Carl Asplund

Temperature sensitivity of the threshold current of long-wavelength InGaAs-GaAs VCSELs with large gain-cavity detuning

Record-long emission wavelengths up to 1.3 /spl mu/m have recently been demonstrated from highly strained InGaAs-GaAs double-quantum-well (DQW) vertical-cavity surface-emitting lasers (VCSELs). The operation of InGaAs VCSELs at such long wavelengths has relied on a large detuning between the spectral positions of QW gain maximum and cavity resonance. This detuning also affects the high-temperature performance and temperature sensitivity of such devices. In this paper, we present and evaluate the threshold current-temperature characteristic of such lasers in relation to the gain-cavity detuning at room temperature (RT). For a near-zero gain peak offset from the emission wavelength at RT, the minimum threshold current is found at the temperature where the gain peak wavelength and the cavity resonance are approximately aligned. This is well in line with a common design rule for GaAs-based VCSELs. However, we show that this design rule fails in the case of larger gain-cavity misalignment at RT. Instead, a minimum threshold current is obtained considerably below the temperature of zero gain offset. We propose a conceptual model that relates the gain-cavity detuning at RT to the temperature sensitivity of the active region performance, which qualitatively describes the threshold current-temperature characteristic typical of VCSELs. The results demonstrate the importance of improving the temperature characteristic of the active region in order to reduce the high temperature sensitivity of devices with large detuning.

Origin of photocurrent in lateral quantum dots-in-a-well infrared photodetectors

Interband and intersubband transitions of lateral InAs/In0.15Ga0.85As dots-in-a-well quantum dot infrared photodetectors were studied in order to determine the origin of the photocurrent. The main intersubband transition contributing to the photocurrent (PC) was associated with the quantum dot ground state to the quantum well excited state transition. By a comparison between intersubband PC measurements and the energy level scheme of the structure, as deduced from Fourier transform photoluminescence (FTPL) and FTPL excitation spectroscopies, the main transition contributing to the PC was identified. (c) 2006 American Institute of Physics.

Structural effects of the thermal treatment on a GaInNAs/GaAs superlattice

We have studied structural changes that occur during annealing of GaInNAs/GaAs multiple quantum wells grown by metalorganic vapor-phase epitaxy (MOVPE). Different thermal treatments led to an improved room-temperature photoluminescence (PL) intensity, but also to room-temperature PL peak splitting. This splitting is related to the appearance of compositional clustering as displayed by transmission electron microscopy (TEM). In addition to this, interfacial layers on each side of the wells have also been observed by TEM and their composition is discussed on the basis of high resolution x-ray diffraction studies. It is suggested that the interface layers are indium deficient, but enriched in nitrogen, degrading the optical quantum well performance and indicating a need for improved switching sequences in the MOVPE growth.

Doping-induced losses in AlAs/GaAs distributed Bragg reflectors

We have studied n- and p-type doping-induced performance degradation of AlAs/GaAs distributed Bragg reflectors (DBRs) for applications in vertical cavity lasers (VCLs). Based on high-accuracy optical reflectance and triple-axis x-ray diffraction measurements on a variety of differently doped DBR structures grown by metalorganic vapor-phase epitaxy, a fitting procedure was employed to extract the doping-dependent optical loss. A striking observation is that the reflectance of these DBRs is much more sensitive to n- than p-type doping incorporation. While in the latter case the loss can be well accounted for by intervalence-band and free-carrier absorption, additional loss mechanisms must be considered for n-type DBRs. We relate the losses to doping-enhanced interdiffusion effects resulting in increased interface scattering. These findings should have important consequences for the design of VCLs, demonstrating the importance of reduced n-type doping concentrations and/or growth temperatures, or the applica...

The nature of optical transitions in Ga0.64In0.36As1−xNx/GaAs single quantum wells with low nitrogen content (x≤0.008)

Abstract Ga 0.64 In 0.36 As 1− x N x /GaAs single quantum wells (SQWs) with low nitrogen content have been investigated by both photoluminescence (PL) and photoreflectance (PR) at low and room temperatures. A huge broadening of the PR features has been observed at low temperature and a decrease in this broadening with the temperature increase was detected. This effect and the nature of the optical transitions observed in absorption and emission can be explained using a model which assumes band gap variation due to different nitrogen nearest-neighbour environments (different configurations). In the framework of this model, the large Stokes shift observed for quantum wells (QWs) with smooth interfaces is explained as originating from the potential fluctuations of conduction band edge in the QW layer.

Low-Temperature Metal-Organic Vapor-Phase Epitaxy Growth and Performance of 1.3-µm GaInNAs/GaAs Single Quantum Well Lasers

GaInNAs/GaAs quantum-well (QW) lasers emitting at 1.3 µm were grown using metal-organic vapor-phase epitaxy (MOVPE) in the limit of very low growth rate and temperature. The material was characterized by photoluminescence (PL) Spectroscopy as well as by implementation in broad-area (BA) edge-emitting lasers. While the PL intensity was found to decrease by more than two orders of magnitude between 1175 and 1350 mn, the corresponding BA laser threshold current showed a much more modest increase. For a 1.28-µm laser the threshold current was 1.2 kA/cm2 (1200 µm long devices), with a slope efficiency 0.24 W/A per facet and T0=100 K. Comparison between PL emission properties and BA laser performance revealed a complex relationship. A high PL intensity does not necessarily lead to low threshold-current lasers. In these cases, the FWHM seems to be the more relevant parameter for QW optimization.

Morphological instability of GaInNAs quantum wells on Al-containing layers grown by metalorganic vapor-phase epitaxy

We report on the optical and structural integrity of metalorganic vapor-phase epitaxy-grown GaInNAs/GaAs single quantum wells on AlGaAs/GaAs distributed Bragg reflectors (DBRs). Photoluminescence and surface morphology as measured by atomic force microscopy were investigated for different numbers of DBR periods and different DBR-growth temperatures. An increasing number of DBR periods may severely degrade the quantum well surface morphology and photoluminescence. Surface secondary-ion mass spectroscopy revealed surface segregation of an Al-containing species. Decreasing the DBR-growth temperature lowers the surface concentration of Al and improves the GaInNAs quantum-well morphology.

Bias and temperature dependence of the escape processes in quantum dots-in-a-well infrared photodetectors

The performance of quantum dots-in-a-well infrared photodetectors (DWELL IPs) has been studied by means of interband and intersubband photocurrent measurements as well as dark current measurements. Using interband photocurrent measurements, substantial escape of electrons from lower lying states in the DWELL structure at large biases was revealed. Furthermore, a significant variation in the escape probability from energy states in the DWELL structure with applied bias was observed. These facts can explain the strong temperature and bias dependence of both photocurrent and dark currents in DWELL IPs.

Photoluminescence and photoresponse from InSb/InAs-based quantum dot structures

InSb-based quantum dots grown by metal-organic vapor-phase epitaxy (MOVPE) on InAs substrates are studied for use as the active material in interband photon detectors. Long-wavelength infrared (LWIR) photoluminescence is demonstrated with peak emission at 8.5 µm and photoresponse, interpreted to originate from type-II interband transitions in a p-i-n photodiode, was measured up to 6 µm, both at 80 K. The possibilities and benefits of operation in the LWIR range (8-12 µm) are discussed and the results suggest that InSb-based quantum dot structures can be suitable candidates for photon detection in the LWIR regime.

Quantum dots-in-a-well infrared photodetectors for long wavelength infrared detection

We report on a quantum dots-in-a-well infrared photodetector (DWELL QDIP) grown by metal organic vapor phase epitaxy. The DWELL QDIP consisted of ten stacked InAs/In0.15Ga0.85As/GaAs QD layers embedded between n-doped contact layers. The density of the QDs was about 9 x 1010 cm-2 per QD layer. The energy level structure of the DWELL was revealed by optical measurements of interband transitions, and from a comparison with this energy level scheme the origin of the photocurrent peaks could be identified. The main intersubband transition contributing to the photocurrent was associated with the quantum dot ground state to the quantum well excited state transition. The performance of the DWELL QDIPs was evaluated regarding responsivity and dark current for temperatures between 15 K and 77 K. The photocurrent spectrum was dominated by a LWIR peak, with a peak wavelength at 8.4 μm and a full width at half maximum (FWHM) of 1.1 μm. At an operating temperature of 65 K, the peak responsivity was 30 mA/W at an applied bias of 4 V and the dark current was 1.2×10-5 A/cm2. Wavelength tuning from 8.4 μm to 9.5 μm was demonstrated, by reversing the bias of the detector.