Toshiya Saitoh

Control of Fermi level pinning and recombination processes at GaAs surfaces by chemical and photochemical treatments

The effects of photochemical oxidation in water, Na2 S deposition and exposure to NH3 and HCl gas ambients on the Fermi level pinning, and the surface recombination process at GaAs surfaces were studied by measuring the band edge photoluminescence (PL) intensity, x‐ray photoelectron spectroscopy spectra, and surface current transport(SCT). Computer simulation of the surface recombination process was also made on the basis of the disorder‐induced gap state model. Marked increase of PL intensity was observed after both photochemical oxidation and deposition of Na2 S as previously reported. However, SCT measurements on n‐type materials detected increase of surface band bending in the dark. Exposure to NH3 resulted in a slight reduction in the band bending with little change in the PL intensity. Exposure to HCl, on the other hand, resulted in marked reduction of the band bending with marked increase of the PL intensity. The computer simulation shows that the contradictory behavior of the PL intensity and band...

In-Situ Characterization of Compound Semiconductor Surfaces by Novel Photoluminescence Surface State Spectroscopy

The recently proposed novel photoluminescence surface state spectroscopy (PLS3) technique is applied for in-situ, non-destructive and contactless characterization of variously processed surfaces of GaAs, InP and InGaAs. Chemically etched, anodized and passivated surfaces, as well as the original as-received surface, give rise to U-shaped surface state density distributions with characteristic charge neutrality energy levels, EHO, which is consistent with the disorder induced gap state (DIGS) model. Annealing of as-received surfaces in hydrogen ambient leads to formation of discrete levels, possibly due to escape of As or P atoms. The effectiveness of a new UHV-based passivation scheme for InGaAs using an ultrathin MBE Si interface control layer (ICL) is also confirmed.

In Situ Surface State Spectroscopy by Photoluminescence and Surface Current Transport for Compound Semiconductors

By a rigorous computer analysis of the surface recombination process, it is shown that the surface state distributions on semiconductor free surfaces can be determined from a measurement of the dependence of band-edge photoluminescence intensity on the excitation intensity. The measurement of the Fermi level pinning position in the dark by the surface current transport measurement avoids the possible ambiguity of the interpretation. The new technique is successfully applied to variously treated GaAs surfaces and to passivated InGaAs surfaces with and without the ultrathin Si interface control layer.

Correlation between Photoluminescence and Surface-State Density on GaAs Surfaces Subjected to Various Surface Treatments

Relationship between band edge photoluminescence (PL) and surface states is rigorously analyzed on computer, using the disorder-induced gap state (DIGS) model, and is compared with experiments on GaAs surfaces subjected to various treatments. It is shown that PL intensity is directly correlated with the effective surface recombination velocity, but is not necessarily directly correlated with the surface-state density (Nss) itself. PL enhancement by photochemical oxidation and by Na2S deposition is proposed to be due to a fixed negative charge, whereas photochemical treatment in HCl leads to significant reduction in Nss.

Regular Array Formation of Self-Assembled InAs Dots Grown on Patterned (111)B GaAs Substrate by MBE

We report, for the first time, self-assembled molecular beam epitaxial (MBE) growth of InAs dots on patterned (111)B GaAs substrates by Stranski-Krastanow growth mode. (111)B GaAs substrates were patterned by using conventional optical lithography process and wet-chemical etching. Self-assembled InAs dot growth was performed on both tetrahedral etch-pit pattern and trapezoidal grooves. InAs dots were found to be selectively grown on the bottom of the tetrahedral etch-pit with appropriate growth conditions. As for the growth on trapezoidal grooves, InAs dots were found to be periodically arrayed on the middle of one side of the slope of grooves patterned along [2*BAR*1*BAR**BAR*1*BAR*] direction and on the top and the bottom surface of grooves patterned along [0*BAR*1*BAR*1] direction. The observed periodic distributions of InAs dots on the patterned substrates are presumably caused by the periodic lattice strain distributions on the patterned surface at the initial stage of highly strained layer growth.

A computer simulation of the recombination process at compound semiconductor surfaces and hetero-interfaces

Abstract The complex recombination process through quantum states at compound semiconductor surfaces and hetero-interfaces is analyzed in a unified manner on the computer, using the unified disorder induced gap state (DIGS) model. Recombination through uniformly distributed states at surfaces and hetero-interfaces, and that through U-shaped surface states at GaAs surfaces subjected to various surface treatments, are specifically analyzed. The result indicates that the effective surface recombination velocity is not constant, but is strongly dependent on the excitation intensity and the location of charge neutrality level, E HO . PL intensity enhancement after photochemical oxidation in water and sulfur treatments (Na 2 S, (NH 4 ) 2 S) is shown to be not due to reduction of the durface states, but due to the generation of a fixed charge, whereas photochemical HCl treatment reduces the surface states significantly.

Relationship among surface state distribution, recombination velocity and photoluminescence intensity on semiconductor surfaces

Abstract Recombination processes through surface states with arbitrary distributions are analyzed on computer, and the behavior of the band edge photoluminescence (PL) intensity. IPL, is studied. In contrast to previous PL analyses based on an assumption of a constant surface recombination velocity Sc, the present result shows that Sc is not a constant, but strongly depends on the intensity of the excitation light, o. The behavior of IPL versus o is investigated in detail for different shapes and densities of surface states. The effect of fixed surface charge is also analyzed. It is shown that Nss distributions on free surfaces can be determined by a detailed measurement of the ratio IPL/oversuso. The result is applied to actual measurement on chemically etched and sulfur-treated GaAs surfaces.

Electronic properties of AlxGa1−xAs surface passivated by ultrathin silicon interface control layer

Abstract The photoluminescence surface state spectroscopy (PLS3) method was applied to a study of the surface state distribution (NSS), effective surface recombination velocity (Seff), electron (EFn) and hole (EFp) quasi-Fermi levels and band bending (VS) on the Al0.33Ga0.67As surface air-exposed and passivated by the Si interface control layer (ICL) technique. Using the detailed measurements of the PL quantum efficiency for different excitation intensities, combined with the rigorous computer simulations of the bulk and surface recombination processes, the behavior and correlation among the surface characteristics under photo-excitation was determined. The present analysis indicated that forming of a Si3N4/Si ICL double layer (with a monolayer level control) on AlGaAs surface reduces the minimum interface state density down to 1010 cm−2 eV−1 and surface recombination velocity to the range of 104 cm/s under low excitations.

In-situ photoluminescence and capacitance-voltage characterization of InAlAs/InGaAs regrown heterointerfaces by molecular beam epitaxy

Abstract This paper characterizes the electronic properties of InAlAs/InGaAs molecular beam epitaxy (MBE) regrown interfaces by combined use of in-situ photoluminescence surface state spectroscopy (PLS 3 ) and capacitance-voltage ( C - V ) techniques. It is shown that the interface state density at the continuously grown InAlAs/InGaAs interface is low and comparable with that of the uninterrupted AlGaAs/GaAs interface, and that the effect of the growth interruption was surprisingly small, whereas the same interruption resulted in almost 10 2 times reduction of the PL efficiency for the AlGaAs/GaAs system. It is also shown that a high density of surface states exists at the MBE InGaAs surface. Interruption after the growth of the bottom InAlAs layer in the InAlAs/InGaAs/InAlAs system also leads to appreciable generation of interface states, but it is much smaller as compared with the case of the AlGaAs/GaAs/AlGaAs system.

Measurement of surface recombination velocity of silicon wafers under sunlight condition by novel photoluminescence surface state spectroscopy

Abstract For a successful realization of super-high efficiency solar cells, reduction of surface or interface recombination is required. Thus, it is important to know the value of the surface recombination velocity ( S ) for the optimization of the passivation technology and for the design of solar cells. In this paper, a photoluminescence (PL) based novel technique for the measurement of the value of S under sunlight is presented, and applied to variously passivated Si surfaces. S was found not to be constant but to depend strongly on the excitation intensity near and above 1 sun condition. The value of S is strongly reduced under concentrated sunlight indicating that concentration of sunlight or use of a thin film is effective for efficiency increase.