Low-Noise Avalanche Photodiodes

Avalanche photodiodes (APDs) have been widely used in many applications, including long-haul telecommunication, quantum communication, light detection and ranging (LIDAR), data centers, high-performance computers, imaging, and biological and chemical agent detection. Owing to the internal multiplication gain, APDs have a higher sensitivity than conventional p-i-n photodiodes. However, the multiplication mechanism, impact ionization, is a stochastics process. This is the source of an additional noise, referred to as the excess noise. This thesis focuses on low-noise performance, and includes several types APDs.
Conventional III-V APDs have numerous benefits, they have direct bandgap, high absorption coefficients, wide spectral response regions, and flexible, complex structure design. Previously the excess noise was not as low as silicon APDs. However, four years ago, our group collaborated with Prof. Seth Bank’s group in the University of Texas at Austin showed that AlInAsSb digital alloy APDs exhibit comparable low excess noise to silicon APDs. This breakthrough makes opens the potential for wider deployment of low-noise APDs. Different from multi quantum well (MQW) and superlattice materials, digital alloys have extremely thin periods, only a few monolayers (MLs) thick, which allows the wave functions to couple through several wells by the resonant tunneling effect. The digital alloy growth method may be the origin of low noise in AlInAsSb digital alloy APDs. Recently, Prof. John P. R. David’s group at the University of Sheffield has demonstrated an extremely low excess noise AlAsSb digital alloy APDs [1]. They proposed that large phonon scattering and large hole effective mass caused by Sb may explain the low excess noise. In Chapters 3 to 5, I report the characteristics of Al0.7InAsSb binary digital alloy, Al0.7InAsSb ternary digital alloy, Al0.8InAsSb digital alloy, InAlAs digital alloy, AlGaAs digital alloy and InGaAs digital alloy APDs. Only the Al0.7InAsSb, Al0.8InAsSb, and InAlAs digital alloys exhibit smaller excess noise than their random alloy counterparts. In order to further explore this low-noise performance, temperature measurements and ionization coefficient measurements have been done, and the experimental results are consistent with Dr. Jiyuan Zheng’s simulations. Recently, Ann Kathryn Rockwell, one of Professor Seth Bank’s graduate students, has grown an Al0.7InAsSb “random-digital alloy” by changing the periods from 4ML to 16ML. The average is 10ML. The Al0.7InAsSb random-digital alloy shows higher excess noise than the Al0.7InAsSb digital alloy. In Chapter 6, I describe the characteristics of digital alloy Al0.7InAsSb APDs in Geiger-mode operation, to achieve single photon detection. These single photon avalanche diodes (SPADs) achieve higher single photon detection efficiency and lower dark count rate than reported InGaAs-InAlAs SPADs.
Another promising research area is low-noise APDs in silicon photonics. Chapters 8 and 9 introduce the low-noise III-V APD and Si-Ge APD on silicon. We demonstrated the first III-V APD grown by heteroepitaxy on silicon. This InGaAs-InAlAs APD exhibits the same small excess noise as the APDs on InP substrate. In the future, high-bandwidth-density optical interconnects, high bandwidth, and high sensitivity APDs are desirable owing to the need for high data rates and low power consumption. I demonstrated a low-voltage, high-speed Si-Ge waveguide
APD that shows superior high-temperatures performance with 100% internal quantum efficiency. Its breakdown voltage, bandwidth, and gain-bandwidth product are also very insensitive to temperature. The excellent temperature characteristics of this Si-Ge APD demonstrates its potential to be used in a high-operating-temperature optical link for future energy-efficient data centers and high-performance computers. Moreover, this Si-Ge APD can obtain higher sensitivity by adding a distributed Bragg reflector (DBR).