III-Nitride Nanowire Based Near-Infrared Optoelectronic Devices on (001) Silicon

Abstract

III-nitride-based optoelectronics research has seen rapid progress since the demonstration of the first blue-emitting diode laser by Nakamura et. al. The research has so far focused mainly on devices emitting in the visible spectrum, and significant work can still be done in the near-infrared (near-IR) to infrared (IR) spectrum. Photonic devices based on the III-nitrides and operating at near-IR can have several advantages over other material systems. Such devices emitting in the near-IR can also be used in various applications including silicon-photonics, fiber optic communications, and various biomedical applications. A photonic integrated circuit consisting of a coherent light source such as a laser and detector directly on (001) silicon will be of great importance for the progress of silicon photonics. This work focuses on the III-nitride-based alloys which have operating wavelengths in the near-IR. The present work successfully demonstrates lasers, detectors, and a complete monolithic photonic integrated circuit all directly grown on (001)Si. The presented work includes some preliminary results on III-nitride-based devices operating in the infrared instead of the near-infrared.

To accomplish the above-mentioned objective, self-assembled III-nitride nanowire arrays have been epitaxially grown on (001)Si instead of using planar III-nitride layers, which cannot be directly grown on Si with good optical quality. The density and diameter of these nanowires have been varied over a wide range by tuning the growth conditions. The nanowires grow strain relaxed and are relatively free of extended defects. Due to the strain relaxation, nanowires exhibit superior properties compared to their planar counterparts such as higher radiative efficiency, and reduction of the polarization field. Strain relaxation has also enabled the In composition in the disks to be increased to 100% forming binary InN disks used as the active region. Formation of quantum dots has also been observed in similar disks which further enhance quantum confinement of the carriers. A nanowire array has been used to demonstrate the first monolithic nanowire laser on (001) silicon emitting at 1.3 micron. The nanowire arrays have also been used to demonstrate near-IR detectors on (001)Si. Both the optoelectronic devices have been fabricated on the same chip to demonstrate the operation of a photonic integrated circuit directly grown on (001)Si. Both the laser and the detector have the same nanowire heterostructure simplifying the fabrication of the photonic integrated circuits.

The active region of the disks-in-nanowire heterostructure arrays, grown using molecular beam epitaxy, is essentially InGaN with the In composition varying from ~85% to ~100%. These InN disk-based half-laser heterostructures demonstrate excellent optical and structural characteristics. Lasers have been characterized which demonstrate high optical output power levels (~7mW above threshold), a differential slope efficiency of ~12%, an ideal emission wavelength of ~1.2-1.3 micron, excellent temperature stability (T_0 > 200K) and a differential gain of ~3x10^-16cm2. The detectors show good responsivity of 0.11A/W at near-IR. The photonic integrated circuits show about a 10-fold increase in the detector photocurrent as a function of laser injection current. All photonic integrated circuit related measurements have been made at room temperature. The disks-in-nanowire heterostructures grown on (001)Si also demonstrate absorption in the mid-IR (MIR) as well as in the long wavelength IR (LWIR) wavelength regimes. Absorption peaks in the wavelength range of ~1.6 to ~20 micron have been measured at room temperature paving the path for future demonstration of long wavelength detectors using the III-nitrides.