Optical characterization of a GaAs/GaAlAs asymmetric microcavity structure Der-Yuh Lin Department of Electronic Engineering, National Changhua University of Education, Changhua 500, Taiwan
[email protected]
Abstract: A GaAs/GaAlAs-based asymmetric microcavity structure was studied by various optical characterization techniques. The angle-dependent reflectance (R) spectra showed that the cavity mode (CM) superimposed on quantum well excitonic transitions. The resonance enhancement effect between the excitonic transitions and the CM in the weak-coupling regime was explored using the angle-dependent differential surface photovoltage spectroscopy (DSPS) and photoluminescence (PL), and temperature-dependent PL. In this work, we have also implemented a new modulation technique, namely, the angle modulation reflectance (AMR) to decouple the CM from the overlapped excitonic transitions. The AMR technique has been demonstrated to be an efficient method for the study of weak coupling effect in the microcavity structure. ©2005 Optical Society of America OCIS codes: (300.6380) Modulation, spectroscopy; (250.5230) Photoluminescence; (230.1480) Bragg reflectors; (230.5590) Quantum-well devices.
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#9233 - $15.00 USD
(C) 2005 OSA
Received 24 October 2005; revised 6 December 2005; accepted 7 December 2005
26 December 2005 / Vol. 13, No. 26 / OPTICS EXPRESS 10865
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1. Introduction In recent years microcavity structures have attracted considerable interest because of both their basic and applied properties. Microcavity structure, which consists of quantum wells surrounded by distributed Bragg reflectors (DBRs), plays an important role in many light emitting devices such as various vertical-cavity surface emitting lasers (VCSELs) [1-3], Fabry-Perot modulators [4], microcavity light-emitting diodes (MCLEDs) [5-6] and resonant cavity light-emitting diodes (RCLEDs) [7]. A DBR mirror is a periodical structure made up of two semiconductor or dielectric materials with different refractive indices. The thickness of each layer is chosen so that the light reflected by all the interfaces interferes within a spectral range further referred to as the stop-band. It can be used to improve the brightness, modulation speed and external quantum efficiency of optoelectronic devices. It is also the key structure of a future generation of optoelectronic devices such as polariton lasers [8]. A microcavity structure consists of two DBR mirrors and a cavity. The cavity photons are confined between two mirrors, and interact with the excitonic transitions of a semiconductor quantum well. The one-dimensional confinement of the optical wave in the microcavity is similar to the confinement of excitonic states in a quantum well structure and results in a so-called cavity mode (CM). The interaction between CM and quantum well excitonic transitions can be divided into a strong- and a weak-coupling regime. For strong coupling, the energies of exciton and CM show a Rabi splitting. For weak coupling, the spontaneous emission can be modified by tuning the CM in and out of resonance with the excitonic transitions. The asymmetric microcavity structure consists of a bottom DBR but no top DBR mirror and a cavity between the air and semiconductor mirror. When the cavity is tuned to resonate with the excitonic transitions, the spontaneous emission [9-12] and absorption [13-14] in the microcavity are enhanced. In order to improve the devices performance and develop the new optoelectronic devices, it is necessary to develop new and simple methods to obtain further insight into the coupling effect between the excitonic transitions and the CM. In this study, due to the absence of top DBR mirror the GaAs/GaAlAs asymmetric microcavity structure operates in the weak-coupling regime and no Rabi splitting is observed. The angle-dependent R spectra are used to inspect the reflectivity at different angle of incidence. It is found that both the fundamental conduction to heavy- and light- hole excitonic transitions of quantum well superimposing on the CM plus a rich interference pattern related to the mirror stacks. In order to distinguish the CM from the mixing reflectance (R) spectra, the angle modulation reflectance (AMR) technique is developed. It is performed as a function of angle of incidence by using the similar experimental setup to that of R measurement and implementing the angle modulation with a microstep stepping motor. Because the photon energy of CM is dependent on the angle of incidence while that of the excitonic states are not, only the feature related to the angle-dependent CM is detected by the AMR spectra and those of excitonic transitions are inhibited. By using the AMR technique the CM can be resolved unambiguously. We have also performed the angle-dependent differential surface photovoltage spectroscopy (DSPS) and photoluminescence (PL) measurements for cross check and comparison purposes as well as studying the resonance enhancement effect between the excitonic transitions and the CM in the weak-coupling regime. The DSPS has been demonstrated to be a good nondestructive characterization tool for VCSEL structures [15]. The angle-dependent PL spectra were detected at different angle of incidence with respect to the axis perpendicular to the device surface within a small solid angle. The temperature-dependent PL has also been performed in the temperature range between 15 K and 300 K. The amplitudes of the related features reveal the coupling effect between the CM and excitonic transitions.
#9233 - $15.00 USD
(C) 2005 OSA
Received 24 October 2005; revised 6 December 2005; accepted 7 December 2005
26 December 2005 / Vol. 13, No. 26 / OPTICS EXPRESS 10866
2. Experimental details The sample used in this study was grown by metalorganic chemical vapor deposition on an n+-GaAs (001) substrate. The high reflectivity DBR was built by 30 pairs of Ga0.08Al0.92As/Ga0.88Al0.12As layers. Five undoped 60 Å GaAs wells and 70 Å Ga0.82Al0.18As barriers served as the active region and were sandwiched by Ga0.7Al0.3As/Ga1-xAlxAs (x=0.3 to 0.6) spacer layers to form a single wavelength cavity. The 613 Å Ga0.02Al0.98As layers are placed above and below the spacer layers for selective lateral oxidation to provide optical and electrical confinement. For R measurement a 150 W tungsten-halogen lamp filtered by a model 270 McPherson 0.35 m monochromator was used as the light source. The monochromatic light used as probe light was modulated at 200 Hz by a mechanical chopper. It was then directed onto the sample at different angles of incidence controlled by a rotary mechanical stage. The reflected light was detected by a Si photodetector and the signal was recorded from an NF model 5610B lock-in amplifier. The AMR measurement system utilizes similar experimental setup to the R measurement. The modulation mechanism is implemented by mounting the sample on the spool of the micro-step stepping motor controlled by a computer to perform the small angle periodic vibration at 25 Hz. A five phase step motor and a micro-step controller are used to perform high precision control of small angle vibration at ~3.6x10-3 degree. The small vibration of the reflected light spot on the detector due to the vibrating reflected angle may result in some spurious signals due to the position sensitivity of the Si detector. The effect can be eliminated by fixing the reflected light spot position on the detector by using a focusing lens to collect and focus the reflected light. A reference signal coming from the controller was fed into the lock-in amplifier to record the reflected light signal. In DSPS, the derivative-like surface photovoltage is measured between the sample and a reference grid electrode in a capacitive manner as a function of the photon energy of the probe beam with a wavelength-modulation technique. The light from a 150 W tungsten-halogen lamp was filtered by a 0.25 m monochromator equipped with wavelength-modulation equipments and focused onto the surface of the sample. In this study the wavelength of the probe light is modulated by a vibrating entrance slit operated by a power amplifier, employing a 2 in. loudspeaker as transducer. A beam splitter was placed in the path of the incident light. The intensity of this radiation was monitored by a power meter and was kept constant by a stepping motor connected to a variable neutral density filter, which was also placed in the path of the incident beam. The incident light intensity was maintained at a constant level of 10-4 W/cm2. The illumination intensity and the amplitude of wavelength modulation were experimentally selected at levels not affecting the measured spectra; typically Δλ / λ was on the order of 10-3. Since our measurements were performed over a rather narrow photon energy range, constant intensity is essentially equal to constant photon flux. The PL measurements were excited by a 6328 Å He-Ne laser with a power density of about 10 mW/cm2 and performed with the same equipments as R measurement. An RMC model 22 close-cycle cryogenic refrigerator equipped with a digital thermometer controller was used for low-temperature measurements. The measurements were made in the temperature range of 15 K
