# Presentation: 164173

## Microwave Growth and Characterization of Zinc Oxide Single Crystal Microtubes for Optoelectronic Applications 

**The ZnO microtubes exhibit desired ultraviolet photoresponse characteristics. **
**(a) The ZnO microtubes have their  maximum UV photoresponse in wavelengths ranging from 300 to 350 nm, with a cut-off wavelength at 370 nm.  **
**(b) The evolution of the photocurrent of the ZnO microtubes upon 365 nm UV illumination revealed reversible switching characteristics and up to five orders of magnitude increase in the photocurrent as the UV light was turned on.**

**Microwave Growth and Characterization of ****Zinc Oxide Single Crystal Microtubes for Optoelectronic Applications**** **

**   **A novel microwave growth technique is developed for fabrication of high quality and high purity ZnO single crystal microtubes. The microwave grown ZnO microtubes are colorless, fully transparent, and of near-perfect crystallinity with unique crystalline structure. The wall thicknesses of the ZnO microtubes are typically between 0.5 and 1 m, with various cross-sectional dimensions ranging from 100 to 250 m and different lengths up to 5 mm.
-   The ZnO single crystal microtubes exhibit some excellent optoelectronic and dielectric properties such as  strong near band-edge emission at room temperature, high ultraviolet (UV) photoresponse, strong electric field emission, and extremely high piezoelectric coefficient, promising potential applications in civil and military areas.

**The growth technique developed in this research features self-encapsulated and contamination-free growing of ZnO single crystal microtubes.**** **

**Microwave grown**
**ZnO microtube**

**(a)**

**(b)**

**Notes:**
Aim of the project: 
     Wide band-gap semiconductor materials, such as GaN, SiC and AlN, have become a focal point of research in semiconductors and optoelectronics during the past decade because of an increasing need for short wavelength photonic devices and high-power, high-frequency electronic devices.  ZnO is a promising material for UV and blue light emitting and UV detection devices as in fact it has several fundamental advantages over its chief competitor GaN, such as much higher binding energy of the free exciton, more resistant to the radiation damage, and lower cost.  ZnO microtubes exhibit some excellent electronic properties similar to nanosized ZnO materials, for example, strong near-band-edge emission at room temperature, high piezoelectric coefficient, and strong electron field emission, which are not observed in ZnO bulk crystals.  Meanwhile, ZnO microtubes are much easier to assemble for device applications than nanosized ZnO materials.  The objective of this project is to develop the self-encapsulated microwave growth technique to fabricate high quality, dimension-controlled ZnO single crystal microtubes, and characterization of the physical properties of ZnO microtubes for their optoelectronic applications.

Experimental methods:
     ZnO has a high melting point of 1975C and vaporizes violently at temperatures above 1400C, which makes it very difficult to achieve high temperature crystal growth.  So far, most of approaches to growing of ZnO crystals were carried out at temperatures below 1100-1150C.  Our investigations had demonstrated that only ZnO crystal rods (with diameters from 50 nanometers to a few microns) were obtained as the growth temperatures were below 1150C; the tubular growth became predominant when the growth temperature exceeded 1300C.  A novel microwave high-temperature heating technique has been developed in this research work to grow highly crystalline ZnO microtubes which features self-encapsulate and contamination-free growth conditions that cannot be achieved by conventional growth methods.
     For the development of ZnO based optoelectronic devices, both high-conductivity n-type and p-type ZnO materials are required. The n-type ZnO is easily available even without any doping, while it has been recognized very difficult to create reproducible p-type ZnO materials.  In this research, we developed a microwave enhanced plasma deposition method to fabricate nitrogen-doped ZnO to realize p-type conductivity.  The microwave plasma can efficiently ionize the nitrogen gas to enhance the solubility of nitrogen (N) in ZnO crystals.  The ellipsometry, electron energy loss spectroscopy (EELS), and electron probe microscopic analysis (EPMA) studies on the chemical state of ZnO:N have been conducted to analyze the nitrogen doped p-type ZnO crystals.
     ZnO crystal is not a conventional dielectric material and its charge separation mechanism is based on both piezoelectricity and free carrier transport.  Thus the direct measurement and the interpretation of the piezoelectric coefficients of ZnO are challenging considering its electric properties and different physical boundary conditions.  An optical laser dilatometry method based on single beam interferometer in Michelson configuration is exploited to measure the effective longitudinal piezoelectric properties of ZnO microtubes.  

Research results:
     In this research project, ZnO microtubes prepared via the self-encapsulated microwave growth technique are grown in a hexagonal hollow tubular with a well faceted end and side surfaces.  The wall thickness of the ZnO microtubes is less than 2 , typically between 0.5-1 icron.  By adjusting microwave growth conditions such as the temperature, time and pressure, the ZnO tubes have been fabricated into different cross-sectional dimensions ranging from 100 to 250 icrons, and different length up to 5 mm.  The microwave grown ZnO microtubes exhibit some excellent physical properties similar to nanosized ZnO materials.  Under optical excitation, a strong near-band-edge emission at room-temperature was obtained at a peak wavelength of 377.8 nm with a full-width at half-maximum of 11 nm. The ZnO microtubes also exhibited highly selective UV light response with a cut-off wavelength at ~370 nm, and good electron field emission properties with an emission current density of 11 mA/cm2 at an applied field of ~20 V/m.
     Using the microwave enhanced plasma deposition method, p-type ZnO region is formed successfully with verification of I-V characteristics and Hall Effect measurement results. The EPMA analysis has determined up to 3 atom% nitrogen concentration on the top layer of the ZnO crystals. 
     We observed extremely high (order of magnitude higher than that of bulk single crystal values) effective d33  of ZnO microtubes under low frequency (1Hz) and 1V/mm electric field conditions.  Such giant d33 value however showed strong frequency dependence tapering off rapidly to the typical bulk crystal values with increasing frequency.  Several possible mechanisms may be contributing to the exceptional piezoelectric response of the ZnO microtubes, such as enhanced longitudinal effect from corner-joined shear mode contribution, charge redistribution, and resonance enhancement.  More systematic investigations are currently ongoing to understand the piezoelectric properties of those ZnO microtubes with unique hexagonally tubular textures.
     The study on the ultraviolet photodetection properties of ZnO microtubes demonstrated that that ZnO microtubes exhibited highly selective UV light response, and relatively fast response time.  The measured average responsivity value is about 6-9 A/W.  The photoresponse property of the ZnO microtubes strongly depends on the ambient atmosphere: higher peak current with slow response time in pure nitrogen or argon atmosphere, and lower peak current with fast response in pure oxygen or air atmosphere, which could be attributed to the different chemisorbed oxygen and low-conductivity depletion layer under different atmospheres.   Under zero bias, the ZnO microtubes exhibited excellent 'on-off' UV photoresponse characteristics, indicating potential applications of ZnO microtubes for solar-blind UV light detection.

Significance of the results:
     The self-encapsulate microwave growth is a novel approach for fabrication of high-purity and high-quality semiconductor crystal materials, which has several fundamental advantages over the conventional crystal growth methods.  A further development of this technology may open up new venues for high-vapor-pressure and high-melting-point semiconductor processing.  The candidates for such approach should include AlN, GaN, and SiC,  whether in rod or in tube form. 
     ZnO single crystal microtubes, synthesized by self-encapsulated microwave growth technique and reported for the first time by the PIs in this research work, possess a unique quality and geometry and may hold special promise for further discoveries.  For UV optoelectronic sensor applications, for instance, due to ZnO microtubes’ large surface area and thin cross sections, highly efficient UV detectors are very much probable.  As microlasers, it is also promising to produce single longitudinal mode lasers with longitudinal thickness naturally 1-2 micron.  In addition, the effective piezoelectric property of ZnO crystals (d33 longitudinal mode coupled with the d13 transverse or d15 shear mode) may make wideband lasing frequency tuning possible under low drive voltages.

Future plans:
     1. Further developments on p-type ZnO crystal materials including bulk crystals and microtubes to develop ZnO based UV/blue light emitting devices.
     2. Further investigation on ZnO microtube UV detectors and their applications.
     3. Optimization the self-encapsulated microwave growth technology for lager scale fabrication of ZnO microtubes and other wide bandgap semiconductor nano/micro and bulk crystal materials.

## Education and Outreach 

-  EE422 *Optical Engineering Lab* (taught by Dr. Guo). A hands-on interferometric measurement set up has been added to the official course offering in an advanced optical engineering lab for the undergraduate students of Department of Electrical Engineering.
-  PIs and the students supported by this grant are active participants of the Annual Penn State University Materials Day. More than 200 people from industries and research institutes attended the Materials Day held in Penn Stater Conference Center Hotel, April 10-11, 2007. Two posters were presented to public at this event.
-  PIs and the students supported by this grant are active participants of the Open House event hosted by Center for Optical Technologies, Commonwealth of Pennsylvania, at Lehigh University. Two posters were presented this past year at the event which showcased our research activity and capability to more than 100 visitors (Pennsylvania industries and high school students).
-  PIs and the students supported by this grant continue to host undergraduate students participating in NSF REU program by mentoring their research and hosting lab tours for EEREU group (Dr. Ruyan Guo serves as PI in EEREU program).

**Jiping Cheng & Ruyan Guo, Pennsylvania State University, DMR-0505946**

**Notes:**
EEREU: Electric Engineering Research Experience for Undergraduates