Fabrication of high-performance 370 nm ultraviolet light-emitting diodes

ARTICLE IN PRESS

Journal of Crystal Growth 264 (2004) 48–52

Fabrication of high-performance 370 nm ultraviolet light-emitting diodes H.X. Wanga,*, H.D. Lib, Y.B. Leec, H. Satoa, K. Yamashitaa, T. Sugaharab, S. Sakaic a Nitride Semiconductors Co., Ltd., 115-7 Itayajima, Akinokami, Seto-cho, Naruto-shi, Tokushima 771-0360, Japan Satellite Venture Business Laboratory, The University of Tokushima, 2-1, Minami-josanjima, Tokushima 770-8506, Japan c Department of Electrical and Electronic Engineering, The University of Tokushima, 2-1, Minami-josanjima, Tokushima 770-8506, Japan b

Received 30 November 2003; accepted 19 December 2003 Communicated by M. Schieber

Abstract High-performance 370 nm ultraviolet light-emitting diodes (UV-LED) with an AlGaN/InGaN single quantum well structure were fabricated on sapphire substrate by using the metal organic chemical vapor deposition (MOCVD) technique. Three group different growth conditions of undoped n-side barrier and p-side barrier were chosen to sandwich the InGaN well layer in the LED samples. By using high-Al content and symmetric composition n- and p-side barriers, the output efficiency of our UV-LED has been greatly improved. An output power of 2.5 mW at an injection current of 20 mA was achieved. r 2003 Elsevier B.V. All rights reserved. PACS: 85.60.Jb; 78.66.Fd; 78.55.Cr Keywords: A1. Characterization; A3. Metalorganic chemical vapor deposition; B1. GaN; B3. Ultraviolet light emitting diodes

1. Introduction GaN and its related materials have been widely used as candidate materials for the realization of visible or ultraviolet (UV) laser diodes (LDs) and light emission diodes (LEDs) [1]. Major developments in the GaN-related materials have recently led to the commercialization of high-brightness *Corresponding author. Present address: Collaboration of Regional Entity, Kochi Industrial Promotion Center, Nunoshida 3992-3, Kochi-City, Kochi, 781-5101, Japan. E-mail address: [email protected] (H.X. Wang).

blue, green, and ultraviolet LEDs, and to the realization of violet LDs with a lifetime of more than 10,000 h [2–5]. Recently, the UV light source is attracting considerable attention for the application to excite various kinds of fluorescent materials, and also, the application in medical equipments, communication equipments, detective sensors and other fields [3]. As demonstrated by GaN-related optical devices, many devices must take advantage of quantum wells (QW) such as GaN/AlGaN, InGaN/GaN and InGaN/AlGaN QWs. In order to optimize the devices design, it is necessary to study and understand QW structure

0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.12.030

ARTICLE IN PRESS H.X. Wang et al. / Journal of Crystal Growth 264 (2004) 48–52

effects on the device performance. Recent works on the QWs-based devices have shown that, in the case of GaN/AlGaN QWs, the optical transition easily changes from direct to spatially indirect due to the internal polarization field, resulting in poor emission [3,6]. There are also some reports on the quantum-confined Stark effect (QCES) in special designed p–i–n structure on the effect of injection current [7], and the piezoelectric-barrier heterostructure designed for efficient all-optical light modulation [8]. In addition, the QW recombination, the transition energy and emission intensity are expected to be highly influenced by strain, piezoelectric fields, and carrier localization [9]. Tight carrier confinement on the output efficiency was also reported [10]. However, there are few discussions on the barrier composition balance on the output efficiency. The emission mechanism is still not well known. In this paper, a growth of 370 nm LED with an InGaN well layer and different group undoped n- and p-side barriers was carried out by using the MOCVD technique. It was found that the light-output efficiency was seriously influenced by the balance of the barrier composition. Also, by optimizing the LED structure, an output power of 2.5 mW at an injection current of 20 mA was achieved.

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layer superlattices (SLSs). The undoped single quantum well (SQW) active layer was then grown at a temperature of 800 C, which consisted of an n-side undoped AlGaN barrier, InGaN well, and p-side undoped AlGaN barrier. The active layer was then capped with 30 periods of p-doped GaN(2.5 nm)/AlGaN(2.5 nm) SLS. Finally, a 20 nm p-doped GaN layer was deposited for the p-contact. In the active layers, both n- and p-side AlGaN barriers were grown with three group different TMA flow rates, the details of which are shown in Table 1. For simplicity, these samples were denoted as samples A, B, and C. UV-LED chips were fabricated using standard chip-processing technology, the details of which have been described elsewhere [11]. The electrical and optical characteristics of three samples were measured under the condition of a CW injection current at room temperature. In each case, the device was in the bare chip geometrical form with a size of 300  300 mm2. Electro-luminescence spectrum of the LEDs was obtained by collecting current-injection-induced emissions into a UV p-electrode SQW

p-GaN

p-AlGaN/GaN SLS

p-side Un-AlGaN InGaN well n-side Un-AlGaN

2. Experimental procedure

n-electrode

n-AlGaN/GaN SLS n-GaN

The samples were grown on (0 0 0 1) sapphire substrates by a MOCVD system with a special designed reactor. Trimethylgallium (TMGa), trimethylindium (TMIn), trimethylaluminum (TMA), and ammonia were used as the Ga, In, Al, and N precursors, respectively. Silane and biscyclopentadienyl magnesium (Cp2Mg) were used as n- and p-type dopants, respectively. Hydrogen and nitrogen were used as the carrier gas. Fig. 1 shows a schematic drawing of the sample structure. The substrates were firstly treated in hydrogen ambient at 1150 C, followed by the growth of about 25 nm thick GaN buffer layer at 500 C. Then, the temperature was increased to 1075 C to grow 2 mm undoped GaN and 1.5 mm n-doped GaN, followed by the growth of 50 periods of ndoped GaN(2.5 nm)/AlGaN(2.5 nm) strained-

Un-GaN LT buffer layer

Sapphire Substrate

Fig. 1. Schematic drawing of the structure of SQW UV-LED grown on the sapphire substrate.

Table 1 Different TMA flow rates of three groups Sample no.

A (mmol/ min)

B (mmol/ min)

C (mmol/ min)

n-Side barrier p-Side barrier

2.6 2.6

2.6 4.3

4.3 4.3

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fiber placed near the top of the planar device. Similarly, the UV output power was measured by a silicon detector placed in immediate proximity to the top of the LED.

3. Results and discussion Fig. 2 shows the I2L characteristics of LEDs of samples A, B, and C, which have the SQW active layer with different barrier compositions. When both side barriers were grown with the TMA flow rate of 2.6 mmol/min, the output power of sample A saturates even at a low injection current of 50 mA. When the n-side barrier was grown with a TMA flow rate of 2.6 mmol/min, and that of the p-side was 4.3 mmol/min, the output power of sample B shows a very small value with the injection current below 20 mA. Then sample B shows almost linear I2L characteristics with the injection current above 20 mA. When the injection current was above 50 mA, the output power of sample B was higher than that of sample A. When both side barriers were all grown with a TMA flow rate of 4.3 mmol/min, sample C shows a higher output power even at the low injection

current, and also addresses an almost linear I2L characteristics. In the case of sample A, the low Al content barriers have the small band offset which cannot confine a larger injection current. Therefore, injection current was very easily spilled over from the well region to the other sides, as shown in Fig. 3(a). The reason why sample B with this asymmetric barrier shows such a low output power at a low injection current is not well known. One possibility is that, while the barrier compositions are changed from symmetry to asymmetry, the leakage current would be increased. Since the n- and p-side barriers were grown with different TMA flow rates, the barrier would have a different band gap, which means that there are two different band offsets in both sides of well. In this case, the injection carriers would have different injection manners to the well. As shown in Fig. 3(b), compared with that of the case with the symmetry barriers, only a certain part of the holes can be injected into the well, and the other part of the holes directly flows to the other side of the barrier. Therefore, the net injection ratio of holes to the well would be smaller, i.e., compared with the case of SQW LED with symmetric barrier, only a certain part of the injection hole can be injected into well of SQW LED with asymmetric barrier, leading to a decrease in the light emission efficiency. With increase in the injection current,

Output Power (a.u.)

(c)

(b) (a)

(a) 0

20

40

60

80

(b)

(c)

100

Injection Current (mA) Fig. 2. I2L characteristics of UV-LED samples with a SQW active layer with different compositions of undoped n- and pside barriers. (a) An InGaN well layer sandwiched by low Alcontent symmetry composition barriers, (b) an InGaN well layer sandwiched by a low Al-content n-side barrier and a high Al-content p-side barrier, (c) an InGaN well sandwiched by a high Al-content symmetry composition barrier.

Fig. 3. Schematic band diagrams. (a) Low Al-content and composition symmetry n- and p-side barriers; (b) low Alcontent n-side and high Al-content p-side composition asymmetry barriers; (c) high Al-content and composition symmetry n- and p-side barriers.

ARTICLE IN PRESS H.X. Wang et al. / Journal of Crystal Growth 264 (2004) 48–52

Intensity (a.u.)

the net carrier in the well will be increased, resulting in an increase of output power at a low speed. In addition, since the p-side barrier is grown with a TMA flow rate of 4.3 mmol/min, it shows a better electron block function which can decrease the electron overflow to the p-layer, leading to an enhancement of injection ratio of carriers to the well. Therefore, the light emission efficiency of sample B increases very faster with increase in the injection current to above 20 mA, and the output power of sample B shows a higher valve than that of sample A when the injection current is above 50 mA. Therefore, low Al-content composition symmetric barriers shows a high carrier confinement at low injection current, and a large carrier spillover at large injection current. Composition asymmetry barriers indicate a large leakage at low current, but a higher performance at a large injection current. In the case of sample C, the higher performance could be contributed to the larger band-offset to decrease the carriers’ spillover, and also to the symmetric composition barrier to increase the carrier confinement, leading to an enhancement of the light emission efficiency, as shown in Fig. 3(c). Compared with the cases of samples A and B, it could be assumed that higher Al content and composition symmetry undoped n- and p-side balanced composition AlGaN barriers in our

300

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SQW LED structure are very important to obtain a high performance. I2L measurement of sample C was carried out at room temperature under a bare-chip geometry, that is, without any coating, mirror, or molding. The output power is 2.5 mW at an injection current of 20 mA. Fig. 4 shows the electro-luminescence (EL) spectra of sample C at the injection current of 20 mA. A strong emission peak can been observed at the wavelength of about 370 nm with a fullwidth at half-maximum (FWHM) of approximately 14 nm.

4. Conclusion In this paper, a high-performance UV-LED with an emission wavelength has been successfully fabricated. Our data indicate that low Al-content composition symmetric barriers show a higher confinement at a low injection current, and a large carrier spillover at a large injection current. The composition asymmetric barrier addresses a large leakage at low injection current, but a higher performance at a large injection current. High Alcontent and composition symmetric undoped nand p-side AlGaN barriers in our SQW UV-LED structure are very important to enable emission with a high output power. The optimized sample presents an output power of 2.5 mW at an injection current of 20 mA.

References

400

500

600

Wavelength (nm) Fig. 4. Electro-luminescence spectrum of sample C with a FWHM of 14 nm at a 20 mA injection current at room temperature.

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