Research Of The Epitaxial Growth Of Yellow-green LED And The Carrier Transport Mechanism In The Quantum Wells

Nowadays,the main materials adopted for light emitting diodes(LEDs)are AlGaInP and InGaN.AlGaInP-based LEDs mainly emit red and yellow light while InGaN-based LEDs have a high quantum efficiency at blue and green light spectra range.However,both of the two materials have extremely low quan-tum efficiency at the 530nm?570nm spectra range,which causes the“Green Gap”problem.According to the quantum mechanics and semiconductor physics theory,multiple quantum wells(MQWs)are designed to restrict carriers to raise quantum efficiency.Nevertheless the MQWs sandwiched with a p-n junction will generate photovoltage between the p and n electrodes under resonant excitation condition.Connecting the p and n electrodes by a copper wire,as called short circuit condition,the external circuit will generate photocurrent and the pho-toluminescence(PL)will suffer a drastically decay compared with open circuit condition.Under short circuit condition the photoluminescence peak position exhibits a little blue shift from which under open circuit condition.Such phe-nomena imply the carries' transit mechanism cannot be well explained by the existed theory.In this thesis,we fabricate a hybrid MQWs structure to realize high light output power yellow-green LED with the emission wavelength at?550nm by metal organic chemical vapor deposition(MOCVD)epitaxial method.We also propose some mechanisms to explain the carrier transport process in the MQWs,and visualize the phenomena consistent with our mechanisms.The main contents are listed as follows:(1)We fabricate a hybrid MQWs structure and gain high luminous efficien-cy yellow-green LEDs in the“Green Gap”range.Under 20mA pulsed current injection,compared with conventional MQWs LED the hybrid MQWs LED per-forms suitable forward voltage,similar spectra curve,but about 3 times light output power,which shows superior electroluminescence(EL)luminous efficien-cy.Temperature dependent resonant excitation photoluminescence results reveal the yellow-green quantum well in the hybrid LED maintains more localized s-tates.It suffers less strain and retains a better crystal quality with less defect density and quantum confined stark effect(QCSE).(2)We propose physics mechanisms to explain the absorption and transition processes of photogenerated carriers in the MQWs.At non-resonant absorption condition,the photogenerated carriers will escape out of the MQWs immediately under the effect of p-n junction built-in electric field.The excess carriers accumu-late at the p and n sides,forming a photovoltage between p,n electrodes.Under short circuit condition,the external circuit will conduct a photocurrent and the PL intensity will reduce a large scale compared with open circuit condition due to the drastically reduction of restricted carriers in the MQWs.Such mecha-nisms are validated by continuously changing excitation wavelength PL under open and short circuit conditions and photocurrent measurement results.The experimental results show in non-resonant absorption regime,the PL intensities under open and short circuit conditions present a great difference,and the pho-tocurrent retains at a relative high value.While in resonant absorption regime,the PL intensities are almost the same under open and short circuit conditions,and the photocurrent decreases sharply.Moreover,we also point out that the photoluminescence process occurs after the built-up of the photovoltage,which coincides with the ultrafast measurement results.