Michael D. Camras

Wafer bonding of 50‐mm diameter GaP to AlGaInP‐GaP light‐emitting diode wafers

The feasibility of wafer bonding 50‐nm diameter wafers consisting of GaP‐AlGaInP light‐emitting diode epitaxial films to GaP substrates is demonstrated. Wafer bonding over the entire wafer area is achieved while maintaining optical transparency and low‐resistance electrical conduction at the wafer‐bonded interface. Using this technique, visible‐spectrum transparent‐substrate GaP‐AlGaInP/GaP light emitting diodes (LEDs) are fabricated across an entire 50‐mm wafer with typical operating voltages <2.1 V at 20 mA and twice the flux of absorbing‐substrate GaP‐AlGaInP/GaAs LEDs. This large‐area wafer‐bonding method is further shown to be capable of producing very high efficiency emitters, with an external quantum efficiency of 23.7% (300 K, 20 mA, dc) at 635.6 nm.

Evidence for voltage drops at misaligned wafer-bonded interfaces of AlGaInP light-emitting diodes by electrostatic force microscopy

Electrostatic force microscopy (EFM) with phase detection has been applied to cleaved cross sections of wafer-bonded transparent substrate (TS) AlGaInP light-emitting diode (LED) structures. EFM was performed with the LED under active bias to image the voltage drops across the device layers. Measurements on a nonwafer-bonded, absorbing substrate (AS) AlGaInP LED wafer, showed a voltage drop only at the p–n junction. A TS wafer with high forward voltage (Vf ) showed a much larger voltage drop at the wafer-bonded interface, compared with a normal TS LED wafer. Secondary ion mass spectrometry profiles of these wafers revealed ∼1×1013 cm−2 of carbon at the bonded interface in the high Vf sample, compared to ∼3×1012 cm−2 in the normal wafer. The unwanted voltage drop at the bonded interface was likely caused by a combination of carbon acting as a p-type dopant and the presence of interface states due to a ∼3° in-plane rotational misalignment at wafer bonding.

Pressure dependence of AlxGa1−xAs light emitting diodes near the direct‐indirect transition

High‐pressure studies on high quality AlxGa1−xAs double heterostructure light emitting diodes (LEDs) grown by liquid phase epitaxy (LPE) are presented. The AlxGa1−xAs active region varies in composition from x∼0.25 to x∼0.53, i.e., through the important region of the direct‐indirect crossover (x≡xc≊0.45). The pressure coefficient of the Γ conduction band is observed to decrease (∼1 meV/kbar for x∼0.25 to x∼0.53) with increasing Al concentration, which is in accord with alloy disorder and band‐edge bowing. Indirect‐gap (X) recombination radiation of significant intensity is observed and provides evidence for the high quality of the LPE diodes. High‐pressure measurements, and the corresponding increase in energy of the direct band edge and decrease in the indirect band edge, show that the light emission is a strong function of the carrier distribution in the Γ and X conduction‐band minima. Comparison LEDs fabricated from a crystal (x∼0.37) grown by metalorganic chemical vapor deposition exhibit nearly simil...

High-performance AlGaInP light-emitting diodes

A new class of LEDs based on the AlGaInP material system first became commercially available in the early 1990s. These devices benefit from a direct bandgap from the red to the yellow-green portion of the spectrum. The high efficiencies possible in AlGaInP across this spectrum have enabled new applications for LEDs including automotive lighting, outdoor variable message signs, outdoor large screen video displays, and traffic signal lights. A review of high-brightness AlGaInP LED technology will be presented.