GaN Based p-n Structures Grown on SiC Substrates
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 29
GaN Based p-n Structures Grown on SiC Substrates V.A. Dmitriev Cree Research, Inc. This article was received on June 4, 1996 and accepted on October 30, 1996.
Abstract Wide band gap nitrides(InN, GaN, AlN) have been considered promising optoelectronics materials for many years [1]. Recently two main technological problems in the nitrides were overcome: (1)high quality layers has been grown on both sapphire and SiC substrates and(2) p-type GaN and AlGaN material has been obtained. These achievements resulted in the fabrication of bright light emitters in the violet, blue and green spectral regions [2].First injection laser has been demonstrated [3]. This paper reviews results obtained over the last few years on nitride p-n junctions, particularly on GaN based p-n junctions grown on SiC substrates. We will consider GaN p-n junctions, AlGaN p-n junctions, GaN and AlGaN p-i-n structures, and, finally, GaN/SiC p-n structures.
1. Introduction For all structures discussed bellow 6H-SiC and 4H-SiC wafers were used as substrates. All structures except GaN/SiC heterostructures were grown by metal organic chemical vapor epitaxy (MOCVD) [4]. Hydride vapor phase epitaxy (HVPE) [5] was employed for GaN/SiC p-n heterojunction fabrication. Usually GaN was deposited on (0001)Si face of the substrate (the p-n junction plane was perpendicular to the c crystal axis). In MOCVD grown structures, Si and Mg were used as donor and acceptor impurities, respectively. Ni, Ti or Pd metals was employed to form ohmic contact top-GaN [6] and Al, Ni or Ti/Ni ohmic contacts were formed to n-GaN [7]. Mesa structures were formed by reactive ion etching (RIE) [8].
2. GaN p-n junctions In this section we will discuss characteristics of GaN p-n homojunctions. The first GaN p-n junction was reported in 1989 [9]. Since, significant progress has been achieved on fabrication of GaN p-n structures [10] [11] [12] [13] [14] [15] [16]. However, published information on the main properties of these junctions(i.e., impurity distribution in the junction, built-in voltage, current flow mechanism, breakdown characteristics) still is very limited. We will describe two types of the junctions fabricated on silicon carbide substrates: (1) linear graded p-n junctions and (2) abrupt p-n junctions. Mesa structure diameter ranged from 100 to 300 µm. The p-n junctions were characterised in terms of capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The position of the p-n junction in the structure was determined by electron beam induced current. The concentration of electrically active uncompensated impurities, [Nd-Na], was measured using C-V characteristic of GaN p-n junction or Shottky barrier.
2.1. Linearly graded GaN p-n junctions GaN layers of n- and p-type were grown subsequently in the same epitaxial run (Figure 1). A typical thickness for nand p-layer was ~2 µm and ~0.5 µm, respectively. The concentration Nd - Na in n-layer was ~ 2x1018 cm-3.Na - Nd concentration in p-G
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