1230 IEEE TRANSACTIONS O N ELECTRON DEVICES, VOL. ED-26, NO. 8 , AUGUST 1979

with convex mirror windows were fabricated. The lp~ak emission wavelength was 1.17 pm and the full width at half 12] maximum was about 1000 A. The I-L characteristics ex- hibited fairly good linearity.

[31 ACKNOWLEDGMENT

The authors wish to thank Prof. Y. Suematsu, Prof. 13. [4i Fukuyo, and Prof. S. Furukawa, as well as Associate I? .of. N. Oura, Tokyo Institute of Technology, and T. Yamamloto, KDD Research Laboratory, for helpful discussion of :his work.

[61 REFERENCES

[ l ] H. Osanai, T. Shioda, T. Moriyama, S. Araki, M. Horiguchi, T. [7] Izawa, and H. Takata, “Effect of dopants on transmission loss of low-OH-content optical fibers,” Electron. Lett . , vol. 12, pp.

Comparison of Surface- and Use in Fiber-Optical

549-550, Oct. 1976. T. P. Pearsall, B. I. Miller, R. J. Capik, and K. J. Bachmann, “Efficient lattice-matched double-heterostructure LED’s at 1.1

499-501, May 1976. pm from GaxInl-xAsyP1-y,” Appl. Phys. Let t . , vol. 28, pp.

A. G. Dentai, T. P. Lee, C. A. Burrus, and E. Buehler, “Small- area, high-radiance C.W. InGaAsP L.E.D.s emitting at 1.2 to 1.3 pm,” Electron. Lett . , vol. 13, pp. 484-485, Aug. 1977. K. Oe, S. Ando, and K. Sugiyama, “Surface emitting LED’s for the 1.2-1.3 pm wavelength with GaInAsP/InP double hetero- structures,” Japan. J. Appl. Phys., vol. 16, pp. 1693-1.694, Sept. 1917. H. Nagai and Y. Noguchi, “InP/Ga,Inl-,As P1-,, double- heterostructure for 1.5 pm wavelength,” Appl. h y s . Lett., vol. 32, pp. 234-236, Feb. 1978. I. Umebu, 0. Hasegawa, and K. Akita, “InGaAsP/InP D.H. L.E.D.s for fiber-optical communications,” Electron. Lett . , vol. 14, pp.

K. Wakao, K. Moriki, T. Kambayashi, and K. Iga, “GaInAsP/InP DH laser grown by newly designed vertical LPE furnace,” Japan.

499-500, Aug. 1978.

J. Appl. Phys., V O ~ . 16,pp. 2073-2074,Nov. 1977.

Edge-Emitting LED‘s for Communications

Abstract-The performance of state-of-the-art double-heterojunclion (DH) surface and edge emitters are compared with respect to their use in high-data-rate fiber-optical communication systems. Thick-window (20-25-pm) surface emitters with 2-2.5-pm thick active layers and emitting up to 15-mW optical power at 300 mA have been fabricated. For edge emitters, we use very-high-radiance-type devices with 5(313-8 thick active layers. For these two types of LED’s we examine differ- ences in structure and light coupling efficiency to fibers of various nu- merical apertures (NA). For typically good devices we compare the diodes’ output power capabilities, the powers coupled into step- :md graded-index fibers of various NA, and their respective frequency re- sponse. For the same drive current level, we find that edge emitters couple more power than surface emitters into fibers with NA 50.3. The edge emitters also have a 5 times larger bandwidths. We estimate that an edge emitter can couple 5-6 times more power into low nu- merical aperture (NA 50.2) fibers than a surface emitter of the same bandwidth. We conclude that edge emitters are preferred to surface emitters for optical data rates above 20 Mbits/s.

Manuscript received January 2, 1979;revised January 30, 1979. ::his work was supported in part by the Department of the Air Force, Rmme Air Development Center, Hanscomb Air Force Base, MA, and in pas’ by RCA Laboratories, Princeton, NJ.

The authors are with RCA Laboratories, Princeton, NJ 08540.

H I. INTRODUCTION

IGH-RADIANCE light-emitting diodes (LED’s) have been the subject of intensive research and development

for fiber-optical communications, due to their linearity, small temperature sensitivity, and inherently small sensitivity to gradual degradation (compared to injection lasers). As a result of these efforts, two basic types of diodes have emerged: sur- face-emitting LED’s [ l ] and edge-emitting LED’s [ 2 ] . We present in this paper a comparison between state-of-the-art surface and edge emitters fabricated in our laboratory. The edge emitters are of the “very-high-radiance” type on which we reported previously [3] . The surface emitters are “thick- window” type diodes whose structure and performance are presented below.

The two types of diodes are first compared from a general point of view; differences in structure, emission angular dis- tribution, and the consequent differences in coupling effi- ciency to optical fibers. For typically good diodes we com- pare the optical powers emitted into air, the powers coupled into optical fibers of various numerical apertures (NA), as well

0018-9383/79/0800~1230$00.75 0 1979 IEEE

BOTEZ AND ETTENBERG: COMPARISON OF SURFACE- AND EDGE-EMITTING LED’S 1231

5 0 p m METAL CONTACT 0 0.2 0.4 X

0.3& p+ - GaAs - I-I.SpE 2-2.5pm

T- p - AlxGal-xAs- p - AIXGaI.XAs-

20-25pm

i n - A\xGal+As-

-70pm -nc - GaAs-

DOPING (cm-3,

Ge IOl9 Ge IOi8 Ge 5x10’’

Sn , p l O ” 1 . 5 ~ 1 0 ”

si 2 x 1 0 ‘ ~

- 1 M K T A L CONTACT I C P O O p m J

Fig. 1. “Thick-window” DH surface emitter:structure; A1 content (x) variation; and doping concentrations.

as their respective frequency response. We find that a surface emitter can couple almost as much power as an edge emitter into low NA fibers, but at a high price in bandwidth. A dis- cussion of the frequency response for the two types of diodes is also presented.

11. THICK-WINDOW (NGa)As SURFACE EMITTERS The structure of our surface-emitting LED is shown in Fig. 1 .

The four-layer double heterojunction (DH) was grown by liquid-phase epitaxy (LPE) between 900 and 840°C in a pre- viously described LPE multibin boat [4]. For most surface emitters the n-(A1Ga)As confining layer of the DH struc- ture is also the window for the radiation generated in the active layer. We use the name “thick-window” for our diodes, since the n-AlGaAs layer was grown much thicker than for a regular DH laser structure (i.e., 20-25 pm versus 2-3 pm). The relatively thick window layer is grown by cooling between 900 and 855°C. As a result of A1 depletion from the solution, as well as the difference in segregation coefficients between Al and As [5] a graded Al,Gal-,As layer results with an Al concentration x varying be.tween 0.34 and 0.22 over -22 pm (see Fig. 1). The thick-window layer provides a sturdy diode base, which in turn considerably eases diode handling and po- tentially gives a more reliable device. Furthermore, the thick- ness of the window layer does not seem to affect the diode external efficiency since the light emitted in the active layer (x = 0.05-0.07) is only negligibly absorbed in n-Al,Ga,-,As material with x > 0.2 and n < 1 X 10” ~ m - ~ . Abe et al. have reported [6] on a similar thick-window structure, which they fabricate by using a modified LPE boat [7].

The active layer thickness in our structure is in the 2.0- 2.5-pm range, which has been shown experimentally [8] as well as predicted theoretically [8], [9 ] to be optimal for maxi- mum diode external quantum efficiency. The doping concen- tration in the active layer was chosen to be 5 X 10’ cm-3 (Ge) in order to improve the device frequency response with- out affecting its external quantum efficiency. The doping concentrations of the other layers are a compromise between the requirements of good current confinement [lo] and low device series resistance. The difference in Al content between the active layer and the passive layers was kept at relatively small values (Ax 0.17) since for our relatively thick active layers (i.e., 2 2 pm) the leakage currents are negligible when normally operating current densities are considered [8]. In turn, the relatively low AI content of the passive p layers in- sures a low device series resistance [ 111. By using a thermally

THICK- WINDOW

2 2 o c SURFACE EMITTER

1 5 -

/ I

3 5 0 p W COUPLED INTO 0.14 NA FIBER

OO 100 200 C U R R E N T ( m A )

300

/ 1 ~ 1 ~ 1 , 1 , 1 , 1 , ~ 8800 8700 8600 8500 8400 8300 8200 8100 8000 7900

WAVELENGTH, X (&

(b) Fig. 2. (a) Output optical power (into air) and voltage drop as a func-

tion of dc drive current for the best “thick-window” surface LED (Fig. 1). At the 12-mW output level 350 p W were coupled into flat- ended 0.14 NA step-index fiber of 90-Mm-diameter core. (b) Typical spectrum for “thick-window” surface LED’s (Fig. 1).

deposited SiOz mask and an evaporated metallic trilayer (Ti/Pt/Au), 50-pm-diameter dot contacts were placed on the p side of the wafer. The wells were etched into the n-type GaAs substrate with the mixture 95HzO2 (30 percent): 5 N H 4 0 H (29 percent NH,), which stops at the GaAs-n- Alo.3Gao.,As interface.

The performance of the best “thick-window” surface emitters thus fabricated is shown in Fig. 2(a) and (b). The output optical power was measured by placing a l-cm diameter cali- brated Si photodiode in close proximity to the etched-well rim of the LED. The output powers at 100 and 300 mA were as high as 5.8 and 15 mW, respectively. Above 200 mA, the power versus current curve deviates from a straight line due to Joule heating. In the linear output regime, an external quan- tum efficiency as high as 4.5 percent is obtained. These results are comparable to the best results reported in the literature for surface emitters with an active layer dopant concentration in the middle 1017 cm-3 range [6], [8], [12]-[14]. At the 12-mW output power level, 350 pW were coupled into 0.14 NA flat-ended fiber of 9O-pm core diameter (i.e., - 15.4-dB coupling loss), without the use of index-matching fluids. Plotted in the same figure is the diode I-V curve, which shows that the device electrical characteristics were quite good (i.e., 1.7-V potential drop at 100 mA and =,2-Sl series resistance). Fig. 2(b) shows a typical spectrum of our surface emitters. The spectral halfwidth is approximately 350 A. Many diodes

1232 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-26, NO. 8 , AUGUST 1979

SURFACE EMITTER EDGE EMITTER t

n*-GaAj *-CONTACT

P-CONTACT

GOAS -1AIGO)AL AlCalAs

- C O N T A C ~

--8 ACTIVE LAYER

EMITTED LIGHT ANGULAR DISTRIBUTIONS

Fig. 3. Surface- and edge-emitting LED's. Top: Schematic represcnta- tions. Bottom: Isolated active layers with light-generating v o l ~ mes marked by dashed lines, and the diodes' respective emission angular distributions.

showed interference peaks similar to the ones previously re- ported by King et al. [12] and Shimano et al. 1151, Tllese peaks can be represented by the relation

where Ah is the spacing between the interference peaks, h is the vacuum wavelength, ne&) is an effective index of ref rac- tion (which takes into account dispersion effects), and L is the difference in optical path length between the interfering beams. For Ah EE 34 A, h = 8380 A, and n,ff 2 3.9 we obtain .I, = 53 pm from (1). This corresponds to interference between transmitted beams and twice-reflected (at the air-window interface and metal-cap interface) beams, in agreement v i th the findings of Shimano et al. [ 151 . However, our result:; as well as the ones reported by King et al. [12], disprove those authors' assertion that interference peaks cannot occur for active layer thicknesses larger than the electro1uminesce:lce wavelength (0.6 pm). Interference patterns should occur as long as the grown layers interfaces are parallel to one another.

111. SURFACE VERSUS EDGE EMITTER: GENERAL CONSIDERATIONS

Fig. 3 displays the geometry and emission distribution for the two types of LEDs. For (A1Ga)As DH structure surface emitters, a well must be etched through the GaAs substrate to prevent heavy absorption of the emitted radiation. The lab- rication also requires precise alignment of the dot contact >u,.ith the etched well. After etching, the relatively fragile diode base is usually strengthened by electroplating a thick Au layer li:.2] on the diode p side. By contrast, the edge-emitting LEI) is fabricated by standard oxide-stripe laser technology without post-contact etching and careful contact alignment. The icm- tact stripes for the edge emitters are 50-70pm wide, to maich typical fiber diameters (50-90 pm). The diodes are usudly 100-150 pm long. The front emitting facet has an antirefk ct- ing (AR) coating. Similar coatings and/or index-matching epoxies are used in surface emitters for more efficient light extraction [12] . We generally find that the fabrication of

BEAM ANGLE FROM NORMAL

(a)

t

" 90' 4 5 O 0 45' 90'

B E A M A N G L E F R O M N O R M A L

(b) Fig. 4. Typical far-field intensity patterns in the junction plane (611)

and in a plane perpendicular to the junction (@I) for: (a) very-high- radiance edge emitters [ 31 ; (b) surface emitters.

surface emitters is more difficult and of much lower yield than that of edge emitters.

The active layers for both types of diodes are shown isolated at the bottom of Fig. 3. The respective light generating vol- umes for both diodes are indicated with dashed lines. In a surface emitter the generated light spends little time in the active region and thus is not seriously subjected to reabsorp- tion. By contrast, in the edge emitter light must travel most of the diode length before being emitted into the air. As a consequence, edge emitters have lower external quantum effi- ciencies than surface emitters. However, the effect of absorp- tion in the active layer can be drastically reduced in the edge emitter by making symmetric DH structures of very thin active layers (300-500 A) [3]. Thus the randomly excited fundamental transverse modes have relatively little overlap of the field intensity with the absorbing active region (as shown in Fig. 3); consequently, the diode external quantum efficiency is increased. The effect can be modeled with the help of the radiation confinement factor ro [I61 applied to a wavelength-averaged background absorption loss coeffi- cient [ 171 .

The most striking difference between the two diodes appears when comparing the angular distributions of their emission. For a surface LED, the emitted light has a two-dimensional Lambertian distribution with a 120" full width at half power (FWHP) beamwidth (Fig. 3 and 4(b)). The corresponding radiation pattern is thus virtually spherical (Fig. 3). For the edge emitter, the presence of a transverse waveguide has the effect of funneling most of the emitted radiation in relatively narrow angles in the plane perpendicular to the junction [20] . As shown in Fig. 4(a) in the plane parallel to the junction the far-field intensity profile is still Lambertian with 120" beam-

BOTEZ AND ETTENBERG: COMPARISON OF SURFACE- AND EDG .E-EMITTING LED’S 1233

SURFACE OR “BURRUS” LED

,EDGE EMITTING L E D

,INJECTION LASER

01 I 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

FIBER N U M E R I C A L APERTURE

11.0

Fig. 5. Relative coupling loss to optical fibers of various numerical apertures for surface emitters, edge emitters, and lasers. The data points are: 0 and ., surface and edge emitters, this work (Fig. 9); 0, surface emitters, from [ 11 , [ 61 , [ 121.

width (FWHP), while in the plane perpendicular to the junc- tion far-field patterns with beamwidths of -30’ can be obtained [3]. The two-dimensional radiation pattern of the edge emitter looks then as an oblate spheroid (Fig. 3). It must be stressed that as opposed to large-optical-cavity-like edge emitters [I 71 , [18] , for DH edge emitters the light escaping the active layer gives only a negligible contribution to the de- vice angular distribution [20]. Possible explanations are light absorption in the “cap” layer and substrate and/or spontaneous- emission-rate enhancement for the guided modes [20].

The difference in radiation patterns between surface and edge emitters strongly affects their relative coupling to optical fibers of various NA. Fig. 5 shows the estimated coupling effi- ciencies for surface emitters, edge emitters, and injection lasers as a function of the NA of flat-ended optical fiber. Coupling efficiency calculations were done for the case when the source size is smaller than the fiber core area. For surface emitters with Lambertian angular distributions the coupling efficiency is simply given by [12] , [21] . For edge emitters and lasers one has to numerically compute the coupling integral [21] (i.e., the percentage of the emitted power contained within the fiber acceptance cone angle). The widths of the shaded regions shown in Rg. 5 represent estimated variations in coupling efficiency introduced by several factors: the differ- ence between fiber core and source diameters when the latter is the larger of the two [12] ; deviations from a Lambertian angular distribution for the emission of surface emitters; and variations in beamwidth for edge emitters and lasers.

For low NA fibers (i.e., NA < 0.2) the’ coupling efficiency for edge emitters can be 5 to 7 dB better than for surface emitters (Fig. 5 ) . We also plot experimental data (circles for surface emitters and squares for edge emitters) obtained by us (solid points) or by other workers [ l ] , [6] , [12] (open points). As can be seen, the experiments agree reasonably well with the calculated curves. While edge emitters have higher

I 2 r - - - - - l

‘ O t 8 -

6 -

-

-

0 50 100 150 200 250 300 350 lzcl *EDGE EMITTER

J= 2.3 kA/cm2

0 50 100 150 200 250 300 350

CURRENT ( m A 1

Fig. 6. Output optical power versus dc drive current for typically good surface and edge emitters. The surface emitters have 50-pm-diameter dot contacts and the edge emitters have 65-pm wide stripe contacts of 100-pm length.

fiber coupling efficiencies than surface emitters it is also true that surface emitters have higher external quantum efficiencies. Thus it is more relevant to compare the absolute coupled optical powers for typically good surface and edge emitters.

Iv. SURFACE VERSUS EDGE EMITTERS: TYPICALLY GOOD DEVICES

A. Power-Current Characteristics Within the same batch and/or from wafer to wafer the fabri-

cated diodes display a certain variation in performance. We find that the scatter in device external efficiency is especially pronounced for surface emitters. We believe that the differ- ence in yield between surface and edge emitters is due on one hand to intrinsically simpler processing to access the generated light in the edge emitter, and on the other hand we find that dot contacts are less adherent than stripe contacts. Lee and Dentai [8] also have reported surface LED efficiency varia- tions (up to 50 percent) for devices from the same batch. The choice of a typically good surface emitter was based on results from a large number of diodes obtained from nine wafers. Specifically, the chosen diode has -70 percent of the best diode efficiency (Fig. 2(a)), which, as mentioned previously, compares very well with the best results reported in literature [6] , [8] , [I21 -[14] for middle lo” cm-3 active region doping levels. We would like to point out that, although lower doping levels (e.g., 10l6 cm-3 and n-type) would provide somewhat higher diode efficiencies [13], such devices would not be relevant for our comparison since their relatively low (electrical) bandwidths (i.e., < 6 MHz) prohibit their use in fiber-optical communications applications [22] .

For edge emitters a typically good device was also chosen. The edge-emitting LEDs had undoped active layers -500 A thick, spectral halfwidths between 200-250 A [3], [21], and 100-pm diode lengths. Fig. 6 shows the output power capa-

1234 IEEE TFLiNSACTIONS ON ELECTRON DEVICES, VOL. ED-26, NO. 8 , AUGUST 1979

SURFACE EMlTTEFl

5 q 2 Fig. 7 . Near-field photographs (top) and intensity profiles (botto n) for

the surface and edge emitter chosen for comparison (Fig. 6 )

bilities of the two devices. At 150-mA bias the surface LED emits 2.6 times more power than the edge-emitting LED. That occurs, however, at approximately three times more current density for the surface emitter. No index matching epoxjes or conformal coatings were used. The near-field photographs and intensity profiles for the two devices are shown in Fig. 7. The surface emitter had a 50-pm contact diameter while the edge emitter had a 65-,um wide contact stripe. In both cases, the emission intensity is quite uniform over most of the emitting region. For coupling to optical fibers both devices were operated at a dc drive current of 150 mA.

B. Coupling to Optical Fibers o f Various NA Both graded- and step-index fibers were used in the coupling

experiments. Mode-stripped fibers were placed with the: help of a micropositioner inside the etched well of the su~face emitters and in close proximity to the air-n-AlGaAs intedace. For edge emitters the fibers were positioned close to the diode cleaved facet. The micropositioner was then used to maximize the light coupling. No matching lenses were employed. With one exception (i.e., a lensed 0.28 NA graded-index fiber) all coupling experiments were made with flat-ended (cleared) fibers.

Fig. 8 shows the results of light coupling to a 1-km-iong graded-index fiber of 60-bm core diameter and 0.28 NA. The optical power detected at the end of the 1-km fiber is plo :ted versus drive current for surface and edge emitters in two cases: flat-ended fiber and lensed-end fiber. The use of a long flber ensures that the measured power is only that which propag,ites in the fiber core. Then, by using the fiber loss paramete- (5 dB/km in this case) we extrapolate to find the core-coupled powers (right scale in Fig. 8). Over the whole range of drive currents (0-300 mA) the edge emitter is found to couple =:20 percent more power than the surface emitter. Lensing of the fiber receiving end brings about a IO-percent improvemen: in coupling efficiency for both LED types, This relatively snlall difference between the lensed and the flat-ended fibers is most probably due to the fact that the fiber core size is comparable to the size of the sources. Also, due to the radial variatiort of the refractive index, lensing of a graded-index fiber is less effective in enhancing the coupling efficiency than in the case

LENSED FIBER EDGE EMITTER WITH LENS

k/

CLEAVED END NO LENS +

I P I I I I 1 I I 0 0 50 100 150 200 250 300 350 0

CURRENT (mA)

Fig. 8. Light detected at the end of a 1-km 0.28 NA graded-index fiber (left scale) as a function of the drive current applied to the diodes of Fig. 6 . Solid and dashed lines correspond to flatended fiber (bottom right inset) and lensedend fiber (top left inset). The right scale shows the coupled power computed by using the fiber loss coefficient (5 dB/km).

SURFACE EMITTER (50prn 0IA.I - - - E D G E E M I T T E R (65pm S T R I P E ) -

@ m G R A D E D INDEX F I B E R

I

t 1 6 2 0 2 0.2 0.4 0.6

FIBER N U M E R I C A L A P E R T U R E

Fig. 9. Coupled optical powers versus fiber numerical aperture for the LED's of Fig. 6, at the 150-mA drive current level. The photore- sponse curves correspond to powers measured as the photodetector was moved at various distances away from the sources. The equiva- lent NA was calculated by using (2). The data for 0.28 NA graded- index fiber are obtained from Fig. 8.

The difference in emitted power angular distribution be- tween surface and edge emitters is evidenced in Fig. 9 by results of coupling experiments to fibers of various NA, and by pho- toresponse measurements of a 1-cm-diameter detector at various distances from the two sources. For the latter experi- ment we calculated the solid cone angle subtended by the detector at a given distance from the source, and translated it - -

of a step-index fiber [23]. into an equivalent NA. The relation used is

BOTEZ AND ETTENBERG: COMPARISON O F SURFACE- AND EDGE-EMITTING LED'S 1235

0 = 8, = sin-' (NA) (2)

where 8 is the half-angle of the cone covered by the detector area and 8, is the fiber critical angle, which from a coupling point of view is the fiber acceptance half-angle. For a step- index fiber NA = where n l , n 2 are the refractive indices of the fiber core and cladding, respectively. The photoresponse curves (solid for the edge emitter and dashed for the surface emitter) cross around 0.25 NA, which corre- sponds to a 14" acceptance half-angle. For NA < 0.25 the edge emitter provides more power than the surface emitter into the angle covered by the measurement. The opposite is true for NA > 0.25. These results are supported by light coupling experiments for the two types of diodes to several different NA fibers (round data points for the surface emitter and square data points for the edge emitter). The data points shown in Fig. 9 were obtained for flat-ended step- and graded- index fibers. Specifically, we used 50-pm core diameter step- index fibers of NA values 0.22, 0.28, 0.3, and 0.4; a 90-pm core diameter 0.14 NA step-index fiber; and graded-index fibers of 0.14 and 0.28 NA with 90- and 6Oym core diam- eters, respectively. The fibers were relatively short (3-4 ft) and mode stripped so as to eliminate cladding modes from the measurement. As can be seen from Fig. 9 the surface and edge emitters coupled the same amount of power for NA = 0.3. Just as in the case of the photoresponse curves, for NA values lower than the cross point, the edge emitter couples increas- ingly better than the surface emitter. Thus for 0.14 NA fibers, the edge LED couples ~ 4 0 percent more power than the sur- face LED. Similar ratios are obtained when coupling into 0.14 NA graded-index fibers. The coupling efficiencies for these graded-index fibers are about 30 percent of those for step- index fibers of the same NA and core diameter. We conclude that for NA < 0.3 edge emitters couple slightly better than surface emitters and that the roles are reversed for NA > 0.3. The differences, however, are not very dramatic. I t will be shown in the next subsection that the real difference between the two emitters lies in their frequency response. We note that our experimentally found crossover point for edge and surface emitters (i.e., NA = 0.3) is in good agreement with theoretically calculated values by Gloge [24] (i.e., NA = 0.25) and Marcuse [9] (i.e., NA = 0.23).

C. Frequency Response The frequency response of the two types of emitters was

measured at a bias level of 150 mA and the results are pre- sented in Fig. 10. The measurements were made with a 30-mA peak-to-peak signal. The electrical bandwidth (i.e., the 1.5-dB rolloff point in the modulated light output) is 17 MHz for the surface emitter and 100 MHz for the edge emitter.

To understand this difference, we must explore the factors which control the bandwidth. To first order the electrical bandwidth (f) is a measure of the injected carrier lifetime (7)

~ 2 6 1 1 f = -

2717

which, in turn, is composed of a radiative (7,) and a non-

Lo 7 - W '

EMITTER

2 3 5 7 IO 20 30 5 0 70 100 MODULATION FREQUENCY ( M H z 1

Fig. 10. The frequency response for the LED's of Fig. 6, at the 150- mA drive current level. A 30-mA peak-to-peak signal was used.

radiative (rn,.) part

The nonradiative component for relatively lightly doped active regions and active regions thicknesses (d) less than or equal to a carrier diffusion length is dominated by interfacial recombi- nation velocity (s) at the Alo.22Gao.,8As/Alo,05Gao.9sAs interface [27]

The radiative component for a p-type active region is [21]

- 1 = B(po + An) (6) r

where B is the recombination coefficient, p o is the active region hole concentration, and An is the injected carrier concentration.

At injection levels such that the injected carrier density is much lower than the doping density (An << p o ) and/or the active region thickness is larger than the carrier diffusion length the lifetime is solely determined by the doping level [81 , [211

1 - = Bpo, An <<po 7

At the other extreme, we have the case An >> p o when bi- molecular recombination (i.e., direct recombination of in- jected electrons and holes) occurs and then [21], [28], [29]

+= (z)'", A n > > p o .

For heavily doped active layers the lifetime is determined (at most current levels) by the doping density level (i.e., (7)). For lightly doped active layers the situation can be quite compli- cated. Thus as shown by Lee and Dentai for surface emitters [8], as the drive level is increased the lifetime is successively determined by the doping level; by a combination of doping and bimolecular recombination [8] , [21] ; and by bimolecular recombination (see (8)). For our surface emitters, which are optimized for high external quantum efficiency, the band- width is relatively small due to two factors: on one hand we have thick active layers (2-2.5 pm) which considerably reduce

1236

50mBIT

35mBIT

2 0 m B I T

IOmBlT

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-26, NO. 8 , AUGUST 1979

I1 ‘ ‘ IO 20 50 100 200 400

ELECTRICAL BANDWIDTH (MHz1

E D G E E M I T T E R S U R F A C E E M I T T E F Fig. 11. Optical pulse response of the LED’s compared in Figs. 6-,10,

when subjected to current pulse trains of various data rates.

the effects of bimolecular recombination (i.e,, (8)), and on t.le other hand, the doping level is moderately low (5 X 1017 cm-”,I.

The edge emitters studied here (i.e., of undoped and vlery thin active layers) operate in a regime where the lifetime is controlled almost completely by injected carrier density [3] ,

Due to strong bimolecular recombination, the edge emitter operates with a short carrier lifetime, which results in a hi1;b modulation frequency.

The effect of modulation bandwidth on optical pulse r:., sponse is illustrated in Fig. 11 for lo-, 20-, 3 5 , and 50-Mbit/s data rate pulse trains applied to our edge and surface emitters. While the edge emitter can handle each of these data rates comfortably, the surface emitter produces intersymbol inter- ference at rates much in excess of 20 Mbitsls.

For surface emitters, the most common way to increase the bandwidth is to heavily dope the active layer [6] , [8] , [ l ; ] since, as shown in (7), this decreases the carrier lifetime. Hal\+ ever, a bandwidth increase by heavy doping occurs at the expense of external quantum efficiency [6] , [8], [ 121, [30 1 . The decrease in diode external efficiency with increased doping is commonly attributed to an increase in the density of nonradiative recombination centers as the active layer d0pir.g density is increased [8] , [30], [31]. The tradeoff between J9,

the output optical power, and f, the electrical bandwidth is best illustrated by the fact that the power-bandwidth product (at the same drive level) is found to be almost constant for all surface emitters of active layer doping concentrations p o 10l8 cm-3 [32] -[35]. For lower doping levels the P X ,f

product actually decreases with respect to its heavy-doping almost constant value [ 121 , [ 131 . We show in Fig. 12 output power versus bandwidth results of Lee and Dentai [8] for DH surface emitters at the drive level I = 300 mA, and with active layer doping levels in the range 2 X 1017 ~ m - ~ - 2 X 10’’ ~ m - ~ . Their data are seen to be bordered by two parallel lines corre- sponding to P - f - o . 8 . Here, it is apparent that the light out- put of the surface emitter cannot be increased without a corre- sponding sacrifice in frequency response.

In Fig. 12, we also include our data points for surface and edge emitters versus the results of Lee and Dentai [8] , For the reader’s benefit we show a scale of the equivalent optical bandwidth, i.e., the frequency at which the diode light output drops to half its dc value. Our surface emitters (round dot) fall in the range defined by those authors. The edge emitters (square dot) of 120-MHz electrical bandwidth, provide 30-60 percent more power (into air) then surface emitters of same bandwidth. If we now take into account the diodes’ respective coupling efficiency to optical fibers (Fig. 5) it becomes obvious that for low NA fibers (i,e,, NA < 0.2) the edge emitters couple 5-6 times more power than surface emitters of same bandwidth. Furthermore, this occurs at three times less current density for the edge-emitting LED’s.

The question that arises is: can surface emitters be improved to have the same radiance-bandwidth product as that of edge emitters? One approach for increasing the surface LED band- width is tight current confinement (e.g., proton bombard- ment) [13] . Although the bandwidth is increased, the diode external efficiency is reduced and thus the power-bandwidth product is lowered or remains the same as before the current confining process [ 131 . As in the edge-emitter case, the band- width can also be improved by reducing the active layer thick- ness, since 7, - fi (see (8)). However, as d is decreased from 2.5 to 0.5 pm, the nonradiative interfacial surface recombina- tion becomes a factor (see (S)), and reduces the diode external quantum efficiency. For instance, the authors of [8] show a reduction in power by as much as a factor of 2 as they decrease

BOTEZ AND ETTENBERG: COMPARISON OF SURFACE- AND EDGE-EMITTING LED’S 1237

the active layer thickness from 2.5 to 0.5 pm. For layers thinner than 0.5 pm, they find some relative improvement in the diode efficiency, which they attribute to a reduction of the interfacial recombination velocity? in good agreement with the previously reported studies by Ettenberg and Kressel [27] . Thus it is conceivable that surface emitters of very thin active layers (d N- 500 A) could achieve higher radiance-bandwidth products than the best surface emitters reported to date.

V. CONCLUSION State-of-the-art surface-emitting LED’s were fabricated and

compared to previously reported [3] state-of-the-art edge- emitting LED’s. For typically good devices, it is found that at the same drive level, the surface emitter provides 2.5-3 times more power into air than the edge emitter. For evaluation of diode coupling to optical fibers, we used surface emitters with 50-pm-diameter dot contacts and edge emitters with contact stripes 65 pm wide and 100 pm long. Thus at the same drive currents, our edge emitters were operated at a current density approximately 3 times smaller than that of our surface emitters. For fiber NA less than 0.3, edge emitters couple sli&tly more power than surface emitters (a maximum ratio of 1.4 is ob- tained for NA = 0.14). For NA > 0.3, the situation is re- versed. At the same drive level, the bandwidth of the edge emitter is found to be more than 5 times larger than the sur- face emitter’s (i.e., 100 versus 17 MHz). The power versus bandwidth results are compared to extensive data by previous workers [8]. From that comparison, it is inferred that edge emitters can couple 5-6 times more power in low NA (NA 5 0.2) fibers than surface emitters of comparable bandwidth. Based on these observations, we conclude that edge emitters have a clearly superior performance over surface emitters when considering optical communications data rates between 20 and 100 Mbits/s, where low NA fibers are usually required. Sur- face emitters, in turn, are preferable to edge emitters for short optical data links with high NA fiber [36], [37] and data rates less than 20 Mbits/s. Finally, we find surface emitters to be more difficult to fabricate, and thus potentially more costly than edge emitters.

ACKNOWLEDGMENT The authors gratefully acknowledge helpful discussions with

H. Kressel, C. J. Nuese, and I. Ladany, and the excellent tech- nical expertise of I. J . Hegyi. They also thank M. Harvey, D. B. Gilbert, D. Marinelli, T. Purman, F. Hawrylo, and H. Kowger for technical assistance.

REFERENCES [ 1 ] C. A. Burrus, “Efficient small-area GaAs-Gal,Al,As hetero-

structure electroluminescent diodes coupled to optical fibers,” Proc. IEEE,vol. 59, pp. 1263-1264, Aug. 1971.

[ 2 ] M. Ettenberg, K. C. Hudson, and H. F. Lockwood, “High radi- ance light-emitting diodes,” IEEE J. Quantum Electron., vol. QE-9, no. 10, pp. 987-991, Oct. 1973.

[ 3 ] M. Ettenberg, H. Kressel, and J. P. Wittke, “Very high radiance edgeemitting LED,” IEEE J. Quantum Electron., vol. QE-12, p. 360, 1976.

[ 4 ] H. F. Lockwood and M . Ettenberg, “Thin solution multiple layer epitaxy,” J. Cryst. Growth, vol. 15,pp. 81-83, 1972.

[5 ] I. Crossley andM. B. Small, “The application of numerical methods to simulate the liquid phase epitaxial growth of Gal,Al,As from

an unstirred solution,” J. Cryst. Growth, vol. 15, pp.’ 268-214, 1972. M. Abe, I. Umebu, 0. Hasegawa, S. Yamakoshi, T. Yamaoka, T. Kotani, H. Okada, and M. Takanashi, “High-efficiency long- lived GaAlAs LED’s for fiber-optical communications,” IEEE Trans. Electron. Devices, vol. ED-24, pp. 990-994, July 1971. S. Isozumi, Y. Komatsu, M. Abe, T. Yamaoka, and T. Kotani, “Liquid phase epitaxial growth of Ga+,Al,As for LEDs with a 5 0 W r n thick window layer,” Paper C.1. presented at the 3rd IEEE Specialist Conf. on the Technology of Electroluminescent Diodes, San Francisco, CA, Nov. 1978. T. P. Lee and A. G. Dentai, “Power and modulation bandwidth of GaAs-AlGaAs high radiance LED’s for optical communication systems,” IEEE J. Quantum Electron., vol. QE-14, no. 3, pp. 150-159, Mar. 1978. D. Marcuse, “LED fundamentals: Comparison of front- and edge- emitting diodes,” IEEE J. Quantum Electron., vol. QE-13, no.

J. Zucker and R. B. Lauer, “Optimization and characterization of high radiance (Al, Ga)As double-heterostructure LED’s for opti- cal communication systems,” IEEE Trans. Electron. Devices, vol. ED-25, pp. 193-199, Feb. 1978. A. J. SpringThorpe, F. D. King, and A. Becke, “Te and Ge- Doping studies in Gal-,Al,As,” J. Electron. Mater., vol. 4 , pp. 101-118, Feb. 1975. F. D. King, A. J. SpringThorpe, and 0. I. Szentesi, “High-power long-lived double heterostructure LED’s for optical communica- tions,” in Int. Electron Devices Meeting, Tech. Dig., Washington, DC, 1975, paper 21.2, p. 480. J. C. Dyment, A. J. SpringThorpe, F. D. King, and 5 . Straus, “Proton bombarded double heterostructure LED’s,’’ J. Electron. Mater., vol. 6, no. 2, pp. 113-193, 1977. J. C. Dyment, F. P. Kapron, and A. J. SpringThorpe, “Top and side emission from double heterostructure LED’s and lasers,” IEEE Trans. Electron Devices, vol. ED-24, pp. 995-1000, July 1977. N. Shimano, T. Yamamoto, T. Kawasaki, H. Takano, and Y. Morimoto, “Observation of internal interference in emission spec- trum of the Burrus type GaAs-GaAlAs DH LED’s with thin active layer,” Japan. J. Appl. Phys., vol. 17, no. 1, pp. 237-238, 1918. D. Botez, “Analytical approximation of the radiation confine- ment factor for the TEo mode of a double heterojunction laser,” IEEE J, Quantum Electron., vol. QE-14, pp. 230-232, API. 1978. D. Botez, P. Zory, and M. J. Brady, “Simple expression for the light output of edgeemitting DH LED’s,’’ Electron. Lett., vol. 14, no. 22, pp. 716-718, Oct. 1978. Y. Horikoshi, Y. Takanashi, and G. Iwane, “High radiance light- emitting diodes,” Japan J. Appl. Phys., vol. 15, pp. 485-492, Mar. 1976. Y. Seki, “Light extraction efficiency of the LED with guide layers,” Japan J. Appl. Phys., vol. 15, pp. 327-338, Feb. 1976. J. P. Wittke, “Spontaneousemission-rate alteration by dielectric and other waveguiding structures,” RCA Rev., vol. 36, pp. 655- 666, Dec. 1975. J. P. Wittke, M. Ettenberg, and H. Kressel, “High radiance LED

June 1976. for single-fiber optical links,” RCA Rev., vol. 37, pp. 159-183,

K. C. Kao and M. E. Collier, “Fiber optical system in future tele- communications networks,” Telecommun., p. 25,1977. D. Kato, “Light coupling from a stripe-geometry GaAs diode laser into an optical fiber with spherical end,” J. Appl. Phys., vol. 44, pp. 2756-2758, June 1973. D. Gloge, “LED design for fibre systems,” Electron. Lett. vol. 13, no. 4, pp. 399-400, July 1977. Y. S. Liu and D. A. Smith, “The frequency response of an ampli- tude-Modulated GaAs luminescence diode,” Proc. IEEE (Lett.), vol. 63, pp. 542-544, Mar. 1975. H. F. Lockwood, J. P. Wittke, and M. Ettenberg, “LED for high data rate, optical communications,” Optics Commun., vol. 16, no. 1, pp. 193-196, Jan. 1976. M. Ettenberg and H. Kressel, “Interfacial recombination at AlGaAs/GaAs heterojunction structures,” J. Appl. Phys., vol. 47,

H. Namizaki, H. Kan, M. Ishii, and A. Ito, “Current dependence of spontaneous lifetimes in GaAs-AlGaAs double heterostructure lasers,” Appl. Phys. Lett., vol. 24, pp. 486-487, May 1974. J. Zucker, R. B. Lauer, and J. Schlafer, “Response time of Ge-

10, pp. 819-827, Oct. 1977.

pp. 1538-1544, Apr. 1976.

XEEE TWANSCiCTIONS ON ELECTRON DEVICES, VOL. ED-26, NO. 8, AUGUST 1979

doped (Al, Ga) As-GaAs double-heterostructure LED’s,” J. Ay>,7l. Phys., vol. 47, no. 5, pp. 2082-2084, May 1976.

[30j K. Ikeda, S. Horiuchi, T. Tanaka, and W. Susaki, “Design paran- eters of frequency response of GaAs-(Ga, A1)As double hete~ 0- structure LED’s for optical communications,” IEEE Trans. Elc’e- tron Devices, vol. ED-24, pp. 1001-1005, July 1977.

[31j T. Yamaoka, M. Abe, and 0. Hasegawa, “GaAlAs LED’s for fibtz- optical communication systems,” Fujitsu Sci. Tech. J., vol. 14, no. 1, pp. 133-146, Mar. 1978.

[32] M. Honda, T. Kihara, and M. Endo, “Design and performance of an optical fiber digital link for widespread applications,”Fujitw Sei. Tech. J., vol. 14, no. 3, pp. 1-24,Sept. 1978.

[33] F. D. King, J. Straus, 0. I. Szentesi, and A. J. SpringThorpe, “High-radiance long-lived LED’s for analogue signalling,” Prc c.

Inst. Elec. Eng., vol. 123, no. 6, pp. 619-622, June 1976. [34] R. C. Goodfellow and A. W. Mabbitt, “Wide-bandwidth high-

radiance gallium-arsenide light-emitting diodes for fibre-optic communication,” Electron. Lett . , vol. 12, no. 2, pp. 50-51, Jan. 1976.

[35] J. Heinen, W. Huber, and W. Harth, “Light emitting diodes with a modulation bandwidth of more than 1 GHz,” Electron. Lett., vol. 12,110. 21 ,p . 553, 1976.

[36] P. B. O’Connor, J. B. MacChesney, and C. M. Melliar-Smith, “Large-core high-N.A. fibres for data-link applications,” Electron. Lett., vol. 13, no. 7, pp. 170-171, Mar. 1977.

[37] U. Daido, “The estimated coupling efficiency of a light emitting diode with large-core high N.A. fibers,” Fujitsu Sci. Tech. J. , vol. 14, no. 3, pp. 25-36, Sept. 1978.

LED Pulse Response Analysis Considering the Distributed CR Constant in the

Peripheral Junction ISAO HINO AND KUNIAKIRA IWAMOTO

Absstruct-An improved equivalent circuit of a current crowding type LED is proposed to explain the LED pulse response behavior, takblg into account the junction capacitance and the spreading resista1n’:e distributed over the entire junction area. Calculated results bawd on this model can consistently explain the experimentally observ1:d LED behavior, including dc-bias effects and pattern effects, which occllr in high-speed pulse modulation.

R I. INTRODUCTION

ECENT developements in high-radiance and high-speed light-emitting diodes (LED’s) has made possible their 9)-

plication to high-bit-rate PCM optical communications. For such applications, proper understanding of transient behavhr of an LED is needed. LED transient behavior is mainly go.(- erned by junction space-charge capacitance and injected carrb r lifetime. A simple equivalent circuit, in which an LED is r(:- placed by discrete circuit elements of a single junction includ- ing diffusion capacitance, space-charge capacitance, and norl- linear resistance, was previously proposed to analyze the LEllIl transient behavior. By solving this equivalent circuit, ca1cu’l;I- tions of the LED transient behavior were performed numer.- cally [1] , [ 2 ] and also in closed-form expressions [3]. Tlhe

Manuscript received November 13, 1978. The authors are with Central Research Laboratories, Nippon Electr .c

Company, Ltd., Takatsuku, Kawasaki, Japan.

pulse response of a large emitting area LED, whose emitting area is as large as an entire p-n junction, can be essentially understood by this model [2] . An LED for optical communi- cations has a small emitting area in the center of a large p-n junction to obtain sufficient fiber coupling. In this structure, it does not seem reasonable to treat emitting area junction and peripheral junction surrounding the emitting area as a single- body junction. In fact, the transient behavior of this type LED, such as the turn-on delay and emission transient, cannot be fully explained by the calculations based on the simple dis- crete circuit model.

In this paper, an improved equivalent circuit of a current crowding type LED is proposed, taking into account the junc- tion distributed capacitance connected by the spreading re- sistance spread over the entire junction area. Simulation of the LED pulse response, based on this model, can consistently ex- plain the experimental results, including dc-bias effects and pattern effects.

11. MODEL Fig. 1 shows the etched-well structure LED [4], [5] to be

investigated in this paper. The double heterostructure, in which a p-type active Al,,,sGao.95As layer is sandwiched by n-Al,,,,Ga,.,,As and p-A10,3sGao.6sAs cladding layers, is grown on an n-GaAs substrate. The small circular emitting re-

0018-9383/79/0800- 1238$00.75 0 1979 IEEE