United States Patent [191
`Stevenson et al.
`
`[54] GALLIUM NITRIDE
`METAL-SEMICONDUCTOR JUNCTION
`LIGHT EMITTING DIODE
`[76] Inventors: David A. Stevenson, 331 Lincoln
`Ave., Palo Alto, Calif. 94301;
`Walden C. Rhines, 9321 Forest Ln.,
`Apt. 1096, Dallas, Tex. 85231;
`Herbert P. Maruska, 2326
`California St., No. 39, Mountain
`View, Calif. 94040
`Mar. 12, 1973
`[22] Filed:
`[21] Appl. No.: 340,539
`
`[52] U.VSY._ CL... 313/499, 317/235 UA,371”7]235 AD
`[51] Int. Cl. ......................................... .. H05b 33/14
`[58] Field of Search ........ .. 313/108 D; 317/235 UA,
`317/235 AD; 331/94.5 H
`
`[56]
`
`3,404,305
`
`References Cited
`UNITED STATES PATENTS
`10/1968
`Wright .......................... .. 313/108 D
`
`3,819,974
`[111
`[45] June 25, 1974
`
`3,462,630
`
`'8/1969
`
`Cuthbert et al ............... .. 313/108 D
`
`Primary Examiner-Herman Karl Saalbach
`Assistant Examiner—Siegfried H. Grimm
`Attorney, Agent, or Firm—Flehr, Hohbach, Test, Al
`britton & Herbert
`
`[57]
`
`ABSTRACT
`
`A light emitting diode comprising a ?rst layer of gal
`lium nitride, a second, substantially intrinsic layer of
`magnesium doped gallium nitride forming a junction
`therewith, a metallic rectifying contact to the second
`layer, an ohmic contact to the ?rst layer, and means
`for applying a voltage across said contacts and said
`junctions whereby to bias thev device and generate
`light.
`
`4 Claims, 7 Drawing Figures
`
`l8
`
`lN DIU M
`CONTACT
`
`SAPPHIRE
`SUBSTRATE
`
`n-Go N
`
`Pea N: Mg
`
`lNDlUM
`CONTACT
`
`21
`
`TCL 1032, Page 1
`
`

`

`PATENTEDsux25 iar
`
`|
`SHEET 1 OF 2
`
`3°8191974
`
`FIG 2
`
`SAPPHIRE
`SUBSTRATE —
`
`
`Z
`aah p+ An
`CZ
`8
`CONTACT
`124
`tL n-GaN
`
`(RRS i-Ga N: Mg
` LS
`
`
`CONTACT
`
`INDIUM |
`
`TCL 1032, Page 2
`TCL 1032, Page 2
`
`

`

`PATENTEDJun25 i974
`
`SHEET 2 OF 2
`
`3,819,974
`,
`
`+14V,,2.0ma-———F
`
`+13V., |.8ma
`
`+I1V.,0.8ma (ARB.
`
`RELATIVEINTENSITY
`UNITS)
`
`20
`
`25
`
`3.0
`hv (eV)
`
`16 20 2A
`35 Wel—a—— 12
`INPUT CURRENT (mA)
`
`FIG.
`
`FIG. §
`
`15
`
`20
`25.
`hv (eV)
`
`i
`30
`
`qt
`=
`
`,
`
`08
`04
`
`0
`-0.2
`
`-8.0
`
`0
`VOLTS
`
`+8.0
`
`FIG. 6
`
`|
`
`FIG. 7
`
`>K
`
`E
`
`~2
`Ew
`Z2
`aan
`a>
`o>
`cen
`a
`im
`
`TCL 1032, Page 3
`TCL 1032, Page 3
`
`

`

`3,819,974
`
`2
`the n-type layer is typically 100 microns with an ap
`proximate range of thickness between 50 and 200 mi
`crons (it). The gallium nitride is formed by the reaction
`
`5
`
`2.0
`
`25
`
`45
`
`1
`GALLIUM NITRIDE METAL-SEMICONDUCTOR
`JUNCTION LIGHT EMITTING DIODE
`GOVERNMENT CONTRACT
`The invention described herein was made in the per
`formance of work under a research grant from the Ad
`vanced Research Projects Agency.
`BACKGROUND OF THE INVENTION
`This invention relates generally to light emitting di
`odes and more particularly to a violet light emitting di- _
`ode.
`Undoped gallium nitride always occurs highly n-type
`(n > 1018 cm‘”) and thus far has not been made con
`ducting p-type. However, a deep acceptor such as zinc
`has been utilized to compensate the donors and pro
`duce insulating gallium nitride crystals. This dopant
`can be introduced during the growth of the gallium ni
`tride crystal. When the dopant is introduced after ini
`tial deposition of undoped material, an i-n junction is
`formed. In the prior art, red, yellow, green and blue
`light emitting diodes have been obtained with zinc
`doped insulating regions forming i-n junctions.
`OBJECTS AND SUMMARY OF THE INVENTION
`It is a general object of the present invention to pro
`vide a violet light emitting diode.
`It is another object of the present invention to pro
`vide a violet light emitting diode formed by a rectifying
`metal contact to an intrinsic magnesium doped layer of
`gallium nitride forming a junction with a gallium nitride
`layer.
`The foregoing and other objects of the invention are
`achieved by a light emitting diode comprising a ?rst
`layer of gallium nitride, a second layer of magnesium
`doped gallium nitride forming a junction therwith, a
`metal layer forming a rectifying junction with the sec
`ond layer, and means for applying a voltage across said
`junctions to generate and emit light.
`BRIEF DESCRIPTION OF THE DRAWING
`FIG. 1 shows the steps in the growing of a layered de
`vice in accordance with the invention.
`'
`FIG. 2 shows the step of forming ohmic contacts with
`the device regions.
`FIG. 3 shows a device in accordance with the inven
`tion mounted in a metallic support.
`FIG. 4 shows the electroluminescence spectrum with
`forward bias.
`'
`FIG. 5 shows the shift of forward bias electrolumi
`nescent peak with input current.
`FIG. 6 shows the electroluminescence spectrum with
`reverse bias.
`FIG. 7 shows typical current voltage characteristics
`for the device.
`DESCRIPTION OF PREFERRED EMBODIMENT
`Referring to FIG. 1, the steps of forming a junction
`gallium nitride light emitting diode are illustrated. A
`wafer or slice of single crystal ?ame-fusion-grown sap
`phire may be used as the substrate 11. A layer of highly
`n-type gallium nitride 12 is formed on one surface of
`the wafer 11 by transporting gallium as its gaseous
`monochloride and introducing nitrogen into the growth
`zone in the form of ammonia, both at an elevated tem
`perature (approximately 900°-950°C.) whereby there
`is epitaxially grown the GaN layer 12. The thickness of
`
`GaCl + NI-Ia = GaN + I-ICl + H2
`After growth of the region 12, the atmosphere is'doped
`by introducing metallic magnesium while the layer is
`being grown to form a magnesium doped gallium ni
`tride layer 13. The dopant atoms compensate the nor
`mally n-type growth to forma substantially intrinsic
`GaNzMg layer 13. The layer 13 forms an i-n junction
`14 with the layer 12. The magnesium is added by plac
`ing magnesium in a graphite crucible and maintaining
`it at approximately 710°C while passing thereover ni~
`trogen gas. This transports the elemental magnesium
`atoms into the growth zone where they deposit as an
`impurity or dopant with the gallium nitride to form the
`intrinsic GaNzMg region 13. The introduction of Mg
`produces an energetically deep (many kT above the va
`lence band) acceptor level which compensates the na
`tive donors in GaN, thus making it intrinsic. The thick
`ness of this intrinsic, i, layer 13 is typically 10p, with a
`possible range of 5-20p. and the magnesium concentra
`tion in the layer is typically 0.15 weight percent (1020
`atoms cm‘‘*) as determined by electron microprobe
`analysis, with a possible range of 5 X 1019 to 1021 atoms
`cm'i‘.
`After the formation of the slice shown in FIG. 1C, the
`slice is cut up or diced to form devices of predeter
`mined size. A metal layer 17, 100p. thickness or larger,’
`is deposited onto the surface of the intrinsic layer to
`form a second m-i rectifying junction 15 with the intrin- .
`sic layer. Various metals and deposition techniques
`may be utilized. For example, an indium-mercury amal
`gam may be painted on the surface of the magnesium
`doped gallium nitride region 13. The chip is then
`heated for about a minute at 400°C to drive off the
`mercury. This leaves a solid indium layer. Other metals,
`such as Al, Au, Pt and Ag, may be deposited as a layer
`17 by vacuum evaporation, chemical vapor deposition
`or by sputtering. Similar techniques are used to pro
`duce a metal ohmic contact 16 on the edge of the n
`layer 12. A variant in this structure consists of the re
`moval of a portion of the sapphire substrate or the in
`trinsic layer whereby contact may be made to a portion
`of the surface of the n-type layer 16.
`gThe device may be placed in a holder 18 such as
`shown in FIG. 3 comprising a cup-shaped metal holder.
`One surface of the indium contact 17 forms ohmic con
`nection with the holder. Leads 19 and 21 provide elec
`trical connection to the indium contact 16 and holder
`18 for application of voltage across the region 13 and
`junctions l4 and 15.
`In a device constructed in accordance with the fore
`going, electroluminescence or light generation is ob
`tained both with forward and reverse bias, that is, with
`the i-layer bias either positive or negative. The forward
`bias voltage is more e?icient. In the forward direction,
`substantial conduction begins at 10 volts and the violet
`light is readily seen in a well lit room at 20 volts. Under
`reverse bias, conduction occurs in 40 - 60 volt range
`and produces a greenish light. Emission under forward
`bias electroluminescence peaked in the region of 2.86
`- 2.98 electron volts in various samples. The spectral
`width of half maximum is about 400 me\/; A typical
`spectrum is shown in FIG. 4. It is seen that the peak
`
`TCL 1032, Page 4
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`

`

`3,819,974
`
`.,
`4
`to develop different colors for aesthetic purposes, but
`also to produce light in a spectral iange of greater sen
`sitivity for the human eye. By use of different phos
`phors, all the primary colors may be developed from
`this same basic device. An array of such devices may
`be used for color display systems; for example, a solid
`state TV screen.
`We claim:
`'
`
`-
`
`1. A light emitting diode comprising a ?rst region of
`gallium ‘nitride, a second region of magnesium doped
`gallium nitride on one surface thereof forming a recti
`fying junction therewith, a metal forming a rectifying
`junction with the second region; and means forming
`ohmic contact to said ?rst region whereby a voltage
`can be applied to said metal and said means forming
`ohmic contact to apply a voltage across said junctions.‘
`
`3
`shifts to shorter wavelength with increasing current
`until a saturation value is reached; an example of the
`saturation is shown in FIG. 5 wherein emission peak
`versus input current is shown. The voltage ‘current
`characteristics of a device constructed in accordance
`with the foregoing is shown in FIG. 7. The reverse bias
`light emission is shown in FIG. 6. Although the emitted
`light appears uniform to the unaided eye, it actually
`consists of an array of spots in the size range of 5 - 25a
`with an inter-spot distance of 100 — 200;.t, as deter
`mined by high resolution optical microscopy. The lumi
`nescence is believed to be the result of ?eld-emission
`of electrons trapped by the Mg acceptor levels, with
`subsequent recombination of electrons to the then
`empty levels left by the ?eld-emission. This process oc
`curs in regions of high electric ?eld. It has been deter
`mined by scanning electron microscopy that a high
`electric ?eld occurs at the i-n junction with forward
`bias, and at the m-i junction with reverse bias. The fact
`that these two junctions are expected to have different
`20
`characteristics is responsible for the shift in the peak of
`the luminescence in going from forward to reverse bias.
`
`2. A light emitting diode as in claim 1 wherein said
`magnesium has a concentration in the range of 5 X 10m
`to l02l atoms/cm“.
`3. A light emitting diode as in claim 2 wherein'said
`second layer has a thickness in the range of 5 to 20 mi
`crons.
`4. The method of generating violet light which com
`prises the steps of trapping electrons in magnesium ac
`ceptors in an intrinsic gallium nitride layer, causing re
`moval of said electrons from said acceptors by applying
`.an electric ?eld of sufficient magnitude to remove the
`electrons, and causing electrons to recombine with said
`magnesium acceptors whereby to generate violet light.
`
`Thus, it is seen that there has been provided an im
`proved light emitting diode capable of emitting light in
`the violet region of the spectrum. This device may be
`used as a source of violet light for applications where
`this spectral range is appropriate. This light may be
`converted to lower frequencies (lower energy) with
`good conversion efficiency using organic and inorganic
`phosphors. Such a conversion is appropriate not only
`
`25
`
`30
`
`35
`
`40
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`45
`
`50
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`55
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`65
`
`I
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`* * * =t=
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`*
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`TCL 1032, Page 5
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