ICTON 2010 Tu.P.10

978-1-4244-779 - /10/$26.00 ©2010 IEEE

Stimulated Emission of Radiation at 2.5 µm Wavelength at Room

Temperature from Optically Excited CdxHg1- xTe Structures

A. A. Andronov 1, Yu. N. Nozdrin

1 and A. V. Okomel’kov

1

N. N. Mikhailov 2, G. Yu. Sidorov

2 and V. S. Varavin

2

1 Institute for Physics of Microstructures, Russian Academy of Sciences, Nizhi Novgorod, 603950 Russia

e-mail: okom@ipm.sci-nnov.ru 2Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences

pr. Academika Lavrent’eva 13, Novosibirsk, 630090 Russia

ABSTRACT

The main requirements for the MCT (CdxHg1-xTe) structures optimization for increase of the stimulated emission wavelength under optical pumping are discussed. The stimulated emission at wavelengths 2 – 2.5 µm at room temperature from optimized MCT structures is observed experimentally. The obtained experimental data are the first observation of stimulated emission from the MCT structures at these wavelengths at room temperature.

1. INTRODUCTION

Investigation of mercury–cadmium–tellurium (MCT) CdxHg1-xTe material and MCT solid-state films and structures historically associated with development of infrared (IR) range photodetectors (see, for example, Refs. [1, 2]). Only few attempts to use this material for IR lasers development have been made. However, substantial advancement into the long wavelength range was not attained, which seems to be associated with insufficiently high quality of the samples and with the use of not optimal samples for the laser emission. There were mainly reports only about low-temperature experiments with these laser structures in literature.

Up to now, we have studied the possibility of obtaining the stimulated emission from the MCT structures rather well (see, for example, [3-5]). However, all these results has been obtained for samples not optimized for obtaining the stimulated emission (thickness of the samples was too large in order to they could be efficiently “pumped” by optical pumping). An essential part of this paper has been devoted to optimization of the MCT samples to effectively obtain the stimulated emission. For aims of the optimization we suggested the two following schemes of the spatial distribution of the composition and, therefore, the band structure.

The first structure is shown in Fig. 1a; this is a structure with rather thin MCT homogeneous layer. The thickness d of the structures should be as small as about 0.6 – 1 µm to secure the penetration of the optical pumping field into the sample. This condition makes it possible to “pump” well the MCT layer by the optical pumping (in thick samples which we have used before, the active medium is apparently formed due to the diffusion of carriers). The use of the thin samples should results in substantially decreasing of pumping intensity threshold for arising of stimulated emission. The presence of the surface passivation layer of a wide gap material can increases the lifetime of nonequilibrium carriers.

Figure 1. Geometry of cross section composition for the MCT samples optimization: (a) variant without covering

layer, and (b) variant with covering layer on the free surface; d is the thickness of MCT layer, and z is the

coordinate in the perpendicular to the film direction (z is counted from the buffer layer).

Since the existence of the surface wide gap layer should increase the lifetime, it is also of interest to study the samples with a sufficiently thick surface “coating” layer which is transparent for the pumping optical wave. A structure of this type is depicted in Fig. 1b. The thickness of the surface layer h is apparently not essential if it is transparent for the pumping wave. The important thing is here to make the width of the intermediate layer between the surface layer (with thickness h) and active MCT layer (with thickness d) as narrow as possible.

ICTON 2010 Tu.P.10

2

2. OPTIMIZED SAMPLES AND EXPERIMENTAL SETUP

When carrying out this study, we fabricated the MCT samples with the composition profile shown in Fig. 2a. We had in our disposal structures after growth, structures after annealing at 260°C for 3 h, and structures after annealing at 290°C for 1 h. We have studied samples with covering CdTe layer (about 0.1 µm, on the right in Fig. 2a) and without it.

Figure 2. Spatial profiles of the structures. The variation in the

composition x versus the film thickness:

(a) MCT 0811085 1 and (b) MCT 081114 1. The coordinate is counted

from a buffer CdTe layer deposited on the

substrate.

We have also investigated the structures which composition is displayed in Fig. 2b (after growth, after annealing at 250°C for 3 h and without wide gap surface CdTe layer). The width of the gap for the active MCT layer (in the central “well”) in these structures corresponds to the emission wavelength by 2.81 µm (it was determined at T = 300 K by the threshold of photoconductivity). Stimulated emission for them at T = 300 K has been observed at wavelengths by 2.5 µm (somewhat lower or higher, depending on the intensity of optical pumping) and only from the structures annealed at 250°C for 3 h. From the structures after growth (without annealing), we observed only spontaneous emission at room temperature. This demonstrates that the annealing process is very important for the development of laser structures.

In our experiments, we have used pulsed optical pumping at wavelengths 400 2500 nm with repetition frequency of the pulses of 10 Hz and duration of the emission pulse of 6–8 ns. The pumping intensity can be varied from 300 W/cm2 up to 10 – 30 kW/cm2. The use of this experimental setup allowed us to make the pulsed pumping of semiconductor structures and to study the values of the absorption coefficient in them.

3. EXPERIMENTAL RESULTS

3.1. Stimulated Emission Spectra

For the structures depicted in Fig. 2a we have observed stimulated emission at pump intensities above the threshold. This stimulated emission has been observed at wavelength by 2 µm with the width of the spectral line by 1.5–2% (see also Fig. 3a). We believe that this spectral broadening is mainly caused by the spatial inhomogeneity of the samples. We have find that the highest intensity of the stimulated emission is observed for the samples annealed at 260°C for 3 hours, which contained a thin surface CdTe layer (similarly to the case displayed in Fig. 2a). Therefore, the presence of the surface layers is important for the laser structures. These surface wide gap layers are known to increase the lifetime of nonequilibrium carriers in the structures. The gap for the active MCT layer in the structures (see Fig 2a) corresponds to the emission wavelength of by 2 µm, which was determined at T = 300 K by the edge of photoconductivity.

The use of a thin MCT samples with a thickness by 0.5 – 1 µm provides good pumping of the structures by the wave of optical pumping. The use of these rather thin MCT structures gave us possibility to receive stimulated emission from optically pumped MCT samples at wavelengths of 2 2.5 µm at room temperature. The spectra of stimulated emission for the described structures MCT 081105 1 (Fig. 3a) and MCT 081114 1(Figs. 3b, 3c) at T = 300 K are displayed in Fig. 3. The spectra for the MCT 081114 1 sample after annealing at 250°C for 3 hours (see Fig. 3b, 3c) illustrate the stimulated emission from these samples at wavelength of 2.5 µm at room temperature.

ICTON 2010 Tu.P.10

3

Figure 3. Stimulated emission spectra under

optical pumping at a wavelength of 650.7 nm at T = 300 K for various MCT samples and

pumping intensities: (a) for MCT 081105_1,

(b) for MCT 081114_1, and (c) for MCT 081114_1; with pumping intensity is smaller

than in the case (b). The case (c) corresponds

to the pumping intensity just above the threshold for the stimulated emission arises.

The observation of such long wavelength stimulated emission from MCT samples at room temperature has not been reported before to our knowledge. Previous our achievement in observation of the stimulated emission in these conditions corresponds to wavelength of 1.7 µm (see [3, 5]). It should be noted that stimulated emission always arises at a wavelength somewhat smaller than the wavelength corresponding to the gap. This distinction we associate with the fact that the probability of the stimulated emission is known to be proportional to the product of the density of states in the band and the population of the energy levels. The population of energy levels has maximum at the bottom of the conduction band, but the density of states is closed to zero there.

It is important to note that the thickness of the active layer in the MCT 081114 1 sample is about 0.2 µm (see Fig. 2b). This thickness of the active layer is thinner than the characteristic absorption depth of the pumping wave, so pumping is used inefficiently in this sense. Study of the MCT 081114 1 structure allows us to conclude that the samples with too thin active layers are also not optimal for obtaining stimulated emission. Because of this reason, the stimulated emission was obtained from the MCT 081114 1 structures for only optimal annealing conditions (3 h at 250°C). To attain the high level of amplification and stimulated emission in them, it is necessary to increase the intensity of optimal pumping in order to increase the general number of photoexcited carriers. This is gives enhancement of the concentration of electrons and their quasi Fermi level in the conduction band, which results in decrease of the stimulated emission wavelength.

It is interesting to compare the spectra b and c in Fig. 3 obtained from the same MCT 081114 1 sample at various intensities of pumping. Spectrum c corresponds to intensity somewhat higher than the threshold value but less than that in the case b. The instability of pumping intensity in our experiments was about 5%. It manifests in the form of noise in spectral dependences. This noise is most intense at pump intensities just near the threshold for the stimulated emission arises (see Fig. 3c). For the case b, the emission wavelength corresponding to the maximum is somewhat smaller than that in the case c, which is caused by a higher concentration of photoexcited electrons and, therefore, by the larger value of the quasi Fermi levels for electrons in the conduction band.

It also should be pointed out that for the MCT 081114 1 sample (see Fig. 2b) after the growth (without annealing of the sample) only spontaneous emission has been observed at room temperature.

ICTON 2010 Tu.P.10

4

3.2. Measurement of the Amplification Coefficient

We also have studied the amplification coefficient in these systems. For measurement of the amplification coefficient we used the procedure of exposure of narrow strip with the variation of its length. The similar idea has been described in Ref. [6]. Carrying out the measurements it is important to remember of the large values of the amplification coefficient in these systems (up to 102 cm–1).

We have formed a strip of light on the surface of the sample from the pumping wave using nontransparent screen. During the exposure, the local temperature in the strip is increases. This results in a certain decrease of gap in the region of the exposed strip. Therefore, this leads to a certain increase in the wavelength of the stimulated emission compared with the wavelength for band-to-band transitions in the unexposed region. Due to this, the regions of the sample surrounding the exposure strip become transparent for the stimulated emission. That is why the stimulated emission can propagate over the sample without absorption. This makes difficulties to study the amplification coefficient in these systems. In the measurements of large values of the amplification coefficient of the medium it is necessary to avoid all reflections from the boundaries. It is need if we want to measure the amplification coefficient of the medium.

Figure 4. Dependence of the stimulated emission intensity versus the length of the lighted strip l for the MCT

081105-1 at T=300 K. At the exposure length of about 1mm, the amplification coefficient value is about 50 cm–1.

We have studied the amplification coefficient in the MCT structures using the following experimental scheme. We used the MCT films with linear sizes by 10 mm. In order to avoid the mirror reflections of the waves from the sample boundaries, the edges of the sample were artificially made rough and with a slope of the film and substrate (about 45°) at the edge. With the help of opaque screen we have formed stripe of light on the surface of the MCT film with the wide of 100 – 300 µm. We have changed the length of the stripe usually from 0.5 to 7 – 10 mm. The direction of the stripe we have chose not perpendicular to the boundaries of the sample to avoid re-reflections. We have measured then the output intensity from the stripe versus the length of the stripe. Figure 4 shows the dependence of the stimulated emission intensity versus the length of the lighted strip l for the MCT 081105-1 sample (with frequency of the stimulated emission about 2 µm, see Fig. 3a). Evaluations show that the maximum value of the amplification coefficient is attained at those lengths of the lighted strip when the stimulated emission arises (in this case, this length is about 1 mm). The evaluation consists simply in finding the exponential factor under the assumption of exponential increase of the stimulated emission versus distance. Evaluations gives us that maximal value of the amplification coefficient is 50 cm–1 in this case.

4. CONCLUSION

The obtained experimental data are the first results on observation of the stimulated emission from MCT structures at wavelengths 2 – 2.5 µm at room temperature. The values of the amplification coefficient in the active medium measured are very large and attain by 50 cm–1 in the case of the emission wavelength 2 µm. This allows us to hope for the receiving the stimulated emission at more long wavelengths from MCT structures at room temperature. It should be pointed out that even not so large increase in the wavelength (up to wavelengths larger than 3 µm) can lead to important applied consequences. The window of the atmospheric transparency is known to be located at wavelengths 3 – 5 µm. So the shift of the operating frequency into this region can allow us to develop active devices of IR range at room temperature, for which the atmosphere is transparent.

REFERENCES

[1] R. Dornhaus and G. Nimtz, Springer Tracts Mod. Phys. 78, 1 (1978). [2] Yu. G. Sidorov, S. A. Dvoretskii, V. S. Varavin et al. Semiconductors 35, 1045 (2001). [3] Yu. N. Nozdrin, A. V. Okomel’kov et al., Pis’ma JETP 80 (1), 29 (2004) [JETP Lett. 80, 23 (2004)]. [4] A. A. Andronov, Yu. N. Nozdrin, A. V. Okomel’kov et al., Proc. SPIE 6636, 66360U (2007). [5] A. A. Andronov, Yu. N. Nozdrin, A. V. Okomel’kov et al., in Proc. ICTON-2008, v. 1, p. 235. [6] L. D. Negro, P. Bettotti, M. Cazzanelli et al., Opt. Commun. 229, 337 (2004).