Electrical and optical properties of defects and impurities in ZnO

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Physica B 340–342 (2003) 32–38

Electrical and optical properties of defects and impurities in ZnO D.C. Looka,b,c,*, C. Cos-kunc,1, B. Claflina,b, G.C. Farlowc a

Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OH 45433, USA b Semiconductor Research Center, Wright State University, Dayton, OH 45435, USA c Physics Department, Wright State University, Dayton, OH 45435, USA

Abstract Advancements in ZnO device applications have fostered much interest in the electrical and optical activities of various defects and impurities in the material. Although it has long been known that Group III dopants, such as Al, make efficient donors, the roles of other impurities, such as H and N, are only recently being elucidated. The same is true of the simplest point defects, such as Zn and O vacancies and interstitials. Theoretical work has been essential in identifying and understanding various defects and impurities. For example, theory has shown that H is always a donor (not amphoteric), that the O vacancy is a deep donor, not shallow, and that the Zn interstitial is a shallow donor, in agreement with electron-irradiation (EI) experiments. Recent irradiation studies show that significant defect annihilations take place, even at low temperatures, thus showing why ZnO is so resistant to radiation effects. To develop applications involving electroluminescence, it will be necessary to identify a reliable acceptor dopant, and N, P, and As have been most thoroughly investigated so far. In fact, p-type samples with resistivities o1 O-cm have been demonstrated, but certain questions remain unanswered. r 2003 Elsevier B.V. All rights reserved.

1. Introduction For at least four decades, wide-band gap semiconductors have been investigated for possible applications in blue and UV photonics, and in high-power, high-temperature electronics. However, it was not until 1993 that a commercial ‘‘breakthrough’’ occurred, namely, a blue, GaNbased light-emitting diode. Since then, GaN-based blue laser diodes have been developed, and will *Corresponding author. Semiconductor Research Center, Wright State University, Dayton, OH 45435, USA. E-mail address: [email protected] (D.C. Look). 1 Permanent address: Physics Department, Ataturk . University, 25240 Erzurum, Turkey.

soon be in DVD players and laser printers, while GaN-based heterostructure field effect transistors have already demonstrated record power performances at high frequencies. Other wide-band gap materials, such as ZnSe, 6H–SiC, and ZnO have also been proposed as potential candidates for some of these same applications. Unfortunately, SiC does not produce a very bright emitter, and ZnSe is subject to defect formation under high current drive. However, ZnO is an even brighter emitter than GaN, because its excitons have a 60meV binding energy, as compared with 24 meV for GaN. It also should offer competition in the electronic field, because it has a higher theoretical saturation velocity than GaN, as well as the availability of large-area substrates, amenability

0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.188

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to wet chemical etching, a high radiation resistance, and relatively low materials costs [1,2]. However, ZnO has suffered from one major disadvantage: the lack of good, reliable, p-type material. There are several possible reasons for this dilemma: (1) lack of shallow-acceptor dopants; (2) low solubility of such dopants; or (3) compensation by increased levels of impurity and nativedefect donor species. Thus, it is very important to understand the electrical activities of relevant impurities and defects. In particular, recent work has concentrated on impurities N and H, and defects VO (O vacancy), VZn (Zn vacancy), and ZnI (Zn interstitial). In this report, we will investigate the roles of each, particularly as they affect temperature-dependent Hall-effect (T-Hall), and photoluminescence (PL) measurements. We will also discuss the production of VO, VZn, and ZnI in room-temperature and low-temperature 1.5-MeV electron irradiations (EIs), and compare with the GaN case.

2. Undoped ZnO

-3

n (cm )

Perhaps the purest ZnO commercially available at present is that grown by a seeded vapor phase (VP) technique. In the last few years, many groups have investigated such material by various techniques [3–11]. The first T-Hall measurements [4] are shown in Fig. 1, and can be well fitted with two donors D1 and D2, of concentration ND1 ¼ 9  1015 cm3 and ND2 ¼ 1  1017 cm3,

10

16

10

14

10

12

10

10

10

Vapor phase: ED1=31 meV ED2 =61 meV

Vapor phase, annealed ED =63 meV

Hydrothermal ED =340 meV 8

0

5

10 3

15

20

-1

10 /T (K )

Fig. 1. Typical carrier concentration vs. temperature curves in unannealed and annealed ZnO grown by the vapor-phase and hydrothermal methods.

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respectively, and a total acceptor concentration NA of only 2  1015 cm3. It turns out that this NA is as low as that of the best GaN, although the ND ’s are slightly higher than those in very pure GaN [12]. The donor energies in this case, ED1 ¼ 31 meV and ED2 ¼ 61 meV, are quite representative of those measured in other samples by many different groups. In fact, the range for the shallow donor (D1) is about 30–40 meV, and that for the deeper donor (D2), 60–70 meV. The expected effective-mass (hydrogenic) donor can be calculated as ED ¼ 13:6m=e2stat ¼ 65 meV, using an effective mass of 0.318m0 ; and a static dielectric constant of 8.12e0 : Interestingly, when the same material has been annealed in water vapor at 950 C for 4 h (cf. Fig. 1), or in air at 700 C for 0.5 h, the shallower donor concentration D1 greatly decreases [13]. Several workers have recently found that H diffuses from the ZnO lattice at temperatures of about 600 C or higher [14], and thus it is very tempting to associate D1 with H. Even so, however, H is not the dominant donor because the room-temperature carrier concentration is controlled mainly by D2, not D1 [4]. Some ZnO crystals grown by the hydrothermal method do not show either D1 or D2, but instead a deeper donor level D3, with an energy ED3 of about 340 meV (cf. Fig. 1). That is not to say that hydrothermal material does not contain D1 or D2, because the PL spectra, indeed, are very similar to those of the VP material. A more likely scenario is that there are enough acceptors to compensate D1 and D2, thereby revealing the next deeper donor, D3. The PL spectra from VP wafers exhibit three groups of sharp, intense excitonic features, centered at about 3.36, 3.33, and 3.32 eV, respectively, as seen in the ‘‘undoped bulk’’ curve of Fig. 2. The group at 3.36 eV, shown in more detail in Fig. 3, is itself often split into three main lines, at 3.363, 3.360, and 3.357 eV, respectively. These lines have been studied for over 30 years, and in fact have been given designations I4, I6, and I9, respectively. Almost all workers consider I4 and I6 to be DBEs, but some consider I9 to be an acceptor-bound exciton [15]. None of these lines has been conclusively identified, but recent annealing experiments (Fig. 3) find that I4, at 3.363 eV, greatly

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PL Intensity (arb. units)

1600

0

0

TES

N-doped MBE A X undoped bulk

DX

1200 x16

x1300

x1

0

800

A X - 1 LO, 0 0 and D A

400 0 3.20

3.25

3.30

3.35

E (eV)

Fig. 2. PL spectra, at 4 K, for an undoped, VP grown, bulk ZnO sample, and a N-doped, MBE-grown epitaxial layer.

PL intensity (arb. units)

15

10

TES + 29.7 meV

0

DX

600 °C anneal TES + 29.7 meV

5

0

DX no anneal

0 3.356

3.358

3.360

3.362

3.364

E (eV)

Fig. 3. PL D0X and TES spectra of unannealed, and 600 Cannealed ZnO samples grown by a VP technique. The TES lines are shifted up by 29.7 meV, in order to align the largest D0X line (3.3630 eV) and largest TES line (3.3332 eV) for the unannealed sample. Note that the 3.363-eV D0X line disappears in the annealed sample, but the corresponding 3.333-eV TES line does not.

diminishes at about 600 C [13], the temperature at which H leaves the lattice, and I4 can be reintroduced by exposure to an H plasma [11]. Thus, I4 likely represents an exciton bound to an H donor. Unfortunately, the emission energy of a donorbound exciton (DBE) does not directly lead to the energy of the donor itself. Sometimes indirect methods, such as Haynes’ rule, can be established to provide approximate donor energies in a particular semiconductor material, but it is not yet known whether this rule applies to bound

excitons in ZnO. However, the best method for determining a particular donor energy is to find a ‘‘two-electron satellite’’ (TES) line associated with that donor [7,13]. For the DBE emission, the final configuration of the donor is the ground state (n ¼ 1), whereas for the TES emission, the final configuration is the n ¼ 2 state. In a hydrogenic model, the n ¼ 1 and 2 states are related by En¼1 ¼ 4=3ðEn¼1  En¼2 Þ; where all energies are referred to the conduction-band edge. Indeed, a strong line appears at 3.3332 eV, which some think is the TES line of I4 [7]. If so, then ED ¼ 4=3ð3:36323:333Þ ¼ 40 meV. Support for such an energy assignment comes from the Hall results, in which a 37-meV donor also disappears with annealing [13]. However, a comparison of the top and bottom dashed lines in Fig. 3 shows that the 3.3332 ‘‘TES’’ line seems only to slightly shift (by about 0.06 meV), not disappear, with annealing. It might be that this ‘‘shifted’’ line is really a completely different line, and that the 3.3332-eV line has indeed disappeared. If that is the case, then it is very plausible that the 3.3332-eV TES line is related to the 3.3630-eV DBE line, and that both are associated with a 40-meV H donor. However, at this stage, it is not at all clear that the 3.3332-eV TES line is connected with the 3.3630-eV DBE line. In summary, then, there is good evidence that H is a shallow donor in ZnO, as predicted by theory [16], and that it is associated with the 3.3630-eV PL line I4. There is also some evidence that its energy is about 40 meV, since a 37-meV donor tracks the disappearance of I4 with annealing. Finally, even if H is the 40-meV donor in VP ZnO, still it is not the dominant background donor in this material.

3. N-, P-, and As-doped ZnO A glance at the periodic table would suggest N as the best acceptor candidate for ZnO, because it has about the same ionic size as that of O, and thus should fit in well on the O site. Indeed, annealing a ZnO sample in either air or N2 gas produces a clear NO electron paramagnetic resonance signal [6,9], and using an RF-plasma source for N in

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Concentration (cm-3)

molecular-beam epitaxial growth gives a high surface concentration of N, as seen in the secondaryion mass spectroscopy results of Fig. 4 [17]. Such high concentrations of N are sufficient to overcome the background shallow donors, and produce a p-type sample, as first reported in 1997 [18]. Typical hole mobilities in p-type ZnO are very low, about 1–3 cm2/V-s, as also found to be the case in p-type GaN. A recent N-doped, p-type MBE ZnO sample was reported to have a mobility of 2 cm2/ V-s, a hole concentration of 9  1016 cm3, and a resistivity of 40 O-cm [17]. More recently, other groups have reported 1-O-cm, P-doped, p-type ZnO [19], and 2-O-cm, As-doped, p-type ZnO [20], both of which should be sufficient for LED applications. The PL of N-doped MBE ZnO shows broad peaks at 3.363, 3.315, and 3.238 eV, respectively, as seen in Fig 2 [17]. The peak at 3.363 is just the broadened envelope of the usual DBE lines, while the peak at 3.238 eV probably has contributions from donor–acceptor (D–A) pairs as well as the LO-phonon replica of the DBE lines. However, the peak at 3.315 eV is not common, and may well be an acceptor-bound exciton associated with the NO acceptor. Another group has observed a very similar line at 3.322 eV in N-doped MBE ZnO, although they attribute it to D–A pair recombination, involving NO [21]. Since their sample remained n-type, the line at 3.315–3.322 eV is not necessarily associated only with p-type ZnO, but it does seem to be generated by the presence of N.

10

22

10

20

10

18

10

16

10

14

bulk, undoped

0

MBE, N-doped

1

2

3

Depth (µm)

Fig. 4. Secondary ion mass spectroscopy measurements of the N concentration in two ZnO samples: (1) undoped bulk; and (2) N-doped MBE.

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Although the results discussed above show that N is evidently a good acceptor dopant, that does not suggest that it is necessarily the most common background acceptor, as will be discussed below.

4. Point defects in ZnO Since the mid-1980s, density functional theory (DFT) has proven to be instrumental in helping to identify point defects in semiconductor materials. For example, in GaAs, the As vacancy was predicted to have a (0/+) shallow donor level, the As antisite (EL2), a deep (0/+) donor level, and the Ga vacancy, a relatively shallow (0/) acceptor level, all of which are in fairly good agreement with experiment [22]. In fact, in most II–VI and III–V semiconductors, the cation vacancy is expected to have one or more acceptor levels, and very recently it has been shown that the Zn vacancy VZn in ZnO is no exception. In a sample irradiated with 2-MeV electrons, which would be expected to create VZn at an estimated rate 2 cm1, Tuomisto et al. have used positron annihilation spectroscopy to measure a rate 0.1 cm1 [23]. The lower experimental rate is expected because of annihilations during the room-temperature irradiation. On the other hand, in as-grown VP ZnO, the concentration of VZn is at least 40% of the total acceptor concentration determined by T-Hall measurements [23]. Thus, VZn is clearly an important compensating acceptor center in n-type ZnO, but not necessarily the only acceptor center. Perhaps NO is also present, but more studies will be necessary to completely understand the compensation problem. Interestingly, in n-type GaN, it appears that the Ga vacancy can account for nearly all of the compensating acceptors, at least in GaN grown by hydride VP epitaxy [24]. We now turn to the two defects most commonly invoked in the past as the dominant background donors in ZnO, namely, the O vacancy VO, and the Zn interstitial, ZnI. Here, DFT finds that both are donors, but that only one of them, ZnI, is a shallow donor [25–28]. Indeed, T-Hall measurements show that 2-MeV EI creates both shallow donors and acceptors (of unknown energy), and

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the donors have been attributed to ZnI, rather than VO [5]. This assignment is supported by the previously discussed fact that 2-MeV EI produces stable VZn, the Frenkel partner of ZnI [23]. The measured energy of the donor defect attributed to ZnI is about 25–30 meV, perhaps within range of the 30–40 meV background-donor energy usually seen in as-grown material. Could the background donors, usually at the 1015–1016 cm3 level, then include ZnI? There is controversy over whether or not this much ZnI could be present in as-grown, ntype ZnO, because of the expected high formation energy of this defect [25–28]. However, certain complexes involving ZnI might have lower formation energies, and indeed it has been reported that stacking faults in epitaxial ZnO are likely decorated with ZnI [29]. The low production rate of Zn vacancies, mentioned above, is consistent with many experiments that have shown ZnO to be ‘‘radiation hard’’ [8,30]. For example, as seen in Fig. 5, an irradiation by 1-MeV electrons to a fluence of 4  1016 cm2 (at each point) strongly increases the acceptor concentration of GaAs, and even GaN, but not ZnO. Assuming that the dominant defect acceptor in GaAs is VGa, and that in ZnO is VZn, the calculated production rates of these vacancies at 1 MeV are nearly equal. Thus, defect annihilation processes in ZnO must be stronger than those in GaAs and GaN. To study this problem in greater detail, we have recently set up a variable

temperature, in situ T-Hall apparatus [31]. With this unique system, we can irradiate the sample at any temperature between 100 and 500 K and continuously measure mobility m and carrier concentration n; while the beam is on. Conversely, we can irradiate for a given time and then shut the beam off while we follow m and n as a function of time or temperature. Unfortunately, the beam itself can heat the sample, so it is necessary to measure the temperature at all times with a thermometer in intimate contact with the sample. As an example, we show in Fig. 6 mobility and temperature vs. time for a sample irradiated with a 1.5-MeV, 0.9 mA/cm2 electron beam for 120 min, after which time the beam is turned off. Also, an external heater is turned on and then off at the 40 and 80-min points. Note that the temperature starts at 130 K in this case, rises to a value as high as 310 K, due to the beam heating and external heating, and then falls back to 130 K when the beam is turned off. We can also use this same data set to plot m vs. T; as shown in Fig. 7, and it is interesting to compare this curve with a ‘‘regular’’ m vs. T curve, obtained immediately after the irradiation, by using the external heater alone. Above about 150 K, there is no significant difference between the two curves, showing that the defects created by the beam must be annihilating within a few minutes at these temperatures. In other words, there is very little permanent damage being created at 150 K or higher. This is not the

10 ZnO - Zn face

5

GaAs

ZnO - O face

0

0.5

µ

500

2

GaN

2

1

µ (cm /V s); T (K)

NA (normalized)

1000

1.0

1.5

2.0

E (MeV)

Fig. 5. The evolution of acceptor concentration NA with electron bombardment energy for GaAs, GaN, and ZnO samples irradiated with 4  1016 cm2 electrons at each step. In each case, NA was calculated from temperature-dependent mobility curves.

200 T 100 0

40

80

120

160

Time (min)

Fig. 6. Mobility m and temperature T vs. time for a ZnO sample being irradiated by a 1.5 MeV, 0.9 mA/cm2, electron beam. Also, a heater is turned on and off at various intervals during the irradiation. The beam itself is turned off after 120 min.

ARTICLE IN PRESS D.C. Look et al. / Physica B 340–342 (2003) 32–38 1000 during irradiation after irradiation

600

2

µ (cm /V s)

800

400 200 0 100

150

200

250

300

T (K)

Fig. 7. Mobility vs. temperature using the same data set as that displayed in Fig. 6. Also overlaid is a m vs. T curve taken by using the heater alone to raise temperature, beginning after the sample had cooled down following the irradiation illustrated in Fig. 6 (i.e., at the 160-min point). Note that the irradiation is not significantly affecting the mobility at temperatures above 160 K.

case for GaAs or GaN. Efforts are underway to study damage annihilation at even lower temperatures, and obtain activation energies. However, for temperatures greater than 150 K, it can be stated that ZnO is radiation hard because of rapid defect annihilations.

5. Summary As-grown ZnO is always n-type. Although Group III atoms, such as Al, have long been known to provide good shallow donors in ZnO, recent theoretical and experimental investigations have established that H is also a shallow donor in ZnO. However, it is evidently not the dominant donor, at least not in VP grown material. Furthermore, EI experiments have shown that the Zn interstitial is a shallow donor, but a high formation energy may limit its concentration in asgrown n-type material. Optical fingerprints of H include a DBE PL line at 3.3630 eV, and possibly a two-electron satellite line at 3.3332 eV. As far as we know, there is no reported optical fingerprint of the Zn interstitial. Among the possible acceptor candidates, N has been used most frequently for achieving p-type ZnO, since it can be doped to high levels and it incorporates on the proper (O) site. However,

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good p-type material has also been successfully produced with P and As, even though their ionic radii are larger than that of O. In spite of these successes, it is probably fair to say that a reliable, repeatable p-ZnO technology has not yet been established. Another acceptor found in as-grown VP material is the Zn vacancy, and it appears to account for about half or more of the measured acceptor concentration. Possibly, N also contributes to the acceptors in this material. A PL line at 3.315 eV seems to be associated with N-doped ZnO, and it may arise from an exciton bound to an NO acceptor. Research on this important problem needs to continue. ZnO is radiation hard because of rapid defect annihilations that take place at temperatures above 150 K. This property could be advantageous for electronic and photonic applications in radiation environments.

Acknowledgements We wish to thank Z-Q. Fang, D.C. Reynolds, and C.W. Litton for helpful discussions, and T.A. Cooper, J.E. Hoelscher, and W. Rice for technical assistance. DCL and BC were supported by US Air Force Contract F33615-00-C-5402.

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