UC San Diego

To explore novel physical mechanisms for detection and internal amplification of light signals in semiconductor, this dissertation reports three distinctive photodiode devices to demonstrate desired characteristics such as efficient detection, single-photon sensitivity, high speed, low noise, inherently stable and CMOS compatible.

Impact ionization, a prevailing intrinsic signal amplification mechanism widely used in Avalanche Photodiodes (APDs), was sought after to complement the thermal noise limitations of transistor amplifiers. However, due to the high operation voltage and large excess noise, APD has been out of favor for most optoelectronic systems. For the first device, a heavily doped partially compensated Si p/n junction photodiode is presented, with a new internal gain mechanism. The abundant localized impurity states greatly enhance the Auger excitation rate, by relaxing the k-selection rule. Furthermore, through electron-phonon interaction, the carrier excitation occurs in a cyclic manner with internal regulation, and hence this unique gain mechanism is called Cycling Excitation Process (CEP). Si photodiodes using this concept show ultrahigh gain, low operation voltage, CMOS compatibility, and, above all, quantum limit noise performance that is 30 times lower than devices using impact ionization. Furthermore, the gain, frequency response, and noise characteristics of CEP junction are modeled by a set of rate equations.

Realizing that CEP effect depends greatly on Auger excitation involving localized states, we made the counter-intuitive hypothesis that disordered materials like amorphous silicon, with plenty of localized states, can produce strong CEP effects with high gain and speed at low noise, despite their extremely low mobility and large number of defects. For the second device, an amorphous silicon photodiode will be demonstrated, with low noise and a gain-bandwidth product of over 2 THz, based on a very simple structure. This device is very impactful because amorphous silicon, as the primary gain medium, is a low-cost, easy-to-process material that can be formed on many rigid or flexible substrates.

Conventional semiconductor single photon detectors are Geiger-mode avalanche photodiodes made of high-quality crystalline semiconductors and require external quenching circuits. By a novel combination of CEP and impact ionization gain, the third device is a single photon detector having dual gain sections to obtain mesoscopic cycling excitation and an amorphous/crystalline heterointerface to form an electron transport barrier that suppresses gain fluctuations. The dual gain sections are comprised of a crystalline silicon n/p junction and a thin layer of amorphous silicon. At 100 MHz, the device shows single photon detection efficiency greater than 11%, self-recovery time of less than 1ns, and an excess noise factor of 1.22 at an average gain around 75,000 under 8.5V bias.