The group Ⅲ nitrides of InN, GaN, AlN possess direct bandgaps of ≒1.9eV ≒3.4eV and ≒6.2eV, respectively. Because of their direct band gap, wide-band-gap III-Ⅴ nitrides are of increasing interest for applications in short-wavelength optical devices, or high power and high frequency electronics. Due to the lack of appropriate substrate, GaN is generally grown on sapphire (α-Al2O3(0001)) or silicon carbide (SiC(0001)) heteroepitaxially. However large lattice mismatch and coefficient of thermal expansion between GaN and sapphire lead to the high defect density in the GaN films causing degradation of optical property. And difficulties with p doping of GaN has slowed the development of GaN technology and delayed the application of this material in devices. Mg is the best candidate among all possible dopants to obtain p doping, but thermal annealing needs to be applied to dissociate Mg-H complexes and activate the Mg atoms. However, general understanding of the behavior of this dopant is still not satisfactory.
In this paper, effects of Mg doping level on structural properties of metal organic chemical vapor deposition (MOCVD) grown GaN-based heterostructure samples in a LED structure were investigated. As increasing the Mg flux, hexagon-like islands were observed on the surfaces and surface roughness is increased. The size and density of these islands were increased with the Mg flux. Full width at half maximum (FWHM) values of (002) Ω rocking curves for the samples were increased and decreased again as increasing the Mg flux. These values were belonging to a narrow range of from 250 to 290 arcsec. However, FWHM values of $(10 \bar{1} 1)$ Ω rocking curves were continuously decreased as increasing the Mg flux, These values were ranged from 840 to 490 arcsec. The trend of Ω rocking curves with the Mg flux is different from previous observations generally reported. Structural properties including mosaicity angles and dislocation densities were investigated in detail for a series of samples in a LED structure with various Mg doping levels.
In addition, for a Mg concentration above a certain threshold, inversion domain boundaries (IDBs) were formed. So the surface of Mg doped GaN layer is inverted to the N polarity GaN from the Ga polarity GaN. Maybe N polarity surface GaN caused the hexagon-like islands in the surface of Mg doped GaN layer.
3족 질화물은 직접천이형 와이드 밴드갭 물질로 광전자 소자로의 활용가능성이 기대되고 있다. 그러나 GaN는 GaN 성장을 위한 적절한 기판의 부재로 인해 사파이어 기판 위에서 성장시키게 되는데 이 때 GaN와 사파이어 기판 사이의 열팽창 계수의 차이와 격자상수의 차이로 인해 수많은 결함이 존재하게 된다. 또한 고효율의 발광소자 제작을 위해서는 낮은 저항값과 높은 홀 농도를 가지는 p-type GaN가 필수적이고 이는 보통 Mg을 도핑함으로서 얻어진다. 본 연구에서는 LED구조를 가지는 GaN 박막의 Mg 도핑 농도를 달리하여 Mg의 도핑농도에 따른 GaN의 미세구조를 살펴보았다. 그 결과 Mg의 도핑 농도가 높아짐에 따라 나선 전위의 밀도는 증가하다 다시 감소하였지만 그 변화폭은 크지 않았다. 그러나 칼날 전위의 전위 밀도는 Mg의 도핑 농도가 증가함에 따라 지속적으로 감소하는 것으로 나타났다. 또한 Mg의 도핑 양이 어떤 임계값을 넘어서면 inversion domain boundary가 생성되었으며 그 결과 GaN의 극성이 Ga 극성에서 N 극성으로 바뀜을 알 수 있었다. 이러한 N 극성의 GaN 형성으로 인해 GaN의 표면에 육각형 형태의 hillock이 생성됨을 알 수 있었다.
