A.S. Terekhov

Evolution of electronic properties at the p‐GaAs(Cs,O) surface during negative electron affinity state formation

The evolution of surface band bending and surface photovoltage was monitored in situ by photoreflectance spectroscopy during activation of the surface of epitaxial GaAs to the state of negative electron affinity by successive deposition of cesium and oxygen in a standard ‘‘yo‐yo’’ technique. Considerable variations of the band bending (by approximately 0.3 eV) and surface photovoltage (by three orders of magnitude) were observed. It was found that the maximum of photoemission quantum yield corresponded to unexpectedly small value of the band bending φs=0.3 eV, as compared to widely accepted value of approximately 0.5 eV.

Atomic structure and electronic properties of HCl–isopropanol treated and vacuum annealed GaAs(100) surface

The HCl-isopropanol treated and vacuum annealed GaAs(100) surface was studied by means of AES, XPS, HREELS and LEED. The chemical treatment and sample transfer into UHV were performed under N 2 atmosphere. The HCl-isopropanol treatment removes gallium and arsenic oxides from the surface leaving behind about two monolayers of excess arsenic. The residual carbon contamination was around 0.2-0.4 ML. Bending and stretching C-H modes were observed in HREELS spectra and attributed to the CH 2 and CH, radicals. This hydrocarbon contaminations were removed from the surface together with the excess arsenic by vacuum annealing at 300-420°C. LEED measurements of the chemically treated surface showed spotty (1 × 1) patterns even before annealing. These patterns became sharper after annealing in the temperature range 250-400°C. At higher temperatures LEED patterns changed to (2 X 4)/c(2 × 8), (2 X 6), (3 X 1)/(3 X 6), (4 X 1) and (4 X 2)/c(8 X 2) reconstructions. The XPS results indicated weak regular movement of the Fermi-level within about 150 meV in the 250-600°C annealing temperature range. We observed systematic variations of work function and electron affinity up to 550 meV with the maximum and minimum for the (2 X 4)/c(2 X 8) and (4 × I )/c(8 X 2) structures, respectively. These variations are due to a charge transfer between the top two surface layers (Ga and As) of the crystal.

Step-terraced morphology of GaAs(001) substrates prepared at quasiequilibrium conditions

“Step and terrace” surface morphology with regular arrays of atomically flat terraces separated by monatomic steps was obtained by annealing GaAs(001) substrates in the conditions close to equilibrium. These conditions were realized under molecular hydrogen ambient in a narrow slit (“capillary”) between two adjacent GaAs substrates or in a graphite cassette containing saturated Ga–GaAs melt. The evolution of the step and terrace morphology was characterized by the length of monatomic steps, which was proved to be a more adequate characteristic of flattening process as compared to the root mean square roughness.

Unpinned behavior of the electronic properties of a p-GaAs(Cs,O) surface at room temperature

The evolution of the surface band bending, photovoltage, and recombination rate is experimentally studied in situ by means of photoreflectance and photoluminescence techniques under deposition of cesium and oxygen on the surface of epitaxial p- and n-type GaAs layers at room temperature. The evolution of the band bending is explained in terms of Fermi level pinning by initial and adatom-induced donor-like and acceptor-like surface states. For the surface of p-type GaAs multiple reversible variations of the electronic properties are observed under alternate Cs and O 2 deposition. This unpinned behavior proves that adatom-induced surface states dominate over defect-induced states. Hysteresis in the dependences of photovoltage and photoluminescence intensity on the band bending shows that variations of the concentrations or cross sections of surface capture and recombination centers take place at the first cycles of Cs and O 2 deposition.

Unpinned behavior of the Fermi level and photovoltage on p-(100) GaAs surface facilitated by deposition of cesium

Abstract The preparation technique of (100)GaAs surface with unpinned electronic properties is further developed in this work. It was proved that the unpinned behavior of the Fermi level and surface photovoltage on p-(100)GaAs surface was facilitated by the deposition of cesium. The surface preparation technique included removal of oxides in the solution of HCl in isopropanol and transfer to a UHV set-up in nitrogen atmosphere. The variations of band bending and surface photovoltage were measured by means of photoreflectance spectroscopy after anneals at successively increasing temperatures. Without Cs deposition, the band bending varied only by 50–100 meV as the annealing temperature increased from 300 to 600°C. Deposition of a monolayer of Cs, followed by annealing at 400°C, caused a pronounced decrease of band bending. The evolution of surface photovoltage, which depends both on band bending and on the rates of capture and recombination of the photogenerated carriers on the surface, was more diverse and sensitive to the details of surface treatment. Possible mechanisms of cesium influence on the electronic properties of the surface are discussed.