Alex Siew-Wan Lee

Quantum well intermixing in InGaAsP laser structures using a low temperature grown InP cap layer

Quantum well intermixing (QWI) in a 1.55 µm InGaAsP laser-like structure has been enhanced using the defects incorporated in an InP capping layer grown at low temperature (below the congruent sublimation temperature) by molecular beam epitaxy and subsequently subjected to rapid thermal annealing. The structures used had quantum wells (QWs) and barrier layers with identical group III compositions so the inter-diffusion occurs only on the group V sub-lattice. This inter-diffusion is induced by the diffusion of P-interstitials that result from the dissociation of PIn anti-site defects that are present in large concentrations in the low temperature InP (LT-InP) layer. The magnitude of the QWI is determined by measuring the blueshift in the wavelength of room temperature photoluminescence emission from the QWs. It was found that the magnitude of the blueshift is dependent on the growth conditions of the LT-InP such that larger blueshifts are observed for LT-InP layers either grown at lower temperatures or with increasing P2 overpressures. These features correlate with the expected changes in the concentration of PIn defects with these changes in growth conditions. Also, there is a change in the rate of change in blueshift with the thickness of the LT-InP layer. For thin layers the rate of change of blueshift with thickness is rapid, but at a certain thickness a transition occurs to a lower rate of change with thickness. This transition thickness is temperature dependent such that the transition to the reduced rate occurs at larger thicknesses at higher anneal temperatures. This transition is interpreted as re-trapping of the P-interstitials in the LT-InP by the In-vacancies resulting from the PIn dissociation which leads to a reduced rate of supply of P-interstitials into the underlying laser structure.

New methods of defect-enhanced quantum well intermixing and demonstrated integrated distributed-feedback laser modulator

New methods of implementing quantum well intermixing (QWI) in InP-based materials using defect-enhanced diffusion are presented and compared to the widely reported technique employing dielectric (usuall SiO2) capping with subsequent rapid thermal anneal (RTA) treatments. The new methods discussed use InP layers grown either at low temperature by gas-source molecular beam epitaxy (GSMBE) or using He-plasma-assisted GSMBE where growth surface is subjected to a continuous low energy He-plasma generated in an electron cyclotron resonance (ECR) source. The two new QWI processes, and the SiO2 capping method, are applied to a 1.55(mu) m InGaAsP multiple quantum well laser structure. For application of the QWI process the laser structure growths are interrupted in a manner and location appropriate to carrying out the QWI process and subsequent grating etch for the fabrication of a distributed feedback (DFB) laser. After implementing the QWI and grating etch, growth on the top cladding and contact layers completes the device structure. Finally, the fabrication of a DFB laser with an integrated electro-absorption (EA) modulator is described and the resultant characteristics given.

Comparison of quantum well intermixing in GaAs structures using a low temperature grown epitaxial layer or a SiO2 cap

Studies of quantum well intermixing (QWI) have been performed on Al-free GaAs based structures in which InGaAs quantum wells (QWs) have either GaAs barriers or InGaAsP quaternary barriers such that the barrier-QW compositional change consists solely of a group III change (GaAs barrier) or a group V change (quaternary barrier). These structures permit identification of the sublattice upon which intermixing occurs when the point defects responsible for the QWI are created by annealing in the presence of a (conventional) dielectric (SiO2) cap layer versus an InGaP cap layer grown at low temperature (LT-InGaP). QWI occurs on the group III sublattice via vacancy diffusion in both the LT-InGaP and SiO2 capped samples with identical group V compositions in the QW and barrier layers. For the samples with identical group III compositions for the QW and barriers, QWI is only observed with the LT-InGaP capping and occurs via group V interstitial diffusion and P–As exchange in the QW.

Thermal stability of 1.55 μm quantum well laser structures grown by gas-source molecular beam epitaxy

Full and partial InGaAsP quantum well, laser structures grown by gas source molecular beam epitaxy, have been subjected to various thermal anneal treatments. Room temperature photoluminescence has been used to measure changes in the quantum well emission wavelength. In all cases, the wavelength decreases (blue-shifts) after the anneal treatment and the details of this blue-shift with anneal time and temperature have been used to establish how specific grown in defects and defects produced through surface dissociation can be used to explain the thermally induced changes.

Evolution of composition modulations in InGaAs/InGaAsP quantum well structures due to quantum well intermixing

The presence of the composition modulation affects the quality of thin films and their optical properties. This paper reports on TEM observations of compositional modulations that take place during rapid thermal annealing (RTA) in structures involving InGaAs QWs with InGaAsP barriers with identical In/Ga ratio. A similar compositional behavior was also observed by high resolution X-ray diffraction and photoluminescence measurements.

Two-section tunable laser using impurity free intermixing in InGaAsP multiple quantum well structures

We report on controlled group V intermixing in a compressively strained In0.76Ga0.24As0.85P0.15/In0.76Ga0.24As0.52P0.48 multi-quantum well laser structure using different encapsulating layers followed by rapid thermal annealing, and the two-section tunable laser made by using this technique. The sample used is a laser structure with emission wavelength at 1.55micrometers . The active region consisting of three In0.76Ga0.24As0.85P0.15 quantum wells with In0.76Ga0.24As0.52P0.48 barriers grown by metal organic chemical vapor deposition. At the same thermal treatment, the blueshift of band gap energy was enhanced most efficiently by capping the sample with an InP layer grown at low temperature and less efficiently by a SiO2 film. While the blueshift was suppressed by a SixNy film with a refractive index of about 2.1. The suppression effect was independent of the SixNy film thickness from 30 nm to 2400 nm. Time of flight secondary ion mass spectra showed that the quantum well intermixing was caused by the interdiffusion of group V atoms between the wells and barriers that have the same group III compositions. A group V interstitial diffusion model was proposed to be responsible for the enhanced intermixing. A 1.55 micrometers two section ridge waveguide laser was fabricated using this technique. The energy transition level of the phase tuning section was tuned to be transparent to the emission wavelength of the active section. A tuning range of about 10 nm can be achieved by simply tuning the bias current for the phase tuning section.

Infrared photodetectors based on Sb materials

Strained layer superlattices based on Sb semiconductor material are proposed for long wavelength photodetector applications. Theoretical studies on the band structure of AlGaSb/GaSb, AlInSb/InSb, and GaInSb/InAs superlattices show that the wavelength coverage can be extended from 0.5 to 30 micrometer. Optical absorptions of the superlattices are calculated taking into account the intermixing effect at different diffusion lengths. Responsivity and detectivity of the GaInSb/InAs superlattice detector are also analyzed. Blue shift of responsivity is observed for increased intermixing and the detectivity D* at 77 K is increased by more than one fold in magnitude as R0A increases linearly with intermixing.