Scanning Tunneling Microscope-Induced Luminescence Studies of Defects in GaN Layers and Heterostructures
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The STL of GaN has been reported using both tunnel injection [6-9] and field emission operation. [10] We have recently demonstrated the STL imaging of GaN layers [7,8] and InGaN/GaN multiple quantum wells (MQW). [9] In this proceeding we combine those findings with previously unpublished data, and emphasize their relevance as proof of concept of the STL analysis of defects. We first discuss the cross-sectional imaging of InGaAs/GaAs quantum dots (QDs). We then particularly report the first nanometer-scale luminescence imaging of threading dislocations in GaN layers, and the imaging of 100 nm-scale sub-surface fluctuations of luminescence in InGaN/GaN quantum wells. EXPERIMENT The experimental system consists of a JEOL 4500 low temperature UHV STM fitted with single photon counting electronics. [7,8] The system also features a liquid helium cooled finger allowing sample temperatures of T= 20-30 K. Tungsten tips are prepared using a NaOH electrochemical etch technique. The insertion of interference filters featuring bandpasses of Ak, = 50-70 nm provides a coarse spectral resolution. Unless otherwise noted, all STM/STL results are obtained from the top surface under liquid helium cooling. The luminescence morphology is initially assessed with CL using a modified Zeiss 960 SEM. [1 ] Samples are grown on sapphire by metalorganic chemical vapor deposition (MOCVD). The GaN sample consists of a first 20 nm layer of unintentionally doped (u.i.d.) GaN buffer, followed by a 0.5 pm layer of u.i.d. GaN, and a 2 pm thick Si-doped GaN. The sample features a doping concentration of n = 2 x 1018 cm-3, and a mobility in the vicinity of R = 200 cm2 IVs. The InGaN/GaN MQW sample consists of 12 successive repetitions of 30 A of In 0 .2Ga 0 .8N and 45 A of Si-doped GaN. A 100 nm u.i.d. GaN layer caps the heterostructure. CROSS-SECTIONAL STM/STL OF InGaAs/GaAs QUANTUM DOTS Figure 1 shows cross-sectional images of 1n0.3Ga 0.7As/GaAs quantum dots obtained at room temperature with a tip bias of Vt=-2.3 V and a constant tunnel current of I,=2 nA.
a)b) 0
Topography
584nm Luminescence (max=1OOOcps)
Fig. 1. Cross-sectional STM/STL images of Ino.3Gao. 7As\GaAs QDs at roomtemperature. a) Topography. b) Unfiltered luminescence. Image acquired on insitu cleaved facet. Growth direction is from bottom-left to top-right. The QDs are visible as a string of beads in the top right of the topographic image. These protrusions are attributed to an increase of density of states. The detector is not sensitive to the wavelength emitted by the InGaAs dots. The QDs therefore appear as dark features in the resulting luminescence image. The non-luminescent areas are larger than the topographical features, suggesting the lateral diffusion and trapping of carriers in the dots. 20
Carrier diffusion still limits our ability to resolve nearby features. However, the issue remains an intrinsic material property rather than imposed by a technique limitation such as the size of generation volume. The luminescence map therefore contains factual information about carrier
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