Spatially Resolved Electroluminescence of InGaN-MQW-LEDs
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DUCTION EL is the major characterization technique for LEDs probing the active region of the whole device under operation. However, as an usually integral measurement it averages over the whole LED. An even deeper insight can be obtained employing spatially and spectrally resolved EL (µ-EL). This technique is used to gain valuable insights into GaN-based LEDs. It is a non-destructive and fast method which provides information about the full device including epitaxial growth and processing. The resulting EL data yield full spectra at each point of investigation at a 1 µm resolution. The recorded data set allows to extract the distribution of the peak wavelength, the intensity, the linewidth and the presence of other recombination paths (DAP, non-radiative, etc. ). Those data can be correlated with a conventional microscope image to compare morphology and defects from growth and/or processing with the EL distribution. Furthermore, EL features such as peak wavelength and intensity are depending on local device temperature and current injection, respectively. The disclosed device properties might be used to improve spatial and spectral emission characteristics of LEDs. Within the present work EL-homogeneities, In-fluctuations, heat generation due to ohmic losses, current injection homogeneity, and transparent contacts are investigated. Experimental InGaN/GaN-LEDs emitting near the UV are grown by low pressure MOVPE in a horizontal reactor (AIXTRON AIX 200 RF) on c-plane sapphire substrates. The structure consists of 2000 nm GaN and 1000 nm Si-doped GaN followed by the active region, a
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100 nm Mg-doped Al0.08Ga0.92N layer, and finally 300 nm p-doped GaN. The active region containes either a 50 nm thick In0.09Ga0.91N-layer (double heterostructure) or 3 periods of In0.09Ga0.91N/GaN (MQW structure). Conventional photolithography is used to define the mesa structure. Chemically-assisted ion-beam etching (CAIBE) transferred the pattern using a conventional photoresist mask. A second lithographic step defines the n- and p-contact area, using lift-off technique and subsequent Ni/Au metallization for ohmic contact formation. The resulting LEDs reveal narrow dominating peak emission at about 400-410 nm. The devices have series resistances of approximatly 30 Ω. The spatially resolved µ-EL system is based on an optical microscope using UVtransparent lenses with a long working distance and computer controlled scanning stages. A DC-motor driven scanning stage enables the scanning with a resolution of 250 nm. The µ-EL is spatially resolved detected. For µ-EL an overall spatial resolution of 1 µm is obtained. The electroluminescence is dispersed in a 0.5 m spectrometer and detected by a liquid nitrogen cooled Si-CCD camera. The spectral resolution for µ-EL measurements reported here is 0.5 nm. All µ-EL measurements are performed at room temperature. LEDs are investigated from both the p-contact- and substrate-side (hereafter called front- and backside). A complete spectrum is recorded at each pixel (x,y) and stored during the
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