Mattias Hammar

Adsorption of Sn onSi(111)7 × 7: reconstructions in the monolayer regime

Different monolayer phases of Sn on Si(111)7 × 7 have been studied by means of scanning tunneling microscopy (STM), core-level photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). The STM results show that 3 × 3 reconstructions are obtained for room-temperature deposition of 13 ML of Sn followed by sample annealing in a broad temperature range. A T4 Sn adatom 3 ×3 phase is formed for temperatures between 500 and 800°C, with a concentration of defects that is strongly dependent on the temperature and which is as high as 25% for the lowest temperatures. Above 825°C a second 3×3 adatom reconstruction is formed, a mosaic-like phase with a 1:1 mixture of Si and Sn atoms in T4 positions. The results from investigations of the higher coverage 23 × 23 reconstruction by XPS and RBS support the theory that this phase is a two-layer epitaxial Sn structure with all Si(111) dangling bonds saturated. The Sn coverage for this phase was determined to be between 1 and 1.2 ML.

Photoreflectance investigations of the energy level structure in GaInNAs-based quantum wells

In this paper, we present the application of photoreflectance (PR) spectroscopy to investigate the energy level structure of GaInNAs-based quantum wells (QWs). Series of single GaInNAs/GaAs QWs wit ...

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.

Large-area InP-based crystalline nanomembrane flexible photodetectors

Large-area (3x3 mm(2)) flexible photodetectors were realized, based on crystalline InP semiconductor nanomembranes transferred to flexible polyethylene terephthalate substrates. Very low dark curre ...

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.

Highly strained InGaAs∕GaAs multiple quantum-wells for laser applications in the 1200–1300 nm wavelength regime

We report on metalorganic vapor-phase epitaxy growth optimization and properties of highly strained InGaAs∕GaAs quantum-well (QW) structures with emission wavelength beyond 1200 nm. It is observed that a sufficiently high V/III ratio in combination with low growth temperature is critical for preserved layer integrity when increasing the strain. Multiple QWs with up to five wells are realized without any degradation in photoluminescence intensity or broad-area laser performance at an emission wavelength of 1240 nm with threshold current density below 70A∕cm2 per well.

Suppression of Higher Order Transverse and Oxide Modes in 1.3-

It is shown, by a systematic variation of design parameters, that the use of an inverted surface relief is very effective for suppressing higher order transverse modes in oxide confined 1.3-mum InGaAs vertical-cavity surface-emitting lasers (VCSELs). Single-mode emission is achieved for a large variety of oxide aperture and surface relief diameters, with optimum designs, having a surface relief with a diameter half of that of the oxide aperture, producing 1.1-1.3 mW of single-mode power. It is also shown that the anti-phase layer employed to enable the use of an inverted surface relief is effective for suppressing oxide modes that otherwise appear in oxide confined VCSELs with a large detuning between the gain peak and the cavity resonance

\mu

This article reports on the fabrication and characterization of wafer fused heterojunctions between p-InP and p-GaAs. Secondary ion mass spectroscopy was used to characterize doping profiles across the interface as well as the interface contamination with oxygen or carbon. The crystalline quality of the fused material was characterized using cross section and plan-view transmission electron microscopy. The electrical properties of the fused interface were studied as a function of various doping elements such as Be and Zn in InP or Zn and C in GaAs as well as for different acceptor concentrations in GaAs. Finally, the electrical characteristics were analyzed using a numerical model that includes thermionic emission and tunneling across the heterobarrier.

m InGaAs VCSELs by an Inverted Surface Relief

The properties of 1.2-μm highly strained InGaAs quantum wells (QWs) grown on GaAs substrates have been analyzed. Optical gain spectra versus injection current and temperature, transparency current density, as well as other figures of merit were assessed from measurements on broad-area and ridge-waveguide lasers based on these QWs. Such active regions are of interest for a range of applications, including GaAs-based high-power lasers and vertical-cavity lasers for wavelengths beyond 1.2 μm.

Wafer fused p-InP/p-GaAs heterojunctions

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...