U. W. Pohl

Temporal evolution of GaSb/GaAs quantum dot formation

The formation of GaSb quantum dots in a GaAs matrix in the Stranski–Krastanow growth mode under metalorganic chemical vapor deposition conditions is investigated. Transmission electron microscopical images and photoluminescence measurements show the islands to nucleate during the GaSb deposition and to grow subsequently by mass transfer from the two-dimensional wetting layer. The evolving surface morphology indicates local equilibria between quantum dots and the surrounding wetting layer regions.

Coexistence of planar and three-dimensional quantum dots in CdSe/ZnSe structures

Two well distinguishable classes of nanoscale islands were identified in CdSe/ZnSe quantum dot structures by optical spectroscopy and transmission electron microscopy. For 2.1 to 3.1 monolayer CdSe deposition, coherent three-dimensional (3D) islands, formed in the Stranski–Krastanow (SK) mode, are found with typical diameters of ∼16 nm and a coverage-dependent density of up to 3×1010 cm−2. Simultaneously, small islands with lateral extensions below 10 nm and a density of ∼5×1011 cm−2 are formed by strain-modified island growth. Whereas the 3D SK islands dominate the emission properties at room temperature, the latter smaller islands determine the optical properties at temperatures below 120 K.

Structure and intermixing of GaSb/GaAs quantum dots

We present cross-sectional scanning tunneling microscopy results of GaSb quantum dots in GaAs, grown by metalorganic chemical vapor deposition. The size of the optically active quantum dots with base lengths of 4–8 nm and heights of about 2 nm is considerably smaller than previously published data obtained by other characterization methods. The local stoichiometry, obtained from atomically resolved images, shows a strong intermixing in the partly discontinuous wetting layer with an average GaSb content below 50%, while the GaSb content of the partly intermixed quantum dots is between 60% and 100%.

Temperature-stable operation of a quantum dot semiconductor disk laser

We demonstrate temperature-independent output characteristics of an optically pumped semiconductor disk laser (SDL) based on quantum dots (QDs) grown in the Stranski-Krastanow regime. The gain structure consists of a stack of 7×3 QD layers, each threefold group being located at an optical antinode position. The SDL emits at 1210nm independent of the pump power density. Threshold and differential efficiency do not dependent on heat sink temperature. Continuous-wave operation close to 300mW output power is achieved using the ground-state transition of the InGaAs QDs.

Nanovoids in InGaAs∕GaAs quantum dots observed by cross-sectional scanning tunneling microscopy

We present cross-sectional scanning tunneling microscopy data of a type of InGaAs∕GaAs quantum-dot structure characterized by a hollow center. This void structure develops during a long growth interruption applied after deposition of a quantum dot layer and a thin cap layer, resulting in an eruption of indium-rich material. Subsequent fast overgrowth does not fill the void completely. This growth behavior demonstrates limitations of current strategies to grow large quantum dots.

Low temperature MOVPE growth of ZnSe with ditertiarybutylselenide

Abstract The results on the synthesis of the selenium alkyl ditertiarybutylselenide and its application in atmospheric pressure MOVPE are presented. In combination with dimethylzinc-triethylamine, single crystalline ZnSe layers were grown on GaAs at temperatures lower than 350°C. Good morphology, crystalline and interface quality are demonstrated by optical and electron microscopy, X-ray diffraction and Raman spectroscopy. Photoluminescence at 2 K reveals chlorine as an impurity. The electron mobility of 500 cm 2 /V·s at room temperature supports a fairly low compensation.

Ultrahigh-brightness 850 nm GaAs/AlGaAs photonic crystal laser diodes

One-dimensional photonic crystal lasers emitting in the 850 nm range show high internal quantum efficiencies of 93% and very narrow vertical beam divergence of 7.1° (full width at half maximum). 50 μm broad area lasers with unpassivated facets exhibit a high total output power of nearly 20 W in pulsed mode with a divergence of 9.5°×11.3° leading to a record brightness of 3×108 W cm−2 sr−1, being presently the best value ever reported for a single broad area laser diode. 100 μm broad devices with unpassivated facets show continuous wave operation with an output power of 1.9 W.

Shallow impurity- and defect-related complexes in undoped ZnSe crystals

Abstract The electronic structure of bound excitons involving native-as well as point-defects is investigated by optical spectroscopy using highly crystalline ZnSe samples grown by the Markov method. We demonstrate that the interactions of attendant single particles is resposible for the splitting of the (D 0 , X) and (A 0 , X) states. From these splitting the hole-hole interaction parameter γ and the cubic crystal-field parameter β for the Li bound exciton complex has been determined unambigously for the first time. The I d 1 emission line shows a fine structure which is similar to the I Li 1 exciton line. I d 1 is therefore assigned to an acceptor-bound exciton recombination with an associate formed by a zinc vacancy and an impurity donor combined acting as an acceptor.

Arrangement of nitrogen atoms in GaAsN alloys determined by scanning tunneling microscopy

The pair distribution function of nitrogen atoms in GaAs 0.983N0.017 has been determined by scanning tunneling microscopy. Nitrogen atoms in the first and third planes relative to the cleaved (1 0) surface are imaged. A modest enhancement in the number of nearest-neighbor pairs particularly with [001] orientation is found, although at larger separations the distribution of N pair separations is found to be random. Considerable interest has developed in recent years concerning GaAsN and InGaAsN alloys with low N content, typically a few %. The large predicted band gap bowing in this system of highly mismatched anions leads to the possibility of considerable band gap reduction with modest N content [1,2]. Important applications include lasers with wavelength in the 1.3‐1.55 μm range, as well as solar cells with band gap around 1.0 eV [3]. Generally speaking the GaAsN and InGaAsN alloys have displayed evidence of inhomogeneities, such as broad photoluminescence (PL) line widths, variable PL decay times, and short minority carrier diffusion lengths [4-7]. Such observations are often taken as an indicator of compositional fluctuations in the materials, although direct structural characterization of such fluctuations is lacking. In this work we use cross-sectional scanning tunneling microscopy (STM) to directly probe the arrangement of N atoms in GaAs 0.983 N 0.017 alloys. Nitrogen atoms of two distinct contrast levels are imaged, which we assign to occupation in the first and third surface planes relative to the (1 0) surface. From an accurate determination of the position of about 1000 N atoms in a continuous strip of alloy material, we compute the distribution function of pair separations. The arrangement of N atoms is found to be quite consistent with that expected from random occupation, with the exception that an enhanced occurrence of nearest-neighbor N pairs is found. The GaAsN alloys studied here were grown on GaAs(001) substrates by metal organic vapor phase epitaxy (MOVPE) at temperatures between 530 and 570 C using TMGa, TBAs or AsH3, and tertiarybutylhydrazine (TBHy) under hydrogen carrier gas. Additional details of the growth and characterization of the material can be found in Ref. [8]. The particular film studied here consists of a GaAs buffer layer followed by a GaAs 0.983 N 0.017 layer, a 52 nm thick GaAs spacer layer, a GaAs 0.972 N 0.028 layer, and a 370 nm thick GaAs cap layer. The thickness of the GaAsN layers was determined by high-resolution x-ray diffraction (HRXRD) to be about 18 nm; STM measurements of their thickness gave results of 14‐19 nm depending on location in the wafer. The N contents quoted above were also determined by HRXRD; STM measurements for those quantities gave similar results. The GaAs substrate, buffer layer, and cap layer were doped with Si at a con1

Room-temperature lasing of strain-compensated CdSe/ZnSSe quantum island laser structures

Efficient resonant excitonic waveguiding is achieved in laser structures, grown by metallorganic chemical vapor deposition, with stacked CdSe quantum islands which were separated by ternary ZnSSe barriers. Plastic relaxation within the stack is shown to be suppressed by adjusting the sulfur content in the barriers to compensate the strain. Excitonic lasing with low threshold intensities is demonstrated well above room temperature with Ith77 K=0.8 kW/cm2 and Ith300 K=55 kW/cm2.