ing coils mounted in the same liquid-helium cryostat (Fig. ... helium gas pressure. This tube was ..... 12, edited by R. K. Willarson and A. C. Beer (Aca- demic ...
A far-infrared spectrometer emission sources
based on cyclotron
resonance
W. Knap,a) D. Dur, and A. Raymond G. E. S., Universitk des Sciences et Techniques du Languedoc, Place Eug&e Bataillon, F 34060 Montpellier, Cedex, France
C. Meny and J. Leotin Laboratoire de Physique des Solides, INSA, Complexe ScientiJque de Rangueil, 31077 Toulouse Cedex, France
S. Huant SNCI/CNRS,
BP 166 X, 38042 Grenoble Cedex, France
B. Etienne L2M/CNRS,
196, avenue Henri Ravera, 92220 Bagneux Cedex, France
(Received 30 December 1991; accepted for publication 6 February 1992) Practical realization of the spectrometer based on the cyclotron emission sources is presented. It is shown that it can be used for transmission measurements in the range from 35 to 110 cm- ’ with resolution up to 1.3 cm- ‘. Performances of the spectrometer are demonstrated by its application to the studies of impurity and free-electron states in two-dimensional structures.
1. INTRODUCTION Cyclotron emission sources of far-infrared radiation (FIR) have unique features to be continuously (magnetic field) tunable and narrow band ones. They were proposed to be used in FIR spectroscopy since their discovery.‘,2 Although their possibilities were demonstrated in a few photoconductivity or transmission measurements” they did not gain a genuine interest. Here we show practical realization of a transmission spectrometer based on bulk and two-dimensional cyclotron resonance emitters. We show that use of these sources together with uniaxially stressed Ge:Ga and GaAs detectors substantially improves spectrometer parameters and makes it useful for FIR experiments. The unusual spectrometer feature is that it works immersed in the liquid helium which provides ultralow background radiation conditions. Another remarkable feature of our spectrometer is that it allows to perform both fixed-field and swept-field measurements which can be very powerful for many studies, e.g., experiments on polarons in semiconductors.5 The performance of the spectrometer is demonstrated by its application to studies of impurity states in selectively doped multiple quantum wells (MQWs) and to studies of the cyclotron resonance in GaAs/GaAlAs heterojunctions. II. THE SPECTROMETER The experimental setup consisted of two superconducting coils mounted in the same liquid-helium cryostat (Fig. 1) The emitter was placed in the 8-T lower coil and the investigated sample in the 8.5-T upper coil. The radiation which was generated by the emitter passed through the sample and reached the detector placed outside the magnetic fields. The emitter, the sample, and the detector were enclosed in a copper tube which served simultaneously as a *‘Warsaw University, Hoza 69, 00681 Warsaw, Poland.
light guide for FIR radiation and a screen of a thermal radiation from the warm parts of the cryostat. The light guide was placed in the stainless-steel tube with regulated helium gas pressure. This tube was directly immersed in the liquid helium. The emitters were biased by a rectangular pulse voltage generator with adjustable frequency and duty ratio. The changes of the detector resistance synchronous with the emitter FIR pulses were recorded in a circuit comprising connected in series a dc voltage source and a current preamplifier. A low noise current preamplifier with a gain of 10 - * A/V was used. The preamplified signal was next passed to a lock-in amplifier, digitized, and then stored in the memory of a computer. The detector signal was recorded as function of the magnetic field applied either to the emitter or to the investigated sample. GaAs and uniaxially stressed Ge:Ga detectors were used to cover a wide spectral range. The GaAs detector was a high-purity epitaxial layer of Nd - N, = 3X lOI cmw3 and mobility 2.5 X lo5 cm2/V s at 77 K. Its spectral response starts at 35 cm - ’ which corresponds to photon energy equal to the 13-2~ shallow donor transition. Its high-energy limit is determined at about 60 cm - ’ by a decrease of the probability of the Is-continuum transition. The Ge:Ga photoconductive detector uniaxially stressed up to 7 kbar along the (100) direction was used to cover spectral range above 50 cm - 1.6 The spectral characteristic of the spectrometer as obtained with the GaAs emitter is shown in Fig. 2. It is a convolution of the detector and the emitter characteristics. The dotted line in Fig. 2 shows the spectral response of the unstressed Ge:Ga detector. It is seen that without the uniaxial stress, we cannot have a continuous detection in the whole spectral region. The rapid decrease of the signal above 80 cm - ’ is due to the decrease of the detector sensitivity and to the decrease of the emitter efficiency. The latter decreases because the electron heating to higher
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!!%I
XW
GaAs
-
InSb
---
AcaJcIIsmoN
FIG. 1. The experimental setup.
Landau levels starts to be less eflicient in higher magnetic fields. Another reason for this is that due to the emitter magnetoresistance, the input power injected into the emitter decreases with magnetic field (we use for excitation voltage pulses of constant amplitude). The high-energy limit of the spectral range is about 110 cm - ’ and is due to the maximum magnetic field (8 T) available in our system for the emitter. The GaAs cyclotron resonance emitter gives the rate of tunability of about 14 cm - l/T. To increase the high-energy limit an InSb cyclotron resonance emitter which has about five times higher tunability was used. Comparison of these two emitters is shown in Fig. 3. Unfortunately the high-energy limit of the InSb emitter is only slightly higher (about 20 cm - ‘) than that of the GaAs emitter. The physical reason for this is a decrease of the In% emitter efficiency due to the increase of the nonradiative recombinations as the cyclotron resonance energy approaches longitudinal-optic phonon energy.’ This energy is equal to 197 cm - ’ as to be compared with 295 cm - ’ for GaAs.7 The maximum resolution which can be obtained with our spectrometer is determined by the width of the cyclotron resonance emission line. Comparison of different emitters were performed with a high quality GaAs selective
I
I Stressed
Ge:Ga
Ge:Ga -\
10 Energy
40
so
60 Energy
70
80
Qo
loo
110
120
( c~rn-~ )
FIG. 2. Spectral characteristics of the spectrometer with bulk GaAs and uniaxially stressed Ge:Ga detectors. Dotted line corresponds to the unstressed Ge:Ga detector.
)
FIG. 3. Photoconductivity signal of the uniaxially stressed Ge:Ga detector for the bulk GaAs (continuous line) and the bulk InSb emitter (dotted line).
detector placed in the magnetic field.2*8 The spectral characteristics of this detector, as determined by Fourier transform spectroscopy, is shown in Fig. 4(a). It consists mainly of three lines corresponding to the 1%2p - Js-2po,ls-2p +, shallow impurity transitions. The linewidth is smaller than 0.5 cm - ‘. To compare the CR linewidth of different emitters, the detector was placed in the upper coil (the sample place in Fig. 1). Its magnetic field was kept constant and its signal was recorded as a function of the magnetic field applied to the emitter. Three emitters were tested: InSb, an epitaxial GaAs layer, and a GaAs/GaAlAs heterojunction. The results are shown in Fig. 4. To compare the intensities the spectra were normalized and multiplication factors are indicated for each emitter. The InSb emitter is the most powerful but it gives a CR emission with large linewidth of about 10 cm - ‘. GaAs bulk emitter provides less power but gives a better linewidth of 2.2 cm - I. The highest resolution ( 1.3 cm - ‘) was obtained by using the GaAs/GaAlAs heterojunction emitter. This is due to the spatial separation of free electrons from impurities giving mobilities of the order lo6 cm’/V s. In order to reach a higher resolution one can choose emitters of higher mobility operating in lower temperatures or use lower electric fields for FIR excitation. III. TRANSMISSION
30
(WI-’
EXPERIMENTS
The GaAs/GaAlAs heterojunction was used in the first transmission measurements. GaAs emitter providing a resolution of about 2 cm - * served as a FIR source. The sample was placed in the upper coil (see Fig. 1) and the Ge:Ga stressed detector was used. The spectrometer can be operated in two different ways (modes) depending on the choice which magnetic field is kept constant and which is swept. In the transmission experiments with the heterojunction we used both of them.
Rev. Sci. Instrum., Vol. 63, No. 6, June 1992 3294 Infrared spectrometer 3294 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 147.173.61.3 On: Sun, 02 Aug 2015 11:40:58
TV 0
Bdt1.96‘
a
100
i
2
100
B
cm-l cm-l
c
d
Sample
f
6
5 ( Tesia)
50
*-+
same experiment was repeated without the sample and the appropriate spectra were divided to get a real value of the transmission. Results for four different emitter photon energies are collected in Fig. 5. It is worth noting that in contrast to laser based magneto-optical experiments, in our case the photon energy can be tuned continuously to desired value. From the amplitude of noise (in Fig. 5) we can estimate that transmission changes down to a few percent can be easily measured. In the second operating mode we swept the emitter magnetic field and kept constant the sample magnetic field. This mode is analogous to the magnetospectroscopic measurements with the use of a Fourier-transform spectrometer. The detector signal was recorded versus magnetic field of the emitter (FIR photon energy). Results for three different sample magnetic fields are shown in Fig. 6. In this case a higher noise level was observed. Its origin is not clear up to now. One of the reasons could be that in order to get narrow emission lines, the heating electric fields was chosen in the vicinity of the impact ionization value.
Bd - 1.94 T
Xl
1
r\ 2.2 ml-
0
a
Bd -1.97 T
(d
50
80
B(a) =4.8T
0 w 20
= 75.6
FIG. 5. Transmission spectra of cyclotron resonance in a GaAs/GaAlAs heterojunction for four different emitter FIR photon energies.
,. ( 2
100
E (c)
E (d ) = 83.2
b
4
3
ii s bc 0 iE
= 60.9cm-1 = 68.3 cm-1
20!
5 cd T 50 3z / c 0 3 iz
E(a) E (b)
30
40
50 Energy
60 ( cm-’
70 )
FIG. 4. Photoconductivity spectra: (a) GaAs detector at with use a Fourier transform spectrometer resolution 0.1 cm InSb emitter with GaAs detector at 1.90 T. (c) Bulk GaAs GaAs detector at 1.94 T. (d) Heterojunction GaAs/GaAlAs GaAs detector at 1.97 T.
B(b)
80
= 5.31
be 1.96 T taken - ‘. (b) Bulk emitter with emitter with
In the first operating mode the emitter magnetic field was kept constant and the sample magnetic field was swept. This is analogous to the magnetospectroscopic measurements with the use of a FIR laser. The detector signal as a function of the magnetic field applied to the sample was recorded, digitized, and stored by the computer. The
g sot 2 i $ ’ f E 40
50
60 Energy
70 (cm-l
80
90
’ IO
)
FIG. 6. Transmission spectra of cyclotron resonance in a GaAs/GaAlAs heterojunction vs emitter FIR photon energy.
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801 8(a)=
3.9
8(b)=4.8 B(C)“58T
gt$ tI z2 E
B(d)=6.8
T T T
601
4012 30
a
50
b
70 Energy
cd
90 (cm
110
130
150
4T
201 30
170
50
’ 70
90
110
Energy
-‘)
130
150
1
( cm-’ )
FIG. 7. Study of impurity states in selectively doped MQW with the use of the CR spectrometer. Transmission spectra vs emitter FIR photon energy are plotted for four different magnetic fields.
FIG. 8. Transmission spectra of the MQW taken with the Fourier transform spectrometer.
Therefore, changing the magnetic field could lead to emitter instabilities. Minima observed on the transmission spectra (Figs. 5 and 6) correspond to a cyclotron resonance of the electrons in the heterojunction. The effective mass of the twodimensional determined to be electrons was m* =O.O71m c, slightly higher than the bulk value m* = 0.0665mo. This is due to band nonparabolicity effects.“” The spectrometer was also applied to study impurity states in MQWs. Transmission measurements were performed for 100 A/l00 A GaA.s/GaAIAs MQW doped with Si in the middle of the barriers ( 10” cm - 2). The InSb emitter was used in these experiments. Examples of transmission spectra versus photon energy (emitter magnetic field) for four different magnetic fields applied to the sample are shown in Fig. 7. Minima in the transmission curves correspond to 1.~2~ * transitions for donors located at the middle of the barriers.” Tlhe same spectra have been measured with the Fourier transform spectrometer (Brucker IFS 113V) connected to a 13-T superconducting magnet. Results are shown in F?g. 8. Both methods give similar noise on the spectra. Lines obtained by CR spectrometer are broader, because the InSb emitter of a linewidth of about 10 cm - t was used. However, the energy of the ls2p + transition versus magnetic field, determined by this method, agrees well with values obtained by Fourier transform spectrometer. In Fig. 9 Fourier transform spectrometer results are compared with the results obtained with the use of the CR spectrometer operating in both modes. The dotted line shows the energy of ls-2p + transition for donors in the bulk GaAs. The difference in the bulk and MQW energies is due to confinement effects.”
high sensibility bolometer. The high-energy limit of the spectrometer is determined by the maximum magnetic field available ( 8 T in our system). To reach higher energies one has either to increase the magnetic field or to use an emitter with a smaller effective mass. InSb is not a good candidate because it increases only slightly the high-energy limit ( lO20 cm - ’) decreasing prominently the resolution. Resolution of the measurements is determined by the width of the CR line of the emitter. The best resolution can be obtained by using a high mobility GaAs heterojunction as an emitter. As can be seen in Fig. 4 the increase of resolution when passing from InSb to GaAs and then to GaAs/GaAlAs heterojunction is associated with a decrease of the intensity of emitted FIR radiation. Therefore a compromise between the resolution and the signal-tonoise ratio required in the experiment is always needed. The signal-to-noise ratio determines the minimal
.--
/ / / /
3 6 6 t li
120 .’ , /
/
90 I I ,I’ > /
6Or
i
501 3
/
4
/
/
,
/
The spectral range over which the spectrometer can be operated is 35-110 cm - ‘. The lower energy is determined by the low-energy cut off of the GaAs detector (see Fig. 2). To reach lower energies one has to use InSb detector’ or
/
/
h . .
: A - Fourier
.JJ 4u
0 - Magnetotransmission
+J
n
u
4
5
6 Magnetic
IV. DISCUSSION
A
/
- Transmission
7 Field
8 (Tesla
9
10
1
11
)
FIG. 9. The energy of the Is-2p+ transition in the MQW vs magnetic field. Results of the measurements of the transmission (full rectangles) and magnetotransmission (open rectangles) are compared with the results obtained with use of the Fourier transform spectrometer (triangles). Dotted line shows the energy of ls-2p+ transition for donors in bulk GaAs.
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changes in transmission that can be measured. The best values were obtained with a GaAs emitter and were equal to 50. That means that changes of the transmission higher than 2% can be detected. The detector signal after being processed by a lock-in amplifier was stored in the computer. Therefore noise of the sample spectra and background spectra can be easily decreased by taking many scans and numerical averaging. The unique features that differ the FIR spectrometer based on the CR source from standard Fourier transform or laser based spectrometers are as follows: ( 1) It allows to perform simultaneously magnetospectroscopic measurements (by sweeping sample magnetic field) or standard spectroscopic measurements (by sweeping emitter frequency). This can be used to test possible dielectric artifacts in resonant polaron experiments.5 (2) It operates with very low flux of the background photons because the whole system is closed in the copper light pipe which is cooled to the liquid-helium temperature. The possibility of the effective screening of the background thermal radiation is the important feature of the spectrometer and it has already been used in the tests of the low background detectors prepared for astrophysics experiments.6 (3) It can be operated in very high modulation frequencies because the FIR source is driven by electrical pulses. It can be also adapted to perform the time-resolved measurements.’ In conclusion, we have shown practical realization of a transmission spectrometer based on cyclotron resonance emission sources. Performances of the spectrometer are demonstrated by its application to the studies of impurity and free-electron states in two-dimensional structures. Its unique features show that it can be a new and very useful tool in the far-infrared measurements.
ACKNOWLEDGMENTS
The authors would like to thank A. Dubois and J. Ortega for the help in the construction of the spectrometer, and Professor J. L. Robert for his constant interest and encouragements. ’E. Gornik, Proceedings of the Conference on Physics and Application of Narrow Gap Semiconductors, Nimes 1979, edited by W. Zawadski, published in Lecture Notes in Physics (Springer, Berlin, 1979), Vol. 133; E. Gornik, in Proceedings of the Conference on Application of High Magnetic Fields, Grenoble 1982, edited by G. Landwehr, Lecture Notes in Physics (Springer, Berlin, 1982), Vol. 177. 2G. E. Stillman, C. M. Wolfe, and J. 0. Dimmock, Semiconductors and Semimetals, Vol. 12, edited by R. K. Willarson and A. C. Beer (Academic, 1977). 3E. Gornik, W. Muller, and F. Gaderer, Infrared Phys. 16, 109 (1976). 4J. Waldman, T. S. Chang, H. R. Fetterman, G. E. Stillman, and C. M. Wolfe, Solid State Commun. 15, 1309 ( 1974). ‘K. Karrai, S. Huant, G. Martinez, and L. C. Brunel, Solid State Commun. 66, 355 (1988). “C. Meny, K. Karpierz, J. Lusakowki, W. Knap, R. Barbaste, J. Radostitz, J. Ltotin, M. Witzany, and E. Gomik, in The 14th International Conference on Infrared and Millimeter Waves, Wurzburg 1989, Conf. Digest, edited by M. Von Ortenberg, (SPIE, 1989), Vol. 1240, p. 460; C. Laverny, J. Leotin, C. Villarzel, and J. R. Birch, SPIE Proc. 588,69 (1985). ‘Landoh Bornstein, Numerical Data and Functional Relationships in Science and Technology, edited by 0. Modeling (Springer, Berlin, 1982). *W. Knap, J. Lusakowski, K. Karpierz, B. Orsal, and J. L. Robert, J. Appl. Phys. (to be published). 9F. Thiele, U. Merkt, J. P. Kotthaus, G. Lommer, F. Malcher, U. Rossler, and G. Weiman, Solid State Commun. 62, 84 (1987); K. Ensslin, D. Heitmann, H. Sigg, and K. Ploog, Phys. Rev. B 36, 8177 (1987). ‘“C. Chaubet, A. Raymond, W. Knap, J.Y. Mulot, M. Baj, and J. P. Andre, Semicond. Sci. Technol. 6, 160 ( 1991). “S. Huant , S . P. Najda, W. Knap, G. Martinez, B. Etienne, C. Langerak, J. Singleton, R. Thomeer, G. Hai, F. M. Peeters, and J. T. Devreesse, in Proceedings of the Twentieth International Conference on the Physics of Semiconductors, edited by E. M. Anastassakis and J. D. Joannopoulos (World Scientific, Singapore, 1990), p. 1369.
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