Low resistance ohmic contacts to n- and p-InP

The contact properties of various metal combinations, deposited by vacuum evaporation on InP, were studied. Among these metal combinations, Au/Ge + Ni and Au/Zn proved to be most suitable. The former on n-InP (n = 8 × 10¹⁷/cm³) and the latter on p-InP (p = 9 × 10¹⁷/cm³) exhibited specific contact resistances as low as 1.2 × 10^?6 and 1.1 × 10^?4 Ωcm², respectively. The specific contact resistances were analyzed using a four-point method which also accounts for the spreading resistance. Furthermore, the resistances of metal contacts to InP were calculated as a function of doping concentration and were compared with the experimental results. The described contacting technique was successfully applied to the preparation of quaternary lasers.     

Ohmic contact to p-type GaP

A study was made of the contact properties of a AuBe eutectic and a AuBeNi alloy on p-type GaP. The specific contact resistance varied from 1 × 10^?3 to 7.5 × 10^?5 ω cm² in the acceptor concentration range of 9 × 10¹⁶ to 2 × 10¹⁸ cm^?3. In the sintering temperature range resulting in good ohmic behaviour and low contact resistance the AuBe contacts do not form drops, whereas the AuBeNi contacts became molten; even after melting they wetted the surface of the GaP well. At the temperature of sintering Be diffuses from the contact into the GaP. The diffusion of Be gives rise to an additional acceptor concentration of 5 × 10¹⁸ to 1 × 10¹⁹ cm^?3 beneath the contact surface. Taking this into consideration the concentration-specific contact resistance relationship appears to support a field emission FE conduction mechanism.     

Enhanced Performance of GaN-Based Green Light-Emitting Diodes with Gallium-Doped ZnO Transparent Conducting Oxide

Ga-doped ZnO (GZO) transparent conducting oxide was grown by oxygen plasma-enhanced pulsed laser deposition. GZO grown in the presence of oxygen radicals had resistivity of 1 × 10^?3 Ω cm and average visible (500–700 nm) transmittance of 92.5%. A low specific contact resistance of 6.5 × 10^?4 Ω cm² of GZO on p-GaN was achieved by excimer laser annealing (ELA) treatment of p-GaN before GZO electrode deposition. The ELA-treated light emitting diode (LED) fabricated with the GZO electrode as a current-spreading layer resulted in light-output power enhanced by 56.2% at 100 mA compared with that fabricated with a conventional Ni/Au metal electrode. The high-light output and low degradation of light-output power were attributed to the decrease in contact resistance between the p-GaN layer and the GZO electrode and uniform current spreading over the p-GaN layer. In addition, low contact resistance results in a decrease of self-heat generation during current drive.     

Microstructure and Electrical Properties of Low Temperature Processed Ohmic Contacts to p‐Type GaN

With Ni/Au and Pd/Au metal schemes and low temperature processing, we formed low resistance stable Ohmic contacts to p‐type GaN. Our investigation was preceded by conventional cleaning, followed by treatment in boiling HNO₃:HCl (1:3). Metallization was by thermally evaporating 30 nm Ni/15 nm Au or 25 nm Pd/15 nm Au. After heat treatment in O₂ + N₂ at various temperatures, the contacts were subsequently cooled in liquid nitrogen. Cryogenic cooling following heat treatment at 600 ·C decreased the specific contact resistance from 9.84·10^?4 Ωcm² to 2.65·10^?4 Ωcm² for the Ni/Au contacts, while this increased it from 1.80·10^?4 Ωcm² to 3.34·10^?4 Ωcm² for the Pd/Au contacts. The Ni/Au contacts showed slightly higher specific contact resistance than the Pd/Au contacts, although they were more stable than the Pd contacts. X‐ray photoelectron spectroscopy depth profiling showed the Ni contacts to be NiO followed by Au at the interface for the Ni/Au contacts, whereas the Pd/Au contacts exhibited a Pd:Au solid solution. The contacts quenched in liquid nitrogen following sintering were much more uniform under atomic force microscopy examination and gave a 3 times lower contact resistance with the Ni/Au design. Current‐voltage‐temperature analysis revealed that conduction was predominantly by thermionic field emission.     

Influence of Annealing on Electrical Properties of an Organic Thin Layer-Based n-Type InP Schottky Barrier Diode

The electrical properties of a fabricated Au/polymethylmethacrylate (PMMA)/n-InP Schottky barrier diode have been analyzed for different annealing temperatures using current–voltage (I–V) and capacitance–voltage (C–V) techniques. It is observed that the Au/PMMA/n-InP structure shows excellent rectifying behavior. The extracted barrier height and ideality factor of the as-deposited Au/PMMA/n-InP Schottky contact are 0.68 eV (J–V)/0.82 eV (C–V) and 1.57, respectively. However, the barrier height (BH) of the Au/PMMA/n-InP Schottky contact increases to 0.78 eV (J–V)/0.99 eV (C–V) when the contact is annealed at 150°C for 1 min in nitrogen atmosphere. Upon annealing at 200°C, the BH value decreases to 0.72 eV (J–V)/0.90 eV (C–V) and the ideality factor increases to 1.48. The PMMA layer increases the effective barrier height of the structure by creating a physical barrier between the Au metal and the n-InP. Cheung’s functions are also used to calculate the series resistance of the Au/PMMA/n-InP structure. The interface state density (N _ss) is found to be 6.380 × 10¹² cm^?2 eV^?1 and 1.916 × 10¹² cm^?2 eV^?1 for the as-deposited and 150°C-annealed Au/PMMA/n-InP Schottky contacts, respectively. These results indicate that the interface state density and series resistance have a significant effect on the electrical characteristics of Au/PMMA/n-InP Schottky barrier devices. Finally, it is noted that the diode parameters change with increasing annealing temperature.     

The ohmic contact formation mechanism of Au/Pt/Pd ohmic contact to p-ZnTe

The ohmic contact formation mechanism and the role of Pt layer of Au(500Å) Pt(500Å)/Pd(100Å) ohmic contact to p-ZnTe were investigated. The specific contact resistance of Au/Pt/Pd contact depended strongly on the annealing temperature. As the annealing temperature increased, the specific contact resistance decreased and reached a minimum value of 6×10^?6 Θcm² at 200°C. From the Hall measurement, the hole concentration increased with the annealing temperature and reached a maximum value of 2.3×10¹⁹ cm^?3 at 300°C. The Schottky barrier height decreased with the increase of annealing temperature and reached a minimum value of 0.34 eV at 200°C and it was due to the interfacial reaction of Pd and ZnTe. Therefore, the decrease of contact resistance was due to the increase of doping concentration as well as the decrease of Schottky barrier height by the interfacial reaction of Pd ZnTe. The specific contact resistances of Au Pd, Au/Pt/Pd and Au/Mo/Pd as a function of annealing time was investigated to clarify the role of Pt layer.     

Doping dependence of the properties of GaAsAlxGa1-xAs single heterostructure luminescent diodes

GaAs luminescent diodes with single heterostructure have been fabricated. The dependence of light power output and time behaviour on doping of the active layer has been investigated. An optimal doping of p = 2.5 × 10¹⁸ cm^?3 with respect to light power and 8 × 10¹⁸ cm^?3 with respect to the power bandwidth product has been achieved.     

Effect of annealing on electrical properties of radio-frequency-sputtered ZnO films

We report on the electrical properties of ZnO films and devices grown on different substrates by radio-frequency magnetron sputtering. The films grown on c-plane sapphire were annealed in the range 800–1,000°C. The electron concentration increased with annealing temperature reaching 1.4×10¹⁹ cm^?3 for 1,000°C. Mobility also increased, however, reaching its maximum value 64.4 cm²/V · sec for 950°C anneal. High-performance Schottky diodes were fabricated on ZnO films grown on n-type 6H-SiC by depositing Au/Ni(300/300 Å). After annealing at 900°C, the leakage current (at ?5 V reverse bias) decreased from 2.2 × 10^?7 A to ～5.0 × 10^?8 A after annealing at 900°C, the forward current increased by a factor of 2, and the ideality factor decreased from 1.5 to 1.03. The ZnO films were also grown on p-type 6H-SiC, and n-ZnO/p-SiC heterostructure diodes were fabricated. The p-n diode performance increased dramatically after annealing at 950°C. The leakage current decreased from 2.0×10^?4 A to 3.0×10^?7 A at ?10 V reverse bias, and the forward current increased slightly from 2.7 mA to 3.9 mA at 7 V forward bias; the ideality factor of the annealed diode was estimated as 2.2, while that for the as-grown sample was considerably higher.     

Electrical properties of InAs irradiated with protons

The results of studying the electrical properties of InAs irradiated with 5-MeV H⁺ ions at a dose of 2×10¹⁶ cm^?2 are reported. It is shown that, independently of the doping level and the conductivity type of the as-grown InAs, InAs always has the n⁺-type conductivity after irradiation (n≈(2–3)×10¹⁸ cm^?3). The phenomenon of pinning of the Fermi level in the irradiated material is discussed. The thermal stability of radiation damage in InAs subjected to postirradiation annealing at temperatures as high as 800°C was studied.     

Current-voltage characteristics of electric contacts on p-type ZnSe

The current-voltage characteristics of electric contacts made of different materials on p-type ZnSe that form Schottky barriers from 0.3 to 1.2 eV are studied theoretically using the formula $$J = \frac{{A^* T}}{k}\int_0^\infty {T(E)[F(E) - F(E - eV)]dE,} $$ where T(E) is the energy-dependent quantum tunneling probability and F(E) is the Fermi distribution function. The contribution to the total current of both the thermionic emission and the tunneling are therefore included. The net doping concentrations under study range from 1.0×10¹⁷ cm^?3 to 1.0×10¹⁹ cm^?3. The reverse bias voltage across the barrier at a current density of 200 A/cm² is used to assess whether the barrier is reduced to an ohmic contact. A barrier of 0.3 eV is already an ohmic contact at doping concentration p=1.0×10¹⁷ cm^?3, while a barrier of 1.2 eV still behaves like a diode event at p=1.0×10¹⁹ cm^?3.     

Thermophotovoltaic cells based on In0.53Ga0.47As/InP heterostructures

Reflection of infrared radiation from n-InP substrates with a rear MgF₂/Au mirror is investigated in the wavelength range 1000–2200 nm. It is found that the reflectance weakly depends on substrate thickness and free-carrier concentration in the (0.1–6) × 10¹⁸ cm^?3 range. Thermophotovoltaic cells based on the InP/In_0.53Ga_0.47As lattice-matched heterostructure of p-n and n-p are fabricated by liquid-phase epitaxy and Zn and P diffusion from a gas phase. The characteristics of p-n and n-p thermophotovoltaic cells with an identical configuration of the contacts of 1 cm² area are determined. These characteristics are the open-circuit voltage U _oc = 0.465 V, the filling factor FF = 64% at the current density of 1 A/cm², and the reflectance R = 76–80% for wavelengths longer than 1.86 μm.     

Photoelectric and luminescence properties of GaSb-Based nanoheterostructures with a deep Al(As)Sb/InAsSb/Al(As)Sb quantum well grown by metalorganic vapor-phase epitaxy

The luminescence and photoelectric properties of heterostructures with a deep Al(As)Sb/InAsSb/Al(As)Sb quantum well grown on n-GaSb substrates by metalorganic vapor-phase epitaxy are investigated. Intense superlinear luminescence and increased optical power as a function of the pump current in the photon energy range of 0.6–0.8 eV are observed at temperatures of T = 77 and 300 K. The photoelectric, current-voltage, and capacitance characteristics of these heterostructures are studied in detail. The photosensitivity is examined with photodetectors operating in the photovoltaic mode in the spectral range of 0.9–2.0 μm. The sensitivity maximum at room temperature is observed at a wavelength of 1.55 μm. The quantum efficiency, detectivity, and response time of the photodetectors were estimated. The quantum efficiency and detectivity at the peak of the photosensitivity spectrum are as high as η = 0.6–0.7 and D _λmax ^* = (5–7) × 10¹⁰ cm Hz^1/2 W^?1, respectively. The photodiode response time determined as the rise time of the photoresponse pulse from 0.1 to the level 0.9 is 100–200 ps. The photodiode transmission bandwidth is 2–3 GHz. Photodetectors with a deep Al(As)Sb/InAsSb/Al(As)Sb quantum well grown on n-GaSb substrates are promising foruse in heterodyne detection systems and in information technologies.     

Ta/Au ohmic contacts to n-type ZnO

Ta/Au ohmic contacts are fabricated on n-type ZnO (∼1 × 10¹⁷ cm⁻³) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). After growth and metallization, the samples are annealed at 300°C and 500°C for 30 sec in nitrogen ambient. The specific contact resistance is measured to be 3.2×10⁻⁴ Ωcm² for the as-deposited samples. It reduces to 5.4×10⁻⁶ Ωcm² after annealing at 300°C for 30 sec without significant surface morphology degradation. When the sample is annealed at 500°C for 30 sec, the specific contact resistance increases to 3.3 × 10⁻⁵ Ωcm². The layer structures no longer exist due to strong Au and Ta in-diffusion and O out-diffusion. The contact surface becomes rough and textured.     

Calculation of avalanche breakdown voltage and depletion layer thickness in a P−N junction with a double error function doping profile

We have calculated the avalanche breakdown voltage and the extent of the depletion region for a pn-junction with a double error function doping profile which could be useful in the design of high voltage thyristors. The results are presented as a set of curves corresponding to various surface concentrations and depths of the two diffusions. The parameter range covered is as follows: surface concentrations: first diffusion 10¹⁸–10²⁰ cm^?3, and second diffusion 2 × 10¹⁵–10¹⁷ cm^?3; diffusion depths: first diffusion 60–100 μm, and second diffusion 40–160 μm; background doping density: 10¹³–10¹⁴ cm^?3.     

Ohmic contacts to Si-implanted InP

Ohmic contacts to Si-implanted, n⁺ layers on semi-insulating InP are investigated on the basis of the transmission line model. It is found that Au/Ni/AuGeNi/InP system shows a good ohmic behaviour with the specific contact resistance $\text{ρ}{}_{\mathrm{c}}\text{of}\text{2 × 10}{}^{\mathrm{?5}}\text{Ω}\text{cm}{}^2$ and the minimum contact resistance $\text{Z}{}_{\mathrm{c}}\text{of}\text{～ 2 × 10}{}^{\mathrm{?3}}\text{Ω}\text{cm}$ for a Si-dose higher than 2 × 10¹⁴ cm^?2 at 100 or 200 keV. The results indicate that, in the FET fabrication, at least 120 μm in length is necessary in order to obtain source and drain electrodes with the minimized resistance.     

Electronic properties of p-GaN(Mg) irradiated with reactor neutrons

The effect of irradiation with full-spectrum reactor neutrons and predominantly fast reactor neutrons (up to a fluence of 8 × 10¹⁸ cm^?2) on the electrical properties of epitaxial p-GaN(Mg) films at different initial doping levels (in the range of hole concentrations p = 10¹⁷–10¹⁹ cm^?3) is analyzed. It is found that neutron irradiation induces an increase in the resistivity of the initial material to 10¹⁰ Ω cm at 300 K. It is shown that, at high neutron fluences, the resistivity of the material decreases because of the hopping conduction of charge carriers over radiation defect states. The study of isochronous annealing at 100–1000°C reveals stages of donor-defect (100–300°C, 500–700°C, 750–850°C) and acceptor-defect (300–500°C, 650–800°C) annealing in the neutron-irradiated p-GaN(Mg) samples.     

Highly Emissive and Electrochemically Stable Thienylene Vinylene Oligomers and Copolymers: An Unusual Effect of Alkylsulfanyl Substituents

The synthesis, unexpected efficient photoluminescence, and reversible electrochemical p‐ and n‐doping of new conjugated thienylene vinylene materials functionalized with alkylsulfanyl substituents poly(trithienylene vinylene) (PTTV) and poly(dithienylvinyl‐co‐benzothiadiazole) (PDTVB) along with dithienylvinylene‐based oligomers is reported. The materials are studied by thermal and X‐ray diffraction analysis, optical spectroscopy, cyclic voltammetry, and spectroelectrochemistry. Organic field‐effect transistors (OFETs) are fabricated with PTTV and PDTVB. The polymers, prepared by Stille polycondensation, exhibit good thermal stability and a photoluminescent quantum yield in the range 34%–68%. Low bandgaps (1.5–1.8 eV), estimated by optical and electrochemical measurements along with high stability of both redox states, suggest that these structures are promising materials for photovoltaic applications. OFETs fabricated with PDTVB reveal a hole mobility of 7 × 10^?3 cm² V^?1 s^?1 with on/off ratio 10⁵, which are comparatively high values for completely amorphous polymer semiconductors.     

Calculations and measurements of contact resistance of semi-transparent Ni/Pd contacts to p-GaN

Calculations of specific contact resistance as a function of doping and barrier height were performed for p-type GaN. These calculations took into account two valence bands, each with different effective masses, and show that at low doping, the heavy hole band accounts for most of the conduction, whereas at heavier doping, the light hole band dominates conduction. These calculations also indicate the barrier height for typical contacts to p-GaN is between 0.75 eV and 1 eV. Specific contact resistance measurements were made for oxidized Ni/Au, Pd, and oxidized Ni/Pd ohmic contact metal schemes to p-GaN. The Ni/Pd contact had the lowest specific contact resistance, 6×10⁻⁴ Ω cm². Auger sputter depth profile analysis showed some Ni diffused away from the GaN surface to the contact surface with the bulk of the Pd located in between two areas of Ni. Both Ni and Pd interdiffused with the GaN at the semiconductor surface. The majority of the oxygen observed was with the Ni as NiO. Angle-resolved-x-ray photoelectron spectroscopy (AR-XPS) analyses showed the formation of predominantly NiO and PdO species, with higher Ni and Pd oxides at the contact surface.     

A study of the electrical characteristics of various metals on p-type GaN for ohmic contacts

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 × 10¹⁷, 1.5 × 10¹⁷ and 4.7 × 10¹⁷ cm⁻³ are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm². In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.     

Microstructural and electrical investigation of Ni/Au ohmic contact on p-type GaN

Electrical properties of Ni/Au ohmic contacts on p-type GaN were interpreted with the change of microstructure observed under transmission electron microscopy. The contact resistivity was decreased from 1.3×10⁻² to 6.1×10⁻⁴ Ωcm² after annealing at 600°C. The reduction is due to the dissolution of Ga atoms into Au−Ni solid solution formed during annealing, via the generation of Ga vacancies. Thus, net concentration of holes increased below the contact, resulting in the reduction of contact resistivity. At 800°C, N atoms decomposed; reacted with Ni, and forming cubic Ni₄N. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to the increase of contact resistivity to 3.8×10⁻² Ωcm².