Litton C.W., . Reynolds D.C., Collins T.C. Zinc Oxide Materials for Electronic and Optoelectronic Device Applications

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peak was observed at 3.3206 eV with LO phonon replicas at 3.2505 eV and 3.180 eV,

which is attributed to donor-acceptor transition.

[43]

As for the defect-related features, only

the broad band with the maximum at about 2.4 eV (green band) is resolved. The low-

temperature PL spectrum of the Cermet sample is nearly identical to that of the Hanscom

sample. However, the FWHM of the 3.36 eV peak is lower approximately by a factor of

three than that of the Hanscom sample. The broad defect related band was observed at

about 2.1 eV instead of the green band. In the Eagle Piche r sample, the PL spectra from

two faces (Zn and O) were compared at room temperature, in an effort to identify the

transitions. Now it is easy to analyze PL spectra of GaN deposited on ZnO, if any traces of

ZnO or mixed phase of GaN and ZnO appeared. The low-temperature PL spectru m of GaN

grown on ZnO compared with GaN grown on other substra tes is shown in Figure 9.22. The

FWHM of the excitonic peak was found to be about 13 meV for GaN/ZnO and a sharp

Figure 9.20 CL spectra of sample 4 (n

¼5 10

2

) at different interface distances.

Reprinted from X. L. Sun, et al., Depth-dependent investigation of defects and impurity doping

in GaN/sapphire using scanning electron microscopy and cathodoluminescence spectroscopy.

J. Appl. Phys., 91, 6729. Copyright (2002) with permission from American Institute of Physics

Optical Properties 243

peak at 3.358 eV is due to excitonic emiss ion from the underlying ZnO substrate in

PL spectra. Gu et al. reported that the high radiative efﬁciency and weaker yellow

luminescence was obser ved in GaN/ZnO, as compared with other substrates.

[18]

The exciton peak of GaN/ZnO was very slightly blue-shifted while that for GaN/SiC

3.63.22.82.42

PL Intensity (a.u.)

Photon Ener

y (eV)

T = 10 K

Bulk ZnO

3.3653.363.355

Figure 9.21 PL spectra of ZnO substrate (produced by Cermet Inc.) at 10 K. Theinset shows

the enlarged ﬁne structure in the near-band-edge region

Figure 9.22 Low temperature PL spectra of GaN grown without buffer on different substrates.

Reprinted from F. Yun, et al., Mat. Res. Soc. Symp. Proc., 719, F8.21. Copyright (2002) with

permission from MRS

244 Growth and Characterization

is red-shifted. It is a clear indication that the blue and red shifts are due to compressive and

tensile strains of GaN/ZnO and GaN/SiC, respectively.

Hamdani et al.

[21]

demonstrated that the growth temperature is one of the important

factors while depositing GaN on ZnO by using NH

in the MBE system. Room

temperature PL spectra of GaN grown on the O-face ZnO subst rates at different

temperatures, i.e. 720, 750, and 780



C, are shown in Figure 9.23. At room temperature

the FWHM of the band edge emission increased from 36 to 60 meV with increasing

growth temperature from 720 to 780



C, respectively, due to increasing etching rate by

on the substrates with effect of temperature. The FWH M was found to be 8 meV at

4.2 K for the ﬁlms grown at 720



C. Moreover, any GaN growth was not observed at higher

growth temperatures such as 800



C. At low temper ature, the intensity ratio of the free

exciton peak and the donor bound exciton of GaN ﬁlms grown on the O-fac e ZnO is two

orders of magnitude higher than that of ﬁlms on the Zn-face ZnO substrates. However, Gu

et al. also achieved good quality GaN layers on Zn-face ZnO substrates.

[18]

Figure 9.24 shows the low temperature PL and reﬂectivity spectra of the ﬁlms grown on

O-face ZnO substra tes at growth temperature of 750



C. A, B and C excitons at 3.476,

3.489 and 3.511 eV, respectively, and interference fringes due to multiple reﬂections of

emission beam between the surface of GaN and GaN/ZnO interface are seen in the

reﬂectivity spectrum. In the PL spectrum, the main peaks were attributed to free A exciton,

a bound exciton and DAP.

[21,44]

Similarly, the PL and reﬂectivity spectra were recorded for

GaN ﬁlms grown on the Zn-face of ZnO substrates,

[21]

as shown in Figure 9.25. A and B

excitons only pronou nced at 3.481 and 3.496 eV, respectively, and the broadening of free

exciton (A) in the reﬂectivity spectrum reﬂects the lower crystalline quality of the GaN

ﬁlms. Moreover, the donor bound exciton, DX, with a binding energy of 12 meV occurred

instead of the free A exciton peak in the PL spectrum when comparing with Figure 9.24.

Figure 9.23 Room temperature PL spectra of GaN grown without buffer on O-face ZnO at

different growth temperatures. Reprinted from F. Hamdani, A. E. Botchkarev, H. Tang, W. Kim,

and H. Morkoc¸ , Effect of buffer layer and substrate surface polarity on the growth by molecular

beam epitaxy of GaN on ZnO, Appl. Phys. Lett. 71, 3111. Copyright (1997) with permission

from American Institute of Physics

Optical Properties 245

Figure 9.24 Low temperature (a) differential reﬂectivity and (b) PL spectra of GaN grown

without buffer on O-face ZnO. A, B and C, free excitons; AX, bound exciton; and DA,

donor–acceptor pair. Reprinted from F. Hamdani, A. E. Botchkarev, H. Tang, W. Kim, and H.

Morkoc¸, Effect of buffer layer and substrate surface polarity on the growth by molecular beam

epitaxy of GaN on ZnO, Appl. Phys. Lett. 71, 3111. Copyright (1997) with permission from

American Institute of Physics

Figure 9.25 Low temperature (a) differential reﬂectivity and (b) PL spectra of GaN grown

without buffer on Zn-face ZnO. DX, bound exciton. Reprinted from F. Hamdani, A. E.

Botchkarev, H. Tang, W. Kim, and H. Morkoc¸ , Effect of buffer layer and substrate surface

polarity on the growth by molecular beam epitaxy of GaN on ZnO, Appl. Phys. Lett. 71, 3111.

246 Growth and Characterization

An additional shift of the free exciton peak on about 5 meV indicates that the GaN ﬁlms

grown on the Zn-face ZnO substrates are under more compressive stress than the ﬁlms on

the O-face ZnO due to the difference in interface bonding between ZnO and GaN. The

reﬂectivity derivative spectra of the GaN grown on the O-face ZnO with different buffer

layers such as GaN, AlN and In

0.20

0.80

N are given in Figure 9.26. A, B and C excitons

clearly appeared and shifted to lower photon energies for GaN/ZnO with the In

0.20

0.80

buffer layer whereas in other cases no red shift was observed. This might be due to the

strain relaxation between the In

0.20

0.80

N buffer layer and GaN.

[21]

Figure 9.27 shows the

reﬂectivity derivative spectra of ZnO and GaN/ZnO in which the ground and excited levels

of free excitons labeled A

, etc., occurred for GaN/ZnO, when In

0.20

0.80

was used as a buffer layer indicating better quality of the GaN epilayers.

[21]

Hamdani et al.

[16]

found from PL spectra that the FWHM of the room temperature band

edge emission was 80 and 36 meV for GaN/ZnO and ZnO substrate, respectively, and the

observation of free exciton in the low-temperature PL spectrum is conﬁrmation of the high

quality of both materials. The intensity ratio of the free and bound excitons is higher for GaN/

ZnO compared with ZnO in the low-temperature PL spectra, and thus it can be concluded that

the quality of the epitaxial layers is higher, when ZnO is used as the substrates. The positions

of A, B, C excitons in GaN grown on a ZnO substrate obtained from the reﬂectivity spectra

slightly blue-shifted compared with the positions in bulk GaN. It indicates small compressive

strain between these layers. However, the excitonic positions, in the case of InGaN buffer

which is closely lattice matched to the GaN, shifted to lower energy levels compared with the

positions of GaN grown directly on ZnO. On the other hand, the yellow luminescence did not

appear at low temperatures, further proving the high quality of the samples, though at very

low excitation levels it has been observable. The excitonic peaks were not observed in GaN

Figure 9.26 Reﬂectivity derivative spectra of GaN grown on O-face ZnO with different buffer

layers: (a) AlN; (b) GaN; and (c) In

0.20

0.80

N. The growth temperature of the buffer layers was

650



C.ReprintedfromF.Hamdani,A.E.Botchkarev,H.Tang,W.Kim,andH.Morkoc¸, Effect of

buffer layer and substrate surface polarity on the growth by molecular beam epitaxy of GaN on ZnO,

Appl. Phys. Lett. 71, 3111. Copyright (1997) with permission from American Institute of Physics

Optical Properties 247

grown on AlN buffer layers.

[21]

In PL spectra, A (3.375 eV), B (3.386 eV) free exciton and

X (3.364 eV) peaks were seen in GaN/ZnO whereas A, B and A2 exciton peaks with

binding energy of 67 meVappeared in ZnO substrate.

[44]

The segregation of free exciton is a

sign of low carrier concentration and high crystalline quality of the ZnO substrate. The

properties of the grown GaN layers depend not only on the type of the buffer or substrate used

for the growth but also on the polarity of the buffer or substrate. One can see the segregation of

different peaks from GaN, if the Zn-face or the O-face ZnO substrates were used,

[16]

as shown

in Figure 9.28.

In the PL spectra at 77 K from GaN grown on sapphire substrate with ZnO buffer layer,

the BL band is seen.

[12]

Note that the band edge emission peak at 3.45 eV segregated in

both type s of samples with and without buffer layers. However, in the ZnO/GaN layers,

a wide peak at 2.8 eV is due to the diffusion of Zn into GaN. Detchprohm et al.

[13]

studied

the low temperature PL properties of GaN ﬁlms grown on sput ter deposit ed Zn O at

different ambients such as Ar and O

. The peaks at 3.486 and 3.479 eV are due to free and

donor bound excitons, respectively, observed in both GaN samples. However, the FWHM

(2.58 meV) of PL peak due to exciton bound to neutral donor for GaN grown in Ar is less

than that of the GaN grown in O

. It was concluded that the ﬁlms grown using ZnO buffer

showed high crystalline quality. The donor bound excitons and free excitons are observed

in the PL spectra of GaN grown on sapphire with ZnO buffer layer. The very weak yellow

luminescence at 2.2 eV was also present.

[14]

In the Raman spectra, the coupled plasmon-phonon mode, in addition to two E

modes

at 570 and 143 cm

1

, was a broad line at approximately 510–530 cm

1

[12]

The broad nature

of this mode indicates that GaN ﬁlms might have a higher carrier concentration. The

Figure 9.27 Reﬂectivity derivative spectra of ZnO substrate and GaN grown on ZnO with

thickness of 2 mm with InGaN buffer layer. Reprinted from F. Hamdani, et al., Microstructure

and optical properties of epitaxial GaN on ZnO (0001) grown by reactive molecular beam

epitaxy, J. Appl. Phys., 83, 983. Copyright (1998) with permission from American Institute of

Physics

248 Growth and Characterization

measured carrier concentration is in the order of 10

3

. Regarding the surface, terrace-

like ﬂat features were observed when growing GaN ﬁlms on ZnO buffers.

[12]

9.7 Electrical Properties

Perhaps the most sensitive characterization method, one which also bodes well for

electronic devices, is electrical characterization, such as Hall measurements. In this

vein, Detchprohm et al.

[13]

found that the growth of GaN on sputtered ZnO in O

better than that of sputtered ZnO in Ar. The measured room temperature Hall mobility is in

Figure 9.28 PL spectra of GaN grown on (a) O-face ZnO, (b) Zn-face ZnO, (c) with AlN buffer

on O-face ZnO and (d) with GaN buffer on O-face ZnO. A

, free exciton; DX, donor bound

exciton; and AX, acceptor boundexciton. The inset shows the spectrum over a wider energy range

including red and green wavelengths. Reprinted from F. Hamdani, et al., Microstructure and

optical properties of epitaxial GaN on ZnO (0001) grown by reactive molecular beam epitaxy, J.

Appl. Phys., 83, 983. Copyright (1998) with permission from American Institute of Physics

Electrical Properties 249

the range of 420–520 cm

1

for GaN ﬁlms grown directly on sapphire. At room

temperature, the carrier concentration of 9 10

–410

3

for GaN ﬁlms grown on

ZnO is two or three orders of magnitude lower than that of the ﬁlms grown on sapphire. A

relatively lower mobility value of 60 cm

1

and a higher carrier conce ntration of

6 10

3

for GaN ﬁlms grown on ZnO by PLD are reported in the literature.

[7]

The

carrier concentration and mobility of In

0.23

0.77

N ﬁlms grown on ZnO substrates at

800



C are 1 10

3

and 100 cm

1

, respectively.

[22]

The electron concentration

and the Hall mobility are 7 10

3

and 880 cm

1

at 293 K and 4 10

3

and

2248 cm

1

at 120 K for 74 mm-thick ZnO pretreated GaN sample. A little variation in

electrical properties was noted for over 2 inch sample, showing homogeneity of the sample

grown by Molnar et al.

[14]

Electrical measurements, which were carried out on

n-GaN grown by HVPE and treated by either GaCl, or ZnO, effectively showed thickness

dependence. As depicted in Figure 9.29, the Hall mobility increased with increasing

thickness of the GaN ﬁlm/ZnO/sapphire. This is a clear indication that the defect

concentration decreased with increasing thickness of the ﬁlm ranging from 4.5 to 74 mm.

A two-layer model was proposed by Look and Molnar,

[45]

and extended by G

€

otz et al.,

[46]

to obtain more accurately the bulk mobility and electron concentration when a highly

conductive interfacial layer is inserted. The effective areal density of free electrons n

s,eff

and the effective Hall mobility m

eff

are related to the free electron concentration n(x) and

electron mobility m (x):

s;eff

ðdÞ¼

nðxÞmðxÞdx

nðxÞmðxÞ

ð9:1Þ

Figure 9.29 (a) Variation of electron concentration vs inverse temperature and (b) electron

mobility vs temperature for GaN layers grown by HVPE. Reprinted from R. J. Molnar, W. G

€

otz,

L. T. Romano, N. M. Johnson, Growth of gallium nitride by hydride vapor-phase epitaxy, J.

Cryst. Growth, 178, 147. Copyright (1997) with permission from Elsevier

250 Growth and Characterization

and

eff

ðdÞ¼

nðxÞmðxÞ

dx=

nðxÞmðxÞdx ð9:2Þ

The ﬁnal product can be derived from Equations (9.1) and (9.2):

s;eff

eff

ðdÞ¼

nðxÞmðxÞdx ð9:3Þ

For layer 1: 0 G x d

, n(x) ¼n

, m(x) ¼m

and for layer 2: d

G x  d

þd

¼d,

n(x) ¼n

, m(x) ¼m

Equation (9.3) becomes (d

d

d)

s;eff

eff

ðdÞn

þn

ð9:4Þ

Equation (9.4) predicts a linear dependence of n

s,eff

eff

on ﬁlm thickness (d). Experi-

mentally, linearity was observed for the product of n

s,eff

eff

vs thickness for GaCl-treated

samples, whereas for the ZnO-treated samples, a slightly nonlinear behavior was observed,

as shown in Figure 9.30. One could believe that there might be some disturbance at the

interface, for which some arbitrar y decay parameters are introduced in the equations:

nðxÞ¼n

expðx=d

Þþn

ð9:5Þ

(a)

HVPE GaN

GaCI pretreatment

T=300 K

HVPE GaN

ZnO pretreatment

T=300 K

0510

15 20

020 406080

Thickness (µm) Thickness (µm)

seff

eff

(10

–2 +

–1

)

seff

eff

(10

–2 +

–1

)

(b)

Figure 9.30 Product of electron density (n

s,eff

) and mobility (m

eff

) vs thickness of GaN layer for

(a) GaCl-treated samples and (b) ZnO-treated samples. The solid and dotted lines are the linear

ﬁt to the experimental data. Reprinted from W. G

€

otz, L. T. Romano, J. Walker, N. M. Johnson,

and R. J. Molnar, Hall-effect analysis of GaN ﬁlms grown by hydride vapor phase epitaxy, Appl.

Phys. Lett., 72, 1214. Copyright (1998) with permission from American Institute of Physics

Electrical Properties 251

mðxÞ¼m

m

expðx=d

Þð9:6Þ

where n

and m

represent the electron concentration and mobility for a thick GaN ﬁlm,

respectively, and n

þn

and m

m

are roughly related to electron conce ntration and

mobility near the GaN/sapphire interface, respectively. The terms d

and d

represent the

increase and decrease of the electron concentration and mobility, respectively. From

Equations (9.1), (9.2), (9.5) and (9.6):

s;eff

ðdÞ¼AðdÞ

=BðdÞ and m

eff

ðdÞ¼BðdÞ=AðdÞð9:7Þ

where

AðdÞ¼n

d þn

½expðd=d

Þ1n

½expðd=d

Þ1

and

BðdÞ¼n

d1 = 2n

½expð2d=d

Þ1þ2n

½expðd=d

Þ1

n

½expðd=d

Þ1

ð9:8Þ

The theoretically determined electron concentration a nd mobility are: n

¼3.5 

3

and m

¼903 cm

1

for a thick ﬁlm; and n

þn

¼4.3 10

3

and

m

¼216 cm

1

for t he interface between sap phire and GaN. The reporte d

highest mobility value of GaN grown on sapphire with ZnO buffer is 2248 cm

1

at 120 K.

[14]

TEM analysis revealed that there is no interface between sapphire and

GaN when ZnO is used as a buffer. These results solely support the electrical analysis.

9.8 GaN/ZnO Hybrid Devices

Because p-type ZnO had been relatively hard to attain, hybrid het erostructures in which

the p-type material was of some other semiconductor have been explored. Furthermore,

for some device features integration of GaN and ZnO technologies paves the way for

enhanced performance and/or make certain device functions possible. To reiterate,

semiconductor ZnO has a direct band gap of 3.3 eV with a wurtzite structure. In many

respects ZnO competes wit h GaN for device applications. It is also clear that GaN h as

been shown to be capable of producing very high performance electronic and optical

devices. Issues such as reliability, efﬁciency, etc., are being explored which is indicative

of a success story. Power ﬁeld effect transistors are capable of producing over 500 W of

CW power in the communication band, LEDs have become very popular with current

emphasis on efﬁciency while they are already more efﬁcient than ﬂuorescent bulbs, and

blue lasers are already used in the Sony Play Station III for high deﬁnition viewing. The

same, however, cannot necessarily be said about ZnO at this point. As already

mentioned, the applications of ZnO overlap a good deal with those of GaN. On the

electronic side, the relatively low mobilit y of ZnO compared with GaN and stronger

electron–phonon coupling together with relatively low thermal conductivity are serious

shortcomings for ZnO. However, transparent thin ﬁlm transistors built in poly-ZnO

252 Growth and Characterization