Fabrication of n^+ Polysilicon Ohmic Contacts with a Heterojunction Structure to n-Type 4H-Silicon Carbide

Polysilicon ohmic contacts to n-type 4H-SiC have been fabricated. TLM （transfer length method） test patterns with polysilicon structure are formed on n-wells created by phosphorus ion （P^＋） implantation into a Si-faced p-type 4H-SiC epilayer. The polysilicon is deposited using low-pressure chemical vapor deposition （LPCVD） and doped by phosphorous ions implantation followed by diffusion to obtain a sheet resistance of 22Ω/□. The specific contact resistance pc of n^＋ polysilicon contact to n-type 4H-SiC as low as 3.82 × 10^-5Ω· cm^2 is achieved. The result for sheet resistance Rsh of the phosphorous ion implanted layers in SiC is about 4.9kΩ/□. The mechanisms for n^＋ polysilicon ohmic contact to n-type SiC are discussed.

Ni/Pd-based ohmic contacts to p-GaN through p-InGaN/p^(+)-GaN contacting layers

Specific contact resistance to p-GaN was measured for various structures of Ni/Pd-based metals and thin(20-30 nm thick)p-InGaN/p^(+)-GaN contacting layers.The effects of surface chemical treatment and annealing temperature were ex-amined.The optimal annealing temperature was determined to be 550°C,above which the sheet resistance of the samples de-graded considerably,suggesting that undesirable alloying had occurred.Pd-containing metal showed~35%lower com-pared to that of single Ni.Very thin(2-3.5 nm thick)p-InGaN contacting layers grown on 20-25 nm thick p^(+)-GaN layers exhib-ited one to two orders of magnitude smaller values of compared to that of p^(+)-GaN without p-InGaN.The current density de-pendence of,which is indicative of nonlinearity in current-voltage relation,was also examined.The lowest achieved through this study was 4.9×10^(-5)Ω·cm^(2)@J=3.4 kA/cm^(2).

Ti-Al based ohmic contacts to n-type 6H-SiC with ion implantation

The Ti-Al ohmic contact to n-type 6H-SiC has been fabricated. An array of TLM （transfer length method） test patterns with Au/Ti/A1/Ti/SiC structure is formed on N-wells created by P^＋ ion implantation into Si-faced p-type 6H-SiC epilayer. The specific contact resistance pc as low as 8.64×10-6Ω·cm^2 is achieved after annealing in N2 at 900℃ for 5min. The sheet resistance Rsh of the implanted layers is 975Ω. X-ray diffraction （XRD） analysis shows the formation of Ti3SiC2 at the metal/n-SiC interface after thermal annealing, which is responsible for the low resistance contact.

Effects of annealing temperature on the electrical property and microstructure of aluminum contact on n-type 3C–SiC

The 3C-SiC thin films used herein are grown on Si substrates by chemical vapor deposition. A1 contacts with differ- ent thickness values are deposited on the 3C-SiC/Si （100） structure by the magnetron sputtering method and are annealed at different temperatures. We focus on the effects of the annealing temperature on the ohmic contact properties and mi- crostructure of A1/3C-SiC structure. The electrical properties of A1 contacts to n-type 3C-SiC are characterized by the transmission line method. The crystal structures and chemical phases of A1 contacts are examined by X-ray diffraction, Raman spectra, and transmission electron microscopy, respectively. It is found that the A1 contacts exhibit ohmic contact behaviors when the annealing temperature is below 550 ℃, and they become Schottky contacts when the annealing tem- perature is above 650 ℃. A minimum specific contact resistance of 1.8 × 10-4 Ω cm2 is obtained when the A1 contact is annealed at 250 ℃.

Transparent conducting indium-tin-oxide(ITO) film as full front electrode in Ⅲ–Ⅴ compound solar cell

The application of transparent conducting indium-tin-oxide （ITO） film as full front electrode replacing the conven- tional bus-bar metal electrode in III-V compound GalnP solar cell was proposed. A high-quality, non-rectifying contact between ITO and 10 nm N＋-GaAs contact layer was formed, which is benefiting from a high carrier concentration of the terrilium-doped N＋-GaAs layer, up to 2×10^19 cm^-3. A good device performance of the GalnP solar cell with the ITO electrode was observed. This result indicates a great potential of transparent conducting films in the future fabrication of larger area flexible III-V solar cell.

Ohmic Contact Property of Ti/Al/Ni/Au on AlGaN/GaN Heterostructures for Application in Ultraviolet Detectors

Ohmic contacts of Ti/Al/Ni/Au multi-layer metal on A10.27 Ga0.73N/GaN heterostructures were fabricated. Specific contact resistivities were measured by the linear transmission line method （LTLM） and the circular transmission line method （CTLM） ,respectively. A minimum specific contact resistivity of 1.46× 10 ^-5Ω· cm^2z was obtained by evaporating a Ti（10nm）/Al（100nm）/Ni（40nm）/Au（100nm） multi-layer and annealing for 30s at 650℃ in ultra-high purity N2 ambient. A10.27 Ga0.73 N/GaN photoconductor ultraviolet （UV） photodetectors were prepared. The dark current-voltage （I-V） characteristics of the detectors were measured and the result shows that the I- V curve was linear. Experimental results indicate that good ohmic contact on the Al0.27 Ga0.73 N/GaN heterostructure is obtained and it can be applied in high- performance AlGaN/GaN UV photodetector fabrications.

High quality non-rectifying contact of ITO with both Ni and n-type GaAs

We report the specific contact resistance for ITO with both metal and a semiconductor. Good quality ITO was deposited by electron beam evaporation with the resistivity of 2.32×10^-4 Ω.cm and an averaged transmittance of 92.8% in the visible light region. The circular transmission line model （c-TLM） method was used to evaluate and compare the properties of the ITO/metal and ITO/semiconductor ohmic contacts. The lowest specific contact resistance of the ITO/Ni is 2.81×10^-6 Ω.cm^2, while that oflTO/n-GaAs is 7×10^-5Ω.cm^2. This is the best ohmic contact between ITO and n-GaAs ever reported. These results suggest that good quality ITO has strong potential to be used to realize highly efficient solar cells.

NiO removal of Ni/Au Ohmic contact to p-GaN after annealing

The Ni/Au contact was treated with oxalic acid after annealing in 02 ambient, and its 1-V characteristic showed the property of contact has been obviously improved. An Auger electron spectroscopy （AES） depth profile of the contact as-annealed showed that the top layer was highly resistive NiO, while an X-ray photo-electron spectroscopy （XPS） of oxalic acid treated samples indicated that the NiO has been removed effectively. A scanning electron microscope （SEM） was used to observe the surface morphology of the contacts, and it was found that the lacunaris surface right after annealing became quite smooth with lots of small Au exposed areas after oxalic acid treatment. When the test probe or the subsequently deposited Ti/Au was directly in contact with these small Au areas, they worked as low resistive current paths and thus decrease the specific contact resistance.

Ni(Pt) germanosilicide contacts formed on heavily boron doped Si_(1-x)Ge_x substrates for Schottky source/drain transistors

The electrical properties of Ni0.95Pt0.05-germanosilicide/Si1_xGex contacts on heavily doped p-type strained Sil-xGex layers as a function of composition and doping concentration for a given composition have been investigated. A four-terminal Kelvin-resistor structure has been fabricated by using the conventional com- plementary metal-oxide-semiconductor （CMOS） process to measure contact resistance. The results showed that the contact resistance of the Ni0.95Pt0.05-germanosilicide/Sil-xGex contacts slightly reduced with increasing the Ge fraction. The higher the doping concentration, the lower the contact resistivity. The contact resistance of the samples with doping concentration of 4×10^19 cm^-3 is nearly one order of magnitude lower than that of the sam- ples with doping concentration of 5 × 10^17 cm^-3. In addition, the influence of dopant segregation on the contact resistance for the lower doped samples is larger than that for the higher doped samples.

Erbium germanosilicide Ohmic contacts on Si_(1-x)Ge_x(x=0-0.3) substrates

We have studied erbium germanosilicide (ErSiGe) Ohmic contacts on n-type Si_(1-x)Ge_x substrates with differing Ge concentrations (0≤x≤0.3).Thin layers of Ti (20 nm)/Er (20 nm) were deposited on Si_(1-x)Ge_x substrates and then post-annealed at 600°C for 60 s to form a stable ErSiGe film.The structures of the ErSiGe films and ErSiGe/Si_(1-x)Ge_x interfaces were characterized by Transmission Electron Microscopy measurements (TEM).The TEM images showed that the thicknesses of ErSiGe films and the Si_(1-x)Ge_x substrates were about 60 and 50 nm,respectively.The ErSiGe/Si_(1-x)Ge_x structure had a smooth interface.Moreover,no agglomeration or Ge segregation was observed.The contact resistivity of the ErSiGe/Si_(1-x)Ge_x structures was measured by the specially designed four-terminal Kelvin structures.When the Ge concentration of Si_(1-x)Ge_x substrates increased from 10% to 30%,the specific contact resistivity (c) slightly decreased from 9.0×10 7 ·cm 2 to 7.4×10 7 ·cm 2,indicating that the Ge concentration is not the main effect on the c of the ErSiGe/Si_(1-x)Ge_x Ohmic contacts.

Optimization of ohmic contact for InP-based transferred electronic devices

The effect of the annealing time and annealing temperature on Ni/Ge/Au electrode contacts deposited on the n-type InP contact layer has been studied using a circular transmission line model. The minimum specific contact resistance of 3.210 7 cm2was achieved on the low-doped n-type InP contact layer with a 40 s anneal at 425 ℃. In order to improve the ohmic contact and reduce the difficulty in the fabrication of the high doped InP epi-layer, the doping concentration in the InP contact layer was chosen to be 51018cm 3in the fabrication of transferred electronic devices. Excellent differential negative resistance properties were obtained by an electron beam evaporating the Ni/Ge/Au/Ge/Ni/Au composite electrode on an InP epi-layer with a 60 s anneal at 380 ℃.