Photoelectrochemical Oxygenation of GaN Epilayers

Journal of the Korean Physical Society, Vol. 39, December 2001, pp. S313∼S317

Photoelectrochemical Oxygenation of GaN Epilayers D. J. Fu, Sh. U. Yuldashev, Y. H. Kwon, N. H. Kim, S. H. Park and T. W. Kang∗ Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul 100-715

K. S. Chung Department of Electronic Engineering, Kyunghee University, Yongin 499-701 (Received 1 November 2000) GaN epilayers grown by molecular beam epitaxy were photoelectrochemically (PEC) oxidized in aqueous KOH solution. The PEC treated GaN layers have decreased steady-state photoresponse, enhanced D0 X photoluminescence (PL), and decreased luminescence around 3.4 eV, compared with those of the as-grown samples. The concentration of deep states determined from the light intensitydependence of the rise and decay time-constants was decreased by several times after the PEC treatment. Temperature-dependence photoconductivity (PC) measurement showed that shallow electron traps existing in the epilayers were also passivated by the PEC treatment. The decrease in the PL intensity of the 3.4 eV band for the PEC treated sample suggests that the PL band is not related to oxygen impurities. Raman measurements showed upward shifts in GaN E2 mode for the thermally oxidized samples, whereas little change was detected in PEC oxidized GaN. The passivation is found to be efficient at the initial phase of PEC treatment and agrees well with the time evolution of the photocurrent that monitors the PEC process. The results suggest PEC processing to be promising for realization of oxide/GaN interfaces with low density of defects.

I. INTRODUCTION GaN and related III-nitride materials has attracted much attention since they are promising for applications in optoelectronics in the ultraviolet-visible spectrum [1] and in high-temperature and high-power electronics [2]. The device performances depend strongly on localized states in the bandgap and also on the role of oxygen and oxides. Oxygen impurities are believed to contribute to the high background conductivity in undoped GaN. They are also related to the yellow luminescence around 2.2 eV [3] and the 3.4 eV luminescence in oxygen-doped GaN epilayers [4]. Compared with conventional III-V semiconductors, GaN surfaces have large numbers of unsaturated dangling bonds, which are active to oxygen and other impurities detrimental to device fabrication. In fabrication of metal-oxide-semiconductor structures, defects existing at the oxide/semiconductor interface is a major issue of interest, which is not well resolved even for GaAs. Pure Ga oxide deposited on GaAs by evaporation does not passivate the surface, so that complicated technologies and complex compounds such as (Ga2 O3 )1−x (Gd2 O3 )x has to be employed [5]. Little work has been done on Ga oxide deposited on GaN ∗

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epilayers. Foreign dielectrics such as SiO2 and Si3 N4 demonstrate degradation of GaN properties [6]. Several authors have studied native oxide and intentional oxidation [7–9]. While exposure of GaN surfaces to oxygen at room temperature results in a maximum of one monolayer of oxide, thermal oxidation at temperatures lower than 750 ◦ C proceeds at a rate only slightly higher than adventitious oxidation. Considerable thermal oxidation is only achievable at temperatures in excess of 900 ◦ C. At this high temperature GaN begins decompose, which may be partly responsible for the appreciable oxidation. It has been shown that high temperature oxidation introduces compressive strain in GaN. Degradation of the optical and electrical properties of the GaN epilayer is then unavoidable. On the contrary, studies on photoelectrochemical processing of GaN revealed high oxidation rates (exceeding 200 nm/h), improved bandedge PL and PC, suggesting that the GaN surface can be passivated by the chemical wet oxidation [10,11]. However, the mechanism for this passivation is not clear and to our best knowledge systematic investigations of the effect are still absent. Encouragingly, GaN-based MOS structures based on PEC oxidation was reported to demonstrate strong rectifying behavior [11]. A clear understanding of the PEC passivation would be beneficial to both electronic and optoelectronic device applications. This paper presents results of defect-related PC and PL

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Journal of the Korean Physical Society, Vol. 39, December 2001

Fig. 1. Time evolution of the etch current measured during PEC processing of GaN. The inset shows a cross section SEM image of the sample PEC treated for 2 h.

measurements for as-grown and PEC treated GaN epilayers. Passivation of deep as well as shallow states is evidenced by the extrinsic PC measurement. The treatment is found to enhance the PL peak at 3.47 eV and suppress the 3.4 eV emission, relating the latter to structural defects rather than oxygen impurities.

Fig. 2. Room-temperature photoconductivity transients of GaN epilayers. The labels 30 sec and 2 min refer to PEC oxidation time.

scattering was measured at room temperature by using a Jobin Yvon T64000 micro-Raman system with the 514 nm line of a Ar+ laser. The Raman spectra were recorded in backscattering geometry with a spectral resolution of 0.65 cm−1 .

III. RESULTS AND DISCUSSION II. EXPERIMENTAL The GaN epilayers were grown on c-plane sapphire by molecular beam epitaxy (MBE) using a Riber compact 21 s MBE system under Ga-rich conditions. An AlN buffer was deposited at 600 ◦ C prior to the epilayer growth. The epilayers were nominally undoped, 2 µm thick, and with a Hall concentration in range of 1016 1017 cm−3 and mobility of 43-74 cm2 V−1 s−1 . The GaN epilayers were degreased in TCE, cleaned in acetone and methanol, and rinsed in de-ionized water. Next they were etched in HCl:H2 O = 1:1 for 3 min. A bi-layer of 30 nm Au/60 nm Ti was evaporated on each sample and patterned by metal masking. The contacts are 2×0.9 mm2 in area and separated by an inter-spacing of 0.9 mm. The samples were annealed at 900 ◦ C for 30 s to improve electrical contact and adhesion between the metal and GaN epilayer. The PEC treatment was performed in KOH with pH=11-13 by clipping the sample to a Teflon base using an Au/Ni coated glass slide. UV illumination was provided by a 10 W Hg lamp. The extrinsic PC transient signals were generated by using a 2.9 eV blue GaN/SiC LED. A pulse generator was employed for control of the light intensity. A dc voltage of 8 V was applied to the sample through a 66 kΩ resistor from which the PC output was measured and recorded by a computer. The sample temperature was controlled by using a liquid nitrogen cryostat. The PL measurement was carried out at 60 K using a 75 cm monochromator with an RCA 31034 photomultiplier tube. The excitation source was the 325 nm line of a He-Cd laser. Raman

Fig. 1 shows the photocurrent that monitors the PEC process. The current decreases rapidly within a few minutes of oxidation, followed by a further but slower decrease and saturation. The inset shows a cross section scanning electron microscopy (SEM) image of a sample PEC treated for 2 hours. One sees an overgrown layer which forms a clear interface with the GaN epilayer. The composition of the over layer was determined by X-ray photoelectron spectroscopy (XPS) to be predominantly Ga and oxygen. From the SEM image we estimate the oxidation rate to be 200 nm/h. PEC etching of GaN is known to proceed via photo carrier-assisted oxidation of GaN followed by dissolution of the oxide. Both oxidation and dissolution consume OH− and contribute to the etch current, but the two contributions are different during different stages of PEC process. The success in oxide growth on GaN suggests a limited dissolution rate and that the rapid decrease in photocurrent is a result of reduction in oxidation rate. At the beginning of PEC treatment, the GaN surface is exposed to a large number of OH+ , oxidation proceeds most efficiently. Consequently a thin oxide layer is formed and this layer poses a kinetic barrier for diffusion of OH+ through the oxide to the GaN surface, slowing down the oxidation process. The saturation of photocurrent corresponds to a balance between the oxidative reaction and dissolution processes. From Fig. 1 we believe the GaN surface to be effectively covered by an oxide layer within a few minutes. Fig. 2 shows roomtemperature extrinsic PC transients of the as-grown and

Photoelectrochemical Oxygenation of GaN Epilayers – D. J. Fu et al.

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Table 1. Physical parameters of deep level states for the as-grown and PEC oxidized GaN epilayers. PEC time (min) As-grown 0.5 2

Photoionization cross-section (cm2 ) 4.77 × 10−15 4.39 × 10−15 5.0 × 10−15

Concentration of states (cm−3 ) 3.91 × 1014 1.91 × 1014 7.91 × 1013

treated GaN layers. A dramatic decrease (30 mV) in the photoresponse is observed for the sample after a 30 sec treatment. Further decreases of about 10 mV are observed after longer-time treatments, but the decreases are not so quick as the 30-sec treated sample. This corresponds well to the slow-down of oxidation rate. As the excitation energy from the blue LED is lower than the GaN bandgap, the room-temperature PC is attributed to electron excitation from localized deep levels within the gap. We also measured the PC transients using a 2.2 eV yellow GaAsP LED, and much smaller PC responses were observed. Therefore the PC transient is attributed to compensated acceptors. The rise and decay timeconstants, τr and τd , were determined from exponential fitting of the PC transient. Under the low excitationlevel approximation, the photoionization cross-section, q, and concentration of deep levels for all the samples are obtained by plotting the reciprocal rise and decay times as functions of light intensity, I, and fitting the data with 1/τr − 1/τd = qI and ∆σ = qIm0 eµτr . The conductivity, ∆σ, is determined by ∆σ = ∆G S1 , where ∆G, l, and S are the photoconductance, inter-contact spacing, and area of the contact pad. The electron concentration at the deep level states in dark, m0 , is a measure of the concentration of deep states. The results obtained are presented in Table 1, from which we see decreases of the concentration by 2 and 5 times after the samples were treated for 30 sec and 2 minutes, respectively. However, the 5 min treated sample shows a concentration

Fig. 3. Logarithmic PC magnitude as a function of reciprocal temperature for the as-grown GaN epilayer and samples PEC treated for 30 sec and 2 min, respectively.

Relaxation time (s) 6.4 × 10−3 6.0 × 10−3 3.2 × 10−3

Thermal activation energy (eV) 0.21 0.19 0.16

higher than that of the 2 min treated sample. This is due to deviations from the low excitation-level approximation which assumes the density of photo-generated carriers to be much lower than the concentration of defect states. Fig. 3 shows the temperature dependence of the PC amplitude of decay curves. In the high temperature range of 218-360 K the PC amplitudes of all the samples decrease as the temperature increases. Exponential fitting of the data yields a thermal activation energy of about 0.2 eV for the deep levels, as summarized in Table 1. In general the PC amplitude is controlled by two factors, the concentration of deep levels and relaxation time, and is proportional to the number of electrons excited to the conduction band. When there exist shallow electron traps near the conduction band, part of the photo excited electrons will be trapped and not contribute to PC. From Fig. 3 we see that the PC magnitude of as-grown sample increases with temperature in the lower temperature range. This is indicative of the existence of shallow levels. These levels trap electrons but do not serve as recombination centers. Therefore they are activated at higher temperatures and no longer influence the PC amplitude. The 30 sec and 2 min treated samples show little change in PC amplitude in the same temperature range, indicating that the shallow traps are effectively passivated. Fig. 4 shows PL spectra for the as-grown and PEC oxidized samples. The as-grown sample shows a peak at 3.47 eV attributed to donor bound excitons (D0 X)

Fig. 4. Photoluminescence spectra of GaN epilayers at 60 K. The labels 30 sec and 2 min refer to PEC oxidation time.

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Fig. 5. Room-temperature Raman scattering spectra of PEC and thermally oxidized GaN epilayers. The labels 2 min and 1 h refer to PEC oxidation time. The thermal oxidation was performed at 850 ◦ C in air for 40 minutes.

and a broad band centered at 3.4 eV. The exact position of D0 X peak is 3.395 eV, lower reported values of 3.40-3.42, possibly caused by the high measuring temperature. This peak has a high intensity relative to the 3.47 eV peak for the as-grown sample. After the 30 sec treatment, however, the D0 X line becomes more intense than the 3.4 eV line. The sample that has been treated for 2 minutes shows a more pronounced D0 X peak and flatter 3.4 eV band. The origin of the 3.4 eV band has been addressed by several authors [4,12–14]. It was first attributed to the recombination of a valence-band hole with an electron on a deep donor due to oxygen. But this attribution contradicts the fact that oxygen donors are not deep in GaN and that the 3.4 eV band was not always observed in oxygen-doped GaN. An alternative approach was to attribute it to excitons bound to extended structural defects, e. g., a screw dislocation or stacking fault, as supported by the observation of the band in Ar implanted GaN and from the substrate side of high quality GaN. As the PEC oxidation suppresses the 3.4 eV band, correlation of the band to oxygen cannot be supported. Comparing with the PC data, we see the decreases in the intensity of the 3.4 eV line and the enhancement of the D0 X line are in good agreement with the decreases in extrinsic PC and concentration of deep states. Therefore the 3.4 eV band should be related to deep defects rather than oxygen impurities. The defects are associated with the compensated acceptors within the bandgap. The suppression of the 3.4 eV band is therefore a result of surface passivation, which also contribute to the enhancement of the D0 X line. For comparison, we conducted thermal oxidation using a different set of samples having a weaker emission at 3.4 eV and stronger D0 X line. We found that the D0 X line was degraded while the 3.4 band was enhanced as a result of thermal oxidation at 850 ◦ C for 10-60 min, opposite to the variations introduced by the PEC processing.

Journal of the Korean Physical Society, Vol. 39, December 2001

Similar enhancement of the 3.4 eV line was observed by Hayes et al and attributed to diffusion of oxygen into GaN [9]. This in-diffusion of oxygen is also responsible for the ambiguous oxide/GaN interface. Degradation of the D0 X line was also detected for SiO2 -capped GaN and attributed to oxygen atoms incorporated in GaN as nonradiative recombination centers [6]. The incorporation of oxygen strains the GaN lattice, leading to the defectrelated luminescence. This may also happen in oxygendoped GaN, though the most important source for the defect creation is ion-induced damage when oxygen doping is made by ion implantation. It is also possible that oxygen passivate some defects created during doping. In that case the 3.4 eV band would be invisible in oxygendoped GaN. Fig. 5 shows room-temperature Raman spectra for the thermally and PEC treated samples. The spectra are characterized by the GaN phonon modes of E2 (high) at 568 cm−1 , A1 (TO) at 534 cm−1 , A1 (LO) at 734 cm−1 . The peak at 750 cm−1 is due to the Eg mode of sapphire. While the A1 (LO) phonon depends on strain and free carrier concentration, the E2 frequency is known to be shifted by stress only. The E2 frequency of the asgrown sample agrees with the values reported for GaN epilayers but is higher than the standard value of 566.2 cm−1 for bulk GaN. The difference is due to strain caused by the lattice mismatch between GaN and sapphire. For annealed sample one sees a shift of the E2 mode by up to 4 cm−1 toward the high frequency, indicative of the presence of compressive strain in the annealed GaN. Samples annealed for shorter time also show upward but smaller Raman shifts. In sharp contrast, no shift of the E2 line is observed for all the PEC treated samples within the spectral resolution. In addition, the thermally oxidized sample displays a broad band around 609 cm−1 attributable to disorder-induced Raman scattering [15]. In summary we have studied the PEC oxygenation effect on GaN by the defect-related PC and PL. The PC measurement shows that deep and shallow defect states present in GaN are passivated by PEC oxidation in KOH. The concentration of deep states is considerably decreased as a result of the PEC passivation, or oxygenation. The extent of passivation characterized by PC response is related to the time evolution of the PEC process. The PEC treatment results in enhancement of the D0 X emission, while the 3.4 eV band was restrained by the treatment relating the band to structural defects. The results show the possibility for making oxide/GaN structures with low density of defects by native oxidation. ACKNOWLEDGMENTS This work was supported by the Korea Science and Technology Foundation through the QuantumFunctional Semiconductor Research Center at Dongguk University. The authors thank K. J. Kim and C. M. Lee of the Korea Research Institute of Standards and Sci-

Photoelectrochemical Oxygenation of GaN Epilayers – D. J. Fu et al.

ence and M. K. Choi of Ewha Woman’s University for characterization of the samples. [9]

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