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Spatially resolved photoluminescence at oval defects in molecular beam epitaxial GaAs layers
472
Journal of Crystal Growth 66 (1 954) 472—474
North—Holland. Amsterdam
LETTER TO THE EDITORS SPATIALLY RESOLVED PHOTOLUMINESCENCE AT OVAL DEFECTS IN MOLECULAR BEAM EPITAXIAL GaAs LAYERS M. BAFLEUR and A. MUNOZ-YAGIJE Lahoratoire CIAutoniatique cC CIA na/i’ve c/es Si’st!~niesc/u (‘entre National c/c Ia Recherche Scientifique, 7 A i enue c/u ( n/one! Roche. F- 31400 Toulouse, France
and
N. LAU RET and J.C. BRABANT Laboratotre c/c Physique des So/ides, Associe au C’NRS. INSA, Ac ‘enue c/c Ranguecl. F - 31077 Toulouse ( ‘cc/er, Francs’ Received 26 December 1983~manuscript received in final form 31 January 1954
The influence of crystal defects on the photoluminescence properties of GaAs molecular beam epitaxial lasers was investigated. Spatially resolved ( 2 gm) photoluminescence measurements were carried Out and special attention was paid to oval defects which seriously affect the morphology of the layers. The results presented show that the photoluminescence spectra are dominated by emissions involving complex defects over an are much larger than the crystallographic defect itself,
The most common macroscopic defect found in molecular beam epitaxy (MBE) of 111—V semiconductors is the hillock or oval defect. These defects are easily identified on the surface of the layer as (110) oriented overgrowths. For 1—5 jam thick layers, the hillock length ranges from 3 to 15/tm. In a given layer, defects of two different sizes are sometimes observed, the larger ones presenting a 2000 A high overgrowth, with a less regular edge. Attention is now paid to such defects as they perturb the layer morphology and thus represent a serious drawback for lithography [1] and the manufacture of integrated microelectronic devices. The aim of this paper is to show that oval defects should also be eliminated for optoelectronic applications. Indeed, it will be shown that the photoluminescence emission of GaAs MBE layers is seriously affected by the larger oval defects. For this purpose, spatially resolved photoluminescence measurements were carried out on MBE layers presenting oval defects. The ultrahigh vacuum system used for this study was equipped with a vacuum interlock system for
rapid substrate exchange. pyrolitic boron nitride evaporation cells and analytical apparatuses (AES. QMS, RHEED). The substrate used, chemical preparation procedure and the growth conditions have been described elsewhere [21. The layers used for the study were Ge-doped GaAs with thicknesses ranging from 1 to 5 p.m. Photoluminescence (PL) spectra and PL maps of these layers were recorded at 77 K using an experimental setup allowing the scanning of a 2 region by an excitation spot of 2 100>< 100 p.m jam diameter; a He—Ne laser was used (X = 6328 A, 5 mW) as light source. Spot scanning and data collection and processing were assured by a computer. The experimental setup incorporates an optical microscope allowing the scanned region to be accurately chosen. PL maps for a given wavelength emission can be obtained as well as PL spectra from a 2 p.m diameter region of the surface of the layer. In the first place. the influence of dislocations and impurity microprecipitates on PL efficiency was investigated. For this purpose the layers were
0022-0248/84/$03.00 3~Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M, Baf/eur et a!.
/
473
Spatial/i revolved photo/uminescence at oval defects
~‘4
__
Fig. I. PL map of ihe near handgap emission on a MBE 3) previously revealed and GaAs:Ge layer (n = 4xlO~ cm’ presenting dislocations (A) and microprecipitates (B). The map area is 60x60 pm2.
chemically etched following the DABL procedure [3] and PL maps were recorded in the regions including the defects revealed. The results reported by Bohm and Fischer [4] were confirmed: a reduction of the near band gap PL, and an enhancement of the emission involving deep acceptors were observed around the dislocations normal to (100). Fig. 1 shows a map of the near band gap PL of a region including a dislocation normal to (100) and two impurity microprecipitates, all defects previously revealed by a shallow (1000 A) chemtcal etching. A drastic reduction of PL intensity is observed, which affects a region of about 12 p.m diameter around the dislocation and of about 5 p.m diameter around the impurity microprecipitates. The main concern in this study was oval defect influence on PL efficiency of GaAs MBE layers. In this case no previous chemical etch was done as the typical morphology of the defects allows a correct positioning of the sample in the experimental setup. Fig. 2a shows a micrograph of a layer containing a particularly large density of such defects which was used in this study. The crystallographic structure of large oval defects has already been reported [5]. It was shown by transmission electron microscopy (TEM) that the defect consists of a perturbed region of about 5 x 5 p.m2 limited by microtwins and including a small (— I
‘~
.--~-.-
Ii
~ tt~tt1::::~~: ~.
~
~f
tt~i tt:t
Fig. 2. (a) contrast micrograph of the morphology of Nomarksi a layer presenting a large number of surface oval defects. (b) PL map of the near bandgap emission on the same layer at an oval defect. The map area is 60x60 pm2.
jam) polycrystal. Most of the overgrowth observed on the surface thus consists of a slightly perturbed or even perfect crystal. Nevertheless, the near band gap PL maps (fig. 2b) on these defects show that the optical characteristics of the layer are perturbed on a much more extended area (25 x 15 jam2), which overlaps all the overgrowth observed on the surface of the layer (15 x 10 jam2). There is therefore a net increase in size of the perturbation in the sequence: crystal defect, morphology, optical performance degradation. Complete spectra from points in and around the defects were recorded. Fig. 3 shows three of such spectra: (a) in a defect free region of the
474
/
M. BaJleur ci a!.
Spatial/i’ resolved photo/urninescenc’e at oval c/cfects
relative to the crystallographic perturbed area 1475
sv
could be interpreted by an increase in crystal point
~
-~
1,410eV
°
l
~1475eV
only dislocations, stacking faults or even no extended defects are observed by TEM, the PL emis-
sion suffers only a slight decrease in intensity without noticeable variations in the relative inten-
1,440 eV~ ~1440eV
>-
sity of the emissions involved.
1 410 eV
/
if
_ C
a
10 ~‘
C o~
10 Co
I
= a
WAVELENGTH
10 a
C
~n
0 a
defect density. It can be considered that the concentration of such defects increases in the transition region between the unperturbed layer and the extended defects (polycrystal, microtwins, dislocations) located at the center of the oval defect. As far as small oval defects are concerned, in which
/
ui ~—
a
If 1
o’ o’
nm)
Fig. 3. (a) PL spectrum on a point of a defect free region of the layer of fig. 2. (b), (c) PL spectra at different points on an oval defect.
The experimental results presented show that the near band gap photoluminescence emission of GaAs MBE layers is locally reduced by the presence of oval defects. For larger oval defects, it has been shown that these reduction affects a much 2) than the crystal defect jam .larger area (~350 Ti itself (‘—5 p.m) and even than the surface overgrowth (— 150 jam2). It was also observed that in this area, the PL spectra are dominated by emissions involving point defect — doping impurity complexes.
On the basis of these results it can be concluded that the reduction of oval defect density in GaAs
layer, (b) and (c) on two different points on the oval defect. An overall decrease of the integrated intensity (—‘ 50%) is observed at the defect. On spectrum (a). the emission band observed at 77 K is dominated by the donor—acceptor (GeA,,) emission (1.47 eV) owing to the high doping level
(6
MBE layers is a relevant point both in improving performance and yield in microelectronic as well as in optoelectronic devices.
References
X
10t8 cm ~ Ge atoms) and the high electrical compensation of the layer under study. In the spectra (c) and (b) obtained on the defect, the emission is dominated by less energetic transitions at 1.44 and 1.41 eV, and the relative intensity of
these emissions vary from point to point. The two latter emissions have been observed and studied in MBE GaAs: Ge [2] and have been tentatively ascribed to complex defects involving native defects (vacancies, interstitials, etc.) and GeAS [2.6.7]. The large area in which a PL decrease is observed
111
J.C.M. Hwang. T.M. Brennan and AX. Cho, J. Electrodiem. Soc. 130 (1983) 493. [2] M. Bafleur, A. Munoz-Yague. iL. C’astano and J. Piqueras, J. Appl. Phys. 54 (1983) 2630 [3] A, Munoz-Yague and M. Bafleur, J. Crystal Growth 53 (1981) 239. [~l K, Bohm and B. Fisher. J. AppI. Phys. 50 (1979) 5453. [5] M. Bafleur, A. Munoz-Yague and A. Rocher. J. C’rystal Growth 59 (1982) 531.
161
I-I. Kressel, F.Z. Hawrylo and P. Lefur. J. Appl. Phys. 39 (1968) 4059
17] E.W. Williams and CT. 1657.
Elliott, Brit. J. AppI. Phys. 2(1969)
Journal of Crystal Growth 66 (1 954) 472—474
North—Holland. Amsterdam
LETTER TO THE EDITORS SPATIALLY RESOLVED PHOTOLUMINESCENCE AT OVAL DEFECTS IN MOLECULAR BEAM EPITAXIAL GaAs LAYERS M. BAFLEUR and A. MUNOZ-YAGIJE Lahoratoire CIAutoniatique cC CIA na/i’ve c/es Si’st!~niesc/u (‘entre National c/c Ia Recherche Scientifique, 7 A i enue c/u ( n/one! Roche. F- 31400 Toulouse, France
and
N. LAU RET and J.C. BRABANT Laboratotre c/c Physique des So/ides, Associe au C’NRS. INSA, Ac ‘enue c/c Ranguecl. F - 31077 Toulouse ( ‘cc/er, Francs’ Received 26 December 1983~manuscript received in final form 31 January 1954
The influence of crystal defects on the photoluminescence properties of GaAs molecular beam epitaxial lasers was investigated. Spatially resolved ( 2 gm) photoluminescence measurements were carried Out and special attention was paid to oval defects which seriously affect the morphology of the layers. The results presented show that the photoluminescence spectra are dominated by emissions involving complex defects over an are much larger than the crystallographic defect itself,
The most common macroscopic defect found in molecular beam epitaxy (MBE) of 111—V semiconductors is the hillock or oval defect. These defects are easily identified on the surface of the layer as (110) oriented overgrowths. For 1—5 jam thick layers, the hillock length ranges from 3 to 15/tm. In a given layer, defects of two different sizes are sometimes observed, the larger ones presenting a 2000 A high overgrowth, with a less regular edge. Attention is now paid to such defects as they perturb the layer morphology and thus represent a serious drawback for lithography [1] and the manufacture of integrated microelectronic devices. The aim of this paper is to show that oval defects should also be eliminated for optoelectronic applications. Indeed, it will be shown that the photoluminescence emission of GaAs MBE layers is seriously affected by the larger oval defects. For this purpose, spatially resolved photoluminescence measurements were carried out on MBE layers presenting oval defects. The ultrahigh vacuum system used for this study was equipped with a vacuum interlock system for
rapid substrate exchange. pyrolitic boron nitride evaporation cells and analytical apparatuses (AES. QMS, RHEED). The substrate used, chemical preparation procedure and the growth conditions have been described elsewhere [21. The layers used for the study were Ge-doped GaAs with thicknesses ranging from 1 to 5 p.m. Photoluminescence (PL) spectra and PL maps of these layers were recorded at 77 K using an experimental setup allowing the scanning of a 2 region by an excitation spot of 2 100>< 100 p.m jam diameter; a He—Ne laser was used (X = 6328 A, 5 mW) as light source. Spot scanning and data collection and processing were assured by a computer. The experimental setup incorporates an optical microscope allowing the scanned region to be accurately chosen. PL maps for a given wavelength emission can be obtained as well as PL spectra from a 2 p.m diameter region of the surface of the layer. In the first place. the influence of dislocations and impurity microprecipitates on PL efficiency was investigated. For this purpose the layers were
0022-0248/84/$03.00 3~Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M, Baf/eur et a!.
/
473
Spatial/i revolved photo/uminescence at oval defects
~‘4
__
Fig. I. PL map of ihe near handgap emission on a MBE 3) previously revealed and GaAs:Ge layer (n = 4xlO~ cm’ presenting dislocations (A) and microprecipitates (B). The map area is 60x60 pm2.
chemically etched following the DABL procedure [3] and PL maps were recorded in the regions including the defects revealed. The results reported by Bohm and Fischer [4] were confirmed: a reduction of the near band gap PL, and an enhancement of the emission involving deep acceptors were observed around the dislocations normal to (100). Fig. 1 shows a map of the near band gap PL of a region including a dislocation normal to (100) and two impurity microprecipitates, all defects previously revealed by a shallow (1000 A) chemtcal etching. A drastic reduction of PL intensity is observed, which affects a region of about 12 p.m diameter around the dislocation and of about 5 p.m diameter around the impurity microprecipitates. The main concern in this study was oval defect influence on PL efficiency of GaAs MBE layers. In this case no previous chemical etch was done as the typical morphology of the defects allows a correct positioning of the sample in the experimental setup. Fig. 2a shows a micrograph of a layer containing a particularly large density of such defects which was used in this study. The crystallographic structure of large oval defects has already been reported [5]. It was shown by transmission electron microscopy (TEM) that the defect consists of a perturbed region of about 5 x 5 p.m2 limited by microtwins and including a small (— I
‘~
.--~-.-
Ii
~ tt~tt1::::~~: ~.
~
~f
tt~i tt:t
Fig. 2. (a) contrast micrograph of the morphology of Nomarksi a layer presenting a large number of surface oval defects. (b) PL map of the near bandgap emission on the same layer at an oval defect. The map area is 60x60 pm2.
jam) polycrystal. Most of the overgrowth observed on the surface thus consists of a slightly perturbed or even perfect crystal. Nevertheless, the near band gap PL maps (fig. 2b) on these defects show that the optical characteristics of the layer are perturbed on a much more extended area (25 x 15 jam2), which overlaps all the overgrowth observed on the surface of the layer (15 x 10 jam2). There is therefore a net increase in size of the perturbation in the sequence: crystal defect, morphology, optical performance degradation. Complete spectra from points in and around the defects were recorded. Fig. 3 shows three of such spectra: (a) in a defect free region of the
474
/
M. BaJleur ci a!.
Spatial/i’ resolved photo/urninescenc’e at oval c/cfects
relative to the crystallographic perturbed area 1475
sv
could be interpreted by an increase in crystal point
~
-~
1,410eV
°
l
~1475eV
only dislocations, stacking faults or even no extended defects are observed by TEM, the PL emis-
sion suffers only a slight decrease in intensity without noticeable variations in the relative inten-
1,440 eV~ ~1440eV
>-
sity of the emissions involved.
1 410 eV
/
if
_ C
a
10 ~‘
C o~
10 Co
I
= a
WAVELENGTH
10 a
C
~n
0 a
defect density. It can be considered that the concentration of such defects increases in the transition region between the unperturbed layer and the extended defects (polycrystal, microtwins, dislocations) located at the center of the oval defect. As far as small oval defects are concerned, in which
/
ui ~—
a
If 1
o’ o’
nm)
Fig. 3. (a) PL spectrum on a point of a defect free region of the layer of fig. 2. (b), (c) PL spectra at different points on an oval defect.
The experimental results presented show that the near band gap photoluminescence emission of GaAs MBE layers is locally reduced by the presence of oval defects. For larger oval defects, it has been shown that these reduction affects a much 2) than the crystal defect jam .larger area (~350 Ti itself (‘—5 p.m) and even than the surface overgrowth (— 150 jam2). It was also observed that in this area, the PL spectra are dominated by emissions involving point defect — doping impurity complexes.
On the basis of these results it can be concluded that the reduction of oval defect density in GaAs
layer, (b) and (c) on two different points on the oval defect. An overall decrease of the integrated intensity (—‘ 50%) is observed at the defect. On spectrum (a). the emission band observed at 77 K is dominated by the donor—acceptor (GeA,,) emission (1.47 eV) owing to the high doping level
(6
MBE layers is a relevant point both in improving performance and yield in microelectronic as well as in optoelectronic devices.
References
X
10t8 cm ~ Ge atoms) and the high electrical compensation of the layer under study. In the spectra (c) and (b) obtained on the defect, the emission is dominated by less energetic transitions at 1.44 and 1.41 eV, and the relative intensity of
these emissions vary from point to point. The two latter emissions have been observed and studied in MBE GaAs: Ge [2] and have been tentatively ascribed to complex defects involving native defects (vacancies, interstitials, etc.) and GeAS [2.6.7]. The large area in which a PL decrease is observed
111
J.C.M. Hwang. T.M. Brennan and AX. Cho, J. Electrodiem. Soc. 130 (1983) 493. [2] M. Bafleur, A. Munoz-Yague. iL. C’astano and J. Piqueras, J. Appl. Phys. 54 (1983) 2630 [3] A, Munoz-Yague and M. Bafleur, J. Crystal Growth 53 (1981) 239. [~l K, Bohm and B. Fisher. J. AppI. Phys. 50 (1979) 5453. [5] M. Bafleur, A. Munoz-Yague and A. Rocher. J. C’rystal Growth 59 (1982) 531.
161
I-I. Kressel, F.Z. Hawrylo and P. Lefur. J. Appl. Phys. 39 (1968) 4059
17] E.W. Williams and CT. 1657.
Elliott, Brit. J. AppI. Phys. 2(1969)