V Grankin, S Alioshin, S Voloshchuk - Electronic processes on the surface of wide-gap semiconductors via excitation by thermal energy atoms and ultra-violet light - страница 1
ВІСНИК ЛЬВІВ. УН-ТУ
Серія фізична. 2006. Вип. 39. С. 273-281
VISNYKLVIV UNIV. Ser. Physic. 2006. N39. P.273-281
PACS number(s): 34.50.Dy
ELECTRONIC PROCESSES ON THE SURFACE OF WIDE-GAP SEMICONDUCTORS VIA EXCITATION BY THERMAL ENERGY ATOMS AND ULTRA-VIOLET LIGHT
V. Grankin, S. Alioshin, S. Voloshchuk
Priazovskiy State Technical University e-mail: email@example.com, firstname.lastname@example.org, email@example.com
The electronic channel of energy accommodation of chemical reactions by atomic particles of thermal energy (H, O) on the surface of wide-gap semiconductors Zn2SiO4-Mn, ZnS,CdS-Ag, ZnS-Tm, ZnS-Cu,Al are surveyed. The kinetic mechanism of radical-recombination luminescence excitation of semiconductors, for which phenomenon - high-efficiency electronic heterogeneous accommodation (HEHA) is found out are determined. The mathematical model is written. The effective involvement of electronic states in energy accommodation of heterogeneous plasmochemical reactions is demonstrated. Usage of an phenomenon HEHA for definition of the electronic traps energy depth are surveyed.
Key words: electronic trap, accommodation of energy, heterogeneous plasmochemical reactions, luminescence.
The chemical reactions proceeding on a surface of semiconductors in process for example of epitaxial growth, plasma etching, adsorption or recombinations of atomic particles, in some cases are accompanied by considerable liberation of energy (up to several eV for the elementary act). This energy at the first moment is concentrated on the reformed chemical bond and has the form of high-excited vibrational movement of nucleus of reaction product. Then the molecule relaxes due to transfer of energy to the semiconductor (the process of accommodation). The basic channel of energy accommodation of plasmochemical interactions on semiconductors and dielectrics is the phonon one. Molecules, which formed on a surface (H^OjfLand others) in case of
participation of light particles (H, O and others) in reaction, have the vibration frequencies laying outside of a crystal phonon spectrum, and energy of vibrational quantums is much greater than optical phonons energy in the semiconductor. Therefore process of relaxation is a multiphonon or proceeding with a rather small speed. It enables to participate of electronic states of the semiconductor in an accommodation of energy of heterogeneous plasmochemical reactions.
The whole series of the non-equilibrium chemoeffects connected to generation of electron-hole pairs in semiconductors was found out in the 70s. That is a chemomagnitoelectrical effect on Ge  (analogue of Kikoin-Noskov photomagnetoelectric effect), valve chemoeffect on p-n transition in Ge at excitation by
© Grankin V., Alioshin S., Voloshchuk S., 2006
H atoms , abnormal chemovoltaic effect on CdTe . Non-equilibrium chemoconductivity raised due to energy of atoms recombination was discovered in  (system: atoms H - CdS), and non-equilibrium adsorbconductivity (H - ZnO) was discovered in . The electronic accommodation is exhibited in excitation of luminescence of a wide-gap semiconductors and dielectrics (radical-recombination luminescence (RRL) and adsorboluminescence (AL)) in visible area at processing of heterogeneous chemical reactions  and electrons and ions chemoemission from a surface . In other words the chemical reactions on the semiconductor surface reduce to forming of the non-equilibrium electronic effects similar to effects at photoexcitation. The luminescence and the other electronic chemoeffects are the product of the chemical reaction on the surface and consequently carry the information about the semiconductor surface, physical and chemical processes on it and the gas environment.
The probability of wide-gap semiconductors electronic excitation to the act of chemical interaction is not too large (< 10-2) as distinct from the photoexcitation, and consequently the electronic channel of energy accommodation was neglected. The new non-equilibrium phenomenon - high-efficiency electronic heterogeneous accommodation of energy of chemical reaction on the surfaces of electron-excited wide-gap semiconductors was revealed in . That phenomenon consists in increase in some times of the heterogeneous reaction speed at excitation of samples by ultra-violet (UV) light owing to a relaxation of energy of the vibrational-excited molecules along the electronic channel and in increase on some orders of electrons generation speed in a conductivity band (or holes in a valence band) due to reaction energy in comparison with the unexcited semiconductors by UV-light. The HEHA allows approaching to the explanation of catalytic action of surface and photostimulated growth of semiconductors in a new way and allows operating with the speed and selectivity of reaction. Owing to superficial character of radical-recombination excitation the HEHA may be used for diagnostics of electronic conditions on the surface of luminescencing semiconductors.
Measurements of RRL were carried out on a high-vacuum setup with the realized method of an atomic probe . For RRL excitation was used hydrogen or oxygen atoms, which were generated of non-electrode HF discharge or pyrolysis source (for creation of the pulse atomic stream normalized in intensity). A mercury lamp was used to photoexcite of the sample. An UV quantums stream onto surface was changed with the help of diaphragm. FEU-97 or FEU-118, calibrated in absolute units, registered the luminophore radiation. The oil-free pump-down by magnetic-dischargeable pump NMD-016 was used.
Powdery semiconductors Zn2SiO4-Mn; ZnS,CdS-Ag (Xmax =520 nm); ZnS,CdS-Cu,Al (540 nm); ZnS-Tm (478 nm) and self-activated monocrystals ZnS (480 nm) were used as samples. The stream density of excitation light was < 1015 cm-2s-1. Preliminary irradiated sample in a reactionary cell was probed with the pulse stream of Н (O)-atoms (j~1014 sm-2s-1), which was normed in intensity, from the atomic pyrolysis source. The pulse stream was switched on for 2s during the process of de-excitation of the accumulated light sum by the semiconductor. The intensity flash of RRL at the moment
of probing by atomic stream and also intensity of afterglow IA were registered. The low density and short time of atomic probing stream did not change the surface condition and the character of de-excitation of the accumulated light sum. It has been found the
magnitude of RRL intensity flash IeRRL at the probing moment was proportional to the concentration of electrons on traps at current moment and exceeded almost on two
orders of the RRL intensity, which was excited by the same atomic stream on the unexcited surface by UV light (fig. 1). The value of concentration of electrons on traps was determined by the afterglow intensity magnitude of the sample.
The dependence of flash magnitude IeRRL in experiments was directly proportional
to the concentration of the filled electronic traps n e . That specifies participation of the
excited electronic conditions in the accommodation of energy of the heterogeneous reaction. The increase of RRL intensity at pulse switching on of an atomic stream on the ZnS,CdS-Ag sample is explained with the electron transfer from the electronic trap to the crystal conductivity band by the vibrational energy of molecule formed in the recombination reaction, and the subsequent recombination of this electron with a hole, located on the center of a luminescence (the ionization mechanism of excitation of
heterogeneous chemiluminescence ). The dependence of IRRL on concentration of electron traps on ZnS-Tm sample (for which the intercenter mechanism of RRL excitation is characteristic ) testifies to it. Therefore IRRL of ZnS-Tm sample should not depend on the concentration of excited electronic conditions, as it was observed in experiment.
The dependence of IeRRL of ZnS,CdS-Ag sample from intensity of a crystal phosphorous photoluminescence (IPL) excited by a stationary UV light stream, at pulse probing by Н-atoms stream is represented on fig. 2. The values of ne and IPL at small UV light Ф streams are proportional to Ф, and the given technique enabled to investigate the dependence of IRRL from ne. The IRRL is proportional to Ф at Ф < 1012 quantum/(cm2s), and it grew approximately by 5 orders of magnitude at Ф = 6D1011 quantum/(cm2s) in comparison with IpPI of the unexcited sample by
light . The increase of IRRL was more than 3 orders of magnitude for ZnS,CdS-Ag.
The IRRLof ZnS,CdS-Ag and monocrystal ZnS at Ф >1012 quantum/(cm2s) and
Т = 295 K almost did not achieve the maximal value and varied at the further increase of electronic excitation level by UV light. The increase of Т resulted to decreasing of
IeRRL at constant values of the light stream and atomic stream density with activation
energy of 0,17 and 0,62 eV for ZnS,CdS-Cu,Al; 0,17 eV for ZnS,CdS-Ag; 0,17 and 1,16 eV for self-activated ZnS, that coincided with energy depth of electronic traps in researched samples . In its turn, the UV light illumination of ZnS-Tm sample with the
direct mechanism of RRL excitation did not influence to the IeRRL .
The obtained dependence of RRL intensity at pulse probing of sample by atoms on the level of semiconductor electronic excitation (concentration of the filled electronic traps), and also high efficiency of the excitation of irradiated sample with UV light (increase on 3 - 5 orders of the value) due to energy of heterogeneous reaction enable to use HEHA as sensitive and practically inertialless tool for semiconductors research usage. Owing to surface character of radical-recombination excitation HEHA may be used for studying actually surfaces, in particular for diagnostics of surface electronic
conditions in luminescent semiconductors (an optical method of heterogeneous surface spectroscopy).
The semiconductor band model that contains one type of electronic traps (TL) and the luminescence centers (AL) is represented on fig. 3. Electronic traps (TeL) filling and ionization of luminescence centers (ApL) (transitions 1 and 2, accordingly) happens during the preliminary photoexcitation of the semiconductor. At switching on of atomic stream (for example the stream of hydrogen Н) to the surface there is an adsorption and recombination of atoms, resulting in to formation of a vibrational-excited
molecule . Due to anharmonicity of vibrational bond in transitions through
some vibrational levels at once (fig. 3, transition 3) are possible already in the first order of decomposition of quadruple moment of molecule on internuclear distance . Due
to interaction of quadruple (HVL) - dipole (electron in the electronic trap) the direct
switchover of energy (which one was liberated in acts of H2V L vibrational relaxation) to
the excitation energy of electron on the electronic trap is possible. Then transfer of the electron into conduction band (transition 4) is possible . The delocalization of electron from an electronic trap is also possible as a result of thermal excitation (transition 5). Electrons from a conductivity band are grasped by the empty electronic traps (transition 6) or recombined with the holes located on the ApL luminescence centers (transition 7) with emission of luminescence quantum.
The kinetic mechanism of RRL excitation of semiconductors, for which HEHA is found out, includes the following basic stages: adsorption, desorption of R atoms and R2 molecules on a surface centers L, recombination of atoms from a gas phase with atoms on a surface (the Rideal-Eley mechanism of recombination), ionization of traps and generation of electron-hole pairs due to reaction and light irradiation.
R + L < V > RL TL + eL < k > TeL
R + RL - > AL +pL < 2 > ApL
R2 + L —-— R2L ApL + eL—^ A*L
+ L0 ——- R2L + ph + L0 AL -
R2X + L0 <2 > R2L + eL + pL + L0
-— AL + hv -—2-- AL + ph
+ TeL <- > R2L + TL + eL
hv + L0-4—» eL + pL + L0
Here ph - heating of a sample - phonons; v , vj, kj are probabilities of
appropriate processes referred to a unit time; ri (i = 1^3), k (j = 1^3) are constants of
appropriate stages speeds; ?] is a probability of radiating relaxation of the luminescence center. We shall enter designations for concentration of the surface and volume centers in the moment t: [l] — N (t), [RL]-> N1 (t), [R2L]- N2 (t), [r2l] — N2 (t),
[tl]- n (t), [TeL] —» ne (t), [al]-m (t), [ApL]-mp (t), [eL]-(t), [pL] — n L (t). The following system of the kinetic equations corresponds to the given model:
N 1 =VjN -v'j Nr
NN v :V3N-V3N2 +(riN0 +r2N0 +Г3Пе )NV2 = = V2N1 -(riN0 +r2N0 +Г3Пе )N2, (1)
ii = k,nTn-(r\NV + k')n ,
el eL \ 3 2 1/ e'
in = k0n T m - k^n T m ,
p2 pL 3 eL p'
n eL = (Г 2N0 +г3пє )NV + k'ne +V 4N0 - klneLn - k3neLmp' npL =r2N0N2 -k2npLm + V4N0-
As time of the excited conditions life is small the quasi-equilibrium N2 = 0, n = n = 0 takes place, and then n = m . It allows to simplify system (1) and to
eL pL ep
reduce it to the following kind:
N, = VJN■V1 Nr
N2 =V3N -V3N2 +V2N1'
n e =(Г 2N0NV +V 4N0)
(Г 3neNV + kine)
k,n + k3m
1 3 p
Here the next designation is supposed:
г1n0 +г 2n0 +г 3пє
The HCL intensity is equal to
1 = ^kn ^kpm (Г 2N0NV +Г 3neNV + k1ne + V4N0 ) ■
At that the condition of a constancy of full number adsorption places N0 on surface N + N + N = N and the charge conservation law n = m are fulfilled.
12 0 ep
The decision of the receiVed equations system (2) with expression for luminescence intensity (3) together allows to determine the dependence of stationary RRL intensity on temperature, density of atoms and molecules streams and also to describe kinetics of RRL inflaming at switching-on of atoms aboVe the clean surface; enables to describe the non-equilibrium ionization of traps, the changing of RRL intensity at modulation of atomic stream density and dependences of RRL intensity on time at switching-on of atoms after "dark" pauses.
The intensity of the luminescence arising at "switching-on" of atomic stream on the semiconductor surface in a metastable condition is equal I = I + Iе +1 .
Here It, is an intensity of radical-recombination luminescence; Ie
contribution to the luminescence intensity, caused by excitation of electrons on electronic traps by Vibrational-excited molecules energy; IA is an intensity of the sample
afterglow. At absence of exciting atomic stream and Ф switching off we haVe for afterglow intensity from (3):
At switching-on of pulse atoms stream j of small density (t < 2 s) in process of de-excitation of the accumulated light sum, as it follows from (3), IRrl will be equal
IRRL =n-—— Г3п N2
RRL 'k,n + k3m 3 e 2
Here considered that owing to HEHA г3neNV <<г2N0NV- Due to (4) expression (5) will be written as p = nvI^Ii , that is IRrl is directly proportional to т , as it
RRL "2 k' A
also was observed in experiments (fig. 1). We shall estimate the relative increase A of intensity of the chemiluminescence due to HEHA:
A = TRrL/TRRL =Г 3V (П 2N0k1)
Fig. 1. Dependence of afterglow intensity and RRL Fig. 2. Dependence of RRL intensity at pulse
excited by hydrogen atomic probe at the moment ti of switching-on of a hydrogen atomic stream on
ZnS, CdS-Ag sample on time after UV excitation Zn2SiO4-Mn surface on the sample photolumi-
switching off. On an insert - the ie dependence nescence intensity, T=3°0 К
on electrons concentration on traps, T=295 К
Since Ц = • exp(-E/kT) (where is the frequency factor of an
electronic trap thermal relaxation; E is an electronic trap depth; T is the semiconductor temperature) the expression (6) will become: a = -
From here e = k (ln ) ln (a 2)) 1/T1 - 1/T2
Fig. 3. Electronic processes on the surface of semiconductor
Here A1 and A2 are the relative increase of chemiluminescence intensity due to
HEHA at two Various Valuations of semiconductor temperatures T1 and T2. For ZnS,CdS-Cu,Al sample the following values of depth of electronic traps occurrence are received: 0,17 eV, 0,68 and 0,72 eV. For ZnS, CdS-Ag the basic were fine traps with E = 0,20 eV, and besides them for ZnS monocrystal in the high concentration were the traps with depth of 1,11 eV. Electronic conditions of semiconductors, obtained with the help of the giVen method, coincide with the data receiVed by method of curVe of thermostimulated luminescence (TSL) and on dependence of photoluminescence attenuation speed after preliminary UV excitation switching off from the sample temperature.
The reViewed results testify that for a series of electronic-excited semiconductors a main channel of energy accommodation of heterogeneous reaction is electronic instead of phonon one. Designed HCL-based method of definition of electronic states energetic places in semiconductors can be applied for a broad spectrum of luminescent semiconductors and dielectrics. The giVen method can be realized for study of electronic states of surfaces purely. HEHA allows managing of epitaxial growth speed of semiconductors and films. There is a possibility of control by cross-section, speed and selectiVity of reaction that is releVant in chemical physics, plasmochemistry, and adsorptiVe isotope separation.
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