Light -emitting device

Details Light -emitting device Light -emitting device

2001-03-01

2004-09-21

Vu, David (Department: 2828)

Coherent light generators

Particular active media

C372S043010, C372S045013, C372S046012

Reexamination Certificate

active

06795463

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-emitting device using electroluminescence (EL).
2. Description of the Related Art
Semiconductor lasers have been used as a light source for optical communications systems. Semiconductor lasers excel in wavelength selectivity and can emit light with a single mode. However, it is difficult to fabricate the semiconductor lasers because many stages of crystal growth are required. Moreover, types of light-emitting materials used for semiconductor lasers are limited. Therefore, semiconductor lasers cannot emit light with various wavelengths.
Conventional EL light-emitting devices which emit light with a broad spectral width have been used in some application such as for displays. However, EL light-emitting devices are unsuitable for optical communications and the like, in which light with a narrow spectral width is required.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a light-emitting device which can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices, and can be applied not only to displays but also to optical communications and the like.
Light-emitting Device of First Embodiment
A light-emitting device according to a first embodiment of the present invention comprises a substrate and a light-emitting device section,
wherein the light-emitting device section includes:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
an optical section for propagating light emitted in the light-emitting layer in a specific direction; and
an insulation layer disposed between the pair of electrode layers, having an opening formed in part of the insulation layer and capable of functioning as a current concentrating layer for specifying a region through which current supplied to the light-emitting layer flows through a layer in the opening,
wherein the optical section forms photonic bandgaps capable of inhibiting three dimensional spontaneous emission of light and includes a defect section which is set so that an energy level caused by a defect is within a specific emission spectrum, and
wherein the light emitted in the light-emitting layer is emitted with spontaneous emission being inhibited in three dimensions by the photonic bandgaps.
According to this light-emitting device, electrons and holes are injected into the light-emitting layer respectively from the pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by allowing the electrons and holes to recombine in the light-emitting layer. At this time, light with a wavelength in the photonic bandgaps cannot be propagated through the optical section. Only light with a wavelength equivalent to the energy level caused by the defect is propagated through the optical section. Therefore, light with a narrow emission spectrum width with an inhibited three-dimensional spontaneous emission can be obtained with high efficiency by specifying the width of the energy level caused by the defect.
According to the first light-emitting device, since the insulation layer functions as a current concentrating layer in the light-emitting device section, the region through which current supplied to the light-emitting layer flows can be specified. Therefore, current intensity and current distribution can be controlled in the region from which light is to emit, whereby light can be emitted with high emission efficiency.
In the case where the insulation layer functions as cladding, assuming that the waveguide formed of a light-emitting layer as a core and an insulation layer as cladding, the guide mode of light propagated toward the waveguide section through the optical section can be controlled by specifying the opening of the insulation layer. Specifically, the guide mode of light propagated through the light-emitting layer (core) can be set at a specific value by specifying the width of the region in which light is confined (width perpendicular to the direction in which light is propagated) by the insulation layer (cladding) The relation between the guide mode and the waveguide is generally represented by the following equation.
N
max+1≧
K
0
·a
·(
n
1
2
−n
2
2
)
1/2
/(
&pgr;
/2)
where:
K
0
:2
&pgr;
/
&lgr;
,
a: half width of waveguide core,
n
1
: refractive index of waveguide core,
n
2
: refractive index of waveguide cladding, and
Nmax: maximum value of possible guide mode.
Therefore, if the parameters of the above equation such as the refractive indices of the core and cladding have been specified, the width of the light-emitting layer (core) specified by the width of the opening of the current concentrating layer may be selected depending on the desired guide mode. Specifically, the width (
2
a
) of the light-emitting layer corresponding to the core in a desired guide mode can be calculated from the above equation by substituting the refractive indices of the light-emitting layer provided inside the current concentrating layer and the insulation layer (current concentrating layer) for the refractive indices of the core and cladding of the waveguide, respectively. The suitable width of the core layer of the waveguide section to which light is supplied from the light-emitting device section can be determined taking into consideration the resulting width of the light-emitting layer, calculated value obtained from the above equation based on the desired guide mode, and the like. Light with a desired mode can be propagated from the light-emitting device section toward the waveguide section with high combination efficiency by appropriately specifying the width of the light-emitting layer, width of the core layer, and the like. In the light-emitting device section, light-emitting layer in the current concentrating layer formed of the insulation layer may not uniformly emit light. Therefore, the specific values for each member such as the light-emitting layer, optical section, and waveguide section can be suitably adjusted based on the width (
2
a
) of the core (light-emitting layer), determined using the above equation, so that each member exhibits high combination efficiency.
The guide mode of the light-emitting device can be set to 0 to 1000. In particular, when used for communications, the guide mode can be set to about 0 to 10. Light with a specific guide mode can be efficiently obtained by specifying the guide mode of light in the light-emitting layer in this manner.
As described above, according to the present invention, a light-emitting device which substantially has three-dimensional photonic bandgaps structure can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also optical communications and the like, can be provided.
Light-emitting Device of Second Embodiment
A light-emitting device according to a second embodiment of the present invention comprises comprising a light-emitting device section and a waveguide section propagating light from the light-emitting device section which are integrally formed on a substrate,
wherein the light-emitting device section includes:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
an optical section for propagating light emitted in the light-emitting layer in a specific direction; and
an insulation layer disposed between the pair of electrode layers and capable of functioning as a cladding layer,
wherein the waveguide section includes:
a core layer continuously formed with part of the optical section; and
a cladding layer continuously formed with the insulation layer, and
wherein the optical section forms photonic bandgaps capable of inhibiting three dimensional spontaneous emission of light and includes a defect s

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