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. 2017 Jul 10;7(1):4968.
doi: 10.1038/s41598-017-05391-0.

Ultraviolet GaN Light-Emitting Diodes with Porous-AlGaN Reflectors

Feng-Hsu Fan  1 , Zun-Yao Syu  1 , Chia-Jung Wu  1 , Zhong-Jie Yang  1 , Bo-Song Huang  1 , Guan-Jhong Wang  1 , Yung-Sen Lin  2 , Hsiang Chen  3 , Chyuan Hauer Kao  4 , Chia-Feng Lin  5
Affiliations

Ultraviolet GaN Light-Emitting Diodes with Porous-AlGaN Reflectors

Feng-Hsu Fan et al. Sci Rep. .

Abstract

A GaN/AlGaN ultraviolet light emitting diode (UV-LED) structure with a porous AlGaN reflector structure has been demonstrated. Inside the UV-LED, the n+-AlGaN/undoped-AlGaN stack structure was transformed into a porous-AlGaN/undoped-AlGaN stack structure through a doping-selective electrochemical etching process. The reflectivity of the porous AlGaN reflector was 93% at 374 nm with a stop-bandwidth of 35 nm. In an angle-dependent reflectance measurement, the central wavelength of the porous AlGaN reflector had blueshift phenomenon by increasing light-incident angle from 10° to 50°. A cut-off wavelength was observed at 349 nm due to the material absorption of the porous-AlGaN/u-AlGaN stack structure. In the treated UV-LED structure, the photoluminescence emission wavelength was measured at 362 nm with a 106° divergent angle covered by the porous-AlGaN reflector. The light output power of the treated UV-LED structure was higher than that of the non-treated UV-LED structure due to the high light reflectance on the embedded porous AlGaN reflector.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1
OM images of the (a) UV-LED and (b) EC-UV-LED were observed. (c) The cross-sectional SEM micrograph of the EC-UV-LED structure was observed. (d) A 40.8nm-thick porous-AlGaN layer and 37.7nm-thick u-AlGaN layer were measured in the 12-pair reflector structure. SEM images of the porous AlGaN reflector structure were observed at (e) the cleaved region and (f) the peeling region.
Figure 2
Figure 2
Power-dependent μ-PL spectra of the (a) UV-LED and (b) EC-UV-LED were measured at room temperature by varying the laser excited power density of the 325 nm HeCd laser.
Figure 3
Figure 3
PL emission spectra of (a) the UV-LED and (b) the EC-UV-LED were measured through the angle-resolved PL measurements using a 325 nm diode laser as an excitation laser source. (c) The PL spectra of both of the LED structures were measured at normal direction (at 90°). (d) Normalized PL far-field radiation pattern of both of the LED structures were measured.
Figure 4
Figure 4
Angle-dependent reflectance spectra of (a) the porous AlGaN reflector and (b) the EC-UV-LED structures were measured by varying the detected angles from 10° to 50°. The central wavelength and the band-width at 10° and 50° detected angles were labeled. The reflectance spectra of the non-treated DBR epitaxial structure (DBR-epi), the non-treated UV-LED epitaxial structure (UV-LED epi), and the flat Al2O3 substrate were measured.
Figure 5
Figure 5
The EL emission spectra of the (a) UV-LED and (b) EC-UV-LED were measured by varying the injection current at room temperature.
Figure 6
Figure 6
(a) Light output power and the peak wavelength of the EL spectra of both of the LED structures were measured. After the embedded reflector structure was formed, the EL emission peak wavelengths of the EC-UV LED were slightly redshifted compared with the non-treated UV-LED structure. (b) The I–V curves and the turn-on voltage of both devices are measured.
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