Out-of-well carrier screening in a strainedInxGa1−xN/GaNmultiple quantum well structure

Abstract

Steady state and time-resolved differential transmission spectroscopy in a strained In xGa12xN/GaN multiple quantum well structure embedded within a p-i-n structure has been studied to identify the dominant screening mechanisms and measure the carrier sweep-out time. The results are consistent with a long-range out-of-well carrier screening where the photoexcited carriers escape the wells and separate, creating a space-charge field that induces an increase of the in-well field and shifts the excitonic transition to lower energies. A redshift of excitonic resonance under photoexcitation suggests that the direction of in-well field is opposite to that in the barriers due to the piezoelectric polarization in this strained In xGa12xN well. Compared with GaAs multiple quantum wells, a long carrier sweep-out time of 650 ps was observed in this structure. DOI: 10.1103/PhysRevB.68.233304 PACS number~s!: 78.67.De, 71.35.2y, 78.47.1p Time-resolved differential transmission measurements on GaAs multiple quantum wells ~MQW’s! embedded within a p-i-n structure have shown that the photoexcited electronhole pairs in the quantum well ~QW! under the influence of the electric field escape the well and travel in opposite directions to produce a space-charge field which opposes the built-in field. 1‐ 4 This out-of-well carrier screening mechanism, through the quantum-confined Stark effect ~QCSE!, shifts the excitonic resonance and modifies the absorption of the QW. Measurements of the change in the absorption as a function of time delay and wavelength provide a unique way to determine the dynamics of perpendicular carrier transport and carrier sweep-out times in p-i(MQW)-n structures. It is well known that hexagonal wurtzite III-nitride semiconductors have pronounced piezoelectric constant. 5,6 For InxGa12xN/GaN QW’s, the strength of the in-well electric field generated by spontaneous polarization and piezoelectric polarization can be as large as several MV/cm. Several groups have attributed the blueshift of the photoluminescence peak energy in undoped InxGa12xN/GaN QW’s with increasing carrier injection to the reduction of the QCSE due to in-well carrier screening. 7,8 Furthermore, built-in electric fields due to the p-n junction are present in these p-i(MQW)-n structures. Therefore, it is of interest to determine if the dominant carrier screening mechanism is in-well or long range out-of-well in such structures. A clear understanding of these carrier dynamics can provide great value for the design and optimization of InxGa12xN based emitters as well as for the optimization of material growth conditions. More importantly, such a study is expected to determine the carrier sweep-out times and provide insight into the potential profile in the strained InxGa12xN/GaN MQW’s. In particular, the sweep-out rate, a key property that limits the response time of nitride based p-i-n photodetectors, can be determined. In this Brief Report, we report experimental results on steady state and time-resolved differential transmission measurements of an InxGa12xN/GaN p-i(MQW)-n structure at room temperature. The differential transmission spectra are used to identify the dominant screening mechanisms and simultaneously to estimate the spatial band potential profile. A redshift of excitonic resonance immediately following pulsed excitation and a carrier sweep-out time of 650 ps were observed, indicating that photoexcited carriers sweep out of the well and screen the entire MQW structure. The increase of the in-well field under photoexcitation indicates that the direction of the in-well field is opposite to that in the barriers due to the piezoelectric polarization in the strained InxGa12xN wells. The sample described in this work was grown using metalorganic vapor phase epitaxy. Initially, a 4 mm thick GaN:Si layer was deposited on a c-face sapphire substrate. This layer was followed by the growth of ten periods of 7 nm thick GaN barriers and 2.5 nm thick In0.2Ga0.8N wells. The last QW was capped with a GaN barrier region followed by a 0.2 mm thick GaN:Mg p-contact layer. In order to determine the magnitude of the field distribution, we solved the Poisson equation for a p-i(MQW)-n structure within the depletion approximation. Neglecting the screening effects of background carriers, the net electric field within the wells ~barriers! Ew (Eb )i s