Thermal Conductivity of Bulk ZnO after Different Thermal Treatments

Thermal conductivities (??) of melt-grown mass ZnO samples thermally treated below different conditions were measured using scanning thermal microscopy. Samples annealed in air at 1050?°C for 3 h and treated with N-plasma at 750?°C for 1 min. exhibited ?? = 135 ?± 008 W/cm-K and ?? = 147 ?± 008 W/cm-K respectively. These are the highest values reported for ZnO. Atomic force microscopy (AFM) and conductive-AFM measurements revealed that surface carrier concentration as well as surface morphology affected the thermal conductivity.

Key words: ZnO, thermal conductivity, atomic force microscopy (AFM), conductive AFM

INTRODUCTION

The semiconductor ZnO has lately gained substantial interest in the research community, firing materialed and fanned by its scenes in optoelectronics applications owing to its direct wide bandgap (E^sub g^ ~ 33 eV at 300 K) and large exciton binding might (60 meV).1 Some optoelectronic applications of ZnO overlap with that of GaN, another wide bandgap semiconductor (E^sub g^ ~ 34 eV at 300 K) that is widely used for production of virid blue-ultraviolet, and white light emitting devices.2 However, ZnO has a certain number of advantages over GaN, among which are the availability of fairly high-quality ZnO dimensions single crystals and a abundant simpler crystal growth technology, resulting in a potentially lower require to be paid [i]or[/i] undergone for ZnO-based devices. ZnO is also considered as a promising candidate for GaN epitaxy mainly to be paid to the fact that the lattice mismatch between GaN and ZnO is real small compared to that with the greatest in quantity commonly used substrate, sapphire. Despite having a relatively small lattice mismatch with GaN, SiC is a actual expensive substrate to produce, and owed in part to its different stacking order from that of GaN, structural deficiencys such as inversion domain boundaries and stacking mismatch boundaries become inevitable. Using the wurtzite manner of making ZnO as a substrate is wait fored to avoid such structural deficiencys Besides the crystal structure, the thermal mismatch between the GaN and ZnO is also relatively small, which is about half of that between GaN and SiC. to be paid to the lack of large area and affordable GaN volume substrates, ZnO remains the single isomorphic substrate for GaN epitaxy. Device design also can be relatively simple because ZnO substrates are usually conductive, allowing vertical electrical injection in potential vertical devices. greatest in quantity of the applications for the above-mentioned wide bandgap materials are related to high power electronic and optoelectronic devices; therefore, it is essential to understand the thermal characteristics. It has been shown that using different surface treatments, flat ZnO surfaces can be prepared for GaN epitaxy.

To date, there have been real limited studies of thermal conductivity (??) in ZnO.3-7 greatest in quantity of the works were mainly upon powder ZnO, which reported true low thermal conductivities at sweep temperature.3-6 Later on, availability of better quality volume ZnO samples led to larger values for thermal conductivity for the greatest part due to improved sample quality.7 Here, we report upon the effects of different thermal treatments upon the thermal conductivity ?? of several volume ZnO (0,001) samples grown by dint of Cermet, Inc.8 using the fuse growth technique.

EXPERIMENTAL conduct

In this study, three high-quality volume ZnO single-crystal samples of wurtzite form produced by Cermet, Inc. (Atlanta, GA) were investigated after different forms of thermal treatments. Each single of these samples (Zn-face polished) was make subordinateed to one of the following treatments: (1) air annealing at 1050?°C for 3 h in a ceramic furnace, (2) forming gas (FG) annealing (5-10% H^sub 2^ and 90-95% N^sub 2^) at 600?°C for 10 min. in a conventional quartz tubular furnace, and (3) N-plasma treatment at 750?°C for 30 min. in an ultra-high-vacuum chemical vapor deposition reactor. Another sample (O-face polished), which was not make subordinateed to any treatment, was used as mastery

These samples were previously characterized through low-temperature photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy9 In all cases, the post-treatment was observ to strengthen the intrinsic excitonic features significantly to be paid to the improvement of crystal quality at the surface. The FG-annealed sample showed the highest intensity PL (4 times higher optical efficiency compared to as received) and narrower excitonic peaks with the narrowest linewidth (07 meV) main donor-bound exciton emission. Biexponential fits to TRPL data also revealed that compared to the as-received sample (170 ?± 2 p and 864 ?± 15 ps) the decay times increased single after FG annealing (359 ?± 9 p and 2470 ?± 256 ps)9 N-plasma did not show much of an effect (232 ?± 4 p and 785 ?± 24 ps) while air annealing reduc the decay times (141 ?± 1 p and 428 ?± 30 ps) The springs are summarized in Table I.

Atomic force microscopy (AFM) measurements were performed upon the samples to analyze the results of different surface treatments. These measurements were then correlated with the thermal conductivity measurements performed by the agency of scanning thermal microscopy SThM, which is purported to provide nondestructive, absolute measurements with a high spatial resolution of about 2-3 ?µm10 Thermal imaging is achieved using a resistive thermal simple body incorporated at the end of a cantilever/AFM stamp feedback, where the resistive tip forms individual element of a Wheatstone bridge. on contact with the sample, the tip take care ofs to cool due to heat conduction into the sample, which is related to its thermal conductivity, ?? The bridge circuit applies a compensating voltage (U^sub out^) to maintain its target operating temperature. The feedback signal for constant resistance is a measure of the thermal conductivity of the material with which the tip is in contact; specifically, U^sup 2^^sub out^ is proportional to ?? since power dissipation is the mechanism here. Measurement of the absolute values of ?? is based upon a calibration procedure. This simply comprises calibrating the feedback signal, U^sup 2^^sub out^ for a constant thermal simple body resistance against that for samples with known conductivities of the like kind as GaSb, GaAs, InP, Si, and Al metal. The influence of the surface roughnes upon the effective thermal conductivity is of belong to For a perfectly flat surface, the contact between the probe tip (radius of curvature ~1 ?µm) and the sample surface is real small. However, for rough surfaces, the tip could impinge upon a valley- or hillock-like feature with the originate that a valley/hillock will lead to increased/decreased thermal signal with a corresponding change in the measured effective thermal conductivity. Florescu et al.7 have shown that for a height (h) variation of 20 nm 10 ?µm this difference is smooth less.