Generation of coherent acoustic phonons in strained GaN thin films
Yue-Kai Huang, Gia-Wei Chern, and Chi-Kuang Suna)
Department of Electrical Engineering and Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei, Taiwan, 10617 Republic of China
Yulia Smorchkova, Stacia Keller, Umesh Mishra, and Steven P. DenBaars Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106
共Received 25 May 2001; accepted for publication 4 September 2001兲
Coherent acoustic phonon oscillations were generated and studied in strained GaN thin films. Inside the bulk GaN film, the longitudinal interference of an ultraviolet femtosecond pump pulse created periodic carrier distribution that screened out the strain-induced piezoelectric field and initiated the coherent longitudinal acoustic phonon oscillations corresponding to the carrier periods. The created coherent phonon oscillation modulated the piezoelectric field thus modified the absorption property of the GaN thin film through Franz–Keldysh effect. This time-dependent absorption modulation was reflected in the transmission variation of the followed probe pulses, resulting a long decay time ⬃300 ps for the initiated coherent phonon oscillations. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1418450兴
Progress in femtosecond lasers and ultrafast spectros-copy technology has enabled us to generate and directly ob-serve the coherent oscillation of phonon modes. Optically excited coherent phonon mode can behave like a classical oscillator and have nonzero time-dependent displacement. The corresponding modulation of the material dielectric con-stant could then be observed by changes in the intensities of transmitted or reflected probe light pulses. In semiconduc-tors, carriers were excited by ultrashort pump pulses in a spatial area much larger than one lattice unit cell and the excited carrier populations 共with wave vector q⬇0兲 are coupled to the corresponding optical phonon modes 共also with q⬇0兲.1,2For acoustic phonon excitations, due to wave vector constraint, only low frequency 共low q兲 acoustic wave was directly excited in bulk materials. Higher frequency co-herent acoustic phonon oscillations were previously ob-served in artificial semiconductor heterostructures such as AlAs/GaAs superlattices,3,4 GaAs/AlxGa1⫺xAs and Ge/Si
multiplayer structures,5,6and PbTe and PbS quantum dots.7,8 Due to the artificial structures, acoustic waves corresponding to the structure periodicity or zone-folded acoustic phonon modes3,4were observed. Recently we demonstrated coherent acoustic phonon oscillation in InGaN/GaN multi-quantum wells共MQWs兲, with phonon oscillation frequency tuned by MQW period width.9Different from most previous studies, piezoelectric field inside the MQW was used to initiate the observed displacive oscillation. Due to the modulation of piezoelectric field properties, strong transmission variation on the order of 1% can be generated and observed. Even though the specific phonon mode was suggested to be se-lected by the coupling between the periodic carrier distribu-tion and the corresponding acoustic phonon mode, there are discussions about the unavoidable contribution of zone-folded acoustic mode in InGaN MQWs.10
One way to generate q⫽0 coherent longitudinal acoustic phonon oscillation without any contributions of zone-folded mode from artificial periodic structures is to generate coher-ent phonon oscillations directly in bulk materials. In this let-ter, we report the generation of coherent longitudinal acous-tic phonon oscillations in strained GaN thin films with longitudinal interferometric techniques. The principle of the reported coherent acoustic phonon oscillation is similar to previous traveling coherent acoustic wave generation using laser induced gratings.11,12Surface acoustic wave was previ-ously generated by lateral interference of two excitation laser pulses with stimulated Brillouin scattering mechanisms. Thus demonstrated acoustic wave had frequency from 30 MHz up to 2 GHz, with a possible acoustic frequency up to 30 GHz proposed with a counter-propagating excitation scheme. In our experiment of coherent acoustic phonon ex-citation, it is the longitudinal self-interference of an ultravio-let femtosecond pump pulse that created periodic carrier dis-tribution in a strained GaN thin film. This periodic carrier distribution screened out the local piezoelectric field, and a displacive coherent longitudinal acoustic phonon oscillation was initiated corresponding to the photoexcited carrier pe-riod width, with a momentum q in the sample growth direc-tion perpendicular to the surface. Combining UV excitadirec-tion, the method of longitudinal interference, and the nature of high sound velocity in GaN c axis, coherent acoustic phonons induced in the demonstrated experiment can have frequency higher than 100 GHz, which can be tuned by the pumping wavelength. With a much thicker total layer thick-ness compared with previous MQW samples, oscillation de-cay time longer than 200 ps can be observed.
The experiments were performed on a 0.60-m-thick strained GaN thin film grown on top of an AlGaN substrate. After annealing the c-plane sapphire substrate at 1050 °C, a 20-nm-thick buffer layer of GaN was deposited using metal-organic chemical vapor deposition共MOCVD兲. The tempera-ture of the MOCVD was then raised to grow a 1.8-m-thick
a兲Author to whom correspondence should be addressed; electronic mail:
sun@cc.ee.ntu.edu.tw
APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 20 12 NOVEMBER 2001
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AlGaN layer. The Al content was around 5%. The uninten-tionally doped GaN layer with a thickness of 0.60 m was then grown on top of the AlGaN substrate layer using mo-lecular beam epitaxy. Room temperature photoluminescence measurements indicate a 3.42 eV共362 nm兲 band gap energy for the GaN layer and a 3.53 eV band gap energy for the AlGaN substrate. A conventional transmission-type pump-probe setup was used in the room-temperature experiment. Femtosecond pulses with a pulse width around 150 fs were generated using a Ti sapphire laser. The output pulse was frequency doubled by a BBO crystal of 0.5-mm-thick to the UV wavelength range. The UV beam, with a pulse width of 250 fs, was then separated into a pump and a probe that would both be focused on the sample with a spot size around 7 m. With a normal incident pump beam, self-longitudinal interference can be generated inside the GaN sample due to high interface reflection. With photoexcitation around the band tail states,13 periodic carrier distribution corresponding to the optical interference pattern can be generated with the resulted transmission change detected by the probe beam at the same wavelength. A series of probe transmission mea-surements with different pump/probe wavelengths, from 370 to 390 nm, were recorded.
When we photoexcited the bulk sample with 390 nm wavelength, coherent acoustic phonon oscillation induced transmission modulation was observed as shown in Fig. 1共a兲. With a pump fluence of 1.9⫻10⫺5J/cm2 and an average
photocarrier density of 8.4⫻1017cm⫺3in GaN, transmission modulation larger than 0.01% can be observed. It is interest-ing to notice its cosinusoidal nature, which indicates a dis-placive oscillation. With 390 nm wavelength and 2.6 index of GaN,14 the pump self-longitudinal interference spacing was 75 nm, which was half of the UV pump wavelength inside sample. For this particular interference pattern, an os-cillation frequency of 106 GHz was observed. Figure 1共b兲 demonstrates the initiated coherent acoustic phonon oscilla-tions by pumping at a wavelength of 370 nm. With a shorter
interference pattern spacing of 71 nm, a higher oscillation frequency of 117 GHz could be observed. By performing the experiment with different pump wavelengths between 370 and 390 nm, a longitudinal acoustic phonon dispersion curve in bulk GaN c axis can be obtained. In Fig. 2 we find a linear relation between the phonon wave vector 共which is the in-version of interference pattern spacing times 2by Fourier series analysis兲 and the observed phonon oscillation fre-quency. Estimating the slope by a linear fitting will give a bulk GaN c-axis LA sound velocity of 8160⫾200 m/s, which is close to the recently reported value of 8020 m/s.15
A comparison experiment in a 5-m-thick bulk GaN thin film without AlGaN substrate13 shows no acoustic phonon oscillation, indicating the importance of the strain-induced piezoelectric field in the initiation of the coherent oscillation. Similar to our previous experimental results,9 the observed coherent acoustic phonons here should also be activated by the carrier-induced piezoelectric field screening in the strained GaN thin film. Thus activated coherent phonon os-cillation with nonzero time-dependent displacement would then modulate the strain-induced piezoelectric field and re-sult in the observed probe absorption oscillation. This phe-nomenon could be described by the frequency domain Franz–Keldysh effect,16 with a characteristic of absorption modulation sign change around the band gap. By differenti-ating traces in Figs. 1共a兲 and 1共b兲 关shown in inset in Fig. 1共a兲 as normalized oscillation periods兴, we found an obvious sign change 共or 180° phase change兲 between these induced oscil-lations, which is attributed to the Franz–Keldysh effect due to different probing wavelengths. The Franz–Keldysh effect states the relation between probe absorption ␣ and probe energy បwhen a uniform electric field F is applied in the material with ␣共ប兲⫽ A0 2
冉
2mr ប2冊
3/2冑
បF关⫺Ai2共gh兲⫹Ai⬘
2共兲兴, 共1兲 where បF⬅(ប2e2F2/2mr)1/3 and ⬅(Eg⫺ប)/បF, mr is the reduced effective mass and Eg is the GaN band gap energy. Ai() represents the Airy function of variableand Ai⬘
() is its derivative. By plotting the normalized absorp-tion modulaabsorp-tion versus pump wavelength with a fixed carrier density of 1⫻1018cm⫺3 in Fig. 3, the zero-crossing wave-length is found to be ⬃370 nm. This zero-crossing wave-length is close to but larger than the expected GaN band gap value of 362 nm, probably due to existing band tail states that lower the value of thus resulting a redshift in zero-FIG. 1. Transmitted probe signal with a pump/probe wavelength of共a兲 390nm and共b兲 370 nm. Also noted is that a 180° phase difference existed in the oscillations of the two traces, as shown in the inset of共a兲 by differentiating the two traces.
FIG. 2. Dispersion curve of coherent longitudinal acoustic phonon shows a
c-axis sound velocity of GaN about 8160⫾200 m/s.
crossing probe energy. By comparing the experimental result of Fig. 3 with a theoretical simulation model based on Eq. 共1兲,16 –19 an order of 10⫺2% of strain modulation in bulk
GaN can be extracted under our experimental condition. According to fitting, the acoustic oscillation decay time in Fig. 1 is⬃300 ps, longer than any coherent phonon oscil-lations reported before. This increase of decay time supports our previous hypothesis that the coherent phonon oscillation decay time in InGaN MQWs was dominated by dephasing between different oscillators instead of acoustic phonon lifetime.9 Dephasing in current acoustic phonon oscillation might have several causes; one was due to excitation line-width. With a 3.6 nm spectral full width at half maximum 共FWHM兲, a thus created self-longitudinal interference pat-tern would have an uncertainty in its spacing width, with 0.39 m⫺1spread in phonon wave vector. This would turn out to be negligible compared with another factor: the finite sample thickness. With a total sample thickness ⬃2.4 m, the 1.56 m⫺1 uncertainty in phonon wave vector corre-sponds to a dephasing time ⬃200 ps,9 close to our experi-mental results.
Our result indicates that the observed short oscillation decay time in previous experiments might be limited by
fi-nite sample width of artificial quantum structures. Bulk ma-terials with larger sample thickness can avoid this disadvan-tage. With the method of longitudinal interference, our demonstration thus provides the direction toward achieving long-lasting high-frequency coherent acoustic phonons.
This work is supported by National Science Council of Taiwan, R.O.C., under Grant No. 90-2112-M-002-051.
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