GaN and InGaN Light-Emitting Diodes Grown on Sapphire

Research Article www.acsami.org

Semipolar (202̅1̅) GaN and InGaN Light-Emitting Diodes Grown on Sapphire Jie Song,*,†,‡ Joowon Choi,† Kanglin Xiong,† Yujun Xie,§,∥ Judy J. Cha,§,∥ and Jung Han† †

Department of Electrical Engineering, Yale University, New Haven, Connecticut 06520, United States Saphlux Inc., Branford, Connecticut 06405, United States § Department of Mechanical Engineering & Materials Science, Yale University, New Haven, Connecticut 06520, United States ∥ Energy Sciences Institute, Yale University, Yale West Campus, West Haven, Connecticut 06516, United States ‡

S Supporting Information *

ABSTRACT: We have demonstrated growing uniform and purely nitrogen polar semipolar (202̅1̅) GaN epilayers on 2 in. patterned sapphire substrates. The as-grown surface of (2021̅ )̅ GaN is composed of two stable facets: (1010̅ ) and (101̅1̅). A chemical mechanical polishing process was further used to planarize the surface with a final surface root-mean-square roughness of less than 1.5 nm over an area of 10 × 10 μm2. InGaN light-emitting diodes were grown on a polished (2021̅ )̅ GaN/sapphire template with an electroluminescence emission at around 490 nm. Our work exhibits the potential to produce highquality nitrogen-polar semipolar GaN templates and optoelectronic devices on large-area sapphire substrates with economical feasibility.

KEYWORDS: N-polar, semipolar GaN, MOCVD, patterned sapphire substrate, selective area growth, light-emitting diodes

■

substrates.11−13 In these reports, Ga-polar (0001) GaN was selectively grown on stripe-etched c-plane sapphire sidewalls following the established process of two-step growth of GaN.11−14 However, there are many fewer reports of growing N-polar semipolar GaN on sapphire3 because of primarily two challenges. First, because of the low surface energy of the Npolar GaN (0001̅) facet, it is very difficult to grow N-polar GaN selectively and uniformly on a patterned sapphire substrate.15 Second, it appears that the selective area growth of N-polar GaN can easily induce an inversion in the polarity, and it is very challenging to obtain single-polarity N-polar semipolar GaN over a sufficiently large area.3,16,17 In this work, we study the selective growth of N-polar GaN on a patterned sapphire substrate and demonstrate polaritypure semipolar (202̅1̅) GaN epilayers over entire 2 in. sapphire substrates. The surface of the (202̅1̅) GaN templates appears microfaceted and is made atomically flat by a chemical mechanical polishing (CMP) process. Indium gallium nitride (InGaN) LEDs are prepared on the planar (202̅1̅) GaN wafers. Our work suggests the potential for fabricating high-performance semipolar GaN-based devices on low-cost and large-area sapphire substrates.

INTRODUCTION Nonpolar and semipolar orientations of gallium nitride (GaN) have the potential to address long-standing problems due to a high polarization field in light-emitting diodes (LEDs). Such a high electrostatic field has been attributed to the cause of deleterious effects such as a reduced recombination efficiency, an increased barrier for hole injection, increased electron leakage, and Auger recombination due to increased carrier densities.1,2 Among all the semipolar and nonpolar orientations, semipolar GaN with “nitrogen-polar orientations”,3 and in particular the (2021̅ )̅ orientation, has been demonstrated to be particularly beneficial for high-efficiency LEDs and laser diodes (LDs) because of a favorable direction of the internal polarization field and surface atomic configuration.4−10 Compelling results such as low-droop LEDs4,5 and highpower LDs10 point to the need for further explorations for future lighting and display applications. However, all of these high-performance devices were realized on bulk GaN substrates produced by the cross slicing of (0001)-oriented GaN crystals prepared by either hydride vapor phase epitaxy (HVPE) or ammonothermal growth. These bulk substrates tend to be expensive, limited in dimension, of irregular shapes, and incompatible with mass-production facilities in epitaxy and processing.4−10 It is therefore highly desirable to produce highquality semipolar GaN films on large-diameter sapphire substrates. Recently, it was reported that semipolar GaN growth with gallium-polar (Ga-polar) directions such as (1122̅ ) and (202̅1) can be achieved using patterned sapphire © 2017 American Chemical Society

Received: January 25, 2017 Accepted: March 31, 2017 Published: March 31, 2017 14088

DOI: 10.1021/acsami.7b01336 ACS Appl. Mater. Interfaces 2017, 9, 14088−14092

Research Article

ACS Applied Materials & Interfaces

■

EXPERIMENTS

The process begins with metal−organic chemical vapor deposition (MOCVD) growth of a (202̅1̅)-oriented GaN layer on patterned sapphire substrates. The sapphire substrates were chosen such that the [0001] sapphire direction with respect to the substrate surface corresponds exactly to the [0001] GaN direction with respect to the (202̅1̅) GaN surface, that is, the c-plane offcut 75.09° toward the adirection [sapphire (224̅3) offcut 0.45°]. Trenches are patterned across the wafer with a period of 6 μm, with 3 μm wide stripes by photolithography, and with the reactive-ion etched to a depth of 1 μm. Except for the desired c-plane sapphire sidewall, all of the exposed surfaces are masked by dielectric SiO2, fabricated with a technique of shadow evaporation at the properly inclined angle. The schematic drawing showing the process is shown in Figure S1a−c in Supporting Information. Details about stripe-patterned sapphire substrates can be found in our previous publication.3,13 After preparation, the sapphire substrates are loaded into the MOCVD reactor chamber for growing (202̅1̅) GaN. During the MOCVD growth, trimethylgallium (TMG), trimethylaluminum (TMA), and ammonia (NH3) were used as Ga, Al, and N sources, respectively. Before the growth, a nitridation process was conducted on the sapphire substrate at 950 °C in a mixture of N2 and NH3.18 A thin AlN buffer layer was then deposited on the sapphire substrate at high temperature, followed by GaN grown at temperature, pressure, TMG flow rate, and NH3 flow rate of 1030 °C, 100 mbar, 110 nmol/min, and 1 slm (standard liters per minute) NH3, respectively. We have found that the growth temperature of the AlN buffer plays a very important role in achieving N-polarity of the GaN epilayer, and the detailed study has been published elsewhere.19 After growth, the samples were characterized by Nomarski optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The polarity of (202̅1̅) GaN was examined by wet-etching with potassium hydroxide (KOH) (4.5 M) solution at room temperature for 5 min. After the single-crystalline (202̅1̅) GaN growth on sapphire was achieved, the wafer was taken out of the reactor and planarized by a Strasbaugh CMP machine. The growth of LEDs started with about 2 μm Si-doped GaN regrown on the CMP-processed (202̅1̅) GaN/ sapphire template, followed by three pairs of undoped InGaN (3 nm)/ GaN (8 nm) multiple quantum wells (MQWs). After that, 200 nm ptype GaN was grown with a Mg doping level of about 5 × 1019 cm−3. As it was reported earlier, N2 carrier gas is preferred during the regrowth of Si-doped GaN and InGaN MQWs in order to retain smooth, facet-free surface morphology after the CMP process.13 Here, all layers in the LEDs’ structures were also grown in the N2 carrier gas. LED devices were fabricated by standard photolithography with Clbased inductively coupled plasma etching20 and with each die area of 600 × 600 μm2. Ni (20 nm)/Au (50 nm) was used as both n- and ptype contacts. Room-temperature electroluminescence (EL) measurements were performed under pulsed conditions with a duty cycle of 2% to prevent self-heating and with a current pulse width and repetition rate of 10 μs and 2 kHz, respectively.

Figure 1. Cross-sectional SEM images of (2021̅ )̅ GaN stripes under (a) low and (b) high magnifications. The 45° tilted-view SEM images of (202̅1̅) GaN stripes (c) before and (d) after KOH etching. The roughened facets after etching (false-colored with yellow) indicate a uniform nitrogen-polarity.

polarity of the GaN layer because KOH is found to be effective in etching N-polar GaN but does not etch Ga-polar GaN.22 Figure 1c shows a 45° tilted-view SEM image of the as-grown GaN stripes with smooth facets. The inclined front facets became rough with pyramids appearing after the KOH etching (they are marked with false color in Figure 1d) confirming that these facets are N-polar (0001)̅ and that we have achieved uniform growth of N-polar GaN selectively from the inclined cplane sapphire. According to crystallography, when the sidewall of the GaN stripes points into the [0001̅] direction, the normal direction of the GaN surface will be the [2021̅ ]̅ as shown in Figure 1a. An XRD 2θ/ω scan was conducted with a scan range from 30° to 90° to further confirm the crystal orientation, as shown in Figure 2a. Only two peaks corresponding to GaN (2021̅ )̅ and sapphire (2243̅ ) diffraction are visible, indicating that single (202̅1̅)-orientation GaN has been achieved with the direction parallel to the sapphire (224̅3) orientation. We further note that, by performing multiple XRD samplings from the

■

RESULTS AND DISCUSSION The selective growth of GaN stripes with a nitrogen-polar growth direction was studied by SEM. Figure 1a shows a crosssectional SEM image of the initial growth off the c-plane (0001) sidewalls of etched sapphire substrates. The observed crosssectional profile (Figure 1a,b) is distinctly different from the triangular profile in the early stage of (2021̅ ) GaN growth, which is usually bounded by two (101̅1) planes, an optional (101̅0) plane, and a N-polar (0001̅) facet.12,13,21 The GaN stripes in this study are bounded by a (0001̅) facet, two (101̅1̅) planes, a (1010̅ ) plane, a (1011̅ ) plane, and a Ga-polar (0001) plane, as marked in the zoomed-in cross-sectional SEM image shown in Figure 1b. The GaN stripes were etched in a KOH (4.5 M) solution at room temperature to further confirm the

Figure 2. (a) XRD 2θ/ω scan showing single orientation GaN has been achieved, with the (202̅1̅) plane parallel to the sapphire substrate. (b) Cross-sectional SEM images of 10 μm (202̅1̅) GaN grown on a patterned sapphire substrate. (c) XRD rocking curves of the on-axis (202̅1̅) plane with rocking axis perpendicular (red curve) and parallel (blue curve) to patterned stripes. (d) Cross-sectional TEM image under two-beam condition taken along a diffraction vector of g = ⟨101̅0⟩. 14089

DOI: 10.1021/acsami.7b01336 ACS Appl. Mater. Interfaces 2017, 9, 14088−14092

Research Article

ACS Applied Materials & Interfaces center to the edge of a 2 in. sapphire wafer, we could confirm that a polarity-pure (202̅1̅) GaN has been achieved from the selective sidewall growth over the entire wafer. After the polarity and the desired crystal orientation were confirmed, thick layers were grown on patterned sapphire substrates. Figure 2b shows the cross-sectional SEM image of a 10 μm (202̅1̅) GaN grown on the patterned sapphire substrate. Since the (202̅1̅) plane is a higher-index plane that is likely to have a higher surface energy,23 the resultant surface after the stripe coalescence grown under H2 carrier gas is stabilized by two low-index facets: (101̅0) and (101̅1̅). It is conceivable that alternative conditions may exist that can help to stabilize the (202̅1̅) plane. The crystalline quality of 10 μm thick GaN layers was also characterized by XRD. On-axis XRD rocking curves with (202̅1̅) diffraction are shown in Figure 2c, with the rocking axis perpendicular (red curve) and parallel (blue curve) to patterned stripes. The full-width-at-half-maximums (FWHMs) of (202̅1̅) with the rocking axis perpendicular and parallel to patterned stripes are 352 and 504 arcsec, respectively. A complete diffraction analysis, including many off-axis diffractions such as (0002n), (n0n0), ̅ and (1122̅ (n − 1)) for n = 1−3, shows that all of these yield rocking curve line widths below 700 arcsec. The comprehensive XRD characterizations with line widths around 0.1−0.2° exhibit that semipolar (202̅1̅) GaN, with a microstructural quality comparable to that of c-plane GaN directly grown on sapphire, has been achieved. With the combination of flexible slicing of sapphire substrates and good control of the polarity of GaN selectively grown on the c-plane sapphire sidewalls, the dream of producing device-quality, largearea semipolar GaN in arbitrary surface orientations is becoming more realistic. The microstructural quality of (202̅1̅) GaN was examined by TEM. Figure 2d shows a cross-sectional TEM image under two-beam condition taken along a diffraction vector of g = ⟨101̅0⟩. The majority of the N-polar growth region (above the trenches) exhibits a low density of defects. However, the Gapolar growth direction (above the sapphire terrace) has a high density of straight dark contrast (or lines) inclined at an angle of ∼15° with respect to the surface normal to the sapphire terrace. This contrast is attributed to the presence of basal plane stacking faults (SFs). Usually, basal plane stacking faults are generated in the N-polar GaN region over a dielectric mask during heteroepitaxy.24−26 However, the majority of SFs in our (202̅1̅) GaN grown on the patterned sapphire substrates is generated in the Ga-polar GaN region. The specific mechanism for the generation of SFs is still under investigation. As seen in Figure 2b, the “surface” of a (202̅1̅)-oriented GaN layer is composed of (101̅0) and (101̅1̅) facets. These facets need to be eliminated so that the bona fide GaN (202̅1̅) surface can be accessed and AlGaInN heterostructures can be grown at the atomistic scale on the (202̅1̅) plane. Here, the zigzagged surface of the (202̅1̅) GaN was planarized by performing a direct CMP. Figure 3a,b shows the Nomarski optical microscope images of the (202̅1̅) GaN before and after the CMP process, respectively, where approximately 4 μm of GaN was removed. As seen from Figure 3a,b, the surface of the (202̅1̅) GaN template becomes optically flat after the process of CMP. Specularly reflective 2 in. (202̅1̅) GaN on the sapphire wafer was obtained and is shown in Figure 3c. Cross-sectional SEM was used to estimate the remaining thickness of GaN after CMP, and the remaining thickness is in the range 5−6 μm across the 2 in. wafer. The surface morphology after the CMP

Figure 3. Nomarski optical microscope images of (202̅1̅) GaN grown on the patterned sapphire substrate: (a) before and (b) after the CMP process. (c) A photo of a 2 in. (202̅1̅) GaN/sapphire after the CMP process. (d) AFM image with a scanning area of 10 × 10 μm2.

process was further examined by atomic force microscopy (AFM), as shown in Figure 3d. A root-mean-square (RMS) roughness of