QUALIFICATION OF OMVPE ALGAAS/GAAS HBT STRUCTURES USING NONDESTRUCTIVE PHOTOREFLECTANCE SPECTROSCOPY

High performance heterojunction bipolar transistor (HBT) integrated circuits are extremely dependent upon the uniformity and quality of the III-V compound heteroepitaxial materials used in their fabrication. HBT requirements include the need for excellent control over layer thickness, uniformity, alloy composition, and intentional impurity concentration over a wide range of carrier concentrations. We have used the contactless technique of photoreflectance (PR) to evaluate HBT structures of various epitaxial layer design and correlated the measured PR results with transistor performance. PR was performed in the range 1.3-2.0 eV which is ideally suited to probing AlGaAs/GaAs HBT structures. The resulting signals gave information about the gap energy of the material probed (and thus its composition) and the built-in DC electric fields within the various epitaxial layers. Measurement of the spectral energy of the observed Franz-Keldysh oscillation extrema near the GaAs and AlGaAs edges, were used to determine the built-in electric field in the collector and the emitter regions. These measured fields were in good agreement with electric field distribution calculations obtained by solving the Poisson and the continuity equations for an HBT structure. Run-to-run uniformity of HBT structures from a multi-wafer OMVPE reactor were correlated to measured transistor characteristics.