Very High Efficiency GaP Green Light Emitting Diodes

Abstract

GaP green light emitting p-n junction material with an average quantum efficiency for uncontacted diced diodes in air at 7 A/cm 2 of 0.101% has been reproducibly grown in a vertical dipping system using a modified overcompensation process. This value corresponds to an average efficiency of 0.16% for epoxy encapsulated mesa diodes, which is 60% higher than previously obtained. Mesa diode efficiencies as high as 0.230% at 5 A/cm 2 and 0.67% at ~3'0:0 A/cm 2 were obtained. The low current value is the highest reported to date while the high current value is equal to the highest previously reported. These efficiencies were obtained through the introduction of several modifications in the previous overcompensation growth procedure. It was determined that a large cooling interval, from 900 ~ to 700~ with a corresponding decrease in the temperature at which the p-n junction is formed (850~ was the most important modification that resulted in the efficiency improvement. Minority carrier lifetime measurements revealed spatial variations of the lifetimes in both n-and p-layers. The lifetimes increased with decreasing growth temperature, indicating that LPE growth temperatures as low as possible should be used. Typical values were Th ----200 nsec and Xe ~ 100 nsec at the junction. An analysis of the exit gas from the LPE growth system lets one estimate that as much as ~5 ppm 02 could typically be present in the LPE growth system. Deliberate additions of small oxygen concentrations >10 ppm to the inlet gas produced a pronounced reduction in the quantum efficiency and minority carrier lifetimes while oxygen additions < 10 ppm had no effect on the efficiency or lifetimes.

Oxygen Gettering in Green GaP : N LED's Grown by Overcompensated LPE

“…The refinement of our LPE technique with O getters increased the diffusion lengths still further. This is consistent with the recent results of Lorimor et al (15) who report that deliberate addition of small amounts of oxygen to the inlet gas produced a pronounced reduction in the quantum efficiency and minority carrier lifetimes.…”

“…However, associated complexes of oxygen such as Si-O (13), VG~-O (14), etc., may play a role in the recombination efficiency in both the n-and p-layers of an LED. Evidence to support this hypothesis has been reported by a number of workers (15)(16)(17)(18). It is therefore clear that in order to improve the EL and generate purer spectra in green LED's all traces of oxygen must be removed from both the n-and p-layers.…”

“…Residual traces of oxygen in the Ga melt or oxygen present in the gas stream could be further reduced by using O getters such as Mg, A1, or St. Reactions between these getter elements and 02, H20, and Ga203 are also listed in Table I (reactions 9, 10, [14][15][16][18][19][20].…”

Effect of dislocations on green electroluminescence efficiency in GaP grown by liquid phase epitaxy

“…4, the etch-pit denSity in the substrate can range from (a) -5X 10 4 cm-2 near the center , to (c) ~7X105 cm-2 near the edge of a slice o Moreover, the localized dislocation distribution is often nonuniform o Dislocations tend to form a cellular substructure, with walls of high dislocation density surrounding nearly dislocation-free areas o This behavior is particularly conspicuous for regions (b) midway between the center and the edge; within the walls, the dislocation density can be ~ 1 x 10 6 cm-2 o (Note the Similarity to the dislocation configurations in Fig. 10 ) In addition, bands of very high dislocation density (locally up to ~ 3 X 10 6 cm-2 ) lying along (211) directions and originating at intervals around the slica periphery are observed (d); it is believed that these bands may serve as sites for fracture initiation which can occur during chemical polishingo 20 The ordered dislocation arrays (observed also by x-ray topography20) in the LEC GaP wafers represent lower energy configurations than random dislocations and are reminiscent of substructures in metals which have been extenSively discussed o 21 Formation of the ordered arrays requires dislocation climb and thus necessarily takes place at elevated temperatures where gallium and phosphorus vacancies and/or interstitials have appreciable concentrations and mobilities 0…”

Effect of n--layer thickness on green-light-emitting efficiency in nitrogen-doped GaP grown by liquid phase epitaxy

“…As already discussed in an earlier paper [7], the photoluminescence intensity is proportional to the minority-carrier lifetime, majority-carrier concentration and the nitrogen concentration in the considered doping range. The measured minority-carrier lifetime [8] of holes in the n-type crystal near the p-n junction is τ h = 110-250 ns, which is about 1.3-2 times of the minority-carrier lifetime of an electron in the p-type crystal (τ e = 85-185 ns). While the majority-carrier (Zn) concentration in the p-type region is about 10 times that of the majority carrier (S) in the n-type region, the photoluminescence intensity of the p-type region will be distinctively higher than that of the n-type region when there is a comparative nitrogen concentration doped on both sides of a p-n junction.…”