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Laser direct writing of GaN-based light-emitting diodes—The suitable laser source for mesa definition
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Image of FIG. 1.
FIG. 1.

Schematic diagram of the LED layer sequence used for laser processing. The epitaxial layers were grown in a single wafer reactor by metalorganic vapor phase epitaxy. The QW active region is designed for an emission wavelength of 400 nm.

Image of FIG. 2.
FIG. 2.

(a) Schematic drawing of a laser processed LED-device, comprising p- and n-contact metallization, mesa definition for electrical isolation, and trenches for mechanical separation. For simplicity, only the n- and p-GaN layers of the LED heterostructure are shown. (b) Simplified LED-processing for fast analysis of relevant parameters. Trenches, n-contact opening, and n-contact metallization are omitted. For electrical characterization, an n-contact in the form of an indium bump is applied.

Image of FIG. 3.
FIG. 3.

Transmission spectra of the 4.4 μm thick LED layer sequence epitaxial grown on a 330 μm thick sapphire substrate (red) and transmission spectra of a sapphire substrate (black). The vertical blue line indicates the laser wavelength of the PLS and NLS, and the vertical violet line that of the EWS.

Image of FIG. 4.
FIG. 4.

Transmission versus axial focus position of the laser beam with respect to the surface of the LED heterostructure. Data were measured using the PLS (λ = 355 nm, τ = 10 ps) for three different pulse energies.

Image of FIG. 5.
FIG. 5.

White light interferometry images of mesa-trenches fabricated around p-contacts using different laser systems. Wavelength and pulse length as well as size of the focus area have been varied as indicated in the figure. The images in the upper row showing expanded views of the corresponding lower image.

Image of FIG. 6.
FIG. 6.

(a) Current-voltage characteristics of a LED with mesa-trenches fabricated by the PLS after different cleaning procedures. (b) Corresponding output power vs. current characteristics.

Image of FIG. 7.
FIG. 7.

(a) Spectrum of the defect luminescence from a mesa-trench fabricated using the PLS under −20 V reverse bias conditions. The luminescence spectrum was measured before applying a cleaning or annealing procedure. (b) Intensity of the defect luminescence at −20 V before and after different cleaning procedures and the corresponding leakage current at −5 V. The upper image on the right hand side shows a microscope image of a p-metal contact, surrounded by a mesa-trench. Spotty blue defect luminescence is seen at the trenches at −20 V. The lower image shows the same device after the cleaning and annealing procedure. At −20 V, no defect luminescence was observable.

Image of FIG. 8.
FIG. 8.

Comparison of the reverse bias leakage currents at −5 V for different mesa definition methods. Each data point shows the average over data taken from 8 devices; the error bars indicate the resulting standard deviation. By annealing the LEDs, a further reduction of the leakage current is achieved. The leakage current of the samples processed at 355 nm can be lowered to that of conventional dry etched ones, whereas the samples processed at 248 nm show even without annealing leakage currents as low as dry etched reference devices.

Image of FIG. 9.
FIG. 9.

(a) Microscope image of p-metal contacts deposited onto a LED wafer with mesa-trenches fabricated by the EWS. All five LEDs have the same area of the contact, whereas the perimeter is increasing from left to right. (b) I-V-characteristics of the LEDs shown in (a). (c) Corresponding P-I-characteristics and the emission spectrum as an inset. The LED with the spiral layout was not functional when fabricated using the EWS process and had thus to be omitted here in (b) and (c).

Image of FIG. 10.
FIG. 10.

Leakage current at −5 V vs. perimeter for LED devices containing mesa-trenches fabricated by the PLS (355 nm, 10 ps) (a) and (b) and by the EWS (248 nm, 20 ns) (c). The straight line represents a linear least square fit through the data to obtain the slope and the y-intercept. After annealing the devices fabricated by the PLS, a distinct reduction in leakage current is observable, whereas annealing of devices processed by the EWS is not necessary.

Image of FIG. 11.
FIG. 11.

Comparison of a conventional fabricated mesa LED and two fully laser processed LEDs, whereas the mesa-trench of the first one was fabricated using the PLS (355 nm, 10 ps), the second one using the EWS (248 nm, 20 ns). (a) I-V-characteristics and (b) P-I-characteristics of all three devices. The inset shows the electroluminescence image observed through the sapphire substrate for the LED processed using the PLS.

Image of FIG. 12.
FIG. 12.

Electroluminescence spectra of the conventional fabricated mesa LED and the two fully laser processed LEDs at a driving current of 20 mA.


Generic image for table
Table I.

Leakage currents for varying mesa definition methods, separated into areal leakage current through the p-n-junction and leakage at the mesa sidewall. Values marked by (*) were taken from the results obtained by fitting the devices processed by EWS (248 nm, 20 ns) and were fixed during linear least square fit.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Laser direct writing of GaN-based light-emitting diodes—The suitable laser source for mesa definition