Light-Emitting Devices Based on Gallium Nitride and Related Compound Semiconductors
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889 Mat. Res. Soc. Symp. Proc. Vol. 395 0 1996 Materials Research Society
In this paper, high efficiency GaN-based blue and UV LEDs including active layers of undoped GaInN/GaN MQW are reported. The superlattices of GaInN/GaN MQWs are evaluated by SIMS and TEM. The thickness and number of GaInN quantum wells are described in terms of the MQW optical efficiency. EXPERIMENT
GaN epitaxial films were grown on a sapphire substrate by MOVPE using an AIN buffer layer. The Al, Ga, In, and N source gases were trimethylaluminum (TMA), trimethylgallium (TMG), trimethylindium (TMI), and ammonia (NH 3 ), respectively. Silane (SiH 4 ) was used as the Si source gas. The Mg source gas was biscyclopentadienylmagnesium (bis-Cp2Mg). Other growth conditions were the same as the previous report [8]. The Si-doped n÷-GaN layer was approximately 5 ym thick and the carrier concentration was about 1x10 18 cm3. The emitting layer of GalnN was doped with donors or acceptors as recombination centers. The InN molar fraction of GaInN active layer was approximately 0.08. Mg-doped p-AIGaN and p-GaN layers were deposited sequentially on the bulk GaInN or the GalnN/GaN MQW layers and were converted to p-type conduction by LEEBI treatment after the growth. The GaN-barrier and GalnN-well MQW was grown by controlling growth conditions for MOVPE such as the flow rate of gas sources and the growth temperature. Ohmic contacts to the p- and n-GaN were Au/Ni. For evaluating the optical properties of GaInN bulk layer and GaInN/GaN QW layers, cathodoluminescence (CL) and PL measurements were applied mainly at room temperature. The penetration depth of the incident electrons was estimated to be around 100 nm at an accelerating voltage of 5 kV. RESULTS High efficient ADH blue LEDs
The ADH of p-AIGaN/GaInN/n-GaN was fabricated for highly efficient blue LEDs. The detailed structure of the LED chip is shown in Fig. 1. The active layer of GalnN was doped with Zn, etc. Figure 2 shows the electroluminescence (EL) spectra of devices described in Fig. 1 at room temperature. The main peak wavelength was blue emission of 450 nm and the full width at half-maximum (FWHM) of the spectra was 70 nm at room temperature. Ultra-violet (UV) emissions also appeared in the peak wavelength of 385 nm as shown in Fig. 2. The intensity ratio of UV to blue emissions increased with increasing forward current. The UV emission originated from recombinations between shallow donors and the valence band. The multi-peak spectra of blue emission could be understood by the interference of blue light within the GaN-based epitaxial layers. The emission built up at about 1 mA and increased linearly with forward current up to about 25 mA. The luminous intensity was typically 2 cd at 20 mA at a viewing angle of 160 The highest brightness was 2.5 cd at 20 mA and its total output power was 3.6 mW at 20 mA, 3.5 V. The external quantum efficiency was 5.1 % maximum. Figure 3 shows the current and voltage (I-V) characteristic of the ADH blue LED. The lowest forward voltage at 20 mA was 3.3 V and the re
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