The disclosure herein pertains to the preparation of semiconductor materials and solid-state devices fabricated therefrom. More particularly, the disclosure pertains to a vapor phase process for the preparation of electroluminescent materials, particularly GaAs1-x Px, doped with isoelectronic impurities, particularly nitrogen, and to electroluminescent devices fabricated therefrom.
|
6. A process for the preparation of electroluminescent materials for light-emitting diodes, said process comprising:
a. providing in a reaction chamber a substrate of a single-crystal compound formed from Ga and one of the elements selected from the group consisting of As and P; b. combining in a vapor phase reactant stream reactant material including impurity atoms, said materials being adapted for the formation of GaAs1-x Px, wherein x has an initial value within the range of from 0-1 inclusive and a final value within the range of about 0.4 to about 0.92; c. introducing said reactant stream into said reactant chamber; d. epitaxially depositing on said substrate from said stream said reactant materials to form a first epitaxial layer of a first electrical conductivity type on said substrate, said layer constituting a bulk region substantially free of isoelectronic impurities; e. introducing into said stream an isoelectronic impurity in vapor form; f. epitaxially depositing on said substrate from said stream said reactant materials to form a second epitaxial layer of said first conductivity type on said first epitaxial layer, said second epitaxial layer providing a surface region containing said isoelectronic impurity atoms; and g. forming a junction in said surface region by introducing electronic impurity atoms of an electronic conductivity type opposite to that of said first conductivity type.
1. A process for the preparation of electroluminescent materials for light-emitting diodes, said process comprising:
a. providing in a reaction chamber a substrate of a single-crystal compound formed from Ga and one of the elements selected from the group consisting of As and P; b. combining in a vapor phase reactant stream reactant material including impurity atoms, said materials being adapted for the formation of GaAs1-x Px, wherein x has an initial value within the range of from 0-1 inclusive and a final value within the range of greater than 0.2 to less than 1∅]. about 0.4 to about 0.9; c. introducing said reactant stream into said reactant chamber; d. epitaxially depositing on said substrate from said stream said reactant materials to form a first epitaxial layer of a first electrical conductivity type on said substrate, said layer constituting a bulk region substantially free of isoelectronic impurities; e. introducing into said stream an isoelectronic impurity in vapor form; f. epitaxially depositing on said substrate from said stream said reactant materials to form a second epitaxial layer of said first conductivity type on said first epitaxial layer, said second epitaxial layer providing a surface region containing said isoelectronic impurity atoms; and g. forming a junction in said surface region by introducing electronic impurity atoms of an electronic conductivity type opposite to that of said first conductivity type.
3. The process according to
4. The process according to
5. The process according to
|
|||||||||||||||||||||
This is a division of application Ser. No. 158,312, filed June 30, 1971, now U.S. Pat. No. 3,725,749.
This invention pertains to the field of semiconductor material preparation and device fabrication. In preferred embodiments, the invention pertains to the field of electroluminescent materials and devices.
As pertinent to this invention, it is known in the prior art that nitrogen may be introduced into gallium phosphite (GaP) to create isoelectronic traps which function as radiative recombination centers for enhancement of the emission of green light when fabricated into junction devices. The prior art processes specifically designed for introducing nitrogen into the GaP, whether used as substrate or as an epitaxial film or both in the fabricated device, has been limited, apparently, to solution growth or liquid phase epitaxial processes. Typical of these prior art processes is that described, for example, in U.S. Pat. No. 3,462,320, where electroluminescent GaP devices are prepared by adding gallium nitride (GaN) and polycrystalline GaP containing a dopant of one conductivity type to a melt of elemental gallium . also as can be seen from FIG. 6, the improvements are obtained up to a value for x of about 0.92.
By way of comparison, a second sample of the identical material produced as above was processed to remove the nitrogen-doped layer (region 4 in FIG. 1C) prior to diffusion of the Zn dopant under the same conditions described above for diffusing Zn into the nitrogen-doped layer to produce the P-N junction. The average brightness of a batch of 10 diodes fabricated from the nitrogen-free material was only 58 foot-Lamberts at a current density of 20 A/cm2 at 5800 A as shown by reference to the solid curve in FIG. 6 for nitrogen-free diodes.
This example exemplifies an embodiment of the invention wherein a GaAs substrate is used and the P-N junction is formed by using zinc arsenide (ZnAs2) as the diffusant.
The process operation here follows that described in the preceding example, again having reference to the steps and structure shown in FIGS. 1A-F. The reactant gas was produced by passing 5.4 cc/min. of HCl in 50 cc/min. of H2 over elemental Ga at 780° C and combining the resultant mixture with 450 cc/min. of H2 containing 2.6 cc/min. of AsH3 and 1.4 cc/min. of PH3 at a reaction temperature of about 925° C. About 0.4 cc/min. of a 100 ppm diethyl telluride mixture in H2 was added to the main H2 stream to produce a net donor concentration of about 6 × 1016 cm-3. The reaction mixture then contacted a single crystal GaAs substrate 1 oriented within 2° of the (100) plane at a deposition temperature of 840°C By adjusting the relative concentration of PH3 and AsH3 in the vapor phase, a graded composition layer 2 was grown on the substrate to a thickness of about 65μm, at which level the alloy composition was GaAs.525 P.475. An epitaxial layer 3 of this composition was then grown to a thickness of about 192μm. During the final minutes of the growth period, 300 cc/min. of a 10% NH3 -in-H2 mixture was substituted for 300 cc/min. of H2 to produce a nitrogen-doped surface layer (region 4 prior to diffusion) about 12 μm thick.
Material of the above composition was then fabricated into diodes by diffusion with 8 mg. of ZnAs2 at 800° C for 45 minutes in an evacuated and sealed ampoule to form a P-region 4b and P-N junction 5 about 5 μm below the surface. After lapping to a thickness of about 5 mils and attaching ohmic contacts and leads as before, a series of light-emitting diodes (LED's) thus fabricated produced an average brightness of about 1,100 foot-Lamberts at a current density of 20 A/cm2 as shown on the upper (nitrogen-doped) curve in FIG. 6. By way of comparison a second series of LED's fabricated from the same alloy composition, except for removal of the nitrogen-doped layer (region 4 in FIG. 1C) and a re-diffusion with ZnAs2 in the manner described in Example 1, produced an average brightness of only 490 foot-Lamberts.
At a reduced current density of 10 A/cm2, nitrogen-doped LED's fabricated from the alloy composition of this example, produced an average brightness of 470 foot-Lamberts at a wavelength of 6,650 A, which is of the same order magnitude of brightness produced by the non-nitrogen-doped LED's at 20 A/cm2. This performances is an order of magitude better than that typically obtained for this alloy composition (which is in the indirect energy bandgap region) and is comparable in brightness to that of red-emitting LED's from non-nitrogen-doped alloys of the composition GaAs.64 P.36, which is in the direct energy bandgap region. Thus, with the addition of nitrogen according to this invention, LED's of generally equivalent brightness can be fabricated throughout the spectral range from 6,500 A to 5,600 A. This is particularly important in the yellow portion of the spectrum, because high brightness yellow-emitting LED's have not been available heretofore.
The improved efficiency performance of the nitrogen-doped electroluminescent devices of this invention, as compared with nitrogen-free devices is shown by reference to FIGS. 2-4. The external quantum efficiencies referred to herein were obtained using epoxy-encapsulated diodes (epoxy lens not shown in FIG. 1) which were mounted on TO-18 headers using Au/Ge preforms.
Referring to FIG. 2, it will be noted that the addition of nitrogen causes a shift in the peak emission energy (eV) hence, wavelength, for a given GaAs1-x Px composition. To convert from wavelength in Angstom units (A) to peak emission energy, in electron volts (eV), the wavelength value is divided into the conversion factor 12395, thus eV = .Badd.12305/A..Baddend. 12395/A. The separation between emission peaks in nitrogen-doped and nitrogen-free LED's changes as a function of alloy composition. It will be noted that the separation between the peak emission energies of the nitrogen-doped and undoped LED's increases with decreasing x, reaching a maximum separation of about 0.15 eV in the region of 0.5<x<0.6. The peak position and band width changes with current density and the nature and degree of the change is dependent upon the alloy composition and temperature. The peak emission energies plotted in FIG. 2 were obtained at a relatively low injection current density of 10 A/cm2.
In FIG. 3, the external quantum efficiency is plotted as a function of the GaAs1-x Px composition. The efficiency of the LED's increases with decreasing x. This increase in efficiency is believed to be due largely to two factors. First, the increasing depth of the nitrogen center results in increased thermal stability of the trapped exciton. Second, the fact that the separation between the (100) and (000) minima is decreasing with decreasing x is expected to give rise to an increase in the transition probability for the A-line emission.
In FIG. 4 are shown curves for nitrogen-doped and nitrogen-free LED's with external efficiencies plotted against peak emission wavelengths for various alloy compositions. It will be seen that the efficiencies for the nitrogen-doped LED's is greater than those of nitrogen-free LED's throughout the spectrum shown on the graph. The greatest separation between the curves, representing the greatest improvement in external efficiencies of the nitrogen-doped over the nitrogen-free LED's is generally in the yellow region of the spectrum.
Referring to FIG. 3 it will be noted that for the alloy composition region 0.5<x<0.6, the efficiency of the nitrogen-doped LED's is more than 20 times greater than that for te nitrogen-free LED's. Another way to express this increased efficiency is shown in FIG. 5 wherein the efficiency ratio, (GaAs1-x P:N)/GaAs1-x Px), of nitrogen-doped to nitrogen-free LED's is plotted against alloy composition.
Although the quantum efficiency of the nitrogen-doped diodes is a strong function of alloy composition, the luminous efficiency and brightness are nearly independent of alloy composition in the region x>0.4. The reason for this is that the sensitivity of the human eye decreases sharply as x decreases and the color changes from green through yellow to red. Typical brightness performance obtained with and without nitrogen doping are shown in FIG. 6 wherein brightness is plotted as a function of alloy composition.
In preferred embodiments of the invention, referring now to FIGS. 1B-1D, the graded alloy composition, layer 2, can be from 1 to 300μm or more, although best results to date are obtained with layers on the order of about 25μm. The region 3 of constant alloy composition is preferably about 100μm thick, but can have thicknesses with the range 0-300μm or more. The N-type region 4a of the nitrogen-doped surface layer preferably snould be about 5μm, but more broadly, can have thicknesses within the range 0-300μm or more. The P-type region 4b of the nitrogen-doped layer preferably should be about 5-10μm thick and, more broadly can be from 1 to 25μm or slightly more. Thus, it will be noted that in some embodiments, either one or both of the constant composition alloy layer 3 and/or nitrogen-doped 4a can be omitted from the epitaxial GaAs1-x Px structures and LED's of this invention. However, in preferred embodiments as exemplified in the above examples, the epitaxial GaAs1-x Px structure is as shown in FIG. 1F, with layers 1 and 2 removed by lapping.
The conductivity type determining impurity used in doping the epitaxial film may be introduced initially into the region 2 of graded composition and continuously added throughout the remainder of the growth period, or the impurity may be first introduced at the beginning of growth of the constant composition layer 3. In preferred embodiments, the epitaxial film is doped with N-type impurities and diffused with P-type impurities to form the P-N junction. Suitable impurities include those conventionally used in the art, e.g., S, Se. Te or Si for N-type doping and Be, Zn or Cd for P-type doping. The N-type impurity concentration range is broadly, from about 2.0 × 1016 - 2.0 × 1017 cm-3 and, preferably, about 7.0 × 1016 cm-3. The surface concentration of P-type impurities is typically on the order of 1019 atoms/cm3.
With respect to the nitrogen dopant, as indicated above, in preferred embodiments, the nitrogen is selectively introduced into the growing epitaxial film only in the region in which the P-N junction is to be formed, typically in the upper 5-20μm surface region (layer 4 in FIG. 10). The nitrogen concentration in this surface region is typically about 1 × 1018 - 1 × 1019 atoms/cm2. However in less preferred embodiments of the invention, the entire epitaxial film (layers 2-4b) may be doped with nitrogen, but in must lower concentrations below layer 4a. The isoelectronic impurity may be introduced from any suitable source, e.g., elemental nitrogen, gaseous or volatile compounds thereof.
The graded composition alloy layer 2 may be either linearly or non-linearly graded, but in preferred embodiments is linearly graded from the composition of the GaAs or GaP substrate to the desired final composition.
The electroluminescent devices of this invention may be fabricated as discrete LED's or as arrays thereof by conventional photolithographic techniques.
The nitrogen-doped GaAs1-x Px alloy compositions of the present invention are particularly suitable for use in the fabrication of LED's in the visible portion of the spectrum. Although visible light is generated in materials within the range x> 0.2 to <1.0 a preferred range from the LED's of the invention is where x is between about 0.3 and 0.9. For red light-emitting LED's, x preferably is between 0.4 and 0.6, and for yellow LED's x is between 0.6 and 0.9. As can be seen in general, the improvement utilizing the present invention is gained in the range of x having a value from about 0.4 to about 0.9.
With further respect to the LED's of this invention, the presence of initial layers (1 and 2 in FIG. 1) essential in producing the desired material is not essential to the operation of the final device and they may be removed in reducing the thickness of the semiconductor chip to a convenient value of 100 to 150μm. In the embodiment using GaAs as a substrate it is desirable to remove the substrate 1 and the region of graded composition 2 to minimize absorption losses and gain radiation reflected from layer 7.
Various modifications may be made in the present invention without departing from the spirit and scope thereof. For example, still other modifications within the purview of this invention include the use of other substrates whose lattice structure is compatible with epitaxial growth of the GaAs1-x Px film, e.g., Ge, Si, etc. It is also contemplated that other alloy systems are also amenable to doping with nitrogen and other isolectronic impurities in the manner of this invention for fabrication in electroluminescent devices.
It is also contemplated that vapor epitaxial deposition processes other than those specifically used in the working examples above may be employed for this invention.
Groves, Warren O., Herzog, Arno H., Craford, Magnus G.
| Patent | Priority | Assignee | Title |
| 4902356, | Jan 21 1988 | Mitsubishi Kasei Corporation | Epitaxial substrate for high-intensity led, and method of manufacturing same |
| Patent | Priority | Assignee | Title |
| 3634872, | |||
| 3646406, | |||
| 3687744, |
| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Mar 07 1977 | Monsanto | (assignment on the face of the patent) | / |
| Date | Maintenance Fee Events |
| Date | Maintenance Schedule |
| May 30 1981 | 4 years fee payment window open |
| Nov 30 1981 | 6 months grace period start (w surcharge) |
| May 30 1982 | patent expiry (for year 4) |
| May 30 1984 | 2 years to revive unintentionally abandoned end. (for year 4) |
| May 30 1985 | 8 years fee payment window open |
| Nov 30 1985 | 6 months grace period start (w surcharge) |
| May 30 1986 | patent expiry (for year 8) |
| May 30 1988 | 2 years to revive unintentionally abandoned end. (for year 8) |
| May 30 1989 | 12 years fee payment window open |
| Nov 30 1989 | 6 months grace period start (w surcharge) |
| May 30 1990 | patent expiry (for year 12) |
| May 30 1992 | 2 years to revive unintentionally abandoned end. (for year 12) |