342 IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 3, MARCH 2011

Improvement of ESD Level of GaN-Based LEDsUsing Antiparallel Ga- and N-Polar Domains

in p-GaN LayerJenn-Bin Huang, Lu-Sheng Hong, and Chen-Chia Chou

Abstract—Electrostatic discharge (ESD) endurance ability ofInGaN blue light-emitting diodes is significantly improved whenantiparallel Ga- and N-polar domains coexist within the p-typeGaN-layer region. The inversion of Ga to N polarity in this region,which is verified using convergent-beam electron diffraction, wasinduced by the stress accumulated in the underlying layers. Atypical p-type GaN layer, which is composed of a periodical ar-rangement of Ga-polar domains (width, 450 nm) and antiparallelGa- and N-polar domains (width, 150 nm), improves the negativehuman-body mode ESD 4000 V pass yields to greater than 90%.

Index Terms—Antiparallel domain, electrostatic discharge(ESD), InGaN light-emitting diode (LED), p-type GaN layer.

N ITRIDES are semiconductors possessing wide and di-rect energy gaps. They have attracted much interest

as materials for use in short-wavelength optoelectronic andhigh-temperature electronic applications, such as visible light-emitting diodes (LEDs), laser diodes, and photodetectors.Because nitride-based LEDs are applied in traffic signals, back-light units, and general lighting, their reliability is an importantissue. In particular, the electrostatic discharge (ESD) stress,which induces latent damage or sudden failure in LED devices,is now one of the most important product specifications ofLEDs. The high density of inherent threading dislocations inGaN-based layers grown on lattice-mismatched and insulatingsapphire substrates makes these LEDs susceptible to the gen-eration of poor ESD properties during the device encapsulationprocess. Several attempts have been made to improve the ESDproperties of GaN-based LEDs, including combining the LEDswith internal Schottky diodes [1], internal protection diodes [2],[3], or a metal–oxide–semiconductor capacitor [4]; insertinga floating metal ring in the LEDs [5]; employing a devicebonding-pad design [6] or thicker p-GaN or p-AlGaN layers

Manuscript received November 8, 2010; accepted November 16, 2010. Dateof publication January 6, 2011; date of current version February 23, 2011.The review of this letter was arranged by Editor K.-L. Yu.

J. B. Huang is with the Graduate Institute of Engineering, NationalTaiwan University of Science and Technology, Taipei 106, Taiwan (e-mail:D9414902@mail.ntust.edu.tw).

L. S. Hong is with the Department of Chemical Engineering, NationalTaiwan University of Science and Technology, Taipei 106, Taiwan (e-mail:hongls@mail.ntust.edu.tw).

C. C. Chou is with the Department of Mechanical Engineering, NationalTaiwan University of Science and Technology, Taipei 106, Taiwan (e-mail:ccchou@mail.ntust.edu.tw).

Digital Object Identifier 10.1109/LED.2010.2095828

[7], [8]; varying the p-GaN layer growth temperature [6]; de-veloping modulation-doped AlGaN-GaN superlattice structures[9]; and varying the internal-capacitance structure of the LED[10]. We are unaware, however, of any reports describing the re-lationship between the p-layer polarity and the ESD propertiesof nitride-based LEDs.

In this letter, we achieved a significant improvement in theESD properties of an LED through the growth of an elaboratebundle of antiparallel Ga- and N-polar domains within thep-type GaN layer region. We have also developed a plausiblemodel to explain this improvement in the ESD properties.

The epitaxial layers of the InGaN LED were grown on (0001)patterned sapphire substrates in a vertical close-coupled show-erhead rotating-disk reactor (Thomas Swan Scientific Equip-ment, Cambridge, U.K.). A 30-nm-thick buffer layer was grownat 550 ◦C, and then, a 1.5-μm-thick undoped GaN layer, a5-μm-thick Si-doped GaN n-type layer, and a 10-nm-thickn-type AlGaN cladding layer were grown at 1040 ◦C. Next, a260-(sample A) or 460-nm-thick (sample B) heavily Si-dopedGaN space layer was grown at a lower temperature of 840 ◦C.Subsequently, eight pairs of InGaN/GaN multiple-quantum-well (MQW) active regions were grown, with each periodconsisting of a 3.5-nm-thick InGaN well layer (deposited at760 ◦C) and a 10-nm-thick barrier layer (deposited at 920 ◦C).Thereafter, an 18-nm-thick undoped GaN space layer, a23-nm-thick Mg-doped AlGaN cladding layer, and a 0.9-μm-thick Mg-doped GaN contact layer were sequentially grown at920 ◦C, 950 ◦C, and 930 ◦C, respectively. After the epitaxialgrowth, the LEDs were fabricated using a standard lateral-typeLED constitution (dimensions of 254 μm × 584 μm). The mi-croscopic features of the cross-section of the LED device wereinvestigated using transmission electron microscopy (TEM;Philips Tecnai G2 F20 FEI-TEM), together with a convergent-beam electron diffraction (CBED) technique, to determinethe polarity for the sample zone. The ESD characteristics ofthe device were evaluated using an ESD simulator (Wei-MinESD-800).

Fig. 1(a) and (b) present the cross-sectional bright-field TEMimages of the LEDs featuring 260- (sample A) and 460-nm-thick (sample B) space layers, respectively. In sample A,V-shaped defects appeared in the MQWs, with a density ofca. 3.96 × 108 cm−2; they originated not only from thread-ing dislocations (TDs) but also from stacking faults (SFs).Cho et al. [11] and Chen et al. [12] reported that the formationof V-shaped defects from SFs is due to the strain relaxation

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HUANG et al.: IMPROVEMENT OF ESD LEVEL OF LEDS USING Ga- AND N-POLAR DOMAINS 343

Fig. 1. Cross-sectional TEM images of LEDs featuring (a) a 260-nm-thickSi-doped space layer (sample A) and (b) a 460-nm-thick Si-doped space layer(sample B). White arrows highlight n-type AlGaN cladding layers; black arrowshighlight the coexisting p-type Ga- and N-polar GaN regions.

Fig. 2. (a) Expanded view of the image in Fig. 1(b), highlighting the interfacebetween the MQWs and the p-type layer. (b) Ga-polar CBED pattern ofthe region marked A; (c) N-polar CBED pattern of the region marked B;(d) Ga-polar CBED pattern of the region marked C.

at a high-indium content, whereas the formation of V-shapeddefects associated with TDs results from a decrease in thelocal energy. These V-shaped defects were filled, and the wafersurface flattened when we grew the p-type AlGaN and/or GaNlayers at a higher temperature (950 ◦C) [13]. In contrast, sampleB, with its thicker space layer, exhibited quite different cross-sectional characteristics. First, the V-shaped defects, includingthose originating from SFs and TDs (not shown here), werepresent at a lower density (1.68 × 108 cm−2). Second, in con-trast with the entirely planar textures of the p-type AlGaN andGaN layers in sample A, bundlelike domain structures appearedin the valleys of the saw-tooth region in sample B. Fig. 2(a)presents a magnified image of the domain structure existing atthe interface between the MQWs and the p-type layer. Verytiny V-shaped pits appeared at the bottom of the bundlelikeregion, with facet angles ranging from 45◦ to 50.7◦. A domainstructure investigation performed using the CBED [14]–[17],measured along the 〈11̄00〉 zone axis of GaN, revealed thatdomain A featured a Ga-polar structure [see Fig. 2(b)], whichhad the same polarity as that of flat domain C [see Fig. 2(d)].An inversion of the domain structure from Ga to N polarity isclearly evident in region B in the form of the reverse CBEDpatterns [see Fig. 2(c)], that is, antiparallel Ga- and N-polar

Fig. 3. HBM-ESD pass yields of samples A and B plotted as a function of theapplied ESD voltage.

domains coexist in this bundlelike p-type GaN region. Themechanism of the formation of such an antiparallel domainstructure is vague. One possibility is the relaxation of the strain.The relatively low growth temperature (840 ◦C) and heavy Sidoping concentration (1 × 1019 atoms cm−3) of the underlyingspace layer enhance the formation of V-shaped pits, which canbe attributed to the small diffusion length of the Ga atom thathinders its migration to the proper location at low temperature[7]. A high-resolution X-ray diffraction θ/2θ scan did reveal alarger Bragg angle θ position (17.373◦) shifting to the higherangle side for sample B, compared with that for sample A(17.346◦), indicating that a larger tensile stress prevails insample B [19]. Compared with sample A, with its thinner spacelayer (260 nm) and where most of the strain in the film wasrelaxed through the formation of large-scale V-shaped defects[see Fig. 1(a)], the formation of tiny-scale V-shape pits on thep-AlGaN cladding-layer surface in sample B [see Fig. 2(a)]presumably resulted from the necessity to release more strainresulting from the thicker space layer. The V-shape represents amultifacet boundary, thereby enhancing the polarity inversion.

At a forward current of 20 mA, both samples exhibitedalmost the same forward bias voltage (3.1 V); the light outputpower from sample B was 7.30 mW, which is 9.3% less thanthat from sample A (7.98 mW). Under reverse biases, sampleA (sample B) exhibited reverse currents of 2.195 × 10−8 and9.382 × 10−7 A (6.466 × 10−9 and 7.595 × 10−7 A) at reversebiases of −5 and −10 V, respectively. A lesser effective MQWregion [due to the larger scale of the SF in Fig. 1(b)] and carrierconcentration may be responsible for the light output powerdrop in sample B. Nevertheless, the ESD level for the samplewith the coexistence of antiparallel Ga- and N-polar domainsin p-type GaN was greatly improved. Fig. 3 presents human-body mode (HBM) ESD measurements recorded for the LEDswith various space layer thicknesses. Sample B, with the thickerspace layer of 460 nm, provided significantly greater negativeHBM-ESD 4000 V pass yields (> 90%) presumably becauseof its antiparallel polar domains coexisting in the p-type GaN-layer region or/and the thicker space layer by which the internalcapacitance of devices is increased [10]. This improvementin ESD properties can be explained by considering surfacecharge models depicted in Fig. 4. A typical GaN film grown by

344 IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 3, MARCH 2011

Fig. 4. (a) Schematic representation of the GaN polarity under a polarization-induced fixed charge, a compensation surface charge, and an electric-fielddirection E for (upper) Ga-polar and (lower) N-polar regions. (b) Schematicrepresentation of coexisting antiparallel Ga- and N-polar domains positionedbetween typical Ga-polar p-type GaN regions, forming a model exhibitingneutralized positive and negative charges.

metal–organic chemical vapor deposition exhibits Ga polarity[e.g., see Fig. 4(a)], thereby inducing positive charges at theinterface [18]. For N polarity, all charges and directions areinverted, that is, many positive charges will accumulate atthe Ga-polar surface, potentially facilitating charge penetrationwhen the negative electric field is applied during the ESD test.On the other hand, the coexisting antiparallel Ga- and N-polardomains will neutralize their positive and negative charges, asindicated in the schematic representation in Fig. 4(b), resultingin the high ESD endurance. Another effect that introducing athicker space layer may suppress the formation of V-shaped pit-related TDs with their dislocations penetrating to the p-layerregion should not be excluded in reasoning the improvement ofthe ESD level [7]. The proposed model in Fig. 4 neverthelesshighlights a possibility of improving the ESD level throughactively introducing antiparallel Ga- and N-polar domain struc-tures to the GaN-based LEDs.

The ESD endurance ability of InGaN blue LEDs can beefficiently improved by introducing coexisting antiparallel Ga-and N-polar domains in the p-type GaN-layer region. Vary-ing the thickness of the underlying low-temperature-grown(840 ◦C) Si-doped GaN space layer allows the scale of theantiparallel domain to be controlled. A periodic array of450-nm-wide Ga-polar regions and 150-nm-wide antiparallelGa- and N-polar domain regions was induced by a 460-nm-thick space layer; this structure provided negative HBM-ESD4000 V pass yields greater than 90%. Further studies towardenhancing both the ESD properties and the brightness of theLEDs, through optimizing the dimension of the active regionwith antiparallel Ga- and N-polar domains, are now underway.

ACKNOWLEDGMENT

The authors would like to thank the SuperNova Optoelec-tronics Corporation for their equipment and technical supportand the National Science Council of Taiwan for supporting thisresearch financially (NSC 98-2221-E-011-027-MY3).

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