Joakim Lindahl

Stark effect in individual luminescent centers observed by tunneling luminescence

We have measured photon emission from individual luminescent states in GaInP/InP heterostructures, containing InP dots, using local injection from a scanning tunneling microscope tip. By changing the tip‐sample bias we are able to see the Stark shift of the emission peaks, as well as the onset of impact ionization. We find that the exciton diffusion length is about 1 μm, while the minority carrier diffusion length is much less in our samples. Below the threshold for impact ionization the excitation is extremely local, limiting the excitation to one or a few quantum dots.

Nano-Optical Studies of Individual Nanostructures

Optical techniques play a significant role in studies of nano-structures. The electronic structures of quantum dots, for example, vary with the geometric sizes in an ensemble, resulting in broadened spectral lines. Recently, different forms of local spectroscopic techniques have been applied to investigate such inhomogeneously broadened emission lines. In this paper we report on three methods for local spectroscopy : cathodo-luminescence, luminescence induced by a scanning tunnel microscope and microphotoluminescence. Each of these techniques is shown to have the capacity to investigate single quantum dots, with linewidths in the range 40-1000 μev. Besides demonstrating the possibility of imaging and spectroscopically studing individual dots, we also demonstrate the possibility of investigating single impurity atoms, in imaging as well as in emission spectroscopy modes.

Tunnel-induced photon emission in semiconductors using an STM

The need for higher resolution in the study of semiconductor nanometer structures has led to the development of sharper probes for imaging of the geometrical structures, all the way down to the atomic level as in scanning tunnelling microscopy (STM). Simultaneously there is a pressing need for a possibility to make spectroscopic investigations of the electronic properties of such nm-structures, especially for the excitations of individual nm-features. In this paper we describe results from investigations of spectral analysis of photons emitted as a consequence of tunnel-injection of minority carriers in semiconductor bulk or quantum-well (QW) samples. We denote this technique scanning tunnelling luminescence (STL). Two different types of STL-experiments will be described: One where electrons are injected from states having energies close to the Fermi-level of a metal STM-tip into the conduction-band of p-type InP where they recombine, partly radiatively, as minority carriers. In the other type of experiment we employ a tip of a large band-gap semiconductor, p-type GaP, to which electrons tunnel from an n-type InP/GaInAs/InP QW sample. We report the first observation of such minority carrier injection where the sharply defined carrier (hole) energy distribution of the emitting tip is employed for almost mono-energetic injection and, again for the first time, on the observation at higher bias levels of how electrons are being injected from the InP surface into the tip where they recombine radiatively with the emission spectrum having the characteristic energy distribution of the tip material.

STM-based luminescence spectroscopy on single quantum dots

Abstract The ability to fabricate quantum dots using the Stranski-Krastanow growth technique has improved dramatically during the last number of years. Due to the large number of dots formed (typically 10 8 –10 9 cm −3 ) the emission linewidth is inhomogeneously broadened. We have grown quantum dots of InP in between barriers of GaInP, as well as on top of GaInP, having a low density of dots in order to perform single-dot spectroscopy. These dots have been studied by photoluminescence and scanning tunneling luminescence. We find in photoluminescence that the fully formed dots have well-defined, sharp (0.04–1 meV) emission lines, which are very similar from dot to dot. In scanning tunneling luminescence we find that we can very locally excite only a few partially formed dots which have sharp emission lines (0.1 meV at 77 K). These emission lines display a quantum confined Stark effect when the applied tip to sample bias is varied. We can carefully determine the onset of exciton formation as a function of applied bias. The applied bias which is necessary for impact ionization is found to agree well with simple theory.

Sharp line injection luminescence from InP quantum dots buried in GaInP

InP quantum dots embedded in Ga0.5In0.5P are investigated by injection luminescence. By using a masking technique we have improved the spatial resolution. At 77 K, the luminescence peak of the fully formed InP dots occurs at about 1.62 eV. In addition, in the 1.7–1.8 eV energy range, we observe a rich structure in the spectra with several sharp lines typically 3 meV in width. The origin of this luminescence is attributed to the partially formed InP quantum dots. This injection luminescence band also exhibits spatial variations both in the envelope as well as in the fine structure.

Quiet cryoliquids achieved by diffusion through porous material

A quiet state of cryoliquid is of extreme importance for sensitive optical and scanning probe microscope investigations. A special cryostat was designed and constructed for preventing bubbling of cryoliquid by cooling the liquid diffusing it through the walls of porous material.

Luminescence spectroscopy on individual nanostructures and impurity atoms using STM and SEM

In the exploration of the nano-world of semiconductors there is a strong focus on low-dimensional structures and ultra-small devices. Two fundamental problems, which challenge progress in this field are: (1) large ensembles of nano-objects, like Quantum Dots (QDs), do not have identical geometrical shapes and electronic properties, and, (2) the properties of a low-dimensional structure can be dominated by a few impurity atoms, whereas the properties of a macroscopic structures is determined by the quasi-continuous background of dopant impurities. To allow QDs and discrete impurities to be studied, novel experimental techniques are required. In this paper the authors describe how local luminescence has been excited from single QDs using electrons injected from a Scanning Electron Microscope (SEM), from the tip of a Scanning Tunneling Microscope (STM) or using highly focused photons for excitation. They present images of QDs as well as characteristic spectra of individual QDs. They finally show how the local character of the excitation enable them to excite and image individual impurities in low-dimensional structures, including the measurement of characteristic emission spectra from a single impurity atom in GaAs.

Spectrally resolved luminescence of InP at low temperatures using minority carrier injection from a scanning tunneling microscope tip

The need for higher resolution in the study of materials has led to the development of sharper probes for imaging of the geometrical and surface structures. This development has led to the Scanning Tunnelling Microscope (STM), and variations thereof such as the atomic force microscope. Simultaneously there is a strong need for the possibility of making spectroscopic investigations of nm-structures. In this paper we describe results from spectral analysis of photons emitted as a consequence of tunnel-injection of minority carriers in semiconductor bulk or Quantum-Well (QW) samples. We denote this techtiique Scanning Tunneling Luminescence (STL), which is yet a new type of tunneling microscopy. Two different types of STL-experiments will be described, differing in the type of tip being used. In the first experiment we use a metallic tip to inject electrons into p-type JnP, where the charge carriers recombine, partly radiatve1y. In the second experiment we employ a tip of a large band-gap semiconductor, p-type GaP, from which electrons tunnel into an n-type InP/GaInAs/InP QW sample. We report on the first observation of such minority carrier injection where the sharply defined carrier (hole) energy distribution of the emitting tip is employed for almost monoenergetic injection and on the observation at higher bias levels of how electrons are also being injected from the InP surface into the tip where they recombine radiatively.