The donor nature of the main electron traps in electron-irradiated n-type GaAs

Solid State Communications, Vol. 64, No. 5, pp. 805-807, 1987. Printed in Great Britain.

0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd.

T H E D O N O R N A T U R E O F T H E MAIN E L E C T R O N TRAPS IN E L E C T R O N - I R R A D I A T E D n-TYPE GaAs D.C. Look University Research Center, Wright State University, Dayton, OH 45435, USA

(Received 9 February 1987 by J. Tauc) Of all the electron (E) and hole (H) traps measured by DLTS in 1 MeV electron-irradiated GaAs, two of them, E1 and E2, have much higher production rates (,-~ 2cm -~) than any of the others. By carefully analyzing nine samples which exhibit the type-conversion phenomenon, we show that the net production rate of below-mid-gap acceptors over above-mid-gap donors in n-type samples is ~'net ~ rA -- ~o ~---0.4 + 0. l cm -~. This number is consistent with recent temperature-dependent Hall-effect data only if E1 is a donor. We then show that other evidence also strongly favors E2 as a donor, while E3 remains uncertain. The donor nature of El and E2 is much more consistent with their identification as As-vacancy related than are other proposed models.

T H E POINT D E F E C T S C R E A T E D by 1 MeV electron irradiation in GaAs have been studied extensively by many characterization techniques, especially deeplevel transient spectroscopy (DLTS) in the last decade [1, 2]. The DLTS results may be summarized as follows [2]: in n-type material, five primary electron traps, El, E2, E3, E4 and E5, are produced at approximate rates of 1.5, 1.5, 0.4, 0.1 and 0.1 cm ~, respectively; and in p-type material, two primary hole traps, H0 and H1, are produced at rates of 0.8 and 0.1 cm-~, respectively. A quick glance at these results shows that E1 and E2 are dominant, and that if both are donors, the rest of the traps combined, even if all acceptors, could not explain why the Fermi level drops well below E 2 = Ec - 0.15 eV as n-type material is irradiated. Explanations generally take one of two forms: (1) either or both E1 and E2 are acceptors [3]; or (2) H0 and/or H1 must be acceptors and must be produced at a much higher rate in n-type material than in p-type material [2, 4]. Generally explanation (1) is favored, for various reasons, and thus many models are based on the acceptor nature of El and/or E2. However, another problem then arises, namely, that E1 and E2 appear to be associated with the arsenic vacancy [2], which is expected, on intuitive as well as strong theoretical grounds [5], to be a donor. This puzzle has been partially resolved from a recent, extensive temperature-dependent Hall-effect (TDH) study [6]. In this study, it was shown that two centres, C2 and C3, are produced at rates of 2.0 and 0.5 ± 0.2cm -~, with energies o f E c - 0.148 and

Ec - 0.295 _+_ 0.002 eV, respectively, in good agreement with the DLTS data for the centres E2 and E3. However, the T D H study also clearly showed that "'shallow" acceptors (i.e., below E3), are being produced at a very high rate, 4 + I cm -~, with the uncertainty in this rate including all possibilities for the donor/acceptor (D/A) natures of El, E2 and E3. This result is significant in that the Hall-effect (Fermi level) data are now entirely explained without invoking an acceptor nature for E1 and/or E2. However, the T D H study, by itself, does not specifically prohibit the assignment of such acceptor nature. In this study we use the type-conversion phenomenon [7, 8] to show that El is a donor, and then argue that other data strongly suggest that E2 is also a donor. We cannot judge E3 on the basis of the evidence here. It was first shown by Dresner [7] that the Fermi level (~F) not only drops in electron-irradiated n-type GaAs, but that it can drop below mid-gap, rendering the sample p-type. However, this sample exhibited a very low hole mobility and was interpreted as being highly inhomogeneous. Later, Farmer and Look [8] demonstrated on the basis of several, pure, vaporphase epitaxial (VPE) layers that high-mobility n-type GaAs could indeed be converted to high-mobility p-type GaAs. An example is shown in Fig. l, and it is seen that as soon as eFdrops to near the valence band, it immediately begins to rise again, an identical behavior to that of samples which begin as p-type. This phenomenon will be discussed more fully elsewhere, but here we will simply exploit the fluence at

805

806

E L E C T R O N - I R R A D I A T E D n-TYPE G a A s

lO ! 08

-z

pe 06

04

02

1014

I

I

I I 10 '5

I

I

I

I I 1016

I

I

I

I 1017

~(e,,crn 2)

Fig. I. The Fermi energy as a function of fluence in an n-type VPE G a A s sample. Parts of these data previously appeared in [8]. which the rapid drop takes place to calculate a net production rate, tnc, -- tA -- tD. The VPE GaAs sample shown in Fig. 1 was 21 #m thick, and thus was well suited for both uniform damage and small depletion corrections. The initial 296 K carrier concentration was no -~ 3.6 × 10~4cm 3, and in this case, no = N~ - N A , where N o is the shallow donor concentration (probably Si), and N A is the total acceptor concentration, mostly below mid-gap. The 1MeV irradiation produces both donors and acceptors, but evidently the acceptor production rate, tA, is higher, because ~:r is always observed to drop in n-type GaAs. In fact, the acceptors must mostly lie below Ev + 0.5 eV since ~;F eventually approaches that value, according to Fig. 1. The net production rate of acceptors over donors, rnc, =- rA - r~, causes ~F to precipitously drop below midgap at a critical fluence 05, such that 05, ~- no/r,,. In Fig. l, it is seen that 05~ _~ 8 × 10~4ecm2, s o t h a t t,e~ ~ 3.6 × 10~4/8 × 10 14 ~ 0.4cm i. We have also examined eight other samples, with different n0's, and then calculated the expected 05,'s from the

Vol. 64, No. 5

relationship 05, " n0/0.4. Experimentally, it is difficult to find the exact point at which each sample changes from n-type to p-type, because our data are limited in several cases, and also because the electrical properties and generally quite inhomogeneous in the transition region. In view of these facts, we have chosen to illustrate the accuracy of the calculated 05,'s by showing that the carrier concentration has dropped by several orders of magnitude at 0 = 05, in every case, but has not dropped greatly at 05 = 05,/2. These results are presented in Table 1, in which samples covering an no range of nearly two orders of magnitude can all be well characterized by the relationship 05, = n0/0.4, or conversely, tnct "" 0.4 + 0.1 cm -L. At this point, we know simply that the, -- tA -- rO -----0.4cm ~, but we do not know the individual values tA and rD. For this information, we apply the results of a recent T D H study [6] which related to and tA to various D/A choices for the centres El, E2, and E3. As seen in Table 2, only four of the eight possible D/A choices are consistent with the results of the present type-conversion study. Each of these four choices requires El to be a donor. We will now show that independent evidence strongly suggests that E2 is also a donor. First of all, the E2 electron capture cross section is large [2], about 1 × 10 ~3cm 2, which indicates that the electron is likely being captured by a positively charged (donor) ion. Secondly, the production rates of E1 and E2 are always equal, or nearly so, which is generally taken as evidence that they are two charge states of the same center [2]. If so, then the shallower state, E l, could not be a donor while E2 is an acceptor; the states would have to be reversed, Thirdly, there is strong evidence that El and E2 are both associated with the As vacancy [2]. This defect would intuitively be expected to be a double or triple donor. Thus, the evidence highly favors both El and E2 being donors, probably, in fact, the ( 0 / + ) and ( + / + + ) states, respectively, of the As vacancy. This model would further suggest that

Table 1. Verification of the critical fluenee formula, 05,. = n0/0.4 Sample

no (1 0 14c m - 3)

(9, (cm ~1)

n (05,/2)/n0

A

16.2

40

B

3.6

9

C D E F G H I

3.5 2.5 1.9 1.7 1.6 0.9 0.23

9 6 5 4 4 2 0.5

0.6 0.1 0.03 0.3 0.5 0.05 (not meas.) 0.01 0.4

n (05c),/no 10 - 6

10 _7 10 7 10 3 10 _7

10 5 10 7 10 -~ 10 2

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E L E C T R O N - I R R A D I A T E D n-TYPE G a A s

807

Table 2. Net production rate, r,et - r A - rD, f ° r various D/A choices of defect centers El, E2, and E3. The values o f rA and ~ are calculated from data in [6]. The units for ~ are cm i El

E2

E3

ro

rA

r,ct

Consistent with rnct = 0.4cm 1?

D A D A D A D A

D D D D A A A A

D D A A D D A A

4.5 2.5 4.0 2.0 2.5 0.5 2.0 0.0

5.4 4.5 4.5 3.6 3.5 2.6 2.7 2.1 2.7

0.9 + 0.5 2.0 0.5 1.6 1.0 2.1 0.7 2.1-2.7

yes no yes no yes no yes no

the El capture cross section should be much smaller than that of E2, and this indeed is the case [2]. Finally, we note that the assignment of donor nature to El and E2 allows the shallow (below E3) acceptor production rate to be fixed at rA = 5.0 + 0.5cm ~ (see Table 2), a more accurate value than the 4 _+ I cm ~figure given in [6]. It should further be pointed out that for the case of centers with multiple ionization states, as e.g., E1 and E2 appear to be, the r's must not be added together in determining the production rate of the centre in question. However, the r's are still added together to reflect the total number of electrons a donor can provide, or an acceptor can trap. In conclusion, we have shown that the results of two separate studies require El to be a donor, and other evidence then strongly indicates that E2 is also a donor. A model consistent with almost all available data is that E1 and E2 are the ( 0 / + ) and ( + / + + ) states of the As vacancy. If so, then As-vacancy donor electrons are produced at a rate of at about 4 cm ~ in 1 MeV-electron-irradiated, n-type GaAs [6]. Furthermore, acceptors (or electron-trapping states) below E3 are produced at a rate of about 5.0 cm 1. The possible

identification of these acceptors will be discussed separately. Acknowledgements - - This work was performed at the Avionics Laboratory, Wright-Patterson Air Force Base, under contract F33615-86-C-1062. We wish to thank P. Schwenke for manuscript preparation. REFERENCES !. 2. 3. 4. 5. 6. 7. 8.

D.V. Lang, Radiation Effects in Semiconductors, Inst. Phys. Conf. Ser. 31, 70 (1977). Other references cited therein. D. P o n s & J.C. Bourgoin, J. Phys. C: Solid State Phys. 18, 3839 (1985). Other references cited therein. S. Loualiche, A. Nouailhat, G. Guillot & M. Lannoo, Phys. Rev. B30, 5822 (1984). D. Stievenard, X. Boddaert & J.C. Bourgoin, Phys. Rev. B34, 4048 (1986). G.A. Baraff & M. Schluter, Phys. Rev. Lett. 55, 1327 (1985). D.C. Look, J. Appl. Phys. (in press). J. Dresner, J. Appl. Phys. 45, 4118 (1974). J.W. Farmer & D.C. Look, J. Appl. Phys. 50, 2970 (1979).