| dc.description.abstract |
Defects decline the light output efficient and life time of GaN-based LED. In our study, as the defect density of device is 2.17×10-9 cm-2, 3.636×10-9 cm-2, 4.654×10-9 cm-2, respectively, the external quantum efficient is 20.546 %, 19.563%, 16.785%, respectively. To establish a high accuracy and convenient defect measuring system is necessary. However, the defect measuring methods that commonly used have many limits. For example, XRD spectrum is only suitable for wafer size sample. TEM and EPD evaluating method are destructive measurement that waste many sample and cost much time. We established a defect measuring method in this study by analyzing anelasticity behavior of GaN-based LED after applying stress or external electric field.
Defects develop Anelasticity behavior in solid. It causes the strain delay-responding after applying stress on material. Although there has complete study about anelasticity behavior of bulk material, the research of thin-film materials and the influence of semiconductors is lack. In this study, the anelasticity of the GaN layer in the GaN light-emitting-diode device was discussed. And, establish defect measurement system can be established. The studied chip was applied by thermal stress, piezoelectric stress, and external stress. The present results show that the decrease of forward-voltage of GaN LED is due to the degree of energy-level change as forward external electric field applied. After removing external electric field, the forward voltage increase with time. We found that the increment of the forward-voltage with time attributes to the delay-response of the piezoelectric fields (internal electrical fields in GaN LED device). We applied -0.5 V reverse bias and heat the chip to 100 oC, respectively. Thus, the forward voltage increases gradually due to the energy-level in GaN LED is flattened with time by thermal stress and external electric field. Furthermore, reverse bias (-1 V) flatten the energy level much more, so, the forward voltage enhancement is higher. As -5 V reverse bias is applying on the GaN LED, the voltage dropped due to the higher reverse bias tilted the energy-level in GaN LED again. Using the correlation of strain-piezoelectric-forward voltage, a plot of strain of the GaN layer against time can be obtained by measuring the forward-voltage of the studied GaN LED against time. The key anelasticity parameter, characteristic relaxation time τ, and anelastic strain Uan of the GaN would be analyzed by the curves of the thermal strain in GaN epi-layers versus time. For estimating the dislocation density, the studied GaN LED chips were etched by H3PO4 in 260 oC. The etching pits on GaN LED after H3PO4 reacted were calculated. GaN LED chip with higher dislocation density has higher τ and Umax, furthermore, the dislocation density and anelastic strain is linear relation. The results show the anelasticity behavior is attributed by defect. In other side, it is possible for establishing a defect measurement system by analyzing anelasticity behavior of GaN LED. The relation of defect density and anelastic strain can be expressed as Vf,max=0.00168×Ddis+0.00329. Vf,max is the forward-voltage difference induced by anelastic strain of GaN-based LED. Ddis is the density of dislocation in GaN-based LED. | en_US |