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InP epitaxial layers
Materials Science and Engineering, B9 ( 1991 ) 109-113
109
The consequences of dislocations and thermal degradation on the quality of InGaAsP/InP epitaxial layers B. Sartorius, F. Reier and P. Wolfram Heinrich-Hertz-lnstitut fiir Nachrichtentechnik Berlin GmbH, Einsteinufer 37, W-IO00, Berlin 10 (F.R.G.)
Abstract
Thermal degradation is identified as the origin for a variety of defects in epitaxial layers. The consequences of these defects are more critical than those of dislocations. It turns out that thermal degradation affects the layer quality more than dislocations.
1. Introduction
The performance of InP-based optoelectronic devices has been improved in the last few years. Nevertheless, growing reproducibly high quality epitaxial layers is still a problem. Dislocations in the substrate are considered to be mainly responsible for the moderate layer quality. However, another important phenomenon is defect formation due to thermal degradation, caused by evaporation of phosphorus during high temperature processes. In this paper the consequences of both defect sources on the quality of epitaxial layers are analysed. 2. Quality inspection by means of luminescence microscopy In this paper we investigate localized defects, for which purpose luminescence microscopy is a powerful technique [1]. Defects reduce the luminescence intensity and thus are visible in luminescence (LUM) images as dark features on a bright background (see the bottom part of Fig. 1 ). We use an optical microscope for resolving small defects and an image processing system with zero suppression and gain for the detection of even slight intensity variations by weak defects. The optical microscope has a second advantage: additional optical inspection modes can be applied for quasi-simultaneous inspection of the identical sample section. Nomarsky interference contrast (IC) is the best method for revealing surface structures. Thus the correlation between morphological defects (e.g. Fig. 1, top part) and 0921-5107/91/$3.50
Fig. 1. Luminescence and interference contrast images of the identical section of an MOVPE-grown InP layer, showing crystallographic (LUM) and morphological (IC) defects.
crystallographic defects (Fig. 1, lower part) can be recognized. Infrared transmission (TRANS) at a wavelength where the substrate is transparent and the layer is absorbing is a third important analyti© Elsevier Sequoia/Printed in The Netherlands
110 cal tool. This method shows thickness inhomogeneities in the layer (hidden pinholes in Fig. 2 which appear as bright spots) as well as metallic indium inclusions which are less transparent and appear as dark structures (see later examples).
3. Dislocations in substrate and layer Dislocations are generally considered to be the most critical defects in III-V materials, and the dislocation density is the only quality parameter specified by substrate suppliers. One reason for their importance is that the growth of dislocations from the substrate into the layer cannot be avoided. This can be recognized in Fig. 3, which shows InP layers grown on a substrate with high dislocation density (lnP:Fe, lower image) and on one with a low dislocation density (InP:S, upper image). It is clearly seen that the dislocation density in the layer is a replication of that of the substrate.
Fig. 2. Hillocks (IC), hidden pinholes (TRANS, bright points) and their local matching (IC +TRANS)in an LPEgrown lnGaAs layer.
4. Material problems in InP Typically the dislocation density of substrates (except InP:S) is about 5 x 104 cm :. T h e probability that a dislocation is threading the active region of a laser about (300/zm x 3/~m in area) is 50% for this density. It is known that in GaAs the existence of any dislocation in the active area causes rapid degradation of the device. This sensitivity of GaAs devices is the second reason for the high attention given to dislocations. T h e origins of the catastrophic device failures are stimulated climb and multiplication processes of dislocations. In InP these processes are much less probable. In the absence of multiplication, however, a dislocation in the laser will only reduce the pumping level along a few /~m of the cavity length. T h e resulting slight reduction in output power is not critical for device operation. Although dislocations themselves are not critical in InP there exist material problems, as dem-
Fig. 3. Dislocations in InP grown by MOVPE on InP:S with low dislocation density (upper image) and on InP:Fe with high dislocation density (lower image).
111 onstrated in Figs. 1 and 2. The dark disc defect can be very effective as a consequence of its large size. Surface unevenness hinders following processes like lithography. Threading or hidden pinholes may cause electrical short circuits. The occurrence of these defects cannot be correlated with dislocations. A possible mechanism for their origin is thermal degradation, which arises by incongruent evaporation of phosphorus during high temperature processes.
5. Thermal degradation of InP substrates Thermal degradation was studied on samples which were heat treated in a way similar to that used for epitaxial growth [2]. Figure 4 shows the surface of a sample that was heated for 1 h at 700 °C with a protecting phosphorus pressure generated by an InP counter substrate. Most imposing are the crystallographic large dark disc defects, visible in luminescence. The white features in the superposition IC and LUM are
caused by surface peaks, revealed in the IC mode. Any dark disc has such a peak at its centre, but the peaks are also found without dark discs. The peaks consists of InP, sometimes with an indium cap on top. They arise by material transport out of the crystal, similar to the generation of a volcano (see ref. 2). If protection methods against phosphorus evaporation are not sufficient, then epitaxial growth will start on a surface containing the defects described. The consequences for the quality of the epitaxiai layer are analysed in the following.
6. Correlation between substrate degradation and defects in the layer The typical geometrical features of thermal degradation effects as described in the last section are the key for identifying correlations with defects in the epitaxial layer. In the IC image of Fig. 5 two hillocks with holes exactly at their centres are visible. The luminescence image shows two dark disc defects in the layer, and transmission reveals an indium inclusion at the centre of both dark discs. The superposition of IC and LUM shows the local matching of all the features described. We conclude that a dark disc defect in the substrate has grown into the layer, and additionally this defect has generated the hillock growth (compare with the model in Fig. 6). The InP volcano at the centre of the dark disc has an indium cap (visible as the dark points in TRANS). It is known that epitaxial growth is blocked above indium droplets, and indeed we observe as a consequence the hole visible in IC above the InP/In volcano.
7. Modifications of degradation induced defects
Fig. 4. Crystallographic (LUM) and morphologic (IC + LUM)defectsin a substrate after heat treatmentssimilar to LPE growth.
Not in all cases do the thermal degradation induced defects exhibit simultaneously all of the symptoms discussed. In Fig. 1 for example the dark disc defect has grown into the layer, but hillock growth has not been generated. In Fig. 2 and in Fig. 7 the pinholes at the centres of the hillocks are of the hidden type, caused by InP peaks without an indium cap. In Fig. 7 three hidden pinholes without correlation to a hillock are also detected. They are generated by InP peaks without a dark disc defect. Figure 8 shows that the dark disc defect can increase the growth rate (hillock) as well as block the growth (large hole at the centre). The InP volcano (IC) and the
112
I
Fig, 5. Correlation between typical features of thermal degradation in a suhstratc and dcfccls in an InGaAsP layer grown by I.PE (see text for a discuss on)
Hi[rocks
InP Peak
_%_ .............. ~iiiiiiiiiiiii~i~i~i~i~ik
l!iiiiiii!iigiiiii iiiiiligiiiiiiiii!iiiiiii i iiiiiiiiii!ii!!!! ] iiiiiiii ! iii!iii!!iMiiiiiiiiiiiiiiiiiiiii i!!i!i Ternary Layer t
...............................
0 0 0 0 0 " O ~ - ~ D e g faded 0 0 0 0 0 0 Oommn 0
0
0
0
Hidden
0
0 Threading
Pinho(es
Fig. 6. Model for lhe correlation between thermal degradation induced defects in the substrate (dark disc, lnP/|n peaks) and defects in the epitaxial layer.
indium cap (TRANS) are visible at the centre of the hole. Figure 9 shows that different modifications of thermal degradation induced defects can be present in one sample: a hillock with hidden (upper left) and threading (middle right) pinholes, a large hole (middle low), and indium inclusions without hillocks or dark discs (two dark points in the middle of the transmission image). 8. Conclusion
The examples presented demonstrate that thermal degradation is responsible for different
Fig. 7. Hidden pinholes with and without (three brighl poinls in the middle) hillock growth (lnGaAsP/InP, LPE).
113
~d
Fig. 8. Hillock with a large crater (blocked growth) and an lnP/ln volcano at the centre (InGaAs, MOVPE).
types of defects in epitaxial layers. Some of these defects (crystallographic dark disc defects, hidden pinholes) are not detectable by inspection with visible light and are thus often ignored. Other defects are morphological. They are a known phenomenon, but their origin has yet not been identified. Consequently, avoiding these defects was a trial and error optimization of epitaxial growth. The defects can be critical themselves, for example dark disc defects in the active layer of a laser, or electrical short circuit at a pinhole, or they can become critical through interference with consecutive technological processes,
Fig. 9. Simultaneous appearance of different types of degradation-induced defects in one layer (lnGaAsE LPE). See the discussion in the text.
for example lithography at an unevenness or diffusion at pinholes or along phosphorus vacancies. In summary, the crystal quality in lnGaAsP layers seem to be more affected by thermal degradation than by the dislocation density of the substrate. References I B. Sartorius, D. Franke and M. Schlak, J. CO,st. Growth, 83 (1987)238. 2 B. Sarlorius, M. Schlak, M. Rosenzweig and K. P~irschke, J. AppI. I'hys., 63 (1988) 4677.
109
The consequences of dislocations and thermal degradation on the quality of InGaAsP/InP epitaxial layers B. Sartorius, F. Reier and P. Wolfram Heinrich-Hertz-lnstitut fiir Nachrichtentechnik Berlin GmbH, Einsteinufer 37, W-IO00, Berlin 10 (F.R.G.)
Abstract
Thermal degradation is identified as the origin for a variety of defects in epitaxial layers. The consequences of these defects are more critical than those of dislocations. It turns out that thermal degradation affects the layer quality more than dislocations.
1. Introduction
The performance of InP-based optoelectronic devices has been improved in the last few years. Nevertheless, growing reproducibly high quality epitaxial layers is still a problem. Dislocations in the substrate are considered to be mainly responsible for the moderate layer quality. However, another important phenomenon is defect formation due to thermal degradation, caused by evaporation of phosphorus during high temperature processes. In this paper the consequences of both defect sources on the quality of epitaxial layers are analysed. 2. Quality inspection by means of luminescence microscopy In this paper we investigate localized defects, for which purpose luminescence microscopy is a powerful technique [1]. Defects reduce the luminescence intensity and thus are visible in luminescence (LUM) images as dark features on a bright background (see the bottom part of Fig. 1 ). We use an optical microscope for resolving small defects and an image processing system with zero suppression and gain for the detection of even slight intensity variations by weak defects. The optical microscope has a second advantage: additional optical inspection modes can be applied for quasi-simultaneous inspection of the identical sample section. Nomarsky interference contrast (IC) is the best method for revealing surface structures. Thus the correlation between morphological defects (e.g. Fig. 1, top part) and 0921-5107/91/$3.50
Fig. 1. Luminescence and interference contrast images of the identical section of an MOVPE-grown InP layer, showing crystallographic (LUM) and morphological (IC) defects.
crystallographic defects (Fig. 1, lower part) can be recognized. Infrared transmission (TRANS) at a wavelength where the substrate is transparent and the layer is absorbing is a third important analyti© Elsevier Sequoia/Printed in The Netherlands
110 cal tool. This method shows thickness inhomogeneities in the layer (hidden pinholes in Fig. 2 which appear as bright spots) as well as metallic indium inclusions which are less transparent and appear as dark structures (see later examples).
3. Dislocations in substrate and layer Dislocations are generally considered to be the most critical defects in III-V materials, and the dislocation density is the only quality parameter specified by substrate suppliers. One reason for their importance is that the growth of dislocations from the substrate into the layer cannot be avoided. This can be recognized in Fig. 3, which shows InP layers grown on a substrate with high dislocation density (lnP:Fe, lower image) and on one with a low dislocation density (InP:S, upper image). It is clearly seen that the dislocation density in the layer is a replication of that of the substrate.
Fig. 2. Hillocks (IC), hidden pinholes (TRANS, bright points) and their local matching (IC +TRANS)in an LPEgrown lnGaAs layer.
4. Material problems in InP Typically the dislocation density of substrates (except InP:S) is about 5 x 104 cm :. T h e probability that a dislocation is threading the active region of a laser about (300/zm x 3/~m in area) is 50% for this density. It is known that in GaAs the existence of any dislocation in the active area causes rapid degradation of the device. This sensitivity of GaAs devices is the second reason for the high attention given to dislocations. T h e origins of the catastrophic device failures are stimulated climb and multiplication processes of dislocations. In InP these processes are much less probable. In the absence of multiplication, however, a dislocation in the laser will only reduce the pumping level along a few /~m of the cavity length. T h e resulting slight reduction in output power is not critical for device operation. Although dislocations themselves are not critical in InP there exist material problems, as dem-
Fig. 3. Dislocations in InP grown by MOVPE on InP:S with low dislocation density (upper image) and on InP:Fe with high dislocation density (lower image).
111 onstrated in Figs. 1 and 2. The dark disc defect can be very effective as a consequence of its large size. Surface unevenness hinders following processes like lithography. Threading or hidden pinholes may cause electrical short circuits. The occurrence of these defects cannot be correlated with dislocations. A possible mechanism for their origin is thermal degradation, which arises by incongruent evaporation of phosphorus during high temperature processes.
5. Thermal degradation of InP substrates Thermal degradation was studied on samples which were heat treated in a way similar to that used for epitaxial growth [2]. Figure 4 shows the surface of a sample that was heated for 1 h at 700 °C with a protecting phosphorus pressure generated by an InP counter substrate. Most imposing are the crystallographic large dark disc defects, visible in luminescence. The white features in the superposition IC and LUM are
caused by surface peaks, revealed in the IC mode. Any dark disc has such a peak at its centre, but the peaks are also found without dark discs. The peaks consists of InP, sometimes with an indium cap on top. They arise by material transport out of the crystal, similar to the generation of a volcano (see ref. 2). If protection methods against phosphorus evaporation are not sufficient, then epitaxial growth will start on a surface containing the defects described. The consequences for the quality of the epitaxiai layer are analysed in the following.
6. Correlation between substrate degradation and defects in the layer The typical geometrical features of thermal degradation effects as described in the last section are the key for identifying correlations with defects in the epitaxial layer. In the IC image of Fig. 5 two hillocks with holes exactly at their centres are visible. The luminescence image shows two dark disc defects in the layer, and transmission reveals an indium inclusion at the centre of both dark discs. The superposition of IC and LUM shows the local matching of all the features described. We conclude that a dark disc defect in the substrate has grown into the layer, and additionally this defect has generated the hillock growth (compare with the model in Fig. 6). The InP volcano at the centre of the dark disc has an indium cap (visible as the dark points in TRANS). It is known that epitaxial growth is blocked above indium droplets, and indeed we observe as a consequence the hole visible in IC above the InP/In volcano.
7. Modifications of degradation induced defects
Fig. 4. Crystallographic (LUM) and morphologic (IC + LUM)defectsin a substrate after heat treatmentssimilar to LPE growth.
Not in all cases do the thermal degradation induced defects exhibit simultaneously all of the symptoms discussed. In Fig. 1 for example the dark disc defect has grown into the layer, but hillock growth has not been generated. In Fig. 2 and in Fig. 7 the pinholes at the centres of the hillocks are of the hidden type, caused by InP peaks without an indium cap. In Fig. 7 three hidden pinholes without correlation to a hillock are also detected. They are generated by InP peaks without a dark disc defect. Figure 8 shows that the dark disc defect can increase the growth rate (hillock) as well as block the growth (large hole at the centre). The InP volcano (IC) and the
112
I
Fig, 5. Correlation between typical features of thermal degradation in a suhstratc and dcfccls in an InGaAsP layer grown by I.PE (see text for a discuss on)
Hi[rocks
InP Peak
_%_ .............. ~iiiiiiiiiiiii~i~i~i~i~ik
l!iiiiiii!iigiiiii iiiiiligiiiiiiiii!iiiiiii i iiiiiiiiii!ii!!!! ] iiiiiiii ! iii!iii!!iMiiiiiiiiiiiiiiiiiiiii i!!i!i Ternary Layer t
...............................
0 0 0 0 0 " O ~ - ~ D e g faded 0 0 0 0 0 0 Oommn 0
0
0
0
Hidden
0
0 Threading
Pinho(es
Fig. 6. Model for lhe correlation between thermal degradation induced defects in the substrate (dark disc, lnP/|n peaks) and defects in the epitaxial layer.
indium cap (TRANS) are visible at the centre of the hole. Figure 9 shows that different modifications of thermal degradation induced defects can be present in one sample: a hillock with hidden (upper left) and threading (middle right) pinholes, a large hole (middle low), and indium inclusions without hillocks or dark discs (two dark points in the middle of the transmission image). 8. Conclusion
The examples presented demonstrate that thermal degradation is responsible for different
Fig. 7. Hidden pinholes with and without (three brighl poinls in the middle) hillock growth (lnGaAsP/InP, LPE).
113
~d
Fig. 8. Hillock with a large crater (blocked growth) and an lnP/ln volcano at the centre (InGaAs, MOVPE).
types of defects in epitaxial layers. Some of these defects (crystallographic dark disc defects, hidden pinholes) are not detectable by inspection with visible light and are thus often ignored. Other defects are morphological. They are a known phenomenon, but their origin has yet not been identified. Consequently, avoiding these defects was a trial and error optimization of epitaxial growth. The defects can be critical themselves, for example dark disc defects in the active layer of a laser, or electrical short circuit at a pinhole, or they can become critical through interference with consecutive technological processes,
Fig. 9. Simultaneous appearance of different types of degradation-induced defects in one layer (lnGaAsE LPE). See the discussion in the text.
for example lithography at an unevenness or diffusion at pinholes or along phosphorus vacancies. In summary, the crystal quality in lnGaAsP layers seem to be more affected by thermal degradation than by the dislocation density of the substrate. References I B. Sartorius, D. Franke and M. Schlak, J. CO,st. Growth, 83 (1987)238. 2 B. Sarlorius, M. Schlak, M. Rosenzweig and K. P~irschke, J. AppI. I'hys., 63 (1988) 4677.