- Email: [email protected]
InP laser wafers
Journal of Crystal Growth 78 (1986) 549—557 North-Holland, Amsterdam
549
IDENTIFICATION OF THE SOURCE OF PINHOLE DEFECTS IN InGaAsP/InP LASER WAFERS D.G. KNIGHT Bell-Northern Research Ltd., P.O. Box, 3511 Station C, Ottawa, Ontario, Canada K] Y 4H7 Received 23 April 1986; manuscript received in final form 23 June 1986
Results on the origin and formation of morphological defects on planar double heterostructure InGaAsP/InP wafers grown by Liquid Phase Epitaxy are reported. These defects are in the form of pinholes propagating to the surface from the first InP confining layer or the same type of pinholes filled with ternary material. Using interrupted growth sequences, it is shown that these defects nucleate at In-rich droplets deposited at meniscus lines on only the InP surfaces. Although these droplets are found to originate from the InP growth melts, thermal decomposition of the InP surface does play a role in their formation. Surfaces of droplets are also found to be contaminated with impurities, suggesting a possible mechanism for non re-absorption by following growth melts and subsequent defect propagation through all layers.
1. Introduction
on the surface of the InP substate form pits in the substrate. The droplets in the pits then seem to
Double heterostructure In1 ~Ga~As~P1_~/InP LED’s and lasers emitting in the 1—1.5 ~m region are the dominant light sources in fiber optic communication systems. The use of liquid phase epitaxy (LPE) to produce high quality epitaxial layers for these devices is well known and understood [1]. One of the remaining problems with the use of LPE is the prevention of morphological defect formation occuring during the growth of epitaxial layers. This situation is to be distinguished from defects in the crystal lattice; where dislocations in the substrate propagate through the epitaxial layers, or originate in the layers themselves [2]. This work reports on morphological defects consisting of physical pits or pinholes in grown epilayers. These defects are correlated with the presence of In-rich droplets that are deposited from the melt on intermediate InP layer surfaces during the process of InP melt interchange in the growth sequence. Mahajan et al. [3] have reported on epilayer defects arising from indium droplets that originate from the indium rinse melt used to etch back the InP substrate prior to growth of the InP first confining layers. The mechanism they propose is one where the In-rich droplets left from the melt -
prevent the deposition of the InP during layer growth. These pits, called dissolution pits by the authors, can then propagate upwards through all layers. The authors also mention that incomplete wipe-off of phosphorus-poor solutions at any stage in the crystal growth process can also start the propagation of these pits. Recently, Di Giuseppe et al. [4] examined another factor that affects the quality of grown LPE layers. They have determined that impurity contamination greatly increases the occurrence of In-rich inclusions along the surface and walls of mesas in buried heterostructure lasers. Hersee et al. [5] also noticed heavily decorated bands of In-rich droplets near the meniscus lines of complete double heterostructure wafers. They attribute these droplets as arising from possible thermal decomposition of the epilayers near the wafer edge and subsequent formation of In-rich droplets. This mechanism is in disagreement with the droplets originating from the melts. The decomposition of InP surfaces in the presence of hydrogen and subsequent formation of In-rich droplets which form dissolution pits and tracks has been discussed by Chu et al. [6]. In this work physical holes originating at all
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
550
D.G. Knight
/ Source ofpinhole defects in
JnGaAsP/InP laser wafers
epitaxial layer interfaces are studied. It is found that In-rich droplets which are shown to cause these defects originate from the melts and only those used to grow InP epilayers, but that thermal decomposition of the InP surface does play a role in the droplet formation mechanism. No droplets are found on ternary or quaternary surfaces. In addition we find that impurities such as sulfur and silicon present on the surface of the droplets seem to prevent their reabsorption by subsequent LPE
melts were equilibrated at 660°C for 1 h, with the substrate protected from thermal degradation by an InP cover block. The ramp cooling LPE growth technique was then used at a ramp rate of 0.12°C/mm. At 635.5°C,the substrate was etched back by a pure In rinse melt for 30 s to remove any thermally decomposed surface, and the appropriate epitaxial layers were then grown. All epitaxial growth was conducted between 635.5 and 626.0°C. The amounts of 02 and H20 in the LPE
growth melts, leaving pinholes which propagate through layers.
system during crystal growth were monitored using a Research Inc 64800 02 monitor and a Panametrics model 2100 H20 monitor, where typical values were ~ 0.08 and ~ 0.2 ppm~ (dew point —80°C)respectively. Surface features were studied using Nomarski optical microscopy and scanning electron microscopy. Analysis of defects and related impurity contaminants was performed using energy dispersive X-ray analysis (EDX), wavelength dispersive X-ray analysis (WDX) and Auger electron spectroscopy. After examination of a complete wafer, chemical etching of a complete structure was used to examine the surface of the InP first confining layers. In the case where it was desirable to reveal the memscus lines on InP epilayers, a 2000 A gold coating was sputtered on the exposed InP epilayer of the wafer.
2. Experimental To conduct this study, etch-back of planar In1~Ga~As~P1~/inP heterostructures using chemical etches to expose layer interfaces, was performed similar to the work of Mahajan et al., but examination of the surface of epitaxial structures grown layer-by-layer is also used. The observation of the surface of each epitaxial layer in the structure allows for an assessment of the quality of wipe-off for each layer. Also, the droplets present on each layer can be subjected to microanalysis using energy and wavelength dispersive X-ray spectroscopy, and Auger electron spectroscopy. Layer-by-layer examination of wafers used to make planar 1.3 ~tm double heterostructure lasers was conducted by observing wafers with: (a) a 3 ~tm thick Sn-doped3); InP(b) firsttheconfining layera above plus (Nd ~smthick Na = 4 xIn 10l~cm 0.2 1~Ga5As~P1~~ (X = 1.3 ~tm) active layer lattice matched (L~a/a ~3); 0.02%) the (c) theto two buffer plus layera (Nd 1017Zncm~ layers 1.5 ~smNathick doped InP second confining layer (Na Nd = 6 x iO’~cm3); (d) the three layers as in (c) plus a 1.0 p~m thick Sn doped In 053Ga0 47As capping layer lattice matched 3). (zla/a ~ 0.02%) to InP(Nd Na =oriented 1 x 1018S-doped cm All layers were grown on (100) InP with an etch pit density of ~ 2000 cm2, which was polished in Br 2 : MeOH solution followed by a MeOH rinse just before graphite loading into conventional horizontal multiwell boat.a After purging in a Pd purified H 2 ambient, the —
3. Results 3.1. First InP confining layers
—
—
—
—
—
Fig. 1 shows the surface features of a typical LPE grown first particles confining on layer, ence of minute the where surfacetheofpresthe epilayer can be seen. Examination of the layer surface under high magnification using Nomarski interference microscopy reveals that the particles on There the epilayer are distinct in the form droplets. are two typesof of droplet coverage. The first is a general coverage region where the droplets range in size from 1—5 ~smin diameter with a typical size of 2 ~tm, and have a2.fairly Oceven coverage valuewith of IasXlow iO~droplets/cm casionally, patches as iO~or as high as 2 could be seen. Fig. 2 shows 106 droplets/cm
D.G. Knight
/ Source ofpinhole defects in
551
InGaAsP/InP laser wafers
layer by the droplet, similar to the mechanism reported by Chu et a!. [6]. This coverage in the band region is fifty times the value for the general
2.0mm ..~.
Fig. 1. Surface features of a single Sn-doped InP epitaxial layer. The epilayer surface is decorated with minute particles which occur in general coverage and band regions. The band regions are most evident on the right hand side of the wafer.
-~
I ....
I
50 ~m
representative droplets from the general coverage area. The second type of coverage is a droplet band region with a coverage of 5 x 106 droplets/cm2 in the bands, and very few droplets between the bands. When these bands are observed under high magnification as in fig. 3, the droplets are associated with tracks in the layer. It is felt that these tracks are due to etching of the —
I-
I
15 ~m
I
I
25 j.~m Fig. 2. SEM micrograph of a Sn-doped InP epilayer, showing etch tracks on the layer surface. The droplets in the upper portion of the figure were damaged by handling of the wafer, Exposed holes under these damaged droplets are shallow, indicating that the droplets do not propagate up from the substrate.
Fig. 3. (a) Micrograph of the droplet band region. The lines are made up of In-rich droplets and their etch tracks. (b) A droplet band under higher magnification, showing individual droplets and etch tracks. (c) A droplet band, with etch tracks originating at the meniscus line of the InP epilayer.
552
D.G. Knight / Source ofpinhole defects in InGaAsP/InP laser wafers
coverage area, and the bands have a spacing of 100 ~sm. Fig. 3c shows that the etch tracks for droplets in the band region originate at the meniscus lines of InP epilayers. This indicates that —
band droplets originate at meniscus lines, as suggested by Hersee et al. [5]. It should be noted that in addition to the droplets described above, there was the occasional 2 ~.imsquare pit seen on InP epilayers which was filled with solid which analysis shows to be In-rich. These are probably droplets which became imbedded in the epilayer. None of these pits or droplets seemed to originate from holes which extended to the surface of the substrate, which means that the layers grown in this study are relatively free of the dissolution pits described by Mahajan et al. [3]. Evidence for the melt origin of the droplets was obtained by microanalysis. Analysis of InP epilayers by EDX shows that the major component of the droplets is indeed indium. Analysis can also be used to determine the origin of the droplets. The melt used to grow the InP epilayer contained 5.2 at% Sn for an n-type dopant, so this Sn concentration would be expected of droplets which originated from the melt. The doping the 3 (1 xlevel 10-2inAt%) grown epilayer was 4 x 1018 cm and the ratio of total to electrically active Sn is 2.3 : 1 [7], so detection of 0.02 at% Sn for In as a product of a thermally degraded epilayer is expected. The large difference in the Sn content for In produced by the two mechanisms will allow the determination of the origin of the droplets, where the detection limit of 0.1 at% for Auger and WDX spectroscopy means that Sn should be detected if wipe-off is the origin but not if the epilayer degraded. Fig. 4 shows the WDX spectra for In-rich droplets, and a deliberately degraded InP epilayer that had been previously cleaned of droplets by an HNO 3 etch. The signal from Sn can be clearly seen for the droplets, while no signal is seen for In-rich pits in the degraded sample. The presence of Sn in droplets has also been confirmed by Auger spectroscopy. Therefore it is concluded that melt wipe-off is the source of In-rich droplets •1 on mr episayers. Consistent patterns in the travel of In-rich droplets were observed, and can be partially ex—
plained in terms of the results of Chu et a!. [6] who studied the thermal decomposition of InP surfaces. These authors found that exposure of (100) oriented InP substrates to H2 resulted in the
In
Sn
In
a) -
OOX
I
J
I
b)
—
-
-
20X
—
-
3.50
3.60
3.70
3.80
X-ray wavelength (nm) Fig. 4. WDX spectra for a deliberately degraded Sn-doped InP epilayer (a), and In-rich droplets (b). Arrows on the spectra denote the expected positions of spectral peaks for indium and tin, where increased sensitivity for the detection of tin is used. A Sn spectral peak is present for In-rich droplets, while only a drifting baseline for the high sensitivity used is present for the indium present in decomposition pits.
D. G. Knight / Source ofpinhole defects in InGaAsP/InP laser wafers
553
nucleation of In-rich droplets of a few thousand A diameter at nucleation temperatures of 600— 680°C, depending on the H2 flow rate. These droplets then induced the formation of pits which lengthened along a unique <110) direction, where a (111) phosphorus plane at the end of the pit was rapidly attached by the In-rich liquid. For the droplets described in this work etch tracks which tend to travel along the (011) direction are observed. The presumed droplet motion is not influenced by the direction of slider motion, since etch tracks are oriented along the (011) direction regardless of the orientation of the substrate. However, the etch tracks are not strictly confined to travel along the (011) direction as can be seen in fig. 2, and are frequently noted to veer from this direction when approaching a terrace edge on the InP epilayer. Also, the etch tracks are very shallow (~c0.2 ~sm), where the bulk of the droplet lies above the surface of the epilayer. This is opposed to the observations noted for a decomposing substrate where the indium in a decomposition pit lies within the pit, and the pit is always rectangular in shape with the long axis along the (011) direction. These differences are consistent with droplets deposited on the surface of the epilayer which, due to phosphorus loss to the vapour, then attack the epilayer during the initial stages of the cool-down after crystal growth. A large portion of the droplet volume would lie above the surface allowing for greater ease of travel in directions other than (011). Travel along this direction will still be favoured because droplets can attack the (111) phosphorus plane in this case. In fact, attack of the (111) phosphorus plane explains differing etch track lengths, which can be observed in figs. 3b and 3c (12 ~im and 55 ~tm respectively). In the former case, uneveness in the polishing of the substrate has caused a slight misorientation from the (100) surface, so the droplets bury themselves in the exposed (111) phosphorus plane. In the latter, the surface is more exactly aligned and the droplets travel a greater distance along the surface. Assuming an approximately 30 s time interval above 600°C during cool-down, etch rates of about 0.5—2 p.m/s are obtained. Since attack of InP epilayers similar to thermal
15 ~m Fig. 5. Micrograph of a droplet band from an InP epilayer protected by phosphorus overpressure. Note that the droplets reside on the meniscus line, and no etch tracks are present.
decomposition might play a role in the travel of In-rich droplets on InP epilayers, the following experiments were performed. The first series of experiments involved placing a quartz holder contaming Sn and InP in a 10: 1 ratio, in the well of the LPE boat where the grown InP epilayer would be placed after crystal growth. This will provide a large overpressure of phosphorus within this well, which has been proven effective in preventing the thermal decomposition of InP substrates before crystal growth [8,9]. Observation of InP epilayers protected by phosphorus overpressure show no
______
____________________________________________
15 ~
A
Fig. 6. Surface the first confining of a have doublebeen heterostructure laser of wafer, where surface layer features enhanced by gold deposition. A faint meniscus line can be seen to connect the marks on the epilayer surface.
D.G. Knight / Source ofpinhole defects in JnGaAsP/InP laser wafers
554
droplets over large areas of the wafer if the InP: Sn mixture is fresh, but droplet coverage including the general coverage and band regions were noted once the same mixture was used 2—3 times. After
retreating melt after crystal growth, a larger amount of phosphorus should be present in droplets which were not allowed to lose phosphorus to the H2 ambient when compared to droplets which
a few times at least some of the phosphorus in the melt (InP: Sn) has been depleted. A striking difference between the droplet coverage in this case, as opposed to an exposed epilayer, can be seen in fig. 5. Here, band droplets can be seen at the meniscus line with no etch tracks. For some droplets movement has occurred, but the etch tracks are very faint. This result shows that In-rich droplets in the band region do originate at the meniscus lines of InP epilayers, and that the motion of the droplets is a result of droplets which become undersaturated with respect to phosphorus. For a full epilayer growth sequence however, one would expect that the phosphorus overpressure would be sufficient to prevent droplet motion. To confirm this, a complete double heterostructure wafer was etched down to the surface of the first confining layer which then had 2000 A of gold depOsited to enhance surface features. Fig. 6 shows that bands of marks on the top of the epilayer are indeed present, and that InP meniscus lines can be seen to connect the marks in the bands that are seen. The marks indicate the location where In-rich droplets were deposited, showing that the phosphorus overpressure was sufficient to prevent droplet motion. These experiments also indicate that some small amount of thermal degradation of the epilayer is necessary to initiate the poor wipe-off process that gives droplet coverage, since a very high phosphorus overpressure will give a droplet-free layer. Coverage of the epilayer by droplets is observed when an overpressure of phosphorus still high enough to prevent attack of the epilayer by droplets is present. Therefore thermal decomposition did not produce the droplets directly, since a phosphorus overpressure high enough to prevent undersaturation of the droplets should be high enough to prevent large scale degradation of the epilayer surface. Confirmation of the role of phosphorus overpressure arises from Auger studies concerning the amount of phosphorus present in the In-rich droplets. Since the droplets were deposited by the
produce etch tracks on epilayers. This is indeed the case: the P: In peak height ratio for etch track free droplets preserved under a phosphorus overpressure was 0.21 : 1, while a 0.07: 1 ratio was found for droplets at the end of etch tracks. Auger spectroscopy of the droplets also reveals that there is a difference in the level of impurity contaminants on the droplets for the band and general coverage regions on the epilayer. The largest signals were from carbon and oxygen contaminants, but signals were also seen from 5, Si and to a lesser extent from K and Zn. It was found that the C: In peak height ratio changed from 0.3: 1 to 1.5 : 1 when the general coverage and band regions were probed respectively. This result varied when droplets from different areas on the epilayer were probed, but the tendency of droplets in the band regions to be more heavily contaminated was consistently observed. It has been assumed that In-rich pits and now In-rich droplets can cause defects in double heterostructure laser wafers, but until the present, a mechanism where the In-rich features are preserved and prevented from being absorbed into subsequent LPE melts has been missing. A coating of contaminants could provide just such a barrier; allowing the In-rich features t.o remain, and form a defect when epitaxial material grows around the feature. The need for contaminants has been found by Di Giuseppe et al. [4], who found that InP could be grown over mesa structures without Inrich inclusions if the In used for crystal growth is etched immediately before use. For the epitaxial growth in this study the band regions have the largest amount of contaminants, which is consistent with finding the largest number of defects due to In-rich droplets in this region.
—
3.2. Multiple layer growth
Observation of the surface of the two layer structure (n-InP plus A = 1.3 p.m quaternary) reveals that the wipe-off of the quaternary melt was excellent, with no droplets produced on the
D. G. Knight / Source ofpinhole defects in InGaAsP/lnP laser wafers
______
15
~t m
555
melt. It should be noted that not all of the droplets found in bands escaped absorption, since the occurrence of pinhole bands is less than that of bands seen on the first confining layer. Analysis of the bright areas in the pinholes by EDX spectroscopy shows that they are In-rich, confirming the origin of the pinholes as imbedded In-rich droplets. Auger analysis reveals that contaminants can be seen on the surface of the imbedded droplets, with sulfur being the impurity most often detected. For three layer structures, the exact same surface feature noted on single first confining layers were observed for these surfaces. The general coverage was 1 x iO~droplets/cm2, which is the value obtained for Sn-doped InP. Holes caused by the propagation of pinhole defects from the first InP confining layer seen on the two layer structures were also detected. Examination of the ternary top surface of a complete double heterostructure laser wafer shows features similar to those of a two layer structure with a quaternary top surface. The wipe-off is —
15j~m Fig. 7. (a) Pinholes in a two layer n-InP plus A =1.3 ~m quaternary epitaxial wafer. The bright areas are the tops of In-rich droplets in the pinholes, and these pinholes form a band structure. A meniscus line on the quaternary layer can be seen running along this band. (b) A two layer structure that has been etched for 30 s in ferricyanide etch, showing the indium droplets in the pinholes.
quaternary epilayer. However, minute pinholes were detected which were noted in bands on the layer surface (see fig. 7a). A 30 s K 3Fe(CN)6: KOH: H20 etch of a portion of a wafer containing pinholes was performed, and a micrograph of the layer surface after etching appears in fig. 7b. It can be seen that the pinholes are caused by droplets that become buried in the quaternary epilayer. Since the pinholes appeared in bands, the source of the In-rich droplets which caused the pinholes are the remnants of the bands of In droplets produced at the meniscus lines of the first confining layer. The general coverage area of In-rich droplets presumably had insufficient contaminants to prevent their absorption by the quaternary
I, _________________________________________
~
Fig. S. (a) A cleaved cross section through a defect band in a complete double heterostructure wafer showing a pinhole defect extending to the first confining layer. (b) A “closed pinhole”, where the droplet responsible for the defect was absorbed by the ternary layer melt. Ternary material extends all the way to the first confining layer surface.
556
D.G. Knight
/ Source ofpinhole defects in InGaAsP/InP laser wafers
good producing no droplets, and In-filled pits in the epilayer can be seen. These pits likely propagated from the surface of the first confining layer.
estimated for InP in vacuum at 700°C [10], so it is reasonable to assume that a few monolayers of an InP epilayer will degrade in H2 during the 0.1 s
Removal of In from the epilayer surface by a 450: 5 : 3 H2O: HNO3 : HF acid etch reveals holes in the epilayer, and EDX analysis of the holes indicated the presence of phosphorus. This phosphorus signal comes from the exposed second InP confining layer, showing that the defect has propagated up through this layer from the first confining layer. As is the case with the two layer structure, the holes in the epilayer occur in bands which have a 100 p.m spacing. By cleaving through these bands of defects, they can be studied using the SEM. Fig. 8 shows SEM micrographs of the two types of defects that were noted. The first is a pinhole which can be seen from the epilayer surface; but the second is a “closed pinhole” where ternary material has grown into the pit. Both defects originated from the first confining layer surface, and all of the defects seen •were noticed on the cleaved bar where the cleavage plane intersected a defect band.
required for melt interchange, since this ambient gas enhances the thermal decomposition process with respect to inert gases [11—13].A patch of thermally degraded layer will have an In-rich surface which is capable of wetting the growth melt, and could easily cause growth melt to cling to such a patch. The fact that the stick-slip motion of the melt meniscus lines correlates strongly with a high density of droplets indicates that the detailed interactions of the decomposing InP surface with the liquid melt in the interface region results in droplet formation. The complete process of pinhole defect formation in double heterostructure wafers can now be described. After the epitaxial growth of the InP first confining layer; minute In-rich droplets are deposited in bands at the meniscus line of the retreating InP growth melt. The melt also deposits some droplets after retreat from the meniscus line, resulting in a general coverage area on the InP epilayer. The droplets deposited at the meniscus lines seem to have a coating of contaminants which prevent their absorption when coming into contact with the quaternary melt. The quaternary material than grows around the droplet forming a pinhole defect. This defect then propagates upwards through subsequent layers; although the ternary material has been seen to grow in some of the defects, producing a closed pinhole. Fig. 9 illustrates how defects would form for a contaminated LPE system, and how growth would differ for a clean system. This mechanism of defect formation is different from that of Mahajan et al. [3], since wipe-off from subsequent InP epilayers instead of the substrate is the origin of the pinhole defects. These authors mention that poor wipe-off of undersaturated melt from subsequent melts may cause defects. However, it is found that only InP melts have poor wipe-off due to the thermal decomposition which triggers this process, and it is specifically the retreat of the melt from a sticking position that produces the most In-rich droplets. These droplets do not have to be undersaturated, but only need to have a coating of contaminants to form defects in subse-
—
4. Discussion and conclusion In-rich droplets on InP epilayers were found to originate from the growth melt but formation was initiated by a small amount of thermal decomposition of the epilayer surface. Thermal decomposition of InP surfaces was ruled out as the sole origin of In-rich droplets, since: (a) Analysis of the droplets by WDX and Auger spectroscopy detect amounts of dopant that can only come from the melt. (b) Phosphorus overpressure maintained in the well of a graphite boat which was high enough to prevent loss of phosphorus by a droplet was insufficient to prevent droplet formation itself. (c) Differences in the appearance of droplet etch tracks and dissolution pits caused by thermal decomposition were noted. Nevertheless thermal decomposition of InP surfaces does play a role in triggering droplet formation from the melt InP interface region. A decomposition rate of 9 monolayers per second is —
D.G. Knight / Source ofpinhole defects in !nGaAsP/InP laser wafers Contaminated System
a
b
Clean System
1st Con). Layer
Active Layer
______________—
Pinhole
d
“
remove impurities from the surface of the indium used in crystal growth, which could very well be a major source of these contaminants.
The microanalytica! work of C.C. Tan and G. Smith (EDX, WDX spectroscopy), and S. Ingrey (Auger spectroscopy) is gratefully acknowledged. Also acknowledged are useful discussions with I. Turlik and H.W. Willemsen, and the encouragement and assistance of K.D. Chik. The original design for the quartz holder was provided by A.J. SpringThorpe and A. Majeed.
____________
~
clusions around mesa lasers. This procedure will
Acknowledgements
I
—
557
2nd Con) Layer
Capping Layer
Closed Pinhole ~
Fig. 9. Illustration of LPE growth for clean and contaminated systems. After withdrawal of the first confining layer melt, clean and impurity covered droplets are deposited on the epilayer respectively (a). The quaternary melt will form around droplets coated with contaminants (b). These pinholes can then propagate upwards through all subsequent layers, or are filled in by ternary material if the droplet is absorbed by the ternary melt (c, d).
References [1] K. Nakajima, in: GaInAsP Alloy Semiconductors, Ed. T.P. Pearsall, (Wiley, New York, 1982) pp. 43—60, and references therein. [2] A discussion of misfit dislocations in GaInAsP epitaxial layers is presented on pp. 54—56 of ref.~1]. [3] S. Mahajan, D. Brasen, MA. DiGiusepLpe, V.G. Keramidas, H. Temkin, CL. Zipfel, WA. Bonner and G.P. Schwartz, Appl. Phys. Letters 41(1982) 266. [4] MA. DiGiuseppe, AK. Chin, B.H. Chin, J.A. Lourenco and I. Camlibel, J. Crystal Growth 67 (1984) 1. [5] S.D. Hersee, AC. Carter, R.C. Goodfellow, G. Hawkins and I. Griffith, Solid State Electron Devices 3 (1979) 179. [6] S.N.G. Chu, CM. Jodlauk and W.D. Johnston, Jr., J. Electrochem. Soc. 130 (1983) 2398.
quent epilayers. To prevent pinhole defects in double heterostructure wafers, it is obvious that the LPE system must be kept free of contaminants, since contaminants allow the propagation of these defects. From the study of two layer structures, sulfur should be particularly avoided since this impurity was detected on the surface of imbedded droplets. Di Giuseppe et al. [4] have found that an indium etch immediately before crystal growth is useful in preventing In-rich in-
[7] B.H. Chin, RE. Frahm, T.T. Sheng and WA. Bonner, J. Electrochem. Soc. 131 (1984) 1373. [8] GA. Antypas, AppI. Phys. Letters 37 (1980) 64. [9] P. Besomi, RB. Wilson, W.R. Wagner and Ri. Nelson, J. AppI. Phys. 54 (1983) 535. [10] P.K. Gallagher and S.N.G. Chu, J. Phys. Chem. 86 (1982) 3246. [11] T. Nishinaga, K. Pak and S.U. Chiyama, J. Crystal Growth 42 (1977) 315. [12] J.A. Lourenco, J. Crystal Growth 59 (1982) 563. [13] J.L. Benchimol. M. Quillec and S. Slempkes, J. Crystal Growth 64 (1983) 96.
549
IDENTIFICATION OF THE SOURCE OF PINHOLE DEFECTS IN InGaAsP/InP LASER WAFERS D.G. KNIGHT Bell-Northern Research Ltd., P.O. Box, 3511 Station C, Ottawa, Ontario, Canada K] Y 4H7 Received 23 April 1986; manuscript received in final form 23 June 1986
Results on the origin and formation of morphological defects on planar double heterostructure InGaAsP/InP wafers grown by Liquid Phase Epitaxy are reported. These defects are in the form of pinholes propagating to the surface from the first InP confining layer or the same type of pinholes filled with ternary material. Using interrupted growth sequences, it is shown that these defects nucleate at In-rich droplets deposited at meniscus lines on only the InP surfaces. Although these droplets are found to originate from the InP growth melts, thermal decomposition of the InP surface does play a role in their formation. Surfaces of droplets are also found to be contaminated with impurities, suggesting a possible mechanism for non re-absorption by following growth melts and subsequent defect propagation through all layers.
1. Introduction
on the surface of the InP substate form pits in the substrate. The droplets in the pits then seem to
Double heterostructure In1 ~Ga~As~P1_~/InP LED’s and lasers emitting in the 1—1.5 ~m region are the dominant light sources in fiber optic communication systems. The use of liquid phase epitaxy (LPE) to produce high quality epitaxial layers for these devices is well known and understood [1]. One of the remaining problems with the use of LPE is the prevention of morphological defect formation occuring during the growth of epitaxial layers. This situation is to be distinguished from defects in the crystal lattice; where dislocations in the substrate propagate through the epitaxial layers, or originate in the layers themselves [2]. This work reports on morphological defects consisting of physical pits or pinholes in grown epilayers. These defects are correlated with the presence of In-rich droplets that are deposited from the melt on intermediate InP layer surfaces during the process of InP melt interchange in the growth sequence. Mahajan et al. [3] have reported on epilayer defects arising from indium droplets that originate from the indium rinse melt used to etch back the InP substrate prior to growth of the InP first confining layers. The mechanism they propose is one where the In-rich droplets left from the melt -
prevent the deposition of the InP during layer growth. These pits, called dissolution pits by the authors, can then propagate upwards through all layers. The authors also mention that incomplete wipe-off of phosphorus-poor solutions at any stage in the crystal growth process can also start the propagation of these pits. Recently, Di Giuseppe et al. [4] examined another factor that affects the quality of grown LPE layers. They have determined that impurity contamination greatly increases the occurrence of In-rich inclusions along the surface and walls of mesas in buried heterostructure lasers. Hersee et al. [5] also noticed heavily decorated bands of In-rich droplets near the meniscus lines of complete double heterostructure wafers. They attribute these droplets as arising from possible thermal decomposition of the epilayers near the wafer edge and subsequent formation of In-rich droplets. This mechanism is in disagreement with the droplets originating from the melts. The decomposition of InP surfaces in the presence of hydrogen and subsequent formation of In-rich droplets which form dissolution pits and tracks has been discussed by Chu et al. [6]. In this work physical holes originating at all
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
550
D.G. Knight
/ Source ofpinhole defects in
JnGaAsP/InP laser wafers
epitaxial layer interfaces are studied. It is found that In-rich droplets which are shown to cause these defects originate from the melts and only those used to grow InP epilayers, but that thermal decomposition of the InP surface does play a role in the droplet formation mechanism. No droplets are found on ternary or quaternary surfaces. In addition we find that impurities such as sulfur and silicon present on the surface of the droplets seem to prevent their reabsorption by subsequent LPE
melts were equilibrated at 660°C for 1 h, with the substrate protected from thermal degradation by an InP cover block. The ramp cooling LPE growth technique was then used at a ramp rate of 0.12°C/mm. At 635.5°C,the substrate was etched back by a pure In rinse melt for 30 s to remove any thermally decomposed surface, and the appropriate epitaxial layers were then grown. All epitaxial growth was conducted between 635.5 and 626.0°C. The amounts of 02 and H20 in the LPE
growth melts, leaving pinholes which propagate through layers.
system during crystal growth were monitored using a Research Inc 64800 02 monitor and a Panametrics model 2100 H20 monitor, where typical values were ~ 0.08 and ~ 0.2 ppm~ (dew point —80°C)respectively. Surface features were studied using Nomarski optical microscopy and scanning electron microscopy. Analysis of defects and related impurity contaminants was performed using energy dispersive X-ray analysis (EDX), wavelength dispersive X-ray analysis (WDX) and Auger electron spectroscopy. After examination of a complete wafer, chemical etching of a complete structure was used to examine the surface of the InP first confining layers. In the case where it was desirable to reveal the memscus lines on InP epilayers, a 2000 A gold coating was sputtered on the exposed InP epilayer of the wafer.
2. Experimental To conduct this study, etch-back of planar In1~Ga~As~P1~/inP heterostructures using chemical etches to expose layer interfaces, was performed similar to the work of Mahajan et al., but examination of the surface of epitaxial structures grown layer-by-layer is also used. The observation of the surface of each epitaxial layer in the structure allows for an assessment of the quality of wipe-off for each layer. Also, the droplets present on each layer can be subjected to microanalysis using energy and wavelength dispersive X-ray spectroscopy, and Auger electron spectroscopy. Layer-by-layer examination of wafers used to make planar 1.3 ~tm double heterostructure lasers was conducted by observing wafers with: (a) a 3 ~tm thick Sn-doped3); InP(b) firsttheconfining layera above plus (Nd ~smthick Na = 4 xIn 10l~cm 0.2 1~Ga5As~P1~~ (X = 1.3 ~tm) active layer lattice matched (L~a/a ~3); 0.02%) the (c) theto two buffer plus layera (Nd 1017Zncm~ layers 1.5 ~smNathick doped InP second confining layer (Na Nd = 6 x iO’~cm3); (d) the three layers as in (c) plus a 1.0 p~m thick Sn doped In 053Ga0 47As capping layer lattice matched 3). (zla/a ~ 0.02%) to InP(Nd Na =oriented 1 x 1018S-doped cm All layers were grown on (100) InP with an etch pit density of ~ 2000 cm2, which was polished in Br 2 : MeOH solution followed by a MeOH rinse just before graphite loading into conventional horizontal multiwell boat.a After purging in a Pd purified H 2 ambient, the —
3. Results 3.1. First InP confining layers
—
—
—
—
—
Fig. 1 shows the surface features of a typical LPE grown first particles confining on layer, ence of minute the where surfacetheofpresthe epilayer can be seen. Examination of the layer surface under high magnification using Nomarski interference microscopy reveals that the particles on There the epilayer are distinct in the form droplets. are two typesof of droplet coverage. The first is a general coverage region where the droplets range in size from 1—5 ~smin diameter with a typical size of 2 ~tm, and have a2.fairly Oceven coverage valuewith of IasXlow iO~droplets/cm casionally, patches as iO~or as high as 2 could be seen. Fig. 2 shows 106 droplets/cm
D.G. Knight
/ Source ofpinhole defects in
551
InGaAsP/InP laser wafers
layer by the droplet, similar to the mechanism reported by Chu et a!. [6]. This coverage in the band region is fifty times the value for the general
2.0mm ..~.
Fig. 1. Surface features of a single Sn-doped InP epitaxial layer. The epilayer surface is decorated with minute particles which occur in general coverage and band regions. The band regions are most evident on the right hand side of the wafer.
-~
I ....
I
50 ~m
representative droplets from the general coverage area. The second type of coverage is a droplet band region with a coverage of 5 x 106 droplets/cm2 in the bands, and very few droplets between the bands. When these bands are observed under high magnification as in fig. 3, the droplets are associated with tracks in the layer. It is felt that these tracks are due to etching of the —
I-
I
15 ~m
I
I
25 j.~m Fig. 2. SEM micrograph of a Sn-doped InP epilayer, showing etch tracks on the layer surface. The droplets in the upper portion of the figure were damaged by handling of the wafer, Exposed holes under these damaged droplets are shallow, indicating that the droplets do not propagate up from the substrate.
Fig. 3. (a) Micrograph of the droplet band region. The lines are made up of In-rich droplets and their etch tracks. (b) A droplet band under higher magnification, showing individual droplets and etch tracks. (c) A droplet band, with etch tracks originating at the meniscus line of the InP epilayer.
552
D.G. Knight / Source ofpinhole defects in InGaAsP/InP laser wafers
coverage area, and the bands have a spacing of 100 ~sm. Fig. 3c shows that the etch tracks for droplets in the band region originate at the meniscus lines of InP epilayers. This indicates that —
band droplets originate at meniscus lines, as suggested by Hersee et al. [5]. It should be noted that in addition to the droplets described above, there was the occasional 2 ~.imsquare pit seen on InP epilayers which was filled with solid which analysis shows to be In-rich. These are probably droplets which became imbedded in the epilayer. None of these pits or droplets seemed to originate from holes which extended to the surface of the substrate, which means that the layers grown in this study are relatively free of the dissolution pits described by Mahajan et al. [3]. Evidence for the melt origin of the droplets was obtained by microanalysis. Analysis of InP epilayers by EDX shows that the major component of the droplets is indeed indium. Analysis can also be used to determine the origin of the droplets. The melt used to grow the InP epilayer contained 5.2 at% Sn for an n-type dopant, so this Sn concentration would be expected of droplets which originated from the melt. The doping the 3 (1 xlevel 10-2inAt%) grown epilayer was 4 x 1018 cm and the ratio of total to electrically active Sn is 2.3 : 1 [7], so detection of 0.02 at% Sn for In as a product of a thermally degraded epilayer is expected. The large difference in the Sn content for In produced by the two mechanisms will allow the determination of the origin of the droplets, where the detection limit of 0.1 at% for Auger and WDX spectroscopy means that Sn should be detected if wipe-off is the origin but not if the epilayer degraded. Fig. 4 shows the WDX spectra for In-rich droplets, and a deliberately degraded InP epilayer that had been previously cleaned of droplets by an HNO 3 etch. The signal from Sn can be clearly seen for the droplets, while no signal is seen for In-rich pits in the degraded sample. The presence of Sn in droplets has also been confirmed by Auger spectroscopy. Therefore it is concluded that melt wipe-off is the source of In-rich droplets •1 on mr episayers. Consistent patterns in the travel of In-rich droplets were observed, and can be partially ex—
plained in terms of the results of Chu et a!. [6] who studied the thermal decomposition of InP surfaces. These authors found that exposure of (100) oriented InP substrates to H2 resulted in the
In
Sn
In
a) -
OOX
I
J
I
b)
—
-
-
20X
—
-
3.50
3.60
3.70
3.80
X-ray wavelength (nm) Fig. 4. WDX spectra for a deliberately degraded Sn-doped InP epilayer (a), and In-rich droplets (b). Arrows on the spectra denote the expected positions of spectral peaks for indium and tin, where increased sensitivity for the detection of tin is used. A Sn spectral peak is present for In-rich droplets, while only a drifting baseline for the high sensitivity used is present for the indium present in decomposition pits.
D. G. Knight / Source ofpinhole defects in InGaAsP/InP laser wafers
553
nucleation of In-rich droplets of a few thousand A diameter at nucleation temperatures of 600— 680°C, depending on the H2 flow rate. These droplets then induced the formation of pits which lengthened along a unique <110) direction, where a (111) phosphorus plane at the end of the pit was rapidly attached by the In-rich liquid. For the droplets described in this work etch tracks which tend to travel along the (011) direction are observed. The presumed droplet motion is not influenced by the direction of slider motion, since etch tracks are oriented along the (011) direction regardless of the orientation of the substrate. However, the etch tracks are not strictly confined to travel along the (011) direction as can be seen in fig. 2, and are frequently noted to veer from this direction when approaching a terrace edge on the InP epilayer. Also, the etch tracks are very shallow (~c0.2 ~sm), where the bulk of the droplet lies above the surface of the epilayer. This is opposed to the observations noted for a decomposing substrate where the indium in a decomposition pit lies within the pit, and the pit is always rectangular in shape with the long axis along the (011) direction. These differences are consistent with droplets deposited on the surface of the epilayer which, due to phosphorus loss to the vapour, then attack the epilayer during the initial stages of the cool-down after crystal growth. A large portion of the droplet volume would lie above the surface allowing for greater ease of travel in directions other than (011). Travel along this direction will still be favoured because droplets can attack the (111) phosphorus plane in this case. In fact, attack of the (111) phosphorus plane explains differing etch track lengths, which can be observed in figs. 3b and 3c (12 ~im and 55 ~tm respectively). In the former case, uneveness in the polishing of the substrate has caused a slight misorientation from the (100) surface, so the droplets bury themselves in the exposed (111) phosphorus plane. In the latter, the surface is more exactly aligned and the droplets travel a greater distance along the surface. Assuming an approximately 30 s time interval above 600°C during cool-down, etch rates of about 0.5—2 p.m/s are obtained. Since attack of InP epilayers similar to thermal
15 ~m Fig. 5. Micrograph of a droplet band from an InP epilayer protected by phosphorus overpressure. Note that the droplets reside on the meniscus line, and no etch tracks are present.
decomposition might play a role in the travel of In-rich droplets on InP epilayers, the following experiments were performed. The first series of experiments involved placing a quartz holder contaming Sn and InP in a 10: 1 ratio, in the well of the LPE boat where the grown InP epilayer would be placed after crystal growth. This will provide a large overpressure of phosphorus within this well, which has been proven effective in preventing the thermal decomposition of InP substrates before crystal growth [8,9]. Observation of InP epilayers protected by phosphorus overpressure show no
______
____________________________________________
15 ~
A
Fig. 6. Surface the first confining of a have doublebeen heterostructure laser of wafer, where surface layer features enhanced by gold deposition. A faint meniscus line can be seen to connect the marks on the epilayer surface.
D.G. Knight / Source ofpinhole defects in JnGaAsP/InP laser wafers
554
droplets over large areas of the wafer if the InP: Sn mixture is fresh, but droplet coverage including the general coverage and band regions were noted once the same mixture was used 2—3 times. After
retreating melt after crystal growth, a larger amount of phosphorus should be present in droplets which were not allowed to lose phosphorus to the H2 ambient when compared to droplets which
a few times at least some of the phosphorus in the melt (InP: Sn) has been depleted. A striking difference between the droplet coverage in this case, as opposed to an exposed epilayer, can be seen in fig. 5. Here, band droplets can be seen at the meniscus line with no etch tracks. For some droplets movement has occurred, but the etch tracks are very faint. This result shows that In-rich droplets in the band region do originate at the meniscus lines of InP epilayers, and that the motion of the droplets is a result of droplets which become undersaturated with respect to phosphorus. For a full epilayer growth sequence however, one would expect that the phosphorus overpressure would be sufficient to prevent droplet motion. To confirm this, a complete double heterostructure wafer was etched down to the surface of the first confining layer which then had 2000 A of gold depOsited to enhance surface features. Fig. 6 shows that bands of marks on the top of the epilayer are indeed present, and that InP meniscus lines can be seen to connect the marks in the bands that are seen. The marks indicate the location where In-rich droplets were deposited, showing that the phosphorus overpressure was sufficient to prevent droplet motion. These experiments also indicate that some small amount of thermal degradation of the epilayer is necessary to initiate the poor wipe-off process that gives droplet coverage, since a very high phosphorus overpressure will give a droplet-free layer. Coverage of the epilayer by droplets is observed when an overpressure of phosphorus still high enough to prevent attack of the epilayer by droplets is present. Therefore thermal decomposition did not produce the droplets directly, since a phosphorus overpressure high enough to prevent undersaturation of the droplets should be high enough to prevent large scale degradation of the epilayer surface. Confirmation of the role of phosphorus overpressure arises from Auger studies concerning the amount of phosphorus present in the In-rich droplets. Since the droplets were deposited by the
produce etch tracks on epilayers. This is indeed the case: the P: In peak height ratio for etch track free droplets preserved under a phosphorus overpressure was 0.21 : 1, while a 0.07: 1 ratio was found for droplets at the end of etch tracks. Auger spectroscopy of the droplets also reveals that there is a difference in the level of impurity contaminants on the droplets for the band and general coverage regions on the epilayer. The largest signals were from carbon and oxygen contaminants, but signals were also seen from 5, Si and to a lesser extent from K and Zn. It was found that the C: In peak height ratio changed from 0.3: 1 to 1.5 : 1 when the general coverage and band regions were probed respectively. This result varied when droplets from different areas on the epilayer were probed, but the tendency of droplets in the band regions to be more heavily contaminated was consistently observed. It has been assumed that In-rich pits and now In-rich droplets can cause defects in double heterostructure laser wafers, but until the present, a mechanism where the In-rich features are preserved and prevented from being absorbed into subsequent LPE melts has been missing. A coating of contaminants could provide just such a barrier; allowing the In-rich features t.o remain, and form a defect when epitaxial material grows around the feature. The need for contaminants has been found by Di Giuseppe et al. [4], who found that InP could be grown over mesa structures without Inrich inclusions if the In used for crystal growth is etched immediately before use. For the epitaxial growth in this study the band regions have the largest amount of contaminants, which is consistent with finding the largest number of defects due to In-rich droplets in this region.
—
3.2. Multiple layer growth
Observation of the surface of the two layer structure (n-InP plus A = 1.3 p.m quaternary) reveals that the wipe-off of the quaternary melt was excellent, with no droplets produced on the
D. G. Knight / Source ofpinhole defects in InGaAsP/lnP laser wafers
______
15
~t m
555
melt. It should be noted that not all of the droplets found in bands escaped absorption, since the occurrence of pinhole bands is less than that of bands seen on the first confining layer. Analysis of the bright areas in the pinholes by EDX spectroscopy shows that they are In-rich, confirming the origin of the pinholes as imbedded In-rich droplets. Auger analysis reveals that contaminants can be seen on the surface of the imbedded droplets, with sulfur being the impurity most often detected. For three layer structures, the exact same surface feature noted on single first confining layers were observed for these surfaces. The general coverage was 1 x iO~droplets/cm2, which is the value obtained for Sn-doped InP. Holes caused by the propagation of pinhole defects from the first InP confining layer seen on the two layer structures were also detected. Examination of the ternary top surface of a complete double heterostructure laser wafer shows features similar to those of a two layer structure with a quaternary top surface. The wipe-off is —
15j~m Fig. 7. (a) Pinholes in a two layer n-InP plus A =1.3 ~m quaternary epitaxial wafer. The bright areas are the tops of In-rich droplets in the pinholes, and these pinholes form a band structure. A meniscus line on the quaternary layer can be seen running along this band. (b) A two layer structure that has been etched for 30 s in ferricyanide etch, showing the indium droplets in the pinholes.
quaternary epilayer. However, minute pinholes were detected which were noted in bands on the layer surface (see fig. 7a). A 30 s K 3Fe(CN)6: KOH: H20 etch of a portion of a wafer containing pinholes was performed, and a micrograph of the layer surface after etching appears in fig. 7b. It can be seen that the pinholes are caused by droplets that become buried in the quaternary epilayer. Since the pinholes appeared in bands, the source of the In-rich droplets which caused the pinholes are the remnants of the bands of In droplets produced at the meniscus lines of the first confining layer. The general coverage area of In-rich droplets presumably had insufficient contaminants to prevent their absorption by the quaternary
I, _________________________________________
~
Fig. S. (a) A cleaved cross section through a defect band in a complete double heterostructure wafer showing a pinhole defect extending to the first confining layer. (b) A “closed pinhole”, where the droplet responsible for the defect was absorbed by the ternary layer melt. Ternary material extends all the way to the first confining layer surface.
556
D.G. Knight
/ Source ofpinhole defects in InGaAsP/InP laser wafers
good producing no droplets, and In-filled pits in the epilayer can be seen. These pits likely propagated from the surface of the first confining layer.
estimated for InP in vacuum at 700°C [10], so it is reasonable to assume that a few monolayers of an InP epilayer will degrade in H2 during the 0.1 s
Removal of In from the epilayer surface by a 450: 5 : 3 H2O: HNO3 : HF acid etch reveals holes in the epilayer, and EDX analysis of the holes indicated the presence of phosphorus. This phosphorus signal comes from the exposed second InP confining layer, showing that the defect has propagated up through this layer from the first confining layer. As is the case with the two layer structure, the holes in the epilayer occur in bands which have a 100 p.m spacing. By cleaving through these bands of defects, they can be studied using the SEM. Fig. 8 shows SEM micrographs of the two types of defects that were noted. The first is a pinhole which can be seen from the epilayer surface; but the second is a “closed pinhole” where ternary material has grown into the pit. Both defects originated from the first confining layer surface, and all of the defects seen •were noticed on the cleaved bar where the cleavage plane intersected a defect band.
required for melt interchange, since this ambient gas enhances the thermal decomposition process with respect to inert gases [11—13].A patch of thermally degraded layer will have an In-rich surface which is capable of wetting the growth melt, and could easily cause growth melt to cling to such a patch. The fact that the stick-slip motion of the melt meniscus lines correlates strongly with a high density of droplets indicates that the detailed interactions of the decomposing InP surface with the liquid melt in the interface region results in droplet formation. The complete process of pinhole defect formation in double heterostructure wafers can now be described. After the epitaxial growth of the InP first confining layer; minute In-rich droplets are deposited in bands at the meniscus line of the retreating InP growth melt. The melt also deposits some droplets after retreat from the meniscus line, resulting in a general coverage area on the InP epilayer. The droplets deposited at the meniscus lines seem to have a coating of contaminants which prevent their absorption when coming into contact with the quaternary melt. The quaternary material than grows around the droplet forming a pinhole defect. This defect then propagates upwards through subsequent layers; although the ternary material has been seen to grow in some of the defects, producing a closed pinhole. Fig. 9 illustrates how defects would form for a contaminated LPE system, and how growth would differ for a clean system. This mechanism of defect formation is different from that of Mahajan et al. [3], since wipe-off from subsequent InP epilayers instead of the substrate is the origin of the pinhole defects. These authors mention that poor wipe-off of undersaturated melt from subsequent melts may cause defects. However, it is found that only InP melts have poor wipe-off due to the thermal decomposition which triggers this process, and it is specifically the retreat of the melt from a sticking position that produces the most In-rich droplets. These droplets do not have to be undersaturated, but only need to have a coating of contaminants to form defects in subse-
—
4. Discussion and conclusion In-rich droplets on InP epilayers were found to originate from the growth melt but formation was initiated by a small amount of thermal decomposition of the epilayer surface. Thermal decomposition of InP surfaces was ruled out as the sole origin of In-rich droplets, since: (a) Analysis of the droplets by WDX and Auger spectroscopy detect amounts of dopant that can only come from the melt. (b) Phosphorus overpressure maintained in the well of a graphite boat which was high enough to prevent loss of phosphorus by a droplet was insufficient to prevent droplet formation itself. (c) Differences in the appearance of droplet etch tracks and dissolution pits caused by thermal decomposition were noted. Nevertheless thermal decomposition of InP surfaces does play a role in triggering droplet formation from the melt InP interface region. A decomposition rate of 9 monolayers per second is —
D.G. Knight / Source ofpinhole defects in !nGaAsP/InP laser wafers Contaminated System
a
b
Clean System
1st Con). Layer
Active Layer
______________—
Pinhole
d
“
remove impurities from the surface of the indium used in crystal growth, which could very well be a major source of these contaminants.
The microanalytica! work of C.C. Tan and G. Smith (EDX, WDX spectroscopy), and S. Ingrey (Auger spectroscopy) is gratefully acknowledged. Also acknowledged are useful discussions with I. Turlik and H.W. Willemsen, and the encouragement and assistance of K.D. Chik. The original design for the quartz holder was provided by A.J. SpringThorpe and A. Majeed.
____________
~
clusions around mesa lasers. This procedure will
Acknowledgements
I
—
557
2nd Con) Layer
Capping Layer
Closed Pinhole ~
Fig. 9. Illustration of LPE growth for clean and contaminated systems. After withdrawal of the first confining layer melt, clean and impurity covered droplets are deposited on the epilayer respectively (a). The quaternary melt will form around droplets coated with contaminants (b). These pinholes can then propagate upwards through all subsequent layers, or are filled in by ternary material if the droplet is absorbed by the ternary melt (c, d).
References [1] K. Nakajima, in: GaInAsP Alloy Semiconductors, Ed. T.P. Pearsall, (Wiley, New York, 1982) pp. 43—60, and references therein. [2] A discussion of misfit dislocations in GaInAsP epitaxial layers is presented on pp. 54—56 of ref.~1]. [3] S. Mahajan, D. Brasen, MA. DiGiusepLpe, V.G. Keramidas, H. Temkin, CL. Zipfel, WA. Bonner and G.P. Schwartz, Appl. Phys. Letters 41(1982) 266. [4] MA. DiGiuseppe, AK. Chin, B.H. Chin, J.A. Lourenco and I. Camlibel, J. Crystal Growth 67 (1984) 1. [5] S.D. Hersee, AC. Carter, R.C. Goodfellow, G. Hawkins and I. Griffith, Solid State Electron Devices 3 (1979) 179. [6] S.N.G. Chu, CM. Jodlauk and W.D. Johnston, Jr., J. Electrochem. Soc. 130 (1983) 2398.
quent epilayers. To prevent pinhole defects in double heterostructure wafers, it is obvious that the LPE system must be kept free of contaminants, since contaminants allow the propagation of these defects. From the study of two layer structures, sulfur should be particularly avoided since this impurity was detected on the surface of imbedded droplets. Di Giuseppe et al. [4] have found that an indium etch immediately before crystal growth is useful in preventing In-rich in-
[7] B.H. Chin, RE. Frahm, T.T. Sheng and WA. Bonner, J. Electrochem. Soc. 131 (1984) 1373. [8] GA. Antypas, AppI. Phys. Letters 37 (1980) 64. [9] P. Besomi, RB. Wilson, W.R. Wagner and Ri. Nelson, J. AppI. Phys. 54 (1983) 535. [10] P.K. Gallagher and S.N.G. Chu, J. Phys. Chem. 86 (1982) 3246. [11] T. Nishinaga, K. Pak and S.U. Chiyama, J. Crystal Growth 42 (1977) 315. [12] J.A. Lourenco, J. Crystal Growth 59 (1982) 563. [13] J.L. Benchimol. M. Quillec and S. Slempkes, J. Crystal Growth 64 (1983) 96.