Shu-Cheng Mai

Room-temperature operation type-II GaSb/GaAs quantum-dot infrared light-emitting diode

A GaSb/GaAs quantum-dot light-emitting diode (QD LED) with a single GaSb QD layer is investigated in this paper. The room-temperature photoluminescence peak blueshift with increasing excitation power densities suggests a type-II alignment of the GaSb/GaAs heterostructures. Significant electroluminescence (EL) is observed for the device under forward biases, which suggests that pronounced dipole transitions occur at the GaSb/GaAs interfaces. With increasing forward biases, the observed EL peak blueshift confirms that the origin of luminescence is from the type-II GaSb/GaAs QD structures. A model is established to explain the operation mechanisms of the type-II QD LED.

Influence of as on the Morphologies and Optical Characteristics of GaSb/GaAs Quantum Dots

The influence of As atoms on the morphologies of GaSb quantum dots (QDs) is investigated. Without any special treatment, GaSb quantum rings (QRs) are observed in the embedded GaSb layer even when the uncapped layer reveals QD like morphologies. With intentional As supply after the uncapped GaSb QD deposition, a QD to QR transition is observed. The phenomenon suggests that insufficient Sb atoms on the GaSb QDs would lead to the QD to QR transition as in the case of embedded GaSb layers. With extended Sb soaking time following GaSb deposition, QD structures could be well maintained for the embedded GaSb layers. A light-emitting diode operated at room temperature is fabricated based on the GaSb/GaAs QD structure. Identical peak positions in photoluminescence and electroluminescence (EL) spectra of the device show that type-II GaSb QDs are responsible for the observed EL.

Voltage-tunable two-color quantum-dot infrared photodetectors

A two-terminal quantum-dot infrared photodetector with stacked five-period InAs/GaAs and InGaAs-capped InAs/GaAs quantum-dot (QD) structures is investigated. The device has exhibited distinct responses at mid-wavelength and long-wavelength infrared regions under positive and negative biases, respectively. The results suggest that the QD confinement states near the anode side are completely filled, such that selective responses at different wavelength ranges would be observed for the stacked structure under different voltage polarities. Also observed are the similar absorption ratios of the device under different incident light polarizations at the two response regions.

InGaAs-Capped InAs–GaAs Quantum-Dot Infrared Photodetectors Operating in the Long-Wavelength Infrared Range

A ten-period InAs-GaAs quantum-dot infrared photodetector (QDIP) with 8-nm In0.15Ga0.85 As capping layer grown after quantum-dot (QD) deposition is investigated. With reduced InAs QD coverage down to 2.0 mono-layers, responses at 10.4 and 8.4 mum are observed for the device under positive and negative biases, respectively. The phenomenon is attributed to the large Stark effect resulted from the asymmetric band diagrams of the device under different voltage polarities. The demonstration of long-wavelength infrared detections with the simple structures of the InGaAs-capped QDIP is advantageous for the development of multicolor QDIP focal-plane arrays.

High-Temperature Operation GaSb/GaAs Quantum-Dot Infrared Photodetectors

A ten-period GaSb/GaAs quantum-dot infrared photodetector (QDIP) is investigated in this letter. A broad detection window 2-5 μm with peak responses at ~ 3.7 μm is observed. Compared with the 4- to 8-μm detection window of a standard InAs/GaAs QDIP, the detection wavelengths of the GaSb/GaAs QDIP are shifted to the 2- to 5- μm range such that water absorption is avoided. The enhanced normal incident absorption of the GaSb QDIP is attributed to its smaller sizes compared with InAs QDs. Without additional high-bandgap barrier layers, the 200 K spectral response of the simple stacked GaSb/GaAs QDIP has already been observed, which has demonstrated the potential for practical applications of the GaSb/GaAs QDIPs.

The influence of In composition on InGaAs-capped InAs/GaAs quantum-dot infrared photodetectors

The influence of an additional InGaAs-capped layer on the performance of InAs/GaAs quantum-dot infrared photodetectors (QDIPs) is investigated. For the device with a 15% InGaAs-capped layer, a significant response at 7.9 μm is observed for the QDIP device. The results suggest that with the additional InGaAs-capped layer, the detection wavelengths of the InAs/GaAs QDIPs could be shifted to a longer-wavelength infrared range. A further increase in the In composition will not help to obtain an even longer-wavelength detection, which is attributed to the cancellation of a lower InGaAs state, and InAs-QD bandgap shrinkage resulted from the relaxed compressive strains of the InGaAs layer with a higher In composition.

Transition mechanism of InAs/GaAs quantum-dot infrared photodetectors with different InAs coverages

In this article, the authors investigate the influences of different InAs coverages on the photoluminescence excitation (PLE) spectra and spectral responses of InAs/GaAs quantum-dot infrared photodetectors (QDIPs). An increase in InAs coverage would lead to an increase in energy separation between heavy-hole state and light-hole state in the wetting layer (WL) region in the QD PLE spectra. The results suggest that most of the strain resulted from the InAs/GaAs lattice mismatch may be accumulated in the WL instead of the QD region. Also observed are the similar energy separations of energy levels responsible for the intraband absorption in the PLE spectra of the QDIPs such that similar detection wavelengths are observed for the devices.

Wavelength-Tunable InGaAs-Capped Quantum-Dot Infrared Photodetectors

Quantum-dot infrared photodetectors (QDIPs) with InGaAs capping layers are investigated. Compared with the standard QDIP with 2.5-mono-layer (ML) InAs QDs, the detection wavelength is shifted from 6 to 7.9 ¿m for an 8-nm InGaAs-capped QDIP. By decreasing the QD coverage from 2.5 to 2.0 ML, an even longer detection wavelength 10.4 ¿m is observed, which is attributed to the higher energy levels of the QD excited states resulted from the smaller QDs. By further increasing the capping layer thickness to 12 nm, longer detection wavelengths with broad response 10-18 ¿m is observed for the InGaAs-capped QDIP.