Observation and Study of Dislocation Etch Pits in Molecular 

Beam Epitaxy Grown Gallium Nitride With the Use of 

Phosphoric Acid and Molten Potassium Hydroxide 

by Fred Semendy and Unchul Lee 

ARL-TR-4164 June 2007 

Approved for public release; distribution unlimited. 
NOTICES 

Disclaimers 

The findings in this report are not to be construed as an official Department of the Army position unless 

so designated by other authorized documents. 

Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of the 

use thereof. 

Destroy this report when it is no longer needed. Do not return it to the originator. 
Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TR-4164 June 2007 

Observation and Study of Dislocation Etch Pits in Molecular 

Beam Epitaxy Grown Gallium Nitride With the Use of 

Phosphoric Acid and Molten Potassium Hydroxide 

Fred Semendy and Unchul Lee 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
ii

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid 

OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

June 2007 

2. REPORT TYPE 

Final 

3. DATES COVERED (From - To) 

FY 2004 to FY 2005 

5a. CONTRACT NUMBER 

5b. GRANT NUMBER 

4. TITLE AND SUBTITLE 

Observation and Study of Dislocation Etch Pits in Molecular Beam Epitaxy 

Grown Gallium Nitride With the Use of Phosphoric Acid and Molten Potassium 

Hydroxide 

5c. PROGRAM ELEMENT NUMBER 

5d. PROJECT NUMBER 

5e. TASK NUMBER 

6. AUTHOR(S) 

Fred Semendy and Unchul Lee 

5f. WORK UNIT NUMBER 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

U.S. Army Research Laboratory 

ATTN: AMSRD-ARL-SE-EI 

2800 Powder Mill Road 

Adelphi, MD 20783-1197

8. PERFORMING ORGANIZATION 

 REPORT NUMBER 

ARL-TR-4164 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S) 

U.S. Army Research Laboratory 

2800 Powder Mill Road 

Adelphi, MD 20783-1197

11. SPONSOR/MONITOR'S REPORT 

 NUMBER(S) 

12. DISTRIBUTION/AVAILABILITY STATEMENT 

Approved for public release; distribution unlimited 

13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Defects continue to challenge the functionality and reliability gallium nitride (GaN)-based devices. GaN grown on sapphire by molecular 

beam epitaxy was investigated by wet etching in hot phosphoric acid (H3PO4) and molten potassium hydroxide (KOH). Hexagonally shaped 

etch pits were formed on the etched sample surfaces. Etched samples were characterized with the use of atomic force microscopy (AFM) and 

scanning electron microscopy (SEM) and SEM cathode luminescence (SEM-CL). AFM images show dark spots indicating mixed 

dislocations. The densities of the mixed dislocations are almost ~3 x 108 cm–2. Observations were made about the three different types of etch 

pits distinguished as α, β, and γ. By comparing SEM and AFM, we made observations about a relationship between etch pits and dislocations. 

The origin of etch pits is the mixed dislocation, and the combination of KOH etching and AFM is found to be a better approach for a twodimensional evaluation of mixed dislocations. Results showed that both H3PO4 and molten KOH are good wet etchants for GaN and the pits 

created by H3PO4 were smaller and numerous when compared to the pits created by molten KOH.

15. SUBJECT TERMS

GaN, etch pits, phosphoric acid, molten KOH 

16. Security Classification of: 

19a. NAME OF RESPONSIBLE PERSON

Fred Semendy 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified 

c. THIS PAGE 

Unclassified 

17. LIMITATION

OF 

ABSTRACT 

SAR 

18. NUMBER 

OF PAGES 

18 19b. TELEPHONE NUMBER (Include area code) 

(301) 394-4627 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18 
Contents 

List of Figures iii

Acknowledgments iv

1. Introduction 1

2. Experiment 2

3. Results and Discussion 2

4. Conclusions 7

5. References 9

Distribution List 11 

List of Figures 

Figure 1. SEM images of GaN before (a) and after (b) etching in H3PO4 for 5 minutes at 

180 °C. .......................................................................................................................................3 

Figure 2. (a) SEM image of GaN etched in H3PO4 for 5 minutes at 170 °C panchromatic 

SEM-CL image at RT. ...............................................................................................................3 

Figure 3. SEM images of GaN (a) and (b) after etching in KOH (a) for 10 minutes at 300 °C 

and (b) at 300 °C for 20 min. .....................................................................................................4 

Figure 4. SEM images of GaN (a) and (b) after etching in KOH for 10 minutes at 300 °C. ........4 

Figure 5. Schematic view of etch pits in GaN. ...............................................................................5 

Figure 6. (a) SEM image of GaN etched in molten KOH for 5 minutes at 350 °C (b) 

panchromatic SEM-CL image at RT .........................................................................................5 

Figure 7. (a) and (b) AFM image of GaN etched in H 3PO 4 for 5 minutes at 170 °C..................6 

Figure 8. (a) and (b) AFM images of GaN etched in molten KOH at 350 °C................................6 

Figure 9. (a) and (b) AFM images of GaN etched in molten KOH at 350 °C................................7 

iii
Acknowledgments 

The authors would like to thank Dr. Anand Sampath, Sensor and Electron Devices Directorate, 

Army Research Laboratory for graciously supplying Molecular Beam Epitaxy fron gallium 

nitride samples for this study. 

iv
1. Introduction 

Recently, wideband material gallium nitride (GaN)-based materials have been the subject of 

intensive research for opto-electronic devices and high temperature/high-power electronic 

devices. Ultraviolet detectors (1), aluminum nitride (AlN)/GaN field-effect transistors (2, 3) 

visible emitting diodes (4) and high mobility transistors (5) have been demonstrated. GaN and 

aluminum gallium nitride (AlGaN) materials are generally grown by metal organic vapor phase 

epitaxy (MOCVD), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE) 

(6). Because of the large lattice mismatch between GaN and sapphire and the mismatch in 

thermal expansion coefficient, GaN films contain dislocation densities as high as 108

 to 

1010 cm

–2. High dislocation density results in poor device characteristics and shorter life times 

(7). Two-dimensional surface evaluation for defects and dislocations is important in order to 

characterize the relationship between device performances and GaN crystal qualities. 

Characterizations of dislocations (mostly threading dislocations in GaN) are mainly conducted 

through transmission electron microscopy (TEM). It is a process that will reveal the microscopic 

structure of dislocations. However, the procedure is cumbersome and time consuming. Thus, it 

is important that the crystal characterization be done via simple procedure, namely, chemical 

etching to obtain valuable information about dislocations. 

Chemical etching is a useful method for crystal characterization. Etching in general is a 

destructive procedure. No suitable etchant has been found for GaN because of its chemical 

stability against various mineral acids and aqua regia. Molten sodium hydroxide, potassium 

hydroxide, and potassium pyrosulfate and 50 % sodium hydroxide aqueous solution (8) were the 

only known etchants. GaN grown on sapphire was investigated for etch pits with the use of 

molten KOH (9). Kozawa et al. found that all etch pits were hexagonal pyramids that were 

tentatively ascribed to dislocations in GaN layers. 

Ono et al. (10) found that the etch pit density (EPD) of a GaN surface etched by a solution of 

H3PO4 and H2SO4 was reduced from 4x107 to 6x106

 cm–2 when a thin layer of InGaN was 

inserted in the GaN layer. Yamamoto et al. (11) studied the etch pits in Si-doped GaN formed by 

molten KOH. They found hexagonally shaped etch pits with three different sizes of varying 

densities. The large and medium size pits were on the order of 6x106

cm–2 and for the smaller 

size, 3x108

cm

–2. From all previous work on the etch pit studies of GaN, we can conclude that 

the EPD is usually 102 to 104 lower than dislocation density evaluated by TEM. Etch pit varies 

even in same sample, etch pit has no relation to threading dislocation, and etch pits might be 

related to nano pipes or threading screw dislocations. Some reports indicate that the commonly 

observed hexagonally shaped etch pits to dislocations (12, 13); no convincing evidence has been 

presented yet. 

1
2. Experiment 

GaN samples were grown on (0001) sapphire by MBE. For the experiment, samples were cut 

into small pieces. The layer thicknesses of these samples were less than 6 μm with unintentional 

doping levels of less than 1015 cm–3. Commercial 85% phosphoric acid was heated to an etching 

temperature in a beaker and GaN samples were etched for various times. The etching 

temperature between 160 °C and 180 °C was monitored with a Pt-PtRh thermocouple. In the 

case of using molten KOH, the samples were held in a platinum basket and immersed in the 

etchant. In this case, the experiments were performed between 350 and 450 °C, lasting for 10 to 

20 minutes. Etching was performed in a ceramic furnace. A nickel crucible was used for the 

molten KOH, and thermocouples were used to monitor the temperature. Etching was performed 

after 30 minutes after the melting temperature reached the process temperature. The etching rate 

was calculated from the thickness difference before and after etching. The surface topography 

analysis was performed with AFM, SEM, cathodeluminescence, and surface profiling in some 

cases. 

3. Results and Discussion 

The etchants remarkably affect the etch rate and the size and shape of etch pits. Figures 1 (a) and 

(b) show SEM images of GaN before and after etching, respectively, in H3PO4 for 5 minutes at 

180 °C. Many darkly etched spots are observed and the dark spot density is about 4 x 108

 cm

–2, 

which is closer to the lower limit of the threading dislocation (TD) density. The scale used is 

1 μm. There are mainly three types of TDs in GaN epilayers: pure edge TDS with a Burgers 

vector of 1/3 (11 2 0) , pure screw TDs with Burgers vector of <001>, and mixed type TDs with 

Burgers vectors of 1/3 (11 2 3). Pure screw TDs with typical densities more than one order of 

magnitude lower than that of the pure edge or mixed type TDs would have an average distance 

of less than 3 μm among each other. Pure TDs do not have a component in the C-direction and 

are emerged into terraces. 

2
 

(a) (b) 

 

Figure 1. SEM images of GaN before (a) and after (b) etching in H3PO4 for 5 minutes 

at 180 °C. 

The etched GaN samples in phosphoric acid were used to obtain panchromatic SEM-CL and the 

image is shown in figure 2 (b). Figure 2 (a) is an SEM image of a GaN-etched sample in 

phosphoric acid. The dislocations act as additional radiative recombination centers, giving rise 

to increased or reduced panchromatic CL intensity in the micrographs. The dislocations are 

clearly visible in the CL image and present all over the surface. 

 (a) (b) 

 

Figure 2. (a) SEM image of GaN etched in H3PO4 for 5 minutes at 170 °C panchromatic 

SEM-CL image at RT. 

Figure 3 shows the hexagonal etch pits after etching the GaN samples in molten KOH at 300 °C 

for 10 minutes and (b) for 20 minutes. Here, the hexagonal pits seen on the surface reflect the 

crystal symmetry of GaN. The method indicates that the molten KOH etching is a useful method 

for revealing the etch pits and for characterizing the GaN surface. 

3
(a) (b) 

 

Figure 3. SEM images of GaN (a) and (b) after etching in KOH (a) for 10 minutes at 300 °C and (b) at 

300 °C for 20 min. 

Figure 4 gives the SEM images of GaN with couple of different angular views of the hexagonal 

etch pit. Figure 5 shows the schematic view of the etch pits. All pits formed hexagonal 

pyramids, the side of the base being along {1120} and the facet being (3032) face of GaN. In 

general, the shape and density of etch pits were independent of the etching temperature (300 to 

400 °C). The shape of the etch pits was identical and uniform throughout the surface. When 

etching with phosphoric acid is compared, the shape and density were different when etched in 

molten KOH. 

 (a) (b) 

 

Figure 4. SEM images of GaN (a) and (b) after etching in KOH for 10 minutes at 300 °C. 

4
Figure 5. Schematic view of etch pits in GaN. 

Samples etched in molten KOH were used for SEM-CL for the panchromatic view as shown in 

figure 6. These views indicate that the molten KOH etching is aggressive in nature and creates 

larger pits with hexagonal shapes. 

 (a) (b) 

 

Figure 6. (a) SEM image of GaN etched in molten KOH for 5 minutes at 350 °C (b) panchromatic 

SEM-CL image at RT. 

Figure 7 shows the AFM images of GaN with (a) and (b) showing the standard surface scan of 

the H3PO4 etched for 5 minutes at the 170 °C sample. In the first case, (a) a size of 10 μm area 

of the sample was scanned at 1 Hz. As can be seen, several shapes are observed when the 

sample is etched in hot phosphoric acid (13). Unclear etch pits can be seen in the AFM scanned 

(a) image. These are mainly truncated pyramid pits and conical pyramid pits along with some 

regularly shaped pits. 

〈 〉 12 10

 

<0001 > 

〈10 10〉

{1120} (3032)

5
 (a) (b) 

 

Figure 7. (a) and (b) AFM image of GaN etched in H 3PO 4 for 5 minutes at 170 °C. 

Figure 8 gives the AFM analysis of GaN etched on molten KOH at 350 °C. In the case of (a) 

roughness analysis, the rms (root mean square) roughness for a 1-μm scan was 1.7 nm with an 

rms maximum value of 10 nm with an rms average value of 1.3 nm. 

 (a) (b) 

 

Figure 8. (a) and (b) AFM images of GaN etched in molten KOH at 350 °C. 

6
Figure 9 gives the AFM images of GaN etched in molten KOH at 450 °C and because of the 

anisotropic character steps tend to align with one of three sides of the hexagon. Subsequently, 

dark spots disappear and hexagonal pits appear on the GaN surfaces. These pits are larger than 1 

μm and smaller pits are in the nano-meter scale. However, the density of the small pits is more 

than that of the larger ones. 

(a) (b) 

Figure 9. (a) and (b) AFM images of GaN etched in molten KOH at 350 °C. 

These etch pits might be produced by threading dislocations propagating to the top surface. 

According to Hino et al. (14), there are three types of etch pits in GaN. The first type is 

hexagonal which is well defined with a large black core called α type, the second one is the 

unclear hexagon with a small black core called β type, and the last is a polygon called γ type. 

The α type has a triangular shape and has a facet. The β type has a combination of triangular and 

trapezoidal shapes, which seems to be a combination of α and β types, and the γ type has a 

polygonal shape. 

4. Conclusions 

In conclusion, both H3PO4 and molten KOH chemical etching were performed on MBE grown 

GaN samples to investigate the nature of the etch pits. GaN grown on sapphire by MBE was 

investigated by wet etching in hot phosphoric acid H3PO4 and molten potassium hydroxide 

(KOH). Hexagonally shaped etch pits were formed on the etched sample surfaces. Etched 

samples were characterized by AFM and SEM and SEM-CL. Mixed dislocations were indicated 

7
by dark spots as shown by AFM images. The densities of the mixed dislocations are almost ~3 x 

108

 cm–2. Observations were made about the three different types of etch pits distinguished as α, 

β, and γ. By comparing SEM and AFM, we made observations about a relationship between 

etch pits and dislocations. The origin of etch pits is the mixed dislocation, and the combination 

of KOH etching and AFM is found to be a better approach for 2-D evaluation of mixed 

dislocations. Results showed that both phosphoric acid and molten potassium hydroxide are 

good wet etchants for GaN and the pits created by phosphoric acid etching were smaller and 

numerous when compared to the pits created by molten KOH. 

8
5. References 

1. Khan, M.A.; Kuzznia, J.N.; Olsen, D.T.; Van Hovve, J.M.; Blaasingame, M.; Reitz, L.F. 

Appl. Phys. Lett 1992, 60, 2917. 

2. Khan, M.A.; Kuzznia, J.N.; Bhattarai, A.R.; Olsen, D.T. Phys. Appl. Phys. Lett 1993, 62, 

1786. 

3. Binari, S.C.; Rowland, L.B.; Kruppa, W.; Kelner, G.; Doverspike, K.; Gaskill, D.K. 

Electron. Lett 1994, 30, 1248. 

4. Nakamura, S.; Mukai, T.; Senoh, M. Appl. Phys. Lett 1994, 64, 1687. 

5. Khan, M.A.; Kuzznia, J.N.; Olsen, D.T.; Schaff, W.; Burn, J.W.; Shur, M.S. Appl. Phys. 

Lett 1994, 65, 1121. 

6. Lester, S.D.; Ponce, F. A.; Craford, M.G.; steigerwaid, D.A. Appl. Phys. Lett 1995, 66, 

1149. 

7. Nakamura, Shuji; Semoh, Masayuki; Nagahama, Shin-ichi; Iwasa, Naruhito; Iwasa, Yamada, 

Takao; Matsushita, Toshio; Kiyoku, Hiroyuki, Sugimoto, Yasunobu; Kozaki, Tokuya; 

Umemoto, Hitoshi; Sano, Masahiko; Chocho, Kazuyuki Appl. Phys. Lett 1998, 72, 211. 

8. Chu, T.L. J. Elechem. Society 1971, 118, 1200. 

9. Kozawa, T.; Kaaxhi, T.; Ohwaki, T.; Taga, Y.; Kiode, N.; Koide, M. J. Electrochem. Soc

1971, 118, 1200. 

10. Ono, Y.; Iyechika, Y.Y.; Takada, T.;INUI, K.; Matsye, T. J. Crys. Growth 1998, 189/190, 

133. 

11. Yamamoto, K., Ishikawa, H.; Egawa, T.; Jimbo, T.; Umeno, M. J. Crys. Growth 1998, 

189/190, 575. 

12. Heta, M.; Kunisato, T.; Kano, T.; Matsushita, Y.; Ito, S.; Ueda, Y.; Yegi, K.; Ibaraki. A. 

Ibaraki, Extended Abstract of 57th Autumn Meeting of Japanese Society of Applied Physics

10 A –ZG-2, 302 ( 1996), JSAP Catalog Number, AP961119-01. 

13. Shintani, A.; Minagawa, S. J. Electrochem. Soc 1976, 123, 706. 

14. Hino, T.; Tomiya, S.; Miyajima, S.; Yaanashima, Y.; Hashimoto, S.; Ikeda, M.; Kawagachi, 

K. Appl. Phys. Lett 2000, 76, 3421. 

9
INTENTIONALLY LEFT BLANK

10
Distribution List

ADMNSTR 

DEFNS TECHL INFO CTR 

ATTN DTIC-OCP (ELECTRONIC COPY) 

8725 JOHN J KINGMAN RD STE 0944 

FT BELVOIR VA 22060-6218 

DARPA 

ATTN IXO S WELBY 

3701 N FAIRFAX DR 

ARLINGTON VA 22203-1714 

OFC OF THE SECY OF DEFNS 

ATTN ODDRE (R&AT) 

THE PENTAGON 

WASHINGTON DC 20301-3080 

US ARMY TRADOC 

BATTLE LAB INTEGRATION & TECHL 

DIRCTRT 

ATTN ATCD-B 

10 WHISTLER LANE 

FT MONROE VA 23651-5850 

SMC/GPA 

2420 VELA WAY STE 1866 

EL SEGUNDO CA 90245-4659 

US ARMY INFO SYS ENGRG CMND 

ATTN AMSEL-IE-TD F JENIA 

FT HUACHUCA AZ 85613-5300 

US ARMY NATICK RDEC ACTING 

TECHL DIR 

ATTN SBCN-TP P BRANDLER 

KANSAS STREET BLDG 78 

NATICK MA 01760-5056 

COMMANDER 

US ARMY RDECOM 

ATTN AMSRD-AMR W C MCCORKLE 

5400 FOWLER RD 

REDSTONE ARSENAL AL 35898-5000 

US ARMY RSRCH LAB 

ATTN AMSRD-ARL-CI-OK-TP TECHL 

LIB T LANDFRIED (2 COPIES) 

BLDG 4600 

ABERDEEN PROVING GROUND MD 

21005-5066 

SANDIA NATL LAB 

ATTN R J SHUL 

ALBUQUERQUE NM 87185 

US GOVERNMENT PRINT OFF 

DEPOSITORY RECEIVING SECTION 

ATTN MAIL STOP IDAD J TATE 

732 NORTH CAPITOL ST., NW 

WASHINGTON DC 20402 

DIRECTOR 

US ARMY RSRCH LAB 

ATTN AMSRD-ARL-RO-EV W D BACH 

PO BOX 12211 

RESEARCH TRIANGLE PARK NC 27709 

US ARMY RSRCH LAB 

ATTN AMSRD-ARL-CI-OK-T TECHL 

PUB (2 COPIES) 

ATTN AMSRD-ARL-CI-OK-TL TECHL 

LIB (2 COPIES) 

ATTN AMSRD-ARL-SE 

ATTN AMSRD-ARL-SE-D 

ATTN AMSRD-ARL-SE-DC 

ATTN AMSRD-ARL-SE-DE 

ATTN AMSRD-ARL-SE-DP 

ATTN AMSRD-ARL-SE-DS 

ATTN AMSRD-ARL-SE-E 

ATTN AMSRD-ARL-SE-EE 

ATTN AMSRD-ARL-SE-EI 

ATTN AMSRD-ARL-SE-EI U LEE 

ATTN AMSRD-ARL-SE-EM 

ATTN AMSRD-ARL-SE-EM 

F SEMENDY (10 COPIES) 

ATTN AMSRD-ARL-SE-EO 

11
US ARMY RSRCH LAB (cont’d) 

ATTN AMSRD-ARL-SE-L 

ATTN AMSRD-ARL-SE-R 

ATTN AMSRD-ARL-SE-RE 

ATTN AMSRD-ARL-SE-RL 

ATTN AMSRD-ARL-SE-RM 

ATTN AMSRD-ARL-SE-RU 

ATTN AMSRD-ARL-SE-S 

ATTN AMSRD-ARL-SE-SA 

ATTN AMSRD-ARL-SE-SE 

ATTN AMSRD-ARL-SE-SG 

ATTN AMSRD-ARL-SE-SH 

ATTN AMSRD-ARL-SE-SS 

ATTN IMNE-ALC-IMS MAIL & 

RECORDS MGMT 

ADELPHI MD 20783-1197 

12