The Use of Micro-Raman Spectroscopy to Monitor High-Pressure High-Temperature Annealing of Ion-Implanted GaN Films
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INTRODUCTION The family of III-V nitrides (GaN, InGaN, AlGaN) has recently attracted great interest because of their wide spectrum of applications ranging from opto-electronic devices for the blue-ultraviolet spectral region [1] to high-temperature electronic devices [2]. A large number of processing steps is required for the III-V nitride device fabrication, for example, reactive ion etching (RIE) or focused ion beam (FIB) etching [3], annealing to achieve a low contact resistance to n-type and p-type GaN [4], annealing to activate the Mg-acceptors for p-doping [5]. The non-invasive monitoring of the processing of III-V nitrides is of great interest to gain good control over the III-V nitride device fabrication. In this paper, we demonstrate the use of Raman scattering to monitor non-invasively the processing of GaN, illustrated on the example of the high-pressure high-temperature annealing of ion-implanted GaN films. Ion-implantation is highly attractive for the integration of III-V nitride devices into circuits, however, introduces severe lattice damage [6]. Annealing at temperatures in excess of 1300ºC is needed to recover the crystalline quality of ion-implanted GaN films. AlN cap layers [7] or highpressure nitrogen atmospheres [8,9] have to be employed to prevent the GaN surface decomposition at such high temperatures. Raman scattering provides information on the vibrational states of GaN, which track noninvasively the crystalline quality and the stress in ion-implanted GaN films. The crystalline quality was determined from the E2 phonon
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linewidth of GaN, whilst the stress from the E2 phonon frequency [10]. We restrict this paper on the monitoring of the crystalline quality and the stress in GaN. Results on the free carrier concentration in ion-implanted GaN films monitored by Raman scattering will be reported elsewhere. High optical efficiency micro-Raman systems nowadays make the recording of Raman spectra with very short integration times possible. Fast feedback on the material properties of III-V nitride layers using Raman scattering is therefore possible.
EXPERIMENT Visible and ultraviolet (UV) micro-Raman spectra were recorded from the top surface of annealed Mg/P-implanted GaN films in backscattering Z(X,.)Z geometry, i.e., XQSRODUL]HGGHWHFWLRQZLWK
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(UV) Renishaw micro-Raman system with the 514nm- and 488nm-line of an Ar+-laser and with the 325nm-line of a HeCd-laser as excitation source, respectively. The spectral resolution of the Raman setup was 2-3cm-1 for all excitation wavelengths used. The GaN ILOPVXVHGIRUWKLVVWXG\ZHUH
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deposition (MOCVD) on sapphire (0001) substrates (commercially available - CREE). Mg- and P-ions were implanted into the GaN by a multistep process of increasing the ion energy up to 1MeV. As a result, a uniform (across the whole layer thickness) concentration of Mg and P equivalent to 2×1019cm-3 was achieved [9]. The samples were annealed at temper
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