Gallium phosphide luminescent device

Details Gallium phosphide luminescent device Gallium phosphide luminescent device

2001-06-27

2002-11-12

Sherry, Michael (Department: 2829)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Compound semiconductor

C257S087000, C257S101000, C438S046000, C438S093000, C438S761000, C438S767000, C438S918000

Reexamination Certificate

active

06479312

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a gallium phosphide light emitting device and a method for fabricating thereof, and in more detail, a gallium phosphide light emitting device with a high luminance and a method for fabricating such device.
DESCRIPTION OF THE BACKGROUND ART
Light emitting devices such as light emitting diodes are generally obtained by stacking a plurality of semiconductor layers on a semiconductor substrate to thereby fabricate a multi-layered semiconductor wafer having a p-n junction, and processing such wafer into the devices. Among such devices, those having a dominant emission wavelength (dominant wavelength) within a range from 555 to 580 nm are obtained by stacking on an n-type gallium phosphide (which may simply be noted as “GaP” hereinafter) substrate at least one each of n-type and p-type gallium phosphide layers in this order, and processing the obtained wafer into the devices. A method for determining the dominant emission wavelength (dominant wavelength) is defined by JIS(Japanese Industrial Standard)-Z8701 (1995).
While GaP having no doped nitrogen (N) serving as an emission center can emit a green light with a dominant emission wavelength of 559 nm or longer and shorter than 560 nm, the light emission efficiency is considerably low since the emission is ascribable to indirect transition. Adding nitrogen (N) to such GaP can now significantly increase the light emission efficiency. It has been considered that such increase is mainly contributed by a mechanism mentioned below. That is, N capable of substituting phosphorus (P) in GaP, will become an electrically neutral impurity since N is a group V element same as P. N has, however, a larger electron affinity than P has, so that it can trap a neighboring electron. Such impurity is known as an isoelectronic trap. Once an electron is trapped, Coulomb's force appears and is exerted far beyond to attract a hole, which results in generation of an exciton. Dissipation by recombination of such exciton emits a green to yellow-green light with a dominant emission wavelength between 560 nm and 580 nm, where energy of such light nearly corresponds to the band gap. Light having a wavelength in the above range and in particular between 560 nm and 564 nm is substantially recognized as a green light. It is now generally known that a potential of N relative to an electron is expressed as a narrow and deep short-distance force in the real space, so that a wave function of the electron can extend widely in the momentum space. Such extension of the wave function in the momentum space desirably increases direct transition component with a null wave vector, so as to allow GaP to have a relatively large emission-recombination probability despite of its nature of indirect transition.
FIGS. 9A and 9B
respectively show an exemplary sectional constitution and a carrier concentration profile of the individual layers of a conventional GaP light emitting device. As shown in
FIG. 9A
, the conventional GaP light emitting device has on an n-type GaP single-crystalline substrate
40
an n-type GaP crystallinity improving layer
41
, a non-N-doped n-type GaP layer
42
, an N-doped n-type GaP layer
43
, an N-doped p-type GaP layer
44
, and a non-N-doped p-type GaP layer
45
stacked in this order. A p-n junction is formed between the N-doped n-type GaP layer
43
and the N-doped p-type GaP layer
44
. The n-type GaP substrate
40
and the n-type GaP layers
41
to
43
are added with silicon (Si) as an n-type dopant, and the p-type GaP layers
44
and
45
with zinc (Zn) as a p-type dopant.
Such constitution of the GaP light emitting device shown in
FIG. 9A
, having the N-doped p-n junction for composing a light emissive zone allows a higher light emission efficiency and accordingly a higher luminance as compared with a device having no doped N. It should now be noted that luminance refers to brightness per unit area of a light emitting body. Since luminance considerably varies depending on the morphology of the light emitting body or measurement methods, the value will be evaluated on a relative basis in the context of the present invention.
The light emission efficiency of the GaP light emitting device having the N-doped p-n junction portion is, however, still smaller than that, for example, of a GaP: Zn—O red LED, so that it is important to grow a p-n junction made of high-quality crystal having a minimum amount of defects in order to raise the luminance. The N-doped p-type GaP layer
44
is most commonly grown by the so-called impurity compensation process. The process is such that growing the n-type GaP layer
43
, adding zinc (Zn) or other acceptors into the gallium solution, and then growing the p-type GaP layer
44
while compensating the donor. The process is advantageous in that allowing formation of the p-n junction portion using a single solution.
It is now noted that liquid-phase growth of the N-doped n-type GaP layer
43
composing the p-n junction requires ammonia gas as an source to be introduced into the growth vessel containing the Si-added gallium solution. On the other hand, growth of the N-doped p-type GaP layer
44
is proceeded by further adding Zn to the above Si-added gallium solution to thereby compensate the donor impurity (Si), and by similarly introducing ammonia gas. Adding N both to the n-type GaP layer
43
and the p-type GaP layer
44
can principally ensure both layers with a probability of light emission. The p-type GaP layer
44
grown by the foregoing impurity compensation process will, however, inevitably has the acceptor concentration higher than the donor concentration in the n-type GaP layer
43
, so that the emission from the n-type GaP layer
43
upon hole injection will become dominant.
In the process of the N doping using ammonia gas, Si reacts with ammonia to produce a stable compound to thereby lower the Si content in the gallium solution, which results in lowered Si concentration (donor concentration, accordingly carrier concentration) in the N-doped n-type GaP layer
43
as shown in FIG.
9
B. Since a donor that presents in the light emissive zone acts as a non-emissive center, it is important to prevent as much as possible the donor concentration from being lowered, in order to extend a lifetime of the carrier. The foregoing introduction of ammonia gas during the growth of the n-type GaP layer
43
in order to reduce the concentration of Si as a donor is thus advantageous in terms of correspondingly raising the injection efficiency of holes responsible for light emission.
As described in the above, adding N to the GaP layer composing the p-n junction successfully raises the light emission efficiency. Raising the N concentration, however, cause red-shift of the dominant emission wavelength to intensify yellow component, so that it has been difficult to further raise luminance at a stable wavelength.
In addition, while the ammonia gas introduction during the growth of the n-type GaP layer
43
inevitably reduces the Si (donor) concentration, to thereby advantageously raise the injection efficiency of holes responsible for the light emission, this will raise another problem in that the donor concentration of the n-type GaP layer
43
will always vary in association with the concentration of N introduced in a form of ammonia gas and serves as a light emissive center, so that it will become difficult to control the carrier concentration independently from the N concentration.
It is therefore an object of the present invention to solve the foregoing problems and to provide a gallium phosphide light emitting device with a high luminance and a method for fabricating such device.
SUMMARY OF THE INVENTION
A major concept of the present invention resides in that providing a nitrogen-doped low carrier concentration layer having both of a donor concentration and an acceptor concentration controlled below 1×10
16
/cm
3
at a p-n junction portion between an n-type GaP layer and a p-type GaP layer, to thereby raise the luminance by as much as 20 to 30% over the c

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