Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular dopant concentration or concentration profile
Reexamination Certificate
2002-01-28
2004-11-09
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular dopant concentration or concentration profile
C257S079000, C257S085000, C257S094000, C257S102000, C257S103000, C438S087000, C438S231000
Reexamination Certificate
active
06815730
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor light-emitting devices (including light-emitting diode (LED) and laser diode (LD)) and particularly to improvements in operating voltage, luminous efficiency, lifetime, and yield of nitride-based semiconductor light-emitting devices.
2. Description of the Background Art
Nitride semiconductor materials such as GaN, InN, AlN, and mixed crystals thereof have a band gap where direct interband transition occurs. In particular, a mixed crystal of InGaN can emit radiation in the wavelength range from red to ultraviolet, and accordingly attracts attention as a material for short-wavelength radiation. A light-emitting diode capable of emitting radiation in the wavelength range from ultraviolet to green has already got practicability by utilizing the mixed crystal of InGaN. Further, a bluish violet laser diode achieves a lifetime longer than 10,000 hours under a condition of continuous lasing at the room temperature. As such, semiconductor light-emitting devices for short-wavelength radiation are making rapid progress toward commercialization thereof.
One of factors for such rapid progress is that ELOG (Epitaxial Lateral Over Growth) technique can reduce the dislocation density in a nitride-based semiconductor layer. That is, it has been found in recent years that application of ELOG technique to growth of a GaN layer on a sapphire substrate is effective in reduction of dislocations which are generated when the GaN layer is grown by HVPE (Hydride Vapor Phase Epitaxy) method. The GaN layer grown by ELOG technique includes less defects of threading dislocations and the like. It is accordingly reported that an LD produced by using such a GaN layer can exhibit a longer lifetime. On the other hand, it is proposed to use a thick film of GaN produced by HVPE as a substrate. The substrate of such a thick GaN film can be used to reduce crystal defects in a nitride-based semiconductor layer grown on the substrate by metal-organic chemical vapor deposition (MOCVD) etc., promising a longer lifetime of a resultant nitride-based semiconductor light-emitting device.
Although currently produced GaN-based substrates include dislocation defects reduced to some degree by utilizing the ELOG technique etc., they still have a considerably higher dislocation density than that of other group III-V compound semiconductor substrates such as GaAs substrate. Moreover, N and Ga are likely to escape out of the GaN substrate, especially out of the substrate interface, due to a high equilibrium vapor pressure of nitrogen, which causes an increased defect density. Therefore, a nitride-based semiconductor light-emitting device formed by MOCVD on a GaN substrate still contains lots of crystal defects. Such defects act as centers for non-radiative recombination, and the defective portions serve as current paths to cause current leakage. Here, a problem is that light-emitting devices containing lots of crystal defects need higher drive voltage and result in less yield.
In particular, crystal defects in an LD increase the threshold current density and then shorten the lifetime of the LD, and thus reduction of the defect density is important. There also exists a problem that light-emitting devices fabricated on a wafer produced by ELOG have respective emission outputs greatly different from each other depending upon their position on the wafer, because the dislocation density in the wafer is higher in some regions and lower in the other regions. Then, emission patterns were observed in light-emitting devices with emission outputs more than 2 mW and light-emitting devices with emission outputs less than 0.5 mV that were fabricated on the same wafer. It was found that the devices of lower outputs cause non-uniform radiation in which dark and bright portions were mixed. In addition, the lower-output devices had their shorter lifetimes and 90% thereof stopped emission shortly after electric current is supplied. Due to this, the total yield of the devices was as low as about 45%. The dark portions in the lower-output devices correspond to regions with high dislocation density in the GaN substrate, and it is considered that the defects in the GaN substrate affect the dark portions.
SUMMARY OF THE INVENTION
In view of the problems in the prior art discussed above, an object of the present invention is to improve the operating voltage, luminous efficiency, lifetime, and yield in the nitride-based semiconductor light-emitting devices.
A nitride-based semiconductor light-emitting device according to the present invention includes a semiconductor stacked-layer structure including a plurality of nitride-based semiconductor layers grown on a GaN-based substrate by vapor phase deposition. An interface region of the GaN-based substrate contacting the semiconductor stacked-layer structure contains oxygen atoms at a concentration n in the range of 2×10
16
≦n≦10
22
cm
−3
, and then the semiconductor stacked-layer structure has a lower crystal defect density as compared with that in the case that the interface region does not contain oxygen atoms at such a concentration n.
The GaN-based substrate may contain at least one of chlorine and oxygen. A nitride-based semiconductor layer included in the semiconductor stacked-layer structure, which is in direct contact with the GaN-based substrate, may contain oxygen.
With reference to
FIG. 1
, an explanation is here given regarding oxygen doping in the interface region of the GaN substrate that is in contact with the nitride-based semiconductor stacked-layer structure.
FIG. 1
shows SIMS (secondary ion mass spectrometry) profiles obtained by oxygen doping in the vicinity of the interface between the GaN substrate and a nitride semiconductor layer grown thereon by MOCVD. In this graph, the horizontal axis represents layer thickness (nm) and the vertical axis represents concentration (cm
−3
) of oxygen atoms. The layer thickness of 0 nm represents a surface when SIMS is started, and the oxygen atom concentration of 10
16
cm
−3
corresponds to the concentration without positive or effective doping of oxygen atoms. Any ion concentration lower than 10
16
cm
−3
is difficult to identify due to noise in SIMS.
The oxygen doping in the present invention is effective in relaxing strain caused in the interface region between the substrate and the crystal growth layer and preventing deterioration of crystallinity from being caused by N escape, Ga escape, etc. in the vicinity of the interface. In this case, the interface region of the substrate that contacts the crystal growth layer may have a thickness of single-atom layer to be doped with oxygen. However, the interface region is preferably doped in a thickness range that is likely to suffers damage during new crystal growth. Specifically, the advantage discussed above becomes clear when the interface region is doped in a thickness of at least 1 nm and becomes clearer when doped in 20 nm thickness. The interface region may be doped in a thickness exceeding 20 nm, but the doping effect with such a large thickness does not show much difference.
FIG. 1
shows SIMS profiles obtained by measuring oxygen distribution near the interfaces when oxygen atoms are added to the interface regions of at least 15 nm thickness in the substrates. SIMS measurement does not have a high accuracy with respect to the thickness direction and thus it is considered that oxygen would be observed in a range slightly greater than that of the region to which oxygen atoms are actually added.
The profile represented by curve A in
FIG. 1
is obtained actually by adding oxygen atoms to a GaN buffer layer formed directly on a GaN substrate for fabricating a light-emitting device, and thus oxygen atoms are not directly added into the substrate. Regarding curve A, therefore, it is considered that the oxygen atoms diffuse into the formed substrate due to thermal hysteresis of heating during fabricating the light-emitting device on the substrate. Similarly, th
Louie Wai-Sing
Morrison & Foerster / LLP
Pham Long
Sharp Kabushiki Kaisha
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