Electrically manipulating the optical sensitivity ...

Electrically manipulating the optical sensitivity function in quantum wells for nanoacoustic wave detection Pei-Hsun Wang, Yu-Chieh Wen, Shi-Hao Guol, Chih-Ming Lai, Hung-Cheng Lin et al. Citation: Appl. Phys. Lett. 95, 143108 (2009); doi: 10.1063/1.3243988 View online: http://dx.doi.org/10.1063/1.3243988 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v95/i14 Published by the American Institute of Physics.

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APPLIED PHYSICS LETTERS 95, 143108 共2009兲

Electrically manipulating the optical sensitivity function in quantum wells for nanoacoustic wave detection Pei-Hsun Wang,1 Yu-Chieh Wen,1 Shi-Hao Guol,1 Chih-Ming Lai,2 Hung-Cheng Lin,3 Peng-Ren Chen,3 Jin-Wei Shi,3 Jen-Inn Chyi,3 and Chi-Kuang Sun1,4,a兲 1

Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan 2 Department of Electronic Engineering, Ming Chuan University, Taoyuan 333, Taiwan 3 Department of Electrical Engineering, National Central University, Taoyuan 32001, Taiwan 4 Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan

共Received 1 July 2009; accepted 15 September 2009; published online 6 October 2009兲 We demonstrate electrical control of the optical sensitivity function in multiple quantum wells 共MQWs兲 for nanoacoustic wave detection. This is realized by bias controlling the quantized level and the quasi-Fermi level of carrier-populated InGaN/GaN MQWs. Experimentally, a strongly bias-dependent optical sensitivity was observed when the optical probe transition was near the quasi-Fermi level, which agrees well with the theoretical prediction. © 2009 American Institute of Physics. 关doi:10.1063/1.3243988兴 Coherent acoustic phonons 共CAPs兲 are generated at terahertz frequencies when semiconductor quantum wells are illuminated by femtosecond laser pulses.1–4 These terahertz CAPs—also known as nanoacoustic waves 共NAWs兲— typically have wavelengths of tens of nanometers, which prove useful in nanoultrasonic imaging applications.5–7 For nanoultrasonic engineering, a previous study7 spatially manipulated the phonon generation process so that NAWs were emitted with lateral dimensions much smaller than the laser wavelength restrictions. Besides controlling the phonon generation process, another key for nanoultrasonic engineering is to control the optical sensitivity function for NAW detection, which can serve as a switch mechanism in the opticalbased terahertz piezoelectric transducers. Previously the optical sensitivity functions with varying photon energy and in different materials had been studied, while most reports discussed only the interaction between light and CAPs.8,9 There is only one bias-dependent report showing that the observed CAP oscillation amplitude was independent of the bias voltage.10 The sensitivity function modification for NAW detection under external bias is still lack of exploration, while electronic control of the sensitivity could be the key for future nanoultrasonic applications. In this paper, we experimentally demonstrate the electrical control of the optical sensitivity function for detecting NAWs in piezoelectric multiple quantum wells 共MQWs兲 through altering quasi-Fermi levels. With electrical manipulation, the optically detected NAW oscillations can be found to decrease significantly in magnitude and the optical sensitivity can be even turned off. At the same time, we have confirmed that the photogenerated NAW amplitude is almost independent of the bias voltages. Our experiment provides the solution to electrically manipulate the sensitivity function for NAW detection and for nanoultrasonic engineering. Due to its piezoelectric property, we have previously demonstrated that the GaN material system is an ideal candidate for photogeneration and photodetection of NAWs.3,4,7 a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0003-6951/2009/95共14兲/143108/3/$25.00

It is also well-known that by applying reverse bias on InGaN/GaN MQW light emitting diodes 共LEDs兲, significant quantized energy shift can be observed.11,12 This effect results from the piezoelectric field compensation, thus shifting the quantized energy and the quasi-Fermi levels. To take advantage of this energy shift to modify the optical sensitivity function for NAW detection, we considered the “carrier distribution modulation” 共CDM兲 effect, which was proved to be one of the dominant mechanisms for NAW detection.13,14 When NAWs with a wavelength equal to the period of the piezoelectric MQW travel through the carrier-populated quantum wells, the longitudinal strain waves will induce quantized energy modulation through piezoelectric and deformation forces14 and thus modify the optical absorption. When the optical probe energy is positioned around the quasi-Fermi surface of populated carriers, a fixed-amplitudestrain-wave induced optical transmission T modulation ⌬T / T, which defines the optical transmission sensitivity for NAW detection, at a specific probe photon energy Epr was found to be affected by the CDM and was proportional to df e共E兲 / dE 兩E=Epr as discussed in a previous paper,13

冉冊 ⌬T T

⬀ − ␦Efn CDM

冏冏 df e共E兲 dE

− ␦Efp E=Epr

冏冏 df h共E兲 dE

, E=Epr

共1兲 where ␦Efn and ␦Efp are the quasi-Fermi level modulation induced by a fixed-amplitude strain-wave for electrons and holes respectively. f e共E兲 and f h共E兲 are the Fermi–Dirac distributions for electrons and holes. When the probe energy position is near the electron quasi-Fermi level, the optical sensitivity for detecting stain waves will then be easily influenced through the bias-controlled quasi-Fermi level shift. We adopted an In0.25GaN/ GaN MQW LED as our sample. The sample sequentially consisted of the following layers on a patterned sapphire substrate along the crystal c-axis: an undoped 4 ␮m GaN buffer layer, a 2.5 ␮m n-GaN layer, five periods of In0.12GaN/ GaN 共3/12 nm兲 strain release layers, ten periods of In0.25GaN/ GaN 共3/12 nm兲 MQWs, a 10 nm p-AlGaN layer, a 0.125 ␮m p-GaN layer, and a GaN/ In0.07GaN/ GaN 共3.3/3.3/3.3 nm兲 top layer to re-

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FIG. 1. The calculated energy differential of the electron Fermi–Dirac distribution df e共E兲 / dE 兩E=Epr as a function of applied bias at different probe energy positions of 2.76 eV 共solid line兲, 2.73 eV 共dashed line兲, and 2.70 eV 共dotted line兲.

FIG. 2. Backward Brillouin oscillation amplitude under different bias voltages. The probe energy was set at 2.76 eV 共solid squares兲, 2.73 eV 共open circles兲, and 2.70 eV 共open triangles兲. The inset shows the temporally differentiated reflectivity signals at the pump/probe energy of 2.76 eV with no applied bias.

duce p-type Ohmic contact and to improve the efficiency of hole generation.15 To observe the bias dependency of the quantized energy levels, a photoluminescence 共PL兲 measurement was performed under reverse bias. The room temperature PL was with a 2.61 eV 共475 nm兲 peak when no external voltage was applied and the PL peak energy blueshifted 60 meV when the applied voltage was ⫺25 V. In In0.25GaN quantum wells, since the effective mass of heavy holes is 9.7 times larger than that of electrons,16 the quasi-Fermi energy of holes is comparatively insensitive to the applied bias voltages. To bias shift the quasi-Fermi level with energy greater than that of the room-temperature thermal distribution 共kT = 26 meV兲 in our device, we have to count on electrons. Assuming a photoinduced twodimensional 共2D兲 carrier density 5 ⫻ 1012 cm−2 in the quantum wells, the position of quasi-Fermi level is calculated based on the 6 ⫻ 6 Hamiltonian for wurtzite structures using self-consistent potentials.17 To simplify our simulation, optical probe position was determined by the C1 共electronic state 1兲 to HH1 共heavy hole state 1兲 subband transition. The calculated probe energy corresponding to the electron quasiFermi surface shifts from 2.78 to 2.82 eV when applying 0 to ⫺25 V bias. Figure 1 shows the calculated energy differential of the electron Fermi–Dirac distribution df e共E兲 / dE 兩E=Epr at the corresponded probe position as a function of applied bias for probe energies of 2.76 eV 共solid line兲, 2.73 eV 共dashed line兲, and 2.70 eV 共dotted line兲. According to Eq. 共1兲, we thus expect that the strain-wave induced optical transmission modulation will change with bias significantly through the CDM effect if the probe energy is positioned close to the electron quasi-Fermi surface. The experiment setup is a femtosecond pump-probe system. A frequency-tunable Ti:sapphire laser provided the femtosecond optical pulses, and then the pulses were frequency doubled by a 0.7 mm thick ␤-barium borate crystal. The pump/probe energy was set at 2.76, 2.73, and 2.70 eV with a fixed 30 mW incident pump power and a measured focal diameter of 19 ␮m, resulting in a corresponding optical fluence of 1.09⫻ 10−4 J / cm2. The spectral full width at half maximum of the pulses was measured to be 4 nm. The pump-induced 2D carrier density at the specific pump/probe energy was measured to be 4.97⫻ 1012 cm−2 共2.76 eV兲, 4.84⫻ 1012 cm−2 共2.73 eV兲, and 4.78⫻ 1012 cm−2 共2.70 eV兲 without observable bias dependency. To calibrate the relative photogenerated NAW amplitude under different bias voltages, we performed reflectivity pump-probe measurement to observe the oscillation amplitude of the backward Brillouin

scatterings,18,19 which reflects the strain magnitude of propagating NAWs. The inset of Fig. 2 shows an example transient reflectivity trace taken at 2.76 eV pump/probe energy when no bias was applied. The measured oscillation frequency at different probe energy positions of 2.76, 2.73, and 2.70 eV were 88, 87, and 86 GHz, respectively, indicating the nature of the backward Brillouin scatterings. The shape of the reflectivity oscillation envelope was a wave packet with a peak and an abrupt phase change around 31 ps, indicating that the observed strain-wave was from the MQW and traveling toward the sample surface. To avoid the possible detection sensitivity contribution of the MQW to the analyzed NAW amplitudes, we Fourier transformed the measured reflection traces after 83 ps time delay, when the NAWs propagated completely in the n-GaN layer. Figure 2 shows the analyzed backward Brillouin oscillation amplitude as a function of bias for three experimental wavelengths. We found that the observed backward Brillouin oscillation amplitude, which was directly dependent on the NAW amplitude and had no relation with the sensitivity for NAW detection in the MQWs to be discussed below, was almost independent of the bias voltages. To study the MQW sensitivity function for NAW detection, transient transmission measurements were performed at the same condition as the reflectivity measurements. The inset of Fig. 3共a兲 shows a typical transient transmission trace measured at the pump/probe energy of 2.76 eV when no bias was applied. Similar to our previous study, a transmission modulation oscillated at 0.54 THz and decayed in 20 ps can be observed, corresponding to the 15 nm MQW period assuming an 8085 m/s sound velocity. This indicates that the 0.54 THz NAW was generated in the In0.25GaN MQW and affected their optical transmission through strains, which propagated out of the MQW region in 20 ps. By Fourier analyzing the probe detected transmission oscillation signals at 0.54 THz between the 2–20 ps time delay, the amplitude of the NAW induced optical transmission modulation ⌬T / T as a function of applied bias for three different probe energies of 2.76, 2.73, and 2.70 eV can then be determined, as shown in Fig. 3共a兲. To calibrate the pump wavelength and bias-dependent NAW amplitude, we further divided the results in Fig. 3共a兲 by the bias-dependent NAW amplitude reflected by the backward Brillouin oscillation amplitude shown in Fig. 2. According to the calculation in Ref. 20, the sensitivity of the dielectric function to strain could depend on the probe photon energy when near the GaN energy gap. For three different probe energies of 2.76, 2.73, and 2.70 eV,


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cillations under the same experimental condition 共Fig. 2兲, our study indicated that the MQW transmission sensitivity for NAW detection can be even turned off due to the completely filled electron population at the corresponding probing position. In summary, we demonstrate electrical control of the optical transmission sensitivity function in piezoelectric MQWs for terahertz NAW detection. This is realized by biascontrolling the quantized level and the quasi-Fermi level of carrier-populated InGaN/GaN MQWs, while the dominant physical mechanism is the so called CDM. With further improvement on the bias-controlled energy shift, our proposed mechanism can provide an efficient electrical control to manipulate the NAW detection sensitivity or even to switch off the sensitivity for nanoultrasonic engineering. This project is sponsored by the National Science Council of Taiwan under Grant No. NSC97-2120-M-002-010. 1

FIG. 3. 共a兲. Detected transmission oscillation amplitude ⌬T / T as a function of applied bias for three different probe energies of 2.76 eV 共solid squares兲, 2.73 eV 共open circles兲, and 2.70 eV 共open triangles兲 with a fixed optical pump fluence. The inset shows the measured transient transmission trace after temporal differentiation at 2.76 eV probe energy with no bias voltage. 共b兲 Calibrated transmission sensitivity in MQW for 0.54 THz NAW detection.

away from the GaN energy gap, we assumed that the straininduced refractive change is nearly the same for a fixed amplitude strain-wave. The calibrated transmission oscillation amplitude, as shown in Fig. 3共b兲, thus represents the probeenergy- and bias-dependent transmission sensitivity in MQW for detecting 0.54 THz NAWs. The observed probe-energyand bias-dependency trends agree well with the calculated energy differential of the electron Fermi–Dirac distribution as shown in Fig. 1. Our results shown in Fig. 3 not only indicate that we can manipulate the optical sensitivity for NAW detection in piezoelectric MQW through electrical bias, but also support the CDM effect as the dominating switching mechanism. For the applied probe energies, the electron quasi-Fermi surface shifted away from the probe position toward higher energy when applying reverse bias, thus reducing the optical sensitivity for NAW detection. Combined with the observed strong backward Brillouin os-

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