~ ) Solid State Communications, Vol. 85, No. 3, pp. 199-202, 1993. Printed in Great Britain.

0038-1098/9356.00+.00 Pergamon Press Ltd

Optical Properties of Si/Sil.xGex Heterostructure Based Wires

Y.S.Tang, C.D.W.Wilkinson and C.M.Sotomayor Torres Nanoelectronics Research Centre, Department of Electronics and Electrical Engineering, University of Glasgow,

Glasgow G12 8QQ, UK

D.W.Smith, T.E.Whall and E.H.C.Parker Department of Physics, University of Warwick, Coventry CV4 7AL, UK

(Received 5 October 1992, accepted for publication 23 October 1992 by D. Van Dyck)

We report for the first time a study of the optical properties of nanostructured wires fabricated by using electron beam lithography and reactive ion etching in SiCI 4 from a modulation doped p+-Si/Sil.xGex heterojunction structure. Both photoreflectance and photoluminescence at 4K show a fabrication process-induced partial strain relaxation and a quasi-one dimensional (1D) behavior when the wire width:reduces to less than about 40nm. A strong emission from electron-hole droplets in the quasi-1D wires is also detected.

Recently, there is a growing interest in studying the physical properties of strained layer S i /S i l .xGex heterostructure systems,[ 1"5] grown by molecular beam epitaxy (MBE) or metal-oganic chemical vapour deposition (MOCVD), due to their potential applications in electrical and optical devices. The characterisation techniques used include photoluminescence (pL),l l ,2] modulation spectroscopy such as photoreflectance (PR),[ 3] Raman scattering spectroscopy,[ 4,5] etc. Although substantial achievements in understanding the physics of those structures, which leads to improved quality control in the material growth, have been reached, to our knowledge, there has been no report on nanostructure studies in this new material system. In this communication, we report for the first time a study of the optical properties of a series of nanostructured wires fabricated by using electron beam lithography in combination with reactive ion etching in SiCI4 from a quasi-two dimensional (2I)) modulation doped p+-Si/Sil.xGex heterostructure. The results from both PL and PR measurements show that , in addition to a dry etching induced partial strain relaxation process, the wires start to exhibit quasi-one dimensional (1D) behavior when the physical wire width is less than about 40nm. A strong emission from electron-hole droplets (EHD) in the epitaxial layers is also observed.

The modulation doped structure, as schematically shown in Fig. 1, was grown by MBE in a VG V90S system on a (100) silicon substrate heated to 5509C. The evaporation of Si and Ge using an electron beam yields a growth rate of 0.1nm/s. The sample,contains a 50nm single Si0.8Geo.2 quantum well layer sandwiched between a 300nm undoped Si buffer layer and a 50nm heavily boron doped Si layer with a doping concentration of 5xl018cm -3. A 30nm of nominally undoped Si layer was used as a spacer layer to

199

reduce the effect of impurity scattering in the 2D hole gas region.

The MBE material was first scribed into a series of 5ram x 5mm chips, and then cleaned in methanol, acetone and isopropanol for five minutes in an ultrasonic bath followed by rinsing in de-ionized water and finally blown dry in nitrogen gas. The samples were then patterned with a series of wires of 2.5~tm long and 10nm to 500nm wide by using electron beam lithography and lift-off technique followed by reactive ion etching (RtE) in SiC14.16, 71

The RIE process was performed on a Plasma Technology RIE80 machine with a planar reaction chamber,

50nm Si:B 5 x 1018 cm -3

30nm undoped Si spacer

50rim Si0.sGe0.2

300nm Si buffer

(100) Si substrate

Fig. 1 Schematic diagram of the modulation doped p+-Si/ Sil.xGex heterojunction structure

2110

which was operated at an rf frequency of 13.56MHz. The etching gas was 99.99% of pure SiCI4. After optimizing the etching process for achieving vertical sidewalls,[ 71 we achieved a set of optimized processing parameters on our system for etching Si/Sil.xGex heterostructures, i.e. a gas flow rate of 2.25seem with a chamber pressure of 10.7reTort at an rf power level of 100W, which gives a stable etching rate of',A5nm/min. The fabricated wire dimensions were then ctlecked by using a Hitachi S-900 scanning electron microscope.

The photoreflectance (PR) measurements were carried out at room temperature on a system similar to that described elsewhere[ 8] by using a monochromatic tungsten lamp source mechanically chopped at a frequency of 290I-Iz

Si/Sil_xGx HETEROSTRUCTURE BASED WIRES

30[ o

20

r~ 1

ol . o

10

Vol. 85, No. 3

I00 I000 Wire Width (nm)

Fig.3 PL spectra of a control sample and some etched wires

as the modulation beam. The signal was detected by a Si p- i-n diode detector. The photoluminescence (PL) was performed at 4K on a standard system under the irradiation of the 488nm line of a Ar + ion laser with its power density of about 100W/cm 2.

Fig.2 shows the PR spectra of a control sample and a set of etched wires, where 1 lh represents F-like 2D interband transition between the,fwst electron subband and the first heavy hole subband. I~an be seen that with the reduction of wire width down to aboiJt 100nm, the l l h transition first shifts to lower energies indicating a swain relaxation process occured during the wire fabrication. As the width is further deduced to about 40nm or less, the 1 lh transition shifts to higher energies, and the spectral line shape shows a systematic evolution from a typical 2D interband transition to a 1D line shape indicating a clear dimensional transition with decreasing wire width.[ 9]

2.70

i

T=300K

11h

x 1 J ~ /,~- control sample \ /

- ~ " f 2.5~tm x 50Onm 1 X

x 2 ~ ~ 2.5~m x 200rim

2 - ~ / ' ' - - 2.5~tm x 100nm

x 5 - - ~ ~/"--- 2.5~m x 40nm

x 5 - / ~ t 2 . ~ m x 15nm

I I I

2.72 2.74 2.76 2.78 Photon Energy (eV)

Fig.2 PR specwa of a conu~l sample and different etched wires

at 4K. a) control sample; b) 2.51am x 500nm wires; c) 2.51am x 100rim wires; d) 2.511m x 40rim wires; e) 2.51am x 15am wires; f) etched subswate

To confmn the behavior of the 1 lh peak observed in PR, we have further measured PL on the same samples. As shown in Fig.3, in addition to the well known sharp bound exciton no-phonon fine (BE~'s~s~), the TO phonon assisted free exciton (FET°si.si) and the bound exciton (BET°siosi) transitions, ll,2] a wirewidth dependent feature (marked by arrows in Fig.3) is observed which might be related to the no-phonon assisted exciton transition (X~si.¢.) from the SiGe layer. This line has a width of about 8meV due to alloy scattering and strain. The TO phonon assisted transition from the SiGe layer is not resolved because it is masked by the intense electron-hole droplets (EHD) emission from the epitaxial layers. It can be seen that with the decrease of wire width, a blue shift of the XNest.o, transition appears, which is comparable to that observed in the PR measurements.

In Fig.4 the energy shift of the XSPsi.~ transition to higher energies with reducing physical wire width is shown. The solid curve is a theoretical energy shift of the X~si_c~ transition calculated by taking into account the effect of strain.[ 1] The corresponding curve for the X~si.c, transition energy as a function of wire width without strain is about 80meV above this curve (not shown). In the calculations, we choose a zero conduction band discontinuity at the Si/SiGe interface for simplicity. It can be seen that the experimental results follow the strained line for the control sample and wider wires, but shift upwards for wire width less than about 40nm. Although the explanation of this phenomena could be purely dry etching induced strain relaxation or the introduction of extra-lateral confinement, by comparing it with the PR results, we can conclude that it is due to a combined effect of. both partial strain relaxation and the expected 1D confine~ient. Since the transition detected in PR is F-like, strain relaxation will reduce the transition energy, but meanwhile, the introduction of extra- dimensional confinement will increase the transition energy. We interpret the red shift of the 1 lh transition from the 100rim wide wire as resulting from purely strain relaxation during the wire fabrication. This interpretation was further confirmed by a PL study of similar wires and control

Vol. 85, No. 3

Energy (meV)

1150 1120 1090 1060

T=4K ~ ,.~ ,z

1.07 1.09 1.11 1.13 1.15 1.17

Si/Sil_xG x HETEROSTRUCTURE BASED WIRES

Energy (meV)

1180 1080 980 880

Wavelength (~tm)

Fig.4 Wire width dependence of the X~sl.c~ peak observed in PL experiments. The open circles are experimental data and the solid curve is calculated by including the effect of strain.

201

.=_ e .

t .

.=

780

T=4K

X

~'~ ~ _~

~,~ ~ .-~e, j

Z_ i I t i i i i

1.05 1.15 1.25 1.35

a l

w i r e s

' ' .5"5' ' 1.45 1 1.65

Wavelength (l~m)

Fig.5 PL spectra of a control Si/Si0.s7Ge0.i3 multiple quantum wells and a half etched 2.51an x 100rim wire sample at 4K. The splitting of the Si0.87Ge0.13 layers related peaks is due to strain relaxation induced by the reactive ion etching.

samples of a undoped Si/Si0.87Ge0.13 multiple quantum well system. An example is shown in Fig.5, the disappearance of the strain/dislocation related structures and the splitting of all the other structures related to the Sio.87Geo.13 layers in the multiple quantum wells are due to the effect of partial strain relaxation intrnduoed during the process of reactive ion etching. The peak FE'l°sisi in the wire sample is TO plmnon assisted transition from the Si substrate. The peak located at about 1.21.tm is from an unknown origin related to the SiGe layers, and the transition marked as Xsi~(D) is from a direct gap related to the SiGe layers, which might be from the surface or interface regions. This transition Xsi.c~(D) was also observe.din further PR measurements where only direct gap transitions should be s e e n .

Except for those discussed above, Fig.3 also shows a strong emission from the EHD in the wires. It can be seen that this emission is from the epitaxial layers, and the existence of this emission is independent of the wire width except for the 40nm wide wires, where the EI-ID emission disappears, which might be due to the imperfection of the fabrication process. We have later found that most of the 40nm wide wires were non-uniform and even broken into dots.

Another important result from this study is that optical transitions due to 1D quantum confinement are still observable even when the wire width goes down to about

15rim, which is much better than a GaAs system, where dry etching induced damage inhibits emission from quantum confined states when the lateral dimension of a wire reduces to about (50~100)nm.[ 10l This indicates a significant less damage of SiGe system during our PIE process.

In conclusion, the authors have for the first time studied the optical properties of a 1D hole gas system fabricated based on a modulation doped p+-Si/Sil.xGex heterojunction structure. Both photoluminesoence and photoreflectance indicate a systematic blue shifts of the interband transition energy with reducing wire dimension. By comparing the experimental results with theoretical calculations based on a simple model, we have found that the above properties are due to the introduction of an extra-dimensional confinement in the system (wire width) and a strain relaxation process arising from the dry etching process. In addition, a strong emission from electron-hole droplets in the wires has been detected.

Acknowledgement-The authors wish to thank the technical staff of the Ultrasmall Research Laboratory of the University of Glasgow for their assistance during the course of this study. This work was partially supported by the British Science and Engineering Research Council (SERC) and the Department of Trade and Industry (DTI) under the collabarative Si-2000 project.

202 Vol. 85, No. 3 Si/Sil_xG x HETEROSTRUCTURE BASED WIRES

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