The Research On N-Zno/P-GaN Heterojunction Light Emitting Diodes With A Polarization-induced Electron Blocking Layer

In the recent years, people have showed a great interest on doing research on ZnO/GaN heterojunction. As the donor defects, such as oxygen vacancy and zinc interstitial, are in undoped-ZnO, self-compensation effect has made it difficult to grow ZnO thin film. And, p-GaN has been used as an ideal substitution for p-ZnO. However, the disadvantages of n-ZnO/p-GaN LEDs are the large range emission originated from both of p-GaN and n-ZnO sides generally and n-ZnO/p-GaN LEDs always stimulate the defect emission. Thus, it is necessary to involve an electron blocking layer to enhance the emission from ZnO. AlGaN has similar structure with GaN and ZnO, which can be a well electron blocking layer. However, AlGaN/GaN interface has strongly polarization effect, we use a polarization-induced graded-p-AlxGa1-xN layer to reduce the polarization electrical field.In this paper, we describe the process of fabricating different devices and analyze the distribution of polarization charges at the AlGaN/GaN interface in theory. Then we characterize and analyze different devices by using hall-effect, current-voltage (1-V) characteristics, scanning electron microscope. X-ray diffraction, photoluminescence spectra and electroluminescence spectra.Through crystal structure, Ga atom and Al atom is positive and N atom is negative. For GaN wurtzite structure, because the Al0.4Ga0.6N and GaN layer is deposited along Ga-face direction, the directions of polarization are from Ga atom to N atom. Therefore, the positive polarization charges,the density of which is almost3.7x10-6C/m2, are formed and gathered at lower Al0.4Ga0.6N/GaN interface. In this paper, n-ZnO/p-GaN LED with polarization-induced graded-p-A]NGa1-xN electron blocking layer was fabricated. For comparison, n-ZnO/p-GaN LEDs with and without a p-Al0.4Ga0.6N electron blocking layer were also prepared. The turn-on voltages are3.75V,7.5V and4.6V for n-ZnO/p-GaN, n-ZnO/p-Alo.4Gao.6N/p-GaN and n-ZnO/graded-p-AlxGa1-xN/p-GaN. respectively. The turn-on voltage of n-ZnO/p-GaN with a graded-p-AlxGa1-xN is obviously lower than that of n-ZnO/p-Alo4Gao6N/p-GaN heterjunction and all of them present a nonlinear rectifying behavior. According to the surface EL spectrum, the peak position of n-ZnO/p-Al0.4Ga0.6N/p-GaN and n-ZnO/graded-p-AlxGa1-xN/p-GaN is at388nm. But n-ZnO/graded-p-AlxGa1-xN/p-GaN has the larger emission under the same measurement condition. From the EL spectrum of cross-section. n-ZnO/graded-p-AlxGa1-xN/p-GaN shows a sharp peak at390.5nm and a weak peak at370nm with the increasing of current. On the contrary, the n-ZnO/p-Al0.4Ga0.6N/p-GaN device exhibits a sharp peak at368nm and a weak peak at385nm. The result shows that the polarization induced electron blocking layer can effectively block the electron and reduces holes barrier caused by energy-band bending and valance band offset△EvAlGaN/GaN, leading to enhancement of recombination in ZnO. From these results, it is suggested that the polarization induced electron blocking layer will be helpful for application of ZnO LEDs and LDs...