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Hemispherical GaP:N green electroluminescent diodes
Solid-Store
Electronics,
1973, Vol.
16, pp. 1037-1042.
Pergamon
Press.
Printed in Great Britain
HEMISPHERICAL GaP : N GREEN ELECTROLUMINESCENT DIODES R. Z. BACHRACH,
R. W. DIXON
and 0. G. LORIMOR
Bell Laboratories,MurrayHill, New Jersey 07974, U.S.A. (Received
2 January
1973; in revisedform,
26 Feburaty
1973)
Abstract -
Hemispherically shaped, reduced active junction area, GaP green light-emitting diodes have been fabricated by mechanical polishing and electrochemical etching from p - n junctions grown by double liquid phase epitaxy on pulled crystal substrates. The hemispherical structure with the lightemitting area restricted to the central portion of the flat surface represents an idealized geometry in which to study luminescent generation and optical coupling phenomena in green-emitting material and devices. Experiments reported here show that the efficiency with which internally generated light is extracted from these GaP structures is at least twice as large as in encapsulated reduced junction area slurry cut cubical dice or mesa diodes. The junction reduction procedure is described and the properties of reduced junction area cubical diodes examined. A maximum external quantum efficiency of 0.41 per cent has been achieved with the hemispherical structure from a crystal which for other geometric devices yielded a maximum external efficiency of 0.17 per cent. Green d&e
1. INTRODUCTION
fabrvxtlon
geNIetry
THE
standard design of green-emitting batchfabricated GaP electroluminescent diodes closely follows that developed for red diodes [ll and typically utilizes rough sided slurry-diced diodes
Sl-nucontact
with junction areas of approximately 1.6 X 10m3cm2 (0.4 x 0.4mm2). Figure l(a) shows the structure schematically. There are at least two inadequacies of this structure for optimum green diode design [2] caused by intrinsic differences in the properties of red- and green-emitting Gap. The first relates to quantum efficiency as a function of current density and is exemplified in Fig. 1(b) which shows the green and red quantum efficiencies obtained with this structure as a function of current density. The green quantum efficiency peaks at current densities of several hundred amperes/centimetre2, instead of at the approximately 10 A/cm2 available with the standard jun&ion areas for the many applications where lo-20 mA driving currents are available. The second inadequacy relates to the efficiency with which interally generated light may be coupled out of the green diodes[3]. The large dielectric discontinuity which exists between GaP and known encapsulants reflects a large fraction of randomly directed light incident on the interface and greatly increases the average path length before photon escape. The room temperature absorptivities encountered in optimally nitrogendoped material, which range between 5-500 1037
nnP -
LEC LPE
(S,N)
LPE
(Zn,Nl
~,,one-d,str,buted W-c&COn+oct preform Current denslty,
A/U?
$ g I-O E ;t 2 o_, z f 001 O-I
I-O
10
Current.
100
1000
10.000
mA
Fig. 1. (a) Line drawing illustrating the basic geometry of the standard green GaP batch fabricated diode. Note the player is adjacent to the header. (b) Relative efficiency vs current for standard 0.4 X 0.4mm2 batch fabricated diodes. 1OmA is approximately 7 A/cm*.
1038
R. Z. BACHRACH,
R. W. DIXON
cm-‘, [4] imply that a significant fraction of the internally generated light can be reabsorbed in a device with characteristic dimensions of order 0. I to 1.O mm, especially when the multiple pass nature of the photon escape process is considered. Optical coupling experiments in fact show that in rough-sided encapsulated cubical devices, approximately 7.5 per cent of the generated light is lost due to self absorption. With respect to optical coupling efficiency, green-emitting GaP : N diodes represent an intermediate case between red-emitting GaP, where the self absorption is sufficiently small that even in nonoptimized rough-sided encapsulated cubical dice more than 60 per cent of the internally generated light escapes;[3] and a direct bandgap material such as GaAs or GaAsP, where the selfabsorption is sufficiently large that only the surface near the p-n junction couples near-band-edge emission from the device. These considerations indicate that it is desirable to fabricate diodes with active junction areas smaller than those presently considered standard and to investigate geometries which couple out the generated light more efficiently. The most appropriate such geometry is a polished, hemispherically shaped dome with the emitting area confined near the center of the flat face. This structure, or the related Weierstrass section of a sphere, has often been investigated previously, [5-91 and was first used to study recombination radiation in Ge [5]. In this geometry, the light emitted from the vicinity of the junction is incident within the critical angle at the semiconductor-encapsulant interface. The coupling efficiency for this “first-pass” light can be made much higher than that for randomly directed light and this should lead to more efficient devices. Except for certain minor variations, the hemisphere extracts the most light possible and thus establishes, experimentally, the limit of optical coupling efficiency which one may approach with other cheaper designs, i.e., it thus provides an emperical standard by which the efficiency of other designs may be judged. A prerequisite for using the dome geometry is that the dome material not be strongly absorbing for the emitted radiation[7-91. In the case of GaP :N, undoped GaP is a reasonably suitable dome material since most of the emission occurs below the intrinsic absorption edge [3]. In Sections 2 and 3 the fabrication of the GaP hemispherical diodes and the junction reduction process are described. The properties of reduced
and 0. G. LORIMOR
active junction area cubical diodes are discussed in Section 4. Section 5 discusses the hemisphere results. 2. FABRICATION The hemispherically shaped GaP : N green lightemitting diodes were fabricated by mechanical polishing in cylindrical races. The starting junction material was grown by double-liquid phase epitaxy on pulled crystal substrates in a vertical open tube dipping system [lo, 111. The doping parameters in these layers were similar to those given by Logan [12] as resulting in optimal diodes. Both epitaxial layers were approximately 30 pm thick and had nitrogen concentrations near 10’” crne3 as determined from optical absorption measurements. The doping favored injection into the n-layer. The p-LPE side of the starting slice was first polished and then the substrate side was lapped and polished to a final thickness of 0.4 mm. A distributed Be-Au contact was evaporated over a silane mask and alloyed into the p-layer, after which the into 0.8 X 0.8 X 0.4 mm” slice was slurry-cut dice. Pairs of dice were selected and bonded with p-sides together to form cubes approximately 0.8 mm on a side. The cubes were mechanically ground to form spheres by circulating them sequentially in several cylindrical races which had finer and finer grinding compounds impregnated into their peripheries. After the final polishing, the spheres were etched briefly in half-strength aqua regia and then separated and cleaned. A contact was wire bonded to the n-layer on the side of the dome and the electrical connections completed by bonding the die to the center of a TO- 18 header with a Si-Au preform. Junction restriction was achieved by selectively electrochemically etching the p-layer (which is adjacent to the TO-l 8 header). Previous experimental results on hemispherical light-emitting diodes achieved the reduced active area by forming the p-n junction with dopant in-diffusion[6,9]. This type of junction formation does not yet produce as efficient diodes in GaP [ 131 as the double LPE structure and therefore the electrochemical etching techniques were developed to reduce junction areas. These techniques allow the necessary junction areas to be reliably obtained. Diode spreading resistance [ 141 or proton bombardment might be possible alternative junction restriction techniques, but preferential etching has
GaP : N GREEN
ELECTROLUMINESCENT
the advantage of removing nitrogen containing material from regions of the diode where no light emission takes place. Initial-to-final junction diameter ratios greater than four were achieved in order to permit the emitted light to be incident on the domed surface within the critical angle. The minimum ratio needed is determined by the index ratio, or 3.4: 1 in air and 2.3 : 1 in Castolite. The diodes were etched in a 3 : 2 : 1 (60°C) solution of sulfuric acid-water-H,O, to clean the junction and reduce the surface-related current components. This concentration of etchant was found to degrade the polish of the dome less than the standard 3 : 1: 1 solution. Finally, the diodes were encapsulated with Castolite.
junction area by an order of magnitude. Since the etching rate changes by at least a factor of lo4 when the current flows through the etching solution, [ 151 the etching procedure can be electrically timed. This possibility makes the process sufficiently noncritical that it may be performed in a controlled manner on “batches” of diodes. Figure 2 shows a scanning electron micrograph of a reduced junction area cubical diode.6 The preferential etching has resulted in a very sharp boundary between the pand n-material. Note also the relatively uniform etching of the p-material and the absence of significant undercutting along the interface between the p-material and the header. 4. PROPERTIES
3. PREFERENTIAL
ETCHING
PROCEDURES
The diodes were etched in an electrochemical 1.25 N NaOH bath [ 151 consisting of a 250 ml beaker with a stainless steel electrode. The diodes were mounted with p-layer adjacent to the header. This configuration provides a mask so that the p-type selective etching proceeds only parallel to the junction. The distributed dot pattern of 1 mil dots on 2 mil centers insured that a uniform contact distribution was likely after area reduction. The selective etching procedures were similar to those reported by McGahan, [ 161 but with the important difference that the n-contact was connected to the solution electrode or left floating. With this configuration, deterioration of the n-electrical contact of the diode could be avoided. This is an important observation since it allows the preferential etching to be performed after diode assembly. The etching procedure can be realiably followed by measuring the junction capacitance which is proportional to the diode area. For a uniform etch rate, the capacitance or area should change according to
where /3 is the etching rate (assumed constant), C, is the initial diode capacitance and Lo2 the initial diode area. Using ultrasonic agitation to break up gas bubbles which form under the interface and disrupt the etching procedure, agreement with the expected relation is found. Etching rates are typically p = 10-4cm/sec; thus etching times of the order of 6-10 min are sufficient to reduce the
1039
DIODES
OF REDUCED
AREA
CUBICAL
ACTIVE
JUNCTION
DIODES
The reduced active junction area slurry diced cubical diode is an intermediary geometrical structure between the hemisphere and full area cube. This structure is analogous to the commonly used handmade sandblasted mesa structure, and, in fact, we find that the two have similar optical efficiency. Several effects are observable as the junction area is progressively reduced. First, the current density at fixed current increases as the junction area is reduced and an increase in the external efficiency results. One can see from Fig. l(b), that for a 10 mA operating current, an order of magnitude increase in current density gives approximately a factor of 2 increase in external efficiency. The active area in such a diode is approximately 0.15 X 0.15 mm2. In addition to current density effects, optical coupling efficiency changes occur as the junction area is reduced. The effect varies from a decrease in optical coupling efficiency when the n-contact is centered to an increase in coupling efficiency when the n-contact is off-center. The cases are distinguished by the fact that with the top contact centered and with a small junction area, a significant fraction of the first pass light is blocked by the contact. The effects occurring can be seen most clearly by plotting the efficiency versus current density. If no optical coupling changes occur, all the points should lie on the same curve. Figure 3 illustrates the effects seen for; (a) the contact on center and; (b) the contact off-center. The curves A, B, C, . . . represent the junction reduction shown and the curve De in (b) represents the results of encapsulation. Typically an unreduced diode shows a 100
R. Z. BACHRACH,
1040
Centered
Current
Off
contact
dens,+y
center
and 0. G. LORIMOR
R. W. DIXON
.
AhIll
COntaCt
Photon
1
_A
09
-
08
-
07
-
06
-
energy,
eV
Ubl
1000
Current density.
Ah2
Fig. 3. Quantum efficiency vs current density as a function of the junction area for slurry diced 0.4 X 0.4 mm* diodes. (a) 5-mil dia. centered n-contact: (b) Smil dia. off-center n-contact.
za c 2 I c_
A B
A, A,/2
05.
J
per cent increase in external efficiency upon encapsulation while the reduced area diodes show a 50 per cent increase. One sees in Fig. 3 that the reduced area diode with the contact in the center shows a reduced coupling efficiency while diodes with off-center contacts show an increased coupling efficiency. The separation of the curves in Fig. 3(b) indicate the optical coupling efficiency increases by approximately 100 per cent. Overall, the encapsulated reduced area geometry yields an external efficiency gain for lo-20 mA operating currents of 200-250 per cent relative to encapsulated full area diodes. This results from the enhanced efficiency with increased current density and from the optical coupling efficiency increase. One expects that the changes in optical coupling efficiency would be spectrally dependent because of the large variation of absorption over the emission spectra. In order to investigate these effects, the spectral composition of the light emitted by the green diodes at each stage of the fabrication process was measured. Typical data are presented in Fig. 4. Since the spectral output of irregularly shaped green-emitting GaP diodes may vary somewhat with direction, these spectra were measured with the diode in an integrating
‘; z LL
04.
03
-
02.
01
0
2.00
2 04
2 08
2 12 Energy,
2 20
2 16
2 24
2 28
2 32
ev
Fig. 4. The relative intensity vs energy as shown in; (a) on a logarithmic scale and in; (b) on a linear scale as a function of junction area. Note that as the junction area is reduced, the spectra shifts to lower energy. On encapsulation, the high energy portion of the spectra increases much more than the low energy.
sphere. The sphere has the property of weighting all emission angles equally. It should be noted that no spectral dependence on current density has been observed in these diodes for the near bandedge emission [ 171. Several features of the data in Fig. 4 are noteworthy. The first is that the intensities of the low energy portions of the spectra increase faster as the
GaP:N
GREEN ELECTROLUMINESCENT
junction area is decreased than do the higher energy components. We attribute this to the fact that the selective etching removes highly absorbing, nitrogen containing GaP, as well as modifying the interface reflectivity. The second feature is the greater percentage increase in optical coupling on encapsulation for the higher energy components. We attribute this effect to the fact that the decreased dielectric discontinuity resulting from encapsulation favors the extraction of first pass light which would otherwise be rather strongly absorbed in the diode. These spectral changes are seen when the n-contact is in either position. Table 1 compares the efficiences of encapsulated 10 mil mesas and reduced junction area slurry diced diodes. Because of severe current crowding in the mesa diodes, a comparison of the current densities in these diodes is not meaningful. The efficiency numbers are, however, representative of the efficiency plateau. One should observe that the handmade mesa structure and the slurry diced structure are now very similar, and one concludes from this table that they have comparable optical coupling efficiency. Tuble 1. Comparison of encapsulated green eficiencies of mesa and reduced urea model shop diodes Mesa
(o/o) Sample 1 Sample 2 Sample 3 Sample 4
0.156 0.205 0.145 0.139
Reduced area slurry diced (%I 0.156 0.222 0.150 0.146
5. PROPERTIES OF HEMISPHERICAL REDUCED ACTIVE JUNCTION AREA DIODES Figure 5 shows the efficiency vs current density curves typically obtained for a hemispherically shaped diode as the junction ares is reduced. The inset of the figure shows the junction area reduction relative to the initial junction area A. The change E + Ee results with encapsulation. The comparison of optical coupling efficiencies in different structures is complicated by the fact that the fractional increase in quantum efficiency on encapsulation is a function of the particular structure. In the necessarily brief discussion given here the comparisons of optical coupling
1041
DIODES
9 C
A/l-O6 A/l94
E
A/II
I
P c5 01
100
IO
current density
)
A
Wn2
Fig. 5. Green quantum efficiency vs current density followed in a hemispherical diode as the junction area is reduced. The separation between curves at a constant current density measures the optical coupling efficiency increase. The inset in the figure shows the fractional change in junction area relative to curve A. The change E+ k? resulted when the device was encapsulated in Castolite. efficiencies are intended to be applicable for epoxy encapsulated diodes. One finds in the case of hemispheres optical coupling efficiencies four or five times larger than those of batch fabricated slurry cut cubical dice or approximately double the best previously achieved. This includes an efficiency increase on encapsulation of the hemisphere of approximately 20-30 per cent. The latter increase is consistent with the trend found from cubical dice and reduced junction area cubical dice where the efficiency increases on encapsulation were 100 and 50 per cent. This trend is related to the increased efficiency with which light is coupled from the reduced junction area structures [3]. As the active junction area is reduced on a hemispherical diode and more of the emitted light is brought within the critical angle, the spectra from domed diodes shift continuously to higher energies. This is consistent with the enhanced extraction efficiency for the first-pass component in the hemispherical geometry and is a desirable effect not only from the viewpoint that it signifies an increased optical coupling efficiency, but also because it makes the diodes “greener”. This spectral trend is opposite that found in the reduced area cubical diodes where reduction of the junction area favored the low energy random path components. Figure 6 gives a comparison of normalized spectra from an encapsulated reduced
R. Z. BACHRACH,
1042
R. W. DIXON
and 0. G. LORIMOR 6. SUMMARY
,011 21
2.2
Energy.
2.3
eV
Fig. 6. Spectral comparison of an encapsulated dome and an encapsulated cubical die both with reduced area junctions. The curves show the relative increase in the coupling of high energy photons out of the domed diode.
area cube and reduced area dome diodes made from the same layer. The data are shown on a logarithmic scale to accentuate the line shape differences. The reduced area cube spectra has been normalized to the low energy tail of the dome spectra. These spectra show the enhancement of the high energy spectral component due to the improved first-pass optical coupling. The hemispherical dome properties just described were observed only on carefully polished structures. If the dome surface has a poor polish, the incident photon angles become randomized, and the advantages of the hemispherical shape are lost. The behavior observed is then similar to that of the rough cube. In general, the optical coupling enhancement achieved with the dome geometry depends sensitively on achieveing a first-pass condition and requires high quality sphericity and surface polish. The primary experiments confirming the optical coupling results discussed above essentially depend on comparing a given diode with itself at various stages of fabrication. Using this method, the optical coupling enhancement cited is typically seen if a finished device is achieved with a highly polished, spherical surface with the reduced junction in the center. The highest external efficiency achieved while operating on the high current density efficiency plateau attained to date with encapsulated hemispherical domed diodes is 0.41 per cent. The best encapsulated mesa or reduced junction area cubical die made from this crystal had a high current density external efficiency of 0.17 per cent.
We have demonstrated that the reduced junction area hemispherically shaped green GaP diodes achieve approximately 2-2*5-times the optical coupling efficiency of other diode geometries and more than four times the quantum efficiency at lo-20mA currents compared with the standard encapsulated cubical slurry diced diodes. The optical properties of the hemispheres relative to full area and reduced area cubes was described and discussed. Acknowledgemenf-Continuing discussions on optical coupling phenomena with W. B. Joyce have been extremely helpful. Discussions with P. D. Dapkus and W. H. Hackett, Jr., concerning electrochemical etching in III-V’s, the cooperation of G. W. Hammlott in taking the scanning electron micrograph, and the technical assistance of J. W. Mann, J. V. Rigo and N. J. Engelmann are much appreciated. REFERENCES 1. N. E. Schumaker and G. A. Rozgonyi, J. Elect. Chem. Sot. 119,1233 (1972). 2. R. Z. Bachrach, W. B. Joyce and R. W. Dixon, Paper 10.3, IEDM, Washington, DC, October 197 1. 3. W. B. Joyce, R. Z. Bachrach, and R. W. Dixon, Optical Diodes,
Extraction
E@ciency
of Electroluminescent
Paper 10.4, IEDM, Washington, D.C., October 197 1. 4. P. J. Dean, M. Gershenzon and G. Kaminksy, J. appl. Phys. 38,5332
(1967).
5. P. Aigrain, Physica, 20, 1010 (1954). 6. W. N. Carr and G. E. Pittman, Appl. Phys. Letts 3, 173 (1963).
7. S. V. Galginaitis, J. aml. Phvs. 36,460 (1965). 8. H. J. Heiko and G.. Ziegler, Solid-St. Electron. 10, 158 (1967). 9. E. d. Dierschke, L. E. Stone and R. W. Harsty, Appl. Phys. Lefts
19,98 (1971).
10. 0. G. Lorimor, L. R. Dawson, R. Z. Bachrach, D. D. Roccasecca, and R. G. Sobers (unpublished) 11. 0. G. Lorimor, W. H. Hackett, and R. Z. Bachrach J. Elect. Chern. (to be published). 12. R. A. Logan, H. G. White and W. Wiegmann, Solid-St.
Electron.
14,55 (1971).
13. L. C. Luther, H. C. Casey, Jr., S. E. Haszko, A. S. Jordan, 0. G. Lorimor, G. A. Rozgonyi, J. Elect. Mat. 1,54 (1972). 14. W. B. Joyce and S. H. Wemple, J. appl. Phys. 41, 3818 (1970).
15. R. L. Meek and N. E. Schumaker, .I. Elect. Chem. Sot. 119, 1148 (1972). 16. W. H. Hackett, Jr., T. E. McGahan, R. W. Dixon, G. W. Kammlott, J. Elect. Chem. Sot. 119, 973 (1972). 17. R. Z. Bachrach and 0. G. Lorimor, Phys. Rev. B7, 700 (1973).
Electronics,
1973, Vol.
16, pp. 1037-1042.
Pergamon
Press.
Printed in Great Britain
HEMISPHERICAL GaP : N GREEN ELECTROLUMINESCENT DIODES R. Z. BACHRACH,
R. W. DIXON
and 0. G. LORIMOR
Bell Laboratories,MurrayHill, New Jersey 07974, U.S.A. (Received
2 January
1973; in revisedform,
26 Feburaty
1973)
Abstract -
Hemispherically shaped, reduced active junction area, GaP green light-emitting diodes have been fabricated by mechanical polishing and electrochemical etching from p - n junctions grown by double liquid phase epitaxy on pulled crystal substrates. The hemispherical structure with the lightemitting area restricted to the central portion of the flat surface represents an idealized geometry in which to study luminescent generation and optical coupling phenomena in green-emitting material and devices. Experiments reported here show that the efficiency with which internally generated light is extracted from these GaP structures is at least twice as large as in encapsulated reduced junction area slurry cut cubical dice or mesa diodes. The junction reduction procedure is described and the properties of reduced junction area cubical diodes examined. A maximum external quantum efficiency of 0.41 per cent has been achieved with the hemispherical structure from a crystal which for other geometric devices yielded a maximum external efficiency of 0.17 per cent. Green d&e
1. INTRODUCTION
fabrvxtlon
geNIetry
THE
standard design of green-emitting batchfabricated GaP electroluminescent diodes closely follows that developed for red diodes [ll and typically utilizes rough sided slurry-diced diodes
Sl-nucontact
with junction areas of approximately 1.6 X 10m3cm2 (0.4 x 0.4mm2). Figure l(a) shows the structure schematically. There are at least two inadequacies of this structure for optimum green diode design [2] caused by intrinsic differences in the properties of red- and green-emitting Gap. The first relates to quantum efficiency as a function of current density and is exemplified in Fig. 1(b) which shows the green and red quantum efficiencies obtained with this structure as a function of current density. The green quantum efficiency peaks at current densities of several hundred amperes/centimetre2, instead of at the approximately 10 A/cm2 available with the standard jun&ion areas for the many applications where lo-20 mA driving currents are available. The second inadequacy relates to the efficiency with which interally generated light may be coupled out of the green diodes[3]. The large dielectric discontinuity which exists between GaP and known encapsulants reflects a large fraction of randomly directed light incident on the interface and greatly increases the average path length before photon escape. The room temperature absorptivities encountered in optimally nitrogendoped material, which range between 5-500 1037
nnP -
LEC LPE
(S,N)
LPE
(Zn,Nl
~,,one-d,str,buted W-c&COn+oct preform Current denslty,
A/U?
$ g I-O E ;t 2 o_, z f 001 O-I
I-O
10
Current.
100
1000
10.000
mA
Fig. 1. (a) Line drawing illustrating the basic geometry of the standard green GaP batch fabricated diode. Note the player is adjacent to the header. (b) Relative efficiency vs current for standard 0.4 X 0.4mm2 batch fabricated diodes. 1OmA is approximately 7 A/cm*.
1038
R. Z. BACHRACH,
R. W. DIXON
cm-‘, [4] imply that a significant fraction of the internally generated light can be reabsorbed in a device with characteristic dimensions of order 0. I to 1.O mm, especially when the multiple pass nature of the photon escape process is considered. Optical coupling experiments in fact show that in rough-sided encapsulated cubical devices, approximately 7.5 per cent of the generated light is lost due to self absorption. With respect to optical coupling efficiency, green-emitting GaP : N diodes represent an intermediate case between red-emitting GaP, where the self absorption is sufficiently small that even in nonoptimized rough-sided encapsulated cubical dice more than 60 per cent of the internally generated light escapes;[3] and a direct bandgap material such as GaAs or GaAsP, where the selfabsorption is sufficiently large that only the surface near the p-n junction couples near-band-edge emission from the device. These considerations indicate that it is desirable to fabricate diodes with active junction areas smaller than those presently considered standard and to investigate geometries which couple out the generated light more efficiently. The most appropriate such geometry is a polished, hemispherically shaped dome with the emitting area confined near the center of the flat face. This structure, or the related Weierstrass section of a sphere, has often been investigated previously, [5-91 and was first used to study recombination radiation in Ge [5]. In this geometry, the light emitted from the vicinity of the junction is incident within the critical angle at the semiconductor-encapsulant interface. The coupling efficiency for this “first-pass” light can be made much higher than that for randomly directed light and this should lead to more efficient devices. Except for certain minor variations, the hemisphere extracts the most light possible and thus establishes, experimentally, the limit of optical coupling efficiency which one may approach with other cheaper designs, i.e., it thus provides an emperical standard by which the efficiency of other designs may be judged. A prerequisite for using the dome geometry is that the dome material not be strongly absorbing for the emitted radiation[7-91. In the case of GaP :N, undoped GaP is a reasonably suitable dome material since most of the emission occurs below the intrinsic absorption edge [3]. In Sections 2 and 3 the fabrication of the GaP hemispherical diodes and the junction reduction process are described. The properties of reduced
and 0. G. LORIMOR
active junction area cubical diodes are discussed in Section 4. Section 5 discusses the hemisphere results. 2. FABRICATION The hemispherically shaped GaP : N green lightemitting diodes were fabricated by mechanical polishing in cylindrical races. The starting junction material was grown by double-liquid phase epitaxy on pulled crystal substrates in a vertical open tube dipping system [lo, 111. The doping parameters in these layers were similar to those given by Logan [12] as resulting in optimal diodes. Both epitaxial layers were approximately 30 pm thick and had nitrogen concentrations near 10’” crne3 as determined from optical absorption measurements. The doping favored injection into the n-layer. The p-LPE side of the starting slice was first polished and then the substrate side was lapped and polished to a final thickness of 0.4 mm. A distributed Be-Au contact was evaporated over a silane mask and alloyed into the p-layer, after which the into 0.8 X 0.8 X 0.4 mm” slice was slurry-cut dice. Pairs of dice were selected and bonded with p-sides together to form cubes approximately 0.8 mm on a side. The cubes were mechanically ground to form spheres by circulating them sequentially in several cylindrical races which had finer and finer grinding compounds impregnated into their peripheries. After the final polishing, the spheres were etched briefly in half-strength aqua regia and then separated and cleaned. A contact was wire bonded to the n-layer on the side of the dome and the electrical connections completed by bonding the die to the center of a TO- 18 header with a Si-Au preform. Junction restriction was achieved by selectively electrochemically etching the p-layer (which is adjacent to the TO-l 8 header). Previous experimental results on hemispherical light-emitting diodes achieved the reduced active area by forming the p-n junction with dopant in-diffusion[6,9]. This type of junction formation does not yet produce as efficient diodes in GaP [ 131 as the double LPE structure and therefore the electrochemical etching techniques were developed to reduce junction areas. These techniques allow the necessary junction areas to be reliably obtained. Diode spreading resistance [ 141 or proton bombardment might be possible alternative junction restriction techniques, but preferential etching has
GaP : N GREEN
ELECTROLUMINESCENT
the advantage of removing nitrogen containing material from regions of the diode where no light emission takes place. Initial-to-final junction diameter ratios greater than four were achieved in order to permit the emitted light to be incident on the domed surface within the critical angle. The minimum ratio needed is determined by the index ratio, or 3.4: 1 in air and 2.3 : 1 in Castolite. The diodes were etched in a 3 : 2 : 1 (60°C) solution of sulfuric acid-water-H,O, to clean the junction and reduce the surface-related current components. This concentration of etchant was found to degrade the polish of the dome less than the standard 3 : 1: 1 solution. Finally, the diodes were encapsulated with Castolite.
junction area by an order of magnitude. Since the etching rate changes by at least a factor of lo4 when the current flows through the etching solution, [ 151 the etching procedure can be electrically timed. This possibility makes the process sufficiently noncritical that it may be performed in a controlled manner on “batches” of diodes. Figure 2 shows a scanning electron micrograph of a reduced junction area cubical diode.6 The preferential etching has resulted in a very sharp boundary between the pand n-material. Note also the relatively uniform etching of the p-material and the absence of significant undercutting along the interface between the p-material and the header. 4. PROPERTIES
3. PREFERENTIAL
ETCHING
PROCEDURES
The diodes were etched in an electrochemical 1.25 N NaOH bath [ 151 consisting of a 250 ml beaker with a stainless steel electrode. The diodes were mounted with p-layer adjacent to the header. This configuration provides a mask so that the p-type selective etching proceeds only parallel to the junction. The distributed dot pattern of 1 mil dots on 2 mil centers insured that a uniform contact distribution was likely after area reduction. The selective etching procedures were similar to those reported by McGahan, [ 161 but with the important difference that the n-contact was connected to the solution electrode or left floating. With this configuration, deterioration of the n-electrical contact of the diode could be avoided. This is an important observation since it allows the preferential etching to be performed after diode assembly. The etching procedure can be realiably followed by measuring the junction capacitance which is proportional to the diode area. For a uniform etch rate, the capacitance or area should change according to
where /3 is the etching rate (assumed constant), C, is the initial diode capacitance and Lo2 the initial diode area. Using ultrasonic agitation to break up gas bubbles which form under the interface and disrupt the etching procedure, agreement with the expected relation is found. Etching rates are typically p = 10-4cm/sec; thus etching times of the order of 6-10 min are sufficient to reduce the
1039
DIODES
OF REDUCED
AREA
CUBICAL
ACTIVE
JUNCTION
DIODES
The reduced active junction area slurry diced cubical diode is an intermediary geometrical structure between the hemisphere and full area cube. This structure is analogous to the commonly used handmade sandblasted mesa structure, and, in fact, we find that the two have similar optical efficiency. Several effects are observable as the junction area is progressively reduced. First, the current density at fixed current increases as the junction area is reduced and an increase in the external efficiency results. One can see from Fig. l(b), that for a 10 mA operating current, an order of magnitude increase in current density gives approximately a factor of 2 increase in external efficiency. The active area in such a diode is approximately 0.15 X 0.15 mm2. In addition to current density effects, optical coupling efficiency changes occur as the junction area is reduced. The effect varies from a decrease in optical coupling efficiency when the n-contact is centered to an increase in coupling efficiency when the n-contact is off-center. The cases are distinguished by the fact that with the top contact centered and with a small junction area, a significant fraction of the first pass light is blocked by the contact. The effects occurring can be seen most clearly by plotting the efficiency versus current density. If no optical coupling changes occur, all the points should lie on the same curve. Figure 3 illustrates the effects seen for; (a) the contact on center and; (b) the contact off-center. The curves A, B, C, . . . represent the junction reduction shown and the curve De in (b) represents the results of encapsulation. Typically an unreduced diode shows a 100
R. Z. BACHRACH,
1040
Centered
Current
Off
contact
dens,+y
center
and 0. G. LORIMOR
R. W. DIXON
.
AhIll
COntaCt
Photon
1
_A
09
-
08
-
07
-
06
-
energy,
eV
Ubl
1000
Current density.
Ah2
Fig. 3. Quantum efficiency vs current density as a function of the junction area for slurry diced 0.4 X 0.4 mm* diodes. (a) 5-mil dia. centered n-contact: (b) Smil dia. off-center n-contact.
za c 2 I c_
A B
A, A,/2
05.
J
per cent increase in external efficiency upon encapsulation while the reduced area diodes show a 50 per cent increase. One sees in Fig. 3 that the reduced area diode with the contact in the center shows a reduced coupling efficiency while diodes with off-center contacts show an increased coupling efficiency. The separation of the curves in Fig. 3(b) indicate the optical coupling efficiency increases by approximately 100 per cent. Overall, the encapsulated reduced area geometry yields an external efficiency gain for lo-20 mA operating currents of 200-250 per cent relative to encapsulated full area diodes. This results from the enhanced efficiency with increased current density and from the optical coupling efficiency increase. One expects that the changes in optical coupling efficiency would be spectrally dependent because of the large variation of absorption over the emission spectra. In order to investigate these effects, the spectral composition of the light emitted by the green diodes at each stage of the fabrication process was measured. Typical data are presented in Fig. 4. Since the spectral output of irregularly shaped green-emitting GaP diodes may vary somewhat with direction, these spectra were measured with the diode in an integrating
‘; z LL
04.
03
-
02.
01
0
2.00
2 04
2 08
2 12 Energy,
2 20
2 16
2 24
2 28
2 32
ev
Fig. 4. The relative intensity vs energy as shown in; (a) on a logarithmic scale and in; (b) on a linear scale as a function of junction area. Note that as the junction area is reduced, the spectra shifts to lower energy. On encapsulation, the high energy portion of the spectra increases much more than the low energy.
sphere. The sphere has the property of weighting all emission angles equally. It should be noted that no spectral dependence on current density has been observed in these diodes for the near bandedge emission [ 171. Several features of the data in Fig. 4 are noteworthy. The first is that the intensities of the low energy portions of the spectra increase faster as the
GaP:N
GREEN ELECTROLUMINESCENT
junction area is decreased than do the higher energy components. We attribute this to the fact that the selective etching removes highly absorbing, nitrogen containing GaP, as well as modifying the interface reflectivity. The second feature is the greater percentage increase in optical coupling on encapsulation for the higher energy components. We attribute this effect to the fact that the decreased dielectric discontinuity resulting from encapsulation favors the extraction of first pass light which would otherwise be rather strongly absorbed in the diode. These spectral changes are seen when the n-contact is in either position. Table 1 compares the efficiences of encapsulated 10 mil mesas and reduced junction area slurry diced diodes. Because of severe current crowding in the mesa diodes, a comparison of the current densities in these diodes is not meaningful. The efficiency numbers are, however, representative of the efficiency plateau. One should observe that the handmade mesa structure and the slurry diced structure are now very similar, and one concludes from this table that they have comparable optical coupling efficiency. Tuble 1. Comparison of encapsulated green eficiencies of mesa and reduced urea model shop diodes Mesa
(o/o) Sample 1 Sample 2 Sample 3 Sample 4
0.156 0.205 0.145 0.139
Reduced area slurry diced (%I 0.156 0.222 0.150 0.146
5. PROPERTIES OF HEMISPHERICAL REDUCED ACTIVE JUNCTION AREA DIODES Figure 5 shows the efficiency vs current density curves typically obtained for a hemispherically shaped diode as the junction ares is reduced. The inset of the figure shows the junction area reduction relative to the initial junction area A. The change E + Ee results with encapsulation. The comparison of optical coupling efficiencies in different structures is complicated by the fact that the fractional increase in quantum efficiency on encapsulation is a function of the particular structure. In the necessarily brief discussion given here the comparisons of optical coupling
1041
DIODES
9 C
A/l-O6 A/l94
E
A/II
I
P c5 01
100
IO
current density
)
A
Wn2
Fig. 5. Green quantum efficiency vs current density followed in a hemispherical diode as the junction area is reduced. The separation between curves at a constant current density measures the optical coupling efficiency increase. The inset in the figure shows the fractional change in junction area relative to curve A. The change E+ k? resulted when the device was encapsulated in Castolite. efficiencies are intended to be applicable for epoxy encapsulated diodes. One finds in the case of hemispheres optical coupling efficiencies four or five times larger than those of batch fabricated slurry cut cubical dice or approximately double the best previously achieved. This includes an efficiency increase on encapsulation of the hemisphere of approximately 20-30 per cent. The latter increase is consistent with the trend found from cubical dice and reduced junction area cubical dice where the efficiency increases on encapsulation were 100 and 50 per cent. This trend is related to the increased efficiency with which light is coupled from the reduced junction area structures [3]. As the active junction area is reduced on a hemispherical diode and more of the emitted light is brought within the critical angle, the spectra from domed diodes shift continuously to higher energies. This is consistent with the enhanced extraction efficiency for the first-pass component in the hemispherical geometry and is a desirable effect not only from the viewpoint that it signifies an increased optical coupling efficiency, but also because it makes the diodes “greener”. This spectral trend is opposite that found in the reduced area cubical diodes where reduction of the junction area favored the low energy random path components. Figure 6 gives a comparison of normalized spectra from an encapsulated reduced
R. Z. BACHRACH,
1042
R. W. DIXON
and 0. G. LORIMOR 6. SUMMARY
,011 21
2.2
Energy.
2.3
eV
Fig. 6. Spectral comparison of an encapsulated dome and an encapsulated cubical die both with reduced area junctions. The curves show the relative increase in the coupling of high energy photons out of the domed diode.
area cube and reduced area dome diodes made from the same layer. The data are shown on a logarithmic scale to accentuate the line shape differences. The reduced area cube spectra has been normalized to the low energy tail of the dome spectra. These spectra show the enhancement of the high energy spectral component due to the improved first-pass optical coupling. The hemispherical dome properties just described were observed only on carefully polished structures. If the dome surface has a poor polish, the incident photon angles become randomized, and the advantages of the hemispherical shape are lost. The behavior observed is then similar to that of the rough cube. In general, the optical coupling enhancement achieved with the dome geometry depends sensitively on achieveing a first-pass condition and requires high quality sphericity and surface polish. The primary experiments confirming the optical coupling results discussed above essentially depend on comparing a given diode with itself at various stages of fabrication. Using this method, the optical coupling enhancement cited is typically seen if a finished device is achieved with a highly polished, spherical surface with the reduced junction in the center. The highest external efficiency achieved while operating on the high current density efficiency plateau attained to date with encapsulated hemispherical domed diodes is 0.41 per cent. The best encapsulated mesa or reduced junction area cubical die made from this crystal had a high current density external efficiency of 0.17 per cent.
We have demonstrated that the reduced junction area hemispherically shaped green GaP diodes achieve approximately 2-2*5-times the optical coupling efficiency of other diode geometries and more than four times the quantum efficiency at lo-20mA currents compared with the standard encapsulated cubical slurry diced diodes. The optical properties of the hemispheres relative to full area and reduced area cubes was described and discussed. Acknowledgemenf-Continuing discussions on optical coupling phenomena with W. B. Joyce have been extremely helpful. Discussions with P. D. Dapkus and W. H. Hackett, Jr., concerning electrochemical etching in III-V’s, the cooperation of G. W. Hammlott in taking the scanning electron micrograph, and the technical assistance of J. W. Mann, J. V. Rigo and N. J. Engelmann are much appreciated. REFERENCES 1. N. E. Schumaker and G. A. Rozgonyi, J. Elect. Chem. Sot. 119,1233 (1972). 2. R. Z. Bachrach, W. B. Joyce and R. W. Dixon, Paper 10.3, IEDM, Washington, DC, October 197 1. 3. W. B. Joyce, R. Z. Bachrach, and R. W. Dixon, Optical Diodes,
Extraction
E@ciency
of Electroluminescent
Paper 10.4, IEDM, Washington, D.C., October 197 1. 4. P. J. Dean, M. Gershenzon and G. Kaminksy, J. appl. Phys. 38,5332
(1967).
5. P. Aigrain, Physica, 20, 1010 (1954). 6. W. N. Carr and G. E. Pittman, Appl. Phys. Letts 3, 173 (1963).
7. S. V. Galginaitis, J. aml. Phvs. 36,460 (1965). 8. H. J. Heiko and G.. Ziegler, Solid-St. Electron. 10, 158 (1967). 9. E. d. Dierschke, L. E. Stone and R. W. Harsty, Appl. Phys. Lefts
19,98 (1971).
10. 0. G. Lorimor, L. R. Dawson, R. Z. Bachrach, D. D. Roccasecca, and R. G. Sobers (unpublished) 11. 0. G. Lorimor, W. H. Hackett, and R. Z. Bachrach J. Elect. Chern. (to be published). 12. R. A. Logan, H. G. White and W. Wiegmann, Solid-St.
Electron.
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13. L. C. Luther, H. C. Casey, Jr., S. E. Haszko, A. S. Jordan, 0. G. Lorimor, G. A. Rozgonyi, J. Elect. Mat. 1,54 (1972). 14. W. B. Joyce and S. H. Wemple, J. appl. Phys. 41, 3818 (1970).
15. R. L. Meek and N. E. Schumaker, .I. Elect. Chem. Sot. 119, 1148 (1972). 16. W. H. Hackett, Jr., T. E. McGahan, R. W. Dixon, G. W. Kammlott, J. Elect. Chem. Sot. 119, 973 (1972). 17. R. Z. Bachrach and 0. G. Lorimor, Phys. Rev. B7, 700 (1973).