TEM and PL characterisation of MBE-grown epitaxial GaN/GaAs
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Ej,. PL spectra of epitaxial GaN grown on (001), (l1l)Aand (iii)B GaAs substrate.
a ý UBill Ejg..2(a) Bright field TEM cross-sectional image of GaN/[Ii})B GaAs showing high density of threading defects; [1120] SAD pattern is inset.
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Ejg,.2(b) HRTEM image showing the interfacial region. The initial deposit is wurtzite GaN, while small domains of zincblende GaN are present.
Fg. (c) Plan view image of GaN epilayer showing mosaic structure.
slightly nitrogen rich growth. Epitaxial GaN was also grown on (001IGaAs under an As flux at the same temperature. In this instance the substrate was cleaned in the presence of N flux at 620"C. Growth with As was then initiated by opening the Ga and N shutters, with the As flux being held constant throughout the growth run. For the photoluminescence (PL) measurements, the samples were mounted on the cold finger of a continuous-flow He cryostat in which the sample temperature could be varied between 7K to 300K. PL from the GaN samples was excited using a Kimmon He-Cd laser at 325nm, dispersed with a 0.75m monochromator and detected by a bi-alkai photomultiplier. The PL resolution was typically in the range of 1-2meV. TEM characterisation was performed in cross-section using a JEOL 4000EX-II for high resolution electron microscopy (HRTEM), and a Philips CM30 microscope for conventional microscopy and Energy Dispersive X-ray (EDX) analysis. The crystal polarity for all of the samples examined was determined using convergent beam electron diffraction (CBED). 312
Results and Discussion
PL spectra
from the three samples grown on
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(001), (111)A and I III)B GaAs are shown in VM aI Fig. 1. For the GaN grown on the (III)B substrate, the PL is dominated by the recombination of excitons bound to shallow donors, indicative of the good overall quality of the layer. For the other samples, however, additional defect/impurity related features are I clearly present. However, it should be noted that we did not observe the typical deep (yellow) emission in any of the three samples. The microstructure of the epilayer grown on {III }B, illustrated in Fig. 2a, comprises a very high density of threading defects. The [1120] selected area diffraction (SAD) pattern inset in Fig. 2a confirms the epilayer to be mainly 50 nm b wurtzite single crystal. The striations parallel to Fig...3(a) Low magnification bright field TEM the growth surface in the top left corner of this images of GaN/t 111) A GaAs showing the figure correspond to a small region of zincblende presence of an amorphous interlayer. (b) Bright GaN. Fig. 2b shows an HRTEM image of the field image of the epilayer showing a high density GaN/{III})B GaAs interface region. It is of threading defects. The [1120] SAD pattern is evident that the substrate surface is rough, with inset. undulations on the scale of several nanometres. Alth
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