Abstract
In this dissertation, we investigate the fabrication and characterization of high-frequency modulation of GaAs/AlGaAs near-infrared (NIR) light-emitting diodes (LEDs) by using gallium-doped zinc oxide (GZO) prepared by atomic layer deposition (ALD) as the current-spreading layer. The GZO film reveals a low resistivity of ~ 3.3 x 10-4 Ω-cm, a high sheet resistance of 7.8 Ω/□ in the lateral direction. For the GZO contacts to p+-type GaAs, the minimum specific contact resistance of 1.7 x 10-5 Ω-cm2 is obtained. In order to obtain the device RC-limited bandwidth, we performed the current-voltage (I-V) and capacitance-voltage (C-V) measurements on devices with different aperture diameters. The LEDs with the aperture diameter of 59, 84, 109, 134, 159, and 184 μm, and the same bonding pad diameter of 80 μm have a forward voltage of 1.6-1.7 V and the series resistance of 5.3-6.1 Ω at 20 mA with decreasing the aperture diameter. The total capacitance (CT) in the LEDs with different aperture areas at the reverse bias of 5 V is about 11.6, 14.4, 17.9, 21.8, 26.5, and 31.8 pF, respectively. Since the total capacitance is proportional to the area of junction, the pad capacitance (Cp) with aperture diameter of 80 µm is calculated as about 8.8 pF. And the LED reveals a light output power of 4.6, 5.4, 5.7, 5.8, 6.2, and 6.3 mW at 50 mA with increasing the aperture diameter, respectively. The measured peak wavelength (λpeak) of the device biased at 25 mA is about 757 nm. By the design of a ring-shaped electrode overlapping with GZO film, the 3-dB frequency (f3dB) for the NIR LED biased at 50 mA decreases with aperture diameter, and achieves a maximum of 107.8, 90.6, 74.6, 66.8, 48.3, and 45.4 MHz owing to the increase of injected current density into the confined region. However, we evaluate the current spreading of GZO on NIR LEDs from the aspect of minority carrier lifetime. We also analyze the effects of aperture area of the NIR LEDs on minority carrier lifetime and further investigate the required injection current to achieve the 3-dB frequency bandwidth of 100 MHz. As to our fabricated NIR LEDs, the aperture area required to achieve the 3-dB frequency bandwidth of 100 MHz at a driving current of 10, 20, 30, 40, and 50 mA is 1.4 x 10-6, 5.1 x 10-6, 1.7 x 10-5, 3.4 x 10-5, and 5.1 x 10-5 cm2, respectively. At the same injection current density, the LEDs with different aperture areas always exhibit the same minority carrier lifetime, which is inversely proportional to the square root of current density, but not depend on the aperture diameter. It means that the GZO layer plays a good current spreading in the lateral direction from the ohmic contact of NIR LEDs, which shows to the lower contact resistance and lower forward voltage.