V.L. Alperovich

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.

Photon-enhanced thermionic emission from p-GaAs with nonequilibrium Cs overlayers

Photon-enhanced thermionic emission (PETE), which is promising for increasing the efficiency of solar energy conversion, is studied during cesium deposition on the As- and Ga-rich p-GaAs(001) surfaces and subsequent relaxation in the nonequilibrium Cs overlayer by means of photoemission quantum yield spectroscopy adapted for systems with time-variable parameters. Along with direct photoemission of “hot” electrons excited by light above the vacuum level, the spectra contain PETE contribution of “thermalized” electrons, which are excited below the vacuum level and emit in vacuum due to thermalization up in energy by phonon absorption. Comparing the measured and calculated spectra, the effective electron affinity and escape probabilities of hot and thermalized electrons are obtained as functions of submonolayer Cs coverage. The minima in the affinity and pronounced peaks in the escape probabilities are observed for Cs deposition on both the As- and Ga-rich surfaces. Possible reasons for the low mean values of the electron escape probabilities and for the observed enhancement of the probabilities at certain Cs coverages are discussed, along with the implications for the PETE device realization.

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.

Thermal Smoothing and Roughening of GaAs Surfaces: Experiment and Monte Carlo Simulation

GaAs thermal smoothing at temperatures T ≤ 650°C in the conditions close to equilibrium yields surfaces with atomically smooth terraces separated by steps of monatomic height. At higher temperatures surface smoothing is changed to roughening. Possible reasons of surface roughening at elevated temperatures are studied by means of Monte Carlo simulation and compared with the experimental results on GaAs. It is proved that GaAs roughening at elevated temperatures is caused by kinetic instabilities due to deviations from equilibrium towards growth or sublimation. The microscopic mechanisms of kinetic-driven roughening are discussed.

Photoemission properties of flat and rough GaAs surfaces with cesium and oxygen layers

The evolution of surface band bending and the probabilities of electron emission in a vacuum are studied under cesium and oxygen deposition on atomically flat and rough As-rich and Ga-rich GaAs (001) surfaces by means of photoreflectance spectroscopy and photoemission quantum-yield spectroscopy. On the rough surface, as compared to the flat one, the suppression of non-monotonic Cs-induced band bending variations is observed, along with the overall band bending increase. Multiple repeated activations of the GaAs (001) surface by cesium and oxygen followed by vacuum anneals led to an increase in the root-mean-square roughness from Rq ˜ 0.1-0.2 nm up to ˜ 3 nm. As a result, the probability of electron escape into vacuum decreased by half.