Effects of TD on the reverse-bias leakage current

Chapter 5 The Roles of Threading Dislocation on the Electrical Properties of

5.3.3 Effects of TD on the reverse-bias leakage current

Another important parameter of HEMT device is the reverse-bias leakage current at the gate contact. This current can be originated from both the surface and bulk material of AlGaN/GaN structure. In this work, only factors controlling the bulk component will be investigated and the surface leakage currents are assumed similar for all samples since they were grown under the same condition. Bulk leakage mechanism play the dominant role in the reverse-bias current [66] and it is believed that the threading dislocation is the major factors controlling this property, as suggested by the literatures [61, 67]. The leakage currents of

AlGaN/GaN structures were determined from the fabricated SBDs. For useful comparison, only sample A, B, C, M and N, with measurable sheet resistances and current densities, have been fabricated into SBDs. Leakage current in SBDs is flowed vertically across the AlGaN layer as in contrast to Hall measurement with electrons moving transversely in the 2DEG. The quality of AlGaN layer is, therefore, crucial under this circumstance. In the present study, crystal qualities of the AlGaN layers are too thin to be determined using HRXRD. Since they were all grown continuously on GaN without applying a growth stop, the dislocations in AlGaN are presumed to be similar to or imitate most of those from the GaN layers. Strain induced from the lattice mismatches between these two materials was believed only to bend TDs slightly in the growth direction but did not obstruct dislocation from propagating to the sample surface. The leakage currents measured at reverse-bias for AlGaN/GaN samples are shown in Fig. 5.4. For sample A, B and C, all with AlGaN thickness of 20nm, the amount of leakage current increases with increasing screw TD density. Insert of Fig. 5.4 shows the values of leakage current measured at the reverse-bias of 5V for these samples. For MBE grown GaN materials, the screw dislocations acted as leakage path for reverse-bias current as suggested by Hsu et al. [61]. Their study also revealed that the screw dislocations, which are likely to be associated with excess Ga under Ga-rich growth condition, have a relaxed core and become electrically active [63]. For sample M and N, with AlGaN thickness of 25nm, their leakage also increased with screw TD density but with much slower rate. This has

suggested that the longer leakage paths in thicker AlGaN layers have increased the resistance for leakage current.

On the other hand, the influence of edge TD density on leakage currents is not following a clear trend. At first thought, the availability of free carrier density in the 2DEG (decrease with edge TD density) seems to limit the amount of leakage current as shown in sample A, B and C (with thinner AlGaN layer). But it is not the case for sample M and N (with thicker AlGaN layer). Although the edge TD densities are much higher than the screw TD densities, the role of the latter is dominant in this case. If only sample A, B and C are compared, the slope of the insert of Fig. 5.4 is much steeper than the one in Fig. 5.5. This has suggested that the screw dislocation plays a more important role than the edge dislocation in affecting the reverse-bias leakage current of AlGaN/GaN structure.

5.4 Conclusions

The effects of different TD types on the electrical properties of AlGaN/GaN structure include electron mobility in the 2DEG and reverse-bias leakage current at the gate contact have been investigated. As the dominant type of dislocation in GaN, the edge TD is shown to affect the electron mobility in the 2DEG. They tended to trap carriers in the channel and in turn, acted as the scattering center to reduce the electron mobility. As a result, the availability of free carrier density was reduced while the channel resistance was increased with the increase of the edge TD density. On the other hand, the screw TD seemed not affecting the

electron mobility significantly but served as the major player in controlling the reverse-bias leakage current. Each of the screw TD may provide a conducting path in the AlGaN layer for the leakage current and therefore a thicker barrier layer could be used to suppress the leakage current. This is because the current resistance was increased due to the longer leakage paths in the thicker AlGaN layer. The availability of free carrier density in the 2DEG, which is a function of edge TD density, also seems to affect the leakage current for sample with thinner AlGaN layer, but the effect is relatively insignificant as compared to the effect of screw TD density.

Table 5.1FWHM of GaN (0002) and (10-12) planes prepared on buffers with different growth conditions.

Sample

AlN buffer growth temperature (^oC)

FWHM of rocking curve

(arcsec) AlGaN

thickness (nm) (0002) plane (10-12) plane

A 740 71 2404 20

B 700 85 2276 20

C 550 264 1890 20

D 800 75 2584 20

M 800 100 1460 25

N 750 233 1221 25

Fig. 5.1 AFM images of the typical as-grown (a) GaN and (b) AlGaN surfaces. Inserts show their corresponding RHEED patterns.

Fig. 5.2. Hall electron mobility of AlGaN/GaN samples as a function of edge TD density.

Insert shows the effect of screw TD density on electron mobility. Open symbols in the diagrams represent the values for sample D.

Fig. 5.3. Sheet resistance and carrier concentration of the AlGaN/GaN samples as a function of edge TD density. Open symbols represent the values for sample D.

Fig. 5.4. The reverse bias currents of AlGaN/GaN Schottky barrier diode samples. Insert shows the reverse bias current for sample A, B and C at -5V as a function of screw TD.

density.

Fig. 5.5. The reverse bias currents of AlGaN/GaN Schottky barrier diode samples as a function of sheet carrier density in the 2DEG.

Chapter 6 Conclusions

The effects of buffer layer structure on the threading dislocation (TD) density in GaN film grown on sapphire by plasma-assisted molecular beam epitaxy (PA-MBE) were investigated. For AlN buffer layer, the growth temperature has contrary effect on the screw and edge TD densities. On a relatively rough AlN surface grown at low temperature (LT), the edge TD density in GaN was reduced through the recombination and annihilation processes of the inclined TDs. However, the screw TD density was increased with the use of lower-temperature AlN because many screw TDs would be generated on the rough buffer surface. Besides, the AlN thickness also affects the TD density especially for the edge TD.

More stress and higher edge TD density were generated in the GaN film grown on the AlN with thickness other than the optimum value (15 nm).

An effective method to reduce both screw and edge TDs was also introduced. This method combined an AlN buffer layer grown at high temperature (HT) and a GaN buffer prepared at slightly N-rich condition (Ga-lean condition). The HT-AlN layer minimized the generation of screw TD while the Ga-lean GaN buffer could reduce the edge TD effectively.

The Ga-lean GaN surface was covered with truncated mesa separated by deep trenches. The inclined surfaces on the trench walls were found useful in bending the edge TD growth direction to enhance the dislocation recombination and annihilation. As a result, the edge TD

density was reduced by approximately two orders of magnitude to 2x10⁸ cm^-2. Furthermore, the Ga-lean GaN buffer was suppressed the formation of new screw TDs. The rough surface of Ga-lean buffer was then recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N₂ radicals during the MBE growth. By using a sufficient number of MEE cycles, the GaN surface could be fully recovered with atomically-flat morphology.

Finally, the roles of different TDs on the electrical properties of AlGaN/GaN structure were also studied. AlGaN/GaN samples with different defect structures and densities were prepared and measurements were taken from the same sample to study the correlative behaviors of different TDs. The edge TDs tended to trap carriers in the two dimensional electron gas (2DEG) channel to act as Coulomb scattering centers. Therefore, it reduced the carrier mobility and increased the channel resistance. On the other hand, the screw TDs played a much significant role than edge TDs in controlling the reverse-bias leakage current at the gate. Each of the screw TD may provide a conducting path in the AlGaN layer for the leakage current. The leakage current was increased with screw TD density but could be reduce by using thicker AlGaN layer. This is because the current resistance was increased due to the longer leakage paths in such layer.

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Vita

Name：Yuen-Yee Wong (黃延儀) Gender: Male

Date of Birth: 10 Aug 1975 Nationality: Malaysian

Email: [email protected]; [email protected]

Education Background

1. Doctor of Philosophy, Materials Science and Engineering, National Chiao Tung University, Taiwan (Feb 2006 – July 2011)

2. Master of Science, Applied Physics, University of Malaya, Malaysia (July 2000 – August 2003)

3. Bachelor of Science, Applied Physics, University of Malaya, Malaysia (July 1996 – September 1999)

Thesis of Doctor of Philosophy:

Epitaxial Growth of AlGaN/GaN Heterostructure by Plasma-assisted Molecular Beam Epitaxy for High Electron Mobility Transistor Applications

(以電漿輔助式分子束磊晶成長應用於高電子遷移率電晶體之氮化鋁鎵/氮化鎵異質結構)

84 Al2O3/n-InAs Metal–Oxide–Semiconductor Capacitors With Various Surface Treatments, Electron Device Letters, IEEE , vol.32, no.6, pp752-754, 2011.

2. Yuen-Yee Wong, Edward Yi Chang, Yue-Han Wu, Mantu K. Hudait, Tsung-Hsi Yang, Jet-Rung Chang, Jui-Tai Ku, Wu-Ching Chou, Chiang-Yao Chen, Jer-Shen Maa_, Yueh-Chin Lin, Dislocation reduction in GaN film using Ga-lean GaN buffer layer and migration enhanced epitaxy, Journal of Thin Solid Film, vol. 509, issue 19, pp 6208-6213, 2011.

3. Yu, H. W., Chang, E. Y., Nguyen, H. Q., Chang, J. T., Chung, C. C., Kuo, C. I., Wong, Y.

Y., Wang, W. C., Effect of substrate misorientation on the material properties of GaAs/Al0.3Ga0.7As tunnel diodes, Applied Physics Letters, vol.97, no.23, 231903, 2010.

4. Lin, Y. C. , Shie, S.-L., Shie, T.-E., Wong, Y.-Y., Chen, K.S., Chang, E.Y., Study of the formation mechanism of Cu/Ge/Pd ohmic contact to n-type InGaAs. Journal of Electronic Materials, vol. 40, issue 3, pp 289-294, 2010.

5. Hai-Dang Trinh, Edward Yi Chang, Yuen-Yee Wong, Chih-Chieh Yu, Chia-Yuan Chang, Yueh-Chin Lin, Hong-Quan Nguyen and Binh-Tinh Tran, Effects of Wet Chemical and Trimethyl Aluminum Treatments on the Interface Properties in Atomic Layer Deposition of Al2O3 on InAs. Japanese Journal of Applied Physics, 49, 111201, 2010.

6. H. D. Trinh, E. Y. Chang, P. W. Wu, Y. Y. Wong, C. T. Chang, Y. F. Hsieh, C. C. Yu, H.

Q. Nguyen, Y. C. Lin, K. L. Lin, and M. K. Hudait, The influences of surface treatment and gas annealing conditions on the inversion behaviors of the atomic-layer-deposition Al2O3 /n-In0.53Ga0.47As metal-oxide-semiconductor capacitor. Applied Physics Letter, 97, 042903, 2010.

7. Yuen-Yee Wong, Edward Yi Chang, Tsung-Hsi Yang, Jet-Rung Chang, Jui-Tai Ku, Wu-Ching Chou, Micheal Chen and Kung-Liang Lin. The Roles of Threading Dislocations on Electrical Properties of AlGaN/GaN Heterostructure Grown by MBE.

在文檔中以電漿輔助式分子束磊晶成長應用於高電子遷移率電晶體之氮化鋁鎵/氮化鎵異質結構 (頁 80-100)