Effect of O Impurity on the Properties of InGaN/GaN Multiple Quantum Well and Light Emitting Diode Structures

ABSTRACT

               InGaN/GaN light emitting diode (LED) structures with different GaN barrier growth temperatures have been grown by metalorganic chemical vapor deposition (MOCVD). Atomic Force Microscopy (AFM)-Conductive AFM (CAFM) analysis has been performed to study the submicron scale electrical properties of the LED structures. The effect of high O impurity concentration, introduced during the low temperature barrier layer growth, on the MQW LED structure has been explored. It is observed that (V_Ga-O_N)^2- point defects formed in the MQW layer serve as deep-level traps and lead to defect-assisted tunneling. Increasing barrier growth temperature decreases O impurity incorporation and thus improves the device performance. 

INTRODUCTION

               InGaN/GaN MQW structures grown by MOCVD have been widely studied in the solid state lighting field for the past few decades. The role of introduced impurities and point defects formed during the epitaxial growth process and their effects on the optical properties of MQW structures have drawn a great deal of attention. Several previous investigations have been done on the effects of GaN barrier growth temperature on the characteristics of InGaN/GaN MQWs [1-3], in which the InGaN quantum wells were grown at low temperature from 700-760^oC, while the growth temperature of the GaN barriers was varied over the range of 700-950^oC. It was suggested that GaN barriers grown at temperatures below 800^oC leads to the generation defects and In inclusions and higher barrier growth temperature up to 910^oC improves the crystal quality of GaN barrier by suppressing the formation of deep level related defects.

               In the present work, the electrical properties of InGaN/GaN LED structure with same barrier growth temperature as the well temperature is studied. The effects of induced O impurity are discussed.

EXPERIMENT

               The InGaN/GaN MQW and LED samples were grown on (0001) sapphire substrates with low temperature GaN buffer layers using Veeco K465i MOCVD reactor. 2.2µm undoped GaN layer and 2.7µm n-GaN layer were grown at 1085^oC. 4 pairs of In_0.14Ga_0.86N/GaN (2.6 nm/10 nm) quantum wells were grown at 760^oC for LED structure Al. 6 pairs of such quantum wells were grown at 760^oC and 860^oC for the InGaN wells and GaN barriers, respectively, for LED structure B1. 20nm p-AlGaN layer and 95nm p-GaN layer were grown at 980^oC on top of the MQW structures to form the LED structures.     

               AFM analysis was done on a Bruker Icon system and by using TESPA probe. CAFM module is mounted to the AFM system for conductivity studies of the sample. Extended-tunneling AFM tip holder and PFTUNA probes are used for AFM-CAFM analysis.

EXPERIMENT RESULTS

               An AFM-CAFM study on LED structure with various reverse biases has been done on A1 and shown in Figure 2. Notice that in Figure (b) and (c), the color bar is from 450nA to 300nA, while 450nA to -400nA in Figure (d). High density of spots of early breakdown has been observed when the reverse bias increases. The high reverse current at those spots may owe to the defect-assisted tunneling by deep-level traps located in the MQW active layer [4]. This is consistent with the high O concentration and high point defects density in the MQW layer reported in Ref. [3]. These point defects cause leakage and deteriorate the device performance [5].

CONCLUSION

               In summary, the InGaN/GaN MQW LED structure with 760^oC quantum barrier growths has been investigated by AFM-CAFM system to study the submicron electrical properties of the sample. High density of spots of early breakdown caused by defect-assisted tunneling by (V_Ga-O_N)^2- complex point defects located in the MQW layer. These point defects cause leakage and deteriorate the device performance.