MOCVD多晶ZnO的生长

ABSTRACT: A growth model for the low pressure chemical vapor deposition (LPCVD) of polvcrystalline Zu0 thin films isproposed. This modelis based on experimental observations of the surface morphology and crystallographic orientations of theavers at different thicknesses and growth temperatures. It is shown that the films preferred orientation evolves from C-axis to aaxis as the growth temperature is increased from 110 to 220  and then goes back to c-axis at 380 C. At the same time. when thefilm thickness increases. the surface morphology evolves from small rounded grains to large pyramids at a growth temperatureof 150 °C. The selection of various preferential orientations under different deposition conditions is attributed to growthcompetition between clusters initially formed with different crystallographic orientations.

Introduction

Because of its wide bandgap and high exciton bindingenergy, zinc oxide (ZnO) has attracted much attention in thepast years, notably for opto-electronic applications such asUV diodes andlasing devices operating at room temperature.Yet another important prospect for ZnO is its use as atransparent conductive oxide (TCO) for thin film (TF) solarcells applications.2 Regarding the ZnO growth, it can besynthesized by various techniques such as metal organicchemical vapor deposition (MOCVD), sputtering," molecu-or even sol-gel methods.6 Forlar beam epitaxy (MBE)photovoltaic applications, one of the preferred depositiontechniques is the low pressure chemical vapor deposition(LPCVD). When used as front TCO electrodes, such LPCVDgrown layers lead to high currents and efficiencies for amor-phous.'microcrystalline solar cells and a high matched cur-rent can be obtained in tandem micromorph cells.8,9 Indeed ZnO layers grown with this technique, under given growthconditions, are polycrystalline films constituted oflarge grainswith a pronounced preferential orientation (PO) along the a-axis loThis leads to natural staircase-faced pyramids at thetop of the film, which gives to LPCVD grown ZnO good lightscattering ability, a prerequisite for TF solar cell applicationsIndeed, because of the relatively small values of the amorphous and microcrystalline silicon (Si) absorption coefficients, the optical path of the light inside the active layers ofTF Si photovoltaic cells has to be increased to enhance thephotogenerated current.! This can be achieved by introdu.cing rough interfaces that induce a light trapping effect withinthe device. Another important parameter that qualifies anefficient TCO is a low resistivity, which has also been provento be related to the size of the ZnO grains.12 Therefore, theunderstanding and control ofZnO growth is of major interestto develop efficient layers to be used in photovoltaic cells.

The different phenomena that might govern polycrystallinegrowth ofZnO have already been investigated in ref 13, whichis mainly based on results from sputtering experiments. However, little light has been shed up to now on the specificgrowing mechanisms of the LPCVD ZnO used in photovoltaic applications. In this letter, such a growth mechanism willbe proposed on the basis of observations of ZnO growth ofthin (40 nm) and thick (above 1.65 um) layers at low (110 C)intermediate (150C), and high (220 and 380 °C) growthtemperatures (Tgrowth).

Experimental Details

ZnO films were deposited by LPCVD process on 4 x 4 cm% 0.5 mmthick AF45 Schott glass substrates. Before deposition, the substrateswere chemically cleaned with acid and base in ultrasonic bathsDiethylzinc (DEZ) and water (H,O) vapors were used as precursorsand their flows were set to 16.2 and 32 sccm, respectively. Diborane(B,H6) was used as doping gas, diluted at 1% in argon. The totalpressure was kept at 0.5 mbar (~0.37 Torr)inside the reactor and thegrowth temperature was varied from 110 to 380 °C. The differentsamples were characterized with AFM and scanning electron micro-scopy to study the surface morphology, whereas XRD was used todetermine the crystallographic orientations present at the differentgrowth stages of the films.

Results and Discussion

It is well-known that several mechanisms, such as prefer-ential nucleation and growth rate anisotropy of differentcrystal planes, can explain the apparition of texture in poly-crystalline films. It has also been shown that the origin oftextured surface is to be found either in the initial (nucleation)stage or in the growth stage.13 In this work, the evolution ofthe crystallographic orientation with deposited thickness offilms grown at 110 °C is first investigated. In Figure la, thelogro of the X-ray diffraction (XRD) intensity of LPCVDZnO films with various thicknesses is plotted and eachspectrum has been normalized to the maximum of the(0002) peak. Note that we have used the logarithm of theintensities in order to be able to identify even smaller featuresIt can be seen that for both thin and thick (1.7 and 3.5 um)layers, the PO growth is along c-axis.In addition to the (0002peak, several lower intensity features, such as (1011) and(1010), can also be observed in the thicker layer spectra. Thesepeaks are ascribed to non c-axis oriented grains, which are