The First Nitride Laser Diode on Silicon Carbide
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layers. The devices were fabricated into index-guided structures and cleaved into bars corresponding to a laser cavity length of 500 jtm prior to delivery to NCSU. Laser testing at NCSU consisted of (a) spectral optical emission versus current measurements, (b) light output versus current measurements, and (c) polarization measurements of the light output versus current. The laser samples were cleaved at Cree Research to produce bars containing 15 to 20 laser test structures. Each bar was cemented to a copper heat sink using conducting epoxy and transferred to a micro-manipulator platform equipped with the necessary electrical probe tips. Spectral light emission studies versus current were conducted by focusing the light emitted from the test structure onto an optical fiber connected to an Acton 0.5 meter spectrometer equipped with a diode array detector and with 150, 600, and 2400 line/umm gratings for low, medium and high resolution scans, respectively. The spectrometer wavelength range scan and the output spectrum from the diode array detector was controlled and stored by computer. With this arrangement, spectral emission measurement at a given current could be obtained in 1 to 2 s. A Hewlett Packard 214B pulse generator was used to electrically drive the laser test structures. All of the measurements reported here were obtained using a duty cycle of 10-3 and a pulse frequency of 104 Hz. A calibrated resistor in series with the laser structure, along with an oscilloscope, were used to measure the current in the laser circuit. The voltage across the laser sample was also monitored with the dual channel oscilloscope. Care was taken in the design of this measurement circuit in order to minimize ringing effects associated with impedance mismatch. A calibrated UV-enhanced Si photodiode placed directly in front of the laser sample was used to monitor the light output versus current from the device. The current produced by the device-illuminated photodiode was measured using an EG&G PARC lock-in amplifier, which was converted to an equivalent de current using the standard conversion formula. These measurements were cross-calibrated with similar measurements conducted independently at Cree Research to insure that accurate absolute values of the laser light emission versus current were obtained. Polarization measurements were made by placing a linear polarizer between the laser sample and the photodiode detector. These measurements were taken at 5-degree increments as the polarizer was rotated through a full 360 degrees. Far-field light emission patterns from the laser samples were obtained by placing a Polaroid film holder 4 cm in front of the laser structure. The exposed film was then scanned by computer and enhanced to show maximum detail. The scanned images were then used to obtain estimates of the horizontal and vertical beam divergences. RESULTS AND DISCUSSION Prior to June 5, 1997 a number of laser test structures were measured but failed to produce laser action. Fig. 1 shows a typical light emission spectrum reco
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