Au and Ag electrical contacts to p-ZnSe

The reverse bias break-down voltage and resistance of Au and Ag contacts sputter deposited on nitrogen doped p-type ZnSe, having free hole concentrations in the low to mid 10¹⁷ cm⁻³ range, have been studied. Samples were heat treated over the range of 150—400°C for times of 15—45 min. The minimum break-down voltage for the Au contacts (3.0 V) was found to occur following heat treatments at 350—400°C for 30 min and for Ag contacts (2.3 V) following heat treatments at 150°C for 45 min. Secondary ion mass spectrometry and Auger electron spectroscopy were used to identify changes in the contacts induced by heat treatments. No evidence was found for the formation of new interfacial compounds, but Au diffused into the ZnSe at T >350°C. The data suggest that conduction through the Au/ZnSe contacts was dominated by avalanche breakdown assisted by deep acceptor levels formed by the diffusion of Au into the ZnSe. The results from the Ag contacts suggest that interfacial O forms a highly doped region in the ZnSe leading to conduction dominated by field emission currents.     

Current transport mechanisms and deep level transient spectroscopy of Au/n-Si Schottky barrier diodes

The temperature-dependent electrical characteristics of the Au/n-Si Schottky diodes have been studied in the temperature range of 40-300 K. Current density-voltage (J-V) characteristics of these diodes have been analyzed on the basis of thermionic emission theory with Gaussian distribution model of barrier height. The basic diode parameters such as rectification ratio, ideality factor and barrier height were extracted. Under a reverse bias, the conduction process at low voltage is determined by Schottky emission over a potential barrier but at higher voltage the Poole Frenkel effect is observed. The capacitance-voltage (C-V) features of the Au/n-Si Schottky diodes were characterized in the high frequency of 1 MHz. The barrier heights values obtained from the J-V and C-V characteristics have been compared. It has been seen that the barrier height value obtained from the C-V measurements is higher than that obtained from the J-V measurements at various temperatures. Possible explanations for this discrepancy are presented. Deep level transient spectroscopy (DLTS) has been used to investigate deep levels in Au/n-Si. Three electron trap centers, having different emission rates and activation energies, have been observed. It is argued that the origin of these defects is of intrinsic nature. A correlation between C-V and DLTS measurements is investigated.     

Annealing temperature effect on electrical and structural properties of Cu/Au Schottky contacts to n-type GaN

Schottky contacts were fabricated on n-type GaN using a Cu/Au metallization scheme, and the electrical and structural properties have been investigated as a function of annealing temperature by current-voltage (I-V), capacitance-voltage (C-V), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) measurements. The extracted Schottky barrier height of the as-deposited contact was found to be 0.69 eV (I-V) and 0.77 eV (C-V), respectively. However, the Schottky barrier height of the Cu/Au contact slightly increases to 0.77 eV (I-V) and 1.18 eV (C-V) when the contact was annealed at 300 °C for 1 min. It is shown that the Schottky barrier height decreases to 0.73 eV (I-V) and 0.99 eV (C-V), 0.56 eV (I-V) and 0.87 eV (C-V) after annealing at 400 °C and 500 °C for 1 min in N₂ atmosphere. Norde method was also used to extract the barrier height of Cu/Au contacts and the values are 0.69 eV for the as-deposited, 0.76 eV at 300 °C, 0.71 eV at 400 °C and 0.56 eV at 500 °C which are in good agreement with those obtained by the I-V method. Based on Auger electron spectroscopy and X-ray diffraction results, the formation of nitride phases at the Cu/Au/n-GaN interface could be the reason for the degradation of Schottky barrier height upon annealing at 500 °C.     

Electrical characteristics of high performance Au/n-GaN schottky diodes

Au/n-GaN Schottky diodes with the Au electrode deposited at low temperature (LT=77K) have been studied. In comparison, the same chip of GaN epitaxial layer was also used for room temperature Schottky diodes. The low temperature Schottky diodes exhibit excellent performance. Leakage current density as low as 2.55×10⁻¹¹ A·cm⁻² at −2.5 V was obtained in the LT Schottky diodes. The linear region in the current-voltage curve at forward bias extends more than eight orders in current magnitude. Current-voltage-temperature measurements were carried out to study the characteristics of the LT Schottky diodes. A typical barrier height of about 1.32 eV for the LT diode, which is the highest value ever reported, was obtained. The obvious enhancement in electrical performance makes the LT processing a very promising technique for GaN device application although the detailed mechanisms for the LT Au/n-GaN Schottky diodes are still under investigation.     

Schottky barrier height studies of Au/4H-SiC(0001) using photoemission and synchrotron radiation

The Schottky barrier height (SBH) of Au on 4H-SiC(0001) has been studied using photoemission and synchrotron radiation. The Au was deposited in-situ on clean and well-ordered √3×√3 R30° reconstructed SiC surfaces prepared by in situ heating at ∼950°C. The SBH was determined from the shift observed in the Si 2p core level, in addition to the initial band bending determined for the clean surface. The results were compared with values obtained by electrical, capacitance-voltage (C-V), and current-voltage (I-V) characterization methods. A favorable comparison between the three independent, SBH determination methods was found.     

Enhancement of the Schottky Barrier Height using a Nitrogen-Rich Tungsten Nitride Thin Film for the Schottky Contacts on AlGaN/GaN Heterostructures

Tungsten, stoichiometric W₂N, and nitrogen-rich W₂N films were used as Schottky contacts on AlGaN/GaN heterostructures. The nitrogen content in the film was controlled by varying the nitrogen-to-argon gas flow ratio during the reactive sputter deposition. The diode with the nitrogen-rich film exhibited a higher Schottky barrier height and the leakage current was comparable to that of the Ni/Au Schottky contact. Analysis suggested that this was due to the increase of the tungsten nitride work function as the result of higher nitrogen incorporation. Furthermore, after 600°C thermal annealing, the diode was stable and showed no change in the leakage current.     

Electrical and interfacial properties of Au/P3HT:PCBM/n-Si Schottky barrier diodes at room temperature

In this study, a gold/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/n-type silicon (Au/P3HT:PCBM/n-Si) metal-polymer-semiconductor (MPS) Schottky barrier diode (SBD) was fabricated. To accomplish this, a spin-coating system and a thermal evaporation were used for preparation of a P3HT/PCBM layer system and for deposition of metal contacts, respectively. The forward- and reverse-bias current–voltage (I–V) characteristics of the MPS SBD at room temperature were studied to investigate its main electrical parameters such as ideality factor (n), barrier height (Φ_B), series resistance (R_s), shunt resistance (R_sh), and density of interface states (N_ss). The I–V characteristics have nonlinear behavior due to the effect of R_s, resulting in an n value (3.09) larger than unity. Additionally, it was found that n, Φ_B, R_s, R_sh, and N_ss have strong correlation with the applied bias. All results suggest that the P3HT/PCBM interfacial organic layer affects the Au/P3HT:PCBM/n-Si MPS SBD, and that R_s and N_ss are the main electrical parameters that affect the Au/P3HT:PCBM/n-Si MPS SBD. Furthermore, a lower N_ss compared with that of other types of MPS SBDs in the literature was achieved by using the P3HT/PCBM layer. This lowering shows that high-quality electronic and optoelectronic devices may be fabricated by using the Au/P3HT:PCBM/n-Si MPS SBD.     

Thermal reliability and performances of InGaP schottky contact with Cu/Au and Au/Cu-MSM photodetectors

We report the Schottky performance and thermal reliability of a wide bandgap InGaP layer in contact with a Cu/Au metallic system. An effective Schottky barrier height of 0.97 eV and an ideality factor of 1.21 can be achieved. The thermal reliability of the resultant Schottky barrier diodes was analyzed using Auger electron spectroscopy and atomic force microscopy. The thermal reliability could be main tained up to 450°C. The failure mechanism was attributable to the decomposition of the InGaP layer and the interdiffusion of the chemical elements at higher temperature. Insensitive photoresponsivity with the in cident optical power was found for the resultant Au/Cu-metal-semiconductor-metalphotodetectors (MSM-PDs). According to the measured temporal response of the Au/Cu-MSM-PDs, the operation frequency could be above 10 GHz.     

The determination of the interface-state density distribution from the capacitance-frequency measurements in Au/n-Si schottky barrier diodes

The Au/n-Si Schottky barrier diodes (SBDs) with 200-μm (sample D200) and 400-μm (sample D400) bulk thicknesses have been fabricated. The ideality factor and the barrier height have been calculated from the forward-bias current-voltage (I-V) characteristics of D200 and D400 SBDs. The energy distribution of the interface states and relaxation time are found from the capacitance-frequency (C-f) characteristics. The density of interface state and relaxation times have a (nearly constant) slow exponential rise with bias in the range of E_c −0.77 and E_c −0.47 eV from the midgap toward the bottom of the conductance band. Furthermore, the energy distribution of the interface states obtained from C-f characteristics has been compared with that obtained from the forward-bias I-V characteristics.     

Thermally stable Nb and Nb/Au ohmic contacts to p-GaN

We have investigated Nb single and Nb/Au metallization schemes for the formation of thermally stable ohmic contacts to p-GaN. It is shown that the asdeposited Nb and Nb/Au contacts exhibit rectifying behavior. However, both the contacts produce ohmic characteristics when annealed at 850°C. Measurements show that the 850°C Nb/Au and Nb contacts yield a specific contact resistance of 1.9×10⁻⁸ and 2×10⁻² ωcm², respectively. Schottky barrier heights are found to decrease with increasing annealing temperature. A comparison of the XRD and electrical results shows that the formation of gallide phases such as Ga-Nb and Ga-Au compounds, play a role in forming ohmic contacts. Atomic force microscopy results show that the surface morphology of the Nb contacts is fairly stable up to 850°C, while the Nb/Au contacts are slightly degraded upon annealing at 850°C.     

Au/Ge/Ni ohmic contacts to n-Type InP

The relationship between the electrical properties and microstructure for annealed Au/Ge/Ni contacts to n-type InP, with an initial doping level of 10¹⁷ cm^-3, have been studied. Metal layers were deposited by electron beam evaporation in the following sequence: 25 nm Ni, 50 nm Ge, and 40 nm Au. Annealing was done in a nitrogen atmosphere at 250-400‡C. The onset of ohmic behavior at 325‡C corresponded to the decomposition of a ternary Ni-In-P phase at the InP surface and the subsequent formation of Ni₂P plus Au₁₀In₃, producing a lower barrier height at the InP interface. This reaction was driven by the inward diffusion of Au and outward diffusion of In. Further annealing, up to 400‡C, resulted in a decrease in contact resistance, which corresponded to the formation of NiP and Au₉ln₄ from Ni₂P and Au₁₀In₃,respectively, with some Ge doping of InP also likely. A minimum contact resistance of 10^-7 Ω-cm² was achieved with a 10 s anneal at 400‡C.     

Gaussian distribution of inhomogeneous barrier height in tungsten/4H-SiC (000-1) Schottky diodes

The Gaussian distribution model have been used to analyze the anomalies observed in tungsten (W)/4H-SiC current voltage characteristics due to the barrier inhomogeneities that prevail at the metal-semiconductor interface. From the analysis of the forward I-V characteristics measured at elevated temperatures within the range of 303-448 K and by the assumption of a Gaussian distribution (GD) of barrier heights (BHs), a mean barrier height of 1.277 eV, a zero-bias standard deviation σ₀ = 0.092 V and a factor T₀ of 21.69 K have been obtained. Furthermore the modified Richardson plot according to the Gaussian distribution model resulted in a mean barrier height and a Richardson constant (A^∗) of 1.276 eV and 145 A/cm² K², respectively. The A^∗ value obtained from this plot is in very close agreement with the theoretical value of 146 A/cm² K² for n-type 4H-SiC. Therefore, it has been concluded that the temperature dependence of the forward I-V characteristics of the W/4H-SiC contacts can be successfully explained on the basis of a thermionic emission conduction mechanism with Gaussian distributed barriers. In addition, a comparison is made between the present results and those obtained previously assuming the pinch-off model.     

Current transport and formation of energy structures in narrow Au/n-GaAs Schottky diodes

The current transport and formations of potential barrier height in narrow Au/n-GaAs Schottky diodes (SD) with a contact surface in length of 200 μm, width of 1 and 4 μm have been investigated.It was determined that features of current transport are in good agreements with the thermionic emission theory in the forward bias as like high-quality conventional (flat) SD. Features of current transport in the reverse bias also is well described by thermionic emission theory, but it has specific features unlike I–V characteristics flat SD.Forward bias of narrow SD current–voltage (I–U) characteristics are represented by straight lines in semi-logarithmic scale in a wide range, nearly nine order of current up to 0.7 V with near unit ideality factor. In the beginning of the reverse voltage, the current practically was extremely low, by increasing in voltage the current jump in steps approximately for 3–4 order in voltage of 3–4 V, then current increases linear for 3–5 order in semi-logarithmic scale by increasing in voltage up to nearly 7 V.Numerical values of parameters such as the saturation currents, the operating barrier height, ideality factor, dimensionless factor are obtained. The correlations between ideality factor and dimensionless factor were meaningful.The energy diagrams of narrow SD have been drawn in absence and presence of forward and reverse voltage. It is found that electronic processes in narrow SD are well described by energy model of real narrow metal–semiconductor contacts. The additional electric field arising in near contact area of the semiconductor because of creating contact potential difference between contact surface and to it adjoining free surfaces of the metal and semiconductor.     

Schottky diode performance of an Au/Pd/GaAs device fabricated by deposition of monodisperse palladium nanoparticles over a p-type GaAs substrate

Au/Pd/p-GaAs Schottky diodes were fabricated by simple assembly of monodisperse Pd nanoparticles on a p-type GaAs semiconductor. Monodisperse 5-nm Pd nanoparticles were synthesized via reduction of palladium(II) acetylacetonate in oleylamine using a borane tert-butylamine complex. The Au/Pd/p-GaAs Schottky diodes provided a barrier height of 0.68 eV, which is higher than room-temperature values reported in the literature. A double distribution was observed for the barrier height for the Schottky diodes from I–V–T measurements. A decrease in temperature lowered the zero-bias barrier height and increased the ideality factor. These observations were ascribed to barrier height inhomogeneities at the interface that altered the barrier height distribution. Values of the series resistance obtained by the Norde method decreased with increasing temperature. Understanding the temperature dependence of the current–voltage characteristics of Au/Pd/p-GaAs devices might be helpful in improving the quality of Pd deposited on GaAs for future device technologies.     

Au/Ge/Pd ohmic contacts to n-GaAs with the Mo/Ti diffusion barrier

The influences of the As-outdiffusion and Au-indiffusion on the performances of the Au/Ge/Pd/n-GaAs ohmic metallization systems are clarified by investigating three different types of barrier metal structures Au/Ge/Pd/GaAs, Au/Ti/Ge/Pd/ GaAs, and Au/Mo/Ti/Ge/Pd/GaAs. The results indicate that As-outdiffusion leads to higher specific contact resistivity, whereas Au-indiffusion contributes to the turnaround of the contact resistivity at even higher annealing temperature. For Au/Mo/Ti/Ge/Pd/n-GaAs samples, they exhibit the smoothest surface and the lowest specific contact resistivity with the widest available annealing temperature range. Moreover, Auger electron spectroscopy depth profiles show that the existing Ti oxide for the Mo/Ti bilayer can very effectively retard Au-indiffusion, reflecting the onset of the turnaround point at much higher annealing temperature.     

Effect of annealing temperature on the electrical characteristics of Platinum/4H-SiC Schottky barrier diodes

Pt/4H-SiC Schottky barrier diodes have been fabricated to investigate the effect of annealing on the electrical characteristics of the fabricated devices. The parameters such as barrier height, ideality factor and donor concentration were deduced from the current–voltage (I–V) and the capacitance–voltage (C–V) measurements at room temperature. Diodes showed non-ideal behaviour like high value of ideality factor and lower value of barrier height. A barrier height of 1.82?eV was obtained from C–V measurements and it was 1.07?eV when obtained from the I–V measurements with ideality factor 1.71 for as-deposited diodes at room temperature. The diodes, therefore, were annealed in the temperature range from 25°C to 400°C to observe the effect of annealing temperature on these parameters. Schottky barrier height and ideality factors were found to be temperature-dependent. After rapid thermal annealing upto 400°C, a barrier height of 1.59?eV from C–V measurements and the value of 1.40?eV from I–V measurements with ideality factor 1.12 were obtained. Barrier heights deduced from C–V measurements were consistently larger than those obtained from I–V measurements. To come to terms with this discrepancy, we re-examined our results by including the effect of ideality factor in the expression of the barrier height. This inclusion of ideality factor results in reasonably good agreement between the values of barrier height deduced by the above two methods. We believe that these improvements in the electrical parameters result from the improvement in the quality of interfacial layer.     

Performances of novel Pd/Sn and Pd/Sn/Au ohmic metallizations to n-GaAs

Performance of novel Pd/Sn and Pd/Sn/Au Ohmic metallizations to n-GaAs have been investigated. Metallizations were deposited using a resistance heating evaporator and annealings were performed utilizing a conventional graphite strip annealer (cGSA). Metallization samples were characterized using scanning tunneling microscopy (STM), secondary ion mass spectrometry (SIMS) and current–voltage (I–V) measurements. Contact resistivities, ρ_c, of the metallizations were measured utilizing conventional transmission line model (cTLM) method. Novel Pd/Sn and Pd/Sn/Au Ohmic contacts exhibit better thermal stability compared to non-alloyed Pd/Ge metallization. In order to investigate the effectiveness of novel Pd/Sn and Pd/Sn/Au Ohmic metallizations in device applications, gallium arsenide metal-semiconductor field-effect transistors (GaAs MESFETs) have been fabricated. MESFETs fabricated with Pd/Sn/Au Ohmic contacts show a extrinsic transconductance, g_me, of more than 133 mS/mm for a gate length, L_G, of 2 μm.     

Dielectric properties and ac electrical conductivity studies of MIS type Schottky diodes at high temperatures

Dielectric properties and ac electrical conductivity of the Al/SiO₂/p-Si (MIS) Schottky diodes were studied in the frequency and temperature range of 10 kHz-1 MHz and 300-400 K, respectively. Experimental results show that the dielectric constant (ε′), dielectric loss (ε″), loss tangent (tan δ), ac electrical conductivity (σ_ac) and the electric modulus were found a strong function of frequency and temperature. The values of the ε′, ε″ and tan δ decrease with increasing frequencies due to the interface states capacitance and a decrease in conductance with increasing frequency. Also, these values increase with increasing temperature. The σ_ac is found to increase with increasing frequency and increasing temperature. The variation of conductivity as a function of temperature and frequency reveals non-adiabatic hopping of charge carriers between impurities localized states. In addition, the experimental dielectric data have been analyzed by considering electric modulus formalism.     

Effect of 6 MeV electron irradiation on electrical characteristics of the Au/n-Si/Al Schottky diode

Electron irradiation of the Au/n-Si/Al Schottky diode was performed by using 6 MeV electrons and 3 × 10¹² e⁻/cm² fluency. The current-voltage (I-V), capacitance-voltage (C-V) and capacitance-frequency (C-f) characteristics of the unirradiated and irradiated Schottky diode were analyzed. It was seen that the values of the barrier height, the series resistance, and the ideality factor increased after electron irradiation. However, there was a decrease in the leakage current with electron irradiation. The increase in the barrier height and in the series resistance values was attributed to the dopant deactivation in the near-interface region. The interface states, N_ss, have been decreased significantly after electron irradiation. This was attributed to the decrease in recombination centre and the existence of an interfacial layer. A decrease in the capacitance was observed after electron irradiation. This was attributed to decrease in the net ionized dopant concentration with electron irradiation.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.