Hall-Effect Analysis of Persistent Photocurrents inn-GaAs Layers

Abstract

The buildup of persistent photoconductivity, presently a controversial phenomenon, is observed by measuring densities and mobilities of photoinduced excess electrons in thin ^-GaAs layers between successive illuminations. Evidence from this novel type of analysis supports a model assuming charge separation by macroscopic potential barriers. We explain quantitatively how the photon dose logarithmically increases the number, but not necessarily the density, of persisting carriers and ascribe mobility enhancements to screening of ionized impurities.Many semiconductors exhibit persistent photocurrents (PP): A photoinduced conductivity increment persists after the illumination, often with immeasurably long time constants/"'^ Two conflicting interpretations presently exist. The first assumes macroscopic potential barriers, such as junctions or surface barriers, which separate spatially the photogenerated electron/hole pairs to suppress their recombination.^"^ The second interpretation postulates microscopic barriers against recombination due to impurity atoms with large lattice relaxations.^"'^ Interest in this unusual phenomenon has been recently rekindled. Studies of two-dimensional electron gases at semiconductor interfaces rely on PP to enhance electron densities.^ Interpretations of atomistic properties of deep impurities in semiconductors have been based on PP observations.^*"^ Enhancement of carrier mobility by spatial separation of liberated carriers from their dopant ions ("modulation doping")^ niay be related to PP. Quantitative information is needed to describe lateral charge transport in very thin semiconductor layers near surfaces. This would be of interest for quantum effects from reduced dimensionality^^ as well as for applications, such as charge-coupled devices^^ or memories.^ This paper describes PP in well-defined layers of GaAs at low photon excitation levels. We are able to observe and explain for the first time the transient buildup of PP by using the Hall effect with high resolution to measure both density n and mobility /i of the excess electrons in thin wtype layers on high-resistivity substrates. This technique, which yields more information than simple conductance measurements, is shown here to furnish quantitative details about geometric structure and electronic transport. The method thus promises to become a novel technique for analysis of semiconductors interfaces. We obtain clear evidence for the model, assuming macroscopic barriers.

Coexistence of electron-glass phase and persistent photoconductivity in GeSbTe compounds

“…To the best of our knowledge, this is the first occurrence of PPC in a system with carrier-concentration n>10 20 cm -3 . Typical values for n in systems exhibiting PPC range from10 15 cm -3 to 10 19 cm -3 [3,7]. These relatively low carrier-concentrations made it feasible to demonstrate, via Hall effect measurements, that the effect is associated with increase of carrier-concentration [3,7].…”

“…like lightly-doped semiconductors [1][2][3][4][5][6][7][8][9][10][11][12][13]. Intrinsic electron-glass effects were never observed in these lowdensity systems [26].…”

“…There are two specific phenomena that exhibit sluggish conductance relaxation following excitation by a light-source: persistent-photoconductivity (PPC) that has been studied extensively as of 1968 [1][2][3][4][5][6][7][8][9][10][11][12][13], and the less familiar (but just as controversial) electron-glass [14][15][16][17][18][19][20][21][22][23]. In addition to the similar way these effects are reflected in the sample conductance once triggered by light, both become more prominent at lower temperatures and both exhibit a non-exponential relaxation.…”

Dynamics of photogenerated nonequilibrium electronic states inAr+-ion-irradiatedSrTiO3

“…Theoretical approaches used to explain the phenomenon invoke either atomic scale microscopic barriers for recombination existing around defect centers with large lattice relaxation (LLR) 51 , or a spatial separation of photo-generated electrons and holes by macroscopic barriers due to band bending at the surface or interfaces. 52 The ultraviolet photoemission spectroscopy studies on ion-bombarded STO show essentially zero band bending. 53 While reduced STO is prone to chemisorption of oxygen onto its surface which may lead to a non-zero band bending, the dominant role of thermal energy in PPC in high T-region along with a substantially week temperature dependence of the relaxation time in low T-region strongly suggest the existence of LLR dominated PPC.…”

Persistent photoconductivity in semiconducting III-V compounds

“…¬ÄÂÌÔÇÓ Ë´ÇÑAEÑÓÖ [8,9] ÑÒÖÃÎËÍÑÄÂÎË ÒÑAEÓÑÃÐÞÇ AEÂÐÐÞÇ Ñ ¥² Ä ÔÎÑâØ GaAs, ÔÑAEÇÓÉÂÜËØ ÏÂÍÓÑÔÍÑÒËÚÇ-ÔÍËÇ ÒÑÕÇÐÙËÂÎßÐÞÇ ÃÂÓßÇÓÞ. …”