Suppressing Surface Recombination Research Articles

Reliability of InP-based HBTs at High Current Density

AbstractInP-based HBTs for ultrahigh speed optical communications systems operation at over 40 GHz require a long-term stability under high current injection conditions, such as current densities of 2 or 5 mA/μm2. We achieved high reliability by suppressing surface recombination and emitter-metal-related crystalline degradation.Changes in the electric properties of devices due to temperature and bias stress were evaluated. The reduction in DC current gain due to surface recombination had the activation energy of 1.7 eV without current density dependence, and the lifetime of HBTs for this degradation mode is predicted to be over 1×108 hours at 125°C. The emitter metal diffusion and disruption of uniformity of the atomic composition were observed by transmission electron microscopy and energy dispersive X-ray spectroscopy in HBTs with the conventional Ti/Pt/Au emitter, whereas suppression of those degradations was observed in HBTs with refractory metal of Mo and W. The emitter resistance was estimated to evaluate the contact layer degradation. The critical time was one order larger for HBTs with refractory metal than for HBTs with conventional metal. The activation energies for resistance increases were 2.0 and 1.65 eV for the current density of 2 and 5 mA/μm2, respectively, for all types of emitter electrodes.The effectiveness of the refractory metal electrode for improving device reliability was confirmed, especially in high-current-density operation, which is essential for applying InP HBTs in high-speed ICs.

Bulk carrier lifetime measurement by the microwave reflectance photoconductivity decay method with external surface electric field

We attempted to measure the bulk carrier recombination lifetime of Si wafers by the microwave reflectance photoconductivity decay (PCD) method. Voltage was applied between an external electrode and a Si wafer to suppress surface recombination. Before the measurement, the surface state density was reduced by a chemical treatment using NH4OH–H2O2–H2O and diluted HF solutions. Carrier lifetime as long as 1 ms was measured by the present method for a wafer with a bare surface. Comparison with results for oxidized wafers show that the present method can suppress surface recombination more effectively than thermal oxidation, which has been often used for surface passivation in PCD measurements.

Optical studies of GaN and GaN/AlGaN heterostructures on SiC substrates

We present the results of spectroscopic studies on GaN based epitaxial materials on SiC substrates by metalorganic chemical vapor deposition. A variety of techniques has been used to study the optical properties of GaN epilayers and GaN/AlGaN heterostructures. Sharp spectral structures associated with the intrinsic free excitons were observed by photoluminescence and reflectance measurements from GaN based materials grown on SiC substrates. The residual strain was found to have a strong influence in determining the energies of exciton transitions. Picosecond relaxation studies of exciton decay dynamics suggest that an AlGaN cladding layer with a small mole fraction of AlN can be relatively effective in enhancing the radiative recombination rate for excitons by reducing the density of dislocations and suppressing surface recombination velocity in the GaN active layer for the GaN/AlGaN heterostructure samples.