III-nitride optoelectronic devices containing wide quantum wells - unexpectedly efficient light sources

In this paper we review the recent studies on wide InGaN quantum wells (QWs). InGaN QWs are known to suffer from an extremely high built-in piezoelectric polarization, which separates the electron and hole wavefunctions and causes the quantum-confined Stark effect. It is shown, both by means of modeling and experimentally, that wide InGaN QWs can have quantum efficiency superior to commonly used thin QWs. The high efficiency is explained by initial screening of the piezoelectric field and subsequent emergence of optical transitions involving the excited states of electrons and holes, which have a high oscillator strength. A high pressure spectroscopy and photocurrent measurements are used to verify the mechanism of recombination through excited states. Furthermore, the influence of QW width on the properties of optoelectronic devices is studied. In particular, it is shown how the optical gain forms in laser diodes with wide InGaN QWs.

Optical Properties of GaN-Based Green Light-Emitting Diodes Influenced by Low-Temperature p-GaN Layer

GaN-based green light-emitting diodes (LEDs) with different thicknesses of the low-temperature (LT) p-GaN layer between the last GaN barriers and p-AlGaN electron blocking layer were characterized by photoluminescence (PL) and electroluminescence (EL) spectroscopic methods in the temperature range of 6–300 K and injection current range of 0.01–350 mA. Based on the results, we suggest that a 20 nm-thick LT p-GaN layer can effectively prevent indium (In) re-evaporation, improve the quantum-confined Stark effect in the last quantum well (QW) of the active region, and finally reduce the efficiency droop by about 7%.

Luminescence Properties of InGaN/GaN Green Light-Emitting Diodes with Si-Doped Graded Short-Period Superlattice

The optical properties of InGaN/GaN green light-emitting diodes (LEDs) with an undoped graded short-period superlattice (GSL) and a Si-doped GSL (SiGSL) were investigated using photoluminescence (PL) and time-resolved PL spectroscopies. For comparison, an InGaN/GaN conventional LED (CLED) without the GSL structure was also grown. The SiGSL sample showed the strongest PL intensity and the largest PL peak energy because of band-filling effect and weakened quantum- confined stark effect (QCSE). PL decay time of SiGSL sample at 10 K was shorter than those of the CLED and GSL samples. This finding was attributed to the oscillator strength enhancement by the reduced QCSE due to the Coulomb screening by Si donors. In addition, the SiGSL sample exhibited the longest decay time at 300 K, which was ascribed to the reduced defect and dislocation density. These results indicate that insertion of the Si-doped GSL structure is an effective strategy for improving the optical properties in InGaN/GaN green LEDs.

Comparative Study of Si-Ge-Sn Resonant Cavity Enhanced Heterojunction Bipolar Phototransistor (RCE-HPT) under Quantum Confined Stark Effect (QCSE) and Franz Keldysh Effect (FKE) at 1.55 µm

Group-IV and their alloy based Heterojunction Bipolar Phototransistors (HPTs) are of immense interest in recent day optical communication. In this paper first resonant cavity enhanced heterojunction bipolar phototransistor (RCE-HPT) with Ge0.992Sn0.008/Si0.30Ge0.61Sn0.09 Quantum Well/barrier structure under Quantum Confined Stark Effect (QCSE) has been evaluated. Further the bulk GeSn absorption region has been considered instead of QW/barrier structure and estimated the Franz Keldysh Effect (FKE). Finally different RCE-HPT related parameters such as quantum efficiency-bandwidth product, responsivity, collector current and optical gain have been studied and compared under QCSE and FKE.