US20050247948A1

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Publication number: US20050247948A1
Application number: US11/184,190
Authority: US
Inventors: Tetsuji Moku; Kohji Ohtsuka; Masataka Yanagihara; Masaaki Kikuchi
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-02-21
Filing date: 2005-07-19
Publication date: 2005-11-10
Also published as: EP1187229A1; US20020158253A1; TW586243B; WO2001061766A1; EP1187229A4; US6979844B2; US20030183835A1

Abstract

A low-resistance silicon baseplate (11) has formed thereon a buffer layer12in the form of an alternating lamination of AlN sublayers (12a) and GaN sublayers (12b). On this buffer layer there are formed an n-type semiconductor region (13) of gallium nitride, an active layer (14) of gallium indium nitride, and a p-type semiconductor region (15) of galliumnitride, in that order. An anode (17) is formed on the p-type semiconductor region (15), and a cathode (18) on the baseplate (11).

Description

TECHNICAL FIELD

This invention relates to a light-emitting semiconductor device composed primarily of gallium nitride (GaN)-based semiconductors, and to a method of making the same.

BACKGROUND ART

The compound semiconductors composed primarily of GaN have been used extensively for fabrication of light-emitting devices such as diodes that are capable of glowing in blue. Examples of such compound semiconductors include, in addition to GaN itself, gallium aluminum nitride (GaAlN), indium gallium nitride (InGaN), and indium gallium aluminum nitride (InGaAlN).

A typical prior art light-emitting device of the kind under consideration comprises a baseplate of electrically insulating material such as sapphire, a buffer layer overlying the baseplate and composed for example of Ga_xAl_1-xN, where x is greater than zero and not greater than one (as taught by Japanese Unexamined Patent Publication No. 4-297023), an n-type semiconductor region of GaN or other compound semiconductor composed principally of GaN and grown epitaxially on the buffer layer, an active layer of another compound semiconductor composed principally of GaN (e.g. InGaN) and grown epitaxially on the n-type semiconductor region, and a p-type semiconductor region grown epitaxially on the active layer. The n-type semiconductor region is connected to a cathode, and the p-type semiconductor region to an anode.

The common practice in the manufacture of light-emitting devices is first to form wafers on which there are fabricated matrices of desired devices, and to cut them into the individual devices as by dicing, scribing, or cleavaging. The noted sapphire baseplate of the light-emitting devices has been a cause of trouble in such dicing of the wafers because of its extreme hardness. Sapphire itself is expensive, moreover, adding much to the manufacturing costs of the light-emitting devices.

There have been additional difficulties in connection with the sapphire baseplate. Being electrically insulating, the sapphire baseplate makes it impossible to form a cathode thereon. This inconvenience was conventionally circumvented by exposing part of the n-type semiconductor region through the active layer and p-type semiconductor region for connection to a cathode. The results were a greater surface area of the semiconductor and a corresponding increase in the costs of the light-emitting devices.

A further inconvenience arose from the fact that current flows through the n-type semiconductor region not only vertically (normal to the plane of the sapphire baseplate) but horizontally (parallel to the sapphire baseplate plane). The dimension of the n-type semiconductor region for the horizontal current flow is as small as four to five micrometers, so that the resistance of the horizontal current path of the n-type semiconductor region was very high, adding substantively to the current and voltage requirements of the prior art devices.

A still further inconvenience concerns the etching-away of parts of the active layer and p-type semiconductor region in order to expose part of the n-type semiconductor region for connection to the cathode. The n-type semiconductor region had to be dimensioned sufficiently large to allow for some errors in etching, necessitating a correspondingly elongated period of time for it to be grown epitaxially.

It has been suggested to use a conductive baseplate of silicon carbide (SiC) in substitution for the sapphire. Permitting a cathode to be formed thereon, the SiC baseplate offers such advantages over the sapphire baseplate as a smaller surface area and easier separation of the wafer by cleavaging. Offsetting these advantages, however, is the fact that SiC is even more expensive than sapphire. Another shortcoming is the difficulty of placing the n-type semiconductor region in low-resistance contact with the SiC baseplate, so that the current and voltage requirements of the light-emitting device incorporating the SiC baseplate were just as high as those of the device with the sapphire baseplate.

The present invention aims at the provision of a light-emitting device, and a method of fabrication thereof, such that the device is efficiently manufacturable at a lower cost than heretofore and is improved in performance too.

DISCLOSURE OF THE INVENTION

The present invention will be briefly summarized with use of the reference characters used in the subsequent detailed description of the best mode of carrying out the invention. As used in this summary, however, the reference characters are meant purely for an easier understanding of the invention and should not be taken in a limitative sense.

Briefly stated in its perhaps broadest aspect, the light-emitting semiconductor device according to the invention comprises a baseplate (11) of low resistivity composed of a silicon compound or silicon with impurities, a buffer layer (12) formed on the baseplate and having a first sublayer (12a) of Al_xGa_1-xN, where x is greater than zero and not greater than one, and a second sublayer (12b) of GaN or Al_yGa_1-yN, where y is less than x and greater than zero and less than one, a semiconductor region (10) formed on the buffer layer and having a plurality of sublayers of compounds composed primarily of GaN or GaN-based compound semiconductors for emission of light, a first electrode (17) formed on the semiconductor region, and a second electrode (18) formed on the baseplate.

As stated in claim2, the sublayers of the semiconductor region (10) may include a first semiconductor sublayer (13) of a first conductivity type formed on the buffer layer (12) and made of a compound composed primarily of GaN, an active sublayer (14) on the first sublayer, and a second semiconductor sublayer (15) of a second conductivity type, which is opposite to the first conductivity type, formed on the active layer and also made of a compound composed primarily of GaN.

As stated in claim3, the buffer layer (12) may consist of an alternation of a first set of sublayers (12a) of Al_xGa_1-xN, and a second set of sublayers of GaN or Al_yGa_1-yN.

As stated in claim4, the first set of sublayers (12a) of thebuffer layer12 should each be from 5×10⁻⁴to 100×10⁻⁴micrometers, and the second set of sublayers thereof from 5×10⁻⁴to 2000×10⁻⁴micrometers.

As stated in claim5, the light-emitting semiconductor device of the above summarized configuration may be fabricated by a method comprising the steps of providing a baseplate (11) of a single crystal of silicon containing impurities and having a low resistivity, forming by a vapor phase growth on the baseplate (11) a buffer layer (12) in the form of an alternation of a first set of sublayers (12a) of Al_xGa_1-xN, where x is greater than zero and not greater than one, and a second set of sublayers (12b) of GaN or Al_yGa_1-yN, where y is less than x and more than zero and less than one, forming by vapor phase growth on the buffer layer a semiconductor region (10) containing a plurality of GaN-based compound semiconductor layers for emission of light, and forming a first electrode (17) on the semiconductor region (10) and a second electrode (18) on the baseplate (11).

The invention as set forth above yields the following advantages:

- 1. Use of silicon or a silicon compound as the baseplate enables substantive reduction in the manufacturing cost of the light-emitting device.
- 2. The buffer layer, an alternation of Al_xGa_1-xN sublayers and GaN or Al_yGa_1-yN sublayers, conduces to improvement of the crystallinity and flatness of the overlying GaN-based compound semiconductor layers. The result is high efficiency with which light is emitted despite use of the cheap baseplate.
- 3. Being compounded of Al_xGa_1-xN sublayers and GaN or Al_yGa_1-yN sublayers, the buffer layer has a coefficient of thermal expansion intermediate that of the silicon or silicon compound baseplate and that of the GaN-based compound semiconductor region, thereby preventing or limiting the warping of the device due to a difference in coefficient of thermal expansion between the baseplate and the semiconductor region.
- 4. The two electrodes are disposed opposite each other, resulting in the lower resistance of the current path and in less current and voltage requirements.
- 5. Connected directly to the, the second electrode is easy of fabrication.

The invention of claim2 provides a light-emitting device of even more favorable performance characteristics.

According to the invention of claim3, the buffer sublayers of Al_xGa_1-xN, which is relatively small in difference in lattice constant from silicon, are provided one directly on the baseplate and another between the buffer sublayers of GaN or Al_yGa_1-yN, resulting in improvement in the flatness of the buffer layer and the crystallinity of the semiconductor region.

According to the invention of claim4, the first set of buffer sublayers are each so determined in thickness as to provide a tunnel effect in terms of quantum mechanics, limiting the resistance of the buffer sublayers and reducing the power and voltage requirements of the device.

The invention of claim5 enables an easy and inexpensive fabrication of the light-emitting semiconductor device of the improved performance characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section through the light-emitting diode embodying the principles of the present invention.

FIG. 2 is a perspective view of the light-emitting diode ofFIG. 1.

FIG. 3 is a section through a semiconductor device similar to that ofFIGS. 1 and 2 but having another semiconductor device formed in its substrate.

BEST MODE OF CARRYING OUT THE INVENTION

The light-emitting semiconductor device according to the invention will now be described in detail in terms of the blue-light-emitting GaN-based compound diode illustrated inFIGS. 1 and 2. The exemplified blue LED comprises asemiconductor region10 composed of a plurality of GaN-based compound semiconductor layers for emission of light, a substrate orbaseplate11 of a silicon semiconductor having a crystal plane (111), and abuffer layer12. The light-emittingsemiconductor region10 comprises an n-type semiconductor region13 composed of GaN, a p-type light-emitting oractive layer14 composed of InGaN, and a p-type semiconductor region15 composed of GaN.

The lamination of the light-emittingsemiconductor region10, thebaseplate11 and thebuffer layer12 constitutes a substrate orbase body16. Ananode17 is formed on one of the two opposite major surfaces, or on the top as seen in the attached drawings, of thebase body16, or on thesemiconductor region15, and acathode18 on the other major surface, or on the bottom, of the base body. Thebuffer layer12, the n-type semiconductor region13, theactive layer14, and the p-type semiconductor region15 are grown epitaxially on thebaseplate11, in that order and with their crystal orientation aligned.

Thebaseplate11 is made of a single crystal of silicon containing impurities that determine its conductivity type. Thebaseplate11 has an impurity concentration ranging from 5×10¹⁸cm⁻³to 5×10¹⁹cm⁻³, and a resistivity ranging from 0.0001 ohm-cm to 0.01 ohm-cm. Made from n-type silicon into which is introduced arsenic, thebaseplate11 is low in resistivity that it serves as a current path betweenanode17 andcathode18. Additionally, being as thick as approximately 350 micrometers, thebaseplate11 functions as a support for thesemiconductor region10 and thebuffer layer12.

Thoroughly covering one surface of thebaseplate11, thebuffer layer12 is shown as an alternation of twobuffer sublayers12aand another twobuffer sublayers12b. In practice, however, the buffer layer may be constituted of as many as fiftysublayers12aand another fiftysublayers12bin alternation.

The first set ofbuffer sublayers12aare made from substances that can be defined by the chemical formula, Al_xGa_1-xN, where x is greater than zero and equal to or less than one. Examples of such substances are aluminum nitride (AlN) and aluminum gallium nitride (AlGaN). The first set of buffer sublayers12aare made from AlN (x being one in the general formula above) in this particular embodiment of the invention. Eachsuch sublayer12ais an extremely thin, electrically insulating film.

The second set ofbuffer sublayers12bare extremely thin, insulating films of an n-type semiconductor that is either GaN or any of substances expressed by the formula, Al_yGa_1-yN, where y is less than x and greater than zero and less than one. In use of Al_yGa_1-yN for the second set ofbuffer sublayers12b, it is recommended that y be made greater than zero and less than 0.8, in order to prevent an increase in the resistance of these sublayers.

The first set of buffer sublayers12ashould each have a thickness ranging from 5×10⁻⁴micrometers to 100×10⁻⁴micrometers, or 5-100 Angstroms, preferably from 10×10⁻⁴micrometers to 80×10⁻⁴micrometers. If less than five Angstroms in thickness, the first set of buffer sublayers12awould fail to keep the overlying n-type semiconductor layer13 sufficiently flat, and, if more than 100 Angstroms in thickness, would fail to provide the desired quantum-mechanical tunnel effect, resulting in an undue increase in the resistance of thebuffer layer12.

The second set ofbuffer sublayers12bshould each have a thickness ranging from 5×10⁻⁴micrometers to 2000×10⁻⁴micrometers, or 5-2000 Angstroms, preferably 10-300 Angstroms. If each less than five Angstroms in thickness, the second set ofbuffer sublayers12bwould fail to provide a desired degree of electrical connection between the neighboringsublayers12a, causing an undesired increase in the resistance of thetotal buffer layer12. If each more than 2000 Angstroms in thickness, on the other hand, the second set ofbuffer sublayers12bmight fail to hold the overlying n-type semiconductor layer13 sufficiently flat.

Speaking more strictly, and in this particular embodiment of the invention, the two sets of

sublayers

12aand12bare each 50 Angstroms. The total thickness of thebuffer layer12 is therefore 5000 Angstroms.

A method of fabricating the light-emitting semiconductor device according to the invention will now be explained on the assumption that the first set of buffer sublayers12aare of AlN, and the second set ofbuffer sublayers12bare of GaN.

The known metal organic chemical vapor deposition (MOCVD) method is recommended for alternate fabrication of the AlN and GaN buffer sublayers. A monocrystalline silicon substrate orbaseplate11 may first be placed in a MOCVD reaction chamber to have oxide films removed from its surfaces by thermal annealing. Then afirst buffer sublayer12aof AlN may be formed to a thickness of approximately 50 Angstroms on one of the major surfaces of thebaseplate11 by introducing trimethyl aluminum (TMA) and ammonia (NH₃) gases into the reaction chamber for approximately twenty-seven seconds. Actually, after heating thebaseplate11 to 1120° C., the TMA gas, or aluminum in effect, was supplied at a rate of approximately sixty-three micromoles per minute, and the NH₃gas, or NH₃itself, at a rate of approximately 0.14 micromoles per minute.

Then, with the heating temperature of thebaseplate11 maintained at 1120° C., the supply of the TMA gas suspended, and the gases of trimethyl gallium (TMG), NH₃, and silane (SiH₄) were introduced instead into the reaction chamber for approximately fifteen seconds. There will thus be created asecond buffer sublayer12bof n-type GaN to a thickness of fifty Angstroms in overlying relationship to thefirst buffer sublayer12aon thebaseplate11. The SiH₄gas is intended for introduction of Si, an n-type impurity, into the sublayer being formed. The TMG gas, or Ga in effect, was introduced at a rate of approximately sixty-three micromoles per minute; the NH₃gas, or NH₃itself, at approximately 0.14 moles per minute; and the SiH₄gas, or Si in effect, at approximately twenty-one nanomoles per minute.

In the case where there are fifty first buffer sublayers and fifty second buffer sublayers, as in this embodiment of the invention, the foregoing process of AlN sublayer creation may be repeated fifty times, and that of GaN sublayer creation as many times, in order to form abuffer layer12 consisting of one hundred alternating AlN and GaN sublayers. These numbers should not, however, be taken in a limitative sense: The buffer layer may be constituted of, for instance, fifty alternating such sublayers.

Next comes the step of successively fabricating the n-type semiconductor region13,active layer14, and p-type semiconductor region15 on thebuffer layer12 by the MOCVD method.

First, for formation of the n-type semiconductor region13, thebaseplate11 with thebuffer layer12 thereon was put into the MOCVD reaction chamber, into which were then introduced TMG, NH₃, and SiH₄gases. The SiH₄gas is intended for introduction of Si, an n-type impurity, into the n-type semiconductor region13. More specifically, thebaseplate11 with thebuffer layer12 thereon was heated to 1040° C. Then the TMG gas, or Ga in effect, was introduced at a rate of approximately 4.3 micromoles per minute; the NH₃gas, or NH₃itself, at approximately 53.6 millimoles per minute; and the SiH₄gas, or Si in effect, at approximately 1.5 nanomoles per minute. The n-type semiconductor region13 was thus formed to a thickness of approximately two micrometers.

It may be noted that the n-type semiconductor region13 is very thin compared with the thickness, from four to five micrometers or so, of the conventional LEDs. The impurity concentration of thesemiconductor region13 was approximately 3×10¹⁸cm⁻³, sufficiently less than that of thebaseplate11. The formation of thesemiconductor layer13 at as high a temperature as 1040° C. is possible thanks to the interposition of thebuffer layer12.

Then theactive layer14 of p-type InGaN was formed on the n-type semiconductor layer13. To this end, with the heating temperature of thebaseplate11 set at 800° C., there were introduced into the reaction chamber both trimethyl indium gas (hereinafter referred to as the TMI gas) and bis-cyclo pentadienylmagnesium gas (hereinafter referred to as the Cp₂Mg gas) in addition to TMG and NH₃gases. The Cp₂Mg gas was intended for introduction of Mg, a p-type impurity, into theactive layer14.

More specifically, for the fabrication of theactive layer14 as above, the TMG gas was introduced at a rate of approximately 1.1 micromoles per minute; the NH₃gas at approximately sixty-seven millimoles per minute; the TMI gas, or In in effect, at approximately 4.5 micromoles per minute; and the Cp₂Mg gas, or Mg, at approximately twelve nanomoles per minute. Theactive layer14 thus formed had a thickness of approximately 20 Angstroms and an impurity concentration of approximately 3×10¹⁷cm⁻³.

Then the p-type semiconductor region15 of p-type GaN was formed on theactive layer14. The heating temperature of thebaseplate11 was raised to 1040° C. toward this end, and there were introduced into the reaction chamber TMG, NH₃, and Cp₂Mg gases. The TMG gas introduced at approximately 4.3 micromoles per minute; the NH₃gas at approximately 53.6 micromoles per minute; and the Cp₂Mg gas at approximately 0.12 micromoles per minute. The thus-formed p-type semiconductor region15 has a thickness of approximately 0.5 micrometers and an impurity concentration of approximately 3×10¹⁸cm⁻³.

The MOCVD growth method set forth above has proved to make possible the fabrication of LEDs such that the crystal orientation of the monocrystalline silicon substrate orbaseplate11 is favorably followed by thebuffer layer12. Additionally, the n-type semiconductor region13,active layer14, and p-type semiconductor layer15 are all aligned with thebuffer layer12 in crystal orientation.

Then, for formation of the first electrode oranode17, nickel and gold were vacuum-deposited on the top of thesemiconductor body16, that is, on the p-type semiconductor region15 in low-resistance contact therewith. Disc-like in shape as depicted inFIG. 2, theanode17 is disposed centrally on thesemiconductor body16. Thatpart19 of the top surface of thesemiconductor base body16 which is left exposed by theanode17 lends itself to emission of light.

The second electrode or cathode was formed on the entire bottom surface of thebaseplate11, as indicated at18, rather than on the n-type semiconductor region13. Vacuum deposition of titanium and aluminum was used for cathode formation.

In use of the blue LED fabricated as above, thecathode18 may be mechanically and electrically connected, as by soldering or with use of an electrically conductive adhesive, to, for instance, an electrode on a circuit board. Theanode17 may be electrically coupled to an external electrode as by wire bonding.

Constructed and manufactured as in the foregoing, the blue LED according to the invention gains the following advantages:

- 1. The manufacturing costs of GaN-based semiconductor LEDs are reduced as the baseplates are made from silicon, which is far less expensive and far more easier of working upon than sapphire.
- 2. Thesilicon baseplate11 permits the fabrication of another electronic device or devices therein, making possible the provision of integrated semiconductor circuits in which GaN LEDs are incorporated with other semiconductor devices on one and the same semiconductor baseplate. InFIG. 3 is shown the LED largely constructed as inFIGS. 1 and 2 and having another semiconductor device such as a diode or atransistor20 formed in thesilicon baseplate11 via a p-type semiconductor region21. (The capitals B, C and E denote the base, collector, and emitter, respectively, of the transistor.) Such integration of light-emitting devices according to the invention with other semiconductor devices will provide smaller and cheaper composite devices.
- 3. The LED according to the invention is favorable in light-emitting characteristics and low in power requirement and operational resistance. Reasons for these performance characteristics will be set forth in the following:
  - 3-1. Shown as an alternation of thefirst sublayers12aof AlN and thesecond sublayers12bof GaN, thebuffer layer12 favorably conforms to the crystal orientation of theunderlying silicon baseplate11. On thisbuffer layer12, moreover, the GaN-basedsemiconductor region10 comprising the n-type semiconductor region13,active layer14, and p-type semiconductor region15 is formed with all their crystal orientation in alignment. Hence the favorable performance characteristics of theGaN semiconductor region10 and so of the light-emitting characteristics of the LED
  - 3-2. Thesemiconductor region10 has its flatness improved by being formed on thebaseplate11 via thebuffer layer12 of the multiple AlN and GaN sublayers12aand12b. Should the buffer layer consist of a GaN semiconductor alone, no GaN semiconductor region of favorable flatness would be created on that buffer layer by reason of too much difference in lattice constant between silicon and GaN. The improved flatness of the GaN-basedsemiconductor region10 according to the invention owes to the provision of the AlN sublayers12ain alternation with the GaN sublayers12b, the difference in lattice constant between silicon and AlN being much less than that between silicon and GaN. Improvement in the flatness of the GaN-basedsemiconductor region10 leads directly to improvement in light-emitting characteristics.
  - 3-3. Upon application of a forward voltage betweenanode17 andcathode18, such that the anode potential is higher than the cathode potential, forward current will flow between the two electrodes in the thickness direction of thesemiconductor body16. There will therefore be no current component flowing through the n-type semiconductor region13 in a direction parallel to the plane of thebaseplate11, as has taken place in the prior art LEDs with the sapphire baseplates. Furthermore, as theanode17 is disposed centrally of thesemiconductor body16, and thecathode18 all over the underside of the semiconductor body, current will flow fromanode17 tocathode18 all through the body of thesemiconductor body16, affording curtailment of current and voltage requirements.
  - 3-4. Thebuffer layer12 has its resistance value minimized as the AlN sublayers12aof thebuffer layer12 are each so determined in thickness as to give rise to a quantum-mechanical tunnel effect. Being electrically insulating, the AlN sublayers12awould make thebuffer layer12 inconveniently high in resistance if they were thicker than taught by the instant invention. According to the invention, however, thebuffer layer12 is sufficiently low in resistance partly because the AlN sublayers12aare so thin as above and partly because they are laminated alternately with the electrically conductive GaN sublayers12b. The results are low power requirement and low operational resistance.
- 4. The warping of the device due to a difference in the coefficient of thermal expansion between GaN-basedsemiconductor region10 andbaseplate11 is minimized. Silicon and GaN are so different in the coefficient of thermal expansion that a considerable deformation of the device would arise if they were placed in direct contact with each other. In the blue LED disclosed above, however, thebuffer layer12 is constituted of the AlN sublayers12aand GaN sublayers12bthat are so different in the coefficient of thermal expansion that the buffer layer has a mean coefficient of thermal expansion intermediate those of the GaN-basedsemiconductor region10 and thesilicon baseplate11. Thus, thanks to thebuffer layer12, the LED is prevented from warping due to the difference in the coefficient of thermal expansion between GaN-basedsemiconductor region10 andbaseplate11.
- 5. Thecathode18 is easier to form than with the prior art light-emitting devices having sapphire baseplates. Conventionally, the equivalents of theactive layer14 and p-type semiconductor region15 have had to be partly removed to expose part of the n-type semiconductor region13, and a cathode formed on this exposed part of the semiconductor region. The prior art devices have thus had the drawbacks of greater trouble in formation of the cathode and a greater exposed surface area of the n-type semiconductor region resulting from the cathode formation. The present invention defeats these drawbacks.

Possible Modifications

Notwithstanding the foregoing detailed disclosure, it is not desired that the present invention be limited by the exact details of such disclosure. The following is a brief list of possible modifications of the illustrated embodiments which are believed to fall within the purview of the instant invention:

- 1. Thebaseplate11 could be made from polycrystalline silicon, instead of from monocrystalline silicon, or from a silicon compound such as silicon carbide.
- 2. The various layers of thesemiconductor body16 could be opposite in conductivity type to those specified above in connection with the illustrated embodiments.
- 3. The n-type semiconductor region13,active layer14, and p-type semiconductor region15 could each be constituted of two or more semiconductor regions.

INDUSTRIAL APPLICABILITY

The present invention provides LEDs and like light-emitting devices of low resistance and low power loss.