Low emitter resistance GaAs based HBT's without InGaAs caps

Low emitter resistance is demonstrated for AlGaAs/GaAs heterojunction bipolar transistors using Pd/Ge contacts on a GaAs contact layer. The contact resistivity to 2-10×10¹⁸ cm ^-3 n-type GaAs is 4-1×10^-7 Ω-cm² . These are comparable to contact resistivities obtained with non-alloyed contacts on InGaAs layers. The non-spiking Pd/Ge contact demonstrates thermal stability and area independent resistivity suitable for scaled devices. The substitution of Pd/Ge for AuGe/Ni GaAs emitter and collector contacts reduced by an order of magnitude the emitter-base offset voltage at high current densities and increased f_t by more than 15% with significantly improved uniformity for devices with 2 and 2.6 μm wide emitters having lengths two, four and six times the width     

Laser annealed Ta/Ge and Ni/Ge ohmic contacts to GaAs

Refractory metal ohmic contacts to n-type GaAs have been developed using epitaxial Ge films and pulsed laser annealing. Laser annealing was carried out with a 22 ns pulse from a Q-switched ruby laser operating in the TEM00mode. The specific contact resistivity of the contacts Ta/Ge and Ni/Ge on 2 × 10¹⁷cm^-3dopes GaAs exhibited sharp minima as a function of laser energy density at 1 × 10^-6Ω-cm²and 2 × 10^-6Ω-cm², respectively, which occurred near the melting point of the layered contacts. Auger electron sputter profiles revealed Ge migration into the GaAs surface after laser annealing at sufficient energy density to form ohmic contact. The contacts have applications to high temperature devices and to devices which experience high channel or contact temperatures, such as power FETs and TEDs.     

Improved uniformity of contact resistance in GaAs MESFET usingPd/Ge/Ti/Au ohmic contacts

Improved contact resistance uniformity, with a low resistance on high-low doped GaAs MESFET, was demonstrated using a Pd/Ge/Ti/Au ohmic contact. The lowest contact resistivity obtained was 2.8×10^-6 Ω-cm². The average value and standard deviation (ΔRc) of the contact resistance (Rc) were 0.73 and 0.07 Ω-mm, respectively, which were more uniform than those for AuGe/Ni contacts with an average Rc of 0.77 Ω-mm and ΔRc of 0.16 Ω-mm. The improved uniformity was attributed to the uniform penetration of the ohmic junction into the buried high-doped channel layer by solid-state reactions, resulting in the improved uniformity of device performance     

一种在砷化镓上形成欧姆接触的新型六层金属系统

研究了在砷化镓上欧姆接触形成机理,并提出了一种新型的欧姆接触六层金属系统(Ni/Ge/Au/Ge/Ni/Au).在n型GaAs样品上实验了在380～460℃合金温度和50～120 s合金时间下形成欧姆接触,在400℃、60 s下典型的欧姆接触值为2.1x10-7Ω·cm2,同时合金后表面形貌光滑、平整.     

Laser Annealed and Thermal Annealed Refractory Ohmic Contacts to GaAs

Ohmic contacts to n-type GaAs have been developed for high-temperature device applications up to 300°C. Refractory metallizations were used with epitaxial Ge layers to form the contacts TiW/Ge/GaAs, Ta/Ge/GaAs, Mo/Ge/GaAs, and Ni/Ge/GaAs. Contacts with high dose Si or Se ion implantation (1012 to 1014/cm2) of the Ge/GaAs interface were also investigated. The purpose of this work was to develop refractory ohmic contacts with low specific-contact resistance (~10-6 ?cm2 on 1 x 1017cm-3GaAs) which are free of imperfections, resulting in a uniform n+ doping layer. The contacts were fabricated on epitaxial GaAs layers (n = 2 x 1016 to 2 x 1017 cm-3) grown on n+ ( 2 x 1018 cm-3) or semi-insulating GaAs (at strates. Ohmic contact was formed by both thermal annealing ( at temperatures up to 700°C) and laser annealing (pulsed Ruby). Examination of the Ge/GaAs interface revealed Ge migration into GaAs to form an n+layer. Under optimum laser anneal conditions, the specific contact resistance was in the range 1-5 x 10-6 ?-cm2 (on 2 x 1017cm-3GaAs). Thermally annealed TiW/Ge had a contact resitivity of 1 x 10-6 ? cm2 on 1 x 1017 cm-3 GaAs under optimum anneal conditions. The contacts also showed improved thermal stability over conventional Ni/AuGe contacts at temperatures above 300°C.     

Thermally stable Ge/Ag/Ni Ohmic contact for InAlAs/InGaAs/InP HEMTs

Excellent annealed ohmic contacts based on Ge/Ag/Ni metallization have been realized in a temperature range between 385 and 500/spl deg/C, with a minimum contact resistance of 0.06 /spl Omega//spl middot/mm and a specific contact resistivity of 2.62 /spl times/10/sup -7/ /spl Omega//spl middot/cm/sup 2/ obtained at an annealing temperature of 425/spl deg/C for 60 s in a rapid thermal annealing (RTA) system. Thermal storage tests at temperatures of 215 and 250/spl deg/C in a nitrogen ambient showed that the Ge/Ag/Ni based ohmic contacts with an overlay of Ti/Pt/Au had far superior thermal stabilities than the conventional annealed AuGe/Ni ohmic contacts for InAlAs/InGaAs high electron mobility transistors (HEMTs). During the storage test at 215/spl deg/C, the ohmic contacts showed no degradation after 200 h. At 250/spl deg/C, the contact resistance value of the Ge/Ag/Ni ohmic contact increased only to a value of 0.1 /spl Omega//spl middot/mm over a 250-h period. Depletion-mode HEMTs (D-HEMTs) with a gate length of 0.2 /spl mu/m fabricated using Ge/Ag/Ni ohmic contacts with an overlay of Ti/Pt/Au demonstrated excellent dc and RF characteristics.     

Ohmic contact penetration and encroachment in GaAs/AlGaAs and GaAsFETs

Hypoeutectic AuGe/Ni ohmic contacts on GaAs and GaAs/AlGaAs were characterized by SEM, TEM, and energy dispersive X-ray analysis. The two main components of the contact were confirmed to be NiGeAs islands distributed in an Au-Ga alloy. The NiGeAs islands were larger and more evenly distributed on GaAs MESFETs than on GaAs/AlGaAs MODFETs. Vertical penetration was impeded but not halted by AlGaAs. Since the contacts did not melt during the alloying cycle, there was no lateral encroachment observed     

Evolution of surface morphology of alloyed AuGe/Ni/Au ohmic contacts to GaAs microwave FETs

During post-deposition alloying of AuGe/Ni/Au ohmic contacts to microwave transistors, there is interdiffusion of alloy materials and GaAs into each other. Outdiffusion from substrate greatly influences the surface roughness of the contacts as a function of alloying temperature. During our experiments, we have observed that the RMS roughness of the contact surface followed the trend of contact resistance with alloying temperature. We seek to explain this evolution of surface morphology using a model involving the phenomena of coalescence and outdiffusion occurring simultaneously.     

Molybdenum germanide ohmic contact to n-GaAs

GeMo refractory ohmic contacts for n-type GaAs with a specific contact resistivity as low as 10-6?cm2 have been obtained on 1018cm-3 epitaxial layers. This low resistivity was obtained by contact annealing under As overpressure. Contacts using As-doped Ge layers and annealed without As overpressure have also been realised; in this case the obtained resistivity was 5 × 10-6?cm2. The ohmic contact formation resulted from the creation of an n + layer by Ge overdoping and the formation of a molybedenum germanide stable phase.     

Cu3Ge ohmic contacts to n-type GaAs

We show that Cu-Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400°C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 20 to 40 at.%. A contact resistivity of (4-6) x 10^-7 Ω cm² is obtained on n-type GaAs with doping concentrations of～ 1 x 10¹⁷ cm^-3. The contact resistivity is affected only slightly by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. In addition, the contacts are electrically stable during annealing at 450°C after contact formation. Structure and properties of Cu-Ge contact layers having lower and higher Ge concentrations from the stoichiometric Cu₃Ge composition are compared. High-resolution transmission electron microscopy and x-ray diffractometry have been used to study the ordering in the ε₁-Cu₃Ge (average lattice parameters: a₀= 5.30Å, b₀= 4.20Å, c₀= 4.56Å) which is responsible for orthorhombic distortion of the parent hexagonal ζ-phase. The results suggest that the formation of theξ and ε-Cu₃Ge phases creates a highly doped n⁺-GaAs surface layer which leads to the low contact resistivity.     

Metallurgical and electrical properties of alloyed Ni/AuGe films on n-type GaAs

An experimental study of the alloying characteristics of a composite thin-film structure which is often used as an ohmic contact to GaAs is presented. A AuGe layer of eutectic composition covered by a thin-film of Ni and deposited on n-type epitaxial GaAs is investigated in order to better understand the relationship between the alloying behavior and the electrical properties of the contact. The barrier energy ?_Bn and the specific contact resistance of the Ni/AuGe/GaAs system is measured for a wide range of alloy temperatures and times. The metallurgical properties of the Ni/AuGe/GaAs system are obtained with Auger electron spectroscopy and scanning electron microscopy. Auger spectroscopy combined with in situ sputter etching is used to determine depth-composition profiles for all constituents of both as-deposited and alloyed Ni/AuGe/GaAs contacts. In samples heat-treated below the AuGe eutectic temperature, Ni is found to move rapidly through the intervening AuGe layer to collect at the GaAs interface, and the effective value of ?_Bn rises to the value characteristic of Ni/GaAs Schottky diodes. For heat-treatment above the AuGe eutectic temperature, ohmic contact behavior is observed, and uniform alloyed contact surfaces are found to result from the presence of Ni at the GaAs interface. Ga outdiffusion and surface accumulation resulting from GaAs dissociation occurs for all alloy temperatures and times. The Ga outdiffusion appears to be characterized by a very low activation energy.     

An improved AuGeNi ohmic contact to n-type GaAs

The relation of contact resistance to heat treatment and microstructure was examined for the three following ohmic contact metallizations on n-GaAs: 200 Å Ge/200 Å Ni/2000 Å Au, 200 Å AuGe/200 Å Ni/2000 Å Au and 1000 Å AuGe/200 Å Ni. After an initial anneal at 440°C, the Ge/Ni/Au metallization had the lowest contact resistance and the highest proportion of the NiGeAs phase at the GaAs/contact interface. It was also the most stable with subsequent ageing at 330°C, this stability being attributed to the NiGeAs phase acting as a diffusion barrier at the GaAs/contact interface.     

Rapid thermal alloyed ohmic contact on inp

The results of a study of the electrical and metallurgical properties of thin metallic layers deposited on InP for use as an ohmic contact are presented. A rapid thermal annealing system was used to alloy AuGe/Ni/Au contacts ton-type ion implanted InP. Rutherford backscattering and contact resistivity measurement were used to evaluate the structural and electrical characteristics of these rapid thermal alloyed thin films. Varying degrees of mixing between the metals and the semiconductor were found depending on the temperature and temperature-time cycle. These results were compared to furnace and graphite strip-heater alloying techniques. A correlation between the interface structure and the contact resistance was found. Temperatures between 430 and 450° C and alloying time of 2 sec have produced the best electrical results, with specific contact resistance as low as 2*10^?7 Ω cm² on semi-insulating InP which was Siimplanted with a peak concentration about 2*10¹⁸ cm^?3. The optimum alloy temperature is marked by the onset of substantial wrinkling of the contact surface, whereas essentially smooth surfaces are obtained at temperatures below optimum. The depth of the alloyed ohmic contact is controlled by the time of heating and could be less than 1000Å.     

Optimization of AuGe-Ni-Au ohmic contacts for GaAs MOSFETs

GaAs-based metal-oxide-semiconductor field-effect transistors (MOSFETs) are promising devices for high-speed and high-power applications. One important factor influencing the performance of a GaAs MOSFET is the characteristics of ohmic contacts at the drain and source terminals. In this paper, AuGe-Ni-Au metal contacts fabricated on a thin (930 /spl Aring/) and lightly doped (4/spl times/10/sup 17/ cm/sup -3/) n-type GaAs MOSFET channel layer were studied. The effects of controllable processing factors such as the AuGe thickness, the Ni/AuGe thickness ratio, alloy temperature, and alloy time to the characteristics of the ohmic contacts were analyzed. Contact qualities including specific contact resistance, contact uniformity, and surface morphology were optimized by controlling these processing factors. Using the optimized process conditions, a specific contact resistance of 5.6/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ was achieved. The deviation of contact resistance and surface roughness were improved to 1.5% and 84 /spl Aring/, respectively. Using the improved ohmic contacts, high-performance GaAs MOSFETs (2 /spl mu/m/spl times/100 /spl mu/m) with a large drain current density (350 mA/mm) and a high transconductance (90 mS/mm) were fabricated.     

Lateral diffusion effects in AuGe based source-drain contacts to AllnAs/InGaAs/InP doped channel MODFETs

We have investigated the formation of source-drain AuGe/Au and Ni/AuGe/Ni/Au alloyed ohmic contacts to AlInAs/InGaAs/InP doped channel MODFETs, and observed lateral diffusion of the contact system after the standard annealing procedure at the temperature range of 185 to 400°C. Auger depth profiling of contacts annealed at 250°C, revealed that Au(Ge) diffused through the top InGaAs and AlInAs layers into the active InGaAs layer, but had reduced penetration into the AlInAs buffer layer. This reduction in diffusion along the depth axis at the AlInAs buffer layer boundary is believed to result in enhanced lateral diffusion and the observed lateral encroachment of the contacts. Both Au and Ni containing contact systems showed similar behavior in terms of lateral diffusion with encroachment extending between 0.25 and 0.5 μm at the periphery of the contacts for annealing temperatures between 300 and 400°C. A controlled ramp-to-peak temperature annealing procedure is developed to suppress such lateral diffusion effects. Low temperature annealing (250°C) using this procedure resulted in equally low contact resistance values (∼0.1Θ-mm) and no lateral diffusion. It is concluded that in thin multilayered structures the modified annealing procedure presented here, is necessary for optimal ohmic contact formation.     

Study of the influence of the sulfide and ultraviolet treatment of the <Emphasis Type="Italic">n</Emphasis>-<Emphasis Type="Italic">i</Emphasis>-GaAs surface on the parameters of ohmic contacts

The possibility of improving the parameters of AuGe/Ni- and Ge/Cu-based ohmic contacts to n-i-GaAs by modifying the preliminarily oxidized GaAs surface in a sulfide-containing solution, as well as via the effect of ultraviolet radiation generated with a KrCl excimer lamp on a chalcogenated surface, is studied. It is shown that preliminary oxidation of the n-i-GaAs surface with subsequent chalcogenation makes it possible to decrease the density of the surface states, to increase the reproducibility of the passivation of the surface, and to decrease the reduced contact resistance of the AuGe/Ni ohmic contacts by a factor of 1.5. The treatment of the chalcogenated surface of n-i-GaAs with an ultraviolet radiation with wavelength λ = 222 nm and emission power density W = 12 mW cm⁻² performed in vacuum before the deposition of metal layers of the Ge/Cu ohmic contacts makes it possible to decrease the reduced contact resistance by 25–50% andimprove the morphological characteristics of the surface of the contact area.     

Hot-plate alloying for ohmic contacts to GaAs

The fabrication and optimization of ohmic contacts to GaAs prepared by heating the wafers on a hot plate is described. The method offers high throughput and is production adaptable. The apparatus consists of a hot plate constructed of a heat pipe with a high surface temperature uniformity (±2°C over 4-in diameter) located inside a glove box with an ambient controlled to 5 percent H2/95 percent N2forming gas. Specific resistance and morphology of AuGe/Ni/Au contacts were characterized as a function of hot-plate surface temperature. At the optimum alloy temperature, specific resistances of less than 10^-6Ω-cm²were obtained repeatably for VPE GaAs with active layers of n ∼ 10¹⁷cm^-3and thicknesses of ∼0.4 µm as well as for VPE GaAs with an n⁺surface layer greater than 10¹⁸cm^-3. The contact surface morphologies were smoothest for those alloyed at temperatures below the optimum, and, for those alloyed at or above the optimum, they were increasingly less smooth for increasingly higher alloy temperatures. A general discussion of this method's potential application to high-volume GaAs processing is given.     

AuGe/Au ohmic contacts to n-type InP by hot-plate alloying

The authors report on the fabrication and characteristics of AuGe/Au ohmic contacts to ion-implanted n-type InP. The contacts have smooth surface morphology, excellent adhesion and good contact resistance uniformity. For samples with carrier concentrations of 1.7 * 10/sup 17/ and 1.6 * 10/sup 18/ cm/sup -3/, contact resistances of 0.07 and 0.03 Omega mm, respectively, have been obtained. These values are among the lowest contact resistances reported for n-type InP.<>     

Sintered ohmic contacts to n-and p-type GaAs

Previously, all known ohmic contacts to n-GaAs have involved a so-called "alloying" procedure which consists of melting a Au-Ge eutectic or Sn-based alloy films on GaAs. We describe here a new contact metallization scheme consisting of Pd/Ge/n-GaAs which requires sintering rather than melting in order to produce ohmic contacts. The sintering is done at temperatures ranging from 350°C, 15 min to 500°C, 2 h depending on the doping level of n-GaAs (10¹⁸-10¹⁶cm^-3. For n-10¹⁶cm^-3GaAs, a specific contact resistance of 3 × 10^-4Ω.cm²was achieved. Sintering leads to the formation of some PdGe and intermetallics associated with the Pd/GaAs interaction, namely, PdAs2and PdGa. Ohmic behavior is attributed to a combination of the doping action of Ge (as donor) and fast in-diffusion kinetics of Pd. Sintered contacts to n⁺and p⁺GaAs (ND.A∼ 10¹⁸cm^-8) made by Au, Pt, and Ti Were also investigated for ohmic behavior. Each of these three metals was at least partially effective in forming ohmic contacts to p⁺GaAs; the degree of effectiveness increases on going from Ti to Au to Pt. It is proposed that a reasonable guideline to follow When searching for ohmic contacts is Xm≳ Xdwhere Xmis the width of the metallurgical junction and Xdis the ideal depletion layer width. This condition should favor ohmic contacts by promoting a micro 3-dimensional current flow at the conductor-semiconductor interface and thereby maximizing field-emission probability.     

Ti/WSi/Ni ohmic contact to n-type SiCN

Ti/WSi/Ni contact to n-type SiCN was investigated using the circular transmission line method. Current-voltage characteristics, X-ray diffraction and X-ray photoelectron spectroscopy were used to characterize the contacts before and after annealing. It is shown that the conducting behavior of the contacts is dependent on the annealing temperature. After annealing at 900 ℃ or above, ohmic contacts with specific contact resistivity were achieved. The 1000-℃-annealed contact exhibits the lowest specific contact of 3.07 ×10-5 Ω·cm2. The formation of ohmic contact with low specific contact resistivity was discussed.