Characteristics of near-band-edge absorption processes in bulk GaAs

ARTICLE IN PRESS

Physica E 25 (2004) 55–63 www.elsevier.com/locate/physe

Characteristics of near-band-edge absorption processes in bulk GaAs S. Tu¨zemen, T. Yıldırım Department of Physics, Faculty of Arts and Sciences, Atatu¨rk University, Erzurum 25240, Turkey Received 31 March 2004; accepted 10 June 2004 Available online 25 August 2004

Abstract Some important characteristics of near band-edge absorption and photo-current were investigated in both n-type and semi-insulating GaAs. A remarkable absorption process peaking at 1.499 meV (at 10 K) with full-width at halfmaximum of about 4 meV was extracted from the absorption spectra of both materials. Parellel photo-current and absorption spectra extends from 1.495 eV towards the band-edge at sample temperature of 10 K. As the sample temperature is increased, this extension shifts towards the band-edge and disappears at just above 100 K. Extrapolation of E g 2E max versus temperature plots show that this absorption level becomes degenerate with the conduction band at sample temperature just above 200 K. We propose that all the absorption is associated with ionization and that a model of photo-ionization of a deep level near the conduction band by an electron (or electrons) from the valance band may describe the observed phenomena. r 2004 Elsevier B.V. All rights reserved.

1. Introduction Since the introduction of semi-insulating (SI) GaAs to the production of high speed digital and microwave devices, there has been concern regarding the uniformity of subsrate wafers. One of experimental techniques used to assess uniformity, 1000 nm wavelength near midgap optical absorption imaging has become very popular because it is inherently non-destructive [1]. Such imaging usually maps different concentrations of the deep donor Corresponding author. Fax: +90-442-236-0948.

E-mail address: [email protected] (S. Tu¨zemen).

level EL2 in either of its charge states and this data relates directly to maps of the resisitivity in SI GaAs [2]. Further investigations revealed that extra absorption was present at energies within 50 meV of the bandedge in GaAs that was cooled below 120 K [3–5]. Maps of this absorption, using strong monochromatic light and samples held in cryostat, was reversed compared to the EL2 images. Therefore, the images were called ‘‘Reverse Contrast (RC)’’ images. Interest in RC images was increased when it was presented that they showed good agreement with low temperature luminescence maps [6–9]. This agreement was emphasized in ingot-annealed

1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.06.042

ARTICLE IN PRESS S. Tu¨zemen, T. Yıldırım / Physica E 25 (2004) 55–63

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material where, although the EL2 concentrations are rendered nearly uniform, the RC and luminescence images remained strongly non-uniform. More recent work on the RC images has demonstrated that the spectral form of the absorption and the ability of the defects that produce the absorption to be ‘‘photo-quenched’’, i.e. photoexcited into a meta-stable state where the absorption is (reversibly) destroyed [10,11]. Moreover positron annihilation measurements have shown that the near band-edge absorption is accompanied by increased positron trapping, probably at As vacancies [12], and that this can be photoquenched in a similar way with the absorption [13]. Until now, the spectral form of the photocurrent and absorption has not been investigated in detail. In this paper, we demostrate that all the absorption is associated with ionization and that a model of photo-ionization of a deep level near the conduction band edge by an electron (or electrons) from the valance band can describe the observed phenomena.

2. Experimental details SI- and Te-doped n-type GaAs samples grown by the Liquid Encapsulated Czochralski (LEC) technique were cleaned and polished on both surfaces. In order to observe absorption spectra, the samples were placed on the cold finger of a closed cycled helium cryostat with an optical tail and cooled to 10 K. Absorbance values as a function of incident light energy or wavelength were recorded using an IR sensitive Perkin Elmer spectrometer with a wavelength resolution better than 2 nm. Absorbance data were converted to absorption coefficient values using the following conventional expression: ð1  RÞ2 expðatÞ 1  R2 expð2atÞ ¼ expðAÞ;

T¼

ð1Þ

where T is the transmission, R the reflectivity, a the absorption coefficient, t the sample thickness, and A the observed absorbance value at a certain wavelength. Reflection coefficient R is determined

from the fact that the absorption coefficient a in the midgap region becomes absolute zero after the entire photo-quenching of the EL2 centers. In this case Eq (1) can be written as ð1  RÞ2 1  R2 ¼ expðAÞ

T¼

ð2Þ

and reflectivity R is calculated to be 31–38% depending on the surface conditions of the samples. Photo-quenching experiments were performed at two different wavelengths of 1000 and 830 nm at low sample temperatures. Near band-edge photoquenching at 830 nm were performed after the entire bleaching of EL2 absorption. For EL2 photo-quenching, a 1000 nm incident light from the spectrometer corresponding to the midgap energy was used which is supposed to be efficient in EL2 photo-quenching [14,15]. For RC photoquenching, an 830 nm wavelength incident light corresponding to the near bandedge absorption was used which is supposed to be efficient for RC photo-quenching [13]. Although the choice of two wavelengths of 830 and 1000 nm does not sufficiently restrict the range of possible photoquenching events, as can be seen in Fig 1 of Ref. [13], the EL2 photo-quenching efficiency is nearly zero at 830 nm and the RC photo-quenching efficiency is about 10 times less than that of EL2 at 1000 nm wavelength. On the other hand, because the RC photo-quenching is performed after the entire photo-quenching of EL2 centers, the EL2 absorption has no additional effect on RC absorption bleached during the latter photoquenching. EL2 and RC concentration measurements were made using a technique based on the absorption coefficient difference before and after photoquenching of these defects. Because there exists EL2 absorption tail at wavelengths close to the band-edge, the RC absorption measurements were made after the entire photo-quenching of EL2 centers. In the case of EL2 absorption coefficient measurements, there is no such conflict, because the RC absorption does not extend towards the midgap. EL2 and RC concentrations were deter-

ARTICLE IN PRESS S. Tu¨zemen, T. Yıldırım / Physica E 25 (2004) 55–63 1.4

λ=1000 nm T=15 K n-GaAs

1.2

α (cm-1)

1 0.8 0.6 0.4 0.2 0 0

100

(a)

200

300 400 Time (min)

500

600

57

independent detector replaced with the sample. During the photo-current measurements, applied voltage was kept constant at about 500 V and current recorded using a Kiethley picoammeter. In SI material no remarkable current was observed in the dark because of its high resistivity (approx 1  107 O-cm). In n-type material because of higher carrier concentration, the current remains constant between the dark and under illumination cases and there is no observable photo-current in n-type material.

3 λ=835 nm, T=15 K n-GaAs

2.5

3. Experimental results As demonstrated in Figs. 1 and 2 for both ntype and SI materials, EL2 and RC concentrations calculated from the photo-quenching results at, respectively l ¼ 1000 and 835 nm, are shown in Table 1, using previously driven calibrations

-1

α (cm )

2 1.5 1 0.5 0

2.5

(b)

100

200

300 Time (min.)

400

500

Fig. 1. Photo-quenching of (a) EL2 and (b) RC absorption at, respectively 1000 and 830 nm wavelength light incident on nGaAs at 15 K.

2 α (cm-1)

0

λ =1000 nm Τ=15 Κ SI-GaAs

1.5 1 0.5 0 0

100

(a)

200 Time (min.)

300

400

4.5

λ=835 nm T=15 K SI-GaAs

4 3.5 3

α (cm-1)

mined using respectively Martin’s [16] and Tu¨zemen’s [17] calibration plots, respectively. For the photo-current measurements, ohmic contacts were formed on the corners of SI GaAs sample by indium evaporation in a vacuum of 105 Torr. Indium dot contacts were sintered at 400 1C for 3 min under nitrogen gas flow. The light was incident on the front of the sample, at appropriate wavelengths. Simultaneous photocurrent and absorption measurements were made on the area between the two point contacts of the sample at 80 K at which temperature the characteristic peak is still observable. Due to the carrier freeze out, the photo-current experiments cannot be performed at temperatures below about 70 K. In a preliminary experiment, it was shown that the intensity of incident light coming from the spectrometer remains approximately constant in the near band edge wavelength range using a wavelength

2.5 2 1.5 1 0.5 0 0

(b)

50

100

150

200

250

Time (min.)

Fig. 2. Photo-quenching of (a) EL2 and (b) RC absorption at, respectively 1000 and 830 nm wavelength light incident on SIGaAs at 15 K.

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Table 1 Some basic values for SI and n-type samples Sample

mH (cm2/V s) at 300 K

mH (cm2/V s) at 10 K

n (cm3) at 300 K

n (cm3) at 10 K

[EL2] (  1016 cm3)

[RC] (  1014 cm3)

n-GaAs SI-GaAs

4900a 7500b

200a —

8.5  1014a o105b

5.5  1013a —

1.5 2.7

1.0 1.2

a

As measured in Ref. [21]. As provided by the manufacturer.

b

25

SI-GaAs at 10 K

α (cm-1)

20

15

Plot1

Plot2

10

5

Plot3(x10)

0 1.490

1.495

(a)

1.500

1.505

Energy (eV) 16

n-GaAs at 10 K

14 12 10

α (cm-1)

[16,17]. Although, as shown by Kretzer and Alt [34] that absorption coefficient at a wavelength of 1.0 mm differs from the expected values of EL2 absorption for the samples with high acceptor concentrations in the order of 1016 cm3, the samples we use in this study has low acceptor concentrations of about 1014 cm3 so that such effect does not interfere with our results. A typical near band-edge absorption spectra of SI sample is shown in Fig. 3(a) as plot 1 at sample temperature of 10 K and is identical for the n-type sample as seen in Fig. 3(b). Fitting the data in plot (1) except for the peak region into an appropriate secondorder polynomial as a function of photon energy gives plot (2) in Fig. 3 with a total square error less than 5%. This also shows that near band-edge tail of GaAs generally obeys square law except for an additional characteristic absorption. Subtracting the data of plot (2) from the data of plot (1), it results in a characteristic absorption peak as shown in plot (3). The peak starts at a photon energy of 1.494 eV which is approximately 30 meV less than the bandgap energy and peaks at 1.499 eV at a sample temperature of 10 K. Similar peak values are also observed for n-type material as shown in Fig. 3(b). Near band-edge absorption measurements were repeated by 10 K intervals up to higher sample temperatures at which the bump over the spectrum disappears. It was shown that this characteristic bump is disappeared in both samples at temperatures above 100 K. As the temperature is increased the intensity of the near band-edge absorption decreases and the peak energy shifts towards the band gap energy. A typical absorption spectrum at a sample temperature of 150 K is shown in Fig. 4. As seen in this plot overall spectrum seems to have no character-

8

Plot1

Plot2

6 4 2 0 1.490

(b)

Plot3(x10)

1.495

1.500

1.505

Energy (eV) n-GaAs

Fig. 3. Typical absorption spectra of (a) semi-insulating and (b) n-type GaAs at 10 K.

istic absorption, i.e. the bump cannot be observed at this sample temperature. Figs. 5 (a) and (b) shows the plots of E g and E max as functions of temperature in SI and

ARTICLE IN PRESS S. Tu¨zemen, T. Yıldırım / Physica E 25 (2004) 55–63 0.010

40 n-GaAs T=150 K

35

0.009 0.008

30

0.007 Eg-Emax (eV)

α (cm-1)

59

25 20

0.006 0.005 0.004

15

0.003

10

0.002 0.001

5

0.000 0

0 1.45

1.455

1.46

1.465

1.47

Energy (eV)

50

100

150

200

250

Temperature (K) SI-GaAs

(a) 0.006

Fig. 4. Typical absorption spectra of n-type GaAs at 150 K.

1.510

1.510

1.505

1.505

1.500

1.500

1.495

1.495

1.490

1.490

1.485

1.485

1.480

1.480 10

20

30

Emax (eV)

(a)

40 50 60 70 Temperature (K) SI-GaAs

80

1.505

1.500

1.500

1.495

1.495

1.490

1.490

1.485

1.485

30

40 50 60 70 Temperature (K) n-GaAs

80

0.002 0.001

0

(b)

1.480 20

0.003

30

60

90 120 Temperature (K) n-GaAs

150

180

210

Fig. 6. The plots of E g and E max versus temperature for (a) semi-insulating and (b) n-type GaAs.

1.505

10

0.004

0.000

90 100

1.480

(b)

Eg-Emax (eV)

1.515

Eg (eV)

1.515

Eg (eV)

Emax (eV)

0.005

90

Fig. 5. The plots of E g and E max as functions of temperatures in (a) semi-insulating and (b) n-type materials.

n-type material, respectively. Both values decrease as the temperature is increased. However, as seen in these figures E max values get

closer to E g as the temperature is increased. We assume that E g and E max becomes equal at a certain temperature. We plotted E g 2E max values as a function of temperature as seen in Figs. 6 (a) and (b). This also decreases with increasing sample temperature and shows a linear behavior. Extrapolations of these plots show that E g 2E max reaches to zero at sample temperature around 200 K. Simultaneous measurements of absorption coefficient and photo-current at sample temperature of 80 K are illustrated in Fig. 7. As seen in this figure, both data accompany each other very well. It follows that both phenomena are due to electron transition of the same kind at the near band-edge.

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60

80 T=80 K SI-GaAs

Current Alpha

70

30

30

20

20

10

10

0

0 49

50

49 1.

1.

1.

50

50 1.

1.

51

51 1.

1.

2 1. 48 9 1. 48 5 1. 48 2 1. 47 8 1.4 75 1. 47 1 1. 46 8 1. 46 4 1. 46 1

40

6

40

0

50

3

50

7

60

0

60

4

α (cm-1)

70

Current (nA)

80

Energy (eV)

Fig. 7. Near band-edge spectral distribution of absorption coefficient and photo-current in SI GaAs.

4. Discussion 4.1. Direct absorption process An absorption band similar to the EL2 intracentre transition band is observed at the near band edge of both SI and n-type material. However this is much less broader than that of EL2 and can only be observed at sample temperatures below 120 K. The FWHM of the band is about 4 meV which is less than 11 meV of photon energy intervals in GaAs at sample temperature of 4.2 K (see Ref. [14]). Therefore we assume that the band cannot include any phonon interaction. This direct transition peaks at 1.499 eV at sample temperature of 10 K. Unless this is an intra-center transition, two things are possible, one is a transition from the valance band to a level close to the conduction band-edge, the other is a transition from the level close to the valance band-edge to the conduction band. Both of these possibilities should be accompanied with a photo-current unlike the case of intra-center transition. As observed in Fig. 7, the absorption data well accompanies with the photo-current data measured for SI sample at 80 K. Therefore we propose that this is not an intra-center transition and discuss the other two possibilities.

Considering this is a transition from a level close to the conduction band-edge, this would be from carbon acceptor which is situated at 27 meV above the valance band-edge [18]. If this was the case this should have been observed at all sample temperatures and should have resulted in an observable photo-current in n-type material too, since its higher quantum efficiency has been observed by PL measurements [19,20]. However such photocurrent can only be observed in SI material. We propose that one possibility is a transition from the valence band to the level near the conduction band. Such a transition would result in p-type conductivity and cannot be observed in n-type material. Because the level gets degenerate with the conduction band as the band gap energy gets narrower by increasing temperatures. This transition may not be observed at higher sample temperatures. Such behavior has been observed by many authors using different experimental techniques [3,4,22–24]. This was called RC by some of these authors who are commonly agreed that this is a point defect related to V As . Especially the authors using positron annihilation technique that is directly sensitive to the vacancy type defects comes out with such conclusions [12,24,25]. One can speculate that this absorption band is due to the chemical defects such as carbon and oxygen that are unintentionally contaminated in the order of 1015–1016 cm3. However no signs of such defects in this wavelength range are observed in the literature and that they do not have photoquenching properties. Most of the data observed in the literaure show that this absorption in this wavelength range has one to one correlation with a native defect related to V As [12]. In the early papers, oxygen was thought to be the main electron trap instead of EL2 and has negative-U behavior. However it was proved that oxygen has no effect on compensation mechanism of SI GaAs [26]. 4.2. Association with other experimental results 4.2.1. Positron annihilation Illumination of SI GaAs samples with near band-edge light (1.32 and 1.42 eV ) shows that the vacancy related defects become observable by

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positron annihilation measurements [12]. These defects cannot be observed in the dark. There is a strong evidence that the defect exist in a more negative charge state under illumination. This is quite in agreement with the proposal that the absorption occurs by a direct transition of an electron (or electrons) from the valance band into the defect center. The fact that the defect cannot be observed in dark by positron annihilation may indicate that the defect is in positive charge state in SI material in detailed balance. In this case, the RC centers have donated an electron in SI material. On the other hand Corbel’s positron annihilation experiments [12,24,27] in n-type material show two transition levels (/0 and 0/+) of the arsenic vacancy situated at approximately 30 and 140 meV below the conduction band minimum, respectively. The first transition from neutral to the negative charge state should correspond to the direct transition peaking at 1.499 eV. This means that the defect level is in neutral charge state in ntype material and transfers to negative charge state by 1.499 eV absorption process. If this is the case, the level has accepted an electron in n-type material in detailed balance. This must be the reason why it releases an electron (or electrons) into the conduction band after photo-quenching which was shown by Tu¨zemen and Brozel [17] as the only electrical activity associated with the RC defect observed only in very lightly n-type material in which the concentrations of RC centers are comparable to the free carrier concentrations in the order of 1014 cm3 (see also Ref. [21]). From the discussion invoked in the above two paragraphs, we have come to the following situation: In SI material the RC centers are in the positive charge state and behaves as an acceptor. The fact that the absorption band is quite similar in both materials launches the following question; why are they exhibiting the same transition as they are in different charge states in the n-type and SI materials? We propose the following explanation to this conflict in the light of the positron annihilation data observed by Corbel et al. [27] and Le Berr et al. [12]. Positron lifetime for negative charge state in n-type material is 257 ps and is quite different from that for

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neutral charge state (295 ps). Positron lifetime for negative charge state (257 ps) is quite comparable with the positron life times (254–263 ps with an average 257 ps) observed in SI material under near band-edge illumination conditions [12]. This means that the vacancyrelated defect which was found to be in one to one correlation with the RC absorption in SI material is in the negative charge state under illumination conditions. Since the absorption measurements have to be performed under illumination conditions, it means that the RC centers are in the same charge state in both materials when subject to absorption measurements. This is to propose that the RC centers are in the positive charge state in SI material in dark and situated at 140 meV below the conduction bandedge. However, when performing absorption measurements due to the simultaneous illumination of the incident light, they become neutral in the quasi-equilibrium condition. This allows the transition to the state of 30 meV below the conduction band-edge and then 0/ transition occurs, resulting in the near band-edge absorption spectra observed in this study. 4.2.2. Deep level transient spectroscopy The only correlation between the RC absorption and the deep levels observed by deep level transient spectroscopy (DLTS) is a level situated at 147 meV below the conduction band with a capture cross-section of 3.6  1015 cm2 and of concentrations about 1.5  1013 cm3 in n-type material [28,29]. In n-type material the RC centers are in the neutral charge state and supposedly situated at 30 meV below the conduction bandedge. However when the Schottky barrier formed, the depletion region under the metal contact behaves as intrinsic or SI. As a result of the band bending in the depleted region, the Fermi level gets below the RC level and they become positively ionized, donating electrons. It follows that this level should be the arsenic vacancies in the positive charge state which is 140 meV below the conduction band according to the positron annihilation measurements. Therefore when the pulse is applied to the reverse biased Schottky diode, the electrons in the conduction band have to fill into the

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positively charged level situated at 140 meV below the conduction band. The absorption measurements in the present work show that there exists no or very little lattice interaction. Therefore this should result in around 140 meV peaks in DLTS as a result of thermal excitation. The energy level observed in DLTS peaking at about 85 K gives energies very close to 140 meV obtained from the Arhenious plots at which temperature the RC absorption is still observable. Similarity of energy levels and observation temperature show that this level observed by DLTS is very likely to be associated with RC in the positive charge state in the depletion region which also acts as a donor in DLTS measurements. However correlation between the concentrations must be performed in due course.

process of a deep level near the conduction bandedge by electrons from the valance band and that no intra center transition is involved in the absorption process. Other experimental results such as positron annihilation, DLTS, deep level PL and Hall effect are in favor of this proposal and that this absorption is possibly due to Asvacancies.

4.2.3. Deep level photoluminescence Deep level PL results have shown three peaks at 0.63, 0.68 and 0.8 eV [30,31]. Of these levels, 0.63 and 0.68 eV peaks are attributed to the EL2 centers. The 0.8 eV band has been reported to anti-correlate with the band to band and band to acceptor transitions across wafers [32]. The intensity of this band also anti-correlates with the lifetime and EL2 related (0.63 and 0.68) luminescence [31]. This is the quite expected behavior for the RC distribution because the RC centers play a major role in the non-radiative recombination mechanism. Therefore it seems that the 0.8 eV PL band and the near band-edge absorption band are due to the same defect. However, although The Frank–Condon shift of approximately 0.7 eV is unexpected for a trap causing a direct absorption process, there may be a strong coupling with the factors other than lattice coupling. It means that the RC defect that causes near band edge absorption cannot be considered as isolated V As . The only Frank–Condon shift of that kind is attributed to EL6 in the literature [33].

References

5. Conclusions Parallel optical absorption and photo-current measurements show that characteristic near bandedge absorption is a direct photo-ionization

Acknowledgments The authors are grateful to Dr. M.R. Brozel of UMIST, England for very helpful discussions and for providing the samples.

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