calculated without taking into account series resistance (R_S) has increased exponentially with bias from 4.235 × 10¹² cm⁻²eV⁻¹ in (E_SS - 0.62) eV to 2.371 × 10¹³ cm⁻²eV⁻¹ in (E_SS - 0.39) eV of p-Si, the N_SS obtained taking into account the series resistance has increased exponentially with bias from of 4.235 × 10¹² to 1.671 × 10¹³ cm⁻²eV⁻¹ in the same interval. This behaviour is attributed to the passivation of the p-doped Si surface with the presence of thin interfacial insulator layer between the metal and semiconductor.     

Characterization of current-voltage (I-V) and capacitance-voltage-frequency (C-V-f) features of Al/SiO2/p-Si (MIS) Schottky diodes

The current-voltage (I-V) characteristics of metal-insulator-semiconductor Al/SiO₂/p-Si (MIS) Schottky diodes were measured at room temperature (300 K). In addition, capacitance-voltage-frequency (C-V-f) characteristics are investigated by considering the interface states (N_ss) at frequency range 100 kHz to 1 MHz. The MIS Schottky diode having interfacial insulator layer thickness of 33 Å, calculated from the measurement of the insulator capacitance in the strong accumulation region. At each frequency, the measured capacitance decreases with increasing frequency due to a continuous distribution of the interface states. From the I-V characteristics of the MIS Schottky diode, ideality factor (n) and barrier height (Φ_b) values of 1.766 and 0.786 eV, respectively, were obtained from a forward bias I-V plot. In addition, the interface states distribution profile as a function of (E_ss − E_v) was extracted from the forward bias I-V measurements by taking into account the bias dependence of the effective barrier height (Φ_e) for the Schottky diode. The diode shows non-ideal I-V behaviour with ideality factor greater than unity. This behaviour is attributed to the interfacial insulator layer, the interface states and barrier inhomogeneity of the device. As expected, the C-V curves gave a barrier height value higher than those obtained from I-V measurements. This discrepancy is due to the different nature of the I-V and C-V measurement techniques.     

Effects of temperature on series resistance determination of electrodeposited Cr/n-Si/Au-Sb Schottky structures

Temperature dependences of the series resistance in the Cr/n-Si/Au-Sb Schottky structures prepared by electrodeposition method have been studied using current-voltage (I-V) characteristics in the 80-320 K temperature range by steps of 20 K. However, the values of series resistance obtained from Cheung functions were compared with each other, and it was seen that there is a good agreement between the values of the series resistance. A modified Norde’s function combined with conventional forward I-V method was used to extract the parameters including barrier height and the series resistance. The barrier height and series resistance obtained from Norde’s function were compared with those from Cheung functions. The values of barrier height and series resistance have very different especially towards to the lower temperatures. This is attributed to non-ideal I-V characteristics of the Cr/n-Si/Au-Sb Schottky structure and non-pure thermionic emission theory due to the low temperature effects.     

Hafnium-ntype silicon Schottky barriers

This paper describes the electrical properties of hafnium-/n-type/silicon contacts. These contacts were found to be Schottky barriers with a low barrier height. Polished and chemically cleaned 〈111〉 silicon wafers with a donor concentration N_d = 7 × 10²² m^?3 were used to fabricate experimental Schottky barrier structures. For the Schottky barrier height φ_bn and the ideality factor n values were found of 0.47 V and 1.07–1.11, respectively. It is concluded that due to their low forward voltage drop and good rectifying properties, Hf-nSi contacts can be applied in microwave Schottky barrier diodes.     

A computer-aided simulation model for the I–V characteristic of M-n-p silicon Schottky-barrier diodes produced by use of low-energy arsenic-ion implantation

Current-transport properties of Al-n-p silicon Schottky-barrier diodes have been studied both experimentally and theoretically. An analytical model for the I-V characteristic of a metal-n-p Schottky barrier diode has been developed by using an interfacial layer-thermionic-diffusion model. Assuming a Gaussian distribution for the implanted profile, the barrier-height enhancement and ideality factor have been derived analytically. Using low energy (25 KeV) arsenic implantation with the dose ranged form 8 × 10¹⁰/cm² to 10¹²/cm², Al-n-p silicon Schottky barrier diodes have been fabricated and characterized. Comparisons between the experimental measurements and the results of computer simulations have been performed and satisfactory agreements between these comparisons have been obtained. The reverse I–V characteristics of the fabricated Al-n-p silicon Schottky barrier diodes can also be well simulated by the developed model.     

Barrier characteristics of gold Schottky contacts on moderately doped n-InP based on temperature dependent I-V and C-V measurements

The temperature dependences of current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the gold Schottky contacts on moderately doped n-InP (Au/MD n-InP) Schottky barrier diodes (SBDs) have been systematically investigated in the temperature range of 60-300 K. The main diode parameters, ideality factor (n) and zero-bias barrier height (apparent barrier height) were found to be strongly temperature dependent and while the decreases, the n and the increase with decreasing temperature. According to Thermionic Emission (TE) theory, the slope of the conventional Richardson plot [In(J₀/T²) vs. 1000/T] should give the barrier height. However, the experimental data obtained do not correlate well with a straight line below 160 K. This behaviour has been interpreted on the basis of standard TE theory and the assumption of a Gaussian distribution of the barrier heights due to barrier inhomogeneities that persist at the metal-semiconductor interface. The linearity of the apparent barrier height vs. 1/(2kT) plot that yields a mean barrier height of 0.526 eV and a standard deviation (σ_s0) of 0.06 eV, was interpreted as an evidence to apply the Gaussian distribution of the barrier height. Furthermore, modified Richardson plot [ vs. 1/T] has a good linearity over the investigated temperature range and gives the and the Richardson constant (A^∗) values as 0.532 eV and 15.90 AK⁻²cm⁻², respectively. The mean barrier heights obtained from both plots are appropriate with each other and the value of A^∗ obtained from the modified Richardson plot is close to the theoretical value of 9.4 AK⁻²cm⁻² for n-InP. From the C-V characteristics, measured at 1 MHz, the capacitance was determined to increase with increasing temperature. C-V measurements have resulted in higher barrier heights than those obtained from I-V measurements. The discrepancy between Schottky barrier heights(SBHs) obtained from I-V and C-V measurements was also interpreted. As a result, it can be concluded that the temperature dependent characteristic parameters for Au/MD n-InP SBDs can be successfully explained on the basis of TE mechanism with Gaussian distribution of the barrier heights.