Avalanche photodiodes (APDs) are widely used to sense and amplify optical signals into electric signals based on their high quantum efficiency of photon detection and desirable internal gain. The high internal gain associated with APDs, however, may also lead to increased levels of excess noise that can negatively impact the signal-to-noise ratio achieved with such APDs. In particular, avalanche multiplication that occurs as electron-hole pairs are created in the active area, or multiplier, of the APD by carriers accelerated by the electric field can result in increased gain compared with other photodiodes. However, this increased gain may in turn lead to increased excess noise. For example, the typical optical gain is limited to less than 50 for typical III–V compound semiconductor APDs due to the high excess noise factor. In most systems, the APDs are operated at a gain of only 10 due to reliability concerns. Significantly, at the same time, the gain bandwidth product is expected to increase, which would be beneficial across many applications.

This invention is directed to methods and materials used with photodiodes to form high-avalanching probability layers in the form of quantum wells to achieve low excess noise and high optical gain. The photodiode has one or more high-avalanching probability layers that contribute to low excess noise and high gain in a tile photodiode. The methods and apparatus provide a system for creating one or more quantum wells in an APD to improve gain and reduce excess noise; in particular, an enhancement to the multiplier of the APD (namely, a thin layer of material) that improves upon optical gain and excess noise reduction of the APD.

The APD features a contact layer of InAlAs, an absorber layer of InGaAs, and a layer of InGaAlAs. The APD also includes a multiplier with one or more layers of quantum wells formed substantially of a first material, and one or more spacer layers formed substantially of a second material. There are two quantum wells formed of InGaAlAs with alternating spacer layers formed of InAlAs that are between the wells. Each quantum well has a bandgap of about 1.3 eV. The structure may include a different reverse-bias voltage applied across the APD; the carriers in the APD are accelerated by the electric field in the multiplier. Most of the avalanche events in the AFD occur inside the quantum wells.

The localization of the avalanche events in the quantum wells results in low excess noise. When the carriers enter one of the quantum wells, they have a high probability to avalanche and provide gain. Thus, with the presence of the multiple quantum wells, the overall APD optical gain becomes very high at moderate electric fields.

Another benefit to the APD structure is that the quantum wells are not intentionally doped with other materials. In APDs that require doping the components, particularly the multiplier, with additional materials, the doping must be carefully controlled for a high level of process accuracy in order to achieve favorable gain with the APD structure. This doping process can lead to high production costs, and limited efficiency and throughput. Eliminating the doping process thus can lead to increased production and efficiency, as high gain in this structure does not rely on accurate doping.

This work was done by Rengarajan Sudharsanan, Ping Yuan, and Xiaogang Bai of The Boeing Company for Goddard Space Flight Center. GSC-16809-1


Photonics Tech Briefs Magazine

This article first appeared in the November, 2015 issue of Photonics Tech Briefs Magazine.

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