Tunable lasers and detectors in the FIR

In]}'ared Phys. TechnoL Vol. 36, No. I, pp. 113 122, 1995

Pergamon

1350-4495(94)00100-6

Copyright ~(') 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 1350-4495/95 $9.50 + 0.00

T U N A B L E LASERS A N D D E T E C T O R S IN T H E F I R E. GORNIK, V. ROSSKOPF and W. HEISS Institut f/Jr Festk6rperelektronik, TU Wien Floragasse 7, A-1040 Wien, Austria

(Received 18 July 1994) Abstract--The presently available tunable and narrowband FIR detectors use radiative transitions between Landau levels as selective photoconductive process. Significant improvements in the performance of InSb detectors can be achieved by special surface treatment and meander structuring. High resolution photoconductive spectrometers can be realized with high purity GaAs bulk and two-dimensional material. A narrowband photoconductive detector signal is demonstrated with quantum wire structures which have the potential for voltage tunable and narrowband detectivity. The demonstration of stimulated emission from p-Ge in crossed electric and magnetic fields provides a magnetically tunable narrow line (0.2cm -~) source of considerable power for spectroscopic applications. High resolution transmission spectroscopy is demonstrated by measuring the absorption coefficient of shallow donors in bulk Ge.

I. I N T R O D U C T I O N In semiconductors with low effective masses of carriers significant energetic splittings of Landau levels can be achieved with moderate magnetic fields in the Tesla range. For the application as detectors or emitters based on radiative transition between Landau levels an additional requirement has to be met: they have to be available with high purity. This requirement is necessary for sufficiently long electronic lifetimes and consequently for narrow absorption and emission line widths. Radiative transitions between Landau levels have been used in InSb to demonstrate the first magnetic field tunable detector in the FIR. "~ Since then significant improvements in the performance of InSb detectors have been reported, c2~New materials have been used to achieve narrow band tunable detection. The best results have been obtained with high purity n-GaAs. °~ With this material a resolution of 0.2 c m - l has been demonstrated over a wide range of frequencies. 141Most recent new detector concepts use local properties of quantum structures to obtain tunable narrowband detection without magnetic field. The demonstration of stimulated emission from p - G e in crossed magnetic and electric fields has had a strong impact on F I R spectroscopy. Laser action based on radiative transitions between Landau levels of light holes has first been demonstrated by Vasil'ev and Ivanov 15~and since reported by many groups. ~6~7~Lasing has been observed over a wide frequency range from 25 to 85 cm t, tunable with magnetic fields between 1.5 and 4.5 T. The laser output spectrum consists of a single line with a line width of about 0.2 cm -1. The maximum output power is in the order of 100 mW for a pulse width of 1 t~s. The basic features of the laser can be explained in good agreement with band structure calculations taking into account the valance band structure in crossed electric and magnetic fields, tsJ Recently, the p - G e cyclotron resonance laser has been applied for magneto absorption measurements. Such experiments were performed for the analysis of shallow impurities in bulk Ge ~9~ as well as for the investigation of light hole Landau levels under intense crossed electric and magnetic fields. "°~ INF36I

1

113

114

E. GORNIKet al. II. FIR D E T E C T O R S

The development of faster and more sensitive solid state detectors has been one of the most important aspects of FIR research in the last few years. FIR detectors are conveniently divided into two categories, thermal and photoconductive. In thermal detectors the radiation is absorbed by the bulk of the material or by a suitable blackened surface, which is heating the bulk of the detector by conduction. The temperature change produces a conductivity change in the material, which is measured. These detectors are equally effective at all wavelengths as long as they are sufficiently absorbing, but rather slow in response, typically in the range of ms. Photoconductive detectors are based on the fact that incident radiation produces either a change in the distribution of the electrons or a change in carrier population, which in turn changes the conductivity. Both types of operation are wavelength sensitive, and fast as only free carriers are involved. The typical response times vary between l0 -7 and 10 -9 s. The direct absorption of the incident radiation by charge carriers can lead to intrinsic excitation (direct transition from the valence to the conduction band, HgCdTe-compounds, InSb, wavelength range 2-20/~m), extrinsic excitation or impurity ionisation (germanium doped with shallow-level impurities, very pure n-type GaAs in epitaxial layers, 10-400/~m), free carrier absorption within the conduction band (broad band InSb detector, 300-1000/am) and cyclotron resonance absorption in the presence of a magnetic field (tunable InSb detector, 20-750/~m). t~l) H.

1. H i g h - p u r i t y

GaAs

detectors

High-purity GaAs layers with mobilities from 90000 to 200000 cm2/Vs at 77 K can be used as highly sensitive and narrowband detectors. They are tunable over a wide range with magnetic fields up to 8 T (40-180cm ~)tJ2,13)using transitions between Zeeman-split impurity transitions. Carrier concentrations vary from some 1013 to some 1015 cm -3 Photoconductivity measurements are performed in a fast Fourier Transform Spectrometer (resolution 0.05cm-I). The samples are

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Tunable lasers and detectors in the FIR

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mounted in an external cryostat and connected via a light pipe to the spectrometer. Without magnetic field the photoconductive spectra show a broad peak (ls-2p at 35cm ~) with the peak weakly sample dependent (central cell effect). When a magnetic field is applied the p-states degeneracy is lifted and the Is-2p line splits into three lines: ls-2p - , Is-2p0, and Is-2p+. In Fig. 1 photoconductivity spectra of the I s - 2 p . transition of sample 10052 (two impurities) and sample R717 (three different impurities) are shown. A comparison with data from Low et al/H n6~and Bose et al. ~7~ leads to an identification of the impurities. Besides the identification of the impurities an estimation of the total number of individual impurities is of great interest for material science. From the analysis of the cyclotron resonance transmission and emission spectra Lindemann eta/. ~TM have lbund that the cyclotron resonance linewidth is proportional to the square root of total impurity content Ac,) ~ v"N~. Assuming that the area of each impurity peak is proportional to the number of corresponding impurities, a similar relation as for the cyclotron resonance is applied for the impurity transitions: k(,) ~mv" N/. By using this relation one can correlate the areas under the photoconductive lines to impurity concentrations. The calibration is done by performing cyclotron emission and photoconductivity on one and the same sample. As a result the concentration of the dominant impurity in the samples shown in Fig. 1 is about 0.9 × 10]4cm ~. 7--

II. 2. Improved tunable lnSb F I R Detectors

Putley (1964) investigated lnSb detectors in great detail. (~92c~ The broad band response of the material due to free carrier absorption becomes narrowband when a magnetic field is applied. Transitions between Landau levels and associated impurity levels are responsible for the two lines in photoconductive spectra at 4.2 K. Both lines shift linearly with magnetic field. At lower temperature only the impurity line remains, since the Landau level line disappears due to carrier freeze out. 121)Previously reported responsivity values are in the order of 5 × 103 V/W with linewidth values of A¢o = 8 10 cm ~ at 4.2 K and 4 c m ~ at 2 K. Wasilewski et al. ~22) and Davidson et al. ~'~ have performed photoconductive measurements under hydrostatic pressure and have found a significant narrowing of the linewidth and an increase in sensitivity. Here we report about improved InSb-FIR detectors which reveal a linewidth of 2 cm ~ at 2 K and one order of magnitude higher responsitivity and a tunable detection range from 10 to 190 and from 210 to 500cm ~. We use InSb with an electron concentration of 5.5 x 10 ]3 cm 3 and a mobility of 6 × 10s cm2/Vs at 77 K to fabricate the photoconductive detectors. The F I R photoconductivity measurements are performed with a Fourier Transform Spectrometer and an optically pumped F I R gas laser. The light is guided into a cryostat containing a magnet system. We have prepared detectors with varying thickness and found that the signal-to-noise ratio of InSb detectors is independent of the detector thickness324) This is a clear evidence that surface processes are the dominating effects. To reduce the surface states the detectors are dipped in a bromium methanol solution. The surface states are saturated by this procedure and an improvement of the signal-to-noise ratio by up to two orders of magnitude is achieved. This chemical treatment passivates the surface and can also be applied when ageing of the detectors occurs. In Fig. 2 photoconductivity spectra of a 50/~m thick detector at 4.2 K are shown. The magnetic fields for the different spectra vary from 1 to 7 T. The Landau level and impurity level transitions for energies below the Reststrahlenband are clearly separated for higher magnetic fields only the impurity line is observed. For very low magnetic fields the Landau level and the impurity level transition broaden and melt together to one transition line. Below 2 K the spectra consists only of the impurity line due to the carrier freeze out. The linewidth of the Landau level transitions and the impurity transition versus magnetic field broadens dramatically above the Reststrahlenband and becomes narrower again at higher magnetic fields. In Fig. 3 the linewidth of an optimized InSb detector is shown over a wide range of magnetic fields. Between 1 and 3 T the linewidth is smaller than 2 cm ~. An optimization of the detector geometry uses the fact that an increasing responsivity

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also leads to better noise equivalent power (NEP). This can be realized with long and thin detectors, fabricated by processing and etching a meander structure into InSb platelets. This meander detectors, mounted on chromium steel, show a seven times improved signal-to-noise ratio as compared to not structured detectors having the same area. {24) Due to the thermal expansion of the substrate, some stress is induced on the meander detector. As a consequence carrier freeze out is stronger than for the non-structured detector and the cyclotron resonance line is narrower. Figure 4 shows the signal-to-noise ratios vs magnetic field for two different detectors one structured and one unstructured: the structured detector clearly shows an improvement of the signal-to-noise ratio at 4.2 K at lower energies in a wavelength range where detector performance is usually low. This better performance at increasing temperatures and decreasing energies makes InSb meander detectors a very suitable tool for the detection of long wavelength FIR radiation. Insb l i n e w i d t h

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H. 3. N e w detector concepts

Quantum wire arrays have been prepared by laser holography and ultrafine wet chemical mesa etching of modulation doped high mobility AlxGa~-xAs/GaAs heterostructures. Due to the narrow geometrical dimensions ( < 300 nm) quantum confinement arises and leads to the formation on one-dimensional electronic subband with typical energy separation of 1-4 meV, which corresponds to a wavelength in the region from 10 to 40 cm -~. FIR transmission spectra of the quantum wire structure show one strong resonance, which can be described with a harmonic oscillator model, which assumes a parabolic confining potential. Applying contacts to the sample, we are able to measure a photoconductive signal (PC) of significant size with the Fourier spectrometer and the FIR gas laser system. The position of the PC line is correlated with the position of the magneto-plasmon resonance transition. "5~ This magneto-plasmon resonance is shifted from the cyclotron resonance according to the relation 0,) ~

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With the demonstration of photoconductivity in wires a whole new avenue of detector configurations and applications is possible where the properties can be adjusted by geometrical design parameters together with a gate voltage. Ill. p-Ge CYCLOTRONRESONANCE LASER In p-type Ge under intense crossed electric and magnetic fields two types of lasers are established. The light to heavy hole laser is characterized by a broadband emission spectrum while the cyclotron resonance laser exhibits a single, narrow, magnetic field tuneable emission line. This emission line is caused by optical transitions between the Landau levels of the light holes. An important requirement for the built of population inversion and thus stimulated emission is an unequal spacing between Landau levels. In the valenceband of Ge this is automatically given due to the degeneracy of the F 8 band which gives rise to the Luttinger (or "quantum") effects,t26) An inverted carrier distribution was first predicted by Vosilious and Levinson~27)and Kurosawa and Maeda t28) for the case of streaming motion in crossed fields. To achieve stimulated emission in p-Ge lasers the applied electric fields have to be chosen in such a way that the light holes are in accumulation while the heavy holes are in streaming motion. In the accumulation region the lifetime of carriers is limited only by acoustical phonon and impurity scattering. The lifetime of the carriers in streaming motion is considerably shorter due to the strong interaction with optical phonons. In streaming motion the heavy holes are repeatedly accelerated collisionless to the optical phonon edge where they are scattered back immediately into the origin of the momentum space, eg) There is a finite probability that the heavy holes are scattered into the Landau levels of the light holes. This process leads to an increased population of the light hole Landau levels. A population inversion between higher lying light hole Landau levels is built up. Ionized impurity scattering and the mixing between the light hole and heavy hole Landau levels at the bottom of the valence band leads to a depopulation of low lying light hole Landau levels. Depending on the impurity concentration stimulated emission of Ge-CR lasers is found in a low (1.5 T < B < 2.5 T) and a high magnetic field range (3.5 T < B < 4.5 T ) J 6) In the present paper only results in the high magnetic field and high frequency range are presented. To optimize the laser

Tunable lasers and detectors in the FIR

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the dependence of the emission on the orientation of the total electric field is investigated. From the detailed analysis of spectrum the dominant laser transition is identified as n = 1 and n = 0 light hole Landau level transition. Further a direct application of the CR laser for far infrared spectroscopy is demonstrated.

III. 1. Sample characterization and experimental setup The laser samples are parallelepipeds (5 x 7 × 25 m m 3) cut from a Ge crystal with an acceptorconcentration ofNA = 6 x 10 ~3cm -3. All faces of the samples were polished and were parallel within 30". Ohmic contacts were fabricated by evaporating In (with 5% gold) on the 7 x 25 mm 2 large sides of the sample and annealing at 400°C. Electric field pulses with a duration of 1/as and a repedition rate of 3 Hz were applied along a crystalic axis which encloses an angle ~ to the [1-10] direction. A range of samples were fabricated with different angles • between 0 and 42'. The magnetic field was applied parallel to the longitudinal axis of the sample and parallel to a [1 10] direction. Two mirrors were mounted at the endfaces of the sample to serve as external resonator. The mirrors consisted of 50/~m thick sheets of mylar coated by 100 nm gold. The radiation was coupled out through a centre bore with a diameter of 1 m m in one of these mirrors. The integral intensity of the laser was measured with a broadband G e : G a detector with a peak sensitivity at 90 c m - ~ while the spectra of the lasers were analysed with the cyclotronresonance transition of an epitaxial bulk n-GaAs detector. The Ge laser and the photoconductive detectors were mounted in the centre of a superconducting solenoid and immersed in liquid helium.

IlL 2. Influence of the orientation of the electric field We have studied the influence of the orientation of the electric field direction on the stimulated emission. In order to estimate the total electric field present in the laser samples, we have performed Hall measurements using samples with the same carrier concentration and the same crystallographic orientation as the laser crystals. The length of the samples was about I mm. For the Hall field measurements two additional contacts were fabricated perpendicular to the contacts for the applied field. With these experiments the angle q~ between the total electric field and the [1-10] direction was deduced as well as the amount of the resulting electric field (sum of applied and Hall field). For several angles of the total electric field • the regions of electric and magnetic fields within which laser emission can be observed are shown in Fig. 6. These measurements were performed using the broadband Ge detector. For angles of • = 13 ° and q~ = 35 ° no laser activity is found. For a sample with ~ = 0 ° the Hall angle was deduced to be 55 °. With these samples stimulated emission was found for magnetic fields between 3.2 and 4.2 T and within an electric field range between 1.7 kV/cm and 3.7. The highest output intensity with this resonator configuration was found at an angle q~ = 75 °. At this angle the emission region extends between 2.4 and 4.4 T. At q~ = 83 ~"the active lasing region is decreased again. No dependence of the emission frequency on the orientation of the electric field was found.

IlL 3. Spectral behaviour and tuning characteristics In Fig. 7 spectra of the sample D 1 ( 4 = 75 °) are shown for a wide range of magnetic fields. For each value of the magnetic field the electric field was adjusted to achieve maximum output intensity. With increasing magnetic field the emission peak is tuned to higher frequencies. The measured emission spectra consist of a single line with a F W H M about 0.5 cm ~. This measured linewidths are due to a folding of the detector cyclotron resonance line with the emission lines of the Ge laser. The laser linewidth is estimated to 0.2 cm -~. The maximum output power of about 100 m W is reached at a frequency of 72 cm -~ by applying a magnetic field of 3.6 T. With this laser stimulated emission is observed in a frequency range between 55 and 80 cm-~.

120

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Tunable lasers and detectors in the FIR

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F o r an identification o f the o b s e r v e d emission lines the frequency o f the emission m a x i m a (solid circles) are c o m p a r e d with theoretical c a l c u l a t e d t r a n s i t i o n energies in Fig. 8. T h e straight lines in this figure represent t r a n s i t i o n s between the light hole L a n d a u levels o f the b-set (mj = I/2-3/2). These t r a n s i t i o n energies are t a k e n from Ref. (31). T h e y are c a l c u l a t e d in the f r a m e w o r k o f an e x t e n d e d P i d g e o n a n d B r o w n m o d e l °°'3~ t a k i n g into a c c o u n t the influence o f the c o n d u c t i o n b a n d a n d the spin split off b a n d as well as the influence o f the t o t a l electric field. T h e L a n d a u levels o f the a-set are n o t expected to s u p p o r t a p o p u l a t i o n i n v e r s i o n f l ~ By the c o m p a r i s o n o f the o b s e r v e d emission frequencies with the c a l c u l a t e d t r a n s i t i o n energies a close coincidence o f the laser t r a n s i t i o n with the n = 1 to n = 0 t r a n s i t i o n a n d the n = 2 to n = 1 t r a n s i t i o n is found. The small difference between the o b s e r v e d t r a n s i t i o n energies a n d the c a l c u l a t e d values m a y be caused by the fact that higher kn states within the L a n d a u levels will be p o p u l a t e d due to the presence o f high electric fields ts) (kB is the m o m e n t u m in the direction o f the m a g n e t i c field). Therefore, the optical t r a n s i t i o n s are n o t t a k i n g place at kB = 0 as a s s u m e d for the c a l c u l a t i o n o f the t r a n s i t i o n energies. The t r a n s i t i o n s n = 0 to n = - l a n d n = 3 to n = 2 o f the b-set are clearly b e l o w the o b s e r v e d t r a n s i t i o n energies. F o r that r e a s o n the transitions are t a k i n g place either between the n = 2 a n d the n = 1 L a n d a u level or between the n = 1 a n d n = 0 L a n d a u level o f the light hole b-set. H o w e v e r , the tuning o f the emission m a x i m a with m a g n e t i c field is parallel to the tuning o f the n = l to n = 0 t r a n s i t i o n which crosses the n = 2 to n = l transition at a m a g n e t i c field o f 3.6 T. Therefore, we suggest t h a t the emission line is caused due to a transition o r i g i n a t i n g at the n = l level a n d ending at the n = 0 level.

III. 4. Application of the CR laser for FIR spectroscopy In a first a t t e m p t we have a p p l i e d the C R - G e laser for t r a n s m i s s i o n m e a s u r e m e n t s on a G e crystal w i t h o u t a p p l i e d external fields. Therefore, a t r a n s m i s s i o n s a m p l e was p r e p a r e d with a length o f 9 m m . T h e a b s o r p t i o n s a m p l e was n o m i n a l l y d o p e d by T1 with an a c c e p t o r c o n c e n t r a t i o n o f NA = 9 × 1013 cm -3. T h e G e laser C6 was used as emission source t u n e a b l e between 64 a n d 84 cm while the t r a n s m i t t e d r a d i a t i o n was m e a s u r e d with a b r o a d b a n d G e detector. In Fig. 9 the d e t e r m i n e d a b s o r p t i o n coefficient is shown. T w o significant p e a k s are observed with a linewidth a b o u t 0.6 c m - 1. These two a b s o r p t i o n p e a k s c o r r e s p o n d to certain i m p u r i t y transitions.

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122

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T h e a b s o r p t i o n m a x i m u m at 71.5 c m - t is c a u s e d by the G t r a n s i t i o n o f the TI a c c e p t o r s p e c t r u m w h i l e the m a x i m u m at 72.8 c m - i is d u e to a n u n i n t e n t i o n a l AI a c c e p t o r . T h i s line c o i n c i d e s w i t h the C t r a n s i t i o n o f the AI a c c e p t o r s p e c t r u m . C o m p a r i n g the a b s o r p t i o n coefficient w i t h d a t a o f J o n e s et al. ~32) we e s t i m a t e a c a r r i e r c o n c e n t r a t i o n o f 2 x 10t4cm -3 f o r t h e T1 a c c e p t o r a n d o f 7 x 10 ~2c m -3 for t h e u n i n t e n t i o n a l AI i m p u r i t y . F o r a s a m p l e f r o m the s a m e G e crystal w i t h a l e n g t h o f 4 c m the s a m e results a r e o b t a i n e d . T h i s d e m o n s t r a t e s t h a t the p - G e c y c l o t r o n r e s o n a n c e laser is a p o w e r f u l t o o l for F I R s p e c t r o s c o p y . Acknowledgements--The authors thank E. E. Hailer for supplying the p-Ge crystal, G. Weinmann and L. Richard for supplying the GaAs layers. The work was supported by the Fonds zu Frrderung der wissenschaftlichen Forschung (P9301TEC), the Jubil~iumsfond der Olsterreichischen Nationalbank and by the "Bundesministerium fiir Wissenschaft und Forschung", Austria. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

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