W. T. Anderson

Ion induced charge collection in GaAs MESFETs

Charge-collection measurements on GaAs MESFET test structures demonstrate that more charge can be collected at the gate than is deposited in the active layer and more charge can be collected at the drain than the total amount of charge produced by the ion. Enhanced charge collection at the gate edge has also been observed. The current transients produced by the energetic ions have been measured directly with about 20-ps resolution. The significance of this work is that it shows charge-collection phenomena in GaAs MESFETs to be very complex with important implications for modeling SEU (single-event upset) phenomena and developing techniques to mitigate SEU effects. >

Reliable Ti/Al and Ti/Al/Ni/Au ohmic contacts to n-type GaN formed by vacuum annealing

In this work, we investigated Ti/Al and Ti/Al/Ni/Au ohmic contact metallizations to n-GaN which were formed by a vacuum annealing method. Our effort was focused on minimizing the oxidation of the Al, which limits the performance of the Al-based contacts. We have also investigated the effect of the Ni/Au overlayers on the performance and morphological characteristics of the contacts. Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. It was found that the Ni/Au overlayer on the Ti/Al reduces the specific contact resistivity values slightly and at the same time causes an upward shift, by ∼200 °C, in the contact formation temperature. For the Ti/Al and Ti/Al/Ni/Au on n-GaN doped at 1.1×1018 cm−3, a minimum specific contact resistivity of 1.2×10−5 Ω cm2 was achieved after 950 and 1100 °C anneals, respectively. Auger electron spectroscopic studies revealed that o...

Transient radiation study of GaAs metal semiconductor field effect transistors implanted in Cr‐doped and undoped substrates

Flash x‐ray measurements have been made on GaAs metal semiconductor field effect transistor structures fabricated in both Cr‐doped and undoped semi‐insulating substrate material in order to characterize the levels responsible for long term transients in these devices. Prominent electron and hole traps with activation energies in the 0.76–0.90‐eV range have been identified. The main electron trap, which is present in both Cr‐doped and undoped substrates, appears to be primarily responsible for the observed transient response. However, traps related to Cr‐doping also contribute to the transient behavior.

Cr/Al and Cr/Al/Ni/Au ohmic contacts to n-type GaN

In this work we investigate the performance of Cr/Al and Cr/Al/Ni/Au ohmic contacts on n-type GaN. Annealing of the contacts was achieved by using a low temperature conventional quartz tube furnace in an Ar ambient and a new vacuum annealing technique using a tungsten strip heater. Low specific contact resistivity (ρc) metallizations were achieved with furnace annealing at considerably lower temperatures (550–600 °C) than those typically required for GaN contacts by halogen lamp rapid thermal annealing (∼900 °C). Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. For the Cr/Al bilayer on GaN with n doping of 1018 cm−3, minimum specific contact resistivities of 1.6×10−4 Ω cm2 and 2.3×10−5 Ωcm2 were achieved for furnace annealing and vacuum annealing, respectively. Our experiments showed that, when Cr was used as a contact material, the simultaneous presence o...

Reduction of Long-Term Transient Radiation Response in Ion Implanted GaAs FETs

The long-term transient radiation response in ion implanted GaAs FETs induced by flash x-ray or electron pulses has been reduced by the incorporation of a deep buried p-layer beneath the active n-layer. Reductions in the change in drain current of up to two orders of magnitude were measured in FETs with the buried p-layer compared to FETs with only an n-implanted channel following a 100 rad x-ray pulse. This improvement in radiation tolerance is attributed to shielding of the active n-layer by the conducting p-layer from the effects of radiation induced trapped substrate charge.

Smooth and continuous Ohmic contacts to GaAs using epitaxial Ge films

Ohmic contacts to n‐type GaAs have been developed using epitaxial Ge films on GaAs alloyed with Ni overlayers by solid‐state diffusion at temperatures of 450–550 °C. The contacts are smooth and continuous, showing no evidence of phase separation or other surface structure. Interdiffusion at the Ge‐GaAs and Ni/Ge‐GaAs interfaces was examined by Auger electron spectroscopy (AES) sputter profiling techniques. An abrupt profile is observed at both the as‐deposited and sintered Ge‐GaAs interface. With the presence of a Ni overlayer, significant interdiffusion between Ge and GaAs is revealed by AES profiles. These results, together with the current‐voltage (I‐V) characteristics of similar contacts prepared on p‐type GaAs, indicate the presence of a Ge‐doped n+ layer at the Ni/Ge‐GaAs interface.

GaAs MMIC Technology Radiation Effects

A comprehensive study was made of radiation effects in the component devices that comprise a particular technology developed by Texas Instruments for GaAs MMICs. Total dose, transient, and neutron radiation effects were measured in FETs. Transient effects were measured in capacitors, resistors, Schottky barrier diodes and the MWICs. Results are compared with predictions of radiation effects models.

Amorphous thin film diffusion barriers on GaAs and InP

Abstract Thin films amorphous WSi and TiWSi diffusion barriers have been studied on GaAs and InP surfaces for the purpose of establishing their reliability for ohmic contacts and Schottky barriers, particularly under high temperature stress. The amorphous films were formed by a new method in which alternate layers of tungsten or TiW and silicon were sputter deposited to a total thickness of about 1300 A and subsequently annealed near the glass transition temperature Tg(≈500°C). Electron channeling and reflection electron diffraction were used to determine the amorphous nature of the films as deposited and after 4 h anneals near Tg. The as-deposited films had interfacial amorphous regions with compositions determined by interfacial reactions during the sputtering process. As-deposited WSi films showed a weak channeling pattern which came from the unreacted polycrystalline tungsten layers. From Auger electron spectroscopy (AES) sputter profiles, it was concluded that the amorphous regions were at the WSi interfaces which had the required tungsten-to-silicon composition ratio. After annealing at 500 °C for 4 h, the films were completely amorphous with no marked evidence of crystallization, indicating interfacial reactions extended completely into the tungsten layers. High magnification scanning electron microscopy (by a factor of 20 000) examination of the films after annealing revealed smooth and continuous surfaces with no evidence of grain boundaries. Diffusion along grain boundaries between gold and GaAs or InP in these amorphous thin films was thus almost completely eliminated. Interdiffusion of gold in layered structures (e.g. Au/(WSi)/GaAs) was studied by AES sputter profiling techniques. No interdiffusion of gold or GaAs was observed after 16 h anneals at 400 °C. With Au/(WSi)/InP structures, no interdiffusion was observed after 8 h anneals at 450 °C. These results are significant improvements over those for previous polycrystalline diffusion barriers (e.g. TiPt) which degrade after 1 h at 350 °C. Based on the AES sputter profiles, the diffusion coefficients in WSi amorphous thin films were found to be less than 3 × 10−18 cm2 s−1 at 400 °C for gold, gallium and arsenic and less than 6 × 10−18 cm2s−1 at 450 °C for gold, indium and phosphorus.

Ensemble Monte Carlo particle investigation of hot electron induced source‐drain burnout characteristics of GaAs field‐effect transistors

The lattice heating rate has been calculated for GaAs field‐effect transistors of different source‐drain channel design by means of the ensemble Monte Carlo particle model. Transport of carriers in the substrate and the presence of free surface charges are also included in our simulation. The actual heat generation was obtained by accounting for the energy exchanged with the lattice of the semiconductor during phonon scattering. It was found that the maximum heating rate takes place below the surface near the drain end of the gate. The results correlate well with a previous hydrodynamic energy transport estimate of the electronic energy density, but shifted slightly more towards the drain. These results further emphasize the adverse effects of hot electrons on the Ohmic contacts.

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 TEM<inf>00</inf>mode. 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.