XI INTERNATIONAL CONFERENCE AND SEMINAR EDM'201O, SECTION II, JUNE 30 - JUL Y 4, ERLAGOL 181

Investigation of Doping Profile Effect on Characteristics of Ion-implanted GaAs MESFET

Alexander K. Shestakov\ Konstantin S. Zhuravlev\ Vadim S. Arykov2, Valery A. Kagadee 1 Institute of Semiconductor Physics, SB RAS, Novosibirsk, Russia

2 Scientific Research Institute of Semiconductor Devices, Tomsk, Russia 3 RPC "Micran" Tomsk Russia , ,

Abstract - Ion-implanted GaAs MESFET was modeled and dependence of MESFET characteristics on doping profile parameters was found. Dependence of transistors characte­

ristics on doping profile parameters variation was investi­gated.

Index Terms - MESFET, modeling, TeAD.

I. INTRODUCTION

ARSENIC GALLIUM FIELD effect transistors with the Schottky barrier (MESFET) are widely used in

different areas, such as commercial microwave communi­cations, high speed data transfer, wireless data transfer and radio location. Structures for the MESFETs are pro­duced by two methods: the ion implantation technique and the epitaxial growth. Advantages of ion-implanted MESFETs are ease of fabrication and low cost. In addi­tion, MESFET's characteristics in a frequency range up to 10 GHz are quite comparable with that of transistors based on epitaxial structures [I]. Therefore, ion-implanted MESFETs are widely used in high-frequency apparatus.

Characteristics of ion-implanted transistors (saturation current, pinch-off voltage, breakdown voltage and tran­sconductance) depend on parameters of initial substrates and parameters of the doping profile (peak concentration of doping impurity, width of doping profile and doping peak position), which is formed by implantation of silicon ions [2]. Dependence of MESFET characteristics on sub­strate parameters have been studied in few papers [I], [3]. The doping profile can be reliably calculated in case of ions implantation directly in a bare substrate. However, a transistors channel is frequently fabricated by implanta­tion through a dielectric mask (Si02 or Si3N4 film). The mask allows shifting the channel to a gate of transistor formed on the surface of substrate and in that way to im­prove parameters of transistors. In case of ion implanta­tion through the dielectric mask it is difficult to predict precisely shape of the doping profile because of chaotic scattering of ions in the amorphous dielectric film. There­fore, additional adjustment of the channel doping profile is required to obtain desired MESTET's parameters.

II. PROBLEM DEFINITION

This study is focused on the effect of doping profile pa­rameters on GaAs MESFET static characteristics and at determination of doping profile parameters that ensure the

best match calculated transistors parameters with experi­mental data. Dependences of transistors characteristics on doping profile parameters variation were also investi­gated.

III. THEORY

A. Geometry and Structure of Ion-implanted Tran­sistor

Ion-implanted MESFETs were fabricated on base of semi-insulated GaAs substrates of SOO !lm thickness by formation of channel and contact regions with the Gaus­sian shape profiles by ions implantation techniques. For calculation we accept thickness of substrate only at 2.S !lm, since the remaining part of the substrate has a weak effect on transistor characteristics. According to the litera­ture, data concentration of background shallow donors in the substrate was taken at 1016 cm-3 [4]. To decrease con­ductivity of the substrate the shallow donors were com­pensated by deep acceptors. Activation energy of the Cr­acceptor equals to Eat = 0.64 eV. To tune the channel pro­file a deep acceptors concentration was varied from I.Sx1Ol6 cm-3 to 4x1OI6 cm-3 and activation coefficient (ratio concentration of ionized ions to total concentration of implanted ions after annealing) was varied from 0.8 to I. The typical doping profile formed below source or drain electrodes by ion implantation in bare substrate (ion implantation energy Eimpn+ = SO keY, ion implantation dose Nimpn+ = 0.9 !lC/cm2, activation coefficient Kact =

0.8) is shown in Fig. 1. The channel was formed by im­plantation of silicon ions with energy of Eimp = 40 keY and dose of Nimp = 1.1 !lC/cm2 through a IS nm thick Si02 mask.

Transistors have a planar structure with following pa­rameters: gate length LG = 0.6 !lm, source-gate and gate­drain lengths LSG = LGD = I !lm. Experimentally deter­mined specific resistance of source and drain Ohmic con­tacts was R = SxIO-6 Qxcm and the Schottky barrier height was <Pb = 0.6 V. Fabricated transistors have follow­ing static characteristics: the saturation current is equal to 300 mA/mm and the threshold voltage is equal to -2.3 V. Measurements were done in the pulse mode to prevent the self-heating effect. These results were obtained by the Research Institute of Semiconductor Devices (Tomsk) by averaging big amount of MESFET devices measurements. To achieve the best coincidence of calculated and experi-

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182 XI INTERNATIONAL CONFERENCE AND SEMINAR EDM'2010, SECTION II, JUNE 30 - JULY 4, ERLAGOL

mental data we varied the parameters of the channel pro­file: peak concentrations from 8. 5xlO17 cm-3 to 1.06xlO18

cm-3, peak width from 0.062 !lm to 0.078 !lm, and peak distance from the surface from 0.026 !lm to 0.036 !lm.

E 2.

10"

10'1

0.05 0.1 0.15 Depth (flm)

Fig. 1 Doping profile: dependence ionized donor doping concentra­tion on distance from transistors surface

B. Model Parameters and Simulation Process

I-V characteristics of transistors were calculated using a two-dimensional hydrodynamic model [5], which takes into account the dependence of the charge-carrier mobili­ty on the electric-field strength, doping concentration, scattering at the boundaries, and the avalanche generation and recombination of charge carriers. The calculation using the model was performed for the following key pa­rameters: band gap width, dielectric permeability, mobili­ties of electrons and holes, and their effective mass, which were taken from the database included in the Synopsys Sentaurus TCAD simulator [6]. A computational grid was generated, and the following parameters were evaluated at the grid nodes: electrostatic potential, electric-field strength, electron and hole concentrations, electron and hole temperature, electron and hole velocities and mobili­ties, electron and hole current densities, rates of genera­tion and recombination of electrons and holes, rates of generation and recombination of electrons and holes via deep levels, filling of traps with electrons and holes, con­duction- and valence-band energies. In the regions of ab­rupt changes in the parameters being calculated, for ex­ample, in the electric field or charge-carrier concentration region, the grid size was decreased. The resulting static I­V characteristics were used to determine the saturation current, breakdown voltage, transconductance, and pinch­off voltage.

IV. MODELING RESULTS

Recently, we have showed [3] that transconductance and breakdown voltage of GaAs MESFET weakly depend on the substrate parameters. In this study we show that transconductance and breakdown voltage of the MESFET also weakly depend on doping profile parameters.

The most sensitive characteristics of transistor to varia­tion of the doping profile are saturation current and thre­shold voltage. These characteristics defme main transis-

tors parameters: output power that is proportional to satu­ration current and efficiency that is inversely proportional to threshold voltage. Increase in transistors output power and efficiency is important task for design of its structure.

The effect of doping profile parameters (spatial position of concentration peak, profile wide and activation coeffi­cient) on saturation current and threshold voltage was investigated on the basis of numerical computer model­ing. Impurity concentration in maximum of the doping profile was constant. Transistor characteristics obtained from the calculation were compared with experimental data. To present our results more vividly we also calcu­lated relative changes in saturation current and threshold voltage variation of doping profile parameters. These viewgraphs can be used for optimization of ratio between power and efficiency of transistor by choosing suitable substrate (compensation coefficient) and doping profile parameters.

As initial values of the doping profile were chosen pa­rameters of doping profile obtained without using dielec­tric: position of doping peak concentration 39 nm, doping profile width 71 nm, and activation coefficient 0.8 - 0.9. Compensation coefficient (K) was defmed as ratio deep impurity concentration to shallow impurity concentration; it was equal to K = 1. 5, 2. 5, 3, 4. At very low concentra­tion of deep acceptors (K < 1. 5) saturation current and threshold voltage significantly exceed the experimentally obtained values because of current flows through sub­strate. At very high concentration of deep acceptors (K >

4) saturation current and threshold voltage are considera­bly lower than the experimentally obtained values, which is explained by decrease in concentration and mobility of electrons in a channel.

A. Investigation of Effect of Doping Peak Position

In the first part of the study saturation current and thre­shold voltage dependences on doping peak position with different values of compensation coefficient and fixed values of doping width and activation coefficient were calculated. The peak position counts off the transistors surface (the surface, in which source, drain and gate are placed). The results obtained from the calculation of satu­ration current and threshold voltage are shown in Fig. 2a, where each curve corresponds to defmite compensation coefficient and each point on the curve corresponds to a defmite position of the peak profile. The initial position of the doping concentration peak was assumed to be 26 nm, which was equal to the doping peak position in case of implantation without dielectric film on the surface (39 nm) minus the dielectric layer thickness (15 nm) and mi­nus a correction shift inward of the substrate because of lesser run of implanted silicon ions in dielectric film than in GaAs because of lower scattering silicon ions by light silicon and oxygen atoms composing dielectric film by comparison with scattering at more heavy gallium and arsenic atoms composing gallium arsenide. The peak po­sition was shifted in depth of the substrate up to 36 nm with step 2 nm. Profile width and activation coefficient were taken to be equal to the doping profile parameters obtained without dielectric film: 68 nm and 0.8, respec-

SHESTAKOV et al: INVESTIGATION OF DOPING PROFILE EFFECT . . .

tively. As implanted impurity deepens, saturation current and threshold voltage increase.

.1.7

·2Jl

-K=1.S -K = 2.S

v K '" 3 '*K =4

200 220 240 260 280 300 320 340 360 380 hal.mAfmm

Fig. 2a Threshold voltage on saturation current with different doping peak position and different compensation coefficient (K). Peak distance from surface varied from 26 nm to 36 nm with step 2 nm. Compensation coefficient was equal to 1.5, 2. 5, 3, 4. Experimental values (Exp): satu-

ration current - 300 mA/mm, threshold voltage - 2.3 V.

It is seen from this Figure that there are three pairs of parameters, which are closest to experiment values: peak position of 26 nm and compensation coefficient of 1. 5; peak position of 30 nm and compensation coefficient of 2. 5; peak position of 32 nm and compensation coefficient of 3, that is deepening of doping profile and compensa­tion influence on saturation current and threshold voltage in a contrary way.

For calculation of relative alternation of saturation cur­rent and threshold voltage their relatively changing initial values (26 nm) were determined. Results are shown on Fig. 2b.

40

35

30

25 ·K=1.5

� 20 "K:2.5 � 9' K=3

15 +K"'4

10

o o 5 10 1S 20 2S 30 3S 40

dInt, %

Fig 2b. Value of relative changes in threshold voltage on relative changes in saturation current with variation of doping peak position and

different compensation coefficients (K). Peak distance from surface varied from 26 nm to 36 nm. Compensation coefficient was equal to 1.5,

2.5, 3 and 4.

It is seen from this Figure that for all compensation coefficients the saturation current increases slower than the threshold voltage (on 1-6%) with shifting of the dop­ing profile peak deeper into the substrate. Thus, output power of transistor increases slower than the efficiency with deepening of the doping profile peak.

B. Investigation of Effect of Doping Profile Width

In the next part of the study dependences of saturation current and threshold voltage on the doping profile width were calculated. With variation of the profile width value of maximum concentration in doping peak was kept con-

183

stant; therefore full concentration of implanted impurity changes with variation of the peak width. Values of peak position and activation coefficient were constant. Profile width was defmed on half of doping concentration peak height. The calculation results are shown in Fig. 3a. The initial value of doping profile width was equal to width of doping profile implanted without of dielectric film and was equal to 71 nm, after which it varied from 62 nm to 78 nm with an average step of 3 nm (doping width varied non-uniformly). The doping peak position was equal to 32 nm, activation coefficient was equal to 0.8, which also conforms to the doping profile parameters obtained with­out dielectric film. With increasing the doping profile width the saturation current and the threshold voltage increase.

-K-1.S ·2.8 +K -2.5 '" .. " K - 3 '"

·3.3 '*K -4

190 240 290 340 390 440

".t-mAl....." Fig. 3a. Threshold voltage on saturation current with different doping

width and different compensation coefficient (K). Doping profile width varied from 26 nm to 36 nm with average step 3 nm. Compensation

coefficient was equal to 1.5, 2.5, 3 and 4. Experimental values (Exp): saturation current 300 - mA/mm, threshold voltage - 2.3 V.

It is seen from this Figure that there are three pairs of parameters, which are closest to experiment values: dop­ing width of 62 nm and compensation coefficient of 1. 5; doping width of 68 nm and compensation coefficient of 2. 5; doping width of 32 nm and compensation coefficient of 3, that is widening of the doping profile and the com­pensation effect on saturation current and threshold vol­tage in a contrary way.

For calculation of relative changing in saturation cur­rent and threshold voltage their relatively changing initial values (62 nm) were determined. Results are shown in Fig. 3b.

90

90

10

90

30

20

10

o o g � � � � � H � �

dlut.%

_K*1.S "'K :0:2.5 v K =3

... K :0:4

Fig 3b. Value of relative changes in threshold voltage on relative changes in saturation current with variation of doping width and differ­ent compensation coefficients (K). Profile width varied from 68 nm to

78 nm. Compensation coefficient was equal to 1.5, 2.5, 3 and 4.

It is seen from this Figure that for all the compensation coefficients the saturation current increases slower than the threshold voltage with widening of the doping profile.

184 XI INTERNATIONAL CONFERENCE AND SEMINAR EDM'2010, SECTION II, JUNE 30 - JULY 4, ERLAOOL

Thus, the output power of transistor increases slower than its efficiency with widening of doping profile.

C. Investigation of Effect of Activation Coefficient

In next part of the study dependences of saturation cur­rent and threshold voltage on activation coefficient of implanted impurity were calculated. Doping peak position and doping width were constant parameters. The calcula­tion results are shown in Fig. 4a. Experimentally esti­mated activation coefficient was equal to 0.8 or 0.9. To check this estimation through the calculation the activa­tion coefficient varied from 0.8 to 1 with a step of 0.0 5. Peak position was equal to 32 nm, profile width was equal to 68 nm, which conforms to doping profile parameters implanted without dielectric film.

·2.2 ."�Exp

·2,4

" ·2.6

:> -2.8 > .,

·3.2

-3.4

240 290 340 390 ·440 lsat,mAlrrm

"K-1.5 '-K-2.S '9' K"'3 "'K-4

Fig. 4a Threshold voltage on saturation current with different activation coefficient and different compensation coefficient (K). Activation coef­

ficient varied from 0.8 to 1 with step 0.05. Compensation coefficient was equal to 1.5, 2.5, 3 and 4. Experimental values (Exp): saturation

current - 300 mA/mm, threshold voltage - 2.3 V.

35

3D

25

10

5 10 15 20 25 30 35 40 45 50 dlsat,%

.K=1.5 +K = 2.5 "K = 3

wK =4

Fig 4b. Value of relative changes in threshold voltage on relative changes in saturation current with variation of activation coefficient and

different compensation coefficients (K). Activation coefficient varied from 0.8 to 1. Compensation coefficient was equal to 1.5, 2.5, 3, 4.

It is seen from this Figure that there are two pairs of pa­rameters which are closest to experiment values: activa­tion coefficient of 0.8 and compensation coefficient of 2. 5; compensation coefficient of 0.8 and compensation coefficient on.

The results obtained from the calculation of relative changes in saturation current and threshold voltage are shown in Fig. 4b. The initial value of activation coeffi­cient was equal to 0.8.

In this case for different compensation coefficients the saturation current and the threshold voltage dependences on compensation coefficient variation are different, but for all the compensation coefficients saturation current increases faster than the threshold voltage. Thus, output

power can be significantly increased (up to 45 %) with lesser decreasing in efficiency (up to 30%)

V. CONCLUSION

The dependence of saturation current and threshold vol­tage of ion-implanted MESFET on doping profile para­meters of channel were found on the basis of the numeri­cal computer calculations. The following doping profile parameters ensured matching of calculated characteristics and experimental parameters of transistor were found on the basis of the calculation results: doping concentration peak position - 30 nm from surface, profile width - 68 nm, activation coefficient 0.8 for compensation coeffi­cient of 2. 5 and doping concentration peak position - 32 nm from surface, profile width - 68 nm, activation coeffi­cient 0.8 for compensation coefficient of 3. Thus, for dif­ferent compensation coefficients doping profiles were placed at different distances from the surface. Due to comparison of the calculated profile with the profile im­planted without dielectric mask, we can conclude that the profile implanted through dielectric film shifts closer to the surface of transistor and converged.

The investigation of relative changes in the saturation current and the threshold voltage show that the saturation current increases slower than the threshold voltage with deepening and widening of the doping profile; thus, the output power increases slower than the efficiency de­creases. But with increase in the activation coefficient the saturation current increases much faster than threshold voltage decreases. Therefore, with increasing this parame­ter the output power may be significantly increased at lesser reduction of efficiency.

REFERENCES

[1] B. L Seleznev, V. A. Dmitriev, A. P. Shtaingart, "Investigation of OaAs microwave transistors with Schottky barrier structures" Jour­nal of Nov go rod State University, N219, 2001 (in Russian).

[2] OaAs Fet Principles and Technology by DiLorenzo, Khande\wel, Artech House Publishers, 1982.

[3] A. K. Shestakov, K. S. Zhuravlev, V. A. Arykov, V. A. Kagadey "Impurities and doping influence on characteristics of OaAs field effect transistor with Schottky barrier", Internano-2009 (Interna­tional School and Seminar on Modem Problems of Nanoelectronics, Micro- and Nanosystem Technologies Proceedings), Novosibirsk, Russia - October 28-31, 2009.

[4] A. M. Bobreshov, A. V. Dyboy, Y. N. Nesterenko, Y. Y. Rasuvaev , "Bulk charge structure in OaAs MESFET on boundary between ac­tive layer and substrate doped by chromium", Vestnik YOU, Series Physics. Mathematics, 2008, N2 1, pp. 5-10 (in Russian).

[5] K. Blotekjaer, "Transport Equations for Electrons in Two-Valley Semiconductors," IEEE Transactions on Electron Devices, vol. ED-17, no. 1, pp.38-47, 1970.

[6] TCAD Sentaurus Manual Version C-2009.06.