Analysis of V-Shaped Pits Originated from Threading Dislocation in III-Nitrides Compound for Light Emitting Diodes

Light emitting devices (LEDs) made with III-nitrides compound on c-plane sapphire substrate using metal organic chemical vapor deposition (MOCVD) have so many threading dislocation defects due to the lattice mismatch. [1, 2] Threading dislocations were caused some problems such as poor luminescence and leakage current. In order to reduce the problems from threading dislocation defects, v-shaped pit initiated from threading dislocations used to construct in III-nitrides compound of LEDs. [3] V-shaped pits were made using different growth rate of etch lattice plane at specific growth temperature. (called mid-temp GaN in Figure 1) Furthermore v-shaped pits grown this method were served as insulator due to a lack of magnesium doping density in pGaN layer. Therefore leakage current through the threading dislocation defects can be prevented by characteristic of v-shaped pits. However some v-shaped pits still have a problem of leakage current. In this paper, dimension and electrical properties of v-shaped pits were separated for identify the causes of leakage current. Figure 1 shows a diagram of v-shaped pit originated from threading dislocation which has an open hexagonal inverted pyramid. The ‘g’ samples show lower current at reverse bias voltage than that of ‘ng’ samples which shown in Figure 2. It means the ‘ng’ samples are acting like leakage source at reverse voltage. Although the ‘g’ samples go to saturation around 5 reverse voltage, the ‘ng’ samples cannot be saturated even at 15 reverse voltage. (Figure 3) It means the ‘ng’ samples have higher carrier concentrations than ‘g’ samples. SEM images are shown in Figure 4(a)-4(d). The mean diameter and depth of the ‘g’ samples are 330.4nm and 306.7nm, respectively. (Figure 4(a), 4(c)) On the other hand, the mean diameter and depth of ‘ng’ samples are 417.7nm and 387.1nm, respectively. (Figure 4(b), 4(d)) As can be seen, the pits of ‘g’ samples are smaller than that of ‘ng’ samples. It means etch samples had a different starting point to making the v-shaped pits. The ‘g’ samples were started to make the v-shaped pits during the mid-temperature GaN growth. However the v-shaped pits of the ‘ng’ sample were made as soon as began of the mid-temperature GaN growth. Some of threading dislocation defects were subjected to tensile stress due to heat budget during high-temperature GaN growth, after that v-shaped pits began to form immediately due to the lower growth temperature. As a result, leakage current was expected to occur the v-shaped pits enlarged by the tunneling effect. In order to solve this problem, evaluation was conducted the step by step decreasing the growth temperature during high-temperature GaN growth. Eventually, a advanced result was obtained to control the dimension of v-shaped pits.