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Physica B 376–377 (2006) 901–906

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Superdiffusion in semiconductors: Dynamics ofradiation-enhanced superdiffusion

Takao Wadaa, Hiroshi Fujimotob,�

aNagoya Sangyo University, Owariasahi, Aichi, JapanbDaido Institute of Technology, Minami-ku, Nagoya, Japan

Abstract

A three-layered system composed of GaAs (layer 3)/Si(Zn)//Si(Zn)/GaAs (layer 1) was used in this study. Si(Zn)/GaAs consists of

Si(Zn) evaporation-deposited on GaAs wafers. The overlying layer was in contact with only one other Si(Zn) layer. The surface of layer 3

(GaAs) was irradiated with 750 keV and 7MeV electrons, respectively. Electron irradiation of the wafer (layer 3) created Frenkel defects

and electron–hole pairs. Interstitials of displaced atoms in the overlayer migrated to the substrate interface. Mobility-enhanced diffusion

by recombination of electron–hole pairs can also occur. Residual defects in layers 1 and 3 after irradiation were athermally annealed by

electron–hole recombination-enhanced defect reactions. Furthermore, new, sharp photoluminescence spectra of the neutral acceptor-

bound exciton peaks appear in both layers 1 and 3, without thermal annealing.

r 2006 Elsevier B.V. All rights reserved.

Keywords: Electron beam doping; Photoluminescence; Kick-out mechanism; Radiation-enhanced superdiffusion

1. Introduction

The basis of physical properties of semiconductorsirradiated using an electron beam has been studied bymany researchers [1,2]. Electron irradiation avoids thecomplications associated with the generation of complex-damaged regions observed in ion-implantation of semi-conductors [1]. Charge-state effects may occur when thecharacteristic process activation energy changes with thecharge of defects.

Recombination enhancement was first proposed by Seitz[3] to account for defect production rates in alkali halides.In this mechanism, the energy liberated during a non-radiative electron–hole recombination event activates theprocess.

In this paper, 750 keV electron beam radiation-enhancedsuperdiffusion (RES) of silicon and zinc impurities intoundoped GaAs have been carried out using the kick-outmechanism, which is a combination of interstitialcy anddirect interstitial diffusion. The RES of Zn and Si atoms

e front matter r 2006 Elsevier B.V. All rights reserved.

ysb.2005.12.225

ng author. 10-3 Takiharu, Minami, Nagoya 457-8530,

1 52 612 6111; fax: +81 52 612 5623.

ss: fujimoto@daido-it.ac.jp (H. Fujimoto).

was investigated using secondary-ion mass spectrometry(SIMS) and photoluminescence (PL) measurements. In theexperiments using the three-layered structures, the elec-tron-irradiated regions resulted in annealing enhancementof defect reaction rates via electron–hole reactions. Whenwafers stacked in layers were irradiated with 750 keVelectrons, the residual native defects in all the wafers wereathermally annealed due to electron–hole recombination.

2. Experimental procedure

The wafers used in the experiments were (1 0 0)-orientedundoped semi-insulating GaAs (dimensions of 8� 8�0.4mm3) grown by the liquid encapsulated Czochralski(LEC) method. The GaAs wafers were chemically polishedby a conventional method [4]. Layers of Si (t�1665 A) andZn (99.9999%, t�2325 A) were deposited on the substrateby vacuum evaporation. In the RES system, two Si(Zn)layers were deposited by evaporation on the GaAs wafers.One underlying Si(Zn) surface on the GaAs wafer was incontact with only one other Si(Zn) surface. In other words,this can be represented as GaAs (layer 3)/Si(Zn)//Si(Zn)/GaAs (layer 1). The surface of layer 3 (GaAs) was

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Fig. 1. PL spectra (a) for A as-grown GaAs sample before RES, for B Si-doped GaAs layer 3, and C Si-doped GaAs layer 1. SIMS concentration profiles

of Si diffusion into GaAs wafers for (b) the back surface of layer 3, and (c) the front surface of layer 1.

T. Wada, H. Fujimoto / Physica B 376–377 (2006) 901–906902

irradiated with a fluence of 3.7� 1017 electrons cm�2 at750 keV at about 100 1C in a N2 gas atmosphere with amean current density of 8.1 mAcm�2. The irradiation wascarried out using a Van de Graaff accelerator provided byNissin High Voltage Co., Ltd. After electron beam dopingEBD, the remaining unreacted Si and Zn layers werecarefully removed [4]. The diffusion of Si(Zn) atoms intothe GaAs substrate by electron irradiation was confirmedusing SIMS and PL spectroscopy. The PL measurementswere also performed at 4.2K using a focused argon laserbeam (100mW, 514.5 nm) as the excitation source.

3. Experimental results

Typical PL spectra for Si EBD at 4.2K are shown inFig. 1(a) for GaAs layers 3 and 1, respectively. The PLspectra for Zn EBD at 4.2K are shown in Fig. 2(a) for

GaAs layers 3 and 1, respectively. The peak energies of Znand Si EBD spectra for layer 1 EBD were about 1.49335and 1.4934470.0002 eV, respectively, and were shiftedabout 30meV below the energy gap of pure GaAs to evenlower energies, which is similar to the results reportedpreviously by Pankove [5]. These peaks attributed toneutral acceptor-bound exciton transitions are labeled(A0,x) [6,4]. The full-width at half-maximum (FWHM) ofthis spectrum was determined to be about 5–6meV, whiledifferent PL peak energies were observed for the as-grownsamples of 1.4929 and 1.49011 eV. The peaks of theas-grown samples can be attributed to conductionband-to-acceptor transitions for defects of C(e,A0) andC(D0,A0) [6].The concentration profiles of Si and Zn atoms as

measured by Quadrupole SIMS in a GaAs wafer oflayers 3 and 1 after EBD are shown in Figs. 1(b), (c),

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Fig. 2. PL spectra (a) for A as-grown GaAs sample before RES, for B Zn-doped GaAs layer 3, and C Zn-doped GaAs layer 1. SIMS concentration

profiles of Zn diffusion into GaAs wafers for (b) the back surface of layer 3, and (c) the front surface of layer 1.

T. Wada, H. Fujimoto / Physica B 376–377 (2006) 901–906 903

2(b), (c), respectively, as functions of depth from the GaAssurface.

The sample list and results of PL and SIMS measure-ments are provided in Table 1.

4. Discussion

4.1. Two main kick-out mechanisms for impurity doping

The experimental results of RES processes in a non-equilibrium condition are schematically shown in Fig. 3.The incident electrons do not reach all the GaAs layers inlayer 1 because the penetration depth electrons at 750 keVin GaAs and Zn is estimated to be about 0.43 and 0.30mm,respectively [7].

4.1.1. First type of kick-out mechanism

The previous paper [7] proved that the kick-outmechanism [8–10] of atoms induced by electron irradiationat the interface of the three-layered system played animportant role in the modified EBD processes. For the Zndoping, thermal equilibrium between Gai and GaZn may beestablished via self-interstitial Zn (Zni) at the interface ofthe Zn sheet and GaAs wafer as

GaAsðsÞ Frenkel defects ðGai;VGa;Asi;VAsÞ;

Gai$GaZnþZni, (1)

where GaZn represents Ga atoms on a Zn site, and Gairepresents a Ga atom on an interstitial site.

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before irradiationunder irradiation

(750 keV electrons, pulse operation)

dynamics of electron hole pairsand Frenkel defects (Gai, Asi)

GaAs (layer 3) (t=0.4 mm)evap. Si(Zn) (2,000Å)

GaAs (layer 1)(as-grown C(A°), C(D°))

Waf

erR

ES

for

thre

e-la

yere

d sy

stem

displaced atoms(ε>150keV)

electron-hole pairs(ε>4eV)

range of incidentelectron (0.43 mm)

electron-holerecombination-enchanced defectreaction

after irradiation

Si(Zn) layer : removedas-irradiatedeven in layer 3 and 1

SiAs

ZnGa

Gai + CGa → GaGa + Ci

Asi + CAs → AsAs + Ci

Gai ⇔ GaZn + Zni, Zni ⇔ ZnGa + Gai

Asi ⇔AsSi + Sii, Sii ⇔ SiAs + Asi

Fig. 3. Experimental results of RES processes in a non-equilibrium condition.

Table 1

Sample list and experimental results of PL and SIMS

Sample (1 0 0)-oriented undoped GaAs dimension

80� 80� 0.45mm3750keV

electron

irradiation

GaAs wafer after EBD

PL SIMS

System Layer 2 (Zn or

Si thickness)

Layer No. Fluence

(e cm�2)

Peak energy

(eV) 70.0002

Line width

(meV)

Specimen

figure

Surface conc.

(cm�3)

Diffusion

depth ( � )

GaAs/Zn//Zn/GaAs 2325 ( � ) 3 3.7� 1017 1.49371 6.2 3(a) 3� 1018 (Zn) 200

1 Unirradiated 1.49335 6.6 3(a) 1019 (Zn) 200

GaAs/Si//Si/GaAs 1665 ( � ) 3 3.7� 1017 1.4938 5.9 2(a) 1018 (Si) 50

1 Unirradiated 1.49344 6.6 2(a) 1019 (Si) 100

For original as grown sample, PL results indicate the peak energy of 1.4929 and 1.49011 eV.

T. Wada, H. Fujimoto / Physica B 376–377 (2006) 901–906904

4.1.2. Second kickout mechanism in layer 1

A number of Zni migrated by surface (interface)diffusion. These were then doped into the GaAs layer 1by another kickout mechanism [10].

Zni ZnGaþGai: (2)

Roughly 1–70% of high concentration impurities of atomsarriving at the surface layers induce the kick-out mechan-ism reactions, and these impurities are doped into thesubstrate [11]. From the ratio of (total number of dopedatoms)/(total number of displaced atoms), the net dopingefficiency, which includes the influence of recombinedinterstitials and interstitial diffusion could be estimated tobe 0.5–80% for each sample.

A Zn sheet was sandwiched between two wafers ofGaAs, and the surface of the GaAs was irradiated with7MeV in a vacuum. Net acceptor concentration profile inthe GaAs wafer (layer 1) was measured with an electro-chemical C–V profiler as a function of depth from the front(upper) surface after 20min annealing at 750 1C using aconventional furnace [12].

4.2. Outline of RES (radiation-enhanced superdiffusion)

The rate of generation, G, of electron–hole pairs (EHPs)per unit time by an incident electron can be estimated as [13]:

G ¼1

edE

dx

dFdt

, (3)

where e is the energy for the formation of the EHPs (�4.63,�3.88 and �2.79 eV for GaAs, Si and Ge, respectively), dE/

dx is the electron energy loss per cm of the path by afast electron [6.99� 106 eVcm�1 e�1 for GaAs, 3.62�106 eVcm�1 e�1 for Si and 6.74� 106 eVcm�1 e�1 for Ge,respectively], dF/dt is the electron dose rate of 3.1�1014 e cm�2 s�1 during electron irradiation. The irradiationresults in values of G of 5.5� 1020, 3.1� 1020 and 10� 1020

EHPs cm�3 s�1 for GaAs, Si and Ge, respectively. G producesan electron–hole pair concentration of

n ¼ Gt

where t is the carrier lifetime. Assuming that t is 10�5 s duringirradiation, then,

nA1015 cm�3.

ARTICLE IN PRESST. Wada, H. Fujimoto / Physica B 376–377 (2006) 901–906 905

Carrier densities of about 1015 cm�3 relate the enhanceddefect annealing of electron–hole recombination processes atthe defects [14–16]. For an energy-release mechanism [17], thenumber of jumps, R, is obtained using

R ¼ nsV exp �ðEc þ EHÞ

kT

� �, (4)

where s is the cross section, V is the thermal velocity, Ec is thethermal activation energy for trapping and EH is the thermalactivation energy of the recombination-enhanced defectreaction (EH ¼ 0 for the Bourgoin mechanism). In the caseof Ge//Si, Ec þ EH was obtained as �0.3 eV near the surfacefrom SIMS measurements for various irradiation tempera-tures. The effective diffusivity for recombination-enhanceddiffusion is roughly given by

Deff � RðDxÞ2

4, (5)

where Dx is the jump distance. By using their values, Deff isestimated to be about 10�15 cm2 s�1, which is roughly inagreement with experiments near the surface [18]. AssumingEc þ EHA0:17 eV, Deff becomes 10�12 cm2 s�1.

4.3. Athermal annealing process of residual defects due to

750 keV electrons

The mechanism of the RES consists of two mainprocesses. The first is the migration of displaced atoms,and the second is an ionization effect which enhancesathermally the annealing of carrier trapping centers. Inthese processes, a pile of one wafer on top of another isessential for the RES. When electron energy is sufficient toproduce displaced atoms, a number of electron–hole pairsare also generated in the wafers, as shown in Fig. 3. The PLpeaks obtained from the wafer grown by the LEC methodrepresent transitions from neutral donor/conduction bandto acceptor-like (shallow) defects [19], such as C(e,A0) andC(D0,A0), as shown in Figs. 1(a) and 2(a). Duringirradiation, as shown in Fig. 3, Frenkel defects (displacedatoms) are introduced into the wafers. These defects areathermally annealed by another kick-out mechanism evenin both layers 1 and 3, whose defects may react as follows:

GaAsðsÞ Gai; VGa; Asi; VAs

GaiþCGa GaGaþCi (6)

AsiþCAs AsAsþCi (7)

During irradiation, as shown in Figs. 1(a) and 2(a), new PLspectra for the neutral acceptor-bound exciton peaks ofZnGa and SiAs appear in layers 1 and 3, respectively.

However, 7MeV electron irradiations for three-layeredstructure of GaAs//Zn//GaAs introduce a number ofdefects and complexes. Although PL spectrum measuredat 77K for as-grown GaAs wafer showed a strong band-edge emission, no PL was observed from the GaAssubstrate of one (GaAs) and two layer (Zn//GaAs)structures after 7MeV electron irradiation. This is becausea large number of defects created by the irradiation act asnonradiative recombination centers. On the other hand, aPL signal at 77K was observed with a FWHM value of�100meV from the irradiated GaAs (layer 1) in GaAs//Zn//GaAs. Peaks at 1.485 eV and 1.445 eV are attributed tothe Zn acceptor (ZnGa) and Ga antisite defect (GaAs),respectively [20]. Therefore, their results suggested thatnonradiative recombination centers could be decreased byadopting sandwiched structures. Then, the thermal anneal-ing processes at higher temperatures are generally neces-sary to fabricate a conductive p-type GaAs of fairly goodquality [20]. More detailed studies for energy dependencyof RES are now underway.

5. Conclusions

In the three-layered structure of GaAs(layer 3)/Zn(Si)//Zn(Si)/GaAs(layer 1), RES of Zn(Si) atoms into theunirradiated GaAs layer 1 and into the irradiated GaAslayer 3 at 750 keV was carried out at room temperature.Such a condition of 750 keV electron irradiation of a three-layered structure is athermally favorable for recombina-tion-enhanced defect reactions. Residual native defects inlayers 1 and 3 were annealed by electron–hole recombina-tion-enhanced defect reactions. There were several kick-outmechanisms caused by displaced atoms and Frenkel defectsat the interface and beneath the GaAs wafer.RES has advantages over alternate doping techniques,

because it allows for the possibility of doping even inregions without damage and at room temperature.

Acknowledgment

We are indebted to Professor Emeritus Rob. Ammer-laan, University of Amsterdam, for helpful discussions andadvice.

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