High quality Gan films are usually produced at high growth temperatures (> 1OOO°C) with the use of substrates such as sapphire (Ah03) or silicon carbide (SiC).Therefore, for a low production cost purpose, there has been a growing interest in producing lower growth temperatures Gan films as well as Gan based devices with low cost substrates such as silicon

Abdullah, M. J.

F., K. Yam

Ibrahim, K.

Y, C.Lee

Z., Hassan

English

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(1-A Comparative Study of the Electrical Characteristics of Metal-Semiconductor-Metal (MSM) Photodiodes Based on GaN Grown on SiliconYC.Lee l , Z. Hassan', F. K. Yam l , M. 1. Abdullah l, K. Ibrahim I ,lSchool of Physics, Universiti Sains Malaysia, Penang, Malaysia2Lab. of Electronic Materials Physics, Department of Physics,Institute Technology Bandung, IndonesiaE-mail: zai(a).usm.myABSTRACT: I jHigh quality GaN films are usually produced at high growth temperatures (> 1OOO°C)with the use of substrates such as sapphire (Ah03) or silicon carbide (SiC). Therefore,for a low production cost purpose, there has been a growing interest in producing lowergrowth temperatures GaN films as well as Ga based devices with low cost substratessuch as silicon. Growth of Ga onto silicon substrates offers a very attractiveopportunity to incorporate GaN devices onto silicon-based integrated circuits. Thedevelopment of device quality films grown at low temperatures could also open upopportunities for large area devices such as UV detector arrays at low cost.In this paper, we report on the electrical characterization of metal-semiconductor-metal (MSM) photodiodes fabricated on GaN grown on silicon by electron cyclotronresonance (ECR) plasma-assisted metalorganic chemical vapor deposition (PA-MOCVD)with different growth temperatures (200°C and 600°C). Electrical characterization werecarried out by using current-voltage (1-V) measurements to study the electricalcharacteristics of the fabricated device. Structural analysis of the Ga samples used forthe photodiodes fabrication were performed by using x-ray diffraction (XRD), atomicforce microscopy (AFM), scanning electron microscopy (SEM), and energy dispersive x-ray analysis (EDX) to analyze the crystalline quality of the samples used for photodiodesfabrication.PACS: 6I.I0.-i; 61.43.Dq; 68.55.-a; 73.40.SxKeywords: AI. Characterization, AI. Crystalline Structure, BI. III-V Nitrides,B3. Photodiodes.INTRODUCTIONGalium nitride (GaN), a wide and direct bandgap semiconductor is a vastly studiedsemiconductor material due to its potential as an excellent candidate for the application inoptoelectronic and high power/temperature electronic devices [1]. Great attention havebeen received in recent years for the development of visible-blind ultraviolet (UV)photodetectors based on III-V nitride which have application in both civil and militaryindustries such as engine control, flame sensing, source calibration, solar UV monitoring,UV astronomy, secure space-to-space communications, and missile plume detection [2].GaN grown on silicon (Si) based photodetectors are still very few especially ofthose grown at low temperature due to the large quantities of defect densities which isattributed to the high lattice and thermal mismatch of Si and Ga . Therefore more workhas to be done to investigate its significance for the application in device fabrication asGaN films grown on Si is expected to be an excellent candidate for the integration ofGaN-based optoelectronic devices with Si-based electronic technology [3]. Moreover the2advantage of low temperature grown films is the capability to deposit the materialinexpensively over large areas, which could open up opportunities for the development oflow cost and large-area devices such as UV detector arrays.Photodetectors in the form of metal-semiconductor-metal (MSM) structure aresubjected to keen interest among various kind of detectors due to its many attractivefeatures like the ease of fabrication, low dark current, large bandwidth, small capacitance,and the suitability for integration in an optical receiver [4].In this paper, we report on the comparative study of the electrical characteristicsof the MSM photodiodes based on ECR PA-MOCVD grown Ga at growth temperaturesof 200°C and 600°C. Structural analysis of the Ga samples used for the photodiodesfabrication was also carried out by using x-ray diffraction (XRD), atomic forcemicroscopy (AFM), scanning electron microscopy (SEM), and energy dispersive x-rayanalysis (EDX) to analyze its crystalline quality.EXPERIMENTALGaN films grown on Si(lOO) at 200°C and 600°C by ECR PA-MOCVD wereused for this study. Details of the reactor and growth technique have been describedelsewhere [5]. The two samples used for the detector fabrication have a transparent Gaepitaxial layer of about O.51lm thick and are unintentionally doped n-type. Theunintentionally doped n-GaN samples used in this study are most commonly thought tobe nitrogen vacancies [6] but until recent investigation, it has been revealed that nitrogenvacancies are high energy defects in n-type Ga [7] and their high formation energymakes it very unlikely that they would form during growth of undoped or n-type GaN,3and instead, the proposed idea in explaining the actual cause of unintentional n-typedoping are unintentional impurities such as oxygen and silicon [8].The structure of the MSM photodiodes consist of two interdigitated Schottkycontact (electrode) with fingers width of 230llm, finger spacing of 400llm, and the lengthof each electrode was about 3300 11m. It consists of 4 fingers at each electrode.X-ray diffraction (XRD) was used to assess the structure of the deposited layers.These data were collected on a Rigaku diffractometer with a wide angled automatedgoniometer and computer based data acquisition and analysis system. Scanning electronmicroscopy (SEM), atomic force microscopy (AFM), and energy dispersive x-rayanalysis (EDX) were used to observe the surface morphology and to identify the elementspresent in the samples.Nickel (Ni) Schottky contacts were deposited on the GaN samples by thermalevaporation where the pattern of metallization was defined by using a metal mask. Priorto metal contact deposition, the GaN sample which was grown at 600°C was cleaned inboiling aqua regia (HCI:HN03 = 3: 1) as a way to etch away surface contaminants andnative oxides. Then it was followed by thermal annealing at 400°C in flowing nitrogenenvironment for 10 minutes. As the existence of a thin contamination layer cannot betotally ruled out upon cleaning unless the samples are cleaved in an ultra-high vacuumcondition [9], thus annealing the Ni-GaN at 400°C in flowing nitrogen environment for10 minutes was done as a way to remove the contamination layer from the GaNfNiinterface [10] and also to reduce the leakage current, increase the Schottky barrier height,and to improve the uniformity of the barrier height [11]. For the GaN sample grown at200°C, the method of cleaning prior to contact deposition was the same but the aqua4regia solution was not at its boiling point, instead it was at room temperature. The samplewas soaked inside the aqua regia solution for about 1 minute. The reason for the slightdifferences in cleaning treatment was mainly due to the amorphous like structure of thesample grown at 2000 e, which might have a lower thermal and chemical stability.RESULTS AND DISCUSSIONSFigure I shows the x-ray diffraction spectra of GaN samples grown on Si(lOO)substrates. For the GaN sample grown at 200 0 e, the peaks at 33.4° and 69.6° correspondto Si(200) and Si (400) planes respectively, with the (400) peak intensity much higherthan the intensity of the (200) peak by a few hundred times. We suggest that the Gasamples grown at 2000 e were amorphous in nature due to the absence of a diffractionpattern which indicate a crystalline phase of GaN. However, for the Ga samples grownat 6000 e, we observed the (0002) peak for Ga at 34.6° which suggest the existence of acrystalline or microcrystalline phase due to the low intensity of this peak. X-raydiffraction peaks for GaN should be observed at around 32.3°, 34.6° and 36.8° whichcorrespond to (l0 10), (0002) and (l 011) planes of the hexagonal crystalline GaN. Withthe absence of strong and sharp GaN crystalline peaks and a broadening centered at thecrystalline region for films grown at 6000 e, we would conclude that this could be anindication of a mixed phase of crystalline and amorphous structure, and therefore, wewould suggest that this could be a sign of microcrystalline phase in the Ga samplesgrown at 6000 e.5in these GaNThe transparent GaN films have a thickness of about O.5flm and exhibit smoothsurface morphology. The measured root mean square (rms) surface rouglmess by AFM(Figure 2) on a 2x2flm2 was 1.86nm for Ga grown at 200°C and 4.95nm for GaN grownat 600°C. Here, we suggest that the formation of small grains (microcrystallites) on thesubstrate surface at this temperature was the main cause of the increase in rouglmess forthe GaN samples grown at 600°C. Finally, the presence of Ga andsamples is confirmed with EDX measurements.Figure 3 and 4 illustrates the I-V characteristics of the photodiodes based on GaNgrown at 600°C and 200°C respectively. In these graphs, a) is the dark current while b) isthe current under illumination condition (photocurrent). The amorphous GaN photodiodeshows a very small fluctuation in the dark current which is nearly constant with theapplied bias, with a magnitude of about O.18flA at lOV much smaller when compared tothe dark current characteristics of the photodiode based on Ga grown at 600°C or themicrocrystalline GaN, which has a magnitude of about l8)-lA at lOV. This behaviour ismost probably due to the resistive nature of amorphous Ga under dark condition. Theconductivity of the amorphous GaN photodiode rises drastically under illumination(white light photoresponse) without any sign of saturation commonly observed in typicalMSM photodiodes [12].The lack of saturation for the dark current and photocurrent characteristics of thephotodiode based on the microcrystalline GaN was also observed. The lack of currentsaturation in Schottky contacts is commonly attributed to various factors like image forcelowering, tunneling mechanism which is more substantial during reverse bias, generationof electron-hole pairs in the depletion region which is more pronounced at low6temperature, and the dependency of the barrier height on the electric field strength in thebarrier as a result of the existence of an interfacial layer between the metal and thesemiconductor. We still can assume that there is a negligible thickness of interfaciallayer at the interface of the microcrystalline GaNlNi Schottky contact as it has beenannealed to remove any possible interfacial/contamination layer, but we cannot ruled outthe existence of a contamination layer at the amorphous GaN since it was cleaned in aslightly different condition. Thus according to Ref. [9], the reverse current of a Schottkycontact with a fairly thick contamination layer may actually be greater than that of aSchottky contact with a very thin contamination layer. This can be observed in theamorphous Ga based photodiode under illumination when compared to themicrocrystalline-based photodiode under the same condition. A large magnitude ofleakage current can mainly be attributed to a high amount of defect densities usuallyfound in Ga materials [13 -16] especially for those grown on Si substrates due to theexistence of defect generated trap-assisted tunnel current [17] which enhances the darkcurrent. This can be seen in the drastic rise in the current magnitude for both of thesamples under illumination.In addition, the drastic nse 111 current without safuration at elevated biasing(>-5V) when illuminated in the amorphous GaN photodiode is unexpected. Usuallytypical MSM photodiodes under dark condition will normally have a flat band pattern intheir dark current characteristics, which are almost independent of the bias voltage atfurther biasing or in other words, the dark current at first rise with voltage and thenbecome saturated. When illuminated, typical photodiodes will behave the same, with aflat band pattern but at a higher magnitude of saturation current. This could be due to the7highly resistive nature of the amorphous GaN under dark condition. Thus, whenilluminated, the change in the resistivity allows electrical conduction to occur at a normalbehaviour.The other factor which is responsible for the large increase in the magnitude ofthe dark current after being illuminated, may be attributed to the contribution from the n-type Si substrate to the current transport mechanism of the Schottky contacts due to theabsence of a buffer layer between the substrate and the Ga film. Usually, structuralproperties of nitride compounds can be improved by a two-step growth method wheremonocrystalline GaN epilayers are grown on top of a thin AlN buffer/nucleation layer[18]. J. L. Pau et al has stated that the role of the Al buffer layer is not only to improvethe crystalline quality, but also to electrically insulate the epitaxial film from theconductive Si substrate [19].CO CLUSIOThe electrical characteristics of MSM photodiodes based on unintentionally doped n-typeamorphous GaN grown at 200°C and microcrystalline GaN grown at 600°C on Sisubstrates were investigated. The photodiode based on amorphous GaN tends to be veryresistive at dark condition and normal conduction can only take place with the assistanceof light illumination. The high dark current characteristics of the MSM photodiode basedon microcrystalline GaN can be attributed to factors like high defect densities present inthe Ga samples which generate a high value of trap-assisted tunnel current as well asleakage current and some contributions from the silicon substrates. The structuralanalysis of the GaN films showed that these films exhibit smooth surface morphology, as8revealed by scanning electron microscopy (SEM), and atomic force microscopy (AFM)measurements.ACKNOWLEDGEMENTThis work was conducted under AUN/SEEDNet and IRPA RMK-8 Strategic Researchgrant. The authors wish to thank M. E. Kordesch (Ohio University) and SpireCorporation for providing the samples in our study. The support from Universiti SainsMalaysia is gratefully acknowledged.REFERE CES[1] S. Strite and H. Morkoc, 1. Vac. Sci. Techno!. B 10 (1992) 1237.[2] Eva Monroy, Fernando Calle, Jose Luis Pau, Elias Munoz, Franck Omnes,Bernard Beaumont, and Pierre Gibart, Mat. Res. Soc. Symp., Vol. 622 (2000)T3.7.1.[3] Z. M. Zhao, R. L. Jiang, P. Chen, 0.1. Xi, Z. Luo, R. Zhang, B. Shen, Z. Z. Chen,and Y. D. Zheng, Appl. Phys. Lett., 74(3) (2000) 444.[4] Eva Monroy, Fernando Calle, Jose Luis Pau, Elias Munoz, Franck Omnes,Bernard Beaumont, and Pierre Gibart, Mat. Res. Soc. Symp., Vol. 622 (2000)T3.7.4.[5] Z. Hassan, M. E. Kordesch, W. M. Jadwisienzak, H. 1. Lozykowski, W.Halverson, P. C. Colter, Mat. Res. Soc. Symp. Proc. 536 (1999) 245.[6] S. Strite and H. Morkoc, 1. Vac. Sci. Technol. B 10 (1992) 1243.[7] 1. Neugebauer, C. G. Van de Walle, Phys. Rev. B 50 (1994) 8067.9[8] J. Neugebauer, C. G. Van de Walle, Proc. ICPS-22, World Scientific, Singapore,(1995) 2327.[9] E. H. Rhoderick and R. H. Williams, Metal-Semiconductor Contact, 2nd ed.(Oxford University Press, New York, 1998).[10] Hidenori Ishikawa, Setsuko Kobayashi, Y. Koide, S. Yamasaki, S. agai, 1.Umezaki, M. Koike, and Masanori Murakami, J. Appl. Phys. 81 (3) (1997) 1318.[11] T. Sawada, Y. Ito, K. Imai, K. Suzuki, H. Tomozawa, and S. Sakai, Appl. Surf.Sci. 159-160 (2000) 449.[12] Kwok K. g, Complete Guide To Semiconductor Devices, 2nd ed. (Wiley-IEEEPress, ew York, 2002) ch. 60, p. 472.[13] S. Tomiya, K. Funato, T. Asatsuma, T. Hino, S. Kijima, T. Asano, and M. Ikeda,Appl. Phys. Lett. 77 (2000) 636.[14] T. L. Tansley, E. M. Goldys, M. Godlewski, Zhou and H. Y. Zuo, in: Ga andRelated Materials, edited by Stephen J. Pearton, (Gordon and Breach Science,Netherlands, 1997) ch. 8, p. 251.[15J W. Gotz, N. M. Johnson, D. P. Bour, C. Chen, H. Liu, C. Kuo, and W. Imler,Mater. Res. Soc. Symp. Proc. 395 (1996) 443.[16J P. Hacke, T. Detchprohm, K. Hiramatsu, N. Sawaki, K. Tadatomo, and K.Miyake, J. Appl. Phys. 76 (1994) 304.[17J 1. C. Carrano, T. Li, P. A. Grudowski, C. 1. Eiting, R. D. Dupuis, and J. C.Campbell, Appl. Phys. Lett. 72(5) (1997) 542.10[18] T. L. Tansley, E. M. Goldys, M. Godlewski, Zhou and H. Y. Zuo, in: GaN andRelated Materials, edited by Stephen 1. Pearton, (Gordon and Breach Science,Netherlands, 1997) ch. 8, p. 253-255.[19] 1. L. Pau, E. Monroy, M. A. Sanchez-Garcia, E. Calleja, E. Munoz, Mater. Sci.Eng. B93 (2002) 159.11List of figure captions:Fig. 1. X-ray diffraction spectra of GaN grown on Si at (a) 200 0 e and (b) 6000 eFig. 2. Atomic force microscopy images of GaN grown on Si at (a) 200 0 e and (b) 600 0 eFig. 3. The I-V characteristics of the MSM photodiode based on the GaN/Si samplegrown at 200 0 e under (a) dark (coincident with the x-axis) and (b) illumination.Fig 4. The I-V characteristics of the MSM photodiode based on the GaN/Si samplegrown at 600°C under a) dark and b) illumination.12Set of original figures:~ r Si(200) Si(400)30 40 50 6028 (degrees)70 80Fig. 1. X-ray diffraction spectra of GaN grown on Si at (a) 2000 e and (b) 6000 e1347.0:;loml""05~""".....1.00~''',200 1. 2.00 luml Z-Max 47 02[nml(a)0.002.00 < 2.00 lum) Z-MBY. 84 .05[nm)(b)0.00Fig. 2. Atomic force microscopy images ofGaN grown on Si at (a) 200°C and (b) 600°C14b)6 8 10 12 14 16 18 20::jo +-.--,.-.,..-c;==-------,------;--=..!....----,---,--024400350300<' 250...'E 200~::lUReverse Bias MFig 3. The I-V characteristics of the MSM photodiode based on the GaN/Si samplegrown at 200°C under (a) dark (coincident with the x-axis) and (b) illumination.250200~2: 150C~ 100::lU502 4 6 8 10 12 14 16 18 20Reverse Bias (V)Fig 4. The I-V characteristics of the MSM photodiode based on the GaN/Si samplegrown at 600°C under a) dark and b) illumination.15

A comparative study of the electrical characteristics of metal-semiconductor-metal (MSM) photodiodes based on gan grown on silicon.

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A comparative study of the electrical characteristics of metal-semiconductor-metal (MSM) photodiodes based on gan grown on silicon.

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