Structural Characterization of Laser Lift-Off Gan

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STRUCTURAL CHARACTERIZATION OF LASER LIFT-OFF GaN

ERIC A. STACH,* M. KELSCH,*,#

W.S. WONG,,

E.C. NELSON,* T. SANDS

AND N.W.

CHEUNG

* National Center for Electron Microscopy, Materials Science Division, Lawrence Berkeley

National Laboratory, Berkeley, CA 94720: email: [email protected] ; http://ncem.lbl.gov Department of Materials Science and Engineering, University of California, Berkeley, CA

94720;

Department of Electrical Engineering and Computer Science, University of California,

Berkeley 94720.# On leave from the Max Plank Institute fr Metallforschung, Stuttgart, Germany.

Present address: Xerox Palo Alto Research Center, Palo Alto, CA 94304.

ABSTRACT

Laser lift-off and bonding has been demonstrated as a viable route for the integration of III-

nitride opto-electronics with mainstream device technology. A critical remaining question is the

structural and chemical quality of the layers following lift-off. In this paper, we present detailed

structural and chemical characterization of both the epitaxial layer and the substrate using

standard transmission electron microscopy techniques. Conventional diffraction contrast and

high resolution electron microscopy indicate that the structural alteration of the material is

limited to approximately the first 50 nm. Energy dispersive electron spectroscopy line profiles

show that intermixing is also confined to similar thicknesses. These results indicate that laser

lift-off of even thin layers is likely to result in materials suitable for device integration.

Additionally, because the damage to the sapphire substrate is minimal, it should be possible to

polish and re-use these substrates for subsequent heteroepitaxial growths, resulting in significant

economic benefits.

INTRODUCTION

III-nitride semiconductor alloys are promising materials for opto-electronic devices in the

ultraviolet to blue/green spectrum. This is because the III-nitrides form a continuous alloy

system with direct band gaps over the range of 1.9 eV (InN) to 3.4 eV (GaN) to 6.2 eV (AlN).

This has resulted in the successful creation of blue and green laser diodes, as well as the full

color spectrum of light emitting diodes.1,2

However, because of the low decomposition

temperature of GaN (on the order of 900 C), significant problems remain in the growth ofmaterials of high crystal quality. This is because this low decomposition temperature makes

bulk crystal growth difficult using standard methods. Additionally, the dissociation of nitrogen

from typical carrier gases used in metallorganic chemical vapor deposition (MOCVD) requires

high temperatures that are often incompatible with growth on conventional substrates. As a

result, the majority of III-nitride devices are grown heteroepitaxially onto either sapphire (singlecrystal (0001) Al2O3), or less frequently, SiC. These two materials provide a hexagonal template

for the growth of wurtzite GaN, and can easily withstand high crystal growth temperatures.

However, both sapphire and SiC have electrical and thermal conductivity constraints which may

limit the functionality of III-nitride devices grown on these materials.

Recently, Kelly et al.3

and Wong et al.4,5

have demonstrated a method of epilayer lift-off and

bonding that permits direct integration of III-nitride devices with most substrate materials. This

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method takes advantage of the different band gaps of GaN and sapphire. A pulsed excimer KrF

laser at 5 eV ( = 248 nm) is used to thermally decompose the heteroepitaxial interface betweenthe two materials. At this wavelength, the sapphire substrate is transparent, but this wavelength

is also well above the absorption edge of GaN. With sufficient laser fluence ( > 400 mJ/cm2)

this absorption results in heating local to the GaN / sapphire interface which causes

decomposition of the GaN into metallic Ga and N2 gas. Slight warming of the material abovethe Ga melting point thereafter releases the GaN layer from the substrate. Prior work has

characterized the bulk properties of the resulting materials using scanning electron microscopy,5

x-ray diffraction,5

channeling Rutherford backscattering spectroscopy6

and photoluminescence.6

Additional experiments have shown that device layers can function following lift-off and

bonding.5,7

In this paper, we present detailed electron microscopy characterization of both the

laser lift-off (LLO) GaN layer and the remaining sapphire substrate. We find that the damage to

both the epilayer and substrate is quite minimal, with both structural alteration and chemical

intermixing confined to approximately the first 50 to 100 nm of the epilayer and the substrate.

EXPERIMENTAL

The GaN layers were grown heteroepitaxially on (0001) oriented sapphire to a thickness of 12.5m using hydride vapor phase epitaxy (HVPE). No growth buffer layer was used. Prior to laser

irradiation the substrates were polished to " t u r v t q v h q h r r q p r u r

scattering of the laser light. The samples were irradiated from the back side of the

heterostructure with a single pulse of the KrF laser at a fluence of 600 mJ/cm2. The

heterostructures were then warmed to 40 C on a hot plate to melt the decomposed Ga interfaciallayer and complete the lift-off process.

Cross sectional TEM sample preparation of the thin GaN layers proved difficult due to the

different ion thinning rates of GaN, sapphire and the glue used in the preparation process. In

order to obtain electron transparent regions of the LLO GaN layers at the location of the prior

heterointerface, a modification of an existing TEM sample preparation method was used. A

total of seven LLO GaN layers were glued together in succession in a miniature vice (i.e. two

layers were glued together first, followed by a third, then a fourth, etc.). This resulted in very

thin glue layers on the order of 0.1 m to 0.25 m. The resulting GaN sandwich was then

glued between Si (001) substrate material for support. TEM preparation thereafter followed the

method of Bravman and Sinclair.8

Final ion milling was performed using a Technorg Linda low

angle, low voltage ion mill. Initial thinning was done at 10 kV and 5 incidence until

perforation, followed by a polish at 500 eV and 5 for a half hour. High resolution electronmicroscopy was performed using the NCEM Atomic Resolution Microscope at 800 kV and

analytical electron microscopy was performed using a Philips CM200 field emission microscope

equipped with a Kevek atmospheric thin-window energy dispersive spectrometer and the

Emispec control and analysis software. Conventional diffraction contrast microscopy wasperformed using a JEOL 200CX microscope at 200 kV, as well as the ARM at 800 kV.

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Figure 1 (a) Large area HREM image of the HVPE GaN / sapphire heterostructure prior to

laser lift-off. Arrows point to amorphous regions at the interface. (b) Computed diffractogram

of the interfacial region showing the orientation relationship between the two materials.

RESULTS AND DISCUSSION

Figure 1 shows a large area high resolution micrograph (HREM) of the GaN / sapphire

heterostructure prior to laser lift-off. Inset is a computed fast Fourier transform (FFT)

diffractogram taken from the interfacial region. This diffractogram indicates that the orientation

relationship between the substrate and heteroepitaxial layer is 0001( )GaN// 0001( )Al 2O3 and

011 2[ ]GaN

// 011 2[ ]Al2O3

. Although this orientation relationship has been observed,9,10

it is

much more common to observe 0001( )GaN// 0001( )Al2O3 and 011 0[ ]GaN// 011 2[ ]Al2O3 inheteroepitaxial GaN / sapphire.

11Visible at the interface between the two layers are pockets of

amorphous material; this amorphous material is a true feature of the heterostructure and not a

TEM preparation related artifact. Additionally, numerous stacking faults are observed within

the first 30 to 40 nm of the layer. These features, along with the high dislocation densities

present in the layer (

10cm

-2, not visible in this image), act to accommodate the

heteroepitaxial strain.

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Figure 2 (a) Large area HREM image of the GaN layer following LLO. (b) Diffractogram

from region away from LLO interface. (c) A typical diffractogram from the LLO region.

Defect spots are arrowed.

Figure 2 shows a similar large area HREM micrograph of the GaN layer following laser lift-off.

This image was taken from the thinnest area of the LLO GaN TEM sample that still had

specimen preparation glue remaining (the amorphous feature which lines the bottom of the

sample throughout the image). This indicates that we are in fact imaging the sample at the LLO

surface. Again, stacking faults are visible in the image within the first 40 to 50 nm of the new

surface. This shows that lift-off occurred atthe site of the GaN / sapphire interface, and not

within either the GaN or sapphire bulk. The inset diffractogram (Figure 2.b) from a region away

from the LLO surface shows that the bulk of the GaN layer has a structural perfection equivalent

to that observed in the sample prior to lift-off (See Figure 1.b for comparison). However, at the

newly created surface, the diffractograms indicate that although the majority of the material is

wurtzite GaN (the strong reflections in Figure 2.c), there is significant formation of structuraldefects consistent with twinning. (Due to a lack of resolution in the computed FFTs the exact

crystallography of these defects could not be determined.) It is apparent from this image that the

structural alteration to the layer is minimal, and confined to the first 50 or so nanometers of the

LLO GaN layer.

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Figure 3Dark field image of the sapphire

substrate following laser lift-off.

Figure 3 presents a diffraction contrast

dark-field electron micrograph of the

sapphire substrate following laser lift-off.

The mottled contrast at the uppermost

region of the sample nearest the newly

created surface is consistent with thepresence of damage. Unfortunately, the

sample preparation process did not

produce a region thin enough for high

resolution electron microscopy. As a

result the exact nature of the damage

(amorphization, etc.) is not certain.

Further investigation is in progress.

Again, though, it is apparent that the

structural damage is confined to a region

very near ( u r r s h p r

In Figure 4 we present the results of the chemical characterization of both layers. The energy

dispersive spectroscopy (EDS) analyses were performed on the Phillips CM200 FEG-TEM, and

the spectra were obtained by scanning a 1.6 nm diameter electron probe along the [0001] growth

direction of the samples. Each scan consisted of 100 points, with a 10 second dwell time at each

point. In Figure 4.a, the EDS analyses of the as-grown GaN / sapphire structure indicates that

the initial interface is very abrupt, with the slight spread of the data across the interface a result

of both finite probe effects and x-ray fluorescence. In Figure 4.b through 4.d the EDS profiles of

both the GaN LLO layer (4.b and 4.c) and the sapphire substrate (4.d) are presented. The

difference in signal counts between the aluminum and gallium and that of oxygen and nitrogen

is due to differences in detector efficiency for the light elements. In each of these spectra it is

apparent that intermixing is not significant, and that it is confined to the first $

CONCLUSIONS

Conventional, high resolution and analytical electron microscopy have been used to characterize

laser lift-off HVPE GaN layers and the remaining sapphire substrate. It is observed that

structural damage and chemical intermixing resulting from laser processing is minimal and that

is confined to approximately the first $ s u r r y v t h r v h y U u r r r y v q v p h r

that LLO of III-nitride opto-electronic devices represents a viable route for materials integration.

ACKNOWLEDGEMENTS

The samples were provided by James Ren of American Xtal Technology, Fremont CA. The

work at NCEM was supported by the Director, Office of Energy Research, Office of BasicEnergy Sciences, Materials Science Division of the U.S. Department of Energy under Contract

No. DE-AC03-76SF000098. This work was also supported in part by the University of

California MICRO Program (Award #98-133). The authors would like to thank C. Kisielowski

and C.J. Echer at NCEM and K.T. Moore at Johns Hopkins Univ. for helpful commentary.

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Figure 4(a) Energy dispersive spectroscopy line profiles of the as grown heterostructure. (b)

EDS from the LLO GaN layer showing distribution of gallium and aluminum. (c) EDS from

LLO GaN layer showing all elements. (d) EDS from the sapphire substrate showing all

elements.

1 S. Nakamura, et al. MRS Int. J. Nitride Semi. Res., 4S1, 1999.

2 For a review, see S.P Denbaars, Proc. IEEE; 85, 1740 1997.

3 M.K. Kelly, O. Ambacher, R. Dimitrov, R. Handschuh, M. Stutzmann; phys. stat. sol. a. 159,

R3-4, 1997; Kelly, et al.; Jap. J. Appl. Phys., Part 2.38, (3A) p.L217, 1997.

4 W.S. Wong, T. Sands, and N.W. Cheung, Appl. Phys. Lett. 72, 599 (1998).

5 W.S. Wong, T. Sands, N.W. Cheung, M. Kneissl, D.P. Bour, P. Mei, L.T. Romano and N.M.

Johnson, Appl. Phys. Lett. 75, 1360 (1999).

6 W.S. Wong, Y. Cho, E.R. Weber, T. Sands, K.M. Yu, J. Krger, A.B. Wengrow and N.W.

Cheung, Appl. Phys. Lett. 75, 1887 (1999).

7 W.S. Wong, A.B. Wengrow, Y. Cho, A. Salleo, N.J. Quitoriano, N.W. Cheung, and T. Sands,

J. Electron. Mater. 28,1409 (1999).

8 J.C. Bravman and R. Sinclair, J. Elect. Mic. Tech. 1, 53, 1984.

9 J.A. Wolk, K.M. Yu, E.D. Bourret-Courchesne and E. Johnson, Appl. Phys. Lett. 70, 2268

(1997).

10 H. Selke, S. Einfeldt, U. Birkle, D. Hommel and P.L. Ryder, in Microscopy of

Semiconducting Materials, 10th vol. Oxford (1997).

11 V. Potin, P. Vermaut, R. Ruterana and G. Nouet, J. Elect. Mat. 27, 266 (1998).

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Structural Characterization of Laser Lift-Off Gan