Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2000-03-27
2003-12-16
Thomas, Tom (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S078000, C257S094000, C257S096000, C257S103000, C257S743000, C257S744000, C372S043010, C438S602000, C438S603000
Reexamination Certificate
active
06664570
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the p-type contact electrode devices usable for the light emitters such as semiconductor lasers and light emitting diodes, and to the light emitters using such p-type contact electrode devices.
2. Related Art Statement
ZnSe-based semiconductor lasers have been investigated as blue and green light emitting elements (“Zn-Se based semiconductor lasers” in OPTRONICS, No. 4, pp 116-119, 1997). The semiconductor lasers and the light emitting diodes with short wavelengths ranging from green to blue have attracted attention as light sources for highly bright displays and optical recording apparatuses.
Group II-VI compounds composed of an element in Group II such as Zr, Be, Cd or Mg and an element in Group VI such as S, Se or Te are now expected to be practically used for semiconductor light emitters in the green-blue color range, such as semiconductor lasers and the light emitting diodes with short wavelengths ranging from green to blue.
Such a light emitter is usually formed by depositing thin layers such as p-type and n-type guide layers, and clad layers, an active layer, a contact layer and a cap layer on a substrate made of GaAs or the like according to a molecular beam epitaxy (MBE) method or a metal organic chemical vapor deposition (MOCVD) method.
In the former ZnSe-based light emitters, a p-ZnSe—ZnTe based device is used in a p-contact layer of a ZnSe-based II-VI blue-green laser diode to overcome the low conductivity of the p-type ZnSe and reduce the threshold voltage to less than 5 volts. However, as much as 7% lattice mismatch exists between ZnSe and p-ZnTe, so that misfit dislocation and strain occurs to raise a defect density thereof.
Former investigations on the degrading mechanism of the ZnSe laser diodes suggest that the strain caused by this contact layer plays an important role, and that a long life span is ensured by reducing the strain induced by an electrode. Based on such experiences, it can be presumed that the strain caused due to such a large lattice mismatching (7.1%) between ZnSe and ZnTe plays an important role for a degrading mechanism of the light emitting diode. For example, even if no dislocation exists in the layer at the beginning, it is anticipated that the misfit dislocation is caused by the large lattice mismatch during flowing current, when a long time passes, so that the misfit dislocation is introduced into the lower layer.
Be-based laser structural bodies have been proposed to prolong the life span of the II-VI ZnSe based bluish green laser devices. It is described that ZnSe/BeTe super lattices are used instead of ZnSe/p-ZnTe (Journal of Crystal Growth, 184/185, pp 1-10, 1998).
The present inventors examined the use of a single BeTe epitaxial layer as a p-type contact layer. The BeTe epitaxial layer has a high p-type doping property (about 10
20
cm
−3
) and a lower lattice mismatch to a GaAs substrate (0.54%).
However, if a single BeTe layer is used as a p-type contact layer, it was clarified that the following problem existed.
For example, when an intermediate product was obtained by forming a BeTe epitaxial layer on a p-type clad layer of a light emitting layer and then this intermediate product was left in air for one month, a BeTe peak of a (004) X-ray locking arm curve completely disappeared. This is considered that since the Be chalcogenide is hygroscopic, the intermediate product was oxidized after absorption of moisture.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a p-type contact electrode device in a ZnSe-based II-VI compound semiconductor in which a BeTe layer having a high p-type doping property and a lower lattice mismatch to a GaAs substrate is used as a contact layer to prevent the oxidation thereof in air.
(Countermeasure to Solve the Problems)
The present invention relates to the p-type contact electrode device in a ZnSe-based II-VI compound semiconductor, said p-type contact electrode device comprising a contact layer composed of a p-BeTe, a cap layer of a p-ZnSe on the contact layer and an electrode on the cap layer.
The present inventors' succeeded in preventing the oxidation of the contact layer by providing the BeTe contact layer and further providing the p-ZnSe cap layer on the contact layer through utilizing a high p-type doping property and a lower lattice mismatching to a GaAs substrate and an n-ZnSe substrate of the p-BeTe. By so doing, it was clarified that no degradation occurred in the contact layer in 2 to 3 weeks.
The thickness of the cap layer is particularly preferably 30 Å or more from the standpoint of view of preventing the oxidation of the contact layer. On the other hand, in order that the p-type contact electrode device may exhibit a preferable ohmic current-voltage characteristic, the thickness of the cap layer is preferably not more than 70 Å, more preferably not more than 60 Å.
A material for the electrode to directly contact the cap layer is preferably gold, platinum, palladium or an alloy thereof.
REFERENCES:
patent: 6147365 (2000-11-01), Fischer et al.
P.M.Mensz, BeTe?ZnSe graded band gap phmic contacts to p-ZnSe, Appl. ohys. Lett., 64 (16), Apr. 18, 1994, pp 2148-2150.
Cho Meoung-Whan
Yao Takafumi
Burr & Brown
Kang Donghee
Thomas Tom
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