Electroluminescence (EL) efficiency of a bright blue (In,Ga)N quantum-well (QW) diode has been studied in comparison with a high quality GaAs QW diode over a wide temperature range and as a function of current. For the red diode the EL intensity increases in directly proportional to the current at 20 K, indicating a nearly unity external quantum efficiency, although the EL efficiency is influenced by the transport of electrically injected carriers and nonradiative processes at higher temperatures. For the blue diode, however, the room temperature EL efficiency is surprisingly high, although the low-temperature EL efficiency is found to be quite low at high injection and significantly varied with current. These variations of the EL efficiency with current and temperature for the blue diode are attributed to the carrier capture and escape processes influenced under the internal piezo-field effects as a function of forward bias voltage.
The vertical tunneling transport of selectively and resonantly photogenerated carriers in a composite quantum-well system, consisting of dual multiple-quantum-wells (MQWs) with different heterostructures, i.e., GaAs/AlGaAs (MQW1) and strained InGaAs/AlGaAs (MQW2), is experimentally investigated at 15 K as a function of electric field by photocurrent (PC) response and photoluminescence (PL) measurements. In PC spectra of this novel structure, distinct pseudo-negative peaks are observed at the exciton resonance wavelengths of the front MQW1. Origins of the pseudo-negative PC peaks are explained by shadowing effects on the number of photons absorbed in the MQW1 layer and by assuming the dominance of electron transport. In the PC spectra, however, the excitonic peaks of the rear MQW2 shift to the lower energy side with increasing the electric field, while no significant shifts are observed for the negative PC peaks of MQW1. In addition, the applied reverse bias dependence of the PL intensities indicates faster decreases for the MQW2 layer, while the strong PL intensity persists at much higher fields for the MQW1 layer. These results clearly indicate that selective carrier injection into the MQW layers causes a non-uniform distribution of the applied electric field, which is useful to realize the charge-induced optical switching between the two MQW layers.
A novel new type of superlattice (SL) structure which consists of strongly coupled asymmetric double-well (ADW) in one period have been investigated to introduce a new degree of freedom for the device funtionality. The GaAs/A1As ADS-SL contained in a p-i-n diode structure was grown by molecular beam epitaxy, and the electroabsorption properties were measured by low temperature photocurrent spectroscopy. It is found that the introduction of the confinement potential asymmetry with respect to electric field will lead to the selectivity of spatially indirect Stark-ladder transitions associated with two different types of the localized hole states, thus providing a new way of modulating the oscillator strengths. Assignment of the possible optical transitions from the miniband to the Stark-ladder regimes as a function of field strength is elucidated in detail by transfer matrix calculations.
Detailed tunneling current measurements using a tunneling microscope tip have been performed on semi-insulating GaAs surfaces as a function of illumination power, Pt/Ir tip- surface distance, and separation between the tip and In/Ga electrode on the sample surface to elucidate previously unsolved problems of illumination-induced thermal expansion effects on the probe and of the surface depletion effects. We show that the tip-sample distance to detect a constant tunneling current is extended with increasing the otpical excitation power. It is also found that the photo-induced tunneling current as high as 8 nA driven by a 746 nm laser diode is linearly proportional to the optical excitation power. This photo-induced carriers conduction is also confirmed by studying the transient photocurrent responses, which is slowed down by increasing the tip-to-electrode distance. These results reveal that, in our case, the thermal effects are negligible and photogenerated electron tunneling is a dominant mechanism for the increased tunneling current from the samples surface biased at negative voltages relative to the tip.
We have studied the field dependence of the absorption coefficient of three GaAs-AlAs superlattices using a new modulation technique, the wavelength modulated photocurrent spectroscopy. At small applied electric fields we observe transitions corresponding to the edges of the joint miniband density of states between electrons and heavy holes as well as light holes. At intermediate fields Franz-Keldysh oscillations appear at the lower and upper band edges of the heavy and light hole joint miniband. With further increasing electric field these oscillations transform gradually into Wannier-Stark ladder transitions. The experimentally observed features are well reproduced by numerical calculations.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.