Analysis of Contact Interfaces for Single GaN Nanowire Devices

Summary

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Abstract

Single GaN nanowire (NW) devices fabricated on SiO₂ can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO₂ interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO₂ surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO₂ substrate.

Introduction

Single-NW devices are made by dispersing a NW suspension onto an insulating substrate and forming contact pads on the substrate via conventional photolithography and metal deposition, which results in randomly formed two-terminal devices. A thick SiO₂ film on a Si wafer is typically used as an insulating substrate^1,2. For metals deposited on a SiO₂ surface, a common problem resulting from heat treatment is the occurrence of void formation at the metal/SiO₂ interface. In addition to cracking and delamination of the metal film, this void formation can negatively affect device performance from an increase in resistance caused b....

Protocol

The GaN NWs used in these experiments were grown by catalyst-free molecular beam epitaxy (MBE) on Si(111) substrates⁹. The general procedure for preparing the NW suspension from the substrate with the as-grown NWs is illustrated in Figure 1.

1. Nanowire Suspension Preparation

1. Cleave a small (<5 mm x 5 mm) piece of the as-grown NWs on the substrate.
2. Fill a small capped vial with about 1 ml of isopropanol (IPA).
3. Place the cleaved piece into the vial, close the cap and sonicate for about 30 sec in order to remove the NWs from the substrate. Once prepared, the NW suspension re....

Results

An example of SEM analysis on annealed Ni/Au films removed from the SiO₂ substrate using carbon tape is shown in Figure 4. The surface of a Ni/Au contact prior to removal is shown in Figure 4A. The underside of the same area of that particular Ni/Au film after removal is shown in Figure 4B. Comparison of the surface and underside morphology can help determine if there is a relationship between the two. For example, when the two images are compared, it can be s.......

Discussion

The technique presented allows for analysis of the contact/substrate and contact/NW microstructure of single NW devices. The main advantages of this technique are its low cost and simplicity. It allows for qualitative and quantitative analysis of the contact interface on a large scale with the substrate as well as on a submicrometer scale with individual NWs. The use of carbon tape for the film removal and SEM pin stubs for sample mounting make it possible for analysis using characterization techniques that require clean.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
REAGENTS and MATERIALS
Lift-off resist	MicroChem	LOR 5A	Varies according to application
Photoresist	Shipley	1813	Varies according to application
Developer	Rohm and Haas Electronic Materials	MF CD-26	Varies according to application
Photoresist stripper	MicroChem	Nano Remover PG	Varies according to application
Ni source	International Advanced Materials	99.999% purity
Au source	International Advanced Materials	99.999% purity
SiO2/Si wafers	Silicon Valley Microelectronics	3-inch <100> N/As 0.001-0.005 ohm-cm, 200 nm thermal oxide
Carbon tape	SPI Supplies	5072, 8 mm wide
Solvents are standard semiconductor or research grade. Vendor is not important for the experimental outcome.
Reactive ion etch gases and thermal annealing gases are high purity grade. Vendor is not important for the experimental outcome.
EQUIPMENT
Ultrasonic cleaner	Cole-Palmer	EW-08849-00	Low power
Micropipette	Rainin	PR-200	Metered, disposal tips
Reactive ion etcher	SemiGroup	RIE 1000 TP	Other vendors also used with different process parameters
Mask aligner	Karl Suss	MJB3	Other vendors also used with different process parameters
UV ozone cleaner	Jelight	Model 42	Other vendors also used with different process parameters
E-beam evaporator	CVC	SC-6000	Other vendors also used with different process parameters
* Manufacturers and product names are given solely for completeness. These specific citations neither constitute an endorsement of the product by NIST nor imply that similar products from other companies would be less suitable.