Low-temperature-grown gallium arsenide photomixers designed for increased terahertz output power

Terahertz radiation can be generated by optical heterodyne conversion in a low-temperature-grown GaAs (IT GaAs) MSM photoconductive switch. In photomixing, two frequency-offset laser beams are used to illuminate voltage biased inter-digitated electrodes, resulting in an AC current at the difference frequency that drives a miniature planar antenna. In this work, the factors limiting photomixer performance have been analyzed and a novel MBE growth structure is presented which provides as much as 5 times greater power in the THz range compared to previous work.; The basic approach used in this work is to reduce the thermal impedance of the photomixer. Because the photomixer active area is so small (10mu m x 10mum) a large temperature gradient is formed within the first several microns of the substrate material and in the LT GaAs itself. The thermal conductivity of the IT GaAs material will, therefore, affect the overall thermal impedance of the photomixer. This implies that a significant improvement in the thermal impedance of the photomixer can be made by reducing the thickness of the LT GaAs layer. A novel layer structure for an improved photomixer is analyzed using finite element analysis for the heat flow simulation and a photoconductive model for the gain calculation. It was found that a large increase in the maximum power should be possible when using a thin LT GaAs layer grown on a relatively thick AlAs layer.; Devices were fabricated on several MBE grown layer structures. Measurement of the photomixers showed record power levels at 850 GHz and 1.6 THz. Compared to a reference sample of 1mum LT GaAs grown directly on GaAs, the maximum power before failure was 4--5 times greater for the devices with an AlAs layer. The optically resonant designs displayed record efficiencies which were 6 times higher than for a sample grown at the same temperature on GaAs.; Analysis of the bias dependence of the external quantum efficiency and the radiated THz power revealed a clear field dependence to the photocarrier lifetime. The increased photocarrier lifetime caused a superlinear I-V characteristic and a super-quadratic bias dependence of the THz power. This field-enhanced lifetime also causes a pronounced peak in the maximum power at failure vs. bias curve. The bandwidth of a broad-band photomixer was decreased at high field, also suggesting a field-dependent lifetime. (Abstract shortened by UMI.)...