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A new approach to the cap layer thinning of GaAs based heterostructures with near surface quantum wells
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectrouic Engineering 35 (1997) 83-86
A n e w a p p r o a c h to the cap layer thinning o f G a A s based h e t e r o s t r u c t u r e s with near surface q u a n t u m wells. T.B.Borzenko ", Y.I.Koval~, L.V.Kulik b and A.V.Larionovb alnstitute of Microelectronics Technology, Russian Academy of Sciences, 142432 Chernogolovka Moscow distr., Russia blnstitute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka Moscow distr., Russia Low energy ion beam etching (IBE) at oblique angle at liquid nitrogen temperature has been applied for thinning of the cap layer of GaAs/InGaAs/GaAs (GaAs/AIGaAs/GaAs) heterostructures with near surface quantum wells (QWs) to study dielectric confinement effects [1]. It was shown that this etching provides the decreased radiation damage of such structures that results in smaller decreasing of photoluminescence (PL) quantum yield in comparison with other IBE techniques tested. This etching also saves a smooth surface of structures. Due to this fact the QW PL peaks are well defined and only slightly broadened even in a close approach to the QW layer position.
INTRODUCTION In heterostructures such as GaAs/InGaAs the surface layer of GaAs usually has a thickness of 1530 nm and serves as a cap layer for the thin 3-10 nm layer of InGaAS (quantum well - QW). It is known that the QW photoluminescence (PL) peak position depends on the thickness of this top layer. Chemical etching is frequently used to reduce the thickness of the cap layer to study optical properties of such structures. It should be noted that chemical etching is very susceptible to the prehistory of the sample, to the quality and cleanness of the reagents and water as well as to etching temperature because strongly diluted solutions are used for precise shallow etching ( - tens of angstroms) of GaAs top layers. All these reasons also cause an increase of surface roughness as a result of chemical etching. Dry etching methods, in particular ion beam etching (IBE), are very attractive from the point of view of getting a smooth surface at a given depth with a good accuracy. However up to now this technique was considered not to be applicable for this purpose because of the serious damage caused by energetic ions that results in a degradation of optical properties of the structures. A great number of articles are devoted to degradation of semiconductor properties as a result 0167-9317(97)/$17.00 © 1997 Elsevier Science B.X~ All rights reserved. PII: S0167-9317(96)00159-1
of dry etching [e.g., 2-4]. It is generally recognized now that during ion bombardment a damaged layer is formed that generates defects into the bulk. This layer is expanded into the depth considerably exceeding the estimated ion penetration depth. This can be due to both ion channeling and radiationstimulated diffusion during etching [51 and partially after it. The articles, devoted to etching with noble gas ions, considered the case of ion normal incidence. In this case, etching efficiency is not high and almost all ion energy is consumed for sample heating and defect formation as a result of elastic collisions between ions and atoms of the subsurface layer. However, the sputtering coefficient is known to increase several times when normal incidence is changed into the grazing one. Taking into account that at low energies the ion penetration depth upon etching at an oblique angle becomes smaller and can be of a few monolayers one can expect that defect density at this kind of etching will be less than that at normal incidence. This work presents the results of investigations of the applicability of the [BE with low energy (less than 500eV) ions at oblique angle with simultaneous sample cooling (close to liquid nitrogen temperature) for the GaAs based heterostructures with near surface quantum wells to observe changes in optical properties of such structures.
84
T.B. Borzenko et al. / Microelectronic Engineering 35 (1997) 83-86
~
EXPERIMENTAL DETAILS
The investigations were carried out on GaAs/InGaAs/GaAs heterostructures with a 5 am near surface quantum well (see the inset in Fig. 1). The 20 nm cap layer of the structures was thinned in four different ways: 1) in diluted polishing solution H2SO4+H~O2+H20 [6]; 2) by 1BE at normal incidence at liquid nitrogen temperature; 3) by IBE at oblique angle at room temperature and 4) by IBE at oblique angle at liquid nitrogen (LN2) temperature. In the etching the oblique angle was equal to 75 ° (the angle between the normal to the surface and direction of ion incidence). IBE was performed in a set-up using a Kaufman-type ion source with 500 eV Ar ÷ ions. PL measurements were carried out at 4.2 K and an Ar laser with the wave length 514.5 nm was used for excitation. Sample radiation was analyzed with a grating monochromator and detected with a photomultiplier in the photon counting regime. EXPERIMENTAL RESULTS
In the first series of the experiments, the samples were thinned by 5 nm with four techniques described above. PL spectra of the samples are presented in Fig.l. The remaining 15 nm thickness of the top GaAs layer should be sufficient for the quantum yield and QW peak position to be invariable unless radiation damage is introduced by etching (the case of [BE - curves 2,3,4). However, a decrease in the qtumtum yield of all the samples is observed, although varying in the extent. The strongest (by 3 orders of the magnitude) decrease in the quantum yield was observed for the sample which had been etched at a normal ion incidence. Of the IBE techniques used the minimum quantum yield decrease (by a factor of 10) was observed for sample 3 which had been etched at grazing angle at LN2 temperature. At this depth the chemical etching results in a minimum decreasing of PL quantum yield. This etching was successfully applied for a similar task when the cap layer of the samples was thinned right after their MBE growth [6]. Our attempts to apply chemical etching in the diluted H2SO4+H202+H20 solution to study optical properties of heterostructures which were stored few months after their MBE growth were not successful because further etching, when approaching the QW layer, resulted in PL QW spectra broadening, and "blue
4
xl000 - 20 nmGaAs
o=s
3 xl0
~
2
. . . .
x200
i
.
.
.
.
.
.
.
.
t
.
,
,
i
I
t
i
i
t
I
J
1.420 1.425 1.430 1.435 1.441) 1.445 Energy, eV
Figure 1. Photoluminescence spectra from QW of GaAsflnGaAs/GaAs heterostructure before etching (dashed) and after etching: 1) in the diluted polishing solution, 2) at oblique angle at room temperature, 3) at oblique angle at LN2 temperature, 4) at normal incidence at LN2 temperature. shift" of QW peak position was not observed as it has been predicted by the theory [7]. The surface study after chemical etching by means of careful measurements by Talystep profilometer has shown considerable increasing in surface roughness. Hence, in spite of the fact that there is some decreasing in quantum yield after oblique IBE at LN2 temperature this technique was applied to study dielectric confinement effects on such structures because it saves or even improve the surface smoothness. The PL spectra of heterostructures when the cap layer was thinned approximately by 2 nm (curve 1), by 16 nm (curve 2), by 18 nm (curve 3) and by more than 20 nm (curve 4) are shown in Fig.2. One can see a shift of the PL peak position to the high energy region on curves 2 and 3 when -4 nm and -2 nm of the cap layer was remained after etching, although some decrease in the quantum yield (by 2 orders of the magnitude) is observed. DISCUSSION
The results of grazing etching at different temperatures (curves 3 and 4, Fig. 1) suggest that the thickness of a damaged layer is largely determined by radiation stimulated diffusion during etching.
T.B. Borzenko et al. / Microelectronic Engineering 35 (1997) 83-86
I 0 ~5
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1021
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85
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1.426 1.430 1.434 1.438 1.442 1.446 Energy, eV
"~ 10~s lOiS
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Figure 2. Photoluminescence spectra from QW of GaAs/InGaAs/GaAs heterostructure before etching (dashed) and after etching at oblique angle at LN2 temperature 1) by 2 nm, 2) by 16 nm, 3) by 18 nm and 4) by more than 20 nm.
Figure 3. Absorbed energy density distribution in GaAs during IBE by 500 eV Ar + ions 1) at normal incidence and 2) at 75 ° oblique angle.
Although the diffusion coefficient of vacancies as well as that of interstitials is not large at room temperature [8], a high non-equilibrium concentration of defects in the subsurface layer can bring about the diffusion of the defects from the surface into the bulk. This diffusion was considered to be responsible for the appearance of defects at the depth remarkably exceeding the ranges of ions and recoil atoms. Radiation-stimulated diffusion can be suppressed at low temperatures, which evidently takes place at 77K etching. Defects in cooled samples lie in a narrow subsurface layer whose thickness is determined by the depth of Ar + penetration into GaAs. In this case, the behavior of defects during warming from LN2 to nitrogen temperature as well as the thickness and degree of disordering of a damaged subsurface layer should be of importance. At normal ion incidence the damaged layer is greater than at grazing one. For 500 eV Ar÷ ions incident normally onto the GaAs surface, their projective range Rp is equal to 1.6 nm and its straggling ARp is equal to 1.4 nm. At 75 ° angle ion bombardment, Rp and ARp are equal to 0.4 and 1.5 nm, respectively. The damaged layer depth is obviously larger in the first case. However, the difference in the observed etching results cannot be explained by the difference of ion ranges at normal and grazing incidence alone. Apparently, the etching
efficiency factor plays a significant role. At ion etching, the major part of energy scattered in the subsurface layer is involved in the interactions with the target atoms, which results in the defect formation. This energy can be characterized by the distribution of the absorbed energy density (AED) along the depth into the bulk. At low irradiation doses, the AED distribution is approximately defined by gaussian distribution with Dp and ADp parameters. At high doses of ion bombardment when the thickness of a sputtered layer becomes comparable or exceeds Dp and ADp (the case of ion etching), the distribution of AED should be described taking into account the surface movement. We have calculated AED distribution in GaAs in equilibrium regime during etching by Ar+ ions with energy 500 eV at normal incidence and at oblique angle incidence (curve 1 and 2, Fig.3). The values of sputtered coefficients were determined with using the measured etching rates at normal and oblique angle incidence. The reflection coefficients, D v and ADp were estimated according to the TRIM program [9]. All the initial data and the estimated results are presented in Table 1. It is seen that the maximum AED is 5 times less and the total absorbed energy is 20 times less in etching at oblique angle than at normal etching. Thus, at normal ion incidence during etching both the thickness of a damaged layer and a damage degree are higher than those at oblique incidence.
86
T.B. Borzenko et al. / Microelectronic Engineering 35 (1997) 83-86 Table 1 Etchin$ parameters
Sputtering yield accomodation coefficient Dp, nm ADp, nrn
Surface absorbed energy, 8~, eV/cm3 Total absorbed energy, 8, eV/cm2
1.3 4.3 0.88 0.6 0.37 -4.4 1.17 1.18 1.5x1025 3. lxl024 1-6x10~8 8.6x1016
These results and considerations suggest that the radiative damage degree of GaAs etching can be decreased if etching is carried out at low temperature at an angle close to that of the maximum sputtering coefficient. Under such conditions, etching to the depth larger than 5 nm gave rise to a sharp decrease of PL quantum yield more than by a factor of 50 in comparison with the pristine sample. However, further etching causes virtually no further decrease in the PL quantum yield until -2 nm before the QW is remained. This observation implies that there is a thin subsurface layer of about 2 nm thick which undergoes almost complete destruction upon ion bombardment. Below this layer is the semiconductor region which is subjected to minor changes. This region extends to a depth of 4-5 nrn. When less than 5 nm of the top layer is removed, the damaged layer is too far to appreciably influence the PL quantum yield. Further thinning of the top layer brings about an increase in the influence of a weakly damaged layer and, as a consequence, to a decrease in the PL quantum yield. The QW layer does not undergo any degradation until the top layer becomes -2 nm thin. When removing the top GaAs layer, the QW peak position shifts to the short-wave region in the PL spectra. The shift is connected with surface potential effect as well as with dielectric confinement effects on the QW properties, and was described in [ 1, 7]. CONCLUSIONS Possible application of [BE by Ar+ ions with energy less than 500 eV at oblique angle and at liquid nitrogen temperature to study optical properties of GaAs based heterostructures is shown. A low degradation of the material properties is explained by several factors. The oblique ion
incidence provides the smaller penetration depth and disordering of the subsurface layer and the higher etching efficiency in comparison with the case of the normal ion incidence. Sample cooling suppresses radiation-stimulated diffusion during etching. The aging effect of the heterostructure properties seems to be of large importance and is under investigation. The technique used appears to be applicable to the other AraBv and AHBw semiconductors as well to any other materials of interest. Moreover, it is possible to apply this technique for low-defective fabrication of quantum wires and dots of semiconductor materials. In this case the direction of ion incidence should coincide with a length of wires. Dots can be produced by twice repeating wire fabrication, thereby the second wire patterning should cross the first one. IBE gives the smoother edges and smaller dimension straggling of fabricated structures in comparison with wet chemical methods. ACKNOWLEDGMENTS
We thank Prof. V.D.Kulakovskii from Institute of Solid State Physics RAS for fruitful discussions of the results. This work was supported in part by Russian national program "Physics of solid state nanostructures", project No.2-029/4. REFERENCES
1. P.Ils, Ch.Greus, A.Forchel, V.D.Kulakovskii, N.A.Gippius, and S.G.Tikhodeev, Phys.Rev., B51(7) (1995) 4272. 2. S.W.Pang, Solid State Technology, 27 (1984) 249. 3. M.Kawabe, N.Kanzald, K.Masuda, and S.Namba, Appl.Optics, 17(16) (1978) 2556. 4. K.Nagata. O.Nakajima, and Y.Ishibashi, Jpn.J.Appl.Phys., 25 (1986)L510. 5. C.-H.Chen, D.L.Green, and EL.Hu, J.Vac.Sci.Technol., B13(6) (1995) 2355. 6. J.Dreybrodt, A.Forchel, and J.P.Reithmaier, Phys.Rev., B48 (1993) 14741. 7. L.V.Kulik, V.D.Kulakovskii, M.Bayer, A.Forchel, N.A.Gippius, and S.G.Tikhodeev, submitted to Phys.Rev B. 8. S.K.Ghandhi, VLSI Fabrication Principles. Silicon and Gallium Arsenide, John Wiley&Sons, 1982. 9. J.P.Biersack and l.G.Haggmark, Nucl. Instr. Meth. Phys. Res., 174 (1980) 257.
ELSEVIER
Microelectrouic Engineering 35 (1997) 83-86
A n e w a p p r o a c h to the cap layer thinning o f G a A s based h e t e r o s t r u c t u r e s with near surface q u a n t u m wells. T.B.Borzenko ", Y.I.Koval~, L.V.Kulik b and A.V.Larionovb alnstitute of Microelectronics Technology, Russian Academy of Sciences, 142432 Chernogolovka Moscow distr., Russia blnstitute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka Moscow distr., Russia Low energy ion beam etching (IBE) at oblique angle at liquid nitrogen temperature has been applied for thinning of the cap layer of GaAs/InGaAs/GaAs (GaAs/AIGaAs/GaAs) heterostructures with near surface quantum wells (QWs) to study dielectric confinement effects [1]. It was shown that this etching provides the decreased radiation damage of such structures that results in smaller decreasing of photoluminescence (PL) quantum yield in comparison with other IBE techniques tested. This etching also saves a smooth surface of structures. Due to this fact the QW PL peaks are well defined and only slightly broadened even in a close approach to the QW layer position.
INTRODUCTION In heterostructures such as GaAs/InGaAs the surface layer of GaAs usually has a thickness of 1530 nm and serves as a cap layer for the thin 3-10 nm layer of InGaAS (quantum well - QW). It is known that the QW photoluminescence (PL) peak position depends on the thickness of this top layer. Chemical etching is frequently used to reduce the thickness of the cap layer to study optical properties of such structures. It should be noted that chemical etching is very susceptible to the prehistory of the sample, to the quality and cleanness of the reagents and water as well as to etching temperature because strongly diluted solutions are used for precise shallow etching ( - tens of angstroms) of GaAs top layers. All these reasons also cause an increase of surface roughness as a result of chemical etching. Dry etching methods, in particular ion beam etching (IBE), are very attractive from the point of view of getting a smooth surface at a given depth with a good accuracy. However up to now this technique was considered not to be applicable for this purpose because of the serious damage caused by energetic ions that results in a degradation of optical properties of the structures. A great number of articles are devoted to degradation of semiconductor properties as a result 0167-9317(97)/$17.00 © 1997 Elsevier Science B.X~ All rights reserved. PII: S0167-9317(96)00159-1
of dry etching [e.g., 2-4]. It is generally recognized now that during ion bombardment a damaged layer is formed that generates defects into the bulk. This layer is expanded into the depth considerably exceeding the estimated ion penetration depth. This can be due to both ion channeling and radiationstimulated diffusion during etching [51 and partially after it. The articles, devoted to etching with noble gas ions, considered the case of ion normal incidence. In this case, etching efficiency is not high and almost all ion energy is consumed for sample heating and defect formation as a result of elastic collisions between ions and atoms of the subsurface layer. However, the sputtering coefficient is known to increase several times when normal incidence is changed into the grazing one. Taking into account that at low energies the ion penetration depth upon etching at an oblique angle becomes smaller and can be of a few monolayers one can expect that defect density at this kind of etching will be less than that at normal incidence. This work presents the results of investigations of the applicability of the [BE with low energy (less than 500eV) ions at oblique angle with simultaneous sample cooling (close to liquid nitrogen temperature) for the GaAs based heterostructures with near surface quantum wells to observe changes in optical properties of such structures.
84
T.B. Borzenko et al. / Microelectronic Engineering 35 (1997) 83-86
~
EXPERIMENTAL DETAILS
The investigations were carried out on GaAs/InGaAs/GaAs heterostructures with a 5 am near surface quantum well (see the inset in Fig. 1). The 20 nm cap layer of the structures was thinned in four different ways: 1) in diluted polishing solution H2SO4+H~O2+H20 [6]; 2) by 1BE at normal incidence at liquid nitrogen temperature; 3) by IBE at oblique angle at room temperature and 4) by IBE at oblique angle at liquid nitrogen (LN2) temperature. In the etching the oblique angle was equal to 75 ° (the angle between the normal to the surface and direction of ion incidence). IBE was performed in a set-up using a Kaufman-type ion source with 500 eV Ar ÷ ions. PL measurements were carried out at 4.2 K and an Ar laser with the wave length 514.5 nm was used for excitation. Sample radiation was analyzed with a grating monochromator and detected with a photomultiplier in the photon counting regime. EXPERIMENTAL RESULTS
In the first series of the experiments, the samples were thinned by 5 nm with four techniques described above. PL spectra of the samples are presented in Fig.l. The remaining 15 nm thickness of the top GaAs layer should be sufficient for the quantum yield and QW peak position to be invariable unless radiation damage is introduced by etching (the case of [BE - curves 2,3,4). However, a decrease in the qtumtum yield of all the samples is observed, although varying in the extent. The strongest (by 3 orders of the magnitude) decrease in the quantum yield was observed for the sample which had been etched at a normal ion incidence. Of the IBE techniques used the minimum quantum yield decrease (by a factor of 10) was observed for sample 3 which had been etched at grazing angle at LN2 temperature. At this depth the chemical etching results in a minimum decreasing of PL quantum yield. This etching was successfully applied for a similar task when the cap layer of the samples was thinned right after their MBE growth [6]. Our attempts to apply chemical etching in the diluted H2SO4+H202+H20 solution to study optical properties of heterostructures which were stored few months after their MBE growth were not successful because further etching, when approaching the QW layer, resulted in PL QW spectra broadening, and "blue
4
xl000 - 20 nmGaAs
o=s
3 xl0
~
2
. . . .
x200
i
.
.
.
.
.
.
.
.
t
.
,
,
i
I
t
i
i
t
I
J
1.420 1.425 1.430 1.435 1.441) 1.445 Energy, eV
Figure 1. Photoluminescence spectra from QW of GaAsflnGaAs/GaAs heterostructure before etching (dashed) and after etching: 1) in the diluted polishing solution, 2) at oblique angle at room temperature, 3) at oblique angle at LN2 temperature, 4) at normal incidence at LN2 temperature. shift" of QW peak position was not observed as it has been predicted by the theory [7]. The surface study after chemical etching by means of careful measurements by Talystep profilometer has shown considerable increasing in surface roughness. Hence, in spite of the fact that there is some decreasing in quantum yield after oblique IBE at LN2 temperature this technique was applied to study dielectric confinement effects on such structures because it saves or even improve the surface smoothness. The PL spectra of heterostructures when the cap layer was thinned approximately by 2 nm (curve 1), by 16 nm (curve 2), by 18 nm (curve 3) and by more than 20 nm (curve 4) are shown in Fig.2. One can see a shift of the PL peak position to the high energy region on curves 2 and 3 when -4 nm and -2 nm of the cap layer was remained after etching, although some decrease in the quantum yield (by 2 orders of the magnitude) is observed. DISCUSSION
The results of grazing etching at different temperatures (curves 3 and 4, Fig. 1) suggest that the thickness of a damaged layer is largely determined by radiation stimulated diffusion during etching.
T.B. Borzenko et al. / Microelectronic Engineering 35 (1997) 83-86
I 0 ~5
,t'x
4
--
3
/
.../
\\
i/~\
x150
>u
10 ~'3
~" ~
1021
xlO0
"o
e-
2
85
// "~\ e~
\~
1017
1.426 1.430 1.434 1.438 1.442 1.446 Energy, eV
"~ 10~s lOiS
\
2
\ k\ \
,
0
\\
2~
u~
xl
\\\
\,
I°'~
2
4
,
i
6
8
',\ .....
J
10
depth,nm
Figure 2. Photoluminescence spectra from QW of GaAs/InGaAs/GaAs heterostructure before etching (dashed) and after etching at oblique angle at LN2 temperature 1) by 2 nm, 2) by 16 nm, 3) by 18 nm and 4) by more than 20 nm.
Figure 3. Absorbed energy density distribution in GaAs during IBE by 500 eV Ar + ions 1) at normal incidence and 2) at 75 ° oblique angle.
Although the diffusion coefficient of vacancies as well as that of interstitials is not large at room temperature [8], a high non-equilibrium concentration of defects in the subsurface layer can bring about the diffusion of the defects from the surface into the bulk. This diffusion was considered to be responsible for the appearance of defects at the depth remarkably exceeding the ranges of ions and recoil atoms. Radiation-stimulated diffusion can be suppressed at low temperatures, which evidently takes place at 77K etching. Defects in cooled samples lie in a narrow subsurface layer whose thickness is determined by the depth of Ar + penetration into GaAs. In this case, the behavior of defects during warming from LN2 to nitrogen temperature as well as the thickness and degree of disordering of a damaged subsurface layer should be of importance. At normal ion incidence the damaged layer is greater than at grazing one. For 500 eV Ar÷ ions incident normally onto the GaAs surface, their projective range Rp is equal to 1.6 nm and its straggling ARp is equal to 1.4 nm. At 75 ° angle ion bombardment, Rp and ARp are equal to 0.4 and 1.5 nm, respectively. The damaged layer depth is obviously larger in the first case. However, the difference in the observed etching results cannot be explained by the difference of ion ranges at normal and grazing incidence alone. Apparently, the etching
efficiency factor plays a significant role. At ion etching, the major part of energy scattered in the subsurface layer is involved in the interactions with the target atoms, which results in the defect formation. This energy can be characterized by the distribution of the absorbed energy density (AED) along the depth into the bulk. At low irradiation doses, the AED distribution is approximately defined by gaussian distribution with Dp and ADp parameters. At high doses of ion bombardment when the thickness of a sputtered layer becomes comparable or exceeds Dp and ADp (the case of ion etching), the distribution of AED should be described taking into account the surface movement. We have calculated AED distribution in GaAs in equilibrium regime during etching by Ar+ ions with energy 500 eV at normal incidence and at oblique angle incidence (curve 1 and 2, Fig.3). The values of sputtered coefficients were determined with using the measured etching rates at normal and oblique angle incidence. The reflection coefficients, D v and ADp were estimated according to the TRIM program [9]. All the initial data and the estimated results are presented in Table 1. It is seen that the maximum AED is 5 times less and the total absorbed energy is 20 times less in etching at oblique angle than at normal etching. Thus, at normal ion incidence during etching both the thickness of a damaged layer and a damage degree are higher than those at oblique incidence.
86
T.B. Borzenko et al. / Microelectronic Engineering 35 (1997) 83-86 Table 1 Etchin$ parameters
Sputtering yield accomodation coefficient Dp, nm ADp, nrn
Surface absorbed energy, 8~, eV/cm3 Total absorbed energy, 8, eV/cm2
1.3 4.3 0.88 0.6 0.37 -4.4 1.17 1.18 1.5x1025 3. lxl024 1-6x10~8 8.6x1016
These results and considerations suggest that the radiative damage degree of GaAs etching can be decreased if etching is carried out at low temperature at an angle close to that of the maximum sputtering coefficient. Under such conditions, etching to the depth larger than 5 nm gave rise to a sharp decrease of PL quantum yield more than by a factor of 50 in comparison with the pristine sample. However, further etching causes virtually no further decrease in the PL quantum yield until -2 nm before the QW is remained. This observation implies that there is a thin subsurface layer of about 2 nm thick which undergoes almost complete destruction upon ion bombardment. Below this layer is the semiconductor region which is subjected to minor changes. This region extends to a depth of 4-5 nrn. When less than 5 nm of the top layer is removed, the damaged layer is too far to appreciably influence the PL quantum yield. Further thinning of the top layer brings about an increase in the influence of a weakly damaged layer and, as a consequence, to a decrease in the PL quantum yield. The QW layer does not undergo any degradation until the top layer becomes -2 nm thin. When removing the top GaAs layer, the QW peak position shifts to the short-wave region in the PL spectra. The shift is connected with surface potential effect as well as with dielectric confinement effects on the QW properties, and was described in [ 1, 7]. CONCLUSIONS Possible application of [BE by Ar+ ions with energy less than 500 eV at oblique angle and at liquid nitrogen temperature to study optical properties of GaAs based heterostructures is shown. A low degradation of the material properties is explained by several factors. The oblique ion
incidence provides the smaller penetration depth and disordering of the subsurface layer and the higher etching efficiency in comparison with the case of the normal ion incidence. Sample cooling suppresses radiation-stimulated diffusion during etching. The aging effect of the heterostructure properties seems to be of large importance and is under investigation. The technique used appears to be applicable to the other AraBv and AHBw semiconductors as well to any other materials of interest. Moreover, it is possible to apply this technique for low-defective fabrication of quantum wires and dots of semiconductor materials. In this case the direction of ion incidence should coincide with a length of wires. Dots can be produced by twice repeating wire fabrication, thereby the second wire patterning should cross the first one. IBE gives the smoother edges and smaller dimension straggling of fabricated structures in comparison with wet chemical methods. ACKNOWLEDGMENTS
We thank Prof. V.D.Kulakovskii from Institute of Solid State Physics RAS for fruitful discussions of the results. This work was supported in part by Russian national program "Physics of solid state nanostructures", project No.2-029/4. REFERENCES
1. P.Ils, Ch.Greus, A.Forchel, V.D.Kulakovskii, N.A.Gippius, and S.G.Tikhodeev, Phys.Rev., B51(7) (1995) 4272. 2. S.W.Pang, Solid State Technology, 27 (1984) 249. 3. M.Kawabe, N.Kanzald, K.Masuda, and S.Namba, Appl.Optics, 17(16) (1978) 2556. 4. K.Nagata. O.Nakajima, and Y.Ishibashi, Jpn.J.Appl.Phys., 25 (1986)L510. 5. C.-H.Chen, D.L.Green, and EL.Hu, J.Vac.Sci.Technol., B13(6) (1995) 2355. 6. J.Dreybrodt, A.Forchel, and J.P.Reithmaier, Phys.Rev., B48 (1993) 14741. 7. L.V.Kulik, V.D.Kulakovskii, M.Bayer, A.Forchel, N.A.Gippius, and S.G.Tikhodeev, submitted to Phys.Rev B. 8. S.K.Ghandhi, VLSI Fabrication Principles. Silicon and Gallium Arsenide, John Wiley&Sons, 1982. 9. J.P.Biersack and l.G.Haggmark, Nucl. Instr. Meth. Phys. Res., 174 (1980) 257.