A comparative study of the thermal stability of Ti-Pt-Au, Ti, W-Au and WSi2-Ti-Pt-Au Schottky contacts on GaAs is presented. Various reliability tests at temperatures up to 400°C have been performed on these contacts leading to the conclusion that Schottky contacts containing refractory metal silicide diffusion barriers such as WSi2-Ti-Pt-Au remain stable even after long term reliability tests at 400°C. Contacts containing only refractory metal diffusion barriers (Ti-Pt-Au, Ti-W-Au) show severe degradations if operated at temperatures around or above 300°C.

Suitability of GaAs Schottky metallizations for continuous device operation at elevated temperatures up to 300°C: a comparative study

The letter reports the results of rapid thermal annealing of W, Ta and TaSix on GaAs. The results indicate that the rapid thermal annealing process results in contact stability to higher temperatures than is seen in conventional annealing. Furthermore, the stability of tantalum is greatly improved when silicon is added to the system, forming a TaSix contact. The preliminary results of TaSix on In-doped GaAs indicate that the process of Ga and/or As outdiffusion is inhibited up to a temperature of 1050°C.

High-temperature metallisation systems for transient annealing GaAs

A study is made of the thermal stability of a range of refractory metals and their silicides on GaAs. Particular emphasis is laid on the effects of transient annealing with temperatures up to 1050 °C. Using the technique of Rutherford backscattering the results for the metallisation systems Ta, W, TaSix, WSix, and VSix are compared and indicate the superior properties of the refractory silicides.



Die thermische Stabilitat einer Reihe von schwer schmelzbaren Metallen und ihrer Silizide auf GaAs wird untersucht. Insbesondere die Einflusse von nichtstationarer Temperung bei Temperaturen bis zu 1050 °C. Mit der Rutherfordruckstreutechnik werden die Ergebnisse fur die Metallisationssysteme Ta, W, TaSix, WSix und VSix verglichen und zeigen die uberlegenen Eigenschaften der schwer schmelzbaren Metallsilizide.

High temperature metallisation for GaAs device processing

Ageing tests on PHEMTs, ohmic contacts and heterojunction test samples have been performed. During moderate temperature life tests ( 210°C) a third degradation mechanism is evident, possibly attributed to field enhanced instabilities in the AlGaAs/InGaAs heterojunction or to Si diffusion, at the drain edge of the gate. A worst case activation energy of 1.49 eV has been deduced.

The long‐term stability of n‐AlGaAs/InGaAs/GaAs pseudomorphic HEMTs

A process has been demonstrated for the fabrication of scaled GaAs based metal-semiconductor field effect transistors (MESFETs) suitable for high frequency characterisation with gate-lengths down to 40nm. MESFETs were fabricated with gate-lengths in the range 40 to 300nm on molecular beam epitaxy (MBE) grown layers with GaAs and AlGaAs buffers. The MESFETs were characterised electrically at direct current (DC) and high frequency. The MESFETs have very good DC and high frequency performance, even down to the shortest gate-lengths. The performance is characterised by figures of merit such as transconductance and unity current gain cut-off frequency. MESFETs with a DC extrinsic transconductance of 720mS/mm and unity gain cut-off frequency of 150GHz (for a 40nm gate-length) have been demonstrated. However, the performance of the MESFETs is limited by scaling effects. Some of these effects are of a technological nature such as buffer layer current, a large gate resistance, surface depletion effects, and the high active layer doping. More importantly, the performance of the shortest gate-length MESFETs is restricted by the fundamental short-channel effects of punch-through and hot-electrons. The origin and function of many of these effects are introduced through a review of the operation and modelling of short-channel FETs. The MESFET design included features to minimise the technological scaling effects, i.e. 1) AlGaAs buffer layers to suppress buffer layer current. Experimental comparison was made with the GaAs buffer case. 2) High active layer doping to reduce the effect of surface depletion and maintain the channel aspect ratio, at the cost of deteriorated carrier transport. Devices with three different doping concentrations were compared. 3) A reduced gate-width with shorter gate-length to compensate for the larger series gate resistance. The experimental work described in this thesis investigates the scaling effects by electrical characterisation of the devices at DC and high frequency. The measurements presented and discussed include: 1) DC open channel and subthreshold transfer and output characteristics. 2) DC threshold voltage shift, equivalent to output conductance. 3) High frequency (up to 26.5GHz) S-parameters which yield the short-circuit current gain, maximum frequency of oscillation, and parasitic carrier transit delays through the channel. The design strategy was successful to the extent of overcoming many of the detrimental scaling effects for MESFETs with gate-lengths down to 200nm. The buffer layer current was suppressed by the AlGaAs buffer, and it was shown that interrupted growth of the AlGaAs buffer can surmount the problem of degradation of active layer quality associated with MBE growth of GaAs on AlGaAs. On the negative side, the technological problems of large gate resistance, surface depletion effects and very high active layer doping (which inhibits carrier transport) were found to significantly degrade the high frequency performance of the devices in this work. These are all issues which could be addressed by modifications to the device design. In addition, the parasitic carrier transit delays became more significant in degradation of high frequency performance as the gate-length was reduced. As for the more fundamental effects, the main conclusion of this thesis is that the ultimate scaling limits of hot-electron and punch-through effects govern the behaviour of the MESFETs with very short gate-lengths (in this case, less than 100nm). The most severely affected parameters are output conductance and subthreshold current. For example, the benefits of AlGaAs buffer are greatly reduced for 40nm gate-length MESFETs. Reduction of gate-length can still give an improvement in high frequency performance, but less than predicted by simple scaling of the carrier transit time and the beneficial hot-electron effect of velocity overshoot. No evidence of velocity overshoot effects has been found in these MESFETs.

Fabrication and Scaling Effects of Very Short Gate-Length GaAs MESFETs

The stability of Schottky barrier gates in GaAs MESFETS has been investigated to allow device processing at elevated temperatures. The incorporation of a Pt diffusion barrier into the TiAu structure gives stability of electrical characteristics up to 350°C and a Ti-W-Au contact is stable up to 400°C. To achieve higher temperature stability for use in a self aligned gate (SAG) technology, a more stable Schottky barrier material such as a TiW (30 at % Ti) alloy has been used. This has been found to be unreliable above 750°C and it has been found necessary to use a tungsten silicide contact W1.6Si (37 at % Si) to achieve reproducible Schottky contacts at the temperature used for ion implantation activation (800°C). SAGFETs have been fabricated using W1.6Si and initial results indicate good yields (91%), high gm (190 mS/mm) and good uniformity (standard deviation = 6.8% for 293 devices).

Stability of Schottky barriers at high temperatures for use in GaAs MESFET technology

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Stability of Schottky barriers at high temperatures for use in GaAs MESFET technology