The e�ect of stoichiometric disturbance on activation of MeV Siimplantation in GaAs

Yanwen Zhang a,*, E-Jiang Ding b,c,d, Tonghe Zhang b

a Department of Nuclear Physics, Lund Institute of Technology, Box 118, S-221 00 Lund, Swedenb Institute of Low Energy Nuclear Physics, Beiging Normal University, Beijing 100875, PeopleÕs Republic of China

c CCAST (World Laboratory) P.O.Box 8730, Beijing 100080, PeopleÕs Republic of Chinad Institute of Theoretical Physics, Academia Sinica, Beijing, 100080, PeopleÕs Republic of China

Received 10 November 1998; received in revised form 2 February 1999

Abstract

MeV Si implantation into semi-insulating (SI) GaAs is a common technique used to create a deep doping layer. It is

observed that the maximum of the carrier concentration is at a shallower depth than that of the implanted atom

concentration. This unexpected phenomenon has previously been explained as depth measurement error. Based on our

theoretical calculations and experiments, the phenomenon can be attributed to the e�ect of non-uniform stoichiometric

disturbances (NSD) on the activation of implanted atoms. The e�ect of NSD in GaAs substrate is caused by MeV

implantation. After implantation, an excess concentration of the heavier element As exists at shallower depth, while an

excess concentration of the lighter element Ga is seen at greater depth. It is likely that Si atoms move more easily to Ga

sites when they are heated in an As-rich environment. Excess lattice interstitials build up around the depth of the peak

in the implanted Si� concentration and prevent Si atoms from becoming activated there. Ó 1999 Elsevier Science B.V.

All rights reserved.

PACS: 61.70.Tm; 34.10.+x

Keywords: Si implantation; Semi-insulating GaAs; Stoichiometric disturbances

1. Introduction

High-energy n-type implantation into III±Vcompound is of interest for monolithic microwaveintegrated circuits (MMICs) and opto-electronicdevices because of high electron mobility in thesematerials [1±8]. In practical applications, many

devices, such as ®eld-e�ect transistors, mixer di-odes and p±i±n photodiodes, require a deep n�

contact region. Because of its low mass, Si has themaximum penetration and produces the least lat-tice damage of all the n-type dopants. Therefore,MeV Si implantation into GaAs to fabricate adeeply buried n� layer has attracted popularinterest in recent years. Since the quality of theactivated region and the position of the maximumcarrier concentration are vital to the design of in-

Nuclear Instruments and Methods in Physics Research B 152 (1999) 307±313

* Corresponding author. Tel.: +46-46-2227682; fax: +46-46-

2224709; e-mail: yanwen.zhang@nuclear.lu.se

0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 1 7 4 - 3

tegrated circuit (IC) devices, considerable atten-tion is paid to theoretical calculations and experi-mental measurement techniques for the range anddamage distributions of MeV Si� implantationinto GaAs.

In our experiments [9], it was found that thepeak of the carrier distribution is not in accor-dance with that of the implanted atom distribu-tion. The maximum of the carrier concentrationalways appears at a shallower position than that ofthe implanted atom concentration. The di�erenceincreases with increasing implantation energy, andoften leads to an incorrect design of the depth ofthe buried layer in high-energy implantation. Onthe other hand, high-voltage electron microscope(HVEM) observation reveals that the residual de-fects are scattered around the position of maxi-mum atomic concentration [9] after rapid thermalannealing (RTA).

For Si� implantation into GaAs or InP, therehave been several publications [10±12] describingthis unique feature that the maximum atomicrange determined using secondary ion mass spec-trometry (SIMS) is deeper than the maximumrange of charge carriers determined from electro-chemical capacitance±voltage (C±V) pro®les.Thompson considered [10,11] that this di�erencebetween the SIMS and C±V pro®les was within themutual uncertainty of the depth measurementtechniques. The uncertainty in the absolute depthof the SIMS pro®les, measured using surfacepro®lometry, is 7%. The uncertainty in the depthscale of the C±V pro®les, which depends upon thecalibration of the contact area, is estimated to be10%. So the deviation between the two concen-tration pro®les shows no di�erence within the 17%systematic errors from di�erent techniques.

However, as the deviation has been seen fromdi�erent laboratories in di�erent countries, it isquestionable if it can be simply attributed to theuncertainty in the measurements. The di�erentchemical and physical properties of the di�erentlattice atoms, may give rise to a non-uniformstoichiometric disturbance (NSD) in the substrateafter ion implantation. As a result, the electricalactivity of amphoteric dopant Si may be in¯u-enced. In the past, little work has been directed tothe e�ects of stoichiometry on the activation of

implanted dopants, probably because such e�ectsare di�cult to measure or calculate. In this paper,the Boltzmann transport equation for stoichio-metry distribution (BTESD) are developed tocalculate the local NSD. The experimental tech-niques are described in Section 2 and results of thecalculation are presented in Section 3. In Section 4,the calculation results are compared with the ex-perimental data. A brief concluding remark is in-cluded in Section 5.

2. Experimental techniques

Undoped, semi-insulating (SI), (1 0 0)-oriented,liquid-encapsulated-Czochralski (LEC) grownGaAs substrate was implanted with 28Si� at roomtemperature. The ion energy ranged from 0.6 to 6MeV, and dose from 3 ´ 1013 to 2.9 ´ 1014 cmÿ2. Toavoid channeling e�ects, the samples were rotatedmechanically during implantation.

Rapid thermal annealing (RTA) was carriedout using a halogen lamp system. In order tominimize As loss from the surface, the wafer washeat treated in a capless face-to-face con®gurationwith another bare GaAs wafer in a high-purity N2

atmosphere. The samples were annealed at 950°Cfor 5 s. There was no observable change in thesurface topography after annealing.

Sheet carrier concentration, sheet resistanceand bulk mobility were obtained from Hall mea-surements in the Van der Pauw con®guration.Carrier concentration pro®les were measured byC±V pro®ling and electrochemical etching. Tomonitor lattice recovery and observe the residualdefects, cross-sectional high-voltage electron mi-croscopy was used. Implanted Si depth pro®les ofsome samples were measured using SIMS.

3. Stoichiometric distributions in Si implanted GaAs

When an energetic ion enters a target, it collideswith the nuclei and electrons of the target atomsgenerating recoils. Thereafter, the substrate is leftwith residual damage because of recoil processesduring implantation. In compound semiconduc-tors, however, because of the target atomic num-

308 Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 307±313

ber, displacement energies and masses, the di�er-ent elements in the semiconductor will have dif-ferent collision cross-sections, maximum energytransfers and recoil range distributions. Conse-quently, lighter atoms have greater recoil rangesthan heavier atoms for the same incident ion en-ergy. There may also be a slightly higher dis-placement energy of element Ga (6 4%) thanelement As for the implantation conditions [13].As a result, after implantation the compound lat-tice is not only damaged, but also left with a non-uniform stoichiometric distribution. As Si is anamphoteric dopant in GaAs, it can capture a Gavacancy (VGa), which acts as a donor (SiGa), orcapture an As vacancy (VAs) to act as an acceptor(SiAs). When the concentration of one element of acompound semiconductor exceeds that of theother by an amount comparable to the dopantconcentration, amphoteric dopants may changethe local doping from one type to another. Becausethe availability of vacancies of each element isdetermined by the local stoichiometry of the lat-tice, one might then expect a possible in¯uence onthe electrical activation of dopants by the stoic-hiometric imbalance at that point.

For the case of MeV Si implantation intoGaAs, we are interested in the spatial distributionsof both the primary ion and the resulting damage,especially the non-uniform stoichiometric distri-bution which can critically a�ect amphoteric Siperformance. Because it is di�cult to experimen-tally measure the e�ect of NSD, it is therefore verydesirable to be able to calculate directly both pri-mary ion and recoil atom distributions, and toexplore the relationship between implantationdamage and implant electrical activity.

Binary collision approximation (BCA) andmolecular dynamic computer codes, which areconventionally used for implantation range anddamage calculations, are computationally ine�-cient for the calculation of NSDs. This is becausethe NSD is generally very small. Therefore wehave preferred to use the Transport equation ap-proach. The theory of Lindhard, Schar� andSchiott (LSS) [14] has been thoroughly developedfor calculation of both range and damage pro®lesfor low-energy ion implantation in a large varietyof targets. It gives satisfactory agreement with

experimental results in semi-in®nite substrates.For high-energy ions, because the electronic stop-ping dominates the slowing down process and themajority of nuclear collisions occur within a nar-row region, some theoretical changes to the cal-culation procedure are needed. An extended formof a transport equation (TE) approach was ®rstintroduced by Smith and Gibbons [15, 16]. Basedon Smith and GibbonsÕ model, it has been furthermodi®ed for our purpose. Such a calculation ismore complex than the calculation of primary ionrange distribution based on the LSS theories. Theresults are shown in Fig. 1.

In Fig. 1 open symbols indicate net vacancyconcentrations of lattice atoms, while closed sym-bols indicate net interstitial atom concentrations oflattice atoms. Circles denote the element Ga andtriangles denote the element As. During ion im-plantation, as the lattice elements Ga and As recoilunequally, there is an imbalance in stoichiometryresulting from the implantation. The result shownin Fig. 1 indicates that the recoiling of Ga and Asfrom the near-surface region results in net vacancyconcentration there (open circles and triangles)while Ga and As atoms pile up deeper into thesubstrate producing local net excess (closed circlesand triangles). The implants Si (solid line), recoilGa (closed circles) and As (closed triangles) atomspreferentially reside at interstitial sites, causing the

Fig. 1. Calculated depth stoichiometric distributions in GaAs

implanted with 2 MeV Si to a dose of 2 ´ 1014 cmÿ2 obtained

from BTESD calculations.

Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 307±313 309

supersaturated concentration of interstitials(closed squares). Net excess element distribution isobtained by subtracting the net displaced As dis-tribution from the net displaced Ga distributionwhich is called here the stoichiometric imbalancehere. A positive value indicates excess As element(dotted line), whilst a negative value implies excessGa element (dashed line). From Fig. 1, it can beseen that net Ga vacancies exceed net As vacanciesat shallow depth which leads to an excess of thenet Ga vacancies (As-rich) from the surface depthssomewhat shallower than the maximum of theimplanted atom concentration (Rm). The lighterelement Ga recoils further, leading to a heavierexcess of Ga (Ga-rich) at greater depths (aroundRm, dashed line). Similar results were obtained forimplantation over a large energy range from 0.6 to6 MeV (see Table 1).

ThecalculationresultsaresummarizedinTable1.The peak positions of concentration distributionsfor implanted Si (RTE

m ), excess As (RTEmAs) and excess

Ga (RTEmGa), the turning point from As-rich region

to Ga-rich region, and projected range RTEp are

obtained using our BTESD code. RTRIMm and RTRIM

p

are calculated using the program of the (TRans-port of Ions in Matter) TRIM-95 [17]. Values ofRSIMS

m measured from SIMS are also listed in Table1. The starred data come from reference [11]. Asshown in Table 1, the positions of the atomicpro®les (RTE

m ) coincide well with that of the pro®lesmeasured by SIMS (RSIMS

m ). Moreover RTEm and

RTEp are also in agreement with the calculation

results from TRIM (RTRIMm and RTRIM

p ), respec-tively. It follows that the calculation results ob-tained from the BTESD approach are reliable.Data in Table 1 indicate that RTE

mAs is positioned ata depth which is shallower than RTE

m and RTEmGa. In

other words, the As-rich region is wider andshallower than the Ga-rich region. Moreover,RTE

mGa is close to RTEm in all cases.

4. Experimental results and transport equation

calculation

Fig. 2 shows the depth pro®les of implantedatoms (from BTESD calculations) and active car-riers (from C±V measurements) in GaAs implant-ed with 2 MeV Si and subsequently rapidthermally annealed at 950°C for 5 s (see sample 1in Table 2). A cross-sectional HVEM micrograph(Fig. 2B) of the same sample, enlarged to matchthe depth scale, is shown in the lower part of the®gure. The electrical properties of sample 1 andRTA condition are listed in Table 2. An importantfeature to note is that carrier pro®le (open circles)does not in general coincide with the implanted ionpro®le (solid line). Near the surface, the donoractivation rate of Si is higher, and it reducesquickly in the deeper region around Rm. Anotherfeature worthy of examination is that the concen-trations of the As-rich region (dashed line) andGa-rich region (dotted line) can be compared tothe dopant Si (solid line) and carrier Si (open cir-

Table 1

The positions of the maximum of implanted Si, excess As, excess Ga concentration distribution and turning point (lm below the

surface)

E (MeV) 0.6 1 1.3 2 2.5 3 4 5 6

RTEm (lm) 0.63 0.94 1.13 1.52 1.78 2.01 2.33 2.67 2.97

RTEp (lm) 0.58 0.87 1.05 1.43 1.64 1.89 2.24 2.57 2.87

RTEmAs (lm) 0.50 0.86 0.98 1.38 1.62 1.86 2.18 2.52 2.82

Turning point 0.60 0.92 1.08 1.50 1.77 1.98 2.29 2.64 2.94

RTEmGa (lm) 0.69 1.01 1.18 1.60 1.87 2.08 2.40 2.74 3.06

Interstitial (lm) 0.64 0.96 1.13 1.54 1.81 2.02 2.35 2.70 3.00

RTRIMm (lm) 0.62 0.94 1.16 1.55 1.80 2.02 2.31 2.67 2.96

RTRIMp (lm) 0.56 0.85 1.04 1.41 1.62 1.82 2.16 2.49 2.76

RSIMSm (lm) 0.67 1.05a 1.19 1.64a 1.81 2.21 2.52a 2.68 3.11a

CarrierC ±V 0.53 0.86 1.08 1.33 1.67 1.77 2.10 2.51 2.79

a From Ref. [11]

310 Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 307±313

cles) concentrations. Since Si can easily occupy Galattice site (SiGa) and acts as a donor in the net Gavacancy (VGa) region (As-rich region) [18], higheractivation rates can be achieved in this region. InGa-rich region around the peak of implant pro®leRm, if Ga vacancies are saturated with Si, theelectrical activation can be in¯uenced by As va-cancy (VAs) sites. In other words, once the Galattice is saturated with Si, then the remaining Sieither goes into the As vacancy (SiAs) acting as acompensating acceptor or stays in an interstitialposition. As vacancies created by ion implantationcan thus play a signi®cant role in this situation.Moreover, the Si interstitials (SiI), excess Ga in-terstitials (GaI) and As interstitials (AsI) make asupersaturated interstitial concentration distribu-tion (closed squares) in a region slightly deeperthan Rm. These supersaturated interstitials willform dislocation loops to reduce the lattice strainafter annealing. Fig. 2B shows that no defects are

visible in the near-surface region but a buried bandof dislocation loops exists near Rm. The loops re-main extending from about 1.0 to 1.8 lm belowthe surface, where they are obviously correlatedwith the low active carrier pro®le (open circles)and supersaturated interstitials (closed squares).This suggests that the loops have a negative e�ecton the behavior of electronic activation. They maytrap the Si atoms in electrically inactive sites orattract point defects that lead to compensation ofSi dopants. Both those mechanisms result in lownet donor activation in a position around Rm.

The activation using higher energy Si implan-tation was investigated with the goal of achieving awide deep n� layer. In order to produce a uniformburied layer, multiple-energy MeV Si implantationwas employed. Such a region extending from 0.3lm below surface to 2.5 lm can be created byimplantation at four energies (4 MeV, 8 ´ 1013

cmÿ2 + 2.7 MeV, 6 ´ 1013 cmÿ2 + 1.3 MeV, 5 ´ 1013

cmÿ2 + 0.6 MeV, 3 ´ 1013 cmÿ2). The Si distributionpro®le is shown in Fig. 3 (circles). The electricalproperties of sample 2 are listed in Table 2. Theaverage carrier concentration (measured from C±V shown in Fig. 3 with a solid line) is about4 ´ 1017 cmÿ3. It is noteworthy that the peaks ofcarrier pro®le (solid arrows) are also shallowerthan their Rm (dashed arrows). A narrow Ga-richdistribution computed using the BTESD code

Fig. 3. The closed circles indicate the atomic pro®le of GaAs

implanted with Si at four energies (sample 2: 4 MeV, 8 ´ 1013

cmÿ2 + 2.7 MeV, 6 ´ 1013 cmÿ2 + 1.3 MeV, 5 ´ 1013 cmÿ2 + 0.6

MeV, 3 ´ 1013 cmÿ2). The solid line denotes the carrier con-

centration pro®le (C±V) of the implanted specimen activated by

950°C for 5 s RTA. A dashed line indicates the Ga-rich dis-

tribution.

Fig. 2. A: the solid line shows the atomic pro®le (TE) of 2

MeV, 2 ´ 1014 cmÿ2 Si implanted GaAs. Open circles denote the

carrier concentration pro®le (C±V) of the implanted specimen

activated by 950°C for 5 s RTA. The dotted line indicates the

As-rich distribution and dashed line is the Ga-rich distribution.

Closed squares denote a supersaturated concentration of in-

terstitials. B: HVEM micrograph of the sample. The enlarge-

ment times of HVEM photos was 25 000´.

Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 307±313 311

appears in a region where the activation rate dropsquickly.

The atomic distribution and HVEM photoof another four-energy Si-implanted specimens(sample 3: 5 MeV, 2.9 ´ 1014 cmÿ2 + 2.8 MeV,2.1 ´ 1014 cmÿ2 + 1.5 MeV, 1.8 ´ 1014 cmÿ2 + 0.75MeV, 1.3 ´ 1014 cmÿ2) is shown in Fig. 4. Higheraverage carrier concentration of about 1.2 ´ 1018

cmÿ3 can be obtained by 950°C, 5 s RTA. Thedi�erent electrical properties are listed in Table 2.The four interstitial peak positions from theBTESD calculations are indicated by arrows. Asshown in Fig. 4B, four bands of dislocation loopsindividually coincide with the four arrows. It is

worth noting that the centers of two heavy dislo-cation bands (C, D) and two arrows (c, d) have aposition slightly deeper than the peak position ofthe Si pro®les (3, 4) in the high-energy region, re-spectively. The results indicate that the dislocationloops result from not only supersaturated SiI butalso GaI and AsI.

A comparison of experimental results and the-oretical results is presented in Table 1. Our ex-perimental results carrierC±V obtained using C±Vexperimental measurement are also listed in Table1. In each case, the maximum carrier concentra-tion (carrierC±V ) is positioned in an As-rich area,which is obviously shallower than the peak ofatomic pro®les (RTE

m ). Ga-rich areas exist in theregion around the peak of the implantation Sidistribution (Rm). As mentioned above, Si atomscan be activated easier in an As-rich environmentthan in a Ga-rich environment. Moreover, excesslattice interstitials build up around Rm and preventSi atoms from activation there. Therefore the non-uniformity of activation and the peculiarity ofcarrier pro®le which is shallower than implantedatom pro®le for Si implantation in SI-GaAs can bereasonably explained.

5. Concluding remarks

The e�ect of the stoichiometry on activation ofMeV Si� implantation into compound semicon-ductor GaAs has been theoretically investigatedusing BTESD. After Si implantation into GaAs,the recoiled Ga atoms are distributed deeper be-cause of their smaller atomic number and lightermass than for As atoms. Therefore, they form lo-cally an As-rich band at a depth shallower than thepeak of implanted Si distribution. In order to act

Fig. 4. A: closed circles denote the atomic pro®le of sample 3

implanted with Si at four energies (5 MeV, 2.9 ´ 1014 cmÿ2 + 2.8

MeV, 2.1 ´ 1014 cmÿ2 + 1.5 MeV, 1.8 ´ 1014 cmÿ2 + 0.75 MeV,

1.3 ´ 1014 cmÿ2). B: The residual defects (left edge is specimen

surface) of the implanted region. The enlargement times of

HVEM photos was 20 000´.

Table 2

Electrical properties of samples. A is 4 MeV, 8 ´ 1013 cmÿ2 + 2.7 MeV, 6 ´ 1013 cmÿ2 + 1.3 MeV, 5 ´ 1013 cmÿ2 + 0.6 MeV, 3 ´ 1013 cmÿ2.

B is 5 MeV, 2.9 ´ 1014 cmÿ2 + 2.8 MeV, 2.1 ´ 1014 cmÿ2 + 1.5 MeV, 1.8 ´ 1014 cmÿ2 + 0.75 MeV, 1.3 ´ 1014 cmÿ2

Sample Energy Dose Carrier Mobility Resistance Annealing condition

(MeV) (1014 cmÿ2) (1013 cmÿ2) (cm2/v.s) (X/h)

1 2 2 4.2 2351 64.2 950, 5 s

2 A A 8.5 3000 30 950, 5 s

3 B B 16.9 1981 15.8 950, 5 s

312 Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 307±313

as a donor, Si has to occupy a Ga lattice site, hencethe activation of Si depends on the availability ofGa vacancies. In the region near to the surface, theover stoichiometry of As leads to donor activationof Si implants. On the other hand, once the Galattice is saturated with Si in the Ga-rich regionaround the peak of implant pro®le, the amphotericSi dopant forms either a compensating acceptor(SiAs) or an interstitial (SiI). Furthermore, the su-persaturated concentration of interstitials formsmany discrete residual dislocation loops in thepeak region of the implanted Si atomic pro®le af-ter heat treatment. Both SiGa±SiAs pairs and dis-location loops will prevent implanted Si fromdonor activation there. Combining these two as-pects, the non-uniformity of activation can be ex-plained.

For the same reasons there is a non-uniformstoichiometric distribution in keV energy Si im-planted in GaAs. The higher energy and heavierthe dose that are used in implantation, the greaternon-uniform stoichiometry disturbance that canbe obtained. The NSD e�ect should be consideredin device design. The proper annealing, such astwo-step RTA [19], may partly remove suchdamage and so promote the donor activation rate.

In summary, the phenomenon that maximumconcentration of the carrier pro®les appears at ashallower position than that of the implanted atomconcentration is not caused by systematic errorsfrom measurement techniques. The stoichiometricdisturbances caused by ion implantation play animportant role in the dopant activation process.

Acknowledgements

Yanwen Zhang would like to express her grat-itude to Docent Harry J. Whitlow and Assoc.Professor Peter N. Johnston for taking the time toread through the article and making valuable hints

and discussions. Yanwen Zhang would also like toacknowledge Professor Z.Q. Huang and ProfessorC.Z. Ji for helpful discussion.

This project is ®nancially supported by theSwedish Institute (SI) and State Education Com-mittee of China through the Foundation of Doc-toral Training.

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