Carrier screening effect in AlGaN quantum-well avalanche photodiode

Carrier-screening effect in an AlGaN quantum-well avalanche photodiode was investigated. The avalanche photodiode is a p-i-n diode consisting of three periods of Al0.1Ga0.9N/Al0.15Ga0.85N multiple quantum wells (MQWs) as the active region. Avalanche electroluminescence (EL) spectra were measured at different reverse bias voltages. By measuring the quantum Stark induced red-shift of the EL peak at different bias values, it was found that carrier screening decreases the local electric field across the AlGaN quantum wells, resulting in a reduced red shift of the EL peak, while enhancing the EL intensity. The carrier screening was found to be strong at the onset of avalanche breakdown of the diode and become weaker with increasing of the applied electric field. This is explained by considering the strong polarization-induced internal field in the barriers of the quantum wells. The polarization-induced electric field in the barriers has a direction opposite to the applied field thus producing a potential barrier to block the carrier transport. This leads to accumulation of carriers in the quantum wells, thus producing the screening effect. The direction of electric field in the barriers inverts when the applied bias increases to be larger than an inverse threshold. Carriers can then be smoothly transported and carrier screening disappears. Our study shows that carrier-screening effect can play an important role in III-Nitride heterojunction devices due to the existence of strong polarization fields.