A study of the profile of the E3 electron trap in GaAs

0038-1098/94 $6.00 + .00 Pergamon Press Ltd

Solid State Communications, Vol. 89, No. 1, pp. 45-49, 1994. Printed in Great Britain.

A STUDY OF THE PROFILE OF THE E3 ELECTRON TRAP IN GaAs* C.D. Kourkoutas, l B. Kovacs,2 P.C. Euthymiou, B. Szentpali, 2 K. Somogyi2 and G.E. Giakoumakis 3 Solid State Section, Physics Department, Athens University, Solonos 104, Athens 106 80, Greece l Department of Physics Chemistry and Material Technology of the TEl Athens, od Ag Spyridonos, Egaleo 122 43, Athens 122 43, Greece 2Research Institute for Technical Physics, P.O. Box 76, H-1325, Budapest, Hungary 3physics Department, University of Ioannina, P.O. Box 1186, Ioannina 451 10, Greece

(Received 14 December 1992 by D. Van Dyck) Electron irradiation at room temperature introduces in GaAs a donor type electronic state Tx at 0.18 eV, which is associated with the E3 electron trap. The presence of Tx is observed at depths d > 1.5#m, which correspond to the limits of the depletion region under the highest applied reverse bias voltage, while the E3 trap concentration drops off into the same region. determining the profile of this state up to 2/zm below the surface in association with the other defects introduced by electron irradiation.

1. INTRODUCTION AS IT IS known, fast electron irradiation creates several defects in GaAs which act as electron traps and affect thus its electric properties. At low tempertures the irradiation produces a family of stable defects, which are usually denoted as E traps [1], but other defects can be produced also, when the irradiation is performed at room temperature, or higher [2, 3]. In a recent work [4] GaAs MESFETs with 0.3 #m thick active layer were irradiated with various doses of 0.65MeV electrons. The results obtained by analysis of the scattering mechanisms show the increasing role of non-lattice scattering versus depth, i.e. charged centers and clusters, which are formed after the irradiation. Though the atomic displacements due to the electron irradiation are created homogeneously at large depths (up to 100#m below the surface), experimental results obtained from capacitance measurements indicate the existence of a nonuniform distribution in electron irradiated GaAs layers 5#m thick [5], and it is apparently shown that at depths corresponding to the substrate in usual FETs, the capacitance is affected by a donor type electronic state. In the present work our interest is focused on

2. EXPERIMENTAL PROCEDURE Schottky contacts of a Cr (70 nm)/Au (400 nm) layer were on n-type sulphur doped GaAs VPE layers 5 #m thick. The free electron concentration was 24E + 15 cm -3 and the drift mobility 6800 cm 2 Vs -1 at room temperature. The specimens were irradiated at room temperature with 0.72-2.40E+ 15 electrons cm -a doses of 0.65 MeV electrons. Capacitance and conductance measurements were performed at temperatures 190 to 300 K before and after irradiation and after heat treatment. Heat treatment was carried out by exposing the specimens for 5min in flowing forming gas (70% N2 + 30% H2). DLTS measurements were performed in irradiated and heat treated specimens. The experimental procedure for each specimen is summarized in Table 1. 3. RESULTS The apparent concentration N and conductivity tr profiles before and after irradiation are shown in Figs. 1(a) and 1(b). We observe that after irradiation the conductivity decreases in the whole measured region, while the concentration increases at depths greater than 1.5 #m. The temperature dependence of the measured concentration changes after irradiation as well.

* This work was supported by the National Committee for Technological Development, Department of the International Relations of Hungary and the Greek General Secretariat of Research and Technology. 45

46

A S T U D Y OF T H E P R O F I L E OF T H E E3 E L E C T R O N T R A P I N G a A s

Vol. 89, No. 1

Table 1

Sample

A5 Al A2 B1

Dose

Thermal treatment

(10E + 15 el cm 2)

200 °C

400 °C

0.72 1.32 1.60 2.40

Yes -

Yes Yes

Before irradiation the apparent concentration N at four temperatures fluctuates randomly around a mean value [Fig. 2(a)], while after irradiation it increases systematically, when the temperature is increased from 190 to 300 K [Fig. 2(b)]. As it is known, the electron concentration n t of a trap, which is lying in the upper half of the gap decreases, when a reverse bias voltage is applied, according to, n, = N r e x p ( - e n t ) ,

(1)

where NT is the trap concentration and e. is the emission rate of this trap given by, en = trn(Vn ) N c exp ( - E T / k T ) .

(2)

Here an is the cross section for capture of an electron, vn is the mean thermal velocity of an electron, Nc is the density of states in the conduction band and E r is the energy of the trap measured from the bottom of the conduction band. The results show that besides the two main electron traps E2 and E3 they are affected by an mtermediate donor type electronic state Tx. In our equipment the frequency of the small bias

DLTS

Yes Yes -

signal was 1 MHz. This is a low enough frequency for the shallow sulfur donors, so that all electrons occupying the donor level are practically immediately released, when the reverse bias voltage is applied. It is thus suggested, that before irradiation we measure the net donor concentration, which is constant m the temperature region 190-300K. On the contrary, the E3 trap, or any other deeper, is far from the bottom of the conduction band into the energy gap and has a capture cross section crn such that during each period of the reverse biasing only a negligible fraction of the trapped electrons can be released into the conduction band. On the other hand, the E2 trap has a very large cross section and releases almost all its trapped electrons during each cycle of the small bias signal. (The activation energy of E2 is 0.14eV and the capture cross section 1 . 2 E - 13cm 2. The corresponding values of E3 are 0.30eV and 6 . 2 E - 15cm 2 [1].) It is thus suggested that none of these traps can produce the effect observed in Fig. 2(b). Taking (1) into account we find that the measured concentration after irradiation is given by, N = N n - N T q- [1 -- exp (--enr)]NTx

(b)

Ca)

where N r

5

is the total deep trap concentration

(b)

(a)

4

35

35

30

(3)

-o

go

/"

•

O,~,-

f

/*

°~o

o , ~ O ~@ 0

2

z

2

25 0 I 1.0

I 1.5 d (IA.m)

I 2.0

I 1.0

I 1.5 d (tun)

I 2.0

Fig. 1. Sample A2. (a) Apparent concentration N profile and (b) conductivity tr profile: (©) before irradiation; (O) after irradiation.

25

L

200

I

250 T (K)

I 3OO

I

200

t

250 T (K)

I

300

Fig. 2. Sample A2. Temperature dependence of the measured concentration N at d = 1.40 #m: (a) before irradiation; (b) after irradiation. The continuous line through the experimental points is the theoretical curve from equation (3).

Vol. 89, No. 1

A STUDY OF THE PROFILE OF THE E3 ELECTRON TRAP IN GaAs

(a)

(b)

8

f-

-e-15

15 "7,

z

-0 01 10

-o- IO

47

=. -0.02

Z

O-~B • 0 J.O.~

05

05

o4r~e I 10

eOo l

--

"~ [ 15

I 10

I 20

d (p.m)

i 15

I 20

-0 03

t-~ -0 04

d (l~m)

Fig. 3. Introduction rate profiles: (a) donor state Tx; (b) deep traps E3 + E4 + E5.

-0 05

-0 06

N(E3 + E4 + E5), NTx is the concentration of the intermediate state Tx and r the time interval, which corresponds to the small reverse bias signal. A tentative estimation obtained from the best fit of equation (3) with the experimental points (N, T) yields ETx = 0.18eV and aTx = 1.2E - 15cm 2 for the activation energy and the capture cross section of the Tx. These values are close to those of the traps denoted J5, or 16 by Stievenard and Bourgoin [3], which were produced by 1 MeV electron irradiation at high temperature. For all four specimens the concentration NTx of Tx increases vs depth, while the total concentration NT of the deep traps decreases after 1.5 #m below the Schottky contact (Nr does not include E2). The profiles of the introduction rates NTx/~ and Nr/c~ are shown in Figs. 3(a) and 3(b). (q~ is the irradiation dose). We must notice that the presence of the electron state denoted here as Tx is clearly associated with the irradiation, at its concentration is constantly decreas-

I 200 T (K)

I 300

Fig. 5. Sample A5. DLTS signal corresponding to E3 electron trap: (A) reverse bias 5V; (B) reverse bias 9.5V. ing after heat treatment at 200°C and 400°C. However, the profile remains the same (Fig. 4). The different heights of the DLTS signals of the E3 trap at different bias voltage [ - 5 V , - 9 . 5 V (Fig. 5)] correspond with different depletion lengths of 1.3/zm and 1.8 #m respectively. Making use of the conductivity data we attempted to calculate the concentrations of E2 and E3 which are of interest from the point of view that they concern the active channel instead of the space charge region. Knowing the donor concentration No before irradiation we can calculate the free electron concentration, which in combination with the appropriate mobility #, will result in the conductivity cr after irradiaiton. The mobility was calculated as a function of the Fermi level position [6] and the ionized centers. In our approximation we considered all traps

I

,'p

3.0-

/

2 "7, E ] "7, E 0

$ /

,Do 0 50

~o ~

/

x-

x

~1

[ 10 15 d (Ixm)

0

I 20

Fig. 4. Sample A2. Concentration profile of Tx: (0) after irradiation; (©) after heat treatment at 200°C for 5min; ( + ) after additional heat treatment at 400°C for 5 min.

x...x ~ X - X o0.0.

x~

xf

0"

,t~+ .+ •

-/

b

e~ ~x"

-

+1 200

I 250

I 300

x (K) Fig. 6. Sample B1. Conductivity a as function of temperature T at various depths: (x) 1.8/zm; (©) 1.6/zm; (O) 1.4#m; ( + ) 0.8 #m. The continuous lines through the experimental points are obtained from the theoretical fit.

48

A STUDY O F T H E P R O F I L E O F T H E E3 E L E C T R O N T R A P IN GaAs 50 30 ~20

10 v ~ Z

-6- e D O0-~LDM-O

05

02

K 10

1

I 15 d

,20

O,m)

Fig. 7. Sample BI. Concentration profiles N of: (O) E2; ( e ) E3. except E2 being occupied by electrons as the Fermi level is above them at any temperature. The occupancy of E2 was calculated from, f=

I/{1 + K e x P l ( E r - EF)/kT]}

(4)

with IErl = 0 . 1 4 e V as the commonly accepted actwation energy for E2. As in [7] the degeneracy factor was set at K = 2 in spite of the fact that E2 is a donor type trap [1,8], but this value is in accordance with the results obtained by Look [9]. The results of the fits are shown in Fig. 6. The calculated cncentrations of E2 and E3 are shown m Fig. 7. The concentration of E2 is as expected homogeneously distributed versus depth. The decrease of the E3 concentration after 1.5 #m below the surface confirms the results shown in Figs. 3(b) and 5. 4. DISCUSSION The experimental results show that the state Tx is associated with one of the deeper levels E3, E4 and E5. Because of the magnitude of the related effects this trap must be E3, as E4 and E5 have a rather small introduction rate. It is beheved [!] that the E traps are pairs of the

Oo

Acknowledgements - We are greatly indebted to Dr P.C. Banbury of the University of Reading (England) for many valuable discussions. We wish also to thank Mr Alan Holman for the assistance in the use of electron accelerator of the J.J. Thomson Physical Laboratory of Reading University.

REFERENCES I. 2.

4.

O0 o

~

form VAs + As, of the As sublattice with As, being the mobile defect. In the case of E3 the distance between the two components of the pair is of the order of few neighbours. It has been shown experimentally [10] that the E3 trap is stable into the space charge region. In our case the Tx concentration is significantly lower near the surface, where the electric field strength is always larger. At the same time the E3 concentration is found to be constant within this region. Tx thus exhibits characteristics similar to those of an unstable state, which is produced by electron irradiation, but tends to be transformed into E3 within the strong electric field of the space charge region. It appears therefore near the limits of the depletion region, which corresponds to the highest applied reverse bias voltage. The nature of Tx is not known, except that it behaves like a donor. Summing the introduction rates of Tx and E ~ ( j = 3,4,5) we find that the total introduction rate v profile is much closer to the expected homogeneous distribution, if Tx is accepted as double donor (Fig. 8). The average introduction rate is then 0.80 cm-l + 6%, whde the corresponding value obtained from [!1] for 0.65MeV electron irradiauon ~s 0.6 cm -~ .

3.

15

0 o

I0

0000000000

5. >

05

I 10 d

I 15

I 20

(p.m)

Fig. 8. Total introduction rate v of Tx and E3 + E4 + E5 vs depth: (©) Tx accepted as single donor; ( e ) Tx accepted as double donor.

Vol. 89, No. 1

6. 7. 8.

D. Pons & J.C. Bourgom, J. Phys. C." Solid State Phys. 18, 3839 (1985). D. Stievenard, J.C. Bourgoin & D. Pons, Physica I16B, 394 (1983). D. Stievenard & J.C. Bourgom, J. Appl. Phys. 59, 743 (1986). B. Szentpali, B. Kovacs, D. Huber, C.D. Kourkoutas, P.C. Euthymiou & G.E. Zardas, Solid State Commun. 80, 321 (1991) C.D. Kourkoutas, B. Kovacs, P.C. Euthymtou, B. Szentpali, K. Somogyi, P.C. Banbury & G.E. Zardas, Phys. Status Solidi (a) 135, K21 (1993). C.D. Kourkoutas, P.C. Euthymiou & G.J. Papaioannou, Solid State Commun. 74, 999 (1990). M. Yamaguchl & C. Uemura, J. Appl. Phys. 157, 604 (1985). D.C. Look, Solid State Commun. 64, 805 (1987).

Vol. 89, No. 1 9. 10.

A STUDY OF THE PROFILE OF THE E3 ELECTRON TRAP IN GaAs

D.C. Look, Electrical Characterization of GaAs Materials and Devices, Chapter 1 p. 130 Wiley, New York . (1989). D. Pons, Defects and Radiation Effects in

ll.

49

Semiconductors, Chapter 6, p. 269, Inst. Phys. Conf. Serv. 59 (1981). D. Ports, P.M. Mooney & J.C. Bourgoin, J. Appl. Phys. 51, 2038 (1980).