Low Quantum Wells Research Articles

Dual-color charge-sensitive infrared phototransistors with dynamic optical gate

Infrared multispectral photodetectors with high performance show great potential in a broad range of applications. Here, sensitive and controllable dual-color photodetection at 10.6 and 15.7 μm is demonstrated by using a charge sensitive infrared phototransistor (CSIP) with dynamical optical gate. The CSIP device is fabricated in a GaAs/AlGaAs double quantum well (QW) crystal with both upper and lower QWs (7 and 11 nm thick, respectively) being photosensitive via intersubband absorption and, hence, each QW corresponding to one target wavelength (10.6 and 15.7 μm, respectively). Moreover, the upper QW serves as a photosensitive floating gate (FG), while the lower QW functions as the conducting channel of the phototransistor. By periodically lifting-up (lowering-down) the electrostatic potential of FG, the photoresponse at 10.6 (15.7 μm) associated with photoexcitation in upper (lower) QW can be achieved. This electrically controllable photoresponse together with intrinsically high photoconductive gain (∼102) provides a scheme to realize sensitive dual-color photodetection for infrared optoelectronic applications.

Unlocking the origin of compositional fluctuations in InGaN light emitting diodes

The accurate determination of the compositional fluctuations is pivotal in understanding their role in the reduction of efficiency in high indium content $In_{x}Ga_{1-x}N$ light-emitting diodes, the origin of which is still poorly understood. Here we have combined electron energy loss spectroscopy (EELS) imaging at sub-nanometer resolution with multiscale computational models to obtain a statistical distribution of the compositional fluctuations in $In_{x}Ga_{1-x}N$ quantum wells (QWs). Employing a multiscale computational model, we show the tendency of intrinsic compositional fluctuation in $In_{x}Ga_{1-x}N$ QWs at different Indium concentration and in the presence of strain. We have developed a systematic formalism based on the autonomous detection of compositional fluctuation in observed and simulated EELS maps. We have shown a direct comparison between the computationally predicted and experimentally observed compositional fluctuations. We have found that although a random alloy model captures the distribution of compositional fluctuations in relatively low In ($\sim$ 18%) content $In_{x}Ga_{1-x}N$ QWs, there exists a striking deviation from the model in higher In content ($\geq$ 24%) QWs. Our results highlight a distinct behavior in carrier localization driven by compositional fluctuations in the low and high In-content InGaN QWs, which would ultimately affect the performance of LEDs. Furthermore, our robust computational and atomic characterization method can be widely applied to study materials in which nanoscale compositional fluctuations play a significant role on the material performance.

A simulation study on the enhancement of the efficiency of GaN-based blue light-emitting diodes at low current density for micro-LED applications

We numerically investigated the optical/electrical characteristics of blue light-emitting-diodes (LEDs) and developed some optimization strategies. Our aim is to increase the optical performance of LEDs at low current density (0.02–2 A cm−2) for micro-LEDs used in display applications. First, we investigated the effects of quantum well (QW) number in blue In0.225Ga0.775N/In0.10Ga0.90N LEDs. We found that at low current density LEDs with lower QW number have much higher internal quantum efficiency (IQE) than those with higher QW number. Second, we also numerically investigated the effects of the electron blocking layer, n-side quantum barrier (QB) doping concentration, QB material and QW width with single quantum well (SQW) structure. For SQW LEDs at low current density, the electron injection efficiency was nearly one. In terms of the QB doping concentration, we found that an appropriate n-side QB doping can ensure Shockley-Read-Hall recombination appropriately equal Auger recombination, which ultimately results in high IQE. For QB material, we choose In0.05Ga0.95N as QB material to achieve both high IQE and low forward voltage. Regarding the QW width, we found that although LEDs with wide QW had high IQE, there were also two peaks in the electroluminescence (EL) spectrum, thus 3 nm was used as an optimal QW width. The original 5-QW structure showed a lower IQE range from 50 to 80% at the current density range from 0.01 to 2 A cm−2 and a forward voltage of 3.06 V at 1 A cm−2. In contrast, our optimized LED structure achieved an IQE of 92.3% at the current density range from 0.01 to 2 A cm−2 and a forward voltage of 2.72 V at 1 A/cm2.

Semiconductor yellow light-emitting diodes

The development of semiconductor light-emitting diode (LED) in the visible emission range is very unbalance, as the power efficiency of yellow LED is far below other colors. Based on the GaN/Si technology, the authors and his team made a systematic research from the aspect of material growth, chip fabrication, device physics and equipment design, resolved the problems of epi-film cracking, high dislocation density, large strain in quantum well (QW), phase separation in QW, low QW growth temperature, low hole concentration, light absorption by substrate and light blocking by electrode, successfully made a breakthrough in fabricating efficient yellow LED. The yellow LED chip achieves a power efficiency of 26.7% at 20 A/cm<sup>2</sup> with 565 nm wavelength and efficacy of 164 lm/W, and the power efficiency goes up to 42.8% at 1 A/cm<sup>2</sup> with 577 nm wavelength and efficacy of 248 lm/W. New LED light source with multi-colors and without phosphor was developed based on the efficient yellow LEDs, opened up a new direction of pure LED healthy lighting.

The atomic structure of polar and non-polar InGaN quantum wells and the green gap problem

We have used high resolution transmission electron microscopy (HRTEM), aberration-corrected quantitative scanning transmission electron microscopy (Q-STEM), atom probe tomography (APT) and X-ray diffraction (XRD) to study the atomic structure of (0001) polar and (11-20) non-polar InGaN quantum wells (QWs). This paper provides an overview of the results. Polar (0001) InGaN in QWs is a random alloy, with In replacing Ga randomly. The InGaN QWs have atomic height interface steps, resulting in QW width fluctuations. The electrons are localised at the top QW interface by the built-in electric field and the well-width fluctuations, with a localisation energy of typically 20meV. The holes are localised near the bottom QW interface, by indium fluctuations in the random alloy, with a localisation energy of typically 60meV. On the other hand, the non-polar (11-20) InGaN QWs contain nanometre-scale indium-rich clusters which we suggest localise the carriers and produce longer wavelength (lower energy) emission than from random alloy non-polar InGaN QWs of the same average composition. The reason for the indium-rich clusters in non-polar (11-20) InGaN QWs is not yet clear, but may be connected to the lower QW growth temperature for the (11-20) InGaN QWs compared to the (0001) polar InGaN QWs.

Stacking faults and interface roughening in semipolar (202¯1¯) single InGaN quantum wells for long wavelength emission

The microstructure of InGaN single quantum wells (QWs) grown in semipolar (202¯1¯) orientation on GaN substrates was studied by transmission electron microscopy. Stress relaxation in the lattice mismatch InxGa1−xN layer was realized by forming partial misfit dislocations associated with basal plane stacking faults (BPSFs). For given composition x = 0.24, BPSFs formation was observed when the QW thickness exceeded 4 nm. The high density of partial threading dislocations that bound the BPSFs is detrimental to light-emitting device performance. Interface roughening (faceting) was observed for both upper and lower QW interfaces (more pronounced for upper interface) and was found to increase with the thickness of the QW. BPSFs had a tendency to nucleate at roughened interface valleys.

Development of Charge Sensitive Infrared Phototransistors for the Far-Infrared Wavelength

Ultra-highly-sensitive far-infrared detectors are developed for potential application to astronomy. The detectors exploit a novel mechanism called Charge Sensitive Infrared Phototransistors (CSIPs), in which an upper quantum well (QW) in GaAs/AlGaAs double QW structures is positively charged up by photo-excitation via inter-subband transition. This causes the conductance of the lower QW channel to increase. The device is effectively a phototransistor, in which the upper QW serves as a photo-sensitive gate to the source-drain channel provided by the lower QW. Resultant extraordinary high photoconductive gain makes CSIPs so sensitive as to detect single photons. CSIPs are well established in the mid-infrared (\(\lambda \) = 12–20 \(\upmu \)m), achieving noise equivalent power around 1.9 \(\times \) 10\(^{-19}\) W/Hz\(^{1/2}\) with a quantum efficiency of 7 %. CSIPs have been demonstrated to work in longer wavelengths up to 45 \(\upmu \)m, but the sensitivity was not as high as in the shorter wavelengths, probably due to lower quantum efficiency. Reported here is a remarkable improvement in the performance of longer wavelength CSIPs (45 \(\upmu \)m), achieved primarily by optimizing the doping concentration in the upper QW. This work indicates that longer wavelength CSIPs are promising detectors for the astronomical application.

Novel ultra-sensitive detectors in the 10-50 μm wavelength range.

We have developed novel single-photon detectors in the 10–50 μm wavelength region. The detectors are charge-sensitive infrared phototransistors (CSIPs) fabricated in GaAs/AlGaAs double quantum well (QW) structures, in which a photo-generated hole (+e) in the floating gate (upper QW) modulates the conductance of a capacitively-coupled channel located underneath (lower QW). The excellent noise equivalent power (NEP = 8.3 × 10−19 W/Hz1/2) and specific detectivity (D* = 8 × 1014 cm Hz1/2/W) are demonstrated for 15 micron detection up to 23 K, which are by a few orders of magnitude better than those of other state-of-the-art high-sensitivity detectors. The dynamic range exceeds 106 (∼aW to pW) by repeatedly resetting the accumulated holes in the upper QW. Simple device structure makes the detectors feasible for array fabrication: Furthermore, monolithic integration with reading circuits will be possible.

Donor-related cathodoluminescence of p-AlGaN electron blocking layer embedded in ultraviolet laser diode structure

Donor-related cathodoluminescence of p-Al0.11Ga0.89N electron blocking layer (EBL) embedded in a laser structure was investigated by low-energy e-beam scanning along the ridge of a vertical taper both at 82 and 300 K. When e-beam probed the close vicinity of multiquantum well from p-side, an emission at 3.313 eV only appeared at 82 K and represented a prominent increase in intensity with a blueshift up to 23 meV. Besides, its intensity exhibited a linear dependence on quantum well (QW) emission intensity at low QW excitation and a sublinear dependence at high QW excitation. Combined with the annealing behaviors of the emission at elevated temperatures, these results were ascribed to the EBL capture of the holes tunneling from neighboring QW under built-in junction field and the subsequent donor-acceptor pairs transition involving a donor level of 116 meV below conduction band, which was presumably related to nitrogen vacancy (VN).

Selective etching of independent contacts in a double quantum-well structure: Quantum-gate transistor

Double GaAs quantum wells (QWs) embedded between modulation-doped AlGaAs barriers with different aluminium (Al) contents were grown by molecular beam epitaxy. Independent electric contacts to each well were realized by applying different etching techniques without substrate removal. In particular, the lower quantum well was electrically pinched off by a local undercut of the lower AlGaAs barrier exploiting an Al selective etching process. In contrast, the upper quantum well was locally depleted by top etched trenches. Transistor operation of quantum wires defined in such bilayers is demonstrated at room temperature with one GaAs layer used as conducting channel controlled by the other nearby GaAs layer as efficient quantum gate.

Ultrafast optical studies of diffusion barriers between ferromagnetic Ga(Mn)As layers and non-magnetic quantum wells

In recent years, ferromagnetic Ga(Mn)As has emerged as a highly interesting material for semiconductor spintronics. One possible application is to use Ga(Mn)As as an injector layer to inject spin-polarized carriers into a non-magnetic semiconductor heterostructure. As Ga(Mn)As layers are typically grown at much lower substrate temperatures than high-mobility GaAs heterostructures, a combination of both requires that the ferromagnetic layer is grown last. We have prepared samples by molecular beam epitaxy which consist of two quantum wells (QWs) of different widths grown at high substrate temperature. The upper QW is separated by a thin barrier (few nm) from a ferromagnetic Ga(Mn)As layer grown at low substrate temperature, while the lower QW is widely separated (more than 100 nm) from the Ga(Mn)As. We observe that the photoluminescence of the upper QW is red-shifted and partially quenched as compared to a control sample without a Ga(Mn)As layer, and time-resolved Faraday rotation measurements reveal that the spin lifetime in the upper QW is up to 50 times longer than the one in the lower QW. We attribute these observations to Mn back-diffusion into the upper QW during sample growth. Both, the PL and the Faraday rotation technique, are highly sensitive to small quantities (below 0.05%) of Mn and allow us to study the effectiveness of different types (e.g., a short-period superlattice) and thicknesses of barrier layers in suppressing Mn diffusion.

Reset Operation of Quantum-Well Infrared Phototransistors

An improved operation of charge-sensitive infrared (IR) phototransistor (CSIP) is demonstrated by adding a reset function. The phototransistor is fabricated in a GaAs/AlGaAs double quantum-well (QW) structure. The upper QW is lithographically defined to form an isolated island. Under IR illumination (lambda ~ 14 mum), excited electrons escape from the isolated QW island. The QW island is thereby positively charged up, which, in turn, causes the conductance through the lower QW to increase. The performance of the detector, however, was restricted by the reduction of sensitivity, which arises from a distortion in the electrostatic potential profile caused by the accumulation of positive charges on the upper QW island. This drawback is removed by introducing a front gate on a channel leading to the isolated QW island. Applying positive pulses (duration 1 mus) to the gate, neutralizes the isolated QW island and reset the CSIP to the highly sensitive state. Typically, a current responsivity on the order of ~104 A/W is realized along with a dynamical range as large as > 109.

A sensitive double quantum well infrared phototransistor

An infrared phototransistor (∼14.5μm) on a GaAs∕AlGaAs double quantum well (QW) heterostructure is studied. A confined upper QW behaves as a photoactive gate to a conducting channel formed by the lower QW. By properly biasing the narrow gates for isolating the upper QW island, the lateral tunneling rate of cold electrons on upper QW can be tuned and hence the lifetime of photocarriers on the QW island can be controlled. Associated with this controllable lifetime, photoresponse takes a sharp maximum, which reaches as high as ∼103A∕W. Analysis in terms of a simple model suggests that the peak response originates from the interplay∕trade-off between the lifetime of photocarriers and the efficiency of photodetection process. The photodetection efficiency substantially varies as a consequence of large band bending induced by the 300K thermal background radiation. The long (approximately millisecond order) and controllable lifetime in our device paves the way for future development of photon counters in the long wavelength range. In addition, our device has a good compatibility with standard GaAs integrated circuit technology.

Analysis of reflectance and modulation spectroscopic lineshapes in optoelectronic device structures

We discuss the spectral lineshapes of reflectance and modulated reflectance (MR) measurements on optoelectronic device structures such as epi-layers, quantum wells (QWs), vertical-cavity surface emitting-lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs). We consider the various methods for the extraction of built-in electric fields and band-gap energies from Franz-Keldysh oscillations (FKO), using the example of a tensilely strained InGaAs QW system, whose InGaAsP barriers yield strong FKO. We describe how critical point transition energies can be easily obtained by eye from Kramers-Kronig (KK) transforms of low field or QW modulation spectra, using the example of the modulated transmittance spectra of dilute-nitrogen InGaAsN p-i-n structures. We also discuss how the ordinary reflectivity spectrum, usually acquired at the same time as the MR signal, may also be exploited to extract layer thicknesses and compositions, and information about the active QW absorption spectrum in VCSEL and RCLED structures.

Drift mobility in quantum nanostructures by scanning probe microscopy

We used scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) to determine the drift hole mobility in nanometer thick Si∕Si0.75Ge0.25∕Si quantum wells (QWs), from the carrier concentration profiles (obtained by SCM) and the local resistivity values (obtained by SSRM). A relevant decrease of the hole mobility at room temperature was observed when reducing the QW width from 10 nm down to 1 nm. This effect has been explained in terms of the increasing role of surface scattering at the Si∕SiGe interface for lower QW widths.

Quantum well intermixing enhanced by InP grown by He-plasma assisted GaS source MBE

Studies have been carried out on the enhancement of quantum well intermixing (QWI) using the defects present in an epitaxial layer of InP grown by He-plasma assisted gas source molecular beam epitaxy (He*–InP). The structures investigated were single and double InGaAsP quantum wells (QW) with an InGaAsP barrier. QWI was implemented using rapid thermal annealing (RTA) and measured using low-temperature photoluminescence (PL) spectroscopy. Changes in the PL emission wavelength and intensity were investigated as functions of anneal temperature and time, He*–InP layer thickness, He*–InP layer to QW distance and separation of the QWs in the two QW structure. The PL emission from the single QW structure exhibits a blueshift which increases monotonically with increasing anneal temperature and time, and decreases with the distance between the He*–InP layer and the QW. For the double QW samples, initially only the upper QW emits PL but as the anneal conditions increase in temperature and time the PL emission switches to the lower QW. The defect mechanisms responsible for the interdiffusion processes due to the defects present in the He*–InP layer are discussed, and a tentative explanation of the PL blueshift and intensity behaviour is proposed.

Cathodoluminescence characteristics of InGaN/GaN quantum wells grown by MOCVD

Phase separation and In segregation in InGaN/GaN quantum wells (QWs) grown by metalorganic chemical vapor deposition (MOCVD) were studied by cathodoluminescence (CL) imaging and spectroscopy. The spatial homogeneity of optical properties in InGan/GaN QW was studied. It is found that the CL images taken at different wavelengths show very little difference for the In low QW, while some parts are complementary for the In low QW. This result strongly suggests that the additional intensity fluctuation which appears with increasing indium composition is due to phase separation in the InGaN layers. Especially, In 0.16Ga 0.84N quantum wells with In compositional non-uniformity and a small number of structural defects originating at the InGaN/GaN interface showed sharp and intense InGaN emission in CL spectroscopy. Increasing the In composition caused surface roughness in the InGaN layer and the formation of stacking faults/dislocations originating at InGaN/GaN interface. This resulted in a reduction of the InGaN emission peak intensity.