Zbigniew Sobiesierski

Correlation of photoluminescence measurements with the composition and electronic properties of chemically etched CdTe surfaces

We have performed photoluminescence (PL) measurements on chemically etched single‐crystal p‐CdTe. In addition, x‐ray photoemission measurements have been used as a guide to surface stoichiometry for each chemical treatment. The relative intensities of the 0.875±0.005 eV and 1.125±0.005 eV PL bands are seen to be linked to the preferential depletion of either Cd or Te from the CdTe surface. Furthermore, the energies of these deep level transitions measured at T=4 K show remarkable agreement with the two values of Schottky barrier, Φb=0.72±0.02 eV and Φb=0.93±0.02 eV, normally obtained at room temperature for Au and Sb contacts to n‐CdTe.

Aspects of reflectance anisotropy spectroscopy from semiconductor surfaces

There currently exists a wide range of powerful techniques for probing surfaces, mainly involving the use of electron or ion beams under high- or ultra-high-vacuum conditions. Recently there have been major efforts to develop surface sensitive optical probes that have the inherent advantage that they can be applied in more challenging environments such as in high pressures or under liquids and in real time. The most powerful of these techniques to emerge ( years ago) is reflection anisotropy spectroscopy (RAS), which early on demonstrated its ability to distinguish different reconstructions of GaAs(001) and to detect monolayer-growth-related oscillations similar to those routinely obtained using reflection high-energy electron diffraction. This article describes some aspects of the development of the RAS technique since that time, focusing on our own theoretical and experimental studies concerning the (001) surfaces of cubic semiconductors which have been prepared by molecular beam epitaxy. These studies demonstrate that in surface chemistry, structure and electronic properties RAS has made powerful contributions to the study of such surfaces.

IN SITU MONITORING OF THE SURFACE RECONSTRUCTIONS ON INP(001) PREPARED BY MOLECULAR BEAM EPITAXY

Reflection anisotropy spectroscopy (RAS) and reflection high-energy electron diffraction (RHEED) were applied to study clean InP(001) surfaces prepared by molecular beam epitaxy (MBE). At phosphorus beam equivalent pressures (BEPs) between 3.5×10−7 and 3.5×10−6 mbar and substrate temperature (Ts) falling from 590 to 150 °C, (2×4), (2×1), (2×2), and c(4×4) RHEED patterns are observed. The main RAS features, observed at 1.7–1.9 and 2.6–2.9 eV are assigned to In and P dimers, respectively. The above reconstruction sequence is associated closely with transformations identified in RAS signatures that are induced by progressively increasing the P surface coverage. The RAS results also imply the existence of (2×4)α and (2×4)β phases. A surface-phase diagram for MBE-grown (001) InP, in the whole range of Ts and phosphorus BEPs is proposed.

Hydrogen activated radiative states in GaAs/GaAlAs heterostructures and InGaAs/GaAs multiquantum wells

Direct observation of optical emission from H‐related complexes in molecular beam epitaxy grown bulk GaAs and GaAlAs, as well as InGaAs/GaAs strained multiquantum wells (MQWs), is obtained from liquid He photoluminescence experiments. Hydrogenation is achieved by low‐energy ion irradiation from a Kaufman source. The volume incorporation of hydrogen, for equal treatment, is dependent upon the density of impurities and defects where H can bind. For moderate H treatment, in addition to passivation of shallow acceptors, in GaAs we observe novel emission bands, δ, peaking at 1.360, and γ, peaking at 1.455 eV. After heavy hydrogen treatment in GaAs of low radiative efficiency−even p type originally−there appears a deeper structure α at ∼1.20 eV, of the kind known for ‘‘internal’’ transitions in the Ga vacancy‐donor complex. Equivalent bands are found in bulk GaAlAs and also in InGaAs/GaAs MQWs. The results allow an approximate estimate of the various optically active Ga‐vacancy levels, as affected by the different degrees of hydrogenation of the dangling bonds, and a comparison with theoretical values. Moreover, they provide evidence for the creation of a H‐related donor whose binding energy is of order 25 meV. Finally, the γ band is suggestive of a transition between localized conduction and valence states associated with the local distortion that is introduced in the lattice when H binds to impurities, defects, and lattice atoms.

The molecular beam epitaxial growth of GaAs/GaAs(111)B: doping and growth temperature studies

A series of investigations are presented which address various aspects of the growth, by molecular beam epitaxy, of n‐type (Si doped) on‐axis GaAs/GaAs(111)B. In situ characterization by reflection high‐energy electron diffraction has identified four surface phases on the static (zero growth rate) surface, and three reconstructions which occur, depending upon the substrate temperature, during growth. The n‐type doping properties of GaAs/GaAs(111)B epilayers have been compared with n‐GaAs/GaAs(100) structures. Hall effect and low‐temperature photoluminescence measurements have demonstrated that it is possible to dope GaAs/GaAs(111)B with Si in the 6×1014 to 1018 cm−3 range. A variable growth temperature study is also presented which examines the surface structural, electrical, optical, and surface morphological properties of n‐GaAs/GaAs(111)B grown in the 400 to 650 °C temperature range. The onset of electrical conduction, and optically active material, was found to be directly related to changes in the dynamic surface structure. The variable growth temperature study also revealed a temperature regime within which it was possible to significantly improve the surface morphology of on‐axis GaAs/GaAs(111)B structures whilst retaining good electrical and optical properties.

AS/P EXCHANGE ON INP(001) STUDIED BY REFLECTANCE ANISOTROPY SPECTROSCOPY

Reflectance anisotropy spectroscopy (RAS) has been used to investigate the As/P exchange reaction for group V stabilized InP(001) surfaces exposed to As2 and/or P2, under molecular beam epitaxy conditions. By comparing RAS spectra taken before, during, and after As2 exposure it is possible to confirm that the As/P exchange reaction is exactly reversible over a range of temperatures from 420 to 560 °C. Time-resolved RAS measurements of the reaction rate, monitored at an energy of 2.65 eV, indicate that the activation energy for the exchange is 1.23±0.05 eV.

Observation of photoluminescence from InAs surface quantum wells grown on InP(100) by molecular beam epitaxy

Photoluminescence (PL) measurements are presented for thin epitaxial layers of InAs, 2.5 A<d <36 A, grown on InP(100) by molecular beam epitaxy. The combination of efficient carrier capture and PL redshift with increasing InAs thickness clearly indicate the formation of InAs quantum wells on the InP surface. Data are also presented for InAs/InP structures capped with strained layers of either GaAs or In0.5 Al0.5 As. Since radiative recombination within the InAs layers can be distinguished from PL arising from both bulk and surface defects, this system allows us to monitor the quality of both the InAs/InP and InAs/air interfaces via their influence on the InAs quantum well luminescence.

Optical monitoring of the development of InAs quantum dots on GaAs(001) by reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) in combination with reflection high‐energy electron diffraction (RHEED) was used to study in situ the initial steps of molecular beam epitaxial growth of InAs on GaAs(001). Due to the large lattice mismatch InAs is known to grow in Stranski–Krastanov mode leading to the formation of quantum dots after the transition from two‐ to three‐dimensional growth mode. In this article the precise determination of the growth mode transition and the subsequent development of the islands have been of particular interest. During the growth of the two‐dimensional InAs layer, the RHEED‐pattern changed from the c(4×4) of the clean GaAs to a (1×3) surface reconstruction. Accordingly, the RAS‐spectra, taken every 0.2 ML, indicate changes of the As‐dimer configuration. At 1.8 ML (spotty RHEED‐pattern) a saturation of the intensity of the dimer related RAS‐signal around 2.6 eV was found. The relaxation of the InAs layer and the formation of the quantum dots was followed by time‐resolved RAS at 2.6 and 4 eV. It is shown here, that the time constant of this process, the thickness of the InAs wetting layer and the equilibrium morphology of the islands are strongly temperature dependent. The remaining equilibrium InAs wetting layer thickness at the surface was estimated to be about 1 ML (0.8 ML at 625 K and 1.2 ML at 725 K).

Processes of quantum dot formation in the InAs on GaAs(001) system: A reflectance anisotropy spectroscopy study

Reflectance anisotropy spectroscopy (RAS) has been used to monitor the formation and development of InAs islands grown onto GaAs(001) substrates by molecular beam epitaxy. Time resolved measurements show entirely different responses at different photon energies, corresponding to different aspects of the islanding process. At a photon energy of 2.6 eV RAS is sensitive to the onset of islanding, whereas the 4.0 eV signal appears to be sensitive to the continuous interisland wetting layer. Thus, by using the 4.0 eV signal it is possible to follow the real time development of the islanded surface. In particular, it is found that (1) during growth and immediately beyond the point of islanding, a fixed fraction of the incoming flux is directly transferred to the islands, and this fraction increases with increasing growth temperature up to 100%; (2) as a consequence of this, the wetting layer can increase in thickness beyond the 1.6 monolayers islanding thickness, to at least ∼2 monolayers; (3) the importance of islands acting as sinks for the acquisition of material from the wetting layer is revealed; (4) the dynamic equilibrium set up between the islands and the wetting layer is such that the thickness of the latter increases significantly with sample temperature.

The homoepitaxial growth of on-axis GaAs(111)A, (111)B and (201) compared with GaAs(100): doping and growth temperature studies

Si-doping and variable growth temperature studies have been performed in the on-axis homoepitaxial GaAs(111)A, (111)B and (201) materials systems. These studies are compared with the already extensively characterized GaAs(100) orientation. It was demonstrated that the Si-doped n-type GaAs(111)B and (201) could be grown across the mid-1014 to 1018 cm-3 range, whereas the Si-doped GaAs(111)A was p-type and could be doped from ≌ 2×1015 to ≌ 5×1017 cm-3. The growth temperature studies revealed similar conducting/non-conducting transitions in the GaAs(111)A, (111)B and (201) specimens as observed for GaAs(100).