Methodologies for low excess noise measurement in wide bandgap materials.

This work aims at investigating the avalanche excess noise characteristics of the AlxGa1-xInP (x from 0 to 1) avalanche photodiode (APDs) for practical applications such as underwater detection, oil well logging and space exploration. Normally, Si APDs are used to operate in the visible part of the spectrum, however this has a broad spectral response and requires optical band-pass filters to avoid the detection of extraneous light sources at other wavelengths, which adds cost and complexity to the system.
To enable these measurements to be undertaken, a high sensitivity system for measuring multiplication and the excess noise is described. The system is capable of measuring the multiplication and excess noise power of devices, even when the photocurrent is low (approximately 10 nA). The signal to noise ratio (SNR) of the system is more than two orders of magnitude better than previously reported systems. The ability to characterise APD performance with such low photocurrents enables the use of low power light sources such as light emitting diode (LED) rather than lasers to investigate their noise performance.
Multiplication and avalanche excess noise measurements have been undertaken on a series of AlInP homo-junction PIN and NIP diodes with i region widths ranging from 0.04 µm to 1 µm, using 442 nm (laser) and 460 nm (LED) wavelength light. Low dark currents of < 170 nA cm-2 at 95% of breakdown voltage were obtained in all the devices because of its wide bandgap and there was no tunneling dark current present even at high-fields > 1000 kV/cm. For a given multiplication factor, the excess noise decreased as the avalanche width decreased due to the ‘dead-space’ effect. Avalanche excess noise measurements were also performed on a separate absorption multiplication avalanche photodiode (SAM-APD) with a nominal multiplication region width of 0.2 µm at 460 nm wavelength and it had an effective k (hole to electron ionization coefficient ratio) of approximately 0.3.
There is also a requirement for photodetectors capable of high temperature operation. A high temperature measurement system is described. The system can measure temperature dependence of the noise characteristics in AlInP devices. AlInP devices are found to be suitable for operating in high temperature environments. Avalanche multiplication and excess noise from 298 K to 475 K have been measured on AlInP diodes under 460 nm LED illumination. These devices have i region widths ranging from 0.04 μm to 1 μm. The impact ionization coefficients have been extracted as the function of the temperature. The experiment results show the excess noises of the AlInP diodes are very similar at different temperatures and vary only with the avalanche width and the value of multiplication.