Characteristics of III-Nitride photodiodes with self-assembled quantum dots

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Characteristics of III-nitride photodiodes with self-assembled quantum dots

Liang-Wen Ji

a

, Te-Hua Fang

b,

⁎

, Sheng-Joue Young

c

, Chi-Chung Liu

a

, Yin-Lai Chai

d

a

Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan

b

Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan

c

Institute of Microelectronics, National Cheng Kung University, Tainan 701, Taiwan

d

Department of Resources Engineering, Dahan Institute of Technology, Hualien 971, Taiwan Received 5 March 2006; accepted 22 July 2006

Available online 11 August 2006

Abstract

In this work, it has been demonstrated that metal–semiconductor–metal (MSM) photodiodes (PDs) with InGaN self-assembled quantum dots (QDs) were fabricated and compared with conventional InGaN MSM photodiodes. The scanning near-field optical microscope (SNOM) results revealed that such InGaN nanostructures could have better absorption for the near-field light with the wavelength of 457–514 nm. It was found that the InGaN QD photodiode with lower dark current can operate in the normal incidence mode; we could achieve a much larger photocurrent to dark current contrast ratio from MSM photodiodes with nanoscale InGaN quantum dots. It was also found that the measured responsivity of MSM photodiodes with QDs and without QDs approximated to the same in the range of 390–460 nm. Furthermore, the photodiodes with QDs showed higher spectral response than that of the photodiodes without QDs at wavelengthsb350 nm and N480 nm.

Keywords: Photodiodes; Quantum dots; Scanning near-field optical microscope (SNOM); Self-assembled

Recently, low-dimensional carrier confinement nanostructures such as quantum wires and quantum dots (QDs) are quite attractive for the application of optoelectronic devices. Laser diodes, light-emitting diodes and photodiodes with nanostructures show many particular properties and reveal novel physics[1]. So far, there have been many literatures on GaAs-based or Si-based nano-structure devices[2].

Nitride nanostructures can be self-assembled using the strain-induced Stranski–Krastanov (S–K) growth mode without any sub-strate patterning process[3]. It has also been shown that nitride nanostructures can be self-assembled using growth interruption during the metal-organic chemical vapor deposition (MOCVD)[4]. Although the size fluctuations of self-assembled quantum dots (SAQDs) resulted in inhomogeneous optical and electrical charac-teristics, the self-assembly of strain-induced islands provides the means for creating zero-dimensional quantum structures without having to overcome the current limitations of lithography[5]. These self-assembled quantum dots could also be used to study novel device physics[6]. However, the reports on III-nitride devices with

nanostructured materials are still very rare, photodiodes (PDs) especially[7,8]. In this work, we fabricated a novel device: nitride metal–semiconductor–metal (MSM) photodiodes with InGaN na-nostructures, and characterized the fabricated MSM photodiodes.

The samples used in this study were grown on (0001)-oriented 2-inch sapphire (Al2O3) substrates in a vertical low-pressure MOCVD reactor with a high-speed rotation disk. Briefly, the gallium, indium and nitrogen sources were trimethyl-gallium (TMGa), trimethylindium (TMIn), and ammonia (NH3), respectively. Biscyclopentadienyl magnesium (CP2Mg) and disilane (Si2H6) were used as the p-type and n-type doping sources, respectively. In this work, we have prepared three samples for the MSM photodiode fabrication. Sample A with InGaN quantum dot (QD) structures (2.4-nm-thick InGaN layer) was grown by an interrupted growth method[4]. In other words, we first deposited a 1.2-nm-thick InGaN layer, stopped the growth for 12 s, and then deposited another 1.2-nm-thick InGaN layer so as to achieve a 2.4-nm nominal thickness of the InGaN layer. The 2.4-nm-thick and 100-nm-thick InGaN layers were directly grown on GaN as samples B and C, respectively. The schematic structures of the three samples are shown in Fig. 1. In brief, samples A, B and C are InGaN/GaN/sapphire (with QD

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An HP-4155B semiconductor parameter analyzer was then used to measure the current–voltage (I–V ) characteristics of these MSM PDs in the dark and under illumination. For photocurrent measurements, a gas Ar+ laser coupled with a fiber, vertically illuminating on the fabricated PDs was used as the light source. Spectral responsivity of these MSM PDs was measured using a Xe arc lamp and a calibrated monochromator as the light source. Output power of the monochromatic light was measured with a calibrated Si photodiode and then illuminated onto the front side of fabricated MSM PDs.

Fig. 2shows the cross-sectional profiles of the shear-force and SNOM for InGaN QD structures of fabricated MSM PD I. The weaker SNOM reflectional intensity appeared in the QD region. As a result, that such a near-field light (with blue-green color) could be partially absorbed by the InGaN nanostructures on the surface, resulted in lower SNOM reflectional intensity. In other words, the InGaN nanostructures were sensitive to such a near-field light in this investigation.

Fig. 3shows photocurrent to dark current contrast ratio of these three PDs. With an applied bias of 10.1 V, it was found that photocurrent to dark current contrast ratio (PC/DC) of PDs I, II and III equaled 400, 11 and 31, respectively. The large photo-current to dark photo-current contrast ratio observed from PD I could again be attributed to the formation of nanoscale InGaN SAQDs by growth interruption. It is possible that InGaN QD as a good quantum capture system will get more photogenerated carriers under such illumination (SNOM laser with the blue-green color); hence the photocurrent easily increases when an external bias (electric field) was applied onto the device. In fact, the SNOM results which the InGaN QD structures have better absorption efficiency for such near-field laser, directly confirm this viewpoint (Fig. 2). PD III could merely provide less effective photogenerated carriers under such illumination, exhibiting poor photoelectric performance. Therefore, a larger photo response from PD I with InGaN nanostructures was observed.

Fig. 4shows the spectral responsivity comparison between PD I (InGaN QD) and PD III (100-nm-thick InGaN), at biases 1 V and 2 V. It was found that the minimum responsivity occurred at around 475 and 490 nm for PDs I and III, respectively. It was also found that the measured responsivity of PDs approximated to the same in the range of 390–460 nm. In addition, we can find that the spectral response of PDs increased while the wavelength (N480 nm) increased. The difference of

spectral response between PD I and PD III occurred at wavelengthsb350 nm and N480 nm, noting that the magnitudes of responsivity measured from PD I are always larger than those measured from PD III in the wavelength range b350 nm or N480 nm. This result also agreed with the data shown inFig. 3, noting the wavelength of light source was in the range of 457– 514 nm for photocurrent measurements.

In summary, it has been demonstrated that MSM photo-diodes with InGaN SAQDs were fabricated and compared with conventional InGaN MSM photodiodes. The SNOM results revealed such InGaN nanostructures had better absorption for the near-field light with the wavelength of 457–514 nm. It was found that the InGaN QD photodiode with lower dark current could operate in the normal incidence mode; we could achieve a much larger photocurrent to dark current contrast ratio from MSM photodiodes with nanoscale InGaN SAQDs. It was also found that the measured responsivity of MSM PDs (PD I with QDs and PD III without QDs) approximated to the same in the range of 390–460 nm. Furthermore, the photodiodes with QDs showed higher spectral response than that of the photodiodes without QDs at wavelength ranges ofb350 nm and N480 nm. Acknowledgment

This work was financially supported by the National Science Council of Taiwan under project Nos. NSC 94-2215-E-150-009 and NSC 94-2215-E-150-045.

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