The production of high quality optical devices based on porous silicon relies on having precise control over the refractive
index and thickness of each porous silicon layer. Until now this has been achieved by pre-calibrating each growth
system and making sure that parameters such as wafer doping, electrolyte concentration and temperature are kept constant
with each fabrication. However low doped silicon required for IR based silicon photonics has significant non-uniformity
in the index and growth rate during formation of the porous silicon. The solution we have developed is based on realtime
in-situ monitoring of low-doped silicon during porous silicon growth. This process rapidly measures the optical
interference between the porous silicon film and the backside silicon surface. The optical light source comes from six
coarse-wavelength-division-multiplexed lasers, with rapid switching between wavelengths achieved using a
microelectromechanical switch. The system permits rapid measurement (<1 sec) of the reflection spectra from all lasers,
enabling real-time thickness and refractive index of each layer to be determined during growth. Our aim is to enable
growth of high quality multi-layer films such as those required for Bragg Reflectors and high-Q Fabry-Perot microcavities.
In this paper we briefly describe the instrument, the numerical models developed to gather the measurements,
and show preliminary results gathered from this instrument during growth. The results show a good agreement with
theoretical optical modelling, and also direct measurements of the porous silicon layers.
This paper will detail investigations into rapid thermal annealing (RTA) treatment of ohmic contacts to reactive ion etch (RIE) damaged p-type GaN. It was found that annealing at moderate temperatures in N2 atmosphere can improve the ohmic nature of contacts to RIE-damaged p-GaN. After chlorine-based RIE treatment of the p-GaN surface the sheet resistance and contact resistivity of the ohmic contact metallisation scheme increased, and the contacts became extremely non-ohmic. After RTA treatment in N2 atmosphere at 550°C, linearity of the I-V curves was substantially improved, and the contact resistivity decreased. This improvement is most likely related to improvements in the metal-GaN interface and/or improvements in the bulk material when protected by the contact metal. Unprotected surfaces were further damaged (manifested as higher sheet resistance) by the annealing procedure.
The effect of 60Co gamma-irradiation on the device characteristics of Al0.35Ga0.65N-GaN heterojunction field effect transistors (HFET) has been investigated using DC and geometrical magnetoresistance measurements. Cumulative gamma-ray doses up to 20 Mrad(Si) are shown to induce drain current degradation, negative threshold voltage shifts and reverse gate leakage current degradation. Analysis of drain magneto-conductance characteristics measured at 80 K indicated an increase in two-dimensional electron gas (2DEG) sheet concentration with accumulated radiation dose. More importantly, the 2DEG mobility-concentration characteristics are noted to remain aproximately constant for total gamma-radiation doses up to 20 Mrad(Si), indicating that the areal density of radiation-induced defects at the heterointerface is likely to be negligible. The threshold voltage shifts are therefore attributable to the introduction of relatively shallow radiation-induced defects in the AlGaN barrier region and/or to defects introduced at the gate-barrier interface. Although the drain conductance characteristics manifested similar degradation trends at 80 and 300 K, the 2DEG parameters obtained at 300 K exhibited significant scatter with increasing dose, possibly a manifestation of device instabilities induced by radiation-induced surface defects in the ungated access region near the edge of the gate. Device failure due to severe gate leakage and loss of gate control over the 2DEG charge, occurred after a total dose of 30 Mrad(Si).
We discuss investigations into a contactless UV-enhanced wet etching technique for GaN. The technique utilizes the oxidising agent potassium persulfate to consume photogenerated electrons, thus avoiding the need for an electrical contact to an external cathode. The etch rate is strongly dependent on illumination intensity and uniformity and on the pH of the KOH solution, as is the roughness of the etched surface. The implementation of a dual illumination scheme whereby an additional UVC lamp was used to illuminate only the solution and not the wafer, resulted in an increased etch rate and smoother etched surface. Finally, the ohmic nature of contacts deposited on n-type GaN that had been etched in this manner was found to be improved compared to contacts on the unetched surface.
In this paper, we report transient capacitance measurements performed on MOCVD-grown nominally undoped n-GaN Schottky diodes exposed to 60Co gamma irradiation. Three radiation-induced defect levels are identifiable at an accumulated dose of 21 Mrad(Si) with thermal activation energies of 88+/- 7 meV, 104+/- 12 meV and 144+/- 13 meV, produced at a rate of 2.2x10-3 cm-1 per 1.25 MeV photon. The isochronal annealing behavior of these defects indicates that they are of similar nature, stable at temperatures < 100 C and disappear for annealing temperatures > 350 C. The carrier emission and annealing characteristics of these defects are consistent with previously identified nitrogen-vacancy related defects. Three deep-level defects present before irradiation exposure with activation energies of 254, 363 and 586 meV were found to remain unaffected for cumulative gamma-ray doses up to 21 Mrad(Si).
This paper reports the observation of defect-related anomalous low temperature drain current-voltage characteristics in AlGaN/GaN MODFETs. We have performed our study on devices with a relatively large number of lattice defects, generally referred to as nanopipes, in their active area. The observed low temperature anomalies appear as 'kinks' in the Ids-Vds characteristics and are observable at temperatures < 210 K. In a device with a large density of defects, we observe current collapse and large threshold voltage shifts at 80 K, which depend on bias history. We attribute the observed behavior to impact ionization of charge accumulated in the AlGaN layer by high- energy electrons injected from the 2DEG via real space transfer. The existence of these mechanisms indicates that device self-heating is not the solely responsible for the negative differential resistance at high electric fields in AlGaN/GaN MODFETs. These mechanisms may have significant influence on the high frequency performance of power transistors and on our current understanding of high electric field parallel transport phenomena in III-nitride heterojunctions.