Characteristics of 3-dimensional Semipolar GaN fabricated by Selective Area Growth

Abstract

Optoelectronic devices based on a wide band gap semiconductor GaN been received plenty of interest due to their potential to realize next light source. In the wurzite GaN structure, most of conventional GaN-based light emitting diodes (LEDs) are grown on c-plane[0001] sapphire (Al2O3) substrate by using metal organic chemical vapor deposition (MOCVD). This polar GaN shows the strong polarization filed(spontaneous and piezoelectric field) as large as ~ 1MV. This strong internal electric field causes the separation of electrons and holes wave functions and thus reduces the radiative recombination rate and internal quantum efficiency (IQE). This phenomena is quantum stark confined stark effect (QCSE). To reduce such polarization effects, therefore, the growth of nonpolar or semipolar GaN is of great interest. However, heteroepitaxy of nonpolar and semipolar GaN on sapphire substrate shows significantly high defect density which can deteriorate the device performance. Thus, selective area growth (SAG) method is has received great attentions for creating semipolar GaN with low defect density. 3-dimentional (3D) GaN structure, formed by SAG method, provides both large active area and enhanced extraction efficiency. In addition, semipolar GaN exhibits high indium incorporation due to its lattice configurations. However, there have been some difficulties in fabricating the 3D semipolar GaN based “Green. LEDs” It is well known that in 3D semipolar GaN structure, the device performance is limited by poor current injection due to its lower Mg incorporation rate. However, the 3D structure limits the application of conventional analysis techniques such as Secondary ion mass spectrometry (SIMS) and Hall effect measurement. Generally, high quality p-GaN with low impurity level is obtained under the high growth temperature, high temperature growth condition (>1000ºC) result in both thermal degradation of active layer and anisotropic growth of 3D GaN structure. It is obvious that the p-GaN grown under the low growth temperature contains a relatively lower Mg concentration and a plenty of impurity which can play a complicated role. There remain challenges to evaluate quantitative distribution of impurity elements including Mg, C and precisely understand the role of impurity elements in p-GaN for realization of high quality p-GaN. The aim of present study is that fabrication of 3D semipolar GaN LEDs for efficient longer wavelength emission. Also we investigate the effect of indium surfactant on material properties of p-GaN by quantifiying the impurity elements and investigating the role of impurity elements in p-GaN. In order to quantify the dilute impurity and find out the role of impurity in 3D GaN structure, various advanced techniques such as Cs-corrected STEM, APT and low temperature PL and EL were employed. In present study, 3D semipolar (10-11) GaN based-LEDs is fabricated on c-plane sapphire by using SAG method. The LED structures comprise the InGaN/GaN multi quantum wells that were grown on the (10-11) side facets. The emission wavelength of fabricated semipolar and c-plane polar LEDs is 489 nm and 494 nm respectively, when a constant current of 20mA was injected. The center electroluminescence peak of semipolar LED and was blue-shifted from 489 to 484 nm, when the injection current was increased from 1 mA to 10mA. However, no further shift was observed although the injection current was increased up to 120 mA. Compared to semipoalr LED, the c-plane polar LED gradually blue shifted with increasing injection current. This result represents that the quantum-confined Stark effect and the band-filling effect are not too much critical in the semipolar GaN-based LEDs. Low-resistance ohmic contact is important factor for device performance. However, the low doping efficiency of 3D semipolar p-GaN cause the poor current injection of 3D semipolar GaN LEDs. The limited current injection degrade the device performance of 3D semipolar GaN LEDs. is low doping efficieny of 3D semipolar p-GaN. In this chapter, we employed the indium surfactant to modify the growth mode and enhance the electrical property of 3D semipolar p-GaN. In order to investigate the effects of indium surfactant on the growth mode of 3D semipolar p-GaN, growth rate and shape of 3D semipolar p-GaN was investigated by using STEM HAADF. The additions of In surfactant greatly affect the vertical(0001) growth rate of 3D semipolar GaN. This is due to more reduced the surface energy of semipolar plane(1-101) than that of polar(0001) by adding the indium surfactant. The contact property of 3D semipolar p-GaN using CTLM mehod is investigated. The contact resistance of 3D semipolar p-GaN with optimized indium surfactant shows relatively low contact resistance. The lowest contact resistance is obtained with In/Mg ratio ~ 0.15. This enhanced contact resistance is attributed to enhanced doping efficiency and reduced carbon related acceptor-compensator. By combining the Low temperature PL and quantitative analysis of impurity elements in p-GaN, the main compensating center of our 3D semipolar p-GaN is confirmed as CN-ON. The light output power of 3D semipolar p-GaN with In/Mg ratio ~ 0.15 show a 130~140% increase compared to that of conventionally grown 3D semipolar p-GaN without indium surfactant. We attribute the improved output for the indium surfacted growth of 3D semipolar p-GaN to the enhanced doping efficiency and reduced compensating center.