Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
1998-06-03
2001-05-15
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S047000, C438S681000
Reexamination Certificate
active
06232137
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a semiconductor light emitting element and its manufacturing method. More particularly, the invention relates to a semiconductor light emitting element and its manufacturing method preventing thermal deterioration of nitride compound layer containing indium and deterioration of interfaces, and thereby promising growth of a high-quality nitride compound semiconductor light emitting element, by restricting materials of layers overlying the nitride compound layer containing indium to specific materials or by restricting growth temperatures within a predetermined range upon stacking a plurality of semiconductor layers involving the nitride compound layer containing indium on a substrate.
Most of nitride compound semiconductors are optically direct-transitional and capable for highly efficient radiative recombination. Their bandgap energy widely ranges from 1.89 to 6.2 eV. For these favorable natures, development is under progress for using them as high-efficient light emitting elements, such as various kinds of short-wavelength semiconductor lasers and high-luminous visible LEDs.
In the present application, the term “nitride compound” pertains to any compound semiconductor which can be expressed by the chemical formula B
x
In
y
Al
z
Ga
1−x−y−z
N (0≦x<1, 0≦y≦1, 0≦z≦1, x+y+z≦1) with any values of the mole fractions x, y and z in their respective ranges. For example, InAlN (x=0, y=0.4,z=0.6) is also regarded as one of nitride compound semiconductors.
Nitride compound semiconductors can be expressed as combinations of gallium nitride, aluminum nitride, indium nitride and boron nitride which are binary semiconductors. Among them, gallium nitride (GaN) has been a subject of active developments. Gallium nitride has a melting point as high as 1700° C., the equilibrium vapor pressure of nitrogen under a growth temperature is very high. Therefore, it is difficult to grow gallium nitride in bulk single crystal, and its crystalline growth mainly relies on hydride chemical vapor deposition (HCVD) or metal organic chemical vapor deposition (MOCVD). Among them, crystal growth technique by MOCVD has been most actively developed, and has succeeded in growing ternary mixed crystals, such as gallium indium nitride (GaInN) made by adding indium to gallium nitride, gallium aluminum nitride (GaAlN) made by adding aluminum to gallium nitride, and indium aluminum nitride (InAlN) made by adding aluminum to indium nitride.
By utilizing a heterojunction of these materials, light emitting efficiency can be improved. When using a double hetero structure effective for confinement of injected carriers or light, highly luminous LED or short-wavelength semiconductor lasers can be realized.
As to gallium indium nitride which is a ternary mixed crystal, its band gap energy can be changed from 3.4 eV of gallium nitride (GaN) to 1.89 eV of indium nitride (InN) by changing the mole fraction of indium, and it can be used as active layers of light emitting elements over wide visible wavelength bands.
Gallium indium nitride can be made by combining gallium nitride and indium nitride. However, gallium nitride needs a growth temperature of 1000° C. or more to ensure an acceptable crystallographic quality whereas indium nitride must be grown under a lower temperature because of a high vapor pressure of indium.
These growth temperatures are taught in greater detail, for example, in Appl. Phys. Lett., 59, (1991) p2251. As taught there, the growth temperature for crystalline growth of gallium indium nitride containing a relatively higher rate of indium must be lower than that of gallium nitride.
As to the growth temperature of gallium aluminum nitride, there is a teaching in Appl. Phys. Lett., 64, (1994) p1535. As taught there, a temperature as high as that for gallium nitride is preferable for the epitaxial growth of gallium aluminum nitride.
As reviewed above, the optimum temperature is different for individual compound semiconductors. Therefore, in order to grow a double-heterostructure sandwiching an active layer made of gallium indium nitride between cladding layers made of gallium nitride or gallium aluminum nitride, for example, the growth temperature must be changed at the interfaces.
However, if the layers are grown in this mode, indium having a high vapor pressure evaporates from the surface of the gallium indium nitride layer already grown, when the temperature is raised after the growth of the gallium indium nitride active layer. It results in deteriorating the crystallinity of the gallium indium nitride layer and its interfaces with the cladding layers, and hence results in deterioration of operative characteristics of the device.
It would be a possible approach for preventing evaporation of indium to decrease the composition ratio of indium. However, composition of indium is an essential factor which determines band gaps and other basic properties, and any change in its value may invite undesirable changes in essential characteristics such as emission wavelength.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a semiconductor light emitting element and its manufacturing method permitting a hetero structure to be made without degrading the crystalline quality of nitride compound layer containing indium, such as gallium indium nitride, having a predetermined composition ratio of indium.
According to the invention, there is provided a method for manufacturing a semiconductor light emitting element having a first layer of a first nitride compound semiconductor containing indium and a second layer stacked on the first layer, comprising the step of:
stacking the second layer under conditions inside the closed region defined by connecting points plotted at x and y coordinates (
364
,
600
), (
364
,
1010
), (
550
,
1010
), and (
650
,
600
) on a graph taking emission wavelengths based on band-to-band transition of said first layer in nanometer on the x axis and taking growth temperatures of said second layer in ° C. on the y axis. In this manner, a high-performance light emitting element can be made without inviting thermal deterioration of the first layer.
When using MOCVD for stacking the layers and stacking a cap layer made of a nitride compound semiconductor having a low mole fraction of indium to cap the first layer, a layer above the cap layer can be stacked under conditions outside the closed region on the graph without inviting thermal deterioration of the first layer.
The first layer may be made of gallium indium nitride, and the cap layer may be made of gallium aluminum nitride, and the layer above the cap layer may be made of nitride compound.
The first layer and the second layer preferably have thicknesses not exceeding critical thicknesses in terms of generation of crystallographic defects caused by lattice mismatching, and the gallium indium nitride stacked as the first nitride compound semiconductor may have a mole fraction within the miscibility gap under an operation of a distortion energy by lattice mismatching.
According to the invention, there is further provided a semiconductor light emitting element including a substrate and a multi-layered structure of nitride compound semiconductors stacked on the substrate, wherein the multi-layered structure includes at least a first layer of gallium indium nitride, a second layer of gallium aluminum nitride stacked on said first layer, and a third layer of gallium nitride stacked on said second layer, the first and second layers having thicknesses not exceeding critical thicknesses in terms of generation of crystallographic defects caused by lattice mismatching, and the gallium indium nitride forming the first layer having a mole fraction within its miscibility gap.
According to the invention, there is also provided a semiconductor light emitting element having a substrate and a multi-layered structure of gallium nitride compound semiconductors stacked on the substrate, wherein the multi-layered structur
Ishikawa Masayuki
Sugawara Hideto
Hogan & Hartson LLP
Kabushiki Kaisha Toshiba
Pham Long
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