Analysis of InGaN/GaN Multiple Quantum Well Heterostructures by Means of Photoconductivity Measurements

A novel method for the characterization of multiple quantum well(MQW) light emitting diodes(LEDs) is investigated. This method involves the measurement of photocurrent generation in an LED when irradiated with photons of energy near the absorption edge of the quantum wells. In this situation the I-V curve of the LED reveals a number of photocurrent oscillation zones corresponding to spatial variations in the heterostructure (Figure 1). 

The mechanism leading to the voltage-bias dependence of these features is hypothesized to be the expansion of the depletion (space-charge) region. Electrons and holes, from the N and P regions respectively, are drawn to the P-N junction as the reverse bias-voltage is increased. The area occupied by the ionized dopant atoms left behind by these charges is referred to as the ‘depletion region’. As the magnitude of the bias increases this area of depleted atoms becomes larger, engulfing the quantum wells in series. An electric field forms between the charges gathered at the P-N junction and the ionized atoms, giving rise to a potential gradient across the depletion region. The I-V features are then caused by the dynamics of the excited electrons as the potential gradient traverses each quantum well. 

A preliminary model is constructed by using the SIMS data (Figure 2) for the LED heterostructure supplied by Veeco. The spatial extent of the depleted region is calculated as a function of the applied bias using a doping profile derived from the SIMS data. This allows the estimation of the voltage at which each quantum well will become depleted and thus when a feature will be expected. The resulting curve is compared to the experimentally determined I-V feature spacing. 

The wavelength and temperature dependencies of this phenomenon are discussed, citing examples from existing literature. The benefits of this method of analysis are examined in terms of acquisition speed, measurement resolution and hardware cost with respect to current techniques such as capacitance-voltage measurement. 

Further research into this effect would involve the spectral transmission of the MQW devices as a function of voltage bias and the possible applications in spectrometry or optical filtering. A more complete picture of the quantum mechanical interaction between the incident photons, quantum wells, and potential gradient would also be valuable for the interpretation of the I-V curves and possible engineering applications.