Achim Dörnen

Self-assembled InAs/GaAs quantum dots under resonant excitation

The energy structure and the carrier relaxation in self-assembled InAs/GaAs quantum dots (SADs) is investigated by photoluminescence excitation spectroscopy (PLE) and photoluminescence (PL) at resonant excitation (below the GaAs and the wetting layer bandgap). In PLE measurements we find a clear resonance from the first excited hole state as well as resonances from a relaxation via different phonons. From a comparison of the PL-rise times in time resolved spectroscopy, we conclude on a fast electron relaxation (⩽50 ps) and a slow hole relaxation with a time constant of about 400 ps. Different relaxation paths are observed in the InAs/GaAs quantum dot system and allow us to identify the hole relaxation in the SADs as multiphonon assisted tunneling. The PL-decay time in the SADs after resonant excitation (about 600 ps) is attributed to the lifetime of the quantum dot exciton. In agreement with theoretical predictions, we find a constant lifetime of about 600 ps for temperatures below 50 K and a linear incre...

In incorporation efficiency and composition fluctuations in MOVPE grown GaInN/GaN hetero structures and quantum wells

Abstract GaInN layers play a key role in short wavelength optoelectronic devices for the visible spectrum. However, the epitaxial growth of In containing nitrides is more problematic than that of GaN and AlGaN. In order to increase the In incorporation efficiency, lower growth temperatures of around 700–800°C are needed. We have optimized the metalorganic vapor-phase epitaxial growth of GaInN by decreasing the H2/N2 ratio in the gas-phase and increasing the growth rate. However, the deposited films showed strong indications for compositional fluctuations. Besides a large miscibility gap predicted for GaInN, the mismatch induced strain for GaN may play a major role in these growth problems.

GaInN/GaN-Heterostructures and Quantum Wells Grown by Metalorganic Vapor-Phase Epitaxy

The dependence of the In-incorporation efficiency and the optical properties of MOVPE-grown GaInN/GaN-heterostructures on various growth parameters has been investigated. A significant improvement of the In-incorporation rate could be obtained by increasing the growth rate and reducing the H 2 -partial pressure in the MOVPE reactor. However, GaInN layers with a high In-content typically show an additional low energy photoluminescence peak, whose distance to the band-edge increases with increasing In-content. For GaInN/GaN quantum wells with an In-content of approximately 12%, an increase of the well thickness is accompanied by a significant line broadening and a large increase of the Stokes shift between the emission peak and the band edge determined by photothermal deflection spectroscopy. With a further increase of the thickness of the GaInN layer, a second GaInN-correlated emission peak emerges. To elucidate the nature of these optical transitions, power-dependent as well as time-resolved photoluminescence measurements have been performed and compared to the results of scanning transmission electron microscopy.

MOVPE of GaInN heterostructures and quantum wells

Abstract GaInN layers play a key role in short wavelength optoelectronic devices for the visible spectrum. However, the epitaxial growth of In containing nitrides is more problematic than that of GaN and AlGaN. This requires a detailed understanding of the growth procedure and the material properties. We have grown GaInN heterostructures and quantum wells by low-pressure metalorganic vapour-phase epitaxy. They have been further analysed by X-ray diffraction, optical spectroscopy and atomic force microscopy. Our results indicate a strong difference in GaInN quality depending on the composition and thickness of the grown layers and the growth temperature. Possible reasons for these problems are shortly discussed. Besides thermodynamic limitations predicted for GaInN, the mismatch induced strain to GaN may play a major role for these growth problems.

Tensile strained InGaAs/InP multiple-quantum-well structures studied by magneto-optical spectroscopy

Low-temperature (1.8 K) magneto-optical absorption experiments have been performed on two tensile strained In1−xGaxAs/InP multiple-quantum-well samples. By using derivative spectra and the effective-mass equation of a quasi-two-dimensional exciton, we are able to determine the exciton ground state energy more accurately and to identify higher excited exciton states up to the 5S level. Data of the effective masses for the light hole and the heavy hole are given. Furthermore, the effect of nonparabolicity is discussed.

Optical characterization of MOVPE grown GaInP layers

Abstract Ordered and disordered GaInP layers have been grown by MOVPE. The complex refractive index below and above the band edge has been determined by applying conventional absorption spectroscopy, transmission experiments and ellipsometry. We have observed that ordered and disordered samples only differ near the fundamental band gap with respect to these properties, whereas at higher energies, no differences could be detected.

Effective mass and exciton binding energy in ordered (Al)GaInP quantum wells evaluated by derivative of reflectivity

Low-temperature (1.8 K) magneto-optical reflectivity spectroscopy together with derivative operation has been employed in evaluating the reduced effective mass and exciton binding energy in GaxIn1−xP/AlGaInP quantum wells (QWs). Low uncertainty of the method in identifying excitonic transition energy (⩽0.3 meV) enables a simultaneous determination of both the heavy-hole (hh) and light-hole (lh) excitons in the QWs with ordering. The obtained effective masses manifest to be ordering-dependent, the dependence of which is compared with that reported for the GaInP2 bulk material. Results from the lattice-matched QWs suggest that the magnetic-field enhancement of the oscillator strength is stronger for the hh exciton than for the lh exciton.

Tm Doping of III-V Semiconductors by MOVPE

Abstract We have epitaxially grown various semiconductor hosts with Tm as rare earth element dopant to evaluate the excitation and decay mechanisms of its inner-atomic transitions in semiconductors. Electrical measurements revealed no characteristic influence of the Tm doping, besides a slight decrease of the carrier mobilities. We have detected Tm 3+ related emissions at 1.9 and 1.2 μm wavelenght. Highest intensities have been observed in GaInP:Tm samples. The spectra are similar in all semiconductor hosts under investigation. We have found indication that Tm is not only incorporated in regular lattice sites, but probably in the form of complexes. Further, we have done co-doping experiments with S (for n-type doping) and Zn (p-type doping) in combination with Tm. We have found only a slight influence of the doping partner on the optical properties, but no indication for optical transitions of Tm 2+ .

Photoluminescence and absorption identification of Ti3+ in zinc telluride

Low-temperature photoluminescence (PL) and optical absorption are carried out on a Ti-doped ZnTe bulk crystal with carefully optimized experimental conditions. The influence of excitation energy in the PL is checked. Two zero-phonon lines (ZPLs) are clearly identified and phonon sidebands are evidenced in the PL and in the absorption, respectively. A relation is established between the PL and the absorption for the intensity ratio of the two ZPLs. Thereby, the Ti ion is identified as Ti3+ and the ZPLs in the PL spectrum are from the Γ7 and Γ8 levels of the 2T2 excited state to the well-separated Γ8 level of the 2E ground state. The separation of the two excited levels is 2.4 cm−1. The population numbers are shown to be fully thermalized in the 2T2 excited state in the PL process.

Forbidden transitions and the effective masses of electrons and holes in In1-xGaxAs/InP quantum wells with compressive strain

Two types of forbidden transitions are identified in In1−xGaxAs/InP undoped quantum wells (QWs) with compressive strain by low-temperature (1.8 K) magneto-optical absorption. One of them is due to the interband transitions with different principal quantum numbers and is observable mainly in a low magnetic field and the other corresponds to P- and D-type exciton states and gets stronger as the magnetic field increases. By analyzing the forbidden transitions the in-plane effective masses of electrons (me,ρ*) and heavy holes (mh,ρ*) are simultaneously determined, together with the z-direction (growth-direction) ones (me,z*,mh,z*). The theoretically predicted relation among the electron effective masses [Sugawara et al., Phys. Rev. B 48, 8102 (1993)], me,Γ6<me,ρ*<me,z*, where me,Γ6 is the band-edge electron effective mass of bulk material, is therefore verified. The difference between the values of me,z* and me,ρ* is found to decrease as the strain in the QW drops.