Abrupt degradation of three types of semiconductor light emitting diodes at hightemperatureOsamu Ueda, Hajime Imai, Takao Fujiwara, Shigenobu Yamakoshi, Tomonobu Sugawara, and ToyoshiYamaoka Citation: Journal of Applied Physics 51, 5316 (1980); doi: 10.1063/1.327445 View online: http://dx.doi.org/10.1063/1.327445 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/51/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Degradation mechanism beyond device self-heating in high power light-emitting diodes J. Appl. Phys. 109, 094509 (2011); 10.1063/1.3580264 Influence of temperature and drive current on degradation mechanisms in organic light-emitting diodes Appl. Phys. Lett. 80, 3430 (2002); 10.1063/1.1476704 Kinetics of pressuredependent gradual degradation of semiconductor lasers and lightemitting diodes Appl. Phys. Lett. 55, 1170 (1989); 10.1063/1.101687 Kinetic model for gradual degradation in semiconductor lasers and lightemitting diodes Appl. Phys. Lett. 53, 2135 (1988); 10.1063/1.100297 Hightemperature degradation of InGaAsP/InP light emitting diodes J. Appl. Phys. 52, 5377 (1981); 10.1063/1.329398

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Abrupt degradation of three types of semiconductor light emitting diodes at high temperature

Osamu Ueda, Hajime Imai, Takao Fujiwara, Shigenobu Yamakoshi, Tomonobu Sugawara, and Toyoshi Yamaoka Fujitsu Laboratories. Ltd .. 10J 5 Kamikodanaka. Nakahara-ku. Kawasaki. Japan

(Received 27 February 1980; accepted for publication 19 June 1980)

Abrupt degradation of three types of semiconductor light emitting diodes (LED's), Gal _ x At As double-heterostructure (DH) LED's, Gal _ x Alx As/GaAs DH LED's, and LED-operated Gal _ x Alx As/GaAs DH lasers operated at high temperature, was studied by electroluminescence topography, photoluminescence topography, and transmission electron microscopy. The following results were obtained from all types ofthe diodes. Dark regions which included (100) dark-line defects (DLD's) and/or (110) DLD's were often observed in the electronluminescence or photoluminescence patterns of the active regions. Two phenomena were associated with this type of degradation as follows: (i) generation of high density of dislocations and dislocation loops (in some cases, stacking faults) by dislocation glide motion due to the relaxation of the stress concentrated in the active region. (ii) subsequent development of dislocation dipoles from the glided dislocations (often accompanied with many small dislocation loops). In some of the degraded diodes, the phenomenon (ii) did not occur. There might be a certain threshold for the phenomenon (ii).

PACS numbers: 61.70.Jc, 85.60.Jb, 78.60.Fi

I. INTRODUCTION II. EXPERIMENTAL

There have been many reports on the defect structure of rapidly degraded optical devices, such as Gal _ xAlx As/ GaAs double-heterostructure (DH) lasers,I-7 GaAs photo lamps,8 GaAs light emitting diodes (LED's), 9 Gal _ x Al x As LED's, 10.1 I GaP LED's,12 and GaAsl_xPx LED's.l3 The authors have previously reported that (100) dark-line de­fects (DLD's), (110) DLD's, and dark-spot defects (DSD's) were observed in the electroluminescence (EL) patterns of rapidly degraded Gal _ xAlxAs LED's and that the follow­ing results were obtained by transmission electron micro­scope (TEM) observation of these dark defects. lO

.l1

Three types of LED's that degraded abruptly during operation at high temperature were investigated in this experiment.

(100) DLD's and DSD's have (i) interstitial type dislo­cation dipoles accompanied with small dislocation loops or (ii) large zigzag-shaped interstitial type dislocation loops. (110) DLD's have (i) half dislocation loops or (ii) multiple stacking faults. These defects were similar to those of Gal _xAtAs/GaAs DH lasers. 1-4

Dark defect-free Gal _ x Alx As LED's were selected after 100 h of room-temperature operation with good repro­ducibility.14 However, at high temperature, three types of dark defect-free LED's, Gal _ x At" As LED's, Gal_xAlxAs/GaAs DH LED's, and LED-operated Gal _ xAlxAs/GaAs DH lasers, sometimes degraded ab­ruptly and dark regions were newly revealed in the EL pat­terns. From the viewpoint of the degradation mechanism, it is quite important to compare the defect structures which correspond to the dark regions in the different types (struc­tures and operating conditions) of LED's.

In this paper, these three types of LED's that degraded abruptly at high temperature were investigated and com­pared by electroluminescence topography, photolumines­cence (PL) topography, and TEM.

04

(GaAs sub)

• I 0.1 I

X~..,--L-"""'--'-i emitti ng area

(I)

n-electrode

(II) -_ ,---' (III)

window layer (n-Gal-XAI XAs) active layer

1==:;:::;:;ieIIi~=;:;::::~carrier confining I===:::::::;~~====~ layer

5i02 (I) Te-doped (IT) Ge-doped (III) Ge-doped

(a)

EL light

t ~-'--------'---'1 n-electrode

active n-GaAs sub region ill) ~------1n-GaAs

(40fJ n-Ga7AL3As ~=~===}- p-GagA1 1As

(b)

P-Ga7AI3As ~~~~~~P-GaAs

p-el ect rode 5-ddfused regIOn

I=======r n-electrode active -n-GaAs sub. region p...~-----I __ n-GaAs

(stripe) n-Ga7AL3As t==~m::::=3::::.P-Ga.9All As ~~<;+--f%~~ p-Ga7AL3As ~~~~~~,p-GaAs

p-electrode 5-ditfused region

(c)

plated Au heat sink

FIG. I. Structure of three types of semiconductor LED·s. (a) Ga" AI, As DH LED. (b) Ga, xAlxAs/GaAs DH LED. (c)Ga, ,AlxAs/GaAs DH

laser (LED operation).

5316 J. Appl. Phys. 51(10). October 1980 0021-8979/80/105316-10$01.10 © 1980 American Institute of Physics 5316

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The structures of these diodes were schematically shown in Figs. l(a)-l(c). Figure l(a) shows the Gal_xAlxAs DH LED (details of the fabrication were re­ferred to Ref. 15). The Gal_xAlxAs epitaxial layers with the DH structure were grown on (O()l)-oriented GaAs sub­strate by liquid phase epitaxial (LPE) growth. The profile of aluminum composition X is presented on the left of Fig. l(a). The active layer is about 1 f.lm thick and was doped with Ge (p = 2-5 X 1018 cm-3

). The emitting area in the active layer is about 40 f.lm in diameter. Figure l(b) shows also the Gal _ x Alx As/GaAs DH LED. However, these diodes were fabricated from the Gal_xAlxAs/GaAs DH materials for lasers. These DH structures were also grown on (OOl)-ori­ented GaAs substrates by LPE growth. The active layer is 0.15 f.lm thick and doped with Si (p = 1 X 1017 cm-3

), and

(a)

C --......;...=- [110] 10 1-1,

(001) (b)

FIG. 2. EL patterns of the degraded Ga, _ x Alx As DH LED's. (a) Ring­shaped dark region which includes many small dark defects in diode 1. (b) Anomalous dark region in the diode 2.

5317 J. Appl. Phys., Vol. 51, No.1 0, October 1980

the Al content of X in the active layer is 0.1. Sulphur diffu­sion was performed from the p side to the center of the Gao 7 Ala.3 As clad layer in the whole area except for the cen­tral region of 40 f.lm in diameter (emitting region). Figure l(c) shows the Gal _ x AlxAs/GaAs DH laser. The structure is internal stripe planar and the stripe width and the cavity length are 7 and 300 f.lm, respectively. These diodes were fabricated from the same type of DH material as the one shown in Fig. l(b). The sulphur diffusion was performed from the p side except for the striped region performed by the same method as described above.

Six diodes were used in this experiment. The diodes 1 and 2,3, and 4, and 5 and 6 are Gal_xAlxAs DH LED's, Gal_xAlxAs/GaAs DH LED's, and Gal_xAtAs/GaAs DH lasers, respectively. The diodes 1 and 2 were operated at 300 and 100 rnA (dc), and 120 and 180°C, respectively. The diodes 3 and 4 were operated at 50 rnA (dc) and 300 dc. The laser diodes 5 and 6 were LED operated at 100 rnA (below the threshold current) and 110 ·C.

EL patterns of the diodes 1-4 were observed using a Si vidicon microscope. PL patterns of diodes 5 and 6 were ob­served by the optical excitation of the active region by Kr ion laser (4 = 6471 A), after removing the p electrode, the p­GaAs cap layer, and a part of the p-Gao7Ala.3As clad layer by Ar ion etching.

The thin specimens for TEM were prepared by chemi-I h· d . h" d 'b d . I 10 11 16 ca etc mg an Ion t mnmg as escn e prevIous y. . .

TEM observation was carried out by JEM 120 operated at 120 kV, with a specimen tilting and rotating mechanism. Relative rotations between the electron diffraction patterns and the electron micrographs were taken into account, and the micrographs were printed emulsion side up in order to avoid any confusion over the inversion.

III. RESULTS

A. Gat_xAlxAs DH LED's

The output power of the diodes 1 and 2 decreased ab­ruptly by 97% and 53% after 380 and 4390 h of operation at 120· and 180 ·C, respectively.

Figure 2(a) shows an EL pattern of the diode 1. A ring­shaped dark region was revealed in the pattern. This dark region corresponded to the one between the edge of the p electrode and the edge of the Si02 mask. Many DSD's exist­ed in this region. Several DSD's were also observed both inside the edge of the Si02 mask and outside the p electrode. These types of dark defects have not been observed in de­graded Gal _ x Alx As DH LED's that were operated at room temperature.

Another dark region was observed in the diode 2 as shown in Fig. 2(b). Although this dark region included sev­eral DLD's, these DLD's were neither (100) DLD's nor (110) DLD's, they inclined slightly to the (100) direction, i.e., (120) direction.

Figure 3(a) shows a transmission electron micrograph of a part of the ring -shaped dark region in diode 1, obtained by the 220 reflection with a positive s. Numerous disloca­tions and dislocation loops were observed. The density of dislocations and dislocation loops were approximately

Uedaeta/. 5317

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(a)

(c)

FIG. 3. Transmission electron micrographs of the dark region shown in Fig. 2(a). (a) Numerous dislocations and dislocation loops which correspond to a part of the dark region, where g = 220 and s > O. (b) High-magnification electron micrograph of the region P in Fig. 3(a) where g = 220 and s> O. Several dislocation loops were nucleated in a row from the dislocation X. (c) Diffraction contrast of the defects shown in Fig. 3(a) where g = 040 and s> O. The defects show double contrast.

1 X 109/cm3 and 1 X 1013 Icm3, respectively, considering the

foil thickness of 0.5 f..Lm. Most of the dislocations had many kinks. They must have cross-slipped with each other during the slip motion, or have been pinned at the dragging points.

5318 J. Appl. Phys., Vol. 51, No.1 0, October 1980

In the vicinity of the region denoted by P, several dislocation loops were nucleated in a row from the dislocation denoted by X as shown in Fig. 3(b). These dislocation loops may be nucleated by local pinning of the moving dislocations at dragging points.

These defect structures, which were accompanied by numerous dislocations and dislocation loops, were consid­ered to be caused by dislocation glide motion due to the the relaxation of the local stress concentrated in the vicinity of the p electrode. They were quite different from those includ­ing dislocation dipoles caused by dislocation climb motion via the absorption of interstitial atoms. They are similar to the half dislocation loops which correspond to the (110) OLD's.!! However, the former defect structures include much more dislocations and dislocation loops. Although we could not fully perform the contrast analysis because of the thick foil (about l/lm), they had double contrast when g = 040, and s > 0 as shown in Fig. 3(c). Therefore, these dislocations were 60· -type ones having Burgers vectors of the type al2 (011) and prismatic dislocation loops lying on the ! 110 J plane. These defects were very often generated by the (011) I! 111 J glide systems in zinc-blende structure.

FIG. 4. Transmission electron micrographs of the active region shown in Fig. 2(b) (diode 2). (a) Low-magnification electron micrograph of all of the active region where g = 220 ands > O. The regions denoted by Ao, Bo and Co correspond to A, B, and C in Fig. 2(b), respectively. (b) High magnification transmission electron micrograph of the defects in region Bo in Fig. 4(a) where g = 220 and s> O.

Uedaetal 5318

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(a)

le)

Figure 4(a) shows a transmission electron micrograph of the active region of diode 2. Complete correlation between the regions denoted by Ao, Bo' and Co in Fig. 4(a), and the regions denoted by A, B, and C in Fig. 2(b) can be obtained, respectively. Figure 4(b) is a high-magnification transmis-

5319 J. Appl. Phys., Vol. 51, No.1 0, October 1980

(b)

FIG. 5. High-magnification transmission electron micrographs of small dislocation loops in the region Co. (a) g = 400, s> 0; (b) g = 040, s> 0; (c) g = 040, s> 0; (d) g = 220, s> 0, and B = 1 16; (e) g = 220, s> 0, and B = 116, where B is the direction of the electron beam. These electron micrographs were taken in the same magnitude.

sion electron micrograph of Fig. 4(a). Dislocation dipoles which were considered to be generated by dislocation climb motion, and dislocation loops were observed. Contrary to this, the density of the dislocation loops was very high as compared with the case of diode 1. The diameter of the dislo-

Uedaetal 5319

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(a) (001) (b)

(001) (c) (d)

FIG. 6. Variation of the EL patterns of diodes 3 and 4 during operation. Diode 3: (a) after 660 h of aging at 300 'C; (b) after subsequent 290 h of operation at 180 'C and 14 h of operation at 300 'C. Diode 4: (c) after 660 h of aging at 300 'C; (d) after subsequent 5 h of operation at 300 'Co

cation loops varied in the range 100-500 A. Several irregular circular dislocation loops were also observed. Figures 5(a)-5(e) show the variation of the diffraction contrast of these dislocation loops. These electron micrographs were ob­tained by the 400, 040, and 040, with a positive s, respective­ly. All of these dislocation loops were out of contrast when g = 400, s> O. The loops show outside contrast when g = 040, s> O. According to Gever's rule, a dislocation loop shows outside contrast when (g·b)s> O. Therefore, b l = a/2[011) and b2 = a/2[01 1] are possible for the b of the dislocation loops. Figures 5(d) and 5(e) are stereoelectron pair of the dislocation loops. These were obtained by the 220 reflection, where the beam directions were [116] and [116], respectively. From these figures, the loops were proved to lie on the plane close to the (011) plane. From these results, the b of the dislocation loops was determined to be a/2[O 11]. The loops and b in an upward direction. Therefore, they were extrinsic in character.

B. Ga 1 _ xAlxAs/GaAs DH LED's

Diode 3 was aged at 300 DC for 660 h (not operated), and then it was operated at 180 DC for 290 h. Finally, it was oper­ated at 300 DC for 14 h. Figure 6(a) shows an EL pattern of diode 3 after 660 h of aging at 300 DC. No OLD's and DSD's were observed. However, two dark regions were revealed in the EL pattern after 290 h of operation at 180 DC. At this

5320 J. Appl. Phys., Vol. 51, No.1 0, October 1980

time, the output power decreased by 50%. The dark regions extended after subsequent 14 h of operation at 300 DC as shown in Fig. 6(b). The output power decreased by 70%.

The output power of diode 4 decreased by 97% after 660 h of aging at 300 DC and 5 h of operation at 300 DC. Fig­ure 6( c) shows an EL pattern of diode 4 after 660 h of aging at 300 DC. No dark defects were observed. However, after only 3 h of operation at 300 DC, cross-grid-shaped dark region containing many (100) OLD's and (110) OLD's were re­vealed in the EL pattern. After 5 h of operation, the dark region extended to the whole of the active region as shown in Fig.6(d).

Figure 7 shows a high-magnification transmission elec­tron micrograph of the dark region of diode 3, obtained by the 220 reflection with a positive s. Dislocation tangles about 1 f.Lm in diameter were observed. These defects consist of many dislocation dipoles and dislocation loops (100-500 A in diameter). Some of the dislocations have dipoles and/or jogs. The dislocation tangles were not distributed densely. This may be due to the fact that the foil did not include all of the active layer. A stacking fault denoted by S and a disloca­tion dipole denoted by 0 were also observed. The stacking fault may originate from the interface between the epitaxial layer and the substrate, and dislocation dipoles and disloca­tion loops generated in the vicinity of the fault. The active area was dislocation-free before operation. Therefore, the

Uedaetal. 5320

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origins of these dislocation dipoles are considered to be half dislocation loops which were generated by dislocation glide motion, not threadin~ dislocations.

Figure 8(a) shows an optical micrograph of the chemi­cally etched surface of the first layer of diode 4. bv the et­chant of 1 NH40HI 10HzOz' Circular and cross-grid-shaped

FIG. 8. (a) Chemically etched surface of the first layer of diode 4. (b) Trans­mission electron micrograph of many groups of dislocations which corre­spond to the center of diode 4, where g = 220 and s > O.

5321 J. Appl. Phys., Vol. 51, No.1 0, October 1980

0.5 f-I ! I

FIG. 7. High-magnification trans­mission electron micrograph of dis­location networks which corre­sponds to the dark region shown in Fig. 6(b). Stacking fault denoted by S accompained by dislocation dipoles and dislocation loops, and disloca­tion dipole denoted by D having many jogs, where observed. g = 220 ands>O.

etch patterns were revealed in the figure. This pattern corre­sponds to the EL pattern shown in Fig. 6(d). The area which included the circular area, was thinned down to the thick­ness suitable for TEM (nearly 0.5 J.lm for a TEM whose ac­celerating voltage is 120 kV). Figure 8(b) shows a transmis­sion electron micrograph of the dislocation network corresponding to the center of the active region, obtained by the 220 reflection with a positive s. Many groups of disloca­tions were observed. Each group of dislocations generally lay in the (100) or (110) directions. These groups were simi­lar to the cell structures which are often observed in work­hardened metals. 17 Almost all of the dislocations were half­dislocation loops caused by dislocation glide motion. Dislo­cation dipoles were observed very rarely. Figures 9(a) and 9(b) were high-magnification transmission electron micro­graphs of the groups of dense dislocations. These micro­graphs were obtained by the 220 and 220 reflection and a positive s, respectively. Faulted loops denoted by SI' S2' S3' and S4 were observed. SI and Sz were out of contrast when g = 220, and S3 and S4 were out of contrast when g = 220. These faulted loops were different from the stacking faults which originate from the interface between the layer and the substrate. There were two types of dislocations having Burg­ers vectors of the type a/2(011) inclined at 45° to the (001) junction plane or al2 (110) parallel to the junction plane. Many precipitate-like defects denoted by P were observed very close to the dislocation lines. The origins of these de­fects were not clear.

C. LED-operated Ga1 _ x Alx As/GaAs DH lasers

Diodes 5 and 6 degraded after 55 and 78 h of LED operation at 110 ·C, respectively. The EL intensity of both of the diodes decreased abruptly by 90% after the LED oper­ation. Figure lO(a) shows a PL pattern of diode 5. Dark regions were observed in both the middle part of the stripe and one edge of the stripe. The dark regions extended to the outside the stripe, and included several (100) DLD's. As shown in Fig. 100b), several dark regions were also observed in the PL pattern of diode 6.

Figure 11 shows a transmission electron micrograph of a part of the dark region in the PL pattern of diode 5, ob-

Uedaetal. 5321

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(a) (b)

FIG. 9. Variation of the diffraction contrast of a part of the groups of dislocations shown in Fig. 8(b). (a) g = 220, s> 0; (b) g = 220, s> 0. These electron micrographs were taken in the same magnitude. S" S2' S3' and S4 are faulted loops.

tained by the 220 reflection, and a positive s. Dislocation dipoles, helical dislocations, glide dislocations, and disloca­tion loops were generated densely. The dipoles elongated in two equivalent directions of (100) and (010), and they ex­isted among the dipoles. The diameter of these dislocation loops was in the range 100-500 A. Arrays of dislocation loops were observed outside the stripe and lay in the (l00) direction.

Figures 12(a)-12(c) are transmission electron micro­graphs showing the dark region close to the area shown in

k region

10) ~ 501J ~

FIG. 10. PL patterns of diodes 5 and 6 degraded abruptly after LED oper­ation. (a) diode 5; (b) diode 6.

5322 J. Appl. Phys., Vol. 51, No.1 0, October 1980

Fig. 11. In these figures, the defects were classified into two groups. The one group was out of contrast when g = 040 and s> O. Therefore, this group had Burgers vectors of the type a/2 ( 101). The other one was out of contrast when g = 400 and s > O. Therefore, this group had a al2 (011). The direc­tions of al2 ( 101) and al2 (011) were equivalent ones in­clined at 45° to the (001) junction plane. These results indi­cate that the dislocation dipoles elongated from the dislocation halfloops caused by the dislocation glide motion in two equivalent slip systems of (011) I /111 J and ( 101) I /111 J . The dislocation dipoles were determined to be extrinsic in character by the inside-outside contrast experi­ment as described in Sec. III A. The dislocation loops out­side the dipoles, and inside the dipoles were extrinsic and intrinsic in character, respectively.

FIG. 11. Transmission electron micrograph of the defects associated with the dark region M of diode 5. This micrograph was obtained by 220 reflec­tion with a positive s.

Uedaetal. 5322

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(a)

(b)

FIG. 12. Variation ofthe diffraction contrast of the defects existing close to the area shown in Fig. II. (a) g = 220, s> 0; (b) g = 040, s > 0; (c) g = 400, s> O. These micrographs were taken in the same magnitude.

Figure 13 shows a transmission electron micrograph of a part of the dark regions in the EL pattern of diode 6, where g = 220 and s> O. In this case, numerous dislocation loops

5323 J. Appl. Phys., Vol. 51, No.1 0, October 1980

were observed. Very few dislocation dipoles denoted by ar­rows were observed among the loops.

IV. DISCUSSION

From the results obtained in Sec. III A, III B, and III C, the following two processes are considered to be asso­ciated with the abrupt degradation of semiconductor LED's during operation at high temperature; (i) High density of dislocations and dislocation loops (in rarely case, stacking faults) are generated. (ii) Subsequent development of dislo­cation dipoles from the slip dislocations.

Dislocation glide motion caused by nonradiative re­combination of the minority carrier in the active region is expected to be associated with process (i).

Process (ii) is due to the dislocation climb motion via the absorption of interstitial atoms, or the mechanism as proposed by Matsui et al. 18

In most of the degraded diodes used in this experiment, the degradation was caused by both of these processes. How­ever, as shown in the case of diodes 1 and 4, no dislocation dipoles were generated from the glide dislocations at all.

There might be a certain threshold for process (ii). Sev­eral parameters can be considered to be the threshold as fol­lows: (a) light output power, (b) current density during LED operation, (c) structure of the diode, (d) impurities doped in the active layer, (e) carrier concentration in the active layer, (f) density of glide dislocation and (g) density of defects in the active region which can be the pinning sources for dislo­cation climb motion.

Recently, B. Monemar et al. 19 have shown that there is a sharp threshold for the process of optically induced glide in Gal _ x Alx As/GaAs DH materials at which the velocity changes by more than a factor of 103 when the excitation intensity changes only 20%. They assumed that the difficul­ty in causing the dislocation to glide once it has climbed could be explained by the reduction in carrier lifetime or by a pinning effect of the climb network upon the threading dislo-

FIG. 13. Transmission electron micrograph of numerous dislocation loops and few dislocation dipoles associated with a part of the dark regions in diode 6 where g = 220, s> O.

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cation. And they concluded that the effect is shown not to be related to recombination enhanced motion or local heating.

In their experiment, Gal _ x Alx As/GaAs DH wafers whose surfaces were scratched gently were used. Therefore there were sources for multiplication of dislocations half dis­location loops caused by the mechanism proposed by F.e. Frank and W. T. Read. 20 The samples in their experiment were not lasing.

In this experiment, the driving force which caused the generation of glide dislocations in the crystal was the elastic stress in the vicinity of the active area (no scratches were introduced) and the diodes were LED operated. Thus the difference between the two processes cannot be explained only by the optical output which may correspond to the exci­tation intensity. The difference was not correlated to the pa­rameters (b), (c), (d), and (e) in this experiment.

In the diodes whose degraded area included climbed dislocation dipoles, comparatively low densities of disloca­tions were observed (107 cm -2). While, in those diodes whose degraded area had no dislocation dipoles, the disloca­tion density was much higher (109/cm2

). There might be a certain threshold for the density of the glide dislocations above which dislocation climb motion does not occur. The nucleation of jogs at the dislocations may occur only with difficulty when a high density of dislocations is generated in the crystal.

Next, we discuss the formation of the high density of dislocation loops during the dislocation climb process. The small dislocation loops were observed only in the vicinity of the dislocation dipoles, and have the same character (extrin­sic). Therefore, these dislocation loops were generated dur­ing the dislocation climb process.

The dopants in the active layer of Gal _ x Alx As DH LED's Gal _xAtAs/GaAs DH LED's and LED-operated Gal _ xAlxAs/GaAs DH lasers are Ge, Si, and Si, respec­tively. Ge atoms were comparatively highly doped in the active layer of Gal _ xAlxAs DH LED's. High dopingofTe, Ge, and Si into GaAs or Gal _ x At As LPE layers results in the generation of high density of interstitial-type faulted loops or prismatic dislocation loops (above 1 X 1010 cm .\) in the matrix. 21-24 However, the average diameter of the dis­location loops which were observed in this experiment was smaller than the average diameter of those observed in as­grown epitaxial layers; moreover, the generation of small dislocation loops was observed in all of the three types of degraded diodes. Therefore this phenomenon is quite inde­pendent on the kind of dopant and the density of the dopant in the active layer.

From these phenomena, the generation process of the high density of small dislocation loops in the vicinity of the dislocation dipoles might be described as follows: (i) intersti­tials of the host atoms are generated especially in the vicinity of the glide dislocations due to the effect of the optical excita­tion on the crystal, (ii) the interstitials migrate towards the dislocations and are absorbed by the dislocations (disloca­tion climb motion), (iii) some of the migrating interstitials are trapped by certain nucleation centers, i.e., impurities in the matrix, and small dislocation loops are formed, and (iv) as the dislocation climb motion develops, the number of the

5324 J. Appi. Phys., Vol. 51, No.1 0, October 1980

interstitials increases and numerous dislocation loops are generated at the time.

v. CONCLUSIONS

Abrupt degradation of three types of semiconductor light emitting diodes (LED's), Gal ._ xAlxAs double-hetero­structure (DH) LED's, Gal .. x Alx As/GaAs DH LED's, and LED-operated Gal _ xAlx As/GaAs DH lasers operat­ed at high temperature was studied by electro luminescence topographY, photoluminescence topography, and transmis­sion electron microscopy.

The following results were obtained from all types of the diodes.

Dark regions which included (100) dark-line defects (OLD's) and/or (110) OLD's were often observed in the electroluminescence or photoluminescence patterns of the active regions. Two phenomena were associated with this type of degradation as follows: (i) Generation of high density of dislocations and dislocation loops (in some cases, stacking faults) by dislocation glide motion due to the relaxation of the stress concentrated in the active region. (ii) Subsequent development of dislocation dipoles from the glided disloca­tions (often accompained with many small dislocation loops). In some of the degraded diodes, the phenomenon (ii) did not occur. There might be a certain threshold for the phenomenon (ii).

ACKNOWLEDGMENT

The authors would like to thank O. Ryuzan, T. Kotani, and M. Takusagawa for their encouragement. They also thank M. Abe, S. Komiya, and S. Isozumi for their valuable discussions. Helpful suggestions on the electron microscopic study with Professor Y. Murakami, K. Osamura, and H. Yamanaka of Kyoto University are greatly appreciated.

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