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Kelvin probe force microscopy – An appropriate tool for the electrical characterisation of LED heterostructures
Microelectronics Reliability 46 (2006) 1736–1740 www.elsevier.com/locate/microrel
Kelvin Probe Force Microscopy – An appropriate tool for the electrical characterisation of LED heterostructures W. Bergbauera,b, T. Lutzb*, W. Frammelsbergera, G. Benstettera** a
Department of Microelectronic, University of Applied Sciences Deggendorf, Edlmairstr. 6+8, 94469 Deggendorf, Germany b OSRAM Opto Semiconductors GmbH, Leibnizstrasse 4, 93055 Regensburg, Germany
Abstract Light Emitting Diodes (LEDs) are commercially important devices in opto-semiconductor industry. The light emitting properties of LEDs degrade with time of operation and may lead to device failure. Even though the stability and reliability of LEDs are important topics, they are not well researched with AFM to date. This work demonstrates that Kelvin Probe Force Microscopy (KPFM) is an appropriate method to identify specific sites of increased degradation in a semiconductor heterostructure. Furthermore, the study shows that KPFM provides the metrological basis for further investigations with respect to the progress of degradation and its physical background. In this study, KPFM has been used to measure the potential gradient over cross-sectioned LED heterostructure in operation at different states of degradation. The results show significant differences between new and aged LEDs, markedly at specific layers of the LED heterostructure.
1. Introduction LEDs consist of heterostructures of different semiconductor materials. The degradation of these structures is a well known problem and has been researched in many papers with common methods up to now [1,2,3]. Degradation and device failure of LEDs are frequently associated with decreased forward bias and reduced light emission due to non-radiative recombination [1-5]. The decrease of the forward bias is not a homogenous process over the active area of an LED, but takes place at specific layers or heterointerfaces [5]. Even though this phenomenon may in part be observed by conventional measurement methods (Fig. 1), its cause and its physical background *
**
[email protected] [email protected]
0026-2714/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.microrel.2006.07.064
may not be assessed by this means. The reason is that, the characterisation of LED hetrostructures requires a lateral resolution in the sub-micrometer range, which is not offered by conventional methods. In general, Atomic Force Microscopy (AFM) based techniques are able to address this problem [58]. Scanning Spreading Resistance Microscopy (SSRM) measurements have shown that the degradation of LEDs may be associated with differences in the carrier distribution within certain heterostructures [5]. Kelvin Probe Force Microscopy (KPFM) is an AFM method that enables to measure the electrostatic surface potential with a lateral resolution in the nanometer range [6-8]. The electrostatic potential over a cross-sectioned LED heterostructure may provide important information concerning the electrical characteristics of single LED structures.
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740
In this research degradation mechanisms of AlGaInP LEDs are investigated, which are the most common solid state lighting products in the wavelength range between red and yellow. The structure consists of an undoped (AlxGa1-x)0.5In0.5P multiple quantum well active layer, which is embedded between doped Al0.5In0.5P confinement layers. On the top of the structure a GaP current spreading layer is grown. An n–doped AlGaAs Bragg reflector between GaAs substrate and the LED structure is integrated. The chips are 200 μm by 200 μm and were mounted on TO18 or in a TOPLED package before preparation.
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Table 1 Used probe tips for KPFM measurements Material
CoCr
PtIr
Type fRes (kHz) Force constant (N/m) Tip radius (nm)
MFM 75 2.8 40-70
EFM 75 2.8 <25
The tip – sample distance was held constant at about 5 nm above the surface to reduce surface influences to the measured signal. The force to the tip was nullified by varying the voltage of the tip equal to the surface potential by using a feedback loop [10,11]. The probe tips used are shown in Table 1. Measurements were performed with Electric Force Microscopy (EFM) tips because of the better topography images. The surface potential was almost equal with both tips. 2.2. Sample preparation and mounting
Fig. 1: Increased forward current (If) for a certain forward bias (Uf) and reduced light emission for an aged and an unaged LED. As may be seen for the aged LED, the severity of degradation is not distributed homogenously over the device area.
The LEDs were embedded in epoxy and electrically contacted. After waiting for 24 hours the LED was cross sectioned by a grind and polishing machine down to the active layer. To minimize topography influences the LEDs were polished until the roughness of the surface was below 5 nm After the preparation the samples were mounted on the chuck to minimize drift effects due to the mounting system.
The electrical characteristics of the reference and the aged diode were both measured with an analyzer and showing significant differences in the low current range (Figure 1). 2. Experimental 2.1. AFM equipment The AFM measurements were performed with a Digital Instruments Dimension D 3100 Scanning Probe Microscope [9], equipped with a NanoScope IIIa controller, operating in normal atmospheric environment. KPFM measurements were performed in sub sequent steps: Each line was scanned twice, firstly to determine the topography of the surface and secondly to determine the surface potential in constant distance of the measured topography, the so called Lift Mode.
Fig. 2: Topography image of a well prepared LED surface. The Distributed Bragg Reflector could be seen in the topography image due to oxidation and material characteristics
1738
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740
a)
b)
c)
Fig. 3: a) Topopraphy image versus the potential image b) with grounded pins and c) in normal forward operation at 4 μA. At the right side is a 3D potential image of the same potential measurements. 3. Measurements
velocities. A lift scan height of about 0 nm to 5 nm led to the best potential images.
Due to the huge voltage difference between the aged and the reference diode in the low current range, the surface potential was measured at 4 μA. To obtain the external applied voltage over the cross sectioned LED layers each measurement was made twice, firstly in normal forward operation and secondly in short circuit (Figure 3). Thus the measured potential was normalised to the work function and the energy of the space charge region of the MIS structure between tip and sample [6,12]. The potential measurements were performed at different localisations at the layer cross section, especially at the chip edges. Each measurement had 64 potential lines which were averaged to one potential line. To identify differences much more accurately between the aged and the reference diode it’s very useful to calculate the electrical field strength E which is the derivation of the potential Φ. E = - grad Φ
(1)
The modulated AC voltage (drive amplitude) in the interleave mode was set up between 2000 mV and 4000mV depending on each single tip characteristic. The tip velocity was kept below 10 μm/s because of an increased noise in the measured signal at higher scan
4. Results Voltage differences between the aged and the reference diode could be shown very effectively with KPFM. External voltage alterations ΔV are also visible in the difference between the potentials (Figure 1, Figure 4). In this case of degradation internal voltage alterations near the interface between confinement and active area are getting visible very obvious (Figure 4). Due to the extraordinary DBR topography (Figure 2, Figure 6), it is possible to superpose the KPFM image exactly with the image of the Secondary Electron Microscope (SEM). In proximity to the electrical field strength it is possible to determine alterations in the potential more precisely (Figure 5). In this visualisation mode it is also possible to observe the disappearance of the electrical field which is the disappearance of the potential over this layer. Generation of non radiative centres in InGaAlP layers or heterointerfaces could decrease both light output efficiency and forward voltage in low current range. The localisation of the pn junction of the LED which is at the maximum of the electrical field could also be determined. A lateral resolution down to 50 nm is reachable in
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740
a)
1739
b)
Fig. 6: Superposition of an a) SEM image with the b) KPFM signal due to the extraordinary topography of the reflector
Fig. 4: Superposition of an SEM image with the KPFM signal of the aged and the reference diode. Voltage alterations are visible near the confinement – active region interface
The tip geometry is a fundamental factor which must be regarded [13]. Fluctuations up to 30% between different tips were observed. For an objective comparison between an aged and a reference diode it is necessary to use the same tip. Because of low tip wear in the intermitted contact mode during the topography measurement it is possible to measure more than 20 potential maps without a visible loss of resolution. The location of the junction is also visible with different tips.
5. Summary In conclusion KPFM is an appropriate tool to determine internal voltage fluctuations. Due to the extraordinary topography of the Distributed Bragg Reflector in high optical power LEDs it is possible to superpose KPFM and SEM image in the nm range. It was shown that resolutions down to 50 nm in the lateral direction and 50 mV in the voltage range are possible.
References Fig. 5: Superposition of an SEM image with the derivation of the KPFM signal of the aged and the reference diode. In the electrical field a decreased voltage drop is getting visible very easily
normal atmospheric environment. In addition a relative voltage resolution down to 50 mV was shown in the measurements. Because of the topography signal in the SPM image and the material contrast in the SEM both images could be superimposed precisely (Figure 6). Due to an increasing of the temperature in active forward operation over 4 mA the sample is thermally expanding in the nm range. This drift in z direction is a problem especially in Lift Mode. But after waiting for a balanced temperature distribution the measurement is also possible in the high current range.
[1] O. Pursiainen, N. Linder, A. Jaeger, R. Oberschmid, K. Streubel: „Identification of aging mechanisms in the optical and electrical characteristics of light-emitting diodes“, Applied Physics Letters 79(18) (2001), 28952897. [2] T. Yanagisawa: “The degradation of GaAlAs red lightemitting diodes under continuous and low-speed pulse operation“, Microelectronics Reliability 38 (1998), 16271630. [3] P. Altieri, T. Lutz, A. Jaeger, P. Stauss, K. Streubel:“ Influence of doping on the reliability of AlGaInP LEDs“, Applied Physics Letters, submitted for publication, 2005 [4] X. A. Cao, J. M. Teetsov, M. P. D'Evelyn, D. W. Merfeld: „Electrical characteristics of InGaN/GaN lightemitting diodes grown on GaN and sapphire substrates“, Applied Physics Letter 85(1) (2004), 7-9. [5] K. Katayama, T. Nakamura: “Degradation mechanism
1740
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740 limiting the lifetime of ZnSe-based white light-emitting diodes”, Journal of Applied Physics 95(7) (2004), 35763581. [6] U. E. Behnke: „Dynamische Spannungsmessungen an Submikrometerleitungen mittels der elektrischen Kraftmikroskopie“, Dissertation, Gerhard-MercatorUniversität - Gesamthochschule Duisburg, 2002 [7] M. Nonnenmacher, M. P. O’Boyle, H. K. Wickramasinghe: “Kelvin probe microscopy“, Applied Physics Letter 58(25) (1991), 2921-2923. [8] H. O. Jacobs, P. Leuchtmann, O. J. Homan, A. Stemmer: “Resolution and contrast in Kelvin probe force microscopy“, Journal of Applied Physics 84(3) (1998), 1168-1173 [9] Veeco 2005. “Dimension 3100 Scanning Probe Microscope (SPM)” Online. Available from: http://www.veeco.com/html/datasheet\_d3100.asp [Accessed 9 September 2005] [10] F.M. Serry, K. Kjoller, J.T. Thornton, R.J. Tench, D. Cook “Electric Force Microscopy, Surface Potential Imaging and Surface Electric Modification Application Note AN27” Available from www.veeco.com [Accessed from 17.05.2006], Digital Instruments Veeco Metrology Group, 2004 [11]Bharan Bushan, Anton V. Goldade “Kelvin probe microscopy measurements of surface potential change under wear at low loads”. Elsevier, WEAR, Wear 244 (2000) 104-117 [12]S. Hudlet, M. Saint Jean, B. Roulet, J.Berger and C. Guthmann “Electrostatic forces between a metallic tip and semiconductor surfaces” Journal de Physique I, France 4 (1994) 1725 - 1742 [13]H.O. Jacobs, P. Leuchtmann, O.J. Homan, A. Stemmer “Resolution and contrast in Kelvin probe force microscopy”. Journal of Applied Physics, Volume 84, Number 3, August 1998
Kelvin Probe Force Microscopy – An appropriate tool for the electrical characterisation of LED heterostructures W. Bergbauera,b, T. Lutzb*, W. Frammelsbergera, G. Benstettera** a
Department of Microelectronic, University of Applied Sciences Deggendorf, Edlmairstr. 6+8, 94469 Deggendorf, Germany b OSRAM Opto Semiconductors GmbH, Leibnizstrasse 4, 93055 Regensburg, Germany
Abstract Light Emitting Diodes (LEDs) are commercially important devices in opto-semiconductor industry. The light emitting properties of LEDs degrade with time of operation and may lead to device failure. Even though the stability and reliability of LEDs are important topics, they are not well researched with AFM to date. This work demonstrates that Kelvin Probe Force Microscopy (KPFM) is an appropriate method to identify specific sites of increased degradation in a semiconductor heterostructure. Furthermore, the study shows that KPFM provides the metrological basis for further investigations with respect to the progress of degradation and its physical background. In this study, KPFM has been used to measure the potential gradient over cross-sectioned LED heterostructure in operation at different states of degradation. The results show significant differences between new and aged LEDs, markedly at specific layers of the LED heterostructure.
1. Introduction LEDs consist of heterostructures of different semiconductor materials. The degradation of these structures is a well known problem and has been researched in many papers with common methods up to now [1,2,3]. Degradation and device failure of LEDs are frequently associated with decreased forward bias and reduced light emission due to non-radiative recombination [1-5]. The decrease of the forward bias is not a homogenous process over the active area of an LED, but takes place at specific layers or heterointerfaces [5]. Even though this phenomenon may in part be observed by conventional measurement methods (Fig. 1), its cause and its physical background *
**
[email protected] [email protected]
0026-2714/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.microrel.2006.07.064
may not be assessed by this means. The reason is that, the characterisation of LED hetrostructures requires a lateral resolution in the sub-micrometer range, which is not offered by conventional methods. In general, Atomic Force Microscopy (AFM) based techniques are able to address this problem [58]. Scanning Spreading Resistance Microscopy (SSRM) measurements have shown that the degradation of LEDs may be associated with differences in the carrier distribution within certain heterostructures [5]. Kelvin Probe Force Microscopy (KPFM) is an AFM method that enables to measure the electrostatic surface potential with a lateral resolution in the nanometer range [6-8]. The electrostatic potential over a cross-sectioned LED heterostructure may provide important information concerning the electrical characteristics of single LED structures.
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740
In this research degradation mechanisms of AlGaInP LEDs are investigated, which are the most common solid state lighting products in the wavelength range between red and yellow. The structure consists of an undoped (AlxGa1-x)0.5In0.5P multiple quantum well active layer, which is embedded between doped Al0.5In0.5P confinement layers. On the top of the structure a GaP current spreading layer is grown. An n–doped AlGaAs Bragg reflector between GaAs substrate and the LED structure is integrated. The chips are 200 μm by 200 μm and were mounted on TO18 or in a TOPLED package before preparation.
1737
Table 1 Used probe tips for KPFM measurements Material
CoCr
PtIr
Type fRes (kHz) Force constant (N/m) Tip radius (nm)
MFM 75 2.8 40-70
EFM 75 2.8 <25
The tip – sample distance was held constant at about 5 nm above the surface to reduce surface influences to the measured signal. The force to the tip was nullified by varying the voltage of the tip equal to the surface potential by using a feedback loop [10,11]. The probe tips used are shown in Table 1. Measurements were performed with Electric Force Microscopy (EFM) tips because of the better topography images. The surface potential was almost equal with both tips. 2.2. Sample preparation and mounting
Fig. 1: Increased forward current (If) for a certain forward bias (Uf) and reduced light emission for an aged and an unaged LED. As may be seen for the aged LED, the severity of degradation is not distributed homogenously over the device area.
The LEDs were embedded in epoxy and electrically contacted. After waiting for 24 hours the LED was cross sectioned by a grind and polishing machine down to the active layer. To minimize topography influences the LEDs were polished until the roughness of the surface was below 5 nm After the preparation the samples were mounted on the chuck to minimize drift effects due to the mounting system.
The electrical characteristics of the reference and the aged diode were both measured with an analyzer and showing significant differences in the low current range (Figure 1). 2. Experimental 2.1. AFM equipment The AFM measurements were performed with a Digital Instruments Dimension D 3100 Scanning Probe Microscope [9], equipped with a NanoScope IIIa controller, operating in normal atmospheric environment. KPFM measurements were performed in sub sequent steps: Each line was scanned twice, firstly to determine the topography of the surface and secondly to determine the surface potential in constant distance of the measured topography, the so called Lift Mode.
Fig. 2: Topography image of a well prepared LED surface. The Distributed Bragg Reflector could be seen in the topography image due to oxidation and material characteristics
1738
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740
a)
b)
c)
Fig. 3: a) Topopraphy image versus the potential image b) with grounded pins and c) in normal forward operation at 4 μA. At the right side is a 3D potential image of the same potential measurements. 3. Measurements
velocities. A lift scan height of about 0 nm to 5 nm led to the best potential images.
Due to the huge voltage difference between the aged and the reference diode in the low current range, the surface potential was measured at 4 μA. To obtain the external applied voltage over the cross sectioned LED layers each measurement was made twice, firstly in normal forward operation and secondly in short circuit (Figure 3). Thus the measured potential was normalised to the work function and the energy of the space charge region of the MIS structure between tip and sample [6,12]. The potential measurements were performed at different localisations at the layer cross section, especially at the chip edges. Each measurement had 64 potential lines which were averaged to one potential line. To identify differences much more accurately between the aged and the reference diode it’s very useful to calculate the electrical field strength E which is the derivation of the potential Φ. E = - grad Φ
(1)
The modulated AC voltage (drive amplitude) in the interleave mode was set up between 2000 mV and 4000mV depending on each single tip characteristic. The tip velocity was kept below 10 μm/s because of an increased noise in the measured signal at higher scan
4. Results Voltage differences between the aged and the reference diode could be shown very effectively with KPFM. External voltage alterations ΔV are also visible in the difference between the potentials (Figure 1, Figure 4). In this case of degradation internal voltage alterations near the interface between confinement and active area are getting visible very obvious (Figure 4). Due to the extraordinary DBR topography (Figure 2, Figure 6), it is possible to superpose the KPFM image exactly with the image of the Secondary Electron Microscope (SEM). In proximity to the electrical field strength it is possible to determine alterations in the potential more precisely (Figure 5). In this visualisation mode it is also possible to observe the disappearance of the electrical field which is the disappearance of the potential over this layer. Generation of non radiative centres in InGaAlP layers or heterointerfaces could decrease both light output efficiency and forward voltage in low current range. The localisation of the pn junction of the LED which is at the maximum of the electrical field could also be determined. A lateral resolution down to 50 nm is reachable in
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740
a)
1739
b)
Fig. 6: Superposition of an a) SEM image with the b) KPFM signal due to the extraordinary topography of the reflector
Fig. 4: Superposition of an SEM image with the KPFM signal of the aged and the reference diode. Voltage alterations are visible near the confinement – active region interface
The tip geometry is a fundamental factor which must be regarded [13]. Fluctuations up to 30% between different tips were observed. For an objective comparison between an aged and a reference diode it is necessary to use the same tip. Because of low tip wear in the intermitted contact mode during the topography measurement it is possible to measure more than 20 potential maps without a visible loss of resolution. The location of the junction is also visible with different tips.
5. Summary In conclusion KPFM is an appropriate tool to determine internal voltage fluctuations. Due to the extraordinary topography of the Distributed Bragg Reflector in high optical power LEDs it is possible to superpose KPFM and SEM image in the nm range. It was shown that resolutions down to 50 nm in the lateral direction and 50 mV in the voltage range are possible.
References Fig. 5: Superposition of an SEM image with the derivation of the KPFM signal of the aged and the reference diode. In the electrical field a decreased voltage drop is getting visible very easily
normal atmospheric environment. In addition a relative voltage resolution down to 50 mV was shown in the measurements. Because of the topography signal in the SPM image and the material contrast in the SEM both images could be superimposed precisely (Figure 6). Due to an increasing of the temperature in active forward operation over 4 mA the sample is thermally expanding in the nm range. This drift in z direction is a problem especially in Lift Mode. But after waiting for a balanced temperature distribution the measurement is also possible in the high current range.
[1] O. Pursiainen, N. Linder, A. Jaeger, R. Oberschmid, K. Streubel: „Identification of aging mechanisms in the optical and electrical characteristics of light-emitting diodes“, Applied Physics Letters 79(18) (2001), 28952897. [2] T. Yanagisawa: “The degradation of GaAlAs red lightemitting diodes under continuous and low-speed pulse operation“, Microelectronics Reliability 38 (1998), 16271630. [3] P. Altieri, T. Lutz, A. Jaeger, P. Stauss, K. Streubel:“ Influence of doping on the reliability of AlGaInP LEDs“, Applied Physics Letters, submitted for publication, 2005 [4] X. A. Cao, J. M. Teetsov, M. P. D'Evelyn, D. W. Merfeld: „Electrical characteristics of InGaN/GaN lightemitting diodes grown on GaN and sapphire substrates“, Applied Physics Letter 85(1) (2004), 7-9. [5] K. Katayama, T. Nakamura: “Degradation mechanism
1740
W. Bergbauer et al. / Microelectronics Reliability 46 (2006) 1736–1740 limiting the lifetime of ZnSe-based white light-emitting diodes”, Journal of Applied Physics 95(7) (2004), 35763581. [6] U. E. Behnke: „Dynamische Spannungsmessungen an Submikrometerleitungen mittels der elektrischen Kraftmikroskopie“, Dissertation, Gerhard-MercatorUniversität - Gesamthochschule Duisburg, 2002 [7] M. Nonnenmacher, M. P. O’Boyle, H. K. Wickramasinghe: “Kelvin probe microscopy“, Applied Physics Letter 58(25) (1991), 2921-2923. [8] H. O. Jacobs, P. Leuchtmann, O. J. Homan, A. Stemmer: “Resolution and contrast in Kelvin probe force microscopy“, Journal of Applied Physics 84(3) (1998), 1168-1173 [9] Veeco 2005. “Dimension 3100 Scanning Probe Microscope (SPM)” Online. Available from: http://www.veeco.com/html/datasheet\_d3100.asp [Accessed 9 September 2005] [10] F.M. Serry, K. Kjoller, J.T. Thornton, R.J. Tench, D. Cook “Electric Force Microscopy, Surface Potential Imaging and Surface Electric Modification Application Note AN27” Available from www.veeco.com [Accessed from 17.05.2006], Digital Instruments Veeco Metrology Group, 2004 [11]Bharan Bushan, Anton V. Goldade “Kelvin probe microscopy measurements of surface potential change under wear at low loads”. Elsevier, WEAR, Wear 244 (2000) 104-117 [12]S. Hudlet, M. Saint Jean, B. Roulet, J.Berger and C. Guthmann “Electrostatic forces between a metallic tip and semiconductor surfaces” Journal de Physique I, France 4 (1994) 1725 - 1742 [13]H.O. Jacobs, P. Leuchtmann, O.J. Homan, A. Stemmer “Resolution and contrast in Kelvin probe force microscopy”. Journal of Applied Physics, Volume 84, Number 3, August 1998